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We report on an experimental study of current-induced switching in perpendicular magnetic random-access memory (MRAM) cells with variable resistance-area products (RAs). Our results show that in addition to spin-transfer torque (STT), current-induced self-heating and voltage-controlled magnetic anisotropy also contribute to switching and can explain the RA dependencies of the switching-current density and STT efficiency. Our findings suggest that thermal optimization of perpendicular MRAM cells can result in significant reduction of switching currents.

We present an analytical model for calculating energy barrier for the magnetic field-driven domain wall-mediated magnetization reversal of a magneto-resistive random access memory (MRAM) cell and apply it to study thermal stability factor $\Delta$ for various thicknesses of W layers inserted into the free layer (FL) as a function of the cell size and temperature. We find that, by increasing W thickness, the effective perpendicular magnetic anisotropy (PMA) energy density of the FL film monotonically increases, but at the same time, $\Delta$ of the cell mainly decreases. Our analysis shows that, in addition to saturation magnetization $M_s$ and exchange stiffness constant $A_\mathrm{ex}$ of the FL film, the parameter that quantifies the $\Delta$ of the cell is its coercive field $H_c$, rather than the net PMA field $H_k$ of the FL film comprising the cell., Comment: 8 pages, supplementary material included

The critical current density $J_{c0}$ required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (STT-MRAM) cell is proportional to the product of the damping parameter, saturation magnetization and thickness of the free layer, $\alpha M_S t_F$. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of sub-monolayer thickness, we have reduced $t_F$ while minimizing $\alpha$ and maximizing $M_S$, ultimately leading to lower $J_{c0}$ while maintaining high thermal stability. Bottom-pinned MRAM cells with device diameter in the range of 55-130 nm were fabricated, and $J_{c0}$ is lowest for the thinnest (1.2 nm) FLs, down to 4 MA/cm$^2$ for 65 nm devices, $\sim$30% lower than 1.7 nm FLs. The thermal stability factor $\Delta_{\mathrm{dw}}$, as high as 150 for the smallest device size, was determined using a domain wall reversal model from field switching probability measurements. With high $\Delta_{\mathrm{dw}}$ and lowest $J_{c0}$, the thinnest FLs have the highest spin-transfer torque efficiency., Comment: 6 pages, 6 figures

The writing quality of magnetic recording improves with increasing sharpness of the written transitions. Ultimately, the transition sharpness is limited by the grains themselves, and no longer depends on the head field gradient. In this study, we varied the head field gradient by changing the head-media spacing and characterized the write quality by transition jitter and equalized signal-to-noise (eSNR) measurements. Consistent with a recording in the grain size limit, we discovered that the write quality did not improve with smaller head to medium spacings. We also found that the measured values for the jitter and the eSNR are close to what is estimated for grain size limited recording.

The recording behavior of exchange coupled composite (ECC) media is investigated. It is found experimentally that the change of the written track width is less pronounced as a function of linear density when the write-ability is controlled by varying the vertical exchange coupling between the various layers in the medium stack as opposed to varying the lateral exchange coupling between the grains. The results are explained using a simple three-spin model which captures the essential physics. It is pointed out that the traditional concept of an effective field that separates out the field angle effect does not apply to the recording process of ECC media.

We present the measurements of the picosecond magnetization dynamics of Co/Pd multilayer films. The dynamic magnetization properties of sputtered multilayer films were analyzed as a function of Co layer thicknesses and applied bias field. Both the eigenfrequencies of the magnetization precession in the multilayers and the associated Gilbert damping exhibit extreme sensitivity to the magnetic layer thickness on an atomic monolayer scale. The eigenfrequency increases more than threefold when the Co thickness decreases from 7.5 to 2.8 A, mainly due to the changes in effective saturation magnetization and perpendicular anisotropy constant. A concomitant 2.6-fold increase in the damping of the oscillations is observed and attributed to stronger interface dissipation in thinner Co layers. In addition, we introduce a quasi-1D micromagnetic model in which the multilayer stack is described as a one-dimensional chain of macrospins that represent each Co layer. This model yields excellent agreement with the observed resonance frequencies without any free parameters, while being much simpler and faster than full 3D micromagnetic modeling.

