Magnetic microscopies: the new additions
Introduction
The long history of imaging magnetic domains in ferromagnetic materials started with Bitter, Hamos, and Theissen using Bitter solutions in 1931 [1], [2], [3]. Since this time many other powerful techniques have been developed which include magnetic force microscopy (MFM), electron holography (EH), scanning electron microscopy with polarization analysis (SEMPA), Lorentz microscopy, magneto-optic microscopy (MO) which includes both Kerr and Faraday effects, magnetic circular dichroism microscopy, and scanning SQUID microscopy. It is interesting to note that all of the above listed microscopies, including Bitter solutions, are currently employed by magnetics researchers as each has its own particular strengths for solving particular magnetic problems. A second interesting point to note is that more than half of the above microscopies, are relatively young, on the order of a decade or less. In the present work, we will provide a limited introduction to the newer magnetic microscopies, MFM, EH, SEMPA, and scanning SQUID, and magneto-optic microscopy (which has benefited from technological advances).
Section snippets
Magnetic force microscopy (MFM)
The modern magnetic force microscope (MFM) is an offspring of the atomic force microscope using noncontact imaging with a magnetically sensitive cantilever to sense or image the magnetic field (for an earlier magnetic microscope design, see Refs. [4], [5]). The MFM offers sub-100 nm resolution without the need for involved sample preparation [6], [7], [8], [9], [10] and can image the fields from magnetic structures with protective overcoats. These advantages make it very useful for a variety of
Electron holography
The combination of the high spatial resolution inherent in scanning electron microscopes and the absolute phase measurements extracted from electron holograms allows magnetometry to be performed on nanometer-sized regions with high precision [26], [27], [28], [29]. Electron holography is an indirect (M is not directly measured), transmission-based method limited to thin (200 nm maximum for 100 keV microscopes) conducting specimen. Typical phase images reconstructed from electron holograms have 5–7
Scanning electron microscopy with polarization analysis (SEMPA)
In the process of scanning electron microscopy, there are secondary electrons ejected from the specimen. These secondary electrons retain the spin polarization information of their initial state. Initial studies [31] of the energy distribution of the spin polarized electrons from a ferromagnetic specimen suggested measurements of this type may be useful for magnetic imaging. From this pioneering work, magnetic imaging with SEMPA has developed into a very respected tool for micromagnetics
Kerr microscopy
Magneto-optic imaging is another technique for imaging the magnetic domains in magnetic materials. It originates from the interaction of a photon with the spin–orbit coupling of the magnetic electrons which provides image contrast proportional to the magnetization. This makes the technique non-invasive allowing one to probe direction of the magnetization without any alterations of the magnetization. In addition, it has a relatively large light amplitude penetration depth of about 20 nm [38], [39]
Scanning SQUID magnetometry
The scanning SQUID microscope measures magnetic flux with a sensitivity of about 1×10E-6(hc/2e) Hz−1/2 where hc/2e is the flux quantum. In practical terms, this means there is more or less a linear trade off between the field sensitivity and spatial resolution with larger loops giving increased field sensitivity and decreased spatial resolution.
Of the various magnetic imaging techniques described here, scanning SQUID magnetometry is the most sensitive in terms of absolute magnetic field. As an
Acknowledgements
The authors would like to thank J. Wittborn, K.V. Rao, M. Scheinfein, D. Pierce, A. Hubert, R. Schaefer, and J. Kirtley for their help in generating this work. In many respects we acted more as editors than authors and thank them for their input. This work was supported by the ONR under grant no. N00014-94-1-0123.
References (50)
- et al.
J. Magn. Magn. Mater.
(1998) - et al.
J. Magn. Magn. Mater.
(1987) Phys. Rev.
(1931)Phys. Rev.
(1932)- et al.
Z. Physik
(1932) Czechosl. J. Phys.
(1955)- et al.
Czechosl. J. Phys.
(1956) - et al.
J. Appl. Phys.
(1993) - M. Lederman, S. Schultz, M. Ozaki, Phys. Rev. Lett. 731986...
- et al.
J. Appl. Phys.
(1996)
J. Appl. Phys.
Mater. Res. Soc. Symp. Proc.
Appl. Phys. Lett.
Appl. Phys. Lett.
J. Appl. Phys.
Appl. Phys. Lett.
Phys. Stat. Sol. (b)
Appl. Phys. Lett.
Appl. Phys. Lett.
Appl. Phys. Lett.
J. Appl. Phys.
IEEE Trans. Magn.
Appl. Phys. Lett.
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