Magnetoacoustic waves near the points of spin-reorientation phase transitions and magnetoelastic oscillations of domain walls

Abstract

The symmetry of magnetically ordered crystal states is broken as compared with the symmetry of equilibrium (macroscopic) magnetic properties. The degenerated magnetic states of crystals are fenced by energy barriers from each other (as soon as these states are connected with each other only by a discrete set of symmetry elements). This is just a reason why the Symmetry Breaking Effects (SBE) in crystals under magnetic ordering are manifested near the phase transition points mainly where the barriers disappear. The magnetoacoustic SBE such as a magnetoelastic energy gap of the quasi-magnon mode, and an anomalous softening of the corresponding quasi-phonon mode for various ferro- and antiferromagnets near the spin-reorientation phase transition points (after magnetic field, temperature, and one-directional pressure) are discussed in detail [1]. Domain walls and domains break the translation symmetry as well. It gives additional quasilocal (resonance) modes of Magneto-Elastic Domain Wall (MEDW) oscillations. At the excitement of these modes by microwave magnetic field the hypersound flux is generated by domain walls. It is shown that the MEDW modes may be used for the detection of domain walls in antiferromagnets [2]. The SBE mentioned above are discussed on the level of the first principles of physics. Some analogies are established with the SBE of the other physics regions (the Higgs effect in the theory of elementary particles, and the Meissner effect in the theory of superconductivity).