Se ha denunciado esta presentación.
Utilizamos tu perfil de LinkedIn y tus datos de actividad para personalizar los anuncios y mostrarte publicidad más relevante. Puedes cambiar tus preferencias de publicidad en cualquier momento.

2018.06.12 javier tejada ub NanoFrontMag

27 visualizaciones

Publicado el

II Jornada científica NanoFrontMag - 12 de junio de 2018 - IMDEA Nanociencia
Javier Tejada
Universidad de Barcelona

Publicado en: Ciencias
  • Sé el primero en comentar

  • Sé el primero en recomendar esto

2018.06.12 javier tejada ub NanoFrontMag

  1. 1. Experiments on Quantum and Classical magnetization reversal process in nanomagnets MADRID 2018 Professor Javier Tejada. Dept. Física Fonamental, Universitat de Barcelona.
  2. 2. Contenidos Single Domain Particles Molecular Magnets Quantum magnetic deflagration Superradiance Nanospheres and Mg-12 acetate triclinic Effects of SAW: Classical reversal Conclusions Content
  3. 3. TítuloMacroscopic solid to single domain particles 53 1010 −≈ an ex E E • Domains and domain walls: • Tipically • If the particle has then no domain wall can be formed. This is a SDP: • The probabilty of an individual spin flip is: with The exchange energy is so high that it is difficult to do any non-uniform rotation of the magnetization ( ). Hence, at low T, the magnetic moment is a vector of constant modulus:
  4. 4. Single domain particles (SDP) • Classical description: energy barrier of height • Blocking temperature is defined via the condition which leads to: Analogously, we can also define the Blocking volume: Anisotropy constant Volume ( )TVU e /)(− =Γ ν U mt/1=Γ ( )mB tKVT νln/= ( )mB t K T V νln= Microscopic attempt frequency
  5. 5. Important aspects of SDPs
  6. 6. • The particles relax toward the equilibrium state: SDP: magnetic relaxation Initial remanent magnetization • The dependence of S on T shows two different regimes: 1) Thermal regime: at high temperatures it is easier to “jump” the barrier. In this regime, 2) Quantum regime: at low temperatures, magnetic relaxation is due to tunnel effect. In this regime S is of T. ( ) ( ) ( )( )tSMtM RR νln10 −=
  7. 7. Quantum Classical Empirically, the magnetic moment is considered to behave quantumly if |M| ≤ 100μB holds. Quantum magnetic entities • Their magnetic moment M is a quantum operator: it verifies the commutation relation which yields
  8. 8. [ ] 0,ˆ ,ˆˆ 10Spin 2 ≠ +−= = ⊥ ⊥ z z SH HDSΗ S Molecular Magnets (MM): example of Mn12 acetate Quantum counterpart of a SDP. Discrete projection of the spin onto the easy axis. -3/2 +2 -3/2-3/2 -3/2 +2 +2 +2 +2 +2 +2 +2
  9. 9. Magnetic bistability of Mn12 acetate Degenerate ground states for the Mn12 acetate molecule. There exists an anisotropy energy barrier between these two spin orientations. The effect of an external magnetic field applied along the easy axis.
  10. 10. Título • Application of an external field: adds a Zeeman term Longitudinal component of the field (H // easy axis) Shifts the levels. Transverse component of the field (H easy axis) Allows tunnel effect. • The tunnel effect is possible for certain values of the field: the resonant fields. Resonant spin tunneling on MM
  11. 11. -10 -9 -8 -7 -6 -5 -4 -3 -2-10 1 2 3 4 5 6 7 8 9 10 H=0 Magnetic field Resonant spin tunneling on MM
  12. 12. -10 -9 -8 -7 -6 -5 -4 -3-2-10 1 2 3 4 5 6 7 8 9 10 H = 0.5HR Magnetic field Resonant spin tunneling on MM
  13. 13. H = HR -10 -9 -8 -7 -6 -5 -4 -3-2 1 2 3 4 5 6 7 8 9 10 Magnetic field Resonant spin tunneling on MM
  14. 14. -10 -9 -8 -7 -6 -5 -4 -3-2-10 1 2 3 4 5 6 7 8 9 10 H = 2HR Magnetic field Resonant spin tunneling on MM
