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Magnetic anisotropy of spin tetramer system SeCuO3 studied by torque magnetometry and ESR spectroscopy

Magnetic interactions in S=1/2 systems are usually well described by isotropic Heisenberg Hamiltonian and magnetic anisotropy comes mostly from the g factor anisotropy. Another source of magnetic anisotropy in S=1/2 systems is small anisotropy of exchange interaction. In low - dimensional systems this source of magnetic anisotropy can have a profound influence on their ground state. Here we present an experimental study of magnetic anisotropy of a novel spin S=1/2 tetramer system SeCuO3 using torque magnetometry and ESR spectroscopy. In the previous work [1] it was suggested that isolated spin tetramers exist in this system with intratetrahedral interactions described by Hamiltonian H = J12 (S1 · S2 + S3 · S4) + J11 S2 · S3 with J11 ≈ 220K and J12 ≈ 150K. However, temperature dependence of magnetic susceptibility could not be satisfactorily described using this model [2]. Temperature dependence of g factor and large rotation of macroscopic magnetic axes was also observed [1]. In this work we show that the disagreement of measured susceptibility with the predictions of theory is not a consequence of the temperature dependence of g factor. We establish the connection between macroscopic rotation of magnetic axes observed by torque and the rotation of g tensor with temperature measured by ESR in the temperature range T > 50K. Below that temperature g tensor is temperature – independent, while the rotation of magnetic axes observed by torque is very large. Magnetic susceptibility anisotropy measured by torque cannot be reconciled with the model of isotropic interactions. Estimated contributions to the ESR linewidth from dipole-dipole interaction, anisotropic Zeeman interaction and hyperfine coupling are too small to explain the large observed linewidth, thus revealing presence of anisotropic exchange interactions. All this strongly suggests that anisotropic exchange interactions need to be included in the spin Hamiltonian of SeCuO3. Below T_N = 8K the system orders antiferromagnetically with easy axis along ≈<101>* direction.

strongly correlated electron systems; quantum magnetism; low-dimensional spin systems

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