Radu Tanasa

Hysteretic behavior of Fe(phen)2(NCS)2 spin-transition microparticles vs. the environment: A huge reversible component resolved by first order reversal curves

We discuss the influence of the embedding matrix on the thermal hysteretic behavior of spin transition microparticles of Fe(phen)2(NCS)2 by using a series of experimental first order reversal curves (FORCs). The shape of FORCs supports the hypothesis considering additional interactions between the spin-transition microparticles and the embedding matrix, which compares to a negative pressure on the particles. A mean-field approach based on negative variable external pressures, together with a cut off/switch on of particles-matrix interactions accounts for the experimental features.

Rate-dependent light-induced thermal hysteresis of [Fe(PM-BiA)2(NCS)2] spin transition complex

The light-induced bistability under permanent irradiation (LITH: light-induced thermal hysteresis), due to the competition between light irradiation and self-accelerated relaxation, shows crucially rate-dependent hysteresis. Major and minor hysteresis loops for [Fe(PM-BiA)2(NCS)2] spin crossover compound have been measured. Using appropriate master equations, distribution data from the first-order reversal curves diagrams of the purely thermal hysteresis, and parameter values previously derived from relaxation curves, we could simulate the LITH experimental data. The tunneling contribution to the relaxation rate revealed to be a key factor for the final agreement.

Theoretical investigations on an axial next nearest neighbour Ising-like model for spin crossover solids: one- and two-step spin transitions

We investigate the thermodynamical properties of an adapted ANNNI-like (axial next nearest neighbour Ising-like) model for 1D spin crossover solids. We performed an exact treatment within the framework of the well known transfer matrix method and derived the thermal evolution of the high spin fraction, the correlation function and the heat capacity. We show that a model of competition of the interactions between similar spin state ions reproduces qualitatively the main features of experimental data: gradual, abrupt and two-step spin crossover transitions. In addition, we found that the intermediate region of the twostep transition exhibits two types of spatial organization, according to the sign of the interactions. Indeed, antiferro-like and ‘antiphase’ (called the phase � 2� in the ANNNI model) structures are obtained. We also found that these two configurations can be distinguished through the thermal evolution of their corresponding correlation functions. The existence of the phase � 2� , predicted by the present model, is in good agreement with recent observations on a spin crossover sample of [{Fe(CNBH3)(4phpy)}2(μ-bpypz)2] for which this phase was evidenced for the first time, in these materials. (Some figures in this article are in colour only in the electronic version)

Matrix-assisted relaxation in Fe(phen)2(NCS)2 spin-crossover microparticles, experimental and theoretical investigations

In this study, we present the influence of the embedding matrix on the relaxation of Fe(phen)2(NCS)2 (phen = 1,10-phenanthroline) spin-transition microparticles as revealed by experiments and provide an explanation within the framework of an elastic model based on a Monte-Carlo method. Experiments show that the shape of the high-spin → low-spin relaxation curves is drastically changed when the particles are dispersed in glycerol. This effect was considered in the model by means of interactions between the microparticles and the matrix. A faster start of the relaxation for microparticles embedded in glycerol is due to an initial positive local pressure acting on the edge spin-crossover molecules from the matrix side. This local pressure diminishes and eventually becomes negative during relaxation, as an effect of the decrease of the volume of spin-crossover microparticles from high-spin to low-spin.

A first order reversal curve investigation of pressure hysteresis in multiferroics spin transition compound

Spin crossover multiferroic [Fe(PM-BiA)2(NCS)2] is investigated for the first time with the first order reversal curve (FORC) diagram method for its hysteretic pressure behavior. The experimental setup allows the measurements of the high spin fraction as the function of temperature, light intensity, and pressure. The experimental FORC diagrams obtained in compression and releasing modes show significant differences which are in disagreement with the classical Preisach model. Also, stronger kinetic effects are evidenced in the compression mode and possible explanations of this effect considering the viscoelastic behavior are provided.

