Two-Staged Magnetoresistance Driven by the Ising-Like Spin Sublattice in ${\mathrm{SrCo}}_6{\mathrm{O}}_{11}$

A two-staged, uniaxial magnetoresistive effect has been discovered in ${\mathrm{SrCo}}_6{\mathrm{O}}_{11}$ having a layered hexagonal structure. Conduction electrons and localized Ising spins are in different sublattices but their interpenetration makes the conduction electrons sensitively pick up the stepwise field dependence of magnetization. The stepwise field dependence suggests two competitive interlayer interactions between ferromagnetic Ising-spin layers, i.e., a ferromagnetic nearest-layer interaction and an antiferromagnetic next-nearest-layer interaction. This oxide offers a unique opportunity to study nontrivial interplay between conduction electrons and Ising spins, the coupling of which can be finely controlled by a magnetic field of a few Tesla.

Figure 1

(a) Out-of-plane magnetization curves ($H//c$) at 5, 15, and 25 K. In-plane magnetization at 5 K is also appended. (b) Normalized out-of-plane resistivity, ${\rho }_c(H)/{\rho }_c(0)$, and (c) normalized in-plane resistivity, ${\rho }_{ab}(H)/{\rho }_{ab}(0)$, at 5, 15, and 25 K under a magnetic field applied parallel to the $c$ axis. For clarity, the resistivity curves at 5 and 15 K have been shifted downward. In order to indicate the hysteresis, the resistivity curves obtained on reducing magnetic fields are denoted with the dashed curves, and the sequential numbers with arrows showing the order of measurements are appended. Schematic crystal structure and sample configurations for the resistivity measurements were illustrated as insets.

Figure 2

Temperature dependence of the magnetic and electronic properties. (a) The dc magnetization along the $c$ axis divided by a field, $M/H$, obtained on field heating after zero-field cooling and field cooling in a magnetic field of 0.01 and 1 T. (b) The in-plane and the out-of-plane resistivities, ${\rho }_{ab}$ and ${\rho }_c$, respectively, with or without the magnetic field of 5 T along the $c$ axis.

Figure 3

Co-NMR spectra measured at 4.2 K with a frequency of 92.3 MHz for the $3/3$-plateau state (a) and with a frequency of 24.4 MHz for the $1/3$-plateau state (b). The blue area represents a spectral component featured by a large shift caused by the presence of a transferred internal magnetic field, while the yellow component with a small shift is affected by the nuclear quadrupole interactions only. The insets are schematic illustrations of the corresponding magnetic states.

Figure 4

Calculated PDOS per Co muffin-tin sphere for Co(1), Co(2) (a), and Co(3) (b) in the antiferromagnetic solution ($\mathrm{Co}(1)\downarrow \mathrm{Co}(2)\downarrow \mathrm{Co}(3)\uparrow$). (c) Schematic illustration of a spin-valve effect in the $1/3$-plateau state. The thick arrows on the pillars indicate directions of the Co(3) moments, while the small circled arrows denote spin polarization of the conduction electrons. The scattering rate along the out-of-plane is enhanced when the moment of Co(3) is antiparallel to that of the conduction electrons.