Magnetic Overlayer Research Articles

Surface Study of Fe3O4 Nanoparticles Functionalized With Biocompatible Adsorbed Molecules.

Surfaces of iron oxide of ferrimagnetic magnetite (Fe3O4) nanoparticles (MNPs) prepared by Massart's method and their functionalized form (f-MNPs) with succinic acid, L-arginine, oxalic acid, citric acid, and glutamic acid were studied by dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FTIR-S), UV-vis, thermogravimetric analysis (TGA)/differential scanning calorimetry (DSC), X-ray photoelectron spectroscopy (XPS), and reflection electron energy loss spectroscopy (REELS). The XPS analysis of elements and their chemical states at the surface of MNPs and f-MNPs revealed differences in chemical bonding of atoms, content of carbon–oxygen groups, iron oxide forms, iron oxide magnetic properties, adsorbed molecules, surface coverage, and overlayer thickness, whereas the Auger parameters (derived from XPS and Auger spectra) and elastic and inelastic scattering probabilities of electrons on atoms and valence band electrons (derived from REELS spectra) indicated modification of surface charge redistribution, electronic, and optical properties. These modified properties of f-MNPs influenced their biological properties. The surfaces biocompatible for L929 cells showed various cytotoxicity for HeLa cells (10.8–5.3% of cell death), the highest for MNPs functionalized with oxalic acid. The samples exhibiting the largest efficiency possessed smaller surface coverage and thickness of adsorbed molecules layers, the highest content of oxygen and carbon–oxygen functionalizing groups, the highest ratio of lattice O2− and OH− to C sp2 hybridizations on MNP surface, the highest ratio of adsorbed O− and OH− to C sp2 hybridizations on adsorbed molecule layers, the closest electronic and optical properties to Fe3O4, and the lowest degree of admolecule polymerization. This high cytotoxicity was attributed to interaction of cells with a surface, where increased content of oxygen groups, adsorbed O−, and OH− may play the role of additional adsorption and catalytic sites and a large content of adsorbed molecule layers of carboxylic groups facilitating Fenton reaction kinetics leading to cell damage.

Nondegenerate valleys in the half-metallic ferromagnet Fe/WS2

We present a first principles investigation of the electronic properties of monolayer WS$_2$ coated with an overlayer of Fe. Our ab initio calculations reveal that the system is a half-metallic ferromagnet with a gap of $\sim 1$ eV for the majority spin channel. Furthermore, the combined effect of time-reversal symmetry breaking due to the magnetic Fe overlayer and the large spin-orbit coupling induced by W gives rise to non-degenerate K and K$'$ valleys. This has a tremendous impact on the excited state properties induced by externally applied circularly polarized light. Our analysis demonstrates that the latter induces a singular hot spot structure of the transition probability around the K and K$'$ valleys for right and left circular polarization, respectively. We trace back the emergence of this remarkable effect to the strong momentum dependent spin-noncollinearity of the valence band involved. As a main consequence, a strong valley-selective magnetic circular dichroism is obtained, making this system a prime candidate for spintronics and photonics applications.

Growth and characterization of hybrid insulating ferromagnet-topological insulator heterostructure devices

We report the integration of the insulating ferromagnet GdN with epitaxial films of the topological insulator Bi2Se3 and present detailed structural, magnetic, and transport characterization of the heterostructures. Fabrication of multi-channel Hall bars with bare and GdN-capped sections enable direct comparison of magnetotransport properties. We show that the presence of the magnetic overlayer results in suppression of weak anti-localization at the top surface.

Asymmetric critical current of niobium microbridges with ferromagnetic stripe

A highly asymmetric current-voltage (IV) characteristic is reported in long Nb microbridges with a ferromagnetic cobalt overlayer. In the hysteretic regime, asymmetry as high as 100% was observed between forward and reverse critical currents. Such an asymmetry is a result of the competing magnetic fields produced at the edge of the bridge due to the bias current and the magnetic overlayer. While the temperature dependence of the critical current in the shallow branch of hysteretic IV curves (Ic−) followed a typical Ginzburg–Landau (GL) behavior, Ic+, the critical current of the other branch increased more sharply at low temperature. Local heating of the bridge due to the bias current was found to have no significant effect on the nature of critical current asymmetry.

