Physica B 183 (1993) 379-384 North-Holland
On the spin reorientation A.V. AndreePb,
and related compounds
and E.N. Taraso$
“Permanent Magnets Laboratory, Institute of Physics and Applied Mathematics, Ekaterinburg, Russian Federation
Ural State University,
bInstitute for Solid State Physics, Universily of Tokyo, Japan
Received 14 September
The ferrimagnet TbFe,,Ti (LL, = llh at 4.2 K, T, =56OK) with the tetragonal ThMn,,-type of crystal structure undergoes a first-order spin-reorientation phase transition from basal-plane (at low temperatures) to uniaxial (at high temperatures) magnetic anisotropy. The temperature of the transition is extremely sensitive to the magnetic field in the low-field range (from 250 K at 0.3 mT to 32.5K at 0.15 T internal field).
1. Introduction In the binary R-T alloy systems (R = rareearth metal, T = 3d metal) only Mn forms intermetallic compounds with the ThMn,, type tetragonal crystal structure. This structure can be stabilized by a third component M in the highFe-content alloys RFe,,_,M, (M = Si, Ti, V, Cr, MO, W, Re; x = l-3). In these compounds, the Fe sublattice has uniaxial magnetocrystalline anisotropy. At low temperatures, the contribution from the R sublattice to the total anisotropy may be either uniaxial (R = Sm, Ho, Tm) or multiaxial (of basal-plane or cone type, R = Pr, Nd, Tb, Dy, Er). In the latter compounds, spontaneous spin reorientations have been observed with increasing temperature due to the competition between the anisotropies of the R and Fe sublattices. For the compound under present investigation, the ferrimagnet TbFe,,Ti, four different results about the spin reorientation exist, presented as tentative phase diagrams in fig. 1. In ref. [l], as well as in several other papers of these authors, the compound is found to have magnetocrystalto: A.V. Andreev, Permanent Magnets Laboratory, Institute of Physics and Applied Mathematics, Ural State University, Ekaterinburg 620083, Russian Federation.
line anisotropy of the cone type at low temperatures up to 220 K. Then it exhibits basal-plane anisotropy up to 450 K, where it transforms directly to uniaxial anisotropy up to the Curie temperature of 554 K. In all other studies only one spin-reorientation phase transition is found. From measurements on polycrystals [2,3] we have concluded that the easy-cone structure exists at low temperatures, then the angle of cone
Fig. 1. Tentative phase diagrams of TbFe,,Ti the results of various studies.
fQ 1993 - Elsevier Science Publishers B.V. All rights reserved
et (11. ! Spin
decreases and at 330 K the cone-axis transition takes place. In refs. [4-61 the transition is claimed to be of the plane-axis type; however, the temperature of spin reorientation differs considerably: 327 K in refs. [4,6] and 285 K in ref. . Such a discrepancy cannot be explained by the difference in composition of the samples within the possible homogeneity range of the compound, since the obtained values of T, are practically the same (554-558 K) and also there is rather good agreement in the lattice-parameter data. All the above-mentioned studies were carried out on polycrystals. For highly anisotropic magnetic materials, the interpretation of the observed results is always questionable in the case of polycrystalline samples. In the present paper, the results of a spin-reorientation study on TbFe, ,Ti single crystals are presented.
in 7’t~F‘e,~ Ti
was measured between 77 and 350 K by a standard technique in fields with amplitude 0.34 mT.
In fig. 2, the temperature dependences of the magnetization along the u- and c-axes in various magnetic fields are presented. The different M(T) behaviour along a- and c-axes manifests a large uniaxial magnetocrystalline anisotropy. At low temperatures, the magnetization measured in the same field is higher along the u-axis and at high temperatures along the c-axis. This shows that the compound undergoes a spontaneous spin reorientation. A large anisotropy was found 100
The TbFe,,Ti alloy was prepared by melting the components (Tb 99.8% purity, Fe and Ti 99.99%) in an induction furnace under a protective helium atmosphere. In order to increase the grain size, the ingot was remelted in an electric-resistance furnace with a high temperature gradient and cooled slowly through the melting point. According to X-ray and metallographic analyses, the ingot contained less than 3% of impurity phase (a-Fe). The values of the lattice parameters as well as the Curie temperature of the compound are in good agreement with previous results [1,2,4]. Samples were cut out of large grains of the ingot and polished perpendicular to the ( 10 0) and (0 0 1) axes into 2 mm cube shape. In the samples used for the magnetization measurements, the misorientation of the subgrains was less than 3” and the orientation of the surfaces was better than l-2”. Magnetization measurements along the (10 0) (u-axis) and (0 0 1) (c-axis) directions as well as in the plane (0 10) between these axes were carried out with a vibrating sample magnetometer in magnetic fields up to 1.3 T in the temperature range 4.2-800 K. The AC susceptibility
Fig. 2. Temperature dependences of magnetization of a TbFe,,Ti single crystal along the ( 10 0) (u-axis) and (0 0 1) (c-axis) directions as measured in different magnetic fields.
