Origin Of Room Temperature Ferromagnetism Research Articles

Origin of room temperature ferromagnetism in optically transparent 2D graphene/Co-doped ZnO/graphene

Despite extensive efforts to find room temperature ferromagnetic (FM) two-dimensional (2D) materials, only a few 2D FM materials have been experimentally synthesized. Particularly, it is a still challenging task to find optically transparent room temperature 2D FM systems. Recently, room temperature ferromagnetism was confirmed in 2D graphene/Co doped ZnO/graphene multilayer structure. However, in our calculations, the Co doped ZnO (ZCO) monolayer showed an antiferromagnetic (AFM) ground state. Moreover, both ZCO/graphene bilayer and graphene/ZCO/graphene trilayer systems also exhibited an AFM state. Thus, we investigated the possibility of hydrogen-induced ferromagnetism. After the introduction of the H atom (H-ZCO), we found a ferromagnetic (FM) ground state in both H-ZCO monolayer and graphene/H-ZCO/graphene trilayer systems. We obtained that the FM state appeared only when the H atom stayed on the same ZCO layer. Thus, we propose that the ferromagnetism in the graphene/ZCO/graphene originates from the H effect. Also, we obtained a Curie temperature (TC) of 500 K in H-ZCO and graphene/H-ZCO/graphene trilayer systems. Moreover, we find that the graphene/H-ZCO/graphene shows optically transparent behavior in the visible frequencies.

Room-temperature ferromagnetism in two-dimensional transition metal chalcogenides: Strategies and origin

Room-temperature (RT) ferromagnetic atomically thin transition metal chalcogenides (TMCs) provide a novel platform for discovering new physical phenomena in the two-dimensional (2D) limit and developing the next-generation spintronic applications. Recent progress in exploring the RT ferromagnetism in 2D TMCs have attracted significant interest from experimental and theoretical scientists; However, the semiconducting TMCs are non-magnetic. In parallel, the inconsistency of magnetism between density functional theory (DFT) calculations and experimental results persist in both TMC semiconductors and metals. We review the strategies for RT ferromagnetism in 2D TMC semiconductors and the origin of RT ferromagnetism in 2D TMC metals, followed by the discussion of promising future directions in the area of RT ferromagnetic 2D TMC materials.

Improved Room-Temperature Ferromagnetism in Nd-Doped SnO2 Nanostructures through Defect Engineering

Many efforts have been made to introduce room-temperature ferromagnetism (RTFM) in metal oxide semiconductors doped with either magnetic or nonmagnetic ions for their suitable application in spintronics in the past few years. However, it is not yet well understood whether the origin of RTFM in these systems is intrinsic or because of magnetic clusters. Hence, we report detailed experimental investigations on RTFM in Nd-doped SnO2 nanostructures synthesized using a sol–gel process, which has potential spintronics application. In the present work, we observe that defects/oxygen vacancies play a crucial role in improving the RTFM in SnO2 doped with Nd ions. The presence of oxygen vacancies in the prepared samples was confirmed by Raman scattering, X-ray photoelectron spectroscopy (XPS), and photoluminescence (PL) spectroscopy. The enhancement of the density of oxygen vacancies was further quantified by the deconvoluted core-level O 1s XPS spectra. All samples exhibited RTFM with a small contribution of paramagnetic (PM) ordering. These phases (FM + PM) were established by fitting all M–H curves using a two-phase theoretical model. Furthermore, the Curie–Weiss and spin wave (CWSW) model fitted the M–T curve well, which also indicated the existence of the two phases in our systems. The overlapping of bound magnetic polarons occurred because of the exchange interaction between Nd3+ ions and electrons trapped in oxygen vacancies; this overlap is responsible for introducing RTFM in Nd-doped SnO2. Hence, the origin of RTFM can be mostly explained by the bound magnetic polaron (BMP) model.

Origin of room-temperature ferromagnetism in Co-doped CeO2

Co-doped CeO2 fabricated using the conventional solid-state reaction method demonstrates ferromagnetism at room temperature (RT). To investigate the origin of ferromagnetism at RT, powder X-ray diffraction analysis is first performed, in which pattern changes due to Co doping are not indicated. Subsequently, local environment analysis is carried out using X-ray absorption near-edge structure (XANES) measurements at the Co K-edge, demonstrating the metallic valence state of Co. Hence, it is confirmed that Co exists as nanosized small particles in CeO2, which demonstrates ferromagnetism at RT.

