Theory of Antiferromagnetic Resonance in CuCl₂-2H₂O

After giving a short review of Nagamiya-Yosida’s theory of antiferromagnetic resonance, it is shown that this theory is equivalent to Gorter-Ubbink’s at absolute zero, so far as the external field H employed in the former is replaced by H′ defined by 2Hx′=gaHx, etc. and the parallel and perpendicular susceptibilities are related with observed susceptibilities in a suitable way (equations (1.9)), where ga is the value of the g-tensor in the direction of the crystalline a-axis, etc. (§1) It is then shown that the small anisotropy of the Weiss molecular field can be taken into account by introducing an anisotropy energy of orthorhombic symmetry and that in this way the anisotropic Weiss field can be replaced by an isotropic one (§2). By using this anisotropy energy, the hyperbola in the ac-plane that defines the critical field strength is derived in a simple way for an arbitrary temperature, without making use of Weiss approximation (§2). In §4, then, a summarizing review of Yosida’s theory of resonance is given, supplementing it with a few more resonance formulas. In § 5, it is shown that in a certain range of temperature resonance takes place at the critical field strength. There is a case such that the resonance absorption curve is semi-infinitely broad, being cut off for the field strengths inferior to the critical field strength, and this occurs at one end of that temperature range. This is in accord with Ubbink’s experiment. In § 6 it is shown that all the observed resonance peak positions obtained by varying the temperature and the direction of the applied field are quantitatively well explained, if one determines the three parameters entering the theory (two anisotropy constants, the ratio between them being independent of temperature, and the ratio between the parallel and perpendicular susceptibilities) from a part of the resonance data and the measurements of the susceptibilities. Some discussions are given about the change of the width of resonance curve with the change of the direction of the applied field and also about the polarization effect. In § 7 are given further predictions from the theory.

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