Magnetocaloric effect and the heat capacity of ferrimagnetic nanosystems in magnetic fluids

The specific heat capacity of a magnetite-based magnetic fluid and changes in the magnetic part of the molar heat capacity of its magnetic phase in magnetic fields of 0–0.7 T were determined calorimetrically over the temperature range 288–353 K. The temperature dependence of changes in the magnetic part of entropy in an applied magnetic field was calculated. It was found that the field dependence of heat capacity had a maximum in fields of 0.3–0.4 T, and the temperature dependences of changes in the magnetic part of heat capacity ΔC _p (H) and entropy ΔS _m(H) had maxima at the magnetic phase transition temperature.     

Magnetocaloric properties of dendrimer complexes of Fe(III) with substituted Schiff base

In dendrimer complexes of iron (III) with Schiff base (three complexes of iron (III) based on azomethine 4,4′-dodecyloxybenzoyloxybenzoyl-4-salicylidene-2-aminopyridine, a significant magnetocaloric effect (MCE) and heat capacity was found the first time. It was found that the magnitude of MCE depends on the nature of the counter-ion of the complex. MCE were measured with a microcalorimeter over the temperature range of 278–320 K and in a magnetic induction of 0–1.0 T. The temperature dependences of the MCE dendrimer complexes of iron (III) with Schiff base were obtained for the first time. For all the samples studied, the existence of extreme temperature dependence of MCE in the range of temperatures 300–350 K, which is possibly the result of the magnetic phase transition, is shown. The correlation between the thermotropic mesomorphism with the magnetic phase transition in complexes has been established.     

The magnetothermal properties of substituted (tetraazoporphynato)manganese(III) in aqueous suspension

The magnetocaloric effect (MCE), heat capacity, and enthalpy and entropy of magnetization of the high-spin (acetato)(2,3,7,8,12,13,17,18-octa-para-tert-butylphenyltetraazaporphynato)manganese(III) complex in a 6% aqueous suspension were determined microcalorimetrically at 298 K in magnetic fields of 0.1–1.0 T, that is, under the conditions comparable with those used with (2,3,7,8,12,13,17,18-octaethylporphynato)chloromagnanese(III) studied earlier. High-dispersity complex particles were found to have paramagnetic properties. Positive MCE values were obtained. These values grew as magnetic field induction increased. MCE sensitivity to the nature and electronic structure of the aromatic macroring was studied. The presence of aza groups in the structure of the complex decreases the MCE value compared with the porphyrin complex. The specific heat capacity of the complex strongly depended on the magnetic field value; field dependences had a maximum close to 0.3 T. Changes in the ΔS _m(H, T) molar entropy part were also extremal and had maxima at 0.3–0.4 T. Prospects for the use of magnetothermal properties in the quantitative determination of thermodynamic characteristics and for revealing trends of changes in the magnetic activity of porphyrin complexes are discussed.     

Thermal characteristics of thermosets formed by free radical photocuring

We have studied the compound MnAs with a giant magnetocaloric effect, which has a magneto-structural transition at 316 K. The magnetic phase diagram has been deduced from adiabatic heat-capacity measurements and from cooling and heating curves. The ferroto paramagnetic transition changes with field from 316 K at 0 T to 335.3 K at 6 T. There is a strong thermal hysteresis between 10 and 5 K, depending on field. Direct measurements of the adiabatic temperature change caused by the application of magnetic field were made and compared with the values deduced from heat-capacity data.Moreover, adiabatic field cycles were performed quasi-statically between 0 and 6 T around the phase coexistence region, showing the strength of the effect in each phase and on the coexistence lines.     

The magnetocaloric effect and heat capacity of ferrimagnetic nanosystems: High-dispersity magnetite

The magnetocaloric effects of aqueous and ethanolic high-dispersity magnetite suspensions and the magnetite magnetic liquid were determined calorimetrically over the temperature range 15–80°C. The temperature dependence of the magnetocaloric effect of suspensions was evidence of the thermal oxidation of magnetite to maghemite. The temperature dependences of the magnetocaloric effect of the magnetic liquid passed extrema related to the second-order magnetic phase transition.     

