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Self-forming, vertically-aligned, ZnO moth-eye-like nanoarrays were grown by catalyst-free pulsed laser deposition on a-SiO2/Si (1 1 1) substrates. X-Ray Diffraction (XRD) and Cathodoluminescence (CL) studies indicated that nanostructures were highly c-axis oriented wurtzite ZnO with strong near band edge emission. The nanostructures were used as templates for the growth of non-polar GaN by metal organic vapor phase epitaxy. XRD, scanning electron microscopy, energy dispersive X-ray microanalysis and CL revealed ZnO encapsulated with GaN, without evidence of ZnO back-etching. XRD showed compressive epitaxial strain in the GaN, which is conducive to stabilization of the higher indium contents required for more efficient green light emitting diode (LED) and photovoltaic (PV) operation. Angular-dependent specular reflection measurements showed a relative reflectance of less than 1% over the wavelength range of 400–720 nm at all angles up to 60°. The superior black-body performance of this moth-eye-like structure would boost LED light extraction and PV anti-reflection performance compared with existing planar or nanowire LED and PV morphologies. The enhancement in core conductivity, provided by the ZnO, would also improve current distribution and increase the effective junction area compared with nanowire devices based solely on GaN.

We present here a study of exploiting Lorentz transmission electron microscopy to observe the magnetic structure in a nanobridge formed by confining a magnetic domain wall. The domain wall is nucleated in an elliptical pad by magnetization reversal and is driven to be pinned at the constriction to form a complex magnetic structure. High-resolution Lorentz microscopy shows that such a structure is composed by a mixture of S and C states of smooth rotation of the magnetization through the nanobridge rather than a conventional single wall as observed previously. This confinement contributes a domain wall resistance observed via anisotropic magnetoresistance presented previously. A serial transformation from S state to S/C mixture, to C state and finally to S state of the topology of the flux is evidently confirmed.

Electron microscopy has been used to determine directly the effect of artificially introduced roughness on the micromagnetic processes that occur in soft high moment multilayer films. The nanodefects, introduced to roughen the substrate, and the local magnetic domain structure were identified in the same images, leading to unambiguous information on the role played by the former. Characteristic of the micromagnetic state of the samples was a high density of 360° wall segments that showed quite remarkable resistance to annihilation. Following a description of the domain walls generated in both easy and hard axis magnetization cycles, a model is proposed for the way the observed domain walls respond when their ends are strongly pinned and, using this, we account for their continued existence over an extended number of magnetization cycles. Finally, the implications for device performance are discussed briefly.

We demonstrate that for multilayered magnetic nanowires, where the thickness and composition of the individual layers has been carefully chosen, domain walls can be pinned at non-topographic sites created purely by ion irradiation in a focused ion beam system. The pinning results from irradiation induced alloying leading to magnetic property modification only in the affected regions. Using Lorentz transmission electron microscopy, we have studied the pinning behavior of domain walls at the irradiation sites. Depending on the irradiation dose, a single line feature not only pinned the domain walls but also acted to control their structure and the strength of their pinning., Comment: 5 pages, 4 figures

Analytical electron microscopy is a powerful tool for investigating the structural and chemical properties of magnetic materials with high spatial resolution. This is illustrated by results from two specific examples. Firstly, we show that while the MnPt layer in CoFe/MnPt exchange biased bilayers transforms from the non-magnetic fcc phase to the magnetic fct phase during annealing, the annealed MnPt layers also exhibit an undesirable increase in grain complexity. Secondly, we use electron energy loss spectroscopy to obtain elemental profiles in spin tunnel junctions being developed for MRAM application. In particular, we use the near edge fine structure present on the O K-edge to separate the signals from the AlOx barrier layer and other oxides present in the sample.

