Yoshihiro Doi

Crystal structures and magnetic properties of ordered perovskites Sr2LnRuO6 (Ln = Eu-Lu)

The perovskite-type compounds Sr2LnRuO6 (Ln = Eu-Lu) have been synthesized, and their crystal structures and magnetic properties have been investigated. Powder x-ray diffraction measurements and the Rietveld analysis show that they are monoclinic with space group P21/n and that Ln3+ and Ru5+ ions are structurally ordered. These compounds show complex magnetic transitions at low temperatures. These transitions are considered to be antiferromagnetic ones from large negative Weiss constants. Below the transition temperatures, there is a large difference in the temperature dependence of the magnetic susceptibility measured under the zero-field cooled condition and under the field cooled condition, which shows the existence of a weak ferromagnetic moment associated with the antiferromagnetism.

Specific Heat Study on Sm-based Filled Skutterudite Phosphides SmT4P12(T=Fe, Ru and Os)

Sm-based filled skutterudite phosphides SmFe 4 P 12 , SmRu 4 P 12 and SmOs 4 P 12 synthesized at high temperatures and high pressures have been studied by specific heat measurements in magnetic fields. SmFe 4 P 12 shows an anomaly at 1.6 K attributed to a ferromagnetic transition. SmRu 4 P 12 shows a metal–insulator transition at 16 K. This transition consists of two successive transitions, which suggests orbital and antiferromagnetic orderings. SmOs 4 P 12 shows a sharp anomaly at 4.5 K due to an antiferromagnetic ordering. We discuss the characteristic behavior of those phase transitions and the crystalline electric field state in Sm T 4 P 12 ( T =Fe, Ru and Os). Interestingly, the magnetic entropy of SmFe 4 P 12 have only 0.16 R ln 2 below the ferromagnetic ordering temperature. This suggests a strong spin fluctuation effect. On the other hand, the variation of magnetic entropy in SmRu 4 P 12 and SmOs 4 P 12 reaches to nearly R ln 4 below the ordering temperature.

Magnetic properties of the antiferromagnetic double perovskite Ba2PrRuO6

The ordered perovskite compound Ba2PrRuO6 is prepared and its magnetic properties are investigated. The Rietveld analysis of the neutron diffraction profiles measured at 150 K shows that the Pr3+ and Ru5+ ions are arranged with regularity over the six-coordinate B sites of the perovskite ABO3 and that Ba2PrRuO6 belongs to space group P21/n, with a = 6.0063(5), b = 5.9863(4), c = 8.4677(7) A and 90.04(2)°. The magnetic susceptibility and the heat capacity measurements show that this compound transforms to an antiferromagnetic state below 117 K. From the neutron diffraction patterns measured at 7 K, the magnetic structure is determined to be of Type I and the magnetic moments of Pr3+ and Ru5+ are estimated to be 2.2(1) and 2.0(2) µB, respectively. Their values are discussed on the basis of theoretical calculations for the crystal field splitting.

Magnetic properties of ordered perovskites Ba2LnTaO6 (Ln = Y, lanthanides)

Magnetic properties of the quaternary oxides Ba2LnTaO6 (Ln = Y or lanthanides) are reported. Their powder x-ray diffraction measurements and Rietveld analysis show that they have an ordered perovskite structure and are monoclinic (Ln = La-Tb) with space group P21/n or cubic (Ln = Y, Dy-Lu) with space group Fmm. Magnetic susceptibilities of Ba2LnTaO6 were measured. All compounds in this work are paramagnetic down to 5 K. Ba2EuTaO6 and Ba2SmTaO6 show Van Vleck paramagnetism; their spin-orbit coupling constants are determined to be 331 cm-1 and 287 cm-1, respectively. 151Eu Mossbauer measurements for Ba2EuTaO6 were carried out at 12, 100, 200 and 300 K. The Eu ion is in the trivalent state, and the symmetry of the Eu site is distorted from the octahedral symmetry. The Debye temperature of Eu3+ was determined to be 335 K from the temperature dependence of the absorption area of intensity curves.

Crystal structures and magnetic properties of magnetically frustrated systems BaLn2O4 and Ba3Ln4O9 (Ln = lanthanide)

The crystal structures and magnetic properties of the ternary oxides BaLn2O4 (Ln = Pr, Nd, Sm–Ho) and Ba3Ln4O9 (Ln = Dy–Lu) have been investigated. These two series of compounds have the CaFe2O4-type structure with space group Pnma and a perovskite-related structure with space group R3, respectively. In both structures, the Ln ions locally adopt a triangle-based array, which has the possibility of resulting in a geometrically frustrated magnet. From the magnetic susceptibility measurements, it was found that some compounds show a magnetic anomaly at 2.2–4.0 K and the others are paramagnetic down to 1.8 K. The results of the specific heat measurements show that these compounds show a long-range antiferromagnetic ordering of Ln ions at 1.7 K (for BaNd2O4), 2.6 K (BaGd2O4), 0.8 K (BaHo2O4), and 4.0 K (Ba3Er4O9).

