Giant Magnetoresistance

1. Giant Magnetoresistance Kómár PéterSolid state physics seminar25/09/2008

2. Types of magnetoresistance • Ordinary MagnetoResistance • Anisotropic MR • Giant MR • Tunneling MR • Colossal MR • Ballistic MR • Extraordinary MR

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

5. I Iρ I’ B  0 B = 0 Corbino-disk

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

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

8. Barber’s pole magnetic sensor • Barber’s pole: • The sensor: • permalloy base (Fe20Ni80) • Au-Al strips  current flows in 45° → R(B) linear near 0 (2a) (2b) (2 a,b)Dr. Andreas P. Friedrich, Helmuth Lemme, "The Universal Current Sensor”, Sensors weekly (May 1, 2000)

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

10. Manufacturing multilayered samples • 1970s epitaxial growth technology: • laser evaporation • molecular beam • sputtering • chemical deposition • Features: • Si, SiO2, semiconductorbase • compatible lattice parameters(!) • good reproductivity

11. 1 EA HA 12 12 [nm] EA: Results of Grünberg et al. I. • Fe-Cr-Fe sample: • GaAs base (epitxial growth, bcc) • AF coupling between Fe-s • [100] easy-(EA), [110]hardaxis(HA) • Checking: • MOKE (Magneto-opticalKerr effect) • light scattering on spin-waves 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. EA: HA: Results of Grünberg et al. II. • Change of resistance(T = TRT) • 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. 7

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. EA 30 (1.8nm) HA 35 (1.2nm) EA 60 (0.9nm) Results of Fert et al. II. • Change of resistance(T = 4.2 K) • ΔR/R (-50%) andHS (2 T) was measured • influence of temperature (TRT : -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

15. Theory of GMR I. • RKKY interaction( Ruderman, Kittel (1954), Kasuya (1956), Yosida (1957) ) • Coupling between atomic and conducting electrons (exchangeint.,2nd 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. N↓(EF)=N↑(EF) N↓(EF)>N↑(EF) B R↓=R↑ R- =R↓<R↑= R+ Theory of GMR II. • Spin-dependentresistance • scattering in FM, and at FM/NM interlayer • R-1~σ ~ N(EF) • Fermi-surface changes as an effect ofB Class for physics of the Royal Swedish Academy, “Discovery of the Giant Magnetoresistance” (9 October 2007)

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

18. Theory of GMR IV. • Half metals • ↓ - conducting, ↑ - insulator (eg. CrO2) • spin polarization: 100% Class for physics of the Royal Swedish Academy, “Discovery of the Giant Magnetoresistance” (9 October 2007)

19. Application– HDD read heads • Construction • layers withdifferingcoercivity • + AFM layer (Bruce Gurney) • Rmeasuring • 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. 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. 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 La2/3Ba1/3MnOx Ferromagnetic Films”, Pys. Rev. Letters Vol. 71, No. 14

22. Spintronics I. • Manipulating both charge and spin • Spin sources: GMR, TMR (Current In Plane, CPerpendicular P) • Manipulation: Spin Torqe Transfer (spin of current → magnetization of layer) • Reading (in semiconductors):light scattering, electroluminescence, spin valve, ballistic spin filtering

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

24. Thank you for the attention!