nanoscale magnetic imaging - people @ eecs at uc berkeley

Peter Fischer EE 290F Synchrotron Radiation for Materials Science Applications April 17, 2007 1

Nanoscale Magnetic Imaging

Peter Fischer

LBNL/CXRO, Berkeley CA USAemail:[email protected]

http://www.cxro.lbl.gov/~pjfischer/index.php


Outline

• history

• magnetism on the nanoscale

• ultrafast spin dynamics

• soft X-ray transmission microscopy

• summary


lodestone =magnetite Fe3O4

History of magnetism


• ~400BC in China• spoon made of magnetic lodestone• not used for navigation but for geomancy

First use of magnetism


Animal Magnetismlat. animus = soul

• mineral magnetism• cosmic magnetism• planetary magnetism.

1734-1850

Scientific examination of Mesmer's magnetic fluid by a French Royal Commission set up by Louis XVI in 1784: Majault, Franklin, Bailly, Le Roy, Sallin, d'Arcet, de Borey, Guillotin, Lavoisier, Poissonnier, Caille, Mauduyt de la Varenne, Andry, and de Jussieu.

“no evidence of the existence of his magnetic fluid, … its effects derived from either the imaginations of its subjects or through charlatanry”


transport

transformers

generators

electric motors

medicine

data storage

every day´s magnetism


The future of magnetic data storage technology

- http://www.hitachigst.com/hdd/hddpdf/tech/hdd_technology2003.pdf- D. A. Thompson and J. S. Best, IBM J. Res. Develop. vol. 44 No. 3 May 2000

problems:-grain size (thermal self erasure)-superparamagnetic limit (kBT wins over KuV)-head-disk spacing to atomic dimensions-timing of read/write critical

problems:-grain size (thermal self erasure)-superparamagnetic limit (kBT wins over KuV)-head-disk spacing to atomic dimensions-timing of read/write critical

patterned media

holographic datastorage

heat / thermalassistedrecording

SOMA

S. Sun et al, Science (2000)

courtesy D. Weller (Seagate)


Spin-Electronics

S. Parkin et al (2005)

race track memory

current induced switching

GMR/TMR

courtesy G. Meier (U Hamburg)

spin transistor

domain wall logic

D. Allwood et al., Science (2005)

Magnetic Tunnel Junction for MRAM(Magnetic Random Access Memory)

GMR read head


Novel magnetic materials

Diluted Magnetic Semiconductors

• combining properties of both ferromagnet and semiconductor for spintronics

• alloys of nonmagnetic semiconductor and magneticelements (e.g. Co-doped ZnO)

• possible room-temperature ferromagnetism

multiferroics

→ controlling magnetism through electric fields(e.g. BiFeO3)

courtesy R. Ramesh (UCB)


Micromagnetic structures

10:1

sample: FeTb(Dy) layer (Terfenol-D)

1µm

d=70nmlayer thickness: d=1000nm

JHJHmFmAg sexan

rrrrrr⋅−⋅−+∇=

212 )()(

exchange anisotropy ext. field stray fieldGibbs´sfree energy density

MicromagnetismM

MMrr

rrr

0

/

µJ

m

=

=


Micromagnetism

rotation :field external ⇒⋅− JHex

rr

domains 21:strayfield ⇒⋅− JH

s

rr

axiseasy )( :anisotropy ⇒mFan

r

parallell spins )( :exchange 2 ⇒∇mAr

m

g

MH eff r

r

δ

δ1−=

→ equilibrium configuration of magnetization = energy minimum


Length scales of micromagnetism

202

,S

SK J

Al

KAl

µ==

Magnetic exchange lengths = range of interactions

1.32.7Nd2Fe14B

5.73.2Co

23.33.7Fe

161.25.7Permalloy Ni80Fe20

lK (nm)lS (nm)material

magn. hardness

stray field

anisotropy

A exchange constantK anisotropy constantJS saturation polarisation


Fast magnetisation dynamics

Landau-Lifshitz-Gilbert

dampingα damping constant

anisotropymorphologygeometry

[ ]

×+×−=

dt

MdM

MHM

dt

Mdeff

rrrr

rα

γ

for 1T: 90° in 10pstypically α<<1, ~100ps

precessionγ gyromagnetic ratio

courtesy: J. Stoehr


How fast can we switch the magnetization?

