Quasi-static and dynamic switching of exchange biased micron-sized TMR junctions

We study the quasi-static and dynamical switching of magnetic tunnel junction patterned in micron-sized cells with integrated field pulse line. The tunnel junctions are CoFe/AlO/CoFe with an exchange biasing layer of MnIr. Quasi-static characterizations have been used to determine anisotropy, coercive as well as exchange bias fields. Dynamic switching measurements are done by applying fast-rising magnetic field pulses (178 ps–10 ns) along the hard axis of the junction with a quasi-static easy-axis applied field. We identify the field conditions leading to no-switching, to direct-writing and to toggle switching. We identify these field conditions up to the precessional limit, and construct the experimental dynamical astroïd. The magnetization trajectories leading to direct-writing and to toggle switching are well described by macrospin simulations.     

High-speed creep effects in magnetic films

Creep measurements were made on single domain walls in thin magnetic films using high-speed pulses with variable rise and fall times (0.4 ns to>1 mus) and durations (<1 ns to 3 μs). Combinations of these pulses and dc fields were applied along the hard axis while simultaneously easy-axis dc fields were applied. The two basic measurements that were made were onset of creep and the distance the wall crept per pulse as a function of applied fields. Definite rise-time effects were found, the exact behavior depending on the domain wall structure. For Bloch walls, gyromagnetic effects of the total wall (similar to wall streaming) are present for rise timeslsim 20ns, whereas for longer rise times the Bloch to Néel wall transition appears to be responsible. For films thinner than 900 Å the existence of a cross-tie structure was found to be necessary for creep. For this wall structure the exact mechanism which causes creep enhancement for rise times <100 ns is unknown.     

Anisotropic Superconductivity and Vortex Dynamics in?Magnetically Coupled F/S and F/S/F Hybrids

Magnetically coupled superconductor?Cferromagnet hybrids offer advanced routes for nanoscale control of superconductivity. Magnetotransport characteristics and scanning tunneling microscopy images of vortex structures in superconductor?Cferromagnet hybrids reveal rich superconducting phase diagrams. Focusing on a particular combination of a ferromagnet with a well-ordered periodic magnetic domain structure with alternating out-of-plane component of magnetization, and a small coherence length superconductor, we find directed nucleation of superconductivity above the domain wall boundaries. We show that near the superconductor-normal state phase boundary the superconductivity is localized in narrow mesoscopic channels. In order to explore the Abrikosov flux line ordering in F/S hybrids, we use a combination of scanning tunneling microscopy and Ginzburg?CLandau simulations. The magnetic stripe domain structure induces periodic local magnetic induction in the superconductor, creating a series of pinning?Canti-pinning channels for externally added magnetic flux quanta. Such laterally confined Abrikosov vortices form quasi-1D arrays (chains). The transitions between multichain states occur through propagation of kinks at the intermediate fields. At high fields we show that the system becomes nonlinear due to a change in both the number of vortices and the confining potential. In F/S/F hybrids we demonstrate the evolution of the anisotropic conductivity in the superconductor that is magnetically coupled with two adjacent ferromagnetic layers. Stripe magnetic domain structures in both F-layers are aligned under each other, resulting in a directional superconducting order parameter in the superconducting layer. The conductance anisotropy strongly depends on the period of the magnetic domains and the strength of the local magnetization. The anisotropic conductivity of up to three orders of magnitude can be achieved with a spatial critical temperature modulation of 5% of T _c. Induced anisotropic properties in the F/S and F/S/F hybrids have a potential for future application in switching and nonvolatile memory elements operating at low temperatures.     

Switching processes in thin ferromagnetic film memory elements

The switching properties of small, rectangular areas of thin ferromagnetic films, relevant to their utilization as memory elements in digital computers, are discussed. An attempt is made to derive the switching properties theoretically by superposition of the calculated demagnetizing field effects upon the known intrinsic film properties. Experiments performed using homogeneous quasi-static applied fields show good agreement with the theory. In the case of high easy direction applied fields the complexity of the magnetization distributions necessitates a number of simplifying assumptions in the theoretical treatment, and here the agreement is poorer. The treatment is sufficiently accurate to yield relationships between the intrinsic film properties, the dimension of the element, and the available output flux. In practical configurations the element is switched by inhomogeneous fields produced by fast current pulses in strip lines, and its state is ascertained by the observation of an EMF induced by the rotating magnetization. The differences between the configuration observed here and the practical one are discussed.     