Magnetocaloric effect and heat capacity of high-spin manganese complexes in a disperse state

Magnetothermal properties of high-spin chloro(2,3,7,8,12,13,17,18-octaethylporphyrinato)manganese(III), chloro(5,10,15,20-tetraphenylporphyrinato) manganese(III), bromo(5,10,15,20-tetraphenylporphyrinato)manganese(III), and (acetato)(5,10,15,20-tetraphenylporphyrinato)manganese(III) complexes as 6% water suspensions were determined by the microcalorimetric method at 298 K in a magnetic field of 0–1.0 T. It was established that when the magnetic field was applied, the temperature of the systems increases, leading to positive values of the magnetocaloric effect: the higher the magnetic field induction, the higher the values. It is shown that the dependences of the heat capacity of the complexes’ solid particles on the magnetic field induction are of an extreme nature with a heat capacity in the area above 0.6 T less than that in the zero field. The regularities of the dynamics of the numerical values of the change in enthalpy and magnetic entropy of the manganese complexes when a growing magnetic field was applied and the regularities of the influence of the acidoligand in pentacoordinated complexes on their magnetothermal properties were considered.     

The magnetocaloric effect and the heat capacity of aqueous suspensions of porphyrin complexes of rare earth elements according to microcalorimetric data

The magnetocaloric effect (MCE) and heat capacity during the magnetization process of (5,10,15,20-tetraphenylporphyrinato)acetatogadolinium(III), (AcO)GdTPP, and (5,10,15,20-tetraphenylporphyrinato)chlorogadolinium(III) complexes, (Cl)GdTPP, in the form of 6%-aqeous suspensions are determined by the microcalorimetric method in a range of temperatures from 278 to 318 K and magnetic fields from 0 to 1 T. It is found that MCE for all the complexes are positive, i.e., at applying a magnetic field in the adiabatic conditions temperature of a suspension of complex increases. It is established that MCE increases with an increase in magnetic induction at all temperatures and decreases with an increase in temperature at all magnetic fields. It is shown that the substitution of chloride ligand by acetate in (X)GdTPP leads to a significant increas in MCE and its temperature dependence; in the case of (Cl)GdTPP actually MCE does not depend on temperature. Relationships between magnetothermal properties and structure of the complexes are analyzed. The conclusion is argumented that the reason of changes in magnetothermal properties after the replacement of axial ligand in gadolinium complexes and complexes of lanthanides with an unsymmetrically filled f-shell is non-planar geometry of the coordination site and specific electronic properties of the central ion. It is concluded that heat capacity of the complexes slightly increases with an increase in temperature and more noticeably in the case of (AcO)GdTPP; a magnetic component of heat capacity is revealed only in (AcO)GdTPP at temperatures above 298 K, which is connected perhaps with a temperature change in the crystal lattice of the complex and influence of the magnetic properties of gadolinium ion on this change.     

A microcalorimeter for studying the magnetocaloric effect and the heat capacity in magnetic fields

The design of an automated microcalorimeter for determining the magnetocaloric effect and the heat capacity of suspensions and magnetic colloids in magnetic fields of from 0 to 1 T over a wide temperature range and the corresponding experimental procedure were described.     

Magnetocaloric effect and heat capacity of ferrimagnetic nanosystems: Magnetite-based magnetic liquids and suspensions

The magnetocaloric effect (MCE) and heat capacity of magnetite-based magnetic liquids and suspensions of magnetite in cyclohexane and water were studied calorimetrically at various temperatures and magnetic inductions. It was found that the magnetocaloric effect in the systems under study increases nonlinearly with the magnetic induction. In contrast to monocrystalline magnetite, the inverse temperature dependence was observed for the MCE in the nanosystems studied over the entire temperature range covered; i.e., the effect decreases with increasing temperature. It was found that the dependence of the specific heat on the magnetic induction passes through a maximum for all the systems at all temperatures tested; its height increases with the temperature. The extremal character of the dependence can be explained by the formation of chain structures of magnetite nanoparticles in the presence of a magnetic field.     

