Showing papers on "Single domain published in 2010"

Detection of noninteracting single domain particles using first-order reversal curve diagrams

Abstract: We present a highly sensitive and accurate method for quantitative detection and characterization of noninteracting or weakly interacting uniaxial single domain particles (UNISD) in rocks and sediments. The method is based on high-resolution measurements of first-order reversal curves (FORCs). UNISD particles have a unique FORC signature that can be used to isolate their contribution among other magnetic components. This signature has a narrow ridge along the Hc axis of the FORC diagram, called the central ridge, which is proportional to the switching field distribution of the particles. Therefore, the central ridge is directly comparable with other magnetic measurements, such as remanent magnetization curves, with the advantage of being fully selective to SD particles, rather than other magnetic components. This selectivity is unmatched by other magnetic unmixing methods, and offers useful applications ranging from characterization of SD particles for paleointensity studies to detecting magnetofossils and ultrafine authigenically precipitated minerals in sediments.

Three-dimensional ferroelectric domain visualization by Čerenkov-type second harmonic generation

Abstract: We show that focusing a laser light onto the boundary between antiparallel ferroelectric domains leads to the non-collinear generation of two second harmonic (SH) beams. The beams are emitted in a plane normal to the domain boundaries at the angles that satisfy the Cerenkov-type phase matching condition. Moreover, these beam disappear when the laser light is focused on a homogenous part of a single domain. We utilize this effect for 3-dimensional visualization of fine details of the ferroelectric domain pattern with a submicron accuracy.

Electric field controlled manipulation of the magnetization in Ni/BaTiO3 hybrid structures

Abstract: The manipulation of the ferromagnetic magnetization via electric fields is investigated in Ni/BaTiO3 hybrid structures. The application of an electric field to the ferroelectric BaTiO3 induces elastic strain. In the Ni/BaTiO3 hybrids, this strain is transferred into the ferromagnetic Ni layer, affecting its magnetization due to inverse magnetostriction. Two approaches to electrically alter the Ni magnetization are investigated. One approach exploits the strain-induced change of the nickel magnetic coercivity, and allows for both reversible and irreversible magnetization control. The other is based on irreversible ferroelectric domain effects in BaTiO3, and yields two different electro-remanent magnetization states.

Spin-glass-like freezing and enhanced magnetization in ultra-small CoFe2O4 nanoparticles

Abstract: The magnetic properties of ultra-small (3 nm) CoFe2O4 nanoparticles have been investigated by DC magnetization measurements as a function of temperature and magnetic field. The main features of the magnetic behaviour are blocking of non-interacting particle moments (zero-field-cooled magnetization Tmax≈40 K), a rapid increase of saturation magnetization (up to values higher than for the bulk material) at low T and an increase in anisotropy below 30 K due to the appearance of exchange bias. The low temperature behaviour is determined by a random freezing of surface spins. Localized spin-canting and cation distribution between the two sublattices of the spinel structure account quantitatively for the observed increase in saturation magnetization.

Bennett clocking of nanomagnetic logic using multiferroic single-domain nanomagnets

Abstract: The authors show that it is possible to rotate the magnetization of a multiferroic (strain-coupled two-layer magnetostrictive-piezoelectric) nanomagnet by a large angle with a small electrostatic potential. This can implement Bennett clocking [Int. J. Theor. Phys. 21, 905 (1982)] in nanomagnetic logic arrays resulting in unidirectional propagation of logic bits from one stage to another. This method is potentially more energy efficient than using spin-transfer torque for magnetization rotation. For realistic parameters, it is shown that a potential of ∼0.2 V applied to a multiferroic nanomagnet can rotate magnetization by nearly 90° to implement Bennett clocking.

Synthesis and characterization of Fe3O4 magnetic nanofluid

Abstract: Ferrofluids are colloidal systems composed of single domain of magnetic nanoparticles dispersed in a liquid carrier. In the present work, Fe 3O4 magnetic ferrite nanoparticles were synthesized by chemical coprecipitation method, and were coated with oleic acid as surfactant agent. Magnetic properties of nanoparticles in ferrofluids were investigated with the aid of a vibrating sample magnetometer (VSM) at room temperature. Superparamagnetic behavior, characteristic of magnetic nanoparticles, was determined from the hysteresis loop of M vs. H measurements. The sample as powder was characterized by means of X-ray diffraction. XRD pattern result shows the presence of the most intense peak corresponds to the (311) crystallographic orientation of the spinel phase of Fe 3O4 magnetic nanoparticles. The mean size of the nanoparticles was determined from the X-ray diffraction pattern by using the Scherrer approximation. The particle size was calculated to be 9.64 nm. Atomic Force Microscopy was used to visualize the morphology of nanoparticles and to measure their diameter. The AFM method showed an average nanoparticles diameter of D N = 15.3 nm. FTIR absorption spectroscopy was used to confirm the formation of Fe–O bonds, allowing to identify the inverse ferrites spinel structure, as well as, the presence of other chemical substances adsorbed on the surface of particles.

