Nanotesla magnetoresistance in π-conjugated polymer devices

We demonstrate submicrotesla sensitivity of organic magnetoresistance in thin-film diodes made of the conducting polymer poly(styrene sulfonate)-doped poly(3,4-ethylenedioxythiophene) (PEDOT:PSS). The magnetoresistance sensitivity is shown to be better than 20 parts per billion (ppb). As for other conjugated polymers, magnetoresistance can be separated into two regimes of field strength: the nonmonotonic ultrasmall magnetic field effect on magnetic field scales below 2 mT, and the monotonic intermediate magnetic field effect on scales over several tens of mT. The former gives the PEDOT:PSS magnetoresistance curve a characteristic W-shaped functionality, with inverted turning points compared to those found in conventional organic light-emitting diode (OLED) devices. We succeed in resolving the ultrasmall magnetic field effect of the PEDOT:PSS layer incorporated within an OLED structure, which is responsible for an additional magnetoresistive feature on the ppm scale. Such a device shows unprecedented complexity in magnetoresistance with a total of four extrema within a field range of ±1 mT. We propose that these unique characteristics arise from spin-spin interactions in the weakly bound carrier pairs responsible for the spin-dependent recombination probed in magnetoresistance.

Figure 1

Magnetoresistance of polymer-based diodes under a constant current of $I=5\mathrm{mA}$ at room temperature. The left-hand insets show the device geometries and the relative orientation of the magnetic field and device stack schematically. (a) Magnetoresistance of a device structure incorporating only PEDOT:PSS as the active layer between two electrodes. A zoom into the ultrasmall magnetic field effect close to the origin is shown in the right-hand inset. The orange line is a fit of the outer data region to the function $f\left(B\right)\propto \mathrm{const}+{\left(\frac{B}{\left|B\right|+B_0}\right)}^2$, with $B_0$ the fitting constant. Because of the high conductivity of the PEDOT:PSS layer, the absolute resistance at zero field is quite low $[R_{(B=0)}=119\mathrm{\Omega }]$. (b) Magnetoresistance of a conventional OLED structure with PEDOT:PSS and MEH-PPV polymer layers $[R_{(B=0)}=697\mathrm{\Omega }]$. The right-hand inset, plotted on the same absolute scale as in (a), shows that the ultrasmall magnetic field effect is much weaker in this case. For both devices, the dominant magnetoresistive response occurs in the intermediate-field regime of several tens of mT. This effect is generally associated with hyperfine coupling [11].

Figure 2

Magnetoresistance of an MEH-PPV OLED under a constant current of 250 μA at liquid-helium temperature $[R_{(B=0)}=34.5\mathrm{k}\mathrm{\Omega }]$. On field scales of $\pm 1\mathrm{mT}$ a total of five extrema are observed, at $B_1=0\mathrm{mT},B_2\cong \pm 150\mu \mathrm{T},B_3\cong \pm 700\mu \mathrm{T}$, giving rise to a nonmonotonic double-W functionality. The magnetoresistance on field scales of up to 55 mT shows no unexpected changes in functionality, as displayed in the inset.

Figure 3

Nanotesla magnetoresistance of PEDOT:PSS devices under a constant current of $250\mu \mathrm{A}$ at liquid-helium temperature $[R_{(B=0)}\sim 10\mathrm{k}\mathrm{\Omega }]$. (a) The ultrasmall magnetic field effect, shown in the $\pm 2\mathrm{mT}$ range, as a function of the total magnetic field $B=B_{\mathrm{sol}}+B_{\mathrm{earth}}$, where $B_{\mathrm{sol}}$ is the solenoid's magnetic field and $B_{\mathrm{earth}}$ is the local geomagnetic field. The inset shows the effect on higher magnetic field scales ($\pm 60\mathrm{mT}$). (b) High-resolution measurement, averaged over 300 consecutive sweeps of the solenoid field, showing an effective magnetic field sensitivity of $\pm 250\mathrm{nT}$ around the local geomagnetic field. Error bars indicate the standard error of the mean.