Transmission Electron Microscopy on Magnetic Phase Transformations in Functional Materials
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Transmission Electron Microscopy on Magnetic Phase Transformations in Functional Materials Y. MURAKAMI, T. YANO, D. SHINDO, R. KAINUMA, and T. ARIMA This article reports our recent studies on the magnetic imaging by electron holography or Lorentz microscopy, which is combined with other supplementary techniques related to transmission electron microscopy (TEM). By combining with dark-field imaging, the ‘‘one-to-one correspondence’’ between the magnetic domain walls and antiphase boundaries (APBs) in a ferromagnetic shape memory alloy (SMA) Ni2Mn(Al,Ga) was revealed. In order to examine both magnetism and conductivity in a nanoscale area, we have developed a double probe piezodriving holder, by which microprobes can be brought in contact with the portion of interest. The conductivity measurement can be simultaneously performed with Lorentz microscopy or electron holography. I. INTRODUCTION
MAGNETIC imaging is particularly important in the studies of magnetic phase transformations in alloys and ceramics. Among the several techniques of magnetic imaging established so far, both electron holography[1,2] and Lorentz microscopy[2,3] have advantages over the other methods. It is possible to obtain a clear magnetic flux map by electron holography, wherein the phase shift of electrons traversing a thin foil specimen is analyzed. The position or thickness of magnetic domain walls can be determined easily by a defocus mode of Lorentz microscopy. Because these methods are related to transmission electron microscopy (TEM), the spatial resolution is sufficient for the imaging of magnetic nanostructures. Another significant advantage of the electron holography or Lorentz microscopy is that each of them can be combined with other methods of TEM, which provide considerable information on the crystallographic microstructure. For example, in the research and development of ferromagnetic shape memory alloys (SMAs),[4–8] which supply large magnetostriction in an applied magnetic field, it is essential to understand the correlation between the magnetic domains and crystallographic microstructures, e.g., twinrelated martensite variants and antiphase boundaries (APBs) due to chemical ordering. As described later in detail in Section III–A, it is effective to combine dark-field imaging and electron holography in order to examine the domain structure in ferromagnetic SMAs such as a Ni2Mn(Al,Ga) alloy.[9] The Ni2Mn(Al,Ga) alloys undergo a cubic-to-monoclinic martensitic transformation with specific compositions. The cubic parent phase has an L21ordered structure, which contains APBs due to the chemical ordering from the disordered B2 state (disordered state with respect to Mn, Al and Ga). It is noted that the APBs have attracted renewed interest with respect to the development of excellent permanent magnets;[10–13] thermally formed Y. MURAKAMI, Lecturer, T. YANO, Student, D. SHINDO, R. KAINUMA, and T. ARIMA, Professors, are with the Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan. Contact e-
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