S. McVitie

Modified differential phase contrast Lorentz microscopy for improved imaging of magnetic structures

A modification to the differential phase contrast mode of Lorentz microscopy is proposed in which an annular quadrant detector is used instead of the standard solid quadrant detector. The new imaging mode allows a high degree of control over the relative efficiencies with which low and high spatial frequency information can be transferred from the specimen to the image. This makes it possible to effect a considerable separation of contrast arising from magnetic and nonmagnetic origins in thin polycrystalline magnetic films and thus reveals finer magnetic detail than was hitherto possible. Experimental details of the implementation of the technique, together with examples of its use for investigating thin-film recording media, are presented. >

Coherent magnetic imaging by TEM

A novel transmission electron microscope optimised for the study of magnetic material is described. Using this instrument, a new method for revealing magnetic structures, coherent Foucault imaging, can be realised. Images appear directly in the form of magnetic interferograms and provide immediate access to a quantitative description of the induction distribution across the specimen. A simple analytical approach is given to the underlying theory and the main features are confirmed by computer modelling. Experimental images of small regular permalloy elements illustrate the power of the technique for studying domain structures. >

Switching of nanoscale magnetic elements

We have investigated the magnetic properties of ultra-small-patterned elements of Co and NiFe thin films. The elements were rectangular with an aspect ratio in the range 3.75–20. The smallest were 200×40 nm2 with 50 nm gaps between them, corresponding to an areal density of 27 Gbit/in2 if used as discrete-patterned media for magnetic recording. The elements were fabricated by electron-beam lithography and lift-off patterning and high-resolution magnetic images were obtained by Lorentz microscopy in a transmission electron microscope. In situ magnetization reversal experiments showed that the strong dependence of the switching field on element width extended to the smallest elements of both materials. The switching field for 40-nm-wide Co elements was 1200 Oe and for 40-nm-wide NiFe elements was 800 Oe. Element length and aspect ratio had little effect.

Investigation of the influence of edge structure on the micromagnetic behavior of small magnetic elements

Thin film elements of a soft magnetic nickel–iron alloy have been fabricated with structured edges in order to determine their effect on the magnetization reversal processes. Lorentz microscopy was used to study acicular elements with different edge structures and these were compared with standard elements with straight edges. The presence of the structured edges results in deviations of the magnetization along the length of the elements in the remanent state. Switching processes are described for a number of different elements and the effect of structuring the edges is discussed.

Direct observation of magnetization reversal processes in micron-sized elements of spin-valve material

Simple calculations suggest that when continuous films of spin-valvematerial are patterned into micron-sized elements the magnetic properties should change markedly, depending on the element shape and size. We have used the differential phase contrast imaging mode of transmission electron microscopy to study directly the magnetization distributions supported by such elements in zero field and when subjected to an applied field in the pinning direction. For elements whose long axis is parallel to the pinning direction a parallel alignment of the free and pinned layers is favored. When subjected to a field a complex domain structure evolves and different irreversible paths are followed as the element is taken from negative to positive saturation and back again. By contrast, when the pinning direction is parallel to the short axis an antiparallel arrangement, where the magnetostatic contribution to the energy is effectively suppressed, can be preferred and simpler reversal mechanisms, with a higher degree of reversibility, are frequently seen.

In-situ magnetising experiments on small regularly shaped permalloy particles

The magnetisation process in small regular permalloy particles is investigated using the Foucault mode of Lorentz electron microscopy. Results are presented for particles of three different thicknesses, 17, 60 and 95 nm, with in-plane dimensions lying in the range 0.25 to 4.00 μm. From a study of Foucault images recorded under an applied field, hysteresis loops of individual particles are constructed. Examples of these are given together with a qualitative explanation of how the transition between solenoidal and non-solenoidal magnetisation distributions is dependent on particle dimensions.

Magnetic properties of magnetite arrays produced by the method of electron beam lithography

Arrays of magnetite particles in the submicron range (0.1–4.0 µm) have been produced. A novel method involving the utilisation of Electron Beam Lithography techniques often employed in the engineering design of integrated circuits of microchips was used. The fabrication process involved first producing arrays of iron (Fe) particles and then converting them to magnetite (Fe3O4) by thermal treatment. The fabricated magnetite particles have well controlled parameters including inter-particle spacing, an impossible task to achieve using artificially produced powders often employed in rockmagnetic studies. Two methods of converting Fe to Fe3O4 by annealing were used. One method led to Fe3O4 grains with high coercivities, typical of stressed grains and the other low coercivities in agreement with those for laboratory grown crystals. The crystal unit cell edge. Curie temperature, and saturation isothermal remanent magnetisation (SIRM) intensity observed at the Verwey transition are all consistent with stoichiometric magnetite.

Controlled domain wall injection into ferromagnetic nanowires from an optimized pad geometry

The authors present an improved geometry for a micron-scale pad for the injection of vortex domain walls (VDWs) into ferromagnetic nanowires. The pad supports a single vortex magnetization state, the chirality of which can be controlled simply by field saturation along a specific direction. We show, using Lorentz transmission electron microscopy, that utilization of such pads allows the chirality of VDWs injected into the attached wire to be predetermined. Furthermore, the pad vortex state is highly stable and survives repeated injection and depinning of VDWs from an asymmetric notch located some distance along the wire.

Quantitative imaging of magnetic domain walls in thin films using Lorentz and magnetic force microscopies

Images of a thin film permalloy element taken with Lorentz and magnetic force microscopies are compared with those from a simulation of the expected magnetic structure of the element. Measurements taken from the domain walls present in the element allow a quantitative comparison to be made. In the case of magnetic force microscopy, quantification is made possible by using a nonperturbative approach based on an extended charge model for the magnetic probe. Excellent agreement between experiment and simulation is observed for both imaging techniques.

Magnetic structure determination in small regularly shaped particle using transmission electron microscopy

The magnetic domain structures supported by regularly shaped permalloy (Ni/sub 82.5/Fe/sub 17.5/) particles whose in-plane dimensions are in the micrometer and submicrometer range are investigated. Two modes of Lorentz microscopy are used in this study; the Fresnel mode of conventional transmission electron microscopy and the differential phase contrast mode of scanning transmission electron microscopy. Results from particles of varying size and shape are presented and the transition from multidomain to near-single-domain behavior is discussed. Finally, some details are given of how the domain-wall structure varies with particle thickness and the angle through which the magnetization rotates. >