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Scientific Reports 5 12054 2015 Kurshid
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Some of the main experimental observations related to the occurrence of exchange bias in magnetic
systems are reviewed, focusing the attention on the peculiar phenomenology associated to
nanoparticles with core/shell structure as compared to thin film bilayers. The main open questions
posed by the experimental observations are presented and contrasted to existing theories and models
for exchange bias formulated up to date. We also present results of simulations based on a
simple model of a core/shell nanoparticle in which the values of microscopic parameters such as
anisotropy and exchange constants can be tuned in the core, shell and at the interfacial regions,
offering new insight on the microscopic origin of the experimental phenomenology. Adetailed study
of the magnetic order of the interfacial spins shows compelling evidence that most of the experimentally
observed effects can be qualitatively accounted within the context of this model and allows also
to quantify the magnitude of the loop shifts in striking agreement with the macroscopic observed
values.
This article reviews the 40+ year old spin-glass field and one of its earliest model interpretations as a spin density wave. Our description is from an experimental phenomenological point of view with emphasis on new spin glass materials and their relation to topical problems and strongly correlated materials in condensed matter physics. We first simply define a spin glass (SG), give its basic ingredients and explain how the spin glasses enter into the statistical mechanics of classical phase transitions. We then consider the four basic experimental properties to solidly characterize canonical spin glass behavior and introduce the early theories and models. Here the spin density wave (SDW) concept is used to explain the difference between a short-range SDW, i.e. a SG and, in contrast, a long-range SDW, i.e. a conventional magnetic phase transition. We continue with the present state of SG, its massive computer simulations and recent proposals of chiral glasses and quantum SG. We then collect and mention the various SG 'spin-off's'. A major section uncovers the fashionable unconventional materials that display SG-like freezing and glassy ground states, such as (high temperature) superconductors, heavy fermions, intermetallics and Heuslers, pyrochlor and spinels, oxides and chalogenides and exotics, e.g. quasicrystals. Some conclusions and future directions complete the review.
A comparative study of the static and dynamic magnetic properties of polycrystalline hollow c-Fe2O3 nanoparticles with two distinctly different sizes of 10.361.3 nm and 14.860.5 nm has been performed. High-resolution TEM images confirmed the crystalline structure and the presence of the shell thickness of 2.1760.28 nm and 3.2560.24nm for the 10nm and 15 nm particles, respectively. Quantitative fits of the frequency dependent ac susceptibility to the Vogel-Fulcher model, s¼so exp[Ea/k(T�To)], show stronger inter-particle interactions in the 15 nm nanoparticles than in the 10 nm nanoparticles. A systematic analysis of the room-temperature magnetic loops using the modified Langevin function indicates a stronger effect of disordered surface spins in the 10nm
hollow particles as compared to the 15 nm hollow particles. Our study suggests that while the effect of disordered surface spins dominates the magnetic behavior of the 10 nm hollow particles, both the disordered surface spins and inter-particle interactions contribute to the magnetism of the 15nm hollow particles.
The development of positive magnetic entropy change in the case of ferromagnetic (FM) nanostructures is a rare occurrence. We observe positive magnetic entropy change in core/shell (Fe/γ-Fe2O3) and hollow (γ-Fe2O3) nanoparticles and its origin is attributed to a disordered state in the nanoparticles due to the random distribution of anisotropy axes which inhibits any long range FM ordering. The effect of the energy barrier distribution on the magnetic entropy change and its impact on the universal behavior based on rescaled entropy change curves for core/shell and hollow nanostructures is discussed. Our study emphasizes that the magnetic entropy change is an excellent parameter to study temperature and field dependent magnetic freezing in such complex nanostructures.
The aging and memory effects of Fe3O4 nanoparticles have been studied using a series of zero-field-cooled (ZFC) and field-cooled magnetization measurements at various aging protocols. The genuine ZFC magnetization after the ZFC procedure with a single stop and wait process shows an aging dip at the stop temperature on reheating. The depth of the aging dip is dependent on the wait time. The frequency dependence of the ac magnetic susceptibility is indicative of critical slowing down at a freezing temperature Tf (=30.6±1.6K) . The relaxation time tau is described by a power-law form with a dynamic critical exponent x (=8.2±1.0) and a microscopic relaxation time tau0 [=(1.33±0.05)×10-9s] . The ZFC-peak temperature decreases with increasing magnetic field (H) , forming a critical line with an exponent p=1.78±0.26 , close to the de Almeida-Thouless exponent (p=3/2) . These results indicate that the superspin-glass phase occurs below Tf .
Magnetite Fe3O4 nanoparticles of controlled size within 6 and 20nm in diameter were synthesised by thermal decomposition of an iron organic precursor in an organic medium. Particles were coated with oleic acid. For all samples studied, saturation magnetisation Ms is size-independent, and reaches a value close to that expected for bulk magnetite, in contrast to results in small particle systems for which Ms is usually much smaller due to surface spin disorder. The coercive field for the 6nm particles is in agreement with coherent rotation, taking the bulk magnetocrystalline anisotropy into account. Both results suggest that the oleic acid molecules covalently bonded to the nanoparticle surface yield a strong reduction in the surface spin disorder. However, although the saturated state may be similar, the approach to saturation is different and, in particular, the high-field differential susceptibility is one order of magnitude larger than in bulk materials. The relevance of these results in biomedical applications is discussed.
