Dina Tobia

Size-dependent passivation shell and magnetic properties in antiferromagnetic/ferrimagnetic core/shell MnO nanoparticles.

The magnetic properties of bimagnetic core/shell nanoparticles consisting of an antiferromagnetic MnO core and a ferrimagnetic passivation shell have been investigated. It is found that the phase of the passivation shell (gamma-Mn(2)O(3) or Mn(3)O(4)) depends on the size of the nanoparticles. Structural and magnetic characterizations concur that while the smallest nanoparticles have a predominantly gamma-Mn(2)O(3) shell, larger ones have increasing amounts of Mn(3)O(4). A considerable enhancement of the Néel temperature, T(N), and the magnetic anisotropy of the MnO core for decreasing core sizes has been observed. The size reduction also leads to other phenomena such as persistent magnetic moment in MnO up to high temperatures and an unusual temperature behavior of the magnetic domains.

Size effects in bimagnetic CoO/CoFe2O4 core/shell nanoparticles

The control of the size of bimagnetic nanoparticles represents an important step toward the study of fundamental properties and the design of new nanostructured magnetic materials. We report the synthesis and the structural and magnetic characterization of bimagnetic CoO/CoFe2O4 core/shell nanoparticles. The material was fabricated by a seed-mediated growth high-temperature decomposition method with sizes in the range of 5-11 nm. We show that the core/shell morphology favours the crystallinity of the shell phase, and the reduction of the particle size leads to a remarkable increase of the magnetic hardening. When the size is reduced, the coercive field at 5 K increases from 21.5 kOe to 30.8 kOe, while the blocking temperature decreases from 388 K to 167 K. The size effects on the magnetic behaviour are described through a phenomenological model for strongly ferri-/antiferromagnetic coupled phases.

Evolution of the magnetic anisotropy with particle size in antiferromagnetic Cr2O3 nanoparticles

We report the magnetic properties of antiferromagnetic Cr2O3 from bulk material down to 6 nm nanoparticles. We have found a decrease in the lattice parameters and the cell volume when the size diminishes. On the contrary, the magnetic anisotropy constant, Keff, shows a nonmonotonic behavior. The Keff decreases from its bulk value, shows a minimum near ϕ∼30 nm and displays an important increase for smaller sizes. We analyzed the size dependence of Keff in terms of the magnetocrystalline and surface contributions, and we fitted the Keff size evolution on the basis of a phenomenological model taking into account the magnetically ordered core and the surface contribution.

Origin of magnetic anisotropy in ZnO/CoFe2O4 and CoO/CoFe2O4 core/shell nanoparticle systems

ZnO-core/CoFe2O4-shell nanoparticles of 7.4 nm average size have been synthesized and their magnetic properties have been compared to those of CoO-core/CoFe2O4-shell nanoparticles with similar morphology. The coercive field values are much lower than those for CoO/CoFe2O4 nanoparticles (e.g., at 5 K: Hc = 7.8 kOe for ZnO/CoFe2O4; Hc = 27.8 kOe for CoO/CoFe2O4). The nature of the coercive field values is explained by a phenomenological model for the free energy of a non-magnetic core, or an antiferromagnetic core, encapsulated by a hard ferrimagnetic shell.

Exchange bias of Co nanoparticles embedded in Cr2O3 and Al2O3 matrices

The magnetic properties of ∼1.5 nm Co nanoparticles embedded in a diamagnetic Al2O3 or antiferromagnetic (AFM) Cr2O3 matrix were investigated. For Co nanoparticles in Al2O3 matrix, a typical behavior of weakly interacting nanoparticles is observed, characterized by a superparamagnetic regime and a progressive blocking of particle moments centered at ⟨TB⟩=14 K. On the other hand, when the Co nanoparticles are immersed in a Cr2O3 matrix a very different magnetic behavior was found. The system shows large irreversibility in field-cooling/zero-field-cooling magnetization curves and much larger coercivity was observed even up to room temperature. Hysteresis loop shift is present when the system is field-cooled from a temperature above the Cr2O3 Neel temperature. We found that the exchange bias field follows a Brillouin type temperature dependence and goes to zero at TN. These results evidence the enhancement of thermal stability of the Co nanoparticle moments, associated to the increase of anisotropy due to th...

Temperature evolution of the effective magnetic anisotropy in the MnCr2O4 spinel

In this work, we present a study of the low temperature magnetic phases of polycrystalline MnCr2O4 spinel through dc magnetization and ferromagnetic resonance spectroscopy (FMR). Through these experiments, we determined the main characteristic temperatures: T(C) ∼ 41 K and T(H) ∼ 18 K corresponding, respectively, to the ferrimagnetic order and to the low temperature helicoidal transitions. The temperature evolution of the system is described by a phenomenological approach that considers the different terms that contribute to the free energy density. Below the Curie temperature, the FMR spectra were modeled by a cubic magnetocrystalline anisotropy to the second order, with K1 and K2 anisotropy constants that define the easy magnetization axis along the <1 1 0> direction. At lower temperatures, the formation of a helicoidal phase was considered by including uniaxial anisotropy axis along the [11¯0] propagation direction of the spiral arrange, with a Ku anisotropy constant. The values obtained from the fittings at 5 K are K1 = -2.3 × 10(4) erg cm(-3), K2 = 6.4 × 10(4) erg cm(-3) and Ku = 7.5 × 10(4) erg cm(-3).

Sowing crystals: crystal growth activities in Bariloche, Argentina

Adriana Serquis1, Virginia Tognoli2, Martin Saleta2, Dina Tobia2, Mauricio D Arce1, Morena Robles3, Jordana Dorfman3, Patricia Mateos3 1Materials Characterization Department CNEA-CONICET, Bariloche, Argentina, 2Magnetic Resonance Department CNEA CONICET, Bariloche, Argentina, 3Sección Divulgación Científica y Tecnológica, CAB-IB CNEA CUYO, Bariloche, Argentina E-mail: aserquis@cab.cnea.gov.ar