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Imaging the Magnetic Spin Structure of Exchange Coupled Thin Films Ralf Röhlsberger Hamburger Synchrotronstrahlungslabor (HASYLAB) am Deutschen Elektronen.

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Presentation on theme: "Imaging the Magnetic Spin Structure of Exchange Coupled Thin Films Ralf Röhlsberger Hamburger Synchrotronstrahlungslabor (HASYLAB) am Deutschen Elektronen."— Presentation transcript:

1 Imaging the Magnetic Spin Structure of Exchange Coupled Thin Films Ralf Röhlsberger Hamburger Synchrotronstrahlungslabor (HASYLAB) am Deutschen Elektronen Synchrotron (DESY), Notkestr. 85, D Hamburg

2 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, (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

3 Production of Hard-Magnetic FePt Films then annealing for 20 min at 700 K: Si wafer FePt alloy, L1 0 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

4 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

5 Imaging the Spin Structure of Exchange-Coupled Thin Films M H Fe FePt 20 mm 11nm Scattering plane 0.7 nm 57 Fe Isotopic probe layers of 57 Fe can be used to determine the depth dependence of the spiral twist in the soft-magnetic layer: An ultrathin layer of 57 Fe 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 57 Fe. A synopsis of the method is given on the next slide.

6 beats T emporal From the beat pattern the magnetization direction in the sample can be derived. Hyperfine interaction of the 57 Fe nucleus in magnetic materials Energy spectra Time spectra Radioactive 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

7 Imaging the Internal Spin Structure of Exchange-Spring Magnets Time spectra of nuclear resonant scattering Time (ns) log(intensity) R. Röhlsberger et al, Phys. Rev. Lett. 89, (2002) 160 mT 20 mm 11 nm Scattering plane 0.7 nm 57 Fe x 11 nm Fe 30 nm FePt

8 The field dependence of the internal spin structure in exchange- spring layer systems 500 mT240 mT 160 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

9 Minimize the magnetic free energy of the system Simulation of Exchange-Spring Layer Systems Divide 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, (1998)) Exchange Anisotropy Dipolar energy This equation is iterated with the values obtained from dE/d i until 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


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