1

Exploring magnetic behavior in 4d transition

metals: Could Pd being ferromagnetic?

Fernando C. Lussani1, J.J. S. Figueiredo1, F. Garcia2, R. Landers1,2 and Abner de Siervo1,2

Lisandro J. Giovanetti3 and Félix G. Requejo3

1Dep. de Física Aplicada, Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, Campinas Brazil

2Laboratório Nacional de Luz Síncrotron, Campinas, Brazil

3Instituto de Física La Plata (IFLP) , Facultad de Ciencias Exactas ,Universidad Nacional de La Plata, La Plata, Argentina

2

Motivation: Magnetism in non-magnetic materials

Is it possible to obtain ferromagnetic behavior from non-magnetic bulk materials?

Yes!

• Induced ferromagnetism in Ag, Ru, Mo, Pd ultrathin films sandwiched by Fe films. M.A. Tomaz, et al. J. Vac. Sci. Tech. A 16, 1359 (1998) �

• 1ML Pd on Fe(100): Rader et al. PRB 45, 13823 (1992); PRL 72, 2247 (1994) �

In all the cases the ferromagnetism in the substrate drives the non-magnetic film into a ferromagnetic ground state

Structural changes also can play a lot to the magnetic properties of the materials.

3

Ferromagnetism induced by structural transitions

through ion bombardment patterning

1.5 nm Fe film on Cu(100) capped with 2nm Au layer.

(fcc) Fe is transforms to bcc Feinduced by Ar ion bombardment.

Work done by P. Vargas Group ���� Rupp et al, APL 93, 063102 (2008)

MFM Ferromagnetic pattern

4

Motivation: Magnetism in non-magnetic materials

Is it possible to obtain ferromagnetic behavior from non-magnetic bulk materials?

Yes!

• Induced ferromagnetism in Ag, Ru, Mo, Pd ultrathin films sandwiched by Fe films. M.A. Tomaz, et al. J. Vac. Sci. Tech. A 16, 1359 (1998) �

• 1ML Pd on Fe(100): Rader et al. PRB 45, 13823 (1992); PRL 72, 2247 (1994) �

In all the cases the ferromagnetism in the substrate drives the non-magnetic film into a ferromagnetic ground state

But, is there a possibility to induce ferromagnetism purely by lattice distortions, size effect, or by electronic structure modifications in non-magnetic materials?

-Several theoretical results claim for ferromagnetic stability in some 4d and 5d metal when they are specially tailored as nanoparticles or ultrathin films.

5

The Stoner Criterion

There are theoretical predictions of

ferromagnetism in Pd hcp particles ,

and in films with great distortions of

the lattice constant [1-3].

I N(EI N(EFF) > 1) > 1DOS at EF

Pd is close to satisfying the Stoner Criterion

I N(EI N(EFF) ) ≈≈≈≈≈≈≈≈ 0.8 for bulk Pd (0.8 for bulk Pd (fccfcc)) �� ���� ��

Stoner parameter(exchange interaction)�

[1] S. Bouarab et al., Phys. Lett. A 151, 103 (1990).[2] F. Aguilera-Granja et al., Phys. Rev. B 73 , 115422 (2006).[3] T. Shinohara et al., Phys. Rev. Lett. 91, 197201 (2003) .[4] Y. Sun, et al. PRB 81, 064413 (2010)

6

Size effect and more …

-Twin boundaries breaks the cubic symmetryno splitting of the 4d levels �local enhanced N(E)�� Stoner criterion satisfied

Pd nanoparticles Pd nanowires

- Hcp packed structure or larger distorted latticeparameter � enhanced N(E) � Stoner criterion satisfied A. Delin et al. PRL 92,

057201 (2004).

S.S. Alexandre, et al.

PRL 079701 (2006)�.

Capped nanoparticles

- Charge transfer from Au[5d] to S[3p] � increases d holes density in Au surface atoms. - Mechanism: localized magnetic moments on the S-Au bond. P. Crespo et al. PRL 93, 087204 (2004)�

7

Experimental physicist doing theory ...

