Showing papers on "Single domain published in 2014"

Ultrafast magnetization switching by spin-orbit torques

Abstract: Spin-orbit torques induced by spin Hall and interfacial effects in heavy metal/ferromagnetic bilayers allow for a switching geometry based on in-plane current injection. Using this geometry, we demonstrate deterministic magnetization reversal by current pulses ranging from 180 ps to ms in Pt/Co/AlOx dots with lateral dimensions of 90 nm. We characterize the switching probability and critical current Ic as a function of pulse length, amplitude, and external field. Our data evidence two distinct regimes: a short-time intrinsic regime, where Ic scales linearly with the inverse of the pulse length, and a long-time thermally assisted regime, where Ic varies weakly. Both regimes are consistent with magnetization reversal proceeding by nucleation and fast propagation of domains. We find that Ic is a factor 3–4 smaller compared to a single domain model and that the incubation time is negligibly small, which is a hallmark feature of spin-orbit torques.

Tailoring the topology of an artificial magnetic skyrmion.

Abstract: Despite theoretical predictions, it remains an experimental challenge to realize an artificial magnetic skyrmion whose topology can be well controlled and tailored so that its topological effect can be revealed explicitly in a deformation of the spin textures. Here we report epitaxial magnetic thin films in which an artificial skyrmion is created by embedding a magnetic vortex into an out-of-plane aligned spin environment. By changing the relative orientation between the central vortex core polarity and the surrounding out-of-plane spins, we are able to control and tailor the system between two skyrmion topological states. An in-plane magnetic field is used to annihilate the skyrmion core by converting the central vortex state into a single domain state. Our result shows distinct annihilation behaviour of the skyrmion core for the two different skyrmion states, suggesting a topological effect of the magnetic skyrmions in the core annihilation process.

Magnetic properties of graphite oxide and reduced graphene oxide

Abstract: Graphite oxide (GO) and reduced graphene oxide (RGO) have been prepared using standard chemical methods. The formations of the oxides are characterized by X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) studies. Both the oxides exhibit weak superparamagnetism and hysteresis for the first time at room temperature. Magnetic moment for RGO is comparatively smaller than that of GO sample. The superparamagnetism in these oxides is attributed to the presence of single domains, each domain being cluster of defect induced magnetic moments coupled by ferromagnetic interaction. Apart from these single domain clusters there are other defect induced moments coupled by ferromagnetic interaction which show ferromagnetism and hysteresis.

Modeling of magnetoelastic nanostructures with a fully coupled mechanical-micromagnetic model

Abstract: Micromagnetic simulations of magnetoelastic nanostructures traditionally rely on either the Stoner-Wohlfarth model or the Landau?Lifshitz-Gilbert (LLG) model, assuming uniform strain (and/or assuming uniform magnetization). While the uniform strain assumption is reasonable when modeling magnetoelastic thin films, this constant strain approach becomes increasingly inaccurate for smaller in-plane nanoscale structures. This paper presents analytical work intended to significantly improve the simulation of finite structures by fully coupling the LLG model with elastodynamics, i.e., the partial differential equations are intrinsically coupled. The coupled equations developed in this manuscript, along with the Stoner-Wohlfarth model and the LLG (constant strain) model are compared to experimental data on nickel nanostructures. The nickel nanostructures are 100???300???35 nm single domain elements that are fabricated on a Si/SiO2 substrate; these nanostructures are mechanically strained when they experience an applied magnetic field, which is used to generate M vs H curves. Results reveal that this paper?s fully-coupled approach corresponds the best with the experimental data on coercive field changes. This more sophisticated modeling technique is critical for guiding the design process of future nanoscale strain-mediated multiferroic elements, such as those needed in memory systems.

Visualized effect of oxidation on magnetic recording fidelity in pseudo-single-domain magnetite particles

Abstract: Magnetite (Fe3O4) is an important magnetic mineral to Earth scientists, as it carries the dominant magnetic signature in rocks, and the understanding of its magnetic recording fidelity provides a critical tool in the field of palaeomagnetism. However, reliable interpretation of the recording fidelity of Fe3O4 particles is greatly diminished over time by progressive oxidation to less magnetic iron oxides, such as maghemite (g-Fe2O3), with consequent alteration of remanent magnetization potentially having important geological significance. Here we use the complementary techniques of environmental transmission electron microscopy and off-axis electron holography to induce and visualize the effects of oxidation on the magnetization of individual nanoscale Fe3O4 particles as they transform towards g-Fe2O3. Magnetic induction maps demonstrate a change in both strength and direction of remanent magnetization within Fe3O4 particles in the size range dominant in rocks, confirming that oxidation can modify the original stored magnetic information.

