Spin valves have revolutionized the field of magnetic recording and memory devices. Spin valves are generally realized in thin film heterostructures, where two ferromagnetic (FM) layers are separated by a nonmagnetic conducting layer. Here, we demonstrate spin-valve-like magnetoresistance at room temperature in a bulk ferrimagnetic material that exhibits a magnetic shape memory effect. The origin of this unexpected behavior in Mn(2)NiGa has been investigated by neutron diffraction, magnetization, and ab initio theoretical calculations. The refinement of the neutron diffraction pattern shows the presence of antisite disorder where about 13% of the Ga sites are occupied by Mn atoms. On the basis of the magnetic structure obtained from neutron diffraction and theoretical calculations, we establish that these antisite defects cause the formation of FM nanoclusters with parallel alignment of Mn spin moments in a Mn(2)NiGa bulk lattice that has antiparallel Mn spin moments. The direction of the Mn moments in the soft FM cluster reverses with the external magnetic field. This causes a rotation or tilt in the antiparallel Mn moments at the cluster-lattice interface resulting in the observed asymmetry in magnetoresistance.
We predict the existence of a new ferromagnetic shape memory alloy Ga 2 MnNi using density functional theory. The martensitic start temperature (T M ) is found to be approximately proportional to the stabilization energy of the martensitic phase (δE tot ) for different shape memory alloys. Experimental studies performed to verify the theoretical results show that Ga 2 MnNi is ferromagnetic at room temperature and the T M and T C are 780 K and 330 K, respectively. Both from theory and experiment, the martensitic transition is found to be volume conserving that is indicative of shape memory behavior.
7M orthorhombic modulated structure in the martensite phase of Ni 1.8 Pt 0.2 MnGa is reported by powder neutron diffraction study, which indicates that it is likely to exhibit magnetic field induced strain. The change in the unit cell volume is less than 0.5% between the austenite and martensite phases, as expected for a volume conserving martensite transformation. The magnetic structure analysis shows that the magnetic moment in the martensite phase is higher compared to Ni 2 MnGa, which is in good agreement with magnetization measurement. PACS numbers: 75.50.Cc, 81.30.Kf Ni 2 MnGa is a ferromagnetic Heusler alloy, which shows a large magnetic field induced strain (MFIS) and fast actuation in the martensite phase. 1-3 These properties make Ni 2 MnGa a material with high potential for application as magnetic actuators. However, brittleness and low transition temperature of this material has necessitated the search for new alloys with similar MFIS, but with higher transition temperatures and ductility. 4 In the Ni-Mn-Ga family, an increased magnetic transition temperature (T C ) of 588 K as well as 4% MFIS has been reported for Mn 2 NiGa. 5 Generally, MFIS is observed in structures that exhibit modulation, since that leads to lower twinning stress. 2 The modulation can be described as a shuffling of the (110) planes along the [110] direction. 7-9 A modulated structure in Mn 2 NiGa has been recently reported by x-ray diffraction study. 6 In the case of Ga 2 MnNi, although T C is lower than Ni 2 MnGa, a large martensitic start temperature (M s ) and modulated structure have been observed. 10,11 Other ferromagnetic shape memory alloys such as Ni-Mn-Al, 12 Ni-Mn-Fe-Ga, 13,14 and Ni-Fe-Ga-Co 15 showing MFIS with improved ductility have been reported. A modulated structure has also been reported for off-stochiometric Ni-Mn-In compositions. 16,17 However, although the above mentioned materials exhibit MFIS, the magnitude is much smaller compared to Ni 2 MnGa (10%). Of late, ab-initio density functional theory (DFT) has played an important role in predicting new ferromagnetic shape memory alloys. 10,18 Taking cue from an earlier experimental work, 19 a recent DFT study has put forward Pt doped Ni 2 MnGa to be an alternative to Ni 2 MnGa. 20The theoretical estimate of maximum MFIS is about 14% that is higher than Ni 2 MnGa. 20In this letter, we report the existence of a modulated structure in the martensite phase of Ni 1.84 Pt 0.2 MnGa 0.96 from neutron diffraction studies, which is strongly suggests that it would exhibit MFIS. Our analysis shows that the magnetic moment in the martensite phase is higher than Ni 2 MnGa.The specimen has been prepared by melting appropriate quantities of Ni, Pt, Mn and Ga of 99.99% purity in an arc furnace. Less than 1% weight loss was observed after melting. The ingot was then annealed at 1173 K for 3 days for homogeneization and then slow cooled to room temperature. X-ray diffraction at room temperature showed a single phase L2 1 structure, as expected for the austenite phase. The compos...
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