AFM Structure Research Articles

The effect of oxygen isotope substitution on the phase diagram of nearly half-dopedR1−xSrxMnO3 manganites(R = Sm,NdTb, NdEu)

The effect of isotope substitution on the electrical resistance, magneticsusceptibility and neutron diffraction patterns of nearly half-dopedR1−xSrxMnO3 manganites was studied for ceramic samples withR = Sm (x = 0.425–0.525),Nd/Tb (x = 0.45)and Nd/Eu (x = 0.45). In Nd/Tb and Nd/Eu manganites, the composition was chosen to yield the same mean ionicradius as in the Sm compound, but the samples differed in the cation disorder parameterσ. The oxygen isotope substitution leads to fundamental changes in the phase diagram of themanganites studied, favouring enhanced inhomogeneity, stabilizing the insulatingantiferromagnetic (AFM) state and even causing a metal–insulator transition (MIT). TheMIT induced by the oxygen isotope substitution was found here for the first time inmanganites with A-type AFM; earlier this unusual phenomenon was observed only forcompounds with the CE-type AFM structure. This suggests that such a MIT is closelyrelated to the ferro–AFM crossover independently on the specific nature of the AFM phaseand related orbital order.

Synthesis, Cation Ordering, and Magnetic Properties of the (Sb1-xPbx)2(Mn1-ySby)O4 Solid Solutions with the Sb2MnO4-Type Structure

Single phase (Sb1-xPbx)2(Mn1-ySby)O4 (0.0 ≤ x ≤ 0.608, 0.0 ≤ y ≤ 0.372) samples with the Sb2MnO4−type structure were prepared at 650 °C by solid-state reaction in evacuated sealed silica tubes. A replacement of Sb by Pb results in the oxidation of Sb3+ to Sb5+, which in turn replaces Mn2+ cations in octahedrally coordinated positions within the infinite rutile-type chains. The crystal structures of Pb0.44Sb1.64Mn0.92O4, Pb0.75Sb1.48Mn0.77O4, Pb1.07Sb1.26Mn0.67O4, and Pb1.186Sb1.175Mn0.639O4 were refined from X-ray powder diffraction data. Increasing the Pb content leads to a decrease of the a parameter from a = 8.719(2) Å to a = 8.6131(8) Å and to an increase of the c parameter from c = 5.999(2) Å to c = 6.2485(7) Å (for Sb2MnO4 and Pb1.216Sb1.155Mn0.628O4, respectively). This occurs due to increasing average cation size at the Pb/Sb position and decreasing cation size at the Mn/Sb position that leads to strong deformation of the (Mn/Sb)O6 octahedra. Starting from the Pb0.75Sb1.48Mn0.77O4 composition a modulated structure with q = γc* was observed by electron diffraction. Hig-resolution electron microscopy observations revealed that Mn and Sb ions order forming layers of octahedrally coordinated positions filled either by Mn2+ or by Sb5+ cations and alternating along the c axis. The dilution of the magnetic Mn2+ cations by nonmagnetic Sb5+ entities leads to a suppression of the antiferromagnetic intrachain interaction and disappearance of long-range magnetic order at high doping level. At T = 20 K the Axy spin component was found to be dominant in the AFM structure of the Pb0.44Sb1.64Mn0.92O4 sample by neutron diffraction.

Spin dynamics and spin disorder in frustrated TbCo xNi 1− xC 2

The orthogonal dicarbides RTC 2 ( R= rare earth , T= transition metal ) show ferromagnetic (fm) order of the R ions below T C ≈30 K for T=Co with a being the easy axis and antiferromagnetic (afm) order (with about the same transition temperature) for T=Ni with c as easy axis. The spin turning is caused by changes in the crystalline electric field (CEF) interaction. In the mixed TbCo x Ni 1− x C 2 alloys, fm and afm exchange on the one side and different CEF interactions on the other compete as a function of x. These competing interactions are a source of magnetic frustration. They also cause the materials to undergo several magnetic transitions into fm, afm or spin modulated structures which are well documented by neutron diffraction. Zero, longitudinal and transverse field μSR was carried out on TbCo x Ni 1− x C 2 with x=0.3, 0.5 0.7 and 0.85 using the same samples as in the neutron studies. Marked local spin disorder and persistent spin dynamics were seen in all magnetically ordered states as a consequence of frustration, but distinct differences exist for different values of x. Not all reported phase transition show clearly in the μSR data. In TbCo 0.85Ni 0.15C 2, a short-range dynamically correlated spin state having a strong fm polarizability exists above T C =30 K extending for about 50 K .