Theoretical predictions for magnetism in Pd

A. de Siervo et al.,PRB 76 075432 (2007).

B. Sampedro et al. Phys. Rev. Lett , 91, 237203-1(2003).

5 x10 - 3 µB

Only the surface atoms contribute.

twin boundaries

Stacking faults

QUANTUM ESPRESSO Package, http://www.pwscf.org.

9

Testing systems

-Twin boundaries breaks the cubic symmetryno splitting of the 4d levels �local enhanced N(E)�� Stoner criterion satisfied

Pd nanoparticles Pd nanowires

- Hcp packed structure or larger distorted latticeparameter � enhanced N(E) � Stoner criterion satisfied A. Delin et al. PRL 92,

057201 (2004).

S.S. Alexandre, et al.

PRL 079701 (2006)�.

Capped nanoparticles

- Charge transfer from Au[5d] to S[3p] � increases d holes density in Au surface atoms. - Mechanism: localized magnetic moments on the S-Au bond. P. Crespo et al. PRL 93, 087204 (2004)�

10

Pd and AuxPd(1-x) dodecanethiol-capped nanoparticles

1 2 3 4 5

1E-5

1E-4

1E-3

0.0 1.5 3.0 4.5 6.0 7.50.0

0.2

0.4

0.6

0.8

1.0

I(q)

/ A

rb. U

nits

q / nm-1

Au3Pd

1

V(D

) / N

orm

aliz

ed

NP diameter / nm

1 2 3 4 5

1E-4

1E-3

0.01

0.0 1.5 3.0 4.5 6.0 7.50.0

0.2

0.4

0.6

0.8

1.0

Au1Pd

3

I(q)

/ A

rb. U

nits

q / nm-1

V(D

) / N

orm

aliz

ed

NP diameter / nm

1 2 3 4 51E-5

1E-4

1E-3

0.0 1.5 3.0 4.5 6.0 7.50.00.20.40.60.81.0

Au1Pd

1

I(q)

/ A

rb. U

nits

q / nm-1

V(D

) / N

orm

aliz

ed

NP diameter / nm

SAXS measurements by Lisandro J. Giovanetti (UNLP).

D= 2.2 nm.Au3Pd1

Au1Pd1

Au1Pd3

D= 1,56 nm.

D1= 1nm.

D2= 3 nm.

Pd0

TEM measurements Daniel B. Roa (UFMG)

Nanoparticles prepared by chemical synthesis (Dr. Young S. Shon (California State University))

SQUID MeasurementsPd metallic

Au3Pd1 Au1Pd1 Au1Pd3

XPS does not show any contamination by Fe, Co or Ni !!!

12

Nice . . .

. . . Let's right another paper!

But we have hundreds of papers saying that almost everything “nano” is magnetic!

First, we should answer some questions.

13

Experimental results and possible source of errors

- Most of the experimental results in the literature are based on SQUID measurements.

Angew. Chem. Int. Ed. 47, 2055 (2008), PRL 91, 197201(2003), and MANY OTHERS!!!

- Total magnetization are small and could be due contamination or experimental artifacts:for example: ferromagnetic contaminants from solvents, manipulation tools, exposition to air, etc …

“Sources of experimental errors in the observation of nanoscale magnetism”M. A. Garcia et al. JAP 105, 013925 (2009)

Element specific magnetic measurements are need for one unquestionable proof of the origin of the ferromagnetic signal.

- XMCD (X-ray magnetic circular dichroism) J. Stöhr, JMMM 200 470(1999)

The size effect: Everything could be ferromagnetic?“Ferromagnetism as a universal feature of nanoparticles of the otherwise nonmagnetic oxides”A. Sundaresan et al. PRB 74 161306(R) (2006) ���� and more than 100 papers in the same subject!