A stage has been constructed to supply a variable strength in situ magnetizing field for use in a focused ion beam system. Ion irradiation experiments were performed in the presence of the magnetic field in order to investigate its effect on the localized exchange bias field in thin CoFe/IrMn bilayers. Squares with dimensions 10 ? 10??m2 were directly irradiated by Ga+ ions of energy 30?keV at doses between 1 ? 1013 and 1 ? 1015?ions?cm?2. TEM studies of the magnetization reversal behaviour of the squares showed that bias field reversal could be achieved with the maximum achievable bias field strength being > 50% of that along the original direction.

Exchange bias of bottom-pinned bilayers and tunnel junctions (TJs) containing MnPt as the antiferromagnet was set by rapid thermal annealing. An annealing duration of 1 min was sufficient to induce the structural phase transition to obtain antiferromagnetic behaviour in MnPt without cool down in a magnetic field. For Mn51.5Pt48.5(20 nm)/Co90Fe10(5 nm) bilayers the exchange coupling was found to be (Jex = 0.4 erg cm−2). Higher rapid thermal annealing temperatures led to increased diffusion in the layers reducing the pinned layer magnetic moment. Structural characterization by x-ray diffraction and transmission electron microscopy clearly showed the phase transition in the MnPt film. Exchange bias, coercive and free layer coupling field of a rapid thermal annealed and a standard annealed TJ were compared.

Magnetic multilayer structures are key components in devices used for magnetic information storage and sensing. Application in such devices relies on the ability to precisely control the switching characteristics and the magnetic coupling between the layers. The magnetic and transport properties of the device are governed by the composition, structure and thickness of the various layers which make up the multilayer stack as well as the quality of the interfaces between the layers. Advanced analytical electron microscopy techniques are used to investigate the structure and composition of layers in two different types of spin tunnel junction stacks and in perpendicular spin valve stacks. High angle annular dark field scanning transmission electron microscopy imaging combined with electron energy loss spectroscopy spectrum imaging provides rapid coverage of large areas of material along with precise chemical information from specific regions of interest. This combination of techniques is shown to be particularly useful for examining the oxide barrier layers and the extent of any oxidation defects within the layer stacks.

The influence of 30 keV Ga + ion irradiation on the magnetic properties of polycrystalline NiFe/FeMn exchange bias layers was investigated. In this context, it was of particular interest to determine whether a previously observed enhancement of the bias field value for the irradiation with 5 keV He + ions is a material-specific or an ion-specific effect. Moreover, the capability of magnetic micro-patterning using Ga + ions from a focused ion beam source was tested. It is demonstrated that magnetic patterning with a lower width limit in the range of 100–1000 nm is possible.

Exchange bias and blocking temperature were studied in MnPt based bottom-pinned bilayers and synthetic antiferromagnets (SAF) prepared by magnetron sputtering. The structure and magnetic properties were determined as a function of the MnPt layer thickness. Exchange coupling was found to be (Jex = 0.4 erg/cm2) for a MnPt (t ≤20 nm)/CoFe (5 nm) bilayer. The distribution of the blocking temperature TB was analyzed and its width ΔTB and center point TB,center determined. TB is about 280 ○C for thinner MnPt films, and increases to 330 °C for thick films. ΔT B is constant for thick MnPt but steadily increases as the thickness decreases. SAF structures show higher exchange bias and higher TB,center at thin layer thickness (tMnPt = 8.5 nm) compared to bilayers.

Controlled and reproducible fabrication of nano-structured materials will be one of the main industrial challenges in the next few years. We have recently proposed exploitation of the nano-structuring potential of a high resolution Focused Ion Beam Tool, to overcome basic limitations of current nano-fabrication techniques. The aim of this article is to present some new routes for material patterning, which benefit from ion-induced local property modifications or damage. In the experiments we describe hereafter an ultra-sharp pencil of 30 keV gallium ions is used to tailor the characteristics of several materials at a scale of a few nanometres. The experimental results are then compared to simulations. First, we simulate the control of collisional defects generated in a thin magnetic layer under FIB irradiation. The results explain the stable magnetic structures we have obtained experimentally. This was achieved with a low surface ion dose (1012 to 1014 ions/cm2). In addition we have explored the promising direction of “Bottom-up” or “self-organization” processes using a FIB instrument. We have defined artificial surface defects. These defects created by the impact of an 8-nm FWHM probe were used to pin the diffusion and to organize nanometre-sized gold clusters on a graphite surface.

