Less does not mean worse: skyrmions and domain walls in ferr-magnets

You have probably heard more than once how someone made the biggest cake in the world, or the biggest pizza or the biggest burger. These records are funny, sometimes very funny, and in the case of the above options, they are also delicious. But they are not useful. The scientific world also likes to set records in the size of something, but lately diametrically opposed. Researchers from around the world are trying to use the smallest objects for the benefit of humanity and technology. Today we will talk about the prospect of using domain walls and skyrmions inside a ferrimagnet for storing and transmitting information. To say that these “carriers” are small is to greatly exaggerate. What and how does it work, what are the prospects for this research and why precisely ferrimagnetic? We will look for answers in the report of the research group. Go.

Theoretical basis of the study
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First of all, it should be noted that most studies, which are based on magnetism and its aspects in one way or another, mostly use a ferromagnet, rather than a ferrimagnet. One letter in the word actually changes not only the name, but the whole point.

Ferromagnet is what we see most often. If you have a magnet hanging from the last vacation on the fridge, you should know that it hangs there due to ferromagnetism. A ferromagnet is a substance that is magnetized without the use of an external magnetic field and at a temperature below the Curie point. If we talk about room temperature, then 4 substances have ferromagnetic properties: nickel (Ni), iron (Fe), cobalt (Co) and ruthenium (Ru) .


Neodymium magnets (rare earth neodymium + iron + boron) against a smartphone. People with nomophobia, please do not look.

If we change the letter “o” to “and”, we will get a completely new type of substance. Ferrimagnetics are somewhat similar to their brothers ferromagnets, at least all magnetic characteristics are applicable to both, as well as to those and others that “work” at temperatures below the Curie point. The most important difference is the fact that ferrimagnets have magnetic moments of the atoms of the sublattices that are antiparallel. Why is that? In fact, ferrimagnetics is a cocktail of several chemical elements, and not one, like ferromagnets. Due to this, they consist of several sublattices, the structure of which differs either by the number of atoms or their origin (different chemical elements). Chief among the owners of ferrimagnetic features are ferrites, which are based on iron oxide (Fe ₂ O ₃ ).


Comparison of the directivity of the magnetic moments of a ferromagnet ( a ) and a ferrimagnet ( b ).

And now let's look even deeper and try to understand what these domain walls are.

So, a domain wall is almost literally a wall between two magnetic domains, a kind of feature or border point. Continuing the last analogy, these magnetic domains are like North and South Korea, that is, opposite to each other. More precisely, they have different directions of magnetization.


Magnetic domains: black and white areas differ in the directivity of the vectors of their magnetic moments.

The domain, if not deepened, is part of a magnetic crystal, a microscopic region in which the magnetization vectors are strictly ordered relative to the vectors in the neighboring region.

In order not to repeat once again, you can find an explanation of what a magnetic skyrmion is in one of the previous articles . Only briefly, I will say that these are a kind of atomic spin craters, which are named after the physicist Tony Skyrme.


Image a - skyrmion "hedgehog", b - spiral-shaped skyrmion.

With the theory, we figured out a bit, now let's see what our today's heroes have made of all this.

The essence of the study

Above, we considered ferromagnets and ferrimagnetics, and also their differences for a reason. Researchers believe that even though ferromagnets have surprisingly useful characteristics and properties, they are still limited in speed and size, or rather, you can transfer data more slowly with their help, and each bit will be “bigger” than if you use ferrimagnetics. It sounds very promising, but requires proof. What scientists and engaged in this study.

The real base of the experiment was the Pt / Gd ₄₄ Co ₅₆ / TaOx compound, or rather a thin film of it.


Image number 1

To begin with, the investigators decided to study the statics and dynamics of the spin structure of Gd ₄₄ Co ₅₆ (image 1a ), which is an amorphous ferrimagnetic alloy. The antiferromagnetically coupled sublattices of this alloy have a similar g-factor, therefore, TA (temperature compensation of angular momentum) is very close to TM (temperature compensation magnetization).

As we already know, the main protagonist of the experiments was Pt / Gd ₄₄ Co ₅₆ / TaOx. The film thickness of each component was as follows: Ta - 1 nm; Pt - 6 nm; Gd ₄₄ Co ₅₆ - 6 nm; TaOx - 3 nm. All films were perpendicularly magnetized and deposited on a Si / SiO ₂ substrate by sputtering.

The lower layer (Pt) was the main source of spin-orbit vortexes (hereinafter SOW ) and constantly generated a strong Dzyaloshinsky-Moriya interaction (hereinafter VDM ), which is responsible for the weak manifestations of ferromagnetism in antiferromagnetic dielectrics. The top layer (TaOx) is protective.

Graph 1b shows two indicators as a function of temperature: the coercive force (squares) necessary for the complete demagnetization of a ferrimagnet (or ferromagnet) and magnetic saturation (circles). The first indicator was obtained using the method of vibration magnetometry, and the second - the method of polarimetry of the magneto-optical Kerr effect.

