Magnetic Heat Capacity Research Articles

Ferromagnetic Kondo lattice behavior in a novel compound Ce4Rh4Ge3

The formation of a ternary cerium germanide Ce4Rh4Ge3 was established. The crystal structure of this novel compound was determined by mean of single crystal and powder X-ray diffraction techniques to be of the monoclinic Nd4Rh4Ge3-type (space group C2/c, No.15, Pearson symbol mS44, Z = 4) with the lattice parameters: a = 21.027(6) Å, b = 5.7248(16) Å, c = 7.999(3) Å, β = 110.39(3) deg. The low-temperature physical properties of Ce4Rh4Ge3 were studied by means of magnetic susceptibility, magnetization, heat capacity, electrical resistivity and magnetoresistance measurements. The compound was found to order ferromagnetically at TC = 4.2(2) K. Its physical behavior in the paramagnetic state was found to be governed by interplay of crystalline electric field effect and Kondo interactions, with possible contribution due to valence fluctuations associated with one of the two inequivalent cerium atoms located in the crystallographic unit cell of Ce4Rh4Ge3.

Cluster-Based Haldane State in an Edge-Shared Tetrahedral Spin-Cluster Chain: Fedotovite K_{2}Cu_{3}O(SO_{4})_{3}.

Fedotovite K_{2}Cu_{3}O(SO_{4})_{3} is a candidate of new quantum spin systems, in which the edge-shared tetrahedral (EST) spin clusters consisting of Cu^{2+} are connected by weak intercluster couplings forming a one-dimensional array. Comprehensive experimental studies by magnetic susceptibility, magnetization, heat capacity, and inelastic neutron scattering measurements reveal the presence of an effective S=1 Haldane state below T≅4 K. Rigorous theoretical studies provide an insight into the magnetic state of K_{2}Cu_{3}O(SO_{4})_{3}: an EST cluster makes a triplet in the ground state and a one-dimensional chain of the EST induces a cluster-based Haldane state. We predict that the cluster-based Haldane state emerges whenever the number of tetrahedra in the EST is even.

Valence fluctuations in Ce2Rh3+xGe1-x

Ternary intermetallic alloys Ce2Rh3+xGe1-x (0 ≤ x ≤ 0.33) have been found to crystallize in a rhombohedral structure of the Mg2Ni3Si-type (space group R-3m). With varying the stoichiometry, the lattice parameters significantly vary from a = 5.5887(2) Å, c = 11.9557(5) Å (hexagonal setting) for x = 0 to a = 5.5127(2) Å, c = 12.2790(5) Å for x = 0.33, while the unit cell volume remains nearly unchanged. By means of magnetic susceptibility, heat capacity and electrical resistivity measurements, the Ce2Rh3+xGe1-x alloys have been shown to be intermediate valence systems with highly unstable 4f shell of Ce ions. Notably, the ferromagnetic ordering, claimed in the literature to coexist with the valence fluctuations in Ce2Rh3Ge, has been revealed to be an extrinsic effect due to impurity contribution.

Hopping conduction and spin glass behavior of La2FeMnO6

We report the existence of a charge localized state in a distorted, low-temperature phase of the perovskite La2FeMnO6 below a charge localization temperature (TCL) ∼133 K, from the detailed analysis of dc resistivity, supported by magnetic and specific heat capacity measurements. We observe that variable range hopping conduction is active below TCL while small polaron hopping conduction governs dc transport both above and below TCL, however with different activation energies. Evidence for the presence of Fe2+ is obtained from the X-ray photoelectron spectroscopic analysis, along with Fe3+, Mn3+, and Mn4+ ions which eventually result in various types of magnetic exchange interactions within the system. As a result, La2FeMnO6 exhibits a spin glass state, which is confirmed by ac susceptibility measurements, zero-field cooled memory and aging tests.

Analysis based on scaling relations of critical behaviour at PM-FM phase transition and universal curve of magnetocaloric effect in selected Ag-doped manganites.

