Spin transport in a thin graphite flake

We have studied the spin transport on a 30-nm thick and several micrometer long oriented graphite flake using a spin-valve configuration with four ferromagnetic Co electrodes of different widths and several \upmum\upmu\hbox{m} separation. A 5-nm thin Pt layer has been introduced in between the ferromagnetic Co injector/detector and the graphite surface. In spite of the conductivity mismatch problem, efficient electrical spin injection and detection in graphite has been achieved. The magnetoresistance in the local and half-local electrodes shows clear maxima with symmetry around zero field. The spin transport can be detected up to 150 K.     

Cross-Tie Domain Wall Ground State in Thin Films

No Heading Cross-tie domain wall is a complex two dimensional magnetic domain wall structure, often found experimentally in thin soft ferromagnetic films with thicknesses of about several exchange lengths. The structure of such a wall can roughly be imagined as a sequence of magnetic vortices and anti-vortices, arranged along a straight line. In this work, equilibrium energies of different one-dimensional magnetic domain wall configurations, existing in thin soft ferromagnetic films, are calculated (fully taking into account the exchange and long range dipolar interaction), based on the various classical Ritz models. They are compared to the energy of two-dimensional cross-tie domain wall, thus, estimating the region of the film thicknesses, where the cross-tie structure is the lowest energy domain wall configuration. The upper boundary of this region is estimated here for the first time. Convenient approximate analytical energy expressions in terms of elementary functions are given here both for classical and for cross-tie domain walls.PACS numbers: 75.75.+a, 75.25.+z, 75.60.–d.     

Effect of induced shape anisotropy on magnetic properties of ferromagnetic cobalt nanocubes

We report on the synthesis of ferromagnetic cobalt nanocubes of various sizes using thermal pyrolysis method and the effect of shape anisotropy on the static and dynamic magnetic properties were studied. Shape anisotropy of approximately 10% was introduced in nanocubes by making nanodiscs using a linear chain amine surfactant during synthesis process. It has been observed that, ferromagnetism persisted above room temperature and a sharp drop in magnetic moment at low temperatures in zero-field cooled magnetization may be associated with the spin disorder due to the effective anisotropy present in the system. Dynamic magnetic properties were studied using RF transverse susceptibility measurements at different temperatures and the singularities due to anisotropy fields were probed at low temperatures. Symmetrically located broad peaks are observed in the frozen state at the effective anisotropy fields and the peak structure is strongly affected by shape anisotropy and temperature. Irrespective of size the shape anisotropy gave rise to higher coercive fields and larger transverse susceptibility ratio at all temperatures. The role of shape anisotropy and the size of the particles on the observed magnetic behaviour were discussed.     

Controlling the Magnetic Properties of LaMnO3/SrTiO3 Heterostructures by Stoichiometry and Electronic Reconstruction: Atomic‐Scale Evidence

Interface‐driven magnetic effects and phenomena associated with spin–orbit coupling and intrinsic symmetry breaking are of importance for fundamental physics and device applications. How interfaces affect the interplay between charge, spin, orbital, and lattice degrees of freedom is the key to boosting device performance. In LaMnO₃/SrTiO₃ (LMO/STO) polar–nonpolar heterostructures, electronic reconstruction leads to an antiferromagnetic to ferromagnetic transition, making them viable for spin filter applications. The interfacial electronic structure plays a critical role in the understanding of the microscopic origins of the observed magnetic phase transition, from antiferromagnetic at 5 unit cells (ucs) of LMO or below to ferromagnetic at 6 ucs or above, yet such a study is missing. Here, an atomic scale understanding of LMO/STO ambipolar ferromagnetism is offered by quantifying the interface charge distribution and performing first‐principles density functional theory (DFT) calculations across this abrupt magnetic transition. It is found that the electronic reconstruction is confined within the first 3 ucs of LMO from the interface, and more importantly, it is robust against oxygen nonstoichiometry. When restoring stoichiometry, an enhanced ferromagnetic insulating state in LMO films with a thickness as thin as 2 nm (5 uc) is achieved, making LMO readily applicable as barriers in spin filters.     

