NiFe Thin Films Research Articles

Note: Electrical detection and quantification of spin rectification effect enabled by shorted microstrip transmission line technique

We describe a shorted microstrip method for the sensitive quantification of Spin Rectification Effect (SRE). SRE for a Permalloy (Ni80Fe20) thin film strip sputtered onto SiO2 substrate is demonstrated. Our method obviates the need for simultaneous lithographic patterning of the sample and transmission line, therefore greatly simplifying the SRE measurement process. Such a shorted microstrip method can allow different contributions to SRE (anisotropic magnetoresistance, Hall effect, and anomalous Hall effect) to be simultaneously determined. Furthermore, SRE signals from unpatterned 50 nm thick Permalloy films of area dimensions 5 mm × 10 mm can even be detected.

Precise determination of the spectroscopic g-factor by use of broadband ferromagnetic resonance spectroscopy

We demonstrate that the spectroscopic g-factor can be determined with high precision and accuracy by broadband ferromagnetic resonance measurements and by applying an asymptotic analysis to the data. Spectroscopic data used to determine the g-factor are always obtained over a finite range of frequencies, which can result in significant errors in the fitted values. We show that by applying an asymptotic analysis to broadband datasets, precise values of the intrinsic g-factor can be determined with errors well below 1%, even when the exact form of the Kittel equation (which describes the relationship between the frequency and resonance field) is unknown. We demonstrate this methodology with measured data obtained for sputtered Ni80Fe20 (Permalloy) thin films of varied thicknesses, where we determine the bulk g-factor value to be 2.109 ± 0.003. Such an approach is further validated by application to simulated data that include both noise and an anisotropy that is not included in the Kittel equation that was used in the analysis. Finally, we show a correlation of thickness and interface structure to the magnitude of the asymptotic behavior, which provide insight into additional mechanisms that may lead to deviations from the Kittel equation.

The effect of strain induced by Ag underlayer on saturation magnetization of partially ordered Fe16N2 thin films

Partially ordered Fe-N thin films were grown by a facing target sputtering process on the surface of a (001) Ag underlayer on MgO substrates. It was confirmed by x-ray diffraction that the Ag layer enlarged the in-plane lattice of the Fe-N thin films. Domains of the ordered α″-Fe16N2 phase within an epitaxial (001) α′-FexN phase were identified by electron diffraction and high-resolution aberration-corrected scanning transmission electron microscopy (STEM) methods. STEM dark-field and bright-field images showed the fully ordered structure of the α″-Fe16N2 at the atomic column level. High saturation magnetization(Ms) of 1890 emu/cc was obtained for α″-Fe16N2 on the Ag underlayer, while only 1500 emu/cc was measured for Fe-N on the Fe underlayer. The results are likely due to a tensile strain induced in the α″-Fe16N2 phase by the Ag structure at the interface.

Effect of composition on magnetic softness and magnetoimpedance of electrodeposited NiFe/Cu

A systematic variation of film composition was done during electrodeposition of NiFe thin films onto 100 μm diameter copper wire. Current density of deposition was maintained at 20 mA/cm2 to ensure constant film thickness. Pure Ni film as well as Fe rich NiFe film were not magnetically soft enough and showed poor magnetoimpedance effect. As Fe concentration was systematically varied, softest magnetic properties and magnetoimpedance value were found for the film composition Ni79Fe21. Soft magnetic properties induced and magnetoimpedance effect observed at this composition was attributed to the lowering of magnetoelastic anisotropy as well as magnetocrystalline anisotropy.

Improving the morphological and magnetic properties of permalloy films by adopting ultra-low-pressure sputtering

A series of permalloy (NiFe) thin films of fixed thickness were grown by DC-sputtering at different argon pressures and without buffer layers. When the pressure was lowered from 1.2Pa to 0.004Pa, the coercivity decreased remarkably form 1353.5A/m to 119.4A/m. In addition, the NiFe film fabricated at ultra-low-pressure showed low damping, and its zero-field linewidth approached zero. Based on ferromagnetic resonance (FMR) and atomic force microscopy (AFM) observations, we attribute the excellent magnetic properties of NiFe film fabricated under ultra-low pressure to its improved morphological and magnetic uniformity.

