Unconventional Colossal Magnetoresistance in Sodium Chromium Oxide with a Mixed-Valence State (original) (raw)

2012, Angewandte Chemie International Edition

The unusual properties of mixed-valence transition-metal oxides have been challenging the condensed matter community for decades. These include the Verwey transition in iron oxides, metal-insulator transitions in MagnØli-type titanium and vanadium oxides, high T C superconductivity in copper oxides, and colossal magnetoresistance (CMR) in manganese oxides. On the other hand, chromium oxides have made little contribution to this important field, mainly because mixed-valence chromium oxides are rare. Cr 3+ ions in octahedral coordination are very stable owing to the halfoccupied t 2g states that tend not to coexist with tetravalent chromium ions. Instead of forming a 3d 2 state, Cr 4+ donates a hole to the oxygen ligand, thus forming a 3d 3 L state with negative charge-transfer energy, as found in CrO 2 for example. This electronic configuration is expected in any oxide containing Cr 4+ ions because of the single-ion effect. The recent discovery of potassium chromium hollandite with a unique metal-insulator transition in the ferromagnetic phase suggests that mixed-valence chromium oxides may demonstrate anomalous physical properties related to their unusual electronic states. Herein we present calcium ferrite-type NaCr 2 O 4 , a new member of the chromium oxide family with mixed-valence Cr 3+ /Cr 4+ ions. This material demonstrates an unusual CMR effect that is closely related to its unconventional magnetic structure caused by spin frustration. The CMR of this compound is unique from several aspects. First, it is observed in a chromium oxide, not in a manganese oxide. Second, it is found in a single-phase material having an insulating ground state. Third, the CMR is not limited to the vicinity of the magnetic phase transition but becomes progressively more prominent with decreasing temperature down to 0 K. The discovery of the NaCr 2 O 4 with a novel CMR mechanism will stimulate further search for new transition-metal CMR compounds other than the manganese oxides. Finding novel CMR materials is important not only from the viewpoint of fundamental science, but also from the viewpoint of leading technology. The tunnel magnetoresistance (TMR) is a key technology for position sensors, acceleration sensors, and magnetic switches: therefore, the TMR-based devices have been used in a wide variety of machines, including hard disc drives, cameras, computers, mobile phones, cars, and virtual reality systems, including video game consoles and robots. The CMR materials are expected to drastically enhance the performance of these systems by replacing ferromagnetic metal layers in conventional TMR devices. NaCr 2 O 4 was obtained using a high pressure, hightemperature technique. It remained highly stable even upon quenching to room temperature. X-ray diffraction studies revealed that NaCr 2 O 4 crystallizes in the calcium ferrite type structure (Pnma space group with a = 9.01873(5), b = 2.913776(15), c = 10.41549(6) ) and is isostructural with b-CaCr 2 O 4 . The fit to the diffraction pattern and the structural parameters obtained from Rietveld refinement are shown in and the Supporting Information, , respectively. The crystal structure of NaCr 2 O 4 is depicted in . The double chains formed by edgesharing CrO 6 octahedra form important structural units that define the electronic properties of this compound. Both onedimensional correlation and spin frustration are expected in this structure.