Spin Reorientation Effects Research Articles

Spin reorientation and magnetocaloric effect study in HoAl2 by a microscopic model Hamiltonian

The magnetocaloric properties in HoAl2 were fully investigated using a model Hamiltonian that takes into account the exchange magnetic interaction and the crystalline electrical field anisotropy. We have shown that spin reorientation has an influence on the magnetocaloric effect. An anomalous valley was predicted to exist in the magnetocaloric curves, and it was ascribed to the spin reorientation. All theoretical results were obtained using the proper model parameters found in the literature for this compound.

AMagnetic control of spin reorientation and magnetodielectric effect below the spin compensation temperature in TmFeO3

The onset of antiferromagnetic transition, spin reorientation, and spin compensation of TmFeO3 single crystals were investigated by the magnetic and heat capacity measurements. Control of spin reorientation by magnetic field and anomalous hysteretic behavior in domain switching were clarified. No appreciable magnetodielectric effect was observed in spin reorientation temperature region. On the other hand, below the spin compensation temperature both a dielectric anomaly along the c axis and a concomitant magnetodielectric effect up to ∼4% at 80kOe were observed. This suggests that rare-earth orthoferrites can be another candidate for magnetodielectric system through the mediation of spin compensation phenomena.

Spin reorientation effects in GdFe3(BO3)4 induced by applied field and temperature

Magnetic properties of GdFe3(BO3)4 single crystals were investigated by 57Fe-Mossbauer spectroscopy and static magnetic measurements. In the ground state, the GdFe3(BO3)4 crystal is an easy-axis compensated antiferromagnet, but the easy axis of iron moments does not coincide with the crystal C3 axis, deviating from it by about 20°. The spontaneous and field-induced spin reorientation effects were observed and studied in detail. The specific directions of iron magnetic moments were determined for different temperatures and applied fields. Large values of the angle between the Fe3+ magnetic moments and the C3 axis in the easy-axis phase and between Fe3+ moments and the a2 axis in the easy-plane phase reveal the tilted antiferromagnetic structure.

Magnetic behavior ofEuCu2As2: A delicate balance between antiferromagnetic and ferromagnetic order

The Eu-based compound, EuCu2As2, crystallizing in the ThCr2Si2-type tetragonal structure, has been synthesized and its magnetic behavior has been investigated by magnetization (M), heat-capacity (C) and electrical resistivity (rho) measurements as a function of temperature (T) and magnetic field (H) as well as by 151Eu Moessbauer measurements. The results reveal that Eu is divalent ordering antiferromagnetically below 15 K in the absence of magnetic field, apparently with the formation of magnetic Brillouin-zone boundary gaps. A fascinating observation is made in a narrow temperature range before antiferromagnetism sets in: That is, there is a remarkable upturn just below 20 K in the plot of magnetic susceptibility versus T even at low fields, as though the compound actually tends to order ferromagnetically. There are corresponding anomalies in the magnetocaloric effect data as well. In addition, a small application of magnetic field (around 1 kOe at 1.8 K) in the antiferromagnetic state causes spin-reorientation effect. These results suggest that there is a close balance between antiferromagnetism and ferromagnetism in this compound

Magnetic properties of single-crystalDyAl2

We measured the magnetic properties and heat capacity of three $\mathrm{Dy}{\mathrm{Al}}_{2}$ single crystals with the magnetic field oriented along the three principal crystallographic directions: [100], [110], and [111]. The isothermal entropy change versus temperature curves were obtained from heat capacity and magnetization data for these directions. The experimental results were successfully explained by a mean field model that includes spin reorientation, exchange interactions, and crystalline electric field effects. The anomalous magnetocaloric effect along the [111] direction predicted by theory was confirmed experimentally.

