Magnetization reversal behavior in complex shaped Co nanowires: a nanomagnet morphology optimization

A systematic micromagnetic study of the morphological characteristic effects over the magnetic static properties of Co-based complex shaped nanowires is presented. The relevance of each characteristic size (i.e. length L, diameter d, and size of the nanowires head T) and their critical values are discussed in the coercive field optimization goal. Our results strongly confirms that once the aspect ratio (L/d) of the nanowire is bigger than around 10, the length is no more the pertinent parameter and instead the internal diameter and the shape of the nanowires play a key role. We attribute this behavior to the non uniform distribution of the demagnetizing field which is localized in the nanowires head and acts as a nucleation point for the incoherent magnetization reversal. Finally, angular dependence of the magnetization are simulated and compared to the case of a prolate spheroid for all considered morphologies.Comment: 7 pages, 6 figure

Voltage-induced strain control of the magnetic anisotropy in a Ni thin film on flexible substrate

Voltage-induced magnetic anisotropy has been quantitatively studied in polycrystalline Ni thin film deposited on flexible substrate using microstrip ferromagnetic resonance. This anisotropy is induced by a piezoelectric actuator on which the film/substrate system was glued. In our work, the control of the anisotropy through the applied elastic strains is facilitated by the compliant elastic behavior of the substrate. The in-plane strains in the film induced by the piezoelectric actuation have been measured by the digital image correlation technique. Non-linear variation of the resonance field as function of the applied voltage is found and well reproduced by taking into account the non linear and hysteretic variations of the induced in-plane strains as function of the applied voltage. Moreover, we show that initial uniaxial anisotropy attributed to compliant substrate curvature is fully compensated by the voltage induced anisotropy.Comment: 7 pages, 6 figures, published in the Journal of Applied Physic

Current-driven magnetization decrease in single crystalline ferromagnetic manganese oxide

The electrical and magnetic response to a bias current has been investigated in a singlecrystalline ferromagnetic manganese oxide $\Pr_{0.8}$Ca$_{0.2}$MnO$_3$ . A significant decrease of the magnetization is observed at the same threshold current where a non-linearity of V-I characteristics appears. Such a behavior cannot be understood in the framework of the filamentary picture usually invoked for the non linearity of the other manganese oxides. Instead, an analogy with spintronic features might be useful and experimental signatures seem to be in agreement with excitations of spin waves by an electric current. This provides an example of a bulk system in which the spin polarized current induces a macroscopic change in the magnetization.Comment: 3 pages, 4 figure

Morphology control of the magnetization reversal mechanism in Co80Ni20 nanomagnets

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 morphological 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 morphologyComment: 7 pages, 5 figure

Control of the colossal magnetoresistance by strain effect in Nd0.5_{0.5}Ca0.5_{0.5}MnO3_{3} thin films

Thin films of Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ manganites with colossal magnetoresistance (CMR) properties have been synthesized by the Pulsed Laser Deposition technique on (100)-SrTiO$_{3}$. The lattice parameters of these manganites and correlatively their CMR properties can be controlled by the substrate temperature $T_{S}$. The maximum CMR effect at 75K, calculated as the ratio $\rho (H=0T)/\rho (H=7T)$ is 10$^4$ for a deposition temperature of $T_{S}=680$ degC. Structural studies show that the Nd$_{0.5}$Ca$_{0.5}$MnO$_{3}$ film is single phase, [010]-oriented and has a pseudocubic symmetry of the perovskite subcell with a=3.77$\AA$ at room temperature. We suggest that correlation between lattice parameters, CMR and substrate temperature $T_{S}$ result mainly from substrate-induced strains which can weaken the charge-ordered state at low temperature.Comment: 9 pages, 4 figures. To be published in Applied Physics Letter

Micro-strip ferromagnetic resonance study of strain-induced anisotropy in amorphous FeCuNbSiB film on flexible substrate

The magnetic anisotropy of a FeCuNbSiB (Finemet) film deposited on Kapton has been studied by micro-strip ferromagnetic resonance technique. We have shown that the flexibility of the substrate allows a good transmission of elastic strains generated by a piezoelectric actuator. Following the resonance field angular dependence, we also demonstrate the possibility of controlling the magnetic anisotropy of the film by applying relatively small voltages to the actuator. Moreover, a suitable model taking into account the effective elastic strains measured by digital image correlation and the effective elastic coefficients measured by Brillouin light scattering, allowed to deduce the magnetostrictive coefficient. This latter was found to be positive $(\lambda=16\times10^{-6}$) and consistent with the usually reported values for bulk amorphous FeCuNbSiB.Comment: 9 pages, 8 figure

Microphase separation in Pr0.67Ca0.33MnO3 by small angle neutron scattering

We have evidenced by small angle neutron scattering at low temperature the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in Pr0.67Ca0.33MnO3. The results are compared to those obtained in Pr0.80Ca0.20MnO3 and Pr0.63Ca0.37MnO3, which are F and AF respectively. Quantitative analysis shows that the small angle scattering is not due to a mesoscopic mixing but to a nanoscopic electronic and magnetic ''red cabbage'' structure, in which the ferromagnetic phase exists in form of thin layers in the AF matrix (stripes or 2D ''sheets'').Comment: 4 figure

Irreversible Effects in LaGa1-xMnxO3

Quasi-irreversible increase in the electrical conductivity is observed in single crystals of LaGa1-xMnxO3. The effect lasts for long time at room temperature and can be erased by heating of the crystal above the phase transition temperature. We explain the observed effects in terms of ionization and local lattice distortion processes.Comment: 8 pages, 4 figure