The structural, spectroscopic and magnetic properties of the two-dimensional (2D) molecule-based magnets of [Mn(II)(TCNE)(NCMe)2]X (X = PF6, AsF6, SbF6; TCNE = tetracyanoethylene, NCMe = acetonitrile) composition are reported. It is shown that the alteration of the interlayer distance by increasing the anion size has little effect on the critical magnetic ordering temperature, Tc, suggesting that it depends predominantly on the intra-plane magnetic exchange. The observed field-induced irreversibility in static magnetization, a slow decay of isothermal remanence below Tc, and the dynamic susceptibility data are in accord with a re-entrant spin-glass nature of the ground state of all materials. In contrast to the isostructural Fe-based magnets, in which strong magnetocrystalline anisotropy facilitates the finite temperature magnetic ordering with the magnetization easy axis perpendicular to the μ4-TCNE(•-) plane, in the studied Mn-based magnets the easy axis is canted away from the normal direction, due to a small magnetocrystalline anisotropy. The two magnetic transitions observed on cooling are assigned to the ferrimagnetic long-range ordering of the normal magnetization component followed by the re-entrant spin-glass type transition resulting from a random freezing of the in-plane magnetization component.
We observe an enormous spontaneous exchange bias (�300–600 Oe)—measured in an unmagnetized
state following zero-field cooling—in a nanocomposite of BiFeO3 (�94%)-Bi2Fe4O9 (�6%) over a temperature range 5–300 K. Depending on the path followed in tracing the hysteresis loop—positive (p)
or negative (n)—as well as the maximum field applied, the exchange bias (HE) varies significantly with HEp> HEn. The temperature dependence of HE is nonmonotonic. It increases, initially, till �150 K and then decreases as the blocking temperature TB is approached. All these rich features appear
to be originating from the spontaneous symmetry breaking and consequent onset of unidirectional
anisotropy driven by ‘‘superinteraction bias coupling’’ between the ferromagnetic core of Bi2Fe4O9 (of
average size �19 nm) and the canted antiferromagnetic structure of BiFeO3 (of average size �112 nm)
via superspin glass moments at the shell.
γ-Fe2O3/SiO2 core-shell nanoparticles with different shell thicknesses were prepared to elucidate the condition for superspin-glass (SSG) dynamics. As the shell thickness decreases, the contribution of interparticle dipolar interaction becomes apparent in the magnetic dynamics of nanoparticle assembly. The frequency dependence of peaks in ac-magnetic susceptibility in samples with strong interactions slows down, which is characterized as the emergence of a spin-glasslike phase. Aging in magnetization relaxation is found in a strongly interacting sample with an interparticle distance of L⩽14 nm but is scarce in a sample with L = 18 nm. Scaling analysis reveals an increase in superparamagnetic properties with an increase in L. Therefore the critical interparticle distance necessary for SSG transition is 15–18 nm with 11-nm γ-Fe2O3 nanoparticles. This corresponds to the ratio of interparticle-interaction energy to the magnetic-anisotropy energy Edip/Ea of 6–12%.
We have demonstrated a low temperature surface spin-glass layer in the ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ high-pressed nanocompacts. The related core-shell magnetic structure arises during the compaction, which is evident by the field dependence of the freezing temperature following the de Almeida\char21{}Thouless relationship and obvious exchange bias effect. Moreover, we have also found a critical cooling field, above which both the surface spin-glass behavior and the exchange bias effect abruptly disappeared.
A detailed study is presented on Fe/γ-Fe 2 O 3 core-shell structured nanoparticles (mean size ∼10 nm) to understand the spin dynamics of the core and shell independently and their role in triggering exchange bias (EB) phenomena. The particle dynamics critically slow down at T g ∼ 68 K, below which they exhibit memory effect in field-cooled and zero-field-cooled protocols associated with a superspin glass state. The field dependence of mean blocking temperature fits the de Almeida-Thouless line and shows two different linear responses in the low and high field regimes corresponding to the core and shell, respectively. We show that the energy barrier distribution estimated from the temperature decay of isothermal remanent magnetization shows two maxima that mark the freezing temperatures of the core (T f-cr ∼ 48 K) and shell (T f-sh ∼ 21 K). Last, hysteresis measurements after field cooling reveal strong EB indicated by a loop shift associated with unidirectional anisotropy. The onset of EB is at 35 K when the ferromagnetic core is frozen and the moments in the ferrimagnetic shell begin to block, resulting in enhanced exchange coupling.
We report a comparative study of the magnetic properties of polycrystalline hollow γ-Fe2O3 nanoparticles with two distinctly different average sizes of 9.2 ± 1.1 nm and 18.7 ± 1.5 nm. High-resolution transmission electron microscopy images reveal the presence of a shell with thickness of 2 nm and 4.5 nm for the 9.2 nm and 18.7 nm nanoparticles, respectively. The field-cooled hysteresis loops show interesting features of enhanced coercivity and horizontal and vertical shifts associated with the polarity of the cooling field for both types of nanoparticles. While the anomalously large horizontal shifts and open hysteresis loop in a field as high as 9 T observed for the 9.2 nm nanoparticles corresponds to a “minor loop” of the hysteresis loop, the loop shift observed for the 18.7 nm nanoparticles manifests an intrinsic “exchange bias” (EB). Relative to the 18.5 ± 3.2 nm solid nanoparticles, a much stronger EB effect is achieved in the 18.7 nm hollow nanoparticles. Our studies point to the importance of inner and outer surface spin disorder giving rise to surface anisotropy and EB and reveal a perspective of tuning EB in hollow magnetic nanoparticle systems.
