1. Magnetism in ultrathin films W. Weber IPCMS Strasbourg

2. Orbital and Spin moment Intuitive : Orbital moment Mysterious : Spin moment

3. Ferromagnetism Paramagnetic behavior: usually one has to apply an external magnetic field in order to align the magnetic moments Ferromagnetic behavior: magnetization without an external magnetic field at non-zero temperatures possible H

4. Ferromagnetism How can we explain magnetic order up to temperatures of 1000 K? Curie temperature

5. Ferromagnetism Early explanation (1907): Weiss’ molecular field A molecular field exists within the ferromagnet which orders the moments against the thermal motion. It is so large that the ferromagnet can be saturated even without an external magnetic field. Order of magnitude:

6. Ferromagnetism What is the physical interaction responsible for it? Dipole-dipole interaction? Strength Too weak!

7. Ferromagnetism Interplay of Pauli principle and Coulomb interaction Two electrons of opposite spin can share the same orbital and come close Two electrons of same spin cannot  farther apart  Lower Coulomb energy This interaction does not act like a real magnetic field J positive : parallel orientation (ferromagnetic) J negative : anti-parallel orientation (anti-ferromagnetic) Exchange interaction in a solid The strength of the interaction depends on the orbital overlap between neighbouring atoms  decreases exponentially with distance

8. Indirect exchange coupling in multilayers FM1 FM2 nonmagnetic metal

9. Indirect exchange coupling Unguris et al., Phys.Rev. B 49, 14 (1994)

10. Indirect exchange coupling in multilayers Amplitude of the coupling strength decreases with thickness Parkin et al., Phys.Rev. B 44, 7131 (1991) Co 80 Ni 20 / Ru / Co 80 Ni 20

11. Indirect exchange coupling RKKY interaction (Ruderman, Kittel,Kasuya, Yosida). It explains for instance the coupling in rare earth systems Virtually no overlap between magnetic 4f-orbitals Indirect exchange through conduction electrons

12. RKKY interaction distance Spin density

13. RKKY interaction distance Spin density

14. RKKY interaction distance Spin density

15. RKKY interaction distance Spin density

16. RKKY interaction distance Spin density

17. RKKY interaction

21. Giant magnetoresistance Baibich et al., PRL 61,2472 (1988) Magnetic Non magnetic Fe Cr Fe

22. Spinfilter effect E EFEF Paramagnet

23. Spinfilter effect E EFEF Ferromagnet strong scattering weak scattering

24. Giant magnetoresistance r r R R

25. R R r r

27. GMR read head

30. voltage

38. Spin-resolved photoemission spectroscopy on MnPc/Co(001): spin-polarized interface states

39. Insulating spacer layer : tunneling MR P i = polarisation = Jullière’s model

40. De Teresa et al., Science 286 (1999) The polarisation depends on the interface !! STOLSMOCo//ALOLSMOCo/STO//

41. Mn(II)-phthalocyanine : Mn-C 32 H 16 N 8 MnPc Cu(001) Co(001) Advantage: large spin diffusion length expected due to weak spin-orbit coupling in low-Z materials.

42. Photoemission spectroscopy

43. Spin detector Au foil

44. Spin-resolved spectra

45. E EFEF

54. Interface state

55. Difference spectra

58. first layersecond layerthird layer contribution of the different Pc layers to the interface states

59. Polarization of difference spectra

60. Character of interface states Determination of the character by exploiting the variation of the cross section with photon energy. By going from 20 to 100 eV the cross sections change by the following factors: Co 3d: 1.4 Mn 3d: 0.7 C 2p: 1/40 N 2p: 1/20

61. Character of interface states

62. EFEF CoInterface Pc/Co EFEF C. Barraud et al., Nature Phys. (2010)

63. EFEF CoInterface Pc/Co EFEF C. Barraud et al., Nature Phys. (2010)

64. EFEF CoInterface Pc/Co EFEF

65. Electron spin motion: a new tool to study ferromagnetic films

66. M Spin up Spin down

67. x y z P0P0 M

68. x y z M

69. x y z M

70. x y z M

71. x y z ε M

72. M Spin up Spin down

73. x y z ε M

74. ε x y z M

75. ε x y z M

76. ε x y z M

77. ε x y z M 

78. Experiment

79. Spin-dependent band gaps and their influence on the electron-spin motion

80. Typical electronic band structure

81. Theory Experimental results Joly et al., PRL 96, 137206 (2006)

82. Spin-dependent band gaps and their influence on the electron-spin motion Fabry-Pérot experiments with spin-polarized electrons

