Ferromagnetic Transition Metals Research Articles

PHASE SEPARATION INDUCED BY CATION DISORDER IN (La, Y)2/3(Sr, Ca)1/3MnO3

Polycrystalline manganite oxides with different A-site cation disorder were synthesized. The hole concentration and average radius of A-site cation are kept unchanged while the A-site cation disorder increases linearly by Y and Sr co-doped in La 2/3 Ca 1/3 MnO 3. All these samples exhibit a paramagnetic insulator to ferromagnetic (FM) metal transition, and the transition temperature T C and T MI decrease while the transition width increases with increasing the A-site cation disorder. T MI is usually 10 K higher than T C for the small cation disorder samples. However, with increasing the cation disorder, T MI is about 60 K lower than T C , and the resistivity increases in magnitude while the magnetization decreases. The metal-to-insulator transition for samples with small cation disorder can be well described by a percolative model, which indicates FM-metal state and antiferromagnetic insulator state assuredly coexist in the transition interval. It reveals that cation disorder suppresses the long-range FM-metal state which results in the phase separation behavior in samples with large cation disorder.

Ferromagnetism of transition metals and screened exchange interactions

We have investigated the magnetic properties of Fe, Co, and Ni at finite temperatures on the basis of the first-principles dynamical coherent potential approximation (CPA) in order to clarify the role of the exchange interaction energy ($J$) screened by {\it sp} electrons and its applicability to finite-temperature magnetism. With use of the atomic $J$, we obtained the Curie temperatures ($T_{\rm C}$) 1930 K for Fe and 2550 K for fcc Co, which are overestimated by a factor of 1.8 as compared with the experimental values, while we obtained $T_{\rm C}=620$ K for Ni being in good agreement with the experiment. Calculated effective Bohr magneton numbers also quantitatively agree with the experimental values. By comparing the results with those obtained by the screened $J$ and by comparing them with the experiments, we found that the screened $J$, which are reduced by 30% as compared with the atomic ones, improve the ground-state magnetizations and densities of states at low temperatures in Fe and fcc Co, as well as $T_{\rm C}$ in fcc Co. But the screened $J$ yield worse results for finite temperature properties of Fe and underestimate both the ground-state magnetization and $T_{\rm C}$ in case of Ni. We discuss possible origins for these inconsistencies.

Spin Density Distribution of YTiO<sub>3</sub> Observed by X-Ray Magnetic Diffraction

An X-ray magnetic diffraction (XMD) is applied to ferromagnetic transition metal oxide YTiO3. The spin magnetic form factors are measured for 81 reciprocal lattice points of h0l, hk0 and 0kl series. Three-dimensional spin density distribution in the real space of YTiO3 is successfully obtained by using Maximum Entropy Method (MEM). This is the first result of spin density in the real space by the XMD.

ChemInform Abstract: Scanning Electron Microscopy with Polarization Analysis: Studies of Magnetic Microstructure

When a beam of electrons with energies greater than several hundred eV is incident upon a ferromagnetic metal, spin polarized secondary electrons are emitted. The polarization of these secondary electrons is related to the polarization of the electrons in the ferromagnet. In the case of transition metal ferromagnets, the polarization of the secondary electrons is directly proportional to the magnetization. Spin polarization analysis of the secondary electrons, therefore, provides a direct measurement of the magnetization in the region probed by the incident electron beam. Scanning electron microscopy with polarization analysis (SEMPA), illustrated schematically in Fig. 11.1, combines the finely focused beam of the scanning electron microscope with secondary electron spin polarization analysis to obtain a technique that provides high resolution images of the surface magnetic microstructure of ferromagnetic materials. The purpose of this chapter is to review the SEMPA technique and to present several examples of magnetic microstructures that were studied using SEMPA.

