Fragile singlet ground-state magnetism in the pyrochlore osmates $R_2{\mathrm{Os}}_2{\mathrm{O}}_7$ ($R=\mathrm{Y}$ and Ho)

The singlet ground-state magnetism in pyrochlore osmates ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ is studied by dc and ac susceptibility, specific heat, and neutron powder diffraction measurements. Despite the expected nonmagnetic singlet in the strong spin-orbit coupling (SOC) limit for ${\mathrm{Os}}^{4+}$ ($5d^4$), ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ exhibits a spin-glass ground state below 4 K with weak magnetism, suggesting possible proximity to a quantum phase transition between the nonmagnetic state in the strong SOC limit and a magnetic state in the strong superexchange limit. ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ has the same structural distortion as in ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$; however, the Os sublattice in ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ shows long-range magnetic ordering below 36 K. The sharp difference of the magnetic ground state between ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ signals that the singlet ground-state magnetism in $R_2{\mathrm{Os}}_2{\mathrm{O}}_7$ is fragile and can be disturbed by the weak $4f-5d$ interactions.

Figure 1

Neutron powder diffraction pattern of ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ collected at 60 K on HB-2A with $\lambda =1.54\AA$. The red line on the top is the refined pattern including three phases. The vertical ticks are the Bragg positions of ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ ($Fd\overline{3}m$), ${\mathrm{Ho}}_2{\mathrm{O}}_3$ ($Fm\overline{3}m$), and ${\mathrm{Ho}}_2{\mathrm{O}}_3$ ($Ia\overline{3}$) phases, respectively from top to bottom. The blue line on the bottom is the difference between the observed and calculated intensities.

Figure 2

Temperature dependence of electrical resistivity below room temperature in zero magnetic field. Solid lines are 2D VRH fittings.

Figure 3

The dc magnetic susceptibility, magnetization, and specific heat of (a)–(c) ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ and (d)–(f) ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. The red solid line in the inset of (a) is the Curie-Weiss fit and $\mathrm{\Delta }\chi =\chi -{\chi }_0$. $C_{\mathrm{p}}$ of ${\mathrm{Y}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ in (c) is scaled by multiplying by a factor of 0.9 to match the high-temperature phonon specific heat of ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. Inset of (c): Magnetic specific heat of ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. The solid line is a linear fit. Inset of (d): The derivative of the ZFC susceptibility. Inset of (f): Specific heat with a two-level Schottky fit (solid line) at low temperatures.

Figure 4

Real part of the ac susceptibility (${\chi }^{\prime }$) of (a) ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ and (b) ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ measured with $H_{\mathrm{ac}}=12$ Oe and $H_{\mathrm{dc}}=0$. Inset: ${\chi }^{\prime }$ of the ${\mathrm{Os}}^{4+}$ moments in ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. Contribution from the ${\mathrm{Ho}}^{3+}$ moments is subtracted from the total after fitting the Curie-Weiss tail below 6 K. (c) The normalized frequency dependencies of the inverse of the transition temperatures for both compounds.

Figure 5

(a) Neutron diffraction patterns of ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ collected on HB-1A with $\lambda$ = 2.36 Å and ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ collected on HB-2A with $\lambda =2.41\AA$. The intensity of ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ patterns are shifted for clarity. The intensity is given using a log scale. (b) Temperature dependence of the intensity for ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ at $Q=0.68{\AA }^{-1}$ ($2\theta ={15}^{\circ }$) (solid circles) and $Q=3.7{\AA }^{-1}$ ($2\theta ={90}^{\circ }$) (open circles). The scaled integration of the diffuse scattering within $0.22\le Q\le 1.3{\AA }^{-1}$ is also plotted on the right scale as the star symbols. The dashed line is a guide for the eyes, illustrating that the diffuse scattering starts to increase below 30 K.

Figure 6

Comparison of local distortion between ${\mathrm{Y}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ and ${\mathrm{Ho}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$ around 4 K. ${\mathrm{OsO}}_{6/2}$ octahedra are connected by sharing the oxygen (red).