Magnetic Properties Of Transition Metals Research Articles

Photoluminescence and Electron Paramagnetic Resonance Studies of Bulk GaN Doped with Gadolinium

Theoretical predictions of possible room temperature ferromagnetism in GaN diluted with manganese strongly motivated studies of nitrides based diluted magnetic semiconductors (DMS) with transition metals or rare-earths. Over the last years, above room temperature ferromagnetism for MBE grown GaN layers weakly doped with Gd has been reported by different researchers [for example see ref.1]. The estimated average value of magnetic moment per Gd ion has been obtained as high as 500 times that of Gd atomic value. It has been argued that the observed enhancement of the average magnetic moment resulted from strong contribution of GaN host lattice to macroscopic magnetization of the whole material [1]. The origin of such behavior remains still puzzling and more studies of GaN:Gd are required to clarify this issue. In this paper we present photoluminescence (PL) and electron paramagnetic resonance (EPR) experiments on strain free GaN bulk crystals of wurtzite structure doped with gadolinium. The samples were grown from the solution of nitrogen in liquid gallium under high (1.5GPa) pressure (HP) of N2 and at elevated temperatures of about 1500 0 C. Doping with Gd was obtained by adding this element to the solution. Both PL and EPR techniques are well known as a powerful experimental tools to study electronic and magnetic properties of transition metals and rare-earth in semiconductors, and they have not been exploited up to now for GaN:Gd system. In EPR experiment sharp absorption lines were detected, which could be attributed to Gd 3+ (4f 7 ) state. Identification of the lines was possible by comparison with other semiconductors doped with Gd. The observed Dysonian shape of the lines originated from presence of high concentration of free electrons in measured GaN. However, the estimated value of free electron concentration was definitively lower than in undoped HP GaN, what indicated purification role of Gd from unintentional donors (presumably oxygen). PL studies revealed intensive spectra in two energy range: close to GaN band gap (3.13.6 eV) and in the “red” range of 1.6-1.8 eV. Band-to band transitions, typical for GaN doped with free electron concentration at a high 10 18 cm -3 level, were clearly seen. They were followed by several sharp peaks in direction of lower energy. In the “red” range sharp structure with main line at 1.77eV energy was detected. The FWHM of this line was as small as 5meV. This structure was related to Gd 3+ (4f 7 ) state. Judging from EPR and PL results Gd dopant was present in the studied samples. However, no ferromagnetic behavior of the samples was observed by means of EPR. This creates some doubts if ferromagnetism observed by others was really related to GaGdN system.

Ab initio cluster calculations of the magnetic properties of ZnO doped with transition metal ions

The magnetic properties of transition metal (TM) doped ZnO (TM = Mn 2+, Co 2+, and Ni 2+) were determined by an embedded cluster approach using ab initio methods. We performed CASSCF (complete active space self-consistent field) calculations on clusters with one and two transition metal centers, respectively. In particular, we were interested in the magnetic exchange coupling between TM centers which are directly connected by an oxygen bridge. Two Mn as well as two Co centers are weakly antiferromagnetically coupled. For the investigation on the magnetic properties of Ni 2+ doped ZnO it was necessary to include spin orbit coupling in the calculations. While without inclusion of spin orbit coupling the ground state of an isolated Ni 2+ ion in ZnO is a 3E state, which should show a first order Zeeman interaction with the magnetic field, Ni 2+ has a non-magnetic A 1 ground state, when spin orbit coupling is included. The results of our calculations are in good agreement with experimental data for low temperatures and low concentrations of the transition metal centers.

Magnetic properties of transition metal atoms doped in silicon nanotubes with hexagonal prism structure

Magnetic properties of magnetic transition metals doped in the infinite Si nanotubes (SiNTs) with hexagonal prism structure (Si/sub 12/M/sub n/, M=Fe, Co, Ni, n=1, 2) have been investigated for two different numbers of dopants per hexagonal prism by using the localized basis calculational method. The decrease (increase) of spin-down (spin-up) electrons for the n=2 case results in the increase of magnetic moments compared with the n=1 case. The calculated magnetic moment per dopant Fe atom (2.39 /spl mu//sub B/) is larger than both of the bulk value (2.22 /spl mu//sub B/) and previous result for finite nanotube (1.7 /spl mu//sub B/). For Co and Ni atoms doped in the tube, the magnetic moments are smaller than those of bulk metals. The Si-Si bond lengths for the hexagonal prisms decrease compared with that of the nanotube without transition metals, but there is no dependency on different dopants. The distances between the transition metals and Si atoms decrease as the atomic number of transition metals increases, which is the same trend for the atomic radii of transition metal atoms. The doping of transition metal atoms leads to the increase of the binding energy (BE).

