Bulletin of the American Physical Society
APS March Meeting 2023
Volume 68, Number 3
Las Vegas, Nevada (March 5-10)
Virtual (March 20-22); Time Zone: Pacific Time
Session M57: Nanomagnetic Systems: Artificial Spin Ice and Exchange BiasFocus
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Sponsoring Units: GMAG DMP Chair: Vinay Sharma, Morgan State University Room: Room 303 |
Wednesday, March 8, 2023 8:00AM - 8:36AM |
M57.00001: Quantum mechanical characteristic of magnetic charges in artificial magnetic honeycomb lattice Invited Speaker: Deepak K Singh The intriguing physics of magnetic honeycomb lattice has drawn lot of attention in both the bulk and the two-dimensional nanostructured specimens. The artificially engineered honeycomb system manifests the two-dimensional projection of spin ice where magnetic charges are confined to the vertices due to magnetic Coulomb’s interaction. It provides unique opportunity to unravel both the classical and the quantum magnetic properties in reduced dimensionality. While the macroscopic spin of individual element renders the occurrence of entropy driven classical phenomenology, the Pauli matrix representation of magnetic charge quasi-particle enables the exploration of emergent quantum mechanical ground state properties. Quantum disordered state of magnetic charges is one such example. In addition to the inherent geometrical frustration, the competing nature of exchange interactions (J1, J2 terms) in thermally tunable artificial magnetic honeycomb lattice of single domain size elements provides a disorder-free environment to the exploration of quantum disorderness at low temperature. Our detailed neutron-centric research works have revealed massively degenerate dynamic ground state of magnetic charges in permalloy (Ni0.81Fe0.19) honeycomb that remain unperturbed to magnetic field application. The charges relax by emitting or absorbing a net magnetic charge defect or magnetic monopole between the vertices. In an important finding, for the first time we have not only quantitatively determined the magnetic charge relaxation rate, ~ 20 ps, in an artificial spin ice but have also demonstrated the intrinsic quantum mechanical nature of magnetic charge dynamics, thus establishing its quasi-particle characteristic. Besides the fundamental importance, the quasi-particle has direct implication to the spintronics research. We have discovered that charge relaxation propels electrical conduction via indirect interaction with electric carriers in the recently reported magnetic diode effect in permalloy honeycomb lattice. It elucidates a practical aspect of magnetic charge physics in the design of next generation spintronic devices. |
Wednesday, March 8, 2023 8:36AM - 8:48AM |
M57.00002: Linking magnetic memory and computational capacity in artificial spin ice Alex Vanstone, Jack C Gartside, Kilian Stenning, Will R Branford Artificial spin ice (ASI) is a metamaterial composed of nanomagnetic islands that are used as functional platforms for neuromorphic computing. The global field amplitude during minor field loops has been used to input information to the microstate of ASI and has been used to demonstrate reservoir computing in both simulation and experiment [1,2]. The memory effects emergent in ASI are crucial to understanding the computational capacity. In previous studies, ASI has been shown to exhibit return point memory (RPM) after several ‘training’ field loops [3]. We explore, using dipolar simulations, the temporal harmonics and subharmonics that emerge from these field loops as we vary the interaction strength of the system and the quenched disorder. Linking these physical memory effects to the ASI’s computational potential via the memory capacity and nonlinearity. |
Wednesday, March 8, 2023 8:48AM - 9:00AM |
M57.00003: Field and temperature tuning of magnetic diode in permalloy honeycomb lattice George Yumnam, Moudip Nandi, Pousali Ghosh, Amjed Abdullah, mahmoud almasri, Eric Henriksen, Deepak K Singh We report the observation of magnetic diode behavior with an ultra-low forward voltage of 5 mV. This renders a new venue for energetically efficient spintronic device research in the unconventional system of the two-dimensional permalloy honeycomb lattice. A detailed understanding of temperature and magnetic field tuning of diode behavior is imperative to any practical application. Here, we performed a comprehensive study by performing electrical measurements on magnetic diode samples as functions of temperature and magnetic field. The magnetic diode is found to persist across a broad temperature range and the application of an external magnetic field unveils a peculiar reentrant characteristic where diode behavior is suppressed in remnant field but reappears after warming to room temperature. Analysis of the current-voltage data suggests a modest energy gap, ~0.03 - 0.1 eV, comparable to magnetic Coulomb's interaction energy between emergent magnetic charges on honeycomb vertices in the reverse biased state. Our observations reaffirm the role of magnetic charge correlation in unidirectional conduction in 2D honeycomb lattices. These experimental results are expected to pave the way for using magnetic diodes in next-generation spintronic device applications. |
