Strong Room Temperature Ferromagnetism Research Articles

Room-Temperature Ferromagnetism with Strong Spin-Orbit Coupling Achieved in CaRuO3 Interfacial Phase via Magnetic Proximity Effect.

Recently, theoretical and experimental research predicted that ferromagnets with strong spin-orbit coupling (SOC) could serve as spin sources with dramatically enhanced spin-orbit torque (SOT) efficiency due to the combination of spin Hall effect and anomalous Hall effect (AHE), presenting potential advantages over conventional nonmagnetic heavy metals. However, materials with a strong SOC and room-temperature ferromagnetism are rare. Here, we report on a ferromagnetic (FM) interfacial phase with Curie temperature exceeding 300 K in the heavy transition-metal oxide CaRuO3, in proximity to La0.67Sr0.33MnO3. Electron energy loss and polarized neutron reflectometry spectra reveal the strong charge transfer from Ru to Mn at the interface, triggering antiferromagnetic exchange interactions between interfacial Ru/Mn ions and thus transferring magnetic order from La0.67Sr0.33MnO3 to CaRuO3. An obvious advantage of such interfacial phase is the enhanced anomalous Hall effect at temperatures from 150 to 300 K. Compared to the most promising room-temperature ferromagnetic oxide La0.67Sr0.33MnO3, the anomalous Hall conductivity σxyAHE (or anomalous Hall angle θH) of CaRuO3/La0.67Sr0.33MnO3 superlattices is increased by 30 (or 31) times at 150 K and 10 (or 3) times at 300 K. This work demonstrates a special approach for inducing ferromagnetism in heavy transition-metal oxides with strong SOC, offering promising prospects for all-oxide-based spintronic applications.

Controllable syntheses and magnetic and photoelectric properties modulations of CuFeS2 nanoplates

As an antiferromagnetic material, chalcopyrite CuFeS2 has both magnetic and semiconductor properties, which brings great prospects for the application of spintronics devices. Here, chalcopyrite CuFeS2 nanoplates were successfully synthesized through a template mediated method. The CuFeS2 exhibits a tetragonal phase while largely maintaining the hexagonal morphology of the CuS template, with particle sizes ranging between 90 and 120 nm. Specifically, the magnetic properties of CuFeS2 can be tuned by varying the Fe content. The strong room-temperature ferromagnetism is attributed to the coexistence of the ferromagnetic Fe7S8 and the antiferromagnetic CuFeS2, as well as to the unsaturated spin states of Fe on the surfaces of the nanoplates. Furthermore, the photoresponses of CuFeS2 nanoplates were confirmed by I-V measurements under dark and light illumination. The stable photoresponse capability is attributed to its semiconductor properties. This novel room-temperature ferromagnetism and photoelectric effect promotes the development of CuFeS2 nanoplates, and has great significance for the synthetic pathways of other ternary magnetic compounds.

Profiles of oxygen and titanium point defects in ferromagnetic TiO2 films

Experimentally it is shown that without any oxygen manipulation for TiO2, a strong room temperature ferromagnetism could be expected only in ultra-thin films, with the ideal thickness below 100 nm. Both bulks and nano-powders of TiO2 are diamagnetic, indicating that the surface and its nano-sublayers play very important roles in tailoring the magnetic properties in this type of compound. To shed a new light on the defect-related magnetism in the typical case of anatase TiO2 surfaces, we have performed a series of quantum-mechanical calculations for TiO2 slabs containing Ti or O vacancies in different distances from the (001) surface. The lowest formation energies were obtained for the Ti vacancies in the first sub-surface layer and the O vacancies within the surface. The computed magnetic states reflect complicated structural relaxations of atoms influenced by both the surface and vacant atomic positions. O atoms cannot contribute much to magnetic moment when Ti vacancies are isolated and far from the surface. Ti vacancies in TiO2 are only metastable. The formation energy of Ti interstitials is lower than for Ti vacancies since high-temperature annealing, especially with a lot of O2 available that would fill up O-related defects, and as a result, eliminate most of Ti vacancies. Lower temperatures, less O2, and shorter exposure times may enable not only partial elimination of Ti vacancies but also can facilitate their diffusion into different states of aggregations. In the ferromagnetic films (i.e. thin films below 100 nm), it looks like that the O atoms are located closer to the Ti vacancies.

Strong Room-Temperature Ferromagnetism of MoS2 Compound Produced by Defect Generation.

