Small In-plane Magnetic Field Research Articles

Stable oscillation in spin torque oscillator excited by a small in-plane magnetic field

Theoretical conditions to excite self-oscillation in a spin torque oscillator consisting of a perpendicularly magnetized free layer and an in-plane magnetized pinned layer are investigated by analytically solving the Landau-Lifshitz-Gilbert equation. The analytical relation between the current and oscillation frequency is derived. It is found that a large amplitude oscillation can be excited by applying a small field pointing to the direction anti-parallel to the magnetization of the pinned layer. The validity of the analytical results is confirmed by comparing with numerical simulation, showing good agreement especially in a low current region.

Differential conductance oscillations in asymmetric quantum point contacts

Small differential conductance oscillations as a function of source-drain bias were observed and systematically studied in an asymmetric quantum point contact (QPC). These oscillations become significantly suppressed in a small in-plane magnetic field ($\sim~0.7~T$) or at higher temperatures ($\sim~800~mK$). Qualitatively, their temperature evolution can be simulated numerically based on smearing of the Fermi distribution, whereas features near zero-bias cannot. Single particle scenarios are unsatisfactory in accounting for the oscillations, suggesting that they are likely caused by electron and spin correlation effects.

Nonexponential tunneling and control of microwave absorption lineshapes via Fano resonance for electrons on helium

We consider the application of a small in-plane magnetic field to electrons on a helium surface in a perpendicular magnetic field. Certain states that were bound to the helium surface then dissolve into the continuum, turning into long-lived resonances. As a result microwave absorption lines acquire an asymmetric Fano lineshape that is tunable by varying the microwave polarization or the in-plane magnetic field. Electrons trapped in a formerly bound state will tunnel off the surface of helium; we show that under suitable circumstances this ``radioactive decay'' can show damped oscillations rather than a simple exponential decay. The mechanism for oscillatory exponential decay is not specific to electrons on helium and this effect may also be relevant elsewhere in physics.

Non-collinear magnetization configuration in interlayer exchange coupled magnetic thin films

Element specific magnetic hysteresis loops of the interlayer exchange coupled CoFeB/Ru/[Co/Ni]4 structure were measured utilizing x-ray magnetic circular dichroism. It was found that the Co/Ni multilayer and the CoFeB layer have Ru thickness dependent oscillatory interlayer coupling. Due to its interlayer coupling with the perpendicularly magnetized Co/Ni multilayer, the CoFeB magnetization direction is slightly tilted out-of-plane from its in-plane magnetic easy axis. Quantitative measurements show that the tilting angle is small (<12°) and that a small in-plane magnetic field (∼50 Oe) applied to this structure will result in a completely in-plane CoFeB magnetization.

Quantized anomalous Hall insulator in a nanopatterned two-dimensional electron gas

We propose that a quantum anomalous Hall insulator (QAHI) can be realized in a nanopatterned two-dimensional electron gas (2DEG) with a small in-plane magnetic field and a high carrier density. The Berry curvatures originating from the in-plane magnetic field and Rashba and Dresselhaus spin-orbit coupling, in combination with a nanoscale honeycomb lattice potential modulation, lead to topologically nontrivial insulating states in the 2DEG without Landau levels. In the bulk insulating gaps, the anomalous Hall conductivity is quantized $\ensuremath{-}{e}^{2}/h$, corresponding to a finite Chern number $\ensuremath{-}1$. There exists one gapless chiral edge state on each edge of a finite-size 2DEG.

Control of magnetic domain wall displacement using spin current in small in-plane magnetic field in Permalloy nanowires

We microscopically investigate the magnetic domain wall motion induced by current pulse application in a small in-plane magnetic field in U-shaped Permalloy wires by means of Lorentz microscopy together with simultaneous transport measurement. An in-plane magnetic field less than 7 Oe parallel to the wire direction in U-shaped geometry effectively works to impede bidirectional motion of the domain wall induced by current pulse application, i.e. to suppress the stochastic nature of the domain wall displacement. The present finding will provide practical and reliable ways of controlling and manipulating the domain wall dynamics, which are widely applicable in spintronic devices, especially when stochastic nature causes serious problems in device operation. Reliable manipulation of the magnetic state is discussed using the current-driven domain wall motion and domain nucleation in the magnetic wire device.

