EFFECTS OF GA SUBSTITUTION ON STRUCTURE AND MAGNETOCRYSTALLINE ANISOTROPY OF TM2FE17

A detailed investigation of the structure and magnetic properties of T m2Fe17-xGax (x = 0, 1, 2, 3, 4, 5, 6, 7, and 8) compounds has been per formed by means of x-ray-diffraction, neutron-diffraction, magnetizati on, and ac-susceptibility measurements. Crystal-structure studies have shown that the prepared samples are single phase with the hexagonal T h2Ni17 for x less than or equal to 3 and the rhombohedral Th2Zn17 stru cture for x greater than or equal to 5, In Tm2Fe13Ga4 the Th2Zn17 stru cture coexists with the Th2Ni17 structure. Substitution of Ga for Fe i n Tm2Fe17 leads to an increase of the unit-cell volume, which is linea r with the Ga concentration. In Tm2Fe17-xGax, the saturation magnetiza tion at 1.5 K decreases linearly with increasing Ga content with a rat e of 2.3 mu(B), per substituted Ga atom. The Curie temperature is foun d first to increase with increasing Ga content, going through a maximu m value of 485 K at about x = 3, then to decrease. Between x = 6 and 7 , a minimum value of T-C is reached and for higher x values T-C increa ses again. X-ray-diffraction measurements on magnetically aligned Tm2F e17-xGax powders show that the compounds with x less than or equal to 6 have an easy-plane type of magnetic anisotropy, whereas the compound s with x greater than or equal to 7 exhibit easy c-axis anisotropy at room temperature. All Tm2Fe17-xGax compounds exhibit a spin-reorientat ion transition at low temperature, except for the sample with x = 6, w hich shows an easy-magnetization direction perpendicular to the c axis in the temperature range from 5 to 300 K. For x less than or equal to 5, the spin-reorientation temperature is found first to increase with x and then to decrease, having a maximum value of 211 K at about x = 3. In the samples with x greater than or equal to 7, an easy-plane ani sotropy was observed at low temperature, but an easy-axis preference o f the magnetization at room temperature was observed. The results obta ined for Tm2Fe17-xGax indicate that the mutually competing Tm- and Fe- sublattice anisotropies both change their sign with increasing Ga conc entration. (C) 1998 American Institute of Physics. [S0021-8979(98)0031 1-9].