Dion Song

Microwave generation in MgO magnetic tunnel junctions due to spin transfer effects (invited)

Low resistance-area product and high spin polarization lead to current-driven precession of the magnetization in CoFeB∕MgO∕CoFeB magnetic tunnel junctions due to spin-transfer torque (STT) effects. Current-driven precession of the magnetization leads to pronounced narrow peaks in the frequency range of 4–7GHz with a full width at half maximum as low as 21MHz. The peak amplitudes have a threshold dependence on the dc bias current. Experimental results show that the STT-driven microwave generation can also occur in MgO-based junctions at maximum resistance state but at opposite current polarity, which corresponds to precession of the magnetization of the reference layer (RL) electrode. This conclusion is supported by the peak frequency dependence on magnetic field. The maximum generated power was 35nW at a peak frequency of about 6GHz. The estimated maximum angle change of the RL in-plane magnetization rotation is 19° and corresponds to a large angle precession.

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We review the 2 Tbit/in2 reader design landscape based on existing knowledge and projection. We found that the reader signal-to-noise ratio (SNR) requirement will be highly challenging due to the rapid increase in noise and the additional requirements from assisted writing. An acceptable level of channel bit density can be achieved in spite of a slow head-to-media spacing (HMS) reduction provided that both the shield-to-shield (SS) spacing and the ¿a¿ parameter scale with the bit length. We expect the side reading control for high ktpi to be difficult, and potentially a reader side shield will be required. The reader will likely use a higher quality MgO tunneling giant magnetoresistance (TGMR) stack with improved permanent-magnet coercivity. Certain new structures such as the differential reader or the trilayer will likely be part of the solution.

{\hbox{Tbit/in}}^{2}

The resistance–area product (R*A) and the magnetoresistance (MR) of NiFe/AlOx/NiFe spin-dependent tunnel junctions exhibit a strong dependence on the thickness of Al before oxidation. We obtain these data from wafers where we uniformly oxidize an Al layer with a wedged thickness profile, enabling us to reliably characterize the effect of Al thickness variations with subangstrom precision. The R*A drops from 104 to 102 Ω μm2 as the Al thickness decreases from 9 to 4 A, respectively. The MR is highest (21%) for an Al thickness of 7 A, where the Al layer is fully oxidized and the oxidation of the bottom NiFe electrode is minimal.

Reader Design Outlook

Heater-induced head-disc contacts can cause highly localized wear of the pole tip. We found that scanning electron microscopy and low beam-current focused ion beam imaging are suited at providing a map of the entire pole tip surface and quickly identify the location and shape of heater-induced wear. For longitudinal heads, the imaging can reveal detailed structure near the writer gap. By measuring the topography near the pole tip using atomic force microscopy (AFM), one can retrieve quantitative wear information. Using unburnished parts as controls, the accuracy of the AFM method reported here is about 1nm. We found transmission electron microscopy and Auger electron spectroscopy to be less effective in quantifying wear.

Magnetic tunnel junction performance versus barrier thickness: NiFe/AlOx/NiFe junctions fabricated from a wedged Al layer

Electrostatic force has been proposed for use in fly height reduction and control. The dominant failure mode is electrical discharge at the head-disk interface (HDI) due to field emission. Ballast resistor films have been used for limiting field emission. We applied this idea to the HDI by depositing a thick coating of diamond-like carbon (DLC) on the slider. A typical slider has 25 /spl Aring/ of DLC and exhibits breakdown voltage of less than 3 V on product media. When the coating thickness was increased to 430 /spl Aring/, it was sufficient to prevent discharge up to 6 V and allowed approximately 33% clearance reduction without crashing or discharging. The result is in good agreement with fly height modeling, which takes into account the head-disk electrostatic force.

Inspection of pole tip diamondlike carbon wear due to heater-induced head-disc contact

The lithographic requirements for the thin film head industry are comparable to the semiconductor industry for certain parameters such as resolution and pattern repeatability. In other aspects such as throughput and defectivity, the requirements tend to be more relaxed. These requirements match well with the strengths and weaknesses reported concerning nanoimprint lithography (NIL) and suggest an alternative approach to optical lithography. We have demonstrated the proof of concept of using NIL patterning, in particular Jet and FlashTM Imprint Lithography (J-FILTM) 1 , to build functional thin film head devices with performance comparable to standard wafer processing techniques. An ImprioTM 300 tool from Molecular Imprints, Inc. (MII) was modified to process the AlTiC ceramic wafers commonly used in the thin film head industry. Templates were produced using commercially viable photomask manufacturing processes and the AlTiC wafer process flow was successfully modified to support NIL processing. Future work is identified to further improve lithographic performance including residual layer thickness uniformity, wafer topography, NIL→NIL overlay, and development of a large imprint field that exceeds what is available in optical lithography.

Discharge mechanism for electrostatic fly control

The Bottom Spin Valve (BSV) geometry has been the standard HDD Reader design for well over 10 years [1].

Hard disk drive thin film head manufactured using nanoimprint lithography

We showed a new method of measuring air bearing take-off performance. The procedure involves spindle speed ramp up as during CSS. The data revealed the intrinsic behaviors for different air bearing designs and are in good agreement with both air bearing modeling and CSS tests over magnetic media. The method is an effective tool in evaluating air bearing take off performance.