Magnetoresistencia Final

MAGNETORESISTANCE

November 10, 2014

Contents 1 Introduction

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2 Technical Background 2.1 What is Magnetism? . . . . . . . . . . . . . . . . . . . 2.1.1 Magnetic Behavior Of Solids . . . . . . . . . . . . 2.2 Electrical Resistivity . . . . . . . . . . . . . . . . . . . . . 2.3 Magnetoresistance Historical Background . . . . . . . . . 2.3.1 Ferromagnetic Materials . . . . . . . . . . . . . . . 2.4 Anisotropic Magnetoresistance . . . . . . . . . . . . . . . 2.4.1 Application of Anisotropic Magnetoresistance . . . 2.5 Giant Magnetoresistance and Colossal Magnetoresistance

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3 Giant Magnetoresistance 3.1 Historical Background . . . . 3.1.1 Grown of supperlatice 3.1.2 Interlayer Coupling . . 3.1.3 Spintronics . . . . . . 3.2 Experiments . . . . . . . . . . 3.2.1 Discovery . . . . . . . 3.2.2 Theorical Models . . . 3.2.3 Aplications . . . . . . 3.2.4 GMR Influence . . . . 3.3 2007 Nobel Prize . . . . . . .

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4 Colossal Magnetoresistance 4.1 Technical Background . . . . . 4.2 Negative Magnetoresitance . . 4.3 Manganese Oxides Perovskites 4.4 Substrates . . . . . . . . . . . .

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5 Conclusions 24 5.1 Future Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

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Abstract I review different information and articles about Magnetoresistance, which is the phenomena of the materials to change their electrical resistance when a magnetic field is applied. The discover of this property was in the XIX century, but it was until 70 year ago that the scientists found real application for it, they need the help of the develop of new technologies as techniques to grown thin layers know as molecular deposition. The first magnetoresistance was the Anisotropic Magnetoresistance which was amply used in the sensoring field. The Giant Magnetoresistance which is stronger than AMR, can achieve a change in the resistivity until 50%, and it is use in the data storage world, his develop has been very important in the grew of modern computers. At last we will see the Colossal Magnetoresistance, it is one of the newest type of Magnetoresistance, the different to the others is that the material suffer a dramatic decrease of resistance (negative magnetoresistance) when a strong magnetic field is applied. We are going to see important application and the way of produced the MR effect.

Chapter 1

Introduction Magnetism or electron spin has always been important for information storage. Apart from charge and mass, electrons possess another degree of freedom; the spin. As the charge of an electron decides how it behaves in an electric field, the spin of an electron contributes to its behaviour in a magnetic field. Spintronics is the acronym for spin transport electronics, where the spin carries the information, unlike in semiconductor electronics, where the charge carries the information. These properties are very important in the development of new research and innovation, a whole new of electronic devices has been created due these properties. They are so important basic science, such as scientific research and context for hence innovation. The development of the giant magnetoresistance was due to two scientists, on of them is Albert Fert and the other one is a Brazilian Mario Baibich, in the year of 1983, and this was a very rapid development because it took less than 20 years to do it. They observed what happens when you have two magnetic layers rather thin, what happens when you apply a magnetic field, the resistance change found with the magnetic field, in that moment they did not found an application for their new discover, but that was only the beginning. They spent a lot of time in laboratories where they worked, Mario Baibich Brazil went to work in France with Fert, only to see the behavior of this magnetic field structure. Well today that research lead to the development of sensors that read heads of hard drives. It is how we can undertand why a present-day hard drive is capable of storing standard densities in the order of gigabytes per square centimeter and this discovery was recognized with award Nobel physics in 2007 being one of the greatest impact on technology and change radically the way how we store information and how we read exactly and was considered the first mass application of nanotechnology. The Magnetoresistance (MR) effects has been of substantial importance technologically, especially in connection with reading heads for magnetic dics and as magnetic sensor. The most useful material has been an alloy of iron and nickel, Fe20Ni80(permalloy). In general, however, there was hardly any improvement of the performance of magnetoresitansive materials since the work 1

