Soft Report - Dipartimento di Fisica - Sapienza

Scientific Report – Elastic and inelastic scattering of neutrons and X-raysFemtosecond dynamics in Ferromagnetic MetalsIn electronic Resonant Raman Scattering (RRS) thesample is resonantly excited at a core threshold anda photon is emitted bringing the sample to the finalstate. This process sets a characteristic time scaleconnected with the lifetime of the intermediate statecore hole. If there is an evolution of the systemalong this time scale the effect is seen in the spectralfunction [1]. For this reason this approach to thestudy of fast processes is called core hole clock. Thepresent work is to our knowledge the firstimplementation of the core hole clock in the study ofmagnetism. This is done in ferromagnetic metals(Fe-Co-Ni) in the scattering channel 2p 6 3d n + hν in →2p 5 3d n+1 → 2p 6 3d n+1 3s 1 + hν out, where n is the 3doccupation number in the ground state and hν in andhν out are the incident and outgoing photon energies(in this case hν out is about 100 eV lower than hν in).We use the so called Integrated RRS (IRRS) [2]where one measures, as a function of hν in, theintegral of the scattered intensity in the scatteringchannel. The geometry is shown in the upper panelof Figure 1. The IRRS spectra show an effect notpresent in ferromagnetic insulators. A magneticcircular dichroism (MCD) is seen in IRRS spectra alsoin those regions between L 3 and L 2 and above L 2where the usual XMCD gives no signal (see the lowerpanels on Co in the figure where the red areas showthis effect on the IRRS dichroism given in green).This is due to the spin dependent screening takingplace in the intermediate state [3] so that the corehole becomes polarised. In order to take place, thisprocess requires a non zero magnetic moment of thesystem. In the fully relaxed theoretical model of ref.[3] the dichroism (more exactly the flipping ratio)depends on the local magnetic moment of theexcited site once the core hole is fully screened. Thecrucial experimental information given here is thetrend of this dichroism (flipping ratio) along thesequence Fe-Co-Ni and in particular the fact that Ni-100.20770 790 810 770 790 810Fig.1.L 3 L 13LL 22XMCDCo metalIRRS-MCDA0Fig. 2.metal shows clearly this effect. This is importantsince in a fully relaxed situation we should see noeffects because a well screened core hole in Ni givesessentially a 3d 10 configuration with almost zeromagnetic moment. Thus along the characteristic timescale of the scattering (typically 1-2 femtoseconds)Ni has not the time to develop the spin dependentscreening of the Fermi gas.More can be seen from Figure 2 where thehistograms give the magnetic moments of therelaxed and unrelaxed sites [3] in comparison withthe flipping ratio of the IRRS-MCD given by the reddots. The data are normalised to one in the case ofFe where a relaxed model accounts for the effect asseen in Auger spectroscopy [3]. The trend showsthat Co is somewhere in between the fully screenedand unscreened situation whereas Ni is basicallyunrelaxed. Thus Ni is much slower in building up thescreening and we attribute this behaviour to thenarrow band nature of the Ni-3d holes while the Feand Co case are certainly much more itinerant. Inthis sense we see a link between the correlationproperties and the magnetic dynamics along shorttime scales. In conclusion the use of the core holeclock in soft x-ray resonant scattering points out asubstantially slower dynamics in Ni in comparisonwith Fe and Co.References[1] P.A. Brühwiler, O. Karis, and N. Mårtensson,Rev. Mod. Phys. 74, 703-740 (2002) and referencesquoted therein.[2] L. Braicovich et al. , Phys. Rev. Lett. 90, 117401(2003).[3] A. Chassé et al. , Phys. Rev B 68, 214402(2003) and references quoted therein.AuthorsL. Braicovich (a), G. Ghiringhelli (a), A. Tagliaferri(a), G. van der Laan (b), E. Annese (c), and N. B.Brookes (d)(a) INFM-CNR and Dip. di Fisica, Politecnico diMilano, Italy; (b) Magnetic Spectroscopy, DaresburyLaboratory, UK; (c) TASC, Trieste, Italy; (d)European Synchrotron Radiation Facility, Grenoble,France.SOFT Scientific Report 2004-06100