Magnetocaloric effect in melt-spun rare earth intermetallic compound ErAl₂

The quest to identify suitable materials for magnetic cooling applications over a wide temperature range has triggered numerous studies on rare earth intermetallic compounds and alloys.^1–3 In this context, dimensional effects on magnetic and magnetocaloric properties of materials are being understood.⁴ It is known that a non-equilibrium technique such as melt-spinning could be used to stabilize metastable phases as well as to synthesize highly crystalline phases. Melt-spinning led to production of rare earth based giant magnetocaloric materials with improved magnetic properties.^5–7 Cubic, Laves phase intermetallic compounds RAl₂ (R = heavy rare earth) have been widely studied for their magnetocaloric properties and these materials indeed exhibit large magnetocaloric effect near their ferromagnetic ordering temperature.^8,9 In fact, RAl₂ compounds are considered as prototypical systems to understand the role of various magnetic interactions namely, indirect Rudermann-Kittel-Kasuya-Yoshida (RKKY) exchange, crystalline electric field, magnetoelastic interaction and magnetocrystalline anisotropy on magnetic and transport properties.¹⁰ Magnetic and magnetocaloric properties of several melt-spun rare earth intermetallic compounds have been reported in the recent past.^11–13 Often melt-spinning process leads to synthesis of polycrystalline samples with micron-sized or nano-sized grains and texture. The melt-spun samples also exhibit reasonable magnetocaloric effect when compared to that in the corresponding arc-melted samples. Melt-spun nanostructured GdAl₂ compound (of ∼48 nm crystallite size) undergoes a broad paramagnetic to ferromagnetic transition around 136 K with equivalent magnetocaloric effect as the arc-melted GdAl₂.¹⁴ In the present work, another member of RAl₂ family namely ErAl₂ has been prepared by melt-spinning. Melt-spun ErAl₂ has relatively larger grains and therefore the ferromagnetic transition temperature (T_C ∼10 K) remains almost the same as the arc-melted ErAl₂ compound (T_C ∼14 K). The isothermal magnetic entropy change near T_C is found to be moderate.

Laves phase intermetallic compound ErAl₂ has been prepared by melt-spinning under argon atmosphere. The starting material has been prepared by arc-melting stoichiometric amounts of pure elements [Er (3N pure), Al (4N pure), Goodfellow, UK] under inert argon gas atmosphere. To improve homogeneity, the sample was remelted a few times. The mass loss after remelting is less than 0.5%. The spinning copper wheel used for melt-spinning was rotating at a linear speed of about 17 m/s. The melt-spun sample was studied in as-spun state without any further heat treatment. Powder X-ray diffraction (XRD) (Rigaku, CuK_α radiation, λ=1.5406 Å) has been employed to characterize the samples. The lattice parameters were calculated from Rietveld refinement of the powder XRD data using the Rietan-program in the isotropic approximation at room temperature.¹⁵ Transmission electron microscopy (Technai G2 T20) has been used to further characterize the melt-spun ErAl₂ sample. DC magnetization data have been collected using commercial magnetometers (MPMS SQUID and a SQUID based vibrating sample magnetometer, MPMS 3, Quantum Design) in the temperature range of 2 K to 300 K in applied magnetic fields up to 70 kOe on both arc-melted and melt-spun samples. In the manuscript, magnetic field (H) values are given in CGS units. 1 Oe = 10³/4π A/m and 1 Oe corresponds to the strength of magnetic field of 10⁻⁴ T. Magnetization is given in emu/g and Bohr magneton (μ_B) units (1 emu/g = 1 Am²/kg and 1 μ_B = 9.274009 × 10⁻²⁴ JT⁻¹).

Room temperature powder X-ray diffraction data of arc-melted and melt-spun ErAl₂ confirm the formation of cubic structure (Space group Fd-3m) [Fig. 1]. The lattice parameters and the unit cell information are provided in Table I. The melt-spun process leads to minor modification of the lattice with a decrease in lattice parameter (a) and unit cell volume (V). The Bragg peaks in the powder XRD data of melt-spun ErAl₂ are slightly broadened. Using Scherrer formula, the average crystallite size is found to be about 54 nm. The energy dispersive X-ray analysis confirms the sample composition. The transmission electron microscopy image reveals the presence of agglomerated nanoparticles of size ∼ 70 nm [Inset in Fig. 1a].