Magnetic properties of ultrathin Ni81Fe19 films with Ta and Ru capping layers

Magnetic properties of Ni81Fe19 (permalloy) ultrathin films with Ru and Ta capping layers (CLs) were investigated for applications to magnetic random access memory units (MRAM). The sample structure, which simulated an MRAM free layer, is Si- sub./SiO2/Ni81Fe19/Ru(Ta). The Ni81Fe19 thin films less than 3 nm thick with Ru CL show low coercive fields compared with the Ta capping layer. Both systems showed loss of momentum equivalent to magnetically dead layers of thickness (δ) ∼0.6 nm for Ru cap layer and ∼1.4 nm for Ta cap layer, respectively. Moreover, after annealing the thicknesses are slightly increased to an equivalent magnetic dead layer thickness of δ ∼ 0.84 nm and ∼1.80 nm for Ru and Ta CL, respectively. Our calculations showed that the presence of only 11% Ta concentration at the interface reduced the Ni momentum to zero, with the Ni–Ta coupling being anti-ferromagnetic; while 50% Ru intermixing at the interface reduced the Ni momentum to zero with the coupling between Ru and Ni being ferromagnetic. To find out more about the intermixing at the interface, the composition and chemical states were characterized by the x-ray photoelectron spectroscopy and peak decomposition technique. The result showed that the peak positions were different from the pure metallic case at the interface region, mainly because of the intermixing between two layers. In conclusion, the Ru capping layer might be important for MRAM use in terms of low coercive field and small δ layer thickness if compared with the Ta capping layer.

Atomic-Layer Alignment Tuning for Giant Perpendicular Magnetocrystalline Anisotropy of3dTransition-Metal Thin Films

The magnetocrystalline anisotropy (MA) of Fe-based transition-metal thin films, consisting of only magnetic 3d elements, was systematically investigated from full-potential linearized augmented plane-wave calculations. The results predict that giant MA with a perpendicular magnetic easy axis (PMA) can be achieved by tuning the atomic-layer alignments in an Fe-Ni thin film. This giant PMA arises from the spin-orbit coupling interaction between occupied and unoccupied Ni dx2-y2,xy bands crossing the Fermi level. A promising 3d transition-metal thin film for the MgO-based magnetic tunnel junctions with the giant PMA was, thus, demonstrated.

Ultrafast dynamics in NiFe alloy thin films by two times of transient reflection

The femtosecond optical pumped transient reflection technique is an effective tool to investigate the ultrafast dynamics in metal thin films. In this paper, both the transient reflectivity response and nonequilibrium relaxation time of electron–phonon interaction in NiFe alloy thin films are studied by using femtosecond laser pump–probe transient reflection experimental technology. The result shows that the heating time of NiFe thin films with different thickness is independent of the thickness of the films, while the thermalization time by electron–phonon relaxation from nonequilibrium state to equilibrium state is proportional to the thickness of the films. In addition, two transient reflectivity signals on glass substrate films are found which is different with previous researches.

