Suppression of martensitic transformation in ...

Suppression of martensitic transformation in Fe50Mn23Ga27 by local symmetry breaking Tianyu Ma, Xiaolian Liu, Mi Yan, Chen Wu, Shuai Ren, Huiying Li, Minxia Fang, Zhiyong Qiu, and Xiaobing Ren Citation: Applied Physics Letters 106, 211903 (2015); doi: 10.1063/1.4921928 View online: http://dx.doi.org/10.1063/1.4921928 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/106/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Temperature-induced martensite in magnetic shape memory Fe2MnGa observed by photoemission electron microscopy Appl. Phys. Lett. 100, 032401 (2012); 10.1063/1.3677939 Large inverse magnetic entropy changes and magnetoresistance in the vicinity of a field-induced martensitic transformation in Ni 50 − x Co x Mn 32 − y Fe y Ga 18 Appl. Phys. Lett. 97, 062505 (2010); 10.1063/1.3467460 Magnetic-field-induced transformation in FeMnGa alloys Appl. Phys. Lett. 95, 222512 (2009); 10.1063/1.3269590 Martensitic transformation and magnetic field-induced strain in Fe–Mn–Ga shape memory alloy Appl. Phys. Lett. 95, 082508 (2009); 10.1063/1.3213353 Macroscopic pattern formation preceding martensitic transformation in a ferromagnetic shape memory alloy Ni 51 Fe 22 Ga 27 Appl. Phys. Lett. 92, 102512 (2008); 10.1063/1.2896645

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APPLIED PHYSICS LETTERS 106, 211903 (2015)

Suppression of martensitic transformation in Fe50Mn23Ga27 by local symmetry breaking Tianyu Ma,1,2,a) Xiaolian Liu,1 Mi Yan,1 Chen Wu,1 Shuai Ren,2,3 Huiying Li,3 Minxia Fang,3 Zhiyong Qiu,4 and Xiaobing Ren2,3,b)

1 State Key Laboratory of Silicon Materials, School of Materials Science and Engineering, Key Laboratory of Novel Materials for Information Technology of Zhejiang Province, Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou 310027, China 2 Ferroic Physics Group, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0047, Japan 3 Multi-disciplinary Materials Research Center, Frontier Institute of Science and Technology, Xi’an Jiaotong University, Xi’an 710049, China 4 WPI Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan

(Received 21 February 2015; accepted 20 May 2015; published online 28 May 2015) Defects-induced local symmetry breaking has led to unusual properties in nonferromagnetic ferroelastic materials upon suppressing their martensitic transformation. Thus, it is of interest to discover additional properties by local symmetry breaking in one important class of the ferroelastic materials, i.e., the ferromagnetic shape memory alloys. In this letter, it is found that local symmetry breaking including both tetragonal nano-inclusions and anti-phase boundaries (APBs), suppresses martensitic transformation of a body-centered-cubic Fe50Mn23Ga27 alloy, however, does not affect the magnetic ordering. Large electrical resistivity is retained to the low temperature ferromagnetic state, behaving like a half-metal ferromagnet. Lower ordering degree at APBs and local stress fields generated by the lattice expansion of tetragonal nanoparticles hinder the formation of long-rangeordered martensites. The half-metal-like conducting behavior upon suppressing martensitic transformation extends the regime of ferromagnetic shape memory materials and may lead to potential C 2015 AIP Publishing LLC. applications in spintronic devices. V [http://dx.doi.org/10.1063/1.4921928]

Ferroelastic materials that undergo martensitic transformation (MT, diffusionless structure transformation) are technologically important due to their unique shape memory effect and superelastic behavior.1,2 The martensitic transformation behavior, however, is significantly affected by crystallographic defects, including point defects,3 dislocations,4 grain boundaries or anti-phase boundaries (APBs),5,6 and nanoprecipitates.7 Such defects serve as the kinetic hindrance for MT. When the defect concentration exceeds a critical value, the spontaneous MT becomes kinetically forbidden.8,9 For instance, the B2 parent phase in a Ti50Pd50 alloy can be stabilized when APBs are formed under rapid solidification conditions.6 The existence of APBs, on one hand, decreases the degree of ordering, and on the other hand, generates internal stress to inhibit the subsequent development of long-range B19 martensite ordering. Alternatively, as reported in Ti48Ni52 and Ti48.7Ni51.3 alloys, both point defects3 and nanoprecipitates9 suppress the MT but give rise to a glassy state, i.e., the strain glass. Being physically parallel to the clusterspin glass10 and ferroelectric relaxor,11 the strain glass exhibits frequency dependent dynamic mechanical anomalies, which also conforms to the Vogel-Fulcher relationship.3,9,12–14 The key point to prohibit the formation of long-range strainordered martensite is the random local stress fields generated by the point defects (excessive soluting atoms) or nanoprecipitates. Consequently, a physical picture for the strain glass with local symmetry breaking is established, which involves a)

