Staggered spin order of localized π-electrons in the insulating state of the organic conductor κ-(BETS)2Mn[N(CN)2]3

Abstract

Magnetic properties of the conduction π-electron system of κ-(BETS) 2 Mn[N(CN) 2 ] 3 have been probed using 13 C NMR. At ambient pressure, the metal-insulator transition observed in the resistivity measurements below T ≃ 23 K is shown to be accompanied by ordering of the π-spins in a long-range staggered structure. As the metal-insulator transition is suppressed by applying a small pressure of ∼ 0.5 kbar, the π spin system maintains the properties of the metallic state down to 5 K. PACS numbers: 74.70.Kn, 71.30.+h, 76.60.Jx 1 The organic charge-transfer salt κ-(BETS) 2 Mn[N(CN) 2 ] 3 [1] (hereafter abbreviated as BETS-Mn) is a quasi-two-dimensional metal consisting of insulating layers of Mn[N(CN) 2 ] − 3 polymer anions alternating with conducting layers of dimers of organic radical cations BETS[bis(ethylenedithio)tetra-selenafulvalene, C 10 S 4 Se 4 H 8 ] molecules (Fig. 1). The bulk magnetic properties of the system are dominated by the paramagnetic 3d electron spin moments of Mn 2+ ions (S d = 5/2, g ≈ 2) located in the anion layers [1,2]. The (BETS) + 2 dimers accommodating one hole per dimer, form an effectively half-filled quasi-two-dimensional πelectron conduction band [3]. The compound undergoes a metal-insulator (MI) transition at cooling below T MI ∼ 23 K at ambient pressure [1]. The transition temperature rapidly decreases at applying pressure, P , so that at P ≥ 1 kbar the system is metallic in the whole temperature range [3]. Moreover, it goes superconducting with maximum T c = 5.8 K at P = 0.6 -1.0 kbar.Much effort has been done recently to understand the influence of paramagnetic d-metal ions of the anion layers on the transport properties of quasi-two-dimensional systems [4,5].It has been undoubtedly established now that the field-induced superconductivity in λ-(BETS) 2 FeCl 4 and κ-(BETS) 2 FeBr 4 arises when the external magnetic field compensates the exchange field created by localized 3d moments of Fe 3+ on the spins of itinerant πelectrons [6]. The role of π-d interactions in driving the MI transition observed in the systems with magnetic anions is not as clear. For example, at ambient pressure and zero field, non-magnetic (BETS) 2 Ga(Cl,Br) 4 systems are metallic down to lowest temperatures, whereas the iron-containing κ-(BETS) 2 Fe(Cl,Br) 4 are metallic below the antiferromagnetic (AF) transition, and λ-(BETS) 2 FeCl 4 is a uniaxial Néel-type AF insulator below T N = T MI = 8 K [5]. It has been suggested [7] that in the abovementioned Fe-containing systems the AF state is initiated within the system of 3d electron spin moments of Fe 3+ ions, which in the case of λ-(BETS) 2 FeCl 4 drives the MI transition as well. This reasonable assumption has been doubted however in later communications [8,9].Despite the similarity of the title compound to the listed systems with FeX 4 (X=Cl, Br) anions, including the presence of the 3d ions in the anion layer and the MI transition, its phase diagram is different. First, at ambient pressure the system remains insulating in the external magnetic field up to...

Spin Vortex Crystal Order in Organic Triangular Lattice Compound

“…sublattice chiral order analogous to the spin-vortex crystal (SVC) observed in some Fe-based superconductors [37][38][39]. Finally, we show that the 13 C NMR [27,28] and magnetic torque [26,29] experiments are only compatible with this chiral order, thus confirming the ground state of κ-Mn.…”

“…Magnetic order in the BETS layer onsets at T N ∼ 22 K in conjunction with a metal-insulator transition (MIT). This transition is marked by a significant broadening of the 13 C NMR resonances [27,28] and the appearance of a field-induced spin reorientation detected via magnetic torque [26,29]. However, as we elaborate in this work, the angle-dependence of the torque and specific pattern of NMR resonances are incompatible with conventional collinear magnetic orders.…”

“…In this letter, we consider the magnetic ground state of κ-(BETS) 2 Mn[N(CN) 2 ] 3 (κ-Mn) [22][23][24][25][26][27][28][29][30], which we demonstrate to lie in a parameter region conducive to chiral magnetic order. This material has a layered structure (Fig.…”

New radical cation salt κ-(BETS) 2 Co 0.13 Mn 0.87 [N(CN) 2 ] 3 with two magnetic metals: Synthesis, structure, conductivity and magnetic peculiarities

“…As for the anion sublattice, below the MI transition temperature the d-electron Mn 2+ spins show a tendency towards AF ordering. However, no static long-range order as in -(BETS) 2 FeCl 4 [18], occurs in the anion layer [23,24], probably due to frustration in the triangular arrangement of Mn 2+ ions. Details of the atypical structure of Mn 2+ spins in the insulating state, neither purely paramagnetic nor AF, which is presumably induced by the AF-ordered π-spins through the π-d interaction [24,25], is thus far unclear.…”

Shubnikov–de Haas oscillations and electronic correlations in the layered organic metal κ-(BETS)2 Mn[N(CN)2]3

“…The electrical conductivity is provided by delocalized π electrons of fractionally charged BETS donors arranged in two-dimensional (2D) sheets, whereas magnetic * mark.kartsovnik@wmi.badw.de properties are dominated by localized d-electron spins of Mn 2+ in the insulating anionic layers [18]. In addition to the interesting, still not understood crosstalk between the two subsystems [19][20][21], the narrow, half-filled conduction band is a likely candidate for a Mott instability [22]. The material undergoes a metal-insulator transition at ≈ 21 K [18,22].…”