Polycrystalline Permalloy Films Research Articles

Pinning and gyration dynamics of magnetic vortices revealed by correlative Lorentz and bright-field imaging

Topological magnetic textures are of great interest in various scientific and technological fields. To allow for precise control of nanoscale magnetism, it is of great importance to understand the role of intrinsic defects in the host material. Here we use conventional and time-resolved Lorentz microscopy to study the effect of grain size in polycrystalline permalloy films on the pinning and gyration orbits of vortex cores inside magnetic nanoislands. To assess static pinning, we use in-plane magnetic fields to shift the core across the island while recording its position. This enables us to produce highly accurate two-dimensional maps of pinning sites. Based on this technique, we can generate a quantitative map of the pinning potential for the core, which we identify as being governed by grain boundaries. Furthermore, we investigate the effects of pinning on the dynamic behavior of the vortex core using stroboscopic Lorentz microscopy, harnessing a new photoemission source that accelerates image acquisition by about two orders of magnitude. We find characteristic changes to the vortex gyration in the form of increased dissipation and enhanced bistability in samples with larger grains.

Magnetization loops of submicron ferromagnetic permalloy dot arrays

Polycrystalline Permalloy films were deposited onto silicon substrates by rf sputtering. Arrays of cylindrical dots with diameter d between 150 and 550 nm were patterned by optical holographic lithography. The dots were arranged on a square lattice with period p, ranging from 310 to 1030 nm. The height h of dots, given by the thickness of the permalloy films, ranged between 10 and 80 nm. The shape of the magnetization loops of the dot arrays depends strongly on the aspect ratio r=d/h. The in-plane saturation field is given by the intrinsic demagnetization field of single, isolated dots. The dipolar interaction between the dots is negligible. For cylinders with elliptical base plane, the magnetization loop depends on the field direction. The measured magnetization loops of the arrays are the superposition of the magnetization loops of the single dots without dipolar interaction. A model of magnetization reversal in applied fields is given.

Cross-tie walls and magnetic singularities on the surface of permalloy films (abstract)

An understanding of the surface magnetic microstructure of thin polycrystalline permalloy films is important for the development of improved magnetoresistive sensors. Scanning electron microscopy with polarization analysis (SEMPA) was used to image the surface magnetic domain structure of permalloy films in ultrahigh vacuum. The SEMPA system uses a compact Mott electron spin polarimeter with a Th foil (operating at 25 keV) that has been attached to the back of a hemispherical energy analyzer. Two orthogonal in-plane components of the electron spin polarization were measured to obtain magnetic domain images with excellent contrast. 350 Å Ni83Fe17 films, deposited by Honeywell-Micro Switch using dc magnetron sputtering, were studied. The samples were demagnetized along the easy axis by an ac magnetic field with decreasing amplitude. Using SEMPA, zigzag domain walls separating two large approximately head-on domains were observed. Cross-tie walls were observed with a periodic vortex structure along the straight edges of the zigzag domain walls. The cross-tie walls occur at the points where the magnetization is reversed by 180° across the straight edges of the wall. At high magnification, the elliptical and hyperbolic singularities at the cross-tie walls were clearly observed. In addition, the Néel part and the Bloch part of the cross-tie were distinguished This is a detailed study of cross-tie walls on sputter deposited thin permalloy films using SEMPA and our results are in good agreement with theoretical calculations.

Epitaxial growth and characterization of (100) and (110) permalloy films

High-quality, single-crystal fcc (100) and (110) permalloy films were epitaxially grown on MgO(100) and MgO(110) substrates, respectively, while polycrystalline structures were established on Si(111) substrates. The excellent crystal growth of the permalloy films on the MgO substrates was evidenced by fine streaks in reflection high energy electron diffraction and X-ray diffraction. Low-temperature magnetic hysteresis measurements show that the polycrystalline permalloy films grown on the silicon substrates are magnetically hard with coercive fields of ∼ 15–20 Oe. On the other hand, (100) and (110) permalloy films are magnetically softer with coercive fields of about 1 and 6 Oe, respectively.

