Probing Magnetism with X-ray Techniques J an Lüning Sorbonne University, Paris (France) and Helmholtz-Zentrum Berlin (Germany) jan.luning@helmholtz-berlin.de Lecture topics: 1) A brief introduction to X-rays - The basics of the interaction of X-rays with matter - Origin and properties of synchrotron radiation 2) X-ray based techniques - X-ray absorption spectroscopy - Types of magnetic dichroism - XMCD and Sum Rules - XMLD - Resonant magnetic scattering 3) X-ray microscopy - STXM - XPEEM - Lensless microscopy 1
Reference for synchrotron radiation 2
Discovery of X-rays 8 November 1895 (Würzburg, Germany) 1901 : Nobel price in Physics 3
First applications 1914 A chest X-ray in progress at Professor Menard's radiology department at the Cochin hospital, Paris, 1914. (Jacques Boyer/Roger Viollet—Getty Images) 4
Why are X-rays so useful J. Stöhr 5
Electromagnetic Waves Only difference: the wavelength! 6 J. Stöhr
Soft & Hard X-rays as part of the electromagnetic spectrum Three ‘common’ definitions: Soft: Grating UHV electronic structure Hard: Crystal 'Air' crystal structure Soft X Hard X nano atomic Wavelength Energy C, N, O K-edges 3d TM: strong magnetic L-edge resonances 7
Origin of synchrotron radiation Emission angle Θ = 1/γ rad ~ 100 10 mm μrad Small angle: tan (Θ [rad]) = Θ [rad] 100 m 8
Brilliance of X-ray sources UNE grandeur caractéristique pour évaluer la qualité d’une source est la luminance. (Liouville : A∙Ω = const) Photons/second Brilliance = Source size x Divergence x ΔE Note: The coherence of a source is proportional to its brilliance! 9
Bending magnet and insertion device radiation Bend magnet Wiggler Undulator 10
50+ SR sources world-wide 13 SR sources in Europe … and 4 XUV/X-FELs 11
Photon – Matter Interaction 12
Photon – Matter Interactions 10 8 N Total Total Carbon Gold Vacuum K M Absorption Absorption 10 6 level Photon Cross Section (barn) Coherent Coherent L empty Incoherent Incoherent 10 4 K Pair Pair Valence Band gap Triplet Triplet States 10 2 filled Binding Energy 10 0 10 -2 ~ ~ Core C 1s 10 -4 10 1 10 3 10 5 10 7 10 9 10 11 10 13 10 1 10 3 Level 10 5 10 7 10 9 10 11 10 13 Photon Energy (eV) Photon Energy (eV) Absorption → dominates in X-ray photon energy range Coherent = Thomson scattering → X-ray scattering/diffraction Incoherent = Compton scattering # of absorbed photons per second Photon Absorption Cross Section: = X # of incident photons per second per area -24 2 [ ] = barn = 12 cm 13
Photon Matter Interactions 10 8 N Total Total Carbon Gold K M Absorption Absorption 10 6 Coherent Coherent L Photon Cross Section (barn) Incoherent Incoherent 10 4 K Pair Pair Triplet Triplet 10 2 10 0 10 -2 10 -4 10 1 10 3 10 5 10 7 10 9 10 11 10 13 10 1 10 3 10 5 10 7 10 9 10 11 10 13 Photon Energy (eV) Photon Energy (eV) Vacuum level empty Valence Band gap Binding Energy States filled Binding Energy N (n = 4) M (n = 3) ~ ~ L (n = 2) Core C 1s K (n = 1) Level 14
Interaction strengths of different probes Electrons Visible light Charged probe (Metals) Section efficace (barn / atome) Photoemission Diffusion Visible light Neutrons (Insulators) No charge, but a spin 15
Mean free path of electrons in materials 16
Absorption and Photoemission in atoms and solides Atome Solide Vacuum level empty Valence Band gap States filled Absorption : hν < E B [+ E T.d.Sortie ] L’électron excité ne sort pas de l’atome/du solide. Binding Energy Photoémission : hν > E B [+ E TdS ] L’électron excité sort de l’atome/ du solide. E Kin = hν - E B [- E TdS ] ~ ~ Core C 1s Level 17
X-ray absorption spectroscopy 18
X-ray absorption resonances Elemental Specificity N i F e C o empty Valence States filled Binding Energy 700 800 900 Photon Energy (eV) ~ ~ h Chemical Sensitivity Fe 2p ~ ~ Core Co 2p Levels ~ ~ Ni 2p 19
X-ray absorption: Transmission versus Yield detection “Photons lost” Volume (< 1 - 10 microns) “Electrons generated” Surface (1 – 2 nanometer) 20
Studying complex materials layer by layer Total electron yield detection to render X-ray absorption spectroscopy surface sensitive Fe 21
L 2,3 edge absorption spectroscopy on 3d transition metals 22
L 32 ‘white line’ intensity of transition metals (3d) J. Stohr et al. 23
X-ray magnetic circular dichroism (XMCD) in X-ray absorption spectroscopy 24
