Coupling In Magnetic Multilayers Research Articles

Origin of noncollinear magnetization coupling across RuX layers

We present a simple atomistic model for the description of noncollinear coupling in magnetic multilayers with hybrid spacer layers made of Ru alloyed to ferromagnetic atoms such as Fe. In contrast to previous analytical and micromagnetic models that explain the noncollinear coupling by means of lateral fluctuations in the coupling constant, the presented model accounts for atom-atom coupling in all three spatial dimensions within the spacer layer. The model is able to accurately predict the dependence of the macroscopic bilinear and biquadratic coupling constants on the spacer-layer composition and thickness, showing much better quantitative agreement than lateral-fluctuation models. Moreover, it predicts that the noncollinear coupling is virtually independent of the exchange stiffness in the ferromagnetic layers, which goes beyond the predictions of previous models. This prediction is validated by experimental measurements.

Giant magnetocaloric effect driven by indirect exchange in magnetic multilayers

Indirect exchange coupling in magnetic multilayers, also known as the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction, is known to be highly effective in controlling the interlayer alignment of the magnetization. This coupling is typically fixed at the stage of the multilayer fabrication and does not allow ex-situ control needed for device applications. It is highly desirable, in addition to the orientational control, to also control the magnitude of the intralayer magnetization, ideally switch it on/off by switching the relevant RKKY coupling. Here we demonstrate a magnetic multilayer material, incorporating thermally -- as well as field-controlled RKKY exchange, focused on to a dilute ferromagnetic alloy layer and driving it though it's Curie transition. Such on/off magnetization switching of a thin ferromagnet, performed repeatably and fully reproducibly within a low-field sweep, results in a giant magnetocaloric effect, with the estimated isothermal entropy change of -10 mJ/K cm^3 under an external field of ~10 mT, which greatly exceeds the performance of the best rare-earth based materials used in the adiabatic-demagnetization refrigeration systems.

Spin-orbit torques from interfacial spin-orbit coupling for various interfaces.

We use a perturbative approach to study the effects of interfacial spin-orbit coupling in magnetic multilayers by treating the two-dimensional Rashba model in a fully three-dimensional description of electron transport near an interface. This formalism provides a compact analytic expression for current-induced spin-orbit torques in terms of unperturbed scattering coefficients, allowing computation of spin-orbit torques for various contexts, by simply substituting scattering coefficients into the formulas. It applies to calculations of spin-orbit torques for magnetic bilayers with bulk magnetism, those with interface magnetism, a normal metal/ferromagnetic insulator junction, and a topological insulator/ferromagnet junction. It predicts a dampinglike component of spin-orbit torque that is distinct from any intrinsic contribution or those that arise from particular spin relaxation mechanisms. We discuss the effects of proximity-induced magnetism and insertion of an additional layer and provide formulas for in-plane current, which is induced by a perpendicular bias, anisotropic magnetoresistance, and spin memory loss in the same formalism.

Ferromagnetic resonance and interlayer exchange coupling in magnetic multilayers with compositional gradients

Ferromagnetic resonance (FMR) in magnetic multilayers of type F1/f/F2, where two strongly ferromagnetic layers F1 and F2 are separated by a weakly magnetic spacer f with a compositional gradient along its thickness, is investigated. The method allows to detect the weak signal from the spacer in additional to the more pronounced and readily measured signal from the outer strongly-magnetic layers, and thereby study the properties of the spacer as well as the interlayer exchange interaction it mediates. Variable temperature FMR measurements, especially near the relevant Curie points, reveal a rich set of properties of the exchange interactions in the system. The obtained results are useful for designing and optimizing nanostructures with thermally-controlled magnetic properties.

Temperature dependence of RKKY interaction

The temperature dependence of RKKY coupling is shown to decrease much slower than the conventional zero-temperature RKKY coupling for a certain range of the distance between two magnetic ions. The magnitude of the coupling, but not the sign, can change significantly as the temperature arises. We discuss implication of such temperature dependence on laser-induced magnetization dynamics and predict a temperature dependence of the interlayer coupling in magnetic multilayers.

Enhancement of antiferromagnetic coupling in magnetic multilayers by low energy ionbeam substrate nanopatterning

Ion beam irradiation has been shown to be an interesting tool for tailoring the magneticproperties of thin films and multilayers. The modified properties include magneticanisotropy, interlayer exchange coupling, exchange bias, magnetic domain structureand magnetization reversal. In this work, new results are shown concerning theenhancement, by one order of magnitude, of the antiferromagnetic coupling strengthin amorphous CoSi/Si multilayers by irradiating Si(100) substrates with 1 keVAr+ ions. The ion beam exposure induces an increase of the substrate roughness, from 0.07 to0.88 nm, which enhances antiferromagnetic coupling in the magnetic multilayers grown ontop. One possible mechanism governing this enhancement is discussed, related to theformation of magnetic/non-magnetic regions where dipolar interactions could stabilize theantiferromagnetic alignment. The presence of non-magnetic regions is suggestedby the observed trend to superparamagnetism, and is expected since the Curietemperature of the amorphous CoSi alloy used is slightly above but very closeto room temperature. Accordingly, small fluctuations in the local composition,leading to an enrichment of Si, would produce non-magnetic regions enablingdipolar interactions to take place. Furthermore, the ion beam induced increase ofroughness makes surface diffusion of the atoms arriving at the sample difficult,favoring the formation of local non-magnetic inhomogeneities. Finally, the role ofother possible mechanisms to enhance antiferromagnetic coupling is also brieflydiscussed.

