Magnetic-field-induced martensitic transformation in MnNiGa:Co alloys

APPLIED PHYSICS LETTERS 92, 032509 共2008兲

Magnetic-field-induced martensitic transformation in MnNiGa:Co alloys L. Ma, H. W. Zhang, S. Y. Yu, Z. Y. Zhu, J. L. Chen, and G. H. Wua兲 Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, China

H. Y. Liu, J. P. Qu, and Y. X. Li School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, People’s Republic of China

共Received 16 October 2007; accepted 8 January 2008; published online 25 January 2008兲 With a high Curie temperature and low entropy change, the magnetic-field-induced martensitic transformation has been obtained in ferromagnetic shape memory alloys MnNiGa by doping a small amount of Co. Due to the ferromagnetic activation effect of Co, a large amount of antiferromagnetically aligned Mn moments are turned into ferromagnetic ordering, which is verified by our electronic structural calculation and experimental observation. Consequently, the magnetization rises up to 70 emu/ g and the magnetization difference between two phases increases about ten times, resulting in a considerable dT / dH of 4 K / T and a well-defined reversed transformation induced by a magnetic field. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2838343兴 Ferromagnetic shape memory alloys 共FSMAs兲 have become a very attractive candidate for high performance magnetic actuator materials, since large magnetic-field-induced strains were observed by the rearrangement of martensite variants.1,2 In recent years, the magnetic-field-induced martensitic transformation has been obtained in several FSMA systems, such as NiMnX 共X = In, Sn, Sb兲,3–6 NiCoMnIn,7 NiCoMnSn,8 NiCoMnSb,9 NiCoMnGa,10 and there have been many reports on their magnetic properties, martensitic transition, inverse magnetocaloric effect, magnetic superelasticity, and magnetically controlled shape memory effect,11–18 exhibiting a great perspective for multifunctional applications. In the FSMA family, magnetic-field-induced transformation has been an interesting property that highlights the advantage of ferromagnetism. In the process of development of such materials, it has been widely accepted that a large magnetization difference ⌬M between two phases is of great importance. The ⌬M enhancement has been observed by adding Co into those Mn-rich FSMA.7–10 However, the mechanism of the ⌬M enhancement is still open. Furthermore, the Heusler alloy-type FSMAs focused on by many works are with L21 structure, whose Curie temperature 共TC兲 is usually low. In this paper, we attempt to realize the magnetic-fieldinduced martensitic transformation in MnNiGa alloy, based on its two advantages: high TC and low entropy change.19,20 A well-defined field-induced transformation can be obtained by adding 5.5 at. % Co in MnNiGa alloy. The ferromagnetic activation effect of Co atoms in this quaternary system has been verified by both the experimental observation and the theoretical calculation. Mn48CoxNi32−xGa20 共x = 0, 2, 4, and 5.5兲 ingots were prepared by arc-melting high-purity metals under argon atmosphere. The martensitic transformation temperature 共Tm兲, the Curie temperature 共TC兲, and the magnetization were mea-

sured by physical property measurement system 共Quantum Design equipped兲. The temperature dependence of magnetic susceptibility of Mn48CoxNi32−xGa20 共x = 0, 2, 4, and 5.5兲 alloys is presented in Fig. 1 and the related parameters are collected in Table I. Apparently, TC increases monotonously with the increase of Co content, which indicates an enhancement of magnetic exchange interaction, and Tm decreases by doping Co. As a result, the highest TC of 530 K and the lowest Tm of 145 K have been simultaneously achieved in the sample of Mn48Co5.5Ni26.5Ga20, in which the ⌬M is maximized. Figure 2 shows M共T兲 curves of Mn48CoxNi32−xGa20 through the martensitic transformation. It is interestingly observed that the magnetization of the parent phase is enhanced dramatically by doping Co, while that of martensite remains almost unchanged. Consequently, the ⌬M across the transformation significantly increases with the increase of x, as listed in Table I. The maximum ⌬M 共18.7 emu/ g兲 is exhibited in Mn48Co5.5Ni26.5Ga20 sample. It is almost ten times larger than that of the sample without Co 共only 1.89 emu/ g兲, For Mn2NiGa of Hg2CuTi structure, our previous works19,20 have verified by both experimental observation and the first principle calculation that its parent phase is fer-

a兲

FIG. 1. Temperature dependence of ac susceptibility for Mn48CoxNi32−xGa20 samples.

Author to whom correspondence should be addressed. Electronic mail: [email protected].

