In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

We review recent advances in biosensors based on one-dimensional (1-D) nanostructure field-effect transistors (FET). Specifically, we address the fabrication, functionalization, assembly/alignment and sensing applications of FET based on carbon nanotubes, silicon nanowires and conducting polymer nanowires. The advantages and disadvantages of various fabrication, functionalization, and assembling procedures of these nanosensors are reviewed and discussed. We evaluate how they have been used for detection of various biological molecules and how such devices have enabled the achievement of high sensitivity and selectivity with low detection limits. Finally, we conclude by highlighting some of the challenges researchers face in the 1-D nanostructures research arena and also predict the direction toward which future research in this area might be directed.

Nanowire‐Based Electrochemical Biosensors

TOPICAL REVIEW: Organic spintronics

The authors perform nonlocal four-probe spin-valve experiments on graphene contacted by ferromagnetic Permalloy electrodes. They observe sharp switching and often sign reversal of the nonlocal resistance at the coercive field of the electrodes, indicating the presence of a spin current between injector and detector. The nonlocal spin-valve signal changes magnitude and sign with back-gate voltage, and is observed up to T=300K. The gate voltage variation of the spin-valve signal may result from quantum-coherent transport, as evidenced by Fabry-Perot-like oscillations of the current.

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Gate-tunable graphene spin valve

Spintronics aims to develop electronic devices whose resistance is controlled by the spin of the charge carriers that flow through them1,2,3. This approach is illustrated by the operation of the most basic spintronic device, the spin valve4,5,6, which can be formed if two ferromagnetic electrodes are separated by a thin tunnelling barrier. In most cases, its resistance is greater when the two electrodes are magnetized in opposite directions than when they are magnetized in the same direction7,8. The relative difference in resistance, the so-called magnetoresistance, is then positive. However, if the transport of carriers inside the device is spin- or energy-dependent3, the opposite can occur and the magnetoresistance is negative9. The next step is to construct an analogous device to a field-effect transistor by using this effect to control spin transport and magnetoresistance with a voltage applied to a gate10,11. In practice though, implementing such a device has proved difficult. Here, we report on a pronounced gate-field-controlled magnetoresistance response in carbon nanotubes connected by ferromagnetic leads. Both the magnitude and the sign of the magnetoresistance in the resulting devices can be tuned in a predictable manner. This opens an important route to the realization of multifunctional spintronic devices.

Electric field control of spin transport

Spin polarized transport over 5μm long carbon nanotubes (CNTs) is observed in magnetically assembled spin valves with electrochemically deposited ferromagnetic (FM) electrodes. An annealing procedure is developed to produce stable devices with highly transmissive FM/CNT contacts. The devices exhibit magnetoresistance (MR) with magnitudes up to 4%, and the sign of MR is consistently negative. The temperature dependence is investigated and MR is observed up to 14K. Bias dependence is also investigated, with MR persisting up to ∼300mV.

Spin-polarized transport in magnetically assembled carbon nanotube spin valves

Random telegraph noise (RTN) measurements are typically carried out at the device level using standard probe station based electrical characterization setup, where the measured current represents a cumulative effect of the simultaneous response of electron capture/emission events at multiple oxygen vacancy defect (trap) sites. To better characterize the individual defects in the high-κ dielectric thin film, we propose and demonstrate here the measurement and analysis of RTN at the nanoscale using a room temperature scanning tunneling microscope setup, with an effective area of interaction of the probe tip that is as small as 10 nm in diameter. Two-level and multi-level RTN signals due to single and multiple defect locations (possibly dispersed in space and energy) are observed on 4 nm HfO2 thin films deposited on n-Si (100) substrate. The RTN signals are statistically analyzed using the Factorial Hidden Markov Model technique to decode the noise contribution of more than one defect (if any) and estimate t...

Single vacancy defect spectroscopy on HfO2 using random telegraph noise signals from scanning tunneling microscopy

A facile method for assembling carbon nanotubes (CNT) on ferromagnetic metal contacts is described. Multiwalled carbon nanotubes (MWNT) with a magnetic cap were fabricated by thermally evaporating nickel on top of a vertical array of MWNTs grown on silicon. Magnetic interaction between the magnets on the nanotubes and lithographically patterned ferromagnetic electrodes caused the relative alignment and directed placement of nanotubes. The lithographically patterned electrodes were further modified using electrochemical deposition to form asymmetric ferromagnetic electrodes as well as improve the electrical and magnetic interactions at the contacts. MWNTs thus assembled showed characteristics that have been established for various nanotube device configurations. This validates the assembling technique for exploring charge- and spin-based electronic devices.

Magnetically assembled multiwalled carbon nanotubes on ferromagnetic contacts

We have studied ultrathin Fe x Mn 1 - x films on Cu(100) for Fe contents ranging from 45% to 80%. In the bulk the fcc structure displays antiferromagnetic order in this concentration regime. The growth was investigated viareflection high energy electron diffraction and structural properties were investigated with low energy electron diffraction. From lattice mismatch arguments one would have expected to observe a p( I × 1) pattern. However we find a c(2 × 2) structure for thicknesses below 5 ML. Above this coverage it transforms into a p(1 x 1) structure. The c(2×2) structure is not present when the alloys are grown on a Co/Cu(100) surface. With the use of Auger spectroscopy we find clear evidence of Fe surface segregation. At 54% Fe content we estimate an enhancement of the surface content of Fe by ∼ 10%. The amount of excess Fe agrees well with the observation of 'uncompensated' Fe spins in Co/Fe 5 0 Mn 5 0 structures. Further we were able to detect magnetic signals for coverages below ∼5 ML.

Structural and magnetic properties of ultrathin fccFexMn1−xfilms on Cu(100)

Two-probe electronic transport measurements on a Molybdenum Disulphide (MoS_2) surface were performed at low temperature (30K) under ultra-high vacuum conditions. Two scanning tunneling microscope tips were precisely positioned in tunneling contact to measure the surface current-voltage characteristics. The separation between the tips is controllably varied and measured using a high resolution scanning electron microscope. The MoS_2 surface shows a surface electronic gap (E_S) of 1.4eV measured at a probe separation of 50nm. Furthermore, the two- probe resistance measured outside the electronic gap shows 2D-like behavior with the two-probe separation.

R. Thamankar

Papers

Spin-polarized transport in magnetically assembled carbon nanotube spin valves

Single vacancy defect spectroscopy on HfO2 using random telegraph noise signals from scanning tunneling microscopy

Magnetically assembled multiwalled carbon nanotubes on ferromagnetic contacts

Structural and magnetic properties of ultrathin fccFexMn1−xfilms on Cu(100)

Low Temperature Nanoscale Electronic Transport on the MoS_2 surface