Magnetism and giant magnetocaloric effect in rare-earth-based compounds R3BWO9(R=Gd,Dy,Ho)*

Lu-Ling Li(李炉领) Xiao-Yu Yue(岳小宇) Wen-Jing Zhang(张文静) Hu Bao(鲍虎)Dan-Dan Wu(吴丹丹) Hui Liang(梁慧) Yi-Yan Wang(王义炎)Yan Sun(孙燕) Qiu-Ju Li(李秋菊) and Xue-Feng Sun(孙学峰)

1Institute of Physical Science and Information Technology,Anhui University,Hefei 230601,China

2Hefei National Laboratory for Physical Sciences at Microscale,Department of Physics,and Key Laboratory of Strongly-Coupled Quantum Matter Physics(CAS),University of Science and Technology of China,Hefei 230026,China

3School of Physics&Material Science,Anhui University,Hefei 230039,China

Keywords: magnetocaloric effect,short-range spin correlation

1. Introduction

Recently, the rare-earth-based systemsR3BWO9(R=Gd, Dy, Ho) have attracted attention as searching for quantum spin liquid candidates.[27]These compounds crystallize in hexagonal structure with the space groupP63, where the magnetic ions are connected by oxygen ions and form a distorted Kagome lattice in theabplane. Along thecaxis, the nearest neighboring rare-earth ions form a frustrated zigzag chain structure. The distance of interlayer rare-earth ions is comparable with the intralayer distance, constituting a three-dimensional framework. The magnetic property studies showed paramagnetic ground states without thermal or magnetic hysteresis in these systems.[27]Under a small applied magnetic field, the weakly coupled spins can easily rotate towards the direction of the magnetic field, accompanying a large magnetic entropy change in the magnetization process. Thus,a considerable MCE is expected in the series compounds.

In this paper, we studied the magnetism and MCE ofR3BWO9(R=Gd, Dy, Ho) compounds by using magnetic susceptibility, isothermal magnetization, and specific heat measurements. The isothermal magnetic entropy changes have been calculated according to the Maxwell thermodynamic relations. Due to the absence of long-range magnetic ordering at low temperature, a large MCE was observed in these compounds.

2. Experimental details

The polycrystallineR3BWO9(R=Gd,Dy,Ho)were synthesized using standard solid-state reaction method by mixing stoichiometric amounts of high purity rare-earth oxides,H3BO3, WO3, and MoO3. In order to remove hydroxide impurity,the rare-earth oxides were dried at 900°C for 12 hours before mixing. Then, the powders were ground together and sintered in air at 1200°C for 72 hours with several intermediate regrindings. The purity and morphology of samples were examined by powder x-ray diffraction(XRD)at room temperature. The relative Rietveld powder diffraction profile fitting software FullProf was used to refine the crystal structure.[28]Magnetic susceptibility and isothermal magnetization were measured by using a SQUID-VSM (Quantum Design). The temperature dependence of the specific heat was measured by using a physical property measurement system(PPMS,Quantum Design)from 50 K to 2 K.

3. Results and discussion

Figure 1 shows the Rietveld refinements of powder XRD patterns forR3BWO9(R=Gd,Dy,Ho)at room temperature.All samples are single phase without any detectable impurities.Table 1 lists the unit-cell parameters obtained by Rietveld refinements, which are in good agreement with the results of previous report.[27]Due to the lanthanide shrinkage,the lattice parameters decrease gradually with the decrease of the rareearth ionic radius.

The temperature dependence of magnetic susceptibilityχ(T) curves measured at different applied fields are shown in Fig. 2. At 0.1 T, no obvious magnetic transition can be observed down to 2 K for the three compounds. The corresponding inverse susceptibilityχ-1(T)exhibits a typical linear behavior above 50 K.Compared with Gd3BWO9,the relatively obvious slope changes ofχ-1(T)can be found around 50 K in Dy3BWO9and Ho3BWO9, indicating the possible existence of short-range spin correlations at low temperature.Table 1 exhibits the Weiss temperaturesθCWfitted by Curie-Weiss lawχ(T)=C/(T-θCW) and the calculated effective magnetic momentsμeffat two different temperature regimes of 100-300 K and 2-25 K.For all temperature regions,these compounds exhibit negative Weiss temperatures,revealing the dominant antiferromagnetic (AFM) exchanges between the rare-earth ions. The calculated effective magnetic moment of Gd3BWO9is 7.97μBat low temperature,which is close to the theoretical value 7.93μBof free Gd3+ions(S=7/2,g=2.0).For Dy3BWO9and Ho3BWO9, the calculated effective magnetic moments are 10.44μBand 10.23μB,respectively,which are slightly larger than the theoretical values of free Dy3+and Ho3+ions.

Fig. 1. The Rietveld refinements of XRD data for R3BWO9 (R=Gd, Dy,Ho)with experimental and calculated XRD patterns and difference between them. The vertical bars represent the expected Bragg reflection positions.

