Showing papers on "Single domain published in 2019"

Realisation of a frustrated 3D magnetic nanowire lattice

Abstract: Patterning nanomagnets in three-dimensions presents a new paradigm in condensed matter physics and allows access to a plethora of fundamental phenomena including robust spin textures, magnetic metamaterials that are home to defects carrying magnetic charge and ultrahigh density devices that store information in three dimensions. However, the nanostructuring of functional magnetic materials into complex three-dimensional geometries has thus far proven to be a formidable challenge. Here we show magnetic nanowires can be arranged into 3D frustrated magnetic nanowire lattices by using a combination of 3D polymer nanoprinting and metallic deposition. The fabricated nanowires are single domain and they switch via nucleation and propagation of domain walls. Deep nanoscale magnetic imaging and finite element simulations elucidate the spin texture present on the 3D nanostructured lattice. Our study demonstrates a generic platform for the production of 3D nanostructured magnetic materials allowing the realisation of racetrack memory devices and 3D nanostructured systems that mimic bulk frustrated crystals.

Effect of Nb3+ Substitution on the Structural, Magnetic, and Optical Properties of Co0.5Ni0.5Fe2O4 Nanoparticles

Abstract: Co0.5Ni0.5NbxFe2−xO4 (0.00 ≤ x ≤ 0.10) nanoparticles (NPs) were prepared using the hydrothermal approach. The X-ray powder diffraction (XRD) pattern confirmed the formation of single-phase spinel ferrite. The crystallite size was found to range from 18 to 26 nm. The lattice parameters were found to increase with greater Niobium ion (Nb3+) concentration, caused by the variance in the ionic radii between the Nb3+ and Fe3+. Fourier transform infrared analysis also proved the existence of the spinal ferrite phase. The percent diffuse reflectance (%DR) analysis showed that the value of the band gap increased with growing Nb3+ content. Scanning electron microscopy and transmission electron microscopy revealed the cubic morphology. The magnetization analyses at both room (300 K, RT) and low (10 K) temperatures exhibited their ferromagnetic nature. The results showed that the Nb3+ substitution affected the magnetization data. We found that Saturation magnetization (Ms), Remanence (Mr), and the Magnetic moment ( n B ) decreased with increasing Nb3+. The squareness ratio (SQR) values at RT were found to be smaller than 0.5, which postulate a single domain nature with uniaxial anisotropy for all produced ferrites. However, different samples exhibited SQRs within 0.70 to 0.85 at 10 K, which suggests a magnetic multi-domain with cubic anisotropy at a low temperature. The obtained magnetic results were investigated in detail in relation to the structural and microstructural properties.

Domain State Diagnosis in Rock Magnetism: Evaluation of Potential Alternatives to the Day Diagram

Abstract: Author(s): Roberts, Andrew P; Hu, Pengxiang; Harrison, Richard J; Heslop, David; Muxworthy, Adrian R; Oda, Hirokuni; Sato, Tetsuro; Tauxe, Lisa; Zhao, Xiang

Study of spinel ferrites with addition of small amount of metallic elements

Abstract: Spinel ferrites MPr0.1Fe1.9O4 (M = Mn, Cu, Mg) were prepared by sol-gel technique. X-ray Diffraction (XRD) showed that along with spinel phase, secondary phase (PrFeO3) appeared for all composition. Scanning electron microscopy showed the inhomogeneous grain size. Temperature dependence normalized AC susceptibility and Curie temperature of spinel ferrites revealed that in all the samples transitions from multi domain (MD) to single domain (SD) occurred. Magnetic properties exhibited the soft nature of all the samples at room temperature. Temperature dependent resistivity of all the samples increased as the temperature was enhanced, representing the semiconducting behavior. Dielectric constant and complex dielectric constants were determined at the high-frequency range of 1 MHz to 3 G Hz. Impedance analysis clearly demonstrated the role of grains and grain boundary in the spinel ferrites. Cole-cole plots of all the samples showed only one semi-circle at high-frequency. All the samples elaborated the good picture of Koop’s theory and Maxwell-Wagner model. The low value of coercivity and high magnetization of Mn-based spinel ferrites made it suitable for hyperthermia applications.

