Metastable Magnetic States Research Articles

Metamagnetic transition and meta-stable magnetic state in Co-doped Fe3GaTe2.

The transition between the ferromagnetic (FM) and anti-ferromagnetic (AFM) phases in van der Waals (vdW) magnets has been extensively studied since the discovery of vdW magnets, due to the importance of both transitions within a single material. Recently, among vdW magnets, Fe3GaTe2 (FGaT) has garnered significant attention for its robust FM properties that remain stable above room temperature. Also, the FM to AFM phase transition in this material has been achieved through substitutional Co-atom doping at Fe sites. Here, we have reconfirmed the FM to AFM phase transition in FGaT and observed the metamagnetic transition between the two magnetic phases. Furthermore, the meta-stable magnetic state in 19-22% Co-doped FGaT in a certain field range was noted, which vanishes when the doping level increases further. Interestingly, when measuring the minor loop during the phase transition, its magnetization under a field-sweep reversing field is maintained in a meta-stable magnetic state region. The persistence of magnetization, which indicates the co-existence of AFM and FM domains in this meta-stable magnetic region, creates multi-level configurations that enable advanced applications in multi-level logic devices, neuromorphic computing, and applications involving magnetic domains. Our findings can expand the application scope and the utilization methods of vdW magnets.

Quantitative estimation of coercive field in a ferromagnetic grain using field sweep simulation

High coercivity is an important property of permanent magnets for application in energy conversion devices. The Nd magnet, ${\mathrm{Nd}}_{2}{\mathrm{Fe}}_{14}\mathrm{B}$, is a typical material. Because coercivity is a long-time relaxation phenomenon, which originates from a strong metastable magnetic state, it is difficult to estimate coercive field (coercive force) studying the time evolution dynamics simulation of a model with atomistic parameters under the limitation of the simulation time. In our recent study [M. Nishino et al., Phys. Rev. B 102, 020413(R) (2020)], we presented a method to estimate coercivity using a statistical method to extend the limitation of simulation time and evaluated appropriately the coercive field of a single grain of the Nd magnet. In the present study, we propose an alternative method to estimate coercivity more conveniently using the field-dependent survival (nonreversal) probability generated by a time evolution simulation under a field sweep. We demonstrate that the coercive field of the single grain can be estimated. In this method, not only coercive field but also the zero-field energy barrier and field for the zero-energy barrier can be estimated. We discuss detailed features of the estimation of these quantities.

Antisite disorder mediated exchange bias effect and spin-glass state in La[formula omitted]Sm[formula omitted]NiMnO[formula omitted

We report the synthesis of La2−xSmxNiMnO6 (x = 0, 0.1, 0.2) polycrystalline compounds through the sol–gel process using all-aqueous precursors. The influence of Sm-substitution on magnetic properties of La2−xSmxNiMnO6 (LSNMO) compounds are investigated through X-ray diffraction (XRD), dc-magnetization (Mdc), ac-magnetic susceptibility measurements, and Raman spectroscopy. XRD analysis confirms that the phase stability of LSNMO is improved significantly upon Sm-substitution as evident from a structural phase transformation of the compound from a biphasic (monoclinic + rhombohedral) into an aphasic (monoclinic) structure. The magnetic measurements indicate a formation of a metastable magnetic state with random ferromagnetic (FM) and antiferromagnetic (AFM) interactions below 100 K. The metastable spin configuration exhibits critical slow-down dynamics and memory effect that signifies a spin-glass (SG) state. The influence of Sm-doping reduces the Curie temperature (Tc) from 284 K → 245 K and enhances saturation magnetization (Ms). These findings are attributed to the combined effect of a reduced degree of antisite disorder upon the addition of Sm. A profound variation (45.5 → 72.9 K) in SG transition temperature (TSG) is also reported. Magnetization loops M(H) confirm the presence of an exchange bias (EB) effect that value suppressed prominently from 120 → 28 Oe upon Sm-substitution. The temperature-dependent magnetization M(T) and frequency-dependent ac-susceptibility χ[ω, T] are used to study the antisite-driven SG phase, spin relaxation dynamics, magnetic memory, and rejuvenation effect. Raman spectroscopy is employed to probe the degree of antisites disorder that reduces after Sm-addition. The phase stability, lower Tc, tunable Ms, and EB in the single-phase compound make LSNMO a potential candidate for its applications in realizing energy-efficient devices.

