Index of content:
Volume 34, Issue 4, 01 April 1963
- THIN FILMS—2
34(1963); http://dx.doi.org/10.1063/1.1729435View Description Hide Description
When a magnetostrictive Permalloyfilm is stressed in the direction of its preferred axis of magnetic orientation, an effective anisotropy is induced in the film causing a change in measured Hk . Magnetoelastic measurements were made on evaporated and plated films 100–4000 Å thick and varying in composition from 70% Ni, 30% Fe to 85% Ni, 15% Fe. Composition was determined by x‐ray fluorescence. It was found that the magnetoelastic coupling constant B = (2/M) (dHk/de) was significantly less for plated than for evaporated films of the same composition. There is evidence of both intercrystallite or internal crystallite slip and film‐to‐substrate slip, but neither is large enough to account for the observed differences.
34(1963); http://dx.doi.org/10.1063/1.1729436View Description Hide Description
Magnetic film memories and other devices depend upon the relaxation from a state of hard direction magnetization to an easy direction state in the presence of a small easy direction ``tipping'' field HL . The relaxation times of five Permalloyfilms of thicknesses 0.1, 0.25, 0.4, 0.5, and 1.0 μ have been measured with an apparatus of switching field turn‐off time <0.5 nsec and a detection risetime of 0.85 nsec. The relaxation time is defined as the time for a 10–90% easy direction flux change. It is found that the relaxation time varies roughly inversely with easy direction field and is to some extent independent of the initial hard direction field (8 Oe for the following remarks).
The tangent slope Sw to the 1/τ s vs HL curve at a field HL =1 Oe fits the relation Sw =1+10d 2 Oe nsec, where d is the film thickness in μ, within experimental error (∼20%). The Sw predicted from the Landau‐Lifshitz equation for the rotational model with λ=108 cps is 1 Oe nsec. The rotational model also predicts that τ s approaches 2 nsec for HL →0. The limiting τ s for the real films varied from about 7 nsec for the 0.4‐μ film to about 40 nsec for the 0.25‐μ film with no thickness correlation. In addition, the 0.1‐, 0.25‐, and 0.5‐μ films exhibited a distinct slow‐switching component for |HL | <H disp (H disp is the field HL necessary for a 90% relaxation in one direction). Observation of the final state of the 0.1‐ and 0.25‐μ films by the Bitter technique revealed a fine‐scale domain structure for |HL | <H disp and a single domain state for |HL | ≳H disp.
The two thicker films were electrodeposited on a BeCu substrate, while the three thinner films were evaporated on a glass substrate. Since all five fit the same Sw vs d relation, it would appear that substrate‐induced damping is not large.
34(1963); http://dx.doi.org/10.1063/1.1729437View Description Hide Description
The phenomenon of rotatable anisotropy (RA) in Ni–Fe films refers to a reorientation of the easy axis along the direction of a previously saturating magnetic field in a time <1 sec at room temperature. The effect was first unambiguously observed by Lommel and Graham in Nifilms and attributed by them to ferromagnetic‐antiferromagnetic interaction between Ni and NiO. The present study shows that the RA effect occurs under the same conditions of film thickness and composition as a previous studied ``mottling'' effect reported by Huber and Smith. In both cases the effects only occur for compositions with negative magnetostriction λ and for thicknesses greater than a critical thickness tc which is nearly inversely proportional to λ. These thickness and compositional dependencies suggest that magnetostriction and strain are responsible for both effects; a detailed microdomain model is being developed.
34(1963); http://dx.doi.org/10.1063/1.1729438View Description Hide Description
Coupled films were made either by sandwiching striplines between two films on different substrates, or by multilayer evaporation. The first structure was found to be inadequate in reducing the demagnetizing field because the airgap was appreciable. The switching speed of coupled films was calculated with the Landau‐Lifshitz equation, taking into account the magnetostatic coupling energies. Nearly identical rotational switching speeds were found for single and coupled films. The results were verified by measurements. Skew, dispersion, and creeping were measured. It was found that coupled films were less susceptible to the word field disturb from neighboring lines. It was also found that the orthogonal drive scheme could be realized, using coupled films with small diameter and large thickness, which would prevent their use as single films.
