Electron spin resonance study on room-temperature ferromagnetic La0.9Sr0.1MnO3 nano-particles

La_1-xSr_xMnO₃ half-metal manganites attract much attentions due to the colossal magnetoresistance (CMR) effect related to the Griffiths singularity with a critical percolation threshold (x=~0 .08) . For a ferromagnetic (FM) insulating homogeneous phase of bulk La_0.9Sr_0.1MnO₃ (T_C ~ 150 K), the Griffiths phase which contains both FM and paramagnetic (PM) domains could be induced by intrinsic inhomogeneity at a transition temperature T_G (275K) above T_C . Such Griffiths phase could also be affected by the size of sample when the sample dimension is downscaled to nanometers . Previous studies reported many interesting phenomena related to size effects including the size-induced structural transition, core-shell phase separation and the enhancement of magnetization and exchange bias . However, the studies on nano-size effects are mostly focused on the doped metallic phase . Recently, room temperature ferromagnetism is reported in nano-sized insulating La_0.9Sr_0.1MnO₃, which remains to be understood . In this work, temperature dependent magnetization (M-T) and electron spin resonance (ESR) are investigated in La_0.9Sr_0.1MnO₃ nano-particles . ESR is a very sensitive probe to detect the core-shell phase separation and FM signal in Griffiths phase . The field dependent M-T results show the coexistence of multiple magnetic phases which is similar to Griffiths phase in bulk samples . Multiple lines appear in ESR spectra . By fitting with Lorenz/Gaussian functions, the multiple phases are identified as FM and PM states for 270 <; T <; 310 K while they are antiferromagnetic (AFM) and FM states for T <; 270 K as shown in Fig . 1 . Our results indicate that both TC and TG are enhanced in nano-sized La_0.9Sr_0.1MnO₃ comparing with bulk one . Based on our analysis, the size effect - n the crystal structure of nanoparticle could be the cause for room-temperature ferromagnetism .