Domain structure in ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ and ${\mathrm{HoNi}}_2{\mathrm{B}}_2\mathrm{C}$ single crystals observed by a high-resolution Bitter decoration technique

Peculiarities of the superconducting vortex lattice structures in ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ and ${\mathrm{HoNi}}_2{\mathrm{B}}_2\mathrm{C}$ single crystals were studied by the high resolution Bitter decoration technique. Instead of forming uniform vortex lattices, rows of vortices, associated with pinning by magnetic twin boundaries, were observed in the superconducting state, below the antiferromagnetic transition temperatures, in both compounds. Crystallographic aspects of the vortex pinning by the magnetic twin boundaries in the ${\mathrm{RNi}}_2{\mathrm{B}}_2\mathrm{C}(\mathrm{R}=\text{rare earth})$ are discussed.

Figure 1

Representative decoration patterns for ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ for $T_d<T_N$ for different magnetic fields $H\parallel c$ : (a) $H=8\mathrm{Oe}$, $T_d=5.5\mathrm{K}$; (b) $H=72\mathrm{Oe}$, $T_d=5.8\mathrm{K}$; (c) $H=200\mathrm{Oe}$, $T_d=5.8\mathrm{K}$.

Figure 2

Magnetic field dependence of the width of the vortex bands. Decoration performed below $T_N$, at $T_d=5.5\text{−}5.8\mathrm{K}$. Each point gives an average value from a separate decoration experiment. Dashed line is a guide to the eye.

Figure 3

Decoration pattern for ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ in magnetic field $H=25\mathrm{Oe}$ and decoration temperature $T_d=7.6\mathrm{K}>T_N$.

Figure 4

Representative decoration patterns for ${\mathrm{HoNi}}_2{\mathrm{B}}_2\mathrm{C}$ for the decoration temperature below the incommensurate-commensurate magnetic transition for different magnetic fields $H\parallel c$ : (a) $H=17\mathrm{Oe}$, $T_d=4.3\mathrm{K}$; (b) $H=145\mathrm{Oe}$, $T_d=4\mathrm{K}$.

Figure 5

Decoration pattern containing triangular vortex lattice and vortex arrays along [100] in ${\mathrm{HoNi}}_2{\mathrm{B}}_2\mathrm{C}$ in magnetic field $H=72\mathrm{Oe}$ and $T_d=5.8\mathrm{K}$ (in the incommensurate antiferromagnetic phase).

Figure 6

(a) Vortex structure of ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$ in magnetic field $H=90\mathrm{Oe}$ tilted by $ϴ=72°$ from $c$ axis, $T_d=4.6\mathrm{K}$; (b) vortex arrays in ${\mathrm{HoNi}}_2{\mathrm{B}}_2\mathrm{C}$ in magnetic field $H=72\mathrm{Oe}$ tilted by $ϴ=60°$ from $c$ axis, $T_d=4.3\mathrm{K}$. In both cases the dense vortex lines defining the bands are clearly preserved while the vortex lines within the “bulk” of the bands are faded out. Absence of the vortex lines images within the bands is possibly related to high values of the angles $ϴ$.