Defect-induced Ferromagnetism Research Articles

Interface-induced enhanced room temperature ferromagnetism in hybrid transition metal dichalcogenides

Magnetic semiconductors with both electron charge and spin features exhibit tremendous potential in spintronics. Although defective transition-metal dichalcogenides are promising with induced room temperature (RT) magnetic moments, impacts of the defect type and underlying mechanisms remain unclear. Herein, two strategies involving elemental substitution and epitaxial growth have been explored to synthesize alloyed and hybrid MoSe2-xSx with lattice distortion and artificial interfaces respectively. Both experimental measurements and first-principle calculations demonstrate induced magnetism in the resultant MoSe2-xSx with the magnetization intensity closely associated to the atomic structure. The alloyed MoSe2-xSx exhibits satisfactory structural stability and atomic magnetic moments due to the Mo 4d orbital splitting induced by lattice distortion. Nevertheless, both enhanced RT ferromagnetism and thermomagnetic stability can be achieved for the hybrid MoSe2-xSx resulted from stronger localized spin polarization at the MoSe2/MoS2 interfaces. As such the work not only sheds light on the mechanisms underlying defect-induced ferromagnetism in transition-metal dichalcogenides, but also proposes an interface engineering strategy to induce significant ferromagnetism for multi-fields including spintronics, multiferroics and valleytronics.

Surface-disorder and cationic-site defect-induced ferromagnetism and correlated Raman vibrational modes in Sn0.9 In0.1O2 nanocrystalline thin-films

Exploration of magnetism in otherwise nonmagnetic wide-band oxides through defect-engineering has been become a new challenge to achieve the goal of room-temperature ferromagnetic semiconductor for potential applications in spintronics. In this backdrop, we report surface-disorder and cationic-site defect-induced FM in series of Sn0.9In0.1O2 nanocrystalline thin films prepared using pulsed laser deposition different Argon (Ar) pressure. With the increase of deposition pressure, surface morphology of Sn0.9In0.1O2 nanocrystalline thin-films changes as nanoclusters, nanofibres, nanoflakes and nanowires respectively, crystallite size decreases and surface disorder increases gradually. Raman, X-ray photoelectron and Photoluminescence spectroscopic analysis confirm the formation tin-vacancy (VSn) and associated combinational defects that significantly increase the ferromagnetic signal in In-doped SnO2 thin-films. The substitution of trivalent In3+ in Sn site also cause serious lattice distortion which transform the Raman inactive modes A2g and Eu modes into Raman active modes and thus appeared at 574 cm−1 and 354 cm−1 respectively. The nanowires of Sn0.9In0.1O2 are found to possess highest concentration of VSn defects and therefore it exhibits strongest ferromagnetic moment (MS) ∼ 11.1 emu/cm3 and highest Curie temperature (TC) ∼ 572 K. The gradual decrease in carrier concentration with the increasing deposition pressure suggests stabilization of hole-mediated VSn-induced room temperature FM in series of In-doped SnO2 nanocrystalline thin-films.

Defect-induced edge ferromagnetism and fractional spin excitations of the SU(4) π-flux Hubbard model on honeycomb lattice

Recently, a SU(4) π-flux Hubbard model on the honeycomb lattice has been proposed to study the spin-orbit excitations of α-ZrCl3 (Yamada et al., 2018 [27]). Based on this model with a zigzag edge, we show the edge defects can induce edge flat bands that result in a SU(4) edge ferromagnetism. We develop an effective one-dimensional interaction Hamiltonian to study the corresponding SU(4) spin excitations. Remarkably, SU(4) spin excitations of the edge ferromagnet appear as a continuum covering the entire energy region rather than usual magnons. Through further entanglement entropy analysis, we suggest that the continuum consists of fractionalized spin excitations from the disappeared magnons, except for that from the particle-hole Stoner excitations. Moreover, in ribbon systems with finite widths, the disappeared magnons can be restored in the gap formed by the finite-size effect and the optical branch of the restored magnons are found to be topological nontrivial.