Oersted switching

today: ~1ns

current induced switching

T. Gerrits et al, Nature 418, 509 (2002)

precessional switching

tomorrow: „much faster“


Challenges to modern magnetic microscopies

magneticmicroscopy

fundamental magneticlength scales

exchange length

nm10sub~K

A~lK −

fundamental magnetic time scales

exchange interactions

)eV(E4~)fs(t

multicomponent magnetic materials

element specificity/sensitivity


– Optical microscopies– Kerrmicroscopy

– Electron microscopies– Scanning Electron Microscopy with Polarization Analysis (SEMPA)

– Lorentz (TEM) microscopy

– Spin polarized SPLEEM

– X-PEEM (X-ray-in/ Photoelectron-out)

– Spin polarized SP-STM

– Scanning force microscopies– Magnetic Force Microscopy (MFM)

Magnetic microscopy techniques

500 nm

V V

H. Ding, et al., PRL 94 (2005)

R. Schäfer, et al.IEEE TransMag 39 (2003)

courtesy J. Unguris, NIST

A. Wachowiak et al, Science 298 (2002)

courtesy J. Zweck, U Regensburg

LaFeO3

Co

F. Nolting et al, Nature (2000)


Imaging with X-rays

„dass man mit Linsen die X-Strahlen

nicht concentrieren kann“

(„it is impossible to focus the X-rays with lenses“)W. C. Röntgen, Sitzungsberichte der physikal.-medizin. Gesellschaft, 132 (1895)


E = 250eV - 1.8 keVλ = 0.7 nm - 5 nm

Imaging with soft X-rays

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba Lu Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Lr Rf Db Sg Bh Hs Mt Ds Rg Uub Uut Uuq Uuh

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb

Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No

K edge

L2,3 edge

M4,5 edge


Fresnel Zone Plate Lenses

• spatial resolution ~ ∆r

• focal length ~N(∆r)2/λ

• spectral bandwidth ∆λ/λ ~ 1/N

courtesy of E. Anderson (LBNL)


Soft X-ray Microscope XM-1 (BL 6.1.2 @ ALS)

http://www.cxro.lbl.gov/BL612/

X-ray source condensor sample objective 2dim-detector


Magnetic imaging with XM-1 at the ALS

E = 250eV - 1.8 keVλ= 0.7 nm - 5 nm∆E/E=500

Hmax= 5kOe (perp.)= 2kOe (long.)

CCD 2048x2048 px2

Mag ~ 2000FOV ~ 10-15µm

∆t<70ps

3rd generationsynchrotron source

element specificity

lateral resolution

time resolution

polarization


XMCD as magnetic (absorption) contrast

1µm

• element specific

• large contrast

• M·σPhoton• imaging local magnetic moment

(spin-orbit information)

1.45852.7Ni1.59778.1Co1.75706.8Fe

λ (nm)E (eV)L3 edge

from http://xdb.lbl.gov

0.35µm PI

59 nm Gd Fe25 75

10nm Al


P. Gambardella et al., NATURE 416, 301 (2002)

XMCD of Co structures: 3D ⇒ 0D


2.5mm above orb. pl.2.5mm below orb. pl.

Modulation of circ. polarization

-1,0

-0,5

0,0

0,5

1,0

-4 -3 -2 -1 0 1 2 3 4

-0,10

-0,05

0,00

0,05

0,10

Exp

erim

en

tal m

ag

ne

tic c

on

tra

st

Vertical distance from orbital plane (mm)

Experimental magnetic contrast

2mm slit width

5mm slit width

avg.

de

gre

e o

f p

ola

rizatio

n

Calculated PC

2mm slit width

5mm slit width

� reducing non-magnetic

background

� increase magnetic contrast

� fast switching scheme

B.-S. Kang et al., J Appl. Phys 98 (2005) 093907


Magnetic coupling in Gd-Fe systems

0.35µm PI

59 nm Gd Fe25 75

10nm Al

L+S

1µm

Fe L3(706eV)

1µm

⇒ antiparallel coupling

Gd M5(1190eV)

1µm


(Pt/[Pt 0.75nm/Co 0.35nm]*50/Pt 3nm/Tb45Fe55 25nm)/Pt 5nm)

Fe L3 edge (706eV) Co L3 edge (777eV)

