Ferromagnetic Signal Research Articles

Strain-mediated ferromagnetism and low-field magnetic reversal in Co doped monolayer WS_2

Strain-mediated magnetism in 2D materials and dilute magnetic semiconductors hold multi-functional applications for future nano-electronics. Herein, First principles calculations are employed to study the influence of biaxial strain on the magnetic properties of Co-doped monolayer WS_2. The non-magnetic WS_2 shows ferromagnetic signature upon Co doping due to spin polarization, which is further improved at low compressive (-2 %) and tensile (+2 %) strains. From the PDOS and spin density analysis, the opposite magnetic ordering is found to be favourable under the application of compressive and tensile strains. The double exchange interaction and p-d hybridization mechanisms make Co-doped WS_2 a potential host for magnetism. More importantly, the competition between exchange and crystal field splittings, i.e. (Delta _{ex}>Delta _{cfs}), of the Co-atom play pivotal roles in deciding the values of the magnetic moments under applied strain. Micromagnetic simulation reveals, the ferromagnetic behavior calculated from DFT exhibits low-field magnetic reversal (190 Oe). Moreover, the spins of Co-doped WS_2 are slightly tilted from the easy axis orientations showing slanted ferromagnetic hysteresis loop. The ferromagnetic nature of Co-doped WS_2 suppresses beyond pm 2~% strain, which is reflected in terms of decrease in the coercivity in the micromagnetic simulation. The understanding of low-field magnetic reversal and spin orientations in Co-doped WS_2 may pave the way for next-generation spintronics and straintronics applications.

Nanometers-Thick Ferromagnetic Surface Produced by Laser Cutting of Diamond.

In this work, we demonstrate that cutting diamond crystals with a laser (532 nm wavelength, 0.5 mJ energy, 200 ns pulse duration at 15 kHz) produced a ≲20 nm thick surface layer with magnetic order at room temperature. We measured the magnetic moment of five natural and six CVD diamond crystals of different sizes, nitrogen contents and surface orientations with a SQUID magnetometer. A robust ferromagnetic response at 300 K was observed only for crystals that were cut with the laser along the (100) surface orientation. The magnetic signals were much weaker for the (110) and negligible for the (111) orientations. We attribute the magnetic order to the disordered graphite layer produced by the laser at the diamond surface. The ferromagnetic signal vanished after chemical etching or after moderate temperature annealing. The obtained results indicate that laser treatment of diamond may pave the way to create ferromagnetic spots at its surface.

Paramagnetic transitions and weak-diamagnetism in sulfur-doped buckypapers and graphene-oxide composites

The appearance of coexisting ferromagnetic and superconductive phenomena in graphite‑sulfur and amorphous carbon‑sulfur composites has recently attracted an important attention. In this work we propose a novel re-investigation of the carbon‑sulfur doping mechanism performed by employing carbon nanotube networks (cm-scale buckypapers) and graphene-oxide films as host-materials. In the buckypaper-case, the presence of multiple sulfidation processes involving formation of 1) carbon‑sulfur and 2) metal-sulfide phases was demonstrated. Presence of carbon‑sulfur bonding was identified by employing both Raman spectroscopy and X-ray photoelectron spectroscopy. The conductive and magnetic properties of the sulfur-rich areas within the buckypaper were also investigated. An enhanced carbon‑sulfur bonding was then identified in sulfur-doped graphene-oxide films. In this latter case we demonstrate an almost complete annihilation of ferromagnetic-signals. ESR-spectroscopy of this second-type of system revealed the appearance of a paramagnetic transition for g ~ 2.08 at T ~ 77 K, possibly originating from the carbon‑sulfur bonding. A weak enhancement in the diamagnetic component could be interestingly detected below T ~ 60 K as a consequence of sulfur doping, after subtraction of the percolative ferromagnetic signals.