Phase formation in the system CaO–Al 2O 3–B 2O 3–H 2O at 23±1°C

Soluble borates are known to interfere with the hardening of cement. Phase formation in the quaternary system CaO–Al 2O 3–B 2O 3–H 2O at 23±1°C was studied and the range of mole ratios permitting the removal of soluble borates from solution has been established. Hydrating the powder composition with molar ratio Ca(OH) 2:H 3BO 3:C 3A 1 In cement nomenclature, C 3A=3CaO·Al 2O 3 (tricalcium aluminate). 1 =3:4:1 resulted in the formation of phase pure 6CaO·Al 2O 3·2B 2O 3·39H 2O after 3 months of equilibration. During the early stages of hydration, though, the hexagonal calcium aluminate hydrate 4CaO·Al 2O 3·1/2B 2O 3·12H 2O and the calcium borate hydrate CaO·B 2O 3·6H 2O crystallized but were later consumed by the reaction to form phase pure 6CaO·Al 2O 3·2B 2O 3·39H 2O. At powder ratios of 3:3:1 and 3:2:1, 6CaO·Al 2O 3·2B 2O 3·39H 2O and 4CaO·Al 2O 3·1/2B 2O 3·12H 2O were present together at equilibrium. When powders were mixed in the molar proportion of 1:2/3:1, well crystallized 4CaO·Al 2O 3·1/2B 2O 3·12H 2O formed directly as a single phase within 5 h. The present study also demonstrates that boron may take on two coordinations, depending on which hydrate forms. Similarities between the hexagonal hydrates 4CaO·Al 2O 3·1/2B 2O 3·12H 2O and 4CaO·Al 2O 3·1/2CO 2·12H 2O suggest boron to be three-coordinated in the AFm structure, whereas the literature shows boron to be four-coordinated in 6CaO·Al 2O 3·2B 2O 3·39H 2O, which has the ettringite structure.

Neutron diffraction studies of nuclear magnetic ordering in silver

Nuclear antiferromagnetism in fcc silver metal, already investigated by NMR measurements, has been studied in a single crystal of109Ag by neutron absorption and diffraction techniques. Below the Neel temperature TN, a (001) Bragg reflection with a resolution limited width demonstrates long range order in a simple type-I AFM structure with the ordering vectork = (2π/a)(001) (“up-down” structure). The entropy at the transition,\({\text{S}}_c = 0.54R\) ln 2 in zero magnetic field, corresponding to a critical polarization Pc= 0.75 and TN= 700 ± 80 pK. Magnetic field B versus entropy S phase diagrams of the (001) structure have been constructed for two directions ofB: [001] and \([\overline {0.8} {\text{ }}\overline {{\text{0}}{\text{.8}}} {\text{ }}1]{\text{.}}\) The critical field extrapolated to S = 0 is 100 ± 10 μT, and for both field directions Scis highest around B = 30 μT. The transition to the paramagnetic state is presumably of second order. The nuclear magnetization was measured by transmission of unpolarized neutrons, and the dimensionless static volume susceptibility in SI units was found to be 0.36 ± 0.01 in the ordered state independently of B and S. The ac susceptibility at 7.9 Hz showed a kink at the transition only when the sample was not exposed to neutrons.

Classification of magnetic structures in compounds with the Fe 2P-structure

A study is made of possible spin configurations in Fe 2P permitted by the symmetry considerations provided that the magnetic and crystallographic cells coincide. In the exchange approximation a number of magnetic structures is revealed. It is found that alongside with the standard ordering types: ferromagnetic (FM), ferrimagnetic (FIM), antiferromagnetic with the triangular spin lattice (AFM), there are non-trivial structures: antiferromagnetic-paramagnetic (AFM-PM) and antiferromagnetic-ferrimagnetic (AFM-FIM). The account of relativistic interactions leads to the upset of collinearity between the sublattice magnetic moments in the FM and FIM structures of the “easy-plane” type and in the basal plane of the AFM-FIM and AFM-PM structures. In the latter case a moment on the paramagnetic ion appears. These processes result in the change of the net moment and appearance of the weak ferromagnetic moment in the AFM-FIM and AFM-PM as well as in some AFM structures.