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Since the initial discovery of 2D van der Waals (vdW) materials, significant effort has been made to incorporate the three properties of magnetism, band structure topology, and strong electron correlations-to leverage emergent quantum phenomena and expand their potential applications. However, the discovery of a single vdW material that intrinsically hosts all three ingredients has remained an outstanding challenge. Here, the discovery of a Kondo-interacting topological antiferromagnet is reported in the vdW 5f electron system UOTe. It has a high antiferromagnetic (AFM) transition temperature of 150 K, with a unique AFM configuration that breaks the combined parity and time reversal (PT) symmetry in an even number of layers while maintaining zero net magnetic moment. This angle-resolved photoemission spectroscopy (ARPES) measurements reveal Dirac bands near the Fermi level, which combined with the theoretical calculations demonstrate UOTe as an AFM Dirac semimetal. Within the AFM order, the presence of the Kondo interaction is observed, as evidenced by the emergence of a 5f flat band near the Fermi level below 100 K and hybridization between the Kondo band and the Dirac band. The density functional theory calculations in its bilayer form predict UOTe as a rare example of a fully-compensated AFM Chern insulator.

Spin waves in magnetic materials are promising information carriers for future computing technologies due to their ultra-low energy dissipation and long coherence length. Antiferromagnets are strong candidate materials due, in part, to their stability to external fields and larger group velocities. Multiferroic antiferromagnets, such as BiFeO3 (BFO), have an additional degree of freedom stemming from magnetoelectric coupling, allowing for control of the magnetic structure, and thus spin waves, with the electric field. Unfortunately, spin-wave propagation in BFO is not well understood due to the complexity of the magnetic structure. In this work, long-range spin transport is explored within an epitaxially engineered, electrically tunable, 1D magnonic crystal. A striking anisotropy is discovered in the spin transport parallel and perpendicular to the 1D crystal axis. Multiscale theory and simulation suggest that this preferential magnon conduction emerges from a combination of a population imbalance in its dispersion, as well as anisotropic structural scattering. This work provides a pathway to electrically reconfigurable magnonic crystals in antiferromagnets.

A family of exchange-coupled core–shell (CS) nanoparticles composed of an antiferromagnetic (AFM) core (Co0.3Fe0.7O) and a ferrimagnetic (FiM) shell (Co0.6Fe2.4O4) was investigated to unravel the role played by the dimension of the two components on the magnetic properties of the system. The series comprises three samples with different core diameters (2, 5, and 16 nm) and fixed shell thickness of ∼2 nm. Although a strong core and shell magnetic coupling occurs in all the samples, the final properties of the hybrid nanosystems are greatly influenced by the size of the two counterparts. Indeed, while the larger sample can be described as a classic TC > TN exchange-bias, where TC and TN denote the ordering temperature of the FiM and AFM phases, respectively, on reducing the size, the blocking transition of the FiM shell decreases to values well below the TN of the AFM. In the first case, the FiM-AFM exchange-bias effect is determined by the magnetic ordering of the AFM core; in the other cases, it is due to the reduction of the thermal-driven magnetic fluctuations of the ordered FiM shell. On the other hand, the AFM properties of the core regions also are extremely sensitive to the particle size reduction, showing, for the smallest sample, the effect of the coupling between the two phases to appear at temperature well below TN displayed by the bulk system, indicating the potential presence of a blocking transition in the AFM core for small particles. These findings highlight the significant influence of the size of the AFM and FiM components on the hybrid system's ultimate properties. This result is potentially relevant for defining the working conditions of nanodevices exploiting exchange-bias phenomena, which have been recently proposed in the literature for application in several technological fields, ranging from rare-earth free magnets, spintronics, optoelectronics, and magnetic-refrigeration. [ABSTRACT FROM AUTHOR]

