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Showing papers on "Ferromagnetism published in 2008"


Journal ArticleDOI
TL;DR: A one-to-one mapping of the ferroelectric and ferromagnetic domains is discovered, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic.
Abstract: Multiferroics are of interest for memory and logic device applications, as the coupling between ferroelectric and magnetic properties enables the dynamic interaction between these order parameters. Here, we report an approach to control and switch local ferromagnetism with an electric field using multiferroics. We use two types of electromagnetic coupling phenomenon that are manifested in heterostructures consisting of a ferromagnet in intimate contact with the multiferroic BiFeO(3). The first is an internal, magnetoelectric coupling between antiferromagnetism and ferroelectricity in the BiFeO(3) film that leads to electric-field control of the antiferromagnetic order. The second is based on exchange interactions at the interface between a ferromagnet (Co(0.9)Fe(0.1)) and the antiferromagnet. We have discovered a one-to-one mapping of the ferroelectric and ferromagnetic domains, mediated by the colinear coupling between the magnetization in the ferromagnet and the projection of the antiferromagnetic order in the multiferroic. Our preliminary experiments reveal the possibility to locally control ferromagnetism with an electric field.

1,242 citations


Journal ArticleDOI
TL;DR: N nanoscaling laws for magnetic components are found to be critical to the design of optimized magnetic characteristics of hybrid nanoparticles and their enhanced applicability in the biomedical sciences including their utilizations as contrast enhancement agents for magnetic resonance imaging (MRI), ferromagnetic components for nano-bio hybrid structures, and translational vectors for magnetophoretic sensing of biological species.
Abstract: Magnetic nanoparticles, which exhibit a variety of unique magnetic phenomena that are drastically different from those of their bulk counterparts, are garnering significant interest since these properties can be advantageous for utilization in a variety of applications ranging from storage media for magnetic memory devices to probes and vectors in the biomedical sciences. In this Account, we discuss the nanoscaling laws of magnetic nanoparticles including metals, metal ferrites, and metal alloys, while focusing on their size, shape, and composition effects. Their fundamental magnetic properties such as blocking temperature (Tb), spin life time (τ), coercivity (Hc), and susceptibility (χ) are strongly influenced by the nanoscaling laws, and as a result, these scaling relationships can be leveraged to control magnetism from the ferromagnetic to the superparamagnetic regimes. At the same time, they can be used in order to tune magnetic values including Hc, χ, and remanence (Mr). For example, life time of mag...

744 citations


Journal ArticleDOI
TL;DR: A surface magnetoelectric effect is revealed by density-functional calculations that are applied to ferromagnetic Fe(001), Ni(001, and Co(0001) films in the presence of an external electric field.
Abstract: A surface magnetoelectric effect is revealed by density-functional calculations that are applied to ferromagnetic Fe(001), Ni(001), and Co(0001) films in the presence of an external electric field. The effect originates from spin-dependent screening of the electric field which leads to notable changes in the surface magnetization and the surface magnetocrystalline anisotropy. These results are of considerable interest in the area of electrically controlled magnetism and magnetoelectric phenomena.

627 citations


Journal ArticleDOI
Feng Pan1, Cheng Song1, X. J. Liu1, Yuchao Yang1, Fei Zeng1 
TL;DR: In this paper, the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics, are discussed.
Abstract: This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent ferromagnetic ordering of TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics.

614 citations


Journal ArticleDOI
TL;DR: A high value for the spin wave stiffness D=2100 meV A2 and a spin-collinear domain wall creation energy E(dw)=114 meV accompanied by low magnetic anisotropy is found and possible ways of increasing the range of magnetic order and effects of edge roughness on it are discussed.
Abstract: Magnetic zigzag edges of graphene are considered as a basis for novel spintronics devices despite the fact that no true long-range magnetic order is possible in one dimension. We study the transverse and longitudinal fluctuations of magnetic moments at zigzag edges of graphene from first principles. We find a high value for the spin wave stiffness D=2100 meV A2 and a spin-collinear domain wall creation energy E(dw)=114 meV accompanied by low magnetic anisotropy. Above the crossover temperature T(x) approximately 10 K, the spin correlation length xi proportional, variantT(-1) limits the long-range magnetic order to approximately 1 nm at 300 K while below T(x), it grows exponentially with decreasing temperature. We discuss possible ways of increasing the range of magnetic order and effects of edge roughness on it.