Transmission electron microscopy and micromagnetic modeling have been used to study magnetization processes in small elongated magnetic elements with different end shapes. Their response to fields applied both parallel and perpendicular to the element long axes has been investigated with particular emphasis on the latter. A rich variety of magnetic states have been revealed and it is shown that very similarly shaped elements can respond quite differently; moreover the same element need not respond in the same way when subjected to repeated field cycles. The importance of thermal excitation and the resulting stability at remanence of the various metastable states observed is discussed.

The effects of focused beam Ga+ ion irradiation on the physical microstructure and magnetic properties of CoFe/IrMn exchange biased bilayers have been investigated by transmission electron microscopy. Only for Ga+ ion doses >1014 ions cm−2 was the microstructure detectably altered with increases in both the bilayer mean grain size and texture being observed. At larger doses (>1015 ions cm−2) radical alterations to the grain morphology and polycrystalline nature of the film were observed including formation of a remarkable needle-like phase at 3×1015 ions cm−2. The magnetic properties and magnetization reversal behavior of the bilayer were studied using the Fresnel mode of Lorentz microscopy. Ga+ ion doses >1013 ions cm−2 were found to progressively reduce the bias field strength and coercivity with the former reaching half original strength at 2×1014 ions cm−2. Higher doses were found to alter the reversal mechanism accompanied by progressive degradation of the magnetic properties. It is likely that the o...

The reversal process of the free layer of two spin valves (SVs) with crossed anisotropy was studied as a function of temperature by transmission electron microscopy. In situ magnetizing experiments were carried out using the Fresnel imaging mode at room temperature, 100 °C, and 200 °C. The two SVs studied had slightly different synthetic antiferromagnetic (SAF) structures, the magnetic layers being thicker in one than the other. It was found that the SV with the thinner SAF layer was less thermally stable. Moreover, a change in the reversal process was observed at elevated temperature while no changes were seen for the SV with thicker SAF. Insight into why the reversal mode varied in the way it did was obtained using a modified Stoner–Wohlfarth model.

A new stage for carrying out in situ magnetising experiments in the transmission electron microscope has been designed, constructed and tested. The principal advantages of the stage are that it delivers horizontal fields with negligible perturbation to the illumination and is suitable for operation in pulsed or continuous field mode. Details of its performance, including field calibration, are given. The paper concludes with a description of where the stage is likely to be of most use.

We have studied how the switching field of permalloy elements with sub-micron widths varies as a function of the orientation of the applied field. Switching fields for individual elements were determined using Lorentz microscopy, the studies being facilitated greatly by the use of a newly designed magnetizing stage. Brief details of the stage are given. For elements with widths ~400 nm, switching fields increased modestly as the angle the applied field made with the long axis of the element increased. In contrast, as the element width was reduced to 160 nm, little variation with field orientation was observed. The results suggest that the residual domain structures close to the ends of the elements played a major role in determining their switching characteristics, even for the smallest elements investigated here.

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.

The reversal of the free layer of a spin-valve with crossed anisotropy was studied by transmission Lorentz electron microscopy. In situ magnetizing experiments were carried out using the Fresnel imaging mode for various applied field orientations. Reversal could be by simple magnetization rotation or by rotation combined with complex domain processes. Asymmetric magnetization reversals whereby the modes on the outward and return paths of a magnetization cycle were seen on occasion. Insight into why the reversal mode varied in the way it did was obtained using a modified Stoner-Wohlfarth model. The model provided a good description of the various modes and there was reasonable agreement between predicted and observed fields at which key stages of the reversals took place. Even though a single variable parameter model of the kind used cannot describe a multi-domain state, its use in inferring the nature of domain configurations that arise is discussed as are its other strengths and weaknesses.

