Magnetic ground state of geometrically frustrated Mn_2-xFe_1+xAl heusler alloys

Heusler alloys with a general formula X₂YZ (X and Y: transition metals, Z: main group element) have become a topic of tremendous interest in recent times primarily due to their diverse range of physical properties that are tunable using different constituents and annealing conditions. This tenability makes them highly competitive candidates for applications in various fields, including spintronics. Among the large family of Heusler alloys, Mn-based ones are especially intriguing because of their complex physical properties and abundance. Many recent studies are focused on such a class of Heusler alloys to explore the magnetic ground state of novel Mn-based Heusler alloys. A systematic investigation of electronic, structural, and magnetic properties of Mn_2-xFe_1+xAl (x=0, 0.5) Heusler alloys is carried out, both theoretically and experimentally, to understand the magnetic ground state. Previously, L21 (regular) and X (inverse) Heusler structures have been reported for Mn₂FeAl Heusler alloy. However, the present electronic structure calculations propose a more stable β-Mn crystal structure which is in excellent agreement with the obtained experimental results. Polycrystalline ingots with a β-Mn structure are prepared by arc melting technique using high-purity elements for experimental study. Detailed magnetic analysis suggests a geometrically frustrated system with a spin glass feature. A sharp peak appears in the zero-field-cooled magnetization curves at low temperatures (T = 42 K for x = 0.0 and T = 34.5 K for x = 0.5). This peak behavior signifies strong antiferromagnetic correlations with a high degree of frustration, indicating the presence of competing magnetic interactions. The complex magnetic structure of the β-Mn with a high degree of frustration and competing magnetic interactions leads to a spin glass state in these alloys. The characteristics signatures of spin glass behavior are verified from frequency-dependent AC susceptibility study using the Critical Power Law and Vogel Fulcher Law, magnetic relaxation, and magnetic memory effect. (author)