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 symm
etry. 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.
The structural and magnetic phase transitions have been studied on NdFeAsO single crystals by neutron and x-ray diffraction complemented by resistivity and specific heat measurements. Two low-temperature phase transitions have been observed in additi
on to the tetragonal-to-orthorhombic transition at T_S = 142 K and the onset of antiferromagnetic (AFM) Fe order below T_N = 137 K. The Fe moments order AFM in the well-known stripe-like structure in the (ab) plane, but change from AFM to ferromagnetic (FM) arrangement along the c direction below T* = 15 K accompanied by the onset of Nd AFM order below T_Nd = 6 K with this same AFM configuration. The iron magnetic order-order transition in NdFeAsO accentuates the Nd-Fe interaction and the delicate balance of c-axis exchange couplings that results in AFM in LaFeAsO and FM in CeFeAsO and PrFeAsO.
Neutron diffraction measurements have been carried out to investigate the magnetic form factor of the parent SrFe2As2 system of the iron-based superconductors. The general feature is that the form factor is approximately isotropic in wave vector, ind
icating that multiple d-orbitals of the iron atoms are occupied as expected based on band theory. Inversion of the diffraction data suggests that there is some elongation of the spin density toward the As atoms. We have also extended the diffraction measurements to investigate a possible jump in the c-axis lattice parameter at the structural phase transition, but find no detectable change within the experimental uncertainties.
We report polarized neutron scattering and piezoresponse force microscopy studies of millimeter-sized single crystals of multiferroic BiFeO$_3$. The crystals, grown below the Curie temperature, consist of a single ferroelectric domain. Two unique ele
ctric polarization directions, as well as the populations of equivalent spiral magnetic domains, can be switched reversibly by an electric field. A ferroelectric monodomain with a single-$q$ single-helicity spin spiral can be obtained. This level of control, so far unachievable in thin films, makes single-crystal BiFeO$_3$ a promising object for multiferroics research.
We use neutron scattering to study the spin and lattice structure on single crystals of SrFe2As2, the parent compound of the FeAs based superconductor (Sr,K)Fe2As2. We find that SrFe2As2 exhibits an abrupt structural phase transitions at 220K, 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 in SrFe2As2 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 elements) families of materials and on BaFe2As2, and therefore suggest that static antiferromagnetic order is ubiquitous for the parent compound of these FeAs-based high-transition temperature superconductors.
ZnCr2O4 undergoes a first order spin-Peierls-like phase transition at 12.5 K from a cubic spin liquid phase to a tetragonal Neel state. Using powder diffraction and single crystal polarized neutron scattering, we determined the complex spin structure
of the Neel phase. This phase consisted of several magnetic domains with different characteristic wave vectors. This indicates that the tetragonal phase of ZnCr2O4 is very close to a critical point surrounded by many different Neel states. We have also studied, using elastic and inelastic neutron scattering techniques, the effect of nonmagnetic dilution on magnetic correlations in ZnCr_{2-2x}Ga_{2x}O_4 (x=0.05 and 0.3). For x=0.05, the magnetic correlations do not change qualitatively from those in the pure material, except that the phase transition becomes second order. For x= 0.3, the spin-spin correlations become short range. Interestingly, the spatial correlations of the frozen spins in the x=0.3 material are the same as those of the fluctuating moments in the pure and the weakly diluted materials.
