Field-induced Magnetic Transition in Cobalt-Ferrite
Martin Kriegisch, Weijun Ren, Reiko Sato-Turtelli, Herbert Müller, Roland Grössinger, Zhidong Zhang
备注：
This Paper is already accepted for the conference proceedings of the MMM 2011 (http://www.magnetism.org). The proceedings will be published in April 2012 in Journal of Applied Physics
发表信息：
Journal of Applied Physics (2012) Vol.111, Issue 7, 07E308
We present magnetostriction and magnetization measurements of a cobalt ferrite Co0.8Fe2.2O4 single crystal. We observe unusual behaviour in the magnetic hard axis of the single crystal which manifests in a jump of the magnetization curve at a critical field. This first order magnetization process (FOMP) which is explained as an anisotropy driven transition is visible at temperatures lower than 150 K. By analyzing the anisotropy constants we found that the higher order anisotropy constant K2 dominates the anisotropy energy. In the magnetostriction measurements the FOMP is clearly visible as a huge jump in the [111] direction, which can be explained by means of a geometric model.
Zinc oxide and Aluminum Ferrite were prepared Chemical route. The samples were characterized by XRD and VSM. Simulation of M-H plots of Co/CoO thin films were performed. Effect of parameters was observed on saturation magnetization.
Cobalt ferrite nanoparticles have been synthesized using a co-precipitation approach. Co:Fe precursor ratio is varied over a certain window (1.05 to 1.2). It is observed that the 1.15:2 precursor ratio gives better phase control but poor coercivity. On the other hand, 1.05:2 precursor ratio results in substantially better coercivity values (274kA/m; almost 300% the value reported for co-precipitated cobalt ferrite by Praveena et al.), but moderate BH product maximum (2.25 kJ/m3; ~ comparable to many reports on cobalt ferrite nanoparticles so far). The nanoparticles with best coercivity are ann…
Chromium substituted cobalt ferrite with grain size less than the single domain (approx. 70 nm) has been prepared by the sol-gel method. XRD analysis reveals that the samples crystallize to cubic symmetry with spacegroup number 227. Two transition temperatures (TD (approx. 450 K) and TM (approx. 600 K) have been observed from the impedance verses temperature measurement. TD increases with the increase in frequency due to dipole response to the frequency. TM is comparable with the para-ferrimagnetic transition temperature of cobalt ferrite, which is independent of frequency. This result is well…
In this work, a successful synthesis of magnetic cobalt ferrite (CoFe2O4) nanoparticles is presented. The synthesized CoFe2O4 nanoparticles have a spherical shape and highly monodisperse in the selected solvent. The effect of different reaction conditions such as temperature, reaction time and varying capping agents on the phase and morphology is studied. Scanning transmission electron microscopy showed that the size of these nanoparticles can be controlled by varying reaction conditions. Both X-ray diffraction and energy dispersive X-ray spectroscopy corroborate the formation of CoFe2O4 spine…
The mixed ferrite-cobaltites LnFe0.5Co0.5O3, with Ln = Eu & Dy have been prepared by a sol-gel method and the samples have been characterized using X-ray diffraction and electron microscopy. The magnetic investigations reveal that both samples ordered in canted antiferromagnetic structures near room temperature. The Dzyaloshinskii-Moriya or antisymmetric exchange interaction induces weak ferromagnetism due to canting of the antiferromagnetically ordered spins. In the case of Ln-Fe-Co orthoferrites, two magnetic sublattices (Ln3+-4f and Fe3+/Co3+-3d) generally align in opposite directions and i…
CoFe2O4 thin films (5 nm and 20 nm thick) were grown by oxygen assisted molecular beam epitaxy on Pt(111) at 523~K and subsequently annealed at 773 K in vacuum or oxygen. They were characterized in-situ using Auger Electron Spectroscopy, Low-Energy Electron Diffraction, Scanning Tunneling Microscopy and Conversion Electron Mössbauer Spectroscopy. The as-grown films were composed of small, nanometric grains. Annealing of the films produced an increase in the grain size and gave rise to magnetic order at room temperature, although with a fraction of the films remaining in the paramagnetic state.…
High coercivity induced by mechanical milling in cobalt ferrite powders
在这项工作中，我们报告的研究亚铁磁性氧化物CoFe 2 O 4的磁行为处理的机械研磨与不同的磨球。采用简单的水热法制备了钴铁氧体纳米粒子，并在500 ℃下进行了退火处理。未研磨的样品呈现约1.9kOe的磁化率、69.5emu/g的饱和磁化强度和0.42的剩磁比。球磨后，两种样品的磁化强度分别为4.2和4.1kOe，饱和磁化强度分别为67.0和71.4emu/g。这些样品的剩磁比MR/MS分别增加到0.49和0.51。为了研究这些样品的磁行为的微观结构的影响，我们使用X射线粉末衍射（XPD），透射电子显微镜（TEM），和振动样品磁强计（VSM）。通过Williamson-Hall图的XPD分析用于估计样品中由机械研磨引起的平均微晶尺寸和应变。
