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
1Dep. de Física Aplicada, Instituto de Física “Gleb Wataghin”, Universidade Estadual de Campinas, Campinas Brazil
2Laboratório Nacional de Luz Síncrotron, Campinas, Brazil
Lisandro J. Giovanetti3 and Félix G. Requejo3
3Instituto
de Física La Plata (IFLP) , Facultad de Ciencias Exactas ,Universidad Nacional de La Plata,
La Plata, Argentina
1
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.
2
Ferromagnetism induced by structural transitions
through ion bombardment patterning
Work done by P. Vargas Group Rupp et al, APL 93, 063102 (2008)
1.5 nm Fe film on Cu(100) capped with
2nm Au layer.
(fcc) Fe is transforms to bcc Fe
induced by Ar ion bombardment.
MFM
Ferromagnetic pattern
3
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.
4
The Stoner Criterion
I N(EF) > 1
DOS at EF
Stoner parameter
(exchange interaction)
Pd is close to satisfying the Stoner Criterion
I N(EF) ≈ 0.8 for bulk Pd (fcc)
There are theoretical predictions of
ferromagnetism in Pd hcp particles ,
and in films with great distortions of
the lattice constant [1-3].
[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)
5
Size effect and more …
Pd nanoparticles
-Twin boundaries breaks the
cubic symmetry
no splitting of the 4d levels local enhanced N(E)
Stoner criterion satisfied
Pd nanowires
- Hcp packed structure or
larger distorted lattice
parameter 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)
6
Experimental physicist doing theory ...
7
Theoretical predictions for magnetism in Pd
QUANTUM ESPRESSO Package, http://www.pwscf.org.
twin boundaries
Stacking faults
A. de Siervo et al.,PRB 76 075432 (2007).
Only
the
surface
atoms contribute.
5 x10 - 3 µB
B. Sampedro et al. Phys. Rev.
Lett , 91, 237203-1(2003).
Testing systems
Pd nanoparticles
-Twin boundaries breaks the
cubic symmetry
no splitting of the 4d levels local enhanced N(E)
Stoner criterion satisfied
Pd nanowires
- Hcp packed structure or
larger distorted lattice
parameter 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)
9
Pd and AuxPd(1-x) dodecanethiol-capped nanoparticles
0.8
0.6
0.4
1E-3
0.2
0.0
0.0 1.5 3.0 4.5 6.0 7.5
NP diameter / nm
1E-4
1E-4
Au3Pd1
1E-5
2
3 -1
I(q) / Arb. Units
0.01
1E-3
4
q / nm
V(D) / Normalized
1
V(D) / Normalized
I(q) / Arb. Units
1E-3
1.0
I(q) / Arb. Units
V(D) / Normalized
Nanoparticles prepared by chemical synthesis (Dr. Young S. Shon (California State University))
5
1E-5
1.0
0.8
0.6
0.0
0.0 1.5 3.0 4.5 6.0 7.5
1
1
2
3 -1
4
5
3 -1
q / nm
4
D= 2.2 nm.
Au1Pd1
D= 1,56 nm.
Au1Pd3
Au1Pd3
2
Au3Pd1
NP diameter / nm
1E-4
Pd0
Au1Pd1
0.4
0.2
1.0
0.8
0.6
0.4
0.2
0.0
0.0 1.5 3.0 4.5 6.0 7.5
NP diameter / nm
5
D1= 1nm.
D2= 3 nm.
TEM measurements
Daniel B. Roa (UFMG)
q / nm
SAXS measurements by Lisandro J. Giovanetti (UNLP).
10
SQUID Measurements
Pd metallic
XPS does not show
any contamination by
Fe, Co or Ni !!!
Au3Pd1
Au1Pd1
Au1Pd3
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.
12
Experimental results and possible source of errors
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!
- 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)
13
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>
14
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
15
XMCD-UHV (1.5T, 6-350K)
Installed at SXS beamline
Sample holder
Detector
TEY- Total electron yield
A
electrometer
ring
J.J. S. Figueiredo et al., J. Synchrotron Radiation V16, 346 (2009) 16
XMCD at Pd L edges: Ferromagnetic Polarization in Pd/Fe multilayers
(a)
Si(100)[Fe(40Å)/Pd(20Å)]20C(20Å)
C
Pd
Fe
Pd
Fe
20
Pd
(b)
morb
mspin
Pd
Fe
Fe
= 0.095
J. Vogel et al. Journal of Magnetism and Magnetic Materials 165 (1997)
96
17
XMCD at Pd L edges: PdCo nanoparticles Encapsulated on CNT
VSM
D. Bretas-Roa et al., (submitted APL 2010)
18
Thiolated Pd NP´s (2.6 nm)
No clear evidence
of ferromagnetism
with XMCD
Signal beyond the
Detection limite?
