Exchange-coupled Multilayers Research Articles

NiMn, IrMn, and NiO Exchange Coupled CoFe Multilayers for Microwave Applications

We compare the static and microwave properties of AF/CoFe/AF multilayers with NiMn, IrMn, and NiO as AF-layers, respectively. These exchange-coupled multilayers combine very high saturation magnetization (4piM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) with soft rotational behaviors. NiMn has the best exchange coupling and exhibits very high f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FMR</sub> (5-10 GHz) and good thermal stability up to 150degC. However, its resistivity is rather low (175 muOmegamiddotcm), and requires very large thickness to be efficient leading to low filling ratio. IrMn exhibits an intermediate exchange coupling leading to f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FMR</sub> in the range of 3-6 GHz. It is a good compromise regarding resisitivity (275 muOmegamiddotcm), and the reduced thickness for mu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">r</sub> ~100 at such frequencies. The high resistive NiO exhibits permeability spectra with moderate f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FMR</sub> around 3-5 GHz but allows the possibility to stop the eddy currents by lamination. However, NiO and IrMn both exhibit linear decrease of their FMR versus temperature. This work shows that NiMn, IrMn, and NiO multilayers provide suitable magnetic properties for microwave applications, and exhibit complementary properties on selected bandwidths which allow to address the 3-10 GHz range

Mn – Ir ∕ Fe – Si exchange-coupled multilayer film with plural ferromagnetic resonance absorptions for wideband noise filter

Mn – Ir ∕ Fe – Si exchange-coupled multilayer films for a wideband and gigahertz band noise filters are fabricated and evaluated. The Fe–Si film has characteristics of high saturation magnetization of 19kG, low magnetostriction of the order of 10−6, and high resistivity of 73μΩcm. When the exchange bias magnetic field is introduced to the Fe–Si layers by the effect of Mn–Ir∕Fe–Si exchange-coupling, the ferromagnetic resonance (FMR) frequency of the multilayer film is higher than the natural frequency of the single Fe–Si film. In addition, the multilayer film which has different thicknesses in each Fe–Si layer has plural FMR frequencies. The multiple FMR frequency gives the film characteristic of wideband energy absorption. The FMR absorption bandwidth of the multilayer film which has three FMR frequencies is about three times wider than that of a single FMR film. Therefore, the multilayer film with plural FMR frequencies has an advantage in wideband noise absorption and will be a useful material for the wideband and gigahertz frequency range noise filterings.

Swift ion irradiation of magnetostrictive multilayers

The modification of the magnetic and magnetostrictive properties of TbFe2/Co exchange-coupled multilayers irradiated with 700MeV Pb ions was investigated as a function of ion fluence. After irradiation, the magnetostrictive properties of the sample are first strongly improved up to a fluence of 1×1012ionscm−2 where the magnetoelastic susceptibility is six times higher than for the as-deposited sample. This effect is ascribed to stress relaxation and Fe–Co intermixing at the interfaces. For higher fluences, the magnetostrictive properties do not vary further, indicating a stationary state of mixing.

Preparation and characterization of Mn-Ir/Fe-Si exchange-coupled multilayer film with Ru underlayer for high-frequency applications

Fe-Si magnetic film and Mn-Ir/Fe-Si exchange-coupled film with an Ru underlayer were fabricated and evaluated. Both coercivity and anisotropy dispersion were reduced due to the Ru underlayer effect on the magnetic properties of Fe-Si magnetic film and Mn-Ir/Fe-Si exchange-coupled film. Even in the Mn-Ir/Fe-Si exchange-coupled multilayer film with an Ru underlayer, the coercivity of the upper Fe-Si layer was reduced.

Combination of ultimate magnetization and ultrahigh uniaxial anisotropy in CoFe exchange-coupled multilayers

