Off-Stoichiometric Ni-Mn-Ga Alloys
7.1.3. Phase Transition Observed by STM
This section discusses temperature-dependent STM measurements revealing an austenite-martensite phase transition of an off-stoichiometric Ni-Mn-Ga epitaxial film sample (EF4). Figure 7.10 demonstrates a series of topographic STM images of a Ni-Mn-Ga surface obtained during a continuous cooling pro-cedure. While the image surface area was repeatedly scanned (starting at the bottom of the image) by the STM tip the temperature was slowly reduced with a cooling rate of≈ 10K/h. Hence every recorded linescan maps the sample surface at a different temperature. Red-blue color bars on the left-hand side of STM images indicate the sample temperature for a certain linescan. Fig. 7.10a displays an austenitic Ni-Mn-Ga surface, where several atomic steps can be recognized. The austenitic state can be detected due to the absence of meso-scopic surface corrugation caused by twin variant formation. However, in a small region highlighted by a black rectangle a surface corrugation can be detected. Also small periodic corrugation lines can be detected (marked by arrows). Based on preceding observations of the martensitic Ni-Mn-Ga sur-face this feature can be assigned to mesoscopic twin variants. It is remarkable that this feature is linked to a crystal lattice defect, namely screw dislocations, which are visible in the surface area highlighted by a black rotated square.
Fig. 7.10a displays the same surface at lower sample temperatures. The lower
7.1. Martensitic Phase
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Figure 7.10.: Caption next page.
Figure 7.10.:(Previous page.) A series of STM topography images of a276× 276nm2large area obtained while continuously cooling the epitaxial film sample (EF4) with a cooling rate of≈10K/h. The measurements show the modifica-tion of the Ni-Mn-Ga surface during the martensitic transformamodifica-tion (UT= 1V, IT = 0.15nA). (a)Sample is in the austenitic state with small region exhibit-ing twinnexhibit-ing and superstructure (marked by arrows) features in the vicinity of screw dislocations. (b)At a sample temperature of ≈ 289.5K the sample transforms locally to martensite at ms time scale (see scan profile 2).(c)and(d) Upon further cooling the observed sample surface area completely transforms to martensite.
part of the image still shows an austenitic surface. Yet, for a sample temperature of≈289.5K an abrupt dramatic change of the sample surface topography can be observed. A zig-zag shaped surface topography is formed, indicating the transition to the martensitic state. Also the formation of the nanoscale super-structure can be observed in the topography gradient image. It is remarkable how fast this transition proceeds. The presented height profiles allow an esti-mation of the twinning foresti-mation speed. Profile shows the the surface in the austenitic state. During the linescan the transition happens and twin vari-ants are formed. It can be recognized as a strong peak of the topography signal.
It must be assumed that this signal does not resemble the surface topography, but appears due to the abrupt change of the tunneling condition (surface-tip distance) and the associated regulation of the STM tip. However, it can be seen that the formation of martensitic twin variants happens on a ms scale, since the time interval between the data points is just≈1ms. Fig. 7.10c shows the surface area after the observed transformation during the ongoing cooling procedure.
The image reveals the surface in a mixed martensite/austenite state. It means that the transition observed in Fig. 7.10b took place only for the upper right part of the observed area. The complete transformation of the scanned area happened after the temperature was lowered further to≈287K (Fig. 7.10d).
The presented results show that the martensitic transformation proceeded locally stepwise, with discrete areas of the Ni-Mn-Ga surface transforming to martensite at a time scale of ms. The whole observed area transformed in a temperature interval of∆TM = 2.5K. This observation goes hand in hand with
7.1. Martensitic Phase literature reports on transformation behavior of film samples. It was reported that martensitic domain growth is limited by the speed of sound and hence occurs on a time scale of ps [162]. Also the transformation of film samples was reported to proceed in a much larger temperature interval compared to single crystal bulk samples [31, 163]. It was also observed that the martensitic phase nucleates at a crystallographic defect site, in that case a screw dislocation.
However, it should be also mentioned that prior to the presented measurement the sample was heated and therefore transformed to the austenitic phase. It can therefore not be ruled out that the the observed martensitic variants are a residual feature. In this case the screw dislocation served as a pinning site and still locally influenced the transformation behavior. The presented measure-ment show that STM is a powerful technique to perform temperature and time resolved studies on martensitic transformation behavior. However, one should take into account that this is only possible for thin film samples. STM tip can follow sudden height changes of only several nanometers. Therefore, bulk sam-ples which exhibit large volume changes during the martensitic transformation are not suited for this measurement procedure.
7.1.4. Summary
In summary, the study of epitaxial off-stoichiometric martensitic Ni-Mn-Ga films by means of scanning tunneling microscopy and low energy electron diffraction revealed the existence of anincommensurately modulated 7M mar-tensitic phase. A hierarchical microstructure, which is also termed as “twins within twins”, was observed in the experiments. The martensitic nanotwinned superstructure becomes apparent as a distinct surface topography line pat-tern. The linear pattern is oriented perpendicular to the orientation of twin boundaries of mesoscopic 7M twin variants. The modulation periodicity is found to be very irregular and frequently appearingstacking faultscould be de-tected. The modulation topography signal shows a stacking type modulation character.
In contrast to the 5M phase of the stoichiometric Ni2MnGa sample, where a harmonic displacement of (110) planes was identified in the martensitic phase,
the observed superstructure of 7M martensitic phase was identified as a nano-twinned structure. The superstructure could be described as an incommensu-rately modulated martensitic phase constructed from nanotwinned variants which coincides well with the predicted modulation of an adaptive martensitic phase. In accordance with the adaptive approach, coarsening of the nanotwins in discrete steps was observed. A disagreement with the adaptive concept was observed, namely a nanotwin width ratiod1/d2 which is much larger then a theoretically expected one. Also a strong deviation from the ideal(5¯2)stacking sequence was identified.
One can definitely conclude that the character of the modulation of off-stoichiometric samples differs fundamentally from the modulation nature of the stoichiometric Ni2MnGa sample. However, also the question arises whether the type of the studied samples might play a role. Off-stoichiometric Ni-Mn-Ga samples presented so far are epitaxial film samples grown on MgO substrates.
The rigid substrate is an ideal habit plane, a prerequisite for the formation of an adaptive phase, which is present also at temperatures belowTM. The ques-tion is whether the presence of the rigid substrate triggers the formaques-tion of an adaptive phase. Figure 7.11 shows STM images performed on two bulk off-stoichiometric single crystal samples. Surface topography features clearly show close resemblance with the superstructure of off-stoichiometric film samples:
a peak-to-peak signal in the order of several tens of picometers and a zig-zag shaped corrugation. The modulation period is≈3.1nm and≈4.1nm, which largely exceeds the modulation periods expected for the five-fold as well as for the seven-fold modulation. Hence, the deviation of the chemical composition from the stoichiometric Ni2MnGa chemical order must play a decisive role defining the superstructure nature.
7.1. Martensitic Phase
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Figure 7.11.: STM images of the Ni-Mn-Ga surface obtained in the martensitic state at room temperature for two off-stoichiometric single crystals with dif-ferent compositions: ((a): SC1,UT = 0.1V,IT = 1.48nA, (b): SC2,UT = 1V, IT = 0.2nA). Height profiles extracted along the lines indicated in STM images are shown in the lower part of the figure. Measurements were performed on twin variants showing out-of-plane surface corrugation and indicate a stacking type superstructure.