Nonmagnetic Surface Layer Research Articles

Effect of Surface Oxidation and Crystal Thickness on the Magnetic Properties and Magnetic Domain Structures of Cr2Ge2Te6.

van der Waals (vdW) magnetic materials, such as Cr2Ge2Te6 (CGT), show promise for memory and logic applications. This is due to their broadly tunable magnetic properties and the presence of topological magnetic features such as skyrmionic bubbles. A systematic study of thickness and oxidation effects on magnetic domain structures is important for designing devices and vdW heterostructures for practical applications. Here, we investigate thickness effects on magnetic properties, magnetic domains, and bubbles in oxidation-controlled CGT crystals. We find that CGT exposed to ambient conditions for 5 days forms an oxide layer approximately 5 nm thick. This oxidation leads to a significant increase in the oxidation state of the Cr ions, indicating a change in local magnetic properties. This is supported by real-space magnetic texture imaging through Lorentz transmission electron microscopy. By comparing the thickness-dependent saturation field of oxidized and pristine crystals, we find that oxidation leads to a nonmagnetic surface layer that is thicker than the oxide layer alone. We also find that the stripe domain width and skyrmionic bubble size are strongly affected by the crystal thickness in pristine crystals. These findings underscore the impact of thickness and surface oxidation on the properties of CGT, such as saturation field and domain/skyrmionic bubble size, and suggest a pathway for manipulating magnetic properties through a controlled oxidation process.

Local Kondo scattering in 4d-electron RuOx nanoclusters on atomically-resolved ultrathin SrRuO3 films.

Perovskite SrRuO3 is a unique 4d transition metal oxide with coexisting spin-orbit coupling (SOC) and electron-electron correlation. However, the intrinsic, non-reconstructed surface structure of SrRuO3 has not been reported so far. Here we report an atomic imaging of the non-reconstructed, SrO-terminated SrRuO3 surface by scanning tunneling microscopy/spectroscopy. Moreover, a Kondo resonant behavior is revealed in RuOx clusters located on top of the nonmagnetic SrO surface layer. The density functional theory calculations confirm that RuOx clusters possess localized 4d-electron-involved spin moments and hybridize with the conduction electrons in the metal host, resulting in the appearance of the Kondo resonance features around the Fermi level. Our work demonstrates that artificially-engineered transition metal oxides provide new opportunities to explore the Kondo physics in 4d multi-orbital systems.

Synthesis and Three-Dimensional Magnetic Field Mapping of Co2FeGa Heusler Nanowires at 5 nm Resolution.

We present the synthesis of Co2FeGa Heusler nanowires and the results of our investigations on their three-dimensional (3D) electric and magnetic internal and external fields mapped by electron holographic tomography (EHT). These fields will be of great importance in next-generation nanomagnets integrated in spintronics and memory devices. The Co2FeGa nanowires with a L21 ordered structure are prepared by a SBA-15 silica-assisted method. The magnetic dipole-like stray fields of several Co2FeGa nanowires are revealed by holographically reconstructed phase images. Based on the measured magnetic phase shifts of an individual nanowire and its 3D reconstruction using EHT, we obtain an internal magnetic induction with a magnitude of 1.15 T and a nonmagnetic surface layer of 10 nm thickness. Furthermore, we also reconstruct the 3D distribution of the electrostatic potential of the same nanowire.

The effects of polydispersity on the initial susceptibilities of ferrofluids

The effects of particle-size polydispersity on the initial susceptibilities of concentrated ferrofluids are analyzed using a combination of theory and computer simulation. The study is focused on a model ferrofluid with a prescribed magnetic-core diameter distribution, a fixed non-magnetic surface layer (corresponding to a demagnetized layer and adsorbed surfactant) and a combination of dipolar and hard-core interactions. The non-trivial effects of polydispersity are identified by comparing the initial susceptibilities of monodisperse and polydisperse ferrofluids with the same Langevin susceptibility. The theory is based on a correction to the second-order modified mean-field theory arising from a formal Mayer-type cluster expansion; this correction is dependent on a parameter similar to the normal dipolar coupling constant, except that it contains a complicated double average over the particle-size distribution, which means that the initial susceptibility should depend significantly on polydispersity. Specifically, the theory predicts that the initial susceptibility is enhanced significantly by polydispersity. This prediction is tested rigorously against results from Monte Carlo simulations and is found to be robust. The qualitative agreement between theory and simulation is already satisfactory, but the quantitative agreement could be improved by a systematic extension of the cluster expansion. The overall conclusion is that polydispersity should be accounted for carefully in magnetogranulometric analyses of real ferrofluids.

