Papers by Chiara Pernechele
We report on the synthesis and characterization of BiFe 0.5 Mn 0.5 O 3 , a potential type-I multi... more We report on the synthesis and characterization of BiFe 0.5 Mn 0.5 O 3 , a potential type-I multiferroic compound displaying temperature-induced magnetization reversal. Bulk samples were obtained by means of solid-state reaction carried out under the application of hydrostatic pressure of 6 GPa at 1100 • C. The crystal structure is a highly distorted perovskite with no cation order on the B site, where, besides a complex scheme of tilt and rotations of the TM-O 6 octahedra, large off-centering of the bismuth ions is detected. Below T 1 = 420 K the compound undergoes a first weak ferromagnetic transition related to the ordering of iron-rich clusters. At lower temperatures (just below RT) a complex thermally activated mechanism induces at first an enhancement of the magnetization at T 2 = 288 K, then a spontaneous reversal giving rise to a negative response. The complementary use of powder neutron diffraction, superconducting quantum interference device magnetometry, and Mössbauer spectroscopy allowed us to propose as a possible interpretation of the overall magnetic behavior the presence of an uncompensated competitive coupling between nonequivalent clusters of weakly ferromagnetic interactions characterized by different critical temperatures and resultant magnetizations.
were synthesized via sol-gel method, with calcination processes at 400 and 500 in air. The effect... more were synthesized via sol-gel method, with calcination processes at 400 and 500 in air. The effects of the different calcinations in the formation of the crystal structure and magnetic and optical properties were studied with X-ray diffraction (XRD), electron microscopy, SQUID magnetometer, and optical absorption. The XRD studies reveal the formation of a phase corresponding to cubic spinel structure in both samples and the presence of a second phase identified as , in the case of the sample with higher temperature treatment. The TEM images of the first sample show small nonuniform nanoparticles with a mean size of 7.8 nm, with a strong tendency to form agglomerates. Magnetization studies as a function of temperature were carried following field-cooled (FC)-Zero-field-cooled (ZFC) routines, where the ZFC curves exhibit blocking temperatures close to 250 K in both cases, and the behavior of the samples below this temperature suggests strong interaction between the particles. In the magnetization as a function of magnetic field studies, the curves display a tendency to saturate at low temperatures and the system shows superparamagnetic behavior above the blocking temperature. Saturation magnetization values (at low temperatures) are low compared to the expected ones, according to the Néel model of collinear spins, this can be attributed to canting effects or the presence of a second antiferromagnetic phase, specifically in the sample treated at 500 . No significant differences were observed in the magnetic behavior of the samples. Semiconducting characteristics of the ferrites were confirmed by optical absorption measurements, obtaining an energy gap value close to 2.23 eV at room temperature.
We report on the optimisation of the growth conditions of manganite La 0.7 Sr 0.3 MnO 3 thin film... more We report on the optimisation of the growth conditions of manganite La 0.7 Sr 0.3 MnO 3 thin films prepared by Channel Spark Ablation (CSA). CSA belongs to pulsed electron deposition methods and its energetic and deposition parameters are quite similar to those of pulsed laser deposition. The method has been already proven to provide manganite films with good magnetic properties, but the films were generally relatively rough (a few nm coarseness). Here we show that increasing the oxygen deposition pressure with respect to previously used regimes, reduces the surface roughness down to unit cell size while maintaining a robust magnetism. We analyse in detail the effect of other deposition parameters, like accelerating voltage, discharging energy, chamber pressure and substrate temperature and provide on this basis a set of optimal conditions for the growth of atomically flat films. The thicknesses for which atomically flat surface was achieved is as high as about 10-20 nm, corresponding to films with room temperature magnetism. We believe such magnetic layers represent appealing and suitable electrodes for various spintronic devices.