NiFe 박막의 두께에 따른 강자성 공명 특성 분석

본 연구에서는 NiFe 박막 시편을 마그네트론 스퍼터링 방법으로 제조하여 박막면 기준으로 수직면(out-of-plane) 자기장 방향과 수평면(in-plane) 자기장 방향에 따른 강자성 공명 자기장을 측정하였다. 수직면 자기장 방향에 따른 강자성 공명 자기장으로부터 유효자화량(<TEX>$M_{eff}$</TEX>)을 도출하였으며, NiFe 두께에 따른 <TEX>$M_{eff}$</TEX>의 감소는 <TEX>$K_s=-0.23\;erg/cm^2$</TEX>의 값을 갖는 표면 이방성 상수에 기인하였다. 또한 수평면 자기장 방향에 따른 강자성 공명 자기장으로부터 수평면에서의 일축 이방성 자기장을 도출하였다. 한편, 일축 이방성 에너지의 자화 용이축이 두께가 감소함에 따라 시편 제조 시 인가한 자기장의 반대 방향으로 회전하고 있었으며, 이러한 현상은 시편 표면에 형성된 NiFeO의 반강자성 특성에 의한 현상으로 설명하였다. The out-of-plane and in-plane angular dependence of ferromagnetic resonance field was measured in NiFe thin films fabricated by magnetron sputtering. The effective magnetization was obtained from the out-of-plane angular dependence of ferromagnetic resonance field, which was well agreed with calculated one. The decrease of effective magnetization with NiFe thickness was due to the surface anisotropy constant of <TEX>$K_s=-0.23\;erg/cm^2$</TEX>. The in-plane uniaxial anisotropy fields were obtained from the in-plane angular dependence of ferromagnetic resonance field. The easy axis of in-plane uniaxial anisotropy field was rotated to the reverse direction of applied magnetic field during sample fabrication, which was explained by the antiferromagnetic NiFeO layer at sample surface.

Well-designed rectangular cavity resonator for FMR experiment

The unloaded quality factor of the cavity resonator is the ratio between the stored energy of the cavity resonator to the power loses in the cavity resonator. The homemade rectangular cavity resonator in X-band shows higher unloaded quality factor compare with standard cavity resonator in the TE102 mode. Because the inner walls of rectangular cavity resonator are treated through high quality polishing and high purity Au plating. Also the inner walls are made by printed circuit board which has thin Cu foil, two problems such as mechanical vibration and thermal expansion can be solved by minimizing unwanted eddy current. Through the ferromagnetic resonance measurement by using our rectangular cavity resonator, we can be obtained reasonable values of resonance frequency and linewidth by using NiFe thin film. As a result, the Gilbert damping constant from the experimental result is in good agreement with the typical value of damping parameter of the NiFe thin film.

Strain induced giant magnetism in epitaxial Fe16N2 thin film

We report a direct observation of giant saturation magnetization in Fe16N2. By exploiting thin film epitaxy, which provides controlled biaxial stress to create lattice distortion, we demonstrate that giant magnetism can be established in Fe16N2 thin film coherently grown on MgO (001) substrate. Explored by polarized neutron reflectometry, the depth-dependent saturation magnetic induction (Bs) of epitaxial Fe16N2 thin films is visualized, which reveals a strong correlation with the in-plane lattice parameter and tensile strain developed at near substrate interface. With controlled growth process and dimension adjustment, the Bs of these films can be modulated over a broad range, from ∼2.1 Tesla (T) (normal Bs) up to ∼3.1 T (giant Bs).

Effects of ruthenium seed layer on the microstructure and spin dynamics of thin permalloy films

The spin dynamics and microstructure properties of a sputtered 12 nm Ni81Fe19 thin film have been enhanced by the use of a ruthenium seed layer. Both the ferromagnetic resonance field and linewidth are enhanced dramatically as the thickness of ruthenium seed layer is increased. The surface anisotropy energy constant can also be largely tailored from 0.06 to 0.96 erg/cm−2 by changing the seed layer thickness. The changes to the dynamics magnetization properties are caused by both ruthenium seed layer induced changes in the Ni81Fe19 structure properties and surface topography properties. Roughness induced inhomogeneous linewidth broadening is also seen. The damping constant is highly tunable via the ruthenium thickness. This approach can be used to tailor both the structure and spin dynamic properties of thin Ni81Fe19 films over a wide range. And it may benefit the applications of spin dynamics and spin current based devices.