S.Mangin, A. Berger, E. Fullerton (HGST), D.-H. Kim, P. Fischer (2006) unpublished

Element specificity = layer resolving(with high sensitivity)


Reducing the dimensions

1µm

• sample:multilayered system75x(0.4nm Fe/ 0.4nm Gd)• perpendicular anisotropy• stripe width: 300nm

• recorded@ Fe L3 edge• field perp. to surface

sample: M. Scholz (U Regensburg)

γ

300 nm


Magnetic imaging at 15nm spatial resolution

0 20 40 60 80 100 1200,96

0,98

1,00

1,02

intensity (a.u.)

distance (nm)

50nm thick

nanogranular

(Co84Cr16)87Pt13 film

15nm

EPh = 777eV (Co L3)

D.H. Kim et al, J. Appl. Phys. 99, 08H303 (2006) and Virt. J. Nanoscale Sci.&Technol., 13(17) May 1, 2006

0 10 20 30 40 50 600,1

1

10

numberdensity(%

)

grain size (nm)


Reversal behaviour on the nanoscale

D.H. Kim et al, J. Appl. Phys. 99, 08H303 (2006) and Virt. J. Nanoscale Sci.&Technol., 13(17) May 1, 2006

• microsopic M(H,T)• RPM/CPM• stochasticity• switching field distribution

• microsopic M(H,T)• RPM/CPM• stochasticity• switching field distribution


Magnetic nanospheres

� self-assembled 2d arrays of

monodisperse spherical

polysterene particles

� deposition of a magnetic film

(Co and Pd)

� strong PMA to the (curved)

surface

deposition110nm particles with Co/Pt ML(3nm total Co thickness)

→ switching field distributionI L Guhr, S van Dijken, G Malinowski, P Fischer, F Springer, O Hellwig, M Albrecht, J. Phys D Appl Phys (2007) in print


I L Guhr, S van Dijken, G Malinowski, P Fischer, F Springer, O Hellwig, M Albrecht, J. Phys D Appl Phys (2007) in print


1 mµ

SiN(70nm)/Tb25(Fe75Co25)75(50nm)/SiN(20nm)/Al(30nm)/SiN(20nm)

Thermomagnetically written bits in MO media

N

S

v=1m/s

laser

magn. field

T

Tc

P. Fischer,et al., J. Magn. Soc. of Japan 25(3-2), 186 (2001)


Domain structures in patterned PY elements

PY: Ni80Fe20 (50 nm) @ Ni L3 edge

1µm

1.0

µm

1.3

µm

1.5

µm

2.0

µm

2.5

µm

3.0

µm

P. Fischer, J. Phys. D Appl. Phys. 35(19) (2002) 2391-97

M

M.k

Photon

1x1µm2


Comparison with simulations

+Hext

1µm x 3.2µm

x 80nm1µm x 2µm

x 50nm

P. Fischer, et al., IEEE Trans Mag 38(5) (2002) 2427-2431


Spin current induced DW motion

1 mµ contact pad

current pulse

•20 nm thin, 1000nm wide

permalloy (Ni80Fe20) ring

segment

•1ns short pulses

current density <1012 A/m2

EPh = 854 eV

(Ni L3)DW

G. Meier, M. Bolte, R. Eiselt, B. Krüger, D.-H.Kim, P. Fischer, Phys. Rev. Lett (2007) accepted

Hext


Composite particle vs. transformation

→ vDW= 110m/s in agreement with micromagnetic simulations

→ strong indications for a stochastic motion of the DW (Barkhausenjumps?), explanation for low vDW seen with long pulses?

1 mµ contact pad

current pulse

DVW V-AV

G. Meier, M. Bolte, R. Eiselt, B. Krüger, D.-H.Kim, P. Fischer, Phys. Rev. Lett. (2007) accepted


Stroboscopic pump-probe setup for time resolved soft X-ray microscopy

storage ring

pulser

t+ t∆

0< t<328ns∆

t

<70ps

rise time 100ps≈

f=3MHz

sample

waveguide IH

⇒ perfect repeatability of the dynamics


Spin dynamics upon magnetic field excitation

D.H. Kim (CNU), B.L. Mesler, K.Y. Lee (KU), P. Fischer et al, (2006) unpublished

1µm

∆t = 0 – 5.3ns (steps of 50ps)