34(1963); http://dx.doi.org/10.1063/1.1729439View Description Hide Description
Domain wall motion is expected to be the fundamental flux reversal mechanism for thin permalloy films in the low drive region H<HK . The velocity of a wall when a switching field H is applied is given by V=G(H‐H 0), where G is the mobility and H 0 is the starting field. For films with thicknesses on the order of few hundred angstroms, where Néel walls are expected, the mobility is limited by intrinsic damping to a value much smaller than would be expected if only eddy current damping were present. Calculations have been made using a value for the damping constant, α=.015, obtained from coherent rotation studies, and the experimental values for the thickness of the wall obtained by Fuchs in order to obtain G as a function of film thickness. The mobility was measured for seven permalloy films ranging from 128 to 495Å thick. The agreement between the calculations and experiment was within 30%, indicating that the value of α for a 100‐Å film is nearly the same as for thicker films.
34(1963); http://dx.doi.org/10.1063/1.1729440View Description Hide Description
An optical strobing apparatus has been developed which makes it possible for the first time to observe magnetization reversal in thin films with reversal taking place during a period of the order of a microsecond. Studies of the details of magnetization reversal are important for understanding how reversal takes place in films. In addition, work on strobing of magnetic films will be of value in developing nondestructive magneto‐optic readout for thin‐film computer memories.
34(1963); http://dx.doi.org/10.1063/1.1729441View Description Hide Description
Faraday rotation measurements on ferrimagnetic oxides have so far been limited to single crystals which are rendered transparent by grinding. In the present studies films of NiFe2O4 and CoFe2O4, of thickness 1000–8000 Å, suitable for optical studies are prepared by oxidizing films of the alloys NiFe2 and CoFe2 sputtered on silica substrates. The Faraday rotation is determined with the film magnetized along axes both normal and parallel to its surface. Using the latter arrangement, optical hysteresis loops are recorded and checked against those obtained with a vibrating sample magnetometer. The dependence of loop shape and coercive force on film thickness and oxidation temperature is studied.
34(1963); http://dx.doi.org/10.1063/1.1729442View Description Hide Description
Ferrite films of a nickel‐zinc‐cobalt composition have been deposited by pyrolytic hydrolysis of metal inorganic salts. The object of the research described in this paper was to deposit these films with a composition like that of a bulk ferrite of particular magnetic properties, and to evaluate the films in terms of structure, composition, and magnetic properties.
The films are prepared by atomizing a mixed solution of the metal chlorides into a reaction tube heated to 800°C. The metal ion concentration in the starting solution is compensated to allow for excessive and varying loss of the constituents during the film‐forming process. The method used for determining the amount of starting solution compensation will be discussed along with a detailed description of the experimental apparatus.
The films prepared by this technique exhibit the ferrite spinel structure, as is evidenced by x‐ray diffraction data.
A value of 100 for the initial permeability of the films was measured by observing the film B‐H relationship. A study of film density and porosity was made from photomicrographs of the film cross section and from the weight of film deposited. These properties were compared to those of the bulk ferrite of like composition, and similar values were obtained.
34(1963); http://dx.doi.org/10.1063/1.1729443View Description Hide Description
A wide range of thin ferritefilms has been prepared by a chemical‐deposition process. Alcoholic solutions of ferric nitrate and other metal nitrates were combined in necessary proportions to yield stoichiometric ratios of the desired ferrites, which were then deposited on substrates. Firing the coated substrates between 900–1100°C in a controlled atmosphere resulted in spinel or garnetferrite formation. Magnetic properties were evaluated at X band by cavity‐resonance measurements. Ferrimagneticlinewidths obtained ranged between 125 and 1950 Oe. Crystal structure and composition were verified by x‐ray diffraction analysis. Experimental data are given. A nickel film was measured in an experimental resonance isolator in the 35‐Gc region resulting in a maximum reverse to forward loss ratio of 22.7 to 1. Data and results obtained suggest applications in millimeter and submillimeter ferrite devices.
