Substrate Strain Induced Effects on Multiferroic Epilayers

Abstract

Multiferroics are materials that simultaneously possess more than one ferroic ordering. The magnetoelectric coupling if any present, of these multiferroic systems are known to be strain mediated. This chapter deals with substrate strain induced effects on multiferroic systems mainly with major focus on BiFeO3 (BFO), a room temperature multiferroic. Under varying strain conditions, BFO undergoes a series of structural transformations. In this chapter, epitaxial strain experienced by BFO under artificial superlattice construction and homogeneous growth of thin films of BFO grown on LaAlO3 (LAO (001)) single crystal substrates using pulsed laser ablation are discussed. The strain relaxation phenomenon observed in many epitaxial layers is utilized to engineer the strain experienced by BFO layer and hence the structural stability of BFO is altered. The strain conditions are altered such that, a morphotropic phase diagram like behavior was achieved by altering strain conditions. The associated theoretical studies using phase field modeling was carried out and the thermodynamic stability of the rhombohedral, tetragonal and the mixed phase (R + T with monoclinic distortion) were simulated. The polarization domains of every phases were also studied and eventually a huge increment in the piezoelectric coefficient d33 of ∼46 Pm/V for the 20 nm film with mixed phase was obtained. The enhancement of the piezo/ferroelectric properties is similar to the enhanced piezo properties observed in systems possessing morphotropic phase boundary. Thus in this chapter, both experimental and theoretical studies of structural tunability of BFO under varying strain conditions and a property enhancement due to the same is discussed.