Current in plane (CIP) and current perpendicular to plane (CPP) structures
Giant Magnetoresistance (GMR) effect was discovered while measuring the resistance of magnetic superlattices of current flowing in the plane of the system.
In 1991, the resistance change of magnetic metallic superlattices has been found by changing the relative orientation of the magnetic layers for the current flowing perpendicular to the plane of the layers.
In multilayergate heterostructures ferromagnetic metal and normal metal giant magnetoresistance effect appeared for the current perpendicular to plane (CPP) and the current in plane (CIP) geometry. In CIP structure current is parallel to the planes of the layered magnetic films and in CPP structure current flows perpendicular to the film plane as shown in Figure 1.14.
Magnetic tunnel junction (MTJ) structure
In other generations of spintronics, structures based on the phenomenon of the Magnetic Tunnel Junction (MTJ) had been included. The MTJ consists of two layers of ferromagnets, separated by an ultra-thin layer of insulator (see Figure 1.15).
Moreover, if the thickness of the insulator is smaller than 2 nm, the electron can leak through the barrier. This process is called tunneling due to the wave nature of electrons. The probability of tunneling depends on the wavelength or energy of the electron.
In a ferromagnetic material, the energy of an electron depends upon its spin orientation up or down. This leads to a spin-dependent tunneling effect. If the magnetic moments of adjacent layers are parallel, the conductivity of the magnetic tunnel junction is great, and if the magnetization is antiparallel, the tunneling probability is small. The maximum value of the magnetoresistive effect observed in the structures is approximately 50 % at room temperature.
Bioapplication of magnetic nanoparticles
Nanometer is a unit of measurement equal to 10 ̶ 9 m and an average of 3 to 6 atoms (depends on kind of atom) together to make a nanometer. All objects and creatures that range in size from 1 to 100 nm called the nanoscale. Nanomaterials are significant in medicine due to the many biological agents such as viruses, bacteria, antibodies and proteins are classified in nanoscale . Figure 1.26 reports the nanoscale and other small scale.
Brief historical aspects of ferrofluid and their applications
The first magnetic fluid or magnetic liquid was obtained by an American, Solomon Stephen Papell in 1960 , as the product of mechanical grinding of magnetite particles in ball mills.
The ferrofluid consists of a fluid that contains ultra-fine magnetic or other particle compounds containing iron, nickel or cobalt with a particle size of 50 nanometers to a few micrometers, stabilized in a polar (water or organic solvent) and nonpolar (hydrocarbons and silicones) media, using surfactants or polymers. Magnetic fluids produce a strongly expressed black color in the fluid due to the presence in them of a highly dispersed magnetic phase of magnetite. Magnetic fluids are unique in that high turnover of particles is combined with high magnetization greater than that of ordinary liquids. Each particle is coated with a thin layer of protective membrane that prevents the adhesion of the particles to each other. The thermal motion of the liquid scatters them throughout the entire volume of the liquid . Also magnetic fluids retain long term stability for many years and have a good flow in conjunction with their magnetic properties.
Methods for preparing magnetic fluids are varied; some are based on the combination of iron, nickel, and cobalt of hundreds of a micron size in mills, with arc or spark discharge, some – on the use of sophisticated equipment.
Today, magnetic fluids are used in various applications  of technology such as in the chemical industry (as magnetic lubricants, in the cooling systems), medicine (magnetic cell separation, cancer treatment, magnetic drug delivery and immunoassay), in mechanical engineering (in NASA research projects on the use of a ferromagnetic fluid in a closed ring as the basis for the stabilization system of spacecraft in space , and in the oil and the printing industry), electronic devices, mining industry and military (to seal the sealing and insulation gaps between the moving parts of machines, reduce friction).
Other significant usage of ferrofluids are in medicine. As we know anticancer drugs are harmful to healthy cells. But if you mix them with magnetic fluid and enter them into the blood the medicine becomes concentrated in the tumor without harming the body. You can also move the body enzymes. Magnetic magnetite particles are used to treat cancer (hyperthermia). This method of treatment is based on the fact that under the influence of an alternating magnetic field magnetite particles are heated and inhibit the growth of cancer cells .
