Magnetocaloric Effect In La0 Research Articles

The hydrogen absorption properties and magnetocaloric effect of La0.8Ce0.2(Fe1−xMnx)11.5Si1.5Hy

The effect of hydrogenating process on the homogeneity of hydrogen absorption in the La0.8Ce0.2(Fe1−xMnx)11.5Si1.5 compounds was investigated. It is attractive that the distribution of hydrogen atoms in the compounds becomes more uniform with increasing hydrogen pressure and annealing temperature. Homogeneous hydrides of La0.8Ce0.2(Fe0.985Mn0.015)11.5Si1.5 were obtained by annealing them at the temperature of 773 K and under the hydrogen atmosphere of 0.5 MPa. With changes of Mn content, the Curie temperatures (TC) of the hydrides of La0.8Ce0.2(Fe1−xMnx)11.5Si1.5 can be adjusted in the room temperature range from 279 to 312 K. Large magnetic entropy changes due to the itinerant-electron metamagnetic transition are obtained in the room temperature range for the hydrides of La0.8Ce0.2(Fe1−xMnx)11.5Si1.5.

Effect of Fe substitution on magnetocaloric effect in La0.7Sr0.3Mn1−xFexO3 (0.05≤x≤0.20)

We have studied the effect of Fe substitution on magnetic and magnetocaloric properties in La0.7Sr0.3Mn1−xFexO3 (x=0.05, 0.07, 0.10, 0.15, and 0.20) over a wide temperature range (T=10–400K). It is shown that substitution by Fe gradually decreases the ferromagnetic Curie temperature (TC) and saturation magnetization up to x=0.15 but a dramatic change occurs for x=0.2. The x=0.2 sample can be considered as a phase separated compound in which both short-range ordered ferromagnetic and antiferromagnetic phases coexist. The magnetic entropy change (−ΔSm) was estimated from isothermal magnetization curves and it decreases with increase of Fe content from 4.4Jkg−1K−1 at 343K (x=0.05) to 1.3Jkg−1K−1 at 105K (x=0.2), under ΔH=5T. The La0.7Sr0.3Mn0.93Fe0.07O3 sample shows negligible hysteresis loss, operating temperature range over 60K around room temperature with refrigerant capacity of 225Jkg−1, and magnetic entropy of 4Jkg−1K−1 which will be an interesting compound for application in room temperature refrigeration.

Field Dependence of the Refrigerant Capacity for La<sub>0.6</sub>Ca<sub>0.4</sub>MnO<sub>3</sub> Manganite

The magnetocaloric effect in La0.6Ca0.4MnO3 manganite has been investigated. The isothermal magnetization versus applied magnetic field at various temperatures in the vicinity of Curie temperature was measured, and the temperature dependence of magnetic entropy change was determined using Maxwell’s relation. This value is comparable to that in Gd. Nevertheless, the relative cooling power of La0.6Ca0.4MnO3 was shown to be considerably lower than that of Gd. The experimental results have been analyzed in frames of a phenomenological statistical model. This model considers explicitly Jahn-Teller interactions and allows prediction of the field dependences of the magnetic entropy change and the relative cooling power.

Oxygen deficiency as a driving force for metamagnetism and large low field magnetocaloric effect in La0.7Ca0.3−xSrxMnO3−δ manganites

We report the importance of metamagnetism for the giant magnetocaloric effect in lanthanum manganites and the key role of oxygen deficiency for this phenomenon. It is found that La0.7Ca0.3−xSrxMnO3−δ samples with x=0.0, 0.025, 0.05, 0.075, 0.1, and 0.125 are orthorhombic (O) phase, while those with x=0.15, 0.175, 0.2, and 0.3 are rhombohedral (R) phase at room temperature. The samples possess relatively high oxygen deficiency δ. With increasing x, the oxygen content increases from about 2.92 to 2.94 in the O phase and from 2.93 to 2.97 in the R phase, while it decreases in the vicinity of the O-R phase transition. The Curie temperature increases with increasing x in both the structural phases and shows a positive jump around the O-R phase transition. The x=0 sample shows S-shape magnetization, which is typical for metamagnetic materials. The S shape is continuously suppressed with increase in Sr and disappears for x≥0.075. Interestingly, the magnetic entropy change, adiabatic temperature change, and refrigerant capacity achieved for the present samples are about 20%–50% larger than those previously reported for manganites prepared using different processing routes. The large magnetocaloric effect in the present samples originates from the sharp change in magnetization with respect to temperature at the Curie temperature and the presence of S-shape magnetization near this transition temperature caused by the oxygen deficiency.

Metamagnetic transition and extremely large low field magnetocaloric effect in La0.7Ca0.3MnO3 manganite

Metamagnetic transition was observed in low magnetic field for La0.7Ca0.3MnO3−δ lanthanum manganite prepared by solid-state technology. Magnetization shape causes giant low field magnetocaloric effect at Curie temperature TC=242K. Magnetic entropy changes ΔSm, deduced with isothermal magnetization curves, are equal to 7.0 and 8.0J∕kgK in the field change from 0 to 1.0 and 2.0T, respectively. The obtained ΔSm are essentially higher than that reported before for lanthanum manganites with similar TC. The observation shows the way for the construction the magnetic refrigerants, working in the field of permanent magnets, which is beneficial for household and automotive applications.

