Magnetic properties and magnetocaloric effects of RNiSi 2 ( R = Gd , Dy , Ho , Er , Tm ) compounds

Orthorhombic CeNiSi2-type polycrystalline RNiSi2 (R=Gd, Dy, Ho, Er, Tm) compounds were synthesized and the magnetic and magnetocaloric properties were investigated in detail. The transition temperatures of RNiSi2 compounds are all in a very low temperature range (<30 K). As temperature increases, all of the compounds undergo an AFM to PM transition (GdNiSi2 at 18 K, DyNiSi2 at 25 K, HoNiSi2 at 10.5 K, ErNiSi2 at 3 K and TmNiSi2 at 3.5 K, respectively). ErNiSi2 compound shows the largest (ΔSM)max (maximal magnetic entropy change) among these compounds. The value of (ΔSM)max is 27.9 J/kgK under a field change of 0-5 T, which indicates that ErNiSi2 compound is very competitive for practical applications in low-temperature magnetic refrigeration in the future. DyNiSi2 compound shows large inverse MCE (almost equals to the normal MCE) below the TN which results from metamagenitic transition under magnetic field. Considering of the normal and inverse MCE, DyNiSi2 compound also has potential applications in low-...

Orthorhombic CeNiSi 2 -type polycrystalline RNiSi 2 (R=Gd, Dy, Ho, Er, Tm) compounds were synthesized and the magnetic and magnetocaloric properties were investigated in detail.The transition temperatures of RNiSi 2 compounds are all in a very low temperature range (<30 K).As temperature increases, all of the compounds undergo an AFM to PM transition (GdNiSi 2 at 18 K, DyNiSi 2 at 25 K, HoNiSi 2 at 10.5 K, ErNiSi 2 at 3 K and TmNiSi 2 at 3.5 K, respectively).ErNiSi 2 compound shows the largest (∆S M ) max (maximal magnetic entropy change) among these compounds.The value of (∆S M ) max is 27.9 J/kgK under a field change of 0-5 T, which indicates that ErNiSi 2 compound is very competitive for practical applications in low-temperature magnetic refrigeration in the future.DyNiSi 2 compound shows large inverse MCE (almost equals to the normal MCE) below the T N which results from metamagenitic transition under magnetic field.Considering of the normal and inverse MCE, DyNiSi 2 compound also has potential applications in low-temperature multistage refrigeration.© 2018 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/10.1063/1.5007018

I. INTRODUCTION
The magnetocaloric effect (MCE), a kind of physical phenomenon found more than 130 years ago, is one of the intrinsic properties of magnetic materials.Commercial and residential refrigeration, based on conventional gas compression/expansion technology, is a mature industry.2][3][4][5] In the last twenty years great progress has been made on exploring large MCE materials for applications at room temperature such as refrigerators and air conditioners. 6,7The typical room temperature MCE materials mainly include Gd 5 Si 2 Ge 2 , 8,9 La(Fe, Si) 13 , 10-14 MnAs 1-x Sb x , 15 MnFeP 1-x As x , 16 Heusler alloys, 17,18 etc. MCE materials are generally evaluated by the following parameters: the maximum value of magnetic entropy change ((∆S M ) max ), the full width at half maximum of ∆S M − T curve (δT FWHM ), refrigerant capacity (RC) and adiabatic temperature change (∆T ad ).For most MCE materials, the value of the ∆S M is negative (normal MCE).But the positive ∆S M which called inverse MCE also can be found in some materials such as TbNiGe 2 , 19 TbMn 0.33 Ge 2 , 20 etc.Recently, much attention has also been paid on the MCE materials with low transition temperature because these materials are promising to be used for gas liquefaction in magnetic cooling cycle or combined magnetic-gas cooling cycle. 6,21These low temperature MCE materials mainly include the rare earth based intermetallic compounds such as RCo 2 , 22,23 RNi, 24 etc.Rare earth based intermetallic compounds have shown interesting magnetic properties and excellent performance on MCE.6][27] The results show that the compounds with R= Pr and Nd are ferromagnets and those with R= Gd, Tb, Dy, Ho, Er are antiferromagnets.Experimental results of electrical resistivity, Hall coefficient, magnetic susceptibility and specific heat for CeNiSi 2 support a theory of the Anderson lattice. 28][32][33] Considering the lack of research work on MCE of RNiSi 2 compounds, further study on magnetic properties and MCE will be performed.In this paper, the polycrystalline RNiSi 2 (R=Gd, Dy, Ho, Er, Tm) compounds were synthesized.The magnetic properties and MCE which both contains normal and inverse effects were investigated in detail.

