Magnetic properties of Tb2Pd2In; single crystal study

S. Mašková1,a, A. Kolomiets1,2, L. Havela1, A. V. Andreev3, P. Svoboda1, Y. Skourski4

1Department of Condensed Matter Physics, Charles University, Ke Karlovu 5, 12116 Prague 2, The Czech Republic

2Department of Physics, Lviv Polytechnic National University, 12 Bandera Str., 79013 Lviv, Ukraine

3Institute of Physics, AVCR, Na Slovance 2, 18221 Prague 8, The Czech Republic

4 Hochfeld-Magnetlabor, FZ Dresden-Rossendorf, D-01314 Dresden, Germany

amaskova@mag.mff.cuni.cz (corresponding author)

Keywords: intermetallic compounds, magnetic frustration, magnetic properties, single crystal

Abstract

A pilot polycrystalline magnetization study of Tb2Pd2In in comparison with new single-crystal data reveal interesting magnetocrystalline anisotropy with field induced spin-flip transitions both in the c- and a-axis of the tetragonal structure. Introduction

For the majority of the known rare-earth compounds with the R2T2X stoichiometry (T-transition metal, X-p-metal) only the crystal structures have been reported so far [1]. Our preliminary studies of the magnetic properties and the crystal structure of several tetragonal R2T2X compounds, which crystallize in the Mo2FeB2 structure type, and their hydrides have revealed complex magnetic nature and an enormous hydrogen absorption capability in these materials. The complex magnetic nature is related to the triangular arrangement of the R ions, which brings geometrical frustration affecting the magnetic ground state. For instance, Nd2Ni2In is an antiferromagnet with TN = 8 K, but the frustration leads to metamagnetic transition into the ferromagnetic state in weak magnetic fields (µ0H < 0.3 T) [2]. Nd2Ni2In forms also a series of hydrides with the hydrogen concentrations of 2, 4 and 7 H/f.u. The crystal lattice becomes orthorhombically distorted (space group Pbma) once the full H-occupancy is reached, which also affects the magnetic properties. Structural anisotropy and the 2D geometric frustration in R2Pd2In make the single-crystal studies of these compounds highly desirable. In this paper we describe the results of both polycrystalline and single-crystalline studies of magnetic properties of Tb2Pd2In, which is another representative of the Mo2FeB2 structure type. Recently, we have succeeded to grow this compound in the single-crystalline form and the detailed study is still in progress. Tb2Pd2In was already reported to be an antiferromagnet with the Néel temperature of TN = 32 K and the spin-reorientation transition at µ0Hc = 4 T [3]. Based on neutron powder diffraction studies, the magnetic structure has been reported as cos-modulated wave of Tb magnetic moments aligned along the c-axis with the propagation vector k = (1/4, 1/4, 1/2) [4]. Considering the lattice symmetry of the R2Pd2In compounds and the metamagnetic behavior of most of them [4] together with rather large magnetic unit cell in Tb2Pd2In [4], one may assume that this compound belongs also into a Shastry-Sutherland system [5,6]. To shed more light on this compound, we undertook a pilot study of magnetic properties of the polycrystalline Tb2Pd2In in various magnetic fields. The results we obtained encouraged us to grow a single crystal of this compound, in which effort we finally succeeded. Crystal structure

A polycrystalline sample of Tb2Pd2In was prepared by arc melting of stoichiometric amounts of the elemental constituents. The Rietveld analysis of the powder X-ray diffraction pattern has confirmed that the crystal structure of Tb2Pd2In is of the Mo2FeB2-structure type (space group P4/mbm).

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Obtained lattice parameters and atomic positions are in a good agreement with those reported in [3] (Table 1). We can also compare the lattice parameters with slightly non-stoichiometric samples reported in [7]. They are in a good agreement showing that our sample is probably closer to the ideal composition 221 compared to compounds reported there. Table 1. Crystal lattice parameters of our Tb2Pd2In samples in comparison with those reported in [3].

a [Å] c [Å] x y z Tb2Pd2In [3] 7.6632(2) 3.7239(1) Tb 4h 0.1739(2) 0.6739(2) 1/2 Pd 4g 0.3724(3) 0.8724(3) 0 In 2a 0 0 0 Tb2Pd2In 7.6728(2) 3.7188(1) Tb 4h 0.1740(2) 0.6740(2) 1/2 Pd 4g 0.3725(3) 0.8725(3) 0 In 2a 0 0 0

Pilot polycrystalline study

First we have studied magnetization and magnetic susceptibility of polycrystalline Tb2Pd2In in low-temperature region and in applied magnetic fields up to 14 T. Temperature dependences of the magnetic susceptibility exhibit a maximum indicating an antiferromagnetic ordering at about TN = 33 K, which shifts to lower temperatures with increasing magnetic field (see Fig. 1), quite in agreement with [3]. The different behavior of the 14 T-dependence suggests, assuming random orientation of the crystallites, that part of the sample is already above the metamagnetic transition.

