Neutron spectroscopy study of magnons in the austenite and martensite phases of a Ni-Mn-Ga Heusler alloy

The low energy region of magnon excitations of an off-stoichiometric Ni49.1Mn29.4Ga21.5 single crystal has been investigated by neutron spectroscopy. The lowest magnetic exchange stiffness constant D of 97±2 meV A2 has been found in the cubic austenite phase. In the two martensitic phases the exchange stiffness constants are significantly larger with values of 149±4 meV A2 in the tetragonal phase and 198±7 meV A2 in the low temperature martensite. The large value of D in the low temperature phase compared to the other phases cannot be explained solely by renormalization effects due to magnon-magnon interaction and is attributed to a stronger magnetic coupling. In both the martensitic phases a gap of magnon excitation at the Γ-point of about 0.2 meV was observed.