Electrical control of a single magnetoelastic domain structure on a clamped piezoelectric thin film—analysis

Abstract: This paper presents an analytical model coupling Landau-Lifshitz-Gilbert micromagnetics with elastodynamics and electrostatics to model the response of a single domain magnetoelastic nano-element attached to a piezoelectric thin film (500 nm). The thin film piezoelectric is mounted on a Si substrate, globally clamping the film from in-plane extension or contraction. Local strain transfer to the magnetoelastic element is achieved using patterned electrodes. The system of equations is reduced to eight coupled partial differential equations as a function of voltage (V), magnetic potential ϕ, magnetic moments (m), and displacements (u), i.e., fully coupled material. The weak forms of the partial differential equations are solved using a finite element formulation. The problem of a Ni single domain structure (i.e., 150 nm × 120 nm × 10 nm) on a thin film (500 nm) piezoelectric transducer (PZT)-5H attached to an infinite substrate is studied. Discretization in the single domain structure is on the order of the ...

Giant magnetic anisotropy and tunnelling of the magnetization in Li2(Li1−xFex)N

Abstract: Large magnetic anisotropy and coercivity are key properties of functional magnetic materials and are generally associated with rare earth elements. Here we show an extreme, uniaxial magnetic anisotropy and the emergence of magnetic hysteresis in Li₂(Li(1-x)Fe(x))N. An extrapolated, magnetic anisotropy field of 220 T and a coercivity field of over 11 T at 2 K outperform all known hard ferromagnets and single-molecular magnets. Steps in the hysteresis loops and relaxation phenomena in striking similarity to single-molecular magnets are particularly pronounced for x≪1 and indicate the presence of nanoscale magnetic centres. Quantum tunnelling, in the form of temperature-independent relaxation and coercivity, deviation from Arrhenius behaviour and blocking of the relaxation, dominates the magnetic properties up to 10 K. The simple crystal structure, the availability of large single crystals and the ability to vary the Fe concentration make Li₂(Li(1-x)Fe(x))N an ideal model system to study macroscopic quantum effects at elevated temperatures and also a basis for novel functional magnetic materials.

Unique single-domain state in a polycrystalline ferroelectric ceramic

Abstract: Non-180\ifmmode^\circ\else\textdegree\fi{} ferroelectric domains are also ferroelastic domains; their existence in polycrystalline materials is to relieve internal stresses generated during solid-solid phase transitions and minimize the elastic distortion energy. Therefore, grains with random orientations in polycrystalline ceramics are always occupied by many domains, especially in the regions close to grain boundaries. In this Rapid Communication, we report the observation of a single-domain state in a BaTiO${}_{3}$-based polycrystalline ceramic at intermediate poling electric fields with in situ transmission electron microscopy. The grains in the virgin ceramic and under high poling fields are found multidomained. The unique single-domain state is believed to be responsible for the ultrahigh piezoelectric property observed in this lead-free composition and is suggested to be of orthorhombic symmetry for its exceptionally low elastic modulus.

Coercivity enhancement in V2O3/Ni bilayers driven by nanoscale phase coexistence

Abstract: We studied the temperature dependence of coercivity and magnetization of V2O3/Ni bilayers across the Structural Phase Transition in V2O3. We found a coercivity peak that coincides with the V2O3 phase transition on top of an overall increase of the coercivity with decreasing temperature. We propose that this sharp increase arises from a length scale competition between magnetic domains of Ni and phase coexistence during the V2O3 phase transition. This model is supported by micromagnetic simulations and shows that magnetic properties of ferromagnetic films are strongly affected by a proximal first order phase transition.