Nickel ferrite nanoparticles dispersed in SiO2 matrix have been synthesized by sol-gel method. Structural analysis has been performed by using x-ray diffraction and transmission electron microscopy. Magnetic properties have been investigated by using superconducting quantum interference device magnetometry. In addition to the average blocking temperature peak at TB = 120 K measured by a zero field cooled temperature scan of the dc susceptibility, an additional hump near 15 K is observed. Temperature dependent out-of-phase ac susceptibility shows the same features: one broad peak at high temperature and a second narrow peak at low temperature. The high temperature peak corresponds to magnetic blocking of individual nanoparticles, while the low temperature peak is attributed to surface spin-glass freezing which becomes dominant for decreasing particle diameter. To prove the dynamics of the spin (dis)order in both regimes of freezing and blocking, the frequency dependent ac susceptibility is investigated under a biasing dc field. The frequency shift in the “frozen” low-temperature ac susceptibility peak is fitted to a dynamic scaling law with a critical exponent zv = 7.5, which indicates a spin-glass phase. Exchange bias is turned on at low temperature which signifies the existence of a strong core-shell interaction. Aging and memory effects are further unique fingerprints of a spin-glass freezing on the surface of isolated magnetic nanoparticles.
The nanomagnetism of monodisperse 7 nm
γ-Fe2O3
nanoparticles exhibits unique features due to a significant amount of surface spin disorder.
To correctly characterize the superparamagnetism of a dilute dispersion requires including
the effects of the magnetic anisotropy and a shell of disordered spins surrounding
the ordered core. The nanoparticle shell's disordered spin structure is exchange
coupled to that of the ordered core. This enables an exchange bias loop shift,
Hex, when the nanoparticle dispersion is field cooled. The surface spin disorder also leads to an
unusual exponential-like decrease of the nanoparticle's total saturation magnetization with
increasing temperature.
The dynamic properties of a system constituted by small interacting particles (iron grains dispersed in an amorphous alumina matrix) have been studied by AC susceptibility measurements at low frequency ( nu =2*10-2 and 2*10-3 Hz). Maxima have been found for the in-phase susceptibility chi ' (at TB) and for the out-of-phase susceptibility chi ". The frequency dependence of TB is analysed over a large frequency range, including previous results from AC susceptibility measurements (17<or= nu <or=104 Hz) and Mossbauer spectroscopy. The results are compared with (i) the predictions from a superparamagnetic model, which takes into account the effect of magnetic interactions on the relaxation time, and (ii) the dynamic scaling laws proposed for spin glasses (the Fulcher law, the generalised Arrhenius law (transition at Tc=0) and the power law (transition at Tc not=0)). The results are not compatible with a transition at a finite temperature. They are satisfactorily explained by the authors superparamagnetic model, including inter-particle interactions, which implies a transition at a temperature of 0 K.
We report on zero-field–cooled magnetization relaxation experiments on a concentrated frozen ferrofluid exhibiting a low-temperature superspin glass transition. With a method initially developed for spin glasses, we investigate the field dependence of the relaxations that take place after different aging times. We extract the typical number of correlated spins involved in the aging dynamics. This brings important insights into the dynamical correlation length and its time growth. Our results, consistent with expressions obtained for spin glasses, extend the generality of these behaviours to the class of superspin glasses. Since the typical flipping time is much larger for superspins than for atomic spins, our experiments probe a time regime much closer to that of numerical simulations.
Co-based nanostructures ranging from core/shell to hollow nanoparticles were prepared by varying the reaction time and the chemical environment during the thermal decomposition of Co(2)(CO)(8). Both structural characterization and kinetic model simulation illustrate that the diffusivities of cobalt and oxygen determine the growth ratio and the final morphology of the nanoparticles. Exchange coupling between Co and Co-oxide in core/shell nanoparticles induced a shift of field-cooled hysteresis loops that is proportional to the shell thickness, as verified by numerical studies. The increasing nanocomplexity, when passing from core/shell to hollow particles, also leads to the appearance of hysteresis above 300 K due to an enhancement of the surface anisotropy resulting from the additional spin-disordered surfaces.
The dynamics of a magnetic particle system consisting of ultrafine Fe-C particles of monodisperse nature has been investigated in a large time window, 10(-9)-10(4) s, using Mossbauer spectroscopy, ac susceptibility, and zero field cooled magnetic relaxation measurements. By studying two samples from the same dilution series, with concentrations of 5 and 6 x 10(-3) vol%, respectively, it has been found that dipole-dipole interaction increases the characteristic relaxation time of the particle system at all temperatures investigated. The results for the most concentrated particle assembly are indicative of collective magnetic dynamics and critical slowing down at a finite temperature, T-g approximate to 40 K. Close to and below the transition temperature, an aging phenomenon is observed, another manifestation of collective magnetic dynamics.
We have systematically studied the magnetic properties of ferrite nanoparticles with 3, 7, and 11 nm of diameter with very narrow grain size distributions. Samples were prepared by the thermal decomposition of Fe(acac)3 in the presence of surfactants giving nanoparticles covered by oleic acid. High resolution transmission electron microscopy (HRTEM) images and XRD diffraction patterns confirms that all samples are composed by crystalline nanoparticles with the spinel structure expected for the iron ferrite. ac and dc magnetization measurements, as well in-field Mössbauer spectroscopy, indicate that the magnetic properties of nanoparticles with 11 and 7 nm are close to those expected for a monodomain, presenting large MS (close to the magnetite bulk). Despite the crystalline structure observed in HRTEM images, the nanoparticles with 3 nm are composed by a magnetically ordered region (core) and a surface region that presents a different magnetic order and it contains about 66% of Fe atoms. The high saturation and irreversibility fields in the M(H) loops of the particles with 3 nm together with the misalignment at 120 kOe in the in-field Mössbauer spectrum of surface component indicate a high surface anisotropy for the surface atoms, which is not observed for the core. For T
Magnetization relaxation measurements show that aging occurs in a discontinuous metal-insulator multilayer (DMIM) $[{\mathrm{Co}}_{80}{\mathrm{Fe}}_{20}(0.9\mathrm{nm})/{\mathrm{Al}}_{2}{\mathrm{O}}_{3}(3\mathrm{nm}){]}_{10}$ below the spin-glass transition temperature, ${T}_{\mathrm{g}}$\approx${}44\mathrm{K}.$ Furthermore, the DMIM system memorizes the structure of a quasiequilibrium state reached after an intermittent stop-and-wait protocol during cooling. These results unambiguously corroborate the collective nature of the low-temperature spin dynamics. The correlation length achieved at ${T=0.95T}_{\mathrm{g}}$ after a wait time of ${10}^{4}\mathrm{s}$ is estimated to extend to ${10}^{5}$ superspins, which seems to imply a crossover from three- to two-dimensional growth of the correlation length.
