Magnetic properties and giant reversible magnetocaloric effect in GdCoC2

The crystal structure, magnetic properties and magnetocaloric effect (MCE) of GdCoC2 have been studied. The compound crystallizes in an orthorhombic CeNiC2-type structure which belongs to the Amm2 space group. A giant reversible MCE is observed in GdCoC2 accompanied by a second-order paramagnetic to ferromagnetic (PM–FM) phase transition around the Curie temperature ∼15 K. For the magnetic field change of 0–5 T, the maximum values of the magnetic entropy change (−ΔSmaxM), relative cooling power (RCP), and refrigerant capacity (RC) are 28.4 J kg−1 K−1, 566 J kg−1 and 369 J kg−1, respectively. The present results indicate that GdCoC2 is a promising candidate for low temperature magnetic refrigeration.


Introduction
Materials with large/giant magnetocaloric effect (MCE) have gained more and more interest due to their applications for magnetic refrigeration. Compared with traditional gas refrigeration, magnetic refrigeration has signicant advantages in conversion efficiency, low noise and environmental protection. [1][2][3][4][5] The MCE manifests as DS M (isothermal magnetic entropy change) and DT ad (adiabatic temperature change). Finding special materials with a large MCE value is considered to be the most important work of magnetic refrigeration, since the MCE is an inherent characteristic of magnetic materials. In recent years, rare-earth based compounds have been widely investigated with respect to their MCE properties. [6][7][8][9][10][11][12][13][14][15][16][17][18][19] And some of them exhibit large/giant MCE at 10-25 K, which is around the boiling point of hydrogen. [6][7][8][9][10][11][12][13][14][15] Considering the problems in application and storage of liquid hydrogen at room temperature, these materials are eligible to be applied for magnetic refrigeration for hydrogen liquefaction.
In the present study, we have investigated the MCE in GdCoC 2 which belongs to an existing ternary system RETC 2 (RE stands for heavy rare earth element and T stands for transition metal Co and Ni). Schafer et al. have reported that all the RECoC 2 compounds order ferromagnetically, whereas, compounds of RENiC 2 are antiferromagnetic. [20][21][22][23][24][25][26] Up to the present, only the MCE in TbCoC 2 belonging to this series compounds has been reported and the maximum magnetic entropy change (ÀDS max M ) is 15.3 J kg À1 K À1 for the magnetic eld change of 0-5 T. 15 For the presently studied GdCoC 2 , the values of ÀDS max M are 16.0 J kg À1 K À1 , 28.4 J kg À1 K À1 , and 32.9 J kg À1 K À1 for magnetic eld changes of 0-2 T, 0-5 T and 0-7 T, respectively.

Experimental details
The polycrystalline GdCoC 2 sample was synthesized by the method of arc-melting. First, stoichiometric amounts of highpurity components were weight and 3% extra carbon was added to compensate the loss during arc-melting. The sample was turned over and melted for four times under an argon atmosphere to ensure good homogeneity. Then the sample was annealed at 900 C for six days in evacuated quartz tubes and rapidly quenched in ice water. The sample was characterized by X-ray powder diffraction (XRD) measurement by Rigaku D/MAX 2550 using Cu Ka radiation. The magnetization measurements were done with a commercial superconducting quantum interference device (SQUID) based vibrating sample magnetometer (VSM) (Quantum Design Company, USA).

