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.     

Soft magnetic thin-film memory materials

Thin-film deposition techniques as applied to magnetic memory applications are reviewed. The relationship between the microstructure of the film and the fabrication technique and pertinent parameters is discussed in terms of the atomistics of film nucleation and growth. The dependence of the magnetic properties governing switching of a magnetic storage element is qualitatively discussed in terms of the microstructure and ripple theory; literature data is used to elucidate this by showing the effects of composition, temperature, thickness, and substrate morphology on the magnetic properties. The literature values for the anisotropy field, coercivity field, and dispersion of various classes of ternary alloys is reviewed, extending previous papers reviewing elemental and binary alloys.     

The electrical properties of cadmium phosphate glasses at high electric fields

The cadmium phosphate glasses previously described by the authors are further examined in relation to their electrical conduction properties at high applied fields and also when the fields are sufficiently high for memory switching to occur. The conduction at high electric fields (>2×10⁴ V cm⁻¹) is believed to be due to thermally activated hopping of electrons and to field-lowering at the electrodes as predicted by Schottky. The relative significance of the Schottky and Poole-Frenkel processes is discussed. The memory switching observed in thin samples at high applied fields is shown by electron diffraction studies to be associated with field-induced crystallization of a localized region of the glass film.     

Calculation of the switching constant of magnetic recording media

We studied the dynamic switching time in two classes of media by considering two different particle orientation distribution functions. We calculated the switching constant directly from the Landau-Lifshitz-Gilbert equation of motion, which was chosen to simulate the dynamic properties of the media. A strong linear relation between the reciprocal of the switching time and the difference between the applied and anisotropy fields is illustrated. In media for which experimental results are available, the values we obtained here agree within a factor of 2     

Analysis of the Anisotropy Field and the Saturation Magnetization for Ultrathin Ferromagnetic Films

We study the magnetic in-plane anisotropic fields for a two-dimensional film by including magnetic field in the basal plane for an easy axis film. We present the balance between the applied field and in-plane anisotropic field at equilibrium. We have also investigated the approach to saturation of magnetization and numerically solving the nonlinear equation for equilibrium, and results are discussed in connection with experimental data reported for Co films.     

Magnetic behavior in an ordered Co nanorod array

The magnetization reversal process of an ordered Co nanorod array is shown using the images obtained from successive in-field magnetic force microscope (MFM) measurements. The magnetization reversal model is discussed according to local and whole magnetization reversal properties measured by the polar magneto-optical Kerr effect (PMOKE) and an alternating gradient magnetometer (AGM), respectively. Additionally, the dipolar field was probed using in-field MFM measurements. By removing the effect of the dipolar field, an intrinsic switching field distribution (SFD) is shown in a map with a hexagonal array. A detailed study of the dipolar field in ordered nanorod arrays with various diameters and pitches was carried out by numerical calculations.     

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.     

The mechanism of magnetic relaxation in Fe-based nanocrystalline alloy ribbons investigated by permeability spectroscopy

In the present work, in order to elucidate the mechanism of magnetic relaxation in Fe-based nanocrystalline alloy ribbons at low fields and low frequencies, the dependence of the dynamic magnetization properties of nanocrystalline alloy ribbons on the thickness of ribbons has been investigated by permeability spectroscopy. The agreement of the experimental results with the domain wall motion equations confirms that domain wall bulging between the pinning edges is the dominant magnetization mechanism at low fields and low frequencies.     

Magnetization distribution in flat and cylindrical films subject to nonuniform hard direction fields

The magnetization distribution, resulting from the application of a nonuniform field along the hard axis of a uniaxially anisotropic thin film, is calculated. Planar and cylindrical film geometries are considered. We note 1) that there is a simple solution to the nonuniform field problem, provided the applied field is spatially periodic along the hard axis and 2) that many fields of interest are strongly localized and may be treated by the periodic calculation by assuming that the field is repeated at intervals along the hard axis. The interval is taken large enough that the resulting magnetization distribution does not depend on the periodicity so introduced. The calculation becomes invalid when appreciable saturation of magnetization occurs. Experimental measurements on plated wires and planar films, carried out for a variety of drive strap configurations using the Kerr effect probe, are in good agreement with theory.     

Principles of a three-dimensional recording model for short wavelength magnetic recording

Principles are provided for a method of evaluating the magnetization of a magnetic recording tape element running in front of a recording gap. The distributions of switching fields P(Hc) and magnetization axes P(θ, φ) used in the calculations are derived from simple experimental results. The output level is calculated by means of the reciprocity principle. The first application of the method concerns certain phenomenological aspects of magnetic recording. As a practical example, a qualitative explanation of the output level curve versus the recording current for short wavelength signals is given; the advantage of microgaps for recording are demonstrated and finally, the circular Bitter patterns discovered by S. Iwasaki are shown to be predicted by the model. This study is a first step toward a more general solution including the demagnetizing field.     

The Spin Wave Gap and Switching Field in Thin Films with In-Plane Anisotropy

In this paper, we have calculated the spin wave gap and the angular dependence of magnetization reversal in a single-layer thin magnetic film that includes the strong perpendicular magnetic anisotropy and in-plane anisotropy. The film is assumed to be under the influence of the out-of-plane direction of the applied magnetic field at zero temperature. Using the quantum model, it is shown that the calculated equations present a nonzero spin wave gap at zero magnetic field which is strongly affected by anisotropies. The effects of the in-plane anisotropy and the role of the applied field were examined. We also discussed a simple theoretical model for the angular variation of switching field by using a quasi-classical argument. We used some constants in connection with experimental data which are reported for chromium telluride thin films grown by molecular beam epitaxy.     

