Magnetic Fringe-Field Control of Electronic Transport in an Organic Film

Abstract

Random, spatially uncorrelated nuclear-hyperfine fields in organic materials dramatically affect electronic transport properties such as electrical conductivity, photoconductivity, and electroluminescence. The influence of these nuclear-hyperfine fields can be overwhelmed by a uniform externally applied magnetic field, even at room temperature where the thermodynamic influences of the resulting nuclear and electronic Zeeman splittings are negligible. As a result, even in applied magnetic fields as small as 10 mT, the kinetics of exciton formation, bipolaron formation, and single-carrier hopping are all modified at room temperature, leading to changes in transport properties in excess of 10% in many materials. Here, we demonstrate a new method of controlling the electrical conductivity of an organic film at room temperature, using the spatially varying magnetic fringe fields of a magnetically unsaturated ferromagnet. (The fringe field is the magnetic field emanating from a ferromagnet, associated with magnetic dipole interactions or, equivalently, the divergence of the magnetization within and at the surfaces of the ferromagnet.) The ferromagnet's fringe fields might act as a substitute for either the applied magnetic field or the inhomogeneous hyperfine field. The size of the effect, the magnetic-field dependence, and hysteretic properties rule out a model where the fringe fields from the ferromagnet provide a local magnetic field that changes the electronic transport properties through the hyperfine field, and show that our effects originate from electrical transport through the inhomogeneous fringe fields coming from the ferromagnet. Surprisingly, these inhomogeneous fringe fields vary over length scales roughly 2 orders of magnitude larger than the hopping length in the organic materials, challenging the fundamental models of magnetoresistance in organic layers which require the correlation length of the inhomogeneous field to correspond roughly to the hopping length.

Singlet-to-triplet interconversion using hyperfine as well as ferromagnetic fringe fields

“…Note that it is a characteristic of perfectly flat, saturated, perpendicularly magnetized films that they produce no fringe fields above the films far from the edges; fringe fields above the film originate from the presence of domains in unsaturated states. Transmission X-ray microscopy based on the X-ray magnetic circular dichroism (XMCD) effect is used to determine the microscopic magnetic domain structure of the magnetic film [28]. Several example images are shown in figure 2, which show the magnetic film transitioning from a state with mostly down domains (light grey) to mostly up domains (dark domains) as the applied magnetic field is increased as part of a magnetization loop.…”

“…they cannot sense the field direction), OMAR sensors based on an externally supplied field-landscape may be sensitive to different field-magnitude ranges, be hysteretic and anisotropic. Such 'extrinsic' OMAR devices were recently demonstrated [28] using fringe fields emitted from a nearby ferromagnetic film as the source of the magnetic-field landscape. Figure 2.…”

“…The strength and the spatial-correlation length of fringe fields from a ferromagnet also depend sensitively on the distance from the ferromanget. Wang et al [28] used a film of alternating Co and Pt layers with perpendicular magnetic anisotropy (PMA) as the source of magnetic fringe fields. This magnetic film with PMA can be magnetized (i.e.…”

“…The fields are applied in a direction perpendicular to the magnetic film plane. For details, see [28]. (Adapted from [31].)…”

“…XMCD images are used to determine the magnetic fringe fields at different distances above the ferromagnetic layer. The fringe-field strength anywhere in space can be calculated from the domain geometry [28].…”