In this chapter the exchange bias (EB) properties of bulk Mn-rich Ni50-xMn37+xSn13 (0 ≤ x ≤ 4) Heusler alloys has been discussed by changing the Ni-Mn concentration. In these alloys the exchange bias field increases with the excess Mn concentration, but exchange bias blocking temperature (TEB) decreases from 149 to 9 K. The hysteresis loop for Ni46Mn41Sn13 alloy shows a maximum shift of 377 Oe. The exchange bias property is strongly influenced by varying Ni-Mn concentration in Ni-Mn-Sn alloys then the variation of Mn/Sn. We have observed in these alloys that the TEB would show a decreasing value either by changing the Ni or Sn concentration while the Mn content is above 37% in Ni-Mn-Sn alloys.
- exchange bias
- blocking bias temperature
- coercive field
- exchange bias field
From the discovery of exchange bias (EB) in CoO particle , more work has been done on this area both experimentally and theoretically due to its potential applications in various fields such as spintronic devices, permanent magnets, magnetic recording, read head and giant magnetoresistive sensors, etc. [2, 3, 4, 5]. EB arises in the presence of applied magnetic field after cooling the materials and it is connected with the exchange anisotropy formed at the interface between an antiferromagnetic (AFM) and ferromagnetic (FM) materials. The whole phenomenon at low temperature shifts the hysteresis loops along the field axis. This similar kind of phenomenon is observed in multilayer films, small oxide particles, nanostructures and inhomogeneous materials [6, 7, 8, 9]. In addition to this, the EB phenomenon is also observed in materials which contains spin glass phase . Recently, Ni-Mn-X (X = Ga, Sb, In, Sn) Heusler-based alloy systems achieved great attention due to their immense applications in magnetic refrigeration, magnetic actuated devices and spintronic devices [11, 12]. The different composition of Ni-Mn-Sn alloy shows a wide physical properties such as magnetic field-induced transition, inverse magnetocaloric effect (IMCE), giant magnetoresistance, giant Hall effect, giant magnetothermal conductivity, magnetic superelasticity effects, exchange bias and shape memory effect [13, 14, 15, 16, 17].
The recent observation of EB in the Ni-Mn-based alloys shows an intense interest in the further study of magnetic properties. Due to the different occupations of Mn atoms in the Sn sites as well in the Ni sites, the Ni-Mn-Sn alloy will have excess content of Mn atom. Hence the EB property is very sensitive to the excess Mn. The Ni2MnSn Heusler alloy crystallizes in L21 structure, in which the Ni atoms occupy in the (1/2, 1/2, 1/2) and (0, 0, 0) sites, Mn atoms occupy in the (1/4, 1/4, 1/4) site and Sn atoms occupy in the (3/4, 3/4, 3/4) site . In the Mn-rich alloys, the excess Mn occupying Ni and Sn sites couples antiferromagnetically to surrounding Mn atoms on the regular Mn sites . Also the decrease of Mn-Mn distance may lead to AFM exchange between each other in the martensite phase at low temperature. The EB behaviour has been studied in Ni-Mn-X (X = Sb, Sn, In) alloys by several authors [20, 21, 22, 23]; particularly in Ni-Mn-Sn alloy, the EB behaviour has been investigated either by varying the Ni/Sn or Mn/Sn concentration [24, 25]. The structural effects, magnetic property and magnetic entropy change have been studied by varying Ni-Mn concentration in the Ni50-xMn37+xSn13 (0 ≤ x ≤ 4) Heusler alloy system . In the Ni50-xMn37+xSn13 alloy system, the cubic austenite phase was stabilized by the excess Mn content at room temperature. The martensitic transition temperature decreases from 305 to 100 K by increasing the Mn concentration. The exchange bias blocking temperature (TEB) was found to decrease drastically from 149 to 9 K with increasing Mn concentration. In this work, we have taken up a detailed study on the effect of varying Ni-Mn concentration on EB properties in the bulk Ni50-xMn37+xSn13 alloys. This chapter explains the EB behaviour by varying Ni-Mn concentration in Ni-Mn-Sn alloys.
