ΔSM Values Research Articles

Inverse magnetocaloric effect in sol–gel derived nanosized cobalt ferrite

The magnetocaloric properties of cobalt ferrite nanoparticles were investigated to evaluate the potential of these materials as magnetic refrigerants. Nanosized cobalt ferrites were synthesized by the method of sol–gel combustion. The nanoparticles were found to be spherical with an average crystallite size of 14 nm. The magnetic entropy change (ΔSm) calculated indirectly from magnetization isotherms in the temperature region 170–320 K was found to be negative, signifying an inverse magnetocaloric effect in the nanoparticles. The magnitudes of the ΔSm values were found to be larger when compared to the reported values in the literature for the corresponding ferrite materials in the nanoregime.

Influence of hydrogenation on volume dependence of the Curie temperature and entropy change in La(Fe0.86Si0.14)13

To elucidate the influence of hydrogenation on the volume dependence of the Curie temperature TC and the entropy change ΔSm in La(Fe0.86Si0.14)13, magnetic properties under hydrostatic pressure have been investigated. In the thermomagnetization curves of La(Fe0.86Si0.14)13H1.1, the first-order phase transition becomes clearer due to the decrease of volume, as observed before hydrogenation. By hydrogenation, the reduction of the pressure dependence of TC by about 20% is caused by the magnetovolume contribution to the free energy. Meanwhile, the maximum value of ΔSm is increased in magnitude from -9 to -18 J/kgK by applying pressure of 1 GPa. Accordingly, the decrease of volume is also effective to enhance ΔSm after hydrogenation.

Room-temperature large refrigerant capacity of Gd6Co2Si3

The magnetic properties and magnetocaloric effects of the Gd6Co2Si3 alloy are investigated. Gd6Co2Si3 undergoes a reversible second-order magnetic transition with a Curie temperature at room temperature (TC=295 K). A broad distribution of the magnetic entropy change ΔSm peak is observed, and the full width at half maximum of the ΔSm peak is found to be about 111 K under a magnetic field of 5 T. The large value of the refrigerant capacity (RC) is found to be comparable with those of pure Gd, which have maximal RC values for all the magnetocaloric materials reported previously. The maximal value of ΔSm is 6.3 J/kg K for a field change of 0–5 T. Excellent magnetocaloric properties, especially considerable values of RC, indicate the applicability of Gd6Co2Si3 for magnetic refrigeration in the room-temperature range.

The existence of large magnetocaloric effect at low field variation and the anti-corrosion ability of Fe-rich alloy with Cr substituted for Fe

The influence of Cr substituted for Fe in Fe78-xCrxSi4Nb5B12Ag1 ribbons on the crystallization and magnetic properties including the magnetocaloric effect have been investigated. DSC measurements showed that the crystallization temperature and the crystallization activation energy increased with Cr content. Thermomagnetic curves measured in low applied field indicated that there is a sharp ferromagnetic-paramagnetic phase transition at Curie temperature, TC, of the amorphous phase. In addition, substitution of Cr for Fe led to approximate linear decrease of TC with Cr content, namely from 450K to 280K for x = 0 and x = 8, respectively. From a series of the isothermal magnetization curves M(H) measured at different temperatures, magnetic entropy change ΔSm was determined at magnetic field variations of 13.5, 10.0 and 5.0 kOe. Maximum value of ΔSm (occurred near TC) decreased with increasing Cr content and these values established are quite large at low field variation. Namely, at moderate low magnetic field variation of 5.0 kOe, |ΔSm|max is of 4.4 and 1.9 J/kg.K for x = 0 and x = 8, respectively. Our studied alloys system could be considered as the best magnetocaloric material candidates for magnetic refrigeration because of colossal magnetic entropy change at low field variation and working temperature could be controlled in large region by substitution effect. The anti-corrosion ability of alloys was also examined.

Magnetocaloric effect in Ho2In over a wide temperature range

The compound Ho2In exhibits two successive magnetic phase transitions: a spin-reorientation transition at TSR=32 K and a magnetic-ordering transition at TC=85 K. The maximum reversible −ΔSM values are 6.3 and 11.2 J/kg K at TSR and TC, respectively, for a field change of 5 T. These two −ΔSM peaks with the same sign are partly overlapping, which results in a wide temperature interval with appreciable magnetocaloric effect. The results on Ho2In indicate that materials with successive SR and magnetic-ordering transitions may constitute an important new class of magnetic refrigerants since they work in a wider temperature range than the conventional refrigerant materials.

