Spin Reorientations in (Er x Y 1-x ) 2 Fe 14 B and (Er x Pr 1-x ) 2 Fe 14 B Systems
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SPIN REODRI
ATI0NS IN (ErxYl1x) 2Fel4B AND (Er Frl
E. B.BOLTICa), A.T. Z
) 2Fel4B SYSTEMS
'c) ANDW E.WALACE
MEMS Department and Magnetics Technology Center, Carnegie-Mellon University, 15213 Pittsburgh, PA ABSTRACT The bulk magnetic properties of (ErxYlix) 2 Fel 4 B and (ErxPrl.X) 2 Fel4B systems were studied over the temperature range 4.2 - 1100 K. Lattice parameters, saturation magnetizations, Curie temperatures and spin reorientation temperatures were determined. Theoretical description of the detailed magnetic behavior is presented, based on a crystal field model. The (ErxYlx) 2 Fel 4 B compounds were all found to exhibit plane-to-axis spin reorientations similar to that observed for Er2Fel4B, with the transition temperature decreasing with increasing Y content. In contrast, the spin reorientations in the (ErxPrl-x) 2 Fel 4 B systems appear to be of the cone-to-axis type. Since higher 3 order crystal field terms appear to be significant only in the cases of Nd + 3 and Ho +, the results are discussed in terms of a crystal field Hamiltonian involving only 2nd order terms. Using known values of the exchange field, Fe anisotropy and the ratios of the crystal field coefficients, the multi-ion cr 6 .tal field problem was formulated in terms of a single adjustable parameter (B2 (f). It is shown that 2nd order crystal field terms are capable, not only of explaining the conical anisotropy of the (ErxPrl.x)2Fel4B systems, but also the decrease in the Er moment upon passing through the spin reorientation (as has been observed for Er 2 Fel 4 B). The magnetic structure of Erl. 5 Pr 0 . 5 Fel 4 B is also predicted. INTRODUCTION Two different types of spin reorientations have been observed in the R2 FeI 4 B compounds (R = Nd, Ho, Yb, Er and Tm) with increasing temperature[l-12]: 1) transitions attributable to higher order terms in the crystal field Hamiltonian of the rare earth ions, as in Nd2 FeI 4 B near 135 K and Ho2 FeI 4 B near 65 K and 2) transitions resultingfrom the competition of various sublattices possessing different anisotropies as in Er 2 Fel 4 B, Tm2FeI 4 B and Yb2 Fel 4 B near 330, 315 and 115 K, respectively. Both types of spin reorientations are evidenced experimentally by the appearance of irregularities in the magnetization versus temperature (M vs. T) curves measured on rough chunks in low applied fields[13]. Transitions from conical to axial structures (and vice versa) appear as steps in the M vs. T curves, while transitions from planar to axial structures (and vice versa) appear as spikes. Spin reorientations in R2 FeI 4 B systems have been the subject of a great deal of interest within the magnetics community. Early point charge calculations led to the conclusion that the conical anisotropy observed in Nd2 Fel 4 B at low temperatures was the result of competing anisotropies of the two rare earth sites[14]. However, it has since been shown that the conical structure of this system is due to the presence of higher order terms in the crystal field In the potential of the rare earth ions and not to co
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