arXiv subjects

Raghvendra Posti

Publications and source records attributed to Raghvendra Posti.

8 recordsLinked to original sources

arXiv2026

Symmetry Breaking by Interfacial Dead Layers: Observation of Forbidden Self-Induced Spin-Orbit Torque in Symmetric Ferromagnets

Conventionally, spin-orbit torques (SOTs) in ferromagnets require heavy-metal layers or engineered structural asymmetry to break inversion symmetry. In this work, we report the observation of robust, self-generated SOTs in a nominally symmetric, heavy-metal-free MgO/NiFe/MgO trilayer - a geometry where such torques are theoretically forbidden. By combining harmonic Hall measurements with SQUID magnetometry and X-ray photoelectron spectroscopy, we identify the symmetry-breaking origin: a 1.8 nm magnetic dead layer at the bottom interface. Crucially, we demonstrate a quantitative agreement between our data and the drift-diffusion theory predicted by Kim and Lee, yielding a theoretically extracted dead-layer thickness (1.2 nm) which matches structural characterization. Furthermore, density-functional calculations confirm that NiFe possesses sufficient intrinsic spin Hall conductivity to support the observed spin currents. These results reframe the parasitic dead layer as a functional spintronic component, establishing a universal, all-ferromagnetic route to SOTs in standard magnetic heterostructures.

cond-mat.mes-hall↗

arXiv2026

Element-Specific Visualization of Layer-Parity and Twist-Dependent Magnetism in CrSBr

Van der Waals (vdW) based antiferromagnets (AFMs) are an ideal platform for probing and understanding thickness- and twist-angle-dependent emergent spin phenomena. However, element-specific nanoscale characterization of the spin structure in atomically thin vdW-based AFMs systems and layer-parity effects remain elusive, making them crucial for both fundamental insight into low-dimensional magnetism and the rational design of spintronic devices based on these materials. Here, we utilize X-ray magnetic circular and linear dichroisms paired with photoemission electron microscopy to resolve the magnetic order in atomically thin CrSBr. Our comprehensive measurements reveal CrSBr magnetic structure at the nanoscale and its dependence on the layer number, surface encapsulation, temperature, and applied field. Moreover, in the orthogonally twisted bilayer configuration, obtained by twisting two CrSBr ferromagnetic monolayers by 90$^\circ$, the magnetic easy axis fundamentally differs from the individual monolayers, unlocking a new pathway for moir\'e magnetism.

cond-mat.mes-hall↗

arXiv2026

Enabling Electrical Readout of N\'eel vector reversal in a van der Waals Antiferromagnet

Owing to its robustness against external perturbations and intrinsically ultrafast dynamics, the N\'eel vector in antiferromagnets (AFMs) can enable the development of next-generation spintronic and magnonic devices for memory and computing applications. To realize AFM-based magnetic memory devices, one of the key requirements is to demonstrate electrical readout of 180-degree reversal of N\'eel vector in thin film AFMs, which remains critically missing. In this work, we report experimental demonstration of a novel transport methodology to detect N\'eel vector reversal in atomically thin films of a van der Waals (vdW) based A-type AFM. For this, we utilize spin-dependent electronic band properties of CrSBr by coupling it to a spin-polarized layer, separated by a tunnel barrier. In this configuration, the spin-dependent tunnelling magnetoresistance (MR) becomes sensitive to the relative orientation between the magnetization of the reference electrode and the interfacial sublattice magnetization of the AFM layer, in turn enabling electrical detection of the N\'eel vector orientation. Importantly, the observed MR can also reveal 180-degree reversal of N\'eel vector in even-layers of CrSBr, wherein adjacent sublattice magnetic layers are exactly compensated and the net magnetization vanishes and thus establishes a broadly applicable strategy for electrical detection of N\'eel vector in vdW-based AFMs.

cond-mat.mes-hall↗

arXiv2024

Reconfigurable Spin Logics and High-density Multistate Memory in a Single Spin-orbit Torque Device

Nonvolatile devices based on the spin-orbit torque (SOT) mechanism are highly suitable for in-memory logic operations. The current objective is to enhance the memory density of memory cells while performing logic operations within the same memory unit. Present study demonstrates that integrating SOT with an out-of-plane magnetic field effectively achieves multiple magnetic states in perpendicularly magnetized heterostructures. This study further explores this approach, experimentally demonstrating reconfigurable logic operations within a single SOT device using W/Pt/Co/AlOxheterostructures. Our results show that multistate tuning by SOT integration with out-of-plane magnetic field enables reconfigurable logic operations, including AND, OR, NOR, NAND, and Always ON, within a single device. Additionally, we found that careful selection of input logic operations allows multiple configurations to achieve the same logic function within a single memory device. To enhance multistate memory density, we proposed and experimentally verified a two-step writing process, achieving the highest reported multistate memory density in SOT-based memory devices. These findings highlight the potential of integrating SOT and magnetic field effects to realize high-density, multifunctional in-memory logic devices.

