Magnetic ordering and exchange interactions in the rare-earth gallium compounds R Ga₂

Authors

T.-H. Tsai, University of Nebraska - Lincoln
David J. Sellmyer, University of Nebraska-LincolnFollow

Date of this Version

December 1979

Comments

Published by American Physical Society. Phys. Rev. B 20, 4577 (1979). http://prb.aps.org. Copyright © 1979 American Physical Society. Permission to use.

Abstract

Magnetic ordering and indirect exchange interactions were studied in the series of hexagonal rare-earth intermetallic compounds R Ga₂, where R denotes Ce, Pr, Nd, Gd, Tb, Dy, Ho, and Er. The magnetic susceptibility (χ) of polycrystalline samples was measured at low fields, from 1.5 K to about 300 K; the magnetization (M) versus applied magnetic field (H) of these polycrystalline samples and a HoGa₂ single crystal was measured up to 80 kOe at low temperatures; the resistivities (ρ) of CeGa₂, GdGa₂, DyGa₂, and HoGa₂ were measured from about 1.5 K to about 300 K, at zero field. The χ(T) and M (H) results indicated that the samples order antiferromagnetically, with ordering temperatures T_N ranging from 6.4 K for DyGa₂ to 14.8 K for TbGa₂. M (H) results for single-crystal HoGa₂ show that the [100] direction is the easy direction and the [001] direction is the hard direction. High-temperature χ(T) results were fitted to the Curie-Weiss formula and the resulting effective moments are in good agreement with those expected for trivalent rare-earth ions. The paramagnetic Weiss temperatures (Θ) are positive for all the samples measured except GdGa₂. The effects of magnetic ordering were observed below T_N in the resistivity results. Letting the spin-disorder part of the resistivity ρ_s be proportional to Tⁿ, n was determined to be about 2.0, 4.3, and 3.3 for HoGa₂, GdGa₂, and DyGa₂, respectively. In view of the theory of Mannari for ρ_s, these differences may be connected with differences in the spin structures of these compounds. Ruderman-Kittel-Kasuya-Yosida (RKKY) sums have been calculated for these compounds. The negative sign of the sum is in agreement with the anitiferromagnetic properties observed in various measurements. For HoGa₂, the dressed moment, due to polarization of conduction electrons, has been estimated. The theory of Nagamiya was applied to three of the compounds which can be considered as a layer compound with long-range ferromagnetic intralayer coupling and antiferromagnetic coupling between a layer and its nearest-neighbor layers. With one possible exception, the data are consistent with the theory which contains three exchange coefficients. Crystal-field-induced anisotropy appears to be important in determining the spin structures of these compounds.