We report visualization of spin polarized states in macroscopic ensembles of biologically-produced
ferromagnetic palladium nanoparticles using the Faraday effect-based technique of magneto-optical
imaging. The ferromagnetic palladium only exists in the form of nanoparticles. Large quantities of
palladium nanoparticles may be synthesized via biomineralization from a Pd2+ solution. The ferromagnetic
Pd nanoparticles are formed in the periplasmic space of bacteria during the hydrogen-assisted reduction of
Pd2+ ions by hydrogenases. The ferromagnetism in Pd comes from itinerant electrons. A high Curie
temperature of ferromagnetic palladium, about 200 degrees centigrade above room temperature, would
allow for a range of room-temperature magnetic applications. The processes of the isolation of electron
spins in separate nanoparticles, spin hopping, spin transport and spin correlations may even form a basis
of quantum computing. So far, measurements of the magnetic properties of Pd nanoparticles (PdNP) have
been limited by integral techniques such as SQUID magnetometry, magnetic circular dihroism and muon
spin rotation spectroscopy ( SR). In the present study, ferromagnetic Pd nanoparticles are characterized
using the technique of magneto-optical imaging. This allows visualization of the spin polarization by the
variations in the intensity of polarized light. To perform measurements at relatively low magnetic fields, a
spin injection from a colossal magnetoresistive material has been used.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3533

Attard, G.A.

Jenkins, P.

Johansen, T.H.

Macaskie, L.E.

Mikheenko, I.

Mikheenko, P.

