High Spin Moment Research Articles

Diminution of surface spin disorder in a ferromagnet via proximity effect of single-molecule magnets

Abstract Field-dependent resistance of a magnetic layer is often used as the basic tunable operational entity in several spintronics applications. In this work we demonstrate that the magnetoresistance (MR) of a classic ferromagnetic Ni layer can be tuned significantly by controlling the surface spin scattering of electrons, by manipulating disordered surface spins via proximity effect. This was achieved by drop casting molecules of a well-known single molecule magnet Mn 12-ac on micro-tracks of a thin (~12 nm) Ni film. We found significant changes in the width and sign of the magnetoresistance of the Ni micro-tracks. This is explained in the light of possible locking of the disordered surface spins of the Ni layer with the high spin moment of adjacent Mn 12-ac molecules, causing reduction in the surface spin scattering. As expected, the spin moments of Mn 12-ac molecules were found to effectively pin the surface spins only below the blocking temperature of Mn 12-ac. Above the blocking temperature, the Mn 12-ac molecules had no effect on the MR behavior of Ni micro-tracks. This observation offers a simple route to achieve on-demand low or high resistance states of a magnetic layer, very relevant to the field of spintronics.

Critical behavior, structural, electronic, and magnetic properties of the Heusler alloy: CoFeCrP

In this work, we study the equiatomic quaternary Heusler alloy (EQHA) CoFeCrP using two methods: density functional theory (DFT) and Monte Carlo simulations. The DFT method allowed us to illustrate the structural, electronic, and magnetic properties of this alloy. The ground state phase diagrams have been presented to show the stable configurations in different physical parameter planes. On the other hand, the Monte Carlo simulations, performed under the Metropolis algorithm, permitted to deduce the critical the behavior of the EQHA CoFeCrP. The structural properties results show that the phase of type I, of this alloy is the most stable configuration. In addition, the band structures and density of states calculations results show that this compound exhibits a half-metallic character with a 100% of spin polarization (SP) at the Fermi-level. The total magnetic moment of the Heusler compound is found to be 4.00 µB. Moreover, it is found that the Slater-Pauling is well described for this alloy. Our results show that this material is a potential candidate for the spintronic applications. This is due to its half-metallicity, its high spin moments, its complete SP polarization, and its high Curie temperature.

Nickelate superconductors: Multiorbital nature and spin freezing

Superconductivity with a remarkably high $T_c$ has recently been found in Sr-doped NdNiO$_2$ thin films. While this system bears strong similarities to the cuprates, some differences, such as a weaker antiferromagnetic exchange coupling and possible high-spin moments on the doped Ni sites have been pointed out. Here, we investigate the effect of Hund coupling and crystal field splitting in a simple model system and argue that a multiorbital description of nickelate superconductors is warranted, especially in the strongly hole-doped regime. We then look at this system from the viewpoint of the spin-freezing theory of unconventional superconductivity, which provides a unified understanding of unconventional superconductivity in a broad range of compounds. Sr$_{0.2}$Nd$_{0.8}$NiO$_2$ falls into a parameter regime influenced by two spin-freezing crossovers, one related to the emergent multi-orbital nature in the strongly doped regime and the other related to the single-band character and square lattice geometry in the weakly doped regime.

Syntheses and Characterization of Diammine-Nickel/Cobalt(II)-Bisdicyanamide M(NH3)2[N(CN)2]2 with M = Ni and Co.

Crystals of M(NH3)2[N(CN)2]2 with M = Ni and Co were obtained from the reaction of stoichiometric amounts of Na[N(CN)2] with NiCl2·6H2O or CoCl2·6H2O in aqueous, ammoniacal solutions. X-ray single-crystal structure analyses show that M(NH3)2[N(CN)2]2 with M = Ni and Co crystallize isotypically to each other and adopt the monoclinic space group P21/ c (no. 14). The lattice parameters of Ni(NH3)2[N(CN)2]2 are a = 5.8498(9) Å, b = 10.6739(12) Å, and c = 6.8089(17) Å, β = 98.037(3)° and Z = 2, while those of Co(NH3)2[N(CN)2]2 are a = 5.8303(11) Å, b = 10.746(2) Å, c = 6.7773(13) Å, and β = 96.422(3)°. In addition, the crystal structure of the nickel compound was refined from neutron powder diffraction, augmented by DFT calculations as regards atomic displacement parameters. The IR spectra of the title compounds exhibit modes typical for the dicyanamide anion and ammonia. The UV/vis spectrum of Ni(NH3)2[N(CN)2]2 shows that the dicyanamide moiety is a medium-field ligand. Additional superconducting quantum interference device (SQUID) magnetic susceptibility measurements of Ni(NH3)2[N(CN)2]2 and Co(NH3)2[N(CN)2]2 confirm not only significant high-spin moments of χm T = 1.24 cm3·K·mol-1 (μeff = 3.15 μB) and 2.89 cm3·K·mol-1 (μeff = 4.81 μB), respectively, at 290 K and 0.1 T but also an absence of magnetic ordering.

