Showing papers on "Single domain published in 2011"

Simple models for dynamic hysteresis loop calculations of magnetic single-domain nanoparticles: Application to magnetic hyperthermia optimization

Abstract: To optimize the heating properties of magnetic nanoparticles (MNPs) in magnetic hyperthermia applications, it is necessary to calculate the area of their hysteresis loops in an alternating magnetic field. The separation between “relaxation losses” and “hysteresis losses” presented in several articles is artificial and criticized here. The three types of theories suitable for describing hysteresis loops of MNPs are presented and compared to numerical simulations: equilibrium functions, Stoner–Wohlfarth model based theories (SWMBTs), and a linear response theory (LRT) using the Neel–Brown relaxation time. The configuration where the easy axis of the MNPs is aligned with respect to the magnetic field and the configuration of a random orientation of the easy axis are both studied. Suitable formulas to calculate the hysteresis areas of major cycles are deduced from SWMBTs and from numerical simulations; the domain of validity of the analytical formula is explicitly studied. In the case of minor cycles, the hysteresis area calculations are based on the LRT. A perfect agreement between the LRT and numerical simulations of hysteresis loops is obtained. The domain of validity of the LRT is explicitly studied. Formulas are proposed to calculate the hysteresis area at low field that are valid for any anisotropy of the MNP. The magnetic field dependence of the area is studied using numerical simulations: it follows power laws with a large range of exponents. Then analytical expressions derived from the LRT and SWMBTs are used in their domains of validity for a theoretical study of magnetic hyperthermia. It is shown that LRT is only pertinent for MNPs with strong anisotropy and that SWMBTs should be used for weakly anisotropic MNPs. The optimum volume of MNPs for magnetic hyperthermia is derived as a function of material and experimental parameters. Formulas are proposed to allow to the calculation of the optimum volume for any anisotropy. The maximum achievable specific absorption rate (SAR) is calculated as a function of the MNP anisotropy. It is shown that an optimum anisotropy increases the SAR and reduces the detrimental effects of the size distribution of the MNPs. The optimum anisotropy is simple to calculate; it depends only on the magnetic field used in the hyperthermia experiments and the MNP magnetization. The theoretical optimum parameters are compared to those of several magnetic materials. A brief review of experimental results as well as a method to analyze them is proposed. This study helps in the determination of suitable and unsuitable materials for magnetic hyperthermia and provides accurate formulas to analyze experimental data. It is also aimed at providing a better understanding of magnetic hyperthermia to researchers working on this subject.

Magnetic properties of sedimentary greigite (Fe3S4): An update

Abstract: Greigite (Fe3S4) is an authigenic ferrimagnetic mineral that grows as a precursor to pyrite during early diagenetic sedimentary sulfate reduction. It can also grow at any time when dissolved iron and sulfide are available during diagenesis. Greigite is important in paleomagnetic, environmental, biological, biogeochemical, tectonic, and industrial processes. Much recent progress has been made in understanding its magnetic properties. Greigite is an inverse spinel and a collinear ferrimagnet with antiferromagnetic coupling between iron in octahedral and tetrahedral sites. The crystallographic c axis is the easy axis of magnetization, with magnetic properties dominated by magnetocrystalline anisotropy. Robust empirical estimates of the saturation magnetization, anisotropy constant, and exchange constant for greigite have been obtained recently for the first time, and the first robust estimate of the low-field magnetic susceptibility is reported here. The Curie temperature of greigite remains unknown but must exceed 350°C. Greigite lacks a low-temperature magnetic transition. On the basis of preliminary micromagnetic modeling, the size range for stable single domain behavior is 17–200 nm for cubic crystals and 17–500 nm for octahedral crystals. Gradual variation in magnetic properties is observed through the pseudo-single-domain size range. We systematically document the known magnetic properties of greigite (at high, ambient, and low temperatures and with alternating and direct fields) and illustrate how grain size variations affect magnetic properties. Recognition of this range of magnetic properties will aid identification and constrain interpretation of magnetic signals carried by greigite, which is increasingly proving to be environmentally important and responsible for complex paleomagnetic records, including widespread remagnetizations.

