NiFe Films Research Articles

Relation of planar Hall and planar Nernst effects in thin film permalloy

We present measurements of the planar Nernst effect (PNE) and the planar Hall effect (PHE) of nickel-iron (Ni–Fe) alloy thin films. We suspend the thin-film samples, measurement leads, and lithographically-defined heaters and thermometers on silicon-nitride membranes to greatly simplify control and measurement of thermal gradients essential to quantitative determination of magnetothermoelectric effects. Since these thermal isolation structures allow measurements of longitudinal thermopower, or the Seebeck coefficient, and four-wire electrical resistivity of the same thin film, we can quantitatively demonstrate the link between the longitudinal and transverse effects as a function of applied in-plane field and angle. Finite element thermal analysis of this essentially 2D structure allows more confident determination of the thermal gradient, which is reduced from the simplest assumptions due to the particular geometry of the membranes, which are more than 350 μm wide in order to maximize sensitivity to transverse thermoelectric effects. The resulting maximum values of the PNE and PHE coefficients for the Ni–Fe film with 80% Ni we study here are and , respectively. All signals are exclusively symmetry with applied field, ruling out long-distance spin transport effects. We also consider a Mott-like relation between the PNE and PHE, and use both this and the standard Mott relation to determine the energy-derivative of the resistivity at the Fermi energy to be , which is very similar to values for films we previously measured using similar thermal platforms. Finally, using an estimated value for the lead contribution to the longitudinal thermopower, we show that the anisotropic magnetoresistance (AMR) ratio in this Ni–Fe film is two times larger than the magnetothermopower ratio, which is the first evidence of a deviation from strict adherence to the Mott relation between Seebeck coefficient and resistivity.

Ferromagnetic resonance of facing-target sputtered epitaxial γ′-Fe4N films: the influence of thickness and substrates

The microstructure and high frequency properties of facing-target sputtered epitaxial γ′-Fe4N films were investigated in detail. It was found that the eddy current in ultrathin γ′-Fe4N films is too small to influence the ferromagnetic resonance (FMR) linewidth, where the linewidth is mostly determined by intrinsic damping and the two-magnon scattering (TMS) process. In relatively thick films, the TMS process can significantly affect the linewidth due to the roughness on the sample surface. However, the TMS process in a thin film is quite weak because of its smooth surface. The Gilbert damping constant of about 0.0135 in our γ′-Fe4N films is smaller than the experimental value in the previous work. Moreover, substrates can also influence the FMR linewidth of the γ′-Fe4N films by the TMS process. Besides, the resonance field of polycrystalline γ′-Fe4N film is larger than the epitaxial ones because of the lack of a magnetic anisotropic field, but the linewidth of the polycrystalline γ′-Fe4N film is smaller.

Epitaxial Fe16N2 thin film on nonmagnetic seed layer

Metastable α″-Fe16N2 has attracted much interest as a candidate for rare-earth-free hard magnetic materials. We demonstrate that Fe16N2 thin films were grown epitaxially on Cr seed layers with MgO (001) substrates by facing-target sputtering. Good crystallinity with the epitaxial relation MgO (001)[110] ∥ Cr (001)[100] ∥ Fe16N2 (001)[100] was obtained. The chemical order parameter, which quantifies the degree of N ordering in the Fe16N2 (the N-disordered phase is α′-Fe8N martensite), reaches 0.75 for Cr-seeded samples. Cr has a perfect lattice constant match with Fe16N2, and no noticeable strain can be assigned to Fe16N2. The intrinsic saturation magnetization of this non-strained Fe16N2 thin film at room temperature is determined to be 2.31 T by polarized neutron reflectometry and confirmed with vibrating sample magnetometry. Our work provides a platform to directly study the magnetic properties of high purity Fe16N2 films with a high order parameter.