Beam-broadening due to inelastic scattering of fast electrons in polycrystalline permalloy films

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Evidence for a Dependence of the Long-Range Order Developed in Ni–Fe Thin Films on Grain Size

Uniaxial anisotropy energy measurements on polycrystalline Permalloy films following 3He irradiation indicate that the development of NiFe and Ni3Fe long-range order in thin films has a strong dependence on the grain size. The results of these measurements show that Ku due to the development of NiFe long-range order is at least a factor of four larger in the larger-grain-size films (grain size ∼4000 Å) than in the smaller-grain-size films (grain size ∼200–300 Å); films with compositions around Ni3Fe show a much faster decrease in Ku, due to the development of Ni3Fe long-range order, in the larger-grain films than in the smaller-grain films. Such a grain-size dependence is in part attributed to the fact that the defects responsible for the ordering process can participate in more atomic replacements in the larger grains than in the smaller grains before they are annihilated at grain boundaries. Thus, the larger the crystallite, the greater the amount of long-range order developed during irradiation.

M-Induced Anisotropy in Electrodeposited Polycrystalline Permalloy Films

M-induced uniaxial anisotropy, Ku, is investigated for electrodeposited polycrystalline permalloy films in a range from 40 to 100% Ni. The observed Ku is composed of two terms which are respectively sensitive and insensitive to annealing treatment below 200°C. These terms correspond to the terms due to magnetostrictively induced anisotropic strain and directional ordering of iron pairs, respectively. The latter separated from the observed Ku by annealing below 200°C is in good agreement with the data obtained for bulk permalloy. The observed Ku can be quantitatively fitted to the sum of the former deduced theoretically and the latter obtained experimentally except for near 100% Ni films, if inhomogeneous composition of film along the thickness is evaluated. In a case of near 100% Ni, the values of Ku are considerably smaller than that deduced from the theoretical model. This may be related to the high angular dispersion of easy axes of the films.

Evidence of Long-Range Order in 3He-Irradiated Permalloy Films

Room-temperature measurements of the uniaxial anisotropy energy Ku have been made on a series of vacuum-deposited thin polycrystalline Permalloy films following 3He particle irradiations at temperatures of 125°, 200°, 250°, and 300°C. The results indicate the development of two types of long-range order, in addition to the so-called Fe–Fe atom pair short-range directional ordering: an NiFe-type order similar to that reported for neutron-irradiated bulk Permalloy, which enhances Ku for compositions around 50% Ni–50% Fe, and the well-known Ni3Fe-type order, which develops at the expense of Fe–Fe atom pair ordering, for compositions in the neighborhood of 75% Ni–25% Fe. The maximum value of Ku obtained due to NiFe long-range order is at least an order-of-magnitude less in the thin film than in the neutron-irradiated bulk Permalloy; also, the irradiation temperature at which Ku ceases to be enhanced by the development of NiFe long-range order in the thin film is well below the ordering temperature reported for bulk samples. For films with compositions off the NiFe and Ni3Fe stoichiometry, the temperature dependence of Ku was found to be in good agreement with the (Tc-Ta) behavior as suggested for Fe–Fe atom pair short-range directional ordering.

Lorentz Microscopic Examinations of Polycrystalline Films with Biaxial Properties

Polycrystalline Permalloy films with biaxial properties have been examined in detail by Lorentz microscopic techniques. The stable configuration for type I films, consisting of an array of discrete small squares, is a simple arrangement of four domains with complete flux closure and 90° walls along the diagonals. At high fields, a single domain configuration is obtained in which the spins along the edge deviate from the field direction due to the ``boundary anisotropy.'' The magnetization curling distances D along the edge are found to be proportional to MT/H, independent of the dimension of the square. The critical field HA for the flux closure structure is proportional to MT/L, where T is the film thickness. For type II films, composed of discrete thick squares superimposed on an isotropic continuous film, the conditions for flux closure are the same as for type I films. Type III continuous films with nonmagnetic square voids have also been examined, and their biaxial properties are similar to those of type II films.