Soft Ferromagnetic Thin Films Research Articles

Electroless Deposition of Ni-Fe Alloys on Scaffolds for 3D Nanomagnetism.

3D magnetic nanostructures are of great interest due to the possibility to design novel properties and the benefits for both technological applications such as high-density data storage, as well as more fundamental studies. One of the main challenges facing the realization of these three-dimensional systems is their fabrication, which includes the deposition of magnetic materials on 3D surfaces. In this work, the electroless deposition of Ni-Fe on a 3D-printed, non-conductive microstructure is presented. The deposited films exhibit low coercivity, with the saturation magnetization and composition corresponding to the archetypal soft magnetic material permalloy. For fundamental studies of 3D micromagnetism, this new development in fabrication offers the possibility to combine the flexibility of 3D nanofabrication techniques such as two-photon lithography for the fabrication of 3D scaffolds with a homogeneous soft ferromagnetic thin film, and thus represents an important step toward exploring the rich physics of complex 3D magnetic architectures with tailored properties and the development of advanced applications.

Quantitative magnetic force microscopy using calibration on superconducting flux quanta

We present a new procedure that takes advantage of the magnetic flux quantization of superconducting vortices to calibrate the magnetic properties of tips for magnetic force microscopy (MFM). Indeed, a superconducting vortex, whose quantized flux is dependent upon Plank constant, speed of light and electron charge, behaves as a very well defined magnetic reference object. The proposed calibration procedure has been tested on new and worn tips and shows that the monopole point-like approximation of the probe is a reliable model. This procedure has been then applied to perform quantitative MFM experiments on a soft ferromagnetic thin film of permalloy, leading to the determination of the local out-of-plane component of the canted magnetization, together with its spatial variations across a few μm2 scan area.

Large Uniaxial Anisotropy Induced in Soft Ferromagnetic Thin Films by Oblique Deposition of Underlayer

We produced a large uniaxial anisotropy $(H_{k})$ in ultrathin (1.2–2.4 nm) soft ferromagnetic layers through the oblique deposition of a nanometer-thick metallic underlayer. Fe, Ni, and Ni80Fe20 (Permalloy) films deposited on a 5 nm thick Nb layer grown at an oblique angle (up to 60 ${}^{\circ}$ ) exhibit an $H_{k}\approx 40-500$ Oe and have properties that are nearly ideal for use as a fixed layer in magnetic devices, such as for magnetic memory. A much higher induced $H_{k}$ is observed in Ni (500 Oe) and Fe (300 Oe) compared to Ni-Fe (40 Oe). We used this technique to increase the switching field ( $H_{{\rm{sw}}}$ ) by 3–5 times in an array of pattered Fe or Ni-Fe bits, and achieved an excellent relative switching distribution of about 0.05–0.06. We also controllably enhanced the $H_{k}$ in the fixed layer of patterned spin-valve devices for superconducting Josephson-junction magnetic random-access memory and produced a large difference in $H_{{\rm{sw}}}$ between the fixed and free layers. Different spacer layers between the Nb and the magnetic layers either propagate (for Ru) or reduce (for Cu, Ru/Al, ion-milled Ru) the effect of the oblique Nb. This technique was also effectively used in synthetic antiferromagnet structures and, therefore, has the potential to eliminate the need for an antiferromagnetic pinning layer when it degrades magnetic device performance.

Spin dynamics and frequency dependence of magnetic damping study in soft ferromagnetic FeTaC film with a stripe domain structure

Perpendicular magnetic anisotropy (PMA) and low magnetic damping are the key factors for the free layer magnetization switching by spin transfer torque technique in magnetic tunnel junction devices. The magnetization precessional dynamics in soft ferromagnetic FeTaC thin film with a stripe domain structure was explored in broad band frequency range by employing micro-strip ferromagnetic resonance technique. The polar angle variation of resonance field and linewidth at different frequencies have been analyzed numerically using Landau-Lifshitz-Gilbert equation by taking into account the total free energy density of the film. The numerically estimated parameters Landé g-factor, PMA constant, and effective magnetization are found to be 2.1, 2 × 105 erg/cm3 and 7145 Oe, respectively. The frequency dependence of Gilbert damping parameter (α) is evaluated by considering both intrinsic and extrinsic effects into the total linewidth analysis. The value of α is found to be 0.006 at 10 GHz and it increases monotonically with decreasing precessional frequency.