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b)

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gradual formation, growth, and freezing (at Tg) of nano-sized strain domains or martensite clusters without changing the average structure of the parent phase. Interestingly, the suppression of MT has led to unusual properties that are totally different from the martensitic systems, such as superelasticity with narrow hysteresis over a wide temperature range,15 high damping and low modulus,16 as well as Invar and Elinvar effect in cold-rolled b-Ti alloys.17 In one important member of the ferroelastic material family, i.e., the ferromagnetic shape memory alloys (FSMAs), martensitic transformation and magnetic transition coexist.18–20 The large magnetic-field-induced-strain (MFIS), giant magnetocaloric effects (MCEs), large magnetoresistance (MR), and exchange bias (EB) behavior found in FSMAS trigger more interest in the technology applications. Due to the strong interaction between the strain and magnetic degrees of freedom, an interesting question then arises: Can special functions be obtained when the martensitic transformation in FMSAs is suppressed? To answer this question, we selected the nearstoichiometric Fe2MnGa Heusler alloys that can undergo MT from a paramagnetic body-centered-cubic (BCC) phase to a ferromagnetic body-centered-tetragonal (BCT) phase, facilitating large MFIS,21 large shape memory effect,22 and anomalous Hall-effect.23 The MT in these alloys is accompanied with a large thermal hysteresis (70–80 K), a comparatively large volume expansion (DV 0.7%–1.35%), and a significant reduction in resistivity (20%). In this work, Fe50Mn23Ga27 alloy is prepared, which has a slight difference in Mn/Ga ratio from Fe50Mn22.5Ga27.5 that undergoes apparent MT.22 It is found that the MT is suppressed but the magnetic ordering is retained by

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local symmetry breaking, i.e., the formation of APBs and nanoscale tetragonal particles. As a consequence, the large resistivity at the high temperature paramagnetic state is retained to the low temperature ferromagnetic state, which makes this alloy behave like a half-metal ferromagnet that is promising for spintronic devices. Fe50Mn23Ga27 alloy was prepared by induction melting under the protection of argon atmosphere. Sheet specimen with a thickness of 2 mm was obtained by hot rolling at 1273 K. After polishing, small pieces were annealed for 2 h at 1273 K, followed by quenching into ice water. X-ray fluorescence analysis revealed the final composition of Fe50.28Mn22.33Ga27.39. Electrical resistivity was measured with the standard four-probe technique. In-situ heating X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were performed to detect the possible structural change. Dynamical mechanical properties were conducted on a dynamic mechanical analyzer DMA (Q800 TA Instruments) with the three-point bending mode. Magnetic properties were measured using a physical properties measurement system (PPMS) and a Lakeshore vibrating sample magnetometer (VSM), respectively. Specimen for transmission electron microscope (TEM) observation was prepared utilizing the standard techniques of grinding, dimpling, and argon ionbeam thinning. Ion-beam thinning was carried out on both sides of the specimen at an inclination angle of 8 for the ion-beam with respect to the specimen surface. TEM characterization was performed using a TOPCON EM-002B and a JEM-2100F microscopy (for high-resolution observations) equipped with a GATAN CCD slow scan camera, respectively. HR-TEM images were analyzed by DigitalMicrograph software. Figure 1 shows the microstructure of Fe50Mn23Ga27 at room temperature. The alloy has a BCC average structure, as revealed by the selected area electron diffraction (SAED) pattern with the [011] zone axis (Fig. 1(a)). The {111} and {002} superlattice reflections suggest the existence of highly ordered L21 phase (aL21 0.5825 nm). The intensity of {111} spots, however, is weaker than that of the {002} ones, indicating that the ordering degree is lower than ideal L21

