Permalloy Core Research Articles

Anhysteretic Magnetization of Ni–Fe Tape Cores

Results of an experimental investigation of the anhysteretic magnetization of ``square-loop'' Ni–Fe tape cores are presented. Anhysteretic remanence and magnetization curves are identical below saturation remanence and can be reproduced with an accuracy of about ±10%. The initial slope of the anhysteretic magnetization does not depend on core geometry for a wide range of sizes, and takes on values from 3.6 to 29 H/m for different 50% Ni–Fe and square Permalloy cores. It is shown, by comparing hysteresis loops taken from an initial anhysteretic state, with minor loops taken conventionally, that the anhysteretic state seems to correspond to a state of about equal average flux at each cross section of the tape. On the other hand, saturation in opposite directions for the inner and outer wraps of the tape seems to occur when partial flux reversal is obtained by slowly increasing an applied dc field.

Efficiency of High-Frequency Mixing in a Permalloy Toroid

A theoretical harmonic analysis is made of the output voltage from a small, etched Permalloy sheet core which is energized by the sum of two low-amplitude currents of different frequencies. The magnitudes of the two fundamentals, the sum and the difference frequencies, are predicted by the analysis. The two-frequency analysis of a hysteretic device is complicated by the multivaluedness of the input—output relation. The small signal behavior of a Permalloy core is further complicated by the lack of a model for high-frequency drive (∼1.0 Mc/sec). The analysis of mixing begins, therefore, with an experimentally measured high-frequency minor loop of a saturated core. The amplitude and frequency of the applied current is selected to insure nonswitching behavior. Once the flux—current relation is established, multiple Fourier analysis is used to calculate the output voltage waveform. The theoretical amplitudes of the frequency components are compared with measured values from a small Permalloy core (o.d.=23 mil, i.d.=17 mil, thickness=0.25 mil).

Demonstration of Displacement Current

An apparatus is described in which the displacement current in a barium titanate capacitor forms the primary of a toroidal transformer with a Permalloy core surrounding the capacitor. The voltage output of the transformer increases with the square of the frequency. At f = 2kcps and a supply voltage of 50 V an output voltage of 50 mV is obtained.

Sendust Sheet-Processing Techniques and Magnetic Properties

A procedure has been developed whereby the brittle magnetic alloy of silicon, aluminum, and iron (Sendust) can be produced in sheet from through metal powder rolling techniques. Ring laminations stamped from this sheet at 0.014 in. thickness have the following dc properties: μ20=16 130; μmax=36 625; Hc=0.062 Oe; Br=3450 G; Bm=8960 G. High frequency (<1 Mc/sec) loss values were lower than in laminated molybdenum Permalloy cores of the same thickness. The nominal Sendust composition is 9.5% silicon, 5.7% aluminum, and balance iron. This new process should make possible more extensive use of Sendust in applications that can take advantage of its remarkable intrinsic characteristics such as high initial permeability, high resistivity, low magnetostriction, and low magnetocrystalline anisotropy.

All-Magnetic Logic Elements Using Strained Permalloy Wire

An element for performing digital logic which employs Permalloy wire, solenoids, and resistors and utilizes a two-phase current pulse source has been studied experimentally. The reentrant hysteresis loop exhibited by a torsionally strained Permalloy wire provides the mechanism for power gain. A balanced input circuit assures unidirectional information flow and provides means for obtaining logic functions. Each element consists of two 0.6-in. bifilar-wound solenoids for bias and reset pulse currents, one or several 0.025-in. solenoids to which logic inputs are applied, an output resistor and a Permalloy wire core. The application of a bias current pulse to the logic element will cause the magnetization to change from the zero to the one state provided a signal is present simultaneously at the input to nucleate the magnetization reversal. The reset current pulse switches the magnetization back again and produces an output across the bias coil which is used to control one or several following elements. A buffer zone, the magnetization of which is controlled by the reset pulse, separates adjacent logic elements to minimize element interaction. Measurements on elements employing a 1.5-mil, 5–79 Permalloy core show that a switching time of 15 μsec and a power gain of 10 can be achieved. Preliminary tests indicate satisfactory operation over the temperature range of −55° to +185°C. These elements have been used to construct flip-flops, binary counters and a serial adder which employs majority logic. The circuits operated successfully when the pulse currents from the power supply were varied by ±20%. Because an open flux structure is used, it is believed that similar techniques will have application in circuits employing deposited magnetic films.