Prediction of topological phases in metastable ferromagnetic MPX3 monolayers

Density functional theory calculations are carried out to study the electronic and topological properties of $M$P$X_3$ ($M$ = Mn, Fe, Co, Ni, and $X$ = S, Se) monolayers in the ferromagnetic (FM) metastable magnetic state. We find that FM MnPSe$_3$ monolayers host topological semimetal signatures that are gapped out when spin-orbit coupling (SOC) is included. These findings are supported by explicit calculations of the Berry curvature and the Chern number. The choice of the Hubbard-$U$ parameter to describe the $d$-electrons is thoroughly discussed, as well as the influence of using a hybrid-functional approach. The presence of band inversions and the associated topological features are found to be formalism-dependent. Nevertheless, routes to achieve the topological phase via the application of external biaxial strain are demonstrated. Within the hybrid-functional picture, topological band structures are recovered under a pressure of 15% (17 GPa). The present work provides a potential avenue for uncovering new topological phases in metastable ferromagnetic phases.

Impact of Magneto-Electric Coupling on Metastable Magnetic States in Thin Disks

The role of the electric field on the magnetic ground state in thin disks is investigated. The equation governing the magnetic equilibrium in the case of rotational symmetry is derived and solved by varying electric and magnetic fields. The equilibrium states so obtained are classified in a phase diagram, which divides the plane of the electric and magnetic fields in regions where the magnetization profile has certain shape. Full micromagnetic simulations are used to confirm our analysis.

Efficient Method for Prediction of Metastable or Ground Multipolar Ordered States and Its Application in Monolayer α-RuX_{3} (X=Cl, I).

Exotic high-rank multipolar order parameters have been found to be unexpectedly active in more and more correlated materials in recent years. Such multipoles are usually dubbed "hidden orders" since they are insensitive to common experimental probes. Theoretically, it is also difficult to predict multipolar orders via abinitio calculations in real materials. Here, we present an efficient method to predict possible multipoles in materials based on linear response theory under random phase approximation. Using this method, we successfully predict two pure metastable magnetic octupolar states in monolayer α-RuCl_{3}, which is confirmed by self-consistent unrestricted Hartree-Fock calculations. We then demonstrate that these octupolar states can be stabilized in monolayer α-RuI_{3}, one of which becomes the octupolar ground state. Furthermore, we also predict a fingerprint of an orthogonal magnetization pattern produced by the octupole moment that can be easily detected by experiment. The method and the example presented in this Letter serve as a guide for searching multipolar order parameters in other correlated materials.

Effect of nonmagnetic substituent Zn on the phase competition and multiferroic properties in the polar magnet Fe2Mo3O8

The polar magnet, Fe2Mo3O8 (FMO), with linear magnetoelectric (ME) coupling, is a promising candidate for multiferroic applications in advanced spin devices. However, a giant magnetic bias (Hb) is needed for optimizing the inverse ME effect, i.e., electric field (E) modulation of magnetization (M), which is still a core issue. Herein, we utilize the chemical doping route to enhance the sensitivity of controlling the competitive magnetic interactions and/or multiferroic phases by means of introducing nonmagnetic Zn2+ ions into FMO crystals. Compared with FMO, the Zn-doped composition (Fe0.95Zn0.05)2Mo3O8 (FZMO) generates three metastable magnetic states in the middle of antiferromagnetic and ferrimagnetic states, along with obvious ferroelectric polarization. The inverse ME effect of FZMO is intact with a relative change of ΔM ≈ 0.06 μB/f.u. responding to an E value of ± 20 kV/cm at 52 K. Most interestingly, the exciting Hb is dramatically dropped to 0.8 T for FZMO from 5.1 T for FMO, which is in favor of the application of ME coupling. It is suggested that the perturbation of magnetic interactions via substituting specific sites by nonmagnetic ions plays a key role in decreasing the exciting Hb without deteriorating the inverse ME coupling in this polar M2Mo3O8 family.

