Magnetoelastic Waves in Ferromagnets in the Vicinity of Lattice Structural Phase Transitions

2018 ◽  
Vol 63 (9) ◽  
pp. 836 ◽  
Author(s):  
V. G. Bar’yakhtar ◽  
A. G. Danilevich
Keyword(s):  
Phase Transitions ◽  
Structural Phase ◽  
Martensitic Phase ◽  
Spin Reorientation ◽  
Dissipative Function ◽  
Relaxation Processes ◽  
Magnetoelastic Interaction ◽  
Degenerate Ground State ◽  
Martensitic Phase Transformations ◽  
Magnetoelastic Waves

The dispersion laws for coupled magnetoelastic waves in ferromagnets with uniaxial or cubic symmetry have been calculated. The features of obtained dispersion laws in the vicinity of spin-reorientation phase transitions are analyzed. The interaction between elastic and spin waves is shown to depend on the direction of the ferromagnet magnetic moment. The influence of the magnetoelastic interaction on the dispersion law of quasispin waves in the degenerate ground state of a uniaxial “easy plane” ferromagnet is studied. The results of calculations show that the magnetoelastic interaction eliminates the degeneration and leads to the appearance of a magnetoacoustic gap in the ferromagnet spectrum. The behavior of the spectra of coupled magnetoelastic waves in the vicinity of lattice phase transitions, namely, in the vicinity of martensitic phase transformations in materials with the shape memory effect, is analyzed. The obtained results are used to interpret experimental data obtained for the Ni–Mn–Ga alloy. The phenomenon of a drastic decrease of the elastic moduli for this alloy, when approaching the martensitic phase transition point is explained theoretically. It is shown that the inhomogeneous magnetostriction is the main factor affecting the elastic characteristics of the material concerned. A model dissipative function describing the relaxation processes associated with a damping of coupled magnetoelastic waves in ferromagnets with cubic or uniaxial symmetry is developed. It takes the symmetry of a ferromagnet into account and describes both the exchange and relativistic interactions in the crystal.

scholarly journals Damping of Magnetoelastic Waves

2020 ◽  
Vol 65 (10) ◽  
pp. 912
Author(s):  
V. G. Bar’yakhtar ◽  
A. G. Danilevich
Keyword(s):  
Significant Role ◽  
Easy Axis ◽  
Dissipative Function ◽  
Relaxation Processes ◽  
Magnetoelastic Interaction ◽  
Dissipative Processes ◽  
Dispersion Law ◽  
Magnetoacoustic Waves ◽  
Magnetoelastic Waves ◽  
General Method

A general method for constructing a model of the dissipative function describing the relaxation processes induced by the damping of coupled magnetoacoustic waves in magnetically ordered materials has been developed. The obtained model is based on the symmetry of the magnet and describes both exchange and relativistic interactions in the crystal. The model accounts for the contributions of both the magnetic and elastic subsystems to the dissipation, as well asthe relaxation associated with the magnetoelastic interaction. The dispersion law for coupled magnetoelastic waves is calculated in the case of a uniaxial ferromagnet of the “easy axis” type. It is shown that the contribution of the magnetoelastic interaction to dissipative processes can play a significant role in the case of magnetoacoustic resonance.

Release of Tetrahydrofuran, Structural Phase Transitions and Dynamic Relaxation Processes in Ca (BH4)2

2012 ◽  
Vol 184 ◽  
pp. 24-32 ◽  
Author(s):  
Annalisa Paolone ◽  
O. Palumbo ◽  
P. Rispoli ◽  
Rosario Cantelli ◽  
E. Rönnebro ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Room Temperature ◽  
Structural Phase ◽  
Young Modulus ◽  
Relaxation Processes ◽  
Structural Phase Transitions ◽  
Exponential Factor ◽  
Β Phase ◽  
Α Phase ◽  
Calcium Borohydride

