arXiv subjects

Adam L. Friedman

Publications and source records attributed to Adam L. Friedman.

At least 19 recordsLinked to original sources

arXiv2026

Demonstrating magnetic memory in iron-rhodium structures using a quantum diamond microscope

Iron-rhodium (FeRh) has a first-order phase transition near room temperature between antiferromagnetic (AFM) and ferromagnetic (FM) phases, making it a promising material for magnetic memory technologies like heat-assisted magnetic recording (HAMR). It has a comparatively sharper phase transition and lower writing temperature than alternative materials, implying less thermal engineering constraints and an increase in write/read head lifetime. Despite great effort, however, AFM-based magnetic memory using FeRh has not yet been realized. Here, we employ both wide-field and scanning nanoscale quantum diamond microscopes (QDMs) to image directly the magnetic field of a patterned FeRh thin film structure under ambient conditions, demonstrating a magnetic recording technique that is reliable and robust. We experimentally identify coupling between the N\'eel and magnetization vector directions; and also, that the magnetic orientation of the FM phase uniquely determines the N\'eel vector in the AFM phase, due to pinned uncompensated magnetic moments (UMMs) in the FeRh structure. Thus, the magnetic orientation is maintained when the system is cycled between AFM and FM phases, providing the foundation for a practical, AFM-based magnetic memory.

cond-mat.mtrl-sci↗

arXiv2026

Evidence of a Hybridized Topological State in Weyl Semimetal/Topological Insulator Mn$_{3+x}$Sn$_{1-x}$/Bi$_{0.85}$Sb$_{0.15}$ Heterostructures

We report magnetotransport evidence of a hybridized Weyl semimetal (WSM) Fermi arc/topological insulator (TI) surface state at the interface of a ferromagnetic Mn$_{3+x}$Sn$_{1-x}$/Bi$_{0.85}$Sb$_{0.15}$ heterostructure. High target utilization sputtering (HiTUS) was used to grow polycrystalline Mn$_{3+x}$Sn$_{1-x}$ films and Mn$_{3+x}$Sn$_{1-x}$/Bi$_{0.85}$Sb$_{0.15}$ heterostructures on thermally oxidized Si/SiO$_2$ (100) substrates that exhibit the negative coefficient anomalous Hall effect (AHE) resulting from topological Weyl node transport. When various defects and impurities are introduced into these Mn$_{3+x}$Sn$_{1-x}$ films, a ferromagnetic (FM) phase develops that practically eliminates the topological Weyl node conduction. These FM Mn$_{3+x}$Sn$_{1-x}$ films exhibit large exchange bias effects below T=200 K that we attribute to the coexistence of a FM phase and the triangular antiferromagnetic (AFM) WSM phase. When Bi$_{0.85}$Sb$_{0.15}$ overlayers are grown on the FM Mn$_{3+x}$Sn$_{1-x}$, the magnetotransport signal of Weyl node topological transport is restored, an effect we do not observe when replacing the Bi$_{0.85}$Sb$_{0.15}$ TI with heavy metal overlayers. We attribute the restoration of the Weyl node topological transport to the formation of a hybridized topological state at the WSM/TI interface.

cond-mat.mtrl-sci↗

arXiv2026

Transport Enhancement and In Situ Control of Electronic Correlation via Photoinduced Modulation Doping of van der Waals Heterostructures

Modulation doping, a well-established technique for traditional semiconductor heterostructures, is a promising approach for tailoring carrier concentration in 2D materials devices. In this letter we report on photoinduced modulation doping in hBN-graphene-hBN-SiO2 heterostructures utilizing standard white light sources and no additional fabrication complexity. We establish the use of this technique to both dope the channel material and to photoanneal devices, providing control over electronic doping and disorder in the graphene channel. We analyze the transport properties by employing Drude and Landauer transport models, highlighting the ability to reversibly tune the mobility and mean scattering length of the graphene with a high degree of accuracy. This tunability allows us to switch our device between the diffusive and quasi-ballistic transport regimes in situ. We utilize the exceptional control our technique provides over local disorder to realize quantum Hall isospin ferromagnetic states in a device whose initial quality would otherwise leave such states unobservable. These results demonstrate precise manipulation of carrier density and charge disorder in van der Waals heterostructures, providing a highly accessible approach to creating high-quality devices capable of realizing correlated electronic states.

