William Ratcliff

Electric field control of the magnetic state in BiFeO3 single crystals

Single crystals of multiferroic BiFeO3 were investigated using neutron scattering. Application of an electric field reversibly switches ferroelastic domains, inducing changes in the magnetic structure which follows rotation of the structural domains. In addition, electric fields can be used to control the populations of the equivalent magnetic domains within a single ferroelastic domain, possibly via field-induced strain.

Spin and lattice structures of single-crystalline Srfe2As2

We use neutron scattering to study the spin and lattice structure of single-crystal SrFe2As2, the parent compound of the FeAs-based superconductor (Sr,K)Fe2As2. We find that SrFe2As2 exhibits an abrupt structural phase transition at 220 K, where the structure changes from tetragonal with lattice parameters c>a=b to orthorhombic with c>a>b. At almost the same temperature, Fe spins develop a collinear antiferromagnetic structure along the orthorhombic a axis with spin direction parallel to this a axis. These results are consistent with earlier work on the RFeAsO (R=rare earth) families of materials and on BaFe2As2, and therefore suggest that static antiferromagnetic order is ubiquitous for the parent compounds of these FeAs-based high-transition temperature superconductors.

Atomically engineered ferroic layers yield a room-temperature magnetoelectric multiferroic

Materials that exhibit simultaneous order in their electric and magnetic ground states hold promise for use in next-generation memory devices in which electric fields control magnetism. Such materials are exceedingly rare, however, owing to competing requirements for displacive ferroelectricity and magnetism. Despite the recent identification of several new multiferroic materials and magnetoelectric coupling mechanisms, known single-phase multiferroics remain limited by antiferromagnetic or weak ferromagnetic alignments, by a lack of coupling between the order parameters, or by having properties that emerge only well below room temperature, precluding device applications. Here we present a methodology for constructing single-phase multiferroic materials in which ferroelectricity and strong magnetic ordering are coupled near room temperature. Starting with hexagonal LuFeO3—the geometric ferroelectric with the greatest known planar rumpling—we introduce individual monolayers of FeO during growth to construct formula-unit-thick syntactic layers of ferrimagnetic LuFe2O4 (refs 17, 18) within the LuFeO3 matrix, that is, (LuFeO3)m/(LuFe2O4)1 superlattices. The severe rumpling imposed by the neighbouring LuFeO3 drives the ferrimagnetic LuFe2O4 into a simultaneously ferroelectric state, while also reducing the LuFe2O4 spin frustration. This increases the magnetic transition temperature substantially—from 240 kelvin for LuFe2O4 (ref. 18) to 281 kelvin for (LuFeO3)9/(LuFe2O4)1. Moreover, the ferroelectric order couples to the ferrimagnetism, enabling direct electric-field control of magnetism at 200 kelvin. Our results demonstrate a design methodology for creating higher-temperature magnetoelectric multiferroics by exploiting a combination of geometric frustration, lattice distortions and epitaxial engineering.

Low Energy Spin Waves and Magnetic Interactions in SrFe2As2

We report inelastic neutron scattering studies of magnetic excitations in antiferromagnetically ordered SrFe2As2 (T_{N}=200-220 K), the parent compound of the FeAs-based superconductors. At low temperatures (T=7 K), the magnetic spectrum S(Q,Plancks omega) consists of a Bragg peak at the elastic position (Plancks omega=0 meV), a spin gap (Delta< or =6.5 meV), and sharp spin-wave excitations at higher energies. Based on the observed dispersion relation, we estimate the effective magnetic exchange coupling using a Heisenberg model. On warming across T_{N}, the low-temperature spin gap rapidly closes, with weak critical scattering and spin-spin correlations in the paramagnetic state. The antiferromagnetic order in SrFe2As2 is therefore consistent with a first order phase transition, similar to the structural lattice distortion.

Structural anomalies at the magnetic and ferroelectric transitions in RMn 2 O 5 (R=Tb,Dy,Ho)

Strong anomalies of the thermal expansion coefficients at the magnetic and ferroelectric transitions have been detected in multiferroic

Temperature-dependent properties of the magnetic order in single-crystal BiFeO 3

RMn_2O_5

Single ferroelectric and chiral magnetic domain of single-crystalline BiFeO 3 in an electric field

. Their correlation with anomalies of the specific heat and the dielectric constant is discussed. The results provide evidence for the magnetic origin of the ferroelectricity mediated by strong spin-lattice coupling in the compounds. Neutron scattering data for

Tailoring exchange couplings in magnetic topological-insulator/antiferromagnet heterostructures

HoMn_2O_5

3:1 magnetization plateau and suppression of ferroelectric polarization in an Ising chain multiferroic

indicate a spin reorientation at the two low-temperature phase transitions.

Short-range Incommensurate Magnetic Order Near the Superconducting Phase Boundary in Fe1+δTe1−xSex

We report neutron diffraction and magnetization studies of the magnetic order in multiferroic BiFeO3. In ferroelectric monodomain single crystals, there are three magnetic cycloidal domains with propagation vectors equivalent by crystallographic symmetry. The cycloid period slowly grows with increasing temperature. The magnetic domain populations do not change with temperature except in the close vicinity of the N{\P}eel temperature, at which, in addition, a small jump in magneti- zation is observed. No evidence for the spin-reorientation transitions proposed in previous Raman and dielectric studies is found. The magnetic cycloid is slightly anharmonic for T=5 K. The an- harmonicity is much smaller than previously reported in NMR studies. At room temperature, a circular cycloid is observed, within errors. We argue that the observed anharmonicity provides important clues for understanding electromagnons in BiFeO3.