Isotropic and anisotropic magnetic behavior of the frustrated spin-chain compound $\beta -{\mathrm{TeVO}}_4$ is reported. Three magnetic transitions observed in zero magnetic field are tracked in fields applied along different crystallographic directions using magnetization, heat capacity, and magnetostriction measurements. Qualitatively different temperature-field diagrams are obtained below 10 T for the field applied along $a$ or $b$ and along $c$, respectively. In contrast, a nearly isotropic high-field phase emerges above 18 T and persists up to the saturation that occurs around 22.5 T. Upon cooling in low fields, the transitions at $T_{\mathrm{N}1}$ and $T_{\mathrm{N}2}$ toward the spin-density-wave and stripe phases are of the second order, whereas the transition at $T_{\mathrm{N}3}$ toward the helical state is of the first order and entails a lattice component. Our microscopic analysis identifies frustrated $J_1\text{−}{J}_2$ spin chains with a sizable antiferromagnetic interchain coupling in the $bc$ plane and ferromagnetic couplings along the $a$ direction. The competition between these ferromagnetic interchain couplings and the helical order within the chain underlies the incommensurate order along the $a$ direction, as observed experimentally. While a helical state is triggered by the competition between $J_1$ and $J_2$ within the chain, the plane of the helix is not uniquely defined because of competing magnetic anisotropies. Using high-resolution synchrotron diffraction and ${}^{125}\mathrm{Te}$ nuclear magnetic resonance, we also demonstrate that the crystal structure of $\beta -{\mathrm{TeVO}}_4$ does not change down to 10 K, and the orbital state of ${\mathrm{V}}^{4+}$ is preserved.

• Received 25 January 2016
• Revised 9 July 2016

DOI:https://doi.org/10.1103/PhysRevB.94.064403