~ Journal of magnetism ,~ and magnetic materials ELSEVIER Journal of Magnetism and Magnetic Materials 139 (1995) 102-108 Magnetic and structuralproperties of KCoxMg l_xF3 compounds D. Skrzypek *, J. Heimann Institute of Physics, Universityof Silesia, Uniwersytecka4, 40-007 Katowice, Poland Received 2 May 1994; in revised form 6 July 1994 Abstract The mixed crystals KCoxMg 1_xF3 with 0.25 < x < 1 were studied by static susceptibility and X-ray diffraction methods, in the temperature range 4.2-350 K. From these measurements, the variations of several quantities, viz. TN, t = TN (x)/T N (1), 0 (Curie-Weiss temperature), /zef f (effective moment), and a, c (lattice parameters) with concentration or temperature were determined. The exchange constant was evaluated using theoretical results obtained for random, dilute magnetic systemg with magnetic ions having nonvanishing orbital momenta. From the temperature evolution of the cubic (400) Bragg line, the values of temperature for the structural phase transition and tetragonal deformation lattice with c/a < i were estimated. A strong correlation was found between the magnetic and structural phase transition, with the lattice deformation resulting from the magnetic properties of the crystals studied. I. Introduction In this paper we report the results of our study carried out using temperature-dependent magnetic susceptibility measurements and temperature-depen- dent measurements of lattice parameters. Five sam- ples were investigated in the composition range 0.25 < x < 1 and in the temperature range 4.2-350 K. From these studies, the variations of several quanti- ties, viz. Try, t= TN(x)/TN(1), 0 (Curie-Weiss temperature), /zef f (effective magnetic moment) and a, c (lattice parameters) with x or temperature were determined. The magnetic properties of KCoxMgl_xF 3 are of particular interest because the orbital angular mo- mentum of the Co 2+ ion is not quenched by the * Corresponding author. octahedrally coordinated crystal field. The free CO 2+ ion has seven d electrons so that L = 3 and S = 3/2. The octahedral crystal field produces, as the lowest state, a T 1 triplet within which the matrix elements of L may be described in terms of those of an effective orbital angular momentum l = 1. The pro- portionally constant L--* all is --3/2 when the mixing with an upper state is neglected. In the perovskite structure the spin-orbit interaction gives, as the lowest, a doublet state with s' = 1/2. The crystal KCoF 3 is a G-type antiferromagnet with T N = 114 K [1]. In the antiferromagnetic region the spins are arranged along the z-axis. The ex- change interactions for cations having cubic-field ground state terms T2g of Tlg have been discussed by Eremin et al. [2]. It follows from the studies of the effective Hamiltonian, which contains an or- bitally dependent part expressed in terms of fictitious orbital moments, that in the case of KCoF 3 the anisotropic exchange is smaller by an order of mag- 0304-8853/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD1 0304-8853(94)00457-9