The critical behaviour of Pr0.5Sr0.5−xAgxMnO3 (0 ≤ x ≤ 0.2) samples around the paramagnetic–ferromagnetic phase transition is studied based on isothermal magnetization measurements. The assessments based on Banerjee's criteria reveal the samples undergoing a second-order magnetic phase transition. Various techniques such as modified Arrott plot, Kouvel–Fisher method, and critical isotherm analysis were used to determine the values of the ferromagnetic transition temperature TC, as well as the critical exponents of β, γ and δ. The values of critical exponents, derived from the magnetization data using the Kouvel–Fisher method, are found to be (β = 0.43 ± 0.002, 0.363 ± 0.068 and 0.328 ± 0.012), (γ = 1.296 ± 0.007, 1.33 ± 0.0054 and 1.236 ± 0.012) for x = 0.0, 0.1 and 0.2, respectively. This implies that the Pr0.5Sr0.5−xAgxMnO3 with 0 ≤ x ≤ 0.2 systems does not belong to a single universality class and indicates that the presence of magnetic disorder in the system must be taken into account to fully describe the microscopic interaction of these manganites. With these values, magnetic-field dependences of magnetization at temperatures around TC can be well described following a single equation of state for our samples. From magnetic entropy change (ΔSM), it was possible to evaluate the critical exponents of the magnetic phase transitions. Their values are in good agreement with those obtained from the critical exponents using a modified Arrott plot (MAP). We used the scaling hypotheses to scale the magnetic entropy change and heat capacity changes to a universal curve respectively for Pr0.5Sr0.5−xAgxMnO3 samples.

Synthesis and characterization of the heavy-fermion compound CePtAl4Ge2

We report the synthesis of the Ce-based quaternary compound CePtAl4Ge2 that crystallizes in the trigonal structure (space group R3¯m, 166) with unit cell parameters, a = 4.1995(5) Å, c = 31.851(7) Å, and γ = 120°. Powder X-ray diffraction and energy dispersive X-ray spectroscopy show that CePtAl4Ge2 (LaPtAl4Ge2) is in a single, homogeneous phase. Magnetic susceptibility, electrical resistivity, and heat capacity measurements of CePtAl4Ge2 show that it exhibits antiferromagnetic behavior below 2.3 K. The magnetic susceptibility for the magnetic field applied perpendicular (χab) and parallel (χc) to the crystalline c-axis is very anisotropic, and the susceptibility ratio (χab/χc) reaches a maximum value of 10, indicating that the spin easy axis is within the Ce plane. The entropy recovered at TN is consistent with the doublet ground state of the crystal field split J = 5/2 multiplet of Ce3+ ions.

Effect of crystalline electric field on heat capacity of LnBaCuFeO5 (Ln = Gd, Ho, Yb)

Structural, magnetic and thermodynamic properties of layered perovskite compounds LnBaCuFeO5 (Ln = Ho, Gd, Yb) have been investigated. Unlike the iso-structural compound YBaCuFeO5, which shows commensurate antiferromagnetic to incommensurate antiferromagnetic ordering below ∼200 K, the studied compounds do not show any magnetic transition in measured temperature range of 2–350 K. The high temperature heat capacity of the compounds is understood by employing contributions from both optical and acoustic phonons. At low temperature, the observed upturn in the heat capacity is attributed to the Schottky anomaly. The magnetic field dependent heat capacity shows the variation in position of the anomaly with temperature, which appears due to the removal of ground state degeneracy of the rare earth ions, by the crystalline electric field.

Low temperature synthesis, structure and magnetic properties of Mn2[VO4

The compound Mn2[VO4]F was synthesized using a hydrothermal synthesis route at low temperature and its crystal structure was determined from single crystal X-ray diffraction data. Mn2[VO4]F was characterized by magnetic susceptibility and specific heat capacity measurements. Mn2[VO4]F crystallizes with the triplite-type structure, space group C2/c, a = 13.451(3) Å, b = 6.6953(16) Å, c = 10.126(3) Å, β = 116.587(4)°, V = 815.6(3) Å3 and Z = 8. The structure consists of a 3D-framework built up of VO4 tetrahedra, and manganese (II) polyhedra which form chains running along the [101] and [010] directions. The coordination of the manganese cations and the connectivity between the manganese polyhedra are not defined clearly due to the disorder of the fluoride anions which form zigzag chains along [001]. The magnetic susceptibility follows a Curie–Weiss behavior above 50 K with Θ = −88 K indicating that predominant magnetic interactions are antiferromagnetic. The specific heat capacity and magnetization measurements show that Mn2[VO4]F undergoes a three-dimensional magnetic ordering at TN = 30 K and a canted weak ferromagnetism due to mixed-anion effect.

