Ultrathin Ferromagnetic Layers Research Articles

Voltage Control of Magnetization and Its Chemical Properties

Magnetization control using an electric field [1] will be useful because of its expected ultra-low power consumption and coherent behavior. Several experimental approaches to realize it have been done using ferromagnetic semiconductors [2], materials with magnetostriction together with piezo-driver [3], multiferroic materials [4], ferromagnetic metal films sintered in a liquid electrolyte [5], and ultra-thin ferromagnetic layer in solid-state junctions[6-8].One of the critical issues in the electric field switching is a realization of bi-stable switching. Since the electric field does not break time reversal symmetry, it does not remove degeneracy of two magnetic states with opposite magnetization. Therefore, a selection of an arbitral magnetic state is not straightforward [9]. Here, we demonstrate a realization of the bi-stable switching using a coherent precessional magnetization toggle switching in nanoscale magnetic cells with a few atomic FeCo (001) epitaxial layers adjacent to MgO barrier [10].The control of the magnetization by voltage requires well controlled interface structure. We have investigated chemical states of the ferromagnetic atoms at interface with MgO using XAS and XMCD, and found several problems that may happen at the interfaces in voltage devices, i.e. segregation [11] and voltage controlled reversible oxidization/reduction [12]. We discuss the significance of those effects and the intrinsic effect.

Curie temperature of ultrathin ferromagnetic layer with Dzyaloshinskii-Moriya interaction

We investigate the effect of the Dzyaloshinskii-Moriya interaction (DMI) on the Curie temperature of the ultrathin ferromagnetic layers. It has been known that the Curie temperature of the ferromagnet depends on spin wave excitation energies, and they are affected by DMI. Therefore, the ferromagnetic transition temperature of the ultrathin ferromagnetic layer must be sensitive on the DMI. We find that the Curie temperature depends on the DMI by using the double time Green's function method. Since the DMI is arisen by the inversion symmetry breaking structure, the DMI is always important in the inversion symmetry breaking ultrathin ferromagnetic layers.

Rashba Spin-Orbit Anisotropy and the Electric Field Control of Magnetism

The control of the magnetism of ultra-thin ferromagnetic layers using an electric field, rather than a current, has many potential technologically important applications. It is usually insisted that such control occurs via an electric field induced surface charge doping that modifies the magnetic anisotropy. However, it remains the case that a number of key experiments cannot be understood within such a scenario. Much studied is the spin-splitting of the conduction electrons of non-magnetic metals or semi-conductors due to the Rashba spin-orbit coupling. This reflects a large surface electric field. For a magnet, this same splitting is modified by the exchange field resulting in a large magnetic anisotropy energy via the Dzyaloshinskii-Moriya mechanism. This different, yet traditional, path to an electrically induced anisotropy energy can explain the electric field, thickness, and material dependence reported in many experiments.

Current-induced torques and interfacial spin-orbit coupling

In bilayer systems consisting of an ultrathin ferromagnetic layer adjacent to a metal with strong spin-orbit coupling, an applied in-plane current induces torques on the magnetization. The torques that arise from spin-orbit coupling are of particular interest. Here, we calculate the current-induced torque in a Pt-Co bilayer to help determine the underlying mechanism using first principles methods. We focus exclusively on the analogue to the Rashba torque, and do not consider the spin Hall effect. The details of the torque depend strongly on the layer thicknesses and the interface structure, providing an explanation for the wide variation in results found by different groups. The torque depends on the magnetization direction in a way similar to that found for a simple Rashba model. Artificially turning off the exchange spin splitting and separately the spin-orbit coupling potential in the Pt shows that the primary source of the "field-like" torque is a proximate spin-orbit effect on the Co layer induced by the strong spin-orbit coupling in the Pt.

