Magnetocaloric effect and magnetic ordering in GdFe1-xTxSi, T = Cr, V, Ni.

The intermetallic compounds GdFe1-xCrxSi, x = 0-0.8, GdFe1-xVxSi, x = 0-0.4, and GdFe1-xNixSi, x = 0-0.4 with a tetragonal CeFeSi (P4/nmm) structure type have been synthesized. The Curie temperature, TC, sharply increases from 130 K to 255 K and 250 K for the GdFe1-xCrxSi and GdFe1-xVxSi compounds and decreases to 104 K for GdFe1-xNixSi. Within the framework of the model of effective d-f exchange interaction in R3̄d intermetallics, these changes in TC can be caused by the corresponding changes in the density of states. The electronic structure, magnetic moments and types of magnetic orderings of the GdFe1-xTxSi, T = Cr, V, Ni intermetallic compounds were calculated using the DFT+U theoretical method. For the GdFe1-xNixSi system, the transformation of a ferromagnet with the composition x = 0 into an antiferromagnet with the composition x = 0.3 was established experimentally and using first-principles calculations. The correlation of the ferromagnetic or antiferromagnetic type of the magnetic state in the GdFe1-xNixSi compounds with the value of the lattice parameter c to greater or less than the critical value c = 6.72 Å for the GdCoSi antiferromagnet has been experimentally established. The magnetic structures of the antiferromagnets GdFe0.7Ni0.3Si and GdCoSi were found to be different. GdFe0.7Ni0.3Si is characterized by a collapse in the magnetocaloric effect via a change in the isothermal magnetic entropy ΔSM(TC). The compounds GdFe0.4Cr0.6Si with -ΔSM(TC) = 2.37 J kg-1 K-1 at TC = 255 K and RC = 82.07 J kg-1 and GdFe0.7V0.3Si with -ΔSM(TC) = 2.06 J kg K-1 at TC = 250 K and RC = 96.02 J kg-1 in a field changing to 17 kOe could be of practical interest due to the TC being close to room temperature.

Giant kinks in the entropy change temperature dependence of the magnetocaloric effect in layered phase-separated metals.

In this work, the validity of standard magnetocaloric (MCE) scenarios is revisited for the Hubbard model for a square (two-dimensional) lattice to describe a layered metal. Different types of magnetic ordering (ferrimagnetic, ferromagnetic, Néel and canted antiferromagnetic states) with magnetic transitions between them are considered to minimize the total free energy. The phase-separated states formed by such first-order transitions are also considered consistently. We employ the mean-field approximation to focus attention on the vicinity of a tricritical point, where the order of the magnetic phase transition changes from first to second and phase separation bounds merge. Two types of first-order magnetic transition can be found: PM-Fi, Fi-AFM; with further temperature growth, the phase separation boundaries between them merge and a second order transition, PM-AFM, is observed. The temperature and electron filling dependencies of the entropy change in the phase separation regions are investigated in detail in a consistent way. The dependence of the phase separation bounds on the magnetic field results in the existence of two different characteristic temperature scales. These temperature scales are indicated by giant kinks in the temperature dependence of the entropy, which are an exceptional attribute of phase separation in metals.

Disorder-order and order-order phase transformations in Ta5C4 phases predicted using the evolutionary algorithm and symmetry analysis.

A search for stable ordered phases in the nonstoichiometric cubic tantalum carbide TaC0.8 has been performed by use of the evolutionary algorithm and symmetry analysis. Four stable Ta5C4 superstructures with tetragonal, monoclinic, orthorhombic, and triclinic symmetry have been predicted for the first time. The DOS values of these Ta5C4 superstructures and stoichiometric TaC1.00 carbide have been calculated. All the tantalum carbide superstructures and stoichiometric TaC1.00 carbide have metal conductivity. The disorder-order phase transition channels TaCy → Ta5C4 associated with the formation of the considered model superstructures include superstructural vectors of non-Lifshitz stars {k1}, {k2}, and {k4}. The distribution functions of carbon atoms over the sites of the tetragonal, monoclinic, orthorhombic, and triclinic Ta5C4 superstructures have been calculated. For the first time, the physically permissible sequence of disorder-order and order-order phase transitions is established for the detected phases of the Ta5C4 family. Based on the formation enthalpy and the cohesion energy magnitudes, the triclinic Ta5C4 superstructure is the most favorable among all Ta5C4 phases predicted. The composition of the predicted Ta5C4 superstructures corresponds to TaC0.80 which possesses the highest melting temperature and hardness.

Magnetism of 3d and 4d doped Mn0.7T0.3NiGe (T = Fe, Co, Ru and Rh): bulk magnetization and ab initio calculations.

