Uniaxial Shape Anisotropy Research Articles

Magnetic Properties of the Densely Packed Ultra-Long Ni Nanowires Encapsulated in Alumina Membrane.

High-quality and compact arrays of Ni nanowires with a high ratio (up to 700) were obtained by DC electrochemical deposition into porous anodic alumina membranes with a distance between pores equal to 105 nm. The nanowire arrays were examined using scanning electron microscopy, X-ray diffraction analysis and vibration magnetometry at 300 K and 4.2 K. Microscopic and X-ray diffraction results showed that Ni nanowires are homogeneous, with smooth walls and mostly single-crystalline materials with a 220-oriented growth direction. The magnetic properties of the samples (coercivity and squareness) depend more on the length of the nanowires and the packing factor (the volume fraction of the nanowires in the membrane). It is shown that the dipolar interaction changes the demagnetizing field during a reversal magnetization of the Ni nanowires, and the general effective field of magnetostatic uniaxial shape anisotropy. The effect of magnetostatic interaction between ultra-long nanowires (with an aspect ratio of >500) in samples with a packing factor of ≥37% leads to a reversal magnetization state, in which a “curling”-type model of nanowire behavior is realized.

Exact Analytical Solutions for Kinetic Equations Describing Thermochemical Remanence Acquisition for Single‐Domain Grains: Implications for Absolute Paleointensity Determinations

AbstractThe magnetic record, preserved by igneous rocks in the form of thermoremanent magnetization (TRM) or thermo‐chemical remanent magnetization (TCRM), is essential to reconstruct Earth's absolute paleointensity (API) but strongly depends on the kinetic conditions in which the remanence was acquired. In this paper, we present exact analytical solutions describing the time‐dependent processes of acquisition and thermal demagnetization of various kinds of thermally activated remanences for non‐interacting single‐domain grains with uniaxial shape anisotropy. Our solutions, derived in less‐restrictive conditions than previous studies, are also valid for TCRMs acquired either by growth of grain volume or by increase of the Curie point Tc. We first show that TCRMs by Tc increase and TRMs are of comparable intensity whereas TCRMs by volume growth are significantly less intense. We then model Arai‐Nagata diagrams for assemblies of coercivity‐variable grains and find that all Thellier‐type protocols yield reliable API determinations for TRMs and TCRMs by Tc increase, with the peculiarity of the IZZI protocol to produce small zigzags in the Arai‐Nagata diagram. In contrast, TCRMs by volume growth yield convex Arai‐Nagata diagrams. The most conspicuous kinetic effect is the influence of cooling rate on API determinations due to a ∼5% increase of the remanent magnetization for a 10‐fold increase in cooling time. We show that the situation is problematic when the cooling time of natural samples coincides with the geomagnetic secular‐variation time scales. Natural samples with cooling times sufficient to average out secular variation conversely yield reliable API determinations provided a cooling‐rate correction is applied.

Field-free spin–orbit-torque switching of perpendicular magnetization aided by uniaxial shape anisotropy

It has been demonstrated that the switching of perpendicular magnetization can be achieved with spin–orbit torque (SOT) at an ultrafast speed and low energy consumption. However, to make the switching deterministic, an undesirable magnetic field or unconventional device geometry is required to break the structure inverse symmetry. Here we propose a novel scheme for SOT-induced field-free deterministic switching of perpendicular magnetization. The proposed scheme can be implemented in a simple magnetic tunnel junction (MTJ)/heavy-metal system, without the need of complicated device structure. The perpendicular-anisotropy MTJ is patterned into elliptical shape and misaligned with the axis of the heavy metal, so that the uniaxial shape anisotropy aids the magnetization switching. Furthermore, unlike the conventional switching scheme where the switched final magnetization state is dependent on the direction of the applied current, in our scheme the bipolar switching is implemented by choosing different current paths, which offers a new freedom for developing novel spintronics memories or logic devices. Through the macrospin simulation, we show that a wide operation window of the applied current pulse can be obtained in the proposed scheme. The precise control of pulse amplitude or pulse duration is not required. The influences of key parameters such as damping constant and field-like torque strength are discussed as well.