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 1 No 4, 04MFPN03(4pp) (2012)   2304-1862/2012/1(4)04MFPN03(4) 04MFPN03-1  2012 Sumy State University Visualization of Spin Polarized States in Biologically-Produced Ensembles of Ferromagnetic Palladium Nanoparticles  P. Mikheenko*,1,2, I. Mikheenko†,3, P. Jenkins4, G.A. Attard4, L.E. Macaskie3 T.H. Johansen1,5,6  1 Department of Physics, University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway 2 School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 3 School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 4 Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, UK 5 Centre for Advanced Study at the Norwegian Academy of Science and Letters, 0271 Oslo, Norway 6 Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia  (Received 06 June 2012; published online 19 August 2012)  We report visualization of spin polarized states in macroscopic ensembles of biologically-produced ferromagnetic palladium nanoparticles using the Faraday effect-based technique of magneto-optical imaging. The ferromagnetic palladium only exists in the form of nanoparticles. Large quantities of palladium nanoparticles may be synthesized via biomineralization from a Pd2+ solution. The ferromagnetic Pd nanoparticles are formed in the periplasmic space of bacteria during the hydrogen-assisted reduction of Pd2+ ions by hydrogenases. The ferromagnetism in Pd comes from itinerant electrons. A high Curie temperature of ferromagnetic palladium, about 200 degrees centigrade above room temperature, would allow for a range of room-temperature magnetic applications. The processes of the isolation of electron spins in separate nanoparticles, spin hopping, spin transport and spin correlations may even form a basis of quantum computing. So far, measurements of the magnetic properties of Pd nanoparticles (PdNP) have been limited by integral techniques such as SQUID magnetometry, magnetic circular dihroism and muon spin rotation spectroscopy ( SR). In the present study, ferromagnetic Pd nanoparticles are characterized using the technique of magneto-optical imaging. This allows visualization of the spin polarization by the variations in the intensity of polarized light. To perform measurements at relatively low magnetic fields, a spin injection from a colossal magnetoresistive material has been used.  Keywords: Nanoparticles, Palladium, Ferromagnetic, Bacteria, Magneto-Optical Imaging, Spin Injection.   PACS numbers: 75.50.Tt, 75.76.+j, 78.20.Ls                                                             * pavlo.mikheenko@fys.uio.no † i.mikheenko@bham.ac.uk 1. INTRODUCTION  Future quantum computing will strongly depend on spin transfer between clusters of spin-polarized electrons. Before a quantum computer is even designed, there is an opportunity to investigate spin-polarized transport between isolated sites using arrays of spin-polarized nanoparticles. Such an array could be based on palladium nanoparticles. Palladium is a metal on the verge of ferromagnetism. It can be driven into the ferromagnetic state when produced in the form of nanoparticles [1]. It is believed that several factors contribute to ferromagnetism of Pd nanoparticles (PdNP), including strain [2] and the influence of surface states [3]. Pd nanoparticles lose ferromagnetism when grown larger than ~7 nm, when the influence of surface states becomes small and the crystal lattice parameters relax to their equilibrium bulk values. X-ray magnetic circular dichroism revealed that ferromagnetism in Pd nanoparticles is mainly of spin origin with a small orbital component [4]. Since it comes from itinerant electrons, one could expect spin polarization of nanoparticles and thereby a range of spin-transport phenomena. One such phenomenon, magneto-resistance of a two-dimensional Pd nanoparticle superlattice, has already been observed revealing conduction-electron polarization in Pd of about 4 % [5].  In addition to chemical methods, ferromagnetic Pd nanoparticles may be produced biologically [6]. It was found that they are formed enzymatically in the periplasmic space of bacteria suggestive of spin-polarization assisted reduction of the metal ion. This process involves hydrogenases [7-9] that may allow spin transport during formation of the nanoparticles. The spin-polarized properties of biological Pd nanoparticles could be different from those produced by other methods. In addition to this, the biological route offers a possibility to produce a large amount of nanoparticles in a reasonably short period of time.  Another important advantage of a biological route is that each nanoparticle is surrounded by organic matter that protects it from coalescing with other nanoparticles. An excess of organic matter, however, complicates spin exchange between the nanoparticles. Several methods are already developed to partially remove the biomass material. To improve the contact between nanoparticles, one can press them into a pellet, or disperse them in a solvent and deposit them onto a suitable substrate. To monitor the PdNP spin polarization, a visualization technique would be ideal. In this work we report the application of such a technique together with an exploration of spin-injection from a colossal magnetoresistive material.    