Magnetic Multilayer Systems Research Articles

Stripe glasses in ferromagnetic thin films

Domain walls in magnetic multilayered systems can exhibit a very complex and fascinating behavior. For example, the magnetization of thin films of hard magnetic materials is in general perpendicular to the thin-film plane, thanks to the strong out-of-plane anisotropy, but its direction changes periodically, forming an alternating spin-up and spin-down stripe pattern. The latter is stabilized by the competition between the ferromagnetic coupling and dipole-dipole interactions, and disappears when a moderate in-plane magnetic field is applied. It has been suggested that such a behavior may be understood in terms of a self-induced stripe glassiness. In this paper we show that such a scenario is compatible with the experimental findings. The strong out-of-plane magnetic anisotropy of the film is found to be beneficial for the formation of both the stripe-ordered and glassy phases. At zero magnetic field the system can form a glass only in a narrow interval of fairly large temperatures. An in-plane magnetic field, however, shifts the glass transition towards lower temperatures, therefore enabling it at or below room temperature. In good qualitative agreement with the experimental findings, we show that a moderate in-plane magnetic field of the order of $30~{\rm mT}$ can lead to the formation of defects in the stripe pattern, which sets the onset of the glass transition.

Elemental Distribution in Multilayer Systems by Laser-Assisted Atom Probe Tomography with Various Analysis Directions.

Elemental distributions in a magnetic multilayer system with the structure Si substrate/Ta/NiFe/Ru/CoFeB/Ru/NiFe were studied using atom probe tomography (APT) along different analysis directions. The distributions of Ru and B atoms, which require a high evaporation field, were strongly influenced by the APT analysis direction. In particular, B in the CoFeB layer appeared near the interface with the lower Ru layer when the analysis was anti-parallel to the film growth direction, while B atoms were observed at the other side of the CoFeB layer when the analysis was parallel to the film growth direction. Moreover, when the analysis was perpendicular to the film growth direction, a homogenous distribution of B atoms was found within the CoFeB layer. Owing to this B behavior, the underlying Ru layer was affected in both of these analysis directions. In APT measurements of such a multilayer system composed of a stack of different evaporation field materials, evaluation of the elemental distribution around interfaces should be performed from more than one analysis direction.

Thickness dependence of the interfacial Dzyaloshinskii-Moriya interaction in inversion symmetry broken systems.

In magnetic multilayer systems, a large spin-orbit coupling at the interface between heavy metals and ferromagnets can lead to intriguing phenomena such as the perpendicular magnetic anisotropy, the spin Hall effect, the Rashba effect, and especially the interfacial Dzyaloshinskii–Moriya (IDM) interaction. This interfacial nature of the IDM interaction has been recently revisited because of its scientific and technological potential. Here we demonstrate an experimental technique to straightforwardly observe the IDM interaction, namely Brillouin light scattering. The non-reciprocal spin wave dispersions, systematically measured by Brillouin light scattering, allow not only the determination of the IDM energy densities beyond the regime of perpendicular magnetization but also the revelation of the inverse proportionality with the thickness of the magnetic layer, which is a clear signature of the interfacial nature. Altogether, our experimental and theoretical approaches involving double time Green's function methods open up possibilities for exploring magnetic hybrid structures for engineering the IDM interaction.

Normal and inverse magnetocaloric effect in magnetic multilayers with antiferromagnetic interlayer coupling

The thermodynamics of a spin-1/2 magnetic multilayer system with antiferromagnetic interplanar couplings is studied using the pair approximation method. Special attention is paid to magnetocaloric properties, quantified by isothermal entropy change. The multilayer consists of two kinds of magnetic planes, one of which is diluted. The intraplanar couplings in both planes have arbitrary anisotropy ranging between Ising and isotropic Heisenberg interactions. The phase diagram related to the occurrence of magnetic compensation phenomenon is constructed and discussed. Then the isothermal entropy change is discussed as a function of interaction parameters, magnetic component concentration and external magnetic field amplitude. The ranges of normal and inverse magnetocaloric effect are found and related to the presence or absence of compensation.

Reduced spin transfer torque switching current density with non-collinear polarizer layer magnetization in magnetic multilayer systems

Through micromagnetic simulations, it was found that the spin transfer torque (STT) switching current density is reduced with non-collinear polarizer layer magnetization. The dependence of the switching current density on the polarizer layer magnetization angle was investigated, and a typical magnetic tunneling junction structure with an exchange biased synthetic ferrimagnetic polarizer layers was considered. The easy axis of the polarizer layer was varied through controlling the exchange bias field direction in the ellipse cross-section nano-pillar structures. It was found that the switching current density was reduced by 39% when the exchange bias field was 10° from the long axis of the ellipse without the perpendicular STT. When the effect of the perpendicular STT was included, the switching current reductions were general for the non-collinear polarizer layer.

