Correlated Electrons and Complex Spin Systems
Multiferroicity from Complex Spin Structures
Manipulating magnetic moments using an electrical field is a promising approach for fast device applications that operate without heat-producing currents. This requires material exhibiting a strong magneto-electric coupling. So-called ‘multiferroica’, with their combined long range order of magnetic and electric dipoles are promising candidates. Recently, new compounds have been discovered, where the ferro-electric order appears in conjunction with a chiral magnetic structure. In this case, the magnetic structure does not only break the time-reversal symmetry but also the inversion symmetry, needed for the formation of spontaneous polarization. Using neutron and synchrotron radiation scattering, we concentrate our investigations on the structure and dynamics of multiferroic materials. In particular, the application of polarization analysis provides new insights into chiral magnetic interactions.
Interplay of Different Electronic Degrees of Freedom in Correlated Oxides
Strong electron correlations in transition metal oxides "activate" several electronic degrees of freedom with ordering tendencies: charge order, orbital order, spin order. A subtle interplay of these degrees of freedom leads to complex behaviour and new phenomena which are of interest in terms of applications. Our studies involve electronic order, correlations, and excitations using synchrotron x-ray and neutron scattering methods. As an example, the mixed-valent borate Fe2OBO3 has been identified using x-ray scattering combined with Mössbauer spectroscopy as a model charge ordering system with intriguing room temperature properties that are highly influenced by geometrical frustration. Indications for strong electron-lattice coupling are now probed by nuclear resonance scattering and inelastic x-ray scattering.
Multiferroicity from Charge Order
Multiferroic materials in which magnetic order and ferroelectricity are coupled offer great potential for applications in information technology, but the traditional mechanisms of magnetism and ferroelectricity are incompatible. A potential solution to this difficulty is ferroelectricity from specific spin structures, which has the drawback, however, that the achievable electric polarization is too limited for many applications. A recently proposed alternative mechanism compatible with both large polarizations and strong magnetoelectric coupling is ferroelectricity originating from charge ordering. For this mechanism, only very few examples have yet been proposed, e.g. LuFe2O4. Our studies focus on magnetic order, and charge order as the origin of ferroelectricity, as well as coupling between these orders on a microscopic basis by means of neutron and X-ray scattering.
Unravelling Complex Ordering Phenomena with Resonant Soft X-ray Scattering (RSXS)
Strongly correlated electron systems such as 3-D transition-metal oxides present exciting fundamental properties, e.g. the extraordinary giant magnetoresistance effect or multiferoic behaviour in manganites. Their rich structural, magnetic and electronic properties are governed by the interplay of lattice, spin, charge and orbital degrees of freedom. Identification of the ordered phases is a prerequisite for understanding the physical properties and unusual phenomena. The emerging RSXS technique is a powerful method for probing these phases individually since the RSXS process depends on the anisotropy of the empty electronic levels as determined not only by the orbital filling on the resonant atom, but also by the geometrical distribution of its neighbours. Utilizing this method, we have successfully observed huge resonant enhancements of expected superlattice reflections, supporting direct and definitive evidence for ordering of charge, orbital and spin degrees of freedom in various single crystals, e.g. La1-xSrxMnO3, La2-2xSr1+2xMn2O7, Fe3O4, etc.
Complex Crystal and Magnetic Structures
Complex transition metal oxides show a strong interdependence of physical properties and atomic and magnetic structure, giving rise to new functionalities. As an example, the nature of the unusual ferromagnetic insulating phase in lightly doped La1-xSrxMnO3 with x ≅0.10-0.15 has attracted much attention and created controversies. A consensus on the importance of charge and orbital ordering (CO/OO) has been reached. However, the exact and detailed CO/OO model is still a puzzle, due to the bewildering structures involved. A combined Rietveld refinement with high-resolution neutron and x-ray powder data together with resonant x-ray scattering from a single crystal can identify the correct structural model. Utilizing this method, we have resolved the correlation between structural and magnetic properties of polycrystalline (air sintered, Ar and O2 annealed) and single-crystal La1-xSrxMn1±γO3±δ (x ≅ 1/8), etc.
Kagomé Spins Dance a Rondo in the Cold
Geometrical frustration may prevent the settling of even strongly-coupled magnetic moments of atoms into an ordered state in a crystal. Using polarized diffuse neutron scattering, we have studied the spin frustration in a new cobalt oxide, a close realization of the famous 2-dimensional Kagomé lattice.
We have observed zero energy modes of spins, where hexagons of spins undergo entropy -driven collective motion, gradually freezing into an incompletely ordered state with only short range chiral (non-collinear) order.
Longitudinal Spin Fluctuations in Antiferromagnets
Spin waves are transversal modes observed during single magnon scattering. Their complete dynamic behaviour also includes, however, longitudinal fluctuations. Using polarized neutrons, we have investigated longitudinal spin fluctuations in 3D antiferromagnets, which were excited with two-magnon and higher order stimulants. Anisotropism plays an essential role in the shielding of longitudinal modes as stimulation energy is reduced.
Critical Chiral Spin Fluctuations
Contrary to ordinary collinear (anti-)ferromagnets, the order in spin spirals is described by the spin chirality (C=SxS) as an additional order parameter. It is connected with a four-spin correlation function inaccessible to conventional scattering experiments. However, polarized neutron scattering in an external field can give access to the critical behaviour of the spin chirality. It could thus be shown that chiral magnetic systems belong to their own universality class. Their unusual magnetic behaviour is characterized by new chiral critical exponents.
Frustrated Magnetic Systems
Frustrated magnetic systems can lead to a variety of cooperative spin states such as spin glass, spin ice (such as Ho2Ti2O7) and spin liquid (such as Tb2Ti2O7) which behave like glass, ice and liquid in nature. These interesting magnetic properties all result from the delicate balance of frustrated spin-spin interactions. We are therefore studying geometrically frustrated magnetic systems with the intention of investigating their novel ground and excited states and the interactions between spins.