Large low field magnetocaloric effect in first-order phase transition compound TlFe3Te3 with low-level hysteresis

Magnetic refrigeration based on the magnetocaloric effect (MCE) is an environment-friendly, high-efficiency technology. It has been believed that a large MCE can be realized in the materials with a first-order magnetic transition (FOMT). Here, we found that TlFe3Te3 is a ferromagnetic metal with a first-order magnetic transition occurring at Curie temperature TC = 220 K. The maximum values of magnetic entropy change (Δ) along the crystallographic c-axis, estimated from the magnetization data, reach to 5.9 J kg−1K−1 and 7.0 J kg−1 K−1 for the magnetic field changes, ΔH = 0–1 T and 0–2 T, respectively, which is significantly larger than that of MCE materials with a second-order magnetic transition (SOMT). Besides the large ΔSM, the low-level both thermal and field hysteresis make TlFe3Te3 compound an attractive candidate for magnetic refrigeration. Our findings should inspire the exploration of high performance new MCE materials.

Scientific RepoRts | 6:34235 | DOI: 10.1038/srep34235 measurements. We found that this compound exhibits a large MCE with a small magnetic field change, Δ H, and with a low-level thermal and field hysteresis, thus identifying it to be another class of solids for the magnetic refrigerants. Figure 1 presents the powder x-ray diffraction (XRD) pattern of TlFe 3 Te 3 and its Rietveld refinement. All the diffraction peaks could be indexed by a hexagonal structure with space group P6 3 /m. The lattice parameters a = 9.355(1) Å and c = 4.224(5) Å were obtained by the refinement, which are in good agreement with previous reports 44 . The electron probe micro-analyzer (EPMA) experiments performed on several single crystals verified that the sample composition (the average atomic ratio) is of Tl : Fe : Te = 0.99(1) : 2.95(2) : 3.00 (1), which is in consistent with the nominal composition. The temperature dependence of electrical resistivity along c-axis, ρ(T), for a TlFe 3 Te 3 crystal is shown in Fig. 2(a). In the whole measuring temperature range, the positive resistivity-temperature coefficient of ρ(T) indicates its metallic behavior. The resistivity has a very sharp drop at 220 K with detectable thermal hysteresis [see the inset of Fig. 2(a)], which is associated with the first-order ferromagnetic transition. The resistivity at 300 K and 1.8 K are of 120 μΩ cm and 1.8 μΩ cm, respectively. The small resistivity should be viewed as a merit since a good thermal conductivity is required for a high performance magnetic refrigerant material 45 . Both a rather low residual resistivity and a considerable large residual resistivity ratio (RRR) = 67 indicate that our crystals are of high quality. Figure 2(b) shows the magnetization as a function of temperature, M(T), measured from 2 to 300 K in an applied magnetic field H = 1000 Oe, aligned both || and ⊥ the c-axis, with a field cooling process. A sharp increase of M for both directions at the Curie temperature, T C ~ 220 K, confirms the occurrence of a ferromagnetic transition. Larger magnetization along c-axis suggests that the easy axis of magnetization is in the c axis. As discussed by Uhl et al. 43 and Pelizzone et al. 44 , the strong magnetic anisotropy observed in the ferromagnetic state is certainly related to its peculiar structure being composed of |Fe 3 Te 3 | ∞ chains, whose central part is a column of edge-sharing octahedral Fe clusters. The Fe-Fe distance of 2.6 Å within the clusters are comparable to the interatomic distance in metallic iron, while the nearest two Fe atoms belong to different |Fe 3 Te 3 | ∞ chains are 6.7 Å apart. Thus, a strong anisotropy of the exchange coupling is to be expected. As shown in Fig. 2(c,d), it is clear that the M(T) curves near T C exhibit a small thermal hysteresis for both directions, which is in contrast to that reported by Uhl et al. 43 and Pelizzone et al. 44 , who did not observe any hysteresis in their measurements. We observed a distinguishable but very small hysteresis, (i.e., the hysteresis temperature Δ T hy = 0.2 K for H || c-axis and 0.1 K for H ⊥ c-axis), which suggests that a first-order ferromagnetic transition occurs at ~220 K.

