Nanowires with very different size, shape, morphology and crystal symmetry can give rise to a wide ensemble of magnetic behaviors whose optimization determines their applications in nanomagnets. We present here an experimental work on the shape and m
orphological dependence of the magnetization reversal mechanism in weakly interacting Co80Ni20 hexagonal-close-packed nanowires. Non-agglomerated nanowires (with length L and diameter d) with a controlled shape going from quasi perfect cylinders to diabolos, have been studied inside their polyol solution in order to avoid any oxidation process. The coercive field HC was found to follow a standard behavior and to be optimized for an aspect ratio L/d > 15. Interestingly, an unexpected behavior was observed as function of the head morphology leading to the strange situation where a diabolo shaped nanowire is a better nanomagnet than a cylinder. This paradoxical behavior can be ascribed to the growth-competition between the aspect ratio L/d and the head morphology ratio d/D (D being the head width). Our experimental results clearly show the importance of the independent parameter (t = head thickness) that needs to be considered in addition to the shape aspect ratio (L/d) in order to fully describe the nanomagnets magnetic behavior. Micromagnetic simulations well support the experimental results and bring important insights for future optimization of the nanomagnets morphology
Driving a two-dimensional superconductor normal by applying a high magnetic field may lead to Cooper pair localization. In this case, there should be a quantum critical point associated with specific scaling laws. Such a transition has been evidenced
in a number of low critical temperature superconducting thin films and has been suggested to occur also in high temperature cuprate superconductors. Here we show experimental evidence for two distinct quantum critical regimes when applying perpendicular magnetic fields to underdoped La2-xSrxCuO4 thin films. At intermediate values of the magnetic field (18T-20T), a ghost QCP is observed, for which the values of the related critical exponents point towards a fermionic -as opposed to bosonic- scenario. At higher (about 37 T) magnetic field, another QCP is observed, which suggests the existence of either a 2D/3D or a clean/dirty temperature crossover.
The magnetic field driven superconductor/insulator transition is studied in a large variety of $La_{2-x}Sr_xCuO_4$ thin films of various Sr dopings. Temperature dependence of the resistivity down to 4.2 or 1.5 K under high pulsed magnetic field (up t
o 57 T) is analyzed. In particular, the existence of plateaus in the resistance versus temperature curves, in a limited range of temperature, for given values of the magnetic field is carefully investigated. It is shown to be associated to scaling behaviour of the resistance versus magnetic field curves, evocative of the presence of a quantum critical point. A three-dimensional (H,x,T) phase diagram is proposed, taking into account the intrinsic lamellar nature of the materials by the existence of a temperature crossover from quantum-two-dimensional to three-dimensional behavior.
