Permalloy Films Research Articles

Specific features of observing magnetization inhomogeneities on the surface of permalloy thin films by means of highly sensitive magnetic-force-microscopy probes

The results of studying regions of inhomogeneous magnetization on the surface of permalloy thin films with the use of fabricated highly sensitive probes of magnetic force microscopy (MFM) are presented. The technological features of manufacturing MFM probes with a high sensitivity to magnetic-field gradient are analyzed. Regions of ordering of the vertical component of the magnetic field are revealed, and domain walls are visualized in the thin films under study. Nanoscale measurements of the domain-wall thicknesses are performed.

Visualization and Interpretation of Magnetic Configurations Using Magnetic Charge

The calculation and graphic imaging of magnetic charge density provides fresh insights into old problems in micromagnetics. (1) The full saturation magnetization $M_{\mathrm{s}}$ of polycrystalline iron can be used in a transformer when magnetic charge at the grain boundaries suppresses the adverse effects of anisotropy. A single domain wall separates two regions of opposite magnetization $\vert {{\mathbf M}} \vert = M_{\mathrm{s}}$ in toroidal geometry. This is a three-dimensional expression of the ripple effect, long known in polycrystalline thin films of Permalloy. (2) Helical patterns of ${{\mathbf M}}$ result from interaction of regions of opposite magnetic charge developed by the introduction of some $\nabla \cdot {{\mathbf M}}$ when the exchange energy primarily invokes $\nabla \times {{\mathbf M}}$ to accommodate to the boundary conditions set by geometry and surface charge density ${{\mathbf n}} \cdot {{\mathbf M}}$ . The helical structures resolve 40-year-old mysteries in the appearance of periodic stripe domains associated with the propagation of edge solitons in a hexagonal [1 1 1] oriented iron whisker.

Effect of Perpendicular Anisotropy and Eddy Currents on the Microwave Performance of Single-Layer and Multi-Layer Permalloy Films

In this letter, microwave and magnetostatic measurements of single- and multilayer Permalloy films are made to find physical reasons causing the decrease of the microwave permeability of the films with increase in film thickness. The permeability is measured in the frequency range of 0.1–10 GHz by a coaxial technique. From the measured permeability, a contribution to the effective field from the magnetoelastic effect is determined. The decrease of microwave permeability in thick films is shown to arise mainly due to the perpendicular magnetic anisotropy rather than from the effect of eddy currents, as is frequently suggested. It is also shown that both these effects are reduced in multilayer films comprising alternating magnetic and dielectric layers. Therefore, such films are promising materials for developing magnets that possess high permeability at high frequencies.

Stability of standing spin wave in permalloy thin film studied by anisotropic magnetoresistance effect

We have investigated the stability for the resonant spin precession under the strong microwave magnetic field by a specially developed detection method using the anisotropic magnetoresistance effect. The electrically separated excitation and detection circuits enable us to investigate the influence of the heating effect and the nonuniform spin dynamics independently. The large detecting current is found to induce the field shift of the resonant spectra because of the Joule heating. From the microwave power dependence, we found that the linear response regime for the standing spin wave is larger than that for the ferromagnetic resonance. This robust characteristic of the standing spin wave is an important advantage for the high power operation of the spin-wave device.

Dependence of transverse magnetothermoelectric effects on inhomogeneous magnetic fields

Transverse magneto-thermoelectric effects are studied in permalloy thin films grown on MgO substrates. We find that small parasitic magnetic fields below 1 Oe can produce artifacts of the order of 1 % of the amplitude of the anisotropic magneto-thermopower which is also detected in the experiments. The measured artifacts reveal a new source of uncertainties for the detection of the transverse spin Seebeck effect. Taking these results into account we conclude that the contribution of the transverse spin Seebeck effect to the detected voltages is below the noise level of 20 nV.

Thermal and electrical conductivity of approximately 100-nm permalloy, Ni, Co, Al, and Cu films and examination of the Wiedemann-Franz Law

We present measurements of thermal and electrical conductivity of polycrystalline permalloy (Ni-Fe), aluminum, copper, cobalt, and nickel thin films with thickness $l200$ nm. A micromachined silicon-nitride membrane thermal-isolation platform allows measurements of both transport properties on a single film and an accurate probe of the Wiedemann--Franz (WF) law expected to relate the two. Through careful elimination of possible effects of surface scattering of phonons in the supporting membrane, we find excellent agreement with WF in a thin Ni-Fe film over nearly the entire temperature range from 77 to 325 K. All other materials studied here deviate somewhat from the WF prediction of electronic thermal conductivity with a Lorenz number, $L$, suppressed from the free-electron value by $10%\phantom{\rule{4.pt}{0ex}}\text{to}\phantom{\rule{4.pt}{0ex}}20%$. For Al and Cu we compare the results to predictions of the theoretical expression for the Lorenz number as a function of $T$. This comparison indicates two different types of deviation from expected behavior. In the Cu film, a higher than expected $L$ at lower $T$ indicates an additional thermal conduction mechanism, while at higher $T$ lower than expected values suggests an additional inelastic scattering mechanism for electrons. We suggest the additional low-$T\phantom{\rule{4pt}{0ex}}L$ indicates a phonon contribution to thermal conductivity and consider increased electron-phonon scattering at grain boundaries or surfaces to explain the high-$T$ reduction in $L$.