Structural and magnetic properties of DyFe 12–x Nb x compounds

Spin reorientation effects in DyFe12–xNbx compounds with x=0.6, 0.7 and 0.8 have been investigated by ac magnetic susceptibility and 57Fe Mössbauer effect measurements over the temperature range 4.2–300 K. Refinements of the X-ray diffraction patterns show that DyFe12–xNbx compounds crystallize in the ThMn12-type structure and that the Nb atoms occupy the 8i sites. A spin reorientation from easy axis to easy cone at Tsr1, then to easy plane at Tsr2 with decreasing temperature has been detected by ac susceptibility. Both Tsr1 and Tsr2 decrease monotonically with increasing Nb content (from Tsr1 ∼ 236 K for x=0.6 to Tsr1 ∼ 204 K for x=0.8 and from Tsr2 ∼ 154 K for x=0.6 to Tsr2 ∼ 94 K for x=0.8). Analyses of the Mössbauer spectra (4.2–300 K) reveal a discontinuity in the temperature dependences of the hyperfine parameters at Tsr2 for the 8i Fe. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropic materials: A way to increase the efficiency of magnetic refrigeration

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%.

Magnetic behaviour of quasi-one-dimensional oxides, Ca 3Co 1+ xMn 1− xO 6

The results of ac and dc magnetization and heat capacity measurements on the oxides, Ca 3Co 1+ x Mn 1− x O 6, forming in a K 4CdCl 6-derived rhombohedral quasi-one-dimensional crystal structure, are reported. As far as Ca 3Co 2O 6 is concerned, the results reveal a truly complex nature of the two magnetic transitions, identified to set in at 24 and 12 K in the previous literature. However, partial replacement of Co by Mn apparently results in a long magnetic ordering of an antiferromagnetic type (below 13 and 18 K for x=0.0 and 0.25, respectively), instead of spin-glass freezing in spite of the fact that there is Co–Mn disorder; in addition, interestingly there are hysteretic spin reorientation effects as revealed by the isothermal magnetization behavior.

Local Spin Configurations and Magnetic Ordering in Competing-Exchange System (Rh, Pd)90Fe10

Mössbauer spectroscopy technique has been used to investigate the evolution of the hyperfine field distributions and the local spin configurations of Fe atoms in the magnetically ordered (Rh1—xPdx)0.9Fe0.1 alloys over the concentration range 0 ≤ x ≤ 1. In the whole composition range, the magnetic behavior of the alloys is governed by the competition between two exchange interactions opposite in sign: the direct antiferromagnetic coupling between the nearest-neighbor Fe atoms and the ferromagnetic interaction due to the matrix polarization. For Rh-rich alloys, this leads to a spin-glass-like magnetic structure, where the ferro- and antiferromagnetic local spin configurations are realized with nearly equal probabilities. For the concentration range x ≤ 0.3, the magnetic ordering temperature is equal to about 32 K independent of the Pd concentration. In the same concentration range, a sharp increase of the magnetic moment of Fe atoms was observed. The severe influence of rhodium on the Curie temperature of the ferromagnetic PdFe alloy is explained by the spin reorientation effect, leading to a destruction of the ferromagnetic spin structure. The strong diffuseness of the magnetic transition in the intermediate concentration range 0.4 < x < 0.8 is due to the presence of Fe sites with both Rh-rich and Pd-rich local environments.

Magnetic anisotropy and spin reorientation effects in Gd/Fe and Gd/(FeCo) multilayers for high density magneto-optical recording

We have investigated the anisotropy behavior and temperature dependent changes of the magnetic anisotropy in Gd/Fe and Gd/(FeCo) multilayers. The spin reorientation effects are very important for the super resolution readout in new methods for high-density magneto-optical recording. Gd/(Fe, Co) multilayered films are a good alternative to the common Gd(Fe, Co) alloy films, because the magnetic anisotropy and also spin reorientation effects can be comfortably adjusted by varying the interface and volume anisotropy components and the composition using experimental parameters such as the periodicity λ=tGd+tTM [tGd,tTM: the sublayer thicknesses of the Gd and transition metal Fe, FeCo (TM)] and the ratio of the sublayer thicknesses tGd/tTM. We have found the mechanisms for spin reorientation, which are explained qualitatively with a new model.

Different mechanisms of spin reorientation in exchange coupled double rare earth-transition metal layers with in-plane and perpendicular magnetic anisotropy

The objective of this article is to clarify the spin reorientation effects in exchange coupled layers based on rare earth and transition metals with in-plane and perpendicular anisotropy. These systems are proposed for high density magneto-optical recording with super resolution readout. Two mechanisms are found to be responsible for this change of the magnetization direction with the temperature. The first mechanism is an intrinsic spin reorientation in the GdFeCo layer and the second is the exchange coupling force between the perpendicularly magnetized TbFeCo film and the in-plane GdFeCo film. With appropriate adjustment of the compensation temperatures and the thicknesses of the films, it is possible that the one or the other mechanism dominates. The experimental results are in agreement with a numerical calculation of the magnetization profiles in the coupled system.

