Showing papers in "IEEE Magnetics Letters in 2014"

Nanometer-Thick Yttrium Iron Garnet Films With Extremely Low Damping

Abstract: Yttrium iron garnet (YIG) films that are in the nanometer thickness range and show extremely low damping are reported. The films were deposited via sputtering at room temperature and were then annealed in O 2 at high temperature. A 22-nm-thick YIG film showed a Gilbert damping constant α = (8.58 ± 0.21) × 10 -5 , which represents the lowest damping ever reported for nanometer-thick magnetic films. The film had a gyromagnetic ratio of |γ| = 2.83 MHz/Oe and a saturation induction of 4π M s = 1766 G, which are both very close to those of single-crystal YIG bulk materials. The film had a very smooth surface, with an rms surface roughness of about 0.13 nm.

CoFeB-Based Spin Hall Nano-Oscillators

Abstract: We demonstrate magnetization auto-oscillations driven by pure spin currents in spin Hall nano-oscillators based on CoFeB/Pt bilayers. Despite the very low anisotropic magnetoresistance of CoFeB, a substantial microwave signal power can be detected, even at room temperature, indicating that a sizable spin wave amplitude is generated. Spin torque ferromagnetic resonance measurements reveal that the generated auto-oscillation frequency lies below the ferromagnetic resonance frequency of CoFeB and is therefore well described by a self-localized spin wave bullet mode.

Multimode Propagation of Magnetostatic Waves in a Width-Modulated Yttrium-Iron-Garnet Waveguide

Abstract: An irregular tapered ferrite waveguide with a periodically width-modulated region was investigated. By using space- and time-resolved Brillouin light scattering spectroscopy, we measured the features of the intermodal interaction of width modes and their scattering at the boundaries of the waveguide. Near the band-gap frequency region, the spatial pattern of the spin-waves depends on the mode interaction in the periodically width-modulated yttrium-iron-garnet waveguide. These experimental results are important for controlling spin-wave propagation in width-modulated magnetic structures for future spintronic devices.

De Magnete et Meteorite: Cosmically Motivated Materials

Abstract: Meteorites, likely the oldest source of magnetic material known to mankind, are attracting renewed interest in the science and engineering community. Worldwide focus is on tetrataenite, a uniaxial ferromagnetic compound with the tetragonal L1 0 crystal structure comprised of nominally equiatomic Fe-Ni that is found naturally in meteorites subjected to extraordinarily slow cooling rates, as low as 0.3 K per million years. Here, the favorable permanent magnetic properties of bulk tetrataenite derived from the meteorite NWA 6259 are quantified. The measured magnetization approaches that of Nd-Fe-B (1.42 T) and is coupled with substantial anisotropy (1.0-1.3 MJ/m 3) that implies the prospect for realization of technologically useful coercivity. A highly robust temperature dependence of the technical magnetic properties at an elevated temperature (20-200 °C) is confirmed, with a measured temperature coefficient of coercivity of -0.005%/K, over one hundred times smaller than that of Nd-Fe-B in the same temperature range. These results quantify the extrinsic magnetic behavior of chemically ordered tetrataenite and are technologically and industrially significant in the current context of global supply chain limitations of rare-earth metals required for present-day high-performance permanent magnets that enable operation of a myriad of advanced devices and machines.

Self-Modulated Soliton Modes Excited in a Nanocontact Spin-Torque Oscillator

Abstract: Self-modulated bubble-like solitons, namely droplets, recently observed in spin-torque nanooscillators with a perpendicular free layer, are promising for applications in spintronics, magnonics, and magnetic logic devices. This letter presents a micromagnetic analysis on soliton dynamics. Our results identify the role of physical parameters (i.e., external field, saturation magnetization, and exchange constant) in achieving experimental findings such as hysteretic, linear and nonlinear spin-wave excitations. The modes with different excitation frequencies have nonuniform spatial distributions of power, and the power of frequency sidebands is mostly located near the outer border of the nanocontact. At high currents, a wavelet-based analysis highlights that the magnetoresistive signal due to the sideband modes is not stationary, providing a possible origin of the asymmetric sidebands observed in spintronic self-modulators. We also identify the thermal field as the key ingredient for the excitation of linear modes in a subcritical regime.

