Magnetic properties of a multicomponent intermetallic compound Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2

Polycrystalline multicomponent Laves phase rare earth intermetallic compound Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 has been synthesized by arc-melting and characterized by powder X-ray diffraction, magnetization and heat capacity measurements. The sample has MgCu 2 -type cubic structure (space group Fd-3m) at 300 K. The compound Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 orders ferromagnetically at 50 K (T C ). This value is almost equal to the average of the ferromagnetic ordering temperatures of the individual RAl 2 (R = Tb, Dy, Ho and Er) compounds. Field dependence of magnetization below T C indicates soft-ferromagnetic nature. From the magnetization vs field data obtained near T C and the temperature dependence of heat capacity, low field magnetocaloric effect has been estimated.


Introduction
Rare-earth intermetallic compounds RAl 2 (R= Gd, Tb, Dy, Ho and Er) crystallize in MgCu 2type, cubic Laves phase structure at room temperature and display a second-order paramagnetic to ferromagnetic magnetic transition at Curie temperature ranging from 13 K (for ErAl 2 ) to 167 K (for GdAl 2 ) [1]. These RAl 2 compounds are well-known as model systems to study the influence of magnetic exchange interaction, crystalline electric field and magnetic anisotropy on the magnetocaloric effect (MCE). Several binary and pseudo-binary RAl 2 compounds display large MCE around the magnetic transition temperature [2] [3]. Using three or more magnetic rare earth elements in the rare-earth sublattice will help to modify Curie temperature and also magnetocrystalline anisotropy [4]. Large MCE spread over a wide temperature range may also result. Reports on multicomponent systems and high entropy alloys [5] [6] have motivated us to study the magnetic properties of a multicomponent RAl 2 system where R site is occupied by four equimolar heavy rare earth elements.

Experimental details
Polycrystalline Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 compound has been prepared by arc melting under argon atmosphere, starting from stoichiometric amounts of pure elements. The sample was remelted several times to improve homogeneity. The sample was characterized using powder X-ray diffraction (XRD) at 300 K (Cu-Kα) followed by energy dispersive X-ray analysis (EDX). Magnetization (M) measurements have been performed using a vibrating sample magnetometer (Lake Shore 7410-S) in fields up to 15 kOe in a temperature (T) range of 15 K to 300 K. Heat capacity (C) has been measured as a function of temperature from 2 K to 300 K by relaxation technique using a physical property measurement system (PPMS, Quantum Design) [7].

Results and discussion
Powder X-ray diffraction data confirm that the sample crystallizes in an MgCu 2 -type structure (cubic, space group Fd-3m) [ Figure 1]. The lattice parameter (a) obtained by Rietveld refinement of the powder XRD data is 7.8394(3) Å. The value is close to the mean value of the lattice parameters of TbAl 2 , DyAl 2 , HoAl 2 and ErAl 2 [ Table 1]. The compound Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 is found to be isostructural with the parent compounds RAl 2 . Nominal composition of the sample is verified by the EDX analysis.
Magnetization measurements of Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 have been carried out in zerofield-cooled (ZFC) and field-cooled (FC) modes during warming from 15 K to 300 K in a field of 5 kOe. The M(T) data show a transition from paramagnetic to ferromagnetic state at 50 K (T C ) [ Figure 2(a)]. The ferromagnetic ordering temperature has been identified by plotting first derivative of magnetization with respect to temperature [Inset in Figure 2(a)]. No thermomagnetic irreversibility between ZFC and FC magnetization is observed. The compounds TbAl 2 , DyAl 2 , HoAl 2 and ErAl 2 order ferromagnetically at about 105 K, 65 K, 29 K and 13 K respectively [8]. Interestingly, the mean value of these ordering temperatures is nearly equal to the ordering temperature of the multicomponent system under study. Similar behaviour has also been observed in five-component Laves phase intermetallic compound Gd 0.2 Tb 0.2 Dy 0.2 Ho 0.2 Er 0.2 Al 2 [9].
Paramagnetic susceptibility obeys Curie-Weiss law. From the fit, effective paramagnetic moment (µ e f f ) and the paramagnetic Curie temperature (θ p ) values are calculated as 10.1

013.2
obtained from the thermodynamic Maxwell relation [10]. Here, H i and H f stand for the initial and final applied field values and µ 0 is the permeability of free space. The multicomponent system Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 exhibits normal magnetocaloric effect below T C as expected in ferromagnetically ordered materials [ Figure 3(b)]. The maximum isothermal magnetic entropy change (∆S ma x m ) is about -3.6 Jkg −1 K −1 at 47.5 K for a field change of 10 kOe. It is comparable with single component RAl 2 [ Table 1]. The corresponding relative cooling power is large and is about 69 J/kg for 10 kOe field change. The second order nature of the transition has been confirmed by Arrott plots [ Figure 3(c)].
Heat capacity of Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 compound has been measured as a function of temperature in zero magnetic field and in applied field of 10 kOe. The data reveal a λ-type transition around T C in zero field [ Figure 4(a)]. This corresponds to the ferromagnetic ordering observed in the magnetic data. However, the transition is not as sharp as the one observed in end member RAl 2 (R = Tb, Dy, Ho and Er) compounds. The peak gets diminished upon the application of 10 kOe field and shifts to higher temperature as evidenced in ferromagnets. At room temperature, heat capacity approaches the Dulong and Petit's limit (3NR). Isothermal magnetic entropy change is calculated from the heat capacity data using the equation and is compared with the value calculated using magnetization data [ Figure 4(b)]. Magnetocaloric effect around T C has also been quantified in terms of adiabatic temperature change 013.3 The maximum adiabatic temperature change of about 2.4 K at T = 52.5 K has been observed for Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 compound for 15 kOe magnetic field change. This is of the same order of adiabatic temperature change observed in the RAl 2 (R = Gd, Tb, Dy, Ho and Er) compounds. Thus, substitution of multiple principal elements at the rare earth site of RAl 2 compound helps in tuning the ferromagnetic transition temperature in accordance with the de Gennes factor [13]. Multicomponent Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 forms in the same crystal structure and shows an average T C of individual rare earth containing RAl 2 compounds and substantial magnetocaloric effect.

Conclusions
Multicomponent rare earth intermetallic compound Tb 0.25 Dy 0.25 Ho 0.25 Er 0.25 Al 2 crystallizes in cubic structure and orders ferromagnetically around 50 K. Magnetocaloric effect has been calculated using both the heat capacity and magnetization data and the compound shows a moderate magnetocaloric effect. A large isothermal entropy change of about -3.6 J/Kg −1 K −1 at 47.5 K is observed for a field change of 10 kOe.