NiFe Films Research Articles

Construction of FeN alloy films with ultra-strong magnetism and tunable magnetic anisotropy for spintronic application

FeN alloy film is a promising spintronic material with the theoretically ultra-strong magnetism (saturation magnetization MS and magnetic anisotropy Keff) and high spin polarization, which relies on the degree of N ordering interstice occupancy (S). However, due to the high activation energy for N ordering, the S value of an actual FeN film is mostly lower than 35% and this restricts the achievable magnetism and transportation property. Thus, the construction of a FeN alloy film with well-controlled magnetism and efficient electronic transportation remains a long-standing challenge. Here, we tackle the problem by strain engineering. Using an Fe/Cr underlayer, we introduced a considerable epitaxial strain in the FeN lattice. The strain is proven to effectively promote the S value to over 60%, resulting in remarkable enhancement of MS value from 2.18T to 2.81T (30% increment) and effective tunability of Keff value ranging 1.3∼2.2 × 106 J/m3. Besides, the matched energy band symmetry (Δ5) between Cr and Fe16N2 facilitates the efficient electronic transportation for spintronic applications. By simulating interstice distribution with the first-principles calculations, the lattice strain is found to decrease the activation energy for N interstitial migration, which serves as a thermodynamic driving force for the magnetism tunability.

Achieving high magneto-impedance effect: Controlling the evolution of textures in FeNi thin films by thermal annealing

The controlled growth and tailorable magnetic properties of FeNi thin films are of critical importance for controlling the magneto-impedance (MI) effect. In this study, a Ti layer was introduced to eliminate thickness-driven perpendicular anisotropy in thick FeNi films and thereby preserve their magnetic softness. Furthermore, the temperature-driven anisotropy determined the in- and out-of-plane magnetization behavior in FeNi/Ti-based multilayers, and so dominated the MI properties of these films. The oriented (111) texture of the out-of-plane crystal structure was enhanced with increasing annealing temperature. This supported the formation of perpendicular anisotropy in FeNi films, and reduced their magnetic softness. The in-plane texture changed from a dominant (111) orientation to a (220) orientation with increasing annealing temperature. This appear to be related to magnetic properties of FeNi thin films, because a minimum in coercivity occurred at the critical temperature of the texture transition. Thus, the largest MI effect was obtained in the FeNi-based multilayer at this critical temperature. Moreover, the interdiffusion at higher temperature between the Ti layer and the FeNi layer should be considered, because the coercivities and saturation magnetization of permalloy films increase and decrease sharply at 520°C.

Dependence of anomalous Nernst effect on crystal orientation in highly ordered γ′-Fe4N films with anti-perovskite structure

The anomalous Nernst effect (ANE) in highly ordered Fe4N films was examined in the measurement configuration with an in-plane temperature gradient (∇Tx). The anomalous Nernst coefficient (Qs) in the case of ∇Tx ∥ Fe4N[110] was clearly larger than that in the case of ∇Tx ∥ Fe4N[100]. Note here that no reports on the Qs in Fe4N films have been presented. This anisotropic Qs (Qani) was also observed in a different configuration with an out-of-plane temperature gradient. Unlike Fe4N, pure Fe and A2-FeAl systems showed a negligible Qani. This Qani might be attributed to the specific hybridization effects between the 3d orbitals of Fe and the 2p orbitals of N.

Anisotropic magnetothermoelectric power of ferromagnetic thin films

In this article, we report the measurements of the magnetothermoelectric power (MTEP) in metallic ferromagnetic thin films of Ni80Fe20 (Permalloy; Py), Co and CrO2 at temperatures in the range of 100K to 400K. In 25nm thick Py films and 50nm thick Co films both the anisotropic magnetoresistance (AMR) and MTEP show a relative change in resistance and thermoelectric power (TEP) of the order of 0.2% when the magnetic field is reversed, and in both cases there is no significant change in AMR or MTEP after the saturation field has been reached. Surprisingly, both Py and Co films have opposite MTEP behaviour although both have the same sign for AMR and TEP. The data on half metallic ferromagnet CrO2 films show a different picture. Films of thickness of 100nm were grown on TiO2 and on sapphire. The MTEP behavior at low fields shows peaks similar to the AMR in these films, with variations up to 1%. With increasing field both the MR and the MTEP variations keep growing, with MTEP showing relative changes of 1.5% with the thermal gradient along the b-axis and even 20% with the gradient along the c-axis, with an intermediate value of 3% for the film on sapphire. It appears that the low-field effects are due to the magnetic domain state, and the high-field effects are intrinsic to the electronic structure of CrO2 and intergarian tunnelling magnetoresistance that contributes to MTEP as tunnelling-MTEP. Our results will stimulate the research work in the field of spin dependent thermal transport in ferromagnetic materials to further develop spin-Caloritronics.