of Lord Kelvin. The general consensus in the 1980s was that it was possible to significantly improve on the performance of the magnetic sensors based on mangnetoresistance. The development of all this kind of technology needed a lot of time and hard work to be created, the magnetoresistance is an old topic, but in the last years has an significance importance, we can not imagine the whole data world with out the magnetoresistance that made possible the storage of information and the readers lenses behavior in many systems. The new discoveries open a gate for future new technology and also to go far away in the field of resistance, all started with a difference of 5 percent of resistance, the giant magnetoresistance can make a 40 percent of difference. It is important to focus in the investigation, research, the most of the greatest innovations, was that, innovations, and then the people in the word can use than innovation and find an use for it. This research work has the objective of inform about MR, we are going to know how it started, when it was discovered and by who was it, but we will also read about their development as a new property of materials. There is a lot of things under investigations, and are making experiments continually. I pretend that the people who read this achieves understand what is Magnetoresistance and how it works.

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Chapter 2

Technical Background 2.1

What is Magnetism?

The magnetic forces is one of the oldest physic phenomenons known for the human being. The origin of the word ”Magentism” comes from a magnetic mineral called Magnetite (F e304), that mineral was found in Magnesia, the name of a region in the Middle East, nowadays in Turkey. Now we know that iron is not the only material that is easily magnetized when placed in a magnetic field; others include nickel and cobalt. Magnetism is a class of physical phenomena that attracts some materials as iron, nickel, cobalt, that kind of materials were called Magnets. The most powerful intense of the magnets are in the outsides of the magnet, and it is called dipole. The same electric polarity reject each other, and different polarity attract each other. In the year of 1820, there was not connection found between electrical and magnetic phenomena. In 1820 Danish Scientist Hans Christian Oersted observed the magnetic effects of current and thus showed a connection between electricity and magnetism. Twelve years later,Michael Faraday found that a momentary current existed in a closed circuit while the current in the nearby circuit was being started or stopped. The same effect was also observed by motion of a magnet toward or away from the circuit. Thus we see that the work of Oersted demonstrated the magnetic effects by moving electric charges and that of Faraday the electric effects (production of current) by moving magnets. Nowadays it is believed that all so-called magnetic phenomena result from forces between electrostatic charges in motion and the magnetism can not be considered as a separate entity. The possibility that the magnetic properties of matter were the result of tiny atomic currents (due to continuous rotation of electrons in atoms about the axis passing through the atomic nuclei) was first suggested by Ampere.

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2.1.1

Magnetic Behavior Of Solids

In an atom, the magnetic field is due the sum of the magnetic momentum and the magnetic of the spin. The magnetic field of the spin is associated to the movement of the electron in his own axe while the magnetic field of the orbital is associated to the movement of the electron around the atom’s core. The combination of the orbital magnetic momentum and the spin moment due as result the magnetic properties of the materials. We can affirm, that the magnetic momentum of a free atom has 2 contributions for each atom. 1. The angular momentum of the electrons going round the core, this is an Ampere molecular current and it is known as orbital contribution. 2. The twist of the spin itself.

Figure 2.1: Orbital and Spin magnetic momentum

2.2

Electrical Resistivity

We define the electrical resistivity as the whole opposition to the current flow in a close circuit, this decrease or block the free electric flow of electric charges or electrons. We can say that every single devices connected in to a close circuit is represent a charge, and they all have some resistance to the free flow of electrons, but we have materials who have a very small electrical resistant, we called that materials are good conductors of electricity, and the materials who have a very high resistance to free electric flow are called insulating, or non-conducting. Usually the electrons try to flow through the circuit in a more or less organized way, but it depends of the resistance of the material. If the resistance is poor, the order than they keep is higher; but when they foun a strong resistance, they start to hit each other and they released energy (heat). That is why the temperatures is higher in a material with a big resistance when there is a current on it. 4

Figure 2.2: A.- Electrons flowing in a good conductor material. B.- Electrons flowing in a bad conductor material.