Induction of coherent magnetization switching in a few atomic layers of FeCo using voltage pulses

The magnetization direction of a metallic magnet has generally been controlled by a magnetic field or by spin-current injection into nanosized magnetic cells. Both these methods use an electric current to control the magnetization direction; therefore, they are energy consuming. Magnetization control using an electric field is considered desirable because of its expected ultra-low power consumption and coherent behaviour. Previous experimental approaches towards achieving voltage control of magnetization switching have used single ferromagnetic layers with and without piezoelectric materials, ferromagnetic semiconductors, multiferroic materials, and their hybrid systems. However, the coherent control of magnetization using voltage signals has not thus far been realized. Also, bistable magnetization switching (which is essential in information storage) possesses intrinsic difficulties because an electric field does not break time-reversal symmetry. Here, we demonstrate a coherent precessional magnetization switching using electric field pulses in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to a MgO barrier. Furthermore, we demonstrate the realization of bistable toggle switching using the coherent precessions. The estimated power consumption for single switching in the ideal equivalent switching circuit can be of the order of 10(4)k(B)T, suggesting a reduction factor of 1/500 when compared with that of the spin-current-injection switching process.     

Deterministic Magnetization Switching Using Lateral Spin–Orbit Torque

Current-induced magnetization switching by spin–orbit torque (SOT) holds considerable promise for next generation ultralow-power memory and logic applications. In most cases, generation of spin–orbit torques has relied on an external injection of out-of-plane spin currents into the magnetic layer, while an external magnetic field along the electric current direction is generally required for realizing deterministic switching by SOT. Here, deterministic current-induced SOT full magnetization switching by lateral spin–orbit torque in zero external magnetic field is reported. The Pt/Co/Pt magnetic structure is locally annealed by a laser track along the in-plane current direction, resulting in a lateral Pt gradient within the ferromagnetic layer, as confirmed by microstructure and chemical composition analysis. In zero magnetic field, the direction of the deterministic current-induced magnetization switching depends on the location of the laser track, but shows no dependence on the net polarization of external out-of-plane spin currents. From the behavior under external magnetic fields, two independent mechanisms giving rise to SOT are identified, i.e., the lateral Pt–Co asymmetry as well as out-of-plane injected spin currents, where the polarization and the magnitude of the SOT in the former case depends on the relative location and the laser power of the annealing track.     

Ultrafast Magnetization Manipulation Using Single Femtosecond Light and Hot‐Electron Pulses

Current‐induced magnetization manipulation is a key issue for spintronic applications. This manipulation must be fast, deterministic, and nondestructive in order to function in device applications. Therefore, single‐ electronic‐pulse‐driven deterministic switching of the magnetization on the picosecond timescale represents a major step toward future developments of ultrafast spintronic systems. Here, the ultrafast magnetization dynamics in engineered Gd_x [FeCo]_1?x ‐based structures are studied to compare the effect of femtosecond laser and hot‐electron pulses. It is demonstrated that a single femtosecond hot‐electron pulse causes deterministic magnetization reversal in either Gd‐rich and FeCo‐rich alloys similarly to a femtosecond laser pulse. In addition, it is shown that the limiting factor of such manipulation for perpendicular magnetized films arises from the formation of a multidomain state due to dipolar interactions. By performing time‐resolved measurements under various magnetic fields, it is demonstrated that the same magnetization dynamics are observed for both light and hot‐electron excitation, and that the full magnetization reversal takes place within 40 ps. The efficiency of the ultrafast current‐induced magnetization manipulation is enhanced due to the ballistic transport of hot electrons before reaching the GdFeCo magnetic layer.     

Switching windows for a nanostructured cell of synthetic ferrimagnets

The magnetization switching window of nanostructured synthetic ferrimagnets with lateral dimension of 160 nm x 80 nm under combined in-plane magnetic fields along the longitudinal and transverse directions is investigated by numerical calculation using an analytical equation for the total energy. The considered total energy equation precisely accounts for the magnetostatic energy, which is significantly large in nanostructured magnetic cells. Due to the complex magnetization alignment of synthetic ferrimagnets, a different switching criterion based on the reversibility of magnetization process is used, instead of the simple criterion frequently used for single magnetic layers. Synthetic ferrimagnets with various thickness asymmetries are considered, and switching windows are calculated both in static and dynamic conditions. The static switching windows show a smaller dependence on the thickness asymmetry than the dynamic switching windows do. The dynamic switching window at a large thickness asymmetry resembles that of a single magnetic layer. The results are discussed in terms of energy profiles that can be obtained by locating the lowest energy path linking the two stable states from the total energy surface.     

