RESEARCH NEWS
Ferromagnetic
Tunneling electrons leave atom in a spin
CHARACTERIZATION
origins of TiO2
 Our new technique provides this information
MAGNETIC MATERIALS
in much more detail and precision than has
Daniel R. Gamelin and colleagues at
been possible before.
the University of Washington and
The team measured the spin excitation
Pacific Northwest National Laboratory
spectra of single Mn atoms adsorbed on
Al2O3 islands on a NiAl surface using have observed strong ferromagnetism
inelastic electron tunneling spectroscopy.
at room temperature in Co2+-doped
Above a certain voltage, tunneling electrons
TiO2 nanocrystal films prepared by
transfer energy to spin-flip excitations of the
direct chemical methods [Bryan et al.,
Mn atoms. This additional tunneling channel
J. Am. Chem. Soc. (2004) 126 (37),
results in a step increase in the conductance
11640].
detected at the STM tip.
Ferromagnetic semiconductors are
The spin excitation spectra, measured under
desirable as key components of spin-
ultrahigh vacuum at 0.6 K, shows shifts in
Conceptual diagram showing electrons tunneling from an STM tip
based semiconductor devices, but
the conductance above a certain voltage.
through an Mn atom on a surface. Electrons above a certain
developments have been hindered by
energy can flip the spin of the Mn atom. (Courtesy of IBM.) Under a magnetic field of 7 T, the energy
the cryogenic temperatures required
required to flip the spin of a single Mn atom
Andreas J. Heinrich and coworkers at IBM s is ~0.8 meV. This energy varies with applied for ferromagnetic ordering in existing
Almaden Research Center have used a field, as expected for Zeeman splitting of Mn
materials.
scanning tunneling microscope (STM) to spin states. The team was able to determine
Co-doped TiO2 (Co2+:TiO2, anatase)
measure the energy needed to flip the spin of the magnetic moments of individual Mn
attracted great interest when it was
a single atom [Heinrich et al., Sciencexpress atoms and showed that they vary depending
reported that thin films of the material
(2004) doi: 10.1126/science.1101077]. The on the atom s local environment.
are ferromagnetic above 300 K.
ability to study individual magnetic moments The site specific study of magnetic moments,
However, the ferromagnetism is widely
could be very important for future coupled with STM s ability to fabricate
believed to arise from phase-
information technologies from spintronics to atomically precise structures, could provide
segregated Co metal nanocrystals.
quantum computing.  We will need a powerful tool for investigating the local
In order to address this issue, colloidal
fundamental knowledge of the magnetic properties of engineered magnetic
suspensions of Co-doped TiO2
properties of small numbers of atoms in nanostructures, say the researchers.
various environments, says Heinrich. Jonathan Wood nanocrystals capped with
trioctylphosphine oxide ligands were
prepared using an inverse micelle
Quantum optics on a chip procedure. Thin, nanocrystalline films
were then produced by spin coating.
MAGNETIC MATERIALS
These films, prepared under oxidative
Getting a single photon to interact with a single microwave photons to the system. A
conditions that preclude the formation
atom has been a focus of atomic physics research superconducting Josephson tunnel junction placed
of cobalt metal, are strongly
for a number of years. Now, researchers from Yale within this cavity acts as a qubit with two energy
ferromagnetic. X-ray absorption
and Indiana Universities have achieved this in a states that differ by the transfer of a single electron
studies confirm the majority of the
solid-state system that has many desirable features or Cooper pair. The energy difference can be tuned
cobalt is in the Co2+ oxidation state.
for a future quantum computer [Wallraff et al., by varying the gate voltage and applied magnetic
 These results provide strong support
Science (2004) 431, 162]. field. When the excitation energy matches the
for the existence of intrinsic
Strong coupling can be achieved between a confined resonant frequency of the cavity, strong coupling
ferromagnetism in this material and
photon and an isolated atom within in a cavity. This between the photon and the qubit occurs. In this
setup results in the rapid exchange of energy special case, transmission of microwaves through bode well for the future of high-
between the photon and atom, and the state of the the cavity no longer occurs at the resonant
temperature ferromagnetic
system becomes a superposition of two possibilities: frequency, but at two different frequencies reflecting
semiconductors in spintronic
the energy is both an excitation of the atom and a the two energy states of the superposed artificial
technologies, says Gamelin. The
photon. atom-microwave photon system.
colloidal nanocrystals could be used as
Rather than use an atom and visible light photons The ability to couple qubits to photons could allow
building blocks for the assembly of
from a laser, Andreas Wallraff and coworkers use a qubits to be wired together on a chip via a  quantum
spintronic devices by soft lithography
superconducting two-level system, which acts as an information bus carrying single photons, which is
or self-assembly processes.
artificial atom, and microwave photons. A highly desirable for building a quantum computer.
superconducting waveguide resonator confines the Jonathan Wood Jonathan Wood
22 November 2004