Specific Heat Measurements Research Articles

Magnetic Field–Temperature Phase Diagram of CeCoSi Constructed on the Basis of Specific Heat, Magnetoresistivity, and Magnetization Measurements: Single Crystal Study

A Ce-based metallic compound CeCoSi with a tetragonal structure exhibits successive phase transitions: the one whose order parameter is unidentified at T_0 = 12 K and the antiferromagnetic one at T_N = 9.4 K. We performed specific heat, magnetoresistivity (MR), and magnetization measurements for a single crystal CeCoSi at low temperatures in magnetic fields B of up to 14 T and constructed detailed magnetic field--temperature phase diagrams for both B || [100] and [001]. The longitudinal MR measured for B || [100] shows a sign change from negative to positive across T_0 = 13 K updated in the present sample, indicating a clear change in an electronic state. In addition, the constructed magnetic phase diagrams for both the field directions have a B-induced region in each ordered state. The presence of the newly found regions would be attributed to a change in the symmetry of the order parameter or domain alignment by applying B.

Investigating the limits of superconductivity in UTe2

Spin-triplet bulk superconductors are a promising route to topological superconductivity, and UTe2 is a recently discovered contender. The superconducting properties of UTe2, however, vary substantially as a function of the synthetic route, and even nonsuperconducting single crystals have been reported. To understand the driving mechanism suppressing superconductivity, we investigate UTe2 single crystals grown close to the nonsuperconducting boundary (growth temperature ∼710 ∘C) through a combination of thermodynamic and x-ray diffraction measurements. Specific heat measurements reveal a sharp decrease in the superconducting volume and a concomitant increase in the residual specific heat coefficient close to the nonsuperconducting boundary. Notably, these crystals are inhomogeneous and show an apparent double transition in specific heat measurements, similar to samples grown at much higher temperatures (∼1000 ∘C). Our single crystal x-ray diffraction measurements reveal that there are two important tuning parameters: uranium vacancies and the atomic displacement along the c axis, which shows a twofold increase in samples with a reduced superconducting volume. Our results highlight the key role of local disorder along the uranium-uranium dimers and suggest that the apparent double superconducting transition is more likely to emerge close to the superconducting limits of UTe2.

Superconductivity of high-entropy-alloy-type transition-metal zirconide (Fe,Co,Ni,Cu,Ga)Zr2

We synthesized a new high-entropy-alloy-type (HEA-type) superconductor (Fe,Co,Ni,Cu,Ga)Zr2 with a T c of 2.9 K. The EDX analyses revealed that the actual composition of the transition-metal site (Tr-site) is Tr = Fe0.17(1)Co0.18(2)Ni0.21(2)Cu0.25(1)Ga0.19(1), which gives the configurational entropy of mixing ΔS mix = 1.60 R for the Tr site. Neutron powder diffraction revealed that the sample has a tetragonal CuAl2-type (space group: #140). The lattice constant of a monotonically decreases with decreasing temperature, but the lattice constant of c does not exhibit a clear shrinkage. Isotropic displacement parameter for both the Tr and Zr sites are large, which is probably caused by the HEA-type Tr site. The small temperature dependences of U iso for both sites also indicate the presence of the local structural disorder in (Fe,Co,Ni,Cu,Ga)Zr2. From electrical resistivity, magnetic susceptibility, and specific heat measurements, bulk superconductivity was confirmed.

Second harmonic AC calorimetry technique within a diamond anvil cell.

Tuning the energy density of matter at high pressures gives rise to exotic and often unprecedented properties, e.g., structural transitions, insulator-metal transitions, valence fluctuations, topological order, and the emergence of superconductivity. The study of specific heat has long been used to characterize these kinds of transitions, but their application to the diamond anvil cell (DAC) environment has proved challenging. Limited work has been done on the measurement of specific heat within DACs, in part due to the difficult experimental setup. To this end, we have developed a novel method for the measurement of specific heat within a DAC that is independent of the DAC design and is, therefore, readily compatible with any DACs already performing high pressure resistance measurements. As a proof-of-concept, specific heat measurements of the MgB2 superconductor were performed, showing a clear anomaly at the transition temperature (Tc), indicative of bulk superconductivity. This technique allows for specific heat measurements at higher pressures than previously possible.