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

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

Prediction and observation of defect-induced room-temperature ferromagnetism in halide perovskites

The possibility to induce a macroscopic magnetic moment in lead halide perovskites (LHPs), combined with their excellent optoelectronic properties, is of fundamental interest and has promising spintronic applications. However, these possibilities remain an open question in both theory and experiment. Here, theoretical and experimental studies are performed to explore ferromagnetic states in LHPs originated from lattice defects. First-principle calculations reveal that shallow-level Br vacancies in defective CsPbBr3 can produce spin-splitting states and the coupling between them leads to a ferromagnetic ground state. Experimentally, ferromagnetism at 300 K is observed in room-temperature synthesized CsPbBr3 nanocrystals, but is not observed in hot-injection prepared CsPbBr3 quantum dots and in CsPbBr3 single crystals, highlighting the significance played by vacancy defects. Furthermore, the ferromagnetism in the CsPbBr3 nanocrystals can be enhanced fourfold with Ni2+ ion dopants, due to enhancement of the exchange coupling between magnetic polarons. Room-temperature ferromagnetism is also observed in other LHPs, which suggests that vacancy-induced ferromagnetism may be a universal feature of solution-processed LHPs, which is useful for future spintronic devices.

The relationship between the doping concentration and d ferromagnetism in n-type 4H-SiC

To date, research on defect-induced magnetism in SiC has been conducted to clarify the relationship between the ferromagnetic signal and the carrier concentration. It has been experimentally proven that there is an interaction between the d0 magnetic moment and the hole carrier in p-type 4H-SiC. However, for n-type SiC, the existing theoretical predictions are insufficient to explain the variation in magnetization with the doping concentration. To solve this problem, we prepared 4H-SiC epitaxial layers with different nitrogen doping concentrations and introduced defects by ion implantation. By measuring and analyzing the magnetic properties, we found that the ferromagnetic composition depends on both the implantation dose and the doping concentration. By performing first-principles calculations, we studied the magnetic moments and interactions of defects with different charge states, which is related to defect-induced ferromagnetism. These defects include not only the paramagnetic centers reported in previous studies, such as silicon vacancies and divacancies, but also the NCVSi complex defect of recent interest, which are indicated by positron annihilation experiments. Combining experimental observations with theoretical calculations, we explained the relationship between magnetic properties and the nitrogen doping concentration in the epitaxial samples. Our research will help us to better understand the physical mechanism of defect-induced magnetism in doped semiconductors and provide a potential platform for the control of defect-induced magnetism by carrier density modulation and the fabrication of SiC spintronic devices without transition metals.

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.

Tuning room temperature ferromagnetism of ‘in-situ’ inkjet printed Fe-doped ZnO films

ZnO is a wide-band gap semiconductor widely used in optical and electric devices, associating with ferromagnetism at low dimension endowing its possibility for functional applications with magneto-optical and magneto-electric properties. We prepared ZnO and Fe-doped ZnO thin films ‘in-situ’ on substrate by inkjet printing, and tuned the room temperature ferromagnetism (RTFM) of the film by Fe-doping concentration, film thickness and post annealing temperature. It was found that by Fe doping the saturation magnetization (MS) of the film can be enhanced by more than 4 folds comparing with the un-doped film, i.e. from 0.9 emu g−1 for the ZnO film to 3.8 emu g−1 for the Fe-doped ZnO film with comparable thickness. The enhancement was attributed to the introduction of un-paired 3d electrons which formed long range ferromagnetic ordering, as well as the consequent structure changes with smaller grains which increased the interface induced magnetism. By changing the annealing temperature and the film thickness, the defect-induced ferromagnetism was investigated. The RTFM shows thickness dependence with peak saturation magnetization value of 4.44 emu g−1 for the 45 nm thick film. The work provides an effective way of tuning magnetism in ZnO based films for functional device applications.

57Fe Mössbauer study of epitaxial TiN thin film grown on MgO (1 0 0) by magnetron sputtering

The properties and performance of TiN thin films are closely related to the concentration and mobility of lattice defects in the thin film structures of TiN. This makes a local atomic scale study of TiN thin films an ever-growing demand. Emission 57Fe Mössbauer spectroscopy (eMS) is a powerful tool in this regard, which we apply here to study an ultrathin TiN film epitaxially grown on MgO (1 0 0). With the help of theoretical calculations, our results show that most implanted Fe ions adopt a 2+ valence state and locate at the Ti sublattice in the bulk-like single crystalline grains, with the rest Fe residing at the grain boundaries as interstitials. A small percentage of nitrogen point defects (vacancy VN and interstitial NI) are observed in the bulk-like crystalline grains. A temperature-dependent, interstitial NI mediated site-exchange between NI and VN inside the crystal grain are deduced via a N2 dimmer like diffusion of NI through the crystal grains in the temperature range of 540–620 K. This is interesting in the perspective of exploring the catalytic property of TiN nanostructures. The titanium vacancy (VTi) is only detected at the grain boundaries. Annealing up to 813 K, both the VN and NI are annihilated in the crystalline grains and the VTi is fully recovered with healing of the grain boundaries. However, no evidence of ferromagnetism due to dilute implantation of 57Mn/57Fe and or structural defects in the film is obtained. This suggests that the so far reported dilute magnetism and defect-induced ferromagnetism in TiN nanostructures requires a further systematic investigation.

