Unusual Magnetization Research Articles

Unusual magnetism from lead adsorption on the surface of brookite titanium dioxide thin film

The adsorption of Lead (Pb) on brookite titanium dioxide thin films is determined using spin-polarized density functional theory. Band structures as well as density of states are shown. Magnetic irregularities are observed in thin layers that Pb atoms adsorb. The thin film has been identified to have a pivotal thickness of 1.6 nm. Beyond the essential thickness, Pb is unable to cause magnetic anomalies. The atypical magnetic properties stem from the quantum confinement phenomenon and the relaxation of the surface. The film’s long-range sequence presents complexity. The positioning of defects is dictated by the concentration of Pb volume rather than its concentration on the surface. The thin film exhibits a pivotal interaction range of 1.0 nm. A gap between two neighboring spins can lead to fluctuations in the magnetic configuration’s stability or instability, contingent on the Pb volume concentration. If not, the impact of long-range order is minimal, and the stability between magnetic and non-magnetic setups is nearly identical.

Ultrafast magnetization enhancement via the dynamic spin-filter effect of type-II Weyl nodes in a kagome ferromagnet.

The magnetic type-II Weyl semimetal (MWSM) Co3Sn2S2 has recently been found to host a variety of remarkable phenomena including surface Fermi-arcs, giant anomalous Hall effect, and negative flat band magnetism. However, the dynamic magnetic properties remain relatively unexplored. Here, we investigate the ultrafast spin dynamics of Co3Sn2S2 crystal using time-resolved magneto-optical Kerr effect and reflectivity spectroscopies. We observe a transient magnetization behavior, consisting of spin-flipping dominated fast demagnetization, slow demagnetization due to overall half-metallic electronic structures, and an unexpected ultrafast magnetization enhancement lasting hundreds of picoseconds upon femtosecond laser excitation. By combining temperature-, pump fluence-, and pump polarization-dependent measurements, we unambiguously demonstrate the correlation between the ultrafast magnetization enhancement and the Weyl nodes. Our theoretical modelling suggests that the excited electrons are spin-polarized when relaxing, leading to the enhanced spin-up density of states near the Fermi level and the consequently unusual magnetization enhancement. Our results reveal the unique role of the Weyl properties of Co3Sn2S2 in femtosecond laser-induced spin dynamics.

A Gadolinium(III) Complex Based on Pyridoxine Molecule with Single-Ion Magnet and Magnetic Resonance Imaging Properties.

Pyridoxine (pyr) is a versatile molecule that forms part of the family of B vitamins. It is used to treat and prevent vitamin B6 deficiency and certain types of metabolic disorders. Moreover, the pyridoxine molecule has been investigated as a suitable ligand toward metal ions. Nevertheless, the study of the magnetic properties of metal complexes containing lanthanide(III) ions and this biomolecule is unexplored. We have synthesized and characterized a novel pyridoxine-based GdIII complex of formula [GdIII(pyr)2(H2O)4]Cl3 · 2 H2O (1) [pyr = pyridoxine]. 1 crystallizes in the triclinic system and space group Pī. In its crystal packing, cationic [Gd(pyr)2(H2O)4]3+ entities are connected through H-bonding interactions involving non-coordinating water molecules and chloride anions. In addition, Hirshfeld surfaces of 1 were calculated to further investigate their intermolecular interactions in the crystal lattice. Our investigation of the magnetic properties of 1, through ac magnetic susceptibility measurements, reveals the occurrence of a slow relaxation in magnetization in this mononuclear GdIII complex, indicating an unusual single-ion magnet (SIM) behavior for this pseudo-isotropic metal ion at very low temperatures. We also studied the relaxometric properties of 1, as a potential contrast agent for high-field magnetic resonance imaging (MRI), from solutions of 1 prepared in physiological serum (0.0-3.2 mM range) and measured at 3 T on a clinical MRI scanner. The values of relaxivity obtained for 1 are larger than those of some commercial MRI contrast agents based on mononuclear GdIII systems.

