Nonmagnetic Spacer Layer Research Articles

Perpendicular giant magnetoresistance of CoFeB/Cu single and dual spin-valve films

We present giant magnetoresistance measurements in the current-perpendicular-to-plane mode on single and dual spin valves whose areas are from 0.1 to 1.0 μm2. Dependence on the magnetic layer thickness and the structure was investigated. It was revealed that magnetoresistance change increases 20% by insertion of a nonmagnetic spacer layer in the free layer. The results were analyzed within the framework of the two-current series-resistor model, and proved to be consistent with the theory.

Interlayer exchange coupling in M/N/M multilayers

The interlayer exchange coupling (IEC) of two local momentferromagnetic layers separated by a non-magnetic spacer layer(a M/N/M multilayer) is studied using the modified Ruderman-Kittel-Kasuya-Yosida (RKKY) methodalong with the s-f model. The IEC exhibits oscillatorybehaviour with respect to the spacer layer thickness and itoscillates between ferromagnetic and antiferromagnetic configurations.The conventional RKKY method is also used to obtain the IECand the results are compared with those obtained from the modified RKKY method which incorporates the electron correlationeffects. We find significant correlation effects on the IEC andin fact the correlations alter the nature and magnitude of themagnetic coupling. Hence this study indicates the importanceof the inclusion of correlation effects to the understanding ofthe IEC in multilayer systems with local moment ferromagneticsublayers and to offering a satisfactory explanation for theexperimental results.

Dependence of the ferromagnetic resonance modes on the coupling strength in exchange-coupled trilayer structures

The dependence of the dispersion relation and the magnetization on the exchange coupling strength was calculated for a system consisting of two ferromagnetic layers exchange-coupled through a nonmagnetic spacer layer. The magnetic layers are characterized by both uniaxial and cubic magnetocrystalline anisotropies. A minimization procedure was developed which allows the resonance modes to be obtained for any magnetic field orientation and strength, as well as for any exchange coupling strength. If the antiparallel coupled film is unsaturated at resonance, the dispersion relation for both acoustic and optic modes could be rather complex, especially when the field is applied in the plane of the film.

The Spin-Wave Contribution to the Heat Capacity of Ferromagnetic Thin Films

A Heisenberg model is solved within the spin-wave theory for thin films in which ferromagnetic monolayers are separated by nonmagnetic spacer layers. The interface interaction is assumed to be ferromagnetic. We have included also a magnetic anisotropy in each monolayer. The temperature dependence of the spin-wave contribution to the heat capacity C m in such composite systems is derived for several sets of material parameters and for different film thicknesses.

Spin-valve heads with self-stabilized free layer by antiferromagnet

We demonstrate the feasibility of a spin-valve read head using an adjacent antiferromagnetic (AFM) layer to provide a stabilization Held, eliminating the hard bias permanent magnet layer. The head stack consists of seed/AFM/CoFe/Ru/CoFe/Cu/CoFe (and/or NiFe)/Cu/IrMn/cap where the AFM is PtMn or IrMn. The abutted junction is about 0.1 /spl mu/m wide at the free layer and no permanent magnet materials are used. Micromagnetic modeling indicates that a proper bias field is needed to have a stable head response. The bias field from the AFM layer can be modified by a nonmagnetic spacer layer between AFM and FM. The coupling field decays exponentially with the thickness of the spacer layer, consistent with a pinhole-dominated mechanism. Unshielded sensors show much higher sensitivity as,compared with a standard PM abutted junction head. Microtrack profile based on the spin-stand test shows a regular shape without apparent lumps. The electrical reader width is about 6 /spl mu/" (0.15 /spl mu/m), corresponding to a track density of 112 ktpi. Combined with a linear density of 591 kbpi the heads in this work are capable of areal density around 66 Gb/in/sup 2/.

The spin-wave contribution to the heat capacity of ferromagnetic thin films with variable interlayer couplings

A Heisenberg model is solved within the spin-wave theory for thin films in which ferromagnetic monolayers are separated by nonmagnetic spacer layers. The interface interaction is assumed to be ferromagnetic. We have included also a magnetic anisotropy in each manolayer. The temperature dependence of the spin-wave contribution to the heat capacity C m in such composite systems is derived for different interlayer couplings and for different film thickness.