DC magnetization measured in applied field of 5 kOe in the temperature range of 300 K to 5 K indicate a paramagnetic to ferromagnetic transition at 10 K (T_C) and 14 K respectively for the melt-spun and arc-melted samples [Fig. 2a-b]. Arc-melted ErAl₂ is known to order ferromagnetically around this temperature through a second order transition.¹⁶ The zero-field-cooled and field-cooled magnetization data overlap with each other in both the samples. Since the melt-spun sample has relatively larger grains, the nature of the transition and the ordering temperature are only marginally modified. The paramagnetic susceptibility follows Curie-Weiss law. The fit parameters yield paramagnetic Curie temperature (θ_p) and the effective paramagnetic moment values as +16 K and 9.4 μ_B/f.u. respectively for melt-spun ErAl₂. For the arc-melted ErAl₂ compound, these values are +18 K and 10 μ_B/f.u. respectively. Positive θ_p confirms the presence of dominant ferromagnetic interactions in the sample. Isothermal magnetization measured at 5 K shows a tendency towards saturation with negligible hysteresis as expected for a soft ferromagnet [Fig. 3a-b]. The magnetization value at 5 K in 70 kOe field is about 7 and 7.9 μ_B/f.u. respectively for the melt-spun and arc-melted ErAl₂ samples. Since the melt-spinning process has not reduced the crystallite size much, the magnetic properties are comparable to that of bulk ErAl₂ phase.

Using the magnetization vs field data obtained in the vicinity of ferromagnetic transition, the isothermal magnetic entropy change (ΔS_m) is calculated as a function of temperature for both melt-spun and arc-melted ErAl₂ samples [Figs. 4a and 5a]. From thermodynamic Maxwell relation one obtains the following expression:

Here, H_i and H_f are the initial and final values of applied magnetic field and μ₀ is the permeability of free space.¹⁷ The maximum value of ∆S_m is ∼ −34 Jkg⁻¹K⁻¹ for 70 kOe field change at 14 K for the melt-spun ErAl₂ [Fig. 4b]. This value is little less than −42 Jkg⁻¹K⁻¹ obtained for the arc-melted ErAl₂ compound at 14 K for the same field change of 70 kOe [Fig. 5b].

The maximum isothermal magnetic entropy change values for a given field change (ΔH) is fitted to a power law i.e. ΔS_m^max ∝ ΔHⁿ [Fig. 6a-b]. The value of ‘n’ is found to be 0.65 and 0.63 respectively for the melt-spun and arc-melted ErAl₂ samples at 14 K. This is close to the value expected for a mean-field ferromagnet (n = 2/3).¹⁸ The exponent does not vary significantly from the mean-field value indicating that role of micro-graining is minimal in melt-spun ErAl₂.

In literature, mechanically milled, nanograined GdAl₂ is known to show a disorder broadened magnetic entropy change spanning over a broad range of temperatures.^19–21 However, such disorder effects are not seen in the melt-spun RAl₂ samples where one has submicron size particles. The magnetic and magnetocaloric properties of the melt-spun ErAl₂ sample are comparable to that in the bulk ErAl₂ sample. It will be interesting to study the heat transfer properties of such melt-spun samples.

Crystalline ErAl₂ (cubic, Fd-3m) compound has been prepared by melt-spinning method. Melt-spun ErAl₂ has submicron size particles and it orders ferromagnetically around 10 K and this value is close to the ordering temperature (14 K) of the corresponding arc-melted bulk sample. Using magnetization vs field isotherms, magnetocaloric effect has been estimated. The magnetic field dependence of ∆S_m value indicates that the melt-spun sample is a mean-field ferromagnet.

Authors thank Ganesh Jangam, TIFR for the help during magnetic measurements.

The authors have no conflicts to disclose.

The data that support the findings of this study are available within the article.