X-ray magnetic circular dichroism (XMCD) spectroscopy is a key technique for studying magnetic materials, providing element specificity and significantly high detection sensitivity to magnetic mome...

To investigate the possible sources of the switching field distribution in bit-patterned media, we applied a scanning hard-X-ray nanoprobe technique based on X-ray magnetic circular dichroism spectroscopy to an array of Co-Pt dots with the typical diameters of 200 nm. Element-specific magnetization curves at the Pt L-3 edge were measured for individual dots isolated from each other, and the magnetization switching field (coercivity) values were determined for more than 100 individual dots. To assess the effect of dot diameters as a possible source of switching field distribution, a statistical analysis of the coercivity values and dot diameters measured for many dots was performed. The resulting switching field distribution had a mean of < HSWi > = 1.80 kOe and a standard deviation of sigma H-SW = 0.64 kOe. The relative deviation of sigma HSW/< HSWi > = 36% was not in good agreement with the relative dispersion in a dot diameter of sigma D/< Di > = 2.7%, and no clear correlation between the coercivity and dot diameter was observed. These results may suggest other possible sources of switching field distribution than dot diameter, such as dispersion in the c-axis orientation and in magnetocrystalline anisotropy. Published by AIP Publishing.

The fabrication of FePt nanodots with a high structural quality and the control of their switching fields are key issues in realizing high density bit pattern recording. We have prepared FePt dot patterns for dots with 15–300nm diameters by electron beam lithography and re-annealing, and studied the relation between magnetization reversal process and structure of FePt nanodots. The switching field (Hsw) of dot patterns re-annealed at 710°C for 240min showed a bimodal distribution, where a higher peak was found at 5–6T, and a lower peak was found at ~2T. It was revealed by cross-sectional TEM analysis that the structure of dots in the pattern can be classified into two groups. One group has a high degree of order with well-defined [001] crystalline growth, and the other group includes structurally-disturbed dots like [111] growth and twin crystals. This structural inhomogeneity causes the magnetic switching field distribution observed.

We have studied the coercive force of fine magnetic CoPt dot enhanced by nanometer-sizing of the dot. The 39-to 106-nm-sized CoPt magnetic dot arrays were formed by30-keV-electron beam (EB) drawing with thin calixarene resist and 200-eV-Ar ion milling. Using the fine magnetic dot arrays, we measured coercive forces of the dot by x-ray magnetic circular dichroism (XMCD) with an energy of 11.56 keV, which corresponds to an energy edge of Pt-L3. It is experimentally demonstrated that a coercive force of the nanometer-sized magnetic column increased as the dot diameter decreased. Keywords: Coercive force, nanomagnetic column, XMCD, magnetic storage, EB lithography

We upgraded the hard X-ray spectroscopy beamline BL39XU at SPring-8 by installing Kirkpatrick-Baez mirrors to equip it with submicron-focusing capability. At the new experimental end-station located 74 m from the undulator source, an X-ray nanoprobe with a size of 120 × 100 nm2 has become available at 5–16 keV. This upgrade is fully compatible with the X-ray polarization tunability and helicity-switching between right- and left-circular polarizations; these are unique features of the beamline, which is equipped with a diamond X-ray phase retarder. A spatial resolution of ≈100 nm for X-ray absorption fine structure and X-ray magnetic circular dichroism (XMCD) spectroscopy has been achieved. We show a magnetization reversal process of an individual magnetic dot in bit-patterned perpendicular recording media using element-specific XMCD magnetization measurements to demonstrate the performance of the new station.

Deposition of inclined anisotropy film for bit-patterned media was studied using oblique incidence collimated sputtering. Co-Pt20 films with a thickness of 10 nm deposited on an annealed Pt/Ru under layer exhibited an inclination angle of the anisotropy axis of around 10° from the film normal corresponding to that of crystalline orientation. The anisotropy field and the inclination angle were estimated by comparing measured hysteresis loops with simulated loops. The estimated anisotropy field of the film, μ0Hκ, was around 1.2 T which indicated an expected anisotropy energy density of 6×105 J/m3. It was indicated that oblique incidence collimated sputtering is useful to fabricate inclined anisotropy recording media with high anisotropy.

