arXiv subjects

Joseph H. Ross Jr.

Publications and source records attributed to Joseph H. Ross Jr..

3 recordsLinked to original sources

arXiv2003

Ferromagnetism in Fe-doped Ba6Ge25 Chiral Clathrate

We have successfully synthesized a Ba6Ge25 clathrate, substituting 3 Fe per formula unit by Ge. This chiral clathrate has Ge sites forming a framework of closed cages and helical tunnel networks. Fe atoms randomly occupy these sites, and exhibit high-spin magnetic moments. A ferromagnetic transition is observed with Tc = 170 K, the highest observed Tc for a magnetic clathrate. However, the magnetic phase is significantly disordered, and exhibits a transformation to a re-entrant spin glass phase. This system has a number of features in common with other dilute magnetic semiconductors.

cond-mat.mtrl-sci↗

arXiv2003

Ferromagnetism and Metamagnetism in Copper-doped Germanium Clathrate

Cu-doped type-I germanium clathrate can exhibit dilute magnetism, including ferromagnetism, antiferromagnetism, and metamagnetic transitions up to 90 K. 63Cu NMR measurements confirm that these transitions are due to a dilute composition of magnetic defects coupled by conduction electrons, behavior similar to that of magnetic semiconductors. Magnetic measurements indicate localized magnetic moments, attributed to clusters of magnetic ions, with competition between ferromagnetic and antiferromagnetic exchange, and also indications of glassy behavior in the ferromagnetic phase. NMR Knight shifts and relaxation times show that the conduction band is metallic with a large Korringa ratio. Comparison to a mean-field theory for the ordering behavior gives a good accounting for the ferromagnetic transition.

cond-mat.mtrl-sci↗

arXiv2003

NMR and Mossbauer study of spin dynamics and electronic structure of Fe{2+x}V{1-x}Al and Fe2VGa

In order to assess the magnetic ordering process in Fe2VAl and the related material Fe2VGa, we have carried out nuclear magnetic resonance (NMR) and Mossbauer studies. 27Al NMR relaxation measurements covered the temperature range 4 -- 500 K in Fe(2+x)V(1-x)Al samples. We found a peak in the NMR spin-lattice relaxation rate, 27T1^-1, corresponding to the magnetic transitions in each of these samples. These peaks appear at 125 K, 17 K, and 165 K for x = 0.10, 0, and - 0.05 respectively, and we connect these features with critical slowing down of the localized antisite defects. Mossbauer measurements for Fe2VAl and Fe2VGa showed lines with no hyperfine splitting, and isomer shifts nearly identical to those of the corresponding sites in Fe3Al and Fe3Ga, respectively. We show that a model in which local band filling leads to magnetic regions in the samples, in addition to the localized antisite defects, can account for the observed magnetic ordering behavior.

cond-mat.mtrl-sci↗