Publications

2001

• Magnetization process in FePd thin films
  
  • Klein O.
  • Samson Y.
  • Marty A.
  • Guillous S.
  • Viret M.
  • Fermon C.
  • Alloul H.
  Journal of Applied Physics, American Institute of Physics, 2001, 89 (11), pp.6781-6783. A custom made magnetic force microscope is used to study the magnetization process in thin films of FePd throughout the entire hysteresis loop. The 40 nm thick sample has a strong perpendicular anisotropy, which leads to a maze of 80 nm wide stripes of opposite polarity in the remanent state. The growth of M, when H increases, happens through an unwinding of the reversed domain along their axis. Together with the length recession, the reversed domain width also contracts with increasing field. The later effect is estimated by comparison of our images with magneto-optical Kerr measurements. A large disorder in the propagation process of the domain walls is observed. It is also found that the bubble configuration near the saturation field is unstable. (10.1063/1.1355326)
  DOI : 10.1063/1.1355326
• Inverse' melting of a vortex lattice
  
  • Avraham Nurit
  • Khaykovich B.
  • Myasoedov Yuri
  • Rappaport Michael
  • Shtrikman Hadas
  • Feldman D.E.
  • Tamegai Tsuyoshi
  • Kes Peter H.
  • Li Ming
  • Konczykowski M.
  • van Der Beek C.J.
  • Zeldov Eli
  Nature, Nature Publishing Group, 2001, 411 (6836), pp.451-454. Inverse melting is the process in which a crystal reversibly transforms into a liquid or amorphous phase when its temperature is decreased. Such a process is considered to be very rare(1), and the search for it is often hampered by the formation of non-equilibrium states or intermediate phases(2). Here we report the discovery of first-order inverse melting of the lattice formed by magnetic flux lines in a high-temperature superconductor. At low temperatures, disorder in the material pins the vortices, preventing the observation of their equilibrium properties and therefore the determination of whether a phase transition occurs. But by using a technique(3) to 'dither' the vortices, we were able to equilibrate the lattice, which enabled us to obtain direct thermodynamic evidence of inverse melting of the ordered lattice into a disordered vortex phase as the temperature is decreased. The ordered lattice has larger entropy than the low-temperature disordered phase. The mechanism of the first-order phase transition changes gradually from thermally induced melting at high temperatures to a disorder-induced transition at low temperatures. (10.1038/35078021)
  DOI : 10.1038/35078021
• Magnetization reversal by uniform rotation (Stoner-Wohlfarth model) in f.c.c. cobalt nanoparticles
  
  • Wernsdorfer Wolfgang
  • Thirion Christophe
  • Demoncy N.
  • Pascard H.
  • Mailly D.
  , 2001. The combination of high sensitive superconducting quantum interference device (SQUID) with high quality nanoparticles allowed to check the simplest classical model describing the magnetisation reversal by uniform rotation which were proposed more than 50 years ago by Neel, Stoner and Wohlfarth. The micrometer sized SQUIDs were elaborated by electron beam lithography and the nanoparticles were synthesised by arc-discharge. The measured angular dependence of switching fields of nearly all f.c.c. Co nanoparticles revealed a dominating uniaxial magnetic anisotropy. This result suggests that twin boundaries and stacking faults strongly alter the cubic magnetocrystalline anisotropy leading to dominating uniaxial anisotropy. However, few particles were sufficiently \"perfect\" in order to show a more complex switching field surface and a field path dependence of the switching field which is the important signature of the cubic magnetocrystalline anisotropy.
• Dose, dose rate and irradiation temperature effects in β-irradiated simplified nuclear waste glasses by EPR spectroscopy.
  
  • Boizot B.
  • Petite G.
  • Ghaleb D.
  • Calas G.
  Journal of Non-Crystalline Solids, Elsevier, 2001, 283, pp.179-185. The influence of dose, dose rate and irradiation or annealing temperature on the formation of paramagnetic centers has been investigated by electron paramagnetic resonance (EPR) spectroscopy on a series of β-irradiated 4-, 5- and 6-oxide glasses in order to study the use of external β-irradiation relative to simulate actual conditions during the storage of nuclear waste glasses. An unusual saturation of defect concentration occurs above 104 Gy. There is no dose rate dependence for the irradiation in a range between 2×103 and 2×104 Gy/s. The increase of irradiation temperature leads to an increase of Zr3+, a decrease of Fe3+ concentration, the formation of magnetic clusters and a strong decrease in the total concentration of paramagnetic defects. In addition, annealing and irradiation temperatures do not produce the same effects on the concentration of the various defects. (10.1016/S0022-3093(01)00338-6)
  DOI : 10.1016/S0022-3093(01)00338-6
• Symmetry properties of two-dimensional photonic crystals
  
  • Botti Silvana
  , 2001, pp.71.
• Electronic states and optical properties of GaAs/AlAs and GaAs/vacuum superlattices by the linear combination of bulk bands method
  
  • Botti Silvana
  Physical Review B: Condensed Matter and Materials Physics (1998-2015), American Physical Society, 2001, 63, pp.235313. The linear combination of bulk bands method recently introduced by Wang, Franceschetti, and Zunger [Phys. Rev. Lett. 78, 2819 (1997)] is applied to a calculation of energy bands and optical constants of (GaAs)n/(AlAs)n and (GaAs)n/(vacuum)n (001) superlattices with n ranging from 4 to 20. Empirical pseudopotentials are used for the calculation of the bulk energy bands. Quantum-confinement-induced shifts of critical point energies are calculated and are found to be larger for the GaAs/vacuum system. The E1 peak in the absorption spectra has a blueshift and splits into two peaks for decreasing superlattice period; the E2 transition instead is found to be split for large-period GaAs/AlAs superlattices. The band contribution to linear birefringence of GaAs/AlAs superlattices is calculated and compared with recent experimental results of Sirenko et al. [Phys. Rev. B 60, 8253 (1999)]. The frequency-dependent part reproduces the observed increase with decreasing superlattice period, while the calculated zero-frequency birefringence does not account for the experimental results and points to the importance of local-field effects. (10.1103/PhysRevB.63.235313)
  DOI : 10.1103/PhysRevB.63.235313