  15. 15. • As we only have a single barrier height, relaxation goes exponential. Relaxation MM Mn12 Ac relaxation measurements from the remanent state at different temperatures. ( ) ( ) ( )[ ] ( )tH eq eTMtM Γ− −= 1
  16. 16. • Peaks of the relaxation rate Γ(H) at the resonant fields Relaxation MM Relaxation rates of Mn12 acetate at different fields
  17. 17. Landau-Zener effect m 'm tW EEW mm ν= −= ' m m m'm 'm 'm 'm m +E −E 22 ∆+=− −+ WEE Permanence probability Size of magnetization step νπ h2/2 ∆− = eP P−∝1
  18. 18. TítuloResonant spin tunneling on MM
  19. 19. 19 Energy released  ∆E Ignition (barrier overcoming)  ∆U Thermal diffusion  k Characteristic length of propagation  δ Two important characteristic timescales: • Thermal diffusion • Burning timescale Metastable State ∆ U Deflagration is a technical term describing subsonic combustion that usually propagates through thermal conductivity ∆ E Stable State 19 τb= τd Deflagration Flame width What is a deflagration?
  20. 20. 20 Molecule magnets Field jumps 1999 Deflagration-like description 2005 20 From magnetization jumps to magnetic Deflagration (MD)
  21. 21. Magnetic deflagration: Propagation of a front of reversing spins at constant velocity along the crystal Problem: Sweeping H we cannot control the magnetic field at which it occurs. Y. Suzuki et. al. PRL 95, 147201 (2005) A. Hernández-Mínguez et. al. PRL 95 17205 (2005) H ΔE First evidences of MD
  22. 22. Avalanche ignition produced by SAW: IDT LiNbO3 substrate Conducting stripes Coaxial cable Mn12 crystal c-axis Hz Coaxial cable connected to an Agilent microwave signal generator Change in magnetic moment registered in a rf-SQUID magnetometer Surface Acoustic Waves (SAW) are low frequency acoustic phonons (below 1 GHz) Quantum magnetic deflagration
  23. 23. • The speed of the avalanche increases with the applied magnetic field • At resonant fields the velocity of the flame front presents peaks. • The ignition time shows peaks at the magnetic fields at which spin levels become resonant.         −= fB0 T2k U(H) exp τ κ v This velocity is well fitted: κ = 0.8·10-5 m2/s Tf (H = 4600 Oe) = 6.8 K Tf (H = 9200 Oe) = 10.9 K Quantum magnetic deflagration
  24. 24. Quantum Magnetic Deflagration
  25. 25. Quantum Magnetic Deflagration
  26. 26. I t τ1 Luminescence Superradiance I t τSR This kind of emission (SR) has characteristic properties that make it different from other more common phenomena like luminescence L λL ~ λ Superradiance Proposed by Robert H. Dicke in 1954. NI ∝ 2 NI ∝ N is the number of dipoles
  27. 27. All spins decay to the fundamental level coherently, with the emission of photons. -10 -9 -8 -7 -6 -5 -4-3-2-1012 3 4 5 6 7 8 9 10 B = 2B0 Superradiance
  28. 28. Superradiance?? Sharp peak shows a signal which is equivalent to the sample being at 20 K (the expected self heating is about 3 K).
  29. 29. Magnetic deflagration in pulsed fields Δt dB/dt (kT/s) 1.61.92.53.23.74.87.0 200 400 0 2 4 6 dM/dt t (s) coil 1 coil 2
  30. 30. 1000 2000 3000 4000 5000 6000 7000 -2 0 2 4 6 8 10 12 14 16 Time-difference between the observation of a magnetisation-reversal in a coil on the left and on the right of a Mn12Ac-sample in function of (high) magnetic field-sweeprates. All observations were done in pulsed fields at a temperature of about 500mK (in liquid 3He). ∆t(µs) dHz /dt (T/s) Quantum magnetic detonation t∆
  31. 31. S. Lendínez et al., PRB 91, 024404 (2015) Resonant spin tunneling in Mn12-acetate nanospheres Randomly oriented nanospheres (nanospheres)
  32. 32. Resonant spin tunneling in Mn12-acetate S. Lendínez et al., PRB 91, 024404 (2015) Randomly oriented powder
