Around 0.5 K, the entropy of the spin-ice Dy$_2$Ti$_2$O$_7$ has a plateau-like feature close to Paulings residual entropy derived originally for water ice, but an unambiguous quantification towards lower temperature is prevented by ultra-slow thermal
equilibration. Based on specific heat data of (Dy$_{1-x}$Y$_x$)$_2$Ti$_2$O$_7$ we analyze the influence of non-magnetic dilution on the low-temperature entropy. With increasing x, the ultra-slow thermal equilibration rapidly vanishes, the low-temperature entropy systematically decreases and its temperature dependence strongly increases. These data suggest that a non-degenerate ground state is realized in (Dy$_{1-x}$Y$_x$)$_2$Ti$_2$O$_7$ for intermediate dilution. This contradicts the expected zero-temperature residual entropy obtained from a generalization of Paulings theory for dilute spin ice, but is supported by Monte Carlo simulations.
In this study the magnetic order of the spin-1/2 XXZ chain system Cs$_2$CoCl$_4$ in a temperature range from 50 mK to 0.5 K and in applied magnetic fields up to 3.5 T is investigated by high-resolution measurements of the thermal expansion and the sp
ecific heat. Applying magnetic fields along a or c suppresses $T_textrm{N}$ completely at about 2.1 T. In addition, we find an adjacent intermediate phase before the magnetization saturates close to 2.5 T. For magnetic fields applied along b, a surprisingly rich phase diagram arises. Two additional transitions are observed at critical fields $mu_0 H_{SF1}simeq 0.25$ T and $mu_0 H_{SF2}simeq 0.7$ T, which we propose to arise from a two-stage spin-flop transition.
The elementary excitations of the spin-ice materials Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$ in zero field can be described as independent magnetic monopoles. We investigate the influence of these exotic excitations on the heat transport by measuring
the magnetic-field dependent thermal conductivity $kappa $. Additional measurements on the highly dilute reference compounds HoYTi$_2$O$_7$ and DyYTi$_2$O$_7$ enable us to separate $kappa $ into a sum of phononic ($kappa_{ph}$) and magnetic ($kappa_{mag}$) contributions. For both spin-ice materials, we derive significant zero-field contributions $kappa_{mag}$, which are rapidly suppressed in finite magnetic fields. Moreover, $kappa_{mag}$ sensitively depends on the scattering of phonons by magnetic excitations, which is rather different for the Ho- and the Dy-based materials and, as a further consequence, the respective magnetic-field dependent changes $kappa_{ph}(B)$ are even of opposite signs.
BaCo$_2$V$_2$O$_8$ is a one-dimensional antiferromagnetic spin-1/2 chain system with pronounced Ising anisotropy of the magnetic exchange. Due to finite interchain interactions long-range antiferromagnetic order develops below $T_{rm N} simeq 5.5$ K,
which is accompanied by a structural distortion in order to lift magnetic frustration effects. The corresponding temperature $vs. $ magnetic-field phase diagram is highly anisotropic with respect to the magnetic-field direction and various details are still under vivid discussion. Here, we report the influence of several substitutions on the magnetic properties and the phase diagrams of BaCo$_2$V$_2$O$_8$. We investigate the substitution series Ba$_{text{1-x}}$Sr$_{text{x}}$Co$_{text{2}}$V$_{text{2}}$O$_{text{8}}$ over the full range $0le x le 1$ as well as the influence of a partial substitution of the magnetic Co$^{2+}$ by small amounts of other magnetic transition metals or by non-magnetic magnesium. In all cases, the phase diagrams were obtained on single crystals from magnetization data and/or high-resolution studies of the thermal expansion and magnetostriction.
Comparing high-resolution specific heat and thermal expansion measurements to exact finite-size diagonalization, we demonstrate that Cs$_2$CoCl$_4$ for a magnetic field along the crystallographic b axis realizes the spin-$frac{1}{2}$ XXZ chain in a t
ransverse field. Exploiting both thermal as well as virtual excitations of higher crystal field states, we find that the spin chain is in the XY-limit with an anisotropy $J_z/J_perp approx 0.12$ substantially smaller than previously believed. A spin-flop Ising quantum phase transition occurs at a critical field of $mu_0 H_b^{rm cr} approx 2$ T before around 3.5 T the description in terms of an effective spin-$frac{1}{2}$ chain becomes inapplicable.
