Spin dependent transport in nanostructures David Halley, O. Bengone and W. Weber, Institut de Physique et Chimie des Matériaux de Strasbourg Seoul 2009

Plan I Introduction: magneto-resistive effects II Basis of spintronics Principles of Giant Magneto-Resistance Some typical metallic systems and applications Technological requirements III Tunnel Magneto-Resistance Spin-conserving tunneling of electrons. Julliére’s formula Typical TMR systems Electronic symmetry in monocristalline tunnel junctions Application to Fe/MgO/Fe systems How can chromium become insulating? IV Conclusion and perspectives New materials New effects: spin torque and resistive switching

Interplay between magnetism and electrical resistivity of a solid Magneto-resistive effects: The Lorentz force due to magnetic field modifies the trajectory of electrons : Ordinary Magneto Resistance (OMR (Lord Kelvin, 1856: DR/R < 5%) ): the scalar resistivity is given by: rxx=1/s0 (1+(mB)2) where B is the magnetic field, m the electron mobility, Bulk effect

Plan…. I Introduction: magneto-resistive effects II Basis of spintronics Principles of Giant Magneto-Resistance Some typical metallic systems and applications Technological requirements III Tunnel Magneto-Resistance Spin-conserving tunneling of electrons. Julliére’s formula Typical TMR systems Electronic symmetry in monocristalline tunnel junctions Application to Fe/MgO/Fe systems How can chromium become insulating? IV Conclusion and perspectives new materials new effects: spin torque and resistive switching

from a few microns to nanometers Spintronics Playing with the spin polarisation of electrons in different ferro-magnetic materials. Devices mostly play with interface effects: Need of nanostructured devices: e- a few nanometers lateral size: from a few microns to nanometers Starting point : discovery of Giant Magneto-Resistance (P. Grünberg and A.Fert)

Principle of Giant Magneto-Resistance Giant Magneto Resistance ( GMR): injection through a thin spacer layer into a second ferromagnetic layer: Rap Rp e- e- Rp Rap Ni/Cu/Py system magnetisation Spacer: metal (Fe/Cr/Fe for instance) or insulator: Tunnel Magneto Resistance

The density of states is different for both spins. Physical idea of GMR Electronic band structure and spin polarisation Polarisation of the spin of the conducting electrons: majority electrons ( spin down relatively to the magnetisation) minority electrons (spin up) Density of electronic states for majority electrons (spin down) Fermi level Energy states for minority electrons (spin up) The density of states is different for both spins. It can lead to different mean free path for minority and majority spins.

For which applications? Sensors…. Sensors for magnetic field. Position sensors. Read heads in hard disks. Memories Magnetic Random Access Memories (MRAM) in competition with: Floatting gates (USB flash memories) Phase Change Random Access Memories (PRAM) Ferro-electricity Resistive switching ….

Magnetic recording: writing and reading magnetic bits Conventional memories: H Inductive writting or reading: Magnetic bits

GMR system sensitive to the stray Using GMR in read heads of hard disks V Reading: GMR system sensitive to the stray field of magnetic bits R( ) < R( )

Using GMR junctions as memories? Magnetic Random Access Memorie (MRAM ) Stray field Iwritting Iw Iw Iw ireading ir ir ir Soft layer Hard layer Adressing each bit:ireading+Iwritting

Technological requirements for GMR Decoupling of the magnetic layers: parallel and antiparallel configurations in a low field. Making the resistance measurable : nanostructuration of devices. Obtaining high GMR values….

Technologicaly important Decoupling of the magnetic layers How to obtain two different coercitive fields? Different coercivities: one hard and one soft magnetic layers (NiFe and Co for instance) Interlayer Exchange Coupling Spin valves: magnetic pinning of one of the layers by antiferro-magnets (PtMn). Due to interfacial exchange coupling. Technologicaly important From S. Yuasa et al., J. Phys. D., 40, R337, (2007)

Lithography of metallic GMR junctions Reistivity of a metal: r < 100 mW.cm Assuming a 100 x 100 x 50 nm junction: R = r.l/S# 1 W …..very low!!! Lithography is required to have low lateral dimensions … and measurable resistances From Y. Jiang et al, Nature Materials 3, 361 - 364 (2004) But the GMR does not exceed a few tens of percents…..except with magnetic tunnel junctions.

