Showing papers on "Single domain published in 1999"

Electric field dependence of piezoelectric properties for rhombohedral 0.955Pb(Zn1/3Nb2/3)O3– 0.045PbTiO3 single crystals

Abstract: The electric field dependence of the piezoelectric properties of rhombohedral 0.955Pb(Zn1/3Nb2/3)O3–0.045PbTiO3 crystals were investigated as a function of orientation with respect to the prototypic (cubic) axes. For 〈111〉 oriented fields, depolarization and subsequent domain reorientation resulted in an apparent maximum in the piezoelectric coefficients occurring at ∼5 kV/cm, followed by nonhysteretic dij saturation, indicating a single domain state under bias. By extrapolation, single domain values for the piezoelectric coefficients d33 and d31 were determined to be 125 and −35 pC/N, respectively. The hydrostatic piezoelectric coefficient dh for single domain crystals was calculated to be ∼55 pC/N, coincident with the experimentally determined values under hydrostatic pressure. For 〈001〉 oriented fields, piezoelectric coefficients d33〈001〉 and d31〈001〉 as high as 2250 and −1000 pC/N were determined, respectively. Although a high value of dh〈001〉 (∼250 pC/N) was expected, the experimentally determined va...

Magnetic properties of submicron co islands and their use as artificial pinning centers

Abstract: We report on the magnetic properties of elongated submicron magnetic islands and their influence on a superconducting film. The magnetic properties were studied by magnetization hysteresis loop measurements and scanning-force microscopy. In the as-grown state, the islands have a magnetic structure consisting of two antiparallel domains. This stable domain configuration has been directly visualized as a $2\ifmmode\times\else\texttimes\fi{}2$-checkerboard pattern by magnetic-force microscopy. In the remanent state, after magnetic saturation along the easy axis, all islands have a single-domain structure with the magnetic moment oriented along the magnetizing field direction. Periodic lattices of these Co islands act as efficient artificial pinning arrays for the flux lines in a superconducting Pb film deposited on top of the Co islands. The influence of the magnetic state of the dots on their pinning efficiency is investigated in these films, before and after the Co dots are magnetized.

Writing and reading of single magnetic domain per bit perpendicular patterned media

Abstract: By fabricating patterned media with a large number of nanoscale single domain magnetic particles embedded in a nonmagnetic substrate, and by writing the magnetization for each of these particles in a desired direction, nonvolatile magnetic storage of information could reach densities much higher than what is currently thought possible for longitudinal continuous media. We have fabricated high aspect ratio perpendicular nickel columnar nanoparticles embedded in a hard Al2O3/GaAs substrate. We show that the magnetization states of the individual magnets can be controlled by demonstrating that prototype patterned "single magnetic domain per bit" data tracks can be written and read back using current magnetic information storage technology.

Resistivity due to Domain Walls in Co Zigzag Wires

Abstract: We report a clear manifestation of the negative magnetoresistance due to the domain wall of Co submicron zigzag wires in which the domain structures are artificially controllable by changing the orientation of the magnetic fields. The resistivity in the remanent state discontinuously drops when the domain configuration switches from the single domain to the multidomain. We attribute this to the domain wall, and deduce the decrease in the resistivity of $\ensuremath{-}1.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\ensuremath{\Omega}\mathrm{cm}$ assuming the wall thickness of 15 nm. The origin of the negative resistive contribution due to the domain wall is also discussed compared with the existing theories.

Engineered domain configuration in rhombohedral PZN-PT single crystals and their ferroelectric related properties

Abstract: The domain configuration of rhombohedral PZN-8%PT single crystals has been observed as a function of electric-field and crystallographic orientation using polarizing microscope. Although a single domain state could be achieved by applying an E-field along the rhombohedral polar direction [111], a multidomain state was observed with the removal of the E-field. This domain instability was associated with large hysteresis of the strain vs. E-field behavior. In contrast, an engineered domain configuration of [001] oriented rhombohedral crystals was found to be stable with no domain motion detectable under DC-bias, resulting in hysteresis minimized strain vs. E-field behavior. The stable engineered domain configuration under bias in multidomain [001] oriented crystals was suggested as an evidence of macroscopic symmetry 4mm out of 3m rhombohedral crystals.