Dipolar superferromagnetism with reentrant low-temperature superspin glass behavior is observed on a randomly distributed ferromagnetic nanoparticle systems in discontinuous metal-insulator multilayers Co 80 Fe 20 (t)/Al 2 O 3 3 nm 10 with nominal thickness 1.1t1.3 nm by use of ac susceptometry and dc mag-netometry. At t1.0 nm, superspin glass-like freezing is evidenced by the criticality of dynamic and nonlinear susceptibilities.
Metal oxide gas sensors are predominant solid-state gas detecting devices for domestic, commercial and industrial applications, which have many advantages such as low cost, easy production, and compact size. However, the performance of such sensors is significantly influenced by the morphology and structure of sensing materials, resulting in a great obstacle for gas sensors based on bulk materials or dense films to achieve highly-sensitive properties. Lots of metal oxide nanostructures have been developed to improve the gas sensing properties such as sensitivity, selectivity, response speed, and so on. Here, we provide a brief overview of metal oxide nanostructures and their gas sensing properties from the aspects of particle size, morphology and doping. When the particle size of metal oxide is close to or less than double thickness of the space-charge layer, the sensitivity of the sensor will increase remarkably, which would be called "small size effect", yet small size of metal oxide nanoparticles will be compactly sintered together during the film coating process which is disadvantage for gas diffusion in them. In view of those reasons, nanostructures with many kinds of shapes such as porous nanotubes, porous nanospheres and so on have been investigated, that not only possessed large surface area and relatively mass reactive sites, but also formed relatively loose film structures which is an advantage for gas diffusion. Besides, doping is also an effective method to decrease particle size and improve gas sensing properties. Therefore, the gas sensing properties of metal oxide nanostructures assembled by nanoparticles are reviewed in this article. The effect of doping is also summarized and finally the perspectives of metal oxide gas sensor are given.
We report the systematic dc and ac susceptibility studies on the particle blocking and carrier fluid freezing effects on the magnetization and relaxation processes in two different ferrofluids composed of Fe <sub>3</sub> O <sub>4</sub> nanoparticles (mean size of ∼14 nm ) suspended in hexane and dodecane, which respectively have freezing temperatures below (178 K) and above (264 K) the blocking temperature of magnetic nanoparticles (∼200 K ) . Experimental results reveal that these effects play a key role in the formation of glasslike peaks and magnetic anomalies in ferrofluids. Quantitative fits of the frequency dependent ac susceptibility to the Vogel–Fulcher model τ=τ<sub>o</sub> exp [E<sub>a</sub>/k(T-T<sub>o</sub>)] clearly indicate that the blocking of magnetic nanoparticles in the frozen state significantly affects the interparticle dipole-dipole interaction, causing characteristic spin-glass-like dynamics.
We present a short overview of the influence of inter-particle interactions on the properties of magnetic nanoparticles. Strong magnetic dipole interactions between ferromagnetic or ferrimagnetic particles, that would be superparamagnetic if isolated, can result in a collective state of nanoparticles. This collective state has many similarities to spin-glasses. In samples of aggregated magnetic nanoparticles, exchange interactions are often important and this can also lead to a strong suppression of superparamagnetic relaxation. The temperature dependence of the order parameter in samples of strongly interacting hematite nanoparticles or goethite grains is well described by a simple mean field model. Exchange interactions between nanoparticles with different orientations of the easy axes can also result in a rotation of the sub-lattice magnetization directions.
In this work, we report a detailed study of the formation of hollow nanostructures in iron
oxides. Core/shell Fe/Fe-oxide nanoparticles were synthesized by thermal decomposition of
Fe(CO)5
at high temperature. It was found that 8 nm is the critical size above which the particles
have a core/shell morphology, whereas below this size the particles exhibit a hollow
morphology. Annealing the core/shell particles under air also leads to the formation of
hollow spheres with a significant increase in the average particle size. In the case of
the thermally activated Kirkendall process, the particles do not fully transform
into hollow structures but many irregular shaped voids exist inside each particle.
The 8 nm hollow particles are superparamagnetic at room temperature with a
blocking temperature of 70 K whereas the core/shell particles are ferromagnetic.
Core-shell Fe@Fe(3)O(4) nanoparticles exhibit substantial exchange bias at low temperatures, mediated by unidirectionally aligned moments at the core-shell interface. These spins are frozen into magnetic alignment with field cooling, and are depinned in a temperature-dependent manner. The population of such frozen spins has a direct impact on both coercivity (H(C)) and the exchange-bias field (H(E)), which are modulated by external physical parameters such as the strength of the applied cooling field and the cycling history of magnetic field sweeps (training effect). Aging of the core-shell nanoparticles under ambient conditions results in a gradual decrease in magnetization but overall retention of H(C) and H(E), as well as a large increase in the population of frozen spins. These changes are accompanied by a structural evolution from well-defined core-shell structures to particles containing multiple voids, attributable to the Kirkendall effect. Energy-filtered and high-resolution transmission electron microscopy both indicate further oxidation of the shell layer, but the Fe core is remarkably well preserved. The increase in frozen spin population with age is responsible for the overall retention of exchange bias, despite void formation and other oxidation-dependent changes. The exchange-bias field becomes negligible upon deliberate oxidation of Fe@Fe(3)O(4) nanoparticles into yolk-shell particles, with a nearly complete physical separation of core and shell.
