Showing papers on "Single domain published in 2012"

Current-induced torques in magnetic materials

Abstract: The magnetization of a magnetic material can be reversed by using electric currents that transport spin angular momentum. In the reciprocal process a changing magnetization orientation produces currents that transport spin angular momentum. Understanding how these processes occur reveals the intricate connection between magnetization and spin transport, and can transform technologies that generate, store or process information via the magnetization direction. Here we explain how currents can generate torques that affect the magnetic orientation and the reciprocal effect in a wide variety of magnetic materials and structures. We also discuss recent state-of-the-art demonstrations of current-induced torque devices that show great promise for enhancing the functionality of semiconductor devices.

Electric field-induced magnetization reversal in a perpendicular-anisotropy CoFeB-MgO magnetic tunnel junction

Abstract: The electric field-induced ∼180° magnetization reversal is realized for a sputtered CoFeB/MgO-based magnetic tunnel junction with perpendicular magnetic easy axis in a static external magnetic field. Application of bias voltage with nanoseconds duration results in a temporal change of magnetic easy axis in the free layer CoFeB to in-plane, which induces precessional motion of magnetization in the free layer. The magnetization reversal takes place when the bias voltage pulse duration is adjusted to a half period of the precession. We show that the back and forth magnetization reversal can be observed by using successive application of half-period voltage pulses.

Electric-field control of magnetic domain wall motion and local magnetization reversal

Abstract: Spintronic devices currently rely on magnetic switching or controlled motion of domain walls by an external magnetic field or spin-polarized current. Achieving the same degree of magnetic controllability using an electric field has potential advantages including enhanced functionality and low power consumption. Here we report on an approach to electrically control local magnetic properties, including the writing and erasure of regular ferromagnetic domain patterns and the motion of magnetic domain walls, in CoFe-BaTiO3 heterostructures. Our method is based on recurrent strain transfer from ferroelastic domains in ferroelectric media to continuous magnetostrictive films with negligible magnetocrystalline anisotropy. Optical polarization microscopy of both ferromagnetic and ferroelectric domain structures reveals that domain correlations and strong inter-ferroic domain wall pinning persist in an applied electric field. This leads to an unprecedented electric controllability over the ferromagnetic microstructure, an accomplishment that produces giant magnetoelectric coupling effects and opens the way to electric-field driven spintronics.

Magnetic and electrical properties of In doped cobalt ferrite nanoparticles

Abstract: Nanoparticles of CoFe2O4 and CoIn0.15Fe1.85O4 ferrites were prepared by citrate gel route and characterized to understand their structural, electrical, and magnetic properties. X-ray diffraction and Raman spectroscopy were used to confirm the formation of single phase cubic spinel structure. The average grain sizes from the Scherrer formula were below 50 nm. Microstructural features were obtained by scanning electron microscope and compositional analysis by energy dispersive spectroscopy. The hysteresis curve shows enhancement in coercivity while reduction in saturation magnetization with the substitution of In3+ ions. Enhancement of coercivity is attributed to the transition from multidomain to single domain nature. Electrical properties, such as dc resistivity as a function of temperature and ac conductivity as a function of frequency and temperature were studied for both the samples. The activation energy derived from the Arrhenius equation was found to increase in the doped sample. The dielectric cons...

Thermal fluctuations of magnetic nanoparticles: Fifty years after Brown

Abstract: The reversal time, superparamagnetic relaxation time, of the magnetization of fine single domain ferromagnetic nanoparticles owing to thermal fluctuations plays a fundamental role in information storage, paleomagnetism, biotechnology, etc. Here a comprehensive tutorial-style review of the achievements of fifty years of development and generalizations of the seminal work of Brown [Phys. Rev. 130, 1677 (1963)] on thermal fluctuations of magnetic nanoparticles is presented. Analytical as well as numerical approaches to the estimation of the damping and temperature dependence of the reversal time based on Brown's Fokker-Planck equation for the evolution of the magnetic moment orientations on the surface of the unit sphere are critically discussed while the most promising directions for future research are emphasized.

Thermal fluctuations of magnetic nanoparticles

Abstract: The reversal time (superparamagnetic relaxation time) of the magnetization of fine single domain ferromagnetic nanoparticles owing to thermal fluctuations plays a fundamental role in information storage, paleomagnetism, biotechnology, etc. Here a comprehensive tutorial-style review of the achievements of fifty years of development and generalizations of the seminal work of Brown [W.F. Brown, Jr., Phys. Rev., 130, 1677 (1963)] on thermal fluctuations of magnetic nanoparticles is presented. Analytical as well as numerical approaches to the estimation of the damping and temperature dependence of the reversal time based on Brown's Fokker-Planck equation for the evolution of the magnetic moment orientations on the surface of the unit sphere are critically discussed while the most promising directions for future research are emphasized.

