Magnetic order in2p-electron systems: Electron paramagnetic resonance and antiferromagnetic resonance in the alkali hyperoxides KO2, RbO2, and CsO2

Abstract

The low-temperature phases of the alkali hyperoxides K${\mathrm{O}}_{2}$, Rb${\mathrm{O}}_{2}$, and Cs${\mathrm{O}}_{2}$ have been investigated by means of electron paramagnetic resonance (EPR) and antiferromagnetic resonance (AFMR). The knowledge of their structural and magnetic properties could be enlarged substantially. The orientation of the ${\mathrm{O}}_{2}^{\ensuremath{-}}$ molecule ions in the phases IV and V of K${\mathrm{O}}_{2}$ was determined accurately. It changes considerably at the phase transition IV \ensuremath{\leftrightarrow} V (11 K). For K${\mathrm{O}}_{2}$, Rb${\mathrm{O}}_{2}$, and Cs${\mathrm{O}}_{2}$ the experimental results are compatible with antiferromagnetic order at low temperatures. The magnetic properties are discussed within the framework of a mean-field approximation. The pertinent parameters describing the spin-spin interactions were determined for Rb${\mathrm{O}}_{2}$ and Cs${\mathrm{O}}_{2}$ and some of them for K${\mathrm{O}}_{2}$ using the N\'eel temperature, the Curie-Weiss temperature, the spin-flop field, the temperature dependence of the EPR resonance field, and the AFMR frequencies. In Rb${\mathrm{O}}_{2}$ and Cs${\mathrm{O}}_{2}$ the anisotropy of the mean-field parameters originates predominantly in the anisotropic exchange interaction. The combination of electron paramagnetic resonance and antiferromagnetic resonance data yields information about the magnetic structure: The easy axis of magnetization is perpendicular to the internuclear axis of the ${\mathrm{O}}_{2}^{\ensuremath{-}}$ ions, and the hard axis is parallel to it for all three compounds.