Search Results

A new millimetre wave magnetic resonance spectrometer, using broad-band components, is described. A `magic tee' provides both a balanced harmonic generator and a detector of the harmonic signal which has passed through the sample in the spectrometer. Studies have been made of a simple reflection system with no resonant cavity at about 4.7 mm wave-length, using standard 8 mm components. With a power of about 30 MW incident on the harmonic generator crystals, a time constant of 1 sec and a magnetic field modulation frequency of 630 c/s, approximately 9 × 10 ¹⁵ paramagnetic spins of 1 gauss line-width can be detected. The device is primarily intended for antiferromagnetic resonance studies and is in principle capable of operating at much shorter millimetre wavelengths.

Antiferromagnetic resonance over a range of temperature, frequency and orientation has been observed at millimetre wavelengths in single crystals of orthorhombic Cr ₂BeO ₄ ( T _N = 28 °K). Critical-field resonances occur along both b and c axes, at magnetic fields of 29.5 kG and 47.7 kG respectively, and a non-collinear spin array is inferred. The possibility of a spiral spin structure is considered.

Millimetre wave antiferromagnetic resonance experiments have been made in α-Fe ₂O ₃ using both d.c. and pulsed magnetic fields. Both the weak-ferromagnetic modes above the Morin transition temperature and the usual low-temperature antiferromagnetic resonance were examined. The theoretical expression for the frequency dependence of the low-frequency weak-ferromagnetic mode has been verified up to 40 kG. The angular dependence of this mode at 68·7 Gc/s shows a departure from theory, however, although lower-frequency measurements do not. The angular dependence of the high-frequency weak-ferromagnetic mode observed at 142 Gc/s and 273 °K is in satisfactory agreement with theory. The low-temperature antiferromagnetic resonance for H ₀ along the c axis is similar to previous results; however, off-axis the resonance is observed to greater angles than expected, possibly as a result of a non-zero canting field when the spins deviate from the

Describes a simple electronic arrangement which can be used to protect klystrons and other devices against overheating.

Electron spin resonance and magnetic susceptibility measurements on polycrystalline Mn ₃TeO ₆ are reported. The results indicate an antiferromagnetic transition below about 20K. Curie-Weiss behaviour is observed above 100K, with theta =-125+or-20K;.

Electron paramagnetic resonance at 36 GHz has been studied in the layer compound NaCrS ₂. Below 120 K, the linewidth diverges as (T-T _N) ^-p, with p//=0.71+or-0.07 and p perpendicular to =0.67+or-0.05 parallel and perpendicular to the c-axis. The temperature-dependent part of the linewidth ( Delta B- Delta B _infinity ) is also proportional to (T-T _N) ^-p, with a change of exponent at T/T _N approximately=1.7. Lineshifts of the EPR are also observed. The dipolar fields arising from the bulk magnetization can account for these shifts at temperatures down to at least 50 K, but within 10 to 20 K of T _N there are extra shifts presumably arising from short range order.

The electron spin resonance spectra of Gd _1-xDy _xAl ₂ have been observed for compounds with 0<or=x<0.2. The linewidths of the spectra may be interpreted in terms of a modified Hasegawa theory and provide evidence for the existence of large anisotropic exchange contributions at the Dy site. A negative g shift is found which is difficult to interpret on the basis of presently available theories. The lattice parameters and Curie temperatures of the compounds have been measured for 0<or=x<or=1.

Electron spin resonance measurements are reported for the (Gd, Y)Ir ₂ pseudobinary series. The results indicate that the ESR bottleneck of gadolinium can be broken by dilution with yttrium. The parameters relating to the relaxation rates have been determined by fitting the data to Hasegawa's theory (1959). The results are shown to be consistent with the behaviour of other gadolinium-transition metal compounds.

Electron spin resonance (ESR) measurements have been performed on Gd _0.5Dy _yY _0.5Ir ₂ with y<or=0.007 in order to investigate the effect of dilute concentrations of Dy on the Gd resonance. The ESR bottleneck is opened due to a large relaxation rate of conduction electron spins to Dy. This reveals the significance of anisotropic interactions associated with Dy. Addition of Dy also increases the unbottlenecked limits of the g shift and the temperature gradient of the linewidth.

The crystal structure and magnetic properties of the pseudobinary system Gd(Co _xIr _1-x) ₂ are reported. The compounds crystallise in the C15 Laves phase with a sudden lattice contraction at x=0.4. Magnetic measurements indicate that Co develops a magnetic moment for x>0.4. The onset of this moment is coincident with a sudden contraction of the lattice. Electron spin resonance measurements of g shift and linewidth also show sharp increases at x=0.4. The correlated changes in the various properties are interpreted as due to a movement of the Fermi level as Co is substituted for Ir.