Ferromagnetic materials have been attracting great interest in the last two decades due to their application in spintronics devices. One of the hot research areas in magnetism is currently the two-dimensional materials, transition metal dichalcogenides (TMDCs), which have unique physical properties. The origins and mechanisms of transition metal dichalcogenides (TMDCs), especially the correlation between magnetism and defects, have been studied recently. We investigate the changes in magnetic properties with a variation in annealing temperature for the nanoscale compound MoS2. The pristine MoS2 exhibits diamagnetic properties from low-to-room temperature. However, MoS2 compounds annealed at different temperatures showed that the controllable magnetism and the strongest ferromagnetic results were obtained for the 700 °C-annealed sample. These magnetizations are attributed to the unpaired electrons of vacancy defects that are induced by annealing, which are confirmed using Raman spectroscopy and electron paramagnetic resonance spectroscopy (EPR).

Realization of Robust and Ambient-Stable Room-Temperature Ferromagnetism in Wide Bandgap Semiconductor 2D Carbon Nitride Sheets.

Due to their weak intrinsic spin-orbit coupling and a distinct bandgap of 3.06 eV, 2D carbon nitride (CN) flakes are promising materials for next-generation spintronic devices. However, achieving strong room-temperature (RT) and ambient-stable ferromagnetism (FM) remains a huge challenge. Here, we demonstrate that the strong RT FM with a high Curie temperature (TC) up to ∼400 K and saturation magnetization (Ms) of 2.91 emu/g can be achieved. Besides, the RT FM exhibits excellent air stability, with Ms remaining stable for over 6 months. Through the magneto-optic Kerr effect, Hall device, X-ray magnetic circular dichroism, and magnetic force microscopy measurements, we acquired clear evidence of magnetic behavior and magnetic domain evolutions at room temperature. Electrical and optical measurements confirm that the Co-doped CN retains its semiconductor properties. Detailed structural characterizations confirm that the single-atom Co coordination and nitrogen defects as well as C-C covalent bonds are simultaneously introduced into CN. Density functional theory calculations reveal that introducing C-C bonds causes carrier spin polarization, and spin-polarized carrier-mediated magnetic exchange between adjacent Co atoms leads to long-range magnetic ordering in CN. We believe that our findings provide a strong experimental foundation for the enormous potential of 2D wide bandgap semiconductor spintronic devices.

Facet-engineering enhanced room-temperature ferroelectric control of magnetism in semiconducting β-FeSi2″

The electric-field control of the magnetism of magnetic semiconductors is highly desirable for spintronic-integrated circuits with ultralow power consumption. However, the low Curie temperature, small magnetic moments and the limited screening thickness hinder the practical application of the devices. Herein, we propose a novel strategy to enhance electrical control of magnetic semiconductors at room temperature by facet engineering. Taking β-FeSi2 as a prototype material, which has strong facet-induced room-temperature ferromagnetism, significant nonvolatile room-temperature control of magnetism is realized at the interface between ferromagnetic β-FeSi2 and ferroelectric P(VDF-TrFE). Because β-FeSi2 is ferromagnetic at the surface and non-magnetic in the bulk, the surface ferromagnetism can be successfully modulated through the remarkable carrier density variation at the interface caused by large remanent ferroelectric polarization. The facet-engineering enhanced room-temperature ferroelectric control of magnetism paves the way towards future nonvolatile and low-power-consumption spintronic devices.

Enhancement of magnetic moment of Er-implanted ZnO films by post-annealing in Ar and vacuum

The Er ions have been introduced into ZnO films prepared by radio-frequency magnetic sputtering (RFMS) on sapphire substrates by ion implantation. The annealing effect on structural and magnetic properties for Er-implanted ZnO films under air, Ar, and vacuum atmospheres at 650 °C have been investigated systematically by a combined characterization of X-ray diffractions, Raman spectroscopy, Photoluminescence, X-ray photoelectron spectroscopy and Superconducting Quantum Interference Device. Strong room-temperature ferromagnetism has been observed for the as-implanted and annealed films. We found that the saturation magnetization varied greatly for different annealing conditions. A considerable enhancement in magnetization is observed for Er-doped ZnO annealed in vacuum, which is accompanied by an enhancement in the concentration of oxygen vacancy. Our results give clear evidence that the enhancement of ferromagnetism is strongly correlated with the increase of oxygen vacancies in Er-doped ZnO.

Realization of Strong Room-Temperature Ferromagnetism in Atomically Thin 2D Carbon Nitride Sheets by Thermal Annealing.

