Magnetic properties of thin epitaxial Gd films on Nb

We investigated the magnetic properties of Gd films by magnetization measurements with the transverse magnetooptic Kerr effect in the temperature range from 2 to 295 K and for magnetic field up to 0.3 T. The thicknessd _Gd of the Gd films was varied between 11 and 1000 Å. For thick films (d _Gd500 Å) the form of the hysteresis curves and especially the nonmonotonic temperature dependence of remanence and coercivity can be qualitatively reproduced with a simple model employing theT dependence of the easy-axis direction. TheT dependence of the saturation magnetizationM _s of the thick films (d _Gd>=150 Å) strongly resembles the behavior of bulk crystals, whereas for the thinnest filmsM _s approximately depends linearly onT up to the Curie temperatureT _C . The thickness dependence ofT _C follows approximately a power law with an exponent 1.6 close to the theoretical value.     

Temperaturabhängigkeit von Knight-shift und Suszeptibilität von Vanadium und Niob

Knight-shiftK and susceptibility have been investigated on single-crystals of vanadium and niobium between 4 K and 290 K.In the case ofV the changes ofK and are found to be of opposite sign and proportional toT ² with good approximation. This can be explained by theT-dependence of spin-paramagnetism of 3d-electrons. The hyperfine field due to oned-electron spin is found to be — 90 kG. Small deviations from theT ²-law are explained by minor changes in the orbital susceptibility, which gives the main constituents toK and. In the case of Nb only is found to be proportional toT ². TheT-dependence ofK is very small and shows a maximum at 140 K. This can be explained by theT-dependence of spinparamagnetism of 4d-electrons as well as 5s-electrons.     

Two-magnon scattering and the collective excitation of spin waves

The excitation of strongly coupled spin waves is studied on a model of coupled oscillators. In particular, the question of a uniform mixing of degenerate spin waves is discussed. Attention is also paid to the _k -dependence of the frequency intervals in which the spin waves are appreciably excited.     

Temperature dependence of SSW linewidth in metallic thin films

Measurements of the standing spin wave linewidth H _n at 17·6 GHz from 6 °K to 400 °K are presented for two permalloy films whose structure is either polycrystalline or epitaxial. It is shown that in both films, H _n is proportional ton ² (n is the mode number). Although H _n is little modified by temperature (20% variation maximum over the entire temperature range), we experimentally state that the slopep of H _n (n ²) is a linear function of the saturation magnetizationM which agrees with a planar inhomogeneity model.The authors wish to thank H. Pascard and F. Hoffmann who have prepared and selected the films as also E. Villain and A. Gatineau for invaluable technical assistance.     

Geometry of the space of triangulations of a compact manifold

In this paper we study the spaceT _M of triangulations of an arbitrary compact manifoldM of dimension greater than or equal to four. This space can be endowed with the metric defined as the minimal number of bistellar operations required to transform one of two considered triangulations into the other. Recently, this space became and object of study in Quantum Gravity because it can be regarded as a toy discrete model of the space of Riemannian structures onM.Our main result can be informally explained as follows: LetM be either any compact manifold of dimension greater than four or any compact four-dimensional manifold from a certain class described in the paper. We prove that for a certain constantC>1 depending only on the dimension ofM and for all sufficiently largeN the subsetT _M(N) ofT _M formed by all triangulations ofM with N simplices can be represented as the union of at least [C ^N] disjoint non-empty subsets such that any two of these subsets are very far from each other in the metric ofT _M. As a corollary, we show that for any functional from a very wide class of functionals onT _M the number of its deep local minima inT _M(N) grows at least exponentially withN, whenN.This work was partially supported by the New York University Research Challenge Fund Grant, by Grant ARO-DAAL-03-92-G-0143 and by NSERC Grant OGP0155879.     

Temperature-dependent electronic structure of gadolinium

We investigate the electronic structure of the ferromagnetic 4f-metal Gadolinium by use of a many-body evaluation of a generalized model of magnetism, the one-particle part of which is derived from an ASW-LSDA bandstructure calculation. A striking temperature-dependence of the conduction band states is traced back to a 4f-(5d, 6s) interband exchange. The conduction electron polarization (0.63 _B atT=0) decreases forTT _c very similar to the 4f-magnetization. A red shift of the lower -band edge of about 0.25 eV appears upon cooling fromT=T _c toT=0. — The quasiparticle band-structure exibits a remarkable non uniform magnetic behaviour at different positions in the Brillouin zone, and in particular for different subbands. Weakly correlated (s-like) dispersions show a Stoner-likeT-dependence of the exchange splitting. On the other hand, stronger correlated (d-like) dispersions split belowT _c into four branches, two for each spin direction. TheirT-dependence mainly concerns the spectral weights of the quasiparticle peaks and not so much the energetic positions. An exchange caused splitting remains even forT<T _c .     

