Room Temperature Magnetization Research Articles

Electrostatic Interactions between Barium Hexaferrite Nanoplatelets in Alcohol Suspensions

In a room-temperature liquid magnet, barium hexaferrite (BHF) nanoplatelets suspended in 1-butanol spontaneously order and form a ferromagnetic nematic phase. In such concentrated suspension, the nanoplatelets align in large macroscopic regions, forming magnetic domains. The key parameter for the suspension stability and the formation of the ferromagnetic nematic phase is electrostatic interaction, which can be influenced by the solvent and the concentration of surfactant, i.e., dodecylbenzenesulfonic acid (DBSA). In this study, we investigated electrostatic interactions of the DBSA-functionalized nanoplatelets’ suspensions in different alcohols. We prepared suspensions in tert-butanol, 1-hexanol, 1-butanol, and 2-propanol and measured conductivity, small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), and electrophoretic mobility. SAXS results and electrophoretic mobility measurements confirmed the colloidal stability of the suspensions, which was not affected by the variation in concentra...

Structural, electrical and magnetic properties of cobalt ferrite with Nd3+ doping

A systematic study on the influence of Nd3+ substitution on structural, magnetic and electrical properties of cobalt ferrite nanopowders obtained by sol–gel auto-combustion route was reported. The formation of spinel phase was confirmed by X-ray diffraction (XRD) data, and percolation limit of Nd3+ into the spinel lattice was also observed. Fourier transform infrared spectroscopy (FTIR) bands observed ≈ 580 and ≈ 390 cm−1 support the presence of Fe3+ at A and B sites in the spinel lattice. The variation in microstructure was investigated by scanning electron microscopy (SEM), and the average grain size varies from 5.3 to 3.3 µm. The substitution of Nd3+ significantly affects the formation of pores and grain size of cobalt ferrite. Room-temperature saturation magnetization and coercivity decrease from 60 to 30 mA·m2·g−1 and 19.9–17.8 mT, respectively, with Nd3+ substitution increasing. These decreases in magnetic properties are explained based on the presence of non-magnetic nature of Nd3+ concentration and the dilution of super-exchange interaction in the spinel lattice. The room-temperature direct-current electrical resistivity increases with Nd3+ concentration increasing, which is due to the unavailability of Fe2+ at octahedral B sites.

Room temperature magnetism of ordered porphyrin layers on Fe

We propose a method to grow metal tetraphenyl porphyrin (MTPP) molecular layers where a long-range structural and magnetic order can be achieved simultaneously and at room temperature by a proper treatment of the ferromagnetic substrate. We focus in particular on the oxygen-passivated Fe(001)-p(1 × 1)O surface, where MTPP molecules (with M=Co and Ni) arrange by forming square commensurate overlayers. Spin-resolved photoemission detects a clear spin-splitting of CoTPP electronic states, while no magnetic response is obtained from NiTPP, as expected from the electronic configuration of the respective free molecules. We link these observations to the decoupling action of oxygen at the interface, whose effect is to enhance the molecular diffusivity and tune the electronic interaction with the substrate electronic structure.

Unusual Ferromagnetic to Paramagnetic Change and Bandgap Shift in ZnS:Cr Nanoparticles

We report the chemical synthesis of Cr-doped ZnS nanocrystallites without using any capping ligand/surfactant. An unusual increase in bandgap observed for Cr-doped samples was understood in terms of the Burstein–Moss effect. Additionally, 4A2(F) → 4T1(F) and 4A2(F) → 4T2(F) transitions in the absorption spectra confirmed the substitution of Cr3+ ions at the octahedrally coordinated interstitial sites. Photoluminescence study showed a large amount of Zn vacancies owing to the incorporation of Cr3+ ions which intensified the self-activated blue–green luminescence. Room-temperature magnetization showed FM ordering up to the 2% Cr doping, which gradually transformed into paramagnetic behavior at higher values and became more enhanced in the low-temperature range. Thus, in the present synthesis method, the FM order could only be observed up to the 2% Cr doping concentration. Also, the activation energy was observed to increase with the increase of Cr doping concentration.