Large reversible magnetocaloric effect in Tb3Co compound

A large reversible magnetocaloric effect has been observed in Tb3Co compound. Under a magnetic field change of 5T, the maximum value of magnetic entropy change ΔSM is −18Jkg−1K−1 at 84K and the relative cooling power is 738Jkg−1 with no hysteresis loss. In particular, the large reversible ΔSMmax, −8.5Jkg−1K−1, is achieved for a low magnetic field change of 2T. The magnetic anisotropy and the texture of the material greatly affect ΔSM. The large reversible magnetocaloric effect (both the large ΔSM and the high relative cooling power) indicates that Tb3Co could be a promising candidate for magnetic refrigeration.

Magnetism in Gd–W films

Vapor condensation techniques are useful to prepare magnetic alloys whose components have low or even negligible equilibrium mutual solubility. In this work, one of these techniques—sputtering—was used to obtain GdxW1−x alloys whose magnetic properties were investigated as a function of the Gd atomic concentration x. Gadolinium and various Gd-based alloys are promising materials for magnetic refrigeration and this was one of the motivations for this study. The Gdx–W1−x films were sputter deposited from Gd and W targets with x ranging from 0 to 1 as determined by x-ray energy-dispersive spectroscopic analyses. X-ray diffraction patterns indicate that crystalline structures were formed at low and high Gd concentrations, while at intermediate concentrations, the films were amorphous. Magnetization measurements, performed as a function of temperature and with static and alternating applied fields, reveal a spin glasslike behavior in all the W-containing samples for temperatures below the freezing temperature Tf. For low and intermediate Gd concentrations, and for T>Tf, the films were paramagnetic, while a ferromagnetic phase was observed in the Gd–W alloy of the highest Gd content. The magnetocaloric effect was investigated from the magnetization isotherms M versus H, from which the isothermal magnetic entropy variation ΔSM as a function of T, for the removal of an applied field of 50kOe, was determined. It was observed that the maximum value of ΔSM for each ΔSM versus T curve and the temperature at which these maxima occur, are strongly dependent on x.

Giant room-temperature magnetocaloric effect in Mn1−xCrxAs

A giant magnetocaloric effect was observed at room temperature in Mn1−xCrxAs compounds with x=0.006 and 0.01. The Cr dopant reduces (or even eliminates) the large thermal hysteresis of MnAs, while it lowers the first-order transition temperature from 313K for MnAs to 265K for Mn0.99Cr0.01As. Near the Curie temperature, a magnetic field induces a first-order phase transition from a ferromagnetic hexagonal phase to a paramagnetic orthorhombic phase, leading to a maximum value of ΔSM of 20.2J∕kgK at 267K for a 5T field change for Mn0.99Cr0.01As. The study on the Mn1−xCrxAs system may open an important field in searching proper materials for room-temperature magnetic refrigeration.

Large magnetocaloric effect and enhanced magnetic refrigeration in ternary Gd-based bulk metallic glasses

The magnetocaloric effect and refrigeration capacity (RC) of Gd55Co20Al25 and Gd55Ni25Al20 bulk metallic glasses (BMGs) have been investigated. Large magnetic entropy changes ΔSM of 11.2 and 10.8 J kg−1 K−1 and large RC values of 846 and 920 J kg−1 are obtained for Gd55Co20Al25 and Gd55Ni25Al20, respectively, at a field change of 7 T. The RC value (640 J kg−1 at 5 T or 920 J kg−1 at 7 T) of Gd55Ni25Al20 BMG is larger than that reported for all magnetocaloric materials, including crystalline and amorphous materials measured under the same conditions. The large RC value is due to the broad ΔSM peak (more than 100 K), which is caused by the disordered structure of an amorphous material. The large ΔSM and RC values make these Gd-based ternary BMGs attractive candidates for magnetic refrigeration applications.

Giant low-field magnetic entropy changes in Ni45Mn44−xCrxSn11 ferromagnetic shape memory alloys

A series of Ni45Mn44−xCrxSn11 (x = 0, 1, 2) ferromagnetic shape memory alloys were prepared. With slight doping of Cr, the martensitic transition temperatures decrease rapidly. The magnetic entropy changes at a low magnetic field were investigated in these alloys. The maximum value of ΔSM is 23.4 J kg−1 K−1, which was observed in Ni45Mn43CrSn11 alloys. The origin of the magnetic entropy changes in these alloys has been discussed. The giant low-field magnetic entropy changes and low cost make Ni45Mn44−xCrxSn11 alloys a promising candidate for magnetic refrigeration.

