nanomagnetismo, GMR

la magnetoresistenza colossale (GMR)
Carlo Mariani
Giant Magnetoresistance (GMR) ~ Premio Nobel per la Fisica 2007 ad
Albert FERT e Peter GRÜNBERG
Université Paris-Sud; Unité Mixte
de Physique CNRS/THALES
Orsay, France
Forschungszentrum Jülich
Jülich, Germany
Parallel Spin Filters → Low
Resistance
Opposing Spin Filters → High
Resistance
Albert FERT in Roma, EPS - CMD 22 Conference, August 2008
Magnetoresistenza (MR), le origini
Lord Kelvin (W. Thomson), Proceedings of the Royal Society of London, 8, pp. 546–550
(1856–1857)
…nel Fe immerso in un campo magnetico aumenta la
resistenza elettrica lungo le linee di flusso del campo,
diminuisce quella lungo la direzione normale al campo…
Il magnetismo
Materiale non-magnetico
(paramagnetico), polarizzazione
totale di spin P
Materiale magnetico
(ferromagnetico), polarizzazione
totale di spin P
P = (N↑ – N↓)/(N↑ + N↓) = 0
P>0
La conducibilità elettrica nei metalli di transizione ÅÆ legame con le proprietà
magnetiche
Processi di
diffusione degli
elettroni di
conduzione “s”
in stati “d” vuoti
Sir Nevill Mott, Proc. Roy. Soc. (London) A 153, 699 (1936)
La conducibilità nei metalli di transizione ÅÆ effetto delle impurezze
eq. del trasporto, urti e tempi di rilassamento τ anche
con accoppiamento fra I due tipi di elettroni
resistività complessiva
spin up-spin up scattering >> spin up-spin down scattering
H > Hsat Æ current carried by spin down electrrons
with low resistivity
aumento della resistività con le impurezze nonmagnetiche in materiale magnetico 3D
Magnetoresistenza anisotropa (AMR)
magnetoresistenza anisotropa (AMR)
I.A. Campbell and A. Fert, “Transport Properties of Ferromagnets” in
Ferromagnetic Materials, ed. E.P. Wohlfarth, North-Holland, Amsterdam, Vol. 3, p.
747 (1982)
…a causa dell’accoppiamento spin-orbita degli elettroni… anisotropia
della MR con campo // o normale agli strati magnetici… fenomeno in
generale dell’ordine del qualche %…
Giant Magneto Resistance (GMR)
Albert FERT
Magnetic multilayers:
alternating nm-thick ferromagnetic and non-magnetic layers
up to 60-times repeated Fe/Cr multi-layer slabs along the (001) direction
magnetization in the layers’ planes
antiferromagnetic (AF) coupling
M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen van Dau, F. Petroff, P. Eitenne, G. Creuzet, A. Friederich, and J. Chazelas,
“Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlat-tices”, Phys. Rev. Lett. 61, 2472 (1988)
domini magnetici in antifase nei multilayer
ruolo determinante degli elettroni
degli strati non-magnetici poiché
danno luogo al meccanismo di
accoppiamento fra gli strati
magnetici
• l’interazione di scambio in funzione della distanza dal sito atomico
Density of states in a confined system
Periodic changes
in the density of electron states
when the thickness of a Cu film
is changed.
Electrons in the Cu film
are acting like waves and give
maximum intensity when
an integer number of oscillations
fits into the film.
These structures can
be viewed as the smallest
man-made interferometers,
at the atomic limit.
The density-of-states maxima are correlated with oscillations of the magnetic coupling
in multilayers (bottom panel, from Qiu et al., Phys. Rev. B 46, 8659 (1992)).
The magnetoresistance oscillates with the same period. This gives us the clues for understanding
the magnetic phenomena as the effects of quantized electron levels in nanostructures.
