Giant Magnetic Moment Research Articles

Giant magnetic moment at open ends of multiwalled carbon nanotubes**Project supported by the National Natural Science Foundation of China (Grant Nos. 10774032 and 51472057) and the Instrument Developing Project of the Chinese Academy of Sciences (Grant No. Y2010031).

The attractions of cantilevers made of multiwalled carbon nanotubes (MWNTs) and secured on one end are studied in the non-uniform magnetic field of a permanent magnet. Under an optical microscope, the positions and the corresponding deflections of the original cantilevers (with iron catalytic nanoparticles at the free end) and corresponding cut-off cantilevers (the free ends consisting of open ends of MWNTs) are studied. Both kinds of CNT cantilevers are found to be attracted by the magnet, and the point of application of force is proven to be at the tip of the cantilever. By measuring and comparing deflections between these two kinds of cantilevers, the magnetic moment at the open ends of the CNTs can be quantified. Due to the unexpectedly high value of the magnetic moment at the open ends of carbon nanotubes, it is called giant magnetic moment, and its possible mechanisms are proposed and discussed.

Electronic and magnetic properties of BiFeO3 with intrinsic defects: First-principles prediction

The electronic structure, magnetism, and dielectric functions of BiFeO3 with intrinsic vacancies, including Bi-, Fe-, and O-vacancies (denoted as VFe, VBi, and VO, respectively) are investigated using the first-principles density functional theory plus U calculations. It is revealed that the structural distortions associated with those vacancies impose significant influences on the total density of state and magnetic behaviors. The existence of VBi favors the excitation of the O2p state into the band gap at 0.4 eV, while the O2p and Fe3d orbitals are co-excited into the band gap around 0.45 eV in VFe. Consequently, a giant net magnetic moment of 1.96 μB is generated in VFe, and a relatively small moment of 0.13 μB is induced in VBi, whereas VO seems magnetically inactive. The giant magnetic moment generated in VFe originates from the suppression of the spatially modulated antiferromagnetic spin structure. Furthermore, VFe and VBi have strong influences on dielectric function, and induce some strong peaks to occur in the lower energy level. In contrast, VO has a small effect.

New Co–Pd–Zn γ-Brasses with Dilute Ferrimagnetism and Co2Zn11 Revisited: Establishing the Synergism between Theory and Experiment

A synergism between electronic structure theory and the targeted synthesis of new ternary γ-brass compounds is demonstrated in the Co–Zn system. Co2Zn11, which adopts a cubic γ-brass structure, is shown to be at the Zn-rich end of a homogeneity range that varies from 15.4 to 22.1 atom % Co. Four samples were examined by single-crystal diffraction, all of which crystallize in space group I4̅3m with the lattice parameter ranging from 8.9851(1) to 8.8809(1) Å as the Co content increases. In the 26-atom γ-brass clusters, Co atoms preferentially occupy the outer tetrahedron (OT) sites and then replace Zn atoms at the octahedron (OH) sites at higher Co concentrations. In addition, a small fraction of vacancies occurs on the inner tetrahedron (IT) sites. The electronic structure of Co2Zn11 shows two distinct pseudogaps near the Fermi level: one at 292 valence electrons per primitive unit cell and the other at 302–304 valence electrons per primitive unit cell. Using molecular orbital arguments applied to the body-centered cubic packing of the 26-atom Co4Zn22 γ-brass cluster, these pseudogaps arise from (i) splitting among the valence s and p orbitals, which gives rise to the Hume–Rothery electron counting rule, and (ii) splitting within the manifold of Co 3d orbitals via Co–Zn orbital interactions. Co2Zn11 is Pauli paramagnetic, although the density of states at the Fermi level is large, whereas Curie–Weiss behavior emerges for higher Co concentrations. Because Pd has a size and an electronegativity similar to those of Zn, and inspired by the pseudogaps in the electronic density of states curve of Co2Zn11, Pd-doped γ-brass compounds were designed and two new γ-brass compounds were obtained: Co0.92(2)Pd1.08Zn11 and Co2.50(1)Pd2.50Zn8. In these, the site preferences for Co and Pd can be rationalized by electronic structure calculations. The densities of states indicate that Co 3d states are the major contributors near their Fermi levels, with the Pd 4d band lying ∼2–3 eV below this. The magnetic properties of the Co–Pd–Zn γ-brasses are quite different from those of Co2Zn11: a giant magnetic moment on the Co atom is induced by the Pd atom, and Co2.50(1)Pd2.50Zn8 shows magnetization consistent with a dilute ferrimagnet. The results of first-principles calculations on two different models of the 26-atom γ-brass clusters indicate that intracluster Co–Co exchange is ferromagnetic, whereas intercluster Co–Co exchange is antiferromagnetic. These different magnetic exchange interactions provide rationalization for the high-temperature magnetization behavior of Co2.50(1)Pd2.50Zn8.

