Imaging the Magnetic Spin Structure
of Exchange Coupled Thin Films
Ralf Röhlsberger
Hamburger Synchrotronstrahlungslabor (HASYLAB)
am Deutschen Elektronen Synchrotron (DESY),
Notkestr. 85, D-22603 Hamburg
E-mail: ralf.roehlsberger@desy.de
Permanent Magnets: Evolution of the Energy Product
The magnetic energy product of
permanent magnets can be significantly
enhanced via the mechanism of
exchange hardening in nanostructured
two-phase systems:
Such materials consist
of a hard-magnetic
phase with high
coercivity and a softmagnetic phase with
high magnetization
R. Skomski and J. Coey:
PRB 48, 15812 (1993)
Magnet volumes at constant
magnetic energy
An important aspect
that determines the
properties of such
materials is the
interfacial coupling
between the different
magnetic phases
Production of Hard-Magnetic FePt Films
FePt alloys in the desired composition can
be produced in the following fashion:
FePt alloy, L10 phase
Deposition of a Fe/Pt multilayer:
(0.5 nm Fe/0.5 nm Pt)30,
then annealing for 20 min at 700 K:
Si wafer
Magnetic hysteresis
20 nm Ta
Direct resistive heating of the Si substrate via
current flow through a thin Ta layer allows for
high heating and cooling rates, thus preventing
excessive grain growth during alloy formation
Magneto-Optical Kerr Effect (MOKE)
The Magnetic Structure of Hard/Soft – Magnetic Bilayers
Fe on FePt
An exchange-spring magnet
A soft – magnetic film (Fe) is
deposited on a hard-magnetic film
(FePt) with uniaxial anisotropy
External field
Exchange coupling at the interface
leads to parallel alignment of Fe and
FePt moments in that region.
Fe
With increasing distance from the
interface the magnetic coupling
becomes weaker.
An external field H perpendicular to
the anisotropy direction thus induces
spiral magnetization in the film.
FePt
The moments in the soft-magnetic
film return to parallel alignment when
the external field is switched off.
Direction of anisotropy (remanent
magnetization after saturation)
Due to this spring-like behavior, such systems are called exchange-spring
magnets (E. F. Kneller and R. Hawig, IEEE Trans. Magn. 27, 3588 (1991))
Imaging the Spin Structure of Exchange-Coupled Thin Films
Isotopic probe layers of 57Fe can be used to determine the depth dependence of the
spiral twist in the soft-magnetic layer: An ultrathin layer of 57Fe is embedded in the Fe
film as shown below. Thus, transverse displacement Dx of the sample relative to the
200 mm wide beam of synchrotron radiation allows to probe the magnetic properties of
the film as a function of depth D.
H
Scattering
plane
11nm
Fe
FePt
20 mm
M
0.7 nm
57Fe
The sample is illuminated in grazing incidence geometry with synchrotron radiation tuned
to the 14.4 keV resonance of 57Fe. A synopsis of the method is given on the next slide.
Nuclear Resonant Scattering of Synchrotron Radiation
Hyperfine interaction of the
57Fe nucleus in magnetic
materials
Superposition of wavetrains with
slightly different frequencies leads
to characteristic temporal beats
Temporal
beats
Radioactive source
Energy spectra
Synchrotron radiation
Time spectra
From the beat pattern the
magnetization direction
in the sample can be
derived.
Due to the enormous
brilliance of the
synchrotron radiation,
data acquisition times can
be as short as a few
minutes
Imaging the Internal Spin Structure of Exchange-Spring Magnets
Time spectra of nuclear resonant scattering
Dx
Scattering
plane
log(intensity)
11 nm
20 mm
0.7 nm 57Fe
160 mT
11 nm Fe
30 nm FePt
0
50
100
Time (ns)
150
200
R. Röhlsberger et al,
Phys. Rev. Lett. 89,
237201 (2002)
The field dependence of
the internal spin
structure in exchangespring layer systems
The figures on the right show
the results of the measurements
for different values of the
external field. These results are
plotted in the graph below. The
solid liens are results of
simulations according to the
model explained on the next
slide.
160 mT
240 mT
Ag
500 mT
FePt
Simulation of Exchange-Spring Layer Systems
The equilibrium configuration of such
layer systems is found by application of
the following micromagnetic model
(E. Fullerton et al., Phys. Rev. B 58,
12193 (1998))
Divide the layer system into N sublayers of
thickness d
Minimize the magnetic
free energy of the system
The total magnetic energy of a system
consisting of N layers is given by:
Exchange
Anisotropy
Dipolar energy
The method allows one to determine magnetic depth
This equation is iterated with the values
profiles. Here the exchange constant changes near the
obtained from dE/dji = 0 until equilibrium is
upper interface due to oxidation/interdiffusion
reached for each sublayer.