Large Positive Magnetoresistance of Insulating Organic Crystals in the Non-Ohmic Region

We report a large positive magnetoresistance ratio in insulating organic crystals $\theta \mathrm{\text{−}}(\mathrm{ET}{)}_2\mathrm{CsZn}(\mathrm{SCN}{)}_4$ at low temperatures at which they exhibit highly nonlinear current-voltage characteristics. Despite the nonlinearity, the magnetoresistance ratio is independent of the applied voltage. The magnetoresistance ratio depends little on the magnetic field direction and is described by a simple universal function of ${\mu }_{B}B/k_{B}T$, where ${\mu }_B$ is the Bohr magneton. The positive magnetoresistance may be caused by magnetic-field-induced parallel alignment of spins of mobile and localized electrons, and a resulting blockade of electrical conduction due to the Pauli exclusion principle.

Figure 1

In-plane ($c$ axis) $I-V$ curves of a CsZn crystal at several temperatures in magnetic fields of $B=0$ and 17.5 T ($B\parallel a$). The electrode spacing is $20\mu \mathrm{m}$.

Figure 2

$d\ln (I)/d\ln (V)$ for $T=1.08\mathrm{K}$ plotted as a function of voltage (a) and current (b).

Figure 3

Magnetic-field dependence of current measured while voltage was kept constant in a CsZn crystal. The current is normalized to the value obtained in zero magnetic field at each temperature. Both the voltage and magnetic field were applied along the $c$ (in-plane) axis so that orbital effects would be minimized. For $T=0.135\mathrm{K}$, data measured with $B\parallel b$ (out-of-plane) are also shown. Two curves for each temperature correspond to increasing and decreasing magnetic field. Inset: data for another CsZn crystal at higher temperatures; $V\parallel a$ and $B\parallel b$.

Figure 4

Plots of the $I/I(B=0)$ shown in the main panel of Fig. 3 as a function of ${\mu }_{B}B/k_{B}T$. The curves for small $B$ fall on a universal curve represented by $I/I(B=0)=[\cosh ({\mu }_{B}B/k_{B}T){]}^{-1/2}$ (dotted line) or $I/I(B=0)=1/[1+({\mu }_{B}B/2k_{B}T{)}^2]$ (dashed line). Insets: plots of the $I/I(B=0)$ shown in the inset of Fig. 3 as a function of ${\mu }_{B}B/k_{B}T$ before (left inset) and after (right inset) subtracting a $B$-independent constant $I_{\mathrm{off}}$ at each temperature (see text).

Figure 5

Magnetic-field dependence of current measured while voltage was kept constant in a RbZn crystal. $V\parallel a$ and $B\parallel b$. Inset: the same data are plotted as a function of ${\mu }_{B}B/k_{B}T$.

Figure 6

(a) Dependence of $I/I(B=0)$ of the CsZn salt on the magnetic field direction. The voltage was applied along the $a$ axis, and the magnetic field was tilted by $\theta$ from $b$ towards $a$. The solid lines are fits to the formula, $I(B,\theta )/I(B,\theta =90°)=1+A(B)|\cos (\theta )|$. (b) The coefficient $A$ plotted as a function of the magnetic field.