Alexey V. Nazarov

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.

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.

Mobility enhancement effect in heavily doped junctionless nanowire silicon-on-insulator metal-oxide-semiconductor field-effect transistors

The effective electron mobility in long-channel silicon-on-insulator junctionless multigate metal-oxide-semiconductor transistors is experimentally studied. It is found that the mobility in heavily doped narrow nanowire (NW) devices at low to moderately high carrier densities significantly exceeds that in wide (planar) devices with the same silicon thickness and doping and, in a certain range of carrier densities, it exceeds the mobility in bulk silicon with the same doping concentration. This effect increases when decreasing the NW width. The possible origins of this effect are discussed. These results are extremely encouraging for the development of junctionless NW transistors.

Tunable ferromagnetic resonance peak in tunneling magnetoresistive sensor structures

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.

High-frequency magnetic-force microscopy characterization of magnetic recording writer poles

Measurements of longitudinal writer poles at the air bearing surface were performed using high-resolution magnetic-force microscopy (MFM) with low coercivity tips. Two-dimensional MFM maps were obtained for various write currents. The modeling results indicate that the MFM maps are related more to the field than to its second derivative. Two techniques were used, dc MFM and high-frequency (HF) MFM. The results show that the HF-MFM technique can distinguish between different writer designs. The writers with the best high-frequency performance showed gradual decrease of the maximum MFM signal with frequency up to 1.5GHz.

Dynamics of laminated write elements

One concern as storage technology moves to perpendicular recording is the remnant state of the writer top pole. In principal, a remnant state with a substantial magnetization density perpendicular to the recording medium can lead to the unwanted erasure of data. Also, it is desirable to have the writer reach a nonerasing remnant state as quickly as possible. One technique to reduce the magnetization in the remnant state is to laminate the pole tip with some nonmagnetic material [Y. Satoh, A. Ohtsubo, and Y. Shimada, IEEE Trans. Magn. 21, 1551 (1985); S. Wang et al., IEEE Trans. Magn. 30, 3897 (1994)]. We have performed fully micromagnetic simulations of write elements with eight, five, four, and two laminates coupled antiferromagnetically. Results are presented for recording fields, as well as for the decay of the magnetization to a remnant state. The two- and four-laminate write elements typically have a vortex induced in the pole tip, and this vortex tends to survive, even in the remnant state. This can...

Imaging of isolated magnetic cluster switching in thin CoCrPt films

Thin film permanent magnet materials are a vital component of magnetic recording read elements. However, local variations in the magnetic microstructure inherent in such devices can have numerous consequences on the magnetic state of the films. In this study, magnetic force microscopy is used to image the domains in thin nanocrystalline CoCrPt films that are part of a patterned read sensor device. The films were imaged before and after being subjected to stress fields of 1000–2000Oe (less than the sheet-film coercivity of the CoCrPt) transverse to the original magnet set direction. Subtraction of the images reveals that the magnetization of isolated magnetic clusters irreversibly rotates in the film. These data show that the mechanism for net moment rotation in such films is not a uniform grain moment rotation. The change in net magnetization occurs in discrete local moment switching similar to Barkhausen jumps, where moments of weakly coupled grains irreversibly rotate at fields that are lower than the b...

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.