Electrochemical Synthesis of Soft Ferromagnetic Thin Films and Nanostructures for Magnetic Recording and MEMS Application

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Room temperature ferromagnetism down to 10 nanometer Ni–Fe–Mo alloy films

Magnetic behavior of a few pulsed laser deposited soft ferromagnetic thin films of Ni–Fe–Mo alloys of different thickness on sapphire single crystals is interpreted on the basis of their structural characteristics. Highly textured thin films have high void density due to island-like growth. X-ray reflectivity (XRR) of the thin films indicate that instead of a uniform density there are effectively three layers with density gradient across the thickness, which is further supported by atomic force microscopy and cross-sectional scanning electron microscopy. Rutherford backscattering spectroscopy and energy dispersive spectrum measurements reveal that the composition in the films is not too far from that of the bulk target with a trend of enhanced Fe yield in the films. The structural disorder strongly affected the magnetic property of the films resulting in much higher values of the Curie temperature TC and coercive field HC than those of the bulk targets. Bifurcations of low-field zero-field-cooled and field-cooled magnetization reflect the disorder-induced anisotropy in the thin films. The spin wave stiffness constants D are higher than their bulk counterparts which are supportive of the enhanced Fe yield in the films. The saturation magnetization, M calculated from measurements in field transverse to the films strongly supports the thickness found from XRR. Finally, even the 10nm thin films have sizable M and HC and TC >300K, making them good candidates for magnetic applications. Overall, the magnetic behavior and the structural characteristics have reasonably complemented each other.

Array anisotropy in structured thin film arrays: Influence on the magnetodynamics

The dynamic magnetic properties of soft-ferromagnetic thin film element arrays are strongly influenced by long range interelement magnetostatic interaction. In order to estimate the effective array dipolar field a quantitative model is presented, which is based on the superposition of stray fields that arise from the neighborhood of a reference element. Kittel's equation, that describes the magnetodynamics, is generalized for magnetically saturated arrays by additional array dipolar field terms. Measurements of the magnetodynamic response of quasi-saturated arrays with a rectangular base agree with theoretical predictions. Thus, our model allows the estimation of the frequency of the uniform precessional mode in mesoscopic thin film arrays with non-negligible magnetostatic interaction.

Exchange Bias Training Effect in Coupled All Ferromagnetic Bilayer Structures

Exchange coupled bilayers of soft and hard ferromagnetic thin films show remarkable analogies to conventional antiferromagnetic/ferromagnetic exchange bias heterostructures. Not only do all these ferromagnetic bilayers exhibit a tunable exchange bias effect, they also show a distinct training behavior upon cycling the soft layer through consecutive hysteresis loops. In contrast with conventional exchange bias systems, such all ferromagnetic bilayer structures allow the observation of training induced changes in the bias-setting hardmagnetic layer by means of simple magnetometry. Our experiments show unambiguously that the exchange bias training effect is driven by deviations from equilibrium in the pinning layer. A comparison of our experimental data with predictions from a theory based upon triggered relaxation phenomena shows excellent agreement.

Secondary peaks on microwave permeability spectra of soft ferromagnetic thin films with in-plane anisotropy

Complex permeability spectra of CoNbZr thin films have been measured up to 4GHz by a single coil pertubation technique. In addition to the well-known ferromagnetic resonance, multiple resonant modes have been evidenced in the increasing branches of the spectra. The number of “secondary peaks” observed depends on the sample thickness. Their resonance frequencies follow a nonlinear dispersion law versus the ratio of the mode number over the sample thickness with a power factor close to 1.5.