Giant magnetic anisotropy energy and Os–Ir coupling driven variations in the physical properties of Y[formula omitted]NiIrO[formula omitted

Spintronics is a prosperous domain that comprises colossal applications executing both the charge and spin of the electrons. So double perovskite oxides (DPO) having unusual magnetism are very relevant for these objectives. Therefore, we systematically examine the effect of strong coupling between Ir and Os 5d orbitals on the structural stabilities, electronic structure, and magnetic properties of the Y2NiIrO6 DPO using ab-initio calculations by replacing Os with one of the Ir ion (Os@Ir). We predict that the antiferromagnetic spin–orbit coupling between half-filled Ni 3d and partially-filled Ir 5d in the pristine structure, results in a ferrimagnetic (FiM) Mott-insulating phase due to unusual Jeff=12 state of Ir+4 having an energy band gap (Eg) of 0.43 eV. The calculated partial spin moments (ms) on the Ni+2 and Ir+4 ions are 1.68 and −0.49μB with electronic configurations of t2g3↑t2g3↓eg2↑eg0↓ (S = 1) and t2g3↑t2g2↓eg0 (S =12), respectively. Furthermore, our estimated Curie temperature (TC) of 198 K employing the Heisenberg model is near to the experimentally examined value of 192 K. The easy magnetic axis yields to be the b-axis with a giant magnetocrystalline anisotropy energy (MAE) constant of 1.7×108 erg/cm3, which is the origin of a huge coercive field of 1.1 T. In the case of the Os@Ir motif, Eg closed where the filled Ir t2g and partially filled Os t2g orbitals are mainly responsible for conductivity. Surprisingly, the ms on the Ir ion almost becomes zero (+0.01μB), turns out to be in a +3 state with t2g3↑t2g3↓eg0 (S = 0) despite +4 along with 1.68/−1.55μB on the Ni+2/Os+5. Finally, MAE value and TC in Os-doped structure increase due to large structural distortions as compared to pristine one which makes the system technologically important.

Probing magnetoelectric effect in the spin-modulated magnet Fe2GeO4

The distinct spin amplitude wave was reported in a highly frustrated magnetic compound Fe2GeO4, which is very different from observations on other members of the M2GeO4 (M = Fe, Co, and Ni) family, raising interest in this compound for some additional emergent phenomena. In particular, this non-uniform spin order allows the intrinsic connection between ferroelectric polarization and magnetically gradient structure to probe the potential linear magnetoelectric (ME) effect. In this work, we address this issue and investigate the magnetism of Fe2GeO4 single crystal that hosts two successive anomalies at antiferromagnetic (AFM) Néel temperatures T N1 ∼ 7.5 K and T N2 ∼ 6.7 K, respectively. Our results reveal a remarkable metamagnetic transition in the magnetization as a function of the magnetic field, occurring at a critical magnetic field H c ∼ 4.1 T when applied along the [110] and [1–10] directions, while such transition along the [001] direction is pointedly absent. Further exploration uncovers two predominant off-diagonal ME coefficients αyz and αzy in the incommensurate AFM phase between T N1 and T N2. Additionally, all components of the linear ME tensor remain non-vanishing in the canting AFM phase below T N2. This indicates the ME mechanisms for the two phases that may be driven by different magnetic structures. All these presented results are sufficient for us to draw a non-trivial ME phase diagram, which is beneficial to understanding the ME behavior of Fe2GeO4. Therefore, our study implies that Fe2GeO4, an unusual frustrated magnet, provides a platform for manipulating the fascinating ME effect in the spinel structure.

Magnetization steps at the ferromagnetic transition of (Mn,Fe)2(P,Si) single crystals

Single crystals are highly desirable to study intrinsic properties of phase transitions, unfortunately due to an embrittlement at the transformation, (Mn,Fe)2(P,Si) studies were so far limited to polycrystalline samples. Here, we report the growth of (Mn,Fe)2(P,Si) single crystals which can be cycled across their first-order ferromagnetic transition. In single crystals with optimized composition, detailed magnetization measurements reveal a ferromagnetic transition whose shape is significantly different from polycrystalline samples, with unusual discontinuous magnetization steps induced by temperature or magnetic field leading to a stair-like transition. In contrast to the “virgin effect” of (Mn,Fe)2(P,Si) samples, this behavior is reproducible. This unique magnetic response of some (Mn,Fe)2(P,Si) crystals can be correlated to a microstructure very different from that of the bulk and preserving the clamping/release role played by strains and stresses at the ferro-paramagnetic transformation. On top of offering more details on how the first-order magnetic transition develops in time, space and as a function of the driving parameter, which is of importance for designing functional magnetocaloric materials, the growth of cyclable crystals opens new possibilities for future experiments.

