Showing papers on "Single domain published in 2013"

Chiral spin torque at magnetic domain walls

Abstract: Spin-polarized currents provide a powerful means of manipulating the magnetization of nanodevices, and give rise to spin transfer torques that can drive magnetic domain walls along nanowires. In ultrathin magnetic wires, domain walls are found to move in the opposite direction to that expected from bulk spin transfer torques, and also at much higher speeds. Here we show that this is due to two intertwined phenomena, both derived from spin–orbit interactions. By measuring the influence of magnetic fields on current-driven domain-wall motion in perpendicularly magnetized Co/Ni/Co trilayers, we find an internal effective magnetic field acting on each domain wall, the direction of which alternates between successive domain walls. This chiral effective field arises from a Dzyaloshinskii–Moriya interaction at the Co/Pt interfaces and, in concert with spin Hall currents, drives the domain walls in lock-step along the nanowire. Elucidating the mechanism for the manipulation of domain walls in ultrathin magnetic films will enable the development of new families of spintronic devices. The influence of magnetic fields on the current-driven motion of domain walls in nanowires with perpendicular anisotropy shows that two spin–orbit-derived mechanisms are responsible for their motion.

Single domain spin manipulation by electric fields in strain coupled artificial multiferroic nanostructures.

Abstract: We demonstrate in situ 90\ifmmode^\circ\else\textdegree\fi{} electric field-induced uniform magnetization rotation in single domain submicron ferromagnetic islands grown on a ferroelectric single crystal using x-ray photoemission electron microscopy. The experimental findings are well correlated with micromagnetic simulations, showing that the reorientation occurs by the strain-induced magnetoelectric interaction between the ferromagnetic nanostructures and the ferroelectric crystal. Specifically, the ferroelectric domain structure plays a key role in determining the response of the structure to the applied electric field, resulting in three strain-induced regimes of magnetization behavior for the single domain islands.

Increase of magnetic hyperthermia efficiency due to dipolar interactions in low-anisotropy magnetic nanoparticles: Theoretical and experimental results

Abstract: When magnetic nanoparticles (MNPs) are single domain and magnetically independent, their magnetic properties and the conditions to optimize their efficiency in magnetic hyperthermia applications are now well understood. However, the influence of magnetic interactions on magnetic hyperthermia properties is still unclear. Here, we report hyperthermia and high-frequency hysteresis loop measurements on a model system consisting of MNPs with the same size but a varying anisotropy, which is an interesting way to tune the relative strength of magnetic interactions. A clear correlation between the MNP anisotropy and the squareness of their hysteresis loop in colloidal solution is observed: the larger the anisotropy, the smaller the squareness. Since low anisotropy MNPs display a squareness higher than the one of magnetically independent nanoparticles, magnetic interactions enhance their heating power in this case. Hysteresis loop calculations of independent and coupled MNPs are compared to experimental results. It is shown that the observed features are a natural consequence of the formation of chains and columns of MNPs during hyperthermia experiments: in these structures, when the MNP magnetocristalline anisotropy is small enough to be dominated by magnetic interactions, the hysteresis loop shape tends to be rectangular, which enhances their efficiency. On the contrary, when MNPs do not form chains and columns, magnetic interactions reduce the hysteresis loop squareness and the efficiency of MNPs compared to independent ones. Our finding can thus explain contradictory results in the literature on the influence of magnetic interactions on magnetic hyperthermia. It also provides an alternate explanation to some experiments where an enhanced specific absorption rate for MNPs in liquids has been found compared to the one of MNPs in gels, usually interpreted with some contribution of the brownian motion. The present work should improve the understanding and interpretation of magnetic hyperthermia experiments.

Definition of Magnetic Exchange Length

Abstract: The magnetostatic exchange length is an important parameter in magnetics as it measures the relative strength of exchange and self-magnetostatic energies. Its use can be found in areas of magnetics including micromagnetics, soft and hard magnetic materials, and information storage. The exchange length is of primary importance because it governs the width of the transition between magnetic domains. Unfortunately, there is some confusion in the literature between the magnetostatic exchange length and a similar distance concerning magnetization reversal mechanisms in particles known as the characteristic length. This confusion is aggravated by the common usage of two different systems of units, SI and cgs. This paper attempts to clarify the situation and recommends equations in both systems of units.

Maximizing hysteretic losses in magnetic ferrite nanoparticles via model-driven synthesis and materials optimization.

