Magnetoresistance in ferromagnetic shape memory alloy NiMnFeGa

Journal of Magnetism and Magnetic Materials 323 (2011) 2192–2195

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Magnetoresistance in ferromagnetic shape memory alloy NiMnFeGa Z.H. Liu a, X.Q. Ma a, Z.Y. Zhu b, H.Z. Luo b, G.D. Liu b, J.L. Chen b, G.H. Wu b,n, Xiaokai Zhang c, John Q. Xiao c a b c

Department of Physics, University of Science and Technology Beijing, Beijing 100083, China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 April 2010 Received in revised form 26 December 2010 Available online 1 April 2011

The magnetoresistance (MR){ ¼[R(H) R(0)]/R(0)} properties in ferromagnetic shape memory alloy of NiMnFeGa ribbons and single crystals, and NiFeGa ribbons have been investigated. It is found that the NiMnFeGa melt-spun ribbon exhibited GMR effect, arising from the spin-dependent scattering from magnetic inhomogeneities consisting of antiferromagnetically coupled Mn atoms in B2 structure. In the absence of these magnetic inhomogeneities, Heusler alloys seem to show a common linear MR behavior at around 0.8TC, regardless of sample structures. This may be explained by the s–d model. At low temperatures, conventional AMR behaviors due to the spin–orbital coupling are observed. This is most likely due to the diminished MR from s–d model because of much less spin fluctuation, and is not associated with martensite phase. MR anomaly at intermediate field (r? 4 r99) is also observed in single crystal samples, which may be related to unique features of Heusler alloys. & 2011 Elsevier B.V. All rights reserved.

Keywords: Magnetoresistance Heusler alloys Ferromagnetic shape memory alloys

1. Introduction Ferromagnetic materials of high spin polarization have been attracting great interest due to their potential applications in spintronic devices [1]. Among them, Heusler alloys are particularly interesting since their electronic structures show 100% spin polarization [2]. In addition, certain Heusler alloys, such as Ni2MnGa and NiMnFeGa, show large reversible strains in the presence of magnetic field [3–5]. The potential new functionality combining both magnetoresistance and magnetic shape memory behaviors motivated us to investigate the magnetotransport properties of Ni2MnGa and NiMnFeGa Heusler alloys. Previous studies have shown negative MR in several Heusler alloys. For example, NiMnGa film deposited by pulsed laser deposition (PLD) on Al2O3 and Si substrates showed negative MR of about 3–5% over a wide temperature range [6]. It was proposed that the main contribution to MR is due to the transport between the areas with different orientation of magnetic moments at low magnetic fields, while at high fields it is an electron scattering of spin-disordered areas. In another example, nonstoichiometric polycrystalline Cu–Al–Mn shape memory Heusler alloy also show a large negative MR of about 7% at 5 K and 1% at 300 K in a magnetic field of 5 T. The MR ratio decreases after thermal annealing [7]. It was speculated that the MR originates from the spin-dependent scattering of conduction electrons from ferromagnetic clusters embedded in a nonmagnetic matrix. Negative MR of about 1–3% at room temperature in

n

Corresponding author. E-mail address: [email protected] (G.H. Wu).

0304-8853/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2011.03.029

80 kOe field has also been observed in NiMnSb and PtMnSb [8], and is attributed to the inelastic s–d scattering. Our group has also reported large negative MR of 9% in Fe-doped NiMnGa meltspun ribbons [9]. Recently, a negative MR of about 5% in 80 kOe field has been discovered in bulk Ni2 þ xMn1 xGa polycrystals at room temperature [10]. These results show complexity of the magnetotransport properties in Heusler alloys and the exact mechanism remains elusive. In this paper, the origin of a large MR in NiMnFeGa ribbon sample is discussed by comparing the MR and magnetic behaviors with those of NiMnFeGa single crystal and Ni2FeGa ribbon samples.

2. Experimental Nominal Ni50Mn19Fe6Ga25 and Ni2FeGa ribbons were prepared by melt-spun method with the same spinning linear velocity of 25 m/s. The detailed fabrication procedure is outlined elsewhere [11]. The single crystal Ni50Mn19Fe6Ga25 is grown by Czochralski method in Crystalox MCGS-3 cold crucible system. A 20-mm/h growth rate and a 30-rpm rotation rate were used for the growth process. The resistance and magnetization are measured along the ribbon direction and [1 0 0] direction for single crystal samples using a superconducting quantum interference device (SQUID) magnetometer (Quantum Design MPMS-5 S).