Large intrinsic magnetization in an epitaxial BiFeO3/NdGaO3system

We report a comparative study of the crystal structure and magnetic properties between (001) oriented epitaxial films grown on and substrates without any magnetic parasitic phase. The 15 nm film deposited on a substrate shows substantially high magnetization as compared to an identically thick film deposited on a substrate. Detailed structural analysis revealed that the high magnetization is associated with a monoclinic (Cm) like distortion of the crystal structure. We corroborated our experimental findings with first principles density functional theory calculations which suggested a saturation magnetization (266 emu/cc) close to the experimentally obtained one and a ferromagnetic ground state with the high spin moment in the Cm phase.

Synthesis of birnessite with adjustable electron spin magnetic moments for the degradation of tetracycline under microwave induction

Manganese dioxides (MOs) are excellent microwave absorbing materials and have been used as substrates or catalysts for enhanced degradation of organic compounds. In this study, we extended our research on using birnessite (Birn), a type of MOs, for the degradation of tetracycline (TC) in aqueous solution under microwave induction (MI). The Birns were fabricated to achieve different ratios of Mn(II)/Mn(III)/Mn(IV) in the crystal structure. Their physical and chemical characteristics were determined by X-ray diffraction, field-emission environmental scanning electron microscopy, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption fine structure spectra, and a microwave network analyzer. Their removal of TC under MI was evaluated under different physicochemical conditions. Lower solution pH, higher MI power, and higher Mn(II) and Mn(III) in the structure resulted in more TC removal. And the Birns are regenerable. LC–MS analyses confirmed the presence of degradation intermediates. The better performances of TC removal by Birns under MI was attributed to the high magnetic spin moment of Mn(II) and Mn(III) in the structure that has higher MI response.

Spin-phonon coupling in scandium doped gallium ferrite

We embarked on a study of Scandium (Sc) doped (onto Ga site) gallium ferrite (GaFeO3) and found remarkable magnetic properties. In both doped as well as parent compounds, there were three types of Fe3+ ions (depending on the symmetry) with the structure conforming to space group Pna21 (Sp. Grp. No. 33) below room temperature down to 5 K. We also found that all Fe3+ ions occupy octahedral sites, and carry high spin moment. For the higher Sc substituted sample (Ga1−xScxFeO3: x = 0.3), a canted magnetic ordered state is found. Spin-phonon coupling below Néel temperature was observed in doped compounds. Our results indicated that Sc doping in octahedral site modifies spin-phonon interactions of the parent compound. The spin-phonon coupling strength was estimated for the first time in these Sc substituted compounds.

Spin-crossover thermal hysteresis and light-induced effect on iron(II) complexes with tripodal tris(2-pyridyl)methanol

The X-ray crystal structure of Me4N[Fe(py3COH)(NCS)3](1-PrOH)0.5(H2O)2 [1; py3COH stands for tris(2-pyridyl)methanol] was successfully determined, clarifying that the complex has a pseudo-trigonal molecular arrangement, though the space group was P1¯. Possible π–π interaction is operative between the pyridine rings from neighboring [Fe(py3COH)(NCS)3]− ions. After annealing of 1 at 400K, the resultant compound was characterized to be Me4N[Fe(py3COH)(NCS)3](H2O) (1′), which showed a relatively abrupt spin-crossover (SCO) with thermal hysteresis. Combining the differential thermal analysis results, the SCO temperatures were determined as T1/2↓=224K and T1/2↑=246K. Compound 1′ also showed light-induced excited spin-state trapping (LIESST), as indicated with the χmT increased to a 32% level of the high-spin moment upon Xe-lamp irradiation at 5K. The LIESST effect disappeared at around 26K on heating. The complex ion [Fe(py3COH)(NCS)3]− is a rare anionic SCO and LIESST building block.