Antiferromagnetic (AF) nanostructures from Co3O4, CoO and Cr2O3 were prepared by the nanocasting method and were characterized magnetometrically. The field and temperature dependent magnetization data suggests that the nanostructures consist of a core-shell structure. The core behaves as a regular antiferromagnet and the shell as a two-dimensional diluted antiferromagnet in a field (2d DAFF) as previously shown on Co3O4 nanowires [Benitez et al., Phys. Rev. Lett. 101, 097206 (2008)]. Here we present a more general picture on three different material systems, i.e. Co3O4, CoO and Cr2O3. In particular we consider the thermoremanent (TRM) and the isothermoremanent (IRM) magnetization curves as "fingerprints" in order to identify the irreversible magnetization contribution originating from the shells. The TRM/IRM fingerprints are compared to those of superparamagnetic systems, superspin glasses and 3d DAFFs. We demonstrate that TRM/IRM vs. H plots are generally useful fingerprints to identify irreversible magnetization contributions encountered in particular in nanomagnets. Comment: submitted to PRB
In the present work, we investigate the magnetic properties of ferrimagnetic and noninteracting maghemite (g-Fe2O3) hollow nanoparticles obtained by the Kirkendall effect. From the experimental characterization of their magnetic behavior, we find that polycrystalline hollow maghemite nanoparticles are characterized by low superparamagnetic-to-ferromagnetic transition temperatures, small magnetic moments, significant coercivities and irreversibility fields, and no magnetic saturation on external magnetic fields up to 5 T. These results are interpreted in terms of the microstructural parameters characterizing the maghemite shells by means of an atomistic Monte Carlo simulation of an individual spherical shell model. The model comprises strongly interacting crystallographic domains arranged in a spherical shell with random orientations and anisotropy axis. The Monte Carlo simulation allows discernment between the influence of the structure polycrystalline and its hollow geometry, while revealing the magnetic domain arrangement in the different temperature regimes. Comment: 26 pages, 8 figures. In press in Phys. Rev. B
Eisencarbidnanopartikel mit graphitischer Hulle sind bei der Reduktion von Sauerstoff aktiv. In ihrer Zuschrift auf S. 3749 ff. berichten W. Xing, Q. F. Li et al., dass hohle Kugeln aus diesen Nanopartikeln ohne Oberflachenstickstoff oder funktionelle metallische Gruppen sowohl im sauren als auch im alkalischen Milieu eine hohe Aktivitat und Stabilitat zeigen. Die Katalysatoren konnen in einem einstufigen Prozess hergestellt werden.
A simple strategy involving desilication and recrystallization of silicalite-1 in tetrapropylammonium hydroxide (TPAOH) solution was successfully developed to prepare hollow zeolite nanocubes and three-dimensionally macroporous zeolite monoliths. Large voids were introduced to silicalite-1 crystals by controlled silicon leaching with OH– and thin intact shells were formed by the recrystallization of silicon with TPA+. The size of nanocubes could be easily controlled from 150 nm to 600 nm by simply adjusting the size of parent silicalite-1. Apart from template function to increase the yield of hollow silicalite-1, TPA+ adsorbed on the zeolite protects the parent crystal surface where the recrystallization occurred. The size of the mesopores and/or macropores in the hollow zeolite shell can be controlled by varying the amount of competitive Na+ adsorbent added to the TPAOH solution. Furthermore, three-dimensional macroporous zeolite monoliths can be formed when an electrolyte, such as NaCl, was added to the TPAOH solution. When the sample was used as the support for iron-based catalyst for hydrogenation of CO2 to hydrocarbons, both the conversion of CO2 and the selectivity of C5+ higher hydrocarbons were improved.
Magnetic nanoscale structures are intriguing, in part, because of the
exotic properties that emerge compared with bulk. The reduction of
magnetic moment per atom in magnetite with decreasing nanoparticle size,
for example, has been hypothesized to originate from surface disordering
to anisotropy-induced radial canting, which are difficult to distinguish
using conventional magnetometry. Small-angle neutron scattering (SANS)
is ideal for probing structure, both chemical and magnetic, from nm to
microns across an ensemble of particles. Adding polarization analysis
(PASANS) of the neutron spin orientation before and after interaction
with the scattering particles allows the magnetic structure to be
separated into its vector components. Application of this novel
technique to 9 nm magnetite nanoparticles closed-packed into
face-centered crystallites with order of a micron revealed that at
nominal saturation the missing magnetic moments unexpectedly interacted
to form well-ordered shells 1.0 to 1.5 nm thick canted perpendicular to
their ferrimagnetic cores between 160 to 320 K [1]. These shells
additionally displayed intra-particle "cross-talk", selecting a common
orientation over clusters of tens of nanoparticles. However, the shells
disappeared when the external field was removed and interparticle
magnetic interactions were negligible (300 K), confirming their magnetic
origin. This work has been carried out in collaboration with Ryan Booth,
Julie Borchers, Wangchun Chen, Liv Dedon, Thomas Gentile, Charles Hogg,
Yumi Ijiri, Mark Laver, Sara Majetich, James Rhyne, and Shannon
Watson.[4pt] [1] K.L. Krycka et al., Phys. Rev. Lett. 104, 207203 (2010)
Spin-canting in spinel nanoparticles of maghemite has multiple origins, among which surface effects, finite-size effects or chemical disorder effects. XMCD at the Fe L2,3L2,3 edges allows to separate the contributions of the magnetic moments of Fe3+Fe3+ ions in tetrahedral and octahedral sites of γγ-Fe2O3Fe2O3. We investigate three powders of γγ-Fe2O3Fe2O3 synthetized via aqueous precipitation: particles of average diameter 2.7, 8 nm and particles of average diameter 8 nm coated with phosphoric acid. The relative contributions of the spins of the FeTd3+ and the FeOh3+ ions are observed, varying the external magnetic field. Under high magnetic fields, a reduction of the magnetic contribution of the FeOh3+ ions occurs for 8 nm phosphate-coated particles by comparison with the uncoated ones. A similar reduction appears at lower magnetic fields for the small 2.7 nm particles.
