Magnetic order, metamagnetic transitions, and low-temperature spin freezing in Ca3Co2O6: an NMR study

TL;DR

This NMR study of Ca3Co2O6 reveals detailed magnetic structures, phase transitions, and spin dynamics, highlighting the system's strong Ising character, the nature of magnetic phases, and the slow spin fluctuations associated with metamagnetic transitions.

Contribution

The paper provides the first precise NMR measurements of electric field gradients, chemical shifts, and internal magnetic fields in Ca3Co2O6, clarifying magnetic interactions and spin dynamics.

Findings

Identification of distinct internal fields for ferri- and ferromagnetic phases

Disproof of Co3+(I) orbitals participating in superexchange

Detection of glassy spin relaxation dynamics near critical fields

Abstract

We report on a 59Co NMR investigation of the trigonal cobaltate Ca3Co2O6 carried out on a single crystal, providing precise determinations of the electric field gradient and chemical shift tensors, and of the internal magnetic fields at the non-magnetic Co I sites, unavailable from former studies on powders. The magnetic field-induced ferri- and ferromagnetic phases at intermediate temperature (e.g. 10 K) are identified by distinct internal fields, well accounted for by purely dipolar interactions. The vanishing transferred hyperfine field at the Co I site indicates that the Co3+(I) orbitals do not participate in the intra-chain superexchange, in disagreement with a previous theoretical model. The strong Ising character of the system is confirmed experimentally by the field dependence of the resonance lines, indicating that local moments are saturated even at the phase boundaries. In the vicinity of the critical fields, nuclear spin-spin relaxations detect the spin reversal dynamics of large magnetic assemblies, either Ising chain fragments or finite size domains, which drive the metamagnetic transitions. Such collective excitations exhibit a glassy behavior, slowing down to subacoustic frequencies and freezing at low temperature. The relevance of such slow fluctuation modes for the low-temperature multi-step behavior reported in the magnetization is discussed.