Fabrication of L10-FeNi phase by sputtering with rapid thermal annealing

FeNi films were directly deposited on MgO(001) substrates by co-sputtering and rapid thermal annealing (RTA). The formation of the L10 phase was investigated for films with different thicknesses and different annealing conditions by grazing incidence X-ray diffraction. For the FeNi films with a thickness of 5 nm, superlattice 001 and 110 peaks were observed after annealing at a heating rate of 50 °C/s, which indicates that three variants of L10 grains were formed in the films. The maximum long-range order parameter, which corresponded to a volume-averaged parameter, was approximately 0.11. For the FeNi films with a thickness of 30 nm, a superlattice 110 peak was only observed after annealing at a heating rate of 50 °C/s, and the formation of 001 textured grains was clarified. Magnetic properties also changed depending on the FeNi film structures. The formation mechanism of L10-FeNi is discussed based on the strain caused by RTA in the FeNi films.

Collective magnetic response of inhomogeneous nanoisland FeNi films around the percolation transition

By using superconducting quantum interference device (SQUID) magnetometry, we investigated anisotropic high-field (H ≲ 7T) low-temperature (10 K) magnetization response of inhomogeneous nanoisland FeNi films grown by rf sputtering deposition on Sitall (TiO2) glass substrates. In the grown FeNi films, the FeNi layer nominal thickness varied from 0.6 to 2.5 nm, across the percolation transition at the d c ≃ 1.8 nm. We discovered that, beyond conventional spin-magnetism of Fe21Ni79 permalloy, the extracted out-of-plane magnetization response of the nanoisland FeNi films is not saturated in the range of investigated magnetic fields and exhibits paramagnetic-like behavior. We found that the anomalous out-of-plane magnetization response exhibits an escalating slope with increase in the nominal film thickness from 0.6 to 1.1 nm, however, it decreases with further increase in the film thickness, and then practically vanishes on approaching the FeNi film percolation threshold. At the same time, the in-plane response demonstrates saturation behavior above 1.5–2T, competing with anomalously large diamagnetic-like response, which becomes pronounced at high magnetic fields. It is possible that the supported-metal interaction leads to the creation of a thin charge-transfer (CT) layer and a Schottky barrier at the FeNi film/Sitall (TiO2) interface. Then, in the system with nanoscale circular domains, the observed anomalous paramagnetic-like magnetization response can be associated with a large orbital moment of the localized electrons. In addition, the inhomogeneous nanoisland FeNi films can possess spontaneous ordering of toroidal moments, which can be either of orbital or spin origin. The system with toroidal inhomogeneity can lead to anomalously strong diamagnetic-like response. The observed magnetization response is determined by the interplay between the paramagnetic- and diamagnetic-like contributions.

Effect of tungsten (W) on structural and magnetic properties of electroplated NiFe thin films for MEMS applications

Electrodeposition of nanocrystalline NiFe and NiFeW thin films were carried out from ammonium citrate bath at a constant current density and controlled pH of 8 by varying the bath temperature from 40 °C to 70 °C. The surface morphology and chemical composition of the electrodeposited NiFe and NiFeW soft magnetic thin films were studied by using SEM and EDAX. The SEM micrographs of the films coated at higher electrodeposited bath temperature have no micro cracks and also the films have more uniform surface morphology. The existence of crystalline nature of the coated films were analysed by XRD. The presence of predominant peaks in x-ray diffraction pattern (compared with JCPDS data) reveal that the average crystalline size was in the order of few tens of nano meters. The magnetic properties such as coercivity, saturation magnetization and magnetic flux density have been calculated from vibrating sample magnetometer analysis. The VSM result shows that the NiFeW thin film synthesised at 70 °C exhibit the lower coercivity with higher saturation magnetization. The hardness and adhesion of the electroplated films have been investigated. Reasons for variation in magnetic properties and structural characteristics are also discussed. The electroplated NiFe and NiFeW thin films can be used for Micro Electro Mechanical System (MEMS) applications due to their excellent soft magnetic behaviour.

