Magneto-optical behaviour of EuIn₂P₂

EuIn₂P₂ belongs to the broad class of materials known as Zintl compounds. The Zintl concept for the formation of compensated-valence intermetallic compounds is based on a complete charge transfer from an alkali or alkaline-earth element to a post-transition element of the groups 13 (IIIA) or 15 (VA) in the periodic table. The concept was later extended to synthesize more complicated intermetallic compounds with new types of structures and a variety of physical properties, such as magnetic order and superconductivity. Our compound orders magnetically at T_C = 24 K and exhibits concomitant large magnetoresistance effects. The main panel of Fig. 1.6 presents the field dependence of σ₁(ω) at 30 K and at frequencies in the FIR range, while its inset emphasizes σ₁(ω) across the entire covered spectral range. Besides the sharp IR phonon mode at 270 cm^-1 and the effective metallic component emerging at ω ≤ 200 cm^-1, σ₁(ω) also reveals a strong absorption peaking at about 600 cm^-1. This latter absorption is reminiscent to what has been seen in a large variety of so-called Kondo materials and ascribed to excitations across the hybridization gap. The most important qualitative observation is that the metallic (Drude) component as well as the feature at 600 cm^-1 increase with magnetic field, i.e. they both gain spectral weight. The trend of the magnetic field dependence of σ₁(ω) shown for 30 K in Fig. 1.6 is representative for all T ≤ 50 K. The enhancement of these respective mode strengths with magnetic field is only marginal in the σ₁(ω) spectra evaluated at temperatures above 50 K. The observed gain of Drude weight upon increasing magnetic field suggests an enhancement of the itinerant charge-carrier concentration due to the increasing magnetization, a phenomenon that was previously observed in other compounds, like e.g. in the Eu_1-xCa_xB₆ series, which exhibit colossal magnetoresistive effects. The similarities of the magneto-optical response in Eu_1-xCa_xB₆ and EuIn₂P₂ provide strong support to recently developed theoretical concepts based on the double exchange scenario.