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Anisotropic and large low-field magnetic entropy change in a La 4/3Sr 5/3Mn 2O 7 single crystal
Abstract
We have studied the magnetocaloric effect (MCE) in a bilayered La4/3Sr5/3Mn2O7 single crystal with applied field along both ab-plane and c-direction. Due to the quasi-two-dimensional structure, the crystal exhibits a strong anisotropy in the MCE. The difference of magnetic entropy change between two crystallographic directions depends on external magnetic fields and has a maximum of 2J/kgK. A large low-field magnetic entropy change, reaching 3.2J/kgK for a magnetic field change of 15kOe, is observed when the applied field is along ab-plane. This large low-field magnetic entropy change is attributed to the rapid change of magnetization in response to external magnetic fields in the easy magnetizing plane.
... Previous studies show that the rare earth element Gd has a large MCE. However, the use of Gd as a magnetic refrigerant is limited due to its high price [4][5][6]. Doped manganese oxides have attracted some attention in the field of magnetic refrigeration research. Studies reveal that the double-doped manganese oxide with the formula R 1-x A x MnO 3 , where R is a trivalent rare-earth cation, and A is a divalent alkaline earth cation, has a large MCE. ...
Polycrystalline La0.9-xEuxSr0.1MnO3 (x = 0.000, 0.025) samples were prepared using a traditional high temperature solid-state reaction method. The effects of double doping of cationic Eu3+ and Sr2+ on the preformed clusters, critical field behavior, and change of magnetic entropy of LaMnO3 were studied systematically. The results show that polycrystalline La0.9-xEuxSr0.1MnO3 (x = 0.000, 0.025) samples are orthogonal crystal structures with a good unidirectionality and the space group of the polycrystalline samples is Pbnm. As the temperature increased from 15 to 380 K, the polycrystalline La0.9-xEuxSr0.1MnO3 (x = 0.000, 0.025) sequentially displayed the characteristics of cluster spin glass, ferromagnetic phase, preformed cluster, and paramagnetic phase. For both La0.9-xEuxSr0.1MnO3 samples, the second-order phase transition occurs near Curie temperature. The critical index of the undoped parent compound and doped sample fitted best to the mean-field model and 3D-Heisenberg model, respectively. At an applied magnetic field of 7 T, the maximum absolute values of the magnetic entropy changes of the two samples were 2.76 ± 0.002 J·kg−1·K−1 and 2.81 ± 0.002 J· kg−1· K−1, respectively, and the temperature corresponding to the maximum absolute values of the magnetic entropy change was approximately 130 K. The magnetic refrigerant capacities (RC) of the two La0.9-xEuxSr0.1MnO3 samples were 436.01 ± 0.002 J· kg−1 and 438.09 ± 0.002 J· kg−1, respectively. Our results revealed that the changes in magnetic entropy and magnetic refrigerating capacity of the doped sample increased compared to the parent sample, suggesting that the double-doped manganese oxide La0.9-xEuxSr0.1MnO3 samples can be used as magnetic refrigerants in the medium temperature range of 80 to 250 K.
... The doped manganese perovskites with the general formula A 1-x B x MnO 3 (A=La, Nd, etc., and B=Ca, Sr, etc.) not only exhibit colossal magnetoresistsnce (CMR) effect, but also exhibit significant magnetocaloric effect (MCE)-external magnetic field can induce temperature change [1] . Their remarkable MCE and CMR properties have been widely researched from preparation to performance of the materials [2][3][4][5][6][7][8] . The commonly used method is substitution of other elements at sites A or B of different perovskite crystals. ...
