Magnetocaloric effect in textured rare earth intermetallic compound ErNi

A non-equilibrium rapid solidification technique, namely, melt-spinning, has been used to obtain a highly crystalline giant magnetocaloric material La(Fe, Si)₁₃ with superior magnetic and magnetocaloric properties.¹ This has triggered a series of studies on melt-spun, crystalline intermetallic compounds and alloys.^2,3 The equiatomic rare earth intermetallic compounds RNi (R = Gd, Tb, Dy, Ho and Er) are known to exhibit interesting structural, magnetic and transport properties.^4,5 The RNi compounds with light rare earths (i.e. R = La to Gd) crystallize in orthorhombic CrB-type structure (Space group Cmcm) while those with heavy rare earths such as Dy, Ho and Er stabilize in orthorhombic FeB-type structure (Space group Pnma). The compound TbNi is dimorphic. Interesting magnetoelastic behaviour such as giant linear magnetostriction with negligible volume magnetostriction has been reported for ferromagnetic GdNi.⁶ A large magnetocaloric effect is observed in RNi (R = Gd, Tb, Dy, Ho and Er) close to the ferromagnetic ordering temperatures. In particular, ErNi orders ferromagnetically around 10 K⁴ and hence is suitable for low temperature cooling applications. In fact, polycrystalline ErNi has been considered as a potential regenerator material for a Gifford-McMahon refrigerator at temperatures less than ∼15 K because the specific heat of ErNi is larger than that of the conventional regenerator materials such as Er₃Ni and lead.⁷ Recently, we have studied the magnetism and magnetocaloric effect of a few RNi (R = Gd, Tb, Dy and Ho) compounds prepared by melt-spinning.^8,9 In this work, the magnetic and magnetocaloric properties of melt-spun ErNi have been studied. While the melt-spun ErNi is found to be textured, the magnetic ordering temperature and the magnetocaloric behaviour is almost the same as that of the arc-melted sample. Thus melt-spinning is not only a faster, energy saving synthesis technique but also yields highly crystalline specimens. These melt-spun ribbon shaped specimens have larger surface area which may help communicate the heat/cold more efficiently than the arc-melted buttons.

Polycrystalline ErNi has been synthesized by (i) arc melting stoichiometric amounts of high pure Er and Ni elements in argon atmosphere, followed by annealing at 1173 K for 12 h in an evacuated quartz tube and by (ii) melt-spinning in argon atmosphere, starting from the molten liquid of the arc-melted ErNi, using a single roller cold copper wheel rotating at a constant speed of 34 m/sec. The melt-spun materials were studied as-prepared. The samples were characterized by room temperature powder X-ray diffraction (XRD). The microstructure was studied by scanning electron microscopy (SEM) and the sample composition was checked using energy dispersive analysis of X-rays (EDAX). DC magnetization studies were performed using commercial magnetometers (MPMS XL SQUID and PPMS-VSM, Quantum Design) in magnetic fields up to 70 kOe/140 kOe at temperatures between 2 to 300 K.

Room temperature X-ray diffraction experiments confirm that both the arc-melted and melt-spun ErNi samples are in single phase, crystallizing in orthorhombic FeB-type structure (Space group Pnma, no. 62). Comparing relative intensity of the Bragg reflections in the XRD data obtained for the wheel-side and free surfaces of the melt-spun sample with those of the powdered arc-melted and melt-spun samples, one can observe the presence of texture and preferred crystal orientation in the melt-spun ErNi [Fig. 1]. The XRD data obtained from the free surface of the melt-spun ErNi show large intensity enhancement for (1 0 2) Bragg reflection. The lattice parameters and other relevant structural details are provided in Table I.

The scanning electron microscopy image of the free surface of the melt-spun ErNi depicts a granular microstructure with grains of average size of ∼1 μm. The micrograph of the cross-section of the melt-spun ErNi sample indicates a columnar grain growth along the thickness (∼32 μm) of the ribbon. EDAX analysis confirmed the 1:1 nominal composition of the constituent elements Er and Ni.

Temperature (T) dependent magnetization (M) measurements performed in applied magnetic field of 5 kOe indicate that arc-melted and melt-spun ErNi compounds order ferromagnetically at 11 K and 10 K (T_C) respectively [Fig. 2]. The T_C value matches with that reported earlier for ErNi.⁵ The difference between the low field (100 Oe) zero-field-cooled (ZFC) and field-cooled (FC) magnetization below T_C for ErNi signifies the magnetic anisotropy inherent in the sample. The dM/dT vs T plot of ErNi shows inflection points around T_C (which is ∼10 K) and also at spin-reorientation transition temperature (T_SR) of ∼ 7 K.

