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Low temperature paramagnetic resonance studies of the rare earth group ions using a new high sensitivity spectrometer Buckmaster, Harvey Allen

Abstract

A high sensitivity, wide or narrow band, double field modulation paramagnetic resonance spectrometer operating at a wavelength of 1.25 cm. for use at liquid helium temperatures has been developed which is described in detail. This spectrometer employs a transmission type cylindrical resonant cavity operated in the H₁₁₁ mode. In wide band operation, the magnetic field is modulated at 60 cps. to an-amplitude in excess of the resonance line width and at 462.5 kcs. to an amplitude less than or equal to the line width. For narrow band operation only, the high frequency modulation is employed and the static magnetic field is linearly swept. A signal from 10⁻⁹ grams of diphenyltrinitro phenyl hydrazyl [(C₆H₅)₂•N-N•C₆H₂•(NO₂)₃] has been observed at 290°K. using wide band operation (8 kcs.) indicating that sensitivities of the order of 10⁻¹² grams can be achieved with it at 4.2°K. in narrow band operation (1 cps.). This sensitivity is close to the theoretical value predicted by Bleaney of 10⁻¹³ grams and is several orders of magnitude greater than that reported for any other paramagnetic resonance spectrometer. Using this improved sensitivity, higher order transitions ΔM = ± 2, ± 3 in dilute gadolinium ethyl sulphate [4f⁷; ⁸s₇͵₂] which were not previously observable, have been studied at 90°K. as a function of the orientation of the magnetic field with respect to the axis of symmetry of the crystal. These transitions show that the effect of off-diagonal terms in spin-Hamiltonian have a greater effect on the energy levels than had previously been appreciated from measurements of the ΔM = ±1 transitions and help to explain the discrepancies between the calculated and observed zero field splittings. An excited state in dilute dysprosium ethyl sulphate [4f⁹; ⁸H₁₅͵₂] has been observed at 4.2°K. with g₁₁ =5.85 ± 0.05 which had previously only been observed at 20°K. with g₁₁ = 5.80 ± 0.02 and g⊥ = 8.40 ± 0.2. The line width of dilute praseodymium ethyl sulphate [4f²; 3H₄] has been measured at 4.2°K. and found to be 35 ± 5 gauss showing that the previously observed line width of 200 gauss at 20°K. is due to spin-lattice broadening.

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