Antiferromagnets exhibit high-speed spin responses in the terahertz frequency range and robustness against external magnetic fields, making them promising for nextgeneration high-speed, high-density spintronic devices. Recently, two-dimensional van der Waals magnetic systems, which possess rich antiferromagnetic ground states, have gained significant attention and serve as ideal platforms for studying low-dimensional antiferromagnetic physics. Detecting and controlling ultrafast spin dynamics in two-dimensional antiferromagnetic systems will lay the foundation for high-speed spintronic device applications. Antiferromagnets have no net macroscopic magnetization, making traditional optical methods, such as magneto-optical effects, challenging for detecting antiferromagnetic order in equilibrium states. However, in non-equilibrium states, the instantaneous magnetization generated by antiferromagnetic spin dynamics allows the use of time-resolved magneto-optical Kerr effect to detect coherent spin precession in antiferromagnets. Additionally, techniques such as linear dichroism spectroscopy, terahertz emission spectroscopy, and second harmonic generation have been employed in studying the dynamics of two-dimensional antiferromagnets. This paper introduces recent experimental progress on ultrafast spin dynamics in two-dimensional van der Waals antiferromagnetic systems, and briefly describes the coherent magnon excitations and corresponding mechanisms in two-dimensional antiferromagnets, including inverse magneto-optical effects/stimulated Raman scattering, orbital excitations, and exciton coupling. Furthermore, this paper discusses the critical slowing down of spin dynamics due to spin-lattice coupling effects and the amplification of coherent acoustic phonons. [ABSTRACT FROM AUTHOR]

Chromia (Cr2O3) is a magnetoelectric oxide that permits voltage‐control of the antiferromagnetic (AFM) order, but it suffers technological constraints due to its low Néel Temperature (TN ≈307 K) and the need of a symmetry‐breaking applied magnetic field to achieve reversal of the Néel vector. Recently, boron (B) doping of Cr2O3 films led to an increase TN >400 K and allowed the realization of voltage magnetic‐field free controlled Néel vector rotation. Here, the impact of B doping is directly imaged on the formation of AFM domains in Cr2O3 thin films and elucidates the mechanism of voltage‐controlled manipulation of the spin structure using nitrogen‐vacancy (NV) scanning probe magnetometry. A stark reduction and thickness dependence of domain size in B‐doped Cr2O3 (B:Cr2O3) films is found, explained by the increased germ density, likely associated with the B doping. By reconstructing the surface magnetization from the NV stray‐field maps, a qualitative distinction between the undoped and B‐doped Cr2O3 films is found, manifested by the histogram distribution of the AFM ordering, that is, 180° domains for pure films, and 90° domains for B:Cr2O3 films. Additionally, NV imaging of voltage‐controlled B‐doped Cr2O3 devices corroborates the 90° rotation of the AFM domains observed in magnetotransport measurement. [ABSTRACT FROM AUTHOR]

In this study, we suggest a method to amplify spin waves (SWs) in antiferromagnets (AFMs). By introducing a non-uniform Dzyaloshinskii–Moriya (DM) interaction, the potential barrier forms a resonant cavity. SWs with an opposite chirality undergo scattering and are resonantly amplified at a phase-matching condition. The calculation is performed in the insulating AFMs where the electric-field-induced DM interaction and pseudo-dipole anisotropy broaden the parabolic-like SW band for multiple resonant modes. Using a transfer matrix method, we also show numerically that scattering between SWs contributes significantly to the SW amplification. Since the electric field selectively amplifies the SWs with resonant frequencies, the proposed device works as an SW transistor and rectifier. This finding will contribute to insulating AFM-based magnon devices where Joule heating is, in principle, avoided. [ABSTRACT FROM AUTHOR]

2D van der Waals antiferromagnets have emerged as excellent candidates for studying novel low‐dimensional magnetism. The recently discovered spin–orbit‐entangled excitonic bound state appearing below the Néel temperature of 150 K in 2D antiferromagnetic NiPS3 (the so‐called Zhang–Rice (ZR) exciton) has drawn considerable attention in terms of exploring the strong correlation of spins, orbitals, and charges in a localized many‐body state. However, the formation mechanism of ZR excitons remains unclear, and its ultrafast dynamics have yet to be fully explored. Here, utilizing broadband transient reflectivity spectroscopy, the strong correlation between the charge‐transfer state excitation and the dynamic evolution of ZR excitons is investigated. Through systematic tuning of the pumping photon energy across the charge‐transfer state in NiPS3, the results reveal a short radiative lifetime of tens of picoseconds and a long nonradiative lifetime of up to nanoseconds for ZR excitons following an ultrafast charge transfer of ≈2 ps from the charge‐transfer state. The findings provide evidence of charge‐transfer‐induced ZR excitonic states in NiPS3, where the intriguing phenomena of strongly coupled spins and charges can be explored. [ABSTRACT FROM AUTHOR]

