UC Berkeley

The electric field control of ferromagnetism has been a long sought after effect, due to the large number of potential applications in electronic/magnetic devices. Large currents (and hence large powers) are required to generate the large magnetic fields needed to control the magnetization of a ferromagnetic material in a thin film electronic device, which is incompatible with planar integrated circuit technology. Alternatively, creating large electric fields at these scales requires minimal current (and hence minimal power) and is already well established. By exploiting a magnetoelectric material, magnetization can be manipulated in a scalable planar low-power device through the application of electric field. Using this type of device architecture could lead to huge advances in magnetic memory and storage, as well as provide a crucial first step to creating low-power spintronic devices as a replacement for traditional electronics that are reaching the limit of scaling. One possible way to achieve the electric control of ferromagnetism is by controlling exchange bias, the shift of a magnetic hysteresis curve along the applied field axis due to interface interactions between coupled antiferromagnetic (AFM) and ferromagnetic (FM) materials. If it is possible to shift exchange bias through the coercive field of the FM, magnetization can be reversed. Reaching this goal requires a careful understanding of antiferromagnetism, ferromagnetism and the interactions between the two when coupled (exchange bias). The main focus of this thesis will be the design, fabrication, characterization, and understanding of an electric field effect device where we are able to reversibly modulate between two exchange bias states with opposite polarity in a thin film ferromagnet by coupling it to a multiferroic (ferroelectric/antiferromagnetic) material.

The multiferroic material BiFeO₃ (BFO), an AFM and ferroelectric (FE) with coupled AFM/FE order parameters, is a prime candidate material for affecting change in exchange bias systems. When coupled to a thin film ferromagnet such as the colossal magnetoresistive manganite La_0.7Sr_0.3MnO₃ (LSMO), one can envision a system where FE order is switched in BFO, which induces a change in AFM order in BFO, which induces a change in exchange bias in LSMO. Here, an electric field effect device is created using BFO as the dielectric and LSMO as the conducting channel to realize just such a system. Heteroepitaxially deposited BFO (3 nm)/LSMO (200 nm) heterostructures are grown on SrTiO₃ (100) substrates and subsequently patterned into field effect devices using a fabrication process involving photolithography and argon ion milling. These devices are then characterized through magnetotransport measurements to characterize the magnetic properties of the LSMO channel with respect to BFO FE polarization. Through these measurements this thesis shows, for the first time, exchange bias is directly controlled with electric field without temperature cycling or any electric or magnetic field cooling/biasing. This effect is reversible and comes concurrently with the modulation of channel resistance (sometimes over 300%), the modulation of magnetic coercivity, and magnetic Curie temperature.

Based on these results and the current understanding of exchange bias we propose a model to understand the electric control of exchange bias. In this model the coupled antiferromagnetic/ferroelectric order in BFO along with the modulation of interfacial exchange interactions due to ionic displacement of Fe³⁺ in BFO relative to Mn^3+/4+ in LSMO cause exchange bias modulation.