Ferromagnetic coupling and the effect of Fe-dt2g state on ferromagnetism in half-metallic ZnO:Fe

Based on first principle calculations using GGA + U approach, we have studied the origin of room-temperature ferromagnetism in Fe-doped ZnO. The ferromagnetic coupling and the contribution of Fe-d states on the ferromagnetism of ZnO are also studied. The p–d exchange interaction between the Fe-d and O-p states is responsible for the ferromagnetism at lower Fe concentration in ZnO; whereas at the higher concentration, the enhanced short-range antiferromagnetic coupling between Fe ions dominates over ferromagnetism. The DOS results depict that the Fe-dt2g state predominantly contributes the hybridization at the fermi level resulting the magnetism in the Fe-doped ZnO system. The total energy calculations reveal the existence of short-range ferromagnetic coupling in the system. The band structure results depict the half-metallic character of the system with spin-polarized nature.

Effect of dilute co-doping of Ni and Cr on physical properties of TiO2 nanoparticles

The present study focuses on the dilute co-doping effect of Ni and Cr ions on the structural, optical, electrical, and magnetic properties of TiO2 nanoparticles synthesized by the sol-gel method. A systematic analysis of the results indicates the formation of anatase phase for dilute concentrations of Ni and Cr ions with no sign of the secondary phases. X-ray diffraction measurements indicate that the average crystallite sizes vary in the range of 15–35 nm while SEM measurements elucidate the spherical nature of these nanoparticles. It has been found that the band gap reduces while temperature-dependent electrical conductivities increase with co-doping of Ni and Cr ions. The room temperature ferromagnetism exists in all the co-doped samples with the maximum saturation magnetization of 0.21 emu/gm. The origin of room temperature ferromagnetism is accounted to the creation of shallow levels with doping or co-doping and the bound magnetic polarons (BMPs) mediated exchange interactions. These observed results govern that Ni and Cr co-doped TiO2 nanaoparticles can be the potential candidates for spintronic applications.

Ferromagnetic behavior of native point defects and vacancy-clusters in ZnO studied by first principle calculation

The origin of room temperature ferromagnetism in undoped ZnO is still a question of debate. Experimental and theoretical findings are inconclusive as to the predominant contributor for the magnetic behavior of undoped ZnO. First principle calculation pseudopotential method was used to systematically determine the relaxed atomic geometry, the formation energies and the magnetic properties of the native point defects (vacancies, interstitials and antisites), and vacancy clusters (VZnVO, VZn − 2VO and 2VZn − VO) in ZnO. The results show that ZnO cells consisting of the VZn and the Oi have non-zero magnetic moments, energetically favoring ferromagnetic states and close-to-room-temperature Curie temperatures (294 K). VZn and Oi are also characterized by their low formation energies, in particular in the case of n-type (i.e. Fermi level close to the conduction band minimum) and O-rich conditions. The energy differences between the ferromagnetic state and anti-ferromagnetic state for VZn and Oi are larger than kT at room temperature but still relatively small (∼34 meV). Although VZn and Oi would contribute for the room temperature ferromagnetism, the ferromagnetism states would not be robustly stable for thermal excitation to the anti-ferromagnetic states.

Enhancing the saturation magnetisation in Ni doped ZnO thin films by TOPO functionalization

Abstract The saturation magnetization in Ni doped ZnO thin films have been triggered by altering the electronic configuration of ZnO by surface functionalization with Trioctail Phosphine Oxide (TOPO). Initially, Ni doped (3, 5 and 7 mol %) ZnO thin films have been grown by RF magnetron sputtering and the same were functionalized with Trioctylphosphine oxide (TOPO) later by spin coating. The XRD analysis confirms the hexagonal wurtzite structure of grown films. It has been found that lower grain size for both unfunctionalized and functionalized 5 mol% Ni doped ZnO films. PL emission at around 530 nm reveals the presence of oxygen vacancies which is actually cause for origin of ferromagnetism. The saturation magnetization values of 5.70 emu/cm3, 10.14 emu/cm3 and 8.51 emu/cm3 obtained for Ni doped ZnO films from VSM have been enhanced to 7.81 emu/cm3, 11.51 emu/cm3 and 9.61 emu/cm3 upon TOPO functionalization. The high saturation magnetization in 5 mol% Ni doped ZnO film is attributed to lower crystallite size (high grain boundaries) and higher oxygen vacancies. The Zn-P bond observed in the XPS spectrum reveals the formation of the surface layer between Zn and P atom which is responsible for the enhancement of saturation magnetization. The presence of peak at 1146 cm−1 in Raman spectra also confirms the presence of functionalized element, P from TOPO. The simple mechanism based on the bound magnetic poloran and charge distribution has been proposed to explain the origin of room temperature ferromagnetism.

