Magnetic properties of carbon nanocluster

Transcription

1 Magnetic properties of carbon nanocluster Roberto Escudero Departamento de Materia Condensada y Criogenia Instituto de Investigaciones en Materiales Universidad Nacional Autónoma de México Symposium en honor del prof. Ivan Schuller October 2011

2 Colaboradores en este estudio: R. Caudillo. J.B. Goodenough. M. José Yacaman. X. Gao. The University of Texas At Austin

3 Carbon-encapsulated Ag nanoparticles: Magnetic behavior in C. Recent reports of ferromagnetic (FM) behavior in a variety of carbon materials have sparkled interest into magnetic properties of carbon. Doubs on the intrinsic FM with 500 K Curie temperature in polymerized rhombohedral C 60 Detailed chemical analysis reveled iron content. For C 60, FM was reported in tetrakis (dimethylamino(ethylene-c 60, and 3-aminopheny-methano- C 60 with T C below 17 and 19 K, respectively. In graphite proton irradiated, nanographite, graphite containing topographic defects, negative curavture Schwarzite-like carbon nanofoams, fullerene related carbons, microporous carbon, and carbon nanohorns. A clear explanation related to the origin of the magnetic behavior is not complete. Suggestions are: that hydrogen may play an important role in determining the magnetic propertiies, or that oxygen (in nanohorns). Other research points to the importance of edge-states or topographic defects or negative curvature in the graphene sheets.

4 Characteristics of the Ag-C samples 100 nm SEM images of Ag-C samples at 3 different magnifications. It seems that Ag-C powder consists of spherical particles connected in necklace-like structures. 10 nm

5 Figure 2 HRTEM of Ag-C nanoparticles surrounded by graphitic carbon. C nanospheres show areas with an expanded interplanar spacing of 3.7 A. Inset shows the X-ray diffractogram for Ag-C sample indexed for cubic Ag phase Fm3m- Peak broadening has an average Ag crystallite size of about 10 nm.

6 Removal of Ag from the carbon nanospheres By electron irradiation in TEM. Ag-C samples before Ag removal Same area after Ag removal. 50 nm

7 SEM image of a 10 ton pressed sample. The Ag nanoparticles coalesce, into larger particles, showing marked faceting.

8 Magnetic Behavior Ag-C samples and pressed samples analyzed with a MPMS SQUID QD. Magnetization Temperature (M-T), and M-H measurements were performed at different temperatures. M-T measurements performed in ZFC and FC modes. M-H performed in order to analyze the coercive field behavior, and to determine the Curie temperature. Isothermal M-H measurements performed from K.

9 Magnetic characteristics and possible contamination. The compound presents ferromagnetic behavior. The possibility that this magnetism is due to 3d metal contamination is discarded, two studies were performed: The first one is analysis with chemical analysis with EDS, and trace metal analysis with ICP-MS. Those analysis indicated that there are no magnetic contaminants in the sample. The second study was related to the pressure applied to the sample, and changes observed when the sample was measured after the applied pressure: Under pressure ferromagnetism disappear, and the sample turned diamagnetic.

10 Magnetic measurements in pristine Carbon-encapsulated Ag nanoparticles as show in Figure 2 FC, and ZFC indicate field cooling and zero field cooling set of measurements. 1. ZFC: the sample is cool down to the minimum temperature, and once in thermal equilibrium the magnetic field is applied. Measurement is performed increasing T. 2. FC: The sample is under an applied magnetic field at high temperature. Measurement is performed with decreasing T.

11 Ag-C: Note the non-curie-weiss behavior, and the peak in ZFC (only) at about K. Two possible explanation for this magnetic peak behavior. Oxygen presence and/or negative curvature. In general, in magnetic measurements (M-T) a peak may be observed at around K. This could be due to oxygen impurities (trapped oxygen in the sample). Oxygen at those temperatures presents many magnetic anomalies. The peak is observed when the oxygen contamination is big, or when the magnetic susceptibility is small.

12 Different behavior with samples pressed and non-pressed Ferromagnetic 4 M( 10-4 emu) ZFC warming FC warming H = 1T, 1 ton T (K) Diamagnetic: (note the minus sign in the magnetization).

13 STM figures at 3 different magnifications. After pressure by 10 Tons.

14 Magnetization measurements (M-T) in ZFC and FC after 1 ton, applied pressure into the sample. Note the magnetic behavior, changing from Ferromagnetism to Diamagnetism Diamagnetism is the normal carbon behavior. The peak at K clearly is due to oxygen contamination.

15 Typical magnetic characteristic of Ag-C sample Coercive field is the signature of ferromagnetism. The coercive field is at 2 K, about 665 Oe. The saturation magnetization is reached at 1 Tesla.

16 Model of medium field behavior, displaying that the Curie Temperature is about 430 K, for Ag-C samples.

17 ICP-MS elemental analysis performed in Ag-C samples. The amount of impurities is very small, and accordingly the Curie temperature does not fit to any 3d elements. Ni was the only possibility, however, Tc of Ni is about 627 K.

18 Physical interpretation sp 2 bonding gives a graphene sheet its mechanical properties p π electrons are responsible for its electronic and magnetic properties. In a flat graphene sheet the pπ electrons are itinerant, but in a narrow p π band. Introduction of curvature to the graphene sheets, as in Ag-C narrows the p π band sufficiently to result in a ferromagnetic behavior.

19 Physical interpretation Graphite configuration of carbon in different planes. Schematic illustration showing the α and β-sites in graphite.

20 Physical interpretation Importance of curvature Schematic representation of the transition from p π delocalized spins in a planar Graphene sheet toward sp 1 localized spins in a curved graphene sheet. In graphite sp 2 +p π bonding gives metallic behavior from itinerant p π electrons both in the ab-plane and in a narrow c-axis dispersion bands, introduction of curvature changes the sp 2 +p π towards sp 3

21 Schematic of the ferromagnetic superexchange interation between β-sites through a strongly correlated ¾ filled α band (sp) 1 indicates a half filled (sp) orbital and (sp) 2 is a filled (sp) orbital, U is the correlation splitting of the (sp) 1 and (sp) 2 states.

22 Influence of hydrostatic pressure in Carbon foam silver. Using a diamond cell.

23 Conclusion of this study: Carbon structures may present magnetic behavior due to Vacancies in graphitic sites. Graphitic structures with negative curvature give rise to Ferromagnetic behavior. This study indicates that Ag-C and C samples presented a ferromagnetic behavior up to 430 K.