Nonmagnetic Surface Research Articles

Non-Planar Dzyaloshinskii Spirals and Magnetic Domain Walls in Non-Centrosymmetric Systems with Orthorhombic Anisotropy

The basic micromagnetic models of Landau, Lifshitz, and Dzyaloshinskii, are extended by an anisotropy term with two independent parameters. The resulting ground states of the magnetic domains and the domain-wall profiles are discussed for crystal lattices with orthorhombic unit cells. In these simple geometries, the magnetization is not confined to a single plane. Depending on the relations between the spin-stiffness, anisotropy, and Dzyaloshinskii-Moriya interaction several different zero-temperature phases of the magnetic structure are found. The corresponding phase diagrams are obtained numerically. Analytical results are given for some special cases. The studied model is of particular relevance for magnetic wires, nanostripes and ultrathin magnetic films deposited on non-magnetic surfaces.

A compact ultra-precision air bearing stage with 3-DOF planar motions using electromagnetic motors

We designed a novel surface motor stage supported by air bearings and driven by linear electromagnetic motors. This compact and simple planar stage is proposed for compact-sized precision machine systems, such as micro-machine tools or measurement systems requiring minimum X-Y stage height. Four single-phase linear motors with coils and iron cores are located under the base plate, and air bearings and cores with permanent magnets are attached under the moving table. The hard, non-magnetic alumina-ceramic base plate surface acts as a planar guide for the air bearings. The attractive magnetic force between the magnets and motor cores preload the air bearing to increase vertical stiffness. By simultaneously combining actuations of the motors, linear X and Y motion can be controlled, and angular motions can also be generated. A grid encoder is used to control planar motion, and the system is run by a programmable numerical controller. The thrust and attractive force were calculated using a magnetic circuit model. The designed prototype is 220 (L) × 220 (W) × 66 (H) mm3 in size with a 20 mm × 20 mm range of motion. After fabrication, basic aspects of the prototype, such as vertical stiffness and thrust force, were evaluated. Twenty nanometers of positioning resolution was obtained for the X and Y axes, and the three motions could be controlled independently.

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.

A hybrid DFT description of the (0001) surface of americium-I

Using a periodic slab model for the (0001) surface of americium-I (Am-I) and the full-potential linear augmented plane wave (FP-LAPW) plus local basis implementation of hybrid density functional theory (HYB-DFT), we show that with a fraction of 0.40 Hartree-Fock (HF) exchange, the non-magnetic nature of the Am-I surface is well described and the electronic properties are in good agreement with recent photoemission spectroscopy data. The work function and surface energy of the non-magnetic semi-infinite surface are predicted to 3.46 eV and 1.03 J/m2, respectively. As a function of slab thickness, the surface properties, namely the surface energy, work function, and slab incremental energy converge after five layers. A five layer slab is thus deemed to be sufficient to very accurately model the adsorption of impurities on the surface and adsorbate-induced changes in the work function. A layer-by-layer examination of the nine-layer non-magnetic slab clearly indicates that the 5f electrons are uniformly localized, that is, the 5f electron localization is independent of the position of the layer.

Impact on Interface Spin Polarization of Molecular Bonding to Metallic Surfaces

We have studied the repercussion of the molecular adsorption mechanism on the electronic properties of the interface between model nonmagnetic or magnetic metallic surfaces and metallo-organic phthalocyanines molecules (Pcs). Our intertwined x-ray absorption spectroscopy experiments and computational studies reveal that manganese Pc (MnPc) is physisorbed onto a Cu(001) surface and retains the electronic properties of a free molecule. On the other hand, MnPc is chemisorbed onto Co(001), leading to a dominant direct exchange interaction between the Mn molecular site and the Co substrate. By promoting an interfacial spin-polarized conduction state on the molecule, these interactions reveal an important lever to tailor the spintronic properties of hybrid organic-metallic interfaces.