General Solution of the Hollow Cylinder and Concentric DC Surface Current

Abstract: We provide a general solution to the problem of an infinitely long, hollow cylinder of uniform, linear permeability in the presence of any continuous dc current distribution residing on a concentric cylindrical surface. The solution is expressed in terms of the spatial Fourier components of the applied surface current. Applications of this study include the design of dc magnetic shields and shield-coupled coils, as well as a benchmark for finite-element analysis. Examples involving canonical current structures-loops, solenoids, and saddle-shaped coils-are presented.

Transcranial Magnetic Stimulation Device With Reduced Acoustic Noise

Abstract: Transcranial magnetic stimulation (TMS) is widely used for noninvasive activation of neurons in the brain for research and clinical applications. The strong, brief magnetic pulse generated in TMS is associated with a loud (>100 dB) clicking sound that can impair hearing and that activates auditory circuits in the brain. We introduce a two-pronged solution to reduce TMS noise by redesigning both the pulse waveform and the coil structure. First, the coil current pulse duration is reduced which shifts a substantial portion of the pulse acoustic spectrum above audible frequencies. Second, the mechanical structure of the stimulation coil is designed to suppress the emergence of the sound at the source, diminish down-mixing of high-frequency sound into the audible range, and impede the transmission of residual sound to the coil surface but dissipate it away from the casing. A prototype coil driven with ultrabrief current pulses (down to 45-μs biphasic duration) is demonstrated to reduce the peak sound pressure level by more than 25 dB compared to a conventional TMS configuration, resulting in loudness reduction by more than 14-fold. These results motivate improved mechanical design of TMS coils as well as design of TMS pulse generators with shorter pulse durations and increased voltage limits with the objective of reducing TMS acoustic noise while retaining the neurostimulation strength.

Ultrahigh Sensitivity Ferromagnetic Resonance Measurement Based on Microwave Interferometer

Abstract: An ultrahigh sensitivity ferromagnetic resonance (FMR) measurement technique based on a microwave interferometer was developed. A comparison between conventional and interferometer-based FMR showed a signal-to-noise ratio (SNR) improvement of as much as 42 dB. This largely improved SNR allowed clear resolution of the FMR signal of the uniform mode on a 100 nm diameter, 5 nm thick CoFeB single nanodot, and even higher order modes on a 400 nm diameter CoFeB single nanodot. A system noise measurement confirmed that the minimum noise level of this technique is set by Johnson noise at the input of the low noise amplifier, which is more than one order of magnitude smaller than the FMR signal obtained from the 100 nm CoFeB single nanodot.

Fe–Ni–Mn Nanoparticles for Magnetic Cooling Near Room Temperature

Abstract: We have studied the magnetocaloric effect in high-energy ball-milled (Fe 70 Ni 30 ) 95 Mn 5 alloy nanoparticles. The partial substitution of Fe and Ni by Mn decreases the Curie temperature (T C ) of the alloy to 338 K from 443 K. The change in entropy (Δ S M ) occurs over a broad range of temperatures, which results in high relative cooling power (RCP). RCP increases from 26 to 470 J · kg -1 for a field change of 0.5 T and 5 T, respectively; these values are comparable to the benchmark magnetocaloric material, gadolinium. The RCP is proportional to field H to the power 1+1/δ, with a critical exponent δ of 4.34.