14

X-Ray Magnetic Circular Dichroism - XMCD

- Element specific magnetic spectroscopy - Degree of polarization : Pc- Expected value for <m> of the 3d shell- Angle between k and <m>

15

Sum rules for XMCD analysis

In principle relevant for a single channel transition,for example, 2p → 3d. (3d metals)2p � 4d (4d metals)

2p → 4s

16

ringelectrometer

A

TEY- Total electron yield

Detector

Sample holder

XMCD-UHV (1.5T, 6-350K)

Installed at SXS beamline

J.J. S. Figueiredo et al., J. Synchrotron Radiation V16, 346 (2009)

17

(a)

(b)

spin

orb

m

m= 0.095

Si(100)[Fe(40Å)/Pd(20Å)]20C(20Å)

20

Fe

Fe

Fe

FePd

Pd

Pd

Pd

C

J. Vogel et al. Journal of Magnetism and Magnetic Materials 165 (1997) 96

XMCD at Pd L edges: Ferromagnetic Polarization in Pd/Fe multilayers

18

XMCD at Pd L edges: PdCo nanoparticles Encapsulated on CNT

VSM

D. Bretas-Roa et al., (submitted APL 2010)

19

Thiolated Pd NP´s (2.6 nm)

XRF : Fe contaminationat ppm level!

No clear evidenceof ferromagnetismwith XMCD

Signal beyond the Detection limite?

J.J. S. Figueiredo (to submit)

SQUID

20

AuxPd(1-x) dodecanethiol-capped nanoparticles

Au3 Pd1 Au1 Pd1

21

Conclusions for Pd and PdAu Nanoparticles

- Several theoretical studies predict Ferromagnetic behavior for Pd and other 4d transition metals, specially low dimensional systems such as nanoparticles.

- Most of the experimental results are based only in conventional macroscopic magnetization measurements (ex. SQUID). Those findings are controversial.

- Special care should be taken to avoid different source of errors (contaminations and experimental artifacts).

- Element specific measurements such as XMCD are necessary for a unquestionable proof for the origin of the ferromagnetic signal.

J.S. Garitaonandia et al.NanoLetters 8, 661 (2008)

- XMCD confirmation already done for Au-thiolated capped nanoparticles but not for Palladium!

22

Strained Pd ultra thin films

Bulk fcc

12% expandedfcc

1% expanded hcp

A. de Siervo et al. , PRB 76, 075432 (2007)

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2

2

1

0

-1

-2

-2

-1

0

1

2

-2

-1

0

1

2

(a)

Energy (eV)

(b)

Den

sity

of s

tate

s (1

/ato

m e

V s

pin)

spin Up spin Down

(c)

23

Pd ultrathin films on C(0001), Ru(0001) and W(100)

Controlled conditions for Pd growth in UHV environment. High purity materials No magnetic materials (Fe, Ni, Co) present at all !

Substrates: Ru(0001) and C(0001) both with hexagonal structure. W(100) and Nb(100)

C(0001) � lattice parameter smaller then Pd(fcc) by -10.5%island (particle) formation ?

Ru(0001) � lattice parameter smaller then Pd(fcc) by -1.8% layer by layer growth ?

W(100) and Nb(100) � Literature suggest dhcp growth of Pd ultrathin films, which could induce ferromagnetic contributions from surface atoms.

Analysis: XPS and DFT (electronic structure); LEED, XPD, RHEED and EXAFS (surface atomic structure); MOKE, SQUID and XMCD (magnetism).

24

Pd on HOPG

Several results show that hcp Pd nanoparticles can be grown on C(0001) ���� HOPGFor example: J. Water et al, Advanced Materials 12, 31 (2000); J. Phys.: Condens. Matter 11 (1999) L317–L322

360 350 340 330 320 310 300 290 280

hν= 400eV at SGM (LNLS)

SCPd 3d

5/2

(336.5 eV)

C1s(284.6 eV)

Clean C(0001) 1-2 ML Pd (as grown)

Inte

nsity

(ar

b. u

nit.)