The Switching Characteristics of 4-79 Permalloy Cores with Different Anneals

The magnetic properties of 4-79 Permalloy cores, which have been annealed at different temperatures, are discussed. Cores annealed at relatively low temperatures are characterized by high coercivity, slower switching, insensitivity to strain, magnetization difficult to rotate, and insensitivity to an applied transverse field. Some cores exhibit a preference to one remanent state over the opposite state.

Switching Properties of Permalloy Cores of Varying Coercivity

In attempting to develop a 4–79 molybdenum Permalloy core for use in a memory with nondestructive readout, studies have been made of switching properties of Permalloy tape cores with coercivities ranging from 0.1–1.0 oe, corresponding to heat treatments over a temperature range of 300–900°C. The switching time of a core can be shortened by means of auxiliary field pulses which help to nucleate domains of reverse magnetization, but have little effect on their rate of propagation. In general, auxiliary field pulses applied perpendicular to the direction of remanent magnetization are more effective than those applied antiparallel to it provided the latter are not large enough to cause switching by themselves. The timing of the auxiliary pulses is important. Many of the cores have a ``preferred'' direction presumably caused by small domains of extremely high coercivity. The experimental results are interpreted in the light of domain wall motion theory.

An Alternating Current Probe for Measurement of Magnetic Fields

A method of measuring magnetic field strengths is described which utilizes the incremental permeability of a small Permalloy core. Alternating-current excitation is used which in turn permits amplification and hence a relatively high sensitivity and accuracy.

A Sensitive Induction Magnetograph

A SENSITIVE induction magnetograph for measuring the time-rate dH/dt of the horizontal component of the earth's magnetic field is described by H. Nagaoka and T. Ikebe in Scientific Papers of the Institute of Physical and Chemical Research (Tokyo) of August 1939. The magnetograph embodies a design of a special shape for collecting the lines of force into a permalloy core enclosed in a cylindrical coil. A record is given of complicated magnetic disturbances caused by electric trams. Even at a distance of several hundred kilometres from a great city, the disturbances can be detected as minute ripples on the traces. The selection of an observing station for observing true geophysical phenomena is therefore a difficult problem. The observed effect of weak earthquake shocks is discussed. Apart from the purely mechanical effects of shock, there are probably some true magnetic effects due to the varying magnetic susceptibility of erupting lava (the subject of study being the Asawa volcano). The authors think that it may be possible to predict violent eruptions half an hour before they occur. It is also anticipated that the new magnetograph will provide data to test whether short-period disturbances of a few seconds’ duration exist during magnetic storms. Provided that a suitable site can be obtained, the magnetograph has other applications in the study of the characteristics of the E- and F-layers of the ionosphere with respect to the height of origin of the current system producing the diurnal variation of the earth's magnetic field.

Study of Magnetic Losses at Low Flux Densities in 35 Permalloy Sheet

Energy losses in ferromagnetic materials subject to alternating fields have long been considered as due solely to hysteresis and eddy currents. However, at the low flux densities encountered in certain communication apparatus, a further loss is observed which has been variously termed ``residual,'' ``magnetic viscosity,'' or ``square law hysteresis.'' The search for an explanation of this loss has led to precise measurements of hysteresis loops with a vacuum ballistic galvanometer, and of a.c. losses with inductance bridges. From these results, it appears that that part of the a.c. effective resistance of a coil on a ferromagnetic core which is proportional to the coil current is strictly identified with the hysteresis loop area as measured by a ballistic galvanometer, or as indicated by harmonic generation in the coil. The hysteresis loop can now be constructed in detail as to size and skewness on the basis of a.c. bridge measurements. This conclusion was reached previously on a compressed iron powder core, and is now confirmed on an annealed laminated 35 permalloy core. Observed eddy current losses for this core exceed those calculated from classical theory by 20 percent. This excess is ascribed to the presence of low permeability surface layers on the sheet magnetic material. The a.c. residual loss per cycle (nominally independent of frequency, like hysteresis) is not observed by ballistic galvanometer measurements, although it indicates an energy loss some eight times the hysteresis loss for the smallest loop measured (Bm = 1.3 gauss). Analysis of the residual loss shows that it increases with frequency up to about 500 cycles, and remains constant at higher frequencies (to 10,000 cycles per second). Concurrently with the increase of residual loss, the permeability of the alloy is observed to decline with increasing frequency about 1 percent below the value predicted from eddy current shielding. This effect is most noticeable at frequencies below 1000 cycles.