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Magnetic domain structure in Ni53.6Mn23.4Ga23.0 shape memory alloy films studied by electron holography and Lorentz microscopy
Acta Materialia 51 (2003) 485–494 www.actamat-journals.com
Magnetic domain structure in Ni53.6Mn23.4Ga23.0 shape memory alloy films studied by electron holography and Lorentz microscopy Y. Murakami ∗, D. Shindo, M. Suzuki, M. Ohtsuka, K. Itagaki Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Received 23 June 2002; received in revised form 17 September 2002; accepted 17 September 2002
Abstract Magnetic domain structure in Ni53.6Mn23.4Ga23.0 films containing nonmagnetic MnO precipitates was examined by electron holography and Lorentz microscopy. Close experiments discovered peculiar zig-zag-shaped lines of magnetic flux, which are presumably typical configuration in the martensite accompanied by many twin plates. Near the edge of the films, this configuration was modified into a distinct form that was favorable to the reduction of magnetic charges. Inside the martensite plates as crystallographic domains, the lines of magnetic flux were wavy due to the electromagnetic interaction with the nonmagnetic MnO precipitates. The observations have promoted an understanding of the magnetic domain structure in ferromagnetic shape memory alloy films, and also provided essential information for developments of magnetic field-driven microactuators using these films. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Transmission electron microscopy; Martensitic phase transformation; Shape memory alloy; Magnetic domain; Microstructure
1. Introduction Ferromagnetic Ni2MnGa alloys have attracted keen attention of researchers because of the martensitic transformation that can be controlled by a magnetic field [1,2], abnormal softening of the [ζζ0]-TA2 phonon [3], and appearance of the intermediate structure prior to the martensitic transformation upon cooling [4,5] etc. Among these fas∗ Corresponding author. Tel.: +81-22-217-5169; fax: +8122-217-5211. E-mail address: [email protected] (Y. Murakami).
cinating phenomena, the magnetic field-induced shape deformation [6–8] is of special interest from both academic and industrial viewpoints. Some research groups have reported that huge strain, which is comparable with the attainable strain of a giant magnetostrictive material Terfenol-D, can be obtained by the application of a magnetic field to Ni2MnGa alloys [9–11], although the magnitude of the applied field is quite large, e.g. over 10 kOe. This huge strain is presumably achieved by the rearrangement of twin-related martensite plates, i.e. energetically favored martensite plates with a specific orientation grow at the expense of unfavorable plates under the applied magnetic field. From
1359-6454/03/$30.00 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(02)00431-7
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a viewpoint of applications, the magnetic fieldinduced shape deformation has some advantages over the conventional deformation driven by heating. For example, the response to the magnetic field can be quicker than that to temperature. Moreover, the deformation can be done without touching the specimen if the magnetic field is used. Ni2MnGa alloys are quite brittle in a polycrystalline bulk state, and thereby the deformation into a required shape is sometimes in failure. However, the ductility is likely to be improved in a state of thin films [12] that can be obtained by the sputtering method etc. Thus, Ni2MnGa alloy films are hopeful candidates for microactuators driven by a magnetic field. Control of the microstructure is one of the key operations in the development of shape memory alloys, e.g. the microstructure affects the mobility of martensite plates and/or the critical stress to introduce permanent strain in the martensite [13]. In the case of ferromagnetic shape memory alloys like Ni2MnGa, understanding of the magnetic domain structure is important not only for the actuator applications but also for fundamental research. For example, it is of great interest to explore the feature of magnetic domains in a martensitic phase, which exhibits complicated microstructure accompanied by many twins, while this essential problem is likely to remain controversial. On the other hand, when one fabricates the Ni2MnGa alloy films by sputtering, nonmagnetic MnO precipitates are sometimes generated in the ferromagnetic matrix [14]. Effects of the nonmagnetic MnO precipitates on the magnetic domain walls (DWs) and/or the magnetic flux in the martensite films are also important issues, where intensive studies have not yet been carried out. Magnetic domains in Ni2MnGa alloys were discussed by some research groups to date. Rearrangements of twin-related martensite plates under an applied magnetic field were observed by optical microscopy experiments by Likhachev and Ullakko [15], and by Chopra and Ji [16]. Heczko et al. [17] performed scanning electron microscopy studies combined with the Bitter technique. They claimed that the magnetization process in the martensite was governed by the magnetization rotation, although the rotation process was replaced by the
field-driven motion of martensite plates when the direction of the applied field was close to the hard axis. Pan and James [18] observed peculiar fir tree patterns near the martensite plate boundaries by magnetic force microscopy. More recently, de Graef et al. [19] carried out Lorentz microscopy studies in which typical shapes of magnetic domains in the parent phase were discussed. They also mentioned that a change in the easy axis, from [111] in the parent phase to [001] in the tetragonal martensite, did not occur at the same time as the growth of the martensite plates. Despite these accumulated studies, which are likely to focus on the behavior of magnetic domains under an applied magnetic field or during the martensitic transformation, relations between the magnetic domain structure and the microstructure of martensite are not yet fully understood. Probably, an effective approach is electron holography, by which the lines of magnetic flux within the martensite can be visualized with a spatial resolution comparable to the size of thin martensite plates (twin plates) and/or fine precipitates. Moreover, it is possible to compare the observed magnetic domain structure with the conventional transmission electron microscope image for the same area. In the present work, the magnetic domain structure in sputtered Ni2MnGa films, which contained fine precipitates of MnO, were examined by electron holography, Lorentz microscopy, and conventional transmission electron microscopy. The targets are twofold: (1) to reveal the magnetic domain structure in the martensite films accompanied by many twins; and (2) to locate the effects of nonmagnetic MnO precipitates on the magnetic domain structure.
