Electric dipole induced bulk ferromagnetism in dimer Mott molecular compounds

Abstract

Magnetic properties of Mott–Hubbard systems are generally dominated by strong antiferromagnetic interactions produced by the Coulomb repulsion of electrons. Although theoretical possibility of a ferromagnetic ground state has been suggested by Nagaoka and Penn as single-hole doping in a Mott insulator, experimental realization has not been reported more than half century. We report the first experimental possibility of such ferromagnetism in a molecular Mott insulator with an extremely light and homogeneous hole-doping in π-electron layers induced by net polarization of counterions. A series of Ni(dmit)2 anion radical salts with organic cations, where dmit is 1,3-dithiole-2-thione-4,5-dithiolate can form bi-layer structure with polarized cation layers. Heat capacity, magnetization, and ESR measurements substantiated the formation of a bulk ferromagnetic state around 1.0 K with quite soft magnetization versus magnetic field (M–H) characteristics in (Et-4BrT)[Ni(dmit)2]2 where Et-4BrT is ethyl-4-bromothiazolium. The variation of the magnitude of net polarizations by using the difference of counter cations revealed the systematic change of the ground state from antiferromagnetic one to ferromagnetic one. We also report emergence of metallic states through further doping and applying external pressures for this doping induced ferromagnetic state. The realization of ferromagnetic state in Nagaoka–Penn mechanism can paves a way for designing new molecules-based ferromagnets in future.

Role of the Anion Layer’s Polarity in Organic Conductors β″-(BEDT-TTF)2XC2H4SO3 (X = Cl and Br)

“…Figure shows the relationship between the difference of the formula charges between both independent donor layers and the effective voltage. At present, two metallic (orange circles) , and three semiconducting (pink circles) ,, type I salts have been reported, the parameters of which are plotted in Figure . Roughly, each set of metallic and semiconducting data points shows linear tendencies, shown as orange and pink straight lines, respectively.…”

“…The orange and pink circles indicate metallic and semiconducting salts, respectively . The points were obtained from the salts, *1: κ H -(DMEDO-TSeF) 2 [Au­(CN) 4 ]­(THF), *2: κ-β″-(BEDT-TTF) 2 (PO-CONHC 2 H 4 SO 3 ) where PO is 2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl, , *3: α′-α′-(BEDT-TTF) 2 (PO-CONH- m -C 6 H 4 SO 3 )·H 2 O, *4: (TTF) 3 (PO-CON­(CH 3 )­C 2 H 4 SO 3 ) 2 , and *5: the midpoint of (Et-2,5-DBrP)­[Ni­(dmit) 2 ] 2 and (Et-2I-5BrP)­[Ni­(dmit) 2 ] 2 where Et-2,5-DBrP is ethyl-2-iodo-5-bromopyridinium and Et-2I-5BrP is ethyl-2,5-dibromopyridinium …”

“…In fact, it is clear that only one of the two crystallographically independent [Ni(dmit) 2 ] 2 layers becomes a ferromagnet. 17 Thus far, type I, type II, 9,11 and type III salts have been provided from ions bearing aminoxyl radicals from our group, but we have not yet reported any type IV salts. Reports of type I salts from other groups suggest that nonmagnetic, smaller, and simpler anions can also provide polar anionic layers in organic conductors.…”

“…This means that electron-doped layers do not become a ferromagnet. In fact, it is clear that only one of the two crystallographically independent [Ni­(dmit) 2 ] 2 layers becomes a ferromagnet …”

A Memristive Oscillator

“…The temperature coefficient, 𝜶 T , of our memristive sample is ≈−3.3 between 30 and 40 K, which is three orders of magnitude greater than that of semiconductors such as Si and Ge. [10] The heat capacity of sample B is estimated to be C p = 2.3 × 10 −4 J K −1 at 30 K. [4] Next, a linear frequency slope of f = (2346.4) •I bias is expected from the estimated C p , |𝜶 T |= 3.3, and C = 33 𝜇F (red line in Figure 2b). The observed frequency is in agreement with the prediction by Mauro.…”