Phase transitions reflected in HF-EPR spectra

Abstract

EPR is a technique limited to unpaired electrons. Current state of the art set-ups operate at frequencies of 130 GHz or higher. The benefits are increased resolution and the ability to measure large D systems. Using EPR data combined with other techniques systems in which (magnetic) phase transitions occur are studied. Multifrequency EPR measurements on Ni2+ ions above and below the phase transition temperature of the Zn(en)3(NO3)2 matrix showed that the signal observed alongside the S=1 multiplet was not the assumed double quantum resonance was in fact an S = 3/2 impurity. Pure manganocene is a salt with an elusive ground state that can exist in both a low- and high spin state. By combining magnetization and HF-EPR measurements at various temperatures both ground states appear to be detected. Closer examination showed that the alleged S = u signal can be attributed to an antiferromagnetic resonance. At low field and temperature this is the predominant signal in the EPR spectrum. The ground state of solid molecular oxygen was assumed to be a ridged S=1 spin system. HF-EPR measurements on molecular oxygen at various concentrations in inert gas host matrices indicated that the S=1 spin system exhibits a sharp decrease of the total intensity when the temperature is raised to 25 K. Simultaneously a broad quasi continuum around g=2 could be observed up to the melting point of the host matrix. Combined with literature data this indicates that under certain conditions the S=1 ground state is disrupted. The resulting S = u spinsstate are exchange broadened due to their close proximity.