Ferromagnetic Surface Research Articles

Engineering Magnetic Hybridization at Organic–Ferromagnetic Interfaces by C60-Adsorption-Induced Fe(001) Surface Reconstruction

Large magnetoresistance has been reported for C60-based vertical spin valves. But for the underlying organic–ferromagnetic interfacial atomistic structures, both experimental and theoretical works have assumed unreconstructed atomic structures for the magnetic surfaces, although organic molecule (e.g., fullerene and thiolate) adsorption frequently induces nonmagnetic surfaces reconstruction. Here we report that C60 adsorption can induce a prototype ferromagnetic surface, Fe(001), reconstruction, via thorough structural search from first-principles calculations. We propose that Fe(001) surface reconstruction should already occur under the reported annealing temperature in literature. Reconstruction solidifies C60/Fe interface bonding and enhances C60 spin-polarization. More importantly, only our reconstructed structure can explain the experimental observation of an inversion of C60 spin-polarization around Fermi-level relative to that of Fe substrate, which is attributed to the C60 lowest unoccupied molecu...

Surface Magnetism in αPbO Induced by Fe Interstitials

We predict surface ferromagnetism and a high Curie temperature in αPbO doped with Fe interstitials using first-principles calculations. Our results provide a new route on designing dilute magnetic semiconductors using magnetic atoms as interstitials in layered oxides such as PbO.

Chemically functionalized magnetic exchange interactions of hybrid organic-ferromagnetic metal interfaces

We theoretically explore through systematic multiscale ab initio and Monte Carlo calculations how the surface magnetism of a ferromagnetic surface can be fine-tuned by nonmagnetic organic molecules containing a single $\ensuremath{\pi}$ bond. We demonstrate that a magnetic hardening or softening can be induced depending on the electronegativity of the heteroatom or when the $\ensuremath{\pi}$-bond ``bridges'' the magnetic surface atoms. Finally, the Monte Carlo simulations revealed taylored macroscopic hysteresis loops corresponding to soft and hard molecule-surface magnets.

Performance Analysis of Ball End Magnetorheological Finishing Process with MR Polishing Fluid

A ball end magnetorheological finishing (BEMRF) process was developed for finishing a flat as well as 3D workpiece surfaces. The BEMRF process has a wide scope in today's advanced manufacturing systems for finishing 3D complex surfaces. Magnetorheological (MR) polishing fluid is used as finishing medium in BEMRF process. The constituent of MR polishing (MRP) fluid includes ferromagnetic carbonyl iron powder, abrasives, and base fluid medium. The workpiece surface is mainly finished by abrasives contained in MRP fluid. Therefore, the different mesh size from 400 to 1200 and volume percent concentration from 5% to 25% of abrasives in MRP fluid were chosen as factors to study their effects on the developed process performance in terms of percent change in roughness values. Silicon carbide abrasives were chosen in the present experimental investigation. Experiments were performed on the ferromagnetic ground surfaces whose initial roughness values (Ra) were measured in the range of 0.428 to 0.767 µm. The experimental results revealed that the MRP fluid with 15 vol % abrasives of mesh number 400 demonstrated better improvements in surface texture of finished surface as compared to other MRP fluid compositions. The finished surface characteristics and textures were studied at microscopic level using scanning electron microscopy and atomic force microscopy.

Performance modulation of α-MnO₂ nanowires by crystal facet engineering.

Modulation of material physical and chemical properties through selective surface engineering is currently one of the most active research fields, aimed at optimizing functional performance for applications. The activity of exposed crystal planes determines the catalytic, sensory, photocatalytic, and electrochemical behavior of a material. In the research on nanomagnets, it opens up new perspectives in the fields of nanoelectronics, spintronics, and quantum computation. Herein, we demonstrate controllable magnetic modulation of α-MnO2 nanowires, which displayed surface ferromagnetism or antiferromagnetism, depending on the exposed plane. First-principles density functional theory calculations confirm that both Mn- and O-terminated α-MnO2 (1 1 0) surfaces exhibit ferromagnetic ordering. The investigation of surface-controlled magnetic particles will lead to significant progress in our fundamental understanding of functional aspects of magnetism on the nanoscale, facilitating rational design of nanomagnets. Moreover, we approved that the facet engineering pave the way on designing semiconductors possessing unique properties for novel energy applications, owing to that the bandgap and the electronic transport of the semiconductor can be tailored via exposed surface modulations.