Size-Dependent Nonmagnetic Surface Layer of Ferromagnetic Nanoparticles

Based on the size-dependent melting temperature, we have deduced the analytic models for size-dependent Curie temperature Tc and saturation magnetization at room temperature Ms for ferromagnetic nanocrystals. The Tc(D) and Ms(D) functions decrease with decreasing size D. Agreements between model predictions and the corresponding experimental results enable us to determine the size-dependent thickness of nonmagnetic surface layer phi(D) and lambda(D) in describing Tc(D) and Ms(D), respectively. It is found that the surface layer should be an intrinsic property of ferromagnetic crystals since the nonmagnetic surface layer exists in the whole size range rather than vanishes as D approaches infinite.

Surface magnetic states of Ni nanochains modified by using different organicsurfactants

Three powder samples of Ni nanochains formed of polycrystalline Ni nanoparticleswith an estimated diameter of about 30 nm have been synthesized by a wetchemical method using different organic surfactants. These samples, havingmagnetically/structurally core–shell structures, all with a ferromagnetic Ni core, areNi@Ni3C nanochains,Ni@NiSG nanochains with a spin glass (SG) surface layer, andNi@NiNM nanochains with a nonmagnetic (NM) surface layer. The average thickness of the shell forthese three samples is determined as about 2 nm. Magnetic properties tailored by thedifferent surface magnetism are studied. In particular, suppression in the saturationmagnetization, usually observed with magnetic nanoparticles, is revealed to arise from thesurface magnetic states with the present samples.

Synthesis of Fe 3O 4 nanoparticles at 100 °C and its magnetic characterization

Superparamagnetic iron oxide nanoparticles were synthesized by a novel, simple and cost-effective gel-to-crystalline method by alkalizing ferrous chloride with ammonium hydroxide at 80–100 °C under refluxing conditions. Average crystallite size is determined as 11 nm from XRD and 11.4 nm from TEM: magnetic domain size is 9.7 nm. The saturation magnetization is 390 emu/cm 3 at 300 K, and T B is 134 K. The reduction of magnetic particle size and M s is attributed to the presence of non-magnetic (dead) surface layer, compositional variations, superparamagnetic relaxation and spin canting because of the ultrafine nature of the material. The maximization in magnetization near the blocking temperature, T B, is attributed to a large degree of inversion of the Fe 3O 4 particles. Results also indicated the existence of interparticle interactions in the thermomagnetic characteristics of spinels at lower field strengths.

Does the Nonmagnetic Surface Layer Exist Throughout Ferromagnetic Nanoparticles?

The analytic models for size-dependent Curie temperature Tc and saturation magnetization at room temperature Ms of Fe3O4 nanoparticles have been proposed in terms of the size-dependent melting temperature. The Tc(r) and Ms(r) values decrease with a decreasing of the particle radius r. Agreement between model predictions and the corresponding experimental results can be found, which enable us to determine the size dependences of the thickness of nonmagnetic surface layer δ(r) and t(r) in respectively describing Tc(r) and Ms(r) functions. It is found that both δ(r) and t(r) increase with a decreasing of r and t(r) is always twice as large as δ(r). Moreover, the surface layer should be an intrinsic property of ferromagnetic crystals since the nonmagnetic surface layer exists in the whole size range rather than vanishes as r approaches infinite.

Preparation and characterization of self-formed CoFe2O4 ferrofluid

An ionic ferrofluid is prepared using coprecipitation and acid treatment techniques. In the ferrofluid, Co3+ and Fe2+ dissolved from CoFe2O4 nanoparticles via suitable acid attack, are absorbed on the surface of the magnetic particles and thus prevent them from aggregating. We call this self-formed ferrofluid. The colloidal particles in the ferrofluid are constructed with a magnetic core and a nonmagnetic surface layer about 2 nm. The size and the volume fraction of particles in the ferrofluid are described with geometric diameter d and volume fraction ϕv, as well as magnetic diameter dm and magnetic volume fraction ϕm. The viscous and magnetic properties of this ferrofluid depend on these parameters. The viscosity can be tunable for any volume fraction of particles by adding poly(ethylene glycol) into the carrier liquid.