We reported the magnetic properties and magnetocaloric effects (MCE) of (La0.8Ho0.2)2/3Ca1/3MnO3 and (La0.5Ho0.5)2/3Ca1/3MnO3 nanoparticles by sol-gel technique. With this method, we were able to obtain the samples with particle diameters ranging from 50 to 200 nm. In the (La1-xHox)2/3Ca1/3MnO3 compound, an external magnetic field induced a magnetic transition from an paramagnetic phase to a ferromagnetic phase above Ts=105-135 K, leading to magnetocaloric effects. The maximum value of ΔSM was 1.19 J/(kg·K) at 100 K and 2.03 J/(kg·K) at 152 K for a magnetic field change of 5 T. Because both samples had large relative cooling power (RCP) and wide δTFWHM, the study on systems with the (La1-xHox)2/3Ca1/3MnO3-related magnetic transitions may open an important field in searching good magnetic materials.
Effects of Eu doping at the A site on the magnetic properties of the perovskite manganite La0.8-xEuxSr0.2MnO3(x = 0 and 0.05)polycrystalline samples were investigated in this paper. The polycrystalline La0.8-xEuxSr0.2MnO3 (x = 0,0.05)samples were synthesized using a traditional solid-phase reaction method. X-ray diffraction (XRD) analysis of the polycrystalline samples did not indicate any impurities, confirming that the samples are single phase. The magnetization vs. temperature curve (M-T), as well as the magnetization vs. magnetic field curves (M-H) of the two samples, show that both synthesized structures are paramagnetic in the high-temperature region, while near the Curie temperature (Tc), the material is ferromagnetic. The Tc of both samples, La0.8-xEuxSr0.2MnO3 with x = 0 and the Eu3+-doped sample with x = 0.05, increased to 283 K and 284 K, respectively. The hysteresis loops of the samples show insignificant coercive force and no remanence. The inverse magnetic susceptibility vs. temperature curves (χ−1-T, χ−1 = H/M) indicate that the Griffiths phase temperature remains unchanged, while the Curie temperature decreases. The trends confirm that \({Eu}^{3+}\) doping changed the ferromagnetic coupling of the system. The average field model provides a good fit for the critical parameter response of both the parent and the doped synthesized material. At an applied field of 7 T, the change in magnetic entropy of the doped sample reaches its highest value at 4.19 J/kg·K, and the refrigeration efficiency is 445 J/kg. Our results show that the newly synthesized polycrystalline samples could be used as magnetic refrigeration materials.
We report the structure, magnetocaloric effect, and critical phase transition in the manganite La2Sm0.4Sr0.6Mn2O7 (LSSMO) synthesized by a sol-gel method. X-ray diffraction together with Rietveld refinement show that the sample crystallizes in a Sr3Ti2O7-type tetragonal structure with a space group of I4/mmm. This compound undergoes a second-order ferromagnetic (FM) to paramagnetic phase transition at TC=348 K and shows strong FM properties below the TC. Based on the data of isothermal magnetization measured around the TC and Maxwell's relation, we calculated the maximum magnetic entropy change (-ΔSMmax) to be 4.69 J kg⁻¹ K⁻¹ and the relative cooling power to be 233.9 J kg⁻¹ for a μ0ΔH=5 T magnetic field variation. These results indicate that LSSMO can be considered as a potential candidate material for application in magnetic refrigeration above room temperature. The critical behavior near the TC was studied through the analysis of the magnetic field dependence of the magnetic entropy change and Widom's scaling relation. The exponent values estimated in this work are fairly close to those theoretically predicted by mean field theory (β=0.5, γ=1.0, and δ=3.0), revealing that long-range FM ordering exists in LSSMO. Scaling law theory also confirms the validity of the deduced critical exponents.
Pr1.4Sr1.6Mn2O7 compound was prepared by a standard ceramic route, and its structure and magnetocaloric effect have been studied by using an X-ray diffractometer and magnetic measurements. This compound exhibits a single ferromagnetic (FM) to paramagnetic (PM) transition at the Curie temperature (TC) of 265 K. The maximum values of magnetic entropy change reach 2.43 and 5.06 J kg⁻¹ K⁻¹ for the magnetic field change of 2 and 5 T. The corresponding values of relative cooling power are 94.26 and 233.32 J kg⁻¹. Such results demonstrate that the Pr1.4Sr1.6Mn2O7 manganese oxide could be an excellent candidate for working materials in the household application of the magnetic refrigeration. Moreover, according to Banerjee’s criteria and Franco’s universal curves, we have demonstrated the existence of a second-order magnetic transition in the Pr1.4Sr1.6Mn2O7 compound.
