Interactions in magnetic arrays for storage and computation

Patterned magnetic arrays have been proposed for applications in storage and computation. We have fabricated arrays of rectangular, rhombic and triangular elements on silicon substrates using electron beam lithography. The elements were at the micron scale to allow detailed and ready characterisation by magnetic force microscopy. This allowed the magnetic state of each element to be probed individually as part of the array. Arrays were also probed at high resolution using Lorentz transmission electron microscopy. The measurements show that effects of shape and coupling in patterned magnetic arrays may give a richer range of phenomena than might initially be suspected

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 degreesC 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 and magnetoresistance in Co/Cu multilayers as a function of the Cu layer thickness

We have studied Co(15 Angstrom)/Cu(t) multilayers with nominal Cu spacer layer thicknesses of 7, 8, 9, and 10 Angstrom. 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 Co-hcp 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

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

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 nm2200×40 nm2 with 50 nm gaps between them, corresponding to an areal density of 27 Gbit/in<sup>2</sup> 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 Lorentzmicroscopy in a transmission electron microscope.In situmagnetization 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

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 15x1 mu m(2) to 1x1 mu m(2) 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

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

Micromagnetic disorder in antiparallel biased spin valves

The reorientation of antiferromagnetically coupled Co layers comprising the pinned layers of an antiparallel biased spin valve is reported. Initially, the lower Co layer is saturated in the growth field in the deposition chamber, but it reorients as the upper Co layer grows to be thicker than the lower one. We have investigated the nature of this reorientation by ex situ transport measurements and Lorentz microscopy, and found it highly inhomogeneous, leading to a complex in-plane domain pattern. This results in a reduction of the giant magnetoresistance of the spin valves close to the balance point, where the benefits of the antiparallel biasing are greatest

High resolution measurement and modelling of magnetic domain structures in epitaxial FePd (001) L1(0) films with perpendicular magnetisation

Magnetic domain structures in two 50 nm thick chemically- ordered FePd (001) epitaxial films with different perpendicular anisotropies have been studied using Lorentz microscopy. Domain and domain wall structures vary significantly according to the magnitude of the anisotropy. For lower anisotropy films, a stripe domain structure with a period of approximate to 100 nm is formed in which there is a near-continuous variation in orientation of the magnetisation vector. By contrast, in the film with higher anisotropy, a maze-like domain structure is supported. The magnetisation within domains is perpendicular to the film plane and adjacent domains are separated by narrow walls, less than 20 nm wide. Micromagnetic modelling is generally in good quantitative agreement with experimental observations and provides additional information on the domain wall structure