2.3

Magnetoresistance Historical Background

Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. Any material experience a change in the resistance when a magnetic field is applied. The Lorentz force due the applied of magnetic field, the electrons are forced to go inside of the material. Hence, the path of the electrons is affected, and that cause a change in their resistance. Magnetoresistance is defined by:

MR

= 4(R)/R

(2.1)

• MR is magnetoresistance • (R) is the value of the resistance with the magnetic field • R is the initial resistance of the material The effect was first discovered by William Thomson (better known as Lord Kelvin) in 1851, but he was unable to lower the electrical resistance of anything by more than 5 percent. He experimented with pieces of iron and discovered that the resistance increases when the current is in the same direction as the magnetic force and decreases when the current is at 90 to the magnetic force. He then did the same experiment with nickel and found that it was affected 5

in the same way but the magnitude of the effect was greater. This effect is referred to as anisotropic magnetoresistance (AMR). More recent researchers discovered materials (and multilayer devices) showing giant magnetoresistance (GMR), colossal magnetoresistance (CMR), tunnel magnetoresistance (TMR) and extraordinary magnetoresistance (EMR). Researchers at Princeton University recently discovered a new semimetal, tungsten ditelluride, with no saturation point to the limits of testability and a magnetoresistance of 13,000,000 percent; they have tentatively chosen the name large magnetoresistance (LMR) for the effect. Generally, resistance can depend either on magnetization (controlled by applied magnetic field) or on magnetic field directly. The next paragraph is a part of the article that Lord Kelvin write to the Royal Society about his discovered. I have already communicated to the Royal Society a description of experiment by which I found that iron, when subjected to magnetic force, acquires an increase of resistance to the conduction of electricity along, and a diminution of resistance to the conduction of electricity across, the lines of magnetization. By experiments more recently made, I have ascertained that the electric conductivity of nickel is similar influenced by magnetism, but to a greater degree, and with a curious difference form iron in the relative magnitudes of the transverse and longitudinal effects. In these experiments the effects of transverse magnetization was first tested on a little rectangular piece of nickel 1.2 inch long, .52 of an inch broad, and .12 thick, being the ”keeper” of the nickel horseshoe( 143) belonging to the Industrail Museum of Edinburgh, and put at my disposal for experimental purposes through the kindness of Dr. George Wilson. Exactly the method described in 175 of my previous communication referred to above, was followed, and the result, readily found on the first trail, was as stated. The effect of longitudinal magnetization on nickel was first found with some difficulty, by an arrangement with the horse-shoe itself, and magnetizing helix ( 143), the former furnished with suitable electrodes for the powerful current through itself, and the system treated in all respects (incluiding cooling by streams of cold water) as described in 156, for a corresponding experiment on iron. The result, determined by but a very slight indication, was, as stated above, that longitudinal magnetization augmented the resistance. The magnetization of the small piece of metal between the poles of the Ruhmkorff electro-magnet being obviously much more intense than that of the large piece under the influence merely of the small helix, I recurred to the plan of experiment 175 by which the effect of

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transverse magnetization on the little tectangular piece of nickel was first tested, and I had an equal and similar piece of iron, and another of brass, all prepared to be tested, as well as the nickel, with either longitudinal or transverse magnetic force.

2.3.1

Ferromagnetic Materials

Iron, Nickel and Cobalt show a unique magnetic behavior named Ferromagnetism. That name is because of the ironv (ferrum in latin) which the one that present this property is more common. In their atomic structure they haver a long-range order, and because of that they have order in the same direction in the grains. The magnetig field inside the grain is strong, but all of the grains have different orientation, that cause that in general the material is not magnetise. The ferromagnetism is shows up when a little magnetic field is apply to the material, that magnetic field makes that the whole grains turned in one direction, then we say that the material is magnetise. The ferromagnetic materials conserve some of the magnetism after they got magnetise, that is called hysteresis.

Figure 2.3: Internal view on a Ferromagnetic Material

2.4

Anisotropic Magnetoresistance

The discovery of anisotropic magnetoresistance (AMR) in ferromagnetic materials was made by William Thompson in Glasgow in 1857, Thompson actually was honored as Lord Kelvin for his engineering contributions and his scientific achieves. But it was after a century that anisotropic magnetoresistance could finally found an engineering use, it was a detection element in magnetic recording. 7

In the next example we have a ferromagnetic film (showed in the figure 2.4 ) the magnetization can be only parallel to the plane. The shape anisotropic makes us be sure that the magnetization is going to be in the same direction. If we send a current in the plane of the film, the resistance (R) of the material is read. R depends on the angle between the flow of the current and the magnetization. R = ∆R0 + 4 Rcos2 θ Where: • R0 is a constant • ∆R is the change in resistance • θ=0, the magnetism and the current are parallel, R is higher, when θ=90, the R is lower.

