Change In Magnetic State Research Articles

Classification of Major Solar Flares from Extremely Imbalanced Multivariate Time Series Data Using Minimally Random Convolutional Kernel Transform

Solar flares are characterized by sudden bursts of electromagnetic radiation from the Sun’s surface, and are caused by the changes in magnetic field states in active solar regions. Earth and its surrounding space environment can suffer from various negative impacts caused by solar flares, ranging from electronic communication disruption to radiation exposure-based health risks to astronauts. In this paper, we address the solar flare prediction problem from magnetic field parameter-based multivariate time series (MVTS) data using multiple state-of-the-art machine learning classifiers that include MINImally RandOm Convolutional KErnel Transform (MiniRocket), Support Vector Machine (SVM), Canonical Interval Forest (CIF), Multiple Representations Sequence Learner (Mr-SEQL), and a Long Short-Term Memory (LSTM)-based deep learning model. Our experiment is conducted on the Space Weather Analytics for Solar Flares (SWAN-SF) benchmark data set, which is a partitioned collection of MVTS data of active region magnetic field parameters spanning over nine years of operation of the Solar Dynamics Observatory (SDO). The MVTS instances of the SWAN-SF dataset are labeled by GOES X-ray flux-based flare class labels, and attributed to extreme class imbalance because of the rarity of the major flaring events (e.g., X and M). As a performance validation metric in this class-imbalanced dataset, we used the True Skill Statistic (TSS) score. Finally, we demonstrate the advantages of the MVTS learning algorithm MiniRocket, which outperformed the aforementioned classifiers without the need for essential data preprocessing steps such as normalization, statistical summarization, and class imbalance handling heuristics.

Antiferromagnetic ground state and evidence for metamagnetic transition in itinerant-electron compounds YCo12−xFexB6 (x = 1.5, 2, 2.5)

In contrast to the parent compound YCo12B6, the Fe-doped YCo12−xFexB6 (1.5 ≤ x ≤ 2.5) alloys exhibit an antiferromagnetic ground state. It is further revealed that the antiferromagnetic state gets transformed into a forced ferromagnetic state by way of a magnetic-field-induced metamagnetic transition. The results demonstrate a progressive reinforcement of the antiferromagnetic interactions upon increasing Fe content along the YCo12−xFexB6 series of compounds. The magnetic phase diagram of each (x = 1.5, 2, and 2.5) composition is determined by combining isothermal and isofield magnetization measurements. The composition and temperature dependencies of the critical field are derived and discussed. Whereas for x = 2 and 2.5, the magnetic state changes from antiferromagnetic (AFM) to ferromagnetic (FM) and then to the paramagnetic (PM) state upon heating, exhibiting three distinct temperature regions, a multicritical point of magnetic origin is proposed for the x = 1.5 compound at a temperature of about 117 K. The ordering temperature is found to decrease upon increasing Fe concentration, while the critical field of the AFM-FM metamagnetic phase transition shows the opposite trend. The critical transition field diminishes upon increasing temperature.

Topological magnon–polaron transport in a bilayer van der Waals magnet

The stacking of intrinsically magnetic van der Waals materials provides a fertile platform to explore tunable transport effects of magnons, presenting significant prospects for spintronic applications. The possibility of having topologically nontrivial magnons in these systems can further expand the scope of exploration. In this work, we consider a bilayer system with intralayer ferromagnetic exchange and a weak interlayer antiferromagnetic exchange and study the topological magnon-polaron excitations induced by magnetoelastic couplings. Under an applied magnetic field, the system features a metamagnetic transition, where the magnetic ground state changes from antiparallel layers to parallel. We show that the metamagnetic transition is accompanied by a transition of the topological structure of the magnon polarons, which results in discernible changes in the topology induced transport effects. The magnetic-field dependence of the thermal Hall conductivity and spin Nernst coefficient is analyzed with linear response theories.

Functionalized molybdenum disulfide (MoS2) monolayer as an efficient nanosensor towards toxic nitrogen containing gases

Selective detection of toxic nitrogen-containing gases (e.g. NO and NO2) is achieved when special transition metal (TM) atoms are selected in doping molybdenum disulfide (MoS2) monolayer. The change of magnetic state is the secret of that selectivity.

