Ferromagnetic Metal Layers Research Articles

The effect of the ferromagnetic metal layer on tunnelling conductance and magnetoresistance in double magnetic planar junctions

Based on the free-electron approximation, we investigate the effect of the ferromagnetic metal layer on the tunnelling magnetoresistance (TMR) and tunnelling conductance (TC) in the double magnetic tunnel junctions (DMTJs) of the structure NM/FM/I(S)/NM/I(S)/FM/NM, where FM, NM and I(S) represent the ferromagnetic metal, nonmagnetic metal and insulator (semiconductor), respectively. The FM, I(S) and inner NM layers are of finite thickness, while the thickness of the outer NM layer is infinite. The calculated results show that, due to the spin-dependent interfacial potential barriers caused by electronic band mismatch between the various magnetic and nonmagnetic layers, the dependences of the TMR and TC on the thicknesses of the FM layers exhibit oscillations and a much higher TMR can be obtained for suitable thicknesses of FM layers.

Electrical spin injection from ferromagnetic MnAs metal layers into GaAs

The spin injection into GaAs has been studied for the ferromagnetic metal MnAs. Evidence for preferential minority-spin injection is obtained from the circular polarization of the electroluminescence in GaAs/(In,Ga)As light-emitting diodes (LED). The spin-injection efficiency of 6% at the MnAs/GaAs interface is estimated on the basis of spin-relaxation times extracted from time-resolved photoluminescence measurements. This efficiency, as well as the preferential spin orientation, resembles very much the injection behavior found for epitaxial Fe layers. The results do not depend on the azimuthal orientation of the epitaxial MnAs injection layer.

Motion of domain walls under the action of spin-polarized current in a magnetic junction

The effect of spin-polarized current on a domain structure in a magnetic junction consisting of two ferromagnetic metallic layers separated by an ultrathin nonmagnetic layer is studied within a phenomenological theory. The magnetization of one ferromagnetic layer (layer 1) is assumed to be fixed, while that of the other ferromagnetic layer (layer 2) can be freely oriented both parallel and antiparallel to the magnetization of layer 1. Layer 2 can be split into domains. Charge transfer from layer 1 to layer 2 is not attended with spin scattering by the interface but results in spin injection. Due to s-d exchange interaction, injected spins tend to orient the magnetization in the domains parallel to layer 1. This causes the domain walls to move and “favorable” domains to grow. The average magnetization current injected into layer 2 and its contribution to the s-d exchange energy are found by solving the continuity equation for carriers with spins pointing up and down. From the minimum condition for the total magnetic energy of the junction, the parameters of the periodic domain structure in layer 2 are determined as functions of current through the junction and magnetic field. It is shown that the spin-polarized current can magnetize layer 2 up to saturation even in the absence of an external magnetic field. The associated current densities are on the order of 105 A/cm2. In the presence of the field, its effect can be compensated by such a high current. Current-induced magnetization reversal in the layer is also possible.

Version 7/21/01 current-perpendicular-to-plane (CPP) magnetoresistance

Giant magnetoresistance (GMR) in magnetic multilayers composed of alternating layers of ferromagnetic (F) and non-magnetic (N) metals is of both fundamental and technological interest. Technologically, GMR is already being used for sensors and read-heads. Scientifically, much has been learned about the physics underlying GMR, but new insights are still being generated. We describe some important information learned about GMR from measurements in the current-perpendicular-to-plane (CPP) geometry. This geometry has the attractive features that the CPP-MR is often larger than the more usual current-in-plane (CIP)-MR, and often allows more direct access to the physics underlying GMR. Topics covered include: anisotropic scattering in F-layers and at F/N interfaces, spin-memory-loss within F- and N-metal layers and at interfaces, whether mean-free-paths are scaling lengths in the CPP-MR, and current-driven magnon excitation and magnetization reversal.