Transverse susceptibility of nickel thin films with uniaxial anisotropy

A finite temperature Stoner–Wohlfarth model has been used to calculate the transverse susceptibility of an ensemble of ferromagnetic particles with distributed anisotropy. The simulated transverse susceptibility is in excellent agreement with data acquired from thin film samples of elemental nickel, deposited on 128° Y-cut LiNb03. A strong, well-defined, uniaxial anisotropy is induced in the nickel film by low temperature annealing. Three peaks in the transverse susceptibility are observed in both the measured and simulated data when the applied field is misaligned with the hard axis by a few degrees. Two broad, reversible peaks occur when the applied field is equal to the anisotropy field. A single, sharp irreversible peak occurs when the absolute value of the applied field is less than the anisotropy field, and is associated with a metastable magnetic state. The irreversible peak disappears when the applied field is well aligned with the hard axis. The observed transverse susceptibility is consistent with the theoretical predictions of Aharoni et al. and is therefore consistent with the Stoner–Wohlfarth model.

Restoration of ion beam irradiation induced metastable magnetic states and lattice structures of FeRh thin films by heat treatments at elevated temperatures

The restoration process of irradiation-induced metastable magnetic states of FeRh thin films during the heat treatments at 373–1073 K was investigated by measuring the magnetization-magnetic field (M−H) curves and the magnetization-temperature (M−T) curves for the FeRh thin films irradiated with 50 keV Ar ions to various fluences. The recovery of lattice structures accompanying the magnetic restoration was also examined by an x-ray diffraction (XRD) measurement. For the ferromagnetic state induced by low fluence irradiation, the magnetic restoration to the thermal equilibrium antiferromagnetic state occurs during the heat treatment. Whereas for the paramagnetic state induced by high fluence irradiation, it once changes to the ferromagnetic state, and then, is restored to the antiferromagnetic (i.e., thermal equilibrium) state during the heat treatment. The above magnetic transformation occurs by the heat treatments at higher temperatures for the higher fluence irradiation. Moreover, the magnetic transition temperature from the antiferromagnetic state, which has been thermally restored from the irradiation induced ferromagnetic or paramagnetic state, to the ferromagnetic state decreases with increasing the irradiation fluence. This result implies that the antiferromagnetic state with larger amount of lattice defects is more unstable against the thermal vibration of lattice atoms.

Realizing both short- and long-term memory within a single magnetic tunnel junction based synapse

Synaptic plasticity forms the basis of memory retention in the human brain. Whereas a low “rehearsal” rate causes short-term changes in the synaptic connections such that the synapse soon “forgets,” a high rehearsal rate ensures long-term retention of memory in the brain. In this paper, we propose an artificial short- and long-term memory magnetic tunnel junction (SALT-MTJ) synapse. Changes in the synaptic strength are mapped to the SALT-MTJ conductance, which is varied stochastically via spin-transfer torque resulting from input current stimuli. A meta-stable intermediate magnetic state of the SALT-MTJ synapse provides short-term synaptic plasticity and the associated forgetting behavior as in a biological synapse. Repeated spin-current stimulations, while the SALT-MTJ remains in the short-term state, then can cause a near-permanent change in the magnetic state and associated conductance to provide long-term potentiation. The synaptic weight sensitivity to the input stimulus and the forgetting behavior of these short- and long-term states can be controlled via shape engineering of the artificial synapse.