Low-temperature giant coercivity in Co6.2Ga3.8−x Ge x (x = 2.4 to 3.2)

The observation of giant coercivity exceeding 20 kOe at low temperatures in several transition-metal-based compounds has attracted significant attention from a fundamental perspective. This research is also relevant to developing rare-earth-free permanent magnets, wherein cobalt is one of the primary elements used. To facilitate easy fabrication, rare-earth-free and Co-based inorganic bulk magnets that exhibit giant coercivity are highly demanded but rarely reported. Herein, we report the observation of low-temperature giant coercivity in polycrystalline metallic Co6.2Ga3.8−x Ge x (x = 2.4 to 3.2) with the hexagonal Fe13Ge8-type structure composed of Kagome and triangular lattices. As the Ge content x decreases from 3.2, the magnetic ground state changes from ferrimagnetism to ferromagnetism at x = 2.6. In the ferrimagnetic state, we observed a signature of spin frustration arising from the Kagome and/or triangular lattices of Co atoms. The ferromagnetic ordering temperatures for the x = 2.6 and 2.4 samples are 46 K and 60 K, respectively. The coercive fields rapidly increase upon cooling and reach values of 26 kOe and 44 kOe in the x = 2.6 and 2.4 samples, respectively, at 2 K.

Effects of Fe doping on Martensitic Transformation and magnetic properties of Ni-Mn-Ti All-d-metal Heusler Alloy

<sec>Ni-Mn-Ti-based all-d-metal Heusler alloys have become a hot research topic in the field of metal functional materials due to their excellent mechanical properties and elastocaloric effect. However, the relatively large critical stress and transition hysteresis limit its practical applications. Some researchers have found that doping Fe in Ni-Mn-based alloys can not only reduce hysteresis, but also greatly improve the mechanical properties of alloys. Based on this, the effects of Fe doping on phase stability, martensitic transformation and magnetic properties of Ni<sub>50–<i>x</i></sub>Mn<sub>37.5</sub>Ti<sub>12.5</sub>Fe<sub><i>x</i></sub> (<i>x</i> = 3.125, 6.25, 9.375) Heusler alloys are systematically studied by first principles calculation. The corresponding magnetic states of the austenite and martensite of the alloy systems are determined according to the results of the formation energy. The variations of the lattice constants and the phase stability of the austenite and martensite with the increase of Fe content in the alloy systems are revealed, and the associated mechanism is elucidated. The atomic and total magnetic moments of the austenite and martensite in the Ni<sub>50–<i>x</i></sub>Mn<sub>37.5</sub>Ti<sub>12.5</sub>Fe<sub><i>x</i></sub> (<i>x</i> = 3.125, 6.25, 9.375) systems are calculated. Based on the results of electronic structure, the essential reasons for the magnetic state changes of the alloys are further explained.</sec><sec>In the Ni<sub>50–<i>x</i></sub>Mn<sub>37.5</sub>Ti<sub>12.5</sub>Fe<sub><i>x</i></sub> alloy system, the lattice constant of austenite decreases gradually with the increase of Fe doping amount. The stability of austenite phase and martensite phase decrease with the increase of Fe doping amount. Under the different compositions, the formation energy of martensite is always lower than that of austenite, indicating that the alloy can undergo martensite transformation. The energy difference Δ<i>E</i>, electron concentration <i>e</i>/<i>a</i> and density of electrons <i>n</i> of the alloy show a decreasing trend, indicating that the driving force of martensitic transformation decreases, and the corresponding martensitic transformation temperature decreases with the increase of Fe atom doping.</sec><sec>The austenite of the alloy is ferromagnetic and the martensite is antiferromagnetic. After the martensitic transformation, the distance between Mn-Mn atoms decreases, and the magnetic moments of Mn<sub>Mn</sub> and Mn<sub>Ti</sub> atoms are arranged in antiparallel manner, resulting in the total magnetic moments being almost zero. The magnetic properties of the two phases are little affected by the amount of Fe atom doping. The peak density of electronic states in the Fermi surface of martensite phase is lower than that of austenite phase, indicating that martensite phase has a more stable electronic structure than austenite phase. During the transition from austenite to martensite, there is a Jahn-Teller splitting effect at the peak of the down-spin density of states near the Fermi surface. The aim of this paper is to provide guidance for designing the composition design and optimizing the property of the Ni-Mn-Ti-Fe alloy.</sec>

Magnetic, thermal, and magnetocaloric properties of the holmium trialuminide HoAl3 with polytypic phases