Magnetization Reversal by Electric-Field Decoupling of Magnetic and Ferroelectric Domain Walls in Multiferroic-Based Heterostructures

Abstract: We demonstrate that the magnetization of a ferromagnet in contact with an antiferromagnetic multiferroic (${\mathrm{LuMnO}}_{3}$) can be speedily reversed by electric-field pulsing, and the sign of the magnetic exchange bias can switch and recover isothermally. As ${\mathrm{LuMnO}}_{3}$ is not ferroelastic, our data conclusively show that this switching is not mediated by strain effects but is a unique electric-field driven decoupling of the ferroelectric and antiferromagnetic domain walls. Their distinct dynamics are essential for the observed magnetic switching.

Memory effect in magnetic nanowire arrays.

Abstract: www.advmat.de www.MaterialsViews.com Xiaoming Kou , Xin Fan , Randy K. Dumas , Qi Lu , Yaping Zhang , Hao Zhu , Xiaokai Zhang , Kai Liu , and John Q. Xiao* Magnetic materials are widely used for information storage because of their large capacity and low cost. [ 1 ] Storage medium technologies have evolved from analog recording with mag- netic tapes to high fidelity digital recording with magnetic hard disks. Nevertheless, both techniques use a magnetic medium consisting of magnetic particles, whose sizes have also evolved from micrometers in magnetic tapes to nanometers in modern hard disks. In analog recording, signals are converted into mag- netic fields which change the magnetization of a group of mag- netic particles (bit). The magnetization variations represent the stored information which can subsequently be read out. The magnetization, and therefore the stored information, could be changed by an external magnetic field and/or thermal effects. In digital recording, the bit magnetization can be aligned either left or right in parallel recording or up and down in perpen- dicular recording. [ 2 ] The information is stable as long as the medium is not subjected to a magnetic field higher than the coercivity, or a temperature higher than the superparamagnetic limit, of the constituent magnetic particles. In order to clearly distinguish one bit from another it is advantageous to minimize the dipolar interaction among magnetic particles, which is typi- cally achieved by creating boundaries between particles. Since the magnetic dipolar interaction is particularly pronounced in a collection of magnetic entities, such as magnetic particles and nanowires, it is scientifically interesting to question whether such a degree of freedom can be exploited in order to create additional memory functions. To answer this question, one needs a magnetic system with a sizable and preferably control- lable dipolar interaction. The magnetic nanowire array is an ideal system for this purpose. Magnetic nanowire arrays embedded in an insulating Al 2 O 3 matrix have been intensively studied. [ 3–12 ] When the magnetoc- rystalline anisotropy is negligible, the magnetization direction of the nanowires is preferably aligned along the length of the nanowire because of the shape anisotropy. When nanowires are very close to each other, dipolar interactions play a significant X. Kou, Dr. X. Fan, Q. Lu, Y. Zhang Prof. J. Q. Xiao Department of Physics and Astronomy University of Delaware Newark, DE, 19716, USA E-mail: jqx@udel.edu Dr. R. K. Dumas, Prof. K. Liu Department of Physics University of California Davis, CA, 95616, USA Dr. H. Zhu, Dr. X. Zhang Spectrum Magnetics LLC, 1210 First State Blvd, Wilmington, DE, 19804, USA DOI: 10.1002/adma.201003749 Adv. Mater. 2011, 23, 1393–1397 role in the magnetic behavior of the nanowire array, leading to rich physical phenomena and great application potentials. [ 7–12 ] Recently, it was demonstrated that the dipolar interaction among magnetic nanowires could provide zero field ferromagnetic res- onance (FMR) tunability, which has potential applications in a variety of microwave devices. A double FMR feature caused by the dipolar interaction in a magnetic nanowire array was also predicted [ 13 ] and verified. [ 14–17 ] In this manuscript, we demon- strate how dipolar interactions can induce an analog memory effect in magnetic nanowire arrays. Through this effect, the magnetic nanowire array has the ability to ‘memorize’ the maximum magnetic field that the array has been exposed to. A novel, low cost, and robust electromagnetic pulse detecting method is proposed based on this memory effect. Nanowire arrays of Ni 90 Fe 10 and Ni were synthesized by elec- trodeposition into anodized alumina templates. The diameter, center-to-center interpore distance, and length of the nanowires are 35 nm, 60 nm, and 30 μ m, respectively. Figure 1 a shows the hysteresis loop, with a coercivity of 1080 Oe, of a Ni 90 Fe 10 nanowire array with a magnetic field parallel to the wire (open squares). The loop with the field perpendicular to the wire is shown in the inset. Clearly, a well defined easy axis exists along the wire axis because of the dominant shape anisotropy. The memory effect was demonstrated using a vibrating sample magnetometer. The Ni 90 Fe 10 nanowire array was satu- rated along the wire prior to the measurement. The magnetic moment of the array was monitored as a series of magnetic field pulses were applied parallel to the nanowires. Figure 1b displays the series of magnetic pulses with different magni- tudes and directions. The corresponding change of the mag- netic moment is illustrated in Figure 1c. We find that the magnetic moment decreases monotonically as the magnitude of the negative pulses increases, while the moment remains the same after the positive pulses. This demonstrates that the maximum negative magnetic field can be recorded into the nanowire array. However, this is violated for the 800 and 900 Oe field pulses, and this discrepancy will be explained later. The result is also plotted in the magnetic moment verses applied field ( M – H ) graph, displayed in Figure 1d. Similar prop- erties are also observed in Ni nanowire arrays. This phenomenon is attributed to the dipolar interactions among the nanowires. Previously, using a theoretical model, two assumptions were proposed. [ 13 ] First, each nanowire is a single domain cylinder with a uniform magnetization pointing up or down parallel to the wire. The second assumption is that the number of nanowires with up magnetizations ( N ↑ ) and down magnetizations ( N ↓ ) is determined by the total magnetization M(H) , i.e. (N ↑ – N ↓ )/(N ↑ + N ↓ ) = M(H)/M s , where M s is the saturation magnetization. According to these assumptions, the dipolar field among the nanowires can be written as [ 13 ] © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com COMMUNICATION Memory Effect in Magnetic Nanowire Arrays

