Magnetic skyrmions are small whirling topological defects in a texture magnetization state. Their stabilization and dynamics depend strongly on their topological properties. Skyrmions are induced by non-centrosymmetric crystal structure of magnetic compounds and thin films. Skyrmions are extremely small, with diameters in the nanometer range, and behave as particles that can be created, moved and annihilated. This makes them suitable for information storage and logic technologies. Skyrmions had been observed only at low temperature, and mostly under large applied magnetic fields. An intense research in this field has led to the identification of skyrmions in thin-film and multilayer structures in these heterostrutres skyrmions are able to survive at room temperature and can be manipulated by electrical currents. Utilizing interlayer magnetic exchange bias with synthetic antiferromagnet with can be used to isolated antiferromagnetic skyrmions at room temperature. The development of skyrmion-based topological spintronics holds promise for applications in the writing, processing and reading functionalities at room temperature and can be extended further to all-electrical manipulation spintronics.
- thin films
Skyrmions in magnetic materials are topological spin textures that are stabilized in various types of magnetic interactions. Non-centrosymmetric bulk magnetic materials exhibit chiral, polar, or symmetry which provide a good area to study the topological spin structures and emergent electromagnetic responses arising from them [1, 2]. In these materials without the presence of inversion symmetry, Dzyaloshinskii–Moriya interaction (DMI) can gradually twists the ferromagnetic spin arrangement, giving rise to helimagnetic structure in zero field as well as skyrmions in presence of a magnetic field. The stabilization of Skyrmions is not only due to deformation of helical spin structure but the topologically protected state from external perturbations and thus appropriate for carrying information in robust manner. Skyrmions can be different in to three types of as schematically shown in Figure 1 have been observed experimentally, which were theoretically predicted Such as Bloch, Néel and Anti skyrmions .
Bloch-type skyrmions Bloch-type skyrmion is the first category in chiral magnetic materials, where the spins are lying within the tangential plane of the system, as shown in Figure 1(a). It was first experimentally observed in the A-phase of MnSi with B20-type chiral magnetic structure. The skyrmion structure is described as a superposition of three screw-type helices, by small angle neutron scattering (SANS)  and later by Lorentz transmission electron microscopy (LTEM) technique . Later on other bulk systems, in 2012, skyrmions were observed in multiferroic Cu2OSeO3 . In 2015, -Mn type Co-Zn-Mn alloy showed metastable skyrmion at zero magnetic field and room temperature , as displayed in Figure 2. The formation of the skyrmions was confirmed in the real space by Lorentz transmission electron microscopy (LTEM) along the (110) direction of the sample and it thickness was 150 nm. The skyrmions state was seen at 370 K at the 0.07 T magnetic field and it was persistant till 290 K during the field cooled process. The Skyrmions state was not influenced by the field cooling protocol. The characteristic swirling of the magnetic moments in a skyrmion is observed by the angular color coding in Figure 2. In the zero field protocol the magnetic skyrmions are seen also at 350 K and the helical state was seen at 370 K.
Néel-Type skyrmions Néel-type of skyrmion is the second class of skyrmions. It is observed in polar magnets with the spins are lying within a radial plane, as shown in Figure 1(b), and their lattice is described as a superposition of three cycloidal helices. The bulk material of GaV4S8 identified to show Néel-type skyrmion which is a polar compound with a Lacunar spinel structure (Figure 3) .
