Lifeng Yin

Dynamics of a first-order electronic phase transition in manganites

By reducing an electronically phase-separated manganite (La{sub 1-y}Pr{sub y}){sub x}Ca{sub 1-x}MnO{sub 3} single-crystal thin film to dimensions on the order of the inherent phase domains, it is possible to isolate and monitor the behavior of single domains at a first-order transition. At this critical point, it is possible to study the coexistence, formation, and annihilation processes of discrete electronic phase domains. With this technique, we make several observations on the mechanisms leading to the metal-insulator transition in manganites. We observe that domain formation is emergent and random, the transition process from the metallic phase to the insulating phase takes longer than the reverse process, electric field effects are more influential in driving a phase transition than current-induced electron heating, and single domain transition dynamics can be tuned through careful application of temperature and electric field.

High Tunability of the Surface-Enhanced Raman Scattering Response with a Metal-Multiferroic Composite

We demonstrate active control of the plasmonic response from Au nanostructures by the use of a novel multiferroic substrate-LuFe(2)O(4) (LFO)-to tune the surface-enhanced Raman scattering (SERS) response in real time. From both experiments and numerical simulations based on the finite-difference time-domain method, a threshold field is observed, above which the optical response of the metal nanostructure can be strongly altered through changes in the dielectric properties of LFO. This offers the potential of optimizing the SERS detection sensitivity in real time as well as the unique functionality of detecting multiple species of Raman active molecules with the same template.

Tuning the Metal-Insulator Transition in Manganite Films through Surface Exchange Coupling with Magnetic Nanodots

In strongly correlated electronic systems, the global transport behavior depends sensitively on spin ordering. We show that spin ordering in manganites can be controlled by depositing isolated ferromagnetic nanodots at the surface. The exchange field at the interface is tunable with nanodot density and makes it possible to overcome dimensionality and strain effects in frustrated systems to greatly increasing the metal-insulator transition and magnetoresistance. These findings indicate that electronic phase separation can be controlled by the presence of magnetic nanodots.

Visualization of a ferromagnetic metallic edge state in manganite strips

Recently, broken symmetry effect induced edge states in two-dimensional electronic systems have attracted great attention. However, whether edge states may exist in strongly correlated oxides is not yet known. In this work, using perovskite manganites as prototype systems, we demonstrate that edge states do exist in strongly correlated oxides. Distinct appearance of ferromagnetic metallic phase is observed along the edge of manganite strips by magnetic force microscopy. The edge states have strong influence on the transport properties of the strips, leading to higher metal-insulator transition temperatures and lower resistivity in narrower strips. Model calculations show that the edge states are associated with the broken symmetry effect of the antiferromagnetic charge-ordered states in manganites. Besides providing a new understanding of the broken symmetry effect in complex oxides, our discoveries indicate that novel edge state physics may exist in strongly correlated oxides beyond the current two-dimensional electronic systems.

Growth diagram of La0.7Sr0.3MnO3 thin films using pulsed laser deposition

An experimental study was conducted on controlling the growth mode of La0.7Sr0.3MnO3 thin films on SrTiO3 substrates using pulsed laser deposition (PLD) by tuning growth temperature, pressure, and laser fluence. Different thin film morphology, crystallinity, and stoichiometry have been observed depending on growth parameters. To understand the microscopic origin, the adatom nucleation, step advance processes, and their relationship to film growth were theoretically analyzed and a growth diagram was constructed. Three boundaries between highly and poorly crystallized growth, 2D and 3D growth, stoichiometric and non-stoichiometric growth were identified in the growth diagram. A good fit of our experimental observation with the growth diagram was found. This case study demonstrates that a more comprehensive understanding of the growth mode in PLD is possible.

Manipulating electronic phase separation in strongly correlated oxides with an ordered array of antidots

Significance Electronic phase separation (EPS) is one of the most intriguing properties in complex materials. Great efforts have been made to understand or manipulate EPS, but how these phases are created and grow during percolation, let alone artificial control/fabrication of these phases, is still mysterious. In this work, we use a conceptual approach, i.e., fabricating antidots in manganites, and use their ferromagnetic metallic edge states to control the nucleation and growth of EPS domains. Consequently, we are able to tune the physical properties of the system without using external fields or changing doping concentration. The interesting transport and magnetic properties in manganites depend sensitively on the nucleation and growth of electronic phase-separated domains. By fabricating antidot arrays in La0.325Pr0.3Ca0.375MnO3 (LPCMO) epitaxial thin films, we create ordered arrays of micrometer-sized ferromagnetic metallic (FMM) rings in the LPCMO films that lead to dramatically increased metal–insulator transition temperatures and reduced resistances. The FMM rings emerge from the edges of the antidots where the lattice symmetry is broken. Based on our Monte Carlo simulation, these FMM rings assist the nucleation and growth of FMM phase domains increasing the metal–insulator transition with decreasing temperature or increasing magnetic field. This study points to a way in which electronic phase separation in manganites can be artificially controlled without changing chemical composition or applying external field.