First model of the GMR needle probe sensor
The giant magnetoresistance sensor is conditioned in a thin needle with two sensing elements of 75 μm×75 μm. They are placed at the top and the end of the needle and connected in a Wheatstone bridge configuration. A first model is showed on Figure 2.1.
This Wheatstone bridge configuration permits to measure voltage variations induced by magnetic field with removing of DC voltage part due to the GMR polarization. The resistances of the Wheatstone bridge should have values close to that of the GMR elements, in order to have the same DC value of voltage in Vref and Vout. This configuration allows to directly amplify (Vout−Vref) without necessity of a high pass filter for removing the DC component.
The length of the needle inserted into the cavity containing the ferrofluid sample is 3 cm with an approximate diameter of 300 μm. The needle is made of aluminum titanium carbide (AlTiC), a hard material and sintered material such as aluminum oxide (Al2O3) and titanium carbide (TiC). Furthermore, for this type of GMR the maximum admissible current is 9.7 mA. The schematic of the sensor is presented in Figure 2.1.
Second model of GMR needle probe sensor
A second model of the GMR sensor consists also on two main parts: a needle with four GMR elements instead of two as in GMR I and a Wheatstone bridge conditioning circuit.
The needle type SV−GMR sensor consists of four sensing elements: the first sensing element (GMR1) is at the tip of the needle and the other sensing elements (GMR2, GMR3 and GMR4) are at the end of the needle. All four sensing elements are connected in a Wheatstone bridge configuration.
As we mentioned previously for the first model GMR I, the first sensing element at the top of sensor is in contact with the magnetic liquid in the container (inside magnetic flux density B1) and the other sensing elements are embedded to sense magnetic flux density (B0) at the outside of the ferrofluid’s cavity.
The needle length is 17 mm with a cross section 300×300 μm and the material used for fabrication is the same as for the first model. The sensing direction of GMR sensor is parallel to the needle. It should be noted that the maximum allowable current for this type of the GMR sensor is 5 mA.
Determination of the sensitivity value for different magnetic flux densities between two GMR elements
As noted in previous part, this model of sensor has two sensing elements at the top and at the end of needle with 75 μm×75 μm dimensions for each GMR element. The power supply is important part to safe each electronic equipment, therefore it is extremely important to realize this configuration carefully. The power supply configuration and the measurement setup of the GMR sensor is presented in Figure 2.24.
The Helmholtz coil is alimented by a variable gain amplifier through a two serial transformers, the amplifier stage produces a phase shift of the signal. As shown in Figure 2.24, two PCB terminal blocks embedded into interface card of the GMR facilitate changing the value of circuit resistance and allow to find the appropriate amount of resistance, needed to protect the GMR sensor and to have the highest sensitivity.
In this experience, we have used different resistances in the interface card to find the maximum value of sensitivity and to protect the sensor. In other words, the resistances are added to adjust the bias current of a power sensor which fixed in ± 6 V for all GMR sensor types. The optimal experimental result was obtained for 809 Ω resistances (the value of resistances is measured with a LCR meter model HP 4284A). The schematic circuit of power supply of the GMR sensor with the two resistances is presented in Figure 2.25.
