Magnetization switching modes in nanopillar spin valve under the external field

The current-induced magnetic switching is studied in Co/Cu/Co nanopillar with an in-plane magnetization traversed under the perpendicular-to-plane external field.Magnetization switching is found to take place when the current density exceeds a threshold.By analyzing precessional trajectories,evolutions of domain walls and magnetization switching times under the perpendicular magnetic field,there are two different magnetization switching modes:nucleation and domain wall motion reversal;uniform magnetization ...     

Anomalous magnetization processes and non-symmetrical domain wall displacements in L10 FePt particulate films

Anomalous magnetization processes and non-symmetrical domain wall displacements in the minor loop of L1₀ FePt particulate films were investigated by magnetization measurements and in situ magnetic force microscopy. Magnetization (M) decreases dramatically on increasing the magnetic field to ∼3 kOe after which M becomes small and constant in the range of 5–20 kOe as observed in the successive measurement of minor loops. The domain wall displacement is non-symmetrical with respect to the field direction. The anomalous magnetization behavior was attributed to the non-symmetrical domain wall displacement and large magnetic field required for domain wall nucleation. Energy calculations from modeling suggest that non-symmetrical domain wall displacement is caused by the existence of metastable domains in which the domain edges are stuck to the particle boundaries.     

Influence of three-dimensional inhomogeneities of the magnetic parameters on the dynamics of vortex-like domain walls

The interaction of a vortex-like domain wall moving in an external magnetic field with a three-dimensional periodic chain of cubic volumes with high values of the saturation magnetization and magnetic anisotropy constant has been investigated theoretically. It has been found that the result of the interaction depends on the initial distance between the wall and the region of inhomogeneity of magnetic parameters at the moment of turning on the external magnetic field. The pinning of domain walls near the regions with high values of the saturation magnetization and magnetic anisotropy constant has been investigated, and the anisotropy of the corresponding depinning fields has been revealed. The method of investigation is the numerical micromagnetic simulation.     

Thermal activation of magnetization in Pr_2Fe_(14)B ribbons

The aftereffect field of thermal activation,which corresponds to the fluctuation field of a domain wall,is investigated via specific measurements of the magnetization behavior in Pr2Fe14B nanocrystalline magnets.The thermal activation is a magnetization reversal arising from thermal fluctuation over an energy barrier to an equilibrate state.According to the magnetic viscosity and the field sweep rate dependence of the coercivity,the calculated values of the fluctuation field are lower than the aftereffect field and in a range between those of domain walls and individual grains.Based on these results,we propose that the magnetization reversal occurs in multiple ways involving grain activation and domain wall activation in thermal activation,and the thermal activation decreases the coercivity by~0.2 kOe in the Pr2Fe14B ribbons.     

Asymmetric magnetization reversal on exchange biased CoO/Co bilayers

We study magnetic hysteresis loops after field cooling of a CoO/Co bilayer by MOKE and polarized neutron reflectivity. The neutron scattering reveals that the first magnetization reversal after field cooling is dominated by domain wall movement, whereas all subsequent reversals proceed essentially by rotation of the magnetization. In addition, off-specular diffuse scattering indicates that the first magnetization reversal induces an irreversible change of the domain state in the antiferromagnet.     

Effect of a planar magnetic field on the domain wall dynamics in a double-layer uniaxial magnetic film

Motion of an isolated domain wall in a double-layer uniaxial magnetic film, where the film layers differ in characteristic length, saturation magnetization and damping parameter, is investigated by solving the Slonczewski equation. A planar magnetic field is applied normal to the domain-wall plane. The dependences of the threshold field and limiting velocity of disruption of the steady-state motion of the domain wall on the planar magnetic field value are obtained. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 60–63, December, 2008.     