P. MIKHEENKO, I. MIKHEENKO, P. JENKINS, ET AL. PROC. NAP 1, 04MFPN03 (2012)   04MFPN03-2 2. EXPERIMENTAL  2.1 Samples preparation  Several batches of Pd nanoparticles grown in bacterial cells of Desulfovibrio desulfuricans ATCC 29577 and Escherichia coli MC4100 on the different stages of removing organic material around nanoparticles have been investigated. Pre-grown bacteria were transferred under N2 atmosphere into a Pd2+ solution. The suspension was allowed to stand for about one hour at 30 C for biosorption in order to form the Pd nucleation sites in the biomass. The electron donor was then introduced by letting H2 bubble through the suspension. The preparations of biologically reduced Pd (Bio-Pd) were centrifuged and washed with distilled water and acetone, and finally dried in the air.  The removal of organic matter and particle cleaning included phenol-chloroform Bio-Pd treatment which allowed separation of cell wall bound PdNPs and extraction of protein-bound PdNPs (I. Mikheenko, unpublished). Alternatively an organic layer was removed by suspending the Bio-Pd powder in 6 M NaOH [10] increasing the amount of Pd in the sample up to 70 %. The prepared powdered samples were pressed into pellets; liquid samples were deposited onto the substrates in the form of a suspension, and then dried in the air.  2.2 Experimental technique   Magneto-optical imaging (MOI) is used to visualize spin polarization in ferromagnetic Pd nanoparticles. The method is based on the ability of certain materials to change the polarization of light in the presence of a local magnetic field known as the Faraday effect. Specially prepared in-plane magnetized bismuth-substituted ferrite garnet films [11] were used as sensors. The indicator film was placed directly on a flat surface of the pellet or the substrate with a deposited layer of PdNPs. The whole assembly was positioned on the cold finger of a continuous He-flow cryostat [12]. An optical window on the cryostat allowed for sample observation using a polarized light microscope. A DC magnetic field was applied to magnetize the colossal magneto-resistive material (La0.67Ca0.33 MnO3) together with the PdNPs. The brightness of the images obtained with crossed polarizers gives the magnitude of spin polarization in the samples. Magnetic properties of the samples were measured using a SQUID-based Quantum Design MPMS XL system.  3. RESULTS AND DISCUSSIONS   An electron microscopy image of a typical Bio-Pd preparation is shown in Fig. 1. The Pd nanoparticles are seen as black dots and these are localized in the periplasm (insert). The magnetic moment as a function of magnetic field of a bacterial powder containing Pd nanoparticles at a temperature of 200 K is shown in Fig. 2. To fully magnetize an ensemble of separated Pd nanoparticles, a magnetic field of a few Tesla is necessary. Twelve different samples were examined by MOI at different temperatures down to 3.7 K using magnetic fields up to 85 mT. This field is close to the saturation field of the used garnet indicator films. None of the samples showed variation in the intensity that would be linked to spin-polarization of Pd. Thus the applied magnetic field was not sufficient to create a detectable change in the magnetic moment in the Pd nanoparticles.    Fig. 1 – An electron microscopy image of bacteria loaded with Pd nanoparticles (black dots). Insert shows localization of Pd NPs in the periplasm, bar size is 50 nm    Fig. 2 – Magnetic moment as function of magnetic field of a batch of non-treated Bio-Pd nanoparticles produced by bacteria at 200 K  To overcome this obstacle we used the effect of spin injection. A well-known class of easily magnetized 100 % spin-polarized colossal magneto-resistive materials (CMR) was already proved to be effective for injecting spins into conductive materials [13, 14]. In the case of the PdNPs-containing pellets, we used a CMR thin film in tight contact with the pellet. The magnetic field available in the MOI set-up was used to magnetize the CMR film. The spin-polarized electrons were driven from CMR to Pd by the eddy currents of a magnetic field parallel to the surface of the film. At low temperatures, this parallel field was created by a superconducting film placed below the CMR film. The result of the experiment at different temperatures for a segment of a pellet composed of PdNPs associated with bacterial cell walls is shown in Fig. 3. The organic matter was only partially removed in this preparation. The average distance between Pd nanoparticles in the sample is relatively large and their spin correlations are weak. That is why the effect is seen at low temperatures only and vanishes at about 60 K.  