Magnetic State Research Articles

Key Ingredients for the Modeling of Single‐Atom Electrocatalysts

AbstractSingle‐atom catalysis is gaining interest also because of its potential applications in a broad spectrum of electrochemical reactions. The reactivity of single‐atom catalysts (SACs) is typically modeled with first principles approaches taking insight from heterogenous catalysis. An increasing number of studies show that the chemistry of SACs is more complex than often assumed, and shares many aspects in common with coordination chemistry. This evidence raises challenges for computational electrocatalysis of SACs. In this perspective we highlight a few fundamental ingredients that one need to consider to provide reliable predictions on the reactivity of SACs for electrochemical applications. We discuss the role of the local coordination of the metal active phase, the need to use self‐interaction corrected functionals, in particular when systems have magnetic ground states. We highlight the formation of unconventional intermediates with respect to classical metal electrodes, the need to include the stability of SACs in electrochemical conditions and the role of solvation in the analysis of new potential catalytic systems. This brief account can be considered as a tutorial underlining the importance of treating the reactivity of SACs. In fact, neglecting some of these aspects could lead to unreliable predictions failing in the design of new electrocatalysts.

The mysterious magnetic ground state of Ba14MnBi11 is likely self-doped and altermagnetic

Magnetism in the Zintl compound Ba14MnBi11 is rather poorly understood. Experimental claims are largely inconsistent with ab initio calculations, much beyond typical errors of the latter. We revisit this old problem, assuming that the root of the problem may be in nonstoichiometry of existing samples. Our key finding is that the magnetic ground state is indeed very susceptible to charge doping (band filling). Calculations for stoichiometric Ba14MnBi11 give a rather stable ferromagnetic metallic state, in agreement with previous publications. However, by adding exactly one electron per Mn, the system becomes semiconducting as expected, and becomes weakly antiferromagnetic (AF). On the other hand, upon small amount of hole doping, the system transitions to a special type of AF state known as altermagnetism. Furthermore, hole and electron doping-induced phase transitions result from different underlying mechanisms, influencing different exchange pathways. We propose that the inconsistency between experiment and theory is not a failure of the latter, but results from a nontrivial ramification of nonstoichiometry. The possibility of doping-stabilized altermagnetism is exciting.

Unusual Anomalous Hall Effect in Two-Dimensional Ferromagnetic Cr7Te8

Two-dimensional (2D) materials with inherent magnetism have attracted considerable attention in the fields of spintronics and condensed matter physics. The anomalous Hall effect (AHE) offers a theoretical foundation for understanding the origins of 2D ferromagnetism (2D-FM) and offers a valuable opportunity for applications in topological electronics. Here, we present uniform and large-size 2D Cr7Te8 nanosheets with varying thicknesses grown using the chemical vapor deposition (CVD) method. The 2D Cr7Te8 nanosheets with robust perpendicular magnetic anisotropy, even a few layers deep, exhibit a Curie temperature (TC) ranging from 180 to 270 K according to the varying thickness of Cr7Te8. Moreover, we observed a temperature-induced reversal in the sign of the anomalous Hall resistance, correlating with changes in the intrinsic Berry curvature. Additionally, the topological Hall effect (THE) observed at low temperatures suggests the presence of non-trivial spin chirality. Our findings about topologically non-trivial magnetic spin states in 2D ferromagnets provide a promising opportunity for new designs in magnetic memory spintronics.

Development of High-Aspect-Ratio Soft Magnetic Microarrays for Magneto-Mechanical Actuation via Field-Induced Injection Molding

Magnetorheological elastomers (MREs) are in demand in the field of high-tech microindustries and nanoindustries such as biomedical applications and soft robotics due to their exquisite magneto-sensitive response. Among various MRE applications, programmable actuators are emerging as promising soft robots because of their combined advantages of excellent flexibility and precise controllability in a magnetic system. Here, we present the development of magnetically programmable soft magnetic microarray actuators through field-induced injection molding using MREs, which consist of styrene-ethylene/butylene styrene (SEBS) elastomer and carbonyl iron powder (CIP). The ratio of the CIP/SEBS matrix was designed to maximize the CIP fraction based on a critical solids loading. Further, as part of the design of the magnetization distribution in micropillar arrays, the magnetorheological response of the molten composites was analyzed using the static and dynamic viscosity results for both the on and off magnetic states, which reflected the particle dipole interaction and subsequent particle alignment during the field-induced injection molding process. To develop a high-aspect-ratio soft magnetic microarray, X-ray lithography was applied to prepare the sacrificial molds with a height-to-width ratio of 10. The alignment of the CIP was designed to achieve a parallel magnetic direction along the micropillar columns, and consequently, the micropillar arrays successfully achieved the uniform and large bending actuation of up to approximately 81° with an applied magnetic field. This study suggests that the injection molding process offers a promising manufacturing approach to build a programmable soft magnetic microarray actuator.