High-entropy design and its influence on glass-forming ability in Zr–Cu-based metallic glass

The glass-forming ability of Zr–Cu-based metallic glass with high entropy is investigated. Samples are prepared by adding elements, such as Ag, Hf, and Ni, to ternary Zr48Al7Cu45 to systematically evaluate the multicomponent effect. The glass structures are fabricated by tilt casting. In quaternary and senary alloys with increased glass-forming ability, new competitive liquid crystalline phases are generated with the addition of elements. Compared with the quaternary alloy, the quinary alloy does not form a new crystalline phase, and the added Hf is highly soluble in the base crystalline phase. The driving force for crystallization, which is evaluated based on specific heat measurements, is the largest for the quinary alloys. This suggests that stabilization of the competitive phase by the high-entropy effect leads to a decrease in the glass-forming ability. From the kinetics point of view, the relationship between the liquid-phase fragility and glass-forming ability is clarified, and the addition of Ag and Ni, which strengthens the liquid properties, is found to improve the glass-forming ability. Based on the high-entropy strategy, a new high-entropy metallic glass Zr35Hf13Al11Ag8Ni8Cu25, with a maximum vitrification diameter of 20 mm, is fabricated.

Large positive and negative magnetoresistance in the magnetic EuCu4As2 crystal

In this article, we studied the magnetic and transport properties by magnetization, resistivity, and specific heat measurements for EuCu4As2 crystals with a rhombohedral structure. Strongly anisotropic magnetism and rich magnetic phases were observed under magnetic field in this compound. Interestingly, we observed two types of magnetoresistance: a large negative magnetoresistance (−55%, at 35 K, 9 T) near the magnetic transition temperature, due to reduction of magnetic scattering under magnetic field; a large positive magnetoresistance (60% at 2 K, 9 T) at lower temperatures when Eu2+ moments ordering ferromagnetically, similar to that observed in the non-magnetic isostructural analogue SrAg4As2 with possible nontrivial topological bands, which may be related to its special band structure.

Influence of In-induced resonant level on the normal-state and superconducting properties of Sn1.03Te

Normal-state transport properties (2--300 K) of the polycrystalline series ${\mathrm{Sn}}_{1.03\ensuremath{-}\ensuremath{\delta}\ensuremath{-}x}{\mathrm{In}}_{x}\mathrm{Te} (0\ensuremath{\le}x\ensuremath{\le}0.07$; $\ensuremath{\delta}\ensuremath{\le}0.0025$) were investigated by means of electrical resistivity, thermopower, Hall effect, and thermal conductivity measurements. The distortion of the valence-band structure by the In-induced resonant level (RL) has a profound influence on the evolution of the normal-state properties with $x$ and on the emergence of superconductivity evidenced by specific-heat measurements down to 0.35 K. In addition to a nearly 40-fold increase in the residual electrical resistivity ${\ensuremath{\rho}}_{0}$ on going from $x=0.0$ to 0.05, the thermopower $\ensuremath{\alpha}$ shows a nonlinear, complex behavior as a function of both temperature and $x$. While Hall measurements indicate a dominant holelike response across the entire composition and temperature ranges, $\ensuremath{\alpha}$ changes sign below about 100 K and remains negative down to 5 K for $0.0015\ensuremath{\le}x\ensuremath{\le}0.0045$. Additional measurements under magnetic fields ${\ensuremath{\mu}}_{0}H$ of up to 14 T further shows that $\ensuremath{\alpha}({\ensuremath{\mu}}_{0}H)$ gradually shifts towards positive values, suggestive of a dominant holelike contribution to $\ensuremath{\alpha}$. Superconductivity emerges for $x=0.02$ at a critical temperature ${T}_{\mathrm{c}}=0.67$ K, with ${T}_{\mathrm{c}}$ increasing with $x$ to reach 1.73 K for $x=0.07$. The variations in the superconducting parameters with $x$, notably the specific-heat jump at ${T}_{\mathrm{c}}$, confirm the results reported in prior studies and suggests a nontrivial role of the RL on the electron-phonon coupling strength. The striking similarities between this series and the canonical resonant system ${\mathrm{Pb}}_{1\ensuremath{-}x}{\mathrm{Tl}}_{x}\mathrm{Te}$ provide an excellent experimental opportunity to gain a deeper understanding of the close interplay between resonant level, anomalous transport properties, and superconductivity.

