本文对低维磁性系统Y-$_2$-BaCuO-$_5$多晶样品的比热进行了研究。施加高达14 T的磁场，并允许提取（$T$，$H$）相图。低于$μ_0H^*\simeq2$ T时，与三维反铁磁长程有序有关的内尔温度是恒定的，等于$T_N=15.6$ K。在$H^*$之上，$T_N$随$H$线性增加，并且在$T_N$处的场诱导熵增加与在$T_X\simeq20$ K处的等吸光点的存在有关，在该等吸光点处所有比热曲线交叉。本文比较了Y-$_2$-BaCuO-$_5$和BaNi $_{2}$ V $_{2}$ O $_{8}$、Sr-$_2$-CuO-$_2$-Cl-$_2$和Pr-$_2$-CuO-$_4$等准二维磁系的相图。提出了一种有效的场致磁各向异性来解释这些相图。
英文摘要：
A study by specific heat of a polycrystalline sample of the low-dimensional magnetic system Y$_2$BaCuO$_5$ is presented. Magnetic fields up to 14 T are applied and permit to extract the ($T$,$H$) phase diagram. Below $μ_0H^*\simeq2$ T, the Néel temperature, associated with a three-dimensional antiferromagnetic long-range ordering, is constant and equals $T_N=15.6$ K. Above $H^*$, $T_N$ increases linearly with $H$ and a field-induced increase of the entropy at $T_N$ is related to the presence of an isosbestic point at $T_X\simeq20$ K, where all the specific heat curves cross. A comparison is made between Y$_2$BaCuO$_5$ and the quasi-two-dimensional magnetic systems BaNi$_{2}$V$_{2}$O$_{8}$, Sr$_2$CuO$_2$Cl$_2$, and Pr$_2$CuO$_4$, for which very similar phase diagrams have been reported. An effective field-induced magnetic anisotropy is proposed to explain these phase diagrams.
We study the anisotropic quantum Heisenberg antiferromagnet for spin-1/2 that interpolates smoothly between the one-dimensional (1D) and the two-dimensional (2D) limits. Using the spin Hartree-Fock approach we construct a quantitative theory of heat capacity in the quasi-1D regime with a finite coupling between spin chains. This theory reproduces closely the exact result of Bethe Ansatz in the 1D limit and does not produces any spurious phase transitions for any anisotropy in the quasi-1D regime at finite temperatures in agreement with the Mermin-Wagner theorem. We study the static spin-spin c…
Heat capacity studies of Ce and Rh site substitution in the heavy fermion antiferromagnet CeRhIn_5;: Short-range magnetic interactions and non-Fermi-liquid behavior
Motivated by current research efforts towards exploring the interplay between magnetism and superconductivity in multiband electronic systems, we have investigated the effects of Eu substitution through thermodynamic measurements on the superconducting filled skutterudite alloys Pr$_{1-x}$Eu$_x$Pt$_4$Ge$_{12}$. An increase in Eu concentration leads to a suppression of the superconducting transition temperature consistent with an increase of magnetic entropy due to Eu local moments. While the low-temperature heat capacity anomaly is present over the whole doping range, we find that in alloys wi…
The metallic compound EuCo2P2 with the body-centered tetragonal ThCr2Si2 structure containing Eu spins 7/2 was previously shown from single-crystal neutron diffraction measurements to exhibit a helical antiferromagnetic (AFM) structure below TN = 66.5 K with the helix axis along the c axis and with the ordered moments aligned within the ab-plane. Here we report crystallography, electrical resistivity, heat capacity, magnetization and magnetic susceptibility measurements on single crystals of this compound. We demonstrate that EuCo2P2 is a model molecular-field helical Heisenberg antiferromagne…
Field induced first order antiferromagnetic to ferromagnetic transition in Al-doped CeFe$_2$: a calorimetric investigation
研究了Ce（Fe $_{0.094}$ Al $_{0.04}$）$_2$多晶样品在固定温度下的场致热容变化，以确定场致一级相变的起源。在4.5K和3.5K下的热容在不同的场循环中表现出滞后，原始曲线停留在包络曲线之外。这类似于在该系统中观察到的磁化和磁阻行为，其中磁滞的量和零场不可逆性的大小分别归因于过冷/过热的程度，以及在场减少循环中从铁磁到反铁磁状态的反向转变的动力学停滞的程度。然而，相反的磁化和磁阻，在热容量这两个功能都随着温度的降低而降低，这意味着结构贡献的重要性。
The gadolinium pyrochlores, Gd$_2B_2$O$_7$, are amongst the best realizations of antiferromagnetically coupled Heisenberg spins on a pyrochlore lattice. We present a magnetic characterization of Gd$_2$Pt$_2$O$_7$, a unique member of this family. Magnetic susceptibility, heat capacity, and muon spin relaxation measurements show that Gd$_2$Pt$_2$O$_7$ undergoes an antiferromagnetic ordering transition at $T_N = 1.6$ K. This transition is strongly first order, as indicated by the sharpness of the heat capacity anomaly, thermal hysteresis in the magnetic susceptibility, and a non-divergent relaxat…