Magnetic properties of chromium-doped Ni80Fe20 thin films

This paper investigates the properties of thin films of chromium-doped Ni80Fe20 (Permalloy) that could potentially be useful in future low-power magnetic memory technologies. The addition of chromium reduces the saturation magnetization, Ms, which is useful for low-energy switching, but does not significantly degrade the excellent switching properties of the host material even down to 10 K, the lowest temperature measured, in films as thin as 2.5 nm. As an example, an alloy film composed of 15% chromium and 85% Ni80Fe20 has an Ms just over half that of pure Ni80Fe20, with a coercivity Hc less than 4 Oe, an anisotropy field Hk less than 1 Oe, and an easy-axis remanent squareness Mr/Ms of 0.9 (where Mr is the remanent magnetization). Magnetodynamical measurements using a pulsed inductive microwave magnetometer showed that the average Landau Lifshitz damping λ was relatively constant with changing Cr content, but increased significantly for thinner films (λ ≈150 MHz for 11 nm, λ ≈250 MHz for 2.5 nm), and at low bias fields likely due to increased magnetic dispersion. Density functional theory calculations show that chromium reduces Ms by entering the lattice antiferromagnetically; it also increases scattering in the majority spin channel, while adding almost insignificant scattering to the minority channel.

Gap-dependent magnetic anisotropy and high-frequency property of micro-patterned NiFe film

To adjust the high-frequency properties of the soft magnetic film, micro-patterned NiFe films were fabricated on a (100) silicon substrate by using photolithography and magnetron sputtering. Their gap-width were changed from 15 to 60 µm with the strip-length of 5 mm and strip-width of 30 µm. The morphology and magnetic properties of the films were examined by scanning electron microscopy, vibrating sample magnetometer and vector network analysis, respectively. Compared with the continuous NiFe film, the in-plane magnetic anisotropy field (H k ) of micro-patterned NiFe film improved from 0 to 43 Oe. And as the designed gap-width (d) increased from 15 to 60 µm, H k increased from 43 to 80 Oe. The resonance frequency f r is increased up to 8.6 GHz, which is mainly from the demagnetization field and the magnetostatics surface wave energy in our study system. Therefore, the patterned films controlled by various stripe gap-width will also be essential in future high-frequency device design procedures.

Anomalies in Ni-Fe nanogranular films growth

Thin Ni-Fe films were produced via electrodeposition onto silicon substrate using direct current and pulse (with different pulse durations) regimes. The correlation between technological regimes and chemical composition and microstructure for Ni-Fe nanogranular films was discussed. Analysis of the microstructure evolution revealed the changing mechanism of the films growth from the Volmer-Weber (describes island film growth) to the Stranski-Krastanov (layer by layer at the growth beginning) with the shortening of the pulse duration less than 10 μs. This anomalous behavior was explained by an increase of the binding energy between the initial atoms and the substrate surface. The results showed that using electrodeposition regimes, which ensure growth with the average crystal size less than the critical value (10 nm), provides less roughness, defectiveness and greater film uniformity in thickness, and, therefore, stable properties.

Long-range transverse spin Seebeck effect in permalloy stripes using Sagnac interferometer microscopy

Coupling of spins and phonons in ferromagnets (FM) may persist up to mm length scale, thus generating macroscopic spatially distributed spin accumulation along the direction of an applied thermal gradient to an FM slab. This typical feature of transverse spin Seebeck effect (TSSE) has been demonstrated so far using electrical detection methods in FM films, in particular in a patterned structure, in which FM stripes grown onto a substrate perpendicular to the applied thermal gradient direction are electrically and magnetically isolated. Here we report optically detected TSSE response in isolated FM stripes based on permalloy deposited on SiN substrate, upon the application of a thermal gradient. For these measurements we used the magneto-optic Kerr effect measured by an ultrasensitive Sagnac interferometer microscope that is immune to thermo-electrics artefacts. We found that the optical TSSE coefficient in the NiFe stripes geometry is about one order of magnitude smaller than that in the continuous NiFe film, which is due to the limited phonons path in the FM stripes along the thermal gradient direction. Our results further confirm the existence of TSSE response in conducting FM compounds.

