Permalloy Films Research Articles

(Invited) On-Wafer Microwave Devices Based on Magnetic Materials

We present a review of theoretical and experimental results for on-wafer microwave devices. Specifically, we discuss construction of on-wafer inductors designed to operate in frequencies below 1-2 GHz. We also present recent progress in tunable microwave band-stop filters, band-pass filters, phase shifters, and a signal to noise enhancer, all based on a microstrip geometry and using a variety of magnetic thin films and layered structures. These devices are compatible in size and growth process with on-chip high-frequency electronics. For tunable devices based on metallic ferromagnetic films of Fe and Permalloy, the operational frequency ranges from 5 to 35 GHz for external fields below 5 kOe. For the band-stop filters, we observed power attenuation up to ∼100 dB/cm, and an insertion loss on the order of ∼2–3 dB, for both Permalloy and Fe-based structures. We also will discuss the use of thin films of hexagonal ferrites, and liquid crystals/magnetic nano-rods colloids, and show these materials can be used to make on-wafer microwave devices. The devices based on hexagonal ferrite operate in the frequency range 35-70 GHz while devices based on liquid crystal/magnetic nano-rods colloids operate in the frequency range 5 - 110 GHz.

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.

Структурно-фазовий стан, магнеторезистивні та магнетні властивості плівок пермалою

Структурно-фазовий стан, магнеторезистивні та магнетні властивості плівок пермалою

Mapping the Landscape of Domain-Wall Pinning in Ferromagnetic Films Using Differential Magneto-Optical Microscopy

The propagation of domain walls in a ferromagnetic film is largely determined by domain wall pinning at defects in the material. In this letter we map the effective potential landscape for domain wall pinning in Permalloy films by raster scanning a single ferromagnetic vortex and monitoring the hysteretic vortex displacement vs. applied magnetic field. The measurement is carried out using a differential magneto-optical microscopy technique which yields spatial sensitivity $\sim 10$ nm. We present a simple algorithm for extracting an effective pinning potential from the measurement of vortex displacement vs. applied field. The resulting maps of the pinning potential reveal distinct types of pinning sites, which we attribute to quasi-zero-, one-, and two-dimensional defects in the Permalloy film.

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.

Annealing influence on the exchange stiffness constant of Permalloy films with stripe domains

An investigation of the annealing influence on the stripe domains structure in Permalloy films has been performed by comparing the static and dynamic magnetic properties. An increasing exchange stiffness constant Aex is found when the annealing temperature rises. The magnetic force microscopy images show that the width of stripe domain increases with the increase of annealing temperature, and the increasing exchange stiffness constant is considered to be the most prominent reason. A perpendicular standing spin-wave model is used to deduce the Aex from the evolution of resonance frequencies, and the Aex increases nearly twice from 7.2 × 10−7 ergs cm−1 for the as-deposited film to 12.9 × 10−7 ergs cm−1 for the film annealing at 400 °C. Micromagnetic calculation is also used to compute the magnetization configuration, and the results agree well with the experimental conclusions.

Towards a six-state magnetic memory element

We pattern permalloy films into three crossing elongated ellipses with an angle of 60° between the major axes of any pair of ellipses. Planar Hall effect measurements show that the magnetization in the area of overlap of the ellipses has six stable magnetic orientations parallel to the major axes of the three ellipses. We determine the effective anisotropy field for small magnetic deviations from the easy axis and the switching field between the easy axes as a function of magnetic field orientation. We compare our results with micromagnetic simulations and present an effective Hamiltonian that captures the magnetic response. We show how such magnetic structures in a magnetic tunnel junction would result in a magnetic memory element with six distinct resistance states that could be written using spin-orbit torques.

Direct imaging of coexisting ordered and frustrated sublattices in artificial ferromagnetic quasicrystals.

We have used scanning electron microscopy with polarization analysis and photoemission electron microscopy to image the two-dimensional magnetization of permalloy films patterned into Penrose P2 tilings (P2T). The interplay of exchange interactions in asymmetrically coordinated vertices and short-range dipole interactions among connected film segments stabilize magnetically ordered, spatially distinct sublattices that coexist with frustrated sublattices at room temperature. Numerical simulations that include long-range dipole interactions between sublattices agree with images of as-grown P2T samples and predict a magnetically ordered ground state for a two-dimensional quasicrystal lattice of classical Ising spins.

A broadband ferromagnetic resonance dipper probe for magnetic damping measurements from 4.2 K to 300 K.