We compare the magnetic properties of 3d (Fe and Co) and 4d (Ru and Rh) transition metals doped MnNiGe using the combined results of magnetization and ab initio calculations. The alloys crystallize in austenite Ni2In-type hexagonal phase (space group: P63/mmc) with insignificant difference in the lattice parameters. Mn0.7Fe0.3NiGe and Mn0.7Co0.3NiGe exhibit spin-glass behavior, resulting from the competing ferro- and antiferromagnetic interactions. These alloys exhibit spontaneous exchange bias field of about [Formula: see text] Oe and 323 Oe, respectively. From the 4d-metal doped alloys, Mn0.7Ru0.3NiGe shows glassy behavior while long-range ferromagnetic order is confirmed in Mn0.7Rh0.3NiGe. In Mn0.7Rh0.3NiGe, in agreement with experiment and the theoretical calculations, the ground state is confirmed to be ferromagnetic because of the FM exchange interactions of the Mn magnetic moments. But in Mn1-x (Fe,Co,Ru) x NiGe alloys the calculations revealed the competing and comparable FM and AFM exchange interaction parameters, resulting in the formation of spin-glassy characteristics.

Impression of magnetic clusters, critical behavior and magnetocaloric effect in Fe3Al alloys.

Herein, we report unusual magnetic behavior in arc melted bulk stoichiometric Fe3Al alloy with a D03 structure. The temperature variation in the magnetization measurements revealed two transitions, i.e. one at ∼763 K and another at ∼830 K. Below 763 K, it exhibits ferromagnetic ordering and the nature of the transition is second order. However, the second transition is more complex and detailed analysis of the magnetic data suggested the coexistence of ferromagnetic and paramagnetic phases (two-phase: α + D03/α + B2) and structural transitions triggered by temperature. We observed dual peaks in the magnetic entropy change curve, in accordance with the magnetization results, which corroborate the occurrence of a two-phase system. We believe that the concurrent magnetic ordering and the complex two-phase are associated with the evolution of short-range ordered magnetic clusters having a magnetic moment of ∼103µB in the host matrix. A cluster hypothesis is proposed to explain the observed intricate magnetic behavior of this alloy at high temperature. The estimated critical exponents using a modified Arrott plot, Kouvel-Fisher plot and critical isotherm analysis lie in between the 3D-Heisenberg and 3D-Ising model, indicating a short-range interaction and magnetic inhomogeneity, which again are consistent with the magnetization results. The obtained critical exponents follow the universal scaling behavior, which indicates the renormalization of interactions around the magnetic ordering transition (TC). Despite the obvious larger thermal entropy at very high temperature, our synthesized Fe3Al alloy showed enhanced magnetic entropy changes. The obtained magnetic entropy change for binary Fe3Al alloy shows twice the value of that of other binary/ternary Fe-based alloys.

Revelation of spin glass behavior in Ru doped MnNiGe: experiment and theory.

We report on the nature of the magnetism in Ru substituted MnNiGe using the combined results of x-ray diffraction, dc-magnetization, ac-susceptibility and ab initio calculations. Mn0.7Ru0.3NiGe crystallizes in Ni2In-type hexagonal structure (P63/mmc) at room temperature with lattice parameters a = b = 4.099 [Formula: see text] and c = 5.367 [Formula: see text]. From the dc-magnetization; a broad peak around 46.55 K, separation between zero-field cooled and field-cooled warming state and non-saturating isothermal magnetization with typical S-type hysteresis indicate glassy behavior. A cusp in [Formula: see text] is observed to shift toward high temperatures with increasing frequency. Mydosh parameter ([Formula: see text]), single-relaxation time ([Formula: see text] s) obtained through critical slowing-down analysis, [Formula: see text] from the Vogel-Fulcher law and Tholence criterion [Formula: see text], confirm that Mn0.7Ru0.3NiGe belongs to the short-range interaction spin-glass systems with strong coupling between the magnetic clusters. LSDA+U calculations confirmed the competing exchange interactions between large magnetic moments of the Mn ions in Mn0.7Ru0.3NiGe compound resulting in the formation of spin-glassy characteristics.

Magnetization, resistivity, specific heat and ab initio calculations of Gd5Sb3.

We report on the combined results of the structural, magnetic, transport and calorimetric properties of Mn5Si3-type hexagonal Gd5Sb3, together with ab initio calculations. It exhibits a ferromagnetic (FM)-like transition at 265 K, antiferromagnetic (AFM) Néel transition at 95.5 K followed by a spin-orientation transition at 62 K. The system is found to be in AFM state down to 2 K in a field of 70 kOe. The FM-AFM phase coexistence is not noticeable despite large positive Curie-Weiss temperature ([Formula: see text] K). Instead, low-temperature AFM and high-temperature FM-like phases are separated in large temperatures. Temperature-magnetic field (H-T) phase diagram reveals field-driven complex magnetic phases. Within the AFM phase, the system is observed to undergo field-driven spin-orientation transitions. Field-induced tricritical and quantum critical points appear to be absent due to the strong AFM nature and by the intervention of FM-like state between paramagnetic and AFM states, respectively. The metallic behavior of the compound is inferred from resistivity along with large Sommerfeld parameter. However, no sign of strong electron-correlations is reasoned from the Kadowaki-Wood's ratio [Formula: see text] [Formula: see text] cm · (mol · K)2(mJ)-2, despite heavy γ. Essentially, ab initio calculations accounting for electronic correlations confirm AFM nature of low-temperature magnetic state in Gd5Sb3 and attainable FM ordering in agreement with experimental data.