Critical property in relaxor-PbTiO3 single crystals - Shear piezoelectric response

Abstract: The shear piezoelectric behavior in relaxor-PbTiO3 (PT) single crystals is investigated in regard to crystal phase. High levels of shear piezoelectric activity, d15 or d24 >2000 pC N−1, has been observed for single domain rhombohedral (R), orthorhombic (O) and tetragonal (T) relaxor-PT crystals. The high piezoelectric response is attributed to a flattening of the Gibbs free energy at compositions proximate to the morphotropic phase boundaries, where the polarization rotation is easy with applying perpendicular electric field. The shear piezoelectric behavior of pervoskite ferroelectric crystals was discussed with respect to ferroelectric-ferroelectric phase transitions and dc bias field using phenomenological approach. The relationship between single domain shear piezoelectric response and piezoelectric activities in domain engineered configurations were given in this paper. From an application viewpoint, the temperature and ac field drive stability for shear piezoelectric responses are investigated. A temperature independent shear piezoelectric response (d24, in the range of −50°C to O-T phase transition temperature) is thermodynamically expected and experimentally confirmed in orthorhombic relaxor-PT crystals; relatively high ac field drive stability (5 kV cm−1) is obtained in manganese modified relaxor-PT crystals. For all thickness shear vibration modes, the mechanical quality factor Qs are less than 50, corresponding to the facilitated polarization rotation.

High-field magnetization of Ho 2 Fe 17

Abstract: The magnetization of a Ho${}_{2}$Fe${}_{17}$ single crystal has been measured along the principal crystallographic directions in pulsed magnetic fields up to 60 T. Stepwise discontinuities in the magnetization occur at 45 and 55 T along the [120] and [100] directions, respectively. The data allowed us to deduce the molecular field at the Ho site. As a cross check, the molecular field was determined as well from a magnetization measurement when the Ho${}_{2}$Fe${}_{17}$ single crystal was let rotate freely. Both values are in good agreement with each other.