Injection of a magnetic domain wall into a submicron magnetic wire

Abstract: Two types of magnetic wires (150 nm width) with trilayer structure consisting of NiFe (20 nm)/Cu (20 nm)/Co (20 nm) were prepared. One was connected to a square pad (0.5×0.5 μm2) at one end, while the other has a symmetrical shape with two flat ends. Magnetization reversal was detected sensitively by magnetoresistance measurement. Switching field of the Co layer for the wire with a pad was much smaller than that for the wire without a pad. This indicates that a domain wall nucleates initially in the pad and is injected into the wire at the switching field. This model for the magnetization reversal process is supported by the angular dependence of the switching field.

Magnetization process of noninteracting ferromagnetic cobalt nanoparticles in the superparamagnetic regime: Deviation from Langevin law

Abstract: Magnetization measurements were performed and analyzed on two systems of noninteracting superparamagnetic cobalt nanoparticles displaying narrow size distributions. The experiments were carried out above the blocking temperature, i.e., in the superparamagnetic regime. Several deviations from classical Langevin behavior were pointed out, in particular, at high field and near the blocking temperature. These deviations were interpreted in terms of anisotropy effects on the magnetization process and analyzed using theoretical expressions including uniaxial anisotropy energy. The effect of the anisotropy on the theoretical magnetization curves plotted versus applied field divided by the temperature are characterized by: (i) superposition at low fields, (ii) deviations in the approach to saturation area, and (iii) decrease of the magnetization when lowering the temperature. These three characteristics are present in our experimental curves. It allows us to determine the magnetic moment of the particle in the lo...

Negative Domain Wall Resistance in Ferromagnets

Abstract: The electrical resistance of a diffusive ferromagnet with magnetic domain walls is studied theoretically, taking into account the spatial dependence of the magnetization. The semiclassical domain wall resistance is found to be either negative or positive depending on the difference between the spin-dependent scattering lifetimes. The predictions can be tested experimentally by transport studies in doped ferromagnets.

Enhanced coercivity in submicrometer-sized ultrathin epitaxial dots with in-plane magnetization

Abstract: We present measurements, analysis, and modeling of magnetization reversal processes in ultrathin magnetic dots with in-plane magnetization. This study is performed on a model system: Arrays of tens of millions of epitaxial (110) Fe dots. We clearly demonstrate the link existing between the increase of dot thickness and the decrease of reversal field as compared to coherent rotation (CR) predictions. For 1-nm-thick films ${H}_{r}(\ensuremath{\theta})$ is very close to CR law, although magnetic relaxation experiments clearly show that nucleation volumes are by far smaller than an individual dot volume. This apparent discrepancy is discussed. An analytical model is proposed to describe magnetization reversal in this kind of dots.

Enhancement of coercivity in nanometer-size CoPt crystallites

Abstract: In this study, we showed that a magnetic coercivity (Hc) as high as 37 kOe was obtained in a CoPt thin film that contains separated nanometer-size CoPt crystallites. We prepared CoPt thin films with thicknesses of 5 and 175 nm by magnetron sputtering. After annealing in an Ar/H2 atmosphere at temperatures from 650 to 750 °C for 3–12 h, we measured the magnetic properties and found that magnetic Hc relates to annealing temperature, annealing time and film thickness. From atomic force microscopy and magnetic force microscopy studies, the magnetic single domain size of CoPt is in the range of 100–200 nm. The high Hc is likely due to the well-separated nanometer-size crystallites and the well-ordered fct phase of CoPt alloy.

Lattice distortion and uniaxial magnetic anisotropy in single domain epitaxial (110) films of SrRuO3

Abstract: Single domain epitaxial (110) films of SrRuO3 exhibit uniaxial magnetic anisotropy instead of the biaxial anisotropy observed in the bulk material. The magnetic easy axis for the film is along the orthorhombic [010] direction below TC, and it rotates toward the [110] perpendicular direction as temperature decreases. The [100] direction, which is also magnetically “easy” in the bulk, becomes “hard” in the film. X-ray diffraction experiments show that this unique transformation of magnetic anisotropy is related to a distortion from the bulk orthorhombic lattice into a triclinic structure in the epitaxial film, such that the lattice along the [010] direction expands while its [100] counterpart contracts. The distortion appears to arise from rotation and tilt of RuO6 octahedra. The finding indicates that the magnetic anisotropy in epitaxial SrRuO3 films is rooted in the crystalline anisotropy influenced by strong spin–orbit interactions.

