Temperature-induced Spin Reorientation Research Articles

Field-induced magnetic transitions in Pr 2(Co,Fe) 17

A study of the different magnetic field-induced transitions has been performed for Pr 2Co 17− x Fe x compounds in the temperature range 78–300 K. By using the singular point detection technique, it has been shown that all compounds with x>0 exhibit at least one field-induced transition, which has been identified as a first-order magnetization process (FOMP). Different types of FOMP (A1, A2, P1) are observed, and the change of the type can be induced by varying either temperature or composition. Intermediate Co–Fe compositions show a P1-type FOMP which persists even above room temperature, while a double transition has been found in iron-rich compounds with x⩾13.6. It has also been noticed that, for the particular values of x when a temperature-induced spin reorientation is present, the change from A1 to P1 type FOMP (or vice versa) exactly occurs at the spin reorientation temperature, with a vanishing critical field value. A comparison with the behavior of Y 2Co 17− x Fe x compounds leads to the conclusion that the contributions of the Pr sublattice are essential for the development of these field-induced transitions (some of which are still present at room temperature), even if it is inferred that the overall anisotropy is mainly determined by the 3d sublattice.

Magnetic Anisotropy in Rare-Earth - Transition Metal Intermetallic Compounds

ABSTRACTIn this work the rare-earth anisotropy in the rare-earth - transition metal intermetallic compounds is investigated by studying the temperature-induced spin reorientation phenomena on aligned powder samples and by analyzing the temperature dependence of the magnetization measured along different principal axes of single crystals. From such investigations the crystalline-electric-field parameters of the rare-earth ions in these compounds have been derived. The results on different hard magnetic materials of the compositions R2Fe14B, RFe11 and their substituted compounds as well as R2Co17 are summarized.

FMR study of Co-substituted yttrium iron garnet films

Ferromagnetic resonance has been studied in Y 3− z Ca z Fe 5− x-y Co x Ge y O 12 films grown on the (001) plane of a GGG substrate. Temperature-induced spin-reorientation phase transitions were observed. On cooling the sample, the easy magnetization axes orientations changed from directions near [111] to [100], and the [001] direction had the lowest energy. The FMR signal was practically non-measurable below 60 and 120 K when the magnetic field was applied along the [110] and [100] axes, respectively.

Influence of hydrogen on the structural and magnetic properties of the RFe10.5Mo1.5 compounds (R=rare earth)

The structural and magnetic properties of RFe10.5Mo1.5 alloys and their hydrides were studied using X-ray diffraction and magnetic measurements. Hydrogen occupies the octahedral interstitial 2b site of the tetragonal ThMn12-type structure (space group I4/mmm, Z = 2), which allows a maximum uptake of one hydrogen atom per formula unit. The unit cell volume expands about 1% and the Curie temperature is increased about 10% by the hydrogen uptake. The mean magnetic moment of the iron sublattices is increased. On hydrogenation, (i) the magnetocrystalline anisotropy of the 3d sublattice is enhanced, (ii) in the compounds containing the R elements with a positive second-order Stevens factor αJ, the c-axis is the easy axis, and (iii) for the compounds with R having a negative second-order Stevens factor, the easy direction lies in the ab-plane. The Dy and Tb alloys show temperature induced spin reorientation transitions (SRT). On hydrogen uptake, their hydrides have a basal plane behaviour. This change in the series can be understood by the donor character of H together with a narrowing of the 3d metal bands induced by the volume expansion.

Magnetic anisotropy and spin-reorientation transitions in RFe 11Ti (R = Nd, Tb, Dy, Er) rare-earth intermetallics

RFe 11Ti rare-earth intermetallics (R = Nd, Tb, Dy, Er) which show temperature-induced spin-reorientation transitions (SRT) were investigated. The temperature dependences of the rare-earth anisotropy constants were calculated using the single-ion model. By fitting the experimental data a set of crystal-field and exchange-field parameters for R 3+ ions wassb deduced. The temperature dependence of the first-order magnetization process fields was calculated for NdFe 11Ti and ErFe 11Ti within the model. The calculated spin-reorientation transition in NdFe 11Ti observed at T s = 189 K is of first order with a discontinuous jump of the magnetization angle. Present calculations show that a first-order transition occurs in DyFe 11Ti at low temperatures. A second-order SRT from ‘easy axis’ to ‘easy cone’ exists in ErFe 11Ti. A first-order SRT ‘easy axis’-‘easy plane’ is predicted theoretically in TbFe 11Ti. The calculated curves of the temperature dependence of the anisotropy fields and the critical fields of the first-order magnetization processes for RFe 11Ti using the fitted crystal- and exchange-field parameters are in good agreement with the experimental data over a wide temperature range.

Magnetic anisotropy and magnetization processes in 3:29 and 1:12 Nd(FeTi)-based compounds

The temperature behavior of the magnetic anisotropy and magnetization processes in the tetragonal Nd(FeTi)12 and monoclinic Nd3(FeTi)29 have been studied and compared. Both compounds are characterized by the occurrence of temperature-induced spin reorientation transitions, as well as field-induced magnetization processes of different types, which have been interpreted as first-order magnetization process. In both systems the resultant anisotropy of Nd favors the occurrence of an easy magnetization direction out of the principal axes at low temperature. On the contrary the overall Fe sublattice anisotropy favors the tetragonal c-axis in the 1:12 and seems to prefer the monoclinic a-axis in the 3:29 compound.

Effects of iron substitution on the magnetic anisotropy of YCo4B

The effects of iron substitution on the magnetization, the magnetic anisotropy, the spin-reorientation temperature, and the Curie temperature of Y(Co1−xFex)4B have been studied on the cobalt-rich side for x values below 0.4. Temperature-induced spin-reorientation phenomena may be observed in compounds with x values below 0.02 by inspecting the low-field magnetization curves of bulk polycrystalline samples and by observing the change in the easy direction in field-oriented powdered samples. The spin-reorientation temperature shifts from 145 K for YCo4B to higher values with increasing x and the phenomenon disappears for values larger than 0.03. By analyzing the magnetization curves of field-oriented powder samples, the magnetic anisotropy has been evaluated. The value for the first anisotropy constant K1 at 4.2 K decreases in a nonlinear way with increasing x values and reaches a minimum around x=0.3.

Theory of spin reorientation in rare-earth orthochromites and orthoferrites

Mechanisms of the temperature-induced spin-reorientation in rare-earth orthochromites (and orthoferrites) are examined. It is concluded that the anisotropic parts of the magnetic interactions between Cr 3+ (or Fe 3+) and rare-earth ions, the antisymmetric and the anisotropic-symmetric exchange interactions, are generally responsible for both the rotational and the abrupt types of the spin-reorientations. These anisotropic exchange interactions produce an effective field for the Cr 3+ up-spins in the direction perpendicular to that of these spins and an effective field for the Cr 3+ down-spins in the direction opposite to the above. These effective fields favor rotation of the Cr 3+ spins, retaining their original antiferromagnetic configuration. Thus, as the temperature is lowered, this effective field increases due to the increase of the rare-earth magnetization, and when the interaction energy of the Cr 3+ spins with these effective fields exceeds the anisotropy energy of the Cr 3+ ion, spin-reorientation. takes place. At the beginning and ending of the spin-reorientation a second-order phase-transition occurs. The first-order nature of the abrupt spin-reorientation is stressed. Anisotropic exchange interactions between Cr 3+ and rare-earth ions also play an important role in inducing the abrupt spin-reorientation.