Andreev reflections in ferrimagnetic CoFe 2 O 4 spin filters

1. Andreev reflections in ferrimagnetic CoFe2O4 spin filters Group meeting 26/05/2010

2. In collaborations... • F.Rigato, M. Foester and J. Fontcuberta Institut de Ciència de Materials de Barcelona, Spain • F.Giubileo and A.M. Cucolo Physics Department and CNR-SPIN Salerno, Italy Phys. Rev. B 81, 174415 (2010)

3. Outline • Introduction: Spin polarisations and ferrimagnets • Tunneling Spectroscopy: Normal metal/Superconductor interface and Ferromagnet/Superconductor interface • Experimental data and results • Conclusions

4. Determination of the spin polarisation The spin polarisation is the key quantity for spintronics • Photoemission: measures the spin of the electron emitted from a region close to the surface of the ferromagnet of the order of 5-20A; relatively poor energy resolution (~ 1meV) ; sensitive to the surface preparation • Planar tunnel junction: The superconducting density of the states is Zeeman-split by the application of the magnetic field of several Tesla need for a simple and direct technique to measure P • Metallic Point Contact between the sample and a superconductor:based on the Andreev Reflection at a point contact between ferromagnetic material and a superconductor P. D.Johnson, in Core Spectroscopies for Magnetic Phenomena, edited by P.S. Bagus et al. (1995) P. Tedrow and R. Meservey, Phys.Rep. 238, 173 (1994) R.J. Soulen, et al. Science 282, 85 (1998)

5. Injected electron Transmitted electron Reflected electron Fermi level Cooper Pair Reflected hole N S N/s-wave interface In the Point Contact experiments tunneling of quasi-particle (high barrier) and Andreev reflection (low barrier) processes can occur. To take into account these effects Blonder, Tinkham and Klapwijk (BTK model) proposed a model to analyse the different dI/dV(V)-V characteristics introducing a dimensionless parameter Ztomodel the strength of the barrier G. E. Blonder, M. Tinkham, and T. M. Klapwijk , Phys. Rev. B 25, 4515 (1982)

6. Z=0 P=0 Z=0.3 P= 0.2 P=1 is the polarization of the metal unpolarized current for which Andreev Reflection is allowed polarized current for which Andreev Reflection probability is zero S/F interface Soulen et al., Science 282, 85 (1998) I. I. Mazin, A. A. Golubov, and B. Nadgorny, J. Appl. Phys. 89, 7576 (2001)

7. Configuration PCAR I+ V+ Nb tip I- V- Silver paste SRO is a metallic ferromagnet TCurie~120K Nb is a superconductor TC~9.2K CoFe2O4 3 nm SrRuO3 25 nm (111) SrTiO3 substrate

8. Ferrimagnetism • Ferrimagnetic materials include oxides of iron, nickel, or cobalt. • The magnetic moments of adjacent atoms are aligned opposite to each other, but there is incomplete cancellation of the moments because they are not equal. • Thus, there is a net magnetic moment within a domain. M Ferrimagnets (ferrites) behave similar to ferromagnets

9. CoFe2O4 B sites (Fe3+, Co2+) A sites (Fe3+) (111) • Ferrimagnetic oxide (Hc=3000 Oe) – 3.7 µB/F.U. • Inverse spinel structure. a = 8.39 Å => • misfit with MgO : 0,4% (001)//(001) • misfit with Al203 : 7 % (111)//(0001) • Tc ~ 800K Fe3+ Co2+

10. Ferrimagnetic CoFe2O4 spin filters P~39(1)% P~31(3)%

11. Junction in series:

12. SrRuO3: Spin polarisation Nb tip SrRuO3 25 nm (111) SrTiO3 From the literature negative spin polarization is reported that arises from the difference of Fermi velocities for spin-up and spin-down electrons emerging from SRO.

13. Spin-filter efficiency X X

14. Conclusions • Point contact measurements on SrRuO3/CoFe2O4 • Evidence of Andreev reflections at ferrimagnetic tunnel barriers • Spin-filtering of about 13%