M. S. Cohen

Anisotropy in Permalloy Films Evaporated at Grazing Incidence

The magnetic and optical anisotropies observed in oblique-incidence Permalloy films show anomalous behavior for incidence angles greater than 60° (grazing incidence). In particular, the easy axis and the direction of greatest optical absorption are parallel to the vapor beam for incidence angles greater than about 70°, while they are perpendicular to this direction for smaller angles. The anomalies for grazing-incidence films are caused by a shape anisotropy differing from that previously found in lower angle oblique-incidence films. This new shape anisotropy is attributed to the elongation in the beam direction of the particles composing the crystallite chains which are found in all oblique-incidence films; such shape anisotropy has been directly observed by electron microscopy.

Anomalous Magnetic Films

A study of three types of anomalous nickel‐iron magnetic films has been made: high coercive‐force films, which have unusually large hysteresigraph values of wall and rotational coercive forces; rotatable initial‐susceptibility films, for which the value of the initial susceptibility as found from low‐field hysteresigraph measurements is determined by previously‐applied high magnetic fields; and mottled films, whose high‐field hysteresis loops show low remanence and a high wall coercive force, and whose Bitter patterns have a spotted or mottled appearance. Anomalous films have been produced by a variety of techniques. In general, for a given technique, a mild treatment yielded high coercive‐force films, a moderate treatment gave rotatable initial‐susceptibility films, and a drastic treatment produced mottled films. The techniques used were evaporation at slow rates at high substrate temperatures, evaporation on aggregated metal deposits, electroplating on annealed gold films, and annealing normal films. Ac...

WAVE OPTICAL ASPECTS OF LORENTZ MICROSCOPY

The customary defocused and Foucault modes of Lorentz microscopy of magnetic films are usually described in terms of geometric optics. However, Wohlleben has shown that geometric optics has a restricted range of validity; a more fundamental approach is provided by wave optics. The defocused and Foucault modes may be discussed in terms of wave optics, and for the defocused mode, it can be shown explicitly that the geometric theory is simply the first approximation to the wave‐optics theory. Consideration of wave optics also leads to the proposal of two additional modes of Lorentz microscopy: Zernike phase‐contrast and interference microscopy; these modes cannot be described on the basis of geometric optics. The most fundamental problems in magnetic films which are amenable to study by Lorentz microscopy are investigations of the fine structures of domain walls and magnetization ripple. These problems are discussed in terms of wave optics for all four modes of Lorentz microscopy; in particular, the intensit...

Influence of Anisotropy Dispersion on Magnetic Properties of Ni–Fe Films

The dispersion of the magnetic anisotropy was monotonically increased in several Ni–Fe films by subjecting them to increasingly severe heat treatments, and the room‐temperature magnetic properties were monitored after each anneal by Lorentz electron microscopy. A gradual transition towards increasingly anomalous properties in a single film was thereby observed. An increase, with annealing, of the magnetization ripple intensity, the measured angular anisotropy dispersion, and the wall coercive force Hw was found. After sufficient annealing, locking was seen upon reversal parallel to the easy axis; further annealing increased the locking‐wall density. The anisotropy field Hk monotonically decreased with annealing until the uniaxial character of the film was lost and it became a rotatable‐initial‐susceptibility (RIS) film. Reversal at azimuths away from the easy axis proceeded by labyrinth propagation after the early anneals, but by partial rotation after more severe annealing.

Anisotropy and Inversion in Permalloy Films

A phenomenological model of field-induced and oblique-incidence anisotropy in Permalloy films is proposed. It is assumed that the field-induced structure does not introduce any local spatial dispersion in the macroscopic magnetization M; the opposite is assumed for oblique-incidence structure. In addition it is assumed that the oblique-incidence dispersion is anisotropic, being least when M is perpendicular to the depositing beam.Support for the model comes from anisotropies, found for oblique-incidence films only, in the following measurements: resonance line width, transmission of polarized light, and resistivity. A primary success of the model is the prediction of a correlation between anisotropy and inversion (Hw/Hk>1). Inverted films can be made by crossing the field-induced and oblique-incidence anisotropies at 90°. Such films exhibit a “locked” state in which opposite rotation of M in local regions occurs; this implies centers of spatial dispersion and provides the connection with oblique-incidence...

