–Sujan Budhathoki (Graduate Research Assistant University: The University of Alabama, Tuscaloosa)
Ever since the development of mankind, there has been a search for new kinds of materials that would simply make life easier. Basically, talking about electronics, the first thing that comes into one’s mind are computers, cell phones, and whatnot. But how do these operate and what has been done to improve their performance? Many of us might recall the big computer from IBM and if we compare those to present days computers, we see the dramatic reduction in size and computing capacity. Cell phones are not an exception some of which can even outperform some of the present-day computers. Starting from the vacuum tubes, to transistors, it has taken a lot to miniaturize the size of computers, improve storing and information computing speed. So far past 4 -5 decades much work has been done in the field of transistors which has been the building blocks of computers. Today’s computers have billions of transistors chips. With a decrease in size and an increase in the number of silicon chips the information processing speeds have increased a lot.
A very popular law from the industrial point of view, Moore’s law posits that the size of transistors would decrease by half every 18-24 months, and law has held true for the past 4-5 decade , creating a tremendous increase in the magnetic storage capacity and information transfer speeds. One can always dream of decreasing the size of silicon-based transistors and fabricate it out of few atoms and there have been citations for it as well. These transistors function basically dictating the presence or absence of electrons so to call the conventional flow of electric current which indeed produces Joule heating that unfortunately leads to power loss, an undesirable evil. In addition to that below transistor sizes of ~14nm, further miniaturization is limited by the quantum tunneling effect of the electron. It turns out that even if the transistor is in the ‘OFF’ state because of electron quantum tunneling, one cannot avoid leakage current. That is to say, use of silicon-based transistors are taking us nowhere as we have exploited as much it can offer. So, what next? Is that all that we can get? The answer is simple, NO. So far concerning the electrons, we have seen the exploitation of charge of electrons and not its intrinsic spin which is indeed a lot useful. Basically, there has been a lot of research going on harnessing spin in addition to the charge of an electron; in short, ‘spintronics’.
Spintronics is a new way of thinking of electronics whereby one can exploit both the charge as well as a spin degree of freedom of the electrons. The discoveries of Giant Magnetoresistance (GMR) (Figure 1(b)), thin-film structure fabricated out of alternating ferromagnetic and nonmagnetic layers whereby one observes the spin-dependent scattering of the electrons as it passes through the layers and electrons can pass through easily if the magnetization of the alternating layer is parallel and Tunnel Magnetoresistance (TMR) (Figure 1(c)), led to the field of spintronics. In 2007 Albert Fert and Peter Grünberg for the discovery of GMR effect. The GMR effect has been exclusively used in hard disk drives.
In 1996 – Slonczewski and Berger proposed the idea of spin-transfer torque [2, 3], leading to the concept of racetrack memory with information processing speed ~ 105 times faster than the conventional hard disk drives. Put forward by Stuart Parkin, the magnetic domain walls respond to the spin-polarized current. However, domain walls are not robust against the impurities and a high current density (~ 1011 Am-2 ) is required to overcome it . In 1996 – Slonczewski and Berger proposed the idea of spin-transfer torque [2, 3], leading to the concept of racetrack memory with information processing speed ~ 105 times faster than the conventional hard disk drives. Put forward by Stuart Parkin, the magnetic domain
Low power consumption and faster information processing speed are the key prospects of memory devices. One possible solution would be realizing Skyrmions, topologically protected magnetic vortex-like textures that are largely transparent/robust to any impurities in memory devices. They dwell in non-centrosymmetric magnets (Bloch Skyrmion) or magnetic thin film structures (Neel Skyrmion) where the inversion symmetry is broken and is typically induced by chiral interactions between the magnetic spins. Skyrmions can be manipulated via ~5 orders of magnitude less current density than current technology and can be easily driven by spin-polarized currents. Room temperature realization of Skyrmions in the absence of the magnetic field would be useful for magnetic storage devices.
History, discovery, and formation of Skyrmions
Skyrmions, named after Tony Skyrme, are topologically protected magnetic spin textures with chiral helicity where the magnetic moment at the center of the vortex is antiparallel to the magnetic moments at its periphery (Figure 2(a)). He proposed that particles i.e. the wave excitations are topologically protected his nonlinear field theory. The theoretical existence of these chiral magnetic spin textures; Skyrmions was suggested in 2006  and later in 2009 the first experimental observation of these topological particles as hexagonal Skyrmion lattice was made in B20-type chiral material MnSi by small-angle neutron scattering technique (SANS)  (Figure 2 (b)).