Tanke plasti zlitine Ni–Fe za izdelavo AMR-senzorjev

Tanke plasti zlitine Ni–Fe za izdelavo AMR-senzorjev

Effect of a CoFeB layer on the anisotropic magnetoresistance of Ta/CoFeB/MgO/NiFe/MgO/CoFeB/Ta films

The anisotropic magnetoresistance (AMR) and magnetic properties of NiFe films can be remarkably enhanced via CoFeB layer. In the case of an ultrathin NiFe film having a Ta/CoFeB/MgO/NiFe/MgO/CoFeB/Ta structure, the CoFeB/MgO layers suppressed the formation of magnetic dead layers and the interdiffusions and interface reactions between the NiFe and Ta layers. The AMR reached a maximum value of 3.56% at 450°C. More importantly, a single NiFe (111) peak can be formed resulting in higher AMR values for films having CoFeB layer. This enhanced AMR also originated from the significant specular reflection of electrons owing to the crystalline MgO layer, together with the sharp interfaces with the NiFe layer. These factors together resulted in higher AMR and improved magnetic properties.

Thickness dependence of the magnetic anisotropy and dynamic magnetic response of ferromagnetic NiFe films

We investigate the thickness dependence of the magnetic anisotropy and dynamic magnetic response of ferromagnetic NiFe films. We go beyond quasi-static measurements and focus on the dynamic magnetic response by considering three complementary techniques: the ferromagnetic resonance, magnetoimpedance and magnetic permeability measurements. We verify remarkable modifications in the magnetic anisotropy, i.e. the well-known behavior of in-plane uniaxial magnetic anisotropy systems gives place to a complex magnetic behavior as the thickness increases, and splits the films in two groups according to the magnetic properties. We identify magnetoimpedance and magnetic permeability curves with multiple resonance peaks, as well as the evolution of the ferromagnetic resonance absorption spectra, as fingerprints of strong changes of the magnetic properties associated to the vanishing of the in-plane magnetic anisotropy and to the emergence of non-homogeneous magnetization configuration, local anisotropies and out-of-plane anisotropy contribution arisen as a consequence of the non-uniformities of the stress stored in the film as the thickness is increased and/or to the columnar growth of the film. We interpret the experimental results in terms of the structural and morphological properties, quasi-static magnetic behavior, magnetic domain structure and different mechanisms governing the magnetization dynamics at distinct frequency ranges.

Ferromagnetic resonance in a system of magnetic films with different Curie temperatures

The peculiarities of absorption of rf electromagnetic radiation (ferromagnetic resonance) in multilayer NiFe/Ni0.65Cu0.35(d)/CoFe structures in a wide temperature range are analyzed. It is shown that the type of interaction of the NiFe and CoFe ferromagnetic films via a “weak” ferromagnetic Ni0.65Cu0.35 interlayer changes from antiferromagnetic to ferromagnetic upon cooling and a decrease in interlayer thickness d. The detected temperature dependence of the interlayer interaction indicates the possibility of observation of a strong magnetocaloric effect in the structures under investigation.

Effect of thermal annealing on the spin Seebeck effect in Pt/Ni81Fe19 at room temperature

The spin Seebeck effect was observed in Pt/Ni81Fe19. Hysteresis-like behavior was discovered, and the saturated voltage was proportional to the temperature gradient. The measured voltage increased as the Pt thickness was decreased, because the injected spin current intensity drastically decreased. Thermal annealing temperature also strongly affected the observed voltage, which was lower at higher thermal annealing temperatures. The anomalous Nernst effect was also observed in Ni81Fe19 films of various thicknesses. The voltage was nearly constant when the Ni81Fe19 film was thicker than 30nm but drastically decreased for lesser thicknesses.

Role of Ta-spacer layer on tuning the tilt angle magnetic anisotropy of L11-CoPt/Ta/NiFe exchange springs

L11-CoPt/Ta/NiFe trilayers are chosen as model films for probing the role of spacer layer on tuning the tilt angle magnetization (θM) in such exchange springs. For this purpose, a non-magnetic layer (Ta) with varying thickness (tTa) from 0 to 2.5nm was inserted between 10-nm thick CoPt film exhibiting strong perpendicular magnetic anisotropy (PMA) and 4-nm thick NiFe film having in-plane magnetic anisotropy (IMA). With the insertion of Ta-spacer, the magnetic hysteresis loops become more and more tilted as tTa increases. Upon increasing the tTa from 0 to 2.5nm, the estimated SQR⊥ (=Mr⊥/Ms⊥) from the M–H loops is found to decrease moderately; while the θM increases significantly from 43° to 77°. MFM images revealed maze-like domain patterns and the domain size tends to increase at the expense of magnetic phase contrast with increasing tTa. Micro-magnetic simulation of tilt in the anisotropy axis with respect to the bare CoPt-layer showed a trend similar to that of those observed with the M–H loops obtained by VSM measurements. The results of present study suggest that the insertion of Ta-spacer is not only beneficial in terms of preserving the competing anisotropies such as PMA and IMA of CoPt and NiFe-layers respectively through weakened exchange coupling; but also, act as an appropriate means for realizing tunable tilted magnetic anisotropy in the L11-CoPt/NiFe exchange springs.

