Magnetic ground state, field-induced transitions, electronic structure, and optical band gap of the frustrated antiferromagnet GeCo2O4

Abstract

Systematic studies of magnetic ordering, magnetic-field-induced transitions, electronic structure, and optical properties of the frustrated spinel GeCo 2 O 4 (GCO) are reported. Our results reveal that GCO orders antiferromagnetically (AFM) at T N = 20.4 K but with significant short-range ferromagnetic (FM) order up to T ∼ 5 T N . The paramagnetic susceptibility ( χ ) fits the modified Curie–Weiss law, χ = χ o + C /( T − θ ), with θ = + 51 K for 100 K < T < 800 K. The fit to high-temperature-series expansion of χ ( T ) yields J 1 / k B = 14.7 K as the dominant FM exchange coupling for the pyrochlore lattice of Co 2 + spins consisting of alternate planes of Kagomé (KGM) and Triangular (TRI) spins lying perpendicular to [111] direction. From the analysis of the M-H plots at 2 K and published results, three critical fields are identified: H d ∼ 11 kOe due to AFM domains, H C 1 ≈ 44 kOe related to spin-flips and FM ordering of the TRI spins, and H C 2 ≈ 97 kOe related to FM ordering of the KGM spins. For H > H C 2 , GCO is a forced ferromagnet with some canting of the spins. Magnetic field dependence of T N follows the relation T N ( H ) = T N (0)– D 1 H 2 valid for antiferromagnets with D 1 = 6.63 × 10 − 10 K/ Oe 2 . This magnitude of T N (H) along with the temperature dependence of H d , H C 1 , and H C 2 are used to construct the H-T phase diagram. From the magnitudes of the Curie constant ( C ) and the saturation magnetization at 2 K it is shown that Co 2 + ions in GCO have the ground state with effective spin S = 1/2. High resolution x-ray photoelectron spectra of 2p and 3d orbitals of Co and Ge confirm the divalent and tetravalent electronic states of Co and Ge, respectively, in GCO. The energy band gap ( E g = 3.28 eV) evaluated using DFT+U calculations is in good agreement with the experimental results ( E g = 3.16 eV) obtained from the diffuse reflectance spectroscopy.