A dipper probe for broadband Ferromagnetic Resonance (FMR) operating from 4.2 K to room temperature is described. The apparatus is based on a 2-port transmitted microwave signal measurement with a grounded coplanar waveguide. The waveguide generates a microwave field and records the sample response. A 3-stage dipper design is adopted for fast and stable temperature control. The temperature variation due to FMR is in the milli-Kelvin range at liquid helium temperature. We also designed a novel FMR probe head with a spring-loaded sample holder. Improved signal-to-noise ratio and stability compared to a common FMR head are achieved. Using a superconducting vector magnet we demonstrate Gilbert damping measurements on two thin film samples using a vector network analyzer with frequency up to 26 GHz: (1) A Permalloy film of 5 nm thickness and (2) a CoFeB film of 1.5 nm thickness. Experiments were performed with the applied magnetic field parallel and perpendicular to the film plane.

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)

Parameter-free determination of the exchange constant in thin films using magnonic patterning

An all-electrical method is presented to determine the exchange constant of magnetic thin films using ferromagnetic resonance. For films of 20 nm thickness and below, the determination of the exchange constant A, a fundamental magnetic quantity, is anything but straightforward. Among others, the most common methods are based on the characterization of perpendicular standing spin-waves. These approaches are however challenging, due to (i) very high energies and (ii) rather small intensities in this thickness regime. In the presented approach, surface patterning is applied to a permalloy (Ni80Fe20) film and a Co2Fe0.4Mn0.6Si Heusler compound. Acting as a magnonic crystal, such structures enable the coupling of backward volume spin-waves to the uniform mode. Subsequent ferromagnetic resonance measurements give access to the spin-wave spectra free of unquantifiable parameters and, thus, to the exchange constant A with high accuracy.

Nickel-zinc ferrite/permalloy (Ni0.5Zn0.5Fe2O4/Ni-Fe) soft magnetic nanocomposites fabricated by electro-infiltration

Magnetically soft NiZn ferrite (Ni0.5Zn0.5Fe2O4) nanoparticles are embedded within a permalloy (Ni-Fe) matrix via an electro-infiltration process as thin films intended for use as on-chip inductor cores in the MHz frequency regime. A layer of NiZn ferrite nanoparticles is first deposited, and then permalloy is electroplated through the voids to encapsulate the particles and form three-dimensional ferrite/alloy nanocomposites. The composites are estimated to contain 37% ferrite by volume and exhibit a relative permeability of ∼320, a saturation of ∼1.15 T, and an operational bandwidth of 93 MHz. Compared to a permalloy thin film of similar thickness, the nanocomposite exhibits 39% higher electrical resistivity and 50% higher bandwidth.

Optimization of polarizer azimuth in improving signal-to-noise ratio in Kerr microscopy.

The magneto optical Kerr effect (MOKE) is a widely used technique in magnetic domain imaging for its high surface sensitivity and external magnetic compatibility. Optimization of Kerr microscopy will improve the detecting sensitivity and provide high-quality domain images. In this work, we provide a method to optimize the polarizer azimuth in improving the signal-to-noise ratio (S/N) in longitudinal Kerr microscopy with the generalized magneto optical ellipsometry. Detailed analysis of the MOKE signal and the noise components are provided to study the optimum polarizer and analyzer azimuth combinations. Results show that, for a fixed polarizer angle 1°, the laser intensity noise and the shot noise, which vary with the input laser power, have a similar amplitude and decline with the analyzer azimuth increasing. When the analyzer is set at the extinction place, the Johnson noise plays a dominate role in the total noise. Then, the S/N values are calculated to find the optimum polarizer and analyzer azimuth. Results show that the optimum polarizer and analyzer azimuth combination for Permalloy is (18.35°, 68.35°) under an incident angle of 45°. After that, the S/N of 200 nm Permalloy at different analyzer angles with the polarizer azimuth set at 18.35° is measured to verify the validity of the simulation results. At last, the S/N at different incident angles is calculated. Results show that the optimum incident angle of 200 nm Permalloy film to improve the S/N is 70.35° under the polarizer and analyzer angles set at the optimal combinations (18.35°, 68.35°).

Compact spherical neutron polarimeter using high-T(c) YBCO films.

We describe a simple, compact device for spherical neutron polarimetry measurements at small neutron scattering angles. The device consists of a sample chamber with very low (<0.01 G) magnetic field flanked by regions within which the neutron polarization can be manipulated in a controlled manner. This allows any selected initial and final polarization direction of the neutrons to be obtained. We have constructed a prototype device using high-T(c) superconducting films and mu-metal to isolate regions with different magnetic fields and tested device performance in transmission geometry. Finite-element methods were used to simulate the device's field profile and these have been verified by experiment using a small solenoid as a test sample. Measurements are reported using both monochromatic and polychromatic neutron sources. The results show that the device is capable of extracting sample information and distinguishing small angular variations of the sample magnetic field. As a more realistic test, we present results on the characterization of a 10 μm thick Permalloy film in zero magnetic field, as well as its response to an external magnetic field.