Perpendicular magnetic anisotropy (PMA) energy up to $K_{\mathrm{u}}=6.1\pm0.8$ MJ m$^{-3}$ is demonstrated in this study by inducing large lattice-distortion exceeding 3% at room temperature in epitaxially distorted cobalt ferrite Co$ _{x} $Fe$ _{3-x} $O$ _{4} $ (x = 0.72) (001) thin films. Although the thin film materials include no rare-earth elements or noble metals, the observed $ K_{u} $ is larger than that of the neodymium-iron-boron compounds for high-performance permanent magnets. The large PMA is attributed to the significantly enhanced magneto-elastic effects, which are pronounced i…
The magnetization of thin films of cobalt ferrite frequently falls far below the bulk value of 455 kAm-1, which corresponds to an inverse cation distribution in the spinel structure with a significant orbital moment of about 0.6 muB that is associated with the octahedrally-coordinated Co2+ ions. The orbital moment is responsible for the magnetostriction and magnetocrystalline anisotropy, and its sensitivity to imposed strain. We have systematically investigated the structure and magnetism of films produced by pulsed-laser deposition on different substrates (TiO2, MgO, MgAl2O4, SrTiO3, LSAT, La…
Room temperature spin filtering in epitaxial cobalt-ferrite tunnel barriers
我们报告的直接实验证据，室温自旋过滤的磁性隧道结（MTJ）含有CoFe 2 O 4隧道势垒通过隧道磁阻（TMR）测量。Pt（111）/CoFe_2 O_4（111）/γ-Al_2 O_3（111）/Co（0001）全外延MTJ的生长是为了获得能够在室温下工作的高质量系统。自旋极化输运测量显示，在2K和290 K下，TMR值分别为-18%和-3%。此外，TMR比遵循一个独特的偏置电压依赖性，这在理论上已经被预测为在MTJ中包含磁势垒的自旋过滤的签名。因此，CoFe 2 O 4隧道势垒提供了一个模型系统，以调查在很宽的温度范围内的自旋过滤。
CoFe2O4 nanoparticles were synthesized using gelatin as a polymerizing agent. Structural, morphological and magnetic properties of samples treated at different temperatures were investigated by X-ray diffraction, scanning electron microscopy, Mossbauer spectroscopy and magnetization measurements. Our results revealed that the samples annealed at 623 K and temperatures above 973 K have a cation distributions given by (Co0.19Fe0.81)[Co0.81Fe1.19]O4 and (Co0.06Fe0.94)[Co0.94Fe1.06]O4, respectively. The particle sizes varied from 73 to 296 nm and the magnetocrystalline anisotropy, K1, has values r…
Cobalt ferrite ultrathin films with inverse spinel structure are among the best candidates for spin-filtering at room temperature. We have fabricated high-quality epitaxial ultrathin CoFe2O4 layers on Ag(001) following a three-step method: an ultrathin metallic CoFe2 alloy was first grown in coherent epitaxy on the substrate, and then treated twice with O2, first at RT and then during annealing. The epitaxial orientation, the surface, interface and film structure were resolved combining LEED, STM, Auger and in situ GIXRD. A slight tetragonal distortion was observed, that should drive the easy …
In this study we have compared magnetic, magnetostrictive and piezomagnetic properties of isotropic and anisotropic cobalt ferrite pellets. The isotropic sample was prepared by the ceramic method while the sample exhibiting uniaxial anisotropy was made by reactive sintering using Spark Plasma Sintering (SPS). This technique permits to induce a magnetic anisotropy in cobalt ferrite in the direction of the applied pressure during SPS process. Sample with uniaxial anisotropy revealed a higher longitudinal magnetostriction and piezomagnetism compared to the isotropic sample, but the transversal ma…
Magnetoelectric Coupling in Nickel-Cobalt Ferrite and Lanthanum Ferrite Heterostructure Composites: Experimental Evidence and Simulation-Driven Insights
A detailed correlative structural, magnetic and chemical analysis of non-stoichiometric cobalt ferrite micrometric crystals was performed by x-ray magnetic circular dichroism combined with photoemission microscopy, low energy electron microscopy, and atomic force microscopy. The vector magnetization at the nanoscale is obtained from magnetic images at different x-ray incidence angles and compared with micromagnetic simulations, revealing the presence of defects which pin the magnetic domain walls. A comparison of different types of defects and the domain walls location suggests that the main s…
In-depth investigations of size and occupancies in cobalt ferrite nanoparticles by joint Rietveld refinements of X-ray and neutron powder diffraction data