  33. 33. Derivatives along descending branches 1/H2 scaling Derivatives vs. 1/H2 along descending branches S. Lendínez et al., PRB 91, 024404 (2015) Resonant spin tunneling in Mn12-acetate Randomly oriented powder
  34. 34. FE-SEM image average length = 3.2 ± 1.9 µm average width = 0.8 ± 0.3 µm triclinic crystallographic structure (ALBA synchrotron beamline) Resonant spin tunneling in Mn12-acetate Ribbon-shaped particles J.Espin et al.,JACS (2016)
  35. 35. Resonant spin tunneling in Mn12-acetate Ribbon-shaped particles The magnetic moments (arrows) do not form chains along a single magnetic anisotropy axis
  36. 36. Resonant spin tunneling in Mn12-acetate Ribbon-shaped particles I. Imaz et al. JACS (2016),
  37. 37. Resonant spin tunneling in Mn12-acetate Ribbon-shaped particles I. Imaz et al.,JACS (2016)
  38. 38. m 'm tW EEW mm ν= −= ' m m m'm 'm 'm 'm m +E −E 22 ∆+=− −+ WEE Staying probability Size of magnetization step νπ h2/2 ∆− = eP P−∝1 Resonant spin tunneling in Mn12-acetate Ribbon-shaped particles Landau-Zener effect
  39. 39. Resonant spin tunneling in Mn12-acetate Ribbon-shaped particles J.Espin et al.,JACS(2016) Landau-Zener effect
  40. 40. EFFECTS OF SAW: CLASSICAL REVERSAL Fe3O4 nanoparticles ZFC TEM
  41. 41. EFFECTS OF SAW: CLASSICAL REVERSAL
  42. 42. EFFECTS OF SAW: CLASSICAL REVERSAL
  43. 43. EFFECTS OF SAW: CLASSICAL REVERSAL
  44. 44. • Energy barrier between opposite orientations of the magnetic moment is formed by weak relativistic interactions whether stable molecular magnets can ever break liquid nitrogen temperature of 77K. • Making identical molecules comparable to mesoscopic magnetic particles will be a challenging task for chemists. • Another challenging question would be whether magnetic molecules can ever become ultimate memory units of conventional computers or even elements of quantum computers. • We have demonstrated experimentally that surface acoustic waves accelerate transitions between magnetic states of magnetite nanoparticles. • The Einstein–de Haas mechanism of the magnetization reversal by SAWs comes from the shear deformation of the substrate and the resulting rotation of the particle as a whole when its size is small compared to the SAW wavelength. • I hope to see answers to these questions in the near future!! Future
  45. 45. References [1] E. M. Chudnovsky, J. Tejada, Macroscopic Quantum Tunneling of the Magnetic Moment (Cambridge Univ. Press, 1998). [2] E. M. Chudnovsky, J. Tejada, Lectures on Magnetism (Rinton Press, 2006). [3] J.R. Friedman, M.P. Sarachik, J. Tejada and R. Ziolo. Phys. Rev. Lett. 76, 3830–3833 (1996). [4] A. Hernández-Mínguez et al. Phys. Rev. Lett. 95, 217205 (2005). [5] Macià et al. Phys. Rev. B 76, 174424 (2007). [6] Macià et al. Phys. Rev. B 77, 012403 (2008). [7] F. Macià et al. Phys. Rev. B 79, 092403 (2009). [8] S. Vélez et al. Phys. Rev. B 81, 064437 (2010). [9] W. Decelle et al. Phys. Rev. Lett. 102, 027203 (2009). [10] P. Subedi et al. Phys. Rev. Lett. 110, 207203 (2013). Physics 6, 55 (2013). [11] R. Zarzuela, S. Vélez, J.M. Hernandez, J. Tejada and V. Novosad. Phys. Rev. B 85, 180401(R) (2012). [12] R. Zarzuela, E. M. Chudnovsky, and J. Tejada. Phys. Rev. B 87, 014413 (2013). [13] R. Zarzuela, E. M. Chudnovsky, J. M. Hernandez, and J. Tejada. Phys. Rev. B 87, 144420 (2013). [14] E.M. Chudnovsky, S. Vélez, A. García-Santiago, J.M. Hernandez and J. Tejada. Phys. Rev. B 83, 064507 (2011). [15] J. Tejada, E. M. Chudnovsky, R. Zarzuela, N. Statuto, J. Calvo-de la Rosa, P. V. Santos and A. Hernández-Mínguez. EPL, 118, 37005 (2017)

×