Gang Xiao

The muon-spin-relaxation rate $\ensuremath{\sigma}$ has been measured in sixteen specimens of high-${T}_{c}$ cuprate superconductors (the 2:1:4, 1:2:3, 2:2:1:2, and 2:2:2:3 series). This has allowed us to study the magnetic field penetration depth $\ensuremath{\lambda}$ and thus the superconducting carrier density ${n}_{s}$ divided by the effective mass ${m}^{*}(\ensuremath{\sigma}\ensuremath{\propto}\frac{1}{{\ensuremath{\lambda}}^{2}}\ensuremath{\propto}\frac{{n}_{s}}{{m}^{*}})$. A universal linear relation between ${T}_{c}$ and $\ensuremath{\sigma}(T\ensuremath{\rightarrow}0)\ensuremath{\propto}\frac{{n}_{s}}{{m}^{*}}$ has been found with increasing carrier doping. In heavily doped samples, however, ${T}_{c}$ shows saturation and suppression with increasing $\frac{{n}_{s}}{{m}^{*}}$. This saturation starts at different values of $\frac{{n}_{s}}{{m}^{*}}$ for materials with different multiplicities of CuO planes.

The role of grain boundaries in the magnetoresistance (MR) properties of the manganites has been investigated by comparing the properties of epitaxial and polycrystalline films of ${\mathrm{La}}_{0.67}{D}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}} (D=\mathrm{C}\mathrm{a},\phantom{\rule{0ex}{0ex}}\mathrm{S}\mathrm{r},\phantom{\rule{0ex}{0ex}}\mathrm{o}\mathrm{r}\phantom{\rule{0ex}{0ex}}\mathrm{v}\mathrm{a}\mathrm{c}\mathrm{a}\mathrm{n}\mathrm{c}\mathrm{i}\mathrm{e}\mathrm{s})$. While the MR in the epitaxial films is strongly peaked near the ferromagnetic transition temperature and is very small at low temperatures, the polycrystalline films show large MR over a wide temperature range down to 5 K. The results are explained in terms of switching of magnetic domains in the grains and disorder-induced canting of Mn spins in the grain-boundary region.

Andreev reflection at a $\mathrm{Pb}/{\mathrm{CrO}}_{2}$ point contact has been used to determine the spin polarization of single-crystal ${\mathrm{CrO}}_{2}$ films made by chemical vapor deposition. The spin polarization is found to be $0.96\ifmmode\pm\else\textpm\fi{}0.01$, which confirms that ${\mathrm{CrO}}_{2}$ is a half-metallic ferromagnet, as theoretically predicted.

We have used a simple self-aligned process to fabricate magnetic tunnel junctions down to submicron sizes. Optical and electron-beam lithographies were used to cover a range of areas spanning five orders of magnitude. The bottom magnetic electrodes (Co or permalloy) in our junctions were exchange biased by an antiferromagnetic layer (MnFe). The top electrodes were made of soft magnetic materials (Co or permalloy). We have consistently obtained large magnetoresistance ratios (15%–22%) at room temperature and in fields of a few tens of Oe. The shape of the field response of the magnetoresistance was varied from smooth to highly hysteretic by adjusting the shape anisotropy of one junction electrode.