Jhen-Yong Hong

Mapping polarization induced surface band bending on the Rashba semiconductor BiTeI

Surfaces of semiconductors with strong spin-orbit coupling are of great interest for use in spintronic devices exploiting the Rashba effect. BiTeI features large Rashba-type spin splitting in both valence and conduction bands. Either can be shifted towards the Fermi level by surface band bending induced by the two possible polar terminations, making Rashba spin-split electron or hole bands electronically accessible. Here we demonstrate the first real-space microscopic identification of each termination with a multi-technique experimental approach. Using spatially resolved tunnelling spectroscopy across the lateral boundary between the two terminations, a previously speculated on p-n junction-like discontinuity in electronic structure at the lateral boundary is confirmed experimentally. These findings realize an important step towards the exploitation of the unique behaviour of the Rashba semiconductor BiTeI for new device concepts in spintronics.

Interfacial spectroscopic characterization of organic/ferromagnet hetero-junction of 3,4,9,10-perylene-teracarboxylic dianhydride-based organic spin valves

We report interfacial characterization of 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA)-based organic spin valves (OSV) dusted with a thin layer of partially oxidized alumina at the organic semiconductor (OSC)/ferromagnet (FM) interfaces. Up to 13.5% magnetoresistance is achieved at room temperature. X-ray photoelectron spectroscopy measurements reveal interfacial electronic interaction between PTCDA and FM while the application of a thin alumina layer at the PTCDA/FM interfaces prevents the electronic hybridization and effectively preserves the spin injection into the OSC spacer. This finding demonstrates the critical effect of interfacial structure on magnetotransport behavior in OSV.

X-ray photoelectron spectroscopic investigation on Fe geometrical sites of iron nitride thin films

A partially ordered Fe16N2 thin film, which exhibits a higher saturation magnetization than a bcc-Fe thin film, was grown on a Au(001) texture on a GaAs(001) substrate for studies of crystalline structure, electronic structure, and magnetic properties. Fe 2p3/2 and 2p1/2 X-ray photoelectron spectroscopies (XPS) reveal the electronic hybridization between the Fe atoms and the adjacent N atoms, whereas a multipeak analysis suggests the charge-transfer-induced electronic rearrangement of electronic configuration in Fe(8h) and Fe(4e) geometrical sites. These results are consistent with the previous model and help explain the saturation magnetization enhancement in the α-FeN system.

Superpoissonian shot noise in organic magnetic tunnel junctions

Organic molecules have recently revolutionized ways to create new spintronic devices. Despite intense studies, the statistics of tunneling electrons through organic barriers remains unclear. Here, we investigate conductance and shot noise in magnetic tunnel junctions with 3,4,9,10-perylene-teracarboxylic dianhydride (PTCDA) barriers a few nm thick. For junctions in the electron tunneling regime, with magnetoresistance ratios between 10% and 40%, we observe superpoissonian shot noise. The Fano factor exceeds in 1.5–2 times the maximum values reported for magnetic tunnel junctions with inorganic barriers, indicating spin dependent bunching in tunneling. We explain our main findings in terms of a model which includes tunneling through a two level (or multilevel) system, originated from interfacial bonds of the PTCDA molecules. Our results suggest that interfaces play an important role in the control of shot noise when electrons tunnel through organic barriers.

Tuning of magnetism in ferromagnetic thin films by reversing the functional groups of molecular underlayer

We demonstrate a molecular approach of tuning the magnetic properties of ferromagnetic (FM) thin films by reversing the functional groups of the organic underlayer. For the CoFe/Langmuir–Blodgett (LB) film system, we find that the coercivity of CoFe thin films (from 4 to 10 nm) made on hydrophobic surfaces is significantly enhanced whereas that on hydrophilic surfaces remains unchanged, as compared with the films directly on glass substrates. These findings suggest an alternative way for tuning the magnetic properties of the FM layer by LB film in which the functional groups play an important role.

HIGH-FREQUENCY MAGNETOCAPACITANCE EFFECT IN ORGANIC SPIN VALVE WITH A 3,4,9,10-PERYLENE-TERACARBOXYLIC-DIANHYDRIDE SPACER

The frequency-dependent impedance of a series of ferromagnet (FM)/organic semiconductor (OSC)/FM tri-layered organic spin valves (OSV) is investigated in the frequency range of 10 Hz–1 MHz. An equivalent resistor–capacitor (RC) parallel network model is employed to analyze the magnetoresistance (MR) and magnetocapacitance (MC) effects. Fitting with the model yields field-dependent parameters and the resistive parameters agree with the experimental results. The analysis of the impedance spectra indicates an effective magnetotransport mechanism dominated by the charge accumulation at the organic–FM interfaces.

Electron transport and noise spectroscopy in organic magnetic tunnel junctions with PTCDA and Alq3 barriers

The possible influence of internal barrier dynamics on spin, charge transport and their fluctuations in organic spintronics remains poorly understood. Here we present investigation of the electron transport and low frequency noise at temperatures down to 0.3K in magnetic tunnel junctions with an organic PTCDA barriers with thickness up to 5 nm in the tunneling regime and with 200 nm thick Alq3 barrier in the hopping regime. We observed high tunneling magneto-resistance at low temperatures (15-40%) and spin dependent super-poissonian shot noise in organic magnetic tunnel junctions (OMTJs) with PTCDA. The Fano factor exceeds 1.5-2 values which could be caused by interfacial states controlled by spin dependent bunching in the tunneling events through the molecules.1 The bias dependence of the low frequency noise in OMTJs with PTCDA barriers which includes both 1/f and random telegraph noise activated at specific biases will also be discussed. On the other hand, the organic junctions with ferromagnetic electrodes and thick Alq3 barriers present sub-poissonian shot noise which depends on the temperature, indicative of variable range hopping.

Magnetocapacitive response in organic spin valve with a 3,4,9,10-perylene-teracarboxylic-dianhydride spacer

Summary form only given. The frequency-dependent impedance characterization has been carried out in ferromagnet (FM)/ organic semiconductor (OSC)/FM tri-layered organic spin valves (OSVs) over the frequency range of 10 Hz-1 MHz with the objective of studying interfacial properties between ferromagnetic materials and organic semiconductors. The magnetoresistance and magnetocapacitance effects is investigated by fitting the field-dependent impedance spectra with an equivalent resistor-capacitor (RC) parallel network model. The analysis validates the RC parallel network model for OSVs and the extracted field dependent resistive parameters agree with the experimental values, indicating an effective magneto-transport characteristic in OSVs dominated by the charge accumulation at the organic-ferromagnet interfaces.