VISUALIZATION OF SPIN POLARIZED STATES IN BIOLOGICALLY-PRODUCED… PROC. NAP 1, 04MFPN03 (2012)   04MFPN03-3   a    b   c  Fig. 3 – Color-coded representations of MOI images of a section of pellet containing Pd nanoparticles at three different temperatures: 20 K (a), 45 K (b) and 60 K (c). The images were obtained using spin injection from the underlying CMR film  The images produced by the MOI technique are most sensitive to the layer of material positioned directly below the indicator film. We were not able to see the effect from the superconducting or CMR film that are situated several millimeters below the indicator film neither at increasing nor decreasing external magnetic field. It is worth noting that at higher temperatures the outline of the segment becomes sharper. The pictures in Fig. 3 are produced by intensity-light color coding of monochrome MOI images. The blue to red colors correspond to an increase in the intensity of polarized light. One could expect that well-rectified samples with small distances between Pd nanoparticles would be more susceptible to spin injection from CMR than the sample in Fig. 3. To check this, we attempted MOI imaging in the best rectified sample at room temperature.    a    b  Fig. 4 – a) Optical image of NaOH-cleaned PdMPs deposited to a CMR substrate. The area covered by Pd is a nearly circular disk with white NaOH crystals on the rim. b) MOI image of the sample shown in a)  The Bio-Pd powder in this preparation was cleaned in 6M NaOH and deposited directly on the CMR film as shown in Fig. 4a. The result of MOI imaging is shown in Fig. 4b. In this experiment the indicator film was shifted to partially reveal a nearly circular area covered by Pd NPs. Due to the crystallization of NaOH in the process of drying a range of its crystals appeared on the periphery of the covered area. The Pd content is lowest there. It created the corresponding dark rim in the MOI image in Fig. 4b. The successful magnetic imaging of the array of Pd  P. MIKHEENKO, I. MIKHEENKO, P. JENKINS, ET AL. PROC. NAP 1, 04MFPN03 (2012)   04MFPN03-4 nanoparticles, which are in electrical contact with the CMR, indicates strong spin polarization in biological nano-Pd and demonstrates the possibility to move spins from one material to another enhancing its magnetic moment. The resolution achieved in Fig. 4a is already sufficient to see the features on the sub-millimeter scale. The experiments shown in Fig. 3 and 4 can be treated as the visualization of the spin-injection from CMR to ferromagnetic Pd. When injected, the electrons in Pd retain direction of their spins in a locked state defined by the spin interactions between the separate nanoparticles. It is not difficult to extend this experiment to the visualization of the spin transport between Pd nanoparticles. For this, one can make a gap in the CMR film revealing the underlying substrate, and deposit a Pd nanoparticle film across it. By injecting spin-polarized electrons in the Pd layer on one side of the gap and removing them from the other it is possible to achieve spin transport across the gap along the connected Pd nanoparticles that are not in contact with CMR. The MOI technique can provide resolution down to about one micrometer [11]. The better resolution, which may be needed to monitor processes related to spin-based quantum computing, could be provided by spin-polarized electron microscopy or polarized x-ray microscopy.  4. CONCLUSIONS  Magneto-optical imaging was used to visualize spin polarization in biologically produced ensembles of ferromagnetic Pd nanoparticles. The high level of polarization, sufficient for the detection by the indicator films, was achieved at relatively low applied magnetic fields by the spin injection from a colossal magneto-resistive material. The visual monitoring of spin polarization would help to understand spin transport and may be useful in designing quantum computers.  AKNOWLEDGEMENTS  We acknowledge, with thanks, the support of EPSRC (Grant No EP/I007601/1 and EP/1007806/1), Center for Advanced Study (CAS) at the Norwegian Academy of Science and Letters and the Research Council of Norway.  REFERENCES  1. T. Taniyama, E. Ohta, T. Sato, Europhysics Lett. 38 195 (1997). 2. Y. Oba, T. Sato, T. Shinohara, Phys. Rev. B 78, 224417 (2008). 3. T. Shinohara, T. Sato, T. Taniyama, Phys. Rev. Lett. 91, 197201 (2003). 4. Y. Oba, H. Okamoto, T. Sato, T. Shinohara, J. Suzuki, T. Nakamura, T. Muro, H. Osawa, J. Phys. D: Appl. Phys. 41 134024 (2008). 5. T. Okamoto, H. Maki, Y. Oba, S. Yabuuchi, T. Sato, E. Ohta, J. Appl. Phys. 106, 023908 (2009). I.P. Mikheenko, P.M. Mikheenko, C.N.W. Darlington, C.M. Muirhead, L.E. Macaskie, in Biohydrometallurgy: fundamentals, technology and sustainable development (Ed. V.S.T Ciminelli, O.Jr. Garcia) 525–532 (Amsterdam: Elsevier: 2001). 6. L.E. Macaskie, V.S. Baxter-Plant, N.J. Creamer, A.C. Humphries, I.P. Mikheenko, P.M. Mikheenko, D.W. Penfold, P. Yong, Biochem. Soc. Trans. 33, 76 (2005). 7. I.P. Mikheenko, M. Rousset, S. Dementin, L.E. Macaskie, Appl. Eenviron. Microbiol. 74(19), 6144 (2008) 8. K. Deplanche, I. Caldelari, I.P. Mikheenko, F. Sargent, L.E. Macaskie, Microbiology 156, 2630 (2010). 9. K. Deplanche, M.L. Merroun, M. Casadesus, D.T. Tran, I.P. Mikheenko, J.A. Bennett, J. Zhu, I.P. Jones, G.A. Attard, J. Wood, S. Selenska-Pobell, L.E. Macaskie, J. R. Soc. Interface published online 7 March 2012. 10. L.E. Helseth, R.W. Hansen, E.I. Il'yashenko, M. Baziljevich, T.H. Johansen, Phys. Rev. B 64, 174406 (2001). 11. P.E. Goa, H. Hauglin, A.A.F. Olsen, M. Baziljevich, T.H. Johansen, Rev. Sci. Instrum. 74, 141 (2003). 12. P. Mikheenko, M.S. Colclough, C. Severac, R. Chakalov, F. Welhoffer, C.M. Muirhead, Appl. Phys. Lett. 78, 356 (2001). 13. P. Mikheenko, J.S. Abell, J. Phys.: Conf. Series 43, 297 (2006). 

