Magnetic Field Switching Research Articles

Micromagnetic Structure of Co Stripe Arrays With Tuned Anisotropy

An interwire magnetostatic coupling is one of the most important effects in the micro- and nanowire arrays, because of it strongly influences on the magnetization switching, spin-wave dynamics and magneto-transport properties. Manipulation of the magnetostatic coupling is possible not only by variation of a dimension of nanowires and geometry of arrays, but also by changing the magnetic anisotropy. In this paper, we report on an investigation of the magnetic behavior of arrays consisting of finite number of Co microstripes with two mutually perpendicular induced anisotropies. We have studied an effect of the interwire coupling on the switching field of magnetization and analyzed the domain structure of the arrays under an impact of the induced anisotropies.

Simulation Study of Pulse Laser Quality Effects on Recording Performances of Heat-Assisted Magnetic Recording by Short-Pulse Laser Heating

In heat-assisted magnetic recording (HAMR), a higher recording density is pursued. Short-pulse laser heating is a good way to improve the thermal distribution in the media and to reach the full potential of recording density for the HAMR technology. In this paper, the effects of short-pulse laser qualities, such as pulse frequency deviation, pulse power deviation, and pulsewidth deviation, on recording performances for the HAMR with 100 ps pulse laser heating have been analyzed by simulation. The results indicated that the frequency jitter has the most significant effect on the recording performance among these three parameters. The decreases of signal-to-noise ratio (SNR) for 6% deviations of pulse frequency, pulse power, and pulsewidth are 5, 2, and 1.5 dB, respectively. A certain degree phase delay of pulse laser related to magnetic field switching time is beneficial to reducing the effect of pulse frequency deviation on SNR. When the phase delay changes from 0° to 180°, an SNR improvement of $\sim 2$ dB is obtained. The measurement result of a real laser diode output (pulsewidth of 97 ps and pulse frequency of 2 GHz) by gain-switching technology presents a very low frequency deviation (<0.5%). It indicates that the gain-switching technology is a suitable approach for a short-pulse laser generation for the HAMR application.

Engineered Heusler Ferrimagnets with a Large Perpendicular Magnetic Anisotropy.

Synthetic perpendicular magnetic anisotropy (PMA) ferrimagnets consisting of 30-nm-thick D0-MnGa and CoMnSi (CMS) cubic Heusler alloys with different thicknesses of 1, 3, 5, 10 and 20 nm, buffered and capped with a Cr film, are successfully grown epitaxially on MgO substrate. Two series samples with and without post annealing at 400 C are fabricated. The (002) peak of the cubic L2 structure of CMS films on the MnGa layer is observed, even for the 3-nm-thick CMS film for both un-annealed and annealed samples. The smaller remnant magnetization and larger switching field values of CMS (1–20 nm)/MnGa (30 nm) bilayers compared with 30-nm-thick MnGa indicates antiferromagnetic (AFM) interfacial exchange coupling (J) between MnGa and CMS films for both un-annealed and annealed samples. The critical thickness of the CMS film for observing PMA with AFM coupling in the CMS/MnGa bilayer is less than 10 nm, which is relatively large compared to previous studies.

Observation of inverse spin Hall effect in ferromagnetic FePt alloys using spin Seebeck effect

We experimentally observed the inverse spin Hall effect (ISHE) of ferromagnetic FePt alloys. Spin Seebeck effect due to the temperature gradient generated the spin current (Js) in the FePt|Y3Fe5O12 (YIG) structure, and Js was injected from YIG to FePt and converted to the charge current through ISHE of FePt. The significant difference in magnetization switching fields for FePt and YIG led to the clear separation of the voltage of ISHE from that of anomalous Nernst effect in FePt. We also investigated the effect of ordering of FePt crystal structure on the magnitude of ISHE voltage in FePt.

Music-based magnetic resonance fingerprinting to improve patient comfort during MRI examinations.

Unpleasant acoustic noise is a drawback of almost every MRI scan. Instead of reducing acoustic noise to improve patient comfort, we propose a technique for mitigating the noise problem by producing musical sounds directly from the switching magnetic fields while simultaneously quantifying multiple important tissue properties. MP3 music files were converted to arbitrary encoding gradients, which were then used with varying flip angles and repetition times in a two- and three-dimensional magnetic resonance fingerprinting (MRF) examination. This new acquisition method, named MRF-Music, was used to quantify T1 , T2 , and proton density maps simultaneously while providing pleasing sounds to the patients. MRF-Music scans improved patient comfort significantly during MRI examinations. The T1 and T2 values measured from phantom are in good agreement with those from the standard spin echo measurements. T1 and T2 values from the brain scan are also close to previously reported values. MRF-Music sequence provides significant improvement in patient comfort compared with the MRF scan and other fast imaging techniques such as echo planar imaging and turbo spin echo scans. It is also a fast and accurate quantitative method that quantifies multiple relaxation parameters simultaneously. Magn Reson Med 75:2303-2314, 2016. © 2015 Wiley Periodicals, Inc.