Strong Eddy-Current Shielding of Ferromagnetic Resonance Response in Sub-Skin-Depth-Thick Conducting Magnetic Multilayers

Abstract: Exchange-coupled nonmagnetic metal (NM) and ferromagnetic metal (FM) multilayers are crucial for microwave magnonic and spintronic devices. These layered materials usually have total thicknesses smaller than the microwave skin depth. By using a stripline broadband ferromagnetic resonance spectroscopy technique, we experimentally demonstrate that the amplitude of the magnetization precession in the FM layer is strongly diminished by the shielding effect of microwave (612GHz) eddy currents circulating in the NM capping layers.

Strength of Perpendicular Magnetic Anisotropy at Bottom and Top Interfaces in [Pt/Co/Pt] Trilayers

Abstract: The individual strengths of perpendicular magnetic anisotropy (PMA) at the bottom and top interfaces in sputtered [Pt/Co/Pt] trilayers are determined. The PMA strength from the top Co/Pt interface is significantly weaker than that from the bottom Pt/Co interface, indicating a similar difference in the interface quality. The contribution to the total anisotropy strength from the top Co/Pt interface increases with decreasing top Pt layer thickness; for a stack of Co thickness 1 nm, the contribution is only 10% for a top Pt layer of thickness 3.0 nm, but it increases to 32% for a Pt layer of thickness 0.2 nm.

Detection of the DC Inverse Spin Hall Effect Due to Spin Pumping in a Novel Meander-Stripline Geometry

Abstract: The dc voltage obtained from the inverse spin Hall effect (iSHE) due to spin pumping in ferromagnet/normal-metal (NM) bilayers can be unintentionally superimposed with magnetoresistive rectification of ac charge currents in the ferromagnetic layer. We introduce a meander-stripline device in which these spurious rectification voltages vanish while the iSHE voltage is maximized. In this device, a quantitative study of the dc iSHE is performed in a broad frequency range for Ni 80 Fe 20 /NM multilayers with NM = {Pt, Ta, Cu/Au, Cu/Pt}. The experimentally recorded voltages can be fully ascribed to the iSHE due to spin pumping. Furthermore, we measure a small iSHE voltage in single CoFe thin films.

Structural and Magnetic Characterization of FeCoCu/Cu Multilayer Nanowire Arrays

Abstract: A series of [FeCoCu/Cu(x)]10 (7 ≤ × ≤ 40 nm with FeCoCu layer thickness of 300 nm) and [FeCoCu(y)/Cu]10 (120 ≤ y ≤ 900 nm with Cu layer thickness of 15 nm) arrays of multilayer nanowires, 35 nm in diameter, were fabricated by electrodeposition into self-assembled pores of anodic alumina membranes. High-resolution transmission electron microscopy and X-ray diffraction analysis confirm the segregation of layered structures, with well-defined Cu layers (fcc cubic structure) separating FeCoCu-alloy segments (bcc cubic structure). Hysteresis loop measurements indicate an overall magnetization easy axis parallel to the nanowires in all the samples. For constant FeCoCu segment length, the coercivity, the remanence, and especially, the susceptibility increase with the Cu layer thickness, whereas for the series with constant Cu layer thickness, the susceptibility significantly decreases with FeCoCu segment length. Complementary Henkel curves indicate that the net inter/intrananowires magnetostatic interactions always contribute to the demagnetization of the nanowires. The variation of the susceptibility with FeCoCu and Cu layers thickness together with the Henkel plots data indicate that a reduced demagnetizing effect is achieved for multilayer nanowires with the thicker Cu layer and the shorter FeCoCu segment, for which a moderated reduction in saturation magnetization of around 11% is estimated compared to a continuous FeCoCu alloy nanowire array.

Microwave Properties and Damping in [Pt/Co] Multilayers With Perpendicular Anisotropy

Abstract: Magnetic tunnel junctions with perpendicularly magnetized elements are being considered for next-generation, non-volatile, magnetic random access memory (MRAM) elements due to their large thermal stability, low Gilbert damping constant α, and very low critical current density Jc required for spin-transfer-torque (STT), current-induced, magnetization switching. Here we study, by means of static magnetization and field-swept ferromagnetic resonance (FMR), [Pt/Co] multilayers with reduced Pt and Co layer thicknesses, ranging between 0.2 nm and 0.32 nm. Such materials are known to exhibit strong perpendicular magnetic anisotropy due to low interdiffusion, even after annealing up to 500°C. We analyzed our data on FMR frequency versus applied magnetic field with the appropriate Kittel formulas and obtained a high anisotropy field H k and a very low Gilbert damping constant α. This improvement makes the [Pt/Co] system with low overall Pt and Co content a promising candidate for STT-MRAM.

