Evolution of antiferromagnetic domains in the all-in-all-out ordered pyrochlore Nd2_2Zr2_2O7_7

We report the observation of magnetic domains in the exotic, antiferromagnetically ordered all-in-all-out state of Nd$_2$Zr$_2$O$_7$, induced by spin canting. The all-in-all-out state can be realized by Ising-like spins on a pyrochlore lattice and is established in Nd$_2$Zr$_2$O$_7$ below 0.31 K for external magnetic fields up to 0.14 T. Two different spin arrangements can fulfill this configuration which leads to the possibility of magnetic domains. The all-in-all-out domain structure can be controlled by an external magnetic field applied parallel to the [111] direction. This is a result of different spin canting mechanism for the two all-in-all-out configurations for such a direction of the magnetic field. The change of the domain structure is observed through a hysteresis in the magnetic susceptibility. No hysteresis occurs, however, in case the external magnetic field is applied along [100].Comment: Accepted for publication in Phys. Rev. B, 6 pages, 6 figure

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 NaCaCo2_2F7_7

We report high-frequency/high-field electron spin resonance (ESR) and high-field magnetization studies on single crystals of the bond-disordered pyrochlore NaCaCo$_2$F$_7$. Frequency- and temperature-dependent ESR investigations above the freezing temperature $T_f \sim 2.4$ 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 $g$-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 $g$-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 $g$-factors as determined from ESR and the local $g$-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

Splitting of the magnetic monopole pair-creation energy in spin ice

The thermodynamics in spin-ice systems are governed by emergent magnetic monopole excitations and, until now, the creation of a pair of these topological defects was associated with one specific pair-creation energy. Here, we show that the electric dipole moments inherent to the magnetic monopoles lift the degeneracy of their creation process and lead to a splitting of the pair-creation energy. We consider this finding to extend the model of magnetic relaxation in spin-ice systems and show that an electric dipole interaction in the theoretically estimated order of magnitude leads to a splitting which can explain the controversially discussed discrepancies between the measured temperature dependence of the magnetic relaxation times and previous theory. By applying our extended model to experimental data of, various spin-ice systems, we show its universal applicability and determine a dependence of the electric dipole interaction on the system parameters, which is in accordance with the theoretical model of electric dipole formation. © 2020 The Author(s). Published by IOP Publishing Ltd

Evidence of one-dimensional magnetic heat transport in the triangular-lattice antiferromagnet Cs2_2CuCl4_4

We report on low-temperature heat-transport properties of the spin-1/2 triangular-lattice antiferromagnet Cs$_2$CuCl$_4$. Broad maxima in the thermal conductivity along the three principal axes, observed at about 5 K, are interpreted in terms of the Debye model, including the phonon Umklapp scattering. For thermal transport along the $b$ axis, we observed a pronounced field-dependent anomaly, close to the transition into the three-dimensional long-range-ordered state. No such anomalies were found for the transport along the $a$ and $c$ directions. We argue that this anisotropic behavior is related to an additional heat-transport channel through magnetic excitations, that can best propagate along the direction of the largest exchange interaction. Besides, peculiarities of the heat transport of Cs$_2$CuCl$_4$ in magnetic fields up to the saturation field and above are discussed

Inverted hysteresis and negative remanence in a homogeneous antiferromagnet

Magnetic remanence - found in bar magnets or magnetic storage devices - is probably the oldest and most ubiquitous phenomenon underpinning technological applications of magnetism. It is a macroscopic non-equilibrium phenomenon: a remanent magnetisation appears when a magnetic field is applied to an initially unmagnetised ferromagnet, and then taken away. Here, we present an inverted magnetic hysteresis loop in the pyrochlore compound Nd$_2$Hf$_2$O$_7$: the remanent magnetisation points in a direction opposite to the applied field. This phenomenon is exquisitely tunable as a function of the protocol in field and temperature, and it is reproducible as in a quasi-equilibrium setting. We account for this phenomenon in considerable detail in terms of the properties of non-equilibrium population of domain walls which exhibit a magnetic moment between domains of an ordered antiferromagnetic state which itself has zero net magnetisation. Properties and (non-equilibrium) dynamics of topological defects play an important role in modern spintronics, and our study adds an instance where a uniform field couples selectively to domain walls rather than the bulk.Comment: 5 pages, 3 figures in main article and 7 pages, 13 figures in supplementary material

Inverted hysteresis and negative remanence in a homogeneous antiferromagnet

Magnetic remanence -- found in bar magnets or magnetic storage devices -- is probably the oldest and most ubiquitous phenomenon underpinning technological applications of magnetism. It is a macroscopic non-equilibrium phenomenon: a remanent magnetisation appears when a magnetic field is applied to an initially unmagnetised ferromagnet, and then taken away. Here, we present an inverted magnetic hysteresis loop in the pyrochlore compound Nd2Hf2O7: the remanent magnetisation points in a direction opposite to the applied field. This phenomenon is exquisitely tunable as a function of the protocol in field and temperature, and it is reproducible as in a quasi-equilibrium setting. We account for this phenomenon in considerable detail in terms of the properties of nonequilibrium population of domain walls which exhibit a magnetic moment between domains of an ordered antiferromagnetic state which itself has zero net magnetisation. Properties and (nonequilibrium) dynamics of topological defects play an important role in modern spintronics, and our study adds an instance where a uniform field couples selectively to domain walls rather than the bulk.Physic

Large pinning forces and matching effects in YBa2Cu3O7-δ thin films with Ba2Y(Nb/Ta)O6 nano-precipitates

The addition of mixed double perovskite Ba2Y(Nb/Ta)O6 (BYNTO) to YBa2Cu3O7−δ (YBCO) thin films leads to a large improvement of the in-field current carrying capability. For low deposition rates, BYNTO grows as well-oriented, densely distributed nanocolumns. We achieved a pinning force density of 25 GN/m3 at 77 K at a matching field of 2.3 T, which is among the highest values reported for YBCO. The anisotropy of the critical current density shows a complex behavior whereby additional maxima are developed at field dependent angles. This is caused by a matching effect of the magnetic fields c-axis component. The exponent N of the current-voltage characteristics (inversely proportional to the creep rate S) allows the depinning mechanism to be determined. It changes from a double-kink excitation below the matching field to pinning-potential-determined creep above it