(La1-xGdx)(4/3)Sr5/3Mn2O7 (x = 0, 0.05) polycrystalline samples have been prepared by solid state reaction method, and the phase separation phenomena in this samples are investigated by measuring the magnetization-temperature (M-T) curve, electron spin resonance (ESR) curve and resistivity-temperature (rho-T) curve. For both samples, experimental results suggest there exists competition between ferromagnetic and antiferromagnetic interactions in low temperature range, which reflects a characteristic of cluster spin glass. A Griffiths-like phase is observed in temperature ranges 125-375 K and 100-375 K for x = 0 sample and x = 0.05 sample, respectively. It is found that doping contributes to the decrease of three-dimensional long-range ferromagnetic ordering temperature (from T-c0(3D) approximate to 125 K for x = 0 to T-c1(3D) approximate to 100 K for x = 0.05), but has no obvious effect on the Griffiths-like temperature (T-G approximate to 375 K). Above T-G approximate to 375 K, a pure paramagnetic phase appears in both samples. The rho-T curves reveal two insulator-metal transitions in the entire temperature range for x = 0 sample, which is caused by coexistence of the two phases in perovskite manganese oxides. For x = 0.05 sample, however, there exhibits a single insulator-metal transition, indicating that doping can hinder the coexistence phenomenon. It can be seen from the fitted rho-T curves that the electron conduction mechanism in high temperature range is in accordance with the three-dimensional variable range of hopping conduction.
Experimental lattice enthalpies ΔLHθ of lanthanide iron garnets Ln3Fe5O12 (LnIGs) with Ln = Ce − Lu, have been determined from the Born–Haber thermochemical cycle. The results have been compared with those reported by other authors using numerical simulation or an empirical equation. The lattice enthalpy of PmIG has been predicted by linear interpolation between ΔLHθ of the adjacent NdIG and SmIG. It has been found that the partial derivative (∂ΔLHθ/∂Vm) for the series of LnIGs corresponds by magnitude and dimension to an upper limit of their rigidity. New effect of magnetic loops has been noted in the same series in the dependence of the lattice magnetic entropies on the effective magnetic moments (theoretical and experimental values).
Under investigation in this paper is the Hirota–Maxwell–Bloch system which governs the propagation of optical solitons in nonlinear erbium doped fibers with higher order effects. By virtue of the Darboux transformation, the Akhmediev breathers, Ma-breathers, localized solitons, bound solitons, two-breathers and localized solutions are generated for the Hirota–Maxwell–Bloch system. Considering the influences of higher order effects, propagation properties of the breathers, modulation instability conditions for the Akhmediev breathers and interaction scenarios between bound solitons and two-breather solutions are discussed.
Owing to the inhomogeneous state resulting from the doping of a small number of Eu ions into La4/3Sr5/3Mn2O7, from the resulting single crystal (La0.8Eu0.2)4/3Sr5/3Mn2O7 we have observed the magnetization jump, the resistivity jump, as well as the relaxation phenomena. For (La0.8Eu0.2)4/3Sr5/3Mn2O7, it has a very delicate ground state due to the interplays among spin, charge, orbital, lattice degrees of freedom. Consequently, the magnetization state is sensitive to temperature, magnetic field, as well as time. Meanwhile, the evolution of the magnetization with time shows a spontaneous jump when both the temperature and the magnetic field are constant. Similar step-like behaviours are also observed in resistivity. All these results suggest that Eu doping can greatly modulate the physical properties of La4/3Sr5/3Mn2O7 and cause such interesting behaviours.
