Permalloy Thin Films Research Articles

Reciprocal excitation of propagating spin waves by a laser pulse and their reciprocal mapping in magnetic metal films

Focused pulse-laser-induced propagating magnetostatic surface spin waves (MSSWs) were investigated using an all-optical space-time-resolved magneto-optical Kerr effect microscope in 20-nm-thick permalloy thin films to clarify reciprocity and symmetry of MSSW emission. The microscope setup was constructed with variable direction of the applied magnetic field, and the corresponding angular dependence was examined. MSSWs were reciprocally emitted from the laser spot, in contrast to the nonreciprocal emission obtained by the antenna method. Specifically, the observed excitation amplitude and phase were independent of the propagation and magnetization directions within experimental error. By transforming the data into wave number--frequency space, the MSSW dispersion relation was clearly identified within the wave number of 2--3 rad/$\ensuremath{\mu}\mathrm{m}$ and frequency of 10 GHz. These observations are consistent with the model of ultrafast modulation of out-of-plane shape magnetic anisotropy inside the focused laser spot. In addition to confirming the symmetric and reciprocal emission of laser-induced MSSWs, the study demonstrated that this technique can provide all-optical microscopic spectroscopy for MSSW in metals, such as Brillouin light-scattering technique.

Reduction and control of permalloy thin film damping factor under microwave irradiation

Abstract Understand and control of the Gilbert damping, a fundamentally important magnetodynamic characteristic of magnetic materials is crucial for magnonics and high frequency applications. In this paper, we demonstrate the microwave irradiation as an easy and effective technique to control and reduce the damping factor of permalloy (Ni80Fe20) thin films deposited on SiO2 substrate. Based on different irradiation times, damping decreases by ∼16% in a short interval time of few minutes and increases within longer irradiation times. We found that within such short irradiation time, before significant changes occur in its structure, determined by XRD and probed by SEM, AFM, and conductivity measurements, the major influence on the damping factor is the “aspect” of the surface of the film. We found that there is a consistent experimental and theoretical behavior of surface roughness and correlation length that explains the trend of the damping factor. Finally, with irradiation for very long times, cracks at the surface appear due to the different absorption and heat coefficients of permalloy layer and SiO2 substrate, which leads to increment of the resistivity and oxidation of the permalloy and large enhancement of the damping factor. Our achievements are useful not only for control of the damping factor, but for evaluation of microwave irradiation threshold where material properties change which are highly valuable for magnonics devices.

(Invited) Transparent Electrode and Magnetic Permalloy Made from Novel2 Nanopaper

We report a novel partial dissolution strategy to liberate uniform cellulose nanofibers with diameter of 5-10 nm from macroscopic cellulose fibers and promote separation of nanofibers in an aqueous environment by forming water-soluble sodium carboxymethylcellulose (CMC) through heterogeneous sodium acetoxylation of cellulose. With the obtained cellulose nanofibers, we fabricated nanopapers which exhibit a high optical transparency of 90.5 % (550 nm) with promising mechanical properties and high thermal stability. By directly depositing Ag nanowires on a wet nanofiber sheet, we fabricated a flexible transparent electrode with 86.5 % (550 nm) transparency and 26.2 Ohm/sq sheet resistance (Rs). Meanwhile, we studied the magnetic properties of sputter deposited thin film of permalloy on nanopaper which exhibited a similar magnetic coercivity and a close saturation magnetization to conventional silicon dioxide-based permalloy.

Geometrical complexity of the antidots unit cell effect on the spin wave excitations spectra

Three types of antidot lattices (ADLs) based on a permalloy thin film, with increasing complexity of the unit cell: simple square, bi-component square, and wave-like ADLs were studied experimentally using both ferromagnetic resonance and Brillouin light scattering spectroscopy techniques. An additional small antidot placed in the center of the square unit cell induces significant modification of the ADL spin-wave spectrum and changes its dependence on the orientation of the in-plane applied magnetic field. The introduction of air-gaps connecting small and large antidots transforms the system into the wave-like ADL and results in further changes in the spin-wave spectrum, including a strong discrimination between two mutually orthogonal principal directions of the ADL in terms of spin-wave propagation. We also point out the impact of defected holes on the obtained spectra. The micromagnetic simulations and the calculation based on the plane wave method satisfactorily confirmed the experimental results.