Low Energy Magnetic Domain Wall Logic in Short, Narrow, Ferromagnetic Wires

Abstract: We present circuit simulation results of an implementation of universal logic that operates at low switching energy. Information is stored in the position of a single domain wall in a thin, short ferromagnetic wire. The gate is switched by current-driven domain wall motion, and information is read out using a magnetic tunnel junction. The inputs and outputs of the device are currents controlled by voltage clocks, making it compatible with CMOS. Using devices that operate at 100-1 mV, we simulate a shift register circuit and a full-adder circuit. The simulations show that the magnetic logic gates can operate at lower switching energy than CMOS electronics.

Domain Wall Geometry Controls Conduction in Ferroelectrics

Abstract: A new paradigm of domain wall nanoelectronics has emerged recently, in which the domain wall in a ferroic is itself an active device element. The ability to spatially modulate the ferroic order parameter within a single domain wall allows the physical properties to be tailored at will and hence opens vastly unexplored device possibilities. Here, we demonstrate via ambient and ultrahigh-vacuum (UHV) scanning probe microscopy (SPM) measurements in bismuth ferrite that the conductivity of the domain walls can be modulated by up to 500% in the spatial dimension as a function of domain wall curvature. Landau–Ginzburg–Devonshire calculations reveal the conduction is a result of carriers or vacancies migrating to neutralize the charge at the formed interface. Phase-field modeling indicates that anisotropic potential distributions can occur even for initially uncharged walls, from polarization dynamics mediated by elastic effects. These results are the first proof of concept for modulation of charge as a function...

Domain structure in CoFeB thin films with perpendicular magnetic anisotropy

Abstract: Domain structures in CoFeB-MgO thin films with a perpendicular easy magnetization axis were observed by magneto-optic Kerr-effect microscopy at various temperatures. The domain wall surface energy was obtained by analyzing the spatial period of the stripe domains and fitting established domain models to the period. In combination with SQUID measurements of magnetization and anisotropy energy, this leads to an estimate of the exchange stiffness and domain wall width in these films. These parameters are essential for determining whether domain walls will form in patterned structures and devices made of such materials.

Temperature-driven nucleation of ferromagnetic domains in FeRh thin films

Abstract: The evolution of ferromagnetic (FM) domains across the temperature-driven antiferromagnetic (AF) to FM phase transition in uncapped and capped epitaxial FeRh thin films was studied by x-ray magnetic circular dichroism and photoemission electron microscopy. The coexistence of the AF and FM phases was evidenced across the broad transition and the different stages of nucleation, growth, and coalescence were directly imaged. The FM phase nucleates into single domain islands and the width of the transition of an individual nucleus is sharper than that of the transition in a macroscopic average. V C 2012 American Institute of Physics .[ http://dx.doi.org/10.1063/1.4730957]

Electric field induced magnetization rotation in patterned Ni ring/Pb(Mg1/3Nb2/3)O3](1−0.32)-[PbTiO3]0.32 heterostructures

Abstract: Electric field induced magnetoelastic anisotropy is shown to rotate the magnetization of a ring-shaped magnet by 90° in a Ni ring/(011) Pb(Mg1/3Nb2/3)O3](1−0.32)-[PbTiO3]0.32 heterostructure. The 2000 nm diameter ring is initially field annealed forming the “onion” magnetization state. A 0.8 MV/m electric field is applied to the substrate creating anisotropic piezostrain and a perpendicular in-plane easy axis. Magnetic force microscopy confirms the 90° rotation of the vortex-type domain walls from the field annealing direction. Rotations are stable without electric field due to remnant strains induced during the poling process, supporting the viability of strain-based magnetic recording methods.

Thermalized ground state of artificial kagome spin ice building blocks

Abstract: We present a direct magnetic imaging study on the thermal macrospin ordering of artificial kagome spin ice building blocks. Using photoemission electron microscopy, employing x-ray magnetic circular dichroism, we are able to resolve the single domain magnetic nature of the macrospins and determine the states of the combined building block structures. The nano-patterning and material selection allows thermally activated magnetization reversal for the macrospins to occur. The ordering of the macrospins is dominated by the ground state, consistent with a thermal ground state ordering. This work paves the way for the realization of extended artificial spin ice structures exhibiting experimentally accessible thermal behavior.

Trapping and injecting single domain walls in magnetic wire by local fields.

