Room Temperature Magnetization Research Articles

Efficient Discovery of Room Temperature Magnetic Transition Metal Monolayers Assisted by Artificial Neural Network

Efficient Discovery of Room Temperature Magnetic Transition Metal Monolayers Assisted by Artificial Neural Network

Observation of novel coexistence of Kondo effect and room temperature magnetism in AlN/Al/AlN trilayer thin film

In this work for the first time, we are reporting the unusual observation of the Kondo effect with the coexistence of room temperature ferromagnetism in AlN/Al/AlN trilayer thin film. The grown film shows resistivity minimum at a temperature of ∼48K, which shifts to the lower temperature on the application of magnetic fields. After considering various possibilities for an upturn in resistivity, we found that the Kondo scattering is responsible for upturn at low temperature. The simultaneous presence of ferromagnetism and Kondo scattering is explained by spatial variation of nitrogen vacancy defects from the film surface to the Al sandwich layer. Furthermore, magneto-transport properties of the film measured at different temperature exhibits both negative and positive components described by localized magnetic moment model for the spin scattering of carriers and two-band model, respectively. This work provides insight into the novel co-existence of ferromagnetism and Kondo effect in crystalline AlN.

Green synthesis of Nd substituted Co-Ni nanospinel ferrites: a structural, magnetic, and antibacterial/anticancer investigation

Co0.5Ni0.5Nd0.02Fe1.98O4 (CoNiNdFO) nanospinel ferrites (NSFs) with and without plant extracts (cardamom seeds, date fruits, flaxseed, tragacanth gum, lavender seeds, and moringa) were prepared using a sol-gel approach. The impact of utilizing different plant extracts on the structural, morphological, and magnetic features of the final products is discussed. Room temperature (300 K) magnetization findings illustrate superparamagnetic (SPM) characteristics for the products synthesized using the cardamom and moringa plant extracts mediated sol-gel approach, whereas the remaining products display ferromagnetic characteristics. On the other hand, lower temperature (10 K) magnetization curves indicate that all the prepared samples exhibit ferromagnetic (FM) behavior. It was found that the magnetic parameters vary greatly with the synthesis approach, plant extract solution, distribution of cations, morphology, and size of NSFs, spin canting, etc. The obtained results indicate that the different synthesis approaches employed in the present investigation are simple, inexpensive, and eco-friendly, and could lead to obtaining interesting magnetic properties. The biological applications of these NSFs were examined. The impact of NSFs with and without cardamom and moringa extracts on human colorectal carcinoma cells (HCT-116) and cervical cancer cells (HeLa) was assessed by checking cell viability using 3-[4,5-dimethylthiazole-2-yl]-2,5-diphenyltetrazolium bromide (MTT) and 4′,6-diamidino-2-phenylindole (DAPI) staining methods. The treatments of NSFs with and without cardamom and moringa extracts decreased cancer cell viability. We calculated the inhibitory concentration (IC50) values for NSFs with and without cardamom and moringa extracts. The treatment of NSFs with and without cardamom and moringa extracts on HCT-116 cells and HeLa cells induced a significant decrease in cancer viability, as revealed by MTT assay. The treatment of NSFs with and without cardamom and moringa extracts caused a noteworthy decrease in colon cancer cells as the number of DAPI stained cells was found to be less in the treated cells. The evaluation of the anti-staphylococcal activity of NSFs with and without cardamom and moringa extracts via the colony-forming unit (CFU) method showed growth inhibition of S. aureus. It was observed that the nanoparticles synthesized via green synthesis had a marked cell count reduction, proving the microbial toxicity of nanoparticles to S. aureus.