We have used transmission electron microscopy to study directly the way magnetization reversal proceeds in the ferromagnetic layer for sets of bilayers in which the ferromagnetic layer (CoFe) was of constant thickness while the thickness of the antiferromagnetic layer (IrMn) was varied. The first set studied was in the as-deposited state while the second was subjected to rapid thermal processing. For IrMn thicknesses of 20 A, no shift fields were observed, although significant coercivities (∼70 Oe) were recorded and reversal involved rather simple domain processes. By contrast, complex small-scale domains dominated the reversal processes for samples where the IrMn thickness exceeded 60 A; here, strong exchange biasing and higher coercivities were the norm. For all thicknesses of IrMn, an unexpected variation in the dominant orientation of domain walls on the outward and return parts of the magnetization cycle tended to be observed. A possible origin of this, together with the differences between the as-deposited samples and those subjected to rapid thermal processing, are discussed.

TEM has been used to study the partially hydrogen disproportionated NdFeB-type alloys. The lamellae structures, which have distinct features from previously reported, comprise a tetragonal NdFe-containing phase (intermediate lamellae (IL) phase) and α-Fe. The experimental data show strong evidence of well-defined crystallographic relationship between different clusters of lamellae.

He + ion irradiation is an excellent tool to modify the magnitude and direction of the exchange bias field on the sub-micrometer scale without affecting the sample topography. This effect has been utilized to magnetically pattern NiFe/FeMn exchange bias double layers using resist masks patterned by electron beam lithography. Ion irradiation through the masks leads to a local modification of the magnetization reversal behavior and allows to study the magnetization reversal as a function of the exchange bias field strength and the pattern dimensions on a single sample. Results are presented on the macroscopic and microscopic magnetization reversal using the magneto-optic Kerr effect and Lorentz microscopy.

The magnetic properties of Co/Pt films can be modified and even controlled by ion beam irradiation. For example, the coercivity and magnetic anisotropy of a 1.4nm thick Co layer are reduced with increasing the Ga+ ion dose. Images of the demagnetized states and remanent hysteresis loops are obtained for arrays of dots or tracks patterned by the focused ion beam technique. 20 nm wide sharp non-magnetic irradiated lines separate the magnetic dots or the tracks, the size of the dots or the width of the tracks varying from 1.8 μm down to 70 nm. The efficiency of the dipolar interaction is studied, and the use of such dot arrays for magnetic or magneto-optical recorrding discussed.

The magnetisation reversal of the free layer of two different spin-valve (SV) films with crossed anisotropy was studied using Lorentz microscopy in a modified transmission electron microscope. Both SVs were of similar construction, the principal difference being the replacement of Co by CoFe in the layers surrounding the non-magnetic spacer. Marked differences in the reversal mode were apparent. For CoFe SVs reversal was usually by rotation and experimental results were in good agreement with those predicted by a modified Stoner–Wohlfarth model. By contrast, reversal in Co SVs tended to involve complex domain processes with a high degree of irreversibility. The differences are attributed to the much higher magnetostriction constant of Co which gives rise to substantial local property variation.

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.

The local and collective behavior of magnetic arrays fabricated by focused ion beam (FIB) patterning of a Co/Pt multilayer is described. The arrays comprised 1 μm nonirradiated square elements separated by narrow lines which were written using the FIB. While the square elements supported perpendicular magnetization, the ion fluence used to write the lines was chosen to make the local magnetization there lie in-plane. Lorentz microscopy showed that lines were approximately 60 nm wide and that the magnetization had the expected orientation. Application of fields perpendicular and parallel to the array showed that the magnetization in the square elements and in the lines could be controlled essentially independently of each other. Magneto-optic microscopy was used to study the behavior of the arrays as a whole. Frustrated checkerboard patterns were observed, whose detailed properties depended to an extent on the fluence used to write the lines.

