Anomalous Ferromagnetic Phase in the Gd1−xErxB4 Series: Crystal Growth, Thermal, and Magnetic Properties

Abstract

Rare-earth tetraborides RB4 are of great interest due to the occurrence of geometric magnetic frustration and corresponding unusual magnetic properties. While the Gd3+ spins in GdB4 align along the ab plane, Er3+ spins in the isomorphic ErB4 are confined to the c–axis. The magnetization in the latter exhibits a plateau at the midpoint of the saturation magnetization. Therefore, solid solutions of (Gd, Er)B4 provide an excellent playground for exploring the intricate magnetic behavior in these compounds. Single crystals of Gd1−xErxB4 (x = 0, 0.2, and 0.4) were grown in aluminum flux. X-ray diffraction scans revealed single-phase materials, and a drop in the unit cell volume with increasing Er content, suggesting the partial substitution of Er at the Gd sites. Heat capacity measurements indicated a systematic decrease of the Néel temperature (TN) with increasing Er content. The effective magnetic moment determined from the magnetization measurement agreed with the calculated free ion values for Gd3+ and Er3+, providing further evidence for the successful substitution of Er for Gd. The partial substitution resulted in an anomalous ferromagnetic phase below TN, exhibiting significant anisotropy, predominantly along the c-axis. This intriguing behavior merits further studies of the magnetism in the Gd1−xErxB4 borides.

Phase Diagram Mapping out the Complex Magnetic Structure of Single Crystals of (Gd, Er)B4 Solid Solutions

“…We also emphasize that increasing Er concentration in Gd 1−x Er x B 4 solid solutions is accompanied by a systematic decrease in the lattice parameters due to the smaller ionic radius of Er [25]. As far as this point is concerned, we might recall that the frustration in the Shastry-Sutherland lattice model is closely related to the competition between the Zeeman effect and interactions J 1 and J 2 .…”

“…The magnetization curves obtained for 𝐻 ⊥ 𝑐 [25] exhibited just a monotonic increase in magnetization with the applied field for the both x = 0.2 and 0.4 samples (Figure S1). This behavior is consistent with the specific heat measurements shown in Figure 1 (lower panels a and b), for which no field-induced phase transition was observed.…”

“…Considering the results described above, the effects of the partial substitution of Er for Gd in Gd 1−x Er x B 4 solid solutions can be summarized as follows: (i) an increase in the magnitude of magnetization, as shown in Figure S4, and in the experimentally determined µ eff in the PM state (see, for instance, Reference [25]); (ii) a slight decrease in T N for the stabilization of the AF state; (iii) an appreciable anisotropy of the wFM phase along the c-axis; and (iv) the stabilization of the plateau state at ½ M S for the x = 0.4 crystals. However, the transition to the FIP state, certainly related to the presence of Er 3+ ions, occurs at lower applied magnetic fields (H FIP ~4 T) for crystals with x = 0.2, whereas it is H FIP ~5.5 T for x = 0.4, following the emergence of the plateau state at 4.4 T.…”

“…X-ray powder (XRD) analyses and X-ray Laue scans indicated the single-phase nature and high crystallinity of these samples. The characterization of Gd 1−x Er x B 4 single crystals has been presented elsewhere [24,25] and is also summarized in the Supplementary Materials.…”