Structure and magnetostriction in (Tb0.2Pr0.8)xDy1-xFe1.93 Laves compounds synthesised by high-pressure annealing

Accepted Manuscript Title: Structure and magnetostriction in (Tb0 .2 Pr0 .8 )x Dy1−x Fe1 .93 Laves compounds synthesised by high-pressure Authors: G.B. Zhang, Y.D. Liu, C.X. Kan, Y.G. Shi, D.N. Shi PII: DOI: Reference:

S0025-5408(18)30189-2 https://doi.org/10.1016/j.materresbull.2018.12.025 MRB 10326

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MRB

Received date: Revised date: Accepted date:

18 January 2018 27 September 2018 18 December 2018

Please cite this article as: Zhang GB, Liu YD, Kan CX, Shi YG, Shi DN, Structure and magnetostriction in (Tb0 .2 Pr0 .8 )x Dy1−x Fe1 .93 Laves compounds synthesised by high-pressure, Materials Research Bulletin (2018), https://doi.org/10.1016/j.materresbull.2018.12.025 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Structure and magnetostriction in (Tb0.2Pr0.8)xDy1-xFe1.93 Laves compounds synthesised by highpressure

Department of Applied Physics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China 2 Key Laboratory for Intelligent Nano Materials and Devicesof the Ministry of Education, Nanjing 210016, China

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G.B. Zhang,1, 2 Y.D. Liu,1, 2 C.X. Kan,1, 2 Y.G. Shi,1, 2*, and D.N. Shi1, 2†

Electronic address: [email protected]

Electronic address: [email protected]

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Graphical abstract

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Highlights:  A series of new high-Pr content Laves compounds was synthesised.  Structural transition from tetragonal to rhombohedral was observed.

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 Large low-field magnetostriction and anisotropy compensation were realised.

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 Provides a route to synthesise and design a cheap magnetostrictive alloy.

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Abstract

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A series of (Tb0.2Pr0.8)xDy1-xFe1.93 ( 0.3 ≤ x ≤ 1.0 ) Laves compounds was prepared using

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a high-pressure annealing method. These compounds exhibit a multiphase behaviour when prepared by traditional annealing methods whereas they present a single Laves

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phase when synthesised by high-pressure annealing. The Curie temperature of the

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Laves compounds decreases with the increasing atomic ratio x. However, the saturation magnetisation does not increase monotonically but goes through a minimum with increasing x. A structural transition from tetragonal to rhombohedral

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was observed as x increased from 0.6 to 0.7. The magnetostriction decreased slightly with increasing x in the tetragonal phase region and then increased rapidly during the

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transition from tetragonal to rhombohedral phase at x = 0.7. Furthermore, large lowfield magnetostriction and low anisotropy were achieved simultaneously in the

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(Tb0.2Pr0.8)0.7Dy0.3Fe1.93 compound.

I. INTRODUCTION Magnetostrictive compounds are important functional materials because they can facilitate the conversion between electromagnetic and mechanical energies. Nowadays, 2 / 12

magnetostrictive compounds are widely used as acoustic transducers, actuators, or sensors. To achieve large low-field magnetostriction in RFe2 (where R represents a rare earth element) Laves compounds, anisotropy compensation systems with an RxR’1−xFe2 composition were proposed by mixing different RFe2 alloys with opposite signs of the first anisotropy constant K1, such as in the TbxDy1−xFe2 system [1, 2]. In the past, magnetostrictive alloys containing light rare earth Praseodymium have

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gathered more attention because PrFe2 possesses a very large theoretic magnetostriction constant of 5600 ppm at 0 K and mineral sources of Pr are much more abundant than

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those of Tb and Dy [3-7]. In addition, the signs of K1 for PrFe2 and DyFe2 are the same but anisotropy compensation between Pr3+ and Dy3+ ions is expected when the effects

of the different values of the anisotropy constant K2 are considered [8]. For example,

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Ren et al. observed magnetic anisotropy compensation in Tb0.2Dy0.8−xPrx(Fe0.9B0.1)1.93

Laves

compound

through

composition

anisotropy

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Dy0.05Pr0.95(Fe0.8Co0.2)1.9

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alloys [9]. Recently, Shi et al. reported large low-field magnetostriction in

compensation between Pr3+ and Dy3+ [10]. Therefore, we expect composition

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anisotropy compensation in a (Tb,Pr)xDy1−xFe1.93 system given the anisotropy

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compensation between Tb3+ and Dy3+ as well as between Pr3+ and Dy3+. Meanwhile, the cost of the raw materials will be reduced when light rare earth Pr is introduced into a

more

expensive

TbxDy1−xFe2

system.

