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In this paper, we present a derivation of the nonlinear susceptibility tensors for a two-sublattice uniaxial antiferromagnet up to the third-order effects within the standard definition by which the rf magnetization m is defined as a power series expansion in the rf fields h with the susceptibility tensors as the coefficients. The starting point is the standard set of torque equations of motion for this problem. A complete set of tensor elements is derived for the case of a single-frequency input wave. Within a circular polarization frame (pnz) expressions are given for the first-order susceptibility, second-harmonic generation, optical rectification, third-harmonic generation and intensity-dependent susceptibility. Some of the coefficients with representative resonance features in the far infrared are illustrated graphically and we conclude with a brief discussion of the implications of the resonance features arising from the calculations and their potential applications.

Second-harmonic generation of an antiferromagnetic film in Voigt geometry is calculated and analyzed. This calculation is carried out with an assumption of weak nonlinearity in a two-sublattice antiferromagnetic system. Second-harmonic waves are analyzed by the conventional approach of ordinary nonlinear optics.

Spin-wave instabilities in parallel-pumped easy-plane ferromagnets are investigated using the 5-theory formalism of Zakharov et aI. The parameters in the theory are related to the interaction constants in a microscopic Hamiltonian with exchange anisotropy. A numerical study of two-and five-mode systems shows that the nonequilibrium stationary states are ones where all of the spinwave pair correlation functions have the same phase. It is also found that the phases lock to a common value before the stationary state is reached. From the form of the equations it is argued that a similar result holds for a macroscopic number of modes. Results are presented for the stationary magnon population. The approach to a stationary state in the phase-locked regime is governed by two coupled first-order dilerential equations. %hen the equations are linearized about the fixed points, it is found that the approach to the stationary state involves purely exponential decay just above threshold and damped oscillatory decay at higher power levels. Possible experimental tests of the theory are discussed.