We investigate the magnetic, thermal, and magnetocaloric properties of the intermetallic HoAl3 compounds with two different crystal structures. The room-temperature equilibrium trigonal phase HoAl3 undergoes an antiferromagnetic (AFM) transition at the Néel temperature TNtri= 9.8 K. The AFM order of the compound is strong against the magnetic field, so that inverse magnetocaloric effect (MCE) is found even under a magnetic field change of 0–50 kOe. The high-pressure cubic phase HoAl3, prepared by a rapid-solidification process, is antiferromagnetically ordered below TNcub= 15 K. Magnetic and specific heat measurements reveal that this long-range AFM state becomes unstable as the temperature drops and then is replaced by a magnetic glassy state below a spin freezing temperature Tf= 11 K. The successive changes in magnetic state result in complicated temperature and field dependence of the MCE at low temperatures.

Density Functional Theory Study of Oxygen Deficiencies in SrCoO25

We report the effects of oxygen deficiency to the electronic and magnetic properties of SrCoO3 calculated using spin-polarized Density Functional Theory. The correlation effects of the Co 3d orbitals are considered explicitly using the effective Hubbard U eff parameter. The U eff consists of the subtraction between the U value of 3 eV to denote the Coulomb interaction and the Hund’s coupling parameter J of 0.95 eV. Our results show that the localized character of the Co 3d orbitals strongly depends on the oxygen content. The magnetic exchange interaction between two neighboring Co ions is therefore altered. As a consequence, a change of magnetic ground state using the same U eff value from ferromagnetic ordering in full oxygen content to antiferromagnetic ordering in SrCoO2.5 is obtained.

Intermediate spin state and the B1−B2 transition in ferropericlase

Ferropericlase (fp), (Mg$_{1-x}$Fe$_x$)O, the second most abundant mineral in the Earth's lower mantle, is expected to be an essential component of super-Earths' mantles. Here we present an ab initio investigation of the structure and magnetic ground state of fp up to $\sim$ 3 TPa with iron concentrations (x$_{Fe}$) varying from 0.03 to 0.12. Calculations were performed using LDA+U$_{sc}$ and PBE exchange-correlation functionals to elucidate the pressure range for which the Hubbard U (U$_{sc}$) is required. Similar to the end-members FeO and MgO, fp also undergoes a B1 to B2 phase transition that should be essential for modeling the structure and dynamics of super-Earths' mantles. This structural transition involves a simultaneous change in magnetic state from a low spin (LS) B1 phase with iron total spin S=0 to an intermediate spin (IS) B2 phase with S=1. This is a rare form of pressure/strain-induced magnetism produced by local cation coordination changes. Phonon calculations confirm the dynamical stability of the iron B2-IS state. Free energy calculations are then carried out including vibrational effects and electronic and magnetic entropy contributions. The phase diagram is then obtained for low concentration fp using a quasi-ideal solid solution model. For x$_{Fe}$ > 0.12 this approach is no longer valid. At ultra-high pressures, there is an IS to LS spin state change in Fe in the B2 phase, but the transition pressure depends sensitively on thermal electronic excitations and on x$_{Fe}$.

Clar’s Goblet on Graphene: Field-Modulated Charge Transfer in a Hydrocarbon Heterostructure

In certain configurations, the aromatic properties of benzene ring structured molecules allow for unpaired, reactive valence electrons (known as radicals). Clar's goblets are such molecules. With an even number of unpaired radicals, these nanographenes are topologically frustrated hydrocarbons in which pi-bonding network and topology of edges give rise to the magnetism. Clar's goblets are therefore valued as prospective qubits provided they can be modulated between magnetic states. Using first principles DFT, we demonstrate the effects of adsorption on both molecule and substrate in a graphene-Clar's goblet heterostructure. We look at the energy difference bewteen FM and AFM states of the system and discuss underlying physical and chemical mechanisms in reference to the highest occupied molecular orbital (HOMO) and second HOMO (HOMO-1). We find that the HOMO of the molecule in the FM state is right at the Fermi surface, which leads to the hybridization between molecular state and the graphene state near the Dirac point. Furthermore, we investigate qualitative changes in charge realignment and magnetic state under variable electric field. Transitions from FM to AFM and back to FM states are observed.

Electrical conductivity in helical and conical magnetic states

We theoretically study the electrical conductivity in a one-dimensional helimagnet whose spin texture changes from helimagnetic to conical magnetic, and to forced ferromagnetic state while increasing the magnetic field along the helical axis. We find that the conductivity in the helimagnetic state at zero field depends on the electron filling and the coefficient of the spin-charge coupling. We also find that the conductivity in the conical magnetic state changes nonlinearly to the applied field, and the magnetoresistance becomes negative and positive depending on the model parameters.