Thanks to the data obtained ( 1c and 1d ), it was found that TM is approximately 240 K (kelvin), since there is a hysteresis of the magneto-optical Kerr effect.

Through wide-field Kerr microscopy, studies of the motion of the domain wall were carried out. 1e shows several images when nanosecond current pulses were applied on the domain wall, forcing it to move along a given route.

Each of the walls, up-down and down-up (the direction of the magnetization vectors), moved along the current path, which also included Neel's domain walls * controlled by spin-orbit vortices.


Comparison of the Neel wall ( a ) and the Bloch wall ( b ).

Neel's wall * —the rotation of magnetization in this type of wall occurs perpendicular to it, and not in its plane.

Graph 1f is the ratio of the velocity of the domain wall (vDW) and temperature (T). A significant peak is observed precisely at 260 K, which is higher than the previously established TM.

It is worth noting that the discrepancies between the SOW and VDM fields are not the main cause of the increase in the velocity of the domain wall.


Image number 2a

Graph 2a shows an analysis of the effect of the field and current on the velocity of the domain wall by means of a creep diagram. And we see that in both cases the result is identical.

Skyrmions of ferrimagnetics

It is worth noting that ferrimagnets may have much smaller skyrmions than ferromagnets, which is due to their weak demagnetization field. At the same time, these skyrmions exist at room temperature. Previously, the sizes of such skyrmions were in the range of 30 nm - 2 μm at cryogenic temperatures. The large sizes of skyrmions are explained by strong dipole interactions in multilayer structures, usually consisting of heavy metals and ferromagnets.


Comparing skyrmions.

The image a shows the case described above (ferromagnetic multilayer structure), in which there is a direct dependence of the skyrmion energy (E) on its radius ®. In the case of ferrimagnetic materials, the layer can be made much thinner, and there is no need to increase the strength of the demagnetization field (image b ). The researchers also calculated the use of NMR in a zero field * ratio of the size of a skyrmion and the state of VDM (graph c ).

NMR in a zero field * - Nuclear magnetic resonance in a zero field, which is used to analyze magnetically ordered substances, more precisely to determine changes in their crystalline or magnetic structures.

The analysis has shown that the demagnetizing field can destabilize the GMR skyrmion, when the real temperature indicators are very far from the previously established level of the magnetization compensation (TM) temperature level. In this case, the WDM skyrmions can remain stable for a long time with magnetic saturation (Ms) of the order of 150 kA / m ^–1 . And this corresponds to much higher (about 100 K higher than TM) temperatures than in multilayer ferromagnets.


Captured Skyrmions.

These findings are the result of calculations and simulations, but they were fully confirmed by X-ray holography at room temperature of the Pt / Gd ₄₄ Co ₅₆ / TaOx sample.


X-ray holographic images of Pt / Gd ₄₄ Co ₅₆ / TaOx.

As can be seen from the photographs, quite a few skyrmions were found in various parts of the sample. Also, scientists note that there were no signs of a correlation between the position of skyrmions before saturation and re-nucleation. For example, in the 5d image, colored squares mark places where there are no skyrmions, but they were there before (Figures 5a and 5b ). At the same time, all skyrmions disappear when the magnetic field strength reaches 450 mT (millilites).

The size of skyrmions averaged 23 nm ( 5g ). The smallest skyrmion was about 10 nm in diameter. This is important because this size is much smaller than what is observed in skyrmions in ferromagnets at room temperature. Scientists explain the heterogeneity of the sizes of skyrmions by the anisotropy of the sample structure, that is, the presence of differences in properties within a single structure.

It is also worth considering the fact that the size of skyrmions in the pictures was determined by the largest contour of the dark areas. In reality, the skyrmions are even smaller.

Those who wish to get acquainted with the study in more detail I recommend to look into the report of scientists and additional materials to it.

Epilogue

The researchers were able to show that ferromagnets, despite their advantages, will not be able to remain monopolists for a long time. Ferrimagnetics are also capable of showing excellent results. In this case, it was possible to achieve a domain wall displacement with a speed of 1 km / s, and the minimum size of the skyrmion was no more than 10 nm in diameter. And most importantly - all this at room temperature. The latter is particularly attractive for practical use. Many developments at the research stage show good results only in certain conditions (temperature, pressure, humidity, various electromagnetic fields and radiation, etc.), which can only be recreated in the laboratory.

Scientists believe that ferrimagnetics can become the basis for future devices based on spintronics. At the same time, their properties can be monitored, modified and adjusted to the needs of a particular device or process. Moreover, this will make it possible to realize antiferromagnetic spin systems, in which the magnetic state will still be easily detected by optical or electrical methods.

There is still much to learn. Difficulties will also be not enough. But all the technologies and their authors have traveled along a thorny path in due time before they achieve perfection. I remembered one incident, I don’t know how true it is, but still. During the time of the first cars an accident occurred, the perpetrator of which decided to escape from the scene. The police caught up with him on bicycles. And what do we have now? Cars that can accelerate to 350 km / h. There is already a bike for the chase is not suitable.

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