Bibliothek

Notizen: Our neutron-powder-diffraction experiment revealed that URhSi crystallizes in the orthorhombic TiNiSi (space group Pnma) structure and orders ferromagnetically at low temperatures with the U magnetic moments of 0.11 μB aligned along the c axis. Anomalies in the temperature dependence of the magnetic susceptibility, specific heat and electrical resistivity indicate that URhSi orders below 9.5 K. The enhanced Cp/T value (extrapolated to 0 K) of 186 mJ/mol K2 can be partially reduced in magnetic fields, which indicates a considerable magnetic contribution even at very low temperatures. The ferromagnetic ground state is documented also by magnetization measurements at low temperatures. The high-field magnetization data obtained on oriented powder reveal a strong magnetocrystalline anisotropy. All the results obtained on polycrystalline samples classify URhSi as an itinerant 5f ferromagnet with very reduced U magnetic moments. © 1996 American Institute of Physics.

Notizen: By specific-heat, magnetization, electrical resistivity, and neutron-diffraction measurements on a single crystal we have confirmed that UNiGe orders antiferromagnetically below 50.5 K into an incommensurate phase with q=(0,1/2,−1/2)±(0,δ,δ), δ∼0.15. δ decreases continuously with decreasing temperature to ∼0.123 at 41.5 K, where the incommensurate phase vanishes in a first-order phase transition and a commensurate antiferromagnetic structure with q=(0,1/2,1/2) sets in and remains stable down to the lowest temperatures. If a magnetic field sufficient to induce a metalmagnetic transition (1–5 T) is applied along the c axis, both antiferromagnetic phases are transformed to an uncompensated AF phase with q=(0,1/3,1/3) yielding a nonzero magnetization M≈1/3×MS. The latter structure is destroyed and a complete alignment of U moments is achieved in fields above 10 T. The strikingly different B-T diagrams observed for a magnetic field applied along different crystallographic directions reflect strongly anisotropic exchange interactions.

Notizen: The transition from itinerant antiferromagnetic behavior in UNiAl to ferromagnetism in UNiGa was studied by means of magnetic and specific-heat measurements on the pseudoternary series UNi(Al,Ga). The results are discussed in terms of a trend to increasing localization of the 5f electrons when proceeding to higher Ga concentrations.

Notizen: We created helical magnetic structures by depositing in sequence layers of lanthanum and iron (each 30 Å thick) on a substrate rotating in a fixed field of a few Oe. The magnetic field was sufficient to define the magnetization of each layer during growth without disturbing the magnetization of the layers already formed. As a result, the imprinted helical magnetic structure had a chirality given by the direction of rotation of the substrate; the pitch was given by the speed of rotation and the rate of deposition. This finding indicates that it is possible to engineer mesoscopic magnetic structures. © 1998 American Institute of Physics.

Notizen: Electron spin resonance has been studied in single crystal spheres of EuxSr1−xS with samples of nominal composition x=0.4, 0.5, and 0.54. The emphasis has been on the field for resonance of x=0.4 material at 11.65 GHz at temperatures between 4.2 and 1.65 K. Even at the lowest temperatures the lines do not vary with crystal orientation. In all samples there is a depression of the field for resonance beginning at about 10 K, and increasing to the lowest temperatures. This is interpeted as an "internal'' field Hi. Measurements at 17 GHz reveal that Hi for all three compositions decreases with increasing frequency. Surprisingly, at 1.65 K, this seems to vary with sphere size for the x=0.4 composition. In addition to the experiments on spheres, a small plate-shaped specimen of x=0.4 was studied. It too showed a large depression of the field for resonance at 1.75 K.

Notizen: Neutron-spin-echo (NSE) experiments were performed on polycrystalline samples of EuxSr1−xS for x=0.4 and 0.54 in the temperature range of 1.2〈T〈10 K and for 0.036〈Q〈0.18 A(ring)−1. The x=0.4 sample exhibits a paramagnetic (PM) to spin-glass (SG) transition near Tg ∼2 K. In the x=0.54 sample, large ferromagnetic correlations develop below 5 K and a SG state appears at lower temperatures. In the NSE experiment, the spin-spin correlation function S(Q,t) is measured directly for times between 0.03〈t〈5 nsec. At low temperatures, both materials exhibit a weak Q dependence in the dynamics and the spins are essentially frozen over the time range explored. On heating the x=0.4 sample, the spins start to fluctuate more rapidly, but no dramatic change occurs around Tg. On heating the x=0.54 sample, S(Q,t) decreases rapidly with time. Near 5 K, S(Q,t) is exponential (e−Γt) with Γ being strongly Q dependent. Measurements of the depolarization of the scattered beam confirms the absence of true long-range ferromagnetic order below Tc.

Notizen: Raman measurements in all magnetic phases of Eux Sr1−xS show a new type of q≠0 magnetic scattering at low temperatures. The intensity can be quantitatively described by using in the scattering cross section a modified Ornstein–Zernike ansatz for the spin correlation function. A consistent interpretation of the Raman data in correspondence with the magnetic phase diagram has been achieved. Eux Sr1−xS shows a rather unusual magnetic phase diagram. Ferromagnetism breaks down below x=0.51. Spin-glass behavior is observed at low temperatures for 0.13〈x〈0.65. Most interest has been attracted recently by the reentrant phase boundary between the ("frustrated'') ferromagnet and the spin glass for 0.51〈x〈0.65. We performed Raman scattering on Eux Sr1−xS for x=0.65, 0.54, and 0.40 at temperatures between 1.5 and 300 K and in applied magnetic fields up to 7 T using 5145-A(ring) laser excitation. Besides the well-known spin-disorder induced scattering from the LO(L) phonons near the zone boundary in EuS, one observes in the (frustrated) ferromagnetic phase and in the spin-glass regime additional scattering intensity which is of magnetic origin. A consistent theoretical description of our results is given by the one-phonon one-spin scattering cross section used for EuS. For the spin correlation function, we used a modified Ornstein–Zernike ansatz, with an additional exponent y which describes the spin system from uncorrelated ( y=0) via statistically coupled ( y=2) to ferromagnetically aligned spins ( y〉2). Systematic variations of the exponent y as a function of composition and temperature in high magnetic fields characterize the spin correlations and yield a consistent interpretation of the Raman data in correspondence with the magnetic phase diagram. For example, for Eu0.54 Sr0.46S, we observe in the (frustrated) ferromagnetic phase with decreasing temperature an increase of y from y=2.0 to y=2.6 near the reentrant phase boundary and a decrease to y=2.0 in the spin-glass phase. The latter value is also found in the spin glass phase of Eu0.40Sr0.60S. In conclusion, we would like to point out that Raman measurements in Eux Sr1−xS show a new type of q≠0 magnetic scattering at low temperatures and in applied magnetic fields, not observed in the ferromagnetic state of EuS. The deduced exponent y of the spin correlation function agrees well with that deduced from diffuse magnetic neutron scattering. In addition, our measurements in external magnetic fields up to 7 T provide information complementary to the zero-field neutron measurements and yield a microscopic understanding of the magnetic phase diagram.