Discovery of Ferromagnetic-Half-Metal–to–Insulator Transition inK2Cr8O16

Abstract

The hollandite chromium oxide K2Cr8O16 has been synthesized in both powder and single-crystal form under high pressure. Combining electrical resistivity, magnetic susceptibility, and x-ray diffraction, we found that K2Cr8O16 is a ferromagnetic metal (or half-metal) with T(C)=180 K and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. K2Cr8O16 is quite unique in three aspects: It has a rare mixed valence of Cr3+ and Cr4+; it has a metal (or half-metal)-to-insulator transition in a ferromagnetic state; and the resulting low-temperature phase is a rare case of a ferromagnetic insulator. This discovery could open a new frontier on the relation of magnetism and conducting properties in strongly correlated electron systems.

Colossal Magnetoresistance and Magnetism of NaCr2O4

“…Because the structure is composed of double rutile chains as shown in Fig. 1, the compound is expected to show interesting electronic properties originating from spin frustrations and one-dimensional electron correlations, as in the case of other compounds with double rutile chain structures [2][3][4][5][6][7][8]. In addition to the structural feature, the compound has mixed-valence state of Cr 3.5+ although less Cr oxides accept a mixed-valence state.…”

“…The ligand holes often cause interesting properties. Indeed, K 2 Cr 8 O 16 with Cr 3.75+ ions show unusual metal-insulator transition in a ferromagnetic state [5]. It may be noticed that the ligand holes are generally observed in transition metal oxides with extremely high valence, such as CaFeO 3 and LaNiO 3 , because of very low energy of 3d orbitals of the transition metal atoms relative to the energy of oxygen 2p orbitals.…”

Incommensurate helical spin ground states on the hollandite lattice

“…[12][13][14][15] One example of such lattice with geometrical frustration is the Hollandite lattice, 16,17 which is the crystal structure of some transition metal oxides as in the case of α-MnO 2 . 16,18,19 This manganese oxide has recently attracted considerable attention from both experimental and theoretical condensed matter research communities [20][21][22][23][24][25][26][27][28][29] because of its large number of applications as a catalyst for oxygen reduction reaction 30 , microbial fuel cells 31 , electrode materials for Li-ion batteries 32 , lithium-air batteries 29,[33][34][35][36] and in supercapacitors 37 , etc.. Hollandite-type compounds display pores with a diameter of ∼4.6 Å, (see Fig.1(a)). These large channels can accommodate cations such as K + , Na + , and Ba 2+ , leading to non-stoichiometric compounds such as K 1.5 19 The presence of these impurities makes it hard to determine their composition, in particular the water content.…”

Magnetic interactions and orbital state in double chain systems investigated by NMR measurements

“…1 (a) and (b), respectively, have been intensively investigated as a new frustrated quasi-1D system with a double chain shown in Fig. 1 (c) [2,3,4,5,6]. Among them, NaV 2 O 4 with the mixed valence of V 3+ :V 4+ =1:1 (the average electron number 3d 1.5 ) was reported to be an antiferromagnetic (AFM) metal with T N =140 K in spit of the strong ferromagnetic (FM) correlation which appears in magnetic susceptibility [4,5] and the Knight shift [7].…”