Ferrimagnetic Resonance

a type of electron magnetic resonance. Ferrimagnetic resonance is exhibited as a sharp increase in the absorption of the energy of electromagnetic radiation by a ferrimagnet at specific (resonance) frequencies v and at a specific strength of an applied (external) magnetic field H₀. The presence of several magnetic sublattices (see) in ferrimagnets leads to the existence of several branches of ferrimagnetic resonance. The branches correspond to the excitation of resonance oscillations of the sublattice magnetization vectors relative both to each other and to the vector of H₀.

The low-frequency branch of ferrimagnetic resonance corresponds to the excitation of a precession of the net magnetization vector J of a specimen in an effective field H_eff, which is determined by the external field, anisotropy fields, and demagnetizing fields. The precession occurs in such a way that the antiparallel orientation of the sublattices is not disturbed; in this case, v = γ_effH_eff. This type of ferrimagnetic resonance does not differ from ferromagnetic resonance, and therefore the term “ferromagnetic resonance” alone is often used in the scientific literature to describe both ferromagnetic and ferrimagnetic resonance. In the low-frequency case, the specific feature of ferrimagnetic resonance is exhibited only in a change in the value of the gyromagnetic ratio γ_eff. In the simplest case of a ferrimagnet with two sublattices having magnetizations M₁ and M₂, γ_eff = (M₁ – M₂)/(M₁/γ₁ – M₂/γ₂), where γ₁ and γ₂ are the gyromagnetic ratios for the sublattices.

The high-frequency branches of ferrimagnetic resonance correspond to precessions of the sublattice magnetization vectors in which the antiparallel orientation of the sublattices is disturbed. These branches are sometimes called exchange resonances. The resonance frequencies are proportional to the exchange fields that act between the sublattices: v = γαJ, where α is the exchange interaction constant. The frequencies lie in the infrared region of the electromagnetic spectrum. The problems of ferrimagnetic resonance in ferrimagnets with noncollinear sublattice magnetization vectors and ferrimagnetic resonance near a compensation point—that is, near a temperature at which the total magnetization of a specimen is equal to zero—are more complicated and less well-studied.