Giant Magnetoresistance Kmr Pter Solid state physics seminar

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

Types of magnetoresistance s s s s Ordinary Magneto. Resistance Anisotropic MR Giant MR Tunneling MR Colossal MR Ballistic MR Extraordinary MR 2

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

Ordinary MR s Lorentz force → change of mobility: } Lorentz force: velocity of charged particles: s Corbino-disk: } Effective mobility: 4

Corbino-disk I B=0 I’ Iρ B 0 5

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

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

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

Giant MR s 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 9 Photos: U. Montan (http: //nobelprize. org/nobel_prizes/physics/laureates/2007/ )

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

Results of Grünberg et al. I. s 1 Fe-Cr-Fe sample: } Ga. As base (epitxial growth, bcc) } AF coupling between Fe-s } [100] easy- (EA), [110] hard axis (HA) s Checking: EA HA 12 12 [nm] EA: } MOKE (Magnetooptical Kerr effect) } light scattering on spin-waves 11 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

Results of Grünberg et al. II. s 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 EA: HA: 12 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

Results of Fert et al. I. s [Fe-Cr]n sample: } Ga. As 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 13

Results of Fert et al. II. s Change of resistance (T = 4. 2 K) } ΔR/R (-50%) and HS (2 T) was measured } influence of temperature (TRT : -25%, 1. 4 T) } EA-HA difference, number of layers, d(Cr) EA 30 (1. 8 nm) HA 35 (1. 2 nm) EA 60 (0. 9 nm) 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

Theory of GMR I. s 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! 15 Class for physics of the Royal Swedish Academy, “Discovery of the Giant Magnetoresistance” (9 October 2007)

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

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

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

Application – HDD read heads s Construction } layers with differing coercivity } + AFM layer (Bruce Gurney) } R measuring s 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) 19

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

Colossal MR s 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) 21 R. von Helmolt, J. Wecker, B. Holzapfel, L. Schultz, K. Samwer (1993) „Giant Negative Magnetoresistance in Perovskitelike La 2/3 Ba 1/3 Mn. Ox Ferromagnetic Films”, Pys. Rev. Letters Vol. 71, No. 14

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

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

Thank you for the attention!