The reciprocal magnetic susceptibility is found to be linear with temperature in the paramagnetic region. From the fit to the Curie-Weiss law, the paramagnetic Curie temperature (θp) and effective paramagnetic moment (μ_eff) values of ∼ 8 K and 9.8 μ_B/F.U. are obtained. The positive value of θp suggests predominant ferromagnetic interactions in the material. The low field data of melt-spun sample evidences development of some short-range order just above T_C and this is in concordance with the enhanced θp value of the melt-spun sample.

Field dependent magnetization (M-H) at 2 K shows saturation behaviour [Fig. 3]. Very little hysteresis and coercivity has been observed in these samples. The saturation magnetization value is 7.2 μ_B/f.u. for the arc-melted ErNi and 7.4 μ_B/f.u. for the melt-spun ErNi. These are somewhat less than the theoretical gJ value expected for Er³⁺ ion. This reduction could be because of the non-collinear magnetic structure of ErNi and crystalline electric fields (CEF). Earlier studies have shown that among RNi compounds, GdNi is a collinear ferromagnet whereas DyNi, HoNi and ErNi are non-collinear ferromagnets. The complex non-collinear magnetic structure is thought to be a result of competition between magnetic exchange interaction and crystalline electric fields.

The magnetocaloric effect is estimated using the field dependent magnetization data measured around T_C and isothermal magnetic entropy change (ΔS_m) close to T_C has been calculated from the M-H data [Fig. 4, Table II]. Just as the temperature derivative of magnetization attains negative maximum near T_C, ΔS_m also reaches negative maximum around this temperature. The maximum isothermal magnetic entropy change, -ΔS_m^max, is ∼27 J/kg K and ∼24 J/kg K for the arc-melted and melt-spun ErNi for 50 kOe field change, near T_C. The small reduction in the ΔS_m^max value of the melt-spun ErNi sample could be attributed to the anisotropy induced due to the texture developed in the sample. The relative cooling power (RCP) is the heat energy transferred by unit mass of the substance from cold sink to the hot reservoir during one cycle of refrigeration and it is calculated as a product of maximum ΔS_m magnitude and full width at half maximum of the ΔS_m vs T curve. These RCP values are ∼440 J/kg and ∼432 J/kg respectively for the arc-melted and the melt-spun ErNi samples. Although a part of ΔS_m is lost to CEF effects, the magnetocaloric effect is substantially large at 10 K and melt-spun ErNi could be useful in low temperature magnetic refrigeration such as helium gas liquefaction. The advantage of the melt-spun ErNi over the bulk, arc-melted ErNi will be its granularity and enhanced surface area which might facilitate better thermal conduction during the heating/cooling cycles. In addition, we note that melt-spinning under same experimental conditions results in amplification of intensity of different Bragg reflection for different heavy rare earth i.e. (0 4 0) for CrB-type orthorhombic GdNi, (3 0 2) for TbNi (orthorhombic, Pnma), (0 2 0) for FeB-type orthorhombic DyNi and HoNi and (1 0 2) for ErNi. Underlying competing exchange and crystal field interactions in these systems and hence the magnetic structure may have a role in establishing such preferred orientation directions in the melt-spun samples. The T_C of the melt-spun TbNi and ErNi remain almost the same as their arc-melted analogues whereas the T_C of GdNi gets enhanced when drawn in melt-spun form. The T_C of the melt-spun DyNi and HoNi decrease compared to that of the corresponding arc-melted samples. A systematic study of magnetism of fine-grained, crystalline melt-spun rare earth intermetallic compounds should lead us to a method of engineering materials with desired magnetocaloric properties.

Melt-spun rare earth intermetallic compound ErNi orders ferromagnetically at 10 K. This sample has the same structure as that of the arc-melted sample, however, is textured. The preferred orientational growth on the ribbon surface and the columnar grain growth along the thickness are the signatures of the presence of texture in the melt-spun ErNi. The melt-spun specimen shows almost the same magnetocaloric effect as that of the arc-melted sample and could be useful for low temperature cooling applications.

R. N. thanks DST-RFBR for a project support. A. S and R. N thank SAIF, IITMadras for the SEM-EDAX facility and institute Squid-VSM facility. A. S thanks Rajivgandhi for the help during vacuum-sealing and XRD experiments and Srinivas for the help during a few magnetization experiments. SKM acknowledges partial financial support from MEC-UFRN (Brazil).