1. MR and Spin Valve
Bae Hae Kyong

2. Contents 1. Introduction 2. AMR AMR ratio of the material
AMR sensor bridges
3. GMR
Electron Spin
GMR heads
Mechanism

3. Introduction
Magnetic sensors have been in use for over 2,000 years. Early applications were for direction finding, or navigation.
The technology for sensing magnetic fields has also evolved driven by the need for improved sensitivity, smaller size, and compatibility with electronic systems.
In recent years, a new type of electronics, spintronics, has emerged, which utilizes both the electron's spin and charge to enable devices to perform novel and superior functions.
Spintronics-based devices are increasingly used in technologies such as computer information storage, magnetic sensing, and non-destructive evaluation.

4. Magnetoresistance (MR)
If a material's electrical resistance increases or decreases when a magnetic field is applied, the change in resistance is know as
Ordinary Magnetoresistance : Hall effect
AMR (anisotropic MR) : This sensor is made of a nickel-iron (Permalloy) thin film deposited on a silicon wafer and is patterned as a resistive strip.
GMR (giant MR) : In one design, two ferromagnetic layers (such as cobalt or iron) sandwich a layer of nonmagnetic metal.

5. Magnetic sensors
One way to classify the various magnetic sensors is by the field sensing range. These sensors can be arbitrarily divided into three categories low field, medium field, and high field sensing.

6. AMR
William Thompson, later Lord Kelvin, first observed the magnetoresistive effect in ferromagnetic metals in 1856.
AMR occurs in ferrous materials. It is a change in resistance when a magnetic field is applied in a thin strip of ferrous material.
AMR effect is thought to originate from the spin-orbital coupling and in case of 3d transition metals it involves the scattering between s conduction electrons and localized d electrons oriented by the applied field.

7. AMR ratio of the materia
The properties of the AMR thin film cause it to change resistance in the presence of a magnetic field.
bulk ternary Fe-Co-Ni alloys : 5%
Ni80Co20 alloy : 6%
Ni81Fe19 thin films : 2-3%
Thickness of thin films < 100Å
: below 1%

8. AMR sensor bridges
The principal of wheatstone bridges is to create two voltage divider elements (half-bridges), each with normally equal electrical impedances at a 'null point', or when a sensor has no stimulus.
With each half-bridge at its null point, the expected voltage across each divider should be half the total bridge supply voltage (Vb).
Positive bridge output node voltage, Vo+ = Vb [R2 / (R1 + R2)]
Negative bridge output node voltage, Vo- = Vb [R4 / (R3 + R4)]
So, offset voltage Voff = (Vo+ - Vo-)= Vb [R2 /(R1 + R2)] - [R4 / (R3 + R4)]

9. GMR
Large magnetic field dependent changes in resistance are possible in thin-film ferromagnet/non-magnetic metallic multilayers. This phenomenon was first discovered in the late 1980s by two European researchers, Peter Gruenberg and Albert Fert, who were working independently.
GMR was discovered because developments in high vacuum and deposition technology made possible the construction of molecular beam epitaxy(MBE) machines capable of laying multiple thin layer only a few atoms thick.
The resistance of two thin ferromagnetic layers separated by a thin non-magnetic conducting layer can be altered by changing whether the moments of the ferromagnetic layers are parallel or antiparallel.

10. Electron Spin
The magnetic moment of a spinning electron is called the Bohr magneton. Magnitude.
Consider an atom of iron,
Note that in shells 1s, 2s, 2p, 3s, 3p, 4s equal numbers of electron spins point up and down so that total electron spin moment is zero; that is, the electron spin are "compensated',
There is an uncompensated spin moment of 4 Bohr magneton.

11. Exchange Coupling
A quantum effect, called exchange coupling, forces all the iron atom's magnetic moments to point in nearly the same direction. Exchange coupling lowers the system's energy by aligning the uncompensated moments.
At absolute zero, the ordering is perfect(ferromagnetism), whereas at higher temperatures, thermal energy causes increasing disorder.

12. Theory of GMR
A schematic diagram representing the density of states(DOS) of sp and d electron bands of a 3d ferromagnet(Fe, Co, Ni) compared with that of a nonmagnetic material

13. Theory of GMR
Up(↑) spin is majority-spin electrons parallel to the magnetization vector of the ferromagnetic layer and down(↓) spin is minority-spin electrons antiparallel to that of the ferromagnetic layer.
The splitting of the 3d-band contributing to the ferromagnetism of ferromagnets, that is asymmetry of DOS dependent upon spin orientation.
While up-spin electrons occupy most DOS of 3d band fully, down-spin electrons do not occupy most of them.
The 3d states do not effectively contribute to the conductivity due to their high effective mass. However, a high 3d density of the Fermi level leads to a high probability of scattering to d states.

14. Spin-dependent scattering
In order to spin dependent scattering to be a significant part of the total resistance, the layers must be thinner than the mean free path of electrons in the bulk material.
Then, The conduction electron arrives at the interface of the adjacent ferromagnetic layer still carrying its original spin orientation.
When the adjacent magnetic layers are magnetized in a parallel, the arriving electron has a high probability of entering the adjacent layer with negligible scattering, because its spin orientation matches that of the adjacent layer's majority spins.

15. Sandwich
GMR materials consist of two soft magnetic layers of iron, nickel and cobalt alloys separated by a layer of a non-magnetic conductor such as copper.
For use in sensors, sandwich material is usually patterned into narrow stripes. The magnetic field rotate the magnetic layers into antiparallel or high resistance alignment.

16. GMR Heads
GMR heads are not named "giant" because of their size. Rather, they are named after the giant MR effect.
By December 1997, IBM had introduced its first hard disk product using GMR heads

17. Structure of GMR Heads
GMR heads are comprised of four layers of thin material sandwiched together into a single structure:
Free Layer : This is the sensing layer, made of a nickel-iron alloy, and is passed over the surface of the data bits to be read. As its name implies, it is free to rotate in response to the magnetic patterns on the disk.
Spacer : This layer is nonmagnetic, typically made from copper, and is placed between the free and pinned layers to separate them magnetically.
Pinned Layer : This layer of cobalt material is held in a fixed magnetic orientation by virtue of its adjacency to the exchange layer.
Exchange Layer : This layer is made of an "antiferromagnetic" material, and fixes the pinned layer's magnetic orientation.

18. Working Mechanism

19. Working Mechanism
Defining the positive field direction as the direction of the magnetization of the pinned layers, the layers have parrel magnetizations for H>0.
In a small field interval close to H=0 the magnetization of the free ferromagnetic layer reverse, whereas the magnetization of the pinned layer remains fixed. Therefore, this induces the antiparallel spin configuration with higher resistance by GMR effect.
Only upon the application of a large negative field(equal to the Hex), the exchange bias interaction is overcome, and the pinned layer also switches, inducing again the low resistance.

20. Sensing
When the head passes over a magnetic field of one polarity, the electrons on the free layer turn to align with those on the pinned layer, creating a lower resistance in the head structure.
When the head passes over a field of opposite polarity, the free layer electrons rotate so that they are not aligned with the electrons on the pinned layer. This causes an increase in the structure's resistance.
Because the resistance changes are caused by changes to the spin characteristics of electrons in the free layer, GMR heads are also known as spin valves.