Coercivity Reduction in Nickel Ferrite (NiFe $_{2}$ O $_{4}$ ) Thin Films Through Surface Patterning

Abstract: NiFe2O4(NFO), a spinel ferrite with high electrical resistivity and favorable magnetic properties, is an interesting material for high-frequency signal and power electronic applications. Here, significant reduction in the coercivity of NFO films is obtained through surface patterning via nanoimprint lithography. Multilayered NFO films are grown on c-plane (0001) sapphire substrates using room temperature chemical solution deposition. Two film variants, layer-by-layer and bulk, are deposited. Prior to crystallization, films are patterned with a polydimethylsiloxane stamp. Good feature transfer to the thin-film surface is confirmed by atomic force microscopy and transmission electron microscopy. Θ-2Θ X-ray diffraction shows that both variants produce single-phase inverse spinel NFO, with better texture in the layer-by-layer samples. Magnetic measurements show substantial reduction in coercivity in the patterned samples due to the surface anisotropy-enhanced demagnetization field. The bulk patterned sample showed the lowest coercivity, ~18 Oe in-plane, albeit with reduced saturation magnetization, whereas the layer-by-layer patterned film maintained the same degree of texture and saturation as unpatterned films, with a ~80% reduction in coercivity. These results show that nanoimprint lithography of chemical solution deposition films is a cost-effective pathway to engineering the coercivity of NFO films while retaining desirable saturation magnetization and texture.

Effect of Excitation Fatigue on the Synchronization of Multiple Nanocontact Spin-Torque Oscillators

Abstract: Nanocontact spin-torque oscillators (NC-STOs) act as intrinsically nanoscale and highly current and magnetic field tunable, ultrawide band microwave signal generators. However, their low output power and high phase noise remain critical obstacles toward actual applications. Mutual synchronization of multiple NCs is one possibility to overcome these shortcomings. This letter presents a detailed study of the mutual synchronization in a NC-STO with two NCs. In particular, the effect of repeated measurements on the synchronization behavior is explored. Repeated measurements at high drive currents are shown to significantly degrade the performance of the devices with the most striking consequence being that the devices can no longer be synchronized. Ferromagnetic resonance measurements reveal a decrease in the saturation magnetization and an increase in the damping coefficient in annealed NiFe films, consistent with Cu diffusion into the NiFe from the adjacent Cu layers. This increase in damping will act to sever the spin wave-mediated communication channel between the NCs necessary for synchronization. These results highlight an important consideration when studying the synchronization behavior of multi-NC devices where Joule heating is expected to scale unfavorably with the number of NCs.

Optimization of Force Between Cylindrical Permanent Magnets

Abstract: We calculated analytically and with the finite-element method the force between two identical coaxial cylindrical magnets for nine different magnetic materials at separations of 1, 3, 5, 7, and 9 m ...

Atmospheric Magnetic Noise Measurements in Urban Areas

Abstract: The theory of atmospheric magnetic noise is reviewed and we discuss how it impacts magnetoinductive communications systems. Magnetic noise measurements are then conducted to obtain quantitative values of the noise levels at both indoor and outdoor locations. The measurements, using an industrial-grade magnetometer, are compared with results from Comite Consultatif International des Radiocommunications (CCIR) measurements conducted using an electric antenna, and then converted via the E-H relationship in vacuum. Both measurements are in agreement other than in a small frequency range, whereas the CCIR electric measurements show pessimistic minimal values, possibly due to electrical interference.

