Experiments are described in which the nuclear Zeeman temperature T n and the temperature of the electron dipole-dipole interaction system T^ in a dilute paramagnetic crystal are measured simultaneously. The spin-lattice relaxation times of these two systems turn out to be equal, thus yielding a direct evidence of the strong coupling between these systems.The concept of spin temperature in a rotating frame as introduced by Redfield 1 has led to a better understanding of the behavior of spin systems in solids subjected to strong rf fields. Redfield's theory, which was only valid in the limit of strong rf saturation, was extended by Provotorov 2 to rf fields of arbitrary intensity. He showed that in strong static magnetic fields two distinct temperatures should be assigned to the spin system: one temperature T e for the Zeeman system and a temperature T-^ to describe the dipole-dipole interaction system. He also derived equations for the time dependence of both temperatures under the influence of an rf field and of spin-lattice relaxation. Further calculations of the shape of the resonance absorption signal when Provotorov's concept of an interaction temperature Tj n t was taken into account were carried out by Rodak. 3 Atsarkin and Morshnev 4 performed line-shape experiments which confirmed the existence of a temperature T in |-different from T e . They were able to observe the time dependence of T e and T int by studying the shape of a paramagnetic resonance line which had been saturated by a slightly detuned microwave field.An extension of these ideas to explain the results of nuclear dynamic polarization and nuclear relaxation experiments in dilute paramagnetic crystals provided further evidence for the existence of an interaction system of the paramagnetic ions. Actually measurements of the proton spin-lattice relaxation times in such crystals led to the idea that the nuclear Zeeman system could be strongly coupled to the interaction system of the paramagnetic ions. 5 Nuclear-dynamic-polarization experiments were carried out in order to confirm these views. 6 In performing these experiments however, only the nuclear resonance signal, which is inversely proportional to the nuclear Zeeman temperature T ny was monitored. Thus only indirect information about the behavior of the interaction system was obtained.In the experiments to be discussed below the nuclear Zeeman temperature T n as well as the interaction temperature T int were measured simultaneously in order to prove that T n and T^njhad the same time behavior, with a relaxation time equal to the nuclear spin-lattice relaxation time. This would be a direct proof of the strong coupling of the nuclear spins to the electronspin interaction system.The experiments were performed at 1.5 K in a single crystal of La 2 Mg 3 (NO 3 ) 12 0.24H 2 O containing 2% Nd. The magnetic field H 0 of about 2.5 kG was directed perpendicular to the c axis. The paramagnetic resonance line of the Nd ions was observed at 9000 MHz, the proton resonance of the crystal waters occured a...

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