
296
MRS BULLETIN
•
VOLUME 38
•
APRIL 2013
•
www.mrs.org/bulletin
NEWS & ANALYSIS RESEARCH/RESEARCHERS
rapidly as the electrons relaxed back into
their original state.
The researchers applied this tech-
nique to the La
1.9
Sr
0.1
CuO
4
fi lms syn-
thesized at BNL. In fi lms with a critical
temperature of 26 K, the researchers
discovered two new short-lived excita-
tions—both caused by fl uctuating CDWs.
The new pulse-pump technique allowed
the researchers to record the lifetime of
CDW fl uctuations, which is just 2 ps
under the coldest conditions, becoming
briefer as the temperatures rose. These
waves then vanished entirely at about
100 K, actually surviving at much higher
temperatures than superconductivity.
The researchers then hunted for those
same signatures in cuprate fi lms with
slightly different chemical compositions
and a greater density of mobile electrons.
“Interestingly, the superconducting sam-
ple with the highest critical temperature,
about 39 K, showed no CDW signatures
at all,” Gedik said.
The consistent emergence of CDWs
would have bolstered the conjecture
that they play an essential role in HTS.
Instead, the research team’s detection of
such electron waves in one sample but
not in another (with even higher critical
temperature) demonstrates that another
mechanism may be driving the emer-
gence of HTS in cuprate superconductors.
Superdiffusive electron transport
mediates laser-induced
demagnetization
T
he mechanics of laser-induced de-
magnetization has attracted consid-
erable attention in efforts to develop fast-
switching optomagnetic logic devices. A
femtosecond laser pulse for instance can
demagnetize a ferromagnet within a few
hundred femtoseconds. Various theories
have been proposed to explain how an
ultrafast laser pulse can demagnetize a
magnetic thin fi lm, ranging from indi-
rect spin-fl ip scattering to direct laser-
induced spin fl ips. Now, A. Eschenlohr,
M. Battiato, and colleagues at Helmholtz
Zentrum Berlin and Uppsala University
propose that a novel mechanism of su-
perdiffusive electron transport drives the
demagnetization.
In the January 27 online edition
of Nature Materials (DOI: 10.1038/
NMAT3546), the researchers describe
a unique pump-probe study to explore
magnetization dynamics in Ni films.
They fi rst deposited Au/Ni/Pt/Al and
Pt/Ni/Pt/Al multilayers using magnetron
sputtering. The former fi lm was grown
with a Au capping layer thick enough to
absorb almost all the laser light, while
the latter features a very thin Pt capping
layer. The top layer of the samples was
irradiated with a 50 fs laser pulse and
the Ni magnetization was measured in a
pump-probe fashion using x-ray magnet-
ic circular dichroism (XMCD) to probe
the Ni response element-selectively.
The results were modeled using
superdiffusive transport theory, which
allowed the researchers to extract the
spatial and temporal dependence of mag-
netization in each sample. The results
show that in the Au / Ni sample, the spin-
majority electrons are mostly conducted
from the Ni to the substrate and the Au
cap, while spin-minority electrons re-
main largely trapped in the Ni layer. This
leads to a very effi cient and ultrafast de-
magnetization of the Ni layer, which is
almost as fast as the response of the Pt/
Ni sample measured in parallel.
From these results, the researchers
conclude that direct optical excitation
does not need to occur for the demag-
netization process to take place. This
also precludes spin fl ips as the dominant
mechanism for demagnetization, at least
for the Au/Ni structure.
The researchers conclude that the
model of laser-induced spin transport
best describes their demagnetization pro-
cess. This new understanding may help
scientists design a new generation of ul-
trafast optomagnetic memory materials.
Steven Spurgeon
Nano Focus
Schematic of the temporal dependence (vertical axis) and spatial dependence (horizontal
axis) change of magnetization (color) in the (a) Au/Ni and (b) Pt/Ni lms. Spin-minority carriers
(blue) remain trapped in the Ni layer, while the spin-majority carriers (red) are conducted to the
substrate. The two samples behave similarly, demonstrating that direct optical excitation is not
a requirement for the demagnetization process to occur. Reproduced with permission from
Nature Mater. (DOI: 10.1038/NMAT3546). © 2013 Macmillan Publishers Ltd.
a
b