Research articles
Phase-transition-induced magneto-elastic coupling and negative thermal expansion in (Hf,Ta)Fe1.98 Laves phase

https://doi.org/10.1016/j.jmmm.2020.167236Get rights and content

Highlights

  • Tc reduced from 303 K at x = 0.125 to 247 K at x = 0.135 in Hf1-xTaxFe1.98 alloys.

  • The nature of phase transition evolved from second-order type to weak first-order type with more Ta substitution.

  • The maximum of magnetic entropy change increased with more Ta substitution.

  • The giant linear negative thermal expansion co-efficiency of −56 ppm/K realized in Hf0.865Ta0.135Fe1.98 alloy.

  • The saturation magnetostriction of 1700 ppm realized in Hf0.865Ta0.135Fe1.98 alloy near its Tc.

Abstract

The effects of Ta substitution on the phase transition have been studied in Hf1-xTaxFe1.98 (x = 0.125, 0.13, 0.135) alloys with kagome-type lattice. Increased Ta substitution leads into the reduced metamagnetic transition temperature (Tt). The nature of such phase transition is also changed from second-order type to the weak first-order one, suggested by the gradually observed hysteresis. The magnetic entropy change (ΔS) is meanwhile enhanced from 3.04 J kg−1 K−1 to 3.4 J kg−1 K−1. Such enhanced magnetocaloric effects are strongly correlated with the enhanced magneto-elastic effect, which is further confirmed by the maximized saturation magnetostriction (λs//) of ~1700 ppm near the Tt of Hf0.865Ta0.135Fe1.98 alloy. The concurrence of maximized ΔSmax and λs// suggests the strong magneto-elastic coupling during phase transition and the high linear co-efficiency of negative thermal expansion can be triggered in Fe-deficient Hf0.865Ta0.135Fe1.98 alloy.

Introduction

AFe2 Laves phase compounds, where A can be early transition elements and rare-earth elements, have been a subject of intensive experimental and theoretical investigations. The interesting relationship between the magnetism and the crystal structure has been found. The magnetic ordering is sensitively dependent on the elements at A site. In MgZn2-type hexagonal structure, ScFe2 and HfFe2 phase exhibit the ferromagnetism [1], [2]. TiFe2 [3] and TaFe2 [4] show the antiferromagnetism. But NbFe2 phase displays the ground state of spin-density wave [5], [6]. As comparison, when rare-earth elements occupy A site, the cubic structure (C15) is formed with interesting magnetism. Particularly, RFe2 (R = Tb, Dy) phase is well known as magnetostriction materials. Among them, HfFe2 system has attracted lots of attentions in the past decades. Its Curie temperature (Tc) is as high as 600 K [2], [7]. By chemical tuning with the other end member with different magnetic ground state, the critical point between ferromagnetic, antiferromagnetic and paramagnetic phases could be realized in recently reported (Hf, Nb)Fe2 [8] and well-studied (Hf,Ta)Fe2 systems. When Ta substitution amount, x in Hf1-xTaxFe2 alloy, is lower than 0.1, the FM ground state is still preserved. For 0.1 ≤ x ≤ 0.3, the first-order phase transformation is observed with increasing temperature without change in the crystal symmetry but significant change in volume [9], [10]. Based on such phase transformation, many investigations on the magnetic properties [11], [12], [13], [14], [15], electronic properties [16], [17], potential applications in giant magneto-resistance [9], [18] have been reported. Due to the interesting magnetic phase transition occurred in Hf1-xTaxFe2 (0.1 ≤ x ≤ 0.3) alloys, the magnetic configuration in FM state and AFM state has been the research topic for a long time and studied by using neutron powder diffraction and Mössbauer spectrum. Dujin et al proposed that all Fe moment carrying 1 μB should be aligned within in basal plane in the FM state [9] in contrary to the alignment along c-axis proposed by Nishihara et al. [12], [13], [14]. In pseudobinary Hf0.82Ta0.18Fe2 alloy, the alignment within basal plane of Fe moments at FM state is proposed by Mössbauer spectrum [19]. Additionally, during phase transition from FM to AFM, the spin flipping from in-plane to c-axis is deduced by Mössbauer spectrum [19].