Antiferromagnetism and mixed valency in the new Kondo lattice compound Ce3Rh4Sn7

Single crystals of a new ternary compound Ce3Rh4Sn7 were synthesized by flux method and its crystal structure and magnetic properties were studied by single crystal X–ray diffraction and magnetic susceptibility, specific heat and resistivity measurements. Ce3Rh4Sn7 crystallizes in a new monoclinic crystal structure, space group P21∕m (No. 11) with Z = 2 and lattice parameters a = 9.8708(5) Å, b = 4.5779(3) Å and c = 14.2381(7) Å, β = 109.535(5)∘, and V = 606.35(6) Å3. The structure exhibits three inequivalent Ce-sites. A striking structural motif is the tetragonal pyramid of Rh-ions surrounding the Ce2 yielding an extreme short Ce2–Rh1 spacing of d = 2.7486(10) Å. This strong hint to mixed valent cerium behavior is supported by the entropy analysis. We argue that the Ce2–ion is in a nonmagnetic Ce3+δ state while the other 2 Ce–sites are in a trivalent state. At low temperatures these latter become partially Kondo screened (TK ≈ 4 K) and order antiferromagnetic below TN = 2.31 K. The ordering is accompanied by a superzone gap-opening in direction of the magnetic propagation vector Q = (0, 0, l).

Phase Transformation in Quasi-Two-Dimensional Quantum Antiferromagnet Cu(tn)Cl2 (tn = 1,3-Diaminopropane)

Cu(tn)Cl2 belongs to family of molecular magnets with low-dimensional magnetism mediated by hydrogen bonds. Here, the X-ray diffraction, electron paramagnetic resonance, specific heat measurements, and ab initio calculations performed within the framework of density functional theory have been employed to investigate the structural phase transition in Cu(tn)Cl2. Satellite reflections in the X-ray diffraction pattern from single crystals, visible anomaly in the specific heat measured in a zero magnetic field, rapid growth of the EPR resonance line width, along with the increase of the g-factors in the vicinity of 160 K are indicative of the structural phase transition in Cu(tn)Cl2. Our results reveal that this system undergoes transition from a disordered structure of the Pnma symmetry to a modulated structure with the Pnma(0β0)s00 superspace group. The transition is reversible and driven by the reorientation of the diaminopropane ligands. The density functional theory studies support results of our specific heat measurements.

Anisotropic spin-glass and magnetic behavior in single-crystalline U2PtSi3

The results of ac and dc susceptibilities, magnetization, magnetic relaxation, electrical resistivity, and specific heat measurements performed on a hexagonal AlB2-type single-crystalline U2PtSi3 are reported. The previously reported spin-glass transition in polycrystalline U2PtSi3 was confirmed to also occur in the single-crystalline sample with spin freezing temperature Tf=12.4 K. It is newly found that the spin-glass effect and magnetic behavior of U2PtSi3 measured along and perpendicular to the hexagonal c-axis show different magnetic field responses. Along the hard magnetization axis (c-axis), both the rate of change of Tf with the magnetic field and the detected magnetic viscosity coefficient are much smaller than those obtained in the c-plane. On the other hand, the low-temperature specific heat measurement yields an enhanced coefficient of the linear temperature term, and the temperature dependence of electrical resistivity illustrates a shallow minimum at the temperature much larger than Tf. The possible mechanisms for these phenomena are discussed by comparing with the corresponding properties observed in other ternary 2:1:3 uranium intermetallic compounds.

Anisotropic magnetic property of single crystals RV6Sn6 (R=Y, Gd−Tm, Lu)

$R$V${_6}$Sn${_6}$ ($R$ = Y, Gd - Tm, Lu) single crystals are synthesized by Sn-flux method and their physical properties are characterized by magnetization, resistivity, and specific heat measurements. Powder X-ray diffraction patterns of all samples can be well indexed with the hexagonal HfFe$_6$Ge$_6$-type structure, where rare-earth atoms form hexagonal layers and vanadium atoms form Kagome layers. At high temperatures, magnetic susceptibility measurements of moment bearing rare-earths ($R$ = Gd - Tm) follow Curie-Weiss behavior. Effective moments estimated from the polycrystalline average of magnetic susceptibility curves are consistent with the values for free $R^{3+}$ ion. Strong magnetic anisotropy due to crystalline electric field effects is observed for moment bearing rare-earths, except GdV$_6$Sn$_6$. The easy magnetization direction is determined to be $c$-axis for $R$ = Tb - Ho and $ab$-plane for $R$ = Er, and Tm. The vanadium ions in $R$V${_6}$Sn${_6}$ possess no magnetic moment. The compounds for $R$ = Y and Lu exhibit typical characteristics of paramagnetic metals. At low temperatures, the magnetic ordering is confirmed from magnetization, specific heat, and resistivity: the highest $T_{N} = 4.9$~K for GdV$_6$Sn$_6$ and the lowest $T_{N} = 2.3$~K for HoV$_6$Sn$_6$. No magnetic ordering is observed down to 1.8~K for $R$ = Er and Tm. A slight deviation of the magnetic ordering temperature from the de Gennes scaling suggests the dominant Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction between rare-earth moments in metallic $R$V${_6}$Sn${_6}$ compounds.