Magnetization reversal through an antiferromagnetic state

Magnetization reversal in ferro- and ferrimagnets is a well-known archetype of non-equilibrium processes, where the volume fractions of the oppositely magnetized domains vary and perfectly compensate each other at the coercive magnetic field. Here, we report on a fundamentally new pathway for magnetization reversal that is mediated by an antiferromagnetic state. Consequently, an atomic-scale compensation of the magnetization is realized at the coercive field, instead of the mesoscopic or macroscopic domain cancellation in canonical reversal processes. We demonstrate this unusual magnetization reversal on the Zn-doped polar magnet Fe2Mo3O8. Hidden behind the conventional ferrimagnetic hysteresis loop, the surprising emergence of the antiferromagnetic phase at the coercive fields is disclosed by a sharp peak in the field-dependence of the electric polarization. In addition, at the magnetization reversal our THz spectroscopy studies reveal the reappearance of the magnon mode that is only present in the pristine antiferromagnetic state. According to our microscopic calculations, this unusual process is governed by the dominant intralayer coupling, strong easy-axis anisotropy and spin fluctuations, which result in a complex interplay between the ferrimagnetic and antiferromagnetic phases. Such antiferro-state-mediated reversal processes offer novel concepts for magnetization control, and may also emerge for other ferroic orders.

Enhancing the Squareness and Bi-Phase Magnetic Switching of Co2FeSi Microwires for Sensing Application.

In the current study we have obtained Co2FeSi glass-coated microwires with different geometrical aspect ratios, ρ = d/Dtot (diameter of metallic nucleus, d and total diameter, Dtot). The structure and magnetic properties are investigated at a wide range of temperatures. XRD analysis illustrates a notable change in the microstructure by increasing the aspect ratio of Co2FeSi-glass-coated microwires. The amorphous structure is detected for the sample with the lowest aspect ratio (ρ = 0.23), whereas a growth of crystalline structure is observed in the other samples (aspect ratio ρ = 0.30 and 0.43). This change in the microstructure properties correlates with dramatic changing in magnetic properties. For the sample with the lowest ρ-ratio, non-perfect square loops are obtained with low normalized remanent magnetization. A notable enhancement in the squareness and coercivity are obtained by increasing ρ-ratio. Changing the internal stresses strongly affects the microstructure, resulting in a complex magnetic reversal process. The thermomagnetic curves show large irreversibility for the Co2FeSi with low ρ-ratio. Meanwhile, if we increase the ρ-ratio, the sample shows perfect ferromagnetic behavior without irreversibility. The current result illustrates the ability to control the microstructure and magnetic properties of Co2FeSi glass-coated microwires by changing only their geometric properties without performing any additional heat treatment. The modification of geometric parameters of Co2FeSi glass-coated microwires allows to obtain microwires that exhibit an unusual magnetization behavior that offers opportunities to understand the phenomena of various types of magnetic domain structures, which is essentially helpful for designing sensing devices based on thermal magnetization switching.

The penta-hexa silicene: A promising candidate for intrinsic room temperature magnetic semiconductor

Performing ab initio calculations, we investigate electronic and magnetic properties of a silicon allotrope (PH-silicene) composed entirely by six silicon pentagons and two silicon hexagons. The dynamically and mechanically stable PH-silicene hosts two-dimensional honeycomb spin structures, which can be antiferromagnetic, ferromagnetic, or ferrimagnetic depending on the applied tensile strain and/or number of stacked layers. In particular, the transition temperature of an in-plane antiferromagnetic ground state and a strain-induced ferromagnetic state of monolayer PH-silicene is found to be around 533 and 80 K, respectively. This unusual metal-free magnetism can be explained by the d0 charge transfer mechanism. On the other hand, we show that the PH-silicene is an indirect semiconductor with the bandgap of 0.585 eV. When stacking up to 4-layers, they vary from the semiconductor, the semimetal to the normal metal. Our findings suggest PH-silicene as a promising candidate for the room temperature magnetic semiconductor and will pave a way for silicon based spintronic devices.

