Current-induced Field Research Articles

OBSERVATION OF NEW SUPERCONDUCTIVITY AND PERFORMANCE IMPROVEMENT BY EMPLOYING CYCLOTRON OF ELECTRON PAIRS

A novel temperature-independent superconducting device that employs a doped semiconductor is presented in this study. The underlying theory of this superconductivity is confirmed by experimental results. Specifically, superconductivity generates a negative electric field with characteristics of both electrostatic and current-induced fields. This type of electric field creates a new paired interaction between two electrons and implies the existence of a new force. The negative electric field also exhibits the Meissner effect. Moreover, magnetic flux quanta are produced in the semiconductor. The Aharonov–Bohm effect is exhibited to create a superconducting current along the electric circuit of the superconducting system. Therefore, a load introduced to the circuit will also become superconductive. This finding has strong potential for practical applications. To solve the problem of critical current, a static magnetic field is applied. This field combines with the new electric field to yield cyclotron motion, which increases superconducting current.

Effect of current-induced magnetic field on magnetization dynamics in spin-torque nano-oscillator with point-contact geometry

We show micromagnetic simulation results for spin-torque nano-oscillator (STNO) with point-contact geometry. Numerical results obtained from three different models are compared in order to investigate the effect of current-induced Oersted field and non-uniform current distribution on the characteristics of STNO. It is found that the Oersted field and non-uniform current distribution affect the frequency and precession mode both qualitatively and quantitatively. An anisotropic emission of spin-waves is observed only when considering the Oersted field properly. Our results suggest that a realistic consideration of the current distribution and consequent Oersted field distribution is of critical important to design STNOs and to understand the characteristics of STNOs.

Layer thickness dependence of the current-induced effective field vector in Ta|CoFeB|MgO

Current-induced effective magnetic fields can provide efficient ways of electrically manipulating the magnetization of ultrathin magnetic heterostructures. Two effects, known as the Rashba spin orbit field and the spin Hall spin torque, have been reported to be responsible for the generation of the effective field. However, a quantitative understanding of the effective field, including its direction with respect to the current flow, is lacking. Here we describe vector measurements of the current-induced effective field in Ta|CoFeB|MgO heterostructrures. The effective field exhibits a significant dependence on the Ta and CoFeB layer thicknesses. In particular, a 1 nm thickness variation of the Ta layer can change the magnitude of the effective field by nearly two orders of magnitude. Moreover, its sign changes when the Ta layer thickness is reduced, indicating that there are two competing effects contributing to it. Our results illustrate that the presence of atomically thin metals can profoundly change the landscape for controlling magnetic moments in magnetic heterostructures electrically.

Chirality control of magnetic vortex in a square Py dot using current-induced Oersted field

We have proposed a method for controlling the vortex chirality in a squared permalloy dot by using the circular Oersted field locally induced by flowing a DC current across a small Py/Cu junctions. The reliability of the chirality control has been evaluated by measuring the nonlocal spin valve signal. The desired vortex chirality has been obtained when the injecting DC current has a moderate magnitude. However, the large DC current is found to reduce the control reliability. Another possibility for controlling the vortex structure using the large DC current injection was also discussed.

Nanoelliptic Ring-Shaped Magnetic Tunnel Junction and Its Application in MRAM Design With Spin-Polarized Current Switching

Nanoring magnetic tunnel junction (NR-MTJs) with outer diameter of 90 nm, and nanoelliptical ring (NER) -shaped MTJ with major outer diameter of 120 nm, and minor outer diameter of 70 nm were fabricated. For both geometrical structures, the ring width was kept at around 30 nm. The magnetic field driven and current-induced magnetization switching of the NR-MTJs and NER-MTJs were investigated. Compared with the NR-MTJs, the NER-MTJs show a sharp magnetization switching and lower coercivity under the sweep of an external magnetic field, which indicates the higher uniaxial anisotropy of NER-MTJs. In addition, the current-induced magnetization switching was achieved in both MTJs which indicated a lower critical current density in sandwiched-type and spin-valve-type NER-MTJs. This reduction of critical current could be resulting from different distribution of current-induced Oersted field in the NR-MTJs and NER-MTJs, which play an assisted role in spin transfer torque switching. This work proposes the NER-MTJs with the low current density, and small stray field, as data storage bits of magnetic random access memory (MRAM), could be a promising candidate for further reducing the power consumption, scaling memory cell size and increasing density of MRAM.