609 citations


Journal ArticleDOI
18 Jan 2008-Science
TL;DR: By dynamically modifying the potential bias between neighboring lattice sites, the magnitude and sign of the superexchange interaction can be controlled, thus allowing the system to be switched between antiferromagnetic and ferromagnetic spin interactions.
Abstract: Quantum mechanical superexchange interactions form the basis of quantum magnetism in strongly correlated electronic media. We report on the direct measurement of superexchange interactions with ultracold atoms in optical lattices. After preparing a spin-mixture of ultracold atoms in an antiferromagnetically ordered state, we measured coherent superexchange-mediated spin dynamics with coupling energies from 5 hertz up to 1 kilohertz. By dynamically modifying the potential bias between neighboring lattice sites, the magnitude and sign of the superexchange interaction can be controlled, thus allowing the system to be switched between antiferromagnetic and ferromagnetic spin interactions. We compare our findings to predictions of a two-site Bose-Hubbard model and find very good agreement, but are also able to identify corrections that can be explained by the inclusion of direct nearest-neighbor interactions.

573 citations


Journal ArticleDOI
TL;DR: The combination of some three-atom bridges with paramagnetic 3d transition metal ions results in the systematic isolation of molecular magnetic materials, ranging from single-molecule and single-chain magnets to layered weak ferromagnets and three-dimensional porous magnets.

486 citations


Journal ArticleDOI
TL;DR: Exchange striction working between adjacent Fe3+ and Dy3+ layers with the respective layered antiferromagnetic components is proposed as the origin of the ferroelectric polarization in the multiferroic phase.
Abstract: Versatile and gigantic magnetoelectric (ME) phenomena have been found for a single crystal of DyFeO3. Below the antiferromagnetic ordering temperature of Dy moments, a linear-ME tensor component as large as alphazz approximately 2.4 x 10(-2) esu is observed. It is also revealed that application of magnetic field along the c axis induces a multiferroic (weakly ferromagnetic and ferroelectric) phase with magnetization [> or =0.5 microB/formula unit (f.u.)] and electric polarization (> or =0.2 microC/cm2) both along the c axis. Exchange striction working between adjacent Fe3+ and Dy3+ layers with the respective layered antiferromagnetic components is proposed as the origin of the ferroelectric polarization in the multiferroic phase.

408 citations


Journal ArticleDOI
TL;DR: To understand the magnetic properties of this Co7 wheel, ab initio calculations for each cobalt fragment at the CASSCF/CASPT2 level were performed, including spin-orbit coupling effects within the SO-RASSI approach.
Abstract: A mixed-valence Co(II)/Co(III) heptanuclear wheel [CoII3CoIII4(L)6(MeO)6] (LH2 = 1,1,1-trifluoro-7-hydroxy-4-methyl-5-aza-hept-3-en-2-one) has been synthesized and its crystal structure determined using single-crystal X-ray diffraction. The valence state of each cobalt ion was established by bond valence sum calculations. Studies of the temperature dependence of the magnetic susceptibility and the field dependence of the magnetization evidence ferromagnetic interactions within the compound. In order to understand the magnetic properties of this Co7 wheel, we performed ab initio calculations for each cobalt fragment at the CASSCF/CASPT2 level, including spin−orbit coupling effects within the SO-RASSI approach. The four Co(III) ions were found to be diamagnetic and to give a significant temperature-independent paramagnetic contribution to the susceptibility. The spin−orbit coupling on the three Co(II) sites leads to separations of ∼200 cm−1 between the ground and excited Kramers doublets, placing the Co7 wh...