Magnetocaloric effect has been studied for antiferromagnetic single crystalline DySb, by measuring the temperature (T) and magnetic field (H) dependences of magnetization in the range and with field along the [100] and the [110] direction. The magnetic entropy change caused by a field change of 5 T is as larger as −36.4 J kg−1 K−1 for H//[110] around 11 K, which is a value much greater than that reported for the polycrystalline sample. The magnetocaloric effect is further found to be anisotropic. Above TN (=10 K), the entropy change is much larger for H//[110] than for H//[100]. In contrast, the entropy change is positive below TN, and it is significantly lower for H//[110] than for H//[100]. Our results indicate that DySb is a typical material possessing giant and highly anisotropic magnetocaloric effect, and suggest that magnetic orientation could be used to enhance magnetic entropy change.
A study of four Gd samples of different purities using ac
susceptibility, magnetization, heat capacity, and direct measurements of
the magnetocaloric effect in quasistatic and pulse magnetic fields
revealed that all techniques yield the same value of the zero-field
Curie temperature of 294(1) K. The Curie temperature determined from
inflection points of the experimental magnetic susceptibility and heat
capacity is in excellent agreement with those obtained from the
magnetocaloric effect and Arrot plots. Above 2 T the temperature of this
transition increases almost linearly with the magnetic field at a rate
of ~6 K/T in fields up to 7.5 T. The spin reorientation transition,
which occurs at 227(2) K in the absence of a magnetic field, has been
confirmed by susceptibility, magnetization, and heat-capacity
measurements. Magnetic fields higher than 2-2.5 T apparently quench the
spin reorientation transition and Gd retains its simple ferromagnetic
structure from the TC(H) down to ~4 K. The nature of anomaly
at T≅132 K, which is apparent from ac susceptibility measurements
along the c axis, is discussed. The presence of large amounts of
interstitial impurities lowers the second-order
paramagnetic<-->ferromagnetic transition temperature, and can
cause some erroneous results in the magnetocaloric effect determined in
pulsed magnetic fields. The magnetocaloric effect was studied utilizing
the same samples by three experimental techniques: direct measurements
of the adiabatic temperature rise, magnetization, and heat capacity. All
three techniques, with one exception, yield the same results within the
limits of experimental error.
In the present work we analyze the magnetic entropy change ΔSM of the Pr1−xCaxMnO3 manganites, for a wide range of Ca concentrations (0.20⩽x⩽0.95). The results for the samples with 0.20<x⩽0.30 present the usual behavior expected for ferromagnetic systems, peaking at the Curie temperature TC. In contrast, for the charge-ordered antiferromagnetic samples (0.30<x<0.90), an anomalous magnetic entropy change starts around the charge-ordering temperature TCO, persisting for lower values of temperature. This effect is associated to a positive contribution to the magnetic entropy change due to the charge-ordering ΔSCO, which is superimposed to the negative contribution from the spin-ordering ΔSspin; that is described using a mean-field approximation. Supposing these contributions, we could also appraise ΔSCOmax as a function of Ca content, which vanishes for the limits x∼0.30 and 0.90 and presents a deep minimum around x∼0.50, with two maxima at x∼0.35 and 0.65. We conclude that for x>0.65 only the magnetic order governs the charge ordering, contrarily to x<0.65, where there are more than one mechanism ruling the charge ordering. Moreover, for the samples with phase coexistence (0.30<x⩽0.40), we found extremely large values for the magnetic entropy change at low temperatures. Finally, for x>0.90, we found usual magnetic entropy change curves, peaking at the Néel temperature TN.