Magnetocaloric effect in the ternary DyCo3B2 compound

A large magnetocaloric (MCE) effect has been observed for the ternary compound DyCo₃B₂. This material shows the magnetic ordering below T_C = 22 K for H = 0 T. MCE has been determined based on the isothermal magnetization curves measurements and the isomagnetic heat capacity dependence on temperature. The maximum magnetic entropy change −ΔS_M = 17.5 J kg⁻¹K⁻¹ and the adiabatic temperature change ΔT_ad = 14 K have been observed in the neighborhood of the magnetic phase transition at the magnetic field change of 9 T. The analysis of the magnetic contribution to the specific heat indicates on the important role of the crystal electric field and the anisotropy for the properties of the DyCo₃B₂ compound.     

Magnetostructural study of 2-(4-N-tert-butylaminoxylphenyl)benzimidazole

The heat capacity of the title organic free radical, PhBABI, was measured over 0.3-300 K by adiabatic calorimetry and relaxation methods in the presence of external magnetic fields up to 9 T. A hump in the magnetic heat capacity was observed with a maximum at about 15 K in zero field, which did not shift at fields up to 9 T. The experimental magnetic entropy was in good agreement with the theoretical value of R ln 2 (= 5.76 J K(-1) mol(-1)) for S = 1/2 systems. The higher temperature, field-insensitive feature was fitted to several antiferromagnetic Heisenberg models. The best fits were obtained using spin ladder and coupled spin bilayer models.     

Co4(OH)2(C10H16O4)3 metal-organic framework: slow magnetic relaxation in the ordered phase of magnetic chains

Reported here are the synthesis and structural and topological analysis as well as a magnetic investigation of the new Co(4)(OH)(2)(C(10)H(16)O(4))(3) metal-organic framework. The structural analysis reveals a one-dimensional inorganic subnetwork based on complex chains of cobalt(II) ions in two different oxygen environments. Long alkane dioic acid molecules bridge these inorganic chains together to afford large distances and poor magnetic media between dense spin chains. The thermal dependence of the χT product provides evidence for uncompensated antiferromagnetic interactions within the cobaltous chains. In zero-field, dynamic magnetic susceptibility measurements show slow magnetic relaxation below 5.4 K while both neutron diffraction and heat capacity measurements give evidence of long-range order (LRO) below this temperature. The slow dynamics may originate from the motion of broad domain walls and is characterized by an Arrhenius law with a single energy barrier Δ(τ)/k(B) = 67(1) K for the [10-5000 Hz] frequency range. Moreover, in nonzero dc fields the ac susceptibility signal splits into a low-temperature frequency-dependent peak and a high-temperature frequency-independent peak which strongly shifts to higher temperature upon increasing the bias dc field. Heat capacity measurements have been carried out for various applied field values, and the recorded C(P)(T) data are used for the calculation of the thermal variations of both the adiabatic temperature change ΔT(ad) and magnetic entropy change ΔS(m). The deduced data show a modest magnetocaloric effect at low temperature. Its maximum moves up to higher temperature upon increasing the field variation, in relation with the field-sensibility of the intrachain magnetic correlation length.     

