20 research outputs found
Evidence for a dynamical ground state in the frustrated pyrohafnate Tb2Hf2O7
We report the physical properties of Tb2Hf2O7 based on ac magnetic
susceptibility \chi_ac(T), dc magnetic susceptibility \chi(T), isothermal
magnetization M(H), and heat capacity C_p(T) measurements combined with muon
spin relaxation (\muSR) and neutron powder diffraction measurements. No
evidence for long-range magnetic order is found down to 0.1 K. However,
\chi_ac(T) data present a frequency-dependent broad peak (near 0.9 K at 16 Hz)
indicating slow spin dynamics. The slow spin dynamics is further evidenced from
the \muSR data (characterized by a stretched exponential behavior) which show
persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data
collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering)
but no magnetic Bragg peaks. The analysis of the diffuse scattering data
reveals a dominant antiferromagnetic interaction in agreement with the negative
Weiss temperature. The absence of long-range magnetic order and the presence of
slow spin dynamics and persistent spin fluctuations together reflect a
dynamical ground state in Tb2Hf2O7.Comment: 11 pages and 8 figure
Magnetic interactions and spin dynamics in the bond-disordered pyrochlore fluoride NaCaCoF
We report high-frequency/high-field electron spin resonance (ESR) and
high-field magnetization studies on single crystals of the bond-disordered
pyrochlore NaCaCoF. Frequency- and temperature-dependent ESR
investigations above the freezing temperature K reveal the
coexistence of two distinct magnetic phases. A cooperative paramagnetic phase,
evidenced by a gapless excitation mode, is found as well as a spin-glass phase
developing below 20 K which is associated with a gapped low-energy excitation.
Effective -factors close to 2 are obtained for both modes in line with
pulsed high-field magnetization measurements which show an unsaturated
isotropic behavior up to 58 T at 2 K. In order to describe the field-dependent
magnetization in high magnetic fields, we propose an empirical model accounting
for highly anisotropic ionic -tensors expected for this material and taking
into account the strongly competing interactions between the spins which lead
to a frustrated ground state. As a detailed quantitative relation between
effective -factors as determined from ESR and the local -tensors obtained
by neutron scattering [Ross et al., Phys. Rev. B 93, 014433 (2016)] is still
sought after, our work motivates further theoretical investigations of the
low-energy excitations in bond-disordered pyrochlores.Comment: 9 pages, 6 figure
Freezing of molecular rotation in a paramagnetic crystal studied by P NMR
We present a detailed P nuclear magnetic resonance (NMR) study of the
molecular rotation in the compound
[Cu(pz)(2-HOpy)](PF), where pz = CHN and
2-HOpy = CHNHO. Here, a freezing of the PF rotation modes is
revealed by several steplike increases of the temperature-dependent second
spectral moment, with accompanying broad peaks of the longitudinal and
transverse nuclear spin-relaxation rates. An analysis based on the
Bloembergen-Purcell-Pound (BPP) theory quantifies the related activation
energies as = 250 and 1400 K. Further, the anisotropy of the
second spectral moment of the P absorption line was calculated for the
rigid lattice, as well as in the presence of several sets of PF
reorientation modes, and is in excellent agreement with the experimental data.
Whereas the anisotropy of the frequency shift and enhancement of nuclear
spin-relaxation rates is driven by the molecular rotation with respect to the
dipole fields stemming from the Cu ions, the second spectral moment is
determined by the intramolecular interaction of nuclear F and P
moments in the presence of the distinct rotation modes.Comment: 9 pages, 5 figures; additionally 2 pages with 3 figures of
supplemental material; typos corrected, references added, supplemental
material adde
Field-tunable Berezinskii-Kosterlitz-Thouless correlations in a Heisenberg magnet
We report the manifestation of field-induced Berezinskii-Kosterlitz-Thouless
(BKT) correlations in the weakly coupled spin-1/2 Heisenberg layers of the
molecular-based bulk material [Cu(pz)(2-HOpy)](PF). Due to the
moderate intralayer exchange coupling of K, the
application of laboratory magnetic fields induces a substantial anisotropy
of the spin correlations. Crucially, this provides a significant BKT regime, as
the tiny interlayer exchange mK only
induces 3D correlations upon close approach to the BKT transition with its
exponential growth in the spin-correlation length. We employ nuclear magnetic
resonance and SR measurements to probe the spin correlations that
determine the critical temperatures of the BKT transition as well as that of
the onset of long-range order. Further, we perform stochastic series expansion
quantum Monte Carlo simulations based on the experimentally determined model
parameters. Finite-size scaling of the in-plane spin stiffness yields excellent
agreement of critical temperatures between theory and experiment, providing
clear evidence that the nonmonotonic magnetic phase diagram of
[Cu(pz)(2-HOpy)](PF) is determined by the field-tuned
anisotropy and the concomitant BKT physics.Comment: 10 pages, 7 figure
Berezinskii—Kosterlitz—Thouless correlations in copper-based quasi-2D spin systems (Review Article)
We present an overview of selected copper-based quasi-2D square-lattice spin-1/2 materials with an easy-plane anisotropy, providing the possibility to study emergent Berezinskii-Kosterlitz-Thouless (BKT) correlations. In particular, in those materials with a comparatively small exchange coupling, the effective XY anisotropy of the low-temperature spin correlations can be controlled by an applied magnetic field, yielding a systematic evolution of the BKT correlations. In cases where the residual interlayer correlations are small enough, dynamical BKT correlations in the critical regime may be observed experimentally, whereas the completion of the genuine BKT transition is preempted by the onset of long-range order. © 2023 Author(s)
Magnetization beyond the Ising limit of Ho2Ti2O7
International audienceWe report that the local Ising anisotropy in pyrochlore oxides-the crucial requirement for realizing the spin-ice state-can be broken by means of high magnetic fields. For the case of the well-established classical spin-ice compound Ho2Ti2O7 the magnetization exceeds the angle-dependent saturation value of the Ising limit using ultrahigh fields up to 120 T. However, even under such extreme magnetic fields full saturation cannot be achieved. Crystal-electric-field calculations reveal that a level crossing for two of the four ion positions leads to magnetization steps at 55 and 100 T. In addition, we show that by using a field sweep rate in the range of the spin-relaxation time the dynamics of the spin system can be probed. Exclusively at 25 ns/T, a new peak of the susceptibility appears around 2 T. We argue, this signals the crossover between spin-ice and polarized correlations
Magnetization beyond the Ising limit of Ho2Ti2O7
International audienceWe report that the local Ising anisotropy in pyrochlore oxides-the crucial requirement for realizing the spin-ice state-can be broken by means of high magnetic fields. For the case of the well-established classical spin-ice compound Ho2Ti2O7 the magnetization exceeds the angle-dependent saturation value of the Ising limit using ultrahigh fields up to 120 T. However, even under such extreme magnetic fields full saturation cannot be achieved. Crystal-electric-field calculations reveal that a level crossing for two of the four ion positions leads to magnetization steps at 55 and 100 T. In addition, we show that by using a field sweep rate in the range of the spin-relaxation time the dynamics of the spin system can be probed. Exclusively at 25 ns/T, a new peak of the susceptibility appears around 2 T. We argue, this signals the crossover between spin-ice and polarized correlations