2. Experimental Ni-rich Ni2MnGa films were fabricated by using a radio-frequency magnetron sputtering apparatus (Shibaura, CFS-4ES). Nominal composition of the targets was Ni52.0Mn24.0Ga24.0 when the oxygen as impurity was disregarded, whereas composition of the deposited films determined by inductivity coupled plasma spectroscopy was Ni53.6Mn23.4Ga23.0. See the literature by Suzuki et
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al. [20] for the technical details. The martensitic transformation start temperature (MS) and the Curie temperature (TC) were about 315 K and 345 K, respectively. Thus, the specimens were in a state of ferromagnetic martensite at room temperature. Both the electron holography and Lorentz microscopy studies were performed at room temperature in the present work. To explore the effects of nonmagnetic MnO precipitates on the magnetic domain structure in Ni53.6Mn23.4Ga23.0 films, three types of specimens (Sp.1, Sp.2, and Sp.3) exhibiting distinct fashions of the precipitation were prepared by using the targets with different oxygen content and/or heat treatments with different conditions, as summarized in Table 1. Dark-field images of the MnO precipitates (Fig. 1) make the distinction between these specimens: in Sp.1, the size of the MnO precipitates is 10–20 nm, and the population is sparse [Fig. 1(a)]; in Sp.2, the size of the MnO precipitates is still 10–20 nm, but the population is dense [Fig. 1(b)]; in Sp.3, the size of the MnO precipitates ranges from 30 to 120 nm, and the population is dense [Fig. 1(c)]. Structure and morphology of the martensite were studied using a JEM-2010⍀ transmission electron microscope equipped with an omega-type energy filter, by which the strong background arising from inelastic electron scattering was effectively reduced [21,22]. Lorentz microscopy and electron holography experiments were carried out with a JEM-3000F transmission electron microscope, to which a field emission gun and a biprism were installed. A special pole piece for magnetic domain observation was utilized, in which the magnetic field near the specimen was about 0.2 mT [23]. Holograms were recorded with conventional films and they were digitized by a film scanner with a resolution 8 µm/pixel.
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Here the essence of electron holography [21,24] is briefly explained. Application of positive voltage to a biprism, which is integrated into the electron microscope, generates interference fringes of the object wave passing through the specimen with the reference wave through the vacuum, as shown in Fig. 2(a). The interference fringes should be straight if a phase shift of the object wave relative to the reference wave is absent. When the phase shift arises due to the magnetic flux of a specimen, the interference fringes deviate from the straight lines, as shown in Fig. 2(a), lower region. [The interference fringes are almost straight outside the specimen in Fig. 2(a), upper region, because of the little stray field of the closure magnetic domains, as described later. Strictly speaking, the in-plane component of magnetic flux, i.e. perpendicular to the incident electrons, can be measured by the holography experiment [25,26]. The electrostatic potential also contributes to the phase shift although this effect is significant when the specimen thickness steeply changes, as in the case of small particles. Change in the specimen thickness is rather gradual in the fields of view of the present specimens, compared with small particles etc.] It is therefore possible to derive the information on the magnetic flux of the specimen by analyzing the phase shift. The phase shift f(x,y) can be evaluated by the Fourier transform of the electron hologram recorded as a digital image, as described elsewhere in detail [21], where x and y represent the coordinates in the hologram. If the phase shift is displayed in terms of cosf(x,y), a reconstructed phase image as shown in Fig. 2(b) is obtained, where the direction and the density of gray lines correspond to the direction and the density of lines of magnetic flux projected along the electron beam, respectively.
Table 1 Size and population of MnO precipitates in the three kinds of specimens. Conditions of heat treatment are also listed Specimen
Size of MnO
Population of MnO
Heat treatment
Sp.1 Sp.2 Sp.3
10–20 nm 10–20 nm 30–120 nm
sparse dense dense
1073K — 36ks 1073K — 36ks 1273K — 36ks
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Fig. 2. (a) Typical electron hologram; and (b) the reconstructed phase image. See text for details as to how the reconstructed phase image was obtained from the hologram. The measured phase shift was amplified three times to give the reconstructed phase image [21]. Observations were done using Sp.2.
Fig. 1. Dark-field images of the MnO precipitates in (a) Sp.1; (b) Sp.2; and (c) Sp.3.
3. Results and discussion 3.1. Morphology and structure of martensite Figure 3 (left-hand side) shows bright-field images of the three types of specimens: (a) Sp.1; (b) Sp.2; and (c) Sp.3, in the martensitic state.
Typical electron diffraction patterns in these specimens are also provided in the right-hand side. All of the specimens show a typical morphology of the martensite consisting of thin plates. These martensite plates are crystallographically equivalent but their orientations are different. In fact the martensite plates can be classified into several groups depending on their orientations, and the groups are generally called “martensite variants” (MVs). Neighbored martensite plates have specific twinning relations [27]. The MnO precipitates can be observed in the bright-field images, e.g. at the positions of open circles, although the contrast is weak. Extra spots as marked by 111MnO in the diffraction patterns also confirm the presence of MnO precipitates. It is clear that the unit reciprocal lattice distance, e.g. between the transmitted beam and the 0014M reflection, is divided into seven in all of the diffrac-
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are observed in the 14M martensite of other shape memory alloys like Ni50.0Al16.0Mn34.0 [31] and Ni63.0Al37.0 [27,32]. 3.2. Characteristic features of magnetic domains in martensite films Figure 4(a) shows a typical bright-field image of Sp.1. The white lines represent grain boundaries. Each grain is filled with martensite plates containing planar defects as mentioned earlier. Figure 4(b) offers a Lorentz microscope image for the same field of view as that of Fig. 4(a), and DWs can be
Fig. 3. Bright-field images (left side) and electron diffraction patterns (right side) observed in (a) Sp.1; (b) Sp.2; and (c) Sp.3.
tion patterns. The result indicates that the crystal structure of the martensite is 14M, which is a long period stacking order structure of basal planes originating from {110} planes of the parent phase. (Although the structure of martensite is the sevenlayer one, it is denoted as 14M in the terminology by Otsuka et al. [13]. See the previous reports [28,29] for more details on the crystal structure.) Hence, all of the specimens were found to show the same crystal structure of martensite, and exhibit typical plate-like morphology as observed in other shape memory alloys, regardless of the fashion of the precipitation. There are many planar defects like stacking faults on the basal planes of martensite, as evidenced by the streak along the c∗ axis in the diffraction patterns and/or striations within the martensite plates in the bright-field images. These planar defects are likely to be due to the lattice invariant shear that is necessary to achieve the invariant plane strain condition of the martensitic transformation [30]. In fact, similar planar defects
Fig. 4. (a) Bright-field image; (b) Lorentz microscope image; and (c) reconstructed phase image for the enclosed area of (b). White lines in (a)–(c) represent grain boundaries. The measured phase shift was amplified three times. Observations were done using Sp.1. See text for details.
