Intermartensitic Transformation and Enhanced Exchange Bias in Pd (Pt) -doped Ni-Mn-Sn alloys

In this work, we studied the phase transitions and exchange bias of Ni50−xMn36Sn14Tx (T = Pd, Pt; x = 0, 1, 2, 3) alloys. An intermartensitic transition (IMT), not observed in Ni50Mn36Sn14 alloy, was induced by the proper application of negative chemical pressure by Pd(Pt) doping in Ni50−xMn36Sn14Tx (T = Pd, Pt) alloys. IMT weakened and was suppressed with the increase of applied field; it also disappeared with further increase of Pd(Pt) content (x = 3 for Pd and x = 2 for Pt). Another striking result is that exchange bias effect, ascribed to the percolating ferromagnetic domains coexisting with spin glass phase, is notably enhanced by nonmagnetic Pd(Pt) addition. The increase of unidirectional anisotropy by the addition of Pd(Pt) impurities with strong spin-orbit coupling was explained by Dzyaloshinsky-Moriya interactions in spin glass phase.


Results and Discussions
show the XRD patterns of Ni 50−x Mn 36 Sn 14 T x (x = 0, 1, 2, 3) alloys at room temperature with T = Pd and Pt, respectively. All samples are of the pure austenitic phase with a cubic Heusler L2 1 -type structure at room temperature, indicating the MT temperature is below room temperature. In the inset of Fig. 1(a), it can be clearly seen that the (220) peaks shift towards low angles with the increasing substitutions of Ni by Pd in Ni 50 Mn 36   Ni 50−x Mn 36 Sn 14 T x (T = Pd, Pt) alloys, which will be discussed in detail later. It can be seen that the value of a increases gradually with the increase of Pd(Pt) content.
It is known that in Ni-Mn-X (X = In, Sn, Sb) alloys the MT temperatures increase with the increase of e/a, which provides a convenient way to modulate the transition temperature. In the case of Ni 50−x Mn 36 Sn 14 T x alloys, since Ni, Pd, and Pt are located within the same main group, the influence of e/a can be eliminated. Figure 2 shows the temperature dependence of magnetization (M-T) for Ni 50−x Mn 36 Sn 14 T x (T = Pd, Pt) alloys. All these data were recorded upon zero field cooling (ZFC), field Cooling (FC), and field warming (FW) with an applied field of 100 Oe in the temperature range between 10 K and 340 K. Normally, the phase transitions in Ni-Mn-X alloys are characterized by the Curie temperature of austenite (T C A ), the martensitic transformation starting temperature (M s ), and the Curie temperature of martensite (T C M ). As can be seen, all curves show typical behavior with  in Ni-Mn-In alloy 12 . Recently, an increase of MT temperature was observed in Ni 2 MnGa alloy by the substitution of Pt for Ni, which has been attributed to enhanced antiferromagnetic correlations with the increase of Pt content 28 . Similar enhancement of antiferromagnetic correlations, could be expected by Pd(Pt) substitution in Ni 50−x Mn 36 Sn 14 T x (T = Pd, Pt) alloys, which will be further discussed in the composition dependence of magnetization at low temperature. Therefore, the competition of two factors may result in the nonmonotonous evolution of MT temperature with Pd(Pt) doping.
A prominent feature in Fig. 2 is the appearance of IMT below M s for Ni 50−x Mn 36 Sn 14 Pd x (x = 1, 2) and Ni 49 Mn 36 Sn 14 Pt alloys. As can be seen in Fig. 2(b,c,e), different from M-T curves of Ni 50 Mn 36 Sn 14 alloy [ Fig. 2(a)], M-T curves of these alloys show a two-step behavior around the MT temperature. This peculiar behavior in M-T may suggest the existence of an intermartensitic phase at temperature T I where T I < M s , as proposed in Ni-Mn-Ga alloys 24 . Nevertheless, one may suspect that inhomogeneous phases may produce a two-step process in M-T curves considering the sensitivity of transformation temperature to composition.
