D. S. Rodbell

Influence of Pulsed Magnetic Fields on the Reversal of Magnetization in Square‐Loop Metallic Tapes

Reversal of magnetization in square‐loop metallic tapes is described in terms of the domain geometry and the dependence of this geometry upon the applied magnetic field wave shape. For a steady field whose amplitude is slightly greater than the coercive force, the reversal is accomplished by growth of a single axial domain originating at the thin tape edge. For steady fields of much greater amplitude, the reversal domain grows in from all tape surfaces. Because of the finite nucleation time required to establish the reversal domain, short duration, high amplitude field pulses can be used to investigate the mechanism of nucleation of the high field reversal domain configuration. A model which accounts for the nucleation process is suggested. The compatability of this model with various experimental results is presented. From these results an understanding of the high field reversal‐domain configuration and the kinetics of the nucleation and growth of these domains in lamellar materials is obtained. An auxi...

Magnetic Critical‐Point Behavior of CrO2

The magnetization σ of the ferromagnetic compound CrO2 was measured as a function of field H and temperature T near the Curie point Tc. From isotherms of σ2 vs H/σ, the initial susceptibility χ0 above Tc was obtained, which when tested against the relationship χ0−1 ∝ (T−Tc)γ gives a constant γ of 1.63±0.02 from just above Tc (386.5°K) up to about 1.15 Tc. This γ value contrasts with the values near43 recently computed for the Heisenberg model and later found experimentally in various ferromagnetic metals and compounds. At higher temperatures the effective γ decreases rapidly towards unity. Up to the highest field used (25 kOe), the critical isotherm obeys the relationship σ∝H1/δ with δ=5.75±0.05, which differs markedly from the theoretical δ values of 3 (molecular field) and 5.2 (3‐dimensional Ising) and from various experimental values. Gradual departure from this relationship below 1.5 kOe is attributed to the magnetocrystalline anisotropy that persists at Tc. Furthermore, we find that all the σ(H, T) d...

Study of Precipitate Particles in Cu–Co Employing Ferromagnetic Resonance

The dependence of the ferromagnetic resonance upon specimen anisotropies (shape and magnetocrystalline) is well known. Observations of the ferromagnetic resonance absorption of submicroscopic particles may be interpreted to yield useful information about these anisotropies. The alloy 2% Co–Cu heat treated to precipitate cobalt containing about ten percent copper from the nonmagnetic solid solution has been studied using ferromagnetic resonance techniques. In the early stages of growth the precipitate particles are shown to be essentially spherical in shape. Further growth of the precipitate particles (by subsequent heat treatment) is accompanied by aspherical shape changes which modify the resonance spectra. The magnetocrystalline anisotropy of the cobalt precipitate in single crystals of this alloy has been determined. This anisotropy is cubic with K1/Ms=−665 oe at room temperature. The cobalt‐rich precipitate retains its cubic crystal structure to at least 4.2°K, apparently stabilized by the copper matrix.

Magnetic First‐Order Phase Transition and Anisotropy in Single‐Crystal MnAs

The experimental and theoretical effects of magnetocrystalline anisotropy upon the magnetic first‐order phase transition in the compound MnAs are here described. A 3°C orientation‐dependent difference in the transition temperature is observed near 20 kOe for a single‐crystal specimen. Anisotropy measurements using Shenkers method give anisotropy sums, ΣnnKn, varying from −11.9×106 ergs/cm3 at 77°K to −5.6× 106 ergs/cm3 at 314°K, where Kn are the anisotropy terms in the energy representation, EK=ΣnKn sin2nθ, that is appropriate for this hexagonal material. A separate method gives an anisotropy field of 18.3 kOe at 35°C. Data analysis indicates that three constants, K1, K2, and K3, are needed to describe the anisotropy. Their values at 35°C are −5.75, +1.5, and −1.15×106 ergs/cm3, respectively. Theoretical checks using the magnetic form of the Clausius‐Clapeyron equation are in close accord with the experimental data and the determined constants.

