P. R. Aitchison

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.

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.

Interactions and switching field distributions of nanoscale magnetic elements

Magnetic nano-elements made from NiFe and Co have been investigated using magnetic imaging in the transmission electron microscope. Nano-elements like these have possible uses for in-plane patterned media or solid state memory. In both cases the elements will need to be patterned into closely spaced arrays and magnetostatic interactions between the elements will begin to become significant. Arrays must therefore be designed so that an element’s interactions with its neighbors will be small compared to its coercivity. Arrays of NiFe elements 300 nm long, 50–100 nm wide, and 26 nm thick, were fabricated by electron beam lithography and lift-off patterning. Their switching behavior and the interactions between them were studied in detail. Magnetization sequences were recorded and hysteresis loops constructed. For rows of NiFe elements with the gap between elements the same as the element width or larger, the interactions turn out to be small, suggesting that denser arrays would be possible.

Influence of end shape, temperature, and time on the switching of small magnetic elements

The switching characteristics of 10 nm thick permalloy elements with submicron widths has been studied by Lorentz microscopy. Of particular interest were the changes that occurred when the end-shape of the elements was modified. Gentle curving of the ends led to a reduction in switching field compared with an equivalent element with square ends. The principal reason for this was the different magnetization configurations found close to the element ends. At a temperature of 150 °C switching fields fell markedly, though those for elements with gently curved ends remained consistently lower. While most of the elements under study supported only high moment remanent states, intermediate low moment remanent states could be induced in elements of a certain size. The conditions under which this occurred are discussed.

MAGNETIZATION PROCESSES IN CO/CU MULTILAYERS WITH LOW MAGNETORESISTIVE HYSTERESIS

We have used transmission electron microscopy to study magnetization processes in Co/Cu multilayers with the Cu spacer layer thickness close to 9 A. The films show giant magnetoresistance (GMR) values ≈25%, saturation fields of 1–2 kOe, and very little magnetoresistive hysteresis; they are of interest as position sensors. While the Cu thickness was chosen to correspond to the first antiferromagnetic maximum, magnetic images taken throughout a magnetization cycle attest to the fact that the antiferromagnetic coupling is far from complete. Detailed analysis of image sequences and the corresponding low angle diffraction patterns suggests that the coupling is dominated by a biquadratic component. This is consistent with the relatively low value of GMR. Furthermore, the well-defined and relatively simple domain processes which are observed over the low field regime (±50 Oe) explain why little hysteresis is observed.

Correlation of domain processes and magnetoresistance changes as a function of field and number of bilayers in Co/Cu multilayers

The understanding of the magnetization processes in Co/Cu multilayers showing substantial giant magnetoresistance (GMR) is incomplete. To gain further insight into the coupling and resultant switching characteristics, a series of Co/Cu multilayers with 3–15 bilayers has been grown by dc magnetron sputtering. Microstructural and micromagnetic studies along with MR measurements were made on the samples. Lorentz microscopy was used to directly observe the evolution of domain structures under the influence of an applied field. It appeared that dissimilar domain structures occurred in different layers. Magnetic structures were generally submicron in size although in samples with few bilayers well defined domain walls were also observed during the reversal process. Their occurrence coincided with abrupt steps in the GMR curves.

Magnetization reversal of patterned spin-tunnel junction material: a transmission electron microscopy study

Electron beam lithography and reactive ion etching have been used to pattern micron-size magnetic elements in the free layer of spin–tunnel junctions. The magnetization reversal processes of elements with dimensions in the range from 15×1 μm2 to 1×1 μm2 have been studied using Lorentz microscopy in the transmission electron microscope. Under the application of a field parallel to the bias direction, elongated elements reverse by the growth and subsequent annihilation of a quasiperiodic domain structure which evolves from the ends of the elements. Similar processes occur in both halves of a magnetization cycle. By contrast, the reversal of square elements involves the formation of more complex domain structures which differ significantly according to the direction in which the field in applied.

Magnetization processes and magnetoresistance in Co/Cu multilayers as a function of the Cu layer thickness.

We have studied Co(15 A)/Cu(t) multilayers with nominal Cu spacer layer thicknesses of 7, 8, 9, and 10 A. For multilayers with identical dimensions, transport measurements showed that the introduction of oxygen during growth increased the magnetoresistance while transmission electron microscopy revealed the effect of the oxygen bleed on the microstructure was reduced grain size, suppression of the Cohcp phase, and reduced texturing. Lorentz microscopy was used to determine the angle between magnetization vectors in adjacent magnetic layers and the values so deduced were found to correlate well with the variation of magnetoresistance within the multilayer sets.

Domain structures supported by micron-sized patterned Co/Cu multilayers with AF and FM coupling

Micron-sized Co/Cu multilayer elements were prepared by dc magnetron sputtering and subsequent lift-off patterning. Two types of patterned multilayers, AF or PM coupled, were produced by controlling the width of the Cu spacer layers. Domain structures supported by these elements during magnetic switching have been imaged directly using the Lorentz imaging mode, differential phase contrast, in TEM. Patterning of the FM coupled multilayers introduces significant differences in the domain structures observed in the films as they adopt flux closure geometries in the remanent state. For AF-coupled multilayers little change occurs.

Visualization of Domain Wall Propagation in Ultra-Thin Co/Pt Films Using Transmission Electron Microscopy

In order to better understand magnetic properties in ultra-thin films, a knowledge of domain wall propagation and of domain wall structure is essential. A way to achieve it is to study hysteresis loops and magnetic after-effects by magneto-optics (MO), and also to look at the domain structure doing time resolved magnetic imaging. Unfortunately, using such optical techniques, it is almost impossible to see details in the domain structure smaller than 0.5 µm. So, Transmission Electron Microscopy (TEM) seemed to be a complementary technique in achieving higher resolution with the great advantage over other high resolution techniques like MFM that one can easily apply a perpendicular field to the sample and perform time dependent experiments.