A new type of antiferrodistortive structure in a double perovskite with Pb

We have found a new structural transition in Pb(2)MnReO(6) at 410 K. Above this temperature, Pb(2)MnReO(6) is cubic with disordered and dynamic atomic displacements manifested in the large thermal parameters of Pb and O atoms. Below 410 K, the antiferrodistortive shift of 2/3 of Pb(2+) cations away from the high-symmetry cubic site produces a new type of monoclinic cell. The unit cell expands at the transition and the heat capacity shows a peak with thermal hysteresis. These features agree with a first order transition. The entropy content of the transition is quite low indicating that the structural disorder has not been completely removed in the low temperature phase. The monoclinic phase of Pb(2)MnReO(6) shows thermally activated conductivity which does not vary when an external magnetic field is applied. A change in the slope of the resistivity curve, observed at the structural phase transition temperature, is related to a slight difference in the activation energy between both phases. It suggests that the condensation of the distortions likely affects the conduction mechanism. The isothermal magnetization measurements reveal the presence of ferromagnetic contributions below 85 K. The ac magnetic susceptibility shows a dynamic peak at 50 K and, in addition, zero-field-cooled and field-cooled magnetization curves diverge strongly below 80 K. These features might be signature of magnetic inhomogeneity. Magnetic loops, obtained at 5 K, do not show saturation in fields up to 9 T. Furthermore, the measured coercivity increases sharply at low temperature indicating an abrupt change in the magnetic anisotropy. We show that all these magnetic properties point out to a ferrimagnetic ordering of Mn and Re atoms in an intermediate valence state.     

D-和L-丙氨酸的低温磁相变——磁场变化下的比热和直流磁化率(英文)

为了解D-和L-丙氨酸单晶晶格在极低温下是否存在磁手性相变,在2-20 K下改变磁场强度(0,1,3,5T)测定其比热.实验结果表明比热和温度之间的函数关系很好地符合C(T)=aT3+b/T2方程,其中aT3项为晶格声子的贡献,可由公式CV=(12/5)π4R(T/ΘD)3来描述(ΘD为德拜温度),b/T2项为磁场对比热的贡献.实验发现,在2-20 K范围内D-和L-丙氨酸单晶在不同磁场强度下均存在Boson峰(在Cp/T3-T曲线中表现为一个最大值).磁的贡献导致D-和L-丙氨酸单晶的四条Cp/T3-T曲线在2-12 K时不重合,且在12-20 K时消失,此即Schottky反常.零磁场下,D-和L-丙氨酸的Boson峰分别为9.44和10.86 K;德拜温度分别为151.5和152.7 K.结合磁场强度1 T下的直流磁化率测定,发现在温度低于5 K时,D-和L-丙氨酸单晶有相反的磁化率行为,反映了核自旋和电子自旋弱相互作用的手性表现.     

Comparison of magnetocaloric properties of the Mn2−xFexP0.5As0.5 (x = 1.0 and 0.7) compounds

The Mn_2−xFe_xP_0.5As_0.5 compounds (x = 0.7 and 1.0) studied exhibit the magnetic phase transitions, which are accompanied by a magnetic entropy change. For x = 1 the PM–FM transition is of the first order one with a weak (2–3 K) thermal hysteresis in the vicinity of T_C = 275 K. The Mn_1.3Fe_0.7P_0.5As_0.5 compound possesses two magnetic transitions: the second-order PM–FM transition at T_C = 190 K, followed by the FM–AFM transition at T_N = 90 K, leading to normal and inverse magnetocaloric effects, respectively. The maximum values of magnetic entropy change are equal to 17 J kg⁻¹ K⁻¹ in MnFeP_0.5As_0.5 and 5 J kg⁻¹ K⁻¹ in Mn_1.3Fe_0.7P_0.5As_0.5 for a field change of 5 T. The magnetic entropy changes were calculated using both the isofield magnetization curves versus temperature and the isothermal magnetization curves versus applied magnetic field. The magnetocaloric effect in MnFeAs_0.5P_0.5 is discussed in the terms of both the thermodynamic Maxwell relation and the Clausius–Clapeyron equation.     

Heat capacity of superfine oxides of iron under applied magnetic fields

Heat capacity is one of the most characteristic and important properties when the peculiarities of magnetic nanosystems are studied. In these systems the magnetic ordering becomes obvious due to the thermal effects such as heat capacity anomalies. It was considered earlier that heat capacity change under magnetic fields applied is slight and it cannot be taken into account in thermodynamic calculations. However the experimental heat capacity data for ferrofluids under magnetic fields applied show that field and temperature heat capacity dependences have a complicated behavior and in magnetic fields an essential heat capacity change takes place. On the other hand in the literature the contradictory data about heat capacity of nanoparticles appear. According to some papers nanoparticles heat capacity can exceed heat capacity of a bulk material a few times.     