Dynamics of solenoidal magnetic structures in soft magnetic thin-film elements

We report on the dynamics of magnetic domain structure conversions exhibited by soft magnetic thin-film elements of elementary geometrical shape (square, disc, triangle) when exposed to a strong external magnetic field. Starting from flux closure vortex patterns, the magnetic structures evolve towards an in-plane saturated state under the influence of an external field. This irreversible and nucleation-free magnetization process occurs on the time scale of picoseconds. The details of this conversion are investigated by means of a time-resolved micromagnetic finite element modeling. We find a sensitive dependence of the temporal evolution of the magnetic structure on the value of the damping parameter in Gilbert's equation of motion. In the case of high damping, domain wall motion dominates the process, while lower damping leads to the formation of a 360° wall which collapses by emitting magnetization waves. It is shown that the mobility of vortices is generally much lower than that of domain walls. The calculations indicate that at a low damping, a magnetic vortex can act almost as a source for concentric waves in ferromagnetic thin-film elements.     

Spatial distribution of magnetic field and persistent currents in Condon domain phase

The distributions of magnetic field above the surface of the sample due to existence of the diamagnetic domain structure are found. It is shown, that the constant magnetic induction splitting inside of a sample is caused by the magnetization current density, localised in the boundaries between adjacent domains, close to the sample surface. The properties of this current are studied. The influence of the domain wall thickness on the spatial distribution of magnetic field and magnetization current density is present. A possibility of detection of the changes in the magnetic field distribution in vacuum, close to the surface of the sample, by means of Hall probes, is discussed. The measurement of the spatial distribution of magnetic field can give lacking information about characteristic sizes of magnetic domain formation at the conditions of the strong de Haas van Alphen effect.     

Explicit solutions to phenomenological models of magnetization reversal of thin ferromagnetic films in the presence of a sawtooth magnetic field

Explicit solutions are derived for several phenomenological models of magnetization reversal in thin ferromagnetic films driven by a sawtooth magnetic field. For a domain wall velocity that is linear in the magnetic field, it is found that the dynamic coercive field follows a square-root power-law in the slope of the magnetic field, shifted by the depinning field. For a more general domain wall velocity different power-law exponents are found, yet the overall form for the scaling of the area of the hysteresis loop remains a power-law shifted by the depinning field. This shifted power-law could be interpreted to be a crossover between adiabatic and dynamic regimes.     

Domain wall dynamics and interlayer interactions in magnetic trilayer systems studied by XMCD-PEEM

We have used time-resolved x-ray magnetic circular dichroism combined with photoemission electron microscopy (XMCD-PEEM) to investigate the layer-resolved microscopic magnetization reversal in FeNi/X/Co (with X=Cu, Al₂O₃) trilayer systems. These measurements were performed in pump-probe mode, synchronizing magnetic pulses with synchrotron x-ray pulses. The good magnetic contrast observed for most samples reveals that in many cases the magnetization reversal is reproducible. We have used the measurements to obtain domain wall propagation speeds as a function of applied magnetic field, and to investigate the influence of domain wall interactions on the magnetic switching.     

Effect of a pulsed magnetic field on the nonlinear dynamics of vortexlike domain walls in magnetic films

The Landau-Lifshitz equation is numerically solved to study the nonlinear dynamic behavior of domain walls with the 2D vortexlike magnetization distribution in magnetically uniaxial films that have in-plane anisotropy and are exposed to a pulsed magnetic field. It is shown that a pulsed magnetic field H _p may induce transitions between various steady wall motions that differ in magnetization distribution. Solitary rectangular pulses, as well as a regular train of rectangular pulses, may be used to control the period of nonlinear dynamic transformations of the wall internal structure and the related period of variation of the wall velocity.     

