LOW-TEMPERATURE SPECIFIC-HEAT OF NDMO3 (M=CO, FE, CR, NI) - MAGNETIC-ORDERING OF ND

A low temperature (200 mK < T < 5 K) specific heat study of NdMO3 (M = Co, Fe, Cr, Ni) reveals that in NdCoO3 the Nd ions order cooperatively at T(N2) = 1.20(1) K in good agreement with the simple cubic S=1/2 XY model. In contrast, in NdFeO3, NdNiO3 and NdCrO3 a Schottky-type anomaly caused by the Nd-M interaction is the dominating feature of the specific heat curves, whereas the Nd ordering peak is greatly suppressed and shifted to lower temperatures in NdFeO3 and NdNiO3 (T(N2) = 1.05(1) and 0.77(2) K, respectively) and does not appear in the Cr compound. These phenomena depend critically on the strength of the Nd-M exchange field experienced by the Nd ions-zero in NdCoO3, intermediate in NdFeO3 and NdNiO3, and large in NdCrO3. Existence of two magnetically different Nd sites in NdNiO3 is demonstrated.