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

Pramanik, P and Ghosh, S and Yanda, P and Joshi, DC and Pittala, S and Sundaresan, A and Mishra, PK and Thota, S and Seehra, MS (2019) Magnetic ground state, field-induced transitions, electronic structure, and optical band gap of the frustrated antiferromagnet GeCo2O4. In: PHYSICAL REVIEW B, 99 (13).

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Abstract

Systematic studies of magnetic ordering, magnetic-field-induced transitions, electronic structure, and optical properties of the frustrated spinel GeCo2O4 (GCO) are reported. Our results reveal that GCO orders antiferro-magnetically (AFM) at T-N = 20.4 K but with significant short-range ferromagnetic (FM) order up to T similar to 5 T-N. The paramagnetic susceptibility (chi) fits the modified Curie-Weiss law, chi = chi(o) + C/(T - theta), with theta = +51 K for 100 K < T < 800 K. The fit to high-temperature-series expansion of chi (T) yields J(1)/k(B) = 14.7 K as the dominant FM exchange coupling for the pyrochlore lattice of Co2+ spins consisting of alternate planes of Kagome (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 similar to 11 kOe due to AFM domains, H-C1 approximate to 44 kOe related to spin-flips and FM ordering of the TRI spins, and H-C2 approximate to 97 kOe related to FM ordering of the KGM spins. For H > H-C2, 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)-D1H2 valid for antiferromagnets with D-1 = 6.63 x 10(-10) K/Oe(2). This magnitude of T-N(H) along with the temperature dependence of H-d, H-C1, and H-C2 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 Co2+ 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.

Item Type:	Journal Article
Publication:	PHYSICAL REVIEW B
Publisher:	AMER PHYSICAL SOC
Additional Information:	Copyright of this article belongs to AMER PHYSICAL SOC
Keywords:	RAY PHOTOELECTRON-SPECTROSCOPY; TOTAL-ENERGY CALCULATIONS; PERFORMANCE; SPECTRA; SUSCEPTIBILITY; SERIES; CO3O4
Department/Centre:	Division of Physical & Mathematical Sciences > Physics
Date Deposited:	29 May 2019 11:32
Last Modified:	29 May 2019 11:32
URI:	http://eprints.iisc.ac.in/id/eprint/62590

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