Magnetoelastic Anisotropy Research Articles

Preparation and properties of glass-coated microwires

Technological aspects of the Taylor–Ulitovsky method for fabricating glass-coated microwires with different structures of metallic nucleus are discussed. The main parameters of the process can be distinguished as follows: a casting rate of microwire and its limits, cooling rate of the metal core, geometrical characteristics and alloy composition. The casting rate is important in determining the upper and lower limits of the diameter of microwire. Depending on the critical quenching rate (10 4–10 7 K/s), metastable, amorphous or supersaturated state phases are obtained. Mixed structures consisting of micro- and nanocrystallites embedded into the amorphous matrix can also be formed. During the fabrication process, strong internal stresses are generated, which determine some of the most interesting properties of these materials. In particular, magnetic properties depend mostly on the magnetoelastic anisotropy arising from the coupling of the internal stresses with magnetostriction constant.

Stress level in Finemet materials studied by impedanciometry

In this work, a study of the stress relief in Finemet ribbons, Fe73.5Cu1Nb3Si16.5B6, as a function of the annealing temperature is presented. The as melt-spun samples are amorphous and become partially crystallized after annealing at appropriate temperatures. For temperatures TA⩾480 °C the samples are nanocrystalline, with a microstructure composed by α-Fe1−xSix (x∼0.2) crystallites (10 nm average diameter) embedded in an amorphous matrix. Nanocrystallization, associated with stress relief effects, improves the soft magnetic properties of this kind of material. The stress level was quantified using magnetorestriction (measured by SAMR), magnetoelastic anisotropy, and domain wall energy data obtained from impedance spectra measurements. A reduction of the internal stress from 15 to 0.2 MPa was verified when comparing the as-cast to the sample annealed at 580 °C. Improvement of the magnetic softness of the samples was also followed by the increase of the domain wall and magnetization rotation contributions to the overall effective permeability.

Strain-induced magnetic anisotropies in ultrathin epitaxial NixPd1−x alloy films

We studied the influence of alloy composition and thickness on the magnetic properties of ultrathin NixPd1−x alloy films epitaxially grown on Cu(100) and Cu3Au(100) substrates. Our experiments show that the magnetoelastic anisotropy plays a seminal role in the competition of the different contributions to the magnetic anisotropy. Its magnitude and sign can be adjusted via the film strain that in turn depends on the alloy composition. At a fixed thickness of 9 monolayers and a Ni concentration of 87% for Cu(100) and of 50% for Cu3Au(100) we observe an inverse spin reorientation transition (iSRT). The occurrence of the iSRT is attributed to the fact that the magnetoelastic anisotropy overcompensates all other contributions to the effective anisotropy Eeff.

Stress dependence of soft, high moment and nanocrystalline FeCoB films

Soft, high moment materials are crucial for the magnetic data recording industry in applications such as write poles and soft underlayers. Ever increasing areal densities have pushed the requirements for materials exhibiting saturation magnetizations above 2 T. In this work we investigate nanocrystalline FeCoB which exhibits a saturation magnetization of ∼2.2 T. FeCoB exhibits a magnetostriction of >50×10−6 which suggests that the magnetoelastic anisotropy is important. Nanocrystalline FeCoB films are rf diode sputtered as a function of Ar pressure. As the pressure is increased, film stress increases from compressive to tensile with a critical pressure of ∼12 mTorr. TEM and XRD show that pressure has no significant effect on microstructure, however, at pressures above a critical pressure, the FeCoB films change from magnetically isotropic to magnetically uniaxial with low coercivity. This is due entirely to the magnetoelastic anisotropy which must be considered as the industry begins to utilize magnetic alloys with nonzero magnetostriction.

Barkhausen noise measurements in materials with vanishing magnetoelastic anisotropies

In this work, Barkhausen noise measurements are shown for a series of amorphous ribbons: Metglas 2605TCA, Metglas 2705M, and Finemet materials with composition Fe73.5Cu1Nb3Si18.5B4 under different stress (σ) and annealing temperature (TA) conditions. The dynamical state of the system is characterized by the power spectra and amplitude distribution functions as well as by the Minkowski–Bouligand dimension for each set of samples. Special attention has been paid to the ranges of vanishing anisotropies: low applied stress for the Metglas samples and the nanocrystallized phase of Finemet materials.

Magnetoelastic coupling in epitaxial Cu/Ni90Fe10/Cu/Si(001) thin films

Magnetic anisotropy energy (MAE) and magnetoelastic (ME) stress in epitaxial Cu(4 nm)/Ni90Fe10/Cu(160 nm)/Si(001) films have been studied at room temperature as a function of the permalloy film thickness (2 nm⩽tPm⩽50 nm). Magnetostatic energy keeps the magnetization within the film plane, although surface and magnetoelastic anisotropy energy favor magnetization normal to the film plane. The direct measurement of the magnetoelastic stress shows the ME coefficients to depend linearly on the strain, ε, for the ME coefficient. As a result, a second-order magnetoelastic contribution, proportional to ε2, has to be included in the MAE. Using both sets of measurement we determine two second-order ME coefficients, M1γ,2=−0.3×107 J/m3 and M2γ,2=8.3×107 J/m3, and the surface magnetic anisotropy constant, Ksur=0.4 mJ/m2.