Abstract: This article develops a set of design guidelines for maximizing heat dissipation characteristics of magnetic ferrite MFe2O4 (M = Mn, Fe, Co) nanoparticles in alternating magnetic fields. Using magnetic and structural nanoparticle characterization, we identify key synthetic parameters in the thermal decomposition of organometallic precursors that yield optimized magnetic nanoparticles over a wide range of sizes and compositions. The developed synthetic procedures allow for gram-scale production of magnetic nanoparticles stable in physiological buffer for several months. Our magnetic nanoparticles display some of the highest heat dissipation rates, which are in qualitative agreement with the trends predicted by a dynamic hysteresis model of coherent magnetization reversal in single domain magnetic particles. By combining physical simulations with robust scalable synthesis and materials characterization techniques, this work provides a pathway to a model-driven design of magnetic nanoparticles tailored to a ...

Measuring the Curie temperature

Abstract: [1] Curie point temperatures (TC) of natural and synthetic magnetic materials are commonly determined in rock magnetism by several measurement methods that can be mutually incompatible and may lead to inconsistent results. Here the common evaluation routines for high-temperature magnetization and magnetic initial susceptibility curves are analyzed and revised based on Landau's theory of second-order phase transitions. It is confirmed that in high-field magnetization curves TC corresponds to the inflection point, below the temperature of maximum curvature or the double-tangent intersection point. At least four different physical processes contribute to the initial magnetic susceptibility near the ordering temperature. They include variation of saturation magnetization, superparamagnetic behavior, magnetization rotation, and magnetic domain wall motion. Because each of these processes may influence the apparent position of TC, initial susceptibility and high-field curves can yield deviating estimates of TC. A new procedure is proposed to efficiently determine the temperature variation of several magnetic parameters on a vibrating-sample magnetometer, by repeatedly measuring quarter-hysteresis loops during a single heating cycle. This procedure takes measurements during the inevitable waiting time necessary for thermal equilibration of the sample, whereby it is not slower than the commonly performed measurements on a Curie balance. However, it returns saturation magnetization, saturation remanence, high-field and low-field slopes, and other parameters as a function of temperature, which provide independent information about TC and other sample properties.

Orientation dependence of piezoelectric properties and mechanical quality factors of 0.27Pb(In1/2Nb1/2)O3-0.46Pb(Mg1/3Nb2/3)O3-0.27PbTiO3:Mn single crystals

Abstract: The complete set of material constants of single domain rhombohedral phase 0.27Pb(In1/2Nb1/2)O3-0.46Pb(Mg1/3Nb2/3)O3-0.27PbTiO3:Mn single crystal has been determined. The orientation dependence of piezoelectric, dielectric, and electromechanical properties was calculated based on these single domain data. The maximum piezoelectric and electromechanical properties were found to exist near the [001]C pseudo-cubic direction. In addition, the piezoelectric properties of [001]C poled crystals with “4R” multi-domain configuration were experimentally measured and compared with the calculated values. Only a small difference (3%) was found between experimental and theoretical values, indicating the high piezoelectric properties in the “4R” state are mainly from intrinsic contributions. The mechanical quality factors Q33 are significantly improved by the Mn-doping for the “4R” domain engineered crystals but almost no change for the single domain “1R” state. On the other hand, Q15 of both single domain and multidoma...

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 $I_c$ as function of pulse length, amplitude, and external field. Our data evidence two distinct regimes: a short-time intrinsic regime, where $I_c$ scales linearly with the inverse of the pulse length, and a long-time thermally assisted regime where $I_c$ varies weakly. Both regimes are consistent with magnetization reversal proceeding by nucleation and fast propagation of domains. We find that $I_c$ 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.

Magnetic nanoparticles: a subject for both fundamental research and applications

Abstract: Single domain magnetic nanoparticles (MNPs) have been a vivid subject of intense research for the last fifty years. Preparation of magnetic nanoparticles and nanostructures has been achieved by both bottom-up and top-down approaches. Single domain MNPs show Neel-Brown-like relaxation. The Stoner-Wohlfarth model describes the angular dependence of the switching of the magnetization of a single domain particle in applied magnetic fields. By varying the spacing between the particles, the interparticle interactions can be tuned. This leads to various supermagnetic states such as superparamagnetism, superspin glass, and superferromagnetism. Recently, the study of the magnetization dynamics of such single domain MNPs has attracted particular attention, and observations of various collective spin wave modes in patterned nanomagnet arrays have opened new avenues for on-chip microwave communications. MNPs have the potential for various other applications such as future recording media and in medicine. We will discuss the various aspects involved in the research on MNPs.

