296

MRS BULLETIN

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VOLUME 38

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APRIL 2013

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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

0.1

CuO

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

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