Wednesday, March 8, 2023 9:00AM - 9:36AM |
M57.00004: Enhanced Magnetoresistance in Nanostructures with a Single Ferromagnet Invited Speaker: Chenghao Shen It is commonly assumed that multiple ferromagnets are required to realize large spin-valve effects. In contrast, we reveal that the interplay between the proximity-induced magnetization and Rashba spin-orbit coupling (SOC) in a two-dimensional electron gas (2DEG), or in a conventional superconductor, can lead to a large magnetoresistance even with one ferromagnet [1-4]. However, such enhanced magnetoresistance in nanostructures is not generic. For an in-plane rotation of magnetization, it is typically negligibly small for 2DEG and depends on the tunneling resonances which emerge from a novel spin-parity-time symmetry of the scattering states, while this symmetry is generally absent from the Hamiltonian of the system [1]. The key difference from considering the normal-state magnetoresistance is the presence of the spin-dependent Andreev reflection at superconducting interfaces. In magnetic nanostructures Rashba SOC can reduce the effective barrier strength for a given helicity [2] and support elusive equal-spin-triplet superconductivity. This mechanism explains an observed huge increase of magnetoresistance in all-epitaxial MgO-based nanostructures with a superconductor [5] as well as in junctions of quasi-2D van der Waals ferromagnets with conventional s-wave superconductors (Fe0.29TaS2/Nb) [3]. Realizing this enhanced magnetoresistance and spin-triplet superconductivity is an important building block for superconducting spintronics [6]. |
Wednesday, March 8, 2023 9:36AM - 9:48AM |
M57.00005: Quantum mechanical nature of magnetic charge dynamics in artificial honeycomb ice Jiasen Guo, Pousali Ghosh, Daniel M Hill, George Yumnam, Yiyao Chen, Laura-Roxana Stingaciu, Piotr A Zolnierczuk, Carsten A Ullrich, Deepak K Singh Magnetic charges, arising due to the non-vanishing magnetic fluxes on Kagome vertices, are at the core of emergent novel phenomena in artificial magnetic honeycomb lattices. We have unveiled the quantum mechanical nature of magnetic charge dynamics in honeycomb ice via neutron spin echo measurements on an artificial magnetic honeycomb lattice of permalloy elements of nanometer size. It is found that the magnetic charge dynamics in honeycomb ice is self-propelled and insensitive to the thickness of the honeycomb in the range studied. The quantized charge dynamics is manifested in both time and space. Magnetic charges relax at around 20 ps time scale, comparable to Dirac's monopole's dynamics in atomistic spin ice. Most importantly, magnetic charge dynamics prevails at low temperature, confirming its temporal quantum characteristic. In addition, magnetic charge relaxations localize at distinct wave-vectors corresponding to the integral multiples of honeycomb element length, suggesting its spatial quantization. The observed quantum nature of magnetic charge dynamics is supplemented by first principal quantum mechanical calculation. Finally, we will discuss the implication of our findings in broader perspective in artificial spin ice research. |
Wednesday, March 8, 2023 9:48AM - 10:00AM |
M57.00006: Antiferromagnetic real-space configuration probed by x-ray orbital angular momentum phase dichroism Margaret R McCarter, Ahmad Us Saleheen, ARNAB SINGH, Ryan Tumbleson, Justin S Woods, Anton S Tremsin, Andreas Scholl, Lance E De Long, Jeffrey T Hastings, Sophie A Morley, Sujoy Roy X-ray beams with orbital angular momentum (OAM) are a promising tool for x-ray characterization techniques. Beams with OAM have an azimuthally varying phase, and new material properties can potentially be probed by utilizing this unique phase structure. Here, we show how OAM beams are created through resonant diffraction from an artificial antiferromagnet with a topological defect. The scattered OAM beams have circular dichroism whose sign is coupled to the phase of the beam [1]. Using magnetic scattering calculations, we show that this dichroism is related to the real-space configuration of the antiferromagnetic ground state. Thermal cycling of the artificial antiferromagnet can change the ground state, as indicated by the changing phase dichroism. These results exemplify the potential of OAM beams to probe matter in a way that is inaccessible using currently available x-ray techniques. |
Wednesday, March 8, 2023 10:00AM - 10:12AM |
M57.00007: The nature of super exchange interactions in transition metal-doped ε-Fe2O3 nanoparticles Rachel Nickel, Michael Shepit, Chengjun Sun, Debora Motta Meira, Richard Rosenberg, Johan Van Lierop ε-Fe2O3 is the least common and understood iron oxide polymorph, isomorphic to GaFeO3 and κ-Al2O3. ε-Fe2O3 has attracted significant interest recently for its unique properties, including a huge coercivity at room temperature, millimeter wave ferromagnetic resonance and magnetoelectric coupling. This research examines transition metal-doped ε-Fe2O3 (M=Cr, Mn, Co, Ni, Cu, Zn) nanoparticles to identify the electronic configurations of the different ion sites and how they impact the overall properties. X-ray diffraction reveal the samples have the expected orthorhombic structure while EDS and ICP-OES measurements confirm successful doping. Temperature dependent susceptibility and hysteresis measurements show the characteristic spin reorientation transition (SR) at 150 K, where the system transforms from an incommensurate state to a high-coercivity canted antiferromagnet. While this SR is preserved in all samples, the type and concentration of dopant ion does alter the magnetism. 57Fe Mössbauer spectroscopy and extended x-ray absorption fine structure analysis quantify the exchange interactions, revealing changes to the super transferred hyperfine interactions and charge ordering. |
