Anomalous organic magnetoresistance from competing carrier-spin-dependent interactions with localized electronic and nuclear spins

We describe a regime for low-field magnetoresistance in organic semiconductors, in which the spin-relaxing effects of localized nuclear spins and electronic spins interfere. The regime is studied by the controlled addition of localized electronic spins to a material that exhibits substantial room-temperature magnetoresistance ($\sim 20\%$). Although initially the magnetoresistance is suppressed by the doping, at intermediate doping there is a regime where the magnetoresistance is insensitive to the doping level. For much greater doping concentrations the magnetoresistance is fully suppressed. The behavior is described within a theoretical model describing the effect of carrier spin dynamics on the current.

Figure 1

Schematic of device structure. Dopants (green, smaller symbols) in an organic host material (larger, gray symbols). Bottleneck in host is denoted by sites 1 and 2. Black arrows (vertically aligned) are electron or hole spin vectors. Red arrows (randomly oriented) are hyperfine field vectors. Lower right: HOMO, LUMO, and SOMO levels for MEH-PPV and galvinoxyl as well as the electron affinity and ionization potential for galvinxoyl [13, 15].

Figure 2

Room-temperature magnetoconductance traces for a constant applied voltage for several galvinoxyl doping concentrations. Current flow through sample was approximately 1.5 $\mu A$. Inset: The current-voltage characteristics of the device studied.

Figure 3

Dependence of the magnetoconductance magnitude at 0.1 T on the galvinoxyl concentration for several different temperatures. The devices were operated at a constant voltage, resulting in a current flow of approximately 1.5 $\mu A$.

Figure 4

Magnetoconductance traces for a device with 0.8% galvinoxyl concentration for several different temperatures at a current flow of 1.5 $\mu A$. The inset shows the $I-V$ traces for the same temperatures.

Figure 5

Experiment (50 K; black symbols connected with black line for an aid to the eye) and theory (solid red line and dotted orange line) clearly demonstrating three regimes of doping effects on magnetotransport. Regime I: galvinoxyl molecule is far from bottleneck such that both exchange couplings are smaller than the hyperfine coupling. Regime II: galvinoxyl molecule is now near one of the bottleneck sites such that its exchange coupling with that site is larger than $E_{hf}$ (inequality denoted by encircling dashed line). The exchange coupling with the further site, however, remains small compared to $E_{hf}$. Regime III: Exchange couplings between galvinoxyl and bottleneck spins now dominate the hyperfine interaction. The galvinoxyl acts as an intermediary for exchange interaction between the MEH-PPV sites which is known [18, 19] to effectively pin the spins thereby quenching the magnetoconductance. Right: alternative image of the three different regimes.