High-Thrust Linear Actuator Based on Double Corner-Pole Airgaps for Proportional Relief Valve

Abstract: This letter presents a high-thrust linear actuator for use in a proportional relief valve, based on double corner-pole airgaps formed by the structure of welding sleeve and step armature. The magnetic flux through the double corner-pole airgaps are divided into three parts of two radial fluxes (Φ1, Φ3) and one axial flux (Φ2), resulting in a linear characteristic with high-thrust force on the armature of the actuator. Parameter improvement in the limited space is analyzed based on modeling, and optimized results for a prototype actuator were obtained. The experimental and simulation results agree well. The prototype actuator can produce a thrust force of 110 N with a rated power of 12 W, and is linear within 4%, with 2% hysteresis over the displacement range of 0.8 to 2.3 mm. The force step response time of the actuator reaches 51 ms, and the force frequency response (-3 dB) reaches 36 Hz for a displacement of 1.5 mm.

Alternating Motion of Single-Domain Walls in Uniaxial Magnetic Wire

Abstract: Fe-Si-B microwires with strong uniaxial anisotropy reverse magnetization under homeogeneous driving magnetic field, H dr , by the nucleation at one end and propagation of a single standard domain wall, DW st , resulting in a giant Barkhausen jump between two stable remanent states with opposite magnetization. Under the additional presence of local field, H loc , a local reverse domain is nucleated, generating a pair of head-to-head and tail-to-tail injected walls, DW inj , that propagate in opposite directions. We performed designed experiments to control the motion of the DW st and DW inj under the action of both fields. The amplitude and frequency of square-shaped in-phase H dr and H loc ac fields are tuned to achieve controlled motion of the DW inj domain wall. By properly selecting the amplitude of H dr and H loc ac fields (i.e., double or multiple frequencies), we experimentally confirm the DW inj alternating motion between stable local positions/magnetic states, where H loc plays the role of a magnetic valve and H dr is the driving field.

Comparative Study of Magnetic Properties of Nanoparticles by High-Frequency Heat Dissipation and Conventional Magnetometry

Abstract: The rate of heating of 15 nm uniformly-sized magnetic aqueous nanoparticles suspension by high-amplitude and high-frequency ac magnetic field induced by the resonating LC circuit was measured. The results are analyzed in terms of specific energy absorption rate (SAR). Fitting field amplitude and frequency dependences of SAR to the linear response theory, magnetic moment per particles was extracted. The value of magnetic moment was independently evaluated from dc magnetization measurements of a frozen colloid by fitting field-dependent magnetization to a Langevin function. The two methods produced similar results, which are compared to the theoretical expectation for this particle size. Additionally, analysis of SAR curves yielded effective relaxation time.

Control of In-Plane Uniaxial Anisotropy of CoPd-CaF $_{2}$ Nanogranular Films by Tandem-Sputtering Deposition

Abstract: In-plane uniaxial anisotropic CoPd-CaF 2 nanogranular soft magnetic films for gigahertz applications were prepared using RF tandem-sputtering deposition with substrate revolution. Strong anisotropy with a large anisotropy field exceeding 50 kA/m was achieved in the centrifugal-centripetal direction of the substrate revolution, regardless of the presence or absence of an applied external dc field for induced anisotropy. X-ray pole-figure analysis demonstrated that the crystalline orientation of CoPd111 as a crystallographically magnetic easy axis corresponds to the easy axis of the films. Thus, the soft magnetic properties and uniaxial anisotropy of nanogranular films are determined by both random-magnetocrystalline and shape anisotropies. Complex permeabilities indicated high ferromagnetic resonance frequency exceeding 5GHz with a low Gilbert damping factor of 0.06 for the nanogranular films, and the applied field caused a change in the damping factor. Thus, the magnetic properties of the films can be controlled by the substrate motion and the applied field under tandem-sputtering conditions.