A recording density of 1.5 Pb m−2 using heat-assisted magnetic recording in a bit-patterned media is demonstrated. This represents a dramatic improvement in track width and optical efficiency over continuous media, owing largely to advantageous near-field optical effects.

A nano-sized oscillator for RF applications is presented which is based on two spintronic effects, the tunneling magnetoresistance (TMR) and the spin momentum transfer torque. The oscillation frequency is several GHz and can be tuned by both a DC bias current and an external DC magnetic field. High compactness, high tunability and full compatibility with standard CMOS process make this spin torque nano-oscillator (STNO) a promising candidate for future RF transceivers. The main issues to be addressed are spectral purity and output power. First measurements on a hybrid built connecting the STNO to a dedicated wideband amplifier show that today's performance in terms of power is close to but not yet compatible with telecommunication standard requirements. Using time domain analysis we show that frequency fluctuations are an issue for spectral purity. Frequency synthesis concepts based on STNOs are also discussed.

Lithographically patterned recording media are one of the approaches to achieving Tb/in 2 and beyond recording densities. This will require fabrication of sub-10 nm discrete magnetic islands covering a full disk with tight spacing and size distributions and a narrow switching field distribution. To become an economically successful technology, this will need to be achieved with high throughput and low cost. The technology to fabricate such patterned media will need to be developed, and may require innovative solutions such as self-assembly and nanoimprinting, along with improved magnetic thin films for achieving high anisotropy and narrow switching field distributions.

Current induced domain wall creation in CPP nanopillars made of Co/Ni films with strong perpendicular magnetic anisotropy is described. We find that stable domain wall states (DWS) can be nucleated with modest current densities of ~107 A cm−2 and further controlled by current to restore the two uniform states. The reproducibility of both the creation and annihilation of such DWS is studied. Experiments have been performed several times for a given nanopillar, but also for different devices. The influence of temperature, size and shape of the devices as well as the distribution of magnetic properties on magnetization reversal is discussed.

One strategy to delay the onset of superparamagnetism and achieve magnetic storage densities approaching 1 Tb/in.2 is the use of lithographically patterned magnetic media. While one of the main advantages enjoyed by magnetic recording is low cost due to use of featureless media, there are several advantages that may be realized by patterning the medium in a hard disk drive. The commercial success of patterned media will of course depend on the relative costs and gains. In particular, there are three main types of disk patterning proposed, each requiring a different length scale of patterned feature. Patterning of servo marks for maintaining the head position on-track, the fabrication of discrete tracks, and the fabrication of discrete bits have all been proposed and will be discussed. For discrete bit recording single domain magnetic islands are required, and one approach to fabricating these, by depositing magnetic films onto prepatterned substrates, is described. The switching characteristics of the islands as compared to those of the full film, along with initial recording results are presented.

Patterned media are a promising approach for future high density magnetic recording. In order to reliably address individual bits, a recording head will be required to reverse a selected island without disturbing neighboring islands. This leads to a requirement for islands with a narrow switching field distribution. We review the switching and recording properties of patterned granular CoPtCr-based films, and present results on the reversal properties of topographically patterned Co/Pd multilayer films, with particular emphasis on switching field distributions. Finally, the use of islands with multiple storage layers is demonstrated as a potential route to even higher storage density without the need for smaller, lithographically defined islands.

Nanofabrication of magnetic storage media, where servo marks, discrete tracks or individual islands are defined, offer the prospect for improved performance and increased areal density. However, this increase in performance will require that new and additional processes be introduced into disk manufacturing. We review here the fundamental patterning and fabrication processes that have been proposed, along with their respective strengths and weaknesses and the potential advantages they may offer for magnetic recording. The increase in data density afforded by nanofabrication may have added significance as more conventional approaches to ever increasing density will encounter physical limitations set by the thermal stability of the recorded bits.

A flexible imprinter can be used to accommodate substrate or template roughness in nanoimprint lithography. The contact mechanics of a multi-layer imprinter incorporating bending and local deformation is described. With the right combination of dimensions, moduli, and viscosity, the imprinter can transfer a pattern evenly to a non-flat substrate. These concepts have been used to pattern magnetic media for high density information storage.