Magnetic relaxation due to spin pumping in thick ferromagnetic films in contact with normal metals

Spin pumping is the most important magnetic relaxation channel in ultrathin ferromagnetic layers in contact with normal metals (NMs). Recent experiments indicate that in thick films of insulating yttrium iron garnet (YIG) there is a large broadening of the ferromagnetic resonance (FMR) lines with deposition of a thin Pt layer which cannot be explained by the known damping processes. Here we present a detailed study of the magnetic relaxation due to spin pumping in bilayers made of a ferromagnetic material (FM) and a NM. Two alternative approaches are used to calculate the transverse and longitudinal relaxation rates used in the Bloch-Bloembergen formulation of damping. In one we consider that the dynamic exchange coupling at the interface transfers magnetic relaxation from the heavily damped conduction electron spins in the NM layer to the magnetization of the FM layer while the other utilizes spin currents and the concept of the spin-mixing conductance at the interface. While in thin FM films, the relaxation rates vary with the inverse of the FM layer thickness; in thick films, they become independent of the thickness because in the FM/NM structure the FMR excitation has a surface mode character. Regardless of the thickness range the longitudinal relaxation rate is twice the transverse rate resulting in damping of the magnetization with constant amplitude characterizing a Gilbert process. The enhanced spin-pumping damping explains the experimental observations in YIG/Pt bilayers.

Dynamic Binding of Driven Interfaces in Coupled Ultrathin Ferromagnetic Layers

We demonstrate experimentally dynamic interface binding in a system consisting of two coupled ferromagnetic layers. While domain walls in each layer have different velocity-field responses, for two broad ranges of the driving field H, walls in the two layers are bound and move at a common velocity. The bound states have their own velocity-field response and arise when the isolated wall velocities in each layer are close, a condition which always occurs as H→0. Several features of the bound states are reproduced using a one-dimensional model, illustrating their general nature.

Spins on the move

Sankey and co-workers are interested in magnetic nanopillars that consist of two ultrathin ferromagnetic layers separated by a non-magnetic spacer. The resistance of such a structure depends on the relative orientation of the magnetization in the magnetic layers – a phenomenon known as magnetoresistance – and the device resistance will oscillate in time if the magnetization in either layer is made to precess.

Perpendicular spin valves with ultrathin ferromagnetic layers: Magnetoelectronic circuit investigation of finite-size effects

We address two finite-size effects in perpendicular transport through multilayers of ferromagnetic and normal metal layers: (i) the transport properties depend on the magnetic layer thickness when of the order or thinner than the spin-flip diffusion length and (ii) magnetic layers with thickness approaching the magnetic coherence length become transparent for spin currents polarized perpendicular to the magnetization. We use magnetoelectronic circuit theory to investigate both effects on angular magnetoresistance (aMR) and spin-transfer torque in perpendicular spin valves. We analyze recent aMR experiments to determine the spin-flip diffusion length in the ferromagnet permalloy as well as the Py\ensuremath{\mid}Co interface spin-mixing conductance and propose a method to measure the ferromagnetic coherence length.

Spin detectors operating in the medium-energy range

Solid-state spin filters consisting of ultrathin ferromagnetic layers can be used as convenient spin polarimeters. A few years ago, a good sensitivity was demonstrated using a Pd∕Fe bilayer deposited onto a n-type ozonized GaAs substrate, the spin detector being operated around 800eV primary energy. From the analysis of the experimental data, a spin-dependent transport scheme is proposed. This provides keys for the practical development of spin filters in the keV range, an energy domain which is well suited for application, and opens perspectives in the study of hot-electron transport in metals.

Thermal processing effects on ultra-thin ferromagnetic Fe layers grown on GaAs(0 0 1)

We used Brillouin light scattering and magneto-optical Kerr effect measurements to study the magnetic properties of ultra-thin 1.5nm Fe layers undergoing various annealing and high temperature growth processes on GaAs(001). We found that the thermal processing has a profound effect on the form and magnitude of the magnetic anisotropy and value of the coercivity. In addition, in situ scanning tunneling microscopy revealed that high temperature post-deposition annealing results in the formation of rectangular pits, while high temperature growth results in a rough faceted microstructure.