Magneto-Electric Control of Surface Anisotropy and Nucleation Modes in L10-CoPt Thin Films

Abstract: The interplay between electric field-controlled surface magnetic anisotropy and micromagnetic nucleation modes for L1 0 -CoPt thin films is investigated with density-functional and micromagnetic model calculations. The electric field redistributes electron states near the Fermi level, which has a fairly strong effect on the surface anisotropy, but due to inversion symmetry, the net anisotropy of the films with odd numbers of layers remains unchanged. By contrast, the micromagnetic nucleation mode is spatially asymmetric even for symmetric thin films with odd numbers of layers. This leads to a reduction of the nucleation field (coercivity) and-for suitably chosen nanostructures-to substantial changes in the hysteretic behavior. In the lowest order, the coercivity reduction is independent of the total film thickness. This counterintuitive feature can potentially be exploited in magneto-electric switching devices.

Strong impact of the eddy-current shielding on ferromagnetic resonance response ofsub-skin-depth-thick conducting magnetic multilayers

Abstract: Exchange-coupled nonmagnetic metal (NM) and ferromagnetic metal (FM) multilayers are crucial for microwave magnonic and spintronic devices. These layered materials usually have total thicknesses smaller than the microwave skin depth. By using a stripline broadband ferromagnetic resonance spectroscopy technique, we experimentally demonstrate that the amplitude of the magnetization precession in the FM layer is strongly diminished by the shielding effect of microwave (6-12 GHz) eddy currents circulating in the NM capping layers.

Microstructures and Magnetic Properties of Electrodeposited Co-Pt Nanowires With Diameters Below 100 nm

Abstract: We have investigated the effect of the microstructure on the magnetic properties of Co-Pt nanowires (NWs) with diameters of 15, 50, and 80 nm. These Co-Pt NWs were fabricated by using polycarbonate membranes with different pore diameters via direct electrodeposition. A detailed transmission-electron-microscopy analysis revealed that the Co-Pt NWs transform from a polycrystalline to a single-crystalline-like structure along the growth direction of the NWs. The selected-area electron-diffraction investigation of the Co-Pt NWs with 50 nm diameters revealed a fcc-dominant crystal structure, while the 15 and 80 nm NWs were composed of an intermixture of fcc and hcp phases. This investigation allows us to understand the magnetic hysteresis loops of the Co-Pt NW arrays. Furthermore, the magnetic domains of the individual Co-Pt NWs were studied with magnetic force microscope (MFM), and the MFM contrasts for the 50 nm and 80 nm diameter NWs are interpreted as consisting of z-vortices.

Role of Thermal Effects on Magnetic Interactions in Stacked Magnetic Layers With Perpendicular Anisotropy

Abstract: The role of thermally excited magnetization reversals on magnetic interactions in stacked magnetic layers with perpendicular anisotropy has been investigated by first-order reversal curves (FORC) and coercive squareness (S*). For this purpose, a study of the effect of systematic variation of exchange interaction in granular perpendicular magnetic anisotropy layers on FORC contours has been carried out along with S* and coercivity measurements at various timescales. The results indicate that the FORC contour peak height is a better parameter than S* to study the exchange interactions in systems such as magnetic recording media and is insensitive to thermal effects.

Linewidth Variation of the Higher Harmonics in Spin-Torque Vortex Oscillators

Abstract: We have studied microwave emission due to vortex gyration in pseudo-spin-valve nanopillar devices. Subgigahertz emission spectra with a large number of higher harmonics are found with linewidth of the fundamental peak as low as ~500 kHz. Several distinct gyration modes exist whose peak frequencies are separated by a few tens of megahertz, likely corresponding to magnetic oscillations with different metastable vortex core trajectories. In a single spectral measurement, however, only one of the distinct modes appears, indicating that the excited mode is stable. The linewidth of higher harmonics, in many cases, increases linearly with the harmonic number n, indicating that the frequency-amplitude coupling is significant. These results show the strong nonisochronous characteristics of spin-torque vortex oscillators.