Plan…. I Introduction: magneto-resistive effects II Basis of spintronics Principles of Giant Magneto-Resistance Some typical metallic systems and applications Technological requirements III Tunnel Magneto-Resistance Spin-conserving tunneling of electrons. Julliére’s formula Typical TMR systems Electronic symmetry in monocristalline tunnel junctions Application to Fe/MgO/Fe systems How can chromium become insulating? IV Conclusion and perspectives new materials new effects: spin torque and resistive switching

Conservation of the spin of the electron Tunnel Magneto Resistance (TMR) metallic electrode insulator metallic electrode Electron potential Bias voltage Conservation of the spin of the electron during tunneling e- Fermi level

One channel for each spin Tunnel Magneto Resistance e- Bias voltage Electron potential Fermi level magnetisation magnetisation magnetisation Density of electronic states for majority electrons (spin down) Fermi level Energy One channel for each spin Density of electronic states for minority electrons (spin up) Difference in resistivity between the parallel and anti-parallel magnetic configurations

Tunneling probability? Tunnel Magneto Resistance Fermi level Electrode 1 Electrode 2 Tunneling probability? Juillère’s model: Considering both electrodes as two isolated systems with different hamiltonians « The tunneling current in each spin channel is proportionnal to the product of the effective density of states at the Fermi level. «  Cf Fermi golden rule So, the conductance in the parallel and anti parallel cases can be written: gP  D1D2 +D1D2 gAP  D1D2+D1D2 

We define the spin polarisation in each electrode: Julliére’s Formula We define the spin polarisation in each electrode: It yields (Juliére, 1975):

Search for half-metals TMR = 2 P1 P2 /(1-P1P2) If P1 = P2 = 1 the TMR can theoretically by infinite. This 100% spin polarisation corresponds to: thus: D = 0 for each electrode Such materials are called half-metals: no transition metal some oxides are good candidates ( manganites). (cf N. Viart)

Growth of TMR systems Growth of continuous insulating layer with a low roughness. Thickness between 1 and 3 nm. Deposition methods: Sputtering ( Alumina barriers): giving amorphous or polycristalline samples. * * Pulsed laser deposition (complex oxides, for instance SrTiO3) e-beam target substrate Molecular Beam epitaxy (MgO barriers). * * Images from wikipedia

SEM image of a 20 nm MgO grown on Fe (001) Defects in TMR systems pinholes in the insulating layer = short circuit localised defects in the barrier ( oxygen vacancies, metallic impurities, dislocations ): Can depolarise the current: the TMR drops E. Fullerton et al, J. Appl.Phys., 81, (2) (1997) SEM image of a 20 nm MgO grown on Fe (001) e- Localised defect

“hot spots” in TMR junctions Roughness of the barrier Fluctuation of the barrier thickness t Exponential dependency of the tunneling probability on t Hot spots Hot spot AFM measurements on an Alumina barrier: Topography (left) current (right) V. Da Costa et al, Eur. Phys. J. B., 13, 297, (2000)

Higher resistance: lateral sizes in the microns range Lithography of tunnel junctions Higher resistance: lateral sizes in the microns range d #microns 12μm

Typical TMR systems Polycristalline electrodes and amorphous insulating barrier Co/Al2O3 barrier/ NiFe, with two different coercitive fields for both electrodes: Typical TMR measurements in a Ferro/insulator/Ferro junction (C. Tiusan, Phys.Rev. B, 64, 104423 (2001)) TMR (%) Co NiFe Al2O3 Magnetic field The TMR does not exceed 70% in polycristalline systems: due to defects in the barrier and to a non 100% spin polarisation in electrodes ( no real half metallic electrodes)