cond-mat.mes-hall↗

arXiv2026

Spatially-resolved voltage-reversal due to Bernoulli potentials in dissipative Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$

We measure magneto-transport and critical currents in Bi$_2$Sr$_2$CaCu$_2$O$_{8+x}$ Hall bar devices. Above critical current in an applied magnetic field, we observe longitudinal differential voltage along one edge comparable in magnitude but opposite in sign to the other edge. This phenomenon is unaffected by reversal of the applied field, and seems unique to devices with invasive voltage contacts. We attribute the source of this behavior to particle-hole symmetry breaking in moving vortices and the formation of opposite Bernoulli potentials due to opposing vortex velocities at the edges where the invasive contacts create hotspots for rapid vortex nucleation and flux flow. These results are fundamental to the composition and flow of dissipative currents in layered superconductors.

cond-mat.supr-con↗

arXiv2026

Magnetic Field-Mediated Superconducting Logic

While superconductors are highly attractive for energy-efficient computing, fundamental limitations in their logic circuit integration have hindered scaling and led to increased energy consumption. We therefore propose and experimentally demonstrate a novel superconducting switching device utilizing the proximity magnetization from a spin-orbit torque-switched magnet to control the resistivity of a superconductor. We further propose a complete logic family comprised solely of these devices. This novel implementation has the potential to drastically outperform existing superconducting logic families in terms of energy efficiency and scalability.

cs.ET↗

arXiv2025

Direct Comparison of Static and Dynamic Measurements of Spin Generation in a Topological Insulator Thin Film

The competition between intrinsic spin-orbit physics, magnetic phenomena, and the quality of materials and interfaces governs the charge-to-spin conversion processes that are essential to the implementation of spintronic devices. Direct comparisons of spin parameters, which serve as metrics of device quality, obtained by different measurement techniques are scarce, leading to uncertainty regarding discrepancies and the reliability of the methods. Here, we directly compare the spin Hall coefficient (${\theta}_{SH}$) in molecular beam epitaxy grown films of $(Bi_{1-x}Sb_{x})_{2}Te_{3-y}Se_{y}$ (BSTS, x = 0.58, y = 1) at room temperature using two complementary techniques: a static method using non-local voltage (NLV) measurements in BSTS Hall bars with DC charge current, and a dynamic method using spin-torque ferromagnetic resonance (ST-FMR) measurement in $BSTS/Ni_{80}Fe_{20}$ heterostructures at GHz frequencies. We obtain comparable spin Hall coefficients in angular-dependent ST-FMR (${\theta}_{SH}$=$4.7\pm1.1$) and in NLV (${\theta}_{SH}$=$2.8\pm0.6$). The complex effects of ferromagnetic interfaces while determining spin Hall coefficients using static or dynamic techniques becomes evident by contrasting our results to literature.

cond-mat.mtrl-sci↗

arXiv2025

Probing Magnetic Properties of RuO$_{2}$ Heterostructures Through the Ferromagnetic Layer

RuO$_{2}$ has been proposed as the prototypical altermagnetic material. However, several reports have recently questioned its intrinsic magnetic ordering, leading to conflicting findings, especially in thin film heterostructures pointing to possible interface effects being convoluted with supposed antiferromagnetic/altermagnetic signatures. Here, extensive magnetometry measurements were performed on two independently grown thin film heterostructures of RuO$_{2}$ interfaced with either NiFe or Fe acting as the ferromagnetic layer. Below about 15 K, both samples exhibit exchange bias fields when cooled to approximately 2 K in a $+$1 T field, and a spin transitional feature is observed around 31 K. Magneto-Raman measurements on RuO$_{2}$ thin films only reveal a magnon mode when there is a NiFe layer, suggesting that RuO$_{2}$ does not intrinsically possess long range magnetic ordering.. When in contact with a ferromagnet, RuO$_2$ displays effects that could be ascribed to antiferromagnetism. However, the lack of intrinsic magnon modes points toward possible diffusion between the layers or spin disorder at the interface as seen by density functional theory (DFT) calculations.