Imaging and manipulating the spin direction of individual atoms

Single magnetic atoms on surfaces are the smallest conceivable units for two-dimensional magnetic data storage. Previous experiments on such systems have investigated magnetization curves, the many-body Kondo effect and magnetic excitations in quantum spin systems, but a stable magnetization has not yet been detected for an atom on a non-magnetic surface in the absence of a magnetic field. The spin direction of a single magnetic atom can be fixed by coupling it to an underlying magnetic substrate via the exchange interaction, but it is then difficult to differentiate between the magnetism of the atom and the surface. Here, we take advantage of the orbital symmetry of the spin-polarized density of states of single cobalt atoms to unambiguously determine their spin direction in real space using a combination of spin-resolved scanning tunnelling microscopy experiments and ab initio calculations. By laterally moving atoms on our non-collinear magnetic template, the spin direction can also be controlled while maintaining magnetic sensitivity, thereby providing an approach for constructing and characterizing artificial atomic-scale magnetic structures.

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.

A novel method to improve finish on non-magnetic surfaces in magneto-rheological abrasive honing process

Magneto-Rheological Abrasive Honing (MRAH) is a recently developed process that makes use of magnetically stiffened abrasive-mixed magneto-rheological fluid and combined rotary and reciprocating motions to finish engineering surfaces. Effect of magnetic field has been investigated on specimens mounted in a holder and subjected to MRAH. It is observed that the change in the surface finish on magnetic specimens is insignificant, while the surface finish on non-magnetic specimens improves appreciably. By a novel method of introducing magnetic specimens along with non-magnetic specimens, the magnetic field near the non-magnetic specimens is enhanced. The results obtained on the non-magnetic specimens by this approach for different magnetic field and process duration confirm that the finishing capability of the process is greatly improved, and yields a maximum improvement in finish of 41.7% and 43.5%, respectively while the previously reported results are 6.7% and 24.2%.

Optimized Steric Stabilization of Aqueous Ferrofluids and Magnetic Nanoparticles

The preparation and properties of an aqueous ferrofluid consisting of a concentrated (>65 wt %) dispersion of sterically stabilized superparamagnetic, iron oxide (maghemite) nanoparticles stable for several months at high ionic strength and over a broad pH range is described. The 6-8 nm diameter nanoparticles are individually coated with a short poly(acrylic acid)-b-poly(acrylamide) copolymer, designed to form the thinnest possible steric stabilizing layer while remaining strongly attached to the iron oxide surface over a wide range of nanoparticle concentrations. Thermogravimetric analysis yields an iron oxide content of 76 wt % in the dried particles, consistent with a dry polymer coating of approximately 1 nm in thickness, while the poly(acrylamide) chain length indicated by electrospray mass spectrometry is consistent with the 4-5 nm increase in the hydrodynamic radius observed by light scattering when the poly(acrylamide) stabilizing chains are solvated. Saturation magnetization experiments indicate nonmagnetic surface layers resulting from the strong chemical attachment of the poly(acrylic acid) block to the particle surface, also observed by Fourier transform infrared spectroscopy.

Theory of Inelastic Electron Tunneling from a Localized Spin in the Impulsive Approximation

A simple expression for the conductance steps in inelastic electron tunneling from spin excitations in a single magnetic atom adsorbed on a nonmagnetic metal surface is derived. The inelastic coupling between the tunneling electron and the spin is via the exchange coupling and is treated in an impulsive approximation using the Tersoff-Hamann approximation for the tunneling between the tip and the sample.

Inorganic magnetic support for sodium cation scavenging

Three different magnetic iron oxide-silica matrices with polyoxaalkyl units have been obtained in a multi-step synthesis. The structures of the matrices were confirmed by Fourier transform infrared spectroscopy, while their surface morphology was analyzed by scanning electron microscopy. The scavenging ability was studied by ultraviolet–visible measurements. The results demonstrate very good scavenging efficiency of compounds studied against sodium cations (Na +). The complexing abilities of the magnetic iron oxide-silica surfaces were compared with those of the previously studied analogously modified non-magnetic silica surfaces.