Dual Control of Giant Field-like Spin Torque in Spin Filter Tunnel Junctions.

We predict a giant field-like spin torque, , in spin-filter (SF) barrier tunnel junctions in sharp contrast to existing junctions based on nonmagnetic passive barriers. We demonstrate that has linear bias behavior, is independent of the SF thickness, and has odd parity with respect to the SF’s exchange splitting. Thus, it can be selectively controlled via external bias or external magnetic field which gives rise to sign reversal of via magnetic field switching. The underlying mechanism is the interlayer exchange coupling between the noncollinear magnetizations of the SF and free ferromagnetic electrode via the nonmagnetic insulating (I) spacer giving rise to giant spin-dependent reflection at the SF/I interface. These findings suggest that the proposed field-like-spin-torque MRAM may provide promising dual functionalities for both ‘reading’ and ‘writing’ processes which require lower critical current densities and faster writing and reading speeds.

Interfacial exchange coupling and magnetization reversal in perpendicular [Co/Ni]N/TbCo composite structures.

Interfacial exchange coupling and magnetization reversal characteristics in the perpendicular heterostructures consisting of an amorphous ferrimagnetic (FI) TbxCo100–x alloy layer exchange-coupled with a ferromagnetic (FM) [Co/Ni]N multilayer have been investigated. As compared with pure TbxCo100–x alloy, the magnetization compensation composition of the heterostructures shift to a higher Tb content, implying Co/Ni also serves to compensate the Tb moment in TbCo layer. The net magnetization switching field Hc⊥ and interlayer interfacial coupling field Hex, are not only sensitive to the magnetization and thickness of the switched TbxCo100–x or [Co/Ni]N layer, but also to the perpendicular magnetic anisotropy strength of the pinning layer. By tuning the layer structure we achieve simultaneously both large Hc⊥ = 1.31 T and Hex = 2.19 T. These results, in addition to the fundamental interest, are important to understanding of the interfacial coupling interaction in the FM/FI heterostructures, which could offer the guiding of potential applications in heat-assisted magnetic recording or all-optical switching recording technique.

Magnetic field control of plasmon polaritons in graphene-covered gyrotropic planar waveguide.

In this Letter, we report about magnetic field switching of plasmon polaritons propagating into a planar gyrotropic waveguide covered by two graphene layers at a deeply subwavelength scale. It is shown that applying an external magnetic field may lead to energy redistribution between two waveguide surfaces. The effect value resonantly depends on the relation between waveguide size and exciting light wavelength. A change in chemical potential of graphene layers may be used for tuning the phase shift between plasmon polaritons at near-resonant wavelengths. Evident effect may be observed at low magnetic fields (less than one tesla) for wavelengths about microns on a scale of tens of nanometers. Such an effect may be used for plasmonics, photonics. and optoelectronics devices, as well as sensing applications.

Magnetic-field-induced switchable exchange bias in NiFe film on (110) Fe3O4 with a strong uniaxial magnetic anisotropy

The exchange bias in a soft ferromagnetic NiFe layer coupled with a hard ferrimagnetic Fe3O4 film grown on a (110) SrTiO3 single-crystal substrate was investigated as a function of the switching magnetic field (HS) as a means to control the magnetization direction of the Fe3O4. The sign of the exchange bias was consistent with the sign of HS, indicating that the exchange coupling constant between the NiFe and (110) Fe3O4 layers was positive. Below |HS| = 1 kOe, the hysteresis behavior of the exchange bias of the soft ferromagnetic NiFe resembled the magnetic hysteresis behavior of the hard ferrimagnetic Fe3O4.

MAGNETIZED PLASMA AS A VERSATILE PLATFORM FOR SWITCHING

We study the magneto-permittivity effect in a magnetized plasma with appropriately designed parameters. We show that at frequencies near the plasma frequency, magneto-optical activity plays an important role to manipulate and control the wave propagations in the magnetized plasma. Such a unique feature can be utilized to establish sensitive magnetic field switching mechanism, which is confirmed by detailed numerical investigations. Switching by magnetic field based on magnetized plasma is flexible and compatible with other optical system; moreover it is applicable to any frequency by tuning the plasma density. For these reasons, our work shows the possibility for developing a new family of high frequency and ultrasensitive switching applications.