Oscillatory Curie temperature of 2D-ferromagnets

The effective exchange interactions of the magnetic overlayer Fe/Cu(0 0 1) covered by a Cu-cap layer of varying thickness were calculated in real space from first principles. The effective two-dimensional Heisenberg Hamiltonian was constructed and used to estimate spin-wave stiffness constants and overlayer Curie temperatures within the random-phase approximation. Oscillatory behavior of the overlayer Curie temperature, spin-wave stiffness, and magnetic moment as a function of the cap-layer thickness was found and explained.

A novel sulfur-passivation method and magnetic overlayers on passivated III–V semiconductor surface

A sulfur passivation method for GaAs, CH 3CSNH 2 treatment has been developed. It is quite effective for removing the surface oxide layer and forming the sulfide passivation layer on GaAs surface, with sulfur atoms bound with Ga and As atoms. After being annealed, a stable sulfur passivation layer is formed. The enhancement of PL intensity reveals the reduction of surface non-radiative recombination and the density of surface states. Moreover, the investigation has been made for the role of S-passivation on interfacial interaction between magnetic overlayer and GaAs. The interdiffusion of As, Ga into overlayer is effectively inhibited, and the magnetization of Fe overlayers is enhanced. In addition, a relationship has been found between the surface chemical structure of the substrates and the magnetic property of overlayers.

Symmetry breaking at magnetic surfaces and interfaces

Examples are presented of how symmetry breaking enters into consideration of the physical properties of magnetic surfaces and ultrathin films. The role of magnetic anisotropy is discussed with a view to understanding: (i) the existence of two-dimensional magnetic long-ranged order at finite temperature; (ii) magnetization scaling behavior at the Curie transition; (iii) the two-dimensional spin reorientation transition; and (iv) step-induced magnetic behavior. Experimental examples cited include ultrathin magnetic Fe and Co overlayer and wedge structures grown onto single crystal substrates that are either flat or curved to produce vicinal surfaces with a continuous gradient in the step density. Also included is an example of an atomically flat manganite intergrowth that appears as a stacking fault in a bulk single crystal of a naturally layered structure.

Increasing coercivity of CoCrTaPt/Cr thin film media by interdiffusion of Mn and Cu overlayers by low temperature annealing

Mn and Cu overlayers were studied with CoCrTaPt/Cr films on glass substrates. Low temperature postannealing reveals an increase in the in-plane coercivity of 700 Oe for Mn and 400 Oe for Cu. X-ray diffraction patterns indicate that the change in magnetic properties is unlikely due to the improvement of crystalline structure. It is suggested that the interdiffusion between magnetic layer and overlayer plays a key role in the coercivity increase. ΔM curves show reduced exchange coupling between grains. It is believed that during low temperature annealing, the diffusion of the overlayer atoms most likely occurs at the grain boundaries, which results in better segregation between grains that may lead to higher coercivity and lower media noise.

Modification of aluminum thin films

We have utilized several processing techniques to locally modify the superconducting transition temperature and the normal state resistivity of aluminum thin films. The techniques of ion implantation, application of a magnetic overlayer, and reactive ion etching have been used to fabricate S-N interfaces with controlled differences in transition temperatures and normal state resistances. Lithographic techniques and CF4 reactive ion etching at low power and pressure reliably produce two-dimensional S-N structures of desired dimensions in the limit where the transition temperatures and normal state electronic properties of the two regions are close to each other.

Magnetic and optical characteristics of bilayered films composed of Tb-Fe-Co layer with overlayers of Co-Cr, Ni-Fe, Ta, and C

Amorphous Tb-Fe-Co single layers and Tb-Fe-Co/M (M: Ta, C, Co82Cr18, Ni81Fe19) bilayers have been prepared by using a facing targets sputtering apparatus. The Tb-Fe-Co films deposited by Kr sputtering exhibited internal stress as low as 1×109 dyn/cm2 even at a pressure as low as 0.2 mTorr. The Tb-Fe-Co/Ta bilayers showed a Kerr rotation angle as large as 0.85° at the thickness of Tb-Fe-Co layer of about 30 nm. The direction of magnetic moments in Tb-Fe-Co layers were arranged antiparallel by a soft magnetic Ni81Fe19 overlayer.

