Magnetic and caloric study of the phase transitions of copper, nickel, manganese and cobalt lanthanum double nitrate

Abstract

Heat capacity measurements on four double nitrates of composition X 3La 2(NO 3) 12. .24H 2O, where X stands for Cu, Ni, Mn and Co respectively, showed anomalies at 0.089 K, 0.393 K, 0.230 K and 0.189 K respectively. The entropy yield involved in magnetic ordering indicated that below these temperatures long range order occurs in the samples. A.c. and d.c. susceptibility measurements parallel and perpendicular to the crystallographic c axis were performed in zero magnetic field and in trasversal and longitudinal fields. These measurements showed that the Cu, Mn and Co compounds are antiferromagnetic, while the Ni compound exhibits ferromagnetic behaviour. From susceptibility measurements in a longitudinal field, a spontaneous magnetization of 1 3 of the total saturation magnetization was deduced for the Ni salt. Specific-heat measurements were performed on this salt in which the Ni ions were partly replaced by diamagnetic Mg ions. From these measurements a critical concentration for ferromagnetism of about 45% was deduced. On basis of the crystal structure and particularly on basis of the experimental results obtained for the Cu compound, a model is proposed in which 2 3 of the divalent ions at crystallographic X sites are antiferromagnetically coupled in pairs, while 1 3 of the ions at crystallographic Y sites are less strongly ferromagnetically coupled with nearest and next nearest neighbours. On basis of this model an exchange constant J/ k = -0.23 K for the Cu-ion pairs was deduced from the specific heat data. EPR measurements on mixed Ni-Mg samples yielded a value for the exchange coupling constant between Ni-ion pairs of J/ k = -0.35 K. Paramagnetic relaxation measurements on these samples showed pronounced cross-relaxation effects, which were in remarkably good agreement with the energy level scheme for Ni-ion pairs as determined by EPR. Adiabatic field variations, yielding isentropes in the H-T phase diagram, strongly support the pair-exchange model. Furthermore, a remarkably large cooling upon adiabatic magnetization was obtained in the Ni salt. Nuclear orientation of 54Mn in the manganese salt was studied under various conditions of magnetic field and temperature; these data indicated a short nuclear spin lattice relaxation time, in contrast to results on other antiferromagnetic or paramagnetic Mn ++ compounds, reported in the literature. Predominance of pairwise superexchange coupling in these salts would explain the inadequacy of a molecular field model for the interpretation of the experimental results.