Memory effect in magnetic nanowire arrays

Abstract: www.advmat.de www.MaterialsViews.com Xiaoming Kou , Xin Fan , Randy K. Dumas , Qi Lu , Yaping Zhang , Hao Zhu , Xiaokai Zhang , Kai Liu , and John Q. Xiao* Magnetic materials are widely used for information storage because of their large capacity and low cost. [ 1 ] Storage medium technologies have evolved from analog recording with mag- netic tapes to high fidelity digital recording with magnetic hard disks. Nevertheless, both techniques use a magnetic medium consisting of magnetic particles, whose sizes have also evolved from micrometers in magnetic tapes to nanometers in modern hard disks. In analog recording, signals are converted into mag- netic fields which change the magnetization of a group of mag- netic particles (bit). The magnetization variations represent the stored information which can subsequently be read out. The magnetization, and therefore the stored information, could be changed by an external magnetic field and/or thermal effects. In digital recording, the bit magnetization can be aligned either left or right in parallel recording or up and down in perpen- dicular recording. [ 2 ] The information is stable as long as the medium is not subjected to a magnetic field higher than the coercivity, or a temperature higher than the superparamagnetic limit, of the constituent magnetic particles. In order to clearly distinguish one bit from another it is advantageous to minimize the dipolar interaction among magnetic particles, which is typi- cally achieved by creating boundaries between particles. Since the magnetic dipolar interaction is particularly pronounced in a collection of magnetic entities, such as magnetic particles and nanowires, it is scientifically interesting to question whether such a degree of freedom can be exploited in order to create additional memory functions. To answer this question, one needs a magnetic system with a sizable and preferably control- lable dipolar interaction. The magnetic nanowire array is an ideal system for this purpose. Magnetic nanowire arrays embedded in an insulating Al 2 O 3 matrix have been intensively studied. [ 3–12 ] When the magnetoc- rystalline anisotropy is negligible, the magnetization direction of the nanowires is preferably aligned along the length of the nanowire because of the shape anisotropy. When nanowires are very close to each other, dipolar interactions play a significant X. Kou, Dr. X. Fan, Q. Lu, Y. Zhang Prof. J. Q. Xiao Department of Physics and Astronomy University of Delaware Newark, DE, 19716, USA E-mail: jqx@udel.edu Dr. R. K. Dumas, Prof. K. Liu Department of Physics University of California Davis, CA, 95616, USA Dr. H. Zhu, Dr. X. Zhang Spectrum Magnetics LLC, 1210 First State Blvd, Wilmington, DE, 19804, USA DOI: 10.1002/adma.201003749 Adv. Mater. 2011, 23, 1393–1397 role in the magnetic behavior of the nanowire array, leading to rich physical phenomena and great application potentials. [ 7–12 ] Recently, it was demonstrated that the dipolar interaction among magnetic nanowires could provide zero field ferromagnetic res- onance (FMR) tunability, which has potential applications in a variety of microwave devices. A double FMR feature caused by the dipolar interaction in a magnetic nanowire array was also predicted [ 13 ] and verified. [ 14–17 ] In this manuscript, we demon- strate how dipolar interactions can induce an analog memory effect in magnetic nanowire arrays. Through this effect, the magnetic nanowire array has the ability to ‘memorize’ the maximum magnetic field that the array has been exposed to. A novel, low cost, and robust electromagnetic pulse detecting method is proposed based on this memory effect. Nanowire arrays of Ni 90 Fe 10 and Ni were synthesized by elec- trodeposition into anodized alumina templates. The diameter, center-to-center interpore distance, and length of the nanowires are 35 nm, 60 nm, and 30 μ m, respectively. Figure 1 a shows the hysteresis loop, with a coercivity of 1080 Oe, of a Ni 90 Fe 10 nanowire array with a magnetic field parallel to the wire (open squares). The loop with the field perpendicular to the wire is shown in the inset. Clearly, a well defined easy axis exists along the wire axis because of the dominant shape anisotropy. The memory effect was demonstrated using a vibrating sample magnetometer. The Ni 90 Fe 10 nanowire array was satu- rated along the wire prior to the measurement. The magnetic moment of the array was monitored as a series of magnetic field pulses were applied parallel to the nanowires. Figure 1b displays the series of magnetic pulses with different magni- tudes and directions. The corresponding change of the mag- netic moment is illustrated in Figure 1c. We find that the magnetic moment decreases monotonically as the magnitude of the negative pulses increases, while the moment remains the same after the positive pulses. This demonstrates that the maximum negative magnetic field can be recorded into the nanowire array. However, this is violated for the 800 and 900 Oe field pulses, and this discrepancy will be explained later. The result is also plotted in the magnetic moment verses applied field ( M – H ) graph, displayed in Figure 1d. Similar prop- erties are also observed in Ni nanowire arrays. This phenomenon is attributed to the dipolar interactions among the nanowires. Previously, using a theoretical model, two assumptions were proposed. [ 13 ] First, each nanowire is a single domain cylinder with a uniform magnetization pointing up or down parallel to the wire. The second assumption is that the number of nanowires with up magnetizations ( N ↑ ) and down magnetizations ( N ↓ ) is determined by the total magnetization M(H) , i.e. (N ↑ – N ↓ )/(N ↑ + N ↓ ) = M(H)/M s , where M s is the saturation magnetization. According to these assumptions, the dipolar field among the nanowires can be written as [ 13 ] © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com COMMUNICATION Memory Effect in Magnetic Nanowire Arrays