Sign reversal of the magnetic anisotropy in La 0.7A 0.3MnO 3 (A = Ca, Sr, Ba, □) films

The magnetic anisotropy of epitaxial manganite La 0.7A 0.3MnO 3 films (A = Ca, Sr, Ba, □=vacancy) is studied as a function of tolerance factor. Within the film plane a biaxial magnetocrystalline anisotropy was observed with an anisotropy constant changing sign as function of doping: whereas La 0.7Ca 0.3MnO 3 and La 1− x □ x MnO 3 have [1 0 0] easy axes, La 0.7Sr 0.3MnO 3 and La 0.7Ba 0.3MnO 3 films were found to have [1 1 0] easy axes. This is related to a symmetry change of structural distortions with increasing tolerance factor. Films strained by the substrate show a large uniaxial magnetoelastic anisotropy along the film normal; the saturation magnetostriction was determined to λ 100=(3–6)×10 −5.

Effect of Applied Mechanical Stressses on the Impedance Response in Amorphous Microwires with Vanishing Magnetostriction

The effect of external (torsion and tensile) stresses on the electrical impedance of nearly-zero magnetostrictive amorphous microwires (conventional and glass covered) is reported. Glass coated Co 68.5 Mn 6.5 Si 10 B 15 microwire exhibits a maximum relative change in magnetoimpedance ratio up to around 130% at a frequency of 10 MHz, magnetic dc field of about 180 A/m and under tension of 60 MPa. This giant magnetoimpedance (GMI) effect of the microwire is affected by the magneto-elastic anisotropy induced in the sample by applying tensile stress. In addition, from the linear variations of the magnetic field H m corresponding to the maximum AZ/Z ratio, with the applied tensile stress, that is the H m (σ) curve, the magnetostriction constant of this amorphous microwire (λ s -2 × 10 -7 ) is estimated. The torsion giant impedance (TGI) ratio (ΔZ/Z) ξ has been investigated in as-cast and annealed (Co 0.94 Fe 0.06 ) 72.5 B 15 Si 12.5 conventional amorphous microwire. As-cast conventional microwire shows a slightly asymmetric Torsion Giant Impedance (TGI) and such asymmetry can be eliminated by current annealing or enhanced by current annealing under torsion. The asymmetry of (ΔZ/Z) ξ in the as-cast state and after torsion annealing could be ascribed to the spontaneous or induced helical magnetic anisotropy, which can be compensated by the application of certain torsional stress.

Magnetic Anisotropy and the Appearance of Stripe Magnetic Domains in Co/Fe Multilayers

Thin Co/Fe multilayers were e-beam evaporated in ultra-high vacuum, keeping constant the Co layer thickness and varying that of Fe in the 0.5÷15 nm range. By increasing the Fe layer thickness, a component of magnetization perpendicular to the film plane rises up, and long and parallel magnetic domains (stripe domains) appear. The phenomenon is explained on the basis of the competition between the magnetoelastic anisotropy induced by stresses at the interfaces, and the shape anisotropy constraining the magnetization in the film plane.

Ｃｏ基アモルファスリボンの熱処理による特異な応力磁化特性変化

The stress-magnetization properties of Co-based amorphous ribbons due to heat treatment were investigated. A sample was annealed with a longitudinal magnetic field applied to its ribbon axis. Examination of the stress-magnetization change revealed a stress-insensitive region that depended on the annealing temperature. To apply further stress, the magnetization of the sample was decreased to zero. However, examination of the magnetization change revealed a unique behavior due to a stress application at a lower exciting magnetic field (< 5 A/m). The stress-magnetization change showed irreversibility, with a maximum slightly larger than the initial magnetization value while the stress was decreased and then restored to the initial value. The results could be interpreted as implying a model of combined effects: a magnetic field parallel to the ribbon axis, an induced anisotropy field due to field annealing, and magnetoelastic anisotropy due to the applied tensile stress.

FMR investigation of the amorphous CoFeSiB glass-covered wires in the presence of mechanical tension

Results on the FMR investigation of the effect of applied tensile stresses on the surface magnetic anisotropy of Co 68.25Fe 4.5Si 12.25B 15 amorphous glass-covered wires are reported. Axial stresses determine a decrease of the resonance field and an increase of the absorption intensity that correspond to the resonance peak, indicating a reinforcement of the circumferential magnetoelastic anisotropy from the wire's surface region.