Magnetoresistance in ferromagnetic-metal/graphene/ferromagnetic-metal lateral junctions

Based on a numerical study of a full-orbital tight-binding model, we show that magnetoresistance (MR) in ferromagnetic metal (FM)/graphene/FM lateral junctions originates from a spin-dependent shift of the Dirac points (DPs) caused by the presence of FM electrodes. Graphene with finite length $L$ is in contact with FM at a zigzag edge and the spin-dependent conductance is calculated in the ballistic transport regime. We find that the tunneling conductance near DPs influences the features of the MR considerably. We further show that a slight change in the electronic structure near the DPs strongly affects the spin-dependent conductance resulting in a large MR ratio for an FM made of certain ferromagnetic transition metal alloys.

ChemInform Abstract: The Orbital Origins of Magnetism: From Atoms to Molecules to Ferromagnetic Alloys

A chemical view of spin magnetic phenomena in finite (atoms and molecules) and infinite (transition metals and their alloys) systems using the concepts of bonding and electronic shielding is presented. The concept is intended to serve as a semiquantitative signpost for the synthesis of new ferromagnets. After a concise overview of the historic development of related theories developed within the physics community, the consequences of spin-spin coupling (made manifest in the exchange or Fermi hole) in atoms and molecules are explored. Upon moving to a paramagnetic state, the majority/minority spin species become more/less tightly bound to the nucleus, resulting in differences in the energies and spatial extents of the two sets of spin orbitals. By extrapolating well-known arguments from ligand-field theory, the paucity of ferromagnetic transition metals arises from quenching the paramagnetism of the free atoms due to strong interatomic interactions in the solid state. Critical valence electron concentrations in Fe, Co, and Ni, however, result in local electronic instabilities due to the population of antibonding states at the Fermi level varepsilon(F). Removal of these antibonding states from the vicinity of varepsilon(F) is the origin of ferromagnetism; in the pure metals this results in strengthening the chemical bonds. In the 4d and 5d transition metals, the valence d orbitals are too well shielded from the nucleus, so a transition to a ferromagnetic state does not result in sufficiently large changes to occur. Thus, the exceptional occurence of ferromagnetism only in the first transition series appears to parallel the special main-group chemistry of the first long period. A connection between ferromagnetism in the transition metals and Pearson's absolute hardness eta is easily established and shows that ferromagnetism appears only when eta<0.2 eV in the nonmagnetic calculation. As expected from the principle of maximum hardness, Fe, Co, and Ni all become harder upon moving to the more stable ferromagnetic state. Magnetism in intermetallic alloys follows the same path. Whether or not an alloy contains ferromagnetic elements, the presence of antibonding states at varepsilon(F) serves as a fingerprint to indicate a ferromagnetic instability. The differences in the sizes of the local magnetic moments on the constituent atoms of a ferromagnetic alloy can be understood in terms of the relative contributions to the density of states at varepsilon(F) in the nonmagnetic calculations. Appropriately parameterized, nonmagnetic, semi-empirical calculations can also be used to expose the ferromagnetic instability in elements and alloys. These techniques, which have become relatively commonplace, can be used to guide the synthetic chemist in search of new ferromagnetic materials.

Theory of giant saturation magnetization in α″-Fe16N2: role of partial localization in ferromagnetism of 3d transition metals

A new model is proposed for the ferromagnetism associated with partially localized electron states in the Fe16N2 system that explains its giant saturation magnetization. It is demonstrated that an unusual correlation effect is introduced within the Fe–N octahedral cluster region and the effective on-site 3d–3d Coulomb interaction increases due to a substantial 3d electron charge density difference between the cluster and its surroundings, which leads to a partially localized electron configuration with a long-range ferromagnetic order. The first-principles calculation based on the LDA+U method shows that giant saturation magnetization can be achieved at sufficiently large Hubbard U values. The feature of the coexistence of the localized and itinerant electron states plays a key role in the formation of giant saturation magnetization.