Visualization of Spin Polarized States in Biologically-Produced Ensembles of Ferromagnetic Palladium Nanoparticles

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 1 No 4, 04MFPN03(4pp) (2012) 
 
 
2304-1862/2012/1(4)04MFPN03(4) 04MFPN03-1  2012 Sumy State University 
Visualization of Spin Polarized States in Biologically-Produced Ensembles of Ferromagnetic 
Palladium Nanoparticles 
 
P. Mikheenko*,1,2, I. Mikheenko†,3, P. Jenkins4, G.A. Attard4, L.E. Macaskie3 T.H. Johansen1,5,6 
 
1 Department of Physics, University of Oslo, P.O. Box 1048, Blindern, 0316 Oslo, Norway 
2 School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 
3 School of Biosciences, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK 
4 Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Park Place, Cardiff CF10 3AT, UK 
5 Centre for Advanced Study at the Norwegian Academy of Science and Letters, 0271 Oslo, Norway 
6 Institute for Superconducting & Electronic Materials, University of Wollongong, NSW 2522, Australia 
 
(Received 06 June 2012; published online 19 August 2012) 
 
We report visualization of spin polarized states in macroscopic ensembles of biologically-produced 
ferromagnetic palladium nanoparticles using the Faraday effect-based technique of magneto-optical 
imaging. The ferromagnetic palladium only exists in the form of nanoparticles. Large quantities of 
palladium nanoparticles may be synthesized via biomineralization from a Pd2+ solution. The ferromagnetic 
Pd nanoparticles are formed in the periplasmic space of bacteria during the hydrogen-assisted reduction of 
Pd2+ ions by hydrogenases. The ferromagnetism in Pd comes from itinerant electrons. A high Curie 
temperature of ferromagnetic palladium, about 200 degrees centigrade above room temperature, would 
allow for a range of room-temperature magnetic applications. The processes of the isolation of electron 
spins in separate nanoparticles, spin hopping, spin transport and spin correlations may even form a basis 
of quantum computing. So far, measurements of the magnetic properties of Pd nanoparticles (PdNP) have 
been limited by integral techniques such as SQUID magnetometry, magnetic circular dihroism and muon 
spin rotation spectroscopy ( SR). In the present study, ferromagnetic Pd nanoparticles are characterized 
using the technique of magneto-optical imaging. This allows visualization of the spin polarization by the 
variations in the intensity of polarized light. To perform measurements at relatively low magnetic fields, a 
spin injection from a colossal magnetoresistive material has been used. 
 
Keywords: Nanoparticles, Palladium, Ferromagnetic, Bacteria, Magneto-Optical Imaging, Spin Injection. 
 