Plan…. I Introduction: magneto-resistive effects II Basis of spintronics Principles of Giant Magneto-Resistance Some typical metallic systems and applications Technological requirements III Tunnel Magneto-Resistance Spin-conserving tunneling of electrons. Julliére’s formula Typical TMR systems Electronic symmetry in monocristalline tunnel junctions Application to Fe/MgO/Fe systems How can chromium become insulating? IV Conclusion and perspectives new materials new effects: spin torque and resistive switching

e- Intrinsic issues in polycrystalline junctions electrode 1 insulator electrode 2 Growth direction When tunneling, electrons change their wave vector relatively to the cristalline directions the spin polarisation should be 100% on the whole Fermi surface ( for all electron wave vectors)

TMR in monocristalline systems W.H. Butler (2001) introduced the concept of coherent tunneling in TMR epitaxial junctions: Tunnel barrier Ferro 1 Ferro 2 Growth direction The periodicity of the crystalline potential is same throughout the sample: The electron can be described by a Bloch wave function in both electrodes and in the barrier.

Symmetry of Bloch functions Bloch theorema: in a period crystaline potential the wave function of electrons can be written: These functions can be classified into different symmetries relatively to the normal to interfaces. Cf orbital wave function characterised by their symmetry: s, p, d…. Bloch wave function can be decomposed into these orbitals: D direction

Symmetry of Bloch functions Ferro 2 Tunnel barrier Coherent tunnelling Ferro 1 The electron keeps its symmetry relatively to the normal to interfaces. For instance D1, D5, D2, D2’ states in [001] Fe is a label for the tunneling electron. The tunnelling of electrons is now determined by their spin and by the symmetry of their wave function Jullière model is no more valid

Symmetry dependent tunneling for Bloch functions This symmetry strongly determines the tunneling probability of electrons: Density of states as a function of the insulator thickness for different symmetries in a Fe/MgO/Fe monocristalline system (from W.H. Buttler *) S. Yuasa et al., J. Phys. D., 40, R337, (2007) *Phys. Rev. B, 63, 054416, (2001)

Amorphous or polycristalline Symmetry dependent tunneling for Bloch functions Amorphous or polycristalline barriers Monocristalline barriers From S. Yuasa et al., J. Phys. D., 40, R337, (2007)

Transmission electron microscopy Monocrystalline Fe/MgO/Fe systems Epitaxial growth of Fe/MgO/Fe systems by Molecular Beam Epitaxy Fe MgO Co Hard layer Soft layer Transmission electron microscopy image of a MgO barrier MgO aMgO aFe Mg O Fe

Coherent tunnelling in Fe/MgO/Fe junctions Fe/MgO/Fe (001) monocristalline junctions Dispersion curves for D1 and D5 electrons in iron, ( k perpendicular to the barrier) 0 p /a Wave vector k Majority electrons Minority EF Regarding electrons dominating the tunnel transport (D1 electrons), Fe is half-metallic!

Very high TMR in MgO-based tunnel junctions Less defects in FeCoB/MgO/FeCoB systems: Best results: up to 1000% at low temperature* *Y.M. Lee et al., Appl. Phys. Lett.90, 212507 (2007)

Very high TMR in MgO-based tunnel junctions Textured junctions grown by sputterring industrial applications coming soon? S. Yuasa et al., J. Phys. D., 40, R337, (2007)

Plan…. I Introduction: magneto-resistive effects II Basis of spintronics Principles of Giant Magneto-Resistance Some typical metallic systems and applications Technological requirements III Tunnel Magneto-Resistance Spin-conserving tunneling of electrons. Julliére’s formula Typical TMR systems Electronic symmetry in monocristalline tunnel junctions Application to Fe/MgO/Fe systems How can chromium become insulating? IV Conclusion and perspectives new materials new effects: spin torque and resistive switching

Insertion of a thin Cr epitaxial layer How can Chromium become insulating? 0 p /a Wave vector k Regarding D1 electrons Chromium is an insulator ( no D1 states at the Fermi level) Dispersion curve for electrons in chromium Fe MgO Cr Insertion of a thin Cr epitaxial layer below the MgO barrier Fe Cr MgO D1 Electrons Potential magnetisation e- Chromium is an insulating barrier in the parallel magnetic configuration

