The Superconductivity in Spin-valves

Feb. 18, 2013 - PRLog -- Background
Spin-valve ferromagnetism is based on theoretical predictions by Slonczewski [1] over a decade ago. Spin-valves comprise alternating nanoscale layers of FMs separated by a NM spacer. FM stands for ferromagnetic and NM for non-magnetic. Spin polarized current is produced by passing un-polarized current through the first FM layer, the polarization unchanged as the current flows through the NM spacer. The second FM having a dis-ordered spin-state is characterized by a giant magneto-resistance known as the GMR. The spin angular momentum from the first FM is thought transferred to the second FM as a physical spin-torque, the process tending to produce parallel spins in both FMs that change the GMR, the local change in resistance allowing data to be stored bit by bit in magnetic recording heads. See http://en.wikipedia.org/wiki/Spin_valve

Problem
The mechanism by which the GMR is lowered by the alignment of spins remains controversial even to this day. The relatively rigid lattice not only shields the electrons but limits the propagation of the spin-torque to phonons having frequencies < 10 GHz and response times > 100 ps. However, spin-valves are observed to respond much faster. Indeed, laser studies in femtomagnetism [2] show nanoscale FMs demagnetize on a sub-picosecond time scale (< 350 fs).

Casting further doubt, Jiang et al. [3] showed spin-transport to be inconsequential in Fe/Alq3/Co spin valves compared to the switching by holes common to non-volatile electrical switching. Alq3 stands for tris-(8-hydroxyquinolate) aluminum representative of organic spin-valves. The fact that non-volatile electrical switching was only recently proposed [4] to coexist with magnetoresistance nevertheless also supports the hole [3] mechanism.  Regardless, observations suggest electron spins have nothing to do with observed switching in spin-valves, and therefore the only relevant question is:

What is the switching mechanism in spin-valves?

Alternative Switching Mechanism
One alternative switching mechanism finds basis in QM and QED. QM stands for quantum mechanics and QED for quantum electrodynamics. The mechanism called QED induced photoconductivity is proposed for resistive switching in spin-valves. See NANOSMAT-Asia Paper and Presentation at http://www.nanoqed.org , 2013.

Finding basis in the QM requirement that the heat capacity of the atom vanishes at the nanoscale, Joule heat caused by either un-polarized or spin-polarized current cannot be conserved by a change in temperature. Instead, conservation proceeds by the QED induced frequency up-conversion of the Joule heat within the FM layers to non-thermal EM radiation at their TIR confinement frequencies. EM stands for electromagnetic and TIR stands for total internal reflection. See earlier spin-valve PR  http://www.prlog.org/12035920-qed-induced-switching-in-sp...

The EM radiation having Planck energy far beyond the UV creates excitons (holon and electron pairs) by the photoelectric effect, the positive holons (or holes) of which act as charge carriers to dramatically increase the photoconductivity of the FMs allowing the GMR to approach the zero resistance of superconductivity even with the spin-valve operating at ambient temperature.
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Simulations
The QED induced switching is simulated for Alq3 film thicknesses of 10, 20, 50, and 100 nm. All films were assumed to have an initial GMR of R_O = 1 megohm. The bias voltage +1 V was assumed for 10 ns followed by reversed bias -1 V for 10 ns. The transient resistance response is shown in the thumbnail.

The QED induced reduction in GMR is observed to change significantly depending on the film thickness d. The 10 nm film resistance ratio R/R_O  is reduced to ~ 0.000624 or (R ~ 624 ohms) in < 1 ns.  As the film thickness increases, R/R_O increases. Note the reversal of voltage Vo shows an abrupt change for the 10 nm film. In contrast, magnetic induced GMR reductions are relatively insignificant, i.e., the horizontal dashed line for 125 nm Alq3 film [4] at 100 K showing a GMR reduction of about 22% corresponding to R/R_O = 0.78.

Conclusions

1. The QED induced photoconductivity in spin-valve switching is so significant to entertain the notion that superconductivity at ambient temperature already exists or at least may be possible.

2. Spin-valve switching by electron-spin is insignificant compared to QED induced superconductivity allowing the possibility that electron-spin was never the mechanism for spin-valve switching [1] a decade ago.

3. Superconductivity in spin-valves of 10 nm Alq3 films is highly supportive of the planned use of nanowires with diameters < 10 nm as ambient temperature interconnects between circuit elements in nanoelectronics.  

References

[1]  J.C. Slonczewski, “Current-driven excitation of magnetic multilayers,” J. Magn. Magn. Mater, 150 (1996) L1.
[2]  C. Boeglin, et al., “Distinguishing the ultrafast dynamics of spin and orbital moments in solids,” Nature, 465 (2010) 458.
[3]  J. S. Jiang, J. E. Pearson, and S. D. Bader, “Absence of spin transport in the organic semiconductor Alq3,” Phys. Rev. B, 77 (2008) 035303.
[4]  M. Prezioso, et al., “Electrically Programmable Magnetoresistance in Multifunctional Organic-Based Spin Valve Devices,” Adv. Mater., vol. 23, pp. 1371–1375, 2011.