2D van der Waals antiferromagnets have emerged as excellent candidates for studying novel low‐dimensional magnetism. The recently discovered spin–orbit‐entangled excitonic bound state appearing below the Néel temperature of 150 K in 2D antiferromagnetic NiPS3 (the so‐called Zhang–Rice (ZR) exciton) has drawn considerable attention in terms of exploring the strong correlation of spins, orbitals, and charges in a localized many‐body state. However, the formation mechanism of ZR excitons remains unclear, and its ultrafast dynamics have yet to be fully explored. Here, utilizing broadband transient reflectivity spectroscopy, the strong correlation between the charge‐transfer state excitation and the dynamic evolution of ZR excitons is investigated. Through systematic tuning of the pumping photon energy across the charge‐transfer state in NiPS3, the results reveal a short radiative lifetime of tens of picoseconds and a long nonradiative lifetime of up to nanoseconds for ZR excitons following an ultrafast charge transfer of ≈2 ps from the charge‐transfer state. The findings provide evidence of charge‐transfer‐induced ZR excitonic states in NiPS3, where the intriguing phenomena of strongly coupled spins and charges can be explored. [ABSTRACT FROM AUTHOR]

Abstract Epitaxially strain‐engineered tetragonal (T)‐like BiFeO3 (BFO) is a multiferroic material with unique crystallographic and physical properties compared to its bulk rhombohedral parent. While the effect of this structural change on ferroelectric properties is understood, the influence on correlated antiferromagnetic (AFM) properties, especially with reduced film thickness, is less clear. Here, the AFM behavior of T‐like BFO films 9–58 nm thick on LaAlO3 (001) substrates fabricated by pulsed laser deposition was studied using conversion electron Mössbauer spectroscopy and X‐ray diffraction. The key findings include: i) Ultrathin T‐like BFO films (

Transition metal dichalcogenide (TMD) monolayers exhibit unique valleytronics properties due to the dependency of the coupled valley and spin state at the hexagonal corner of the first Brillouin zone. Precisely controlling valley spin-polarization via manipulating the electron population enables its application in valley-based memory or quantum technologies. This study uncovered the uncompensated spins of the antiferromagnetic nickel oxide (NiO) serving as the ferromagnetic (FM) order to induce valley spin-polarization in molybdenum disulfide (MoS2) monolayers via the magnetic proximity effect (MPE). Spin-resolved photoluminescence spectroscopy (SR-PL) was employed to observe MoS2, where the spin-polarized trions appear to be responsible for the MPE, leading to a valley magnetism. Results indicate that local FM order from the uncompensated surface of NiO could successfully induce significant valley spin-polarization in MoS2 with the depolarization temperature approximately at 100 K, which is relatively high compared to the related literature. This study reveals new perspectives in that the precise control over the surface orientation of AFMs serves as a crystallographic switch to activate the MPE and the magnetic sustainability of the trion state is responsible for the observed valley spin-polarization with the increasing temperature, which promotes the potential of AFM materials in the field of exchange-coupled Van der Waals heterostructures. [ABSTRACT FROM AUTHOR]

Antiferromagnetic (AF) and ferrimagnetic (FiM) thin films have burgeoning significance in memory and computing applications due to their robustness and ultrafast and energy-efficient switching dynamics. Mn3Ga features a multitude of spin orders that can be meticulously controlled with stoichiometry, temperature, and strain modulations. In this work, we have carefully designed three suitable stacks of Mn3Ga thin films on MgO (111), STO (111) and STO (111)/Ta substrates deposited across varying substrate temperatures up to 500°C. The delicate interplay of strain and temperature tuning is examined by characterizing their magnetic, crystallographic, and morphological properties. The FiM tetragonal τ-Mn3Ga and AF hexagonal ε-Mn3Ga phases display relatively low saturation magnetizations of 10–60 and ≤ 20 kA/m, respectively. No preferential in-plane or out-of-plane magnetic anisotropy is observed for both τ- and ε-Mn3Ga phases. Critically, we observed that the STO strain-regulated τ-phase is stabilized over a wider temperature window and provides more compact, uniformly dispersed grains with average grain size of ~ 100 nm. This work establishes a sturdy methodology in understanding Mn3Ga thin film growth for eventual AF- and FiM-based memory and computing applications. [ABSTRACT FROM AUTHOR]

