Field Dependence Of Specific Heat Research Articles

A Setup for Direct Measurement of the Adiabatic Temperature Change in Magnetocaloric Materials

In order to find a highly efficient, environment-friendly magnetic refrigerant, direct measurements of the adiabatic temperature change Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_adb</i> is required. Here, in this work a simple setup for the Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_adb</i> measurement is presented. Using a permanent magnet Halbach array with a maximum magnetic field of 1.8 T and a rate of magnetic field change of 5 T/s, accurate determination of Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_adb</i> is possible in this system. The operating temperature range of the system is from 100 K to 400 K, designed for the characterization of materials with potential for room temperature magnetic refrigeration applications. Using the setup, the Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_adb</i> of a first-order and two second-order compounds have been studied. Results from the direct measurement for the first-order compound have been compared with Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_adb</i> calculated from the temperature and magnetic field dependent specific heat data. By comparing results from direct and indirect measurements, it is concluded that for a reliable characterization of the magnetocaloric effect, direct measurement of Δ <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T_adb</i> should be adopted.

Numerical Interchain Mean-Field Theory for the Specific Heat of the Bimetallic Ferromagnetically Coupled Chain Compound MnNi(NO2)4(en)2 (en = Ethylenediamine).

We present a detailed study of the field-dependent specific heat of the bimetallic ferromagnetically coupled chain compound MnNi(NO)(en), en = ethylenediamine. For this material, which in zero field orders antiferromagnetically below K, small fields suppress magnetic order. Instead, in such fields, a double-peak-like structure in the temperature dependence of the specific heat is observed. We attribute this behavior to the existence of an acoustic and an optical mode in the spin-wave dispersion as a result of the existence of two different spins per unit cell. We compare our experimental data to numerical results for the specific heat obtained by exact diagonalization and Quantum Monte Carlo simulations for the alternating spin-chain model, using parameters that have been derived from the high-temperature behavior of the magnetic susceptibility. The interchain coupling is included in the numerical treatment at the mean-field level. We observe remarkable agreement between experiment and theory, including the ordering transition, using previously determined parameters. Furthermore, the observed strong effect of an applied magnetic field on the ordered state of MnNi(NO)(en) promises interesting magnetocaloric properties.

Study of magnetization and specific heat of Heisenberg model on Lieb nanolattice: Anisotropic effects

We study the effects of longitudinal magnetic field and temperature on the thermodynamic properties of two dimensional Heisenberg antiferromagnet on the line-centered square lattice in the presence of anisotropic Dzyaloshinskii–Moriya interaction. We have studied the temperature dependence of specific heat and longitudinal magnetization of Lieb lattice for various physical parameters in the model Hamiltonian. Using a hard core bosonic representation, the behavior of thermodynamic properties has been studied by means of excitation spectrum of mapped bosonic gas. The effect of Dzyaloshinskii–Moriya interaction term on thermodynamic properties has also been studied via the bosonic model by exploiting Green’s function approach. Furthermore we have studied the magnetic field dependence of specific heat and magnetization for various Dzyaloshinskii–Moriya coupling strength. At low temperatures, the specific heat is found to be monotonically increasing with temperature for magnetic fields in the gapped field induced phase region. We have found the magnetic field dependence of specific heat shows a monotonic decreasing behavior for various magnetic fields due to increase of energy gap in the excitation spectrum. Also we have studied the dependence of magnetization on Dzyaloshinskii–Moriya interaction strength for different magnetic field values. • The study of thermodynamic properties of Lieb lattice in the context of Heisenberg model. • The study the temperature dependence of specific heat of Lieb lattice. • The study of magnetization with spin orbit coupling. • The study of specific heat of Lieb lattice.

