Chiara Pernechele

Hanle effect missing in a prototypical organic spintronic device

We investigate spin precession (Hanle effect) in the prototypical organic spintronic giant magnetoresistance device La0.7Sr0.3MnO3/tris(8-hydroxyquinoline)/AlOx/Co. The Hanle effect is not observed in measurements taken by sweeping a magnetic field at different angles from the plane of the device. As possible explanations we discuss the tilting out of plane of the magnetization of the electrodes, exceptionally high mobility, or hot spots. Our results call for a greater understanding of spin injection and transport in such devices.

Reverse Magnetostructural Transitions by Co and In Doping NiMnGa Alloys: Structural, Magnetic, and Magnetoelastic Properties

We review the composition dependence of the structural and magnetic properties of the Co-doped Ni–Mn–Ga Ferromagnetic Shape Memory Alloy around the Mn-rich composition Ni50Mn30Ga20. The presence of Co affects 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. The magnetic and structural properties display noticeable discontinuities across the martensitic transformation: remarkable values of the saturation magnetization jump at the transformation (DM), of the field dependence of the martensitic transformation temperature (dTM/dH), and of the crystalline volume change (DV/V) are reported, and are considerably enhanced by additional Indoping of the quaternary alloy. These properties give rise to a remarkable phenomenology which is of interest for multifunctional applications; magnetic superelasticity and high values of reversible strain are found.

Direct magnetocaloric characterization and simulation of thermomagnetic cycles

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.

Direct deposition of magnetite thin films on organic semiconductors

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 (Fe3O4) have been successfully grown directly on top of organic semiconductor layers, tris(8-hydroxyquinoline)aluminium(III) (Alq3), 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 10nm thick films. Charge injection at magnetite-organic interface has been finally demonstrated by detecting electroluminescence from Alq3.

Non-interacting hard ferromagnetic L10 FePt nanoparticles embedded in a carbon matrix

Monodispersed ferromagnetic FePt nanoparticles, partially ordered in the L10 phase, were directly prepared without further annealing by high temperature synthesis (≈300 °C) involving poly(N-vinyl-2-pyrrolidone) 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 L10 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.

Structural and Electric Evidence of Ferrielectric State in Pb2MnWO6 Double Perovskite System

In this paper we describe the new ferri-electric compound Pb2MnWO6 (PMW), a double perovskite that can be considered as a novel structural prototype showing complex nuclear structure and interesting electric properties. According to single-crystal synchrotron data, PMW crystallizes in the noncentrosymmetric polar group Pmc21, in which the two symmetry-independent lead atoms give rise to a ferrielectric arrangement. The accurate crystallographic characterization indicates the presence of a complex distortion of the perovskite lattice driven by the local instability induced by the 6s(2) lone pair of the lead atoms. These peculiar structural features are confirmed by the complete electrical characterization of the system. Dielectric and transport measurements indicate an insulating character of the sample, while pyroelectric measurements point out a ferrielectric state characterized by different contributions. The magnetic transition at 45 K is accompanied by a magnetostrictive effect indicating a probable spin-lattice coupling. The characterizations carried out on PMW, showing the evidence of a coexistence of antiferromagnetism and ferrielectricity at low temperature, could lead to the definition of a new class of multiferroic materials.

Structural, Magnetic, and Optical Characterization of

Nanoparticles of MnFe2O4 were synthesized via sol-gel method, with calcination processes at 400 °C and 500 °C 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 Fe2O3, 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 °C. 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.

{\rm MnFe}_{2}{\rm O}_{4}

The commensurate crystal structure of the magnetic phase occurring below 7 K of the multiferroic Pb2MnWO6 perovskite has been solved by the introduction of the superspace approach. This lead based double perovskite is characterized by a complex ferrielectric non-centrosymmetric nuclear structure with orthorhombic symmetry stable in a wide temperature range. As indicated from the analysis of powder neutron diffraction data, the low temperature antiferromagnetic structure showing a propagation vector κ = [1/4 0 0] is stabilized by a multi-step process involving the evolution from incommensurate to commensurate spin ordering with a concomitant change of the magnetic symmetry. The determination of the Pb2MnWO6 magnetic structure offers a meaningful example of the superspace application and provides a detailed phase diagram of the involved magnetic states. Nowadays this ordered perovskite could be considered as a new type of multiferroic material combining ferrielectric properties and a long period antiferromagnetic structure.

Nanoparticles Synthesized Via Sol-Gel Method

The chemical and physical properties of the double perovskite Pb2FeMoO6 are systematically studied by means of structural and magnetic characterization. The compound crystallizes in the cubic Fmm space group, with partial cation order involving iron and molybdenum at the perovskite B site. Structural and Mossbauer characterization points to the presence of nanometer-sized antiphase domains within the ordered matrix giving rise to two iron populations, characterized by different chemical environments, with the same weight but different valence (0.3 electrons) and inequivalent magnetic anisotropy. This structural feature deeply affects the properties of the compound: Mossbauer and EPR measurements show a high-temperature superparamagnetic-like behavior ascribed to weak magnetic interactions occurring between the antiphase domains and the rest of the sample. However, below 270 K ferrimagnetic ordering of the atomic moments is observed by neutron diffraction and SQUID magnetometry, with the onset of blocked long range magnetic interactions on the Mossbauer timescale involving both the antiphase domains and the ordered matrix below 230 K. The superparamagnetic-like behavior is ascribed to the presence of low anisotropy barriers, giving rise to an extremely thin hysteresis loop at 5 K, with a very small coercive field and remnant magnetization. The observed saturation magnetization of 1.75 μB per f.u. is in agreement with the magnetic structure determined by neutron diffraction, with the two symmetry independent sites producing a ferrimagnetic resultant μS = 1.59 μB.

Superspace application on magnetic structure analysis of the Pb2MnWO6 double perovskite system

We report a comprehensive study of the spontaneous magnetization reversal (MRV) performed on the disordered polycrystalline perovskite BiFe(0.5)Mn(0.5)O(3), an intriguing compound synthesized in high pressure-high temperature conditions. In disordered systems, the origin of MRV is not completely clarified, yet. In BiFe(0.5)Mn(0.5)O(3), compositional disorder involves the ions on the B-site of the perovskite determining the presence of mesoscopic clusters, characterized by high concentrations of iron or manganese and thus by different resultant magnetization. This leads to the observation of two singular fields H(1) and H(2) dependent on the degree of inhomogeneity, unpredictably changing from sample to sample due to synthesis effects. These fields separate different magnetic responses of the system; for applied fields H < H(1), the Fe and Mn clusters weakly interact in a competitive way, giving rise to MRV, while for an intermediate field regime the energy of this weak interaction becomes comparable to the energy of the system under field application. As a consequence, the zero field cooled magnetization thermal evolution depends on the sample degree of inhomogeneity. In this field regime, applied field Mössbauer spectroscopy indicates that the iron rich clusters are highly polarized by the field, while the largest part of the material, consisting of AFM clusters characterized by axial anisotropy and uncompensated moments, shows soft or hard magnetism depending on T. Above the higher singular field, the M(T) curves show the trend expected for a classical antiferromagnetic material and the competitive character is suppressed. The MRV phenomenon results to be highly sensitive on both the thermal and magnetic measurement conditions; for this reason the present work proposes a characterization strategy that in principle has a large applicability in the study of disordered perovskites showing similar phenomenology.