Low frequency hysteresis loops of superparamagnetic nanoparticles with uniaxial anisotropy

Abstract: The low frequency hysteresis loops of superparamagnetic nanoparticles with uniaxial anisotropy are calculated as a function of the particle diameter, alternating magnetic field amplitude H0, frequency, and particle magnetic parameters both for oriented and nonoriented assemblies. The magnetic field frequency is assumed so high, f>50–100 kHz, that the mechanical rotation of a particle in surrounding medium is restricted. Thus, only the Neel–Brown magnetization relaxation process is taken into account. Pronounced dependence of the specific loss power on the particle diameter is found in the linear regime pertaining to small magnetic field amplitudes. For an oriented assembly of Co nanoparticles with optimal diameter D≈6 nm the specific power loss can be as high as 1600 W/g for typical values H0=200 Oe and f=500 kHz. It is three times less for the corresponding nonoriented assembly. Nevertheless, using of particles with low anisotropy field, i.e., particles of soft magnetic type, seems to be preferable for t...

Bennett clocking of nanomagnetic logic using electrically induced rotation of magnetization in multiferroic single-domain nanomagnets

Abstract: The authors show that it is possible to rotate the magnetization of a multiferroic (strain-coupled two-layer magnetostrictive-piezoelectric) nanomagnet by a large angle with a small electrostatic potential. This can implement Bennett clocking in nanomagnetic logic arrays resulting in unidirectional propagation of logic bits from one stage to another. This method of Bennett clocking is superior to using spin-transfer torque or local magnetic fields for magnetization rotation. For realistic parameters, it is shown that a potential of ~ 0.2 V applied to a multiferroic nanomagnet can rotate its magnetization by nearly 900 to implement Bennett clocking.

Domain wall motion on magnetic nanotubes

Abstract: In this paper the dynamical regimes of the motion of domain walls in magnetic nanotubes are studied theoretically. We compare results obtained with a simplified model of the magnetic energy with a detailed one that includes an exact treatment of the dipolar field. We demonstrate that the proper inclusion of dipolar effects changes qualitatively the mobility of a vortex domain wall driven by an applied magnetic field. We report that magnetic nanotubes display the characteristic phenomenology of domain wall motion: at low fields we find a steady motion with almost constant mobility (velocity/field) up to a critical field, where steady motion breaks out and a precessional motion appears. It is also found that the initial chirality of a vortex domain wall determines the dynamic regime of the motion near the Walker critical field.

Dynamics of magnetic charges in artificial spin ice.

Abstract: Artificial spin ice has been recently implemented in two-dimensional arrays of mesoscopic magnetic wires. We propose a theoretical model of magnetization dynamics in artificial spin ice under the action of an applied magnetic field. Magnetization reversal is mediated by domain walls carrying two units of magnetic charge. They are emitted by lattice junctions when the local field exceeds a critical value ${H}_{c}$ required to pull apart magnetic charges of opposite sign. Positive feedback from Coulomb interactions between magnetic charges induces avalanches in magnetization reversal.

Synthesis of nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite and study of its magnetic behavior at different temperatures

Abstract: Ni0.5Zn0.5Fe2O4 ferrite nanocrystals with average diameter in the range of 1–2 nm have been synthesized by reverse microemulsion. X-ray diffraction (XRD), transmission electron microscopy (TEM) and vibrating sample magnetometer (VSM) are used to characterize the structural, morphological and magnetic properties. X-ray analysis showed that the nanocrystals possess cubic spinel structure. The absence of hysteresis, negligible remanence and coercivity at 300 K indicate the superparamagnetic character and single domain in the nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite materials. The nanocrystalline Ni0.5Zn0.5Fe2O4 ferrite were annealed at 600 °C. As a result of heat treatment the average particle size increases from 2 nm to 5 nm and the corresponding magnetization values have increased to 21.69 emu/g at 300 K. However, at low temperature of 100 K, the annealed samples show hysteresis loop which is the characteristic of a superparamagnetic to ferromagnetic transition. In addition, a comparative study of the magnetic properties of Ni0.5Zn0.5Fe2O4 ferrite nanocrystals obtained from reverse microemulsion has been carried out with those obtained from the general chemical co-precipitation route.