Bi-Directional Giant Magneto Impedance Sensor

A thin film giant magneto impedance (GMI) based on magnetic field sensor has been developed using electrodeposited Ni-Fe permalloy. Chemical composition, surface morphology, and magnetic properties of Ni-Fe permalloy were char-acterized as a function of plateup parameters, and process conditions were established to deposit a Ni-Fe thin film with a high permeability (~1000) and a low coercivity (0.6 Oersted). Conventional GMI sensors are uni-directional and are several millimeters long. In this work, a spiral-shaped sensor using electroplated Ni-Fe permalloy to detect bi-directional magnetic field is reported. Excellent bi-directional magnetic field sensing has been demonstrated using the 1 mm2 compact double-spiral structure.

Control and Analysis of Grain Size in Sputtered NiFe Thin Films

In this paper we describe a method for deposition of ferromagnetic thin films with controlled grain size and distri- bution. The close control of grain size without the use of seed layers is demonstrated for NiFe - permalloy films by using a novel sputtering technology. We also present a detailed analysis of the grain size distribution based on the cumulative percentage technique. This is a more convenient mathematical approach to the study of particle size distributions as it gives consistent results even for a small number of particles.

Controlling Anisotropy of NiFe Thin Films During Deposition for Device Applications

Thin film permalloy (with compositions around Ni80Fe20 continues to be an important ferromagnetic material for a range of applications including sensors, inductors and spintronic devices. Very weak magnetostriction and magnetocrystalline anisotropy are key attributes contributing to the favorable magnetic properties required in many such applications. Also in applications the anisotropy of the permalloy is controlled, usually by field-induced anisotropy or lithographic patterning. This work describes the control of anisotropy and the magnetization behavior in Permalloy thin-films by controlling the angle of the substrate with respect to the source during thin-film deposition. Film thicknesses from 5 nm up to 40 nm were investigated and all showed the development of a significant anisotropy and coercivity enhancement as the angle of the substrate was increased. Although this approach produces a well-defined in-plane easy axis, the orthogonal magnetization can be more complex than that of simple hard axis behavior. Deposition induced anisotropy effects were apparent even for a substrate tilt of only ten degrees. The control of anisotropy using this depositional effect was demonstrated for applications with examples showing control of magnetoresistance and via the competition between the deposition anisotropy and shape anisotropy in controlling the magnetization behavior of lithographically patterned structures.

Effect of grain size on magnetic properties and microstructure of Ni80Fe20 thin films

Ni–Fe thin films were deposited from a Ni80Fe20 target on a glass substrate by direct current magnetron sputtering. The film thickness was varied from 300Å to 1000Å and the films are formed at room temperature (RT) and annealed for 1h at 250°C. Plane-view high-resolution transmission electron microscopy was used to examine the microstructure of each NiFe thin film and to determine the grain sizes. Electron diffraction patterns reveal that the as-deposited and annealed NiFe thin film had a face-centered cubic NiFe. It also indicates that the crystallization of the annealed NiFe film is stronger than that of the as-deposited film. The coercivity and magnetic squareness (SQR) and remanent and saturation magnetization ratio (Mr/Ms) ratio of the in-plane magnetic hysteresis loop were measured using a superconducting quantum interference device. The coercitivity as well as the magnetic remanence properties and squareness ratio of the NiFe thin films is related to the distribution of grain sizes. The SQR value of the NiFe thin film increases upon annealing because the ordered spins in the NiFe thin film are randomized by the thermal vibration. Accordingly, Mr is increased and Ms is reduced. Hence, the SQR value decreases as the thickness of thin film increases because the refinement of the grains reduces the effective anisotropy, strengthening the ferromagnetic exchange coupling interaction.

