arXiv subjects

G. J. Omar

Publications and source records attributed to G. J. Omar.

6 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↗

arXiv2023

Direct Determination of Spin-Splitting Energy in Magnetic Graphene by Landau Fan Shifts

Spin-polarized two-dimensional materials with large and tunable spin-splitting energy promise the field of 2D spintronics. While graphene has been a canonical 2D material, its spin properties and tunability are limited. Here, we demonstrate the emergence of robust spin-polarization in graphene with large and tunable spin-splitting energy of up to 132 meV at zero applied magnetic fields. The spin polarization is induced through a magnetic exchange interaction between graphene and the underlying ferrimagnetic oxide insulating layer, Tm3Fe5O12, as confirmed by its X-ray magnetic circular dichroism. The spin-splitting energies are directly measured and visualized by the shift in their landau fan diagram mapped by analyzing the measured subnikov-de-Haas oscillations as a function of applied electric fields, showing consistent fit with our first-principles and machine learning calculations. Further, the observed spin-splitting energies can be tuned over a broad range between 98 and 166 meV by cooling fields. Our methods and results are applicable to other two-dimensional (magnetic) materials and heterostructures, and offer great potential for developing next-generation spin logic and memory devices.

cond-mat.mtrl-sci↗

arXiv2023

Tunable Magnetic Properties in Sr$_2$FeReO$_6$ Double-Perovskite

Double-perovskite oxides have attracted recent attention due to their attractive functionalities and application potential. In this paper, we demonstrate the effect of dual controls, i.e., the deposition pressure of oxygen (P$_O2$) and lattice mismatch ($ε$), on tuning magnetic properties in epitaxial double-perovskite Sr$_2$FeReO$_6$ films. In a nearly-lattice-matched Sr$_2$FeReO$_6$/SrTiO$_3$ film, the ferrimagnetic-to-paramagnetic phase transition occurs when P$_O2$ is reduced to 30 mTorr, probably due to the formation of Re$^{4+}$ ions that replace the stoichiometric Re$^{5+}$ to cause disorders of $B$-site ions. On the other hand, a large compressive strain or tensile strain shifts this critical P$_O2$ to below 1 mTorr or above 40 mTorr, respectively. The observations could be attributed to the modulation of $B$-site ordering by epitaxial strain through affecting elemental valence. Our results provide a feasible way to expand the functional tunability of magnetic double-perovskite oxides that hold great promise for spintronic devices.

cond-mat.str-el↗

arXiv2023

High-quality NiFe thin films on oxide/non-oxide platforms via pulsed laser deposition at room temperature

Soft ferromagnetic NiFe thin films are promising for applications in spintronic devices because of their constituent electrical and magnetic properties. Electron beam evaporation and sputtering techniques have been used to deposit NiFe thin films. For in-situ stacking of NiFe with functional complex oxides, the pulsed laser deposition (PLD) method is highly desirable. However, the growth of high-quality NiFe (and non-oxide thin films in general) by PLD remains a formidable task. Here, we report high-quality NiFe thin films of various thicknesses on oxide/non-oxide substrates with desirable magnetic properties by PLD at room temperature. The magnetic properties are found to be strongly dependent on the laser fluence of the deposition process. The laser fluence of 4 Joule/cm$^2$ produces the highest magnetization of ~547 emu/cc. The small coercivity (few Oersted) and sharp ferromagnetic switching behaviour indicate uniaxial anisotropy with an easy axis along the in-plane direction. In addition, thickness-dependent magnetodynamics characterizations are studied via ferromagnetic resonance. Our findings indicate the ferromagnetic characteristics are sensitive to the quality of the oxide/non-oxide substrate surface. These results offer significant insight into the PLD-based development of thin metal magnetic films.

cond-mat.mtrl-sci↗

arXiv2022

Ionic modulation at the LaAlO$_3$/KTaO$_3$ interface for extreme high-mobility two-dimensional electron gas

Due to the coexistence of many emergent phenomena, including 2D superconductivity and a large Rashba spin-orbit coupling, 5d transition metal oxides based two-dimensional electron systems (2DESs) have been prospected as one of the potential intrants for modern electronics. However, despite the lighter electron mass, the mobility of carriers, a key requisite for high-performance devices, in 5d-oxides devices remains far behind their 3d-oxides analogs. The carriers mobility in these oxides is significantly hampered by the inevitable presence of defects generated during the growth process. Here, we report very high mobility ($\sim$ 22650 cm$^2$V$^{-1}$s$^{-1}$) of 5d-2DES confined at the LaAlO$_3$/KTaO$_3$ interface. The high mobility, which is beyond the values observed in LaAlO$_3$/SrTiO$_3$ and $γ$-Al$_2$O$_3$/SrTiO$_3$ systems in the same carrier-density range, is achieved using the ionic-liquid gating at room temperature. We postulate that the ionic-liquid gating affects the oxygen vacancies and efficiently reduces any disorder at the interface. Investigating density and mobility in a broad range of back-gate voltage, we reveal that the mobility follows the power-law $μ\propto n^{1.2}$, indicating the very high quality of ionic-liquid-gated LaAlO$_3$/KTaO$_3$ devices, consistent with our postulate. Further, the analysis of the quantum oscillations measured in high magnetic fields confirms that the high-mobility electrons occupy the electronic sub-bands emerging from the Ta:5d orbitals of KTaO$_3$.

cond-mat.mtrl-sci↗

arXiv2021

Room-temperature colossal magnetoresistance in terraced single-layer graphene

Disorder-induced magnetoresistance (MR) effect is quadratic at low perpendicular magnetic fields and linear at high fields. This effect is technologically appealing, especially in the two-dimensional (2D) materials such as graphene, since it offers potential applications in magnetic sensors with nanoscale spatial resolution. However, it is a great challenge to realize a graphene magnetic sensor based on this effect because of the difficulty in controlling the spatial distribution of disorder and enhancing the MR sensitivity in the single-layer regime. Here, we report a room-temperature colossal MR of up to 5,000% at 9 T in terraced single-layer graphene. By laminating single-layer graphene on a terraced substrate, such as TiO2 terminated SrTiO3, we demonstrate a universal one order of magnitude enhancement in the MR compared to conventional single-layer graphene devices. Strikingly, a colossal MR of >1,000% was also achieved in the terraced graphene even at a high carrier density of ~1012 cm-2. Systematic studies of the MR of single-layer graphene on various oxide- and non-oxide-based terraced surfaces demonstrate that the terraced structure is the dominant factor driving the MR enhancement. Our results open a new route for tailoring the physical property of 2D materials by engineering the strain through a terraced substrate.

cond-mat.mes-hall↗