Structural and magnetic properties of γ-Fe2O3 have been studied in isometric nanoparticles ranging from 3 to 14 nm with a narrow particle size distribution. Cation vacancy order is observed for particles larger than 5 nm in diameter giving rise to a cubic superstructure, while for the smallest particles these vacancies are disordered. All magnetic properties measured showed a strong dependence on the average crystallite size. For the ordered samples, saturation magnetization was found to decrease linearly with decreasing crystallite size due to a surface spin canting effect. However, a stronger decrease was observed in the disordered samples, suggesting that also an internal spin canting (cation vacancy order−disorder) has to be taken into account to explain the magnetic properties of nanoparticles. The room-temperature coercive field decreases with decreasing crystallite size; however at low temperatures, the coercivity increases as the size decreases, reaching values larger than 3000 Oe. A model to explain the magnetic properties of these particles considering both surface and order−disorder effects is proposed.
The solid solutions of CaBaFe4-xLixO7 (CBFLix) with the "114" hexagonal symmetry (P6(3)mc) could be synthesized for 0 < x < 0.4. The dc magnetization study of these oxides shows that they are ferrimagnetic (FiM). The FiM to paramagnetic transition temperature, T-C, decreases from 265.6 K for x=0 to 97.5 K for x=0.4 in agreement with the introduction of the diamagnetic Li1+ cation at the Fe sublattice. No magnetic saturation is observed and the coercive field H-C is very large, ranging from 0.7 to 0.9 T at 5 K. Importantly, the ac magnetic susceptibility, chi(ac)(T), indicates a frequency-dependent freezing temperature, T-f, increasing with frequency and satisfying the conventional power law, tau/tau(0)=(T-f/T-SG-1)(-z upsilon). For the undoped CaBaFe4O7 (CBFO) (x=0), the best fitting parameters are characteristic of a spin glass with a transition temperature T-SG=176 K, spin-relaxation time tau(0)=4.9x10(-12) s, and critical exponent (z upsilon) of 6.4. In contrast, for CBFLix (0.1 < x < 0.4) series the tau(0) increases from 1.3x10(-14) to 8x10(-16) s as x is increased; the values of z upsilon are in the range of 12-13, which lies much closer to that of cluster glasses (tau(0)=10(-14)-10(-16) sec; z upsilon=12-14). The resistivity behavior is consistent with small polaronic hopping which changes to variable range hopping at low temperatures. The measurements under H similar to 7 T show that only CBFO (undoped) exhibits magnetoresistance, with a maximum value of 4.3% at 264 K.
A spin-glass model consisting of a kinetic Ising model with random nearest-neighbor interactions is studied by Monte Carlo methods. As in real experiments the system is cooled, and a magnetic field is applied and then switched off. Below a freezing temperature Tf both an irreversible and a reversible magnetic susceptibility are observed. A remanent magnetization M occurs which decays very slowly with time t with a power law M∼t-a to the equilibrium value M=0. For different cooling procedures different remanent magnetizations are discussed as a function of temperature and previously applied field. A characteristic difference between field cooled (TRM) and isothermal (IRM) remanent magnetization is observed in the field dependence of the exponent a. Many of the predictions resemble experimental results. In the second part an exactly solvable spin-glass model incorporating a symmetric distribution of random interactions and frustration is introduced. Since the range of the interactions is infinite there exist no local clusters in this model. A phase transition with a cusp in the susceptibility, a remanent magnetization, and a ferromagnet—spin-glass transition are found.
γ-Fe2O3 magnetic nanoparticles, with a very
high surface to volume ratio, exhibit both strong exchange anisotropy
and magnetic training effect. At the same time high field
irreversibility in MH curves and zero field cooled-field cooled (ZFC-FC)
processes has also been detected. A low temperature spin-glass-like
transition is evidenced at TF~42 K with strong
irreversibility even at H = 55 kOe. TFH evolves following the
well known de Almeida-Thouless line
δTF~H2/3. The thermal dependence of the
exchange anisotropy field HE is described by the random-field
model of exchange anisotropy. In the framework of this theory, a surface
spin-glass layer about 0.6 nm thick is determined.