Antiskyrmions Antiskyrmion is the third type in the skyrmion family as shown in Figure 1(c). It’s structure can be understood in terms seeing along the cross-sections of an antiskyrmion with either screw-type helix or cycloidal one, depending on the azimuthal direction of observance. It was discovered in Heusler compounds with crystal symmetry in 2017 and experimental observed using LTEM Figure 4 . Application of magnetic fields stabilizes the antiskyrmion in the lattice in the wide range temperature including room temperature. The antiskyrmions in thesystem in detail, an under-focused LTEM image of a single antiskyrmion taken at a field of 0.29 T applied parallel to the  direction is displayed in Figure 4(a). Most interestingly, it exhibits two bright spots and two dark spots along the  and  directions, respectively. The modulation of the contrast can be discerned from the appearance of two peaks with a trough in the middle of the line profile taken along  (lower inset) and two troughs with a peak in the middle of the line profile taken along  (upper inset). A slight distortion of the lattice along  might be related to the presence of a small amount of in-plane field due to a slight misorientation (by approximately ±3°) of the sample away from the exact  direction. At low in-plane fields, the distortion is mostly in the  direction as the helix propagates along . Owing to the tetragonal D2d symmetry, we expect that the antiskyrmion lattices undergo characteristic distortions in oblique fields, because their axis remains locked to the tetragonal axis, and that the antiskyrmions are consequently distorted, with the perfectly radial core shifting from the centres of the cells of the hexagonal densely packed lattice. in Figure 4(d), illustrating the skewed appearance of the antiskyrmions. This effect occurs because the in-plane component of the magnetic field favors and broadens one edge of the circulating magnetization along the field, at the cost of the opposite edge. As can be seen in Figure 4(e), when a magnetic field was applied at an angle of about 20 degrees to  by rotating the sample along , the antiskyrmions become distorted. We find that the antiskyrmions are stable up to this maximum possible tilt angle. The appearance of double-skyrmion-like features results from the projection of the antiskyrmion tubes that are oriented obliquely to the illuminating electron beam and to the applied field. As the rotation angle decreases (Figure 4e–g), the original quadrupole-like internal magnetization distribution of the antiskyrmions is revealed. Strong bright spots can be seen at the upper half of the antiskyrmions for a rotation angle of = 8 (Figure 3g), whereas they appear at the lower half for = 5° (Figure 4 (h)). In both cases, the antiskyrmion lattice exhibits a large distortion along . Nearly symmetric bright and dark spots in an almost hexagonal lattice can be seen in Figure 4(i), for an applied field of 0.29 T along  (within the ±3° limit). A small increase in the field to 0.33 T perturbs the regular arrangement of the antiskyrmions in the lattice Figure 4 (j). This field corresponds to the stability limit of the equilibrium lattice phase. Owing to their topological stability, a large number of antiskyrmions remain as metastable excitations in the homogeneous field-polarized collinear state. For even higher fields, antiskyrmions disappear from the relatively thinner region of the sample and are stabilized only in the thicker region (Figure 4(k)). Finally, the antiskyrmion lattice evolves into an array of single antiskyrmions, which disappear for fields above 0.49 T at room temperature (Figure 4(l)).
It can be observed that some materials exhibit (anti)skyrmions above room temperature, but continued efforts to has been made in this area to expand the horizon of such materials and necessary understanding of the fundamental physics of the skyrmions is required for their device applications. Further three-dimensional nature of skyrmions in non-centrosymmetric bulk materials can provide unique functionalities where the directionally non-reciprocal transmission of spin excitations can be manipulated. The spin excitations are difficult to observe for interfacial-DMI-based skyrmions in magnetic multilayer systems.
2. Skyrmions in mono layer thin films and multilayer interfaces
From the previous section it can be observed that the magnetic skyrmions were initially identified in single crystals of magnetic materials with a non-centrosymmetric lattice [4, 5], and explained by the existence of DMIs induced by spin–orbit coupling in the absence of inversion symmetry in the crystal lattice.
2.1 Monolayer epitaxial thin films
Skyrmions were observed in epitaxial ultrathin magnetic films grown on heavy metals, which are subject to gaint DMIs induced at the interface that breaks inversion symmetry and the strong spin–orbit coupling with neighboring heavy metal. The first investigated systems in this class were Fe monolayers and PdFe bilayers on Ir(111), huge DMI was seen at the interface of Fe/Ir(111) . Skyrmions observed in these systems are extremely small and was extending for only a few lattice parameters (5(c)). However, the skyrmions are stabilization in large magnetic fields of 1 T and low temperatures and at low temperature of 30 K. Moreover, the skyrmion lattice ground state of an Fe monolayer on Ir(111) does not allow the specific properties of individual skyrmions to be exploited. In PdFe bilayers epitaxially grown on Ir(111), spin spirals are observed at low field with an applied field of about 1 T induces a ferromagnetic state embedding individual metastable skyrmions transition to a .