Ferromagnetic ordering in half metallic Fe doped CdS diluted magnetic semiconductor: A DFT study

A detailed experimental and theoretical investigations on the origin of room temperature ferromagnetism in Fe doped CdS diluted magnetic semiconductor is reported in the present study. X-ray diffraction results reveal the cubic zinc blende CdS structure with an average crystallite size of 2 nm–3 nm which is in concurrence with the Transmission electron microscopy results. UV–visible spectroscopy depict the enhancement of optical band gap of Fe doped CdS nanoparticles compared to the bulk CdS as a result of the quantum confinement effect. Photoluminescence spectra revealed the presence of Cd2+ and S2− defect states in the nanoparticles in addition to the fluorescence quenching effect. FTIR results demonstrate the stretching vibrations of CdS bond. Magnetic study confirms the existence of room temperature ferromagnetic ordering in the nanoparticles for the lower concentration of Fe, as a result of spin polarization induced by the Fe atoms to the nearest neighboring S atoms. At higher concentration of Fe, the enhanced antiferromagnetic interaction between short range Fe ions suppresses the ferromagnetic ordering. Theoretical calculations demonstrate the half metallic nature of Fe doped CdS system with strong p-d hybridization between the Fe-d states and the S-p states which results the room temperature ferromagnetism.

Defect induced room temperature ferromagnetism and optical properties of (Co, Y) co-doped ZnO nanoparticles

Abstract In this work, pristine zinc oxide (ZnO), Co2+ doped zinc oxide (ZCO) and Co-Y co-doped zinc oxides (ZCYO) are synthesized by sol-gel method. To increase the oxygen vacancy, the as-prepared samples are annealed in vacuum furnace. Refinement of the recorded X-ray diffractograms (XRD) shows that all the samples have grown in single hexagonal phase with space group P63mc and the nanocrystallites are grown with average sizes lie in the nanometer regime (10–29 nm). Fourier-transform infrared spectroscopy (FTIR) and photoluminescence (PL) spectroscopy could not detect any secondary phase except the desired one. Interestingly, room temperature ferromagnetism (RTFM) is observed in Co-Y co-doped sample where maximum magnetization is quite high (∼0.34 emu/g) compared with undoped ZnO. Absence of any metal cluster in ZCYO by XRD and optical analyses ruled out the ordering due to clustering. Thus origin of RTFM in ZCYO is intrinsic in nature. The defect mediated RTFM observed in ZCYO nanoparticles due to exchange interaction between dopants ions is explained by the oxygen vacancy together with the presence of the increase of surface defects due to co-doping of Co ion and non-magnetic Y ion.

Structural, Optical, and Magnetic Properties of Hydrothermally Grown Fe-Doped ZnO Nanorod Arrays on Glass Substrate

In this paper, undoped and Fe-doped ZnO nanorod arrays were grown on glass substrate by a facile hydrothermal method. Structural, optical, and magnetic properties of the prepared nanorods were investigated. Structural investigation clearly showed that the prepared nanorods had single phase wurtzite structure without any secondary phases and the prepared undoped and Fe-doped nanorods were grown vertically. The optical characterization revealed that the optical band gap of ZnO nanorods was decreased by increasing the value of Fe concentration. Furthermore, the formation of diluted magnetic semiconductor nanorod arrays was verified. The origin of room temperature ferromagnetism is explained by the bound magnetic polaron concept.

Experimental and theoretical investigations of unusual enhancement of room temperature ferromagnetism in nickel-cobalt codoped CeO2 nanostructures

Room temperature ferromagnetism (RTFM) in Ni/Co codoped CeO2 nanostructures has been studied. The samples are synthesized by hydrothermal method and further characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), Raman spectroscopy (RS), photoluminescence (PL) spectroscopy and vibrating sample magnetometer (VSM) measurements. The undoped CeO2 sample shows RTFM and a consistent enhancement in its RTFM is observed after doping. The Ni (5%)-Co (5%) codoped CeO2 nanostructures revealed interestingly, highest RTFM. The origin of RTFM in all CeO2 samples is explained by electronic density of states and magnetic moments calculated using density functional theory (DFT).