Resource Letter STMN-1: Spin transport in magnetic nanostructures

This Resource Letter provides a guide to the literature in the still rapidly developing area of spin-dependent transport phenomena, which forms the basis for applications in magnetoelectronics. Citations to original papers and review articles are provided for the major topics of giant magnetoresistance, spin-dependent tunneling, exchange coupling between magnetic layers, and micromagnetics of the nanostructures in which these occur. Related phenomena, such as exchange coupling in magnetic multilayers, exchange biasing, and the various experimental techniques developed to observe these effects are covered more briefly, with citations only to historically significant papers and selected review articles.

Interlayer segregation of nonmagnetic metal spacers and its influence on exchange coupling in magnetic multilayers

Two sets of NiFe∕FeMn films with Ta and Ta∕Cu buffer layers were prepared by magnetron sputtering to study the influence of different buffers on the exchange coupling process. The results show that the exchange bias field (Hex) of NiFe∕FeMn films with a Ta∕Cu buffer is lower than that of films with a Ta buffer. There is no apparent difference in the texture and roughness of films both with Ta and Ta∕Cu buffers. However, the segregation of Cu atoms on the surface of NiFe in the trilayer of Ta∕Cu∕NiFe was found by using angle-resolved x-ray photoelectron spectroscopy (XPS). A decrease of Hex for NiFe∕FeMn films with a Ta∕Cu buffer layer is primarily caused by the segregation of Cu atoms to the interface of the NiFe and FeMn layers. In order to suppress the Cu segregation, we deposited Bi insetting layers at the interface of Cu and NiFe in Ta∕Cu∕NiFe∕FeMn∕Ta films and found that Hex of NiFe∕FeMn can be doubled when the proper quantity of Bi is deposited. XPS analysis shows that Bi insetting layers deposited at the interface of Cu and NiFe effectively suppress the Cu segregation on the NiFe layer. As a result, Hex is increased. However, if the insetting Bi is deposited excessively, it will partially migrate to the FeMn layer, damaging the antiferromagnetic property of the FeMn layer, thereby resulting in decreased Hex. When Ag and Pb were deposited at the interface of Cu and NiFe in Ta∕Cu∕NiFe∕FeMn∕Ta films, similar experimental results were obtained.

Competing Exchange Interactions in Magnetic Multilayers

We have studied alloying of the nonmagnetic spacer layer with a magnetic material as a method of tuning the interlayer coupling in magnetic multilayers. We have specifically studied the Fe/V(100) system by alloying the spacer V with various amounts of Fe. For some Fe concentrations in the spacer, it is possible to create a competition between antiferromagnetic Ruderman-Kittel-Kasuya-Yoshida exchange and direct ferromagnetic exchange coupling. The exchange coupling and transport properties for a large span of systems with different spacer concentrations and thicknesses were calculated and measured experimentally and good agreement between observations and theory was observed. A reduction in magnetoresistance of about 50% was observed close to the switchover from antiferromagnetic to ferromagnetic coupling.

A Busy March Meeting Is Brewing in Seattle

A Busy March Meeting Is Brewing in Seattle

Magnetism in one-dimensional Hubbard superlattices

We consider fermions in one-dimensional superlattices (SL's), modeled by site-dependent Hubbard-U couplings arranged in a repeated pattern of repulsive (i.e., $Ug0)$ and free $(U=0)$ sites. Lanczos diagonalization is used to investigate magnetic properties. For each SL configuration, we found that local moments are displaced from repulsive to free sites as the density is reduced and we were able to establish three distinct regions in a ``phase diagram.'' We also determined that spin density waves can be frustrated or restored upon doping with either electrons or holes. A connection between the spacer thickness dependence of the maxima in the magnetic structure factor and the oscillation of the exchange coupling in magnetic multilayers was also established.

Molecular-orbital model of heat-induced effective exchange coupling in magnetic multilayers

Heat-induced effective exchange coupling between two ferromagnets across a semiconductor spacer layer is described by the interaction of localized, weakly bound electron states which are situated at the two interfaces. These states overlap across the spacer layer and form large molecular orbitals. The energies of these orbitals depend on the spin configuration of the electrons and therefore determine the exchange coupling. Thermal repopulation of the levels yields a positive temperature coefficient of the coupling. The results are found to well reproduce the experimental observations.

