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We present an experimental study of macroscopic and microscopic magnetic anisotropy of a spin tetramer system SeCuO 3 using torque magnetometry and ESR spectroscopy. Large rotation of macroscopic magnetic axes with temperature observed from torque magnetometry agrees reasonably well with the rotation of the g tensor above T 50 K. Below 50 K, the g tensor is temperature independent, while macroscopic magnetic axes continue to rotate. Additionally, the susceptibility anisotropy has a temperature dependence which cannot be reconciled with the isotropic Heisenberg model of interactions between spins. ESR linewidth analysis shows that anisotropic exchange interaction must be present in SeCuO 3 . These findings strongly support the presence of anisotropic exchange interactions in the Hamiltonian of the studied system. Below T N = 8 K, the system enters a long -range antiferromagnetically ordered state with easy axis along the <101 > * direction. Small but significant rotation of magnetic axes is also observed in the antiferromagnetically ordered state suggesting strong spin-lattice coupling in this system.

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