Thermal transport and spin–phonon interaction in magnetic Janus Cr2X3S3 (X = Br, I) monolayers

Comprehending the relationship between spin and phonons is essential to regulate the lattice thermal conductivity in 2D magnetic materials. Lattice thermal conductivity is a relevant part to consider in magnetic data storage and the working of spin-based devices. In this article, we examined the origin and effect of spin–phonon coupling (SPC) on the lattice thermal conductivity of pristine CrI3 monolayer-based Janus monolayers Cr2X3S3 (X=Br,I). We find a high SPC in these Janus monolayers due to in-plane Cr–S atomic vibrations. We observe a reduction in the lattice thermal conductivity at Janus monolayer magnetic states (∼52.3% in Cr2Br3S3 and ∼63.9% in Cr2I3S3) compared to its paramagnetic states. The analysis is also conducted to determine which magnetic state has more anharmonicity using potential energy wells. A wide range of 2D magnetic materials can benefit from our results in the future development of spin-based thermal devices.

Tunable Anomalous Hall Effect in Broken–Symmetry Integrated Perovskite/Brownmillerite Superlattices

AbstractThe anomalous Hall effect (AHE), a hallmark of broken time–reversal symmetry, serves as a powerful tool for probing magnetic states and elucidating complex spin–orbit interactions in transition metal oxides. It is reported here that the emergence of AHE signals and interfacial ferromagnetism is directly observed from broken–symmetry integrated perovskite PrCoO3 and brownmillerite SrCoO2.5 superlattices. Through the strategic combinations of these two complex oxide systems onto the substrates with tailored lattice parameters, squared AHE hysteresis loops with perpendicular magnetic anisotropy are found under compressive strain in the optimized superlattices, and the magnitude of the loops is more than three times greater compared to those under tensile strain. The manifestation of AHE in these superlattices is attributed to an extrinsic mechanism involving spin–polarized scattering of charge carriers, with the mismatched perovskite/brownmillerite interfaces acting as the primary scattering centers. These discoveries suggest a promising avenue for the development of artificial materials with controllable AHE properties.

High-Entropy Magnet Enabling Distinctive Thermal Expansions in Intermetallic Compounds.

The high-entropy strategy has gained increasing popularity in the design of functional materials due to its four core effects. In this study, we introduce the concept of a "high-entropy magnet (HEM)", which integrates diverse magnetic compounds within a single phase and is anticipated to demonstrate unique magnetism-related properties beyond that of its individual components. This concept is exemplified in AB2-type layered Kagome intermetallic compounds (Ti,Zr,Hf,Nb,Fe)Fe2. It is revealed that the competition among individual magnetic states and the presence of magnetic Fe in originally nonmagnetic high-entropy sites lead to intricate magnetic transitions with temperature. Consequently, unusual transformations in thermal expansion property (from positive to zero, negative, and back to near zero) are observed. Specifically, a near-zero thermal expansion is achieved over a wide temperature range (10-360 K, αv = -0.62 × 10-6 K-1) in the A-site equal-atomic ratio (Ti1/5Zr1/5Hf1/5Nb1/5Fe1/5)Fe2 compound, which is associated with successive deflection of average Fe moments. The HEM strategy holds promise for discovering new functionalities in solid materials.

