Magnetic characterization of Fe / 57 FeSi / Fe trilayers Francisco Almeida, student from Faculdade de Ci ê ncias da Univer...
Overview <ul><li>Motivation </li></ul><ul><ul><li>Magnetic multilayers, interlayer exchange coupling and Giant Magnetic Re...
Motivation Interlayer exchange coupling Bilinear antiferromagnetic (   = 180 º ) coupling between two ferromagnetic layer...
Motivation Giant Magnetoresistivity <ul><li>Interlayer coupling gives enhances Magnetoresistivity </li></ul><ul><li>Giant ...
Interlayer exchange coupling Phenomenological energy expression 1  Slonczewski, J. C. – “Overview of interlayer exchange t...
Interlayer exchange coupling The RKKY approximation <ul><li>Ruderman-Kittel-Kasuya-Yosida indirect exchange </li></ul><ul>...
Interlayer exchange coupling The RKKY approximation <ul><li>Exchange coupling oscillating as a function of a metallic spac...
Interlayer exchange coupling Temperature and thickness dependence <ul><li>Insulator spacers: </li></ul><ul><ul><li>Monoton...
Interlayer exchange coupling Debate on the coupling  behaviour  of Fe/FeSi trilayers <ul><li>Different groups show contrad...
Samples grown through Molecular Beam Epitaxy <ul><li>Samples consisting of Fe/ 57 FeSi/Fe </li></ul><ul><ul><li>Varying Fe...
Samples grown through Molecular Beam Epitaxy <ul><li>Intended system is MgO subst Fe 80 A  57 Fe 0.5 Si 0.5 Fe 40 A Au cap...
Samples grown through Molecular Beam Epitaxy
Vibrating Sample Magnetometry <ul><li>Measurement of total sample magnetization for low and high temperatures </li></ul><u...
Vibrating Sample Magnetometry Measuring hysterysis curves <ul><li>Characterizing magnetic materials through: </li></ul><ul...
Numerical analysis Simulation through energy minimization <ul><li>Finding the energy minimum </li></ul><ul><li>No analytic...
Numerical analysis Simulation of biquadratic coupling t 1 t 2 (pinned)
Numerical analysis Simulation of bilinear coupling t 1 t 2 (pinned)
Numerical analysis Automatically fitting the data <ul><li>Fitting allows to measure: </li></ul><ul><ul><li>Bilinear coupli...
Results Coupling strength <ul><li>Possible to obtain a trend of the coupling strength as function of temperature and thick...
Results Coupling strength – C1208 (14  Å  FeSi) <ul><li>Use of “easy” and “hard” axis measurements allows to estimate anis...
Coupling strength – C1208 (14  Å  FeSi)
Coupling strength – C1208 (14  Å  FeSi)
Coupling strength Loose spins model (Slonczewski  et al ) Interlayer coupling mediated through “loose spins” in the spacer
Coupling strength Loose spins model fit to data
C1208 (14  Å  FeSi) A strange effect
C1208 (14  Å  FeSi) Antiferromagnetic fraction (AFF) <ul><li>Introducing a normal ferromagnetic term allows to reproduce t...
Thinner samples (C1205 and C1206) Difficulties in finding solutions <ul><li>AFF function quickly drops just below RT or at...
Thinner samples (C1205 and C1206) Difficulties in finding solutions
Thinner samples (C1205 and C1206) Difficulties in finding solutions
Coupling strength All samples <ul><li>C1208 is representitive of what occurs in most coupled samples </li></ul><ul><li>Thi...
Coupling strength All samples
Coupling strength All samples J 0  = Maximum value (null thickness)    = Coeherence length
Conclusions <ul><li>The chosen method for analysis of magnetisation loops obtained through VSM is efficient, but its effic...
References <ul><li>Baibich, M.N.; Broto, J.M.; Van Dau, F.N. -  “ Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Supe...