Effect of aggregates on the magnetization property of ferrofluids: A model of gaslike compression

The effect of field-induced aggregation of particles on the magnetization property of ferrofluids is investigated. From the viewpoint of energy, magnetizability of ferrofluids is more complicated than predicted by Langevin theory because the aggregation, i.e., the transition of ferrofluid microstructure, would consume the energy of the applied magnetic field. For calculating the effect of aggregates on the magnetization of ferrofluids, a model of gaslike compression (MGC) is proposed to simulate the evolution of the aggregate structure. In this model, the field-induced colloidal particles aggregating in ferrofluids is equivalent to the “gas of the particles” being compressed by the applied magnetic field. The entropy change of the ferrofluid microstructure is proportional to the particle volume fraction in field-induced aggregates øH. On the basis of the known behavior of ferrofluid magnetization and the aggregate structure determined from the present experiments, øH is obtained and found to depend on the aggregating characteristic parameter of ferrofluid particles γ in addition to the particle volume fraction in ferrofluids ø and the strength of applied magnetic field H. The effect of the nonmagnetic surface layer of ferrofluid particles is also studied. The theory of MGC conforms to our experimental results better than Langevin theory.

The Synthesis of Silica-Coated Permalloy Nanoparticles Using a Water-in-Oil Microemulsion

A water-in-oil microemulsion method was used for the preparation of silica-coated permalloy (Ni0.80Fe0.20) nanoparticles. The particles were prepared by the reduction of their salts with sodium borohydride NaBH4 in a cationic water-in-oil (w/o) microemulsion of water/cetyltrimethyl- amonium bromide (CTAB) and n-butanol/isooctane at 25oC. According to the TEM and x-ray diffraction analyses, the synthesized material was in the form of nanoparticles. The magnetization of the silica-coated nanoparticles was lower than that of the uncoated particles, reflecting the influence of the nonmagnetic silica surface layer on the magnetization.

Phenomenological magnetic modeling of Au:Fe:Au nano-onions

A new type of materials, the nano-onions, has been shown to exhibit GMR. These nanostructured composites consist of a nonmagnetic core coated with a thin layer of a bulk ferromagnet with a passivating nonmagnetic surface layer. The nano-onion investigated had a 3 nm Au core, a 1 nm Fe layer, and a 2 nm Au coating; all values correspond to the radius. The materials were manufactured using a sequential reverse micelle technique, detailed elsewhere. The sample preparation method produces a powder sample was cold pressed into a pellet. Magnetic investigation of the sample indicated that the material was superparamagnetic with a blocking temperature of 52 K for particles approximately 8 nm in diameter. At 10 K, the coercivity was 420 Oe, indicating a large degree of order. The GMR was measured over the entire temperature range available. At 10 K, in 5 T, a 1% MR was observed. The GMR was modeled using a simple phenomenological magnetic model initially used to study GMR granular thin films. To more accurately match the physical parameters of reverse micelles, the conventional log-normal particle size distribution was replaced with a normal distribution. The model suggested that the system consisted of an ensemble of 1.3 nm particles with a standard deviation of 0.07. The model does not detect the entire 8 nm diam, rather it detects the individual walls of the shell. In this context, it has shown that a simple phenomenological model can accurately predict the magnetic and electronic behavior of nano-onions.

The non-magnetic surface of magnetic particles in nanostructured glass ceramics studied by SANS

Nanosized crystallites of ferrimagnetic magnetite, Fe 3O 4, were prepared by heat treatment of a rapidly quenched silicate glass melt of composition 27.4CaO–42.8SiO 2–6.1B 2O 3–23.7Fe 2O 3 (mol%). SANS experiments of samples annealed at different heat treatment conditions were performed to determine the evolution and nature of the non-magnetic surface layer of the nanocrystals. It is suggested that this layer is formed by the distorted crystalline structure and thermally induced effects at the surface region and by the formation of a segregation shell enriched with Fe 2O 3 and subsequent surface oxidation of the Fe 3O 4 nanocrystals as well.

Formation of magnetic nanocrystals in a glass ceramic studied by small-angle scattering

The formation of nanosized crystallites of magnetite, Fe3O4, by heat treatment of a glass containing iron oxide was investigated. The magnetic properties of the glass ceramic manufactured strongly depend on the heat treatment conditions. The evolution of size distribution and volume fraction of the nanocrystallites formed was studied by small-angle x-ray scattering (SAXS). The size distribution of the nanocrystalline phase turned out to show bimodal shape. The possibility of magnetic contrast variation offered by small-angle neutron scattering (SANS) was utilized in order to distinguish the small-angle scattering of magnetite from the scattering contributions of nonmagnetic iron containing crystallites that can additionally be formed during the heat treatment. The results obtained reveal that both size grades of particles observed in the size distribution are superparamagnetic consisting of magnetite. The evolution of the volume fraction of magnetite in dependence on the heat treatment was found to be correlated with the magnitude of the specific saturation magnetization of the glass ceramic. The volume size distributions derived from magnetic SANS revealed peaks at smaller radii in relation to those from nuclear SANS and SAXS data. Therefore, the existence of a nonmagnetic surface layer is suggested that surrounds the magnetically active core of the magnetite nanocrystals.