Antiferromagnets (AFM) hold significant promise as ideal candidates for high‐density and ultrafast memory applications. Electrical manipulation of exchange bias (EB) has emerged as an effective solution to integrate AFMs into magnetic memories as active elements. In particular, spin‐orbit torque antiferromagnetic random‐access memory (SOT‐ARAM) is recently been demonstrated by using an AFM/FM hybrid free layer, which can simultaneously satisfy field‐free switching and good device scalability. However, the switching current density of the exchange bias in SOT‐ARAM devices is still high, and novel functionalities are exploited in this device scheme. In this study, the all‐electrical manipulation of the ARAM devices through the interplay of SOT and spin‐transfer torque (STT) is reported, both in three‐terminal and two‐terminal configurations. The SOT current density achieves a 40% reduction thanks to the incorporation of the STT current. Macrospin simulations are performed to illustrate the underlying mechanism. Further, a majority gate that can be decomposed into reconfigurable AND/OR functionalities in a single ARAM device is demonstrated, with an operation speed as fast as 2 ns. The results can advance the development of high‐performance memories and in‐memory computing. [ABSTRACT FROM AUTHOR]

Hybrid light–matter coupled states, or polaritons, in magnetic materials have attracted significant attention due to their potential for enabling novel applications in spintronics and quantum information processing. However, most magnon‐polariton studies in the strong coupling regime to date have been carried out for ferromagnetic materials with magnon excitations at gigahertz frequencies. Here, strong resonant photon–magnon coupling at frequencies above 1 terahertz is investigated for the first time in a prototypical room‐temperature antiferromagnetic insulator, NiO, inside a Fabry–Pérot cavity. The cavity is formed by the crystal itself with a thickness adjusted to an optimal value. Terahertz time‐domain spectroscopy measurements in magnetic fields up to 25 T reveal the evolution of the magnon frequency through Fabry–Pérot cavity modes with photon–magnon anticrossing behavior, demonstrating clear vacuum Rabi splittings exceeding the polariton linewidths. These results show that NiO is a promising platform for exploring antiferromagnetic spintronics and cavity magnonics in the terahertz frequency range. [ABSTRACT FROM AUTHOR]

Effective control of domain walls or magnetic textures in antiferromagnets promises to enable robust, fast, and nonvolatile memories. The lack of net magnetic moment in antiferromagnets implies the need for creative ways to achieve such a manipulation. We conducted a study to investigate changes in magnetic force microscopy (MFM) imaging and in the magnon-related mode in Raman spectroscopy of virgin NiO films under a microwave pump. After MFM and Raman studies were conducted, a combined action of broadband microwave (0.01–20 GHz, power scanned from − 20 to 5 dBm) and magnetic field (up to 3 kOe) were applied to virgin epitaxial (111) NiO and (100) NiO films grown on (0001) Al 2 O 3 and (100) MgO substrates, following which the MFM and Raman studies were repeated. We observed a suppression of the magnon-related Raman mode subsequent to the microwave exposure. Based on MFM imaging, this effect appeared to be caused by the suppression of large antiferromagnetic domain walls due to the possible excitation of antiferromagnetic spin oscillations localized within the antiferromagnetic domain walls. [ABSTRACT FROM AUTHOR]

