Magnetic ordering in CsCoBr3

Abstract

The magnetic ordering in CsCo${\mathrm{Br}}_3$ has been studied by neutron diffraction, on both powder and single-crystal specimens, and by specific-heat and magnetic-susceptibility measurements. This compound has the hexagonal CsNi${\mathrm{Cl}}_3$ type of structure, and the susceptibility shows characteristic one-dimensional behavior, while the specific heat indicates that three-dimensional ordering occurs at about 28°K. The neutron single-crystal study shows that between $T_N$ and 14°K the magnetic structure is consistent with the orthorhombic space group $\mathrm{Cmc}{2}_1$, with one-third of the antiferromagnetic cobalt chains being disordered and the other two-thirds antiferromagnetically coupled in the basal plane. Between 4 and 10°K the structure may be described by the space group $C{m}^{\prime }c{2_1}^{\prime }$, and is similar to the collinear arrangement reported for CsCo${\mathrm{Cl}}_3$ and RbCo${\mathrm{Br}}_3$ but with a small canting of about 10°. The transformation from one structure to the other in the region of 10-14°K can be explained equally well by either the formation of an intermediate phase of lower symmetry or by a discontinous process in which the two phases coexist. The powder data reveal a small amount of a second low-temperature phase compatible with the space $C_P{m}^{\prime }c{2_1}^{\prime }$, consistent with negative rather than positive next-nearest-neighbor interactions in the basal plane.