Carrier-induced Magnetic Circular Dichloism in the Magnetoresistive Pyrochlore Tl2Mn2O7

Infrared magnetic circular dichloism (MCD), or equivalently magneto-optical Kerr effect, has been measured on the Tl 2 Mn 2 O 7 pyrochlore, which is well known for exhibiting a large magnetoresistance around the Curie temperature T C ∼120 K. A circularly polarized, infrared synchrotron radiation is used as the light source. A pronounced MCD signal is observed exactly at the plasma edge of the reflectivity near and below T C . However, contrary to the conventional behavior of MCD for ferromagnets, the observed MCD of Tl 2 Mn 2 O 7 grows with the applied magnetic field, and not scaled with the internal magnetization. It is shown that these results can be basically understood in terms of a classical magnetoplasma resonance. The absence of a magnetization-scaled MCD indicates a weak spin–orbit coupling of the carriers in Tl 2 Mn 2 O 7 . We discuss the present results in terms of the microscopic electronic structures of Tl 2 Mn 2 O 7 .