Abstract Antiferromagnets (AFM) hold significant promise as ideal candidates for high‐density and ultrafast memory applications. Electrical manipulation of exchange bias (EB) has emerged as an effective solution to integrate AFMs into magnetic memories as active elements. In particular, spin‐orbit torque antiferromagnetic random‐access memory (SOT‐ARAM) is recently been demonstrated by using an AFM/FM hybrid free layer, which can simultaneously satisfy field‐free switching and good device scalability. However, the switching current density of the exchange bias in SOT‐ARAM devices is still high, and novel functionalities are exploited in this device scheme. In this study, the all‐electrical manipulation of the ARAM devices through the interplay of SOT and spin‐transfer torque (STT) is reported, both in three‐terminal and two‐terminal configurations. The SOT current density achieves a 40% reduction thanks to the incorporation of the STT current. Macrospin simulations are performed to illustrate the underlying mechanism. Further, a majority gate that can be decomposed into reconfigurable AND/OR functionalities in a single ARAM device is demonstrated, with an operation speed as fast as 2 ns. The results can advance the development of high‐performance memories and in‐memory computing.

Abstract 2D magnetic materials offer the opportunity to study and manipulate emergent collective excitations. Among these, spin–phonon coupling is one of the most important interactions correlating charge, spin and lattice vibrations. Understanding and controlling this coupling is important for spintronics applications, control of magnons and phonon by THz radiation, and for strain‐driven magnetoelastic applications. Here, a resonant mode‐selective spin–phonon coupling in several magnetic 2D metal thiophosphates (NiPS3, FePS3, CoPS3 and MnPS3) through multi‐excitation and temperature‐dependent Raman scattering measurements is uncovered. The phonon mode, which is a Raman‐active out‐of‐plane vibrational mode (∼250 cm−1 or 7.5 THz), exhibits an asymmetric Fano lineshape where its asymmetry is proportional to the spin–phonon coupling. The measurements reveal the coupling to be the highest in NiPS3, followed by FePS3 and CoPS3, and least in MnPS3. These differences are attributed to the metal–sulfur interatomic distances, which are the lowest in NiPS3, followed by CoPS3, FePS3 and MnPS3. Finally, the spin–phonon coupling is also observed in exfoliated materials, with a slight reduction between 20 and 30% in the thinnest flakes compared to the bulk crystals.

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Antiferromagnet (AFM) has currently participated in the spin‐orbit torque (SOT) technology due to its great potential to be applied to the field‐free SOT switching and to promote the thermal stability of MRAM. However, the effect of varying AFM ordering on the SOT switching and the associated properties is still not comprehensively understood. This work reports how an AFM ordering modifies the strength of Dzyaloshinskii–Moriya‐interaction (DMI) in a heavy metal (Pt)/FM (Co)/AFM (IrMn) trilayer and its effects on SOT switching. Increasing the AFM ordering reflects the enhanced exchange bias through increasing IrMn thickness appears to significantly reduce the DMI strength of the trilayer. Controlling the IrMn thickness appears to serve as a unique switch to activate memristivity/stochasticity in the devices via tailoring AFM ordering on exchange bias: The strong AFM ordering via increasing IrMn thickness enables to increase the stability of multi‐levels for SOT switching, which promotes the memristivity for neuromorphic application. On the contrary, the weak AFM ordering via reducing IrMn thickness will lead to significant stochasticity for the physically unclonable functionality. This work demonstrates an intrinsic tuning over the AFM ordering will serve as a switch to turn the SOT device into a stochastic/memristive cell to bridge probabilistic and neuromorphic computing. [ABSTRACT FROM AUTHOR]

With conventional atomic-resolution electron microscopes, atomic-scale observation of magnetic materials has been extremely difficult since the samples are inevitably placed in a strong magnetic field of 2 T or higher. To overcome this limitation, we have developed a new objective lens system that enables atomic-resolution observation while maintaining a sample in the magnetic field-free environment. The electron microscope with this new lens system facilitates atomic structure analysis of magnetic materials and is expected to enable ultra-high resolution magnetic field imaging inside materials and devices. In this review, we describe the development of elemental technologies for the Magnetic field-free Atomic-Resolution Scanning transmission electron microscope (MARS) and show the results of real-space observation of the atomic-scale magnetic field distribution inside antiferromagnetic hematite. [ABSTRACT FROM AUTHOR]