Switching through intermediate states seen in a single nickel nanorod by cantilever magnetometry

In-plane to out-of-plane magnetization switching in a single nickel nanorod affixed to an attonewton-sensitivity cantilever was studied at cryogenic temperatures. We observe multiple sharp, simultaneous transitions in cantilever frequency, dissipation, and frequency jitter associated with magnetic switching through distinct intermediate states. These findings suggest a new route for detecting magnetic fields at the nanoscale.     

Novel magnetic tips developed for the switching magnetization magnetic force microscopy

Using micromagnetic calculations we search for optimal magnetic properties of novel magnetic tips to be used for a Switching Magnetization Magnetic Force Microscopy (SM-MFM), a novel technique based on two-pass scanning with reversed tip magnetization. Within the technique the sum of two scans images local atomic forces and their difference maps the local magnetic forces. The tip magnetization is switched during the scanning by a small magnetic field. The technology of novel low-coercitive magnetic tips is proposed. For best performance the tips must exhibit low magnetic moment, low switching field, and single-domain state at remanence. Such tips are equipped with Permalloy objects of a precise shape that are defined on their tilted sides. We calculate switching fields of such tips by solving the micromagnetic problem to find the optimum shape and dimensions of the Permalloy objects located on the tips. Among them, hexagon was found as the best shape for the tips.     

Viscous flux motion in anisotropic magnetic superconductors

Magnetic field penetrates in the form of flux lines or vortex thread into type-II magnetic superconductors (MSC) and induces magnetization of magnetic subsystem over a distance of an order of the London penetration depth surrounding the normal cores. When a flux line moves by, surrounding magnetization moves as a whole through the sample and a free motion of vortices is subjected to magnetic viscous drag, giving rise to dissipation. The flux flow resistance in the mixed state of anisotropic MSC has been studied on the basis of the London theory. Expressions for the dissipation and viscosity coefficient associated with the change of the magnetic subsystem as a vortex moves about are derived.     

Simulation of the perpendicular recording process including image charge effects

This paper presents a complete model for the perpendicular recording process in single-pole-head keeper-layer configurations. It includes the influence of the image-charge distributions in the head and the keeper layer. Based on calculations of magnetization distributions in standstill situations, the model describes the relaxation process that takes place if the activated head is shifted along the recording layer, periodically switching its head field. The magnetization distributions thus derived are used in combination with a model for the readback process to calculate the readback flux and voltage pulses. For the sake of arithmetical convenience, the model was applied to a recording configuration with a thick single-pole head, but it can also be used for calculations with other head shapes, e.g. thin single-pole heads.     

不同直径玻璃包覆非晶微丝的制备及其磁性能

采用熔融拉丝法制备了直径范围分别在6.1～28.0μm和14.0～35.2μm之间的玻璃包覆非晶Fe基和Co基合金微丝, 测试了不同合金直径和不同玻璃包覆层厚度的玻璃包覆合金微丝样品的静磁性能. 结果表明: 轴向矫顽力和轴向剩磁比随着微丝直径的增大而降低, 随着玻璃包覆层的增大而升高; 径向剩磁比的变化趋势则相反. 微丝合金直径和玻璃包覆层厚度改变, 静磁性能变化的主要原因是作用在合金芯上的内应力的变化, 导致了具有不同磁畴结构的合金内芯区和合金外壳区体积比的改变.     

