Exchange bias and magnetoresistance effects in Ni50Mn50-yXy (X = In, Sn) alloys

• Main Author: Wang, Baomin
• Other Authors: Liu Yong
• Format: Theses and Dissertations
• Language: English
• Published: 2012
• Subjects: DRNTU::Science::Physics::Electricity and magnetism
• Online Access: https://hdl.handle.net/10356/48074
• Institution: Nanyang Technological University

Recently, a kind of new ferromagnetic shape memory alloys (FSMAs) has been discovered in Ni50Mn50-y Xy (NiMnX, X = In, Sn, Sb) systems, in which the magnetization in the martensite phase is very low in comparison to that of the austenite phase. This kind of alloys is termed metamagnetic shape memory alloys (MSMAs) to distinguish them from previously discovered FMSAs. Beside producing magnetic-field-induced strain (MFIS) with large stresses of over 100 MPa caused by magnetic field-induced phase transformation, the MSMAs also exhibit several other important sensor capabilities, such as large magnetoresistance effect, large inverse magnetocaloric effect, and exchange bias effect. These properties have attracted much interest in recent years. However, to date, the magnetic properties in the martensitic state of these alloys are not clear and the sensor capabilities of MSMAs are needed to be further explored. In the present work, the magnetic properties in the martensitic state and magnetoresistance effect of NiMnX (X = In, Sn) have been studied. Exchange bias effect is observed in the martensitic state of NiMnX (X = In, Sn) alloys after zero-field cooling from an unmagnetized state, which cannot be expected in the previous exchange bias systems. We propose that this is related to the newly formed interface between different magnetic phases during the initial magnetization process. The magnetic unidirectional anisotropy, which is the origin of EB effect, can be created isothermally below the blocking temperature. Both the value of exchange bias field and its sign can be tuned isothermally or by the amplitude of remanent magnetization after zero-field cooling. This tunability is strongly dependent on the direction of the initial magnetization field for the hysteresis loop measurements. In addition, the value of exchange bias field can also be tuned by the amplitude of the cooling field during field cooling. The magnetoresistance effect in MSMA Ni49.5Mn34.5In16 has also been studied in detail. The large negative magnetoresistance is observed within the transformation regime across the first-order magnetic transition, which is correlated to the magnetic-field-induced phase transformation from antiferromagnetic martensite to ferromagnetic austenite. The irreversibility and “overshooting” behaviors in isothermal magnetoresistance of Ni49.5Mn34.5In16 are observed at temperatures reached by heating and cooling processes, respectively. The strong thermal-history-dependent (THD) magnetoresistance behavior in MSMA Ni49.5Mn34.5In16 is discussed in terms of phase coexistence, strain-induced THD energy diagram and the competition of instable and THD metastable phases.