D. O. Smith

Static and Dynamic Behavior of Thin Permalloy Films

Thin magnetic films (∼1000 A) evaporated onto a heated glass substrate in the presence of a dc magnetic field develop a uniaxial magnetic anisotropy energy of the form Ek=Ksin2φ, with φ measured from the deposition field. Such films promise to outperform ferrite cores in digital‐computer memory‐systems by several orders of magnitude. A detailed static and dynamic theory based on the above anisotropy energy and the Landau‐Lifshitz dynamical equation is presented. The static theory contains the unique feature of the coercive force parallel to the easy axis being dependent on a field in the plane of the film and perpendicular to the easy axis. Experimental verification of the theory is given.The dynamic theory considers both steady‐state and pulse response. The steady‐state solution is well known; the pulse solution must take very careful account of all the important torques acting in a switching experiment, and numerical results are obtained by the use of a digital computer. Experimentally, steady‐state res...

Anisotropy in Permalloy Films

Evaporated films of Permalloy are found to be uniaxially anisotropic under all the conditions of preparation studied so far, including deposition in dc, zero, and rotating‐circular magnetic fields. Three principal effects have been identified as contributing to this anisotropy, namely; (1) the formation of directed Fe pairs in the Ni matrix, (2) an effect due to the angle‐of‐incidence between the substrate and the depositing beam, and (3) anisotropic stress in combination with the isotropic magnetostriction. The evidence for directed pairs comes from the compositional dependence of the anisotropy; the angle‐of‐incidence effect is demonstrated by depositing onto tilted substrates; anisotropic stress effects are shown by detaching the film from the substrate. The simultaneous presence and interaction of the above effects is thought to be the reason for the variability in the anisotropy which is encountered in Permalloy films.

Noncoherent Switching in Permalloy Films

High resolution Bitter pattern studies of the domain structure of permalloy films, with uniaxial anisotropy Hk and under the influence of applied fields in the film plane, are reported, and from these studies are inferred some aspects of noncoherent flux reversal processes. The threshold field for irreversible domain propagation across the film is measured for a variety of films differing in thickness (150 to 1100 A), rate of deposition (30 to 1100 A/min), substrate temperature (100 to 350°C), and composition (80 to 83.5% Ni), as a function of the angle α between the reversing field and the easy axis, after saturating along the opposite easy direction. For α<αc, where αc depends on thickness and preparation variables and varies from 0° to about 60°, reversal takes place by parallel wall displacement, with a wall coercive force Hw characteristic of the film. Two principal structural features have been identified as affecting Hw: Nonmagnetic inclusions, and local easy axes in the film plane normal to the ma...

Development of a Vibrating‐Coil Magnetometer

The vibrating‐coil magnetometer measures the magnetization of a small sample of magnetic material placed in an external magnetizing field by converting the dipole field of the sample into an ac electrical signal. To allow space for temperature‐ or pressure‐generating apparatus around the sample, the measurement is made at distances up to 2 cm from the sample. The measurement is continuous and can be recorded on a chart as a function of time, temperature, crystallographic orientation, or magnetizing field. The present stable sensitivity is such as to provide one percent accuracy for a dipole moment of 8.56×10−4 amp‐m2 (the saturation moment at room temperature of a nickel sphere 1.5 mm in diameter).

Anisotropy in Nickel-Iron Films

The magnetic and optical anisotropies of evaporated iron-nickel films deposited at an angle to the substrate normal are found to be sensitive to the rate of deposition and the orientation of the magnetization M during deposition. It is postulated that crystallite elongation toward the vapor beam or in the direction of M occurs. Study of spatial inhomogeneities in the normal-incidence M-induced anisotropy field Hk is an aid in understanding the origin of this anisotropy and the switching behavior of films. Small-angle dispersion of Hk is attributed to local deviation from isotropy of the isotropic tension present in films; measurements support this model. Magnitude dispersion is estimated from rotational hysteresis experiments. Large-angle dispersion of a special type has been identified wherein certain regions of a film have an easy axis perpendicular to the field present during deposition, i.e., Hk is negative in these regions. For alloy compositions within a few percent of 83% Ni and 17% Fe, the normalized wall coercive force hw = Hw/Hk has significance beyond the defining concepts. Thus, hw is a measure of resonance linewidth and an internal field inferred from resonance data; for hw>1, a unique “locking” phenomenon occurs. Bitter-pattern studies of quasi-static switching show that negative-Hk regions are present and that hw is in some sense a measure of these regions. A tentative model forM -induced anisotropy which qualitatively explains both positive and negative Hk is proposed which invokes directed pairs of iron atoms and strain anisotropy arising from elongated crystallites. In nonlocking films, quasi-static switching by coherent rotation is broken up by labyrinth domains consisting of alternating regions of switched and unswitched M. Propagation of these domains is postulated to be caused by dispersion in the magnitude and orientation of Hk. If a pulsed reversing field exceeds the switching threshold of all regions of the film, a transition from labyrinth switching to coherent switching should occur, as has been observed experimentally.The magnetic and optical anisotropies of evaporated iron-nickel films deposited at an angle to the substrate normal are found to be sensitive to the rate of deposition and the orientation of the magnetization M during deposition. It is postulated that crystallite elongation toward the vapor beam or in the direction of M occurs. Study of spatial inhomogeneities in the normal-incidence M-induced anisotropy field Hk is an aid in understanding the origin of this anisotropy and the switching behavior of films. Small-angle dispersion of Hk is attributed to local deviation from isotropy of the isotropic tension present in films; measurements support this model. Magnitude dispersion is estimated from rotational hysteresis experiments. Large-angle dispersion of a special type has been identified wherein certain regions of a film have an easy axis perpendicular to the field present during deposition, i.e., Hk is negative in these regions. For alloy compositions within a few percent of 83% Ni and 17% Fe, the normali...

