Magnetic ordering of Sm26Co11Ga6-type R26Co9-6Ga8-11 compounds (R = Gd–Ho, Tm)

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Highlights

  • Gd26Co9Ga8 is the soft ferromagnet down to 2 ​K with the Curie temperature of 124 ​K.

  • Tb26Co6Ga11, Tb26Co8Ga9, Dy26Co9Ga8 and Ho26Co9Ga8 show mixed F-AF ordering at 76-16 ​K.

  • They exhibit permanent magnet properties at low temperatures.

  • Novel Tm26Co9Ga8 shows dominant AF ordering at 6 ​K and it is a soft ferromagnet at 2 ​K.

  • The ΔSm of Tb26Co8Ga9 reaches value of −9.2 ​J/kg⋅K at 76–81 ​K in field change of 50 ​kOe.

Abstract

The magnetic ordering of polycrystalline Sm26Co11Ga6-type {Gd, Dy, Ho, Tm}26Co9Ga8 and Tb26Co6-8Ga11-9 (space group P4/mbm, N 127, tP86) has been established using magnetic measurements. Gd26Co9Ga8 exhibits soft ferromagnetic with the Curie temperature of 124 ​K Tb26Co6Ga11, Tb26Co8Ga9, Dy26Co9Ga8 and Ho26Co9Ga8 show mixed ferro-antiferromagnic ordering with the Curie temperatures of 76 ​K, 74 ​K, 28 ​K and 16 ​K, respectively. They display permanent magnet properties at low temperatures. The novel Tm26Co9Ga8 demonstrates dominant antiferromagnetic ordering at 6 ​K, while it is a soft ferromagnet at 2 ​K.

Among these compounds, Tb26Co8Ga9 exhibits largest magnetocaloric effect with magnetic entropy change of −9.2 ​J/kg⋅K at 76–81 ​K and relative large cooling power of ~560 ​J/kg (field change of 50 ​kOe), while Tb26Co6Ga11 is the best permanent magnet with a coercivity of 9 ​kOe, residual magnetization of 3.26 μB per Tb and theoretical energy product of 200 ​kJ/m3 at 2 ​K.

Introduction

The recently reported Sm26Co11Ga6-type {Gd - Dy, Er}26Co11-xGa6+x (x ​= ​1.6–6) [1] compounds (space group P4/mbm, N 127, tP86) supplement series of known isostructural Ho26Co6.7-13.7Ga10.3-3.3, {Ce - Sm}26Ni11-xGa6+x and {La–Sm}26Co11-xGa6+x (x ​~ ​0–5) compounds [[2], [3], [4]].

They exhibit extended region of solid solutions and they are interesting for systematic magnetic study as similar rich rare earth tetragonal Zr3Al2-type Gd3Al2, Tb3Al2 [[5], [6], [7]] (space group P42nm, N 102, tP20) and Cr5B3-type Gd5CoSi2 [8] (space group I4/mcm, N 140, tI32) compounds exhibit large magnetocaloric effect and giant low-temperature coercivity, meanwhile investigated Tb26Co9.4Ga7.6 shows large magnetic entropy change of −9.5 ​J/kg⋅K at 68 ​K and relative cooling power (RCP50kOe) of 570 ​J/kg (field change of 0–50 ​kOe) with low-temperature permanent magnet properties [1].

The structural aspect of these compounds is interesting for consideration, too as formally, the type of rare earth sublattice of compound determines its magnetic ordering.

The obtained magnetic properties of {Gd, Dy - Ho, Tm}26Co9Ga8 compounds are reported herein as addition of these data. Magnetic properties of Tb26Co6Ga11 and Tb26Co8Ga9 investigated for comparison with published Tb26Co9.4Ga7.6 [1] as the difference in Co sublattice may influence for resulting magnetic properties of Co-containing Sm26Co11Ga6-type compounds.

