Spin-glass transition in a La-doped ${\mathrm{Sr}}_2\mathrm{Mn}\mathrm{W}{\mathrm{O}}_6$ double perovskite

Polycrystalline ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Mn}\mathrm{W}{\mathrm{O}}_6$ ($x=0.25$ and 0.5) materials have been prepared by a traditional solid state sintering method and studied by neutron powder diffraction (NPD) and magnetization measurements. Rietveld analysis of the temperature dependent NPD data shows that the compounds crystallize in monoclinic symmetry (space group $P{2}_1∕n$). The unit cell volume decreases with increasing ${\mathrm{La}}^{3+}$ concentration at the $A$ site. The crystal structure contains alternating $\mathrm{Mn}{\mathrm{O}}_6$ and $\mathrm{W}{\mathrm{O}}_6$ octahedra, considerably tilted due to the relative small size of the cations that occupy the $A$ sublattice of the perovskite. ac and dc magnetization measurements show a destruction of the low temperature antiferromagnetic phase by electron doping through substitution of ${\mathrm{Sr}}^{2+}$ for ${\mathrm{La}}^{3+}$. The electron doping creates a mixed tungsten valence $({\mathrm{W}}^{6+}∕{\mathrm{W}}^{5+})$ resulting in a low temperature spin-glass state.

Figure 1

[(a) and (b)] Observed (circles) and calculated (continuous line) NPD intensity profiles for ${\mathrm{Sr}}_{1.75}{\mathrm{La}}_{0.25}\mathrm{Mn}\mathrm{W}{\mathrm{O}}_6$ at 295 and $5\mathrm{K}$. The short vertical lines indicate the angular position of the allowed Bragg reflections. At the bottom in each figure, the difference plot, $I_{\mathit{obs}}-I_{\mathit{calc.}}$, is shown.

Figure 2

The three-dimensional crystal structure of the monoclinic perovskite ${\mathrm{Sr}}_{1.5}{\mathrm{La}}_{0.5}\mathrm{Mn}\mathrm{W}{\mathrm{O}}_6$. $\mathrm{Sr}∕\mathrm{La}$ atoms are shown as large black spheres; oxygens are shown as small black spheres at the corner of octahedra. Mn and W are located inside the white and gray octahedra, respectively.

Figure 3

[(a) and (b)] Observed (dots), calculated (lines), and difference (bottom) plots of NPD Rietveld profiles for ${\mathrm{Sr}}_{1.5}{\mathrm{La}}_{0.5}\mathrm{Mn}\mathrm{W}{\mathrm{O}}_6$ at 295 and $10\mathrm{K}$. The short vertical lines indicate the angular position of the allowed Bragg reflections. At the bottom in each figure, the difference plot, $I_{\mathit{obs}}-I_{\mathit{calc.}}$, is shown.

Figure 4

[(a)–(c)] ${\chi }_{ZFC}$ and ${\chi }_{FC}$ versus temperature for ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Mn}\mathrm{W}{\mathrm{O}}_6$, $x=0$ in (a), $x=0.25$ in (b), and $x=0.5$ in (c), measured with the applied fields $H=20$, 500, and $2000\mathrm{Oe}$.

Figure 5

Magnetization $\left(M\right)$ versus applied field $\left(H\right)$ at $T=10\mathrm{K}$ for ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Mn}\mathrm{W}{\mathrm{O}}_6$; square, circle, and triangular symbols correspond to $x=0.5$, 0.25, and 0, respectively.

Figure 6

[(a) and (b)] Frequency dependent ac susceptibility versus temperature for the ac field frequencies of 0.17, 1.7, 17, and $170\mathrm{Hz}$ for $x=0$.

Figure 7

[(a) and (b)] Frequency dependent ac susceptibility versus temperature for the ac field frequencies 0.17, 1.7, 17, and $170\mathrm{Hz}$ for $x=0.25$.

Figure 8

[(a) and (b)] Frequency dependent ac susceptibility versus temperature for the ac field frequencies 0.17, 1.7, 17, and $170\mathrm{Hz}$ for $x=0.5$.

Figure 9

Observed NPD patterns for ${\mathrm{Sr}}_{2-x}{\mathrm{La}}_x\mathrm{Mn}\mathrm{W}{\mathrm{O}}_6$ ($x=0.25$ and 0.5) at $5\mathrm{K}$ to locate the magnetic reflections. Magnetic reflection is indicated by *.