The structural, dielectric, magnetic, and magnetoelectric (ME) properties of particulate composites containing lead-iron niobate and lead titanate piezoelectric 0.94[PbFe$_{0.5}$Nb$_{0.5}$O$_3$]-0.06[PbTi$_{0.5}$O$_3$] (PFN-PT) and Zn-substituted cobalt ferrite magnetostrictive CoFe$_{2-x}$Zn$_{x}$O$_4$ (CF$_{2-x}$Z$_{x}$O); 0.6(PFN-PT)/0.4(CF$_{2-x}$Z$_{x}$O), x=0, 0.025, 0.1, 0.2, 0.3 (with ratio of 60 Wt\% ferroelectric and 40 Wt\% ferrite); have been investigated. We investigated the ME voltage coefficient as a complex quantity for all composite samples using the dynamic piezomagnetic coef…
We present the low temperature magnetic properties in Ni1.5Fe1.5O4 ferrite as the function of pH at which the material was prepared by chemical route and post annealing temperature. The material is a ferri or ferromagnet, but showed magnetic blocking and random spin freezing process on lowering the measurement temperature down to 5 K. The sample prepared at pH =12 and annealed at 800 ^C showed a sharp magnetization peak at 105 K, the superparamagnetic blocking temperature of the particles. The magnetization peak remained incomplete within measurement temperature up to 350 K for rest of the sam…
Interfaces separating ferromagnetic (FM) layers from non-ferromagnetic layers offer unique properties due to spin-orbit coupling and symmetry breaking, yielding effects such as exchange bias, perpendicular magnetic anisotropy, spin-pumping, spin-transfer torques, conversion between charge and spin currents and vice-versa. These interfacial phenomena play crucial roles for magnetic data storage and transfer applications, which require forming FM nano-structures embedded in non-ferromagnetic matrices. Here, we investigate the possiblity of creating such nano-structures by ion-irradiation. We stu…
In this study, the microscopic origins of the voltage-controlled magnetic anisotropy (VCMA) in 3d-ferromagnetic metals are revealed. Using in-situ X-ray fluorescence spectroscopy that provides a high quantum efficiency, electric-field-induced changes in orbital magnetic moment and magnetic dipole Tz terms in ultrathin Co films are demonstrated. An orbital magnetic moment difference of 0.013μB. was generated in the presence of electric fields of +(-)0.2 V/nm. The VCMA of Co was properly estimated by the induced change in orbital magnetic moment, according to the perturbation theory model. The i…
We present an x-ray magnetic dichroism (XMCD) and soft x-ray absorption spectroscopy (XAS) study to address the nature of emerging magnetic order in metallic Copper as Cobalt is added to the matrix. For this purpose line shape and energy position of XAS and XMCD spectra will be analyzed for a series of Co/Cu alloys as well as a multilayer reference. We observe an increased hybridization between Cu and Co sites as well as increased localization of the Cu d-electrons and an induced magnetic moment in Cu. The emergence of long range magnetic order in non-magnetic materials that are in proximity t…
In this work, we have performed Monte Carlo simulations in a classical model for RFe$_{1-x}$Cr$_x$O$_3$ with R=Y and Lu, comparing the numerical simulations with experiments and mean field calculations. In the analyzed compounds, the antisymmetric exchange or Dzyaloshinskii-Moriya (DM) interaction induced a weak ferromagnetism due to a canting of the antiferromagnetically ordered spins. This model is able to reproduce the magnetization reversal (MR) observed experimentally in a field cooling process for intermediate $x$ values and the dependence with $x$ of the critical temperatures. We also a…
We synthesized pure and Co-doped (6.25 12.5 at.) ZrO$_2$ nanopowders in order to study their magnetic properties.We analyzed magnetic behavior as a function of the amount of Co and the oxygenation, which was controlled by low pressure thermal treatments. As prepared pure and Co-doped samples are diamagnetic and paramagnetic respectively. Ferromagnetism can be induced by performing low pressure thermal treatments, which becomes stronger as the dwell time of the thermal treatment is increased. This behavior can be reversed, recovering the initial diamagnetic or paramagnetic behavior, by performi…
Despite the potential advantages of information storage in antiferromagnetically coupled materials, it remains unclear whether one can control the magnetic moment orientation efficiently because of the cancelled magnetic moment. Here, we report spin-orbit torque induced magnetization switching of ferrimagnetic Co1-xTbx films with perpendicular magnetic anisotropy. Current induced switching is demonstrated in all of the studied film compositions, including those near the magnetization compensation point. The spin-orbit torque induced effective field is further quantified in the domain wall moti…