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observed as bright or dark lines in the Fresnel mode of Lorentz microscopy [21]. It is clear that the martensite plate boundaries (MPBs) themselves correspond to DWs. Moreover, one notices the presence of DWs crossing the martensite plates, e.g. the dark lines marked by “DW” in the upper right part of Fig. 4(b). Thus, the observation has deduced that each martensite plate, as a crystallographic domain, does not correspond to a large magnetic domain but is divided into several small magnetic domains. The result is consistent with that of CoNiAl ferromagnetic shape memory alloys having a distinct crystal structure, L10 [33]. The magnetic domain structure is discussed more in detail with a reconstructed phase image [Fig. 4(c)], which was obtained from the hologram for the enclosed area in Fig. 4(b). Gray lines in Fig. 4(c) represent the lines of magnetic flux projected along the electron beam, and the arrows give directions of magnetization vectors. As shown in the area “Z” of the grain B, the lines of magnetic flux are modulated into peculiar zig-zag lines. The abrupt change in direction occurs when the lines of magnetic flux cross the MPBs. That is, since the magnetocrystalline anisotropy of the martensite is large [7,11,34] and the orientation of the easy magnetization axis is different between the neighbored martensite plates, the direction of the lines of magnetic flux changes steeply at the MPBs. The zig-zag-shaped lines of magnetic flux will be a characteristic feature of the martensite with multiple variants. It is worth noting that the zig-zag lines are obscure near the specimen edge even in the grain B. The lines of magnetic flux are rather parallel to the specimen edge, and thereby the magnetization vector deviates from the easy axis. In fact one sees the lines of magnetic flux that are passing through the grains A, B, and C along the specimen edge, although these grains have distinct orientations. Probably, near the specimen edge, reduction of the magnetostatic energy will be more significant over the magnetocrystalline anisotropy energy, and thereby the lines of magnetic flux tend to be parallel to the edge. As a result, formation of the magnetic charge will be reduced, although some stray field is still observed in Fig. 4(c). The wall marked by “DW” separates the lower area showing zig-zag lines from the upper area exhibiting lines of magnetic
flux that are nearly parallel to the edge in the grain B. In Fig. 4(c), some other DWs are present near the grain boundaries, e.g. the grain C has the DWs as indicated by dashed lines near the boundary to the grain D. These DWs are presumably generated as a result of the balance of the magnetocrystalline anisotropy energy, magnetostatic energy, and the exchange energy near the boundaries etc. Thus, the present observations have revealed essential features of magnetic domains that are produced in the martensite films. Another case of accommodation in the magnetic flux near the specimen edge is observed in Fig. 2(b), which provides magnetic domains within a large martensite plate in Sp.2, i.e. there is no MPB in the field of view. The grain size (about 10 µm) in Fig. 2(b) is much larger than that in Fig. 4 (about 2 µm), and the magnetization vector is almost perpendicular to the surface. The long and slender magnetic domains are separated by the 180° walls as denoted by broken lines. Namely, within a martensite plate, the 180° walls may be produced due to the uniaxial magnetic anisotropy of the martensite. It should be noticed that the lines of magnetic flux are bent with approaching the specimen edge, resulting in a characteristic closure pattern. As a result, formation of magnetic charges at the specimen edge is reduced. Selected area electron diffraction patterns confirmed that the specimen was still crystalline with the 14M structure near the specimen edge. This closure pattern is another style of accommodation of the magnetic flux near the specimen edge observed in the present work. 3.3. Influence of MnO precipitates on the magnetic domain structure We now focus on the effects of MnO precipitates on the magnetic domain structure in the martensite. Since the magnetic domain structure is complicated in the presence of many martensite plates as shown in the previous section, this issue is discussed by observing the magnetic domains in particularly large martensite plates so as to simplify the analysis, i.e. the zig-zag modulation in the magnetic flux due to the MPBs was excluded. Figures 5(a)–(c) show Lorentz microscope images of Sp.1, Sp.2, and Sp.3, respectively, where DWs
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Fig. 5. Lorentz microscope images obtained from (a) Sp.1; (b) Sp.2; and (c) Sp.3. Arrows in (c) indicate the position of particularly large MnO precipitates grown by the heat treatment.