High-quality two-dimensional electron gas in undoped InSb quantum wells

We report on transport experiments through high-mobility gate-tunable undoped InSb quantum wells (QWs). Due to the elimination of any Si modulation doping, the gate-defined two-dimensional electron gases in the QWs display a significantly increased mobility of $260\phantom{\rule{0.16em}{0ex}}000\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{Vs}$ at a rather low density of $2.4\phantom{\rule{4pt}{0ex}}\ifmmode\times\else\texttimes\fi{}{10}^{11}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}2}$. Using magnetotransport experiments, we characterize spin-orbit interactions by measuring weak antilocalization. Furthermore, by measuring Shubnikov--de Haas oscillations in tilted magnetic fields, we find that the $g$ factor agrees with $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ theory calculations at low magnetic fields but grows with spin polarization and carrier density at high magnetic fields. Additionally, signatures of Ising quantum Hall ferromagnetism are found at filling factor $\ensuremath{\nu}=2$ for tilt angles where the Landau level energy equals the Zeeman energy. Despite the high mobility, the undoped InSb QWs exhibit no fractional quantum Hall effect up to magnetic fields of 25 T.

Increased Manganese Solubility and Ferromagnetic Signal in Chalcopyrite CuGaSe2:Mn as a Result of High-Temperature Quenching

Increased Manganese Solubility and Ferromagnetic Signal in Chalcopyrite CuGaSe2:Mn as a Result of High-Temperature Quenching

Chemical and magnetic order in mass-selected large FeRh nanomagnets embedded in a carbon matrix

In this paper, we present some specific chemical and magnetic order results obtained on bimetallic FeRh nanoparticles prepared under non-equilibrium conditions using mass-selected Low Energy Cluster Beam Deposition. Clusters around 7 nm incident diameter are in situ sandwiched between amorphous carbon films before transfer in air, with different surface coverage (from nearly isolated particles to percolating 2D films) in order to artificially increase diameters of the nanoparticles (NPs) by coalescence. On such FeRh@C samples, we observe different NPs morphologies and magnetic responses after UHV thermal treatments. By transmission electron microscopy (TEM), we show that after moderate annealing, nanoalloys evolved from a metastable structure towards the equilibrium CsCl-type (B2) chemically ordered phase with a more spherical shape. While from Superconducting Quantum Interference Device (SQUID) magnetometry measurements on 700 °C-annealed FeRh@C samples, we clearly evidence huge magnetization enhancement for such B2 nanoalloys with robust ferromagnetic (FM) signature even at very low temperature unlike their bulk counterparts.

Magnetic, structural and Mössbauer study of soils from an ancient mining area in Huancavelica-Peru

We present the magnetic, structural and 57Fe Mossbauer characterization of soils collected from an ancient mercury contaminated city named Huancavelica in Peru. The characterization results indicate that silicates and carbonates are the main mineralogical constituents in the samples. In addition, 57Fe Mössbauer spectra at room temperature reveal, the presence of two components: a magnetic component related to magnetic Fe-oxides (magnetite, hematite, goethite) and a high non-magnetic component related to Fe+3 in high spin configuration and tetrahedral coordination in silicates. The magnetization measurements present screening of paramagnetic, ferromagnetic and antiferromagnetic signals, typical from soils containing different silicates and iron minerals. Remarkably the Verwey and Morin transitions corresponding to magnetite and hematite, respectively, are screened by the paramagnetic signal corresponding to the major silicate components in the samples. Overall, the soils are mainly composed of crystalline and amorphous silicates, calcites and iron bearing which are typical from Andean soils.

Hysteretic Magnetoresistance in a Non-Magnetic SrSnO3 Film via Thermal Coupling to Dynamic Substrate Behavior.

Hysteretic magnetoresistance (MR) is often used as a signature of ferromagnetism in conducting oxide films and heterostructures. Here, magnetotransport is investigated in a nonmagnetic La-doped SrSnO3 film. A 12 nm La:SrSnO3/2 nm SrSnO3/GdScO3 (110) film with insulating behavior exhibited a robust hysteresis loop in the MR at T < 5 K accompanied by an anomaly at ∼±3 T at T < 2.5 K. Furthermore, MR with the field in-plane yielded a value exceeding 100% at 1.8 K. Using detailed temperature-, angle- and magnetic field-dependent resistance measurements, we illustrate the origin of hysteresis is not due to magnetism in the film but rather is associated with the magnetocaloric effect of the substrate. Given GdScO3 and similar substrates are commonly used, this work highlights the importance of thermal coupling to processes in the substrates which must be carefully accounted for in the data interpretation for heterostructures utilizing these substrates.