Lorentz transmission electron microscope study of ferromagnetic domain walls in SrRuO3: Statics, dynamics, and crystal structure correlation

Abstract: The magnetic microstructure of SrRuO3 thin films is studied using Lorentz transmission electron microscopy (TEM). The zero-field-cooled magnetic stripe structure shows one-to-one correlation with the crystal domain structure and is used to identify the uniaxial magnetic anisotropy of these films, consistent with results on single domain films. The anisotropy is shown to be primarily magnetocrystalline in nature with the b axis as the easy axis. Lorentz TEM also yields quantitative information about the domain structure and domain wall resistivity, and allows for in situ observation of domain wall formation and dynamic response to an applied field.

Analysis of asymmetric giant magnetoimpedance in field-annealed Co-based amorphous ribbon

Abstract: The asymmetric giant magnetoimpedance (GMI) profile has been realized in weak-field-annealed Co-based amorphous ribbon at the annealing temperature of 380 °C in open air. Asymmetric GMI profiles with respect to applied field become profound as the annealing field increases over 500 mOe. The asymmetric GMI profile at the frequencies of ac current over 0.5 MHz is well ascribed for by the rotational transverse magnetization of single domain under a uniaxial anisotropy in amorphous core and a unidirectional anisotropy due to the exchange coupling with the bias field in the crystalline layer, underlying surface oxidation layer developed during the annealing in open air.

Single-domain critical sizes for coercivity and remanence

Abstract: It is usually assumed that magnetic parameters such as coercivity and saturation remanence are single-domain (SD) over the same size range. In reality, there is a different SD size range for each parameter. We define critical sizes LSDcoerc for coercivity and LSDrem for remanence. In general, LSDcoerc ≤ LSDrem. Up to L = LSDrem, the saturation remanent state is single-domain. If a sufficiently large reverse field is applied, a conventional SD state would reverse by uniform rotation. However, the mode of reversal is nonuniform if the grain size is between LSDcoerc and LSDrem, so in this size range the SD state is less stable. To calculate the critical sizes, we use rigorous nucleation theory and obtain analytical expressions. The analytical form allows us to explore the effect of grain shape, stress, crystallographic orientation and titanium content in titanomagnetites. We adapt the theory to cubic anisotropy with K1 < 0, which allows us to apply the expressions to titanomagnetites. We find that the size range for SD coercivity is always small. The size range for SD remanence can vary enormously depending on the anisotropy. If the easy axes are oriented favorably, the SD state can occur in large x = 0.6 titanomagnetite grains. Ensembles of magnetite grains with aspect ratios greater than 5 have SD-like remanence but low coercivity. However, most synthetic magnetite grains are nearly equant, and the predicted size range for SD remanence is small to nonexistent. This, rather than grain interactions, may be the reason they have properties such as saturation remanence that do not agree well with standard SD theory.

Perpendicular quantized magnetic disks with 45 Gbits on a 4×4 cm2 area

Abstract: Quantized magnetic disks consisting of an array of single domain Ni pillars with a density of 18 Gbits/in.2 were fabricated using nanoimprint lithography (NIL) and electroplating. The total disk area, limited by the NIL mold, is 4 cm×4 cm, leading to a total 45 Gbits. Magnetic force microscope (MFM) images show that all pillars 70 nm in diameter and 400 nm in height are single domain. The magnetostatic interaction between adjacent pillars is fairly strong. The pillars have an average switching field of 360 Oe and can be switched by a MFM tip with a large magnetic moment.

Influence of the cubic anisotropy constants on the hysteresis loops of single-domain particles: A Monte Carlo study

Abstract: The influence of the first and second cubic anisotropy constants on the hysteresis loops of noninteracting single-domain magnetic particles is studied by Monte Carlo simulation, which turns out to be a very powerful method for studying simple magnetic models. Both signs in the anisotropy constants are taken into account. Relevant properties such as coercivity and remanence are studied as a function of temperature when the second anisotropy constant is negligible. The influence of the second term of the anisotropy energy is studied in detail for T=0 K. It is concluded that this term has a big influence on the static magnetic behavior when the first anisotropy constant is negative.

