Curie temperature adjustment in the solid solution Gd_1–xY _x PtMg

1 Introduction

The magnetic ordering in rare earth-based intermetallic compounds can effectively be influenced through the formation of diverse solid solutions [1]. In a general ternary compound RE _x T _y X _z with RE = rare earth element, T = electron-rich transition metal, X = element of the third, fourth or fifth main group, all three sites can be substituted, effecting the magnetic long-range ordering of the rare earth substructure.

To give some examples, substitution of the T element by another one (i) modifies the RE–T hybridization and/or (ii) the electron count. An interesting case concerns the solid solution CeRh_1−xIr _x Ge [2, 3], where a change from antiferromagnetic CeRhGe to intermediate-valent CeIrGe sets in at 0.28 ≤ x ≤ 0.36. Nickel substitution by cobalt decreases the electron count in the solid solution CeNi_1−xCo _x Sn [4]. A first order valence phase transition takes place in the x range from 0.35 to 0.40. As a function of the cobalt concentration, the transition temperature changes from 40 to 80 K [4]. CeRu_1−xPd _x Sn and CeRh_1−xPd _x Sn are examples for solid solutions where the valence change of cerium is accompanied also by a structural phase transition [5].

The X site substitution was e.g. tested for the solid solution CeRhSb_1−xSn _x [6]. The TiNiSi-type structure is stable up to x < 0.2. The Kondo gap disappears by tin substitution and the solid solution switches to a non-Fermi liquid system. A remarkable example with an X site substitution is the solid solution EuAu₄Cd_2–xMg _x . These samples show ferromagnetic ordering at around 16 K and the structural Cd/Mg disorder has no major influence on the ferromagnetic ground state [7].

Substitution of the rare earth site by replacing the paramagnetic rare earth element by a diamagnetic one allows a study of the spin dilution. Typical pairs are Ce³⁺/La³⁺, Eu²⁺/Sr²⁺, Gd³⁺/Y³⁺ and Yb³⁺/Lu³⁺. Especially the equiatomic CeTX compounds were studied with respect to solid solutions Ce_1–xLa _x TX [8], [9], [10]. Examples for the Eu²⁺/Sr²⁺ and Yb³⁺/Lu³⁺substitutions are the ferromagnetic solid solution Eu_1–xSr _x PtIn₂ [11] and the segmented Heisenberg quantum spin chains in Yb_4–xLu _x As₃ [12]. Many other intermetallic and oxydic materials have been studied in this direction.

From the point of view of the very close covalent radii, the Gd³⁺ (161 pm)/Y³⁺ (162 pm) pair is an obvious case [13]. Nevertheless, only a few studies of such solid solutions have been reported, e.g. Gd_1–xY _x Co₂ [14], Gd_1–xY _x Mn₂Ge₂ [15], Gd_1–xY _x TiGe [16], and Gd_1–xY _x Co₂B₂ [17]. Recent investigations showed that the intermetallic compounds GdAuMg and YAuMg form a continuous solid solution Gd_1–xY _x AuMg. The gadolinium substitution leads to a strong decrease of the Néel temperature from T_N = 81.1(1) K for GdAuMg to 3.5(1) K for Gd_0.1Y_0.9AuMg [18]. Dilution of the gadolinium substructure with yttrium drives the magnetization isotherms to nearly Brillouin-type behavior as is known for the free Gd³⁺ ion.

In continuation of these substitution experiments we herein report on the solid solution Gd_1–xY _x PtMg which starts from ferromagnetic GdPtMg with T_C = 97.6 K [19].

2 Experimental

3 Structure refinements

The four data sets showed hexagonal lattices with high Laue symmetry without further systematic extinctions. Space group P 6 ‾ 2m was found to be correct, in agreement with previous work on GdPtMg [19]. The atomic parameters of the pure gadolinium compound [19] and the ratio of Gd to Y of the initial sample compositions were taken as starting values. The structures were then refined by least-squares refinements on F² using the program Jana2006 [26] with anisotropic displacement parameters for all atoms. Separate refinements of the occupancy parameters revealed full occupancies for the platinum and magnesium sites. The ratios Gd:Y were refined as least-squares variable in the final cycles. Refinement of the correct absolute structures was ensured through calculation of the Flack parameter [27], [28], [29]. The final difference Fourier syntheses were flat. Further details of the refinements, the positional parameters and the interatomic distances are listed in Tables 2–4.

CCDC 2102223 (Gd_0.692Y_0.308PtMg), 2102225 (Gd_0.691Y_0.309PtMg), 2102220 (Gd_0.485Y_0.515PtMg) and 2102224 (Gd_0.298Y_0.702PtMg) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

4 Crystal chemistry

The intermetallic phases GdPtMg [19] and YPtMg [23] form a complete solid solution Gd_1–xY _x PtMg. Samples in steps of x = 0.1 were synthesized and characterized through their X-ray powder patterns along with their refined lattice parameters (Table 1). The solid solution Gd_1–xY _x PtMg shows small differences when compared with the isotypic series Gd_1–xY _x AuMg [18]. While both the a and c lattice parameters weakly decrease from the gadolinium to the yttrium compound for Gd_1–xY _x AuMg [18], for Gd_1–xY _x PtMg the c parameter remains more or less constant and only the a lattice parameter decreases by less than 1%. In total we observe a small decrease of the cell volume (Figure 1) in going to the pure yttrium compound. The single-crystal data is not plotted in Figure 1. Differences in the lattice parameters between powder and single-crystal data are mainly due to the use of different techniques (no internal standard for the single-crystal diffractometer).

