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.

Vanadium diboride (VB) with an AlB-type structure has been synthesized at 8 GPa and 1700 K in a D-DIA-type multianvil apparatus. The obtained bulk modulus is B = 262(2) GPa with fixed B' = 4.0 for VB via high-pressure X-ray diffraction measurements. Meanwhile, VB has also been demonstrated to possess a high Vickers hardness of 27.2 ± 1.5 GPa, a high thermal stability of 1410 K in air, among the highest for transition-metal borides, and an extremely low resistivity value (41 μΩ cm) at room temperature. Results from first-principles calculations regarding the mechanical and electronic properties of VB are largely consistent with the experimental observations and further suggest that VB possesses simultaneously the properties of a hard and refractory ceramic and those of an excellent electric conductor.

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We investigate the external hydrostatic pressure effect on the superconducting transition temperature (Tc) of new layered superconductors Bi4O4S3 and NdO0.5F0.5BiS2. Though the Tc is found to have a moderate decrease from 4.8 K to 4.3 K (dTconset/dP = –0.28 K/GPa) for Bi4O4S3 superconductor, the same increases from 4.6 K to 5 K (dTconset/dP = 0.44 K/GPa) up to 1.31 GPa followed by a sudden decrease from 5 K to 4.7 K up to 1.75 GPa for NdO0.5F0.5BiS2 superconductor. The variation of Tc in these systems may be correlated to an increase or decrease of the charge carriers in the density of states under externally applied pressure. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Vertically aligned undoped ZnO nanorod arrays (NRAs) have been fabricated on silicon (111) substrates by radio frequency magnetron sputtering. The structural studies illustrate a hexagonal wurtzite structure of the ZnO NRAs with compressive stress. Raman analysis of the E 2 high phonon mode corroborates the partial relaxation of stress in NRAs by the post growth treatment under oxygen and vacuum atmospheres. The anomalous Raman modes have been attributed to the local vibrations and it corresponds to the silent modes of wurtzite ZnO. The appearance of forbidden modes illustrates the breakdown of the Raman selection rules. The role of point defects on the ferromagnetic behaviour of NRAs was analyzed by optical transitions and correlated with the magnetic properties. Post growth treatment of NRAs under oxygen and vacuum atmospheres significantly suppresses the point defects owing to the enhancement of the crystalline quality. The temperature dependent zero-field cooled and field cooled magnetizations reveal the coexistence of antiferromagnetism and ferromagnetism below 7 K. However, the ferromagnetism is dominant and stable between 7 K and room temperature. The decrease of ferromagnetism in NRAs is directly associated with the compensation of point defects such as zinc and oxygen vacancies as substantiated by the radiative transition between shallow donor and acceptor energy levels. These results confirm that point defects play an important role in enhancing the room temperature ferromagnetism in ZnO NRAs.

Inverse magnetocaloric effect is demonstrated in Mn2NiGa and Mn1.75Ni1.25Ga magnetic shape memory alloys. The entropy change at the martensite transition is larger in Mn1.75Ni1.25Ga, and it increases linearly with magnetic field in both the specimens. Existence of inverse magnetocaloric effect is consistent with the observation that magnetization in the martensite phase is smaller than the austenite phase. Although the Mn content is smaller in Mn1.75Ni1.25Ga, from neutron diffraction, we show that the origin of inverse magnetocaloric effect is the antiferromagnetic interaction between the Mn atoms occupying inequivalent sites.

We report the structure, magnetism and magnetic entropy change in a Mn-rich Ni50−x Mn37+x Sn13 Heusler alloy system in the composition range 0 ⩽ x ⩽ 4. An excess Mn content stabilizes the cubic austenite phase at room temperature. Martensitic transition decreases from 305 to 100 K with increasing Mn concentration (x: 0 → 4) and also it was found to shift to a lower temperature with the application of a higher magnetic field. The exchange bias blocking temperature was found to decrease drastically from 149 to 9 K with increasing Mn concentration. A large magnetic entropy change (ΔS M) of 32 J kg−1 K−1 has been achieved for a field change of 5 T in the x = 3 alloy.

We report upon the effect of hydrostatic pressure on the martensitic transition, magnetic properties, and magnetic entropy change in Ni50-xMn37 + xSn13 (x = 2, 3) Heusler alloys. The application of pressure has significantly shifted the martensitic transition temperature to higher values. A large rate of change of the martensitic transition with a pressure of ∼3.1 K/kbar has been obtained for the x = 2 alloy, whereas the Curie temperature changes marginally with pressure (∼0.3 K/kbar). Magnetization of both the austenite and martensite phases decreases with an increase of pressure. The maximum magnetic entropy change of 34 J kg−1K−1 at ambient pressure and 22.5 J kg−1K−1 at 8 kbar was observed around the martensitic transition temperature for the x = 3 alloy.