Multiferroics are compounds which can be simultaneously magnetized (are ferromagnetic) and electrically charged (are ferroelectric). Such compounds hold promise for applications in devices and lead to unprecedented data-storage capacity. However, the underlying mechanism which leads to such unique properties is poorly understood and needs to be resolved before practical devices can be fabricated. This individual investigator award supports the study of a family of multiferroic manganese oxide compounds known as manganites. The properties of manganites are intimately linked to their crystal structure. The theme of this project is to quantify the role played by crystal structure in multiferroic manganites and suggest methods to design materials better suited for device applications. High quality manganite thin films will be grown using laser ablation. These thin films' response to strain (change in the crystal structure) will be studied by measuring the change in their electrical resistance and magnetism. The microscopic effect of strain will be studied using techniques such as a microscope capable of mapping the local magnetism of the material. Since the research and training program includes both sample preparation and measurement, this award will help train undergraduate and graduate students in various aspects of condensed matter physics research and give them a broad based experience, which will enhance their future career options in the industry or academe.
Perovskite manganese oxides (manganites) display unique properties such as micrometer scale phase separation and multiferroism, which are intimately linked to their crystal structure. Hence, the electrical and magnetic properties of manganites are sensitive to strain. This individual investigator award supports an experimental program to directly measure the effect of strain on the properties of manganites and by comparing the results to the predictions of theoretical models, determine the origin of phase separation and multiferroism in manganites. The experimental results are also expected to reveal methods of enhancing the magnetoelectric coupling in multiferroic manganites and controlling the nanoscale properties of phase separated manganites. High quality manganite thin films will be grown using pulsed laser deposition. The thin films will then be subjected to direct external stress using a three point beam-bending apparatus to induce uniaxial strain in the material. Using a complementary suite of local and bulk measurement techniques such as magnetotransport, magnetization, scanning probe microscopy, and neutron reflectometry, the phase diagram of the thin films will be mapped as a function of strain, magnetic field, electric field, and temperature. Since the research and training program includes both sample preparation and measurement, undergraduate and graduate students will be able to learn about a wide variety of materials and experimental techniques, which will prepare them for academic and industrial careers.
Currently digital data storage is usually achieved by the manipulation of magnetic domains using magnetic fields or control of electric charge using an electric field. The ability to control the magnetism of a material using an electric field (aptly named 'magnetoelectric coupling') holds promise for future device applications and is an exciting challenge in materials physics. Through our experiments we are investigating two different types of magnetoelectric coupling. The first type of coupling that we have observed suggests a reorientation of the fluid-like ferromagnetic metallic regions due to an applied electric field. This effect can be understood by looking at the behavior of a liquid metallic drop in an electric field. The competition between the surface tension and the electrostatic energy of the liquid drop stretches it out in a direction parallel to the electric field. The second type of magnetoelectric effect is observed in the so-called multiferroic materials, such as the compound BiMnO3. Such materials exhibit both spontaneous magnetization (are ferromagnetic) and spontaneous polarization (are ferroelectric). Although the two types of magnetoelectric coupling have different origins, the crystal structure plays an integral role in both phenomena. Hence, it is expected that a change in the crystal lattice parameters i.e. strain, will profoundly influence the properties of such materials and provides us with a tool to investigate the origin of the magnetoelectric coupling. We have studied the effect of strain on a class of materials known as manganites. Our experimental results reveal a coupling between the lattice constants of manganites and their magnetic and electronic properties which can be used to design new materials with stronger magnetoelectric coupling. Competing ground states are characteristic to transition metal oxides. In manganites, the coexistence of a ferromagnetic metallic phase and a charge-ordered insulating phase (phase separation) and the coexistence of magnetic order and ferroelectricity (multiferroism) are of particular interest because of the exciting new physics and possible device applications. For the case of phase separation in manganites, the emerging picture is that competing ferromagnetic metallic and charge-ordered insulating phases form a state with properties analogous to an 'electronic soft matter' which can be manipulated using various external parameters, such as magnetic field, electric field, and strain. Multiferroics are considered to be a small class of materials because the coexistence of magnetic order and ferroelectricity enforces seemingly mutually exclusive requirements on the material. The investigation of the underlying principles which allow manganites to achieve these requirements is a rich and intriguing subject in condensed matter physics. In phase separated as well as multiferroic manganites, the magnetic and electronic properties are intimately linked to their crystal structure and are therefore highly sensitive to strain. Hence, the effect of strain on the phase separation and multiferroism in manganites will reveal important aspects of the origin of magnetoelectric coupling in manganites. We have grown high-quality thin films of manganites for these experiments. We have observed colossal piezoresistance (a very large change in the resistance of a material due to the direct application of strain) and colossal electroresistance in phase separated manganites. Conducting atomic force microscopy and neutron scattering experiments together with transport and magnetization measurements have shown that an electric field/ strain induced reshaping of the metallic regions leads to the piezoresistance and electroresistance. We have also shown that non-uniform strain can be used to tune the transition temperatures of multiferroic manganites which leads to stronger magnetoelectric effects. Our results can be used to design prototype devices in which magnetic information is written using an electric field, or vice versa. Over the past four years this project has supported four graduate and five undergraduate students. Five Research Experiences for Undergraduates (REU) students have also participated in research related to this project. Students working in my group conduct world class research on thin film technology, transport and magnetic measurements, scanning probe microscopy, and nanofabrication technology.