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.     

A study of noncoherent rotational switching for thin magnetic films

Thin magnetic film switching was investigated for fields near those needed for pure rotation. Experimentally the films were switched using <0.4 ns rise time field pulses. The resulting flux changes were detected in the easy and hard direction with a response time of 0.6 ns. Measurements were made for pulses both longer and shorter than the magnetization switching times. By analyzing the voltage waveforms and flux changes, it was concluded that instabilities and rapid rearrangements of the magnetization can occur within a few nanoseconds, causing anomalous results during switching. Equations of existing quantitative switching models-pure rotation, spin-wave, and stripe domain-were solved with a digital computer. To better compare theory and experiment, the solutions were modified to account for the sense system's finite rise time. It was found that none of the existing models adequately described the switching processes for low amplitude magnetic fields. However, qualitatively, the stripe domain model best fit the experimental data.     

Quasi-static switching of plated wires in external magnetic fields

Switching thresholds of plated wires were determined with slowly applied external magnetic fields, using both torquemeter and ac techniques. The samples included wires having helical easy-direction angles of 0°, 24°, 30°, and 60°. A study was made of the dependence of switching threshold HCon the angle θ between applied field and wire axis. Good agreement was found with the relationH_{C}(theta)=H_{C}(0)/cos thetafor all helical angles and for θ as high as 85°. It is concluded that with the cylindrical film geometry only the axial component of external field is effective in quasi-static switching over wide ranges of field direction and helical angle.     

Quantitative Imaging of the Magnetic Configuration of Modulated Nanostructures by Electron Holography

By means of off‐axis electron holography the local distribution of the magnetic induction within and around a poly‐crystalline Permalloy (Ni₈₁Fe₁₉) thin film is studied. In addition the stray field above the sample is measured by magnetic force microscopy on a larger area. The film is deposited on a periodically nanostructured (rippled) Si substrate, which was formed by Xe⁺ ion beam erosion. This introduces the periodical ripple shape to the Permalloy film. The created ripple morphology is expected to modify the magnetization distribution within the Permalloy and to induce dipolar stray fields. These stray fields play an important role in spinwave dynamics of periodic nanostructures like magnonic crystals. Micromagnetic simulations estimate those stray fields in the order of only 10 mT. Consequently, their experimental determination at nanometer spatial resolution is highly demanding and requires advanced acquisition and reconstruction techniques such as electron holography. The reconstructed magnetic phase images show the magnetized thin film, in which the magnetization direction follows mainly the given morphology. Furthermore, a closer look to the Permalloy/carbon interface reveals stray fields at the detection limit of the method in the order of 10 mT, which is in qualitative agreement with the micromagnetic simulations.     

The effect of vertical head field component on the switching of thin metallic film

Magnetometric measurements indicate that when the magnetization of cobalt films is reversed by the application of fields at angles greater than grazing incidence, switching occurs when the field component Hsin the film plane is less than the coercivity HcCurves of Hsversus the vertical field component Hyare shown for cobalt tapes and Co-P films. This effect has been shown to influence the writing process and could be used to facilitate the switching of the high coercivity films necessary for high packing density systems.     

A link between the coercivity and microstructure of high moment Fe films and their use in magnetic tunnel junctions

Magnetron sputtered single Fe films have been “softened” magnetically by controlled N-doping during the sputter deposition. This technique allows a reduction in grain size and coercivity of the Fe films, without decreasing the saturation magnetization and without the formation of any crystalline FeN phases. We describe this effect through a modification of the random magnetocrystalline anisotropy model, by taking the film thickness into account. The coercivities calculated in this way are in good agreement with those obtained experimentally.It is demonstrated that N-doping can be applied beneficially to control the switching field of the ‘free’ layer in magnetic trilayer films of the MTJ type. It is thus possible to construct an all Fe-electrode magnetic tunnel junction (MTJ) that displays the tunneling magnetoresistance (TMR) effect by altering the switching field of one Fe layer using N-doping. The ability to control the magnetic softness of high magnetic moment materials is important in regard to their incorporation into TMR devices.     

Reversal modes and reversal times in submicron sized elements for MRAM applications

The switching behavior of submicron sized NiFe nanoelements was calculated using a hybrid finite element/boundary element method. The numerical integration of the Gilbert equation of motion reveals the transient states during magnetization reversal under the influence of a constant applied field. The reversal mode and the reversal time sensitively depend on the size and the shape of the elements. The 200 × 100 × 10 nm³ elements switch well below 1 ns for an applied field of 80 kA/m and a Gilbert damping constant =0.1. The elements reverse by non-uniform rotation. If an external field is applied the magnetization starts to rotate near the ends, followed by the reversal of the center. This process requires only about 0.1 ns. In what follows, the magnetization component parallel to the field direction shows oscillations, which decay within a time of 0.4 ns. The excitation of spin waves is caused by the precession of the magnetization around the local effective field. A rapid decay of the oscillations is obtained in elements with slanted ends, where surface charges cause a transverse demagnetizing field.     

An over‐current protection module for telephone network line cards—an analysis of electro‐thermal properties

In this paper the results of a finite‐element analysis of the electro‐thermal behaviour of an over‐current protection thick‐film hybrid module are presented. The module was designed for protecting the line card of a telephone network against an abnormal surge of current, resulting from accidental shorts between adjacent power feed lines. The switching time of the device is crucial to its effectiveness as a protective element. A transient finite‐element thermal analysis was performed in order to predict the dynamic temperature states at the critical points of the circuit design and to evaluate the influence on the switching characteristics. A comparison between simulated and practical results is given. Copyright © 2002 John Wiley & Sons, Ltd.     

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.