Microscopic theory on the Gilbert damping due to spin pumping effects in the magnetic multi-layer system

We have calculated the Gilbert damping coefficient α in the magnetic multi-layer structures consisted of ferromagnetic (FM) and non-magnetic (NM) layers which are FM/NM and FM/NM/FM from microscopic theory. In this study, our model Hamiltonian doesn't include the spin-orbit interaction term, but the kinetic electron hopping term. For obtaining α in theory, we used Kubo formula and Projection method. We found α is dependent on structure of systems and both thickness of the FM and NM layer. Spin pumping effect which is predicted by experimental results is confirmed by this calculation in theory. We conclude that transferring conduction electrons carrying angular momentum to adjacent layers are the origin of the Gilbert damping for multi-layer systems.

Out-of-Plane Torque Influence on Magnetization Switching and Susceptibility in Magnetic Multilayers

Based on both the spin diffusion equation and the Landau–Lifshitz–Gilbert (LLG) equation, we demonstrate the influence of out-of-plane spin torque on magnetization switching and susceptibility in a magnetic multilayer system. The variation of spin accumulation and local magnetization with respect to time are studied in the magnetization reversal induced by spin torque. We also research the susceptibility subject to a microwave magnetic field, which is compared with the results obtained without out-of-plane torque.

Coupled Dipolar Spin Waves of Magnetic Multilayer Systems With Cylindrical Geometries

A theory is developed for the dipolar spin waves in cylindrical multilayer systems consisting of a core surrounded by any arbitrary number of concentric layers (or tubes). Each layer may be magnetic (either ferromagnetic or antiferromagnetic) or a nonmagnetic spacer. The long-range dipolar fields provide coupling between magnetic layers across the spacer regions, resulting in a coupling between the dipolar modes of the system. A transfer matrix approach is employed to calculate the dispersion relations for the mode frequencies in terms of a longitudinal wave number. Numerical applications are made to multilayer systems that include several layers of the ferromagnet Ni, or the uniaxial antiferromagnet GdAlO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> , or both. Particular attention is given to the localized interface modes, which are shown to be strongly modified (for example, in their frequency and wave number cutoff) by the multilayer structure, compared to the behavior found in single magnetic wires and tubes.

Effects of capping layer on the spin accumulation and spin torque in magnetic multilayers

We study the effects of the physical properties of the capping layer (spin diffusion length (SDL), resistivity and interface resistance) on the spin transfer torque (STT) and spin accumulation in the free ferromagnetic (FM) layer of a magnetic multilayer system. The STT and spin accumulation are calculated based on a non-collinear spin drift–diffusion (SDD) model under the assumption of quasi-interfacial absorption of transverse spin accumulation in the FM layers. It is found that the out-of-plane component of the STT in the free FM layer is significantly enhanced by having a thick, highly resistive capping layer, with a long SDL. The in-plane torque component is, however, insensitive to variations in the capping layer properties. The implications of these trends to applications, such as current-induced magnetization switching and spin torque oscillations, are discussed.

The early stage of deposition process for Fe–Cu magnetic multilayer systems: molecular dynamics simulation

The deposition behaviour for a Fe–Cu magnetic multilayer system in an early stage of the deposition process was investigated by molecular dynamics (MD) simulation. Specifically, the steering effect was quantitatively investigated through extensive measurements of the trajectory, deposition flux and force of atoms near the artificially structured Fe or Cu step positioned on the Cu(0 0 1) or Fe(0 0 1) surface. Near the step edges of the planar structure at a low incident energy of 0.1 eV, the steering effect for the case of Fe/Cu(0 0 1) was observed more significantly than for Cu/Fe(0 0 1). Additionally, the mechanism of down-diffusion from the step was discussed and the corresponding energetic was calculated using the molecular statics method.

