First-order magnetic phase transition of LaFe11.6Si1.4C0.2 studied by ...

DOI: 10.1556/JRNC.268.2006.1.23

Journal of Radioanalytical and Nuclear Chemistry, Vol. 268, No.1 (2006) 137–140

First-order magnetic phase transition of LaFe11.6Si1.4C0.2 studied by Mössbauer spectroscopy Nai-Li Di,* Zhao-Hua Cheng, Yuan-Fu Chen, Qing-An Li, Bao-Gen Shen State Key Laboratory of Magnetism, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, P.R. China (Received October 13, 2005)

Temperature induced magnetic phase transition in the interstitial compound LaFe11.6Si1.4C0.2 was studied by zero-field Mössbauer spectroscopy. A paramagnetic doublet and a magnetically split sextet co-existed in the zero-field spectra in the temperature region of 213–218 K, and a thermal hysteresis was observed in the heating and cooling cycles. The spectra indicate that the magnetic phase transition induced by temperature is of first order for this sample.

Introduction As potential applicants in magnetic refrigeration, materials with large magnetocaloric effect have been paid research interest.1–5 It has been found that materials having first-order magnetic phase transition show a very great magnetocaloric effect.6–8 The Fe-based rare-earth transition-metal intermetallic compound LaFe11.6Si1.4 is one of the materials which show a large magnetic entropy change induced by the first-order magnetic phase transition.9–12 However, the Curie temperature (TC) of the sample is rather low (TC~195 K) for practical application. To enhance TC and keep the large magnetic entropy change, carbon has been introduced into the lattice of LaFe11.6Si1.4 by solid-solid phase reaction to form interstitial compounds like LaFe11.6Si1.4C0.2.13 The TC value of LaFe11.6Si1.4C0.2 increases from 195 to 218 K due to the introduction of carbon. Although LaFe11.6Si1.4C0.2 has known thermomagnetic curves,13 in order to understand the mechanism of the magnetic entropy change with carbon introduction it is necessary to study the magnetic phase transition induced by temperature change in detail. Therefore, the Mössbauer spectroscopy provides a good opportunity. In this paper, we present the results for LaFe11.6Si1.4C0.2 and discuss the characteristic feature of the magnetic phase transition gained by temperature change around TC in the Mössbauer spectra. Experimental The magnetization of LaFe11.6Si1.4C0.2 was measured using a Quantum Design SQUID magnetometer. The Mössbauer spectra were taken at temperatures from 20 to 300 K using a conventional constant-acceleration spectrometer in a transmission arrangement with a 57Co-Rh source. Low temperature was produced by a closed cycle refrigerator system

made for zero-field Mössbauer spectroscopy. The stability of the temperature was controlled within ±0.05 K. The velocity was calibrated with a natural α-Fe foil absorber at room temperature. The center shift was referred to as that of α-Fe at room temperature. Results and discussion The magnetization curve of LaFe11.6Si1.4C0.2 showed a magnetic phase transition from the paramagnetic state to the ferromagnetic one at TC = 218.5 K. Around TC, a thermal hysteresis was observed for the heating and cooling cycles. The temperature variations of zero-field Mössbauer spectra of LaFe11.6Si1.4C0.2 observed by cooling and heating effects are shown in Fig. 1. The spectra exhibit a typical paramagnetic doublet above 218 K and a typical ferromagnetic sextet below 214 K. From 214 to 218 K, a co-existence of the paramagnetic doublet and the magnetically split sextet was observed. Furthermore, the co-existing spectra show a thermal hysteresis between the cooling and the heating cycles. The LaFe11.6Si1.4C0.2 compound has a cubic NaZn13-type structure like that of the parent alloy LaFe11.6Si1.4. Fe atoms occupy two non-equivalent sites, the 8b site (FeI) and the 96i site (FeII) in the hypothetical compound LaFe13 in a ratio of 1 :12. In our case, the Mössbauer spectra could be fitted by a single Fe site model with an average hyperfine field, i.e., the ferromagnetic phase with one sextet and the paramagnetic phase with a doublet. The fitting parameters are the average value of the hyperfine magnetic field (Hhf), the quadrupole splitting (∆EQ = (1/2) e2qQ), the center shift (δ), and the area ratio of the magnetic spectrum to that of the total absorption (Rmag). On the basis of these parameters, the result obtained is represented in Fig. 1 by a solid line.

