Phase constitution, magnetic ordering and microstructure of the LaFe11.0Co0.8Si1.2 alloy

Microstructure and phase constitution of the LaFe11.0Co0.8Si1.2 bulk and ribbon samples in as-cast state and after annealing, were studied. For both samples in as-cast state the dominant dendritic α-Fe phase was formed. Annealing of the samples resulted in homogenization of the microstructure and change of the phase constitution. The heat treatment resulted in development of almost single-phase of the NaZn13-type structure identified as LaFe11.0Co0.8Si1.2 with minor fraction of the α-Fe. The evolution of microstructure and phase constitution was verified by EDX analysis and Mössbauer spectroscopy.


Introduction
Conventional refrigerators working on gas transformations have negative impact on the natural environment. More friendly alternative is a cooling process that utilizes giant magnetocaloric effect (GMCE) [1,2]. Especially important group of materials that can be used in such devices are those, which reveal both the structural phase transition and the second order transition from ferro-to paramagnetic state near the Curie point T C . These both effects have an important influence on the magnetic entropy change |ΔS M | [3,4]. Well known and widely studied alloy was the Gd 5 Ge 2 Si 2 , discovered by Pecharsky and Gschneidner in 1997, for which magnetic entropy change reach up to 18.5 J kg -1 K -1 in external magnetic field change of 5T around Curie point of 276K [5]. However this alloy is relatively expensive for commercial applications, due to high content of Gd and rigorous conditions of processing. Promising candidates for magnetocaloric applications are the rare-earthtransition metal compounds [6,7]. The La(Fe,Si) 13 -type alloys have been intensively studied, for their possibility of applications as active elements in magnetic refrigerant due to relatively low price and specific magnetic properties. The crystal structure bases on cubic NaZn 13type elementary cell (of the space group Fm3c) [8,9]. In this unit cell the La occupy 8a sites, while Fe atoms are randomly distributed between 8b and 96i sites. Magnetocaloric effect is observed in wide range of temperatures from 195 to 330K and depends on the alloy composition [10,11]. The largest magnetic entropy change was measured for LaFe 11.8 Si 1.2 alloy, for which |ΔS M | reaches 31 J(kg K) -1 at μ 0 ΔH ~ 5T around 202K [12]. Such good magnetocaloric properties were due to the transition from ferro-to paramagnetic state, accompanied by the change of the lattice parameter of the La(Fe,Si) 13 -type phase around T C . The admixture of Co resulted in the increase of the Curie point thus leading to the shift of maximum of |ΔS M | to ~ 280K [13]. The aim of present work was to study the phase constitution and microstructure of the LaFe 11.0 Co 0.8 Si 1.2 alloy produced in a form of bulk and ribbon samples.

Experimental
The ingot samples of the LaFe 11.0 Co 0.8 Si 1.2 alloy were obtained by arc-melting of high purity elements in an Ar atmosphere. Master alloy was prepared for stoichiometric composition corresponding the LaFe 11.0 Co 0.8 Si 1.2 phase. Due to high loss of La during arc-melting the 15 wt.% excess of La was used. The ribbon samples were meltspun under the Ar atmosphere with the linear velocity of the copper wheel of ~35 m/s. Both bulk and ribbons samples were sealed-off in a quartz tubes under low pressure of Ar and annealed at 1323K for 24 hours in case of ribbon and for 28 days for bulk samples. X-ray diffraction data were measured by Bruker D8 Advance diffractometer with CuKα radiation. Simulations of elementary cell and theoretical X-ray patterns were performed using PowderCell 2.4 software [13]. Mössbauer spectra were measured using Polon mössbauer spectrometer with a 57 Co:Rh source in conventional transmission geometry. The samples were subjected to the mechanical polishing and etching for 3 s in 0.5% Nital solution. Microstructures and element distributions in both the as-cast and annealed bulk specimens were revealed using metallographic microscope and scanning electron microscope SEM JEOL JSM 6610LV equipped with the energy dispersive X-ray spectrometer (EDX).

