Linshu Kong

Nanolithographically defined magnetic structures and quantum magnetic disk (invited)

Isolated and interactive arrays of magnetic nanostructures as small as 15 nm are fabricated using nanolithography and related technologies, and are characterized using magnetic force microscopy. It has been demonstrated that manipulating the size, aspect ratio, and spacing of these nanostructures can lead to unique control of their magnetic properties. A quantum magnetic disk based on discrete single‐domain nanomagnetic structures with storage density of 65 Gbits/in.2 is demonstrated along with a low‐cost method for mass producing such disks. Other impacts that nanofabrication can bring to the development of future magnetic storage are discussed.

Large area high density quantized magnetic disks fabricated using nanoimprint lithography

A new low-cost, high throughput method was developed for fabricating large area quantized magnetic disks (QMDs) using nanoimprint lithography (NIL), electroplating, and chemical mechanical polishing. Perpendicular QMDs with a density of 18 Gbit/in.2 and good uniformity over an area of 4 cm×4 cm (total 45 Gbit) have been achieved, as well as longitudinal QMDs of 30 Gbit/in.2 The NIL molds for the perpendicular QMDs were fabricated using double NIL with a grating mold. The magnetic properties of both types of QMDs were studied by magnetic force microscopy.

Quantification of magnetic force microscopy using a micronscale current ring

Metal rings with inner diameters of 1 and 5 μm, fabricated using electron-beam lithography, were used to calibrate magnetic force microscopy (MFM). A MFM tip’s effective magnetic charge, q, and effective magnetic moment along the tip’s long axis, mz, can be determined from the MFM signal of the ring at a different scan height and a different electric current in the ring. The magnetic moments in the directions transverse to the tip’s long axis were estimated by a straight current wire. It was found that for a Si tip coated with 65 nm cobalt on one side, q is 2.8×10−6 emu/cm, mz is 3.8×10−9 emu, and mx and my are in the order of 10−13 emu, which are negligible compared with mz. Furthermore, the MFMs sensitivity to the second derivative of the magnetic field was determined from the minimum ring current for a measurable MFM signal to be 0.1 Oe/nm2.

Perpendicular quantized magnetic disks with 45 Gbits on a 4×4 cm2 area

Quantized magnetic disks consisting of an array of single domain Ni pillars with a density of 18 Gbits/in.2 were fabricated using nanoimprint lithography (NIL) and electroplating. The total disk area, limited by the NIL mold, is 4 cm×4 cm, leading to a total 45 Gbits. Magnetic force microscope (MFM) images show that all pillars 70 nm in diameter and 400 nm in height are single domain. The magnetostatic interaction between adjacent pillars is fairly strong. The pillars have an average switching field of 360 Oe and can be switched by a MFM tip with a large magnetic moment.

Effects of bar length on switching field of nanoscale nickel and cobalt bars fabricated using lithography

The switching behavior of isolated nanoscale nickel and cobalt bars, which were fabricated using electron‐beam lithography, was studied as a function of bar length. The bars have a 35 nm thickness, a 100 nm width, and a length varying from 200 nm to 5 μm. Magnetic force microscopy showed that except for the Ni bars with a length equal to or less than 250 nm, all other as‐fabricated bars were single domain. Unlike the bar width dependence, the switching field of the single‐domain bars was found to first increase with the bar length, then decrease after reaching a peak. The peak switching field and the corresponding bar length are 640 Oe and 1 μm for Ni and 1250 Oe and 2 μm for Co, respectively. The nonmonotonic length dependence suggests that the magnetization switching may be quasicoherent in the short bars and incoherent in the long bars, and that the exchange coupling is much stronger in Co bars than in Ni bars. Furthermore, the switching field of 1‐μm‐long Co bars was found to increase monotonically as the bar width decreases, reaching 3000 Oe at a 30 nm width.

Magnetotransport and domain structures in nanoscale NiFe/Cu/Co spin valve

Nanoscale spin-valve structures with a width as small as 70 nm were fabricated using nanoimprint lithography and ion milling or lift off. The spin-valve multilayers consisting of NiFe(10 nm)/Co(1 nm)/Cu(13 nm)/Co(10 nm)/NiFe(2 nm) were deposited using direct current sputtering. The effects of device size, as well as fabrication process on domain structures, switching fields, switching field variation, and giant magnetoresistive ratio were investigated using scanning electron microscopy, atomic force microscopy, magnetic force microscopy, and magnetoresistance measurements.

Study of magnetic properties of magnetic force microscopy probes using micronscale current rings

Metal rings with inner diameters of 1 and 5 μm, fabricated using electron-beam lithography, were used to calibrate magnetic force microscopy (MFM). A MFM tip’s effective magnetic charge, q, and effective magnetic moment along the tip long axis, mz, can be determined by the current flowing in the ring. The magnetic moments in the directions transverse to the tip’s long axis were estimated by a straight current wire. It was found that for a silicon tip coated with 65 nm thick cobalt on the side, q=2.8×10−6 emu/cm, mz=3.8×10−9 emu, and mx=my=10−13 emu, which are negligible compared with mz. Furthermore, the tip’s sensitivity to the second derivative of the magnetic field was found to be about 0.1 Oe/nm2.

FABRICATION, WRITING, AND READING OF 10 GBITS/IN2 LONGITUDINAL QUANTIZED MAGNETIC DISKS WITH A SWITCHING FIELD OVER 1000 OE

Quantized magnetic disks (QMDs) with a density of 10 Gbits/in2 were fabricated using nanoimprint lithography and a lift-off process. Each bit in the QMD consists of a patterned single domain Co bar with a length of 200 nm, a width of 60 nm, and a bar spacing of 130 nm. Magnetic force microscope (MFM) was used to write and read the QMD. In 2.5 µm×2.5µm MFM window, error-free MFM writing has been achieved, even though there is no feedback control of the writing tip position and the Co bars switching field was as high as 1020 Oe.

Writing Bits of Longitudinal Quantized Magnetic Disk Using Magnetic Force Microscope Tip

Longitudinal quantized magnetic disks (QMDs) consisting of discrete single domain Ni bars on a silicon substrate with a density of nearly 2 Gbis/in.2 were fabricated. A magnetic force microscope (MFM) tip with a large magnetic moment has been used to successfully write individual bits of the QMD by simply positioning the tip near one end of a Ni bar. It was found that the MFM tip can write each individual bit perfectly without writing the neighboring bits. This is attributed to the fact that the magnetization of each QMD bit has only two stable quantized values and therefore the writing process is quantized.

Effects of sample size and field orientation on pseudo-Hall voltage in micronscale nickel thin-film squares

Pseudo-Hall effect (PHE) in Ni thin-film squares of 1–5 μm size is measured with a constant current through two leads along one diagonal of the square and the voltage output from leads along the other diagonal. The PHE voltage in response to an in-plane magnetic field depends on the square size and field orientation. The minimum PHE voltage at low field is close to zero only with the 2 μm square containing four symmetrical closure domains leading to a 600% relative change in PHE voltage. The PHE signal is found the largest when the field direction is along the square side while the smallest when along the square diagonal.