Influence of an ultrathin Mn ‘spy layer’ on the static and dynamic magnetic coupling within FePt/NiFe bilayers

Abstract We explore whether insertion of an ultrathin Mn ‘spy layer’ within a magnetic hard/soft bilayer can enable depth-sensitive element-specific measurements of the static and dynamic magnetization, while avoiding significant disruption of the original magnetic state. MgO(110)/FePt(100 Å)/NiFe(200 Å)/Mn(tMn Å)/NiFe(200 Å) samples with Mn thicknesses of tMn=0, 5, and 10Å were fabricated by magnetron sputtering and studied by element-selective x-ray magnetic circular dichroism (XMCD), vector network analyzer ferromagnetic resonance (VNA-FMR), and x-ray detected ferromagnetic resonance (XFMR). For tMn= 5 Å, the magnetic reversal properties remain broadly similar to tMn= 0 Å. For tMn=10 Å, the two NiFe layers decouple with XMCD hysteresis loops at the Mn edge showing two switching events that suggest the presence of two distinct Mn-containing regions. While the Mn moments within each region have ferromagnetic order, their relative alignment is antiparallel at high field. Analysis of the magnetic data and additional scanning transmission electron microscopy (STEM) measurements point to the presence of a Mn layer at the lower NiFe/Mn interface, and the formation of a NiFeMn alloy at the upper Mn/NiFe interface. The Mn moments of the former region lie antiparallel to those of the underlying NiFe layer. The VNA-FMR data suggests that for tMn= 5 and 10 Å, the interfacial exchange coupling at the FePt/NiFe is suppressed and the in-plane uniaxial magnetic anisotropy of the NiFe is increased, perhaps due to migration of Mn towards the buried interface. The above findings show that Mn is a problematic magnetic spy, and that a Mn thickness of less than 5Å would be required.

Feature extraction and classification of microstructure and magnetic tomography images of an advanced Nd-Fe-B sintered magnet

Abstract Although the microstructure and magnetic tomography images of an advanced Nd-Fe-B sintered magnet were previously reported [Takeuchi, SO., NPG Asia Mater. 14, 70 (2022)], the relationship between these three-dimensional images has not been well analyzed. In this work, a feature extraction method of histogram of oriented gradients and a classification method of uniform manifold approximation and projection are employed for this issue. The microstructural features with superimposing the information of magnetic domain structure are classified into two groups depending on the external magnetic field, resulting in the different microstructural features for the different magnetization states are successfully classified. These differences of the microstructural features are difficult to be found by human recognition. Further detailed analysis of these microstructural features may clarify the key microstructures for the nucleation of reversed domains and their propagations.

Relativistic treatment of hole alignment in noble gas atoms

The development in attosecond physics allows for unprecedented control of atoms and molecules in the time domain. Here, ultrashort pulses are used to prepare atomic ions in specific magnetic states, which may be important for controlling charge migration in molecules. Our work fills the knowledge gap of relativistic hole alignment prepared by femtosecond and attosecond pulses. The research focuses on optimizing the central frequency and duration of pulses to exploit specific spectral features, such as Fano profiles, Cooper minima, and giant resonances. Simulations are performed using the Relativistic Time-Dependent Configuration-Interaction Singles method. Ultrafast hole alignment with large ratios (on the order of one hundred) is observed in the outer-shell hole of argon. An even larger alignment (on the order of one thousand) is observed in the inner-shell hole of xenon.

Characterizing the diffuse continuum excitations in the classical spin liquid h−YMnO3

We extend previous inelastic neutron scattering results on the geometrically frustrated antiferromagnet hexagonal-YMnO3, which has been suggested to belong to the class of classical spin liquids. We expand the energy transfer range of the diffuse signal up to 6.9 meV within a wide temperature range around the ordering temperature, TN. The two distinct diffuse signals in the a−b plane, the signal localized at Γ′ and the scattering intensity connecting Γ′ points over the M′, are shown to be only weakly energy dependent. In addition, an external magnetic field of up to 10.5 T applied along c is shown to have no effect on the diffuse signal. In the orthogonal scattering plane, the signals are shown to be dependent on l only through the magnetic form factor, showing that the correlations are purely two-dimensional, and supporting the origin of this to be the frustrated Mn3+ triangles. This result is corroborated by atomistic spin dynamics simulations showing similar scattering vector and temperature behaviors. Lastly, data for the spin wave scattering in the (h,0,l) plane allow for a discussion of the magnetic ground state where better agreement is found between the data and an ordered structure of the Γ1 or Γ3 symmetry, albeit crystal electric field arguments dismiss the Γ1 as a possibility. Published by the American Physical Society 2024

