Effect of Lattice Relaxations on the Hyperfine Fields of Heavy Impurities in Fe

We present first-principles calculations of hyperfine fields of heavy impurities in bcc Fe. In particular, the effect of lattice relaxations on the calculated hyperfine fields are studied. The calculations are based on a full-potential Korringa–Kohn–Rostoker Green's-function method for defects and employ the local spin-density approximation for the exchange and correlation effects. The calculated lattice relaxations around the 5sp and 6sp impurities in Fe are found to be relatively small, for the first and second nearest neighbor shells less than 6% of the nearest neighbor distance. Also the estimated relative volume changes induced by the sp impurities are found to be around 60–95% of the elementary volume of Fe, surprisingly small in view of the large atomic volumes of the impurities. The calculated hyperfine fields of 5sp and 6sp elements are compared with available experimental data and it is shown that the inclusion of lattice relaxations in the calculation improves the agreement with experiments.     

Adsorbate influence on the coercive field of ultrathin Co/Cu{1 1 0}

We report on experimental coercive field measurements for ultrathin films of Co on Cu{1 1 0}, revealing a complex dependence upon the surface coverage of oxygen and hydrogen adatoms. With reference to first-principles calculations, we rationalise this behaviour in terms of initial reaction with surface contaminants and/or adsorption at defects, followed by (in the case of oxygen) subsequent surface ordering and eventual formation of surface oxide.     

A cluster calculation of the muon hyperfine field in nickel

We have performed a partially self-consistent, first-principles calculation of the hyperfine field at a ⁺ at the octahedral interstitial site in ferromagnetic nickel. Using a package of programs made available by the Department of Theoretical Chemistry of the University of Mexico, various calculations were performed of the spin-dependent charge density in the central region of a spherically averaged superposition of atomic charge densities. In this way it was found that: (1) The electronic configuration of bulk Ni contains a significant amount of 4p-states, in agreement with other calculations¹. (2) The ⁺ produces only a weak perturbation on its nearest-neighbor Ni atoms. (3) The hyperfine field at the ⁺ is opposite to the bulk magnetization and has the value –590 G, in good agreement with the measured² value of –641 G.     

Electrostatic field effect on molecular structures at metal surfaces

We report on the structural transitions of molecules on metal surfaces by external electrostatic field. An electrode–molecule–electrode model is considered to quantify the effect of electrostatic forces at the molecule–electrode interface. Within a quasi-parallel-plate capacitor approach, this model reveals how external electrostatic fields change the delicate balance between molecule–substrate and molecule–molecule interactions, leading to substantial changes in the molecular conformation. The predictions are validated by scanning tunneling microscopy (STM) observations of four different molecules and electrode facets. In addition, first-principles simulations verify the results of our model calculations.     

First-principles calculation of the single impurity surface Kondo resonance

We perform first-principles calculations of the surface and bulk wave functions of the Cu(111) surface and their hybridization energies to a Co adatom, including the potential scattering from the Co. By analyzing the calculated hybridization energies, we find the bulk states dominate the contribution to the Kondo temperature, in agreement with recent experiments. Furthermore, we also calculate the tunneling conductance of a scanning tunneling microscope and compare our results with recent experiments of Co impurities in the Cu(111) surface. Good quantitative agreement is found at short parallel impurity-tip distances (<6 A). Our results indicate the need for a new formulation of the problem at larger distances.     

First application of the real space linear muffin-tin orbital atomic sphere approximation method for calculating Fermi contact hyperfine fields

In this work we apply the recently developed first-principles real space linear muffin-tin orbital atomic sphere approximation (RS-LMTO-ASA) scheme to calculate the hyperfine field for 3d impurities (V, Cr, Mn andFe) in Cu. We obtain the Fermi contact term at the impurities andat four shells of host atoms around the impurities. We compare our results with theoretical values obtained using the KKR-Green-function method andwith NMR measurements. The overall agreement is excellent, confirming the reliability of the RS-LMTO-ASA approach as means of obtaining information on hyperfine fields.     

Magnetic and transport properties of granular and Heusler-type glass-coated microwires

We studied magnetic and structural properties of granular Co_xCu_100−x (5<x<40 at%), Cu₆₃Fe₃₇ and Heusler-type Ni₂MnGa glass-coated microwires. We found that the structure of Co–Cu microwires consists of two phases: fcc Cu for all the samples and fcc α-Co present for higher Co content. In the case of low Co content, Co atoms are distributed within the Cu matrix. The quantity and the size of grains strongly depend on the geometry of the microwire. Co–Cu and Fe–Cu microwires exhibited considerable magnetoresistance (MR). For Co_xCu_100−x microwires at x≥30 the anisotropic contribution to MR has been observed. Temperature dependences of magnetization measured without an external magnetic field (ZFC) and in the presence of a field (FC) show considerable difference below 20 K, indicating the presence of small α-Fe or Co grains embedded in the Cu matrix. Annealed Ni₂MnGa microwires showed ferromagnetic behavior with Curie temperature about 330 K and polycrystalline structure with space group I4/mmm and lattice parameters a=3.75 Å and c=6.78 Å.     

