Saturation of thc microwave electron paramagnetic resonance of impurities in various materials produces enhanced and reversed nuclear polarization throughout the material. This effect was investigated and is discussed in some detail for LiF single crystals. Results for other materials at microwave frequencies between 8 and 10 kMc/sec and temperatures between 1.2 and 1.6 K are presented. The variety of materials yielding a measurable effect suggests that this is a very general way of polarizing nuclei. (W.D.M.)
The hyperfine splitting of 165Ho in holmium hydroxide has been studied by spin-echo NMR at liquid-helium temperatures in fields up to 8 T. The behaviour of the dipolar splitting in fields below 0.5 T confirms that the hyperfine parameters are not thermally averaged and that the NMR signals arise solely from ions in the electronic ground state. The field dependence of the hyperfine splitting is not accurately described by crystal-field parameters derived from magnetic susceptibility measurements on Ho(OH)3, but is in close agreement with calculations based on parameters derived from optical spectroscopy of Ho3+ in Y(OH)3. The following quantities are derived from the measurements: spontaneous magnetisation and longitudinal susceptibility of Ho(OH)3 at T=0:M0=(1.181+or-0.001) MA m-1 and chi /sub ///=0.0146+or-0.0005; ratio of nuclear to electronic anti-shielding factors for Ho3+ in the hydroxide: gamma N/ gamma E=149+or-15.
Spin echo measurements have been made, at liquid helium temperatures, on 165Ho in ferromagnetic alloys of holmium with gadolinium, terbium and dysprosium. Intra and extraionic contributions to the hyperfine interaction are discussed, with particular emphasis on conduction electron effects. The hyperfine field Bp due to conduction electrons polarized by the parent holmium ion is found to be +12 T, and the field Bn due to conduction electrons polarized by neighbouring ions is -8.3 S T, where S is the mean ionic spin of the alloy. The contributions from individual neighbour ions to Bn are found to be additive and of moderately long range, and to depend only on the spin of the polarizing ion. Electric quadrupole splittings in the Ho-Tb and Ho-Dy alloys indicate that the electric field gradient due to conduction electrons exceeds that due to the ionic lattice by a factor of about seven.
The surface resistance of single crystal terbium, dysprosium and holmium has been measured at microwave frequencies and at low temperatures. The results are combined with electrical resistivity measurements to yield an effective microwave permeability μR as a function of temperature. It is found that μR remains close to unity except for pronounced peaks in the neighbourhood of the Curie and Néel temperatures. The data on μR are used to obtain estimates of enhancement factors for nuclear magnetic resonance in the three metals. An anomalous absorption effect in powdered terbium is briefly discussed.
We have studied the field dependence of the hyperfine splitting of holmium as a dilute substituent in a lamina of dysprosium prepared by molecular-beam epitaxy. The measurements were made by spin-echo NMR at temperatures below 2 K and in fields up to 8 T. In fields above 3 T we obtain sharp seven-line spectra which are characteristic of 165Ho in fully polarized holmium ions in a ferromagnetically ordered host medium. The resolution is markedly superior to that obtained in bulk rare-earth alloys and has enabled us to resolve features hitherto unmeasurable. The form of the zero-field spectrum suggests that helimagnetic and ferromagnetic phases coexist in the zero-field-cooled material. Magnetization measurements support this interpretation.
In EPR (and in its close relative NMR) there exists a peculiarly intimate connection between the spectrometer and the system under observation. A book which relates the various techniques of EPR to their manifold applications is therefore to be welcomed.
The hyperfine interaction of praseodymium, as a dilute substitutional impurity in GdAl2 and in NdAl2, has been studied by spin-echo NMR at 1.4K. The electric quadrupole splitting is unresolved in the frequency domain, but has been measured by observing the oscillatory decay of the echo amplitude. From the magnetic dipole splitting the authors deduce, after allowance for extra-ionic contributions, that the Pr3+ moments in GdAl2 and NdAl2 are (2.85+or-0.03) mu B and (2.96+or-0.03) mu B, respectively. Theoretical analysis of these results yields effective exchange constants for Pr3+:GdAl2 and for Pr3+:NdAl2 which differ significantly from the values to be expected on the conventional model of indirect lanthanide-lanthanide exchange.
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