Strains induced by magnetic field and phase transformation in Ni50.5Mn26.2Ga23.4 ferromagnetic shape memory alloy

Abstract

In recent years, a lot of investigations have been focused on ferromagnetic shape memory alloys (FSMAS), such as Ni2MnGa, FePd, CoNi and FeNiCoTi [1–4], due to their large magnetic-field induced strain (MFIS) and high response frequency. Of the above alloys Ni Mn Ga alloy is the only known Heusler alloy exhibiting thermal martensitic phase transformation. It is reported up to now that a maximum 6% of MFIS has been obtained in Ni49.8Mn28.5Ga21.7 single crystal by applying a constant 2 MPa compression stress [5]. However, few results have been reported about the temperature dependence of MFIS in NiMnGa alloy. The purpose of the present paper is to measure the MFIS along the [110] direction of Ni50.5Mn26.2Ga23.4 single crystal at different temperatures. It is interesting to find that the MFIS between the temperature of Ms and Mf is larger than the MFIS below Mf. The specimen used in this study was cut from a bulk single crystal with composition of Ni50.5Mn26.2Ga23.4 (at%). The single crystal was grown in the [110] direction according to the cubic parent phase using a Czochralski instrument with a cold crucible system. This single crystal was annealed at 850 ◦C for 12 h, then quenched into ice water to obtain high L21 ordering [6]. An orientation along the specimen length direction was confirmed to be [110] by Laue X-ray diffraction. The dimensions of specimens for MFIS measurements were 2 mm × 4 mm × 6 mm. The magnetic field induced strain was measured by a strain-gauge attached to the sample which was placed in a furnace between the electromagnets. The external magnetic field was applied parallel to the [110] direction of the sample. The sample was cooled from high temperature to a temperature far below Mf. Then the MFIS was measured at the temperatures of interest. The phase transformation temperatures were determined by a low field ac susceptility method. Fig. 1 shows the χ–T curve of the experimental alloy. It can be seen that, the martensitic transformation start temperature (Ms) and finish temperature (Mf) are 279 K and 274 K respectively. And the reverse transformation temperature (TA) is 283 K. The Curie temperature (Tc) is about 370 K higher than the martensitic transformation and reverse transformation temperatures indicating that both autensite and martensite are ferromagnetic in this alloy. It is seen that the value of Ms–Mf is very small which means that thermal martensitic transformation happens quickly. Fig. 2 shows a typical curve of the MFIS measured along the [110] direction in a reverse magnetic field at 244 K below Mf. It was found that the sample shrinks in the external magnetic field. A small MFIS, about −600 ppm, was obtained with an axial magnetic field up to 2 T. Further, it can been seen the MFIS is reversible and reaches saturation at about 0.8 T. All strain recovers after returning the field to zero. Similar MFIS curves are obtained irrespective of the direction of the applied magnetic field. Fig. 3 depicts the magnetic field induced strains as a function of magnetic field in the temperature range between 244 K and 276 K. According to the different phase states at various temperatures, the experimental results can be divided into two temperature stages: Stage I. Full martensite state which is below Mf. For the specimens tested at 247 K, 255 K and 263 K, the saturated MFIS is only−600 ppm. And with the increasing of temperature only an increase of 100 ppm is observed. In this stage, all the MFIS is attributed to the motion of martensite twin boundaries caused by the magnetic field because only martensite phase exists. It is known that [110] is the hard axis of magnetization for NiMnGa alloy. According to R.C. O’Handley’s model [6] the experimental Ni Mn Ga alloy has weak anisotropy. This means that only a weak driving force could be provided on the twin boundaries regardless of the strength of applied field, which leads to very a small MFIS in the full martensite phase. Stage II. Parent phase and martensite co-exist which is between Ms and Mf. For the specimens tested at 275 K, the strain produced is as large as −900 ppm, meanwhile, of the total strain about −300 ppm strain does not recover when the magnetic field is removed. At 276 K, the saturated MFIS jumps abruptly to −1700 ppm, which is nearly three times as large as that of the specimen measured below Mf, and a greater irreversible MFIS, nearly −1200 ppm, is observed. Clearly, the strain caused by thermal phase transformation plays an important role in the increase of this MFIS. Based on the analysis of experimental results, the temperature dependences of saturated MFIS and the strain recovery