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are seen as bright or dark lines. Stripes of magnetic domains are observed in every specimen, as in the case of Fig. 2(b). In Sp.1, where MnO precipitates are small (10–20 nm) and the population is sparse, DWs are smooth as shown in Fig. 5(a). In contrast, DWs are wavy in Sp.2, where the precipitates are still small but the population is dense, and some of the magnetic domains are branched into two stripes as shown in Fig. 5(b). In Sp.3, where the MnO precipitates have grown to 30–120 nm, DWs are also wavy [Fig. 5(c)]. It is clear that some of the DWs cross the large MnO precipitates as indicated by the arrows in Fig. 5(c). Thus, the shape of DWs was found to be sensitive to the fashion of precipitation. The magnetic domain structure was further studied by electron holography. Figures 6(a)–(c) show the reconstructed phase images of the electron holograms for Sp.1, Sp.2, and Sp.3, respectively, where gray lines represent the lines of magnetic flux, and the arrows the direction of magnetization vectors projected along the electron beam. Broken lines denote the positions of DWs determined by Lorentz microscope observations, whereas dot– dash-lines give the specimen edge. The lines of magnetic flux appear to run along the stripes of magnetic domains in each specimen, although the flux has a tendency to close near the specimen edge to inhibit the formation of magnetic charges as mentioned earlier. The DWs are likely to be 180° walls in every specimen. In Sp.1 with a low density of the precipitates, the lines of magnetic flux are likely to be smooth, i.e. appreciable local modulations are not observed, although periodic small notches in the gray lines due to the artifact of image processing are visible. In Sp.2 with a high density of precipitates, the lines of magnetic flux are wavy, but the modulation is not periodic. This wavy modulation, which is distinct from the artificial notches in Fig. 6(a), is probably attributed to the MnO precipitates. In Sp.3, where the MnO precipitates are quite large, the wavy modulation is also significant. White circles in Fig. 6(c) indicate the positions of large MnO precipitates that were recognized by a bright-field image observation. It is clear that the DWs cross the large MnO precipitates as predicted by the Lorentz microscope observation. Figure 6(c) also demonstrates that the
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direction of the lines of magnetic flux gradually changes near the large MnO precipitates, i.e. the flux is buckled near the precipitates. The wavy modulation as observed in Fig. 6(b) presumably originates from a similar mechanism to that of the buckling. Two principal characters derived from the holography studies are: (1) DWs tend to cross the MnO precipitates; and (2) lines of magnetic flux are buckled near the MnO precipitates, resulting in a peculiar fluctuation of the magnetic flux. These observations appear to be justified by the following mechanisms. To explain the character (1), the magnetostatic energy near the MnO precipitates should be considered. Suppose that a MnO precipitate is present between DWs, i.e. the precipitate does not cross DWs, in the ferromagnetic matrix of the martensite. In this case, magnetic charges appear at the interface between the precipitate and the matrix, and therefore the magnetostatic energy will be significant. However, if the precipitate is across a DW, distribution of the magnetic charges is modified, like a case of dividing a magnetic domain into two parts, so that the magnetostatic energy is much reduced. For example, when a spherical precipitate crosses a DW, the magnetostatic energy is reduced by almost half the magnitude for a precipitate without crossing DWs. Thus, DWs should be stabilized at the positions of the precipitates. The character (2) is likely to be rationalized as follows. Since the MnO is nonmagnetic, its relative permeability should be close to unity. Therefore, the magnetic flux density inside the MnO precipitate (BMnO) is approximated to that of a void in the ferromagnetic matrix. Assuming that the MnO precipitate is spherical and its relative permeability is unity, BMnO can be expressed as Fig. 6. Comparison of the reconstructed phase images for (a) Sp.1; (b) Sp.2; and (c) Sp.3. Closed circles in (c) indicate the positions of MnO precipitates that were recognized in a brightfield image. Positions of the domain walls (DWs) were determined by the Lorentz microscope images. The phase shift in (b) and (c) was amplified three times. The result of (a) was given without the phase amplification since the artificial noise by the image processing was emphasized. See text for details.
BMnO ⫽ BNiMnGa(mNiMnGa ⫹ 2) / 3mNiMnGa
(1)
where BNiMnGa and mNiMnGa represent the magnetic flux density and the relative permeability of the ferromagnetic martensite, respectively. Although mNiMnGa is not yet determined for the sputtered Ni2MnGa films, the relative permeability of ferromagnetic alloys is in general much greater than unity. Hence, BMnO must be much smaller than BNiMnGa. In other words, some flux will turn away the precipitate without passing through it. This
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mechanism reasonably explains the buckled lines of magnetic flux near the precipitates and/or the wavy modulation as seen in Fig. 6(b). Finally, some probable influences of the MnO precipitates to the magnetic field-induced shape deformation in Ni53.6Mn23.4Ga23.0 films are touched on. Since DWs are stabilized at the positions of MnO precipitates, these precipitates will act as the pinning sites against the motion of DWs under an applied magnetic field. Accordingly, the precipitates may impede the rearrangement of magnetic domains by an applied magnetic field, while the rearrangement is likely to be an essential mechanism to the field-induced deformation. From a crystallographic viewpoint, the MnO precipitates with a cubic symmetry are generated in the cubic parent phase when one fabricates the Ni53.6Mn23.4Ga23.0 films. It is reasonable to consider that the precipitates have some orientation relationship to the parent phase so as to reduce the strain energy at the interface. When the martensitic transformation occurs upon cooling, the matrix is heavily distorted to produce the monoclinic martensite, e.g. several percent of elongation and/or contraction depending on the orientation, while the structure of the MnO precipitates remain unchanged. As a result, considerable lattice strain will be produced at the interface between the martensite and precipitates. This lattice strain is thought to hamper the smooth motion of martensite plate boundaries, while the smooth motion is a necessary condition to realize the shape memory effect. Thus, as far as these microscopic observations are considered, it is proposed that formation of the MnO precipitates should be restrained if the Ni2MnGa films are used in actuator applications, while a method to control the MnO precipitates was recently developed by Suzuki et al. [20]. Measurements of the magnetic and mechanical properties of the films are not in contradiction to this proposal [35], which will be reported elsewhere in detail. Nevertheless, the present work has revealed the behavior of magnetic flux in the presence of small nonmagnetic precipitates and/or martensite plates by direct observations with electron holography. The results will be of special importance for development of microactuators by using the films in which the magnetic domain structure may be modified from
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the original configuration in the bulk state, and/or to which applications of conventional techniques to explore magnetic domains are difficult because of the restricted spatial resolutions etc.
4. Conclusions The magnetic domain structure of sputtered Ni53.6Mn23.4Ga23.0 films in the martensitic state, which are hopeful candidates for microactuators driven by a magnetic field, has been investigated by electron holography and Lorentz microscopy. The results are summarized as follows. 1. The sputtered Ni53.6Mn23.4Ga23.0 films were found to produce the 14M martensite with platelike morphology regardless of the fashion of precipitation of MnO. 2. Each martensite plate as crystallographic domain did not correspond to a huge magnetic domain, but it was divided into small magnetic domains. 3. Electron holography studies have displayed peculiar zig-zag lines of magnetic flux, which are due to the high magnetocrystalline anisotropy in the martensite. Near the edge of films, the zigzag lines were obscure, but the lines of magnetic flux appeared to run along the edge so as to suppress the formation of magnetic charges. The observations will be essential when one fabricates small actuators using Ni2MnGa alloy films. 4. Magnetic domain walls tended to exist at the positions of nonmagnetic MnO precipitates, i.e. the domain walls are stabilized by the precipitates. Lines of magnetic flux were found to be buckled or modulated into a wavy shape near the MnO precipitates. These characters were explained by considering some electromagnetic interactions between the nonmagnetic precipitates and the ferromagnetic matrix.
Acknowledgements This work was supported by the Grant-in-Aid for Young Scientists (Y.M.) from the Ministry of
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Education, Science, Sports and Culture of Japan, and by the Special Coordination Funds for Promoting Science and Technology on “Nanohetero Metallic Materials” from the Science and Technology Agency.