FePt nanodot arrays are patterned and investigated by X-ray magnetic circular dichroism measurements and magneto-optic Kerr effect magnetometry combined with pulse magnetic fields. The experimental results on varied timescale of the applied field are analyzed by Sharrock's formula, showing the emergence of hard and soft magnetic FePt grains with distinct perpendicular anisotropy. While the hard grains in L10-phase can construct the FePt dots with intrinsic perpendicular anisotropy fields around 90kOe, the exchange coupling between the hard and soft grains in other dots degrades the dot perpendicular coercivity and widens the array switching field distribution, in both the 100 and 30nm dot arrays. The dot size dependence of the proportion of the hard dot in the array demonstrates that the soft grains originate from the FePt grains of L10-phase with large c-axis misaligning and of fcc phase inside the dots, and the ion etching effects are insignificant.

FePt films that have a high degree of order S in their L10 structure (S>0.90) and well-defined [001] crystalline growth perpendicular to the film plane were fabricated on thermally oxidized Si substrates by the addition of an oxide and successive rapid thermal annealing (RTA). The mechanism of L10 ordering and [001] crystalline growth perpendicular to the film plane arising through the oxide addition and RTA process is also discussed. The L10 ordering (S>0.90) and the [001] crystalline growth were achieved by (1) lowering the activation energy due to in-plane tensile stress and the initiation of L10 ordering at a low temperature, (2) [001] crystalline growth through in-plane tensile stress, and (3) enhancement of atomic diffusion via the addition of an oxide and the resultant lowering of the ordering temperature. Effect (1) was observed in the case of SiO2 addition, effect (2) was generally observed in the case of oxide addition and the RTA process, and effect (3) was prominent in the case of ZnO addition. With the addition of ZnO, the L10 ordering started at below 400°C and was completed at 500°C. Finally, dot patterns were successfully fabricated down to a diameter of 15nm using electron beam lithography, and the magnetic state of the dot pattern was observed by magnetic force microscopy.

A microscopic magnetometer using magneto-optical Kerr effect (MOKE) was developed to characterize the magnetic properties of hard magnetic nano-structures such as a bit-patterned medium and a magnetoresistive random access memory. Our new type magnetometer has a unique feature that adopts the reflective objective instead of the generally used refractive lens to reduce the unnecessary rotation of polarization axis of the light by the lens with Verdet constant in a magnetic field. A Schwarzschild-type objective consisting of two spherical mirrors was applied as the reflective objective in our magnetometer. The objective was designed specifically for our magnetometer. An actual focusing spot diameter at the sample surface was estimated to be 4.7 μm by the knife-edge measurement. Furthermore, a magnetization curve was measured by MOKE for the Co80Pt20 thin-film line with a width of 75 μm, and demonstrated that our magnetometer can reduce the unnecessary rotation of polarization axis compared with the one measured by the magnetometer with a refractive lens.

FePt dot arrays with dot size down to 15nm are fabricated by film annealing and patterning. The array coercivity shows an increase with dot size decreasing from 100 to 30nm, and a slight reduction for the 15nm dot sample. Annealing these dot arrays at higher temperatures results in large enhancements in the coercivities, except the 15nm dot array where the coercivity increases a little. Micromagnetic models of a 15nm FePt dot with uniform and nonuniform edges of soft magnetic defects and with inside defects are calculated to reveal the microstructure origins of the dot magnetic properties. It is found that the volume fraction of the L10-phase FePt with perpendicular c-axis orientation is about 50% in the dot and the switching field distribution of the dot array can be influenced significantly by the defect arrangement in the dots.

We have studied the possibility to form fine magnetic column arrays using 30-keV-electron beam (EB) drawing with thin calixarene resist and 200-eV-Ar ion milling, and nanometer-sizing effect of the magnetic column on the corecive force for patterned media. We achieved 20-nm-sized resist dot arrays on PtCo magnetic and thin metals layers on glass substrate. We formed fine magnetic column arrays with a diameter of 39 to 106 nm and a space of about 100 nm using the resist pattern by the 200-eV Ar ion-milling. Using the nano magnetic column arrays, the hysteresis were measured by X-ray magnetic circular dichroism (XMCD) with an energy of 11.57 keV, which corresponds to an energy edge of Pt-L3. It is clarified that a coercive force of the nanometer-sized magnetic column increased as the diameter decreased.