Effect of size and shape dependent anisotropy on superparamagnetic property of CoFe2O4 nanoparticles and nanoplatelets

Superparamagnetic cobalt ferrite (CoFe2O4) spherical nanoparticles and rhomboidal nanoplatelets were synthesized by co-precipitation at 80°C (S1) and hydrothermal route at 150°C (S2). X-ray diffraction (XRD) pattern confirms formation of cubic inverse spinel structure of as prepared cobalt ferrite samples (S1 and S2) with average crystallite size of 13nm and 18.7nm for S1 and S2 respectively. Transmission electron microscopy (TEM) reveals spherical and rhomboidal shaped with average particle size 16.7nm (S1) and 19.8nm (S2). The zero field cooled magnetization MZFCvs. T exhibit a broad maxima at 400K and 510K for S1 and S2 respectively. The blocking temperature TB is obtained as 310K and 341K for S1 and S2 respectively, by fitting coercive field at different temperatures to T1/2 law. The morphology of S1 and S2 corresponds to shape dependence of continuum approach. The effective demagnetization factors estimated as ΔN1=0 and ΔN2=0.749 for S1 and S2 samples respectively. The uniaxial anisotropy and shape anisotropy observed to be dominant in spherical shaped and rhomboidal shaped CoFe2O4 nanoparticles respectively. The uniaxial anisotropy constant of S1 sample is estimated as 56(kJ/m3) at TB=310K whereas the effective anisotropy constant for S2 sample is 627(kJ/m3) at TB=341K, in which shape anisotropy constant 605(kJ/m3) dominates over contribution from uniaxial anisotropy constant 22(kJ/m3) in S2 sample.

Probing temperature-driven spin reorientation transition of GdFeCo film by Kerr loops and ferromagnetic resonance

The magnetic anisotropy is of both scientific and technological interest for magneto-optical material GdFeCo film. We characterize the magnetic anisotropy of a 20 nm GdFeCo film from 265 K to 320 K via Kerr loops and ferromagnetic resonance. With increasing temperature, both of the first-order uniaxial magnetic anisotropy and shape anisotropy increase. However, the competition between them causes a temperature-driven spin reorientation transition (SRT) and the effective perpendicular magnetic anisotropy decrease from 2.22 × 104 ergs/cm3 (288 K) to −1.56 × 104 ergs/cm3 (317 K). The positive second-order uniaxial magnetic anisotropy determines an easy-cone state as the mediated state during SRT.

Domain Structure and Magnetoresistance in Co2MnGe Zigzag Structures

We report a clear manifestation of the negative contribution to the magnetoresistance due to domain walls in Co2MnGe-Heusler submicron zigzag wires in which the domain structure, domain size and domain wall density can be well controlled. The magnetic behavior of these systems results from the interplay between the intrinsic magneto-crystalline (K4) anisotropy, growth induced uniaxial (KU = 4.7x103 J/m3) anisotropy and shape anisotropy (KS), as observed by magnetic-force microscopy (MFM) and longitudinal Kerr hysteresis loop measurements. Magnetoresistance measurements were performed by the four-point method under a field applied in the plane of the wires at a temperature of 300K. In these structures, domain wall-creation and annihilation occur in a coherent way. As a result, clear jumps of the resistance are detected during the transition from single-domain- to multi-domain states. At room temperature a value RDW = -2.5 mΩ was obtained; this result is the same order of magnitude as other experimental and theoretical findings. The negative resistive contribution due to the domain wall is also discussed and compared with the existing theoretical models.

Biologically controlled synthesis and assembly of magnetite nanoparticles.

Magnetite nanoparticles have size- and shape-dependent magnetic properties. In addition, assemblies of magnetite nanoparticles forming one-dimensional nanostructures have magnetic properties distinct from zero-dimensional or non-organized materials due to strong uniaxial shape anisotropy. However, assemblies of free-standing magnetic nanoparticles tend to collapse and form closed-ring structures rather than chains in order to minimize their energy. Magnetotactic bacteria, ubiquitous microorganisms, have the capability to mineralize magnetite nanoparticles, the so-called magnetosomes, and to direct their assembly in stable chains via biological macromolecules. In this contribution, the synthesis and assembly of biological magnetite to obtain functional magnetic dipoles in magnetotactic bacteria are presented, with a focus on the assembly. We present tomographic reconstructions based on cryo-FIB sectioning and SEM imaging of a magnetotactic bacterium to exemplify that the magnetosome chain is indeed a paradigm of a 1D magnetic nanostructure, based on the assembly of several individual particles. We show that the biological forces are a major player in the formation of the magnetosome chain. Finally, we demonstrate by super resolution fluorescence microscopy that MamK, a protein of the actin family necessary to form the chain backbone in the bacteria, forms a bundle of filaments that are not only found in the vicinity of the magnetosome chain but are widespread within the cytoplasm, illustrating the dynamic localization of the protein within the cells. These very simple microorganisms have thus much to teach us with regards to controlling the design of functional 1D magnetic nanoassembly.

