Measurement at the magnetic a.c. susceptibility at frequencies between 2 MHz and 1 GHz: zero field spin-spin relaxation of cerium- and neodymium-magnesium nitrate at 4.2 K

An apparatus for measuring the a.c. susceptibility of magnetic systems at frequencies 1 MHz<or= nu <or=1 GHz and at liquid helium temperatures is described. The sample is placed in helical resonators or in the coils of LC circuits. The exciting frequency is locked to the resonant frequency of the resonator, allowing continuous recording of chi ' and chi " as functions of magnetic fields parallel or perpendicular to the a.c. magnetic field. Preliminary results on the spin-spin relaxation of R 2 Mg 3 (NO 3 ) 12 .24H 2 O and R=Ce, 142 Nd are reported.