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INTRODUCTION
All materials have an inherent magnetic character arising from the movements of their electrons. Since dynamic electric fields induce a magnetic field, the orbit of electrons, which creates atomic current loops, results in magnetic fields. An external magnetic field will cause these atomic magnetic fields to align so that they oppose the external field. This is the only magnetic effect that arises from electron pairs. If a material exhibits only this effect in an applied field it is known as a diamagnetic material.
Magnetic properties other than diamagnetism, which is present in all substances, arise from the interactions of unpaired electrons. These properties are traditionally found in transition metals, lanthanides, and their compounds due to the unpaired d and f electrons on the metal. There are three general types of magnetic behaviors: paramagnetism, in which the unpaired electrons are randomly arranged, ferromagnetism, in which the unpaired electrons are all aligned, and antiferromagnetism, in which the unpaired electrons line up opposite of one another. Ferromagnetic materials have an overall magnetic moment, whereas antiferromagnetic materials have a magnetic moment of zero. A compound is defined as being ferrimagnetic if the electron spins are orientated antiparrallel to one another but, due to an inequality in the number of spins in each orientation, there exists an overall magnetic moment. There are also enforced ferromagnetic substances (called spin-glass-like) in which antiferromagnetic materials have pockets of aligned spins
Types of magnetism: (A) paramagnetism (B) ferromagnetism © antiferromagnetism (D) ferrimagnetism (E) enforced ferromagnetism
Magnetic character of materials is typically analyzed relative to its magnetic susceptibility (χ). Magnetic susceptibility is the ratio of magnetization (M) to magnetic field (H). The type of magnetic behavior of a compound can be defined by its value of χ (see Table 1 for a comparison of magnetic behavior versus χ and Table 2 for the susceptibilities of some common paramagnetic materials).
Antiferromagnetic materials can be distinguished from paramagnetic substances, in that the value of χ increases with temperature, whereas χ shows no change or decreases in value as temperature rises for paramagnetic compounds. Ferromagnetic and antiferromagnetic materials will lose magnetic character and become paramagnetic if sufficiently heated. The temperature at which this occurs is defined as the Curie temperature (Tc) for ferromagnetic compounds and the Néel temperature (TN) for antiferromagnetic compounds. Some substances, particularly the later lanthanides, will go from paramagnetic to antiferromagnetic to ferromagnetic as temperature decreases (Table 3).
Curie Temperature Néel Temperature Curie Temperature Néel Temperature
Table 3. Curie and Néel temperatures of some lanthanides.1
There are several unique properties of magnetic materials which are exploited. Changing magnetic fields induce an electrical voltage making magnetic materials a central component of nearly all electrical generators. Magnetic materials are also essential components for information storage in computers, sensors, actuators, and a variety of telecommunications devices ranging from telephones to satellites.
Some materials, known as soft magnetic materials, exhibit magnetic properties only when they are exposed to a magnetizing force such as a changing electric field. Soft ferromagnetic materials are the most common of these as they are widely used in both AC and DC circuits to amplify the electrical flux. Magnetic nanopowders have shown great promise in advanced soft magnetic materials.2 Magnetocaloric materials heat up in the presence of a magnetic field and subsequently cool down when removed from the magnetic field. Pure iron, for example will change temperature by 0.5 – 2.0 °C/Tesla. More recently alloys of the formula Gd5SixGe1-x (where x = 0 – 5) will exhibit a 3 – 4 °C/Tesla change.3,4 Some nanomagnetic materials have shown significant magnetocaloric properties.
In general, molecule-based magnets have magnetic properties comparable to traditional magnets. However, being molecular, they have many advantages over metal-based magnets in terms of device fabrication. For example, they can be deposited as thin films by lowtemperature (40 ºC) CVD, are low density, and can be transparent. This makes them ideal candidates for such advanced devices utilizing magnetic imaging, data storage, magnetic shielding, or magnetic induction. In addition, molecule-based magnets can have more specialized properties such as photomodulated magnetization.
Theories likening electron-transfer salts to molecular magnets date back to 1963. The phenomenon was not observed until 1985,however, when Miller and coworkers identified ferromagnetism in decamethylferrocenium tetracyanoethenide [Fe(Cp*)2][TCNE]. Since this time many electron-transfer salts of decamethylmetallocenes with TCNE or TCNQ (7,7,8,8-tetracyano-pquinodimethane) have been reported.
Structures of 7,7,8,8-tetracyano-p-quinodimethane (TCNQ) and tetracyanoethenide (TCNE).
tures of some Electron-Transfer salts.
One aspect of molecule-based magnets that distinguish them from traditional magnetic materials is dimensionality. Molecular magnets are often only magnetic in a single direction or along a single dimension. For example, hexylammonium trichlorocuprate(II) (CuCl3(C6H11NH3) or CHAC) consists of double-bridged chain of CuCl3 units with the hexylamine cations hydrogen bonding parallel chains together . There is a ferromagnetic interaction along the chain within the orthorhombic crystal structure.