Magnetic vortex γ-Fe2O3 nanospheres decorated with gold nanoparticles for dual hyperthermia treatment

In this work, multifunctional systems with potential applications for dual control of thermal heat delivery are developed. For this purpose, the nanosystems produced consist of vortex iron oxide nanospheres (VIONS) decorated with different amounts of gold nanoparticles (AuNPs). The VIONS were synthesized using the solvothermal reduction method, resulting in a 204 ± 24 nm diameter and meeting the requirements for biomedical applications. Magnetic measurements revealed that these systems are composed mainly of maghemite (γ-Fe2O3), exhibiting relatively low coercivity and moderate saturation magnetization, both of which serve as indicative attributes of an exceptional magnetic vortex state as validated by electron holography microscopy. The decoration with gold NPs was achieved through deposition-precipitation, involving the deposition of gold hydroxide on the surface of the previously amine-modified iron oxide NPs, followed by the concurrent reduction of Au3+. Furthermore, the gold NP size modulation was investigated by consecutively adding Au3+ to obtain gold NPs that interact with light differently depending on their size. The study provides crucial information that can aid the decoration of magnetic iron oxide nanospheres with AuNPs with tailored magnetic and optical properties; consequently, they may be promising candidates for magnetic and photothermal hyperthermia based on three-dimensional magnetic vortex structures.

Chirality Control of Magnetic Vortices in Ferromagnetic Disk–Nanowire System

The results of experimental studies and micromagnetic modeling of magnetic states in a one-dimensional array are presented. The array has the form of a chain of ferromagnetic disks coupled with a ferromagnetic nanowire made of the same material. The disks are located on opposite sides of the nanowire, which makes it possible to obtain distributions when the chiralities of the magnetic vortex shells in neighboring disks alternate, which can find application in vortex spin nanooscillators. Using the application in situ of a magnetic field to an excited objective lens using Lorentz transmission electron microscopy, it is shown that in this system it is possible to control the chiralities of the shells of magnetic vortices, which is realized by magnetization in the sample plane along various azimuthal directions. When wire magnetized along a nanowire, vortex states with opposite chiralities are realized in disks located on opposite sides of it. An antivortex is formed in the nanowire itself at the boundary with the disk, since the local direction of magnetization in the wire and in the disk are anticollinear. When magnetized perpendicular to the nanowire, states with the same chirality are realized in all disks. In this case, two perpendicular domain walls are formed between the disks in the nanowire, and the vortex in the disk is shifted to one of the edges along the nanowire.

Versatile Metamaterial: Exploring the Resonances of Symmetry‐Protected Modes for Multitasking Functionality

AbstractIn this work, an experimental study supported by numerical modeling that demonstrates the possibility of exciting Symmetry Protected‐Bound states In the Continuum (SP‐BICs) in a 1D silicon grating fabricated on a lithium niobate substrate is presented. bBoth transverse electric and magnetic polarization states are investigated, leading to the excitation of four quasi‐ Bound states In the Continuum (quasi‐BIC) resonances, exhibiting distinct behaviors. Under standard illumination conditions (plane of incidence perpendicular to the 1D grating lines), two of these resonances are highly sensitive to illumination conditions, while the other two resonances involving unconventional illumination directions (plane of incidence parallel to the grating lines) are more robust to the angle of incidence, but just as sensitive to external stresses in terms of resonance wavelength and quality factor. Additionally, temperature detection is experimentally demonstrated with a Sensitivity of ST = 0.81∼nm °C−1, a state‐of‐the‐art value achieved due to significant electromagnetic field enhancement inside the lithium niobate substrate at the quasi‐BIC resonance. These findings pave the way for their use in various sensing applications (such as biology, electromagnetic, and temperature sensing), as well as nonlinear applications like second harmonic generation, and electro‐ and acousto‐optic modulation.