Growth of giant magnetoresistance multilayers: Effects of processing conditions during radio-frequency diode deposition

The magnetotransport properties of giant magnetoresistance multilayers are significantly effected by the atomic-scale structure of the interfaces between the nonferromagnetic conducting and ferromagnetic (FM) metal layers. The interfacial roughness and the extent of intermixing at these interfaces are both known to be important. A combination of experimental and multiscale modeling studies have been used to investigate control of interface structure during multilayer growth by rf diode deposition and the consequences of such control for magnetotransport. Experiments were conducted to evaluate the dependence of the magnetotransport properties of NiFeCo/CoFe/CuAgAu multilayers upon the growth conditions (background pressure, input power), and to link the roughness of vapor-deposited copper layers to the same process parameters. These experimental studies reveal the existence of intermediate background pressure (20 mTorr) and plasma power (175 W) that resulted in the highest magnetoresistance and a strong sensitivity of copper layer surface roughness to both the power and pressure at which deposition was conducted. By using a combination of modeling technologies, the deposition process conditions have been linked to the atomic fluxes incident upon the sample surface. This was then used to determine the atomic-scale roughness of the film. Energetic metal atoms (and inert gas ions) were found to have very strong effects upon interfacial structure. The models revealed an increase in interfacial roughness when metal (or inert gas ion) translational energy was decreased by either reducing the plasma power and/or increasing the background pressure. Because high-energy metal impacts activated atomic jumping near the impact sites, high plasma power, low background pressure process conditions resulted in the smoothest interface films. However, these conditions were also conducive to more energetic Ar+ ion bombardment, which was shown by molecular dynamics modeling to induce mixing of the FM on the copper interface. Intermediate plasma powers/background pressures result in the most perfect interfaces and best magnetotransport. The insights gained by the modeling approach indicate a need to avoid any energetic ion bombardment during the early growth stages of each new layer. This could be accomplished by operating at low power and/or high pressure for the first few monolayers of each layer growth and may provide a growth strategy for further improvement in magnetotransport performance.

Ferromagnetic metals on II–VI semiconductors: epitaxial growth, and structural and magnetic properties

We have studied the growth conditions to obtain the chemically well ordered L1 0 phase of the FePd alloy deposited on ZnSe and CdZnMgTe II–VI semiconductors. We have observed that the segregation of Se or Te atoms at the surface involved a dramatic effect on the formation of the L1 0 phase. The strong anisotropy exhibited along the growth direction allows to observe, by magnetic force microscopy, the perpendicular magnetic configuration with up and down domains. Comparably, we have grown, on the same materials, Cu/Ni multilayers showing a clear perpendicular magnetic anisotropy. Optical measurements on a semimagnetic heterostructure covered by a ferromagnetic metal layer suggest fluctuations of the Fermi level at the interface between the semiconductor and the metal.

π-phase magnetism in ferromagnet-superconductor superlattices

New 0π and ππ Larkin-Ovchinnikov-Fulde-Ferrell (LOFF) states with antiferromagnetic orientation of magnetizations in the neighboring layers of a ferromagnetic metal (FM) are predicted for FM/superconductor (FM/S) superlattices. Under certain conditions, the critical temperature T c of these states is higher than for the known 00 and π0 LOFF states with ferromagnetic ordering of the FM layers. It is shown that the nonmonotonic behavior of T c in the FM/S superlattices with S-layer thickness d s less than the threshold value d s π is due to the phase transition cascade 0π-ππ-0π At d s >d s π , the T c oscillations are caused by the 00-π0-00 transitions. New logic elements based on the FM/S structures and combining the advantages of the superconducting and magnetic data-record channels in a single sample are proposed.

Point-contact studies of current-controlled domain switching in magnetic multilayers

We present measurements demonstrating current-induced magnetic domain switching, and also other magnetic excitations, in point-contact devices containing alternating ferromagnetic (F) and noble metal (N) layers, for perpendicular currents ∼109 A/cm2. For F/N/F trilayers in which one F layer is much thinner than the other, we can controllably switch the magnetic moments in the two F layers parallel with a current bias of one sign, and switch them antiparallel with a reversed current. For thicker magnetic films, and for thin films in the presence of a saturating magnetic field, we observe nonhysteretic current-induced changes in resistance, which can be understood as current-induced spin-wave excitations. These observations are in agreement with a model of current-induced magnetic reorientations caused by local exchange forces between conduction electrons and the magnetic moments.

