Magnetic Anisotropy and Magneto-Transport Properties of Co–Ni–N Granular Alloys Thin Films

The effect of nitrogen addition on the morphology, magnetic anisotropy, and magnetoresistance properties of Co–Ni–N granular thin films were investigated. The films were grown by electrodeposition onto aluminum substrates at room temperature. By a complex process of cationic catalysis occurring at the cathode/electrolyte interface, nitrogen is adsorbed in the Co–Ni film. Finally, a granular film grows by a tridimensional progressive nucleation mechanism. The nature of the grains and of the interface between them influences exchange interactions between grains, which play an important role in determining the magnetic anisotropy. From the magnetic measurements, we found that the magnetic anisotropy constant varied in the range K _eff=(−21.5÷36.6)×10⁴ J⋅m⁻³ and the coercivity varied between H _c=(13÷67) kA⋅m⁻¹ depending on the sodium nitrate content in the plating bath. The Co–Ni–N granular thin films display large values (∼160%) of magnetoresistance. These large values of magnetoresistance make such structures attractive for applications as sensitive magnetic field sensors.     

Amorphous Gd-Fe alloy films prepared by RF cosputtering technique

Amorphous GdxFe1-x(0 < x < 0.35) thin films were prepared by RF cosputtering using an iron target covered partially with small gadolinium pieces. The composition dependences of saturation magnetization, hysteresis curves, and domain structures were studied. Spark-like domains were observed when the magnetization was reversed from saturation. The uniaxial anisotropy was induced perpendicular to the film plane. The average value of the perpendicular uniaxial anisotropy constant Ku was about 2 × 10⁵erg/cm³. Ku, however, decreases as the saturation moment decreases in the vicinity of room temperature compensation. The stripe domain width was varied from 0.3 to 4 microns as the saturation moment decreased from 300 to 20 gauss. These magnetic properties of amorphous Gd-Fe sputtered films are similar to those of amorphous Gd-Co films.     

Preparation of mullite-based iron magnetic nanocomposite powders by reduction of solid solution

In this article, the preparation of mullite-based iron magnetic nanocomposite powders by hydrogen reduction of Fe-doped mullite solid solution with a nominal composition of Al_5.4Fe_0.6Si₂O₁₃ is reported. The formation process of Al_5.4Fe_0.6Si₂O₁₃ solid solution was analyzed using X-ray diffraction analysis (XRD), Fourier Transform Infrared Spectrum (FT-IR), thermogravimetric, and differential thermal analysis (TG-DTA). It is found that doping with Fe³⁺ cation affects the crystallization temperature of mullite. During the hydrogen reduction process, more than 89% Fe³⁺ cation in solid solution were transformed into α-Fe phase when reduction temperature reached 1200 °C. Microstructure characterization of nanocomposite powders reduced at 1300 °C reveals that there are two types of α-Fe particles in mullite matrix. Fe nanoparticles with a size of approximately 10 nm were precipitated within the mullite grains, while Fe particles larger than hundreds of nanometers were located at the surfaces of the mullite grains. The measurement of the magnetic properties of nanocomposite powders indicates that large particles and nanoparticles of α-iron have the ferromagnetic and superparamagnetic behavior at room temperature, respectively.     

Single crystal growth of actinide compounds

During recent years, the importance of solid state actinide research has been increasingly recognized. Further progress in actinide solid state physics depends on the availability of pure and perfect single crystals. Actinide compounds have large magnetic anisotropy with anisotropy fields of 8 × 10⁷ A·m^?1 or higher. Investigation of the mechanism responsible for such unique behaviour requires large single crystals of high purity for magnetization, neutron diffraction, angular and energy dependent photoemission measurements. Materials of interest for actinide solid state research are the metals and compounds with simple crystal structures like dioxides (CaF₂ structure), monopnictides and monochalcogenides (NaCl structure), and intermetallic compounds (Laves phases).This article gives an overview of actinide single crystal growth facilities in Karlsruhe and Geel. The actinide compounds are prepared by direct synthesis from the purest elements available using non-contaminating techniques. The reaction occurs in vacuum sealed quartz tubes where the actinide metal reacts with the vapour of the other element, or by levitation melting in a Hukin crucible. Different techniques have been developed to grow single crystals of actinide metals and compounds. High temperature solution growth from molten salts is used to prepare actinide dioxide single crystals. Oxides, pnictides and chalcogenides are grown by chemical vapour transport. Large single crystals of the monopnictides and monochalcogenides are obtained with the recrystallization (or mineralization) technique in sealed tungsten crucibles. Single crystals of congruently melting intermetallic compounds are pulled from levitated or semilevitated melts by the Czochralski method. Selected single crystals are characterized, orientated and encapsulated for safe handling during measurements.     