Itinerant-electron metamagnetic transition and giant magnetocaloric effect in La0.8Ce0.2Fe11.4Si1.6 compound

The itinerant-electron metamagnetic (IEM) transition and magnetocaloric effects (MCE’s) in the La0.8Ce0.2Fe13−xSix (x=1.6,1.4,1.2,1.0) compounds have been investigated by x-ray powder diffraction and magnetic measurements. The La0.8Ce0.2Fe11.4Si1.6 compound exhibits giant values of the isothermal magnetic entropy change ΔSM around the Curie temperature TC∼186K in low magnetic fields. The maximum value ∣ΔSM∣max∼78.29J∕kgK is under a field of 3T. Such large MCE’s are attributed to the sharp change of the magnetization, the field-induced IEM transition and a strong temperature dependence of the critical field BC. The substitution of Ce for part of La in La0.8Ce0.2Fe11.4Si1.6 increases the value of a(T)c(T)∕b(T)2 and enhances the property of IEM.

Direct and indirect measurement of the magnetocaloric effect inLa0.67Ca0.33−xSrxMnO3 ± δ ()

The magnetocaloric properties of a series of manganites with the compositionLa0.67Ca0.33−xSrxMnO3 ± δ, (LCSM), have been investigated by direct and indirect measuring techniques. The compoundsshowed a magnetocaloric effect near the Curie temperature, which increased from 267 K forx = 0 to 369 Kfor x = 0.33. Both the adiabatic temperature change and the isothermal magnetic entropy change werefound to decrease upon increased replacement of Ca with Sr, in good agreement withprevious reports. However, all samples showed almost the same relative cooling power,RCP, because the decrease in maximum magnetocaloric effect was accompanied by awidening of the magnetocaloric peaks. The compounds showed RCP values of about300 mJ cm−3 upon a field change of 1.2 T, which is about three times less than the RCP of gadolinium,the prototype material for magnetic cooling at ambient conditions. However, theLCSM materials show a magnetocaloric effect in temperature ranges where themagnetocaloric effect in pure Gd is vanishing. LCSM might therefore have potential as aworking substance in multi-component refrigerant units, where the possibility oftailoring compounds with a specific magnetic transition temperature is essential.

Magnetocaloric effect of La0.8Sr0.2MnO3 compound under pressure

The La0.8Sr0.2MnO3 compound presents a ferromagnetic paramagnetic transition around room temperature to which a reasonably high magnetocaloric effect is associated, turning this material of interest for application in magnetic refrigeration. We synthesized this compound in fiber single crystalline form by the Laser Heated Pedestal Growth method. The sample was characterized by x-ray diffraction and magnetic measurements as a single phase and with the required magnetic properties. We measured the magnetic properties and the magnetocaloric effect under hydrostatic pressure for pressures up to 6 kbar as a function of temperature. Our results indicate that the Curie temperature increases with pressure while the low temperature transition from the orthorhombic to the rhombhoedral structures decreases as pressure increases. This is in close agreement with the literature. Measurement of the magnetocaloric effect at the high temperature transition indicates that the peak of the effect follows the trend of the Curie temperature, but its maximum value remains almost constant as a function of pressure.

Magnetocaloric effect in Er- and Eu-substituted ferromagnetic La-Sr manganites

We have studied the magnetocaloric effect in La0.7Sr0.3MnO3 colossal magnetoresistive manganites, where La is substituted by Er or Eu. The addition of these rare-earth ions allows a tunable decrease in the Curie temperature, TC. For example, in the LaEuSrMnO3 system TC drops from 360 to 308K for 0.14% of Eu substitution. We find quite large values of magnetic entropy change. For an applied magnetic field of 1 T, the maximum values for both studied systems are close to that of the parent La0.7Sr0.3MnO3, near their respective TC's. Relative cooling power value increases notably for higher concentrations of Eu, in contrast to the Er-substituted samples. These results are discussed in terms of magnetoelastic and electron interaction contribution to the magnetic entropy.

Direct and specific heat study of magnetocaloric effect in La0.845Sr0.155MnO3

Magnetocaloric effect and specific heat of a La0.845Sr0.155MnO3 sample were measured over a temperature range of 4.2–300 K, to verify whether large entropy changes induced by magnetic field make possible the use of manganites for magnetic refrigeration and whether a magnetostrictive contribution to magnetocaloric effect is significant. Isothermal entropy change and adiabatic temperature change induced by ΔB=7 T were determined as functions of temperature. Near to the Curie temperature, TC≈234 K, they reach, respectively, ∼−6.6 J/(kg K) and ∼3.4 K, i.e., the values ∼2 and ∼4 times smaller than for Gd. Despite the presence of large magnetostriction, its contribution to magnetocaloric effect was found negligible.