Complex magnetic properties and large magnetocaloric effects in RCoGe ( R = Tb , Dy ) compounds

Complicated magnetic phase transitions and Large magnetocaloric effects (MCEs) in RCoGe (R=Tb, Dy) compounds have been reported in this paper. Results show that the TbCoGe compounds have a magnetic phase transition from antiferromagnetic to paramagnetic (AFM-PM) at TN∼16 K, which is close to the value reported by neutron diffraction. The DyCoGe compound undergoes complicated phase changes from 2 K up to 300 K. The peak at 10 K displays a phase transition from antiferromagnetic to ferromagnetic (AFM-FM). In particular, a significant ferromagnetic to paramagnetic (FM-PM) phase transition was found at the temperature as high as 175 K and the cusp becomes more abrupt with the magnetic field increasing from 0.01 T to 0.1 T. The maximum value of magnetic entropy change of TbCoGe and DyCoGe compounds achieve 14.5 J/kg K and 11.5 J/kg K respectively for a field change of 0-5 T. Additionally, the correspondingly considerable refrigerant capacity value of 260 J/kg and 242 J/kg are also obtained respectively, sugges...

Complicated magnetic phase transitions and Large magnetocaloric effects (MCEs) in RCoGe (R=Tb, Dy) compounds have been reported in this paper.Results show that the TbCoGe compounds have a magnetic phase transition from antiferromagnetic to paramagnetic (AFM-PM) at T N ∼16 K, which is close to the value reported by neutron diffraction.The DyCoGe compound undergoes complicated phase changes from 2 K up to 300 K.The peak at 10 K displays a phase transition from antiferromagnetic to ferromagnetic (AFM-FM).In particular, a significant ferromagnetic to paramagnetic (FM-PM) phase transition was found at the temperature as high as 175 K and the cusp becomes more abrupt with the magnetic field increasing from 0.01 T to 0.1 T. The maximum value of magnetic entropy change of TbCoGe and DyCoGe compounds achieve 14.5 J/kg K and 11.5 J/kg K respectively for a field change of 0-5 T. Additionally, the correspondingly considerable refrigerant capacity value of 260 J/kg and 242 J/kg are also obtained respectively, suggesting that both TbCoGe and DyCoGe compounds could be considered as good candidates for low temperature magnetic refrigerant.© 2017 Author(s).All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/1.5007114

I. INTRODUCTION
Since Thomson firstly predicted magnetocaloric effect (MCE) in 1860, 1 and decades later Weiss and Piccard discovered it experimentally, 2 magnetic refrigeration based on magnetocaloric effect (MCE) has attracted tremendous attention due to its energy-efficient and environment-friendly advantages as compared with the common gas-compression refrigeration technology.Magnetic refrigeration has achieved great success in ultra-low temperature. 3,4Recently, it has been anticipated to be widely used in room temperature in the near future. 51][22][23][24][25][26][27][28] Investigations by power X-ray diffraction, susceptibility measurements and neutron diffraction experiments on the ternary RCoSi (R=La-Sm, Ga, Tb) and RCoGe (R=La-Nd) compounds are reported. 29,30esults show they crystalline in the well-known tetragonal CeFeSi-type structure (space group P4/nmm) which is closely related to the ThCr 2 Si 2 structure and can be described as isolated "ThCr 2 Si 2 blocks" (BaAl 4 slab) connected via R-R contacts (CrB slab).2][33][34][35][36][37] So they display different magnetic properties.In this paper, we will report the large MCEs and the complicated magnetic phase transitions in RCoGe (R=Tb, Dy) compounds.

II. EXPERIMENTAL DETAILS
Polycrystalline DyCoGe and TbCoGe compounds were synthesized by arc-melting the stoichiometric mixture of constituent elements Dy/Tb, Co, and Ge with high-purity under purified argon atmosphere.3 at% excessive rare earths were added to compensate the weight loss during the arcmelting.These ingots were re-melted several times to ensure their homogeneity.Then these samples were annealed in a quartz tube filled with high-purity argon atmosphere for two weeks at 900 K to improve the homogeneity.Phase purity and crystal structure of annealed samples were checked by Powder X-ray diffraction (XRD) using Cu Ka radiation at room temperature.Magnetizations were carried out on a commercial MPMS SQUID VSM magnetometer and a commercial MPMS-XL SQUID magnetometer (Quantum Design).