Imaging and control of surface magnetization domains in a magnetoelectric antiferromagnet.

Abstract: We report the direct observation of surface magnetization domains of the magnetoelectric Cr2O3 using photoemission electron microscopy with magnetic circular dichroism contrast and magnetic force microscopy. The domain pattern is strongly affected by the applied electric field conditions. Zero-field cooling results in an equal representation of the two domain types, while electric-field cooling selects one dominant domain type. These observations confirm the existence of surface magnetization, required by symmetry in magnetoelectric antiferromagnets.

Experimental evidence for an angular dependent transition of magnetization reversal modes in magnetic nanotubes

Abstract: We report on the experimental and theoretical investigation of the magnetization reversal in magnetic nanotubes that have been synthesized by a combination of glancing angle and atomic layer deposition. Using superconducting quantum interference device magnetometry the angular dependence of the coercive fields is determined and reveals a nonmonotonic behavior. Analytical calculations predict the crossover between two magnetization reversal modes, namely, the movement of different types of domain boundaries (vortex wall and transverse wall). This transition, already known in the geometrical dependences of the magnetization reversal in various nanotubes, is found within one type of tube in the angular dependence and is experimentally confirmed in this work.

Magnetization reversal assisted by the inverse piezoelectric effect in Co-Fe-B / ferroelectric multilayers

Abstract: We investigate magnetization reversal assisted by electric fields in a Co${}_{40}$Fe${}_{40}$B${}_{20}/$Co$/$Pd multilayer with soft perpendicular anisotropy deposited onto an epitaxial piezoelectric layer. Both the nucleation and depinning magnetic fields show a strong reduction of up to 30$%$ under the application of an electric field. These results show good prospects for assisting the switching processes in ultra-low-power spintronic devices.

Rotational diffusion of magnetic nickel nanorods in colloidal dispersions.

Abstract: Colloidal dispersions of Ni nanorods were synthesized by pulsed electrodeposition of Ni into nanoporous aluminum oxide layers followed by dissolution of the templates. Geometrical characterization of the nanorods by transmission electron microscopy and scanning electron microscopy allowed us to determine the average length (100-250 nm) and diameter (20-40 nm) of the rods and to estimate the thickness of the polyvinylpyrrolidone surfactant layer. Due to their acicular shape, nanorods of the given size are uniaxial ferromagnetic single domain particles and exhibit a distinct anisotropic polarizability. These two characteristic properties are the physical basis for magnetic field-dependent optical transmission and allow us to investigate the rotational diffusion of the nanorods in liquid dispersion. In the present study, we employed AC magnetization measurements, dynamical light scattering and optical transmission measurements in a rotating magnetic field to determine the rotational diffusion coefficient. The results from all three methods were consistent and agree with theory within a factor of 2.