Antiparallel exchange biased multilayers for low magnetic noise magnetic field sensors

Abstract: High sensitivity magnetoelectric (ME) thin film composite sensors, which enable the detection of picotesla magnetic fields, are improved in terms of magnetic noise performance by the elimination of magnetic domain (MD) activity. Using an antiparallel (AP) exchange biasing scheme, suppression of magnetic noise is obtained. Postsetting of AP biased ferromagnetic/antiferromagnetic multilayers is accomplished by magnetic field free annealing with in-situ MD control. Overcoming the shape and demagnetization effects, stable single MD configurations in the magnetic sensing layers of magnetic multilayers are formed. Magnetic noise contributions are undetectable. The achieved single domain field stability opens the path to ultralow noise ME sensor applications. The demonstrated AP biasing scheme is applicable to other magnetic layer-based field sensing devices.High sensitivity magnetoelectric (ME) thin film composite sensors, which enable the detection of picotesla magnetic fields, are improved in terms of magnetic noise performance by the elimination of magnetic domain (MD) activity. Using an antiparallel (AP) exchange biasing scheme, suppression of magnetic noise is obtained. Postsetting of AP biased ferromagnetic/antiferromagnetic multilayers is accomplished by magnetic field free annealing with in-situ MD control. Overcoming the shape and demagnetization effects, stable single MD configurations in the magnetic sensing layers of magnetic multilayers are formed. Magnetic noise contributions are undetectable. The achieved single domain field stability opens the path to ultralow noise ME sensor applications. The demonstrated AP biasing scheme is applicable to other magnetic layer-based field sensing devices.

Single Domain 10 nm Ferromagnetism Imprinted on Superparamagnetic Nanoparticles Using Chiral Molecules

Abstract: The rapid growth in demand for data and the emerging applications of Big Data require the increase of memory capacity. Magnetic memory devices are among the leading technologies for meeting this demand; however, they rely on the use of ferromagnets that creates size reduction limitations and poses complex materials requirements. Usually magnetic memory sizes are limited to 30-50 nm. Reducing the size even further, to the ≈10-20 nm scale, destabilizes the magnetization and its magnetic orientation becomes susceptible to thermal fluctuations and stray magnetic fields. In the present work, it is shown that 10 nm single domain ferromagnetism can be achieved. Using asymmetric adsorption of chiral molecules, superparamagnetic iron oxide nanoparticles become ferromagnetic with an average coercive field of ≈80 Oe. The asymmetric adsorption of molecules stabilizes the magnetization direction at room temperature and the orientation is found to depend on the handedness of the chiral molecules. These studies point to a novel method for the miniaturization of ferromagnets (down to ≈10 nm) using established synthetic protocols.

Magnetic Domains in Thin Ferromagnetic Films with Strong Perpendicular Anisotropy

Abstract: We investigate the scaling of the ground state energy and optimal domain patterns in thin ferromagnetic films with strong uniaxial anisotropy and the easy axis perpendicular to the film plane. Starting from the full three-dimensional micromagnetic model, we identify the critical scaling for which the transition from single domain to multidomain ground states such as bubble or maze patterns occurs as the film thickness goes to zero and the lateral extent goes to infinity. Furthermore, we analyze the asymptotic behavior of the energy in these two asymptotic regimes. In the single domain regime, the energy Γ-converges towards a much simpler two-dimensional and local model. In the multidomain regime, we derive the scaling of the minimal energy and deduce a scaling law for the typical domain size.

Dielectric and magnetic properties of rare-earth-doped cobalt ferrites and their first-order reversal curve analysis

Abstract: The present study focuses on the effects of rare-earth (R) ion substitutions namely Y, Sm and Pr ions on the structural, magnetic and electrical properties of cobalt ferrite (CFO) and the nature of their magnetic domains using the first-order reversal curve (FORC) analysis. These samples were synthesized using the sol–gel auto-combustion method. The powder X-ray diffraction analysis reveals that the Pr-substituted CFO crystallizes in a single-phase spinel structure, while a few traces of RFeO3 appear as minor phases in Sm- and Y-doped CFO. The scanning electron micrographs show that the substitution of R3+ ions causes a considerable reduction in the grain size. The studies on magnetic properties reveal that the saturation magnetization of R-substituted CFO decreased. The FORC analysis was done to know the domain state of magnetization and the nature of magnetic interactions among the grains. All the FORC diagrams depict a single peak with a single contour suggesting that all the synthesized samples have single domain particles and are comprised only of mono-magnetic phase. The electrical properties of the R-doped CFO compounds exhibit high values of dielectric constant at room temperature with the highest value for Pr-doped CFO. The activation energy determined using electrical properties is found to decrease with R doping.

