Field Dependence Of Specific Heat Research Articles

Electronic properties of UCoAl0.75Sn0.25 single crystal

A single crystal of the UCoAl0.75Sn0.25 quasiternary compound has been grown and studied by measuring the magnetization, specific heat and electrical resistivity at various temperatures and magnetic fields. Similar to the parent compounds UCoAl and UCoSn, the magnetization of their solid solution exhibits uniaxial anisotropy with a strong magnetic response along the c-axis and a weak and nearly temperature independent paramagnetic signal in the basal plane. The evolution of the c-axis magnetization isotherms, as well as the anomalies in the temperature dependence of the specific heat and resistivity point to ferromagnetism in UCoAl0.75Sn0.25 below TC≈5.5K. The low value of the spontaneous magnetic moment, poor saturation of the magnetization, the pronounced negative magnetoresistance with increasing magnetic field and the temperature and magnetic field dependence of specific heat provide indications of strong fluctuations of the U magnetic moment also in the ferromagnetic state.

Antiferromagnetic interactions in the semimetallic Yb 4As 3: field-dependent specific heat and magnetization data

The S= 1 2 antiferromagnetic Heisenberg model with the Dzyaloshinsky–Moriya interaction is applied to a semimetallic compound Yb 4As 3. The field dependent specific heat and the magnetization of this model are calculated by the numerical quantum transfer-matrix method and compared with experiment. New specific heat experimental results for the polydomain sample in the field applied are presented and compared with the theoretical predictions obtained without adjustable microscopic parameters.

Critical examination of heat capacity measurements made on a Quantum Design physical property measurement system

We examine the operation and performance of an automated heat-capacity measurement system manufactured by Quantum Design (QD). QD’s physical properties measurement system (PPMS) employs a thermal-relaxation calorimeter that operates in the temperature range of 1.8–395 K. The accuracy of the PPMS specific-heat data is determined here by comparing data measured on copper and synthetic sapphire samples with standard literature values. The system exhibits an overall accuracy of better than 1% for temperatures between 100 and 300 K, while the accuracy diminishes at lower temperatures. These data confirm that the system operates within the ±5% accuracy specified by QD. Measurements on gold samples with masses of 4.5 and 88 mg indicate that accuracy of ±3% or better can be achieved below 4 K by using samples with heat capacities that are half or greater than the calorimeter addenda heat capacity. The ability of a PPMS calorimeter to accurately measure sharp features in C p ( T) near phase transitions is determined by measuring the specific heat in the vicinity of the first-order antiferromagnetic transition in Sm 2IrIn 8 ( T 0=14 K) and the second-order hidden order (HO) transition in URu 2Si 2 ( T N =17 K). While the PPMS measures C p ( T) near the second-order transition accurately, it is unable to do so in the vicinity of the first-order transition. We show that the specific heat near a first-order transition can be determined from the PPMS-measured decay curves by using an alternate analytical approach. This correction is required because the latent heat liberated/absorbed at the transition results in temperature–decay curves that cannot be described by a single relaxation time constant. Lastly, we test the ability of the PPMS to measure the specific heat of Mg 11B 2, a superconductor of current interest to many research groups, that has an unusually strong field-dependent specific heat in the mixed state. At the critical temperature the discontinuity in the specific heat is nearly 15% lower than measurements made on the same sample using a semi-adiabatic calorimeter at Lawrence Berkeley National Laboratory.

Magnetic anisotropy of the heavy fermion state in PrFe4P12

We report the magnetic field dependence of specific heat on single crystalline PrFe4P12 measured in fields applied along the three main symmetry directions 〈100〉,〈110〉, and 〈111〉. The electronic thermal excitation in the high-field heavy-fermion state is found to be strongly enhanced for fields along the 〈111〉 direction.