Giant ferromagnetism and exchange bias in tensile strained and Cr modified CaRuO3 thin films

We present an unified approach of epitaxial strain and chemical substitution to induce ferromagnetic order in otherwise non-magnetic CaRuO3. The tensile and compressive strained CaRu1−xCrxO3 (0 < x ≤ 0.2) (CRCO) films deposited on SrTiO3 (100) and LaAlO3 (100) substrates, respectively, exhibit a magnetic moment larger than their bulk compositions. The compressive strained x = 0.15 film displays unusually large magnetic moment ∼1.4 μB/f.u. which is about one order of magnitude larger than that of its bulk counterpart. We show that this giant magnetic moment manifests in the realization of (i) a giant exchange bias of ∼700 Oe in CaRu0.85Cr0.15O3/Pr0.5Ca0.5MnO3 bilayer and (ii) a large hysteretic magnetoresistance; both potential attributes for the spintronic applications.

Unusual Magnetic Properties of Functionalized Graphene Nanoribbons

The edge-induced magnetism of graphene nanoribbons (GNRs) has been found to be highly unstable at room temperature, thus making it difficult for spintronics applications under ambient conditions. Using first-principles calculations, we propose a way to overcome this problem by embedding transition-metal atoms into functionalized GNRs. We show that due to low or vanishing reaction barriers, 3d transition-metal atoms can be easily embedded into GNR once they are functionalized by −F or −CN groups. A systematic study of these materials shows fascinating magnetic properties; Co-embedded systems not only have high magnetic anisotropy energies hundreds of times larger than that of pure GNR and bulk Co, but also, the magnetic preferential directions can be switched by an electric field. Mn-embedded systems exhibit giant magnetic moments and half-metallicity with greatly enhanced stability. Under a small amount of biaxial strain, the magnetic moment of every Mn atom in such a Mn-embedded system can be increased by 2.1 μB due to a change in the dxy orbital occupancy, and its Curie temperature is estimated to be over 1000 K.

ChemInform Abstract: Revisiting Magnetism of Capped Au and ZnO Nanoparticles: Surface Band Structure and Atomic Orbital with Giant Magnetic Moment

AbstractReview: 56 refs.

Giant magnetic moments of Pd-doped manganese clusters

For the sake of producing a giant magnetic moment in small manganese clusters, we have performed a density function theory calculation on Mn N Pd (N = 1–5) clusters. The results demonstrate that the doped -Pd atoms do not alter the basic framework, but significantly improve the average binding energy of host Mn clusters. The interaction among Mn atoms for Mn N Pd becomes weaker in comparison with corresponding Mn N clusters which plays an important role in the appearance of giant magnetic moments of the system. Mn4Pd is the most promising in terms of gaining substantial binding energy as well as achieving larger ferromagnetic alignment (20μ B).

Giant magnetic moments of B and C doped cuboctahedral Mn13 clusters

Using first-principles calculations based on gradient corrected density functional theory we show that an otherwise distorted icosahedric Mn(13) ferrimagnetic cluster, when doped with six B or C atoms, transforms into a ferromagnetic cuboctahedral cluster with a magnetic moment that is an order of magnitude larger than that of the pure Mn(13) cluster. The origin of this magnetic transition is attributed to the change in the Mn-Mn interatomic distance resulting from the structural transformation. These doped clusters remain ferromagnetic with giant moments even after the removal of a B or C atom. However, similar doping with N atom does not lead to ferromagnetic ordering and Mn(13)N(6) remains ferrimagnetic with a magnetic moment of only 3 μ(B), just as in its parent Mn(13) cluster.

Revisiting magnetism of capped Au and ZnO nanoparticles: Surface band structure and atomic orbital with giant magnetic moment

AbstractIn this article we review the exotic magnetism of nanoparticles (NPs) formed by substances that are not magnetic in bulk as described with generality in Section 1. In particular, the intrinsic character of the magnetism observed on capped Au and ZnO NPs is analysed. X‐ray magnetic circular dichroism (XMCD) analysis has shown that the magnetic moments are intrinsic and lie in the Au and Zn atoms, respectively, as analysed in Section 2, where the general theoretical ideas are also revisited. Since impurity atoms bonded to the surface act as donor or acceptor of electrons that occupy the surface states, the anomalous magnetic response is analysed in terms of the surface band in Section 3. Finally, Section 4 summarizes our last theoretical proposal.

Giant magnetic moment of the core-shell Co13@Mn20 clusters: First-principles calculations

The core-shell clusters Co(13)@TM(20) with TM = Mn, Fe, Co, and Ni are investigated within first-principles simulations in the framework of density-functional theory. Huge magnetic moments have been found in the Co(13)@TM(20) clusters especially for the Co(13)@Mn(20) cluster with a giant magnetic moment of 113 μ(B). The large magnetic moments are mainly due to the special core-shell structure and the weak interaction between the TM and other atoms.

