PHASE-DIAGRAM AND MAGNETIC-PROPERTIES OF THE DILUTED FCC SYSTEM NIPMG1-PO

Starting from Ni and Mg nitrates, about 20 samples of NipMg1-p O samples (with 0.06 less-than-or-equal-to p less-than-or-equal-to 0.86) were prepared and x-ray-diffraction studies showed the samples to have the NaCl structure with the lattice constant fitting the expression a(p) = 4.2115-0.0340p angstrom. Temperature-dependent magnetic-susceptibility (chi) studies of the samples were carried out between 1.8 and 600 K using a superconducting-quantum-interference-device magnetometer and Neel temperatures T(N)'s were determined from the peak in partial derivative(chi-T)/partial derivative T. The variation of t = T(N)(p)/T(N)(1) vs p is compared with that of CopMg1-pO. For both systems, the variations for p > 0.31 are found to fit the predicted values for a simple-cubic Heisenberg antiferromagnet and a theoretical basis for this anomalous result is advanced. The experimental percolation threshold p(c) = 0.15 +/- 0.01 and for p(c) < p < 0.31, chi below T(N) shows irreversible behavior for the zero-field-cooled and field-cooled cases, suggestive of spin-glass-like behavior also observed in other diluted fcc antiferromagnets, such as CopMg1-pO and EupSr1-pTe. It is suggested that the differences in the t-vs-p variations for p < 0.31 in NipMg1-pO, CopMg1-pO, and EupSr1-pTe may be related to the differences in the ratio of the next-nearest-neighbor to nearest-neighbor exchange constants in these systems.