Magnetovolume instabilities and ferromagnetism versus antiferromagnetism in bulk fcc iron and manganese

Discovery and Implications of Hidden Atomic-Scale Structure in a Metallic Meteorite

Iron and its alloys have made modern civilization possible, with metallic meteorites providing one of the human's earliest sources of usable iron as well as providing a window into our solar system's billion-year history. Here highest-resolution tools reveal the existence of a previously hidden FeNi nanophase within the extremely slowly cooled metallic meteorite NWA 6259. This new nanophase exists alongside Ni-poor and Ni-rich nanoprecipitates within a matrix of tetrataenite, the uniaxial, chemically ordered form of FeNi. The ferromagnetic nature of the nanoprecipitates combined with the antiferromagnetic character of the FeNi nanophases gives rise to a complex magnetic state that evolves dramatically with temperature. These observations extend and possibly alter our understanding of celestial metallurgy, provide new knowledge concerning the archetypal Fe-Ni phase diagram and supply new information for the development of new types of sustainable, technologically critical high-energy magnets.

Magnetic behavior of metastable Fe films grown on Ir(1 1 1)

We investigated the growth of ultra-thin Fe films on Ir(1 1 1) by means of in situ low energy electron diffraction and spin-resolved photoemission techniques. We observe a (1  ×  1) diffraction pattern, characteristic of the fcc substrate, below four monolayers (ML). Then, a complex superstructure starts to develop, compatible with the formation of bcc-like Fe domains aligned with the substrate according to the Kourdjumov-Sachs orientation relationships. The analysis of the diffraction patterns reveals a progressive evolution towards a fully relaxed bcc lattice, characteristic of bulk Fe. Both photoemission (filled states) and inverse photoemission (empty states) results show characteristic features related to the contribution of the Fe layer, evolving towards those observed on the Fe (1 1 0) bcc surface. Spin resolution allows to detect a spectral polarization above 4 ML, corresponding to the formation of bcc Fe, which gradually increases indicating the formation of an in-plane magnetized ferromagnetic layer in thick films. No in-plane net magnetization is detected in thinner films, independent of the sample temperature down to 30 K. Following recent investigations on the Fe/Ir(1 1 1) system with microscopy techniques, we link this observation to the stabilization of a non collinear spin structure yielding an overall nil magnetization.

Structure and magnetism in Cr-embedded Co nanoparticles

We present the results of an investigation into the atomic structure and magnetism of 2 nm diameter Co nanoparticles embedded in an antiferromagnetic Cr matrix. The nanocomposite films used in this study were prepared by co-deposition directly from the gas phase, using a gas aggregation source for the Co nanoparticles and a molecular beam epitaxy (MBE) source for the Cr matrix material. Co K and Cr K edge extended x-ray absorption fine structure (EXAFS) experiments were performed in order to investigate atomic structure in the embedded nanoparticles and matrix respectively, while magnetism was investigated by means of a vibrating sample magnetometer. The atomic structure type of the Co nanoparticles is the same as that of the Cr matrix (bcc) although with a degree of disorder. The net Co moment per atom in the Co/Cr nanocomposite films is significantly reduced from the value for bulk Co, and decreases as the proportion of Co nanoparticles in the film is decreased; for the sample with the most dilute concentration of Co nanoparticles (4.9% by volume), the net Co moment was 0.25 μ B/atom. After field cooling to below 30 K all samples showed an exchange bias, which was largest for the most dilute sample. Both the structural and magnetic results point towards a degree of alloying at the nanoparticle/matrix interface, leading to a core/shell structure in the embedded nanoparticles consisting of an antiferromagnetic CoCr alloy shell surrounding a reduced ferromagnetic Co core.

Carrier-mediated magnetoelectric coupling in functionalized graphene

Materials in which magnetic order and electric fields can be coupled are of high fundamental and technological interests. Electrical control of magnetism is not only important for ultralow power consumption applications, but also enables control over intrinsic material properties that may have a major step in new developments in spintronic and magnetoelectric devices. Here we show that the magnetism induced by aryl-radicals covalently functionalized on top of multilayer graphene is sensitive to external electric fields which coupled to the interlayer charge-imbalance yields a strong magnetoelectric coupling. We used first-principles simulations, taking into account van der Waals dispersion forces, to show that this effect is thickness-dependent: it increases dramatically to thicker graphene structures reaching magnetoelectric coefficients comparable to perovskite interfaces. The interplay between electric fields and magnetism also leads functionalized graphene layers to a fully polarized spin state (half-metallicity). Efficiency nearly to 100% spin-polarization is observed at low electric bias, and the selection of the spin-conducting channel is determined by the field polarization.