In-plane magnetic field dependence of electric field-induced magnetization switching

Abstract: Electric field-induced magnetization switching through magnetization precession is investigated as a function of in-plane component of external magnetic field for a CoFeB/MgO-based magnetic tunnel junction with perpendicular easy axis. The switching probability is an oscillatory function of the duration of voltage pulses and its magnitude and period depend on the magnitude of in-plane magnetic field. Experimental results are compared with simulated ones by using Landau-Lifshitz-Gilbert-Langevin equation, and possible factors determining the probability are discussed.

Magnetization dynamics in permalloy films with stripe domains

Abstract: Through a systematic investigation of the field-dependent dynamic magnetization of a series of NiFe films with and without stripe domains in conjunction with the static magnetization process, we demonstrate that the experimental rotatable anisotropy field is not a fixed value but strongly varied with the external in-plane magnetic field, being qualitatively associated with the emergence of stripe domains. Moreover, the frequency linewidth spectra of the films with stripe domains show an abnormal behavior with three distinct regimes which are strongly correlated with both the static magnetization process and the competition between external magnetic field and dynamic anisotropy field. The results are discussed in terms of the effect of inhomogeneous magnetization associated with the formation of stripe domains and the field-dependent dynamic anisotropy that cause the broadening of frequency linewidth.

Electric-field control of domain wall nucleation and pinning in a metallic ferromagnet

Abstract: The electric (E) field control of magnetic properties opens the prospects of an alternative to magnetic field or electric current activation to control magnetization. Multilayers with perpendicular magnetic anisotropy (PMA) have proven to be particularly sensitive to the influence of an E-field due to the interfacial origin of their anisotropy. In these systems, E-field effects have been recently applied to assist magnetization switching and control domain wall (DW) velocity. Here we report on two new applications of the E-field in a similar material : controlling DW nucleation and stopping DW propagation at the edge of the electrode.

Single Domain SmCo5@Co Exchange-coupled Magnets Prepared from Core/shell Sm[Co(CN)6]·4H2O@GO Particles: A Novel Chemical Approach

Abstract: SmCo5 based magnets with smaller size and larger maximum energy product have been long desired in various fields such as renewable energy technology, electronic industry and aerospace science. However, conventional relatively rough synthetic strategies will lead to either diminished magnetic properties or irregular morphology, which hindered their wide applications. In this article, we present a facile chemical approach to prepare 200 nm single domain SmCo5@Co core/shell magnets with coercivity of 20.7 kOe and saturation magnetization of 82 emu/g. We found that the incorporation of GO sheets is responsible for the generation of the unique structure. The single domain SmCo5 core contributes to the large coercivity of the magnets and the exchange-coupled Co shell enhances the magnetization. This method can be further utilized in the synthesis other Sm-Co based exchange-coupled magnets.

The effect of cooling rate on the intensity of thermoremanent magnetization (TRM) acquired by assemblages of pseudo-single domain, multidomain and interacting single-domain grains

Abstract: SUMMARY Experiments designed to measure the absolute palaeointensity of the geomagnetic field generally do so by comparing the ancient thermoremanent magnetization (TRM) retained by an igneous rock with a new TRM imparted in the laboratory. One problem with this procedure is that the relative magnitudes of the ancient and laboratory TRMs may be influenced, not only by the external field intensities at the time the two coolings took place, but also by the rate at which the coolings themselves occurred. Here, we present new measurements of this ‘cooling rate effect’ obtained from treatments in the laboratory differing in cooling rate by a factor of ∼200. Synthetic samples containing sized ferrimagnetic grains were used in the experiments. Theoretical considerations and previous experiments have indicated the cooling rate effect to be dependent on domain state. Increases in TRM magnitude of more than 7percent per order of magnitude decrease in cooling rate have been reported for assemblages of non-interacting single-domain (SD) grains. Here, we focus on magnetite grains in the less well-studied pseudo-single domain (PSD) and multidomain (MD) states using a range of applied field intensities to impart the TRMs. For the first time, we also measure the cooling rate effect in grains of titanomagnetite that have been oxyexsolved so that they contain strongly interacting SD lamellae. In all cases, the cooling rate effect measured was in the same sense as alreadyobservedinidealmagneticallynon-interactingSDgrainsbutwasconsiderablyweaker. On average, the effect did not exceed ∼3percent increase in TRM per order of magnitude decrease in cooling rate and did not show any systematic dependence on appliedfield intensity. Insomesamples containing coarsergrains,thecoolingrateeffectwasnotdistinguishablefrom zero. The sense and magnitude of the cooling rate effect remain uncertain in truly MD grains as different studies have produced discrepant results. For the more practically relevant case of PSD and interacting SD grains, which commonly dominate the TRM in igneous rocks, however, it appears that we can be more confident in our assertions. The cooling rate effect in such materials is in the same sense as in non-interacting SD grains but smaller: a consequence of long-range ordering. In lavas and small intrusions containing these, it is unlikely to exceed 10percent. Although a correction should always be attempted, the results of palaeointensity studies based upon such samples will generally not be severely biased.

