Spin configuration, magnetic and magnetostrictive properties of Tb_0.27Dy_0.73-xNd_xFe₂ compounds

Highlights

• Tb_0.27Dy_0.73-xNd_xFe₂ Laves phase alloys were prepared by high pressure annealing.

• EMD lying along <111> axis accompanied by a rhombohedral distortion.

• Spin reorientation transition was explained by two sub-lattice model.

• Spin configuration phase diagram was designed.

• Low-field large magnetostriction was found at Tb_0.27Dy_0.63Nd_0.1Fe₂.

Abstract

The polycrystalline Tb_0.27Dy_0.73-xNd_xFe₂ Laves phase compounds were prepared at equilibrium conditions and consequent annealing method at high pressure. Their structural, spin configuration, magnetic and magnetostrictive properties were studied. Rietveld refinement analysis of X-ray diffraction (XRD) reveals that the studied compounds possess predominantly the MgCu₂-type cubic Laves phase, coexisting with a small amount of hcp-type rare-earth (RE)-rich phase and PuNi₃-type REFe₃ phase. The magnetization (M_S) and Curie temperature (T_C) decrease while the lattice parameter increases with the increase in Nd concentration. The direction of easy magnetization (EMD) accompanied by a rhombohedral distortion (λ₁₁₁) was observed along [111] axis at room temperature (RT). The spin reorientation transition (SRT) occurs at low temperature, which was explained by the two-sublattice model. Based on the experimental results, a detailed phase diagram has been designed for the Tb_0.27Dy_0.73-xNd_xFe₂ Laves phase to demonstrate the different spin configurations and crystal structure. The anisotropy compensation between the Nd and Dy sublattices was realized at x = 0.1. The Nd-containing Tb_0.27Dy_0.63Nd_0.1Fe₂ Laves phase compound shows large low-field polycrystalline magnetostriction (λ_S) and large spontaneous magnetostriction coefficient (λ₁₁₁~1.7 × 10⁻³), which may make it a good magnetostrictive material. Our results indicate that the substitution of Nd by Dy in Tb_0.27Dy_0.73Fe₂ compounds is beneficial for the improvement of magnetostrictive properties. The present study might be useful to design Nd-based magnetostrictive materials for technological applications.

Introduction

The rare-earth (RE) based REFe₂ Laves phase compounds have attracted constant interest owing to their exotic magnetic and magnetostrictive properties [[1], [2], [3]], and are considered as suitable candidates for potential applications in permanent magnets, magnetomechanical transducers, sensors, and vibration-control systems, etc. [[4], [5], [6]]. However, they also possess large magnetocrystalline anisotropy at room temperature (RT), which requires a large magnetic field to saturate the magnetization and magnetostriction, and consequently limited their applications [[7], [8], [9]]. Therefore, it is necessary to minimize the anisotropy to exploit the large magnetostriction for practical applications. For this purpose, pseudo-binary alloys like Tb_0.27Dy_0.73Fe₂, Tb_0.15Ho_0.85Fe₂, were designed by the composition anisotropy compensation between Tb (RE) and Dy, Ho (RE^/), where the both of RE and RE^/have same magnetostriction sign but opposite signs of first anisotropy constant (K₁) [10,11]. The renowned Tb_0.27Dy_0.73Fe₂ and Tb_0.15Ho_0.85Fe₂ alloys possess large magnetostriction at room temperature accompanied by low magnetocrystalline anisotropy and are now being used in transducers and actuators. These compounds have one degree of freedom, which permits only the minimization of the K₁. Recently, it has shown that the magnetostrictive properties of these compounds can be improved by further introducing a third light RE element (e.g., Nd or Pr) [[12], [13], [14]]. The basic principle is that the addition of the third element will provide an additional degree of freedom, which minimizes the first-order anisotropy constants (K₁) as well as the second-order anisotropy constants (K₂), and subsequently improve the magnetostrictive properties. Following this approach, Cui and coworker achieved large magnetostriction at a low field in Nd-substituted Tb_0.15Ho_0.85-xNd_xFe₂, which is 50% higher than that of Nd free compound [15]. Recently, Hari et al., also reported low-field large magnetostriction in Tb_0.1Ho_0.75Pr_0.15 (Fe_0·9B_0.1)₂ compounds [14]. Moreover, both the Nd, Pr are cheaper as compare to heavy RE (Tb, Dy); therefore, the addition of Nd or Pr will definitely reduce the cost of the materials, which is beneficial from the economic point of view. Based on these results, we expected that the introduction of Nd, in the Tb_0.27Dy_0.73-xNd_xFe₂ Laves phase system would not only reduces the cost of the material but also improves the magnetostrictive properties effectively. However, Nd-based Laves phase compounds cannot be synthesized easily by ambient pressure method [16], because of the size effect of the Laves phase. Due to the larger atomic radius of Nd; the radius ratio of (Tb, Dy, Nd): Fe ion is not consistent with the ideal ratio (i.e., 1.225) for the Laves compounds [17]. Recently, Shi et. at synthesized the Nd-based Tb_xNd_1-xFe₂ Laves phase system by a high-pressure annealing method [18]. In the present work, a series of polycrystalline Tb_0.27Dy_0.73-xNd_xFe₂ (0 ≤ x ≤ 0.4) compounds were fabricated by this method and their structure, magnetic and magnetostrictive properties were investigated. The main aim of the present work is to investigate whether anisotropy compensation can be realized in this system and find the composition with improved magnetoelastic properties for practical applications. Based on the experimental results of temperature dependent of magnetization, susceptibility and XRD analysis, a phase diagram of Tb_0.27Dy_0.73-xNd_xFe₂ was designed for the first time to demonstrate the spin configuration and crystal structure. The spin reorientation transition (SRT) observed at low temperature has been explained by the two-sublattice model [19]. As expected, the anisotropy compensation was realized in this system at Tb_0.27Dy_0.63Nd_0.1Fe_1.93, which shows large low-field magnetostriction (λ_S) and large magnetostriction coefficient λ₁₁₁. Our work indicates that the introduction of the Nd at relatively low concentrations improves the magnetostrictive properties. Therefore, the light rare earth Nd can be considered as a low-cost alternative for the heavy RE elements to design Nd-based magnetostrictive materials. The results discussed in this paper were obtained from polycrystalline samples, which have an averaging effect from different grains; however, based on the same mechanism, much better results can be achieved in the single-crystal or oriented samples.