Computation of Two-Port Parameters in Magnonic Devices Through Circuit-Field Coupling

This paper presents a model for the computation of two-port parameters from the configuration of a typical Magnonic device involving two coplanar waveguides. A Permalloy thin film is saturated using an appropriate dc field. The first waveguide then performs the function of excitation of spin waves by superposition of an RF field. The second waveguide detects the excited wave vector in Damon–Eshbach mode at resonance frequency. The model is solved using the generalized mean residual method for a linearized approximation of the Landau–Lifshitz–Gilbert equation coupled with Maxwell equations. Complex impedance matrices from the two-port network are extracted for both uniformly saturated and non-uniform initial ground states. Findings report low absorption potential and shift in resonance frequency for highly non-uniform ground states. Validation of the model for uniformly saturated ground states is performed against experimental and semi-analytical findings, showing good agreement.

Anisotropic magnetoresistance and anomalous Nernst effect in exchange biased permalloy/(1 0 0) NiO single-crystal

Abstract We have investigated the anisotropic magnetoresistance (AMR) and the anomalous Nernst effect (ANE) in an exchange-biased bilayer consisting of a thin film of permalloy deposited on a single crystal antiferromagnetic NiO (1 0 0). The exchange bias field ( H EB ) value was obtained by means of AMR, ANE and magnetization hysteresis measurements. The shift of the hysteresis loop, measured with the three different techniques, yield approximately the same value of H EB. In spite of the measurement techniques be based in different physical phenomena, our results confirm the robustness of the exchange anisotropy at the Py/NiO interface. The strength of the anomalous Nernst effect for the exchange-biased permalloy film is compared to values measured in non biased films.

Static and Dynamic Magnetization Investigation in Permalloy Electrodeposited onto High Resistive N-Type Silicon Substrates

The present study reports on the development of permalloy thin films obtained by electrodeposition onto low-doped n-type silicon substrates. While changing from non-percolated clusters into percolated thin films upon increasing the electrodeposition time, the static and dynamic magnetic properties of the as-obtained structures were investigated. We found the experimental magnetic results to be in very good agreement with the simulations performed by solving the Landau-Lifshitz for the dynamics of the magnetic moment. For short electrodeposition times we found the static and dynamic magnetization behavior of the as-formed nanoclusters evidencing vortex magnetization with random chirality and polarization, which is explained in terms of dipolar interaction minimization. Indeed, it is herein emphasized that recent applications of ferromagnetic materials in silicon-based spintronic devices, such as logic and bipolar magnetic transistors and magnetic memories, have revived the possible utilization of low cost and simple electrodeposition techniques for the development of these upcoming hetero-nanostructured devices.

Relaxation mechanism in NiFe thin films driven by spin angular momentum absorption throughout the antiferromagnetic phase transition in native surface oxides

We report an alternative mechanism for the physical origin of the temperature-dependent ferromagnetic relaxation observed in bare permalloy (NiFe) thin films. Through spin-pumping experiments, we demonstrate that the peak in the temperature dependence of NiFe damping can be understood in terms of enhanced absorption of spin angular momentum at the magnetic phase transition in native antiferromagnetic surface-oxidized layers. These results suggest some avenues for the investigation of an incompletely understood phenomenon in physics.

Magnetic coupling of stripe domains in FePt/Ni80Fe20 bilayers

Equiatomic FePt in the A1 soft magnetic phase and Ni80Fe20 (permalloy) thin films form a stripe-like magnetic domain structure above a critical thickness. This critical thickness is considerably different in the two alloys and allows us to study the influence of the magnetic coupling in the domain configuration in bilayers. Using dc magnetron sputtering techniques, we fabricated two different sets of FePt/Ni80Fe20 bilayers, keeping one thickness fixed and varying the other, and investigated the dc magnetic properties and the magnetic domain configuration of the structure. In all cases, magnetization reversal occurred at a single coercive field, indicating a relatively strong magnetic exchange coupling between both layers. The observed stripe period was also consistent with a model of spring magnet-like behavior. However, for certain values of FePt and permalloy thickness a complex nonparallel double stripe structure was found, which may be attributed to the influence of the dipolar stray field of the thicker layer on the stripe structure of the thinner film.

Quantitative separation of the anisotropic magnetothermopower and planar Nernst effect by the rotation of an in-plane thermal gradient

A thermal gradient as the driving force for spin currents plays a key role in spin caloritronics. In this field the spin Seebeck effect (SSE) is of major interest and was investigated in terms of in-plane thermal gradients inducing perpendicular spin currents (transverse SSE) and out-of-plane thermal gradients generating parallel spin currents (longitudinal SSE). Up to now all spincaloric experiments employ a spatially fixed thermal gradient. Thus, anisotropic measurements with respect to well defined crystallographic directions were not possible. Here we introduce a new experiment that allows not only the in-plane rotation of the external magnetic field, but also the rotation of an in-plane thermal gradient controlled by optical temperature detection. As a consequence, the anisotropic magnetothermopower and the planar Nernst effect in a permalloy thin film can be measured simultaneously. Thus, the angular dependence of the magnetothermopower with respect to the magnetization direction reveals a phase shift, that allows the quantitative separation of the thermopower, the anisotropic magnetothermopower and the planar Nernst effect.

