Petronel Postolache

Micromagnetic and Preisach analysis of the First Order Reversal Curves (FORC) diagram

The First Order Reversal Curve (FORC) diagrams of interacting single-domain ferromagnetic particle systems have been found experimentally to contain negative regions. In this paper, we use micromagnetic and phenomenological (Preisach-type) models to help explain the occurrence of these negative regions. In Preisach-type modeling, the position of the negative region is correlated with the sign of the mean-field interactions. In micromagnetic modeling, the position of the negative region is correlated with the spatial arrangement of the particles in the model.

Functional properties of BaTiO3–Ni0.5Zn0.5Fe2O4 magnetoelectric ceramics prepared from powders with core-shell structure

In the present work, diphasic ceramic composites with core-shell nanostructures formed by Ni0.50Zn0.50Fe2O4 core and BaTiO3 shell were investigated. Their properties were compared with those of composites prepared by coprecipitation. The core-shell structure was confirmed by microstructural powder analysis. Homogeneous microstructures with a good phase mixing and percolated dielectric phase by the magnetic one were obtained from coprecipitated powders. Less homogeneous microstructures resulted in ceramics produced from the powder prepared by core-shell method, with isolated small ferrite grains besides large ferrite aggregates embedded into the BaTiO3 matrix. Both the ferroelectric and magnetic phases preserve their basic properties in bulk composite form. However, important differences in the dielectric relaxation and conduction mechanisms were found as result of the microstructural difference. Extrinsic contributions play important roles in modifying the electric properties in both ceramics, causing spa...

First-order reversal curves diagrams for the characterization of ferroelectric switching

A method to describe the switching characteristics of the ferroelectrics is proposed, using the first-order reversal curve (FORC) diagrams. On these diagrams, the reversible and irreversible contributions to the ferroelectric polarization can be clearly separated. They are extremely sensitive to the changes of the hysteresis loops induced by degradation of the ferroelectric polarization, such as fatigue. Sharp in the fresh state, the FORC distribution becomes wide with its maximum shifted towards higher fields after 109 switching cycles. A strong increase of the reversible component was found in the fatigue state. With appropriate interpretation, these diagrams could be valuable as “fingerprints” of the switching characteristics of the ferroelectric systems in a particular state.

Experimental evaluation of the Preisach distribution for magnetic recording media

In this paper, we describe the evaluation of the Preisach distribution for magnetic recording media on the basis of FORC diagrams.

Analysis of the Switching Characteristics of PZT Films by First Order Reversal Curve Diagrams

A new type of characterization of ferroelectric switching using First Order Reversal Curve (FORC) diagrams based on recording of minor hysteresis loops has been recently proposed. We apply this method to polycrystalline and epitaxial PZT thin films measured under various conditions and compare them to PZT ceramics. In each case we have been able to separate the reversible and irreversible contributions to switching and discuss the most dominant of them in relation to hysteresis loop shapes and ferroelectric switching mechanisms. The FORC diagrams of fatigued PZT exhibit features in agreement with its hysteresis probed at nanoscale and confirm a degradation mechanism based on local imprint. The asymmetrical tail present on measured FORC diagrams has been explained in terms of a nucleation-growth switching process triggered by latent nuclei. All these findings attest the usefulness of the FORC diagrams for both characterizing various types of samples as well as the elucidation of ferroelectrics’ physics.

Functional Properties of the (1-x)BiFeO3—xBaTiO3 Solid Solutions

Solid solutions of (1-x)BiFeO 3 –xBaTiO 3 (0 ≤ x ≤ 0.30) were prepared by a two-step solid state reaction procedure. After pre-sintering and sintering at 800°C/1h and slow cooling, pure single phase compositions were obtained for all the analyzed mixtures in the limit of XRD accuracy, including for x = 0. A gradual attenuation of the rhombohedral distortion with the increase of BaTiO 3 content was found. Small dielectric losses (tan δ < 3 %) and permittivities in the range of (180, 240) were found for all the compositions at room temperatures. The magnetic investigations shown a composition-dependent ferromagnetism with a maximum magnetization and coercivity for the composition x = 0.05 and perfect antiferromagnetic behavior for the BiFeO 3 ceramics. The complex impedance is sensitive to the magnetic field at small temperatures below 300 K, with a stronger variation in the range of fields of (0, 2 T) and a tendency of saturation at higher fields, demonstrating a kind of magnetoelectric coupling, with both magnetocapacitance and magnetoresistence phenomena.

Preisach-type model for strongly interacting ferromagnetic particulate systems

In this paper, we present a new Preisach-type model that can be successfully applied to simulate complex magnetization processes for strongly correlated particulate systems. As suggested by micromagnetic simulations, the Preisach distribution of interaction fields has a two peak shape and the amplitude of the peaks is dependent of the magnetic state of the sample. The model is compared with the simulations made with a variable variance Preisach model, and the results are discussed.

BaTiO3–ferrite composites with magnetocapacitance and hard/soft magnetic properties

BaTiO3–ferrite multiphase composites were prepared starting from di-phase mixtures of α-Fe2O3 and BaTiO3 powders. During the sintering step, the formation of small amounts of secondary phases with multifunctional character as BaFe12O19 or Ba12Ti28Fe15O84 was promoted. The resulting multiphase ceramic compounds show interesting dielectric, magnetic and small magnetocapacitance effect at low temperature. The coexistence of different magnetic phases with contrasting coercivities (hard/soft) was detected by the presence of ‘wasp-waisted’ M(H) hysteresis loops and first-order reversal curve analysis. The present approach demonstrates that active materials can be realised by controlling in situ reactions at the interfaces in ferroelectric–magnetic oxide composites.

Novel magnetoelectric ceramic composites by control of the interface reactions in Fe2O3@BaTiO3 core-shell structures

In the present work, composite ceramics of ferroelectric BaTiO3 (BT) with magnetic αFe2O3 were prepared from powders synthesized by two different methods: (a) core-shell powders of Fe2O3@BT and (b) Fe2O3-BT composites from mixed powders with the same composition. The M(H) loops at room temperature show a “wasp-waisted” character, with multiple components, determined as result of the formation of magnetic phases with contrasting coercivities (hard BaFe12O19 and soft Ba12Ti28Fe15O84 phases) in different ratios, as indicated by the magnetic first-order-reversal curves analysis. The core-shell composite ceramics generally showed slightly improved dielectric properties and smaller conductivity. The high field properties of composite ceramics show a strong nonlinearity in both cases, together with a reduction of zero field permittivity, making from these composites possible tunable materials interesting for applications. Their multifunctional character is enhanced by the presence of a complex magnetic character...

First order approximation for interactions in particulate single-domain particle systems

In most cases the interparticle interactions are essential in the evaluation of the magnetic properties of the ferromagnetic particulate systems. Usually, in first approximation, a mean interaction field linearly dependent on the magnetic moment of the sample is considered and other interactions are neglected. Using micromagnetic simulations on a system of interacting uniaxial single-domain particles we have shown that both the mean interaction field and interaction fields distribution dispersion depend on the magnetic moment of the sample. When the magnetic moment of the sample is small the statistical interactions are more important than the mean field ones and cannot be neglected.