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    Brent Van Neste

    Nuclear Resonant Scattering (NRS) is a hyperfine spectroscopy method, similar to Mössbauer spectroscopy but with information retrieved from the time domain. This method, which uses synchrotron radiation, is used in many fields of study, in particular the analysis of the electronic and magnetic structure of small volumes containing micro- and nano-structures.

    The first stage of NRS is exciting nuclear energy levels of a sample system. This is done by exposing atoms to monochromatised synchrotron radiation, with an energy large enough to cross the gap between two nuclear levels of the atom. The nuclear levels are splitted into hyperfine levels due to hyperfine interactions. Exposing the atoms to the radiation excites all available transitions simultaneously.
    The second stage of NRS is the sample decay. Levels which are close in energy will emit quanta. Due to the hyperfine splitting, multiple transitions (at slightly different frequencies) are possible. Due to superposition of the emitted waves from each of the transitions (interference), we wil observe a quantum beat pattern for the intensity in function of time (see graph of figure 1). This “time spectrum” can be numerically analysed (based on Fourier transform) to receive information about the frequencies of the emitted quanta. These frequencies in turn give information about the hyperfine parameters of the system, for example the hyperfinefield B_hf, the quadrupole splitting and the isomeric shift.
    Note that synchrotron radiation was used to excite nuclear levels, and not for example a radioactive source. This is because the synchrotron radiation intensity in function of time consists of very short pulses (see graph in video). The intensities we measure are “delayed” intensities (delayed w.r.t. that initial excitation moment). Because of the pulsed intensity time spectrum of the radiation, the intensity time spectrum we measure from the transitions will have a beat like pattern, which is usefull for determining hyperfine parameters.

    Features of NRS:
    The NRS technique is very sensitive to the direction of magnetisation w.r.t. the polarisation vector of the incoming radiation, as well as to the k vector of the incoming radiation. If the orientation of the magnetisation (black arrow on figure in video) is changed, then the pattern in the time spectrum will be different. If the magnetisation is oriented orthogonal to the beam, then we receive much more complex patterns.

    Application example of NRS:
    A single atomic layer of Fe was deposited on atomically cleaned W(100) (single crystal) at a temperature of 570K. A sequence of deposition steps was done, with each step adding 0.2 monolayers. After every deposition, a time spectrum was made. (zie figuur uit video). This procedure was repeated for the (001) plane. From both experiments we receive analogous results. For 1 ML, we only see the exponential decay form the excited state, because we are in the situation of a ferromagnet with T_Curie < T_room. When the thickness of the layer is increased, we start to see the appearance of quantum beats (for example figure at 1.6 ML). We see this for both the (110) and (001) planes, which are orthogonal to each other. This means that there is a big contribution of perpendicular magnetic anisotropy in the system. When we further increase the thickness, we see a spin orientaiton transition from out-of-plane to in-plane.
    An important remark here is that for these experiments, which were done in UHV, we could neglect the absorption of residual gases. This is important, as absorption of residual gases would kill the anisotropy.

    From this example we can conclude the following. NRS can be used to map the spin structure of the Fe film, not only of the surface layers, but also of buried layers. NRS allows us to see magnetic anisotropy at room temperature and to seea gradual transition from out-of-plane to in-plane direction. The magnetic structure of this transition if related to the morphology of the film (a deviation from the layer-by-layer growth which strongly influences the magnetic structure of the system).

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