Giant magnetostriction in magnetite nanoparticles

Typically the value of the magnetostrictive coefficient (λ) observed for bulk magnetic materials such as cubic ferrites is 10⁻⁶. However, giant magnetostriction (λ ≤ 10⁻³) is only observed in a few bulk intermetallic materials based on alloys of rare earth and iron such as TbFe, TbFe₂, DyFe₂ and Terefenol-D. While giant magnetostriction is known in nanostructured films, we show for the first time, this phenomenon occurs in magnetic nanoparticles. By using in-field small angle X-ray scattering (SAXS) as a tool, we demonstrate that a 4% relative change in dimension of the particle can be observed in 5.0 nm Fe₃O₄ nanoparticles at room temperature with 1 kG magnetic field. Also, we propose that the observed values are due to interaction effects and magnetoelastic coupling of particle magnetic moments and external magnetic field.     

A New Highly Anisotropic Rh-Based Heusler Compound for Magnetic Recording

The development of high-density magnetic recording media is limited by superparamagnetism in very small ferromagnetic crystals. Hard magnetic materials with strong perpendicular anisotropy offer stability and high recording density. To overcome the difficulty of writing media with a large coercivity, heat-assisted magnetic recording was developed, rapidly heating the media to the Curie temperature T_c before writing, followed by rapid cooling. Requirements are a suitable T_c, coupled with anisotropic thermal conductivity and hard magnetic properties. Here, Rh₂CoSb is introduced as a new hard magnet with potential for thin-film magnetic recording. A magnetocrystalline anisotropy of 3.6 MJ m⁻³ is combined with a saturation magnetization of μ₀M_s = 0.52 T at 2 K (2.2 MJ m⁻³ and 0.44 T at room temperature). The magnetic hardness parameter of 3.7 at room temperature is the highest observed for any rare-earth-free hard magnet. The anisotropy is related to an unquenched orbital moment of 0.42 μ_B on Co, which is hybridized with neighboring Rh atoms with a large spin–orbit interaction. Moreover, the pronounced temperature dependence of the anisotropy that follows from its T_c of 450 K, together with a thermal conductivity of 20 W m⁻¹ K⁻¹, make Rh₂CoSb a candidate for the development of heat-assisted writing with a recording density in excess of 10 Tb in.⁻².     

Ferromagnetic Behavior of Fe+ Implanted Si(100) Semiconductor

Fe ions have been implanted into Si (100) single crystals using ion implantation technique. The Fe ions have been accelerated to 45 keV with a dose of 5×10¹⁷ ion/cm² at room temperature. The ions have been sent to the substrate??s surface at normal incidence. The temperature dependence of magnetization measurement was explored at the temperature range of 10?C300 K. The implanted Si substrate was studied with Ferromagnetic Resonance (FMR) technique and Vibrating Sample Magnetometer (VSM). The FMR spectra were recorded by applying external magnetic field in different experimental geometries. FMR spectra were analyzed and the magnetic properties, which are the g-factor, effective magnetization and uniaxial anisotropy parameter, were estimated by simulation of the experimental data. The sample showed two-fold magnetic anisotropic symmetry. By fitting the Si-2p region obtained through XPS measurements it is observed that Fe and Fe compounds are present in the material.     