Efficient magnetic hysteresis model for field and stress application in magnetostrictive Galfenol

Abstract: We present a discrete energy-averaged model for the nonlinear and hysteretic relation of magnetization and strain to magnetic field and stress. Analytic expressions from energy minimization describe three-dimensional rotations of domains about easy crystal directions in regions where domain rotation is the dominant process and provide a means for direct calculation of magnetic anisotropy constants. The anhysteretic material behavior due to the combined effect of domain rotation and domain wall motion is described with an energy weighted average while the hysteretic material behavior is described with an evolution equation for the domain volume fractions. As a result of using a finite set of locally defined energy expressions rather than a single globally defined expression, the model is 100 times faster than previous energy weighting models and is accurate for materials with any magnetocrystalline anisotropy. The model is used to interpret magnetization and strain measurements of ⟨100⟩ oriented Fe79.1Ga20...

Dynamic magnetic hysteresis in single-domain particles with uniaxial anisotropy

Abstract: On the basis of Brown's kinetic equation a consistent study of the regimes of cyclic magnetization reversal of single-domain particles with a uniaxial anisotropy is performed. The applied field is harmonic and linearly polarized, its amplitude equals the maximal coercive force of a Stoner-Wohlfarth particle. The dynamic magnetic hysteresis loops are obtained for a particle (oriented particle ensemble), whose easy-magnetization axis is tilted to the field direction under an arbitrary angle. It is shown that the Stoner-Wohlfarth regime (often termed as quasistatic) is able to describe the behavior of a nanoparticle only in a quite limited material and external parameter range. The developed approach has at least two major merits: it enables one to consider dynamic magnetic hysteresis in the temperature-frequency domains inaccessible with the aid of approximate methods, and provides a tool to test the accuracy of the latter.

High-Field Study of Strong Magnetoelectric Coupling in Single-Domain Crystals of BiFeO3

Abstract: Magnetic and dielectric properties of single-domain crystals of BiFeO 3 were studied in pulsed magnetic fields up to 55 T. At low temperatures, metamagnetic transitions accompanied with sharp changes in electric polarization ( P ) were observed at around 18 T. The angular dependence of the transition field coincides with that of the transition from the cycloidal state to the canted antiferromagnetic spin state studied in the framework of the Landau–Ginzburg theory incorporated with the Lifshitz-like invariant. The parasitic P component caused by the cycloidal spin structure amounts to 210±30 µC/m 2 in terms of the projected component on the pseudocubic principal axis, which can be controlled by applying magnetic fields at least up to 500 K. This result indicates that the microscopic magnetoelectric coupling in BiFeO 3 is not weak: In fact, it is rather strong as compared to that in the canonical multiferroic material TbMnO 3 .

A complete set of material properties of single domain 0.26Pb(In1/2Nb1/2)O3–0.46Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 single crystals

Abstract: Pb(In(12)Nb(12))O(3)-Pb(Mg(13)Nb(23))O(3)-PbTiO(3) (PIN-PMN-PT) single crystals have been developed recently, which can increase the operating temperature by at least 20 degrees C compared to PMN-PT crystals. We have measured a complete set of material properties of single domain PIN-PMN-PT crystal, which is urgently needed in theoretical studies and electromechanical device designs using this crystal. Because the rotated values of d33*=1122 pCN and k33*=89% along [001](c) calculated using the single domain data obtained here are in good agreement with the [001](c) poled multidomain PIN-PMN-PT crystals, one may conclude that the physical origin of the ultrahigh piezoelectric properties mainly come from orientation effect.

Effect of Crystallographic Texture on Magnetic Characteristics of Cobalt Nanowires

Abstract: Cobalt nanowires with controlled diameters have been synthesized using electrochemical deposition in etched ion-track polycarbonate membranes. Structural characterization of these nanowires with diameter 70, 90, 120 nm and length 30 μm was performed by scanning electron microscopy, high-resolution transmission electron microscopy, and X-ray diffraction techniques. The as-prepared wires show uniform diameter along the whole length and X-ray diffraction analysis reveals that [002] texture of these wires become more pronounced as diameter is reduced. Magnetic characterization of the nanowires shows a clear difference of squareness and coercivity between parallel and perpendicular orientations of the wires with respect to the applied field direction. In case of parallel applied field, the coercivity has been found to be decreasing with increasing diameter of the wires while in perpendicular case; the coercivity observes lower values for larger diameter. The results are explained by taking into account the magnetocrystalline and shape anisotropies with respect to the applied field and domain transformation mechanism when single domain limit is surpassed.