Wednesday, March 8, 2023 10:12AM - 10:24AM |
M57.00008: Imaging voltage controlled antiferromagnetic domains’ switching in B doped Cr2O3 epitaxial films using nitrogen-vacancy scanning microscopy Abdelghani Laraoui, Adam D Erickson, Ather Mahmood, Syed Qamar Abbas Shah, Ilja Fescenko, Rupak Timalsina, Christian Binek Antiferromagnetic (AFM) magnetoelectric chromia (Cr2O3) allows voltage-control of the Néel vector in the presence of an applied magnetic field (H) [1]. Boron doping further allows the realization of voltage controlled Néel vector switching at zero H [2], a promising finding to AFM spintronics. B-doping is also believed to break the local symmetry allowing for the formation of polar nanoregions giving rise to transient polarization, and to in-plane 90° rotation of the Néel vector into a new stable state (vs 1800 in undoped film) [2]. However, it is not clear how the voltage reversibly switches the Néel vector from in-plane to out-of-plane. In this study we use nitrogen vacancy (NV) scanning microscopy [3, 4] to image the surface/boundary magnetization in 200-nm thick B-Cr2O3 films grown by pulsed laser deposition on Al2O3 substrates. The acquired BNV images confirm the presence of homogeneously magnetized domains with domain sizes ∼40-500 nm. We discuss the effect of voltage applied along micro-structured Pt Hall bars deposited on B-Cr2O3/V2O3 structures on the switching of AFM domains. [1] N. Wu et al., Phys. Rev. Lett. 106, 087202 (2011). [2] A. Mahmood et al. Nat. Comm. 12, 1674 (2021). [3] P. Appel et al., Nano Lett. 19, 1682 (2019). [4] A. Erickson et al., RSC Advances, Under review (2022). |
Wednesday, March 8, 2023 10:24AM - 10:36AM |
M57.00009: Control of exchange bias using spin-orbit torque through an antiferromagnetic insulator IVAN K SCHULLER, Pavel Salev, ALI C BASARAN, Fernando Ajejas Enabling electrical control of antiferromagnetic (AFM) properties is one of the key requirements for the development of novel spintronic technologies. We present a device concept in which the spin configuration of an AFM insulator (FeF2) can be modified taking advantage of the spin-orbit-coupling existing in heavy metals (HM), such as W or Pt. We consider a trilayer, HM|AFM|FM, where the top ferromagnetic (FM) layer is used to monitor the changes in the AFM spin configuration. We performed magneto-optical Kerr effect measurements to probe the FM hysteresis loops as a function of temperature (T) and applied current (I). We found that the exchange bias (EB) and coercivity (Hc) produced at the top AFM|FM interface can be strongly modified by the I passing at bottom HM layer. We attribute this effect to an active spin-orbit torque generated at the HM|AFM interface. that reaches the AFM|FM top interface modifying amplitude and sign of EB. We found a critical current beyond which the effect on the EB and Hc is irreversible. Temperature-dependent control experiments using normal metals (NM) such as Au in NM|AFM|FM and without AFM in HM|FM confirmed that the effect is produced by the SOT induced by the HM and is not caused by thermal heating, Oersted field or other potentially spurious effects. |
Wednesday, March 8, 2023 10:36AM - 10:48AM |
M57.00010: Large exchange-bias in magneto-ionic Mn4N/Mn3N2 heterostructures Zhijie Chen, Christopher J Jensen, Chen Liu, YIJING LIU, Paola Barbara, Xixiang Zhang, Kai Liu Nitrogen based magneto-ionics offers an alternative to that in the more traditional oxide-based systems [1-3]. Of particular interest is the manganese nitride ferrimagnet Mn4N as an emergent rare-earth-free spintronic material due to its uniaxial anisotropy, small saturation magnetization (Ms), and high thermal stability. We have achieved high quality all-nitride Mn4N/Mn3N2 thin film heterostructures via sputtering onto Si substrates. X-ray diffraction confirms the Mn3N2 and Mn4N phase with (010) and (001) out-of-plane orientation, respectively. Magnetometry shows that the 20nm-Mn4N layer exhibits a Ms of over 120 emu/cm3 at room temperature, the largest so far in Mn4N films grown by sputter deposition. Hall measurements reveal a very large exchange bias field of up to 6 kOe at 5K after field cooling, resulted from nitrogen migration across the interface between the ferrimagnetic Mn4N and the antiferromagnetic Mn3N2 layer. The exchange bias effect is further correlated with sample microstructures, studied by transmission electron microscopy. These results demonstrate a promising new platform for spintronic applications via nitrogen-based magneto-ionics. |
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