Figure 2.4: Principle of reading data in a magnetic cell

2.4.1

Application of Anisotropic Magnetoresistance

How we talk before, the discovery of AMR had to wait more than 100 year before thin film technology can make a sensor for application use. AMR sensors are made of a nickel-iron alloy material in a thin film layer shape. This layer has a magnetization vector in a specific direction and it rotates when an external field is apply to the layer, but it depends on the direction of the magnetic field directions and his magnitude, the magnitude vector is diverted by an angle. The magnetization vector is created by applying a strong magnetic field along the band to magnetize it. Honeywell for example use a ferrous material named permalloy and four resistance elements (FeNi) to become a wheatstone bridge sensor. To create the sensor the four elements are oriented in a diamond shape with the ends connect each other. The top and the bottom receive a direct current stimilus in the form of voltage (Vs), no matter the order of the elements, the remaining side connections to be measured. With no maggnetic field supply (0 gauss), the side contacts should be at the same voltage. 8

Figure 2.5: AMR Bridge

With the AMR elements connected in this way to form the wheatstone bridge, these side contacts will produce a differential voltage(∆V). To have the element magnetization direction to align with an external apply magnetic field, the apply field has to saturate the permalloy element. For a Honeywell’s magnetic position sensor, a minimum of 80 gauss magnetic field must be applied at the bridget for the specified performance characteristic to be measure. With less than 80 gauss, maybe they can have some bridge operation, but the permalloy materials are not going to be saturated.

2.5

Giant Magnetoresistance and Colossal Magnetoresistance

The science is constantly making new technology, new discoveries, new theories, in this case is not the exception, and the growth has been huge, we will talk about that changes. We are going to talk about two other types of Magnetoresistance, one of them is the Giant Magnetoresistance (GMR). GMR is a mechanic quantum effect, we can observed than in a material with different layer of ferromagnetic and no ferromagnetic materials. This effect is similar to the AMR effect, the resistance change considerably when a magnetic field is applied. If the magnetic field is 0, the 2 ferromagnetic layers have an antiparallel magnetise, because between the layers they have a very week ferromagnetic effect. With a magnetic field applied, both of the magnetic elements of the layer get straight and the resistance of the whole element decrease. In the next chapter we a are going to explain in more detail way the importance of the GMR. The other kind of magnetoresistance is called, Colossal Magnetoresistance (CMR). The CMR is the new era about changes in the material resistance. For the last 30 years, the GMR was hardly studied and there was a very important discoveries using his principle, but in the new generation, new problems are getting important, the need of more capacity of store information, even the size

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of new nanotechnology. The CMR is the next generation in devices that use magnetoresistance, we are going to talk about it, the new materials, the new change in resistance.

Figure 2.6: Growth of MR in storage industry

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Chapter 3

Giant Magnetoresistance 3.1 3.1.1

Historical Background Grown of supperlatice

Back in the time, from the beginning of 1970s, the whole development on the technology allowed scientists to created a new method in the word of material. That method was called epitaxial grown. In the first time of humanity they can start to manufacturing new materials, building one atomic layer after the next. Some of the techniques introduced were: sputtering (DC and RF), laser ablation, molecular beam epitaxy and chemical vapour deposition. They started to use molecular beam epitaxy final in the decade of 1960 to make thin semiconducting material, and in the last of 1970 they can manufacture layer about nanometer scale already (Figure 3.1). Their first use was not the magnetic material, but then the ferromagnetic materials were studied and development using these techniques. They were many advances in different fields also, in characterization, they was using for example the magnetic-optic Kerr effect (MOKE) and light scattering from spin waves. That makes possible the manufacture of multilayers of iron to study the magnetic properties. If we want to obtained a well-defined materials, the choice of substrates on where our materials is going to grown is very important. Commonly used materials are silicon, silicon dioxide, magnesium oxide and aluminium oxide. If you need your material has a well-behaved metallic multilayer, it is important that the different metallic layer match each other, also is an advantage if the to metals that form the multilayer have the same crystal structure.

