Weak ferromagnetic spin and charge stripe order in La_5∕3Sr_1∕3NiO₄

We present magnetization and specific heat data of a La_5∕3Sr_1∕3NiO₄ single crystal in high magnetic fields. From the charge and spin stripe ordering temperatures, as well as a magnetic low temperature transition, we have constructed the electronic phase diagram for fields up to 14 Tesla. While the charge stripe ordering temperature T_CO is independent of the magnetic field, there is a significant shift of the spin stripe ordering temperature T_SO of about 1.5 K∕Tesla, if the magnetic fields are applied parallel to the NiO₂-planes. The specific heat measurements indicate a large anomalous entropy change at T_CO. In contrast, no significant entropy change is observed at the spin stripe transition. The high field magnetization experiments reveal the presence of in-plane weak ferromagnetic moments in the charge stripe ordered phase. From a phenomenological analysis, the magnetic correlation length of these moments is determined. We suggest that the weak ferromagnetism is due either to the presence of bond-centered charge stripes or to double exchange interactions across site-centered charge stripes.