This chapter discusses polarization dependent X-ray photoemission electron microscopy (X-PEEM) and its application to coupled magnetic layers, in particular ferromagnet-antiferromagnet structures.

A state of the art X-ray photoemission electron microscope (PEEM2) is operational at the Advanced Light Source at a resolution of typically 50nm for a range of chemical and magnetic surface studies. A new microscope, PEEM3, is under development with an aim of achieving a resolution of 5nm and more than an order of magnitude increase in transmission at the nominal resolution of PEEM2. The resolution and flux improvement is realized by providing geometric and chromatic aberration compensations in the system using an electron mirror and a beam separator magnet. The nearly aberration-free design of the beam separator is critical to the performance of third generation PEEMs. In this paper, we present the optics design model, optimal operation parameters, analyses of aberration impact, as well as the mechanical alignment tolerance for PEEM3 separator prototypes. In particular, we emphasize the importance of a new semi-analytical approach to design complex charged particle optics using the truncated power series algebra. Because of its ability to compute high-order aberrations, this approach allows systematic and comprehensive analyses of any charged particle optics systems with analytical electric and magnetic fields.

We show that under certain conditions, highly Co-enriched TiO2 anatase clusters nucleate on epitaxial TiO2 anatase grown on LaAlO3(001) by oxygen plasma assisted molecular beam epitaxy. In the most extreme cases, virtually all incident Co segregates to the clusters, yielding a nanoscale ferromagnetic phase that is not ferromagnetic in homogeneous films of the same Co concentration. The nucleation of this phase simultaneous with continuous epitaxial film growth must be carefully monitored in order to avoid drawing false conclusions about the film structure.

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 reduced by annealing in inert or reducing atmospheres. A very thin layer (<0.4 nm) of oxide surrounding the particle is sufficient to explain the observed spectra. Structural analysis using XRD showed that a minimum temperature of 450 °C was required to start the formation of the ordered ferromagnetic phase. Annealing for longer times and at higher temperatures led to higher coercivity and a larger fraction of ordered phase but also to the onset of some agglomeration of the nanoparticles.

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 annealing. XRD studies on the assemblies annealed at high temperature revealed the increased atomic order and the formation of the high-anisotropy L10 phase within the particles. However, the high-temperature annealing also resulted in nanoparticle agglomeration.

Summary form only given. Co/sub 70/Cr/sub 18/Pt/sub 12/ perpendicular media has been patterned using a focussed beam of Ga/sup +/ ions (FIB) into arrays of sub -100nm islands suitable for demonstrating high density recording. The islands show single domain behaviour below a critical size, which in this case is approximately 110nm. In order to characterise the change to the magnetic properties introduced by FIB patterning, we have measured reversal properties including switching field distributions and thermal decay rates by magnetic force microscopy (MFM) imaging and Kerr rotation. The MFM was used to measure small arrays of islands by counting reversed islands at remanence as a function of reverse field or decay time. The Kerr measurements were averaged over larger arrays using a /spl sim/20/spl mu/m focussed laser beam. As a comparison unpatterned media was measured both using Kerr rotation and vibrating sample magnetometer (VSM).

The modeling of the optical properties of the acceleration field and objective lens of a photoemission electron microscope (PEEM) is presented. Theory to calculate the aberrations of the extraction field was derived, and extended to include relativistic effects. An analysis of the microscope’s electron optical performance and aberrations has been performed using an analytical model as well as a ray tracing method. Ray tracing has the flexibility needed for the assessment of aberrations where the geometry is too complex for analytical methods. This work shows that in the case of a simple PEEM front end of the acceleration gap and objective lens, the all orders ray tracing and full analytical treatments agree to very high precision. This allows us now to use the ray tracing method in situations where analytical methods are difficult, such as an aberration compensating electron mirror.

Epitaxial (111)-oriented CoPt3 films were deposited on WSe2(0001) substrates at room temperature using molecular beam epitaxy. We observed strong growth induced uniaxial perpendicular magnetic anisotropy which has a maximum of 3.2×106 erg/cm3 and coercivity of about 200 Oe for films with thicknesses <6 nm. At a thickness larger than 6 nm the easy magnetization axis progressively rotates into the plane of the film as the film thickness is increased. The magnetic domain structure in films with perpendicular magnetic anisotropy was investigated by photoemission electron microscopy revealing a characteristic thickness dependence near the reorientation transition.

Patterned magnetic media, where the bit cells are predefined, offer a potential path to ultrahigh density data storage. Here we report results on the magnetic and recording properties of islands with lateral dimensions on the order of 80 nm. Prototype nanometer-scale magnetic structures were made by patterning single-layer Co70Cr18Pt12 perpendicular media using a focused ion beam of Ga+. The effects of patterning were investigated by comparing patterned and unpatterned regions of the same medium. The magnetic reversal properties of the patterned media were obtained from magnetic force microscopy images, while those for the unpatterned media were obtained from vibrating sample magnetometer measurements. At remanence the unpatterned region decays at 1.4% per decade, whereas the patterned region shows no measurable change over the course of the experiment (5×105 s). We have also studied aspects of the recording physics of patterned media using a quasistatic write/read tester with a giant magnetoresistance head. We have written a square wave bit pattern that matches the respective island periodicity and investigated the readback signal for out-of-phase and in-phase writing.