Magnetic anisotropy in Gd/(FeCo) and Gd/Fe multilayers for high density magneto-optical recording

Very recently, new techniques in the magneto-optical recording were proposed which promise the readout of domains smaller than the optical diffraction limit. For this purpose Gd–(Fe,Co) alloy films are used, which change their magnetization direction with temperature. In this experimental study for the first time spin reorientation effects in multilayers of Gd/(FeCo) and Gd/Fe were investigated with varying composition and periodicity. Both Gd/(FeCo) and Gd/Fe films show a compositional dependence of the anisotropy similar to a phase transition. An in-plane to perpendicular spin reorientation near the compensation temperature Tcomp can be obtained in rare earth rich films for small periodicities λ∼1.1 nm, but for increasing λ the anisotropy change occurs at temperatures different from Tcomp. The opposite effect, i.e., spin reorientation from perpendicular to in-plane, can be observed in transition metal rich Gd/(FeCo) multilayers but not in Gd/Fe multilayers. The anisotropy behavior and the reorientation effects are explained qualitatively within a new model.

Low field magnetic studies of some alloys

Polycrystalline pure Gd and (x = 0.15, 0.25, 0.27 and 0.50) alloys have been studied by low field magnetic susceptibility , magnetization and electrical resistivity measurements. Measurements were also performed on samples with low demagnetizing factor geometry in order to study the ground state properties of these systems. For hexagonal close-packed (hcp) samples the effect of spin reorientation is indicated by a broad peak in the susceptibility curve and it shifts rapidly to lower temperatures with increasing La content in Gd metal. The spin reorientation effect is not observed in the samples with La concentration higher than 15%. The and resistivity measurements indicate a ferromagnetic transition at for 15% La concentration. For the Sm-type structure sample, with x = 0.27, two magnetic transition peaks are observed at 135 K and 113 K and helical antiferromagnetic order is suggested. Finally, the results for the x = 0.50 alloy with double hexagonal close-packed (dhcp) crystalline structure indicate a spin-glass magnetic phase type, below . Observed magnetization remanent effects, in this latter case, give additional support for the presence of the spin-glass phase.

Field-induced spin reorientations in TbFeO3 at 4.2 K

We report Mössbauer and magnetisation studies of spin-reorientation transitions in single-crystal samples of TbFeO3 at 4.2 K, induced by fields up to 10 T. Fields were applied along all three crystal axes and various spin-reorientation effects were observed. These may be explained microscopically in terms of FeTb interactions, perturbation of the Tb3+ ion energy levels, and metamagnetic transitions in the Tb3+ spin subsystem.

Synthesis under high pressure of vacancy-ordered phases in the system CaFeO3 –y

The perovskite-related oxide Ca2Fe2O5 takes up additional oxygen under high pressure and elevated temperature. The Mössbauer spectra from a number of preparations establish the existence of new phases which are structurally related to the original lattice. The latter features an ordered arrangement of oxygen vacancies so as to form alternate layers of octahedral and tetrahedral Fe polyhedra. The introduction of oxygen to give CaFeO2.60 initially increases the number of octahedral layers to give a sequence [ootot], and some of the iron in the double-octahedral layers undergoes oxidation to the 5+ oxidation state (by a nominal charge disproportionation of Fe4+ at low temperature). At least six distinct iron sites can be identified, and their coordination numbers established. It is believed that the charge disproportionation is reversed with increasing temperature. Further uptake of oxygen to give CaFeO2.69 produces some layers with five-coordination. The Mössbauer spectra reveal the existence of unusually complicated and subtle magnetic interactions, which include spin-reorientation effects and may be triggered by a change in oxidation state of some cations. Oxidation can extend to at least CaFeO2.80 before any substantial intergrowth of CaFeO3 becomes apparent. It is clear that a wide range of non-stoichiometry can be achieved by the intergrowth of layers of Fe in different coordination, and the extra oxygen is counterbalanced by the generation of Fe5+ cations in octahedral coordination. The charge disproportionation appears to be established within two-dimensional layers of corner-sharing octahedra.

