Ferromagnetic Rods Research Articles

A Gyromagnetic Electron Gyroscope

We recently proposed a detector of absolute angular velocity based on the difference between the gyromagnetic factor of the Cooper pairs in a superconductor, 2mc/e, and that of electron spins in a magnetic material, 2mc/ge with g∼2. The detector, which does not contain moving parts, is based on the measurement of the Barnett magnetization of a ferromagnetic rod enclosed in a corotating superconducting shield. Here we present an extensive experimental demonstration of the principle of operation.

Magnetostatic hole modes

This letter deals with the general concept of magnetostatic modes that are localized around cavities bored in an infinite ferromagnetic medium. The particular case of a circular hole of infinite length inside a material whose magnetization is parallel to the hole axis is studied in some detail. By using an original experimental configuration, it is shown that such a hole supports a very simple fundamental mode which is closely related to the well-known uniform precession of a ferromagnetic rod biased along its axis.

Impressed propagation of irreversible and reversible magnetizing processes — theory and experiment

To investigate the propagation of the elementary magnetizing processes which on the whole form the random Barkhausen field, irreversible and reversible flux changes are impressed on ferromagnetic rods. For the irreversible flux propagation an experimental example is given. The reversible flux transfer under eddy current damping is calculated for a current loop, and the expectation value of the spectral density Φ( k, ω) ( k wavenumber, ω angular frequency) and its cutoff wavenumber are given.

Spatial extent of the flux due to local irreversible changes of the magnetic polarization in ferromagnetic strips and rods

Two experimentally determined functions for 3% silicon-iron strips with Goss texture, part of which have been plastically deformed, are reported. The experiments were done using two short induction coils with variable distance a. The two functions are (i) the normalized cross spectrum K( f, a) of the Barkhausen noise and (ii) the irreverible transfer of locally impressed flux changes along the sample. The normalized cross spectrum was found to be independent of the frequency f at low frequencies, but decaying with the distance a. The transferred flux changes were found to decay exponentially with a characteristic length, which could be changed by plastic deformation or by varying the static homogeneous field. In all these cases, and in the case of an iron rod the characteristic length could be calculated from magnetostatic theory for the large differential permeabilities involved. The low frequency limiting curve of the normalized cross spectrum can be computed quantitatively from the irreversible flux transfer function. The results show that despite the different metallurgical and magnetic domain structures in the investigated cases, the transfer of both externally impressed and spontaneously released irreversible flux changes are governed by magnetostatic principles.

Enhancement effect of magnetostrictive waves by magnetic pulses

It is experimentally shown that the response of the magnetization of a ferromagnetic rod to a low-intensity pulsed magnetic field does not consist of an exponential decay alone, but also of a damped oscillation, if the combination of bias and pulsed fields is used. As a matter of fact, magnetoelastic waves propagating in the solid are observed as well as effects of interference.

Investigation on the domain-domain interactions by the propagation of magnetization changes

We have developed a simple model for the interaction among domains during the magnetization processes in polycrystalline soft materials. This model is based on the experimental results obtained by studying the propagation of reversible or irreversible magnetization changes along ferromagnetic wires or ferrite rods. By assuming that the coupled domains form long chains or «magnetic threads», in order to reduce the free poles at grain boundaries, we arrive at an equation, for the spreading of weak magnetization changes, similar to a diffusion equation. By this model we evaluate a mean interaction field among domains which is directly proportional to the saturation magnetization and inversely proportional to the grain size.

Magnetic properties of directionally solidified MnBi-Bi eutectics

Samples consisting of approximately parallel arrays of ferromagnetic MnBi rods in a Bi matrix have been grown by directional solidification of eutectic Mn-Bi alloys. The rods are of order 1μm in diameter, and very long in comparison to their diameter. Room-temperature intrinsic coercive fields up to 30 kOe have been attained, suggesting that the rods show single-domain behavior. The measured magnetizations are substantially higher than expected from the known moment of MnBi and the predicted volume fraction of MnBi in the eutectic; this means that the MnBi in our samples has an abnormally high moment, or (more likely) that the MnBi content is higher than predicted from the phase diagram. Magnetic measurements on samples cooled to 77 K show a stepped hysteresis loop which is most simply interpreted as a superposition of two loops, indicating the presence of two different magnetic phases. One phase has low coercive field (< 1 kOe) and is probably "normal" MnBi, which is known to have low anisotropy at 77 K. The other phase has a Curie point near 270 K, and is presumably the phase found in similarly prepared samples by Noothoven van Goor and Zijlstra. In some samples, we find this phase has a coercive field of 120 kOe, and is not saturated at our maximum field of 145 kOe (all at 77 k). We have not yet been able to separate this phase from "normal" MnBi, and so do not know its morphology or magnetic properties. It seems likely that it has a magnetization at least as large as that of normal MnBi, and much higher anisotropy.

