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5 7 Fe Emission Mössbauer Study on Gd 3 Ga 5 O 1 2 implanted with dilute 5 7 Mn
Abstract
57Fe emission Mössbauer spectroscopy has been applied to study the lattice location and properties of Fe in gadolinium gallium garnet Gd3Ga5O
12 (GGG) single crystals in the temperature interval 300 – 563 K within the extremely dilute (<10−4 at.%) regime following the implantation of57Mn (T
1/2= 1.5 min.) at ISOLDE/CERN. These results are compared with earlier Mössbauer spectroscopy study of Fe-doped gadolinium gallium garnet Gd3Ga5O
12(GGG), with implantation fluences between 8×1015 and 6×1016 atoms cm−2. Three Fe components are observed in the emission Mössbauer spectra: (i) high spin Fe2+ located at damage sites due to the implantation process, (ii) high spin Fe3+ at substitutional tetrahedral Ga sites, and (iii) interstitial Fe, probably due to the recoil imparted on the daughter57∗Fe nucleus in the β
− decay of57Mn. In contrast to high fluence57Fe implantation studies the Fe3+ ions are found to prefer the tetrahedral Ga site over the octahedral Ga site. No annealing stages are evident in the temperature range investigated. Despite the very low concentration, high-spin Fe3+ shows fast spin relaxation, presumably due to an indirect interaction between nearby gadolinium atoms.
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In search for dilute magnetic semiconductors, the magnetic properties at the atomic-scale of Fe atoms incorporated in ZnO, in a concentration range of more than fi ve orders of magnitude from 1 × 10 −5 to 2.2 at% have been probed using emission 57 Fe Mössbauer spectroscopy on implanted 57 Mn and 57 Co produced at ISOLDE/CERN. In the ultra-dilute regime (10 −5 at%), the system shows isolated paramagnetic Fe 3+ ions with a spin–lattice type of relaxation. At higher concentrations (between 0.02 and 0.2 at%) a transi-tion to spin–spin type of relaxation between neighboring Fe 3+ is observed, without any signature of magnetic ordering up to 2.2 at%. Despite the many reports of dilute magnetism in 3 d -doped ZnO, this atomic level study shows no evidence of any long-range magnetic ordering between isolated Fe atoms incorporated in the ZnO lattice.
A series of R3(Ga1−xFex)5O12 (R=Gd, Dy, Ho; 0
Single crystals of α-Al2O3 and LiNbO3 were implanted with57Co (dose: up to 21015 atoms/cm2) and with57Fe (dose: 21015 atoms/cm2) ions. The Mssbauer spectra revealed the disordered atomic environment. Fe2+ and Fe3+ charge states were observed. The spectra were compared to the spectra of crystals doped with57Co. It was remarkable that in the doped α-Al2O3 Fe3+ states with slow spin-spin relaxation have appeared. The CEMS study of the samples implanted with57Fe resulted in Fe2+ ionic states indicating that a fraction of Co atoms can also be in Co2+ state.
Gadolinium gallium garnet single crystals were implanted with doses of ions in the range . Depending on the dose, iron with or charge states was found to have formed after the implantation. After a subsequent annealing in air, the iron oxidized to . The Mössbauer and channelling measurements showed lattice recrystallization taking place at . After recrystallization, the iron was found to have substituted for gallium ions both at the octahedral and at the tetrahedral positions. The relative concentration of the two types of iron at the two sites shifted towards the equilibrium distribution upon high-temperature annealing.
The spin–lattice relaxation rate of paramagnetic Fe 3+ in single-crystalline ZnO has been determined following low-fluence (< 10 12 cm −2) 60 keV implantation of 57 Mn + (T 1/2 = 1.5 min) and emission Mössbauer spectroscopy on the 57 Fe daughter nucleus in the temperature range from 300 to 664 K. The spin–lattice relaxation of Fe 3+ is found to follow a T 9 temperature dependence, in contrast to the T 2 dependence expected for a two-phonon Raman process determined in both single-crystal MgO and α-Al 2 O 3 using the same analysis method of the Mössbauer spectra measured without an applied external magnetic field. This is an unexpected result since ZnO has a lower Debye temperature than both MgO and α-Al 2 O 3 .
The valence state and annealing reactions of Mn/Fe in single crystalline α-Al2O3 have been determined following low fluence (<1012cm − 2) 60keV implantations of 57Mn + (T1/2 = 1.5min) and emission Mössbauer spectroscopy on the 57m
Fe daughter nuclei in the temperature range from 110–700K. At 110K, most probe atoms are found in the Fe2 + state in amorphous surroundings due to the implantation damage. A fraction of the Fe is found in cubic environment, possibly
nano-precipitates of η-Al2O3. This site is found to disappear from the spectra above 500K. Annealing of the damage sites at increasing temperatures leads
first to increased incorporation of the probe atoms as Fe3 + on Al sites, and, above room temperature, also as Fe4 + . The Fe3 + sub-spectrum is masked by slow paramagnetic relaxations following a T2 dependence, as expected for a two-phonon Raman process. Our data is consistent with data from 57Co and 57Fe implantations, suggesting a general increase in the average Fe valence state with lower implantation dose and negligible
annealing reactions at room temperature.
