Atomic scale engeered spins at a surface
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Atomic-scale Engeered Spins at a Surface. Chiung-Yuan Lin IBM Almaden Research Center. Nanomagnetism and Information Technology. Magnetism is at the heart of data storage. Many novel computations schemes are based on manipulation of magnetic properties. Courtesy of Hitachi.

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Atomic scale engeered spins at a surface

Atomic-scale Engeered Spins at a Surface

Chiung-Yuan Lin

IBM Almaden Research Center


Nanomagnetism and information technology

Nanomagnetism and Information Technology

  • Magnetism is at the heart of data storage.

  • Many novel computations schemes are based on manipulation of magnetic properties.

Courtesy of Hitachi

J.R. Petta et al.

Science309, 2180 (2005)

A. Imre et al.

Science311, 205 (2006)


Nanomagnets

Nanomagnets

  • Fabricated nanomagnets can recreate model spin systems such as spin ice.

  • A small number of atomic spins can be coupled in metal clusters or molecular magnetic structures.

R.F. Wang et al., Nature439, 303 (2006)

Fe8, courtesy ESF.

M.B. Knickelbein

Phys. Rev. B70, 14424 (2004)


Assembly and measurement of nanomagnets

Assembly and Measurement of Nanomagnets


Stm studies of atomic scale spin coupling

|ST,m>

|ST,m>

|1,+1>

|5/2,+5/2>

|5/2,+3/2>

|1,0>

|5/2,+1/2>

Energy

Energy

|5/2,-1/2>

|1,-1>

|5/2,-3/2>

|5/2,-5/2>

|0,0>

Magnetic Field

Magnetic Field

STM Studies of Atomic-Scale Spin-Coupling

  • Manipulation on thin insulators:build individual nanomagnets with an STM

  • Spin Excitation Spectroscopy: collective spin excitations of individual nanostructures

10Mn chain

Mn atom

Science 312, 1021 (2006)


Keep it simple free mn atom

Keep it Simple: Free Mn Atom

4s

  • Half filled d-shell

  • Weak spin-orbit interactions

3d

Mn: S = 5/2, L = 0, J = 5/2


S canning t unneling s pectroscopy ldos

dI/dV

V

0

Scanning Tunneling Spectroscopy: LDOS

Ef

eV

tip

sample

Features in the local DOS are reflected in dI/dV.


Magnetic atoms on surfaces

Thin insulating layer

Magnetic Atoms on Surfaces

Magnetic atom

  • Atom’s spin is screened by conduction electrons (Kondo effect)

  • A thin insulating layer may isolate the atomic spin

Metal surface


I nelastic e lectron t unneling s pectroscopy

Ef

Ef

eV

D

D

eV

X

tip

sample

tip

sample

D

D

|eV| < D

Elastic Channel Open

Inelastic Channel Closed

|eV| > D

Elastic Channel Open

Inelastic Channel Open

Inelastic Electron Tunneling Spectroscopy

Non-magnetic tip

Thin insulator

Magnetic atom

dI/dV

kBT < D

σe+σie

σe

Non-magnetic sample

eV

-D

D

0


Methods of electronic structure calculation

Methods of Electronic-structure Calculation

Plane wave

Atomic spheres

Atomic partial wave

Atomic partial

wave

Interstitial region

  • Full-potential Linearized Augmented Plane Wave basis

  • Periodic-slab geometry

    (5-layer Cu + 8-layer vacuum)

  • Density Functional Theory

    Generalized Gradiant Approximation (GGA)

    PBE96: Perdew et al., PRL 77, 3865 (1996)

  • Structure Optimization


Methods of electronic structure calculation1

Methods of Electronic-structure Calculation

Cu

Cu

Cu

Cu

vacuum

vacuum

vacuum

  • FLAPW basis

  • Periodic-slab geometry

    (5-layer Cu + 8-layer vacuum)

  • Density Functional Theory

    Generalized Gradiant Approximation (GGA)

    PBE96: Perdew et al., PRL 77, 3865 (1996)

  • Structure Optimization


Methods of electronic structure calculation2

Methods of Electronic-structure Calculation

  • FLAPW basis

  • Periodic-slab geometry

    (5-layer Cu + 8-layer vacuum)

  • Density Functional Theory

    Generalized Gradiant Approximation (GGA)

    PBE96: Perdew et al., PRL 77, 3865 (1996)

  • Structure Optimization


Thin insulator cun islands on cu 100

N

a0=Ö2d0

Cu

d0

Thin Insulator: CuN Islands on Cu(100)

d0=2.55Å

a0=3.60Å

1nm

  • Atomic resolution on CuN

  • Mn atoms bind to Cu and N sites

CuN

Mn

Mn

Mn

Mn

Mn

Mn

Cu(100)

CuN monolayer

Cu(100)


Dft calculation of electron density in cun

0.25Å

1.80Å

DFT Calculation of Electron Density in CuN

N atoms are approximately coplanar with Cu atoms on CuN surface.