Section snippets

Materials and methods

The samples were prepared by arc melting stoichiometric amounts of polycrystalline pieces of rare earths (99.9 ​wt %), Co (99.95 ​wt %) and Ga (99.99 ​wt %) in argon (99.998%) purified with a Zr getter. To compensate for the loss during melting, some surplus of Gd and Ga was loaded. The samples were re-melted three times to obtain the full homogeneity of alloys. The samples were annealed at 870 ​K for 900 ​h in argon (99.998%) and subsequently quenched in ice-cold water. The structure, phase

Crystal structure and sample’s quality

X-ray powder diffraction analysis indicates that {Gd - Tm}26Co11-xGa6+x crystallize in the Sm26Co11Ga6-type structure and they exhibit extended region of solid solution from ~R26Co5Ga12 to ​~ ​R26Co9-10Ga8-7 (Table 1 and Fig. 1s). The novel Tm26Co9Ga8 supplements the serial of Sm26Co11Ga6-type compounds. The unit cell data of Sm26Co11Ga6-type compounds follow to the rare earth atomic radii rule with slight anisotropic distortion of unit cell: a, c cell parameters and unit cell volume V

Discussion

Till now, Sm26Co11Ga6-type compounds were only detected in the ternary R-{Co, Ni}-Ga systems [[1], [2], [3], [4]]. The Sm26Co11Ga6-type structural can be viewed as the derivative of Gd3Ga2 structure (I4/mcm, N 140, tI80) [2,18] via transformation of rare earth and gallium sublattices as shown in Fig. 9.

The Gd3Ga2 compound was initially reported to crystallize in the Zr3Al2-type structure [16]. However, the structure of {Sc, Y, Nd, Sm, Gd - Tm}3Ga2 compounds was specified as the Gd3Ga2

Conclusions

The novel Sm26Co11Ga6-type compounds may be detected in ‘Rare earth - Transition metal - Ga’ systems at some synthesis parameters. The magnetic ordering of Sm26Co11Ga6-type {Gd - Ho, Tm}26Co11-xGax are determined by magnetic ordering of rare earth with slight influence of Co sublattice. We suggest that novel Sm26Co11Ga6-type compounds will demonstrate similar magnetic ordering, including permanent magnet properties and magnetocaloric effect.

CRediT authorship contribution statement

A.V. Morozkin: Data curation, Methodology, Investigation, Validation, Software, Writing - original draft, Writing - review & editing. A.V. Knotko: Formal analysis. V.O. Yapaskurt: Formal analysis. Jinlei Yao: Investigation, Methodology, Writing - original draft, Writing - review & editing.

Declaration of competing interest

This work is systematic study of specific features of crystal structure and magnetic properties of rare earth compounds. These compounds exhibit large magnetocaloric effect, relative cooling power and low-temperature coercivity and they are promising as a background of magnetic refrigerators and low-temperature permanent magnets.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (Grant No. 21771136), Jiangsu University 333 Project and M.V.Lomonosov Moscow State University Program of Development. This work is supported by ​Russian Fund for Basic Research ​through the project N° 20-03-00209-a.

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    Citation Excerpt :

    Magnetization is given in emu/g and μB units (1 emu/g ​= ​1 A ​m2/kg, 1 μB ​= ​9.7400968(20)⋅10−24 A ​m2) [13]. X-ray powder diffractions analyses show that the known Tb26Co9.4Ga7.6 and Er26Co9Ga8 compounds [5,6] and the novel Gd26Ni6.5-9Ga10.5-8, Tb26Ni8Ga9 and {Dy, Ho, Er}26Ni6.5Ga10.5 ones crystallize in the Sm26Co11Ga6-type structure (Table 1 and supplementary Figure 1s). The crystal structure of Sm26Co11Ga6-type Er26Co9Ga8 (as an example of crystal structure of these compounds) is shown in Fig. 1 (see atomic sites of Er26Co9Ga8 in the caption of Table 1).

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