The presence of the intrinsic band gap of 3.06 eV makes atomically thin carbon nitride sheets (CNs) a promising spin-based semiconductor material. However, the absence of localized spins makes the pristine CNs intrinsically nonmagnetic. Here we report the realization of strong room-temperature (RT) ferromagnetism with a high Curie temperature of ca. 524.2 K in atomically thin 2D CNs by annealing pristine CNs at 700 °C. In particular, the RT saturated magnetization reaches as high as 0.71 emu/g, which is the highest value reported so far in carbon-based materials. The structural characterization combined with density functional theory calculations reveals that (i) the seven C-C bonds per unit cell were formed after annealing and (ii) the C-C bonds can introduce high-density localized spins and realize the long-range ferromagnetic couplings among these spins.

A magnetic sensor using a 2D van der Waals ferromagnetic material

Two-dimensional (2D) van der Waals ferromagnetic materials are emerging as promising candidates for applications in ultra-compact spintronic nanodevices, nanosensors, and information storage. Our recent discovery of the strong room temperature ferromagnetism in single layers of VSe2 grown on graphite or MoS2 substrate has opened new opportunities to explore these ultrathin magnets for such applications. In this paper, we present a new type of magnetic sensor that utilizes the single layer VSe2 film as a highly sensitive magnetic core. The sensor relies in changes in resonance frequency of the LC circuit composed of a soft ferromagnetic microwire coil that contains the ferromagnetic VSe2 film subject to applied DC magnetic fields. We define sensitivity as the slope of the characteristic curve of our sensor, df0/dH, where f0 is the resonance frequency and H is the external magnetic field. The sensitivity of the sensor reaches a large value of 16 × 106 Hz/Oe, making it a potential candidate for a wide range of magnetic sensing applications.

Defect-induced ferromagnetism in W18O49 nanowires

In this paper, strong room temperature ferromagnetism was observed in non-stoichiometric tungsten oxide nanostructure, sea urchin-like single crystalline W18O49 nanowires prepared by a solvothermal approach. The prepared W18O49 nanowires exhibited a 39-fold enhancement in saturation magnetic moment as compared to the transformed WO3 by annealing in air. By combining x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), x-ray photoelectron spectroscopy (XPS), and electron-spin resonance (EPR) spectroscopy, a defect complex, i.e., W5+ ions accompanied by oxygen vacancies, has been identified in the prepared W18O49. The observed ferromagnetism in W18O49 is qualitatively explained through a model involving bound magnetic polarons which were induced by the coupling between the electron trapped by oxygen vacancy and the unpaired 5d electron of W5+ ion. The results provide us better insight into the defect engineering of room-temperature ferromagnetism in tungsten oxide.

Magnetic and optical properties of new (1 − x) Bi0.5Na0.5TiO3 + x BaMnO3−δ solid solution materials

A solid solution of BaMnO3−δ and Bi0.5Na0.5TiO3 was prepared using a sol–gel method. X-ray diffraction analysis of Bi0.5Na0.5TiO3 − x BaMnO3−δ samples indicated a rhombohedral structure of Bi0.5Na0.5TiO3 materials during distortion due to the random distribution of Ba and Mn cations in the host lattice. The substitution of Ba and Mn at A- and B-sites in the perovskite structure of Bi0.5Na0.5TiO3 changed the optical band gap due to local defect and/or promotion of oxygen vacancies. The samples exhibited strong room temperature ferromagnetism, which overcame the weak ferromagnetism and diamagnetism of Bi0.5Na0.5TiO3 materials. Our study provided a flexible approach to integrate the ferromagnetism properties of ferroelectric materials.

Structural, optical and magnetic properties of Ni1−xZnxO/Ni nanocomposite

Ni1−xZnxO/Ni nanocomposite samples synthesized by wet chemical method were characterized by X-ray powder diffraction, Fourier Transform Infrared (FTIR) spectrometer, UV–Vis spectrometer and Nine Tesla Physical Property Measurement System (PPMS)-based vibrating sample magnetometer (VSM). XRD spectra contained peaks due to NiO and Ni. Rietveld refinement of XRD data indicated the change in lattice parameter is more than twice in NiO than in Ni suggests that Zn goes more favorably into NiO than in Ni. The band gap of NiO in Ni1−xZnxO/Ni samples increased from 3.58 to 3.81 eV with increasing x from 0 to 0.07 and thereafter the band gap slightly decreases. NiO/Ni nanocomposites show strong room temperature ferromagnetism. The ferromagnetic interaction further enhanced with 1% of Zn doping into NiO in NiO/Ni nanocomposite. Beyond this doping concentration, the magnetic interaction does not show any systematic variation. The observation of room temperature ferromagnetism in nanocomposite samples is explained by combining the effect of bound magnetic polaron and the magnetization of system with the distribution of magnetic ions as meagerly dispersed in large clusters. Our study indicated that Ni0.99Zn0.01O/Ni nanocomposite sample could be useful for magnetic recording device application.

