Showing papers on "Single domain published in 2005"

Analytical investigation of spin-transfer dynamics using a perpendicular-to-plane polarizer

Abstract: We studied current-induced magnetic switching and excitations in structures comprising a free layer with in-plane magnetization traversed by a current with perpendicular-to-plane spin polarization. We derived analytical solutions from the Landau–Lifshitz–Gilbert equation including the spin-torque term, and compared them to numerical simulations within the single domain assumption. Taking into account the criterion of thermal stability, the magnetization switching in nanostructures of typical size below 100nm comprising a perpendicular polarizer is found to require larger current density but to be much faster than with a longitudinal polarizer. Furthermore, a steady precession of magnetization can be generated in this geometry; those frequencies can be tuned from about 1 to 20GHz by only changing the current without applying any external field. This opens a promising application as microwave sources.

Single-Domain Wall Propagation and Damping Mechanism during Magnetic Switching of Bistable Amorphous Microwires

Abstract: The mechanism of nucleation and propagation of a single-domain wall is studied as a function of temperature in bistable Fe-based amorphous microwire with a unique simple domain structure An extended nucleation-propagation model is proposed with a negative nucleation field From quantitative analysis of the propagating wall characteristics, a new damping is theoretically introduced as arising from structural relaxation which dominates in the low temperature regime

Nanoscale piezoelectric response across a single antiparallel ferroelectric domain wall

Abstract: Experiments and three-dimensional numerical modeling of nanoscale piezoelectric response across a single domain wall in ferroelectric lithium niobate are presented. Surprising asymmetry in the local electromechanical response across a single antiparallel ferroelectric domain wall is reported. Piezoelectric force microscopy is used to investigate both the in-plane and out-of- plane electromechanical signals around domain walls in congruent and near-stoichiometric lithium niobate. The observed asymmetry is shown to have a strong correlation to crystal stoichiometry, suggesting defect--domain-wall interactions. A defect-dipole model is proposed. The finite-element method is used to simulate the electromechanical processes at the wall and reconstruct the images. For the near-stoichiometric composition, good agreement is found in both form and magnitude. Some discrepancy remains between the experimental and modeling widths of the imaged effects across a wall. This is analyzed from the perspective of possible electrostatic contributions to the imaging process, as well as local changes in the material properties in the vicinity of the wall.

Spin reorientation transition in single-domain (Ga, Mn)As

Abstract: We demonstrate that the interplay of in-plane biaxial and uniaxial anisotropy fields in $(\mathrm{Ga},\mathrm{Mn})\mathrm{As}$ results in a spin reorientation transition and an anisotropic ac susceptibility which is fully consistent with a simple single-domain model. The uniaxial and biaxial anisotropy constants vary, respectively, as the square and fourth power of the spontaneous magnetization across the whole temperature range up to ${T}_{\mathrm{C}}$. The weakening of the anisotropy at the transition may be of technological importance for applications involving thermally assisted magnetization switching.

A high-precision model of first-order reversal curve (FORC) functions for single-domain ferromagnets with uniaxial anisotropy

Abstract: [1] Plots of the first-order reversal curve (FORC) function are used to characterize ferromagnetic particles in rocks. The function is based on classical Preisach theory, which represents magnetic hysteresis by elementary loops with displacement Hu and half width Hc. Using analytical and numerical integration of single-particle magnetization curves, a high-precision FORC function is calculated for a sample with randomly oriented, noninteracting, elongated single-domain (SD) particles. Some properties of the FORC function are independent of the distribution of particle orientations and shapes. There is a negative peak near the Hu axis, and the FORC function is identically zero for Hu > 0. The negative peak, previously attributed to particle interactions, is due to the increasing slope of a reversible magnetization curve near a jump. This peak is seen in experimental FORC functions of SD samples but not of samples with larger particles, probably because of Barkhausen jumps. The second feature is not seen in any experimental FORC function. A spread of the function to Hu > 0 can be caused by particle interactions or nonuniform magnetization.