Emergence of Magnetic Transition in Cobalt Oxide Nanowires on Vicinal Pt Substrate

This letter is devoted to a theoretical study of the magnetic properties of supported pure and oxidized Co nanowires (NWs) on the vicinal surface of a Pt substrate. The main goal is to study the influence of oxygen on the magnetic properties of Co NWs. Transition metal atoms are magnetic, and oxygen effectively interacts with electromagnetic (EM) radiation in the optical range. Thus, by choosing the resonant frequency, it becomes possible to change the magnetic properties of oxide NW as a whole and switch the magnetic state of the wire by using an EM field. Our study has shown that Co oxide (CoO) NWs can exhibit magnetic properties in the ground state. Moreover, we observed the emergence of magnetic transition from a ferromagnetic to an antiferromagnetic state in a CoO NW under surface oxidation. We show that the magnetic properties of oxidized NWs depend strongly on the surface geometry. The study of the electronic structure of CoO NWs explains the reason for the formation of several magnetic states. The analysis of band structures of pure and oxidized Co NW shows that changes in magnetic states can be explained by the changes in band structure due to oxidation and that oxidation decreases the value of magnetic moments of the Co atoms in comparison to the pure NW. However, the values of these moments are still high.

Domain Wall Dynamics in a Ferroelastic Spin Crossover Complex with Giant Magnetoelectric Coupling.

Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [MnIII(3,5-diCl-sal2(323))]BPh4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc → Pc → P1 → P1(1/2). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn–Teller-active Mn3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc → P1 and P1 → P1(1/2) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1→ P1(1/2) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

Investigations of the size distribution and magnetic properties of nanoparticles of Cu2OSeO3

Skyrmions in confined geometries have been a subject of increasing interest due to the different properties that they exhibit compared to their bulk counterparts. In this study, nanoparticles of skyrmion-hosting Cu2OSeO3 have been synthesised using a precipitation method followed by thermal treatment. This enables us to produce nanoparticles whose mean size varies from tens of nanometers to a few micrometers by varying the temperature and duration of the thermal decomposition of the precursor. These sizes span the ∼63 nm diameter of skyrmions in Cu2OSeO3, allowing investigations into how the magnetic state changes when the size of the geometrical confinement is similar to and smaller than the size of an isolated magnetic skyrmion. AC susceptibility measurements performed on nanoparticles with a size distribution from 15 to 250 nm show a change in the magnetic phase diagram compared to bulk Cu2OSeO3.

Tuning the spin-flop transition in perpendicularly magnetized synthetic antiferromagnets by swift heavy Fe ions irradiation

Synthetic antiferromagnets (SAFs) with perpendicular magnetic anisotropy (PMA) have recently attracted intensive attention for their potential applications in domain-wall-based racetrack memory. In this paper, swift heavy Fe ion irradiation is introduced to tune the magnetic properties of Pt/Co/Pt/Ru/Pt/Co/Ta SAFs from spin-flip to spin-flop transition. The evolutions of magnetic domain and domain wall motion are investigated to obtain the microscopic insight into the change of magnetic states. Both the experimental and the simulated results demonstrate that different responses of the effective PMA constant and the interlayer-exchange-coupling strength to ion irradiation determine the magnetic-phase transitions. The structure evolution from experiments and theoretical simulations suggests that the ion-irradiation-induced damage mainly happens at the Pt/Co interface. The damage leads to the different decrease rates of the effective PMA constant and the interlayer-exchange-coupling strength due to their intrinsic direct and Ruderman-Kittel-Kasuya-Yosida interaction nature, respectively. A quite good thermal stability of samples is obtained with ion fluence $<1.5\ifmmode\times\else\texttimes\fi{}{10}^{14}\phantom{\rule{0.16em}{0ex}}\mathrm{ions}/\mathrm{c}{\mathrm{m}}^{2}$. In this paper, we demonstrate that ion irradiation is a promising method to tailor magnetic properties of SAFs. Additionally, the ion-irradiation-induced well-controlled antiferromagnetically coupled domains could show potential applications in spintronic devices.

