1. Giant Magnetoresistance Kómár Péter Solid state physics seminar 25/09/2008

2. 2 Types of magnetoresistance  O rdinary M agneto R esistance  A nisotropic MR  G iant MR  T unneling MR  C olossal MR  B allistic MR  E xtraordinary MR

3. 3 First achievements  1856 Thomson (Lord Kelvin) (AMR)  B ║ I → Increase of resistance  B ┴ I → Decrease of resistance (max. 5%)  1886 Boltzmann, 1911 Corbino  Corbino-disk (OMR)

4. 4 Ordinary MR  Lorentz force → change of mobility:  Lorentz force:  velocity of charged particles:  Corbino-disk:  Effective mobility:

5. 5 Corbino-disk IρIρ I’I’ B  0 I B = 0

6. 6 Anisotropic MR  Angle between I and B  R = max. at parallel alignment  B ┴ I → OMR  Application: magnetic sensors  electronic compass  traffic sensors  non-galvanic current meter B I

7. 7 AMR and Hall-effect  Ohm’s law: j = σ E,where σ is a matrix  Diagonal elements: conductivity + AMR  Off-diag. elements: Hall-effect ( j ┴ B ┴ E H )

8. 8 Barber’s pole magnetic sensor  Barber’s pole:  The sensor:  permalloy base (Fe 20 Ni 80 )  Au-Al strips  current flows in 45° → R(B) linear near 0 (2 a,b) Dr. Andreas P. Friedrich, Helmuth Lemme, "The Universal Current Sensor”, Sensors weekly (May 1, 2000) (2a) (2b)

9. 9 Giant MR  1988 Fert & Grünberg (2007 Nobel prize)  Multilayered samples (Fe-Cr-Fe)  Ferromagnetic. – Antiferromagn. coupling  Decrease in resistance of 10% and 50% Photos: U. Montan (http://nobelprize.org/nobel_prizes/physics/laureates/2007/) Albert Fert Peter Grünberg

10. 10 Manufacturing multilayered samples  1970s epitaxial growth technology:  laser evaporation  molecular beam  sputtering  chemical deposition  Features:  Si, SiO 2, semiconductor base  compatible lattice parameters(!)  good reproductivity

11. 11 Results of Grünberg et al. I.  Fe-Cr-Fe sample:  GaAs base (epitxial growth, bcc)  AF coupling between Fe-s  [100] easy- (EA), [110] hard axis (HA)  Checking:  MOKE (Magneto- optical Kerr effect)  light scattering on spin-waves EA: G. Binasch, P. Grünberg, F. Saurenbach, W. Zinn (1989) „Enhanced magnetoresistance is layered magnetic structures with antiferromagnetic interlayer exchange” Pys. Rev. B Vol 39. No. 7 12 1 [nm] EA HA

12. 12 Results of Grünberg et al. II.  Change of resistance (T = T RT )  B ║EA: GMR (-1.5%)  B ║HA: AMR (-0.13%*) és GMR (-1.5%)  d(Fe) = 8 nm → ΔR/R = 3% * 25 nm Fe plate G. Binasch, P. Grünberg, F. Saurenbach, W. Zinn (1989) „Enhanced magnetoresistance is layered magnetic structures with antiferromagnetic interlayer exchange” Pys. Rev. B Vol 39. No. 7EA:HA:

13. 13 Results of Fert et al. I.  [Fe-Cr] n sample:  GaAs base  5 – 60 layers  changing d(Cr) (6, 3, 1.8, 1.2, 0.9 nm) → change in coupling of Fe layers: Ferromagnetic (6 nm) Antiferromagnetic (0.9 nm) (T = 4.2 K) M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff (1988) „Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattice” Pys. Rev. Letters Vol. 61, No. 21

14. 14 Results of Fert et al. II.  Change of resistance (T = 4.2 K)  ΔR/R (-50%) and H S (2 T) was measured  influence of temperature (T RT : -25%, 1.4 T)  EA-HA difference, number of layers, d(Cr) M. N. Baibich, J. M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff (1988) „Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattice” Pys. Rev. Letters Vol. 61, No. 21 HA EA 60 (0.9nm) 35 (1.2nm) 30 (1.8nm) EA

15. 15 Theory of GMR I.  RKKY interaction ( Ruderman, Kittel (1954), Kasuya (1956), Yosida (1957) )  Coupling between atomic and conducting electrons (exchange int., 2 nd order perturb.)  Based on the Bloch wavefunction  applies only for periodic structures  F-NF-F arrangement: coupling oscillates! Class for physics of the Royal Swedish Academy, “Discovery of the Giant Magnetoresistance” (9 October 2007)

16. 16 Theory of GMR II.  Spin-dependent resistance  scattering in FM, and at FM/NM interlayer  R -1 ~ σ ~ N ( E F )  Fermi-surface changes as an effect of B Class for physics of the Royal Swedish Academy, “Discovery of the Giant Magnetoresistance” (9 October 2007) R↓= R↑R↓ = R↑R↓= R↑R↓ = R↑ N↓ (EF) = N↑ (EF)N↓ (EF) = N↑ (EF)N↓ (EF) = N↑ (EF)N↓ (EF) = N↑ (EF) R - = R ↓ < R ↑ = R + B N↓ (EF) > N↑ (EF)N↓ (EF) > N↑ (EF)N↓ (EF) > N↑ (EF)N↓ (EF) > N↑ (EF)

17. 17 Theory of GMR III.  Spin-valve  d(NM) < λ e → the spin of e - -s is constant  ↓ and ↑ parallel conduction channels Class for physics of the Royal Swedish Academy, “Discovery of the Giant Magnetoresistance” (9 October 2007) B

18. 18 Theory of GMR IV.  Half metals  ↓ - conducting, ↑ - insulator (eg. CrO 2 )  spin polarization: 100% Class for physics of the Royal Swedish Academy, “Discovery of the Giant Magnetoresistance” (9 October 2007)

19. 19 Application – HDD read heads  Construction  layers with differing coercivity  + AFM layer (Bruce Gurney)  R measuring  Efficiency  1991. MR  1997. GMR (Stuart Parkin) Magnet Academy, (http://www.magnet.fsu.edu/education/tutorials/magnetacademy/gmr/), IBM Research, (http://www.research.ibm.com/research/gmr.html)

20. 20 Tunneling MR  Ferromagn. – insulator– ferromagn.  1975: 14%/ -  1982: - / few%  1995: 30% / 18%  2007: >200%  Application:  spintronics  magnetic sensors Class for physics of the Royal Swedish Academy, “Discovery of the Giant Magnetoresistance” (9 October 2007)

21. 21 Colossal MR  1993 von Helmolt et al.  perovskite-like La-Ba-Mn-O  annealing, T = 300 K, B = 7 T  |ΔR| / R > 60% (steep start, no saturation) R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer (1993) „Giant Negative Magnetoresistance in Perovskitelike La 2/3 Ba 1/3 MnO x Ferromagnetic Films”, Pys. Rev. Letters Vol. 71, No. 14

22. 22 Spintronics I.  Manipulating both charge and spin  Spin sources: GMR, TMR (C urrent I n P lane, C P erpendicular P)  Manipulation: Spin Torqe Transfer (spin of current → magnetization of layer)  Reading (in semiconductors): light scattering, electroluminescence, spin valve, ballistic spin filtering

23. 23 Spintronics II.  Application:  MRAM (NVM)  transistor  laser

24. Thank you for the attention!