Electronic structure and magnetic properties of the effective spin Jeff=12 two-dimensional triangular lattice K3Yb(VO4)2

We report the structural, magnetic, specific heat, and electronic structure studies of the material ${\mathrm{K}}_{3}\mathrm{Yb}{({\mathrm{VO}}_{4})}_{2}$, which has two-dimensional triangular layers constituted by rare-earth magnetic ${\mathrm{Yb}}^{3+}$ ions. Magnetic susceptibility data show the absence of magnetic long-range order down to $0.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. No bifurcation is observed between zero-field-cooled and field-cooled magnetic susceptibility data, ruling out the possibility of spin-glassiness down to $0.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. From the fit to magnetic susceptibility data with Curie-Weiss law in the low-temperature region, the observed Curie-Weiss temperature $(\ensuremath{\theta}{}_{\mathrm{CW}})$ is about $\ensuremath{-}1\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, implying an antiferromagnetic coupling between the ${\mathrm{Yb}}^{3+}$ ions. Magnetic field-dependent specific heat fits well with two-level Schottky behavior. The analysis of magnetization and specific heat data confirms that the ${\mathrm{Yb}}^{3+}$ ion hosts the effective spin ${J}_{\mathrm{eff}}=1/2$ state. To provide a microscopic understanding of the ground state nature of the titled material, we carried out state-of-the-art first-principles calculations based on density functional theory $+$ Hubbard U and density functional theory $+$ dynamical mean-field theory approaches. Our calculations reveal that the system belongs to the novel class of spin-orbit driven Mott Hubbard insulators and possesses large in-plane magnetocrystalline anisotropy.

Dependence of Electrical and Thermal Properties on Applied Electric Field in La Doped Barium Titanate Displacive Ferroelectric Perovskites

The high dielectric constant and low dielectric loss are the most desirable characteristics of the dielectric materials. Considering the hazardous nature of lead-based ceramics, many research groups have paid great attention to the research on non-lead-based perovskites. The effect of the electric field on the electrical and thermal properties of the Lanthanum doped barium Titanate, one of the main important members of the ferroelectric Perovskites family is studied using the method of double time thermal Green’s function and Kubo formalism. With the help of the Silverman-Joseph Hamiltonian a general expression is derived. The electric field and temperature dependent dielectric constant and specific heat capacity of Ba1-xLaxTiO3 crystal has been investigated. The dielectric constant decreases with increase in temperature and applied electric field component. The variation shows a consequence of the gradual decrease in the Curie temperature of the doped material with the increase in defect concentrations. The results also compared with some previous studies done by the author and other workers on similar other materials.

Spin dynamics of the quantum dipolar magnet Yb3Ga5O12 in an external field

We investigate ytterbium gallium garnet Yb$_{3}$Ga$_{5}$O$_{12}$ in the paramagnetic phase above the supposed magnetic transition at $T_{\lambda} \approx 54$ mK. Our study combines susceptibility and specific heat measurements with neutron scattering experiments and theoretical calculations. Below 500 mK, the elastic neutron response is strongly peaked in the momentum space. Along with that the inelastic spectrum develops flat excitation modes. In magnetic field, the lowest energy branch follows a Zeeman shift in accordance with the field-dependent specific heat data. An intermediate state with spin canting away from the field direction is evidenced in small magnetic fields. In the field of 2 T, the total magnetization almost saturates and the measured excitation spectrum is well reproduced by the spin-wave calculations taking into account solely the dipole-dipole interactions. The small positive Curie-Weiss temperature derived from the susceptibility measurements is also accounted for by the dipole spin model. Altogether, our results suggest that Yb$_{3}$Ga$_{5}$O$_{12}$ is a quantum dipolar magnet.

Unique magnetic properties of Nd based cuprate francisite: Major enhancement upon Co doping

