B. E. Argyle

Micromagnetics of laminated Permalloy films

The edge curling wall (ECW) is observed in an optical Kerr microscope. Conditions are derived energetically favoring single transverse domains with ECWs over other multiple closure domains or single longitudinal domains that are undesirable because of their low permeability. Computed figures illustrate how the maximum permissible spacer thickness depends on Permalloy sublayer thickness, uniaxial anisotropy, width of the magnetic strip, mismatch of Permalloy thickness, and perpendicular anisotropy. The case with strip width W=100 mu m and vanishing magnetostriction or stress should require only a few sublayers. However, if W is very small, or if stress-induced or some other form of perpendicular anisotropy is excessive, then attainment of single transversely magnetized domains requires many sublayers. Some of these conclusions are supported by microscopic observations. >

Optical imaging of magnetic domains in motion (invited)

We describe a laser magneto‐optic microscope (LAMOM) system, used to observe magnetic domain dynamics in Permalloy thin‐film devices. Optimization of Kerr magneto‐optic contrast is achieved by using laser illumination in a polarized light microscope in combination with video image processing. High magnetic contrast, a diffraction‐limited (0.25 μm) resolution, and motion picture recording capability at up to 30 frames/s are demonstrated by recent results.

Magnetic Properties of Some Divalent Europium Compounds

The compounds of divalent europium form an attractive series for magnetic investigations because the magnetic ions are in S states and the crystal structures are generally simple. Moreover, true ferromagnetism, which is quite unusual in compounds, has been observed in EuO by Matthias, Bozorth, and Van Vleck.We have prepared samples of EuS, EuSe, EuTe, EuCO3, EuTiO3, EuZrO3, EuSO4, and EuCl2, and studied their magnetic properties in the temperature range 4–300°K. The sulfide, selenide, and telluride all have the same NaCl crystal structure as EuO; we find that the first two also become ferromagnetic at low temperature, while EuTe is paramagnetic, or possibly antiferromagnetic, at 4.2°K. The Curie temperatures for EuS and EuSe are about 18° and 7°K, respectively. The saturation moments of EuS and EuSe extrapolated to 0°K correspond to 6.8 and 6.7 μB per Eu++ ion, respectively, in good agreement with the expected value of 7 μB. None of the other compounds were ferromagnetic at liquid helium temperatures, and...

Paramagnetic Faraday Rotation of EuSe

Faraday rotation and absorption measurements were made on a bulk single crystal and on evaporated polycrystalline films of EuSe. Bulk EuSe is ferromagnetic, transparent in the red, and exhibits an unusually large Verdet constant at room temperature: −9.7 min/Oe‐cm at 6660 A where the absorption is 173 cm−1. Measurements at 77°K show that the Verdet constant varies approximately inversely with temperature and thereby correlates with the magnetic susceptibility. These properties are due to the Eu+ + ion and are comparable to those we have observed in other divalent europium compounds such as Eu2SiO4.The evaporated films of EuSe exhibit a broad but well‐defined absorption band at 4600 A which is essentially duplicated in similarly prepared films of EuS and EuO at 5200 and 5800 A, respectively. The wavelength dependence of the Faraday rotation of EuSe exhibits a rather complicated structure with two sign reversals in the region of the absorption band. At 6660 A the Verdet constant measured at room temperature...

MAGNETIC AND OPTICAL PROPERTIES OF TRANSPARENT RbNiF3

Magnetic and optical data on single‐crystal RbNiF3 are presented. We conclude that this material is ferrimagnetic, rather than antiferromagnetic as previously reported, with a transition temperature of 139°K and a saturation moment of 21 emu/g at 4.2°K. The basal plane is the preferred plane and the out‐of‐plane anisotropy is 106 ergs/cc. The Faraday rotation measured along the c axis at 77°K and 80% of magnetic saturation exhibits a peak value of 400° per cm at a wavelength of 4950 A and values of 250° to 50° per cm in the region 5000 to 6000 A where the optical absorption is lowest.