Magnetite Crystal Orientation in Magnetosome Chains

One-dimensional magnetic nanostructures have magnetic properties superior to non-organized materials due to strong uniaxial shape anisotropy. Magnetosome chains in magnetotactic bacteria represent a biological paradigm of such magnet, where magnetite crystals synthesized in organelles called magnetosomes are arranged into linear chains. Two-dimensional synchrotron X-ray diffraction (XRD) is applied to cells of magnetotactic bacteria that are pre-aligned with a magnetic field to determine the crystallographic orientation of magnetosomes relative to the chain axis. The obtained pole figure patterns reveal a [111] fiber texture along the chain direction for magnetospirilla strains MSR-1 and AMB-1, whereas a [100] fiber texture is measured for Desulfovibrio magneticus strain RS-1. The [100] axis appears energetically unfavorable because it represents a magnetic hard axis in magnetite, but can be turned into an effective easy axis by particle elongation along [100] for aspect ratios higher than 1.25, consistent with aspect ratios in RS-1 magnetosomes determined earlier. The pronounced fiber textures can be explained either by a strain-specific biological control on crystal orientation at the chain level or by physical alignment effects due to intra-chain magnetic interactions. In this case, biological control of the axis of elongation would be sufficient to influence the crystallographic texture of the magnetosome chain.

CPP-GMR of Co/Cu Multilayered Nanowires Electrodeposited into Anodized Aluminum Oxide Nanochannels with Large Aspect Ratio

Co/Cu multilayered nanowires with alternating Co and Cu layers of 10 nm in thickness were electrodeposited into extremely long nanochannels of anodized aluminium oxide thick films with 60 μm in thickness. Growth rate of Co/Co multilayered nanowires was around 30 nm/sec and the cylindrical shape was precisely transferred from the nanochannels to the nanowires and the aspect ratio reached up to ca. 1,000 to 1. Co/Cu multilayered nanowires with diameter 60 nm were easily magnetized to the long axis direction of nanowires due to the uni-axial shape anisotropy. 10.5 % of perpendicular giant magnetoresistance effect was observed in Co/Cu multilayered nanowires with 3,000 Co/Cu bilayers.

Anisotropic Magnetoresistance of Ni-Co-Fe Alloy Nanowires Electrodeposited into Anodized Aluminium Oxide Membrane Thin Films

To synthesize a novel anisotropic magnetoresistance (AMR) sensor, Ni-Co, Ni-Fe, Co-Ni and Co-Fe alloy nanowires were electrodeposited into nano-channels of anodized aluminium oxide films with the thickness ranging from 20 μm to 200 μm. The growth rate of the nanowires was around 200 nm/sec at the cathode potential of -1.2 V vs. Ag/AgCl. The aspect ratio of the nanowires reached up to ca. 3,000 to 1 and the cylindrical shape was precisely transferred from the nano-channels to the nanowires. Magnetic hysteresis loops of Ni-Co, Ni-Fe, Co-Ni and Co-Fe alloy nanowires with the diameter of 60 nm showed typical perpendicular magnetization behaviour due to the uni-axial shape anisotropy and the coercive force reached up to 1 kOe. 3.2% of AMR was observed in Co-1.5%Ni alloy nanowires with the aspect ratio of 1,000.