To understand the physical mechanism of intralayer ferromagnetic (FM) and interlayer antiferromagnetic (AFM) interactions in quasi-two-dimensional model of a real three-dimensional system, rare earth (RE)-Nd based cuprate francisite, Cu3Nd(SeO3)2O2Cl (NdCufr) was doped with Co and studied using structural (X-ray diffraction technique), magnetic (temperature and field dependent magnetization and specific heat measurements) and local structural (X-ray absorption technique) techniques supported by first principle theoretical calculation. The structure of the samples is orthorhombic and doped specimens indicate more or less substitutional incorporation of Co at Cu site. The magnetic properties highlighted with two transitions, viz. AFM to paramagnetic (PM) transition around 30 K and a spin reorientation transition (SRT) below 10 K. Two phenomena viz. magnetic hysteresis below SRT and magnetic field induced metamagnetic transition (MMT) were observed. Co doping significantly enhanced these two phenomena in addition to change the transition temperatures. The concomitant change and enhancement of magnetic properties by Co doping has been interpreted using splitting scheme of Cu-d and Nd-f states. The local structure study corroborates the overall findings and detects the cause of unexpected trend of magnetism with increasing Co doping.

Field-dependent specific heat of the canonical underdoped cuprate superconductor hbox {YBa}_2hbox {Cu}_4hbox {O}_8

The cuprate superconductor hbox {YBa}_2hbox {Cu}_4hbox {O}_8, in comparison with most other cuprates, has a stable stoichiometry, is largely free of defects and may be regarded as the canonical underdoped cuprate, displaying marked pseudogap behaviour and an associated distinct weakening of superconducting properties. This cuprate ‘pseudogap’ manifests as a partial gap in the electronic density of states at the Fermi level and is observed in most spectroscopic properties. After several decades of intensive study it is widely believed that the pseudogap closes, mean-field like, near a characteristic temperature, T^*, which rises with decreasing hole concentration, p. Here, we report extensive field-dependent electronic specific heat studies on hbox {YBa}_2hbox {Cu}_4hbox {O}_8 up to an unprecedented 400 K and show unequivocally that the pseudogap never closes, remaining open to at least 400 K where T^* is typically presumed to be about 150 K. We show from the NMR Knight shift and the electronic entropy that the Wilson ratio is numerically consistent with a weakly-interacting Fermion system for the near-nodal states. And, from the field-dependent specific heat, we characterise the impact of fluctuations and impurity scattering on the thermodynamic properties.

High pressure study of magnetocaloric effect in Ho3Co and Tb3Co

The magnetocaloric effect of intermetallic compounds of Tb3Co and Ho3Co is studied under high pressures up to ∼1 GPa using pressure dependent dc magnetisation and specific heat measurements at ambient conditions. The magnetic entropy change (−ΔSM) obtained from magnetisation data and adiabatic change in temperature (ΔTad) determined from zero-field specific heat and magnetisation data are found to be nearly identical within error limits with those deduced from purely field dependent specific heat experiments. With increasing hydrostatic pressure to ∼1 GPa, the −ΔSM and ΔTad, both show a significant enhancement of about 37% and 13%, respectively for 9 T field change in case of Tb3Co. On the other hand, Ho3Co exhibits a decrease of about 8% in both −ΔSM and ΔTad with increasing pressure. The refrigerant capacity (RC) also increases from 650 J kg−1 to 847 J kg−1 in the case of Tb3Co and it goes down from 665 J kg−1 to 615 J kg−1 for Ho3Co for an increase of pressure to 1 GPa. With increasing pressure, the peak widths of both −ΔSM and ΔTad increase in case of Tb3Co, although the increase is more in −ΔSM. However, such noticeable changes in peak widths with pressure were not observed in Ho3Co. At ambient pressure, peak of −ΔSM () scales with for both the compounds, consistent with the prediction of mean field theory (MFT) for second order magnetic transition. However, deviation from MFT was noticed at high pressures as was found to scale with instead of for both the alloys. Further, normalised −ΔSM curves for different ΔH and pressures collapse on a single universal curve in both the compounds thereby indicating that the second order magnetic transition persists even up to ∼1 GPa pressure.