Phenomenological Modeling of Magnetocaloric Effect for Ni58Fe26Ga28 Alloy

The magnetocaloric effect (MCE) of shape memory Ni58Fe26Ga28 has been investigated. A phenomenological model is used for simulation of magnetization dependence on temperature variation to investigate magnetocaloric properties such as magnetic entropy change and heat capacity change. The results indicate the potential of Ni58Fe26Ga28 alloy to achieve the MCE at temperatures near Curie temperature. This alloy presents a good-looking approach for cooling applications in its prospective candidate for a cooling system in a wide temperature interval including room temperature. The results also confirmed that the phenomenological model is useful for prediction of the MCE for magnetic materials.

Phase Competition in the Palmer-Chalker XY Pyrochlore Er_{2}Pt_{2}O_{7}.

We report neutron scattering measurements on Er_{2}Pt_{2}O_{7}, a new addition to the XY family of frustrated pyrochlore magnets. Symmetry analysis of our elastic scattering data shows that Er_{2}Pt_{2}O_{7} orders into the k=0, Γ_{7} magnetic structure (the Palmer-Chalker state), at T_{N}=0.38 K. This contrasts with its sister XY pyrochlore antiferromagnets Er_{2}Ti_{2}O_{7} and Er_{2}Ge_{2}O_{7}, both of which order into Γ_{5} magnetic structures at much higher temperatures, T_{N}=1.2 and 1.4K, respectively. In this temperature range, the magnetic heat capacity of Er_{2}Pt_{2}O_{7} contains a broad anomaly centered at T^{*}=1.5 K. Our inelastic neutron scattering measurements reveal that this broad heat capacity anomaly sets the temperature scale for strong short-range spin fluctuations. Below T_{N}=0.38 K, Er_{2}Pt_{2}O_{7} displays a gapped spin-wave spectrum with an intense, flat band of excitations at lower energy and a weak, diffusive band of excitations at higher energy. The flat band is well described by classical spin-wave calculations, but these calculations also predict sharp dispersive branches at higher energy, a striking discrepancy with the experimental data. This, in concert with the strong suppression of T_{N}, is attributable to enhanced quantum fluctuations due to phase competition between the Γ_{7} and Γ_{5} states that border each other within a classically predicted phase diagram.

Magnetic properties of monoclinic lanthanide metaborates, Ln(BO2)3, Ln = Pr, Nd, Gd, Tb

The bulk magnetic properties of the lanthanide metaborates, Ln(BO2)3, Ln = Pr, Nd, Gd, Tb are studied using magnetic susceptibility, heat capacity and isothermal magnetisation measurements. They are found to crystallise in a monoclinic structure containing chains of magnetic Ln3+ and could therefore exhibit features of low-dimensional magnetism and frustration. Pr(BO2)3 is found to have a non-magnetic singlet ground state. No magnetic ordering is observed down to 0.4 K for Nd(BO2)3. Gd(BO2)3 exhibits a sharp magnetic transition at 1.1 K, corresponding to 3D magnetic ordering. Tb(BO2)3 shows two magnetic ordering features at 1.05 K and 1.95 K. A magnetisation plateau at a third of the saturation magnetisation is seen at 2 K for both Nd(BO2)3 and Tb(BO2)3, which persists in an applied field of 14 T. This is proposed to be a signature of quasi 1D behaviour in Nd(BO2)3 and Tb(BO2)3.

Strange metal from Gutzwiller correlations in infinite dimensions

Recent progress in extremely correlated Fermi liquid theory (ECFL) and dynamical mean field theory (DMFT) enables us to compute in the $d \to \infty$ limit the resistivity of the $t-J$ model after setting $J\to0$. This is also the $U=\infty$ Hubbard model. We study three densities $n=.75,.8,.85$ that correspond to a range between the overdoped and optimally doped Mott insulating state. We delineate four distinct regimes characterized by different behaviors of the resistivity $\rho$. We find at the lowest $T$ a Gutzwiller Correlated Fermi Liquid regime with $\rho \propto T^2$ extending up to an effective Fermi temperature that is dramatically suppressed from the non-interacting value. This is followed by a Gutzwiller Correlated Strange Metal regime with $\rho \propto (T-T_0)$, i.e. a linear resistivity extrapolating back to $\rho=0$ at a positive $T_0$. At a higher $T$ scale, this crosses over into the Bad Metal regime with $\rho \propto (T+T_1)$ extrapolating back to a finite resistivity at $T=0$, and passing through the Ioffe-Regel-Mott value where the mean free path is a few lattice constants. This regime finally gives way to the High $T$ Metal regime, where we find $\rho \propto T$. The present work emphasizes the first two, where the availability of an analytical ECFL theory is of help in identifying the changes in related variables entering the resistivity formula that accompany the onset of linear resistivity, and the numerically exact DMFT helps to validate the results. We also examine thermodynamic variables such as the magnetic susceptibility, compressibility, heat capacity and entropy, and correlate changes in these with the change in resistivity. This exercise casts valuable light on the nature of charge and spin correlations in the strange metal regime, which has features in common with the physically relevant strange metal phase seen in strongly correlated matters.

