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Torque magnetometry study of magnetically ordered state and spin reorientation in quasi-1D S = 1/2 Heisenberg antiferromagnet CuSb2O6

Antiferromagnetically ordered state of monoclinic quasi-one-dimensional S = 1/2 Heisenberg antiferro magnet CuSb2O6 was studied combining torque magnetometry with phenomenological approach to magnetic anisotropy. This system is known to have a number of different twins in monoclinic β phase which differ in orientation of the two CuO6 octahedra in unit cell resulting in different orientation of magnetic axes with respect to crystal axes for each twin. We performed torque measurements in magnetic fields H ≤ 0.8 T on a sample where certain type of twin was shown to be dominant by ESR spectroscopy. The measured data reveal that easy axis is the crystallographic b axis for this sample. Phenomenological magnetocrystalline anisotropy energy invariant to crystal symmetry operations was used to model spin axis direction in zero and finite magnetic field. Our model reproduces the value of the spin flop field H_SF = 1.25 T found in literature. Combination of this approach with our torque results shows that the spin axis will flop in the direction of the maximal value of measured g tensor when magnetic field H > H_SF is applied along easy axis direction. Our analysis of magnetocrystalline anisotropy energy predicts two possibilities for the easy axis direction in this system, b or a, connected to different crystallographic twins that can be realized in CuSb2O6. These results offer possibility to reconcile different reports of easy axis direction found in literature for this system and also nicely demonstrate how combination of torque magnetometry and phenomenological approach to magnetic anisotropy can be used to determine the value of the spin flop field and the direction of spin axis in antiferromagnets in both H < H_SF and H > H_SF by performing measurements in fields significantly smaller than H_SF.

Antiferromagnets; torque magnetometry; magnetic anisotropy

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