Effects of preparation conditions on the magnetic anisotropy of amorphous Fe78B13Si9 alloy

The in-plane magnetic anisotropy of amorphous Fe₇₈B₁₃Si₉ alloy prepared under various forming conditions was investigated. The anisotropy of circular discs was measured using the torque method. The experimental results indicate that the degree of magnetic anisotropy decreases, as does the depth of the surface grooves on the roller side, with the increase of the cooling rate or the melt temperature before jet casting. This shows that the directional surface irregularities are a principal reason for the in-plane anisotropy of the amorphous Fe₇₈B₁₃Si₉ ribbons. The results also indicate that the value of the anisotropy constant is about 10⁴ erg cm⁻³ and that the easy direction coincides with the longitudinal direction (of the amorphous Fe₇₈B₁₃Si₉ alloy). At the same time, the effects of the distribution of stress aligned predominantly along one direction and of the directional structural units or defects on the magnetic anisotropy are discussed.     

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.     

X-ray and electron microscopy studies of polysulphur nitride

(SN) _x crystals have been studied by a combination of scanning electron microscopy, transmission electron microscopy and rotating crystal X-ray diffraction. The studies reveal a structure of considerable complexity. The size of individual crystallites is typically a few nanometres by a few tens of nanometres, and their crystal structure is as previously reported by Mikulski and co-workers rather than that reported by Boudeulle. In bulk crystals there is a mosiac spread of approximately 10° in and about the fibre axis. A new twin mode is proposed where crystals twin about a plane perpendicular to the plane defined by individual chains arid is referred to as a (¯102)^* mode. The possibility of the existence of at least two other twin modes is also discussed. An alternative orientation relationship between the precursor dimer phase and the final polymer is presented.     

Magnetoelectric coupling at metal surfaces

Magnetoelectric coupling allows the magnetic state of a material to be changed by an applied electric field. To date, this phenomenon has mainly been observed in insulating materials such as complex multiferroic oxides. Bulk metallic systems do not exhibit magnetoelectric coupling, because applied electric fields are screened by conduction electrons. We demonstrate strong magnetoelectric coupling at the surface of thin iron films using the electric field from a scanning tunnelling microscope, and are able to write, store and read information to areas with sides of a few nanometres. Our work demonstrates that high-density, non-volatile information storage is possible in metals.     

Magnesioferrite formation in low-concentration Fe/MgO single crystals

Single crystals of MgO containing iron in concentrations from 310 to 12900 ppm have been examined by magnetic resonance and reflection electron diffraction before and after ageing in oxygen in the temperature range 600–800° C. Before ageing, only the e.s.r. spectrum of isolated Fe³⁺ ions in cubic sites was seen and the diffraction patterns revealed MgO alone. After ageing, a strong anisotropic line, centered near g=2.00 and exhibiting the characteristics of ferrimagnetic resonance, was found and its presence was coupled with the appearance of a spinel pattern superimposed on that of the MgO. Analysis of the variation of anisotropy field with ageing time gave estimates of the room-temperature magnetization, first anisotropy constant and Curie temperature of 258 emu cm^–3, –2.7× 10^–3 J cm^–3 and 500±10 K, respectively, which, in view of the close agreement with published data, suggested magnesioferrite formation even at very low iron concentrations. In the initial stages of ageing a strong isotropic resonance line was observed which appeared to be a precursor of the anisotropic ferrimagnetic line.     

Thermally induced changes in the magnetic properties of iron oxide nanoparticles under reducing and oxidizing conditions

Application of iron oxide nanoparticles in the fields of water purification, biomedicine or chemistry often requires controlled magnetic properties that can be modified by changing temperature and redox conditions. Therefore, this work investigates the changes in the magnetic properties of iron oxide nanoparticles in the FeOOH − Fe₂O₃ − Fe₃O₄ system (i.e. hematite, goethite, lepidocrocite, maghemite and magnetite) at heating under reducing and oxidizing conditions. The results show that heat treatment of hematite and goethite in the presence of a reducing agent (5% starch) leads to their conversion into high magnetic magnetite. The starting temperature of transformation is approximately 350 °C for both samples. The magnetization increases to 86 Am²/kg for hematite reduced at 700 °C and to 88 Am²/kg for goethite reduced at 900 °C. An intense reaction occurs within the first 10 min and then the conversion process decelerates. Thermal treatment of lepidocrocite under both oxidizing and reducing conditions leads to an increase in magnetization due to the formation of maghemite and magnetite, respectively. Regardless of the redox conditions, the formation of magnetic phase begins at a temperature of 250 °C and is associated with the formation of maghemite from lepidocrocite. Under oxidizing conditions, the magnetization begins to decrease at 350 °C, which is associated with the conversion of maghemite to hematite. On the contrary, under reducing conditions, the magnetization of lepidocrocite increases up to 900 °C, which is associated with the formation of magnetite. Maximum values of magnetization are 36 Am²/kg for maghemite obtained at 350 °C, and 88 Am²/kg for magnetite obtained at 900 °C from lepidocrocite. With the help of conventional heating, the magnetic properties of IONs can be altered by phase transformations in the FeOOH − Fe₂O₃ − Fe₃O₄ system. Temperature and redox conditions are the two most important factors controlling the transformation pathways and the magnetic properties of the resulting IONs.     

