Relaxation of low-temperature magnetoresistance and magnetization of polycrystalline (La_(0.5)Eu_(0.5))_(0.7)Pb _(0.3)MnO_3

摘要

Hysteresis and relaxation of magnetoresistance and magnetization of substituted (La_(0.5)Eu_(0.5))_(0.7)Pb _(0.3)MnO_3 lanthanum manganite in a low-temperature region (<40 K) are investigated. It is shown that at these temperature features of the magnetoresistive effect are determined mainly by spin-dependent tunnelling of carriers via insulating grain boundaries. As was demonstrated previously, the grain boundaries may be antiferromagnetically ordered. Therefore, relaxation of magnetization and resistance is determined by the processes of relative orientation of the magnetic moments of ferromagnetic domains neighbouring the antiferromagnetic boundary of ferromagnetic domains under the action of temperature fluctuations. It is shown that relaxation follows the logarithmic law within the time interval t ～ 10~2-3×10~3 s. A comparison between time evolutions of the magnetic moment and resistance shows that magnetoresistance and magnetization are related as δR = δM ~n, where n = 2.5. The obtained value n is close to the characteristic value n = 2 for tunnel magnetoresistance of granular ferromagnetic metal/insulator systems.