CoFe exchanged-coupled multilayers are studied versus the thickness of the ferromagnetic (F) layer (eF) and the thickness of the antiferromagnetic (AF) layer (eAF). The F layer consists in Co90Fe10, Co50Fe50, and Co35Fe65, respectively. The AF layer is Ni50Mn50. Very high saturation magnetization (4πMs) are reported with 18.5, 23, and 24kG, respectively, combined with excellent soft rotational behaviors. The AF–F–AF multilayers cumulate top and bottom interfacial exchange coupling and show very high effective energy densities (Jex) of 1.01, 0.81, and 0.98ergcm−2, respectively. Jex increases with eAF up to a maximum for eAF⩾500Å. This leads to a strong bias field (Hex) with a classical 1∕eF dependence, which translates into a large uniaxial anisotropy (Hk) at 90° from the pinning direction. Typically, for eF=150Å, Hk is 741, 650, and 642Oe, respectively. However, this translation is rather complex and eF and 1∕eAF dependences of the ratio Hk∕Hex suggest a contribution of an intrinsic CoFe anisotropy. For all CoFe, it is shown that a ferromagnetic resonance frequency (fFMR) of 10GHz is achievable. Co35Fe65 mulilayers exhibit the highest eF×μ′ product, μ′ being very close to the theoretical ultimate permeability. Finally, the soft rotational behavior is shown unchanged over an angular range of ±30° from the pinning direction which is very unusual.

高周波マイクロ磁気デバイス用Ｍｎ‐Ｉｒ／Ｆｅ‐Ｓｉ交換結合膜の作製と特性評価

To develop micromagnetic devices for microwave applications, it is necessary to develop magnetic films with a ferromagnetic resonance (FMR) frequency higher than 1 GHz. For this purpose, it is essential that such films should have both high saturation magnetization and large magnetic anisotropy. In the present study, an exchange-coupled multilayer film, consisting of Fe89Si11(at.%) with a saturation magnetization of 19 kG and Mn80Ir20(at.%), was fabricated and characterized. From the experimental results, the Mn-Ir/Fe-Si exchange energy Jex was estimated to be about 0.1 erg/cm2. The [Mn-Ir (10 nm)/Fe-Si (7.2 nm)]21/Mn-Ir (10 nm)/Fe-Si (3.6 nm) multilayer film exhibited FMR at around 6 GHz. In addition, μ' was estimated 40-80, and μ” was nearly equal to zero at low frequencies (f < 4.5 GHz). The exchange bias magnetic field was decreased by thermal process. If the decrease of the FMR frequency due to thermal process is allowed by 10 % of as deposited film, the maximum process temperature must be lowered by 250 °C.

Magnetic patterning of exchange-coupled multilayers

The local modification of antiferromagnetic (AF) interlayer exchange coupling by focused ion-beam irradiation has been studied experimentally in the epitaxial Fe/Cr/Fe(001) trilayer systems. Square ferromagnetic (FM) areas of 200×200 μm2 were created in the initially AF trilayer by ion irradiation with a fluence of 1015 ions/cm2. It was found, that in the range of the external magnetic field of about ±200 Oe, the change of magnetic properties at the boundaries separating FM and AF areas occurs within distances of less than 200 nm. This fact allows the use of the technique for magnetic patterning of antiferromagnetically coupled trilayers on the submicrometer scale.

Spin-resolved unpolarized neutron off-specular scattering for magnetic multilayer studies

The capabilities of the method of using unpolarized neutron off-specular scattering for investigation of magnetic structures in exchange-coupled magnetic multilayers are thoroughly examined. It is demonstrated that strong anomalies in spin-flip selective scattering processes originating from magnetic fluctuations enables a straightforward determination of the coupling angle between the magnetization direction of successive Fe layers in Fe/Cr multilayers. A complete quantitative 2-dimensional data analysis of specular and off-specular scattering has been employed to provide detailed information on the lateral and transverse magnetization arrangement in the multilayer.

Antiferromagnetic spin structure and domains in exchange-coupled multilayers

As revealed by the observation of memory effects and domain imaging, the antiferromagnetic (AFM) spin structure in exchange bias is not static. During reversal, the AFM spins form an exchange spring connected with the ferromagnet (FM). We have observed hybrid domain walls consisting of FM and AFM sections and their evolution using the magnetooptical indicator film technique. The external magnetic field moves only the FM section of the hybrid domain walls, leading to the formation of an exchange spring parallel to the interface. The nucleation and unwinding of the exchange spring occur at different locations, and the propagation depends strongly on the chirality of the FM domain walls.

