NiFe Thin Films Research Articles

Control of ferromagnetic properties of Ni80Fe20 thin films by voltage-induced oxidation

We demonstrate large voltage-induced changes of magnetic properties in thin films of Ni80Fe20 (permalloy) when gated using an ionic liquid medium [1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIMTFSI)]. The coercivity and magnetic moment of 5 nm thick permalloy films could be reduced by 75% and 35%, respectively, by using applied voltages. These changes were partially restored by reversing the potential polarity. Electrochemical, time-course magnetometry and surface analysis measurements suggest that the voltage-induced changes are due to changes in the oxidation state at the surface of the film, causing a thinning of the permalloy layer. The control of soft magnetic properties with low voltages may be of use in tuneable devices.

Ferromagnetic resonance in coupled magnetic nanostructured arrays

The modulated field sweep ferromagnetic resonance (FMR) spectroscopy was used to study the magnetization dynamics in large arrays of interacting cubic nanomagnets. A 60nm thick permalloy (Ni80Fe20) thin films were patterned using a lift-off process into several large arrays of 60 x 60 nm2 nanostructures where the spacing between the magnetic nanocubes was varied to control the strength of dipolar coupling. Electron beam lithography was used for device patterning using lift-off. The permalloy films were deposited using magnetron sputtering. DC magnetic properties were evaluated using alternating gradient force magnetometer (AGFM). In the FMR measurements, the orientation of DC bias magnetic field was varied from the in-plane to out-of-plane with respect to the 2D plane of the arrays. The FMR peak splitting and multiple FMR modes were observed in the evolution of the FMR spectra as the function of the bias field orientation, and were strongly influenced by the nanomagnet geometry and the spacing between the nanomagnets. Two resonance modes, shape and lattice, were particularly well pronounced in the observed FMR spectra. These modes are characterized by effective demagnetizing factors representing different symmetries of the system of interacting nanomagnets. Micromagnetic modeling suggest that micromagnetic texture within the nanomagnets modulated by the stray fields from the neighbors correlates with the FMR spectra.

Measurement of Elemental Composition of FeNi and SiGe Thin Films by Electron Probe Microanalysis with Stratagem Software

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Comparison of magnetic and structural properties of permalloy Ni80Fe20 grown by dc and high power impulse magnetron sputtering

We study the microstructure and magnetic properties of Ni80Fe20 thin films grown by high power impulse magnetron sputtering (HiPIMS), and compare with films grown by dc magnetron sputtering (dcMS). The films were grown under a tilt angle of 35° to identical thickness of 37 nm using both techniques, at different pressure (0.13–0.73 Pa) and substrate temperature (room temperature and 100 °C). All of our films display effective in-plane uniaxial anisotropy with square easy axis and linear hard axis magnetization traces. X-ray diffraction reveals that there is very little change in grain size within the pressure and temperature ranges explored. However, variations in film density, obtained by x-ray reflectivity measurements, with pressure have a significant effect on magnetic properties such as anisotropy field (Hk) and coercivity (Hc). Depositions where adatom energy is high produce dense films, while low adatom energy results in void-rich films with higher Hk and Hc. The latter applies to our dcMS deposited films at room temperature and high pressure. However, the HiPIMS deposition method gives higher adatom energy than the dcMS and results in dense films with low Hk and Hc. The surface roughness is found to increase with increased pressure, in all cases, however it showed negligible contribution to the increase in Hk and Hc.

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.

High spin polarization in epitaxial Fe4N thin films using Cr and Ag as buffer layers

Fe4N thin films with (001) texture were prepared by reactive sputtering on MgO substrates, utilizing either a Cr or Ag buffer layer to facilitate the epitaxial growth. X-ray diffraction, atomic force microscopy, and vibrating sample magnetometry measurements show that the Fe4N thin film grown on the Ag buffer layer is superior to that grown on the Cr buffer layer. The point contact Andreev reflection measurement was then conducted, and the spin polarizations were determined to be 61.1% and 81.3% for Fe4N thin films with Cr and Ag buffer layers, respectively. The 81.3% spin polarization is significantly higher than the ratio reported previously for Fe4N and is comparable with that of state-of-the-art Heusler alloys. This result is in agreement with the theoretical prediction on the discrepancy between the two differently defined spin polarizations for Fe4N. Moreover, our study indicates that an optimized growth process for Fe4N thin films is crucial for achieving a high spin polarization and that true half-metallicity could potentially be realized with Fe4N. The high spin polarization of Fe4N combined with its low fabrication temperature and simple composition makes Fe4N a competitive candidate to be a half-metallic ferromagnet in spintronic devices.

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.

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.

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.

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.

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.

Influence of different conducting substrates on magnetic properties of electrodeposited Ni–Fe thin films

Electrodeposition of Ni–Fe soft magnetic alloy on copper and stainless steel substrates was performed in chloride bath. The deposition parameters such as current density, pH, temperature and deposition time have been investigated. From the investigation the optimized deposition parameters were current density 3.5 mA/cm2, pH 3, temperature 30 °C and deposition time 15 min. The Ni–Fe magnetic alloys deposited on copper and stainless steel substrates under optimized deposition parameters are subjected to various characterizations. The structural and surface morphology of the Ni–Fe films were detected by using X-ray diffractogram (XRD) and scanning electron microscope (SEM) respectively. The constituents in the films were determined by energy dispersive X-ray spectroscopy (EDAX) technique. The magnetic properties such as the coercivity (Hc) and saturation magnetization of the films were studied with the help of vibrating sample magnetometer (VSM). From the magnetic studies it is concluded that the grain size are create a considerable impact on magnetic behavior of the films on both the substrates. The films prepared on stainless steel substrate of 0.1 M concentration at optimized deposition parameters exhibits higher coercivity (5010 Oe) which seems to be ideal for magnetic sensor applications.

