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Van der Waals (vdW) magnets are promising, because of their tunable magnetic properties with doping or alloy composition, where the strength of magnetic interactions, their symmetry, and magnetic anisotropy can be tuned according to the desired application. However, so far, most of the vdW magnet-based spintronic devices have been limited to cryogenic temperatures with magnetic anisotropies favoring out-of-plane or canted orientation of the magnetization. Here, we report beyond room-temperature lateral spin-valve devices with strong in-plane magnetization and spin polarization of the vdW ferromagnet (Co_0.15Fe_0.85)₅GeTe₂ (CFGT) in heterostructures with graphene. Density functional theory (DFT) calculations show that the magnitude of the anisotropy depends on the Co concentration and is caused by the substitution of Co in the outermost Fe layer. Magnetization measurements reveal the above room-temperature ferromagnetism in CFGT and clear remanence at room temperature. Heterostructures consisting of CFGT nanolayers and graphene were used to experimentally realize basic building blocks for spin valve devices, such as efficient spin injection and detection. Further analysis of spin transport and Hanle spin precession measurements reveals a strong in-plane magnetization with negative spin polarization at the interface with graphene, which is supported by the calculated spin-polarized density of states of CFGT. The in-plane magnetization of CFGT at room temperature proves its usefulness in graphene lateral spin-valve devices, thus revealing its potential application in spintronic technologies.

The discovery of van der Waals (vdW) magnets opened a new paradigm for condensed matter physics and spintronic technologies. However, the operations of active spintronic devices with vdW ferromagnets are limited to cryogenic temperatures, inhibiting their broader practical applications. Here, the robust room-temperature operation of lateral spin-valve devices using the vdW itinerant ferromagnet Fe5GeTe2 in heterostructures with graphene is demonstrated. The room-temperature spintronic properties of Fe5GeTe2 are measured at the interface with graphene with a negative spin polarization. Lateral spin-valve and spin-precession measurements provide unique insights by probing the Fe5GeTe2/graphene interface spintronic properties via spin-dynamics measurements, revealing multidirectional spin polarization. Density functional theory calculations in conjunction with Monte Carlo simulations reveal significantly canted Fe magnetic moments in Fe5GeTe2 along with the presence of negative spin polarization at the Fe5GeTe2/graphene interface. These findings open opportunities for vdW interface design and applications of vdW-magnet-based spintronic devices at ambient temperatures.

A detailed understanding of the different mechanisms being responsible for terahertz (THz) emission in ferromagnetic (FM) materials will aid in designing efficient THz emitters. In this report, we present direct evidence of THz emission from single layer Co0.4Fe0.4B0.2 (CoFeB) FM thin films. The dominant mechanism being responsible for the THz emission is the anomalous Hall effect (AHE), which is an effect of a net backflow current in the FM layer created by the spin polarized current reflected at the interfaces of the FM layer. The THz emission from the AHE-based CoFeB emitter is optimized by varying its thickness, orientation, and pump fluence of the laser beam. Results from electrical transport measurements show that skew scattering of charge carriers is responsible for the THz emission in the CoFeB AHE-based THz emitter.

Using element-specific measurements of the ultrafast demagnetization of Ru/Fe65Co35 hetero-structures, we show that Ru can exhibit a significant magnetic contrast (3% asymmetry) resulting from ultrafast spin currents emanating from the demagnetization process of the FeCo layer. We use this magnetic contrast to investigate how superdiffusive spin currents are affected by the doping of heavy elements in the FeCo layer. We find that the spin currents are strongly suppressed, and that the recovery process in Ru slows down by Re doping. This is in accordance with a change in interface reflectivity of spin currents as found by the superdiffusive spin transport model.

Topological insulators with high spin-orbit coupling and helically spin-momentum-locked topological surface states (TSSs) can serve as efficient spin current generators for modern spintronics applications. We used the industrial-friendly DC magnetron sputtering technique to fabricate magnetic heterostructures consisting of Bi2Te3 (BT) as a topological insulator and Co60Fe20B20 (CFB) as a magnetic layer and studied the temperature-dependent spin pumping, utilizing out-of-plane ferromagnetic resonance spectroscopy. These results demonstrate that the effective spin-mixing conductance is significantly affected by the contribution of two-magnon scattering (TMS). It is found that the TMS-free effective spin-mixing conductance increases with decreasing temperature. Additionally, results from magneto-transport measurements indicate that the surface coherence length of BT is in accordance with the temperature-dependent effective spin-mixing conductance. This enhancement of effective mixing conductance correlated with the enhancement in the contribution of the TSSs as evaluated using the weak-anti-localization effect. This study provides a deeper understanding of the temperature-dependent spin dynamics in sputtered BT/CFB heterostructures which can serve as a guide for further exploration of such bilayers for topological-based spintronic applications.

Two-dimensional transition metal dichalcogenides (TMDs) have immense potential for spintronics applications. Here, we report atomic layer thickness dependence in WS2/Co-3 FeB heterostructures. The layer dependence is predicted by density functional theory and demonstrated experimentally by the layer dependence of the Dzyaloshinskii-Moriya interaction (DMI). Notably, we have observed the DMI in WS2 to be larger than that for heavy metals such as W and Ta, which is important to stabilize chiral structures. Inversion symmetry is not preserved with an odd number of layers, while it exists with an even number of layers. This symmetry rule is reflected in the temperature dependence of the effective damping parameter of the heterostructure. That the damping parameter decreases (increases) in odd (even) layers can be resolved at low temperature. This suggests that the layer dependence has its origin at the WS2 interface, where the spin-valley coupling and spin-orbit coupling activate these features. Large DMI, pure spin current, and unique layer dependence in TMDs provide valuable information and fundamental understanding for designing TMD-based quantum information storage devices.

