  Kitaev materials often order magnetically at low temperatures due to the
presence of non-Kitaev interactions. Torque magnetometry is a very sensitive
technique for probing the magnetic anisotropy, which is critical in
understanding the magnetic ground state. In this work, we report detailed
single-crystal torque measurements in the proposed Kitaev candidate honeycomb
magnet $\alpha$-RuBr$_3$, which displays zigzag order below 34 K. Based on
angular-dependent torque studies in magnetic fields up to 16 T rotated in the
plane normal to the honeycomb layers, we find an easy-plane anisotropy with a
temperature dependence of the torque amplitude following closely the behaviour
of the powder magnetic susceptibility. The torque for field rotated in the
honeycomb plane has a clear six-fold periodicity with a saw-tooth shape,
reflecting the three-fold symmetry of the crystal structure and stabilization
of different zigzag domains depending on the field orientation, with a torque
amplitude that follows an order parameter form inside the zigzag phase. By
comparing experimental data with theoretical calculations we identify the
relevant anisotropic interactions and the role of the competition between
different zigzag domains in this candidate Kitaev material.