An experimental setup for the direct measurement of the magnetocaloric effect capable of simulati... more An experimental setup for the direct measurement of the magnetocaloric effect capable of simulating high frequency magnetothermal cycles on laboratory-scale samples is described. The study of the magnetocaloric properties of working materials under operative conditions is fundamental for the development of innovative devices. Frequency and time dependent characterization can provide essential information on intrinsic features such as magnetic field induced fatigue in materials undergoing first order magnetic phase transitions. A full characterization of the adiabatic temperature change performed for a sample of Gadolinium across its Curie transition shows the good agreement between our results and literature data and in-field differential scanning calorimetry. © 2013 AIP Publishing LLC. [http://dx.
Co O % nanopowders were prepared by using the autocombustion technique, and the structural, optic... more Co O % nanopowders were prepared by using the autocombustion technique, and the structural, optical and magnetic properties were investigated. X-ray diffraction measurements show that the synthesized samples have a wurtzite structure. The formation of the wurtzite structure in Co doped ZnO nanoparticles was further confirmed by Fourier transform infra-red spectrometry.
The study of two aspects of the magnetocaloric effect (MCE), namely, the matching between isother... more The study of two aspects of the magnetocaloric effect (MCE), namely, the matching between isothermal entropy change and direct adiabatic temperature change, is not straightforward since huge differences between these two quantities have often been reported. Here we put in relation the direct and indirect measurements on the first order magnetostructural martensitic transformation occurring in Ni-Co-Mn-Ga alloys. In order to complete the characterization of the MCE and to find an explanation of these mismatches, differential scanning calorimeter measurements have been performed at different applied magnetic fields.
A careful characterization of the magnetocaloric effect in Co-substituted Ni-Mn-Ga Heusler alloys... more A careful characterization of the magnetocaloric effect in Co-substituted Ni-Mn-Ga Heusler alloys, based both on direct and indirect methods, is presented. In the present paper, adiabatic temperature change values ( T ad ) up to 1.6 K in 1.9 T were measured across the magnetostructural transformations. The studied samples, with similar transformation temperatures and comparable entropy changes, show surprising differences of the T ad . In order to gain better insight into the behavior of the T ad peak values among different samples, a qualitative model is proposed based on magnetization and magnetothermal data.
The iron potassium fluorides with the general formula K x Fe II
Monodispersed ferromagnetic FePt nanoparticles, partially ordered in the L1 0 phase, were directl... more Monodispersed ferromagnetic FePt nanoparticles, partially ordered in the L1 0 phase, were directly prepared without further annealing by high temperature synthesis (z300 C) involving poly(N-vinyl-2pyrrolidone) and Triton X-100 as protective agent and reaction solvent respectively. Depending on the synthesis conditions, nanoparticles with average sizes ranging from 5 to 7 nm and coercive fields reaching 0.1 T at 300 K were obtained, but they invariably aggregate by magnetic dipolar interaction. By increasing the solvent viscosity (using PEG 600), 5 nm superparamagnetic nanoparticles are embedded in an amorphous matrix derived from solvent condensation/decomposition, thus avoiding aggregation. Nanoparticles are then completely converted to the hard tetragonal L1 0 phase, preserving the original size, by annealing in a vacuum at higher temperatures that, at the same time, transform the matrix into amorphous carbon. Annealing at 650 C for 3 h leads to coercive fields of about 0.25 T at RT and 1.3 T at 5 K (without reaching the saturation magnetization) and to a peculiar squeezing of the hysteresis loops. Subsequent treatments at higher temperatures induce a further shrinking of the loop and a reduction of the coercive field. The possible explanation takes into account that, by raising the annealing temperature, an increasing number of nanoparticles becomes free to rotate inside the matrix, aligning like ''nano-compasses'' with the applied magnetic field. However a fraction of nanoparticles remains still locked to the matrix, generating a superimposed magnetically hard contribution.
Journal of Magnetism and Magnetic Materials, 2010
We present a magnetic characterisation of MnAs films grown by MOVPE on semiconductor substrates, ... more We present a magnetic characterisation of MnAs films grown by MOVPE on semiconductor substrates, for comparing the effect of thickness and substrate on their magnetic and magneto-thermal properties, with particular focus on the nature of the magneto-structural transition between the ferromagnetic hexagonal a-phase and the paramagnetic orthorhombic b-phase. The nature of this transformation depends on the presence of strain and turns out to be of second-order type for epitaxial-crystalline MnAs/(0 0 1)GaAs films and closer to a first-order type for polycrystalline MnAs/SiO 2 /Si films. The magnetocaloric effect at the transition has been estimated for thin film samples, resulting appreciably lower than that of bulk MnAs.