Effect of a dc magnetic field on the magnetization relaxation of uniaxial single-domain ferromagnetic particles driven by a strong ac magnetic field

The nonlinear ac stationary response of the magnetization of noninteracting uniaxial single-domain ferromagnetic particles acted on by superimposed dc and ac magnetic fields applied along the anisotropy axis is evaluated from the Fokker-Planck equation, expressed as an infinite hierarchy of recurrence equations for Fourier components of the relaxation functions governing longitudinal relaxation of the magnetization. The exact solution of this hierarchy comprises a matrix continued fraction, allowing one to evaluate the ac nonlinear response and reversal time of the magnetization. For weak ac fields, the results agree with perturbation theory. It is shown that the dc bias field changes substantially the magnetization dynamics leading to new nonlinear effects. In particular, it is demonstrated that for a nonzero bias field as the magnitude of the ac field increases the reversal time first increases and having attained its maximum at some critical value of the ac field, decreases exponentially.     

AC-current-induced magnetization switching in amorphous microwires

We studied the influence of AC current flowing through microwires, on magnetization dynamics. We used a previously developed Sixtus-Tonks modified setup to evaluate the domain wall (DW) velocity within the microwire. However, instead of a magnetizing solenoid, we used a current flowing through the microwire. We observed that the AC current flowing through the annealed Co-rich microwire leads to remagnetization by fast domain wall propagation. The estimated DW velocity was approximately 4.5 km/s, which is similar to and even higher than that reported for the magnetic-field-driven domain wall propagation in Fe- and Co-rich microwires. We measured the DW velocity under tensile stress, and found that the DW velocity decreases under applied stress. An observed DW propagation induced by the current flowing through the microwire is explained considering the influence of an Oersted magnetic field on the outer domain shell. This field has a circular easy magnetization direction and magnetostatic interaction between the outer circumferentially magnetized shell and the inner axially magnetized core.     

Magnetization reversal in a preferred oriented (111) L1(0) FePt grown on a soft magnetic metallic glass for tilted magnetic recording

L1(0) FePt is an important material for the fabrication of high density perpendicular recording media, but the ultrahigh coercivity of L1(0) FePt restricts its use. Tilting of the magnetic easy axis and the introduction of a soft magnetic underlayer can solve this problem. However, high temperature processing and the requirement of epitaxial growth conditions for obtaining an L1(0) FePt phase are the main hurdles to be overcome. Here, we introduce a bilayered magnetic structure ((111) L1(0) FePt/glassy Fe(71)Nb(4)Hf(3)Y(2)B(20)/SiO(2)/Si) in which the magnetic easy axis of L1(0) FePt is tilted by ~36° from the film plane and epitaxial growth conditions are not required. The soft magnetic underlayer not only promotes the growth of L1(0) FePt with the preferred orientation but also provides an easy cost-effective micro/nanopatterning of recording bits. A detailed magnetic characterization of the bilayered structure in which the thickness of (111) L1(0) FePt with the soft magnetic Fe(71)Nb(4)Hf(3)Y(2)B(20) glassy underlayer varied from 5 to 60 nm is carried out in an effort to understand the magnetization switching mechanism. The magnetization switching behavior is almost the same for bilayered structures in which FePt layer thickness is >10 nm (greater than the domain wall thickness of FePt). For FePt film ~10 nm thick, magnetization reversal takes place in a very narrow field range. Magnetization reversal first takes place in the soft magnetic underlayer. On further increase in the reverse magnetic field, the domain wall in the soft magnetic layer compresses at the interface of the hard and soft layers. Once the domain wall energy becomes sufficiently large to overcome the nucleation energy of the domain wall in L1(0) FePt, the magnetization of the whole bilayer is reversed. This process takes place quickly because the domain walls in the hard layer do not need to move, and the formation of a narrower domain wall may not be favorable energetically. Our results showed that the present bilayered structure is very promising for the fabrication of tilted bit-patterned magnetic recording media.     

Nonlinear dynamic structure rearrangement of vortexlike domain walls in magnetic films with in-plane anisotropy

The nonlinear dynamic behavior of vortexlike domain walls in magnetic uniaxial films having an in-plane anisotropy was investigated within a rigorous micromagnetic approach in the framework of a two-dimensional magnetization distribution by numerically solving the Landau–Lifshitz equations (with the Gilbert damping parameter) with allowance for all the main interactions, including the dipole–dipole one. The studies were carried out on magnetic soft films with an anisotropy axis lying in their plane in a dc magnetic field parallel to an easy axis and a pulsed magnetic field normal to it. New possibilities for controlling the nonlinear dynamic rearrangement of the internal structure of domain walls and their velocities in fields both above and below the critical field are established. The wall motion in the field above the critical one is nonstationary.     

