Magnetic Anisotropy and Exchange Interactions in R_n+1Co_3n+5B₂n Compounds

Abstract

The individual Co site contributions to the magnetocrystalline anisotropy in RCo5, RCo4 B and R3Co11B4 compounds have been studied. An analysis is given of the room temperature anisotropy constants of these compounds by using a model of Streever. The values of the rare earth-transition metal exchange coupling JRco/kB derived by mean field analysis of the Curie temperatures are -11.2 K, -i5 K and -20.9 K, respectively. In general, a decrease in JRco/kB with increasing Co concentration is observed.PACS numbers: 75.30.Gw, 75.30.Et IntroductionThe Rn+1CO3n+5B2n type compounds formed between rare earth (R), cobalt and boron are an important and physically interesting class of materials. Compounds in this series are known to exist for n = O (RCo5), n = 1 (RCo4B), n = 2 (R3Co11B4), n = 3 (R2Co7B3) and for n -f co (RCo3B2). These structures are based on the well-known CaCu5 structure. The unit cells are formed by alternative stacking of one layer of RCo5 and n layers of RCo3B 2 unit cells. They all have the hexagonal symmetry and belong to the space group of P 6 / m m m [1]. These compounds exhibit a strong axial anisotropy for R = Sm [2]. Smit et al. [3] determined the crystal field parameters by measuring Mössbauer spectrum of Gdn+1Co3n+5B2n compounds. The strength of the individual contributions to the magnetic anisotropy and magnetic moments of the non-equivalent Co sites in these compounds was studied using nuclear magnetic resonance (NMR) [4,5] and neutron diffraction [6].In the present investigation, the results of magnetocrystalline anisotropy and exchange interactions study of the RCo5, RCo4B and R3Co11B4 compounds are presented. A model proposed by Streever [4] is used to analyse the room temperature anisotropy constants of these compounds.

Magnetism in Pr3Co11B4 compound

“…An explanation for the strong variation in the exchange integrals with changing R atom was given by Belorizky et al [20] who showed that the decrease of in R-T compounds is related to the decrease of the 4f-5d exchange interaction within the R ion. In general, a decrease of J R C~ with increasing Co concentration is observed in R , + I C O~~+~B~, compounds [17]. According to Brooks et al [21] the 3d-5d hybridization strongly governs the interatomic 3d-5d interaction.…”