Measurement of Internal Magnetic Field Distribution in Axially Magnetized YIG Rods Based on Magnetoelastic Resonance Absorption

A new method for measuring internal magnetic field in ferromagnetic rods is described. Magnetoelastic resonance is observed in longitudinally magnetized YIG rods. The separation ΔH of adjacent resonance peaks is varied with signal frequency and applied magnetic field He. It is shown the internal magnetic field is calculated from He-ΔH curves and the results agree well with the theoretical predictions previously published. The shape of axial field distribution depends on the applied external field. In the low external field region the shape is smooth, in the medium field region it becomes square and in the high field region it again becomes rather smooth, approaching Sommerfeld field.

Effect of a joint on the magnetic field of a long ferromagnetic rod

Effect of a joint on the magnetic field of a long ferromagnetic rod

Nonlinear resonant roll motion of magnetically oriented satellites

A systematic analysis of the steady-state roll motion of satellites with passive magnetic attitude control has been made. The roll motion can be described by a nonlinear differential equation of second order with periodic coefficients. Mitropolskii's method has been employed to compute the asymptotic approximation of the roll equation. The stationary solutions can be represented in terms of the solution of a linear Hill-type equation, which is known and tabulated. It was found that apart from the nonresonant rest position there is a large variety of resonant oscillatory and rotational roll modes. The resonance effect is externally excited by the orbital pitch motion of the spacecraft. Inertial crosscoupling provides the mechanism for exchanging energy between the roll and the transverse modes. The frequency of the roll oscillation as well as the mean rate of the rotational roll motion are found to be certain rational fractions of the orbital frequency. Numerical results are presented in form of amplitude characteristics for the most important resonance modes. ECAUSE of its simplicity and reliability the principle of passive magnetic attitude control has been applied to quite a number of national and international European satellite projects in recent years. The principle, first proposed by Fischell1 makes use of a rigidly mounted permanent magnet to align the reference axis of the spacecraft to the local gradient of the geomagnetic field. The nominal performance of this one-axis control is shown in Fig. 1. In order to damp the librational motions of the satellite, a hysteresis damper is applied, consisting of a number of ferromagnetic rods placed perpendicular to the reference axis. Previous studies of the attitude dynamics were mainly concerned with the short periodic librations of the system and optimal design of the magnetic control elements with respect to the transient behavior.1-2 The knowledge about the residual roll motion was rather limited; generally it was assumed that this motion comes to rest after dissipation of the initial kinetic energy. This paper deals with the analysis of the steady-state roll motion, especially with its long periodic response to the external excitation. The extremely nature of these motions allows one to consider the pitch and yaw response as being loosely coupled to the roll motion. As a first approximation the forced pitch and yaw motion are introduced as known time-dependent terms. The roll equation appears now as nonlinear differential form of second order, with periodic coefficients. The analysis of the roll equation was based on the method of asymptotic approximations. HilPs equation was chosen as unperturbed base, since solutions of HilPs equation are available in tabulated form. By a suitable nonlinear transformation the roll equation can be brought into a form convenient for the averaging method developed by Mitropolskii.3 After introduction of a slow timescale and averaging of the standard equations the conditions for the existence of periodic steady-state solutions have been derived. Apart from the purely periodic type of solutions also rotational solutions have been found.

A laminar subsidiary resonance limiter

A laminar subsidiary resonance limiter that reduces the threshold power level for limiting by an order of magnitude has been developed. The design was derived from a computer investigation of the effects of ferrite demagnetizing factors on threshold power level for ferromagnetic limiting. Minimum threshold power level is achieved when the demagnetizing factor in the direction of the applied dc biasing field is zero. This occurs only in a needle geometry. Multiple needles are required to have sufficient ferromagnetic material for effective limiting. A laminar design consisting of alternate ferromagnetic and dielectric rods is used to obtain dc magnetic field isolation between the ferrite rods. This design gives the desired dc magnetic field demagnetizing factors while providing a dominant mode propagation path for the microwave signal. An X-band limiter has been developed. It consists of 11 rectangular YIG rods separated by dielectric rods of the same dimensions and mounted on the narrow side of RG-52/U X-band waveguide. A combination of polycrystal and single-crystal YIG rods are used to increase the dynamic range of the limiter. The waveguide structure also effectively eliminates high-loss magnetostatic modes. The limiter has a threshold power of 2 watts and a dynamic range greater than 30 db. The insertion loss is less than 0.5 db over the frequency range of 8.5 to 9.3 GHz. Techniques for further reducing the threshold, utilizing rf magnetic field concentration techniques, will also be described.

Discussion on “Eddy-current effects in rectangular ferromagnetic rods”

Discussion on “Eddy-current effects in rectangular ferromagnetic rods”

Eddy-current effects in rectangular ferromagnetic rods

Expressions are obtained for the eddy-current distribution and the resulting loss angle for an infinitely long ferromagnetic rod of rectangular cross-section containing a number of domains magnetized parallel and anti-parallel to the axis of the rod and separated by domain walls running perpendicular to the long edge of the cross-section. Results are expressed in terms of the ratio of the lengths of the sides of the rod and the number of walls. Two cases are considered —the low-frequency limit in which the domain walls remain plane and the more general case in which the wall becomes bowed because of eddy-current screening effects.