Metastable Giant Moments in Gd-Implanted GaN, Si, and Sapphire

We report on Gd ion implantation and magnetic characterization of GaN films on sapphire substrates and of bare sapphire and Si substrates to shed light on the mechanism underlying the induced magnetism upon Gd ion implantation. For all three hosts, giant magnetic moments per Gd ion were observed at temperatures of 5 through 300 K. The maximum moment per Gd in GaN was 1800μ B , while the moments in Gd-implanted Si and sapphire were only slightly smaller. The apparent induced ferromagnetic response was found to be metastable, disappearing after on the order of 50 days at room temperature, except for the implanted sapphire. We argue that our findings support a defect-based picture of magnetism in Gd-implanted semiconductors and insulators.

Engineering the magnetic properties of the Mn13cluster by doping

With a goal to produce giant magnetic moment in Mn$_{13}$ cluster which will be useful for practical applications, we have considered the structure and magnetic properties of pure Mn$_{13}$ cluster and substitutionally doped it with X = Ti, V, Cr, Fe, Co, Ni atom to produce Mn$_{12}$X clusters. We find that Ti and V substitutions in Mn$_{13}$ cluster are the most promising in terms of gaining substantial binding energy as well as achieving higher magnetic moment through ferromagnetic alignment of atom-centered magnetic moments. This has been demonstrated in terms of energetics and electronic properties of the clusters. For comparison, we have also studied the effect of N-capping of Mn$_{13}$ cluster, predicted in the earlier work [Phys. Rev. Lett. {\bf 89}, 185504 (2002)] as a means to produce stable giant magnetic moment in Mn clusters upto cluster size of 5 Mn atoms.

On the origin of the giant magnetic moment of the Al-Mn quasicrystals

Ab initio calculations of magnetic moments for icosahedral clusters contained in crystal structures Al10 Mn3 , Al5 Co2 , Al17 Mn4 (Al13 Cr4 Si4 -type) fulfilled in the framework of Density Functional Theory. The AlMn cluster having the trigonal D3h symmetry with the triangle of Mn ions in the interior has the moment being equal to three magnetic moments of a single manganese ion (4.4 μB ), the moment of the tetrahedral Td cluster with the Mn tetrahedron in the interior is equal approximately to twelve magnetic moments of the single manganese ion (15.5 μB ). The magnetic moment of icosahedral Al-Co clusters having the same configuration is equal to zero. The magnetic moments of the rod assembled from the icosahedral clusters with the sequence Td D3h - Td was found to be 20.5 μB . This value permits to explain the giant magnetic moment of icosahedral and decagonal Al-Mn quasicrystals and gives the indirect evidence to the hierarchical model of the quasicrystals structure proposed by the authors recently. An arrangement of magnetic moment carriers in the interior of the aluminum shell of icosahedral clusters permits to suggest the interaction between contacting manganese ions as the main origin of the giant magnetic moment of the Al-Mn quasicrystals.

Tuning Magnetic Properties of CeO2 by Fe Doping via an Electrochemical Deposition Route

Herein Ce(1-x)Fe(x)O(2-δ) nanocomposites were investigated for dilute magnetic semiconductor (DMS) properties. Ce(1-x)Fe(x)O(2-δ) nanospheres and porous nanostructures with high surface areas have been successfully prepared by electrochemical deposition at room temperature and atmospheric pressure. The structures and morphologies of Ce(1-x)Fe(x)O(2-δ) deposits were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and N(2) adsorption-desorption techniques. The magnetic properties of the prepared Ce(1-x)Fe(x)O(2-δ) nanospheres and porous nanostructures were studied, and they showed room-temperature ferromagnetism and giant magnetic moments. In addition, the effects of morphologies and compositions on the magnetic properties of Ce(1-x)Fe(x)O(2-δ) deposits were studied.

Giant magnetic moment enhancement of nickel nanoparticles embedded in multiwalled carbon nanotubes

We report a giant magnetic moment enhancement of ferromagnetic nickel nanoparticles (11 nm) embedded in multiwalled carbon nanotubes (MWCNTs). High-energy synchrotron x-ray diffraction experiment and chemical analysis are used to accurately determine the ferromagnetic nickel concentration. Magnetic measurements show that the room-temperature saturation magnetization of the nickel nanoparticles embedded in the MWCNTs is enhanced by a factor of about $3.4\ifmmode\pm\else\textpm\fi{}1.0$ as compared with what they would be expected to have for free nanoparticles. The giant moment enhancement is unlikely to be explained by a magnetic proximity effect but possibly arise from the interplay between ferromagnetism in nickel nanoparticles and strong diamagnetism in multiwalled carbon nanotubes.