Statics and dynamics in giant magnetostrictive Tb/sub x/Fe/sub 1-x/-Fe/sub 0.6/Co/sub 0.4/ multilayers for MEMS

In the present paper, giant magnetostrictive (GMS) thin films have been investigated for future microelectromechanical systems (MEMS) purposes. To this end, flexural and torsional motions have been studied in low-field anisotropic GMS single-domain state (SDS) exchange-coupled TbFe-FeCo multilayers (ECML). The magnetoelastic (ME) coefficient b/sup /spl gamma/,2/ depends strongly on the ECML structures, compositions, and sputtering deposition parameters. GMS multilayers with a high b/sup /spl gamma/,2/ (18 MPa for TbFe/sub 2//Fe/sub 0.6/Co/sub 0.4/ compared to 11 MPa for TbFe/sub 2//Fe) were obtained with or without an in-plane easy axis with a controlled direction, and without any annealing postprocess. Dynamical excitations of the actuators have been investigated under various conditions. An enhancement up to a factor 5 of the oscillations compared to the TbFe-Fe multilayers is observed with the possibility to tune the flexural/torsional dynamical behavior of these cantilevers. The corresponding very large dynamical ME susceptibility of these improved uniaxial ECML gives the possibility to control GMS MEMS with further reduction of the excitation field down to a few oersteds.

Antiferromagnetic coupled high-frequency carrier-type magnetic field sensor using Ni-Fe/Fe-Mn multilayer

In this paper, we describe the asymmetrical field dependence of a high-frequency carrier-type thin-film magnetic field sensor (or so-called magneto-impedance sensor) consisting of Ni-Fe/Fe-Mn multilayer film. We fabricated a ferro/antiferro exchange-coupled multilayer film constituted of five 100-nm-thick Ni-Fe layers and four 30-nm-thick Fe-Mn layers by RF sputtering. The multilayer film had a unidirectional anisotropy of 600 A/m that was four times larger than that of a Ni-Fe (200 nm)/Fe-Mn (30 nm) bilayer film. Furthermore, the asymmetrical applied field dependence and high sensitivity of 19 k/spl Omega//T (1.9 /spl Omega//Oe) around an applied field of zero were obtained with the multilayer sensor element. The sensitivity was about 15 times larger than that of the bilayer sensor element. We conclude that the sensor element with high sensitivity can be realized by the Ni-Fe/Fe-Mn multilayer film without dc bias field.

Giant magnetostriction of exchange-coupled (TbFe/Fe) ML with high sensitivity after heat treatment

The possibility to improve the magnetoelastic (ME) properties of exchange-coupled (TbFe/Fe) multilayers (ML) after post-deposition annealing treatment is presented. Giant magnetostriction (GMS) improvement has been observed in highly anisotropic single domain state (SDS) multilayers deposited by ion sputtering under a magnetic polarization field H/sub d/ from two points of views corresponding to strong increase of both the magnetoelastic constant b/sup /spl gamma/,2/ by about 75% and the GMS sensitivity db/sub f//dH, by a factor 6 (b/sub f/ is the ME parameter of the flexural motion of the (TbFe/Fe)/spl times/40/glass bimorph). These improvements have been induced for both the flexion and the torsion motions of the bimorphs submitted to either a static or a dynamical magnetic field which can excite linear and nonlinear anharmonic oscillating modes.

Magnetic neutron off-specular scattering for the direct determination of the coupling angle in exchange-coupled multilayers

A new method is presented for the direct determination of the coupling angle in magnetic exchange multilayers. Strong anomalies in the off-specular neutron spin–flip scattering intensity at critical angles give a new tool, which is tested with Fe/Cr multilayers. In addition, a complete two-dimensional data analysis of specular reflection and off-specular scattering has been employed to verify atomic spin correlation in Fe/Cr multilayers, a typical system showing the GMR effect.

Low field anisotropic magnetostriction of single domain exchange-coupled (TbFe/Fe) multilayers

Giant magnetostriction (GMS) reported until now in exchange-coupled multilayers (ECML) present quasi-isotropic magnetostrictive behavior. In the present work it is shown the possibility to induce highly anisotropic GMS at low field with the single domain state in (TbFe/Fe)n multilayers when deposited by ion sputtering under a magnetic polarization field Hd. ECML grown under bias field Hd, applied along the large size of rectangular glass substrates present original magnetic and magnetoelastic (ME) properties associated with strong uniaxial behavior. After saturation by an external field H∥ applied parallel to the easy axis induced by Hd, a single domain state is kept by decreasing H∥ down to zero (Mr/Ms=1) which explains why there is no change of the magnetoelastic coefficient bγ,2 by increasing or decreasing H∥. A different behavior arises along the hard magnetization axis with a strong change of bγ,2 by increasing H∥ up to the saturation. ME anisotropy is the origin of high amplitude flexural and torsional oscillation modes as observed in our ECML.