Large room-temperature magnetocaloric effects in Fe0.8Mn1.5As

In Fe0.8Mn1.5As compound, an external magnetic field induces a metamagnetic transition from an antiferromagnetic phase to a ferrimagnetic phase above Ts=285K, leading to large magnetocaloric effects around room temperature. Instead of showing inverse magnetocaloric effects, the sign of the entropy change ΔSM in the compound is unexpectedly negative, revealing a different mechanism. The maximum value of ΔSM is 6.2J∕kgK at 287.5K for a magnetic field change of 5T. The study on systems with antiferromagnetism-related metamagnetic transitions may open an important field in searching good materials for room-temperature magnetic refrigeration.

Magnetocaloric effect and magnetic-field-induced shape recovery effect at room temperature in ferromagnetic Heusler alloy Ni–Mn–Sb

The martensitic transition, magnetocaloric effect (MCE) and shape memory effect (MSE) of ferromagnetic Heusler alloys Ni50Mn50−xSbx (x = 12, 13 and 14) have been investigated. A large positive magnetic entropy change ΔSM was observed in the vicinity of the martensitic transition. The maximum value of ΔSM is 9.1 J kg−1 K−1 in Ni50Mn37Sb13 at 287 K for a magnetic field change of 5 T. This change originates from the first-order transition from a low-temperature weak-magnetic martensitic phase to a high-temperature ferromagnetic parent phase. A magnetic-field-induced shape recovery strain of about 15 ppm at room temperature and at a relatively low magnetic field (1.2 T) was observed to accompany the reverse martensitic transformation. The large field-induced MCE and MSE in the NiMnSb system make it a promising material for room-temperature application.

Giant magnetocaloric effect in ε-(Mn0.83Fe0.17)3.25Ge antiferromagnet

The magnetic phase transition and magnetocaloric effect are investigated on ε-(Mn0.83Fe0.17)3.25Ge compound. A large positive magnetic entropy change ΔSM is observed, which is accompanied with a field-induced metamagnetic transition from a collinear to a triangular antiferromagnetic configuration in this antiferromagnetic compound. The maximum value of ΔSM is 11.6J∕kgK at 93K for a magnetic field change of 7T. The study on systems with antiferromagnetic phases may open an important field in searching new materials for magnetic refrigeration.

MAGNETIC ENTROPY CHANGE IN A TWO-DIMENSIONAL ANTIFERROMAGNETIC SQUARE LATTICE

In this study, we investigate magnetic entropy variation and magnetocaloric properties as a function of frustration by considering a spin-1/2 antiferromagnetic Heisenberg model on a square lattice with nearest (J1) and next-nearest neighbor exchange interaction (J2). We show that the degree of frustration in the square lattice increases with α=J2/J1. While the square lattice is unfrustrated for α = 0, it becomes fully frustrated for α=0.5. Numerical results show that the entropy of the square lattice approaches zero for the unfrustrated case at T=1 K. In contrast, finite entropy can survive in the frustrated case at the same temperature. We also calculate the magnetic entropy change (ΔSm) in the square lattice and show that the maximum value of ΔSm increases with increasing α. These results indicate that the magnetic entropy change and consequently the magnetocaloric effect can be enhanced by increasing the degree of frustration. We conclude that the enhanced magnetocaloric effect is related to quantum fluctuations and disordered ground state present in the frustrated square lattice.

Contribution of solid-state properties to the aqueous solubility of drugs

This study investigates the influence of the solid-state properties melting point (Tm), enthalpy of melting (ΔHm) and entropy of melting (ΔSm) of a drug on its intrinsic solubility (S0). For this purpose, 26 chemically and structurally diverse drugs covering the oral drug space were selected and the S0, Tm, ΔHm and ΔSm were determined experimentally. The influence of Tm, ΔHm and ΔSm on S0 was studied using regression analysis. The overall improvement of the predictions were 0.3 log units, however, five compounds (astemizole, glyburide, fenbufen, gliclazide and griseofulvin) were improved by more than one log unit. Tm and ΔHm had a larger effect than ΔSm on the solubility predictions. The well-known general solubility equation (GSE) and the Dannenfelser semi-empirical equation for the calculation of ΔSm were evaluated using our data set. While predictions of drug solubility obtained using the GSE were acceptable, the use of the experimental ΔSm values instead of the constant 56.5Jmol−1K−1 improved the accuracy of the prediction. The Dannenfelser equation underestimated the ΔSm for most compounds with on average 15Jmol−1K−1. Our results show that solid-state properties should be considered for improved performance of future models for prediction of drug solubility. In addition our study provides accurate experimental data on intrinsic solubility for 26 compounds, supplying a useful external data set for validation of drug solubility models.