To

this

end,

we

synthesised

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(Tb0.2Pr0.8)xDy1−xFe1.93 Laves alloys using a traditional annealing method. However, composition compensation could not be reached because the second phase appeared at

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x > 0.3 [11]. In the present work, (Tb0.2Pr0.8)xDy1−xFe1.93 single Laves compounds with composition 0.3 ≤ x ≤ 1.0 were synthesised by high-pressure annealing. The structural, magnetic, and magnetostrictive properties of these compounds were investigated.

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During the structural transition from tetragonal to rhombohedral, composition anisotropy compensation was obtained near a composition with x = 0.7 and a large lowfield magnetostriction was achieved.

II. EXPERIMENTAL PROCEDURE 3 / 12

(Tb0.2Pr0.8)xDy1−xFe1.93 ingots (x = 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and 1.0) were prepared by arc melting in an argon atmosphere. The ingots were melted several times to ensure homogeneity. An excess of approximately 5 wt.% of rare earth elements was added to compensate for the loss during arc-melting. The cast ingots (about 2g for each composition) were annealed at 1173 K for 45 min under a pressure of

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6 GPa. The details of the high-pressure annealing apparatus have been described

elsewhere [12]. Since the sample was annealed in a sealed assembly, the weight loss

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of the sample can be neglected during high-pressure annealing. After annealing, disk samples with a 10-mm diameter and 2-mm thick were produced. To check the phase structure, the samples were manually pulverized in an agate mortar and

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analyzed by X-ray powder diffraction (XRD) using the Cu Kα radiation [Rigaku

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D/Max-γA]. The magnetic properties were measured with a commercial vibrating

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sample magnetometer [VSM-175 Yingpu Magnetic Technology Development Co. Ltd]. The magnetostriction measurement were performed on the disk samples as

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synthesized. The strain gauge was bonded to the centre of the disk surface with

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gauge adhesive. The magnetostriction λ|| and λ ⊥ were measured by applying a magnetic field in the plane of the disk either parallel (λ||) or perpendicular (λ⊥) to

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the length of the gauge.

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III. RESULTS AND DISCUSSION

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Fig. 1 Typical XRD powder patterns for (Tb0.2Pr0.8)xDy1-xFe1.93 (a) annealed at 973 K for 7 days in a tube furnace filled with argon of 30 kPa and (b) annealed at 1173 K for 45 min

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under 6 GPa.

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Fig. 1(a) shows the XRD patterns for samples prepared by traditional annealing,

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namely at 973 K for 7 days in a tube filled with argon of 30 kPa. These samples exhibited a multiphase structure in agreement with our previous study [11]. The Laves

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phase showed that an MgCu2-type structure remained the major phase at x = 0.4 but was gradually replaced by a non-cubic phase with a PuNi3-type structure. The

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compound prepared by high-pressure annealing [Fig. 1(b)] presented a single cubic Laves phase with an MgCu2-type structure in contrast with the compounds prepared by

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the traditional annealing method in Fig. 1(a). The introduction of the larger radius Pr3+ into TbxDy1−xFe2 results in a much larger radius ratio between R and Fe than the ideal ratio of √3/2 [13] for a cubic Laves phase. Therefore, the amount of non-cubic phase

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increases with x when a traditional annealing method is used. In contrast, high-pressure annealing is more efficient to obtain a single Laves phase with a high-Pr content within a short annealing time. In addition, the diffraction peaks of the Laves phase considerably broadened after high-pressure annealing. The average crystallite size and strain were estimated by the Williamson-Hall method [14]. The average 5 / 12

crystallite size for the sample with x=0.4 prepared by ambient pressure, was about 1.5 μm and the strain was around 0.12%. In contrast, an average size of 90-120 nm and a strain of 0.15-0.25% were estimated for the samples prepared by highpressure annealing. The changes in the crystallite size and strain are clearly due

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to the high-pressure annealing process.

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Fig. 2 Normalised magnetisation as a function of temperature for different sample

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compositions, x. The inset shows the composition dependence of the Curie temperature, TC.

The temperature-dependence of the magnetisation was recorded at a field of 2 kOe

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to determine the Curie temperature (TC) of the Laves phase. For comparison, the normalised magnetisation as a function of temperature for samples with different values of x is shown in Fig. 2. When the temperature decreases, a paramagnetic to

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ferromagnetic (PM-FM) transition was observed for each curve, corresponding to the TC of the Laves phase. Besides the PM-FM transition of the Laves phase, no other magnetic transition could be observed on the M-T curves. This confirms that magnetic impurities, such as an RFe3 or R2Fe17 phase, were not present in the samples synthesized under high-pressure. The value of TC as a function of x for the Laves phase from the M6 / 12

T curves is shown in the inset to Fig. 2. The TC of the (Tb0.2Pr0.8)xDy1−xFe1.93 Laves phase compounds decreased with increasing x. Here, the decrease in the Curie

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temperature is attributed to the weakened 3d-4f coupling between Pr and Fe [15].