Exchange coupling with the multiferroic compound BiFeO 3 in antiferromagnetic multidomain films and single-domain crystals

Abstract: The exchange coupling observed between a soft ferromagnetic layer and the antiferromagnetic multiferroic compound ${\text{BiFeO}}_{3}$ (BFO) is investigated. Results obtained on BFO ferroelectric and antiferromagnetic multidomain films and monodomain single crystals are compared. A significant interface coupling occurs in the two systems whose anisotropy however differs significantly. In thin film based heterostructures, the measured twofold anisotropy of the FM layer imposed by the magnetic field during deposition is well accounted for using a double macrospin model describing the role of uncompensated spins, pinned or reversible, in the vicinity of the interface. In contrast, no macroscopic bias is observed in thin films deposited on BFO single crystals where the anisotropy direction is imposed by the underlying antiferromagnetic structure. This highlights the fundamental difference between exchange coupling with a single domain antiferromagnet and with a much more magnetically disordered multidomain state.

Magnetization Process in Dilute Magnetic Oxides

Abstract: Thin films of dilute magnetic oxides often exhibit high-temperature ferromagnetism with magnetization curves of the form M ? Ms tanh (H/H0); the curves are practically anhysteretic and temperature-independent below room temperature. The absence of temperature-dependent coercivity indicates that the magnetization process is dominated by magnetic dipole interactions, not by magnetocrystalline anisotropy. In a model of ferromagnetic grain boundaries, H0 ? 0.14M0, where M0 is the magnetization of the ferromagnetic regions. Quantitative analysis of more than 200 films of different oxides reveals that only a few percent of the volume of these films is magnetically ordered.

Simulation of magnetization switching by electric-field manipulation of magnetic anisotropy

Abstract: Electrical anisotropy modulation was recently observed in ferromagnetic semiconductors and metals. The authors have investigated magnetization switching through magnetic anisotropy modulation induced by external electric field by means of simulation. Macrospin simulation using Landau–Lifshitz–Gilbert equation shows that switching is possible by controlling magnetic anisotropy for appropriate sets of parameters. The condition for quasistatic magnetization switching is also presented, in which magnetization direction is determined to minimize the magnetic free energy.

Spin-valve planar Hall sensor for single bead detection

Abstract: The planar Hall effect (PHE) sensor with a junction size of 3 μm × 3 μm for a single micro-bead detection has been fabricated successfully using a typical spin-valve thin film Ta(5)/NiFe(16)/Cu(1.2)/NiFe(2)/IrMn(15)/Ta(5) nm. The PHE sensor exhibits a sensitivity of about 7.2 μV Oe−1 in the magnetic field range of ±7 Oe approximately. We have performed an experiment to illustrated the possibility of single micro-bead detection by using a PHE sensor. A single micro-bead of 2.8 μm diameter size is secluded from 0.1% dilute solution of the Dynabeads® M-280 dropped on the sensor surface and is located on the sensor junction by using a micro magnetic needle. The comparison of the PHE voltage profiles in the field range from 0 to 20 Oe in the absence and presence of a single micro-bead identifies a single Dynabeads® M-280, the maximal signal change as large as ΔV ∼ 1.1 μV can be obtained at the field ∼6.6 Oe. The results are well described in terms of the reversal of a basic single domain structure.

Probing one antiferromagnetic antiphase boundary and single magnetite domain using nanogap contacts.

Abstract: We have probed one antiferromagnetic (AF) antiphase boundary (APB) and a single Fe3O4 domain using nanogap contacts. Our experiments directly demonstrate that, in the case of probing one AF-APB, a large magnetoresistance (MR), high resistivity, and a high saturation field are observed as compared with the case of probing a single Fe3O4 domain. The shape of the temperature-dependent MR curves is also found to differ between the single domain and one of the AF-APB measurements, with a characteristic strong temperature dependence for the single domain and temperature independence for the one AF-APB case. We argue that these observations are indicative of profound changes in the electronic transport across APBs. The investigated APB defects increase the activation energy and disturb the long-range charge ordering of monodomain Fe3O4.

