Magnetic anisotropy in the frustrated spin-chain compound β-TeVO4

In this paper, isotropic and anisotropic magnetic behavior of the frustrated spin-chain compound β-TeVO4 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 TN1 and TN2 toward the spin-density-wave and stripe phases are of the second order, whereas the transition at TN3 toward the helical state is of the first order and entails a lattice component. Our microscopic analysis identifies frustrated J1-J2 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 J1 and J2 within the chain, the plane of the helix is not uniquely defined because of competing magnetic anisotropies. Finally, using high-resolution synchrotron diffraction and 125Te nuclear magnetic resonance, we also demonstrate that the crystal structure of β-TeVO4 does not change down to 10 K, and the orbital state of V4+ is preserved.