Signatures of ferromagnetism in p-type cadmium telluride quantum dots

Signatures of Ferromagnetism in p-type conducting Cadmium Telluride (CdTe) quantum dots (QDs), synthesized by Single Injection Hydrothermal (SIH) method are reported in this article. The Powder X-ray Diffraction (P-XRD) analysis depicts hexagonal structure for the CdTe QDs. X-ray Photoelectron Spectroscopic (XPS) technique is used to determine the 3d5/2, 3d3/2 orbital states of the Cd as well as Te and also to confirm the Cd-Te bond formation in CdTe QDs. The van der Pauw Hall measurement technique is used to identify the p-type conductivity in the CdTe QDs, which are attributed to the Cd vacancy defects formed during the synthesis of the CdTe QDs. Appearance of hysteresis in the Superconducting Quantum Interference Device (SQUID) measurements made at 300 K and 5 K, confirm the ferromagnetic behavior of the CdTe QDs. The signatures of ferromagnetism are attributed to Cd vacancy defects.

Electrical and ferromagnetic properties of p-type ZnO thin films with and without the addition of graphene prepared by the sol-gel method

This study determines the effect of the addition of graphene on the electrical and ferromagnetic properties of the ZnO thin films that are prepared by the sol-gel method. ZnO is a typically oxygen-deficient metal oxide and exhibits intrinsic n-type conductivity. ZnO thin films to which graphene is added and those to which it is not, which are annealed at 500 °C for 30 min in oxygen, exhibit p-type behavior. There is an increase in the hole concentration of the graphene-doped ZnO thin film because of competition between the densities of donor-like defects such as oxygen vacancies and acceptor-like defects such as oxygen interstitials (Oi) and the defect complex that is related to the substitution of carbon for zinc (CZn) and Oi. There is ferromagnetism in a ZnO thin film at room temperature because of the presence of Oi. The intensity of the ferromagnetic signal is increased because CZn and Oi are formed in a graphene-doped ZnO thin film.

Electron-beam floating-zone refined UCoGe

The interplay between unconventional superconductivity and quantum critical ferromagnetism in the U-Ge compounds represents an open problem in strongly correlated electron systems. Sample quality can have a strong influence on both of these ordered states in the compound UCoGe, as is true for most unconventional superconductors. We report results of a new approach at UCoGe crystal growth using a floating-zone method with potential for improvements of sample quality and size as compared with traditional means such as Czochralski growth. Single crystals of the ferromagnetic superconductor UCoGe were produced using an ultra-high vacuum electron-beam floating-zone refining technique. Annealed single crystals show well-defined signatures of bulk ferromagnetism and superconductivity at $T_c \sim$2.6 K and $T_s \sim$0.55 K, respectively, in the resistivity and heat capacity. Scanning electron microscopy of samples with different surface treatments shows evidence of an off-stoichiometric uranium rich phase of UCoGe collected in cracks and voids that might be limiting sample quality.

Introducing magnetic properties in Fe-doped ZnO nanoparticles

Proper correlation among the microstructural, optical and magnetic responses of Fe-doped ZnO nanoparticles has been established in this work. All the Fe-doped ZnO nanoparticles (Zn1−xFexO: x = 0.00, 0.05, 0.10 and 0.15) were prepared using chemical co-precipitation route. Average crystallites size of 18 nm to 28 nm was estimated using Scherrer’s formula. Compressive microstrain was detected in pristine ZnO samples, which moved toward tensile regime upon introducing Fe ions of different weight percentages. Mean crystallites size obtained from Scherrer’s formula was found in almost exact match with the particle size estimated from HRTEM images. Nearly spherical ZnO nanoparticles were seen in HRTEM images and negligible agglomeration among particles was also observed. Direct optical band gaps were found in the range of 2.89–3.24 eV as estimated from Tauc plots. A decent ferromagnetic signature in non-magnetic ZnO nanoparticles was also introduced at room temperature with the doping of Fe ions.

