Long-range Order Interactions Research Articles

Crystallographic, magnetic and magnetocaloric properties in novel intermetallic materials R3CoNi (R = Tb, Dy, Ho, Er, Tm, Lu)

A family of novel intermetallic R3CoNi with heavy rare earth ions has been synthesized (R = Tb, Dy, Ho, Er, Tm, Lu) and a study of the crystal structure of these phases performed. All the compounds adopt the rhombohedral Er3Ni2-type structure [Pearson’s symbol hR45; space group R-3h (N. 148)]. A thorough investigation of their magnetic and magnetocaloric properties has been undertaken. Magnetization and ac-susceptibility measurements as a function of temperature show that the samples with Tb, Dy, Ho, Er, and Tm present a paramagnetic to ferromagnetic (PM-FM) transition at temperatures in the range 96–6 K; different reorientation transitions take place below the respective TC, in most cases with thermomagnetic irreversibility. Thermal and magnetic measurements have been used to retrieve the set of critical exponents (α, β, γ, δ) for the PM-FM transition to assign a universality class to Tb3CoNi, Dy3CoNi, and Ho3CoNi, with the result that, in the first case, it is close to the Mean Field model, in the second one it is between the Chiral Heisenberg and the XY-Chiral, and in the third one it is close to the XY-Chiral model. Therefore, in Tb3CoNi the transition is governed by long-range order interactions, whereas in Dy3CoNi and Ho3CoNi there must be some kind of frustrated non-collinear ferromagnetism. The magnetocaloric measurements in five members of the family indicate that all of them present highly competitive magnetocaloric properties in their respective temperature ranges, with high magnetic entropy changes (from 12.8 to 18.5 J/Kg.K at μ0ΔH = 5 T) and refrigerant capacities (from 412 to 699 J/Kg at μ0ΔH = 5 T). These results assess and highlight the applicative interest of these compounds, besides suggesting the possibility of tuning the range of the operating temperature by modifying the rare earth ion. Finally, universal curves for the magnetocaloric properties have been found for Tb3CoNi, Dy3CoNi, and Ho3CoNi; the scaling of the magnetocaloric variables confirms the validity of the assigned universality classes.

Influence of the PbO-excess on the structural, microstructural and ferroelectric properties of PLZT ceramics

The physical properties of lead lanthanum zirconate titanate (PLZT) ceramics have been investigated as a function of a PbO excess from 0 to 20 mol% in samples of Pb0.94La0.06(Zr0.6895Ti0.2955)O3 (composition that reveals the best performance for piezoelectric applications for materials with rhombohedral symmetry). Results are presented in a compressive way in the frame of an overview on PLZT. Structural, microstructural and ferroelectric properties were investigated as a function of the PbO content. The X-ray diffraction results confirmed the formation of the perovskite structure for all the cases. However, the presence of the Pb2O3 secondary phase has been observed for the sample containing 15 mol% PbO excess. Raman spectroscopy studies revealed a strong influence of the PbO content on the active modes shifts. The ferroelectric properties have shown a decrease of both remnant (PR) and saturation (PS) polarizations for compositions up to 10 mol% PbO, whereas an increase of PR and PS was observed for the higher PbO content samples. Results reveal the effect of the PbO excess on the long-range order interactions for the studied system.

Magnetocaloric properties, magnetic interactions and critical behavior in Ho6(Fe,Mn)Bi2 intermetallics

Four polycrystaline Fe2P-type Ho6(Fe,Mn)Bi2 intermetallic compounds (space group P6‾2m, No. 189, hP9) have been studied using magnetic techniques in order to explore their ability as magnetocaloric materials, and study the critical behavior of the paramagnetic (PM) to ferromagnetic (FM) transitions to obtain a deeper understanding of the range of the magnetic interactions. The obtained critical exponents β, γ and δ for the four compounds studied (Ho6MnBi2, Ho6FeBi2, Ho6(Mn0.5Fe0.5)Bi2, Ho6(Mn0.75Fe0.25)Bi2) point to long-range order interactions, as they are close to those of the Mean Field Universality class. All of the compounds show relevant magnetocaloric properties over a very broad temperature range, limited by the PM-FM transition and a spin-reorientation one, well separated in all cases. They present very high values of the refrigerant capacities (from 520 J/kg to 709 J/kg at 5 T), good magnetic entropy changes (from 3.4 to 5.7 J/(kgK) at 5 T), and a flat and wide temperature span for the working temperature range (nearly 200 K for Ho6MnBi2, 80 K for Ho6FeBi2 at 5T). The change in properties with composition proves that the magnetocaloric properties can be tuned in Fe2P-type compounds to accommodate different refrigeration applications. Finally, the magnetocaloric scaling laws have been successfully tested and universal curves for the magnetic entropy change have also been obtained in the PM-FM transition region.