Evidence of electron-spin wave coupling in [formula omitted] magnetic metal thin film

Ferromagnetic resonance (FMR)-induced voltaic effect and electrically modulated FMR absorption was observed in a Co x Nb y ( x = 81 , y = 19 ) soft ferromagnetic thin film by means of experimental technique based on electrically detected magnetic resonance (EDMR) and EM-FMR (electrical modulation of FMR). Our observations are interpreted as an evidence of electron-spin wave (magnon) coupling, in view of the previously reported case of multi-layers and domain walls. We suggest the experimental tools as an effective probe of spin-dependent transports in magnetic materials and devices.

Systematic control of high-speed damping in doped/undoped Ni81Fe19 (50 nm) bilayer thin films

We show that the relaxation of GHz precessional motion in soft ferromagnetic thin films can be controlled through the inhomogeneous placement of dopants. Bilayers of doped and undoped permalloy (Ni81Fe19), Ni81Fe19/Ni81Fe19:Tb (Tb 5%), varied in net relaxation rate λ according to the thickness fraction of the doped layer. The behavior is roughly independent of total bilayer thickness and deposition order in the range studied (50–100 nm). The technique provides a means to control the high-speed response of magnetoelectronic devices with limited effect on signal, as magnetically active dopants can be separated from the interfaces relevant for spin transport.

Giant magneto-impedance and its applications

The status of the giant magneto-impedance effect is reviewed in wires, ribbons, and multilayered soft ferromagnetic thin films. After establishing the theoretical framework for the description of the effect, and the constraints any material should have in order to show the effect, experimental work in wires, ribbons, and multilayered thin films is described. Existing and potential applications of the effect in electronics and sensing are highlighted.

Magnetic tuning of biquadratic exchange coupling in magnetic thin films.

The effective interlayer coupling between antiferromagnetically coupled hard and soft ferromagnetic thin films is investigated as a function of the magnetic bit length in the hard layer, which is controlled using a magnetic recording system. The interlayer coupling is explored by studying the magnetization reversal of the soft layer. As the bit length decreases, the coupling evolves from antiferromagnetic to biquadratic to uncoupled. These results are reproduced using a micromagnetic model and determine the applicability range of Slonczewski's fluctuation model of biquadratic coupling.

Microwave permeability spectra of ferromagnetic thin films over a wide range of temperatures

Ferromagnetic materials are of great interest today for high frequency applications in microelectronics (M-RAMs, planar inductors, etc.) or magnetic recording systems, taking advantage of the high levels of saturation magnetization of ferromagnetics. We are presenting a setup permitting permeability measurements from 10 MHz up to 6 GHz in the 77–400 K temperature range. An existing single-coil perturbation technique is modified to allow precise and accurate microwave measurements. An experimental validation of the technique is performed. Measurements on a CoFeSiB thin film are presented. The clear effect of the temperature on the whole permeability spectra is shown for soft ferromagnetic thin films, including CoNbZr.

High frequency magnetic properties of Co Zr Nb thin films deposited by dynamic sputtering technique for GHz Si-integrated planar inductors

Soft ferromagnetic zero-magnetostrictive Co Nb Zr thin films have been dynamically deposited by dc planar magnetron sputtering onto continuously transported substrates. No magnetic field has been applied to the films during deposition. The films reveal a well-defined in plane uniaxial anisotropy with an easy axis aligned along the direction of substrate motion. As a result, an anisotropy field of 75 Oe and a ferromagnetic resonance frequency of 2.6 GHz have been measured. The study of the relationship between magnetic properties and film thickness points out a critical thickness around 0.3 μm, above which the films exhibit a stripe-domain-like behavior which is detrimental for the high frequency use. The thermal stability of the films is found to be usual as regular amorphous core materials (stable up to 300°C). Additionally, it is shown that the films do not suffer from rotatable magnetic anisotropy usually observed inconventional statically deposited films under magneticfield. Finally, a different origin of the magnetic anisotropyis suggested in dynamically deposited films.