Magnetization of the compound LaFe11.4Si1.6 with the cubic NaZn13-type structure was measured as functions of temperature and magnetic field around its Curie temperature TC of ∼208 K. It is found that the magnetic phase transition at TC is completely reversible. Magnetic entropy change ΔS, allowing one to estimate the magnetocaloric effect, was determined based on the thermodynamic Maxwell relation. The achieved magnitude of ∣ΔS∣ reaches 19.4 J/kg K under a field of 5 T, which exceeds that of most other materials involving a reversible magnetic transition in the corresponding temperature range. The large entropy change is ascribed to the sharp change of magnetization, which is caused by a large negative lattice expansion at the TC. An asymmetrical broadening of ∣ΔS∣ peak with increasing field was observed, which is resulted from the field-induced itinerant-electron metamagnetic transition from the paramagnetic to ferromagnetic state above the TC. © 2001 American Institute of Physics.
MANGANESE oxides with the cubic perovskite structure (typified by LaMnO3) have stimulated considerable interest because of their magnetoresistive properties1–9; they exhibit extremely large changes in electrical resistance in response to applied magnetic fields, a property that is of technological relevance for the development of magnetic memory and switching devices. But for such applications to be viable, great improvements will be needed in both the sensitivity and temperature dependence of the magnetoresistive response. One approach under consideration for optimizing these properties is chemical substitution10. Here we demonstrate an alternative strategy, in which we synthesize layered variants of the cubic perovskite parent compounds that have a controlled number of MnO2 sheets per unit cell. This strategy is structurally analogous to that employed for the systematic exploration of the high-transition-temperature copper oxide superconductors11. We find that the magneto-resistive properties of these materials depend sensitively on the dimensionality of the manganese oxide lattice. Although the properties of our materials are still far from optimal, further exploration of this series of layered perovskites may prove fruitful.
Magnetocaloric properties of perovskite-type manganese oxides with double Mn-O layers of composition La <sub>2-2x</sub> Sr <sub>1+2x</sub> Mn <sub>2</sub> O <sub>7</sub> ( x=0.33 and 0.4) have been investigated. A broad peak of magnetic entropy change (-ΔS<sub>M</sub>) was observed at the Curie temperature. The shape of -ΔS<sub>M</sub> is strongly dependent on the Sr concentration. In contrast to Ln<sub>1-x</sub>A<sub>x</sub> Mn O <sub>3</sub> perovskites, the distinct curvilinear shape of -ΔS<sub>M</sub> for perovskites with double Mn-O layers shows different magnetic mechanisms arising from magnetocrystalline anisotropy.
We revise some recent results on the nature of the magnetic phase transition of ferromagnetic manganese perovskites with colossal magnetoresistance. It is found that they exhibit first-order magnetic phase transitions mainly due to strong lattice effects. The presence of such first-order transition is strongly linked to the existence of a temperature region above the Curie Temperature with a phase-separated regime of coexisting metallic and insulating clusters. This situation is compared with that of MnAs, paradigm of system with first-order magnetic phase transition, and we observe a parallel phenomenology of these apparently different systems. This serves as a clue for the finding of a phase-separated regime also in MnAs and, moreover, for the finding of a colossal magnetoresistive effect similar to that of manganese perovskites.
Recently, Jonker and Van Santen have found an empirical correlation between electrical conduction and ferromagnetism in certain compounds of manganese with perovskite structure. This observed correlation is herein interpreted in terms of those principles governing the interaction of the d-shells of the transition metals which were enunciated in the first paper of this series. Both electrical conduction and ferromagnetic coupling in these compounds are found to arise from a double exchange process, and a quantitative relation is developed between electrical conductivity and the ferromagnetic Curie temperature.
The magnetocaloric effect was examined for DyCo2, which shows a first-order transition from a ferrimagnetic to paramagnetic state at 142 K. A large magnetic entropy change (\DeltaS(M)\) was observed in this alloy. The maximum value of \DeltaS(M)\ was 2.3, 4.8 and 5.8 J/kg(.)K for H = 0.4, 0.8 and 1.0 T respectively. The origin of the large magnetocaloric effect has also been discussed. The large low-field magnetic entropy change indicates a great potential of DyCo2 as a working material for magnetic refrigerants.