Magnetocaloric effect in Mn5Ge3−xSix pseudobinary compounds

Structure and magnetic properties of Mn5Ge3−xSix pseudobinary compounds with x=0–1.5 were studied by x-ray diffraction and magnetic measurements. The compounds retain the hexagonal D88-type structure with a space group P63∕mcm, and their lattice parameters decrease from a=7.204(2)Å and c=5.028(2)Å to a=7.084(2)Å and c=4.945(3)Å with increasing Si content. Wigner-Seitz cell volume calculations indicate that Mn site volumes and Mn-Mn atomic distances decrease with increasing x, which influences the exchange interaction. All the compounds are ferromagnetic and their Curie temperatures, TC, decrease from 298 to 252K as the Si content is increased. In addition, the average atomic magnetic moment of Mn (at 5K) decreases from 2.64 to 2.35μB with increasing x from 0 to 1.5. The magnetocaloric effect is evaluated by measuring the isothermal magnetic entropy change, ΔSm, using the appropriate thermodynamic Maxwell relation. The peak value of ΔSm ranges from about 9.0to6.3J∕kgK for x=0–1.5 near their TC under an external magnetic field change of 5T. On the other hand, ΔSm peaks broaden with increasing x, consistent with the less sharp change of magnetization near TC. We show that the phenomena can be ascribed to the weakening of the exchange interaction as the Mn-Mn separation is reduced upon the addition of Si atoms.

Tuning of magnetocaloric effect in a La0.69Ca0.31MnO3 single crystal by pressure

We report a study of the effect of hydrostatic pressure on the magnetocaloric effect in a La0.69Ca0.31MnO3 single crystal. The single crystal exhibits a much larger magnetic entropy change (ΔSm) than the corresponding polycrystalline samples, reaching 5.2J∕kgK and 8.5J∕kgK for a magnetic field variation of 1T and 5T, respectively. Under hydrostatic pressure, the peak position of ΔSm significantly moves to higher temperatures due to the shift of the magnetic phase transition, from 213.5K under ambient pressure up to 236.5K under a pressure of 1.1GPa, while the maximum value of ΔSm remains nearly the same. These exceptional results demonstrate that the magnetocaloric effect in magnetic materials with strong spin-lattice coupling can be effectively tuned by pressure.

Magnetocaloric Behaviour in Amorphous and Nanocrystalline FeNbB Soft Magnetic Alloys

We have studied the temperature dependence of the magnetocaloric effect in series of amorphous and nanocrystalline Fe80.5Nb7B12.5 melt-spun ribbons. The maximum entropy change ΔSm ≈ 0.72 J/kgK is found at the Curie temperature of the amorphous material, TC(am) ≈ 363 K, upon a 0.7 T magnetic field change. This ΔSm value is a factor of four less than that of gadolinium, the prototypical high-temperature magnetocaloric material, but it compares favorably with other amorphous Fe-based alloys. The progressive nanocrystallization of amorphous ribbons results in a decrease of magnetic entropy change and at the same time the ΔSm peak becomes broadened.

Peak magnetocaloric effects in Al-Gd-Fe alloys

The magnetocaloric properties of several AlxGdyFez (with x+y+z=100) ternary alloys have been determined between 2 and 300 K. Three distinct peaks in the magnetic entropy change ΔSm versus T were found: a low-temperature peak (near 10 K), an intermediate temperature peak (80–160 K), and a higher temperature peak (210 to 280 K). The low-temperature peak coincides with a field-induced antiferromagnetic-to-ferromagnetic transition; the intermediate and high temperature peaks are associated with other magnetic transitions. Above 60 K, these alloys exhibited superparamagnetic behavior and possessed enhanced ΔSm values, as predicted earlier for magnetic nanocomposites.

Effect of Temperature on the Surface Properties of Aqueous Solution of Arachidonic Acid.

The surface tension of aqueous solution of arachidonic acid (AA) was measured with a Du Nouy tensiometer at pH 7.80 and various temperatures, and the effect of temperature on the surface properties of AA was investigated. The value of critical micelle concentration (cmc) of AA increased and the surface tension at the cmc, γcmc, decreased as the temperature was raised from 30 to 60°C. The Langmuir constant k concerning the strength of adsorption decreased, the saturated adsorption ammount of AA, Γ∞, at the air-water interface decreased, and the area occupied by an AA molecule at the saturated adsorption, A∞, increased as the temperature was raised. The standard free energy for the adsorption, ΔGad, was of negative value, and the absolute value |ΔGad| became larger as the temperature was raised because of the negative value of the standard enthalpy change ΔHad and the positive value of the standard entropy change ΔSad. The standard free energy of micellization, ΔGm, was of negative value, and it was similarly found that |ΔGm|became larger as the temperature was raised because of the negative value of the standard enthalpy change ΔHm and the positive value of the standard entropy change ΔSm.