Field‐Induced Inversion of the Magnetoresistive Effect in the Zintl Phase Eu_5+xMg_18−xSi₁₃ (x=2.2)

Abstract

The discovery of giant magnetoresistance (GMR effect) [1] in magnetic multilayers has led to a rapid technological advance in spintronic research and device building, for example, magnetoresistive random-access memory (MRAM). The operating principle is based on the dependence of the electrical resistivity on the spin alignment in the magnetic domains of the material. [2] All ferromagnetic metals exhibit a finite but small change of the electronic conductivity after application of an external magnetic field. The value of the magnetoresistivity MR under applied magnetic field H is defined as MR = [1(H)À1(0)]/1(0)] 100 % (1 = electrical resistivity) and can be either positive or negative. If the resistance drop is associated with critical ferromagnetic fluctuations, this phenomenon is called colossal magnetoresistance (CMR). This effect was observed for the first time for the manganese perovskites RE 1Àx B x MnO 3 (RE = rareearth metal; B = divalent cation). [3] The discovery of enhanced MR, often described as CMR in the literature, for the ternary Zintl phase [4] Eu 14 MnSb 11 has led to intensive investigations of this class of compounds. [5] The MR effect occurs simultaneously with ferromagnetic ordering due to the parallel alignment of the unpaired 4f electrons of the rare-earth metals. In the case of Eu 2+ ions they should contribute with a local magnetic moment of 7.94 m B.We report herein on the synthesis of the new ternary Zintl phase Eu 5+x Mg 18Àx Si 13 (x = 2.2) [6] which displays an unusual MR effect. The title compound does not show a maximum of the resistivity at low temperatures and no saturation at fields up to 6 T. Furthermore, an inversion of the sign of the MR as a function of applied field and temperature can be observed.The crystal structure of Eu 5+x Mg 18Àx Si 13 (x = 2.2) is depicted in Figure 1. Tetrel compounds such as M 5+x Mg 18Àx T 13 (M = Sr, Ba; Tt = Si, Ge), which crystallize in their own structure type, have been known for some time. [7] The structure usually contains isolated Si 4À anions as well as planar Si 4 clusters for which different valence electron numbers have been found. [7,8] Moreover, the tetrel center of the Tt 4 cluster can be replaced by a metal such as Li or Mg without causing a structural change in the geometrical pattern. Recently, we reported on the synthesis of the phase Eu 5+x Mg 18Àx Ge 13 (x = 0.1), which is isostructural with Sr 6.3 Mg 16.7 Si 13 and isopunctual with Eu 8 Mg 16 Ge 12 . In the new compound the Tt 4 unit collapses into three isolated Tt 4À anions by means of such substitution. [9] This indicates the remarkable flexibility of that structure type, which we have tested now by systematic changes of the composition and investigated the related electronic effects.The electronic structure of the title compound can be interpreted according to the Zintl-Klemm concept as (Eu 2+ ) 5+x (Mg 2+ ) 18Àx (Si 4À ) 9 (Si 4 10À ) with nine isolated silicon anions and a planar [Si 4 ] unit. [6,9] The anisotropic displacement ellipsoid of the central silicon atom...

Crystal and Electronic Structure of the Lithium‐Rich Silver Silicide Li12Ag1−xSi4 (x=0.15)

“…The stacking distance ( c =4.47 Å) is in the typical range, in which a direct σ overlap among π states of tetralide anions takes place and thus directs the mutual packing of cations, too. Such arrangements are frequently found in tetralide Zintl phases that show peculiar metallic conductivity 10. Silver d 10 –d 10 overlap cannot contribute to chemical bonding at a distance of 4.47 Å 11…”

“…Such arrangements are frequently found in tetralide Zintl phases that show peculiar metallic conductivity. [10] Silver d 10 -d 10 overlap cannot contribute to chemical bonding at a distance of 4.47 . [11] Different from the [CuSi 2 ] unit in Li 7 CuSi 2 , the trimeric fragment [Si-Ag-Si] appears to be of D 1h symmetry with 1808 angles, and the Ag À Si distances are exactly of the same lengths.…”

Synthesis, Crystal Structure, Polymorphism, and Magnetism of Eu(CN3H4)2 and First Evidence of EuC(NH)3

“…So far, EuC(NH) 3 was only obtained together with an unidentified side phase. The field-dependent molar magnetization Mm curve at 2.0 K hints at a saturation value of Mm,sat ≤ 6 NA μB, significantly lower than the expected saturation value of 7.0 NA μB for noninteracting Eu 2+ (4f 7 ) ions. The ratio of these saturation magnetizations is different from the squared ratio of the effective moments μeff at room temperature; that is, there is not a common factor that could scale both values to reach the expectation.…”

“…Hence, the data for T > 20 K were corrected as detailed in the Methods section and reference [10]. At room temperature, the μeff value of noninteracting Eu 2+ (4f 7 , gJ = 2, J = 7/2) ions is expected to be close to 7.91 μB, lowered by noticeable spin-orbit coupling contributions from the spin-only value of 7.94 μB [44,45]. A Curie-Weiss fit of the corrected data (T > 90 K) gives an effective magnetic moment of 7.74 μB and a slightly negative Curie temperature with −8(2) K. Although both values imply antiferromagnetic exchange interactions between the Eu 2+ ions, they should be looked at as artifacts of the correction method since exchange interactions between lanthanide centers are usually very small (−2J < 2 cm −1 ).…”

“…Within the recent decades, several technological innovations disrupted the rare-earth market [2], in turn stimulating the scientific quest for future materials. One vibrant field is the study of Eu 2+ compounds whose complex crystal structures are coupled with application-relevant properties including, to name only some recent examples, luminescence [3][4][5][6], field-induced reversal of the magnetoresistive effect [7], and complex magnetism [8,9]. The most renowned magnetic compounds are the europium chalcogenides that are considered ideal 3D Heisenberg systems [10].…”