Magnetic and magnetocaloric properties in La–(Fe–Co)–Si

Since its discovery, magnetocaloric effect (MCE) of material has been attracting many scientists because of the possibility of using it for magnetic refrigeration. The application of magnetocaloric materials in chillers has the advantages of not causing environmental pollution (unlike chillers using compression gases), capability of improving the cooling efficiency (saving energy), compact design, less noise and ability to be used in some special applications. Magnetic refrigeration is based on the principle of magnetic entropy change of the material. To have large magnetic cooling efficient, the MCE of the material should be large. There are three main factors to estimate potential magnetocaloric materials: the magnetic entropy change (ΔS_M), the adiabatic temperature change (ΔT_ad) and refrigerant capacity (RC). An ideal magnetic refrigerant has to possess large values of both the ΔS_M and RC around room temperature in low magnetic field.

Magnetocaloric materials are actually of interest to research nowadays due to new discoveries of both the mechanism and magnitude of MCE. The search for magnetocaloric materials with the possibility for applications in magnetic refrigeration at room temperature region are of more and more interest for scientific research. These materials include As-containing alloys (MnAsSb, MnFePAs, etc), La-containing alloys (LaFeSi), Heusler alloys (CoMnSi, NiMnSn, NiMnGa, etc), and ferromagnetic perovskite maganites [1–10]. Among these materials, amorphous magnetic or nanocrystalline materials are potential candidates [11–19] because they (i) undergo second-order magnetic phase transitions which present a broad ΔS_M peak around the Curie temperature T_C, (ii) possess large RC, (iii) do not exhibit structural changes [15] and (iv) have small magnetic and thermal hysteresis, high electrical resistivity and good mechanical properties [16]. Among this kind of materials, rare-earth-based amorphous alloys have high ΔS_M peak and large RC. Particularly, LaFe₁₃-based alloys have attracted intensive attention due to their low price, giant magnetocaloric effect (GMCE) and high thermal conductivity. But their Curie temperature T_C is under room temperature, making them unsuitable for ambient-temperature applications [17–19]. To enhance the Curie temperature T_C of this material, the addition of Co is the most suitable way. Moreover, Co can improve the glass-forming ability (GFA) of the materials.

In this work we replaced a part of Fe by Co in LaFe_11−xCo_xSi₂ (x = 0, 1, 2, 3 and 4) alloy prepared by using melt-spinning method.

Ingots with nominal compositions of LaFe_11−xCo_xSi₂ (x = 0, 1, 2, 3 and 4) were prepared from pure components of La, Fe, Co and Si on an arc-melting furnace to ensure their homogeneity. The ribbons were then fabricated on a single wheel melt-spinning system. The quenching rate of the ribbons could be changed by changing tangential velocity, v, of the wheel. In this study, the ribbons were prepared with v = 40 m s⁻¹. All the arc-melting and melt-spinning were performed under Ar atmosphere to avoid oxygenation. Structure of the ribbons was analyzed by x-ray diffraction (XRD). Magnetic measurements in the temperature range of 77–400 K were performed on a vibrating sample magnetometer (VSM).

The values of magnetic entropy change ΔS_M caused by a variation of applied magnetic field was calculated by using the formula

$$\begin{equation} \Delta S_{\rm{M}} = \int_0^H {\left( {\frac{{\partial M}}{{\partial T}}} \right)} \,{\rm{d}}H. \end{equation} \tag{ 1 }$$

LaFe_11−xCo_xSi₂ (x = 0, 1, 2, 3 and 4) alloy ribbons were prepared with v = 40 m s⁻¹. This is the highest velocity of the wheel. With this quenching rate, the amorphous-to-crystalline transformation is the lowest during the quenching process. Figure 1 shows XRD patterns of LaFe_11−xCo_xSi₂ (x = 0, 1, 2, 3 and 4) ribbons. We can see that all samples are partly crystallized. The results of structure analyses also show that the phase formation clearly depends on the concentration of Co. Intensities of the diffraction peaks are gradually reduced with Co addition. There are two diffraction peaks of the XRD patterns corresponding to α-Fe phase. Other peaks are those of cubic NaZn₁₃-type phase [19].