Competition between in-plane and out-of-plane magnetization in exchange-coupled magnetic films

The coupling between a Co layer with in-plane easy axis of magnetization and a Ni layer with out-of-plane easy axis across a nonmagnetic Cu spacer layer was studied by layer-resolved magnetic imaging. Photoelectron emission microscopy and magnetic circular dichroism in soft x-ray absorption as the magnetic contrast mechanism were used for the domain imaging. Crossed, wedge-shaped epitaxial Co and Cu layers of 3.5--5.7 atomic monolayers (ML) Co and 2.5--4.1 ML Cu were deposited onto 15 ML Ni/Cu(001). The competition between ferromagnetic interlayer coupling and the magnetic uniaxial anisotropies of the two magnetic layers leads to a noncollinear magnetization configuration at Cu thicknesses above 3.1 ML. The Ni magnetization is thereby in a canted state, whereas the Co magnetization is nearly in plane. The canting angle of the Ni magnetization can be tuned between the surface normal and $\ensuremath{\approx}45\ifmmode^\circ\else\textdegree\fi{}$ by adjusting the Cu thickness. At a Cu thickness of approximately 3.1 ML a transition to a collinear in-plane configuration at lower Cu thicknesses is observed. This can be explained by a phenomenological expression including the fourth-order anisotropy of the Ni layer.

Brillouin light scattering from periodic multilayers composed of very thin magnetic films

A simple treatment of the dipole-dipole interaction between the magnetic films in a multilayer has been used to calculate the low-lying spin-wave modes in multilayer structures. It is assumed that in equilibrium the magnetization in each film is parallel to the applied magnetic field. It is also assumed that the magnetic films are thin enough so that the magnetization density remains uniform across the thickness of each film when the magnetization is thermally excited from equilibrium. Surface pinning energies and interlayer exchange coupling are included in the formalism. The calculated spin-wave modes have been used to investigate the scattered light spectrum in a Brillouin light scattering (BLS) experiment. The method has been applied to the cases of a surface mode on an infinite multilayer stack and to multilayers containing 5, 10, and 25 magnetic films having thicknesses up to 50 \AA{} and for nonmagnetic spacer layers having similar thicknesses. The scattered light spectra for such thin-film structures are very simple for multilayers having a total thickness less than, or comparable to, the optical decay length for light propagating through the multilayer structure. For such multilayers only those one or two modes give rise to an appreciable scattered light intensity for which the magnetization precessional amplitude varies very little from one magnetic film to the next. The frequencies corresponding to these strong BLS modes are insensitive to the interlayer exchange coupling because the magnetizations in neighboring films remain nearly parallel as they precess.

Effects of impurities on the exchange couplingin magnetic metallic multilayers

In this work the variation of the exchange coupling between magneticlayers across a non-magnetic spacer layer, due to impurities of themagnetic material in the spacer, is calculated. The impurities aretreated in the dilute limit and beyond it. It is shown that thechange per impurity is comparable to the unperturbed coupling per atom,and that it oscillates as a function of spacer layer thickness withthe same period as that coupling, though with a different phase.The most favourable magnetic moment orientation of the impurity isfound for some spacer layer thicknesses and impurity positions. Itis also shown that the short-period oscillations of the coupling maybe much more affected by the presence of impurities than thelong-period ones.

Mössbauer study of magnetic Fe/FeSi multilayers

Many useful properties of magnetic multilayers depend on the coupling between the ferromagnetic layers. The coupling often oscillates with the thickness of non-magnetic spacer layers: it is ferro- or antiferromagnetic or even non-collinear near a critical thickness. We investigated the magnetron-sputtered Fe/FeSi multilayers with spacer thickness around 1.7 nm by means of Conversion Electron Mossbauer Spectroscopy with oblique incidence of the γ beam in order to gain information on the orientation of the local magnetic moments in the multilayer plane. The results show that the local moments make an angle of 45°–50° with the direction of the remanent magnetization. This is consistent with strong biquadratic coupling which in turn is expected at this spacer thickness from our magnetic measurements. An analysis of the distribution ofBhf corresponding to different numbers of n.n. Si atoms in the bcc Fe structure points to weak diffusion of Si through the Fe/FeSi interface characterized by a diffusion length of about twice the substrate roughness.

Interplay between magnetism and superconductivity in Nb/Co multilayers

We have used torque magnetometry to investigate the influence of magnetic spacer layers on the superconductivity in Nb/Co multilayers. Compared to similar systems which have nonmagnetic spacer layers, the samples exhibit two dimensional superconductivity for relatively thin magnetic layer thicknesses ${d}_{\mathrm{Co}},$ reflecting the effects of magnetic pair breaking. Moreover, the transition temperature to the superconducting state ${T}_{c}$ shows a nonmonatonic dependence on ${d}_{\mathrm{Co}},$ which we examine within the framework of recent theories.