Results and Discussion
In order to further identify the type of the transition and to explore the MCE, we performed the isothermal magnetization measurements near the T C . Figure 3 shows the magnetization as a function of magnetic field, M(H), measured at various temperatures around T C with both H || c-axis and H ⊥ c-axis, and with both increasing and decreasing magnetic field. A small magnetic hysteresis was again observed. The maximum hysteresis is 50 Oe for H || c-axis [see Fig. 3   .9 J/kg K, respectively, which increases continuously with the increasing field change and tends to almost saturate at higher magnetic field change. It is known that a "table-like" behavior and no strong Δ H dependence of − Δ S M max value are the typical behaviors for FOMT materials 2,45 . Although − Δ S M max values are smaller than that for the some giant MCE materials (see Table 1), these values of TlFe 3 Te 3 are comparable with the most potential magnetic refrigerant materials with the a first-order ferromagnetic transition (see Table 1). For the H ⊥ c-axis case, all the − Δ S M (T) curves with different Δ H values exhibit a peak around T C without table-like behavior, and the maximum value of magnetic entropy change − Δ S M max is smaller than that for the H || c-axis. The anisotropy of MCE may origin from the peculiar magnetic structure, as discussed above.
Another important quality factor of magnetic refrigerant materials is the relative cooling power (RCP) or/and refrigeration capacity (RC), defined 29 usually as the product of − Δ S M max and the full width at half maximum in the − Δ S M (T) curve, as an example, i.e., T hot − T cold for Δ H = 0-1 T in Fig. 4(a). RCP/RC is a measurement of the amount of heat transfer between the cold and hot reservoirs in an ideal refrigeration cycle. Due to the limitation of data measured in our experiments, we only estimated that the RCP values for the Δ H = 0-1, 0-2 and 0-3 T, are of 13, 50, and 74.6 J/kg, respectively. Recently, as a figure of merit for the magnetic refrigerant materials, the dimensionless materials efficiency 47,48 , η = |Q/W|, is taken into consideration, where electrical or mechanical work, W, is done to drive highly reversible caloric effects in an isothermal body, whose entropy is thus modified such that heat, Q, flows to (Q < 0) or from (Q > 0). Here, we estimated the mass-normalized values of |W| by integrating − μ 0 MdH 0 from the M(H 0 ) data at T C , and evaluated the mass-normalized value of heat Q by integrating μ 0 T 0 (∂ M/∂ T) H with respect to H from the M(H 0 ) data at T C , which follows from the Maxwell relation As a comparison of MCE properties, we choose several compounds with a similar magnetic transition temperature, T M , as well as some typical materials with a near room temperature, T M , focusing on the performence under Δ H = 0-2 T (the maximum magnetic field generated by a permanent magnet is about 2 T). As listed in Table 1, although the − Δ S M max of TlFe 3 Te 3 is less than that in the some pronounced materials with FOMT, such as GdSi 2 Ge 2 , MnFeP 0.45 As 0.55 , LaFe 11.7 Si 1.3 and 20-LaFe 11.57 Si 1.43 materials, − Δ S M max of TlFe 3 Te 3 is significantly larger   has some other advantages, such as a rare-earth-free element, a low synthesis temperature, as well as a low-level hysteresis in the as-grown crystals. But it should be pointed out that the toxicity of Tl element is not so good for the commercial utilization, which may be improved by the replacement of In, Ba, K for Tl in the future.
In summary, after successfully growing TlFe 3 Te 3 single crystals, we carried systematically out the measurements of its resistivity and magnetization to investigate the nature of the magnetic phase transition and the MCE. It was found that TlFe 3 Te 3 is a FOMT metal with T C = 220 K and has a small thermal and field hysteresis near T C . The relative large MCE at a low Δ H makes this compound a promising candidate for magnetic refrigeration around 220 K. Further efforts should be done to substitute Tl by other nontoxic elements in order to utilize this type of materials widely.

Methods
Single crystals of TlFe 3 Te 3 were grown using a self-flux method. A mixture with a ratio of Tl:Fe:Te = 1:3:3 was placed in an alumina crucible, sealed in an evacuated quartz tube, heated at 923 K for 5 days. The product was a black powder from which needle-like single crystals with a typical dimension of ~0.4 × 0.4 × 4 mm 3 could be isolated. Powder XRD measurements on crushed single crystals were carried out at room temperature on a PANalytical x-ray diffractometer (Model EMPYREAN) with a monochromatic Cu K α1 radiation to identify the phase purity and the crystal structure. The composition was confirmed by an electron probe micro-analyzer (EPMA) (Jeol JXA-8100). The magnetic measurements were performed on a Quantum Design Magnetic Property Measurement System (SQUID-VSM, MPMS-5) and the resistivity measurements were carried out on a Physical Property Measurement System (PPMS-9).