arXiv subjects

K. H. Khoo

Publications and source records attributed to K. H. Khoo.

5 recordsLinked to original sources

arXiv2024

Angular dependence of hump-shape Hall Effects for distinguishing between Karplus-Luttinger and Geometrical Origins

Among the vast magnetic heterostructures explored in Condensed Matter Physics, two contrasting interpretations of the hump-shaped Hall Effects remain ambiguous and debated, namely, the overlap of two opposite-signed Karplus-Luttinger Hall loops associated with inhomogeneous collinear domains with perpendicular anisotropy, or the Geometrical/Topological Hall Effect emanated from hexagnal close-packed lattice of Skyrmion ground state with smoothly varying non-collinear moments. Their similarity in topology implies difficulty in discrimination via magnetic imaging. Here, this ambiguity is overcome and clarified by the divergence exponent of hump peak fields extracted from Hall measurements with magnetic field rotation on several heterostructures. Their difference in sensitivity to in-plane fields reveals that the former mechanism involves higher uniaxial anisotropy than the latter, departing from the Skyrmion ground state regime by the Ginzburg-Landau framework of triple-q spin-wave superposition. Numerous material systems can be summarized into a single curve of divergence exponent versus the collinear quality factor, bridging the crossover of the two mentioned mechanisms.

cond-mat.str-el↗

arXiv2022

Widely Tunable Berry curvature in the Magnetic Semimetal Cr1+dTe2

Magnetic semimetals have increasingly emerged as lucrative platforms hosting spin-based topological phenomena in real and momentum spaces. Of particular interest is the emergence of Berry curvature, whose geometric origin, accessibility from Hall transport experiments, and material tunability, bodes well for new physics and practical devices. Cr1+dTe2, a self-intercalated magnetic transition metal dichalcogenide, TMD, exhibits attractive natural attributes relevant to such applications, including topological magnetism, tunable electron filling, magnetic frustration etc. While recent studies have explored real-space Berry curvature effects in this material, similar considerations of momentum-space Berry curvature are lacking. Here, we systematically investigate the electronic structure and transport properties of epitaxial Cr1+dTe2 thin films over a wide range of doping, d between 0.33 and 0.71. Spectroscopic experiments reveal the presence of a characteristic semi-metallic band region near the Brillouin Zone edge, which shows a rigid band like energy shift as a function of d. Transport experiments show that the intrinsic component of the anomalous Hall effect, AHE, is sizable, and undergoes a sign flip across d. Finally, density functional theory calculations establish a causal link between the observed doping evolution of the band structure and AHE: the AHE sign flip is shown to emerge from the sign change of the Berry curvature, as the semi-metallic band region crosses the Fermi energy. Our findings underscore the increasing relevance of momentum-space Berry curvature in magnetic TMDs and provide a unique platform for intertwining topological physics in real and momentum spaces.

cond-mat.mtrl-sci↗

arXiv2020

Tailoring Magnetism in Self-intercalated Cr1+δTe2 Epitaxial Films

Magnetic transition metal dichalcogenide (TMD) films have recently emerged as promising candidates to host novel magnetic phases relevant to next-generation spintronic devices. However, systematic control of the magnetization orientation, or anisotropy, and its thermal stability, characterized by Curie temperature (Tc) remains to be achieved in such films. Here we present self-intercalated epitaxial Cr1+δTe2 films as a platform for achieving systematic/smooth magnetic tailoring in TMD films. Using a molecular beam epitaxy (MBE) based technique, we have realized epitaxial Cr1+δTe2 films with smoothly tunable over a wide range (0.33-0.82), while maintaining NiAs-type crystal structure. With increasing δ, we found monotonic enhancement of Tc from 160 to 350 K, and the rotation of magnetic anisotropy from out-of-plane to in-plane easy axis configuration for fixed film thickness. Contributions from conventional dipolar and orbital moment terms are insufficient to explain the observed evolution of magnetic behavior with δ. Instead, ab initio calculations suggest that the emergence of antiferromagnetic interactions with δ, and its interplay with conventional ferromagnetism, may play a key role in the observed trends. To our knowledge, this constitutes the first demonstration of tunable Tc and magnetic anisotropy across room temperature in TMD films, and paves the way for engineering novel magnetic phases for spintronic applications.

cond-mat.mtrl-sci↗

arXiv2017

Tunable Room Temperature Magnetic Skyrmions in Ir/Fe/Co/Pt Multilayers

Magnetic skyrmions are nanoscale topological spin structures offering great promise for next-generation information storage technologies. The recent discovery of sub-100 nm room temperature (RT) skyrmions in several multilayer films has triggered vigorous efforts to modulate their physical properties for their use in devices. Here we present a tunable RT skyrmion platform based on multilayer stacks of Ir/Fe/Co/Pt, which we study using X-ray microscopy, magnetic force microscopy and Hall transport techniques. By varying the ferromagnetic layer composition, we can tailor the magnetic interactions governing skyrmion properties, thereby tuning their thermodynamic stability parameter by an order of magnitude. The skyrmions exhibit a smooth crossover between isolated (metastable) and disordered lattice configurations across samples, while their size and density can be tuned by factors of 2 and 10 respectively. We thus establish a platform for investigating functional sub-50 nm RT skyrmions, pointing towards the development of skyrmion-based memory devices.

cond-mat.mes-hall↗

arXiv2007

Novel Orientational Ordering and Reentrant Metallicity in KxC60 Monolayers for 3 <= x <= 5

We have performed local STM studies on potassium-doped C60 (KxC60) monolayers over a wide regime of the phase diagram. As K content increases from x = 3 to 5, KxC60 monolayers undergo metal-insulator-metal reentrant phase transitions and exhibit a variety of novel orientational orderings. The most striking new structure has a pinwheel-like 7-molecule unit cell in insulating K4+dC60. We propose that the driving mechanism for the orientational ordering in KxC60 is the lowering of electron kinetic energy through maximization of the overlap of neighboring molecular orbitals over the entire doping range x = 3 to 5. In the insulating and metallic phases this gives rise to orbital versions of the superexchange and double-exchange interactions respectively.

cond-mat.mtrl-sci↗