Non-collinear states in magnetic sensors

Certain materials have an electrical conductivity that is extremely sensitive to an applied magnetic field; this phenomenon, termed 'giant magnetoresistance', can be used in sensor applications. Typically, such a device comprises several ferromagnetic layers, separated by non-magnetic spacer layer(s)--a so-called 'super-lattice' geometry. In the absence of a magnetic field, the ferromagnetic layers may be magnetized in opposite directions by interlayer exchange coupling, while an applied external magnetic field causes the magnetization directions to become parallel. Because the resistivity depends on the magnetization direction, an applied field that changes the magnetic configuration may be detected simply by measuring the change in resistance. In order to detect weak fields, the energy difference between different magnetization directions should be small; this is usually achieved by using many non-magnetic atomic spacer layers. Here we show, using first-principles theory, that materials combinations such as Fe/V/Co multilayers can produce a non-collinear magnetic state in which the magnetization direction between Fe and Co layers differs by about 90 degrees. This state is energetically almost degenerate with the collinear magnetic states, even though the number of non-magnetic vanadium spacer layers is quite small.

Spin Density Wave in Epitaxial Cr(001)/Sn and Cr(001)/Au Multilayers with Nonmagnetic Spacer Layers

The magnetic structures of epitaxial Cr(001) thin films with periodic monatomic Sn or Au spacer layers, [Cr(8.0 nm)/Sn(0.2 nm)]×30, [Cr(4.0 nm)/Sn(0.2 nm)]×60 and [Cr(8.0 nm)/Au(0.2 nm)]×30 multilayers, were investigated by neutron diffraction. A commensurate antiferromagnetic structure was stabilized in all these multilayers at 300 K. It was found that highly distorted sinusoidal spin density waves (SDWs) with the propagating vector normal to the film plane were formed in the multilayers with Cr(8.0 nm) at 20 K. The SDW period was twice as large as that of the multilayer. The antinodes of the SDW are probably pinned at the Cr/Sn or Cr/Au interfaces so as to form SDWs with a period much longer than that of bulk Cr.

On the exchange biasing through a nonmagnetic spacer layer

We present results on the magnetic coupling between a ferromagnetic (F) thin film and an antiferromagnetic (AF) thin film through a nonmagnetic metallic spacer (S) layer. Multilayered structures have been grown on silicon substrates using dc magnetron sputtering. We have studied the structure AF/S/F for different materials. AF is Ir22Mn78, F is Fe16Co16, and the spacer S is Al, Ag, Au, Si, Pd, Ru, and Ti. In most cases, both the exchange-bias and the coercive fields decrease exponentially with the spacer thickness with a decay length of a few angstroms, depending slightly on the material. Similar decay lengths are observed for the reversed structure F/S/AF. In some specific cases, we observe a nonmonotonic variation of the exchange-bias field with the spacer thickness. For example, the exchange-bias field increases when a thin Ag layer is inserted between the F film and an AF film grown at high sputter pressure.

Interlayer exchange coupling

The extensive research done on interlayer exchange coupling in transition metal multilayers has resulted in a deep understanding of this coupling and a remarkable agreement between theoretical results and measurements. The coupling between two magnetic layers separated by a non-magnetic spacer layer is mediated by the electrons of the spacer layer. The coupling, which oscillates in sign as a function of the thickness of the spacer layer, is closely related to the well-known RKKY interaction between magnetic impurities. Due to the existence of many high-quality measurements, it has been much more fully developed theoretically than the interaction between impurities. Theory predicts that the periods of the oscillatory coupling should depend on critical spanning vectors of the Fermi surface belonging to the spacer-layer material. There is remarkable agreement for the measured periods and those predicted from the Fermi surfaces. There is also substantial agreement between theory and experiment on the strength of the coupling. This review presents the comparison between theory and experiment in some detail.