Figure 1:

Course of the cell volumes of the samples of the solid solution Gd_1–xY _x PtMg.

GdPtMg and YPtMg crystallize with the ZrNiAl-type structure [30], [31], [32], one of the basic structure types for ternary intermetallic compounds with more than 1600 entries in the Pearson data base [33]. The various crystal chemical facets of the ZrNiAl type have repeatedly been reviewed and we refer to review articles [34, 35] for further details. Only some important aspects for the Gd_1–xY _x PtMg series are discussed herein.

As an example we present a projection of the Gd_0.485Y_0.515PtMg structure in Figure 2. The two basic structural motifs are platinum centered trigonal prisms formed by the rare earth (Gd/Y mixing) and magnesium atoms. The two prism types are shifted by half the c parameter with respect to each other. The Pt1@(Gd/Y)₆ prisms are condensed via common edges leading to six-membered rings. The resulting channels are filled by columns (sharing triangular faces in c direction) of the Pt2@Mg₆ prisms. The (Gd/Y)₃ triangles of the Pt1@(Gd/Y)₆ prisms are a striking structural motif with respect to the magnetic behavior, since such triangles lead to geometrical frustration, often observed in such kagome like networks [36], [37], [38].

Figure 2:

Projection of the Gd_0.485Y_0.515PtMg structure onto the ab plane. Gadolinium, yttrium, platinum and magnesium atoms are drawn as medium gray, green, blue and magenta circles. The Gd/Y mixing is emphasized by segments. The trigonal prismatic coordination around the two crystallographically independent platinum sites is highlighted. The two different prismatic building units are shifted by half the c translation period, emphasized by thin and thick lines.

Similar to the solid solution Gd_1–xY _x AuMg [18], we have also carefully checked the data sets from the Gd_1–xY _x PtMg series with respect to possible Gd/Y ordering. Again, no hints for Gd/Y ordering were observed. Most likely the very similar size of gadolinium and yttrium avoids any ordering. In that view we draw to the data sets of Gd_0.692(1)Y_0.308(1)PtMg and Gd_0.691(2)Y_0.309(2)PtMg with similar compositions but slightly differing lattice parameters. This might be indicative of different clustering in the rare earth substructure.

5 Magnetic properties

GdPtMg [19] becomes ordered ferromagnetically at T_C = 97.6 K. Yttrium-substituted samples in x = 0.1 steps were measured with respect to the temperature dependence of the magnetic susceptibility. The magnetic data of the Gd_0.6Y_0.4PtMg sample is presented in Figure 3. The sample shows Curie-Weiss behavior above 100 K with an experimental magnetic moment of 8.35(3) µ_B per gadolinium atom and a Weiss constant of 59.0(4) K. The magnetic moment is slightly higher than the free ion value [39] of 7.94 µ_B per gadolinium atom for Gd³⁺. This is due to the gadolinium 5d electrons that induce 4f–5d exchange interactions [40, 41]. The Gd_1–xY _x AuMg series [18] also shows this behavior. The precise Curie temperature of 55.3(1) K of Gd_0.6Y_0.4PtMg was determined from a low-field measurement in zero-field-cooled/field-cooled mode (Figure 4).

Figure 3:

(Top) Temperature dependence (χ and χ⁻¹ data) of the magnetic susceptibility of Gd_0.6Y_0.4PtMg measured at 10 kOe. (Bottom) Magnetization isotherms of Gd_0.6Y_0.4PtMg measured at 3, 10, 50 and 100 K.

Figure 4:

Temperature dependence of the magnetic susceptibility of the members of the solid solution Gd_1–xY _x PtMg measured at 100 Oe (zero field-cooling (dots) and field-cooling (thin lines) curves).

The magnetization isotherms of Gd_0.6Y_0.4PtMg are shown at the bottom of Figure 3. At 100 K, well above the Curie temperature, we observe an almost linear increase of the magnetization as expected for a paramagnetic material. At 50 K, only slightly below the Curie temperature, the magnetization rapidly rises and becomes saturated around 5 µ_B per gadolinium atom at 80 kOe. The steep increase and the almost negligible hysteresis classifies Gd_0.6Y_0.4PtMg as a soft ferromagnet. The isotherms at 3 and 10 K are congruent. They show full saturation at 80 kOe (7.17(1) µ_B per Gd atom).

The remaining samples all show very similar behavior. The basic magnetic data derived from the susceptibility and magnetization measurements is summarized in Table 5. The results of all zero-field-cooled/field-cooled measurements are plotted in Figure 4. It is remarkable that the ferromagnetic ground state is retained down to the Gd_0.1Y_0.9PtMg sample, however, with a drastically decreased Curie temperature of 3.7(1) K. The course of T_C as a function of x is presented in Figure 5.

Figure 5:

Course of the Curie temperature of the solid solution Gd_1–xY _x PtMg.

Summing up, the results of the studies of the solid solution presented here nicely underpin that gadolinium spin dilution by diamagnetic yttrium is a suitable tool to adjust magnetic ordering temperatures of gadolinium intermetallics over a wide temperature range.