2. Experimental details
The compositions of Ni50-xMn37+xSn13 (x = 0, 1, 2, 3, 4) alloys were prepared by arc melting technique under argon atmosphere. To ensure the homogeneity, the samples are re-melted four times. These alloys were annealed under high vacuum at 1175 K for 6 h and then quenched with Ar gas. The magnetic data were taken suing the physical property measurement system (PPMS-9 T)—vibrating sample magnetometer (VSM) module (Quantum Design, USA). The measurements were taken into two different modes which can be referred as ZFC and FC modes. The sample was firstly cooled in zero magnetic field, and the data was collected by applying a magnetic field of 5 mT during warming in the temperature range of 4–330 K. This refers to zero-field cooled (ZFC) mode. In field cooled (FC) mode, the data was collected without removing the applied field during cooling in the temperature range between 330 and 4 K. Again, the data was recorded upon warming in the range of 4–330 K (referred as field warming (FW)). Magnetization as a function of magnetic field was recorded up to a field of 5 T in the low temperatures.
3. Exchange bias behaviour of Ni50-xMn37+xSn13 (x = 0, 1, 2, 3, 4) alloys
Figure 1 shows the temperature dependence of magnetization for Ni50-xMn37+xSn13 (x = 0,1,2,3,4) alloys in an applied magnetic field of 5 mT during zero-field cooling, field cooling and field warming conditions. The curve shows several transitions with thermal hysteresis which has been observed between FC and FW. This thermal hysteresis is the indication of first-order structural transition from austenite to martensite phase. The ferromagnetic transition of austenite phase () occurs at 309 K. The decrease of magnetization below martensite start temperature (Ms) in the FC curve indicates the fractional decrease of austenite phase. The ZFC and FC curves split into two at low temperatures and show a step kind of behaviour in ZFC curve. This specifies that the sample is inhomogeneous magnetically. The transition observed at 120 K is referred as the exchange bias blocking temperature (TEB). The observed magnetic inhomogeneity and TEB in the sample can be attributed to the coexistence of FM and AFM interactions. This kind of antiferromagnetic interaction occurs from the antiferromagnetic coupling between the Mn atoms in the Ni/Sn sites and Mn atoms in the Mn sites. The similar exchange bias behaviour is observed in further Ni-Mn-X (X = Sb, Sn, In) alloys [20, 21, 22, 23]. The presence of AFM interaction in this alloy system was verified by neutron diffraction studies .
The ZFC and FC hysteresis loops were measured at 5 K to confirm the exchange bias in Ni50-xMn37+xSn13 (x = 0, 1, 2, 3, 4) alloys. The ZFC and FC loops were measured from −20 kOe to 20 kOe. To measure the FC loop, the sample was cooled in a field of 50 kOe and recorded the loop from −20 kOe to 20 kOe. Figure 2 shows the ZFC and FC hysteresis loops for Ni50-xMn37+xSn13 (x = 0, 1, 2, 3, 4) alloys. The curves from −2 kOe to 2 kOe are shown in the inset in Figure 3 to see the shift clearly. For all the samples, it is found that the ZFC curve does not exhibit any shift, but the FC curve shifts to the negative field from the origin. This specifies the coexistence of AFM-FM interactions in the sample below room temperatures. The ZFC loop shows a double-shifted loop and is symmetric around zero point, which indicates the existence of FM-AFM coupling. The emergence of double-shifted loop indicates that the different regions of AFM magnetic structure couple to the FM in opposite directions. The Ni-Mn-X (X = Sb, Sn, In) alloys also show similar EB behaviour [20, 21, 22, 23].
The temperature dependence of EB was investigated in the temperature range of 5–140 K for Ni50-xMn37+xSn13 (x = 0, 1, 2, 3, 4) alloys. The temperature interval has been taken as 20 K and the typical FC curves for Ni50-xMn37+xSn13 (x = 1) alloy are shown in Figure 3. It is clear from Figure 3 that at temperatures of 5 K and 20 K, the hysteresis loops significantly shifted to the negative field which point to the existence of EB in the sample. With the increase of temperature, the hysteresis loop decreases, and finally at 120 K the field shift almost disappears. The temperature where the loop is symmetric can be defined as the TEB for this sample. Moreover, the temperature 120 K nearly coincides with the TEB observed from the thermomagnetic data as shown in Figure 2.
