Jian Shen

Ferromagnetic percolation in MnxGe1−x dilute magnetic semiconductor

We have studied the magnetic and magnetotransport properties of Mn-doped Ge grown by molecular-beam epitaxy. This group-IV dilute ferromagnetic semiconductor exhibits two magnetic transitions. An upper critical temperature TC* (∼112K for x∼0.05) is evident from the extrapolated Curie–Weiss susceptibility and from the Arrott plot analysis of anomalous Hall effect data. The existence of a lower critical temperature TC (∼12K for x∼0.05) is established from ac susceptibility and magnetotransport data. The data are fully compatible with the existence of bound magnetic polarons or clusters below TC* which percolate at TC⪡TC*.

Room-Temperature Multiferroic Hexagonal LuFeO3 Films

The crystal and magnetic structures of single-crystalline hexagonal LuFeO(3) films have been studied using x-ray, electron, and neutron diffraction methods. The polar structure of these films are found to persist up to 1050 K; and the switchability of the polar behavior is observed at room temperature, indicating ferroelectricity. An antiferromagnetic order was shown to occur below 440 K, followed by a spin reorientation resulting in a weak ferromagnetic order below 130 K. This observation of coexisting multiple ferroic orders demonstrates that hexagonal LuFeO(3) films are room-temperature multiferroics.

Electrophoretic-like Gating Used To Control Metal–Insulator Transitions in Electronically Phase Separated Manganite Wires

Electronically phase separated manganite wires are found to exhibit controllable metal-insulator transitions under local electric fields. The switching characteristics are shown to be fully reversible, polarity independent, and highly resistant to thermal breakdown caused by repeated cycling. It is further demonstrated that multiple discrete resistive states can be accessed in a single wire. The results conform to a phenomenological model in which the inherent nanoscale insulating and metallic domains are rearranged through electrophoretic-like processes to open and close percolation channels.

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.

Active control of magnetoresistance of organic spin valves using ferroelectricity

Summary form only given. Organic spintronic devices have been appealing because of the long spin lifetime of the charge carriers in the organic materials and their low cost, flexibility and chemical diversity. In previous studies, the control of resistance of organic spin valves is generally achieved by the alignment of the magnetization directions of the two ferromagnetic electrodes, generating magnetoresistance. Here we employ a new knob to tune the resistance of organic spin valves by adding a thin ferroelectric interfacial layer between the ferromagnetic electrode and the organic spacer: the magnetoresistance of the spin valve depends strongly on the history of the bias voltage, which is correlated with the polarization of the ferroelectric layer; the magnetoresistance even changes sign when the electric polarization of the ferroelectric layer is reversed. These findings enable active control of resistance using both electric and magnetic fields, opening up possibility for multi-state organic spin valves.

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.

Crystal field splitting and optical bandgap of hexagonal LuFeO3 films

Hexagonal LuFeO3 films have been studied using x-ray absorption and optical spectroscopy. The crystal splitting of Fe3+ is extracted as Ee′−Ee″=0.7u2009eV and Ea1′−Ee′=0.9u2009eV, and a 2.0u2009eV optical bandgap is determined assuming a direct gap. First-principles calculations confirm the experiments that the relative energies of crystal field splitting states do follow Ea1′>Ee′>Ee″ with slightly underestimated values and a bandgap of 1.35u2009eV.

Structural and electronic origin of the magnetic structures in hexagonal LuFeO3

Using combined theoretical and experimental approaches, we studied the structural and electronic origin of the magnetic structure in hexagonal LuFeO

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

_3

Chemically induced Jahn–Teller ordering on manganite surfaces

. Besides showing the strong exchange coupling that is consistent with the high magnetic ordering temperature, the previously observed spin reorientation transition is explained by the theoretically calculated magnetic phase diagram. The structural origin of this spin reorientation that is responsible for the appearance of spontaneous magnetization, is identified by theory and verified by x-ray diffraction and absorption experiments.