P. C. Riedi

Quasi-optical cw mm-wave electron spin resonance spectrometer

We describe a novel cw millimeter-wave electron spin resonance (ESR) spectrometer designed to operate in the frequency range of 80–200 GHz and in the temperature range of 2.5–300 K, which may be easily scaled to higher frequencies. The spectrometer uses a bimodal reflection cavity coupled to a circular corrugated guide and uses Gaussian quasi-optics for most of the front-end signal processing. This technique has very low insertion loss and allows a number of sophisticated measurement techniques to be employed including induction operation, which significantly reduces the effect of microphonics and stray reflections. A number of examples are given illustrating the sensitivity of the instrument and the advantages of using ESR at high fields.

Luminescence sensitivity changes in natural quartz induced by high temperature annealing: a high frequency EPR and OSL study

Quartz undergoes very significant luminescence sensitivity changes after high temperature annealing (0-1200 °C), with particular enhancement occurring between the phase transition temperatures 573 and 870 °C. In order to understand why this occurs, high frequency electron paramagnetic resonance (EPR), operating at 90 GHz, has been used to monitor the structure and population of defects in natural sedimentary quartz, following annealing and -irradiation. The results are compared with the optically stimulated luminescence (OSL) data of the same samples. It is shown that: (i) the structure and population of the dominant [AlO4 ]0 recombination centres are largely unaffected by the annealing process; (ii) the oxygen vacancy E´ centres are destroyed when annealed at temperatures between the phase transitions and; (iii) the numbers of both [TiO4 /H+ ]0 and [TiO4 /Li+ ]0 donors increase between 400 and 700 °C. Photo-EPR spectra are presented, providing evidence that both the Ti associated donors and Al acceptors are directly involved in the OSL process. The heat-induced changes in the population of these EPR defects is mirrored in part by the change in the luminescence sensitivity of several OSL components. Evidence is also presented suggesting that E´ may act as non-radiative centres competing in the OSL process.

A computer controlled spin echo spectrometer for the study of ferromagnetic materials

The computer control of a broad band (10–1,000 MHz) untuned NMR spin echo spectrometer is described with examples of distribution of hyperfine field as a function of frequency and of modulated echo decay obtained automatically.

The volume dependence of the magnetization and NMR of Fe4N and Mn4N

The forced volume magnetostriction and the NMR of ferromagnetic FeN and ferrimagnetic Mn4N have been measured in the ordered state. At 4.2 K the magnetization ( sigma ) per unit mass and the effective field (HI) at the nucleus of the Fe(I) site of Fe4N are found to change with pressure such that delta ln sigma / delta P approximately= delta ln mod HI mod / delta P 0. The result for Mn4N is interpreted as showing that the magnitude of the small (negative) moment on the Mn(II) site decreases more rapidly under pressure than does the large Mn(I) moment. The value of delta ln mod HI mod / delta P for Mn4N was found to be a strong function of temperature such that delta lnTN/ delta P approximately=5.2 Mbar-1. An analysis of the Mn4N NMR data suggests that the discrepancy between the calculated (spin) and measured moment at the Mn(I) site may be due to a small orbital moment at that site.

The magnetic moment at the yttrium site in Y-Fe compounds: pressure dependence of the magnetisation and hyperfine field

The pressure dependence of the magnetisation of Y2Fe17, Y6Fe23 and YFe2 at 4.2 K has been deduced from forced volume magnetostriction measurements in fields up to 12 T. The pressure dependence of the 89Y hyperfine field in the above compounds and YFe3 has been measured using NMR. The results are consistent with a model in which a moment of approximately=-0.4 mu B exists at the Y site of all four compounds as found in computer calculations for YFe2.

