K. J. Kirk

Switching fields and magnetostatic interactions of thin film magnetic nanoelements

Switching fields of magnetic elements with nanometric dimensions have been investigated by Lorentz microscopy using a transmission electron microscope. Acicular elements of Co and Ni80Fe20 were fabricated by electron beam lithography and lift-off techniques. They were 1.6–3.5 μm long, 200 nm wide, and 20–50 nm thick, with flat rectangular ends or triangular pointed ends, and were patterned in linear arrays with center-to-center spacing ranging from 7 μm to 250 nm. Switching fields and reversal behavior of the elements were found to depend strongly on the shape of the ends and, in a closely packed array, on element separation, thereby providing a way of controlling their magnetic properties.

Domain configurations of nanostructured Permalloy elements

The magnetization distributions of an array of small NiFe elements were studied using Lorentz transmission electron microscopy (LTEM) and magnetic force microscopy (MFM). The dependence of the domain configurations at zero field as a function of the aspect ratio was observed using MFM, and confirms the earlier observations using LTEM. Comparison of the images of similar islands using both techniques elucidate the complementarity between the LTEM and MFM measurements which individually show different facets of the magnetization distributions on soft magnetic thin films.

Domain structures and switching mechanisms in patterned magnetic elements

Abstract Domain formation and magnetisation reversal in lithographically fabricated magnetic elements 100–300 nm wide and 1.5–4.0 μm long have been investigated using Lorentz imaging and finite element micromagnetics. The numerical integration of the Gilbert equation of motion resolves magnetisation processes in time and in space. The calculated domain patterns are in qualitative agreement with magnetic images obtained from Lorentz electron microscopy. NiFe elements show a small scale domain structure in the remanent state which may be attributed to a transverse anisotropy. In bars with one pointed end, the formation of the domains starts from the flat ends. Narrow elements with a widhh smaller than 200 nm remain in a nearly single domain state. Pointed ends suppress the formation of domains in NiFe elements and increase the switching field by about a factor of two in Co elements.

Lorentz microscopy of small magnetic structures (invited)

Domains and domain walls in micron and submicron sized magnetic elements can be studied at high resolution using Lorentz microscopy in the transmission electron microscope. In situ magnetizing experiments are possible in which magnetization reversal processes can be viewed directly in the presence of varying magnetic fields. These techniques have been used to investigate small magnetic structures fabricated by electron beam lithography on electron transparent membrane substrates. Patterned elements as small as 200u200a×40u200anm have been imaged magnetically. Detailed studies have been carried out into the properties of high aspect ratio (acicular) elements of Co and a soft NiFe alloy. It has been found that the coercivity increases as the elements become narrower, down to ultrasmall elements with a width of 40 nm. Element length has no effect so long as the aspect ratio is sufficiently high. Magnetization reversal in acicular elements is known to begin from the ends of the elements, therefore the shape of the ends—flat, elliptical, or pointed—has a significant effect on the coercivity. The magnetic environment of an element is also highly important in determining its properties. A one-dimensional array of closely spaced elements has the same average switching field as an isolated element but the spread in values is greatly increased when the gap between elements is made smaller than the width of an element. Adding rows of elements to make a two-dimensional array also has an effect, even if the rows are spaced further apart than the length of the elements.

Quantitative interpretation of magnetic force microscopy images from soft patterned elements

By combining a finite element tip model and numerical simulations of the tip–sample interaction, it is shown that magnetic force microscopy images of patterned soft elements may be quantitatively compared to experiments, even when performed at low lift heights, while preserving physically realistic tip characteristics. The analysis framework relies on variational principles. Assuming magnetically hard tips, the model is both exact and numerically more accurate than hitherto achieved.

Role of vortices in magnetization reversal of rectangular NiFe elements

Vortices are seen in the magnetization distributions of rectangular magnetic elements in both experiments and micromagnetic simulations. To investigate the role of vortices during magnetization reversal, Ni80Fe20 elements 100 nm and 200 nm wide and 5-60 nm thick were fabricated by electron beam lithography and studied by high-resolution magnetic imaging in the transmission electron microscope. During reversal, vortices appeared near the ends of the elements, grew under an increasing reverse field, and disappeared after rapid switching. Maximum switching fields of 400 Oe for 100 nm wide elements and 200 Oe for 200 nm wide elements occurred for film thicknesses of 25-30 nm and above. Simulations showed that reversal in these elements always occurred by means of vortices, however the simulated switching fields were much higher than the experimentally observed values. Lower switching fields were obtained in the simulations when vortex creation was assisted by `defects at the edges of the elements. However, to successfully simulate the magnitude and thickness dependence of the switching fields, it was necessary to start from an initial magnetic state which already contained a vortex.

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.

Simulation of magnetization reversal in polycrystalline patterned Co elements

The influence of the polycrystalline microstructure on the switching mechanisms of acicular shaped Co elements was investigated using finite element micromagnetics. The Gilbert equation of motion with a Gilbert damping constant α=1 was solved using a semi-implicit time integration scheme. The elements were 200×40 nm2 and 25 nm thick. The grain size is approximately 8 nm, leading to edge irregularities of the same size. The competitive effects of the shape anisotropy and the random, magnetocrystalline anisotropy lead to a magnetization ripple structure with a wavelength of about 100 nm for zero applied field. With increasing applied field, the magnetization ripple becomes more pronounced. At an applied field of 95 kA/m a vortex, originally formed near sharp edge irregularities, moves into the width of the element. The vortex reaches the opposite edge after about one ns. Then a transverse domain structure of a head to head domain wall forms and the reversed domain expands through the entire particle. The ma...

Domain wall motion in micron-sized permalloy elements

The magnetization reversal process in an array micron sized NiFe patterns was studied using magnetic force microscopy in the presence of external fields. The behavior of the magnetization distribution was correlated with the aspect ratio and the direction of the applied fields. Magnetizing along the hard axis was found to produce solenoidal magnetization at remanence while applying the field along the easy axis tend to form nonsolenoidal configurations. The micromagnetic evolution, which involved domain wall, crosstie, and vortex displacements, was studied and the correlations were consistent with previously reported M–H loop observations and theoretical predictions.

Micromagnetic simulation of domain structures in patterned magnetic tunnel junctions

The magnetization reversal process of patterned magnetic tunnel junctions was investigated using finite element micromagnetics, taking into account the magnetostatic interactions between the pinned and the free layer. Two different reversal modes were observed in the simulations depending on the domain structure for zero applied field. In order to reduce the magnetostatic energy, end domains form in the free layer either in the S state or the C state. If the system is in the S state, the end domains grow under the influence of a reversed field. The end domains touch each other, leading to the reversal of the center. Finally, the residual domains along the edges parallel to the field direction reverse. If the system is in the C state, the growth of the end domains leads to a four domain flux closure structure. The domain with the magnetization in favor of the field direction expands until the free layer becomes reversed at a field. The S state and the C state were found to differ in energy by less than 0.2%.