Spin–Charge Coupling in the Molecular Conductor (DIETSe) 2 FeBr 4 Mitsuhiko MAESATO 1 , Yoshitomo FURUSHIMA 1 , Gunzi SAITO 1;2 , Hiroshi KITAGAWA 1;3 , Tatsuro IMAKUBO 4 , Andhika KISWANDHI 5 , David GRAF 5 , and James S. BROOKS 5 1 Division of Chemistry, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan 2 Faculty of Agricultre, Meijyo University, Nagoya 468-8502, Japan 3 JST, Chiyoda, Tokyo 102-0076, Japan 4 Department of Materials Science and Technology, Nagaoka University of Technology, Niigata 940-2188, Japan 5 NHMFL/Physics, Florida State University, Tallahassee, FL 32310, U.S.A. (Received January 19, 2013; accepted February 4, 2013; published online March 7, 2013) The quasi-one-dimensional (Q1D) –d hybrid molecular conductor (DIETSe) 2 FeBr 4 undergoes an antiferromagnetic (AF) transition of d-electron spins, associated with a metal–insulator (M–I) transition at 7 K. The existence of a Q1D Fermi surface in the high-temperature metallic state is confirmed by the appearance of Lebed resonances, which disappear below 7 K, suggesting an AF-order-induced spin density wave transition. The application of hydrostatic pressure increases the M–I transition temperature, indicating the enhancement of the –d interaction. Angle-dependent magnetic torque measurements reveal a spin-flop transition of AF d-electron spins at 3.4 T. The magnetic easy axis is tilted 54 deg from the b-axis towards the c-axis. The magnetoresistance (MR) shows an anomaly in the spin-flop field, associated with a small hysteresis. The AF order is suppressed with high magnetic fields on the order of 20 T, above which the MR shows a very sharp and large anomaly. A large hysteresis loop appears in the MR below the critical field of the AF transition. The results indicate a significant interplay between itinerant -electrons and local moments of d-electron spins. KEYWORDS: molecular conductor, –d interaction, spin density wave, high magnetic field, magnetoresistance The coupling between spin and charge degrees of freedom has been an important issue in materials science, because it plays an important role in a wide variety of phenomena such as giant magnetoresistance (GMR), 1,2) multiferroicity, 3) unconventional superconductivity, 4) and spin density wave (SDW) formation. 5) Multifunctional –d hybrid molecular conductors have attracted much attention, because they possess a wide variety of possibilities in materials design and show intriguing spin–charge coupled phenomena due to –d interactions. 6–9) Recently, we reported that the multi- functional –d hybrid molecular conductor (DIETSe) 2 FeCl 4 shows unprecedented magnetotransport phenomena such as spin-flop-induced electrical switching and nonvolatile mem- ory, 10) where DIETSe represents diiodo(ethylenedithio)- tetraselenafulvalene. 11) An important characteristic feature of (DIETSe) 2 FeCl 4 is the coexistence of a SDW in quasi- one-dimensional (Q1D)  electrons with an antiferromag- netic (AF) order of d-electron spins, implying the interplay between the SDW instability and AF moments. Note that the SDW instability can couple to the 2k F AF order of local moments in Q1D –d systems, where k F is the Fermi wavevector. In this Letter, we report the discovery of an anomalous magnetoresistance (MR) associated with large hysteresis in the isostructural (DIETSe) 2 FeBr 4 salt under high magnetic field. (DIETSe) 2 MX 4 [M = Fe, Ga, X = Cl, Br] are isostruc- tural Q1D systems. 11) Both Cl salts undergo a SDW transition below about 12 K due to the nesting instability of the Q1D Fermi surface, which was confirmed by the 77 Se NMR study of GaCl 4 salt. 12) In the case of the FeCl 4 salt, an AF transition of d-electron spins occurs below about 2.5 K. Therefore, SDW and AF orders coexist in the ground state. The substitution of Cl by Br suppresses the nesting instability of the Fermi surface. Actually, (DIETSe) 2 MX 4 with a nonmagnetic GaBr 4  anion shows normal metallic behavior down to 4 K. On the other hand, (DIETSe) 2 FeBr 4 salt shows an AF transition at 7 K, associated with a metal– insulator (M–I) transition. 11) The higher Ne ´el temperature of the FeBr 4 salt implies larger –d interaction than that of the FeCl 4 salt, which motivated us to explore spin–charge coupled phenomena in (DIETSe) 2 FeBr 4 under high mag- netic fields. Single crystals of (DIETSe) 2 MBr 4 [M = Fe, Ga] were synthesized by electrochemical oxidation at 283 K. 11) The typical crystal size is 0:4  0:02  0:08 mm 3 , elongated along the most conducting a-axis direction. The resistivity was measured by a conventional four-probe method. Four gold wires were attached to the crystal with carbon paste as electrodes. Hydrostatic pressure was applied using a BeCu pressure cell. We used Daphne 7373 oil as the liquid pressure-transmitting medium. Pressure at low temperatures was estimated by considering pressure loss during cooling. 13) The magnetic torque of a single crystal was measured using a commercially available piezoresistive microcantilever for atomic force microscopy. 14) We used a single-axis rotator in order to measure the angular dependences of MR and magnetic torque. Figure 1(a) shows the temperature dependence of the interlayer b-axis resistivity of (DIETSe) 2 FeBr 4 under hydrostatic pressure. Normal metallic behavior was ob- served from room temperature down to about 10 K. At ambient pressure, (DIETSe) 2 FeBr 4 undergoes an AF transi- tion of d-electron spins at 7 K, associated with the M–I transition. 11) In contrast, the isostructural (DIETSe) 2 GaBr 4 salt, with nearly the same lattice constants, is metallic down to 4 K. Therefore, the M–I transition is induced by the AF magnetic transition of d-electron spins; otherwise, (DIETSe) 2 FeBr 4 should be a metal. The AF-order-induced M–I transition temperature gradually increases with increas- ing pressure, where transition temperature is defined as the temperature at which d lnðÞ=dð1=T Þ shows a maximum. These results suggest the enhancement of the –d interaction Journal of the Physical Society of Japan 82 (2013) 043704 043704-1 LETTERS #2013 The Physical Society of Japan http://dx.doi.org/10.7566/JPSJ.82.043704