Single crystal growth of Ba2Ni(PO4)2 by using the optical floating-zone method

We report a study on the crystal growth of a frustrated antiferromagnet Ba2Ni(PO4)2 by means of optical floating-zone method. Single crystals of Ba2Ni(PO4)2 were obtained by different growth conditions. It is found that the best crystallization was obtained in pure Ar atmosphere under normal pressure and using slow growth speed of 0.6 mm/h. The X-ray diffraction data and Laue diffraction patterns demonstrate the good crystallinity of the crystals. The physical properties were characterized by magnetic susceptibility, magnetization, specific heat and thermal conductivity measurements.

Magnetic, thermal, and magnetocaloric properties of the holmium trialuminide HoAl3 with polytypic phases

We investigate the magnetic, thermal, and magnetocaloric properties of the intermetallic HoAl3 compounds with two different crystal structures. The room-temperature equilibrium trigonal phase HoAl3 undergoes an antiferromagnetic (AFM) transition at the Néel temperature TNtri= 9.8 K. The AFM order of the compound is strong against the magnetic field, so that inverse magnetocaloric effect (MCE) is found even under a magnetic field change of 0–50 kOe. The high-pressure cubic phase HoAl3, prepared by a rapid-solidification process, is antiferromagnetically ordered below TNcub= 15 K. Magnetic and specific heat measurements reveal that this long-range AFM state becomes unstable as the temperature drops and then is replaced by a magnetic glassy state below a spin freezing temperature Tf= 11 K. The successive changes in magnetic state result in complicated temperature and field dependence of the MCE at low temperatures.

Temperature- and Field-Induced Transformation of the Magnetic State in Co2.5Ge0.5BO5.

A tetravalent-substituted cobalt ludwigite Co2.5Ge0.5BO5 has been synthesized using the flux method. The compound undergoes two magnetic transitions: a long-range antiferromagnetic transition at TN1 = 84 K and a metamagnetic one at TN2 = 36 K. The sample-oriented magnetization measurements revealed a fully compensated magnetic moment along the a- and c-axes and an uncompensated one along the b-axis leading to high uniaxial anisotropy. A field-induced enhancement of the ferromagnetic correlations at TN2 is observed in specific heat measurements. The DFT+GGA calculation predicts the spin configuration of (↑↓↓↑) as a ground state with a magnetic moment of 1.37 μB/f.u. The strong hybridization of Ge(4s, 4p) with O (2p) orbitals resulting from the high electronegativity of Ge4+ is assumed to cause an increase in the interlayer interaction, contributing to the long-range magnetic order. The effect of two super-superexchange pathways Co2+-O-B-O-Co2+ and Co2+-O-M4-O-Co2+ on the magnetic state is discussed.

High-Pressure Synthesis and Physical Properties of a Spinel Compound FeAl2S4.

A spinel compound FeAl2S4 was successfully synthesized under high-pressure and high-temperature conditions and was systematically characterized via the structural, magnetic, and specific heat measurements. It crystallizes into a cubic structure with the space group Fd3̅m (no. 227) and the lattice constant a = 10.0207(2) Å. A Fe/Al site inversion is found; that is, the molecular formula can be rewritten as (Fe1-xAlx)(Al2-xFex)S4, and the inversion parameter x is about 0.22. Magnetic susceptibility measurements indicate that FeAl2S4 undergoes a spin glass behavior, which is confirmed by ac susceptibility and specific heat measurements. The freezing temperature Tf ∼ 10.5 K and Weiss temperature Tθ ∼ -107.4 K lead to a high frustration parameter f = |Tθ/Tf| of about 10, which suggests that FeAl2S4 is a high-frustration magnet. Our results indicate that high pressure can help stabilize the spinel structure with small R̅σ and the cation inversion plays an important role in the formation of the spin glass state.

Successive magnetic orderings in the Ising spin chain magnet DyNi5Ge3

In this report, we investigated a new rare-earth-based one-dimensional Ising spin chain magnet $\mathrm{Dy}{\mathrm{Ni}}_{5}{\mathrm{Ge}}_{3}$ by means of magnetization, specific heat, and powder neutron diffraction measurements. Due to the crystalline electrical field splitting, the magnetic Dy ions share an Ising-like ground doublet state. Owning to the local point symmetry, these Ising moments form into two canted magnetic sublattices, which were further confirmed by the angle-dependent magnetization measurement. In zero fields, two successive antiferromagnetic phase transitions were found at temperatures ${T}_{\mathrm{N}1}=6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{N}2}=5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, respectively. Only part of the moments are statically ordered in this intermediate state between ${T}_{\mathrm{N}1}$ and ${T}_{\mathrm{N}2}$. Powder neutron diffraction experiments at different temperatures were performed as well. An incommensurate magnetic propagation vector of ${\mathbf{k}}_{\mathbf{m}}=(0.5,0.4,0.5)$ was identified. The refined spin configurations through the irreducible representation analysis confirmed that these Ising spins are canted in the crystal $ab$ plane.

