How to enhance the magneto-electric coupling in multiferroics?


Multiferroics, a class of oxide compounds simultaneously displaying two or more ferroic orders (typically ferroelectricity and ferromagnetism), are generating a tremendous flurry of research interest due to their fundamental complexity and potential for applications in nanoelectronics and energy conversion. Finding in nature single-phase compounds with those properties, however, has proved extremely difficult. This scarcity of bulk multiferroic materials has motivated researchers to investigate oxide-based compounds in thin-film and/or superlattices geometries where (i) the properties of the ferroelectric can be tuned almost at will by chosing an appropriate substrate lattice parameter, (ii) the electrostatic coupling between different oxide layers can be exploited, and iii) interfaces, rather than the oxide itself, can show novel multifunctional properties which are absent in either of the bulk constituents. First-principles methods have indeed played a major role in the development of this field, leading to the present situation in which theory often settles the way for the design of new devices. 

The main goal of this research line is to investigate the properties of ferroelectric, piezoelectric and magnetoelectric perovskite systems of fundamental and technological interest, using state-of-the-art first-principles methods. In particular, I focus on the study of thin films, interfaces and superlattices grown in a layered geometry, putting a special emphasis on the separation and understanding of macroscopic "bulk" effects (which result from elastic and/or electrostatic coupling between layers) from "interface" phenomena (which are due to local bonding and/or modification of the atomic/electronic structure in the vicinity of a two-oxides junction). In fact, both types of effects offer viable routes to enhance the properties of oxide materials. The novelty of this project is to simulate complex bulk and interface oxide systems fully ab initio and with direct (and rigorous) control over the macroscopic electrical variables of interest (e.g. electric field, polarization and electric displacement). Using this approach, I (i) investigate how bulk polarization can be controlled via surface polarity, (ii) study electric field-driven phase transitions in films of bismuth ferrite and other related compounds, and (iii) design multiferroic superlattice structures based on the novel concept of "charge interface mismatch" engineering.

1. Physical Review B 85, 075426 (2012)
C. Cazorla and M. Stengel
"First-principles modeling of Pt/LaAlO3/SrTiO3 capacitors under an external bias potential"

2. Physical Review B 88, 214430 (2013)
C. Cazorla and J. Iñiguez
"Insights into the phase diagram of bismuth ferrite from quasi-harmonic free-energy calculations"


3. Physical Review B 90, 020101(R) (2014)
C. Cazorla and M. Stengel
"Ab initio design of charge-mismatched ferroelectric superlattices" 


4. Physical Review B 92, 214108 (2015)
C. Cazorla and M. Stengel
"Electrostatic engineering of strained ferroelectric perovskites from first principles"


5. Scientific Reports 6, 28742 (2016)
M. Acosta, L. A. Schmitt, C. Cazorla, A. Studer, A. Zintler, J. Glaum, H. J. Kleebe, W. Donner, M. Hoffman, J. Rodel, and M. Hinterstein
"Piezoelectricity and rotostriction through polar and non-polar coupled instabilities in bismuth-based piezoceramics"