In an ideal classical pyrochlore antiferromagnet without perturbations, an infinite degeneracy at a ground state leads to absence of a magnetic order and spin-glass transition. Here we present Na$_3$Mn(CO$_3$)$_2$Cl as a new candidate compound where classical spins are coupled antiferromagnetically on the pyrochlore lattice, and report its structural and magnetic properties.The temperature dependences of the magnetic susceptibility and heat capacity, and the magnetization curve are consistent with those of an $S$ = 5/2 pyrochlore lattice antiferromagnet with nearest-neighbor interactions of 2 …
The strongly correlated system Ho11B12 with boron sublattice Jahn-Teller instability and nanoscale electronic phase separation (dynamic charge stripes) was studied in detail by inelastic neutron scattering (INS), magnetometry and heat capacity measurements at temperatures in the range 3-300 K. From the analysis of registered INS spectra, we determined parameters of the cubic crystal field at holmium sites, B4=- 0.333 meV and B6= -2.003 meV (in Stevens notations), with an unconventional large ratio B6/B4 pointing on the dominant role of conduction electrons in the formation of a crystal field p…
Kondo lattice and antiferromagnetic behavior in quaternary CeTAl$_4$Si$_2$ (T~=~Rh, Ir) single crystals
本文报道了CeRhAl $_4$ Si $_2$和CeIrAl $_4$ Si $_2$及其非磁性La类似物的合成和磁性。以Al-Si二元共晶为助熔剂生长了这些四元化合物的单晶。通过磁化率、等温磁化强度、电阻率、磁电阻率和热容的测量，详细研究了铈化合物的各向异性磁性。CeRhAl $_4$ Si $_2$和CeIrAl $_4$ Si $_2$都经历了两次反铁磁转变，第一次是在$T_{\rm N1}$ ~=~12.6~K和15.5~K从顺磁性转变为反铁磁性，第二次是在较低的温度$T_{\rm N2}$ ~=~9.4~K和13.8~K。顺磁磁化率是高度各向异性的，其在磁有序状态下的温度依赖性表明$c$轴是相对容易磁化的轴。同时，在2~K下沿$c$轴的等温磁化强度显示出伴随着相当大的滞后的急剧自旋翻转转变，而沿[100]方向直到我们测量的最高场14~T，它随场几乎线性变化。电阻率提供了近藤相互作用的证据，在这两种化合物中，从它的$-lnT$行为在顺磁区推断。热容数据证实了每种化合物中两个磁跃迁的体积性质，并通过与磁有序相关的熵的降低值进一步证实了近藤相互作用的存在。从1 K以下的热容数据推断，CeRhAl $_4$ Si $_2$和CeIrAl $_4$ Si $_2$的电子热容中的线性项系数$γ$分别为195.6和49.4 mJ/mol K $^2$，属于中等重费米子化合物。
We study low energy excitations in the quantum breathing pyrochlore antiferromagnet Ba$_3$Yb$_2$Zn$_5$O$_{11}$ by combination of inelastic neutron scattering (INS) and thermodynamical properties measurements. The INS spectra are quantitatively explained by spin-1/2 single-tetrahedron model having $XXZ$ anisotropy and Dzyaloshinskii-Moriya interaction. This model has a two-fold degeneracy of the lowest-energy state per tetrahedron and well reproduces the magnetization curve at 0.5 K and heat capacity above 1.5 K. At lower temperatures, however, we observe a broad maximum in the heat capacity ar…
New frustrated antiferromagnetic compounds CuRE2Ge2O8 (RE=Pr, Nd, Sm, Eu) have been investigated using high-resolution x-ray diffraction, magnetic and heat capacity measurements. These systems show different magnetic lattices depending on rare-earth element. The nonmagnetic Eu compound is a S=1/2 two-dimensional triangular antiferromagnetic lattice oriented in the ac plane with geometrical frustration. On the other hand, the Pr, Nd, and Sm compounds show a three-dimensional honeycomb-tunnel-like lattice made of RE^3+ running along the a axis with the characteristic behavior of frustrated antif…
The physical properties of itinerant antiferromagnetic (AFM) SrCr$_2$As$_2$ with body-centered tetragonal ThCr$_2$Si$_2$ structure were investigated in single crystalline and polycrystalline forms by electrical resistivity $ρ$, heat capacity $C_{\rm p}$, magnetic susceptibility $χ$ versus temperature~$T$ and magnetization $M$ versus applied magnetic field $H$ isotherm measurements as well as $^{75}$As and $^{53}$Cr nuclear magnetic resonance (NMR) measurements in the wide temperature range $T$ = 1.6--900 K. From the $χ(T)$ and $^{75}$As NMR measurements, the G-type AFM state below $T_{\rm N}$ …
