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The Magnetoelastic Paradox

The Magnetoelastic Paradox. M. Rotter , A. Barcza, IPC, Universität Wien, Austria H. Michor, TU-Wien, Austria A. Lindbaum, FH-Linz, Austria M. Doerr, M. Loewenhaupt, IFP TU-Dresden, Germany B. Beuneu, LLB – Saclay, France M el Massalami, UFRJ, Brazil. Magnetostriction Measurements

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The Magnetoelastic Paradox

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  1. The Magnetoelastic Paradox M. Rotter, A. Barcza, IPC, Universität Wien, Austria H. Michor, TU-Wien, Austria A. Lindbaum, FH-Linz, Austria M. Doerr, M. Loewenhaupt, IFP TU-Dresden, Germany B. Beuneu, LLB – Saclay, France M el Massalami, UFRJ, Brazil

  2. Magnetostriction Measurements • Magnetostriction in the Standard Model of Rare Earth Magnetism • The Magnetoelastic Paradox (MEP) • Experimental Evidence for the MEP in Gd Compounds • Application of Magnetic Fields - the case of GdNi2B2C • Outlook M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  3. Magnetostriction Measurements Experimental Methods X-ray Powder Diffraction Capacitance Dilatometry • Anisotropic Effects on • Polycrystals (Expansion, • Symmetry-Changes) • bad resolution (10-4 in dl/l) • Good resolution (10-9 in dl/l) • 45 T Magnetic Fields - forced magnetostriction • requires single crystals 1cm Rotter et.al. Rev. Sci. Instr. 69 (1998) 2742 M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  4. GdRu2Si2 (008) Gd Ru Si M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  5. GdRu2Si2 (202) (220) ? ? No sign of distortion of the tetragonal plane ! M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  6. Spontaneous Magnetostriction STANDARD MODEL OF RARE EARTH MAGNETISM Microscopic Origin of Magnetostriction: Strain dependence of magnetic interactions Crystal Field Exchange T T L0 L=0, L0 T<TC(N) + T<TC(N) T>TC(N) e- „exchange-striction“ + Gd3+, S=7/2, L=0 M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  7. Exchange striction on a Square Lattice J1 J1 Ferromagnet: J1>0 dV/V<0 No distortion (dJ1/de) M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  8. J1 J1 Anti-Ferromagnet With small |J1| J2<0 dV/V=0 J2 J2 Tetragonal Distortion (dJ1/de) !!! J1 J1 THE MAGNETOELASTIC PARADOX Antiferromagnets with L=0 below TN: Symmetry breaking distortions are expected but have NOT been found Anti-Ferromagnet with NN exchange: J1<0 dV/V>0 No distortion (dJ1/de) .... but in ALL experiments: distortion  <10-4 M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  9. GdCuSn TN= 24 K q=(0 ½ 0) M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  10. GdAg2 TN= 22.7 K <TR1=21.2K M||[001] <TR2=10.8K M||[110] GdAu2 TN= 50 K q=(0.362 0 1) M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  11. Gd3Ni Gd3Rh TN=112 K TN=100 K Large magnetostrictive effects on lattice constants – but NO distortion M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  12. Volume Magnetostriction Spontaneous Magnetoelastic Effects in Gd Compounds A. Lindbaum, M. Rotter Handbook of Magnetic Materials Vol 14 (Buschow, Elsivier,NL) M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  13. Anisotropic Spontaneous Magnetostriction Ferromagnet Antiferromagnet ε TC(N)[K] Spontaneous Magnetoelastic Effects in Gd Compounds A. Lindbaum, M. Rotter Handbook of Magnetic Materials Vol 14 (Buschow, Elsivier,NL) M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  14. GdNi2B2C ? TN= 20 K: M||[010] <TR= 14 K: M||[0yz] q = (0.55 0 0) small magnetostriction, therefore cap.-dilatometry .... M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  15. GdNi2B2C 2T||a 1.5T TN Orthorh. distortion ! 0.75T 0T 5 10 15 20 25 T (K) Thermal Expansion Forced Magnetostriction Da/a TN= 20 K: M||[010] <TR= 14 K: M||[0yz] q = (0.55 0 0) 10-4 M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  16. GdNi2B2C .... FWHM determined by fitting distortion e=3x10-4 would lead to FWHM (204)+ 0.1° FWHM (211)+ 0.05° at H=0 no distortion can be found ? At H=0: Domains ? Powder Xray Diffraction M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  17. McPhase-theWorldofRareEarthMagnetism McPhase is a program package for the calculation of magnetic properties of rare earth based systems.           Magnetization                       Magnetic Phasediagrams     Magnetic Structures  Elastic/Inelastic/Diffuse                                              Neutron Scattering                                             Cross Section www.mcphase.de M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  18. The magnetic Hamiltonian Isotropic exchange (RKKY,...) Classical Dipole Interaction Zeeman Energy M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  19. Hmag + McPhase ? T=2 K

  20. The Magnetoelastic Paradox for L=0.... demonstrated at GdNi2B2C Orthorhombic Distortion ? Exchange Striction Model Capacitance Dilatometry Standard Model of RE Mag ... McPhase Simulation M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  21. ToDo New Methods • Imaging of AFM domains • with XRMS GdNi2Ge2ab-plane T = 17 K 200 µm Moment direction • Anisotropy Measurements • by ESR Neutron Scattering • Transmutation of Gd • More Experiments • Powder X-ray Diffraction • Magnetic Neutron / X-ray Scattering • Dilatometry in high Fields • More Theory • Apply Standard model of RE Magnetism • Ab initio Calculation on MEP M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  22. Normal thermal Expansion Anharmonicity of lattice dynamics anharmonicPotential Harmonic potential with Debye function + Small contribution of band electrons

  23. Forced Magnetostriction Crystal Field Exchange - Striction L0 L=0, L0 H <0 H + e- H >0 + Gd3+, S=7/2, L=0 M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

  24. Theory of Magnetostriction Crystal field Exchange with + M.Rotter „The Magnetoelastic Paradox“ Planneralm 2006

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