Antiferromagnetic ordering in the novel (two-step) and -type) compounds studied by neutron diffraction and magnetic measurements

Abstract

The magnetic ordering of the novel binary compound with two Tb sites ( structure type, space group Cmcm) has been studied by neutron diffraction and magnetic measurements. orders antiferromagnetically in two steps. In a first step the sublattice orders below with a collinear moment arrangement along the shortest axis a which persists down to . Below a reorientation of the moments takes place. Simultaneously the moments order with a collinear arrangement along c. The ordering of the and sites is associated with the wavevector q = (010) and is described by the monoclinic magnetic space group . At 1.4 K the ordered moment values of and are and , respectively. The moment is tilted away from the x-direction by an angle of and by from the y-direction within the (0yz) plane. The resulting structure corresponds to a three-dimensional canting with six sublattices. The presence of diffuse scattering in the 16.5 - 53 K temperature interval is attributed to short-range ordered magnetic domains of the mean correlation length Å. Fourier analysis of the data reveals that the first- (up to 4.5 Å) and third- (6.5 - 7.5 Å) neighbour correlations are mainly ferromagnetic; the second (4.5 - 6.5 Å) and fourth (7.5 - 8.5 Å) ones are antiferromagnetic. The magnetic ordering of the coexisting novel phase with the structure type (space group P6/mmm) has been also studied. The Tb moments order antiferromagnetically below . This collinear moment arrangement is associated with the wavevector and is described by the magnetic space group . At and is confined to the (001) plane.