Ferromagnetism In ZnO Research Articles

Ab initio prediction of half-metallic and metallic ferromagnetism in ZnO:(Co,Cr) systems

Doping effects on the electronic and magnetic properties of Zn1−x (Co,Cr) x O systems are investigated within Local Spin Density Approximation and Hubbard U methods. Based on Density Functional Theory the spin-polarization band structures, density of states for investigated systems are calculated. Systematic analysis of the electronic properties shows that TM-doped ZnO has generated new energy levels in the vicinity of Fermi energy level. From first-principle calculations we obtained Cr-ZnO and Co-ZnO systems are metallic and half-metallic ferromagnetic materials, respectively. The obtained results for Cr-doped ZnO 128- and 192-atom supercell systems show magnetic properties with higher Curie temperature than room temperature. There are large local moments, ∼2.9 and ∼4.2 for Co and Cr dopants, respectively. Magnetic moments are related with two electron defects in the supercell structure and unpaired electrons of transition metal. The ferromagnetic and antiferromagnetic phases and the total energy are obtained for x = 2.08%, 3.125%, 4.16%, 6.25%, 8.3%, 12.5%, and 25% impurity concentrations for doped ZnO.

Polaron induced ferromagnetism in ZnO

The odd phenomenon of the ferromagnetic moment of n-type ZnO increasing with temperature has been observed in experiments. We proposed an impurity model accompanying the polaron theory to explain this result because the electron-phonon interaction plays a critical role in inducing ferromagnetism in n-type ZnOs. Thus, we propose that the ferromagnetism of ZnOs comes from the polarons in the impurity states, and the strong Coulomb interaction in these states causes the spin of electrons to split. In addition, the proper hybridization interaction between the conduction band and the impurity states promotes the increase of electron density in the impurity states as well as the ferromagnetism. In contrast, a too strong hybridization interaction will delocalize the electrons in the impurity states to suppress the ferromagnetism. The polarons induce the ferromagnetism by the Coulomb excitation in the impurity states. The electron thermal excitation produces the polarons in the impurity states near the ZnO conduction band edge coupling with the phonon. Based on the model, the ferromagnetism increases with the temperature consistent with the experimental results.

Effects of surface polarity on the structure and magnetic properties of Co implanted polar ZnO wafers

Microstructural and magnetic properties of ion implantation method prepared Co doped Zn polar and O polar ZnO wafers are investigated by a combined experimental and theoretical approach. Substantial intrinsic ferromagnetism is observed in all the doped ZnO wafers. The ferromagnetism of Co implanted oxygen polar ZnO wafer is found to be larger than that of zinc polar ZnO. First principle calculations reveal that the magnetic moments mainly come from the Co-3d and O-2p states. The O-2p state has a stronger hybridization with the Co-3d state in oxygen polar ZnO, resulting in higher magnetic moments of Co implanted oxygen polar ZnO. These results not only benefit to a better understanding of the origin of surface-polarity dependent ferromagnetism in ZnO diluted magnetic semiconductors but also may open up the possibility to employ these polar ZnO in spintronic devices.

Improving the Room-Temperature Ferromagnetism in ZnO and Low-Doped ZnO:Ag Films Using GLAD Sputtering.

ZnO and doped ZnO films with non-ferromagnetic metal have been widely used as biosensor elements. In these studies, the electrochemical measurements are explored, though the electrical impedance of the system. In this sense, the ferromagnetic properties of the material can be used for multifunctionalization of the sensor element using external magnetic fields during the measurements. Within this context, we investigate the room-temperature ferromagnetism in pure ZnO and Ag-doped ZnO films presenting zigzag-like columnar geometry. Specifically, we focus on the films’ structural and quasi-static magnetic properties and disclose that they evolve with the doping of low-Ag concentrations and the columnar geometry employed during the deposition. The magnetic characterization reveals ferromagnetic behavior at room temperature for all studied samples, including the pure ZnO one. By considering computational simulations, we address the origin of ferromagnetism in ZnO and Ag-doped ZnO and interpret our results in terms of the Zn vacancy dynamics, its substitution by an Ag atom in the site, and the influence of the columnar geometry on the magnetic properties of the films. Our findings bring to light an exciting way to induce/explore the room-temperature ferromagnetism of a non-ferromagnetic metal-doped semiconductor as a promising candidate for biosensor applications.

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.