Magnetic bacterial protein Mms6 controls morphology, crystallinity and magnetism of cobalt-doped magnetite nanoparticles in vitro

Abstract: Magnetic nanoparticles (MNPs) are in high demand within biomedical and nanotechnological industries. Size, shape, material and crystal quality directly affect the particle's properties, namely their magnetic characteristics, and must be tuned and controlled to meet the specification of the application. A key challenge is to refine synthetic methods to tailor the MNP properties with precision, but using cheap, high-yield, industrially robust and environmentally friendly methods. In this study we compare simple high-yield precipitation methods of producing cobalt-doped magnetite MNPs. We explore the variation of magnetic coercivity and saturation with increasing Co-doping from 0–15% in magnetite MNPs, which increases coercivity from 5–62 mT, but decreases saturation from 91–28 emu g−1. An optimum of 6% was further investigated as this produced the greatest increase in coercivity to 34 mT with a relatively small reduction in saturation magnetisation to 79 emu g−1. The methods compared are refined with the addition of the recombinant biomineralisation protein Mms6 from a magnetic bacterium, as this has been shown to help control magnetite MNP morphology and grainsize distribution in vitro. Similar control is seen here over our Co-doped magnetite synthesis. Mms6 increases the size and decreases the size distribution of room temperature co-precipitated particles from 11.7 nm to 31.7 nm. The affinity tagged protein his6Mms6 also controls the size (23.2 nm) but less effectively than Mms6. Therefore the Mms6 mediated Co-doped MNP particles are found to be single domain and thus give very clear, square magnetic hysteresis with a coercivity of 48 mT at 10 K. Hysteresis of the smaller particles (Co-doped MNP with no protein and with his-tagged protein) clearly shows both superparamagnetic and single-domain magnetic behaviours. Powder X-ray diffraction shows that both the addition of Mms6 and cobalt increases the crystal quality of the MNP. Thus Mms6 protein mediated room temperature co-precipitation offers an environmentally friendly, industrially robust route towards tailored, uniform, single-domain, high-quality Co-doped magnetite MNPs.

Simulation of ferromagnetic resonance spectra of linear chains of magnetite nanocrystals

Abstract: Ensembles of linear chains of stable single domain magnetite crystals, as found in magnetotactic bacteria, exhibit a distinctly asymmetric ferromagnetic resonance (FMR) signal, with a pronounced high-field minimum and two or three low-field maxima in the derivative spectrum. To identify the microscopic origin of these traits, we have simulated FMR spectra of dilute suspensions of linear chains oriented randomly in space by modeling the chain as a Stoner−Wohlfarth-type rotation ellipsoid whose long axis coincides with an easy [111] axis of the cubic magnetocrystalline anisotropy system. The validity of the model is examined by comparing the results with explicit calculations of the interactions among the particles in the chain. The single ellipsoid model reproduces the experimentally observed FMR traits and can be related to the explicit chain model by adjusting the contribution to the uniaxial anisotropy along the chain axis to account for the magnetostatic interactions. Finally, we provide a practical ap...

Thermomagnetic conversion efficiencies for ferromagnetic materials

Abstract: The theory of thermomagnetic generation is reviewed and an efficiency analysis using experimentally measured magneto-thermal properties of 3d transitional and 4f rare earth ferromagnetic elements is presented in this study. While theoretical results suggest that 55% of Carnot efficiency is possible, experimental data indicate values smaller than 25% of Carnot efficiency unless large magnetic field (e.g., Ha ∼ 80 kOe) is applied. For smaller magnetic fields representative of NdFeB permanent magnets (e.g., Ha = 3 kOe), the largest efficiencies are obtained for operating ferromagnetic materials over a smaller temperature difference (ΔT = 5 K). Furthermore, single crystal materials are found to have superior efficiencies, as do elements that undergo an order-to-order phase transition. Both of these later results relate to increased magnetization changes over a given ΔT. These results are subsequently used to postulate that a single domain structure will produce larger efficiencies due to the higher magnetization present over a wide range of magnetic fields when compared to multi-domain materials. Calculations for a Gd single domain suggest efficiencies on the order of 30% are possible, representing a threefold increase from multi-domain Gd at relatively small magnetic fields.