Emerging single-phase state in small manganite nanodisks

Significance Electronic phase separation (EPS) is a common phenomenon in complex oxides systems. However, little is known regarding how EPS responds when the size of the system is smaller than the characteristic size of EPS. This issue is not only important for understanding the physical origin of EPS but also for oxides device applications in which oxides have to be fabricated into small-sized structures. Our work on manganites shows a surprising transition from the EPS state to a single phase state when the spatial size of the system is smaller than the characteristic length scale of EPS. This observation paves a way to manipulate EPS, which is potentially useful for oxides electronic and spintronic device applications. In complex oxides systems such as manganites, electronic phase separation (EPS), a consequence of strong electronic correlations, dictates the exotic electrical and magnetic properties of these materials. A fundamental yet unresolved issue is how EPS responds to spatial confinement; will EPS just scale with size of an object, or will the one of the phases be pinned? Understanding this behavior is critical for future oxides electronics and spintronics because scaling down of the system is unavoidable for these applications. In this work, we use La0.325Pr0.3Ca0.375MnO3 (LPCMO) single crystalline disks to study the effect of spatial confinement on EPS. The EPS state featuring coexistence of ferromagnetic metallic and charge order insulating phases appears to be the low-temperature ground state in bulk, thin films, and large disks, a previously unidentified ground state (i.e., a single ferromagnetic phase state emerges in smaller disks). The critical size is between 500 nm and 800 nm, which is similar to the characteristic length scale of EPS in the LPCMO system. The ability to create a pure ferromagnetic phase in manganite nanodisks is highly desirable for spintronic applications.

Unusual giant anisotropic magnetoresistance in manganite strips

Manganites have been known to exhibit giant anisotropic magnetoresistance (GAMR) near metal-insulator transition temperatures. Interestingly, we observed a second GAMR peak at lower temperatures in manganite strips fabricated from epitaxial thin films. The second low-temperature GAMR peak is highly sensitive to magnetic field and vanishes quickly upon increasing of magnetic field. We attribute the emergent GAMR behavior to spatial confinement effect on electronic phase separation in manganite strips.

Controllable magnetization and resistivity jumps of manganite thin films on BaTiO3 substrate

Manganites thin films grown on ferroelectric BaTiO3 (BTO) exhibit dramatic jumps for both magnetization and resistivity upon cooling in accordance with the temperature-dependent structural transitions of the BTO substrate. Both upward and downward jumps have been reported at the same temperature point where BTO undergoes a structural transition from monoclinic to rhombohedral. Using La5/8Ca3/8MnO3/BaTiO3 as protype system, we solve the puzzle by showing that the direction of the jumps can be controlled by applying an electric field during post growth cooling which determines the orientation of the c-axis of the BTO substrate at room temperature. This offers a convenient way to control the magnetic and transport behavior of manganites films using electric field.

A large enhancement of magnetocaloric effect by chemical ordering in manganites

For conventional ferromagnetic systems, the magnetocaloric effect (MCE) is dominated by the entropy changes upon magnetic phase transitions. For strongly correlated oxides such as manganites, in which competing magnetic orders coexist and respond to an external field differently, the MCE is more complex due to the electronic phase separation (EPS) phenomenon. Taking the well-known (La2/3Pr1/3)5/8Ca3/8MnO3 (LPCMO) manganite as a model system, we investigated how the length scale of EPS phases affects the MCE. Specifically, the EPS length scale can be dramatically reduced by the spatial ordering of Pr dopants in the LPCMO system. Experimental results indicate that the magnetic entropy change of the Pr-ordered LPCMO is considerably larger than that of the Pr-random LPCMO by a factor up to six at the onset temperature of the ferromagnetic phase. A direct relation between the length scale of EPS and MCE has been established based on the experimental results.