403 citations


Journal ArticleDOI
TL;DR: From all-electron fixed-spin-moment calculations, the presence of competing strong antiferromagnetic exchange interactions suggests that magnetism and superconductivity in doped LaFeAsO may be strongly coupled, much like in the high-T(c) cuprates.
Abstract: From all-electron fixed-spin-moment calculations we show that ferromagnetic and checkerboard antiferromagnetic ordering in LaFeAsO are not stable and the stripe antiferromagnetic configuration with is the only stable ground state. The main exchange interactions between Fe ions are large, antiferromagnetic, and frustrated. The magnetic stripe phase breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaFeAsO recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions suggests that magnetism and superconductivity in doped LaFeAsO may be strongly coupled, much like in the high- cuprates.

401 citations


Journal ArticleDOI
TL;DR: In this paper, it was shown that ZnO samples can be magnetic even without transition-metal doping and also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZNO nanostructures.
Abstract: Extensive calculations based on density functional theory have been carried out to understand the origin of magnetism in undoped ZnO thin films as found in recent experiments. The observed magnetism is confirmed to be due to Zn, instead of O, vacancy. The main source of the magnetic moment, however, arises from the unpaired 2p electrons at O sites surrounding the Zn vacancy with each nearest-neighbor O atom carrying a magnetic moment ranging from 0.490 to 0.740 B. Moreover, the study of vacancy-vacancy interactions indicates that in the ground state, the magnetic moments induced by Zn vacancies prefer to ferromagnetically couple with the antiferromagnetic state lying 44 meV higher in energy. Since this is larger than the thermal energy at room temperature, the ferromagnetic state can be stable against thermal fluctuations. Calculations and discussions are also extended to ZnO nanowires that have larger surface to volume ratio. Here, the Zn vacancies are found to lead to the ferromagnetic state too. The present theoretical study not only demonstrates that ZnO samples can be magnetic even without transition-metal doping but also suggests that introducing Zn vacancy is a natural and an effective way to fabricate magnetic ZnO nanostructures. In addition, vacancy mediated magnetic ZnO nanostructures may have certain advantages over transition-metal doped systems in biomedical applications.

Journal ArticleDOI
TL;DR: It is concluded that only single-atom defects can induce ferromagnetism in graphene-based materials and the preserved stacking order of graphene layers is shown to be another necessary condition for achieving a finite net magnetic moment of irradiated graphite.
Abstract: The magnetic properties of disordered graphene and irradiated graphite are systematically studied using a combination of mean-field Hubbard model and first-principles calculations. By considering large-scale disordered models of graphene, I conclude that only single-atom defects can induce ferromagnetism in graphene-based materials. The preserved stacking order of graphene layers is shown to be another necessary condition for achieving a finite net magnetic moment of irradiated graphite. Ab initio calculations of hydrogen binding and diffusion and of interstitial-vacancy recombination further confirm the crucial role of stacking order in pi-electron ferromagnetism of proton-bombarded graphite.

Journal ArticleDOI
Abstract: The transport properties of magnetic tunnel junctions with different (110)-textured Heusler alloy electrodes such as Co2MnSi, Co2FeSi or Co2Mn0.5Fe0.5Si, AlOx barrier, and Co–Fe counterelectrode are investigated. The bandstructure of Co2Mn1−xFexSi is predicted to show a systematic shift in the position of the Fermi energy EF through the gap in the minority density of states while the composition changes from Co2MnSi toward Co2FeSi. Although this shift is indirectly observed by x-ray photoemission spectroscopy, all junctions show a large spin polarization of around 70% at the Heusler alloy/Al–O interface and are characterized by a very similar temperature and bias voltage dependence of the tunnel magnetoresistance. This suggests that these transport properties of these junctions are dominated by inelastic excitations and not by the electronic bandstructure.