 PACS numbers: 75.50.Tt, 75.76.+j, 78.20.Ls 
 
 
                                                          
* pavlo.mikheenko@fys.uio.no 
† i.mikheenko@bham.ac.uk 
1. INTRODUCTION 
 
Future quantum computing will strongly depend on 
spin transfer between clusters of spin-polarized 
electrons. Before a quantum computer is even designed, 
there is an opportunity to investigate spin-polarized 
transport between isolated sites using arrays of spin-
polarized nanoparticles. Such an array could be based on 
palladium nanoparticles. Palladium is a metal on the 
verge of ferromagnetism. It can be driven into the 
ferromagnetic state when produced in the form of 
nanoparticles [1]. It is believed that several factors 
contribute to ferromagnetism of Pd nanoparticles 
(PdNP), including strain [2] and the influence of surface 
states [3]. Pd nanoparticles lose ferromagnetism when 
grown larger than ~7 nm, when the influence of surface 
states becomes small and the crystal lattice parameters 
relax to their equilibrium bulk values. 
X-ray magnetic circular dichroism revealed that 
ferromagnetism in Pd nanoparticles is mainly of spin 
origin with a small orbital component [4]. Since it 
comes from itinerant electrons, one could expect spin 
polarization of nanoparticles and thereby a range of 
spin-transport phenomena. One such phenomenon, 
magneto-resistance of a two-dimensional Pd 
nanoparticle superlattice, has already been observed 
revealing conduction-electron polarization in Pd of 
about 4 % [5].  
In addition to chemical methods, ferromagnetic Pd 
nanoparticles may be produced biologically [6]. It was 
found that they are formed enzymatically in the 
periplasmic space of bacteria suggestive of spin-
polarization assisted reduction of the metal ion. This 
process involves hydrogenases [7-9] that may allow 
spin transport during formation of the nanoparticles. 
The spin-polarized properties of biological Pd 
nanoparticles could be different from those produced by 
other methods. In addition to this, the biological route 
offers a possibility to produce a large amount of 
nanoparticles in a reasonably short period of time.  
Another important advantage of a biological route is 
that each nanoparticle is surrounded by organic matter 
that protects it from coalescing with other 
nanoparticles. An excess of organic matter, however, 
complicates spin exchange between the nanoparticles. 
Several methods are already developed to partially 
remove the biomass material. To improve the contact 
between nanoparticles, one can press them into a 
pellet, or disperse them in a solvent and deposit them 
onto a suitable substrate. To monitor the PdNP spin 
polarization, a visualization technique would be ideal. 
In this work we report the application of such a 
technique together with an exploration of spin-injection 
from a colossal magnetoresistive material. 
 
 
 P. MIKHEENKO, I. MIKHEENKO, P. JENKINS, ET AL. PROC. NAP 1, 04MFPN03 (2012) 
 
 
04MFPN03-2 
2. EXPERIMENTAL 
 
2.1 Samples preparation 
 
Several batches of Pd nanoparticles grown in 
bacterial cells of Desulfovibrio desulfuricans ATCC 
29577 and Escherichia coli MC4100 on the different 
stages of removing organic material around 
nanoparticles have been investigated. Pre-grown 
bacteria were transferred under N2 atmosphere into a 
Pd2+ solution. The suspension was allowed to stand for 
about one hour at 30 C for biosorption in order to form 
the Pd nucleation sites in the biomass. The electron 
donor was then introduced by letting H2 bubble 
through the suspension. The preparations of 
biologically reduced Pd (Bio-Pd) were centrifuged and 
washed with distilled water and acetone, and finally 
dried in the air.  
The removal of organic matter and particle cleaning 
included phenol-chloroform Bio-Pd treatment which 
allowed separation of cell wall bound PdNPs and 
extraction of protein-bound PdNPs (I. Mikheenko, 
unpublished). Alternatively an organic layer was 
removed by suspending the Bio-Pd powder in 6 M 
NaOH [10] increasing the amount of Pd in the sample 
up to 70 %. The prepared powdered samples were 
pressed into pellets; liquid samples were deposited onto 
the substrates in the form of a suspension, and then 
dried in the air. 
 