Giant Magneto Resistance (GMR)
Albert FERT
up to 60-times repeated Fe/Cr(001) slabs
AF coupling dCr < 9 Å
T = 4.2 K
MR arising from spin-dependent transmission of the conduction electrons through the thin Cr layer
Giant Magneto Resistance (GMR)
Peter GRÜNBERG
Fe/Cr/Fe
Magnetic 3-layer:
alternating nm-thick ferromagnetic and non-magnetic layers
magnetization in the layers’ plane (easy axes) >>
magnetization along the growth direction (hard axes)
antiferromagnetic (AF) coupling
G. Binasch, P. Grünberg, F. Saurenbach, and W. Zinn, “Enhanced magnetoresistance in layered magnetic structures with
antiferromagnetic interlayer exchange”, Phys. Rev. B 39, 4828 (1989)
Giant Magneto Resistance (GMR)
Peter GRÜNBERG
Fe/Cr/Fe
magneto-optic Kerr effect
(MOKE) signal
at RT
∆R/R signal
come far crescere multi-layer altamente ordinati
Minimo misfit reticolare per
favorire la crescita
epitassiale: per esempio Cr e
Fe (bcc)
direzione cristallina di
crescita (001)
substrato: GaAs
spessore nanometrico -> cammino libero degli elettroni di conduzione
attraverso gli strati >> spessore degli strati
Giant Magneto Resistance (GMR)
Premio Nobel per la Fisica 2007 ad
Albert FERT e Peter GRÜNBERG
Fe
Fe
Cr
Fe
Fe
Cr
Top: external magnetic field H=0, antiferromagnetic coupling
Bottom: external magnetic field H ≠ 0 -> forcing parallel magnetization
GMR: ∆R = RH – R0 = – (1/2)(R↑ – R↓)2 /(R↑ + R↓)
the larger the difference between R↑ and R↓ the larger the negative magnetoresistance
le applicazioni
5 nm
Magnetic Storage Media
Magnetic Force
Microscope (MFM)
Image
600
nm
17 Gbits/inch2 commercial
Hundreds of particles per bit
Single particle per bit !
50 nm
10 nm particle
further work on GMR
• Spin waves from multilayers crossed by currents, theor.: L. Berger,
Phys. Rev. B 54, 9353 (1996)
• Spin waves excitation, expt.: Katine et al., Phys. Rev. Lett. 84, 3149
(2000)
•M. Tsoi, R.E. Fontana and S.S.P Parkin, “Magnetic Domain Wall
Motion Triggered by an Electric Current”, Appl. Phys. Lett. 83, 2617
(2003)
SEM
MFM
Superparamagnetic Limit
of Particle Size
Energy
Barrier ∆E
Flip Rate ≈ νAttempt
≈109s-1
•
exp[-∆E/kT]
≥40kT
Proprietà magnetiche in funzione delle dimensioni
Magnetic nanostructures
.
A. Wachowiak, J. Wiebe, M. Bode, O. Pietzsch, M. Morgenstern, and R. Wiesendanger, Science 298, 577 (2002):
Internal Spin-Structure of Magnetic Vortex Cores observed by Spin-Polarized Scanning Tunneling Microscopy
nano-magnets on surfaces
Exchange coupling
among couples of Fe
atoms on Cu(111), as
measuredv by spin
resolved-STM
Fe antiferromagnetic
coupling on Cu(111), by
single atom maipulation
Wiesendanger group, Nature Physics 8, 497 (2012)
magnetism in a confined system
M. F. Crommie, Science 309, 1501 (2005)
single-molecule magnetism
STM tip–induced dehydrogenation of a
single CoPc molecule.
Hydrogen atoms 2 and 3 of one lobe were
dissociated in our experiments.
(C) Diagram of the dehydrogenation
induced by the STM current
(D to H) STM images of a single CoPc
molecule during each step of the
dehydrogenation process
Kondo-peak at d-CoPc anchored on Au(111), due to
strong localization of the d-like electronic states
A. Zhao et al., Science 309, 1542 (2005)
single-atom magnetism:
quantum mirage
Kondo-peak at 2 Co atoms anchored on Au(111), due
to strong localization of the d-like electronic states
H. C. Manoharan, C. P. Lutz & D. M. Eigler, Nature 403, 512
from single-molecule magnetism to magnetism in highly-ordered 1D2D molecular arrays
LEED
use of naturally nano-patterned surfaces for
firm anchoring of ordered MPc molecular
arrays with definite magnetic moment
experimental thesis available!
Sara Fortuna et alii, J. Phys. Chem. C 116, 6251 (2012);
and Maria Grazia Betti et alii, Langmuir 28, 13232 (2012)