In situ observation of epitaxial growth of [Au/Co/Cu] and [Cu/Co/Au] superlattices and their magnetic interface anisotropies

We studied crystal growth of [Au/Co/Cu](111) and [Cu/Co/Au](111) superlattices during molecular-beam epitaxy and their magnetic anisotropies, and discussed the relationships between the interface structures and the perpendicular magnetic anisotropies. To study the structure at or near the interface of the superlattices, we continuously observed the change of surface in-plane lattice constant during growth using reflection high-energy electron diffraction (RHEED) on a real-time basis. From the RHEED observations, we deduce that gradually decreasing strain in the thickness direction exists in the Co layers at the Co/Au interfaces in the [Cu/Co/Au] superlattices, in which the Co layers are grown on the Au layers, and that coherency strain due to the Cu underlayers and strain due to the Au overlayers coexist in the Co layers in the [Au/Co/Cu] superlattices. From the magnetic measurements and detailed considerations, we conclude that both the magnetocrystalline interface anisotropy (or the Néel-type magnetic surface anisotropy) and the magnetoelastic interface anisotropy contribute to the total interface anisotropy in both types of superlattices. However, we find that the magnetoelastic interface anisotropy originating from the interface regions of the Co layers on the Au underlayers is larger than that under the Au overlayers. This leads to the perpendicular magnetic anisotropy in the [Cu/Co/Au] superlattice but not in the [Au/Co/Cu] superlattice when the Co layers are five monolayers thick. We also find that the magnetoelastic interface anisotropy originating from the interface regions of the Co layers under the Au overlayers strongly depends on the underlayer material of the Co layers, and is much larger in the [Au/Co/Cu] superlattices than in the [Au/Co/Ag] and the [Co/Au] superlattices. Moreover, we demonstrate that the above two contributions to the interface anisotropy can be separately evaluated in the [Cu/Co/Au] and the [Au/Co/Cu] superlattices. The present result is consistent with that of earlier work on [Au/Co/Ag](111) and [Ag/Co/Au](111) superlattices [T. Kingetsu and K. Sakai, Phys. Rev. B 48, 4140 (1993)].

Surface- and strain-induced anisotropy in epitaxial [formula omitted] films

Magnetic anisotropy energy in epitaxial Cu(4 nm)/Ni 90Fe 10/Cu(160 nm)/Si(0 0 1) films as a function of the permalloy film thickness ( 2 nm⩽t Pm ⩽200 nm ) has been studied at room temperature. Magnetostatic energy keeps the magnetization within the film plane, although surface and magnetoelastic anisotropy energy favor magnetization normal to the film plane. Fitting the anisotropy vs. thickness with magnetostatic, bulk magnetoelastic and surface anisotropy energy requires K s =0.15 mJ/m 2 . Including the second-order magnetoelastic anisotropy in the fitting expression increases K s to 0.4 mJ/m 2 .

Spin engineering in ultrathin Co0.35Pd0.65 alloy films

The easy axis of magnetization in CoxPd1−x alloy films with x=0.35 is controllably engineered by varying the thickness, tPd, of the Pd overlayers directly deposited on the alloy layers. In a Pd(50 Å)/CoPd (20 Å)/Pd (tPd) sample with a 10-Å-height step-wedge Pd layer, the easy axis smoothly changes from in-plane orientation (tPd=0 Å) through canted out of plane (0&amp;lt;tPd&amp;lt;30 Å) to perpendicular (30⩽tPd⩽60 Å). We also demonstrate that the spin switching is controllably reversible between in-plane and perpendicular orientations when the individual constituent layers of CoPd and Pd are alternately deposited. Smoothly continuous spin reorientation in a Pd (50 Å)/CoPd (30 Å)/Pd (tPd) film with increasing tPd in a broad range of 0–150 Å convincingly evidences the magnetoelastic anisotropy origin for the observed spin switching.

Perpendicular magnetic anisotropy in ultrathin yttrium iron garnet films prepared by pulsed laser deposition technique

A series of thin and ultrathin (<500 A thick) polycrystalline yttrium iron garnet (YIG) films has been prepared on quartz substrates by the pulsed laser deposition technique to study the effects of dimensional changes on the magnetic properties. The film thickness was varied from 100 to 3800 A. Saturation magnetization decreased rapidly in ultrathin films with decreasing thickness. The lattice cell of all samples was found to be slightly distorted though the elementary cell volume was one of a bulk crystal. The Curie temperature of the films did not differ from the bulk YIG value. Ferromagnetic resonance experiments show evidence of a change from in-plane magnetization to a perpendicular magnetization in films thinner than 140 A. This reorientation can be explained by the magnetoelastic anisotropy contribution related to thermally induced strain. This contribution is likely to be the principal origin of the perpendicular anisotropy.