Ferromagnetism from non-magnetic ions: Ag-doped ZnO

To develop suitable ferromagnetic oxides with Curie temperature (TC) at or above room temperature for spintronic applications, a great deal of research in doping ZnO with magnetic ions is being carried out over last decade. As the experimental results on magnetic ions doped ZnO are highly confused and controversial, we have investigated ferromagnetism in non-magnetic ion, Ag, doped ZnO. When Ag replaces Zn in ZnO, it adopts 4d9 configuration for Ag2+ which has single unpaired spin and suitable exchange interaction among these spins gives rise to ferromagnetism in ZnO with above room temperature TC. Experimentally, we have observed room temperature ferromagnetism (RTFM) in Ag-doped ZnO with Ag concentration varied from 0.03% to 10.0%. It is shown that zinc vacancy (VZn) enhances the ferromagnetic ordering (FMO) while oxygen vacancy (VO) retards the ferromagnetism in Ag-doped ZnO. Furthermore, the theoretical investigation revealed that VZn along with Ag2+ ions play a pivotal role for RTFM in Ag-doped ZnO. The Ag2+-Ag2+ interaction is ferromagnetic in the same Zn plane whereas anti-ferromagnetic in different Zn planes. The presence of VZn changes the anti-ferromagnetic to ferromagnetic state with a magnetic coupling energy of 37 meV. Finally, it has been established that the overlapping of bound magnetic polarons is responsible for RTFM in low doping concentration. However, anti-ferromagnetic coupling sets in at higher doping concentrations and hence weakens the FMO to a large extent.

Room temperature ferromagnetism in ZnO:Cu and ZnO:Ag co-doped with Al

Abstract The room temperature ferromagnetism in doped and co-doped ZnO is reported here. X-ray photoelectron spectroscopy revealed the mixed oxidation states of Cu and Ag in ZnO:Cu and ZnO:Ag films, respectively. However, co-doping of Cu and Ag with Al does not alter their bonding characteristics in ZnO. Al in +3 oxidation state indicates that substitution of Al to Zn site gives an extra electron which increases the conductivity of ZnO:Ag and ZnO:Cu films. Mott variable range hopping dominates the conduction in ZnO:Cu and ZnO:Ag films whereas thermally activated conduction dominates in Al co-doped ZnO:Ag and ZnO:Cu films. We show that Ruderman–Kittel–Kasuya–Yosida mechanism is not responsible for ferromagnetism in Cu and Ag doped ZnO as the increase in carrier concentration due to Al co-doping does not enhance the ferromagnetism. Finally, possible mechanism behind the ferromagnetism is discussed.

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.

Electronic structure and ferromagnetic properties of Zn vacancies in ZnO screw dislocations: First-principles calculations

First principle calculation, based on density functional theory, is applied to study the configuration and electronic structure of ZnO screw dislocations along [0001] and [112¯0]. O-O bonds can be found in the core of dislocations along [0001], which induces an occupied energy level in forbidden band. The dislocations along [0001] also facilitate the formation and accumulation of Zn vacancies, due to the stress relief in dislocations. Moreover, the dislocations are periodic and continuous, and can mediate the ferromagnetic coupling between Zn vacancies. The antibonding state of O-O bonds around Zn vacancies is responsible for the ferromagnetic state, which is a kind of d0 ferromagnetism. The result also gives us a new view to think the origin of d0 ferromagnetism in ZnO.

Vacancy-induced ferromagnetism in ZnO probed by spin-polarized positron annihilation spectroscopy

We investigated the ferromagnetism of ZnO induced by oxygen implantation by using spin-polarized positron annihilation spectroscopy together with magnetization measurements. The magnetization measurements showed the appearance of ferromagnetism after oxygen implantation and its disappearance during post-implantation annealing at temperatures above 573 K. The Doppler broadening of annihilation radiation (DBAR) spectrum showed asymmetry upon field reversal after oxygen implantation. The obtained differential DBAR spectrum between positive and negative magnetic fields was well-explained with a theoretical calculation considering zinc vacancies. The disappearance of the field-reversal asymmetry of the DBAR spectrum as a result of annealing agreed with the observations of ferromagnetism by magnetization measurements. These results suggest the radiation-induced zinc vacancies to be the source of the observed ferromagnetism of ZnO.

