H. N. Oredson

Measurement of the Easy‐Axis and Hk Probability Density Functions for Thin Ferromagnetic Films Using the Longitudinal Permeability Hysteresis Loop

A new method of measuring the inhomogeneity in easy‐axis orientation and in the magnitude of the uniaxial anisotropy in thin ferromagneticfilms has been found. Correlation between the two can also be obtained by this method. Apparatus has been built to measure the probability density of finding a part of the film with a given easy‐axis orientation, as well as a given Hk , and to plot this probability density function on an x‐y recorder. The Hk probability density function is found to be symmetric, contrary to popular belief. Measured values of easy‐axis inhomogeneity agree within experimental error with values obtained by conventional methods. Previous methods of measuringHk inhomogeneity, based on either radio‐frequency resonance linewidth or on reversible limit, are discussed and found to be questionable.

Local Regions with Biaxial Anisotropy in Thin Polycrystalline Ferromagnetic Films with Uniaxial Anisotropy

Deposited thin ferromagnetic films with uniaxial anisotropy have local regions with admixed biaxial anisotropy, although there is no biaxial component to the average anisotropy field of the whole film. Two methods are presented for measuring the biaxial contribution to the inhomogeneity in the anisotropy field. The effect of this biaxial inhomogeneity is investigated theoretically and found to explain much puzzling data: the various negative anisotropy effects, the relationship between α90 and Δ90 (the inhomogeneity in orientation and in magnitude of the anisotropy, respectively), the internal biasing field, the compositional dependence of the inhomogeneity and negative anisotropy, the deposition substrate temperature dependence of negative anisotropy, the position of the negative anisotropy peak, the shape of the probability density function for films with both large and small inhomogeneities, the ratio of positive to negative anisotropy, and the skew of the Δ90 probability density curves. Theoretical sw...

Transition between Bloch and Néel Walls

The transition between Neel and Bloch walls is shown to be gradual rather than abrupt, resulting in a wall with Neel and Bloch components superposed. For films thick enough so that such a transition can occur, the wall is a pure Neel wall if φf, the acute angle between the wall normal in the plane of the film and the magnetization at the edge of the wall (half the angle between the magnetization vectors in the two domains separated by the wall) is less than a critical value. For φf larger than that critical value but less than π/2, the wall contains both Neel and Bloch components. When φf is π/2, i.e., a 180° wall, the wall is of the pure Bloch type; however, this situation is often unstable, resulting in a transition wall with the Neel component reversing periodically in distance along the wall in a crosstie‐like structure. These conclusions are supported by Lorentz micrographs.

Magnetization Creep of Cross‐Tie Walls

Cross‐tie walls are known to have lower creep thresholds than non‐cross‐tie walls. This indicates that a special creep mechanism exists for cross‐tie walls but does not for non‐cross‐tie walls. Such a mechanism is described: In the presence of a pure dc easy‐axis field the magnetization antiparallel to the field buckles more, while the magnetization on the other side of the wall buckles less. This causes magnetostatic charges on the wall which cause the main wall itself to buckle in a zig‐zag fashion with bends at the Bloch lines between the cross‐ties and at the Bloch lines in the cross‐ties. When a hard‐axis field is applied, the Bloch lines between the (fixed) cross‐ties move along the wall, a situation resulting in magnetostatic charges causing the apex of the zig‐zags to move along with the Bloch lines. This distorts the symmetrical zig‐zags into asymmetrical ones having alternately long and short legs. At this point, each segment of the zig‐zag lies at a different angle from the easy axis than it di...

Calculations of the Structure of Néel, Bloch, and Intermediate Walls and the Influence of Their Stray Fields on Bitter Powder Patterns

A comprehensive computer investigation of Neel, Bloch, and intermediate walls in films 200 A to 2000 A thick was made by letting the computer adjust the magnetization direction in small segments of the wall until the sum of the torques on the magnetization in each segment was minimal. Many parallel walls lying parallel to the easy axis were assumed to exist as in a real film during hard axis remagnetization. The Neel wall width was found to be approximately proportional to φf−1, where φf is the angle between the magnetization in the center of the domain and the hard axis. Intermediate walls were found to exist for 90° > φf > φc, where φc is a critical angle dependent on thickness. For φf ≤ φc pure Neel walls exist and only for φf = 90° can pure Bloch walls exist. Once the magnetization distribution was known, the stray field from the wall was calculated. For Neel walls, the magnitude of the stray field was found to increase slightly as the film thickness increased (500 < T < 2000 A) for a fixed‐angle wall...