NiFe 박막에서 교환 뻣뻣함 상수의 각도 의존성 분석

교환 뻣뻣함 상수 ASUBex/SUB는 스핀파의 공명 자기장 HSUBSWR/SUB과 강자성 공명 자기장 HSUBFMR/SUB의 차에 선형적으로 비례한다. 본 연구에서는 두께가 100 nm인 NiFe 박막에서 ASUBex/SUB의 각도 의존성을 분석하기 위하여 수평면에서 자기장 각도에 따른 HSUBSWR/SUB와 HSUBFMR/SUB를 측정하였다. NiFe 박막의 ASUBex/SUB는 수평면의 자기장 각도와 무관하게 등방성 특성을 보였다. 본 연구에서 측정된 ASUBex/SUB의 값은 10.9 × 10SUP−7/SUP erg/㎝였으며, 이 값은 스핀파를 이용한 소자에 응용될 수 있다.

Magnetization dynamics of weak stripe domains in Fe–N thin films: a multi-technique complementary approach

The resonant eigenmodes of an α′-FeN thin film characterized by weak stripe domains are investigated by Brillouin light scattering and broadband ferromagnetic resonance experiments, assisted by micromagnetic simulations. The spectrum of the dynamic eigenmodes in the presence of the weak stripes is very rich and two different families of modes can be selectively detected using different techniques or different experimental configurations. Attention is paid to the evolution of the mode frequencies and spatial profiles under the application of an external magnetic field, of variable intensity, in the direction parallel or transverse to the stripes. The different evolution of the modes with the external magnetic field is accompanied by a distinctive spatial localization in specific regions, such as the closure domains at the surface of the stripes and the bulk domains localized in the inner part of the stripes. The complementarity of BLS and FMR techniques, based on different selection rules, is found to be a fruitful tool for the study of the wealth of localized magnetic excitations generally found in nanostructures.

Development of flexible Ni80Fe20 magnetic nano-thin films

Flexible magnetic Ni80Fe20 thin films with excellent adhesion, mechanical and magnetic properties have been fabricated using magnetron plasma deposition. We demonstrate that flexible Ni80Fe20 thin films maintain their non-flexible magnetic properties when the films are over 60nm thick. However, when their thickness is reduced, the flexible thin films display significant increase in their magnetic coercive field compared to identical films coated on a solid Silicon substrate. For a 15nm flexible Ni80Fe20 film coated onto 110µm Polyvinylidene fluoride polymer substrate, we achieved a remarkable 355% increase in the magnetic coercive field relative to the same film deposited onto a Si substrate. Experimental evidence, backed by micro-magnetic modelling, indicates that the increase in the coercive fields is related to the larger roughness texture of the flexible substrates. This effect essentially transforms soft Ni80Fe20 permalloy thin films into medium/hard magnetic films allowing not only mechanical flexibility of the structure, but also fine tuning of their magnetic properties.

Deposition and spin polarization study of Fe4N thin films with (111) orientation

We have successfully deposited Fe4N thin films with (111) out-of-plane orientation on thermally oxidized Si substrates using a facing-target-sputtering system. A Ta/Ru composite buffer layer was adopted to improve the (111) orientation of the Fe4N thin films. The N2 partial pressure and substrate temperature during sputtering were optimized to promote the formation of the Fe4N phase. Furthermore, we measured the transport spin polarization of (111) oriented Fe4N by the point contact Andreev reflection (PCAR) technique. The spin polarization ratio was determined to be 0.50 using a modified BTK model. The film thickness dependence of the spin polarization was also investigated. The spin polarization of Fe4N measured by PCAR does not show degradation as the sample thickness was reduced to 10nm.

Building Thin Film Model Materials to Study Active Sites of PGM-Free Electrocatalyst for ORR

Replacement of noble metal catalysts for oxygen reduction reaction (ORR) with transition metals can have a profound impact on clean and economical energy production. Although non-precious metals have been extensively studied as ORR electrocatalysts, there has been a long debate about what active site is responsible for the ORR and what the catalytic mechanism is. The mystery of the chemical nature of the active sites hinders the development of efficient and stable ORR catalyst. The biggest obstacle to resolving the mechanism is the fact that the active sites are buried in the bulk, porous catalyst and the components in the real catalyst are complicated. Here, we propose a thin film model approach to study the active sites of FeN thin film, deposited on top of glassy carbon by magnetron sputtering. Rutherford backscattering, X-ray diffraction and Raman spectroscopy were carried out to characterize bulk films and to determine deposition conditions that result in Fe tetrahedrally coordinated with nitrogen. The significance of this study is that this is the first time active sites are intentionally exposed to the surface and can be directly characterized by x-ray photoelectron spectroscopy (XPS). Bonding between Fe-N in the bulk film and N-C at the interface between FeN and glassy carbon have been confirmed by XPS. Rotating disk electrode experiments have been carried out to measure the ORR activity of Fe-N, C-N, Fe-C the Fe-N-C exposed surfaces by looking at thin Fe-N, C-N, Fe-C, and Fe-N-C films. This study will also provide answers to the controversy about whether carbon, nitrogen or iron is essential in the ORR catalyst.