Rutherford Backscattering Spectrometry (inacabado…) <ul><li>Nuclear technique suited for thin films characterization : </l...
Rutherford Backscattering Spectrometry Stoichiometry of the Fe 1-x Si x  spacer layer (inacabado…) <ul><li>Use of low beam...
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Presentation of Licentiate in Physics Engineering of Francisco Almeida

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This seminar was presented to show the results of my research on magnetic thin films for my Licentiate diploma in Physics Engineering. This is a subset (although the biggest portion) of the analysis performed.
(note: the two last slides are not part of the actual presentation).

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Presentation of Licentiate in Physics Engineering of Francisco Almeida

  1. 1. Magnetic characterization of Fe / 57 FeSi / Fe trilayers Francisco Almeida, student from Faculdade de Ci ê ncias da Universidade de Lisboa ( Erasmus exchange ) EST Á GIO PROFISSIONALIZANTE – LICENCIATURA EM ENGENHARIA F Í SICA <ul><li>Promotor: </li></ul><ul><li>Prof. Dr. Jos é Carvalho Soares (FCUL) </li></ul><ul><li>Magnetic thin films group: </li></ul><ul><li>Dr. Bart Croonenborghs </li></ul><ul><li>Dr. Johan Meersschaut </li></ul><ul><li>Dr. Dominique Aernout </li></ul><ul><li>Dr. Caroline L’Abb é </li></ul><ul><li>Coordenators: </li></ul><ul><li>-Prof. Dr. Andre Vantomme (IKS, KUL) </li></ul><ul><li>Dr. Johan Meersschaut (IKS, KUL) </li></ul>
  2. 2. Overview <ul><li>Motivation </li></ul><ul><ul><li>Magnetic multilayers, interlayer exchange coupling and Giant Magnetic Resistance </li></ul></ul><ul><ul><li>Debate on the coupling behaviour of Fe/FeSi trilayers </li></ul></ul><ul><li>Techniques used </li></ul><ul><ul><li>Molecular Beam Epitaxy </li></ul></ul><ul><ul><li>Vibrating Sample Magnetometry </li></ul></ul><ul><ul><ul><li>Numerical analysis: Simulating and fitting the acquired data </li></ul></ul></ul><ul><ul><li>Structural characterization through several techinques </li></ul></ul><ul><ul><ul><li>Conversion Electron Mossbauer Spectroscopy </li></ul></ul></ul><ul><ul><ul><li>High Resolution X-Ray Diffraction </li></ul></ul></ul><ul><ul><ul><li>Rutherford Backscattering Spectroscopy </li></ul></ul></ul><ul><li>Results </li></ul><ul><ul><li>Coupling evolution throughout different thicknesses </li></ul></ul><ul><ul><li>Quality of samples </li></ul></ul><ul><li>Conclusions </li></ul>
  3. 3. Motivation Interlayer exchange coupling Bilinear antiferromagnetic (  = 180 º ) coupling between two ferromagnetic layers Analogy to Heisenberg type exchange: (Bilinear form Hamiltonian) Energy related to the interlayer exchange coupling:
  4. 4. Motivation Giant Magnetoresistivity <ul><li>Interlayer coupling gives enhances Magnetoresistivity </li></ul><ul><li>Giant Magnetoresistivity is the underlying principle of a variety of sensors and magnetic recording </li></ul><ul><ul><li>Discovered in 1988 in Fe/Cr magnetic multilayers 1 </li></ul></ul><ul><ul><li>First seen in Fe/Cr/Fe trilayers 2 in 1989 </li></ul></ul>1 Baibich, M.N.; Broto, J.M.; Van Dau, F.N. - “ Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices ”, Phys. Rev. Lett. 61 , 2472 (1988) 2 Binash, G.; Grunberg, P.; Saurenbach, F. - “ Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange ”, Phys. Rev. B 39 , 4828 (1989) GMR plot from Fert, A.; Grunberg, P.; Barthelemy, A. – “Layered magnetic structures: interlayer exchange coupling and giant magnetoresistance”, J. Mag. M. Mat. 1 , 140 (1995)