2.2 Experimental technique  
 
Magneto-optical imaging (MOI) is used to visualize 
spin polarization in ferromagnetic Pd nanoparticles. 
The method is based on the ability of certain materials 
to change the polarization of light in the presence of a 
local magnetic field known as the Faraday effect. 
Specially prepared in-plane magnetized bismuth-
substituted ferrite garnet films [11] were used as 
sensors. The indicator film was placed directly on a flat 
surface of the pellet or the substrate with a deposited 
layer of PdNPs. The whole assembly was positioned on 
the cold finger of a continuous He-flow cryostat [12]. An 
optical window on the cryostat allowed for sample 
observation using a polarized light microscope. A DC 
magnetic field was applied to magnetize the colossal 
magneto-resistive material (La0.67Ca0.33 MnO3) together 
with the PdNPs. The brightness of the images obtained 
with crossed polarizers gives the magnitude of spin 
polarization in the samples. 
Magnetic properties of the samples were measured 
using a SQUID-based Quantum Design MPMS XL 
system. 
 
3. RESULTS AND DISCUSSIONS  
 
An electron microscopy image of a typical Bio-Pd 
preparation is shown in Fig. 1. The Pd nanoparticles are 
seen as black dots and these are localized in the 
periplasm (insert). The magnetic moment as a function 
of magnetic field of a bacterial powder containing Pd 
nanoparticles at a temperature of 200 K is shown in 
Fig. 2. To fully magnetize an ensemble of separated Pd 
nanoparticles, a magnetic field of a few Tesla is 
necessary. 
Twelve different samples were examined by MOI at 
different temperatures down to 3.7 K using magnetic 
fields up to 85 mT. This field is close to the saturation 
field of the used garnet indicator films. None of the 
samples showed variation in the intensity that would be 
linked to spin-polarization of Pd. Thus the applied 
magnetic field was not sufficient to create a detectable 
change in the magnetic moment in the Pd nanoparticles. 
 
 
 
Fig. 1 – An electron microscopy image of bacteria loaded with 
Pd nanoparticles (black dots). Insert shows localization of Pd 
NPs in the periplasm, bar size is 50 nm 
 
 
 
Fig. 2 – Magnetic moment as function of magnetic field of a 
batch of non-treated Bio-Pd nanoparticles produced by 
bacteria at 200 K 
 
To overcome this obstacle we used the effect of spin 
injection. A well-known class of easily magnetized 100 % 
spin-polarized colossal magneto-resistive materials 
(CMR) was already proved to be effective for injecting 
spins into conductive materials [13, 14]. In the case of the 
PdNPs-containing pellets, we used a CMR thin film in 
tight contact with the pellet. The magnetic field available 
in the MOI set-up was used to magnetize the CMR film. 
The spin-polarized electrons were driven from CMR to Pd 
by the eddy currents of a magnetic field parallel to the 
surface of the film. At low temperatures, this parallel 
field was created by a superconducting film placed below 
the CMR film. 
The result of the experiment at different 
temperatures for a segment of a pellet composed of 
PdNPs associated with bacterial cell walls is shown in 
Fig. 3. The organic matter was only partially removed in 
this preparation. The average distance between Pd 
nanoparticles in the sample is relatively large and their 
spin correlations are weak. That is why the effect is seen 
at low temperatures only and vanishes at about 60 K. 
 VISUALIZATION OF SPIN POLARIZED STATES IN BIOLOGICALLY-PRODUCED… PROC. NAP 1, 04MFPN03 (2012) 
 
 
04MFPN03-3 
 
 
a 
 
 
 
b 
 
 
c 
 
Fig. 3 – Color-coded representations of MOI images of a 
section of pellet containing Pd nanoparticles at three different 
temperatures: 20 K (a), 45 K (b) and 60 K (c). The images were 
obtained using spin injection from the underlying CMR film 
 
The images produced by the MOI technique are most 
sensitive to the layer of material positioned directly 
below the indicator film. We were not able to see the 
effect from the superconducting or CMR film that are 
situated several millimeters below the indicator film 
neither at increasing nor decreasing external magnetic 
field. It is worth noting that at higher temperatures the 
outline of the segment becomes sharper. The pictures in 
Fig. 3 are produced by intensity-light color coding of 
monochrome MOI images. The blue to red colors 
correspond to an increase in the intensity of polarized 
light. 
One could expect that well-rectified samples with 
small distances between Pd nanoparticles would be 
more susceptible to spin injection from CMR than the 
sample in Fig. 3. To check this, we attempted MOI 
imaging in the best rectified sample at room 
temperature. 
 
