Exchange interactions and magnetic properties of hexagonal rare-earth-cobalt compounds

Abstract

Abstract. The magnetic properties of some GdxY1-xCo4A compounds with A = Co, Si or B are analysed including the pressure effects. Isomorphous structure transitions, parallelly with changes of cobalt moments from high spin states to low spin states, were shown as pressure increases. The magnetic data, obtained from band structures, were compared with those predicted by the mean field model. IntroductionThe magnetic properties of RCo5-based compounds, where R is a rare earth or yttrium, were intensively investigated both from scientific reasons as well as in connection with their technical uses, as permanent magnets [1,2]. These compounds crystallize in a CaCu5-type structure, space group P6/mmm. In this lattice, the R atoms are located in 1a site and cobalt at 2c and 3g positions [1]. The RCo4B compounds have a hexagonal structure, derived from CaCu5 one, by replacing every two layers Co2c atoms by boron. There are two sites for R(1a, 1b) and Co(2c,6i) atoms and one for boron (2d) [3]. When R is a magnetic heavy rare-earth the above compounds are ferrimagnetically ordered, while for light rare earths, a parallel alignment of R and Co moments is shown. The easy axis of magnetization, for RCo5 compounds with R = Y or Gd, is the c-axis, as imposed by the local uniaxial anisotropy of the 2c sites, higher than the planar one favoured by 3g atoms [4], YCo4B shows also uniaxial anisotropy, at ambient temperature, which changes to planar one, at TsR = 159 K [5].The exchange interactions between magnetic rare-earths and cobalt are mediated by R5d band polarization, as suggested by the 4f-5d-3d model [6,7]. The above polarization is the result of both local 4f-5d exchange as well as due to R5d-Co3d short range interactions, hybridization effects, respectively. The exchange interactions between cobalt atoms are of short range as well as those between R5d-R5d orbitals, in addition to those by means of conduction electrons (RKKY-type) [8].In this paper, we analyse the exchange interactions in GdxY1-xCo4A series with A = Co, Si, B in the framework of mean field model [9], as well as based on the data obtained from their band structures. The pressure dependences of the cobalt moments, for end series compounds, are theoretically investigated. A linear dependence of cobalt moments on the exchange splitting energy of their 3d bands was shown.