Gate-tuned anomalous Hall effect driven by Rashba splitting in intermixed LaAlO3/GdTiO3/SrTiO3

The Anomalous Hall Effect (AHE) is an important quantity in determining the properties and understanding the behaviour of the two-dimensional electron system forming at the interface of SrTiO3-based oxide heterostructures. The occurrence of AHE is often interpreted as a signature of ferromagnetism, but it is becoming more and more clear that also paramagnets may contribute to AHE. We studied the influence of magnetic ions by measuring intermixed LaAlO3/GdTiO3/SrTiO3 at temperatures below 10 K. We find that, as function of gate voltage, the system undergoes a Lifshitz transition while at the same time an onset of AHE is observed. However, we do not observe clear signs of ferromagnetism. We argue the AHE to be due to the change in Rashba spin-orbit coupling at the Lifshitz transition and conclude that also paramagnetic moments which are easily polarizable at low temperatures and high magnetic fields lead to the presence of AHE, which needs to be taken into account when extracting carrier densities and mobilities.

Ultrafast Optically Induced Ferromagnetic State in an Elemental Antiferromagnet.

We present evidence for an ultrafast optically induced ferromagnetic alignment of antiferromagnetic Mn in Co/Mn multilayers. We observe the transient ferromagnetic signal at the arrival of the pump pulse at the Mn L_{3} resonance using x-ray magnetic circular dichroism in reflectivity. The timescale of the effect is comparable to the duration of the excitation and occurs before the magnetization in Co is quenched. Theoretical calculations point to the imbalanced population of Mn unoccupied states caused by the Co interface for the emergence of this transient ferromagnetic state.

Optical, photoluminescence and ferromagnetic properties of Ni-doped ZnO for optoelectronic applications

The current work presents the optical, photoluminescence and ferromagnetic properties of Zn1−xNixO samples with (0.00 ≤ x ≤ 0.20). The samples are well crystallizing in hexagonal wurtzite structure without a significant change in their lattice constants, while the crystallite size, grain size and number of unit cells in ZnO particle are increased. The residual stress is negative (compressive) and almost increases as Ni increases up to 0.10, but it is changed from negative to positive (tensile) at Ni = 0.20. The absorption and reflectance of UV spectra are increased as Ni increases up to 0.20. An absorption band centered at a wavelength of 552 nm is also observed and is gradually shifted by Ni towards longer wavelengths. Interestingly, two direct electronic transitions are found for all samples and two induced bandgaps which are decreased by Ni. While the other optical constants such as linear χ1 and non-linear third-order susceptibilities χ3, dispersion parameters (Ed, Eo), refractive indexes (no, n2), lattice dielectric constant eL, N/m*, optical and electrical conductivities (σopt, σele) are increased by Ni, the dissipation factor (tanδ) is decreased. Although some of the continuous peaks of lower PL intensities are recorded at UV band edges and slightly shifted to longer wave lengths (red shift) as Ni increases, strong UV emission peaks at about 390, 420 and 440 nm are recorded. Finally, the signature of a clear weak ferromagnetism ordering at room temperature, with evaluated magnetic parameters such as magnetization Ms, magnetic moments μ and magneto-crystalline anisotropy factor γ, is recorded for Ni ≥ 0.05 doped samples.

Enhancement of low-frequency spin-orbit-torque ferromagnetic resonance signals by frequency tuning observed in Pt/Py, Pt/Co, and Pt/Fe bilayers

DC voltages via spin rectification effect (SRE), VDC, under microwave irradiation are investigated for three platinum (Pt)/ferromagnetic metal (FM) bilayer structures: Pt/Ni80Fe20, Pt/Co, and Pt/Fe. At the microwave frequency region lower than the resonant frequency, large VDC is obtained at zero DC magnetic field for all devices. In frequency dependence just around the resonant frequency, sharp rise and drop of magnitude in VDC are observed. These behaviors are well explained by the numerically calculated magnetic susceptibility. It is also found that the magnitude of VDC is strongly dependent on the slope of magnetoresistance spectrum. These findings lead to developments of sensitive detection technique for nano-scale magnetization switching.