Results and discussion
The XRD pattern for GdCoC 2 together with the Rietveld renement proles which were analyzed by MAUD soware are shown in Fig. 1. The inset of Fig. 1 shows the crystal structure diagram obtained by using the CRYSTALMAKER soware package. The factors of Rietveld renement which were calculated by MAUD soware are R wp (%) ¼ 8.99, R B (%) ¼ 6.69, R exp (%) ¼ 12.0. The results reveal that the sample is crystallized in single phase with orthorhombic CeNiC 2 -type structure (Amm2 space group) and no impurity phases can be detected. According to the Rietveld renement proles, the rened lattice parameters a, b, and c are calculated by Bragg equation and found to be 3.621, 4.506, and 6.062Å, respectively, which are close to the standard values.  Fig. 3 shows the M 2 versus H/M curves (also named as Arrot plot) from 3 K to 45 K. Based on the Banerjee criterion, 27 the present GdCoC 2 undergoes a second-order magnetic phase transition since neither negative slope nor inection can be observed in the whole temperature range.
The isothermal magnetic-entropy change DS M is obtained from magnetization isotherms by integrating the Maxwell relation   The temperature-dependent ÀDS M under magnetic eld changes up to 0-7 T is shown in Fig. 4. ÀDS M increases with the increasing value of DH. The maxima of ÀDS M appears around 17 K, which is close to the paramagnetic to ferromagnetic (PM-FM) phase transition. The values of ÀDS max M are equal to 9.3 J kg À1 K À1 , 20.9 J kg À1 K À1 , 28.4 J kg À1 K À1 , 32.9 J kg À1 K À1 under the eld changes of 0-1 T, 0-3 T, 0-5 T and 0-7 T, respectively. These values are around two times of those for TbCoC 2 and larger than those of most of recently reported giant MCE materials in the same temperature range, indicating that the presently studied GdCoC 2 compound belongs to a class of giant MCE materials. The origin of giant MCE in GdCoC 2 is probably related to the large saturation moment, especially under low magnetic eld as well as its eld and temperature sensitive magnetic phase transition. Additionally, the large value of ÀDS max M of 16.0 J kg À1 K À1 is reached under the eld changes of 0-2 T with a quite wide temperature range for present GdCoC 2 , which is quite benecial to application.
Franco et al. revealed a universal behavior of the eld dependence of DS M for the materials with a second-order transition, i.e. in ref. 28, the DS M versus T curves under different magnetic elds can collapse into a universal curve; therefore the inuence of different magnetic elds can be ignored. All the curves are normalized to their respective maximum value as DS M /DS max M and the axis of temperature is rescaled to q below and above T C , T r1 and T r2 are the temperatures of the two reference points of each curve that corresponds to 0.6 ÀDS max M . The curves of normalized entropy change DS M /DS max M versus the rescaled temperature q under different magnetic elds are shown in the inset of Fig. 4. All the curves under different magnetic elds collapse onto a single line for GdCoC 2 , which further conrms that the present GdCoC 2 undergoes a second order magnetic phase transition.
The relative cooling power (RCP) and refrigerant capacity (RC) are important factors to evaluate refrigeration materials. The values of RCP can be calculated as dT FWHM is the full width at half maximum and ÀDS max M is the maximum of magnetic entropy change, respectively. 4 The values of RCP for GdCoC 2 are 216, 566, 769 J kg À1 for DH ¼ 0-2, 0-5 and 0-7 T, respectively. The values of RC can be calculated as T 1 and T 2 are the temperatures of half-maximum value of ÀDS M peak. 4 The value of RC for GdCoC 2 are equal to 160, 369, 514 J kg À1 for DH ¼ 0-2, 0-5 and 0-7 T, respectively. For a comparison, the transition temperature (T M ), together with the values of ÀDS max M , RCP and RC with the eld change from 0 to 2 T and 5 T for GdCoC 2 and some recently reported large/giant MCE materials with T M around 15 K are shown in Table 1. These values for GdCoC 2 are obviously larger than most of those reported materials in the similar working temperature range.

Conclusions
In summary, a single phase GdCoC 2 compound is synthesized, and its crystal structure, magnetic properties and magnetocaloric properties have been investigated. The compound undergoes a second-order paramagnetic to ferromagnetic (PM-FM) transition at the Curie temperature T C $ 15 K. Accompanied with the transition, giant reversible MCE is observed. The values of ÀDS max M for GdCoC 2 reach 16.0, 28.4, and 32.9 J kg À1 K À1 for the eld changes of 0-2 T, 0-5 T and 0-7 T, respectively. The corresponding values of RCP (RC) are 216 (160), 566 (369),  and 769 (514) J kg À1 . For the reason that the compound has large MCE and small hysteresis in a wide temperature range, it could be considered as potential refrigerant material for low temperature magnetic refrigeration.