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.

Summary form only given. Patterning magnetic media with pre-defined, single-domain magnetic bits is one possible candidate for extending magnetic storage densities beyond the limit set by thermal decay for continuous media. In order to study the writing and read-back characteristics of high density patterned media, prototype nanometer-scale magnetic structures were made by patterning Co/sub 70/Cr/sub 18/Pt/sub 12/ perpendicular medium using a focused ion beam (FIB) of Ga/sup +/. The islands have lateral size of 80 nm and have a 103 nm along track period. For comparison, we have also used the FIB to prepare isolated narrow strips of continuous media. The recording properties of the patterned and unpatterned media were evaluated using a static write/read tester.

We have fabricated arrays of magnetic islands in perpendicular CoCrPt media with ∼100 nm lateral dimension using a focused ion beam. A quasistatic write/read tester was used to study aspects of the recording physics of these patterned media. We present results on the variation of the readback signal as the phase of the written square wave changes with respect to the patterned array as a function of island size and write current. Using an analytic near-field expression for head field we are able to model how the observed dependence between phase shift and readback signal as a function of write current arises. This analysis allows us to gain an insight into the role of the island switching field distribution and the write head magnetic field gradient in the patterned media writing process.

The limits to scaling the relevant physical dimensions required to increase the areal density of magnetic storage devices will be reached soon, if the storage density continues to double annually. Two approaches to overcoming the limit of the minimum particle size required for thermal stability are presented. In the first approach, a narrow particle size distribution is produced using self-assembled layers of magnetic Fe–Pt nanoparticles. The very narrow particle size distribution offers the potential for increased storage density by utilizing a smaller mean particle size and ultimately storage of one bit per individual nanoparticle. The second approach involves patterned magnetic Co–Cr–Pt nanostructures produced using a focused ion beam, which offers the possibility of single bit per island storage on thermally stable sub-100-nm islands.

Using x-ray absorption spectromicroscopy we have imaged the uncompensated spins induced at the surface of antiferromagnetic (AFM) NiO(100) by deposition of ferromagnetic (FM) Co. These spins align parallel to the AFM spins in NiO(100) and align the FM spins in Co. The uncompensated interfacial spins arise from an ultrathin CoNiOx layer that is formed upon Co deposition through reduction of the NiO surface. The interfacial Ni spins are discussed in terms of the "uncompensated spins" at AFM/FM interfaces long held responsible for coercivity increases and exchange bias. We find a direct correlation between their number and the size of the coercivity.

Polarization dependent x-ray photoemission electron microscopy was used to investigate the spin structure near the surface of an antiferromagnetic NiO(001) single crystal in response to the deposition of a thin ferromagnetic Co film. For the cleaved NiO surface we observe only a subset of bulklike antiferromagnetic domains which is attributed to minimization of dipolar energies. Upon Co deposition a spin reorientation near the NiO interface occurs, with the antiferromagnetic spins rotating in plane, parallel to the spins of the Co layer. Our results demonstrate that the spin configuration in an antiferromagnet near its interface with a ferromagnet may significantly deviate from that in the bulk antiferromagnet.

Co/Pt thin film multilayers with strong perpendicular anisotropy and out-of-plane coercivities of 5-11 kOe were magnetically altered in areas of local ion beam interaction. The ion irradiations were performed by ion projection through silicon stencil masks fabricated by silicon on insulator (SOI) membrane technology. The ion projector at the Fraunhofer Institute for Silicon Technology (ISiT) was operated at 73 keV ion energy and with a 8.7- fold demagnification. After exposure to 3 × 1014Ar+/ cm2 magnetic islands smaller than 100 nm in diameter were resolved in the Co/Pt multilayersby means of magnetic force microscopy. The impact of different ion species (He+, Ar+ and Xe+) and ion energies (10 - 200 keV) on the multilayer structure was evaluated using Monte Carlo simulations. The ballistic interface intermixing was used to predict magnetic coercivity changes for various irradiation conditions. The simulations revealed that with 73 keV Ar+ and Xe+ ions the irradiation dose could be reduced by a factor of 100 and 400 respectively in comparison to 73 keV He+which was verified in the experiments. X-ray reflectivity measurements confirmed that the Co/Pt superlattice structure is slightly weakened during the irradiation and that the surface smoothness of the media is preserved. Using the Ion Projection Process Development Tool (PDT) at IMS-Vienna concentric data tracks including head positioning servo informations were patterned onto a 1” IBM microdriveTM glass disk which was coated with Co/Pt multilayers. In a single exposure step several tracks within an exposure field of 17 mm in diameter were structured by 2 × 1015He+/ cm2 at 45 keV using a 4- fold demagnification set-up.