The magnetic phase diagram (Figure 2 (c)) of B20 materials (magnetic field Vs temperature) shows sharp boundaries between different magnetic phases (helical, conical, field polarized and Skyrmionic phase). The A-phase so-called the skyrmionic phase “SkX” is characterized by the hexagonal Bragg reflection, suggested by SANS. It is called A-phase as when this particular phase was observed no one was actually sure about its origin and they just called it Anomalous phase and ever since we call it A-phase. The first direct observation (real-space imaging) of Skyrmions came out in 2010 utilizing Lorentz transmission electron microscopy (LTEM) in FeCoSi thin films  (Figure 2 (d)). The observation of Topological Hall Effect (THE) in the A-phase regime further suggests the existence of Skyrmionic texture [9, 10] (Figure 2 (b)). When the electric current is supplied, Skyrmions will be driven by spin-transfer torque mechanism whereas the electrons are scattered off Skyrmions owing to Lorentz force due to the magnetic field of Skyrmions in a direction opposite to the anomalous Hall Effect which gives rise to THE  (Figure 3 (a)).
One expects a variety of magnetic interactions in a pool of magnetic spins in magnetic systems. Out of different interactions typically Skyrmions formation can be attributed to long-ranged magnetic dipolar interactions, Dzyaloshinskii-Moriya (DM) interaction, frustrated exchange interactions, and four-spin exchange interactions. Skyrmions can be very small but equally mobile and can be manipulated by currents. Basically, dipolar and DM interactions facilitate Skyrmions of size about 5 to 100 nm and the other two typically smaller than 5 nm . Out of these interactions, DM interaction and exchange interaction will be discussed here.
The relative twist of spins is quantified by angle tan = D/J where D and J are coupling constants dictating the relative size of Skyrmions. The periodicity of helicity is given as Q = 2p/ λ where λ = a D/J (λ = 70 nm for FeGe). The cubic B20 structures are the template materials for Skyrmions and among them, FeGe has the highest magnetic ordering temperature ~ 278 K . The unit cell of FeGe contains 4 Fe and 4 Ge atoms where the spin of Fe process along the <111> direction indicating helical magnetic structure .
With these unique properties of these magnetic vortices, we can tune the data processing speed and power consumption in the currently used memory devices. As Skyrmions are mostly stable at low temperature and magnetic field, to realize them in the spintronic devices one needs to tune the magnetic ordering temperature above room temperature and possibly at zero magnetic fields. To exploit the potential of Skyrmions, fabrication of high-quality single-crystal thin film samples becomes the crucial factor, as thin-film properties differ significantly as compared to the bulk. There are different techniques for thin-film fabrication. Magnetron sputter deposition, Molecular beam epitaxy, and chemical vapor deposition are some of the popular ones. In magnetron sputtering (Figure 6 (a)) electric and magnetic fields act simultaneously, the latter being used to concentrate electrons and Argon ions around the sputtering target enhancing deposition rate and sputter yield. The magnetic field is generated by using permanent magnets attached to the cathode where the target is mounted. The free electrons accelerated by electric field will ionize the neutral Argon gaseous atoms into positive Argon ions which then bombards the target material ejecting atoms which is a pure momentum transfer process. The sputtered atoms out of target will then adhere to the substrate and film growth process starts.
Though Skyrmions are topologically protected doesn’t mean they are immune to thermal agitations, the fundamental requirement would be stabilization of Skyrmion by designing materials that can be a playground to produce strong DMI vector. Still, much work has to be done to realize these magnetic quasiparticles in next-generation memory and logic devices where presence and absence of Skyrmions can be assigned a logic of “1” and “0”. Similar to the domain wall racetrack memory, proposed by S.S. Parkin, one can think of Skyrmionice racetrack memory whereby the information can be coded as the presence or absence of Skyrmions which would be identified by the tunneling magneto-resistive devices connected to the racetrack. Skyrmions can be extremely small and are robust against defects which means the storage density can be increased and can be driven nucleation, detection, and stabilization of Skyrmions at room temperature which would bring a proposed Skyrmionic logic device into reality. Typically Skyrmions have been observed at low temperature and large applied magnetic fields which have to be tuned for device applications. Though Skyrmions can be found in thin-film heterostructures concerning the device applications one usually wants to have these textures in non-centrosymmetric magnets as epitaxial growth of hetero-structures by itself is a challenging job and so far skyrmions have been found to be stable at very low temperature in ultra-thin film hetero-structures.
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