Influence of adsorbed surface layer on domain growth in the field produced by conductive tip of scanning probe microscope in lithium niobate

Abstract: The results of investigation of the single domain growth in electric field applied by conductive tip of the scanning probe microscope in thin plates of lithium niobate (LiNbO3) crystals doped with MgO after various surface preparations and at various ambient conditions are presented. It has been shown that the sizes of the produced domain can exceed by several orders of magnitude the value of the tip curvature radius. The observed effect has been explained taking into account the existence of the conductive adsorbed surface layer in all experimental conditions. We have demonstrated that the domain growth decelerates with decreasing of the layer conductivity. The existence of the conductive adsorbed surface layers drastically changes the spatial distribution of electric field. In addition to strongly localized electric field, just in the vicinity of the tip there exists the field component remaining homogeneous over the distance exceeding the radius of any experimentally produced domain. The crucial role of the conductive properties of the adsorbed surface layers on the screening of the depolarization field has been revealed. Within proposed approach the domain growth is controlled by the current in the external circuit including the surface layer with low conductivity. The proposed model allows us to explain time and field dependences of the domain size for various types of surface treatment. V C 2011 American Institute of Physics. [doi:10.1063/1.3624798]

Magnetic field insensitivity of magnetic domain wall velocity induced by electrical current in Co/Ni nanowire

Abstract: We have investigated the velocity of magnetic domain wall (DW) motion induced by electric currents in a Co/Ni nanowire with a perpendicular magnetic anisotropy. The DW velocity increased as current density increased and the maximum velocity of 60 m/s was observed. Furthermore, the DW velocity was found to be almost independent of external perpendicular magnetic fields in the range of −50 to +50 Oe. The mechanism of the observed field insensitivity of the current induced DW motion is also discussed.

Perpendicular magnetic recording medium with single domain exchange-coupled soft magnetic underlayer and device incorporating same

Abstract: A perpendicular magnetic recording disk having a single domain exchange-coupled laminated soft magnetic underlayer (SUL) is disclosed. The SUL is a combination synthetic anti-parallel coupled SUL structure, which has the product (Mst) of saturation magnetization and film thickness of the middle ferromagnetic layer less than the sum of the Mst of the bottom and top ferromagnetic layers. Subjected to a post radial field reset process, this SUL provides single domain state. Moreover, both robustness of stray fields and low permeability are obtained while keeping excellent corrosion resistance and cost effective manufacturability.

Electromechanical properties and anisotropy of single- and multi-domain 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystals

Abstract: Complete sets of elastic, piezoelectric, and dielectric constants of 0.72Pb(Mg1/3Nb2/3)O3-0.28PbTiO3 single crystal poled along [111]c (single domain) as well as non-polar axes [001]c and [011]c (multidomain) have been measured under natural conditions. These data allowed us to evaluate accurately the extrinsic contributions to the superior piezoelectric properties. Very large extrinsic contributions to the unusual anisotropies in multidomain crystals are confirmed. We found that the instability of domain structures is the origin of the low mechanical quality factor Q for the multidomain relaxor-based ferroelectric single crystals. Our results can provide useful guidance in future design of domain engineered materials.

Multiscale magnetic study of Ni(111) and graphene on Ni(111)

Abstract: We have investigated the magnetism of the bare and graphene-covered (111) surface of a Ni single crystal employing three different magnetic imaging techniques and ab initio calculations, covering length scales from the nanometer regime up to several millimeters. With low-temperature spin-polarized scanning tunneling microscopy we find domain walls with widths of 60--90 nm, which can be moved by small perpendicular magnetic fields. Spin contrast is also achieved on the graphene-covered surface, which means that the electron density in the vacuum above graphene is substantially spin polarized. In accordance with our ab initio calculations we find an enhanced atomic corrugation with respect to the bare surface, due to the presence of the carbon ${p}_{z}$ orbitals and as a result of the quenching of Ni surface states. The latter also leads to an inversion of spin polarization with respect to the pristine surface. Room temperature Kerr microscopy shows a stripelike domain pattern with stripe widths of 3--6 $\ensuremath{\mu}$m. Applying in-plane-fields, domain walls start to move at about 13 mT and a single domain state is achieved at 140 mT. Via scanning electron microscopy with polarization analysis a second type of modulation within the stripes is found and identified as 330 nm wide V lines. Qualitatively, the observed surface domain pattern originates from bulk domains and their quasidomain branching is driven by stray field reduction.

