Local Magnetic Order Research Articles

Possibility of various magnetic configurations in the Cr (Fe) monolayer deposited on vicinal surfaces of Fe (Cr).

The local magnetic moments and magnetic order are calculated for a Fe (Cr) monolayer adsorbed on semi-infinite Cr (Fe) substrates with high-index surfaces. These stepped interfaces are present in the wedge-shaped configurations recently analyzed by different experimental groups in order to investigate the interlayer magnetic coupling in Fe/Cr/Fe systems. The spin-polarized electronic distribution is obtained by solving self-consistently a d-band model Hamiltonian in the mean-field approximation within the framework of the tight-binding real-space method. Several magnetic configurations have been found for all the systems investigated. In the most stable configuration of the Cr monolayer on Fe vicinal surfaces, the local moments of most of the Cr atoms are antiferromagnetically coupled with the Fe moments, an exception being the Cr atoms at the edge of the step. Cr atoms at nearest-neighbor positions (located at the kink and edge of the steps) are always antiferromagnetically coupled. For the Fe monolayer on Cr vicinal surfaces, the average magnetization at the surface results is zero for the two most stable solutions. However, other metastable solutions with net average magnetization on the Fe overlayer are found. The Fe atoms at the edge and kink of the steps (first neighbors) are, in all cases, ferromagnetically coupled. These results are in qualitative agreement with recent experimental observations.

Muon-spin rotation study of antiferromagnetic order in hydrogenated YBa2Cu4O8 evidence for a local structural change in the vicinity of Tc

Zero-field muon-spin rotation (ZF-μSR) experiments were performed on hydrogen-doped YBa2Cu4O8. The observation of well defined muon-spin precession frequencies unambiguously demonstrates the presence of local magnetic order. From the magnitude and the temperature dependence of the μSR frequencies we conclude that YBa2Cu4O8 has an antiferromagnetically ordered counterpart similar to that observed in YBa2Cu3O6. A jump of one of the observed μSR frequencies in the temperature around 80 K is discussed in terms of a local structural change of the apical oxygen to which the muon is bound.

Magnetic properties of Mn/Cu(001) and Mn/Ni(001) c(2 x 2) surface alloys.

Solid phase epitaxy was used to grow two-dimensional, ordered alloys of MnCu and MnNi on the surface of Cu(100) and Ni(100) crystals. These surface alloys, which have structures that are not present in the bulk phase diagram, result in highly corrugated surface layers with a c(2\ifmmode\times\else\texttimes\fi{}2) periodicity. The large outward relaxation of Mn atoms has been predicted theoretically to be due to magnetism of the Mn atoms. Using a combination of soft-x-ray absorption spectroscopy and x-ray magnetic circular dichroism, we have found an enhanced local magnetic moment and long-range magnetic ordering in these surface alloys. We find that Mn is in a high-spin ground state, which stabilizes the surface reconstruction, but that long-range magnetic ordering is not always present. Compositionally ordered MnNi films, with the same c(2\ifmmode\times\else\texttimes\fi{}2) surface periodicity, show ferromagnetic coupling of Mn atoms, unlike the antiferromagnetic ordering found in the bulk alloy.

Macroscopic magnetic properties of Ni and Zn substituted YBa 2Cu 3O χ

We present a comparison between Ni and Zn substitutions in YBa 2Cu 3O χ. For χ ≈ 1, besides the −12K/% decrease of T c due to Zn, which is 3 times larger than with Ni, Zn induces a weak magnetic moment μ eff = 0.36 μ B / Zn while in the Ni case, μ eff = 1.5μ B(S ≈ 1 2 ) . In contrast, a deoxydization down to χ ≈ 6. a larger increase of the local moment value for Zn than for Ni. The depression of T c is 2 to 3 times larger than for χ≈1, in both cases. These results support the idea that Zn affects the local magnetic order of nearby Cu on the scale of the magnetic Cu correlation length, which varies with χ. On the contrary, the magnetism associated with the Ni 2+ ion seems to be more local.

Local magnetic order in57Fe doped Y(Ce)Co4B

In order to monitor the peculiar temperature dependence of the magnetization of the compounds YCo4B and CeCo4B from an atornistic point of view, samples doped with 1% 57Fe were studied by MOssbauer spectroscopy. From the two Co-sites present in this structure (2c, 60, only the latter were found to be equipped by Fe. In the case of the Y-compound, the change of direction of the easy axis of magnetization could be confirmed. The broad maximum observed for the magnetization of the Ce-compound is not reflected by the 57Fe hyperfine field.

Dependence of the optical conductivity on the magnetic order

We derive optical sum rules for the upper and lower bands of the large- U Hubbard model. The weight of the upper band is directly related to the local magnetic order.