We investigated magnetic and thermodynamic properties of $S$ = 1/2 quasi-one-dimensional antiferromagnet KCuMoO$_4$(OH) through single crystalline magnetization and heat capacity measurements. At zero field, it behaves as a uniform $S$ = 1/2 Heisenberg antiferromagnet with $J$ = 238 K, and exhibits a canted antiferromagnetism below $T_\mathrm{N}$ = 1.52 K. In addition, a magnetic field $H$ induces the anisotropy in magnetization and opens a gap in the spin excitation spectrum. These properties are understood in terms of an effective staggered field induced by staggered g-tensors and Dzyaloshin…
The magnetic $H$-$T$ phase diagram and magnetocaloric effect in the recently discovered high-temperature heavy-fermion compound YbCoC$_2$ have been studied. With the increase in the external magnetic field YbCoC$_2$ experiences the metamagnetic transition and then transition to the ferromagnetic state. The dependencies of magnetic entropy change -$ΔS_m (T)$ have segments with positive and negative magnetocaloric effects for $ΔH \leq 6$~T. For $ΔH = 9$~T magnetocaloric effect becomes positive with a maximum value of -$ΔS_m (T)$ is 4.1 J / kg K and a refrigerant capacity is 56.6 J / kg.
We construct a new mean-field theory for quantum (spin-1/2) Heisenberg antiferromagnet in one (1D) and two (2D) dimensions using a Hartree-Fock decoupling of the four-point correlation functions. We show that the solution to the self-consistency equations based on two-point correlation functions does not produce any unphysical finite-temperature phase transition in accord with Mermin-Wagner theorem, unlike the common approach based on the mean-field equation for the order parameter. The next-neighbor spin-spin correlation functions, calculated within this approach, reproduce closely the strong…
Spin-1/2 maple leaf lattice antiferromagnets are expected to show interesting phenomena originating from frustration effects and quantum fluctuations. We report the hydrothermal synthesis of a powder sample of bluebellite Cu$_6$IO$_3$(OH)$_{10}$Cl as a first potential candidate. Magnetization and heat capacity measurements reveal moderate frustration with a Curie-Weiss temperature of $-35$ K, and a magnetic transition at $T_N$ = 17 K. Surprisingly, the magnetic susceptibility and heat capacity above $T_N$ are well reproduced by the Bonner-Fisher model, which suggests that a one-dimensional spi…
Exploration of low temperature phase transitions associated with quantum critical point is one of the most mystifying fields of research which is under intensive focus in recent times. In this work, through comprehensive experimental evidences, we report the possibility of achieving quantum criticality in the neighborhood of a magnetic field-tuned tricritical point separating paramagnetic, antiferromagnetic and metamagnetic phases in a magnetic insulator, DyVO4. Magnetic susceptibility and heat capacity indicate to the presence of a long-range second order antiferromagnetic transition at TN ~ …
CeCd$_3$As$_3$ is a rare-earth triangular-lattice antiferromagnet with large inter-layer separation. Our field-dependent heat capacity measurements at dilution fridge temperatures allow us to trace the field-evolution of the spin-excitation gaps throughout the antiferromagnetic and paramagnetic regions. The distinct gap evolution places strong constraints on the microscopic pseudo-spin model, which, in return, yields a close {\it quantitative} description of the gap behavior. This analysis provides crucial insights into the nature of the magnetic state of CeCd$_3$As$_3$, with a certainty regar…
Frustrated three dimensional quantum magnets are notoriously impervious to theoretical analysis. Here we use a combination of three computational methods to investigate the three dimensional pyrochlore $S=1/2$ quantum antiferromagnet, an archetypical frustrated magnet, at finite temperature, $T$: canonical typicality for a finite cluster of $2\times 2 \times 2$ unit cells (i.e. $32$ sites), a finite-$T$ matrix product state method on a larger cluster with $48$ sites, and the numerical linked cluster expansion (NLCE) using clusters up to $25$ lattice sites, which include non-trivial hexagonal a…