Abstract: A single domain wall (DW) moves at linearly increasing velocity under an increasing homogeneous drive magnetic field. Present experiments show that the DW is braked and finally trapped at a given position when an additional antiparallel local magnetic field is applied. That position and its velocity are further controlled by suitable tuning of the local field. In turn, the parallel local field of small amplitude does not significantly affect the effective wall speed at long distance, although it generates tail-to-tail and head-to-head pairs of walls moving along opposite directions when that field is strong enough.

Non-volatile voltage control of magnetization and magnetic domain walls in magnetostrictive epitaxial thin films

Abstract: We demonstrate reproducible voltage induced non-volatile switching of the magnetization in an epitaxial thin Fe81Ga19 film. Switching is induced at room temperature and without the aid of an external magnetic field. This is achieved by the modification of the magnetic anisotropy by mechanical strain induced by a piezoelectric transducer attached to the layer. Epitaxial Fe81Ga19 is shown to possess the favourable combination of cubic magnetic anisotropy and large magnetostriction necessary to achieve this functionality with experimentally accessible levels of strain. The switching of the magnetization proceeds by the motion of magnetic domain walls, also controlled by the voltage induced strain.

Domain wall propagation in micrometric wires: Limits of single domain wall regime

Abstract: We measured magnetic domain propagation and local domain wall(DW) nucleation in Fe-Co-rich amorphous microwires with metallic nucleus diameters from 2.8 to 18 μm. We found that manipulation of magnetoelastic energy through application of applied stresses, changing of magnetostriction constant, and variation of internal stresses through changing the microwires geometry affects DW velocity. We observed uniform or uniformly accelerated DW propagation along the microwire. The abrupt increasing of DW velocity on v(H) dependencies correlates with the location of the nucleation place of the new domain wall.

Strain-induced reversible and irreversible magnetization switching in Fe/BaTiO3 heterostructures

Abstract: Magnetization switching of an Fe film in Fe/BaTiO3 heterostructures is demonstrated due to the interface lattice distortion caused by the structural phase transition of BaTiO3. The temperature dependence of in-plane magnetization of the Fe film in both zero and a small negative applied magnetic field clearly reveals that the reversible and irreversible magnetization switching processes occur in the Fe/BaTiO3 heterostructures. The variation in the magnetization orientation is corroborated by the fact that the symmetry of the magnetic anisotropy changes as the BaTiO3 undergoes the structural phase transition from the tetragonal to orthorhombic phases.

Magnetic and structural properties of ferrofluids based on Cobalt-Zinc ferrite nanoparticles

Abstract: Ferrofluids are colloidal systems composed of a single domain of magnetic nanoparticles with a mean diameter around 10 nm, dispersed in a liquid carrier. Magnetic Co(1-x)ZnxFe2O4 (x = 0.25, 0.50, 0.75) ferrite nanoparticles were prepared via co-precipitation method from aqueous salt solutions in an alkaline medium. The composition and structure of the samples were characterized through Energy Dispersive Xray Spectroscopy and X-ray diffraction, respectively. Transmission Electron Microscopy studies permitted determining nanoparticle size; grain size of nanoparticle conglomerates was established via Atomic Force Microscopy. The magnetic behavior of ferrofluids was characterized by Vibrating Sample Magnetometer; and finally, a magnetic force microscope was used to visualize the magnetic domains of Co(1-x)ZnxFe2O4 nanoparticles. X-ray diffraction patterns of Co(1-x)ZnxFe2O4 show the presence of the most intense peak corresponding to the (311) crystallographic orientation of the spinel phase of CoFe2O4. Fourier Transform Infrared Spectroscopy confirmed the presence of the bonds associated to the spinel structures; particularly for ferrites. The mean size of the crystallite of nanoparticles determined from the full-width at half maximum of the strongest reflection of the (311) peak by using the Scherrer approximation diminished from (9.5 0.3) nm to (5.4 0.2) nm when the Zn concentration increases from 0.21 to 0.75. The size of the Co-Zn ferrite nanoparticles obtained by Transmission Electron Microscopy is in good agreement with the crystallite size calculated from X-ray diffraction patterns, using Scherer’s formula. The magnetic properties investigated with the aid of Vibrating Sample Magnetometer at room temperature presented super-paramagnetic behavior, determined by the shape of the hysteresis loop. In this study, we established that the coercive field of Co1-xZnxFe2O4 magnetic nanoparticles and the crystal and nanoparticle sizes determined by X-ray Diffraction and Transmission Electron Microscopy, respectively, decrease with the increase of the Zn at%. Finally, our magnetic nanoparticles are not very hard magnetic materials given that the hysteresis loop is very small and for this reason Co(1-x)ZnxFe2O4 nanoparticles are considered soft magnetic material.

