Magnetism and magnetocaloric effect of melt-spun, nanostructured GdAl2

Magnetocaloric effect (MCE) of melt-spun rare earth intermetallic compound GdAl2 (Cubic, MgCu2-type) has been studied. The sample becomes nanostructured upon melt-spinning and the crystallite size obtained from the powder x-ray diffraction data is about 48 nm. A sluggish paramagnetic to ferromagnetic transition occurs at a Curie temperature (TC) of about 136 K. This value is about 30 K lower than the ferromagnetic transition temperature of the arc-melted GdAl2. The maximum isothermal magnetic entropy change (ΔSm) is found to be ∼ −8.3 Jkg−1 K−1 at 133 K for 70 kOe field change around TC. This value is quite comparable to that of bulk sample prepared by arc-melting which is about −8.5 Jkg−1 K−1 at 168 K for the same field change. Thus melt-spinning process results in broadening of the peak in the isothermal magnetic entropy change versus temperature plot without compromising on the magnetocaloric effect.


Introduction
Melt-spinning is a non-equilibrium synthesis process and is generally used to stabilize metastable phases. Also this technique is known to yield highly crystalline rare earth based giant magnetocaloric materials with improved magnetic properties [1][2][3]. Laves phase intermetallic compounds RAl 2 (R=heavy rare earth) are considered as model systems to understand the role of various magnetic interactions including indirect Rudermann-Kittel-Kasuya-Yoshida (RKKY) exchange, crystalline electric fields, magnetoelastic interactions and magnetocrystalline anisotropy [4]. These compounds exhibit large magnetocaloric effect near their ferromagnetic ordering temperatures [5]. Recent studies on melt-spun rare earth intermetallic compounds such as RNi, RNi 2 , and RCu 2 [6][7][8] have revealed micro-granularity and preferred orientation while polycrystalline nature of the samples was preserved. Most-importantly, the melt-spun samples displayed comparable magnetocaloric effect with the corresponding arc-melted samples. Since the melt-spun samples are expected to show superior heat transfer properties over the arc-melted analogues, studies on magnetocaloric materials in melt-spun form assume importance. In the present work, the rare earth dialuminide GdAl 2 has been prepared by melt-spun technique and characterized. A nanostructured sample is obtained and it shows a slow paramagnetic to ferromagnetic transition. However, substantial isothermal magnetic entropy change values are obtained near the transition in melt-spun GdAl 2 and these are comparable to that of the arc-melted GdAl 2 .

Experimental details
Polycrystalline GdAl 2 has been prepared by arc-melting starting from stoichiometric amounts of pure elements [Gd (3N pure), Al (4N pure), Goodfellow, UK] under Ar atmosphere. The sample was remelted four times for better homogeneity. The mass loss after melting was less than 0.5%. The arc-melted ingot was used for Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. melt-spinning in the same inert atmosphere. The linear speed of the copper wheel used for melt-spinning is ∼17 m s −1 . The melt-spun samples were studied as-prepared, without any further heat treatment. The samples were characterized by powder x-ray diffraction (XRD) (Rigaku, CuK α radiation, λ=1.5406 Å), scanning electron microscopy and energy dispersive x-ray analysis (SEM-EDAX) (FEI Inspect) and high-resolution transmission electron microscopy (HRTEM) (Tecnai G2 T20). DC magnetization data have been collected using a commercial SQUID magnetometer (MPMS, Quantum Design) and a SQUID based vibrating sample magnetometer (MPMS 3, Quantum Design) in the temperature range of 5 K to 300 K in applied magnetic fields up to 70 kOe.

Results and discussion
Powder XRD data obtained at room temperature confirm the single phase formation (cubic, Fd-3m) and the peaks match well with those of the crystallographic information file # 19923 (figure 1). The broadening of XRD peaks suggests nanoparticle formation upon melt-spinning. The average crystallite size estimated using Scherrer formula is ∼48 nm. The EDAX results confirm the sample composition is indeed 1:2 and the SEM micrograph indicates the presence of nanograins ( figure 2(a)). The HRTEM image shows agglomerated nanoparticles which are polycrystalline in nature ( figure 2(b)).
Magnetization data obtained in the temperature range of 300 K to 5 K indicate progressive transformation from a paramagnetic to a ferromagnetically ordered state at around 136 K (T C ) ( figure 3(a)). It must be noted that the arc-melted, bulk GdAl 2 sample orders ferromagnetically at 167 K through a second order transition [5,9]. Although a small hysteresis is observed between the zero-field-cooled and field-cooled magnetization data measured in 100 Oe field, well below T C , there is no such irreversibility in the data obtained in 5 kOe field. The sluggish nature of the magnetic transition is attributed to the nanostructuring of the sample. Because the particle sizes could limit the long-range magnetic exchange interactions and also there is a non-negligible role played by disorder in the nanoparticle samples.
The paramagnetic susceptibility is fitted to the Curie-Weiss law. From the fit, the paramagnetic Curie temperature (θ p ) and the effective paramagnetic moment values are obtained as+149 K and 8 μ B /f.u. respectively. The magnetization versus field measured at 5 K does reveal the soft ferromagnetic nature of the sample ( figure 3(b)). The hysteresis is negligible and the saturation magnetization (M s ) value at 5 K is about 7.5 μ B /f.u. The M s value is comparable to that of the arc-melted sample. This indicates that the size induced disorder is negligible in the melt-spun sample and the broadening of magnetic transition may be due to the distribution of particle sizes. However, particle size distribution could not be ascertained because the particles observed in HRTEM images are agglomerated ( figure 2(b)). Broadening of the phase transition temperature due to a non-uniform size distribution of nanoparticles has been studied in detail, in order to optimize the performance of particulate matter in various applications [10,11]. The isothermal magnetic entropy change (ΔS m ) has been computed using the magnetization versus field data measured around T C ( figure 4(a)). For this, the following expression obtained from thermodynamic Maxwell relation has been used.