3.1.2

Interlayer Coupling

Interlayer coupling in magnetic thin-film layered structures play an important role in controlling their magnetic properties. The coupling may originate from exchange and from magnetic interactions affecting magnetization of ferromagnetic films across a non-magnetic spacer layer. When you have a defect, impure

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Figure 3.1: Illustration of Superlattice

or a disturbance in the crystal structure, that gives rise to decaying oscillation of the electron density as a function of the distance from the disruption, this is called Friedel oscillation. In the same case, a magnetic impurity atom in a metallic structure gives rise to an induced spin polarization of the electron density. If the distance of the disturbances is big, there is going to be oscillation in the sign of the polarization and the disturbance is going to decrease in the order of magnetism with the distance. Then a second disturbance that is relatively close to the first one, is going to be aligned parallel or antiparallel to the magnetic moment of the first one, it depends of the sign of the induced polarization at that particle distance. This coupling is called interlayer coupling.

Figure 3.2: Behavior of the exchange coupling as a function of the distance

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3.1.3

Spintronics

Nowadays, it is well known that the electron has an essential role in many physical phenomena, such as electricity, magnetism and thermal conductivity. But if we come back in 1838, the concept of a small quantity of electricity charge was first theorized to explain the chemical properties of atoms. Until 1897 the electron was identified as a particle. The idea and concept of spin was first proposed by Pauli. However, till 1928, when Dirac derived his relativistic quantum mechanics, we first met the electron spin, and known as an intrinsic angular momentum, and we give a quantum numer, s=1/2. Since that day, it has been realized that beside mass and charge, electron has an intrinsic spin properties. However, electron charges and spins have been considered apart. For example, in conventional electronics, the electron charges are manipulated by electronic fields, while classical magnetic recording technologies, the electron spins are used through it is macroscopic form, the magnetization of ferromagnetic. Until recent observation of the giant magnetoresistance (GMR) in magnetic multilayers, in which the motion of electrons can be affected by the force of magnetic field acting on the electron spins, it was realized that there are strong coupling between these two intrinsic properties of electrons. Since then exploiting the influence of spins on the mobility of electrons in ferromagnetic materiales and artificial structures becomes one of the intensely researched area in physics, and has introduced the novel idea termed spintronics. Which possibly can be utilized in the future generation high-speed, low-cost electronic devices.. The AMR and GMR are both derived from the interactions between electrical current and the magnetism of the material. Magnetism in the ferromagnetic material comes from the spin-division effects, that is because of quantum exchange interactions between electrons. As a result of the spin-split band structures, the density of states for the spin-up and spin-down electrons at Fermi level is different and exhibits altered conduction properties. GMR is based on the spin dependent scattering effects, it was first proposed to explain the anomalous resistivity trends showed by bulk ferromagnetic materials doped with impurities by Mott. The solution was lying in the two-channel model which talks about spin-up and spin-down electrons as two independent conduction channels, that is, spin-flip scattering between two channel is rejected in the future studies. Because of the spin polarized band structure of ferromagnetic transition metals, the electron mean free path is much smaller, because conduction electrons may make transitions to the partial filled ”d” states, leading to stronger scattering and larger resistivity. Since the unoccupied ”d” states are also responsible for the ferromagnetic, hence, there is a direct connection between the electrical transport and magnetic properties. Later on, some pioneer research on spin dependent scattering was performed by Albert Fert and Ian Campbell.

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3.2 3.2.1

Experiments Discovery

One important fact to the research of GMR was the fast development in thin film preparation technologies, such a sputtering, molecular beam epitaxy, RF sputtering and others. That made possible the control growing of thin layer of ferromagnetic metal in an nanometer order, that is necessary to observed the GMR phenomena. Peter Grunberg first observed that the antiferromagnetic inlayers change their union in the F e/Cr/F e trilayers. Later on that same trilayer, they observed an enhanced room temperature, and the MR was inn 1.5% (see the figure).

Figure 3.3: Measure of the GMR by the group of Grunberg in F e/Cr/F e. The left and the right are the results with a magnetic field applied along the easy and hard axes.(a) and (b) show the magnetization curves, (c) and (d) show the current in plane (CIP) MR measure at room temperature.