Micromagnetic Simulations of Magnetization Reversal in Misaligned Multigrain Magnets With Various Grain Boundary Properties Using Large-Scale Parallel Computing

Abstract: This paper reports on micromagnetic simulations of the magnetization reversal behavior in polycrystalline Nd-Fe-B sintered magnets using large-scale parallel computing. A multigrain model is introduced to calculate the grain alignment dependence of the coercivity of polycrystalline Nd-Fe-B magnets with various magnetic characteristics at grain boundaries (GBs). The magnetic domain wall motion is accurately treated by dividing the analyzed object into extremely small elements. The multigrain model with a soft magnetic GB phase and a reverse domain at the initial state well reproduces experimental results. The calculations of coercivity with several GB widths are also carried out to seek for the origin of the sudden decrease of coercivity with nearly perfect grain alignment.

Achieving single domain relaxor-PT crystals by high temperature poling

Abstract: Single domain relaxor-PT crystals are important from both fundamental and application viewpoints. Compared to domain engineered relaxor-PT crystals, however, single domain crystals are prone to cracking during poling. In this paper, based on the analysis of the cracking phenomenon in [001] poled tetragonal 0.25Pb(In0.5Nb0.5)O3–0.37Pb(Mg1/3Nb2/3)O3–0.38PbTiO3 (PIN–PMN–PT) crystals, the non-180° ferroelastic domain switching was thought to be the dominant factor for cracking during the poling process. A high temperature poling technique, by which the domain switching can be greatly avoided, was proposed to achieve the single domain relaxor-PT crystals. By this poling approach, a quasi-single domain crystal was obtained without cracks. In addition, compared to room temperature poling, the high temperature poled PIN–PMN–PT crystals showed improved electromechanical properties, i.e., a low dielectric loss, a low strain–electric field hysteresis and a high mechanical quality factor, demonstrating a beneficial poling approach.

Visualization of ferroelectric domains in a hydrogen-bonded molecular crystal using emission of terahertz radiation

Abstract: Using a terahertz-radiation imaging, visualizations of ferroelectric domains were made in a room-temperature organic ferroelectric, croconic acid. In as-grown crystals, observed are ferroelectric domains with sizes larger than 50-μm square, which are separated by both 180° and tail-to-tail domain walls (DWs). By applying an electric field along c axis (the polarization direction), a pair of 180° DWs is generated and an each 180° DW oppositely propagates along a axis, resulting in a single domain. By cyclic applications of electric fields, a pair of 180° DWs repeatedly emerges, while no tail-to-tail DWs appear. We discuss the usefulness of the terahertz-radiation imaging as well as the observed unique DW dynamics.

Complete set of material constants of single domain (K, Na)(Nb, Ta)O3 single crystal and their orientation dependence

Abstract: A self-consistent complete set of dielectric, piezoelectric, and elastic constants for single domain Ta modified (K, Na)NbO3 (KNN) crystal was determined. This full set constant for single domain KNN-based crystals allowed the prediction of orientation dependence of the longitudinal dielectric, piezoelectric, elastic coefficients, and electromechanical coupling factors. The maximum piezoelectric and electromechanical properties were found to exist near [001]C. In addition, material constants of [001]C poled domain engineered single crystal with 4 mm symmetry were experimentally measured and compared with the calculated values. Based on this, extrinsic contribution to the piezoelectricity was estimated to be ∼20%.

Micromagnetic analysis of the hardening mechanisms of nanocrystalline MnBi and nanopatterned FePt intermetallic compounds.

Abstract: The uniaxial intermetallic compounds of L10-FePt and the low temperature NiAs structure of MnBi are suitable alloys for application as high-density recording materials or as high-coercivity permanent magnets. Single domain particles of these materials are characterized by coercive fields above 1 T over a large temperature range. In particular MnBi shows a coercive field of 2 T at 450 K. Its extraordinary magnetic properties in the temperature range up to 600 K are due to an increase of the magnetocrystalline anisotropy constant from 1.2 MJ m(-3) at 300 K to 2.4 MJ m(-3) at 450 K. In spite of the large coercivities obtained for both type of materials their experimental values deviate considerably from the theoretical values Hc = 2K1/Js valid for a homogeneous rotation process in spherical particles. As is well known these discrepancies are due to the deteriorating effects of the microstructure. For an analysis of the coercive fields the Stoner-Wohlfarth theory has to be expanded with respect to higher anisotropy constants and to microstructural effects such as misaligned grains and grain surfaces with reduced anisotropy constants. It is shown that the temperature dependence and the angular dependence of Hc for FePt as well as MnBi can be quantitatively interpreted by taking into account the above mentioned intrinsic and microstructural effects.

