Magnetic correlations and Griffith-like phase in Co$_2$TiSi$_{0.5}$Al$_{0.5}$ Heusler alloy

TL;DR

This study investigates the magnetic properties, phase transitions, and electronic structure of Co$_2$TiSi$_{0.5}$Al$_{0.5}$ Heusler alloy, revealing Griffiths-like phase, critical behavior near Curie temperature, and effects of disorder on spin polarization.

Contribution

It provides a detailed analysis of magnetic correlations, phase transitions, and electronic properties of a partially disordered Heusler alloy, including experimental and theoretical insights.

Findings

Identification of two magnetic transitions at 278 K and 270 K.

Observation of Griffiths-like phase above Curie temperature.

Half-metallicity with 100% spin polarization reduced to 8.3% by disorder.

Abstract

We present a comprehensive study aimed at elucidating the complex magnetic correlations in Co$_2$TiSi$_{0.5}$Al$_{0.5}$ Heusler alloy having the partial B2-type structure amid L2$_1$ cubic main phase. The thermo-magnetization measurements at 100 Oe reveal the presence of two magnetic transitions at T$\rm_{C1}=278~K$ and T$\rm_{C2}=270~K$, respectively, with saturation magnetization of around 1.2 $\mu \rm_ B$/f.u. at 5~K. Our magnetic field dependent studies reveal the dominance of T$\rm_{C1}$ transition at lower fields ($\mu_0\rm H \leqslant 0.03~Tesla$); however, at higher fields the T$\rm_{C2}$ transition becomes more pronounced. The observation of remnant magnetization above Curie temperature suggests the development of Griffiths-like phase, which is extensively analyzed through {\it ac} and {\it dc}- magnetic susceptibility ($\chi$) data. The evaluation of magnetocaloric potential indicates second order phase transition with notable $\Delta S_{\rm M}=$ 2.22 Jkg$^{-1}$K$^{-1}$ at 7 Tesla. The low-field ($\mu_0\rm H \leqslant 0.1~Tesla$) magnetic entropy ($\Delta S_{\rm M}$) curves exhibit two non-identical positive peaks. The nature and range of spin interactions near T$\rm_C$ were scrutinized through rigorous critical phenomenon analysis, and the values of exponents $\alpha, \beta, \gamma$ and $\delta$ to be $0.063, 0.361, 1.108$ and $3.943$, respectively, which are found to be slightly deviating from mean-field theory towards 3D Heisenberg model. Additionally, the exchange magnetic interactions are found to decay as $J(r) \sim r^{-4.6}$. Furthermore, the density functional theory results reveal the half-metallic nature exhibiting 100\% spin polarization (SP). However, the electronic and magnetic properties are greatly affected by the incorporation of structural disorder, which results in drastic reduction of SP to mere 8.3\%.