Magnetostructural features of phase transitions in the Mn-=SUB=-1-x-=/SUB=-Co-=SUB=-x-=/SUB=-NiGe system Part 2. Analysis

Abstract

Within the framework of the model of interacting parameters of the magnetic and structural orders, taking into account the internal periodic magnetic field orthogonal to the exchange field, we analyzed the features of magnetostructural transitions in the Mn1-xCoxNiGe system. A qualitative description of changes in the nature of magnetic phase transitions from magnetostructural transitions of the 1st order paramagnetism-antiferromagnetism (x=0.05-0.1) to isostructural transitions of the 2nd order paramagnetism-ferromagnetism (x=0.15-0.8) with a change in the concentration of Co is presented. An explanation is given for the onset of irreversible magnetic-field-induced transitions at temperatures on the order of 5 K in strong magnetic fields, accompanied by a change in the saturation magnetization for samples x=0.15-0.8. The low-temperature inverse magnetocaloric effect at liquid helium temperatures is predicted for these samples. Keywords: irreversible magnetostructural first-order phase transition, helimagnetism, direct and inverse magnetocaloric effects.

Temperature-baric features of magnetic characteristics in systems with structural transitions of the displacement type

“…In this section, we rely on the provisions of [16,17] with the main of them limited to the following assumptions.…”

“…and the measured saturation magnetization with the specified eigenvalue s of spin operator of ŝk n k-th atom of Mn in n-th cell that was initially chosen for the system being described [16,17].…”

“…for a spin subsystem with helimagnetic order (HM) [16] and in displaced harmonic oscillator approximation (dso) [22] for a structural subsystem undergoing transition (hex, P6 3 /mmc) ↔ (orth, Pnma) with applicable parameters which define the type of system. These parameters include combinations of coefficients of the thermodynamic potential, which form baric dependences of structural and magnetic transition temperatures, their behaviors (1th or 2 nd order), initial types of magnetic structures and appropriate effective initial magnetic moments µ = 2µ B s of Mn atoms.…”

“…are formally defined by expressions (4), (5) [16]. For calculation, use distribution function ρ dso (Q n ) in displaced harmonic oscillator (6a) and density matrix ρ k n in mean field approximation (6b),…”

“…For this germanide representative, dependences v 0 (e 1 , e 2 ) and J q a (Q 2 0 , e 1 ) shall provide some features common to germanides, which are discussed in the introduction. For details of selection of the applicable parameters of thermodynamic potential see [15][16][17].…”