Radiative damping in waveguide-based ferromagnetic resonance measured via analysis of perpendicular standing spin waves in sputtered permalloy films

The damping $\alpha$ of the spinwave resonances in 75 nm, 120 nm, and 200nm -thick Permalloy films is measured via vector-network-analyzer ferromagnetic-resonance (VNA-FMR) in the out-of-plane geometry. Inductive coupling between the sample and the waveguide leads to an additional radiative damping term. The radiative contribution to the over-all damping is determined by measuring perpendicular standing spin waves (PSSWs) in the Permalloy films, and the results are compared to a simple analytical model. The damping of the PSSWs can be fully explained by three contributions to the damping: The intrinsic damping, the eddy-current damping, and the radiative damping. No other contributions were observed. Furthermore, a method to determine the radiative damping in FMR measurements with a single resonance is suggested.

Achieving Isotropic Permeability for Integrated Inductors

Partially perpendicular anisotropy in Permalloy thin films was found to produce reliably isotropic permeability in the plane of the film. Isotropic permeability is especially desirable for integrated closed-loop magnetic solenoid inductors in order to create an efficient return path for the flux. Material properties for the Permalloy were verified through hysteresis loop, permeability, and magneto-optic Kerr effect microscopy and utilized to simulate the behavior of the inductor. Laminated Permalloy films with partially perpendicular anisotropy yielded a relatively high permeability value of 860 isotropically in-plane. Simulation results indicate that, depending on the permeability, in-plane closure can yield $2\times $ the inductance or more compared with a rectangular magnetic core with the same permeability. The enhancement due to closure was seen to depend heavily on permeability, increasing at a rate of 6.8 per unit permeability.

Magnetic coupling of vortices in a two-dimensional lattice

We investigated the magnetization reversal of magnetic vortex structures in a two-dimensional lattice. The structures were formed by permalloy (Py) film deposition onto large arrays of self-assembled spherical SiO2-particles with a diameter of 330 nm. We present the dependence of the nucleation and annihilation field of the vortex structures as a function of the Py layer thickness (aspect ratio) and temperature. By increasing the Py thickness up to 90 nm or alternatively by lowering the temperature the vortex structure becomes more stable as expected. However, the increase of the Py thickness results in the onset of strong exchange coupling between neighboring Py caps due to the emergence of Py bridges connecting them. In particular, we studied the influence of magnetic coupling locally by in-field scanning magneto-resistive microscopy and full-field magnetic soft x-ray microscopy, revealing a domain-like nucleation process of vortex states, which arises via domain wall propagation due to exchange coupling of the closely packed structures. By analyzing the rotation sense of the reversed areas, large connected domains are present with the same circulation sense. Furthermore, the lateral core displacements when an in-plane field is applied were investigated, revealing spatially enlarged vortex cores and a broader distribution with increasing Py layer thickness. In addition, the presence of some mixed states, vortices and c-states, is indicated for the array with the thickest Py layer.

Top-down control of dynamic anisotropy in permalloy thin films with stripe domains

Permalloy films with different thickness were prepared for comparison, and the dynamic properties of the stripe domains were investigated by using a vector network analyzer ferromagnetic resonance (VNA-FMR) technique. A top-down approach was used to control the thickness of the films. In the measurement of permeability spectra, an applied magnetic field Happ was employed parallel and perpendicular to the stripe domains, respectively. Dynamic pseudo-uniaxial anisotropy fields are derived. In order to explain the anisotropy variation of permalloy films, a theoretical model has been proposed, and the result shows that the pseudo-uniaxial anisotropy can be attributed to the contributions of saturation magnetization, exchange interaction, perpendicular anisotropy, and film thickness. In addition, with the increase of Happ, the stripe domains gradually rotate towards the direction of the applied field, which can be clearly observed by magnetic force microscopy. Moreover, the dynamic permeability spectra exhibit a rhythmic variation dependent on the rotation of the stripe domains, i.e. the relationship between the rotational stripe domains and resonance frequency.

Large magnetoelastic coupling in NiFe/LiCsSO4 and NiFe/KH2PO4 heterostructures

Temperature dependences of magnetization of 5 nm permalloy films deposited on LiCsSO4 and KH2PO4 ferroelastic substrates were investigated by SQUID magnetometry. Results showed a significant change of thin film longitudinal magnetization at the phase transition temperature of substrates. The evolution of magnetization with temperature was simulated using nonlinear thermodynamic approach and quantitatively compared with experimental data. We conclude that observed magnetization drop is a consequence of a magnetoelastic effect. In NiFe/KH2PO4 sample this effect is additionally interfered by magnetostatic interactions coming from a substrate fine domain pattern.

