Magnetic transitions and spin dynamics in the 3d¹ system K ₂NaCrO₈ investigated by magnetization and ²³Na NMR

We report pulsed magnetization and ²³N a NMR studies of a quantum phase transition and its associated spin dynamics in a 3d¹ system, using the alkali-metal peroxychromate K₂NaCrO₈. This is an example of a Cr⁵⁺-based S=1/2 and I∼0 material with a clearly defined T_N=1.7 K in zero applied field (H). Its magnetization (M) shows a nonlinear increase with H, in contrast to a simple three-dimensional antiferromagnet, and saturates at H_C=7.4 T. This easily accessible H_C allows for a detailed study by ²³N a NMR of the magnetic-field-induced quantum phase transition from the antiferromagnetically ordered to a ferromagnetically polarized phase. The critical exponent of M (T) ∼ (1-T/ TN)^β is 0.44±0.05 at H=5 T but shows a definitive increase to 0.53±0.05 at 6.8 T, close to H_C. This suggests that while the magnetic behavior of K ₂NaCrO₈ is largely describable by a mean-field theory, the field dependence of β remains unexplained.