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Switching behavior of the superconducting spin-valve Py/CuNi/Nb/CuNi/Py/FeMn

Switching behavior of the superconducting spin-valve Py/CuNi/Nb/CuNi/Py/FeMn. Jorina van der Knaap 27 January 2010. Outline. Theory What is a spin valve? AMR Domain effects Experiments Magnetization T c in P and AP state AMR Conclusion. Spin valve. F. S. F. F. S.

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Switching behavior of the superconducting spin-valve Py/CuNi/Nb/CuNi/Py/FeMn

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  1. Switching behavior of the superconducting spin-valvePy/CuNi/Nb/CuNi/Py/FeMn Jorina van der Knaap 27 January 2010

  2. Outline • Theory • What is a spin valve? • AMR • Domain effects • Experiments • Magnetization • Tc in P and AP state • AMR • Conclusion

  3. Spin valve F S F

  4. F S Demler et al. Phys. Rev. B (1997) Proximity effect

  5. 0.5xF F xs S 0.5xF F Superconducting spin switch Tagirov PRL(1999) Prediction: it should be possible to make a spin valve that can switch between superconducting and normal state by rotating M of top layer.

  6. S S F F Why TcP<TcAP? How is this possible? This is a picture of an F/S/F structure. The directions of magnetization are antiparallel. Shown is 1 Cooper pair. A Cooper pair is non-local, it is not only in S but also partially in the F layers. Because the layers are antiparallel, both electrons can enter one of the F layers. They experience the same potential energies and pair breaking is possible but less than in the parallel case. In the parallel case the electrons can still enter the F layer. The electrons experience different potential energies and therefore have different kinetic energies. This leads to very fast pair breaking, much faster than in the antiparallel case. Then the density of Cooper pairs is lower and therefore Tc is lower than in the antiparallel case. F F Antiparallel state, less pair breaking, higher Tc, R lower Parallel state, more pair breaking, lower Tc, R higher

  7. Anisotropic magnetoresistance (1) • Anisotropic magnetoresistance (AMR): resistance of ferromagnet depends on angle between current I and magnetization M. Not on H. • Details material dependent. For simple Ni- based alloys: Rpar>Rperp.

  8. Anisotropic magnetoresistance (2) R high R lower R high

  9. How to get AP state (1) T=300 K: without exchange bias T=100 K: with exchange bias

  10. M H0 AF P F S F -H0 H AP P H R How to get AP state (2) Pinned layer switches Free layer switches AF AMR S H R RH in transition Magnetization

  11. Experiment (Gu & Bader) Gu & Bader, PRL (2002) Thickness:s/Py(4)/CuNi(5)/Nb(18)/CuNi(5)/Py(4)/FeMn(6)

  12. Domain effects Flokstra, PRB (2009)

  13. Nb capping layer FeMn AF Py F2 (pinned layer) CuNi Nb S CuNi F1 (free layer) Py Experiment: sample Thicknesses: s/Py(3)/CuNi(10)/Nb(18)/CuNi(10)/Py(3)/FeMn(6)/Nb(2)

  14. SQUID magnetization measurement SQUID measurement shows nice switching and AP state between -1000 Oe and 0 Oe. Measurements above and below superconducting Tc overlap. F layers not influenced by S layer.

  15. Tc in P and AP state More than 30 mK difference between Tc(P) and Tc(AP)!

  16. AMR AMR at 4.2 K does not show P and AP states. Instead the AP state seems to be a domain state.

  17. Conclusion • Tc(P)> Tc(AP). • 30 mK difference • Effect is not only caused by AP state but also by domains. • Magnetization measurement is not enough to conclude that there is a AP state.

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