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.

Low-temperature magnetic properties of pelagic carbonates: Oxidation of biogenic magnetite and identification of magnetosome chains

Abstract: [1] Pelagic marine carbonates provide important records of past environmental change. We carried out detailed low-temperature magnetic measurements on biogenic magnetite-bearing sediments from the Southern Ocean (Ocean Drilling Program (ODP) Holes 738B, 738C, 689D, and 690C) and on samples containing whole magnetotactic bacteria cells. We document a range of low-temperature magnetic properties, including reversible humped low-temperature cycling (LTC) curves. Different degrees of magnetite oxidation are considered to be responsible for the observed variable shapes of LTC curves. A dipole spring mechanism in magnetosome chains is introduced to explain reversible LTC curves. This dipole spring mechanism is proposed to result from the uniaxial anisotropy that originates from the chain arrangement of biogenic magnetite, similar to published results for uniaxial stable single domain (SD) particles. The dipole spring mechanism reversibly restores the remanence during warming in LTC measurements. This supports a previous idea that remanence of magnetosome chains is completely reversible during LTC experiments. We suggest that this magnetic fingerprint is a diagnostic indicator for intact magnetosome chains, although the presence of isolated uniaxial stable SD particles and magnetically interacting particles can complicate this test. Magnetic measurements through the Eocene section of ODP Hole 738B reveal an interval with distinct magnetic properties that we interpret to originate from less oxidized biogenic magnetite and enrichment of a biogenic “hard” component. Co-occurrence of these two magnetic fingerprints during the late Eocene in the Southern Ocean indicates less oxic conditions, probably due to increased oceanic primary productivity and organic carbon burial.

Chiral solutes can seed the formation of enantiomorphic domains in a twist-bend nematic liquid crystal

Abstract: The twist-bend nematic, an enantiomorphic liquid-crystalline phase, exhibited by the structurally symmetric liquid-crystal dimer CB7CB is induced to form a single domain of uniform handedness, in the bulk, by the addition of the dopant chiral solute (S)-1-phenylethanol. Addition of a nonracemic (or scalemic) mixture of both R and S enantiomers of this solute produced equal volumes of P and M chiral domains for the twist-bend nematic phase. This seeding of the domains in an enantiomorphic nematic conglomerate is revealed using deuterium NMR spectroscopy.

Magnetic dipole and higher pole interaction on a square lattice.

Abstract: We have studied the magnetic interaction of circular magnetic islands with a dipole character on a square lattice. The square pattern consists of lithographically prepared polycrystalline PdFe islands, 150 nm in diameter and a periodicity of 300 nm. Below the Curie temperature at 260 K, the islands are in a single domain state with isotropic in-plane magnetization. Below 160 K, there is an onset of interisland interaction that leads to a change of the shape of the hysteresis, an increase of coercivity, and a development of in-plane anisotropy. Photoemission electron microscopy with circularly polarized incident x rays tuned to the ${L}_{3}$ edge of Fe confirms the increasing correlation of the magnetic islands and the formation of elongated chains, as predicted by Vedmedenko et al. [Phys. Rev. Lett. 95, 207202 (2005)] for contributions from pole interactions of higher order than the dipolar one. Neighboring chains are found to be irregularly oriented either parallel or antiparallel.

Magnetically-controllable zigzag structures as cell microgripper.