Table of contents :
Chapitre 1 : Eléments de base de la magnétorésistance géante
1.2 Aperçu historique et bases de l’effet GMR
1.3 Classes de magnétorésistances géantes
1.3.1 Vannes de spin (SV-GMR)
1.3.2 Courant dans le plan (CIP) et le courant perpendiculaire au plan (CPP)
1.3.3 Structure de jonction tunnel magnétique (JTM)
1.3.4 Structure Magnétoimpédance géante (MIG)
1.4 Les matériaux magnétiques
1.4.2 Propriétés et applications des matériaux magnétiques
Chapitre 2: Caractérisation de capteurs GMR de type aiguille
2.2 Structures et conceptions des sondes GMR à aiguille
2.2.1 Premier modèle de capteur
2.2.2 Deuxième modèle de capteur
2.2.3 Troisième modèle de capteur
2.3 Caractérisation des capteurs à magnétorésistance géante
2.3.1 Caractérisation du premier capteur
2.3.2 Caractérisation du deuxième capteur
2.3.3 Caractérisation du troisième capteur
Chapitre 3: Détection des propriétés des fluides magnétiques par capteurs GMR
3.2 Méthodologie expérimentale de ferrofluide
3.2.1 Préparation de plusieurs concentrations de ferrofluide et montage expérimental
3.2.2 Méthode expérimentale et résultats obtenus par VSM
3.2.3 Comparaison des résultats obtenus par le capteur GMR et par le VSM
3.3 Estimation de la densité de poids de fluide magnétique, de la perméabilité relative et de la susceptibilité magnétique du fluide-D
3.3.1 Spécification et préparation des différentes concentrations de fluide MAG-D
3.3.2 Description de la méthode expérimentale et de la configuration de mesure expérimentaux et théoriques
Chapitre 4: Détection de la bactérie Escherichia coli par capteur GMR
4.2 Résumé sur les Escherichia coli
4.2.1 Définition des Escherichia coli
4.2.2 Spécification d’Escherichia coli
4.2.3 Maladies causées par Escherichia coli
4.3 Détection d’Escherichia coli O157:H7 par des billes magnétiques
4.3.1 Dynabeads Max E. coli O157
4.3.2 Description des méthodes expérimentales pour l’estimation des propriétés magnétiques des Dynabeads
Chapter 1 : The principle of giant magnetoresistance and studies in magnetism
1.2 Brief historical review of GMR effect
1.3 Theoretical framework of giant magnetoresistance (GMR)
1.4 Different types of giant magnetoresistive structures
1.4.1 Spin valve (SV-GMR) structures
1.4.2 Current in plane (CIP) and current perpendicular to plane (CPP) structures
1.4.3 Magnetic tunnel junction (MTJ) structure
1.4.4 Giant magnetoimpedance (GMI) structure
1.5 The basis of magnetism
1.5.1 Classification of magnetic materials
1.5.2 Properties of magnetic materials
1.5.3 Bioapplication of magnetic nanoparticles
1.5.4 Brief historical aspects of ferrofluid and their applications
Chapter 2 : Giant magnetoresistance sensor characterization
2.2 Structure and design of the giant magnetoresistance needle probe
2.2.1 First model of the GMR needle probe sensor
2.2.2 Second model of GMR needle probe sensor
2.3 Setup component of giant magnetoresistance needle probe
2.3.1 Uniform magnetic field generator
2.3.2 Displacement system
2.3.3 Preamplifier and lock-in amplifier
2.4 Characterization of the giant magnetoresistance sensors
2.4.1 GMRI sensor’s characterization
2.4.1.A Determination of the sensitivity value for different magnetic flux densities between two GMR elements
2.4.1.B Sensitivity’s determination by individual variation of magnetic flux density for each GMR element
2.4.2 Second GMR sensor’s characterization
2.5 Structure and characterization of giant magnetoresistance sensor type III
Chapter 3 : Characterization of magnetic fluid properties by GMR sensors
3.2 Experimental methodology of ferrofluid
3.2.1 Determination of the low-concentration of magnetic fluid
3.2.2 Preparation of different concentrations of ferrofluid and experimental setup .
3.2.3 Results obtained by vibrating sample magnetometer (VSM)
3.2.4 Comparison of results obtained by the GMR sensor and by the VSM
3.3 Estimation of magnetic fluid weight density and relative permeability and susceptibility of magnetic fluid-D
3.3.1 Specifications of magnetic fluid-D
3.3.2 Preparation of different concentrations of fluid MAG-D
3.3.3 Experimental measurement setup and method
3.3.4 Analysis of the GMR sensor’s results and comparison of experimental and theoretical results
Chapter 4 : Detection of Escherichia coli by a GMR sensor
4.2 Summary of Escherichia coli
4.2.1 Definition of Escherichia coli
4.2.2 Specification of E. coli
4.2.3 Diseases caused by Escherichia coli
4.3 Detection of Escherichia coli O157:H7 by magnetic beads
4.3.1 A brief description of Dynabeads Max E. coli O157
4.3.2 Experimental methods for estimation the magnetic properties of Dynabeads