Magnetic order in HoNiBC and ErNiBC

Neutron diffraction measurements have been performed to study the nature of magnetic ordering in the compounds HoNiBC and ErNiBC. HoNiBC exhibits antiferromagnetic ordering of the Ho moments with a simple commensurate magnetic structure below T_N = 10 K. The magnetic unit cell, doubled along the c-direction with respect to the chemical unit cell, consists of ferromagnetic planes antiferromagnetically coupled along the c-axis. On the other hand, the Er moments in ErNiBC show ferromagnetic ordering with a Curie temperature of 4.5 K. In contrast to the case for the structurally related superconducting counterparts (HoNi₂B₂C and ErNi₂B₂C) no modulated magnetic structures were observed for any temperatures above 1.4 K.