MRAM using Spintronics
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 unpaireddandfelectrons 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 (C) 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 (seeTable 1for a comparison of magnetic behavior versus χ andTable 2for the susceptibilities of some common paramagnetic materials).
Magnetic Behavior / Value ofχDiamagnetic / small and negative
Paramagnetic / small and positive
Ferromagnetic / large and positive
Antiferromagnetic / small and positive
Table 1.Magnetic behavior versus values of magnetic susceptibility
Compound/ Element / Formula / Mass Susceptibility (χm) (m3/kg) / Mass Susceptibility (χm) (emu/Oe•g) x 10-3Cerium / Ce / 64.84 / 5.160
Chromium(III) oxide / Cr2O3 / 24.63 / 1.960
Cobalt(II) oxide / CoO / 61.57 / 4.900
Dysprosium / Dy / 1301 / 103.500
Dysprosium oxide / Dy2O3 / 1126 / 89.600
Erbium / Er / 556.7 / 44.300
Erbium oxide / Er2O3 / 928.9 / 73.920
Europium / Eu / 427.3 / 34.000
Europium oxide / Eu2O3 / 126.9 / 10.100
Gadolinium / Gd / 9488 / 755.000
Gadolinium oxide / Gd2O3 / 668.5 / 53.200
Iron(II) oxide / FeO / 90.48 / 7.200
Iron(III) oxide / Fe2O3 / 45.06 / 3.586
Iron(II) sulfide / FeS / 13.5 / 1.074
Neodymium / Nd / 70.72 / 5.628
Neodymium oxide / Nd2O3 / 128.2 / 10.200
Potassium superoxide / KO2 / 40.59 / 3.230
Praseodymium / Pr / 62.96 / 5.010
Samarium / Sm / 28.02 / 2.230
Samarium oxide / Sm2O3 / 24.98 / 1.988
Terbium / Tb / 1822 / 146.000
Terbium oxide / Tb2O3 / 984.4 / 78.340
Thulium / Tm / 320.4 / 25.500
Thulium oxide / Tm2O3 / 646.5 / 51.444
Vanadium oxide / V2O3 / 24.83 / 1.976
Table 2.Mass susceptibilities of some common paramagnetic materials [emu = electromagnetic unit (10-3amp·m2), Oe = Oersted 103·4 š-1·amp·m-1)]
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 TemperatureMetal / TC(°C) / TN(°C) / TC(K) / TN(K)
Ce / -260.65 / 12.5
Pr / -248 / 25
Nd / -254 / 19
Sm / -258.35 / 14.8
Eu / -183 / 90
Gd / 20 / 293
Tb / -51 / -44 / 222 / 229
Dy / -188 / -94 / 85 / 179
Ho / -253 / -142 / 20 / 131
Er / -253 / -189 / 20 / 84
Tm / -248 / -217 / 25 / 56
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.2Magnetocaloric 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.3,4Some 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 decamethylferroceniumtetracyanoethenide [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).
[CrCp*2]+[TCNE]- / 22.2 / 3.65
[CrCp*2]+[TCNQ]- / 12.8 / 3.5
[FeCp*2]+[TCNE]- / 16.8 / 4.8
[FeCp*2]+[TCNQ]- / 12.3 / 2.55
[MnCp*2]+[TCNE]- / 22.6 / 8.8
[MnCp*2]+[TCNQ]- / 10.5 / 6.5
Table 5.Curie and critical temperatures 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, hexylammoniumtrichlorocuprate(II) (CuCl3(C6H11NH3) or CHAC) consists of double-bridged chain of CuCl3units with the hexylaminecations hydrogen bonding parallel chains together . There is a ferromagnetic interaction along the chain within the orthorhombic crystal structure.
The CuCl3core of CHAC (chlorine atoms are not labeled).
The interaction parameter (J) along the c axis in figure 5 (JC) is 100 cm-1and ferromagnetic. This is about four orders of magnitude larger than the ferromagnetic interaction along the b axis (Jb≈ 10-1cm-1) and more than five orders of magnitude larger than the antiferromagnetic interaction along the a axis (Ja= < -10-2cm-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 ofgiant magnetoresistanceindependently byAlbert Fertet al.andPeter Grünberget 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 forelectronics, the new 'Ohm's Law' that we've discovered says that the spin of the electron canbe transported without any loss of energy,or dissipation. Furthermore, this effect occurs atroom temperature in materials already widely used in the semiconductor industry,such asgallium arsenide. That's important because it could enable a new generation of computingdevices.''
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 magnetoelectronicsdevices . They rely on the giant magnetoresistance (GMR) or tunnellingmagnetoresistance 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.
Spin based Devices
The present status of spintronics devices at the commercial level is limited to giant magnetoresistance (GMR) based devices. In GMR based memory devises electron spin play passive role . It is limited to detect the change of magnitude of resistance depending on direction of the spin . The change in resistance is controlled by a local or an external magnetic field . But, it is predicted that spintronics can go beyond this passive spin device by integrating electron spin into conventional semiconductors. Thus, the technology based on spintronics may replace conventional semi-conducting devices by introducing active control of electron spin.