Phenomenological theory of a single domain wall in uniaxial trigonal ferroelectrics: Lithium niobate and lithium tantalate

Abstract: A phenomenological treatment of domain walls based on the Ginzburg-Landau-Devonshire theory is developed for uniaxial trigonal ferroelectrics, lithium niobate and lithium tantalate. The contributions to the domain-wall energy from polarization and strain as a function of orientation are considered. Analytical expressions are developed that are analyzed numerically to determine the minimum polarization, strain, and energy configurations of domain walls. It is found that hexagonal $y$ walls are preferred over $x$ walls in both materials. This agrees well with experimental observation of domain geometries in stoichiometric composition crystals.

Anisotropic magnetoresistance and magnetic anisotropy in high-quality (Ga,Mn)As films

Abstract: We have performed a systematic investigation of magnetotransport of a series of as-grown and annealed Ga1-xMnxAs samples with 0.011 = 0.02. This becomes the dominant anisotropy at elevated temperatures, and is shown to rotate by 90o on annealing. We find that the in-plane longitudinal resistivity depends not only on the relative angle between magnetization and current direction, but also on the relative angle between magnetization and the main crystalline axes. The latter term becomes much smaller after low temperature annealing. The planar Hall effect is in good agreement with the measured AMR indicating the sample is approximately in a single domain state throughout most of the magnetisation reversal, with a two-step magnetisation jump ascribed to domain wall nucleation and propagation.

Memory in a magnetic nanoparticle system: polydispersity and interaction effects

Abstract: We report here a theory of relaxation of single domain magnetic nanoparticles, appropriate for analyzing measurements of M\"ossbauer spectra, magnetization response, and hysteretic coercivity. Our special focus of attention in the theoretical formulation is the presence of dipolar interaction between the magnetic particles. We discuss in detail the effect of interaction as well as particle size distribution on the measured relaxation spectra and irreversible, nonequilibrium magnetization response in field-cooled and zero-field-cooled situations. Some of the memory effects, similar to those seen in spin glass systems, may be put to important device applications by tuning the interaction and the particle size.

Perpendicular magnetic recording medium with magnetically resetable single domain soft magnetic underlayer

Abstract: A perpendicular magnetic recording disk with a magnetically resetable single domain soft magnetic underlayer. The perpendicular magnetic recording disk may include a hard magnetic pinning layer disposed above a substrate, a spacer layer disposed above the hard magnetic pinning layer, a soft ferromagnetic film disposed above the spacer layer, and a magnetic recording layer disposed above the soft ferromagnetic film.

Thermochemical remanent magnetization in Precambrian rocks: Are we sure the geomagnetic field was weak?

Abstract: [1] Thellier paleointensity determinations from two dikes of the Early Proterozoic (∼2.46 Ga) Matachewan dike swarm (Canada) yield field values of 2.14 ± 0.18 and 9.81 ± 1.65 μT. Corresponding values of virtual axial dipole moment are very low (0.54 ± 0.05 × 1022 and 2.49 ± 0.42 × 1022 Am2, respectively) when compared with the modern field. The characteristic remanent magnetization (ChRM) was isolated over a narrow range of high unblocking temperatures (∼520–580°C). Detailed rock magnetic analyses indicate that the ChRM is carried by nearly stoichiometric pseudo single domain magnetite. Scanning electron microscopy (SEM) reveals that the magnetite is in the form of fine intergrowths with ilmenite, formed by oxyexsolution during cooling. The high-temperature oxidation defined in the SEM images could have continued at temperatures below the Curie point of magnetite. In this case, the ChRM would be a thermochemical remanent magnetization (TCRM) rather than a thermal remanent magnetization (TRM). Estimates of the TCRM/TRM ratio show that the Thellier data could underestimate the true field value by a factor of 4 without violating experimental selection criteria. This uncertainty in TRM fidelity translates into a potential range of field values that spans that defined by the modern field (∼8 × 1022 Am2) and proposed low Precambrian levels (∼2 × 1022 Am2). Therefore understanding further how TCRM is acquired and records the field represents a major challenge if these and many other similar rocks are to be used in Thellier studies aimed at defining the strength of the Precambrian field.

