Asymmetric Ferromagnetic Criticality in Pyrochlore Ferromagnet Lu${}_2$V${}_2$O${}_7$

Abstract

Critical phenomenon at the phase transition reveals the universal and long-distance properties of the criticality. We study the ferromagnetic criticality of the pyrochlore magnet Lu${}_2$V${}_2$O${}_7$ at the ferromagnetic transition $T_{\text{c}}\approx 70\text{K}$ from the isotherms of magnetization $M(H)$ via an iteration process and the Kouvel-Fisher method. The critical exponents associated with the transition are determined as $\beta =0.32(1)$, $\gamma =1.41(1)$, and $\delta =5.38$. The validity of these critical exponents is further verified by scaling all the $M(H)$ data in the vicinity of $T_{\text{c}}$ onto two universal curves in the plot of $M/|\epsilon {|}^{\beta }$ versus $H/|\epsilon {|}^{\beta +\gamma }$, where $\epsilon =T/T_{\text{c}}-1$. The obtained $\beta$ and $\gamma$ values show asymmetric behaviors on the $T<T_{\text{c}}$ and the $T>T_{\text{c}}$ sides, and are consistent with the predicted values of 3D Ising and cubic universality classes, respectively. This makes Lu${}_2$V${}_2$O${}_7$ a rare example in which the critical behaviors associated with a ferromagnetic transition belong to different universality classes. We describe the observed criticality from the Ginzburg-Landau theory with the quartic cubic anisotropy that microscopically originates from the anti-symmetric Dzyaloshinskii-Moriya interaction as revealed by recent magnon thermal Hall effect and theoretical investigations.

6 pages, 4 figures, submit on the behalf of Dr. J-G Cheng. First two authors contributed equally