Density functional study of transition-metal-encapsulated Si10C10H20 cage-like clusters

Using the density functional theory with generalized gradient approximation, we have studied the geometries, stabilities, and electronic and magnetic properties of 3d transition-metal (TM)-encapsulated hydrogenated Si10C10H20 cage-like clusters. The hydrogenated empty Si10C10H20 cage is very stable with large highest-occupied–lowest-unoccupied molecular orbital (HOMO–LUMO) gap. Endohedral doping of the empty cage with different 3d TM atoms shows that doping can be used to manipulate the HOMO–LUMO gap of the cage. Most interestingly, the TM-doped Si10C10H20 cage can conserve the high spin moment of the TM atom due to weak interaction between the TM atom and the cage. The electronic properties of TM-doped Si10C10H20 cages are characterized by electron transfer from the host empty Si10C10H20 cage to the guest doping TM atoms. First-principles molecular dynamics simulations have shown that the TM-doped Si10C10H20 cages have good thermal stability.

Magnetic Silicon Nanotube: Role of Encapsulated Europium Atoms

A europium-encapsulating silicon nanotube, Eu2@Si30, has been predicted by density functional theory. Electronic structure analysis shows that in Si–Si chemical bonds there exist sp2-like hybridizations induced by the europium atoms, and the hybridizations evidently enhance the stability of the silicon nanotube. Moreover, the nanotube of Eu2@Si30, with D5h symmetry, retains a high spin moment of 10 μB. On the basis of the Eu2Si30Tube, a stable magnetic silicon nanotube (SiNT) was obtained, and it is found to be metallic. Similar to the predictions and speculation of Daedalus, the new magnetic SiNT may have potential applications in the fields of spintronics and high-density magnetic storage.

MSi20H20 Aggregates: From Simple Building Blocks to Highly Magnetic Functionalized Materials

Density-functional theory based global geometry optimization is used to scrutinize the possibility of using endohedrally-doped hydrogenated Si clusters as building blocks for constructing highly magnetic materials. In contrast to the known clathrate-type facet-sharing, the clusters exhibit a predisposition to aggregation through double Si-Si bridge bonds. For the prototypical CrSi$_{20}$H$_{20}$ cluster we show that reducing the degree of hydrogenation may be used to control the number of reactive sites to which other cages can be attached, while still preserving the structural integrity of the building block itself. This leads to a toolbox of CrSi$_{20}$H$_{20-2n}$ monomers with different number of double "docking sites", that allows building network architectures of any morphology. For (CrSi$_{20}$H$_{18}$)$_{2}$ dimer and [CrSi$_{20}$H$_{16}$](CrSi$_{20}$H$_{18}$)$_{2}$ trimer structures we illustrate that such aggregates conserve the high spin moments of the dopant atoms and are therefore most attractive candidates for cluster-assembled materials with unique magnetic properties. The study suggests that the structural completion of the individual endohedral cages within the doubly-bridge bonded structures and the high thermodynamic stability of the obtained aggregates are crucial for potential synthetic polymerization routes $via$ controlled dehydrogenation.

Co/Ni(111) superlattices studied by microscopy, x-ray absorption, andab initiocalculations

We explore the origins of perpendicular magnetic anisotropy in epitaxial and textured Co/Ni(111) superlattices using a combination of thin-film growth, structural characterization, x-ray magnetic circular dichroism (XMCD), and ab initio calculations. Transmission electron microscopy and x-ray diffraction experiments allow us to show that the bulk magnetoelastic contribution to the total magnetic energy is small compared to the interface anisotropy. The magnetic properties are studied by using XMCD at the Co and Ni L 2,3 edges. Hysteresis loops performed at the Co L 3 edge confirm the perpendicular magnetization for Co thicknesses up to four monolayers. The spin and orbital moments were deduced using the XMCD sum rules. The results are explained by considering two kinds of magnetic moments for Co, distinguishing the interfaces from the rest of the layers. Both effective spin and orbital moments of Co atoms are found to be enhanced at the Co-Ni interfaces, whereas the magnetic moment of Co surrounded by Co is similar to the bulk value. Ab initio calculations allow us to show a strong enhancement of the dipole operator contribution on Co atoms at the interface that is partly responsible for this high effective spin moment at the interface. Such a moment enhancement is not observed for Ni, with the dipole operator contribution being close to zero. Finally, we observed a very surprising proportionality between the effective spin and orbital moments, independent of the absorption edge or deposition technique used. We assign this peculiar behavior to the fact that the magnetic dipole operator involved in the sum rules is closely linked to the increase of the Co orbital moment at the interface. Based on XMCD results obtained on both molecular beam epitaxy and sputter-deposited samples, this link allows us to show the extreme sensitivity of the perpendicular anisotropy with the chemical ordering at the interface.

