Phase Coexistence, Dual Transitions, Itinerant Magnetism, and Universal Critical Behavior of Ni-Substituted Mn(Co–Fe)Ge

Abstract

Comprehensive magnetic behavior of the Ni-substituted ferromagnetic Mn(Co–Fe)Ge Heusler alloy is reported using the combined analysis of temperature and field-dependent magnetization. Mn_0.5Ni_0.5Co_0.7Fe_0.3Ge crystallizes in a Ni₂In-type hexagonal structure and undergoes two magnetic transitions at 138.8 K (paramagnetic to ferromagnetic) and 21.2 K (ferromagnetic to antiferro/canted spin). Frequency-independent peaks of the ac-susceptibility χ_ac(T) rule out glass behavior at low temperatures. Coexistence of high-T ferromagnetic and low-T weak antiferromagnetic phases is confirmed by the suppression of the zero-field cooled peak at 21.2 K in field-cooled magnetization and virgin M–H curve lying out of the hysteresis loop at 5 K. The critical exponents estimated through modified Arrott plots and Kouvel–Fisher methods, critical isotherm fitting are self-consistent and exchange correlation function J(r) propound three-dimensional Heisenberg universality class and short-range spin–spin interactions. Notwithstanding, the conformity test displays not-so-well-scaled renormalized Arrott plots (m² versus h/m) below and above T_C, asserting weak first-order phase transition from high-T ferromagnetic to low-T antiferromagnetic state with feeble magnetic phase coexistence. Overall, dual transitions with contrasting magnetic character, short-range interactions, itinerant behavior, perceptible difference between Weiss and magnetic transition temperatures, virgin curve lying outside the envelope, and renormalized Arrott plots advise competing localized 3d Ni–Mn interactions with Co–Ge–Fe-hybridized s-p and s-d orbitals lead itinerant states. This study emphasizes the importance of critical exponent analysis of systems that exhibit feeble phase coexistence to unveil their underlying magnetic interactions.