Results and discussion
The X-ray diffractions measured for as-cast bulk and ribbon samples of the LaFe 11.0 Co 0.8 Si 1.2 alloy togheter with the calculated theoretical diffraction patterns, are shown in figure 1. Microstructure of the LaFe 11.0 Co 0.8 Si 1.2 alloy bulk and ribbon samples in as-cast state are shown in figure 3. In both samples dendritic microstructure was revealed. Much finer dendrites were formed for ribbon, than those for the bulk specimen. Such differences are due to a difference in the cooling conditions during processing bulk and ribbon samples. The microstructures of samples subjected to annealing at 1323K are shown in figure 4. It was revealed that long time heat treatment resulted in homogenization of their microstructures.
In order to confirm the phase transformation accompanied by the change of microstructure, during annealing of both ribbon and bulk specimens, the SEM equipped with EDX was used. In figures 5 and 6 the SEM micrographs together with the maps of constituent element distributions for bulk sample in as-cast state and subjected to annealing at 1323K for 28 days are presented. Occurrence of particular element in the observed area was indicated by the bright color. It was confirmed that dendrites in as-cast samples are formed by the α-Fe phase, while other alloy components were expelled into the dendrite arm spacing. Unlike other elements cobalt was spread uniformly over entire volume of the samples, suggesting that it modifies α-Fe formed during solidification of the alloy. The high temperature annealing resulted in solid state diffusion of constituent elements thus leading to homogenization of the microstructure and change of the phase constitution. The Mössbauer spectra for bulk and ribbon samples of the LaFe 11.0 Co 0.8 Si 1.2 alloy in as-cast state are shown in figure 7. In the Mössbauer spectra analysis the contribution from the α-Fe phase was represented by a single sextet line for which the induction of hyperfine field B hf = 33.1T. Furthermore, another sextet line representing contribution from the ferromagnetic LaFeSi phase was incorporated. For complete fitting additional doublet line was introduced, which suggests presence of nuclei (~4 vol.% for bulk and ~2 vol.% for ribbon) of the LaFe 11.0 Co 0.8 Si 1.2 phase within the as-cast alloy. Contributions from ferromagnetic phases were ~67 and ~29 vol.% for α-Fe and LaFeSi phases, respectively. In case of ribbon slightly higher amount of the α-Fe phase reaching ~72 vol.% and ~26 vol.% for the LaFeSi were obtained.  The rest (~2 vol.%) was accounted for paramagnetic the LaFe 11.0 Co 0.8 Si 1.2 phase. The Mössbauer spectra for annealed bulk and ribbon samples are shown in figure 8.
Here the presence of paramagnetic at room temperature LaFe 11.0 Co 0.8 Si 1.2 phase was represented by a doublet of high intensity. Furthermore, additional sextet corresponding to the α-Fe phase was incorporated. The fraction of paramagnetic phase in the sample reaches ~87 vol.% while ~13 vol.% corresponds to the α-Fe. In the case of ribbon annealed at 1323K for 24 hours, the analysis shown presence of both phases.

Conclusions
It was shown that the LaFe 11.0 Co 0.8 Si 1.2 alloy in as-cast state consist of dendrites of the α-Fe phase while the tetragonal LaFeSi phase is formed in the dendrite arm spacing. Application of melt-spinning resulted in significant reduction of α-Fe grains, thus allowing shortening of the annealing time required for a formation of LaFe 11.0 Co 0.8 Si 1.2 phase. Heat treatment of ribbons at 1323K for 24h resulted in formation of majority of LaFe 11.0 Co 0.8 Si 1.2 phase and minor fraction of α-Fe. However, Mössbauer spectroscopy shown presence of magnetically disordered phase that emerged during annealing due to shortening of annealing time. Therefore one can conclude that even for the rapidly solidified sample with fine dendrite structure the time required for complete transformation to the LaFe 11.0 Co 0.8 Si 1.2 phase has to be longer than 24h.