Structural, magnetic and magnetocaloric effect in the off-stoichimetric Gd₅Ge_2.05−xSi_1.95−xMn_2xalloys

Abstract

and transforms to an anti-ferromagnetically ordered state (θ P = -18.6(2) K) below 33 K. The magnetic spin structure can be described with k = (0, 0, 0) in space group Pbca' and it is similar to the one of the C2/c phase except that it is noncollinear in nature, i.e. there are components of the magnetic moment along all three crystallographic axes. Small magnetoelastic coupling is observed in the orthorhombic phase. More details are reported in [5].[ Compared with conventional refrigeration, magnetic refrigeration technology has many advantages, such as the absence of harmful gas, less noise, low cost and high efficiency. Since the discovery of martensitic transformation with both phases magnetically ordered in Heusler alloys NiMn-Z (Z: Ga, In, Sn and Sb) increasing attention has been paid to study the change in magnetic and electrical properties associated to the first-order reversible magnetostructural transition that originates valuable functional properties such as magnetic superelasticity, large inverse magnetocaloric effect, and large magnetoresistance change [1 and the references therein]. The reversibility and irreversibility of the magnetostructural transition is very important for magnetic actuator materials such as magnetic shape memory alloys. The austenite phase induced by the magnetic field is able to transform back to the initial martensite phase when the magnetic field is removed. A complete recovery of the initial martensite state may bring about magnetoelasticity (two-way magnetic shape memory effect), while the irreversible magnetostructural transition would result in magnetoplasticity (one-way magnetic shape memory effect). We showed that the effects of the irreversibility of the magnetostructural transitions on magnetocaloric effect in the (Ni-Cu)-Mn-Sn compounds was very important by magnetic and resistance measurements under magnetic field [1]. The magnetic entropy change of the Ni 50-x Cu x Mn 36 Sn 14 (x=2 and 4) compounds are estimated by using Maxwell equation and the M(H) curves obtained from noncontinuous heating method. These compounds show the magnetostructural phase transition from cubic to orthorhombic structure with decreasing temperature at around 218 and 168 K, respectively. To see better the type of the magnetostructural transition, we perform the neutron diffraction experiments for these compounds at the different temperatures and under different magnetic fields. According to the neutron diffraction experiment near A S (A S : Austenite start temperature), the Martensite phase of these compounds transforms to Austenite phase with increasing the magnetic field from 0 to 5 T, while these compounds remains in the Austenite phase with decreasing the magnetic field to zero Tesla. This is the evidence of the irreversible magnetostructural transition occurred in these compounds. Because of that, the determination of the magnetic entropy change in alloys which show the irreversible magnetostructural transition has carefully been studied [2].[1] Dincer I., Yüzüak E., Elerman Y., J. Ph...