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10 results on '"Riedi, P. C."'

1. Thermal Expansion and Magnetostriction Studies of a Kondo Lattice Compound: Ceagsb2

Author

Adroja, D. T., Riedi, P. C., Armitage, J. G. M., and Fort, D.

Subjects
Condensed Matter - Strongly Correlated Electrons, Condensed Matter - Materials Science
Abstract

We have investigated a single crystal of CeAgSb2 using low field ac-susceptibility, thermal expansion and magnetostriction measurements in the temperature range 1.5K to 90K. The ac-susceptibility exhibits a sharp peak at 9.7K for both B//c and B perp c due to the magnetic ordering of the Ce moment. The thermal expansion coefficient alpha, exhibits highly anisotropic behaviour between 3K and 80K : alpha is positive for dL/L perp c, but negative for dL/L // c. Furthermore, alpha (for dL/L) perp c (i.e. in ab-plane) exhibits a sharp peak at TN followed by a broad maximum at 20K, while a sharp negative peak at TN followed by a minimum at 20K has been observed for (dL/L //) the c direction. The observed maximum and minimum in alpha(T) at 20K have been attributed to the crystalline field effect on the J=5/2 state of the Ce3+ ion. The magnetostriction also exhibits anisotropic behaviour with a large magnetostriction along the c-axis. The ab-plane magnetostriction exhibits a peak at B=3.3T at 3K, which is consistent with the observed peak in the magnetoresistance measurements., Comment: 4 Pages (B5), 3 figures, submitted to SCES2002

Published
2002

4. Neutron scattering and thermal studies of the Ni-incorporated CeSbNix system

Author

Adroja, D. T., Armitage, J. G. M., Riedi, P. C., Jung, M. H., Tun, Z., Takabatake, Toshiro, Adroja, D. T., Armitage, J. G. M., Riedi, P. C., Jung, M. H., Tun, Z., and Takabatake, Toshiro

Abstract

Neutron scattering, thermal expansion, and magnetostriction measurements have been carried out on the cubic compounds CeSbNix (x=0-0.4). Inelastic neutron scattering studies show a well-defined crystal field (CF) excitation at 4.06 (±0.04) meV and 5.02(±0.05) meV in x=0.08 and 0.15, respectively. The crystal field splitting increases with x compared with the parent compound CeSb (3.19 meV) in spite of the lattice parameter increasing with Ni incorporation. The implication is that the increase in the CF splitting in x=0.08 and 0.15 is due to a collapse of the p-f mixing between the Sb 5p holes and the localized Ce 4f electrons. The analysis of inelastic spectra of x=0.15 shows that the ground state is a doublet (Γ7), which explains the temperature-dependent behavior of the magnetic susceptibility. Thermal expansion shows a dramatic change in behavior with Ni composition. The thermal expansion coefficient exhibits a first-order transition at 15.4 K in CeSb, which disappears for Ni composition as low as x=0.035, as well as in an applied field of 8 T. A large magnetostriction has been observed in CeSb in the magnetic-ordered state as well as in the paramagnetic state. The absolute values of the magnetostriction are reduced considerably in the Ni-incorporated alloys. The volume magnetostriction of x= 0.15 alloy exhibits a scaling behavior in the paramagnetic state from which we have estimated the product of the magnetovolume coupling constant and the isothermal compressibility.

Published
2000

10. Magnetic studies of cobalt based granular thin films

Author

Oates, Colin John and Riedi, P. C.

Subjects
530.4, QC176.84M2O7, Thin films--Magnetic properties
Abstract

The magnetic recording media used for hard disks in laptops and PC's is constantly being improved, leading to rapid increases in data rate and storage density. However, by the year 2010, it is predicted that the superparamagnetic limit will be reached, which is potentially insufficient for data storage. At the beginning of this century, CoCr -based alloys are used in longitudinal media since cobalt has a high magnetocrystalline anisotropy. In this thesis, the static and dynamic properties of longitudinal recording thin films were investigated in order to explain and correlate their magnetic characteristics to their recording properties. The samples in question were test samples and some were in commercial use. Magnetic techniques such as high field ferromagnetic resonance and torque magnetometry were used to determine accurately the crystalline anisotropy field. High field ferromagnetic resonance is an ideal tool to determine the crystalline anisotropy, magnetisation, Lande g-factor and the gyromagnetic damping factor. In contrast to previous work, there are no FMR simulations and so all the relevant parameters were determined directly from measurement. Ideally, there should be no exchange interactions between the neighbouring cobalt grains; however, interactions between the grains within the CoCr-alloy recording layer exist. Previous work on the measurements of interactions in recording media involves measuring the sample's magnetisation. In this thesis, an alternative novel method involves torque magnetometry. Another technique that was used in this thesis is small angle neutron scattering, which aims to determine the size of the magnetic grains and compare that with the physical size determined from TEM, by Seagate. There is an extended section on CoxAg1-x granular thin films, which involves determining the sample's g-factor, effective anisotropy, grain size, exchange constant and comparing the FMR lineshapes at 9.5 and 92GHz.

Published
2002

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