Dynamics of structural phase transition and changes in properties under the influence of elastically anisotropic deforming stresses, regularities of critical lines and points in magnetic semiconductors and magnetic dielectrics

Abstract

The paper deals with a generalizing analysis of elastically deforming regularities by examining the experimental results for magnetic semiconductors and dielectrics. The role of anisotropically deforming elastic (EAD) stresses, determined from the influence of temperature ( T), magnetic field ( H) and hydrostatic pressure ( P), in the formation and changes of the structural phase transitions (PTs) and properties has been estimated. From the analysis of investigations of the resistive and magnetostrictive properties and of PTs in La 0.7Ca 0.3MnO 3, LaMnO 3 the role of EAD in baro-, magneto-, and baromagnetoresistive effects has been determined and it has been deduced that their maximum temperature T PP is equal to the temperature of metal–semiconductor PT T ms. It is noted that the action of EAD stresses in the “cooling”, “heating” effects of T ms( H), T ms( P) and H g( T) change, and in T– H– P (5.1 K–2.42 kOe–1 kbar) influence on the resistive properties, and T– H (5.2 K–2.5 kOe) influence on the magnetostrictive properties is regular. Changes in properties, PT, and T– H– P-induced effects were found to be of the alternating-sign character. From the results of studies of the resonance properties and PTs in CuCl 2·2H 2O the role of EAD stresses has been determined and a correspondence in the T– H– P (1 K–4 kOe–3 kbar) effect on PT change has been revealed. Thermomagnetic and thermobaromagnetic effects have been revealed with the peaks corresponding to T PP=0 K, the temperature which coincides with that of the structural PT T ST. The regularities in “cooling” and “heating” effects of H and P influence have been grounded, as well as those in T P( H), T P( H, P) change going on with sign alternation. The location of point P( H P, T P), where elastic and magnetoelastic anisotropies become conformable to each other at T P=9.2 K, has been found. It considerably differs from the known T N=4.3 K. The results of magnetization and of the field-temperature and field-frequency dependencies investigation have been analyzed, and it has been shown that there is a correspondence in the changes of properties under the influence of T, H, and P before and after the structural PT. The effect of thermally straining striction (compression) (TSS) has been revealed, which is a consequence of structure properties and of the mechanisms of thermoanisotropic strains. The role of EAD stresses in the competitive mechanisms of T and H effects and in changes of the high-frequency properties is shown. The critical lines (i) in magnetic semiconductors, T ms( H), T ms( P) and H g( T) and points T X, P X, PP X, P X′, T PP= T ms, T C, as well as (ii) in magnetic dielectrics, T P( H), T P( H,P) and points P X, PP X, T X= T PP= T ST=0 K, P, T P=9.2 K have been marked out and explained. A relationship between the peculiarities of structure, PTs, properties and mechanisms of EAD stresses from the influence of T– H– P has been found. The role of sign alternation of thermo- and magneto-elasticity and of their anisotropies in changes of properties and PT has been shown. The identity in position (sign alternation) of critical lines H g( T), H P( T) and points P X, PP X, T X, T C, P has been noted. The laws of EAD stresses in changes of properties and PT of magnetic semiconductors and dielectrics have been ascertained.