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

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

The energies of Bloch and Neel walls are calculated as functions of the film thickness and the angle through which the magnetization turns in the wall. It appears that in addition to the well‐known Bloch—Neel wall transition as a function of the film thickness, Bloch—Neel wall and cross tie—Neel wall transitions occur when the angle through which the magnetization rotates is changed by an external field. The new calculations together with a review of previous work give a comprehensive picture of the domain walls in thin Ni—Fe films.

Domain Walls in Thin Ni–Fe Films

Nontrivial spin orders with magnetic chirality emerge in a particular class of magnetic materials with structural chirality, which are frequently referred to as chiral magnets. Various interesting physical properties are expected to be induced in chiral magnets through the coupling of chiral magnetic orders with conduction electrons and electromagnetic fields. One promising candidate for achieving these couplings is a chiral spin soliton lattice. Here, we review recent experimental observations mainly carried out on the monoaxial chiral magnetic crystal CrNb3S6 via magnetic imaging using electron, neutron, and X-ray beams and magnetoresistance measurements, together with the strategy for synthesizing chiral magnetic materials and underlying theoretical backgrounds. The chiral soliton lattice appears under a magnetic field perpendicular to the chiral helical axis and is very robust and stable with phase coherence on a macroscopic length scale. The tunable and topological nature of the chiral soliton lattic...

Symmetry, Structure, and Dynamics of Monoaxial Chiral Magnets

Transmission electron microscopies of magnetic microstructures

Magnetic Domain Observations by Electron Microscopy

Domain walls in thin films of permalloy were previously found, by Huber et al., to frequently exhibit periodically spaced “cross‐ties.” These wall structures have been studied in some detail in a variety of alloys and the model advanced by Huber et al., largely substantiated. Of some interest is the great sensitivity of the position of normal spin orientation between cross‐ties to externally applied fields. A nonmagnetic overcoating on a thin film, and dark‐field illumination with restricted‐azimuth are methods which are valuable in such observations using the Bitter technique. Other characteristic wall configurations peculiar to thin ferromagnetic films have been observed and models have been devised for the magnetization distributions. Powder patterns have been observed on an iron film at intermediate stages of the slow switching of the film. The powder is seen to collect along lines perpendicular to the applied field prior to the formation of domains. A model based on magnetization buckling is suggeste...

Observations Made on Domain Walls in Thin Films

Hysteresis loops in thin films observed with the driving field applied perpendicular to the easy axis often exhibit appreciable squareness of the loop, suggesting the existence of another easy axis along what is presumed to be the hard direction. Measurement of the anisotropy in such films indicates a uniaxial anisotropy with the hard axis along the direction in which the second square loop is found. Observing magnetization reversal in these specimens by the Kerr effect, one notices wall motion which is again characteristic of a second easy axis perpendicular to the first. The use of Bitter powder patterns shows this wall activity with sufficient detail to support the theory that the occurrence of an apparent second easy axis can be normal behavior in a film with uniaxial anisotropy. If the characteristics of the film are such as to cause the magnetization to return to a multidomain state when a saturating field is applied along a hard direction and then removed, this direction can be expected to exhibit ...

Thin-Film Switching in the Hard Direction by Wall Motion

Various properties of cross-tie walls are summarized. Several new Bitter pattern experiments dealing with cross-tie walls are described. The first experiment firmly determines the magnetization distribution about a cross-tie, with the aid of an applied field at 45° to the wall. The second experiment tests the stability of cross-tie walls. It is shown in this experiment that another wall configuration is possible. This is discussed in light of the buckling mechanism of film switching.

Cross-Tie Walls in Thin Films

Dense concentrations of walls were previously observed to appear in very thin magnetic films with the application of an ac demagnetizing field. Other observations showed roughly circular concentric walls around barely discernible imperfections. Particularly clear powder patterns of such walls that were obtained on the thin end of a Permalloy film of tapered thickness are presented. The patterns allow the deduction of the detailed magnetization distribution which takes the form of a pair of antiparallel domains that spiral in toward the central imperfection. A mechanism for the formation of spiral walls is presented, together with confirming experimental evidence. The experiments establish the nature of the central imperfection as a region of abnormally high uniaxial anisotropy where the magnetization is constrained to lie in one direction along the easy axis while the magnetization of the surrounding regions of the film can be rotated with an applied field.

Harvey Rubinstein

Papers

Magnetic Domain Observations by Electron Microscopy

Observations Made on Domain Walls in Thin Films

Thin-Film Switching in the Hard Direction by Wall Motion

Cross-Tie Walls in Thin Films

Spiral Walls in Thin Magnetic Films