Reflection of a Gaussian beam from a magnetized medium in the polar Kerr geometry

Specific features of reflection from a perpendicularly magnetized ferromagnetic metal surface for an oblique incident monochromatic Gaussian beam have been investigated. A numerical analysis of the distortion of the reflected beam profile and polarization distribution over the beam cross section in a wide range of angles of incidence is performed for a linearly polarized incident beam.

Imaging Dirac-mass disorder from magnetic dopant atoms in the ferromagnetic topological insulator Crx(Bi0.1Sb0.9)2-xTe3

To achieve and use the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TIs), it is necessary to open a "Dirac-mass gap" in their spectrum by breaking time-reversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely applied approach. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr0.08(Bi0.1Sb0.9)1.92Te3. Simultaneous visualization of the Dirac-mass gap Δ(r) reveals its intense disorder, which we demonstrate is directly related to fluctuations in n(r), the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of Δ(r) not inconsistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship [Formula: see text] is confirmed throughout and exhibits an electron-dopant interaction energy J* = 145 meV·nm(2). These observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential.

Spin-polarization of organic molecules at the ferromagnetic surface

Study on organic/ferromagnetic interface is helpful for understanding the effects of magnetoresistance in organic spin-valve, because one of the reasons of leading to this phenomenon is due to the spin injection at the interface. However, the interactions at the organic/ferromagnetic interface are complicated and full of possibilities, and the effects are still under debate till now. One possible cause is that the adsorption of organic molecules on the ferromagnetic surface is random, which leads to various adsorbing configurations. Therefore, in this paper we select some typical adsorbing configurations of benzene/Co system to reveal the effect of spin-polarization of organic molecules at the ferromagnetic surface by using first-principles calculations. It is obtained that the spin degenerated electronic states of benzene molecule will be broken due to the coupling between the 3d electrons of Co atoms and the 2p electrons of benzene molecule. The density of states at the Fermi level becomes spin related and a spin polarization appears in the benzene molecule. For both of the configurations T1T2 and T1H12, from the projected density of states we can find that the majority-spin electrons of the benzene molecule is oriented in opposition to the direction of the ferromagnetic electrode at the Fermi level, which means that the organic molecules filter and reverse the original spin direction of the injected electrons from the ferromagnetic electrode. As mentioned above, the adsorbing configurations are different, so we consider three kinds of configurations with different adsorbing distances for further studying the spin polarization at the interface. On the basis of the configuration T1T2, distances of 2.0 Å, 2.2 Å and 2.4 Å are studied, where 2.0 Å is the equilibrium position we obtained with full relaxation. It should be noted that we do not relax the geometric structure of the system in this part of study. It is found that the spin polarization is sensitively dependent on the distance between benzene and Co surface. The spin-polarization near the Fermi level even changes its direction from positive to negative with the increase of the distance in such a small range. Our studies reflect the complexity of organic molecule/ferromagnetic electrode interfaces, and enrich the understanding of this field.

Electrolyte vortex dynamics in the vicinity of a ferromagnetic surface in a direct current magnetic field.

We propose a new method for determining the frequency characteristics of the rotational motion of an electrolyte flow during electrochemical reactions under the influence of an external magnetic field. The main advantage of the proposed method is the possibility to determine the frequency characteristics without introducing marker particles or other changes in the electrolyte or in the nature of the reaction. The effectiveness of this method is demonstrated by measuring the electrolyte rotation frequencies during the corrosion of a steel ball in an external magnetic field. It is shown that at the chosen experimental conditions the typical electrolyte rotation frequencies during etching of the steel ball are 0.88 and 1.7 Hz. The developed method can be used for determining corrosion areas of metallic compounds via in situ testing.