Atomic force microscopy, transmission electron microscopy, optical, and magneto-optical microscopy have been used to study how structural and magnetic properties are changed when a Co/Pt multilayer is quasihomogeneously irradiated with Ga ions. Under low irradiation fluence, both grain size and texture in the multilayer increase. These effects continue for fluences in excess of 1×1015 Ga ions/cm2, but beyond this dose significant thinning of the multilayer is also observed. Three distinct irradiation-induced magnetic regimes with sharp transitions between each were identified. For Ga fluences less than 5×1012 ions/cm2, the irradiated region retains perpendicular uniaxial anisotropy but with coercivity lower than that of the as-grown film. For fluences between 5×1012 and 1×1013 Ga ions/cm2, a transition from perpendicular to in-plane magnetization was experienced. Very little change of the in-plane magnetic properties of irradiated multilayers is then observed until the sample experiences a ferromagnetic t...

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.

Transmission electron microscopy studies have been carried out on partially disproportionated NdFeB-type alloys. A new intermediate magnetic (NIM) phase has been identified. Moreover, the lamella structure which subsequently develops from the tetragonal NIM phase comprises a tetragonal NdFe-containing (IL) phase and α-Fe. The experimental data show strong evidence of a well-defined crystallographic relation between both the NIM and lamella phases and between the IL phase and α-Fe. These observations give insight into how crystallographic texture, and hence anisotropy, can be developed in NdFeB-type powders processed by the hydrogenation, disproportionation, desorption, and recombination route.

We have used the transmission electron microscope to study how the magnetization reversal mechanism of thin NiFe layers exchange-biased by IrMn and FeMn varies over a wide temperature range. The reversal behaviour was qualitatively similar for layers biased by both types of antiferromagnet. At room temperature and below the most striking feature was the scale of the domain structures observed. Very high density domain structures with micron (or sub-micron) wall separations developed. By contrast at elevated temperatures, the reversal mechanism simplified. This is consistent with there being a strong local variation of the pinning strength between the NiFe and the antiferromagnetic layer. The overall temperature variation of the pinning changes much more rapidly than the magnetic properties of an isolated NiFe layer over a similar temperature range. 3Current address: Sun Microsystems, Springfield, Linlithgow, West Lothian, EH49, UK. 4Current address: Veeco Instruments, 80 Las Colinas Lane, San Jose, CA 95119, USA.

Magnetic domain structures in two 50 nm thick chemically-ordered FePd (0 0 1) 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 ≈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.

The transmission electron microscope has been used to study the physical and magnetic microstructures of two HDDR-processed NdFeB-type alloys, one without Zr and the other containing 1.1 at% Zr. Studies were made of the as-produced powders and the solids produced following hot-pressing at 900°C. In the HDDR powders, the principal effects of adding Zr were to reduce the average grain size by ≈ 1 3 and made the grain size distribution more uniform. In the hot-pressed samples, the effect of Zr was more dramatic in that grain growth was very significantly reduced. Zr-containing phases were identified and a simple model, due to Zener, used to provide a plausible explanation of how the small amount of Zr present could stabilise the grain size to ≈0.5 μm. The microstructural results correlated well with measured magnetic properties.

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.

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.

The oscillatory interlayer indirect exchange coupling in Co/Cu multilayers is known to be highly sensitive to structural defects. In this paper the dependence of the antiferromagnetic exchange coupling on the background pressure in the vacuum chamber is investigated. Co/Cu multilayers were grown by dc magnetron sputtering in a system equipped with a leak valve to allow the introduction of low levels of air or ${\mathrm{O}}_{2}.$ Below a relatively narrow band of pressure the samples exhibit excellent antiferromagnetic coupling and consequently a high giant magnetoresistance, $\ensuremath{\sim}40%$ for {Co(8 \AA{})/Cu(8 \AA{})}$\ifmmode\times\else\texttimes\fi{}25$ samples. Above this transitional band of pressure no antiferromagnetic coupling, and hence no giant magnetoresistance, is observed. X-ray diffraction measurements reveal no change of any significance in any of the layer thicknesses or roughnesses. Whilst the high-field magnetic behavior is found to be isotropic in the sample plane, the reversal of the remanent moment around zero field is found to show a varying degree of uniaxial anisotropy. Lower remanent moments are found to be associated with a more isotropic reversal mechanism. A thorough characterization of the physical and magnetic microstructure by means of various modes of transmission electron microscopy is presented. Cross-sectional images reveal subtle changes in the crystallinity and layer quality of the samples as the background pressure is increased. Plan-view Lorentz microscopy reveals that the isotropic reversal mechanism of the low remanence samples involves a complete domain structure. The reversal mechanism of multilayers with a significant remanent moment varies markedly with field direction and can be dominated by rotation or comparatively simple domain processes. Samples with a significant giant magnetoresistance $\ensuremath{\sim}20%$ and remanent fractions $\ensuremath{\sim}0.7$ are found to still show highly anisotropic reversal mechanisms around zero field. Indeed when demagnetized along the easy axis the samples are in a single domain state at remanence. This is compelling evidence for substantial noncollinear ordering of the moments through biquadratic coupling.

