We report pulsed magnetization and ${}^{23}\text{N}\text{a}$ NMR studies of a quantum phase transition and its associated spin dynamics in a $3d^1$ system, using the alkali-metal peroxychromate ${\text{K}}_2{\text{NaCrO}}_8$. This is an example of a ${\text{Cr}}^{5+}$-based $S=1/2$ and $I\sim 0$ material with a clearly defined $T_N=1.7\text{K}$ in zero applied field $\left(H\right)$. Its magnetization $\left(M\right)$ shows a nonlinear increase with $H$, in contrast to a simple three-dimensional antiferromagnet, and saturates at $H_C=7.4\text{T}$. This easily accessible $H_C$ allows for a detailed study by ${}^{23}\text{N}\text{a}$ NMR of the magnetic-field-induced quantum phase transition from the antiferromagnetically ordered to a ferromagnetically polarized phase. The critical exponent of $M\left(T\right)\sim {\left(1-T/T_N\right)}^{\beta }$ is $0.44\pm 0.05$ at $H=5\text{T}$ but shows a definitive increase to $0.53\pm 0.05$ at 6.8 T, close to $H_C$. This suggests that while the magnetic behavior of ${\text{K}}_2{\text{NaCrO}}_8$ is largely describable by a mean-field theory, the field dependence of $\beta$ remains unexplained.

• Received 15 September 2008

DOI:https://doi.org/10.1103/PhysRevB.78.214419