Nanoimprint-lithography patterned epitaxial Fe nanowire arrays with misaligned magnetocrystalline and shape anisotropies

We fabricated large area (>1 × 1 cm2), epitaxial Fe nanowire arrays on MgO(001) substrates by nanoimprint lithography with a direct metallization of epitaxial materials through a metallic mask, which avoided the disadvantageous metal-etching process in conventional methods. The magnetization reversals, as revealed by magneto-optic Kerr effect, showed competing effects between Fe cubic magnetocrystalline anisotropy and lithographically induced uniaxial shape anisotropy. Unlike the weakly induced uniaxial anisotropy observed in continuous films, both the magnitude and direction of the uniaxial shape anisotropy can be easily modulated in the nanowires. Complex magnetization reversal processes including two-step and three-step loops were observed when magnetizing the samples along different Fe cubic easy axes, respectively. These modified magnetization reversal processes were explained by the nucleation and propagation of the domain walls along the non-superimposed easy axes of the competing magnetocrystalline and shape anisotropies.

Functional Nanowires Array Electrodeposited into Nano-porous Membrane Thin Films

Ni, Co, Fe and Co/Cu multilayered nanowires were electrodeposited into a nano-well template to synthesize novel functional ferromagnetic devices. Growth rate of Co/Co multilayered nanowires was around 40 nm•sec−1 and the cylindrical shape was precisely transferred from the nanochannels to the nanowires and the aspect ratio reached up to ca. 150. Magnetic hysteresis loops of Ni, Co and Fe nanowires with diameter 40 nm showed typical perpendicular magnetization behavior due to the uni-axial shape anisotropy and the coercive force reached up to around 1 kOe. 2% of anisotropic magnetoresistance effect was observed in Ni nanowires electrodeposited into anodized aluminum oxide template, while 20% of giant magnetoresistance effect was observed in Co/Cu multilayered nanowires.

Synthesis, structural and magnetic characterization of highly ordered single crystalline BiFeO3 nanotubes

In this work, we report on the fabrication of highly ordered single crystalline BiFeO3 (BFO) nanotubes by a sol–gel technique using two-step anodic aluminum oxide (AAO) as template. We prepared BFO nanotubes with dimensions of 65nm in diameter and 3μm in length, as confirmed by scanning electron microscopy (SEM) measurements. The obtained single crystalline nanotubes present the expected pure phase (BiFeO3) as confirmed by energy-dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED) and high-resolution transmission electron microscopy (HRTEM). In addition to the antiferromagnetic behavior, the magnetization curves of the BFO nanotubes also present a ferromagnetic response, which holds from 2 to 300K. This desirable behavior is associated to the break of the antiferromagnetic helical spin ordering of the BFO nanotubes. Besides the magnetocrystalline anisotropy, the large length-to-diameter ratio induced an uniaxial shape anisotropy, attested by the applied magnetic field angle measurements.

Properties of Dense Assemblies of Magnetic Nanoparticles Promising for Application in Biomedicine

Chemically synthesized iron oxide nanoparticles and magnetosomes produced by magnetotactic bacteria are of great importance for application in biomedicine. In this paper, we discuss the complicated magnetic anisotropy of the nanoparticles, the influence of the magnetostatic interactions, and thermal fluctuations on the behavior of these assemblies. Numerical simulation for dilute assemblies of iron oxide nanoparticles with combined magnetic anisotropy show that the uniaxial shape anisotropy dominates even for small aspect ratios of the particle, L/D≥1.1–1.2. The quasistatic hysteresis loops are calculated for various clusters of bacterial magnetosomes with diameters D=40–60 nm to understand the influence of magnetostatic interactions. The specific absorption rate (SAR) is calculated for assemblies of magnetic nanoparticles dispersed in solid and liquid media. A new electrodynamic method of measurement is used to obtain the SAR of the assembly of bacterial magnetosomes with average diameter D=48 nm.

Metastable state in a shape-anisotropic single-domain nanomagnet subjected to spin-transfer-torque

We predict the existence of a metastable magnetization state in a single-domain nanomagnet with uniaxial shape anisotropy. It emerges when a spin-polarized current, which delivers a spin-transfer-torque possessing a field-like component, is injected into the nanomagnet. At a metastable state, the internal torque due to nanomagnet's shape anisotropy cancels the externally applied spin-transfer-torque and hence the net torque acting on the magnetization becomes zero. Therefore, it prevents spin-transfer-torque from switching the magnetization from one stable state along the easy axis to the other, even in the presence of room-temperature thermal fluctuations.