Bound magnetic polaron driven room-temperature ferromagnetism in Ni doped ZnS nanoparticles

Magnetism in dilute magnetic semiconductors (DMSs) has been a controversial topic since its discovery. There are many models which predict the origin of room temperature ferromagnetism (RTFM) in TM doped wide band gap semiconductors. Here, we report RTFM in chemically synthesized cubic Zn1-xNixS (0 ≤ x ≤ 0.08) DMS nanoparticles of ∼3–5 nm size. Ferromagnetic behavior (at 300 K and 5 K) was found to increase with the increase in Ni doping concentration and was understood due to defect induced ferromagnetism. The low temperature magnetization measurement (ZFC-FC) shows that the nanoparticles are strongly coupled by magnetic interactions. Optical studies showed decrease in the energy bandgap along with the presence of sulfur and zinc vacancies and surface defects. Low temperature resistivity measurement depicted the semiconducting nature of the synthesized samples. With increase in doping concentration, an increase in the resistive behavior was observed which was explained in the realm of defects states created due to doping of the Ni ions.

Theoretical study of room temperature ferromagnetism and band gap energy of pure and ion doped In2O3 nanoparticles

Using the s-d microscopic model including the electron-phonon interaction and the Green’s function theory we have considered the origin of room temperature ferromagnetism (RTFM) in pure and ion doped In2O3 nanoparticles (NPs). The magnetization M increases with decreasing particle size. M of Fe, Tb and Mn doped In2O3 NPs is investigated, which increases, decreases and has a maximum, respectively, with increasing doping concentration. The RTFM is due to surface oxygen vacancies and different ionic radius of the dopants compared to that of the host ions. This differences lead to different strains which changes the exchange interaction constants. We have calculated the dependence of the band gap energy on the particle size in In2O3 NPs and the Fe concentration of Fe doped In2O3 NPs. The results are in good qualitative agreement with the experimental data.

Ferromagnetism in CVT grown tungsten diselenide single crystals with nickel doping

Two dimensional (2D) single crystal layered transition materials have had extensive consideration owing to their interesting magnetic properties, originating from their lattices and strong spin–orbit coupling, which make them of vital importance for spintronic applications. Herein, we present synthesis of a highly crystalline tungsten diselenide layered single crystal grown by chemical vapor transport technique and doped with nickel (Ni) to tailor its magnetic properties. The pristine WSe2 single crystal and Ni-doped crystal were characterized and analyzed for magnetic properties using both experimental and computational aspects. It was found that the magnetic behavior of the 2D layered WSe2 crystal changed from diamagnetic to ferromagnetic after Ni-doping at all tested temperatures. Moreover, first principle density functional theory (DFT) calculations further confirmed the origin of room temperature ferromagnetism of Ni-doped WSe2, where the d-orbitals of the doped Ni atom promoted the spin moment and thus largely contributed to the magnetism change in the 2D layered material.

An insight into the origin of room-temperature ferromagnetism in SnO2 and Mn-doped SnO2 quantum dots: an experimental and DFT approach.

SnO2 and Mn-doped SnO2 single-phase tetragonal crystal structure quantum dots (QDs) of uniform size with control over dopant composition and microstructure were synthesized using the high pressure microwave synthesis technique. On a broader vision, we systematically investigated the influence of dilute Mn ions in SnO2 under the strong quantum confinement regime through various experimental techniques and density functional theoretical (DFT) calculations to disclose the physical mechanism governing the observed ferromagnetism. DFT calculations revealed that the formation of the stable (001) surface was much more energetically favorable than that of the (100) surface, and the formation energy of the oxygen vacancies in the stable (001) surface was comparatively higher in the undoped SnO2 QDs. X-ray photoelectron spectroscopy (XPS) and first-principles modeling of doped QDs revealed that the lower doping concentration of Mn favored the formation of MnO-like (Mn2+) structures in defect-rich areas and the higher doping concentration of Mn led to the formation of multiple configurations of Mn (Mn2+ and Mn3+) in the stable surfaces of SnO2 QDs. Electronic absorption spectra indicated the characteristic spin allowed ligand field transitions of Mn2+ and Mn3+ and the red shift in the band gap. DFT calculations clearly indicated that only the substitutional dopant antiferromagnetic configurations were more energetically favorable. The gradual increase of magnetization at a low level of Mn-doping could be explained by the prevalence of antiferromagnetic manganese-vacancy pairs. Higher concentrations of Mn led to the appearance of ferromagnetic interactions between manganese and oxygen vacancies. The increase in the concentration of metallic dopants caused not just an increase in the total magnetic moment of the system but also changed the magnetic interactions between the magnetic moments on the metal ions and oxygen. The present study provides new insight into the fundamental understanding of the origin of ferromagnetism in transition metal-doped QDs.