Doping-Concentration-Induced Ferromagnetism and Antiferromagnetism in In2S3:Dy3+ Quantum Dots

Diluted magnetic semiconductor (DMS) quantum dots have been researched extensively due to their potential applications in next-generation spin-based devices. Herein, the cubic In2S3:Dy3+ DMS quantum dots (3–5 nm) with different doping concentrations were synthesized via a gas–liquid phase chemical deposition method. The effect of Dy3+ content on the photoluminescence (PL) and ferromagnetism was investigated. The PL emission spectra exhibit a blue-shift compared with those reported previously due to the increased quantum size confinement and enhanced intensity attributed to the Dy3+ doping. The distinct and stronger room-temperature ferromagnetism is observed from vibrating sample magnetometer (VSM) measurement. The coexistence of ferromagnetic (FM) and antiferromagnetic (AFM) phases and antiferromagnetic interation plays a dominant position after a certain doping concentration value can be further confirmed according to the zero field cooling/field cooling (ZFC/FC) curves. As revealed in the magnetic origin study from first-principles calculations, the ferromagnetism obtained arises not only from the Dy atoms but also from the In vacancies. In addition, we also proposed a spontaneous mechanism based on the bound magnetic polaron theory to explain the change of saturation magnetizations along with Dy3+ doping concentration. This work provides experimental and theoretical guidance for designing and synthesizing unique spintronic materials, which can promote development of spintronic applications.

Half Metallic Ferromagnetism in Eu-Doped CdS Nanoparticles

Hexagonal wurtzite structured Eu3+-doped CdS nanoparticles (CdS:Eu NPs) were synthesized via a gas–liquid phase chemical deposition method. It is found that as-prepared CdS:Eu NPs exhibit distinct and stronger room-temperature ferromagnetism (RTFM). The ferromagnetism as observed from the zero-field-cooling and field-cooling curves of CdS:Eu NPs has been further confirmed by our first-principles calculations based on the density functional theory. Because of a coupling between the 4f-related spin polarized states and the carriers, the half metallic ferromagnetic CdS:Eu NPs could be a desired material for the use in spintronics. The origin of ferromagnetism in CdS:Eu NPs is attributed primarily to the Eu atoms and partially to the Cd vacancies as well.

Strong Facet-Induced and Light-Controlled Room-Temperature Ferromagnetism in Semiconducting β-FeSi2 Nanocubes.

Crystalline β-FeSi2 nanocubes with two {100} facets and four {011} lateral facets synthesized by spontaneous one-step chemical vapor deposition exhibit strong room-temperature ferromagnetism with saturation magnetization of 15 emu/g. The room-temperature ferromagnetism is observed from the β-FeSi2 nanocubes larger than 150 nm with both the {100} and {011} facets. The ferromagnetism is tentatively explained with a simplified model including both the itinerant electrons in surface states and the local moments on Fe atoms near the surfaces. The work demonstrates the transformation from a nonmagnetic semiconductor to a magnetic one by exposing specific facets and the room-temperature ferromagnetism can be manipulated under light irradiation. The semiconducting β-FeSi2 nanocubes may have large potential in silicon-based spintronic applications.

Bimagnetic h-Co/h-CoO nanotetrapods: preparation, nanoscale characterization, three-dimensional architecture and their magnetic properties.

Well-defined bimagnetic h-Co decorated wurtzite h-CoO nanotetrapods with uniform size have been successfully fabricated by a one-pot thermal decomposition method for the first time, and their three-dimensional architecture, crystal structure, chemical phase and exchange bias effect are characterized at the nanoscale. It is found that individual bimagnetic h-Co/h-CoO nanotetrapods are made of a h-CoO nanotetrapod skeleton to which multiple nanocrystals of ferromagnetic metallic h-Co are directly attached. The chemical analysis shows that the mass ratio of h-CoO and h-Co is 65 : 35. The detailed investigations of the crystal structure reveal that both the h-CoO nanotetrapod skeleton and h-Co nanoparticles have hexagonal structure. The four pods of individual nanotetrapods are single crystals with the same [001] orientation along with their pod axes and grow together by twinning with (110) the twin interface and the 120° spatial boundary angle. The magnetic measurements reveal that the h-Co/h-CoO nanotetrapods have a surprisingly strong room temperature ferromagnetism and there exists a weak exchange coupling between the h-CoO nanotetrapod skeleton and the decorated h-Co tiny nanoparticles. It is believed that our new structural form of the bimagnetic h-Co/h-CoO nanotetrapods provides not only a smart functional 3D nanoarchitecture as building block in nanoelectronics and nanosensors, but also an ideal specimen for a further understanding of weak antiferromagnetic-ferromagnetic interaction.