Comprehensive mechanism of ferromagnetism enhancement in nitrogen-doped graphene

Realizing the strong ferromagnetism (FM) with both the Curie temperature above room temperature and high magnetization in graphene, is key for its real application in future spintronics. Nitrogen-doping has been confirmed to be one of the most promising ways for experimental realization of room temperature FM in graphene. However, the comprehensive mechanism, which needs to verify the enhancements of both the nitrogen complexes-induced magnetism and the FM coupling among them, remains to be completely explored, which impedes the experimental optimization of the FM in nitrogen-doped graphene (NG). Here, we theoretically perform a comprehensive analysis on the FM enhancement in NG. The enhancement of both the magnetic moments and FM coupling highly depends on the N concentration and relative geometric configuration of the N complexes. Especially, the magnetic moments and FM coupling are attributed to the σ–π magnetic interaction between the localized and itinerant moments, the band reconstruction originated from the proximity of N complexes, and the sublattice match/mismatch. The FM enhancement is high sensibility to the geometric configurations of the N complexes, and only the presence of graphitic-3N combined with trimerized pyridine can realize the sublattice-independent strong FM in the high N-concentration limit. Whereas the low N-concentration limit favors the substantial FM enhancement in all systems, but would unavoidably undergo both the low Curie temperature and low magnetization due to the relatively low N-concentration. We hope that the comprehensive mechanism of FM enhancement we offered can push the optimization of the FM in NG via tuning its N concentration and geometric configuration of the N complexes by the rational design of the specific experimental strategies.

Nd-Fe-B: From sludge waste to powders via purification and modified Ca-reduction reaction process

The oil-based Nd-Fe-B sludge waste was directly recycled to single-phase Nd2Fe14B powders via the combination of purification and modified Ca-reduction reaction method. The impurities and organics in the purified Nd-Fe-B sludge waste were greatly reduced, reducing calcium consumption in the subsequent reduction and diffusion (RD) process, thereby reducing the cost. By further optimizing the Ca-reduction diffusion parameters, especially the mass ratio of CaCl2 and KCl, where the liquid CaCl2, along with the gaseous KCl provide conditions for sufficient and uniform Ca-reduction reaction, high-property Nd-Fe-B magnetic powders with good dispersion, uniform particle sizes, and excellent orientation, were successfully obtained. Using the mixed diffusion medium of CaCl2 and KCl with a mass ratio of 1:1, the room-temperature magnetization of the recycled Nd-Fe-B powders was increased to 157 emu/g at 3 T-magnetic field, which was about 28 % higher than that of the original sludge. The contents of carbon, hydrogen, and oxygen in the recycled magnetic powders were significantly reduced from 6.8, 1.8, and 5.9 wt% to 0.1, 0.19, and 0.56 wt%, respectively. In addition, the uniform grain size with X50 = 3.6 μm and good distribution greatly improved the orientation, which can be beneficial to the preparation of the bonded or sintered Nd-Fe-B-based magnets. The reasons that CaCl2-KCl mixed diffusion medium can lead to well-dispersed, uniform recycled powder with high magnetization arise from a combination of factors for improving the liquid phase reaction environment and the gaseous KCl to prevent agglomeration and inhibit particles merging or growth.

Magnetic properties of doped CaF2 powders

Room-temperature “ferromagnetism” found last decades in nonmagnetic nanoparticles is a puzzle that is not solved yet. In this work we report about the observation of room-temperature magnetism in powders of CaF2 doped with CeO2 as well as in the undoped powder but containing small amount impurity. The studied powders were obtained by grinding of bulk crystals using a sapphire mortar with a pestle. Magnetization dependences on magnetic field for the studied powder samples resemble the dependences observed in nonmagnetic oxide nanoparticles like CeO2. The comparative study of bulk and powder fluorite samples unambiguously confirms the key role of surface defects in the observed features. XRD and TEM measurements show that powder samples have particles of different sizes from the micro down to the submicron scale. The undoped bulk fluorite as well as doped with 0.5at.% (two differently grown crystals), 2at.%, 10at.% of CeO2 revealed only diamagnetic or paramagnetic contribution to magnetization curve whereas the powders of the same samples have ferromagnetic-like contribution on M−H curve. This work may help to get new insight in room-temperature “ferromagnetism” phenomenon of nonmagnetic nanoparticles.