Radial magnetic field reset system for producing single domain soft magnetic underlayer on perpendicular magnetic recording medium

Abstract: A radial magnetic field reset apparatus including a housing having a cavity with a soft magnet rod disposed at its center and a permanent magnet disposed around the cavity The cavity and thickness of the soft magnet rod are sized to permit a cassette of disks to be inserted into the housing with the soft magnet rod disposed within the inner diameter holes of the disks contained in the cassette The soft magnet rod at the center of the housing attracts the magnetic field generated by the permanent magnet, which results in a substantially uniform, radial magnetic field inside the housing

Magnetic domain structure of wires studied by using the magneto-optical indicator film method

Abstract: Experimental studies of the magnetic domain structure of amorphous wires with positive (Fe-rich) and negative (Co-rich) magnetostriction by using the magneto-optical indicator film method have been performed. The following main results have been obtained. (i) Fe-rich wires possess unclosed 180° surface domain structures with magnetization perpendicular to the wire surface, i.e., without closure domains previously assumed for such materials. Such results have been attributed to the large magnetolastic anisotropy induced by the fabrication process. (ii) Domain structures of Co-rich wires consist of rather big (as-compared with Fe-rich wires) circular domains.

From nucleation to coercivity

Abstract: The demagnetization process of an exchange-coupled hard/soft/hard magnetic system and its coercivity mechanism are investigated within a micromagnetic framework. The calculation reveals that the demagnetization is a three-step process: (i) it begins with the nucleation of a transition region, (ii) the subsequent evolution of the domain wall near the interface, and (iii) the irreversible domain-wall motion from the soft phase into hard phase. As the thickness of the soft interlayer decreases, the dominant mechanism of coercivity switches from pinning to nucleation. However, for very thin interlayer, the three-step process occurs at the same applied field.

Pulsed field magnetization of a 36?mm diameter single-domain Sm?Ba?Cu?O bulk superconductor at?30, 35?and 77 K

Abstract: A c-axis oriented single-domain Sm–Ba–Cu–O bulk superconductor 36 mm in diameter was magnetized by the pulsed field magnetization (PFM) method at 30, 35 and 77 K. The trapped field distributions after applying pulsed fields with different amplitudes were measured by scanning a Hall sensor 0.5 mm above the surface of the sample. We also measured the time evolution of magnetic fields during the PFM by using an oscilloscope connected to two Hall sensors mounted on the bulk superconductor surface. Fluxes are found to penetrate into the bulk superconductor and to escape from it by choosing passes through the direction inclined at 45° to growth sector boundaries (GSBs) of the sample. At 35 K, the temperature rise of the sample caused by heat generation due to flux motion becomes more substantial than that at 77 K. Thus, flux jumps occurred through the passes and assisted magnetic fluxes to reach rather easily the centre of the bulk superconductor. As a result, the magnetic field necessary for PFM is lower than that to fully magnetize the sample by means of the static zero-field-cooling magnetization method. The optimized multi-PFM with reducing amplitudes, which was specifically referred to as the IMRA technique, turned out to be very effective in achieving excellent trapped field characteristics by PFM at low temperatures. We could achieve a maximum trapped field of 3.6 T together with a well conical trapped field distribution at 30 K.

Switching superconductivity in superconductor/ferromagnet bilayers by multiple-domain structures

Abstract: We consider the effect of a multiple magnetic domain structure in a superconductor/ferromagnet bilayer, modeled by a ferromagnetic layer with a rotating magnetic moment. The domain walls in this model are of equal size as the domains, and are of N\'eel type. We study the superconducting critical temperature as a function of the rotation wavelength of the magnetic moment. The critical temperature of the bilayer is found to be always enhanced by the domain structure, and exhibits an interesting reentrant behavior. We suggest that this effect can be used for a new device where superconductivity may be controlled by the domain structure of the magnetic layer.

Stable magnetization states under a spin-polarized current and a magnetic field

Abstract: Stable magnetization states of a nanoscale magnet under torque by a spin-polarized current and one under torque by an external magnetic field are studied theoretically. We consider six configurations for the combination of the three directions, that is, the easy axis of the magnet, the spin polarization of the current, and the magnetic field. For each configuration, all the possible stable magnetization states are revealed by constructing a phase diagram in the current-field plane on the basis of the Landau-Lifshitz-Gilbert equation. In addition, analyses on the dynamic behaviors of the magnetization are presented. We find that our system does not exhibit any chaotic behaviors. Nevertheless, the initial condition has a significant effect on the magnetization motion.