N-1

N-1

Cu+0.5

Cu+0.5

Cu+0.5

Cu

Cu


Manipulation of mn on cu 100 cun

Pick up Atom

Manipulation of Mn on Cu(100) / CuN

  • Move tip in

  • Apply 2.0V

  • Pull tip back


Manipulation of mn on cu 100 cun1

Pick up Atom

Drop off

Manipulation of Mn on Cu(100) / CuN

  • Move tip in

  • Apply -0.5V

  • Pull tip back


Spectroscopy of mn dimers

N

Cu

Mn

Mn

Spectroscopy of Mn Dimers

  • Large step at ~6mV splits into three distinct steps at high fields


Coupled spins

|ST,m>

E

|1,+1>

|1,0>

|1,-1>

|0,0>

B

Coupled Spins

5

4

1

0

  • S=5/2 Ä S=5/2  ST =

  • For ST=0 (singlet) the first excited state is ST=1 (triplet)

    • Three excitations around constant energy shift


Chains of mn atoms

IBM Almaden STM Lab has built chains of up to 10 Mn atoms on Cu binding sites

Cu(100)

2

6

1nm

3

7

10Mn

1Mn

4

8

CuN

1nm

5

9

Mn

Mn

Mn

Chains of Mn Atoms

N

Cu


Spectroscopy of mn chains

2

6

1nm

3

7

4

8

5

9

Spectroscopy of Mn Chains

10

Spectra change dramatically with each additional Mn atom.


Heisenberg model of spin coupling

Heisenberg Model of Spin Coupling

J

S

  • Phenomenological Exchange Coupling

    • J = Coupling strength

    • Si = spin of ith atom

  • Assumptions

    • All spins are the same

    • Nearest-neighbor coupling

    • All J are the same

    • J > 0 (antiferromagnetic coupling)


Heisenberg dimer spectrum

Heisenberg Dimer Spectrum

  • SG=0 and SE=1

  • Atomic spin affects numbers of levels but not spacing

  • First excited state at J

J

S


Determination of spin coupling strength

Determination of Spin Coupling Strength

  • From the dimer spectrum J=6.2meV

  • Variations in J of ±5% for different dimers at various locations

J=6.2meV


Determination of atomic spin

Determination of Atomic Spin

  • Using J = 6.2meV, we find S=5/2

  • STM determines both J and S!

S=3

S=5/2

S=2

J=6.2meV


Heisenberg model for longer chains

Heisenberg Model for Longer Chains

  • Use J = 6.2meV and S=5/2

  • Odd chains

    • ground state spin = 5/2

    • excited state spin = 3/2

  • Even chains

    • ground state spin = 0

    • excited state spin = 1


Unit cells used in calculating mn on cun

Unit Cells Used in Calculating Mn on CuN

Single Mn, larger unit cell

Single Mn, smallest unit cell

Mn dimer, smallest unit cell

N

Cu

Mn

Mn

10.80Å

7.20Å

7.20Å


Electron density with an adsorbed mn atom

Electron Density with an Adsorbed Mn Atom

Mn+

N -1.5

N -1.5

Cu+0.5

Cu+0.5

Cu

Cu

Cu

  • N atoms move farther out of surface Cu layer towards Mn atom.

  • Cu atom being pushed into the surface.

  • This “isolates” the free spin of Mn atom.


Mn spin from dft

Mn Spin from DFT

majority ()

minority ()

Free Mn atom

3d 5S=5/2


A new kind of atomic scale magnet

A new kind of atomic-scale magnet

  • Surface N atoms isolate and bridge Mn atoms.

  • This is a “surface” assembled magnet.

Mn

Mn

N

N

N

Cu

Cu

Cu

Cu

Cu


Control of spin coupling strength

Control of Spin Coupling Strength

J=6.2meV

J=2.7meV

STM can switch J by a factor of 2 by selecting the binding site


Gga u

GGA+U

GGA+U (strong Coulomb repulsion on Mn 3d)

Calculating U by constraint GGA

  • Calculating U

    • Lock d-orbital into the atomic sphere

    • Do GGA for Mn d3 d2.5 and d3 d1.5

    • U=Δεd of the above two


Atomic scale engeered spins at a surface

N

Cu

Calculating Exchange Coupling

H=JS1·S2

|±|S=5/2, Sz=±5/2

DFT total energies

= EE

2S2J=++|H|++  +-|H| +-


Atomic scale engeered spins at a surface

Calculating Exchange Coupling

(in meV)


Summary of theoretical work

Summary of theoretical work

  • The nontrivial structure of the engineered spins requires DFT to determine.

  • Calculated structure shows a new kind of molecular magnets.

  • GGA+U produces correct S and very accurate J; very helpful for searching a system of desired S and J.


What s next

What’s Next

  • Can we understand IETS processes?

    • matrix elements, selection rules, transition strengths

  • What is the origin of the exchange coupling?

    • superexchange, delocalized electrons

  • Are other interactions possible?

    • vary distances, shapes, types of atoms

  • Can we control anisotropy effects?

  • Find a way to store and transfer spin information:bits and circuits based on atomic spins


Thanks to

Chris

Lutz

Andreas

Heinrich

Barbara

Jones

CyrusHirjibehedin

Thanks to


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