Mechanical Study of a Superconducting 28-GHz Ion Source Magnet for FRIB

The superconducting electron cyclotron resonance (ECR) source magnet for the facility for rare isotope beams at Michigan State University was designed and built by the Superconducting Magnet Group at Lawrence Berkeley National Laboratory (LBNL) in 2017. The 28 GHz NbTi ion source magnet features a sextupole-in-solenoids configuration which is comparable to the VENUS ECR magnet operated at LBNL. However, the mechanical design of this magnet utilizes a shell-based support structure which allows fine adjustments to the sextupole preload and reversibility of the magnet assembly process. The magnet has been assembled and tested to operational currents at LBNL. This paper describes the mechanical analyses performed to estimate the sextupole's and solenoids' preloads. We will report on the 3-D finite element analysis during room temperature assembly, cool-down, and magnet excitation, and then describe the magnet preload operations. Finally, we will describe the performance of the support structure during the quench training.

Amorphous magnetic semiconductors with Curie temperatures above room temperature

Recently, amorphous magnetic semiconductors as a new family of magnetic semiconductors have been developed by oxidizing ferromagnetic amorphous metals/alloys. Intriguingly, tuning the relative atomic ratios of Co and Fe in a Co-Fe-Ta-B-O system leads to the formation of an intrinsic magnetic semiconductor. Starting from high Curie-temperature amorphous ferromagnets, these amorphous magnetic semiconductors show Curie temperatures well above room temperature. Among them, one typical example is a p-type Co28.6Fe12.4Ta4.3B8.7O46 magnetic semiconductor, which has an optical bandgap of ~2.4 eV, room-temperature saturation magnetization of ~433 emu/cm3, and the Curie temperature above 600 K. The amorphous Co28.6Fe12.4Ta4.3B8.7O46 magnetic semiconductor can be integrated with n-type Si to form p–n heterojunctions with a threshold voltage of ~1.6 V, validating its p-type semiconducting character. Furthermore, the demonstration of electric field control of its room-temperature ferromagnetism reflects the interplay between the electricity and ferromagnetism in this material. It is suggested that the carrier density, ferromagnetism and conduction type of an intrinsic magnetic semiconductor are controllable by means of an electric field effect. These findings may pave a new way to realize magnetic semiconductor-based spintronic devices that work at room temperature.

Effect of WC addition and cooling rate on microstructure, magnetic and mechanical properties of Ti(C0.6,N0.4)-WC-Mo-Ni cermets

Ti(C0.6,N0.4)-8Mo-xWC-25Ni (x = 0, 3, 6 and 9 wt%) cermets were synthesized under different cooling rates by vacuum sintering. The influence of WC addition and cooling rate on microstructure, magnetic and mechanical properties of the as-prepared Ti(C,N)-based cermets was investigated by using X-ray diffraction (XRD), scanning electron microscopy (SEM), vibrating sample magnetometer (VSM) and physical property measurement system (PPMS). The results revealed that the grain size of the Ti(C,N)-based cermets became finer with WC addition. Furthermore, room-temperature saturation magnetization (Ms), remanence (Mr) and Curie temperature (Tc) of the Ti(C,N)-based cermets initially decreased with increasing WC content, followed by a gradual increase. Cermets bacame paramagnetic at x = 6 under the cooling rate of 2 °C/min, x = 6 and 9 under the cooling rate 35 °C/min, respectively. The decrease in magnetic properties could be ascribed to the enhanced solid solubility of alloy elements in Ni-based binder phase. Moreover, the hardness and transverse rupture strength (TRS) of the Ti(C,N)-based cermets initially increased and followed by a gradual decrease, whereas the fracture toughness initially decreased followed by an increase with increasing WC content. At the same value of x, the Ti(C,N)-based cermets exhibited better magnetic and mechanical properties at the cooling rate of 35 °C/min than that at the cooling rate of 2 °C/min, which could be attributed to the grain refinement strengthening and solid-solution strengthening of the binder phase.

Decomposition-Induced Room-Temperature Magnetism of the Na-Intercalated Layered Ferromagnet Fe3-xGeTe2.