Spin crossover of the octahedral Co ion in Co3S4 : Emergence of hidden magnetism

It is well established that the ground-state electron configuration of the octahedral ${\mathrm{Co}}^{3+}$ ion is ${t}_{2\mathrm{g}}^{6}{e}_{\mathrm{g}}^{0}$, which corresponds to a low-spin (LS) state. However, we theoretically demonstrate that the octahedral ${\mathrm{Co}}^{3+}$ ion in ${\mathrm{Co}}_{3}{\mathrm{S}}_{4}$ prefers a high-spin (HS) state with the ${t}_{2\mathrm{g}}^{4}{e}_{\mathrm{g}}^{2}$ configuration, resulting in unusual magnetism. The density-functional theory plus $U$ calculation and ligand-field theory show that weak crystal-field splitting associated with ${\mathrm{S}}^{2\ensuremath{-}}$ induces a spin crossover from the LS to HS state of the octahedral ${\mathrm{Co}}^{3+}$ ion along with a weak Jahn-Teller-like elongation, and as a result, a ferromagnetic (FM) metal phase is energetically stabilized. Nevertheless, this phase is significantly more stable than the antiferromagnetic (AFM) phase experimentally reported at low temperature. Furthermore, phonon calculations suggest that the FM metal phase is possibly one of the ${\mathrm{Co}}_{3}{\mathrm{S}}_{4}$ polymorphs, appearing in the certain experimental growth environment, but it is not expected to appear due to a temperature-dependent transition from the AFM phase. In addition, the Lyons, Kaplan, Dwight, and Menyuk theory shows that this phase is further expected to undergo a phase transition to the frustrated magnetic state. We believe that our work provides insight into magnetism of cobalt compounds and also paves the way to achieve the HS ${\mathrm{Co}}^{3+}$ by design.

Structural and physical characterization of iron-oxide based inks for inkjet printing

Iron-oxide nanopowders, hematite and magnetite, are basic compounds applied in several steps for the preparation of magnetic inks. These steps included ball milling of the nanopowders in dowanol, mixing with polysiloxane binder dissolved in pure ethanol, and final dilution in hexanol to adjust the rheological parameters of the inks applied in the inkjet printing technology under ambient conditions. Various experimental methods, carried out for the samples in all forms (powder, ink, and layer), have yielded basic data concerning morphology, chemical, and physical properties and allowed to follow their changes during individual processing steps. The phase analysis of both the pristine iron-oxides did not confirm the pure single-phase compositions. The hematite contained also a substantial amount of magnetite and maghemite contributing to its unusual high saturation magnetization, Ms, of 66 Am2kg−1. The magnetite contained about 38 % maghemite and its Ms was 117 Am2kg−1. The magnetic parameters of both the iron-oxides in the nanopowder and ink forms were comparable contrary to those measured in the solidified state after printing. Substantial decrease in the Ms values and changes of the remnant magnetization and coercivity were observed in both cases. The obtained results were verified by measurement of a designed magnetite core printed on the flexible foil and tested as a magnetic sensor. Nevertheless, despite the worse magnetic parameters, the relative permeability, approximately 2, was twice as high as that reported by other authors.

Anomalous magnetic behavior in half-metallic Heusler Co2FeSi alloy glass-coated microwires with high Curie temperature

In the current study, we have succeeded to fabricate Co2FeSi Heusler alloy glass-covered microwires with magnetic core nucleus diameter d = 4.36 µm and total diameter D = 17.55 µm, with high Curie temperature (Tc>1100 K) and well-defined magnetic anisotropy for high temperature spintronic devices application. The magnetic properties of as-prepared and annealed at different temperature (873 K, 973 K and 1073 K for 1 h) of Co2FeSi Heusler alloy glass-covered microwires have been investigated. Strong dependence of the magnetic properties on the annealing conditions has been indicated. Anomalous magnetic behavior for annealed samples at 873 K and 973 K has been found and structural properties of such samples have been analyzed. Critical temperatures 155 K and 250 K have been detected for annealed samples at 873 K and 973 K, respectively, where the behavior of M-H loops and coercivity changed. Below the critical point the M-H curve shows “kink or wasp-waisted” magnetic behavior and complex magnetic reversal mechanism is supposed. The anomalous magnetic behavior is due to the martensitic phases induced by annealing conditions below the room temperature. This unusual magnetization behavior provides opportunities to understand the phenomena of different types of magnetic domain structures in the preferred crystallographically oriented Co2FeSi Heusler alloy glass-coated micro-wires, essentially helpful for designing the devices based on magnetization switching.

