with photo-excitation threshold behavior
absent in the picosecond dynamics. This new
paradigm, quantum femtosecond magnetism, means photo-induced femtosecond
magnetic phase transitions driven by laser-excited inter-atomic coherences and quantum
spin flip fluctuations. 1 The laser electric field
and quantum dynamics make electrons oscillate between neighboring atoms to initiate
local FM correlations that compete with
the AFM matrix. A quantum-spin-canted
state thus emerges, with local ferromagnetic
polarons made of an electron surrounded by
a ferromagnetically aligned neighboring spin
cluster. These local magnetic regions grow
with increases in laser intensity, causing a
magnetic order switch during the 100-fs-long
The femtosecond magneto-optics thus
reveals an initial quantum coherent regime
of magnetism that merges quantum non-equilibrium kinetics and exotic phase
competition in complex materials.
This work was supported by NSF contract no.
The challenge to push the gigahertz switch- ing speed of today’s logic and magnetic
memory devices into the terahertz regime
underlies the entire field of information
processing, communication and integrated
devices. The physics of present-day devices
imposes serious limitations on this technological transformation, so we must invent new
paradigms based on quantum dynamics in the
femtosecond regime. This challenge could be
met by all-optical quantum switching based
on femtosecond coherent laser excitation in
strongly correlated magnetic oxides. 1
There is growing evidence that femtosecond laser-induced transient polarization
can be used to manipulate magnetic order
during a laser pulse. 2-4 The idea draws on an
analogy to femtosecond chemistry and photo-synthetic dynamics in which photoproducts
of chemical and biochemical reactions can
be influenced by creating suitable coherent
superpositions of molecular states. Similarly,
femtosecond-laser–excited coherence can
create a controllable transient superposition
of quantum spin/charge states that switches
magnetic order by suddenly breaking the
delicate balance between competing phases of
colossal magneto-resistive (CMR) manganites.
We recently published a potential way to
implement such quantum magnetic switching
with femtosecond laser pulses in a CMR oxide
Pr0.7Ca0.3MnO3.1 Our results demonstrate a
photo-induced switching from antiferromagnetic (AFM) to ferromagnetic (FM) ordering
that completes during 100-fs laser pulses,
while the optical polarization/coherence still
interacts with the spins.
Many 2-D layers and 1-D spin chains can
experience gigantic magnetic changes from
AFM insulating to an FM metallic state during
ultrafast laser excitation less than 100 fs,
(a) Laser-induced magnetic phase transition during 100 fs laser pulses in a CMR
manganese oxide, from antiferromagnetic (green and yellow arrows) to ferromagnetic (red arrows). 1 (b) 3-D view of photo-induced fs-spin generation, measured by
magnetic circular dichroism Δh, with photo-excitation threshold behavior.
Jigang Wang (jwang@
ameslab.gov) Tianqi Li
and Aaron Patz
Iowa State University and
Ames, Iowa, U.S.A.
Ilias E. Perakis and
University of Crete and
Foundation for Research
University of Tennessee,
Knoxville, and Oak Ridge
National Laboratory, Oak
Ridge, Tenn., U.S.A.
Thomas A. Lograsso
1. T. Li et al. Nature 496,
2. J. Wang et al. App.
Phys. Lett. 94, 021101
3. M. D. Kapetanakis et
al. Phys. Rev. Lett.
103, 047404 (2009).
4. M. D. Kapetanakis et
al. App. Phys. Lett. 94,
Quantum Spin Switching