Universal Polar Instability in Highly Orthorhombic Perovskites.

The design of novel multiferroic ABO perovskites is complicated by the presence of necessary magnetic cations and ubiquitous antiferrodistortive modes, both of which suppress polar distortions. Using first-principles simulations, we observe that the existence of quadlinear and trilinear invariants in the free energy, coupling tilts, and antipolar motions of A and B sites to the polar mode drives an avalanche-like transition to a non-centrosymmetric 2 symmetry in a wide range of magnetic perovskites with small tolerance factors, overcoming the above restrictions. We find that the 2 phase is especially favored with tensile epitaxial strain, leading to an unexpected but technologically useful out-of-plane polarization.

The Interplay of Electronic Configuration and Anion Ordering on the Magnetic Behavior of Hydroxyfluoride Diaspores.

We report a new nickel hydroxyfluoride diaspore Ni(OH)F prepared using hydrothermal synthesis from NiCl·6HO and NaF. Magnetic characterization reveals that, contrary to other reported transition-metal hydroxyfluoride diaspores, Ni(OH)F displays weak ferromagnetism below the magnetic ordering temperature. To understand this difference, neutron diffraction is used to determine the long-range magnetic structure.

First-principles investigation of the magnetoelectric properties of BaMnO.

Type-II multiferroics, in which the magnetic order breaks inversion symmetry, are appealing for both fundamental and applied research due their intrinsic coupling between magnetic and electrical orders. Using first-principles calculations we study the ground state magnetic behaviour of BaMnOwhich has been classified as a type-II multiferroic in recent experiments. Our constrained moment calculations with the proposed experimental magnetic structure shows the spontaneous emergence of a polar mode giving rise to an electrical polarisation comparable to other known type-II multiferroics.

Quantum Spin-1/2 Dimers in a Low-Dimensional Tetrabromocuprate Magnet.

This work describes a homometallic spin- tetrabromocuprate adopting a bilayer structure. Magnetic-susceptibility measurements show a broad maximum centred near 70 K, with fits to this data using a Heisenberg model consistent with strong antiferromagnetic coupling between neighbouring copper atoms in different layers of the bilayer. There are further weak intralayer ferromagnetic interactions between copper cations in neighbouring dimers.

Striping of orbital-order with charge-disorder in optimally doped manganites.

The phase diagrams of LaMnO perovskites have been intensely studied due to the colossal magnetoresistance (CMR) exhibited by compositions around the [Formula: see text] doping level. However, phase segregation between ferromagnetic (FM) metallic and antiferromagnetic (AFM) insulating states, which itself is believed to be responsible for the colossal change in resistance under applied magnetic field, has prevented an atomistic-level understanding of the orbital ordered (OO) state at this doping level. Here, through the detailed crystallographic analysis of the phase diagram of a prototype system (AMn[Formula: see text]Mn[Formula: see text]O), we show that the superposition of two distinct lattice modes gives rise to a striping of OO Jahn-Teller active Mn and charge disordered (CD) Mn layers in a 1:3 ratio.