How can Chromium become insulating? Firt principle calculations of the conductance For majority electrons in the parallel magnetic configuration For x=0 (no Cr), and x=6 monolayers Cr The conductance in the parallel magnetic configuration drops with tCr: The tunnel conductance of D1 states is filtered by Cr

How can Chromium become insulating? Voltage (V) The conductance in the parallel magnetic configuration drops with tCr: The tunnel conductance of D1 states is filtered by Cr F. Greullet, Phys. Rev. Lett., 99, 187202 (2007)

Symmetry-resolved quantum wells Cr (2 nm) Electrons Energy Loss Spectroscopy Map: section of a Fe/Cr/Fe/MgO/Fe junction Coll. G. Bertoni EMAT Anvers oxygen chromium iron Fe (20 nm) Fe (1.5 nm) Fe (5 nm) MgO (2 nm) MgO substrate 20 nm Fe Cr Fe MgO Fe D1 electrons potential e- magnetisation Quantum well for D1 electrons only

Symmetry-resolved quantum wells Fe/Cr/Fe/MgO/Fe systems  F. Greuillet et al., Phys. Rev. Lett. 99 , 187202 (2007) Oscillations of the differential conductance: modulations of the density of D1 electronic states in the quantum well Peaks

….and resonant tunnel diodes From T. Niizeki et al.1 1 T. Niizeki et al., Phys. Rev. Lett. 100, 047207 (2008) Ab initio calculation of the position of the peaks as a function of Fe thickness. Cr/Fe/MgO/Fe stacking The amplitude and the energy position of the peaks depends on the width of the Fe quantum well. Changing the voltage selects the resonant condition that is spin-dependant very large TMR expected.

Plan…. I Introduction: magneto-resistive effects II Basis of spintronics Principles of Giant Magneto-Resistance Some typical metallic systems and applications Technological requirements III Tunnel Magneto-Resistance Spin-conserving tunneling of electrons. Julliére’s formula Typical TMR systems Electronic symmetry in monocristalline tunnel junctions Application to Fe/MgO/Fe systems How can chromium become insulating? IV Conclusion and perspectives new materials new effects: spin torque and resistive switching

Conclusion MgO based tunnel junctions very promising for spintronics. industrial ways of deposition are studied by IBM. new concept of «  symmetry-tronics » : artificial way to make half metals. observed in metals, oxides, and …..semi-conductors?

Perspective: organic materials What about organic materials? (High spin diffusion length) Could we combine both aspects: electronic symmetry in organic stacks? From ref. [2] : GMR device based on an Alq3 spacer. The measured GMR reaches 40% 2 Z.H. Xiong, Nature. 427, 821 (2004) From ref [3] : STM observation of ZnPcF8 ( fluorated Phtalocyanine molecules) grown on Ag (111) 3 V. Oison, Phys. Rev. B,75, 35428 (2007) Conclusion: much work to do concerning the growth of epitaxial organics…

Perspective: resistive switching and TMR? Can we play with defects in the barrier ? High electric field across the barrier Tunneling via defects in the insulator Electro-migration of defects: change in the tunneling probability

Fe/Cr/MgO/Fe junctions Perspective: resistive switching and TMR Fe/Cr/MgO/Fe junctions Defining an off and on states… D. Halley et al, Appl. Phys. Lett. 92, 212115 (2008)

Perspective: spin torque Writting bits with a high spin-polarised current density e- M1 Spin polarised of electrons along M1 V e- Magnetic switching of M2 Iwritting Reading: GMR measured with a low current density

Also through thin tunnel barriers….. Perspective: experimental spin torque Injecting the current through small nano-pilars: Also through thin tunnel barriers….. E.B. Myers, et al., Science, 285,868 S. Yuasa et al., J. Phys. D., 40, R337, (2007)

Spin torque and magnetic domain walls: Perspective: spin torque Spin torque and magnetic domain walls: Phys. Rev. Lett., 96, 197207 (2006)