 
 
a 
 
 
 
b 
 
Fig. 4 – a) Optical image of NaOH-cleaned PdMPs deposited 
to a CMR substrate. The area covered by Pd is a nearly 
circular disk with white NaOH crystals on the rim. b) MOI 
image of the sample shown in a) 
 
The Bio-Pd powder in this preparation was cleaned 
in 6M NaOH and deposited directly on the CMR film as 
shown in Fig. 4a. The result of MOI imaging is shown 
in Fig. 4b. In this experiment the indicator film was 
shifted to partially reveal a nearly circular area covered 
by Pd NPs. Due to the crystallization of NaOH in the 
process of drying a range of its crystals appeared on the 
periphery of the covered area. The Pd content is lowest 
there. It created the corresponding dark rim in the MOI 
image in Fig. 4b. 
The successful magnetic imaging of the array of Pd 
 P. MIKHEENKO, I. MIKHEENKO, P. JENKINS, ET AL. PROC. NAP 1, 04MFPN03 (2012) 
 
 
04MFPN03-4 
nanoparticles, which are in electrical contact with the 
CMR, indicates strong spin polarization in biological 
nano-Pd and demonstrates the possibility to move spins 
from one material to another enhancing its magnetic 
moment. 
The resolution achieved in Fig. 4a is already 
sufficient to see the features on the sub-millimeter 
scale. The experiments shown in Fig. 3 and 4 can be 
treated as the visualization of the spin-injection from 
CMR to ferromagnetic Pd. When injected, the electrons 
in Pd retain direction of their spins in a locked state 
defined by the spin interactions between the separate 
nanoparticles. 
It is not difficult to extend this experiment to the 
visualization of the spin transport between Pd 
nanoparticles. For this, one can make a gap in the CMR 
film revealing the underlying substrate, and deposit a 
Pd nanoparticle film across it. By injecting spin-
polarized electrons in the Pd layer on one side of the 
gap and removing them from the other it is possible to 
achieve spin transport across the gap along the 
connected Pd nanoparticles that are not in contact with 
CMR. 
The MOI technique can provide resolution down to 
about one micrometer [11]. The better resolution, which 
may be needed to monitor processes related to spin-
based quantum computing, could be provided by spin-
polarized electron microscopy or polarized x-ray 
microscopy. 
 
4. CONCLUSIONS 
 
Magneto-optical imaging was used to visualize spin 
polarization in biologically produced ensembles of 
ferromagnetic Pd nanoparticles. The high level of 
polarization, sufficient for the detection by the 
indicator films, was achieved at relatively low applied 
magnetic fields by the spin injection from a colossal 
magneto-resistive material. The visual monitoring of 
spin polarization would help to understand spin 
transport and may be useful in designing quantum 
computers. 
 
AKNOWLEDGEMENTS 
 
We acknowledge, with thanks, the support of 
EPSRC (Grant No EP/I007601/1 and EP/1007806/1), 
Center for Advanced Study (CAS) at the Norwegian 
Academy of Science and Letters and the Research 
Council of Norway. 
 
REFERENCES 
 
1. T. Taniyama, E. Ohta, T. Sato, Europhysics Lett. 38 195 
(1997). 
2. Y. Oba, T. Sato, T. Shinohara, Phys. Rev. B 78, 224417 
(2008). 
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Visualization of Spin Polarized States in Biologically-Produced Ensembles of Ferromagnetic Palladium Nanoparticles

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