3. Results and discussions Fig. 1 shows the temperature dependence of resistance for Ni50Mn19Fe6Ga25 ribbon, Ni50Mn19Fe6Ga25 single crystal and

Z.H. Liu et al. / Journal of Magnetism and Magnetic Materials 323 (2011) 2192–2195

Fig. 1. Temperature dependence of resistance for Ni50Mn19Fe6Ga25 as-spun ribbon (a) and single crystal (b), and Ni2FeGa as-spun ribbon (c).

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crystal samples, with the same nominal chemical composition as the ribbon samples and synthesized by different method, the MR reaches only 0.8% in the field of 50 kOe, which is much smaller than that of the ribbon sample. The MR curve shows two regimes, corresponding to an initially rapid decrease followed by a slowly proportional decrease as the field increases from 0 to 50 kOe. For Ni2FeGa ribbon, prepared under the same conditions as NiMnFeGa ribbons, a very small MR of only 1% is observed at 5 K, so the effect of the quenched-in internal stress on the large MR in NiMnFeGa ribbons can be excluded. Clearly, MR behaviors for Ni50Mn19Fe6Ga25 ribbon are very different from those of Ni50Mn19Fe6Ga25 single crystal and Ni2FeGa ribbon samples, and indicate spin-dependent scattering mechanism that is responsible for giant magnetoresistance (GMR). Interestingly, identical room temperature behaviors are observed in Ni50Mn19Fe6Ga25 single crystal and Ni2FeGa ribbon samples, suggesting structural, either chemically or magnetically, independent mechanism in cubic austenite phase. Furthermore in martensite phase at 5 K, single crystal sample shows different style of low and high field MR curves that are absent in Ni2FeGa ribbon samples. To further clarify the MR behaviors, we measured longitudinal (H99I) and transverse (H?I) MR of three samples, as shown in Fig. 3. The isotropic MR behavior in Ni50Mn19Fe6Ga25 ribbon again confirms the spin-dependent scattering mechanism in this sample. Isotropic MR behaviors at 300 K are also observed in Ni50Mn19Fe6Ga25 single crystal and Ni2FeGa ribbon. At 5 K, anisotropic magnetoresistance (AMR) are observed in these two samples, with additional complications for single crystal sample at intermediate field where r? is larger than r99, contrasting to conventional AMR behaviors. Since both GMR and AMR behaviors are related to magnetization, we have measured initial magnetization curves [Fig. 4(a)] and magnetic hysteresis loops at 5 K [Fig. 4(b)] for three samples, respectively. The field is applied along the [1 0 0] direction in single crystals and ribbon direction. All samples are in martensite phases at 5 K, where twinning takes place to reduce the strain. The magnetocrystalline anisotropy constant (K1) is typically large in martensitic phase with easy axis along [0 0 1] direction. The

Fig. 2. MR as a function of magnetic field for Ni50Mn19Fe6Ga25 as-spun ribbon (a) and single crystal (b), and Ni2FeGa as-spun ribbon (c).

Ni2FeGa ribbon samples upon the heating process. With the increase of temperature, three samples undergo a transition from low temperature martensite phase to high temperature austenite phase. The transformation temperatures are 150, 225, and 158 K for Ni50Mn19Fe6Ga25 ribbon and single crystal, Ni2FeGa ribbon samples, respectively. The difference in the transformation temperature between Ni50Mn19Fe6Ga25 ribbon and single crystal sample is attributed to the internal stress and the chemical disorder in the ribbon sample caused by fast-cooling solidification. MR measurements for three samples were performed at 5 and 300 K, corresponding to martensite and austenite phase, respectively. Fig. 2 shows the magnetic field dependence of MR curves for Ni50Mn19Fe6Ga25 ribbon, Ni2FeGa ribbon and Ni50Mn19Fe6Ga25 single crystal samples. The current and the magnetic field are parallel to the longitudinal direction of the ribbons and along the [1 0 0] direction of the single crystal sample. At 5 K, the resistance in Ni50Mn19Fe6Ga25 ribbon sample decreased rapidly with the increase in the magnetic field, showing a large negative MR of 13% under the effect of 50 kOe field. For Ni50Mn19Fe6Ga25 single

Fig. 3. Resistance measured with field parallel and perpendicular to the current direction for Ni50Mn19Fe6Ga25 as-spun ribbon (a) and single crystal (b), and Ni2FeGa as-spun ribbon (c). The current is along the [0 0 1] direction in single crystal case.

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Fig. 5. Magnetization curves at 5 K for Ni50Mn19Fe6Ga25 as-spun ribbon and as-melted ingot samples.

