128 research outputs found
Micromagnetic modelling and imaging of vortex/merons structures in an oxide | metal heterostructure
Using micromagnetic simulations, we have modelled the formation of imprinted merons and anti-merons in cobalt overlayers of different thickness (1-8 nm), stabilised by interfacial exchange with antiferromagnetic vortices in α-Fe2O3. Structures similar to those observed experimentally could be obtained with reasonable exchange parameters, also in the presence of surface roughness. We produce simulated meron/antimeron images by magnetic force microscopy (MFM) and nitrogen-vacancy (N-V) centre microscopy, and established signatures of these topological structures in different experimental configurations
Ferroelectricity Induced by Acentric Spin-Density Waves in YMn2O5
The commensurate and incommensurate magnetic structures of the magnetoelectric system YMn2O5, as determined from neutron diffraction, were found to be spin-density waves lacking a global center of symmetry. We propose a model, based on a simple magnetoelastic coupling to the lattice, which enables us to predict the polarization based entirely on the observed magnetic structure. Our data accurately reproduce the temperature dependence of the spontaneous polarization, particularly its sign reversal at the commensurate-incommensurate transition
Essential Role of the Cooperative Lattice Distortion in the Charge, Orbital and Spin Ordering in doped Manganites
The role of lattice distortion in the charge, orbital and spin ordering in
half doped manganites has been investigated. For fixed magnetic ordering, we
show that the cooperative lattice distortion stabilize the experimentally
observed ordering even when the strong on-site electronic correlation is taken
into account. Furthermore, without invoking the magnetic interactions, the
cooperative lattice distortion alone may lead to the correct charge and orbital
ordering including the charge stacking effect, and the magnetic ordering can be
the consequence of such a charge and orbital ordering. We propose that the
cooperative nature of the lattice distortion is essential to understand the
complicated charge, orbital and spin ordering observed in doped manganites.Comment: 5 pages,4 figure
Cooling rate dependence of the antiferromagnetic domain structure of a single crystalline charge ordered manganite
The low temperature phase of single crystals of NdCaMnO
and GdCaMnO manganites is investigated by squid
magnetometry. NdCaMnO undergoes a charge-ordering
transition at =245K, and a long range CE-type antiferromagnetic state
is established at =145K. The dc-magnetization shows a cooling rate
dependence below , associated with a weak spontaneous moment. The
associated excess magnetization is related to uncompensated spins in the
CE-type antiferromagnetic structure, and to the presence in this state of
fully orbital ordered regions separated by orbital domain walls. The observed
cooling rate dependence is interpreted to be a consequence of the rearrangement
of the orbital domain state induced by the large structural changes occurring
upon cooling.Comment: REVTeX4; 7 pages, 4 figures. Revised 2001/12/0
Ferromagnetic Polarons in La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3
Unrestricted Hartree-Fock calculations on La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3
in the full magnetic unit cell show that the magnetic ground states of these
compounds consist of 'ferromagnetic molecules' or polarons ordered in
herring-bone patterns. Each polaron consists of either three or five Mn ions
separated by O- ions with a magnetic moment opposed to those of the Mn ions.
Ferromagnetic coupling within the polarons is strong while coupling between
them is relatively weak. Magnetic moments on the Mn ions range between 3.8 and
3.9 Bohr magnetons in La0.5Ca0.5MnO3 and moments on the O- ions are -0.7 Bohr
magnetons. Each polaron has a net magnetic moment of 7.0 Bohr magnetons, in
good agreement with recently reported magnetisation measurements from electron
microscopy. The polaronic nature of the electronic structure reported here is
obviously related to the Zener polaron model recently proposed for
Pr0.6Ca0.4MnO3 on the basis of neutron scattering data.Comment: 4 pages 5 figure
Orbital ordering in the manganites: resonant x-ray scattering predictions at the manganese LII and LIII edges
It is proposed that the observation of orbital ordering in manganite materials should be possible at the L II and L III edges of manganese using x-ray resonant scattering. If performed, dipole selection rules would make the measurements much more direct than the disputed observations at the manganese K edge. They would yield specific information about the type and mechanism of the ordering not available at the K edge, as well as permitting the effects of orbital ordering and Jahn-Teller ordering to be detected and distinguished from one another. Predictions are presented based on atomic multiplet calculations, indicating distinctive dependence on energy, as well as on polarization and on the azimuthal angle around the scattering vector
Magnetic fan structures in Ba0.5Sr1.5Zn2Fe12O22 hexaferrite revealed by resonant soft x-ray diffraction
The hexaferrites are known to exhibit a wide range of magnetic structures, some of which are connected to important technological applications and display magnetoelectric properties. We present data on the low magnetic field structures stabilized in a Y-type hexaferrite as observed by resonant soft x-ray diffraction. The helical spin block arrangement that is present in zero applied magnetic field becomes fanlike as a field is applied in plane. The propagation vectors associated with each fan structure are studied as a function of magnetic field, and a new magnetic phase is reported. Mean field calculations indicate this phase should stabilize close to the boundary of the previously reported phases
Phase diagram of the LaCaMnO compound for
We have studied the phase diagram of LaCaMnO for using neutron powder diffraction and magnetization measurements. At
300 K all samples are paramagnetic and single phase with crystallographic
symmetry . As the temperature is reduced a structural transition is
observed which is to a charge-ordered state only for certain x. On further
cooling the material passes to an antiferromagnetic ground state with Neel
temperature that depends on x. For the structural
transformation occurs at the same temperature as the magnetic transition.
Overall, the neutron diffraction patterns were explained by considering four
phase boundaries for which LaCaMnO forms a distinct phase: the
CE phase at , the charge-ordered phase at x=2/3, the monoclinic and
C-type magnetic structure at and the G-type magnetic structure at
x=1. Between these phase boundaries the magnetic reflections suggest the
existence of mixed compounds containing both phases of the adjacent phase
boundaries in a ratio determined by the lever rule
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