Structure and Microscopic Magnetism of Epitaxial Ni–Mn–Ga Films

We report on the structural and magnetic properties of epitaxial thin films of the ferromagnetic shape memory material Ni–Mn–Ga prepared by DC magnetron sputter deposition. Different substrate materials, i.e., MgO(100) and Al₂O₃(11?20) allow for a tailored epitaxial growth. Using a sacrificial chromium buffer layer freestanding epitaxial films are obtained. In combination with photolithography partially freestanding structures such as microbridges are fabricated. The complex martensite crystal structure in substrate‐constrained and freestanding films is studied by means of X‐ray diffraction. The identified asymmetric twin variant configuration is associated with a macroscopic surface pattern observed by optical microscopy. The absence of magnetic‐field induced strain in the (100) oriented samples is explained on basis of the detected twin variant configuration using a simplified model. Taking advantage of the thin film geometry spectroscopic methods are applied to the samples. The measurements provide the first experimental test for changes in the electronic structure of the involved 3d metals during a martensitic transition. Exploiting the X‐ray magnetic circular dichroism quantitative information on the element‐specific spin and orbital magnetic moments are accessed. In addition, angular‐dependent experiments allow us to trace the microscopic origin of the magnetic anisotropy in Ni₂MnGa improving the fundamental understanding of this material.     

Magnetic imaging of a supercooling glass transition in a weakly disordered ferromagnet

Spin glasses are founded in the frustration and randomness of microscopic magnetic interactions. They are non-ergodic systems where replica symmetry is broken. Although magnetic glassy behaviour has been observed in many colossal magnetoresistive manganites, there is no consensus that they are spin glasses. Here, an intriguing glass transition in (La,Pr,Ca)MnO3 is imaged using a variable-temperature magnetic force microscope. In contrast to the speculated spin-glass picture, our results show that the observed static magnetic configuration seen below the glass-transition temperature arises from the cooperative freezing of the first-order antiferromagnetic (charge ordered) to ferromagnetic transition. Our data also suggest that accommodation strain is important in the kinetics of the phase transition. This cooperative freezing idea has been applied to structural glasses including window glasses and supercooled liquids, and may be applicable across many systems to any first-order phase transition occurring on a complex free-energy landscape.     

A general expression of magnetic force for soft ferromagnetic plates in complex magnetic fields

The experiments of ferromagnetic plates in different magnetic environments exhibit two distinct phenomena, i.e. the magnetoelastic instability of a ferromagnetic plate in transverse magnetic fields, and the increase of natural frequency of a ferromagnetic plate with low susceptibility in an inplane magnetic field. Although these two typical phenomena can be predicted separately by two kinds of theoretical models in which the magnetic forces are formulated by totally different expressions, no theoretical model has been found to commonly describe them. This makes it difficult to predict theoretically magnetoelastic interaction of a ferromagnetic structure in complex magnetic environment. A variational principle, here, is proposed to establish the governing equations of magnetoelastic interaction for soft ferromagnetic thin plate structures under complex magnetic fields. The functional is chosen as the summation of the magnetic energy and the strain energy as well as the external work from applied magnetic fields. From manipulations of the variational principle, the governing equations of the magnetic field and mechanical deformation together with an expression of equivalent magnetic force exerted on the ferromagnetic plates are obtained. It is shown that this theoretical model can commonly characterize the experimental phenomena of the magnetoelastic interaction aforementioned.     

A Model for Magnetic Ordering in Quasi-Low-Dimensional Magnet CsDy(MoO4)2

For quasi-low-dimensional magnet CsDy(MoO4)2 a model for the equilibrium spin arrangement in the magnetically ordered state is proposed on the ground of the symmetry analysis of neutron diffraction data. Low crystal symmetry of monoclinic syngony results in several spatial orientations of strongly anisotropic magnetic centers in the crystal and leads to the form of an order parameter more complicated than simply ferromagnetic or antiferromagnetic one. Measurements of magnetization along crystal axes have been performed both above and below the Neel point.     

Magnetoelectric Spin-FET for Memory,Logic, and Amplifier Applications

We propose a ballistic magneto-electric device that permits conductance modulation with both electric and magnetic fields applied perpendicular to its current conduction channel. Fields are applied through the ferromagnetic gates deposited on top of a HEMT heterostructure that contains a 2DEG for current conduction. The minimal-coupling Hamiltonian with spatially uniform electrical potentials, and delta Zeeman splitting is solved in the weak-coupling limit for which the Rashba spin orbit coupling is not considered. Ballistic transmission of electrons through a periodic system of zero-gauge double-pair magnetoelectric barriers is studied. Manipulation of barriers’ geometrical symmetry and configuration leads to the conception of a spin-FET for non-volatile storage and digital logic operations. The linear modulation of electron spin polarization (|P|) is also studied for its relevance to electrical signal amplification. Perpendicular magnetization of the ferromagnetic gates permits modulation of both |P| and electron transmission (T) threshold, the latter is particularly useful for spin logic design.     