High temperature spin cluster glass behavior in Co- and Si-substituted garnet ferrite thin films

In this study, lutetium iron garnet (Lu3Fe5O12; LuIG) thin films have been grown on Y3Al5O12 (1 1 1) substrates by a pulsed laser deposition and are co-substituted with strong magnetically anisotropic ions of Co2+ and nonmagnetic ones of Si4+. LuIG is a garnet-structured ferrimagnet with an exceptionally low Gilbert damping constant as 10−5. The substitution of Co2+ and Si4+ induces a chemical disorder and competing antiferromagnetic and ferromagnetic interactions, which induce a spin glass behavior. When the Co–Si substitution ratio is increased, a spin-freezing behavior is observed below the spin-freezing temperature TG = 190–220 K of the Lu3Fe4Co0.5Si0.5O12 film; this indicates the presence of a cluster glass phase. The film demonstrates a characteristic memory effects in the temperature range 120–180 K, which are below TG; this reveals the presence of a metastable magnetic state in the multi-valley energy landscape with shallow activation energy barriers. Ferromagnetic resonance measurements confirm increased perpendicular anisotropy energy induced by Co substitution and spin-wave characteristics including the inverse spin Hall effect voltage (VISHE). Lu3Fe5-2xCoxSixO12 films show an increase in damping and a decrease in VISHE at the cryogenic temperature, which are attributed to the impurity relaxation mechanism. Therefore, the Co- and Si-substituted films exhibit strong spin-wave damping at a low temperature, which reflects the spin-freezing dynamics of cluster glass behaviors.

Changing the magnetic states of an Fe/BaTiO3 interface through crystal field effects controlled by strain.

The search for better and inexpensive magnetoelectric materials is now commonplace in solid state physics. Intense coupling between technologically viable electric and magnetic properties, embedded in a single material, is still an attribute greatly pursued by the scientific community. Following this line of thought, using DFT, the study of a specific interface between the TiO2 layer of BaTiO3 and a monolayer of Fe atoms is presented, probing different uni-axial strain effects of the considered supercell. Depending on the strain, several different metastable magnetic states are predicted: a perfectly balanced antiferromagnetic state, an unbalanced ferrimagnetic state, a ferromagnetic state, and a non-magnetic state where each atom has its total magnetic moment quenched. Since these multiple magnetic states can be reversibly controlled by strain, under optimized conditions, this interface can switch from the ferromagnetic state (μ≈ 2.2 μB per Featom) to the non-magnetic state (μ = 0 μB per Featom), enabling enticing prospects for technological applications.

Influence of a Strong Magnetic Field on Magnetic Order in Polycrystalline Layered Cobaltites EuBaCo2 –xO5.5 – δ

The magnetic properties of polycrystalline EuBaCo2 –xO5.5 – δ (x ≤ 0.1, δ ≤ 0.14) are studied in the temperature range 2 K < T < 300 K in magnetic fields H ≤ 90 kOe. A comparison is made of the dependencies of magnetization M on temperature T and magnetic field H for polycrystalline and single crystal EuBaCo1.90O5.36, in which only Cu(III) ions are present. It is shown that in the magnetically ordered temperature range of magnetic order, the samples are ferrimagnetic, in which antiferromagnetic interactions between the nearest Co3+ ions with the same spins compete with the ferromagnetic exchange. With a decrease in temperature, a strong magnetic field affects the magnetic order. At T = 2 K in fields H = 50–90 kOe, all samples exhibit the properties of mictomagnetism. There are a shift in the hysteresis loops M(H) and a large difference in the values of the magnetization of the sample cooled in a magnetic field M (FC) and without a field M (ZFC) due to unidirectional magnetic anisotropy. In this case, there is no saturation of magnetization. The field dependences of magnetization taken in the cooling and heating in unequal fields (CHUF) mode indicate a metastable magnetic state. Near T = 20–30 K, an orientational transition characteristic of micromagnets takes place and a complex ferrimagnetic structure is formed. The main reasons for the manifestation of mictomagnetism properties in layered cobaltites are the presence of magnetoactive Co3+ ions in the states with intermediate and high spins at low temperatures and structural defects that affect the exchange interaction between the nearest neighbors. Nonmagnetic Co3+ ions in the low-spin state can also act as defects.