Investigation of real materials with strong electronic correlations by the LDA+DMFT method.

Materials with strong electronic correlations are at the cutting edge of experimental and theoretical studies, capturing the attention of researchers for a great variety of interesting phenomena: metal-insulator, phase and magnetic spin transitions, `heavy fermion' systems, interplay between magnetic order and superconductivity, appearance and disappearance of local magnetic moments, and transport property anomalies. It is clear that the richness of physical phenomena for these compounds is a result of partially filled 3d, 4f or 5f electron shells with local magnetic moments preserved in the solid state. Strong interactions of d and f electrons with each other and with itinerant electronic states of the material are responsible for its anomalous properties. Electronic structure calculations for strongly correlated materials should explicitly take into account Coulombic interactions between d or f electrons. Recent advances in this field are related to the development of the LDA+DMFT method, which combines local density approximation (LDA) with dynamical mean-field theory (DMFT) to account for electronic correlation effects. In recent years, LDA+DMFT has allowed the successful treatment not only of simple systems but also of complicated real compounds. Nowadays, the LDA+DMFT method is the state-of-the-art tool for investigating correlated metals and insulators, spin and metal-insulator transitions (MIT) in transition-metal compounds in paramagnetic and magnetically ordered phases.

Ab initio study on the rare-earth iron-pnictides RFeAsO (R = Pr, Nd, Sm, Gd) in the low-temperature Cmma phase.

We present density functional theory calculations on the iron-based pnictides RFeAsO (R = Pr, Nd, Sm, Gd). The calculations have been carried out using plane waves and the projector augmented wave (PAW) pseudopotential approach. Structural, magnetic and electronic properties are studied within the generalized gradient approximation (GGA) and also within GGA + U in order to investigate the influence of electron correlation effects. The low-temperature Cmma structure is fully optimized by the GGA considering both non-magnetic and magnetic cells. We have found that the spin-polarized structure improves the agreement with experiments on equilibrium lattice parameters, particularly the c lattice parameter and the Fe-As bond-lengths. The electronic band structure, total density of states, and spin-dependent orbital-resolved density of states are also analyzed and discussed in the frameworks of GGA and GGA + U. For all materials, by including an on-site Coulomb correction, the rare-earth 4f states move away from the Fermi level and the Fermi level features of the systems are found to be mostly defined by the 3d electron-electron correlations in Fe.

Electronic structure and magnetic state of transuranium metals under pressure.

The electronic structures of bcc Np, fcc Pu, Am, and Cm pure metals under pressure have been investigated employing the LDA + U method with spin-orbit coupling (LDA + U + SO). The magnetic state of the actinide ions was analyzed in both LS and jj coupling schemes to reveal the applicability of corresponding coupling bases. It was demonstrated that whereas Pu and Am are well described within the jj coupling scheme, Np and Cm can be described appropriately neither in a {mσ}, nor in a {jmj} basis, due to intermediate coupling scheme realization in these metals that requires some finer treatment. The LDA + U + SO results for the considered transuranium metals reveal band broadening and gradual 5f electron delocalization under pressure.

NiO: correlated band structure of a charge-transfer insulator.

The band structure of the prototypical charge-transfer insulator NiO is computed by using a combination of an ab initio band structure method and the dynamical mean-field theory with a quantum Monte-Carlo impurity solver. Employing a Hamiltonian which includes both Ni d and O p orbitals we find excellent agreement with the energy bands determined from angle-resolved photoemission spectroscopy. This brings an important progress in a long-standing problem of solid-state theory. Most notably we obtain the low-energy Zhang-Rice bands with strongly k-dependent orbital character discussed previously in the context of low-energy model theories.

Doped Mott insulator as the origin of heavy-fermion behavior in LiV2O4.

We investigate the electronic structure of LiV2O4, for which heavy-fermion behavior has been observed in various experiments, by the combination of the local density approximation and dynamical mean field theory. To obtain results at zero temperature, we employ the projective quantum Monte Carlo method as an impurity solver. Our results show that the strongly correlated a 1g band is a lightly doped Mott insulator which, at low temperatures, shows a sharp (heavy) quasiparticle peak just above the Fermi level, which is consistent with recent photoemission experiments by Shimoyamada et al. [Phys. Rev. Lett. 96, 026403 (2006)10.1103/PhysRevLett.96.026403].