SNOLAB Journal Club

1. SNOLAB Journal Club Paper: Information Processing with Pure Spin Currents in Silicon: Spin Injection, Extraction, Manipulation and Detection. ArXiv: 0906.5597 (30th June 2009) Discussion lead by Richard Ford (SNOLAB) 19th November 2009

2. How did I pick this paper? • I looked at the 2007 Nobel prize for physics award jointly to Fert and Grunberg for giant magnetoresistance (GMR) discovered independently in 1988. This discovery led to development of the “spin valve” and later the tunnel magnetoresistance effect (TMR) which found application in advanced computer harddrives, and more recently magnetoresistive random access memory (MRAM) (which is non-volatile). However, the bigger revolution in electronics maybe just around the corner, as the emerging field of “spintronics”develops real devices such as transistors.

3. GMR

4. GMR

5. Diskdrive Sensor contact

6. TMR

7. Spintronics • Electronics is about manipulating the flow of electrons – a scaler quantity. • Spintronics is about using the electron spin as an additional state variable: • Net electron flow is zero • Net “spin flow” is non-zero • Invariant under time reversal • Net Angular momentum flow • Vector quantity -> quantum information • Reduced resistive power dissipation • Reduced capacitive time constants

8. Spintronics (cont) • Spin injector • Zeeman effect (but need low temp) • Non-equilibrium cavity effects • Spin valve • Tunnel spin valve • Spin manipulation • magnetic modulation • Spin detector • Spin valve or TMR

9. Spin Pumping Creating a spin current through spin pumping I. A single channel of electrons is formed in a 2D electron system through electrostatic confinement. When out-of-phase ac voltages are applied to the two gates, the channel is perturbed, resulting in a dc electron current. If one of the gates is replaced by an oscillating magnetic field, a spin current is pumped. Creating a spin current through spin pumping II. (Top) A quantum cavity is perturbed through out-of-phase ac voltages applied to the two gates. Electrons entering the cavity scatter off the cavity walls several times before leaving. (Bottom) In a sufficiently large cavity, pumping leads to a spin current with a tunable direction of spin polarization. Both in-plane (green arrow) and out-of-plane (red arrow) polarizations are possible.

10. Fig. 1. Schematic layout of four terminal nonlocal device, a current is applied to contact 3 and 4, and a voltage is measured across contact 1 and 2.

11. Fig. 2. I-V curve of the Fe/Al2O3/Si contacts, measured from Fe to a large Ohmic surface contact. The upper inset shows a modest change in zero bias resistance versus temperature. The lower inset shows the good fit of the data to the Brinkman, Dynes and Rowell model for asymmetric tunnel barriers.

12. Fig. 3. Schematic illustration of (a) injection and (b) extraction of spins from Silicon by means of spin dependent tunneling.

13. Fig. 5. Nonlocal voltage versus inplane magnetic field at 10 K for several values of the injection current, graphs are offset for clarity. For negative bias, electrons are injected from Fe into the Si channel and the change in nonlocal voltage is consistent with majority spin injection. For positive bias, electrons are extracted from the Silicon into the Fe contact. The majority spins are more readily extracted resulting in the accumulation of minority spin in the Silicon. A change in sign is seen for nonlocal voltage peaks for the antiparallel state, consistent with minority spin accumulation. Fig. 4. Nonlocal voltage versus inplane magnetic field, for an injection current of -100 μA at 10 K. Two levels corresponding to the parallel and anti-parallel remanent states are clearly visible.

14. Fig. 6. Hanle measurements at 10 K for positive and negative injector current, with the graphs offset for clarity. The Hanle curve shows a dip or a peak for injection and extraction respectively, consistent with majority and minority spin accumulation. The dashed lines are fits to the Hanle data using g = 2, spin diffusion constant Ds = 10 cm2/s and spin lifetime ts = 0.9 ns.

15. Spintronics Figure 1. Spin-dependent transport structures. (A) Spin valve. (B) Magnetic tunnel junction.

16. Spintronics (devices)

17. Figure 4. Room temperature spin transport across a GaAs/ZnSe heterojunction. Kerr rotation with a probe energy of 2.8 eV detects coherent spins created in GaAs that cross the interface into ZnSe. Results are shown for electron spins precessing in magnetic field B = 0 T (purple curve), 0.025 T (pink curve), and 0.250 T (black curve). [Adapted from (120)] Figure 3. (A) Schematic densities of states N(E) for a concentrated magnetic semiconductor below TC (24). (B) Schematic densities of states N(E) for the half-metallic ferromagnet CrO2 (118, 119). Note that the energy scale is almost 10 times larger in (B).

18. Hall effect Figure 5. Field effect control of hole-induced ferromagnetism in magnetic semiconductor (In,Mn)As field-effect transistors. Shown is magnetic field dependence of the sheet Hall resistance RHall, which is proportional to the magnetization of the magnetic semiconductor layer, as a function of the applied gate voltage VG. RHall is used to measure the small magnetization of the channel. VG controls the hole concentration in the magnetic semiconductor channel. Application of VG = 0, +125, and  125 V results in qualitatively different field dependence of RHall measured at 22.5 K. When holes are partially depleted from the channel (VG = +125 V), a paramagnetic response is observed (blue dash-dotted line), whereas a clear hysteresis at low fields (< 0.7 mT) appears as holes are accumulated in the channel (VG =   125 V, red dashed line). Two RHall curves measured at VG = 0 V before and after application of ±125 V (black solid line and green dotted line, respectively) are virtually identical, showing that the control of ferromagnetism can be done isothermally and reversibly. (Inset) The same curves shown at higher magnetic fields. [Adapted from (23)]