Hard drives and spintronics

Introduction

According to most people, all modern electronics are based on the use of electric current, i.e. directional movement of electrons, well, or charge transfer. In any chip, a huge pile of electrons is working for our blessings. They carry signals, they store in their memory precious for us zeros and ones, do all the work so that our life is comfortable and simple. But in addition to charge transfer, electrons have another important property - spin. And this property is fully exploited by spintronics.

What is spintronics?

Spintronics is a scientific and technical direction focused on the creation of devices in which, apart from the electron charge, its spin is used for the physical representation of information. Spintronics is a well-established term, but there are different interpretations of it: spin transfer electronics (spin transport electronics), spin-based electronics, or simply spin electronics (spin electronics).
For the first time the term “spintronics” was used in a joint report by Bell Laboratories (yes, the Bell Labs) and a scientist from Yale University, dated 07/30/1998. The idea of ​​using single atoms for storing bits of information was first heard in it, and the bits themselves are stored in the form of electron spins.

Here everywhere I say, back and back, and what is it?

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Spin (from the English. Spin - rotation, spinning) - is its own angular momentum of an electron that is not associated with its motion in space. Simplifying a bit, spin can be represented as the rotation of an electron around its axis.

Recall a little math and physics.
In classical physics, a particle, the mechanical moment of momentum (or, as they say, at the moment of impulse), is:

r is the radius vector of the particle;
p is the particle momentum vector.

When p = 0 , the angular momentum of a classical particle is M = 0 . For an electron, p = 0, M ≠ 0.
The electron spin can take two values:

Fig. 1. Electron spins

Generally, the spin is measured in units of h (Planck's constant), and it is said that the spin is equal to . A spin is associated with its own magnetic moment of the electron.

I think that a bunch of mathematical signs above would be enough to torment a few readers. And if so, then we will no longer use formulas.

Unlike classical charges, which create a magnetic moment only in the presence of their current (as, for example, in a solenoid), an electron has a magnetic moment at zero momentum. Not only electrons have a magnetic spin, but also some other elementary particles, as well as the nuclei of some atoms.

The properties of ferrimagnetic materials are used in spintronic effects. These are materials that contain atoms with a magnetic moment (for example, Fe is iron, Co – cobalt, Ni is nickel), and at a temperature below a certain critical (Curie temperature), the magnetic moments of atoms are ordered relative to each other. With a parallel arrangement of the spins, the materials are called ferromagnetic, and with antiparallel - antiferromagnetics.

In 1989, structures consisting of ferromagnetic and nonmagnetic layers were investigated. Their conductivity was studied. Take a look at the picture:


Fig.2. Three-layer ferromagnetic structure

As can be seen from the figure, both structures consist of three layers: ferromagnetic - from the edges of the structure and the nonmagnetic layer in the middle. Fe-Cr-Fe (iron-chromium-iron) or Co-Cu-Co (cobalt-copper-cobalt) can be a real example of such structures. Moreover, the width of the nonmagnetic layer is about 1 nm, or rather the layer width must be less than the electron mean free path, so that there is no scattering and loss of spin during its electron motion. Conductivity in such a structure arises only if the magnetizations of the extreme layers are unidirectional, as can be seen in the right figure. Otherwise, we get a "metal insulator".

And how does this relate to HDD?

I dare to believe that everyone who has read this far does not need to be told what a hard disk is. So how does all the creepiness above apply to hard drives? With the help of the principles shown above, information is recorded on our hard disks. Imagine a HDD partitioned into pieces so that only a recording / read head and a pancake with data are left from it. Something like the picture. The artist is awful to me, so I do everything in a sketchy way.


Fig.3. HDD

Of interest in this article is only the write / read head. I specifically her "gilding" yellow paint (as in the oddity with Petka and Vasily Ivanovich). In general, this is not one device in the head, but as many as two: the recording part and the reading part. Take a look at the reading part closer:


Fig.4. Read head

As you can see, the head consists of four layers: iron, copper, cobalt, and antiferromagnetic AFM. The AFM of words, or as it is also called, the exchange layer, is intended for fixing the magnetic field of the second layer. The second layer is called fixing and here it is made of cobalt. In it, the magnetic field is always directed in one direction. The third layer - conductive, usually copper, serves to separate the ferromagnetic layers. The last layer - sensitive - is also made of a ferromagnet. Unlike the fixing, the direction of its magnetic field depends on the external field - the field of the cell. A hard disk cell contains one bit of information. Depending on the orientation of the cell field, the orientation of the field in the sensitive layer changes. If the orientations of the fields in the sensitive and fixing layers are the same, then the cell, according to the principles discussed above, increases its conductivity, i.e. starts to conduct current. If the orientations of the fields are opposite, then we get a “metal insulator”. This effect of changing conductivity (well, or resistance, because it’s just the reciprocal) was called GMR - Giant Magnetoresistive - the effect of giant magnetoresistance. The GMR effect was first investigated in the laboratories of IBM in the late 80s, but for its industrial implementation it took almost 10 years.

It is very circling that such complex technologies surround us everywhere. To be continued.