Phase field modeling of flexoelectric effects in ferroelectric epitaxial thin films

Abstract: The flexoelectric effect which is defined as the coupling between strain gradient and polarization has long been neglected because it is insignificant in bulk ferroelectrics. However, at nanoscale, the strain gradient can be dramatically increased leading to giant flexoelectric effects. In the present study, the flexoelectric effects in epitaxial nano thin films of a 180° multi-domain structure, which are subjected to a compressive in-plane misfit strain, are investigated by the phase field method. Unlike the case of a single domain structure where the strain gradient is mainly attributed to the formation of dislocation which relaxes the misfit strain, in a multi-domain structure, it is attributed to many factors, such as surface and interface effects, misfit relaxation and domain wall structure. The results obtained show that relatively large flexoelectricity-induced electric fields are produced near the domain wall region. The induced field will not only influence the domain structure of the thin film, but also the hysteresis loops when it is under an applied electric field.

Giant temperature dependence of the spin reversal field in magnetoelectric chromia

Abstract: Magnetic field-induced reversal of surface spin polarization for the magnetoelectric antiferromagnet chromia is studied via magnetometry in (0001)-textured thin films of various thicknesses. Reversal solely by magnetic means has been experimentally evidenced in sufficiently thin films. It sets the field-response of chromia films apart from bulk behavior, where switching between time-reversed single domain states requires the simultaneous presence of electric and magnetic fields. In our detailed experiments, we furthermore observe a giant sensitivity of the coercive field on temperature, thus, indicating the potential of magnetoelectric antiferromagnets as promising candidates for energy assisted magnetic recording media.

Size Dependence of Domain Pattern Transfer in Multiferroic Heterostructures

Abstract: Magnetoelectric coupling in multiferroic heterostructures can produce large lateral modulations of magnetic anisotropy enabling the imprinting of ferroelectric domains into ferromagnetic films. Exchange and magnetostatic interactions within ferromagnetic films oppose the formation of such domains. Using micromagnetic simulations and a one-dimensional model, we demonstrate that competing energies lead to the breakdown of domain pattern transfer below a critical domain size. Moreover, rotation of the magnetic field results in abrupt transitions between two scaling regimes with different magnetic anisotropy. The theoretical predictions are confirmed by experiments on CoFeB/BaTiO3 heterostructures.

Magnetic domain control and voltage response of exchange biased magnetoelectric composites

Abstract: Self-biased magnetoelectric composites, which are realized with the exchange bias effect, hold an increased total anisotropy field compared to systems without exchange bias. Thus, small exchange bias fields are favorable because of a minor reduction of magnetic permeability and magneto-electric voltage coefficient. However, weakly biased magnetoelectric composites lose their self-biasing properties and possibly show an increase of discontinuities in magnetization reversal due to the formation of magnetic domains. By a thickness variation of the ferromagnetic layer, a maximum voltage coefficient αME ≈ 430 V/cm Oe was found for a magnetostrictive multilayer of 3 × (5 nm Ta/3 nm Cu/8 nm Mn-Ir/333 nm Fe-Co-Si-B). Yet, a stable single domain state indicating a well defined magnetization reversal by coherent magnetization rotation was achieved for layer thicknesses up to 100 nm Fe-Co-Si-B with αME ≈ 340 V/cm Oe. This slight reduction is overcompensated by the improved control of the magnetic domain pattern which is highly beneficial for magnetic field sensing applications and of special importance, when frequency conversion techniques are applied.

Mineralogical and chemical composition of magnetic fly ash fraction

Abstract: Magnetic fractions of coal fly ashes from three power plants were obtained by wet magnetic separation method. Quartz and mullite were the crystalline minerals dominating the nonmagnetic fractions. Magnetic fractions contained magnetite, hematite, and, to a lesser extent, quartz and mullite. Iron speciation by Mossbauer spectroscopy indicated the presence of Fe2+ and Fe3+ in aluminosilicate glass in magnetic fractions apart from magnetite and hematite. Chemical analyses revealed that magnetic fractions had about 2.5 times higher concentrations of Co and one to two times higher concentrations of Ni, Cu, Zn, Mo, and Cd. The dominant magnetic minerals were ferrimagnetic, and multi domain and stable single domain grains contributed mainly to the magnetic enhancement of fly ash samples.