physics.app-ph↗

arXiv2024

Correlation of Magnetic State Configurations in Nanotubes with FMR spectrum

Magnetic nanotubes have garnered immense attention for their potential in high-density magnetic memory, owing to their stable flux closure configuration and fast, reproducible reversal processes. However, characterizing their magnetic configuration through straightforward methodologies remains a challenge in both scope and detail. Here, we elucidate the magnetic state details using Remanence Field Ferromagnetic Resonance Spectroscopy (RFMR) for arrays of electrodeposited nanotubes. Micromagnetic simulations revealed distinct spin configurations while coming from saturation, including the edge vortex, onion, uniform and curling states, with chirality variations depending on the preparation field direction. Dynamic measurements, coupled with RFMR spectra analysis, unveiled multiple FMR modes corresponding to these spin configurations. The evolution of spin configurations under bias fields were studied, indicating nucleation within the curling state. Observations revealed opposite RFMR spectra, denoting opposite magnetic spin configurations after removing the positive and negative saturating fields when the magnetic field was applied along {theta_H=0} and perpendicular {theta_H= 90} to the nanotube axis. We observed a mixture of the non-uniform curling states with the end vortex state (onion-like curling state) at the end of the nanotubes for the theta_H=0(90) and uniform magnetization states in the middle of the nanotubes for the theta_H=0 configuration. Building on RFMR information, frequency-swept FMR absorption spectra obtained at different bias fields allowed the characterization of magnetization states. This picture was supported by micromagnetic simulations. These findings were further substantiated with First Order Reversal Curve measurements (FORC).

cond-mat.mes-hall↗

arXiv2024

Initialization-Free Multistate Memristor: Synergy of Spin-Orbit Torque and Magnetic Fields

Spin-orbit torque (SOT)-based perpendicularly magnetized memory devices with multistate memory have garnered significant interest due to their applicability in low-power in-memory analog computing. However, current methods are hindered by initialization problems such as prolonged writing duration, and limitations on the number of magnetic states. Consequently, a universal method for achieving multistate in PMA-based stacks remains elusive. Here, we propose a general experimental method for achieving multistate without any initialization step in SOT-driven magnetization switching by integrating an external out-of-plane magnetic field. Motivated by macrospin calculations coupled with micromagnetic simulations, which demonstrate the plausibility of magnetization state changes due to out-of-plane field integration, we experimentally verify multistate behavior in Pt/Co/Pt and W/Pt/Co/AlOx stacks. The occurrence of multistate behavior is attributed to intermediate domain states with N\'eel domain walls. We achieve repeatable 18 multistate configurations with a minimal reduction in retentivity through energy barrier measurements, paving the way for efficient analog computing.

cond-mat.mes-hall↗

arXiv2023

Odd symmetry planar Hall effect: A method of detecting current-induced in-plane magnetization switching

Type-x device attracts considerable interest in the field of spintronics due to its robust spin-orbit torque (SOT) induced magnetization switching, and easy deposition technique. However, universally applicable and straightforward detection of type-X magnetization reversal is still elusive, unlike type-Z switching, which employs DC-based anomalous Hall effect measurement. Here, we, demonstrated that the odd planar Hall signal (O-PHV) exhibits an odd symmetry with the application of an external magnetic field which motivates us to develop a reading mechanism for detecting magnetization switching of in-plane magnetized type-X devices. We verified our DC-based reading mechanism in the Pt/Co/NiFe/Pt stack where a thin Co layer is inserted to create dissimilar interfaces about the NiFe layer. Remarkably, the current-induced in-plane fields are found to be significantly large in Pt/Co/NiFe/Pt stack. Further, we successfully employed the O-PHV method to detect the current-induced magnetization switching. The pure DC nature of the writing and reading mechanism of our proposed type-X detection technique through O-PHV makes it the easiest in-plane magnetization detection technique. Moreover, the high repeatability and easy detection of our proposed method will open new avenues toward in-plane SOT switching based memory devices and sensors.

cond-mat.mes-hall↗

arXiv2022

Detection of field-free magnetization switching through thermoelectric effect in Ta/Pt/Co/Pt with significant spin-orbit torque and competing spin currents

Application of sufficient lateral current to a heavy metal (HM) can switch the perpendicular magnetization orientation of adjacent ferromagnetic layer (FM) through spin-orbit torques (SOTs). The choice of the HM and its arrangement plays a major role for the SOT induced magnetization switching in magnetic heterostructures. Here, in asymmetric Pt/Co/Pt heterostructures, anti-damping (AD) SOT prevails. Ta addition to this stack (Ta/Pt/Co/Pt) give rise to several compelling effects viz. competing spin currents (due to opposite spin-Hall angles of adjacent Ta and Pt layers), significant AD-SOT, thermoelectric effects (particularly, anomalous Nernst effect (ANE)), and enhanced perpendicular magnetic anisotropy. For this Ta/Pt/Co/Pt stack, the AD-SOT values are stabilized to that of the Pt/Co/Pt stack, which is significant than what is expected for a stack with competing spin currents. Current-induced field-free magnetization switching was absent in uniformly grown Ta/Pt/Co/Pt stack. It was observed that a thickness gradient is essential to assist the field-free magnetization switching in these heterostructures. Further, the thermoelectric effects are utilized to develop a technique to detect the field-free magnetization switching. This technique detects the second harmonic ANE signal as a reading mechanism. Using ANE symmetry with the applied current, the switching can be detected in a single current sweep which was corroborated to the conventional DC Hall method.

cond-mat.mes-hall↗