Single-Shot Time-Domain Studies of Spin-Torque-Driven Switching in Magnetic Tunnel Junctions

We report single-shot measurements of resistance versus time for thermally assisted spin-torque switching in magnetic tunnel junctions. We achieve the sensitivity to resolve the magnetic dynamics prior to as well as during switching, yielding detailed views of switching modes and variations between events. Analyses of individual traces allow measurements of coherence times, nonequilibrium excitation spectra, and variations in magnetization precession amplitude. We find that with a small in-plane hard-axis magnetic field the switching dynamics are more spatially coherent than for a zero field.

Current-driven vortex oscillations in metallic nanocontacts: zero-field oscillations and training effects

We present an experimental and theoretical study of the low-field dynamics of current-driven vortex oscillations in nanocontacts based on spin-valve multilayers. These oscillations appear as low-frequency (250–500 MHz) excitations in the electrical power spectrum which arise from variations in the giant magnetoresistance. We show that the vortex oscillations, once nucleated at large fields applied perpendicular to the film plane, persist at zero applied magnetic fields. Some training effects on the oscillation frequency and linewidth are also observed for small in-plane magnetic fields.

Depairing currents in superconductor ferromagnet Nb/CuNi trilayers close to Tc

In superconductor/ferromagnet (S/F) heterostructures, the exchange field h ex of the F-layer suppresses the superconducting order parameter in the S-layer via the proximity effect. One issue in current research is the effect of a domain state, or, more generally, different directions of h ex, on the superconductivity. We used a pulsed-current technique in order to measure the superconducting transport properties of Cu 1− x Ni x /Nb/Cu 1− x Ni x ( x=0.54) F/S/F trilayers structured in strips of about 2 μm wide and 20 μm long as function of a small in-plane magnetic field. We find that the depairing current is tied to the magnetization behavior. In particular, we show that the suppression of superconductivity in the S-layer is smallest when the external magnetic field equals the coercive field H c of the F-layers.

Dynamic nuclear polarization by electrical spin injection in ferromagnet-semiconductor heterostructures.

Electrical spin injection from Fe into AlxGa1-xAs quantum well heterostructures is demonstrated in small (<500 Oe) in-plane magnetic fields. The measurement is sensitive only to the component of the spin that precesses about the internal magnetic field in the semiconductor. This field is much larger than the applied field and depends strongly on the injection current density. Details of the observed hysteresis in the spin injection signal are reproduced in a model that incorporates the magnetocrystalline anisotropy of the epitaxial Fe film, spin relaxation in the semiconductor, and the dynamic polarization of nuclei by the injected spins.

Bonding and antibonding states in strongly coupled ballistic one-dimensional wires

Low-temperature conductance measurements of strongly coupled ballistic one-dimensional (1D) wires show the formation of bonding and antibonding 1D states. Two parallel 1D wires are defined by a split-gate structure deposited on a GaAs/AlGaAs double quantum well with a 25 Å AlGaAs barrier. A midline gate positioned at the centre of the split-gate allows both the widths and electron densities of the two wires to be tuned. At resonance when the wires are matched, symmetric and antisymmetric 1D energy levels are formed, and are separated by an energy gap Δ SAS. The overlap integral that describes the mixing of the 1D wave functions of different index in the two wires can be altered with a small in-plane magnetic field B. We also present preliminary measurements of the mixing of edge states in the quantum Hall regime.

Magnetic tuning of a microstrip patch antenna fabricated on a ferrite film

A square, single-feed patch, fabricated on a ferrite film, that produced orthogonally polarized, well-formed radiation patterns is described. The application of a small in-plane magnetic field tuned the frequency, and hence phase, of one polarization only. Prior work on patch antennas fabricated on bulk ferrite substrates demonstrated magnetic tuning, but only linear polarization was obtained. The results indicate that (1) thin ferrite films, which are monolithically integrable, may be useful for a magnetically tunable antenna, and (2) the radiation polarization of the patch can be varied by the application of a small in-plane magnetic bias field. >

Measurement of Fermi-surface distortion in double quantum wells from in-plane magnetic fields.

In double quantum wells with interwell tunneling, we find that a small in-plane magnetic field substantially alters the beating observed in Shubnikov--de Haas oscillations. This phenomenon is attributed to distortion of the Fermi surface due to mixing of the symmetric and antisymmetric double-well states. The measured magnitude of the distortion is in good quantitative agreement with a calculation of the symmetric and antisymmetric mixing from the in-plane magnetic field.