Resonance Shift of ESR Lines Due to Low-Symmetric Spin Distribution in Impure Two-Dimensional Systems K₂Mn_cMg_1−cF₄

Abstract

Shifts of ESR lines in impure two-dimensional magnets K₂Mn_cMg_1−cF₄ were investigated in X-band region at room temperature. The theory, which includes the terms arising from the two-dimensionality of spin distribution in the Kubo-Tomita theory, predicts the angular variation of sin² θ(1+9 sin² θ) cos 2φ or sin² θ(3 cos² θ−1) cos 2φ for the shifts according as all q modes or only q=0 mode in the dipolar interaction are concerned. Here, θ and φ are the polar and azimuthal angles, respectively, of the c axis with respect to a coordinate system in which an external field is along the polar axis. The shifts observed in K₂Mn_0.43Mg_0.57F₄ were explained by taking account of the dominancy of the q=0 mode and the effect of the long time tails in the spin correlation function in the impure system.