Tokura Group

Exploration of Transition Metal Oxides and Organic Molecular System

Yoshinori Tokura

Research group led by Y. Tokura (concurrently serving as Professor of Physics at University of Tokyo) aims at exploring new electronic materials and related physics for development of atom technology. The exploratory materials we focus on are (1) 3d transition metal oxides with controlled band filling as strongly correlated electron systems, and (2) organic molecular systems with specialized functions.

■Results in Fiscal Year

We aim at exploring new electronic materials and related physics for development of atom technology. The exploratory materials we focus on are (1) perovskite-type oxides with controlled filling and electron correlationﾕs, and (2) organic molecular systems with magnetoelectronic functions.

(1) We have found first-order phase transitions between the charge-ordered antiferromagnetic insulator (AFI) and the ferromagnetic metal (FM) in single crystals of filling-controlled perovskite-type manganese oxides. The charge-ordered states could be collapsed by a magnetic field. The electronic phase diagram is revealed in the plane of magnetic field and temperature.

　The striction-coupled colossal magnetoresistance (CMR) was observed under a relatively low magnetic field in a (Nd, Sm)1/2Sr1/2MnO3 single crystal with finely controlled its one electron bandwidth. The CMR phenomena are viewed as a first-order phase transition from AFI to FM induced by a magnetic field. Figure 3 shows the comparison with the typical MR materials.

　We could first synthesize single crystals of a series of layered perovskites with two dimensional Mn-O sheets. By controlling the number of Mn-O sheets, CMR without hysteresis is observed in a (La, Sr)3Mn2O7 system. The epitaxal thin films of the Mn-O perovskites are successfully fabricated by pulsed laser deposition method.

(2) In order to construct a molecular system in which conduction electrons are interacting with localized spins, various cation radical salts of organic donor molecules containing halogenometalate as counter ions were prepared by electrocrystallization. New magnetoelectronic functionalities are being explored.

Fig. 2 Single crystal of perovskite-type manganese oxides grown by a floating-zone method: Nd0.55Ca0.45MnO3.

Fig. 3 Comparison of isothermal MR for (Nd, Sm)1/2Sr1/2MnO3 to those for other typical MR materials.

Fig. 1 Pulsed laser deposition apparatus for growth of thin films.