Antiferromagnetic versus ferromagnetic coupling in Fe/Cr(107) and Cr/Fe(107)

We have calculated the local magnetic moments and magnetic order for a Fe(Cr) monolayer adsorbed on a stepped Cr(Fe)(107) substrate. The electronic structure at T=0 K has been self-consistently determined within the unrestricted Hartree–Fock approximation of the Hubbard Hamiltonian in the framework of a real-space tight-binding method. In the Cr/Fe(107) system, two magnetic arrangements have been obtained, the more stable being the less frustrated as obtained in the case of V overlayers on vicinal substrates of Fe. An analysis of both solutions in terms of the total energy calculation and the different degree of frustration is presented. For Fe/Cr(107), a two-step periodicity is obtained. The sign of the magnetization at the Fe overlayer changes from step to step. This spin-flop transition is consistent with the two-layer period oscillation recently observed in Fe/Cr/Fe wedge structures, and with the total magnetization determined from in situ magnetometer measurements during growth of ultrathin Fe films on Cr(001).

Competition between singlet formation and magnetic ordering in one-dimensional spin systems.

We study the behavior of a model of two XXZ spin chains coupled by an isotropic exchange interaction ${\mathit{J}}_{\mathit{K}}$ as the anisotropies and spin velocities in the individual chains are varied. In the limit of large antiferromagnetic ${\mathit{J}}_{\mathit{K}}$ we map the model's low-energy sector onto an Ising model in a transverse field; in the limit of large ferromagnetic ${\mathit{J}}_{\mathit{K}}$ we map the model's low-energy sector onto that of a single spin-one chain. For small ${\mathit{J}}_{\mathit{K}}$, we study the model perturbatively via Abelian and non-Abelian bosonization. We determine the possible phases, approximate locations of the phase boundaries, universality classes of the transitions, and asymptotic behavior of correlation functions. We believe our model captures the essential physics of the interplay between the compensation of local moments and magnetic ordering, which may be important for the heavy-fermion materials.

Influence of magnetic imperfections on the low-temperature properties of d-wave superconductors.

We consider the influence of planar ``magnetic" imperfections which destroy the local magnetic order, such as Zn impurities or $Cu^{2+}$ vacancies, on the low temperature properties of the cuprate superconductors. In the unitary limit, at low temperatures, for a $d_{x^2-y^2}$ pairing state such imperfections produce low energy quasiparticles with an anistropic spectrum in the vicinity of the nodes. We find that for the $La_{2-x}Sr_xCuO_4$ system, one is in the {\em quasi-one-dimensional} regime of quasiparticle scattering, discussed recently by Altshuler, Balatsky, and Rosengren, for impurity concentrations in excess of $\sim 0.16\%$ whereas YBCO$_7$ appears likely to be in the true 2D scattering regime for Zn concentrations less than $1.6\%$. We show the neutron scattering results of Mason et al. \cite{Aeppli} on $La_{1.86}Sr_{0.14}CuO_4$ provide strong evidence for ``dirty d-wave" superconductivity in their samples. We obtain simple expressions for the dynamic spin susceptibility and $^{63}Cu$ spin-lattice relaxation time, $^{63}T_1$, in the superconducting state.

57Fe and 119Sn Mössbauer studies on La1.25Nd0.6Sr0.15CuO4: Evidence for local magnetic ordering below ≂32 K (abstract)

57Fe and 119Sn Mössbauer effect studies on 0.5 at. % 57Fe or 119Sn doped nonsuperconducting La1.25Nd0.6Sr0.15CuO4 in the temperature region 300 K≥T≥4.2 K reveal an onset of local magnetic ordering occurring at TM≂32 K for both 57Fe as well as 119Sn-doped samples. The local magnetic ordering shows up in the presence of a very large transferred hyperfine field of Bthf≂11.0(5) T at the 119Sn nuclei. Since such a large field is present in neither antiferromagnetic La2CuO4 superconducting La1.85Sr0.15CuO4 nor overdoped La2−xSrxCuO4 (x=0.4), the occurrence of such local spin correlations seems to be a signature of the nonsuperconducting low temperature tetragonal phase of La1.25Nd0.6Br0.15CuO4.

Local versus long range magnetic ordering in LaNi 5−xFe x

Magnetic properties of LaNi 5−xFe x has been investigated in magnetic fields up to 15 kG, at temperatures T>1.5 K. Magnetic clustering effects are evidenced, and characterized by the local Fe composition, the concentration and size of the magnetic clusters. The analysis of the data leads to a good understanding of the complex magnetic phase diagram of LaNi 5−xFe x in the whole range of compositions 0.05<x<1.2. In particular the onset of itinerant ferromagnetism takes place at x ≈ 0.5 in addition to the ferromagnetism of iron moments which takes place at the magnetic percolation threshold x ≈ 0.35 ± 0.05.

