Spin memristive systems: Spin memory effects in semiconductor spintronics

Recently, in addition to the well-known resistor, capacitor, and inductor, a fourth passive circuit element, named memristor, has been identified following theoretical predictions. The model example used in such case consisted in a nanoscale system with coupled ionic and electronic transport. Here, we discuss a system whose memristive behavior is based entirely on the electron-spin degree of freedom, which allows for a more convenient control than the ionic transport in nanostructures. An analysis of time-dependent spin transport at a semiconductor/ferromagnet junction provides a direct evidence of memristive behavior. Our scheme is fundamentally different from previously discussed schemes of memristive systems and broadens the possible range of applications of semiconductor spintronics.

Figure 1

(Color online) Semiconductor/half-metal junction. (a) Schematic representation of the circuit made of an interface between a semiconductor and a half metal. (b) Typical dc current-voltage characteristics. Inset: spin-up and spin-down densities in the semiconductor region as a function of the distance from the contact. $i_c=e{N}_0\sqrt{D/\left(2{\tau }_{sf}\right)}$ is the critical current density (Ref. 5).

Figure 2

(Color online) Simulations of ac response of the system. The applied voltages (blue lines) are $V=V_1+V_2\text{sin}\left(2\pi \nu t\right)$ with $V_1=V_2=0.5\text{V}$, $\nu ={10}^9\text{Hz}$ in (a) and $\nu =2\cdot {10}^7\text{Hz}$ in (b). In both cases the current densities (green lines in units of the critical current density) and spin-up electron densities near the contact (red lines) show saturation typical for spin blockade (Ref. 5). It is clearly seen that $i\text{−}V$ hysteresis is significantly reduced in the low-frequency case. The calculations were made using the following system parameters: $L=20\mu \text{m}$, $D=220{\text{cm}}^2/\text{s}$, $\mu =8500{\text{cm}}^2/\left(\text{V}\cdot \text{s}\right)$, ${\tau }_{sf}=2\text{ns}$, $N_0=5\times {10}^{15}{\text{cm}}^{-3}$, and ${\rho }_c^0/\left({\rho }_{s}L\right)=1$.

Figure 3

(Color online) System dynamics excited by step voltages. (a) Unipolar (blue line) and bipolar (red line) step voltage profiles used in our calculations. The spin-injection process (at negative applied voltage) was modeled using a constant interface resistance model. Inset: Total charge flown through the system as a function of time. The profile of bipolar voltage was selected in such a way that the corresponding total charge $q=0$ for $t>2\text{ps}$ (see the red line). (b) Evolution of spin-up electron density near the interface. In the case of bipolar voltage excitation (red line), the final value of spin-up density is close to its initial value (shown by the dotted line). This is a manifestation of a nearly perfect memristor behavior.