Fig. 4. Magnetization curves measured at 5 K: (a) initial magnetization curves and (b) magnetization hysteresis curves.

single crystal sample shows a rather typical magnetization curve, a linear increase of magnetization below 5 kOe and gradual saturation at about 10 kOe, marking the transition from anomaly to normal AMR at high field. Different from the Ni50Mn19Fe6Ga25 single crystal sample, the magnetization of Ni2FeGa ribbon increases much faster due to the much smaller magnetocrystalline anisotropy and random grain structures. By comparing these two samples, it appears that the intermediate field anomaly at 5 K (r? 4 r99) can only be observed in single crystal. Exact mechanism is unclear now. We speculate that the behavior may relate to the reduction of minority spin band and carrier density in this class of materials [8], which will modify the convention transport and magnetotransport behaviors in ferromagnets. In austenite phase at room temperature, where the samples have Heusler structure, linear MR seems to be quite common, observed also in PtMnSb [8], NiMnSb [8], and Ni2 þ xMn1 xGa [10], regardless whether samples are in the form of single crystal, polycrystal, and thin film. The common features in this class of materials are localized spins of magnetic ions, large splitting in spin-up and spin-down band with possible disappearance of the minority band, and high carrier density [12]. The features are preserved in different structural forms and lead to a modified s–d model [12], which produces quite linear MR behaviors when To0.8TC (Fig. 8 in Ref. [12]), a condition that is satisfied in our experiments. At a very low temperature, the spin fluctuation is much reduced due to stable ferromagnetic phase, the MR due to s–d model is pretty much reduced to zero [12], resulting in conventional AMR behaviors due to spin–orbit coupling. However, Ni50Mn19Fe6Ga25 ribbons seems to consist of much larger magnetic inhomogeneities that gives rise to spin-dependent scattering as in the case of magnetic granular materials [13] and multilayers [14] and pins domain wall motion. The former gives rise to isotropic and negative MR and the latter manifested in the pinning mechanism reflected in the initial magnetization curve [Fig. 4(a)] and coercivity [Fig. 4(b)], which are absent in Ni50Mn19Fe6Ga25 single crystal and Ni2FeGa ribbon. We believe these magnetic inhomogeneities are crystallographically constructed by antisite Mn–Mn atoms and magnetically ordered in antiferromagnetic state. As a Heusler alloy, Ni50(Mn, Fe)25Ga25 should be structured in cubic L21 structure in which the Mn and Fe atoms preferentially occupy the 4b sites (Wyckoff notation) of the cubic cell of the Fm3¯m structure [15]. However, Mn and Fe atoms can easily occupy other sites to form antisite B2 structure during the rapid solidification process, similar to Ni2MnGa alloy [16]. These inhomogeneities in B2 structure are

metastable and are relatively easier to be destroyed by thermal treatment [16]. It has been reported that the coercive force in NiMnFeGa ribbons can be reduced to zero after annealing [9]. Similarly, the value of MR can also be greatly decreased [9]. It is also known that the magnetic structure in many ordered X2MnZ (e.g. compounds depend on the nearest Mn–Mn distance, from antiferromagnetic coupling at small separation to ferromagnetic coupling at large separations. For example, in Ni2MnAl, antiferromagnetic coupling occurres in B2 structure in as-cast ingots, whereas ferromagnetic ordering appears in L21 structure after annealing [17]. Fig. 5 compares magnetization behaviors of Ni50Mn19Fe6Ga25 ribbon and melted ingot samples at 5 K. Reduced saturation magnetization in the ribbon sample suggests that the existence of antiferromagnetically coupled Mn ions, which cannot be aligned even at the highest field. These antiferromagnetic clusters have random magnetization directions in zero field, giving rise to strong spin-dependent scattering. An external magnetic field will align the magnetization direction between these clusters and the ferromagnetic matrix. Consequently, spin-dependent scattering is suppressed, resulting in negative MR [14]. It should be pointed out that spins between Mn atoms inside the clusters are still antiparallel, but point to the field direction. It is now nature to raise the question why very different MR behaviors are observed in NiMnFeGa and Ni2FeGa ribbons. In Ni–Fe–Ga ribbons, an increase in both Curie temperature and the saturation magnetization is clearly observed as the excess Fe atoms occupy the normal Ni sites, indicating the decrease of Fe–Fe distance in Heusler structure would not cause antiferromagnetic coupling between Fe atoms, which is opposite to the case in NiMn-based Heusler alloys [18]. Furthermore, we have confirmed that Ni2FeGa ribbon has ordered Heusler structure. The (1 1 1) superlattice reflection corresponding to the nextnearest-neighbor L21 ordering is observed [11]. Different from NiMnFeGa ribbons, there are no magnetic inhomogeneities in Ni2FeGa ribbon sample. The lack of spin-dependent scattering centers in Ni2FeGa ribbon leading to normal AMR behaviors.