Magnetization and low temperature heat capacity of SmFeO3 single crystal

3He heat capacity of SmFeO3 single crystal was carried out from 0.4K to 10K. Magnetization was also checked from 2K to 300K. Magnetization fitting obtains a reasonable Θ=−2.16K, while no signature of magnetic phase transition can be found down to 0.4K from specific heat, showing |Θ/Tc|>5.33. Considering the exchange interactions between Sm3+ and Fe3+, Sm3+ spins are frustrated. This gives a new form of spin frustration.

Magnetic properties, x-ray absorption spectroscopy and electronic structure of GdCrTiO5

We study the electronic structure of GdCrTiO5 polycrystals by means of magnetic susceptibility and heat capacity measurements, as well as x-ray absorption spectroscopies on the Cr 2p, Ti 2p and O 1s levels. The experimental results were interpreted with Charge Transfer Multiplet and Density Functional Theory calculations. Magnetic susceptibility and specific heat measurements indicate a long-range antiferromagnetic ordering at 0.9 K probably related to the Gd sublattice. The results indicate that spin-orbit interaction, rather than hybridization, may be crucial for the stabilization of magnetic ordering in the RCrTiO5 (R = rare earth) family.

A Novel Field Scaling Method in Electromagnetic Railguns

To overcome the shortcoming of existing field scaling method in electromagnetic railgun, a novel field scaling method with different materials in different scales was proposed. The scaling method was derived based on the governing equations of physical field and projectile dynamics using similarity theory. The results show that field scaling and projectile velocity matching can be achieved simultaneously under the condition of linearly scaling the railgun geometry by using two different materials of lower electrical conductivity in full-scale and higher electrical conductivity in subscale. And the magnetic permeability, mass density, and volumetric specific heat capacity in two kinds of materials should be approximately equal. Aiming at the practical considerations of the negative correlation between mechanical properties and electrical conductivities in existing metals, the scaling method proved that materials of good mechanical properties but lower electrical conductivities can be used in full-scale large-caliber railguns.

Thermodynamics of the quantum spin liquid state of the single-component dimer Mott system κ−H3(Cat−EDT−TTF)2

Thermodynamic properties of the proton-mediated single component dimer-Mott insulator of $\ensuremath{\kappa}\text{\ensuremath{-}}{\mathrm{H}}_{3}{(\mathrm{Cat}\text{\ensuremath{-}}\mathrm{EDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}$, which has a two-dimensional triangle lattice structure of $S=1/2$ spins, are reported. The extraordinary large electronic heat capacity coefficient $\ensuremath{\gamma}\phantom{\rule{0.16em}{0ex}}=\phantom{\rule{0.16em}{0ex}}58.8\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}\phantom{\rule{0.16em}{0ex}}\mathrm{mo}{\mathrm{l}}^{\ensuremath{-}1}$ observed by the low-temperature heat capacity measurements up to 6 T suggests the formation of the gapless spin liquid ground state. Although the magnetic interaction $J/{k}_{\mathrm{B}}$ is quite different from those of other dimer-Mott spin liquids, the thermodynamic feature scales well with the Wilson ratio of 1.4\char21{}1.6. The heat capacity measurements also detected that the deuteration of the proton-linkage changes the ground state to the nonmagnetic one with almost vanishing \ensuremath{\gamma}. Using the data of the deuterated compound, the accurate temperature dependence of the magnetic heat capacity reflecting on the low-energy excitations from the gapless spin liquids ground state is discussed.