SQUID
XRF : Fe
contamination
at ppm level!
J.J. S. Figueiredo (to submit)
19
AuxPd(1-x) dodecanethiol-capped nanoparticles
Au3 Pd1
Au1 Pd1
20
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.
- XMCD confirmation already done for Au-thiolated capped nanoparticles but not for
Palladium!
J.S. Garitaonandia et al.
NanoLetters 8, 661 (2008)
21
Strained Pd ultra thin films
2
12% expanded
fcc
Density of states (1/atom eV spin)
1% expanded
hcp
1
(c)
0
-1
-2
2
1
(b)
0
spin Up
spin Down
-1
-2
-2
-1
(a)
0
Bulk fcc
1
2
-8 -7 -6 -5 -4 -3 -2 -1
0
1
2
Energy (eV)
A. de Siervo et al. , PRB 76, 075432 (2007)
22
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).
23
Pd on HOPG
Clean C(0001)
1-2 ML Pd (as grown)
C1s
(284.6 eV)
Intensity (arb. unit.)
hν= 400eV at SGM (LNLS)
SC
Pd 3d5/2
(336.5 eV)
360
350
340
330
320
310
300
290
280
Normalized Total Electron Yield
Several results show that hcp Pd nanoparticles can be grown on C(0001) HOPG
For example: J. Water et al, Advanced Materials 12, 31 (2000); J. Phys.: Condens. Matter 11 (1999) L317–L322
XAFS
~ 1ML of Pd on HOPG
(b)
(a)
(c)
(d)
Pd standard
SA
SB
SC
white line
Binding Energy (eV)
- XPS and XRF No contamination!
- SQUID No FM behavior !
3150 3200 3250 3300 3350 3400 3450
Photon Energy (eV)
- 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). 24
Pd growth on Ru(0001)
LEED
XPS
30
Ru 3d5/2
Evap. rate = 0.421 ± 0.01 Amgstrons/min
20
Ru(0001) 3nm Pd/Ru(0001)
Exp. Ru 3d5/2
linear fit
15
5
4
d (Amgstrons)
d (Amgstrons)
25
10
5
3
2
1
0
-1
0
0
1
2
3
4
5
6
7
8
9
10
Evaporation Time (min)
0
10
20
30
40
50
60
70
Evaporation Time (min)
Model:
 d
I Ru (d ) = I Ru (0) exp − 
 λ
25
PED - Photoelectron Diffraction
Structure ?
26
Surface Structure: Pd on Ru(0001)
Emiter: Pd 3d
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.
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).
27
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)
~ 6ML of Pd on W(100)
Theory
Intensity (arb. units)
Exp.
Pd 3d
as grown
after annealing
hν = 1810 eV
L.S=5.3eV
(Pd metallic)
Pd
metallic
∆E=0.4eV
350 348 346 344 342 340 338 336 334 332
Binding Energy (eV)
Lussani et al. (unpublished)
28
XPD from 6ML of Pd on W(100) : W 4f emitter
Lussani et al. (unpublished)
29
XPD from 6ML of Pd on W(100) : Pd 3d emitter
Lussani et al. (unpublished)
30
DFT Calculations
FCC
BCC
HCP
-57.7
3.16 Å
W lattice parameter
-57.8
-57.9
3.89Å
Pd (fcc)bulk
3.0
2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 4.25
Lattice Constant (Å)
DFT was performed with QUANTUMESPRESSO,
with L(S)DA included in the LDA exchangecorrection ultra-soft pseudopotential.
DOS (states/eV/atom)
Total Energy/Atom (Ry)
-57.6
Pd hcp
Spin Up
Spin Down
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
-3.0
-8
-7
-6
-5
-4
-3
-2
-1
0
1
2
3
4
Energy (eV)
QUANTUM ESPRESSO Package, http://www.pwscf.org.
Lussani et al. (unpublished)
31
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.
32
Acknowledgements
- Flavio Vicentin (SXS – LNLS)
- Paulo de Tarso (SGM – LNLS)
- Carlos Peres (XRF – LNLS)
FAPESP and CNPq of Brazil
33