Modern Electron Paramagnetic Resonance Techniques and Their Applications to Magnetic Systems

Abstract

Ever since its invention in 1940s the electron paramagnetic resonance (EPR) has been one of the most important and versatile spectroscopic methods for studies of new materials. Numerous approaches ranging from conventional continuous wave to more modern pulsedEPRtechniques were introduced to provide insight into their structure, dynamics, or chemical reactivity at the local scale. More recently, efforts to increase sensitivity and enhance resolution led to highly innovative detection methods and expandedEPRto high‐frequency high‐magnetic field experimental regimes. Such intensive experimental instrumentation development together with parallel theoretical breakthroughs in particular enabledEPRto provide unique information about the spin Hamiltonians and the nature of low‐energy excitations in magnetic systems. Here, we review such modern approaches for studies of low‐dimensional and frustrated quantum antiferromagnets and provide several examples where full strength of theEPRtechnique is demonstrated. We provide thorough overview of the “conventional” Kubo–Tomita theory and critically comment its applicability to one‐dimensional spin systems. There, alternative approaches based on exact computations of theEPRresponse and Oshikawa–Affleck field theory have been successfully implemented. Finally, we also review the (anti)ferromagnetic resonance in the magnetically ordered phase. Important instrumental and theoretical advances that we have witnesses in the last couple of decades significantly extend the range of applicability ofEPRand shall lead to many new discoveries in the future.