Dipolar interactions in ferromagnetic thin films

We introduce a discrete model describing the motion of a zigzag domain wall in a disordered ferromagnet with in-plane magnetization, driven by an external magnetic field. The main ingredients are dipolar interactions and anisotropy. We investigate the dynamic hysteresis by analyzing the effects of external field frequency on the coercive field by Monte Carlo simulations. Our results are in good agreement with experiments on Fe/GaAs films reported in literature, and we conclude that dynamic hysteresis in this case can be explained by a single propagating domain wall model without invoking domain nucleation.     

Reversal modes in asymmetric Ni nanowires

We have investigated the evolution of the magnetization reversal mechanism in asymmetric Ni nanowires as a function of their geometry. Circular nanowires are found to reverse their magnetization by the propagation of a vortex domain wall, while in very asymmetric nanowires the reversal is driven by the propagation of a transverse domain wall. The effect of shape asymmetry of the wire on coercivity and remanence is also studied. Angular dependence of the remanence and coercivity is also addressed. Tailoring the magnetization reversal mechanism in asymmetric nanowires can be useful for magnetic logic and race-track memory, both of which are based on the displacement of magnetic domain walls. Finally, an alternative method to detect the presence of magnetic drops is proposed.     

Phase-field simulations of magnetic field-induced strain in Ni2MnGa ferromagnetic shape memory alloy

The magnetization and magnetic field-induced strain behavior of the ferromagnetic shape memory alloy, Ni₂MnGa, under constant compressive stress were studied using the phase-field method. Based on the evolving magnetic domain and martensitic structures, we analyzed the cycling effect, magnetization hysteresis, strain recoveries, and coupling between the domain wall and martensite twin boundaries. We compared the switching behavior of single variant and multivariant martensite structures. We observed three types of magnetic field-induced strain mechanisms, depending on the magnitude of the applied compressive stress. The study revealed that the martensite microstructure of the magnetic shape memory alloy plays an important role in magnetization and strain evolution during loading and unloading of an external magnetic field under different stress conditions. The results are compared with existing experimental observations.     

Magnetostatic energy and stripe domain structure in a ferromagnetic plate of finite width with in-plane anisotropy

The magnetostatic energy and domain structure (DS) in a long ferromagnetic plate of a finite width with in-plane anisotropy are calculated for the case of the domain magnetization vectors lying in the plane of the plate. The situation where the DS period is much shorter than the width but is considerably larger than the thickness of the plate is analyzed in detail. The equilibrium DS period and the width ratio of two adjacent domains are determined as functions of an external magnetic field parallel to the plane of the plate by minimizing the energy. The DS period is found to be proportional to the plate width and the domain wall energy and inversely proportional to the squared saturation magnetization. While the width of the favorable domains (with the magnetization parallel to the field) grows with increasing field, the unfavorable domains, rather than disappearing completely, form relatively narrow transition regions between the favorable domains, i.e., 360° domain walls.     

Dynamics of vortexlike domain walls in triaxial magnetic films with the Goss orientation of the surface

The stationary dynamics of vortexlike domain walls in films with three magnetic axes and Goss orientation of the surface is studied for the first time with a micromagnetic method that exactly takes into account all basic types of interaction (including dipole-dipole interaction). Consideration is carried out using a 2D model of magnetization distribution by numerically solving Landau and Lifshitz’s nonlinear equations with attenuation in the Gilbert form. Dynamic configurations of domain walls are established, and the dependences of the domain wall velocity on an applied magnetic field, damping parameter, and magnetic film thickness are found.