The creation of 2D van der Waals materials with ferromagnetism above room temperature is an essential goal toward their practical utilization in spin-based applications. Recent studies suggest that intercalating lithium in exfoliated flakes of the ferromagnet Fe3-xGeTe2 induces a nonzero magnetization at T ∼ 300 K. However, the nanoscale nature of such experiments precludes precise observations of structural and chemical changes upon intercalation. Here, we report the preparation of sodium-intercalated NaFe2.78GeTe2 as well as the investigation into its structure and magnetic properties. Sodium readily intercalates into the van der Waals gap, as revealed by synchrotron X-ray diffraction. Concurrently, the Fe2.78GeTe2 layer becomes heavily charge doped and strained via chemical pressure, yet retains its structure and ferromagnetic transition temperature of ∼140 K. However, we observe the presence of a ferromagnetic amorphous iron germanide impurity over a wide range of synthetic conditions, leading to room-temperature magnetization. This work highlights the importance of strain and electronic control for manipulating the Curie temperature in 2D ferromagnets, while emphasizing the need for careful chemical analysis when exploring phenomena in exfoliated layers.

Voltage-Driven Room-Temperature Resistance and Magnetization Switching in Ceramic TiO2/PAA Nanoporous Composite Films.

Voltage control of room-temperature ferromagnetism has remained a big challenge which will greatly influence the multifunctional memory devices. In this paper, porous TiO2 thin films were deposited by dc-reactive magnetron sputtering onto ordered porous anodic alumina (PAA) substrates. Voltage-driving room-temperature resistance and magnetization switching without external magnetic field are simultaneously found in an Ag/TiO2/PAA/Al (Ag/TP/Al) device. Further analysis indicates that the formation/rupture of oxygen vacancy defect-based conductive filaments would be responsible for the changes of resistivity and magnetization. Our present results suggest that the TP nanoporous composite film material may therefore be used to achieve voltage control of magnetism and resistance switching in the future multifunctional memory devices. The Ag/TP/Al devices can also be used for new spintronic devices, neuromorphic operations, and alternative logic circuits and computing.

Room-temperature multiferroicity in magnetic field-poled polyaniline and its enhancement via Cu2+-complexation

We report on the discovery of room-temperature multiferroicity in camphorsulfonic acid-doped polyaniline synthesized under an applied magnetic field of 0.5 T, increasingly enhanced via Cu2+-doping up to 8 wt%, probably as a nitrogen-coordinated square planar-like complex revealed by electron paramagnetic resonance spectra. Room-temperature switchable magnetization and polarization hysteresis cycles are presented, reaching saturated magnetization (polarization) of 0.019 emu/g (4.369 μC/cm2) and coercivity (remanence) of 0.051 kOe (2.089 μC/cm2) and revealing strong electric exchange bias. Both real and imaginary parts of permeability and permittivity exhibit resonance feature enhanced by Cu2+-doping, peaking at about the same position in the microwave range of 2–18 GHz and strongly suggesting magnetoelectricity. The proposed model picture, which accounts for both ferroelectricity and exchange bias, assumes ferromagnetic and ferroelectric orders within the same domain represented by crystalline polyaniline islands immersed within the amorphous polyaniline template. Modified Landau-Lifshitz-Gilbert equations account for the matching of permeability and permittivity resonance features.

New Room-Temperature Organic Molecule-Based Magnets

Magnetic ordering at room temperature in organic polymer materials has long remained a challenge for developing new magnetic materials. A recent publication by Wu and colleagues (Chem. 2019;5:1223–1234) describes incorporation of stable radicals in covalently linked organic frameworks to create room-temperature magnets that provide a platform for next-generation magnetic devices.

Room-temperature magnetism and tunable energy gaps in edge-passivated zigzag graphene quantum dots

Graphene is a nonmagnetic semimetal and cannot be directly used as electronic and spintronic devices. Here, we demonstrate that zigzag graphene nanoflakes (GNFs), also known as graphene quantum dots, can exhibit strong edge magnetism and tunable energy gaps due to the presence of localized edge states. By using large-scale first principle density functional theory calculations and detailed analysis based on model Hamiltonians, we can show that the zigzag edge states in GNFs ({mathrm{C}}_{6n^2}H6n, n = 1–25) become much stronger and more localized as the system size increases. The enhanced edge states induce strong electron–electron interactions along the edges of GNFs, ultimately resulting in a magnetic configuration transition from nonmagnetic to intra-edge ferromagnetic and inter-edge antiferromagnetic, when the diameter is larger than 4.5 nm (C480H60). Our analysis shows that the inter-edge superexchange interaction of antiferromagnetic states between two nearest-neighbor zigzag edges in GNFs at the nanoscale (around 10 nm) can be stabilized at room temperature and is much stronger than that exists between two parallel zigzag edges in graphene nanoribbons, which cannot be stabilized at ultra-low temperature (3 K). Furthermore, such strong and localized edge states also induce GNFs semiconducting with tunable energy gaps, mainly controlled by adjusting the system size. Our results show that the quantum confinement effect, inter-edge superexchange (antiferromagnetic), and intra-edge direct exchange (ferromagnetic) interactions are crucial for the electronic and magnetic properties of zigzag GNFs at the nanoscale.