Origin of ferromagnetism in Cu doped rutile TiO2 - An ab-initio approach

Origin of room temperature ferromagnetism in copper doped rutile TiO2 has been studied by varying both copper concentrations as well as oxygen vacancies. Ab-initio calculations have been carried out within the framework of density functional theory. Copper atom has been doped at two different positions to study the effect of distance between the dopants for ferromagnetism in TiO2 system. Spin polarized density of states calculation has been performed for each system and the induced magnetic moment has been calculated along with the total free energy and Fermi energy. Spin-spin interaction studies have been performed for two copper doped at Ti site (Ti22Cu2O48) as well as two oxygen vacancies along with Cu doping at Ti site (Ti22Cu2O46). Spin-spin interaction results have been analyzed to understand the possibility of ferromagnetism in the copper doped rutile TiO2. It has been found that copper doped system with adjacent oxygen vacancies gives ferromagnetic ordering.

Ferromagnetic ordering in chemically synthesized ZnS:Mn diluted magnetic semiconductor: A density functional theory explanation

We have carried out DFT calculations to investigate the origin of room temperature ferromagnetism in experimentally obtained Mn doped ZnS magnetic semiconductor. For experimental investigations, the formation of the cubic ZnS structure of ∼4 nm in size is confirmed by XRD and TEM results. Theoretical calculations reveal that Mn doped ZnS system possesses half metallic characteristics in addition to strong p–d hybridization between the d-states of Mn atom and the p-states of surrounding S atoms, which leads to the strong ferromagnetic coupling and is experimentally realized in the M–H curves of Mn doped ZnS nanoparticles. The ferromagnetic coupling between Mn–Mn atoms are studied by using two dopant model, replacing two Zn atoms by two Mn atoms in ZnS supercell, which depicts that ferromagnetic state is always ground state. In calculating formation energies for single and two Mn dopant ZnS systems, negative values of formation energy are obtained indicating of higher stability of the system.

Effect of oxidation temperature on the ferromagnetism of Co doped ZnO films via thermal oxidation of electrodeposited Co/Zn bilayers

Abstract Room temperature ferromagnetism (RTFM) is detected in the Co doped ZnO films, which are formed via the thermal oxidation of electrodeposited Co/Zn bilayers. The effect of oxidation temperature on the origin of RTFM is examined. Results indicate that various contents of Co 2+ dopants, Zn x Co 3−x O 4 clusters, O vacancy and Zn interstitial are achieved in the films oxidized at 400 and 550 °C. The amount of O vacancy decreases sharply with increasing oxidation temperature. This leads to the saturation magnetization of films oxidized at 400 °C is higher than that of films oxidized at 550 °C. This indicates that the effect of O vacancy on ferromagnetism is more prominent in the Co doped ZnO films.

Magnetic and Electrical Properties of Al3+ Implanted Co–ZnO

Co-doped ZnO ceramic samples sintered at four different temperatures were prepared by solid-state reaction method and implanted by Al ions subsequently. The microstructural, defect, magnetic and electrical properties of the samples were systematically investigated by x-ray diffraction, micro-Raman spectroscope, vibrating sample magnetometer and Hall measurement, respectively. The results show that all the samples exhibit room-temperature ferromagnetism with a saturation magnetization of 0.02–0.03 emu g−1, the value of which is affected by the sintering temperature. The origin of room-temperature ferromagnetism is considered as the formation of Co2+– V O–Co2+ bound magnetic polarons. Even though the implantation of Al3+ slightly reduces the saturation magnetization because of the lattice damage, less V O and larger lattice spacing, however, an appropriate amount of Al3+ implantation and a proper sintering temperature can remarkably improve the electrical properties with the mobility of 200 ∼300 cm2 Vs−1 and the resistivity of 20 ∼40 Ω cm. In general, samples sintered at 1200∘C present more excellent comprehensive performances.