To further investigate the two-step behavior in response to magnetic field, we have looked into the M-T curves and AC susceptibility under different magnetic field. It was found that the two-step process is highly sensitive to the magnitude of field. To demonstrate the field dependence of M-T curves more clearly, we plot the normalized magnetization versus temperature at the field of 100, 200, 500 and 1000 Oe [ Fig. 3(a)] on heating for Ni 49 Mn 36 Sn 14 Pd alloy. Obviously, with the increase of applied magnetic field, the low-temperature step of transition weakened with decreased T I , and was suppressed in the field of 1 kOe. Similar behavior was also observed in Ni 48 Mn 36 Sn 14 Pd 2 and Ni 49 Mn 36 Sn 14 Pt alloys (not shown here). Figure 3(b,c) show the real and imaginary part of ac susceptibility at different magnetic fields. Similar two-step behavior can also be observed in χ ′ (T) curves, and is even more distinct in χ ″ (T) curves. A gradual suppression of low temperature transition by magnetic field was confirmed. Since this suppression behavior should not take place in the case of transition associated with inhomogeneous phase, IMT should account for the two-step transition at low field in Ni 50−x Mn 36 Sn 14 Pd x (x = 1, 2) and Ni 49 Mn 36 Sn 14 Pt alloys. Now let us discuss the physical mechanism for the sensitivity of IMT, i.e. the appearance and diminishment of IMT in response to the change of composition and magnetic field. In the investigation of Ni 2 MnGa single crystal, it has been shown, that the tension along the < 100> direction of the ordered (L2 1 ) parent phase could induce the IMT 29 . Ma et al. observed an IMT in high pressure annealed Ni-Co-Mn-Sn alloy, which was attributed to the enhanced magnetoelastic coupling by the application of pressure 19 . Recently, IMT was observed in Ni-Cu-Mn-Sn alloys, and it was proposed that replacing Ni for Cu generates the internal stress in the alloys, which is responsible for instability in the structure of the martensitic phase 30 . All these results indicate that the stability of martensitic phases with different structure is sensitive to the pressure (external or internal, positive or negative). Looking back to Fig. 1(c), it can be seen that IMT appears with lattice constant in a small range between 5.997 and 6.002 Å for Ni 50−x Mn 36 Sn 14 T x (T = Pd, x = 1,2; T = Pt, x = 1) alloys. Therefore, in the case of Pd(Pt) doped alloys, the small substitution of Pd(Pt) for Ni should induce proper internal tension in the crystal lattice, which makes the intermartensitic phase more stable in the corresponding temperature range. However, further increasing Pd(Pt) content makes the crystal lattice expand and suppress the intermartensitic phase, suggesting that IMT is sensitive to internal tension. The sensitivity of IMT to pressure is also demonstrated by its suppression upon the application of magnetic field in Ni 50−x Mn 36 Sn 14 T x (T = Pd, x = 1,2; T = Pt, x = 1) alloys. This phenomenon can understood by the fact that the application of magnetic field helps align the magnetic moments of the martensitic variants, which may produce internal stress in the martensitic phase and compensate the tension effect generated by Pd(Pt) doping. A similar field dependence of IMT was reported in Ni-Cu-Mn-Sn 30 and Ni-Mn-In-Sb alloys 31 , where IMT vanished at a higher magnetic field.
At low temperature region of martensitic phase, all samples show spin-glass-like behavior characterized by the bifurcation between the FC and ZFC M(T) curves, as shown in Fig. 2. At higher temperature, however, ferromagnetic or ferrimagnetic behavior is present with Curie temperature T C M above the MT temperature, which indicates that the nature of ground state is so-called "reentrant" spin glass 32 . To further study the effect of Pd(Pt) doping on the magnetic ground state, we measured the magnetic hysteresis (M-H) loops at low temperature after field cooling (FC) in a field of 1 T from 300 K. Figure 4(a,b) show the FC M-H loops of Ni 50−x Mn 36 Sn 14 T x (x = 0, 1, 2, 3) alloys at 2 K for T = Pd and Pt, respectively. All samples exhibit the shift of M-H loops to the negative field direction, i.e. exchange bias (EB) effect, which has been observed in Ni-Mn-X (X = In, Sn, Sb) alloys and can be ascribed to the coexistence and competition of FM and AFM interaction at low temperature 25,26,33 . Recently, we proposed, due to the spatial composition fluctuation and competing FM/AFM interactions, a ground state with non-percolated FM domains in SG matrix in Ni 2 Mn 1.4 Ga 0.6 alloy, which accounts for the appearance of zero-field exchange bias effect (ZEB) 27 . As for the case of zero field cooling process in Ni 50−x Mn 36 Sn 14 T x (x = 0, 1, 2, 3), however, M-H loops (not shown here) show no shift along the field axis, that is, no zero-field exchange bias effect was observed in the Ni 50−x Mn 36 Sn 14 T x (T = Pd, Pt) alloys. Combined with the relative large value of magnetization at low temperature, we suggest that the possible ground state can be percolated FM region coexisting with SG phase, and this can result in the formation of unidirectional exchange anisotropy at the interface between FM and SG phases upon FC process.  