Magnetic Layered Structures Research Articles

Magnetic and magnetoresistance studies of the evolution of the magnetic layer structure with Co layer thickness in electrodeposited Co–Cu/Cu multilayers

The structural transformation of electrochemically deposited Co–Cu/Cu multilayers with magnetic layer thickness was monitored via measurements of magnetization and magnetoresistance. For this, electrodeposition of [Co (tCo nm)/Cu (4nm)]50 multilayers were carried out by varying the Co-layer thickness (tCo) down to 0.2nm from a sulfate based single solution electrolyte. Magnetization measurements showed the appearance of anisotropy, increase in remanence magnetization and coercivity with systematic increase of tCo from 0.2 to 1nm. Magnetic field direction dependent magnetoresistance (MR) measurements revealed that the isotropy in MR changes with tCo from tridimensional at 0.2nm to in-plane at 0.4nm to fully anisotropic at 1.0nm. This illustrated that a fully granular magnetic Co-layer structure (tCo=0.2nm) transforms into discontinuous layered one (tCo=0.4nm) due to coalescence of suparparamagnetic regions to a continuous ferromagnetic layer (tCo=1nm).

Spin transport in Au films: An investigation by spin pumping

The thickness and temperature dependence of spin transport in Au has been investigated in multilayer films via the spin pumping effect. To study spin transport in Au, single layer GaAs/16Fe/(d)Au(001) and double layer GaAs/16Fe/(d)Au/12Fe/20Au(001) were investigated using ferromagnetic resonance (FMR), where d = 20, 300, and the numerals preceding Fe and Au indicate the layer thickness in atomic layers (AL). FMR measurements were performed at frequencies ranging from 27.3 to 40.6 GHz and at temperatures ranging from 88 to 295 K. By measuring the total Gilbert damping in the 16Fe layer as a function of d and temperature for both single and double magnetic layer structures and by utilizing the spin diffusion equation, one is able to determine the spin mixing conductance, g↑↓, at the Fe/Au interface, and the spin flip relaxation time, τsf, in Au as a function of temperature. The temperature dependence of the momentum relaxation time, τm, in Au was measured independently by means of electron transport measurements in a van der Pauw configuration. It has been found that the spin flip relaxation time, τsf, in Au is dominated by phonon interactions.

Plasma Edge Transport with Stochastic Magnetic Field Structures

Controlling the transport in the plasma edge of high temperature plasmas has recently been extended by a sophisticated option - the stochastization of the magnetic cage confining the plasma. The idea is to induce a chaotic magnetic field structure in the edge which can act as a magnetic valve to control heat and particle fluxes between the confined plasma and the plasma facing components. This tool is applied in both, stellarators as well as tokamaks. In this lecture an introduction into the topic will be given. The topics are (a) generation and structure of chaotic magnetic edge layers, (b) plasma transport with stochastic magnetic fields including the resulting three-dimensional plasma wall interaction and (c) impact of a plasma response. However, this field is matter of intense ongoing research and hence this lecture gives a systematic introduction into the challenges based on examples from the TEXTOR tokamak.

Spin Polarimetry and Magnetic Dichroism on a Buried Magnetic Layer Using Hard X-ray Photoelectron Spectroscopy

The spin-resolved electronic structure of buried magnetic layers is studied by hard X-ray photoelectron spectroscopy (HAXPES) using a spin polarimeter in combination with a high-energy hemispherical electron analyzer at the high-brilliance BL47XU beamline (SPring-8, Japan). Spin-resolved photoelectron spectra are analyzed in comparison with the results of magnetic linear and circular dichroism in photoelectron emission in the case of buried Co2FeAl0.5Si0.5 layers. The relatively large inelastic mean free path (up to 20 nm) of fast photoelectrons enables us to extend the HAXPES technique with electron-spin polarimetry and to develop spin analysis techniques for buried magnetic multilayers and interfaces.

Spin Polarimetry and Magnetic Dichroism on a Buried Magnetic Layer Using Hard X-ray Photoelectron Spectroscopy

The spin-resolved electronic structure of buried magnetic layers is studied by hard X-ray photoelectron spectroscopy (HAXPES) using a spin polarimeter in combination with a high-energy hemispherical electron analyzer at the high-brilliance BL47XU beamline (SPring-8, Japan). Spin-resolved photoelectron spectra are analyzed in comparison with the results of magnetic linear and circular dichroism in photoelectron emission in the case of buried Co2FeAl0.5Si0.5 layers. The relatively large inelastic mean free path (up to 20 nm) of fast photoelectrons enables us to extend the HAXPES technique with electron-spin polarimetry and to develop spin analysis techniques for buried magnetic multilayers and interfaces.

