Decrease of magnetocaloric effect and magnetic susceptibility of ferromagnet due to magnetostructural phase transition

Abstract In the present article the impact of the first-order magnetostructural phase transition (MSPT) on the spin-exchange energy density has been considered to estimate the influence of MSPT on the magnetic susceptibility of ferromagnetic solid and magnetocaloric effect (MCE). To demonstrate this influence, theoretical model taking into account influence of MSPT on the spin-exchange energy density has been used. The ferromagnetic shape memory alloy Ni-Mn-Ga-Cu undergoing the ferroelastic phase transition from paramagnetic austenitic phase to ferromagnetic martensitic phase has been considered. It has been shown that the MSPT observed in this alloy increases the spin-exchange energy density and due to this decreases the magnetic susceptibility of the alloy and the magnetic-field-induced temperature change. To evaluate the magnetic susceptibility and the magnetic-field-induced temperature change the temperature dependence of saturation magnetization has been computed; the obtained theoretical curve was fitted to experimental points and in such a way the physical values and model parameters predetermining the magnitudes of magnetic susceptibility and magnetic-field-induced temperature change have been estimated. The magnitude of the temperature change induced by the magnetic field of 20 kOe appeared to be close to 1 K, while for the “giant” MCE this value must be greater than 5 K. Therefore, the observation of sharp change of magnetization value in the course of magnetostructural phase transition does not guarantee the availability of large magnetic-field-induced temperature change, characterizing a giant magnetocaloric effect.