A. M. Portis

Nuclear Resonance in Ferromagnetic Cobalt

The observation of nuclear magnetic resonance in ferromagnetic cobalt is reported. The resonance frequency for finely divided face-centered-cubic material has been measured in the intermediate temperature range. The frequency extrapolated to 0°K is 217.2 Mc/sec, and the temperature dependence in this range is in general agreement with that of the magnetization. This frequency implies a hyperfine field of 217 500 oe, which is in good agreement with the field deduced from specific heat measurements on hexagonal cobalt. The agreement in the two structures indicates that there is no dipolar contribution to the hyperfine field. The theoretical implications of this observation are discussed. The resonance line is inhomogeneously broadened with a half width of 400 kc/sec. A pattern of beats is observed at high passage rates which makes it possible to determine a spin-spin time of 25 μsec. By varying the modulation frequency under conditions of intermediate saturation the spin lattice relaxation time is measured ...

Microwave Superheterodyne Induction Spectrometer

The limits of electron spin resonance detection sensitivity are investigated for absorption and induction spectrometers employing superheterodyne detection. The sensitivity of the conventional bridge absorption spectrometer is found to decrease rapidly below the theoretical limit set by the noise figure of the receiver when klystron power is increased above about 10 mw. This decrease is shown to arise from the extreme frequency dependence of the bridge balance. The bimodal cavity induction spectrometer, which has a highly stable frequency independent balance, shows no departure from the theoretical limit. Flicker noise is found to be produced by the i.f. current in the mixer diodes, but it may be greatly reduced by the use of high i.f. gain. The use of a balanced homodyne detector in place of the conventional second detector is found to reduce the over‐all noise figure of the receiver from 13 to 9 db by suppression of carrier noise. The performance of the bridge spectrometer is greatly improved by the hom...

Microwave Faraday Rotation: Design and Analysis of a Bimodal Cavity

The design and analysis of a bimodal cavity for the observation of microwave Faraday rotation is presented. An equivalent circuit of lumped elements is developed and the coupling between degenerate cavity modes is expressed in terms of the elements of the susceptibility tensor of the material producing the rotation. The theory is checked against experimental results with a paramagnetic salt and substantial agreement is obtained. A cavity of this type when used in conjunction with superheterodyne detection appears to provide a high sensitivity spectrometer for the observation of magnetic resonance.

Mössbauer Spectra of 57Fe in Superparamagnetic Nickel

The Mossbauer effect is used to study the superparamagnetic behavior of ultrafine particles of nickel dispersed on a high‐area substrate. In zero applied field, a characteristic hyperfine spectrum is observed at 78°K. As the sample is warmed to room temperature, the spectrum narrows to a single line as the result of rapid fluctuations in the magnetization direction. A fluctuation time of the order of 2 nsec is indicated from the room‐temperature data. With the application of a magnetic field the hyperfine spectrum is developed at room temperature. Although it is not possible to assign a single relaxation time to the magnetic‐field data, there is some indication that the fluctuations are slowed by the application of an external magnetic field.

Transient Excitation of Nuclei in Ferromagnetic Metals

Free precession signals and spin echoes have been observed from Fe57, Co59, and Ni61 nuclei in finely divided multidomain iron, cobalt, and nickel, respectively. Spin-lattice relaxation has been studied in all three metals from temperatures in the liquid helium range to room temperature or above. Although the relaxation is not strictly exponential, the rate of recovery of the magnetization appears to be proportional to the absolute temperature. Spin-spin relaxation has been studied in natural cobalt and in both natural and enriched samples of iron and nickel. The spin-spin coupling is very much stronger in cobalt than in iron or nickel, as expected, because of the very much larger nuclear magnetic moment of Co59. Spin diffusion through the frequency spectrum of cobalt has been investigated at 4.2 and 77°K by the stimulated echo technique. The results are consistent with a theory of one-dimensional diffusion with exchange rate and exchange distance of the order of 1/T2. The spin-echo technique has also bee...

Excitation of Nuclear Magnetic Resonance Modes in Antiferromagnetic KMnF3

The Mn55 nuclear resonance absorption in KMnF3 has been monitored through the observation of antiferromagnetic resonance at 4.2°K and below. It is found that with increasing rf power a second antiferromagnetic resonance grows at a shifted field position. At 4.2°K a shifted resonance can be observed for driving frequencies between 580 and 680 Mc. As the driving frequency is reduced the position of the shifted resonance moves toward that of the unshifted line. These results are interpreted in terms of the nucleation of regions of the sample with nuclear resonance at the driving frequency. Such regions are present in the absence of excitation as the result of electronic pinning. With rf excitation these regions are expected to grow. The power level at which substantial growth takes place is in good agreement with theory.

Direct Mn55 NMR Absorption in Antiferromagnetic KMnF3

The direct observation of Mn55 nuclear magnetic resonance absorption in the nearly cubic antiferromagnet KMnF3 is reported. The striking features of the resonance are its unusual intensity and its strong temperature and field dependence. Both effects are associated with the strong coupling between nuclear and electronic modes in magnetic materials with low anisotropy and large nuclear magnetization. The nuclear linewidth at 4.2°K is 740 kc/sec between points of half‐maximum absorption. This width is in agreement with that expected from the Suhl‐Nakamura indirect coupling between nuclear spins. At lower temperatures the linewidth increases greatly, approaching nearly 3 Mc/sec at 2.1°K. This unexpected increase in linewidth is attributed to a low‐temperature inhomogeneity in the antiferromagnetic excitation spectrum.

Microwave Transmission Through Permalloy Films

A new interference effect has been observed in the transmission of microwave signals through Permalloy films. This effect, which involves interference between the eddy‐current microwave transmission and the magnetic field associated with the excited spin waves, permits identification of the symmetry and surface pinning behavior of the standing spin waves. Previous studies of spin waves in Permalloy films have involved a determination of the surface impedance in the vicinity of spin‐wave excitation. The present measurements show a decrease in transmitted microwave power for the lowest‐order spin‐wave mode and for those higher‐order modes whose magnetic excitation has even symmetry with respect to the plane of the film. However, corresponding to intermediate modes, which have odd symmetry, we observe a transmission maximum. Thus we observe alternate minima and maxima on transmission. The measured fields correspond exactly with the field positions of the alternate strong and weak maxima in surface impedance,...