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Magnetic domain structure in Ni53.6Mn23.4Ga23.0 shape memory alloy films studied by electron holography and Lorentz microscopy Y. Murakami ∗, D. Shindo, M. Suzuki, M. Ohtsuka, K. Itagaki Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan Received 23 June 2002; received in revised form 17 September 2002; accepted 17 September 2002
Abstract Magnetic domain structure in Ni53.6Mn23.4Ga23.0 films containing nonmagnetic MnO precipitates was examined by electron holography and Lorentz microscopy. Close experiments discovered peculiar zig-zag-shaped lines of magnetic flux, which are presumably typical configuration in the martensite accompanied by many twin plates. Near the edge of the films, this configuration was modified into a distinct form that was favorable to the reduction of magnetic charges. Inside the martensite plates as crystallographic domains, the lines of magnetic flux were wavy due to the electromagnetic interaction with the nonmagnetic MnO precipitates. The observations have promoted an understanding of the magnetic domain structure in ferromagnetic shape memory alloy films, and also provided essential information for developments of magnetic field-driven microactuators using these films. 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Transmission electron microscopy; Martensitic phase transformation; Shape memory alloy; Magnetic domain; Microstructure
1. Introduction Ferromagnetic Ni2MnGa alloys have attracted keen attention of researchers because of the martensitic transformation that can be controlled by a magnetic field [1,2], abnormal softening of the [ζζ0]-TA2 phonon [3], and appearance of the intermediate structure prior to the martensitic transformation upon cooling [4,5] etc. Among these fas∗ Corresponding author. Tel.: +81-22-217-5169; fax: +8122-217-5211. E-mail address: [email protected] (Y. Murakami).
cinating phenomena, the magnetic field-induced shape deformation [6–8] is of special interest from both academic and industrial viewpoints. Some research groups have reported that huge strain, which is comparable with the attainable strain of a giant magnetostrictive material Terfenol-D, can be obtained by the application of a magnetic field to Ni2MnGa alloys [9–11], although the magnitude of the applied field is quite large, e.g. over 10 kOe. This huge strain is presumably achieved by the rearrangement of twin-related martensite plates, i.e. energetically favored martensite plates with a specific orientation grow at the expense of unfavorable plates under the applied magnetic field. From
1359-6454/03/$30.00 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(02)00431-7
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a viewpoint of applications, the magnetic fieldinduced shape deformation has some advantages over the conventional deformation driven by heating. For example, the response to the magnetic field can be quicker than that to temperature. Moreover, the deformation can be done without touching the specimen if the magnetic field is used. Ni2MnGa alloys are quite brittle in a polycrystalline bulk state, and thereby the deformation into a required shape is sometimes in failure. However, the ductility is likely to be improved in a state of thin films [12] that can be obtained by the sputtering method etc. Thus, Ni2MnGa alloy films are hopeful candidates for microactuators driven by a magnetic field. Control of the microstructure is one of the key operations in the development of shape memory alloys, e.g. the microstructure affects the mobility of martensite plates and/or the critical stress to introduce permanent strain in the martensite [13]. In the case of ferromagnetic shape memory alloys like Ni2MnGa, understanding of the magnetic domain structure is important not only for the actuator applications but also for fundamental research. For example, it is of great interest to explore the feature of magnetic domains in a martensitic phase, which exhibits complicated microstructure accompanied by many twins, while this essential problem is likely to remain controversial. On the other hand, when one fabricates the Ni2MnGa alloy films by sputtering, nonmagnetic MnO precipitates are sometimes generated in the ferromagnetic matrix [14]. Effects of the nonmagnetic MnO precipitates on the magnetic domain walls (DWs) and/or the magnetic flux in the martensite films are also important issues, where intensive studies have not yet been carried out. Magnetic domains in Ni2MnGa alloys were discussed by some research groups to date. Rearrangements of twin-related martensite plates under an applied magnetic field were observed by optical microscopy experiments by Likhachev and Ullakko [15], and by Chopra and Ji [16]. Heczko et al. [17] performed scanning electron microscopy studies combined with the Bitter technique. They claimed that the magnetization process in the martensite was governed by the magnetization rotation, although the rotation process was replaced by the
field-driven motion of martensite plates when the direction of the applied field was close to the hard axis. Pan and James [18] observed peculiar fir tree patterns near the martensite plate boundaries by magnetic force microscopy. More recently, de Graef et al. [19] carried out Lorentz microscopy studies in which typical shapes of magnetic domains in the parent phase were discussed. They also mentioned that a change in the easy axis, from [111] in the parent phase to [001] in the tetragonal martensite, did not occur at the same time as the growth of the martensite plates. Despite these accumulated studies, which are likely to focus on the behavior of magnetic domains under an applied magnetic field or during the martensitic transformation, relations between the magnetic domain structure and the microstructure of martensite are not yet fully understood. Probably, an effective approach is electron holography, by which the lines of magnetic flux within the martensite can be visualized with a spatial resolution comparable to the size of thin martensite plates (twin plates) and/or fine precipitates. Moreover, it is possible to compare the observed magnetic domain structure with the conventional transmission electron microscope image for the same area. In the present work, the magnetic domain structure in sputtered Ni2MnGa films, which contained fine precipitates of MnO, were examined by electron holography, Lorentz microscopy, and conventional transmission electron microscopy. The targets are twofold: (1) to reveal the magnetic domain structure in the martensite films accompanied by many twins; and (2) to locate the effects of nonmagnetic MnO precipitates on the magnetic domain structure.