Steady‐State B‐H Hysteresigraph for Thin Ferromagnetic Films Using a Torque Magnetometer

An ordinary torque magnetometer can be used to obtain dc B‐H hysteresis loops of thin ferromagnetic films simply by replacing the rotating set of Helmholtz coils with a stationary set of three mutually perpendicular coils and by mounting the film edgewise on the suspension. When the magnetometer is used in this mode, the signal and signal‐to‐noise ratio are directly proportional to the field normal to the film; thus one can measure the anisotropy field of film samples many times smaller than previously possible with a torque magnetometer, as well as the various loop properties previously not obtainable with a torque magnetometer. All sections of the film are sensed uniformly in marked contrast to coil sensing devices. Unlike the usual ac B‐H hysteresigraph, the torque B‐H hysteresigraph is not affected by eddy currents in metal substrates.

Inhomogeneity in the Magnitude of the Anisotropy Field in Thin Nickel‐Iron Films

The inhomogeneity in the magnitude of the anisotropy field of vacuum‐deposited nickel‐iron films has been measured as a function of film composition, inhomogeneity in composition, substrate temperature, time interval of deposition, and diameter of the film. The part of the inhomogeneity in the magnitude of Hk due to the product of the magnetostriction and the nonisotropic strain is found to be twice the part of the inhomogeneity in the direction of Hk due to this cause, as predicted by Crowther. Part of the inhomogeneity in magnitude but not in direction of Hk is attributed to nonuniformity in substrate temperature, and part to magnetocrystalline anisotropy.

Effect of Magnetostatic Interactions on the Real and Imaginary Parts of the Transverse Biased Permeability in Thin Ni‐Fe Films

It has been found that the rapid fall off of μ′ (the imaginary part of the permeability) on the low‐field side of μ′ vs HT is due to the formation of locking domains, and that in this region the probability density function is not proportional to μ′.This fall off can be predicted from μ (the real part of the permeability) by assuming that the fall off in μ is due entirely to the effects of locking domains. When this is done, and Δ90, the inhomogeneity in anisotropy magnitude, is determined from the corrected probability density curves the relationship arc sin Δ90=4α90 is found nearly to hold, but a coefficient of 5 gives a better fit to the data. It is suggested that the 5 comes from anisotropies of order higher than 2 due to higher‐order interactions. These interactions are responsible for triaxial anisotropy introduced by etching a polycrystalline film into small triangles, and for higher‐order anisotropies found by etching small rectangles.A theoretical expression for the permeability has been found as...

Lorentz Microscopy in Polycrystalline Ni–Fe Films 2000‐Å Thick

The use of Lorentz microscopy has been reported on polycrystalline films 1450 A and thinner. In this paper the use of Lorentz microscopy on polycrystalline films in the thickness range between 1200 and 2000 A is presented, and some of the more unexpected results discussed. The intermediate wall cross‐ties observed in a 1450‐A film as reported earlier were not found in a low‐dispersion Ni–Fe film 2000±50 A thick (the thickness measurements were made carefully with torque magnetometer, Tolansky apparatus, and loop checker with calibrated standard), but when the film was annealed in vacua so that the crystallites became enlarged and Hc and α90 increased, the intermediate wall cross‐ties were observed. The conclusion is that the biaxial inhomogenities caused the 180° Bloch wall to become a 170° wall in some segments and a 190° wall in others. Then, according to intermediate wall theory, these non‐180° walls must have a Neel component which alternates along the length of the wall, thereby causing the cross‐tie...

Anisotropy, Inhomogeneity, and Magnetization Creep in Plated Wire

Plated wire, unlike vacuum‐deposited films, can be made to have a reversible limit five times higher than predicted by the Stoner‐Wohlfarth theory. To examine this problem the inhomogeneity in the magnitude of the anisotropy was measured via a variation of the transverse biased permeability method; the measurements indicate the presence of small regions with very high anistropy embedded in a matrix with low anisotropy. HC was found to increase about 20% with increasing field, indicating that these tiny regions have coercivity many times that of the rest of the film. However these regions can be switched with a small easy‐axis field if the magnetization in the rest of the film is rotated with a transverse field; this creates a stray field at the domain boundaries that aids switching. Measurements of creep switching and of switching using a single hard‐axis pulse are presented supporting this view.