But Mario Baibich independently observed a huge MR around 50% in F e/Cr magnetic superlattice grown by molecular beam epitaxy (figure 3.2), that is the result of the spin dependent trasmission of conducting electron between the Fe through the Cr layers. They was surprised because they not just find GMR, they also identify a new spin dependent phenomenon with promising technical applications. They started to make more experiments, one of the parameter was the thickness of the layers in the multilayers coupling, other was the temperature, they make experiments at room temperature (300K) and the other temperature was at 4.2 K, they both was prepared by magnetron sputtering. The Friedel oscillation 14

Figure 3.4: Measure of the group of Albert Fert in the F e/Cr magnetic superllatices at 4.2K

coul be observed at room temperature too (figure 3.5). For the determination of the spin polarization oscillation they made a nuclear magnetic resonance.

Figure 3.5: Cu/Co Layer

After that, they made several experiments with different materials, some of them are: Co − N b/P b, F e/Ag, F e/P d, F e/M o and (Co − Ag)/Ag. But after different studies, they had a particular interest in a Co/Cu system because the system have a good lattice match (between Co and Cu). The resultant system has a low dislocation density at the interface demonstrating low extrinsic spin independent scattering processes. They made and experiment with Co/Cu

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multilayers at room temperature and they achieved a GMR larger than 65%. They realize that both techniques, the GMR at room temperature and the simple sputtering preparation was ready for commercial preparation of GMR devices.

3.2.2

Theorical Models

The GMR can be explaining by the ”two current model” of Mott, we saw that they are two points mentioned by Mott: 1. ”The conductivy in metals can be describe in terms of two independent conducting channels of electrons, that is, channel of spin-up and spin-down electrons”. 2. ”In ferromagnetic metals the scattering rates of the spin-up and the spindown are quite different, that is scattering rates are spin-dependent”. We can see in the figure 3.6 in a collinear magnetic configuration, where the scattering is stronger for electrons with spin antiparallel to the magnetization direction. In the figure 3.6 we can explain in an easy way what is happening in a trilayer system in parallel, and antiparallel. First we can observed in te parallel aligned magnetic layer the up-spin pass trough the structure with a little scattering, but in the down-spin they are strongly scattered by both ferromagnetic layers. The two conduction channels are parallel to each other, the total conduction is determined by the highly conduction spin-up electrons and they have a low resistance. In the second case of the figure 3.6, in the antiparallel aligned magnetic layer, each channel is strongly scattered with one of the ferromagnetic layers and shows a high resistance state. But is not possible to predict the asymptotic behavior of GMR for large layer thickness.

Figure 3.6: Resistor model for GMR. Parallel on the left, Antiparallel on the right

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The first classical GMR model was developed by Bob Camley and Jozef Barnas (Camley-Barnas model), is based in the solving of the Boltzmann transport equation with spin-dependent scattering at the interface. It shows that the GMR depends of the ratio of the layer thickness, to the electron is like a free path, and the asymmetry in the scattering from spin-up and spin-down electrons.

3.2.3

Aplications

Some times a scientific discover has to way many years to find an use application, sometimes their applications just can be predicted, but the GMR has been identified for his practical use. A few years past, but the time between the discovery and the practical use is relatively short. The first commercial GMR sensor was introduced in 1995 and the first commercial GMR read head was introduced in 1997 by IBM. Nowadays the GMR magnetic sensor has found different application in several fields, such a data storage, engineering biology and space science. The principal application of GMR is the data storage industry. The industry first use the AMR to made it, early in the 1990s, the data storage density is about .1 GB/in2 in 1991. Today, it can achieve more than 500Gb/in2 using newer material like Heusler alloys. The next is the big effort of develop a magnetic random access memory (MRAM) which has the advantages of non-volatility, low energy consummation and radiation hardness. The great application in data storage field has new goals, such a high sensitivity, smaller size and higher speed. The next future applications new a better MR sensors: 1. It can be use for an industrial current sensoring, such milliwattmeter, electrical isolators, electronic compasses as well position, angle and rotation sensing. 2. In civil engineering, GMR sensors are related to the detection of the earth’s magnetic field perturbation produced by specifically considered ferreus bodies. 3. The nano granular film with GMR effects can work as a sensor for brushless DC motors. 4. GMR commercial sensors with high operation rate and high temperature dynamic range may found a practical application in Aerospace field.

3.2.4

GMR Influence

The discover of GMR had a great new ideas in the searching field, was the beginning for new studies and discovers of several MR phenomena, for example tunneling magnetoresistance (TMR), MR in ferromagnetic semiconductors and granular films, colossal magnetoresistance (CMR), ordinary magnetoresistance and geometry magnetoresistance.