FIG. 3 (a) Initial magnetisation curves at room temperature. (b) Saturation magnetisation Ms

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as a function of composition, x.

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The initial magnetisation curves measured at room temperature are shown in

Fig. 3(a). The corresponding saturation magnetisation Ms determined from the magnetisation curves is shown in Fig. 3(b). The value of Ms does not increase

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monotonically but first decreases to a minimum at x = 0.6 and then increases with x, which contrasts from the linear increase of Ms in the TbxDy1−xFe2 system [3]. This is because the magnetic moment of Tb/Dy is coupled in an antiparallel manner with that of Fe while Pr is coupled in a parallel manner. Furthermore, it can be deduced from Fig. 3(b) that the magnetisation compensation point in this system should be near x = 7 / 12

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0.6.

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Fig. 4 Typical step-scanned 440C XRD profiles of the (Tb0.2Pr0.8)xDy1-xFe1.93 Laves phase and

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its dependence with composition, x.

The spontaneous magnetostriction in Laves compounds can lead to different types of

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finely distorted structures. Below TC, the cubic Laves structure will transform into a tetragonal structure when the easy magnetisation direction (EMD) is along the < 100 >

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axis and into a rhombohedral structure when the EMD is along the < 111 > axis, as shown in a recent synchrotron XRD study [16, 17]. In fact, previous investigations indicate that these fine structures can also be determined by analysing the 440C

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reflection peak in conventional XRD patterns [18, 19]. Fig. 4 shows the step-scanned 440C XRD profiles of samples with x = 0.6, 0.7, 0.9, and 1.0, where Kα2 was removed. These peaks are slightly shifted to lower Bragg angles with increasing x because of the larger radius of Pr and Tb compared to Dy. The splitting of the peak can be seen for samples with 0.7 ≤ x ≤ 1.0, indicating that these compounds are rhombohedral with an EMD along the < 111 > axis. When x decreases further to 0.6, the splitting of the peak 8 / 12

disappeared indicating that the structure was transformed into a tetragonal form with an EMD along the < 100 > axis. Therefore, we expect that the composition anisotropy

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compensation point of the systems studied should be around x = 0.7 at room temperature.

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Fig. 5. Spontaneous magnetostriction λ111 as a function of the composition, x.

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The spontaneous magnetostriction λ111 was calculated by splitting the 440C peaks

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using the following equation [20]:

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 111  2

Where d404 and

d

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d 404  d 40 4 d 404  d 40 4

(1)

indicate the crystallographic plane distances with the pseudo-

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cubic indices hkl. λ111 calculated as a function of x is shown in Fig. 5. λ111 monotonically increases from about 1160 ppm at x = 0.7 to about 1650 ppm at x = 1.0 because of the larger spontaneous magnetostriction of TbFe2 and PrFe2 compared to that of DyFe2.

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Fig. 6 Room-temperature magnetostriction λ∥-λ⊥as a function of (a) the applied field and (b)

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the composition.

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The measured magnetostriction, λ ∥ -λ ⊥ , for (Tb0.2Pr0.8)xDy1−xFe1.93 compounds is

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shown in Fig. 6(a). The values of λ∥-λ⊥ for samples with 0.3 ≤ x ≤ 0.6 are small and it

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was difficult to reach saturation, corresponding to the magnetostrictive behaviour of compounds with a tetragonal structure. For samples with x ≥ 0.8 with a rhombohedral

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structure and an EMD along < 111 >, λ∥-λ⊥ is large but it is still difficult to achieve saturation. At the anisotropy compensation composition (x = 0.7), the low-field

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magnetostriction is large and it tends to reach saturation. For clarity, Fig. 6(b) presents a plot of λ∥-λ⊥ versus x at magnetic field strengths ranging from 2 to 8 kOe.

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The magnetostriction decreases slightly when x increases from 0.3 to 0.6, which is attributed to the lower Ms value for the rare earth sub-lattice. Interestingly, the magnetostriction jumps from 300 ppm for the sample with x = 0.6 to 700 ppm at x =

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0.7. Therefore, large low-field magnetostriction and anisotropy compensation were simultaneously realised near the phase boundary with the structural transition from tetragonal to rhombohedral at a composition with x = 0.7. The improved magnetostriction comes from the flattened free energy profile and easier magnetisation rotation near the phase boundary [21].

