The concentrations of elements by means of EDS spectrometry (at. %).
Abstract
Ni41Co9Mn31.5Ga18.5 is a re-entrant and metamagnetic Heusler alloy. In order to investigate the magnetic functionality of polycrystalline Ni41Co9Mn31.5Ga18.5, magnetic field-induced strain (MFIS) measurements were performed. A 0.12% MFIS was observed at 340 K and 10 T. Strict MFISs between 330 and 370 K were observed. These magneto-structural variances acted in concert with the metamagnetic property observed by the magnetization measurements and magneto-caloric property observed by the caloric measurements in applied magnetic fields. The MFISs were proportional to the fourth power of the magnetization, and this result is in agreement with Takahashi’s spin fluctuation theory of itinerant electron magnetism. The investigation of time response of the MFIS was performed by means of water-cooled electric magnet, zero magnetic field to 1.66 T in 8.0 s at 354 K. A 2.2×10−4 MFIS was observed, which was 80% of the MFIS in a 60-s mode. This indicates that a high-speed transition has occurred on applying magnetic fields.
Keywords
- magnetostriction
- Heusler alloys
- shape memory alloys
- metamagnetic transition
- itinerant magnetism
1. Introduction
In recent years, the ferromagnetic shape memory alloy (FSMA) was investigated as a candidate of the functional materials widely. Among FSMA, Ni2MnGa is the most famous alloy [1]. The alloy has a cubic
New alloys in the FMSAs of NiMnIn-, NiMnSn-, and NiMnSb-type Heusler alloys have been studied [7, 8]. In these alloys, a metamagnetic transition from paramagnetic martensite phase to ferromagnetic austenite phase occurred, and reverse martensitic transition, which was induced by magnetic fields, occurred under high magnetic fields [9, 10]. These alloys are hopeful as a metamagnetic shape memory alloys with a magnetic field-induced shape memory effect (MSIF) and as magnetocaloric materials which can be cooled down or heated up on applying external magnetic fields. It is noticeable that 3% MFIS has been observed for Ni45Co5Mn36.7In13.3 single crystal in compressive stress–strain measurements [11].
The Co-doped NiCoMnGa-type alloys turned the magnetic order of the parent phase from antiferromagnetic or paramagnetic phase, due to a large magnetization change across the transformation. As a result, it strengthens magnetic field driving force dramatically [12-24]. As for Ni50-
Albertini et al. performed the experimental studies regarding the composition dependence of the structural and magnetic properties of the Ni-Mn-Ga ferromagnetic shape memory alloys substituting Co for Ni atoms around the composition of Ni50Mn30Ga20 [12, 20]. The magnetic and structural properties indicated remarkable discontinuities around the martensitic transition. A metamagnetic transition appeared in the magnetic field around 400 K. The field dependence of the reverse martensitic transition temperature
In our former article [25], we determined the magnetic field dependence of the magnetization of Ni41Co9Mn31.5Ga18.5 around the Curie temperature in the martensite phase in order to investigate the properties of the itinerant electron magnetism according to Takahashi’s spin fluctuation theory of itinerant electron magnetism [26, 27]. The
Takahashi suggested that the anomalous behavior for the magnetostriction can be observed under the influence of the itinerant spin fluctuations around the critical temperature [27]. It is mentioned that the reason is that the magnetostriction is given by the volume derivative of the free energy. By Eq. (6.101) of [27], the magnetostriction is proportional to the fourth power of the magnetization,
In this chapter, we preformed MFIS measurements by means of a 10-T helium-free superconducting magnet and a 1.7-T water-cooled electric magnet. We compared the results of the strain and calorimetric differential scanning colorimetry (DSC) measurements and discussed the irreversibility of the MFIS and the reverse martensitic and metamagnetic transition. We investigated the correlations between magneto-structural variance and the magneto-caloric property observed by the caloric measurements in applied magnetic fields. It is interesting with the investigation of time response of the MFIS for the purpose of industrial use [30]. The time response of the MFIS performed by means of a 1.6-T water-cooled electric magnet and under atmospheric pressure,
2. Sample properties and experimental details
The crystal structure of Ni41Co9Mn31.5Ga18.5 is tetragonal
Ni | Co | Mn | Ga |
---|---|---|---|
40.8 | 9.0 | 31.5 | 18.7 |
MFIS measurements were performed with bulk samples with the size of 0.8 × 3.0 × 4.0 mm3. Strain gauges were used (KFH-02-120-C1–16, size: sensor grid 0.2 mm length × 1.0 mm width, film base 2.5 mm length × 2.2 mm width, Kyowa Dengyo Co., Ltd., Yamagata, Japan). Strain gauge was fixed parallel to the long distance direction (4.0 mm) of the sample.
External magnetic field was applied parallel to the long distance direction of the sample, and elongation of the sample was measured in applied magnetic fields and in atmospheric pressure. Measurements were performed by means of a 10-T helium-free magnet (10 T-CSM) at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. We also performed MFIS measurements by means of a 1.7-T water-cooled electric magnet at Ryukoku University in order to investigate time response of MFIS. The magnetization measurements were performed by using a pulsed-field magnet with the time constant of 6.3 ms. The absolute value was calibrated against a sample of pure Ni.