Design and Analysis of Radiation Hardened Sensing Circuits for Spin Transfer Torque Magnetic Memory and Logic

CMOS downscaling makes advanced memory and computing systems more vulnerable to radiation. Emerging non-volatile memories such as Magnetic RAM (MRAM) are under development to replace SRAM and FLASH memories. Spin Transfer Torque Magnetic Memory (STT-MRAM) is considered as one of the most promising memories for high reliability appli- cations. This is due to its intrinsic hardness to radiation, as storage is based on the spin direction of electrons instead of the charge. Its peripheral CMOS circuits are sensitive to radiation; however we can recharge frequently the non-volatile storage to get correct data. Different from conventional MRAM based on magnetic field switching, STT-MRAM is unable to be used directly for hard radi- ation applications as the current pulses induced by radiation may change the storage due to the stochastic behaviors of spin transfer torque mechanism. In this paper, we analyze firstly this issue and propose a rad-hard design for STT-MRAM sensing circuit with low area overhead and negligible performance degradation. Both transient and Monte-Carlo simulation have been performed to demonstrate its performance and behaviors.

The effect of the thickness ratio of magnetic layers on the microstructure and magnetic properties of (CoCrPt)97.5Nb2.5/Co75Cr13Pt12/Cr thin films

A Co75Cr13Pt12 intermediate magnetic layer was deposited between the (CoCrPt)97.5Nb2.5 upper magnetic layer and chromium underlayer by the magnetron sputtering technique. The effect of the thickness ratio of two magnetic layers and post-annealing treatment on the microstructure and magnetic properties of the films were investigated. The magnetic characteristics of the films were obtained by magnetic force microscopy, hysteresis loops and switching field distribution curves. Although annealing had no significant effect on the layer roughness, it rotated the easy axis of the magnetic layer towards the film plane, thereby enhanced the coercivity. The results showed an impressive effect of the magnetic intermediate layer thickness and post-annealing on the improving of coercivity through isolation of the magnetic grains. Formation of a non-magnetic halo around the magnetic grains reduced the inter-granular magnetostatic interaction.

Maximum magnetic recording density and switching field distribution

An analytical calculation of the parameters of magnetic recording on a square lattice of magnetic granules of the patterned medium has been carried out. The maximum density of the longitudinal and perpendicular recording, which is limited by the thermal instability of the magnetic state and by the magnetic dipole interaction of magnetic granules (bits), has been calculated. The switching field distribution function with the parameters determined by the magnetic dipole interaction of granules and their geometry has been determined for perpendicular recording on rod-shaped granules.

A Self-Aligned Two-Step Reactive Ion Etching Process for Nanopatterning Magnetic Tunnel Junctions on 300 mm Wafers

We demonstrated a self-aligned two-step reactive ion etching (RIE) process to pattern high density magnetic tunnel junction (MTJ) arrays. We did the RIE for the top electrode (TE) and stop in the middle of the tunnel barrier. A nitride conformal film was coated on the device pillars as a dielectric spacer. The conformal spacer protects the tunnel barrier from shorting by redeposition and provides a mask for the bottom electrode (BE) RIE. We used this process and completed perpendicular MTJ devices with our process flow. We tested the devices by measuring magnetic field switching and spin transfer torque switching. We get tunneling magnetoresistance (TMR) up to 100%, switching current as low as 60 μA at 100 ns, switching current density J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c0</sub> as low as 2.5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and endurance above 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">9</sup> for devices as small as 50 nm in diameter. The results are compared with devices from a TE RIE only process, and we find minimum damage was made by the BE RIE. We also discuss the size dependence of MTJ parameters such as TMR and free layer coercive field and offset field, which is very related to the RIE process.

An embedded stress sensor for concrete SHM based on amorphous ferromagnetic microwires.

A new smart concrete aggregate design as a candidate for applications in structural health monitoring (SHM) of critical elements in civil infrastructure is proposed. The cement-based stress/strain sensor was developed by utilizing the stress/strain sensing properties of a magnetic microwire embedded in cement-based composite (MMCC). This is a contact-less type sensor that measures variations of magnetic properties resulting from stress variations. Sensors made of these materials can be designed to satisfy the specific demand for an economic way to monitor concrete infrastructure health. For this purpose, we embedded a thin magnetic microwire in the core of a cement-based cylinder, which was inserted into the concrete specimen under study as an extra aggregate. The experimental results show that the embedded MMCC sensor is capable of measuring internal compressive stress around the range of 1–30 MPa. Two stress sensing properties of the embedded sensor under uniaxial compression were studied: the peak amplitude and peak position of magnetic switching field. The sensitivity values for the amplitude and position within the measured range were 5 mV/MPa and 2.5 μs/MPa, respectively.

