Xianmin Zhang

Room temperature magnetoresistance effects in ferroelectric poly(vinylidene fluoride) spin valves

Ferroelectric poly(vinylidene fluoride) (PVDF) nanofilms have been fabricated by the Langmuir–Blodgett technique, possessing mainly a ferroelectric active phase and a controllable film thickness of 2.3 nm per layer. Atomic force microscopy and Fourier transform infrared spectroscopy were utilized to characterize the film properties. Importantly, the PVDF films could act as barrier layers to prepare spin transport devices using Fe3O4 and Co as bottom and top ferromagnetic electrodes, respectively. Spin-dependent electron transport behaviors were systemically studied in these devices by varying the PVDF film thickness from 3 layers (7 nm) to 13 layers (30 nm). With increasing PVDF layer numbers, the magnetoresistance (MR) response decreases likely due to the change in spin transport from tunneling to hopping transport. We further investigated the MR dependence on operation temperatures (150 K, 200 K, 250 K and 300 K). It is noteworthy that the MR effect was observed even at 300 K with an MR ratio exceeding 2.5%, which is achieved for the first time in such organic devices. The device performance could be further improved at lower operation temperatures. The MR ratios, device resistances and electron transport mechanisms in the present devices were also discussed to analyze the spin transport behaviors. The results indicate that the ferroelectric PVDF nanofilms are promising candidates for spin devices operated at room temperature, thereby shedding light on the design of organic ferroelectric spintronics with a higher performance.

Properties and Applications of the β Phase Poly(vinylidene fluoride)

Poly(vinylidene fluoride), PVDF, as one of important polymeric materials with extensively scientific interests and technological applications, shows five crystalline polymorphs with α, β, γ, δ and ε phases obtained by different processing methods. Among them, β phase PVDF presents outstanding electrical characteristics including piezo-, pyro-and ferroelectric properties. These electroactive properties are increasingly important in applications such as energy storage, spin valve devices, biomedicine, sensors and smart scaffolds. This article discusses the basic knowledge and character methods for PVDF fabrication and provides an overview of recent advances on the phase modification and recent applications of the β phase PVDF are reported. This study may provide an insight for the development and utilization for β phase PVDF nanofilms in future electronics.

Magnetoresistance Effect and the Applications for Organic Spin Valves Using Molecular Spacers

Organic spin devices utilizing the properties of both spin and charge inherent in electrons have attracted extensive research interest in the field of future electronic device development. In the last decade, magnetoresistance effects, including giant magetoresistance and tunneling magnetoresistance, have been observed in organic spintronics. Significant progress has been made in understanding spin-dependent transport phenomena, such as spin injection or tunneling, manipulation, and detection in organic spintronics. However, to date, materials that are effective for preparing organic spin devices for commercial applications are still lacking. In this report, we introduce basic knowledge of the fabrication and evaluation of organic spin devices, and review some remarkable applications for organic spin valves using molecular spacers. The current bottlenecks that hinder further enhancement for the performance of organic spin devices is also discussed. This report presents some research ideas for designing organic spin devices operated at room temperature.

Pressure induced metallization of BaTe

The structural properties and pressure-induced phase transitions of BaTe are theoretically studied using the first-principles pseudopotential method. Our calculations show the phase transition pressure point between B1 and B2 structure is 4.8 GPa, and the metallization pressure for BaTe with B2 phase occurs at 20 GPa The electronic band structures and the density of states for BaTe were further investigated to explore the effect of pressure on its structural variation.

Organic spin valves with poly(vinylidene fluoride) barriers

Poly(vinylidene fluoride) (PVDF) was used as an organic spacer to fabricate spin-valve devices for the first time. Their magnetoresistance (MR) effects with different layer numbers of PVDF were investigated at both room and low temperatures. The device resistances and MR ratios increase with decreasing measurement temperature. It is noted that the MR ratios at room temperature are over 2% and 0.5% for devices with 3 layers and 13 layers of PVDF, respectively.

Organic-inorganic hybrid spin valves with different organic spacers

A series of novel organic spin valve devices with a Fe3O4/Al-O/organic semiconductors (OSs)/Co/Al stacking structure was fabricated. Here, four kinds of OSs, Poly(vinylidene fluoride) (PVDF), rubrene, C60 and Alq3 were tested in the devices. The spin-dependent transport properties and magnetoresistance (MR) effect were systemically studied. Giant MR ratio over 8% was observed at 300 K in C60-based spin valves, which is one of the highest MR ratios reported so far. Moreover, a large SDT length of over 100 nm was experimentally observed in the C60 layer at room temperature. Tunneling MR ratio was achieved over 6% in rubrene and Alq3 based spin valves at room temperature. It is noted that the tunneling MR ratios at room temperature are around 2% for devices with 3 layers of PVDF. The relations between OSs spacer and MR effect were discussed.

Perpendicularly magnetized ferrimagnetic [Mn50Ga50/Co2FeAl] superlattice and the utilization in magnetic tunnel junctions

The ferrimagnetic superlattice (SL) [MnGa/Co2FeAl]n exhibiting perpendicular magnetic anisotropy opened a new method for spintronics materials used in magnetic random access memory, because of the high anisotropy, small damping constant and tunable magnetization. In this work, we fabricated SLs with different MnGa composition and studied the MnGa composition dependence of the structure and magnetic properties of the SLs. Furthermore, we fabricated fully perpendicular magnetic tunnel junctions with SLs as both top and bottom electrodes. A clear tunnel magnetoresistance (TMR) effect with TMR ratio of 1.3% at room temperature was observed.