Defect-induced room-temperature ferromagnetism in single-walled carbon nanotubes

Room-temperature ferromagnetism in single-walled carbon nanotubes (SWNTs) was observed and investigated in this work. Sizeable magnetization was observed in SWNTs aged in air, further increasing by hydrothermal treatment. The magnetic moment appears to arise from a spin-1 species in the pristine sample and from a spin-3/2 species in the hydrothermally treated samples. The spin-1 state may be attributed to the carbon vacancies, whereas the spin-3/2 state may be attributed to hydrogen adsorbed to the carbon vacancies by the hydrothermal treatment.

Synthesis and Characterization of Ferromagnetic Fe3O4–ZnO Hybrid Core–Shell Nanoparticles

Room-temperature synthesis of multifunctional core–shell nanostructures with iron oxide (Fe3O4) core and zinc oxide (ZnO) shell is described. High-resolution transmission electron microscopy revealed the occurrence of heteroepitaxial growth of a ZnO shell over the iron oxide nanoparticle core. The formation of the core–shell nanostructure was also evidenced by x-ray diffraction and x-ray photoelectron spectroscopy analyses. The core Fe3O4 nanoparticles exhibited superparamagnetic behavior at room temperature in an externally applied magnetic field. However, magnetic measurements of the core–shell nanoparticles revealed ferromagnetic behavior, originating from defect-induced ferromagnetism in the ZnO shell. The prepared core–shell nanoparticles also exhibited strong photoresponse while retaining ferromagnetic behavior at room temperature.

Progress of d0 magnetism in SiC

The properties of defect-induced ferromagnetism ( magnetism) in SiC belong to carbon-based material which has been systematically investigated after graphite. In this paper, we reviewed our research progress about magnetism in two aspects, i.e., magnetic source and magnetic coupling mechanism. The divacancies have been evidenced as the probable source of magnetism in SiC. To trace the ferromagnetic source in microscopic and electronic view, the p electrons of the nearest-neighbor carbon atoms, which are around the divacancies, are sourced. For magnetic coupling mechanism, a higher divacancy concentration leads to stronger paramagnetic interaction but not stronger ferromagnetic coupling. So the magnetism can probably be explained as a local effect which is incapable of scaling up with the volume.

Defect-induced weak ferromagnetism in transition metal-doped ZnO nanoparticles

In this work, citric acid-capped and ethylene glycol-stabilized pristine and transition metal (TM=Co, Fe, Mn and Ni)-doped ZnO nanoparticles with the generic formula Zn1−x TM x O, x = 0.01 and 0.02, have been synthesized by sol–gel method. XRD confirmed the phase purity of all the samples. Average crystallite size calculated from Scherrer formula was within the range of 43 ± 25 nm for different doped samples. The Raman spectra of (Co, Mn and Ni)-doped ZnO show strong E 2 (high) and E 1 (LO) modes. The synthesized samples also show strong luminescent emission from inherent Zn and O point defects (interstitial and vacancies) along with a sharp excitonic peak centred at 362 nm. Magnetic studies at 300 K reveal that all samples show weak room-temperature ferromagnetism at low magnetic fields with unsaturated M–H plot up to a measuring field of 5 T.

ChemInform Abstract: Defect‐Induced Ferromagnetism in Semiconductors: A Controllable Approach by Particle Irradiation

AbstractReview: 56 refs.

Anisotropic defect-induced ferromagnetism and transport in Gd-doped GaN two-dimensional electron gasses

Here we report on the effect of rare earth Gd-doping on the magnetic properties and magnetotransport of GaN two-dimensional electron gasses (2DEGs). Samples are grown by plasma-assisted molecular beam epitaxy and consist of AlN/GaN heterostructures where Gd is delta-doped within a polarization-induced 2DEG. Ferromagnetism is observed in these Gd-doped 2DEGs with a Curie temperature above room temperature and an anisotropic spontaneous magnetization preferring an out-of-plane (c-axis) orientation. At magnetic fields up to 50 kOe, the magnetization remains smaller for in-plane configuration than for out-of-plane, which is indicative of exchange coupled spins locked along the polar c-axis. The sample with the lowest Gd concentration (2.3 $\times$ $10^{14}$ cm$^{-2}$) exhibits a saturation magnetization of 41.1 $\mu_B/Gd^{3+}$ at 5 K revealing that the Gd ion spins (7 ${\mu}_B$) alone do not account for the magnetization. Surprisingly, control samples grown without any Gd display inconsistent magnetic properties; in some control samples weak ferromagnetism is observed and in others paramagnetism. The ferromagnetic 2DEGs do not exhibit the anomalous Hall effect; the Hall resistance varies non-linearly with the magnetic field, but does not track the magnetization indicating the lack of coupling between the ferromagnetic phase and the conduction band electrons within the 2DEG.

