Magnetic excitations of spin and orbital moments in cobalt oxideSpecial issue on Neutron Scattering in Canada.

Abstract

Magnetic and phonon excitations in the antiferromagnet CoO with an unquenched orbital angular momentum are studied by neutron scattering. Results of energy scans in several Brillouin zones in the (HHL) plane for energy transfers up to 16 THz are presented. The measurements were performed in the antiferromagnetic ordered state at 6 K (well below TN ∼290 K) as well as in the paramagnetic state at 450 K. Several magnetic excitation modes are identified from the dependence of their intensity on wavevector and temperature. Within a Hund's rule model the excitations correspond to fluctuations of coupled orbital and spin degrees of freedom whose bandwidth is controlled by interionic superexchange. The different <111> ordering domains give rise to several magnetic peaks at each wavevector transfer.

Low-energy magnetic excitations in Co/CoO core/shell nanoparticles

“…7 b,c. In both samples, two distinct peaks were observed at ≈ 20 meV and 30 meV, in reasonable agreement with previously reported spin wave energies [31][32][33][34] . Just as for the 3 meV E 1 peaks, we see that the linewidths of these higher energy magnetic excitations are substantially larger in the nanoparticle sample, although the excitation energies are only slightly shifted.…”

“…Before we begin our discussion of the physical origin of these excitations, we will establish an explicit comparison of intensities between our experiments and previous triple axis neutron scattering studies that were carried out on CoO crystals [31][32][33][34] . These latter experiments were either carried out at much higher energy transfers than ours, or found no excitations at the low energies of our experiments, focussing instead on the magnetic excitations found at energies of 20 -30 meV and above.…”

“…These data make it clear why the 3 meV excitation has not been observed previously in the triple axis experiments on single crystals of CoO, as its intensity in powdered bulk CoO is ≈ 50 times weaker than that of the 20 meV peak. Indeed, the large counting rate of CNCS was absolutely indispensible for observing this excitation in powdered bulk CoO. There has been considerable discussion of the magnetic excitations in CoO over the past 50 years, including both theoretical analyses 38,39,[67][68][69][70] , and studies of the excitations by both inelastic neutron scattering and optical methods [31][32][33][34][35][36][37][71][72][73][74] . The basic model Hamiltonian for understanding the fundamental excitations in CoO was initially proposed by Kanamori 38 , who observed that the orbital moment of Co 2+ is not completely quenched by the crystal fields, so that the further action of the spin-orbit and exchange interactions ultimately determines the level spacing and dispersions that are observed in experiments.…”

“…We show that this excitation is also present in bulk CoO, although it has not previously been observed. CoO is an ideal benchmark system for comparing the excitations of nanoscaled and bulk systems, which have been extensively studied in the latter using triple axis neutron scattering [31][32][33][34] and optical spectroscopy 35-37 measurements, and where theoretical models for the underlying excitation structure are well developed 34,35,37-40 but still controversial.…”

Neutron scattering investigation of thed−dexcitations below the Mott gap of CoO

“…15,16 The nature of the surface of the CoO crystal was examined with optical and scanning electron microscopy and the results showed there was only one growth domain. X-ray scattering measurements showed that the surface had a mosaic spread of about 0.1…”