Evidence of spin-pumping effect in the ferromagnetic resonance of coupled trilayers

The spin dynamics between two ultrathin ferromagnetic layers exchange coupled via a nonmagnetic spacer is studied using in situ ferromagnetic resonance. The angular and the temperature dependence of the linewidth, especially the pronounced changes of the former, can be understood assuming the presence of an additional Gilbert damping resulting from the spin-current pumped through the interface between the ferromagnetic and nonmagnetic material or by considering the dynamic part of the interlayer exchange coupling. The competition between the Gilbert damping inherent in the ferromagnetic film and the Gilbert damping developed by the interface leads to an irregular behavior of the temperature dependence of the resonance linewidth for both the in-phase and out-of-phase precessing modes. It is shown that the oscillatory nature of the coupling is also revealed in the dependence of the resonance linewidth $\ensuremath{\Delta}H$ on the spacer thickness.

Specific features of magnetooptical spectra of Co/SiO2 hybrid multilayers

Magnetooptical spectra of Co(x)/SiO2(y) bilayers and [Co(x)/SiO2(y)]n hybrid multilayers (x and y are the layer thicknesses) are studied in the range 1.5–4.0 eV. In these layers, the ultrathin ferromagnetic layers are not continuous but consist of ellipsoidal Co nanoparticles with a concentration near the percolation threshold. It is found that (a) the sign, magnitude, and spectral shape of the magnetooptical signal depend strongly on the Co particle size; (b) the signal for the [Co(1.6 nm)/SiO2(3.0 nm)]6 multilayers at 3.0 eV has a record-high magnitude for Co-based structures, 3×10−2, which exceeds that for bulk Co about fivefold and about 50-fold that for a uniform Co film of an equivalent thickness; (c) the magnetooptical signal of multilayers depends non-linearly on the number of periods and the thickness of the dielectric layer y; and (d) the dependence of the signal of bilayers and multilayers with x=1.3 and 1.6 nm on the wavelength is nonmonotonic and exhibits clearly pronounced extrema. The data obtained for bilayers are attributed to the strong influence of percolation on the optical and magnetooptical parameters of the structure and interpreted in the framework of the effective medium approximation and macroscopic Fresnel magnetooptics.

Semiconductor Ferromagnetic Structures

Ferromagnetic semiconductor structures such as superlattices or trilayers form a new class of magnetic systems composed entirely of semiconductor materials. The examples are Ga 1- Mn x As—AlGaAs with the ferromagnetic layer of GaMnAs semimagnetic (diluted magnetic) semiconductor or EuS-PbS with the ferromagnetic member (EuS) of the family of europium chalcogenides. We discuss the spectrum of perspective ferromagnetic semiconductor materials, the effect of size and stress on magnetic properties of ultrathin semiconductor ferromagnetic layers, and the effect of interlayer exchange in all-semiconductor systems.

Improvement of crystallinity and magnetic characteristics of ferromagnetic-Co/sub 77/Cr/sub 20/Ta/sub 3//paramagnetic-Co/sub 65/Cr/sub 35/ bilayered films

In order to improve the crystallographic and magnetic characteristics of Co-Cr-Ta recording layers, optimization of the substrate temperature T/sub s/ for the seed layers was investigated. Crystalline orientation and magnetic characteristics of the ferromagnetic-Co/sub 77/Cr/sub 20/Ta/sub 3/ layers were improved by adjusting T/sub s/ of the paramagnetic-Co/sub 65/Cr/sub 35/ seed layer in the range of 150-200/spl deg/C. It was found that deposition of the seed layer with excellent c-axis orientation is effective to enhance the perpendicular magnetic anisotropy and to prepare an ultrathin ferromagnetic recording layer. T/sub s/-optimized deposition of the seed layers will be useful for fabricating perpendicular magnetic recording media.