Strong ferromagnetism of reduced graphene oxide

The ferromagnetism of carbon-based materials has attracted much interest for its possible applications in spintronics devices. In this work, we studied the magnetic properties of reduced graphene oxide (rGO) obtained by high-temperature annealing of graphene oxide (GO) prepared in weak oxidizing conditions. The rGO samples exhibit strong room-temperature ferromagnetism, with saturation magnetization of 0.24 emu/g, about 24 times of the value of rGO prepared in strong oxidizing conditions. Hysteresis negative magnetoresistance (MR) has been observed in rGO, which confirms the intrinsic nature of the observed ferromagnetism. The MR value keeps nearly unchanged below 15 K, and decreases with further increasing temperature, and finally disappears at T ⩾ 50 K. At T = 5 K, the maximum MR ratio of [R(H) − R(0)]/R(0) is about −2.5%. The strong ferromagnetism is attributed to the preserved graphitic structure of weakly oxidized rGO, and the negative MR is due to the spin dependent scattering of defects.

Oxygen vacancy-mediated enhanced ferromagnetism in undoped and Fe-doped TiO2 nanoribbons

We have investigated the structural, optical and ferromagnetic properties of undoped and Fe-doped TiO2 nanoribbons (NRbs) grown by a solvothermal method. A strong room temperature ferromagnetism (RTFM) is observed in both undoped and Fe-doped TiO2 NRbs. Fe-doped TiO2 NRbs exhibited a ∼4.8-fold enhancement in RTFM as compared to the undoped NRbs grown under similar conditions. However, the RTFM decreases at higher Fe concentration, possibly due to antiferromagnetic ordering between nearby Fe3+ ions caused by a super exchange interaction. X-ray diffraction patterns reveal the pure TiO2(B) phase, the TiO2(B)–anatase mixed phase and the anatase–rutile mixed phase of the TiO2 structure. Field emission scanning electron microscopy and transmission electron microscopy observations reveal NRbs with uniform pore distribution and nanopits formed on the surface for both undoped and Fe-doped NRbs. These samples exhibit strong visible photoluminescence associated with oxygen vacancies and the ferromagnetic hysteresis loop, both of which are strongly enhanced after vacuum annealing. Optical absorption, electron spin resonance and x-ray photoelectron spectroscopic analyses are performed to elucidate the origin of RTFM. The observed RTFM in undoped and Fe-doped TiO2 NRbs is qualitatively explained through a model involving bound magnetic polarons, which include an electron locally trapped by an oxygen vacancy with the trapped electron occupying an orbital overlapping with the unpaired electron (3d1) of a Ti3+ ion and/or the unpaired electron (3d5) of a Fe3+ ion. The development of TiO2 NRbs with tunable optical and magnetic properties constitutes an important step towards realizing improved magneto-optical and spintronic devices from novel TiO2 nanostructures.

Formation of different magnetic phases and high Curie temperature ferromagnetism in Fe57-implanted ZnO film

We investigated magnetic properties of ZnO thin film implanted with Fe 57 ions to the fluence of 1.00×10 17 ions/cm 2 . Both vibrating sample magnetometry and magneto-optical Kerr effect measurements revealed strong room temperature ferromagnetism with similar hysteresis loops. Temperature dependent measurements showed a very high Curie temperature around 850 K. Conversion electron Mössbauer spectroscopy experiments proved the existence of paramagnetic Fe +3 and Fe +2 ions and also the presence of substitutional Fe atoms in the hexagonal ZnO crystal resulting in intrinsic ferromagnetic order. • The implanted iron atoms form different magnetic phases in the host ZnO film. • Mössbauer data proved the existence of paramagnetic Fe +3 and Fe +2 ions and substitutional Fe atoms. • The observed RT ferromagnetism is attributed to the iron atoms occupying ZnO crystal lattice. • Temperature dependent measurements showed a very high T C around 850 K.

Peculiarly strong room-temperature ferromagnetism from low Mn-doping in ZnO grown by molecular beam epitaxy

Strong room-temperature ferromagnetism is demonstrated in single crystalline Mn-doped ZnO thin films grown by molecular beam epitaxy. Very low Mn doping concentration is investigated, and the measured magnetic moment is much larger than what is expected for an isolated ion based on Hund's rules. The ferromagnetic behavior evolves with Mn concentration. Both magnetic anisotropy and anomalous Hall effect confirm the intrinsic nature of ferromagnetism. While the Mn dopant plays a crucial role, another entity in the system is needed to explain the observed large magnetic moments.