Soft ferromagnetic thin films for high frequency applications

In this report the fabrication and characterization of different amorphous and nanocrystalline films with special respect to a possible integration into standard semiconductor processes is described. The prepared amorphous FeCoBSi films exhibit excellent high frequencies properties. However, their implementation in established fabrication processes is restricted, since the films start to crystallize at higher annealing temperatures. Soft nanocrystalline films, like FeTaN, are able to overcome this temperature limitation because of their better thermal stability.

Control of magnetization dynamics in Ni/sub 81/Fe/sub 19/ thin films through the use of rare-earth dopants

We show that the magnetization dynamics of soft ferromagnetic thin films can be tuned using rare-earth (RE) dopants. Low concentrations (2 to 10%) of Tb in 50 nm Ni/sub 81/Fe/sub 19/ films are found to increase the Gilbert magnetic damping parameter /spl alpha/ over two orders of magnitude without great effect on easy axis coercivity or saturation magnetization. Comparison with Gd dopants indicates that the orbital character of the Tb moment is important for transferring magnetic energy to the lattice. Structural transformations from the crystalline to the amorphous state, observed over the first 2%-10% of RE doping, may play a contributing but not sufficient role in damping in these films. The approach demonstrated here shows promise for adjusting the dynamical response, from underdamped to critically damped, in thin film materials for magnetic devices.

Effective dynamics for ferromagnetic thin films

In a ferromagnetic material, the dynamics of the relaxation process are affected by the presence of a strong shape or material anisotropy. In this article, we systematically explore this fact to derive the effective dynamical equation for a soft ferromagnetic thin film. We show that, as a consequence of the interplay between shape anisotropy and damping, the gyromagnetic term is effectively also a damping term for the in-plane components of the magnetization distribution. We validate our result through numerical simulation of the original Landau–Lifshitz equation and our effective equation.

Effect of magnetization ripple at high frequencies in amorphous ferromagnetic thin films

An analytical calculation of the permeability frequency spectra using Landau–Lifshitz theory is presented and compared to experimental results obtained on soft amorphous ferromagnetic thin films with an in-plane uniaxial magnetic anisotropy. The disagreement between theory and experiment is attributed to a small magnetic anisotropy dispersion in the film (magnetization ripple). We have measured the real part of the transverse biased permeability μt′ at different frequencies of the exciting field in order to estimate the ripple effects in our films and to improve our model. We have plotted the value of the bias field Hm, at which μt′ versus exciting field frequency reaches a maximum. Hm is found to be frequency independent at low frequencies (up to 280 MHz). Above a critical frequency it increases rapidly, suggesting that ripple effects are strongly frequency dependent.

A remotely interrogatable sensor for chemical monitoring.

A new type of continuously operating, in-situ, remotely monitored sensor is presented. The sensor is comprised of a thin film array of magnetostatically coupled, magnetically soft ferromagnetic thin film structures, adhered to or encased within a thin polymer layer. The polymer is made so that it swells or shrinks in response to the chemical analyte of interest, which in this case is pH. As the polymer swells or shrinks, the magnetostatic coupling between the magnetic elements changes, resulting in changes in the magnetic switching characteristics of the sensor. Placed within a sinusoidal magnetic field the magnetization vector of the coupled sensor elements periodically reverses directions, generating magnetic flux that can be remotely detected as a series of voltage spikes in appropriately placed pickup coils. one preliminary sensor design consists of four triangles, initially spaced approximately 50 micrometers apart, arranged to form a 12 mm x 12 mm square with the triangle tips centered at a common origin. Our preliminary work has focused on monitoring of pH using a lightly crosslinked pH sensitive polymer layer of hydroxyethylmethacrylate and 2-(dimethylamino) ethylmethacrylate. As the polymer swells or shrinks the magnetostatic coupling between the triangles changes, resulting in measurable changes in the amplitude of the detected voltage spirits.