An extremely large magnetic entropy change has been discovered in Gd{sub 5}(Si{sub 2}Ge{sub 2}) when subjected to a change in the magnetic field. It exceeds the {ital reversible} (with respect to an alternating magnetic field) magnetocaloric effect in any known magnetic material by at least a factor of 2, and it is due to a first order [ferromagnetic (I){leftrightarrow}ferromagnetic (II)] phase transition at 276K and its unique magnetic field dependence. {copyright} {ital 1997} {ital The American Physical Society}
Large magnetic entropy change with comparable magnitude to that of pure Gd has been observed in compounds La(Fe1-xCox)11.83Al1.17 (x=0.06,0.08) at their Curie temperatures of ∼273 K and ∼303 K, respectively. These compounds are of a cubic NaZn13-type structure with soft ferromagnetism. The magnetic entropy change is reversible in the whole experimental temperature range from ∼230 to ∼330 K. The most interesting feature is that the Curie temperature can be easily tuned by adjusting the substitution of Co for Fe. It is suggested that the present compounds are suitable candidates for magnetic refrigerants in a wide range near room temperature. The calculated ΔS curve in the molecular field approximation is in a satisfactory agreement with the experimental one.
A giant magnetocaloric effect was found in MnAs, which undergoes a first-order ferromagnetic to paramagnetic transition at 318 K. The magnetic entropy change caused by a magnetic field of 5 T is as large as 30 J/K kg at the maximum value, which exceeds that of conventional magnetic refrigerant materials by a factor of 2–4. The adiabatic temperature change reaches 13 K in a field change of 5 T. The substitution of 10% Sb for As reduces the thermal hysteresis and lowers the Curie temperature to 280 K, while the giant magnetocaloric properties are retained. © 2001 American Institute of Physics.
A breakthrough step in the development of magnetic refrigeration would be to find a way to increase the cooling capacity of the magnetic refrigerant material in order to make this technology even more energy efficient. In this paper, we present a theoretical study which shows how to increase the refrigerant capacity using anisotropic materials. We examine some of the well-known Laves phase compounds that can be described by a Hamiltonian which includes second order and spin reorientation effects. Our results indicate that in some cases it is theoretically possible to increase cooling capacity by up to ∼65%.
Magnetocaloric effect and specific heat of a La <sub>0.845</sub> Sr <sub>0.155</sub> MnO <sub>3</sub> sample were measured over a temperature range of 4.2–300 K, to verify whether large entropy changes induced by magnetic field make possible the use of manganites for magnetic refrigeration and whether a magnetostrictive contribution to magnetocaloric effect is significant. Isothermal entropy change and adiabatic temperature change induced by ΔB=7 T were determined as functions of temperature. Near to the Curie temperature, T<sub>C</sub>≈234 K , they reach, respectively, ∼-6.6 J /( kg K ) and ∼3.4 K , i.e., the values ∼2 and ∼4 times smaller than for Gd. Despite the presence of large magnetostriction, its contribution to magnetocaloric effect was found negligible. © 2003 American Institute of Physics.
This letter reports on the superior magnetocaloric properties of La <sub>0.7</sub> Ca <sub>0.3-x</sub> Sr <sub>x</sub> MnO <sub>3</sub> ( x=0.05 , 0.10, 0.20, and 0.25) single crystals. Upon 50 kOe applied field, the magnetic entropy changes (ΔS<sub>M</sub>) reach values of ∼10.5 , 7.45, 6.97, and 6.86 J / kg K for x=0.05 , 0.10, 0.20, and 0.25 compositions, respectively. The large magnetic entropy changes have been found to occur around 300 K, thus allowing magnetic refrigeration at room temperature. Due to the absence of grains in the single crystals, the ΔS<sub>M</sub> distribution here is much more uniform than that of gadolinium and polycrystalline manganites, which is desirable for an Ericson-cycle magnetic refrigerator. The single crystals have the large magnetic entropy changes induced by low magnetic field change, which is beneficial for the household application of active magnetic refrigerant (AMR) materials. These results indicate that the present single crystals are excellent candidates as working materials for AMR.
