Magnetization steps in the phase-separated Cr-doped Nd0.5Ca0.5MnO3 compounds

The manganite Nd0.5Ca0.5MnO3 has a CE-type antiferromagnetic-insulating (AFI) ground state at low temperatures. This compound undergoes a charge-ordering (CO) transition at T(CO)similar to250 K followed by a paramagnetic to AFI (orbital-ordered) OO phase transition at T(N)similar to170 K. The ferromagnetic (FM) double-exchange interaction between Mn ions is quenched by such a CO effect. Upon Cr doping, as in single crystals of Nd0.5Ca0.5Mn0.96Cr0.04O3, suppression of the quenched CO-OO state occurs at T(C)similar to140 K and is accompanied by a FM-metal-insulator transition. These Cr-doped materials have a low temperature ground state comprised of a mixture of 20-30 nm domains of the FM phase embedded in the CO-OO matrix, as inferred from Lorentz microscopy. Our results show that these Cr-doped materials undergo a field-dependent magnetic transition in a large range of temperature T<50 K, signed by a pronounced magnetization step in M(H) virgin curves. Moreover, from several isothermal hysteresis M(H) curves, we have observed that the FM state remains at H-->0 and does not collapse back to the AFI state in M(H) cycles performed up to 18 T and T<50 K. These results are different from those found in La0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3 compounds, where the induced FM state by H collapses back to AFI at low H. At 1.4 K, the M(H) virgin curves exhibit a sharp DeltaHsimilar to0.1 T magnetization step at H(j)similar to4.3 T, which is followed by saturation of M(H)similar to110 emu/g in magnetic fields up to H=18 T. The temperature dependence of H-j was found to obey an exponential decay up to Tsimilar to50 K. Such a temperature dependence of H-j indicates that the magnetization steps may not be related to a classical metamagnetic transition, as has been proposed for these manganite compounds. (C) 2004 American Institute of Physics.