The a two-dimensional square lattice of skyrmions was observed on a single atomic length scale. The magnetic ground state of a hexagonal Fe film of one-atomic-layer thickness on the Ir(111) surface. The real space image of skrmion was imaged using the Using spin-polarized scanning tunneling microscopy by which we can directly image the non-collinear spin texture in and demonstrate that it is incommensurate to the underlying atomic lattice . To investigate the magnetic ground the measurements were carried out with and without an external magnetic field using a tip sensitive to the in-plane magnetization component of the sample. The SP-STM image measured with an in-plane magnetized tip is shown Figure 5a, shows all three possible rotational magnetic domains due to the combination of square magnetic structure and a hexagonal atomic lattice. Using the of the tip with one particular magnetization they were able to catch different components of the in-plane sample magnetization of the rotational domains with respect to a unique axis in the magnetic unit cell. A closer view of each magnetic domain is shown in Figure 5(
The chiral magnets with archetypal B20 family like MnSi, Fe
2.2 Multilayer thin films
In multilayers for the skyrmions are formed by stacking layers of magnetic and non-magnetic heavy thin metallic films [15, 16]. These multilayer exhibit promising results on the observation of individual skyrmions. Where as in the monolayer a collection of skyrmions are observed. The multilayer approach offers advantages and opportunities for tailoring the skyrmion characteristics. By changing tuning the interactions with different material combination between magnetic metals, heavy metals, ferroelectrics and by varying the number of repetitions in the multilayers.
The multilayered structure combines the interface-driven, out-of-plane magnetic anisotropy and additive DMI at successive interfaces [16, 17]. The room temperature skyrmions have been recently obtained in for example, those formed by 10 repetitions of 0.6 nm of Co sandwiched between 1 nm of Ir and 1 nm of Pt — the additive DMI at the Co/Ir and Co/Pt interfaces [15, 16] induces skyrmions in the Co layers. The skyrmions in successive Co layers are coupled through the ultrathin non-magnetic layers, the large magnetic volume of the resulting along the columns of the thin film of couples. The Monte Carlo simulations for fcc stacking of the Pd overlayer is displayed the low-temperature phase diagram as shown in Figure 8a. At zero magnetic field an spin spiral state with a period of about 3 nm is shown in Figure 8b. In the lattice the distance between adjacent skyrmions amounts to about 3.3 nm and the system undergoes a second phase transition into the saturated ferromagnetic state for magnetic fields above 17 T. The two phase boundaries meta-stable state are mixed and appear on cooling down the sample in a finite magnetic field, for example, composed of spin spirals and single skyrmions in Figure 8c. For larger fields the isolated skyrmions in a homogeneous ferromagnetic background.
The magnetic properties in spin glasses revealed that chiral magnetic interactions between neighboring spins can also play a key role in metallic systems. The RKKY model was extended to 3-sites to the bilayer systems has been predicted by  that a large Dzyaloshinskii–Moriya interaction (DMI) with magnetic film and a heavy material with large spin–orbit coupling. The best solution up to now seems to increase the effective magnetic volume by using multilayer stacks composed of multiple repetitions of thin magnetic metal layers separated by heavy metal nonmagnetic layers grown by sputtering deposition. This approach enables the increase of the thermal stability of columnar skyrmions, that are coupled in the successive layers, leading to the recent observation of sub-100 nm skyrmions stable at RT in different multilayer systems [19, 20]. The quasi static XMCD-STXm images were carried out as shown in Figure 9(a-c), the nucleation and the detection of the protocol employed. The Figure 9(b) shows the nucleation of the magnetic skyrmions by the 5 ns current pulse and the recovery if the initial configuration by the microcoil as shown in Figure 9(c) and the skyrmion nucleation and their window can be observed from these measurement is shown in Figure 9(d). Figure 9(e) shows the XMCD-PEEM image of a magnetic skyrmion with an 630 nm diameter. The larger skyrmion diameter of 190 nm is due to the larger Co thickness. When applying a field 0 Hz = 4 mT as shown in Figure 9(f) in the direction opposite to the skyrmion magnetization the size of the skyrmion decreases to 70 nm. When the external magnetic field is releasing the initial skyrmion structure is recovered as shown in Figure 9(g). This demonstrates that the skyrmion structure is stable and reversible with respect to perturbations and that the skyrmion diameter can be tuned using frequency. The chiral structure of the skyrmion, we also imaged the skyrmion for an X-ray beam direction rotated by 90° in-plane with respect to the sample as shown in Figure 9(h).