Journal ArticleDOI
TL;DR: In this article, the anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnOs, indicating that AHE does not uniquely prove ferromagnetism.
Abstract: ZnO films were prepared by pulsed laser deposition on a-plane sapphire substrates under N2 atmosphere. Ferromagnetic loops were obtained with the superconducting quantum interference device at room temperature, which indicate a Curie temperature much above room temperature. No clear ferromagnetism was observed in intentionally Cu-doped ZnO films. This excludes that Cu doping into ZnO plays a key role in tuning the ferromagnetism in ZnO. 8.8% negative magnetoresistance probed at 5K at 60kOe on ferromagnetic ZnO proves the lack of s-d exchange interaction. Anomalous Hall effect (AHE) was observed in ferromagnetic ZnO as well as in nonferromagnetic Cu-doped ZnO films, indicating that AHE does not uniquely prove ferromagnetism. The observed ferromagnetism in ZnO is attributed to intrinsic defects.

Journal ArticleDOI
TL;DR: This work demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.
Abstract: Films of ZnO doped with magnetic ions Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behavior is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localized spins with static localized states. The magnetism is quenched when carriers in the localized states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.

Journal ArticleDOI
TL;DR: In this paper, a spin transfer switching in the TbCoFe∕CoFeB∕MgO∕ CoFeB ∕TbCo FeB free layer with a large coercive field of 1.2kOe and a large thermal stability factor of 107 at room temperature was studied.
Abstract: Spin transfer (ST) switching in the TbCoFe∕CoFeB∕MgO∕CoFeB∕TbCoFe magnetic tunnel junction (MTJ) was studied. The TbCoFe∕CoFeB free layer with a large coercive field of 1.2kOe and a large thermal stability factor of 107 at room temperature was switched by a 100ns pulse current with a current density of 4.7MA∕cm2. This is the first report of ST switching in a MTJ with perpendicular magnetic anisotropy. The temperature dependence of the coercive field was also investigated to estimate the magnetic anisotropy in the case of rising temperature due to the Joule heating effect. The measured coercive field at 87°C, which was the simulated temperature during the switching pulse current, was about 0.34kOe. The ratio of the switching current density to the coercive field under the switching current in the MTJ with the TbCoFe∕CoFeB free layer is smaller than that in a typical MTJ with an in-plane magnetized CoFeB free layer. This result indicates that a MTJ with perpendicular magnetic anisotropy is advantageous for ...

Journal ArticleDOI
TL;DR: In this article, the spin filtering performance of half-metallic Co-based full-Heusler alloys and a spin filtering device (SFD) using a ferromagnetic barrier have been investigated as highly spin-polarized current sources.

Journal ArticleDOI
TL;DR: It is shown that the exchange field in CoFeB/BiFeO_{3} heterostructures scales with the inverse of the ferroelectric and antiferromagnetic domain size of the BiFeO3 films, as expected from Malozemoff's model of exchange bias extended to multiferroics.
Abstract: We have combined neutron scattering and piezoresponse force microscopy to show that the exchange field in CoFeB/BiFeO_{3} heterostructures scales with the inverse of the ferroelectric and antiferromagnetic domain size of the BiFeO3 films, as expected from Malozemoff's model of exchange bias extended to multiferroics. Accordingly, polarized neutron reflectometry reveals the presence of uncompensated spins in the BiFeO3 film at the interface with CoFeB. In view of these results, we discuss possible strategies to switch the magnetization of a ferromagnet by an electric field using BiFeO3.