• simulation with 40node cluster

• experiment with 25nm lateral and 70ps time resolution

• 50 nm thin 2x4 µm2

permalloy (Ni80Fe20) rectangle

[ ]

×

α+×γ−=

dt

MdM

MHM

dt

Mdeff

rrrr

r


• PY elements 2x4µm2, 60nm thick• j~107-8A/cm2, ns pulses

magnetic element

contact pads

current pulse

Spin dynamics upon spin current excitation

R. Eiselt, M. Bolte, G. Meier (U Hamburg), D.-H.Kim, P. Fischer (2006) unpublished


Imaging spin torque driven dynamics

R. Eiselt, M. Bolte, G. Meier (U Hamburg), D.-H.Kim, P. Fischer (2006) unpublished

[ ] T dt

MdM

MHM

dt

Mdeff

rr

rrrr

+

×

α+×γ−=

1µm


Magnetic phase contrast with Fourier Optics

• XOR pattern = zone plate + transmissiongrating

• direct measurement of real and imaginaryparts of (f´ and f´´)

• differential phase contrast objective

C. Chang, et al., Appl. Optics 41, 7384 (2002) 1mm


Replacing MZP with Fourier Optics

3rd generationsynchrotron source


Magnetic phase contrast with Fourier Optics

• XOR pattern = zone plate + transmissiongrating

• direct measurement of real and imaginaryparts of (f´ and f´´)

• differential phase contrast objective

C. Chang, et al., Appl. Optics 41, 7384 (2002) 1mm

J. Kortright, S.-K. Kim, PRB 62 (2000) 12216

711eV705eV

C. Chang, A. Sakdinawat, P. Fischer, Optics Letters 31(10) (2006) 1564-1566

sample: 59nm Gd25Fe75


J. Kortright, S.-K. Kim, PRB 62 (2000) 12216

C. Chang, et al., Optics Letters 31 (10) 1564 (2006)

Magnetic phase contrast

705eV

711eV

1µm

sample: 59nm Gd25Fe75


Summary

Magnetic soft X-ray microscopy is imaging nanomagnetism

towards fundamental length and time scales

� XMCD provides elemental sensitivity (layer specificity)

� X-ray optics provide lateral resolution (towards 10nm)

� magnetic amplitude and phase contrast

� X-ray source provide time resolution in the 70ps regime (ps

to fs feasible), required: repeatability


TOMORROW?fs imaging with soft X-ray microscopy

- spin fluctuations on the nanoscale

- watching transfer of spin to orbital

moment

- process of exchange interaction


Monographs

J. Stöhr and H. C. Siegmann, Magnetism: From Fundamentals to Nanoscale Dynamics, Springer (2006)

A. Hubert and R. Schäfer, Magnetic Domains, The Analysis of Magnetic Microstructures, Springer (2001)

H. Kronmüller and M. Fähnle Micromagnetism and the Microstructure of Ferromagnetic Solids, Cambridge (2006)

P. Fischer, Magnetic Soft X-ray Microscopy, Springer (2007/2008)


References

• N. Spaldin and M. Fiebig, The Renaissance of Magnetoelectric Multiferroics, SCIENCE 309 (2005) 391

• D. A. Thompson and J. S. Best, The future of magnetic data storage technology, IBM J. Res. Develop. 44 No. 3 (2000) 311

• S. A. Wolf, et al., Spintronics: A Spin-Based Electronics Vision for the Future, SCIENCE 294 (2001) 1488

• http://ischuller.ucsd.edu/


• D.-H. Kim (now Chungbuk U), B.L. Mesler, M.-Y. Im, A.E.

Sakdinawat, W. Chao, R. Oort, B. Gunion, S.B. Rekawa, P.

Denham, E.H. Anderson, D.T. Attwood (CXRO Berkeley USA)

• R. Eiselt, M. Bolte, G. Meier, B. Krüger, D. Pfannkuche, U.

Merkt (U Hamburg, Germany)

• S.-C. Shin (KAIST, South Korea)

• S. Mangin (U Nancy), A. Berger, E. Fullerton (HGST USA)

• C. Chang (U Drexel)

• ALS and CXRO staff

• DOE for funding

Acknowledgement

nanoscale magnetic imaging - people @ eecs at uc berkeley

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