Antiferromagnetic Resonance Absorption

Abstract

The line widths of antiferromagnetic resonance absorption in the vicinity of the Neel point are investigated by the use of a method of relaxation function similar to that employed by the present authors in studying the dynamical behavior of ferromagnetic spins. Above the Neel point, the line width of the high frequency mode is shown to increase rapid.ly when the temperature approaches the Neel point, being proportional to (T-T N)-B/ 4 in the vicinity of this point. This anomalous increase is due to the critical fluctuation of spins and is in agreement with the observation on MnF2• Below the Neel point, the situation is more complicated and the effects of the anomalous fluctuation upon the line widths are discussed. § I. Introduction In a previous paper ,I> hereafter referred to as I, we have presented a theory for dealing with the dynmp.ical behavior of ferromagnetic spins and calculated the damping of the longitudinal spin component above and below the Curie point, The purpose of this paper is to extend this theory to systems with more than one sublattice, in order to investigate the problem of antiferro-magnetic resonance absorption in the vicinity of the Neel point. 2 >,B> The interesting aspect of this problem is that in the vicinity of the Neel point, a certain type of motion of spins slows down due to the enormous ther-modynamic fluctuations associated with this point, which reveals itself through a rapid increase of the line width of the high frequency resonance mode at the Neel point. The same types of phenomena are the vanishing of the spin diffusion constant in ferromagnetics at the Curie point/> and the anomalous increase. of the NMR line width 4 > near the transition points of ferro-, antiferro-, and ferrimagnetics. In § § 2 and 3, we shall discuss the collective motion of antiferromagnetic spins and its damping according to a theory of collective motion at finite temperatures described elsewhere, 15 > introducing the normalized relaxation matrix. The frequency matrix, which determines the frequency spectrum of the collective motion, is defined in terms of the first moment of this matrix. In § 3, the frequency matrix is diagonalized by introducing a transformation matrix, in