Structure and magnetism in Fe/FexPd1-x core/shell nanoparticles formed by alloying in Pd-embedded Fe nanoparticles

We have investigated atomic structure and magnetism in Fe nanoparticles with a diameter of 2 nm embedded in a Pd matrix. The samples for these studies were prepared directly from the gas phase by co-deposition, using a gas aggregation source and an MBE-type source for the Fe nanoparticles and Pd matrix respectively. Extended absorption fine structure (EXAFS) measurements indicate that there is an appreciable degree of alloying at the nanoparticle/matrix interface; at dilute nanoparticle concentrations, more than half of the Fe atoms are alloyed with Pd. This leads to a core/shell structure in the embedded nanoparticles, with an FexPd1-x shell surrounding a reduced pure Fe core. Magnetism in the nanocomposite samples was probed by means of magnetometry measurements, which were interpreted in the light of their atomic structure. These point to a magnetized cloud of Pd atoms surrounding the embedded nanoparticles which is significantly larger than around single Fe atoms in Pd. The coercivities in the Fe/Pd nanocomposite samples are larger than in FexPd1-x atomic alloys of corresponding composition, which is consistent with exchange coupling between the magnetically harder and softer regions in the nanocomposite samples.

Realizing high magnetic moments in fcc Fe nanoparticles through atomic structure stretch

We describe the realization of a high moment state in fcc Fe nanoparticles through a controlled change in their atomic structure. Embedding Fe nanoparticles in a Cu(1-x)Au(x) matrix causes their atomic structure to switch from bcc to fcc. Extended x-ray absorption fine structure (EXAFS) measurements show that the structure in both the matrix and the Fe nanoparticles expands as the amount of Au in the matrix is increased, with the data indicating a tetragonal stretch in the Fe nanoparticles. The samples were prepared directly from the gas phase by co-deposition, using a gas aggregation source and MBE-type sources respectively for the nanoparticle and matrix materials. The structure change in the Fe nanoparticles is accompanied by a sharp increase in atomic magnetic moment, ultimately to values of ~2.5 ± 0.3 μ(B)/atom .

Magnetic Properties of FEXMN1-X/IR(100) Superlattices

We have produced pseudomorphic FexMn1-x/Ir(100) superlattices having different stoichiometry (0.8 ≥ x ≥ 0.3). The alloy crystalline structure is body centered tetragonal with a c/a ratio between 1.18 and 1.26. Iron rich alloys are ferromagnetic when the corresponding bulk alloys are antiferromagnetic. Manganese rich alloys are certainly antiferromagnetic according to bulk magnetization measurements and Mössbauer effect results. The transition from a ferromagnet with a vanishing moment when x = 0.5 to an antiferromagnet is associated with a volume expansion.

Topological catastrophe and isostructural phase transition in calcium

We predict a quantum phase transition in fcc Ca under hydrostatic pressure. Using density functional theory, we find, at pressures below 80 kbar, the topology of the electron charge density is characterized by nearest neighbor atoms connected through bifurcated bond paths and deep minima in the octahedral holes. At pressures above 80 kbar, the atoms bond through non-nuclear maxima that form in the octahedral holes. This topological change in the charge density softens the C' elastic modulus of fcc Ca, while C44 remains unchanged. We propose an order parameter based on applying Morse theory to the charge density, and we show that near the critical point it follows the expected mean-field scaling law with reduced pressure.

Symmetry-induced collapse of ferromagnetism at the α-ε phase transition in iron

The magnetostructural bcc-to-hcp phase transition in iron is analysed theoretically in the framework of the Landau theory of phase transitions. In contrast to recent interpretations which emphasize the driving role of magnetism at the transition, the collapse of the ferromagnetic order in ε-Fe is interpreted as resulting from the large spontaneous strains and the magnitude of the displacive order-parameter involved in the Burgers reconstructive transition mechanism. It yields a direct first-order transition from the ferromagnetic α-phase to the non-magnetic ε-phase, without going across an intermediate magnetic structure.