Magneto-electronics properties of fluorine-passivated “Christmas Tree” graphene nanoribbons

Magneto-electronics properties of fluorine-passivated “Christmas Tree” graphene nanoribbons (F-CTGNRs) are investigated theoretically. It is found that F-CTGNRs have a ferromagnetic ground state and can be a spin metal, a spin semiconductor, a spin half-metal, or a spin gapless semiconductor, completely depending on their geometrical sizes. More importantly, for the F-CTGNR based device, the dual spin-filtering effect with the perfect (100%) spin polarization and high-performance dual spin diode effect with a rectification ratio about 1010 can be achieved. It means that the F-CTGNR can act as an excellent dual spin diode. The magnetoresistance (MR) value larger than 104% can be reached in the calculated bias region, which is comparable to the previously reported experimental values for the MgO tunnel junction. These distinctive features can be attributed to their unique band overlap pattern for two electrodes and particular sensitivity to a switching magnetic field. Our findings suggest that F-CTGNRs have desirable natures for developing magnetic devices.

Magnetoresonance EMF in thin manganite films

The effect of magnetoresonance emf (MREMF) in thin epitaxial films of rare-earth manganites La0.67Sr0.33MnO3 is detected and investigated. The effect is manifested in the occurrence of a constant voltage under the action of microwave pumping in magnetic fields corresponding to ferromagnetic resonance conditions. The MREMF signal includes symmetric and antisymmetric components and changes its polarity upon switching of the external magnetic field. The temperature dependence of the effect (including the range in the neighborhood of the phase transition) is analyzed. The experimental data including the shape of the signal and its dependence of the field orientation are in good agreement with the results obtained in the theoretical model based on the mechanism of anisotropic magnetoresistance. It is shown that the magnetoresistance anisotropy in the manganite under investigation is negative and sharply attenuates as the temperature approaches the Curie point, almost vanishing in the paramagnetic phase.

Three-terminal magnetic tunneling junction device with perpendicular anisotropy CoFeB sensing layer

We demonstrated read and write characteristics of a three terminal memory device with a perpendicular anisotropy-free layer of a strip of [Co/Ni] and a low-switching perpendicular-anisotropy CoFeB/MgO sensing layer. This new design of the cell results in a small cell area. The switching magnetic field of the sensing layer can be decreased by changing sputtering gas for the Ta-cap from Ar to Kr. An electron energy-loss spectroscopy analysis of the cross-section of the magnetic tunneling junction (MTJ) revealed that the boron content in CoFeB with a Kr-sputtered Ta-cap was smaller than that with an Ar-sputtered one. A change in resistance for the MTJ was observed that corresponded to the magnetic switching of the Co/Ni wire and its magnetoresistance ratio and critical current were 90% and 0.8 mA, respectively.

Switching field distribution and magnetization reversal process of FePt dot patterns

The fabrication of FePt nanodots with a high structural quality and the control of their switching fields are key issues in realizing high density bit pattern recording. We have prepared FePt dot patterns for dots with 15–300nm diameters by electron beam lithography and re-annealing, and studied the relation between magnetization reversal process and structure of FePt nanodots. The switching field (Hsw) of dot patterns re-annealed at 710°C for 240min showed a bimodal distribution, where a higher peak was found at 5–6T, and a lower peak was found at ~2T. It was revealed by cross-sectional TEM analysis that the structure of dots in the pattern can be classified into two groups. One group has a high degree of order with well-defined [001] crystalline growth, and the other group includes structurally-disturbed dots like [111] growth and twin crystals. This structural inhomogeneity causes the magnetic switching field distribution observed.

Nonequilibrium steady state of the kinetic Glauber-Ising model under an alternating magnetic field

When periodically driven by an external magnetic field, a spin system can enter a phase of steady entrained oscillations with nonequilibrium probability distribution function. We consider an arbitrary magnetic field switching its direction with frequency comparable with the spin-flip rate and show that the resulting nonequilibrium probability distribution can be related to the system equilibrium distribution in the presence of a constant magnetic field of the same magnitude. We derive convenient approximate expressions for this exact relation and discuss their implications.