Theoretical study of the torque curves in some magnetic multilayer systems

Analytical expressions involving features from torque curves are derived for an exchange coupled [(AF)/(F)] bilayer, a coupled trilayer [(F)/(N-M)/(F)], and a magnetic tunnel junction (MTJ)-like [(AF)/(F)/(N-M)/(F)] system. Included in the model are the exchange anisotropy field ${H}_{E}$, the off-alignment angle $\ensuremath{\beta}$, the bilinear ${J}_{1}$, and the biquadratic ${J}_{2}$ coupling strengths and the in-plane magnetocrystalline anisotropy fields of the two layers whose easy axes make an angle $\ensuremath{\delta}$. It will be shown how these parameters can be analytically derived from the torque curve. For the strong coupling case the MTJ-like system behaves as (AF)/(F) bilayer; for arbitrary $\ensuremath{\delta}$ value, the magnetic anisotropy field of the resulting system has a nonlinear relationship with the individual layer anisotropy fields.

Transport Properties in All Oxide Magnetic Multilayers

The all oxide magnetic multilayer system [LaNiO3/SmFeO3]n (for n = 1 and 2), grown on single crystal SrTiO3(100) substrates, has been produced using the laser MBE (Molecular Beam Epitaxy) technique. We have made a systematic study of the electrical transport properties in the temperature range from 15–300K. As part of this work, we have made a detailed study of the metallic properties of the LaNiO3 layer as a function of the oxygen partial pressure (pO2) and substrate temperature (TS). We have measured magnetic layers of SmFeO3 with LaNiO3 electrodes as a function of the magnetic layer thickness (10 – 470 nm). A non-metallic behaviour is observed with evidence of a “hopping” mechanism at low temperatures. For the n = 2 multilayers, we have measured the temperature dependence of resistance for the sample series with varying LaNiO3 interlayer thickness (t = 2 – 30 nm). We observe an appreciable increase of the low temperature resistance for the interlayer thicknesses between 3.75 – 7.5 nm. This could indicate a change in coupling from ferromagnetic to antiferromagnetic between the magnetic layers.

Atomic Investigation of Fe–Cu Magnetic Thin Films by Molecular Dynamics Simulation

The deposition behavior of an Fe–Cu magnetic metallic multilayer system was investigated at the atomic level using molecular dynamics simulation. It was found that a mixture confined to a single atomic layer at the Cu(001) surface was formed at room temperature. The Fe–Cu system shows mixing characteristics such as layer coverage function and mixing length, that are significantly different from those of the Fe–Al metallic system. The different intermixing behavior could be successfully explained in terms of cohesive energy and atomic size mismatch.

Analysis of results from x-ray magnetic reflectometry for magnetic multilayer systems

To determine the magnetization profiles for magnetic multilayer systems by x-ray magnetic reflectometry, it is generally assumed that the local magnetic contributions $\ensuremath{\Delta}\ensuremath{\delta}(\ensuremath{\omega})$ and $\ensuremath{\Delta}\ensuremath{\beta}(\ensuremath{\omega})$ to the optical constants are proportional to the local atomic magnetic moment. In the present paper, the validity of this basic assumption is explored by general theoretical arguments as well as by ab initio calculations for a Co-Pt multilayer system. It turns out that the assumption may be at least approximately valid under two circumstances: (i) the width of the x-ray line should not be much smaller than the width of the dichroic spectra, and (ii) the spin-orbit coupling should not be too strong.

Transport and STM measurements of HCI modified materials

While more than a decade of work has provided glimpses into the physics of highly charged ion (HCI) neutralization on surfaces, two prominent objectives remain unfulfilled: (1) a unified, quantitative model for separating the kinetic energy response of a wide range of materials classes from the effects of HCIs’ potential energy effects and (2) insertion of HCI technology(s) as a cost-effective processing tool in a high-volume market sector. The National Institute of Standards and Technology (NIST) electron beam ion trap (EBIT) facility has recently incorporated tools for preparing clean, atomically flat surfaces of single crystals from gold to tungsten to silicon and for depositing and patterning thin films that range from high resistivity oxides to transition metals like cobalt and nickel. Current activities are focused on utilizing this unique capability to simultaneously address both of the objectives above by employing technologically important magnetic multi-layer systems to perform transport measurements that provide new insight into the fundamental processes that occur during HCI–surface neutralization. Specifically, we are producing Magnetic Tunnel Junctions (MTJs) critical to both magnetic devices and incorporating HCIs in the processing recipe to adjust critical electronic properties that are currently inhibiting their advancement. In return, the electrical response of the tunnel junction to the HCI processing provides a novel approach to performing ensemble measurements of HCI–surface interactions. By varying the construction of the tunnel junction, critical tests of the role of electron density, densities of states and electronic structure in the HCI–surface charge exchange can be performed.

