Evidence for magnetic polarons in the magnetoresistive perovskites

Manganese perovskites based on the compound LaMnO3 are attracting considerable theoretical and technological interest by virtue of their unusual magnetic and electronic properties1–4. Most notable of these properties is the extremely large change in resistivity that accompanies the application of a magnetic field, an effect known as 'colossal' magnetoresistance. The origin of this effect has been attributed5–7 to the presence of magnetic polarons—charge carriers accompanied by a localized (and magnetically polarized) distortion of the surrounding crystal lattice8,9— but their existence and properties remains a matter of speculation. Here, using a combination of volume thermal expansion (with and without an applied field), magnetic susceptibility and small-angle neutron scattering measurements, we present evidence for the existence of magnetic polarons above the ferromagnetic ordering temperature, Tc. We detect the spontaneous formation of localized ∼12-A magnetic clusters above Tc which, on application of a magnetic field, grow in size but decrease in number. We argue that the response of these magnetic polarons to an applied magnetic field underlies the pronounced magnetoresistive properties in the compounds (La1–xAx)2/3Ca1/3MnO3 (where A is Y or Tb).