Giant Magnetization Research Articles

Electric field driven giant vertical magnetization shift through resistive switching in NiO/Fe bilayers

Electric field driven giant vertical magnetization shift through resistive switching in NiO/Fe bilayers

Structural, optical and magnetic properties of Fe, Co and Co[sbnd]Fe doped SnO2 thin films deposited by a thermionic vacuum arc deposition system

Fe, Co and CoFe doped SnO2 thin films were deposited onto glass, indium tin oxide-coated glass, and Si substrates to investigate the influence of substrate material on the surface and magnetic properties. The motivation of this study is to investigate the magnetic, surface, and optical properties of doped SnO2 semiconductor materials as they vary with different substrate materials. The film surfaces are smooth and compact. Additionally, nano particles were observed. Thin films of SnO2 doped with Fe, Co and CoFe doped SnO2 have a Curie temperature. Magnetization–temperature and magnetization–temperature graphs have shown the predicted magnetic properties. These films have giant magnetization and soft magnetic properties. In separately doped Fe and Co – SnO2 thin films, the M (emu) values are nearly the same. In Co and Fe-doped SnO2 thin films, the M (emu) values for the film deposited onto ITO-coated glass and uncoated glass are nearly the same.

Theoretical study of thermal stability of α″–Fe16N2 against other iron nitrides

α″–Fe16N2 has been investigated as one of promising candidates for environment-friendly magnets. While giant saturation magnetization has previously been experimentally observed in α″–Fe16N2, its magnetic anisotropy and structural stability leave room for improvement. Recent theoretical studies have considered alloying Fe16N2 with various elements to improve the magnetic properties and/or stability against decomposition. However, estimates of stability in particular are typically restricted to simple ground-state-energy comparisons, i.e. effectively taken at 0 K. For a more practical measure of stability, we therefore extend ground-state energies, obtained with the plane-wave density-functional theory code Quantum ESPRESSO, with appropriate empirical and/or statistical corrections to obtain free energies at arbitrary temperature. We then compare the stability of Fe16N2 against the neighboring phases in the Fe-N binary system, to estimate the range of temperatures at which it is stable. We compare against experimental observations of the Fe-N phase diagram.

Synthetic Factors Determining the Curvature and Nuclearity of the Giant Mn70 and Mn84 Clusters with a Torus Structure of ∼4 nm Diameter.

New members of the Mn70 and Mn84 torus-like cluster family have been prepared from a hybrid comproportionation-alcoholysis reaction of [Mn12O12(O2CR)16(H2O)4] in alcohol in the presence of R'CO2H with NnBuMnO4 or MnII salts as initiators. Reactions using MeCO2H in nPrOH or nBuOH gave [Mn70O60(O2CMe)70(OnPr)20(nPrOH)18.5(H2O)21.5] (3) and [Mn70O60(O2CMe)70(OH)3(OnBu)17(nBuOH)7.5(H2O)32.5] (4), respectively, whereas EtCO2H in nPrOH gave [Mn84O72(O2CEt)84(OnPr)24(nPrOH)16(H2O)32] (5). They consist of alternating near-linear [Mn3(μ3-O)4]+ and distorted-cubane [Mn4(μ3-O)2(μ3-OR)2]6+ units bridged by syn,syn-μ-RCO2- and μ3-O2- groups and overall are [Mn14]5 and [Mn14]6 oligomers, the repeating unit containing two Mn3 and two Mn4 units. 3/4/5 possess external diameters (including organic ligands) of 4.0/4.1/4.6 nm, respectively, and crystallize as supramolecular nanotubes but with different packing arrangements. Considering all Mn70/Mn84 tori now available, we conclude that the Mn70 vs Mn84 nuclearity is determined by the relative bulk of the carboxylates vs the alkoxides, their increasing bulk favoring Mn84 and Mn70, respectively, with carboxylates larger than acetate giving Mn84. Alternating current (ac) magnetic susceptibility studies revealed frequency-dependent χ″M signals below ∼2.4 K, indicating 3-5 to be new members of the giant [Mn14]n torus family of giant single-molecule magnets (SMMs), in which Mn84 and Mn70 are the largest homometallic Mn/O clusters and SMMs to date.