Flux Jumps and H-T Diagram of Instability for MgB2

We present a study of magneto-thermal instabilities in polycrystalline MgB2 superconductor, by magnetic hysteresis loop measurements and by investigations of magnetic flux dynamics with a miniature Hall probe. Temperature and magnetic field ranges where the flux jumps may be observed have been determined. On the basis of measurements of the magnetic flux dynamics, an average magnetic diffusivity describing the process of the flux jump is estimated. This parameter is compared with the thermal and magnetic diffusivities calculated on the basis of available data for thermal conductivity, heat capacity and resistivity. It is shown that the estimated value of the field of the first flux jump is influenced significantly by the field dependence of specific heat. In order to explain the observed phenomenon, the temperature reached by the sample during the flux jump at different magnetic fields is calculated.

Ginzburg-Landau theory of a resonating-valence-bond superconductor

We present a Ginzburg-Landau formulation of the bosonic resonating valence bond (RVB) theory of superconductivity. The superconducting order parameter is characterized by phase vortices that describe spinon excitations and the transition to the superconducting state occurs when such phase vortices (un)bind. We show that the boson RVB theory always leads to hc/2e flux quanta and that the presence of a trapped spin-1/2 moment inside a vortex core gives rise to observable consequences for the low temperature field dependent specific heat. We also show that the cores of magnetic fluxoids exhibit enhanced antiferromagnetic correlations.

Superconducting transitions studied by ultrahigh-resolution photoemission

Superconducting transitions of elemental metals and Ni borocarbides are studied using ultrahigh-resolution and low-temperature photoemission spectroscopy. Photoemission spectra of elemental metals (Nb and Pb) show the appearance of a sharp peak just below the Fermi level and a shift of the leading edge, indicating clear opening of the superconducting gap. Furthermore, Pb spectrum shows a dip and hump structure beyond the peak, characteristic of the strong-coupling superconductivity. On the other hand, comparative study of Y(Ni 1− x Pt x ) 2B 2C ( x=0.0 and 0.2) shows a difference in the slope of the leading edge, which can be explained in terms of a highly anisotropic s-wave gap in YNi 2B 2C, consistent to magnetic field dependent specific heat measurements. These observations demonstrate that photoemission spectroscopy is a useful and reliable technique to study electronic structures and phase transitions at very small energy scales.

Experimental observation of quantum coherence in molecular magnetic clusters with half-integer spin

From field-dependent specific heat studies we conclude that magnetic relaxation becomes temperature independent for two Mn 4 compounds, below some temperature T Q. At low temperatures, the specific heat gives evidence for the presence of a tunnel splitting Δ t of the ground state S z =±9/2 for both compounds. The predicted quenching of Δ t for clusters with half-integer spin apparently does not occur, probably due to the joint action of dipolar and hyperfine fields, in combination with the applied perpendicular field of sufficient strength.

Specific heat and magnetocaloric effect study of multiple field-induced phase transitions inHoGa2

Both temperature and field dependences of specific heat C and magnetocaloric effect (MCE) were measured on a ${\mathrm{HoGa}}_{2}$ single crystal in fields up to 8 T at low temperatures down to 0.1 K. The magnetic fields were applied in the [100] direction, for which intermediate magnetic phases appear. Significant Ho nuclear contribution to C is found to dominate below 1 K and its field-independent character reveals that the value of Ho magnetic moments does not change visibly in the measured field range irrespective of the magnetic phases. From anomalies observed in the C and MCE data, the reported field-vs-temperature phase diagram is corroborated thermodynamically. One of the phase transitions occurring at ${T}_{t}=6.6 \mathrm{K}$ in zero field is found to be of first-order transition. Magnetic entropy determined by the MCE measurements shows an enhancement in the intermediate phases and small humps at the phase boundaries. Based on a crystalline-electric-field model, the nuclear specific heat, magnetic entropy of the $4f$ electrons, and anisotropy in the magnetization can be explained consistently. Possible origins of the reported giant magnetoresistance caused by the appearance of the intermediate phases will be discussed.