Observation of Exchange Interaction in Iron(II) Spin Crossover Molecules in Contact with Passivated Ferromagnetic Surface of Co/Au(111)

Spin crossover (SCO) complexes sensitively react on changes of the environment by a change in the spin of the central metallic ion making them ideal candidates for molecular spintronics. In particular, the composite of SCO complexes and ferromagnetic (FM) surfaces would allow spin-state switching of the molecules in combination with the magnetic exchange interaction to the magnetic substrate. Unfortunately, when depositing SCO complexes on ferromagnetic surfaces, spin-state switching is blocked by the relatively strong interaction between the adsorbed molecules and the surface. Here, the Fe(II) SCO complex [Fe^II(Pyrz)₂] (Pyrz = 3,5-dimethylpyrazolylborate) with sub-monolayer thickness in contact with a passivated FM film of Co on Au(111) is studied. In this case, the molecules preserve thermal spin crossover and at the same time the high-spin species show a sizable exchange interaction of > 0.9 T with the FM Co substrate. These observations provide a feasible design strategy in fabricating SCO-FM hybrid devices.     

Probing magnetic anisotropy and spin polarization in spintronic materials

Magnetic anisotropy and spin polarization are fundamental parameters in ferromagnetic materials that have use in spintronic device applications. As the need for screening properties of new magnetic materials rises, it is important to have measurement probes for quantities such as anisotropy and spin polarization. We have developed two unconventional yet powerful techniques to study these parameters. A resonant RF transverse susceptibility method is used to map the characteristic anisotropy and switching fields over a wide range in temperature and magnetic fields. For studies of spin polarization, the phenomenon of Andreev reflection across ferromagnet-superconductor junctions is used to extract values of the transport spin polarization. The effectiveness of these approaches is demonstrated in candidate spintronic materials such as half-metallic CrO/sub 2/ thin films and arrays of monodisperse, single-domain Fe nanoparticles.     

Direct synthesis of shaped carbon nanoparticles with ordered cubic mesostructure

Shaped, mesoporous carbon nanoparticles (MSP-3) were prepared in high yield by a simple direct synthesis using a block copolymer surfactant as the mesopore-directing agent and a colloidal crystal to mold the external shape of the particles. The product consisted of monodisperse nanocubes and uniform nanospheres or tetrapods. The nanocubes contained regularly spaced, cagelike mesopores. The orientation of the cubic unit cell describing the mesopore symmetry coincided with the templated external cube faces.     

Continuous vortex cutting in type II superconductors with longitudinal current

The consequences of stipulating translational symmetry for a type II superconductor to which longitudinal electric current and longitudinal magnetic field are applied are investigated. The magnetic flux lines must cut each other continuously in order to generate an electric field in this symmetry. We describe the steady state by two interpenetrating vortex lattices moving into and out of the sample. We find for the slab and cylinder geometries that cutting, crossjoining, and subsequent straightening of the flux lines reduce the electric field, as compared with the normal conducting state, by a factor which is of the order of one over the total number of flux lines in the cylinder. We conclude that the much larger voltages observed in cylinders of several millimeters diameter can be explained only by a breakdown of translational symmetry. With translational ¥mmetry, the voltage initially increases as the third power of the current. The resulting vortex configuration is force-free. The transverse flux component increases and the longitudinal component decreases from the axis to the surface, leading to a paramagnetic moment. The drift or oscillation velocity of the flux lines is reduced by the same factor as the electric field. We predict low-frequency oscillations of the vortices near the surface of thin superconducting wires.     

Band model approach for the magnetic stiffness within ferromagnetic thin films

The magnetic stiffness within ferromagnetic thin films, considered as a tendency of the film to be homogeneous with respect to the distribution of magnetization directions, is defined on the basis of the domain structure as well as of the spin wave theory. The transversal stiffness parameter in thin films is calculated by means of the band model approach within a Hartree-Fock type of approximation. This parameter depends on the film thickness and, what is particularly interesting, on the surface effects.     

