Absorptive X-ray Studies of Photomagnetic Nanostructures
Daniel M. Pajerowski
Department of Physics, University of Florida
January 2010
Introduction. During my PhD, I studied the magnetic properties of Prussian blue analogues (PBAs).
These compounds have been shown to display an array of novel material properties, and I have focused
on the cobalt hexacyanoferrate analogue (Co-Fe PBA) that shows persistent photoinduced
magnetism [1]. Specifically, my research seeks to further understand what happens when such
materials are incorporated into nanostructures, including photoinducible nanoparticles [2],
photomagnetic thin films [3], atomically mixed powders where chemical composition can tune the sign
of photoinduced magnetization [4], and thin film heterostructures that increase the temperature of
large photomagnetic effects from 18 K in the pure material to above LN2 temperatures [5].
Scientific background. While magnetization is often the quantity of interest for technological reasons,
extensibility of projects requires the deeper understanding provided by microscopic probes. The
understanding of photomagnetic thin film heterostructures of Prussian blue analogues can benefit from
a microscopic study. In pure Co-Fe PBA, light can cause a charge transfer induced spin transition (CTIST)
below 150 K causing a transition from diamagnetic CoIII(S=0)-NC-FeII(S=0) moieties to paramagnetic
CoII(S=3/2)-NC-FeIII(S=1/2) , coupled with a lattice constant change from ~10 Å to ~10.2 Å. Long-range
ferrimagnet order then sets in below ~18 K. By layering photoactive Co-Fe PBA between other Prussian
blue analogs with a higher TC, for example Ni-Cr PBA with TC ~ 75 K, the photoinduced magnetism can be
drastically modified, Figure 1. The ferromagnetic Ni-Cr PBA is not known to be photomagnetic,
however, its magnetic response has dramatic pressure dependence in low fields [6]. Therefore, we can
conclude that the photomagnetic response of the heterostructure is due to the structural change in the
Co-Fe PBA layer coupling to the Ni-Cr PBA, leading to the change in magnetization.
Rb0.8Ni4[Cr(CN)6]3·nH2O
A
1.0
Dark state
Light state
0.8
8.0
10
0.6
Light on
7.5
7.0
5
6.5
0.4
T = 60 K
0.2
0 1 2
Time (hours)
Single phase
0.0
0
0
20
Heterostructure
MAX
B
(iv)
15
-5
Rb0.7Co4[Fe(CN)6]3·nH2O
A
(iii)
2
Rb0.8Ni4[Cr(CN)6]3·nH2O
(ii)
 (10 emu/cm )
(i)
40
60
Temperature (K)
80
0
20
40
60
80
Temperature (K)
The ABA heterostructure. (i) A schema of the heterostructure using a shading gradient between layers to
illustrate regions at the interfaces where there can be mixing of the two phases. (ii) A TEM micrograph
shows a cross-section of a microtomed sample. The scale bar is 100 nm. (iii) The field-cooled magnetic
susceptibility χ(T) in 100 G oriented parallel to the surface of the film, where χ(T) is normalized to the area
of the film. The closed symbols represent the data prior to irradiation (i.e. dark state), and the open symbols
designate the data acquired after 5 hrs of irradiation with white light, but with the light subsequently off,
(i.e. PPIM state). The inset shows the time dependence of χ(T = 60 K) when the irradiation starts at time = 0.
(iv) The absolute value of the photoinduced changes of χ,  = χ(dark)  χ(light), normalized to the maximum
value to show the improvement in ordering temperature resulting from the heterostructured geometry.
1
Why XAS? While I have previously utilized electron magnetic resonance (EMR) and neutron
scattering (NS) to investigate the local environment of samples, these techniques are not suited to
extract the necessary information from the heterostructured photomagnets. Neutron scattering is
impossible even at the world-class SNS facility due to sample size and EMR lacks the requisite atomic
specificity. X-ray Absorption Spectroscopy (XAS) is ideal for this case because it gives information
pertaining to the local environments of specific elements, along with a high experimental flux capable of
detecting small samples.
Specifics of experiment. XAS has already been utilized to investigate the photoinduced magnetism of
the Co-Fe PBA by examining the fine structure of the L2,3 edges of Co and Fe and the K edge of Co before
and after photoirradiation, showing clear changes [7]. In addition, XAS studies of materials analogous to
the Ni-Cr PBA have been undertaken by tracking the K-edges of Ni and Cr constituents [8-9]. However,
no XAS studies have been performed on photomagnetic heterostructures. As Cr(CN)6 units are generally
more rigid, an experiment might consist of Extended X-ray Absorption Fine Structure (EXAFS) and X-ray
Absorption Near Edge Structure (XANES) studies near the K-edge of Ni, before and after photoirradiation
of the heterostructure. Modifications of the Ni K-EXAFS from photoirradiation would support the
hypothesis that the photoinduced Co-Fe PBA distortion is coupling to the Ni-Cr lattice and causing the
novel photoeffect in the heterostructures. These experiments would require a specialized sample
environment consisting of a cell capable of reaching temperatures below ~50 K with access for an
optical fiber to irradiate with visible light from a halogen source. Samples can most easily be obtained
by collaboration with Professor Talham’s chemistry lab and Professor Meisel’s physics lab at the
University of Florida, however in a pinch I have made these samples before using wet chemistry at
standard temperature and pressure using materials purchasable at Acros.
My experimental experience. Although I have no previous synchrotron experience, I believe I am in a
good position to undertake the proposed project. I have experience as a user in a few national labs: the
National Institute of Standards and Technology (NIST) at Gaithersburg, MD, Oakridge National Lab
(ORNL) at Oakridge, TN, and the National High Magnetic Field Lab (NHMFL) at Tallahassee, FL. During an
internship in the Neutron Scattering Science Division (NSSD) at ORNL during August 2009, I spent part of
my time working on probes for neutron scattering with photoexcitation. Locally at the University of
Florida, I’ve worked in labs using x-ray diffraction, transmission microscopy, uv-vis spectroscopy, SQUID
magnetometry, ac-susceptometry, dilution refrigerators, and more. Much of my work was with a SQUID
magnetometer, for which I made a custom optical probe [10].
References.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
2