Magnetic Characteristics of the System of Exchange Coupled Layers

The interlayer exchange coupling of ferromagnetic transition metal layers through non-magnetic spacer has been intensively studied over the last decade (see, e.g. [1-6]). In comparison, little work was devoted to the magnetic characteristics of such systems. An oscillatory interlayer exchange coupling observed in many multilayered systems can be understood as an interference effect of electron waves partially reflected at each interface with spin dependent reflection coefficients [7]. Therefore, one can expect the magnetic properties, which are related to the interlayer exchange coupling parameter, to oscillate as a function of the spacer thickness. However, relatively few results have been reported concerning experimental investigations of basic magnetic properties of exchange coupled layers. Oscillations of the magnetic anisotropy and the Kerr rotation angle in magnetic films covered by non-magnetic overlayers have been reported [8, 9]. It has been also experimentally found that the Curie temperature, ground state magnetic moment, and spin wave stiffness parameter are the oscillatory functions of the thickness of non-magnetic layer [10]. Moreover, in the recent paper the onset of long-range order in exchange coupled epitaxial monolayers is investigated by temperature dependent susceptibility and element resolved magnetization measurements [11].

Structure characterization and magnetic properties of oxide superlatticesLa0.6Sr0.4MnO3/La0.6Sr0.4FeO3

Oxide superlattices composed alternatively of ferromagnetic metal layers of ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{MnO}}_{3}$ and antiferromagnetic insulator layers of ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{FeO}}_{3}$ were fabricated on ${\mathrm{SrTiO}}_{3}$ substrates by pulsed laser deposition with controlling each layer thickness on an atomic scale. The near-perfect superlattice structure with atomically flat interfaces was verified by multiple peaks from Laue function in x-ray diffraction patterns with use of synchrotron radiation. An increase of antiferromagnetic ${\mathrm{La}}_{0.6}{\mathrm{Sr}}_{0.4}{\mathrm{FeO}}_{3}$ layer thickness from 2 to 5 unit cells induces strong magnetic frustration around the superlattice interfaces, leading to reduction in magnetic transition temperature and ferromagnetic volume, while increasing the resistivity and magnetoresistance at low temperatures.

The magnetic stability of spin-dependent tunneling devices

The tunneling resistance between two ferromagnetic metal layers that are separated by a thin insulator depends on the relative orientation of the magnetization M of each layer. In a memory device, independent switching of the magnetically soft layer is achieved by making the other layer either exchange-biased or magnetically hard. The repeated reversal of M of the soft layer by field cycling can demagnetize the other magnetically hard layer and thus erase the tunnel junction's memory. The M of exchange-biased structures was stable at least to 10(7) cycles, whereas in hard structures, M generally decayed logarithmically with the number of field cycles. The decay was very sensitive to the thickness of the hard layer and the composition of the soft layer. However, no decay was observed when the moment reversal was accomplished by coherent rotation, establishing that demagnetization results from the formation and motion of domain walls in the soft layer during field cycling.

Spin-Dependent Transmission of Electrons through the Ferromagnetic Metal Base of a Hot-Electron Transistorlike System

A quasimonoenergetic spin-polarized electron beam, emitted in vacuum from a GaAs photocathode, is injected into a thin ferromagnetic metal layer deposited on an $n$-doped GaAs substrate. The current transmitted through this Schottky barrier is measured. The striking feature of this hot-electron transistorlike system is a current gain spin dependency as high as $20%$. The measured variations of the current gain and its spin dependency with the injection energy are well explained by a very simple analytical model describing the transport of hot electrons in metallic thin films.

The magic of magnetic multilayers

In recent years there has been a renaissance in the study of magnetic multilayers—primarily because of the discoveries of enhanced or giant magnetoresistance [1, 2] and oscillatory interlayer exchange coupling [3] in magnetic multilayered structures consisting of alternating thin ferromagnetic and nonferromagnetic metallic layers. These discoveries have led to intense theoretical and experimental studies of an extraordinary range of structures over the past few years.