Chemically ordered, self-assembled FePt nanoparticle arrays with high magnetic anisotropy are considered as a candidate medium for data storage beyond 1 Tbit/in2. We report comprehensive structural and magnetic studies on thin (three-layer) assemblies of polyethylenimine (PEI) and 4 nm Fe58Pt42 nanoparticles using x-ray diffraction, small angle neutron scattering, and magnetometry. We show that prior to annealing FePt nanoparticles in the PEI-FePt assembly consist of a metallic magnetic core surrounded by a weakly magnetic or nonmagnetic shell. High temperature annealing creates the desired L10 chemical ordering and results in high coercivity FePt nanoparticles. However, we find that the high temperatures necessary to establish full chemical ordering leads to particle sintering and agglomeration. Understanding the magnetic and physical properties of these assemblies allows future research directions to be clarified for nanoparticle arrays as data storage media.

We have produced assemblies of monodisperse 4 nm FePt nanoparticles using polymer-mediated layer–by–layer deposition at room temperature. The process leads to good control of particle assembly thickness and offers great potential for future fabrication of ultra-high density magnetic storage media. Vibrating sample magnetometry with fields up to 9 T was applied to study the magnetic properties of the particle assemblies as a function of annealing condition while near edge x-ray absorption fine structure (NEXAFS) spectroscopy and x-ray diffraction (XRD) were used to investigate the chemical nature and structural properties within particles. It was found that the coercivity can be as high as 22.7 kOe for samples annealed at 800 °C, the moment density (normalized to the particle volume) for the sample annealed at 650 °C is estimated close to the value for bulk FePt, at 1140 emu/cm3. NEXAFS spectroscopy shows that the Fe in the as-deposited assemblies is partly oxidized, and the oxidation is greatly reduced by...

Monodisperse 4 nm FePt magnetic nanoparticles were synthesized by superhydride reduction of FeCl2 and Pt(acac)2 at high temperature, and thin assemblies of FePt nanoparticles with controlled thickness were formed via polymer mediated self-assembly. Adding superhydride (LiBEt3H) to the phenyl ether solution of FeCl2 and Pt(acac)2 in the presence of oleic acid, oleylamine, and 1,2-hexadecanediol at 200 °C, followed by refluxing at 263 °C, led to monodisperse 4 nm FePt nanoparticles. The initial molar ratio of the metal precursors was retained during the synthesis, and the final FePt composition of the particles was readily tuned. Alternately, adsorbing a layer of polyethylenimine (PEI) and the FePt nanoparticles onto a solid substrate resulted in nanoparticle assemblies with tunable thickness. Chemical analysis of the assemblies revealed that more iron oxide was present in the thinner assemblies annealed at lower temperature or for shorter time. Thermal annealing induced the internal particle structure chan...

A lithographically patterned magnetic medium is one of the proposed routes to magnetic recording at a density beyond that thought to be possible using conventional recording media due to thermal instability caused by superparamagnetism. Using a focused ion beam to pattern a granular Co/sub 70/Cr/sub 18/Pt/sub 12/ film, we have fabricated sub-80-nm size islands that are single domain and with a narrowed switching field distribution and an enhanced thermal stability. Magnetic isolation of the islands is shown to be a result of vanishing of magnetic remanence and coercivity in the irradiated region and not a result of sputtering. Recording measurements using a quasi-static giant magnetoresistive head demonstrate the sensitivity to detect single 80-nm islands. The readback jitter from the patterned region is dramatically reduced compared to that measured for continuous media at the same linear density.

The ion projector at the Fraunhofer Institute in Berlin (manufactured by Ion Microfabrication Systems, IMS, Vienna) has been used to pattern thin magnetic films. Disordering of chemically ordered FePt films by patterned ion irradiation (dose: 10^1^6 He^+ ions/cm^2; 75 keV) without a resist mask on the sample is demonstrated. Magnetically altered areas have been investigated by magnetic force microscopy. Magnetic islands of 340 nm width could be detected, limited by the size of the open stencil mask holes. The projector is capable of 50 nm resolution which would result in a storage density of 64 Gb/in^2. In a second experiment the ion projector has been applied for structured ion milling with Xe^+ ions on thin metallic films also without a resist mask. In a polycrystalline Au film 130 nm lines and spaces with a depth of 8 nm could be produced by a dose of 10^1^5Xe^+ions at 75 keV energy.