Structural, optical, antimicrobial and ferromagnetic properties of Zn1−xLaxO nanorods synthesized by chemical route

Nanoparticles of Zn1−xLaxO (x = 0, 0.01, 0.03 and 0.05) were successfully synthesized by a cost-effective chemical method. The influences of La on the microstructural, optical, antimicrobial and magnetic properties of these ZnO nanoparticles were investigated. XRD studies revealed the presence of hexagonal wurtzite phase of ZnO without any impurity. The samples possess nanorod morphology as confirmed by FESEM and TEM analysis. The band gap of ZnO nanorods increased from 3.25 to 3.27 eV with increasing La concentration from 0% to 5%. The presence of various defects in our samples were identified by PL spectroscopy. Magnetic studies exposed the signature of ferromagnetism in all the samples at 300 K. The magnetization of ZnO nanorods is significantly enhanced by La doping. A maximum values of magnetization was observed for 1% La doped ZnO sample which is ~6 times more than for a pure ZnO nanorods. The contributions of oxygen vacancies towards the observed ferromagnetism were confirmed. The origin of ferromagnetism was enlightened on the basis of results of a BMP model. In vitro bioassays showed that synthesized nanorods exhibited narrow-spectrum antibacterial activity against Staphylococcus aureus that compared well with commercial ZnO, nanoparticles (Sigma). Furthermore, the synthesized nanorods when tested for toxicity against Caenorhabditis elegans, but show only low toxicity at 100 µg/mL. Toxicity in zebrafish was also limited. Based on their multifunctional properties, we believe that the synthesized ZnO nanorods could be beneficial for optical device applications in the UV range, spintronic devices, as well as for biomedical purposes.

Anomalous Hall effect in bismuth

We report the occurrence of ferromagnetic-like anomalous Hall effect (AHE) below 30 mT in bismuth crystals. The signatures of ferromagnetism in transport are not corroborated in magnetization measurements, thus suggesting the induction of non-intrinsic magnetism at surfaces and grain boundaries in bismuth. The suppression of the AHE with the increase of magnetic field and temperature coincides with previous reports of superconductivity in Bi, suggesting an interplay between the two phenomena.

A novel study on augmented physical parameters of nickel doped stannic oxide film

A noval study on the fabrication of virgin and nickel (Ni) doped stannic oxide (SnO2) thin films with different doping extent have been conducted to augment the properties of stannic oxide thin films to incorporate into the electric cell which utilizes sun’s energy. The influence of the Ni doping with various extents on the structural, optical and magnetic properties of the different synthesized samples of stannic oxide thin films are investigated byX-ray diffraction (XRD), Scanning electron microscopy (SEM), UV-Visible spectrophotometer and Vibrating Sample Magnetometer (VSM). All the fabricated samples of SnO2: Ni(1at%, 2at%, 3at% and 4at%) exhibited tetragonal structure of stannic oxide. The fusion of Ni into the stannic oxide lattice makes imperfection in the crystal and the presence of additional peaks confirms that the nickel domination is well observed. Increase in the extent of Ni doping causes diminution in optical band gap. The magnetic study reveals that the ferromagnetic signal is gradually enhanced with augment in doping concentration.

Engineering ferromagnetic lines in graphene by local oxidation and hydrogenation using nanoscale lithography

Graphene-based spintronics has attracted much attention owing to the weak spin–orbit interaction and long spin relaxation length in graphene. For implementation of practical and high-density graphene-based spintronic devices, we need to define nanoscale areas with room-temperature ferromagnetism on graphene. Here we report the room-temperature ferromagnetism observed in nanoscale functionalized (oxidized and hydrogenated) graphene using atomic force microscope lithography without involving potential surface contamination and chemical agents. By performing magnetic force microscope (MFM) measurements, we can clearly distinguish the local ferromagnetic signal of selectively functionalized graphene from that of surrounding non-magnetic pristine graphene. The nanoscale functionalized graphene shows experimental evidence of room-temperature ferromagnetism: (1) larger MFM signal than that of graphene; (2) repulsive and attractive interaction with an MFM tip the magnetization of which points into and out of the graphene, respectively; and (3) MFM signal reversal after applying a high magnetic field at an elevated temperature of 400 K. Our first-principles calculations reveal that unpaired spins are present at non-passivated dangling bonds of vacancies on functionalized graphene and the stable ferromagnetic exchange interactions between them are favored. Therefore, nanoscale functionalized graphene is a good candidate for use as the spin injector or detector of high-density graphene-based spintronic devices.