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3.3

2007 Nobel Prize

In 2007 The Royal Swedish Academy of Science decided to award the Nobel Prize in physics to Albert Fert Unit Mixte de Physique CNRS/THALES, Universit Paris-Sud, Orsay, France, and Peter Grunberg Forschungszentrum Julich, Germany for the discovery of GMR. The next paragraph is the press release for the nobel prize in October 8th 2007: This year’s physics prize is awarded for the technology that is used to read data on hard disks. It is thanks to this technology that it has been possible to miniaturize hard disks so radically in recent years. Sensitive read-out heads are needed to be able to read data from the compact hard disks used in laptops and some music players, for instance. In 1988 the Frenchman Albert Fert and the German Peter Grnberg each independently discovered a totally new physical effect Giant Magnetoresistance or GMR. Very weak magnetic changes give rise to major differences in electrical resistance in a GMR system. A system of this kind is the perfect tool for reading data from hard disks when information registered magnetically has to be converted to electric current. Soon researchers and engineers began work to enable use of the effect in read-out heads. In 1997 the first read-out head based on the GMR effect was launched and this soon became the standard technology. Even the most recent read-out techniques of today are further developments of GMR. A hard disk stores information, such as music, in the form of microscopically small areas magnetized in different directions. The information is retrieved by a read-out head that scans the disk and registers the magnetic changes. The smaller and more compact the hard disk, the smaller and weaker the individual magnetic areas. More sensitive read-out heads are therefore required if information has to be packed more densely on a hard disk. A read-out head based on the GMR effect can convert very small magnetic changes into differences in electrical resistance and there-fore into changes in the current emitted by the read-out head. The current is the signal from the read-out head and its different strengths represent ones and zeros. The GMR effect was discovered thanks to new techniques developed during the 1970s to produce very thin layers of different materials. If GMR is to work, structures consisting of layers that are only a few atoms thick have to be produced. For this reason GMR can also be

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considered one of the first real applications of the promising field of nanotechnology.

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Chapter 4

Colossal Magnetoresistance 4.1

Technical Background

We saw in this document, many different explanations about MR, one of the last found was the Colossal Magnetoresistance. This kind of MR was found in the year of 1994 in hole dope manganese (magnesium oxides) oxide perovskites. Then they found some other materials that presented the same properties, those materials were double perovskites, manganese oxides pyrochlores and europium hexaborides. As we can see here, the materials of the CMR are very complex, but it has been object of many experiments, but the entire understand of their properties with different parameters and condition remains incomplete.

4.2

Negative Magnetoresitance

As we know magnetoresistance is defined as the change of resistance in a materials when a magnetic field is applied, it is usually expressed as a porcentage and is given by the next expression (figure 4.1).

Figure 4.1: Where ρ is the resistivity. We can say for this definition that the maximum value of MR is 100%. But in this materials the magnetoresistance is negative, this means that the resistance is going to dramatic fault down when the magnetic field is applied. This is the origin of the thousand-fold magnetoresistance reported by Sungho Jin for manganese oxides perovskites. The problem of this kin od CMR is that usually the magnitude of magnetic field necessary to get this large magnetoresistance range in the order of several Teslas. 20

The term colossal was chosen to make a distinction from the GMR. In the experiments, Jin reported maximum values around −100, 000% for thin film of calcium doped manganese oxides near 77K with applied magnetic field of 6 Teslas. They remark about the importance of the technique to grown the thin films, before they experiment with other manganese oxides perovskites and they did not get that kind of high results.

Figure 4.2: This diagram shows the behavior of a CMR material, first the increase the resistance but then they show a dramatically decrease of resistance