Extraordinary MR effect induced in a non-magnetic metal layer formed on a surface of ferromagnetic metal layers

A curious phenomenon has been observed which can be understood only when assuming that the extraordinary MR does take place in non-ferromagnetic layers formed on surfaces of ferromagnetic metals. In this phenomenon, the maximum value corresponding to the MR ratio of the copper layer formed on a permalloy layer is estimated to be 46% of the permalloy layer.

A New Spin Filter: The Magnetic Schottky Diode

ABSTRACTWe observe spin-dependent transmission of hot electrons through the ferromagnetic metallic layer of a Schottky diode. A 25% spin-polarized electron beam is produced in vacuum by a GaAs photocathode under illumination with circularly polarized light. The photoemitted electrons are then injected from the vacuum into the sample: a 3.5 nm-thick Fe layer deposited on a n-doped GaAs substrate. We measure a transmission coefficient of the order of a few 10-4. Using an incident energy above the Fermi level equal to 5 eV, a 5% asymmetry in the transmitted current is observed by changing the spin-polarization of the incident electrons (i.e. the light polarization) and/or reversing the Fe-layer magnetization. This corresponds to a transmission spin-dependence of 20% for a 100% spin-polarized incident beam.

Oscillatory interlayer couplings as functions of a ferromagnetic metal layer and a semiconducting spacer in magnetic superlattices

Fe layer thickness dependence of the interlayer exchange coupling across a Au spacer is investigated for MBE-growth Au/Fe/Au/Fe/Au(001) films involving two step-shaped Fe layers with each step 1 ML high. The coupling strength oscillates as a function of each Fe layer thickness with a 2 ML Fe period, which is attributed to the resonance state involving the Fe majority spin band. The interlayer coupling is also studied for Fe/Si and Fe/FeSi multilayers as functions of semiconducting spacer thickness and temperature.

Quantum oscillations in the electric field for perpendicular transport through an interface between two metallic layers

We present a quantum statistical theory of the electronic transport through an interface between two metallic layers. It is shown that when an electric current flows in the direction perpendicular to the interfaces, spatial oscillations in the charge density and electric field exist on both sides of the interface. These oscillations are due to quantum interference effects between the incident and reflected electrons. They arise when the electrons have different Fermi wave vectors k F in the two adjacent metallic layers. The dominant term in these oscillations has period π k F and its amplitude decreases as a hyperbolic function of the distance from the interface at short distances (relative to the elastic mean-free paths) and exponentially with a characteristic damping length equal to the mean-free paths at large distances. Furthermore, the presence of a potential barrier of height U at the interface gives rise to a new interfacial resistance proportional to U 2. For the case of an interface between two ferromagnetic transition metal layers, a discussion of the relative role of s and d electrons in the in-plane and perpendicular transport is presented.

Magnetic Tunneling Effect

Spin-polarized electron tunneling between ferromagnetic metal layers is a phenomenon of considerable interest. The tunneling conductance changes with the angle between the magnetization vectors of these ferromagnetic layers (the magnetic valve effect). Although only a small number of experiments on the magnetic valve effect have been reported, it poses interesting possibilities from the standpoint of both basic material science and of applications. This paper reviews briefly the literature on the magnetic valve effect, including some theories and experiments on ferromagnetic tunneling junctions, ferromagnetic multilayer films and ferromagnetic double-layer films.

Removable magnetic domain memory module

The paper describes design, materials and performance of a removable memory module. Information is stored by in-plane- oriented magnetic domains in ferromagnetic metal layers having circular magnetic anisotropy. 49 152 bits are stored in 4 shift registers of 12 kbit each. Maximum data rate is 500 kbit s -1.

The Iron curtain, a batch-fabricated storage array

The iron curtain is a new type of batch-fabricated storage array. The wiring of this array is made from insulated copper wires, covered with a thermobonding lacquer: These wires are glued together by heating, resulting in a wire screen made of one piece. Next the wiring is soldered in a frame and covered with a ferromagnetic metallic layer. Depending on the type of wiring, one obtains an iron curtain with core-type elements (CIC) or one with biax-type elements (BIC). The CIC can be operated in 2 or 21/2 D. The BIC is a word-organized element array and is operated in the flux-tipping mode. Typical outputs are between 5 and 10 mV.