Fabrication of nanometer-scale magnetic features

We fabricate magnetic films with different sizes and shapes of defect features to study their magnetic behavior

Nanostructured thin films can be formed using a number of techniques, including electron beam pattering stamping and replication, holographic lithography, and others. We describe here a few of the methods that were used in our studies.

a. E-Beam Lithography

Fig. 1 Array of micron to submicron Permalloy islands showing domain configurations as a function of the aspect ratio. Sample provided by Jong-Ching Wu at National Changhua University of Education

Magnetic domain patterns of Fe₂₀Ni₈₀ magnetic thin films with different sizes are shown in Fig. 1. The square patterns of the thin films were made by e-beam lithography. It shows that the domain pattern of the elongated shape film is not expected to have a single magnetic domain (The shape anisotropy of a soft magnetic film along the long axis is large). This observation is direct evidence that other factors (such as stress, crystalline defects, etc) may play important role in the nanometer-size magnetic films. The schematic diagrams shown at the left of Fig.1 indicate that the cross-tie-domain structures were formed when the shapes of the films were varied.

b. Self-Assembled Arrays

Recently, considerable research activity has been devoted to electrochemical self-assembly techniques and their application to the fabrication of magnetic nanostuctures. It has been known for many years that when aluminum is anodized in an acid electrolyte, aluminum oxide with a densely packed hexagonal array can be formed. As shown in Fig. 2, a atomic force microscopy image shows the high degree of perfection that can be obtained in an array with 70 nm diameter pores with 100 nm center-to-center separations. We will deposit a soft magnetic film on top of these structures. It will allow us to study magnetic reversal effect by a periodic and nanometer-size array. L. Torres et al. has predicted that the antidot size around 80 nm leading to maximum areal storage density of about 10Gbits/in². Their results were interpreted in terms of demagnetizing, anisotropy, and exchange energies’ balance. It is interesting to compare directly observed domain structures with the micromagnetic simulations.

Fig. 2 A atomic force microscopy image shows in an array with 70 nm diameter pores with 100 nm center-to-center separations. Collaborate with David Sellmyer and Min Zheng at UNL

c. Nanometer Size Permanent magnets

As shown in Fig.3, we have prepared CoPt thin films with thickness of 5nm by magnetron sputtering. After annealing in an Ar/H₂ atmosphere at temperatures at 650^oC for 12 hours, we showed that a magnetic coercivity (H_c) of 20kOe was obtained in a CoPt thin film that contains separated nanometer-size CoPt crystallites. From atomic force microscopy and magnetic force microscopy studies, the magnetic single domain size of CoPt is in the range of 100 to 200nm. The high H_C is likely due to the well-separated nanometer-size crystallites and the well-ordered fct phase of CoPt alloy. These film types can be used to investigate two-phase nanomagnets (CoPt nanometer-size particles with a ferromagnetic over-coating). It will also permit us to study how the magnetic domain structures of soft magnetic layer is affected by a hard magnet.

Fig. 3 (a) the atomic force microscopy image the 5nm-thick film contains well separated nanometer-size crystallites in the range of 100nm to 400nm. The height of crystallites is in the range of 20nm to 80nm. (b) the magnetic force microscopy (MFM) image was obtained using a CoPt MFM tip magnetized parallel to the sample surface. The light and dark contrast corresponds to the strength of the stray-field gradient on the sample surface. The lighter color represents frequency shift in the MFM tip when the magnetization of the sample and that of the MFM tip are repulsive. The crystallites with one light and dark area are single-domain (as indicated by "S"); the grain that may contain a few crystallites with two or more light and dark areas are multi-domain (as indicated by "M"). The size of a single-domain crystallite is between 100-200nm. (c). An H_c value of 20kOe and a saturation magnetization of 668 emu/cm³ were observed in the sample annealed at 650^oC for 12 hours.

d. Lithography using other techniques

Ion beam irradiation has been shown to locally alter the magnetic properties of Co/Pt multilayer films. As shown in Fig. 4, the magnetic film was patterned by irradiated N⁺ ion with a dose of 10¹⁶ ion /cm² at 700 keV through a silicon stencil mask having about 1.3 micrometer diameters holes. Localized modification of the magnetic coercivity and easy magnetization axis with ion irradiation was recently demonstrated by Chappert et al. At a high enough dose, 10¹⁶ ion /cm², the easy axis of Co/Pt multilayer films is rotated into the plane of the film. As shown in Fig. 4(a), the magnetic force microscopy image has roughly circular pattern that is similar to what expected for a 90^o domain wall. The advantage of this process is as follows: (i) It requires no photo- or electron-resists in the process, i.e. it requires no additional clearing process. (ii) The process changes the surface topography to a minimum (about 2nm), as compared to the etching methods. The planization and polishing steps can be avoided.

Fig. 4 (a) A magnetic force microscopy image of a Co/Pt multilayers (10 periods of 3 Å Co and 10 Å Pt deposited on Si) was patterned by irradiated N⁺ at 700 keV through a silicon stencil mask having about 1.3 micrometer diameters holes. (b) Atomic force microscopy image of the same film at same area. The height of irradiated region is approximately increased by 2 nm. Sample provided by B. D. Terris, L. Folks and D. Weller at IBM.