Spin-reorientation transition in magnetic alloy films CoxNi1−x/Cu(100)

With better than 1% control of alloy composition, binary alloy films CoxNi1−x/Cu(100) with x⩽10% were prepared for the study of the spin-reorientation transition at variations of composition, thickness, and temperature. Only the films with a Co concentration less than 10% reveal the spin-reorientation with the film thickness. The critical thickness for the spin-reorientation transition was shifted drastically from 7.5 to 17.5 monolayers for a Co concentration variation from 0% to 8%. These findings indicate a strong influence of the composition on the magnetoelastic anisotropy. A kind of temperature-driven spin-reorientation from in-plane to perpendicular with increasing temperature was also found.

Analysis of the small-angle neutron scattering of nanocrystalline ferromagnets using a micromagnetics model

In ferromagnets with a nonuniform magnetocrystalline and/or magnetoelastic anisotropy, such as nanocrystalline (nc-) or cold-worked (cw-) polycrystalline materials, the static magnetic microstructure gives rise to strong elastic magnetic small-angle neutron scattering (SANS). The paper explores a method for analyzing field-dependent SANS data from such materials in terms of a model based on the theory of micromagnetics. Samples of cw Ni and of electrodeposited nc Ni and nc Co were characterized by x-ray scattering and magnetometry, and were investigated by SANS both with and without polarization of the neutron beam. The variation of the differential scattering cross section with the scattering vector and with the applied magnetic field is well described by the model. Also, experimental results for the exchange stiffness constant A and for the spin-wave stiffness constant D obtained from the analysis are found to agree with literature data obtained by inelastic neutron scattering on single-crystal specimens. The model supplies an ``anisotropy field scattering function'' that contains information on the magnitude of the magnetic anisotropy in the material, and on the characteristic length scales on which the anisotropy changes direction. The results suggest that the anisotropy may be strongly nonuniform in each crystallite, possibly due to twinning, and that some magnetic moments in the Ni samples are strongly pinned at defects.

Magnetoelastic sensor based on GMI of amorphous microwire

The magnetoelastic sensor based on the stress dependence of the GMI effect in Co 68.5Mn 6.5Si 10B 15 amorphous microwire has been introduced. This amorphous microwire after adequate heat treatment shows the maximum GMI ratio (Δ Z/ Z) m up to around 130%. GMI effect of this microwire has been found to be affected by the magnetoelastic anisotropy induced in the sample by the applied tensile stress. This stress dependence of the GMI induces changes on the ac voltage measured between the ends of the sample placed in the magnetic field under applied tensile stress. When the sample is under a load of 3 g such change of the ac voltage across the microwire is about 3.5 V.

Effects of stress, temperature and annealing conditions on the transport properties of amorphous wires

By maintaining a CoSiB wire within a fixed magnetic field these materials provide a measurement of the average stress applied over the full length of the wire. The magneto-impedance (MI) response of CoSiB wire is relatively insensitive to changes in ambient temperature over the range 293–423 K, indicating its suitability for implementation in a number of areas where the measurement of stress at elevated temperature is required. A detailed explanation of this behaviour in terms of circumferential permeability and magnetoelastic anisotropy is given. Optimisation of the giant magneto-impedance (GMI) response is achieved through carefully controlled furnace annealing and this process is also discussed.

Structural origin of perpendicular magnetic anisotropy in Ni–W thin films

The magnetic properties and microstructure of electrodeposited Ni–W thin films (0–11.7 at% W in composition) were studied. The film structures were divided into three regions: an FCC nanocrystalline phase (0–2 at% W), a transition region from FCC nanocrystalline to amorphous phase (2–7 at% W), and an amorphous phase (>7 at% W). In the transition region, (4–5 at% W) films with perpendicular magnetic anisotropy (PMA) were found. The saturation magnetization, magnetic anisotropy field, perpendicular magnetic anisotropy and perpendicular coercivity for a typical Ni–W film (4.5 at% W) were 420 kA/m, 451 kA/m, 230 kJ/m and 113 kA/m, respectively. The microstructure of Ni–W films with PMA is composed of isolated columnar crystalline grains (27–36 nm) with the FCC phase surrounded by the Ni–W amorphous phase. The appearance of the interface between the magnetic core of Ni crystalline grains and the Ni–W non-magnetic boundary layer seems to be the driving mechanism for the appearance of PMA. The origin of PMA in Ni–W films is mainly attributed to the magnetoelastic anisotropy associated with in-plane internal stress and positive magnetostriction. The secondary source of PMA is believed to be the magnetocrystalline anisotropy of 〈1 1 1〉 columnar grains and its shape magnetic anisotropy. It is concluded that Ni–W electrodeposited films (4–5 at% W) may be applicable for perpendicular magnetic recording media.