One-Dimensional Transition Metal−Benzene Sandwich Polymers:  Possible Ideal Conductors for Spin Transport

Abstract

We investigate the electronic and magnetic properties of the proposed one-dimensional transition metal (TM=Sc, Ti, V, Cr, and Mn) -benzene (Bz) sandwich polymers by means of density functional calculations. [V(Bz)]∞ is found to be a quasi-half-metallic ferromagnet and half-metallic ferromagnetism is predicted for [Mn(Bz)]∞. Moreover, we show that stretching the [TM(Bz)]∞ polymers could have dramatic effects on their electronic and magnetic properties. The elongated [V(Bz)]∞ displays half-metallic behavior, and [Mn(Bz)]∞ stretched to a certain degree becomes an antiferromagnetic insulator. The possibilities to stabilize the ferromagnetic order in [V(Bz)]∞ and [Mn(Bz)]∞ polymers at finite temperature are discussed. We suggest that the hexagonal bundles composed by these polymers might display intrachain ferromagnetic order at finite temperature by introducing interchain exchange coupling.PACS numbers: 71.20.Rv, 75.75.+a, 82.35.Lr It is expected that the new generation of devices will exploit spin dependent effects, what has been called spintronics [1,2]. A challenge now facing spintronics is transmitting spin signals over long enough distances to allow for spin manipulation. An ideal device for spin-polarized transport should have several key ingredients. First, it should work well at room temperature and should offer as high a magnetoresistance (MR) ratio as possible. In this sense, a half-metallic (HM) ferromagnet with the Curie temperature higher than room temperature is highly desirable since there would be only one electronic spin channel at the Fermi energy [3]. Second, the size or diameter of the materials should be uniform for large scale applications. Carbon nanotubes (CNTs) were considered as promising one-dimensional (1D) spin mediators because of their ballistic nature of conduction and relatively long spin scattering length (at least 130 nm) [4,5]. Coherent spin transport has been observed in multiwalled CNT systems with Co electrodes. The maximum MR ratio of 9% was observed in multiwalled CNTs at 4.2 K. However, as the temperature increases to 20 K, the MR ratio goes to zero, preventing any room temperature applications [4]. A theoretical work suggested that the ferromagnetic (FM) transition-metal (TM) /CNT hybrid structures may be used as devices for spin-polarized transport to further increase the MR ratio [6]. Unfortunately, although large spin polarization is found in these systems, there is no HM behavior. Another difficulty with CNT is that the devices are unlikely to be very reproducible due to the wide assortment of tube size and helicity that is produced during synthesis. Wide variation in device behavior was reported in the CNT experiment [4].Recently, an experimental study suggested that the unpaired electrons on the metal atoms couple ferromagnetically in the multidecker organometallic sandwich Vbenzene (Bz) complexes, i.e., V n (Bz) n+1 clusters [7]. The FM sandwich clusters are supposed to serve as nanomagnetic building blocks in applications such as recording media or spintronic de...

Revisiting magnetic coupling in transition-metal-benzene complexes with maximally localized Wannier functions

“…Double exchange was proposed to be the mechanism of FM ordering. 19 On the other hand, by examining the elec-tight-binding model constructed now gives more consistent result with that of the direct first-principles calculation.…”

“…17 Inspired by these findings, several theoretical calculations 18,19 have been made on (VBz) n=∞ , an ideal one-dimensional infinite chain. In those works, (VBz) n=∞ is found to have highly stable FM ordering and shows half-metallic behavior.…”

Optical to ultraviolet spectra of sandwiches of benzene and transition metal atoms: Time dependent density functional theory and many-body calculations

“…20 Recently, a theoretical study of the electronic and magnetic properties for ͑MBz͒ ϱ polymers with M = Sc, Ti, V, Cr, and Mn has shown that all chains, except ͑CrBz͒ ϱ , are metallic. 21 Optical and UV spectroscopies are powerful tools for obtaining information on the electronic and atomic structure of clusters. Since the structure of a cluster is generally not directly accessible experimentally, 22 its characterization can be done with a combination of optical spectroscopy and theoretical calculations.…”

Magnetic and Magnetotransport Properties in Nanogranular Co/C<SUB>60</SUB>-Co Film with High Magnetoresistance

“…The spin-dependent transport in the HB region might be associated with the highly spin-polarized molecular-like states generated in the C 60 -Co compound as theoretically predicted for various organic molecule-TM systems. [1][2][3][4] As for the existence of such shallow mid-gap states, the C 60 -Co compounds exhibit a small conductivity explained by the activated transport from the midgap states at less than one À10's meV from the Fermi level, 10,12) which is nearly equivalent to the interparticle voltage drop above a few meV (at V ¼ V T ) in the HB region. Analytical and theoretical approaches to the electronic structure of the C 60 -Co compounds itself and at the interface with Co crystal are indispensable to make obvious the electronic origin of the observed TMR effect, and are in progress.…”

“…The hybrid systems between organic molecules involving carbon nanotubes (CNTs) and transition metals (TMs) has recently attracted attention as a new class of materials systems for spintronics following the theoretical predictions for high spin-polarization. [1][2][3][4] In particular, magnetotransport properties in ferromagnetically contacted CNTs have been investigated. [5][6][7] The reported magnetoresistance (MR) in such CNT-TM systems, however, is as small as less than 10% different from the theoretical predications.…”