Ab initiostudy of3score-level x-ray photoemission spectra in transition metals

We calculate the $3s$- and $4s$-core-level x-ray photoemission spectroscopy (XPS) spectra in the ferromagnetic and nonmagnetic transition metals by developing an ab initio method. We obtain the spectra exhibiting the characteristic shapes as a function of binding energy in good agreement with experimental observations. The spectral shapes are strikingly different between the majority spin channel and the minority spin channel for ferromagnetic metals Ni, Co, and Fe, that is, large intensities appear in the higher binding-energy side of the main peak (satellite) in the majority spin channel. Such satellite or shoulder intensities are also obtained for nonmagnetic metals V and Ru. These behaviors are elucidated in terms of the change of the one-electron states induced by the core-hole potential.

Schottky barriers of rare-earth transition-metal intermetallics on silicon

The ferromagnetic transition metals Fe, Co and Ni have high work functions (4.5 – 5.0 eV) and they invariably form Schottky barriers on Si, often detrimental to efficient spin injection and detection at the interfaces. The rare earths have lower work functions (2.5 – 3.2 eV) but they order magnetically below room temperature. We therefore investigate a series of 12 rare earth metals and alloys of Gd, Ce or Y with Fe or Co, which are evaporated onto (001) Si wafers. The lowest thermionic emission barrier height is found for CeCo5 (48 meV), much less than those found for the 3d ferromagnets or their silicides (~ 500 meV). Y is found to form an ohmic contact after a diffusion anneal at 1300 K. Further alloy additions are needed to lower the barrier height below 25 meV; a target for room temperature operation with significantly increased reverse bias current.

Explaining the paradoxical diversity of ultrafast laser-induced demagnetization

Pulsed-laser-induced quenching of ferromagnetic order has intrigued researchers since pioneering works in the 1990s. It was reported that demagnetization in gadolinium proceeds within 100 ps, but three orders of magnitude faster in ferromagnetic transition metals such as nickel. Here we show that a model based on electron-phonon-mediated spin-flip scattering explains both timescales on equal footing. Our interpretation is supported by ab initio estimates of the spin-flip scattering probability, and experimental fluence dependencies are shown to agree perfectly with predictions. A phase diagram is constructed in which two classes of laser-induced magnetization dynamics can be distinguished, where the ratio of the Curie temperature to the atomic magnetic moment turns out to have a crucial role. We conclude that the ultrafast magnetization dynamics can be well described disregarding highly excited electronic states, merely considering the thermalized electron system.

Ultrafast demagnetization of ferromagnetic transition metals: The role of the Coulomb interaction

The Elliott-Yafet mechanism is arguably the most promising candidate to explain the ultrafast demagnetization dynamics in ferromagnetic transition metals on timescales on the order of 100 femtoseconds. So far, only electron-phonon scattering has been analyzed as the scattering process needed to account for the demagnetization due to the Elliott-Yafet mechanism. We show for the first time that the electron-electron scattering contribution to the Elliott-Yafet mechanism has the potential to explain time-resolved magneto-optical Kerr effect measurements on thin magnetic cobalt and nickel films, without reference to a phononic "spin bath."

Ultra‐fast dynamics in solids: non‐equilibrium behaviour of magnetism and atomic structure

AbstractNon‐equilibrium physics is of fundamental interest, for example, for extensions of statistical mechanics and thermodynamics. In particular, it is important to understand how conservation laws like energy conservation and angular‐momentum conservation in magnetic solids control the time scale of the dynamics. Laser irradiation may cause intense electronic excitations and thus a strong non‐equilibrium state. Results are presented for the ultra‐fast response of magnetism in ferromagnetic transition metals like Ni, Co, Fe, and Gd and furthermore of the atomic structure in semiconductors like Si, Ge, and InSb. Non‐thermal melting is a most spectacular example of ultra‐fast bond breaking. Time‐resolved magnetooptical experiments yielding sub‐picosecond spin dynamics are discussed. The spin dynamics is accompanied by THz light emission. The structural changes in semiconductors, bond changes sp3 → s2p2, and phase transitions occur within about 100 fs. The results also shed light on electron‐transfer processes, on ionization, and on molecular dissociation dynamics, which may occur during fs and as times. We discuss the application of time‐resolved analysis to tunnelling problems and the phase diagram of high‐Tc superconductivity.