Magnetism and site occupancy in epitaxial Y-rich yttrium iron garnet films

Y-rich yttrium iron garnet (Y-YIG) thin films were grown on gadolinium gallium garnet substrates using pulsed laser deposition from a ${\mathrm{YFeO}}_{3}$ target. The films are epitaxial on the substrate, and the unit cell of (001)- and (111)-oriented Y-YIG undergoes a tetragonal or rhombohedral distortion, respectively. The Y-YIG has lower room temperature magnetization but higher low-temperature magnetization than bulk YIG, and its Curie temperature was in the range of 330--400 K, depending on the growth conditions. First-principles calculations predict that ${\mathrm{Y}}_{\mathrm{Fe}}$ antisite defects are more stable in octahedral than tetrahedral Fe sites. Temperature-dependent magnetization measurements together with a superexchange dilution model and composition determined from x-ray photoelectron spectroscopy indicate that nonmagnetic species (${\mathrm{Y}}^{3+}$ and vacancies) are predominantly accommodated in the octahedral sites, but there is a significant occupancy of tetrahedral sites.

Room temperature short-range ferromagnetic order in Ni-doped tetragonal perovskite niobate

Here, we report a strong room-temperature magnetism in Ni-doped BaNbO3. The tetragonal BaNixNb(1-x)O3-δ nano-powders are prepared by the composite-hydroxide-mediated method. The as-prepared BaNixNb(1-x)O3-δ has a saturation magnetization (Msat) of 2.54 emu/g, a remnant magnetization (Mr) of 0.228 emu/g, and a small coercive field (Hc) of 99.14 Oe at room temperature, presenting as a short-range magnetic order. The Ni-doping leads to an incommensurate valence state of Nb, and the oxidation state of Ni in the matrix is + 2. It brings not only strong local spin polarization to the matrix, but also balances the oxygen defects and gives rise to a more doping content. The strong magnetism originated from the existence of short-range magnetic order and the strong local net spin polarization at the Fermi level (EF), which are proved by the first-principle calculations.

Morphologic, structural, and magnetic characterization of cobalt ferrite nanoparticles synthesized at different temperatures

In this study we report on the synthesis and characterization of cobalt ferrite (CoFe2O4) nanoparticles (NPs), synthesized by chemical co-precipitation in alkaline medium. Two samples were synthesized at two different temperatures, 35 and 90 oC. Both samples were characterized by Transmission Electron Microscopy (TEM), x-ray diffraction (XRD), and room-temperature (RT) magnetization. Two samples showed superparamagnetic behavior (SPM) at RT. TEM reveals morphological mean diameter increasing 5.8 nm to 10.4 nm, with the increase of the co-precipitation temperature. XRD confirm the inverse cubic spinel structure. The RT magnetization curves were analyzed by the first-order Langevin function averaged out by a lognormal distribution function of magnetic moments. This analysis showed saturation magnetization and magnetic moment increases from 60.2 to 74.8 emu/g and from 3.9 x 103 to 8.2 x 103 mB, respectively.

Flat epitaxial quasi-1D phosphorene chains

The emergence of peculiar phenomena in 1D phosphorene chains (P chains) has been proposed in theoretical studies, notably the Stark and Seebeck effects, room temperature magnetism, and topological phase transitions. Attempts so far to fabricate P chains, using the top-down approach starting from a few layers of bulk black phosphorus, have failed to produce reliably precise control of P chains. We show that molecular beam epitaxy gives a controllable bottom-up approach to grow atomically thin, crystalline 1D flat P chains on a Ag(111) substrate. Scanning tunneling microscopy, angle-resolved photoemission spectroscopy, and density functional theory calculations reveal that the armchair-shaped chains are semiconducting with an intrinsic 1.80 ± 0.20 eV band gap. This could make these P chains an ideal material for opto-electronic devices.