Artificial multiferroic structures using soft magnetostrictive bilayers on Pb(Mg1/3Nb2/3)–PbTiO3

Artificial multiferroic structures are of great interest as they combine two or more functionalities together. One example of these structures is magnetostrictive films grown on top of piezoelectric substrates; allowing the magnetisation hysteresis loop of the magnetostrictive film to be manipulated using an electric field across the structure rather than a magnetic field. In this paper, we have studied the multiferroic structure NiFe/FeCo/Ti/Pb(Mg1/3Nb2/3)–PbTiO3 (PMN-PT) as a function of the electric and magnetic field. Soft magnetostrictive bilayer films (NiFe/FeCo) are studied, as often applications require soft magnetic properties (small coercive and anisotropy fields) combined with larger magnetostrictive constants. Unfortunately, FeCo films can have coercive fields that are too large, while NiFe films’ magnetostriction constants are almost zero; thus, combining the two together should produce the ‘ideal’ soft magnetostrictive film. It was found that the addition of a thin NiFe film onto the FeCo film reduced the coercive field and remnant magnetisation on the application of an applied voltage in comparison to just the FeCo film. It was also determined that for the NiFe/FeCo bilayer the magnetisation switchability was ~100% on the application of 8 kV cm−m, which was higher than the monolayer FeCo films at the same applied field, demonstrating improvement of the multiferroic behaviour by the soft magnetic/magnetostrictive bilayer.

Spin-Wave Resonance in Chemically Deposited Fe-Ni Films: Measuring the Spin-Wave Stiffness and Surface Anisotropy Constant

Single-layer Fe x Ni1 - x thin magnetic films have been investigated by the spin-wave resonance technique in the entire concentration range. The surface anisotropy and exchange stiffness constants for the films with a Ni content from 30 to 80 at % have been measured from the experimental standing spin wave spectra. The surface exchange spin wave penetration depth δ C = 20–30 nm has been determined from the dependences of the surface anisotropy and exchange coupling constants on the Fe20Ni80 film thickness in the range of 250–400 nm.

Investigation of coercivity for electroplated Fe-Ni thick films

We have already reported Fe-Ni firms with good soft magnetic properties prepared by using an electroplating method. In our previous studies, we prepared the Fe-Ni films from citric-acid-based baths (CA-baths) and ammonium-chloride-based ones (AC-baths), and confirmed that the coercivity for the AC-baths was lower than that for the CA-baths. In the present study, we investigated reasons for the lower coercivity for the AC-baths to further improve the soft magnetic properties. From an observation of magnetic domains of the Fe22Ni78 films, we found that Fe22Ni78 film for AC-bath had a magnetic anisotropy in the width direction, and also found that the coercivity in the width direction was lower than the longitudinal one for the AC-bath. As an annealing for a stress relaxation in the films reduced the difference in the coercivity, we considered that the anisotropy is attributed to the magneto-elastic effect.

Systematic errors in the determination of the spectroscopic g-factor in broadband ferromagnetic resonance spectroscopy: A proposed solution

A theoretical and experimental study of the influence of small offsets of the magnetic field (δH) on the measurement accuracy of the spectroscopic g-factor (g) and saturation magnetization (Ms) obtained by broadband ferromagnetic resonance (FMR) measurements is presented. The random nature of δH generates systematic and opposite sign deviations of the values of g and Ms with respect to their true values. A δH on the order of a few Oe leads to a ∼10% error of g and Ms for a typical range of frequencies employed in broadband FMR experiments. We propose a simple experimental methodology to significantly minimize the effect of δH on the fitted values of g and Ms, eliminating their apparent dependence in the range of frequencies employed. Our method was successfully tested using broadband FMR measurements on a 5 nm thick Ni80Fe20 film for frequencies ranging between 3 and 17 GHz.