Plasmonic near-electric field enhancement effects in ultrafast photoelectron emission: correlated spatial and laser polarization microscopy studies of individual ag nanocubes

Electron emission from single, supported Ag nanocubes excited with ultrafast laser pulses (λ = 800 nm) is studied via spatial and polarization correlated (i) dark field scattering microscopy (DFM), (ii) scanning photoionization microscopy (SPIM), and (iii) high-resolution transmission electron microscopy (HRTEM). Laser-induced electron emission is found to peak for laser polarization aligned with cube diagonals, suggesting the critical influence of plasmonic near-field enhancement of the incident electric field on the overall electron yield. For laser pulses with photon energy below the metal work function, coherent multiphoton photoelectron emission (MPPE) is identified as the most probable mechanism responsible for electron emission from Ag nanocubes and likely metal nanoparticles/surfaces in general.     

Vortex matter in superconductor/ferromagnet hybrids

Vortex pinning in type-II superconducting films can be effectively controlled by combining these films with different ferromagnetic nanostructures. In this article an overview is presented of different types of ferromagnetic pinning centers. The investigated hybrid structures consist of Pb films that are deposited on top of arrays of ferromagnetic dots with in-plane magnetization (IMP) or out-of-plane magnetization (OPM), ferromagnetic films with IPM or OPM that contain arrays of submicron holes (antidots), or continuous films with OPM and a magnetic domain structure. Interesting effects such as field-polarity dependent vortex pinning and the dependence of the pinning strength on the domain structure of the ferromagnet are observed. Our experiments demonstrate that vortex pinning in superconductors is strongly influenced by the magnetic properties of the different ferromagnetic pinning centers.     

Spin filtering in graphene nanoribbons with Mn-doped boron nitride inclusions

We investigate the spin filtering effects in graphene nanoribbons, where inclusions of hexagonal boron nitride were introduced together with substitutional magnetic impurities. The embedded Mn-doped boron nitride regions serve as quasi-0D islands of diluted magnetic semiconductor in the otherwise metallic graphene nanoribbon. Our first principle approach based on non-equilibrium Green's functions gives the polarization of the spin current for structures with one or two Mn impurities as a function of the applied bias. For the two impurity case, ferromagnetic and antiferromagnetic spin configurations of the magnetic impurities are considered. The spin resolved current indicates that the analyzed structures are suitable for spin filter applications or for spin current switching devices.     

Exploring nanoscale magnetism in advanced materials with polarized X-rays

Nanoscale magnetism is of paramount scientific interest and high technological relevance. To control magnetization on a nanoscale, both external magnetic fields and spin polarized currents, which generate a spin torque onto the local spin configuration, are being used. Novel ideas of manipulating the spins by electric fields or photons are emerging and benefit from advances in nano-preparation techniques of complex magnetic materials, such as multiferroics, ferromagnetic semiconductors, nanostructures, etc.Advanced analytical tools are needed for their characterization. Polarized soft X-rays using X-ray dichroism effects are used in a variety of spectroscopic and microscopic techniques capable of quantifying in an element, valence and site-sensitive way basic properties of ferro(i)- and antiferromagnetic systems, such as spin and orbital moments, nanoscale spin configurations and spin dynamics with sub-ns time resolution. Future X-ray sources, such as free electron lasers will provide an enormous increase in peak brilliance and open the fs time window to studies of magnetic materials. Thus fundamental magnetic time scales with nanometer spatial resolution can be addressed.This review provides an overview and future opportunities of analytical tools using polarized X-rays by selected examples of current research with advanced magnetic materials.     

Self-construction of SnO2 cubes based on aggration of nanorods

The SnO₂ cubes with the rutile structure have been successfully synthesized without using any catalyst. Their morphology and microstructure were studied by field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HRTEM), and elected area electron diffraction (SAED). It is revealed that the SnO₂ nanocubes exhibit high crystalline quality. The size of the nanocubes ranges from 100 nm to 300 nm. The side surfaces of nanocubes are {110} planes, while their cube axes are [001] direction. The growth mechanism of SnO₂ nanocubes was discussed and we suggested vapor-solid process should dominate the growth. These SnO₂ nanostructures represent an important example of spontaneous organization.