Quantum oscillations of the spin density in magnetic multilayers

An asymptotic stationary phase formula is derived for the oscillatory spin density induced in a nonmagnetic spacer sandwiched between two semi-infinite ferromagnets. It gives an explicit dependence for the polarization on the spacer layer thickness and on the distance from the ferromagnet-spacer interface. Both dependences are shown to oscillate with the same periods as the exchange coupling between the ferromagnetic layers. The magnitude of the polarization is governed by the degree of confinement of carriers in the spacer quantum well and by the curvature of the spacer Fermi surface. The formula is applied to a Co/Cu/Co ~001! trilayer described by tight-binding bands fitted to an ab initio band structure. Its validity is tested against a fully numerical calculation using the same band structure. As in the case of the oscillatory exchange coupling, the induced polarization is dominated by the contribution of the Cu Fermi surface neck extrema leading to a short period oscillation of 2.6 atomic planes. An interesting non‐Ruderman-Kittel-Kasuya-Yosida initial decay of the induced polarization is discussed. @S0163-1829~99!06309-2# Oscillatory exchange coupling between two ferromagnetic layers separated by a nonmagnetic metallic spacer layer 1 is one of the rare manifestations of quantum interference effects in a metallic system structured on a nanometer length scale. It was proposed early on by Edwards and co-workers 2,3 that the oscillatory exchange coupling is due to size quantization of the energy of electrons confined in a quantum well by the spin-dependent potentials of the magnetic layers. For large spacer layer thicknesses ( ’10 atomic planes!, an analytic formula for the coupling has been derived. 2‐4 The asymptotic formula is based on the stationary phase approximation and is referred to as analytic quantum well ~AQW! formula. It allows us to determine the oscillation periods, amplitudes and rate of decay of the coupling unambiguously and relate them to the details of the band structures of the ferromagnets and nonmagnetic spacer. Applications of the AQW theory 2‐4 using a realistic band structure 5‐7 lead to oscillation periods and amplitudes of the coupling that are in agreement with numerical total energy calculations 6,8‐12 and also with experimental results for multilayers with noble metal spacers. 13 The photoemission experiments of Ortega and Himpsel 14,15 and Segovia et al. 16 confirm the energy quantization of electrons confined in a nonmagnetic spacer layer. Although it is assumed implicitly that quantum confinement of electrons leads also to oscillations of the spin density in the spacer layer, the spin density has never been analyzed analytically. The induced moment in a nonmagnetic spacer has been calculated numerically. 17‐19 A very small moment

Magnetic properties of superlattice-configured thin films with variable interlayer couplings

A Heisenberg model is solved within the spin-wave theory for thin films with a superlattice configuration in which ferromagnetic atomic layers are separated by nonmagnetic spacer layers. The interface interaction is assumed to be ferromagnetic. We have also included magnetic anisotropy and surface effects. The temperature dependence of the magnetization M( T), transition temperature and the energy of magnons in such composite systems are derived for several sets of material parameters.

Hydrogen induced changes of the interlayer coupling in Fe (3)/V ( x) superlattices ( x=11–16)

The effect of hydrogen on the magnetic exchange coupling between iron layers through vanadium spacer layers has been studied with magneto-optical Kerr effect experiments in Fe (3)/V ( x) superlattices. Here x refers to the number of V monolayers varying from 11 to 16 and the Fe layer thickness is fixed at three monolayers. Without hydrogen the superlattice is antiferromagnetic (AFM) for x between 12 and 14 and ferromagnetic (FM) in all other cases. With hydrogen loading the coupling can be switched from AFM to FM and vice versa. As previously observed with neutron reflectivity measurements (Hjörvarsson et al., Phys. Rev. Lett. 79 (1997) 901) the change of the interlayer coupling upon hydrogen uptake is not simply due to the expansion of the non-magnetic vanadium spacer layer but more likely to the distortion of the Fermi surface. Bilinear and biquadratic exchange couplings can be recognized by the magnetic hysteresis loops and their coupling energies have been extracted by fits to the curves. For all samples the easy axis of the magnetization is in the plane without any preferred in-plane direction. Hydrogen loading does not affect the magnetic anisotropy of these samples.

Asymptotic behaviour of the oscillatory exchange coupling across alloy spacers: A first-principles approach

Abstract A theoretical model based on the screened KKR formalism is developed for studying the exchange interaction of two semi-infinite layers separated by a nonmagnetic metallic spacer layer which is either a pure metal or a binary alloy. A first application of the model for pure Cu spacer is also presented.

Brillouin light scattering and ferromagnetic resonance in trilayers with bilinear and biquadratic exchange coupling

We calculate the low-wave-number spin-wave dispersion relations for a trilayer system consisting of two ferromagnetic single-crystal films separated by a nonmagnetic spacer layer, coupled through bilinear and biquadratic exchange interactions, with an applied external field in the plane. The results are successfully applied to interpret Brillouin light scattering and ferromagnetic resonance in epitaxial sputtered (1 0 0)Fe/Cr/Fe trilayers.