Studies of anisotropic magnetoresistance and magnetic property of Ni81Fe19 ultra-thin films with the lower base vacuum

A series of Ta(5nm)/Ni81Fe19(20nm)/Ta(3nm) Ni81Fe19 magnetic thin films were prepared by magnetron sputtering method in a lower test condition, and it won a excellent test result. Effects of substrate temperature and buffer layer thickness on the structure and magnetic property of Ni81Fe19 thin films had been investigated. The results show that the Ni81Fe19 films prepared at the substrate temperatures of 400°C showed a higher AMR value and a lower saturation magnetic field. The highest AMR value is 3.5% and the lowest magnetized saturated field is 739.67A/m. The Ni81Fe19 films prepared at the substrate temperatures of 500°C showed the highest saturation magnetization value. With the increase of the buffer layer thickness AMR value improves first but then decreases. It reaches the maximum when x is 5nm.

Effect of grain size on nanomechanical property Ni80Fe20 thin film

The purpose of this paper is to describe the results of this study into the structure and nanomechanical properties of Ni80Fe20 thin film. The films were sputtered onto glass substrates with thicknesses of 500 Å, 1000 Å, and 1500 Å, respectively. These three thicknesses were tested both at room temperature (RT) and with a post-annealing heat treatment temperature of 250 °C for 1 h. The plane-view microstructure was observed under a high-resolution transmission electron microscope (HRTEM) to determine grain distribution. The selected area diffraction (SAD) pattern was obtained with the HRTEM to investigate NiFe microstructures. Electron diffraction patterns demonstrated that NiFe thin film has a face-centered cubic (FCC) structure and strong NiFe (111) crystallization. Annealing treatment increased the grain size distribution of the thin film. The grain size is increased at the thicker thickness of NiFe thin film. Nano-indentation was used to measure hardness and Young's modulus; based on these results and the grain size, the decline of hardness can be reasonably inferred from an enlarged grain size, which is consistent with the Hall–Petch effect. However, the rising Young's modulus measurement can be reasonably associated with the effect of the sputtered adhesion of NiFe thin film.

Determining the planar Nernst effect from magnetic-field-dependent thermopower and resistance in nickel and permalloy thin films

We present magnetic-field-dependent measurements of thermopower, $\ensuremath{\alpha}$(H), and resistance, $R(H)$, for ${\mathrm{Ni}}_{80}{\mathrm{Fe}}_{20}$ and Ni thin films. We conducted these experiments in fields oriented parallel and perpendicular to the applied thermal gradient, $\stackrel{P\vec}{\ensuremath{\nabla}}T$, for $\ensuremath{\alpha}$(H) and applied current for $R(H)$. We deposited the 20-nm-thick films on 500-nm-thick suspended Si-N thermal isolation platforms that enable in-plane measurements of thermal and electrical properties of thin films. Both $\ensuremath{\alpha}(H)$ and $R(H)$ in Ni-Fe and Ni films exhibit evidence of spin-dependent scattering through an even field dependence and a linear proportionality between $\ensuremath{\alpha}(H)$ and $1/R(H)$. Finally, we use $\ensuremath{\alpha}({\mathrm{H}}_{\ensuremath{\parallel}})$ and $\ensuremath{\alpha}({\mathrm{H}}_{\ensuremath{\perp}})$ to determine the planar Nernst coefficient in Ni-Fe and Ni thin films and use this coefficient to predict the size of the planar Nernst effect (PNE) in the micromachined platform. The measured field dependence of the PNE is well matched by the prediction obtained from our $\ensuremath{\alpha}$(H) results.

Microstructure and magnetic properties of Ni3Fe solid solution thin films deposited by DC-magnetron sputtering

Fe–Ni thin films were co-sputter deposited from Ni and Fe metallic targets on different substrates. The evolution of the coating composition as a function of the discharge current dissipated on each target is studied and the targeted composition for Ni3Fe is obtained by fixing the discharge current applied on Fe target at 0.5A and adjusting the discharge current applied on Ni target at 0.45A. The as deposited coating with convenient composition presents a disordered solid solution FCC structure. Finally, the magnetic properties of the films are investigated as a function of their thickness. We have observed that coercivity decreases slightly and magnetic moment increases which increasing film thickness.