Present and future of magnetooptical recording materials and technology

Erasable optical information storage from amorphous rare earth-transition metal (RE-TM) alloys media has led to impressive improvements towards hyper high density recording in the past five years. Write/erase and readout processes are based on light-induced thermomagnetic switching of magnetic domains and magnetooptical (MO) Kerr effect, respectively, in ferrimagnetic films exhibiting a uniaxial magnetic anisotropy. The route towards ultrahigh optical areal densities was open from the beginning of the nineties from two key technologies related to near field optical techniques and RE-TM exchange-coupled bi, tri- and multilayers. The performances of the MO media depend on the types and states of these multilayers with perpendicular or mixed (perpendicular and planar) anisotropies which change with the temperature and the applied field. The development of the thermomagnetooptical materials is discussed with reference to their magnetic and MO properties to increase the density storage.

180° walls structures in exchange-coupled multilayers with in-plane uniaxial anisotropy

First, the number of irreducible wall configurations is derived, both for an odd and an even number of magnetic layers. Second, the comparison of simulated wall structures in stacks made of three and four magnetic layers serves as an illustration of the interplay between flux–closure mechanisms and interlayer exchange interactions. Flux closure is shown either to compete or accommodate with ferromagnetic-type interlayer exchange interactions owing to its short- or long-range character. Last, an equivalent single layer behavior is shown to develop with an increasing number of magnetic layers, magnetic layer thickness, and interlayer (ferromagnetic-type) exchange parameter.

Dipole-exchange spin wave spectra of exchange-coupled magnetic multilayers calculated by transfer matrix formalism

We employ a transfer matrix formalism to study dipole-exchange spin waves in a perpendicularly magnetized multilayered structure consisting of identical magnetic layers alternated by nonmagnetic spacers, assuming both intralayer and interlayer exchange interactions. The interlayer exchange and the anisotropies at the magnetic layer boundaries are taken into account via linearized Hoffman boundary conditions. The bulk-mode bands for the case of infinite multilayers and the bulk and surface modes for semi-infinite multilayers are derived. Applications are made to recent ferromagnetic resonance data for Co/Pt multilayers. The extension of our theory to finite multilayers is discussed.

Theoretical investigation of magnetization configuration and reversal processes in exchange-coupled multilayers

An analytic study of magnetization profiles and reversal processes is performed in exchange-coupled double and trilayers (ECDL and ECTL) corresponding to mixed parallel and anisotropy structures (M/sub 1//M/sub 2/) and (M/sub 2//M/sub 1//M/sub 2/) where M/sub 1/ corresponds to planar magnetization layers NdCo or GdFeCo and M/sub 2/ are magnetization layers TbFeCo or DyFeCo. The changes of the profiles with magnetic parameters of the layers are calculated from a variational method of the magnetic energies in a continuum model in ECDL and from a noncontinuum model method in ECTL. The effective exchange field induced at the interface by the capping layer M/sub 1/ for thickness higher than a critical value is calculated and its influence on the reversal process of the perpendicular layer M/sub 2/ is determined in ECDL. Both the continuous and noncontinuous models are useful to optimize magnetooptical structures and the results from these models are compared.

Replica behavior in direct overwritable exchange-coupled magneto-optical multilayer system

The process of direct overwriting by light intensity modulation has been simulated for an exchange-coupled multilayer film without an initializing magnetization. Using the mean-field theory, the temperature dependence of the interlayer wall energy has been calculated for a magnetic triple layer, and the effect of interlayer exchange coupling on magnetization reversals has been investigated. The simulation has shown that replica can be easily realized by the arrangement of the Curie and compensation temperatures of the magnetic layers. A new mechanism of achieving replication from the initializing to the writing layers in the writing process is described in terms of subnetwork moments with high laser power.

Overwrite repeatability of magneto-optical exchange-coupled multilayer

Overwrite repeatability of exchange-coupled multilayer for light-intensity-modulated overwrite (OW) was studied by using Arrhenius analysis. A computer simulation showed that typical OW disks and a conventional nonoverwrite (NOW) disk have similar maximum temperature during recording. Nevertheless, the overwrite repeatability of the OW disks was smaller than that of the NOW disk. The degradation mode characteristic to the OW disks was found to have lower activation energy than the conventional NOW single-layer disk. The degradation mode was caused by initialization noise resulting from domain-wall fluctuations under a large initializing field, and was reduced by using a transition-metal dominant memory layer with high-domain-wall coercivity. An overwrite repeatability of more than 106 was achieved.