Since its introduction 40 years ago Lorentz microscopy has been used extensively to study domain structures in magnetic thin films and elements. Here we review the principal imaging modes, with emphasis on those introduced most recently, and discuss their advantages and drawbacks. Special emphasis is given to the development of methodologies which allow the magnetic state of a specimen to be changed in situ and which allow a full quantitative description of the spatial variation of magnetic induction to be extracted. Examples of each are presented. Finally, we discuss areas where further technique development will be beneficial.

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.

Many electrical and mechanical properties of a material are influenced markedly by the distribution of the individual grain orientations, i.e. its texture. In this paper a new TEM technique is described, along with the analysis used, to identify the texture in a film. A single diffraction pattern from a scanned area on a polysilicon film on a cross-sectional TEM specimen is analysed enabling a semi-quantitative description of the texture of that area in the film. A series of these diffraction patterns from linescans parallel to the film surface is used to show how the texture develops through the film.

Transmission electron microscopy has been used to study the physical and magnetic microstructures of thinned sections of four different NdFeB-type alloys. Three of the materials were prepared by the solid-HDDR process and differed in that one was a simple ternary alloy, another contained Co & Ga additions and a third was appreciably richer in Nd than stoichiometric Nd 2 Fe 14 B 1 . For the fourth material HDDR powder had been hot-pressed into a fully dense compact prior to die-upsetting. Isotropic grains with a mean size ≈300 nm were found for the first three materials whilst the die-upset material had an average grain size of ≈300×700 nm 2 . All materials other than the simple ternary alloy showed strong alignment of the c -axis, the clustering of grains with similar alignment being most pronounced in the sample with Co & Ga additions. The sample with the largest region of alignment was the die-upset material, the c -axis orientation being approximately parallel to the pressing direction throughout. It was in this sample that magnetic alignment was found to be most extensive, the domains lying parallel to the mean c -axis across the whole of the visible area. Significant magnetic alignment was also found in the sample with Co & Ga whilst the Nd-rich material had relatively small areas of magnetic alignment. In the three samples with good crystallographic alignment local variations in domain wall orientation between neighbouring grains within an aligned region showed maximum variations that corresponded with the angular spread of spots in the selected area diffraction patterns.

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 200 ×40 nm 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.

Lorentz electron microscopy and micromagnetic modelling have been used to gain insight into the magnetization reversal of CoNi/Pt multilayers supporting perpendicular magnetization. Experimental observations were made in a highly modified transmission electron microscope as a function of the field magnitude and elapsed time. With appropriate selection of parameters, in particular the domain-wall energy and the activation volume, Monte Carlo simulations were able to reproduce most of the experimental results. Both of these quantities are difficult to measure directly. Finally, we note that pinning sites had to be introduced into the model to account for the very irregular manner in which the magnetization reversal proceeded.