Propagation of a magnetic domain wall in magnetic wires with asymmetric notches

Abstract: The propagation of a magnetic domain wall (DW) in a submicron magnetic wire consisting of a magnetic/nonmagnetic/magnetic trilayered structure with asymmetric notches was investigated by utilizing the giant magnetoresistance effect. The propagation direction of a DW was controlled by a pulsed local magnetic field, which nucleates the DW at one of the two ends of the wire. It was found that the depinning field of the DW from the notch depends on the propagation direction of the DW.

Dynamics of magnetization coupled to a thermal bath of elastic modes

Abstract: We study the dynamics of magnetization coupled to a thermal bath of elastic modes using a system plus reservoir approach with realistic magnetoelastic coupling. After integrating out the elastic modes we obtain a self-contained equation for the dynamics of the magnetization. We find explicit expressions for the memory friction kernel and hence, via the fluctuation-dissipation theorem, for the spectral density of the magnetization thermal fluctuations. For magnetic samples in which the single domain approximation is valid, we derive an equation for the dynamics of the uniform mode. Finally we apply this equation to study the dynamics of the uniform magnetization mode in insulating ferromagnetic thin films. As experimental consequences we find that the fluctuation correlation time is of the order of the ratio between the film thickness, h, and the speed of sound in the magnet and that the linewidth of the ferromagnetic resonance peak should scale as B 1 h where B1 is the magnetoelastic coupling constant.

Exchange spring structures and coercivity reduction in FePt∕FeRh bilayers: A comparison of multiscale and micromagnetic calculations

Abstract: Calculations of magnetization reversal mechanism and coercivity reduction in exchange coupled FePt∕FeRh bilayers are presented It is shown by comparison with atomistic model calculations that the use of a standard micromagnetic model leads to an underestimation of the exchange energy at the interface, leading to a reduced coercivity decrease for small interfacial exchange energy constant This is due to the failure of the domain wall (DW) to penetrate the hard FePt phase in the micromagnetic calculations A multiscale model is proposed based an atomic level simulation in the interface region coupled with a micromagnetic approach elsewhere This leads to improved calculations of DW structures at the interface, allowing a detailed study of the magnetization reversal mechanism The new approach predicts a saturation in the coercivity reduction as a function of interface exchange energy at 4% of the bulk value, which is associated with complete continuity of the DW across the interface

Optimization of exchange spring perpendicular recording media

Abstract: A numerical analysis was utilized to optimize the fraction of the hard and soft magnetic material. Also, optimal material parameters are given for single phase media and exchange coupled hard/soft bilayer. Single domain particles are assumed in order to optimize the material parameters such as K/sub 1/ and J/sub s/ of a single phase perpendicular recording media. The finite element micromagnetic calculations (FEM) that take into account the demagnetizing field, the intergrain and intergranular exchange was used to compare this model. It was found that this model is in excellent agreement with FEM. The maximum ratio f which gives the energy barrier divided by the coercive field was found for J/sub s/ = 0.55 T and K/sub 1/ = 2.75 /spl times/ 10/sup 5/ J/m/sup 3/. The FEM was used to optimize the layer thickness of the hard layer and soft layer, K/sub 1/ and J/sub s/ of a magnetic bilayer. If intergranular exchange field of 0.26 T is assumed,the maximum ratio f is obtained for J/sub s/ = 0.7 T, K/sub 1/ in the hard layer of 1.27 /spl times/ 10/sup 6/ J/m/sup 3/ and an equal thickness of the soft layer and hard layer of 7 nm. The energy barrier of 60 k/sub b/T/sub 300/ is obtained for grains with diameter of 6.2 nm. The coercive field is 1.32 T. Compared to the single phase media with the same coercive field, the energy barrier can be increased by more than a factor of two.