Ising-Glauber spin cluster model for temperature-dependent magnetization noise in SQUIDs.

Clusters of interacting two-level-systems, likely due to Farbe+(F(+)) centers at the metal-insulator interface, are shown to self-consistently lead to 1/f(α) magnetization noise [with α(T)≲1] in SQUIDs. Model calculations, based on a new method of obtaining correlation functions, explains various puzzling experimental features. It is shown why the inductance noise is inherently temperature dependent while the flux noise is not, despite the same underlying microscopics. Magnetic ordering in these systems, established by three-point correlation functions, explains the observed flux-inductance-noise cross correlations. Since long-range ferromagnetic interactions are shown to lead to a more weakly temperature dependent flux noise when compared to short-range interactions, the time reversal symmetry of the clusters is also not likely broken by the same mechanism which mediates surface ferromagnetism in nanoparticles and thin films of the same insulator materials.

Erratum: “Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces” [Rev. Sci. Instrum. 85, 073901 (2014)

Erratum: “Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces” [Rev. Sci. Instrum. 85, 073901 (2014)

Magnetic properties of a single transverse Ising ferrimagnetic nanoparticle

Using the effective field theory with a probability distribution technique that accounts for the self-spin correlation function, the thermal and the magnetic properties of a single Ising nanoparticle consisting of a ferromagnetic core, a ferromagnetic surface shell and a ferrimagnetic interface coupling are examined. The effect of the transverse field in the surface shell, the exchange interactions between core/shell and in surface shell on the free energy, thermal magnetization, specific heat and susceptibility are studied. A number of interesting phenomena have been found such as the existence of the compensation phenomenon and the magnetization profiles exhibit P-type, N-type and Q-type behaviors.

Application of strong transverse magneto-optical Kerr effect on high sensitive surface plasmon grating sensors

A high sensitive sensor is demonstrated by exploiting strong transverse magneto-optical Kerr effect on a ferromagnetic surface plasmon grating. The surface plasmon grating, made of a hybridized Au/Fe/Au layer, exhibits a very dispersive Kerr parameter variation near the surface plasmon polariton (SPP) wavelength via coherent scattering of the SPP on the grating structure. Interrogating this Kerr parameter can be utilized for detecting chemical or biological objects in a fluid medium. The experiment results show the minimal detectable mass concentration of sodium chloride in a saline solution is 4.27 × 10(-3) %, corresponding to a refractive index change of 7.60 × 10(-6) RIU. For an avidin-biotin interaction experiment, the sensitivity of avidin detection in PBS solution is 1.97 nM, which is limited by the index fluctuation of flowing media during measurement.

Magnetic Interactions of Vanadyl Phthalocyanine with Ferromagnetic Iron, Cobalt, and Nickel Surfaces

We have investigated the molecular orientation and magnetic properties of vanadyl phthalocyanine (VOPc) deposited on ferromagnetic Fe, Co, and Ni films, which were epitaxially grown on Cu(001) using X-ray absorption spectroscopy and X-ray magnetic circular dichroism. The results reveal that the VOPc framework lies flat on the surfaces with oxygen-up configurations. The spin of the central V atoms in VOPc is antiferromagnetically coupled with those of Fe or Co atoms in the ferromagnetic surface. In contrast, the magnetic coupling between V and Ni is not effective, exhibiting simple paramagnetism of VOPc.

Coupling of single, double, and triple-decker metal-phthalocyanine complexes to ferromagnetic and antiferromagnetic substrates