Effect of quenched disorder on the quantum spin liquid state of the triangular-lattice antiferromagnet 1T−TaS2

To reveal the nature of elementary excitations in a quantum spin liquid (QSL), we measured low temperature thermal conductivity and specific heat of 1T-TaS$_2$, a QSL candidate material with frustrated triangular lattice of spin-1/2. The nonzero temperature linear specific heat coefficient $\gamma$ and the finite residual linear term of the thermal conductivity in the zero temperature limit $\kappa_0/T=\kappa/T(T\rightarrow 0)$ are clearly resolved. This demonstrates the presence of highly mobile gapless excitations, which is consistent with fractionalized spinon excitations that form a Fermi surface. Remarkably, an external magnetic field strongly suppresses $\gamma$, whereas it enhances $\kappa_0/T$. This unusual contrasting behavior in the field dependence of specific heat and thermal conductivity can be accounted for by the presence of two types of gapless excitations with itinerant and localized characters, as recently predicted theoretically (I. Kimchi et al., arXiv:1803.00013 (2018)). This unique feature of 1T-TaS$_2$ provides new insights into the influence of quenched disorder on the QSL.

Quantum criticality and entropy transfer in spin chains and planes—Pyridine oxide copper salts

We present magnetic field-dependent specific heat (C) data for [Cu(pyO)6](NO3)2 (pyO = pyridine oxide) (CPN), a molecular salt shown to be quasi-1D, and for a quasi-2D analogue, [Cu(pyO)6](BF4)2 (CPB). For CPN, a sharp feature indicating 3D ordering is observed at 0.16K in zero-field. As the field, H, is increased, the ordering temperature first increases, as expected for quasi-1D antiferromagnets, before decreasing rapidly for H above 3T. The field also transfers the entropy of short-range ordering (SRO) in the spin chains into the 3D ordering peak. At our lowest accessible temperature, T ∼ 0.096K, C/T exhibits an enhanced peak at the critical field. Qualitatively similar behavior is found in CPB. These results demonstrate a potentially powerful new materials route to study quantum phase transitions.

Ferroelectricity in the 1 μ C cm−2 range induced by canted antiferromagnetism in (LaMn3)Mn4O12

Pyroelectric current and field-dependent specific heat measurements on polycrystalline samples of the quadruple perovskite (LaMn3)Mn4O12 give evidence of ferroelectricity driven by the canted antiferromagnetic ordering of the B-site Mn3+ ions at TN,B=78 K with record large remnant electric polarization up to 0.56 μC cm−2. X-ray diffraction measurements indicate an anomalous behavior of the monoclinic β angle at TN,B, which suggests that the polarization lies in the ac-plane, where the moments are collinear, and that symmetric exchange striction is the mechanism of spin-driven ferroelectricity. Polarization values up to ∼3–6 μC cm−2 are expected in single crystals or epitaxial films, which would enable the development of practical multiferroic applications.

Magnetotransport as a probe of phase transformations in metallic antiferromagnets: The case of UIrSi3

The electrical resistance, Hall resistance and thermoelectric power of the Ising-like antiferromagnet UIrSi3 were measured as functions of temperature and magnetic field. We have observed that the unequivocally different characters of first-order and second-order magnetic phase transitions lead to distinctly different magnetotransport properties in the neighborhood of corresponding critical temperatures and magnetic fields, respectively. The magnetic contributions to the electrical and Hall resistivity in the antiferromagnetic state, and the polarized and normal regimes of paramagnetic state are driven by different underlying mechanisms. Results of detailed measurements of magnetotransport in the vicinity of the tricritical point reveal that the Hall-resistivity steps at phase transitions change polarity just at this point. The jumps in field dependences of specific heat, electrical resistivity, Hall resistivity and Seebeck coefficient at the first-order metamagnetic transitions indicate a Fermi surface reconstruction, which is characteristic of a magnetic-field induced Lifshitz transition. The presented results emphasize the usefulness of measurements of electrical- and thermal-transport properties as sensitive probes of magnetic phase transformations in antiferromagnets sometimes hardly detectable by other methods.