cond-mat.mtrl-sci↗

arXiv2025

Assembly of High-Performance van der Waals Devices Using Commercial Polyvinyl Chloride Films

Control over the position, orientation, and stacking order of two-dimensional (2D) materials within van der Waals heterostructures is crucial for applications in electronics, spintronics, optics, and sensing. The most popular strategy for assembling 2D materials uses purpose-built stamps with working surfaces made from one of several different polymers. However, these stamps typically require tedious preparation steps and suffer from poor durability, contamination, and limited applicability to specific 2D materials or surfaces. Here, we demonstrate significant improvements upon current 2D flake transfer and assembly practices by using mechanically durable stamps made from polyvinyl chloride (PVC) thin films. These stamps are simpler to prepare compared with existing methods and can withstand multiple transfer cycles, enabling greater reusability. We use two commercially available PVC films with distinct pick-up and release temperatures. Together, these films also enable polymer-to-polymer flake transfers and stack-and-flip fabrication of inverted heterostructures in one seamless process. Systematic comparisons of cleaning processes confirm the removal of PVC-derived residue from the assembled structures to create atomically clean interfaces. We demonstrate the utility and versatility of these polymer films and transfer process by fabricating graphene/hexagonal boron nitride heterostructure devices with high-performance electrical characteristics. Further, we demonstrate the ability to pick up and to deposit bulk aluminum gallium arsenide nanostructured films, enabling the creation of heterogeneously integrated devices. This technique increases fabrication rates, improves device quality, and enables more complex structures, thereby facilitating nanomaterial assembly in a broad range of applications.

cond-mat.mtrl-sci↗

arXiv2025

Differentiating anomalous and topological Hall effects using first-order reversal curve measurements

Next generation magnetic memories rely on novel magnetic phases for information storage. Novel spin textures such as skyrmions provide one possible avenue forward due to their topological protection and controllability via electric fields. However, the common signature of these spin textures, the topological Hall effect (THE), can be mimicked by other trivial effects. Competing anomalous Hall effect (AHE) components can produce a peak in the Hall voltage similar to that of the THE, making clear identification of the THE difficult. By applying the first-order reversal curve (FORC) technique to the Hall effect in candidate topological Hall systems we can clearly distinguish between the THE and AHE. This technique allows for quantitative investigation of the THE and AHE in magnetic materials and heterostructures with topologically non-trivial spin textures. We demonstrate the technique and apply it to several examples.

cond-mat.mes-hall↗

arXiv2025

Intrinsic Spin Transport in a Topological Insulator Thin Film

Topological insulators (TIs) are intriguing materials for advanced computing applications based on spintronics because they can host robust spin effects. For instance, TIs have intrinsically large spin generation enabled by their large spin-orbit coupling. Furthermore, topological surface states (TSS) with spin-momentum locking and Dirac dispersion lead to long spin diffusion. Future spintronic device technology will require scalable film growth of high-quality material. We grow epitaxial films of Bi$_{1-x}$Sb$_x$Te$_{3-y}$Se$_y$ (BSTS, $x = 0.58, y = 1$) and confirm the gapless band structure with optimal doping using angle-resolved photoelectron spectra. The temperature dependence of longitudinal resistivity shows bulk transport is suppressed as temperature is decreased, and at low temperature surface transport dominates. We evaluate the spin transport properties in BSTS without using ferromagnetic tunnel contacts via a non-local resistance experiment as a function of temperature and applied charge current. As expected, these experiments reveal the necessity of decreasing the bulk conduction to best enhance the spin transport. In the TSS, we find high efficiency of charge-to-spin conversion (spin Hall angle, $\theta_{SH} \approx 1$) and spin diffusion over several microns. Further development of high-quality TIs will make them viable candidates for efficient and lossless spintronics.

