Guang-jun Wang

Very large magnetic entropy change near room temperature in LaFe11.2Co0.7Si1.1

A very large magnetic entropy change ΔS has been observed in Fe-based cubic NaZn13-type compound LaFe11.2Co0.7Si1.1 near the Curie temperature TC of 274 K. The value of the entropy change is ∼20.3 J/kg K under a magnetic field of 5 T at TC=274 K. It markedly exceeds that of pure Gd at the corresponding temperature range [V. K. Pecharsky & K. A. Gschneidner, Jr., Phys. Rev. Lett. 78, 4494 (1999)]. The great entropy change produced by the sharp change of magnetization is associated with a large negative lattice expansion at TC. The very large magnetic entropy change and low cost suggest that the compound LaFe11.2Co0.7Si1.1 has great potential for applications as magnetic refrigerants near room temperature.

The effect of Mn substitution in LaFe11.7Si1.3 compound on the magnetic properties and magnetic entropy changes

The magnetic properties and magnetic entropy changes of La(Fe1−xMnx)11.7Si1.3 (x = 0–0.03) have been studied. The Curie temperatures TC decrease monotonously with increasing Mn concentration from 188 to 127 K, and the saturation magnetization μS decreases from 23.9 μB/fu to 22.2 μB/fu respectively, as x increases from 0 to 0.03. The maximal magnetic entropy changes |ΔS|, under a magnetic field change of 0–5 T, are 26.0 J kg−1K −1, 25.7 J kg−1K −1, 20.8 J kg−1K −1 and 17.1 J kg−1K −1 for x = 0, 0.01, 0.02 and 0.03, respectively. The appearance of negative slopes in the Arrott plots for all samples confirms the occurrence of a first-order field-induced itinerant-electron metamagnetic (IEM) transition. Furthermore, the full-width at half-maximum (FWHM) of |ΔS| peak, δTFWHM, increases obviously with increasing Mn content, which results in the decrease of the maximum magnetic entropy change.

Magnetism and magnetic entropy change of LaFe11.6Si1.4Cx(x=0−0.6) interstitial compounds

Chemically stable LaFe11.6Si1.4Cx (x=0−0.6) interstitial compounds were prepared. The Curie temperatures increases from 195 to 250 K with an increase in X from 0 to 0.6 due to obvious lattice expansion. The maximal magnetic entropy changes |ΔS| with a change in magnetic field of 0–5 T, are 24.8, 24.2, 18.8 and 12.1 J/kg K for x=0, 0.2, 0.4, and 0.6, respectively, notably exceeding that of Gd (|ΔS|∼9.8 J/kg K at TC=293 K). The magnetic transition varies from first order to second order with an increase in carbon concentration, which has a decisive influence on |ΔS|. The large |ΔS|, chemical stability, and low cost make LaFe11.4Si1.6Cx a promising candidate as a magnetic refrigerant in the corresponding temperature ranges.

Magnetic properties and magnetic entropy change of LaFe11.5Si1.5Hy interstitial compounds

LaFe11.5Si1.5Hy interstitial compounds have been prepared by hydrogen absorption and subsequent desorption. The Curie temperatures are easily tunable for a wide temperature range from ~ 195 to ~ 340 K by hydrogen content. The maximal magnetic entropy changes, under a magnetic field change of 0?5 T, are as large as 16.8? 20.5 J kg?1 K?1, exceeding that of Gd (|?S| ~ 9.8 J kg?1 K?1 at TC = 293 K) by 70?110% in the vicinity of room temperature. The large magnetic entropy change of all hydrides may be attributed to the first-order itinerant-electron metamagnetic transition confirmed by the Arrott plots. Large |?S|, convenient adjustment of TC and small thermal and magnetic hystereses, make LaFe11.5Si1.5Hy interstitial hydrides promising candidates for magnetic refrigerants in the corresponding temperature range.

Strong interplay between structure and magnetism in the giant magnetocaloric intermetallic compound LaFe11.4Si1.6: a neutron diffraction study

Crystallographic and magnetic structures of the cubic NaZn13-type intermetallic compound LaFe11.4Si1.6 have been studied by means of powder neutron diffraction. Rietveld analysis indicates that Si atoms substitute for Fe atoms randomly on two different Fe sites. All spins in the unit cell are aligned ferromagnetically with the FeI?(8b) moment smaller than the FeII?(96i) one. The long-range ferromagnetic ordering induces a drastic expansion of the lattice and the coexistence of the large and small volume phases near the Curie temperature. Even in the ferromagnetic state, the lattice expansion still correlates strongly with the spontaneous magnetic moment, marked by a large positive magnetovolume coupling constant kC = 1.14 ? 10?8 cm6?emu?2. From the temperature dependence of Fe?Fe bond lengths, we suggest that the Fe?Fe exchange interaction between the clusters (each formed by a central FeI atom and 12 surrounding FeII atoms) plays an important role in the magnetic properties of La(Fe1?xAl/Six)13, as does that within the clusters.