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Akadémiai Kiadó, Budapest Springer, Dordrecht

NAI-LI DI et al.: FIRST-ORDER MAGNETIC PHASE TRANSITION OF LaFe11.6Si1.4C0.2

Figures 2a and 2b show the temperature dependence of ∆EQ, δ and Hhf, respectively. The δ has almost the same value as that of the parent alloy. The temperature dependence of Hhf indicates that the phase transition from the paramagnetic to the ferromagnetic state occurred at 218 K as the temperature decreased. The value of ∆EQ abruptly decreased to ~0 below TC, from a value of ~0.48 mm/s above TC. The later value is almost the same as that for La(Fe0.862Al0.138)1314 or for other samples having Fe3+ in their cubic structure. The change of ∆EQ occurred at the phase transition temperature. If the quadrupole interaction is much weaker than the magnetic hyperfine interaction, the quadrupole interaction can be written as:15 EQ =

1 2 e qQ (3cos2θΗ-1+ηsin2θΗcos2φΗ), 8

which are not understood yet, the presence of the thermal hystereses of Hhf and Rmag in itself are typical of a first-order phase transition.

(1)

where the Mössbauer parameters θΗ and φΗ are the polar and azimuthal angles, respectively, defining the orientation of Hhf relative to the principal axes (x, y, and z) of the electric field gradient (EFG) tensor Vij (Vij = 2V/ 2T, i,j = x, y, z), and η (η = (Vxx–Vyy)/Vzz) is the asymmetry parameter of the EFG tensor. When the maximum EFG and η are independent of θΗ and φΗ, the average value of ∆EQ at all directions is: 1 π 2π 1 2 . e qQ = 4π 0 0 8 . (3 cos2θ –1+ η sin2θ cos2φ ).sinθ dθ dφ = 0 (2) Η Η Η Η Η Η The result of =0 means that the average value of the quadrupole shift over all directions is zero, i.e., the observed average in the magnetically ordered state is rather smaller than that in the paramagnetic state. In other words, the abrupt change of comes from the ferromagnetic to the paramagnetic phase transition. In our case, because of the Si atoms are randomly distributed on FeI and FeII sites,16 therefore, the principal axes of the EFG are randomly oriented relative to the magnetic hyperfine field. This explains the observed average value of below TC for the compound investigated. As can be seen in the Mössbauer spectra in Fig. 1, both Rmag and Hhf show a hysteresis around TC. The temperature dependence of Hhf and Rmag near TC are shown in Figs 3a and 3b, respectively. On the heating process, Hhf decreases gradually from ~280 kOe at 20 K to ~200 kOe at 217 K as for a usual ferromagnet, but it suddenly drops from ~200 kOe to zero at 218 K. By the cooling process, Hhf abruptly increased from zero at 217 K to ~200 kOe at 216 K. The area ratio of the magnetic subspectrum, Rmag, decreases abruptly from 100% to zero in the temperature region of 4 K. Although the hysteresis regions of Hhf and Rmag are different,

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Fig. 1. Temperature dependence of zero-field Mössbauer spectra near TC on the cooling and the heating cycles for LaFe11.6Si1.4C0.2. The best fit is shown by the solid line

Fig. 2. Temperature dependence of (a) ∆EQ, δ and (b) Hhf


Fig. 3. Temperature dependence of (a) hyperfine field Hhf and (b) the area ratio of the magnetically split sextet relative to the total absorption area Rmag

Conclusions

The zero-field Mössbauer spectra of the interstitial compound LaFe11.6Si1.4C0.2 have shown a paramagnetic doublet and a co-existed magnetically split sextet around TC. The spectra exhibited thermal hystereses in the heating and cooling cycles, indicating that the phase transition induced by temperature is of first order. The Mössbauer parameters of LaFe11.6Si1.4C0.2 have the same behavior and almost the same values as those of LaFe11.6Si1.4. The carbon atoms introduced into the lattice of LaFe11.6Si1.4Cx up to x = 0.2 did not change the phase transition property of the parent alloy, but increased TC. * This work was supported by the State Key Project of Fundamental Research, and the National Natural Sciences Foundation of China. Z. H. C. thanks the Alexander von Humboldt Foundation for financial support and the generous donation of partial Mössbauer equipments.

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