Local magnetic properties of ultrathin ferromagnetic fcc-Fe-films on Cu(001)

Conversion electron Mössbayer spectroscopy (CEMS) on three monolayers (ML) thick metastable fcc-Fe(001) films grown epitaxially on a Cu(001) substrate under different conditions shows that these films are characterized by a distributionP(B _hf) of magnetic hyperfine fieldsB _hf. The vast majority of⁵⁷Fe nuclei experience relatively large hyperfine fields at low temperature. The temperature dependence of the most probable fieldB _peak was found to follow aT ^3/2 spin-wave law below 300 K. It is shown from the relative line intensities that preferential Fe spin orientation perpendicular to the film plane exists in films grown at 120 K, while preferential in-plane spin orientation is found for a growth temperature of 300 K. Coating a low-temperature grown Fe film by 2 ML of Cu(001) drastically reduces the hyperfine field, in contrast to the case of room-temperature grown Cu-coated films.Dedicated to Professor Ulrich Gonser on the occasion of his 70th birthday     

Spinors and diffeomorphisms

We discuss the action of diffeomorphisms on spinors on an oriented manifoldM. To do this, we first describe the action of the diffeomorphism groupD(M) on the set =H ¹ (M,Z ₂) of inequivalent spin structures and show that it is affine. We argue that in the presence of spinors the gauge group of gravity is a certain double cover ofD(M) which depends on the spin structure. We explicitly compute the action ofD(M) on whenM is a closed Riemann surface; is seen to consist of exactly two orbits, corresponding to even and odd spin structures.On leave of absence from I.F.T., University of Wrocaw, Poland     

Eliashberg theory for disordered superconductors. II. Tunneling into disordered two-dimensional Pb films

A series ofc-axis oriented YBa₂Cu₃O _x -films with different oxygen content were prepared by laser deposition. The oxygen contentx was determined by X-ray diffraction and by resonant Rutherford-back-scattering (RRBS) measurements. Thec-axis length in these films of YBa₂Cu₃O _x is about 0.5% larger compared to bulk values. We describe transport measurements in magnetic fields up to 7 Tesla between room temperature andT _c in samples with an oxygen content between the orthorhombic-to-tetragonal transition (x6.4) and full oxygenation (x7). The ratio /R _H was investigated with respect to the two-dimensional Luttinger liquid theory and the model of the two-dimensional ionic metal. We report on deviations from the expected quadratic temperature behaviour of /R _H , especially in films with high oxygen content.     

Thermoelectric power of heavy-fermion compounds

We calculate the diffusion thermopower of the Anderson lattice as a model for heavyfermion compounds in a semi-phenomenological theory. In this theory, the thermopower is expressed by the dynamical susceptibility which describes spin fluctuations and can be measured by neutron scattering. The Kondo effect is taken into account for a singlef-electron spin which is coupled to all other spins and to the conduction electrons. This approach neglects multiple intesite-scattering of the conduction electrons. We obtain a Kondo termS ₍₁₎ ^d (T) (in which the thermopower of non-interacting spins is multiplied by a factor which describes the spin dynamics) and a resonance termS ₍₂₎ ^d (T) of opposite sign which vanishes for vanishing interactions. The superposition of both terms leads to a broad maximum of the thermopower roughly at the Kondo temperatureT _K and to an additional minimum belowT _K . ForT0 the termS ₍₁₎ ^d vanishes asT ² and the termS ₍₂₎ ^d becomes proportional toT. We also show that the Sommerfeld expansion leads to an incorrect result for the low temperature resistivity of the Anderson lattice and that the Gorter-Nordheim relation does not hold at low temperatures.     

Effect of hydrostatic pressure on phase transition and superconductivity in LaAg x In1−x,LaCd and LaZn

Previous studies of phase transformation and superconductivity on LaAg under pressure are extended to further CsCl-compounds LaAg_0.5In_0.5, LaCd, and LaZn. The cubic-totetragonal transition temperatureT _M in LaAg_0.5In_0.5 and LaCd increases rapidly with pressure, whereas LaZn does not transform to 16 kbar. The present results are used to correct the dependence ofT _M onx in LaAg _x In_1–x to constant volume; the bell-shaped curveT _M (x) with maximum atx0.5 is thereby corrected to one whereT _M rises steeply with In concentration across the entire range. A possible correlation between the magnitude ofT _M and the extent ofd-band occupation is inferred. Above 1.17 K only LaZn is found to be superconducting withT _c (0)=1.65 K;T _c (P) passes through a shallow minimum at 6 kbar.     