Magnetic properties of single crystalline Zn1−xCoxO thin films

We report on the magnetic properties of single crystalline thin films of Zn_1−xCo_xO (x=0.003–0.14) grown by plasma-assisted molecular beam epitaxy. In order to understand the role of intermediate charge carriers in the magnetic properties of this material two types of films were fabricated, with and without Ga-codoping. Magnetic measurements were made between 2 and 300 K in fields up to 5 T with a Quantum Design SQUID magnetometer. We found that all the tested films exhibit paramagnetic behavior following the Curie–Weiss law, χ=C/(T−θ), with negative Curie–Weiss temperatures and that this behavior holds even under strong n-doping. We show that the magnetization data, M(H), in function of the Co content provide additional evidence in favor of the antiferromagnetic Co–Co interaction in this material. We also observe that these data exhibit an ‘easy plane’ magnetic anisotropy for all the studied Co concentrations. Finally, we develop a simple cluster model, in order to describe the magnetic properties of ZnCoO, which is found to be in good agreement with our experiments.     

Local magnetic moments and hyperfine magnetic fields in Fe-M (M = Si, Sn) alloys at small metalloid concentrations

The main tendencies in the formation of local magnetic moments and hyperfine magnetic fields at Fe nuclei in Fe-Sn and Fe-Si alloys at low metalloid concentrations are analyzed on the basis of “first-principles” calculations. The results of calculations are compared with experimental data. The main differences between these alloys were proved to be due to the differences in their lattice parameters. It is shown that a significant contribution to the formation of the hyperfine field comes from the orbital magnetic moment and the Ruderman-Kittel-Kasuya-Yosida polarization, which depend on the impurity concentration and the distance to an impurity atom in the crystal lattice.     

NMR study of some heusler alloys containing Co and Mn

The⁵⁹Co and⁵⁵Mn NMR were measured at 4.2 K by spin echo technique. The hyperfine fields in Co₂ MnAl and Co₂MnSb were determined, and it was suggested that in the Co₂MnZ compounds the Co hyperfine field depends on Z. The magnitude of the hyperfine field at Mn dilutely dissolved in Co₂TZ by replacing T (T=Ti, V, Cr, Fe) was found to be larger by 30–80 kOe than that in Co₂MnZ with the same Z. The hyperfine field at Co dilutely dissolved in X₂ MnZ (X=Ni, Cu) by replacing X had a value quite near to that in Co₂MnZ.     

Calculation of hyperfine interaction parameters in metals-friedel oscillations in dilute Fe and Cu alloys

We review some recent calculations on hyperfine interaction parameters in metals based on density functional theory in the local density approximation. We restrict ourselves to the calculation of Friedel oscillations of the charge and magnetization density in dilute Fe and Cu alloys. In particular, we estimate hyperfine fields, isomer shifts and electric field gradients for a few shells around the impurities. The calculations are performed using the KKR-Green's function method. Especially, we discuss which valence properties determine the hyperfine interaction parameters and what information about the electronic structure can be obtained from their measurement.     

Directional behavior of nuclear quadrupole resonance in YBa2Cu3O6

Powder samples of YBa₂Cu₃O₆ were magnetically aligned and the anisotropies in the systems were studied by means of Cu(1) nuclear quadrupole resonance (NQR) in the absence of external magnetic fields. Our room temperature measurements of the NQR lineshapes and the spin–lattice and spin–spin relaxation times as a function of the aligning magnetic field indicate that full microscopic alignment can be achieved by using a magnetic field of about 4.7 T, for which doublet line patterns arising from a hyperfine splitting were observed.     

Competing polarization mechanisms in the heavy-fermion paramagnet CeCu<Subscript><Emphasis Type="Bold">6</Emphasis></Subscript>

Nuclear magnetic resonance (NMR) and relaxation of ⁶³Cu and ⁶⁵Cu in a powder sample of the heavy-fermion paramagnet CeCu₆ is measured and analysed quantitatively. Five different Cu sites are accessible to a detailed analysis. We derive quadrupolar splitting frequencies, Ce to Cu transferred hyperfine field coupling constants, and transversal as well as longitudinal relaxation behaviour. Only small relaxation anomalies are observed at the orthorhombic to monoclinic structural phase transition of CeCu₆. We point to the different importance of transferred hyperfine interaction and local conduction electron density for static or dynamic part, respectively, of Cu hyperfine interaction. The different sign of the transferred hyperfine interaction from Ce³⁺ to different Cu neighbours reveals the different competing interaction mechanisms, giving rise to the heavy-fermion paramagnetic behavior of CeCu₆. Received 20 November 2001     