DC Electrical Resistivity, Dielectric, and Magnetic Studies of Rare‐Earth (Ho3+) Substituted Nano‐Sized CoFe2O4

The effect of Fe substitution by rare‐earth (Holmium; Ho3+) ions in CoFe2O4(CFO) prepared by glycine nitrate process is investigated from electrical, dielectric, magnetic, and Mössbauer perspectives. The anticipated spinel cubic phase structure is confirmed by X‐ray diffractometry and Fourier transform infrared (FTIR) analysis which support the presence of a secondary phase of HoFe2O3at higher Ho content. Field emission scanning electron microscope (FESEM) and transmission electron microscopic (TEM) micrographs confirmed that the incorporation of Ho3+significantly reduced the grain size of the samples from 50 nm to 20 nm. The complex impedance and modulus spectroscopy are investigated as a function of frequency in the regime of 102–106Hz in the temperature range of 30–300 °C. The conduction mechanism is attributed to the hopping of both electrons and holes. Activation energies (Ea) obtained from the Arrhenius plots of the grain and the grain boundary conductions are ascribed to the hopping of charge carriers. Electrical modulus results specify the existence of the non‐Debye type of dielectric relaxation. A massive reduction in loss factor is successfully achieved. Room temperature magnetization measurements indicate a reduction in saturation magnetization and coercivity (hysteresis losses) with increasing Ho3+content, which entails these materials to be used for magnetic data storage and magneto recording devices.

Room-Temperature Magnets Based on 1,3,5-Triazine-Linked Porous Organic Radical Frameworks

Summary Obtaining room-temperature (RT) molecule-based magnets is a long-sought-after goal in the materials community. However, so far, most of the reported magnets based on charge-transfer salts, pure organic radicals, and coordination polymers have shown low magnetic ordering temperatures. Herein, we propose an alternative approach for magnets by using covalently linked organic radical frameworks, in which neighboring radicals are ferromagnetically coupled. Stable hexacyanotrimethylenecyclopropanide radical anions ([CN6CP]M, M = K+ [1a], n-Bu4N+ [1b]) were found to undergo either thermal polymerization in the solid state at a relatively low temperature (300°C) without the need for ZnCl2 (for 1a) or trifluoromethanesulfonic-acid-mediated polymerization at 60°C (for 1b) to give 1,3,5-triazine-linked porous organic radical framework 2 or 3, respectively. The resulting material 2 exhibited spontaneous magnetization at RT with typical hysteresis of ferromagnets, and the ordering temperature was estimated to be 465 K, whereas the magnetic behavior of 3 is more like superparamagnetism.

Highly Magnetizable Crosslinked Chloromethylated Polystyrene-Based Nanocomposite Beads for Selective Molecular Separation of 4-Aminobenzoic Acid.

In this work, we describe the preparation and characterization of highly magnetizable chloromethylated polystyrene-based nanocomposite beads. For synthesis optimization, acid-resistant core–shelled maghemite (γ-Fe2O3) nanoparticles are coated with sodium oleate and directly incorporated into the organic medium during a suspension polymerization process. A crosslinking agent, ethylene glycol dimethacrylate, is used for copolymerization with 4-vinylbenzyl chloride to increase the resistance of the microbeads against leaching. X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, thermogravimetric analysis, scanning electron microscopy, transmission electron microscopy, and optical microscopy are used for bead characterization. The beads form a magnetic composite consisting of ∼500 nm-sized crosslinked polymeric microspheres, embedding ∼8 nm γ-Fe2O3 nanoparticles. This nanocomposite shows large room temperature magnetization (∼24 emu/g) due to the high content of maghemite (∼45 wt %) and resistance against leaching even in acidic media. Moreover, the presence of superficial chloromethyl groups is probed by Fourier transform infrared and X-ray photoelectron spectroscopy. The nanocomposite beads displaying chloromethyl groups can be used to selectively remove aminated compounds that are adsorbed on the beads, as is shown here for the molecular separation of 4-aminobenzoic acid from a mixture with benzoic acid. The high magnetization of the composite beads makes them suitable for in situ molecular separations in environmental and biological applications.