Unusual Electrical and Magnetic Properties in Layered EuZn2As2

AbstractEu‐based compounds often exhibit unusual magnetism, which is critical for nontrivial topological properties seen in materials such as EuCd2As2. The authors investigate the structure and physical properties of EuZn2As2 through measurements of the electrical resistivity, Hall effect, magnetization, and neutron diffraction. Their data show that EuZn2As2 orders antiferromagnetically with an A‐type spin configuration belowTN = 19 K. Surprisingly, there is strong evidence for dominant ferromagnetic fluctuations aboveTN, as reflected by positive Curie–Weiss temperature and extremely large negative magnetoresistance (MR) betweenTNandTfl≈200 K. Furthermore, the angle dependence of the MRabindicates field‐induced spin reorientation from theab‐plane to a direction ≈45° from theabplane. Compared to EuCd2As2, the doubledTNandTflmake EuZn2As2 a better platform for exploring nontrivial magnetic and electronic properties in both magnetic fluctuation (TN < T < Tfl) and ordered (T < TN) regimes.

Carrier tuning of Stoner ferromagnetism in ThCr2Si2 -structure cobalt arsenides

CaCo$_{2-y}$As$_2$ is an unusual itinerant magnet with signatures of extreme magnetic frustration. The conditions for establishing magnetic order in such itinerant frustrated magnets, either by reducing frustration or increasing electronic correlations, is an open question. Here we use results from inelastic neutron scattering and magnetic susceptibility measurements and density functional theory calculations to show that hole doping in Ca(Co$_{1-x}$Fe$_{x}$)$_{2-y}$As$_{2}$ suppresses magnetic order by quenching the magnetic moment while maintaining the same level of magnetic frustration. The suppression is due to tuning the Fermi energy away from a peak in the electronic density of states originating from a flat conduction band. This results in the complete elimination of the magnetic moment by $x\approx0.25$, providing a clear example of a Stoner-type transition.

Study of interface and its role in an unusual magnetization reversal in 57FeCoB/MgO bilayer

Study of interface and its role in an unusual magnetization reversal in 57FeCoB/MgO bilayer

Unusual magnetization process and magnetocaloric effect in α-CoV2O6 driven by pulsed magnetic fields

In low-dimensional Ising spin systems, an interesting observation is the presence of step magnetization at low temperatures. Here we combine both DC and pulsed magnetic fields to study the 1/3 magnetization plateau and multiple steps in the Ising spin-chain material α-CoV2O6. Magnetization in pulsed fields is quite different from that in DC fields, showing multiple steps in an intermediate range of 4.2–6 K, inverted hysteresis below 4.2 K and asymmetric magnetization in negative fields below 11 K. We demonstrate that these unusual behaviors in magnetization are caused by the spin dynamics and the anomalous magnetocaloric effect (MCE) in α-CoV2O6, i.e., abrupt changes of sample temperature in adiabatic conditions. We successfully separate the influence between the intrinsic slow spin dynamics and the quasi-extrinsic temperature change. From the MCE, we find that some irreversible behavior is originated from the slow spin dynamics. Two different slow dynamics associated with the metastable steps are observed: one is sensitive to the slow field sweep rate at the order of ∼mT s−1 and weakly depends on temperature, while the other responds to the rapid field sweep rate of ∼kT s−1 and dominates at lowest temperature. We also distinguish that the metastable transition at H 4 is the first order and crucial for the ferrimagnetic to ferromagnetic transition. This study is useful to the understanding of multistep magnetization in α-CoV2O6 and sheds light on recent experimental findings of related compounds.

Directional Magnetization Reversal Enables Ultrahigh Energy Density in Gradient Nanostructures.