Binding Energy (eV)

~ 1ML of Pd on HOPG

3150 3200 3250 3300 3350 3400 3450

white line

(d)(c)(b)(a)

SC

SB

SA

Pd standard

Nor

mal

ized

Tot

al E

lect

ron

Yie

ld

Photon Energy (eV)

XAFS

- XPS and XRF ���� No contamination!

- SQUID ���� No FM behavior !

- No hcp packing ? No Twin Boundaries ?

Biasi, de Siervo, Garcia, Landers, Knobel, J. of Elec. Spec. and Rel. Phen. , v.157, p.332 – 335 (2007).

25

Pd growth on Ru(0001) �� ��

LEED

Ru(0001)� 3nm Pd/Ru(0001)�

0 10 20 30 40 50 60 70

0

5

10

15

20

25

30

Exp. Ru 3d5/2

linear fit

0 1 2 3 4 5 6 7 8 9 10-1

0

1

2

3

4

5

d (

Am

gst

ron

s)

Evaporation Time (min)

Ru 3d5/2

Evap. rate = 0.421 ± 0.01 Amgstrons/min

d (A

mgs

tron

s)

Evaporation Time (min)

−=

λ

dIdI RuRu exp)0()(

XPS

Model:

26

PED - Photoelectron Diffraction

Structure � ?

27

Surface Structure: Pd on Ru(0001)

a) experiment

b) Zig-zag (dhcp) model

c) 2 domains fcc(111) model

d) Hcp model

- Lateral lattice parameter identical to

Pd(111).

- Large interlayer expansions ~6% for the

5 ML film.

Emiter: Pd 3d

Ferromagnetism ?

- No ferromagnetism detected by MOKE.

-Could be interesting to measure XMCD at

low temperatures (few Kelvin). A. de Siervo et al.,Phy. Rev. B 76 075432 (2007).

28

Pd on W(100)

W(100) and Nb(100) are suggested as possible surface substrates which would induce Pd to grow as a distorted hcp structure. (K. Osuch et al., Phys. Rev. B, 71 (2005) 165213)

Clean W(100)

Exp.

Theory

350 348 346 344 342 340 338 336 334 332

Inte

nsity

(ar

b. u

nits

)

Binding Energy (eV)

as grown after annealing

hν = 1810 eV

Pd 3d

Pd metallic

∆E=0.4eV

L.S=5.3eV(Pd metallic)

~ 6ML of Pd on W(100)

Lussani et al. (unpublished)

29

XPD from 6ML of Pd on W(100) : W 4f emitter

Lussani et al. (unpublished)

30

XPD from 6ML of Pd on W(100) : Pd 3d emitter

Lussani et al. (unpublished)

31

2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25

-57.9

-57.8

-57.7

-57.6

Tot

al E

nerg

y/A

tom

(R

y)

Lattice Constant (Å)

FCC BCC HCP

3.16 ÅW lattice parameter

3.89Å

Pd (fcc)bulk

-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Energy (eV)

DO

S (

stat

es/e

V/a

tom

) Spin Up Spin Down

Pd hcp

DFT was performed with QUANTUMESPRESSO, with L(S)DA included in the LDA exchange-correction ultra-soft pseudopotential.

QUANTUM ESPRESSO Package, http://www.pwscf.org.

DFT Calculations

Lussani et al. (unpublished)

32

Remarks

- Spontaneous ferromagnetism in Pd is an open and controversial question.

- DFT has successfully described the electronic structure of solids for the ground state. Unfortunately it might be not as good to describe magnetic properties due the presence of significant correlation effects (see for example: J. Stöhr ���� “Magnetism from fundaments to nanoscale dynamics” )

- Nanomagnetism needs element specific magnetic measurements, such as XMCD, to unambiguously address the origin and the physical mechanism of unconventional magnetic behavior.

33

Acknowledgements

- Flavio Vicentin (SXS – LNLS)- Paulo de Tarso (SGM – LNLS)- Carlos Peres (XRF – LNLS)

FAPESP and CNPq of Brazil