We report for the first time a novel precursor-templated conversion method for the controlled synthesis of hierarchically nanostructured magnetic hollow spheres assembled by Fe3O4 or γ-Fe2O3 nanosheets with a relatively high saturation magnetization. We synthesized hierarchically nanostructured hollow spheres organized by nanosheets of a layer-structured ferrous precursor by a microwave-assisted hydrothermal method in ethylene glycol (EG). Ferric chloride (FeCl3·6H2O) was used as the iron source, and EG acted as both a solvent and a reductant to reduce ferric salt to ferrous precursor in the presence of sodium hydroxide (NaOH) and sodium dodecyl benzene sulfonate (SDBS). The precursor was heated to prepare hierarchically nanostructured magnetic hollow spheres assembled by Fe3O4 or γ-Fe2O3 nanosheets, which were surface-modified with poly(ethylene glycol) (PEG). The surface-modified hierarchically nanostructured magnetic hollow spheres were explored as drug carriers. A typical anti-inflammatory drug, ibuprofen, was used for drug loading, and the release behaviors of ibuprofen in a simulated body fluid (SBF) were studied. The results indicate that these hierarchically nanostructured magnetic hollow spheres of Fe3O4 or γ-Fe2O3 have a high drug loading capacity and favorable release property for ibuprofen; thus, they are very promising for application in drug delivery. The samples were characterized by XRD, TEM, SEM, TG/DSC, BET, PPMS, FTIR, and UV−vis.
Manganese-substituted cobalt ferrite nanoparticles coated with triethylene glycol (TREG) have been prepared by the glycothermal reaction. The effect of Mn substitution and coating on temperature-dependent magnetic properties of the TREG-coated Mnx
Co1−x
Fe2O4 nanoparticles (0.0 ≤ x ≤ 0.8) with size of ~5–7 nm has been investigated in the temperature range of 10–300 K in a magnetic field up to 9 T. After the irreversible processes of the magnetic hysteresis curves were completed, the high-field regions of these curves were fitted by using a ‘law of approach to saturation’ to extract the magnetic properties, such as the effective anisotropy constant (K
eff) and the anisotropy field (H
A) etc. High coercive field of 12.6 kOe is observed in pure cobalt ferrite coated with TREG at 10 K. The low temperature unsaturated magnetization behaviour indicates the core–shell structure of the Mnx
Co1−x
Fe2O4 NPs. Zero-field-cooled (ZFC) and field-cooled (FC) measurements revealed superparamagnetic phase of TREG-coated Mnx
Co1−x
Fe2O4 nanoparticles at room temperature. The blocking and irreversibility temperatures obtained from ZFC–FC curves decrease at highest Mn concentration (x = 0.8). The existence of spin-glass-like surface layer with freezing temperature of 215 K was established with the applied field dependence of the blocking temperatures following the de Almeida–Thouless line for the Mn0.6Co0.4Fe2O4 NPs. The shifted hysteresis loops with exchange bias field of 60 Oe and high-field irreversibility up to 60 kOe in FC M–H curve at 10 K show that spin-glass-like surface spins surrounds around ordered core material of the Mn0.6Co0.4Fe2O4 NPs. FMR measurement show that all the TREG-coated Mnx
Co1−x
Fe2O4 nanoparticles absorb microwave in broad field range of about ten thousands Oe. The spectra for all the samples have broad linewidth because of angular distributions of easy axis and internal fields of nanoparticles.
A study was conducted to demonstrate a facile and reproducible technique for synthesizing gold/iron oxide nanoparticles with controllable thickness and morphology of the outer oxide shell. Different aspects of the synthetic approach were analyzed and optical ad magnetic properties of these composite materials were discussed. The study demonstrated the strategy to grow hollow nanoshells encapsulating several nanoparticles. The synthesis of gold nanoparticles encapsulated within hollow iron oxide shell involved three steps, such as synthesizing gold nanoparticles, deposition of iron oxide shell around the gold core, and oxidation of the iron oxide shell, to form a hollow iron oxide shell through Kirkendall effect. It was observed that gold nanoparticles catalyzed the decomposition of Fe(CO)5 and lowered the decomposition temperature from 150-135°C.
Intrinsic properties of magnetic nanoparticles are reviewed, with special emphasis on the e!ects of "nite size on zero-temperature spin ordering, magnetic excitations, and relaxation. E!ects on zero-temperature spin ordering include moment enhancement due to band narrowing in 3d transition metal particles, surface spin disorder in ferrite particles, and multi-sublattice states in antiferromagnetic oxide particles. Magnetic excitations include discretized spin wave modes, and surface modes that have been detected by quasielastic neutron scattering. Thermally activated and quantum tunneling relaxation is discussed in the context of ideal single-domain particles, as well as particles with surface spin disorder. 1999 Elsevier Science B.V. All rights reserved.
Magnetic properties in a magnetically textured ferrofluid made out of interacting maghemite (γ -Fe <sub>2</sub> O <sub>3</sub>) nanoparticles suspended in glycerin have been investigated. Despite the loss of uniform distribution of anisotropy axes, a superspin glass state exists at low temperature in a concentrated textured ferrofluid as in the case of its nontextured counterpart. The onset of superspin glass state was verified from the sample’s ac susceptibility. The influence of the anisotropy axis orientation on the aging behavior in the glassy states is also discussed.
In this letter we show the presence of a spin-glass like phase in single
crystals of magnetoelectric gallium ferrite (GaFeO3) below ~210 K via
temperature dependent ac and dc magnetization studies. Analysis of frequency
dispersion of the susceptibility peak at ~210 K using the critical slowing down
model and Vogel-Fulcher law strongly suggests the existence of a classical
spin-glass like phase. This classical spin glass behavior of GaFeO3 is
understood in terms of an outcome of geometrical frustration arising from the
inherent site disorder among the antiferromagnetically coupled Fe ions located
at octahedral Ga and Fe sites.