2. Experimental Ni-rich Ni2MnGa films were fabricated by using a radio-frequency magnetron sputtering apparatus (Shibaura, CFS-4ES). Nominal composition of the targets was Ni52.0Mn24.0Ga24.0 when the oxygen as impurity was disregarded, whereas composition of the deposited films determined by inductivity coupled plasma spectroscopy was Ni53.6Mn23.4Ga23.0. See the literature by Suzuki et
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al. [20] for the technical details. The martensitic transformation start temperature (MS) and the Curie temperature (TC) were about 315 K and 345 K, respectively. Thus, the specimens were in a state of ferromagnetic martensite at room temperature. Both the electron holography and Lorentz microscopy studies were performed at room temperature in the present work. To explore the effects of nonmagnetic MnO precipitates on the magnetic domain structure in Ni53.6Mn23.4Ga23.0 films, three types of specimens (Sp.1, Sp.2, and Sp.3) exhibiting distinct fashions of the precipitation were prepared by using the targets with different oxygen content and/or heat treatments with different conditions, as summarized in Table 1. Dark-field images of the MnO precipitates (Fig. 1) make the distinction between these specimens: in Sp.1, the size of the MnO precipitates is 10–20 nm, and the population is sparse [Fig. 1(a)]; in Sp.2, the size of the MnO precipitates is still 10–20 nm, but the population is dense [Fig. 1(b)]; in Sp.3, the size of the MnO precipitates ranges from 30 to 120 nm, and the population is dense [Fig. 1(c)]. Structure and morphology of the martensite were studied using a JEM-2010⍀ transmission electron microscope equipped with an omega-type energy filter, by which the strong background arising from inelastic electron scattering was effectively reduced [21,22]. Lorentz microscopy and electron holography experiments were carried out with a JEM-3000F transmission electron microscope, to which a field emission gun and a biprism were installed. A special pole piece for magnetic domain observation was utilized, in which the magnetic field near the specimen was about 0.2 mT [23]. Holograms were recorded with conventional films and they were digitized by a film scanner with a resolution 8 µm/pixel.
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Here the essence of electron holography [21,24] is briefly explained. Application of positive voltage to a biprism, which is integrated into the electron microscope, generates interference fringes of the object wave passing through the specimen with the reference wave through the vacuum, as shown in Fig. 2(a). The interference fringes should be straight if a phase shift of the object wave relative to the reference wave is absent. When the phase shift arises due to the magnetic flux of a specimen, the interference fringes deviate from the straight lines, as shown in Fig. 2(a), lower region. [The interference fringes are almost straight outside the specimen in Fig. 2(a), upper region, because of the little stray field of the closure magnetic domains, as described later. Strictly speaking, the in-plane component of magnetic flux, i.e. perpendicular to the incident electrons, can be measured by the holography experiment [25,26]. The electrostatic potential also contributes to the phase shift although this effect is significant when the specimen thickness steeply changes, as in the case of small particles. Change in the specimen thickness is rather gradual in the fields of view of the present specimens, compared with small particles etc.] It is therefore possible to derive the information on the magnetic flux of the specimen by analyzing the phase shift. The phase shift f(x,y) can be evaluated by the Fourier transform of the electron hologram recorded as a digital image, as described elsewhere in detail [21], where x and y represent the coordinates in the hologram. If the phase shift is displayed in terms of cosf(x,y), a reconstructed phase image as shown in Fig. 2(b) is obtained, where the direction and the density of gray lines correspond to the direction and the density of lines of magnetic flux projected along the electron beam, respectively.
Table 1 Size and population of MnO precipitates in the three kinds of specimens. Conditions of heat treatment are also listed Specimen
Size of MnO
Population of MnO
Heat treatment
Sp.1 Sp.2 Sp.3
10–20 nm 10–20 nm 30–120 nm
sparse dense dense
1073K — 36ks 1073K — 36ks 1273K — 36ks
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Fig. 2. (a) Typical electron hologram; and (b) the reconstructed phase image. See text for details as to how the reconstructed phase image was obtained from the hologram. The measured phase shift was amplified three times to give the reconstructed phase image [21]. Observations were done using Sp.2.
Fig. 1. Dark-field images of the MnO precipitates in (a) Sp.1; (b) Sp.2; and (c) Sp.3.
3. Results and discussion 3.1. Morphology and structure of martensite Figure 3 (left-hand side) shows bright-field images of the three types of specimens: (a) Sp.1; (b) Sp.2; and (c) Sp.3, in the martensitic state.
Typical electron diffraction patterns in these specimens are also provided in the right-hand side. All of the specimens show a typical morphology of the martensite consisting of thin plates. These martensite plates are crystallographically equivalent but their orientations are different. In fact the martensite plates can be classified into several groups depending on their orientations, and the groups are generally called “martensite variants” (MVs). Neighbored martensite plates have specific twinning relations [27]. The MnO precipitates can be observed in the bright-field images, e.g. at the positions of open circles, although the contrast is weak. Extra spots as marked by 111MnO in the diffraction patterns also confirm the presence of MnO precipitates. It is clear that the unit reciprocal lattice distance, e.g. between the transmitted beam and the 0014M reflection, is divided into seven in all of the diffrac-
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are observed in the 14M martensite of other shape memory alloys like Ni50.0Al16.0Mn34.0 [31] and Ni63.0Al37.0 [27,32]. 3.2. Characteristic features of magnetic domains in martensite films Figure 4(a) shows a typical bright-field image of Sp.1. The white lines represent grain boundaries. Each grain is filled with martensite plates containing planar defects as mentioned earlier. Figure 4(b) offers a Lorentz microscope image for the same field of view as that of Fig. 4(a), and DWs can be
Fig. 3. Bright-field images (left side) and electron diffraction patterns (right side) observed in (a) Sp.1; (b) Sp.2; and (c) Sp.3.
tion patterns. The result indicates that the crystal structure of the martensite is 14M, which is a long period stacking order structure of basal planes originating from {110} planes of the parent phase. (Although the structure of martensite is the sevenlayer one, it is denoted as 14M in the terminology by Otsuka et al. [13]. See the previous reports [28,29] for more details on the crystal structure.) Hence, all of the specimens were found to show the same crystal structure of martensite, and exhibit typical plate-like morphology as observed in other shape memory alloys, regardless of the fashion of the precipitation. There are many planar defects like stacking faults on the basal planes of martensite, as evidenced by the streak along the c∗ axis in the diffraction patterns and/or striations within the martensite plates in the bright-field images. These planar defects are likely to be due to the lattice invariant shear that is necessary to achieve the invariant plane strain condition of the martensitic transformation [30]. In fact, similar planar defects
Fig. 4. (a) Bright-field image; (b) Lorentz microscope image; and (c) reconstructed phase image for the enclosed area of (b). White lines in (a)–(c) represent grain boundaries. The measured phase shift was amplified three times. Observations were done using Sp.1. See text for details.
