The Magnetocaloric Properties of Gd 5 Si 4 Alloy Prepared by the New Method

We report on a new method for preparation the magnetocaloric alloy Gd5Si4. By mechanical alloying under argon atmosphere and then melting sample by arc furnace, we produced the Gd5Si4 alloy. The structure and magnetothermal properties of the alloy have been investigated with the help of powder X-ray diffraction and magnetization measurements. This compound crystallized in the orthorhombic structure with space group pnma. In X-ray powder diffraction pattern, a minor orthorhombic GdSi2 phase was observed as a second phase. For this compound, the second order phase transition was observed. The maximum isothermal magnetic entropy change of the Gd5Si4 compound at 348K was found to be -10 J/(kg K) in an applied field of 0.5T.


INTRODUCTION
The magnetocaloric effect (MCE) is the phenomenon of release or absorption of heat by a magnetic material due to a magnetic field change [1].This effect can be quantified either in terms of isothermal magnetic entropy change (ΔSM), or adiabatic temperature change (ΔTad).Magnetic refrigeration based on MCE has attracted ample attention because it has the potential to reduce the global energy consumption, and eliminate or minimize the use of ozone depleting alloys, greenhouse gases, and precarious chemicals [2].
Since the discovery of the giant magnetocaloric effect (GMCE) in intermetallic Gd5Si2Ge2 alloys by Pecharsky and Gschneidner (1997) [4], the Gd5(SixGe1-x)4 alloys have received tremendous attention.The maximum magnetic entropy change near the transition temperature in these alloys is due to their large effective magnetic moment and minimal crystal field effects, which makes these materials a preferred candidate for room temperature magnetic refrigeration [5].
The crystal structures of Gd5(SixGe1-x)4 compounds consist of slabs, where T is a statistical mixture of Ge and Si.Specific T-T dimmers between the slabs can be reversibly broken or reformed by changing concentration (x), temperature and/or by applying magnetic field or pressure, leading to magnetostructural transitions [6].
The Gd silicide crystallizes in the Sm5Ge4-type orthorhombic structure with space group pnma, orders ferromagnetically at a higher Curie temperature Tc=336 K than that of Gd pure metal (Tc=294 K), and has a second order ferromagnetic to paramagnetic transition near Tc [7].This compound is used for the cooling of microelectronic chips [8] and introduced as a candidate for hyperthermia treatment of cancer [9].In recent years Gd silicide nanoparticles because of biocompatibility and high magnetization value has considered as one of the best candidates for self-controlled hyperthermia applications [10].
Most of the research activity to date was focused on the Gd5Si2Ge2 with substitutional doping different atoms [5].The Magnetocaloric properties of the Gd5Si4 system have been studied to a much lesser extent.Also to our knowledge, The usual method which is mostly reported for the synthesis of the current alloys limited to arc melting method.For this reason, we employed for the first time mechanical alloying under argon atmosphere followed to arc melting to synthesize the Gd5Si4 alloy.We discussed its structural and magnetic properties using X-ray powder diffraction and magnetic measurements.We determined the entropy changes near the transition temperature using magnetization data.

EXPERIMENTAL
Sample Preparation:The starting materials for synthesis of Gd5Si4 sample were Gd powder (99%, Aldrich) and Si powder (99+%, ACROS).The metal powders were handled in an Ar-filled dry box.The stoichiometric amounts of the element powders are weighed to an accuracy of approximately 0.01 mg and mixed in hardened steel vials with hardened steel balls (about 10 mm in diameter) using a F5 planetary ball miller under argon atmosphere.The milling was performed for 20 h with balls to powder ratio 14:1 at a speed of 240 rpm.The milled powder was pressed into 10-mm pellets under a load of 10 metric tons and then arc-melted on a water cooled copper hearth in an argon atmosphere.Each sample was remelted three times and after each re-melting the sample was turned over to ensure their homogeneity.The as-cast alloy was annealed at 1373 K for 3h in argon atmosphere with subsequent quenching in cold water.The weight losses after arcmelting were negligible.
The sample was characterized by powder X-ray diffraction experiment at room temperature.Magnetization measurement on this sample was carried out in the temperature range of 323-480 K using vibrating sample magnetometer (Lakeshore 7400).Several magnetizations vs. field isotherms were measured in the vicinity of magnetic transition temperature in fields up to 0.5 T and were used to calculate the magnetic entropy associated with the transition.