Spin-fluctuation-induced superconductivity and normal-state properties of YBa2Cu3O7.

We have carried out strong-coupling calculations using the Eliashberg formalism for ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7}$. We consider the influence of potential impurity scattering on ${\mathit{T}}_{\mathit{c}}$. We find that, as the strength of the impurity potential increases, the unitary limit is reached comparatively quickly. In this (unitary) limit, the influence of isotropic impurity scattering on ${\mathit{T}}_{\mathit{c}}$ is relatively weak. We show, with the aid of a simple model to describe the disruption of the local magnetic order brought about by the substitution of Zn, that the latter has a strong influence on ${\mathit{T}}_{\mathit{c}}$. We consider the competition between antiferromagnetic and superconducting instabilities and find that, for our model parameters, the instability to a superconducting state always comes first. Next, we examine the sensitivity of our results for ${\mathit{T}}_{\mathit{c}}$ to the details of the spin-fluctuation spectrum and hole concentration. When that spectrum is modified so that it is consistent with both NMR ${\mathit{T}}_{1}$ and ${\mathit{T}}_{2}$ measurements, a superconducting transition temperature of 90 K is obtained with a dimensionless coupling contant, \ensuremath{\lambda}1/2. Strong-coupling calculations of the normal state, using these latter parameters and including vertex corrections, yield an in-plane resistivity which varies linearly with temperature, with a magnitude at 100 K of 20 \ensuremath{\mu}\ensuremath{\Omega} cm, and, with minor changes in parameters, a frequency dependence of the optical conductivity in quantitative agreement with experiment for energies 50 meV. With an interlayer hopping ${\mathit{t}}_{\mathrm{\ensuremath{\perp}}}$ of 8 meV, the c-axis resistivity is found to be linear in temperature with a magnitude at 150 K of 2.5 m\ensuremath{\Omega} cm.

Spin polarization at the Fe/V interface.

The local magnetic moments and magnetic order are investigated at the Fe/V interface of thin ${\mathrm{Fe}}_{3}$/${\mathrm{V}}_{\mathit{n}}$ films and thin V films adsorbed on Fe substrate. Different crystallographic directions (001), (101), (111), and a two-atom-width and monoatomic-height-per-step stepped surface [corresponding to the vicinal (103) surface] are considered. We calculate the spin-polarized electronic-charge distribution of these systems as a function of the exchange integral ${\mathit{J}}_{\mathrm{V}}$ of vanadium in the range 0\ensuremath{\le}${\mathit{J}}_{\mathrm{V}}$\ensuremath{\le}0.8 eV using a self-consistent tight-binding real-space model within the unrestricted Hartree-Fock approximation to the Hubbard Hamiltonian. For (001), (011), and (111) crystallographic faces, the spin polarization of the V atoms at the Fe/V interface is nonzero for any ${\mathit{J}}_{\mathrm{V}}$ value, with antiferromagnetic coupling between the Fe and V interface atoms. Due to the hybridization between V and Fe orbitals, the spin polarization at the interface tends to increase in V with respect to the corresponding pure-V system, whereas the opposite happens in Fe, resulting in a magnetic moment induced at the interface V atoms by the Fe substrate. This effect is less pronounced in the (101) orientation. Both results can be qualitatively explained by considering two facts: (a) the stronger electron-electron interaction of Fe with respect to V; (b) Fe atoms at the interface have fewer V atoms at nearest-neighbor positions for the (101) orientation than for the (001) orientation. In the case of V adsorbed on the (103) vicinal surface of Fe, the internal and external V atoms at the surface layer of each step are coupled antiferromagnetically. For this particular case, there is a strong competition between the tendencies of V to couple antiferromagnetically with the ferromagnetic Fe substrate and with its V nearest neighbors.

Magnetism of small vanadium clusters

The onset of magnetism of small V N clusters is studied by using a d-band Hubbard like Hamiltonian in the unrestricted Hartree-Fock approximation. We determine the local moments, magnetic order and average magnetization as a function of the intraatomic exchange integral J. Different structures are considered and charge transfer effects are discussed. We found that the magnetic behavior of small clusters is very different from the bulk behavior. For the considered clusters, two different magnetic configurations are obtained. The role of sp-electrons and spd-hybridization on the magnetic properties is also discussed.

Layer-dependent spin-polarized 3p core-level photoemission from ultrathin Fe films.