Improvement in the Magnetic Detecting Power of Iron-cored Search Coils

SLOWLY changing magnetic fields have usually been measured by means of search coils with iron or mumetal cores, connected either to fluxmeters or to galvanometers. In this communication the term ‘coiled rod’ is employed and means a straight ferromagnetic rod about which a detector-coil is wound. The sensitivity or detecting power of the coiled rod is usually defined as the total number of flux linkages of the winding produced in the coiled rod by a spatially uniform magnetic field of one oersted and is measured in Maxwell-turns per oersted. The advantage of mumetal over iron for the core of the coiled rod has been its high permeability in low magnetic fields, low electrical losses and low coercive force. Any further increase in sensitivity of coiled rods has been obtained either by lengthening the rod and so reducing its demagnetizing factor or by winding on more turns—the latter case being accompanied by a rapid increase in inductance and by increased resistance unless the diameter of the wire in the coil is correspondingly increased, which in turn affects the overall diameter.

THE JOULE MAGNETOSTRICTIVE EFFECT IN A GROUP OF COBALT-IRON ALLOYS

In the course of studying various ferromagnetic rods, a group was found which showed a wide variation in the type and magnitude of the changes in length which occur when they were magnetized longitudinally. This collection included rods of iron, nickel, cobalt and alloys of cobalt-iron. In a later paper it is proposed to correlate the final results in this paper with the mechanical hardness, elastic properties, hysteresis loss and change in resistance due to a magnetic field. In making the following study a new extensometer and recording camera were used. These are described in the following pages.

An apparatus for the determination of a hysteresis curve

A method of measuring the intensity of magnetization of a ferromagnetic rod is described. It is measured by the difference in two currents, passed through the coils of a small Helmholtz galvanometer, necessary in two cases to cause the magnet system to set perpendicularly to the magnetic meridian: first, when the needle is acted upon by the earth's field only; next, when it is acted upon by the earth's field together with the field due to the magnetized rod.

Measurements on magnetostriction vibrators

The study of electromechanical vibrating systems through the measurement of motional impedance was first undertaken by Kennelly and Pierce in 1912, in their work on telephone receivers. The method was applied by K.C. Black in 1928 to the study of ferromagnetic rods centrally clamped and lying along the axis of a coil carrying alternating current superposed upon direct current. The work of Mr Black was done on rods of nickel, stainless steel, and stoic metal (invar steel), and also on nickel tubes. The natural frequencies of the rods he studied lay between 2300 and 3000 cycles per second. The present paper describes similar measurements carried out on rods with natural frequencies near 30,000 cycles per second. The study was made on rods of nichrome, monel metal, stainless steel, stoic metal, and nickel. The dependence of the vibratory characteristics on rod diameter was determined by making measurements on a nickel rod, as its diameter was reduced in successive steps. The rod was annealed before each set of measurements, by heating to redness and cooling in air. Another nickel rod cut from the same bar was bored axially with concentric holes of successively larger diameter, and measurements made at each step. This was done in an endeavor to determine something about the flux distribution across the section of the vibrating rod. The effects of annealing, and of changing the number of turns on the alternating-current coil (in which the rod was supported), were investigated. The effect on the vibratory characteristics of varying the alternating magnetic field on the rod was also measured.

A Dynamic Study of Magnetostriction

In a recent paper by Professor G. W. Pierce1 are described various effects which occur when a magnetic rod has superimposed upon it an alternating axial magnetic field of a frequency in the neighborhood of the natural mechanical frequency of the rod. Rods under the influence of magnetostriction can be made to play a part somewhat analogous to quartz crystals under the influence of piezoelectricity. If a ferromagnetic rod or tube is clamped so that it lies along the axis of a coil, the combination becomes an electrical vibration instru ment, and may be treated in the same way that telephone receivers are treated by Kennelly and Pierce,2 or by Kennelly in his book on vibration instruments.3 The present paper describes the measure ment of the dynamic electrical characteristics of various magnetostric tive rods and tubes, also the magnitude of vibration and other measurements that could be made conveniently at the same time. Method of Measurement.?The motional resistance and reactance are measured by connecting the coil containing the rod into an alternating current bridge, as in the reference2 mentioned above. In this particular experiment, the bridge was designed to pass a direct current as well as the alternating current through the coil, thus it was possible to maintain a known field on the rod at any time. The coil is a long solenoid to insure a uniform magnetic field. The coil is 120 cm. long, has an inside diameter of two inches, and at its center developes a field of 32.5 gauss per ampere. The rods are all about 90 cm. long, and their natural frequencies lie between 2300 and 3000 cycles per second. In all measurements the superimposed alternating field is small compared to the direct or polarizing field. In measurement of amplitude of vibration, an extremely light phosphor bronze strip upon which is mounted a small mirror is fastened from the end of the rod to the stationary frame to which the center of the rod is clamped. The longitudinal vibration of the rod thus gives an angular motion of the mirror. A point source of light reflected from the mirror is observed in a telescope with a