Figure 4 shows the values of EB field (HE) and coercivity (HC) evaluated from the hysteresis loops at various temperatures for a typical Ni50-xMn37+xSn13 (x = 1) alloy. The values of EB field and coercivity field are calculated using HE = −(H1 + H2)/2 and HC = |H1 − H2|/2, respectively, where H1 and H2 denote the negative and positive field at which the magnetization equals zero. It is observed from Figure 4 that with increasing temperature the value of HE decreases linearly and vanishes around TEB. This validates the EB phenomenon to be real in the temperatures below TEB. This is because with the increasing temperature, the FM-AFM coupling gets weakened. The disappearance of HE is due to the domination of FM interaction over the AFM interactions. Conversely, HC value increases in the beginning with temperature and decreases after reaching a maximum value. Due to the pulling of AFM spins by FM, the coercivity below TEB increases. This arises due to the fact that anisotropy of AFM decreases with increasing temperature.
The TEB values derived from the thermomagnetic curves (not shown for all samples) are plotted with Mn concentration for the alloys in Figure 5. From the figure it is seen that with the increase of Mn concentration from 37 to 41% in the Ni50-xMn37+xSn13 alloy series, the TEB decreases drastically from 149 to 9 K. The large fraction of FM phase at low temperature and the weakening of the AFM-FM interaction strength were verified by the experimental results which is shown in Figure 5, where the increase of Mn concentration increases the saturation magnetization (σs) at 5 K marginally. This indicates that the reduction in the FM-AFM interactions occurs due to the increase in Mn content. It can be understood that the weakening of the AFM interactions and the competing AFM and FM interactions led to the decrease of TEB values. Earlier, M. Khan et al. reported the EB properties with varying Mn/Sn concentration in Ni50Mn50-xSnx (11 ≤ x ≤ 17) alloys. In his work he reported that the increase of Mn concentration above 37% decreases the TEB value . Same in Ni50-xMn37+xSn13 alloy series, the TEB value was found to decrease with the increase of Mn content since the Mn content is more than 37% in Ni50-xMn37+xSn13 alloy series. These results indicate that the TEB value would decrease either by varying the Sn or Ni content if the Mn content is above 37% in Ni-Mn-Sn alloys.
The variation of HE with the increase of Mn concentration is shown in Figure 6 for Ni50-xMn37+xSn13 alloy series at 5 K (calculated from Figure 3). It is noticed that with the increases of Mn concentration from 37 to 41%, the value of HE increases linearly from 200 to 377 Oe. M. Khan et al. also observed the similar behaviour in Ni-Mn-Sn alloys and found that the HE increases from ~20 to ~183 Oe with an increase of Mn from 34 to 39% by varying the Mn/Sn concentration . These two results suggest that Ni-Mn variation greatly influences the HE value in contrast to Mn/Sn variation. Recently, Xuan et al. reported EB by varying Ni/Sn concentration in Ni-Mn-Sn [keeping Mn concentration constant (50%)]. He observed that the increase of Sn content decreases the HE field . It indicates that the Mn occupying Sn and Ni sites plays a major role in modifying the magnetic interactions and their strength. It is reported that HE depends on the interface coupling constant and saturation magnetization of the FM . In the present Ni50-xMn37+xSn13 alloy series, the different magnetic moments and interaction strength arises due to the occupation of excess Mn in the Ni sites and Mn atoms occupies Sn and Ni. Hence the increase of HE with increasing Mn could be due to the increase of interaction coupling strength of different moments in Ni50-xMn37+xSn13 alloys. In order to understand the magnetic moment of different sites and their magnetic interactions in the sample, detailed neutron diffraction studies are essential.
In this chapter, we have discussed the exchange bias behaviour in Ni50-xMn37+xSn13 (x = 0, 1, 2, 3, 4) alloys by changing the Ni-Mn concentration. The coexistence of AFM and FM exchange interactions is the reason for the EB phenomenon observed in this alloy series. The temperature strongly influences the HE and HC. The TEB was found to be same for the Mn content more than 37%, even either varying Ni-Mn or Mn/Sn. With the increase of Mn concentration from 37 to 41%, the value of HE increases linearly from 200 to 377 Oe in Ni50-xMn37+xSn13 alloy series.
The authors would like to thank Dr. M. Manivelraja and Dr. N.V. Ramarao, DMRL, Hyderabad, India, for providing the facility for material synthesis.