Magnetic tunneling junction devices, memories, electronic systems, and memory systems, and methods of fabricating the same

Abstract: Provided is a magnetic tunneling junction device including a fixed magnetic structure; a free magnetic structure; and a tunnel barrier between the fixed magnetic structure and the free magnetic structure, at least one of the fixed magnetic structure and the free magnetic structure including a perpendicular magnetization preserving layer, a magnetic layer between the perpendicular magnetization preserving layer and the tunnel barrier, and a perpendicular magnetization inducing layer between the perpendicular magnetization preserving layer and the magnetic layer.

Dynamics of artificial spin ice: a continuous honeycomb network

Abstract: We model the dynamics of magnetization in an artificial analogue of spin ice specializing to the case of a honeycomb network of connected magnetic nanowires. The inherently dissipative dynamics is mediated by the emission and absorption of domain walls in the sites of the lattice, and their propagation in its links. These domain walls carry two natural units of magnetic charge, whereas sites of the lattice contain a unit magnetic charge. Magnetostatic Coulomb forces between these charges play a major role in the physics of the system, as does quenched disorder caused by imperfections of the lattice. We identify and describe different regimes of magnetization reversal in an applied magnetic field determined by the orientation of the applied field with respect to the initial magnetization. One of the regimes is characterized by magnetic avalanches with a 1/n distribution of lengths.

Impact of Various Factors on Relationships Between Stress and Eigen Magnetic Field in a Steel Specimen

Abstract: Many materials which could cause a real threat of a catastrophe due to fatigue, exceeding stress limits or plastic strain have magnetic properties that could affect the local magnetic field. Though active magnetic methods of condition monitoring are quite well-known and widely applied however, passive techniques which are based only on the existence of natural magnetic field of the Earth, still require research and improvement. It is obvious that every physical object within the magnetosphere interacts with Earth's magnetic field and is subjected to special laws of physics. Such objects can attract or deflect magnetic field lines around their matter. Own magnetic field of an object: H= -grad(w), where w is the magnetic potential, is a function of the gradient of magnetization: w = w(div M). Therefore, the measure of magnetic field of an object depends on an object's magnetization and distribution of its volume in the medium (space). Considering magnetoelastic effects (Villari Effect, magnetostriction), the additional stress causes transformation of the material to magnetic state which reflects the magnetization of an object. The magnetization depends on many factors. Magneto-mechanic phenomena have been known for a long time but as the technology developed, there have emerged new possibilities of acquisition, processing and analysis of these phenomena and of their use in technical diagnosis. Following a simple model analysis, a laboratory experiment was proposed and performed. By controlling plastic and elastic range of the specimen's strain, we have investigated the existence of a relation between stress and degree of magnetization, which is strictly connected with deformation and effort. Magnetic anomalies which are generated due to magneto-mechanic effect were collected by the three axial fluxgate magnetometer, which allowed presentation of own magnetic field component, which was least sensitive to the disturbance present in a real world. Experiment included in the paper confirms the existence of a relationship between stress and magnetization degree which additionally depends on the kind of material. In addition the possibility of remote identification of magnetoelastic effects has been contemplated and examined. Finally the paper analyzes the impact of the shape of specimen on the interaction between Earth and eigen magnetic fields during a tension test. Further directions and comments on development of techniques which allow exact stress assessment of technical objects made of ferromagnetic materials have been included.

In situ magnetic force microscope studies of magnetization reversal of interaction domains in hot deformed Nd-Fe-B magnets

Abstract: A hot-deformed Nd-Fe-B sample has been chosen for the investigation of interaction domains by means of magnetic force microscopy. During the imaging process, a magnetic field of up to 6 T was applied in situ along the easy axis of magnetization. The thermally demagnetized state presents a regular pattern of interaction domains with an average width of about 1 μm but with a much larger length scale. Starting from the thermally demagnetized state, magnetization along the initial magnetization curve occurs via sequential switching of neighboring grain columns at the peripheries of the interaction domains. Demagnetization of a saturated sample takes place through the nucleation and expansion of a patchy domain pattern with a much larger extension and a substructure in the lateral range of the underlying grain size. Reversal processes under an applied magnetic field also take place at the borders of the domains.