Domain Wall Profiles in Magnetic Films

An inversion procedure for determining a domain wall profile from the associated Lorentz‐microscopy electron‐density distribution is discussed. This procedure is verified by application to computer‐simulated electron‐density distributions derived from assumed wall profiles. The experimental realization of the inversion procedure is demonstrated for high‐resolution Lorentz micrographs of films ranging in thickness from 100 to 500 A; unreliable profiles were obtained for thicker films. The importance of correcting for the effects of nonmagnetically scattered electrons is emphasized; these effects increase with increasing sample thickness and defocussing distance. The experimental domain wall profiles exhibit first a rapid, then a slow increase of wall angle with distance from the wall center. Wall widths derived from these profiles are consistent with theoretical predictions if corrections are made for nonmagnetic scattering; if such corrections are not made, the large widths reported by other authors are o...

Investigations into the Origin of Anisotropy in Oblique-Incidence Films

Experimental evidence is presented which shows that the magnetic anisotropy of oblique-incidence Permalloy and iron films is not caused by an inclined texture axis or anisotropic strain. Electron diffraction and microscopy have not yet revealed anisotropy in the crystalline microstructure of these films. These techniques have also thus far failed to reveal the existence of agglomeration of the crystallites into small groups having anisotropic geometric shapes. Magnetic anisotropy, on the other hand, was observed in Permalloy films deposited at normal incidence on non-magnetic metal films deposited at oblique incidence; this would suggest such an agglomeration mechanism. There is also some indication that oxygen may play a role in oblique-incidence magnetic anisotropy.

Magnetic Properties of Ni‐Fe‐Cr Films

Films of Ni‐Fe‐Cr were vapor deposited on heated glass substrates in vacuum. In each preparation increasing amounts of Cr were added to the original Ni‐Fe source, and a fresh set of glass substrates was exposed to the vapor beam after each Cr addition. In this way, Cr contents ranging from 0% to about 5% were produced in different films in each preparation, while Ni‐Fe ratios of various preparations ranged from 78% Ni, 22% Fe to 90% Ni, 10% Fe. For all Ni‐Fe ratios the anisotropy field Hk, the magnetization M, and the wall coercive force Hw, decreased monotonically with Cr content, while angular dispersion and electrical resistivity ρ increased with Cr content. The variations of M and ρ with Cr content were consistent with bulk data. The empirical relation Hk=A(M‐B) was found valid except for very small or very large additions of Cr; the constants A and B were independent of the initial Ni‐Fe ratio but dependent on the substrate temperature. Addition of Cr always tended to make the magnetostriction more p...

Parallel Oblique‐Incidence Anisotropy in NiFe Films

Magnetic films exhibiting an easy axis normal to the incidence plane (perpendicular anisotropy) when deposited at oblique incidence are well known. Anisotropic crystallite chains created by self‐shadowing were postulated to explain this effect. One of the physical measurements supporting this model was the observation of optical dichroism. In the present work, an easy axis parallel to the incidence plane (parallel anisotropy) has been found in nonmagnetostrictive films deposited at low (<45°) incidence angles for certain substrate temperatures and film thicknesses. The value of Hk and direction of the easy axis in oblique incidence films was very sensitive to changes in incidence angle, substrate temperature, film thickness, and deposition rate. The magnitude of the magnetic anisotropy Hk was found to be linearly related to the magnitude of the dichroism Δγ by the same constant, dHk/dΔγ = 5.5 × 105 A·Oe, for oblique incidence films having either parallel or perpendicular anisotropy. This result furnishes ...

ELECTRON BEAM DETECTION OF CHARGE STORAGE IN MOS CAPACITORS

A method for destructive detection of the state of charge storage in a metal‐SiO2‐Si capacitor is presented. This method is based on the modulation by the charge of the efficiency of separation of electron‐hole pairs which are generated by the penetration of an electron beam into the capacitor.