Normal Electrochemical Deposition of NiFe Films

Normal Electrochemical Deposition of NiFe Films

Microstructural and magnetic properties of thin obliquely deposited films: A simulation approach

The relation between microstructural and magnetic properties of thin obliquely deposited films has been studied by means of numerical techniques. Using our developed simulation code based on ballistic deposition model and Fourier space approach, we have investigated dependences of magnetometric tensor components and magnetic anisotropy parameters on the deposition angle of the films. A modified Netzelmann approach has been employed to study structural and magnetic parameters of an isolated column in the samples with tilted columnar microstructure. Reliability and validity of used numerical methods is confirmed by a good agreement of the calculation results with each other, as well as with our experimental data obtained by the ferromagnetic resonance measurements of obliquely deposited thin Ni80Fe20 films. The combination of these numerical methods can be used to design a magnetic film with a desirable value of uniaxial magnetic anisotropy and to extract the obliquely deposited film structure from only magnetic measurements.

Synthesis of Ni-Fe thin films by electrochemical deposition technique and characterization of their microstructures and surface morphologies

Microstructural features and surface morphologies of Ni-Fe thin films fabricated by the electrochemical deposition technique have been experimentally studied. Ni-Fe thin films have been deposited under different deposition potentials and FeSO$_{4}$ concentrations. Energy dispersive X-ray measurements demonstrate that the Fe content decreases (increases) as the deposition potential (FeSO$_{4}$ concentration) is enhanced. All of the produced films exhibit an anomalous codeposition behavior. The effects of deposition potential and FeSO$_{4}$ concentrations on the degree of anomalous codeposition have been also characterized. X-ray diffraction studies reveal that the films have face-centered cubic crystallographic structures with [111] preferred crystallographic orientation regardless of the applied deposition conditions. However, the crystallization and the crystallite size of the films are affected by changing of the FeSO$_{4}$ concentration and the deposition potential. The results of scanning electron microscopy analyses verify that the surface structure of the film electroplated from the electrolyte with higher FeSO$_{4}$ concentration under higher deposition potential exhibits a more homogeneous and dense structure with smaller grain sizes.

Periodic surface structures on Ni–Fe film induced by a single femtosecond laser pulse with diffraction rings

This Letter reports the formation of periodic surface structures on Ni–Fe film irradiated by a single femtosecond laser pulse. A concave lens with a focus length of −150 mm is placed in front of an objective (100×, NA=0.9), which transforms the Gaussian laser field into a ring distribution by the Fresnel diffraction. Periodic ripples form on the ablation area after the irradiation of a single femtosecond laser pulse, which depends on the laser polarization and laser fluence. We propose that the ring structure of the laser field leads to a similar transient distribution of the permittivity on the sample surface, which further launches the surface plasmon polaritons. The interaction of the incident laser with surface plasmon polaritons dominates the formation of periodic surface structures.

МАГНИТНЫЕ СВОЙСТВА ТЕКСТУРИРОВАННЫХ ПЛЕНОК NiFe(111) И NiFe(200)

МАГНИТНЫЕ СВОЙСТВА ТЕКСТУРИРОВАННЫХ ПЛЕНОК NiFe(111) И NiFe(200)

Effect of Ni/Fe ratio of electrolyte salts on the magnetic property of electrodeposited Fe–Ni alloy

Electrodeposition of Fe–Ni thin films has been carried out on a copper substrate from simple as well as complex baths containing sulfate salts with Ni/Fe ratio of 1 : 1 and 12 : 1. Complex baths consistedeither all of ascorbic acid, citric acid and saccharine in addition to the salts viz. NiSO4 · 7H2O; FeSO4 · 7H2O; H3BO3 and Na2SO4 in simple bath. The chemical composition of the deposit was determined by an energy dispersive X-ray analyzer. Magnetic properties of the Fe–Ni films were measured by avibrating sample magnetometer (VSM). The X-ray diffraction was done on the electrodeposited thin films to determine Fe–Ni alloy phases. Magnetic properties of films were studied before and after heat treatment of the samples. It is found that the saturation magnetization decreases with increasing Ni content in the films obtained from simple baths with low Ni/Fe ratio (1 : 1) while the saturation magnetization increases with increasing Ni content obtained from complex bath with high Ni/Fe ratio (12 : 1). Among different baths with high Ni/Fe ratio of 12 : 1, the saturation magnetization of deposited film is higher deposited from a bath containing three complexing agents, namely, ascorbic acid, citric acid and saccharine than from a bath containing a single complexing agent–ascorbic acid. The ideal nature of the Ms–H (saturation magnetization vs. applied field) curve was obtained from complex baths with a high Ni/Fe ratio (12 : 1).