Ce1−xCoxO2−δ Nanorods Grown by Electrochemical Deposition and Their Magnetic Properties

Magnetically doped CeO2 is a promising dilute magnetic semiconductor as CeO2 has a face-centered cubic system and a high dielectric constant and is a transparent rare-earth oxide. In this contribution we first report the synthesis of room temperature ferromagnetic Ce1−xCoxO2−δ (0<x < 0.10) nanorods prepared via electrodeposition route. These Ce1−xCoxO2−δ nanorods were thoroughly characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), optical transmission spectroscopy, and magnetic measurements. TEM measurements show these prepared Ce1−xCoxO2−δ nanorods are high crystallinity and single crystalline. The results of optical transmission spectra show the prepared Ce1−xCoxO2−δ nanorods are transparent in the visible regime. The magnetic measurements show the prepared Ce1−xCoxO2−δ nanorods exhibit ferromagnetic property with a high Curie temperature of about 870 K and a giant magnetic moment. In addition, the results in this paper show the ferromagnetism of the prepared Ce1−xCoxO2−δ nanorods can be obviously adjusted by the structural defects, indicating oxygen vacancies could be involved in magnetic coupling between Co ions. The magnetic properties of various Ce1−xCoxO2−δ nanostructures were also studied, and Ce1−xCoxO2−δ nanorods exhibit the larger Hc and Mr values and show promising dilute magnetic semiconductor materials.

Tuning magnetic properties of Mn4 cluster with gold coating

The magnetic properties of transition metal clusters are a unique function of their size and differ from their bulk behavior due to quantum confinement. Here we show that surface modification provides another channel to tune their magnetic properties. This is demonstrated by taking Mn(4) as an example. Although Mn(4) carries a giant magnetic moment of 20 micro(B), the magnetic coupling can be tuned from ferromagnetic to ferrimagnetic by changing the number of gold atoms coated on its surface. We found that 26 gold atoms are needed to fully cover a Mn(4) cluster. When partially coated, the system exhibits ferromagnetic coupling with a total magnetic moment of 18 micro(B) and it becomes ferrimagnetic with a moment of 8 micro(B) when fully coated. This magnetic cross-over is caused by the shrinking of the Mn-Mn bond length, suggesting that the magnetic properties of a Mn(4) cluster can be tuned by controlling the surface coverage.

Stabilizing a 22 karat nanogolden cage

Since the discovery of C(60) fullerene, considerable efforts have been devoted to find other elements with similar hollow cage structures. However, search for hollow metallic cages with a diameter similar to that of C(60) fullerene has been elusive. We describe a procedure for the rational design of metallic cages by suitably choosing their size, composition, and charge state. A 22 karat nanogolden cage with a diameter of about 8.5 A and consisting of 12 Al and 20 Au atoms is found to be metastable, which can be stabilized by embedding a Mn(4) cluster. In contrast to bulk Mn, which is antiferromagnetic, and isolated Mn(4) cluster, which is ferromagnetic with a giant magnetic moment of 20mu(B), the Mn(4)@Al(12)Au(20) endohedral complex exhibits magnetic bistability with 0mu(B) and 14mu(B) configurations being energetically nearly degenerate. These results, based on density functional theory, open the door to design a novel class of endohedral complexes with possible applications.

Room-temperature ferromagnetism in dielectric GaN(Gd)

We present an explanation of recently observed giant magnetic moment and room-temperature ferromagnetism in the dielectric GaN doped with Gd. Our approach uses the polarization mechanism of exchange interaction, which occurs if the d-level of Gd appears in the bandgap close to the valence band edge. Calculated ferromagnetic critical temperature and the value of the magnetic moment well correspond to experimental findings.

Metastable states in the triangular-lattice Ising model studied by Monte Carlo simulations: Application to the spin-chain compoundCa3Co2O6

It is known that the spin-chain compound Ca3Co2O6 exhibits very interesting plateaus in the magnetization as a function of the magnetic field at low temperatures. The origin of them is still controversial. In this paper we study the thermal behavior of this compound with a single-flip Monte Carlo simulation on a triangular lattice and demonstrate the decisive influence of metastable states in the splitting of the ferrimagnetic 1/3 plateau below 10 K. We consider the [Co2O6]n chains as giant magnetic moments described by large Ising spins on planar clusters with open boundary conditions. With this simple frozen-moment model we obtain stepped magnetization curves which agree quite well with the experimental results for different sweeping rates. We describe particularly the out-of-equilibrium states that split the low-temperature 1/3 plateau into three steps. They relax thermally to the 1/3 plateau, which has long-range order at the equilibrium. Such states are further analyzed with snapshots unveiling a domain-wall structure that is responsible for the observed behavior of the 1/3 plateau. A comparison is also given of the exact results in small triangular clusters with our Monte Carlo results, providing further support for our thermal description of this compound.