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Figure 2 represents thermomagnetization curves of the LaFe_11−xCo_xSi₂ (x = 0, 1, 2, 3 and 4) alloy ribbons with an applied magnetic field of 12 kOe. We see that these ribbons have a magnetic phase transition in the range of 200–550 K. The magnetization decreases with increasing temperature and rapidly decreases at the magnetic phase transition. After the magnetic phase transition, the magnetization of all the ribbons does not reduce to zero but keeps at certain values characterizing for the crystalline phase. This is suitable for the above structure analysis. It should be noted that Co-concentration strongly influences the Curie temperature of LaFe_11−xCo_xSi₂ (x = 1, 2, 3, 4 and 5) ribbons. The Curie temperature is enhanced with increasing Co-concentration (see inset of figure 2). Without Co-addition, the magnetic phase transition of alloy is below room temperature. The T_C value determined for the sample x = 0 is only ∼220 K. However, when the concentration of Co is 1 at %, the phase transition temperature is raised to 315 K. As for the samples with x = 2, 3 and 4, the T_C values are 410, 480 and 530 K, respectively (see table 1). The effect of Co-addition on Curie temperature has a significant effect in controlling the working temperature of the magnetic refrigerants.

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Figure 3 shows the dependence of magnetization on external field and Co-concentration of the ribbons at 300 K. From these hysteresis loops, both the saturation magnetization M_S and the coercivity H_c were obtained. All the samples manifest the soft magnetic behavior and their magnetization increases with increasing Co-concentration. Co increases saturation magnetization of the alloy (inset figure 3). This is due to the formation of LaCo₁₃ phase that increased with the addition of Co. The highest M_s value of the alloy is above 100 emu g⁻¹ for x = 4. These two effects of Co furthermore improve capacity for application of the alloy in the magnetic refrigeration.

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With the goal of finding magnetocaloric materials that possess magnetic transition temperature close to room temperature, we chose three samples of LaFe_11−xCo_xSi₂ ribbons with Co concentration in the range of 0–2 at.% to investigate their MCE. We calculate magnetic entropy change (ΔS_M) based on thermomagnetization data (figure 4). We can derive from thermomagnetization curves in different magnetic fields of the samples to be magnetized versus magnetic field curves at various temperatures (figure 5). To check this derivation procedure, we compared data (indirect data) deduced from thermomagnetization curve with data (direct data) of virgin magnetization ones (see inset of figure 4). We found that the data obtained from the two different ways coincided. Then the magnetic entropy change, ΔS_M, is determined from M(H) data by using equation (1).

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By using the proposed method to calculate the magnetic entropy change of magnetocaloric materials, we can save time and expenditure for experiments. This method is also useful to avoid the effect of thermal fluctuation of measurement systems.

Figure 6 shows the plots of the temperature dependences of −ΔS_M for the samples with x = 0, 1 and 2 in a field interval of 0–12 kOe. The results indicate that the maximum entropy change ($\left| {\Delta S_{\rm M}} \right|_{\max }$) is nearly unchanged (above 1.2 J kg⁻¹ K⁻¹ with ΔH = 12 kOe) with various Co concentrations. However, working temperature of the alloy and FWHM of entropy change peak gradually increase with increasing Co concentration. The refrigerant capacity (RC) of the samples, which is defined as the product of maximum entropy change and FWHM of entropy change peak, was also calculated (see inset of figure 6). The $\left| {\Delta S_{\rm M}} \right|_{\max }$ and RC values determined for the samples with x = 0, 1 and 2 are 1.43, 1.25 and 1.26 J kg⁻¹ K⁻¹ and 55, 67 and 70 J kg⁻¹, respectively (see table 1). One can see that the sample with Co concentration of 1 at.% has quite large $\left| {\Delta S_{\rm M} } \right|_{\max }$ and RC at around room temperature. This shows a capability to apply this kind of magnetocaloric materials in practice.

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The magnetic and magnetocaloric properties in LaFe_11−xCo_xSi₂ (x = 0, 1, 2, 3 and 4) alloy ribbons have been studied. Glass forming ability of the alloy can be enhanced considerably by the addition of Co. Curie temperature of the alloy can be regulated in room temperature region by replacement of a part of Fe by Co. The quite high maximum magnetic entropy change (|ΔS_M|_max > 1.2 J kg⁻¹ K⁻¹ with ΔH = 12 kOe), large refrigerant capacity (RC > 80 J kg⁻¹) and wide working range around room temperature (FWHM > 60 K) reveal application potential of this alloy in magnetic refrigeration technology.

This work was supported by the Scientific Research and Technology Development Project for Young Researchers of VAST, the National Foundation for Science and Technology Development (NAFOSTED) of Vietnam under grant number of 103.02–2011.23 and the Converging Research Center Program funded by the Ministry of Education, Science and Technology (2012K001431), South Korea. A part of the work was done in the Key Laboratory for Electronic Materials and Devices, and Laboratory of Magnetism and Superconductivity, Institute of Materials Science, VAST.