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structure. In addition, the SAED pattern contains complex diffuse scatterings around the fundamental reflections, which are also unexpected for ideal L21 structure. The diffuse scattering is known to be related to the correlated atomic position fluctuations, i.e., the local symmetry breaking.3,6,12,24,25 The bright field image (BFI) in Fig. 1(b) taken with the [011]BCC incidence displays the anti-phase domains (APDs) with the size of several hundred nanometers. These irregularly shaped domains are separated by curved interfaces that are the APBs. The enlarged view of APBs in the inset of Fig. 1(b) reveals that their thickness is of several tens of nanometers. The existence of APBs with local lower ordering than the ideal L21 structure is one important contribution to the weak {111} superlattice reflections and diffuse scattering. The development of APBs can be explained by that crystal structure of the almost solidified phase might change from L21 to lower ordered B2 or disordered A2 phases during quenching (both A2 and B2 share the same fundamental spots with L2126). L21 domains sporadically nucleate and grow in the lower-ordered or disordered phases, with the two phases eventually coming into contact with each other during cooling. The dark field image (DFI) (Fig.1(c)) taken by using one {002} reflection in the [011]BCC pattern reveals nanoscale bright contrasts that may have non-cubic symmetry. Fig. 1(d) shows HR-TEM image of the APB region, including one nanoparticle, as highlighted by the blue circle. The fast Fourier transform (FFT) spectrum (inset of Fig. 1(e)) of the APB region (yellow square) shows strong intensity {022} spots, indicating that the neighboring APDs share one {022} plane as their APB. In combination with the inverse FFT (IFFT) result (Fig. 1(e)), the local symmetry is broken at APBs except that the lattice spacing d220 keeps the same with L21 structure. Fig. 1(f) shows the FFT and IFFT results for the red square in Fig. 1(d) that contains the nanoparticle. Additional diffraction spots appear, which can be indexed as a BCT symmetry (inset of Fig. 1(f)) with aBCT ¼ bBCT 0.5401 nm, and cBCT 0.7017 nm. Compared to the BCC phase, the BCT

FIG. 1. TEM characterization of the quenched Fe50Mn23Ga27 sample at 293 K. (a) SAED pattern taken along the [011]BCC zone axis. (b) BFI image taken with the [011]BCC incidence. The inset is an enlarged view. (c) DFI obtained by using the {002} reflection, as indicated by the blue arrow in (a). (d) HR-TEM image of the APB region—one nanoparticle is highlighted by the blue circle. (e) FFT and corresponding IFFT results of the yellow square (APB region) in (d). (f) FFT and IFFT results of the red square (nanoparticle) in (d), revealing the local tetragonal symmetry.


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phase forms with the contraction of a axis and the elongation of c axis simultaneously, assembling a larger unit cell. Accordingly, the lattice spacing for (022) and (202) is larger than that of (220). Consequently, the nanoparticles in the DFI, combined with the diffusing scattering in the SAED pattern and the HR-TEM images (Figs. 1(d)–1(f), are not L21, but have the BCT symmetry. Considering that the martensite in similar alloy Fe50Mn22.5Ga27.5 has the BCT structure,22 the formation of BCT nanoparticles in our alloy can be explained by that the MT occus only at nanoscale. The BCT nanoparticles also contribute to the diffusing scattering in Fig. 1(a), similar to the strain glass alloys Ti-Ni and Ti-Pd-Cr,3,9,12,17 where the diffusing scatterings correspond to nanodomains. Since the volume fraction of BCT nanoparticles is small (Fig. 1(c)), the {111} reflections are not as diffuse as the fundamental ones. In short, the diffuse scattering and HR-TEM characterizations in Fig. 1 demonstrate that the local symmetry is broken, forming tetragonal nanoparticles and APBs. The diffuse scattering is usually considered as an important precursor phenomenon prior to the MT. In the following, possible phase transformation behaviors are investigated, as illustrated in Figs. 2 and 3. The in-situ XRD measurements (Fig. 2(a)) show almost no change and the average BCC structure is kept over the measured temperature range, indicating the absence of global structural transformation. The DSC measurements (Fig. 2(b)) also show no MT features, where no enthalpy peak/dip is observed upon either cooling

FIG. 2. Transition behaviors of the quenched Fe50Mn23Ga27 sample. In-situ cooling XRD patterns (a), DSC curves (b), resistivity results (c), and DMA results (d) show that the spontaneous MT is suppressed. Both heating and cooling rates are 2 K/min.