Competing anisotropies and temperature dependence of exchange bias in Co∣IrMn metallic wire arrays fabricated by nanoimprint lithography

The magnetic behavior of exchange biased Co∣IrMn bilayer metallic wire arrays, fabricated by nanoimprint lithography, was studied and compared with identical thin film heterostructures. A significant uniaxial shape anisotropy, KU-shape, in addition to the unidirectional exchange anisotropy, KE, and the intrinsic uniaxial anisotropy, KU-intrinsic observed in the unpatterned film, was introduced in the wire arrays through wire patterning. The competing anisotropies were shown to modify the angular dependence of exchange bias, HEB, and coercivity, HC, for wire arrays. In addition, an asymmetric behavior is observed for both wire arrays and unpatterned film and is attributed to the noncollinear alignment of uniaxial and unidirectional anisotropies. Temperature dependence of HEB is different for the wire arrays from the unpatterned thin film. This and the large deviation from ideal cubic anisotropy in the antiferromagnet for the wire arrays are both in agreement with Malozemoff’s model of exchange bias.

Crystalline analysis of permalloy narrow wires subject to current pulses

We investigate crystalline structures of a permalloy nanowire subject to current pulses by using standard transmission and scanning electron microscope techniques, together with the analysis of magnetic states by means of Lorenz microscopy. We find that crystalline grains grow slightly larger in a current region around the Curie temperature, which does not affect significantly the uniformly-magnetized state in the wire because of strong uniaxial shape anisotropy given by the wire geometry. When crystalline grains grow large enough to additionally provide in-plane anisotropy comparable to the shape anisotropy by averaging crystalline anisotropy over the grains, the magnetic ripple will be induced in the wire. Such situations can be found in the case when very large current pulses are applied to the wire.

Angle dependence of planar Hall resistance and anisotropic magnetoresistance of (Ga,Mn)As and Permalloy microdevices

We present systematic studies on the angle dependence of planar Hall resistance and anisotropic magnetoresistance of ferromagnetic semiconductor (Ga,Mn)As and metallic Permalloy (Py) microdevices. (Ga,Mn)As shows two distinct planar Hall resistance switchings when the magnetic field is applied in the plane, indicating the magnetization reversal by 90° due to the cubic magnetocrystalline anisotropy. Similar magnetization reversal is observed in metallic Py microdevices. The broader planar Hall resistance jump observed in the Py device is attributed to the uniaxial shape anisotropy and complex domain rotation at the voltage probe. Nevertheless the observed planar Hall effect of Py is comparable to that of (Ga,Mn)As.

Fabrication and Magnetic Anisotropy of the Epitaxial Fe Nanowires/Cu(001)/Si(001)

Fe nanowires 300 nm in width are fabricated from the epitaxial Fe film on Cu(001)/Si(001) by using laser interference lithography and chemical wet etching. The wire axis is parallel to the magnetic hard direction of the film. Scanning electron microscope and electron diffraction spectroscope spot profile analysis indicate that the isolated wires are successively fabricated. Although the Fe film shows intrinsic fourfold symmetry due to its crystallinity, the magnetic properties of the nanowires are strongly dominated by the uniaxial shape anisotropy.

Magnetism of manganite nanotubes constituted by assembled nanoparticles

We present a study on the magnetic properties of manganite ultrafine grains assembled in nanotubes of $800\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ of external diameter and $6--8\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ length. The study includes two homogeneous ferromagnetic compounds, ${\mathrm{La}}_{0.67}{\mathrm{Ca}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ and ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$, and one exhibiting ferromagnetic and charge-ordered phase coexistence, ${\mathrm{La}}_{0.325}{\mathrm{Pr}}_{0.300}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$. From magnetic measurements, we conclude that the grains behave as single magnetic domains. Observations of dipolar interactions between magnetic grains are evidenced by isothermal remanent magnetization and direct current demagnetization experiments. These experiments suggest that the grain magnetic moments should be arranged in a fanning configuration at $H=0$. Also, a uniaxial shape anisotropy was observed on previously aligned ferromagnetic nanotubes by ferromagnetic-resonance experiences. In ${\mathrm{La}}_{0.325}{\mathrm{Pr}}_{0.300}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$, we observe distinctive features related to its inhomogeneous character as compared with the ferromagnetic homogeneous nanotubes.