3. Results and discussion
3.1. Relation between the magnetic field-induced strain and the magnetic entropy of Ni41Co9Mn31.5Ga18.5
In this section, we compared the results of the strain and calorimetric DSC measurements of Ni41Co9Mn31.5Ga18.5. We considered the correlation between the magnetic field-induced strain and the magnetic entropy.
Figure 1 shows the MFIS under steady field by means of the helium-free superconducting magnet. The MFIS measurements in this study were performed under atmospheric pressure and without the compression to make a pre-strain. The point zero of MFIS at each temperature is moved by 1 × 10−4 below 315 K and by 5 × 10−4 above 330 K. The thermal condition was the same as that for the magnetization measurement [23]. When increasing the magnetic field, distinct MFIS was observed. The maximum MFIS was 0.12%, which was approximately the same value as that of the thermal strain for the reverse martensitic transition. The shape of MFIS is similar to that of polycrystalline Ni41Co9Mn32Ga16In2, where the alloy is also a re-entrant metamagnetic Heusler alloy, and 0.30% MFIS was observed [12]. The field dependence of the reverse martensitic transition temperature,
The MFIS of 2, 4, 6, and 8 T is shown in Figure 2. Between 340 and 370 K, large MFIS was observed. Metamagnetic S-shape like
Alloys | Reference | |||
---|---|---|---|---|
Ni41Co9Mn31.5Ga18.5 | 1.1 | 1.2 | This work | |
Ni41Co9Mn32Ga16In2 | 3.5 | 3.0 | [12] |
In the former article, we studied the magneto-caloric properties of Ni41Co9Mn31.5Ga18.5 polycrystalline sample by means of the differential scanning calorimetry (DSC) measurements [25]. Magneto-calorimetric measurements and magnetization measurements of Ni41Co9Mn31.5Ga18.5 polycrystalline ferromagnetic shape memory alloy (FSMA) were performed across the
Now, we compare the results of the strain and calorimetric DSC measurements. Figure 3 shows the temperature dependence of the MFIS and entropy change. The entropy change
3.2. Forced magnetostriction around the critical temperatures
In this section, we offer a topic of forced magnetostriction around the Curie temperature or magneto-structural transition temperature. The spin fluctuation theory of itinerant electron magnetism suggests that the critical index
In this study, we measured the magnetostriction of Ni2MnGa at the Curie temperature in order to investigate the magnetization dependence of the forced magnetostriction. Figure 5 presents the magnetostriction
We studied the magnetostriction at the Curie temperature in the martensite phase,
Further, we investigated the MFIS around the reverse martensitic transition temperature. Figures 8 and 9 show the magnetic field dependences of the magnetization and the MFIS at 330 and 340 K, respectively. These temperatures are around the reverse martensitic transition start temperature. The gradient of the magnetization and magnetostriction has tendencies toward increasing with the magnetic field increases for each temperature. However, the degree of the rate of increase of the magnetostriction is larger than that of the magnetization. From these figures, a correlation between the magnetostriction and magnetization could not be identified. As mentioned in Section 1, Ni41Co9Mn31.5Ga18.5 is an itinerant ferromagnet. The magnetostriction is proportional to
Figure 10 shows the plot of MFIS against
3.3. The time response of the magnetic field-induced strain of Ni41Co9Mn31.5Ga18.5
In order to investigate time response of the MFIS, fast speed sweeping of the magnetic fields was performed at 354 K, as shown in Figure 12. The applied magnetic field increased from the zero magnetic field to 1.66 T in 8.0 s and under atmospheric pressure. Figure 13 shows the MFIS, in which the applied magnetic field increased from the zero magnetic field to 1.66 T in 60 s. As for an 8.0-s mode, 2.2 × 10−4 MFIS was observed, which was 80% of the MFIS in a 60-s mode. This indicates that a high-speed transition has occurred on applying magnetic fields.
The MFIS effect occurs at the temperature between room temperature and 370 K; therefore, it is useful for magnetic sensors, or actuators in the high temperature region,
4. Conclusions
In order to investigate the magnetic functionality of polycrystalline metamagnetic Heus-ler alloy Ni41Co9Mn31.5Ga18.5, magnetic field-induced strain (MFIS) measurements were performed. Strain gauge was fixed parallel to the long distance direction (4.0 mm) of the sample. The external magnetic field was applied parallel to the long distance direction of the sample, and the elongation of the sample was measured. A 0.12% MFIS was observed at 340 K and 10 T. Strict MFISs between 300 and 370 K were observed. These magneto-structural variances acted in concert with the metamagnetic property observed by the magnetization measurements and magneto-caloric property observed by the caloric measurements in the applied magnetic fields. The MFISs were proportional to the fourth power of the magnetization, and this result is in agreement with Takahashi’s spin fluctuation theory of itinerant electron magnetism. The investigation of time response of the MFIS was performed by means of a sweep water-cooled electric magnet, and zero magnetic field to 1.66 T in 8.0 s at 354 K. 2.2
Acknowledgments
The authors thank Mr. M. Okamoto for helping prepare equipment for MFIS measuring system. This experimental study was partly performed at High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University.
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