Vladimir L. Safonov

Kinetic instability and bose condensation of nonequilibrium magnons

Abstract Several models in which a microwave radiation of the frequency ω1 arises from the bottom of spin-wave spectrum under condition of parametric resonance of magnons of the frequency ωp/2 are considered. Weak radiation of the frequency ω2 = ωp + ω1 has been detected in thin ferrite film. This radiation can be treated in favour of both the theory of kinetic instability and the thermodynamic theory of parametric resonance of magnons. Possibility of Bose condensation of nonequilibrium magnons is discussed.

Nonlinear radiation damping of parametrically excited spin waves

We consider the process of microwave parametric excitation of spin waves in a resonator. The interaction of the resonator cavity photon with the parametric magnon pair is expected to cause additional nonlinearity: nonlinear radiation damping which is proportional to the quality factor Q of the resonator. Estimations of this positive nonlinear magnon relaxation in ferromagnets and antiferromagnets (for electronic and nuclear spin waves, magnetoelastic waves) show that it can play an important role in forming the stationary state. The threshold of the autooscillations of microwave absorption in a ferromagnet is found to increase with increasing Q.

Study of Parallel Pumping of Magnetoelastic Waves in an Antiferromagnetic FeBO3.

Parallel pumping of magnetoelastic waves in the sample of FeBO 3 placed in a helix microwave resonator was studied. The resonator is shown to play an important role in the process of parametric excitation of quasiphonon pairs at once above the threshold of pumping. The phenomenon of coupled oscillations of electromagnetic mode of resonator and parametric pair of excited waves is investigated in detail. In order to get correct measurements of absorbed microwave power the equivalent circuit is proposed. We constructed the theory of parametric excitation of waves in nonlinear medium mounted in a resonator and obtained a good agreement with experimental data.

Parametric pair of waves as a nonlinear oscillator

We considered the process of parametric excitation of spin waves and magnetoelastic waves in magnetoordered crystals in a microwave resonator. From our analysis it follows that for the k and - k wave pair there is a single-mode realization (i.e. the pair can be represented as one oscillator). The theory of parametric resonance of waves constructed in terms of these collective variables provides a new perspective on some old spin-wave experiments and helps explain the recently obtained experimental data.

Theory of Bose condensation of magnons excited by noise

Abstract A new type of spin-wave instability under an incoherent external pump is considered. This instability would result in a sharp increase in the number of magnons at the edge of the spectrum as in the case of Bose condensation. It can be detected by the coherent electromagnetic radiation from the sample.

UNITARY TRANSFORMATIONS IN WEAKLY NONIDEAL BOSE GASES

Abstract We developed a method for constructing nonlinear unitary transformations in many-body systems. It is demonstrated how to eliminate the three-boson interaction terms which describe forbidden processes in one- and two-component Bose gases. The corresponding effective four-boson interaction amplitudes are calculated.

Influence of radiation damping on the threshold of auto-oscillations in YIG under parallel pumping

Abstract Nonlinear magnon relaxation above the threshold of magnon instability is influenced by radiation damping as well as internal processes of magnon-magnon and magnon-phonon damping. Parametrically excited magnon pair disappears with creating a microwave cavity photon whose damping is given by a quality factor Q . Influence of microwave cavity mode damping on the instability and auto-oscillations of magnons is discussed on the basis of the S-theory.

Nonlinear Radiation Damping of Nuclear Spin Waves and Magnetoelastic Waves in Antiferromagnets

Parallel pumping of nuclear spin waves in antiferromagnetic CsMnF3 at liquid helium temperatures and magnetoelastic waves in antiferromagnetic FeBO3 at liquid nitrogen temperature in a helical resonator was studied. It was found that the absorbed microwave power is approximately equal to the irradiated power from the sample and that the main restriction mechanism of absortption in both cases is defined by the nonlinear radiation damping predicted about two decades ago. We believe that the nonlinear radiation damping is a common feature of parallel pumping technique of all normal magnetic excitations and it can be detected by purposeful experiments.

Magnetic phase transitions and microwave absorption in rare-earth—Iron garnet

Abstract A microscopic theoretical model is considered to explain the dynamic properties of magnetic system of Ho 0.4 Y 2.6 Fe 5 O 12 . The model hamiltonian contains the exchange interaction between the iron spins, the antiferromagnetic exchange interaction between the iron spins and the rare-earth Ising spins and the Zeeman energy. The experimentally observed anomalies of magnetic resonance are found to appear due to strong fluctuations of the rare-earth Ising spins and their maximal spin-lattice relaxation in the vicinity of magnetic phase transition.

Electromagnetic Emission by a System of Nonequilibrium Magnons in a Ferrite

Spectra of electromagnetic emission of frequencies ≃ω i , 2ω i and 3ω i , where ω i is the bottom of the spin-wave band have been studied in yttrium iron garnet at temperatures of liquid nitrogen and liquid helium. The spin-wave system was strongly excited by parallel pumping with microwaves of frequency ω p > 2 ω i . Line shapes and frequency shifts of the emission depend on temperature, static magnetic field, pump power and time duration of the pump pulse. Discussion of obtained results is given.