Magnetism in bcc and fcc Fe with carbon and manganese

Density functional theory calculations were performed to study the structure and magnetic properties of bcc (α) and fcc (γ) Fe with 3 at.% carbon and manganese impurities. We find that all bcc-based Fe, Fe-C and Fe-Mn-C phases exhibit a ferromagnetic (FM) ground state, while the antiferromagnetic double-layer (AFMD) state is lowest in energy within the collinear spin approach in fcc Fe, Fe-C and Fe-Mn-C phases. However, the carbon and manganese impurities affect the local magnetic interactions significantly. The states with opposite manganese magnetic moments are quasi-degenerate in bcc Fe-Mn alloy, whereas octa-site carbon stabilizes ferromagnetic coupling of the nearest manganese atom with the Fe host. We demonstrate that the antiferromagnetic (AFM) fcc Fe-C and Fe-Mn-C alloys are intrinsically inhomogeneous magnetic systems. Carbon frustrates the local magnetic order by reorientation of magnetic moments of the nearest Mn and Fe atoms, and favors their ferromagnetic coupling. The competition between ferromagnetic and antiferromagnetic Fe-Fe and Fe-Mn interactions and the local magnetovolume instability near carbon may give rise to the spin-glass-like regions observed in austenitic Fe-Mn-C alloys.

Coupling between magnetic and optical properties of stable Au-Fe solid solution nanoparticles

Au-Fe nanoparticles constitute one of the simplest prototypes of a multifunctional nanomaterial that can exhibit both magnetic and optical (plasmonic) properties. This solid solution, not feasible in the bulk phase diagram in thermal equilibrium, can be formed as a nanostructure by out-of-equilibrium processes. Here, the novel magnetic, optical and magneto-optical properties of ion-implanted Au-Fe solid solution nanoparticles dispersed in a SiO(2) matrix are investigated and correlated. The surface plasmon resonance of the Au-Fe nanoparticles with almost equicomposition is strongly damped when compared to pure Au and to Au-rich Au-Fe nanoparticles. In all cases, the Au atoms are magnetically polarized, as measured by x-ray magnetic circular dichroism, and ferromagnetically coupled with Fe atoms. Although the chemical stability of Au-Fe nanoparticles is larger than that of Fe nanoparticles, both the magnetic moment per Fe atom and the order temperature are smaller. These results suggest that electronic and magnetic properties are more influenced by the hybridization of the electronic bands in the Au-Fe solid solution than by size effects. On the other hand, the magneto-optical transitions allowed in the vis-nIR spectral regions are very similar. In addition, we also observe, after studying the properties of thermally treated samples, that the Au-Fe alloy is stabilized, not by surface effects, but by the combination of the out-of-equilibrium nature of the ion implantation technique and by changes in the properties due to size effects.

Magnetic states of transition metal impurities in silicon carbide

Electronic properties of 3C silicon carbide with substitutional transition metal impurities are calculated using an ab initio full-potential linearized augmented plane wave technique. It is shown that transition metal atoms in an SiC host may exist in both magnetic and nonmagnetic states. For different impurity species, the transition from the nonmagnetic to the magnetic state and the corresponding change of the atomic magnetic moments may take place either gradually as the volume of the unit cell changes or when the energy gap between the two states is overcome. The magnetic and nonmagnetic solutions are characterized by significantly different unit cell geometries, which are analysed in detail. Magnetic ordering is studied for a number of transition metal impurities in SiC and mean-field estimates of the Curie temperature are given. Calculated densities of states are used to analyse the nature of the exchange interaction between the impurities. Some properties of transition metal impurities are compared for 3C and 4H silicon carbide substitution.