Evidence of low-energy singlet excited states in the spin- 12 polyhedral clusters {Mo72V30} and {W72V30} with strongly frustrated kagome networks

Magnetization, specific heat, and electron spin resonance (ESR) measurements are carried out to clarify the low-energy excitations for the S = 1/2 polyhedral clusters {Mo$_{72}$V$_{30}$} and {W$_{72}$V$_{30}$}. The clusters provide unique model systems of Kagom\'e network on a quasi-sphere. The linear field variation of magnetization at low temperatures indicates that the ground state is singlet for both clusters. The temperature and the magnetic field dependence of specific heat shows a distinct difference between two clusters with differing structural symmetries. In {W$_{72}$V$_{30}$} with I$_h$ symmetry, the existence of the several tens of low-energy singlet excited states below the lowest triplet excited state is revealed. The specific heat of the slightly distorted {Mo$_{72}$V$_{30}$} with D$_{5h}$ symmetry leads to a drastic decrease of the singlet contribution, which is consistent with the partial lifting of the frustration and the decrease of degenerated low-energy states. The singlet excitation existence is confirmed further by the temperature dependence of the ESR spectra. Comprehensive experimental studies have demonstrated unique low-energy excitations of the spherical Kagom\'e networks and their sensitivity to the cluster symmetry.

Longitudinal magnetization and specific heat of the anisotropic heisenberg antiferromagnet on honeycomb lattice

We study the effects of longitudinal magnetic field and temperature on the thermodynamic properties of two dimensional Heisenberg antiferromagnet on the honeycomb lattice in the presence of anisotropic Dzyaloshinskii-Moriya interaction and next nearest neighbor coupling exchange constant. In particular, the temperature dependence of specific heat have been investigated for various physical parameters in the model Hamiltonian. Using a hard core bosonic representation, the behavior of thermodynamic properties has been studied by means of excitation spectrum of mapped bosonic gas. The effect of Dzyaloshinskii-Moriya interaction term on thermodynamic properties has also been studied via the bosonic model by Green's function approach. Furthermore we have studied the magnetic field dependence of specific heat and magnetization for various anisotropy parameters. At low temperatures, the specific heat is found to be monotonically increasing with temperature for magnetic fields in the gapped field induced phase region. We have found the magnetic field dependence of specific heat shows a monotonic decreasing behavior for various magnetic fields due to increase of energy gap in the excitation spectrum. Also we have studied the dependence of magnetization on Dzyaloshinskii-Moriya interaction strength for different next nearest neighbor coupling constant.

Field dependent specific heat and longitudinal magnetization of the quantum magnet layer Cs2CuCl4: Anisotropy effects

We have addressed the specific heat and magnetization of an anisotropic spin-1/2 triangular Heisenberg antiferromagnet Cs2CuCl4 in the presence of magnetic field at finite temperature. We have investigated the behavior of thermodynamic properties by means of excitation spectrum in terms of a hard core bosonic representation. The effect of in-plane anisotropy on thermodynamic properties has also been studied via the bosonic model by Green’s function approach. This anisotropy is considered for exchange constants that couple spin components perpendicular to magnetic field direction. We have found the temperature dependence of the specific heat and longitudinal magnetization in the gapped field induced spin-polarized phase for various magnetic fields and anisotropy parameters. Furthermore we have studied the magnetic field dependence of specific heat and magnetization for various anisotropy parameters. Our results show temperature dependence of specific heat includes a peak so that its temperature position goes to higher temperature with increase of magnetic field. We have found the magnetic field dependence of specific heat shows a monotonic decreasing behavior for various magnetic fields due to increase of energy gap in the excitation spectrum.

Thermodynamics of anisotropic antiferromagnetic Heisenberg chain in the presence of longitudinal magnetic field

We have addressed the specific heat and magnetization of one dimensional spin-1/2 anisotropic antiferromagnetic Heisenberg chain at finite magnetic field. We have investigated the thermodynamic properties by means of excitation spectrum in terms of a hard core Bosonic representation. The effect of in-plane anisotropy thermodynamic properties has also been studied via the Bosonic model by Green's function approach. This anisotropy is considered for exchange constants that couple spin components perpendicular to magnetic field direction. We have found the temperature dependence of the specific heat and longitudinal magnetization in the gapped field induced spin-polarized phase for various magnetic fields and anisotropy parameters. Furthermore we have studied the magnetic field dependence of specific heat and magnetization for various anisotropy parameters. Our results show temperature dependence of specific heat includes a peak so that its temperature position goes to higher temperature with increase of magnetic field. We have found the magnetic field dependence of specific heat shows a monotonic decreasing behavior for various magnetic fields due to increase of energy gap in the excitation spectrum. Also we have studied the temperature dependence of magnetization for different magnetic fields and various anisotropy parameters.