Influence of nitrogen flow rates on iron nitride thin films prepared by DC reactive magnetron sputtering

Iron nitride (FeN) thin films were deposited on silicon wafer substrates by dc reactive magnetron sputtering techniques. The FeN thin films were prepared at the fixed substrate temperature 200°C under different nitrogen flow rates. The crystal structures, physical morphologies, and surface roughness of the prepared FeN thin films were characterized by the grazing-incident X-ray diffraction (GIXRD), field-emission scanning electron microscopy (FE-SEM), and atomic force microscopy (AFM), respectively. From the results, the GIXRD patterns indicated the change in the film orientations from ξ-Fe2N to γ”-FeN along with the increased nitrogen flow rates. In addition, the FE-SEM micrographs demonstrated the decrease of the FeN film thickness. The effects of the nitrogen flow rates towards the crystallinity, the physical morphology, and surface roughness of the thin films were investigated and discussed.

Modification of Magnetic Characteristics of Polycrystalline Nife Films at the Irradiation Laser Pulses and Formation of Regular Structure of Magnetic Nanoislands

This paper presents the results of experimental measurements of magnetic properties of thin NiFe films after irradiation with nanosecond laser pulses. The results showed that there is an optimal energy in the laser pulse, at which the magnetic film characteristics vary the most. For the film of 500 nm thick at magnetic susceptibility m for a Nd:YAG laser at l=1064 nm increased almost 3 times, and the coercive force Hc decreased about two. For the excimer laser at l=248 nm, we have received an increase in the magnetic susceptibility m almost ten times while a decrease in the coercive force Hc was 5-6 times. The results suggest the importance of non-thermal effects of laser radiation in the process of changing the structure and magnetic properties of magnetic films. All this shows that with the help of laser radiation it is possible to form a regular structure of nanoislands in thin magnetic nanocrystalline films.

Spin galvanic effect at the conducting SrTiO3 surfaces

The (001) surface of SrTiO3 were transformed from insulating to conducting after Ar+ irradiation, producing a quasi two-dimensional electron gas (2DEG). This conducting surface layer can introduce Rashba spin orbital coupling due to the broken inversion symmetry normal to the plane. The spin splitting of such a surface has recently been demonstrated by magneto-resistance and angular resolved photoemission spectra measurements. Here, we present experiments evidencing a large spin-charge conversion at the conducting surface. We use spin pumping to inject a spin current from NiFe film into the surface, and measure the resulting charge current. The results indicate that the Rashba effect at the surface can be used for efficient charge-spin conversion. The conversion efficiency measured by the ratio of the spin flow density to the charge flow density is about 0.23 nm. This value is on the same order as the interfaces in metallic multilayers and 2DEG of LaAlO3/SrTiO3. In this point of view, the conducting SrTiO3 surface provides another solution for oxide spintronics.

Highly oriented epitaxial (α′′+α′)-Fe16N2 films on α-Fe(001) buffered MgAl2O4(001) substrates and their magnetization

We grew epitaxial films of α′-Fe8N by molecular beam epitaxy on α-Fe buffered MgO(001) and MgAl2O4(MAO)(001) substrates. The lattice mismatches of these substrates with α′-Fe8N are −4.0% and 0.08%, respectively. We discuss effects on the crystal orientation of the grown layers in terms of the substrate material and the thickness of an α-Fe buffer layer. The 3-nm-thick α-Fe buffer layer on MAO was compressively strained and provided a more suitable in-plane lattice constant for growth of α′-Fe8N. The full width at half maximum of the x-ray ω-scan rocking curve peak intensity of the α′-Fe8N(002) plane was 0.09° when it was formed on α-Fe(3nm)/MAO. This value was much smaller than that obtained for α-Fe(3nm)/MgO samples (1.85°). (α′′+α′)-Fe16N2 films, which had partly ordered N-sites, were formed by post-annealing of the α′-Fe8N films on α-Fe/MgO(001) and α-Fe/MAO(001) at 165°C. The degree of N-site ordering reached 0.08 after 20h for (α′′+α′)-Fe16N2 grown on α-Fe(3nm)/MAO(001). The saturation magnetizations were approximately 1650emu/cm3, regardless of the annealing time and substrate used. The saturation field of the (α′′+α′)-Fe16N2 films increased by the post-annealing.