Direct and two-phonon Orbach-Aminov type spin-lattice relaxation in molecular magnet V15

In this article we propose a model of spin-phonon relaxation in K6[VIV 15As6O42(H2O)]-8H2O, the so called V15 cluster exhibiting the unique layered magnetic structure. The work is motivated by the recent observation of the Rabi oscillation [1] in this system and aimed to elucidate the role of spin-phonon interaction as a source of decoherence. The spin-phonon coupling is assumed to appear as a result of the modulation of the isotropic and antisymmetric (Dzyaloshinsky-Moriya) exchange interactions in the central triangular layer of vanadium ions by the acoustic lattice vibrations. The relaxation rates are estimated within the Debye model for the lattice vibrations. Within the pseudo-angular momentum representation the selection rules for the direct (one-phonon) transitions between Zeeman levels are derived and a special role of the antisymmetric exchange is underlined. The probabilities of the two-phonon Orbach-Aminov type processes are evaluated as well, while the Raman type relaxation is shown to have a negligible importance at low temperatures at which the Rabi oscillations have been detected.

Non-reciprocal devices using attenuated total reflection and thin film magnetic layered structures

There is need for non-reciprocal devices such as circulators and isolators. Although such devices are common at frequencies below 10 GHz, there is a lack of compact, low-weight, devices at higher microwave frequencies. This paper examines the non-reciprocal behavior associated with attenuated total reflection (ATR) for multi-layered dielectric and magnetic structures. Non-reciprocal behaviors produced by ATR have been explored for semi-infinite magnetic materials. This paper focuses on ATR behavior with magnetic films of finite thickness, from thick layers of around 3 cm to thin layers of about 1 µm. The results show significant non-reciprocity even for magnetic layers less than 0.1 cm thick, with reflection loss differences of more than 30 dB between positive and negative signal propagation. Results are presented for yttrium iron garnet and M type barium hexagonal ferrites. The two materials allow nonreciprocal behavior at different frequencies, 5–20 GHz for the garnet and 45–80 GHz for the hexagonal ferrite.

Damping phenomena in Co90Fe10/Ni multilayers and alloys

We used perpendicular ferromagnetic resonance to measure the damping parameter in Co90Fe10/Ni multilayers over a wide range of layer thicknesses. The magnetic anisotropy within this range varied from in-plane to out-of-plane. We measured (Co90Fe10)xNi1−x alloys of identical thicknesses over the same compositional range of Co90Fe10 and Ni in order to isolate the influence of the multilayer structure. The damping parameter varied from 0.004 to 0.030 and depended only on the relative amounts of Co90Fe10 and Ni and was independent of the magnetic anisotropy and layer structure.

Effect of interfacial scattering on spin-dependent thermoelectric properties in diluted magnetic semiconductor layered structures

We propose a theoretical model to analyze spin-dependent surface and interface roughness effects on the magnetoelectric and thermomagnetoelectric properties of heterostructures based on diluted magnetic semiconductors (DMSs). Our calculations are based on spin-dependent Boltzmann semiclassical equation (BSE) in the relaxation-time approximation. We consider a typical GaMnAs/GaAs/GaMnAs trilayer in current in plane (CIP) geometry, and calculate the currents for parallel and anti-parallel alignments to deduce the magnetoresistance and thermopower. In the boundary conditions, we include the parameter P which denotes the surface roughness probability, while interfacial roughness is characterized by the ratio S, where both are spin-dependent. These kinds of roughness can either increase or decreases the electric and thermoelectric quantities. The magnitude of the effects depends on the surface and interface scattering parameters and the width of the layers. The magnetoresistance (MR) and spin polarization (SP) reach to about 100%, when the interface layer scatters only one type of majority or minority carriers and the influence of outer surfaces on the scattering carriers is infinitesimal. In particular, it is seen that the MR increases when the interface scattering decreases. The presented numerical results are specially useful for designing spin electronics devices.

Interaction of neutrons with layered magnetic media in oscillating magnetic field

New experimental possibilities of investigating layered magnetic structures in oscillating magnetic fields are discussed. Spin-flip and nonspin-flip neutron reflection and transmission probabilities show a frequency dependency near the magnetic neutron resonance condition. This allows to increase the precision of the static magnetic depth profile measurements of the magnetized matter. Moreover, this opens new possibilities of measuring the induction of the oscillating field inside the matter and determining the magnetic susceptibility of the oscillating magnetic field. Refraction of neutrons as they pass through a magnetic prism in the presence of an oscillating magnetic field is also investigated. A non-polarized neutron beam splits into eight spatially separated neutron beams, whose intensity and polarization depend on the strength and frequency of the oscillating field. Also, it is shown that the oscillating magnetic permeability of an angstrom-thick layer can be measured with a neutron wave resonator.