Fast switching behaviour of nanoscopic NiFe- and Co-elements

Three-dimensional micromagnetic simulations were performed to study the magnetisation reversal processes of granular nanoelements using a hybrid finite element/boundary element model. Transient magnetisation states during switching are investigated numerically in granular, thin Ni₈₀Fe₂₀ and Co square shaped nanoelements (100×100 nm²) with 10 nm grain size and a thickness of 10 nm and taking into account a random orientation of the grains. Switching dynamics are calculated for external fields between 80 and 280 kA/m, which were uniformly applied after a rise time of 0.05 and 0.1 ns, respectively, and in comparison for a 10 GHz rotational field. Reversal in the unidirectional field proceeds by the nucleation and propagation of end domains towards the centre of the granular thin film elements. The formation of a vortex magnetisation structure leads to an increase of the switching time in the granular Co element. The switching time strongly depends on the Gilbert damping parameter . Small values of (0.1) lead to shorter switching times at small field strength values (h<0.5 J_s/μ₀). Reversal in rotational fields involves inhomogeneous rotation of the end domains towards the rotational field direction leading to partial flux-closure structures and therefore facilitating the switching by reduced switching times. The micromagnetic study reveals that switching partly occurs already during the rise time of the unidirectionally oriented external field. Shorter switching times are obtained by the application of a half cycle of a 10 GHz rotational field (t_sw=0.05 ns). Precessional oscillation effects after switching off the external field which occurred in the Ni₈₀Fe₂₀ square element, were suppressed by the uniaxial anisotropy of the randomly oriented Co grains. Taking into account thermally activated processes the micromagnetic simulations show that the switching time was reduced by less than 10% at T=300 K for Co and H_ext.=140 kA/m (h=0.1 J_s/μ₀).     

Half-Integer Flux Quantization in a Superconducting Loop with a Ferromagnetic π-Junction

Superconducting loops containing a π-junction are predicted to show a spontaneous magnetic moment in zero external magnetic field. In order to confirm this longstanding prediction experimentally, we performed magnetization measurements on individual mesoscopic superconducting niobium loops with a ferromagnetic (PdNi) π-junction. The loops are prepared on top of the active area of a micro Hall-sensor based on high mobility GaAs/AlGaAs heterostructures. We observe switching of the loop between different magnetization states at very low-magnetic fields, which is asymmetric for positive and negative sweep direction. This is evidence for a spontaneous current induced by the intrinsic phase shift of the π-junction. In addition, the presence of the spontaneous current at zero applied field is directly revealed by an increase of the magnetic moment with decreasing temperature, which results in a half integer flux quantization in the loop at low temperatures. This work is dedicated to H. von L?hneysen on the occasion of his 60th birthday.     

Film-head pulse distortion due to microvariation of domain wallenergy

Experiments show that the location of spurious peaks on the trailing edges of data pulses is strongly influenced by the application of small DC bias currents to the head coil during readback. A simple model of the head response which includes both magnetization rotation and domain wall motion for the case where domain wall coercivity is comparable to signal fields in the head reproduces many of the features of the observations. Calculations show that fields in pole tips during reading can exceed the wall-motion coercive force of permalloy. It is demonstrated that the resulting domain wall motion could give rise to the spurious signals sometimes seen in thin-film heads. It is proposed that such heads contain domain walls parallel to the signal flux path. The observed oscillations in the spurious response are primarily caused by filter dynamics, not domain wall dynamics     

Effects of skew and demagnetizing fields on NDRO performance

Limitations placed upon the NDRO performance of a magnetic film memory device by the detrimental effects of skew and demagnetizing fields are investigated. An expression has been derived that relates these effects to the NDRO critical magnetization angle for devices whose rotational switching can be characterized by a Stoner-Wohlfarth switching astroid. Experimental results for typical devices are presented that substantiate the theory.     

Possible Spin Pumping Effects on Spin Torque Induced Magnetization Switching in Magnetic Tunneling Junctions

Dependence of spin torque induced magnetization switching upon interfacial insulating layers properties of magnetic tunneling junctions (MTJ) are studied. For the same magnetic properties and patterning geometric dimensions, changes in MTJ interfacial insulating layers properties reveal interesting magnetization switching behaviors. These behaviors cannot be explained by conventional Landau-Lifshitz-Gilbert equation with a spin torque term and an intrinsic ferromagnetic relaxation damping. However the magnetization switching dynamics can be understood through assumption of spin pumping effects in magnetic tunneling junctions. This is not only important for fundamental understanding of spin and electronic transport in MTJ but also important for practical trade-offs between critical switching current and MTJ resistance for spin torque random access memory.     

Observations on permanent magnets related to damping effects in levitation devices

Microscopic observations applying high, variable magnetic fields on surfaces of high-coercivity magnets reveal hysteretic magnetization changes by domain wall movements. Lines, formerly believed to be grain boundaries, are moving with changing magnetic fields during the magnetization reversal. Domain wall motions are shown on single micrographs which were selected from sequences of micrographs depicting magnetization cycles on commercial-grade magnets of barium ferrite and of samarium cobalt. The observed processes are believed to consume energy; they can be responsible for the damping of oscillations which was experienced before in magnetic bearings.