Physical Review B, 2011
The micromagnetic continuum theory has been applied to perfect soft/hard multilayers characterize... more The micromagnetic continuum theory has been applied to perfect soft/hard multilayers characterized by antiferromagnetic interface coupling. The soft and hard phases have uniaxial anisotropy with a common direction, along which the external field is applied. The model assumes a nonuniform rotation of the magnetization, and it also considers an interface coupling that is reduced with respect to the strong-limit case. It is found that the deviation of the magnetization from the saturated antiparallel state can occur at two distinct nucleation fields, which mainly involve only one of the two phases. Moreover, in the case of a reduced interface coupling, the saturated parallel state becomes accessible and thus the nucleation from this state is taken into account. The critical equations have been deduced, allowing us to identify the conditions for which the nucleation regime changes from reversible to irreversible as a function of the intrinsic and extrinsic parameters. The results of the model, applied to a typical soft/hard system with planar anisotropy, have been summarized in suitable phase diagrams, as a function of the layer thicknesses and of the strength of the interface coupling. The analysis, supported by additional static and dynamic micromagnetic simulations, shows the occurrence of a rich variety of magnetization curves. As a secondary result we have found that, in the parallel nucleation process, the influence of the interface coupling extends inside the two phases to distances appreciably larger than the corresponding Bloch wall widths.
Journal of Magnetism and Magnetic Materials, 2007
The angular dependence of hysteresis loops of exchange-spring (ES) bilayers constituted by a Fe l... more The angular dependence of hysteresis loops of exchange-spring (ES) bilayers constituted by a Fe layer as the soft phase and a hard layer of L1 0 Fe 53 Pt 47 is presented. FePt thin films of nominal thickness 10 nm have been prepared by RF sputtering on MgO (1 0 0) substrates in order to achieve the epitaxial growth with the c-axis perpendicular to the film plane. The Fe soft layers have been deposited at RT with various thicknesses (from 2 to 10 nm). The angular dependence of the bilayers' magnetic properties is studied by SQUID magnetometry varying the angle between the easy axis of the hard phase and the direction of the applied field from 0 to p/2. The magnetic behavior at different angles is analyzed by a one-dimensional micromagnetic model specifically extended to the case of perpendicular magnetic anisotropy (PMA).
Applied Physics Letters, 2010
The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)... more The effect of the AlOx barrier thickness on magnetic and morphological properties of Ta/Co/(AlOx)/Alq3/Si hybrid structures was systematically studied by means of atomic force microscopy, SQUID magnetometry and nuclear magnetic resonance (NMR). All used techniques pointed out that the barrier thickness of 2 nm is required to obtain a magnetically good cobalt layer on top of Alq3. 59 Co NMR measurements revealed that the AlOx barrier gives rise to the formation of an interface layer with "defective" cobalt favouring growth of "bulk" cobalt with good magnetic properties.
Acta Materialia, 2011
We report the magnetic and magnetocaloric properties in Co-doped Ni-Mn-Ga Heusler alloys around t... more We report the magnetic and magnetocaloric properties in Co-doped Ni-Mn-Ga Heusler alloys around the Mn-rich composition Ni 50 Mn 30 Ga 20 . The presence of Co affects profoundly the critical temperatures and alters the exchange interactions of martensite and austenite to different extents; by varying the composition it is possible to tune the critical temperatures and to induce a paramagnetic gap between the magnetically ordered martensite and magnetic austenite, thus giving rise to a reverse magnetostructural transformation. Contrary to the Co-free alloys, the saturation magnetization moment of austenite is strongly enhanced by Co with respect to the martensitic one: thus the magnetocaloric effect turns from direct into inverse. Remarkable values of the magnetic properties related to the magnetocaloric effect, e.g. the saturation magnetization jump at the transformation (DM) and the field dependence of the transformation temperature (dT/dH) are reported, together with high positive values of the isothermal magnetic entropy change (DS). The conditions for enhancing the magnetocaloric properties and triggering the sign reversal of the magnetocaloric effect are discussed.