Abstract: We designed and fabricated a micro-scaled cell gripper based on two highly flexible magnetic zigzag structures that can be actuated by a magnetic field. Elongated single domain magnetic thin films with high magnetic shape anisotropy were deposited on the zigzag structures. By adjusting the external magnetic field we were able to control the torque applied on the magnetic films that was responsible for the actuation. We measured and discussed the displacement of the zigzag structures under different magnetic fields, and we observed a hysteresis characteristic in the actuation. Furthermore, we demonstrated the ability of gripping a single cell in water solution using the designed cell microgripper. The cell microgripper proposed in this study can provide important information for future biochip and biomedical applications.

Nucleation of reversed domain and pinning effect on domain wall motion in nanocomposite magnets

Abstract: The magnetization behaviors show a strong pinning effect on domain wall motion in optimally melt-spun Pr8Fe87B5 ribbons at room temperature. According to analysis, the coercivity is determined by the nucleation field of reversed domain, and the pinning effect, which results from the weak exchange coupling at interface, makes domain nucleation processes independent and leads to non-uniform magnetization reversals. At a temperature of 60 K, owing to the weak exchange coupling between soft-hard grains, magnetization reversal undergoes processes of spring domain nucleation in soft grains and irreversible domain nucleation in hard grains, and the pinning effect remains strong among hard grains.

Time-dependent conduction current in lithium niobate crystals with charged domain walls

Abstract: We present the experimental study of the increase and decrease of the abnormal conduction current appeared during polarization reversal at elevated temperatures (120–250 °C) in stoichiometric and MgO doped lithium niobate single crystals. It is shown that the conduction current is caused by existence of the through charged domain walls. The time dependence of the conduction current has been measured in low electric field immediately after partial switching. The maximal value of the conduction current in crystal with through charged domain walls is of 4–5 orders of magnitude higher than in initial single domain state. The activation energy is 1.1 eV.

Crystallographically textured self-biased W-type hexaferrites for X-band microwave applications

Abstract: We report the magnetic and structural properties of a series of W-type barium hexaferrites of composition BaZn2−xCoxFe16O27, where x = 0.15, 0.20, and 0.25. The anisotropy field of these barium ferrites (BaW) decreased with the substitution of divalent Co ions, while they maintained crystallographic c-axis texture. The measured anisotropy field was ∼10 kOe, and a hysteresis loop squareness Mr/Ms = 79% was obtained due to well-controlled grain size within the range of single domain scale. These two properties make the BaW suitable for applications in microwave devices at lower frequencies, such as self-biased circulators operated at X-band frequencies.

Measurement of magnetization using domain compressibility in CoFeB films with perpendicular anisotropy

Abstract: We present a method to map the saturation magnetization of soft ultrathin films with perpendicular anisotropy, and we illustrate it to assess the compositional dependence of the magnetization of CoFeB(1 nm)/MgO films. The method relies on the measurement of the dipolar repulsion of parallel domain walls that define a linear domain. The film magnetization is linked to the field compressibility of the domain. The method also yields the minimal distance between two walls before their merging, which sets a practical limit to the storage density in spintronic devices using domain walls as storage entities.

Packing fraction dependence of the coercivity and the energy product in nanowire based permanent magnets

Abstract: The maximum magnetic performance which can be expected from elongated single domain particle based permanent magnets is assessed. The results are derived using micromagnetic calculations to model the behavior of large bundles of aligned nanowires. In particular, we discuss the cases of Co and Fe nanowires and their packing fraction dependence of coercivity, which is the main limiting factor. We show that it is, in principle, feasible to achieve BHmax values close to 300 kJ/m3 in Co nanowires with a packing fraction p = 0.7 and close to 400 kJ/m3 at p = 0.85. The packing fraction limitations are essentially non-existing due to the intrinsic magnetocrystalline anisotropy of Co. On the other hand, if a low anisotropy material such as Fe could be produced in the form of fine, well crystallized wires it could yield a BHmax close to 200 kJ/m3 at an optimum p = 0.7. As the performance of iron nanowires is solely based on shape anisotropy it becomes coercivity limited above p = 0.35.

Dual wavelength magneto-optical imaging of magnetic thin films

Abstract: A magneto-optical imaging approach for the simultaneous imaging of multiple magnetization components is demonstrated. The method is applied to investigate complex magnetization reversal processes in single crystal iron and patterned amorphous magnetostrictive ferromagnetic structures. The use of a splitted optical illumination and observation path allows for the direct extraction of different complementary magnetic information. Real-time in-plane vector magnetization imaging reveals complicated domain arrangement processes in magnetostrictive films due to locally varying stress induced magnetic anisotropy. Magnetic domain features concealed by standard domain imaging techniques are directly exposed.