Peter M. Richards

Quantitative Observation of Slow and Fast Exchange in EPR Spectra of Organic Free Radicals

We have measured effects of electron exchange on the EPR spectrum of the organic free radicals DTBN and DNB/DNB·− in both the low‐concentration and high‐concentration regions. At low concentrations exchange broadens the individual hyperfine lines while at high concentrations exchange narrows the single, merged line. Existing theories are used to calculate the rate constant from both the low‐concentration and high‐concentration data, and consistent results are obtained. Such quantitative agreement has not been previously reported, to our knowledge. In the region of very high concentration, however, intermolecular dipole—dipole interactions contribute to the line breadth making it more difficult to determine the rate constant.

Excitation of Nuclear and Electronic Spin Waves in RbMnF3 by Parallel Pumping

We report observation of the simultaneous excitation of nuclear and electronic spin waves by parallel pumping, an effect predicted by Ninio and Keffer, in RbMnF3 at a pumping frequency of 8.56 GHz. Wavevectors between near zero and 105 cm−1 are excited by varying H. We operate with H and hrf in a [100] direction and H greater than the value required for flopping. From threshold fields, which are about 1.2 Oe minimum, we calculate, at 4.2°K, ηknηke = 6.1×1010 sec−2 for k→0 and 9.5×1010 sec−2 for k = 1.05×105 cm−1 (n stands for nuclear mode, e for electronic). These figures are smaller by two orders of magnitude than what is obtained from ηke inferred by Cole and Courtney from saturation of the main resonance and the value of ηn taken from the half‐width of NMR absorption. To reduce critical‐field data to spin‐wave linewidths we have applied the methods of Ninio and Keffer to obtain a theoretical expression for hcrit in the case of a flopped antiferromagnet.

Direct Excitation of Two Nuclear Spin Waves by Parallel Pumping in CsMnF3

Excitation of two nuclear spin waves, as predicted by Hinderks and Richards, has been accomplished by parallel pumping in CsMnF3 at 988 MHz. This is the first time to our knowledge nuclear spin waves of nonzero wavevector have been excited directly by electromagnetic fields without simultaneous excitation of electronic spin waves or other modes. Wavevectors k from near zero to 105 cm−1 are excited at 4.2° and 1.1°K. Relaxation times are sufficiently long that they can be measured directly from the transient decay, without having to rely on absolute measurement of critical rf fields. Order of magnitude or better agreement is obtained with the theory of Richards on the relaxation rate ηk although the k dependence does not agree at 4.2°K. At 1.1°K, the k dependence is very close to that predicted by theory. The values of ηk at 4.2°K are very close to those inferred by Seavey from more indirect measurements. A second threshold has also been detected at higher powers. This may be the result of nuclear spin and...

Simultaneous fermion and boson spin dynamics in a one-dimensional antiferromagnet. [. cap alpha. -bis (N-methylsabcylamidimato)-copper (II)]

The proton NMR spin-lattice relaxation time T/sub 1/ has been measured in the spin-1/2 one-dimensional antiferromagnet (1DAF) ..cap alpha..-bis (N-methylsalicylamidinato) -copper (II) (CuNSal) at low temperatures and high fields. T/sub 1/ is dominated by electron spin fluctuations of the 1DAF, and we show that these fluctuations are described by Fermi-Dirac statistics for the one-magnon contribution to T/sub 1/ but that the two-magnon part is given by Bose-Einstein statistics.

NMR and AFMR modes in antiferromagnetic MnO

Abstract NMR modes in antiferromagnetic MnO interpreted in terms of a two sublattice model. Observation of AFMR mode at 27.6 GHz which correlates well with this interpretation.

Magnon‐Magnon Interactions as Deduced from Harmonic Generation in YIG

Second‐harmonic generation has been measured in a single‐crystal YIG sphere of ΔH=0.38 Oe at an incident frequency of 8.42 GHz. Both transient and steady‐state effects are reported. The following anomalies exist above the Suhl threshold: (1) there is a sizable variation in the initial part of the harmonic pulse with no corresponding variation seen on the transverse magnetization; (2) after the driving field is turned off, the harmonic power initially increases, and this reradiation effect is again absent in the transverse magnetization; (3) steady‐state harmonic power decreases with increasing incident power over part of the region. Effects (2) and (3) can be explained reasonably well by considering parametric coupling between the spinwaves k initially excited by the Suhl mechanism and other spinwaves k′. In this model, the spinwaves k′ then contribute to the harmonic magnetization π out of phase with that of the uniform mode, accounting for both (2) and (3). These effects cannot be explained simply by lu...