The influence of the internal domain wall structure on spin wave band structure in periodic magnetic stripe domain patterns

Abstract: The magnetization dynamics in periodic magnetic stripe domain patterns in thin ferromagnetic films is summarized. First, a brief theoretical background of magnetization dynamics and spin wave dynamics in the presence of a single domain wall for various configuration of magnetic domains (in-plane and out-of-plane) and domain walls (Bloch- and Neel-type domain walls) is introduced. Then, spin wave dynamics in periodic stripe magnetic domain pattern is studied on an example of a multilayer system composed of Co/Pd. The considered magnetization configuration is non-collinear across both the domain walls and the film thickness. It has the form of a “corkscrew”-like structure that consists of a Bloch wall in the film's center with two Neel caps at the film's surfaces. All domain walls have the same polarity. The Brillouin light scattering measurements were performed to study magnetization dynamics experimentally, and the results were interpreted with the use of micromagnetic simulations. The periodic arrangement of the magnetization increases the number of spin wave bands similarly like a one-dimensional magnonic crystal. The properties of the dynamical excitation related to translational motion of the domain wall (zero-frequency Goldstone modes) are shown. Further, the dynamics of the magnetization configurations with the same and alternating polarities of the neighboring walls are compared. The magnetization dynamics for the propagation along the domain walls direction is analyzed, as well. Here, the interaction between the walls and nonreciprocal properties result in the formation of unidirectional channels, where waves travel in every second wall in the opposite direction.

Periodic boundary conditions for the simulation of 3D domain patterns in tetragonal ferroelectric material

Abstract: Periodic domain patterns in tetragonal ferroelectrics are explored using a phase field model calibrated for barium titanate. In this context, we discuss the standard periodic boundary condition and introduce the concept of reverse periodic boundary conditions. Both concepts allow the assembly of cubic cells in accordance with mechanical and electrical conditions. However, application of the reverse periodic boundary condition is due to an increased size of the RVE and enforces more complex structures compared to the standard condition. This may be of particular interest for other multiphysics simulations. Additionally, we formulate mechanical side conditions with minimal spherical (hydrostatic) stress, or conditions with controlled average strain. It is found that in sufficiently small periodic cells, only a uniform single domain, or the simplest stripe domains constitute equilibrium states. However, once the periodic cells are of order 20 domain wall widths in size, more complex, 3-dimensional patterns emerge. Some of these patterns are known from prior studies, but we also identify other domain patterns with long, ribbon-like domains threaded through them and some vortex-like structures.

Phase evolution and temperature dependent magnetic properties of nanocrystalline barium hexaferrite

Abstract: Barium hexaferrite nanoparticles were prepared by the sol–gel auto combustion method and were annealed at different temperatures in air for 4 h. Structural properties showed the evolution of nanocrystalline Ba-hexaferrite with the increase in annealing temperature. The lattice constants did not show any systematic variation and the grain size increased with the increase in annealing temperature. The magnetization value increased with the increase in annealing temperature and the highest values of 58.9 and 82.7 emu/g were observed at 300 and 60 K respectively for the sample annealed at TA = 1200 °C. The highest coercivity of 5250 Oe was observed at 300 K for the nanoparticle sample annealed at 950 °C with critical single domain size of 48 nm. The coervity decreased with the decrease in measurement temperature from 300 to 60 K. The observed magnetic behavior can be understood on the basis of grain growth, the exchange interaction between the Fe3+ ions sitting at different lattice sites in the magnetoplumbite structure and inter-granular interactions in these nanoparticle samples.