Structural and magnetic properties of transition metal substituted ZnO

Structural and magnetic properties have been studied for polycrystalline Zn1−xTMxO, where TM (transition metal ions)=Mn, Fe, and Co. No bulk ferromagnetism was observed for single-phase materials, contrary to the existing theories. Single-phase samples demonstrate paramagnetic Curie–Weiss behavior with antiferromagnetic interactions, similar to other diluted magnetic semiconductors. Nonoptimal synthesis conditions lead to formation of second phases that are responsible for spin-glass behavior {ZnMnO3 impurity for Zn1−xMnxO [S. Kolesnik et al., J. Supercond. 15, 251 (2002)]} or high-temperature ferromagnetic ordering [Co metal for Zn1−xCoxO with the Curie temperature TC>800 K or (Zn,Fe)3O4 for Zn1−xFexO with TC=440 K].

Superparamagnetism of transition metal nanoparticles in conducting polymer film

Abstract Magnetic properties of transition metal (cobalt, iron, nickel, manganese, chromium) nanoparticles prepared by ion-exchange method in the perfluorinated sulfo-cation polymeric membrane (MF-4SK) have been investigated. While manganese and chromium in MF-4SK exhibited paramagnetic properties, cobalt, iron and nickel particles showed superparamagnetic behaviors. Our experimental evidence suggests that cobalt, iron and nickel nanoparticles in the polymer film obey a single-domain theory.

Structural and magnetic properties of transition metal oxide/metal bilayers prepared by in situ oxidation

Metal oxide/metal bilayers have been prepared by in situ oxidation. Ultrathin layers with antiferromagnetic properties are formed at room temperature by controlled in situ exposure of clean Fe, Co and Ni layers to pure oxygen gas. X-ray diffraction, Auger electron spectroscopy and transmisson electron microscopy have been applied to provide a detailed analysis of the layered structure. The magnetic properties of these antiferromagnetic/ferromagnetic bilayers are characterized by an interfacial coupling (exchange bias) which drastically varies with the type of antiferromagnetic material.

Stabilization ofd-band ferromagnetism by hybridization with uncorrelated bands

We investigate the influence of s-d or p-d hybridization to d-band ferromagnetism to estimate the importance of hybridization for the magnetic properties of transition metals. To focus our attention to the interplay between hybridization and correlation we investigate a simple model system consisting of two non-degenerated hybridized bands, one strongly correlated, the other one quasi-free. To solve this extended Hubbard model, we apply simple approximations, namely SDA and MAA, that, concerning ferromagnetism in the single-band model, are known to give qualitatively satisfactory results. This approach allows us to discuss the underlying mechanism, by which d-band ferromagnetism is influenced by the hybridization on the basis of analytical expressions. The latter clearly display the order and the functional dependencies of the important effects. It is found, that spin-dependent inter-band particle fluctuations cause a spin-dependent band shift and a spin-dependent band broadening of the Hubbard bands. The shift stabilizes, the broadening tends to destabilize ferromagnetism. Stabilization requires relatively high band distances and small hybridization matrix elements. Super-exchange and RKKY coupling are of minor importance.

Electronic Structure and Magnetic Properties of Transition Metal Doped Silicon Carbide

ABSTRACTThe magnetic properties of cubic (3C) silicon carbide (SiC) doped by first row transition metals (TM) are studied within the local spin density functional approach using the linearized muffin-tin orbital (LMTO) method in the atomic sphere approximation (ASA). The magnetic properties are found to depend strongly on the doping site. For the preferred doping site (Si), Cr and Mn exhibit the most pronounced magnetic behavior with Cr favoring ferromagnetic coupling and Mn antiferromagnetic coupling.