II. EXPERIMENTAL PROCEDURE
The polycrystalline RNiSi 2 (R=Gd, Dy, Ho, Er, Tm) compounds were prepared by arc-melting appropriate proportion of constituent components with the purity better than 99.9% in a water-cooled copper hearth in a high-purity argon atmosphere.The ingots were turned over after each melting and re-melted several times to ensure the compositional homogeneity.After arc-melting, the ingots were wrapped by molybdenum foil respectively and sealed in a high-vacuum quartz tube, annealed at 1273K for 25 days, finally quenched into liquid nitrogen.The crystal structure was characterized by powder X-ray diffraction (XRD) method with Cu Kα radiation.Magnetic measurements including the temperature dependence of magnetization (M-T) and the field dependence of magnetization (M-H) curves were performed by employing Vibrating Sample Magnetometer with Quantum Design (SQUID-VSM).Heat capacity measurements were carried out by employing Physical Properties Measurement System (PPMS).

III. RESULTS AND DISCUSSION
The XRD pattern of RNiSi 2 (R=Gd, Dy, Ho, Er, Tm) compounds at room temperature and its crystal structure are shown in Fig. 1.Almost all of the diffraction peaks can be indexed to an orthorhombic CeNiSi 2 -type structure (space group Cmcm #63).The result is in accord with previous work. 26The Bragg positions are marked at the bottom of the picture.It can also be seen that there is a small peak around 37.5 • for all compounds and a peak around 34.5 • for GdNiSi 2 and DyNiSi 2 compound, which indicates that small amount of impurity may exist.The impurity is indexed as RNiSi 3 .However, it does not affect our discussions and conclusions because the amount of impurity is not large.As the atom number of R increases, the position of diffraction peaks moves towards higher angle range, which indicates that the lattice constant becomes smaller from GdNiSi 2 to TmNiSi 2 compound.
The Zero-Field-Cooled (ZFC) and Field-Cooled (FC) magnetization curves for RNiSi 2 compounds were measured under a field of 0.01 T. They are shown in Fig. 2(a)-(d).The MH curves for RNiSi 2 compounds are also shown in Fig. 2(e)-(h).The MT for DyNiSi 2 shows a rapid increase and then a decrease with the increasing temperature, which indicates that this compound undergoes a simple transition from antiferromagnetic (AFM) to paramagnetic (PM) phase.The same conclusion can also be found in MH curves.The transition temperature is determined to be T N =25 K.The overlap of ZFC and FC curves around T N shows a good thermal reversibility in this compound.From MH curve we can see the compound occurs metamagenetic transition with the field increasing.All of the features mentioned above can also be found in other compounds.The transition temperatures along with the effective magnetic moments, which have been calculated according to the Curie-Weiss Law, are shown in Table I.The effective moment and ion moment is almost the same for each compound, indicating only rare earth atoms contribute to the magnetic moments in this series of compounds.It reveals from Fig. 2 that the value of the transition temperature (T N ) shows a decreasing trend when the atomic number of rare earth atom increases.However, there is an exception for TmNiSi 2 , which may result from the complex magnetic coupling in TmNiSi 2 compound.
The MCE materials are generally evaluated by isothermal magnetic entropy change (∆S M ), which is calculated from isothermal magnetization data (M-H curves) by using Maxwell relation:  for each of them.However, HoNiSi 2 shows small inverse MCE below the transition temperature and DyNiSi 2 shows large inverse MCE (almost equals to the normal MCE) below the transition temperature which will be discussed in detail in the following section.It can be observed from Fig. 3 that all of the curves have a large peak and the maximal magnetic entropy change ((∆S M ) max ) occurs around T N .The refrigerant capacity (RC) is another important parameter to evaluate MCE materials.The value of RC can be calculated by using the approach RC = ∫ T 2 T 1 |∆S M |dT , where T 1 and T 2 are the temperatures corresponding to the full width at the half value of (∆S M ) max , respectively.And we call δT FWHM = T 2 T 1 the refrigerant temperature width.The (∆S M ) max , RC and δT FWHM of RNiSi 2 compounds under a field change of 0-2 T and 0-5 T are calculated and shown in Table I.][36][37] For ErNiSi 2 compound, the value of (∆S M ) max is 17.0 J/kgK for 0-2 T and 