Enhanced In-Plane Anisotropy and Ferromagnetic Resonance Frequency in Permalloy Films Laminated With Nitrogen-Doped Tantalum

Soft magnetic materials are used in integrated power magnetic devices (such as micro-inductors and microtransformers) to achieve reasonable inductances at frequencies over a few megahertz. However, the out-of-plane (perpendicular) anisotropy, in thick films produced by vapor deposition, worsens the in-plane soft magnetic properties. This paper reports on the nano-lamination of Permalloy (Ni81Fe19) thin films with the introduction of nitrogen-doped tantalum in between to eliminate the out-of-plane anisotropy and improve the in-plane anisotropy. This significantly improves the in-plane soft magnetic properties, reducing the coercivity from 1352 A/m to 25.5 A/m and increasing the anisotropy field from 180 A/m to 660 A/m. The high-frequency permeability was uniform up to 500 MHz, and the ferromagnetic resonance (FMR) frequency was increased to 1 GHz. These properties are ideal for high efficiency magnetic applications at high frequencies and an extended FMR frequency is imperative for gigahertz-range devices.

Coarsening dynamics of topological defects in thin permalloy films

We study the dynamics of topological defects in the magnetic texture of rectangular Permalloy thin film elements during relaxation from random magnetization initial states. Our full micromagnetic simulations reveal complex defect dynamics during relaxation towards the stable Landau closure domain pattern, manifested as temporal power-law decay, with a system-size dependent cut-off time, of various quantities. These include the energy density of the system, and the number densities of the different kinds of topological defects present in the system. The related power-law exponents assume non-trivial values, and are found to be different for the different defect types. The exponents are robust against a moderate increase in the Gilbert damping constant and introduction of quenched structural disorder. We discuss details of the processes allowed by conservation of the winding number of the defects, underlying their complex coarsening dynamics.

Transparent Electrode and Magnetic Permalloy Made from Novel Nanopaper.

We report a novel partial dissolution strategy to liberate uniform cellulose nanofibers with diameter of 5-10 nm from macroscopic cellulose fibers and promote separation of nanofibers in an aqueous environment by forming water-soluble sodium carboxymethylcellulose (CMC) through heterogeneous sodium acetoxylation of cellulose. With the obtained cellulose nanofibers, we fabricated nanopapers which exhibit high optical transparency of 90.5% (@550 nm) with promising mechanical properties and high thermal stability. By directly depositing Ag nanowires on a wet nanofiber sheet, we fabricated a flexible transparent electrode with 86.5% (@550 nm) transparency and 26.2 Ω/sq sheet resistance (Rs). Meanwhile, we studied the magnetic properties of sputter deposited thin film of permalloy on nanopaper which exhibited a similar magnetic coercivity and a close saturation magnetization to conventional silicon dioxide-based permalloy.

An extension of the Néel-Brown model for systems with multiple switching pathways

Abstract The Neel-Brown model is the most widely accepted model for the description of magnetization reversal by thermal excitation. This model predicts a decreasing average switching field and an increasing width ΔH of switching field distribution as the temperature is increased, and has been found to hold good on several occasions. However, for a few classes of systems, the temperature dependence of ΔH shows the opposite trend, and so far no satisfactory explanation exists. We present here an experimental study of switching field statistics of permalloy (Ni 80 Fe 20 ) thin films on Si(100) grown by pulsed laser ablation. It was seen that the sample deviates from the Neel-Brown behavior in the manner described above. We performed calculations based on a natural extension of the Neel-Brown model, which incorporated multiple reversal pathways characterized by a Gaussian distribution of coercive fields. Calculations based on this model for different values of the width parameter σ HSW show two distinct kinds of behavior. At low values of σ HSW , the total width ΔH is limited by thermal broadening according to the traditional Neel-Brown expression. This regime is characterized by an increasing ΔH with temperature. For high σ HSW , the broadening is dominated by σ HSW , which masks thermal broadening. In this regime, ΔH decreases with increasing temperature. Whereas the experimentally observed temperature dependence of the average switching field was found to be in good agreement with this model, qualitative agreement with regard to the temperature dependence of ΔH could be observed only for relaxation times lower than ~10 −40 s, which is much smaller than Neel-Brown relaxation times (10 −9 –10 −19 s) usually encountered in the literature.

