Structure, exchange interactions, and magnetic phase transition of Er2Fe17-xAlx intermetallic compounds

We present the effect of aluminum substitution on the structure, exchange interactions, and magnetic phase transitions of the:intermetallic compound Er2Fe17. All samples have a hexagonal. Th2Ni17-type structure or a I rhombohedral Th2Zn17-type structure; The-replacement of Fe by Al results in an approximately linear increase in the unit-cell volumes at a rate of 9.3 Angstrom(3) per Al atom. The Al atoms preferentially occupy 12k (18h) and 12j (18f) sites at low Al concentration, while they prefer strongly to occupy 6c (4f) and 18f (12j) sites at high aluminum concentration. The Curie temperature is found to increase at first, form a maximum value at x=3, and then to decrease monotonically with increasing Al concentration: The exchange-coupling constant between 3d and 4f sublattices, J(RT) was obtained from fitting M-T curves for some of the samples. The:The intersublattice molecular-field coefficient n(RT) and hence the R-T exchange-coupling constant J(RT) have been also determined on the basis of magnetization curves at the compensation temperature. The exchange-coupling constant J(RT) shows almost no obvious composition dependence, while the exchange-coupling constant is strongly dependent on the Al concentration. The composition dependence of the 3d sublattice exchange interaction is discussed in terms:of bond lengths and atomic preferential occupancies. It is noteworthy that the substitution of Al has a significant effect on the magnetocrystalline anisotropies of both the Er sublattice and the Fe sublattice in E(2)Fe(17-x)A(x) compounds. The temperature and composition dependence of the easy magnetization direction suggests that the second-order crystal electric-field coefficient A(20) changes its sign from negative to positive with increasing Al concentration up to x>7.