Magnetic and thermodynamic properties of Heusler alloys Ni55Mn26Al19

The temperature dependence of magnetization, M(T), specific heat, Cp(T), and thermal expansion, ΔL/L300K(T) were investigated for the Heusler compound Ni55Mn26Al19 with B2 structure. M(T) has a cusp-type anomaly for the antiferromagnetic (AF) transition at the Néel temperature TN = 280 K that is irreversible between the field-cooled and zero-field-cooled processes below Tf ∼ 60 K, which is characteristic of spin glass. Cp(T) also has an anomaly at TN = 280 K. For temperatures T < 280 K, Cp(T) shows no anomaly without indicating any phase transition. These results mean that with decreasing T Ni55Mn26Al19 has a reentrant spin glass below Tf from the AF state. ΔL/L300K(T) shows no discontinuity indicating a structural transition in the range 5 < T < 300 K, i.e., Ni55Mn26Al19 has no martensitic transformation. TN increases proportionally with pressure, P, because of the enhancement of the AF interaction. The value of its initial rate is estimated to be dTNdP= 5.25 K/GPa. Tf also increases proportionally with P with dTfdP= 2.21 K/GPa, and hence magnetic frustration, which promotes the spin glass system, is enhanced under pressurization.