Oxygen-vacancy-induced memory effect and large recoverable strain in a barium titanate single crystal

Abstract: A phase field model is developed in this paper to study the influences of oxygen vacancies on domain evolutions under electric field in ${\text{BaTiO}}_{3}$ single crystal. The oxygen vacancies are modeled as defect dipoles, enabling the capture of domain memory effect. Three different domain structures are studied, and the distributions of defect density and the electromechanical responses under electric loading are simulated. The simulations reproduce well the reversible domain switching and large recoverable electric-field-induced strain observed in experiments on both engineered domain structure and single domain structure. It is also discovered that double hysteresis loop, stepwise hysteresis loop and shifted hysteresis loop can emerge due to vacancy induced depolarization field, which explains well the previous experimental observations. The results also indicate that domain memory effect and non-$180\ifmmode^\circ\else\textdegree\fi{}$ domain switching are the mechanisms for large recoverable electric-field-induced strain in ferroelectrics with oxygen vacancies.

Charge ordering of magnetic dipoles in artificial honeycomb patterns

Abstract: Artificial spin ice offers the possibility to investigate a variety of dipolar orderings, spin frustrations and ground states. We have investigated magnetic dipoles arranged on a honeycomb lattice as a function of applied field, using magnetic force microscopy. The patterns were prepared by electron beam lithography where the basic units are polycrystalline Fe islands with dimensions length, width, and thickness of 3 μm, 0.3 μm, and 20 nm, respectively. These islands are in a single domain dipolar state at remanence. We have measured the magnetization reversal of the honeycomb patterns with different field directions. For the easy direction with the field parallel to one of the three dipole sublattices we observe at coercivity a maximum of magnetic charge order of alternating charges ±3, where the magnetic charge refers to the number and sign of magnetic poles pointing into any of the vertices.

Atomistic spin model simulation of magnetic reversal modes near the Curie point

Abstract: In order for the current increase in magnetic storage density to continue, one must overcome the so-called magnetic recording trilemma; namely, that smaller grains are required for higher data densities and to ensure their thermal stability, materials with a high anisotropy are required. The higher coercive field that this produces also becomes a limiting factor as the maximum field produced by the recording head is constrained by the saturation magnetization of the pole. One proposed solution to the trilemma is the use of heat assisted magnetic recording HAMR, which utilizes the temperature dependence of the anisotropy to enable writing of materials with a high coercivity. For the highest anisotropy media, this will require heating to the Curie temperature TC of the material. Close to TC, longitudinal fluctuations in the magnetization can have a significant impact on the expected energy barriers and therefore the relaxation time of the magnetization. These effects become especially important when attempting to minimize the time to reverse the magnetization state of the media that will be important at higher storage densities.

Effect of a dc bias field on the dynamic hysteresis of single-domain ferromagnetic particles

Abstract: Dynamic magnetic hysteresis in uniaxial superparamagnetic nanoparticles in superimposed ac and dc magnetic fields of arbitrary amplitude is considered using Brown’s model of coherent rotation of the magnetization. The dependence of the area of the dynamic hysteresis loop on the temperature, frequency, and ac and dc bias fields is analyzed. In particular, the dynamic hysteresis loop of a single-domain ferromagnetic particle is substantially altered by applying a relatively weak dc field. Furthermore, it is shown that at intermediate to low ac field amplitudes, the dc bias field permits tuning of the magnetic power absorption of the particles, while for strong ac field amplitudes the effect becomes entirely analogous to that produced by the exchange biased anisotropy. Simple analytical formulas are provided in the linear response regime for the steady-state magnetization and loop area, exhibiting perfect agreement with the numerical solution of Brown’s Fokker–Planck equation. Comparison with previous result...

Magnetic films on nanoperforated templates: a route towards percolated perpendicular media.

Abstract: We present a study on the magnetization reversal in Co/Pt multilayer films with an out-of-plane easy axis of magnetization deposited onto substrates with densely distributed perforations with an average period as small as 34 nm. Deposition of magnetic Co/Pt multilayers onto the nanoperforated surface results in an array of magnetic nanodots surrounded by a continuous magnetic film. Following the evolution of the magnetic domain pattern in the system, we suggest that domain walls are pinned on structural inhomogeneities given by the underlying nanoperforated template. Furthermore, a series of micromagnetic simulations was performed in order to understand the modification of the pinning strength of domain walls due to the magnetic interaction between nanodots and the surrounding film. The results of the simulations show that magnetic exchange coupling between the nanodots and the surrounding film strongly influences the pinning behavior of the magnetic domain walls which can be optimized to provide maximal pinning.