Surface and interface mediated magnetism in mesoscopic ceria@carbon core–shell structures

CeO2@C submicron core–shell structure was prepared by a two-step synthesis method, and the ceria shell thickness and its microstructure were precisely controlled by the precursor composition and annealing process. The results show that forming the thin ceria shell by aggregating nanoparticles on the surface of the carbon sphere is more favorable for forming ferromagnetism than merely dispersed ceria nanoparticles. The saturation magnetization increases with decreasing shell thickness, indicating the contributions of surfaces and interfaces to the magnetism. Furthermore, high-temperature reduction results in shell densification and the aggregation of defects at the surface. The spectroscopic and microscopic measurements reveal that in such cases, a continuous defect-rich area with a sub-nanometric thickness, doped with electrons extends over the whole surface of the mesostructure. The magnetic strength difference caused by adjusting the thickness of this defective layer from several nanometers to sub-nanometric width can reach 360 times, suggesting that the magnetization of these mesoscopic systems is related to the presence of these quasi-2D electron-rich regions on the surface. This study shows the applicability of giant orbital magnetism to mesoscopic systems, and the results can also provide a reference for the subsequent synthesis of magnetic materials.

Effect of Hubbard U on calculations of magnetic properties of α″–Fe16N2

The ordered iron nitride phase α″–Fe16N2 has been a potential candidate for rare-earth free magnets with giant saturation magnetization, but first-principles electronic-structure calculations have struggled to reproduce recent observations of high magnetic moment, while calculations of magnetocrystalline anisotropy (MCA) vary significantly. Within the framework of density-functional theory (DFT), a common extension to the usual generalized-gradient approximation (GGA) exchange-correlation (XC) functional is the inclusion of Hubbard parameters U (,J) as GGA + U. A number of previous papers have applied this method to Fe16N2, each with their own choice of Hubbard parameters. The plane-wave DFT code Quantum ESPRESSO was employed to more comprehensively study the effect of the value of Hubbard parameters U and J on the system, particularly with respect to its magnetic properties. Various approaches for setting U and J were compared, including self-consistent calculations via the linear-response method.

Magnetization loss of CORC cables under axial tensile loading

As a highly flexible and fully isotropic cable structure, CORC (Conductor on Round Core) cable wound with high-temperature superconducting (HTS) tapes has promise for giant magnets, including fusion magnets, high-energy accelerator magnets, and nuclear magnetic resonance. CORC cables are subjected to mechanical, thermal, and electromagnetic loadings during preparation and operation. Axial tension is one of the basic deformation modes of CORC cables. To investigate the impact of axial tension loads on the electromagnetic performance of CORC cables, a mechanical-electromagnetic model is created in this study. A comparison of numerical simulations and experiments was first performed for single straight tape and single-layer CORC cables to verify the model's reliability, with particular attention to the critical current reduction and the variation of magnetization loss during axial tension. The model developed in this paper may evaluate electromagnetic performance under applied AC magnetic fields with small amplitudes and mechanical response to axial tension loads. On this basis, the strain distribution of CORC cables under axial tension and their magnetization loss characteristics under various strain states are presented. The effects of different cable design parameters on the critical current and magnetization loss are discussed. The results show that the strain affects the magnetization loss of CORC cable significantly, especially for ones with larger winding angles. The magnetization loss can be reduced by choosing a narrower tape, a smaller winding angle, or a cable core material with a higher Poisson ratio. This study can provide a theoretical basis for the structure optimization design and practical application of CORC cables.