Magnetic nanostructures in a glass ceramic charcterized by small angle neutron scattering

Nanocrystalline magnetite Fe 3O 4 embedded in a non-magnetic matrix was formed by heat treatment of CaSiBFeoxide glass. Small Angle Neutron Scattering (SANS) revealed a bimodal volume size distribution for the nanocrystalline phase with maxima centred at radii of 2.5 nm and 10 nm, respectively. The magnetic scattering behaviour is characteristic for superparamagnetism of single-domain particles following the Langevin statistics. Both fractions of nanocrystals are composed by a superparamagnetic core surrounded by a non-magnetic surface layer. Magnetisation measurements indicate different thermal variation of the superparamagnetic moments in both fractions of nanocrystals.

Ferric Substance Formed in Water Circulating in Boilers and Steam Piping, and an Apparatus for Magnetic Treating it

Substances in side the boilers and pipes become attached as scale or precipitate, and these badly effect the thermal efficiency ofthe steam. We made a detailed investigation, particularly on the ferric substances which are present in boilers for heating buildings, generating steam at 100°C and boilers which supply 180°C steam to multistory buildings. In the case of heating buildings, 20°% ofthese substances are made up of Fe 3 O 4 . The average particle size of Fe 3 O 4 is 17∼27 nm, and the saturation magnetization σ s is 65∼85 emu/g, by correcting the content of calcite and non-magnetic surface layer. The substances in boilers which supply multistory buildings contain ∼98% of Fe 3 O 4 and α-Fe 3 O 4 , whose average particle size is 105 and 160 nm, respectively. The σ s of Fe 3 O 4 was 79∼92 emu/g, if we correct the content of the non-magnetic layer, and this value is almost the same as for the bulk material. In our present research we tried to attach a bypass in the pipe and set a magnet from outside, and the substances can be removed from the system while the boiler in still in operation.

Structural and magnetic properties of Ba-ferrite fine particles grown by glass crystallization

The dependence of the specific saturation magnetization on the particle size of pure and Co-Ti-substituted Ba-ferrites was interpreted in terms of a nonmagnetic surface layer parallel to thec-planes of 1.3 nm and 0.7 nm, respectively, in thickness. Electron microscope investigations of the particles demonstrate that this effect arises neither from a sufficient fraction of superparamagnetic particles in the ensemble nor from the Co-Ti substitution or from surface defects such as steps and stacking disorder of the building blocks. Extrapolating this surface effect to small sizes yields a critical size, with the particles losing their ferrimagnetic behaviour. This critical size agrees surprisingly well with the volume calculated for the transition of the single domain state to the superparamagnetic one.

Structural and magnetic properties of BaFe/sub 12-2x/Co/sub x/Ti/sub x/O/sub 19/ powders prepared by the glass crystallization method

The glass crystallization method is shown to be suitable for preparing BaFe/sub 12-2x/Co/sub x/Ti/sub x/O/sub 19/ powders with a nearly perfect crystallographic structure and a narrow particle size distribution. The dependences of the specific magnetization, the coercivity, and the particle size distribution on both the annealing temperature and the Co-Ti content correlate in a characteristic way. Furthermore, the lattice parameters, the specific surfaces and the thickness of the effective nonmagnetic surface layer were determined. The coercivity of the hexaferrite magnetic tapes is more than 8*10/sup 3/ A/m higher than for the corresponding powders.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The magnetic moment of CrO2fine particles

Commercially available, reductively stabilized CrO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> particles have a saturation moment of about 80 emu/g or about 80% of the theoretical maximum (100 emu/g) when extrapolated to infinite field. A reduction process used to stabilize the moment accounts for about 10 emu/g or 10% of the difference from the theoretical. The remaining 10 emu/g or 10% difference from the theoretical moment appears to be due to an effectively non-magnetic surface layer. Spin canting, which was previously suggested to cause the reduction, does not appear to be responsible. In addition, supermagnetism was found in batches of very small particles. For example, 48 m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /g CrO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> particles (< 150 Å diameter with an aspect ratio of 10) contained about 20% superparamagnetic particles.

A non-destructive analysis of the influence of sputter etching on the magnetic surface properties of manganese zinc ferrite

Spectroscopic ellipsometry and the polar magneto-optical Kerr effect (0.6 < hv < 4.5 eV) have been used to study non-destructively the influence of RF sputter etching on the magnetic surface properties of the (100) face of manganese zinc ferrite single crystals. The effect of the sputtering can be described by the formation of a non-magnetic surface layer, which is presumably amorphous.