An investigation of reversible magnetization in NdFeB

Abstract: Magnetization reversal processes in sintered and melt-quenched NdFeB materials have been investigated by measurements of reversible magnetization. For isotropic and anisotropic sintered NdFeB and anisotropic hot pressed die upset NdFeB (MQIII) reversible magnetization is the result of bowing of pinned domain boundary walls. In contrast in isotropic melt quenched NdFeB (MQI) reversible magnetization appears to be the result of the rotation of domain magnetization vectors and not domain wall bowing. The differences in the reversible magnetization processes of the four materials investigated are determined.

Micromagnetics of mesoscopic epitaxial (110) Fe elements with nanoshaped ends

Abstract: The magnetization reversal and magnetic domain configurations of 0.5-μm-wide epitaxial (110) Fe particles with rectangular and needle-shaped ends and competing magnetic anisotropies have been investigated. Magnetic force microscopy imaging and longitudinal Kerr hysteresis loop measurements in conjunction with micromagnetic simulations have been used to elucidate the basic micromagnetic behavior. End shape is shown to be a determining factor for the nucleation of magnetization reversal and the resulting magnetic domain configurations.

Experimental study of Hopkinson effect in single domain CoFe/sub 2/O/sub 4/ particles

Abstract: Nanosize particles of CoFe/sub 2/O/sub 4/ have been synthesized by the citrate precursor technique. Considerably higher coercive force (1.68 kOe) than that obtained by the conventional technique (1.00 kOe) is associated with the nanostructure of CoFe/sub 2/O/sub 4/. These nanosize ferrimagnetic CoFe/sub 2/O/sub 4/ particles exhibit chainlike clusters indicating strong interparticle interactions and reduced magnetic moment, which is attributed to anisotropy and canted spin structure at the surface of the particle. The magnetization shows a peak just below the Curie temperature T/sub c/ during heating in the presence of a small magnetic field (the Hopkinson effect), On the other hand, the magnetization increases monotonically when the system is cooled from T/sub c/. This peak is associated with the single domain behavior of nanocrystalline CoFe/sub 2/O/sub 4/ particles and explained within the mathematical formalism given by Stoner and Wohlfarth in conjunction with other explanations of Hopkinson effect.

Micromagnetic models of pseudo‐single domain grains of magnetite near the Verwey transition

Abstract: Domain structures of small pseudo-single domain (PSD) magnetite near the Verwey transition (Tυ) at ≈120 K were modeled using an unconstrained three-dimensional micromagnetic algorithm. The single-domain (SD) threshold (d0) for the monoclinic phase below Tυ was calculated to be ≈0.14 μm at 110 K. However, it is postulated that as a result of the very high energy barriers in the monoclinic phase, grains near d0 in size and in vortex states are unlikely to denucleate domain walls to become SD. Low-temperature cycling of saturation isothermal remanence (SIRM), thermoremanence (TRM), and partial TRM (pTRM) through Tυ was simulated. Domain structures were found to align along the monoclinic “easy” magnetocrystalline anisotropy axis, i.e., the c axis, on simulated cooling through Tυ. This process was found to “destroy” SIRM structures giving rise to demagnetization; however, for TRM and pTRM structures only “closure” domains were removed increasing magnetostatic leakage giving rise to a reversible anomaly in rough agreement with experimental studies. SIRM displayed a smaller anomaly at Tυ, in agreement with experimental studies.

Magnetization processes and reorientation transition for small magnetic dots

Abstract: A theory for magnetization processes in interacting arrays of small magnetic structures at finite temperatures is presented. Hysteresis and magnetic ordering of weakly coupled arrays of single-domain ferromagnetic particles are examined. The dots are arranged on a planar lattice and the effects of lattice geometry are examined via long-ranged dipolar coupling between magnetic dots. Small clusters of dots arranged in finite arrays are shown to have complicated hysteresis determined by the shape, size, and orientation of the cluster in externally applied fields. One result is an array induced ``shape'' anisotropy that controls how reversal occurs in the array itself. Finite temperature effects are examined and the strength of the dipolar coupling, though weak, can be significant for closely packed particles at low temperatures. A reorientation transition from in plane to perpendicular is shown to occur as the temperature is increased for dots with perpendicular anisotropy.