Heat capacity, enthalpy, entropy, and gibbs energy of terbium diboride as determined from calorimetric measurements within 5–300 K

The heat capacity at constant pressure C _p (T) of terbium diboride synthesized from elements via an intermediate hydride phase was studied experimentally within 5–300 K. A ferromagnetic phase transition manifests itself in the C _p (T) dependence as a sharp maximum at 142.4 ± 0.1 K. The C _p (T) dependence was used to calculate the tempreature dependences of the enthalpy, entropy, and the Gibbs energy and to determine the parameters of the electronic, lattice, and magnetic contributions to the heat capacity of TbB₂.     

Synthesis,structural, magnetic and magnetocaloric properties of La0.8Sr0.2MnO3 nanoparticles

In this work, we reported a detailed study on the synthesis, structural and magnetic properties of nanocrystalline La_0.8Sr_0.2MnO₃. The synthesized nanoparticles were prepared using a sol–gel method and characterized using X-ray diffraction and high-resolution transmission electron microscope. The average particle size was found in the range from 40 to 45 nm. The magnetization versus temperature M(T) measurements as well as magnetization field dependence M(H) have been investigated using vibrating-sample magnetometer. The magnetization as a function of temperature M(T) indicated a broad second-order magnetic phase transition from ferromagnetic state to paramagnetic state in the Curie temperature region (320–340 K). The magnetocaloric effect of the sample has been estimated and presented a maximum magnetic entropy change |ΔS_M|_max?=?0.86 J kg^?1 K^?1 with relative cooling power?=?62.12 J kg^?1 at magnetic field (H)?=?2T. Based on the result of magnetocaloric properties, the investigated sample could be considered as a good refrigerant material for near room temperature magnetic refrigeration.

     

Unusual paramagnetic centers in PbTe undoped crystals

We present the results of the first experimental observation of unusual paramagnetism in solid when magnetic susceptibility of paramagnetic centers doesn't depend on temperature but drastically decreases when the applied magnetic field increases. This unusual combination of the field and temperature dependences of magnetic susceptibility was observed in the studies of magnetization and magnetic susceptibility performed in the wide range of temperatures (1.7–300 K) and magnetic fields (0–5.0 T) on the bulk and surface PbTe powder samples manufactured from crystal ingots grown by Bridgman method out of high-purity Pb and Te. We believe that presence of these features indicate that we are dealing with unknown untypical paramagnetism of paramagnetic centers in solid. We observed that the concentration of such unusual paramagnetic centers in PbTe crystal ingots increases towards their surface. Increase of the concentration of the centers can be so strong that it causes a transition of PbTe from the diamagnetic state to the paramagnetic one in quite wide range of low magnetic fields. Possible nature of the observed unusual paramagnetic centers is discussed.     

Chemical hydrogenation of La(Fe,Si) family of intermetallic compounds

In the present work, the chemical hydrogenation process of La(Fe,Si)₁₃ compounds has been shown. It was found, that the La(Fe,Si) compound can be easily saturated with hydrogen by performing reaction with 0.6 M hydrochloric acid (HCl) for 2 h. After reaction, the heat treatment process is necessary to make hydrogenated powder homogenous. For the LaFe_11.8Si_1.2 micronized (<50 μm) and hydrogenated powder, the strength of the magnetocaloric effect was estimated by means of magnetocalorimetric measurements on plates consolidated with PVDF thermoplastic polymer. Magnetic entropy change was calculated by use of magnetization data acquired at magnetic fields with induction up to 2T. The adiabatic temperature change is equal to 3 K in magnetic field change 0–1.7T at 335 K, while magnetic entropy change is equal 13 J/kg*K at 2T. The structural homogeneity of initial and hydrogenated powders was validated by powder X-ray diffraction method. The amount of hydrogen in the hydrogenated compounds was evaluated using thermogravimetry method (4 H atoms per formula unit LaFe_11.8Si_1.2).