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• Structure-properties (magnetic, electrical and thermal) relationship of Mn3Al alloy. • Observation of weak ferromagnetism buried in an antiferromagnet. • High value of electronic heat capacity. • Disordered system with strong frustration. Numerous interesting features of antiferromagnets open up the possibility of designing new antiferromagnetic materials that can be useful for spintronic device applications. Here we present the structural, magnetic, thermal and electrical transport properties of polycrystalline Mn 3 Al alloys prepared by arc melting. Rietveld refinement of the room temperature X-Ray diffraction patterns reveals that the arc-melted Mn 3 Al crystallizes in a β Mn phase (space group 213). Detailed analysis of the magnetic data suggests for a dominant antiferromagnetic ordering. However, an in-homogenous magnetic disorder is observed in the system having a weak ferromagnetic phase in the antiferromagnetic host. In spite of the strong spin fluctuations in this system, the signature of spin glass-like nature in these alloys are not observed as probed by the frequency-dependent AC-Susceptibility measurements. Furthermore, the heat capacity was insensitive to the applied field of 3 Tesla with a negligible magnetic contribution suggesting a large electronic contribution to the heat capacity in these alloys. This work provides a useful insight into the tunable physical properties of the Heusler alloys for exploring antiferromagnetic based spintronic applications. [ABSTRACT FROM AUTHOR]

• Detailed theoretical and experimental study of arc melted Mn 2 FeAl alloy with a β-Mn structure. • Phase stability of β-Mn structure is verified by comparing the total energies of the other probable Heusler type structures. • With volume expansion, β-Mn structure of Mn 2 FeAl becomes the lowest ground state configuration. • Experimental realization of spin glass features in a frustrated antiferromagnet. The synthesized Mn 2 FeAl alloys crystallize in a geometrically frustrated cubic β-Mn structure (space group: P4 1 32) with an antiferromagnetic ordering whereas the previous theoretical findings suggest for a Heusler structure (L2 1 : regular and X: inverse). The experimental stability of the structure is verified by electronic structure calculations performed for various arrangements of Mn, Fe and Al atoms in the β-Mn-type crystal structure. When compared the energy of the β-Mn structure with the energy of L2 1 and X type structures, it is found that for an expansion of the lattice volume β-Mn structure becomes more preferable in total energy than L2 1 and X-type structures. The calculated theoretical equilibrium lattice parameter value for the β-Mn 2 FeAl is within the accuracy of the experimental value obtained in this work. Additional DFT + U calculations for the optimized crystal structure of the β-Mn 2 FeAl revealed that the electronic correlations in the Mn ions result in the increased total magnetic moment. In the X type structure, Mn 2 FeAl is a half metal, whereas the disordered arrangement of atoms in the β-Mn structure leads to the closure of the semiconductor gap. The β- Mn 2 FeAl alloys exhibit antiferromagnetic ordering (T N ≈ 42 K), which is in excellent agreement with our electronic structure calculations. The detailed analysis of the magnetic and heat capacity measurements suggests a short-range magnetic ordering in the Mn 2 FeAl alloys. Owing to the strong antiferromagnetic spin fluctuation caused by the geometric frustration in β-Mn, a large enhancement in the electronic heat capacity is noticed. Mn 2 FeAl shows the characteristic features of spin glass as verified from the frequency dependent AC susceptibility analysis using critical power law and Vogel-Fulcher law. To the best of our knowledge, this is the first ever report on the theoretically predicted lowest ground state configuration for Mn 2 FeAl with a β-Mn structure and the experimental realization of spin glass features in this geometrically frustrated antiferromagnet. [ABSTRACT FROM AUTHOR]

• Mn 1.5 Fe 1.5 Al alloy with a β-Mn structure exhibits canonical spin-glass behaviour. • Large frustration parameters indicting high degree of frustration is measured. • Hierarchical model of the spin-glass magnetic relaxation is observed. • Unidirectional exchange bias effect and memory effect are also observed. Magnetic properties of magnetically frustrated polycrystalline Mn 1.5 Fe 1.5 Al Heusler type alloy with a β-Mn structure were systematically studied using dc magnetization, ac susceptibility, magnetic relaxation and magnetic memory effect measurements to understand its magnetic ground state. The Mn 1.5 Fe 1.5 Al alloy system exhibits a sharp peak at low temperature around 34.5 K with strong antiferromagnetic correlations (θ CW ∼ −639 K) and a high degree of frustration (f p ∼ 18.7) which suggest the presence of competing magnetic interactions. Despite of strong antiferromagnetic correlations, this peak has spin-glass character as confirmed by the observations of bifurcation of the zero-field-cooled and field-cooled magnetization curves below the irreversibility temperature, field dependent shift of the irreversibility temperature, unidirectional exchange bias effect on the field-cooled M-H hysteresis loop, exponential decay of the temperature variation of remanence and coercivity, frequency dependent shift of the spin glass freezing temperature (T f) as per the dynamic scaling laws given by the critical slowing down model and Vogel-Fulcher law, magnetic relaxation and magnetic memory effect below T f. The observation of a clear asymmetric response with respect to the temperature cycling in the magnetic relaxation measurements validates the hierarchical model of the spin-glass magnetic relaxation in Mn 1.5 Fe 1.5 Al alloy. Since the behaviour exhibited by the Mn 1.5 Fe 1.5 Al alloy has perfect similarity with the well-known atomic spin glasses like CuMn, AuFe, therefore, it may be considered as canonical spin-glass system. [ABSTRACT FROM AUTHOR]