Single-crystal silicon cantilevers 1 /spl mu/m thick have been demonstrated for use in high-density atomic-force microscopy (AFM) thermomechanical data storage. Cantilevers with integrated piezoresistive sensors were fabricated with measured sensitivities /spl Delta/R/R up to 7.5/spl times/10/sup -7/ per /spl Aring/ in close agreement with theoretical predictions. Separate cantilevers with integrated resistive heaters were fabricated using the same basic process. Electrical and thermal measurements on these heating devices produced results consistent with ANSYS simulations. Geometric variants of the cantilever were also tested in order to study the dependence of the thermal time constant on device parameters. Depending on the design, time constants as low as 1 /spl mu/s were achieved. A thermodynamic model was developed based on the cantilevers geometry and material properties, and the model was shown to predict device behavior accurately. A comprehensive understanding of cantilever functionality enabled us to optimize the cantilever for high-speed thermomechanical recording.

Piezoresistive atomic force-microscope (AFM) cantilevers with lengths of 10 /spl mu/m, displacement sensitivities of (/spl Delta/R/R)/A 1.1/spl times/10/sup -5/, displacement resolutions of 2/spl times/10/sup -3/ A//spl radic/Hz, mechanical response times of less than 90 ns, and stiffnesses of 2 N/m have been fabricated from a silicon-on-insulator (SOI) wafer using a novel frontside-only release process. To reduce mass, the cantilevers utilize novel inplane crystallographically defined silicon variable aspect-ratio (INCISIVE) tips with radius of curvature of 40 A. The cantilevers have been used in an experimental AFM data-storage system to read back data with an areal density of 10 Gb/cm/sup 2/. Four-legged cantilevers with both imaging and thermomechanical surface modification capabilities have been used to write 2-Gb/cm/sup 2/ data at 50 kb/s on a spinning polycarbonate sample and to subsequently read the data. AFM imaging has been successfully demonstrated with the cantilevers. Some cantilever designs have sufficient displacement resolution to detect their own mechanical-thermal noise in air. The INCISIVE tips also have applications to other types of sensors.

We have studied the current-driven depinning processes for a narrow 12-nm wide one-dimensional Bloch domain wall (DW) in films exhibiting perpendicular magnetic anisotropy (PMA). High sensitivity magnetotransport measurements allow us to observe the motion of the narrow DW between pinned sites separated by /spl sim/20 nm. Thermal fluctuations are found to play a crucial role. A current-driven depinning force two to three orders of magnitude higher than has been seen in conventional in-plane systems is found, suggesting a more efficient spin transfer mechanism in our PMA system.

Zero-field magnetic viscosity measurements at temperatures between 2 and 30 K were made on a sample of chemically disordered FePt nanoparticles prepared by solution phase chemical synthesis. The data have been used to calculate the apparent magnetic-moment-weighted energy barrier distribution for the FePt. This distribution is composed of a log–normal distribution plus a second distribution that is well described by an exponential decay of barrier frequency with increasing barrier height. The two distributions contribute approximately equally to the overall distribution.

L10 FePt nanoparticle assemblies consisting of a few layers of 4-nm-diameter particles, are a potential data storage medium beyond 1 Tbit/in2. However, annealing at temperatures >500 °C is required to form the high anisotropy L10 phase. Recent studies have shown a substantial drop in magnetization for Tanneal>650 °C. We show that this reduction in magnetization is due to silicide formation as a result of a chemical reaction with the native oxide or Si substrate. We also show that full L10 ordering is established only after annealing at 725 °C for 60 min and note that particle agglomeration occurs under these conditions.

Magnetically isolated single domain islands with perpendicular anisotropy have been prepared by depositing Co/Pd multilayer films on prepatterned sub-50 nm SiO2/Si islands. The island arrays were fabricated by both direct write electron beam lithography and nanoimprinting. Nanoimprinting allows the creation of large area, 4 mm×4 mm, samples appropriate for characterization by conventional measurement techniques. Magnetic force microscopy and vibrating sample magnetometry showed that the reversal behavior of the patterned islands is quite different from that of the continuous films with a large increase in both switching field and switching field distribution. Recording on island arrays with a periodicity of 100 nm, produced from prepatterned substrates, was demonstrated using a quasistatic tester.