Fig. 1. Magnetic susceptibility of polycrystalline Tb2Pd2In in selected magnetic fields.

Quasi-static magnetization measurements using a Quantum Design PPMS magnetometer revealed at least three metamagnetic transitions on the magnetization curve of polycrystalline Tb2Pd2In up to 14 T (Fig. 2a). The transition fields have been determined as the positions of maxima of dM/dH at T = 2 K, which yields: µ0Hc1 = 6.5 T, µ0Hc2 = 9.3 T, and µ0Hc3 = 13.3 T. One has to note that the polycrystalline magnetization curve can reflect transitions of individual grains, which are projected into the field-direction. As the M(H) curves do not show any tendency to saturation even at µ0H = 14 T, we have extended the field range using the pulse-field magnet of the high-field facility at Rossendorf (Fig. 2b). No additional metamagnetic transitions were found in fields between 14 and 60 T.

Single-crystalline study

The ambiguous results from the polycrystalline sample, together with expected large magnetocrystalline anisotropy, have encouraged us to attempt, despite an incongruent formation, a single-crystal growth. Using the Czochralski pulling technique we have succeeded to grow good quality single crystal of Tb2Pd2In by means of a tri-arc furnace. Its stoichiometry will be similar to the polycrystalline sample so as the lattice parameters (a = 7.686(3) and c = 3.717(1)) are quite similar. The grown crystal was then oriented using Laue method and shaped for further measurements. Magnetic properties were then studied applying the magnetic field along [100] (= a-

axis), [110], and [001] (= c-axis) crystallographic directions, respectively.

Solid State Phenomena Vol. 194 59

Fig. 2a. Magnetization curves of polycryst-alline Tb2Pd2In at selected temperatures from 2 K to 25 K.

Fig. 2b. Magnetization curve of polycrystalline Tb2Pd2In at T = 4.2 K in the pulse magnetic field.

The temperature dependence of magnetic susceptibility has confirmed the expected high magneto-crystalline anisotropy in the whole temperature range with the c-axis as an easy axis of magnetization. The dependences of the inverse magnetic susceptibility (Fig. 3) along a- and c- axes are parallel to each other and above 60 K they can be described by the Curie-Weiss law, yielding paramagnetic Curie temperatures θpa = − 46 K and θpc = 3 K, respectively, and the effective magnetic moment µeff = 10.54 µB/Tb. This value is higher than the theoretical one of 9.72 µB/Tb3+, which may be ascribed to the polarization of the 5d states.

Fig. 3. Temperature dependences of inverse magnetic susceptibility of Tb2Pd2In along the principal crystallographic axes in magnetic field µ0H = 2 T. The lines represent the C-W fits with parameters mentioned in the text. Magnetization curves taken at 2 K (Fig. 4a) confirm the high magnetocrystalline anisotropy of Tb2Pd2In even within the basal plane of the tetragonal structure. The easy-axis magnetization curve exhibits two metamagnetic transitions (probably spin-flip type) yielding saturated magnetization 19.1 µB/f.u., slightly less than the full theoretical moment. The magnetization curve along the crystallographic a-axis exhibits surprisingly also a spin-flip transition at about 13 T. Such behavior, together with rather large hysteresis of the metamagnetic transition in a, resembles that observed on TbCu2 [8], which was connected with higher order crystal-field terms significantly contributing to the magnetocrystalline anisotropy. The magnetization above the transition reaches about 2/3 of the maximum saturated value, indicating another probable transition in higher fields. The critical field of the first-order transition for H//a decreases with the increasing temperature. Also its shape changes reaching the same shape as found in the c-direction (2 broad transitions), at approx. T = 25 K (Fig. 4b). The magnetization curve along the [110] direction, which is probably the hard axis of magnetization, does not exhibit any metamagnetic behavior in available fields. Nevertheless, taking into account projection from the a-axis in the basal plane, such transition can be expected in higher fields.

60 Solid Compounds of Transition Elements II

Fig. 4a. Magnetization curves of Tb2Pd2In measured along various crystallographic directions at T = 2 K.

Fig. 4b. Magnetization curves of Tb2Pd2In measured along principal crystallographic axes at selected temperatures.

Summary

Weak positive Θp of Tb2Pd2In for the susceptibility along c-axis indicates ferromagnetic interactions between certain nearest Tb-neighbors. The difference to Θp for field along a, which is about 50 K, gives the estimate of the energy of magnetocrystalline anisotropy. The magnetization plateau at 50% of the saturated magnetization (H//c) indicates a possible link to compounds like TmB4, for which various solutions of the Shastry-Sutherland lattice were attempted [9,10]. To clarify this, a detailed neutron diffraction study on single crystal in magnetic fields should be performed. Acknowledgements

This work was supported by the Grant Agency of the Czech Republic under the grants No. 204/12/0285, 204/10/0330 and P108/10/1006. The work was also supported by the Charles University Grant Agency (project No. 436111). References

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