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IV. CONCLUSION A series of (Tb0.2Pr0.8)xDy1-xFe1.93 (0.3 ≤ x ≤ 1.0 ) Laves compounds with high-Pr content was successfully synthesised. The structural, magnetic, and magnetostrictive properties of these compounds were investigated. Due to the competition of the sublattice magnetisation, the saturation magnetisation first decreases to a minimum at x = 0.6 and then increases with x. The structural and EMD transitions from tetragonal

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(EMD along < 100 >) to rhombohedral (EMD along < 111 >) were observed when x

increases from 0.6 to 0.7. A large spontaneous magnetostriction of approximately 1160

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ppm was obtained for the (Tb0.2Pr0.8)0.7Dy0.3Fe1.93 compound. In addition, this compound is near the anisotropy composition point where a large magnetostrictions λ||λ⊥ was measured at low field intensities. The present work provides an effective route

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to the design and synthesis of promising practical magnetostrictive materials with

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inexpensive Pr.

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References:

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[1] A. E. Clark, R. Abbundi and W. R. Gillmor, IEEE Trans. Magn. 14 (1978) 542. [2] A. E. Clark, in Ferromagnetic Materials, edited by E. P. Wohlfarth (North-Holland, Amsterdam,1980), Vol. 1, pp. 531-589. B.

(1999) 2805

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[3] B.W. Wang, Z. J. Guo, Z. D. Zhang, X. G. Zhao, S. C. Busbridge, J. Appl. Phys. 85

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[4] J. J. Liu, W.J. Ren, J. Li, X. G. Zhao, W. Liu, Z. D. Zhang, Appl. Phys. Lett. 87 (2005) 082506.

[5] Y. G. Shi, S. L. Tang, R. L. Wang, H. L. Su, Z. D. Han, L. Y. Lv, Y. W. Du, Appl.

A

Phys. Lett. 89 (2006) 202503. [6] Y.G. Shi, C.C. Hu, J.Y. Fan, D.N. Shi, L.Y. Lv, S.L. Tang, Appl. Phys. Lett. 101 (2012) 192405 [7] Z.B. Pan, J.J. Liu, P.Z. Si, W.J. Ren, Mater. Res. Bull. 77 (2016) 122-125. [8] W. J. Ren, Z. D. Zhang, X. P. Song, X. G. Zhao, X. M. Jin, Appl. Phys. Lett. 82 11 / 12

(2003) 2664. [9] W. J. Ren, J. J. Liu, D. Li, W. Liu, X. G. Zhao, and Z. D. Zhang, Appl. Phys. Lett. 89 (2006)122506. [10] Y.G. Shi, Z.Y. Chen, L. Wang, C.C. Hu, Q. Pan, D.N. Shi, AIP Advances 6 (2016) 056207.

31 (2018) 2217–2220

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[11] G.B. Zhang, W.G. Zheng, Y. Cui, Y.G. Shi, D.N. Shi, J Supercond. Nov. Magn.

[12] Y.G. Shi, S.L. Tang, L.Y. Lv, J.Y. Fan, J. Alloys. Compd. 506 (2010) 533-536.

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[13] D. Thoma and J. Perepezko, J. Alloys. Compd. 224 (1995) 330-341. [14] G.K. Williamson and W.H. Hall Acta Metall. 1 (1953) 22-31. [15] K. H. J. Buschow, Rep. Prog. Phys. 40 (1977) 1192-1256.

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[16] S. Yang, H.X. Bao, C. Zhou, Y. Wang, X.B. Ren, Y. Matsushita, Y. Katsuya, M.

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Tanaka, K. Kobayashi, X.P. Song, J.R. Gao, Phys. Rev. Lett. 104 (2010) 197201.

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[17] R. Bergstrom, M. Wuttig, J. Cullen, P. Zavalij, R. Briber, C. Dennis, V.O. Garlea,

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M. Laver, Phys. Rev. Lett. 111 (2013) 017203.

[18] A. E. Dwight and C. W. Kimball, Acta Crystallogr., Sect. B: Struct. Crystallogr.

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Cryst. Chem. 30, (1974) 2791-2793.

[19] E. Gratz, A. Lindbaum, A. S. Markosyan, H. Mueller, and A. Y. Sokolov, J. Phys.:

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Condens. Matter 6, (1994) 6699-6709. [20] W. J. Ren, Z. D. Zhang, A. S. Markysan, X. G. Zhao, X. M. Jin, and X. P.

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Song, J. Phys. D 34 (2001) 3024-3027. [21] C.C. Hu, Y.G. Shi, D.N. Shi, S.L. Tang, J.Y. Fan, Y.W. Du, J. Appl. Phys. 113

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(2013) 203906.

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