Properties of Permalloy Films Having a Magnetoelastic Easy Axis Normal to the Film

Internal isotropic strain in Permalloy films having a negative magnetoelastic constant creates a magnetoelastic easy axis normal to the film. Below a critical thickness tc such films are single domain in the film plane and have a square B‐H loop; above this thickness new configurations are established, characterized by a B‐H loop of special form and having relaxation properties. Above tc the Bitter pattern is “mottled” and continues so even in the presence of a 10 000 oe field. A domain structure is proposed in qualitative agreement with experiment; mottling is interpreted in terms of centers of microstrain.

Measured Relaxation Times for the Uniaxial‐Anisotropy Spectrum in Nonmagnetostrictive Permalloy Films

The relaxation time τ for realignment of an atomic anisotropy by 90° has been measured for a number of fast processes (τ<103 sec) occurring in nonmagnetostrictive Permalloy films. The method of measurement utilizes the magnetoresistance effect and is described in detail elsewhere. For films deposited between 23° and 200°C at 10−6 to 10−5 Torr and measured at the same temperature and pressure, three distinct processes plus two probable ones have been found, which together account for about one‐quarter of the total anisotropy. The activation energy Q and period factor τ0 which determine the relaxation time τ=τ0 exp (Q/kT) have been determined for each of these processes.

Steady‐State and Pulse Measurement Techniques for Thin Magnetic Films in the vhf‐uhf Range

A vhf‐uhf bridge consisting of two loops placed symmetrically in a rectangular coaxial cavity has been used to study steady‐state and pulse relaxation in thin (∼1000 A) Permalloy films. For steady‐state measurements a coaxial transformer converts the bridge output from balanced‐to‐ground to unbalanced‐to‐ground; measurements from 100 to 1500 Mc are currently possible. Pulse measurements utilize the ungrounded deflection plates of a wide‐band traveling‐wave oscilloscope; pulse response to 2 mμsec can be resolved. Drive‐pulse calibration is accomplished unambiguously by using some of the unique switching properties of films which have uniaxial anisotropy.A vhf‐uhf bridge consisting of two loops placed symmetrically in a rectangular coaxial cavity has been used to study steady‐state and pulse relaxation in thin (∼1000 A) Permalloy films. For steady‐state measurements a coaxial transformer converts the bridge output from balanced‐to‐ground to unbalanced‐to‐ground; measurements from 100 to 1500 Mc are currently possible. Pulse measurements utilize the ungrounded deflection plates of a wide‐band traveling‐wave oscilloscope; pulse response to 2 mμsec can be resolved. Drive‐pulse calibration is accomplished unambiguously by using some of the unique switching properties of films which have uniaxial anisotropy.

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...

Uniaxial‐Anisotropy Spectrum in Nonmagnetostrictive Permalloy Films

The uniaxial‐anisotropy spectrum in Permalloy films has been studied by: (1) deposition in H∥ on a substrate at temperature Ts, (2) annealing in either a dc field H⊥ or a circular rotating field Hr for specified time intervals, (3) quenching of the substrate holder by water‐cooled coils. Significant aspects of this procedure are: (1) annealing is carried out before exposure to air (oxygen), (2) quenching gives well‐defined annealing times, (3) each experiment involves freshly deposited samples which are in a well‐defined and reproducible state. These experiments have led to the identification of five separate contributions to the anisotropy in nonmagnetostrictive Permalloy, namely: (1) and (2) two types of contributions due to lattice vacancies with rotational activation energies≤0.2 eV and which became locked after exposure to air (oxygen), (3) and (4) two contributions with rotational activation energies∼1 eV, thought to be due to half self‐interstitials and carbon interstitials, respectively, and (5) d...