Structural Properties
Fig. 1 shows the powder X-ray diffraction pattern of as-milled powder after 20 h milling.Except the weak peak at ~ 32 o which is characteristic for Gd5Si4, the as-milled sample is a mixture of free Gd and Si phases.The respective positions of the peaks were identified in accordance with the JCPDS standard data.Fig. 2 represents XRD pattern of the annealed alloy.As can be seen the free Gd and Si phases disappeared.It reveals along with Gd5Si4, GdSi2 and GdSi start to appear.But the Gd5Si4-type orthorhombic structure has been detected as the major/ phase.The remaining two phases could be considered as minor phases corresponding to GdSi2 and GdSi compounds.M a y 1 1 , 2 0 1 4

2. Magnetic Properties
The temperature dependence of the magnetization-M(T) of prepared sample from 300K to 760 K in magnetic field of 0.5 T is presented in Fig. 3.This figure shows obviously how most of the alloys contain two distinct Curie temperatures.The Curie temperature was determined as the minima in the first derivative of the M-T (dM/dT) curve, and the result is shown in the inset of Fig. 3.The first drop of M-T curve at 340 K corresponds to the second order transition for Gd5Si4-type (Tc=340 K) which is in very good agreement with previous experimental results [8].The next drop near 725 K is due to the presence of impurity GdSi2-type structure [18].
Fig. 4 displays the magnetization isotherms of the Gd5Si4 compound between 323 and 383 K with the temperature steps of 10 K.

Magnetocaloric Properties
The magnetocaloric effect was evaluated from the isothermal magnetic entropy change (ΔSM) that can be calculated around the Curie temperature using the Maxwell , s equation [19].
For magnetization measured at discrete field and temperature intervals, the magnetic entropy change defined in Eq. ( 1) can be approximated by Eq. ( 2) [20], The calculated ΔSM from Eq. ( 2) is plotted as a function of temperature in Fig. 5. Since magnetic moments are spontaneously oriented at the Curie temperature, application of the magnetic field can lead to the higher magnetization at this region, thus this shape of the (-ΔSM-T) curve is expected.As seen in Fig. 5, the magnetic entropy change as a function of temperature has the typical λ-shape of systems undergoing a second order transition.The maximum value of ΔSM follows the Curie temperature and has the value of almost -10 J/kg K.This value of magnetic entropy change demonstrates the presence of a conventional magnetocaloric effect and suggests the absence of a first order magnetostructural transition.Our results show an increment in ΔSM as compared to earlier experimental works which synthesized this alloy by arc melting technique [8].This may be due to the presence of GdSi2 phase, appeared in this method.

CONCLUSION
We reported experimental results on structural and magnetocaloric characteristics of the Gd5Si4 compound which were successfully prepared by mechanical alloying under argon atmosphere followed to arc melting technique.The Gd5Si4-type

Figure 1 :
Figure 1: the XRD results of powder as milled at 240 rpm for 20 h milling.

Figure 2 :
Figure 2: the powder XRD pattern of Gd5Si4 alloy

Figure 3 :
Figure 3: Temperature dependency of the magnetization of Gd5Si4 alloy measured in an applied field of 0.5T.

Figure 4 :
Figure 4: The magnetization vs. field (M vs. H) measurements around the Curie temperature for Gd5Si4.

Figure 5 :
Figure 5: Magnetic entropy change for Gd5Si4 as a function of temperature for ΔH=0-0.5 T.