More about X-rays and magnetism J. Stöhr, NEXAFS SPECTROSCOPY , Springer Series in Surface Sciences 25, Springer, Heidelberg, 1992. J. Stöhr and H. C. Siegmann MAGNETISM: FROM FUNDAMENTALS TO NANOSCALE DYNAMICS , Springer Series in Solid State Sciences 152, Springer, Heidelberg, 2006 Many (!) transparencies are taken from Jo Stohr’s 2007 presentation on ‘X-rays and magnetism’ www-ssrl.slac.stanford.edu/stohr 25
Magneto-Optical effects in the visible and X-ray regime 26
Origin of the XMCD effect Relative transition amplitudes are given by the respective Clebsch Gordon coefficients 27
XMCD spectra of ferromagnetic 3d metals Fe, Co and Ni 28
XAS / XMCD sum rules 29
Magnetic moment of 3d transition metals Fe, Co and Ni 30
Spin polarization of valence band states 31
XMCD of transition metal M 2,3 and L 2,3 edges M 2,3 L 2,3 3p 1/2 , 3/2 → 3d 2p 1/2 , 3/2 → 3d S. Valencia et al., New J. Phys. 8, 254 (2006) R. Nakajima et al., Phys. Rev. B 59, 6421 (1999) 33
X-ray magnetic LINEAR dichroism (XM L D) in X-ray absorption spectroscopy 34
X-ray Magnetic Circular and Linear Dichroism XMCD : XMLD : 35
XMLD: Linear dichroism and presence of AFM order La 0.6 Sr 0.4 FeO 3 / SrTiO 3 (110) Above Néel temperature No linear dichroism Warnings: Crystal fields can cause linear dichroism Relationship between orientation of AFM axis and dichroic ratio can depend on crystal orientation Below Néel temperature Strong linear dichroism 36
Orientation of AFM axis in a thin film of LaFeO 3 / SrTiO 3 (110) 37
The next step: Adding spatial resolution to x-ray spectroscopy X-ray spectro-microscopy Application: Co / NiO The interface between a ferromagnetic metal and an antiferromagnetic metal oxide 38
X-ray spectromicroscopy techniques Quantitative imaging with sensitivity to elemental and chemical distribution and charge/spin ordering Condenser Lens 39
Fresnel Zone Plate Lenses 40
Fresnel Zone Plate Principle Spherical Grating with varying line density 41
Principle of a diffraction grating 42
Focussing with a Fresnel zone plate 44
Spatial resolution: resolving two point sources 45
Scanning (STXM) vs fulf field (TXM) X-ray microscopy 47
X-ray spectromicroscopy techniques Quantitative imaging with sensitivity to elemental and chemical distribution and charge/spin ordering Techniques ideally suited to study phenomena Condenser occurring on the nanometer length scale Lens • Thin film and surface sensitivity • Spectroscopic contrast mechanism • Individual parts of complex structures accessible • Spatial Resolution 20 nm – 50 nm routinely - 10 nm and better demonstrated - volume zone plate needed for further improvement 48
X-ray spectromicroscopy techniques Quantitative imaging with sensitivity to elemental and chemical distribution and charge/spin ordering Condenser Lens 49
Photoemission Electron Microscopy Elemental Specificity Schematic layout of the PEEM Topographical Contrast N i F e C o analyzer 700 800 900 Photon Energy (eV) Elemental Contrast Magnetic lenses Chemical Sensitivity Co e - Fe 20 kV La 16° Ti 50
Photoemission Electron Microscopy Topographical Contrast Elemental Contrast Co Fe La Ti 51
XMCD as contrast mechanism in X-ray spectroscopy 52
X-ray microscopy for local X-ray spectroscopy 53
Resolving the spin structure of a magnetic multilayer “Exchange bias” magnetic multilayer Magnetic domain structure in: ferromagnetic metal FM interface AFM antiferromagnetic oxide Spectroscopic Identification and Direct Imaging of Interfacial Magnetic Spins H. Ohldag et al., Phys. Rev. Lett 87 , 247201 (2001). 54
Exchange Bias Phenomenon 55
Spin structure in a magnetic multilayer Co NiO Elemental Specificity N i NiO XMLD Co XMCD F e C o 700 800 900 Photon Energy (eV) XMLD for imaging of XMCD for imaging of antiferromagnetic spin ferromagnetic spin order order in NiO substrate in Co film Parallel alignment of spins on both sides of the FM – AFM interface 56
Interface chemistry X-ray absorption spectroscopy Chemical environment Co Co influences NEXAFS CoO L 3 -edge Co/NiO Model M M x O y 776 778 780 Upon deposition of Ni Ni 2 ML of Co on NiO NiO A L 2 -edge B Co/NiO Model 2 ML CoO (Co oxidized) 2 ML Ni (Ni reduced) Co CoNiO NiO linear combination of metal 868 870 872 874 and oxide spectra possible Photon Energy (eV) 57
Recommend
More recommend