Photo-assisted tunneling at ferromagnet/semiconductor interfaces

We have measured the polarization-dependent photoresponse of ferromagnet-insulator-(III–V) semiconductor thin film tunnel structures as a function of wavelength. When the structures have no interfacial barrier, we find that the response agrees well with calculations of magneto-optical transmission through the magnetic overlayer. However, when a tunnel barrier is present and excitation nearly resonant with the semiconductor bandgap, we observe significant deviations from the magneto-optical effects. These deviations are attributed to spin-dependent electron transmission from the III–V semiconductor acting as a spin-polarized source of tunneling electrons.

Spin-dependent transmission at ferromagnet/semiconductor interfaces

We have measured the polarization-dependent photoresponse of ferromagnet-insulator-(III–V) semiconductor thin film tunnel structures as a function of wavelength. When the structures have no interfacial barrier, we find that the response agrees well with calculations of magneto-optical transmission through the magnetic overlayer. However, when a tunnel barrier is present and the excitation is nearly resonant with the semiconductor bandgap, we observe significant deviations from the magneto-optical effects. These deviations are attributed to spin-dependent electron transmission from the III–V semiconductor acting as a spin-polarized source of tunneling electrons.

Magnetic ordered structures by indirect exchange in metallic overlayers.

We investigate the role of the two-dimensional Ruderman-Kittel-Kasuya-Yosida interaction in determining the ordered structure of a magnetic metallic overlayer. It is found that, under suitable conditions, a noncollinear ordered state may be stable at low temperatures, even when the direct exchange interaction prevails. This effect should be relevant to rare-earth, as well as to transition-metal, epitaxial ultrathin films.

Effects of film growth mode on magnetic exchange coupling in trilayer structures (abstract)

In situ temperature-dependent spin-polarized cascade electron spectroscopy is used to study exchange coupling processes between a magnetic substrate and a magnetic overlayer separated by a nonferromagnetic spacer layer. Relative Auger intensities of the overlayer/substrate configurations as a function of overlayer coverage together with a model by Ossicini are used to evaluate the degree to which these trilayer structures grow by a continuous layer growth mode. Measurement of the spin polarization and Auger spectroscopy are used during film growth as criteria of chemical film purity. Evidence will be given to show that the oscillatory magnetic coupling behavior as a function of Cr thickness of a NiFe/Cr/NiFe trilayer changes significantly depending on the deposition temperature, which may be associated with microstructural changes. Competing coupling mechanisms are thought to account for this behavior. Data will also be presented to suggest how trace quantities of residual gas sorbates, presumably acting as surfactants, can dramatically alter the magnetization behavior of ultrathin films. Significant increases in magnetization of a 5-Å Fe overlayer film in a Fe/Ta/Fe trilayer structure were observed and are attributed to changes in wetting characteristics at the Fe/Ta interface which cause morphological changes in the film structure. Conversion of weakly interacting magnetic islands to a long-range ferromagnetic continuous film medium exhibiting remanence and a steady-state Brillouin functional M(T ) behavior is thought consistent with these observations. The possible significance of these kinds of observations to some conflicting data in the literature will be mentioned.

Magnetic Properties and Micromagnetic Character of RF Sputtered Co Thin Films on Thin Cr Underlayers

ABSTRACTThe effect of thin Cr underlayers on the coercivity, the initial susceptibility, and micromagnetic structure of RF sputtered Co/Cr thin films have been studied. A reciprocal relationship between the initial susceptibility and the coercivity of the RF sputtered Co/Cr films were observed. Micromagnetic images of these Co/Cr films obtained by Fresnel mode Lorentz TEM revealed a refinement of the micromagnetic images correlated with a simultaneous increase in coercivity and decrease in initial susceptibility. Such behavior is expected in cases where the initial magnetization and magnetization reversal are hindered by the variation of local energy with the magnetic moment distribution. The interaction of the Cr underlayers with the Co magnetic overlayer very likely induces fluctuations in local magnetic energy of the Co films. Accordingly, the initial magnetization and magnetization reversal are likely to be impeded by fluctuations of the magnetic energy resulting in the observed increase in coercivity and decrease in initial susceptibility.

