Observation Of Room Temperature Ferromagnetism Research Articles

Observation of room-temperature ferromagnetism in copper-based graphene induced by stress engineering

Two-dimensional (2D) room-temperature ferromagnetism is highly sought after for its great application potential but is challenged by chemical or magnetic structural instability. If room-temperature ferromagnetism can be realized in structurally stable 2D graphene, it will have a broad application prospect and market in flexible spintronics and wearable artificial intelligence. Although symmetry breaking such as vacancies, edges can introduce localized magnetic moments in intrinsically antimagnetic graphene, their weak coupling can only hold long-range ordering at low temperatures. Here, a robust ferromagnetic order has been observed in large-area graphene (centimeter scale) through mechanical folding and successive preprocesses of rapid cooling and heating method, where the Curie temperature of ∼6-layer graphene is close to 100 K and that of ∼10-layer graphene is above room temperature. We propose that the observed ferromagnetism is possibly attributed to the surface sp3-type bonds emergent in the structure phase transition caused by strain effects.

Origin of microscopically coupled ferromagnetic Cu-ions in a distorted system of Cu-doped ZnO and their synchrotron-based electronic structures

Spintronics-based studies have produced significant attention in the last decade while claiming the observation of room temperature ferromagnetism (RTFM). Nevertheless, there is a lack of consensus on a mechanism responsible for this phenomenon. In this study, we focus on Cu-doped ZnO (ZCO) to understand the microscopic origin of RTFM and the role of different oxidation states of Cu in RTFM. We have performed different spectroscopic techniques using synchrotron facilities. The values of spin-moment obtained from x-ray magnetic circular dichroism sum-rule truly exhibit a ferromagnetic interaction in the nanocrystalline powder of ZCO with ∼0.58 μB for 5% of Cu concentration in the total fluorescence yield mode. Such an enhanced magnetization is attributed to the presence of Cu2+, which is mainly localized in the bulk region. Cu in ZCO is mostly dominated by the presence of Cu2+. This is clearly reflected by the profiles of x-ray photoemission spectroscopy. Consequently, the weakly magnetized total electron yield mode is attributed to a state of magnetic frustration as the majority of Cu3+ is found on the surface. Some of these Cu3+ when come in the vicinity of Cu2+ ions result in a highly correlated state of double exchange mechanism, which is the microscopic origin of RTFM in ZCO. The coupling between Cu2+-Cu3+ is mediated via oxygen vacancies (VO), the presence of which is confirmed through the features of electron energy loss spectroscopy over different edges. The confirmation of VO is also supported by the deconvolution of E2high-phonon in the Raman spectra. Moreover, the defects in the local electronic structures of ZCO are demonstrated by the deconvoluted spectra of Cu L3 x-ray absorption spectroscopy. The images obtained from high-resolution transmission electron microscopy confirm the incorporation of Cu into the wurtzite crystal of ZnO. A clear enhancement in magnetization upon an increase in carriers of Cu in ZCO indicates carrier-induced ferromagnetism. Cu2+ and VO are the two attributes of RTFM in ZCO.

Observation of room temperature ferromagnetism in transition metal ions substituted p-type transparent conducting oxide Cr2O3 thin films

In this report, we have performed comparative studies of the structural, optical, electrical, and magnetic behavior of pristine chromium oxide (PCO) and nickel (NCO), manganese (MCO), and cobalt (CCO) substituted Cr2O3 thin films. All thin films were epitaxially grown using the pulsed laser deposition (PLD) technique. X-ray diffraction (XRD) pattern confirmed the single rhombohedral phase of Cr2O3. XPS analysis confirmed the successful incorporation of Ni, Mn, and Co atoms into the Cr2O3 thin films with mixed valence states. The UV–Vis spectra illustrated the optical transparency of 65–72% in the visible region for all thin films. Electrical measurement confirmed the improvement in electrical conductivity of Cr2O3 thin films with substitution of transition metal (TM) ions. The temperature-dependent magnetization depicted ferromagnetic order. On the other hand, a field-dependent study at 5 K exhibits enhanced magnetization due to the carriers-mediated bound magnetic polarons.