Patterning in systems driven by nonlocal external forces.

This work focuses on systems displaying domain patterns resulting from competing external and internal dynamics. To this end, we introduce a Lyapunov functional capable of describing the steady states of systems subject to external forces, by adding nonlocal terms to the Landau Ginzburg free energy of the system. Thereby, we extend the existing methodology treating long-range order interactions, to the case of external nonlocal forces. By studying the quadratic term of this Lyapunov functional, we compute the phase diagram in the temperature versus external field and we determine all possible modulated phases (domain patterns) as a function of the external forces and the temperature. Finally, we investigate patterning in chemical reactive mixtures and binary mixtures under irradiation, and we show that the last case opens the path toward micro-structural engineering of materials.

Possible route to d0 magnetism in α-PbO compound

Using first principles methods, we investigated the possibility of inducing d0 magnetism in α-PbO. The Pbi interstitial defect was found to acquire magnetic properties after its incorporation into the α-PbO crystal structure. The Pbi interstitial defect generates a p localized state with two electrons on-site, the spin alignment of which is governed by Hund׳s rule, thereby leading to the formation of the local magnetic moment 2μB. The magnetic coupling between defects is a function of the occupancy of the defect states. For two defects in their zero charge state Pbi(0), the antiferromagnetic coupling is more stable. The Pb interstitial defects in the charge state Pbi(1+) already have the ferromagnetic ground state. The magnetic coupling was found to be driven by long-range order interactions. The critical limit on the defect concentration required to establish magnetic percolation above room temperature is about 1020cm−3, which may be possible when Pbi defects are formed on the surface of a single crystal. The substitution of the Pb interstitial defect with the non-magnetic elements of the s2px outer shell (the local magnetic moment is defined by 1≤x≤3) was found to be a potential means for tuning the magnetic behavior.

Non-magnetic impurities to induce magnetism in α-PbO crystal structure

A new route to d0 magnetism is established with the help of the first-principles methods. Non-magnetic elements in groups 13 and 14 of the periodic table are found to act as the magnetic centers upon embedding in polycrystalline α-PbO structure. Thus, the local magnetic moment is generated on the impurity site ( and for elements in groups 13 and 14, respectively) due to p orbitals partially filled with electrons whose on-site spin ordering is governed by the first Hund's rule. The magnetic interactions between impurities are controlled by occupation of the p orbitals so that antiferromagnetic (AFM) ordering occurs between impurities of while ferromagnetic (FM) ordering occurs between impurities possessing . With respect to the strength of the magnetic interactions, the atomic radius of impurity is found to be a key element to tune the wave function tails of localized electrons: with the reduction of the atomic radius, the on-site stability of the spin-polarized state grows while losing in the long-range order interactions. However, it has been shown that a suppression of the long-range order interactions can be compensated by higher impurity concentration that is allowed by the shift of the solubility limit to higher magnitude.

Pixel mapping analysis to characterize long-range order interactions in crystals under ion irradiation

High-dose irradiation can cause agglomeration of projectile particles if the impurity concentration exceeds a certain level. In previous classical molecular-dynamics calculations we have investigated the importance of ionimpacts for agglomeration. This has also shown that some long-range network forces exist in a crystal, which may relate phase transition and ion bombardment. In order to investigate the long-range order interactions, we propose a method to describe the lattice interactions macroscopically. We call it pixel mapping (PM). With the help of PM, we can clearly describe details of the intermediate processes and estimate the degree of crystallinity during ion irradiation. B implantation into a B doped Si crystal at different bombarding energies was analyzed by PM. Sudden changes from a crystalline to an amorphous state after a certain number of ion impacts were observed and analyzed by PM. PM described clearly these change as cooperative lattice reactions and revealed the long-range order interactions between target atoms in a crystal. Therefore, we can conclude that impurity agglomeration is cooperatively triggered by long-range interactions of atoms in a mesoscopic regime, and PM is a powerful method to elicit the long-range interactions of atoms in a crystal.