Search for: All records

Abstract Surface segregation is a ubiquitous phenomenon driven by minimization of the total free energy. In this paper we study surface segregation in multicomponent magnetic Bismuth ferrite nanoparticles alloyed with varying amounts of Dysprosium, Zinc and Titanium. We employ surface and bulk sensitive spectroscopic probes to unravel a significant surface segregation of Bismuth oxide and Titanium oxide. High coercive fields of BiFe_0.95Ti_0.05O₃(BFTO) and BiFe_0.96(Zn, Ti)_0.02O₃(BFZTO) at room temperature reveal that they have a strong exchange bias. This suggests that the Titanium oxide is magnetically active, and there is a Ti inducedd^oferromagnetism in action between these nanoparticles. We show, with the addition of Dy₂O₃, the Ti inducedd^oferromagnetism is suppressed making (BDFZTO) superparamagnetic. We observe that all three differently alloyed Bismuth ferrite nanoparticles show a non-saturating paramagnetic background. 

Abstract Topological spin textures (e.g., skyrmions) can be stabilized by interfacial Dzyaloshinskii‐Moriya interaction (DMI) in the magnetic multilayer, which has been intensively studied. Recently, Bloch‐type magnetic skyrmions stabilized by composition gradient‐induced DMI (g‐DMI) have been observed in 10‐nm thick CoPt single layer. However, magnetic anisotropy in gradient‐composition engineered CoPt (g‐CoPt) films is highly sensitive to both the relative Co/Pt composition and the film thickness, leading to a complex interplay with g‐DMI. The stability of skyrmions under the combined influence of magnetic anisotropy and g‐DMI is crucial yet remains poorly understood. Here, we condcut a systematic study on the characteristics of magnetic skyrmions as a function of gradient polarity and effective gradient (defined as gradient/thickness) in g‐CoPt single layers (thickness of 10–30 nm) using magnetic force microscopy (MFM), bulk magnetometry, and topological Hall effect measurements. Brillouin light scattering spectroscopy confirms that both the sign and magnitude of g‐DMI depend on the polarity and amplitude of the composition gradient in g‐CoPt films. MFM reveals that skyrmion size and density vary with g‐CoPt film thickness, gradient polarity, and applied magnetic field. An increased skyrmion density is observed in samples exhibiting higher magnetic anisotropy, in agreement with micromagnetic simulations and energy barrier calculations. 

Abstract We report on the temperature dependent low energy electron diffraction (LEED) studies of 12 nm epitaxial Sr₃Ir₂O₇(001) thin films. The Debye temperature has been extracted from the temperature-dependence of LEED intensity at elevated temperatures and different electron kinetic energies. For the most surface sensitive LEED, obtained at the lowest electron kinetic energies, the extracted surface Debye temperature is 270 ± 22 K, which is much lower than the 488 ± 40 K Debye temperature obtained using higher electron kinetic energies. Surprisingly, the LEED diffraction intensity, at the lowest electron kinetic energies, increases rather than decreases, with increasing sample temperatures up to about 440 K. This anomalous behavior has been attributed to the reduction of the lattice vibrational amplitudes along the surface normal. This damping of the normal mode vibrations with increasing temperature results from the enhanced electronic screening via thermally activated carriers. This scenario is corroborated by the transport measurement, showing that Sr₃Ir₂O₇is a narrow band Mott insulator with a band gap of about 32 meV. We have identified criteria for finding anomalous scattering behavior in other transition metal oxide systems. 

Abstract We report evidence of a finite density of states at the Fermi level at the surface of epitaxial thin films of the narrow bandgap Mott insulator Sr₃Ir₂O₇(001). The Brillouin zone critical points for Sr₃Ir₂O₇(001) thin films have been determined by a comparison of the band mapping from angle-resolved photoemission spectroscopy and low energy electron diffraction. Angle-resolved x-ray photoemission studies reveal the surface termination of Sr₃Ir₂O₇(001) is Sr–O. The absence of dispersion with photon energy, or changing wave vector along the surface normal, indicates the two-dimensional character of the bands contributing to the density of states close to the Fermi level for Sr₃Ir₂O₇(001) thin films. Thus, the finite density of states at the Fermi level is attributed to surface states or surface resonances. The appearance of a finite density of states at the Fermi level is consistent with the increased conductivity with decreasing film thickness for ultrathin Sr₃Ir₂O₇(001) films. 

Abstract The Ruddlesden‐Popper 5diridate Sr₂IrO₄is an antiferromagnetic Mott insulator with the electronic, magnetic, and structural properties highly intertwined. Voltage control of its magnetic state is of intense fundmenatal and technological interest but remains to be demonstrated. Here, the tuning of magnetotransport properties in 5.2 nm Sr₂IrO₄via interfacial ferroelectric PbZr_0.2Ti_0.8O₃is reported. The conductance of the epitaxial PbZr_0.2Ti_0.8O₃/Sr₂IrO₄heterostructure exhibits ln(T) behavior that is characteristic of 2D correlated metal, in sharp contrast to the thermally activated behavior followed by 3D variable range hopping observed in single‐layer Sr₂IrO₄films. Switching PbZr_0.2Ti_0.8O₃polarization induces nonvolatile, reversible resistance modulation in Sr₂IrO₄. At low temperatures, the in‐plane magnetoresisance in the heterostructure transitions from positive to negative at high magnetic fields, opposite to the field dependence in single‐layer Sr₂IrO₄. In the polarization down state, the out‐of‐plane anisotropic magnetoresistanceR_AMRexhibits sinusoidal angular dependence, with a 90° phase shift below 20 K. For the polarization up state, unusual multi‐level resistance pinning appears inR_AMRbelow 30 K, pointing to enhanced magnetocrystalline anisotropy. The work sheds new light on the intriguing interplay of interface lattice coupling, charge doping, magnetoelastic effect, and possible incipient ferromagnetism in Sr₂IrO₄, facilitating the functional design of its electronic and material properties. 