Radio-Frequency Planar Integrated Inductor With Permalloy-SiO$_2$Granular Films

We have designed and fabricated a radio-frequency (RF) planar integrated inductor using Permalloy-SiO$_2$granular film as the magnetic core. By controlling the composition and microstructure, we produced a granular film with excellent soft magnetic properties and high electrical resistivity. The inductance$L$of the inductor with granular Permalloy-SiO$_2$magnetic film increased 6% to 15%, compared to that of an air-core inductor, and the quality factor$Q$value was high, approaching 10 in the frequency range of 2–3 GHz. The controllable anisotropy of the granular film generated in deposition process gives the magnetic inductor a very high self-resonance frequency peak—over 6 GHz.     

Growth of Y<Subscript>3</Subscript>Fe<Subscript>5</Subscript>O<Subscript>12</Subscript> films on Si with AlO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and SiO<Subscript>2</Subscript> buffer layers by ion beam sputtering

Amorphous yttrium iron garnet films ranging in thickness from 100 to 600 nm have been produced on single-crystal silicon substrates by sputtering a polycrystalline target with the composition Y₃Fe₅O₁₂ (yttrium iron garnet) by a mixture of argon and oxygen ions. Before film growth, AlO _x or SiO₂ buffer layers up to 0.8 μm in thickness were grown on the Si surface. The heterostructures were crystallized by annealing in air at a temperature of 950°C for 30 min. The properties of the films were studied by magneto-optical techniques, using Kerr effect and ferromagnetic resonance measurements. The Gilbert damping parameter reached 2.8 × 10^–3 and the effective planar magnetic anisotropy field was independent of the nature of the buffer layer. This suggests that the thin-film heterostructures obtained in this study are potentially attractive for use in spin-wave semiconductor devices.     

Improving Néel Domain Walls Dynamics and Skyrmion Stability Using He Ion Irradiation

Ion irradiation with light ions is an appealing way to finely tune the magnetic properties of thin magnetic films and in particular the perpendicular magnetic anisotropy (PMA). In this work, the effect of He⁺ irradiation on the magnetization reversal and on the domain wall dynamics  of Pt/Co/AlOx trilayers is illustrated. Fluences up to 1.5 × 10¹⁵ ions cm⁻² strongly decrease the PMA, without affecting neither the spontaneous magnetization nor the strength of the interfacial Dzyaloshinskii–Moriya interaction (DMI). This confirms experimentally the robustness of the DMI interaction against interfacial chemical intermixing, already predicted by theory. In parallel with the decrease of the PMA, a strong decrease of the domain wall depinning field is observed after irradiation. This allows the domain walls to reach large maximum velocities with a lower magnetic field compared to that needed for the pristine films. Decoupling PMA from DMI can, therefore, be beneficial for the design of low energy devices based on domain wall dynamics. When the samples are irradiated with larger He⁺ fluences, the magnetization gets close to the out-of-plane/in-plane reorientation transition, where ≈100nm size magnetic skyrmions are stabilized. It is observed that as the He⁺ fluence increases, the skyrmion size decreases while these magnetic textures become more stable against the application of an external magnetic field, as predicted by theoretical models developed for ultrathin films with labyrinthine domains.     