Spin dynamics and spin correlations in the spinS=1 two-dimensional square-lattice Heisenberg antiferromagnet La2NiO4

The static and dynamic spin fluctuations in the spinS=1, two-dimensional (2D) square-lattice antiferromagnet La₂NiO₄ have been studied over a wide temperature range using neutron scattering techniques. The spin correlations in La₂NiO₄ exhibit a crossover from two- to three-dimensional (3D) behavior as the Néel temperature is approached from above. Critical slowing down of the low-energy spin fluctuations is also observed just aboveT _N . The correlation length, (T), and the static structure factor,S(0), have been measured and are compared with recent theoretical calculations for the quantum 2D Heisenberg antiferromagnet using microscopic parameters determined from previous spin-wave measurements. Good agreement for (T) is found with the exact low-temperature result of Hasenfratz and Niedermeyer provided that 2 p _s is renormalized by 20% from the spin-wave value.     

Y1Ba2Cu3O7-δ thin films from KrF laser ablation with substrate heating and in situ patterning by CO2 laser radiation

Y₁Ba₂Cu₃O_7– thin films were deposited by KrF laser ablation while replacing conventional contact heating by cw CO₂ laser irradiation of the substrate front surface. The HTSC films obtained on (100)ZrO₂ showed T _c(R=0)=90 K, T(90–10%)=0.5 K, j _c=2.5 × 10⁶ A/cm², a sharp transition in the ac susceptibility X(T), and pure c-axis orientation. Micrographs of thin films (< 0.5 m) showed a smooth morphology while thick films (>1 m) contained many crystallites sticking in the bulk material. Furthermore, in situ patterning was achieved during deposition by local laser heating of a selected substrate surface area. The resulting planar films contained amorphous, semiconducting parts only 1 mm or less apart from crystalline material showing the above HTSC quality.Presented at LASERION '91, June 12–14, 1991, München (Germany)     

Electrical conductivity and structural behaviour of Al65Cu20Ru15 quasicrystalline alloy

The low temperature conductivity σ (T) and the magnetoresistance (up to 8 T) of a phason disorder free pure Al₆₅Cu₂₀Ru₁₅ quasicrystalline alloy, prepared by melt casting, has been investigated in the temperature range from 1.3 K to 200 K. In the temperature range 1.3 to 20 K the conductivity exhibits a -variation, whereas for the range 20 to 200 K a linearT-dependence has been observed. The analysis of magnetoresistance data reveals the presence of weak localization and electron-electron interaction effects. The positive nature of the magnetoresistance indicates the presence of spin orbit scattering. The σ∞ behaviour prevailing over such a wide temperature range has been observed for the first time for Al₆₅Cu₂₀Ru₁₅. A quantitative analysis of the -variation in terms of electron-electron interaction yields a very low density of states at the Fermi level.     

The magnetism of nickel monolayers

Nickel allows to study the largest variety of phenomena in the magnetism of UHV ultrathin films. The low critical temperature of 630 K for the bulk favors experiments from 0 K to aboveT _c and from one monolayer to infinite thick films. The thickness dependence ofT _c (d) for the (001) and the (111) orientation is compared. Susceptibility measurements in UHV are presented, and from _max the film geometry can be deduced. Ferromagnetic resonance measures the second- and fourth-order anisotropy constants. These give a clear understanding of when and how the reorientation transition from the in-plane to the perpendicular orientation occurs and its nature. Magnetic resonance and circular X-ray dichroism measure the spin and orbital parts of the magnetic moment µ, its anisotropy µ, and the 3d and 4sp contributions. Finally, we show how a 4 Monolayer (ML) Ni(001) film can be transformed into NiO by controlled oxygen dosage and thermal treatment.Paper presented at the annual Spring Meeting of the Deutsche Physikalische Gesellschaft for Condensed- Matter Division, Berlin, Germany. 20–24 March, 1995     

Semi-Inclusive Deep Inelastic Scattering processes from small to large P<Subscript>T</Subscript>

We consider the azimuthal and P_T-dependence of hadrons produced in unpolarized Semi-Inclusive Deep Inelastic Scattering (SIDIS) processes, within the factorized QCD parton model. It is shown that at small P_T values, P _T ≲ 1GeV/c, lowest-order contributions, coupled to unintegrated (transverse-momentum-dependent) quark distribution and fragmentation functions, describe all data. At larger P_T values, P _T ≳ 1GeV/c, the usual pQCD higher-order collinear contributions dominate. Having explained the full P_T range of available data, we give new detailed predictions concerning the azimuthal and P_T-dependence of hadrons which could be measured in ongoing or planned experiments by HERMES, COMPASS and JLab Collaborations.     