Antiferromagnetic Bi_{2}Fe_{4}O_{9} (BFO), lightly substituted by cobalt is studied for magnetodielectricity. The substitution causes a substantial decrease in the Neel temperature (T_N) from 250 K (in parent sample, BFO) to 152 K (in 2% Co substituted sample). At the same time, the substituted samples display a pronounced irreversibility in the ZFC-FC magnetization data for T $<$ 370 K and opening of hysteresis in the M-H plot, thus signifying the onset of weak ferromagnetism (FM) and magnetic glassiness. The induced magnetic glassiness is found to slow down the dynamics such that the magneti…
The work of this thesis comprises extensive Noncontact Atomic Force Microscopy (NC-AFM) characterization of clean metal-oxide (YSZ(100)/(111) and MgO(100)) and graphitic (HOPG) supports as templates for the novel, photochemically induced nucleation of cobalt oxide nanostructures, particularly Cobalt (III) Oxide. The nanostructure-support surfaces were also studied by X-ray Photoelectron Spectroscopy (XPS) to verify the nature of the supported cobalt oxide and to corroborate the surface topographic and phase NC-AFM data. Heteroepitaxial growth of Co2O3 nanostructures proves to exhibit a variety…
Here, we report the catalytic degradation of organic compounds by exploiting the magnetoelectric (ME) nature of cobalt ferrite-bismuth ferrite (CFO-BFO) core-shell nanoparticles. The combination of magnetostrictive CFO with the multiferroic BFO gives rise to a magnetoelectric engine that purifies water under wireless magnetic fields via advanced oxidation processes, without involvement of any sacrificial molecules or co-catalysts. Magnetostrictive CoFe2O4 nanoparticles are fabricated using hydrothermal synthesis, followed by sol-gel synthesis to create the multiferroic BiFeO3 shell. We perform…
Single phase, uniform size (~9 nm) Cobalt Ferrite (CFO) nanoparticles have been synthesized by hydrothermal synthesis using oleic acid as a surfactant. The as synthesized oleic acid coated CFO (OA-CFO) nanoparticles were well dispersible in nonpolar solvents but not dispersible in water. The OA-CFO nanoparticles have been successfully transformed to highly water dispersible citric acid coated CFO (CA-CFO) nanoparticles using a novel single step ligand exchange process by mechanochemical milling, in which small chain citric acid molecules replace the original large chain oleic acid molecules av…
Due to the wide range of applications, theoretical models of Fe3O4 films are found to be important. Ultra thin Fe3O4 films with ferrite structure have been theoretically investigated using second order perturbed modified Heisenberg Hamiltonian. Matrices for ultra thin films with two and three spin layers are presented in this manuscript. Total magnetic energy was expressed in terms of spin exchange interaction, magnetic dipole interaction, second order magnetic anisotropy and stress induced magnetic anisotropy. Magnetic properties were observed for films with two spin layers and variant second…
Implementation of antiferromagnetic compounds as active elements in spintronics has been hindered by their insensitive nature against external perturbations which causes difficulties in switching among different antiferromagnetic spin configurations. Electrically-controllable strain gradient can become a key parameter to tune the antiferromagnetic states of multiferroic materials. We have discovered a correlation between an electrically-written straight-stripe mixed-phase boundary and an in-plane antiferromagnetic spin axis in highly-elongated La-5%-doped BiFeO$_{3}$ thin films by performing p…
Quasiparticle interference on the surface of 3D topological insulator Bi$_{2}$Se$_{3}$ induced by cobalt adatom in the absence of ferromagnetic ordering
用扫描隧道显微镜、角分辨光电子能谱和X射线磁圆二色性研究了钴原子在拓扑绝缘体Bi $_{2}$ Se $_{3}$表面引起的准粒子干涉.研究发现，Co原子被选择性地吸附在Se位的顶部，并在表面作为强散射体，产生各向异性的驻波。一个长程磁序被发现是缺席的，和表面状态狄拉克锥保持无隙。驻波的各向异性归因于未占据态的严重翘曲的等能轮廓，其中散射由于非零的面外自旋而被允许。
We suggest coherent switching of canted antiferromagnetic (AFM) spins using spin-orbit torque (SOT) in small magnet. The magnetic system of orthoferrite features biaxial easy anisotropy and the Dzyaloshinskii Moriya interaction, which is perpendicular to the easy axes and therefore creates weak magnetization (m). A damping-like component of the SOT induces Néel reorientation along one of the easy axes and then exerts torque on m, leading to tilting of the Néel order l. The torque on the magnetization becomes stronger due to coupling with the induced Oersted field or the field-like component of…
