The nature of itineracy in CoV₂O₄: a first-principles study

Abstract

Abstract. Inspired by recent experiments, we have theoretically explored the nature of itineracy in CoV 2 O 4 under pressure and investigated, using first principles density functional theory calculations, if it has any magnetic and orbital ordering. Our calculations indicate that there could be two possible routes to obtain the experimentally observed pressure induced metallicity in this system. One is the spinorbit interaction coupled with Coulomb correlation which can take the system from a semiconducting state at ambient pressure to a metallic state under high pressure. The other mechanism, as indicated by our GGA+U calculations, is based on the presence of two types of electrons in the system: localized and itinerant. An effective FalicovKimball model could then possibly explain the observed insulator to metal transition. Comparison of the two scenarios with existing experimental observations leads us to believe that the second scenario offers a better explanation for the mechanism of insulator to metal transition in CoV 2 O 4 under pressure.

Evolution of the magnetic and structural properties ofFe1−xCoxV2O4

“…The characteristic feature of this OO transition is the accompanied cubic to tetragonal structural transition involving a compressed tetragonal distortion (c < a). This OO transition also results in a non-collinear ferrimagnetic (NCF) ordering below 53 K where the V 3+ moments are canted from the [111] direction; (ii) CoV 2 O 4 [19][20][21] exhibits two magnetic transitions at 150 K and 75 K which are CF and NCF transitions, respectively. This sample also shows no OO transition due to the fact that it is approaching the itinerant electron behavior with the small V-V distance.…”

Variety of elastic anomalies in an orbital-active nearly itinerant cobalt vanadate spinel

“…We note here again that, for Co 1+x V 2−x O 4 , the trigonal crystal field splitting of the V 3d orbitals is present even in the cubic crystal structure [ Fig. 3(d)] [ 33,36,40 ]. Thus the elastic anomaly at T 1 only in C t (T ) suggests that the nearly-itinerant doubly-degenerate e ′ g orbitals [ Fig.…”

“…In Co 1+x V 2−x O 4 , the O octahedra surrounding the V atoms are trigonally distorted from the regular octahedral shape even in the cubic crystal structure [ 33,36,40 ]. Additionally, for the V 3d orbitals, the small trigonal crystal field splits the triply-degenerate t 2g orbitals into a localized a 1g orbital and delocalized doubly-degenerate e ′ g orbitals [ Fig.…”

“…3(d)]. Thus it is expected that the delocalized e ′ g orbitals are responsible for the nearly-itinerant character of the V 3d electrons [ 36 ]. The e ′ g orbitals are also expected to be responsible for the elastic anomalies in the paramagnetic phase [ Fig.…”

“…For AV 2 O 4 (A = Zn, Mg, Cd, Mn, and Fe), the structural phase transition is considered to arise from a longrange ordering of the V t 2g orbitals, where the lowering of the lattice symmetry should result in the release of frustration (magnetic ordering). The orbital order to explain the structural and magnetic orders for AV 2 O 4 is still under debate both theoretically [ [25][26][27][28][29][30][31] and experimentally [ 4,7,12,15 ]; it is considered to be driven by the competition among Jahn-Teller coupling, Kugel- ) is a unique vanadate spinel with its highest electrical conductivity among the insulating or semiconducting AV 2 O 4 , which is considered to arise from the nearly-itinerant character of the V 3d electrons [ [33][34][35][36][37][38][39]. CoV 2 O 4 exhibits a ferrimagnetic order below T C ∼ 150 K, which is similar to AV 2 O 4 with magnetic A = Mn (T C = 57 K) and Fe (T C = 110 K).…”