4.3

Manganese Oxides Perovskites

These materials was studied in the 1950s for the first time by G.H. Jonker and J.H. van Santen, they reported the correlation between ferromagnetic and metallic resistivity in the hole-dope region. This correlation was explained by Zener, he proposed that the ferromagnetism comes from an indirect coupling among the manganese ion spins (core spins), the called double-exchange model (DE). The double-exchange model and manganites were studied for many years, but it was in the 1990s when it has a exponential interest to the scientist, and that was because the recent discovery about the relation between the firsts, and the colossal magnetoresistance. Magnatites have the formula A( 1 − x)B x M nO3 . Where A is a trivalent rare earth and B is a divalent alkaline. The magnetoresistance can be an intrinsic and extrinsic property. Intrinsic magnetoresistance is maximum close to the ferro-paramagnetic transition and it shows up due the intrinsic interaction of the materials. We see in the figure 4.2, the magnetic transition is accompanied by a change in the resistivity due the temperature, the materials behavior is like a metal below the magnetic critical temperature (Tc), and it works as an insulator in the paramagnetic region. Tc range for doubles perovskites can rate more than 500 K. In particular, manganites cover a huge range of Tc: 100-400 K. Getting close to the Tc by the below, the resistance increase dramatically, sometimes by order of magnitude. The height of resistance curve greatly diminishes when a magnetic field is applied leading the CMR. Magnetoresistance can be also created by extrinsic properties. 21

It arises in polycristalline samples and artificially created barriers. In these cases, low magnetic field can achieve a decrease of the resistance in a wide range of temperatures below Tc. Low field magnetoresistance arises due the barriers that interface produce to transport trough them. In particular, the spindependent transport. All the materials who show a extrinsic magnetoresistance have an strong spin polarized. This means that their carriers are mainly of up and down character. These materials are called half-metals because they are metals in one spin orientation of the carriers and insular in the other direction. The spin polarisation is define for the next equation: P =(n↑-n↓)/(n↑+n↓),n↑↓, where n↑↓, are the densities of up and down spins carriers at the Fermi energy If P = 1, the carriers are fully polarized.

Figure 4.3: Schematic representation of the density of states for a ferromagnetic metal (Ni) and half metallic material (La2 /3Sr1 /3M nO3 )

4.4

Substrates

The influence of the substrate is very important in the develop of this new technology, it is a main factor distinguishing the manganite thin film from bulk ceramic samples. A common used substrate is LaAlO3 , his thought lattice is matched with (La, Ca)M nO3 , at room temperature, with an increase of strain in the last layer because of reduction of T. Sungho Jin has observed the effect of several substrates as well as film thickness on MR. They already nottice the strain effect, expressed trough the MR proportion is bigger for the epitaxy films. And they assumed that the stress is better distributed in the thin film because the absence of grains bondaries. For LaAlO3 substrate, the maximum for film thickness is around 1000˚ A. They kept making experiments on different substrates looking what was changing, and making theories about it. Epitaxial film of La1 − xCax M nO3 were grown on SrT iO3 and LaAlO3 substrates using 22

ozono-assisted molecular beam epitaxy in a block by block deposition method. They got a high degree of structural homogeneity on La. 58Ca. 33M nO3 with fully strained film. The magnetic transition at Tc≈ 270 K for this material is wide, presumably due to the off-stoichiometric which was determinated after growth using inductively coupled plasma mass spectroscopy and Rutherford backscattering spectroscopy. Using a similar growth technique, the same group grew La0 .67Ca. 33M nO3 -δ films with a root-mean-square surface roughness of only 2˚ A. The magnetic transition at Tc≈ 170 K is sharper than the off-stoichimetric film.

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Chapter 5

Conclusions The MR phenomena has been strongly studied in the last 60 years, at first, they did not have a direct application, but the studies continued and also the new ideas for making better and extensive chances in the resistance of the material. With the AMR the whole thing about MR sensors started, and many people turn around to look the effects of MR and the interest of scientific community was focus in many research and experiments of MR. The discover of the GMR had a great impact in the field of data storage, but the scientists see the GMR like just the start to develop new technologies, new materials, new techniques to obtain more resistance. The GMR is possibly one of the greatest discovers due the large range of application, and the word as we now today can not be possible with out the application in read-head lector developed using the GMR effect. The colossal magnetoresistance has a different kind of change in the resistivity, and nowadays is difficult to find more application due the great magnetic field that you need to reach that change, but this is just the beginning of the study, many things are not clear yet, and the people of the laboratories are looking for answers, and I am sure that they are going to have answers and application very soon.

5.1

Future Goals

The next generation has the mission of keep working on GMR and AMR, a lot of new applications are showing up, but new materials and better techniques are requires, the speed, the capacity, and other needs. The CMR is pretty young, but it will be necessary the develop of methods that make possible the present of CMR with lower magnetic field intensity.

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