A.V. Andreev et al. I Spin reorientation in TbFe,, Ti
not only between the c-axis and the basal plane, but within the basal plane, as well. The magnetization values measured along the (1 10) direction are lower by a factor 1.4 than those along the u-axis, in good agreement with the symmetry of the crystal (cos-1 45”). Therefore the u-axis is the easiest-magnetization direction at low temperatures. In the present paper we will not further discuss the anisotropy within the basal plane. The Curie temperature obtained from the M( 7’) curve along the c-axis in 20 mT is equal to 560 K. In fig. 2 one can see the tails of magnetization above T, due to the already mentioned impurity o-Fe. After subtraction of this contribution, extrapolated to low temperatures the molecular magnetic moment p,,, at 4.2 K is found to be equal to ll~r,. Assuming k,., = gJh = 9h and a collinear ferrimagnetic coupling between Tb and Fe sublattices, the magnetic moment of the Fe sublattice is equal to 2Oh (the corresponding values obtained on single crystals of RFe,,Ti with Y, Lu, Ho, Er are equal to 1919.6& [3,7]). Figure 2 clearly shows the spin reorientation; however, it is impossible to select a particular temperature as the spin-reorientation temperature. Let us compare the field dependences of the magnetization along the a- and c-axes at different temperatures which are presented in fig. 3 (after subtraction of the o-Fe contribution). No spin reorientation, only a weakening of anisotropy of the basal-plane type can be seen between 4.2K and 320K. The compound becomes nearly isotropic at 320-330 K, then the anisotropy appears at higher temperatures up to the Curie temperature. An additional feature of TbFe,,Ti can be seen in fig. 3 at high temperatures. Due to the low anisotropy a saturation of the magnetization is reached in the hardmagnetization direction, which is higher than in the easy direction. A similar anisotropy of the values of magnetic moment has been observed in a Tb,Fe,, single crystal [S]. In order to determine if this spin reorientation is a first-order transition (‘plane-axis’) or passes through some cone range, we measured the angle dependence of the magnetization in a mag-
Fig. 3. Field dependences of the magnetization along the a-axis (open symbols) and the c-axis (filled symbols) at different temperatures.
netic field 0.4T (which corresponds to almost complete saturation in the easy direction at all temperatures) upon rotation of the sample from the c-axis to the a-axis. These data are presented in fig. 4 in comparison with those for a DyFe,,Ti single crystal. The anisotropy of DyFe,,Ti within the basal plane is negative (in TbFe,,Ti, it is positive), the easy direction in the basal plane is (110). For this reason the rotation from the c-axis to the basal plane of the DyFe,,Ti single crystal has been made in the (1 10) plane instead of the (0 10) plane as in the case of TbFe,,Ti (strictly speaking, DyFe,,Ti has no basal-plane anistropy at all. At lower temperature there is a cone, but the cone angle is BO”, which almost corresponds to the basal plane). In DyFe,,Ti after a rather special first-order transition from a wide BOO-coneto a 45”-cone at 120 K, the secondorder cone-axis transition occurs at 220 K . At low temperature, the maximum magnetization direction for DyFe,,Ti is near the basal plane (fig. 4). Above 220 K it is the c-axis and at
Fig. 4. Angle dependences of the magnetization in a magnctic field of 0.4 T at different temperatures for TbFc, ,Ti (in the (0 I 0) plane) and DyFe,,Ti (in (1 1 0) plane) single crystals.
150 K it makes an angle of 40” with the c-axis. In TbFe, ,Ti the curves below and above the spin reorientation are qualitatively similar to those for DyFe,,Ti; however, no dependence with a maximum for some intermediate angle between the main axes is found. Instead of such a maximum, a practically isotropic state is observed near 320 K (in agreement with fig. 3). Therefore the transition does not go through some cone range but it is first-order type, directly from plane to axis. The presented results evidently confirm the third phase diagram in fig. 1 , which claimed one transition at 327 K from plane to axis without cone. However, the same type of measurements on the same single crystal in a magnetic field of 20 mT shows another result (fig. 5): the isotropic state now occurs at 270 K, 50 K lower than in fig. 4. The data obtained at 20 mT may be considered to be in more or less good agreement with the fourth phase diagram in fig. 1  (T,, = 285 K), but T,, is even lower. Moreover, the curve at 280 K clearly indicates that the
90 0 x 5 il
Fig. 5. Angle dependcnces of the magnetization in a mapnetic field of 20 mT at different temperatures for a ThFe, !Ti single crystal.