Direct and indirect measurements on electrocaloric effect: Recent developments and perspectives

It has been ten years since the discovery of the giant electrocaloric effect in ferroelectric materials showed that it is possible to employ this effect for substantial cooling applications. This last decade has been marked by increasing research interest, especially in characterizing and measuring the electrocaloric effect using both the so-called indirect and direct approaches. In this context, a comprehensive summary and careful reexamination of these approaches are very timely and of great importance to justify the assumptions used in different measurement techniques. This review is therefore dedicated to cover recent important and rapid advances from both the indirect and direct measurements and provides critical insights relevant for quantifying the electrocaloric effect. It involves electrocaloric materials from normal ferroelectrics, antiferroelectrics, and relaxors, and it fundamentally focuses on how the electrocaloric entropy changes in response to electric field in these typical electrocalorics. The article addresses recent developments, especially during the past three years, such as technical selection of proper polarization-electric field loops, negative electrocaloric effect in antiferroelectrics and relaxors, the controversial debate on the indirect method in relaxors, the important role of field dependence of specific heat, kinetic factors, and so on. Moreover, this review also is concerned with extracting reliable data by direct measurements. Four typical techniques and devices used recently, such as thermocouples, differential scanning calorimeters, specifically designed calorimeters, and scanning thermal microscopy, are briefly reviewed, while infrared cameras are emphasized. We hope that our review will not only provide a useful background to understand fundamentally the electrocaloric effect and what one really measures but also may act as a practical guide to exploit and develop electrocalorics towards the design of suitable devices.

Asymmetric field dependence of specific heat in two-leg Heisenberg antiferromagnetic spin ladders with asymmetric leg interactions

Two-leg Heisenberg antiferromagnetic spin-1/2 ladders with asymmetric leg interactions (J1≠J2) are studied using the bond-mean field method. With different J1−J2−J⊥ couplings, magnetization process exhibits distinct properties. For J1:J2:J⊥=4:2:1 and 4:1:2, M–h curves exhibit cusp-type singularities which do not appear for J1:J2:J⊥=2:1:4. Field dependence of specific heat (Cv/T–h curve) has been found to be asymmetric about hm=(h1+h2)/2, h1 and h2 are two critical fields. The cusp in M–h curves and the asymmetric structure of Cv/T–h curves are closely related to their energy spectrums. The linearity of specific heat in the gapless Luttinger-liquid phase and the underlying asymmetric ladder structure can be characterized by Cv/T–h curves.

Suppression of ferromagnetism in the LaVxCr1−xGe3system

We report synthesis of hexagonal LaVxCr1-xGe3 (x = 0-0.21, 1.00) single crystals, and present a systematic study of this series by measurements of temperature and field dependent magnetic susceptibility, magnetization, resistivity, and specific heat. Ferromagnetism has been observed for x = 0-0.21, and the system manifests a strong axial anisotropy in its ordered state. The decrease of the saturated moment and the effective moment per Cr with the increase of V-concentration suggests this is an itinerant ferromagnetic system. The Curie temperature declines monotonically as the V-concentration increases. Single crystalline samples could only be grown for x-values up to 0.21 for which the transition temperature was suppressed down to 36 K. Although we could not fully suppress TC via V-substitution, for x = 0.16, we performed magnetization measurements under pressure. The ferromagnetic state is suppressed under pressure at an initial rate of dTC/dp = -11.7 K/GPa and vanishes by 3.3 GPa.