Extended x-ray absorption fine structure studies of the atomic structure of nanoparticles in different metallic matrices

It has been appreciated for some time that the novel properties of particles in the size range 1-10 nm are potentially exploitable in a range of applications. In order to ultimately produce commercial devices containing nanosized particles, it is necessary to develop controllable means of incorporating them into macroscopic samples. One way of doing this is to embed the nanoparticles in a matrix of a different material, by co-deposition for example, to form a nanocomposite film. The atomic structure of the embedded particles can be strongly influenced by the matrix. Since some of the key properties of materials, including magnetism, strongly depend on atomic structure, the ability to determine atomic structure in embedded nanoparticles is very important. This review focuses on nanoparticles, in particular magnetic nanoparticles, embedded in different metal matrices. Extended x-ray absorption fine structure (EXAFS) provides an excellent means of probing atomic structure in nanocomposite materials, and an overview of this technique is given. Its application in probing catalytic metal clusters is described briefly, before giving an account of the use of EXAFS in determining atomic structure in magnetic nanocomposite films. In particular, we focus on cluster-assembled films comprised of Fe and Co nanosized particles embedded in various metal matrices, and show how the crystal structure of the particles can be changed by appropriate choice of the matrix material. The work discussed here demonstrates that combining the results of structural and magnetic measurements, as well as theoretical calculations, can play a significant part in tailoring the properties of new magnetic cluster-assembled materials.

The role of surface stress in structural transitions, epitaxial growth and magnetism on the nanoscale

We review results on combined stress measurements by the crystal curvature technique and structural investigations on different nanoscale systems. It is shown that stress measurements offer highly sensitive and accurate data which identify even subtle structural changes in the sub-monolayer coverage regime. We discuss the unique potential of stress measurements to complement structural investigations of atomic layers and at surfaces. Our examples reveal that stress measurements enhance, support and clarify the interpretation of quantitative structural data. The role of surface stress and film stress for structural transitions in epitaxial growth, surfactant-mediated growth, surface reconstruction and adsorbate-induced spin reorientation transitions in monolayers is discussed.

Surfactant-assisted epitaxial growth and magnetism of Fe films on Cu(111)

The magnetic properties of thin epitaxial layers of Fe grown on Cu(111) depend sensitively on the films' structure and morphology. A combination of experiments and numerical simulations reveals that the use of a surfactant monolayer (ML) of Pb during molecular beam epitaxy (MBE) growth at room temperature reduces the amount of interdiffusion at the Cu-Fe interface, retards the fcc-to-bcc transformation by about 2 ML and substantially increases the films' coercivity. The origin of all these alterations to the magnetic behavior can be traced back to the structural modifications provoked by the surfactant during the early growth stages. These results open the way for the controlled fabrication of custom-designed materials with specific magnetic characteristics.

Topology of the spin-polarized charge density in bcc and fcc iron

We report the first investigation of the topology of spin-polarized charge density, specifically in bcc and fcc iron. While the total spin-density is found to possess the topology of the non-magnetic prototypical structures, the spin-polarized charge densities of bcc and high-spin fcc iron are atypical. In these cases, the two spin densities are correlated: the spin-minority electrons have directional bond paths and deep minima, while the spin-majority electrons fill these holes, reducing bond directionality. The presence of distinct spin topologies allows us to show that the two phase changes seen in fcc iron (paramagnetic to low-spin and low-spin to high-spin) are different. The former follows the Landau symmetry-breaking paradigm and proceeds without a topological transformation, while the latter involves a topological catastrophe.

Comparison of the Magnetic Properties of Metastable Hexagonal Close-Packed Ni Nanoparticles with Those of the Stable Face-Centered Cubic Ni Nanoparticles

We report the first magnetic study of pure and metastable hexagonal close-packed (hcp) Ni nanoparticles (sample 1). We also produced stable face-centered cubic (fcc) Ni nanoparticles, as mixtures with the hcp Ni nanoparticles (samples 2 and 3). We compared the magnetic properties of the hcp Ni nanoparticles with those of the fcc Ni nanoparticles by observing the evolution of magnetic properties from those of the hcp Ni nanoparticles to those of the fcc Ni nanoparticles as the number of fcc Ni nanoparticles increased from sample 1 to sample 3. The blocking temperature (T(B)) of the hcp Ni nanoparticles is approximately 12 K for particle diameters ranging between 8.5 and 18 nm, whereas those of the fcc Ni nanoparticles are 250 and 270 K for average particle diameters of 18 and 26 nm, respectively. The hcp Ni nanoparticles seem to be antiferromagnetic for T < T(B) and paramagnetic for T > T(B). This is very different from the fcc Ni nanoparticles, which are ferromagnetic for T < T(B) and superparamagnetic for T > T(B). This unusual magnetic state of the metastable hcp Ni nanoparticles is likely related to their increased bond distance (2.665 angstroms), compared to that (2.499 angstroms) of the stable fcc Ni nanoparticles.