EPR investigations on europium(II)-doped aluminates

The europium(II)-doped aluminates, SrAl2O4 and CaAl2O4with β-tridymite structure and BaMgAl10O17 with β-Al2O4 structure, have been investigated by means of EPR spectroscopy. From the complicated X-band EPR spectra observed for SrAl2O4:Eu,Dy and CaAl2O4:Eu,Nd it is deduced that they have large zero-field splittings in contrast to BaMgAl10O17:Eu. High frequency EPR spectroscopy at 180 GHz shows symmetrical weak satellites on both sides of the strongest resonance centred around at 6.45 T. Spectral simulation with second order perturbation has made it clear that for SrAl2O4:Eu,Dy the europium(II) ions occupy three different sites, while for the CaAl2O4 they occupy a single site with trigonal anti-prism symmetry, despite the fact that there are two other sites associated with octahedral symmetry.

Volume magnetostriction and pressure dependence of the Curie point and spontaneous magnetization of weakly ferromagnetic Y(Co1-xAlx)2

The pressure dependence of the Curie point of weakly ferromagnetic Y(Co1-xAlx)2 alloys for x approximately 0.15 has been measured at pressures up to 8 kbar. The pressure dependence of the spontaneous magnetization per unit mass at 0 K ( sigma 0) has been deduced for the same alloys from measurements of the forced volume magnetostriction in fields up to 12 T. It was found that ( delta ln Tc/ delta ln V)=( delta ln sigma 0/ delta P)=120+or-17, one of the largest values known for a ferromagnetic material. The critical pressure for the collapse of ferromagnetism is approximately 9+or-1 kbar for 0.14<x<0.18, while the chemical pressure exerted by the Al in Y(Co0.85Al0.15)2 is approximately -40 kbar, so weak ferromagnetism would not be possible in this Y-Co-Al system if the material was clamped to the volume of YCo2.

Single and double quantum transitions in the multi-frequency continuous wave electron paramagnetic resonance (cwEPR) of three six co-ordinate nickel(II) complexes: [Ni(EtL)2(Me5dien)] and [Ni(5-methylpyrazole)6]X2, X = (ClO4)− or (BF4)−. The single crystal X-ray structure at room temperature of [Ni(5-methylpyrazole)6](ClO4)2

Multi-frequency, variable temperature cw EPR powder spectra of [Ni(EtL)2(Me5dien)] (3) and [Ni(5-methylpyrazole)6]X2, X = (ClO4)− (1) or (BF4)− (2) are reported. The spectra of 1 and 2 show a temperature variation which is consistent with an increase in ∣D∣ as the temperature decreases. The changes in the spectra of 1 are irreversible. The low temperature spectra for each compound exhibit a double quantum transition whose powder dependence is the same as that for the single quantum transitions. The reasons for this behaviour, as well as the simulations of the whole spectra at various frequencies, is discussed. The single crystal X-ray structure of 1 at room temperature is reported, whilst it was found that these crystals disintegrated on cooling. This behaviour is consistent with the change in the EPR behaviour of this compound on cooling.

Pressure dependence of the hyperfine field of YFe2 and ZrFe2

Abstract The hyperfine field (H) of Y, Zr and Fe in YFe2 and ZrFe2 has been measured at 4.2 K as a function of pressure up to 8 kbar. Energy band calculations for H and ∂ ln H/∂P are in good agreement with experiment and provide an explanation of the observed differences in the pressure dependences.

High field ferromagnetic resonance measurements of the anisotropy field of longitudinal recording thin-film media

The average value of the magnetocrystalline anisotropy field, Hk, is an important parameter for the characterization of magnetic recording media but is difficult to measure accurately due in part to the effect of interactions between the grains. In order to evaluate Hk we have studied two model CoCrPtTa magnetic films using a number of complementary techniques: high field ferromagnetic resonance (FMR) (35.0–45.0 kOe), low field (<20 kOe) vector vibrating-sample magnetometry and torque magnetometry. The FMR measurements were performed at a number of discrete frequencies in the range 75–93 GHz using a new quasi-optical spectrometer developed at the University of St. Andrews. The values of Hk derived by FMR (10.8 kOe) are approximately 10% greater than those obtained from conventional magnetometry (9.6 kOe). This difference is believed to be due to the presence of intergranular exchange coupling which reduces the measured value of anisotropy when the applied field is not sufficiently large to completely alig...