Ferromagnetic Resonance of Iron Whisker Crystals

The ferromagnetic resonance absorption characteristics of iron single crystals (grown by K. W. DeBlois of this laboratory) have been examined between −196°C and 850°C. The crystals are of filamentary growth habit and are commonly known as whiskers. The selected crystals have 〈100〉 growth directions, and their square cross sections are bounded by {100} crystal planes. They are typically 10 μ on a side, but sizes ranging from 2 to 40 μ have been examined. Resonance absorption is observed using standard techniques with the dc magnetic field parallel to the length of the whisker. The microwave energy penetrates only a small fraction of the volume due to eddy current limitation. Two resonance modes may be observed. One is driven by the uniform microwave magnetic field, the other by its curl component. The curl mode is equivalent to resonance in a flat plane of infinite extent and no edge corrections are required. The uniform mode subject to an additional field arising from the uncompensated magnetic pole distr...

Some Magnetic First‐Order Transitions

Some transitions between ordered and disordered magnetic states that take place via transitions of first order are tabulated. Among these transitions are some of the order-disorder type. The theoretical basis for such a transition to occur is indicated using a simple physical model that assumes: (1) a strain dependent exchange interaction and (2) a compressible lattice. The necessary condition for first-order behavior is determined, and the theory is compared to the behavior of a real material, MnAs. It is found to be in substantial agreement with the behavior of that compound. The system represented by the simple model employed here possesses ferro-, antiferro-, and paramagnetic states. The equilibrium boundaries between these states are determined in the pressure-temperature plane as an illustration of the possible transitions that may be observed with this model.

Metamagnetic Behavior of Manganese Arsenide

Between 35° and 125°C there exists a threshold magnetic field strength at which the compound MnAs exhibits a large and steep increase of its net magnetization. At 35°C the threshold field is 40 koe. There is a large hysteresis associated with the magnetization change. This behavior resembles that exhibited by a material that is metamagnetic in the sense of Neels description. We believe, however, that in this material the existence of a first-order phase transformation is sufficient to account for the magnetic behavior.

Temperature Dependence of the Magnetocrystalline Anisotropy of Face‐Centered Cubic Cobalt

Single crystals of face-centered cubic cobalt have been examined between 4.2° and 850°K. The usual transformation to the hexagonal structure below 700°K is avoided by using two special forms of samples: (a) thin films evaporated onto MgO substrates, and (b) the precipitated cobalt-rich phase in a 2% Co-Cu single crystal. In both cases the fcc cobalt is stabilized by and has a close correspondence with the host lattice. Standard ferromagnetic (electron-spin) resonance techniques have been used to determine the magneto-crystalline anisotropy parameters K 1/M and K 2/M over the temperature range indicated, and, in addition, the spectroscopic splitting factor g is found to be 2.06±0.03 independent of temperature. The temperature dependence of the anisotropy constants is in accord with the relation K(T)/K(o) = [M(T)/M(o)] n , where for M(T)/M(o) we have taken the determination by V. Jaccarino [Bull. Am. Phys. Soc. 4, 461 (1959)] of the temperature dependence of the cobalt-nuclear magnetic resonance frequency which we assume to be proportional to the magnetization. The results that are obtained indicate that the power n of the dependence noted is nearly 10 for K 1 and much higher for K 2. At 0°K samples of type (a) have −K 1/M=600 oe.

Ferromagnetic Resonance of Single‐Crystal Gadolinium

Gadolinium metal is a system of relative simplicity as far as exchange interactions and ionic states are concerned. The 8S7/2 state of Gd3+ is consistent with the observed 7μb/atom and the moment contributing f electrons are most likely exchange coupled via conduction electrons. The present work concerns the measurement of the ferromagnetic resonance behavior of a single crystal at frequencies of 21 and 35 Gc/sec and at temperatures from above the Curie point (20°C) to 4.2°K. The resonance absorptions are well defined at both frequencies and at all temperatures for most crystal orientations. The minimum linewidth observed is about 400 Oe but the line shape and width degrade when the applied field departs from the easy axis, particularly at the lower frequency and at low temperatures. Strong, well‐defined domain wall resonances are also frequently observed.The deduced magnetocrystalline anisotropy energy is accurately described by the standard uniaxial expansion EA = K1 sin2θ+K2 sin4θ+⋯, about the hexagona...