Ca 2+ /Mg 2+ co-substituted strontium nanohexaferrites: magnetic investigation and Mossbauer analysis

Abstract: We report a comprehensive evaluation of the microstructures, magnetic, and hyperfine traits of Ca2+/Mg2+ co-substituted strontium nanohexaferrites (SHFs) of chemical compositions Sr1–2xCaxMgxFe12O19 (x ≤ 0.1). These SHFs were prepared using sol–gel unified auto-combustion strategy and characterized by diverse analytical measurements to determine the effects of Ca2+/Mg2+ co-substitution on the aforesaid properties. XRD and high resolution transmission electron microscpe (HRTEM) analyses confirmed the nucleation of HF nanocrystallites. UV-visible diffuse reflectance (DR–UV–Vis) spectra were used to evaluate the optical band gap of the proposed SHFs, which was reduced with the increase in co-substitution contents. The 57Fe Mossbauer spectra was exploited to assess the quadrupole splitting, spectral line-width variation, isomer shift, distribution of cations (Ca2+ and Mg2+), and hyperfine magnetic fields. Measured magnetic parameters such as saturation magnetization, remanence, squareness ratio (SQR), coercivity, and magnetic moment were significantly improved due to the Ca2+/Mg2+ co-substitution. All the prepared SHFs revealed ferrimagnetic nature both at 10 and 300 K. Achieved SQR values ≈0.5 of the studied SHFs clearly disclosed their single domain character possessing uniaxial anisotropy.

Magnetic properties of the finite-length biatomic chains in the framework of the single domain-wall approximation

Abstract: A simple analytical method for study the magnetic properties of the finite-length biatomic chains in the framework of Heisenberg model with uniaxial magnetic anisotropy is proposed. The method allows to estimate the reversal time of the magnetization of ferromagnetic and antiferromagnetic biatomic chains. Three cases are considered: the spontaneous remagnetization, the remagnetization under the interaction with a scanning tunneling microscope, and the remagnetization in the external magnetic field. The applicability limits of the method are discussed. Within its limits of applicability the method gives results which are in perfect agreement with the results of the kinetic Monte Carlo simulations. As the examples, two physical systems are considered: biatomic Fe chains on Cu2N/Cu(001) surface and biatomic Co chains on Pt(997) surface. The presented method is incomparably less time-consuming than the standard kMC simulations, especially in the cases of low temperatures or long chains.

Scaling of intrinsic domain wall magnetoresistance with confinement in electromigrated nanocontacts

Abstract: In this work we study the evolution of intrinsic domain wall magnetoresistance (DWMR) with domain wall confinement. Clean permalloy notched half-ring nanocontacts are fabricated using a special ultra-high vacuum electromigration procedure to tailor the size of the wire in-situ and through the resulting domain wall confinement we tailor the domain wall width from a few tens of nm down to a few nm. Through measurements of the dependence of the resistance with respect to the applied field direction we extract the contribution of a single domain wall to the MR of the device, as a function of the domain wall width in the confining potential at the notch. In this size range, an intrinsic positive MR is found, which dominates over anisotropic MR, as confirmed by comparison to micromagnetic simulations. Moreover, the MR is found to scale monotonically with the size of the domain wall, $\delta_{DW}$, as 1/$\delta_{DW}^b$, with $b=2.31\pm 0.39 $. The experimental result is supported by quantum-mechanical transport simulations based on ab-initio density functional theory calculations.

Efficient Magnetic Domain Nucleation and Domain Wall Motion With Voltage Control Magnetic Anisotropy Effect and Antiferromagnetic/Ferromagnetic Coupling

Abstract: The magnetic domain is a promising solution for realizing the next-generation information storage, for example, racetrack memory (RM). However, domain nucleation and domain wall (DW) motion are limited by the magnetic tunnel junctions (MTJs) as the write and read heads. Moreover, the size of a single domain limits the further increase in storage density. In this paper, we report a cross structure for efficient magnetic domain manipulations. With the help of voltage control magnetic anisotropy (VCMA) effect and antiferromagnetic/ferromagnetic (AFM/FM) couplings, a fast, low-power, and highly reliable nucleation can be achieved. We also miniaturize the domain size by applying a voltage on the nanowire, increasing the storage density. An optimized RM device is then proposed combining the data detection via the inverse spin Hall effect (ISHE). The proposed device facilitates the development of high-speed, low power consumption, and high-density RM applications.