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TABLE I. Composition dependence of TC, Tm, ⌬M, and the effect of magnetic field upon Tm 共dT / dH兲 for Mn48CoxNi32−xGa20 samples.

x

TC 共K兲

Tm 共K兲

⌬M 共emu/g兲

dT / dH 共K/T兲

0 2 4 5.5

442 478 503 530

325 275 235 145

2.0 6.8 10.4 18.7

¯ 1.8 2.6 4

rimagnetic due to the antiparallel but unbalanced moments of Mn atoms at A sites 共2.52␮B兲 and B sites 共3.56␮B兲. On the contrary, its martensite is ferromagnetic with the reduced Mn moment of 1.44␮B at A sites and almost diminished one of 0.02␮B at B sites. As a result, the ⌬M of Mn2NiGa alloy is very small. Therefore, the large ⌬M obtained in our samples should be attributed to a change of magnetic structure caused by the addition of Co. In order to reveal the effect of adding Co, the electronic structure calculation using the self-consistent full-potential linearized-augmented plane wave method has been performed based on the local spin-density approximation within the density functional theory.21,22 The calculation results indicate that, when Co atom enters the Hg2CuTi-type lattice, the original two Mn sites become three because the embedded Co breaks the crystallographic symmetry, as shown in Fig. 3. The moments on the nearest neighboring Mn 共all A sites denoted as Mn1兲 are aligned ferromagnetically with those on the next-nearest neighboring Mn 共3 / 4 Mn at B sites denoted as Mn2兲, but the third-nearest neighboring Mn moments 共1 / 4 Mn at B sites denoted as Mn3兲 are still antiferromagnetically aligned with the Mn1 and Mn2 moments. Therefore, the Co atoms work as a “ferromagnetic activator” which turned the antiferromagnetically exchange-coupled Mn1-Mn2 moments in to the ferromagnetically exchangecoupled ones. This theoretical supposition predicts that the net moment should be proportional to the content of Co. As shown in the inset of Fig. 2, a linear relationship between M s共0兲 and x is observed by extrapolating all M共T兲 curves for their parent phase to 0 K, which is well consistent with the above ferromagnetic activation model. This exchange coupling is strongly suggested to be a competition of two kinds of interactions between the spin polarization of transportation electrons among localized Mn moments and the s-p hy-

FIG. 3. Magnetic structure sketch of Co doped Mn2NiGa cubic phase.

bridizing effect involved by Co,20 which will become an interesting subject in the future. The large ⌬M implies that it is possible to use magnetic field to shift the transformation temperature. Figure 4 shows that the Tm of Mn48Co5.5Ni26.5Ga20 sample is obviously shifted down to low temperature by applying magnetic field. With the increase of external field, the Tm decreases in an approximately linear rate of about 4 K / T, as shown in the inset of the Fig. 4. This phenomenon is absent in the Mn48Ni32Ga20 alloy. The dT / dH of 4 K / T in Mn48Co5.5Ni26.5Ga20 sample is comparable to that of Ni45Co5Mn36.6In13.4 共4 K / T兲,7 Ni43Co7Mn39Sn11 共3.6 K / T兲,8 and Ni41Co9Mn39Sb11 共3.5 K / T兲.9 However, the ⌬M of 18.7 emu/ g is much smaller than that in those systems. For example, it is 80 emu/ g in NiCoMnSn and even up to 100 emu/ g in NiCoMnIn.7,8 It implies that the entropy change ⌬S in Mn48Co5.5Ni26.5Ga20 is quite small, which is of benefit to develop a kind of FSMA in which the martensitic transformation can be induced by a relatively low field. Figure 5 shows M共H兲 curves of Mn48Co5.5Ni26.5Ga20 measured at different temperatures. The curve at 100 K reveals the low magnetization in martensitic phase. At temperatures between 140 and 160 K, the curves exhibit a metamagneticlike transformation from low magnetization martensite to high magnetization parent phase. Up to 160 K, the reversed transformation starts in a quite low field and the demagnetizing stage shows a typical MH behavior of parent phase, indicating a complete one-way reversed transformation. These results confirm that a field-induced martensitic

FIG. 2. Temperature dependence of the magnetization of Mn48CoxNi32−xGa20 samples measured in 5 T. The inset is the Co content FIG. 4. M共T兲 curves of Mn48Co5.5Ni26.5Ga20 sample measured in different dependence of saturation magnetization of the parent phase obtained by magnetic fields. The inset is the field dependence of Tm. extrapolating the M共T兲 curves to 0 K. Downloaded 09 Dec 2008 to 159.226.36.154. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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FIG. 5. Magnetic-field-induced reversed Mn48Co5.5Ni26.5Ga20 at various given temperatures.

transformation

in

transformation has been obtained in Mn48Co5.5Ni26.5Ga20. In summary, by doping 5.5 at. % Co, the magnetic-fieldinduced martensitic transformation has been obtained in Mn48Co5.5Ni26.5Ga20. Based on a relatively small ⌬M of 18.7 emu/ g, a considerable dT / dH of 4 K / T has been achieved. Similar to NiMnIn alloys, the field-induced transition is directly evidenced by magnetization measurement. The first principle calculation shows that the Co works as ferromagnetic activator and leads to ferromagnetic alignment of most Mn moments in the parent phase, which explains the dependence of the magnetization of the parent phase on Co content. This work was supported by CNSF 共No. 50531010兲 and NSF 共E2006000063兲. 1

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