Phase relation, crystal structures, and magnetic properties of Nd-Co-B borides

The phase relation of the ternary system Nd-Co-B quenched from 600 degrees C in the Go-rich region has been determined by means of X-ray powder diffraction. In this region there exist 11 ternary borides: Nd3Co13B2, NdCo4B, Nd5Co19B6, Nd3CO11B4, Nd2Co7B3, Nd2Co14B, NdCo12B6, NdCo4B4, Nd2Co5B3, NdCo2B2, and Nd2Co5B2 The first five borides are members of the homologous series Ndm+nCo5m+3nB2n (or NdCo5-xBx). New layered borides Nd3Co13B2 and Nd5Co19B6 have not been reported previously. The space group of them is P6/mmm and their crystal structures are refined by the Rietvled method. All the compounds of this Ndm+nCo5m+3nB2n family magnetically order between Te = 710 K for Nd3Co13B2 and To = 330 K for Nd2Co7B3. The mean cobalt moments of the Ndm+nCo5m+3nB2n, borides determined at 4.2 K are strongly dependent on the composition. On the basis of the experimental results, three kinds of Co sites with different magnetic moments are proposed. It is shown that increasing B content in NdCo5-xBx borides leads to an increase of the second-order crystal-field parameters Ate, which explains the enhancement of the magnetocrystalline anisotropy. Spin reorientations are observed in Nd3Co13B2 and Nd3CO11B4 compounds. The magnetic phase diagram of the Ndm+nCo5m+3nB2n borides is also given, and the behavior of magnetocrystalline anisotropy is analyzed using the single ion model.