Nature of the 550 nm emission and d0 ferromagnetism in ZnO nanocrystals revealed by H2O2 etching

Clarifying the origin of green photoluminescence and the room temperature ferromagnetism in ZnO is crucial for its potential application. In this work, we show that the characteristic 368 nm donor-bound exciton and the g = 1.96 electron spin resonance (ESR) signal in ZnO nanocrystals were eliminated after H2O2 oxidation, along with the appearance of 550 nm photoluminescence, unambiguously evidencing the creation of acceptor defects. Meanwhile, the d0 ferromagnetism was significantly enhanced. In combination with the fact that ambient O2 exposure also resulted in a significant boost of ferromagnetism in ZnO particles with different grain sizes, we surmise that adsorbed oxygen facilitates the d0 ferromagnetism in ZnO while VZn causes the 550 nm green emission. This work illustrates that oxygen exposure is an efficient way to tune the physics in metal oxides through engineering defects.

Investigation of intrinsic defect magnetic properties in wurtzite ZnO materials

Theoretical and experimental investigations of the ferromagnetism induced by intrinsic defects inside wurtzite zinc oxide structures are performed using magnetic field-dependent circular dichroism (MCD-H), direct magnetization measurement (M-H) by superconducting quantum interference device (SQUID) as well as by generalized gradient density functional theory (GGA-DFT). To investigate localized magnetic moments of bulk material intrinsic defects - vacancies, interstitial atoms and Frenkel defects, various-size periodic supercells are calculated. It is shown that oxygen interstitial atoms (Oi) or zinc vacancies (Znv) generate magnetic moments of 1,98 и 1,26µB respectively, however, the magnitudes are significantly reduced when the distance between defects increases. At the same time, the magnetic moments of oxygen Frenkel defects are large (~1.5–1.8µB) and do not depend on the distance between the defects. It is shown that the origin of the induced ferromagnetism in bulk ZnO is the extra spin density on the oxygen atoms nearest to the defect. Also dependence of the magnetization of ZnO (101̅0)and(0001) thin films on the positions of Oi and Znv in subsurface layers were investigated and it is shown that the magnetic moments of both defects are significantly different from the values inside bulk material. In order to check theoretical results regarding the defect induced ferromagnetism in ZnO, two thin films doped by carbon (C) and having Zn interstitials and oxygen vacancies were prepared and annealed in vacuum and air, respectively. According to the MCD-H and M-H measurements, the film, which was annealed in air, exhibits a ferromagnetic behavior, while the other does not. One can assume annealing of ZnO in vacuum should create oxygen vacancies or Zn interstitial atoms. At that annealing of the second C:ZnO film in air leads to essential magnetization, probably by annihilation of oxygen vacancies, formation of interstitial oxygen atoms or zinc vacancies. Thus, our experimental results confirm our theoretical conclusions that ZnO magnetization origin are Oi or Znv defects.

Ferromagnetic behaviour of ZnO: the role of grain boundaries.

The possibility to attain ferromagnetic properties in transparent semiconductor oxides such as ZnO is very promising for future spintronic applications. We demonstrate in this review that ferromagnetism is not an intrinsic property of the ZnO crystalline lattice but is that of ZnO/ZnO grain boundaries. If a ZnO polycrystal contains enough grain boundaries, it can transform into the ferromagnetic state even without doping with “magnetic atoms” such as Mn, Co, Fe or Ni. However, such doping facilitates the appearance of ferromagnetism in ZnO. It increases the saturation magnetisation and decreases the critical amount of grain boundaries needed for FM. A drastic increase of the total solubility of dopants in ZnO with decreasing grain size has been also observed. It is explained by the multilayer grain boundary segregation.

Raman scattering and photoluminescence investigations of N doped ZnO thin films: Local vibrational modes and induced ferromagnetism

N doped ZnO (ZnO:N) thin films are prepared by pulsed laser deposition in an oxygen environment using ZnO:N targets with varying nitrogen doping concentrations (1%–10%). The impact of nitrogen incorporation on the microstructural properties of prepared ZnO:N thin films has been studied using Raman scattering. The Raman shift of E2(high) mode towards lower frequencies indicate the substitution of N at O lattice sites (NO). A local vibrational mode corresponding to Zn–N was observed at 480.3 cm−1 in N doped ZnO thin films and highlights the increased strength of the Zn–N bond in the ZnO lattice. Photoluminescence studies reveal the dominant near band edge emission peak in the ultraviolet region and the absence of deep level emission due to defects. The ZnO:N thin films are found to possess room temperature ferromagnetism. N is found to play a significant role in arising ferromagnetism in ZnO and possess a solubility limit of 8% for uniform and homogeneous atomic substitution in ZnO. The present study confirms the promising application of N doped ZnO (ZnO:N) thin films for room temperature spintronics applications.