Investigation of the different nature of magnetic hyperfine fields of Ce probes in Gd and Co matrices

The magnetic hyperfine fields (MHF) acting on ¹⁴⁰Ce probes diluted in Gd and Co matrices, measured at low temperatures, were compared with those obtained from first-principles electronic structure calculations. It was found that the origin of the MHF for these two situations is completely different even though the total MHF presents similar values on both the cases. For Ce diluted in Gd, the Ce 4f shell is localized and the orbital MHF dominates. On the other hand, for Ce probes in Co matrix the Ce 4f shell is delocalized and, as a consequence, the MHF is dominated by the Fermi-contact contribution.     

Solid-state amorphization reaction in the system zirconium hydride-cobalt

Crystalline multilayer films of zirconium hydride and cobalt were prepared by a dual ion-beam sputtering technique. The structure of the samples was investigated by X-ray diffraction analysis at low and medium scattering angles and by Rutherford backscattering experiments. During suitable heat treatments a solid-state reaction occurs in ZrH_2+x /Co forming a two-phase amorphous Zr–Co–H alloy. The composition of the amorphous phases reveals similarities to the behaviour of crystalline Zr–Co compounds upon dissolution of hydrogen. At small modulation wavelengths and at temperatures above 300 K an in-situ amorphization reaction occurs during deposition, again leading to a compositionally modulated structure with two amorphous phases.     

Observation ofμ − spin precession in ferromagnetic Ni and giant hyperfine anomaly

A ^–SR signal in nickel was found representing the first observation of ^–SR in ferromagnetic materials, and the hyperfine anomaly was determined to be –2.82±0.08% in comparison with the hyperfine field at dilute Co in Ni known from NMR.     

Site preference and elastic properties of ternary alloying additions in B2 YAg alloys by first-principles calculations

First-principles calculations were preformed to study the site preference behavior and elastic properties of 3d (Ti–Cu) transition-metal elements in B2 ductility YAg alloy. In YAg, Ti is found to occupy the Y sublattice whereas V, Cr, Co, Fe, Ni and Cu tend to substitute for Ag sublattice. Due to the addition of 3d transition metals, the lattice parameters of YAg is decreased in the order: V<Cu<Cr<Ni<Co<Fe<Ti. The calculated elastic constants show that Cr, Fe, Co and Cu can improve the ductility of YAg alloy, and Fe is the most effective element to improve the ductility of YAg, while Ti, Ni and V alloying elements can reduce the ductility of YAg alloy, especially, V transforms ductile into brittle for YAg alloy. In addition, both V and Ni alloying elements can increase the hardness of YAg alloy, and Y₈Ag₇V is harder than Y₈Ag₇Ni.     

Hyperfine field calculation for various alloy systems

The hyperfine field and the isomer shift of Fe−X (X=SC, Ti, V, Cr, Mn, Co, Ni, Cu) disordered alloys are calculated by use of the Korringa-Kohn-Rostoker coherent potential approximation (KKR-CPA) combined with the density functional theory in the local density approximation. The agrements with available experimental data are fairly well if the well-known systematic discrepancy arising in the Fe hyperfine field is properly biased. The overall trends of the concentration dependence of the Fe hyperfine field is qualitatively explained in terms of the local and the transferred contributions to the hyperfine field.     

Magnetic hyperfine fields at 111In probes in submonolayer Cu islands on Co(1 0 0) surfaces

Cu islands in the submonolayer coverage regime deposited on Co(1 0 0) have been studied utilizing the PAC spectroscopy. The ¹¹¹In PAC probes are incorporated into the topmost monolayer, which is corroborated by the presence of a rather unique electric field gradient. In addition, weak magnetic hyperfine fields B_hf between 1.9(2) and 0.37(4) T have been observed depending on the Cu coverage. This is attributed to next neighbours of the PAC probes consisting of mixed Co/Cu configurations. A linear decrease of B_hf as a function of the number of Cu atoms replacing Co is found. This revised version was published online in August 2006 with corrections to the Cover Date.     

Electronic stability of magnetic Fe/Co superlattices with monatomic layer alternation

We report a surprising observation that the growth of the [Fe(1 ML)/Co(1 ML)](n) superlattice of L1(0) structure on Cu(100) is stable only up to six atomic layers (n=3), which cannot be rationalized by stress arguments. Instead, first-principles calculations reveal a transition from the L1(0) to the B2 structure due to the effect of dimensionality on the stability of the electronic structure of the superlattice. Whereas the majority-spin electrons are energetically insensitive to the layer thickness, the minority-spin electrons induce the transition at n=3.