Novel Electro-Optical Coupling Technique for Magnetic Resonance-Compatible Positron Emission Tomography Detectors

A new magnetic resonance imaging (MRI)-compatible positron emission tomography (PET) detector design is being developed that uses electro-optical coupling to bring the amplitude and arrival time information of high-speed PET detector scintillation pulses out of an MRI system. The electro-optical coupling technology consists of a magnetically insensitive photodetector output signal connected to a nonmagnetic vertical cavity surface emitting laser (VCSEL) diode that is coupled to a multimode optical fiber. This scheme essentially acts as an optical wire with no influence on the MRI system. To test the feasibility of this approach, a lutetium-yttrium oxyorthosilicate crystal coupled to a single pixel of a solid-state photomultiplier array was placed in coincidence with a lutetium oxyorthosilicate crystal coupled to a fast photomultiplier tube with both the new nonmagnetic VCSEL coupling and the standard coaxial cable signal transmission scheme. No significant change was observed in 511 keV photopeak energy resolution and coincidence time resolution. This electro-optical coupling technology enables an MRI-compatible PET block detector to have a reduced electromagnetic footprint compared with the signal transmission schemes deployed in the current MRI/PET designs.

Rashba effect at the surfaces of rare-earth metals and their monoxides

We present a systematic study of the Rashba-type spin–orbit interaction at the (0001) surfaces of rare-earth metals and their surface monoxides, specifically of Tb metal and the O/Tb, O/Lu and O/Y surfaces. By means of photoemission experiments and ab initio band-structure calculations, we uncover the influence of this interaction on the surface electronic structure. In turn, the dramatic impact of the charge-density distribution of the surface/interface states on the strength of the Rashba-type spin splitting is demonstrated. We discuss the Rashba effect at magnetic and non-magnetic rare-earth surfaces, and compare with cases where it is negligible. The difference between the Rashba effect and magnetic linear dichroism in photoemission is pointed out to help avoid possible confusion in connection with the simultaneous appearance of these two effects at a magnetic surface.

Study of Surface Rashba Effect by Spin- and Angle-Resolved Photoelectron Spectroscopy

Spin-split energy band structure appears in the surface state due to a broken space inversion symmetry at the surface boundary as well as a strong spin-orbit coupling. Spin- and angle-resolved photoemission spectroscopy (spin-resolved ARPES) has been applied to the study of the spin-split energy band structures of non-magnetic solid surfaces. First of all, some explanation of the spin-splitting mechanism for free electrons, so-called Rashba effect, will be given. Next, our spin-resolved ARPES system equipped with the compact Mott-type spin polarimeter will be introduced. We will show how the surface Rashba effect is observed from the spin-resolved ARPES with the recent result for the surface states of Bi(111). Here, we have observed strong and anti-symmetric spin polarizations with respect to the center of surface Brillouin zone in the surface states of Bi(111).

Properties of anisotropic magnetic impurities on surfaces

Using numerical renormalization-group techniques, we study static and dynamic properties of a family of single-channel Kondo impurity models with axial magnetic anisotropy $D{S}_{z}^{2}$ terms; such models are appropriate to describe magnetic impurity atoms adsorbed on nonmagnetic surfaces, which may exhibit surface Kondo effect. We show that for positive anisotropy $D$ and for any spin $S$, the systems behave at low temperatures as regular Fermi liquids with fully compensated impurity spin. The approach to the stable fixed point depends on the value of the spin $S$ and on the ratio $D/{T}_{K}^{(0)}$, where ${T}_{K}^{(0)}$ is the Kondo temperature in the absence of the anisotropy. For $S=1$, the screening occurs in two stages if $Dl{T}_{K}^{(0)}$; the second Kondo temperature is exponentially reduced in this case. More generally, there is an effective spin-1/2 Kondo effect for any integer $S$ if $Dl{T}_{K}^{(0)}$ and for any half-integer $S$ if $Dg{T}_{K}^{(0)}$. For negative anisotropy $D$, the system is a non-Fermi liquid with residual anisotropic exchange interaction. However, the presence of transverse magnetic anisotropy $E({S}_{x}^{2}\ensuremath{-}{S}_{y}^{2})$ restores Fermi-liquid behavior in real systems.