III. RESULTS AND DISCUSSION
The powder XRD patterns of TbCoGe and DyCoGe compounds at room temperature are shown in The temperature dependence of magnetization was measured in zero-field-cooling (ZFC) mode for TbCoGe compounds under the field of 0.01T, as shown in figure 2a.The Neel temperature (T N ) determined by the peak position of M-T curve is 16 K, close to the value reported by neutron diffraction in Ref. 29.Whereas the obvious change at 12 K marked as T t on the M-T curve has not been explained.Neutron diffraction studies revealed the coexistence of noncollinear FM and a sine-modulated antiferromagnetic (AFM) ordering for TbCoGe, the competition of FM and AFM interactions may lead to this abrupt at T t .The inverse susceptibility 1/χ as a function of temperature under 0.01 T is plotted in the inset of figure 2a.It is clear that the inverse magnetic susceptibility of TbCoGe deviates from the Curie-Weiss (CW) law around 160 K, which implies the formation of finite-sized FM clusters in the matrix of the PM phase.
Figure 2b shows the temperature dependence of magnetization for the DyCoGe compound under the fields of 0.01 T, 0.05 T and 0.1 T, respectively.Several cusps indicate DyCoGe undergoes complicated phase changes from 2 K up to 300 K. From M-T curves at 0.01 T, we can speculate that the sharp change at 10 K is corresponding to a magnetic phase from antiferromagnetic to ferromagnetic (AFM-FM).The transition at lower temperature is considered to be caused by many competing forces.In particular, a significant ferromagnetic to paramagnetic (FM-PM) phase transition was found at the temperature as high as 175 K, which is similar to that of HoCoGe and RCoSi (R=Gd, Tb) compounds. 29,32The difference is the cusp of DyCoGe becomes more abrupt with the magnetic field increasing from 0.01 T to 0.1 T. We also suspect a structural phase transition occurs with temperature increasing, but we have found no any peak appearing or vanishing from the powder XRD patterns of DyCoGe around all of the phase transition temperatures, as shown in Fig. 2c.0][31][32][33][34][35][36][37] We will continue to have a deep research about the interesting magnetic phase changes in RCoGe (R=Gd∼Er) compounds in the future.
The magnetization isotherms taken at different temperatures from 0 T to 5 T field are shown in Fig. 3a.The magnetization is not saturated at 5 T field, but it shows a tendency to get saturated at higher field.The significant change in the slope of magnetization isotherms near T N is expected to result in significant magnetic entropy change.To make it clear, partially enlarged view is shown up in the inset of Fig. 3a.Below the temperatures T N , the magnetization curve shows a linear increase and then a sudden jump with increasing magnetic field, indicating the metamagnetic transition from AFM to FM phase.The negative slope of the Arrott plot confirmed the occurrence of a first-order  3b displayed, corresponding to the AFM nature of TbCoGe.The maximum values of ∆SM reach 7.5 J/kg K and 14.5 J/kg K for the field changes of 0-2 T and 0-5 T, respectively.Refrigerant capacity (RC) is considered as another important parameter that characterizes the refrigerant efficiency.It was estimated by using the approach RC= ∫ T hot T cold |∆S M |dT , where T cold and T hot are the temperatures corresponding to the both sides of half maximum value of ∆S M peak.RC values are 80 J/kg and 260 J/kg for the field changes of 0-2 T and 0-5 T, respectively.
Similarly, the magnetocaloric properties of DyCoGe samplers have been analyzed.The magnetization isotherms under typical fields are shown in figure 4a.According to the Banerjee criterion, the plot of M 2 vs. H/M shows negative slope suggesting first-order magnetic phase transition.The crossovers among the curves also verified the AFM ordering in this compound.The values of ∆S M peak reach 6 J/kg K and 11.5 J/kg K for the field change of 0-2 T and 0-5 T, respectively, as shown in figure 4b.It was evident that both the peak values of ∆S M of TbCoGe and DyCoGe compounds were competitive to the other potential magnetic refrigerant materials in low temperature range.

IV. CONCLUSION
In conclusion, both the TbCoGe and DyCoGe compounds crystallizes in TiNiSi-type orthorhombic structure.A competitive MCE and large RC in RCoGe (R=Tb, Dy) compounds were observed.For a field change of 0-5 T, the maximum values of ∆S M are 14.5 J/kg K and 11.5 J/kg K, and RC values are 260 J/kg and 242 J/kg, for TbCoGe and DyCoGe compounds respectively.The excellent performances indicate the RCoGe (R=Tb, Dy) compounds are attractive candidates for magnetic refrigeration in the low temperature range.

Fig. 1 .
FIG. 1.The powder XRD patterns of TbCoGe and DyCoGe compounds at room temperature.

FIG. 3
FIG.3.a shows Magnetic isothermals measured during field increasing for TbCoGe compound around T N .The inset shows the enlarged magnetic isothermals below T N .Fig.3bshows magnetic entropy changes of TbCoGe compound calculated from magnetization under typical magnetic field changes.

TABLE I .
38rie temperature, crystal structure and MCEs of RCoX (R=La-Sm, Gd-Er; X=Si, Ge) compounds.phasetransition.38Generallyspeaking, metamagnetic transition from AFM ground state to FM state belonged to first-order transition.The critical field of metamagnetic transition about the magnetization curve at 2 K was 1.3 T, determined by the maximum value of ∂M/∂H.With temperature increasing to T N , the AFM state change to FM state completely.Magnetic entropy changes of TbCoGe compound in different magnetic fields were calculated using the Maxwell relation ∆S M = ∫ Positive values of ∆S M were found in low temperatures, as figure