Magnetoresistive junctions based on epitaxial graphene and hexagonal boron nitride

We propose monolayer epitaxial graphene and hexagonal boron nitride $(h\text{-BN})$ as ultimate thickness covalent spacers for magnetoresistive junctions. Using a first-principles approach, we investigate the structural, magnetic, and spin transport properties of such junctions based on structurally well-defined interfaces with (111) fcc or (0001) hcp ferromagnetic transition metals. We find low resistance area products, strong exchange couplings across the interface, and magnetoresistance ratios exceeding 100% for certain chemical compositions. These properties can be fine tuned, making the proposed junctions attractive for nanoscale spintronics applications.

Ultra-fast dynamics in solids: non-equilibrium behaviour of magnetism and atomic structure

Non-equilibrium physics is of fundamental interest, for example, for extensions of statistical mechanics and thermodynamics. In particular, it is important to understand how conservation laws like energy conservation and angular-momentum conservation in magnetic solids control the time scale of the dynamics. Laser irradiation may cause intense electronic excitations and thus a strong non-equilibrium state. Results are presented for the ultra-fast response of magnetism in ferromagnetic transition metals like Ni, Co, Fe, and Gd and furthermore of the atomic structure in semiconductors like Si, Ge, and InSb. Non-thermal melting is a most spectacular example of ultra-fast bond breaking. Time-resolved magnetooptical experiments yielding sub-picosecond spin dynamics are discussed. The spin dynamics is accompanied by THz light emission. The structural changes in semiconductors, bond changes sp3 s2p2, and phase transitions occur within about 100 fs. The results also shed light on electron-transfer processes, on ionization, and on molecular dissociation dynamics, which may occur during fs and as times. We discuss the application of time-resolved analysis to tunnelling problems and the phase diagram of high-Tc superconductivity.

The analysis of the knight shift in the μ+ SR technique

We present an analysis and first-principles calculations of the Knight shifts of a positive muon (μ+) in normal and transition metals using a cellular multiple-scattering technique. The main conclusions are the following: (a) The μ+ induces the segregation of states (one of each spin) from the conduction band to lower energies. This cluster bonding state is occupied with two (antiparallel spin) electrons but in general it has less than 35% of μ+1s character. The screening is completed with the conduction electrons, (b) The observed Knight shift is the sum of a diamagnetic shielding (of the order —20 to —50 ppm arising in almost equal parts from the localized bonding cluster states and from the diamagnetic shielding of the conduction electrons) and a spin-enhanced paramagnetic shielding, large for all metals except for the divalent normal metals having a low density of states at the Fermi level which seats at the separation between the s- and the p-conduction bands. A spin-polarized calculation shows that the local spin density at the μ+ site is definitely smaller or even different in sign than the value computed from the local charge density enhancement. The ferromagnetic transition metals, because of the d-electron polarization, present a negative local magnetization at the μ+ site which gives a dominant negative contribution to the observed values.

Transition metal atoms in cubic configurations

Matrix Hartree–Fock calculations are reported for 3d transition metals in a configuration chosen to represent, as closely as is possible with integral occupation numbers, an atom in an fcc metal. The cubic symmetry shell (3dt)6 is assumed to be fully occupied, and electrons are successively removed from the (3de)4 shell to represent atoms from Cu to Mn. A single 4s orbital is included, with independent calculations for the two possible choices of relative spin of the 4s orbital and 3de shell. Orbital energies are computed for each of the nonequivalent 4s and 3d orbitals, and the relationship of these quantities with transition metal energy bands is discussed. Intra-atomic orbital energy splittings are found to be comparable in magnitude to energy band splittings. Radial densities of the 4sα and 4sβ orbitals are computed. These densities are found to be compatible with a proposed explanation of recently observed negative magnetization regions within the unit cell of ferromagnetic transition metals.