Magnetic memory and manufacturing method for the same

Abstract: The present invention to provide a new technique to reduce a variation in switching field of a magnetization free layer in a magnetic memory. The magnetic memory according to the present invention includes a magnetization free layer including a ferromagnetic layer having a shape magnetic anisotropy in a first direction and a magnetic strain constant is positive; and a stress inducing structure configured to apply a tensile stress to said magnetization free layer in a same direction as the first direction.

Hysteresis from antiferromagnet domain-wall processes in exchange-biased systems: Magnetic defects and thermal effects

Abstract: The partial domain-wall theory of exchange bias predicts bias field magnitudes and film thickness dependencies consistent with certain experimental systems. However, the theory does not account for the coercivity enhancement that accompanies the hysteresis loop shift in single domain materials. We show theoretically that the presence of an attractive domain-wall potential in the antiferromagnet, arising from magnetic impurities, for example, can provide an energy barrier for domain-wall processes that control the coercivity. Asymmetric hysteresis loops are observed and modifications to the angular dependence of exchange bias suggest a mechanism of rotational hysteresis in terms of wall pinning. Similar domain-wall processes are also seen at finite temperatures, where an analogous pinning arises from a displacement of the domain wall from the interface due to thermal fluctuations.

Current-driven domain-wall depinning

Abstract: The characteristics of magnetic domain-wall depinning driven by a spin transfer torque are dramatically different compared to those driven by a conventional magnetic field. By using the recently derived formalism of the spin torque, we describe key features in the dynamics of geometrically confined domain walls. We numerically calculated the pinning-depinning phase boundary in the presence of the external field and the current.

Magnetization processes in nanostructured metals and small-angle neutron scattering

Abstract: The magnetization process in nanostructured $(n\text{\ensuremath{-}})$ Fe and Co was investigated via small-angle neutron scattering (SANS). In a zero field, the magnetization exhibits correlations extending over several grains. In intermediate applied magnetic fields around $1\phantom{\rule{0.3em}{0ex}}\mathrm{kOe}$, $n\text{\ensuremath{-}}\mathrm{Fe}$ and $n\text{\ensuremath{-}}\mathrm{Co}$ samples with small grain sizes exhibit an anisotropic scattering profile with an unusual intensity enhancement for scattering vectors parallel to the field direction. Comparing the experimental data with a modeled granular microstructure containing magnetic domains of arbitrary size and orientation, we conclude that magnetic domains extending over several grains are tilted considerably out of the external field direction in intermediate fields. Since the domain size does not change significantly with the magnitude of the external field, we conclude that the magnetization process does not proceed via domain-wall motion. Together with theoretical arguments showing the existence of marginally stable domains within the random-anisotropy model, our SANS data suggests that the magnetization process proceeds by simultaneous reversal of a few adjacent domains, presumably in the form of small avalanches. This resembles the magnetization process predicted for random-field Ising magnets. Our theoretical analysis of SANS data is general and applies to other systems consisting of magnetic nanoclusters embedded in a nonmagnetic matrix.

Electromagneto-optical effects on local areas of a ferrite-garnet film

Abstract: The electromagneto-optical (EMO) effect from separate magnetic domains in the epitaxial films of yttrium-ferrite-garnet is investigated simultaneously with visual control of the film's domain structure. The local EMO effect, both from single domain sites and from the sites with a domain wall is measured. These local effects are different from the EMO from the multidomain area of a film. It was revealed unexpectedly that a local value of the EMO effect for the domain magnetized along the applied magnetic field decreased drastically in the magnetization stage connected with vanishing of the domains with opposite sign of magnetization. In the homogeneously magnetized film, the EMO effect is practically absent. It is concluded that the electric field practically does not modify the film magnetization and the local EMO effect is connected with the influence of the electric field on the magnetic anisotropy parameter of the studied film.

Domain decoration in dipolar coupled ferromagnetic stacks with perpendicular anisotropy

Abstract: The dipolar stray field effects between two nonuniformly magnetized ferromagnetic Co∕Pt stacks with perpendicular anisotropy are investigated by polar magneto-optical Kerr effect microscopy. Decoration of a reversed domain in the hard stack by a domain ring in the soft stack is evidenced and interpreted by magnetostatic calculations. Mirrored 360° domain walls are strongly stabilized by these interactions.