KeywordsEmission Mössbauer spectroscopy–Al2O3
–
57Fe–
57Mn
The working refrigerant material in the majority of magnetic refrigerators has been Gd 3 Ga 5 O 12 (GGG) which has an upper temperature limit near 15 K. In this paper we report on the field‐induced adiabatic magnetic entropy change, ΔS m (H,T), of a series of iron‐substituted gadolinium garnets (GGIG) Gd 3 Ga 5-x Fe x O 12 which have the potential to increase the working temperature range or to reduce the field requirements of cryogenic magnetic refrigeration. Depending on Fe concentration, x, the entropy change of these materials at applied fields of 0.9 and 5.0 T is much greater than that of GGG, especially at temperatures above 15 K. At low Fe concentrations, the results are consistent with formation of magnetically ordered clusters of spins at low temperatures. Room temperature electron paramagnetic resonance measurements show that Fe<sup>3+</sup> ions mediate exchange interactions which are responsible for clustering at low temperatures.
Prompted by the generally poor understanding of the nature of magnetic phenomena in 3d-metal doped ZnO, we have undertaken on-line (57)Fe Mossbauer spectroscopy on ZnO single crystals in an external magnetic field of 0.6 T, following the implantation of radioactive (57)Mn ions at room temperature. The Mossbauer spectra of the dilute Fe impurities are dominated by sextets whose angular dependence rules out an ordered magnetic state (which had been previously proposed) but are well accounted for on the basis of Fe(3+) paramagnetic centers on substitutional Zn sites with unusually long relaxation times (> 20 ns). (C) 2010 American Institute of Physics. [doi:10.1063/1.3490708]
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A series of Gd-x(Ga1-xFex)(5)O-12 (GGIG) compounds for magnetic refrigerants were characterized by magnetization and specific heat measurements For comparison, the base compound gadolinium gallium garnet (GGG) was also measured From the magnetization measurements: the entropy change by external magnetic field was confirmed to be enhanced by the addition of iron, as previously reported by the National Institute of Standards and Technology (NIST) group. the specific heat data obtained in various magnetic fields enabled the establishment of temperature entropy (T-S) diagrams, which are essential for thermal cycle analysis. It has been shown that GGIGs with high iron content have a large specific heat in zero field, and are suitable for regenerative thermal cycle. (C) 2009 The Japan Society of Applied Physics
The work is devoted to an analysis of formation processes of the radiation displacement defects (RDDs) and colour centres (CCs) in complex oxide crystals under irradiation. The calculation results on the displacement process simulation as well as an analysis of the RDD and CC accumulation kinetics are presented. New experimental results on additional absorption spectra induced by neutron irradiation of LiNbO3 (LNO) crystals doped with Fe and Cr and YAlO3 (YAP) crystals doped with Nd and Er as well are presented. Dose dependencies of the additional absorption are compared and their peculiarities are discussed. The obtained results confirm that CCs causing the irradiation induced absorption in LNO and YAP crystals are formed in secondary processes which are affected by impurities. It is also found that the induced absorption dose dependence in LNO crystals has an unexpected slope.
A series of R3(Ga1−xFex)5O12 (R=Gd, Dy, Ho; 0<x<1) compounds for potential magnetic refrigerants were synthesized by chemical routes and characterized by X-ray diffraction, and SQUID magnetometry. Dy and Ho were chosen since they respectively possess increasing orbital contributions to the total angular magnetic moment of the atom over the zero value for Gd. X-ray data showed that garnet structures were obtained and that improvements over the Gd3(Ga0.5Fe0.5)5O12 compound, which was reported in 1992 as possessing enhanced magnetocaloric effects, may be achieved by equilibrating at 1473 K for 15 h, rather than at 1173 K for 15 h as was done in the earlier studies. Magnetometry measurements showed that when Gd was substituted either by Dy or Ho, the material was superparamagnetic, possessing fine magnetic clusters resulting in enhanced magnetocaloric effects (ΔSm) with respect to the basic paramagnetic garnet (i.e., x=0). In addition, with variation in “x”, the optimal ΔSm was measured for the x=0.5 compound, similar to that found for the Gd-containing garnet nanocomposites. The optimal ΔSm values for the Ho- and Dy-containing compounds, respectively, were found to be about the same or smaller than that for the optimal Gd-containing nanocomposite Gd3(Ga0.5Fe0.5)5O12, despite the increased total angular moment. We interpret these results as indicating a reduction in the interaction strength between the rare-earth elements and the Fe as the Gd is replaced by Dy or Ho, and that Dy reduces this interaction strength faster than does Ho.
Ion beams produced by on-line isotope separators are often contaminated by abundantly produced isobars. This significantly complicates, and in a number of cases renders impossible, the realization of physical experiments with such beams. In many cases the purity of the ion beams may be improved on the ionization stage of the isotope separation using a laser resonance ion source. The method of laser resonance ionization of atoms in a hot cavity has been applied for chemically selective production of Mn ion beams at the CERN-ISOLDE facility. Radiation of pulsed dye lasers and copper vapor lasers provided three-step excitation and ionization of Mn isotopes in the ordinary W surface ionizer, connected to the target container. High values of efficiency and selectivity obtained with the laser ion source made it possible to suppress substantially an admixture of 57Fe in the 57Mn ion beam and to perform Mössbauer experiments on implanted impurities in solids.
Displacement Defect Formation in Complex Oxide Crystals
- S B Ubizskii
- A O Matkovskii
- N Mironova-Ulmane
- V Skvortsova
- A Suchocki
- Y A Zhydachevskii
- P Potera
- SB Ubizskii
Spin-lattice relaxations of paramagnetic Fe + 3 $_{+}^{mathrm {3}}$ in ZnO
- T E Mølholt
- H P Gunnlaugsson
- K Johnston
- R Mantovan
- H Masenda
- S Ólafsson
- K Bharuth-Ram
- H P Gislason
- G Langouche
- G Weyer
- TE Mølholt