Preparation of Bi2Fe4O9 particles by hydrothermal synthesis and functional properties

In the present study, particles with different Bi2Fe4O9 micro/nanostructures with a few particular morphologies (flower-like nanoplatelets, hierarchical microstructures, perfectly square platelets single crystals, etc.) obtained under specific hydrothermal synthesis conditions were investigated. The role of the processing parameters (such as NaOH concentration, reaction temperature, and reaction duration time) on the phase formation mechanism and on the microstructural characteristics was investigated. All the Bi2Fe4O9 morphologies showed orthorhombic symmetry with space group Pbam. The photocatalytic properties and magnetic behavior as a function of the micro/nanostructural characteristics of various Bi2Fe4O9 powders were determined. In the presence of Bi2Fe4O9, a degradation rate of Rose Bengal in the range of 52–61% was determined after 180 min under UV light irradiation (λ = 254 nm). Magnetic activity with antiferromagnetic behavior and a transition at ∼240 K slightly dependent on the microstructures was found. The role of Bi2Fe4O9 microstructures in the photocatalytic activity and magnetic properties was discussed.

Kinetic effects on double hysteresis in spin crossover molecular magnets analyzed with first order reversal curve diagram technique

In this paper, we analyze two types of hysteresis in spin crossover molecular magnets compounds in the framework of the First Order Reversal Curve (FORC) method. The switching between the two stable states in these compounds is accompanied by hysteresis phenomena if the intermolecular interactions are higher than a threshold. We have measured the static thermal hysteresis (TH) and the kinetic light induced thermal hysteresis (LITH) major loops and FORCs for the polycrystalline Fe(II) spin crossover compound [Fe1−xZnx(bbtr)3](ClO4)2 (bbtr = 1,4-di(1,2,3-triazol-1-yl)butane), either in a pure state (x = 0) or doped with Zn ions (x = 0.33) considering different sweeping rates. Here, we use this method not only to infer the domains distribution but also to disentangle between kinetic and static components of the LITH and to estimate the changes in the intermolecular interactions introduced by dopants. We also determined the qualitative relationship between FORC distributions measured for TH and LITH.

New statistical method for characterization of structured recording media magnetization processes

In this paper one presents a method for statistical characterization of the magnetization processes in two-dimensional networks of single-domain ferromagnetic particles. These systems are similar with the structured materials that are seen as future high-density recording media. The method allows very interesting developments in the characterization of the stability of the magnetic states which is an important factor for a recording medium.

Statistical characterisation of the FORC diagram

The first-order reversal curves (FORC) diagram method for the characterization of magnetic materials and of other materials showing hysteretic phenomena is gradually replacing the classical use of the major hysteresis loop or of other selection of magnetization curves. The FORC diagram offers to the experimentalist an image of the sample which can be related to the coercivity and interactions fields acting on the magnetic entities within the sample. Of course, this information should be analyzed carefully since errors in interpretation can be possible in the absence of other related physical or phenomenological models. However, in the wider use of the FORC diagram method there are a number of obstacles. One major problem is that the FORC distribution is not usually easily represented by analytical functions and due to that inconvenient are difficult to handle in Preisach-type simulations. Another solution would be to evaluate the geometric properties of the FORC distribution using statistical formulae applied directly to the experimental FORC. In our previous work [Tanasa et al., 2005], we have presented such a statistical approach of the experimental FORC diagram measured on the thermal hysteresis of spin-transition materials. In the full paper we are presenting in details a set of statistical parameters which can be used directly on experimental FORC distribution.

Deterministic and non-deterministic switching in chains of magnetic hysterons

This paper presents a fundamental analysis of a single-domain ferromagnetic particles chain hysteresis in perpendicular geometry as a prototype for ultra-high density memories. Due to magnetostatic long range interactions the system has a complex hysteresis but stable features can be found. The loop has a number of deterministic Barkhausen jumps and consequently a number of stable plateaus that could be used in multistate memories. The fundamental elements that sustain this behavior are shown and discussed.