Surface structure of ultrathin Fe films on Cu(001) revisited

The structure and magnetism of ultrathin Fe films epitaxially grown on a Cu(001) surface are investigated by grazing scattering of fast H and He atoms or ions. By making use of a new variant of ion beam triangulation based on the detection of the number of emitted electrons, we obtain direct information on the structure of the film surface. We observe for room temperature growth a dominant and defined fcc-like structure. Complex surface reconstructions as reported in recent STM and LEED studies are observed only for cooling and H2 dosing.

Frozen low-spin interface in ultrathin Fe films on Cu(111)

In ultrathin film systems, it is a major challenge to understand how a thickness-driven phase transition proceeds along the cross-sectional direction of the films. We use ultrathin Fe films on Cu(111) as a prototype system to demonstrate how to obtain such information using an in situ scanning tunneling microscope and the surface magneto-optical Kerr effect. The magnetization depth profile of a thickness-driven low-spin to high-spin magnetic phase transition is deduced from the experimental data, which leads us to conclude that a low-spin Fe layer at the Fe/Cu interface stays live upon the phase transition. The magnetically live low-spin phase is believed to be induced by a frozen fcc Fe layer that survives a thickness-driven fcc-->bcc structural transition.

Disproportionation, metal-insulator transition, and critical interaction strength in Na(1/2)CoO2

Charge disproportionation (CD) and spin differentiation in Na(1/2)CoO2 are studied using the correlated band local-density approximation + Hubbard U (LDA+U) approach. The simultaneous CD and gap opening seen previously is followed in detail through a first-order charge disproportionation transition 2Co(3.5+)-->Co3++Co4+. Disproportionation in the Co a(g) orbital results in half of the ions (Co3+) becoming electronically and magnetically dead, transforming the quarter-filled a(g) system into a half-filled subsystem that subsequently undergoes the observed charge ordering or metal-insulator transition. Comparing with data in the x approximately 0.3 regime suggests the system has moved into the multiband regime where the effective Coulomb repulsion U-->U(eff)=U/sqrt[3] strongly lessens correlation effects.

Temperature driven reversible breakdown of pseudomorphism in ultrathin Fe/Cu3Au films

We observe that ultrathin Fe/Cu(3)Au(001) films in the 6-13 A thickness range, beyond the thickness of pseudomorphism breakdown at room temperature, exhibit a temperature dependent structural phase transition in the range T(c) approximately 345-380 K. In the high temperature state the Fe film becomes pseudomorphic, while breakdown of pseudomorphism reversibly occurs as the system is cooled below the transition temperature. The difference between substrate and overlayer thermal expansion coefficient is highlighted as the driving force for the observed transition.

Shear instability of gamma-Fe in bulk and in ultrathin films

Using ab initio local-spin-density calculations we demonstrate that along the Bain path describing the transformation of face-centered-cubic (fcc) gamma-Fe into body-centered-cubic (bcc) alpha-Fe, tetragonal Fe is unstable against monoclinic shear deformations producing a nearly bcc structure. In the limit of a monolayer adsorbed on a fcc substrate, the epitaxial constraint suppresses the shear instability, but in ultrathin films with three to six monolayers a striped pattern of near-bcc domains develops, confirming recent observations by scanning tunneling microscopy. A strong correlation between the shear instability and the magnetic state is reported.

Theoretical calculation of magnetic properties of ultrathin Fe films on Cu(100)

The temperature dependence of the magnetization in fcc Fe on Cu(100) is calculated using a self-consistent local mean-field theory. The model reproduces an experimental magnetization oscillation as a function of film thickness and supports a picture where the top two layers are ferromagnetically coupled, and the remaining layers are antiferromagnetically coupled. The origin of the puzzling linear temperature dependence in oscillation amplitude is understood as a "surface phenomena" of the antiferromagnetic layer at the Fe/Cu interface. Proximity effects between a thin antiferromagnet with a low Neel temperature and a neighboring ferromagnet with a higher Curie temperature are discussed.