  5. 5. Interlayer exchange coupling Phenomenological energy expression 1 Slonczewski, J. C. – “Overview of interlayer exchange theory”; Journal of Magnetism and Magnetic Materials, 150 , 13 (1995) Bilinear (AF) coupling Biquadratic coupling Energy power expansion 1 on
  6. 6. Interlayer exchange coupling The RKKY approximation <ul><li>Ruderman-Kittel-Kasuya-Yosida indirect exchange </li></ul><ul><li>Indirect exchange coupling mediated through conduction electrons </li></ul><ul><li>Good approximation for coupling mechanism in metallic spacer trilayers </li></ul>2D decay: 3D (general case) decay:
  7. 7. Interlayer exchange coupling The RKKY approximation <ul><li>Exchange coupling oscillating as a function of a metallic spacer thickness, for several different potencial barrier values: </li></ul><ul><li>V = 0. </li></ul><ul><li>V = 0.3E F . </li></ul><ul><li>V = 0.6E F . </li></ul><ul><li>V = 0.9E F . </li></ul><ul><li>(From Ferreira et al , </li></ul><ul><li>J. Phys.: Cond. Matt. 6 , L619 (1994) ) </li></ul>
  8. 8. Interlayer exchange coupling Temperature and thickness dependence <ul><li>Insulator spacers: </li></ul><ul><ul><li>Monotonous increase with temperature (thermally activated) </li></ul></ul><ul><ul><li>Exponential decay with the thickness </li></ul></ul><ul><li>Conducting metal spacers: </li></ul><ul><ul><li>Weak J 1 and stronger J 2 temperature dependence </li></ul></ul><ul><ul><li>Coupling strength oscillates with thickness (RKKY-type damped oscillation.) </li></ul></ul><ul><li>Semiconductors: </li></ul><ul><ul><li>Possible coupling behaviour depends on band-gap (i.e., state population near Fermi level) </li></ul></ul>
  9. 9. Interlayer exchange coupling Debate on the coupling behaviour of Fe/FeSi trilayers <ul><li>Different groups show contradicting results </li></ul><ul><ul><li>Oscillatory or exponential coupling thickness dependence? </li></ul></ul>de Vries, J.J.; de Jonge, W.J.M. et al – Phys. Rev. Lett. 78 , 3023 (1997) B ü rgler, D.A.; Gareev, R.R. et al - J. Phys. : Condensed Matter 15 S443 (2003)
  10. 10. Samples grown through Molecular Beam Epitaxy <ul><li>Samples consisting of Fe/ 57 FeSi/Fe </li></ul><ul><ul><li>Varying FeSi thickness </li></ul></ul><ul><ul><li>Aiming at Fe 0.5 Si 0.5 stoichiometry </li></ul></ul><ul><li>Trilayers grown epitaxially on an MgO substrate </li></ul><ul><ul><li>Fe (100) axis parallel to MgO (110) </li></ul></ul><ul><ul><li>P~10 -10 mBar, T = 150 º C </li></ul></ul>
  11. 11. Samples grown through Molecular Beam Epitaxy <ul><li>Intended system is MgO subst Fe 80 A 57 Fe 0.5 Si 0.5 Fe 40 A Au cap </li></ul><ul><li>Thicknesses checked through X-Ray reflectivity </li></ul><ul><li>All samples grew with the correct Fe thicknesses, except for C1207 (bottom Fe layer too thick) </li></ul><ul><li>The phase of the iron in the spacer was checked through CEMS, and the Fe in the spacer is in a non-magnetic environment (no Zeeman splitting observed) </li></ul>
  12. 12. Samples grown through Molecular Beam Epitaxy
  13. 13. Vibrating Sample Magnetometry <ul><li>Measurement of total sample magnetization for low and high temperatures </li></ul><ul><li>Physical principle: Faraday Law </li></ul><ul><li>Suited for magnetic thin films </li></ul>