Organic Chiral Charge Transfer Magnets.

In this work, through designing organic helix donor-acceptor complexes, one type of room-temperature chiral magnet was reported. Within these chiral charge transfer magnets, circularly polarized light could induce a larger saturation magnetization compared to linearly polarized light illumination with identical intensity. Moreover, the transmission light polarization from chiral magnets could be tuned via applying the magnetic field. Overall, room-temperature organic chiral magnets with optomagnetic effects will enhance the function of organic magnetochiral materials.

Fabrication and characterization of YIG nanotubes

Abstract We have successfully synthesized Y3Fe5O12 (yttrium iron garnet, YIG) nanotubes (NTs) of different diameters using a conventional sol-gel method in anodized aluminum oxide (AAO) templates. An annealing process at 800 °C is performed to get a pure phase of YIG. The room-temperature magnetization measurements show that the coercivity decreases significantly as the diameter of nanotubes increases. The temperature dependence of magnetic moments in field-cooled (FC) and zero-field-cooled (ZFC) modes indicates that YIG NTs transform from ferromagnetic-like (FM-like) state to superparamagnetic (SPM) state when temperature decreases (in the region of 310 K–400 K). The Blocking temperature dramatically decreases as the size of YIG NTs reduces. In the FM-like state, the temperature-dependent coercivity in three different samples yields a 1/2 power law of the temperature. The temperature dependence of saturation magnetization in three different samples can be well fitted to Bloch’s T3/2 law. Characterization of ferromagnetic resonance (FMR) is conducted, which indicates the Gilbert damping parameter of YIG NTs is around 7–9 × 10−3.

Hydrothermal Synthesis and Characterization of Nanostructured Cobalt-Doped Tin Dioxide

Nanostructured materials based on cobalt-doped SnO2 have been prepared through alkaline hydrothermal treatment of aqueous solutions of inorganic Sn(II) and Co(II) salts. It has been shown that raising the cobalt concentration to 10 wt % leads to significant changes in the morphology and ferromagnetic properties of the synthesis products. The phase composition of the materials corresponds to tetragonal SnO2 with the rutile structure. Calcination above 450°C leads to Co2SnO4 formation. The saturation magnetization and magnetic susceptibility of the materials vary nonlinearly with the amount of cobalt added to the reaction mixture. The nanostructured material obtained in the presence of ~6 wt % Co has an anomalously high room-temperature saturation magnetization, ~3.5 emu/g, which is almost two orders of magnitude higher than that of highly dispersed powders with a similar composition prepared via chemical precipitation.

Enhanced coercivity of NiFe1–xDyxCrO4 ferrites synthesised by glycine-nitrate combustion method

ABSTRACTThe present work was focused on the effect of small amounts of Dy doping on the structural and magnetic properties of Ni-Cr ferrite materials. All the ferrites possess cubic symmetry corresponding to the space group with small amounts of secondary phase DyFeO3. No regular trend in the lattice parameter a with Dy doping has been observed. Room temperature magnetisation results, obtained from Vibrating Sample Magnetometer measurements, showed a decrease in saturation magnetisation with Dy doping. Enhanced coercivity in undoped Ni-Cr ferrite is observed which was several orders of magnitude higher than that of the reported bulk material. Substitution of Dy in Ni-Cr ferrite further enhances coercivity thereby making the materials magnetically more hard.

Electric Field-Controlled Magnetization in GaAs/AlGaAs Heterostructures–Chiral Organic Molecules Hybrids

We study GaAs/AlGaAs devices hosting a two-dimensional electron gas and coated with a monolayer of chiral organic molecules. We observe clear signatures of room-temperature magnetism, which is induced in these systems by applying a gate voltage. We explain this phenomenon as a consequence of the spin-polarized charges that are injected into the semiconductor through the chiral molecules. The orientation of the magnetic moment can be manipulated by low gate voltages, with a switching rate in the megahertz range. Thus, our devices implement an efficient, electric field-controlled magnetization, which has long been desired for their technical prospects.