Author: Josep Lluis

New multiferroics and magnetoelectric oxides and mechanisms

Multiferroics are important functional materials featuring strongly coupled order parameters that can be manipulated by external fields. Magnetoelectric multiferroics  are receiving enormous attention as they open the road to new forms of multifunctional devices. However, they challenge our fundamental understanding of magnetic and ferroelectric order because a strong magnetoelectric coupling is incompatible with traditional mechanisms of ferroelectricity. The recent discovery of a new class of materials (type-II multiferroics) in which the magnetic and electric properties are strongly coupled is attracting very much interest because of the possibility to manipulate magnetism and spins by electric fields and vice-versa, to magnetically control electric charges. Future applications in information technology require new multiferroic materials fulfilling all technological requirements. Along with its technological functionalities, multiferroics are also of great interest in fundamental research into strongly correlated oxides and quantum matter.

Aside from the important strategies to produce magnetoelectricity in complex artificial oxide heterostructures, the discovery of this new class of “single phase” magnetoelectric compounds is stimulating the exploration of a wide range of possible new materials and mechanisms, as well as their better understanding. These activities are encouraging new experimental and theoretical methods. We are particularly interested in the search of novel materials and single-phase compounds of two new types of coupled multiferroicity, based on: (i) Spin driven coupling, where magnetic interaction and magnetic ordering cause ferroelectric polar orders or abrupt electrical anomalies; and (ii) Charge driven coupling, where electronic, orbital or charge orders are responsible for the electric polarization.

Diffraction studies and crystallography of magnetic and electronic materials

The neutron scattering in the field of magnetic and electronic materials presents extraordinary importance. To probe magnetic  properties on atomic scale, neutron diffraction is an established technique and a unique method of choice, which allows perfect quantitative data interpretation. The magnetic moment of the neutron makes it a unique probe for magnetic properties in  condensed matter on atomic scale. It gives a direct access to the spin and orbital distribution in the unit cell. In particular, magnetic structure determination is the foyer to the understanding of many fundamental phenomena in Condensed Matter research.  Neutron and synchrotron techniques can be applied to investigate spin-state transitions, charge and orbital ordering, giant magneto-resistance, magnetoelectric materials as well as other emergent phenomena in frustrated materials such as spin ice, spin liquid behavior or other promising topological defects.

In our group we perform complementary diffraction studies in neutron and synchrotron sources, and apply structural and magnetic crystallography methods to investigate functional magnetic and electronic materials. So, e.g. in magnetically induced ferroelectrics, polarization can originate (but not exclusively) from exotic magnetic ground states (spiral, cycloidal, conical,..orders) that breaks inversion symmetry, thus generating an electric field and ferroelectricity. Moreover, normally these states can be easily modified or manipulated by external parameters (applied fields, pressure, temperature, strain, ..). A wide variety of mechanisms based on internal orders (structural, magnetic, electronic, ..) are now under intense investigation using diffraction techniques with synchrotron and neutron beams. The list of possible mechanisms and materials for multiferroic ground states is very far from complete. Our group is pioneer in applying new crystallographic methods of data analysis to tackle the original magnetic and structural orders that govern this interesting class of materials.

Novel oxides with spin state instabilities for electronic and energy applications

Cobalt oxides present a plethora of very interesting properties like metal-insulator transitions, spin-state changes, giant magnetoresistance, double-exchange, phase separation, high thermoelectric power, oxygen diffusivity, mixed-conduction, charge and orbital ordering or superconductivity among others. These properties are interesting not only from a fundamental point of view but also due to their potential applicability in different fields. One very remarkable characteristic of many cobalt compounds is the ability of Co ions to adopt different spin states. This makes that Co oxides have, in comparison with other transition metal oxides, an extra degree of freedom: the spin state of Co. So, the investigation of novel cobaltites with different structures and prepared in different forms is between the most attractive opportunities within strongly correlated systems: the spin state of Co at selected sites in the structure plays a key role in the structural, magnetic, magnetotransport properties, electronic and ion mobility or the thermoelectric power. This research is inscribed inside the wider objective of understand and control the spin state and electronics degrees of freedom of Co cations, especially with 3+ valence. Trivalent cobalt oxides exhibit unique electronic phases characterized by the interplay between nearly degenerate spin states.

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