Defect-induced magnetism in SiC: Interplay between ferromagnetism and paramagnetism

Defect-induced ferromagnetism has triggered a lot of investigations and controversies. The major issue is that the induced ferromagnetic signal is so weak that it can sufficiently be accounted for by trace contamination. To resolve this issue, we studied the variation of the magnetic properties of SiC after neutron irradiation with fluence covering four orders of magnitude. A large paramagnetic component has been induced and scales up with defect concentration, which can be well accounted for by uncoupled divacancies. However, the ferromagnetic contribution is still weak and only appears in the low fluence range of neutrons or after annealing treatments. First-principles calculations hint towards a mutually exclusive role of the concentration of defects: Defects favor spin polarization at the expense of magnetic interaction. Combining both experimental and first-principles calculation results, the defect-induced ferromagnetism can be understood as a local effect which cannot be scaled up with the volume. Therefore, our investigation answers the long-standing question why the defect-induced ferromagnetic signal is weak.

Antiferromagnetic coupling in Co-doped ZnS

In this paper, we report room-temperature ferromagnetism in chemically synthesized Zn1−x Co x S (0 ≤ x ≤ 0.10) diluted magnetic semiconductor nanoparticles of ~3–5 nm. The incorporation of Co2+ ion for Zn2+ ions in ZnS lattice and the particle size were confirmed by XRD and TEM along with selected area electron diffraction analysis. UV–Vis measurement showed reduction in the bandgap energy with the increase in Co doping. Maximum magnetization was observed for samples with x = 0.04. From photoluminescence, spectra luminescence efficiency was found to get enhanced on Co doping. Magnetization behavior can be understood to be due to defect-induced ferromagnetism; however, for higher doping concentration, the antiferromagnetic coupling of Co–Co interaction in the close proximity results in the decrease of the overall magnetization of the samples. Moreover, magneto-electronic study also showed a maximum negative magneto-resistance of ~43 % for x = 0.04.

Carbon p Electron Ferromagnetism in Silicon Carbide

Ferromagnetism can occur in wide-band gap semiconductors as well as in carbon-based materials when specific defects are introduced. It is thus desirable to establish a direct relation between the defects and the resulting ferromagnetism. Here, we contribute to revealing the origin of defect-induced ferromagnetism using SiC as a prototypical example. We show that the long-range ferromagnetic coupling can be attributed to the p electrons of the nearest-neighbor carbon atoms around the VSiVC divacancies. Thus, the ferromagnetism is traced down to its microscopic electronic origin.

Defect-induced ferromagnetism in undoped In2O3 nanowires

We report the observation of intrinsic room-temperature ferromagnetism in undoped In2O3 nanowires synthesized by a thermal evaporation method. The as-grown sample was characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), temperature-dependent photoluminescence (PL), Ultraviolet–visible spectroscopy (UV/vis), X-ray photoelectron spectroscopy (XPS), and superconducting quantum interference device (SQUID). Structural characterizations show that the as-grown sample is single crystal cubic bixbyite structure with 80–100nm in diameter and 2–4μm long. Both PL and UV/vis results reveal that the sample exhibits large amount of oxygen vacancies and indium vacancies (VO and VIn). XPS results indicate that the In2O3 nanowires are significantly oxygen deficient and non-stoichiometric at surface. Experimentally measured magnetic hysteresis curves for undoped In2O3 nanowires clearly display ferromagnetic behavior at room temperature. We believe the ferromagnetism of undoped In2O3 nanowires is ascribed to the large surface area and intrinsic defects mediating the ferromagnetic ordering.

Retraction: “Study of defect-induced ferromagnetism in hydrogenated anatase TiO2:Co” [J. Appl. Phys. 107, 113916 (2010)

Retraction: “Study of defect-induced ferromagnetism in hydrogenated anatase TiO2:Co” [J. Appl. Phys. <b>107</b>, 113916 (2010)