E. Winkler

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...

Resolving Material-Specific Structures within Fe3O4|γ-Mn2O3 Core|Shell Nanoparticles Using Anomalous Small-Angle X-ray Scattering

Here it is demonstrated that multiple-energy, anomalous small-angle X-ray scattering (ASAXS) provides significant enhancement in sensitivity to internal material boundaries of layered nanoparticles compared with the traditional modeling of a single scattering energy, even for cases in which high scattering contrast naturally exists. Specifically, the material-specific structure of monodispersed Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles is determined, and the contribution of each component to the total scattering profile is identified with unprecedented clarity. We show that Fe₃O₄|γ-Mn₂O₃ core|shell nanoparticles with a diameter of 8.2 ± 0.2 nm consist of a core with a composition near Fe₃O₄ surrounded by a (Mn(x)Fe(1-x))₃O₄ shell with a graded composition, ranging from x ≈ 0.40 at the inner shell toward x ≈ 0.46 at the surface. Evaluation of the scattering contribution arising from the interference between material-specific layers additionally reveals the presence of Fe₃O₄ cores without a coating shell. Finally, it is found that the material-specific scattering profile shapes and chemical compositions extracted by this method are independent of the original input chemical compositions used in the analysis, revealing multiple-energy ASAXS as a powerful tool for determining internal nanostructured morphology even if the exact composition of the individual layers is not known a priori.

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).

Correlation between radiation damage and magnetic properties in reactor vessel steels

Since Reactor Pressure Vessel steels are ferromagnetic, they provide a convenient means to monitor changes in the mechanical properties of the material upon irradiation with high energy particles, by measuring their magnetic properties. Here, we discuss the correlation between these two properties (i.e. mechanical and magnetic properties) and microstructure, by studying the flux effect on the nuclear pressure vessel steel used in reactors currently under construction in Argentina. Charpy-V notched specimens of this steel were irradiated in the RA1 experimental reactor at 275{\deg}C with two lead factors (LFs), 93 and 183. The magnetic properties were studied by means of DC magnetometry and ferromagnetic resonance. The results show that the coercive field and magnetic anisotropy spatial distribution are sensitive to the LF and can be explained by taking into account the evolution of the microstructure with this parameter. The saturation magnetization shows a dominant dependence on the accumulated damage. Consequently, the mentioned techniques are suitable to estimate the degradation of the reactor vessel steel.

Phase coexistence in manganites: doping and structural dependence

We present a study on the phase coexistence (PC) of paramagnetic insulating (PM-I) and ferromagnetic metallic (FM-M) phases in the La(1- y)(Ca(1-x)Sr(x))(y)MnO(3) system with 0.23 ≤ y ≤ 0.45. The study was performed by means of magnetization and electron spin resonance (ESR) measurements. At high temperatures the ESR spectrum consists of a single symmetric PM line. At T(C), a FM asymmetric line is observed shifted to low fields. In a ΔT temperature range both lines are visible, defining a range of PC. For x = 0, we obtained ΔT as a function of the carrier concentration y, finding that the largest ΔT corresponds to y = 0.25. For this y value, the extreme compounds are orthorhombic and rhombohedral for x = 0 and 1, respectively. The rhombohedral to orthorhombic temperature transition (T(RO)) was determined as a function of x. We found that [Formula: see text] only if T(C) < T(RO). The PM-I/FM-M phase coexistence was only observed in the orthorhombic phase while seems to be incompatible with the more symmetric rhombohedral phase.

Unravelling the Elusive Antiferromagnetic Order in Wurtzite and Zinc Blende CoO Polymorph Nanoparticles

Although cubic rock salt-CoO has been extensively studied, the magnetic properties of the main nanoscale CoO polymorphs (hexagonal wurtzite and cubic zinc blende structures) are rather poorly understood. Here, a detailed magnetic and neutron diffraction study on zinc blende and wurtzite CoO nanoparticles is presented. The zinc blende-CoO phase is antiferromagnetic with a 3rd type structure in a face-centered cubic lattice and a Néel temperature of TN (zinc-blende) ≈225 K. Wurtzite-CoO also presents an antiferromagnetic order, TN (wurtzite) ≈109 K, although much more complex, with a 2nd type order along the c-axis but an incommensurate order along the y-axis. Importantly, the overall magnetic properties are overwhelmed by the uncompensated spins, which confer the system a ferromagnetic-like behavior even at room temperature.