Magnetism and spin transport in platinum/scandium-substituted terbium iron garnet heterostructures

Growth, structural, magnetic, and spin transport properties of epitaxial scandium-substituted terbium iron garnet (TbScIG) thin films on gadolinium gallium garnet [GGG, (111) orientation] substrates are reported as a function of Sc content. The films are epitaxial on GGG without strain relaxation up to a thickness of 90 nm and exhibit perpendicular magnetic anisotropy. Sc, a nonmagnetic cation, occupies up to 40% of the octahedral Fe sites. Increasing Sc lowers the compensation temperature and Curie temperature and increases the room-temperature saturation magnetization from 37 kA/m for ${\mathrm{Tb}}_{2.8}{\mathrm{Fe}}_{5.2}{\mathrm{O}}_{12}$ to 59 kA/m for ${\mathrm{Tb}}_{2.8}{\mathrm{Sc}}_{0.8}{\mathrm{Fe}}_{4.4}{\mathrm{O}}_{12}$. Anomalous Hall effect-like spin Hall magnetoresistance measurements were performed to determine the effect of scandium content on spin mixing conductance of the TbScIG|Pt interface. The spin mixing conductance increased significantly with an increase in Sc content, consistent with a dependence of spin transport on the net Fe magnetization instead of the total magnetization of the garnet.

Investigation of Electrical Properties of BiFeO3/LDPE Nanocomposite Dielectrics with Magnetization Treatments.

The purpose of this paper is to study the effect of nano-bismuth ferrite (BiFeO3) on the electrical properties of low-density polyethylene (LDPE) under magnetic-field treatment at different temperatures. BiFeO3/LDPE nanocomposites with 2% mass fraction were prepared by the melt-blending method, and their electrical properties were studied. The results showed that compared with LDPE alone, nanocomposites increased the crystal concentration of LDPE and the spherulites of LDPE. Filamentous flake aggregates could be observed. The spherulite change was more obvious under high-temperature magnetization. An agglomerate phenomenon appeared in the composite, and the particle distribution was clear. Under high-temperature magnetization, BiFeO3 particles were increased and showed a certain order, but the change for room-temperature magnetization was not obvious. The addition of BiFeO3 increased the crystallinity of LDPE. Although the crystallinity decreased after magnetization, it was higher than that of LDPE. An AC test showed that the breakdown strength of the composite was higher than that of LDPE. The breakdown strength increased after magnetization. The increase of breakdown strength at high temperature was less, but the breakdown field strength of the composite was higher than that of LDPE. Compared with LDPE, the conductive current of the composite was lower. So, adding BiFeO3 could improve the dielectric properties of LDPE. The current of the composite decayed faster with time. The current decayed slowly after magnetization.

Enhanced magnetic and magnetodielectric properties of Co-doped brownmillerite KBiFe2O5 at room temperature

We report significant enhancement in magnetic and magnetodielectric (MD) properties of cobalt (Co) doped polycrystalline KBiFe2O5 (KBFO). Rietveld refinement of X-ray diffraction data confirms monoclinic crystal structure with P2/c space group. SEM micrographs confirmed the polycrystalline nature of the samples with grain sizes varying from 1 µm to 5 µm. Raman spectra demonstrate the shifting of vibrational modes due to Co doping which is associated with FeO4 tetrahedral. The shift in vibrational modes due to Co doping is in line with the observed changes in FeO4 tetrahedral bond angles (O1-Fe-O1 & O1-Fe-O2) and bond lengths (Fe-O1 & Fe-O2) obtained from XRD data. Room temperature magnetization data reveal the maximum enhancement in Mr (~103 times) and Ms (~200 times) values for 5% Co-doped KBFO compared with KBFO. This enhancement in magnetization data is due to a spin-canted (Fe3+) antiferromagnetic spin structure and increased lattice strain with increased cobalt substitution. The temperature-dependent dielectric and magnetization data show an anomaly near 770 K (Tc) indicates the MD effect. Compared to KBFO, nearly ten times enhancement in MD and nearly 50 times reduction in magnetic loss (ML) is observed for KBFCO5 measured at 30 kHz frequency. Landau free energy approach confirms the biquadratic nature of MD coupling, and the strength is − 8 (emu/g)2 for 5% Co-doped KBFO and − 5(emu/g)2 for 10% Co-doped KBFO, respectively. The comparative study reveals that Co substitution in KBFO enhances maximum MD% and magnetization in 5% Co substitution. These results establish the coupling between electric and magnetic orders, which can find application in AC/DC magnetic field sensors and ME solar cells.

