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

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

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

Magnetic domain-wall motion study under an electric field in a Finemet thin film on flexible substrate

Influence of applied in-plane elastic strains on the static magnetic configuration of a 530 nm magnetostrictive FeCuNbSiB thin film. The in-plane strains are induced via the application of a voltage to a piezoelectric actuator on which the film/substrate system was glued. A quantitative characterization of the voltage dependence of the induced-strain at the surface of the film was performed using a digital image correlation technique. MFM images at remanence (H=0 Oe and U=0 V) clearly reveal the presence of weak stripe domains. The effect of the voltage-induced strain shows the existence of a threshold value above, which the break of the stripe configuration set in. For a maximum strain of exx~0.5*10-3 we succeed in destabilizing the stripes configuration helping the setting up of a complete homogeneous magnetic pattern.Comment: 5 pages, 4 figures, to appear in J. Mag. Mag. Ma

Non-linear effects and Joule heating in I-V curves in manganites

We study the influence of the Joule effect on the non-linear behavior of the transport I-V curves in polycrystalline samples of the manganite Pr0.8Ca0.2MnO3 by using the crystalline unit cell parameters as an internal thermometer in X-ray and neutron diffraction. We develop a simple analytical model to estimate the temperature profile in the samples. Under the actual experimental conditions we show that the internal temperature gradient or the difference between the temperature of the sample and that of the thermal bath are at the origin of the non-linearity observed in the I-V curves. Consequences on other compounds with colossal magnetoresistance are also discussed.Comment: accepted in Journal of Applied Physic

Field dependence of the electronic phase separation in Pr0.67Ca0.33MnO3 by small angle magnetic neutron scattering

We have studied by small angle neutron scattering the evolution induced by the application of magnetic field of the coexistence of ferromagnetism (F) and antiferromagnetism (AF) in a crystal of Pr$_{0.67}$Ca$_{0.33}$MnO$_3$. The results are compared to magnetic measurements which provide the evolution of the ferromagnetic fraction. These results show that the growth of the ferromagnetic phase corresponds to an increase of the thickness of the ferromagnetic ''cabbage'' sheets

Spin coating and micro-patterning optimization of composite thin films based on PVDF

We optimize the elaboration of very thin film of poly(vinylidene fluoride) (PVDF) polymer presenting a well-controlled thickness, roughness, and nano-inclusions amount. We focused our effiort on the spin coating elaboration technique which is easy to transfer to an industrial process. We show that it is possible to obtain continuous and smooth thin films with mean thicknesses of 90 nm by properly adjusting the concentration and the viscosity of the PVDF solution as well as the spin rate and the substrate temperature of the elaboration process. The electro-active phase content versus the magnetic and structural properties of the composite films is reported and fully discussed. Last but not least, micro-patterning optical lithography combined with plasma etching has been used to obtain well-defined one-dimensional micro-stripes as well as squared-rings, demonstrating the easy-to-transfer silicon technology to polymer-based devices

Anomaly in the dielectric response at the charge orbital ordering transition of crystalline Pr0.67Ca0.33MnO3

The complex impedance of a Pr0.67Ca0.33MnO3 crystal has been measured. The frequency dependence is studied for a wide range of temperatures (50K-403K) and is found to be characteristic of relaxation process with a single Debye time relaxation constant, which is interpreted as a dielectric constant of the material. A strong peak is observed in this dielectric constant (up to two millions) at the charge ordering transition suggesting an interpretation in terms of ordering of electric dipoles at TCO or in term of phase separation. Comparison with Pr0.63Ca0.37MnO3 - in which the phase separation is much smaller and the peak in the dielectric constant is absent - suggests an interpretation in term of phase separation between insulating and metallic states.Comment: pdf fil

Synthesis and characterisation of the vibrational and electrical properties of antiferromagnetic 6L-Ba2CoTeO6 ceramics

Optimal processing conditions for fabrication of dense single-phase 6L-Ba2CoTeO6 ceramics via the solid-state reaction method were determined. These ceramics possess a room-temperature crystal structure described by the centrosymmetric P[3 with combining macron]m1 space group. Polarized Raman spectroscopy enabled the observation of all the 25 predicted Raman modes and assignment of their symmetries. On cooling, BCTO ceramics exhibit two antiferromagnetic transitions at 3 K and 12.5 K, in broad agreement with a recent single-crystal study [P. Chanlert, N. Kurita, H. Tanaka, D. Goto, A. Matsuo and K. Kindo, Phys. Rev. B: Condens. Matter Mater. Phys., 2016, 93, 094420]. Low temperature Fourier-transform infrared reflectivity analyses suggest the antiferromagnetic phase transitions to be driven by small distortions of the CoO6 octahedra, lowering locally their C3v symmetry. This causes splitting of the associated vibrational modes, but without a long-range structural change. AC impedance spectroscopy revealed BCTO ceramics to be leaky insulators with an activation energy for conduction of ∼0.15−0.25 eV, which suggests electron hopping between mixed oxidation states of Co