FePt dot arrays with dot sizes ranging from 100 down to 15 nm were fabricated using sputtering, annealing and patterning techniques. The dot diameter distribution and dot position deviation are greater for smaller dot arrays than for larger dot arrays. The dot arrays produced through a sequence of annealing followed by patterning have a large perpendicular magnetic anisotropy resulting from the highly L1(0)-ordered structure and the perpendicular orientation of the [0 0 1] crystalline axis, whereas samples of annealing after patterning display a magnetic vortex structure. As the dot size reduces from 100 to 29.9 nm, the arrays produced by annealing and then patterning exhibit an increase in the remanent coercivity H-cr from 15.7 to 21 kOe because fewer defects are contained by the smaller dots. This result is explained by nucleation and domain wall propagation mechanisms. For 15.4 nm dot arrays, a model of dot with damaged edge may interpret the decreasing coercivity behaviour.

The switching field width (SFW) in bit-patterned media is one of the important factors affecting the recording performance. The origin of SFW in Co80Pt20 magnetic dot array patterned by electron beam lithography was investigated. The SFWs of dot arrays with a dot size of 29nm and period of 40–100nm were measured from element-specific magnetic hysteresis curve based on xray magnetic circular dichroism. The experimentally obtained SFWs suggested that the intrinsic SFW caused by the distribution in structural and magnetic properties of each dot is 1.2 kOe when additional magnetostatic interaction between the dots is assumed. Distribution in the dot size was investigated as one of the causes of the intrinsic SFW. However, it was estimated that a dot size distribution of 1nm broadens the SFW by 0.14 kOe which is only about 12% of the intrinsic SFW. Primary cause of the intrinsic SFW should be attributed to others.

In order to improve the magnetic intergranular isolation between the magnetic grains in the SmCo5 perpendicular magnetic recording media, the palladium nuclei deposited by an electrochemical process were introduced into a sputter deposition process of the SmCo5 film. A few nanometer size Pd nuclei were electrochemically deposited on the sputtered Cu underlayer by a displacement deposition (chemical plating). The sizes of Pd nuclei were controlled by adjusting the Pd ion concentration in electrolyte solutions. The magnetic domain size in Sm-Co layer deposited on Pd nuclei / Cu / Ti underlayer became smaller and the magnetization reversal process was changed from the wall motion to the coherent rotation. Moreover, the read/write characteristics were improved at higher linear recording densities.

This is a co-publication with Journal of Physics D: Applied Physics. The bulk of the papers, after peer review, are published in Journal of Physics: Conference Series. However a selection of papers, are published separately in a special issue of Journal of Physics D: Applied Physics.

Bit patterned media with inclined anisotropy were proposed along with media design with a higher saturation magnetization and a small thickness for the dot. Inclination of the anisotropy axis was found to reduce broadened switching field distribution of the media caused by the magnetostatic interaction between the dots. Recording simulation at an areal density of 2.6 Tbit/in(2) using a write field of a shielded planar head exhibited substantially increased write shift margins of 6.5 nm and 10 nm in down-and cross track directions, respectively, for the media with inclined anisotropy when the anisotropy dispersion of the media was as small as 2%. Recording density was expected to be increased beyond 4 Tbit/in(2) for the media with additional exchange coupling between the dots.

Dot arrays of Co-Pt with a dot density of 1 Tdot/in2 were fabricated using electron beam writing. Low-energy ion etching was used to prevent the dots from deteriorating the magnetic properties. We found from the magnetic properties measured using x-ray magnetic circular dichroism that etching with low-energy ions not only caused little damage to the magnetization but also that the anisotropy field of the dots resulted in high coercivity. These results demonstrate the effectiveness of etching with low-energy ions to achieve bit-patterned media with an areal density of 1 Tbit/in2. Furthermore, the magnetic dot array with a dot period of 25 and 30 nm was fabricated by electron beam writing and etching with low-energy ion. Coercivity of these dot arrays was found to be more than 5kOe.