Fig. 4(a,b) 35 . Nevertheless, as the change of magnetization is relatively small, we believe this factor should work but is not the dominant reason for the increase of H E . The other reason may be associated with the stronger spin-orbital coupling of Pd(Pt) atom than that of Ni, which gives rise to magnetic anisotropy and consequently increases the value of H E . It has been reported in canonical CuMn SG system, the introduction of nonmagnetic Au(Pt) impurities with strong spin-orbit coupling can largely enhance the magnetic anisotropy 36 . This has been attributed to an additional term in the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction which is of the Dzyaloshinsky-Moriya (DM) type and is due to spin-orbit scattering of the conduction electrons by the nonmagnetic impurities 37,38 . Similarly, in Ni 50−x Mn 36 Sn 14 T x (T = Pd, Pt) alloys, Pd(Pt) doping may also increase the unidirectional anisotropy of SG phase due to DM interaction between the Mn spins, and subsequently lead to the increase of H E . This could also explain why the addition of Pt increases H E more sharply than that of Pd by the fact that the strength of spin-orbital coupling follows Pt > Pd > Ni. Recently, Nayak et al. obtained a giant EB of more than 3 T in the vicinity of the compensation composition in Mn-Pt-Ga system 39 . The large exchange anisotropy has been attributed to the exchange interaction between the compensated host and ferrimagnetic clusters due to intrinsic anti-site disorder. We believe that the effect of strong spin orbital coupling, although not discussed by the authors, should play an important role in the giant EB of Mn-Pt-Ga alloy, considering that the value of H E in Mn-Pt-Ga is much larger that in Mn-Fe-Ga. These results suggest that introducing the elements with strong spin-orbit coupling may provide a general way to enhance the EB effect in Heusler alloys.   Figure 5 shows the temperature dependence of H E and H C for Ni 49 Mn 36 Sn 14 T (T = Ni, Pd, Pt) alloys after FC (H FC = 10 kOe) from 300 K. It can be seen that all alloys show similar temperature dependence of H E and H C : the values of H E decrease almost linearly with increasing temperature and become zero around the blocking temperature (T B = 70 K), where the values of H C reach the maximum value. The similar phenomenon was also found in Co(FM)/CuMn(SG) bilayer as well as convention FM/AFM systems due to the decrease of SG (or AFM) anisotropy close to T B 34,40 . In Ni 49 Mn 36 Sn 14 T (T = Ni, Pd, Pt) alloys, the magnetic anisotropy of SG phase (K SG ) decreases with the increasing temperature, which makes FM phase can drag more SG spins, causing the increase in H C ; until at T B , SG spins can no longer hinder the FM rotation and consequently H E becomes zero.
In summary, we have investigated the effects of Pd(Pt) substitution for Ni on the crystal structure, phase transitions and EB effect in Ni 50−x Mn 36 Sn 14 T x (T = Ni, Pd, Pt) Heusler alloys. With the increase of Pd(Pt) content, the lattice parameter increases gradually, while the MT temperature shows nonmonotonous composition dependence. The appearance of IMT was observed by small Pd(Pt) addition in Ni 50−x Mn 36 Sn 14 T x with x = 1, 2 for T = Pd and x = 1 for T = Pt, and it can be suppressed by the application of magnetic field as well as further Pd(Pt) doping. These results indicate that IMT in Ni 50−x Mn 36 Sn 14 T x alloys is highly sensitive to pressure, such as chemical pressure by doping and internal stress by magnetic field. All samples exhibit a "reentrant" spin glass behavior at low temperature, and a significant enhancement of EB effect after FC treatments was obtained by Pd(Pt) doping. EB effect has been explained in terms of coexistence of percolated FM region and SG phase. The decreased FM proportion and Dzyaloshinsky-Moriya interactions in the SG phase may account for the increase of H E . The latter mechanism plays an important role and provides an effective way to improve the EB effect in Heusler alloys.

Methods
Ni 50−x Mn 36 Sn 14 T x (T = Pd, Pt; x = 0, 1, 2, 3) polycrystalline alloys were prepared by arc melting the appropriate amounts of Ni, Mn, Sn, Pd, Pt in argon atmosphere. These alloys were sealed in quartz tubes and annealed at 1173 K for 72 h followed by quenching in water. The crystal structures were identified by the X-ray diffraction (XRD) using Cu-Kα radiation at room temperature. Magnetic measurements were carried out using a physical property measurement system (PPMS, Quantum Design Evercool-2).