Abstract Antiferromagnet (AFM) has currently participated in the spin‐orbit torque (SOT) technology due to its great potential to be applied to the field‐free SOT switching and to promote the thermal stability of MRAM. However, the effect of varying AFM ordering on the SOT switching and the associated properties is still not comprehensively understood. This work reports how an AFM ordering modifies the strength of Dzyaloshinskii–Moriya‐interaction (DMI) in a heavy metal (Pt)/FM (Co)/AFM (IrMn) trilayer and its effects on SOT switching. Increasing the AFM ordering reflects the enhanced exchange bias through increasing IrMn thickness appears to significantly reduce the DMI strength of the trilayer. Controlling the IrMn thickness appears to serve as a unique switch to activate memristivity/stochasticity in the devices via tailoring AFM ordering on exchange bias: The strong AFM ordering via increasing IrMn thickness enables to increase the stability of multi‐levels for SOT switching, which promotes the memristivity for neuromorphic application. On the contrary, the weak AFM ordering via reducing IrMn thickness will lead to significant stochasticity for the physically unclonable functionality. This work demonstrates an intrinsic tuning over the AFM ordering will serve as a switch to turn the SOT device into a stochastic/memristive cell to bridge probabilistic and neuromorphic computing.

The high‐frequency optical mode of α‐Fe2O3 is examined, and it is reported that Dzyaloshinskii−Moriya (DM) interaction generates a new type of torque on the magnetic resonance. Using a continuous‐wave terahertz interferometer, the optical mode spectra is measured, where the asymmetric absorption with a large amplitude and broad linewidth is observed near the magnetic transition point, Morin temperature (TM ≈ 254.3 K). Based on the spin wave model, the spectral anomaly is attributed to the DM interaction‐induced torque, enabling to extract the strength of DM interaction field of 4 T. This work opens a new avenue to characterize the spin resonance behaviors at an antiferromagnetic singular point for next‐generation and high‐frequency spin‐based information technologies. [ABSTRACT FROM AUTHOR]

The high‐frequency optical mode of α‐Fe2O3 is examined, and it is reported that Dzyaloshinskii−Moriya (DM) interaction generates a new type of torque on the magnetic resonance. Using a continuous‐wave terahertz interferometer, the optical mode spectra is measured, where the asymmetric absorption with a large amplitude and broad linewidth is observed near the magnetic transition point, Morin temperature (TM ≈ 254.3 K). Based on the spin wave model, the spectral anomaly is attributed to the DM interaction‐induced torque, enabling to extract the strength of DM interaction field of 4 T. This work opens a new avenue to characterize the spin resonance behaviors at an antiferromagnetic singular point for next‐generation and high‐frequency spin‐based information technologies. [ABSTRACT FROM AUTHOR]

Antiferromagnetic spin Hall oscillator with delay under action of DC signal is studied numerically. The delay in oscillator can be attributed to internal inertia of the antiferromagnetic lattice, or could be introduced artificially by electrical means. We have shown that as result of this delay system output becomes random. Such behavior arises from nonlinearity of the system, and somewhat resembles Ikeda equation dynamics. We have also calculated Poincare section of the system, to further confirm stochastic nature of the system output. Results show that phase trajectories of the system are scattered all over the phase plane, as simulation time increases. Also we have calculated Fourier transform of the output signal. Obtained spectrum also shows system output random nature. Obtained results are important for further development of antiferromagnetic terahertz-frequency spintronic oscillators and their applications as random signal sources. Such high frequency random signal sources can revolutionize cryptography and probabilistic computing. [ABSTRACT FROM AUTHOR]