Magnetic patterns were created by locally altering the magnetic coercivity of a perpendicular anisotropy film by ion irradiation. The resulting pattern, after dc magnetizing, consists of regions having alternating up/down magnetization patterns in remanence. In this case, the magnetization of the soft magnetic material in the exposed segments is reversed and stabilized by the demagnetization fields of the adjacent unexposed media. A magnetic pattern generated in this manner can be used as a type of servo pattern for recording head positioning. An accuracy in positioning of better than 10 nm was demonstrated and deliberate magnetic patterns were written and read back by addressing individual 80 nm single-domain nanostructures.

Isolated tracks of single-domain magnetic islands have been fabricated using focused ion beam lithography. Islands were fabricated in thin films having either perpendicular or longitudinal magnetic anisotropy, and in both cases writing and reading of individual islands was demonstrated using a static write/read tester. Additionally, we have investigated the synchronization requirements (write margin) needed for writing the different flavors of patterned media. We have found that the write margin is strongly dependent on the switching field distribution of the islands and on the head field gradient. In the presented study, the write margin is much larger for the longitudinal patterned media than for perpendicular patterned media.

Monodisperse FePt nanoparticles were prepared using high-temperature solution phase synthesis. Polymer-mediated layer-by-layer growth leads to precise control of the particle self assembly. The narrow particle size distribution (σ⩽5%) offers the potential for increased data storage density by utilizing a smaller mean particle size and ultimately storage of one bit per individual nanoparticle. We have studied self-assembled multilayers of magnetic FePt nanoparticles. The L10 phase of FePt has a very high magnetic anisotropy that allows the magnetization of particles of about 4 nm diameter to be thermally stable at room temperature. Magnetic measurements using a vibrating sample magnetometer were combined with x-ray diffraction (XRD) and near edge x-ray absorption fine structure (NEXAFS) spectroscopy to study the annealed FePt nanoparticle assemblies and to optimize annealing conditions. NEXAFS spectra showed that a fraction of the iron in the as-deposited particles was oxidized, and this fraction was reduc...

A nanomolding process for producing 55-nm-diameter magnetic islands over 3-cm-wide areas is described. A master pattern of SiO2 pillars is used to form a polymeric mold, which is in turn used to mold a photopolymer resist film. This latter film is used as a resist for etching SiO2, yielding a pattern of pillars. Finally, an 11-nm-CoPt multilayer is deposited. Magnetic force microscopy reveals that the film on top of each pillar is a magnetically isolated single domain that switches independently.

Co/Pt multilayers can exhibit large perpendicular anisotropies and coercivities that are very sensitive to the quality of the Co/Pt interfaces. We have characterized the dependence of coercivity of Co/Pt multilayers on irradiation with various ion species (He+, Ar+, and Ga+), energies (20 keV–2 MeV), and doses (1011–1017 ions/cm2), in order to understand better the nature of the structural changes responsible for the magnetic changes. We find that, in general, the system is much more sensitive to irradiation than expected on the basis of a nearest-neighbor coupling model and simple ballistic ion-beam mixing.

This chapter contains sections titled: Introduction Bit-Patterned Media and Discrete Track Media Fabrication of Patterned Structures Recording Systems Producing a Patterned Media Drive References

The application of radio-frequency (rf) currents in spin-torque devices allows control of magnetization reversal through frequency of the rf current. Using this effect, the impact of the rf currents on switching fields of nanostructured metallic spin valves has been investigated. The rf current injection was observed to change the critical field for free-layer magnetization reversal when the intrinsic spin-transfer-induced dynamics are frequency locked with the injected rf. Experimental results are presented by mapping the switching contour plot, and are compared with simulations in the context of macrospin models of spin transfer in metallic-spin-valve structures. Differences between this effect and the effect of rf currents on direct-current-induced magnetization reversal are highlighted by considering the role of a magnetization dispersion in the two sweep methods, which allows insights for device applications assisted by rf currents.