Theory of interlayer exchange interactions in magnetic multilayers

This paper presents a review of the phenomenon of interlayer exchange coupling in magnetic multilayers. The emphasis is put on a pedagogical presentation of the mechanism of the phenomenon, which has been successfully explained in terms of a spin-dependent quantum confinement effect. The theoretical predictions are discussed in connection with corresponding experimental investigations.

Heat-induced effective exchange coupling in magnetic multilayers with semiconductors

Two ferromagnetic films separated by an amorphous semiconducting spacer layer are exchange coupled across the spacer. The coupling is reversibly temperature dependent with a positive temperature coefficient making such layered systems a 2-D realization of the concept of heat-induced magnetism. By studying ferromagentic Fe layers separated by amorphous Si, Ge, or ZnSe layers we explore the possibilities to generate such an effective exchange coupling and address the question of the mechanism responsible for it.

Short wavelength, spin-polarized quantum-well states in high quality Cu films on FCC-Co(1 0 0)

The electronic structure of ultrathin Cu films deposited on top of an FCC-Co(1 0 0) film has been studied by angle- and spin-resolved photoemission. Deposition at liquid-nitrogen temperature allows the formation of sharp interfaces that produces well-defined quantum-well states in the Cu valence band. Furthermore, short-wavelength quantum-well states originating from the neck of the Cu Fermi surface are only visible after low-temperature deposition. Spin-resolved spectra show that these states, which are predicted to dominate oscillatory magnetic coupling in multilayers, exhibit minority spin character.

Exchange coupling in magnetic multilayers grown on iron whiskers

Meaningful tests of theoretical predictions of magnetic multilayer properties require the fabrication of multilayers with nearly the same atomic scale precision as the theoretical models. Multilayers grown epitaxially on single-crystal Fe whisker substrates come very close to this ideal standard. We have investigated the growth, magnetic structure, and exchange coupling of Fe/(Ag, Au, Cr, Mn, V, Cu, or Al)/Fe (1 1 0) whisker structures primarily using reflection high-energy electron diffraction (RHEED) and scanning electron microscopy with polarization analysis (SEMPA), and in some systems, confocal magneto-optic Kerr effect (MOKE) microscopy. In cases of nearly layer-by-layer growth, the measured oscillatory coupling periods and strengths agree well with theoretical predictions. For rougher growth, less predictable non-collinear coupling is generally observed.

Spin-polarized quantum well states

Abstract Highly-quality Cu/Co/Cu(100) quantum wells are grown in a newly-constructed growth chamber that contains RHEED, LEED, SMOKE, MBE, and sputter deposition facilities. The low defect density of the fcc Co(100) layers is evidenced by a record low coercivity of 0.7 Oe, which indicates minimal pinning of the domains. Spin-polarized, angle-resolved photoemission spectra are obtained using synchrotron radiation from an undulator. They exhibit quantum well states of minority spin character which are derived from states at the neck of the Cu Fermi surface. Such states have been predicted to dominate the magnetic coupling in multilayers. They have been difficult to observe because their short wavelength requires atomically-sharp interfaces. The key to obtaining high quality films with sharp interfaces is deposition at low temperature (≈175 K) and annealing to room temperature.

Magnetic interlayer coupling and interaction between interface states in a quantum-well system

A first-principles Green's function technique has been used to investigate the magnetic interlayer coupling and the electronic structure responsible for the magnetic interaction in a bcc system. We discuss the close relation between the interlayer coupling, the bulk Fermi surface of the spacer material, and the dispersion of the quantum-well states in the two-dimensional Brillouin zone. As a function of spacer thickness, we find an oscillatory long-range energy splitting of Fe/Cu interface states which is shown to be an example of a long-range oscillatory interface interaction transmitted by the spin-polarized quantum-well states of the Cu spacer. These interacting interface states may provide a signature of the magnetic coupling in multilayers.

Fermi surface callipering of chromium alloys

We use maximum entropy to deconvolute the experimental resolution from (projected) 2DACAR (two-dimensional angular correlation of electron-positron annihilation radiation) data for Cr, Mo, V and their alloys. When the original data are subtracted from the deconvoluted data, the difference distribution reveals far more Fermi surface information. It is found that the difference plot may also be used to calliper the N-hole pockets of the projected Fermi surface. The dimensions that we obtain for pure Cr and Mo agree well with those from the literature, although those we obtain for V are smaller. The technique is designed to permit the callipering of the N-hole pocket in certain alloys for eventual comparison with work on oscillatory exchange coupling in magnetic multilayers.

The combined effect of temperature and disorder on interlayer exchange coupling in magnetic multilayers

The combined effect of temperature and disorder on interlayer exchange coupling in magnetic multilayers