A determination of the low energy parameters of the 2-d Heisenberg antiferromagnet

We perform numerical simulations of the 2-d Heisenberg antiferromagnet using a cluster algorithm. Comparing the size and temperature effects of various quantities with results from chiral perturbation theory we determine the low energy parameters of the system very precisely. We finde ₀=–0.6693(1)J/a ² for the ground state energy density, _s = 0.3074(4)/a ² for the staggered magnetization,c=1.68(1)J a for the spin wave velocity andp _s =0.186(4)J for the spin stiffness. Our results agree with experimental data for the precursor insulators of high-T _c superconductors.Supported by the Schweizer Nationalfond     

Exact results for the dynamics of the classical one dimensional easy plane ferromagnet at low temperatures

The in-plane and out-of-plane dynamical correlation functions for the classical one dimensional easy plane ferromagnet are calculated asymptotically exactly at low temperatures. The results are restricted to temperatures much below the crossover temperature at which spins begin aligning in the plane. The long wavelength behavior of the in-plane fluctuations is consistent with dynamical scaling, in contrast to the isotropic case, and agrees with the results of Villain and of Nelson and Fisher. The linewidths for the in-plane fluctuations at short wavelengths are calculated exactly, and approach those of the isotropic model for small anisotropy. The theory of Villain, the theory of Cieplak and Sjolander, and the simulations of Loveluck, Jauslin, Schneider and Stoll all give incorrect results for these linewidths. The out of plane linewidths show an anomalous temperature dependence due to a singularity in the three spin wave density of states that is characteristic of one dimensional systems. The linewidth is proportional toT ² lnT except at the wavevector for which the second derivative of the spin wave frequency with respect to wavevector vanishes (/2 for CsNiF₃) where the linewidth is proportional toT ^5/3. The linewidth has a strong discontinuity as the wavevector increases as a result of a catastrophe occurring in the calculation of the three spin wave density of states. The position and strength of the discontinuity are temperature dependent. The diffusion coefficient is logarithmically dependent on the anisotropy, and diverges as (T ² lnD)^–1, which is consistent with the (lnT)^–1 behavior predicted for the isotropic ferromagnet in earlier work. The results are derived for the case of single ion anisotropy, using a spin wave theory for static correlations and the spin current damping function, and can be readily extended to the case of anisotropic exchange.     

Effect of hydrogen doping on the magnetic properties of Gd2CuO4

We report the results of ac-susceptibility and dc-magnetization measurements for H_yGd₂CuO₄ (0y0.54). It is shown thatH doping lowers the weak ferromagnetic component in the material. The distinct hysteresis loops observed atT=77 K for both non- and hydrogenated samples change its shape withy. The magnetic ordering temperatures T _N ^Cu and T _N ^Gd , as determined from the temperature dependencies of ac-susceptibility, remain unchanged with sample's hydrogenation. This result seems to indicate that extra electrons are not doped onto the Cu-O planes of Gd₂CuO₄. The frequency dependencies ofx(, T) andx(, T) for bothy=0 andy=0.15 samples are analysed., The maximums ofx andx found at about 200K are considered in terms of susceptibility dependence on the spin-lattice relaxation time (). The anomalies in ac-susceptibility found recently in Gd₂CuO₄ atT _a=8 K andT _b=9.5 K decrease significantly withy. Results are discussed in the context of available data on 214T-type compounds.     

Ferromagnetism of monoatomic cobalt films

The magnetic properties of epitaxial Co(111)-films on Cu(111) in the monolayer range were investigated. One group of films, apparently representing the monolayer, is magnetized spontaneously perpendicular to the surface, the magnetizationM _s (T) depending linearly onT up toT _c which results as 0·30T _c () (T _c )() is the Curie-temperature of the bulk).Financial support of this work by the Deutsche Forschungsgemeinschaft is acknowledged.