Perpustakaan Digital ITB

Magnetically frustrated compounds are promising for the emergence of various exotic magnetic states. They can demonstrate spin-glass behavior, generate a variety of spin spirals or even host magnetic skyrmions. Geometrically, magnetic frustration is a situation where most of the magnetic sites in a lattice are subject to competing or contradictory exchange interactions, which usually arises from the geometry or topology of the lattice. One such compound of interest is NaCrO2. Polycrystalline NaCrO2 is reported to be a strongly frustrated two-dimensional triangular lattice antiferromagnet. However, this compound has only been studied for its bulk. Moreover, the method to grow the single crystal of this compound has not been wellestablished yet. Here, we report on several attempts to grow single crystals of NaCrO2 using flux method with various fluxes such as NaCl, PbO – B2O3, and B2O3. The structural properties of the obtained crystals are then characterized using X-Ray Diffraction (XRD), single-crystal XRD, Scanning Electron Microscopy (SEM), dan Energy Dispersive X-Ray Spectroscopy (EDS). The magnetic properties of the crystals are characterized using Magnetic Properties Measurement System (MPMS) and then compared with the magnetic properties of previously investigated polycrystalline NaCrO2. After the solid-state synthesis, dark-green NaCrO2 powder was obtained. The powder was then analyzed using powder XRD and the Rietveld refinement results showed that the composition of NaCrO2 and Na2CrO4 in the sample is 46:4. This is possibly caused by oxidation of NaCrO2 into Na2CrO4 during sample preparation since it can be easily oxidized when exposed to open air. Na2CrO4 is not really interesting to be investigated since it is a non-magnetic compound. The obtained NaCrO2 powder was then recrystallized using the NaCl flux. The obtained crystals have a dark green color surrounded by transparent crystals of NaCl flux. The crystals grow preferentially on the wall of the quartz tube and it is difficult to separate the crystals from the quartz tube. After the washing process, the crystals are still dark green in color, but they are closely attached to each other and cannot be further investigated as a single crystal. Then, two different results were observed from single crystal synthesis using B2O3 flux, black part at the top and dark-green part at the bottom. Powder and single crystal XRD results of the black part confirms that it is mostly consisted of Cr2O3 and a small amount of NaCrO2. After washed with distilled water, the obtained crystals had a flat structure and dark-green color when observed under an optical microscope. Meanwhile, the bottom part consists of Cr2O3 and Na2CrO4 as confirmed by powder and single crystal XRD results. Some of the NaCrO2 compounds may be oxidized during sample preparation. The dark-green part became yellow when ground in open air due to changes in oxidation state from Cr3+ into Cr6+. Several methods have been done in order to grow a single crystal of NaCrO2 and the most suitable method to grow crystal with nice shape is using PbO – B2O3 flux with the actual crystal composition of Na0.86CrO2. The issue faced during the washing process with HNO3 or/and distilled water is the deintercalation of Na+ ions on the surface of the crystal which is affected by the thickness of the crystal. In order to prevent the deintercalation process, a thicker crystal is needed that can be grown using a bigger amount of flux. A bigger amount of flux makes it easier to attain supersaturation and to form the nuclei of the crystalline phase. However, the magnetic properties of the Na0.86CrO2 crystal is different from bulk NaCrO2 and cannot be fitted using Curie-Weiss fit but still have transition around 46 K that can be attributed to the Néel temperature of NaCrO2. The magnetic susceptibility measurement for both of samples grown using PbO – B2O3 flux shows that the spin ordering is maintained above the room temperature different from that of bulk NaCrO2 that the antiferromagnetic ordering vanishes after 46 K. On the other hand, the magnetic field dependence of magnetization curve shows increasing magnetization with increasing applied magnetic field for both samples similar to that of bulk NaCrO2.