UC Berkeley

Multiferroic magnetoelectric (ME) materials, which simultaneously show ferroelectricity and magnetic ordering, have attracted a huge scientific attention due to both the intriguing fundamental importance and the great technological potential, as a result of the coupling between the dual ferroic orderings. However there are only a few single-phase multiferroic materials in nature and the performance is limited by the low ordering temperature, the small polarization/magnetization or the weak coupling efficiency. Another promising pathway to engineer ME coupling is through designing heterostructures. The current studies of ME coupling (electric field control of magnetism) in heterostructures can be divided into 3 routes: (1) Using piezoelectric effects to change the strain (lattice); (2) Using ferroelectric polarization to tune the carriers (charge); (3) Using multiferroic materials to manipulate the moments by magnetic coupling (spin). Previous studies of an all-oxide model heterostructure system that consists of the ferromagnet La0.7Sr0.3MnO3 (LSMO) and the BiFeO3 (BFO) reveals an interesting charge and spin interactions at the interface. Following the same route, the first part of the dissertation focuses on two different pathways to improve the ME coupling in manganite/BFO model system. Chapter 3 demonstrates that the ME coupling in LSMO/BFO is dramatically different by changing the atomic stack-ing sequence at the interface. In chapter 4, another model system La0.5Ca0.5MnO3 (LCMO), which is at the ferromagnetic/antiferromagnetic phase boundary, is studied instead of LSMO to explore the limit within this route. The results complementarily suggest the importance of a more comprehensive design rule at the atomic scale in order to achieve a better ME coupling efficiency.

Furthermore, although the coupling between the lattice, charge and spin has been intensively studied in terms of ME coupling, the orbital degree of freedom has been neglected so far. In principle, orbital is strongly coupled to lattice. Therefore a large ME coupling could be expected if a strong spin-orbit interaction could be established. The second part of the dissertation concentrates on pursuing this new ideal through two different pathways. Chapter 5 presents a systematic study of the in-situ strain effect on the magnetic and orbital orderings of a model system Nd0.5Sr0.5MnO3. The results demonstrate the close correlation between the orbital ordering parameter and the strain. However the magnetic ordering parameter is less sensitive to strain in this system. With the implication from chapter 5, 5d transition metal oxide SrIrO3(SIO), which hosts a strong intrinsic spin-orbit coupling due to the large atomic number, is studied in chapter 6. Superlattice LSMO/SIO shows an interesting novel magnetic state with a large orbital momentum in the nominally paramagnetic SIO, which is likely to be a very promising candidate to engineer the ME coupling at the interface.

In summary, our studies on engineering the ME effects in heterostructures have revealed two key implications: 1. in the traditional routes that have been extensively studied, further improvements are possible by carefully engineering the interface; 2. the coupling between spin and orbital degrees of freedom is likely to be another promising route to investigate the ME coupling in heterostructures.