Magnetocrystalline anisotropy in V– and Cu–doped Fe16N2

While giant saturation magnetization has been observed in α″–Fe16N2, its magnetic anisotropy and structural stability leave room for improvement. Several recent studies have investigated the effect of substitution to improve its magnetic properties and/or its stability; among these, substitution of Fe with V or Cu has shown promise. We thus compare the magnetic properties of such alloys in some more detail using first-principles electronic-structure calculations: The magnetocrystalline anisotropy (MCA) energies of ordered Fe16–nVnN2 and Fe16–nCunN2 alloys (n=1, 2), as well as the co-substituted alloy Fe14VCuN2, are obtained within the plane-wave density-functional theory (DFT) code Quantum ESPRESSO.

Formation of Y3Fe5O12 matrix including ε-phase Fe2O3 with the giant coercive field via optimized sol–gel method

The effect of compression and heat treatment on the microstructural morphology and magnetic characteristics of the imperfectly crystallized yttrium–iron–garnet (Y3Fe5O12, YIG) matrix was observed by optimizing the sol–gel synthesis process. In particular, among the various temperatures when calcination was performed at 850 °C, we observed that an ε-phase Fe2O3 with a giant coercive field but low saturation magnetization was formed in the YIG matrix.

Giant valley-polarized spin splittings in magnetized Janus Pt dichalcogenides

We reveal giant proximity-induced magnetism and valley-polarization effects in Janus Pt dichalcogenides (such as SPtSe), when bound to the Europium oxide (EuO) substrate. Using first-principles simulations, it is surprisingly found that the charge redistribution, resulting from proximity with EuO, leads to the formation of two K and K$^{'}$valleys in the conduction bands. Each of these valleys displays its own spin polarization and a specific spin-texture dictated by broken inversion and time-reversal symmetries, and valley-exchange and Rashba splittings as large as hundreds of meV. This provides a platform for exploring novel spin-valley physics in low-dimensional semiconductors, with potential spin transport mechanisms such as spin-orbit torques much more resilient to disorder and temperature effects.

Re-examining the giant magnetization density in α′′-Fe16N2 with the SCAN+U method

We present an in-depth discussion of the magnetic ground state of α''-Fe16N2 within the framework of the density functional theory (DFT). The exchange-correlation effects are treated using a variety of schemes, including the local-spin-density approximation, the generalized-gradient approximation, and the Strongly-Constrained-and-Appropriately-Normed (SCAN) scheme. We also delineate effects of adding an on-site interaction parameter U on the Fe sites. Among all the schemes considered, only SCAN+U is found to capture the surprisingly large magnetization density in α''-Fe16N2 that has been observed experimentally. Our study shows how the combination of SCAN and self-interaction corrections applied on different Fe sites through the parameter U can reproduce both the correct equilibrium volume and the giant magnetization density of α''-Fe16N2.

Fusion experiments broke records this year

Scientists this year took two big steps on what has been a decades-long, slow march to fusion power. Researchers in California used lasers to trigger a record-breaking fusion reaction, while a team in Massachusetts demonstrated a powerful magnet that could one day be incorporated into an electricity-generating fusion reactor. Whether these and other recent advances lead to fusion power remains to be seen. Even if they do, viable fusion reactors won’t be built for a decade or more. But people involved in both these projects describe a range of chemical and materials science developments that resulted in these breakthroughs, including nearly flawless lenses that focus laser light and superconducting tape coiled inside a giant magnet. The researchers emphasize that at this point, more than half a century after the first investigations of fusion energy, some of the hardest aspects of their work come not in fundamental physics research but in

