imaging the magnetic spin structure of exchange coupled thin films
DESCRIPTION
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: [email protected]. Permanent Magnets: Evolution of the Energy Product. - PowerPoint PPT PresentationTRANSCRIPT
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
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: [email protected]
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Permanent Magnets: Evolution of the Energy Product
Magnet volumes at constant magnetic energy
The magnetic energy product of permanent magnets can be significantly enhanced via the mechanism of exchange hardening in nanostructured two-phase systems:
R. Skomski and J. Coey: PRB 48, 15812 (1993)
Such materials consist of a hard-magnetic phase with high coercivity and a soft-magnetic phase with high magnetization
An important aspect that determines the properties of such materials is the interfacial coupling between the different magnetic phases
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Production of Hard-Magnetic FePt Films
then annealing for 20 min at 700 K:
Si wafer
FePt alloy, L10 phase
Magnetic hysteresis
Magneto-Optical Kerr Effect (MOKE)
(0.5 nm Fe/0.5 nm Pt)30,
20 nm Ta
Deposition of a Fe/Pt multilayer:
FePt alloys in the desired composition can be produced in the following fashion:
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
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
The Magnetic Structure of Hard/Soft – Magnetic Bilayers
Fe on FePt
A soft – magnetic film (Fe) is deposited on a hard-magnetic film (FePt) with uniaxial anisotropy
Exchange coupling at the interface leads to parallel alignment of Fe and FePt moments in that region.
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.
The moments in the soft-magnetic film return to parallel alignment when the external field is switched off.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))
An exchange-spring magnet
External field
Direction of anisotropy (remanent magnetization after saturation)
Fe
FePt
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Imaging the Spin Structure of Exchange-Coupled Thin Films
M
H
Fe
FePt
20 mm
11nm
Scattering plane
0.7 nm 57Fe
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 x of the sample relative to the 200 m wide beam of synchrotron radiation allows to probe the magnetic properties of the film as a function of depth D.
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.
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
beatsTemporal From the beat pattern the
magnetization direction in the sample can be derived.
Hyperfine interaction of the 57Fe nucleus in magnetic materials
Energy spectra Time spectraRadioactive source Synchrotron radiation
Nuclear Resonant Scattering of Synchrotron Radiation
Superposition of wavetrains with slightly different frequencies leads to characteristic temporal beats
Due to the enormous brilliance of the synchrotron radiation, data acquisition times can be as short as a few minutes
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Imaging the Internal Spin Structure of Exchange-Spring MagnetsTime spectra of nuclear resonant scattering
Time (ns)0 50 100 150 200
log(
inte
nsit
y)
R. Röhlsberger et al, Phys. Rev. Lett. 89, 237201 (2002)
160 mT
20 mm
11 nm
Scattering plane
0.7 nm 57Fe
x
11 nm Fe
30 nm FePt
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
The field dependence of the internal spin structure in exchange-spring layer systems
500 mT240 mT160 mT
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.
Ag FePt
“Gütlich, Bill, Trautwein: Mössbauer Spectroscopy and Transition Metal Chemistry@Springer-Verlag 2009”
Minimize the magnetic free energy of the system
Simulation of Exchange-Spring Layer SystemsDivide the layer system into N sublayers of thickness d
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))
Exchange
Anisotropy
Dipolar energy
This equation is iterated with the values obtained from dE/diuntil equilibrium is reached for each sublayer.
The total magnetic energy of a system consisting of N layers is given by:
The method allows one to determine magnetic depth profiles. Here the exchange constant changes near the upper interface due to oxidation/interdiffusion