Table of contents

A review is made of theoretical concepts from nonlinear dynamics of quasi-one-dimensional magnetic materials and of experimental investigations of solitons by inelastic neutron scattering and of studies of anomalies of thermodynamic quantities such as the specific heat, magnetization, susceptibility, etc. The main investigated substances are the quasi-one-dimensional ferromagnet CsNiF₃ and the antiferromagnet tetramethylammonium manganese chloride (TMMC) which share the same crystal structure. They can be regarded as quasi-one-dimensional magnetic materials with easy-plane anisotropy. The spin dynamics of such a system subjected to an external magnetic field applied in the easy plane can be reduced to the sine-Gordon equation. A detailed analysis is given of the recent experiments on CsNiF₃ and TMMC carried out using unpolarized and polarized neutrons and demonstrating that in a certain range of temperatures and fields this dynamics includes nonlinear excitations which can be described qualitatively as solitons of the sine-Gordon equation. Another group of quasi-one-dimensional crystals with easy-axis anisotropy is considered: it belongs to Ising-like magnetic materials. In the absence of an external field these materials exhibit soliton-type nonlinear excitations in the form of antiphase domain walls. The available experimental data confirm the concept of a soliton gas of excitations in quasi-one-dimensional magnetic materials.

The results are presented of the ten-year development of the concepts of anharmonicity of the normal modes that describe the vibrations of a continuous medium having two or more mutually coupled subsystems. The results are discussed of experimental and theoretical studies of the anharmonicity of the magnetoelastic normal waves in antiferromagnetics having weak anisotropy and a large exchange field, such as α-Fe₂O₃, FeBO₃, etc. In these crystals the "effective" anharmonicity of the quasiacoustic branch of vibrations that arises from the magnetostrictive coupling of the elastic and spin subsystems is especially large—larger by a factor of 10³ to 10⁴ than that typical of a pure elastic medium. The corresponding effective moduli are controlled easily, and over a broad range, by a small magnetic field (0.1–2 kOe). The anomalously large ("giant") magnitude of the acoustic nonlinearity has enabled observing many ultrasound analogs of the effects of nonlinear optics (frequency doubling of sound, stimulated combination scattering of sound by sound, etc.). Realization in the near future of other analogs—self-focusing of a sound wave, excitation of magnetoelastic solitons, etc., is expected.

The experimental and theoretical publications on itinerant metamagnetism—the jump-like transition, induced by a magnetic field, of a paramagnetic system of itinerant electrons into a magnetically ordered state—are reviewed. The conditions necessary for the appearance of the metamagnetic transition are formulated on the basis of Stoner's model, and a theory of itinerant metamagnetism for weak itinerant magnets is presented. The effect of fluctuations of the spin density on the properties of an itinerant metamagnet is examined. The results of experimental studies of itinerant metamagnetism, induced by both external and internal effective magnetic fields, in a system of electrons of a number of intermetallides are presented. The systems

Co(Se_1–xS_x)₂,

Y(Co_1–xAl_x)₂,

and

Lu(Co_1–xAl_x)₂,

the compounds

RCO₂⁻¹,

ThCo₅,

CeCo₅,

and others are studied. Some other phenomena associated, like itinerant metamagnetism, with the sharp energy dependence of the density of states near the Fermi level are discussed.

A quantitative study is made of the physical mechanisms responsible for the development of a collective hydrodynamic instability of a charged-particle beam in an isotropic plasma. The corresponding growth rate is calculated through an analysis of the dynamics of the motion of the beam particles in the field of their radiation. The coherence of the beam particles is responsible for a substantial amplification of the collective field excited by these particles. This field forms coherent bunches by the Veksler–MacMillan self-phasing mechanism. This review is addressed to specialists in plasma physics and microwave electronics.

CONTENTS 1. Introduction 763 2. Masers in the local galaxy 763 3. Summary of the characteristics of galaxies with OH-molecule megamasers 764 4. Production of OH molecules, their abundance, and spectroscopic features of low-lying rotational levels 765 5. Possible mechanisms for the formation of a population inversion in OH megamasers; energy estimates 766 6. H₂O and H₂CO megamasers 768 7. Conclusion 769 References 770