GdCuMg with ZrNiAl-type structure – an 82.2 K ferromagnet

1 Introduction

Intermetallic gadolinium compounds are in the focus with respect to their promising magnetocaloric properties, especially when searching for materials for magnetic refrigeration near room temperature. The key feature for characterizing such materials is phase analyses along with a study of the basic magnetic and magnetocaloric data [1], [2], [3], [4], [5], [6], [7], [8], [9]. Another important feature concerns the electron density at the gadolinium nuclei which is readily recordable via ¹⁵⁵Gd Mössbauer spectroscopy [10], [11], [12]; the isomer shift is related to the local electron densities, mainly of s character, giving a finger-print characteristic of the respective phase.

Besides many binary gadolinium intermetallics [9], a huge number of ternary compounds of the general compositions Gd_xT_yX_z (T=transition metal; X=element of the 3^rd, 4^th or 5^th main group) has been characterized with respect to the basic magnetic properties in order to select suitable compounds for further magnetocaloric studies. We have added a number of compounds with X=Mg, Zn and Cd [13], [14], [15], [16] to this family of materials. Magnesium, zinc and cadmium are fully embedded within the three-dimensional polyanionic networks, underlining their covalent bonding character. The important feature for property studies, however, concerns the reduction of the electron count (with respect to group III, IV or V elements), leading to changes of the magnetic ground state and thus the magnetic ordering temperature. To give an example, substitution of indium in GdRhIn by zinc leads to a drastic reduction of the magnetic ordering temperature from T_C=34 K for GdRhIn [17] to T_N=12.7 K for GdRhZn [15], along with a change of the spin alignment in the magnetically ordered state.

Of the magnesium-based ternary systems we already studied the magnetic behavior of the equiatomic compounds GdTMg for T=Pd, Ag, Pt and Au [18], [19], which show comparatively high magnetic ordering temperatures (Table 1). GdAgMg shows the by far lowest Curie temperature of 39.3 K, a consequence of different chemical bonding. We have now closed the gap for this series of compounds and present the synthesis, structural, and magnetic characterization of GdCuMg herein.

2 Experimental

3 Crystal chemistry

GdCuMg crystallizes with the ZrNiAl type structure, space group P6̅2m with the lattice parameters a=749.2(4) and c=403.3(1) pm. So far, only few systematic phase analytical studies have been performed for the RE-Cu-Mg systems, i.e. Y-Cu-Mg [24], La-Cu-Mg [25], Tb-Cu-Mg [26], and Yb-Cu-Mg [27]. Solely the equiatomic compounds YCuMg (a=744.5, c=399.5 pm) [24], LaCuMg (a=773.2, c=417.9 pm) [25] and CeCuMg (a=766.8, c=416.4 pm) [28] have been reported. Already the Tb-Cu-Mg system [26] shows no ZrNiAl type phase. Thus GdCuMg is the last existing phase in the RECuMg series. The smaller rare earth elements do not allow formation of a RECuMg phase. High-pressure high-temperature studies will be performed in order to test if the series can be extended under such conditions, similar to the related series of rare earth stannides [29], [30]. In going from LaCuMg to GdCuMg, the unit cell decreases more or less isotropically as expected from the lanthanide contraction (3.2% decrease in a and 3.5% decrease in c).

The crystal chemistry of ZrNiAl type intermetallics has repeatedly been reviewed, e.g. in [1], [31], [32] and also discussed for equiatomic RETMg intermetallics [13], [16], [19]. We therefore only briefly discuss the GdCuMg structure. A projection of the latter is presented in Fig. 1. Both crystallographically independent copper sites show trigonal prismatic coordination, Cu1@Gd₆ and Cu2@Mg₂. The Cu1@Gd₆ prisms show hexameric units and include the Cu2@Mg₂ prisms. Both prism types are shifted with respect to each other by half the translation period c. Especially the triangular arrangement of the gadolinium atoms calls for frustration of the magnetic moments in the magnetically ordered state (vide infra).

Fig. 1:

Projection of the GdCuMg structure onto the xy plane. Gadolinium, copper and magnesium atoms are drawn as medium grey, blue, and magenta circles, respectively. All atoms lie on mirror planes at z=0 (thin lines) and z=1/2 (thick lines). The trigonal prismatic coordination around the two crystallographically independent copper sites is emphasized.

The shortest interatomic distances in the GdCuMg structure occur between the copper and magnesium atoms, ranging from 271 to 290 pm, slightly longer than the sum of the covalent radii of Cu+Mg of 253 pm [33]. We can therefore assume weaker Cu–Mg bonding. Together, the copper and magnesium atoms build up a three-dimensional network which incorporates the gadolinium atoms. Bonding of gadolinium to the [CuMg] network proceeds through Gd–Cu contacts at 303 and 311 pm.