Evaluation of endohedral doping of hydrogenated Si fullerenes as a route to magnetic Si building blocks

Density-functional theory based global geometry optimization is used to scrutinize the possibility of endohedral doping of hydrogenated Si fullerenes as a route to Si nanostructures with high magnetic moments. In contrast to previous suggestions, our unbiased sampling finds the smallest Si16H16 endohedral cage generally too small to encapsulate 3d metal dopant atoms. For the next larger fullerene-like cage though, we identify perfectly symmetric MSi20H20 (M = Co, Ti, V, Cr) cage structures as ground states. These structures conserve the high spin moment of the dopant atom and therewith underscore the potential of this Si nanoform for novel cluster-based materials with unique magnetic properties.

Nonmagnetic and magnetic thiolate-protected Au25superatoms on Cu(111), Ag(111), and Au(111) surfaces

Geometry, electronic structure, and magnetic properties of methylthiolate-stabilized Au${}_{25}$L${}_{18}$ and MnAu${}_{24}$L${}_{18}$ (L = SCH${}_{3}$) clusters adsorbed on noble-metal (111) surfaces have been investigated by using spin-polarized density functional theory computations. The interaction between the cluster and the surface is found to be mediated by charge transfer mainly from or into the ligand monolayer. The electronic properties of the 13-atom metal core remain in all cases rather undisturbed as compared to the isolated clusters in gas phase. The Au${}_{25}$L${}_{18}$ cluster retains a clear highest occupied molecular orbital/lowest unoccupied molecular orbital energy gap in the range of 0.7 to 1.0 eV depending on the surface. The ligand layer is able to decouple the electronic structure of the magnetic MnAu${}_{24}$L${}_{18}$ cluster from the Au(111) surface, retaining a high local spin moment of close to 5 ${\ensuremath{\mu}}_{B}$ arising from the spin-polarized Mn($3d$) electrons. These computations imply that the thiolate monolayer-protected gold clusters may be used as promising building blocks with tunable energy gaps, tunneling barriers, and magnetic moments for applications in the area of electron and/or spin transport.

Kondo effects in a triangular triple quantum dot with lower symmetries

We study the low-energy properties and characteristic Kondo energy scale of a triangular triple quantum dot, connected to two non-interacting leads, in a wide parameter range of a gate voltage and distortions which lower the symmetry of an equilateral structure, using the numerical renormalization group approach. For large Coulomb interactions, the ground states with different characters can be classified according to the plateaus of $\ensuremath{\Theta}\ensuremath{\equiv}({\ensuremath{\delta}}_{\mathrm{e}}\ensuremath{-}{\ensuremath{\delta}}_{\mathrm{o}})(2/\ensuremath{\pi})$, where ${\ensuremath{\delta}}_{\mathrm{e}}$ and ${\ensuremath{\delta}}_{\mathrm{o}}$ are the phase shifts for the even and odd partial waves. At these plateaus of $\ensuremath{\Theta}$, both $\ensuremath{\Theta}$ and the occupation number ${N}_{\mathrm{tot}}\ensuremath{\equiv}({\ensuremath{\delta}}_{\mathrm{e}}+{\ensuremath{\delta}}_{\mathrm{o}})(2/\ensuremath{\pi})$ take values close to integers, and thus the ground states can be characterized by these two integers. The Kondo effect with a local moment with total spin $S=1$ due to a Nagaoka mechanism appears on the plateau, which can be identified by $\ensuremath{\Theta}\ensuremath{\simeq}2.0$ and ${N}_{\mathrm{tot}}\ensuremath{\simeq}4.0$. For large distortions, however, the high-spin moment disappears through a singlet-triplet transition occurring within the four-electron region. It happens at a crossover to the adjacent plateaus for $\ensuremath{\Theta}\ensuremath{\simeq}0.0$ and $\ensuremath{\Theta}\ensuremath{\simeq}4.0$, and the two-terminal conductance has a peak in the transient regions. For weak distortions, the SU(4) Kondo effect also takes place for ${N}_{\mathrm{tot}}\ensuremath{\simeq}3.0$. It appears as a sharp conductance valley between the $S=1/2$ Kondo ridges on both sides. We also find that the characteristic energy scale ${T}^{*}$ reflect these varieties of the Kondo effect. Particularly, ${T}^{*}$ is sensitive to the distribution of the charge and spin in the triangular triple dot.