Electronic structure of Fe overlayers on Si(1 1 1)

The electronic band structure of Fe overlayers on Si(1 1 1) has been studied by angle resolved photoemission with synchrotron radiation. Formation and evaluation of the interface region have been followed for incremental Fe coverages between 4 Å and 50 Å. The results indicate that films grown on room temperature substrates form an antiferromagnetic buffer layer and silicide formation followed by a pure Fe films (30 Å), while films grown at elevated temperatures (200°C or higher) retain the Fe silicide structure. These results provide information as to the suitability of Fe overlayer on Si(1 1 1) for subsequent magnetic overlayer and heterostructures.

Theory of spin waves in magnetic interfaces, superlattices and disordered layer structures

A general recursion method for calculating the exact local spin-wave Green function in an arbitrary ferromagnetic interface, superlattice and disordered layer structure is developed. The method is applied to magnetic insulator structures described by a nearest-neighbour exchange Hamiltonian. It is shown that the complete response function of an arbitrary layer structure can be generated from a single matrix element of the Green function in the surface plane of a magnetic overlayer. The proposed algorithm for overlayers is very simple, computationally stable and extremely accurate. The method is used to determine the exact exchange stiffness D of a ferromagnetic superlattice and of a disordered layer structure. The range of validity of the exact result for D is discussed in the light of recent experiments on the temperature dependence of the surface magnetisation. The application of the method to ferromagnets with long-range exchange interactions and to metallic layer structures is also discussed.

Temperature dependence of the magnetization in ferromagnetic surfaces and overlayers

The effect of softening of the exchange in a magnetic overlayer (surface) on the temperature dependence of the surface magnetization M s( T) in the spin-wave regime is investigated. A recursion method for calculating the surface density of spin-wave states (DOS) is developed. It is applied to a magnetic overlayer of n atomic layers with softened exchange between the layers. The principal result obtained for an overlayer of any thickness between n = 1 and n = 20 is that the surface DOS and, therefore, the temperature dependence of the surface magnetization M s( T) are determined by the exchange interactions in the overlayer (surface). The classical result of surface magnetism that M s( T) follows the Bloch T 3 2 law but decreases twice as fast as in the bulk breaks down at temperatures as low as 1% of the Curie temperature T c. Above this temperature, there is a rapid crossover to a second extended region of an effective T 3 2 law with a prefactor determined by the surface exchange. This second region of the T 3 2 law which extends up to temperatures T ≈ 0.4 T c provides a perfect explanation for the observed M s( T) which was determined recently by SPLEED for Ni 40Fe 40B 20 surface and by Mössbauer spectroscopy for Fe(110) surface.

Magnetic microcrystals and surface layers in as-quenched and hydrogenized Fe90Zr10 glass

Conversion-electron Mössbauer spectroscopy, energy-dispersive x-ray spectroscopy, and x-ray diffraction were used for surface studies of the as-quenched and hydrogenized metallic glass Fe90Zr10. Surface layers of as-quenched ribbon, paramagnetic at room temperature, were found to be overlaid by a ferromagnetic thin film. The overlayer is composed of α-Fe microcrystals and a crystalline compound (Fe3Zr). Magnetic α-Fe microcrystals are about 500 Å in diameter and are oriented with the (200) plane parallel to the Fe90Zr10 ribbon plane. Mössbauer spectra show that the magnetic moments of the precipitated small particles lie in the ribbon plane. A hydrogen-induced magnetic phase transformation, previously reported for bulk Fe90Zr10, was observed in the amorphous surface layers. At the same time, the magnetic crystalline overlayer disappears on the ribbon shiny surface.

Surface segregation of iron by hydrogenation of FeTi: An 57Fe mössbauer conversion electron study

Mössbauer spectroscopy of 57Fe conversion electrons was used to study FeTi subjected to 51 hydrogen absorption-desorption cycles. This technique probes both the surface and part of the bulk, and it can also distinguish between magnetic and non-magnetic iron in the lattice. Our measurements suggest the presence of a magnetic overlayer on FeTi made up of iron clusters and the absence of magnetic moments on the iron contained in the bulk. This results shows that the magnetic moment observed in bulk magnetization studies in fact originates from this segregated iron layer on the surface of FeTi.