Muons and magnets

Abstract:

We present single-crystal neutron diffraction, powder muon spin rotation, and pulsed-field magnetometry measurements on the Heisenberg quantum chiral chain $\left[\mathrm{Cu}\left(\mathrm{pym}\right){\left({\mathrm{H}}_2\mathrm{O}\right)}_4\right]{\mathrm{SiF}}_6·{\mathrm{H}}_2\mathrm{O}$ (pym = pyrimidine), which displays a fourfold-periodic rotation of the local environment around the Cu() $S=1/2$ ions from site to site along the chain. Previous measurements on this material have shown the absence of magnetic order down to surprisingly low temperatures $\ge 20$  mK, as well as the presence of an energy gap for magnetic excitations that grows linearly with magnetic field. Here we find evidence at dilution refrigerator temperatures for a field-induced transition to long-range magnetic order above an applied magnetic field of 3 T. From the polarization of magnetic moments observed with magnetic fields applied in the $[-1,2,0]$ direction, we can identify the static magnetic structure that best accounts for the data. The proposed model is supported microscopically by the presence of an alternating component of the $g$ tensor, which produces an internal twofold staggered field that dictates both the direction of the ordered moments and the effective coupling between adjacent chains. The observed magnetic structure is contrary to previous proposals for the departure of the magnitude and field dependence of the energy gap from the predictions of the sine-Gordon model.

Abstract:

${\text{YbZn}}_2{\mathrm{GaO}}_5$ is a promising candidate for realizing a quantum spin liquid (QSL) state, particularly owing to its lack of significant site disorder. Pulsed-field magnetometry at 0.5 K shows magnetization saturating near 15 T, with a corrected saturation moment of $2.1(1){\mu }_{\mathrm{B}}$ after subtracting the van Vleck contribution. Our zero-field $\mathrm{\mu }\mathrm{SR}$ measurements down to milliKelvin temperatures provide evidence for a dynamic ground state and the absence of magnetic order. To probe fluctuations in the local magnetic field at the muon site, we performed longitudinal field $\mathrm{\mu }\mathrm{SR}$ experiments. These results provide evidence for spin dynamics with a field dependence that is consistent with a U1A01 Dirac quantum spin liquid as a plausible description of the ground state.

Abstract:

Ruddlesden-Popper oxide phases in the LaxSr2−xCo0.5Ir0.5O4 (0 < x < 1) solid solution can be converted to the corresponding LaxSr2−xCo0.5Ir0.5O4−yHy oxyhydride phases, by topochemical reaction with LiH, in which the hydride ions are substituted exclusively onto the equatorial anion sites of the host framework. Analysis reveals that oxyhydride phases in the range 0.5 < x < 1 adopt LaxSr2−xCo0.5Ir0.5O2+xH2−x compositions which maintain a constant Co1+, Ir3+ oxidation-state combination (confirmed by Co K-edge XANES data), with the presence of low-spin d6 Ir3+ being consistent with the covalent stabilization of the metastable oxyhydride phases via strong Ir–H σ-bonds. Phases at the lanthanum-poor end of the solid solution (x < 0.5) adopt LaxSr2−xCo0.5Ir0.5O4−yHy compositions with lower hydride concentrations (y < 1.5). Magnetisation and μSR data indicate that all the LaxSr2−xCo0.5Ir0.5O4−yHy oxyhydride phases exhibit strong magnetic frustration, attributed to the large-scale cation and anion disorder, and resulting in glassy magnetic behaviour at low temperature.

Abstract:

${\text{CoTi}}_2{\mathrm{O}}_5$ has the paradox that low temperature static magnetic order is incompatible with the crystal structure owing to a mirror plane that exactly frustrates magnetic interactions. Despite no observable structural distortion with diffraction, ${\text{CoTi}}_2{\mathrm{O}}_5$ does magnetically order below $T_N\sim 25\mathrm{K}$ with the breaking of spin ground state degeneracy proposed to be a realization of the spin Jahn-Teller effect in analogy to the celebrated orbital Jahn-Teller transition. We apply neutron and Raman spectroscopy to study the dynamics of this transition in ${\text{CoTi}}_2{\mathrm{O}}_5$ . We find anomalous acoustics associated with a symmetry breaking strain that characterizes the spin Jahn-Teller transition. Crucially, the energy of this phonon coincides with the energy scale of the magnetic excitations, and has the same symmetry of an optic mode, observed with Raman spectroscopy, which atypically softens in energy with decreasing temperature. Taken together, we propose that the energetics of the spin Jahn-Teller effect in ${\text{CoTi}}_2{\mathrm{O}}_5$ are related to cooperative magnetoelastic fluctuations as opposed to conventional soft critical dynamics which typically drive large measurable static displacements. Published by the American Physical Society 2025