cond-mat.mes-hall↗

arXiv2025

Direct-Write Printed Contacts to Layered and 2D Materials

Advancements in fabrication methods have shaped new computing device technologies. Among these methods, depositing electrical contacts to the channel material is fundamental to device characterization. Novel layered and two-dimensional (2D) materials are promising for next-generation computing electronic channel materials. Direct-write printing of conductive inks is introduced as a surprisingly effective, significantly faster, and cleaner method to contact different classes of layered materials, including graphene (semi-metal), MoS2 (semiconductor), Bi-2212 (superconductor), and Fe5GeTe2 (metallic ferromagnet). Based on the electrical response, the quality of the printed contacts is comparable to what is achievable with resist-based lithography techniques. These devices are tested by sweeping gate voltage, temperature, and magnetic field to show that the materials remain pristine post-processing. This work demonstrates that direct-write printing is an agile method for prototyping and characterizing the electrical properties of novel layered materials.

cs.ET↗

arXiv2025

Charge to spin conversion in atomically thin bismuth

We report charge to spin conversion in a hybrid heterostructure comprised of atomically thin bismuth (Bi) confined between a silicon carbide (SiC) substrate and epitaxial graphene (EG). We confirm composition, dimensionality, and a 96.5 \% intercalation coverage using X-ray photolectron spectroscopy, scanning transmission microscopy, low energy electron diffraction, and Raman spectroscopy. Electrical transport measurements show signs of weak antilocalization in the heterostructure, consistent with spin-orbit coupling in this hybrid heterostructure. Spin torque ferromagnetic resonance measurements in permalloy/EG/2D-Bi heterostructures probe charge-to-spin conversion and revealing that an in plane polarization of the spin current, perpendicular to the charge current. The ratio of the in-plane to out-of-plane torque is 3.75 times higher than in hydrogenated graphene control samples.

cond-mat.mes-hall↗

arXiv2024

Quantum Monte Carlo and density functional theory study of strain and magnetism in 2D 1T-VSe$_2$ with charge density wave states

Two-dimensional (2D) 1T-VSe$_2$ has prompted significant interest due to the discrepancies regarding alleged ferromagnetism (FM) at room temperature, charge density wave (CDW) states and the interplay between the two. We employed a combined Diffusion Monte Carlo (DMC) and density functional theory (DFT) approach to accurately investigate the magnetic properties, CDW states, and their response to strain in monolayer 1T-VSe$_2$. Our calculations show the delicate competition between various phases, revealing critical insights into the relationship between their energetic and structural properties. We performed classical Monte Carlo simulations informed by our DMC and DFT results, and found the magnetic transition temperature ($T_c$) of the undistorted (non-CDW) FM phase to be 228 K and the distorted (CDW) phase to be 68 K. Additionally, we studied the response of biaxial strain on the energetic stability and magnetic properties of various phases of 2D 1T-VSe$_2$ and found that small amounts of strain can increase the $T_c$, suggesting a promising route for engineering and enhancing magnetic behavior. Finally, we synthesized 1T-VSe$_2$ and performed Raman spectroscopy measurements, which were in close agreement with our calculated results, validating our computational approach. Our work emphasizes the role of highly accurate DMC methods in advancing the understanding of monolayer 1T-VSe$_2$ and provides a robust framework for future studies of 2D magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

Quantum Hall Transport Measurements of Lateral p-n Junctions Formed via Precise Spatial Photodoping of Graphene/hBN Heterostructures

Doped semiconductors are a central and crucial component of all integrated circuits. By using a combination of white light and a focused laser beam, and exploiting hBN defect states, heterostructures of hBN/Graphene/hBN are photodoped in-operando, reproducibly and reversibly. We demonstrate device geometries with spatially-defined doping type and magnitude. After each optical doping procedure, magnetotransport measurements including quantum Hall measurements are performed to characterize the device performance. In the unipolar (p+-p-p+ and n-n+-n) configurations, we observe quantization of the longitudinal resistance, proving well-defined doped regions and interfaces that are further analyzed by Landauer-Buttiker modeling. Our unique measurements and modeling of these optically doped devices reveal a complete separation of the p- and n-Landau level edge states. The non-interaction of the edge states results in an observed "insulating" state in devices with a bi-polar p-n-p configuration that is uncommon and has not been measured previously in graphene devices. This insulating state could be utilized in high-performance graphene electrical switches. These quantitative magnetotransport measurements confirm that these doping techniques can be applied to any 2D materials encapsulated within hBN layers, enabling versatile, rewritable circuit elements for future computing and memory applications.