Multicomponent flux systems based on both Li2WO4-B2O3-Li2O-CuO-Cr2O3 and Bi2O3-MoO3-B2O3-Na2O-CuO-Cr2O3 were studied in order to grow Cu2CrBO5 crystals. The conditions for Cu2CrBO5 crystallization were investigated by varyingthe component ratios, and the peculiarities of their interaction were characterized by studying the formation sequence of high-temperature crystallizing phases. Special attention was paid to the problem of Cr2O3 solubility. Phase boundaries between CuCrO2, Cu2CrO4, and Cu2CrBO5 were considered. The crystal structure of the obtained samples was studied viasingle crystal and powder X-ray diffraction. The chemical composition of the grown crystals was examined using the EDX technique. Anactual ratio of Cu:Cr = 1.89:1.11 was found for Cu2CrBO5 grown from the lithium-tungstate system, which showed a small deviation from 2:1, implying the presence of a part of bivalent Cr2+ in the samples. Anomalies in the thermal dependence of magnetization were analyzed and compared with the previously obtained data for Cu2CrBO5. The anomaly at TC ≈ 42 K and the antiferromagnetic phase transition at TN ≈ 119 K were considered. Polarized Raman spectra of Cu2CrBO5 were obtained for the first time, and a comparative analysis of the obtained data with other monoclinic and orthorhombic ludwigites is presented. Along with the polarized room temperature spectra, the thermal evolution of Raman modes near the antiferromagnetic phase transition temperature TN ≈ 119 K is provided. The influence of the magnetic phase transition on the Raman spectra of Cu2CrBO5 is discussed. [ABSTRACT FROM AUTHOR]

Exchange bias (EB) is a unidirectional anisotropy occurring in exchange-coupled ferromagnetic/antiferromagnetic systems, such as thin films, core–shell particles, or nanostructures. In addition to a horizontal shift of the hysteresis loop, defining the exchange bias, asymmetric loops and even vertical shifts can often be found. While the effect is used in hard disk read heads and several spintronics applications, its origin is still not fully understood. Especially in nanostructures with their additional shape anisotropies, interesting and often unexpected effects can occur. Here, we provide an overview of the most recent experimental findings and theoretical models of exchange bias in nanostructures from different materials. [ABSTRACT FROM AUTHOR]

Redox-induced magnetic transformation in organic diradicals is an appealing phenomenon. In this study, we theoretically designed twelve couples of diradicals in which two nitroxide (NO) radical groups are connected to the redox-active couplers including p-benzoquinonyl, 1,4-naphthoquinyl, 9,10-anthraquinonyl, naphthacene-5,12-dione, pentacene-6,13-dione, hexacene-6,15-dione, pyrazinyl, quinoxalinyl, phenazinyl, 5,12-diazanaphthacene, 6,13-diazapentacene, and 6,15-diazahexacene. As evidenced at both the B3LYP and M06-2X levels of theory, the calculations reveal that the magnetic reversal can take place from ferromagnetism to antiferromagnetism, or vice versa, by means of redox method in these designed organic magnetic molecules. It was observed that p-benzoquinonyl, 1,4-naphthoquinyl, 9,10-anthraquinonyl, naphthacene-5,12-dione, pentacene-6,13-dione, and hexacene-6,15-dione-bridged NO diradicals produce antiferromagnetism while their dihydrogenated counterparts exhibit ferromagnetism. Similarly, pyrazinyl, quinoxalinyl, phenazinyl, 5,12-diazanaphthacene, 6,13-diazapentacene, and 6,15-diazahexacene-bridged NO diradicals present ferromagnetism while their dihydrogenated counterparts show antiferromagnetism. The differences in the magnetic behaviors and magnetic magnitudes of each of the twelve couples of diradicals could be attributed to their distinctly different spin-interacting pathways. It was found that the nature of the coupler and the length of the coupling path are important factors in controlling the magnitude of the magnetic exchange coupling constant J. Specifically, smaller HOMO-LUMO (HOMO: highest occupied molecular orbital, LUMO: lowest unoccupied molecular orbital) gaps of the couplers and shorter coupler lengths, as well as shorter linking bond lengths, can attain stronger magnetic interactions. In addition, a diradical with an extensively π-conjugated structure is beneficial to spin transport and can effectively promote magnetic coupling, yielding a large |J| accordingly. That is, a larger spin polarization can give rise to a stronger magnetic interaction. The sign of J for these studied diradicals can be predicted from the spin alternation rule, the shape of the singly occupied molecular orbitals (SOMOs), and the SOMO-SOMO energy gaps of the triplet state. This study paves the way for the rational design of magnetic molecular switches. [ABSTRACT FROM AUTHOR]