Influence of NaBH 4 on the size, composition, and magnetic properties of CoFe 2 O 4 nanoparticles synthesized by hydrothermal method

Abstract: Cobalt ferrite nanoparticles are prepared by varying the concentration of reducing agent NaBH4 by hydrothermal method. Transmission electron microscope observations show that particle size increases from 11.47 to 28.05 nm by increasing the concentration of NaBH4 from 30 to 70 mM. Williamson–Hall analysis on X-ray diffraction patterns show that strain increases in nanoparticles with increase in quantity of NaBH4. Energy dispersive X-ray analysis and Inductive coupled plasma atomic emission spectroscopy indicate large amount of oxygen deficiency which increase with amount of NaBH4. The magnetic hysteresis loops measured at room temperature clearly illustrate the influence of concentration of NaBH4 on magnetic properties of CoFe2O4. Saturation magnetization, coercivity, and anisotropy constant increases with the quantity of NaBH4 and they reach the maximum values of 93 emu/g, 2210 Oe, and 5.28 × 105 J/m3, respectively, with addition of 70 mM NaBH4. Estimation of micromagnetic parameters (exchange length, critical single-domain volume, etc.) suggests that prepared nanoparticles of CoFe2O4 are in single domain and size lies between superparamagnetic to critical single domain. The enhancement of magnetization is attributed to oxygen deficiency. This technique is useful in tuning the size and magnetic properties of cobalt ferrite nanoparticles.

Materials process and applications of single grain (RE)–Ba–Cu–O bulk high-temperature superconductors

Abstract: This paper reviews recent advances in the melt process of (RE)–Ba–Cu–O [(RE)BCO, where RE represents a rare earth element] single grain high-temperature superconductors (HTSs), bulks and its applications. The efforts on the improvement of the magnetic flux pinning with employing the top-seeded melt-growth process technique and using a seeded infiltration and growth process are discussed. Which including various chemical doping strategies and controlled pushing effect based on the peritectic reaction of (RE)BCO. The typical experiment results, such as the largest single domain bulk, the clear TEM observations and the significant critical current density, are summarized together with the magnetization techniques. Finally, we highlight the recent prominent progress of HTS bulk applications, including Maglev, flywheel, power device, magnetic drug delivery system and magnetic resonance devices.

Magnetic discrimination between Al-substituted hematites synthesized by hydrothermal and thermal dehydration methods and its geological significance

Abstract: Received 22 June 2011; revised 20 December 2011; accepted 24 December 2011; published 23 February 2012. [1] Hematite, a ubiquitous mineral in aerobic sediments and soils of temperate and warm areas, is weakly magnetic. However, it carries a stable natural remanent magnetization, and thus can reflect paleoenvironment changes. To quantify the influence of Al content in hematite on its magnetic properties, two series of hematite particles were prepared by hydrothermal transformation of ferrihydrite in aqueous suspension (HFh* series) and by thermal dehydration of goethite (HG* series). Crystal morphological and mineral magnetic properties of these two types of hematites differ distinctively. More specifically, the HFh* series samples display oblate (plate-like) morphologies, while the HG* series samples are prolate (highly acicular). HFh* series samples display higher saturation magnetization but lower magnetic coercivity than that of the HG* series. It is tenable that a better lattice ordering of Al substitution occurs during the process of dehydration of goethite than after transformation from ferrihydrite, resulting in weaker saturation magnetization for HG* series samples. The origin of single domain (SD) hematite in nature can be diagnosed by the correlation of unblocking temperature and magnetic coercivity: a positive correlation indicates the presence of pure (Al-free) SD hematite, while a negative correlation indicates a chemical origin of SD Al-substituted hematite. These results bear new information on decoding the complex magnetic properties of SD Al-hematite in nature environments, and thus deepen our understanding of the mechanism of variations in both paleomagnetic and paleoenvironmental signals carried by Al-hematite.

Domain controlled magnetic and electric properties of variable sized magnetite nano-hollow spheres

Abstract: Here, we report the synthesis of variable sized magnetite (Fe3O4) nano-hollow spheres in one step template free solvothermal method and their size dependent magnetic and electrical properties. Size of the hollow spheres is varied from 100 nm to 725 nm by changing the concentration of capping agent. Trace of Verway transition is found for all sets of spheres and the Verway transition temperature (TV) increases with increasing size of the spheres. The domain structure of these spheres changes from pseudo single domain to multi domain state as the size increases from 100 nm to 725 nm as evident from Day plots. This change in domain structure also changes the magnetic and electric properties of these spheres. Temperature dependent of high field magnetization of the hollow spheres can be well explained by Bloch's power law with higher than the bulk value of Bloch constant. The Bloch exponent varies from 1.94 to 1.69 with increasing size of the spheres. Frequency dependence of electrical conductivity (σ) shows ...