Magnetic behaviour of perovskite compositions derived from BiFeO3

The phase content and sequence, the crystal structure, and the magnetic properties of perovskite solid solutions of the (1−y)BiFeO3–yBiZn0.5Ti0.5O3 series (0.05 ≤ y ≤ 0.90) synthesized under high pressure have been studied. Two perovskite phases, namely the rhombohedral R3c and the tetragonal P4mm, which correspond to the structural types of the end members, BiFeO3 and BiZn0.5Ti0.5O3, respectively, were revealed in the as-synthesized samples. The rhombohedral and the tetragonal phases were found to coexist in the compositional range of 0.30 ≤ y ≤ 0.90. Magnetic properties of the BiFe1−y [Zn0.5Ti0.5]yO3 ceramics with y < 0.30 were measured as a function of temperature. The obtained compositional variations of the normalized unit-cell volume and the Néel temperature of the BiFe1−y [Zn0.5Ti0.5]yO3 perovskites in the range of their rhombohedral phase were compared with the respective dependences for the BiFe1−yB 3+yO3 perovskites (where B 3+ = Ga, Co, Mn, Cr, and Sc). The role of the high-pressure synthesis in the formation of the antiferromagnetic states different from the modulated cycloidal one characteristic of the parent BiFeO3 is discussed.publishe

Interplay of Spin and Spatial Anisotropy in Low-Dimensional Quantum Magnets with Spin 1/2

Quantum Heisenberg chain and square lattices are important paradigms of a low-dimensional magnetism. Their ground states are determined by the strength of quantum fluctuations. Correspondingly, the ground state of a rectangular lattice interpolates between the spin liquid and the ordered collinear N&eacute;el state with the partially reduced order parameter. The diversity of additional exchange interactions offers variety of quantum models derived from the aforementioned paradigms. Besides the spatial anisotropy of the exchange coupling, controlling the lattice dimensionality and ground-state properties, the spin anisotropy (intrinsic or induced by the magnetic field) represents another important effect disturbing a rotational symmetry of the spin system. The S = 1/2 easy-axis and easy-plane XXZ models on the square lattice even for extremely weak spin anisotropies undergo Heisenberg-Ising and Heisenberg-XY crossovers, respectively, acting as precursors to the onset of the finite-temperature phase transitions within the two-dimensional Ising universality class (for the easy axis anisotropy) and a topological Berezinskii&ndash;Kosterlitz&ndash;Thouless phase transition (for the easy-plane anisotropy). Experimental realizations of the S = 1/2 two-dimensional XXZ models in bulk quantum magnets appeared only recently. Partial solutions of the problems associated with their experimental identifications are discussed and some possibilities of future investigations in quantum magnets on the square and rectangular lattice are outlined

Syntheses, crystal structure and magnetocaloric effect of [Gd(PDOA)(NO3)(H2O)2]n

The coordination polymer [Gd(PDOA)(NO)(HO)] (1) (PDOA = o-phenylenedioxydiacetato) has been prepared and spectroscopically and structurally characterized. Its crystal structure is formed of a ribbon-like arrangement of Gd(III) atoms linked by bridging carboxylate groups and placed at the cusps of fused triangles with Gd⋯Gd distances of about 6.1 Å. The Gd(III) central atoms exhibit decacoordination by oxygen atoms from hexadentate PDOA ligands with chelating and bridging functions, a chelating nitrato ligand and two aqua ligands. Intra- and intermolecular hydrogen bonds of the O[sbnd]H⋯O and C[sbnd]H⋯O types contribute to the stability of the structure. The temperature dependence of the magnetic susceptibility revealed weak magnetic interactions among Gd(III) ions which may be attributed to dipolar coupling. Investigation of the magnetocaloric effect gives an estimate of the maximum value of isothermal change of magnetic entropy –ΔS ≈ 35 J kg K which suggests that 1 can be a good material for magnetic cooling at low temperatures.This work was supported by the Slovak grants VEGA (grant No. 1/0063/17) and APVV-14-0073. We thank also the European Union Regional Development Fund (Slovakia) (ITMS: 26220120005) for financial support. Funding from the Spanish Ministry of Science and Innovation under grant MAT2015-68200-C2-1-P, from the European Regional Development Fund, and from the Diputación General de Aragón (E16) is gratefully acknowledged.Peer Reviewe

Experimental Study of Magnetocaloric Effect in Tetraaquabis(Hydrogen Maleato)Nickel(II), [Ni(C₄H₃O₄)₂(H₂O)₄]—A Potential Realization of a Spin-1 Spatially Anisotropic Square Lattice with Ferromagnetic Interactions

An experimental study of the magnetocaloric effect in tetraaquabis(hydrogen maleato)nickel(II), [Ni(C4H3O4)2(H2O)4] powder sample is presented. The magnetocaloric properties of the studied sample were investigated using specific heat and magnetization measurements in magnetic fields up to 9 T in the temperature range from 0.4 to 50 K. A large conventional magnetocaloric effect was found at a temperature of about 3.5 K, where −ΔSM = 8.5 Jkg−1K−1 and 11.2 Jkg−1K−1 for a magnetic field of 5 T and 7 T, respectively. Assuming a substantial role of the crystal field, the temperature dependence of the magnetic specific heat in a zero magnetic field was compared with an S = 1 model with single-ion anisotropy parameters D and E (axial and rhombic). The best agreement was found for the parameters D/kB = −7.82 K and E/kB = −2.15 K. On the other hand, the experimental temperature dependence of −ΔSM shows higher values compared to the theoretical prediction for the mentioned model, indicating the presence of additional factors in the system, such as an exchange interaction between magnetic ions. The first exchange pathway can be realized through maleic rings between the nearest Ni(II) ions. The second exchange pathway can be realized through water molecules approximately along the a crystallographic axis. Broken-symmetry DFT calculations performed using the computational package ORCA provided the values of ferromagnetic exchange interactions, J1/kB = 1.50 K and J2/kB = 1.44 K (using B3LYP functional). The presence of such ferromagnetic correlations in the studied system may explain the enhanced magnetocaloric effect compared with the model of an anisotropic spin-1 paramagnet

Phase transformation in quasi-two-dimensional quantum antiferromagnet Cu(tn)Cl-2 (tn=1,3-diaminopropane)

Cu(tn)Cl-2 belongs to fa m i l y of molecular magnets with low-dimensional magnetism mediated by hydrogen bonds. Here, the X-ray diffraction, electron paramagnetic resonance, specific heat measurements, and ab initio calculations performed within the framework of density functional theory have been employed to investigate the structural phase transition in Cu(tn)Cl-2. Satellite reflections in the X-ray diffraction patter n from single crystals, visible anomaly in the specific heat measured in a zero magnetic field, rapid growth of the EPR resonance line width, along with the increase of the g-factors in the vici n i t y of 160 K are indicative of the structural phase transition in Cu(tn)Cl-2. Our results reveal that this system undergoes transition from a disordered structure of the Pnma symmetry to a modulated structure with the Pnma(0 beta 0)s00 superspace group. The transition is reversible and driven by the reorientation of the diaminopropane ligands. The density functional theory studies support results of our specific heat measurements.Web of Science12634145801457

Study of a magnetic-cooling material Gd(OH)CO3_{3}

The magnetocaloric effect of orthorhombic Gd(OH)CO3has been experimentally studied, which exhibits −ΔSmup to 66.4 J kg−1K−1(355 mJ cm−3K−1) for ΔH= 7 T andT= 1.8 K.</p