Improved Scaling of the Magnetic Heat Capacity in La0.85Ag0.15MnO3 Manganite

The heat capacity in a La0.85Ag0.15MnO3 manganite was measured near Curie temperature TC under the applied magnetic fields up to 26 kOe to study scaling critical behavior and obtain universality class. The conventional scaling fails in application to the manganites with a hysteresis and the strong sensitivity of TC to a magnetic field. But the application of the improved scaling procedure designed by us allows of yielding the good scaling the magnetic heat capacity in La0.85Ag0.15MnO3, which may belong to a new universality class for systems with the strong spin-orbital coupling of t2g-electrons–Double 3D-Heisenberg with the critical exponent of the heat capacity α = −0.23 and the critical exponent of the correlation radius ν = 0.7433. This new universality class is consistent with the crystal, magnetic and orbital symmetries for the La0.85Ag0.15MnO3. Scaling failure in the vicinity of TC in the range of t/H1/2ν ≈ [−0.033; 0.024] is understood by finite-size and other disordering effects when T → TC. It is remarkable that finite-size effect is consistent with grain size, L ≈ 50 μm, in the La0.85Ag0.15MnO3. The correlation radius, Ltν ≈ 30.28 A, estimated from the finite-size effect is of the same order of magnitude with the sizes of the ferromagnetic fluctuations and drops in manganites.

Magnetocaloric-Transport Properties Correlation in La0.8 Ca0.2MnO3-Doped Manganites

This investigation is interested in studying the relation between magnetocaloric effect and transport properties i La0.8Ca0.2MnO3 manganite compound. The value of the magnetocaloric effect has been determined from the calculation of magnetization as a function of temperature under different external magnetic fields. This study also provides an alternative method to determine the magnetocaloric properties such as magnetic entropy change and heat capacity change on the basis of M(T, H) measurements. On the other hand, based on magnetic and resistivity measurements, the magnetocaloric properties of this compound were investigated using an equation of the form {Delta } S ,=, - alpha {int limits _{0}^{H}} {left [ {frac {delta Lnleft (rho right )}{delta T}} right ]}_{H} dH, which relates magnetic order to transport behavior of the compounds. As an important result, the values of MCE and the results of calculation are in good agreement with the experimental ones, which indicates the strong correlation between the electric and magnetic properties in manganites.

Exploring quantum Griffiths phase in Ni1\u2212xVx nanoalloys

Metallic Ni1−xVx alloys exhibit a ferromagnetic to paramagnetic disordered quantum phase transition in bulk. Such a phase transition is accompanied by a quantum Griffiths phase (QGP), featuring fractional power-law temperature dependences of physical variables, like magnetic susceptibility and specific heat, at low temperatures. As nanoparticles (NP’s) usually exhibit properties significantly different from their bulk counterparts, it is intriguing to explore the occurrence of quantum Griffiths phase in Ni1−xVx nanoalloys. NP’s of Ni1−xVx (0 ≤ x ≤ 0.17) alloys are prepared by a chemical reflux method. The structure and composition of the nanoalloys are determined by X-ray diffraction, X-ray photoelectron spectroscopy and electron microscopy techniques. Metallicity of the samples has been ensured by electrical resistivity measurements. DC magnetization results suggest that ferromagnetism persists in the NP’s until x = 0.17. Low-temperature upturns in magnetic susceptibility and heat capacity hint at critical fluctuations evolving with V-doping. The fluctuations might stem from isolated Ni-clusters within the ferromagnetic NP, indicating a QGP region ranging from x = 0.085 to x ≫ 0.17.

Heat capacity evidence for proximity to the Kitaev quantum spin liquid in A2 IrO3 ( A=Na , Li)

The honeycomb lattice iridates $A_2$IrO$_3$ ($A =$ Na, Li) are candidates for realization of the Kitaev-Heisenberg model although their proximity to Kitaev's quantum Spin-Liquid (QSL) is still debated. We report on heat capacity $C$ and entropy $S_{mag}$ for $A_2$IrO$_3$ ($A =$ Na, Li) in the temperature range $0.075~{\rm K}\leq T \leq 155$~K\@. We find a well separated two-peak structure for the magnetic heat capacity $C_{mag}$ for both materials and $S_{mag}$ for Na$_2$IrO$_3$ shows a shoulder between the peaks with a value close to ${1\over 2}$Rln$2$. These features signal the fractionalization of spins into Majornana Fermions close to Kitaev's QSL [Phys. Rev. B {\bf 92}, 115122 (2015); Phys. Rev. B {\bf 93}, 174425 (2016).]. These results provide the first thermodynamic evidence that $A_2$IrO$_3$ are situated close to the Kitaev QSL. Additionally we measure the high temperature $T\leq 1000$~K magnetic susceptibility $\chi$ and estimate the Weiss temperature $\theta$ in the true paramagnetic state. We find $\theta \approx -127$~K and $-105$~K, for Na$_2$IrO$_3$ and Li$_2$IrO$_3$, respectively.