The three-dimensional magnetic structure and reversal mechanism of patterned Co/Pt multilayers, were imaged using complementary Lorentz transmission electron microscopy (LTEM) (in-plane component) and magnetic transmission x-ray microscopy (M-TXM) (perpendicular magnetization). The Co/Pt films with perpendicular anisotropy were patterned by ion irradiation through a stencil mask to produce in-plane magnetization in the irradiated regions. The boundaries of the patterns, defined by the transition from out-of-plane to in-plane magnetization, were found to be determined by the stencil mask, whilst the scale of the magnetic reversal by the physical microstructure. The nucleation fields were substantially reduced to 50 Oe for the in-plane regions and 1 kOe for the perpendicular regions, comparing to 4.5 kOe for the as-grown film. The perpendicular reversals were found to always originate at the pattern boundaries.

We describe the concept of using irradiation with light energetic ions in conjunction with a mask to define the edges of giant magnetoresistive sensors without milling. The irradiated material undergoes substantial interfacial mixing, and consequent loss of magnetoresistance (MR), accompanied by small changes in resistivity, and no significant change in magnetic moment. The ion species and energy may be chosen to minimize lateral scattering as they pass through the sensor film. Modeling of a perfect mask edge above a sensor film of thickness /spl sim/240 /spl Aring/ irradiated with a well-collimated ion beam indicates that sensor edge definition of /spl plusmn/20 /spl Aring/ is achievable-a vast improvement over sensor edges generated using ion milling, which may run to hundreds of Angstroms in width. Experimental data from irradiation of typical NiO, IrMn, NiMn and PtMn-based spin valves using 700 keV N/sup +/ ions indicate that doses between 10/sup 15/ and 10/sup 17/ ions/cm/sup 2/ are sufficient to diminish the MR of the films to below 5% of the as-grown value. Changes in MR caused by the irradiation are stable against annealing at temperatures typical for setting these sensors.

We have used a focused ion beam to directly pattern thin-film granular perpendicular Co/sub 70/Cr/sub 18/Pt/sub 12/ media. By cutting trenches /spl sim/6 nm deep in the /spl sim/20 nm media, we have produced square arrays of magnetically isolated islands with periods in the range 65-500 nm. At periods below /spl sim/130 nm we observe only single magnetic domains, which exhibit a rough "checker-board" pattern upon ac demagnetization. We have patterned at densities of over 140 Gbit/in/sup 2/.

Electron beam evaporated Co/Pt multilayers {[Co(tCo nm)/Pt(1 nm)]10, 0.2 250 °C show fine-grained MFM features on the sub-100-nm length scale indicating reversal dominated by localized switching of small clusters. High-resolution cross-sectional transmission electron microscopy (TEM) with elemental analysis shows columnar grains extending throughout the multilayer stack. Co depletion and structural defects at the grain boundaries provide a mechanism for exchange decoupl...

Intrinsic magnetic defects in perpendicularly magnetized nanostructures reduce the predictability of device and developing recording technologies. In addition to a distribution of local anisotropy fields, we show that such defects also exhibit variations in local anisotropy axes. The magnetic defects are identified by the application of in-plane and out-of-plane magnetic fields and magnetic force microscopy imaging. Those defects that control magnetization reversal in arrays of patterned Co/Pd multilayers are highly dependent on applied field orientation. The symmetry of the defects with respect to the applied field direction indicates that the anisotropy consists of a canted axis, deviating from the surface normal. Micromagnetic simulations confirm that variations in anisotropy axis can cause a significant change in reversal field depending on the location and orientation of the defects, consistent with experimental results.

Co/Pt multilayer films with perpendicular magnetic anisotropy and large out-of-plane coercivities of 3.9 - 8.5 kOe have been found to undergo a spin reorientation transition from out-of-plane to in-plane upon irradiation with 700 keV nitrogen ions. X-ray reflectivity experiments show that the multilayer structure gets progressively disrupted with increasing ion dose, providing direct evidence for local atomic displacements at the Co/Pt interfaces. This effectively destroys the magnetic interface anisotropy, which was varied by about a factor of two, between KS@ 0.4 erg/cm2 and KS@ 0.85 erg/cm2 for two particular films. The dose required to initiate spin-reorientation, 6x1014 N+/cm2 and 1.5x1015 N+/cm2, respectively, scales with KS. It is roughly equal to the number of Co interface atoms per unit interface area contributing to KS.

