FORMATION AND MAGNETIC-PROPERTIES OF TBCU7-TYPE RFE7 COMPOUNDS AND THEIR NITRIDES (R=TB AND DY)

DyFex and TbFex alloys with x = 7, 7.7, and 9 were mechanically alloye d followed by annealing in an attempt to prepare TbCu7-type DyFe7 and TbFe7 compounds. X-ray-diffraction and thermomagnetic measurements sho w that single-phase TbCu7-type TbFe7 can be obtained for all three com positions, while DyFe7 was formed for x = 7.7 and 9 when annealed in t he temperature range from 650 to 850 degrees C. The Curie temperature (T-c) of the TbCu7-type phase shows no obvious variation with Fe conte nt, but decreases with annealing temperature. For DyFe7.7 and TbFe7.7, their respective Curie temperatures are reduced from 160 and 185 degr ees C by annealing at 700 degrees C for 3 h to 140 and 160 degrees C a fter annealing at 850 degrees C for 3 h. Magnetometry measurements rev eal that the TbCu7-type phase is a soft phase with a room-temperature coercivity of less than 500 Oe. Nitriding the TbCu7-type phase results in lattice expansions without changing the crystal structure. However , nitriding increases the Curie temperatures dramatically to 465 and 4 50 degrees C for TbFe7.7Ny and DyFe7.7Ny, respectively. Furthermore, n itriding greatly enhanced the coercivities of TbCu7-type compounds, re aching values of 3.75 and 4.0 kOe for TbFe7.7Ny and DyFe7.7Ny, respect ively. The nitrides with optimum coercivities are single-phase TbCu7-t ype compounds with an average grain size of 10-15 nm as estimated from Bragg peak broadening. A coercivity mechanism based on contributions from the random anisotropy of the R3+ ions and the crystalline anisotr opy is proposed to explain the observed effects. (C) 1997 American Ins titute of Physics.