At this point it is interesting to compare GdCuMg with the isotypic indide GdCuIn (a=746.3, c=399.6 pm) [34]. Although indium is larger than magnesium, the indide has smaller lattice parameters and thus also shows stronger Cu–In vs Cu–Mg bonding, similar to the series of RE₂Cu₂In vs RE₂Cu₂Mg compounds [35]. These compounds crystallize with ternary ordered versions of the U₃Si₂ type, again with trigonal prismatic rare earth coordination for the copper atoms.

Finally, in Table 4 we compare the relevant interatomic distances for the whole series of GdTMg (T=Cu, Pd, Ag, Pt, Au) phases. It is readily evident, that GdAgMg shows a pronounced anomaly. Although silver and gold have comparable covalent radii [33], the lattice parameters of the silver compound are distinctly larger than those of GdAuMg. This leads to much weaker Ag–Mg and Gd–Ag bonding and thus strongly influences the gadolinium magnetic ground state (vide infra). Although all of the listed GdTMg phases (Table 1) crystallize with the hexagonal ZrNiAl type, there are distinct differences in chemical bonding and a simple application of a rigid band model for understanding of the structure-property relations is critical.

4 Magnetic properties

The temperature dependence of the magnetic susceptibility (χ and χ⁻¹ data) of GdCuMg measured in zero-field cooled (ZFC) mode with an applied field of 10 kOe is displayed in the top panel of Fig. 2. The inverse magnetic susceptibility was fitted using the Curie-Weiss law in the temperature range of 125–300 K revealing a paramagnetic Curie temperature of θ_P=82.0(1) K and an effective magnetic moment of μ_exp=8.54(1) μ_B per Gd atom, slightly higher than the free ion value of Gd³⁺ (7.94 μ_B). The observed excess magnetic moment can at least partially be ascribed to the gadolinium 5d electrons induced via 4f-5d exchange interactions, also observed for GdAuMg [19] and GdPtMg [18] or the solid solution Gd_xLa_1−x Ni₅ [36].

Fig. 2:

(Top) Temperature dependence of the magnetic susceptibility of GdCuMg measured with an applied field of 10 kOe. The inset shows the ZFC and FC data measured at 100 Oe. (Bottom) Magnetization isotherms measured at 3, 50, 100 and 150 K with a magnetic field strength of up to 80 kOe.

The inset of Fig. 2 depicts the zero-field cooled and field cooled (FC) mode measurement of GdCuMg at 100 Oe. The large bifurcation between the ZFC and the FC curves indicates dominant ferromagnetic interactions. The Curie temperature of T_C=82.2(5) K was determined from the minimum of the derivative dχ/dT of the FC curve. The magnetization isotherms of GdCuMg measured at 3, 50, 100 and 150 K in a field range of 0–80 kOe are shown in the bottom panel of Fig. 2. The isotherms recorded above the Curie temperature exhibit the expected almost linear field dependence of a paramagnetic compound. Below the magnetic ordering temperature, a sharp increase of the magnetization is evident which is due to saturation effects. The observed full saturation value at 3 K and 80 kOe of μ_sat=7.3(1) μ_B per Gd atom is slightly higher than the theoretical value of 7 μ_B per gadolinium atom (according to g×J). The steep increase of the magnetization and the almost negligible hysteresis classify GdCuMg as a soft ferromagnet.

Finally it is interesting to compare GdCuMg with the other isotypic GdTMg (T=Pd, Pt, Ag, Au) [18], [19] phases. The Curie temperature of 82.2 K is comparable to the ordering temperatures published for GdPdMg (T_C=95.7 K), GdPtMg (T_C=97.6 K) [18] and GdAuMg (T_N=81.1 K) [19]. GdAgMg has the much lower Curie temperature of T_C=39.3 K [18]. This is in line with the bonding characteristics discussed above. In the series of GdTMg compounds GdAgMg shows the largest lattice parameters (Table 1) and consequently also larger interatomic distances (Table 4), of which Gd–Ag and Gd–Gd are the important ones with respect to magnetic coupling, resulting in weaker Gd(4f)-Ag(4d) exchange and weaker direct Gd–Gd coupling, leading to the much lower ordering temperature.

A simple correlation of the magnetic ordering temperature with the valence electron count (VEC) of several GdCuX phases is not possible: T_C=82.2 K (GdCuMg; VEC=16), T_C=82 K (GdCuAl [37]; VEC=17), T_N=20 K (GdCuIn [38]; VEC=17) and T_N=14 K (GdCuGe [39]; VEC=18). The magnetic ordering temperature and the type of spin alignment show large differences, indicating that the magnetic ground state is largely influenced by small changes in chemical bonding as discussed above for GdAgMg within the series of GdTMg compounds.