Frequency nonreciprocity of surface spin wave in permalloy thin films

Surface spin waves in thin Permalloy films are studied by means of propagative spin wave spectroscopy. We observe a systematic difference of up to several tens of MHz when comparing the frequencies of counter-propagating waves. This frequency non-reciprocity effect is modeled using an analytical dipole-exchange theory that considers the mutual influence of non-reciprocal spin wave modal profiles and differences in magnetic anisotropies at the two film surfaces. At moderate film thickness (20 nm and below), the frequency non-reciprocity scales linearly with the wave vector and quadratically with the thickness, whereas a more complex non-monotonic behavior is observed at larger thickness. Our work suggests that surface wave frequency non-reciprocity can be used as an accurate spectroscopic probe of magnetic asymmetries in thin ferromagnetic films.

Selective tuning of magnetization dynamics damping in Tb- and Nd-doped permalloy ultrathin films by adjacent copper nanolayers

The mechanism of angular dependence of ferromagnetic resonance linewidth of dilute Tb and Nd doping in permalloy thin films with various thicknesses of adjacent copper layer are investigated by experimental approach and theoretical fitting by considering the contributions from intrinsic spin-orbit coupling, inhomogeneous broadening and two-magnon scattering. The results show that the damping coefficient α, by intrinsic contribution extracted from ferromagnetic resonance linewidth, increases from 0.0153 to 0.0218 for NiFe–Nd films and from 0.0193 to 0.0261 for NiFe–Tb films resulting from the spin pumping effect at the interface of NiFe–Nd(or Tb)/Cu as the thickness of copper layer increases from 1 nm to 15 nm. The surface magnetic anisotropy constant K1 is obtained and shows an decreasing trend from positive to negative, which implies that the copper layer could reduce the surface perpendicular anisotropy. The fitting spin mixed conductivity is (2.72±0.18)×1015 cm−2 at NiFe–Tb/Cu interface and (2.4±0.2)×1015 cm−2 at Cu/NiFe–Nd interface, respectively.

Eigenmodes in thin ferromagnetic film under ferromagnetic resonance and spin-wave resonance conditions at different spin pinning

Ferromagnetic resonance (FMR) and spin-wave resonances (SWRs) have been studied in transmission of an electromagnetic wave through the plate of ferromagnetic metal and in reflection from the plate. The method of calculation of the resonance phenomena has been developed which allows one to study the influence of spin pinning conditions on the amplitudes of eigenmodes. The spatial distribution of eigenmodes is investigated under FMR, SWR and magnetic antiresonance conditions. The transmission coefficient of Fe19Ni81 permalloy films measured as a function of the magnetic field agrees well with the calculated values.

Spin-orbit interaction enhancement in permalloy thin films by Pt doping

Raw data and images used for figures found in Spin-orbit interaction enhancement in permalloy thin films by Pt-doping. The data files are grouped by technique.

Magnetic microstructure analysis of sputter deposited permalloy thin films on a spin-on polyimide substrate

We have performed magneto-optical measurements of sputter deposited permalloy (NiFe 81/19) thin films on a polyimide based spin-on substrate and a silicon oxide substrate. Experiments were performed by means of the magneto-optic Kerr effect in order to complement the findings concerning soft magnetic behaviour on the macroscopic scale presented previously (Rittinger et al., Impact of different polyimide-based substrates on the soft magnetic properties of NiFe thin films, SPIE Microtechnologies. International Society for Optics and Photonics, Bellingham, pp 95171R, 2015) with new insights into the magnetic domain structures of such samples. First, magnetization curves (R---H-, B---H-curves) are compared with corresponding magneto-optical hystereses. Second, by taking into consideration the results of the magnetic microstructure analysis, further aspects of the interactions between the substrate and the functional thin film are revealed in form of analogies between observations on the microscopic scale and variations of macroscopic magnetization curves of analysed samples. Furthermore, the shape and detailed spatial arrangement of domains and their boundaries are determined for the investigated samples, supplying further information on the properties and state of stress of the thin films. Doing so, we aim to provide precise and reliable anisotropic magnetoresistive (AMR) sensors on flexible polymer substrates in the future.