Emergence of ferromagnetism and metallicity in Pr0.5Sr0.5MnO3 nanoparticles

In this paper, we have studied low-temperature magnetic properties and transport dynamics of half-doped Pr0.5Sr0.5MnO3 (PSMO) nano-manganite. Nanoparticles with different average grain sizes are synthesized using chemical pyrophoric reaction route and exhibit same crystal structure. It is reported that particle size reduction in half-doped PSMO manganite interestingly leads to ferromagnetic (FM) metallic behavior which is accompanied by coupled positive magnetoresistance (PMR) (up to 9%) at fields (H ≤ 1kOe at 50K) and negative magnetoresistance (NMR) (∼59% at 100K) at 80kOe fields. Interestingly, maximum PMR is appeared at much lower than the critical field (30kOe) for the melting of insulating -antiferromagnetic (AFM) ordered state. Moreover, it is found that low-temperature AFM-FM phase transition, TN ∼ 135K is smeared out with reduction of particle size; however, the transition is prominent in bulk material. A metastable magnetic state with a mixed fraction of FM and AFM phases is observed below TN. Superparamagnetic (SPM) behavior of PSMO nanoparticles has been revealed by probing ac-susceptibility measurements. Low-temperature transport dynamics has been investigated through four probe resistivity and magneto-transport measurements. A size-induced insulating-metallic phase transition is observed on nano dimension in the range of 125-155K under high magnetic fields; however, the bulk counterpart remains insulating over the entire temperature range. The non-trivial electronic- and magnetotransport properties are explained by the inter-grain spin-polarized hopping mechanism through double exchange (DE) interactions. These issues have been revisited and discussed in the framework of enhanced surface disorder, where surface spins plays the crucial role for inter-grain transport dynamics in nanometric dimension. We believe the present field dependent MR holds excellent potential for future spintronic devices and manifests the exotic properties of strongly correlated materials.

Magnetically induced ferroelectricity in Bi2CuO4

The tetragonal copper oxide ${\mathrm{Bi}}_{2}{\mathrm{CuO}}_{4}$ has an unusual crystal structure with a three-dimensional network of well separated ${\mathrm{CuO}}_{4}$ plaquettes. The spin structure of ${\mathrm{Bi}}_{2}{\mathrm{CuO}}_{4}$ in the magnetically ordered state below ${T}_{\text{N}}\ensuremath{\sim}43$ K remains controversial. Here we present the results of detailed studies of specific heat, magnetic, and dielectric properties of ${\mathrm{Bi}}_{2}{\mathrm{CuO}}_{4}$ single crystals grown by the floating zone technique, combined with the polarized neutron scattering and high-resolution x-ray measurements. Down to 3.5 K our polarized neutron scattering measurements reveal ordered magnetic Cu moments which are aligned within the $ab$ plane. Below the onset of the long range antiferromagnetic ordering we observe an electric polarization induced by an applied magnetic field, which indicates inversion symmetry breaking by the ordered state of Cu spins. For the magnetic field applied perpendicular to the tetragonal axis, the spin-induced ferroelectricity is explained in terms of the linear magnetoelectric effect that occurs in a metastable magnetic state. A relatively small electric polarization induced by the field parallel to the tetragonal axis may indicate a more complex magnetic ordering in ${\mathrm{Bi}}_{2}{\mathrm{CuO}}_{4}$.

Magnetic glass state and magnetoresistance in SrLaFeCoO6 double perovskite

Unusual features in magnetization resembling the kinetic arrest of a magnetic glass state are observed in the La-doped double perovskite, SrLaFeCoO6. Neutron powder diffraction experiments confirm the presence of antisite disorder as well as a lack of long-range magnetic order down to 4 K in this double perovskite which displays spin glass-like features in dc and ac susceptibilities. Magnetic relaxation observed through cooling and heating under unequal fields (CHUF) point towards unusual domain dynamics which is supported by a broad memory effect. Among the two anomalies that are observed at 75 K and at 250 K in the magnetic measurements, the former is associated with a spin-freezing temperature below which the magnetic glass state is experimentally verified. The magnetometric experiments detailed in the paper bring out the non-equilibrium metastable magnetic states in this disordered magnetic system. The magnetic glass state described above manifests in the electrical resistivity through the formation of a ‘hard gap’ because of the spin-exchange energy following the formation of magnetic glass. It is observed that the combination of disorder and magnetic glass state leads to a large, negative magnetoresistance (MR) of ≈47 at 5 K in 8 T.