Single Crystal Growth, Crystallography, Magnetic Susceptibility, Heat Capacity, and Thermal Expansion of the Antiferromagnetic S = 1 Chain Compound CaV2O4
We present the ground state properties of a new quantum antiferromagnet YbBO$_3$ in which the isotropic Yb$^{3+}$ triangular layers are separated by a non-magnetic layer of partially occupied B and O(2) sites. The magnetization and heat capacity data establish a spin-orbit entangled effective spin $J_{\rm eff} = 1/2$ state of Yb$^{3+}$ ions at low temperatures, interacting antiferromagnetically with an intra-layer coupling $J/k_{\rm B} \simeq 0.53$ K. The absence of oscillations and a $1/3$ tail in the zero-field muon asymmetries rule out the onset of magnetic long-range-order as well as spin-…
We present present a quantitative experimental investigation of the scalar chiral magnetic order with in $\rm{Nd_3Sb_3Mg_2O_{14}}$. Static magnetization reveals a net ferromagnetic ground state, and inelastic neutron scattering from the hyperfine coupled nuclear spin reveals a local ordered moment of 1.76(6) $μ_B$, just 61(2)% of the saturated moment size. The experiments exclude static disorder as the source of the reduced moment. A 38(1) $μ$eV gap in the magnetic excitation spectrum inferred from heat capacity rules out thermal fluctuations and suggests a multipolar explanation for the momen…
A quantum spin liquid (QSL) is an exotic state of matter characterized by quantum entanglement and the absence of any broken symmetry. A long-standing open problem, which is a key for fundamental understanding the mysterious QSL states, is how the quantum fluctuations respond to randomness due to quenched disorder. Transition metal dichalcogenide 1T-TaS$_2$ is a candidate material that hosts a QSL ground state with spin-1/2 on the two-dimensional perfect triangular lattice. Here, we performed systematic studies of low-temperature heat capacity and thermal conductivity on pure, Se-substituted a…
We report the low temperature magnetic properties of Nd$^{3+}$ pyrochlore $\rm Nd_2ScNbO_7$. Susceptibility and magnetization show an easy-axis moment, and heat capacity reveals a phase transition to long range order at $T_N=371(2)$ mK with a fully recovered $ΔS = R \ln(2)$, 53\% of it recovered for $T>T_N$. Elastic neutron scattering shows a long-range all-in-all-out magnetic order with low-$Q$ diffuse elastic scattering. Inelastic neutron scattering shows a low-energy flat-band, indicating a magnetic Hamiltonian similar to $\rm Nd_2Zr_2O_7$. Nuclear hyperfine excitations measured by ultra-hi…
The crystal structures and magnetic properties of three previously unreported A2B2F7 pyrochlore materials, NaSrMn2F7, NaCaFe2F7, and NaSrFe2F7 are presented. In these compounds, either S=2 Fe2+ or S=5/2 Mn2+ is on the B site, while nonmagnetic Na and Ca (Na and Sr) are disordered on the A site. The materials, which were grown as crystals via the floating zone method, display high effective magnetic moments and large Curie-Weiss thetas. Despite these characteristics, freezing of the magnetic spins, characterized by peaks in the susceptibility or specific heat, is not observed until low temperat…
We have studied NpPdSn by means of the heat capacity, electrical resistivity, Seebeck and Hall effect, $^{237}$Np Mössbauer spectroscopy, and neutron diffraction measurements in the temperature range 2-300 K and under magnetic fields up to 14 T. NpPdSn orders antiferromagnetically below the Néel temperature $T_N$ = 19 K and shows localized magnetism of Np$^{3+}$ ion with a a doubly degenerate ground state. In the magnetic state the electrical resistivity and heat capacity are characterized by electron-magnon scattering with spin-waves spectrum typical of anisotropic antiferromagnets. An enhanc…