PATTERNED NANOSCALE MAGNETIC TUNNEL JUNCTIONS WITH DIFFERENT GEOMETRICAL STRUCTURES

In this paper, patterned nanoscale magnetic tunnel junctions (MTJs) with different geometrical structures, including nanodisk (ND), nanoellipse (NE), nanoring (NR) and nanoelliptical ring (NER) with the scale of around 100nm and ring width of around 30nm, were fabricated, respectively. The geometrical-shape dependence of magnetic field-driven and current-induced magnetization switching (CIMS) were studied in the nanoscale magnetic tunnel junctions (MTJs). The NER-MTJs showed robust magnetization switching and low critical current density of CIMS, comparing with other geometrical-shaped MTJs. This may be due to the different distribution of current-induced Oersted field in different geometrical structures, which plays an assisted role in CIMS. The present experiments indicate that NER-MTJs may be one of the promising candidates for the cells of high-density and low-consumption spintronic devices.

Current-induced effective field in perpendicularly magnetized Ta/CoFeB/MgO wire

The current-induced effective field in perpendicularly magnetized Ta/CoFeB/MgO wire was investigated. A threshold field decrease of 6.4 kOe/mA was observed by measuring the threshold field of Hall resistance versus the magnetic field curve with various bias currents. The decrease was probably caused by the in-plane effective field, mainly due to the Rashba effect. The effective field of the Ta/CoFeB/MgO wire was smaller and opposite in direction compared to that of Pt/Co/AlOx previously reported.

Study of Noise in Current-Perpendicular-to-Plane Giant Magnetoresistance Devices with a Current Screen Layer

The noise in current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with a current screen layer (CSL) is investigated to clarify the noise generation mechanism. The noise intensity is greatly enhanced in the antiparallel magnetization configuration due to spin torque effects. In addition, the noise intensity increases as the temperature is reduced because thermal spin fluctuations decrease. Furthermore, the noise intensity increases when spins flow to the free layer because the magnetization of the free layer fluctuates easily. These results imply that noise is generated by fluctuations in the magnetization of the free layer caused by spin torque. Moreover, some CPP-GMR devices with a CSL have some peaks in plots of noise intensity against applied magnetic field. These peaks are thought to be related to the current-induced field and magnetization fluctuations at the edge of the device.

Study of Noise in Current-Perpendicular-to-Plane Giant Magnetoresistance Devices with a Current Screen Layer

The noise in current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with a current screen layer (CSL) is investigated to clarify the noise generation mechanism. The noise intensity is greatly enhanced in the antiparallel magnetization configuration due to spin torque effects. In addition, the noise intensity increases as the temperature is reduced because thermal spin fluctuations decrease. Furthermore, the noise intensity increases when spins flow to the free layer because the magnetization of the free layer fluctuates easily. These results imply that noise is generated by fluctuations in the magnetization of the free layer caused by spin torque. Moreover, some CPP-GMR devices with a CSL have some peaks in plots of noise intensity against applied magnetic field. These peaks are thought to be related to the current-induced field and magnetization fluctuations at the edge of the device.

Current-induced thermal stresses in a metal cylinder

Electromechanical coupling has been widely used to actuate and control micromechanical structures and to produce bulk nanostructured materials from micron and submicron particles. The understanding of the mechanical deformation of a mechanical structure carrying an electric current requires the analyses of both electric and current-induced thermomechanical fields. In this work, we analyze the Joule-heating-induced thermoelastic stresses in a metal cylinder which carries an alternating current. Both Joule heating and mechanical stresses are solved analytically for weak skin effect when electric current density varies in radial direction. The thermal stresses created by the Joule heating are found to be a nonlinear function of the angular frequency of the alternating current. The inclusion of the skin effect and radial variation in electric current density in the analyses shows significant quantitative difference in the stress distribution from the thermomechanical deformation of a metal cylinder which carries a direct current.