Snell's Law for Spin Waves.

We report the experimental observation of Snell's law for magnetostatic spin waves in thin ferromagnetic Permalloy films by imaging incident, refracted, and reflected waves. We use a thickness step as the interface between two media with different dispersion relations. Since the dispersion relation for magnetostatic waves in thin ferromagnetic films is anisotropic, deviations from the isotropic Snell's law known in optics are observed for incidence angles larger than 25° with respect to the interface normal between the two magnetic media. Furthermore, we can show that the thickness step modifies the wavelength and the amplitude of the incident waves. Our findings open up a new way of spin wave steering for magnonic applications.

Quantification of a propagating spin-wave packet created by an ultrashort laser pulse in a thin film of a magnetic metal

Coherent spin-wave generation by focused ultrashort laser pulse irradiation was investigated for a permalloy thin film at micrometer scale using an all-optical space and time-resolved magneto-optical Kerr effect. The spin-wave packet propagating perpendicular to magnetization direction was clearly observed, however that propagating parallel to the magnetization direction was not observed. The propagation length, group velocity, center frequency, and packet-width of the observed spin-wave packet were evaluated and quantitatively explained in terms of the propagation of a magneto-static spin-wave driven by ultrafast change of an out-of-plane demagnetization field induced by the focused-pulse laser.

A study on the thickness dependence of static and dynamic magnetic properties of Ni81Fe19 thin films

Abstract A set of Permalloy thin films with thicknesses ranging from 100 nm to 1000 nm have been investigated by in-plane hysteresis loops, magnetic torque, microwave permeability and X-ray diffraction measurements. The frequency evolution of the complex permeability was treated within the Debye relaxation model allowing the obtainment of the resonance frequency, resonance linewidth and the rotational component of the permeability at each applied field. The samples can be separated in three groups according their magnetic properties. Samples with thickness until 150 nm present magnetic properties typical of a system with a well defined in-plane uniaxial anisotropy and just one resonance frequency in the high frequency permeability spectra. Samples with thicknesses above 300 nm present magnetization loops almost isotropic in-plane and two resonance frequencies in the permeability spectra. The samples at the intermediate thickness range present some characteristic from thinner and other of the thicker group. Ferromagnetic resonance and torque measurements have detected the presence of a small uniaxial anisotropy even in the thicker group of samples. The multiple ferromagnetic resonances in the permeability spectra present in the thicker group of samples were treated as non-interacting magnetic systems. These characteristics were attributed to the appearance of stripe domains together with a rotatable anisotropy, due to an out-of-plane magnetization component. The relaxation mechanisms which give rise to the resonance linewidth were discussed considering two possible sources, Gilbert damping and anisotropy dispersion. While the Gilbert damping was almost the same for all samples it was verified the anisotropy dispersion increase with the thickness.

Comparison of the effects of 60 nm and 96 nm thick patterned permalloy thin films on the performance of on-chip spiral inductors

Abstract In our earlier work it was shown that the 60 nm domain patterned Permalloy, incorporated on on-chip spiral inductors, gave better performance at frequencies greater than 5 GHz compared to the bulk Permalloy incorporated inductors, and the control structure. In this paper we compare the effects of 60 nm and 96 nm thick domain patterned Permalloy thin films, on the performance of on-chip spiral inductors. Experimental results show that the 60 nm thick both bulk and patterned Permalloys provide more improvement in inductance and quality factor of inductors, compared to that of 96 nm.

Microstructure investigation and magnetic study of permalloy thin films grown by thermal evaporation

We study the effect of thickness on the structural and magnetic properties of permalloy thin films, evaporated on glass substrate. The films thicknesses range from 16 to 90 nm. From X-ray diffraction spectra analysis, we show that the thinner films present a ‹1,1,1› preferred orientation. However, the thicker films exhibit a random orientation. The grains size increases and the lattice parameter decreases with increasing thickness. The magnetic force microscopy observations display cross-tie walls features only for the two thicker films (60 and 90 nm thick films). The magnetic microstructure, carried out by Kerr microscopy technique, shows the presence of magnetic domains changing with the direction of applied magnetic field. The coercive field, H c , was found to decrease from 6.5 for 16 to 1.75 Oe for 90 nm. All these results will be discussed and correlated.

Publisher's Note: Frequency nonreciprocity of surface spin wave in permalloy thin films [Phys. Rev. B93, 054430 (2016)

Publisher's Note: Frequency nonreciprocity of surface spin wave in permalloy thin films [Phys. Rev. B<b>93</b>, 054430 (2016)