Journal ArticleDOI
TL;DR: This work examines limitations of the band theory approach to stabilization of ferromagnetism in ZnO, and explains the contradictions in previous studies, which drastically overestimate the doping threshold for magnetic ordering.
Abstract: Substitutional cobalt in ZnO has a weak preference for antiferromagnetic ordering Stabilization of ferromagnetism is achieved through $n$-type doping, which can be understood through a band coupling model However, the description of the transition to a ferromagnetic ground state varies within different levels of band theory; issues arise due to the density functional theory underestimation of the band gap of ZnO, and the relative position of the nominally unfilled Co ${t}_{2d}$ states We examine these limitations, including approaches to overcome them, and explain the contradictions in previous studies, which drastically overestimate the doping threshold for magnetic ordering

Journal ArticleDOI
TL;DR: In this paper, a new model for ferromagnetism associated with defects in the bulk or at the surface of nanoparticles is proposed, where a narrow, structured local density of states is associated with the defects, but the Fermi level (which may lie above or below a mobility edge) will not normally coincide with a peak in Ns(E).
Abstract: A new model is proposed for ferromagnetism associated with defects in the bulk or at the surface of nanoparticles. The basic idea is that a narrow, structured local density of states Ns(E) is associated with the defects, but the Fermi level (which may lie above or below a mobility edge) will not normally coincide with a peak in Ns(E). However, if there is a local charge reservoir, such as a dopant cation coexisting in two different charge states or a charge-transfer complex at the surface, then it may be possible for electron transfer to raise the Fermi level to a peak in the local density of states, leading to Stoner splitting of Ns(E). Spontaneous Stoner ferromagnetism can arise in percolating defect-rich regions, such as the nanoparticle surface. The charge-transfer ferromagnetism model may be applicable to a wide range of nanoparticles and thin films of dilute magnetic oxides have previously been regarded as dilute magnetic semiconductors.

Book ChapterDOI
TL;DR: In this article, the most important models for the exchange bias effect are reviewed and the most recent experiments in the light of the presented models are discussed, as well as recent experimental results.
Abstract: The exchange bias effect, discovered more than fifty years ago, is a fundamental interfacial property, which occurs between ferromagnetic and antiferromagnetic materials. After intensive experimental and theoretical research over the last ten years, a much clearer picture has emerged about this effect, which is of immense technical importance for magneto-electronic device applications. In this review we start with the discussion of numerical and analytical results of those models which are based on the assumption of coherent rotation of the magnetization. The behavior of the ferromagnetic and antiferromagnetic spins during the magnetization reversal, as well as the dependence of the critical fields on characteristic parameters such as exchange stiffness, magnetic anisotropy, interface disorder etc. are analyzed in detail and the most important models for exchange bias are reviewed. Finally recent experiments in the light of the presented models are discussed.

Journal ArticleDOI
TL;DR: Using spin-polarized scanning tunneling microscopy, Monte Carlo simulations explain the formation of a nanoscale labyrinth pattern, originating from the coexistence of the two possible rotational domains, that is intrinsic to the system.
Abstract: Using spin-polarized scanning tunneling microscopy we show that the magnetic order of 1 monolayer Mn on W(001) is a spin spiral propagating along $⟨110⟩$ crystallographic directions. The spiral arises on the atomic scale with a period of about 2.2 nm, equivalent to only 10 atomic rows. Ab initio calculations identify the spin spiral as a left-handed cycloid stabilized by the Dzyaloshinskii-Moriya interaction, imposed by spin-orbit coupling, in the presence of softened ferromagnetic exchange coupling. Monte Carlo simulations explain the formation of a nanoscale labyrinth pattern, originating from the coexistence of the two possible rotational domains, that is intrinsic to the system.


Journal ArticleDOI
TL;DR: The total magnetization is reduced by approximately 30%, demonstrating that Co-O-Co pairs are antiferromagnetically coupled and no sign of intrinsic ferromagnetic interactions for isolated or paired Co dopant atoms in Co:ZnO films is found.
Abstract: We report element specific structural and magnetic investigations on Zn(1-x)Co(x)O epitaxial films using synchrotron radiation. Co dopants exclusively occupy Zn sites as revealed by x-ray linear dichroism having an unprecedented degree of structural perfection. Comparative magnetic field dependent measurements by x-ray magnetic circular dichroism and conventional magnetometry consistently show purely paramagnetic behavior for isolated Co dopant atoms with a magnetic moment of 4.8 (mu B). However, the total magnetization is reduced by approximately 30%, demonstrating that Co-O-Co pairs are antiferromagnetically coupled. We find no sign of intrinsic ferromagnetic interactions for isolated or paired Co dopant atoms in Co:ZnO films.