Pyromagnetic Test of Spin Wave Theory in Metallic Nickel

The temperature dependence of the spontaneous magnetization of metallic nickel has been studied between 4.2° and 120°K by a pyromagnetic technique developed by the authors. Fractional changes in magnetization as small as a few parts per million could be detected near 4.2°K. The resultant data were fitted by the method of least squares to a theoretical equation containing terms descriptive of thermal excitation of spin waves in the presence of an effective magnetic field plus a T 2 term descriptive of collective electron behavior. The best fit of the data to this equation is obtained using the spin wave terms alone, provided an intrinsic energy gap is assumed in the spin wave dispersion law of 2.7°K for magnetization parallel to the [111] axis and 1.9°K parallel to the [100] axis. Enhancement of this gap by an externally applied field follows theoretical predictions. It is noted that the measured difference between the gap temperature along the two principal axes has the value theoretically predicted from previous measurements of magnetic anisotropy energy, however the isotropic contribution observed in this experiment has not been theoretically anticipated. A possible origin for the isotropic gap is proposed in terms of interaction of polarized s and d electrons. It is also pointed out, however, that the “isotropic effective field” may be a spurious result, originating in thermal expansion effects not included in the theoretical equation to which the data were fitted. Finally, a new type of pyromagnetic measurement is described which can be used to determine the temperature dependence of the magnetic anisotropy.

Critical Point of the Cubic Antiferromagnet RbMnF3

We have studied the critical region of RbMnF3 (TN=83°K) using specific heat, x‐ray, and strain‐gauge measurements. Because RbMnF3 so closely resembles an ideal Heisenberg antiferromagnet, experimental results may be especially relevant to theoretical predictions. The specific heat is found to diverge logarithmically over three decades for T<TN, but only over two decades for T<TN. No distortion from cubic symmetry is detectable in the range 20° to 300°K. Finally a remarkably small thermal expansion anomaly is found in a narrow region near TN.

Domain conversion under high frequency excitation in inductive thin film heads

A magnetooptic microscope was used to observe the time-averaged magnetization distribution in an inductive thin-film head excited by continuous sine waves. Domain activity in yokes driven with sinusoidal currents (1 to 20 MHz) was observed using the Kerr effect at video frame rates (0 to 30 Hz). Thus, the average location and shape of domains in the top yoke of the head could be recorded. It is shown that the domain pattern generally undergoes significant changes in a slow, repeatable evolution. Some changes lead to abrupt conversions of domain states. Although specific behavior varies from head to head, these conversions follow measurable curves having a common trend in the amplitude versus frequency space. Previous work on analysis of head response has not considered this type of dynamic response, although it appears to be common to many magnetic system. In addition to possible response at the excitation frequency, the wall network can also undergo large changes with a time scale much longer than the excitation period. Three possible mechanisms driving the domain conversions are outlined. >

Time-resolved domain dynamics in thin-film heads

A novel magnetooptic imaging technique was used to investigate the dynamics of magnetization response in thin-film head yokes. Completed head devices were excited with a sinusoidal current applied to the integrated coils. The amplitude (20 to 40 mA pp) and frequency (1 to 50 MHz) of excitation were chosen to simulate the write process. Pulsed laser illumination permitted stroboscopic observation of domain-wall and flux-flow dynamics with a time resolution limited only by the 5-ns pulse width (full width at half maximum). Results suggest that high-frequency write performance is degraded by two mechanisms not considered previously for thin-film heads: first, 180 degrees walls appear to impede the flux-flow across the plane of the wall and, second, the inhomogeneous rotational magnetization response observed is known to be much slower than the typically assumed coherent rotation. The effect of NiFe composition on dynamics was also investigated by comparing responses of two heads, one having positive and the other negative magnetostriction. Flux in the head with positive magnetostriction flows in a constricted path along yoke edges, rather than at its center, which is consistent with the significantly lower efficiency measured electrically for this head. >

Gradientless Propulsion and State Switching of Bubble Domains

We investigate a class of phenomena which we call bubble automotion whereby a bubble or a bubble lattice is propelled by a time‐modulated field which is homogeneous rather than by locally applied field gradients. Linear translation of certain kinds of bubbles occurs when bias modulation Hb+hz(t) having certain amplitudes and large ∣dhz/dt∣ is superimposed on a steady in‐plane field Hip which is less than Hb. Pulsed hz excitations produce characteristic automotion vectors that can uniquely distinguish for the first time among certain wall states (S,L,P) having a common winding number S but different arrangements of L Bloch lines and P Bloch points. A pair of S=1 bubbles we call σ+ and σ−, translate typically one micron per pulse in opposite directions orthogonal to Hip, i.e., parallel to ±Hb⇒ × Hip⇒. A model consistent with our measurements has the static configurations (1,2,0)+ and (1,2,0)− where the ± refers both to the direction of automotion and the two arrangements (N+, S−) and (N−,S+) of Bloch line p...