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.

Structural, electronic and magnetic properties of the surfaces of tetragonal and cubic HfO2

We present ab initio density-functional theory (DFT) calculations of the structure and stability of the monoclinic (m), tetragonal (t) and cubic (c) phases of HfO2 and of the stability and the structural, electronic, and magnetic properties of the polar (001) surface of t-HfO2 and the (100) and (111) surfaces of c-HfO2. We show that on all three surfaces, a termination by Hf leads to a metallic and non-magnetic surface, while surfaces covered by a full monolayer of O are predicted to be half-metallic and ferromagnetic, the magnetisms being induced by the Coulomb repulsion between p-holes in the O-2p valence band. In contrast, the partially reduced surfaces terminated by half a monolayer of oxygen are found to be insulating and non-magnetic. Ab initio statistical mechanics in combination with the DFT total-energy calculations show that the partially reduced surfaces are stable over the entire range of admissible values of the chemical potential of oxygen. Investigations of the formation of Hf vacancies on the Hf- and O-terminated surfaces of tetragonal HfO2 demonstrate that under oxidizing conditions, the formation of Hf subsurface vacancies is energetically favored on the partially reduced O-terminated surface. The formation of Hf vacancies causes the creation of holes in the O-2p valence band and of magnetic moments on the surrounding O atoms. That the formation of near-surface Hf vacancies on the O-terminated surface is energetically favored is in contrast to a high formation energy for neutral Hf vacancies in bulk HfO2 and suggests a cooperative mechanism between surface- and vacancy-formation. We discuss our findings in relation to recent reports on ferromagnetism in ultrathin HfO2 films and other models for the formation of p-wave ferromagnetism.

Low-energy spin-polarized two-electron spectroscopy: a powerful tool for studying exchange correlation and spin-orbit interaction on surfaces

Spin-polarized two-electron spectroscopy-in-reflection has been proven to be a very efficient technique for studying the exchange correlation and spin-orbit interaction on magnetic and non-magnetic surfaces and thin ferromagnetic films. The essence of the technique is the detection of two time-correlated electrons emitted from the sample surface upon an impact of a single incident electron and measurements of their momenta. The coincidence technique has been combined with a time-of-flight electron energy analysis to measure momentum distributions of correlated electron pairs for various orientations of the electron beam polarization relative to the scattering plane and magnetization of the sample. This set of measurements allows to extract the information on exchange correlation and spin-orbit interaction from measured spectra. Energy- and momentum-conservation in the electron-electron scattering allows the valence electron involved into collision to be located in the energy-momentum space of the valence band of the sample. In this way the energy and momentum location of the exchange and SOI can be established. A few examples of the application of this technique for studying ferromagnetic and non-magnetic surfaces are presented.

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.

Absence of local magnetic moments in Ru and Rh impurities and clusters on Ag(100) and Pt(997)

The magnetism of quench-condensed Ru and Rh impurities and metal films on Ag(100) and Pt(997) has been studied using x-ray magnetic circular dichroism. In the coverage range between 0.22 and 2.0 ML, no dichroic signal was detected at the ${M}_{3,2}$ absorption edges of Ru on Ag(100) at a temperature of $5\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in the presence of an applied magnetic field. The same was found for coverages between 0.12 and 0.5 ML of Rh on Ag(100) and Pt(997). It is concluded that the magnetic moments of single impurities, small clusters of various shape, and monolayers of the $4d$ metals are below the detection limit of $0.04\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ per atom. These results provide an unambiguous determination of the local magnetic moment of Ru and Rh deposited on nonmagnetic transition-metal surfaces, which are in contrast with theoretical predictions.