Localized surface plasmon resonance enhanced magneto-optical Kerr effect in Ni80Fe20 thin films coated with Au nanorods

The influence of the localized surface plasmon resonance on the magneto-optical Kerr effect (MOKE) was investigated in Ni80Fe20 thin films coated with gold nanorods. The nanorods are dimensionally tuned to support localized surface plasmon resonance near the incident laser frequency. A significant enhancement of magneto-optical response is observed, in which the MOKE signal is increased by over 40% compared to the reference Ni80Fe20 films. The spectral dependence of MOKE signals exhibits the maximum magneto-optical response centered around the longitudinal surface plasmon resonance wavelength of gold nanorods. Finite-difference time-domain modeling confirms that the excitation of the localized surface plasmons leads to the enhancement of magneto-optical responses.

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.

Retracted Article: Anomalies in growth of electrodeposited Ni–Fe nanogranular films

Thin Ni–Fe films were produced via electrodeposition onto silicon substrates using direct current and pulse current (with different pulse durations) regimes.

Improvement in the Magnetic Properties of Ni–Fe Thin Films on Thick Nb Electrodes Using Oxidation and Low-Energy Ar Ion Milling

We developed a method to engineer the surface topography of Nb underlayers using surface oxidation followed by low-energy Ar ion milling to improve the properties of subsequently deposited magnetic Ni80Fe 20 (Permalloy) thin films. They had reduced coercivity $H_{c}$ , increased remanent squareness, and improved magnetic anisotropy due to the reduced roughness of the Nb underlayer, especially at high spatial frequency ( $> 25\,\mu {\text{m}}^{- 1}$ ). Typical results for 2.4 nm thick Ni80Fe20 films deposited on 100 nm thick Nb were an easy-axis coercivity $H_{{\rm{ce}}}= 3.7$ Oe (compared to 6 Oe without underlayer smoothing), a hard-axis $H_{{\rm{ch}}}= 1.5$ Oe (5.5 Oe without smoothing), an easy-axis remanent squareness (ratio of remanent and saturation magnetizations) $Sq_{e}= 0.92$ , a hard-axis remanent squareness $Sq_{h}= 0.25$ , and a uniaxial anisotropy $H_{k}= 6.0$ Oe, all measured at a temperature of 10 K. This ion-smoothing technique could potentially be used to improve the properties of magnetic layers in superconducting memory and other magnetoelectronic devices that utilize a thick underlayer that serves as an electrical contact.

In-situ Observation of Domain Wall Motion in Electroplated Ni80-Fe20 Thin Film by Lorentz TEM and DPC Imaging

In-situ observations of magnetic domain wall motion in a Ni80-Fe20 thin film electroplated on a Cu film were performed by Lorentz TEM. Detailed magnetization distribution was investigated by differential phase contrast microscopy. The magnetic properties of the coercivity and saturation magnetic flux density were measured to be 6.5 Oe and 1.56 T, respectively. The diffraction ring pattern showed the Ni3Fe FCC structure with nanocrystalline nature (grain size 29 nm) and texture indicating the preferential orientation. We measured the domain width of 1.9 μm and found the fluctuation of magnetization direction coupled with two vortices. With the in-plane and out-of-plane magnetic fields applied, domain wall motions were in-situ observed and discussed.

Self-induced inverse spin-Hall effect in an iron and a cobalt single-layer films themselves under the ferromagnetic resonance

The inverse spin-Hall effect (ISHE) is produced even in a “single-layer” ferromagnetic material film. Previously, the self-induced ISHE in a Ni80Fe20 film under the ferromagnetic resonance (FMR) was discovered. In this study, we observed an electromotive force (EMF) in an iron (Fe) and a cobalt (Co) single-layer films themselves under the FMR. As origins of the EMFs in the films themselves, the ISHE was main for Fe and dominant for Co, respectively 2 and 18 times larger than the anomalous Hall effect. Thus, we demonstrated the self-induced ISHE in an Fe and a Co single-layer films themselves under the FMR.