Magnetic Switching by Oxygen Adsorption in Metal–Organic Framework Systems

In this letter, we address magnetization switching by oxygen adsorption in porous metal-organic framework systems. To this end, we construct a simple localized spin model combined with a Langmuir-type formula for oxygen adsorption and study its finite-temperature properties using Monte Carlo simulation. We successfully explain the main features of this phenomenon, such as the discontinuous changes in magnetic states, sensitivity of the magnetic transition temperatures to oxygen pressure, and absence of singularities in adsorbed oxygen. Based on this model, we also reproduce the observed magnetic transition temperatures for a typical value of oxygen adsorption energy.

Length-induced large magnetoresistance in polyacene molecular spin valves

With the help of first-principles method, an unusual length effect in polyacene molecular spin valves is revealed. It is found that beyond a critical length, the magnitude of tunneling magnetoresistance is significantly enhanced from 10% to 2500%. Theoretical analysis demonstrates that such significant increase of magnetoresistance is related to the nonmagnetic-antiferromagnetic phase transition of polyacene molecules, which is determined by the relative magnetization orientation of the ferromagnetic electrodes. The molecular gap shows distinct length dependence with the change of molecular magnetic state, which keeps decreasing with length in the nonmagnetic state while a fine gap is obtained in the antiferromagnetic state. This work indicates a valid way to obtain considerable tunneling magnetoresistance in long molecular spin valves.

Simulation of Two-Dimensional Images for Ion-Irradiation Induced Change in Lattice Structures and Magnetic States in Oxides by Using Monte Carlo Method

A Monte Carlo method was used to simulate the two-dimensional images of ion-irradiation-induced change in lattice structures and magnetic states in oxides. Under the assumption that the lattice structures and the magnetic states are modified only inside the narrow one-dimensional region along the ion beam path (the ion track), and that such modifications are affected by ion track overlapping, the exposure of oxide targets to spatially random ion impacts was simulated by the Monte Carlo method. Through the Monte Carlo method, the evolutions of the two-dimensional images for the amorphization of TiO2, the lattice structure transformation of ZrO2, and the transition of magnetic states of CeO2 were simulated as a function of ion fluence. The total fractions of the modified areas were calculated from the two-dimensional images. They agree well with the experimental results and those estimated by using the Poisson distribution functions.

Electronic structure, magnetic properties and martensitic transformation of Ga2MnTM (TM = Sc, Y, Lu) Heusler alloys

The atomic site occupation, electronic structure, magnetism, martensitic transformation of Ga2-based Heusler alloys Ga2MnTM (TM = Sc, Y, Lu) have been investigated by first-principles calculations. For three alloys in cubic phase, the L21-type structure is more stable than XA-type structure, and the magnetic state is ferrimagnetic (FIM) at each equilibrium lattice constant. The total magnetic moments are mainly contributed by Mn atoms due to its strong exchange splitting. For Ga2MnY and Ga2MnLu, the magnetic states between ferromagnetic (FM) state and FIM state alternately appear with increasing crystal constant. Potential martensitic transformation can be gained due to the proper thermodynamic driving force (negative values of ΔE) and c/a, together with the analysis of phonon spectra, by considering the tetragonal distortions of cubic Ga2MnTM (TM = Sc, Y, Lu). The energy minima locate at c/a = 1.29, 1.21, 1.27, respectively. The three alloys in martensitic state show different variations of total magnetic moments and still maintain FIM state with changing c/a. The martensitic transformation and abundant changes in magnetic states motivate researchers to further explore new Z2-based Heusler alloys.

Magnetization-affected localized flows in ferromagnetic metallic glass

In this paper, the interaction between magnetization and dynamic relaxation of heterogeneous structure in a ferromagnetic metallic glass (MG) is investigated. It is found that the magnetic state of the MG can be changed through wearing its surface. Accompanying with the change of magnetic state, the local dynamic mechanical behavior of the MG was detected by nanoindentation varies. Through analyzing with combination of the three-parameter viscoelastic model and state-transition theory, it is discovered that the microstructure of the glass can be apparently changed by magnetization with the effective size, viscosity of the liquid-like cores and the relaxation time increased by more than 3 times. Inversely, with cycling loading below the elastic limit of the MG using nanoindentation, it is revealed that localized flows of nano-liquid in the glass erase the effect of magnetization. The interaction between magnetization and localized flows relates to spin-orbit coupling in the MG; the change of magnetic state and liquid-like regions in the MG does not affect its instantaneous shear modulus associated with its ideal glassy state. The studies help understand the dynamic magneto-mechanical properties of amorphous magnetic materials, and have technological importance for their applications.