Residual oxygen driven defect mediated room temperature magnetism in dilute nitrogen incorporated amorphous Al-N-O alloy thin film

In the present work, the effect of residual oxygen during reactive sputtering of Al target in a dilute nitrogen environment has been investigated. We have deposited a thin film by reactive sputtering of high purity Al target (99.999%) using a mixture of ultra-high pure Ar & N2 gas in a percentage ratio of 95% and 5%, respectively. The characterization of as-deposited film using Grazing Incidence X-ray Diffraction (GIXRD), X-ray Photoelectron Spectroscopy (XPS), and Energy-Dispersive X-ray (EDX) spectroscopy reveals that in dilute nitrogen environment Al preferred to react with residual oxygen to form amorphous Al2O3 (a-Al2O3) along with aluminum vacancies (AlV). The dilute nitrogen incorporated in the film occupies the AlV sites in Al2O3 and bonded with nearby oxygen to form Al(NOy)x phase. The bulk of the film mainly consists of Al(NOy)x phase, which decreases as we move towards the film surface. At close to the film surface the oxygen vacancies are significantly present. Magnetic and electrical transport properties measurement of the film shows room temperature (RT) magnetism along with n-type semiconducting behaviour. The origin of RT magnetism and observed transport properties have been investigated using a first-principle calculation based on density functional theory (DFT). The residual oxygen driven defects present in the film are found to play a critical role and discussed in detail. The current study is significant in the sense that it provides a new insight to explore the potentials of Al-N-O alloy-based materials as an n-type magnetic semiconductor at room temperature by manipulating defects and optimizing nitrogen concentration.

High room-temperature magnetization in Co-doped TiO2 nanoparticles promoted by vacuum annealing for different durations

To clarify the contribution of oxygen vacancies to room-temperature ferromagnetism (RTFM) in cobalt doped TiO2 (Co-TiO2), and in order to obtain the high level of magnetization suitable for spintronic devices, in this work, Co-TiO2 nanoparticles are prepared via the sol–gel route, followed by vacuum annealing for different durations, and the influence of vacuum annealing duration on the structure and room-temperature magnetism of the compounds is examined. The results reveal that with an increase in annealing duration, the concentration of oxygen vacancies rises steadily, while the saturation magnetization (M s) shows an initial gradual increase, followed by a sharp decline, and even disappearance. The maximum M s is as high as 1.19 emu/g, which is promising with respect to the development of spintronic devices. Further analysis reveals that oxygen vacancies, modulated by annealing duration, play a critical role in tuning room-temperature magnetism. An appropriate concentration of oxygen vacancies is beneficial in terms of promoting RTFM in Co-TiO2. However, excessive oxygen vacancies will result in a negative impact on RTFM, due to antiferromagnetic superexchange interactions originating from nearest-neighbor Co2+ ions.

Synthesis of Self-Supporting ZnO Nanowire Array Film and its Optical Property and Room Temperature Ferromagnetism

X-ray diffractometer, field emission scanning electron microscope (SEM, Hitachi S-4800), laser confocal micro-region Raman spectrometer and vibration sample magnetometer were used to systematically study the effects of polyethyleneimine concentrations and exposure time on the morphology and size of ZnO nanowire arrays. The photoelectric property and the relationship between the morphology of nanowire arrays and ferromagnetism at room temperature were also analyzed. Under 15 min exposure time, when the polyethyleneimine concentration is 2.25 g / L, the obtained ZnO nanowire array film exhibits the smallest size, the optimal density and vertical orientation. According to the study of luminescence and room temperature magnetism, it is shown that the optical and ferromagnetic property are related to the variation tendency of oxygen defects and surface defects of the ZnO nanowires.