83. Cu (001) Co Quantum interference

84. Cu (001) Co Quantum interference Cu

85. Cu (001) Co Quantum interference Cu

86. Cu (001) Co Quantum interference Cu

87. Cu (001) Co Quantum interference Cu

88. Experimental results and simulations Joly et al., PRL 97, 187404 (2006), Joly et al., PRB 76, 104415 (2007)

90. Spin-dependent band gaps and their influence on the electron-spin motion Fabry-Pérot experiments with spin-polarized electrons Morphology-induced oscillations of the electron- spin precession

91. Tati Bismaths et al., PRB 77, 220405(R) (2008) Fe/Ag(001)

92. A/B without relaxation at the islands edges

93. A/B with relaxation at the islands edges

94. coverage parameter

95. coverage parameter coverage parameter

96. coverage parameter coverage parameter coverage parameter

97. coverage parameter coverage parameter coverage parameter coverage parameter

98. Spin-dependent band gaps and their influence on the electron-spin motion Fabry-Pérot experiments with spin-polarized electrons Morphology-induced oscillations of the electron- spin precession Influence of sub-monolayer MgO coverages on the spin-dependent reflection properties of Fe

99. T. Berdot et al., PRB 82, 172407 (2010) d (ML)     

100. H.L. Meyerheim et al., Phys. Rev. B 65, 144433 (2002) MgO-induced perpendicular relaxation of the Fe surface

101. MgO-induced normal relaxation of the Fe surface H.L. Meyerheim et al., Phys. Rev. B 65, 144433 (2002)

102. MgO-induced normal relaxation of the Fe surface H.L. Meyerheim et al., Phys. Rev. B 65, 144433 (2002)

108. Ab initio calculations based on linear muffin-tin orbital method (LMTO) and the Korringa-Kohn-Rostoker (KKR) method. Ab initio calculations - 9 ML Fe - First interlayer distance is relaxed without actually putting MgO on top of Fe d=d Fe bulk =1,43 Å d = ? Fe(001)

109. T. Berdot et al., PRB 82, 172407 (2010)

110. Spin-dependent band gaps and their influence on the electron-spin motion Fabry-Pérot experiments with spin-polarized electrons Morphology-induced oscillations of the electron- spin precession Influence of sub-monolayer MgO coverages on the spin-dependent reflection properties of Fe Influence of lattice relaxation on the spin precession in Fe/Ag(001)

111. precession angle (degrees) A. Hallal et al., PRL 107, 087203 (2011)

112. precession angle (degrees) A. Hallal et al., PRL 107, 087203 (2011)

113. precession angle (degrees) A. Hallal et al., PRL 107, 087203 (2011)

114. precession angle (degrees) A. Hallal et al., PRL 107, 087203 (2011)

115. precession angle (degrees) A. Hallal et al., PRL 107, 087203 (2011)

116. precession angle (degrees) A. Hallal et al., PRL 107, 087203 (2011)

117. precession angle (degrees) A. Hallal et al., PRL 107, 087203 (2011)

118. MgO/Fe(001) pseudomorphic growth dislocations

119. Ramsauer-Townsend effect Resonance conditionweak scattering

120. on-resonancescattering phase is zero off-resonancescattering phase is non-zero  Energy    E ex

121. A. Hallal et al., PRL 107, 087203 (2011)

122. Spin-dependent band gaps and their influence on the electron-spin motion Fabry-Pérot experiments with spin-polarized electrons Morphology-induced oscillations of the electron- spin precession Influence of sub-monolayer MgO coverages on the spin-dependent reflection properties of Fe Influence of lattice relaxation on the spin precession in Fe/Ag(001) Organic molecules on ferromagnetic surfaces

128. Laser Polarizer Pockels cell Deflector GaAs Coils Electron optics Sample Coils Electron optics Retarding field analyser Spin detector

129. GaAs : Source of polarized electrons