Magnetic properties of silica coated spindle-type hematite particles

Abstract: Magnetic properties of particles are generally determined from randomly oriented ensembles and the influence of the particle orientation on the magnetic response is neglected. Here, we report on the magnetic characterization of anisotropic spindle-type hematite particles. The easy axis of magnetization is within the basal plane of hematite, which is oriented perpendicular to the spindle axis. Two standard synthesis routes are compared and the effects of silica coating and particle orientation on the magnetic properties are investigated. Depending on the synthesis route we find fundamentally different magnetic behavior compatible with either single domain particles or superparamagnetic sub-units. Furthermore, we show that silica coating reduces the mean blocking temperature to nearly room temperature. The mechanical stress induced by the silica coating appears to reduce the magnetic coupling between the sub-units.

Nanostructured MnGa films on Si/SiO2 with 20.5 kOe room temperature coercivity

Abstract: Nanostructured Mn67Ga33 films exhibiting high room temperature coercivity (HC = 20.5 kOe) have been prepared by sputtering onto thermally oxidized Si substrates. Both the morphology and the coercivity of the films can be tuned by varying the growth parameters. The low deposition rate film, sputtered at a reduced power and working pressure, demonstrates a discontinuous island-like growth and the highest HC. The large HC is linked to the presence of the high anisotropy DO22 Mn3Ga phase and the single domain character of the exchange isolated, dipolar interacting, single crystal islands.

Multi-level relaxation model for describing the Mössbauer spectra of single-domain particles in the presence of quadrupolar hyperfine interaction.

Abstract: A multi-level stochastic model taking into account the magnetic anisotropy, precession and diffusion of the uniform magnetization of single-domain particles is developed in order to describe the Mossbauer absorption spectra of an ensemble of magnetic nanoparticles in the presence of quadrupolar hyperfine interaction with an arbitrary orientation of its principal axes. This model allows one to take into account physical mechanisms for forming the hyperfine structure in a real situation and can be easily realized even on a personal computer. In particular, now one can numerically describe qualitative features of temperature evolution of the Mossbauer spectral shape from a ‘symmetric’ magnetic sextet to a quadrupolar doublet of lines, which has been observed in a large number of experimental spectra of 57Fe nuclei in magnetic nanoparticles for almost half a century.

Characterization of single domain Pb(In0.5Nb0.5)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 crystals with monoclinic phase

Abstract: Ternary single crystals Pb(In0.5Nb0.5)O3-Pb(Mg1/3Nb2/3)O3-PbTiO3 with monoclinic phase poled along [011] direction show single domain state (1O state) with macroscopic mm2 symmetry. The complete set of material constants was determined using the combination of impedance and ultrasonic methods. The thickness shear vibrations in the single domain state give the highest piezoelectric coefficients, with values being on the order of d15 = 4550 pC/N and d24 = 4100 pC/N. The rotated value of d33* along [001], calculated using the single domain data, is in good agreement with the [001] poled multidomain crystal, being 1790 pC/N versus 2010 pC/N, exhibiting the high longitudinal piezoelectric in multidomain comes from the high shear properties in single domain state, with the extrinsic contribution about ∼11%, induced by the phase boundary motion.

Enhanced current-induced domain wall motion by tuning perpendicular magnetic anisotropy

Abstract: The effect of perpendicular magnetic anisotropy (PMA) on current-induced domain wall (DW) motion is investigated by micromagnetic simulations. The critical current density JC to drive DWs into periodic transformation and continuous motion by adiabatic spin transfer torque decreases with increasing PMA. Also, with optimized PMA that almost exactly compensates the demagnetizing field, the adiabatic displacement of DWs driven by currents less than JC is strongly enhanced. Since PMA can be controlled easily in magnetic multilayer films, this technique of enhancing current-induced DW motion may be practical for device applications.

Spin-torque oscillator for microwave assisted magnetic recording

Abstract: In a conventional type magnetic head that performs microwave assisted recording, since a difference in a demagnetizing field between an end and the center of a field generation layer (FGL) grows larger when saturation magnetization of the FGL grows larger, the FGL that generates a microwave is not oscillated in a state of a single domain. Then, a spin-torque oscillator according to the present invention used for a magnetic head for microwave assisted recording is provided with at least one fixed layer, one non-magnetic intermediate layer and one alternating-current magnetic field generation layer respectively and is provided with a structure where saturation magnetization at ends of a film except an end in a direction from an air bearing surface to a surface opposite to it is made smaller than saturation magnetization in the center of the film of the alternating-current magnetic field generation layer.