Mechanical detwinning device for anisotropic resistivity measurements in samples requiring dismounting for particle irradiation.

Uniaxial stress is used to detwin the samples of orthorhombic iron based superconductors to study their intrinsic electronic anisotropy. Here, we describe the development of a new detwinning setup enabling variable-load stress-detwinning with easy sample mounting/dismounting without the need to re-solder the contacts. It enables the systematic study of the anisotropy evolution as a function of an external parameter when the sample is modified between the measurements. In our case, the external parameter is the dose of 2.5 MeV electron irradiation at low temperature. We illustrate the approach by studying resistivity anisotropy in single crystals of Ba1-xKxFe2As2 at x = 0.25, where the much discussed unusual re-entrance of the tetragonal C4 phase, C4 → C2 → C4, is observed on cooling. With the described technique, we found a significant anisotropy increase in the C2 phase after electron irradiation with a dose of 2.35 C/cm2.

Magnetic Fluctuations, Precursor Phenomena, and Phase Transition in MnSi under a Magnetic Field.

The reference chiral helimagnet MnSi is the first system where Skyrmion lattice correlations have been reported. At a zero magnetic field the transition at T_{C} to the helimagnetic state is of first order. Above T_{C}, in a region dominated by precursor phenomena, neutron scattering shows the buildup of strong chiral fluctuating correlations over the surface of a sphere with radius 2π/ℓ, where ℓ is the pitch of the helix. It has been suggested that these fluctuating correlations drive the helical transition to first order following a scenario proposed by Brazovskii for liquid crystals. We present a comprehensive neutron scattering study under magnetic fields, which provides evidence that this is not the case. The sharp first order transition persists for magnetic fields up to 0.4 T whereas the fluctuating correlations weaken and start to concentrate along the field direction already above 0.2 T. Our results thus disconnect the first order nature of the transition from the precursor fluctuating correlations. They also show no indication for a tricritical point, where the first order transition crosses over to second order with increasing magnetic field. In this light, the nature of the first order helical transition and the precursor phenomena above T_{C}, both of general relevance to chiral magnetism, remain an open question.

Itinerant and Localized Magnetization Dynamics in Antiferromagnetic Ho.

Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho L_{3} absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x-ray energy to the electric dipole (E1, 2p→5d) or quadrupole (E2, 2p→4f) transition allows us to selectively and independently study the spin dynamics of the itinerant 5d and localized 4f electronic subsystems via the suppression of the magnetic (2 1 3-τ) satellite peak. We find demagnetization time scales very similar to ferromagnetic 4f systems, suggesting that the loss of magnetic order occurs via a similar spin-flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra-atomic 4f-5d exchange coupling. In addition, an ultrafast lattice contraction due to the release of magneto-striction leads to a transient shift of the magnetic satellite peak.

Raman studies of nearly half-metallic ferromagnetic CoS2.

We measured the Raman spectra of ferromagnetic, nearly half-metallic, CoS2 over a broad temperature range. All five Raman active modes Ag, Eg, Tg(1), Tg(2) and Tg(3) were observed. The magnetic ordering is indicated by a change of the temperature dependences of the frequency and the line width of Ag and Tg(2) modes at the Curie point. The temperature dependence of the frequencies and line widths of the Ag, Eg, Tg(1), Tg(2) modes in the paramagnetic phase can be described in the framework of the Klemens approach. Hardening of the Tg(2), Tg(1) and Ag modes on cooling can be unambiguously seen in the ferromagnetic phase. The line widths of Tg(2) and Ag modes behave in a natural way at low exciting laser powers (they decrease with decreasing temperature) in the ferromagnetic phase. At high exciting laser powers the corresponding line widths increase as temperature decreases below the Curie temperature. Then, as will be shown, the line width of the Ag mode reaches a maximum at about 80 K. Tentative explanations of some of the observed effects are given, taking into account the nearly half-metallic nature of CoS2.

Bulk electronic structure of quasicrystals.

We use hard x-ray photoemission to resolve a controversial issue regarding the mechanism for the formation of quasicrystalline solids, i.e., the existence of a pseudogap at the Fermi level. Our data from icosahedral fivefold Al-Pd-Mn and Al-Cu-Fe quasicrystals demonstrate the presence of a pseudogap, which is not observed in surface sensitive low energy photoemission because the spectrum is affected by a metallic phase near the surface. In contrast to Al-Pd-Mn, we find that in Al-Cu-Fe the pseudogap is fully formed; i.e., the density of states reaches zero at E(F) indicating that it is close to the metal-insulator phase boundary.

Spin-valve-like magnetoresistance in Mn2NiGa at room temperature.