Magnetic and calorimetric studies of double perovskites Ba2LnReO6(Ln= Y, Nd, Sm–Lu)

The perovskite-type compounds Ba2LnReO6 (Ln = Y, Nd, Sm–Lu) have been synthesized. Powder X-ray diffraction measurements and Rietveld analysis show that they are monoclinic with space group P21/n and that Ln3+ and Re5+ ions are structurally ordered. Anomalies are found in their magnetic susceptibility and specific heat measurements, which suggest an antiferromagnetic ordering of Re5+ ions for Ba2LnReO6 (Ln = Y, Nd, Sm, Gd–Ho, Lu) and that of Tb3+ ions for Ba2TbReO6.

Magnetic and neutron diffraction studies on double perovskites A2LnRuO6(A = Sr, Ba; Ln = Tm, Yb)

Magnetic properties of double perovskites A2LnRuO6 (A = Sr, Ba; Ln = Tm, Yb) have been reported. Powder neutron diffraction measurements have been performed at 10 K and higher temperatures (≥100 K) to determine their crystal and magnetic structures. As a result of the Rietveld analysis of the diffraction profiles, it is found that they are monoclinic with space group P21/n (A = Sr) or cubic with space group Fmm (A = Ba). From the magnetic susceptibility and specific heat measurements, a magnetic transition at 36–48 K is observed in each compound. The neutron diffraction data collected at 10 K show that this magnetic transition is due to a long range antiferromagnetic ordering involving both Ru5+ and Ln3+ ions. Each of the magnetic moments of Ru5+ and Ln3+ orders in a type I arrangement.

The structural and magnetic characterization of 6H-perovskite-type oxides Ba3LnIr2O9 (Ln = ?Y, lanthanides)

The crystal structures and magnetic properties of quaternary oxides Ba3LnIr2O9 (Ln = Y, lanthanides) are reported. Rietveld analyses of their x-ray diffraction data indicate that they adopt the 6H-perovskite-type structure with space group P63/mmc or, in the case of Ln = La and Nd only, a monoclinically distorted structure with space group C 2/c. They have the valence state of Ba3Ln4+Ir24+O9 (for Ln = Ce, Pr, and Tb) or Ba3Ln3+Ir24.5+O9 (for Ln = Y and other lanthanides). Measurements of the magnetic susceptibility and specific heat were carried out for Ln = Y, Ce, Pr, Tb, and Lu. It was found that the effective magnetic moment of Ir ions is significantly small, which indicates the existence of the strong antiferromagnetic interaction between Ir ions in the Ir2O9 bioctahedra. The Y and Lu compounds with an unpaired magnetic moment remaining in the Ir24.5+O9 show an antiferromagnetic transition at 4.0 and 5.1 K, respectively. For Ba3TbIr2O9, a long-range magnetic ordering of Tb4+ ions was found at 2.0 K.

Crystal Structures and Magnetic Properties of New Europium Melilites Eu2MSi2O7(M = Mg, Mn) and Their Strontium Analogues

Synthesis, crystal structures, and magnetic properties of melilite-type oxides A(2)MSi(2)O(7) (A = Sr, Eu; M = Mg, Mn) were investigated. These compounds crystallize in the melilite structure with space group P4̅2(1)m. The (151)Eu Mössbauer measurements show that the Eu ions are in the divalent state. The Eu(2)MgSi(2)O(7) is paramagnetic down to 1.8 K. Long-range antiferromagnetic ordering is observed at 3.4 K for Sr(2)MnSi(2)O(7). On the other hand, the Eu(2)MnSi(2)O(7) shows a ferrimagnetic transition at 10.7 K. From the magnetization and specific heat measurements, it is found that the Eu(2+) (14 μ(B)) and Mn(2+) (5 μ(B)) sublattices order antiferromagnetically. This result indicates that an interaction between f-d electrons (Eu-Mn) predominantly operate in this compound.

Structural and magnetic properties of pseudo-two-dimensional triangular antiferromagnets Ba3MSb2O9 (M?=?Mn, Co, and Ni)

The crystal structure and magnetic properties of 6H-perovskites Ba3MSb2O9 (M = Mn, Co, and Ni) were investigated. The M = Co and Ni compounds have a hexagonal structure with space group P63/mmc, while the Mn compound has a monoclinically distorted structure with space group C2/c. From the results of magnetic susceptibility and specific heat measurements, it was found that they show an antiferromagnetic transition at 10.0 K (for M = Mn), 3.8 K (Co) and 13.5 K (Ni), and the onset of the magnetic transition was observed much above TN. The powder neutron diffraction measurements for Mn and Co compounds were carried out, and their magnetic structures were determined. Both compounds adopt the non-collinear 120° structure in the ab-plane. These magnetic properties show the existence of a geometric magnetic frustration derived from the triangular array of magnetic M ions in the ab-plane.