Abstract Two-dimensional (2D) ferroelectric and magnetic van der Waals materials are emerging platforms for the discovery of novel cooperative quantum phenomena and development of energy-efficient logic and memory applications as well as neuromorphic and topological computing. This review presents a comprehensive survey of the rapidly growing 2D ferroic family from the synthesis perspective, including brief introductions to the top-down and bottom-up approaches for fabricating 2D ferroic flakes, thin films, and heterostructures as well as the important characterization techniques for assessing the sample properties. We also discuss the key challenges and future directions in the field, including scalable growth, property control, sample stability, and integration with other functional materials. 

Abstract Multiferroic hexagonal rare-earth ferrites (h-RFeO₃, R= Sc, Y, and rare earth), in which the improper ferroelectricity and canted antiferromagnetism coexist, have been advocated as promising candidates to pursue the room-temperature multiferroics, because of strong spin-spin interaction. The strong interactions between the ferroic orders and the structural distortions are appealing for high-density, energy-efficient electronic devices. Over the past decade, remarkable advances in atomic-scale synthesis, characterization, and material modeling enable the significant progresses in the understanding and manipulation of ferroic orders and their couplings in h-RFeO₃thin films. These results reveal a physical picture of rich ferroelectric and magnetic phenomena interconnected by a set of structural distortions and spin-lattice couplings, which provides guidance for the control of ferroic orders down to the nano scale and the discovery of novel physical phenomena. This review focus on state-of-the-art studies in complex phenomena related to the ferroelectricity and magnetism as well as the magnetoelectric couplings in multiferroic h-RFeO₃, based on mostly the recent experimental efforts, aiming to stimulate fresh ideas in this field. 

Abstract A bulk gadolinium (Gd) single crystal exhibits virtually zero remnant magnetization, a common trait among soft uniaxial ferromagnets. This characteristic is reflected in our magnetometry data showing virtually hysteresis free isothermal magnetization loops with large saturation magnetization. The absence of hysteresis allows to model the measured easy axis magnetization as a function of temperature and applied magnetic field, rather than a relation, which permits the application of Maxwell relations from equilibrium thermodynamics. Demagnetization effects broaden the isothermal first-order transition from negative to positive magnetization. By analyzing magnetization data within the coexistence regime, we deduce the isothermal entropy change and the field-induced heat capacity change. Comparing the numerically inferred heat capacity with relaxation calorimetric data confirms the applicability of the Maxwell relation. Analysis of the entropy in the mixed phase region suggests the presence of hitherto unresolved nanoscale magnetic structures in the demagnetized state of Gd. To support this prediction, Monte Carlo simulations of a 3D Ising model with dipolar interactions are performed. Analyzing the cluster size statistics and magnetization from the model provides strong qualitative support of our analytic approach. 

Abstract Two‐dimentional magnets are of significant interest both as a platform for exploring novel fundamental physics and for their potential in spintronic and optoelectronic devices. Recent bulk magnetometry studies have indicated a weak ferromagnetic response in tungsten disulfide (WS₂), and theoretical predictions suggest edge‐localized magnetization in flakes with partial hydrogenation. Here, room‐temperature wide‐field quantum diamond magnetometry to image pristine and Fe‐implanted WS₂flakes of varying thicknesses (45–160 nm), exfoliated from bulk crystals and transferred to NV‐doped diamond substrates, is used. Direct evidence of edge‐localized stray magnetic fields, which scale linearly with applied external magnetic field (4.4–220 mT), reaching up to ±4.7 µT, is observed. The edge signal shows a limited dependence on the flake thickness, consistent with dipolar field decay and sensing geometry. Magnetic simulations using five alternative models favor the presence of edge magnetization aligned along an axis slightly tilted from the normal to the WS₂flake's plane, consistent with spin canting in antiferromagnetically coupled edge states. Thses findings establish WS₂as a promising platform for edge‐controlled 2D spintronics. 

Abstract The study of brain age has emerged over the past decade, aiming to estimate a person’s age based on brain imaging scans. Ideally, predicted brain age should match chronological age in healthy individuals. However, brain structure and function change in the presence of brain-related diseases. Consequently, brain age also changes in affected individuals, making the brain age gap (BAG)—the difference between brain age and chronological age—a potential biomarker for brain health, early screening, and identifying age-related cognitive decline and disorders. With the recent successes of artificial intelligence in healthcare, it is essential to track the latest advancements and highlight promising directions. This review paper presents recent machine learning techniques used in brain age estimation (BAE) studies. Typically, BAE models involve developing a machine learning regression model to capture age-related variations in brain structure from imaging scans of healthy individuals and automatically predict brain age for new subjects. The process also involves estimating BAG as a measure of brain health. While we discuss recent clinical applications of BAE methods, we also review studies of biological age that can be integrated into BAE research. Finally, we point out the current limitations of BAE’s studies.