cond-mat.mes-hall↗

arXiv2023

Spintronic Quantum Phase Transition in a $Graphene/Pb_{0.24}Sn_{0.76}Te$ Heterostructure with Giant Rashba Spin-Orbit Coupling

Mechanical stacking of two dissimilar materials often has surprising consequences for heterostructure behavior. In particular, a two-dimensional electron gas (2DEG) is formed in the heterostructure of the topological crystalline insulator Pb0.24Sn0.76Te and graphene due to contact of a polar with a nonpolar surface and the resulting changes in electronic structure needed to avoid polar catastrophe. We study the spintronic properties of this heterostructure with non-local spin valve devices. We observe spin-momentum locking at lower temperatures that transitions to regular spin channel transport only at ~40 K. Hanle spin precession measurements show a spin relaxation time as high as 2.18 ns. Density functional theory calculations confirm that the spin-momentum locking is due to a giant Rashba effect in the material and that the phase transition is a Lifshitz transition. The theoretically predicted Lifshitz transition is further evident in the phase transition-like behavior in the Land\'e g-factor and spin relaxation time.

cond-mat.mes-hall↗

arXiv2022

High-speed metamagnetic resistive switching of FeRh through Joule heating

Due to its proximity to room temperature and demonstrated high degree of temperature tunability, the metamagnetic ordering transition in FeRh is attractive for novel high-performance computing devices seeking to use magnetism as the state variable. We demonstrate electrical control of the transition via Joule heating in FeRh wires. Finite element simulations based on abrupt state transition within each domain result in a globally smooth transition that agrees with the experimental findings and provides insight into the thermodynamics involved. We measure a 150 K decrease in transition temperature with currents up to 60 mA, limited only by the dimensions of the device. The sizeable shift in transition temperature scales with current density and wire length, suggesting the absolute resistance and heat dissipation of the substrate are also important. The FeRh phase change is evaluated by pulsed I-V using a variety of bias conditions. We demonstrate high speed (~ ns) memristor-like behavior and report device performance parameters such as switching speed and power consumption that compare favorably with state-of-the-art phase change memristive technologies.

cond-mat.mes-hall↗

arXiv2022

Magnetotransport in graphene/Pb0.24Sn0.76Te heterostructures: finding a way to avoid catastrophe

While heterostructures are ubiquitous tools enabling new physics and device functionalities, the palette of available materials has never been richer. Combinations of two emerging material classes, two-dimensional materials and topological materials, are particularly promising because of the wide range of possible permutations that are easily accessible. Individually, both graphene and Pb0.24Sn0.76Te (PST) are widely investigated for spintronic applications because graphene's high carrier mobility and PST's topologically protected surface states are attractive platforms for spin transport. Here, we combine monolayer graphene with PST and demonstrate a hybrid system with properties enhanced relative to the constituent parts. Using magnetotransport measurements, we find carrier mobilities up to 20,000 cm2/Vs and a magnetoresistance approaching 100 percent, greater than either material prior to stacking. We also establish that there are two distinct transport channels and determine a lower bound on the spin relaxation time of 4.5 ps. The results can be explained using the polar catastrophe model, whereby a high mobility interface state results from a reconfiguration of charge due to a polar/non-polar interface interaction. Our results suggest that proximity induced interface states with hybrid properties can be added to the still growing list of remarkable behaviors in these novel materials.

cond-mat.mtrl-sci↗

arXiv2022

Large-Area Intercalated 2D-Pb/Graphene Heterostructure as a Platform for Generating Spin-Orbit Torque

A scalable platform to synthesize ultrathin heavy metals may enable high efficiency charge-to-spin conversion for next-generation spintronics. Here we report the synthesis of air-stable, epitaxially registered monolayer Pb underneath bilayer graphene on SiC (0001) by confinement heteroepitaxy (CHet). Diffraction, spectroscopy, and microscopy reveal CHet-based Pb intercalation predominantly exhibits a mottled hexagonal superstructure due to an ordered network of Frenkel-Kontorova-like domain walls. The system's air stability enables ex-situ spin torque ferromagnetic resonance (ST-FMR) measurements that demonstrate charge-to-spin conversion in graphene/Pb/ferromagnet heterostructures with a 1.5x increase in the effective field ratio compared to control samples.

cond-mat.mtrl-sci↗