Polarization mechanism of LiNbO 3 crystal due to thermoelectric power and current-induced electric fields during growth via micro-pulling-down method

The mechanism of polarization due to thermoelectric power and current-induced electric fields during the growth of LiNbO 3 crystals was studied using a micro-pulling-down method. With no applied electric current, a +c single-domain crystal was grown regardless of the domain orientation of the seed crystal. This +c domain growth was consistent with the direction of the electric field caused by the thermoelectric power in the liquid, despite an opposing electric field in the solid due to the opposite sign of the Seebeck coefficient. Thus, it was the electric field in the liquid that determined the domain structure of the growing crystal. On the other hand, when a current was applied from the melt to the crystal, a −c domain crystal was grown. The electric current required for this domain inversion to occur became larger as the temperature gradient in the solid phase decreased. This shows that the electric field in the solid phase became large enough to induce domain inversion from +c to −c through a combination of the thermoelectric power in the solid phase and current-induced electric field.

Current-induced resonant motion of a magnetic vortex core: Effect of nonadiabatic spin torque

The current-induced resonant excitation of a magnetic vortex core is investigated by means of analytical and micromagnetic calculations. We find that the radius and phase shift of the resonant motion are not correctly described by the analytical equations because of the dynamic distortion of a vortex core. In contrast, the initial tilting angle of a vortex core is free from the distortion and determined by the nonadiabaticity of the spin torque. It is insensitive to experimentally uncontrollable current-induced in-plane Oersted field and thus allows a direct comparison with experimental results. We propose that a time-resolved imaging of the very initial trajectory of a core is a plausible way to experimentally estimate the nonadiabaticity.

Towards beyond 1 GHz NMR: Mechanism of the long-term drift of screening current-induced magnetic field in a Bi-2223 coil

The screening current-induced magnetic field in the (Bi,Pb) 2Sr 2Ca 2Cu 3O x (Bi-2223) insert coil proposed for a beyond 1 GHz nuclear magnetic resonance (NMR) spectrometer may generate a long-term field drift, resulting in a loss of field-frequency lock operation and an inability to make high resolution NMR measurements. The measured screening current-induced magnetic field of a Bi-2223 double-pancake coil exhibits a hysteresis effect at 4.2 K that is reproduced by a numerical simulation based on a finite thickness rectangular superconductor bar model. The screening current-induced field at the coil center is of opposite polarity to that generated by the coil current, and thus the apparent field intensity shows a positive drift with time. On the contrary, the field at a coil end is of the same polarity as the coil field, and the apparent field intensity decreases with time. If we wait for ∼1000 h after coil excitation, the field drift rate approaches the field decay rate of the persistent current of 10 −8 h −1, suitable for a long-term NMR measurement in a beyond 1 GHz NMR spectrometer.

CPP-GMR heads with a current screen layer for high areal density

We investigated the potential for high-areal-density recording in current-perpendicular-to-plane giant magnetoresistive (CPP-GMR) heads with a current screen layer. The current screen layer is a nano-oxide layer with confined current paths. We fabricated the current screen CPP-GMR heads with a narrow sensor width of 40–50 nm, a high MR ratio of 17%, and low-resistance-area (RA) product of 0.2 Ω μm 2. The fabricated heads showed a sufficiently high signal-to-noise ratio (SNR) of 30–40 dB. No extra noise, such as spin-torque noise, was observed. Linear density of 1360 kFCI from the head with the magnetic read width of 45 nm was obtained. Distribution of sensor resistance due to nano-hole area distribution in the current screen layer can be reduced with low-RA film. Spin-torque noise can be suppressed by reducing the current-induced field and controlling the shape anisotropy. Accordingly, the current screen CPP-GMR head is a promising candidate that has the potential for high-areal-density recording.

Field Mapping, NMR Lineshape, and Screening Currents Induced Field Analyses for Homogeneity Improvement in LTS/HTS NMR Magnets.

This paper describes two general methods: field map processing and NMR lineshape analysis, that we are currently using for the on-going research of field improvement techniques specifically applied to our 700 MHz LTS/HTS NMR magnet. The validity of the methods is verified with comparison between calculations and experiments. Also, a simple but effective analysis has been used to identify the principal source of a remanent magnetic field measured in the bore of the 700 MHz LTS/HTS NMR magnet: the Screening Current induced Field (SCF) from the 100-MHz HTS insert, comprised of 48 double pancake coils, each wound with Bi2223 tape.