Domain structure of magnetic layers deposited on patterned silicon

Abstract: Arrays of Si square dots down to 200 nm in size were patterned on silicon substrates, and thin films of different magnetic materials were sputter deposited on these patterned substrates. The magnetic film covers the top of the dots, the bottom of the grooves and to much less extent the sidewalls of the dots. Single domain magnetic dots were obtained for Co/NiO bilayers and Co/Pt multilayers, without significant direct coupling mediated by the magnetic deposit on the sidewalls of the dots. Our results indicate that, in these arrays, the magnetic pinning forces are stronger than the estimated value of the largest demagnetizing magnetostatic field on each individual dot. As a result, any magnetic configuration could in principle be stored in such arrays. This approach seems therefore very promising for the preparation of magnetic storage media with ultrahigh density.

Magnetic structure and microstructure of die-upset hard magnets RE13.75Fe80.25B6 (RE=Nd, Pr): A possible origin of high coercivity

Abstract: In situ transmission electron microscopy magnetizing experiments combined with Lorentz magnetic microscopy in Fresnel–Foucault modes were used to characterize the magnetic structure of die-upset, high energy-product hard magnets Nd13.75Fe80.25B6 and Pr13.75Fe80.25B6. Experimental observations indicate a well-aligned grain structure and quasiperiodic nonaligned “extended defect” layers transverse to press direction. The local remanence of the “defect” layers is far from saturation when the external field is removed. The layers are enriched with inclusions of approximate composition Nd7Fe3, generally with a polygonal shape, and are associated with the original ribbon interfaces. They may be responsible for a high coercivity mechanism, since the motion of reverse domains can be impeded by these layers, even when they are nucleated. Thus, a delayed nucleation of reversed domains seems to be a limiting factor for magnetization reversal and coercivity force. Both Lorentz magnetic imaging and high-resolution mic...

Magnetization of small arrays of interacting single-domain particles

Abstract: We have prepared tailored small arrays of single-domain Ninanomagnets exhibiting an easy-axis perpendicular to the substrate surface. Using ballistic Hall micromagnetometry we probe the magnetization under different tilt angles. For an array of 13 nanomagnets with 700 nm spacing, we observe distinct plateau values in the hysteresis loop indicating the stepwise switching of these particles. The observed hierarchy of reversal comes close to a model on the basis of the magnetostatic interparticle interaction.

New CoPt/Ag films for high density recording media

Abstract: CoPt/Ag nanocomposite films have been prepared with magnetic properties promising for potential application in high density magnetic recording media. The films were initially deposited by sputtering, in a multilayer form and found to crystallize in the magnetically soft face centered cubic phase. Annealing leads to a microstructure consisting of the magnetically hard face centered tetragonal CoPt nanoparticles embedded in a Ag matrix. Microstructure and coercivity can be tailored by adjusting the annealing conditions and the relative CoPt/Ag composition. Values of coercivity in the range of 1–17 kOe with grain sizes in the range of 7–100 nm, respectively, have been obtained. Henkel plots of optimally annealed samples show only weak interactions of dipolar nature, characteristic of magnetically isolated single domain particles.

Domain imaging and local piezoelectric properties of the (200)-predominant SrBi2Ta2O9 thin film

Abstract: The domain structure of the (200)-predominant SrBi2Ta2O9 (SBT) thin film was detected by an atomic force microscope in the piezoelectric mode It was found that the content of the grains split by single domain walls is less than 5% The types of domain walls formed in individual grains were identified by analyzing the dependence of piezoelectric coefficient (d33) on the alternating current driving electric field Several grains larger than 300 nm were found to be split by non-180° domain walls To study the switching properties, the (200)-predominant SBT thin film was polarized and imaged over a large area Unswitchable grains cannot be observed both in the area polarized using +8 V and in the region polarized using −8 V