© 2002 IBM Corporation IBM Research © 2005 IBM Corporation© 2006 IBM Corporation Atomic-scale Engeered Spins at a Surface Chiung-Yuan Lin IBM Almaden Research

© 2002 IBM Corporation

IBM Research

© 2005 IBM Corporation© 2006 IBM Corporation

Atomic-scale Engeered Spins at a Surface

Chiung-Yuan LinIBM Almaden Research Center

IBM Research


Nanomagnetism and Information Technology

A. Imre et al.Science 311, 205 (2006)

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.Science 309, 2180 (2005)

IBM Research


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., Nature 439, 303 (2006)

Fe8, courtesy ESF. M.B. KnickelbeinPhys. Rev. B 70, 14424 (2004)

IBM Research


Assembly and Measurement of Nanomagnets

Top-down Bottom-up

Atomic-scale control Manipulate structures

IBM Research


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

Magnetic Field

Ene

rgy

|5/2,+5/2>

|ST,m>

|5/2,+3/2>

|5/2,+1/2>

|5/2,-1/2>

|5/2,-3/2>

|5/2,-5/2>|0,0>

Magnetic Field

Ene

rgy

|1,-1>

|1,0>

|1,+1>

|ST,m>

Science 312, 1021 (2006)

IBM Research


Keep it Simple: Free Mn Atom

3d

4s

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

Half filled d-shell

Weak spin-orbit interactions

IBM Research


Scanning Tunneling Spectroscopy: LDOS

Ef eV

sampletip

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

dI/dV

V0

IBM Research


Magnetic Atoms on Surfaces

Metal surface

Magnetic atom Atom’s spin is screened by

conduction electrons (Kondo effect)

A thin insulating layer may isolate the atomic spin

Thin insulating layer

IBM Research


dI/dV

eV0

σe

Inelastic Electron Tunneling Spectroscopy

-

σe+σie

kBT <

|eV| < Elastic Channel Open

Inelastic Channel Closed

Ef eV

sampletipX

|eV| > Elastic Channel Open

Inelastic Channel Open

Ef eV

sampletip

Thin insulator Magnetic atom

Non-magnetic tip

Non-magnetic sample

IBM Research


Methods of Electronic-structure Calculation

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

Interstitial regionAtomic partial wave

Plane wave

Atomic spheresAtomic partial wave

IBM Research


Cu Cu Cu Cu

vacu

um

vacu

um

vacu

um

FLAPW basis







IBM Research


rrVm iiieff .

22

rrrr

rrr

XCeff dVV

i

i

2rr

FLAPW basis







IBM Research


Thin Insulator: CuN Islands on Cu(100)

Cu

N

d0

a0=2

d 0

d0=2.55Åa0=3.60Å

Atomic resolution on CuN

Mn atoms bind to Cu and N sites

Cu(100)

CuN

1nm

Cu(100)

CuN monolayerMn Mn

Mn MnMn Mn

IBM Research


DFT Calculation of Electron Density in CuN

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

Cu+0.5 Cu+0.5N-1 N-1

Cu Cu

Cu+0.5

1.80Å

0.25Å

IBM Research


Pick up Atom

Move tip in Apply 2.0V Pull tip back

Manipulation of Mn on Cu(100) / CuN

IBM Research


Pick up Atom

Drop off

Move tip in Apply -0.5V Pull tip back

Manipulation of Mn on Cu(100) / CuN

IBM Research


Spectroscopy of Mn Dimers

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

-10 -5 0 5 100.0

0.5

1.0

1.5

2.0B=7T

B=4T

dI/d

V (

a.u.

)

Voltage (mV)

B=0T

Cu

N

Mn Mn

IBM Research


S=5/2 S=5/2 ST =

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

Three excitations around constant energy shift

Coupled Spins

|0,0>

B

E

|1,-1>

|1,0>

|1,+1>

|ST,m>

54…10

IBM Research


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

Chains of Mn Atoms

2

3

4

5

6

7

8

9

1nm

1nm

1Mn10Mn

CuN

Cu(100)Cu(100)

Cu

N

MnMn Mn

IBM Research


Spectroscopy of Mn Chains

Spectra change dramatically with each additional Mn atom.

2

3

4

5

6

7

8

9

1nm

10

IBM Research


Heisenberg Model of Spin Coupling

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)

ji

jiji SSJH,

,

SJ

1

1

1

N

i

ii SSJH

IBM Research


0111

0111

22222

0111

22222

3333333

0111

22222

3333333

444444444

0111

22222

3333333

444444444

55555555555

0111

22222

3333333

444444444

55555555555

6666666666666

0

5

10

15

20

35/223/21

Ene

rgy

[J]

Atomic Spin1/2

Heisenberg Dimer Spectrum

SG=0 and SE=1

Atomic spin affects numbers of levels but not spacing

First excited state at J

SJ

IBM Research


-25 -20 -15 -10 -5 0 5 10 15 20 251.0

1.5

2.0

2.5

2Mn

dI/d

V (

a.u

.)

Voltage (mV)

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

IBM Research


Determination of Atomic Spin

Using J = 6.2meV, we find S=5/2

STM determines both J and S!

-25 -20 -15 -10 -5 0 5 10 15 20 251.0

1.5

2.0

2.5

3.0

3.5

4.0

3Mn

2Mn

dI/d

V (

a.u

.)

Voltage (mV)

S=2

S=3

S=5/2

J=6.2meV

IBM Research


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

-20 -10 0 10 200

1

2

3

4

5

6

7

6Mn

1Mn

2Mn

3Mn

4Mn

5Mn

dI/d

V (

a.u

.)

Voltage (mV)

IBM Research


N

Cu

Mn

Unit Cells Used in Calculating Mn on CuN

Single Mn, smallest unit cellSingle Mn, larger unit cellMn dimer, smallest unit cell

Mn

7.20Å

10.80Å

7.20Å

IBM Research


Electron Density with an Adsorbed Mn Atom

• 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.

Cu+0.5

Cu

Cu+0.5 N -1.5 N -1.5

Mn+

Cu Cu

IBM Research


Mn Spin from DFT

majority ()minority ()

Free Mn atom

3d 5 S=5/2

IBM Research


A new kind of atomic-scale magnet

Surface N atoms isolate and bridge Mn atoms. This is a “surface” assembled magnet.

Cu

CuCu

N

CuCu

Mn MnNN

IBM Research


Control of Spin Coupling Strength

J=2.7meV

J=6.2meV

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

IBM Research


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 d3 d

2.5 and d3 d

1.5

• U =Δεd of the above two

IBM Research


H=J S1·S2

2S2J= ++|H|++ +- |H| +- = E E

DFT total energies

Cu

N

Calculating Exchange Coupling

|±|S=5/2, Sz=±5/2

IBM Research


Calculating Exchange Coupling

Mn on Cu site Mn on N site

GGA (U=0) 18.5 -1.8 (ferromagnetic!)

GGA + U(calculated) 6.50 ±0.05 2.5

GGA + U(calculated+1ev)

5.4 5.1

STM 6.2±0.3 2.7

(in meV)

IBM Research


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.

IBM Research


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

IBM Research


Thanks to

ChrisLutz

AndreasHeinrich

BarbaraJones

CyrusHirjibehedin

Documents

© 2002 IBM Corporation IBM Research © 2005 IBM Corporation© 2006 IBM Corporation Atomic-scale Engeered Spins at a Surface Chiung-Yuan Lin IBM Almaden Research