4. Conclusions In summary, we have investigated the magnetoresistance properties in NiMnFeGa ribbon and single crystal, as well as in Ni2FeGa ribbon samples. It is found that the NiMnFeGa melt-spun ribbons exhibited GMR effect with a large negative MR up to 13%. The spin-dependent scattering comes from the magnetic inhomogeneities consisting of antiferromagnetically coupled Mn atoms in B2 structure. In the absence of these magnetic inhomogeneities, Heusler alloys seem to show a common linear MR


behavior at around 0.8TC, which is typically around room temperature, regardless of sample structures. This may be explained by the s–d model in ferromagnets with localized spins, large splitting spin-up and spin-down bands, and low carrier concentration, which are unique to Heusler alloys. At low temperatures, conventional AMR behaviors due to the spin–orbital coupling are observed in NiMnFeGa single crystal and Ni2FeGa ribbon samples. This is most likely due to the diminished MR from s–d model because of much less spin fluctuation. MR anomaly at field between 2 and 11 kOe (r? 4 r99) is also observed in single crystal sample, which may be related to unique features of Heusler alloys.

Acknowledgment This work is supported by National Natural Science Foundation of China (Grant no. 50971130). References [1] C. Felser, B. Heikamp, F. Kronast, D. Schmitz, S. Cramm, H.A. Durr, H.J. Elmers, G.H. Fecher, S. Wurmehl, T. Block, D. Valdaitsev, S.A. Nepijko, A. Gloskovskii, G. Jakob, G. Schonhense, W. Eberhardt, J. Phys.: Condens. Matter 15 (2003) 7019.

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[2] R.A. de Groot, F.M. Mueller, P.G. van Engen, K.H.J. Buschow, Phys. Rev. Lett. 50 (1983) 2024. [3] K. Ullakko, J.K. Huang, C. Kantner, R.C. O’Handley, V.V. Kokorin, Appl. Phys. Lett. 69 (1996) 1966. [4] S.J. Murray, M. Marioni, S.M. Allen, R.C. O’Handley, Appl. Phys. Lett. 77 (2000) 886. [5] G.H. Wu, W.H. Wang, J.L. Chen, L. Ao, Z.H. Liu, W.S. Zhan, T. Liang, H.B. Xu, Appl. Phys. Lett. 80 (2002) 634. [6] Vladimir O. Golub, Andriy Ya. Vovk, Leszek Malkinski, Charles J. O’Connor, Zhenjun Wang, Jinke Tang, J. Appl. Phys. 96 (2004) 3865. [7] Jordi Marcos, Antoni Planes, Lluis Manosa, Amilcar Labarta, Bart Jan Hattink, Phys. Rev. B 66 (2002) 054428. [8] J.S. Moodera, D.M. Mootoo, J. Appl. Phys. 76 (1994) 6101. [9] Z.H. Liu, H. Liu, X.X. Zhang, X.K. Zhang, John Q. Xiao, Z.Y. Zhu, X.F. Dai, G.D. Liu, J.L. Chen, G.H. Wu, Appl. Phys. Lett. 86 (2005) 182507. [10] C. Biswas, R. Rawat, S.R. Barman, Appl. Phys. Lett. 86 (2005) 202508. [11] Z.H. Liu, M. Zhang, Y.T. Cui, Y.Q. Zhou, W.H. Wang, G.H. Wu, X.X. Zhang, Gang Xiao, Appl. Phys. Lett. 82 (2003) 424. [12] M. Kataoka, Phys. Rev. B 63 (2001) 134435. [13] John Q. Xiao, J.S. Jiang, C.L. Chien, Phys. Rev. Lett. 68 (1992) 3749. [14] C.L. Chien, John Q. Xiao, J.Samuel Jiang, J. Appl. Phys. 73 (1993) 5309. [15] Z.H. Liu, M. Zhang, W.Q. Wang, W.H. Wang, J.L. Chen, G.H. Wu, F.B. Meng, H.Y. Liu, B.D. Liu, J.P. Qu, Y.X. Li, J. Appl. Phys. 92 (2002) 5006. [16] M. Kreissl, K.U. Neumann, T. Stephens, K.R.A. Ziebeck, J. Phys.: Condens. Matter 15 (2003) 3831. ˜ osa, [17] Mehmet Acet, Eyup Duman, Eberhard F. Wassermann, Lluis Man Antoni Planes, J. Appl. Phys. 92 (2002) 3867. [18] Z.H. Liu, H. Liu, X.X. Zhang, M. Zhang, X.F. Dai, H.N. Hu, J.L. Chen, G.H. Wu, Phys. Lett. A 329 (2004) 214.