Interplay between spin-density wave and 3d local moments with random exchange in a molecular conductor

Abstract

We present the results of high-pressure transport measurements on the anion-mixed molecular conductors (DIETSe)2MBr2Cl2 [DIETSe = diiodo(ethylenedithio)tetraselenafulvalene; M = Fe, Ga]. They undergo a metal-insulator (M-I) transition below 9 K at ambient pressure, which is suppressed by applying pressure, indicating a spin-density-wave (SDW) transition caused by a nesting instability of the quasi-one-dimensional (Q1D) Fermi surface, as observed in the parent compounds (DIETSe)2MCl4 [M = Fe, Ga]. In the metallic state, the existence of the Q1D Fermi surface is confirmed by observing the Lebed resonance. The critical pressures of the SDW, Pc, of the MBr2Cl2 [M = Fe, Ga] salts are significantly lower than those of the the MCl4 [M = Fe, Ga] salts, suggesting chemical pressure effects. Above Pc, field-induced SDW transitions appear, as evidenced by kink structures in the magnetoresistance (MR) in both salts. The FeBr2Cl2 salt also shows antiferromagnetic (AF) ordering of d-spins at 4 K, below which significant spin-charge coupling is observed. A large positive MR change up to 150% appears above the spin-flop field at high pressure. At low pressure, in particular below Pc, a dip or kink structure appears in MR at the spin-flop field, which shows unconventionally large hysteresis at low temperature (T < 1 K). The hysteresis region clearly decreases with increasing pressure towards Pc, strongly indicating that the coexisting SDW plays an important role in the enhancement of magnetic hysteresis besides the random exchange interaction.

Use of Halogen Bonding in a Molecular Solid Solution to Simultaneously Control Spin and Charge

“…The shift of H sf becomes smaller with increasing temperature (Figure S18c), in good agreement with the temperature dependence of magnetic hysteresis . The hysteresis of H sf becomes small when pressure suppresses SDW, suggesting the influence of coexisting SDW …”

“…In addition to the SDW transition, the π–d conductors (DIETSe) 2 FeBr 4 x Cl 4(1– x ) exhibit a rapid resistance increase (Figure b), which is not observed in nonmagnetic analogues, (DIETSe) 2 GaBr 4 x Cl 4(1– x ) , suggesting a π–d-coupled transition. The temperatures of the resistance anomaly coincide with AF ordering temperatures of d spins from magnetic susceptibility χ (Figure d), as discussed below. − ,, The increase in resistance at the Néel temperature T N is associated with reconstruction of the Fermi surface induced by a periodic AF potential of Fe 3+ d spins.…”

“…With increasing Br content x , the SDW transition is suppressed and metallic behaviors are maintained down to 1.9 K in the GaBr 4 salt. , The temperature–Br-content ( T – x ) phase diagram (Figure c) clearly shows the SDW phase shrinks with increasing x . It indicates Br substitution suppresses the nesting instability of Q1D Fermi surfaces by increasing the Fermi surface warping, in other words, enhancing the dimensionality of π electrons, which is quite similar to the application of pressure. ,,, Such a SDW suppression is also observed in the Fe salts (Figure b,d).…”

“…T N becomes higher with increasing Br content x , indicating the enhancement of π–d interactions. It is markedly different from the slight increase in T N by applying physical pressure. − ,, …”

“…Four gold wires (10 μm in diameter) were attached to a single crystal with carbon paste. The b -axis resistivity is larger by a factor of about 10 4 compared to the a -axis resistivity and coherent transport is observed in this system. − ,, The b -axis resistivity seems tougher against microcracks by cooling than the a -axis resistivity, especially in the case of mixed salts. To prevent microcracks from forming, we also applied a moderate hydrostatic pressure (<1.5 kbar) to the crystals at room temperature using a BeCu pressure cell with Daphne 7373 as the pressure medium.…”