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FIG. 3. In-situ heating TEM observations at (a) 110, (b) 175, (c) 245, and (d) 350 K, with the corresponding diffraction patterns shown in each inset, respectively. The nanomartensites are observed over a wide temperature range highlighted by dot lines.

or subsequent heating processes. However, the temperature spectrum of resistivity (Fig. 2(c)) and storage modulus (Fig. 2(d)) exhibit sign change in @q/@T and softening behaviors (more obvious for the heating case), respectively, indicating a local structure transition. The in-situ TEM observations (Fig. 3) upon cooling or heating within the temperature range for the @q/@T change provide direct evidence for such transition. After characterizing the room temperature microstructure, the foil was cooled to 110 K, followed by heating to 175 K, 245 K, and 350 K, respectively. The lattice is mostly the undistorted BCC structure judged by the [111]BCC patterns for each temperature, which also suggests no global structural transformation. The nanoscale Moire modulations due to local lattice mismatch, however, are still observed at all cases. At 110 K, the size of some modulation streaks is over 20 nm. When heating to 175 K (around TC of the BCC matrix, see Fig. 4(a)), the modulated domains become smaller clearly. Upon further heating to 245 K, both the size and density of the nanodomains reduce. At 350 K, the nanodomains still exist but become much sparser. It can be concluded that over a wide temperature range, the low symmetry nanodomains (nanomartensites) still exist. The above results demonstrate the suppression of spontaneous MT, but the magnetic transition is retained, as illustrated in Fig. 4. In Fig. 4(a), the AC susceptibility v0 peaks overlap at 173 K as the Curie temperature TC for the BCC matrix phase in the cooling and heating curves. Fig. 4(b) shows the in-situ cooling magnetization loops measured at 350 K, 230 K, and 140 K, respectively. Although weak, ferromagnetism is observed at 350 K and 230 K (above TC) due to the existence of magnetic nano-inclusions (the BCT phase is ferromagnetic for Fe-Mn-Ga alloys21–23). The coercivity is 0.404 kOe for the measurement taken at 350 K and


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FIG. 4. Magnetic measurement results of the quenched Fe50Mn23Ga27 sample. (a) AC susceptibility, (b) in-situ M-H hysteresis loops, and (c) M-T curves at different magnetic fields measured by PPMS. Each M-T curve is measured from a fresh specimen upon cooling and subsequent heating at 2 K/min. (d) M-T curves measured by VSM upon heating and subsequent cooling at 5 K/min. The small hysteresis in M-T curves indicates that the magnetic transition is associated with local structural transformation. TC (BCC) and TC (M) represent Curie temperatures of the BCC matrix and the nanomartensites, respectively.

0.192 kOe for that at 230 K, respectively. The relative large coercivity excludes the possibility of superparamagnetism even that the magnetic tetragonal particles are smaller than 10 nm. In addition, a larger hysteresis is seen in the M-H loop at 350 K compared with the other two cases, indicating magnetic-field induced structural transition. Further DC M-T measurements present more evidence, as shown in Figs. 4(c) and 4(d). A noticeable thermal hysteresis (DT is 5 K, estimated from the peaks in dM/dT-T curves) is observed in the M-T curves under 2 kOe between cooling and heating processes. The hysteresis shifts to the higher temperature regime under stronger magnetic fields (for the heating case, v0 maximizes at 173 K, dM2kOe/dT, dM20kOe/dT, and dM70kOe/dT minimize at 190 K, 207 K, and 244 K, respectively), suggesting magnetic field-induced structural transition. In comparison with Fe50Mn22.5Ga27.5 and Fe48Mn24Ga28 alloys that exhibit coupled magnetostructural phase transition between ferromagnetic martensite and paramagnetic austenite,21–23 with a DT as large as 70–80 K, the magnetic field-induced structural transition is less apparent for the Fe50Mn23Ga27 alloy. Fig. 4(d) shows the M-T curves measured from 300 to 670 K at 2 kOe. The weak ferromagnetism is maintained up to 625 K, at which the heating and cooling curves reach an inflection. It demonstrates that the residual ferromagnetic nanomartensites exist over a wide temperature range, being consistent with the in-situ TEM observations. One interesting function is that the high resistivity of the paramagnetic state is retained for the low temperature ferromagnetic state. Over the measuring temperature window from 100 to 350 K (Fig. 2(c)), the q is above 200 lX cm with a variation less than 6 2%, indicating a half-metal like conducting behavior. According to theoretical calculations, the high resistivity for stoichiometric Fe2MnGa originates from the half-metal like electronic structure, having a large spin-polarization rate of 68%.27 The negative resistivity

