Magnetic domain structures in thin foils may be revealed in a number of ways in the transmission electron microscope. The most commonly used modes (Fresnel, Foucault, holographic, differential phase contrast and low angle diffraction) are described and intercompared. The Fresnel and Foucault modes are simple to implement but provide a mainly qualitative description of the spatial variation of induction throughout the foil. Quantitative information may be obtained from the other two imaging modes but they require more sophisticated equipment. Electron diffraction is particularly useful for studying periodic magnetic structures. Examples of the use of these, and further specialised techniques, are presented by reference to some applications of current interest within magnetism.

https://iopscience.iop.org/article/10.1088/0022-3727/17/4/003/pdf

The investigation of magnetic domain structures in thin foils by electron microscopy

The recently discovered oscillating exchange effect in iron–chromium multilayers can best be studied on an epitaxial iron sandwich with a wedge shaped chromium interlayer. Domain patterns are analyzed as well as magnetization curves on such samples using magnetooptical techniques. Unusual domains are found in zones around the transitions between ferromagnetic and antiferromagnetic exchange. Also the magnetization curves for larger chromium thickness show some peculiar features. The interpretation of these patterns invokes coupling between the iron layers which favours a right angle between the magnetization directions in both layers. Such a non-collinear interaction can be derived phenomenologi-cally from a biquadratic coupling as introduced earlier in the analysis of spin structures in oxides. A biquadratic coupling can be understood either as a second order Heisenberg or a second order Dzyaloshinskii interaction. In the course of these investigations domain observations proved to be a useful and sensitive method to detect locally the nature of interactions between ferromagnetic layers.



Die kiurzlich entdeckte oszillierende Wechselwirkung in Eisen–Chrom-Vielfachschichten kann am besten an einer epitaxialen Schichtanordnung mit einer keilformigen Chromzwischenschicht studiert werden. Die auf solchen Proben beobachtbaren Domanenstrukturen sowie die Magnetisierungskurven werden mit magnetooptischen Verfahren untersucht. Im Bereich der Ubergange zwischen ferroma-gnetischer und antiferromagnetischer Kopplung werden Zonen ungewohnlicher Muster entdeckt. Auch die Magnetisierungskurven fur grosere Chrom-Schichtdicken zeigten eigentumliche Zuge. Die Beobach-tungen lassen sich erklaren, wenn eine zusatzliche Kopplung der Schichten angenommen wird, die einen rechten Winkel zwischen den Magnetisierungsrichtungen in beiden Schichten begunstigt. Phanomenologisch ergibt sich eine solche nicht-kollineare Kopplung aus der Annahme einer biquadrati-schen Wechselwirkung, wie sie fruher in der Analyse von Spinstrukturen magnetischer Oxide eingefuhrt wurde. Eine biquadratische Kopplung Iast sich entweder als Heisenberg- oder Dzyaioshinskii-Wechselwirkung zweiter Ordnung interpretieren. Domanenbeobachtungen erwiesen sich im Laufe dieser Untersuchung als empfindliche Methode zur Untersuchung des Charakters der Kopplung in Vielfachschichten.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pssa.2211250225

Domain Observations on FeCrFe Layered Structnres. Evidence for a Biquadratic Coupling Effect

Recently, significant interest has emerged in fabricated systems that mimic the behavior of geometrically frustrated materials. We present the full realization of such an artificial spin ice system on a two-dimensional kagome lattice and we demonstrate rigid adherence to the local ice rule by directly counting individual pseudospins. The resulting spin configurations show not only local ice rules and long-range disorder, but also correlations consistent with spin ice Monte Carlo calculations. Our results suggest that dipolar corrections are significant in this system, as in pyrochlore spin ice, and that they open a door to further studies of frustration in general.

https://terpconnect.umd.edu/~cumings/PDF%20Publications/31.PRL77qi.pdf

Direct observation of the ice rule in an artificial kagome spin ice

Some new developments are reported in the technique of observing magnetization in thin ferromagnetic films by transmission electron microscopy, and the results of some observations are described that contribute to an understanding of the magnetic character of thin films. Two modes of operation of a microscope are explained, one using projector magnification alone, and one using both objective and projector lenses. The intensity distribution images of the magnetization distribution in a thin film is derived from the Lorentz force acting on an electron passing through the film. The intensity distributions for some simple domain walls are computed and compared in various ways with experimental observations. A magnetization ripple model is proposed and justified to explain an intensity texture observed principally when projector magnification alone is used. The texture is shown to be an orthogonal map of the magnetization of the film, and to be of use in interpreting wall structures. High magnification and resolution wall micrographs are obtained using objective magnification, and a direct measurement of the wall thickness and shape can be made in this way. 180° walls in a Fe‐Ni alloy film of about 200 A thickness are found to have a width of 2δ = 1500 A, using the wall model cot−1θ = −sinhx/δ. The effect of finite resolution on the measurements is discussed. It is shown that the magnetization distribution of a wall can be obtained from the measured intensity distribution by inverting the parametric equation solution for the intensity distribution.

Determination of Magnetization Distribution in Thin Films Using Electron Microscopy

Magnetic domain observations have been used to clarify the relationship between metallurgical structure and the core loss, permeability, and magnetostriction of grain-oriented 3% Si-Fe. Both the prediction of domain structures based on anisotropy, magnetostatics, stress, etc., and the various methods of making domain observations are reviewed. Both static and dynamic domain structures are shown and used to illustrate the relationships among domain structure, metallurgical structure, and magnetic properties. Core losses are shown to depend on the domain wall spacing and mobility, which in turn are controlled by grain size, stress, and defect structure. The high field permeability ( B at H = 10 Oe) is related to the amount of flux closure structure present, which is determined by the misorientation of the grains in the material. Magnetostriction is caused entirely by the motion of 90° domain walls which are introduced during the formation of flux closure structure and by residual or applied stress.

Magnetic properties and domain structure in grain-oriented 3% Si-Fe

Magnetic Domain Observations by Electron Microscopy

A new method for observing full domains in thin magnetic films by electron microscopy is described. Observations are made with standard transmission instruments utilizing an off‐centered objective aperture diaphragm as a knife edge. The method has the high‐resolution advantage that the microscope is focused on the specimen during domain observations. Limitations of the method and comparisons with the previously reported defocusing technique are presented. Applications to the interpretation of complex domain patterns and cross‐tie walls are demonstrated. The observations were made with electrostatic‐focusing microscopes, the AEG‐Zeiss and Trub‐Tauber instruments, which allow the use of full objective power without influencing the magnetization distribution of low coercive force films. A second method using a knife edge is proposed that would potentially permit a simple measurement of the detailed magnetization distribution of a domain wall in a thin film.

Domains in Thin Magnetic Films Observed by Electron Microscopy

Several modes of operation of a transmission electron microscope are discussed which are useful for the observation of the magnetization distribution in thin films. Gross defocusing of the instrument provides great sensitivity to slow variations in static magnetization direction, while slight defocusing enables the measurement of rapid variations of magnetization at high resolution. The applications of electron microscopy to dynamic wall studies is discussed; this can be demonstrated by motion pictures. Some new ways of observing domains and walls at instrumental focus are described which utilize an opaque knife edge for intercepting a fraction of the deflected electron beam below the specimen.

Murray E. Hale

Papers

Determination of Magnetization Distribution in Thin Films Using Electron Microscopy

Magnetic Domain Observations by Electron Microscopy

Domains in Thin Magnetic Films Observed by Electron Microscopy

Static and Dynamic Studies of Magnetization Distribution in Thin Films by Electron Microscopy

Observation of Néel Walls in Thin Films