A. J. A. de Oliveira

Ferromagnetism induced by oxygen and cerium vacancies above the percolation limit in CeO2

We studied the structural, chemical and magnetic properties of non-doped ceria (CeO(2)) thin films electrodeposited on silicon substrates. Experimental results confirm that the observed room temperature ferromagnetism is driven by both cerium and oxygen vacancies. We investigated ceria films presenting vacancy concentrations well above the percolation limit. Irradiation experiments with neon ions were employed to generate highly oxygen defective CeO(2-δ) structures. X-ray photoelectron spectroscopy and x-ray absorption near-edge structure spectroscopy were used to estimate the concentration of Ce(3+) sites in the films, which can reach up to 50% of Ce(3+) replacing Ce(4+), compared to a stoichiometric CeO(2) structure. Despite the increment of structural disorder, we observe that the saturation magnetization continuously increases with Ce(3+) concentration. Our experiments demonstrate that the ferromagnetism observed in ceria thin films, highly disordered and oxygen-deficient, preserving the fluorite-type structure only in a nanometer scale, remains intrinsically stable at room temperature.

Structural, magnetic and spectroscopic study of a diluted magnetic oxide: Co doped CeO2−δ

A structural, spectroscopic and magnetic study of Co doped CeO2−δ diluted magnetic oxide (DMO) thin films grown by pulsed laser deposition is presented. No secondary phase or metallic clusters could be detected. The samples are ferromagnetic at room temperature and epitaxial films display a large magnetic anisotropy with an out-of-plane easy axis. The evolution of the magnetization with temperature did not reveal any superparamagnetic signal related to nanosized clusters, reinforcing the conclusion that the ferromagnetism is intrinsic in this system. The magnetization at saturation has the same magnitude (~1.4–1.5 μB/Co) in epilayers and textured films and shows no clear dependence on the structural quality, contrary to other DMOs. It is also shown that ferromagnetism is not restricted to a particular region of the films, such as the interface. The ferromagnetic signal depends sensitively on the oxygen pressure during growth and post-growth annealing. The valence of Ce and Co was deduced from x-ray spectroscopies, revealing a predominant Co2+ state. The origin of ferromagnetism in Co doped CeO2−δ is discussed in connection with possible charge-compensating defects and existing models describing indirect exchange in DMOs.

Loss of magnetization induced by doping in CeO2 films

This work reports the effect of Mn, Fe, Co, and Cu low doping (∼3 at. %) on the ferromagnetic behavior of oxygen-defective CeO2−δ films electrodeposited on SiO2/Si(001). Our results indicate that the incorporation of a small number of 3d dopants with unoccupied outermost atomic orbitals, presumably magnetically active, strongly perturb the ferromagnetic ground-state associated with the network of electron clouds surrounding oxygen vacancies. As a consequence, a strong loss of magnetization occurs and saturation magnetization becomes uncorrelated with number of oxygen vacancies.

Anisotropy of Magnetization and Nanocrystalline Texture in Electrodeposited CeO2 Films

Anisotropy of Magnetization and Nanocrystalline Texture in Electrodeposited CeO2 Films V. Fernandes, P. Schio, R. J. O. Mossanek, A. J. A. de Oliveira, W. A. Ortiz, D. Demaille, F. Vidal, Y. Zheng, P. Fichtner,* L. Amaral, M. Abbate, J. Varalda, W. H. Schreiner, and D. H. Mosca Departamento de Fisica, Universidade Federal do Parana, 81531-990 Curitiba PR, Brazil Departamento de Fisica, Universidade Federal de Sao Carlos, 13565-905, S. Carlos SP, Brazil Institut des NanoSciences de Paris, UPMC-Paris 6, CNRS UMR 7588, 75252 Paris Cedex 05, France Escola de Engenharia and Instituto de Fisica, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, Brazil