Microscopic origin of room-temperature ferromagnetism in the double perovskite Sr2FeReO6

The puzzling observation of room temperature ferromagnetism in double perovskites $({A}_{2}{\phantom{\rule{4pt}{0ex}}BB}^{\ensuremath{'}}{O}_{6})$, despite having the magnetic lattice of B ions diluted by nonmagnetic ${B}^{\ensuremath{'}}$ ions, have been examined for ${\mathrm{Sr}}_{2}{\mathrm{FeReO}}_{6}$. Ab initio spin spiral electronic structure calculations along various high symmetry directions in reciprocal space are used to determine the exchange interactions entering an extended Heisenberg model, which is then solved classically using Monte Carlo simulations to determine the ferromagnetic transition temperature ${\phantom{\rule{4pt}{0ex}}T}_{c}$. We find that one must consider on-site Coulomb interactions at the nonmagnetic Re sites $(U)$ in order to obtain a ${T}_{c}$ close to the experimental value. Analysis of the ab initio electronic structure as well as an appropriate model Hamiltonian trace the origin of enhancement in ${T}_{c}$ with $U$ to the enhanced exchange splitting that is introduced at these sites. This in turn destabilizes the antiferromagnetic exchange channels, thereby enhancing the ${T}_{c}$. The role of occupancy at the nonmagnetic sites is examined by contrasting with the case of ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$.

Observation of room temperature ferromagnetism and exchange bias in a 55Mn+ ion-implanted unintentionally doped [formula omitted]-[formula omitted] single crystal

In this manuscript, the structural and magnetic properties of a 55Mn+ ion-implanted unintentionally doped β-Ga2O3 single crystal are investigated. The implanted Mn+ ions cause crystalline degeneration due to the formation of defects in the surface of the substrate. Room temperature ferromagnetism (FM) is observed in the samples with implantation doses of 5 × 1014cm-2, 1 × 1015cm-2 and 5 × 1015cm-2. The zero-field-cooled (ZFC)/field-cooled (FC) measurement of magnetization reveals that the ion implantation process induces an antiferromagnetic (AFM) phase. The temperature evolution of the remanence asymmetry (ME) demonstrates the typical behavior of the exchange bias effect due to the coexistence of FM and AFM phases. Based on the distribution of Mn+ ions and defects and the relationship between the remanence asymmetry (ME) and the exchange bias field (HE), the single domain and core-shell models are used to investigate the exchange bias system. The formation mechanism of the ferromagnetic order is briefly discussed, which is rather different from the one corresponding to the bound magnetic polaron (BMP) model.

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.

Origin of ferromagnetism in Cu-doped ZnO

It is widely reported during last decade on the observation of room temperature ferromagnetism (RTFM) in doped ZnO and other transition metal oxides. However, the origin of RTFM is not understood and highly debated. While investigating the origin of RTFM, magnetic ion doped oxides should be excluded because it is not yet settled whether RTFM is intrinsic or due to the magnetic ion cluster in ZnO. Hence, it is desirable to investigate the origin of RTFM in non-magnetic ion doped ZnO and Cu-doped ZnO will be most suitable for this purpose. The important features of ferromagnetism observed in doped ZnO are (i) observation of RTFM at a doping concentration much below than the percolation threshold of wurtzite ZnO, (ii) temperature independence of magnetization and (iii) almost anhysteretic magnetization curve. We show that all these features of ferromagnetism in ZnO are due to overlapping of bound magnetic polarons (BMPs) which are created by exchange interaction between the spin of Cu2+ ion and spin of the localized hole due to zinc vacancy ({V}_{Zn}). Both the experimental and theoretical investigation show that the exchange interaction between Cu2+-Cu2+ ions mediated by {V}_{Zn} is responsible for RTFM in Cu-doped ZnO.

Room Temperature Intrinsic Ferromagnetism in Epitaxial Manganese Selenide Films in the Monolayer Limit.