27.9 J/kgK for 0-5 T respectively, which are comparable or even larger than those of other listed materials.The RC of ErNiSi 2 compound is approximately calculated to be 75.8J/kg for 0-2 T and 225.3 J/kg for 0-5 T, respectively, where the integration starts with the temperature of 2 K.These values are not accurate because the actual low temperature boundary of δT FWHM is far lower than 2 K for ErNiSi2 compound.According to the model proposed by Oesterrreicher et al. 38 The (∆S M ) max is positively correlated with the total angular momentum quantum number (J) and negatively correlated with magnetic ordering temperature.In this series, HoNiSi 2 compound shows the largest J, but its Neel temperature is much larger than that of ErNiSi 2 compound.Considering that HoNiSi 2 and ErNiSi 2 have a similar J, the value of Neel temperature exerts a main effect on the value of (∆S M ) max .As a result, ErNiSi 2 compound shows the largest (∆S M ) max among RNiSi 2 compounds.The excellent MCE performance of ErNiSi 2 compound indicates its potential applications in low temperature refrigeration.
The Fig. 3 also shows that DyNiSi 2 and HoNiSi 2 compounds both have positive ∆S M below T N , which called inverse MCE.This phenomenon results from the mixed exchange interaction and the applied magnetic field leads to a further spin-disordered state, 39 which occurs metamagenitic transition.This makes ∂M/∂T positive under T N .For HoNiSi 2 compound, the positive ∆S M can only be found under a low field change and the value of positive ∆S M is far smaller than the absolute value of negative ∆S M around T N .When the field change is 5T, the positive ∆S M has been disappeared, which indicates that the AFM ground state of HoNiSi 2 compound below T N is relatively weak.The same phenomenon can also be found in many Heusler alloys and AFM magnetocaloric effect materials. 39However, for DyNiSi 2 compound, the inverse MCE can even be found in a high field change such as 5T and the maximal positive ∆S M is almost equals to the maximal negative ∆S M .For further study, heat capacity was measured and ∆S M also calculated using heat capacity data through the expression ∆S M = ∫ H 0 {[C (T, H) − C (T, 0)]/T}dT .The results all shown in Fig. 4. The Fig. 4 shows that the curves obtained using magnetization data are in good agreement with the corresponding curves obtained using heat capacity.And the field change of the maximal positive ∆S M occurs is about 3.6T.The large inverse MCE results from the strong AFM coupling in DyNiSi 2 compounds which may be related to the large magnetocrystalline anisotropy of Dy atoms.The inverse MCE can also be used for magnetic refrigeration if only the refrigerator works in a reverse process.For example, for DyNiSi 2 compound, magnetic refrigeration can be realized in a usual working recycle around 25K and it can be used in a reverse working recycle around 15K, which seems useful in multistage refrigeration.And the RC of DyNiSi 2 compound is re-calculated by considering of the contribution of inverse MCE at lower temperatures which also shown in Table I.The value of RC for a field change of 0-5 T is modified from 59 J/kg and 90.5 J/kg.FIG. 4. The temperature dependences of ∆S M for DyNiSi 2 compound calculated from magnetization and heat capacity data for field changes of 0-1T, 0-2T, 0-3T, 0-4T, and 0-5T, respectively.The inset shows the field change dependences of (∆S M ) max for inverse MCE at 15.5K.
The excellent performance of DyNiSi 2 compound indicates its potential applications in low temperature refrigeration.

IV. CONCLUSION
In summary, all orthorhombic CeNiSi 2 -type polycrystalline RNiSi 2 (R=Gd, Dy, Ho, Er, Tm) compounds are AFM ordered and T N in a very low temperature.Among these compounds, ErNiSi 2 compound shows the largest (∆S M ) max and RC, which is larger than almost all of the materials in this low temperature range, indicating its potential practical applications in low-temperature magnetic refrigeration in the future.DyNiSi 2 compound shows large inverse MCE (almost equals to the normal MCE) below the T N which results from metamagenitic transition under magnetic field.Considering of the normal and inverse MCE, DyNiSi 2 compound also has potential applications in low-temperature multistage refrigeration.

TABLE I .
The transition temperatures, effective magnetic moments, ion magnetic moments and magnetocaloric parameters of RNiSi 2 (R= Gd, Dy, Ho, Er, Tm) compounds and other compounds.