High-performance ferromagnetic materials are essential for energy conversion and electronic devices. However, the random and nonuniform magnetization reversal in ferromagnetics limits their performance that can be achieved. Here, through both micromagnetism simulations and experiments, a directional magnetization reversal that initiates first from large grains toward smaller ones is discovered by engineering Nd2 Fe14 B/α-Fe gradient nanostructures. Such directional magnetization reversal enables a rare combination of high magnetization and large coercivity, thus leading to a record-high energy density (26 MG Oe) for isotropic permanent magnetic materials, which is ≈50% higher than that of its gradient-free counterpart. The unusual magnetization reversal originates from an ordered arrangement of grain sizes in the gradient material, where the large grains have a lower reversal field than that of the smaller ones. These findings open up new opportunities for developing high-performance magnetic materials.

Helical Homometallic Trinickel String Complexes with Mixed Hard Nitrogen and Sulfur Donors: Structural and Magnetic Studies

Abstract A new tridentate and rigid ligand containing S,N-hetero donor, the 1H-1,8-naphthyridine-2-thione (Hnpt), is designed and developed to build up the first homonuclear nickel string supported by the mixed-donor ligands. Three asymmetric nickel strings possessing the structural feature of the (4,0) configuration are synthesized, namely the (4,0)-Ni3(npt)4(NCS) (1), (4,0)-[Ni3(npt)4(NCS)](PF6) (2) and (4,0)-Ni3(npt)4(NCS)2 (3). Due to the nature of the naphthyridyl group and sulfur donor, complex 1 is composed of one stabilized mixed-valent unit [Ni2]3+ and one low spin nickel thiolate. 1 and 2 are the quasi-1D coordination polymers in the solid-state, in which the molecules are linked by the weak intermolecular Ni⋯SCN interactions. 2 and 3 possess the same Ni36+ oxidized state with the different donation of axial ligands. The weaker donor in 2 yields the low spin state in the middle Ni(II). The stronger axial donor and the rigidity of ligand in 3 lead to the change of torsion angle, creating the unprecedented high spin Ni(II) inside the nickel string. This high spin Ni(II) with the square planar geometry is an unusual structure and magnetism among nickel strings. Detailed magnetic studies allow us to establish the spin state of each nickel in these three complexes. Besides, three different torsion angles with the same ligand and metal provide us with an opportunity to examine the factors governing the helicity of rigid ligands.

The unusual double-shifted magnetization curves in an exchange-biased perpendicular Co/IrMn system

We observed an unusual double-shifted hysteresis loop in a perpendicular exchange-coupled Co/IrMn system at room temperature, which leads to an uneven exchange bias field in the positive and the negative field. With the thickness of the antiferromagnetic layer ranging from 4.8 to 10.4 nm, the negative exchange bias field is approximately 100 Oe larger than the positive exchange bias field. This result is related to the density of net spins in the antiferromagnetic layer that have pinned on the ferromagnetic layer. In addition, the exchange bias field of this unusual double-shifted hysteresis loop can be adjusted by the magnetic field annealing temperature.

Pauli paramagnetism of cubic V3Al , CrVTiAl, and related 18-electron Heusler compounds with a group-13 element

Calculations suggest that ordered Heusler alloys with 18 valance electrons could exhibit a variety of unusual electronic and magnetic states that are absent in the constituent elements. They include magnetic semiconductors, spin gapless semiconductors, compensated ferrimagnetic half-metals, and metallic antiferromagnets. Magnetic order has been predicted at exceptionally high temperature. Any of this would be of interest for spin electronics. Here, we investigate the magnetic properties of bulk, single-phase ${\mathrm{V}}_{3}\mathrm{Al}$, CrVTiAl and the corresponding Ga compounds, with and without $^{57}\mathrm{Fe}$ doping. Results are compared with data on the constituent elements. We conclude that all the as-cast alloys show some degree of B2-type ordering, but all of them are Pauli paramagnets with dimensionless susceptibilities close to the average of the atomic constituents. Prolonged annealing of the single-phase as-cast alloys leads to phase segregation. Density functional theory calculations on ${\mathrm{V}}_{3}X$ and $\text{CrVTi}X$ with $X=B$, Al, Ga, and In confirm that different atomic arrangements on the four interpenetrating face-centered cubic sublattices of the Heusler structure could indeed lead to unusual magnetic properties, but both magnetism and semiconductivity are destroyed by disorder. The energy and entropy differences between different ordered magnetic phases preclude the stabilization of any single one of them. All are metastable and inaccessible in alloys prepared from the melt.