We have used ion beam irradiation through a silicon stencil mask to alter the magnetic properties of Co/Pt multilayer and FePt chemically-ordered superlattice films. In both systems, ion irradiation disorders the as-grown films which results in a reduction of the magnetic anisotropy. Regularly spaced micrometer-sized regions of magnetically altered material have been produced over areas of a square millimeter. These magnetic structures have been observed by magnetic force microscopy. By stepping the mask during irradiation, features at twice the spatial frequency of the mask holes have been produced. Such patterned magnetic films are of interest for application in high-density magnetic recording.

Previously, ion-beam irradiation has been shown to locally alter the magnetic properties of thin Co/Pt multilayer films. In this work, we have used ion-beam irradiation through a silicon stencil mask having 1-μm-diam holes to pattern a magnetic film. Regularly spaced micrometer-sized regions of magnetically altered material have been produced over areas of a square millimeter in Co/Pt multilayers. These magnetic structures have been observed by magnetic force microscopy. The patterning technique is demonstrated with mask–sample spacing as large as 0.5 mm. In addition, smaller regions of magnetic contrast, down to 100 nm, were created by using two masks with partially overlapping micrometer holes. Such patterned magnetic films are of interest for application in high-density magnetic recording.

We demonstrate that magnetic reversal in perpendicularly magnetized nanostructures is highly dependent on the nature and condition of the edges. To understand the impact of edge damage, we compare nanostructures created by ion milling to those prepared on prepatterned substrates. The size- and temperature-dependent reversal properties of 25 nm–1 m diameter nanodots show that reversal in prepatterned nanostructures is controlled by nucleation within the interior, whereas ion milling results in an edge nucleation process with an unpredicted temperature dependence of the reversal field.

We have measured the critical phase change conditions induced by electrical pulses in Ge2Sb2Te5 nanopillar phase change memory devices by constructing a comprehensive resistance map as a function of pulse parameters (width, amplitude and trailing edge). Our measurements reveal that the heating scheme and the details of the contact geometry play the dominant role in determining the final phase composition of the device such that a non-uniform heating scheme promotes partial amorphization/crystallization for a wide range of pulse parameters enabling multiple resistance levels for data storage applications. Furthermore we find that fluctuations in the snap-back voltage and set/reset resistances in repeated switching experiments are related to the details of the current distribution such that a uniform current injection geometry (i.e. circular contact) favors more reproducible switching parameters. This shows that possible geometrical defects in nanoscale phase change memory devices may play an essential role in the performance of the smallest possible devices through modification of the exact current distribution in the active chalcogenide layer. We present a three-dimensional finite element model of the electro-thermal physics to provide insights into the underlying physical mechanisms of the switching dynamics as well as to quantitatively account for the scaling behaviour of the switching currents in both circular and rectangular contact geometries. The calculated temporal evolution of the heat distribution within the pulse duration shows distinct features in rectangular contacts providing evidence for locally hot spots at the sharp corners of the current injection site due to current crowding effects leading to the observed behaviour.

A force microscope has been used to deposit and image localized charge on insulating surfaces. The charge is deposited either by applying a voltage pulse to the microscope tip or by contact charging the insulator surface with the tip. An improved mode of charge imaging is presented, which distinguishes between charge and topography and allows the sign of the charge to be determined in a single scan.

A dual-axis atomic force microscope (AFM) cantilever with independent piezoresistive sensors has been developed for simultaneous detection of vertical and lateral forces. The cantilever consists of a flat, triangular probe connected to a base by four tall, narrow ribs. The vertically compliant triangular probe and the laterally compliant ribs incorporate separate piezoresistors for vertical and lateral force sensing. In the fabrication process, a special oblique ion implant technique is used to produce electrical elements on vertical sidewalls and horizontal surfaces of the cantilever structure at the same time. The dual-axis cantilever has been used to perform microfriction measurements as well as obtain simultaneous vertical-force and lateral-force AFM images.

We demonstrate that the switching field distribution (SFD) in arrays of 50 nm to $5\text{ }\text{ }\ensuremath{\mu}\mathrm{m}$ $\mathrm{Co}/\mathrm{Pd}$ elements, with perpendicular anisotropy, can be explained by a distribution of intrinsic anisotropy rather than any fabrication related effects. Further, simulations of coercivity and SFD versus element size allow the distribution of intrinsic anisotropy to be quantified in highly exchanged coupled thin films where the reversal mechanism is one of nucleation followed by rapid domain wall motion.