Heat-capacity behavior of the alloys Pr 1− xGd xCu 2Si 2

The results of heat capacity measurements (0.3–40 K) on the pseudoternary solid solution Pr 1− x Gd x Cu 2Si 2 (0 < × ≤ 1) are reported. The data show interesting features attributable to magnetic-ordering and spin-reorientation effects. The x dependence of the data indicates heavy-fermion-like behaviour for PrCu 2Si 2.

Effect of spin-reorientation of the domain structure in HoCo 3Ni 2

The domain structure in polycrystalline HoCo 3Ni 2 was studied above (room temperature) and below (5 K) the reorientation range, using the Kerr effect, the powder technique and the oxygen cryocodensation method. The domain wall energy was determined for both temperature regions.

Effect of texture and spin reorientation on the magnetic properties of Nd-Fe-B magnets.

Effects of texture--muniaxial, two-dimensional isotropic, and isotropic--mon the hysteresis loops of fully consolidated ${\mathrm{Nd}}_{15}$${\mathrm{Fe}}_{79}$${\mathrm{B}}_{6}$ and (NdDy${)}_{15}$${\mathrm{Fe}}_{79}$${\mathrm{B}}_{6}$ magnets have been investigated between 4.2 and 273 K. The steps in the loops, due to the spin reorientation have been observed to become more pronounced as the texture symmetry increases. Models based on uniaxial, two-dimensional isotropic, and isotropic magnetization are proposed to describe the loop shapes below the spin-reorientation temperature, ${T}_{\mathrm{sr}}$. The ratio of the extrapolated remanence, before and after spin rotation, has been shown to be indicative of the magnetic texture.

Spin reorientation effect on magnetization - application to Nd2Fe14B

Spin reorientation is a phenomenon observed frequently in magnetic materials and is responsible for changes in magnetization as a function of temperature. In this study an analytical model is described which accounts for such changes in magnetization. Three different magnetization vs. temperature measurements are described which confirm the occurence of spin reorientation and indicate the exact temperature at which it occurs (application to Nd2Fe14B intermetallic compound). The analytical solution of the model enables one to determine the K1/K2 ratio of the first- (K1) and second-order (K2) anisotropy constants as a function of temperature. Also, this model clearly demonstrates the effect of crystal orientation (angle between the c-axis and the applied magnetic field) on the magnetization vs. temperature behavior; one can take advantage of this in order to achieve a maximum or a minimum effect. The effect of the presence of the third-order (K3) anisotropy constant is also discussed.

Magnetic behavior of R 2Fe 14B hydrides (R = Gd, Tb, Dy, HoandEr)

R 2Fe 14B systems, with R = Gd, Tb, By, Ho and Er were hydrogenated to the composition R 2Fe 14BH x where x ranges from 3.7 to 5.4. The pressure-composition isotherms (PCIs) of the hydrides showed only a solid solution behavior. No plateau pressure region was observed between room temperature and 300° C and at pressures down to 10 -2 atm. The absorbed hydrogen leads to an increase of 2.6 to 3.4% in unit cell volume, without a change in crystal structure. Magnetic characteristics of the present compounds were investigated over the temperature range 4.2 to 1100 K and at applied field up to 20 kOe. Saturation magnetization, M s, and magnetic ordering temperature, T c, were enhanced upon hydrogenation. T c, is found to be dependent on the hydriding composition. Hydrogen induces a spin-reorientation effect (SR) in Gd- and Dy-containing compounds, while it has a marked influence in raising the spin-reorientation temperature, T SR, in the Er 2Fe 14B compound. The hydride involving Tb appears to remain uniaxial to the lowest temperature studied. In all cases the anisotropy fields, H A/' were significally reduced by hydrogen absorption. These varied magnetic behaviors can be ascribed to the effects: (1) variations in the interatomic distances, (2) strengthening the 3d-3d and weakening the 4f-3d exchange interactions and (3) the interstitial site occupations of hydrogen in the lattice. The spin-reorientation phenomena observed for Gd 2Fe 14BH x suggest that there is competition among the 6 Fe sublattices in regard to the sign and temperature coefficient of anisotropy.