The derivative dχ/dTcurves under different applied fields have been provided to further analyze the spin correlations below 20 K.As shown in the insets of Figs.2(b),2(d),and 2(f), a common feature is the sharp decrease of dχ/dTwith decreasing temperature whenH ≤1 T for the three compounds. The minimum value of dχ/dToccurred at 2 K for Gd3BWO9and 4 K for Dy3BWO9and Ho3BWO9,confirming the existence of short-range spin couplings at low temperature.Beyond that,with the increase of the applied magnetic fields,the decrease of dχ/dThas been suppressed gradually, supporting a field-induced ferromagnetic(FM)transition in these systems at low temperature.

Table 1. The lattice parameters and magnetic parameters fitted by Curie-Weiss law of R3BWO9 (R=Gd,Dy,Ho).

Fig.2. Temperature dependence of susceptibility χ(T)curves under different applied magnetic fields and inverse susceptibility χ-1(T)curves at 0.1 T of R3BWO9 (R=Gd,Dy,Ho). The pink and green solid lines indicate the Curie-Weiss fitting at different temperature regions. The insets of(a),(d),and(f)exhibit the first-order derivative dχ/dT curves under different applied magnetic fields below 20 K.

Fig.3. The isothermal magnetization curves of R3BWO9 (R=Gd,Dy,Ho)measured at different temperatures.

Fig.4. Temperature dependence of magnetic entropy change-ΔSM for R3BWO9 (R=Gd,Dy,Ho),respectively. The insets of(b)and(c)give the RCP as a function of magnetic field.

In order to further investigate the MCE of Gd3BWO9,the specific heat has been measured from 50 K to 2 K. The temperature dependence of the specific heat under different applied magnetic fields is shown in Fig. 5(a). The zero-field specific heat data indicates the absence of long-range magnetic order down to 2 K, in agreement with the results of magnetic susceptibility, but exhibits an upturn atT ＜8 K.With increasing fields, a broad peak appears around 5 K forH=3 T and shifts to higher temperatures gradually. Similar with GdCrTiO5and BiGdO3, such behaviors indicate the existence of short-range spin correlations at low temperature in this system.[25,29]The background specific heat of phonon contribution can be described byCph=βT3+β5T5+β7T7withβ=112×10-3J/mol·K4,β5=-4.66×10-7J/mol·K6,β7= 7.40×10-11J/mol·K8. By integrating (Cp-Cph)/Tfrom 50 K to 2 K, the temperature dependence of the magnetic entropy is obtained and shown in Fig.5(b). The obtained values of magnetic entropy are 6.4 J/mol·K forH= 0 and 38.5 J/mol·K forH= 7 T, which are 12% and 74% of the theoretically maximum magnetic entropy 3Rln8 for fully polarized Gd3+ions, respectively. The small change of entropy at zero field confirms the existence of spin correlations below 2 K.

Fig. 5. (a) Temperature dependence of the specific heat measured at different magnetic fields for Gd3BWO9. The red line represents the phonon contribution. (b)The temperature dependence of magnetic entropy Smag.

Based on the temperature dependence of the specific heat curves under different applied magnetic fields, the magnetic entropy change of Gd3BWO9can be calculated using the equation

As shown in Fig. 4(a), the temperature dependences of ΔSMcalculated from isothermal magnetizationM(T,H) and specific heatC(T,H)exhibit same tendency. The adiabatic temperature change ΔTadas another important parameter to characterize the MCE materials can be calculated by using the equation

Fig.6.Temperature dependence of adiabatic temperature change-ΔTad for Gd3BWO9.

Table 2.A comparison of the MCE between Gd3BWO9 and some other Gd-based oxides under the magnetic field changes of 1 T,3 T,and 7 T.

Compared with the magnetic order systems, the rareearth ions inR3BWO9(R= Gd, Dy, Ho) are located in a three-dimensional frustrated framework, which may enhance the disorder of spins at low temperature. These weakly coupled spins can easily rotate towards the direction of magnetic field and generate a large magnetic entropy change under a small applied magnetic field. Thus, rare-earth-based oxidesR3BWO9(R=Gd, Dy, Ho) can be considered as the potential magnetic refrigerant materials at low temperature without field and thermal hysteresis in the magnetization process.

4. Summary

In summary we have synthesized a series of rare-earthbased boron tungstate compoundsR3BWO9(R=Gd,Dy,Ho)by the solid-state reaction method. All three compounds exhibit no long-range magnetic order down to 2 K. Under an external magnetic field, the weakly AFM coupled spins can rotate towards the direction of the applied field and yield a large MCE. For a field range of 0-7 T, the maximum values of the magnetic entropy change reach 54.80 J/kg·K at 2 K for Gd3BWO9, 28.50 J/kg·K at 6 K for Dy3BWO9, and 29.76 J/kg·K at 4 K for Ho3BWO9. Especially for Gd3BWO9,