2.3 Antiferromagnetic materials
The skyrmions hosted in ferromagnets are that has two difficulties that is stabilization of ultrasmall skyrmion due to the dipolar interaction and to stabilize individual skyrmions without using the external magnetic fields [17, 19, 21]. In antiferromagnets, two coupled equivalent magnetic subsystems align antiparallel to each other with no net magnetic moment, thus dipolar fields is absent. This forbid dipolar interactions has attracted researchers extremely interesting to use in skyrmions. MFM image obtained for the in the 4 × 3m2 image as shown in Figure 10. The domains in uniform antiferromagnetic configurations with generated the small separation between the two compensating magnetic layers. Perpendicular magnetic ansisotropy and the 1.41 nm is below the 1.47 nm. The spin spiral perodicity constitute the independent measurement of the DMI amplitude for system.
3. Half skyrmions
Fe2O3 is a clasical antiferromagentic materiasl which shows the weak ferromagentic behavior due to the presence of DMI in the material. It crystallizes in the R-3c space group with Néel temperature of 948 K. It also exhibits a Morin transiton in which the spin in-plane at room temperature changes to perpendicular direction . Recently, it was shows that the fabricating the Fe2O3 thin films on the A2O3 substrate showed that three possible antiferromagentic (AFM) domain orientations. From the PEEM measurement has visually confirmeed the formation of the Votex state of the Meron which are the half skyrmions and anti-vertex called the Anti-merons as shown in Figure 11(a) . When Fe2O3 was exchanged coupled with the ferromagentic Cobalt film of 1 nm thick. The interfacial exchange interaction were able to imprint the AFM on the cobalt thin film as shown in the Figure 11(b). The imprinting of the AFM domains on the cobalt by interfacial exchange with the adjacent AFM vortex/anti-vortex in the -Fe2O3 film. Figure 11(c) shows the magnetization in the inplabne of the film. Figure 11(d) shows the smearing of the Co vector map. They have also able to manupuulate the meron pairs by the small magnetic field of 100 mT in plane to the  direction. This proved that the cobalt layer aligns with the -Fe2O3 and with the staggeres magnetization the majority of the merons have seen disappeared.
Recently the Antiferromagnetic half-skyrmions and bimerons at room temperature was observed . In their study the -Fe2O3 and Rh doped Fe2O3 thin films were prepared by pulsed laser deposition and on the top of the film platinum (Pt) was coated on the top of it. For the thin film of -Fe2O3 it showed the Morin transion at 240 K and for Rh doped Fe2O3 thin film it was 298 K . It was AFM textures gradually shrink to become fine bubbles and persist up to room temperature for both the films. They have carried out the details analysis by the Néel vector maps of the IP orientations as shown in Figure 12(a-c). In Figure 12(a) most of the resions are in the out of plane oriented and display in-plane winding, due to the mixing of the Bloch- and Néel-type domain-wall. They also indicates that the small in-plane islands nucleating inside the out of plane matrix have random in-plane orientations consistent with the first-order nature of the Morin transition. When the temperature is near to the morin transition the out of plane regions shrink and in-plane islands widen considerably and merging with each other if in close proximity as shown Figure 12(b). When the temperature is above the morin transition the AFM textures in which spins are predominantly lying along the basal planes, separated by 60 domain wall Figure 12(c). When a region of out of plane spins happens to be encircled by in-plane spins, the out of plane bubble becomes trapped and shrinks to a very small size and does not disappear because it is topologically forbidden from unwinding completely into the plane Figure 12(d).
From this chapter the various presence of the skyrmions in the multilayer, monolayer and non-centrosymmetric layer have been discussed in detail and the finally the recent advances in the observation of the half skyrmion were also discussed. For the device application thin film heterostructures are very important. This will lead to the future skyrmion based devices for the manipulation by optical, current, electric field and also by the spin orbit torque devices. So that the infomation can be used for the racetrack based memory devices and logic implementation.
The author wants to acknowledge IIT Kharapur for SRF funding.
Conflict of interest
The authors declare no conflict of interest.