Semiclassical Magnetization Dynamics and Electron Paramagnetic Resonance in Presence of Magnetic Fluctuations in Strongly Correlated Systems

Semiclassical magnetization dynamics in presence of magnetic fluctuations (including the quantum ones) is derived for a strongly correlated electronic system in the region of the linear magnetic response. Landau-Lifshitz (LL) and Gilbert (G) type equations are obtained with the effective parameters depending on the type of magnetic fluctuations and their magnitude and applied to evaluation of electron paramagnetic resonance (EPR) problem in Faraday geometry. It is shown that in the studied systems LL and G equations may not be equivalent except the case of weak relaxation, where consistent Landau-Lifshitz-Gilbert (CLLG) equation may be considered. Whereas G equation is affected by quantum fluctuations solely, the LL and CLLG equations may be renormalized by magnetic fluctuations of any nature. In contrast to G equation, the LL and CLLG magnetization dynamics may be characterized by the anisotropic relaxation term caused by anisotropic magnetic fluctuations. A consequence of anisotropic relaxation is the unusual polarization effect consisting in strong dependence of the EPR line magnitude on orientation of vector h of the oscillating magnetic field with respect to the crystal structure, so that EPR may be suppressed for some directions of h. In the case of dominating quantum fluctuations, the LL and CLLG equations may lead to a universal relation between fluctuation induced contributions to the EPR line width Delta W and g-factor Delta g in the form Delta W/Delta g = a(0)k(B)T/mu(B), where a(0) is a numerical coefficient of the order of unity and independent of the quantum fluctuation magnitude. The applicability of the proposed semiclassical magnetization dynamics models to the EPR in spin nematic phases and detection by EPR method of a new group of magnetic phenomena - spin fluctuation transitions is discussed.