Monolayer van der Waals (vdW) magnets provide an exciting opportunity for exploring two-dimensional (2D) magnetism for scientific and technological advances, but the intrinsic ferromagnetism has only been observed at low temperatures. Here, we report the observation of room temperature ferromagnetism in manganese selenide (MnSe x) films grown by molecular beam epitaxy (MBE). Magnetic and structural characterization provides strong evidence that, in the monolayer limit, the ferromagnetism originates from a vdW manganese diselenide (MnSe2) monolayer, while for thicker films it could originate from a combination of vdW MnSe2 and/or interfacial magnetism of α-MnSe(111). Magnetization measurements of monolayer MnSe x films on GaSe and SnSe2 epilayers show ferromagnetic ordering with a large saturation magnetization of ∼4 Bohr magnetons per Mn, which is consistent with the density functional theory calculations predicting ferromagnetism in monolayer 1T-MnSe2. Growing MnSe x films on GaSe up to a high thickness (∼40 nm) produces α-MnSe(111) and an enhanced magnetic moment (∼2×) compared to the monolayer MnSe x samples. Detailed structural characterization by scanning transmission electron microscopy (STEM), scanning tunneling microscopy (STM), and reflection high energy electron diffraction (RHEED) reveals an abrupt and clean interface between GaSe(0001) and α-MnSe(111). In particular, the structure measured by STEM is consistent with the presence of a MnSe2 monolayer at the interface. These results hold promise for potential applications in energy efficient information storage and processing.

Ab-initio calculation and experimental observation of room temperature ferromagnetism in 50 keV nitrogen implanted rutile TiO2

Room temperature magnetic properties of 50 keV N4+ ion beam implanted rutile TiO2 have been theoretically and experimentally studied. Ab-initio calculation under the frame work of density functional theory has been carried out to study the magnetic properties of the different possible nitrogen related defects in TiO2. Spin polarized density of states calculation suggests that both Ninst and NO can induce ferromagnetic ordering in rutile TiO2. In both cases the 2p orbital electrons of nitrogen atom give rise to the magnetic moment in TiO2. The possibility of the formation of N2 molecule in TiO2 system is also studied but in this case no significant magnetic moment has been observed. The magnetic measurements, using SQUID magnetometer, results a ferromagnetic ordering even at room temperature for the 50 keV N4+ ion beam implanted rutile TiO2.

Observation of room-temperature ferromagnetism in Co-doped Bi0.5K0.5TiO3 materials

We report the band gap modification and strong room ferromagnetism by substituting the Co ions for the Ti site of Bi0.5K0.5TiO3 materials. The predicted band gap of 2.11 eV and magnetic moment of 2.7 μB/Co are reproduced precisely in UV–Vis spectroscopy and superconducting quantum interference device experiments, respectively. We elucidate the driving mechanisms for these results in terms of the spin-exchange splitting between spin subbands in the presence of substitution ions and high-spin crystal field energy spectrum. This method would provide a promising approach to get single-phase multiferroics and resolve the problem of the scarcity of single-phase multiferroics in nature.

Observation of room temperature ferromagnetism with giant magnetic moment based on Zn1−xCrxO thin films grown on Si(111) substrate via 1,2-dihydroxyethane modified sol-gel dip-coating technique

Dilute magnetic semiconductors have shown a great encouragement from the technological point of view owing to their potential application in multiple spintronics, such as spin light-emitting diode, spin-valve transistor, logic device, nonvolatile memory and ultrafast optical switches. Here, we report on the synthesis of Zn1−xCrxO (0.01≤x≤0.09) thin films grown on Si (111) substrate via 1,2-dihydroxyethane modified sol-gel dip-coating technique for spintronic applications. The influence of partial substitution of Cr ions into the Zn sites on the chemical composition, morphology, crystal structure, and magnetic properties of the prepared films was investigated by the X-ray diffraction (XRD), atomic force microscopy (AFM), high resolution transmission electron microscopy (HR-TEM), dispersive X-ray spectroscopy (EDS), electron diffraction (SAED), X-ray electron spectroscopy (XPS) and vibrating magnetometer (VSM). A single phase and highly crystalline films are obtained. These films showed room temperature ferromagnetism at all Cr ions concentration. The Zn0.95Cr0.05O film showed a remarkable giant magnetic moment of 2.17μB at room temperature. This is the highest magnetic moment value ever achieved among all derived transition elements doped ZnO based dilute magnetic semiconductors till the date.