  14. 14. Vibrating Sample Magnetometry Measuring hysterysis curves <ul><li>Characterizing magnetic materials through: </li></ul><ul><li>Saturation field ( M S ) </li></ul><ul><li>Remanent field ( M R ) </li></ul><ul><li>Coercivity ( H C ) </li></ul>
  15. 15. Numerical analysis Simulation through energy minimization <ul><li>Finding the energy minimum </li></ul><ul><li>No analytical general solution </li></ul><ul><li>Numerical approximations </li></ul>
  16. 16. Numerical analysis Simulation of biquadratic coupling t 1 t 2 (pinned)
  17. 17. Numerical analysis Simulation of bilinear coupling t 1 t 2 (pinned)
  18. 18. Numerical analysis Automatically fitting the data <ul><li>Fitting allows to measure: </li></ul><ul><ul><li>Bilinear coupling J 1 and biquadratic coupling J 2 </li></ul></ul><ul><ul><li>Cubic crystalline anisotropy K 4 </li></ul></ul><ul><ul><li>Ferromagnetic layers thicknesses ( t 1 , t 2 ) </li></ul></ul><ul><ul><li>Angle of magnetisation projection (mismatch from axis) </li></ul></ul><ul><li>Information on “easy” and “hard” axis projections </li></ul><ul><li>Grid Local Search for non-linear least squares fit </li></ul><ul><ul><li>Local  2 minimization </li></ul></ul><ul><ul><li>Parameter error bars </li></ul></ul>[100] Fe // [110] MgO easy hard
  19. 19. Results Coupling strength <ul><li>Possible to obtain a trend of the coupling strength as function of temperature and thickness </li></ul><ul><li>General tendency of decreasing coupling with temperature and spacer thickness </li></ul><ul><li>Strong temperature dependency for both bilinear and biquadratic coupling </li></ul><ul><li>Sample C1210 (with an 18 Å FeSi spacer) looses the interlayer coupling at RT </li></ul><ul><li>Samples with a spacer thicker than ~20 Å are simply not coupled </li></ul>
  20. 20. Results Coupling strength – C1208 (14 Å FeSi) <ul><li>Use of “easy” and “hard” axis measurements allows to estimate anisotropy </li></ul><ul><li>Cubic anisotropy decreases with temperature, from 36 kJ.m -3 to 32 kJ.m -3 </li></ul>
  21. 21. Coupling strength – C1208 (14 Å FeSi)
  22. 22. Coupling strength – C1208 (14 Å FeSi)
  23. 23. Coupling strength Loose spins model (Slonczewski et al ) Interlayer coupling mediated through “loose spins” in the spacer
  24. 24. Coupling strength Loose spins model fit to data
  25. 25. C1208 (14 Å FeSi) A strange effect
  26. 26. C1208 (14 Å FeSi) Antiferromagnetic fraction (AFF) <ul><li>Introducing a normal ferromagnetic term allows to reproduce the measured loops, without loss of continuity in the coupling J 1 and J 2 </li></ul>
  27. 27. Thinner samples (C1205 and C1206) Difficulties in finding solutions <ul><li>AFF function quickly drops just below RT or at even higher temperatures </li></ul><ul><ul><li>This drop multiplies the number of possible solutions of the system </li></ul></ul><ul><ul><li>Complexity of fitting procedure dramatically increases </li></ul></ul><ul><ul><li>Adopted fitting method does not have enough sensitivity </li></ul></ul><ul><li>Only some measurements, at room temperature, might be fitted and used for analysis </li></ul><ul><li>Possible coupling strength distribution causes smeathering of measured data (also seen in other samples, but with a weaker effect and only at low temperature) </li></ul>
  28. 28. Thinner samples (C1205 and C1206) Difficulties in finding solutions
  29. 29. Thinner samples (C1205 and C1206) Difficulties in finding solutions
