Hard drives and spintronics

Introduction

According to the ideas of most people, all modern electronics is based on the use of electric current, i.e. directional motion of electrons, well, or charge transfer. In any microcircuit, a huge bunch of electrons work for our good. They carry signals, they store in our memory precious zeros and ones, do all the work so that our life is convenient and simple. But in addition to charge transfer, electrons have another important property - spin. And this property exploits spintronics with might and main.

What is spintronics?

Spintronics is a scientific and technical direction focused on the creation of devices in which its spin is used in addition to the electron charge for the physical representation of information. Spintronics is an established term, but there are different interpretations of it: spin transport electronics, spin-based electronics, or just spin-electronics.
For the first time, the term "spintronics" was used in a joint report by Bell Laboratories (yes, those same Bell Labs) and a scientist at Yale University, dated July 30, 1998. The idea for the first time to use single atoms to store bits of information was sounded in it, and the bits themselves should be stored in the form of electron spins.

Everywhere I say here, spin and spin, but what is it?


Spin (from the English spin - rotation, spinning) is the intrinsic moment of the momentum of an electron not related to its motion in space. Simplifying a little, the spin can be represented as the rotation of an electron around its axis.

Recall some mathematics and physics.
In classical physics, for a particle, the mechanical moment of the momentum (or as they say, at the moment of momentum) is:

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

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

Fig. 1. Electron spins

In general, the spin is measured in units of h (Planck's constant), and they say that the spin is equal . The spin is associated with the intrinsic magnetic moment of the electron.

I think that a handful of mathematical signs above is enough to torment a few readers. And if so, then we will no longer use the formula.

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

In spintronic effects, the properties of ferrimagnetic materials are used. These are materials containing atoms with a magnetic moment (for example, Fe — iron, Co — cobalt, Ni — nickel), and at temperatures below a certain critical temperature (Curie temperature), the magnetic moments of the atoms are ordered relative to each other. With a parallel arrangement of spins, materials are called ferromagnets, and with antiparallel - antiferromagnets.

In 1989, structures consisting of ferromagnetic and non-magnetic layers were investigated. Their conductivity was studied. Take a look at the figure:


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 a non-magnetic layer in the middle. A real example of such structures can be Fe-Cr-Fe (iron-chromium-iron) or Co-Cu-Co (cobalt-copper-cobalt). Moreover, the width of the non-magnetic layer is of the order of 1 nm, or rather, the width of the layer should be less than the mean free path of the electron, so that there is no scattering and loss of spin during its motion, electron. 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 up to this place does not need to be told what a hard drive is. So how does all the horror described above apply to hard drives? Using the principles shown above, information is recorded on our hard drives. Imagine a HDD partitioned into pieces so that only a recording / reading head and a pancake with data are left from it. Approximately as in the figure. The artist is awful of me, so I do everything schematically.


Fig. 3. HDD Of

interest is only a recording / reading head within the article. I specially “gilded” it with yellow paint (as in that curiosity 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 closer look at the reading part:


Fig. 4. Read head

As you can see, the head consists of four layers: iron, copper, cobalt, and antiferromagnet AFM. AFM words, or as it is also called, an exchange layer, is designed to fix the magnetic field of the second layer. The second layer is called fixing and in our country it is made of cobalt. In it, the magnetic field is always directed in one direction. The third layer is conductive, usually made of copper, serves to separate the ferromagnetic layers. The last layer - sensitive - is also made of a ferromagnet. Unlike the fixing field, the direction of its magnetic field depends on the external field - the field of the cell. A hard drive 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 field orientations in the sensitive and fixing layers coincide, then the cell, according to the principles considered above, increases its conductivity, i.e. begins to conduct current. If the field orientations are opposite, then we get a “metal insulator”. Such an effect of a change in conductivity (well, or resistance, because these are just inverse quantities) was called GMR - Giant Magnetoresistive - the effect of giant magnetoresistance. The GMR effect was first studied in IBM laboratories in the late 80s, but for its industrial implementation it took almost 10 years.

It is very dizzying that such sophisticated technologies surround us everywhere. To be continued.