Spin valves have revolutionized the field of magnetic recording and memory devices. Spin valves are generally realized in thin film heterostructures, where two ferromagnetic (FM) layers are separated by a nonmagnetic conducting layer. Here, we demonstrate spin-valve-like magnetoresistance at room temperature in a bulk ferrimagnetic material that exhibits a magnetic shape memory effect. The origin of this unexpected behavior in Mn(2)NiGa has been investigated by neutron diffraction, magnetization, and ab initio theoretical calculations. The refinement of the neutron diffraction pattern shows the presence of antisite disorder where about 13% of the Ga sites are occupied by Mn atoms. On the basis of the magnetic structure obtained from neutron diffraction and theoretical calculations, we establish that these antisite defects cause the formation of FM nanoclusters with parallel alignment of Mn spin moments in a Mn(2)NiGa bulk lattice that has antiparallel Mn spin moments. The direction of the Mn moments in the soft FM cluster reverses with the external magnetic field. This causes a rotation or tilt in the antiparallel Mn moments at the cluster-lattice interface resulting in the observed asymmetry in magnetoresistance.

Structure and reactivity of Bi allotropes on the fivefold icosahedral Al-Pd-Mn quasicrystal surface.

The growth of Bi on a pseudomorphic Bi monolayer on the fivefold surface of the icosahedral Al-Pd-Mn quasicrystal has been investigated using low energy electron diffraction and scanning tunnelling microscopy. Initially randomly oriented pseudocubic islands are formed with a preference for an even number of layers. Subsequently a morphological transformation takes place to hexagonal Bi islands, which align along high symmetry directions of the substrate. The Bi flux is found to have a strong effect on which island structure is preferred. When C(60) is adsorbed on the three different allotropes of Bi present in this system, hexagonal C(60) islands are formed in each case. On the pseudocubic and hexagonal islands, the C(60) islands are aligned with the substrate. We discuss the energetic, kinetic and geometrical factors which influence the morphological transformation referred to above.

A rapid method to correct objective lens astigmatism in a TEM.

This work describes a rapid method to correct the two-fold astigmatism of transmission electron microscope (TEM) objective lens employing caustic curve when no objective aperture is inserted. The method makes use of rounding the caustic curve via the objective lens stigmators after the condenser lens astigmatism has been corrected. It has many advantages over other methods, it is fast, straightforward, and does not need holes or an amorphous material.

Manganese adlayers on i-Al-Pd-Mn quasicrystal: growth and electronic structure.

Pseudomorphic growth of thin elemental metal films is often observed on a variety of crystalline solids. On quasicrystalline surfaces with their complex structure and the absence of translational periodicity, the situation is different since elemental metals do not exhibit quasicrystalline order, and hence the specific interaction between overlayer and substrate is decisive. Here we study the growth of manganese films on an icosahedral i-Al-Pd-Mn alloy with a view to establishing the growth mode and electronic structure. Although we observe an exponential intensity variation of the adlayer and substrate related x-ray photoemission spectroscopy (XPS) peaks, low energy electron diffraction (LEED) shows that Mn adlayers do not exhibit quasicrystallinity. The detailed structure of the Mn 2p core level line reveals considerable electronic structure differences between the quasicrystalline and elemental metal environment. Evidence of a substantial local magnetic moment on the Mn atoms in the overlayer (about 2.8  µ(B)) is obtained from the Mn 3s exchange splitting.

Heat capacity and thermal expansion of the itinerant helimagnet MnSi.

The heat capacity and thermal expansion of a high quality single crystal of MnSi were measured at ambient pressure at zero and high magnetic fields. The calculated magnetic entropy change in the temperature range 0-30 K is less than 0.1R, a low value that emphasizes the itinerant nature of magnetism in MnSi. A linear temperature term dominates the thermal expansion coefficient in the range 30-150 K, which correlates with an enhancement of the linear electronic term in the heat capacity. A surprising similarity among the variations of the heat capacity, thermal expansion coefficient and temperature derivative of the resistivity is observed through the phase transition in MnSi. Specific forms of the heat capacity, thermal expansion coefficient and temperature derivative of resistivity at the phase transition to a helical magnetic state near 29 K are interpreted as the combination of sharp first-order features and broad peaks or shallow valleys of as yet unknown origin. The appearance of these broad satellites probably hints at a frustrated magnetic state slightly above the transition temperature in MnSi.

Experimental investigation of the electronic structure of Gd(5)Ge(2)Si(2) by photoemission and x-ray absorption spectroscopy.

The electronic structure of the magnetic refrigerant Gd(5)Ge(2)Si(2) has been experimentally investigated by photoemission and x-ray absorption spectroscopy. The resonant photoemission and x-ray absorption measurements performed across the Gd N(4,5) and Gd M(4,5) edges identify the position of Gd 4f multiplet lines, and assess the 4f occupancy (4f(7)) and the character of the states close to the Fermi edge. The presence of Gd 5d states in the valence band suggests that an indirect 5d exchange mechanism underlies the magnetic interactions between Gd 4f moments in Gd(5)Ge(2)Si(2). From 175 to 300 K the first 4 eV of the valence band and the Gd partial density of states do not display clear variations. A significant change is instead detected in the photoemission spectra at higher binding energy, around 5.5 eV, likely associated to the variation of the bonding and antibonding Ge(Si) s bands across the phase transition.