  30. 30. Coupling strength All samples <ul><li>C1208 is representitive of what occurs in most coupled samples </li></ul><ul><li>Thinner samples have a much stronger coupling strength </li></ul><ul><li>AFF tends to reduce below unity at higher temperatures and less, as the thickness decreases </li></ul><ul><li>Bilinear coupling saturates, and in some cases, decreases at low temperature </li></ul><ul><li>Biquadratic coupling generally saturates at low temperatures </li></ul>
  31. 31. Coupling strength All samples
  32. 32. Coupling strength All samples J 0 = Maximum value (null thickness)  = Coeherence length
  33. 33. Conclusions <ul><li>The chosen method for analysis of magnetisation loops obtained through VSM is efficient, but its efficiency depends on the quality of the measurements </li></ul><ul><li>Ambiguous fitting solutions for some of the samples must be resolved through an alternate analysis method </li></ul><ul><li>The best model to explain the temperature dependent coupling behaviour is the loose spins model (Slonczewski et al ), although this system has some peculiarities </li></ul><ul><ul><li>The stoichiometry should be close to Fe 0.5 Si 0.5 , but there might be an excess of Si in the spacer. </li></ul></ul><ul><ul><li>Still ongoing work in adjusting the model to exact the results on a loose spins model interpretation is being done. </li></ul></ul>
  34. 34. References <ul><li>Baibich, M.N.; Broto, J.M.; Van Dau, F.N. - “ Giant Magnetoresistance of (001)Fe/(001)Cr Magnetic Superlattices ”, Phys. Rev. Lett. 61 , 2472 (1988) </li></ul><ul><li>Binash, G.; Grunberg, P.; Saurenbach, F. - “ Enhanced magnetoresistance in layered magnetic structures with antiferromagnetic interlayer exchange ”, Phys. Rev. B 39 , 4828 (1989) </li></ul><ul><li>Fert, A.; Grunberg, P.; Barthelemy, A. – “Layered magnetic structures: interlayer exchange coupling and giant magnetoresistance”, J. Mag. M. Mat. 1 , 140 (1995) </li></ul><ul><li>Ferreira, M.S.; Edwards, D.M. – “The nature and validity of the RKKY limit of exchange coupling in magnetic trilayers”; J. Phys.: Cond. Matt. 6 , L619 (1994) </li></ul><ul><li>B ü rgler, D.A.; Gareev, R.R. – “Exchange coupling of ferromagnetic films across metallic and semiconducting interlayers”; J. Phys. : Condensed Matter 15 S443 (2003) </li></ul><ul><li>Sloncsewski, J.C. – “Origin of biquadratic exchange in magnetic multilayers”; J. Appl. Phys. 73 , 5957 (1993) </li></ul><ul><li>de Vries, J.J.; de Jonge, W.J.M. – “Exponential Dependence of the Interlayer Coupling on the Spacer Thickness in MBE-grown Fe/SiFe/Fe Sandwiches”; Phys. Rev. Lett. 78 , 3023 (1997) </li></ul><ul><li>Strijkers, G.J.; de Jonge, W.J.M. – “Origin of Biquadratic Exchange in FeSiFe” </li></ul>
  35. 35. Rutherford Backscattering Spectrometry (inacabado…) <ul><li>Nuclear technique suited for thin films characterization : </li></ul><ul><ul><li>Concentration and depth sensitive (profiling through ion energy loss) </li></ul></ul><ul><li>Rutherford scattering cross-section (CM approximation) </li></ul><ul><ul><li>Useful for concentration or stoichiometry measurements </li></ul></ul>
  36. 36. Rutherford Backscattering Spectrometry Stoichiometry of the Fe 1-x Si x spacer layer (inacabado…) <ul><li>Use of low beam energy, normal incidence geometry and increased detection solid angle for maximizing the cross-section </li></ul>Sample 0° 80° He + Detector

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