The CuCl3 core of CHAC (chlorine atoms are not labeled).
The interaction parameter (J) along the c axis in figure 5 (JC) is 100 cm-1 and ferromagnetic. This is about four orders of magnitude larger than the ferromagnetic interaction along the b axis (Jb ≈ 10-1 cm-1) and more than five orders of magnitude larger than the antiferromagnetic interaction along the a axis (Ja = < -10-2 cm-1). Many ferromagnetic chain molecules have since been reported among them [MnCu(dto)2(H2O)3•4.5 H2O] (dto = dithiooxalato) was the first characterized.
Spintronics emerged from discoveries in the 1980s concerning spin-dependent electron transport phenomena in solid-state devices. This includes the observation of spin-polarized electron injection from a ferromagnetic metal to a normal metal by Johnson and Silsbee (1985),and the discovery of giant magnetoresistance independently by Albert Fert et al. and Peter Grünberg et al. (1988). The origins of spintronics can be traced back even further to the ferromagnet/superconductor tunneling experiments pioneered by Meservey and Tedrow,[and initial experiments on magnetic tunnel junctions by Julliere in the 1970s. The use of semiconductors for spintronics can be traced back at least as far as the theoretical proposal of a spin field-effect-transistor by Datta and Das
Moore's Law - a dictum of the electronics industry that says the number of transistors that fit
on a computer chip will double every 18 months - may soon face some fundamental
roadblocks. Most researchers think there'll eventually be a limit to how many transistors they
can cram on a chip. But even if Moore's Law could continue to spawn ever-tinier chips, small
electronic devices are plagued by a big problem: energy loss, or dissipation, as signals pass
from one transistor to the next. Line up all the tiny wires that connect the transistors in a
Pentium chip, and the total length would stretch almost a mile. A lot of useful energy is lost as
heat as electrons travel that distance.
Theoretical physicists have found a way to solve the dissipation problem by manipulating a neglected property of the electron - its ''spin,''or orientation, typically described by its quantum
state as ''up'' or ‘‘down.’’.
Electronics relies on Ohm's Law, which says application of a voltage to many materials results
in the creation of a current. That's because electrons transmit their charge through the
materials. But Ohm's Law also describes the inevitable conversion of electric energy into heat
when electrons encounter resistance as they pass through materials.
''Unlike the Ohm's Law for electronics, the new 'Ohm's Law' that we've discovered says that the spin of the electron can be transported without any loss of energy, or dissipation. Furthermore, this effect occurs at room temperature in materials already widely used in the semiconductor industry, such as gallium arsenide. That's important because it could enable a new generation of computing devices.''
A celestial analogy to explain two important properties of electrons - their center
of mass and their spin: ''The Earth has two kinds of motion. One is that its center of mass
moves around the Sun. But the other is that it also spins by itself, or rotates. The way it
moves around the Sun gives us the year, but the way it rotates around by itself gives us the
day. The electron has similar properties.'' While electronics uses voltage to move an electron's
center of mass, spintronics uses voltage to manipulate its spin.
Ferromagnetic metallic alloy based devices are mainly used in memory and information storage. They are also termed as magnetoelectronics devices . They rely on the giant magnetoresistance (GMR) or tunnelling magnetoresistance effect. Magnetic interaction is well understood in this category of devices
Semiconductor spintronics devices combine advantages of semiconductor with the concept of magnetoelectronics. This category of devices includes spin diodes, spin filter, and spin FET. To make semiconductor based spintronic devices, researchers need to address several following different problems. A first problem is creation of inhomogeneous spin distribution. It is called spin-polarisation or spin injection. Spin-polarised current is the primary requirement to make semiconductor spintronics based devices. It is also very fragile state. Therefore, the second problem is achieving transport of spin-polarised electrons maintaining their spin-orientation . Final problem, related to application, is relaxation time. This problem is even more important for the last category devices . Spin comes to equilibrium by the phenomenon called spin relaxation. It is important to create long relaxation time for effective spin manipulation, which will allow additional spin degree of freedom to spintronics devices with the electron charge . Utilizing spin degree of freedom alone or add it to mainstream electronics will significantly improve the performance with higher capabilities.
The third category devices are being considered for building quantum computers. Quantum information processing and quantum computation is the most ambitious goal of spintronics research. The spins of electrons and nuclei are the perfect candidates for quantum bits or qubits. Therefore, electron spin and nuclear based hardwares are some of the main candidates being considered for quantum computers.
Spintronics based devices offers several advantages over conventional charge based devices. Since magnetized materials maintain their spin even without power, spintronics based devices could be the basis of non-volatile memory device. Energy efficiency is another virtue of these devices as spin can be manipulated by low-power external magnetic field. Miniaturization is also another advantage because spintronics can be coupled with conventional semiconductor and optoelectronic devices.
However, temperature is still a major bottleneck. Practical application of spintronics needs room-temperature ferromagnet in semiconductors. Making such materials represents a substantial challenge for materials scientists.