Effects of current on nanoscale ring-shaped magnetic tunnel junctions

We report the observation and micromagnetic analysis of current-driven magnetization switching in nanoscale ring-shaped magnetic tunnel junctions. When the electric current density exceeds a critical value of the order of $6\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.3em}{0ex}}\mathrm{A}∕{\mathrm{cm}}^{2}$, the magnetization of the two magnetic rings can be switched back and forth between parallel and antiparallel onion states. Theoretical analysis and micromagnetic simulation show that the dominant mechanism for the observed current-driven switching is the spin torque rather than the current-induced circular Oersted field.

Model for a collimated spin-wave beam generated by a single-layer spin torque nanocontact

A model of spin-torque-induced magnetization dynamics based on semiclassical spin diffusion theory for a single-layer nanocontact is presented. The model incorporates effects due to the current-induced Oersted field and predicts the generation of a variety of spatially dependent, coherent, precessional magnetic wave structures. Directionally controllable collimated spin-wave beams, vortex spiral waves, and localized standing waves are found to be excited by the interplay of the Oersted field and the orientation of an applied field. These fields act as a spin-wave ``corral'' around the nanocontact that controls the propagation of spin waves in certain directions.

Ｃｏ‐Ｆｅ／Ｃｕ多層膜を用いたＣＰＰ‐ＧＭＲ素子におけるＭＲ特性のセンス電流依存性

The dependence of the MR properties of CPP (Current Perpendicular to the Plane)-GMR (Giant Magneto-Resistive) sensors with a Co-Fe/Cu multilayer on sensing current was investigated. A CPP-GMR sensor with a [Co90Fe10(1.5 nm)/Cu(2.0 nm)]16 multilayer had an MR ratio of 17.5 %. The MR ratio monotonically decreased with increasing sensing current. Split peaks for the MR ratio were also observed in an MR curve, when sensing current was so high as to drastically decrease the MR ratio. We calculated the MR curves with a simple coherent rotation model assuming a circular current induced field, and confirmed that the calculated MR curves were qualitatively in good agreement with experimental ones. The micromagnetic simulation for read-head dimensions also revealed degradation in the MR ratio due to the current induced field. The simulated results indicated that reducing the sensor size would effectively suppress the current induced field, which resulted in high output. The estimated output and head-amp SNR for a size of 50 x 50 nm were over 2 mV and 33 dB, respectively. Therefore, a CPP-GMR sensor with a Co-Fe/Cu multilayer possibly had sufficiently high potential to be a candidate for future recording technology.

Resonant and nonresonant scattering of dipole-dominated spin waves from a region of inhomogeneous magnetic field in a ferromagnetic film

The transmission of a dipole-dominated spin wave in a ferromagnetic film through a localized inhomogeneity in the form of a magnetic field produced by a dc through a wire placed on the film surface was studied experimentally and theoretically. It was shown that the amplitude and phase of the transmitted wave can be simultaneously affected by the current induced field, a feature that will be relevant for logic based on spin wave transport. The direction of the current creates either a barrier or a well for spin wave transmission. The main observation is that the current dependence of the amplitude of the spin wave transmitted through the well inhomogeneity is nonmonotonic. The dependence has a minimum and an additional maximum. A theory was constructed to clarify the nature of the maximum. It shows that the transmission of spin waves through the inhomogeneity can be considered as a scattering process and that the additional maximum is a scattering resonance.

Micromagnetic simulation on effect of oersted field and hard axis field in spin transfer torque switching

The effects of the current-induced Oersted field and of a hard axis applied field on spin transfer torque (STT) switching have been investigated by performing LLG micromagnetic simulations including a STT term and Gaussian thermal fluctuations. In parallel to anti-parallel switching at large currents, the C-like micromagnetic configuration induced by the Oersted field plays an important role in STT switching. In anti-parallel to parallel switching at large currents, the Oersted field induces a complicated micromagnetic configuration with several vortices. In both cases, the magnetization deviates considerably from macrospin behaviour. However, a macrospin-like behaviour is restored when a hard axis field is present.