Evolution of low-energy magnetic excitations pair spectrum in SmMnO3+δ

The identification of low-energy thermal excitations in SmMnO3+δ degenerate states of spin and superconducting quantum liquids in magnetic fields H ≤ 3.5 kOe is presented. In the temperature interval 4.2–12 K, the Landau quantization of the low-energy magnetic excitations pair spectrum of Z2 quantum spin liquid is found in the system spinon-gauge field. The formation of a broad continuum of spinon pair excitations in the “weak magnetic field” regime (H = 100 Oe, 1 kOe) in the FC regime is explained in the framework of the Landau quantization models of the compressible spinon gas with fractional values of the factor ν filling three overlapping bands. In the regime of “strong magnetic field” (H = 3.5 kOe), the quantum oscillations of temperature dependences of “supermagnetization” of the incompressible spinon liquid were observed. They have the form of three narrow steps (plateaus), corresponding to a complete filling of the non-overlapping Landau bands with integer values of the filling factor by spinons. These results are evidence for the existence of vortex gauge field fluctuations with a high density in the magnetic fields H ≥ 100 Oe. The strong growth of vortex fluctuations can be explained by a second-kind phase transition in SmMnO3+δ in the form of the vortices condensation. Growth of the external dc magnetic field strength in the SmMnO3+δ samples in the interval of fields 0 < H ≤ 3.5 kOe leads to a continuous decrease in the giant magnetization jump near the temperature TKT ≅ 12 K of the topological phase transition, Kosterlitz–Thouless dissociation of 2D vortex-antivortex pairs in a local superconducting state. The suppression of the magnetization jump near the TKT temperature with increasing H is explained by the polarization of vortex antivortex pairs at temperatures below TKT by an external dc magnetic field, which weakens the vortex interaction in pairs and leads to their dissociation.

From magnetic cluster glass state to giant vertical magnetization shift induced by ferromagnetic cluster growth in SrFe0.25Co0.75O2.63

A charge ordering phenomenon within the crystallographic sites as following Sr4(Fe0.143+Co0.363+)48h(Fe0.114+Co0.144+Co0.253+)48fO10.52 was previously reported thanks to neutron diffraction coupled with Mossbauer spectroscopy studies. Such distribution supports a natural magnetic layered structure combining both in-plane ferromagnetic super-exchange interactions mainly on the octahedron “8 f” sub-layer alternating and in-plane antiferromagnetic super-exchange interactions on the “8 h” sub-layer containing tetrahedral sites and five-fold symmetry polyhedra (i.e. squared based pyramid and/or trigonal bipyramid). Because of the interfacial magnetic interactions between the two types of layers, we report a detailed study of the intriguing magnetic properties. AC magnetic susceptibility shows a frequency dependent peak suggesting a cooperative character due to inter-cluster interactions resulting in a magnetic cluster glass state. When temperature decreases and/or applied magnetic field increase, the ferromagnetic clusters growth is promoted and results in a large vertical hysteretic shift on the Field Cooled magnetization isotherm in relation with minor loop.

Magnetocrystalline anisotropy of α″–Fe16N2 under various DFT approaches

The magnetocrystalline anisotropy (MCA) energy of the giant saturation magnetization candidate material α″–Fe16N2 was investigated using first-principles electronic-structure calculations. The plane-wave density-functional theory (DFT) code Quantum ESPRESSO was employed to study the effect of different DFT approaches on the system, particularly the influence of exchange-correlation functionals and pseudopotential methods. The MCA energies obtained this way are within the range of previous theoretical and experimental results, while exhibiting significant variation between the different approaches. The role and limitations of these approaches in the view of Fe16N2 band structure are discussed in detail.

Magnetism of Nanosized “Nonmagnetic” Materials; the Role of Defects (Review)

This review analyzes experimental and theoretical works devoted to high-temperature ferromagnetism in nonmagnetic materials, the recently discovered phenomenon. The main attention is paid to nanoparticles of nonmagnetic materials, in which this phenomenon is most pronounced due to the features of the surface defect structure. In addition to the traditional theoretical models of band or state-localized defect-induced ferromagnetism and the nontrivial theory of giant orbital magnetism, the review discusses the ferron model, which is successfully used to fit the properties of advanced magnetic materials with phase separation.