Angular dependence of the magnetic properties of cylindrical diameter modulated Ni80Fe20 nanowires

Abstract: We have investigated numerically the angular dependence of the coercivity and remanence of cylindrical diameter modulated Ni80Fe20 nanowires. We observed that the system always starts reversing its magnetization through the thickest segment by means of a quite complex reversal process, considering the propagation of two vortex domain walls. Furthermore, we observed a transition from vortex domain walls to coherent-mode rotation for the thinnest segment as a function of the angle in which the external magnetic field is applied. In addition, we obtained a non-monotonic behavior for the coercivity and saturation field as a function of the angle at which the external magnetic field is applied. Finally, diameter modulation is an attractive way to handle over the motion of magnetic domain walls, a phenomenon proposed as a future data storage platform.

Magnetic and magnetothermal properties and the magnetic phase diagram of high purity single crystalline terbium along the easy magnetization direction

Abstract: The magnetic and magnetothermal properties of a high purity terbium single crystal have been re-investigated from 15 to 350 K in magnetic fields ranging from 0 to 75 kOe using magnetization, ac magnetic susceptibility and heat capacity measurements The magnetic phase diagram has been refined by establishing a region of the fan-like phase broader than reported in the past, by locating a tricritical point at 226 K, and by a more accurate definition of the critical fields and temperatures associated with the magnetic phases observed in Tb

Influence of cooling rate on thermoremanence of magnetite grains: Identifying the role of different magnetic domain states

Abstract: It is widely accepted that cooling rate can strongly influence the intensity of the thermal remanent magnetization (TRM) acquired by rocks during cooling to ambient temperatures. If ignored, this effect might lead to underestimates or overestimates of the ancient magnetic field intensity. To date, however, the cooling rate dependence of TRM acquired by particles with different domain states has never been systematically analyzed from the theoretical or experimental point of view. In this study, we present measurements of the TRM of synthetic magnetites with well-defined grain sizes that were quenched with constant cooling rates of 0.05, 0.1, 1, 3, 10, and 15 K/min. While single domain (SD) and small pseudo-single domain (PSD) samples are found to show larger TRMs after slow cooling, the TRMs of larger PSD and multidomain (MD) magnetites are not affected by an increase or decrease of the cooling rate. Overall, our results suggest that only smallest magnetite grains acquire a cooling rate-dependent TRM. Therefore, cooling rate corrections of paleointensity determinations are only necessary for samples dominated by SD remanence carriers, while rocks dominated by PSD and MD carriers, such as basalts, which are most commonly used for paleointensity studies, do not require such corrections.

Autoresonant control of the magnetization switching in single-domain nanoparticles

Abstract: The ability to control the magnetization switching in nanoscale devices is a crucial step for the development of fast and reliable techniques to store and process information. Here we show that the switching dynamics can be controlled efficiently using a microwave field with a slowly varying frequency (autoresonance). This technique allowed us to reduce the applied field by more than 30% compared to competing approaches, with no need to fine-tune the field parameters. For a linear chain of nanoparticles the effect is even more dramatic, as the dipolar interactions tend to cancel out the effect of the temperature. Simultaneous switching of all the magnetic moments can thus be efficiently triggered on a nanosecond timescale.

Magnetization states and magnetization processes in nanostructures: From a single layer to multilayers

Abstract: The results of combined (experimental, analytical, and micromagnetic simulations) studies on the evolution of magnetization states and processes in ultrathin films and multilayered systems are presented. We show ways to manipulate magnetization distributions in ultrathin magnetic single or multilayers by tuning: the thickness of the magnetic layer, the thickness of either the non-magnetic cap or spacer layer, the magnetic anisotropy, and the geometrical constrictions of the system. In ultrathin magnetic films, both the magnetization distribution and the critical thickness of the magnetization reorientation phase transition (RPT) between perpendicular and in-plane states can be also controlled by post-growth treatments, e.g., by either ion or light irradiation. By changing the geometrical parameters of the nanostructure, as well as by an applied external magnetic field, one can tune magnetic domain sizes in a giant range (of a few orders of magnitude) and induce the RPT. Transitions between two- and three-dimensional magnetization distributions are discussed.