Neutron diffraction investigation of the crystal and magnetic structure of the new ThCo₄B compound

Abstract

The crystal and magnetic structures of a new ternary phase in the Th-Co-B system have been investigated by high resolution neutron diffraction. The spontaneous magnetization and Curie temperature were determined by magnetic measurements. The ThCo 4 B phase crystallizes in a hexagonal crystal structure, space group P6/mmm with a = 5.088 Å and c = 7.003 Å. ThCo 4 B orders ferromagnetically at 303 K, a temperature much smaller than that of the isotypic RCo 4 B where R is a rare-earth element or yttrium. The Co magnetic moments are found to be aligned along the c-axis of the hexagonal structure in the whole ordered temperature region. The two inequivalent Co crystal sites are found to exhibit different magnetic behaviours. At 2 K a substantial magnetic moment of 1.8 ± 0.1 µ B is observed on the Co 2c site whereas a nearly zero magnetic moment is found on the Co 6i site. A similar behaviour was also found at room temperature, 1.2 ± 0.2 µ B on the Co 2c site and zero µ B on the Co 6i site. The influence of the local environment on the magnitude of the Co moments in ThCo 4 B is discussed. The magnetic properties of ThCo 4 B are compared to that of isotypic YCo 4 B.

Electronic and Magnetic Properties of ThCo4B

“…The Co atoms provide above 80 and 90% contributions to the total DOS at the Fermi level for spin up and down, respectively. ThCo 4 B, contrary to isostructural UCo 4 B compound [5], is magnetically ordered below 303 K [4], and the saturation magnetization obtained from the 4 K isothermal magnetization curve is equal to 1.5±0.1 µ B /f.u. Neutron diffraction measurements gave magnetic moments aligned along the c-axis of the hexagonal structure in the whole ordered temperature region [4].…”

“…The calculations were carried out for the hexagonal structure with 12 atoms per unit cell (two formula units) and experimental values of the lattice constants [4]. We assumed the following electron configurations of atoms: core + semi core electrons (5d6s) + valence electrons (6p7s7p6d5f ) for Th atoms, core + semi core electrons (3s3p) + valence electrons (4s4p3d) for Co atoms, and finally on B atom 1s electrons were treated as semi core, and (2s2p3d) electrons as valence ones.…”

“…It is very useful to compare properties of isostructural systems with and without 5f electrons treating the systems with thorium as reference ones. The ThCo 4 B phase crystallizes in a hexagonal CeCo 4 B type structure and orders ferromagnetically at 303 K [4]. Investigations by high resolution neutron diffraction gave magnetic moments on Co atoms equal to 1.2 and 0.2 µ B /atom for 2c and 6i site, respectively [4].…”

“…The ThCo 4 B phase crystallizes in a hexagonal CeCo 4 B type structure and orders ferromagnetically at 303 K [4]. Investigations by high resolution neutron diffraction gave magnetic moments on Co atoms equal to 1.2 and 0.2 µ B /atom for 2c and 6i site, respectively [4]. In isostructural UCo 4 B no magnetic order was observed [5].…”

Magnetic Properties and Electronic Structure of ThCo<sub>4</sub>B

“…The RCo 4 B structure has two R sites (1a and 1b), two others for Co atoms (2c and 6i) and one for boron (2d). It has been shown that the Co 2c site, in ThCo 4 B is the only site, which exhibits a significant magnetic moment [12], whereas both of the Th and Co 6i sites carry a negligible magnetic moment. On the contrary, in YCo 4 B both Co 2c and Co 6i sites do carry a magnetic moment.…”