Polarized neutron reflectometry – a historical perspective

Born in the early 1980s to study magnetic films, polarized neutron reflectometry (PNR) has enjoyed growing popularity as witnessed by the number of instruments assembled at neutron research centers. PNR has proved its usefulness by providing information as diverse as the penetration depth of the magnetic field in superconductors and the absolute value of the magnetic moments in ultrathin ferromagnetic layers; yet its widest application has become the study of the magnetic configurations in multilayers. Two types of reflectometers have been constructed: time of flight and crystal analyzer. The relative merits of the two types are discussed in the light of present and future applications.

Surface-enhanced magneto-optical effects in ferromagnetic superlattices

Surface-enhanced (surface-plasmon-polariton-mediated) magneto-optical effects are treated for superlattices consisting of alternating ultrathin ferromagnetic layers and noble-metal spacers. Within a developed self-consistent perturbative approach, reflectivity modulation, linear in magnetization, is calculated for the polar Kerr effect in terms of spatially averaged non-local response functions of the layers. A combined enhancement effect in magneto-optics of superlattices is predicted when both surface plasmons are excited and the number of ferromagnetic layers increases.

Giant Magnetoresistance Induced by Ultrathin Magnetic Layers

ABSTRACTWe introduce an interface selective structure, composed of a spin-valve on top of which a thick nonmagnetic back layer is deposited as a straightforward experimental tool to measure the GMR induced by ultrathin magnetic layers. The interface selectivity of spin-dependent scattering is evidenced by calculations and illustrated in both Co/Cu/Co and Ni80Fe20/Cu/Ni80Fe20 spin-valves by an almost discontinuous behavior in the GMR ratio. The temperture dependence of the extremely short length scale associated with this discontinuity is discussed in relation to the structure of ultrathin ferromagnetic layers.

Non-local magneto-optics of ultrathin ferromagnetic layers mediated by surface plasmon-polaritons

A theory is developed for magneto-optical effects in multilayer media including quasi-two-dimensional ferromagnetic layers. In the framework of the Green function approach an ultrathin ferromagnetic layer is described perturbatively, making use of a non-local dielectric permittivity tensor whose components at M = 0 and at M ≠ 0 ( M is the layer magnetization) are taken into account consequently in a self-consistent approximation and in the lowest-order one. The basic consideration concerns a three-medium model with a ferromagnetic layer embedded in a noble metal film whose magneto-optical response is analyzed as functions of the incidence angle and photon energy for different light polarizations. The results of the numerical calculations are presented under attenuated total reflection conditions, in which case the magneto-optical effects are considerably enhanced due to the excitation of surface plasmon-polaritons supported by the noble metal film. The surface-enhanced magneto-optical Kerr effect of this kind has recently been observed in Phys. Rev. Lett. 73 (1994) 3584, where it was treated within the theory developed below.

Spin-dependent transmission of low-energy electrons through ultrathin magnetic layers.

We present an experiment in which the attenuation of a spin-polarized free electron beam is measured by direct transmission through an ultrathin ferromagnetic layer. The self-supported metal target consists of a 1-nm-thick cobalt film sandwiched between 21- and 2-nm-thick gold layers. Measurements are performed throughout a wide energy range (incident electron energies from 4 eV to 50 eV above the Fermi level). At low energy, close to the clean surface vacuum level, we find that the transmission coefficient of minority spin electrons is about 0.7 times that of majority spin electrons.

Magnetic properties of MnSb/Sb superlattices

MnSb/Sb superlattice films containing ultrathin ferromagnetic MnSb layers (0.36–3.0 ML) have been investigated by magnetization measurements. Samples containing MnSb thinner than 3.0 ML show perpendicular magnetic anisotropy. Magnetization vs. temperature curves of samples with MnSb monolayers strongly depend on the external field.