Breather states and compound solitons in a ferromagnet-insulator-metal structure

Nonlinear dynamic states with a fine internal structure, such as breathers, “breather lattices,” kinks, kink-antikink bound states, and their spatially periodic generalizations have been constructed in terms of a modified Korteweg-de Vries equation for a system of magnetostatic waves in a ferromagnet-insulator-metal structure. Specific features of the nucleation, interaction, and transformation of these states have been studied in layered magnetic structures.

Crystallographic anisotropy and distortions in helicoidal magnetic structure

The appearance and development of aperiodic distortions in a helicoidal, layered magnetic structure with increasing crystallographic magnetic anisotropy in the magnetization rotation plane have been theoretically studied. A simple phase diagram for this system is proposed. It is established that, at a weak anisotropy, the spiral splits into regions of various lengths with an approximately uniform rotation of the magnetization in each region and a deviation from uniformity at the boundaries; the stronger the anisotropy, the shorter the regions and the greater the deviations. In the limit of high anisotropy, the minimum energy of the system corresponds (depending on the ratio of interlayer exchange integrals J1 and J2) to either a spiral with constant angular pitch (a multiple of the angle between easy axes) or a double antiferromagnetic structure with a four-layer period. In the case of sixth-order anisotropy with |J1| = −J2, the energies of phases with different periods (four and six layers for J1 > 0; four and three layers for J1 < 0) coincide and the excess boundary energy vanishes. In the case of a fourth- and second-order anisotropy, the analogous anomalies appear at |J1| = −2J2. As a result, the magnetic structure at these points becomes unstable and the phase diagram exhibits the corresponding singularities.

Coercive force and magnetization reversal of Co—P Films obtained by chemical deposition

Results from studying three-layer films with magnetic layers produced on the basis of the Co—P compound and a nonmagnetic Ni—P layer are presented. It is shown that the features of the magnetization reversal of the film are due to the kinetics of the formation of the crystalline structure of the hard magnetic layer. It is established that at low thickness, this layer consists of separate grains of the crystalline phase. With an increase in thickness, this layer transitions to the homogeneous polycrystalline state. These results allow us to explain the anomalous change of the coercive force and the displacement field of the soft magnetic layer depending on the thickness of the hard magnetic layer.

Spin-torque generation by dc or ac voltages in quasi-one-dimensional magnetic layered structures

A general expression of the current induced spin torque in a magnetic layered structure in the presence of external dc or ac voltages is derived in the framework of the scattering matrix approach. A detailed analysis is performed for a magnetic-nonmagnetic-magnetic trilayer connected to external leads in the presence of dc voltage bias in the ballistic regime. Alternatively, the possibility of producing spin torque by means of the adiabatic ac modulation of external gate voltages (quantum pumping) is proposed and discussed.

Crystal Orientation of CO35Fe65 Thin Film for the GHZ Frequency Use with More Than 10 GHZ

The Co35Fe65 magnetic films with Ru underlayer were fabricated using a special RF-magnetron sputtering method and characterized. The underlayer coating effect of improving soft magnetic properties for the GHz frequency use was investigated relating to the surface roughness and thickness of Ru underlayer and the crystalline structure and orientation of as-deposited magnetic layers. The coating of the underlayer with thickness of 10-30 A was so much effective to achieve a high uniaxial magnetic anisotropy field of 1000 Oe with the low coercivity less than 10 Oe for the higher driving frequency (ferromagnetic resonance frequency over 10 GHz), by inducing about 1% elongation of lattice parameter in the c direction of_the bct Co-Fe magnetic phase, preferred crystalline orientation of (110), (200) of the bct Co-Fe magnetic phase, and about 60% decrease of crystallite size in the Co-Fe layer. The (110) plane was preferentially parallel to the film plane and the elongated lattice in the c direction was oriented preferentially parallel to the easy magnetic axis in accord with the direction of the (200) plane space.