Observation of Room Temperature Ferromagnetism in Conducting and Insulating Cu doped ZnO Thin Films

With pulsed laser deposition, the Cu0.04Zn0.96O thin films are grown at 600 °C under three different oxygen pressures, namely PO2 = 0.00, 0.02, and 1.00 Pa. X-ray diffraction shows single-phase material for the samples grown under PO2 = 0.00 and 1.00 Pa and CuO secondary phase for the PO2 = 0.02 Pa grown sample. The observation of satellite structures in the Cu 2p core level X-ray photoelectron spectroscopy (XPS) spectra suggest the presence of Cu2+ and CuO secondary phases in the samples grown at PO2 = 0.02 and 1.00 Pa. The sample grown under vacuum (PO2 = 0.00 Pa) shows mixed Cu oxidation state of 1 + or 2 + . The sample grown without oxygen is n-type and those grown with oxygen are highly insulating. The insulating sample grown at PO2 = 0.02 Pa shows highest magnetization due to possible collective behavior of Cu2+ – O v – Cu2+ network in the form of bound magnetic polaron (BMP) and ferromagnetic superexchange interaction coming from uncompensated surface spins of the Cu ions in the CuO secondary phase. Both delocalized electrons (∼3.32 × 1018) due to oxygen deficient defects and reduced amount of effective Cu2+ ions discredit the BMP model for this vacuum grown sample, and magnetism is suggested due to O v and presence of possible CuO secondary phase.

Ferromagnetism in cluster free, transition metal doped high κ dilute magnetic oxides: Films and nanocrystals

The structural and magnetic properties of high κ dilute magnetic oxides (DMOs) have been studied in two types of samples: thin films and nanocrystals, including Co doped HfO2 films, Co doped Y2O3 films, Co doped Y2O3 nanocrystals, and Mn doped Y2O3 nanocrystals. The characterizations were conducted by extended x-ray absorption fine structure, x-ray absorption near edge structure, and superconducting quantum interference device vibrating sample magnetometer. Oxygen vacancies are shown to play a crucial role in ferromagnetic ordering, as defect centers in the bound magnetic polaron model to account for DMOs with medium band gap and low carrier concentration. The observation of room temperature ferromagnetism in tri-valence oxide of Y2O3 as well as in tetra-valence oxide of HfO2 suggests a generic feature of transition metal doped high κ oxides as good candidates for DMO. The ability to modulate the magnetic behavior of DMO via oxygen vacancy concentration by means of post anneals can be exploited for potential applications in spintronics.

Carrier Concentration Effect of Cu-Doped ZnO Films for Room Temperature Ferromagnetism

Recent reports on the observation of room temperature ferromagnetism in Cu doped ZnO have been highly regarded by the scientific community. There are several contradicting reports where some authors have confirmed the occurrence of ferromagnetism in ZnO while others have ruled it out. Even in studies where room temperature ferromagnetism is reported, the effect of carrier type with the ferromagnetic properties is still unclear. To study this problem, the observation of room temperature ferromagnetism in Cu-doped (5%) single crystalline ZnO films grown on Si substrates was observed. ZnO films were prepared by pulsed laser deposition. Hall measurements showed that the Zn0.95Cu0.05O film was n-type with carrier density around 4.3×1017 cm-3. This film exhibits room-temperature ferromagnetism with a saturation magnetization of 0.31 µB/Cu atom. The predominant valence state of the doped Cu atoms is monovalent. When additional electrons were introduced into the films, the ferromagnetism was weaken and vanished. Our results confirm that the p-type nature of the film is not essential for realizing ferromagnetic characteristics, if only the concentration of n-type carriers were not very high.

Magnetic, structural, electronic, and optical investigations of Ti1−xMnxO2films

Abstract

Room temperature ferromagnetism in Teflon due to carbon dangling bonds

The ferromagnetism in many carbon nanostructures is attributed to carbon dangling bonds or vacancies. This provides opportunities to develop new functional materials, such as molecular and polymeric ferromagnets and organic spintronic materials, without magnetic elements (for example, 3d and 4f metals). Here we report the observation of room temperature ferromagnetism in Teflon tape (polytetrafluoroethylene) subjected to simple mechanical stretching, cutting or heating. First-principles calculations indicate that the room temperature ferromagnetism originates from carbon dangling bonds and strong ferromagnetic coupling between them. Room temperature ferromagnetism has also been successfully realized in another polymer, polyethylene, through cutting and stretching. Our findings suggest that ferromagnetism due to networks of carbon dangling bonds can arise in polymers and carbon-based molecular materials.