We present the results of a spin-polarized photoemission experiment that yields unambiguous evidence of layer-dependent core-level magnetic exchange splitting in ultrathin films. Analysis of the layer dependence of 3p-level spin polarization also establishes the existence of bulklike local magnetic order at 1.5 monolayer thickness.

Exchange interaction and local environment effects on the magnetic properties of FeN clusters.

The magnetism of Fe N clusters is studied by using a d-band Hubbard-like Hamiltonian in the unrestricted Hartree-Fock approximation. The local magnetic moments, magnetic order, and average magnetization per atom are calculated for T→0 as a function of the intra-atomic Coulomb exchange integral J. Different cluster sizes and structures are considered and local enviroment effects are analyzed

On the local magnetic order in La2−xSrxCuO4+δ (0 ⩽x⩽ 0.15)

Abstract The 119 Sn Mossbauer effect technique (300-4.2 K) has been used to study the local magnetic order in La 2− x Sr x (Cu 0.995 Sn 0.005 )O 4+δ for three different Sr concentrations corresponding to the antiferromagnetic state ( x = 0), the spin-glass-like state ( x = .04), and the superconducting state ( x = 0.15). Contrary to previous reports we observe no transferred hyperfine (hf) field at the Sn nuclei in the superconducting state down to 1.7 K. On the other hand, we observe small transferred hf fields in both the antiferromagnetic ( x = 0) and the spin-glass-like state ( x = 0.04). In the spin-glass-like sample ( x = 0.04) we find in the temperature range 10 K ⩽ T ⩽ 200 K a significant increase of the Mossbauer linewidth and an anomalous change of the quadrupole splitting which we attribute to a short range structural disorder as recently reported by perturbed angular correlation measurements of 111 In on La 2− x Sr x CuO 4 (0 ⩽ x ⩽ 0.07). The magnetic behaviour of this sample ( x = 0.04) has been further investigated by 139 La-NQR measurements at low temperatures (1.5 K ⩽ T ⩽ 12 K). Here, we observe an enhancement of the relaxation rate (1/ T 2 ) below about 7 K.

Circularly polarized X-rays probing nuclear magnetic moments and magnetism of solids

It is demonstrated how circularly polarized x-rays can be used in nuclear and solid state physics. Detection of the radiation pattern of circularly polarized x-rays emitted in highly converted decays of oriented radio isotopes allows the determination of nuclear magnetic moments. Studying the absorption profiles of energy-tunable intense circularly polarized x-rays emitted from high-energy electron storage rings provides a powerful method to determine spin structure of electronic states, local magnetic moments and magnetic short range order in ferromagnetic materials.

Magnetic properties of 3d-transition-metal alloy clusters: (Fe x Cr1?x ) n

We present results for the magnetic properties of (Fe x Cr1−x ) n alloy clusters obtained by using a tight-binding Hubbard Hamiltonian in the unrestricted Hartree-Fock approximation. The dependence of the average magnetic moment, local magnetic moments, magnetic order, and cohesive energy on the size and composition of the cluster were determined. In agreement with surface calculations, we find that the average magnetic moment of the alloy is larger than that of the cluster of one element. Forn=15 a transition from ferromagnetic to antiferromagnetic order is obtained forx≈0.4−0.53. We discuss the importance of the overlap interaction in the spin polarized charge distribution of the alloy (e.g., charge transfer, local magnetic moments). The characteristic properties of the mixed clusters are also discussed by comparison with available results for homogeneous clusters.

Mössbauer spectroscopy of CuO and its relevance to high-temperature superconductors.

Moessbauer emission spectroscopy and magnetic susceptibility studies were performed on {sup 57}Co-doped antiferromagnetic CuO. Magnetic susceptibility data show that antiferromagnetic ordering occurs at {ital T}{sub {ital N}}=225 K and that the Co dopant (10--100 ppm) does not influence the bulk magnetic properties of CuO. Moessbauer spectra recorded at {ital T}{gt}244 K show a dominant quadrupole-split site and two minor impurity sites. At {ital T}=244 K, a line broadening of the dominant site is observed indicating the onset of local magnetic order. The hyperfine interaction parameters at {ital T}{lt}{ital T}{sub {ital N}} were determined. In particular, the sign and the asymmetry parameter {eta} of the electric-field gradient ({ital V}{sub {ital z}{ital z}}) and the angle {theta} formed by {ital V}{sub {ital z}{ital z}} and magnetic hyperfine field were derived. We find {eta} to be very small ({eta}{le}0.09) and 21{degree}{le}{theta}{le}28{degree}. The value of {theta} is in accord with neutron-diffraction results. The {sup 57}Fe Moessbauer spectroscopy of CuO is discussed in relation to other copper oxides and Fe containing perovskites.