A neutron powder diffraction study of the helimagnetic structure of TlCo 2Se 2− xS x

The magnetic layer structure of TlCo 2Se 2− x S x has been thoroughly re-investigated with neutron powder diffraction. The cobalt magnetic moments are ferromagnetically arranged within the layers, but the interlayer coupling differs profoundly with varying composition ( x): the spins in TlCo 2Se 2 form a helix along the c-axis with a turning-angle of ∼119° at 1.4 K. This kind of helical structure prevails for 0≤ x≤1.5 with a gradual decrease of the angle with increasing sulphur content, down to 34°, showing an almost linear relationship with the interlayer distance of Co–Co. For x≥1.75 the interlayer coupling changes to ferromagnetic. Unexpectedly, two helices were found to coexist at x=0.5 and x=1.0. The interaction between adjacent cobalt layers is there characterized by an incommensurate angle (106°, resp., 73°) together with a commensurate angle of 90°. The magnetic structures have been refined as two magnetic phases, each having a characteristic wave vector. A tentative model where the symmetry of the structure and the interlayer distance compete is considered for explaining the simultaneous occurrence of the two kinds of diffraction profile satellites.

Magnetization dynamics in the presence of pure spin currents in magnetic single and double layers in spin ballistic and diffusive regimes

In this paper we study the spin transport by using the spin-pumping effect in epitaxial magnetic single and double layer film structures. For the magnetic single layer sample we show the spin-pumping-induced interface damping increases and saturates with the Au capping layer thickness. In addition magnetic double layer structures allowed us to investigate both the spin-pump and spin-sink effects. Coupling of pure spin currents to the magnetization via spin-sink effect is studied using time-resolved magneto-optical Kerr effect. These measurements were used to study the propagation of pure spin currents across a Au spacer layer between the two ferromagnets. The propagation of spin momentum density through the Au spacer layer was well described by spin-diffusion equation, which takes into account electron momentum and spin-flip scattering. The spin-diffusion theory was integrated into modified Landau-Lifshitz equations accounting in self-consistent manner for spin-pump/sink mechanism and spin momentum density propagation. Good agreement between theory and experimental data was found.

Reduction of magnetic grain size of perpendicular recording media with CoCrW seed layer

We report the use of CoCrW seed layer (SL) for making the fine grain granular structure and high crystalline orientation of CoCrPt-oxide magnetic recording layer. It is found that CoCrW SL should be of amorphouslike structure to make fine grain of CoCrPt-oxide magnetic layer. Moreover, the smooth surface of CoCrW SL provides high crystalline orientation of the CoCrPt-oxide magnetic layer.

Phase transitions of the mixed spin-1/2 and spin- [formula omitted] Ising model on a three-dimensional decorated lattice with a layered structure

Phase transitions of the mixed spin- 1/2 and spin- S ( S ≥ 1 / 2 ) Ising model on a three-dimensional (3D) decorated lattice with a layered magnetic structure are investigated within the framework of a precise mapping relationship to the simple spin- 1/2 Ising model on a tetragonal lattice. This mapping correspondence yields for the layered Ising model of mixed spins plausible results either by adopting the conjectured solution for the spin- 1/2 Ising model on the orthorhombic lattice [Z.-D. Zhang, Philos. Mag. 87 (2007) 5309–5419] or by performing extensive Monte Carlo simulations for the corresponding spin- 1/2 Ising model on the tetragonal lattice. It is shown that the critical behaviour markedly depends on a relative strength of axial zero-field splitting parameter, inter- and intra-layer interactions. The striking spontaneous order captured to the ‘quasi-1D’ spin system is found in a restricted region of interaction parameters, where the zero-field splitting parameter forces all integer-valued decorating spins towards their ‘non-magnetic’ spin state.

Current-driven channel switching and colossal positive magnetoresistance in the manganite-based structure

The transport and magnetotransport properties of a newly fabricated tunnel structure manganite/depletion layer/manganese silicide have been studied in the current-in-plane (CIP) geometry. A manganite depletion layer in the structure forms a potential barrier sandwiched between two conducting layers, one of manganite and the other of manganese silicide. The voltage–current characteristics of the structure are nonlinear due to switching conducting channels from an upper manganite film to a bottom, more conductive MnSi layer with an increase in the current applied to the structure. Bias current assists tunnelling of a carrier across the depletion layer; thus, a low-resistance contact between the current-carrying electrodes and the bottom layer is established. Below 30 K, both conducting layers are in the ferromagnetic state (magnetic tunnel junction), which allows control of the resistance of the tunnel junction and, consequently, switching of the conducting channels by the magnetic field. This provides a fundamentally new mechanism of magnetoresistance (MR) implementation in the magnetic layered structure with CIP geometry. MR of the structure under study depends on the bias current and can reach values greater than 400% in a magnetic field lower than 1 kOe. A positive MR value is related to peculiarities of the spin-polarized electronic structures of manganites and manganese silicides.