Effects of carbon on magnetic properties and magnetic entropy change of the LaFe11.5Si1.5 compound

Effects of the interstitial carbon atoms on the magnetic properties, especially on the magnetic entropy change, of the LaFe11.5Si1.5 compound, have been studied. X-ray diffraction patterns reveal a monotonous increase of the lattice constant with the concentration of carbon, while the cubic NaZn13-type structure remains unchanged. The Curie temperatures TC of LaFe11.5Si1.5Cy are ∼195, 225, and 241 K for y=0, 0.2, and 0.5, respectively, increasing with the increase of carbon concentration. The maximal magnetic entropy changes |ΔS| of LaFe11.5Si1.5Cy at the respective TC under a magnetic field change of 0–5 T are ∼24.6, ∼22.8, and ∼12.7 J/kg K for y=0, 0.2, and 0.5, respectively, notably exceeding that of Gd (|ΔS| ∼9.8 J/kg K at TC=293 K). The |ΔS| of LaFe11.5Si1.5C0.2 is nearly as giant as that of the parent alloy LaFe11.5Si1.5 due to the first-order field-induced itinerant-electron metamagnetic transition that occurs in both compounds clearly observed for the LaFe11.5Si1.5C0.5 compound. With the increase ...

Pressure enhancement of the giant magnetocaloric effect in LaFe11.6Si1.4

The authors have studied the effects of pressure on the magnetocaloric effect in a polycrystalline LaFe11.6Si1.4 sample. The Curie temperature TC of the sample rapidly decreases from 191K at ambient pressure to 80K under 8.3kbar pressure. The metamagnetic transition induced by field at temperatures above TC becomes extremely sharp under high pressure and the critical field Hc of the transition increases fast with increasing temperature. As a result, the giant magnetocaloric effect in LaFe11.6Si1.4 is greatly enhanced by pressure, especially at low magnetic fields. For a field variation of 1T only, the maximum value of the entropy change is as high as 34J∕kgK.

Large magnetic entropy change in compound LaFe/sub 11.44/Al/sub 1.56/ with two magnetic phase transitions

It is shown that Fe-based compound LaFe/sub 11.44/Al/sub 1.56/ with cubic NaZn/sub 13/-type structure undergoes both a second order and a first order phase transitions. In small fields, it orders antiferromagnetically at T/sub N/=186 K, and ferromagnetism is established at T/sub 0//spl ap/127 K. At T>T/sub 0/, field-induced metamagnetic transition between the two phases is observed in larger fields. The magnetic entropy change |/spl Delta/S|, estimated as a function of temperature from magnetization curves, has large values near the first-order transition. Metamagnetic transformation leads to an asymmetric broadening of the maximum with increasing field. This compound may be useful for magnetic refrigeration.

Mössbauer effect probe of local Jahn-Teller distortion in Fe-doped colossal magnetoresistive manganites

The local structure of the Fe-doped La1−xCaxMnO3 (x=0.00–1.00) compounds has been investigated by means of Mossbauer spectroscopy. 57Fe Mossbauer spectra provide direct evidence of Jahn–Teller distortion in these manganites. On the basis of the Mossbauer results, the Jahn–Teller coupling was estimated. It is noteworthy that the Ca-concentration dependence of the Jahn–Teller coupling strength is very consistent with the magnetic phase diagram. Our results reveal that Mossbauer spectroscopy cannot only detect the local structural distortion, but also provide a technique to investigate the Jahn–Teller coupling of Fe-doped La1−xCaxMnO3 colossal magnetoresistive perovskites.

Mössbauer effect probe of field-induced magnetic phase transitionin LaFe13−xSix intermetallic compounds

Direct evidence of a field-induced magnetic phase transition in LaFe13−xSix intermetallics with a large magneticaloric effect was provided by Fe57 Mossbauer spectra in externally applied magnetic fields. Moreover, Mossbauer spectra demonstrate that a magnetic structure collinear to the applied field is abruptly achieved in LaFe11.7Si1.3 compound once the ferromagnetic state appears, showing a metamagnetic first-order phase transition. In the case of LaFe11.0Si2.0, the Fe magnetic moments rotate continuously from a random state to the collinear state with increasing applied field, showing that a second-order phase transition is predominant. The different types of phase transformation determine the magnetocaloric effects in response to temperature and field in these two samples.