We report a survey of the magnetic properties of metal–organic complexes coupled to ferromagnetic and antiferromagnetic surfaces. Using element-resolved X-ray magnetic circular dichroism, we investigate the magnetism of single, double, and triple-decker phthalocyanines focusing on MnPc, TbPc2, and Tb2Pc3 deposited on Ni, Mn, and CoO thin films. Depending on the number of Pc ligands, we find that the metal ions within the molecules couple either parallel or antiparallel to a ferromagnetic substrate. Whereas single-decker complexes such as MnPc form a unique magnetic entity with ferromagnetic films, the intrinsic single molecule magnet properties of TbPc2 and Tb2Pc3 remain largely unaltered. TbPc2 deposited on perpendicularly magnetized Ni films exhibits enhanced magnetic stability compared to TbPc2 in molecular crystals, opposite to TbPc2 deposited on in-plane magnetized Ni. Depending on the competition between uniaxial anisotropy, superexchange, and Zeeman interaction, the magnetic moment of TbPc2 can be aligned parallel or antiparallel to that of the substrate by modulating the intensity of an external magnetic field. This occurs also for Tb2Pc3, but the substrate-induced exchange coupling in triple-decker molecules is found to be short-ranged, that is, limited to the Tb ion closer to the ferromagnetic surface. Finally, we discuss the conditions required to establish exchange bias between molecules and antiferromagnetic substrates. We show that TbPc2 deposited on antiferromagnetic Mn thin films exhibits both exchange bias and enhanced coercivity when field cooled parallel to the out-of-plane easy axis. However, exchange bias does not extend to all molecules on the surface. On oxide antiferromagnets such as CoO we find no evidence of exchange bias for either TbPc2 or MnPc.

Direct observation of spin-resolved full and empty electron states in ferromagnetic surfaces.

We present a versatile apparatus for the study of ferromagnetic surfaces, which combines spin-polarized photoemission and inverse photoemission spectroscopies. Samples can be grown by molecular beam epitaxy and analyzed in situ. Spin-resolved photoemission spectroscopy analysis is done with a hemispherical electron analyzer coupled to a 25 kV-Mott detector. Inverse photoemission spectroscopy experiments are performed with GaAs crystals as spin-polarized electron sources and a UV bandpass photon detector. As an example, measurements on the oxygen passivated Fe(100)-p(1×1)O surface are presented.

A Short Review on the Magnetic Effects Occurring at Organic Ferromagnetic Interfaces Formed between Benzene-Like Molecules and Graphene with Ferromagnetic Surfaces

In this article, we briefly summarize our results gained from recent density functional theory simulations aimed to investigate the interaction between organic materials containing π-electrons (i. e., several benzene-like molecules and graphene) with ferromagnetic surfaces. We show how the strong hybridization between the pz -electrons that initially form the π molecular orbitals with the magnetic d-states of the metal influences the spin polarization, the magnetic exchange coupling, and the magnetization direction at hybrid organic-ferromagnetic interface. From a practical perspective, these properties play a very important role for device applications based on organic materials and magnetic surfaces.

Breakdown of the electron-spin motion upon reflection at metal-organic or metal-carbon interfaces

Spin-polarized electron scattering from ferromagnetic metal surfaces with submonolayer coverage by organic molecules shows complete breakdown of the angular spin motion of reflected electrons. This appears to be universal: independent of molecule, surface, electron energy, and other physical parameters.

Low-temperature ferromagnetic properties in Co-doped Ag2Se nanoparticles

β-Ag2Se is a topologically nontrivial insulator. The magnetic properties of Co-doped Ag2Se nanoparticles with Co concentrations up to 40% were investigated. The cusp of zero-field-cooling magnetization curves and the low-temperature hysteresis loops were observed. With increasing concentration of Co2+ ions mainly substituting AgI sites in the Ag2Se structure, the resistivity, Curie temperature Tc, and magnetization increased. At 10 T, a sharp drop of resistance near Tc was detected due to Co dopants. The ferromagnetic behavior in Co-doped Ag2Se might result from the intra-layer ferromagnetic coupling and surface spin. This magnetic semiconductor is a promising candidate in electronics and spintronics.

New constraints on Yukawa-type corrections to Newtonian gravity at short separations

We discuss the strongest constraints on Yukawa-type corrections to Newton’s gravitational law within a submicrometer interaction range, following from measurements of the Casimir force. In this connection, the complicated problems arising when comparing the measurement data with the Lifshitz theory are analyzed. Special attention is paid to the results of two recent experiments on measuring the Casimir interaction between ferromagnetic surfaces and sinusoidally corrugated surfaces at various angles between corrugations.