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temperature coefficient in the metals with high resistivity has been explained by the weak localization at the paramagnetic state, according to the Mooij correlation.28 The sign change of @q/@T for half-metal materials is a feature associated with the paramagnetic-ferromagnetic transition,29 where the material behaves like a semiconductor at the paramagnetic state and like a metal at the ferromagnetic state. The critical temperature at which the sign for @q/@T changes in Fe50Mn23Ga27, however, is higher than the Curie temperature TC, unlike the Co2TiSn half-metallic alloy (Ref. 29). It is ascribed to the existence of ferromagnetic BCT nanoparticles, which exhibit different temperature dependence of resistivity from the BCC matrix phase. The resultant halfmetal-like conducting behaviors may make the alloy a good candidate for spintronic applications. The reported apparent long-range-ordered martensites in similar alloy Fe50Mn22.5Ga27.5,22 however, is not observed here. Change in the resistivity is less than 2%, and small thermal hysteresis is observed between cooling and heating (Fig. 2(c)). Such discrepancy may be attributed to several reasons. First, larger difference in the actual composition may exist between these two alloys than the nominal composition. Second, when critical point defects are introduced into the martensitic system, the spontaneous MT becomes kinetically forbidden and can therefore be suppressed, forming nanodomains or “strain glass.” The Mn with higher concentration in our alloy may serve as point defects to suppress the spontaneous MT. The frequency-independent modulus dip in Fig. 2(d), however, suggests no "strain glass transition" for the present alloy (which has been characterized by the Vogel-Fulcher type frequency-dependent modulus anomaly 3,9,12). Third, APBs are observed in our system. These two-dimensional defects lower the local ordering degree and separate the BCC matrix phase into small regions. The refined grains of the parent phase down to nanoscale could also suppress the spontaneous MT.5,30 Consequently, the existence of APBs may also inhibit the development of longrange martensite ordering. Finally and most importantly, when the local areas of the highly ordered L21 APDs possess slight composition fluctuations, for instance, closer to the ones with spontaneous MT, structural transformation takes place at these local regions (forming tetragonal nanoparticles). Once the MT occurs in nanoscale, i.e., the cubic L21 regions transforms into tetragonal ones (Figs. 1 and 3), their volume expansion generates local stress fields. The local fields then hinder the formation of long-range ordered martensite of the coherent BCC matrix phase (assuming that MT occurs without local stress fields). It is also noted that such local stress fields are available mainly near or at the APBs, which is different from the Ti48.7Ni51.3 strain glass alloy containing randomly distributed nanoprecipitate.9 Consequently, the Vogel-Fulcher type frequency-dependence for a glassy state is not observed. To what extent the APBs and the nanoinclusions dominate the suppression of phase transformation awaits comprehensive investigation. In summary, local symmetry breaking has been observed in Fe50Mn23Ga27, which suppresses the martensitic transformation and retains the magnetic transition. The local symmetry breaking is characterized by diffuse scattering reflections, curved anti-phase boundaries, and tetragonal


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nanoparticles. The development of APBs lowers the degree of local ordering, decreasing the kinetics of martensitic transformation. The local lattice expansion of nano-inclusions near or at APBs can also generate internal stress fields to inhibit the development of long-rang martensitic ordering. This phenomenon is rather different from the strain glass containing nanoprecipitates, in which the local stress fields have randomness effect. The resultant half-metal-like conducting behavior at ferromagnetic state may lead to potential applications in spintronic devices. These findings suggest that unusual function can be obtained in magnetic shape memory alloys by local symmetry breaking. This work was supported by JSPS-P11509, the Foundation for Author of National Excellent Doctoral Dissertation (No. 201037), the 973 Program (No. 2012CB619401), the Project of Nonprofit Technology & Research of Zhejiang Province (No. 2013C31025), and the Grant-in-Aid for Young Scientists (B), Japan (26790038). T.Y. Ma thanks Professor Kazuhiro Otsuka, Professor Yandong Wang, and Professor Guangheng Wu for helpful discussions. 1

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