NATIONAL HIGH MAGNETIC FIELD LABORATORY
2011 RESEARCH REPORT
Magnetic Circular Dichroism (MCD) of Copper Phthalocyanine crystalline thin films in
the Florida HELIX Magnet
M. Furis, Z. Pan, N. Rawat, C. Lamarche (UVM,Physics),T. Tokumoto, D. Semenov, S. McGill (NHMFL)
Research on metal-phthalocyanines (MPC) as archetype for organic semiconductors and optoelectronics
applications has been extensive over the last decade. However, the magnetic studies on MPC, especially in the
solid state phase are sparse. In a crystalline phase MPC, π electrons are highly delocalized through the quasi- 1
D molecular chain, and interactions between localized unpaired d-shell electron spin of central ions could be
mediated by the delocalized π electrons of the PC ring . Understanding the exchange mechanism will be
extremely critical for magnetic applications. In our study, we are particularly interested in copper
phthalocyanine(CuPC) crystalline thin film fabricated by solution processed pen-writing techniques , since in
this spin ½ system, direct exchange is negligible  and we could study pure indirect exchange between through
itinerant carriers. In order to identify the electronic states responsible for the magnetism in the CuPC crystalline
thin film, we performed magnetic circular dichroism (MCD) spectroscopy measurement in high magnetic fields.
MCD measurements was carried out in the split-coil HELIX magnet in cell 5 of NHMFL with B fields up to 27.5
Tesla at 100 and 300 K. Light from an Oriel 300 watt Xenon lamp dispersed by a Cornerstone 260
monochromator with bandwidth of 2 nm was modulated into left and right circularly polarization in 50 kHz and
focused using free space optics onto samples in Faraday geometry ( || ). Signal was collected by a multimode
fiber and focused onto a silicon diode detector.
Results and Discussion
Figure 1 displays 300K MCD spectra from the CuPc film
recorded at different magnetic fields. Each of the Gaussian
features is associated with a distinct transition between states
located at the bandgap of CuPc. All features are significantly
broadened and redshifted in comparison to the ones observed
in monomers. Since the MCD magnitude is proportional to the
time-average of the total change in orbital momentum
associated with a particular electronic transition and the
electronic g-factor, it is expected that MCD increases linearly
with applied magnetic field in the absence of any magnetic
300K interactions. This is precisely what we observe at room
temperature where MCD evolving with B field (inset) can be
very well fitted with a straight line. This dependence remains
0 5 10 15 20 25
linear at 100K with a slight increase in slope which corresponds
to an increase in the g-factor. The results are not surprising
Figure 1. MCD spectra recorded at various magnetic since carrier-mediated exchange is only expected to manifest
fields up to 27.5T from a CuPc crystalline thin film. itself at temperatures lower than 10K.
Lower Inset: Room temperature MCD at 640nm
increases linearly with applied magnetic field reflecting
the expected diamagnetic behavior. Upper Inset: Conclusions
Polarized microscope image of the CuPc thin film We demonstrated the first successful High field MCD
under study. The contrast is the result of different experiment at 27.5T. MCD evolved linearly as B field increase
orientations of the crystalline grains. at both 100 and 300 K with different slope (g-factor). Lower
sample temperatures are needed to reveal the magnetic exchange mechanism in this system.
The Furis group was supported through NSF CAREER award DMR #105658
 Heutz, S., et al., Adv. Mat., 19, 3618-3622 (2007).
 Headrick, R.L., et al., APL, 92, 063302 (2008).  Wu, W., et al., PRB, 77, 184403 (2008).