But the in-plane triangle kagome-like alignment of Fe moments in the AFM state is proposed by neutron powder diffraction [20]. These results indicate that magnetic structure is still not concluded. Besides, the phase transition is also sensitive to the Fe composition. Herbst et al has studied the effects of Fe-rich and Fe-lean composition on the structure, magnetic and magnetocaloric properties of (Hf0.83Ta0.17)Fe2+x alloys. MgZn2-phase can be formed in a wide composition accompanied with the varied phase transition temperature and magnetic entropy changes [21]. The phase transition can be sharpened by Fe vacancies [22]. The effects of Co substitution for Fe in Hf0.8Ta0.2Fe1.98-xCox alloys have also been studied [23]. Recently, Li et al have reported a linear NTE coefficient of −16.3 ppm/K in a wide temperature range of 105 K in a stoichiometric (Hf,Ta)Fe2 system, due to the first-order nature of phase transition [24]. Therefore, in present work, we started from the Fe-deficient Hf1-xTaxFe1.98 (x = 0.125,0.13 and 0.135) system, the magnetic phase transition and the corresponding magnetocaloric effects by Ta substitution had been studied. Meanwhile, due to the first-order nature, the magnetostriction near the phase transition temperature was also studied.

Section snippets

Experiments

The Hf1-xTaxFe1.98 (x = 0.125,0.13 and 0.135) alloys used for present investigation was prepared by arc-melting appropriate raw materials with purity higher than 99.99% in the water-cooled copper crucible under a high purity Ar atmosphere. The sample weight loss after arc-melting was less than 1%. In order to make the alloy homogenization, the samples were melt 4 times. The ingots were then annealed at 1073 K for 4 days followed by the furnace cooling. The room-temperature X-ray powder

Results and discussions

Fig. 1a compares the refined XRD patterns of Hf1-xTaxFe1.98 (x = 0.125, 0.13 and 0.135) alloys. From the refinement, the main MgZn2-type Laves phase is achieved in all ingots. Besides, the additional impurity phase is detected and determined to be HfN [25]. According to the refinement, the volume ratio of HfN is ranged between 2.51% and 3.08%. The lattice parameters are also calculated and shown in Fig. 1b. Compared with those of HfFe2 phase, the lattice parameter a gradually shrinks within

Conclusions

By Ta substitution, the Tc is monotonously decreased from 303 K to 246 K in Hf1-xTaxFe1.98 (x = 0.125, 0.13 and 0.135) alloys. The nature of phase transition is also changed from second-order for x = 0.125 to the first-order for x = 0.13 and 0.135, which is evidenced by the thermal and magnetic hysteresis and further by Arrot plots. The magnetic entropy changes measured under 7 T are also enhanced from 3.04 J kg−1 K−1 to 3.4 J kg−1 K−1 with widened temperature span with increased Ta content. Such

Author statements

All authors are contributing to the work. S. Li performed the experiments of sample preparation and wrote the manuscript. J.C. Yang help the magnetic and magnetostriction measurement. The idea and feasibility of the present work were discussed by X.M. Fan, X.W. Yin, W.B. Cui and Q. Wang. And W.B. Cui and S. Li are responsible for the revised manuscript.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the major projects of National Natural Science Foundation of China (Grant No. 51690161, 51690162 and 51971056), Fundamental Research Funds for the Central Universities (Grant No. N2009001, N2009002, and N2002005), joint funding between Shenyang National Laboratory for Materials Science and State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals (Grant No. 18LHPY014), the open fundings of the State Key Laboratory of Solidification Processing in

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      Unusually, from the 1/χ vs. T plot curve, a small hump is observed at 49 K, which is easily overlooked on the M vs T curve, as shown in the inset of Fig. 6a. By applying 5 T, the peak temperature is elevated to 42 K, indicating the FM state below 22 K. However, the small hump is suppressed by 5 T. The similar phenomenon has been ever observed in (Hf, Ta)Fe2 phase, in which low-temperature FM state in (Hf1-xTax)Fe2 transforms into AFM state followed by the AFM-PM transition represented by a small hump in a heating process [29,30]. Such phase transition observed at 49 K is further confirmed by the temperature-dependent linear expansion as shown in Fig. 6c.

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