Scott Stokes

Spin transfer stimulated microwave emission in MgO magnetic tunnel junctions

In this work, current-driven precession of magnetization in multilayers with Co-Fe-B/MgO/Co-Fe-B MTJ in the form of dots has been investigated.

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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 ability to evaluate high frequency writer performance is crucial to the write-head designs for high data rate application. To quantify the write-head performance at high frequency (∼1 GHz), a new high frequency detection approach has been implemented. The essence of the method is to utilize a two-step signal mixing process to produce the force harmonics, which are not only proportional to the amplitude of the deep-gap field but also fall within the active bandwidth of the probe spectrum. Furthermore, a dual-vibration detection scheme is also described, which can provide superior high spatial resolution and enhance the sensitivity of attractive mode force microscopy as well at high frequencies.

Reader Design Outlook

Noise properties of submicron scale tunneling magnetoresistive (TMR) sensors were investigated at frequencies up to 3 GHz. Noise spectral density was measured as a function of frequency, applied field, and bias current. Noise spectral density versus frequency dependence exhibits a pronounced peak, tunable over a wide frequency range. This peak appears to originate from current-driven precession of magnetization. The peak center frequency can be as low as 200 MHz and has a strong dependence on applied field and bias current. The damping constant α of the main precession mode in the TMR sensor free layer was found to be in the range of 0.05–0.18. It is shown that the magnetic state of a magnetoresistive sensor depends on the bias current and may be characterized by noise properties. The magnetoresistive element can operate as a source of high-frequency radiation with 1 nW emitting power from a 0.1 μm2 junction and signal to noise ratio of 10 dB.

Dual vibrational high frequency magnetic force microscopy

The read sensor bias point plays an important role in the presence of media field as the aspect ratio of the sensor is scaled down. The change in the bias point also affects the asymmetry of the read sensor. The bias point shift is studied experimentally in a spin stand by measuring the change in tunneling magnetoresistive (TMR) resistance in the presence of the media field. The change in the TMR resistance depends on the direction of the media field and the rotation of the free-layer magnetization with respect to the media field. The bias point shift and the associated change in asymmetry in the presence of the media field are calculated from the TMR resistance, and a test based on the bias point shift is proposed to screen the unstable read sensors.

Tunable ferromagnetic resonance peak in tunneling magnetoresistive sensor structures

Magnetic 1/f noise is one of the major noise sources in state-of-the-art magnetic tunneling read heads for hard disk drive applications. The 1/f noise can be induced by thermally activated magnetic switching of antiferromagnetic (AFM) grains in the reader stack. This noise mechanism is studied here by micro-magnetic modeling. The modeling shows that the 1/f like noise power spectrum can come from the addition of several Lorentzian type noise spectra corresponding to the switching of individual AFM moments that are most likely located near the edges of the AFM layer. Additionally, the modeling shows that magnetic glitches can be generated by the switching of the reference layer edge curling state at the top or bottom edge of the layer, which is induced by the switching of one or more AFM moments in those areas.

Bias Point Shift in the TMR Sensor Under the Media Field

In synthetic antiferromagnetic spin valve (SAF–SV) recording heads, as SV sensor dimensions approach the nanometer scale and individual layer thickness shrinks to only a few angstroms, it becomes more challenging to achieve a unity amplitude symmetry (LFA–SYM) and a small variation of LFA–SYM. A better understanding of LFA–SYM, good control of its variation, and an effective measurement of SAF–SV canting angles has become more important. In this study, high field transfer curve (HF–Xfer) was measured to determine head magnetic polarity, bias point, and output linearity. Rotational field transfer curve (RF–Xfer) was measured to determine the canting angles of reference layer and pinned layer. Free layer canting angle, its dependence on magnetic field, and the angle between the reference layer and free layer were consequently determined based on HF–Xfer and RF–Xfer data. A good linear correlation between LFA–SYM and free layer canting angle was observed, indicating that the large variation of LFA–SYM was ma...

Modeling of 1/f noise due to thermally induced magnetic switches of anti-ferromagnetic grains in magnetic tunneling readers

In this work, current-driven precession of magnetization in multilayers with Co-Fe-B/MgO/Co-Fe-B MTJ in the form of dots has been investigated.

Understanding of amplitude symmetry and its variation of synthetic spin valve heads for high density recording

In this paper, the mechanism of permanent magnets induced reader instability is investigated by stray field experiment and micro-magnetic modeling.

Spin Transfer Stimulated Microwave Emission in MgO Magnetic Tunnel Junctions

In this paper, we discuss about the degradation in magnetoresistive properties by short pulse zapping has been investigated for both spin valve (SV) and tunnelling giant magnetoresistive (GMR) heads of about 80 Gb/spl bsol/in/sup 2/ areal density. Simulated ESD transients with pulse from 0.1 nsec to 10 nsec were applied to both spin valve and tunneling GMR devices, and GMR properties of the devices were measure.