Giant Magnetoresistance (GMR) devices
The read heads in modern hard drives and non-volatile, magnetic random access memory (MRAM) are the two application of GMR effect.
In 1988, Albert Fert’s group discovered GMR effect. They observed that when multi layers of alternate magnetic/non-magnetic materials carrying electric current were placed in magnetic field, they exhibit large change in electric resistance, which is also known as magneto resistance.
Figure 2
Figure 2 Giant magneto resistance effect; (a) electron transport takes place when magnetization direction of both ferromagnetic regions aligned parallel to each other, (b)electrons are facinghigh resistance and scattered away near interface when magnetization direction of both ferromagnetic regions are opposite to each other (b).
The change in resistance depends on the relative orientation of the magnetization in magnetic layers . The resistance to passage of current is low when the ferromagnetic layers align in the same direction and transfer of current takes place dynamically (fig 1 (a)). If they align themselves in opposite directions electrons scattering occurs near interface and a high resistance path is produced (fig 1 (b)). The relative orientation of magnetic layers can be altered by the applying external magnetic field . This effect is called spin-valve effect . These multi layers are used to configure the GMR devices.
The read heads in hard disk drives utilize spin-valve effect to read data bits. The data bits are stored as the minute magnetic areas on the surface of HDD . ‘Zero’ is stored, when the magnetic layers align themselves in one direction and ‘one’ when they align in opposite directions . The read head reads the data by sensing a change in voltage corresponding to a change in resistance . It reads 1 when resistance is higher and 0 when resistance is lower . Thus, the ability of read head to sense minute changes in voltage corresponding to small changes in magnetic fields will allow data storage at highest packing densities in small magnetic particles . The expected value of storage densities may reach to 100 gigbites per square inch by using synthetic Ferromagnets. There are three types of GMR.
Spin transistors
The spin-transistors exploit electron spin either by spin-valve effect or by active control of electron spin . The design of transistor is similar to that of GMR devices. It consists of three layers, out of which the non-magnetic layer is sandwiched between the two ferromagnetic layers
Johnson was the first to propose about spin-valve transistor. As per him, the first magnetic layer acts as a spin injector or emitter while the second acts as a spin detector or collector . The non-magnetic layer acts as a base . The magnetization direction of the collector can be changed by the application of an external magnetic field . When the voltage is applied across the emitter-base, it generate electrons with either spin-up or spin-down . When the magnetization direction of emitter and collector is parallel, the current can flow throw the base to the collector . The electrons face high resistance when the relative magnetization direction is opposite. Thus, device acts as one-way switch . Electron spin plays passive role in Johnson’s spin-valve transistor.
Figure 3 Dutta-Das field effect transistor; at zero gate voltage, electron preserves spin state in transport channel (a) it enables current flow from source to drain. With applied gate voltage, electrons change their spin state from parallel to anti parallel to the direction of magnetization of ferromagnetic layer (b) this offers high resistance to flow of current. Therefore, electron scattering occurs at drain and no current flow from source to drain .
The first model of transistor using active control of electron spin was proposed by Datta and Das. In the Datta-Das field effect transistor, the non-magnetic layer acts as a gate while two ferromagnetic layers act as source and drain respectively (fig 2(a)) . The gate plays an important role in Datta-Das field effect transistor. The gate modifies electron spin by generating effective magnetic field and thereby in switching the transistor . When voltage is applied to the gate, it generates effective magnetic field (fig 2(b)). Thus, by modifying gate voltage one can modify electron spin [4]. The electrons ballistically transport in transport channel, if its spin is parallel to the magnetization direction of drain (spin detector) . Otherwise, it will scattered away .
The control of charge current in spin transistor is similar to the conventional transistors [2, 4]. But, the spin transistors possess advantage over conventional transistors. They are smaller in size, and consume less power . Still, the spin-transistors are exist is in prototype models because of theoretical limitation related to spin behavior in different materials.
Manipulation of Electron Spin
Spintronic devices are based on careful manipulation of the electron spin. The spin can be easily manipulated by applying external magnetic field or by shining polarized light . In general, the scheme of spin manipulation works fundamentally on: (1) generation of spin-polarized electron, (2) injection and transportation of the spin-polarized electron, and (3) detection of the spin-polarized carriers with information.
Generation of spin polarization
The generations of spin-polarized electron spins mean generation of spin polarized current. This spin polarized current carries non-equilibrium spin population. The Spintronics devices detect the distribution of spin-up and spin-down electrons in spin polarized current to control the current . This phenomenon of controlling current in spintronic device makes it suitable to act as electronic switch of transistor. Thus, the control of current is then either a control of phase of electron spin or spin-population. It can be generated by transport, optical, and resonance methods or by their combination . Figure 2 shows the schematic representation of generation of spin-polarized current by transport method.