To achieve a large terahertz (THz) amplitude from a spintronic THz emitter (STE), materials with 100% spin polarisation such as Co-based Heusler compounds as ferromagnetic layer are required. However, these compounds are known to loose their half-metallicity in the ultrathin film regime, as it is difficult to achieve L2(1) ordering, which has become a bottleneck for the film growth. Here, the successful deposition using room temperature DC sputtering of the L2(1) and B2 ordered phases of the Co2FeAl full Heusler compound is reported. Co2FeAl is used as ferromagnetic layer together with highly orientated Pt as nonferromagnetic layer in the Co2FeAl/Pt STE, where an MgO (10 nm) seed layer plays an important role to achieve the L2(1) and B2 ordering of Co2FeAl. The THz generation in the Co2FeAl/Pt STE is presented, which has a bandwidth of 0.2-4 THz. The THz electric field amplitude is optimized with respect to thickness, orientation, and growth parameters using a thickness dependent model considering the optically induced spin current, superdiffusive spin current, inverse spin Hall effect, and the THz attenuation in the layers. This study, based on the full Heusler Co2FeAl compound opens up a plethora of possibilities in STE research involving full Heusler compounds.

We report the tuneable spin angular momentum transfer (spin pumping) from Co60Fe20B20 (CFB) amorphous alloy into the Ta heavy metal nanolayers. All the films are grown on Si (100) substrate at room temperature using ion-beam sputtering technique. Structural studies reveal that the grown Ta films over amorphous CFB are crystalline even at ultrathin regime. The bilayers possess very low interface roughness (< 0.5 nm) and are continuous throughout the thickness range. Comparative analysis of the spin pumping in CFB (4, 6 and 8 nm) as a function of the Ta thickness (vary from 1 to 10 nm in step of 1 nm) has been performed employing ferromagnetic resonance (FMR) spectroscopy. It is observed that the effective damping increase exponentially with the increase of Ta, (i.e. follows ballistic spin transport) in two series of CFB (4 nm)/Ta (0-10 nm) and CFB(6 nm)/Ta (0-10 nm) bilayers, which is characteristic of normal spin pumping. However, the anomalous behaviour has been observed for CFB (8 nm)/Ta (0-10 nm) bilayer series where the spin current generated in Ta with the thicker CFB behaves oppositely. The results demonstrate the strong dependence of ferromagnet thickness on the spin pumping into the Ta nanolayers. This study paves the way to choose suitable ferromagnetic layer thickness for spin current-induced switching applications in spintronics.

The transition metal dichalcogenide 1T-TaS₂ is known to exhibit a number of collective electronic states known as charge density wave (CDW) instabilities. Intriguing phenomena such as a large damping-like spin−orbit torque (SOT) have been reported in monolayer 1T-TaS₂ [Nano Lett. 2020, 20 (9), 6372−6380]. Probing of CDWs in monolayer thick 1T-TaS₂ has been an inconceivable task. Here, the temperature-dependent spin dynamics and the effect of CDWs in the 1T-TaS₂(monolayer)/Ni₈₁Fe₁₉(Py) (7 nm) heterostructure are reported. Employing ferromagnetic resonance, the effect of the different commensurate (C) and nearly commensurate (NC) CDW states on the spin dynamics during heating and cooling cycles has been characterized by use of the effective damping constant and the spin mixing conductance of the heterostructure. In addition, these CCDW and NCCDW states, which affect the SOT efficiencies due to damping- and field-like SOTs, have been evaluated by using angle-dependent planar Hall effect measurements in controlled cooling and heating cycles. Our findings provide a fundamental understanding of the effect of different CDW states on the spin dynamics in twodimensional 1T-TaS₂ monolayer interfaced Py.

The transfer of knowledge from one generation to another is key to the intellectualness of mankind. In the present information age, digital technology provides easy access to knowledge and information. However people across the globe simultaneously generate an enormous digital footprint, which demands to store and process the information in a modish way. Spin-based electronics is being considered a prospective candidate beyond complementary metal-oxide-semiconductor technology with several applications in data storage and data communication. The key concept of this technology is the generation, transportation, and detection of spin currents in magnetic heterostructures consisting of ferromagnetic (FM) and non-ferromagnetic (NFM) bilayer thin films.

In this thesis, I describe the concepts of spin dynamics at the nano- to femtosecond timescales and experimental techniques used to extract the spin dynamics properties of magnetic heterostructures. In this regard, we have shown that the Gilbert damping parameter and the number of quantum conductance channels (QCCs) can be enhanced by doping the FM layer with Re in the Ru/Fe₆₅Co₃₅/Ru heterostructure. The same heterostructure was used to evidence superdiffusive spin transport and a proximity induced magnetic moment in the Ru layer. It has also been shown that the number of QCCs can be enhanced by inserting a Cu layer at the interface between the FM and NFM layers in the Co₂FeAl/β-Ta heterostructure where the Gilbert damping parameter of Co₂FeAl depends on its chemical ordering. Further, we have found that the spin torque (SOT) efficiency in the 2D-transition metal dichalcogenide, 1T-TaS₂, based heterostructure is one order larger as compared to Co₂FeAl/β-Ta and Fe/Pd heterostructures. Moreover, it has been shown that crystalline quality and strain engineering can significantly impact the SOT efficiency and emission of terahertz radiation in Fe/Pd and Fe/Pt heterostructures, respectively. Finally, a full Heusler (Co₂FeAl) based spintronic terahertz emitter is presented, which utilizes an optically induced spin current and the inverse spin Hall effect phenomenon. This thesis provides useful insights in the pathway towards power efficient spin logic devices.