DEPARTMENT OF MAGNETIC AND ELASTIC PROPERTIES OF SOLIDS

Elastic, magnetoelastic, magneto-
and piezoelectric properties of solids

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Staff of Research Group

Vyacheslav D. Fil feghataruslpostil@ileghataruslpostt.khareghataruslpostkov.ua Scopus ResearcherID GScholar ORCID	Head of Group, Principal Research Fellow, Prof., DrSc.
Galina A. Zvyagina zvyaeghataruslpostgina@ileghataruslpostt.khareghataruslpostkov.ua Scopus ResearcherID GScholar	Deputy Head of Department, Senior Research Fellow, Dr.rer.nat., PhD.
Konstantin R. Zhekov zeghataruslposthekov@ileghataruslpostt.khareghataruslpostkov.ua Scopus ResearcherID GScholar	Junior Research Fellow, Dr.rer.nat., PhD.
Igor V. Bilych bileghataruslpostich@ileghataruslpostt.khareghataruslpostkov.ua Scopus ResearcherID GScholar	Junior Research Fellow, Dr.rer.nat., PhD.

Main directions of studies

• Phase transitions of different nature.
• Magnetoelasticity, piezo- and magnetoelectric phenomena in magnetic materials.
• Vortex lattice dynamics in type-II superconductors.
• Electronic properties of conducting systems in normal and superconducting states.
• Precision measurements of the elastic modules of single crystals.

Main results

• The extremal dimensions of Fermi surfaces of Al, Mo and Ga have been measured, the energy gap anisotropy in superconducting In and Al has been determined.
• The regimes of strong electron- sound wave interaction have been investigated. It was found that the electronic contribution to the elasticity modulus dynamic for high-purity single crystals could reach about 10-15%.
• For the first time the ballistic electron transport of elastic deformation effects have been detected. The method of precise measurement of extreme Fermi moment with simultaneous the sign of charge carriers determination (Ga) was developed on this basis. The velocity anisotropy of the electron-electron relaxation in Al has been studied.
• Nonlinear electron-acoustic interaction in metals (the so-called pulse linearity) was found.
• New types of electron waves that characterized by a linear dispersion law have been discovered and explored in metals: beam and cyclotron waves, zero-sound, the concentration waves. The excitation of fast magneto-sonic and Alven magneto-plasma waves has been performed in "good metals" at first. These waves previously had been observed only in Bi.
• For the first time in metals it have been registered the following: a) the electric potential connected with the longitudinal sound wave; b) the inertial field accompanying the transverse sound waves. It is the acoustic analog of the Stewart-Tolman effect. Essentially new acousto-electric method for studying the dynamics of vortex lattices in type-II superconductors has been developed. The behavior of the Magnus force has been studied and parameters of the pinning potentials have been measured in borocarbides with its help. For the first time during the long history of the question the Abrikosov vortex mass (~ 10^-17 g/cm, YB₆) has been measured experimentally. It has appeared in two orders of magnitude larger then current theoretical estimates.
• The elastic modules of representative compounds of families of double molybdates, cuprates, borocarbides, hexa- and dodecaborides, ferroborates have been measured. In particular, a huge anisotropy of elastic properties in the basal plane of La₂CuO₄ single crystals was discovered. The evolution of the elastic characteristics, accompanying phase transformations ferroelastic and magnetic nature have been studied in detail for all of these compounds. A lot of these phase transitions have been discovered at first. The corresponding phase diagrams in the plane "magnetic field-temperature" have been constructed.
• Magnetoacoustic investigations of single crystals from rare-earth ferroborate family, which are also belong to multiferroic class, were held. In a number of the family representatives new spontaneous and magnetic field induced spin-oriented transitions were found. In some of them the Neel temperature was determined. For the first time the corresponding H-T diagrams were constructed.
• The sound velocities were measured, the elastic and piezoelectric moduli were calculated in NdFe₃(BO₃)₄ and SmFe₃(BO₃)₄ single crystals. It was shown that the compounds could be characterized by quite a high piezoeffect. The piezomodulus е₁₁ in paraelectric phase (≈1.4 C/m²) is almost an order magnitude higher than in α-quarz.
• For the first time a spin-dependent –magnetopiezoelectric effect was discovered in magnetoordered medium. In samarium borate it is revealed the effective piezomodulus grows, more than two times. The effect is also discovered in NdFe₃(BO₃)₄. The nature of the effect is a joint contribution of both magnetoelectric and magnetoelastic interactions.
• In NdFe₃(BO₃)₄ a new type of the effect was revealed– a direct renormalization of the piezoelectric interaction in the magnetically ordered phase.
• In SmFe₃(BO₃)₄ a piezorespond was discovered in non-piezoactive configuration (from a paraphrase point of view) which is apparently due to surface piezoeffect.
• For FeSe single crystal which belongs to a new family of HTC, all the elasticity modulus components were determined. Significant softening of С₁₁ and С₁₁ - С₁₂ modes were discovered at the region of structural transition. The influence of the superconducting transition on sound velocity and attenuation was studied; it is appeared to be significant. The value of superconducting energy gap was obtained.
• In FeSe single crystals the process of acoustoelectric transformations was studied. Intensive generation of electromagnet radiation in the absence of magnetic field was discovered. Most likely the reason of the effect is piezomagnetism.
• In CoF₂ a new channel of piezomagnetic response which appears in a magnetic field was discovered using the acoustoelectric transformation method. Elastic deformation induces a magnetic moment oriented parallel to shift direction in a transverse acoustic wave. The effect exists both in the antiferromagnetic phase and under the temperatures which singnificaly exceed the Neel temperature.
• For the first time a piezoelectric effect in antiferromagnetics which was predicted more than 50 years ago was observed. The effect was registered using the acoustoelectic transformation experiment on LiCoPo₄ single crystal in magneto ordered state. It reveals itself as a piezoelectric response which exists only in a magnetic field and linearly grows on the amplitude with the field.

Experimental equipment

• One of the major achievements of the department is the development of the unique experimental equipment which permits to carry out high-accuracy measurements of sound velocity and attenuation in strongly absorbing medium. In particular, the "nonius" method, suitable for measuring the sound velocity in samples of sub-millimeter size, was created.
• For the time the method of vortex lattice dynamic was implemented due to acoustic transformation.
• For the first time the technique of detecting and investigation of spin-dependent piezoelectric effects in antiferromagnets was developed.
• Temperature range is 1.7 ÷ 350 K, the magnetic field is up to 5.5 T.
• The development of new and modernization of the present experimental equipment is being proceeded.