Mn doping-induced twofold spin reorientation and multi-step spin switching in NdFeO3 single crystal

Magnetization reversal and magnetic state regulation are the basis of application for magnetic materials in information processing. Here, twofold spin reorientation transitions (SRT) and field-induced multi-step spin switching phenomena were observed in NdMn0.2Fe0.8O3 (NMFO) single crystals grown via the optical floating zone method. Magnetic anisotropy was systematically investigated in both Zero-Field-Cooled (ZFC) and Field-Cooled (FC) modes. The NMFO single crystal exhibited twofold SRTs: from Γ4 (Gx, Ay, Fz) to Γ1 (Ax, Gy, Cz) and from Γ1 (Ax, Gy, Cz) to Γ2 (Fx, Cy, Gz). Furthermore, a magnetic field-induced spin reorientation transition was identified along the c-axis. Additionally, closer proximity to the SRT temperature corresponded to a reduced magnetic field requirement for inducing the spin reorientation transition. These findings contribute to our comprehension of spin dynamics in NMFO, presenting possibilities for future device applications.

Spin dynamics and Phonons of the Chromites CoCr2O4 and MnCr2O4

Abstract Due to the geometric magnetic frustration inherent to their lattice and the resulting complex magnetic states, the spinel compounds are of great interest in both fundamental and application-oriented perspectives. Here, we applied x-ray diffraction, magnetization, heat capacity and powder inelastic neutron scattering measurements, along with theoretical calculations, to study the exotic properties of chromite-spinel oxides, CoCr2O4 and MnCr2O4. The temperature dependence of the phonon spectra provides an insight into the correlation between oxygen motion and the magnetic order, as well as the magnetoelectric effect in the ground state of MnCr2O4. Moreover, spinwave excitations in CoCr2O4 and MnCr2O4 are compared with Heisenberg model calculations. This approach enables the precise determination of exchange energies and offers a comprehensive understanding of the spin dynamics and relevant exchange interactions in complicated spiral spin ordering.

Temperature Studies of the LaMn2Si2 Intermetallide by the Raman Spectroscopy and Magnetic Force Microscopy Methods

Raman spectra of the LaMn2Si2 compound were obtained for the first time by Raman spectroscopy. The change of Raman spectral characteristics in the temperature range of 263–553 K was investigated. The high sensitivity of the Raman spectroscopy method to a change in the magnetic state caused by a temperature influence has been determined. A change in the spectral characteristics of the vibration mode of manganese atoms near the Curie and Neel temperatures has been revealed. The magnetic force microscopy technique was used to investigate the surface features of the LaMn2Si2 compound at room temperature. A change in the type of magnetic domain structure in LaMn2Si2 after cooling from 298 to 263 K has been found.

Non-volatile Fermi level tuning for the control of spin-charge conversion at room temperature

Current silicon-based CMOS devices face physical limitations in downscaling size and power loss, restricting their capability to meet the demands for data storage and information processing of emerging technologies. One possible alternative is to encode the information in a non-volatile magnetic state and manipulate this spin state electronically, as in spintronics. However, current spintronic devices rely on the current-driven control of magnetization, which involves Joule heating and power dissipation. This limitation has motivated intense research into the voltage-driven manipulation of spin signals to achieve energy-efficient device operation. Here, we show non-volatile control of spin-charge conversion at room temperature in graphene-based heterostructures through Fermi level tuning. We use a polymeric ferroelectric film to induce non-volatile charging in graphene. To demonstrate the switching of spin-to-charge conversion we perform ferromagnetic resonance and inverse Edelstein effect experiments. The sign change of output voltage is derived by the change of carrier type, which can be achieved solely by a voltage pulse. Our results provide an alternative approach for the electric-field control of spin-charge conversion, which constitutes a building block for the next generation of spin-orbitronic memory and logic devices.

Tunable multistate field-free switching and ratchet effect by spin-orbit torque in canted ferrimagnetic alloy

Spin-orbit torque is not only a useful probe to study manipulation of magnetic textures and magnetic states at the nanoscale but also it carries great potential for next-generation computing applications. Here we report the observation of rich spin-orbit torque switching phenomena such as field-free switching, multistate switching, memristor behavior and ratchet effect in a single shot, co-sputtered, rare earth-transition metal GdxCo100−x. Notably such effects have only been observed in antiferromagnet/ferromagnet bi-layer systems previously. We show that these effects can be traced to a large anistropic canting, that can be engineered into the GdxCo100−x system. Further, we show that the magnitude of these switching phenomena can be tuned by the canting angle and the in-plane external field. The complex spin-orbit torque switching observed in canted GdxCo100−x not only provides a platform for spintronics but also serves as a model system to study the underlying physics of complex magnetic textures and interactions.

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 perpendicular magnetic anisotropy (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 Néel 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.