Saturation of attenuation length of spin waves in thick permalloy films

The permalloy (Py) thickness dependence of the magnetostatic spin wave (MSSW) propagation was investigated. Large-group velocity is realized for thick Py films and the MSSW can propagate more than 150 µm. However, attenuation length hardly changes for samples with a Py thickness of more than 100 nm, despite the increasing group velocity with increasing thickness. The eddy current effect decreases the wave channel thickness and it could cause the damping enhancement due to intralayer spin pumping in thick Py films.

Effect of the shape and lateral dimensions on the magnetization reversal in permalloy nanofilms

Kinetics of magnetization reversal in patterned permalloy films with nanometer thickness is studied by means of magneto-optic visualization technique, Bitter technique and micromagnetic simulations. The reversal modes and critical dimensions at which a change of the magnetization scenario in the structures takes place are determined. The experimentally observed critical dimensions are shown to differ from the calculated ones by two orders of magnitude. The crucial influence of the ferro-fluid on magnetic patterns and magnetization reversal parameters is found.

Nanoscale switch for vortex polarization mediated by Bloch core formation in magnetic hybrid systems.

Vortices are fundamental magnetic topological structures characterized by a curling magnetization around a highly stable nanometric core. The control of the polarization of this core and its gyration is key to the utilization of vortices in technological applications. So far polarization control has been achieved in single-material structures using magnetic fields, spin-polarized currents or spin waves. Here we demonstrate local control of the vortex core orientation in hybrid structures where the vortex in an in-plane Permalloy film coexists with out-of-plane maze domains in a Co/Pd multilayer. The vortex core reverses its polarization on crossing a maze domain boundary. This reversal is mediated by a pair of magnetic singularities, known as Bloch points, and leads to the transient formation of a three-dimensional magnetization structure: a Bloch core. The interaction between vortex and domain wall thus acts as a nanoscale switch for the vortex core polarization.

Fabrication of Electrodeposited Ni–Fe Cantilevers for Magnetic MEMS Switch Applications

Electrodeposited alloys of nickel and iron are prone to suffer from the development of undesirable-stress gradients resulting from process steps and heat treatments following deposition. As a result, the yield associated with devices using permalloy (Ni–Fe, 80%–20%) films can be severely compromised. Although electrodeposition recipes have been formulated that greatly reduce stress in Ni–Fe films, any stress gradients through the thickness of permalloy can pose considerable problems during the release of cantilevers. The development of such gradients is typically the direct result of the selected design and process architecture and/or problematic material choices. This paper presents an improved architecture for electroplated Ni–Fe freestanding microcantilever structures. This architecture minimizes the development of stress gradients in cantilever beams and facilitates the use of suspended permalloy cantilever structures in magnetically actuated microelectromechanical systems (MEMS) switches. [2014-0170]

Morphology, structure and magnetic study of permalloy films electroplated on silicon nanowires

We report the effect of deposition potential on the morphology, structure and magnetic properties of Ni80Fe20 (Permalloy: Py) deposits, elaborated by electrochemical process onto silicon nanowires (SiNWs). The morphology of SiNWs and Py/SiNWs were performed with scanning electron microscopy (SEM). The SEM micrographs reveal the formation of SiNWs and clearly show a change in the morphology with the deposition potential. The analysis of X-ray diffraction spectra shows a change in the texture with the deposition potential. The grain size, the lattice parameter and the strain were studied as a function of the deposition potentials. From hysteresis loops, we have shown that the magnetization easy axis is the plane of the samples.

Ferromagnetic resonance of Py deposited on ZnO grown by molecular beam epitaxy

We report on the growth of a high-quality single crystal ZnO film on an a-plane sapphire substrate by plasma-assisted molecular beam epitaxy and the properties of a sputtered permalloy (Py) film on the ZnO investigated by ferromagnetic resonance. The results show that one can obtain the Py with a reasonable quality on ZnO, which is expected to provide a testbed system for the investigation of the spin current-related phenomena in materials with a weak spin–orbit interaction.

Detection of Spin Waves in Permalloy Using Planar Hall Effect

Rectification of microwave oscillations of magnetization in a permalloy film is realized using planar Hall effect. Two different rectified signals are obtained: a signal from the linearly excited uniform magnetization precession at the frequency of the external pumping and a signal from the pairs of contra-propagating short-wavelength spin waves parametrically generated at a half of the pumping frequency. The second, most unusual, rectified signal is caused by the uniform component of the dynamic magnetization created due to the interference of the phase correlated pairs of parametric spin waves.

Thickness dependence of spin wave nonreciprocity in permalloy film

The amplitude of the spin wave (magnetostatic surface spin wave) depends on the propagation direction. Nonreciprocity hardly depends on the permalloy (Py) thickness between 100 and 300 nm, and it is determined by the spin wave resonant frequency. The nonreciprocal parameter decreases with increasing frequency, resulting in the large amplitude asymmetry at high frequencies, which is well supported by a result of theoretical analysis. The nonreciprocity of a 25-nm-thick-film, however, is lower than those of thicker films. This is qualitatively explained by a schematic model based on the origin of nonreciprocity.

Magnetic structures of permalloy film microspots

Magnetic structures of permalloy film microspots