Morphology of the interfaces between transition metals and their native oxides: Role of interdiffusion processes

The magnetic coupling phenomena appearing between a ferromagnetic transition metal and its antiferromagnetic oxide crucially depend on the composition and morphology of the interface. We present combined in-situ x-ray absorption spectroscopy and ex-situ x-ray reflectometry studies of the growth of a native-oxide layer and the interface formation with its metal parent. Two model systems, Fe and Ni, are investigated. It is found in the case of Fe that oxidation and reduction processes appear upon deposition of metal on the oxygen-rich oxide. This results in symmetric metal/oxide and oxide/metal interfaces. Comparison with the case of Ni, where no reduction is observed, shows that this symmetry is intrinsically linked to the underlying reduction process. These results have important consequences for the growth of transition-metal/oxide interfaces in general.

Femtosecond electron diffraction: Direct probe of ultrafast structural dynamics in metal films

Femtosecond electron diffraction is a rapidly advancing technique that holds a great promise for studying ultrafast structural dynamics in phase transitions, chemical reactions, and function of biological molecules at the atomic time and length scales. In this paper, we summarize our development of a tabletop femtosecond electron diffractometer. Using a delicate instrument design and careful experimental configurations, we demonstrate the unprecedented capability of detecting submilli-ångström lattice spacing change on a subpicosecond timescale with this new technique. We have conducted an in-depth investigation of ultrafast coherent phonon dynamics induced by an impulsive optical excitation in thin-film metals. By probing both coherent acoustic and random thermal lattice motions simultaneously in real time, we have provided the first and unambiguous experimental evidence that the pressure of hot electrons contributes significantly to the generation of coherent acoustic phonons under nonequilibrium conditions when electrons and phonons are not thermalized. Based on these observations, we also propose an innovative approach to measure the electronic Grüneisen parameter in magnetic materials at and above room temperature, which provides a way to gain new insights into electronic thermal expansion in ferromagnetic transition metals.

Femtosecond time‐resolved reflection spectroscopy in Pr1–xCax MnO3 (x =0.37)

AbstractThe ultrafast dynamics of photoinduced phase transition (PIPT) in a crystal of Pr1–x Cax MnO3 (x = 0.37) has been studied by means of femtosecond time‐resolved spectroscopy. Just after photoirradiation of 1.58 eV light at 100 K, ultrafast change of reflectivity (ΔR/R) was observed within ∼1 ps utilizing the probe light of 0.5 eV due to the ferromagnetic metallic phase transition. While the fast component of ΔR/R gradually increases with increasing the excitation intensity, the slow component observed at the delay time of ∼1 ps shows unusual dependence on the excitation intensity. The signature of ΔR/R changes from negative to positive with the excitaiton power, which cannot be explained by the photo‐thermal heating effect. In addition, we investigated the temperature dependence of the ΔR/R at 0.5 eV and observed the sudden change of ΔR/R around TN, which indicates that the efficiency of PIPT is closely correlated with the ordering of spin and extra electron in Mn4+. (© 2009 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Itinerant ferromagnetism by eg and t2g states in the two-band Hubbard model

Itinerant ferromagnetism derived fromeg and t2g states has been studied by the Gutzwiller variational method based on the two-band Hubbardmodel. The analysis shows that the magnetization value that depends on the orbitalreflects strength of renormalization for each band. As a result, the magnetization of the3d(t2g) band, which is more strongly renormalized in the calculation, has a larger value than that of the3d(eg) band. By changing the atomic interaction and projected orbital density of states(DOS), we have discussed general tendencies that the magnetizations by theeg and t2g states are determined by the relative intensities of the projected orbitalDOS at the Fermi energy, the renormalized kinetic energies relative toU, andstability of atomic multielectron configuration states. The result indicates that increases in Coulomb interactionU and the portion ofHund’s coupling J in the atomic interaction lead to balanced magnetizations between the two states. Variationin the above factors can generate a variety of spin-dependent electronic structures near theFermi energy in 3d transition metal ferromagnets.