Various calcium borohydride samples were investigated by means of combined measurements of thermogravimetry and mass spectrometry, and anelastic spectroscopy. On heating, the release of 2-5% tetrahydrofuran (THF) is detected in all the samples at temperatures below ~480 K, even in those which were previously thermally treated, according to procedures known from the literature, in order to remove the solvent. Dehydrogenation takes place above 480 K. Above room temperature the temperature dependence of the Young modulus of Ca (BH4)2clearly monitors the release of THF and two irreversible structural phase transitions: from the α to the α’ phase around 460 K and from the α’ to the β phase, nearly completely evolved around 590 K. Moreover, the coefficient of elastic energy dissipation presents two dynamic processes below room temperature; a peak around 120 K characterized by an activation energy of 0.20 eV and a pre-exponential factor typical of atom-cluster relaxations, that we attributed to the dynamics of THF molecules retained in the borohydride lattice, and a peak around 200 K, possibly due to the relaxation of H vacancies.

scholarly journals Предпереходные явления в области фазовых переходов первого рода в ионно-молекулярных кристаллах

2020 ◽  
Vol 62 (6) ◽  
pp. 890
Author(s):  
А.Р. Алиев ◽  
И.Р. Ахмедов ◽  
М.Г. Какагасанов ◽  
З.А. Алиев
Keyword(s):  
Phase Transition ◽  
Raman Spectroscopy ◽  
Phase Transitions ◽  
Transition Region ◽  
Diffuse Phase Transition ◽  
Structural Phase ◽  
Study Phase ◽  
Relaxation Processes ◽  
Structural Phase Transitions ◽  
Diffuse Phase

Raman spectroscopy was used to study the molecular relaxation processes in sulfates Li2SO4, Na2SO4, K2SO4, carbonates Li2CO3, Na2CO3, K2CO3, perchlorates NaClO4, KClO4 and nitrates Ca(NO3)2, Sr(NO3)2, Ba(NO3)2. It has been established that in the systems under study, phase transitions of the first kind are stretched in nature (diffuse phase transition): in sulfates, carbonates and perchlorates they are structural phase transitions, and in nitrates this is a crystal-melt phase transition. The existence of a pre-transition region was found in all the crystals studied.

scholarly journals Put Your Trust in Powder: Negative and Zero Thermal Expansion Materials

2014 ◽  
Vol 70 (a1) ◽  
pp. C14-C14
Author(s):  
John Evans
Keyword(s):  
Thermal Expansion ◽  
Phase Transitions ◽  
Powder Diffraction ◽  
Critical Role ◽  
Structural Phase ◽  
Functional Materials ◽  
Relaxation Processes ◽  
In Situ Studies ◽  
Synthetic Routes

Powder diffraction is one of the most powerful structural probes available to the materials chemist. It invariably plays a critical role in the preparation of new phases and is increasingly exploited via in-situ studies to understand and control complex synthetic routes to fleetingly stable materials. Ab initio structure solution followed by Rietveld refinement frequently provides the first structural information on new functional materials, and powder diffraction is often the method of choice for probing structure-property relationships under non-ambient or in-operando conditions, or for following the structures of samples undergoing structural phase transitions. In this presentation I'll show how powder diffraction has been crucial in understanding the properties of so-called negative thermal expansion (NTE) materials – here inorganic oxides which contract on heating. Powder methods have provided key information on the thermodynamic stability of these materials, the low temperature synthetic routes required to prepare them, the average and local distortions that lead to NTE, and on the often complex phase transitions they undergo. I'll also discuss how powder diffraction can probe glass-like relaxation processes which occur over remarkably long timescales in some of these materials and how the synthetic control enabled by in-situ studies has led to the preparation of single-phase isotropic materials whose expansion properties can be systematically tuned from negative to zero to positive values (alpha –8 to +6 * 10–6 K–1) [1]. I'll emphasise how new techniques such as Parametric- [2] and Symmetry-Mode-Rietveld [3] refinement has been crucial in the study of these materials.

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