III–V based magnetic semiconductors

Progress in advanced material growth techniques such as molecular beam epitaxy has made it possible to grow a variety of new materials with novel properties for novel device applications. The magnetic properties of transition metals in semiconductors, for example, are of growing interest for the development of magnetoelectronic devices. As this new research field has emerged, a variety of new materials and artificial structures has been the subject of intensive study. Among them, diluted magnetic III–V semiconductors are expected to combine the properties of both magnetic materials and of semiconductors.

Exchange Stiffness Parameter and Magnetization in Pseudopotential Calculations

We study low temperature magnetic properties of transition metals. The ground state spontaneous magnetization is calculated in a pseudopotenti al approximation. In the calculations, the core electronic orbitals are approxi- mated by asymptotic expressions. The magnetizations are calculated for uni- form spin densities of valence (chemically active) electrons and in the Lind- hard approximation. Our calculations of the Bloch wall parameter and the magnon stiffness constant are based on a study of ferromagnetic and para- magnetic phases. A simple Bragg-Williams approximation between energy of spin polarization and the time-average of the spin moment per atom deter- mines the exchange coupling (Heisenberg-like) for spin-polarized nearest neighbour Wigner—Seitz cells. Ab initio numerical results of the spontaneous magnetization, the Bloch wall parameter, and the magnon stiffness constant are obtained for Fe, Co, and Ni.

Magnetic dichroism in x-ray photoemission

Magnetic dichroism in spin resolved photoemission gives information about the correlation between the valence spin, the core hole orbital moment and the core hole spin. For photoemission from an open shell the sum rules give the expectation value of the orbital magnetic moment and quadrupole moment. Analysis of the angular distribution gives the higher magnetic moments. Therefore, polarized photoemission measurements are important for the understanding of the magnetic properties of transition metal and rare earth materials, such as for ferromagnetic nickel where measurements indicate an enhanced orbital magnetic moment at the surface.

A relativistic spin-polarised band theoretical study of magnetic properties of nickel and iron

In order to study the relativistic effects on the magnetic properties of transition metals, we have performed self-consistent relativistic spin-polarised electronic band structure calculations for two cubic systems: Ni and Fe. The spin-polarised fully relativistic linear muffin-tin orbital method is used. The calculated ground state spin and orbital magnetic moments and magnetic form factors for Ni and Fe are presented. The magneto-crystalline anisotropy in the band structure is studied, by rotating the magnetisation direction from (0 0 1) to (1 1 1) and then to (1 1 0). Finally, a computationally very intensive study of the magneto-crystalline anisotropy energy and moment has been carried out.

Temperature dependence of transition-metal magnetism

A first-principles tight-binding formulation of the paramagnetic spin susceptibility is developed to study the temperature dependence of the magnetic properties of transition metals. The formulation, which includes many-body enhancements, has been successful in predicting magnetic ground-state properties. The present study introduces a temperature-dependent analytical tetrahedron method to perform Brillouin zone sums, and employs accurate Slater–Koster band structures for convenience. Critical temperatures and the temperature dependence of the spin-density wave characterizing antiferromagnetism are determined from instabilities of the paramagnetic state toward magnetic order. The Néel temperature of Cr and the Curie temperature of Fe found in this way are in good agreement with measured values. Nonzero critical temperatures are also found for alternate cubic structures of some transition metals, further supporting the prediction of ferromagnetism in bcc Mn, and antiferromagnetism in fcc Mn and expanded bcc Ti. The computed wave vector of the spin-density wave of Cr is found to increase along the Δ line with increasing temperature following observed trends.

ChemInform Abstract: Spectroscopic and Magnetic Properties of Transition Metal(II) Trifluoromethanesulfonate Compounds with Bridging Asymmetric 3,4‐Dialkyl Substituted 1,2,4‐Triazole Ligands.

AbstractThe dialkyl‐substituted triazoles (III) prepared as shown react with metal(II) trifluoromethanesulfonates in aqueous solution to give the linear tri‐ and binuclear complexes (IV)‐(VIII) in which the metal ions are linked to each other by three bridging triazole ligands.