Transmission electron microscopy has been used to investigate the 'free' layer reversal mechanism of a range of FeMn-biased spin-valves with parallel anisotropy. Fresnel imaging enabled three modes of reversal to be directly observed for different in-plane applied field orientations and interlayer coupling strengths. Low-angle diffraction has provided some quantitative information on the level of dispersion involved in the reversals. Comparison with a modified Stoner-Wohlfarth coherent rotation model is made and magnetic modelling of the free layer energy variation as a function of magnetization orientation is presented. This has helped provide a deeper understanding of the observed discrepancies between the model and experimental data. Finally, a qualitative description of the observed domain processes is given in relation to the free layer energy for the corresponding experimental conditions.

A thinned sample of Nd 12.8 Fe 69.2 Co 11 Ga 1 B 6 after the disproportionation stage of the solid-HDDR process has been examined using advanced transmission electron microscopy techniques. High resolution X-ray mapping is particularly useful for showing the distribution of all elements with the exception of B. Elemental maps show clearly that during the disproportionation reaction, Co and Ga partitioned into the α-Fe and not into the NdH 2 . Domain observations using Lorentz microscopy revealed irregular domain walls whose location was influenced by the non-magnetic NdH 2 inclusions. The walls moved easily under small applied magnetic fields in a series of discrete jumps. The behaviour was consistent with that of a soft magnetic material with low anisotropy, and differed markedly from that of the recombined 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.

The hydrogenation disproportionation desorption recombination (HDDR) process was applied to produce isotropic, submicron powder in 500 g batches in a specially designed HDDR reactor. The hot pressing characteristics of the material were determined and it was shown that the material has an excellent stability against grain growth. The coercivity of 1000-1080 is almost constant over a hot pressing temperature range of 700-, making the material highly suitable for subsequent hot deformation. Die-upset HDDR magnets were prepared in order to study the basic deformation behaviour. A remanence of 1.13 T in the axial direction and a coercivity of were achieved. Similar properties were obtained for the backward extruded magnets produced at and only a small decrease in alignment along the axial direction of the ring was found. Grain sizes were very uniform and on the submicron scale. Platelet-shaped grains were observed in the die-upset magnets. The formation of interaction domains, along the axial and radial directions for the die-upset and backward extruded magnets respectively, were established by high-resolution Kerr microscopy. The high degree of texture in the hot deformed HDDR magnets was also confirmed by Lorentz microscopy revealing continuous equispaced domains extending over the entire thinned sample with only small directional variations.

This paper investigates the possible transitions between transverse and vortex type domain walls in magnetic domain wall trap memory elements, induced by a rotating magnetic field. A phase relation is shown to exist relating the final domain wall configuration to the field strength and field rotation frequency. Thus, it is possible to controllably switch from a transverse domain wall to either a clockwise or counterclockwise one

Antiphase boundaries (APBs) were observed in ${\mathrm{Fe}}_{3}{\mathrm{O}}_{4}$ single crystal films grown on MgO. The APBs are an intrinsic consequence of the nucleation and growth mechanism in films. Across an APB, the intrasublattice superexchange coupling is greatly strengthened, while the intersublattice superexchange coupling is weakened, reversing the dominant interaction from that found in the bulk. Thus the APB separates oppositely magnetized regions, consistent with Lorentz microscopy measurements. The APBs induce very large saturation fields and nearly random magnetization distribution in zero field.

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.

The switching field interval, ¿Hs, of Ni–Fe–Co-based thin films and spin-valve layered structures, sputter-deposited on a Ta-buffer layer, was studied. The switching field interval is the field range in which the magnetization reversal of a ferromagnetic layer takes place. In thin films, ¿Hs is determined by the uniaxial anisotropy, induced by growth in a magnetic field. This anisotropy increases with the ferromagnetic layer thickness and saturates at a thickness of 10–25 nm. It also depends on the alloy composition as well as on the choice of the adjacent layers. In exchange-biased spin valves, an additional contribution to ¿Hs was observed, which increases monotonically with increasing interlayer coupling. We explain this in terms of the effect on the magnetization reversal of the sensitive layer due to a simultaneous small, but temporary, magnetization rotation in the exchange-biased layer and lateral variations of the interlayer coupling. In addition, the effect of biquadratic coupling on ¿Hs is discussed. Finally, the thermal stability of ¿Hs is investigated.