Chemically induced magnetism and magnetoresistance in La(0.8)Sr(1.2)Mn(0.6)Rh(0.4)O(4).

It is shown by magnetometry and microSR spectroscopy that short-range magnetic interactions between the Mn cations in the nonmetallic K(2)NiF(4)-like phase La(0.8)Sr(1.2)Mn(0.6)Rh(0.4)O(4) become significant below approximately 200 K. Negative magnetoresistance (rho/rho(0) approximately 0.5 in 14 T at 108 K) is apparent below this temperature. Neutron diffraction has shown that an applied magnetic field of 5 T is sufficient to induce saturated (3.38(7)mu(B) per Mn) long-range ferromagnetic ordering of the atomic moments at 2 K, and that the induced ordering persists up to a temperature of 50 K in 5 T. Spin glass behavior is observed below 20 K in the absence of an applied field. The induced magnetic ordering is attributed to the subtle changes in band structure brought about by the external field, and to the controlling influence of Rh(3+) over the relative strength of competing magnetic exchange interactions.