The optical properties of rare-earth hexaborides
The synthesis and investigations of physical properties of hexaborides XB6 (with X = Sm, Sr, and Eu), which all crystallize in the very simple CaB6 cubic like structure, is very important. Because these materials show various interesting properties, the electric, magnetic, and thermal properties have been widely investigated by many groups. However, their optical properties have been studied very little. We have embarked in the systematic measurement of the electrodynamic response of XB6 with emphasis on X = Sm, Sr, and Eu. Of particular relevance is EuB6, which is an intrinsic semimetal, and orders ferromagnetically with very large negative magnetoresistance below TC ~ 14 K. The nature of the ferromagnetic phase transition and its origin are still a matter of debate and attract a lot of interest both from the experimental and theoretical side. Although the origin of ferromagnetism and the nature of the EuB6 ground state are not yet fully settled down, there is strong evidence for a ferromagnetic metallic state. The magnetic phase transition is associated with a giant magnetoresistance. Some relationship with the colossal magnetoresistance in the perovskite La1-xSrxMnO3 systems has been also envisaged. Optical investigations are particularly suitable in order to achieve the complete electrodynamic response and to study the influence of the magnetic phase transition on the dynamics of the charge excitation spectrum. We have performed measurements of the optical reflectivity as a function of temperature from the far-infrared up to the ultra violet (i.e. from 2 meV up to 12 eV), as well as in magnetic fields. The most striking result consists in a red shift of the reflectivity plasma edge feature in the paramagnetic phase, followed by a giant blue shift of the plasma frequency at the onset of the ferromagnetic phase transition (Fig. 1.2). Such a blue shift of the plasma edge is further enhanced in magnetic fields (Fig. 1.3). Below TC the unscreened plasma frequency increases considerably, suggesting an increase of the itinerant charge carrier concentration, a reduction of their effective mass or a combination of the two, both driven by the onset of magnetic order.
Furthermore, we have also measured the magneto-optical (MO) Kerr effect, a suitable experimental tool for investigating the dependence of the electronic excitation spectrum on the degree of magnetic order. This spectroscopy yields information about the electronic transitions involving localized magnetic moments as well as the spin polarization of the itinerant charge carriers, especially in relation with the ferromagnetic phase transition (Fig. 1.4). First, we found a giant Kerr rotation occurring in the IR frequency range of the free-electron plasma edge of the reflectivity. It increases in magnitude and shifts to higher frequencies with either decreasing temperature or increasing magnetic field, which indicates that the spin polarized itinerant charge carriers are involved. In addition, a Kerr rotation signal at 1 eV follows the magnetic field and temperature dependence of the magnetic susceptibility and thus is associated with the f-electron response. This offers the possibility of discriminating between itinerant and localized optical responses and indeed opens new perspectives in studying ferromagnetism in metals.
We have measured the optical reflectivity R(ω) of Eu0.6Ca0.4B6 as a function of temperature between 1.5 and 300 K and in external magnetic fields up to 7 T. Recently, dc magneto-transport and magnetization experiments on a series of Eu1-xCaxB6 compounds provided results that reflect the intimate relation between the electronic conductivity and the magnetization. In particular, for the material with x = 0.4, an exponential decrease of the resistivity ρdc(T) as a function of magnetization at constant temperature close to and below the Curie temperature Tc = 4.5 K was reported. Our R(ω) increases with decreasing temperature and increasing field (Fig. 1.5) but the plasma edge feature does not exhibit the same remarkably sharp onset and steep slope that is observed in the binary compound EuB6. The analysis of the magnetic field dependence of the low temperature optical conductivity spectrum confirms the previously observed exponential decrease of the electrical resistivity upon increasing, field-induced bulk magnetization at constant temperature. In addition, the individual exponential magnetization dependences of the plasma frequency and scattering rate are extracted from the optical data.
Because of the potential technological applications, materials exhibiting colossal magnetoresistive (CMR) effects are of high current interest in solid state physics. Europium hexaboride EuB6 and the well known manganites, for which the onset of ferromagnetism is accompanied by a dramatic reduction of the electrical resistivity, are primary examples, that have intensively been studied. We concentrate on the series of cubic Eu1-xCaxB6, which displays interesting correlations between magnetic, transport and optical properties. Substituting Eu by Ca in ferromagnetic EuB6 leads to a percolation limited magnetic ordering. We present and discuss magneto-optical data of the Eu1-xCaxB6 series, based on measurements of the reflectivity R(ω) from the far infrared up to the ultraviolet, as a function of temperature and magnetic field.
Via the Kramers-Kronig transformation of R(ω) we extract the complete absorption spectra of samples with different values of x. The change of the spectral weight in the Drude component by increasing the magnetic field agrees with a scenario based on the double exchange model, and suggests a crossover from a ferromagnetic metal to a ferromagnetic Anderson insulator upon increasing Ca-content at low temperatures (Fig. 1.5b).
In fact, our magneto-optical investigations on the Eu1-xCaxB6 series reveal a phase diagram in support of a scenario based on the close proximity of the Fermi level and a magnetization dependent mobility edge. The ferromagnetic metal-insulator crossover occurs upon increasing the Ca-content in Eu1-xCaxB6 to above xMI. The magneto-optical data suggest a critical Ca-content xMI = 0.5.