Effects of dopants on magnetic properties of Cu-doped ZnO thin films

We report the observation of room temperature ferromagnetism (FM) in Cu-doped ZnO (ZnO: Cu) thin films synthesized by sol–gel technique. While donor Al3+ cations are introduced into the ZnO: Cu films, the saturation magnetization decreases rapidly. Cu 2p core-level X-ray photoelectronic spectra demonstrate that the FM is strongly correlated with Cu2+ cations (3d9 configuration). To further study the relationship between the FM and acceptors, Na+ cations are also introduced into the ZnO: Cu films to increase the saturation magnetization. The enhanced FM in the (Cu, Na)-codoped ZnO is suggested to be due to the hybridization between delocalized holes and spin-split Cu 3d states.

Room Temperature Ferromagnetism in Co-Incorporated TiO<SUB>2</SUB> Thin Films

Observation of room temperature ferromagnetism (RTFM) in nano-crystalline Co-incorporated titanium dioxide [Ti(1-x)Co(x)O2(x = 0.05)] thin films prepared by spray pyrolysis technique is reported. While only the anatase phase was detected in as-deposited 5 at.% Co-incorporated TiO2 film, a small amount of rutile phase developed following its vacuum annealing. Besides, no X-ray diffraction peak corresponding to cobalt metal could be detected in any of the two films. SQUID magnetometry of both pristine and Co-doped thin films at room temperature elucidated distinct ferromagnetic behavior in 5 at.% Co-incorporated as-deposited film with saturation moment M(s) approximately 5.6 emu/cm3 which got enhanced up to 11.8 emu/cm3 on subsequent vacuum annealing. From the zero field cooled magnetization measurement we confirmed the absence of Co-metal clusters. The electrical resistivity was found to be greater than 108 omega-cm for the films. Based on the magnetic and electrical measurements the origin of RTFM has been attributed to the bound magnetic polaron (BMP) model.

Room Temperature Ferromagnetism and Optical Limiting in V2O5 Nanoflowers Synthesized by a Novel Method

We report on the observation of room temperature ferromagnetism as well as optical limiting in V2O5 nanoflower structures synthesized by a simple and novel cost-effective low-temperature method. The flowers are characterized thoroughly by various analytical techniques to ascertain their structure and composition and to confirm the absence of any impurities. The samples exhibit ferromagnetic properties at 300, 200, and 100 K observed from a hysteresis loop. Coercivity for room temperature synthesized V2O5 flowers is 566 Oe at 300 K and is enhanced at 200 and 100 K. We propose a growth mechanism of the flowers and attribute the origin of ferromagnetism to the introduction of oxygen vacancies in accordance with theoretical predictions available on other oxide nanomaterials. The samples also show optical limiting behavior arising from an effective three photon absorption mechanism as demonstrated by a Z-scan experiment for characterization of optical nonlinearity.

Enhancement of ferromagnetism in Ni-implanted HfO2 dielectric thin films

Abstract We report thermal annealing and 100 MeV Si 8+ swift heavy ion irradiation effects on the structural and magnetic properties of Ni-implanted HfO 2 thin films. At low Ni doping concentration (∼1%), HfO 2 thin films show ferromagnetic behavior. We clearly demonstrate the cluster free nature of our film using cross-sectional high resolution transmission microscopy and magnetization vs. temperature data. Rutherford backscattering spectrometry is used to estimate the film thickness and to establish that Ni-ions are placed in the HfO 2 matrix. By comparing the results for the annealed and swift heavy ion irradiated samples, it is concluded that the enhancement in magnetic signal is closely related to the dispersion/diffusion of implanted Ni and defect creation such as oxygen vacancies. The results of magnetic force microscopy supported the observation of room temperature ferromagnetism in Ni-implanted HfO 2 films.