The ferromagnetic behaviour of conducting polymers revisited

The magnetic properties of doped pellets of poly(3-methylthiophene) showing room temperature ferromagnetic behaviour have been discussed in a previous article. The magnetic behaviour was attributed to a weak ferromagnetic phase, due to the superexchange interaction of polarons via the dopant anions. The Dzialoshinsky–Morya interaction among canted spins was proposed to explain the ferromagnetism. In this article the main conclusions of that work concerning the magnetic behaviour are revised. The basic assumption now is that the magnetic moments are spin 1/2 polarons that can interact antiferromagnetically and/or ferromagnetically. In the small crystalline regions of the polymer, which are identified with the polymer portion that remains ferromagnetic at room temperature, the interaction gives rise to S = 0a nd 1 bipolarons and the S = 1 triplet state is lower in energy. In the disordered region, disorder will prevent the complete S = 1 and 0 coupling and bands of polarons ferromagnetically and antiferromagnetically coupled will appear. Using this approach, all the magnetization data can be qualitatively explained, as well as the electron spin resonance data.

Magnetic properties of synthetic eumelanin - Preliminary results

We report an experimental and theoretical study of magnetic properties of synthetic eumelanin. The magnetization curves are determined by using both a vibrating sample magnetometer and a superconducting quantum interferometer device in an extended range of magnetic fields ranging from −10 kOe to 10 kOe at different temperatures. We find that the eumelanin magnetization can be qualitatively explained in terms of a simple model of dipolar spheres with an intrinsic magnetic moment. The latter one is experimentally measured by using X‐band electron paramagnetic resonance. Our findings indicate that synthetic melanins are superparamagnetic.

Weak ferromagnetism in poly(3-methylthiophene)(PMTh)

Ferromagnetic behaviour at 300 K has been observed from SQUID experiments in ClO 4 - doped PMTh. The samples were prepared electrochemically at 25 °C in acetonitrile with 0.1M LiClO 4 and then partially reduced. Atomic absorption analysis discards magnetic particles contamination. Hysteresis curves were observed for pressed pellets in all range of temperatures (300K-2K). The remanence at 300 K and pressed at 250 bar was around 8.06x10 -4 emu/g with coercitivity of 130 Oe. The influence of water content in the solvent during the sample synthesis and the pressure is shown. We discuss a model that explains our data in terms of the anisotropic superexchange Dzialoshinski-Moriya interaction giving rise to weak ferromagnetism.,.

Tunnel magnetoresistance and Coulomb blockade in a planar assembly of cobalt nanoclusters embedded in TiO2

Magnetic and magnetotransport properties of Co nanoclusters embedded in a TiO2 matrix grown by pulsed laser deposition have been studied. The results show a tunneling magnetoresistance associated with spin-polarized multiple quantum tunneling through the Coulomb energy barrier (Coulomb blockade effect). A shift between the fields at the magnetoresistance maxima and the coercive fields in the Coulomb blockade regime is observed. This shift decreases with increasing temperature and disappears when the thermal energy becomes stronger than the Coulomb blockade energy.

Microstructure and magnetism of Fe nanoparticles embedded in Al2O3?ZnO matrix

Nanoscaled Fe particles embedded in a double-oxide matrix were obtained by synthesizing a zinc?aluminium layered double hydroxide intercalated with a hexacyano Fe(III) complex under vacuum thermal annealing. Chemical and structural properties were investigated by Fourier transform infrared spectroscopy, x-ray photoemission spectroscopy, x-ray diffraction and transmission electron microscopy. The magnetic properties of the Fe nanoparticles were investigated using a SQUID magnetometer. Our measurements indicate a system of weakly interacting magnetic nanoparticles that are arranged in a multidomain state at low temperature.

Local moments in the paramagnetic phase of dilute CrV alloys

The magnetic susceptibility of Cr+0.2 at.% V and Cr+0.4 at.% V alloys (nominal concentrations) is found to show a sharp increase at the Neel temperature with increasing temperature and to obey a Curie-Weiss law in the paramagnetic phase up to 400 K, corresponding to a moment per V atom of about one Bohr magneton. The relation of these results to the behaviour of other physical properties of dilute CrV alloys is discussed.