We have prepared crystalline nanowires (diameter ~ 50 nm, length ~ a few microns) of the charge ordering manganite Pr 0.5 Ca 0.5 MnO 3 using a low reaction temperature hydrothermal method and characterized them using X-ray diffraction, transmission electron microscopy, SQUID magnetometry and electron magnetic resonance measurements. While the bulk sample shows a charge ordering transition at 245 K and an antiferromagnetic transition at 175 K, SQUID magnetometry and electron magnetic resonance experiments reveal that in the nanowires phase, a ferromagnetic transition occurs at ~ 105 K. Further, the antiferromagnetic transition disappears and the charge ordering transition is suppressed. This result is particularly significant since the charge order in Pr0.5Ca0.5MnO3 is known to be very robust, magnetic fields as high as 27 T being needed to melt it. a) Electronic mail: svbhat@physics.iisc.ernet.in 1

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In this paper we compare the magnetic and electron paramagnetic resonance (EPR) properties of Pr0.5Ca0.5MnO3 (PCMO) and Bi0.5Ca0.5MnO3 (BCMO) charge ordered manganites. PCMO and BCMO samples were prepared by solid state reaction method. A commercial SQUID magnetometer was used to study the dc-magnetization in the temperature range of 10-300 K. The EPR experiments were carried out using a commercial X-band (frequency ∼ 9.4 GHz) spectrometer in the temperature range 4 K – 300 K. Marked differences are found when the magnetic and EPR properties of rare earth (PCMO) and bismuth based manganites (BCMO) are compared. The temperature dependent resonance fields of PCMO and BCMO samples are seen to be opposite to each other. PCMO shows the resonance field behavior similar to other charge ordered manganites, i.e., decreases with the decrease in temperature whereas the resonance field of BCMO increases with a decrease in temperature. The opposite behavior of resonance field could be due to the different magnetic structures present in the BCMO manganite.

Nanowires of Pr0.57Ca0.41Ba0.02MnO3 (PCBM) (diameter approximately 80-90 nm and length approximately 3.5 microm) were synthesized by a low reaction temperature hydrothermal method. Single-phase nature of the sample was confirmed by XRD experiments. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to characterize the morphology and microstructures of the nanowires. While the bulk PCBM is known to exhibit charge order (CO) below 230 K along with a ferromagnetic transition at 110 K, SQUID measurements on the nanowires of PCBM show that the charge order is completely absent and a ferromagnetic transition occurs at 115 K. However, the magnetization in the nanowires is observed to be less compared to that in the bulk. This observation of the complete 'melting' of the charge order in the PCBM nanowires is particularly significant in view of the observation of only a weakening of the CO in the nanowires of Pr0.5Ca0.5MnO3. Electron paramagnetic resonance experiments were also carried out on the PCBM nanowires using an X-band EPR spectrometer. Characteristic differences were observed in the line width of nanowires when compared with that of the bulk.

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