We report on the discovery of an isothermal structural transition observed in Bi${}_{1-x}$La${}_x$FeO${}_3$ ($0.17⩽x⩽0.19$) ceramics. At room temperature, an initially pure polar rhombohedral phase gradually transforms into a pure antipolar orthorhombic one. The polar phase can be recovered by annealing at $T>300\hspace{0.5em}{}^{°}$C. In accordance with neutron powder diffraction data, an inverse isothermal antipolar-polar transition takes place at $T>300$ ${}^{°}$C, where the polar phase becomes more stable. The antipolar phase is characterized by a weak ferromagnetic state, whereas the polar phase has been obtained in a mixed antiferromagnet–weak ferromagnet state. The relatively low external pressure induces polar-antipolar transition, but there is no evidence of electric-field-driven antipolar-polar transition. The observed large local piezoelectric response is associated with structural instability of the polar phase, whereas local multistate piezoelectric loops can be related to the domain wall pinning effect.

From magnetic, specific heat, ${}^{170}\mathrm{Yb}$ Mössbauer effect, neutron diffraction, and muon spin relaxation measurements on polycrystalline ${\mathrm{Yb}}_2{\mathrm{Sn}}_2{\mathrm{O}}_7$, we show that below the first order transition at 0.15 K all of the ${\mathrm{Yb}}^{3+}$ ions are long-range magnetically ordered and each has a moment of $1.1{\mu }_{\mathrm{B}}$ which lies at $\simeq 10°$ to a common fourfold cubic axis. The four sublattice moments have four different directions away from this axis and are therefore noncoplanar. We term this arrangement splayed ferromagnetism. This ground state has a dynamical component with a fluctuation rate in the megahertz range. The net ferromagnetic exchange interaction has an anisotropy that favors the local threefold axis. We discuss our results in terms of the phase diagram proposed by Savary and Balents [Phys. Rev. Lett. 108, 037202 (2012)] for a pyrochlore lattice of Kramers $1/2$ effective spins.

Magnetization, specific heat, and neutron scattering measurements were performed to study a magnetic transition in jarosite, a spin-$\frac{5}{2}$ kagome lattice antiferromagnet. When a magnetic field is applied perpendicular to the kagome plane, magnetizations in the ordered state show a sudden increase at a critical field $H_c$, indicative of the transition from antiferromagnetic to ferromagnetic states. This sudden increase arises as the spins on alternate kagome planes rotate ${180}^{°}$ to ferromagnetically align the canted moments along the field direction. The canted moment on a single kagome plane is a result of the Dzyaloshinskii-Moriya interaction. For $H<H_c$, the weak ferromagnetic interlayer coupling forces the spins to align in such an arrangement that the canted components on any two adjacent layers are equal and opposite, yielding a zero net magnetic moment. For $H>H_c$, the Zeeman energy overcomes the interlayer coupling causing the spins on the alternate layers to rotate, aligning the canted moments along the field direction. Neutron scattering measurements provide the first direct evidence of this ${180}^{°}$ spin rotation at the transition.

We present comprehensive neutron scattering studies of nonsuperconducting and superconducting electron-doped ${\mathrm{Pr}}_{0.88}{\mathrm{LaCe}}_{0.12}{\mathrm{CuO}}_{4\pm \delta }$ (PLCCO). At zero field, the transition from antiferromagnetic (AF) as-grown PLCCO to superconductivity without static antiferromagnetism can be achieved by annealing the sample in pure Ar at different temperatures, which also induces an epitaxial ${(\mathrm{Pr},\mathrm{La},\mathrm{Ce})}_2{\mathrm{O}}_3$ phase as an impurity. When the superconductivity first appears in PLCCO, a quasi-two-dimensional (2D) spin-density-wave (SDW) order is also induced, and both coexist with the residual three-dimensional (3D) AF state. A magnetic field applied along the $[\overline{1},1,0]$ direction parallel to the ${\mathrm{CuO}}_2$ plane induces a “spin-flop” transition, where the noncollinear AF spin structure of PLCCO is transformed into a collinear one. The spin-flop transition is continuous in semiconducting PLCCO, but gradually becomes sharp with increasing doping and the appearance of superconductivity. A $c$-axis aligned magnetic field that suppresses the superconductivity also enhances the quasi-2D SDW order at (0.5,0.5,0) for underdoped PLCCO. However, there is no effect on the 3D AF order in either superconducting or nonsuperconducting samples. Since the same field along the $[\overline{1},1,0]$ direction in the ${\mathrm{CuO}}_2$ plane has no (or little) effect on the superconductivity, (0.5,0.5,0) and ${(\mathrm{Pr},\mathrm{La},\mathrm{Ce})}_2{\mathrm{O}}_3$ impurity positions, we conclude that the $c$-axis field-induced effect is intrinsic to PLCCO and arises from the suppression of superconductivity.

A neutron-diffraction study of ${\mathrm{HgBa}}_2$ ${\mathrm{CuO}}_{4+\mathrm{\delta }}$ structure and its dependence on extra oxygen content for δ between 0.06 and 0.19 has been carried out with four pure samples using high-resolution TOF and λ=const diffractometers at room temperature and T=8 K. Sample characterization was done by measuring ac susceptibility, x-ray and EPR spectra, and by iodometric titration. A detailed analysis of atomic structure and the occupancy factors of mercury and oxygen was performed and showed the stoichiometric cation content. Extra oxygen was found only in the (1/2,1/2,0) position of the basal plane. ${\mathrm{T}}_{\mathrm{c}}$ varied parabolically with δ and exhibited a maximum of 98 K at δ≈0.12. The data obtained can be considered as evidence of the conventional anion doping mechanism with 2δ holes per ${\mathrm{CuO}}_2$ layer.