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observed as bright or dark lines in the Fresnel mode of Lorentz microscopy [21]. It is clear that the martensite plate boundaries (MPBs) themselves correspond to DWs. Moreover, one notices the presence of DWs crossing the martensite plates, e.g. the dark lines marked by “DW” in the upper right part of Fig. 4(b). Thus, the observation has deduced that each martensite plate, as a crystallographic domain, does not correspond to a large magnetic domain but is divided into several small magnetic domains. The result is consistent with that of CoNiAl ferromagnetic shape memory alloys having a distinct crystal structure, L10 [33]. The magnetic domain structure is discussed more in detail with a reconstructed phase image [Fig. 4(c)], which was obtained from the hologram for the enclosed area in Fig. 4(b). Gray lines in Fig. 4(c) represent the lines of magnetic flux projected along the electron beam, and the arrows give directions of magnetization vectors. As shown in the area “Z” of the grain B, the lines of magnetic flux are modulated into peculiar zig-zag lines. The abrupt change in direction occurs when the lines of magnetic flux cross the MPBs. That is, since the magnetocrystalline anisotropy of the martensite is large [7,11,34] and the orientation of the easy magnetization axis is different between the neighbored martensite plates, the direction of the lines of magnetic flux changes steeply at the MPBs. The zig-zag-shaped lines of magnetic flux will be a characteristic feature of the martensite with multiple variants. It is worth noting that the zig-zag lines are obscure near the specimen edge even in the grain B. The lines of magnetic flux are rather parallel to the specimen edge, and thereby the magnetization vector deviates from the easy axis. In fact one sees the lines of magnetic flux that are passing through the grains A, B, and C along the specimen edge, although these grains have distinct orientations. Probably, near the specimen edge, reduction of the magnetostatic energy will be more significant over the magnetocrystalline anisotropy energy, and thereby the lines of magnetic flux tend to be parallel to the edge. As a result, formation of the magnetic charge will be reduced, although some stray field is still observed in Fig. 4(c). The wall marked by “DW” separates the lower area showing zig-zag lines from the upper area exhibiting lines of magnetic
flux that are nearly parallel to the edge in the grain B. In Fig. 4(c), some other DWs are present near the grain boundaries, e.g. the grain C has the DWs as indicated by dashed lines near the boundary to the grain D. These DWs are presumably generated as a result of the balance of the magnetocrystalline anisotropy energy, magnetostatic energy, and the exchange energy near the boundaries etc. Thus, the present observations have revealed essential features of magnetic domains that are produced in the martensite films. Another case of accommodation in the magnetic flux near the specimen edge is observed in Fig. 2(b), which provides magnetic domains within a large martensite plate in Sp.2, i.e. there is no MPB in the field of view. The grain size (about 10 µm) in Fig. 2(b) is much larger than that in Fig. 4 (about 2 µm), and the magnetization vector is almost perpendicular to the surface. The long and slender magnetic domains are separated by the 180° walls as denoted by broken lines. Namely, within a martensite plate, the 180° walls may be produced due to the uniaxial magnetic anisotropy of the martensite. It should be noticed that the lines of magnetic flux are bent with approaching the specimen edge, resulting in a characteristic closure pattern. As a result, formation of magnetic charges at the specimen edge is reduced. Selected area electron diffraction patterns confirmed that the specimen was still crystalline with the 14M structure near the specimen edge. This closure pattern is another style of accommodation of the magnetic flux near the specimen edge observed in the present work. 3.3. Influence of MnO precipitates on the magnetic domain structure We now focus on the effects of MnO precipitates on the magnetic domain structure in the martensite. Since the magnetic domain structure is complicated in the presence of many martensite plates as shown in the previous section, this issue is discussed by observing the magnetic domains in particularly large martensite plates so as to simplify the analysis, i.e. the zig-zag modulation in the magnetic flux due to the MPBs was excluded. Figures 5(a)–(c) show Lorentz microscope images of Sp.1, Sp.2, and Sp.3, respectively, where DWs
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Fig. 5. Lorentz microscope images obtained from (a) Sp.1; (b) Sp.2; and (c) Sp.3. Arrows in (c) indicate the position of particularly large MnO precipitates grown by the heat treatment.
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are seen as bright or dark lines. Stripes of magnetic domains are observed in every specimen, as in the case of Fig. 2(b). In Sp.1, where MnO precipitates are small (10–20 nm) and the population is sparse, DWs are smooth as shown in Fig. 5(a). In contrast, DWs are wavy in Sp.2, where the precipitates are still small but the population is dense, and some of the magnetic domains are branched into two stripes as shown in Fig. 5(b). In Sp.3, where the MnO precipitates have grown to 30–120 nm, DWs are also wavy [Fig. 5(c)]. It is clear that some of the DWs cross the large MnO precipitates as indicated by the arrows in Fig. 5(c). Thus, the shape of DWs was found to be sensitive to the fashion of precipitation. The magnetic domain structure was further studied by electron holography. Figures 6(a)–(c) show the reconstructed phase images of the electron holograms for Sp.1, Sp.2, and Sp.3, respectively, where gray lines represent the lines of magnetic flux, and the arrows the direction of magnetization vectors projected along the electron beam. Broken lines denote the positions of DWs determined by Lorentz microscope observations, whereas dot– dash-lines give the specimen edge. The lines of magnetic flux appear to run along the stripes of magnetic domains in each specimen, although the flux has a tendency to close near the specimen edge to inhibit the formation of magnetic charges as mentioned earlier. The DWs are likely to be 180° walls in every specimen. In Sp.1 with a low density of the precipitates, the lines of magnetic flux are likely to be smooth, i.e. appreciable local modulations are not observed, although periodic small notches in the gray lines due to the artifact of image processing are visible. In Sp.2 with a high density of precipitates, the lines of magnetic flux are wavy, but the modulation is not periodic. This wavy modulation, which is distinct from the artificial notches in Fig. 6(a), is probably attributed to the MnO precipitates. In Sp.3, where the MnO precipitates are quite large, the wavy modulation is also significant. White circles in Fig. 6(c) indicate the positions of large MnO precipitates that were recognized by a bright-field image observation. It is clear that the DWs cross the large MnO precipitates as predicted by the Lorentz microscope observation. Figure 6(c) also demonstrates that the