Material design in terms of their morphologies other than solid nanoparticles can lead to more advanced properties. At the example of iron oxide, we explored the electrochemical properties of hollow nanoparticles with an application as a cathode and anode. Such nanoparticles contain very high concentration of cation vacancies that can be efficiently utilized for reversible Li ion intercalation without structural change. Cycling in high voltage range results in high capacity (∼132 mAh/g at 2.5 V), 99.7% Coulombic efficiency, superior rate performance (133 mAh/g at 3000 mA/g) and excellent stability (no fading at fast rate during more than 500 cycles). Cation vacancies in hollow iron oxide nanoparticles are also found to be responsible for the enhanced capacity in the conversion reactions. We monitored in situ structural transformation of hollow iron oxide nanoparticles by synchrotron X-ray absorption and diffraction techniques that provided us clear understanding of the lithium intercalation processes during electrochemical cycling.
Discontinuous magnetic multilayers [CoFe/Al2O3] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co80Fe20 nanoparticles are embedded in a diamagnetic insulating a-Al2O3 matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.
This review summarizes recent developments in the theory of spin glasses,
as well as pertinent experimental data. The most characteristic properties
of spin glass systems are described, and related phenomena in other
glassy systems (dielectric and orientational glasses) are mentioned.
The Edwards-Anderson model of spin glasses and its treatment within
the replica method and mean-field theory are outlined, and concepts
such as "'frustration"', "'broken replica symmetry"' "'broken ergodicity"',
etc., are discussed. The dynamic approach to describing the spin
glass transition is emphasized. Monte Carlo simulations of spin glasses
and the insight gained by them are described. Other topics discussed
include site-disorder models, phenomenological theories for the frozen
phase and its excitations, phase diagrams in which spin glass order
and ferromagnetism or antiferromagnetism compete, the Neel model
of superparamagnetism and related approaches, and possible connections
between spin glasses and other topics in the theory of disordered
condensed-matter systems.
The magnetic properties of a spin-glass depend on the time spent in the low-temperature phase : this is the so-called ageing phenomenon. We present measurements of the low-frequency a.c. susceptibility (0.01 to 0.1 Hz) and of the relaxation of the thermo-remanent magnetization in the CdCr1.7 In0.3S4 insulating spin-glass ; we investigate the influence of temperature variations on the ageing processes in the spin-glass phase, in order to get a better insight into their mechanism. The wide spectrum of relaxation times observed in spin-glass dynamics reflects the existence of many energy valleys in the phase space ; a hierarchical organization of these valleys, whose bifurcations appear as the temperature is decreased, is the simplest picture which accounts for our results. Les propriétés magnétiques d'un verre de spin dépendent du temps passé dans sa phase basse température: c'est le phénomène de vieillissement. Nous présentons des mesures de susceptibilité alternative à basse fréquence (0.01 à 0.1 Hz) et de relaxation de l'aimantation thermo-rémanente dans le verre de spin isolant CdCr1.7In0.3S4 ; nous étudions l'influence de variations de température sur le vieillissement dans la phase verre de spin pour mieux interpreter le mécanisme de ce processus. Le large spectre de temps de relaxation observé dans la dynamique des verres de spins reflète l'existence de nombreuses vallées d'énergie dans l'espace des phases ; une organisation hierarchique de ces vallées, dont les bifurcations apparaissent quand la température diminue, est le schéma le plus simple qui prenne en compte l'ensemble de nos résultats.
A phenomenological theory of the ordered phase of short-range Ising spin glasses is developed in terms of droplet excitations and presented in detail. These excitations have free energies with a broad distribution whose characteristic magnitude grows with length scale L as ${L}^{$\theta${}}$. A small fraction of droplets of all scales are thermally active; these dominate much of the physics. The mean-square correlation functions are found to decay with distance as 1/${r}^{$\theta${}}$ for all T${T}_{c}$ and the autocorrelations decay logarithmically with time because of large activation barriers for creation and annihilation of the droplet excitations. A renormalization procedure is sketched in order to define excitations at positive temperature. It is found that the long-distance equilibrium correlation functions are extremely sensitive to small temperature changes, yielding breakdown of certain relations between fluctuations and thermodynamic derivatives. The behavior near to the critical temperature is discussed and some of the ideas are extended to systems with power-law interactions and to spin glasses with X-Y or Heisenberg symmetry. The inequality $\theta${}$\le${}(d-1)/2 is also derived.
We consider the nonequilibrium behavior of the spin-glass ordered phase within the droplet scaling theory introduced previously. The fundamental long-time nonequilibrium process is assumed to be the thermally activated growth of spin-glass ordered domains. The remanent magnetization, m(t), in zero field is found to decay at long times as m(t)∼Rt-λ, where Rt∼(lnt)1/ψ is the linear domain size, ψ is the previously introduced barrier exponent describing the growth of activation-barrier heights with length scale, and λ is a new nonequilibrium dynamic exponent, satisfying the relation λ≥d/2 for d-dimensional systems. The effects of waiting for partial equilibration before making a measurement are studied in various regimes. The effects of quenching first to one temperature and then to another are also examined. Such experiments can, in principle, be used to obtain information about the relative rate of dynamic evolution as well as the overlap between the equilibrium states at different temperatures. In particular, the length scale LΔT, below which equilibrium correlations at temperatures T and T+ΔT are similar, plays an important role. The decay of m(t) and the growth of spin-glass order after a quench are examined in Monte Carlo simulations of the Sherrington-Kirkpatrick model.
The dynamics of the short-range Ising spin-glass Fe0.5Mn0.5TiO3 has been investigated in a SQUID magnetometer. The dynamic spin-correlation function, q(t), as reflected in low-field ac susceptibility and time-dependent magnetization measurements, was studied in the time interval 10-6-104 sec. The functional form of q(t) shows a remarkable agreement with the results obtained from Monte Carlo simulations on a three-dimensional Ising spin-glass. The temperature dependence of the relaxation times above the spin-glass temperature is well described by conventional power-law scaling.