Magnetic silicon fullerene

A metal-encapsulating silicon fullerene, Eu@Si(20), has been predicted by density functional theory to be by far the most stable fullerene-like silicon structure. The Eu@Si(20) structure is a dodecahedron with D(2h) symmetry in which the europium atom occupies the center site. The calculated results show that the europium atom has a large magnetic moment of nearly 7.0 Bohr magnetons. In addition, it was found that a stable "pearl necklace" nanowire, constructed by concatenating a series of Eu@Si(20) units, with the central europium atom, retains the high spin moment. The magnetic structure of the nanowire indicates potential applications in the fields of spintronics and high-density magnetic storage.

Antiphase magnetic boundaries in iron-based superconductors: A first-principles density-functional theory study

Superconductivity arises in the layered iron-pnictide compounds when magnetic long range order disappears. We use first principles density functional methods to study magnetic arrangements that may compete with long range order near the phase boundary. Specifically, we study the energetics and charge density distribution (through calculation of the electric field gradients) for ordered supercells with varying densities of antiphase magnetic boundaries. We quantify the amount by which Fe atoms with low spin moments at the antiphase boundaries have higher energies than Fe atoms with high spin moments away from the antiphase boundaries. These disruptions in magnetic order should be useful in accounting for experimental data such as electric field gradients and hyperfine fields on both Fe and As atoms.

Small [formula omitted] clusters: Magnetic order and magnetic moment

The effects of a manganese atom on the magnetic order and magnetic moments of Fe n Mn ( n = 1 – 12 ) clusters have been investigated using the all-electron density functional theory. The results reveal that the Fe–Mn couplings in the lowest-energy structures of Fe n Mn ( n = 1 – 12 ) clusters undergo a change from ferromagnetic ordering for the smallest ( n = 1 , 2 ) clusters to ferrimagnetic ordering for the intermediate ( n = 3 – 6 ) clusters. Starting from n = 7 , however, ferromagnetic Fe–Mn couplings completely prevail. The low coordination number of the doped Mn atom results in it having a high spin moment in the Fe–Mn binary clusters, which exhibits a marked magnetic moment surface enhancement effect.

Enhanced ferromagnetism in ZnO nanoribbons and clusters passivated with sulfur

Inspired by recent experimental results, the electronic and magnetic properties of sulfur-passivated ZnO clusters and zigzag nanoribbons have been studied using first principles calculations in the framework of the local spin density approximation. In the case of the ZnO nanoribbons, the sulfur atoms or thiol groups were attached in different ways to the zinc or oxygen atoms located at the edges, whereas in clusters, the sulfur atoms were set on the surface, mainly interacting with atoms with low-coordinate number. After an exhaustive atomic relaxation, we found that a magnetic moment emerges in zigzag nanoribbons both with and without sulfur-passivation on the edges. However, the magnitude of the magnetic moment is very sensitive to sulfur passivation. In particular, we found that when sulfur is attached to the zinc atoms in an alternating fashion along the ribbon edges, the magnetic moment is a maximum (1.4 µB/unit cell). In the case of clusters, we found that the Zn15O15 cluster exhibits a high spin moment of 5.5 µB when capped with sulfur atoms. Our calculations indicate that sulfur-passivating of ZnO nanosystems could be responsible for recently observed ferromagnetic responses.

Design of new ferromagnetic materials with high spin moments by first-principlescalculation

We have searched for a new highly spin-polarized ferromagnet which has ahigher spin moment than that of known half-metallic transition metal pnictideswith the zinc-blende structure by first-principles calculations. To generate thehigh spin moment we focus on Gd compounds. Our calculation shows that a(GdN)1/(CrAs)1 structure is a ferromagnetic material. The total magnetic moment of this ferromagnet is over9.9 µB per chemical formula.