A giant magnetocaloric effect (ΔS<sub> mag </sub>) has been discovered in the Gd <sub> 5 </sub>( Si <sub>x</sub> Ge <sub>1-x</sub>)<sub>4</sub> pseudobinary alloys, where x≤0.5. For the temperature range between ∼50 and ∼280 K it exceeds the reversible (with respect to alternating magnetic field) ΔS<sub> mag </sub> for any known magnetic refrigerant material at the corresponding Curie temperature by a factor of 2–10. The two most striking features of this alloy system are: (1) the first order phase transformation, which brings about the large ΔS<sub> mag </sub> in Gd <sub> 5 </sub>( Si <sub>x</sub> Ge <sub>1-x</sub>)<sub>4</sub>, is reversible with respect to alternating magnetic field, i.e., the giant magnetocaloric effect can be utilized in an active magnetic regenerator magnetic refrigerator; and (2) the ordering temperature is tunable from ∼30 to ∼276 K by adjusting the Si:Ge ratio without losing the giant magnetic entropy change. © 1997 American Institute of Physics.
The recent literature concerning the magnetocaloric effect (MCE) has been reviewed. The MCE properties have been compiled and correlations have been made comparing the behaviours of the different families of magnetic materials which exhibit large or unusual MCE values. These families include: the lanthanide (R) Laves phases (RM2, where M = Al, Co and Ni), Gd5(Si1−xGex)4, Mn(As1−xSbx), MnFe(P1−xAsx), La(Fe13−xSix) and their hydrides and the manganites (R1−xMxMnO3, where R = lanthanide and M = Ca, Sr and Ba). The potential for use of these materials in magnetic refrigeration is discussed, including a comparison with Gd as a near room temperature active magnetic regenerator material.
The current-perpendicular-to-plane magnetoresistance (CPP-MR) has been investigated for the layered manganite, La2-2xSr1+2xMn2O7 (x = 0.3), which is composed of the ferromagnetic-metallic MnO2 bilayers separated by nonmagnetic insulating block layers. The CPP-MR is extremely large (10(4) percent at 50 kilo-oersted) at temperatures near above the three-dimensional ordering temperature (Tc approximately 90 kelvin) because of the field-induced coherent motion between planes of the spin-polarized electrons. Below Tc, the interplane magnetic domain boundary on the insulating block layer serves as the charge-transport barrier, but it can be removed by a low saturation field, which gives rise to the low-field tunneling MR as large as 240 percent.
A Comment on the Letter by A. Giguere et al., Phys. Rev. Lett. 83, 2262 (1999). The authors of the Letter offer a Reply.
The study of the manganese oxides, widely known as manganites, that exhibit the ``Colossal'' Magnetoresistance (CMR) effect is among the main areas of research within the area of Strongly Correlated Electrons. After considerable theoretical effort in recent years, mainly guided by computational and mean-field studies of realistic models, considerable progress has been achieved in understanding the curious properties of these compounds. These recent studies suggest that the ground states of manganite models tend to be intrinsically inhomogeneous due to the presence of strong tendencies toward phase separation, typically involving ferromagnetic metallic and antiferromagnetic charge and orbital ordered insulating domains. Calculations of the resistivity versus temperature using mixed states lead to a good agreement with experiments. The mixed-phase tendencies have two origins: (i) electronic phase separation between phases with different densities that lead to nanometer scale coexisting clusters, and (ii) disorder-induced phase separation with percolative characteristics between equal-density phases, driven by disorder near first-order metal-insulator transitions. The coexisting clusters in the latter can be as large as a micrometer in size. It is argued that a large variety of experiments reviewed in detail here contain results compatible with the theoretical predictions. It is concluded that manganites reveal such a wide variety of interesting physical phenomena that their detailed study is quite important for progress in the field of Correlated Electrons. Comment: 76 pages, 21 PNG files with figures. To appear in Physics Reports
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