Quantum size effects in metal thin films grown on quasicrystalline substrates.

We have investigated by scanning tunneling microscopy the growth of Bi and Ag thin films on the fivefold surface of Al63Cu24Fe13 and Al72Pd19.5Mn8.5 quasicrystal, respectively. For both systems, we observe the formation of islands with magic height, corresponding to the stacking of a specific number of atomic layers. We interpret this unusual growth morphology in terms of quantum size effects, arising from the confinement of the electron within the film. The magic island heights are thus a direct manifestation of the electronic structure of the quasicrystalline substrates.

Anisotropy of the magnetoresistance in Gd5Si2Ge2.

The observed magnetoresistance of single crystalline Gd5Si2Ge2 is negative and strongly anisotropic. The absolute values measured along the [100] and [010] directions exceed those parallel to the [001] direction by more than 60%. First principles calculations demonstrate that a structural modification is responsible for the anisotropy of the magnetoresistance, and that the latter is due to a significant reduction of electronic velocity in the [100] direction and the anisotropy of electrical conductivity.

Structure of the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal: experimental evidence of bulk truncations at larger interlayer spacings.

Based on scanning tunneling microscopy of the fivefold surface of the icosahedral Al-Cu-Fe quasicrystal and the refined structure model of the isostructural i-Al-Pd-Mn, we present evidence that the surface corresponds to bulk truncations at the positions where blocks of atomic layers are separated by larger interlayer spacings (gaps). Both step-height distribution and high resolution scanning tunneling microscopy images on terraces reveal bulk truncations at larger gaps.

Origin of magnetic and magnetoelastic tweedlike precursor modulations in ferroic materials.

Based on experimental observations of modulated magnetic patterns in a Co0.5Ni0.205Ga0.295 alloy, we propose a model to describe a (purely) magnetic tweed and a magnetoelastic tweed. The former arises above the Curie (or Néel) temperature due to magnetic disorder. The latter results from compositional fluctuations coupling to strain and then to magnetism through the magnetoelastic interaction above the structural transition temperature. We discuss the origin of purely magnetic and magnetoelastic precursor modulations and their experimental thermodynamic signatures.

Pseudomorphic growth of a single element quasiperiodic ultrathin film on a quasicrystal substrate.

An ultrathin film with a periodic interlayer spacing was grown by the deposition of Cu atoms on the fivefold surface of the icosahedral Al70Pd21Mn9 quasicrystal. For coverages from 5 to 25 monolayers, a distinctive quasiperiodic low-energy electron diffraction pattern is observed. Scanning tunneling microscopy images show that the in-plane structure comprises rows having separations of S=4.5+/-0.2 A and L=7.3+/-0.3 A, whose ratio equals tau=1.618... within experimental error. The sequences of such row separations form segments of terms of the Fibonacci sequence, indicative of the formation of a pseudomorphic Cu film.

Formation of a stable decagonal quasicrystalline Al-Pd-Mn surface layer.

We report the in situ formation of an ordered equilibrium decagonal Al-Pd-Mn quasicrystal overlayer on the fivefold symmetric surface of an icosahedral Al-Pd-Mn monograin. The decagonal structure of the epilayer is evidenced by x-ray photoelectron diffraction, low-energy electron diffraction, and electron backscatter diffraction. This overlayer is also characterized by a reduced density of states near the Fermi edge as expected for quasicrystals. This is the first time that a millimeter-size surface of the stable decagonal Al-Pd-Mn is obtained, studied, and compared to its icosahedral counterpart.

Magnetic EXAFS at Gd L-edges: the spin-pair-distribution function of Gd neighbors.

The purpose of the experiment is to study the normal and the Magnetic EXAFS (MEXAFS) since EXAFS is the method of choice to investigate the local pair- and spin-pair-distribution function. We present MEXAFS and EXAFS measurements at the L-edges of a Gd single crystal in the temperature range of 10 K to 250 K. Therefore we are able to investigate the MEXAFS in a wide range of the reduced temperature t=T/Tc of 0.04 < or = t < or = 0.85 with Tc=293 K. We find a strong decrease of the nearest neighbor EXAFS which retains only about 35% of its 10 K value already at 250 K. This highlights the importance of lattice vibrations. To analyze the individual scattering contributions to the MEXAFS and the EXAFS, ab initio calculations (FEFF code) have been carried out. The comparison of the temperature-dependent damping of the normal EXAFS with the spin-dependent MEXAFS allows us to separate the influence of lattice vibrations (Debye temperature 160 K) from the magnetic ordering (Curie temperature) on the MEXAFS.