K. Le Dang

Competition between Spin-Peierls Phase and Three-Dimensional Antiferromagnetic Order in CuGe1-xSixO3

We have performed magnetic-susceptibility measurements and Cu nuclear-magnetic-resonance studies on single crystals of the quasi-one-dimensional antiferromagnet CuGe1-xSixO3, with 0 ≤ x ≤ 0.1. The Si doping has a drastic effect on the spin-Peierls temperature TSP which decreases at TSP (x)/TSP (0) 1-50x at low Si concentrations. For x ≥ 0.005, three-dimensional antiferromagnetic order with an easy axis along the Cu chains takes place below a well-defined Neel temperature TN. A qualitative interpretation of the phase diagram (T, x) is given.

Magnetic properties of ferromagnetic thin sandwiches correlated to their crystallographic structure and interface morphology: HREM and NMR study

Abstract High resolution electron microscopy (HREM) studies were made on Au/Co/Au sandwiches having ultrathin Co layers. These films were shown previously to possess an important interface anisotropy. We can conclude from the present experiments that there are some fluctuations of the c axis around the film normal, and change in lattice parameter for certain crystallites. (The Co layer is composed of small crystallites of about 7 nm.) All these crystallographic characteristics explain why the Co NMR spectrum is asymmetric with a low frequency tail.

Large magnetoresistance effects in UHV grown fcc (111) Co/Cu multilayers

Abstract Co/Cu multilayers have been grown in ultrahigh vacuum on Au(111) buffer layers. As shown by X-ray diffraction and 59 Co NMR, their structure is fcc (111) with atomically flat interfaces. The studied samples with Cu thicknesses of 3, 5 and 10 atomic monolayers exhibit a large magnetoresistance (MR). This result and the large saturation field observed by MR and by magnetization measurements could be indicative of antiferromagnetic interlayer couplings, previously reported for sputtered Co/Cu multilayers with comparable Cu thickness.

Transport properties and magnetic behaviour of La1-xSrxMnO3 single crystals

A semiconductor-metal transition occurring just below the Curie temperature TC was observed in single crystals of perovskite La1-xSrxMnO3 for x>or=0.175. For x or=0.13 whereas huge MR persists well below TC for the x=0.10 sample, which is probably due to a non-collinear spin configuration.

Magnetic and structural properties of rf‐sputtered Co/Fe and Co/Cr multilayers

Multilayers alternating cobalt with different spacers such as iron and chromium at nanometric scale have been deposited by rf diode sputtering. The structures have been characterized in situ by kinetic ellipsometry and ex situ by grazing x‐ray reflection, x‐ray diffraction, Auger profile analysis, and transmission electron microscopy. Nuclear magnetic resonance (NMR) and high‐field SQUID magnetometry have been used to determine the magnetic properties. In Co/Fe multilayers the structure strongly depends on the cobalt thickness tCo. For tCo below 2 nm, the cobalt layers exhibit a bcc crystalline structure, and the crystalline coherence extends over many periods. For tCo above 2 nm, for Co a mixed fcc‐hcp phase has been observed. In this case the crystalline coherence is destroyed from one iron layer to the other. The NMR frequency (198 MHz), and the magnetic moment (1.58 μB/atom) of bcc cobalt have been evaluated. A similar behavior is observed in Co/Cr multilayers, the bcc phase disappearing around tCo=1....

Influence of crystal structure on the magnetoresistance of Co/Cu multilayers

The saturation magnetoresistance (MR) and Co nuclear magnetic resonance (NMR) spin echo have been measured in a series of sputtered Co/Cu multilayers, with Co thicknesses ranging from 10 to 30 A. The multilayers were grown on 50 A Cu or Fe buffer layers. A clear correlation has been established between the MR and the Co NMR line profile. The sample with the highest MR exhibits a single fcc Co line with weak‐intensity wings. Increasing the number of (111) layer stacking faults and grain boundaries leads to decreasing MR.

Direct experimental evidence of a body-centered-cubic Co phase in radio-frequency-sputtered Co/Fe multilayers

Radio‐frequency‐sputtered Co/Fe multilayers with layer thicknesses varying in the 8–100 A range were studied by extended x‐ray absorption fine structure (EXAFS), x‐ray absorption near edge spectroscopy (XANES), and nuclear magnetic resonance (NMR) techniques. While the Fe environment is body‐centered‐cubic (bcc) in all the samples, the Co environment depends drastically on the Co layer thickness. Under ≂20 A, investigation at the Co K edge by XANES provides a bcc fingerprint in both polarization directions. The XANES profile becomes typical of a full hcp stacking only for Co layer thicknesses larger than 98 A. This behavior is confirmed by EXAFS spectra at the Co K edge and NMR analysis. For Co layer thickness between 6 and 18 A, eight first neighbors at 2.48 A and six second neighbors at 2.83 A are detected which match the bcc phase. Resonance peak at a very low frequency is also observed (198 MHz). The second shell radius appears at 2.78 A only for larger Co layer thicknesses. This effect goes along wit...

Oscillatory behavior of the magnetoresistance versus Cu spacer thickness in UHV-grown Co / Cu(111) multilayers

Abstract The magnetoresistance of Au 3 /Cu 0.3 / Co 1.2 /Cu t /Co 1.2 /Cu 0.3 /Au 25 (111) multilayers (the numbers indicate the thickness grown by evaporation in UHV has been investigated between 1.4 and 300 K. Byusing a stepped wedge-shaped middle Cu layer, eight different t values could be obtained in a single sample. The MR exhibits an oscillatory behaviour up to t = 3.4 nm with a period of 0.8–1 nm, which is consistent with the predictions of RKKY theory for oscillatory interlayer coupling across Cu(111).

Conductivity and magnetoresistance of La1−xSrxMnO3 and La1−xSrxMn1−yMgyO3 single crystals

Abstract We present conductivity and MR measurements on single crystals of the mixed perovskite manganites La 1− x Sr x MnO 3 (0.1 x x = 0.17 and a metallic one at higher Sr concentrations. The MR is maximum near T c , which is strongly concentration dependent. In the sample with x = 0.15, the MR is not saturated even at 20 T. For the same composition, an important anisotropy of the electrical conductivity with respect to the [001] direction appears at T c and increases at lower temperatures. In the compound La 0.825 Sr 0.175 Mn 0.94 Mg 0.06 O 3 , the substitution of Mg for Mn lowers T c , in contrast to what is observed by substituting Sr for La. The crystal exhibits a well defined first order transition from orthorhombic to rhombohedral structure at a temperature T s which can be shifted by a magnetic field.

Anisotropic differential magnetoresistance of Cu/Co(100) multilayers

We report on structural and magnetic properties of diode rf‐sputtered fcc Cu/Co(100) multilayers. All the samples exhibit a pseudo‐epitaxial structure with interface roughness of the order of 6 A. As the Cu layer thickness is varied, we observe oscillations of magnetoresistance corresponding to oscillations of the interlayer coupling. The maximum of magnetoresistance is found for tCu=18.5 A (up to ΔR/R≊6%). We furthermore measure a very high differential magnetoresistance (ΔR/RΔH=1.4 kOe−1). A combination of antiferromagnetic coupling and quadratic in‐plane anisotropy is responsible for this large value of magnetoresistivity.