Spectroscopic and magnetic properties of transition metal(II) triflouromethanesulfonate compounds with bridging asymmetric 3,4-dialkyl substituted 1,2,4-triazole ligands

The spectroscopic and magnetic properties are described of coordination compounds with asymmetric 3,4-diakyl substituted 1,2,4-triazoles. The ligands 3-methyl-4-ethyl-1,2,4-traizole and 3-methyl-4-t-butyl-1,2,4-triazole have been investigated. Using M(CF 3SO 3) 2 (M = Mn, Co, Ni, Cu, Zn) compounds have been obtained with a linear trinuclear structure, in which the metal ions are linked to each other by two pairs of three bridging triazoles. The coordination sphere around the terminal ligands is completed by monodentate ligands and/or water molecules. The structure has been confirmed by an X-ray structure determination of [Co 3(metz) 6(H 2O) 6][Co 3 (metz) 8(H 2O) 4](CF 3SO 3) 12(H 2O) 8. this compound crystallizes in the space group P 1 with lattice constants a = 13.793(4), b = 14.401(3), c = 23.258(4) Å, α = 80.58(2), β = 83.23(2) and γ = 64.33(2)°. The unit cell contains two independent trinuclear clusters of different composition. These two clusters have the same overall structure. Differences are related to the presence of monodentately coordinating ligands as well as to the position of the C3-methyl substituent. The complete refinement of this structure was obstructed by disorder problems in the anions and the non-coordinating water molecules. The magnetic susceptibilities of the compounds have been recorded and could be fitted to theoretical expressions for linear trimers. The compound [Ni 2- (mtbtz) 5(H 2O) 4](CF 3SO 3) 4(H 2O) 4 appears to be a dimeric species as concluded from its magnetic behaviour.

Magnetic Properties of the Transition Metals in a Superconductor

AbstractThe superconductivity as a sensitive probe of the magnetic properties of transition metals is investigated. A theory of magnetic impurities in a superconductor is presented within the Nagaoka‐Suhl approximation (with the double renormalization of energy parameters). Application of Anderson's theory to superconducting host metals permits to determine the relation of the interactions responsibles for magnetic properties of transition metals.

Magnetic properties of transition metal substituted MnP

The magnetic properties of polycrystalline samples of Mn 1− t T t P ( T = V, Cr, Fe and Co for 0.00 ≦ t ≦ 0.50) are studied by magnetic susceptibility, magnetization and neutron diffraction measurements. The magnetic phase diagrams of the Mn 1− t T t P phases exhibit paramagnetic, ferromagnetic, helimagnetic and spin glass regions depending on temperature and substitution ( T, t). The concentrated spin glass regions observed in Mn 1− t V t P and Mn 1− t Co t P (0.30 ≦ t ≦ 0.50) are believed to result from the disorder in the metal sublattice. The variation of the magnetic moment of the ordered Mn 1− t T t P phases with the substitution ( T, t) is discussed.

Correlation effects in the cohesion and magnetic properties of transition metals

The variational method of Stollhoff and Fulde is applied to a model Hamiltonian describing the $d$ band of a transition metal to obtain the correlated ground state. The model used includes the Coulomb and exchange intra-atomic energies and is rotationally symmetric in orbital and spin space. The calculated cohesion energy has a minimum for a half-filled $d$ band, which is connected with gradual localization of a magnetic moment in the presence of the Hund's-rule exchange interaction. At the same time the charge fluctuations are strongly suppressed and spin fluctuations enhanced. We have found a corrected Stoner criterion which gives a ferromagnetic ground state for realistic values of the model parameters for $3d$ metals, and is in agreement with the exact result of Kanamori at very low electron and hole concentrations.

Electron correlation in transition metals

Abstract The role played by electron correlation in determining the magnetic properties of transition metals is reviewed. The successes of band theory, local spin density functional methods and the random phase approximation are discussed. These include calculations of electron and spin density distributions, the form of the Fermi surface, the spin susceptibility at T = 0 and spin wave energies. However, it is shown that the energy dependence of the self-energy, not included in a local potential, is important for determining quasi-particle energies away from the Fermi surface, such as the top of the majority spin band in nickel. This energy dependence is also essential for understanding the low temperature behaviour of the spin susceptibility of paramagnetic metals. The transition to a Curie-Weiss law at higher temperatures is discussed. The experimental observation of spin waves above the Curie temperature in ferromagnetic metals indicates long range spin correlations which may be understood in nickel using a model of three quasi-two-dimensional sub-bands. A new approach to very weak itinerant-electron ferromagnets, based on Hund's rule interactions between strongly-correlated paramagnetic sub-bands, is outlined.

On the magnetic properties of transition metal substituted MnAs

On the magnetic properties of transition metal substituted MnAs