STM and transport measurements of highly charged ion modified materials

Careful measurements of highly charged ions (HCIs) colliding with gases and surfaces have provided glimpses of intense electronic interactions, but a comprehensive model for the interaction mechanisms, time scales, and resultant nano-features that bridges materials systems is yet to be realized. At the National Institute of Standards and Technology (NIST) electron beam ion trap (EBIT) facility, new apparatus is now connected to the HCI beamline to allow preparation of clean, atomically flat surfaces of single crystals, e.g. gold, tungsten and silicon, and deposition and patterning of thin films, e.g. high resistivity oxides, ferromagnetic metals, normal metals and superconductors. Experiments reported here focus on the electronic and morphological structure of HCI induced nano-features. Current activities are focused on using in situ scanning tunneling microscope (STM) on Au(111) and (separately) ex situ transport measurements to study electronic properties within HCI modified magnetic multilayer systems. Specifically, we are fabricating magnetic multilayers similar to magnetic tunnel junctions (MTJs) (important in advanced magnetic field sensors and superconducting Josephson junction devices) and using HCIs to adjust critical electronic properties. The electrical response of the tunnel junction to HCIs provides a novel approach to performing HCI-induced nanostructure ensemble measurements.

Molecular magnetism in thin metallo-supramolecular films: A combined neutron and soft x-ray reflectometry study

A detailed reflectometry study of Langmuir–Blodgett multilayers composed of a polyelectrolyte-amphiphile complex (PAC) is presented. The PAC is self-assembled from metal ions, ditopic bis-terpyridines and amphiphiles. The structure as well as the vacuum stability of the LB multilayer is investigated by soft x-ray reflectometry. The multilayer has a periodicity of 57 Å, which corresponds to an architecture of flat-lying MEPE rods and upright standing amphiphiles (hexadecyl phosphate, DHP). The films remain stable under high vacuum conditions. X-ray magnetic circular dichroism reveals a magnetic moment of 0.5μB/Fe2+ carried by the iron ions within the films. Using spin-resolved neutron reflectometry, Langmuir–Blodgett-films of PAC are identified as magnetic multilayer systems with the same characteristic d-spacing as it is found for the structural multilayer periodicity. The separation of the edges of total external reflection for spin up and spin down neutrons in presence of a magnetic moment is studied by time-of-flight neutron reflectometry.

One-electron Green’s function of multilayer magnetic systems

We propose a method for constructing a symmetric one-electron Green’s function of a magnetic multilayer system with an arbitrary direction of homogeneous magnetization of magnetic layers. We show that the constructed Green’s function allows imposing special boundary conditions.

Magnetoresistivity studies of intermetallic uranium compounds and their impact on the understanding of exhibited phenomena

In recent years the occurrence of a large reduction of the electrical resistivity upon application of a magnetic field, called as a giant magnetoresistvity (GMR) effect, first of all observed in some magnetic-multilayer systems like Fe/Cr or Co/Cu, has attracted much attentions due to the possible applications. It turned out that GMR is also possible in various uranium-based compounds which crystalize in layer-like or atomic-chain structures. For some of antiferromagnetic uranium compounds it is sometimes easy to drive a given compound through a metamagnetic (MTM) transition towards an induced ferromagnetic alignment. This is connected with a reorientation of the magnetic moment, which is also the origin of GMR effects in multilayers. Thus, the MTM transition usually causes a drastic reduction of the electrical resistivity, which often exceeds even that of multilayer systems. The high resistance state in bulk uranium antiferromagnets is attributed to the reduction of effective carrier concentration or change in the Fermi surface topology by arising the Fermi level gapping. The transverse magnetoresistance may give also the overall information on the Fermi surface of such uranium-based systems (having open or closed orbits) allowing, e.g. to distinguish a compensated from an uncompensated kind of metal. For some uranium materials the temperature dependence of MR gives evidence of an enormous electron scattering due to the presence of a special kind of spin (magnetic) fluctuations being responsible among others for their superconducting properties.

Atomic Investigation of Al/Ni(001) by Molecular Dynamics Simulation

Using molecular dynamics simulation, interfacial features of the Ni–Al magnetic metallic multilayer system were quantitatively investigated with regard to substrate temperature and deposition rate. It was found that a mixture confined to a single atomic layer at the Ni(001) surface was clearly formed at relatively high temperatures, 800–1000 K, and that the degree of mixing at the interface was largely dependent on the deposition rate of adatoms. The degree of lateral displacement of Al adatoms on the Ni(001) surface, which is a key parameter for determining the growth mode, was observed to depend on the deposition rate below 700 K.