Specific-heat anomaly of metamagnetism on PrFe 4P 12 and UCoAl

The field dependence of specific heat has been measured at low temperatures in order to study two different types of f-electron metamagnetic systems. The antiferromagnet PrFe 4P 12 with Néel temperature of 6.4 K shows a sharp peak structure in C/ T at the spin flip metamagnetic transition. In the high field state, the magnetic contribution C m/ T shows an anomalous huge value ∼1.4 J/K 2 mol . UCoAl shows the metamagnetic transition from paramagnetic ground state at ∼1 T, where C/ T at 0.5 K shows an apparent decrease reflecting a change of the density of states. In the low-field state, ferromagnetic spin-fluctuation dominates in an anomalous temperature dependence of C.

The experimental study of the magnetic field dependence of specific heat in S = [formula omitted] quantum-spin chain CuSO 4 · 5H 2O

The magnetic specific heat measurements were performed on monocrystalline CuSO 4·5H 2O in a magnetic field from 0 up to T applied in a direction parallel to the easy axis at temperatures 415, 615 and 815 mK. The specific heat as a function of the applied magnetic field exhibits behaviour that cannot be accounted for by the spin-wave approximation.

Metallic ground state of CeNiSn

The electronic ground state of CeNiSn has been investigated by means of specific-heat measurements below 2 K in magnetic field up to 5 T using a high-quality single crystal. Unusual temperature and field dependence of specific heat reveals that the density of states of CeNiSn has a peak at ${E}_{F}$ inside the pseudogap. We claim that CeNiSn is no longer a Kondo insulator but a metal at the lowest temperature.

Metamagnetic-like anomaly in CeCu 6

The field dependences of specific heat C and magnetocaloric effect (MCE) Δ T/Δ B in CeCu 6 have been measured to investigate the thermodynamical aspects of the weak metamagnetic-like anomaly (MA). Around 1.7 T both properties exhibit anomalous behaviors, which have close similarities with those in CeRu 2Si 2. This fact suggests that the quasiparticle density of states in CeCu 6 has a faint peak structure near the Fermi level, which is superposed on the Kondo-resonance peak.

Characterization of a quasi-one-dimensional spin-1/2 magnet which is gapless and paramagnetic forgμBH≲JandkBT≪J

High field mangetization, field-dependent specific heat measurements, and zero field inelastic magnetic neutron scattering have been used to explore the magnetic properties of copper pyrazine dinitrate (Cu(C4H4N2)(NO3)2). The material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with nearest neighbor exchange constant J=0.90(1) meV and chains extending along the orthorhombic a-direction. As opposed to previosly studied molecular-based spin-1/2 magnetic systems, coppyer pyrazine dinitrate remains gapless and paramagnetic for g mu_B H/J at least up to 1.4 and for k_B T/J at least down to 0.03 this makes the material an excellent model system for exploring the T=0 critical line which is expected in the H - T phase diagram on the one-dimensional spin-1/2 Heisenberg antiferromagnet. As a first example of such a study we present accurate measurements of the Sommerfeld constant of the spinon gas versus g mu_B H/J < 1.4 which reveal a decrease of the averages spinon velocity by 32% in that field range. The results are in excellent agreement with numerical calculations based on the Bethe ansatz with no adjustable parameters.

Effect of sample preparation on the magnetic and magnetocaloric properties of amorphous Gd70Ni30

Certain applications of magnetic materials require the knowledge of the magnetocaloric effects, i.e., the adiabatic change of temperature or the isothermal change of the entropy upon application or removal of magnetic field. While the isothermal magnetic entropy change can be calculated from magnetic data only, the calculation of the adiabatic temperature change requires the knowledge of the field-dependent specific heat as well. To compare magnetocaloric effect results obtained from magnetic-only, thermal-only, or combined methods requires the evaluation of the mutual reliability of all methods. In crystalline materials it usually does not present a problem to perform thermal and magnetic measurements on the same sample or at least on samples of identical state. For amorphous materials, unfortunately, thermal measurements are extremely difficult to perform on the ribbons themselves so usually pellets pressed from powdered ribbons, with silver as a binder, are applied. After the magnetic entropy change calculated from thermal measurements performed on such a composite did not agree with the data obtained from magnetic measurements performed on the original ribbon, magnetic measurements were carried out on the pellet and its predecessor powders. Comparison of the results revealed that the pellet’s properties do not reflect the properties of the original ribbon. However, comparison of magnetic and thermal measurements carried out on the pellet itself show good agreement if proper numerical methods are used for evaluation. The study concludes that amorphous ribbons are far more competitive for certain applications than the thermal measurements, performed on the pellet only, would indicate.