In the effective Ising spin-1/2 antiferromagnetic chain system BaCo$_2$V$_2$O$_8, the magnetic-field influence is highly anisotropic. For magnetic fields along the easy axis $c$, the N{e}el order is strongly suppressed already for low fields and an i
ncommensurate order is entered above 4 T. We present a detailed study of the magnetic phase diagrams for different magnetic field directions, which are derived from magnetization data, high-resolution thermal expansion and magnetostriction measurements as well as from the thermal conductivity. Zero-field thermal expansion data reveal that the magnetic transition is accompanied by an orthorhombic distortion within the $ab$ plane. Under ambient conditions the crystals are heavily twinned, but the domain orientation can be influenced either by applying uniaxial pressure or a magnetic field along the [100] direction. In addition, our data reveal a pronounced in-plane magnetic anisotropy for fields applied within the $ab$ plane. For $H || [110]$, the magnetic field influence on T$_N$ is weak, whereas for magnetic fields applied along [100], T$_N$ vanishes at about 10 T and the zero-field N{e}el order is completely suppressed as is confirmed by neutron diffraction data. The second-order phase transition strongly suggests a quantum critical point being present at $Hsimeq 10$ T parallel [100], where the N{e}el order probably changes to a spin-liquid state.
We report a study of the thermal conductivity $kappa$ of the spin-ice material Dy$_2$Ti$_2$O$_7$. From the anisotropic magnetic-field dependence of kappa$ and by additional measurements on the phononic reference compounds Y$_2$Ti$_2$O$_7$ and DyYTi$_
2$O$_7$, we are able to separate the phononic and the magnetic contributions to the total heat transport, i.e. $kappa_{ph}$ and $kappa_{mag}$, respectively, which both depend on the magnetic field. The field dependent $kappa_{ph}$ arises from lattice distortions due to magnetic-field induced torques on the non-collinear magnetic moments of the Dy ions. For $kappa_{mag}$, we observe a highly anisotropic magnetic-field dependence, which correlates with the corresponding magnetization data reflecting the different magnetic-field induced spin-ice ground states. The magnitude of $kappa_{mag}$ increases with the degree of the ground-state degeneracy. This anisotropic field dependence as well as various hysteresis effects suggest that $kappa_{mag}$ is essentially determined by the mobility of the magnetic monopole excitations in spin ice.
Elementary excitations in the spin-ice compound Dy$_2$Ti$_2$O$_7$ can be described as magnetic monopoles propagating independently within the pyrochlore lattice formed by magnetic Dy ions. We studied the magnetic-field dependence of the thermal condu
ctivity {kappa}(B) for B || [001] and observe clear evidence for magnetic heat transport originating from the monopole excitations. The magnetic contribution {kappa}_{mag} is strongly field-dependent and correlates with the magnetization M(B). The diffusion coefficient obtained from the ratio of {kappa}_{mag} and the magnetic specific heat is strongly enhanced below 1 K indicating a high mobility of the monopole excitations in the spin-ice state.
Large single crystals of the new compound SrMn$_2$V$_2$O$_8$ have been grown by the floating-zone method. This transition-metal based oxide is isostructural to SrNi$_2$V$_2$O$_8$, described by the tetragonal space group $I4_1cd$. Magnetic properties
were investigated by means of susceptibility, magnetization, and specific heat measurements. The title compound behaves like a one-dimensional magnetic system above the ordering temperature ($T_N$ = 43 K). The magnetic ground state can be described as a classical long-range ordered antiferromagnet with weak anisotropy.
EuC$_2$ is a ferromagnet with a Curie-temperature of $T_C simeq 15,$K. It is semiconducting with the particularity that the resistivity drops by about 5 orders of magnitude on cooling through $T_C$, which is therefore called a metal-insulator transit
ion. In this paper we study the magnetization, specific heat, thermal expansion, and the resistivity around this ferromagnetic transition on high-quality EuC$_2$ samples. At $T_C$ we observe well defined anomalies in the specific heat $c_p(T)$ and thermal expansion $alpha(T)$ data. The magnetic contributions of $c_p(T)$ and $alpha(T)$ can satisfactorily be described within a mean-field theory, taking into account the magnetization data. In zero magnetic field the magnetic contributions of the specific heat and thermal expansion fulfill a Gruneisen-scaling, which is not preserved in finite fields. From an estimation of the pressure dependence of $T_C$ via Ehrenfests relation, we expect a considerable increase of $T_C$ under applied pressure due to a strong spin-lattice coupling. Furthermore the influence of weak off stoichiometries $delta$ in EuC$_{2 pm delta}$ was studied. It is found that $delta$ strongly affects the resistivity, but hardly changes the transition temperature. In all these aspects, the behavior of EuC$_2$ strongly resembles that of EuO.
