Large variations in the magnetic ordering behavior of EuCu(2)As(2) with the application of external pressure and magnetic field.

The influence of external pressure on the electrical transport and magnetic properties of EuCu(2)As(2), crystallizing in a ThCr(2)Si(2)-type structure, is reported. The system is known to be an antiferromagnet below T(N) ≈ 15 K in the absence of external magnetic fields. We find that there is a gradual reduction of T(N) with the application of a magnetic field with an extrapolated value of the critical field of around 18 kOe which can drive T(N) to zero. Electrical resistivity under pressure (<11 GPa) reveals that the magnetic ordering temperature is pushed up dramatically to higher temperatures which is quite interesting if compared with the behavior in isostructural FeAs-based systems containing Eu. Above 7 GPa, the pressure-induced state appears to be ferromagnetic. The results thus reveal interesting changes in the magnetic ordering behavior of this compound with increasing pressure and magnetic fields.

Effects of Ru doping on the transport and magnetic properties of a La 1.32 Sr 1.68 Mn 2-y Ru y O 7 layered manganite system.

The low temperature magnetization, specific heat, electrical resistance and magnetoresistance have been studied for the Ru-doped La(1.32)Sr(1.68)Mn(2 - y)Ru(y)O(7) (y = 0.0, 0.04, 0.08 and 0.15) layered manganite system. The undoped compound (y = 0.0) shows a sharp ferromagnetic transition (T(C)) accompanied by a metal-insulator transition (T(MI)) at 118 K. The Ru substitution decreases the T(C) and T(MI) temperatures significantly. The temperature dependence of specific heat measurement confirms the decrease in T(C) by observing the anomaly corresponding to T(C). The decreased effective moments from 3.48 µ(B) for the undoped compound to 1.82 µ(B) for the highly doped compound at 5 K indicates the Ru substitution weakens the ferromagnetic order in the low temperature regime and reduces the number of Mn pairs in the highly doped sample. The field dependence of magnetization measurements exhibits an enhancement of the coercive field with increased Ru concentration and gives evidence for the mixed magnetic phase for the highly doped compound. For the undoped sample, a large negative magnetoresistance of 300% at T(C) and 128% at 4.2 K in a 5 T field were observed. The magnetoresistance ratio decreases gradually with increasing Ru substitution. We find that the doped Ru in the Mn site drives the layered manganite system towards a magnetically mixed state. The effects of Ru doping in the transport and magnetic properties will be explained by the antiferromagnetically coupled Ru and Mn sublattices.