Magneto-optical and magnetization studies in the rare-earth orthochromites. VI. NdCrO3

Single crystals of NdCr${\mathrm{O}}_3$ were studied by means of bulk magnetization, susceptibility, and optical-absorption spectroscopy measurements. This compound orders at $T_N=214\mathrm{}$ °K in a ${\Gamma }_2\left(F_x\right)$ canted antiferromagnetic spin structure which persisits down to $T_R=35\mathrm{}$°K. At $T_R$ a first-order Morin-type phase transition occurs and, down to below 4.2°K, the spins are in the antiferromagnetic ${\Gamma }_1\mathrm{}\left(0\right)\mathrm{}$ mode. The temperature dependence of the spontaneous magnetization above $T_R$ is significantly different from that of other rare-earth orthochromites and is reminiscent of that found in some ferrimagnetic materials. Using a single-ion model in the molecular-field approximation, it is shown that this behavior is due to magnetization contributions from the two lowest Kramers's doublets of the ${\mathrm{Nd}}^{3+}$ ground multiplet. The calculated Stark splitting is 97±15°K, in excellent agreement with the absorption spectroscopy measurements. Above $T_R$, the ground-doublet splitting is essentially due to the Nd-Cr interaction, while below $T_R$ the additional polarization due to Nd-Nd coupling becomes significant.