Ultrafast nonthermal photo-magnetic recording in a transparent medium.

Finding a conceptually new way to control the magnetic state of media with the lowest possible production of heat and simultaneously at the fastest possible speeds is a new challenge in fundamental magnetism1–5. Recent results demonstrate that exclusively in metals it is possible to switch the magnetization between two stable states, and thus to record magnetic bits, by femtosecond circularly polarized laser pulses6–8. However, the switching mechanisms in these materials are directly related to laser-induced heating close to the Curie temperature9–12. While several possibilities for nonthermal all-optical switching in magnetic dielectrics have been discussed13,14, no recording was demonstrated. Here we report about ultrafast all-optical photo-magnetic recording in dielectrics. In ferrimagnetic Co-substituted garnet film, a single linearly polarized femtosecond laser pulse resonantly pumps specific d–d transitions in the Co-ions, breaking the degeneracy between metastable magnetic states. By changing the polarization of the laser pulse, we deterministically steer the net magnetization in the garnet, thus writing ”0” and “1” magnetic bits at will. This mechanism outperforms existing alternatives allowing the fastest write-read magnetic recording event (< 20 ps) accompanied by unprecedentedly low heat load (< 6 J cm-3).

Magnetic exchange force microscopy: theoretical analysis of induced magnetization reversals.

In magnetic exchange force microscopy a magnetic tip is scanned over the surface of a solid and an image representing the exchange interaction recorded. Sudden changes in the image corresponding to magnetization switching can be monitored as a function of the tip-surface distance thereby giving important information about the lifetime of metastable magnetic states and how it is affected by the exchange interaction. Here, theoretical calculations are carried out to study the tip-surface interaction and determine the mechanism and rate of transitions in a magnetic exchange force microscopy experiment, and comparison made with reported experimental data on an Fe cluster interacting with an antiferromagnetic Fe overlayer on a W(001) surface. The activation energy was determined from calculations of minimum energy paths and the pre-exponential factor in the Arrhenius rate expression evaluated from harmonic transition state theory, extended to account for zero modes. A noncollinear extension of the Alexander-Anderson model was used to describe the magnetic properties of an atomic scale representation of the system. The calculations reveal how the tip size, tip-surface distance and magnetic field affect the lifetime of the magnetic states.

Domain wall pinning in FeCoCu bamboo-like nanowires.

The three dimensional nature of cylindrical magnetic nanowires has opened a new way to control the domain configuration as well as the magnetization reversal process. The pinning effect of the periodic diameter modulations on the domain wall propagation in FeCoCu individual nanowires is determined by Magnetic Force Microscopy, MFM. A main bistable magnetic configuration is firstly concluded from MFM images characterized by the spin reversal between two nearly single domain states with opposite axial magnetization. Complementary micromagnetic simulations confirm a vortex mediated magnetization reversal process. A non-standard variable field MFM imaging procedure allows us to observe metastable magnetic states where the propagating domain wall is pinned at certain positions with enlarged diameter. Moreover, it is demonstrated that it is possible to control the position of the pinned domain walls by an external magnetic field.

Magnetic states in multiply-connected flat nanoelements

Flat magnetic nanoelements are an essential component of current and future spintronic devices. By shaping an element it is possible to select and stabilize chosen metastable magnetic states, control its magnetization dynamics. Here, using a recent significant development in mathematics of conformal mapping, complex variable based approach to the description of magnetic states in planar nanoelements is extended to the case when elements are multiply-connected (that is, contain holes or magnetic antidots). We show that presence of holes implies a certain restriction on the set of magnetic states of nanoelement.