Journal ArticleDOI
TL;DR: In this paper, the authors used the plane-wave method to determine spin-wave spectra of three-dimensional magnonic crystals (the magnetic counterpart of photonic crystals) composed of two different ferromagnetic materials, and they demonstrated that magnonic gaps in such structures occur at spontaneous magnetization contrast and/or exchange contrast values above a certain critical level, depending on the lattice type.
Abstract: We use the plane-wave method to determine spin-wave spectra of three-dimensional magnonic crystals (the magnetic counterpart of photonic crystals) composed of two different ferromagnetic materials. The scattering centers in the magnonic crystal considered are ferromagnetic spheroids (spheres being a special case) distributed in sites of a cubic (sc, fcc, or bcc) lattice embedded in a matrix of a different ferromagnetic material. We demonstrate that magnonic gaps in such structures occur at spontaneous magnetization contrast and/or exchange contrast values above a certain critical level, which depends on the lattice type. Optimum conditions for magnonic gaps to open are offered by the structure in which the scattering centers are the most densely packed (the fcc lattice). We show that in all three lattice types considered the reduced width of the gap (i.e., the width referred to the gap center) is, in good approximation, a linear function of both the exchange contrast and the magnetization contrast. Also, the gap width proves sensitive to scattering center deformation, and its maximum value to correspond to a scattering center shape close to a sphere. Moreover, our numerical results seem to indicate that dipolar interactions in general result in an effective reduction of the gap width, but their impact only becomes of importance when the lattice constant of the cubic magnonic structure is greater than the ferromagnetic exchange length of the matrix material.

Journal ArticleDOI
TL;DR: In this article, a first-principles calculation of the exchange interactions in semi-and full-Heusler alloys is performed based on the frozen-magnon approach, which shows that the magnetism of the Mn-based Heusler-alloys depends strongly on the number of conduction electrons, their spin polarization and the position of the unoccupied Mn 3d states with respect to the Fermi level.
Abstract: Because of large spatial separation of the Mn atoms in Heusler alloys the Mn 3d states belonging to different atoms do not overlap considerably. Therefore an indirect exchange interaction between Mn atoms should play a crucial role in the ferromagnetism of the systems. To study the nature of the ferromagnetism of various Mn-based semi- and full-Heusler alloys we perform a systematic first-principles calculation of the exchange interactions in these materials. The calculation of the exchange parameters is based on the frozen-magnon approach. The calculations show that the magnetism of the Mn-based Heusler alloys depends strongly on the number of conduction electrons, their spin polarization and the position of the unoccupied Mn 3d states with respect to the Fermi level. Various magnetic phases are obtained depending on the combination of these characteristics. The Anderson's s-d model is used to perform a qualitative analysis of the obtained results. The conditions leading to diverse magnetic behavior are identified. If the spin polarization of the conduction electrons at the Fermi energy is large and the unoccupied Mn 3d states lie well above the Fermi level, an RKKY-type ferromagnetic interaction is dominating. On the other hand, the contribution of the antiferromagnetic superexchange becomes important if unoccupied Mn 3d states lie close to the Fermi energy. The resulting magnetic behavior depends on the competition of these two exchange mechanisms. The calculational results are in good correlation with the conclusions made on the basis of the Anderson s-d model which provides useful framework for the analysis of the results of first-principles calculations and helps to formulate the conditions for high Curie temperature.