anisotropy is already uniaxial whereas in fig. 3 basal-plane anisotropy is found at this temperature. Finally, in fig. 6 the temperature dependences of the AC susceptibilities along both axes are presented in a magnetic field with an amplitude of 0.3 mT. One can see that maxima which are usually considered as corresponding to spin reorientation occur in the temperature range 250-260 K. and there is no trace of an anomaly at 325 K, the T,, value deduced from the full magnetization curves along the main directions (fig. 3). The conclusion which can be made from these results is that the spin reorientation in TbFe, ,Ti is extremely sensitive to the applied field in the low-field region where the sample is multidomain. We should note here that the mentioned values of the magnetic field correspond to the external field. The internal fields in the case of the magnetization values presented in fig. 4 are much lower, about 0.15 T. However, even such a low field shifts the spin reorientation temperature by 70 K compared to the value measured in nearly zero field (0.3 mT). Of course, the magnetic field always influences spin-reorientation
A.V. Andreev et al. I Spin reorientation in TbFe,, Ti
0.3 mT .’ a
00 O0 0 .= oo 00
T WI Fig. 6. Temperature dependences of the AC susceptibility in a field of 0.3 mT along the a-axis (open symbols) and the c-axis (filled symbols).
phenomena in rare-earth compounds with 3d metals (as well as in other magnets), but usually this influence is not so strong. However, the usual transitions belong to the second-order type. The first-order spin reorientation from the axis (at low temperatures) to the basal plane in a single crystal of Er,(Fe,.,Co,.,),, was studied in ref. [lo]. A considerable broadening of the transition was found under the influence of magnetic fields up to 1.5 T; however, the shift of the center of the transition is negligible. The observed strong sensitivity is, probably, a feature of the first-order transition in the RFe,,_,M, compounds. If we compare the data on the spinreorientation temperatures for DyFe,,Ti from refs.  and [ll], the difference in the temperature of the second-order cone-axis transition is acceptable (220 K and 200 K, respectively) but for the low-temperature transition of first-order type it seems too large, 120 K and 58 K, respectively (here it should also be pointed out that there is another disagreement between refs.  and [ll]: in ref. [ll], the easy-magnetization direction is the (10 O} axis and the low-tempera-
ture anisotropy is of the basal-plane type; in ref. , the (1 10) axis and SO’-cone, respectively). Probably, this special first-order transition in DyFe,,Ti is also very sensitive to the magnetic field, which would explain the disagreement. We should also mention that, for other types of the R-3d intermetallics, there never was such great contradiction in the results of the spinreorientation investigation as for RFe,,_,M,. The spin reorientations similar to TbFe,,Ti [l] were deduced from the polycrystal data on TbFe,,Si,  and TbFe,,Cr, . The present paper, together with refs. [2-61, points to only one transition in TbFe,,Ti. The same conclusion, in disagreement with ref. , has been made for TbFe,,Si, in ref. . The most contradictory situation, however, exists for the compounds with R=Ho: (a) a spin reorientation takes place in HoFe,,Ti , HoFe,,V, [ll], HoFe,,Mo, [ll], HoFe,,Si, , HoFe,,Cr, ; (b) there is no spin reorientation in HoFe,,Ti (several papers, including the single-crystal data ), HoFe,,V, (many papers), HoFe,,Mo, (ref.  and the single-crystal data [16,17]). It can be seen from a comparison of singlecrystal magnetization data with polycrystal susceptibility data for HoFe,,Ti and HoFe,,Mo, that anomalies in the x(T) dependences, which are considered as evidence for a spin reorientation, reflect a sharp decrease of uniaxial anisotropy and coercivity with increasing temperature, without a change of the magnetic-anisotropy type. The situation in HoFe,,Si, and HoFe,,Cr,, which still have not been studied on single crystals, could be the same.
4. Conclusion The intermetallic compound TbFe,,Ti is a ferrimagnet with pm = llh at 4.2 K and T, = 560 K. The easy-magnetization axis is (10 0) at low temperatures and (0 0 1) at high temperatures. The spin-reorientation phase transition is of first order, from basal-plane anisotropy to uniaxial anisotropy without a cone range. The temperature of the transition is extremely sensi-
A .V. Andreev
et al. I Spin reorientation
tive to the magnetic field in the low-field range (from 250 K at 0.3 mT to 325 K at 0.15 T internal field), which could be one of the reasons for the large disagreement in the literature on the spin reorientation in this compound.
Acknowledgement The authors thank Dr. Ye.V. Scherbakova for helpful discussions. One of the authors (A.V.A.) thanks the Ministry of Education, Science and Culture of Japan for financial support.
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