Journal of Magnetism and Magnetic Materials, 2007
We report results on epitaxial magnetite (Fe 3 O 4 ) thin films grown by electron beam ablation o... more We report results on epitaxial magnetite (Fe 3 O 4 ) thin films grown by electron beam ablation on (1 0 0) MgAl 2 O 4 substrates. At 120 K magnetite undergoes a structural and electronic transition, the so-called Verwey transition, at which magnetic and conducting properties of the material change. We observed the Verwey transition on epitaxial films with a thickness of 50 nm by comparing zero-field cooling (ZFC) and field cooling (FC) curves measured with a superconducting quantum interference device (SQUID) magnetometer. Observation of the Verwey transition by SQUID measurements in the films is sign of their high crystalline quality. Room temperature ferromagnetism has also been found by magneto-optical Kerr rotation (MOKE) and confirmed by SQUID measurements, with a hysteresis loop showing a coercive field of hundreds of Oe. r 2007 Published by Elsevier B.V.
Journal of Magnetism and Magnetic Materials, 2010
We have developed a simple numerical model for simulating domains as well as remanence and viscos... more We have developed a simple numerical model for simulating domains as well as remanence and viscosity curves in the slow dynamics regime, for thin films characterized by perpendicular magnetization and irregular domain configurations due to strong disorder. The physical system is represented as constituted of identical switching units, described by proper switching field distributions and energy barrier laws for pinning and nucleation processes. The model also includes an effective field which accounts for magnetic forces proportional to magnetization, on average. Simulations of DCD curves show that when the reversal of magnetization is governed by pinning, the coercive field depends on the physical size of the film area on which the external field is applied. In the case of viscosity phenomena described by a linear energy barrier law associated with a single predominant reversal process (pinning or nucleation), universal viscosity curves can be generated by properly transforming the DCD curve of the system. We also demonstrate that a reduction of the maximum viscosity coefficient can coexist with a reduction of the energy barrier heights.
Journal of Magnetism and Magnetic Materials, 2010
We present the study of Co/organic semiconductor (OS) stacks both from the morphological and magn... more We present the study of Co/organic semiconductor (OS) stacks both from the morphological and magnetic point of view. Co has been successfully used up to now as top contact of hybrid vertical devices. While the properties of Co grown on amorphous layers are well established, its deposition on soft materials presents critical aspects such as interfacial damage that affects its electrical and magnetic properties. In this work we focus on the influence of the morphology of the organic underlayer in the magnetic behavior of a Co thin film: tris(8-hydroxyquinoline) aluminum (Alq 3 ) grown in different conditions by molecular beam evaporation have been considered. A further considered aspect is the effect of the presence of a thin oxide barrier (Al 2 O 3 ) on the Co magnetic properties.
Applied Physics Letters, 2008
Technological procedures able to produce high quality electrodes from magnetic oxides in vertical... more Technological procedures able to produce high quality electrodes from magnetic oxides in vertical organic-inorganic hybrid devices is a challenging task in the field of organic spintronics. Thin films of magnetite ͑Fe 3 O 4 ͒ have been successfully grown directly on top of organic semiconductor layers, tris͑8-hydroxyquinoline͒aluminium͑III͒ ͑Alq 3 ͒, by pulsed-electron ablation technique. The films show ferromagnetic behavior and good structural quality, properties detected by magneto-optical Kerr effect, superconductor quantum interference device, micro-Raman spectroscopy, and Atomic Force Microscopy. The ferromagnetic behavior persists even for 10 nm thick films. Charge injection at magnetite-organic interface has been finally demonstrated by detecting electroluminescence from Alq 3 .
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Papers by Chiara Pernechele