Journal ArticleDOI
01 Jun 2008-EPL
TL;DR: In this article, a doping-dependent phase diagram of the LaOM As (M = V-Cu) family is presented, which is characterized as a antiferromagnetic semiconductor around the LaOFeAs side and as a ferromagnetic metal around LaOCoAs.
Abstract: By first-principles calculations, we present a doping-dependent phase diagram of the LaOM As (M = V-Cu) family. It is characterized as a antiferromagnetic semiconductor around the LaOMnAs side and as a ferromagnetic metal around LaOCoAs. Both LaOFeAs and LaONiAs, where superconductivity were discovered, are located at the borderline of magnetic phases. An extensive Fermi surface analysis suggests that the observed superconductivity is of electron type in its origin. We discuss possible pairing mechanisms in the context of competing ferromagnetic phases found in this work and the ferromagnetic spin fluctuations. Copyright c EPLA, 2008

Journal ArticleDOI
TL;DR: Pan et al. as discussed by the authors introduced carbon into ZnO films by ion implantation, and room temperature ferromagnetism has been observed. But the results were limited to the case of C-doped ferromagnetic ZnOs.
Abstract: Unexpected ferromagnetism has been observed in carbon doped ZnO films grown by pulsed laser deposition [H. Pan et al., Phys. Rev. Lett. 99, 127201 (2007)]. In this letter, we introduce carbon into ZnO films by ion implantation. Room temperature ferromagnetism has been observed. Our analysis demonstrates that (1) C-doped ferromagnetic ZnO can be achieved by an alternative method, i.e., ion implantation, and (2) the chemical involvement of carbon in the ferromagnetism is indirectly proven.

Journal ArticleDOI
TL;DR: In this article, the observed room-temperature ferromagnetism in MgO nanocrystalline powders possibly originates from Mg vacancies at/near the surfaces of nanograins.
Abstract: MgO nanocrystalline powders prepared by sol-gel method present room-temperature ferromagnetism, whereas MgO bulk exhibits diamagnetism. The vacuum annealing of MgO nanocrystalline powders reduces ferromagnetism. The observed room-temperature ferromagnetism in MgO nanocrystalline powders possibly originates from Mg vacancies at/near the surfaces of nanograins. Mg vacancies can induce local magnetic moments. Large concentrations of Mg vacancies at the surfaces of nanograins possibly establish magnetic percolation.

Journal ArticleDOI
TL;DR: In this paper, the structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions, and optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration.
Abstract: The structure and magnetic properties of Co-doped ZnO films are discussed in relation to cobalt doping levels and growth conditions Films were deposited by pulsed-laser deposition (PLD) from ZnO targets containing cobalt concentrations from 0 to 30?at% The structure of the films is examined by x-ray diffraction (XRD) and transmission electron microscopy (TEM), and optical absorption is used to infer the substitution of cobalt inside the ZnO lattice Magnetic properties are characterized by superconducting quantum interference device (SQUID) magnetometry Films doped with cobalt concentrations of a few per cent appear to be composed of two magnetic components: a paramagnetic component and a low-field ferromagnetic component Films doped with 30% cobalt show a larger FM signature at room temperature with clear hysteretic shape, but films grown at low pressure are plagued by the precipitation of metallic cobalt nanoparticles within the lattice which can be easily detected by XRD These particles are well oriented with the ZnO crystal structure By increasing the base pressure of the vacuum chamber to pressures above 1?10?5?Torr, metallic cobalt precipitates are undetectable in XRD scans, whereas the films still show an FM signature of ~008??B/Co Depositions in the presence of oxygen background gas at 002?mTorr decreases the magnetization The decreased magnetization with oxygen suggests that the activation of ferromagnetism depends on defects, such as oxygen vacancies, created during growth Optical absorption measurements show a sequential increase in the Co+2 absorption peaks in these films, along with an almost linearly increasing bandgap with cobalt concentration suggesting a large solubility of cobalt in ZnO Bright-field TEM imaging and electron diffraction do not show signs of precipitation; however, dark-field imaging shows circular areas of varying contrast which could be associated with cobalt precipitation Therefore, the possibility that ferromagnetism results from secondary phases cannot be ruled out