Non-Volatile Regulation of Magnetism via Electric Fields in Polycrystal FeSi/(011) PMN-0.32PT Heterostructures

The choice and configuration of the ferroelectric (FE) substrate and the ferromagnetic (FM) layer in FM/FE heterostructures play an important role in magnetism modification with regard to amplitude and efficiency. In this study, we fabricated FeSi films on low crystalline (011) [Pb(Mg1/3Nb2/3)O3]0.7-[PbTiO3]0.3 (PMN-0.32PT) using radio frequency magnetron sputtering. In the annealed FeSi/(011) PMN-0.32PT heterostructures, the FeSi film presented with a (011) preferred orientated polycrystalline structure and low magnetocrystalline anisotropy. Both loop-like and butterfly-like magnetism modifications were observed by applying bipolar electric fields, and the weak and abnormal electrically mediated magnetism behaviors were significantly different from the prominent magnetic anisotropy transition in FeSi/(011) PMN-0.3PT. The comparative analyses suggest that the resulting high-quality single-crystalline PMN-xPT and FM films with low coercivity are of great significance for exploring giant, reversible, and non-volatile magnetism regulation.

Giant Vertical Magnetization Shift Caused by Field-Induced Ferromagnetic Spin Reconfiguration in Ni50Mn36Ga14 Alloy

Vertical magnetization shift (VMS) is a special type of exchange bias effect that may lead to a revolution in future ultrahigh-density magnetic recording technology. However, there are very few reports focusing on the performance of VMS due to the unclear mechanism. In this paper, a giant vertical magnetization shift (ME) of 6.34 emu/g is reported in the Ni50Mn36Ga14 alloy. The VMS can be attributed to small ferromagnetic ordered regions formed by spin reconfiguration after field cooling, which are embedded in an antiferromagnetic matrix. The strong cooling-field dependence, temperature dependence, and training effect all corroborate the presence of spin reconfiguration and its role in the VMS. This work can enrich VMS research and increase its potential in practical applications as well.

Magnetic structure of Fe16N2 determined by polarized neutron diffraction on thin-film samples

Strain-free ${\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ thin films were obtained on MgO (001) single-crystal substrates with Cr seed layers. X-ray diffraction and transmission electron microscopy confirmed that the film is textured in all three spatial directions with crystallographic orientation determined by epitaxial constraints from the substrate. Average grain size was estimated to be in the range of tens of nanometers. Ten pieces of 40-nm-thick ${\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ thin-film samples were stacked together for half-polarized neutron diffraction study at room temperature. A total of six reflections were usable, from which the magnetic structure of ${\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$ was determined and compared to existing models. Contrary to most electronic structure calculations, the present results suggest that the magnetic moment of Fe atoms at the $8h$ site is large and larger than that of the $4d$ site. Our results support the giant magnetization of ${\mathrm{Fe}}_{16}{\mathrm{N}}_{2}$.

Fabrication and Characterization of Fe16N2Micro‐Flake Powders and Their Extrusion‐Based 3D Printing into Permanent Magnet Form

Fe16N2is a compound with giant saturation magnetization approaching or exceeding that of rare‐earth‐based permanent magnets. The abundance of its elements and low‐cost synthesis of this compound has made it highly attractive to replace rare‐earth‐based permanent magnets that are becoming ever more expensive to utilize in applications. Herein, its synthesis from Fe flakes by surfactant‐assisted high energy ball milling is demonstrated. The synthesized Fe flakes are then reduced under forming gas (Ar/H2), followed by nitridation at low temperatures under ammonia (NH3) gas. The formation of Fe16N2phase exceeding 50% by volumetric fraction is observed and confirmed by X‐ray diffraction and Mössbauer analysis. Following the Fe16N2flake synthesis, extrusion‐based 3D printing is used to check the feasibility of incorporation of the flakes into functional polymer matrix composites. For this purpose, an ink of intermixed synthesized powder with photoresist SU8 is used. Using the prescribed method, a prototype Fe16N2permanent magnet composite is successfully produced using an additive manufacturing approach. Such efficient production of Fe16N2powders via routes already applicable to magnet production and the consolidation of the powders with 3D printing are expected to open up new possibilities for next‐generation permanent magnet applications.