34(1963); http://dx.doi.org/10.1063/1.1729444View Description Hide Description
Thin Ni–Fe films specially prepared by vacuum deposition on thin carbon substrates supported on electron‐microscope specimen grids showed inward‐growing spiral, or concentric‐circle walls when rotated in a constant field. For either configuration it was necessary to have a film which was thin enough to support 360° walls, and which possessed high easy‐axis dispersion, particularly near the film edge. Whether spirals or concentric circles were exhibited depended on the value of the easy‐axis dispersion in the center of the film.
34(1963); http://dx.doi.org/10.1063/1.1729445View Description Hide Description
Ohm's law in strongly magnetic, though not necessarily ferromagnetic materials, has leading terms in the presence of a magnetization M and an induction B of the form,which represent the ordinary and extraordinary Hall effects and a magnetoresistance. The M‐dependent terms, in particular, describe interactions due to polarized scattering centers and to the left‐right asymmetry of polarized mobile charge carriers.1
An electromagnetic wave interacting with such a material will, under resonance conditions, cause a precession of M, while at the same time inducing an eddy current density J. According to Eq. (1), the presence of two such terms at microwave frequencies introduces local dc electric fields related to the time average of the various products. These fields will be largest near spin resonance. Such fields are observable only in films thin compared to the electromagnetic skin depth,2 and they may be used either to study spin resonance in very thin films or to determine their transport properties at microwave frequencies. In particular, for a configuration in which the static field is in the film plane and the electromagnetic wave is at normal incidence 〈J×B〉 has no contribution in the film plane; such an experiment allows a separation of R 0 and R 1 even in materials which show no magnetic saturation.
The detailed electromagnetic theory of this configuration has been worked out3 for an isotropic material and experiments have been carried out on nickel films to test the various features of the theory, and to determine the usefulness of the method in studying high‐frequency magnetic and conduction properties in thin films.4 The experimental arrangement consists of placing the sample close to a wave guide or cavity wall, but insulated from it, and measuring the dc voltages, either steady or modulated, induced by the microwaves. The major precaution necessary for a quantitative study is that the microwave field pattern within the film must be well known, so that sample geometry, its electrical boundary conditions, and placement of detection wires is very critical.
The experimental results reproduce all the qualitative features of the theory, with regard to sample thickness, local field configuration, power, and magnetic field orientation. The agreement with the theory is better than reported by Seavey5 for Permalloy films, most likely because of differences in local field configuration, and by Heinz and Silber6 for a ferrite, whose experiment did not permit separation of all parameters. A quantitative analysis of the data7 gives consistent values of the magnetic resonance parameters g, M 0, and τ. It also requires that the films be described at microwave frequencies by a complex conductivity which is generally smaller than at low frequencies. Within the over‐all accuracy of the analysis—about 15%—the magnetoresistance Δρ is the same as at low frequencies, and the increase of R 1 is related to that of ρ, so that no dispersion of these constants is observed.
The same method has been applied to thin films of gadolinium, and while the signals are much smaller, the dc effects clearly persist in the paramagnetic region above room temperature.8 Quantitative analysis of the data has been impeded because the power levels necessary to observe the signals cause considerable heating of the sample. Since both the magnetic and galvanomagnetic properties of gadolinium are very temperature sensitive, a careful determination of the local temperature within the film during the microsecond long pulse of microwaves is required. The data obtained so far indicate that the g value of our gadoliniumfilms is higher than in bulk, and that the extraordinary Hall constant appears to be larger than in bulk material.