Magnetic Interactions in the Double Perovskites R₂NiMnO₆ (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction

R₂NiMnO₆ (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic <i>P</i>2₁/<i>n</i> space group, with an almost complete order between Ni²⁺ and Mn⁴⁺ cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La₂NiMnO₆, with a ferromagnetic structure involving Mn⁴⁺ and Ni²⁺ moments. This spin alignment can be rationalized taking into account the Goodenough–Kanamori rules. The magnetic ordering temperature (<i>T</i>_CM) decreases abruptly as the size of the rare earth decreases, since <i>T</i>_CM is mainly influenced by the superexchange interaction between Ni²⁺ and Mn⁴⁺ (Ni²⁺–O–Mn⁴⁺ angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below <i>T</i>_CM

Half-Metallicity in Pb₂CoReO₆ Double Perovskite and High Magnetic Ordering Temperature in Pb₂CrReO₆ Perovskite

Pb₂MReO₆ (M = Co, Cr) perovskites were prepared by high pressure–high temperature method. Rietveld refinements of synchrotron powder X-ray diffraction show that the crystal structure of Pb₂CoReO₆ is trigonal (space group <i>R</i>-3) with almost complete ordering of Co and Re cations, while Pb₂CrReO₆ (space group <i>Pm-3m</i>) is a cubic perovskite with one single site for Cr and Re atoms. The difference between the symmetry and the degree of order was further clarified by X-ray absorption spectroscopy that establishes formal oxidation states in these phases as Pb₂Co²⁺Re⁶⁺O₆ and Pb₂Cr³⁺Re⁵⁺O₆. Pb₂CrReO₆ is a simple perovskite with a high magnetic ordering temperature of 643 K. Pb₂CoReO₆ is a double perovskite with −23% high field negative magnetoresistance at 10 K and 9 T. First-principles calculations of Pb₂CoReO₆ indicate a half metallic electronic structure

Mn₂MnReO₆: Synthesis and Magnetic Structure Determination of a New Transition-Metal-Only Double Perovskite Canted Antiferromagnet

Transition-metal-only double perovskite oxides (A₂BB′O₆) are of great interest due to their strong and unusual magnetic interactions; only one compound, Mn₂FeReO₆, was reported in this category to date. Herein, we report the second transition-metal-only double perovskite, Mn₂MnReO₆, prepared at high pressure and temperature. Mn₂MnReO₆ crystallizes in a monoclinic <i>P</i>2₁/<i>n</i> structure, as established by synchrotron X-ray and powder neutron diffraction (PND) methods, with eight-coordinated A sites and rock-salt arrangement of the B and B′-site MnO₆ and ReO₆. Both the structural analysis and the X-ray absorption near edge spectroscopy results indicate mixed valence states of the B/B′-site in Mn²⁺₂Mn^2+/3+Re^5+/6+O₆. The magnetic and PND studies evidence an antiferromagnetic (AFM) transition at ∼110 K and a transition from a simple AFM to canted AFM with net ferromagnetic component at ∼50 K. The observed Efros–Shklovskii variable-range-hopping semiconducting behavior is attributed to the three (A-site Mn²⁺, B-site Mn^2+/3+, and B′-site Re^5+/6+) interpenetrating canted AFM lattices. Theoretical calculations demonstrate that the almost fully polarized Mn states in Mn₂MnReO₆ are driven away from the Fermi level by static on-site interactions and open a small gap, which is responsible for the insulating state in such a d-electron-rich system. These results provide insight of the electronic origin of the physical properties of Mn₂MnReO₆ with local electronic structure similar to that of Mn₂FeReO₆

Strong Electron Hybridization and Fermi-to-Non-Fermi Liquid Transition in LaCu₃Ir₄O₁₂

The AA′₃B₄O₁₂-type quadruple perovskite LaCu₃Ir₄O₁₂ prepared at high pressure (9 GPa) and temperature (1523 K) crystallizes in cubic symmetry (<i>Im</i>3̅, <i>a</i> = 7.52418(3) Å) with square planar CuO₄ and octahedral IrO₆ coordination as established from synchrotron powder X-ray diffraction studies. Both crystal structure and X-ray absorption near edge spectroscopy analyses indicate formal oxidation states of LaCu²⁺₃Ir^3.75+₄O₁₂. The temperature dependence of resistivity of LaCu₃Ir₄O₁₂ is metallic down to 10 K, with Femi-liquid behavior above <i>T</i>* ∼ 155 K, and non-Fermi-liquid behavior below <i>T</i>*. The two-fluid behavior of magnetic susceptibility and the dramatic downturn of the resistivity below <i>T</i>* indicate strong Cu²⁺ 3d and Ir^3.75+ 5d orbital hybridization below <i>T</i>*, also supported by an enhanced electronic specific heat coefficient at low temperature. Theoretical calculations are in good agreement with the experimental results and show that the electronic structure of LaCu₃Ir₄O₁₂ is different from that of CaCu²⁺₃Ir⁴⁺₄O₁₂, which is also metallic down to 0.5 K, but presents non-Fermi liquid behavior above <i>T</i>* ∼ 80 K and strong Cu-3d–Ir-5d orbital coupling at significantly lower temperature (<i>T</i> < <i>T</i>* ∼ 80 K)