Anisotropic and high magnetization rare earth transition metal compounds containing metalloids

The structural and magnetic behavior is presented for selected metalloid (B,C) containing hexagonal and tetragonal rare earth-transition metal compounds and compound series. Focus is on materials with high Fe content and resulting high magnetizations. The unusual axial ratios and features of the sheet type structures of these materials have pronounced consequences on such properties as magnetic anisotropy and magnetic hardness. Individual site anisotropy contributions are studied by temperature dependence of magnetization along easy and hard magnetic axes. As an example it is found that tetragonal Nd₂Fe₁₄B has axial anisotropy with H_A = 76 kOe at 300 K but shows tendencies for a spin reorientation around 150 K. Y₂Fe₁₄B has axial anisotropy with H_A = 25 kOe but does not exhibit a similar spin reorientation. This indicates that the two crystallographic Nd sites (4f and 4g) have axis and plane preference respectively, with different temperature dependencies. Axial Nd anisotropy is a consequence of the lack of Nd coordination along the z axis due to intervening thick Fe layers. Both extrinsic (fine particle) and intrinsic magnetic hardness is observed. Crystallographically disordered materials show intrinsic hardness based on domain wall pinning by local fluctuations of magnetic parameters. Strong nucleation phenomena are characteristic for ordered materials in bulk and powder form. The unusually high achievable ratios of extrinsic coercivities to anisotropy fields in the metalloid stabilized materials are related to their chemically relatively inert layer structure. This appears to lead to less corrugated surface structures and is so responsible for the characteristic domain wall nucleation processes.     

Magnetic anisotropy and morphology of Fe epitaxial layers grown on MgO/InAs heterostructures

In-plane magnetic anisotropy and the corresponding morphology of Fe epitaxial layers have been investigated with respect to underlying MgO growth temperature when epitaxial Fe/MgO layers are grown on InAs (001) substrates. Coexistence of three-dimensional Fe islands with strong in-plane textures along <110> and (100) is observed on 4 nm thick MgO layers grown on 200 degrees C, leading to the absence of magnetic anisotropy. Meanwhile, the partially relaxed MgO layers grown above 300 degrees C give rise to two-dimensional Fe layers with cubic magnetic anisotropy. The higher MgO growth temperature accelerates the two-dimensional layer formation of the subsequent Fe as well as the advent of cubic anisotropy by reducing underlying strain within the MgO layer.     

The relationships between microstructure and crystal structure in zincite solid solutions

Single phase (Zn,Fe)_1−x O zincite solid solution samples have been prepared by high temperature equilibration in air and in reducing atmospheres, followed by quenching to room temperature. The Fe²⁺/Fe³⁺ concentrations in the samples have been determined using wet chemical and XPS techniques. Iron is found to be present in zincite predominantly in the form of Fe³⁺ ions. The transition from an equiaxed grain morphology to plate-like zincite crystals is shown to be associated with increasing Fe³⁺ concentration, increasing elongation in <001> of the hexagonal crystals and increasing anisotropic strain along the c-axis. The plate-like crystals are shown to contain planar defects and zincite polytypes at high iron concentrations.     

The nonlinear optical, magnetic, and Mössbauer spectral properties of some iron(III) doped silica xerogels

Iron(III) species dispersed in silica have been synthesized with a sol-gel process. The iron(III) was introduced as the acetylacetonate complex into a solution of tetraethoxysilane to yield, after evaporative drying, pellets or monoliths. Two gels were dried very slowly over a period of five months in order to prepare a defect free monolith useful for nonlinear optical studies. Z-scan experimental studies on the resulting, transparent, monolithic, doped solid revealed an optical Kerr effect, a third order nonlinear optical phenomenon showing a linear dependence of the refractive index on the irradiance with a nonlinear refractive index, n ₂ , of ?1.95 × 10^?11 cm²/W. Magnetic susceptibility studies between 4.2 and 295 K revealed paramagnetic behavior with a Curie constant of 4.433 (emu/mol)K and a Weiss temperature of ?7.1 K. Magnetization studies at 5 K and at applied fields of up to 4 T and Mössbauer spectral studies between 4.2 and 295 K revealed a 50:50 mixture of paramagnetic species and nanoparticles with an average particle radius of 1.3 ± 0.2 nm. A blocking temperature of 70 K and a magnetic anisotropy energy of 2.4 × 10⁵ J/m³ are derived from the Mössbauer spectra.