Preparation of ferromagnetic N–Mn codoped ZnO thin films by ultrasonic spray pyrolysis

N–Mn codoped ZnO films were prepared by ultrasonic spray pyrolysis (USP) to search for the room-temperature ferromagnetism. Structural, magnetic and electrical properties of ZnO-based films were investigated. Rapid thermal processing was conducted to explore the influence of the post-treatment on magnetic and electrical properties of ZnO-based films. The results show that magnetic properties of Mn-doped ZnO films by USP strongly depend on the acceptor doping and annealing treatments, which is consistent with the theoretical prediction that hole carriers might be necessary to induce ferromagnetism in ZnO:Mn system.

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

AbstractReview: 56 refs.

Resonant valence bond states in zinc vacancies induce the ferromagnetism of ZnO

A theoretical model was proposed to investigate the mechanism of ferromagnetism in ZnO as well as to simulate the experimental result that the ferromagnetism can be enhanced by UV irradiation as UV photon energy is equivalent to the band gap. In the model, the spin moments arise from the trapped electrons in oxygen vacancy states and coexist with the itinerant electrons which reside in zinc vacancy states and fall into resonant valence bond states. Charge exchange between the conduction band of ZnO and both vacancy states makes electrons on both vacancy states delocalized and results in a decrease of the ferromagnetism as well.

An alternative approach to investigate the origin of p-type conductivity in arsenic doped ZnO

P-type conductivity in MOCVD grown ZnO was obtained by directional thermal diffusion of arsenic from semi-insulating GaAs substrate. The films were single crystalline in nature and oriented along (002) direction. Ab initio calculations in the framework of density functional theory have been carried out with different chemical states of arsenic in ZnO. Present calculations suggested AsZn–2VZn defect is a shallow acceptor and results in ferromagnetism in ZnO. The magnetic measurements of the samples indeed showed ferromagnetic ordering at room temperature. X-ray photoelectron spectra confirmed the presence of AsZn and VZn. The core level chemical shift in binding energy of AsZn indicated the formation of AsZn–2VZn. Diffused arsenic substitutes zinc atom and creates additional zinc vacancies. The zinc vacancies, surrounding the oxygen atoms, result in unpaired O 2p electrons which in turn induce ferromagnetism in the samples.

Ferromagnetism in Gd doped ZnO nanowires: A first principles study

In several experimental studies, room temperature ferromagnetism in Gd-doped ZnO nanostructures has been achieved. However, the mechanism and the origin of the ferromagnetism remain controversial. We investigate the structural, magnetic, and electronic properties of Zn48O48 nanowires doped with Gd, using density functional theory. Our findings indicate that substitutionally incorporated Gd atoms prefer occupying the surface Zn sites. Moreover, the formation energy increases with the distance between Gd atoms, signifying that no Gd-Gd segregation occurs in the nanowires within the concentration limit of ≤2%. Gd induces ferromagnetism in ZnO nanowires with magnetic coupling energy up to 21 meV in the neutral state, which increases with additional electron and O vacancy, revealing the role of carriers in magnetic exchange. The potential for achieving room temperature ferromagnetism and high TC in ZnO:Gd nanowires is evident from the large ferromagnetic coupling energy (200 meV) obtained with the O vacancy. Density of states shows that Fermi level overlaps with Gd f states with the introduction of O vacancy, indicating the possibility of s-f coupling. These results will assist in understanding experimental findings in Gd-doped ZnO nanowires.

Effect of Strain on the Magnetism of Transition Metal-Doped ZnO: The First-Principles Calculations

The electronic structures and magnetic properties of transition metal (Mn, Fe, Co)-doped ZnO under the in-plane biaxial strains are investigated systematically by the first-principles calculations. It is found that the augmented in-plane biaxial strains can mediate the Fe-doped ZnO ferromagnetism effectively, but have less influence on Mn- and Co-doped systems. Therefore, the strain may be an effective means in inducing ferromagnetism in ZnO:Fe system.