Room temperature magnetization dynamics of Y3Fe5O12 films capped with a Cr2O3 layer

Spin-transport across antiferromagnetic insulators (AFMIs) is a novel way to reduce power consumption in pure spin current based devices. In this paper, room temperature magnetization dynamics of Yttrium-Iron-Garnet (YIG) interfaced with an AFMI Cr2O3 has been investigated in order to study the spin transport across the AFMI. After capping with 20 nm thick Cr2O3, the Gilbert damping parameter of YIG layer is not changed enormously as compared to the non-capped sample. Cr2O3 of 20 nm thickness is in a spin disordered state at room temperature due to finite size scaling effect. So, it was expected that the spin current from YIG layer can be transmitted through the Cr2O3 layer. However, due to large spin diffusion length and large domain size of Cr2O3 (>20 nm), spin current cannot lose its angular momentum during propagation in Cr2O3 as a result of which, no change in Gilbert damping parameter is noticed. This observation will pave a way for further investigation of spin current transport across AFMIs.

Magnetic interactions in graphene decorated with iron oxide nanoparticles

We present the studies of structural and magnetic properties of graphene composites prepared with several quantities of α-Fe2O3 dopant of 5%, 25% and 50% made with either ethanol or acetone. Our studies showed the presence of a weak magnetic order up to room temperature and saturation magnetization close to 0.2 emu g−1 in pure commercial graphene. With regard to magnetic properties of our graphene + iron oxide samples, the solvent used during the preparation of the composite had a significant influence on them. For graphene + Fe2O3 samples made with acetone the magnetic properties of pure graphene played a major role in the overall magnetic susceptibility and magnetization. On the other hand, for graphene + iron oxide samples made with ethanol we observed the presence of superparamagnetic blocking at T < 110 K which was due to the additional appearance of γ-Fe3O4 nanoparticles. Changes in the synthesis solvent played a major role in the magnetic properties of our graphene + Fe2O3 nanocomposite samples resulting in much higher saturation magnetization for the samples made with ethanol. Both the shape and the parameters characterizing magnetization hysteresis loops depend strongly on the amount of iron oxide and changes in the preparation method.

Enhanced Curie Temperature and Critical Exponents of Fe‐Substituted NiCu Alloy

The magnetic moment and the Curie temperature of the NiCu alloy decrease to a large extent with Cu. A Ni60Cu40 alloy with Fe substitutions up to 2 at% using a polyol‐mediated synthesis process resulting in enhanced magnetic properties is obtained. The Ni60Cu39Fe1 and Ni60Cu38Fe2 alloys show fcc phase with the Curie temperatures of 237 and 266 K, respectively. The room temperature magnetization measurement shows paramagnetic behavior, whereas, at 15 K, the 1 at% Fe‐substituted alloy exhibits a saturation magnetization of 20 emu g−1 that increases by 25% with 2 at% Fe. The positive slope in the Arrott plot confirms the second‐order phase transition in the alloys. The maximum magnetic entropy change (MCE) is observed as 0.101 J kg−1 K−1 for the Ni60Cu39Fe1 alloy with the peak temperature of 238 K in an applied magnetic field of 0.5 T. The critical exponent (β) calculated using the MCE method is in the range of 0.576(3)–0.587(3) for the alloys, whereas the values obtained using the Kouvel‐Fisher (KF) method are 0.523(3)–0.565(2). The extracted β values are slightly greater than the mean‐field model suggesting clustering behavior in the alloys.

Cross-Linking and Charging Molecular Magnetoelectronics.

Magnetoelectrics are witnessing an ever-growing success toward the voltage-controlled magnetism derived from inorganic materials. However, these inorganic materials have predominantly focused on the ferroelectromagnetism at solid-to-solid interfaces and suffered several drawbacks, including the interface-sensitive coupling mediators, high-power electric field, and limited chemical tunability. Here, we report a promising design strategy to shift the paradigm of next-generation molecular magnetoelectrics, which relies on the integration between molecular magnetism and electric conductivity though an in situ cross-linking strategy. Following this approach, we demonstrate a versatile and efficient synthesis of flexible molecular-based magnetoelectronics by cross-linking of magnetic coordination networks that incorporate conducting chain building blocks. The as-grown compounds feature an improved critical temperature up to 337 K and a room-temperature magnetism control of low-power electric field. It is envisaged that the cross-linking of molecular interfaces is a feasible method to couple and modulate magnetism and electron conducting systems.