Anomalous field dependence of specific heat of CeNiSn below 1 K

Specific heat measurements have been performed below 2 K in magnetic field up to 5 T using high-quality single-crystalline CeNiSn which shows metallic conduction at low temperatures. Remarkable field dependence has been observed in the specific heat, suggesting the possibility that a new electronic ground state due to a many-body effect is realized in CeNiSn below 0.5 K.

Thermal properties of metamagnetic transition in heavy-fermion systems

In order to investigate the metamagnetic anomaly (MA) observed in several heavy-fermion compounds, field dependence of specific heat C(B) and magnetocaloric effect have been measured in the prototypical 4f-electron compound CeRu 2Si 2, and compared to that of a 5f-electron metamagnet UCoAl. In the C/T versus B curves, an unexpected double-peak structure is found in CeRu 2Si 2, suggesting that MA is closely related to a sharp peak in the density of states of the hybridized quasiparticle bands. On the other hand, for UCoAl, only a small decrease in C/T is observed, although both compounds exhibit similar bulk magnetic properties.

The magnetic field dependence of the electronic specific heat of

The magnetic field dependence of the specific heat was examined for and . The former compound is a simple ferromagnet, while the latter shows an antiferromagnetic-to-ferromagnetic (AF-to-F) transition with increasing temperature and undergoes a metamagnetic transition at around 6 T at low temperatures. The results indicate that the electronic specific heat coefficient, , is larger in the F state than in the AF state. These results suggest the importance of the electronic specific heat as regards the entropy change associated with the AF-to-F transition. In the F state, a decrease of with increasing field was observed for as well as for . The origin of this behaviour is also discussed.

Anomalous behavior of the specific heat and upper critical magnetic field in the superconducting single crystal La1.85Sr0.15CuO4

The specific heat and resistive upper critical magnetic field of the single crystal La1.85Sr0.15CuO4 are investigated in the temperature range 2–50 K in magnetic fields up to 8 T for two directions of the magnetic field, parallel and normal to the ab crystalline plane. For both orientations a nonlinear (close to square root) magnetic field dependence of the mixed-state specific heat and a positive curvature of the temperature dependence of the upper critical magnetic field are observed. Neither of these anomalies is described by standard theories of superconductivity. Within the framework of the thermodynamic relations it is shown that in a type-II superconductor a relationship exists between the temperature dependence of the critical magnetic field and the field dependence of the specific heat. The anomalies observed in these phenomena are interrelated.

Comparison of magnetocaloric properties from magnetic and thermal measurements

The isothermal change of the magnetic entropy of a magnetically ordered material upon application of external magnetic field can be calculated from the temperature and field dependence of the magnetization or of the specific heat. The adiabatic temperature change, i.e., the magnetocaloric effect (MCE) can be measured directly or can be calculated via different methods using the field-dependent specific heat values, or a combination of data obtained via magnetization and thermal measurements. In the present study, magnetic and thermal measurements were carried out on Gd75Y25(TC=232 K) and Gd48Y52(TC=161 K) samples, for applied fields ranging between 0 and 7 T. From both datasets, the magnetic entropy change and MCE values were calculated and compared, in order to assess the mutual reliability of the methods applied. The magnetically or thermally deduced specific heat discontinuities show a reasonable agreement within experimental error. Similar comparison of the calculated magnetic entropy changes reveals that the measured transition temperature and the shape of the curve do not depend on the method selected. It is demonstrated that the choice of an integration constant during entropy calculation has a significant impact on the adiabatic temperature change deduced from the field and temperature dependence of the entropies. For the MCE, a better approximation can be obtained using the magnetically acquired magnetic entropy change and the field-dependent specific heat. The results prove that magnetic measurements carried out in high enough magnetic fields provide reliable information on the isothermal magnetic entropy change and, when combined with field-dependent specific heat measurements, on the magnetocaloric effect as well.