Antiferromagnetic order in Yb4Ru7As6 with the cubic U4Re7Si6-type structure

We investigated the electronic properties of Yb4Ru7As6 using single crystalline samples grown by the Bi-flux method. Yb4Ru7As6 crystallizes in the cubic U4Re7Si6-type structure. From the electrical resistivity, magnetic susceptibility, and specific heat measurements, we have found that Yb4Ru7As6 orders antiferromagnetically below the Néel temperature of 2.5 K with the moderately enhanced electronic specific heat coefficient of 120 mJ/(Yb-mol⋅K2). A rapid increment in magnetization of about 0.25 μB/Yb has been detected at low magnetic field below 4 K, implying a weak ferromagnetic component.

Large Electrocaloric Responsivity and Energy Storage Response in the Lead-Free Ba(GexTi1-x)O3 Ceramics.

Ferroelectric property that induces electrocaloric effect was investigated in Ba(GexTi1−x)O3 ceramics, known as BTGx. X-ray diffraction analysis shows pure perovskite phases in tetragonal symmetry compatible with the P4mm (No. 99) space group. Dielectric permittivity exhibits first-order ferroelectric-paraelectric phase transition, confirmed by specific heat measurements, similar to that observed in BaTiO3 (BTO) crystal. Curie temperature varies weakly as a function of Ge-content. Using the direct and indirect method, we confirmed that the adiabatic temperature change ΔT reached its higher value of 0.9 K under 8 kV/cm for the composition BTG6, corresponding to an electrocaloric responsivity ΔT/ΔE of 1.13 × 10−6 K.m/V. Such electrocaloric responsivity significantly exceeds those obtained so far in other barium titanate-based lead-free electrocaloric ceramic materials. Energy storage investigations show promising results: stored energy density of ~17 mJ/cm3 and an energy efficiency of ~88% in the composition BTG5. These results classify the studied materials as candidates for cooling devices and energy storage applications.

Anisotropic field response of specific heat for a ferromagnetic superconductor UCoGe in magnetic fields

Magnetic-field-angle-resolved specific heat and magnetization measurements were conducted on a ferromagnetic superconductor UCoGe with remarkable anisotropic upper critical field ${H}_{\mathrm{c}2}$. Although ${H}_{\mathrm{c}2}$ reaches a high magnetic field ($\ensuremath{\sim}20\phantom{\rule{4pt}{0ex}}\mathrm{T}$) along the $b$ axis, it is small ($\ensuremath{\sim}0.6\phantom{\rule{4pt}{0ex}}\mathrm{T}$) when a magnetic field is applied along the magnetic easy $c$ axis. This study indicates that the specific heat is abruptly suppressed when the magnetic field is applied toward the $c$ axis from the $a$ and $b$ axes in the ferromagnetic state. The field response of density of states (DOS) is anisotropic, relative to the $c$ axis, and its angle dependence is slightly singular. The Ising-type magnetic anisotropy of the ferromagnetic state is dominant even in the anisotropic reinforced superconducting state. These facts indicate that the suppression of DOS may closely relate to the superconducting state. We theoretically analyze these findings together with URhGe and ${\mathrm{UTe}}_{2}$ by highlighting the common and distinctive features among three compounds.

Magnetic properties of equiatomic CrMnFeCoNi

Magnetic, specific heat, and structural properties of the equiatomic Cantor alloy system are reported for temperatures between 5 kelvin and 300 kelvin, and up to fields of 70 kilo-oersted. Magnetization measurements performed on as-cast, annealed, and cold-worked samples reveal a strong processing history dependence and that high-temperature annealing after cold-working does not restore the alloy to a pristine state. Measurements on known precipitates show that the two transitions, detected at 43 kelvin and 85 kelvin, are intrinsic to the Cantor alloy and not the result of an impurity phase. Experimental and ab initio density functional theory (DFT) computational results suggest that these transitions are a weak ferrimagnetic transition and a spin-glass-like transition, respectively, and magnetic and specific heat measurements provide evidence of significant Stoner enhancement and electron-electron interactions within the material.