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direction of the lines of magnetic flux gradually changes near the large MnO precipitates, i.e. the flux is buckled near the precipitates. The wavy modulation as observed in Fig. 6(b) presumably originates from a similar mechanism to that of the buckling. Two principal characters derived from the holography studies are: (1) DWs tend to cross the MnO precipitates; and (2) lines of magnetic flux are buckled near the MnO precipitates, resulting in a peculiar fluctuation of the magnetic flux. These observations appear to be justified by the following mechanisms. To explain the character (1), the magnetostatic energy near the MnO precipitates should be considered. Suppose that a MnO precipitate is present between DWs, i.e. the precipitate does not cross DWs, in the ferromagnetic matrix of the martensite. In this case, magnetic charges appear at the interface between the precipitate and the matrix, and therefore the magnetostatic energy will be significant. However, if the precipitate is across a DW, distribution of the magnetic charges is modified, like a case of dividing a magnetic domain into two parts, so that the magnetostatic energy is much reduced. For example, when a spherical precipitate crosses a DW, the magnetostatic energy is reduced by almost half the magnitude for a precipitate without crossing DWs. Thus, DWs should be stabilized at the positions of the precipitates. The character (2) is likely to be rationalized as follows. Since the MnO is nonmagnetic, its relative permeability should be close to unity. Therefore, the magnetic flux density inside the MnO precipitate (BMnO) is approximated to that of a void in the ferromagnetic matrix. Assuming that the MnO precipitate is spherical and its relative permeability is unity, BMnO can be expressed as Fig. 6. Comparison of the reconstructed phase images for (a) Sp.1; (b) Sp.2; and (c) Sp.3. Closed circles in (c) indicate the positions of MnO precipitates that were recognized in a brightfield image. Positions of the domain walls (DWs) were determined by the Lorentz microscope images. The phase shift in (b) and (c) was amplified three times. The result of (a) was given without the phase amplification since the artificial noise by the image processing was emphasized. See text for details.
BMnO ⫽ BNiMnGa(mNiMnGa ⫹ 2) / 3mNiMnGa
(1)
where BNiMnGa and mNiMnGa represent the magnetic flux density and the relative permeability of the ferromagnetic martensite, respectively. Although mNiMnGa is not yet determined for the sputtered Ni2MnGa films, the relative permeability of ferromagnetic alloys is in general much greater than unity. Hence, BMnO must be much smaller than BNiMnGa. In other words, some flux will turn away the precipitate without passing through it. This
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mechanism reasonably explains the buckled lines of magnetic flux near the precipitates and/or the wavy modulation as seen in Fig. 6(b). Finally, some probable influences of the MnO precipitates to the magnetic field-induced shape deformation in Ni53.6Mn23.4Ga23.0 films are touched on. Since DWs are stabilized at the positions of MnO precipitates, these precipitates will act as the pinning sites against the motion of DWs under an applied magnetic field. Accordingly, the precipitates may impede the rearrangement of magnetic domains by an applied magnetic field, while the rearrangement is likely to be an essential mechanism to the field-induced deformation. From a crystallographic viewpoint, the MnO precipitates with a cubic symmetry are generated in the cubic parent phase when one fabricates the Ni53.6Mn23.4Ga23.0 films. It is reasonable to consider that the precipitates have some orientation relationship to the parent phase so as to reduce the strain energy at the interface. When the martensitic transformation occurs upon cooling, the matrix is heavily distorted to produce the monoclinic martensite, e.g. several percent of elongation and/or contraction depending on the orientation, while the structure of the MnO precipitates remain unchanged. As a result, considerable lattice strain will be produced at the interface between the martensite and precipitates. This lattice strain is thought to hamper the smooth motion of martensite plate boundaries, while the smooth motion is a necessary condition to realize the shape memory effect. Thus, as far as these microscopic observations are considered, it is proposed that formation of the MnO precipitates should be restrained if the Ni2MnGa films are used in actuator applications, while a method to control the MnO precipitates was recently developed by Suzuki et al. [20]. Measurements of the magnetic and mechanical properties of the films are not in contradiction to this proposal [35], which will be reported elsewhere in detail. Nevertheless, the present work has revealed the behavior of magnetic flux in the presence of small nonmagnetic precipitates and/or martensite plates by direct observations with electron holography. The results will be of special importance for development of microactuators by using the films in which the magnetic domain structure may be modified from
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the original configuration in the bulk state, and/or to which applications of conventional techniques to explore magnetic domains are difficult because of the restricted spatial resolutions etc.
4. Conclusions The magnetic domain structure of sputtered Ni53.6Mn23.4Ga23.0 films in the martensitic state, which are hopeful candidates for microactuators driven by a magnetic field, has been investigated by electron holography and Lorentz microscopy. The results are summarized as follows. 1. The sputtered Ni53.6Mn23.4Ga23.0 films were found to produce the 14M martensite with platelike morphology regardless of the fashion of precipitation of MnO. 2. Each martensite plate as crystallographic domain did not correspond to a huge magnetic domain, but it was divided into small magnetic domains. 3. Electron holography studies have displayed peculiar zig-zag lines of magnetic flux, which are due to the high magnetocrystalline anisotropy in the martensite. Near the edge of films, the zigzag lines were obscure, but the lines of magnetic flux appeared to run along the edge so as to suppress the formation of magnetic charges. The observations will be essential when one fabricates small actuators using Ni2MnGa alloy films. 4. Magnetic domain walls tended to exist at the positions of nonmagnetic MnO precipitates, i.e. the domain walls are stabilized by the precipitates. Lines of magnetic flux were found to be buckled or modulated into a wavy shape near the MnO precipitates. These characters were explained by considering some electromagnetic interactions between the nonmagnetic precipitates and the ferromagnetic matrix.
Acknowledgements This work was supported by the Grant-in-Aid for Young Scientists (Y.M.) from the Ministry of
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Education, Science, Sports and Culture of Japan, and by the Special Coordination Funds for Promoting Science and Technology on “Nanohetero Metallic Materials” from the Science and Technology Agency.
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