1. Krakow, NGCM2004 Change of Co nanoparticles magnetic properties with oxidation Perov Nikolai

2. Krakow, NGCM2004 Participants P.A. Chernavskii, G.V. Pankina, E.V. Pan’kova, N.S. Perov, A.A. Radkovskaya, P.M. Sheverdyaeva

3. Krakow, NGCM2004 1755-2005

4. Krakow, NGCM2004 Applications of magnetic particles Magnetic  Magnetic recording  Sensors  Microelectronics  (Magnets) Non-magnetic  Catalysts  Medicine

5. Krakow, NGCM2004 Sample preparation Porous material (based SiO 2 ) porous size about 3, 6 and 10 nm filled by the solution of Co(NO 3 ) 2 *6H 2 O. Drying in atmosphere at T=353 0 С during 4 hours. Annealing in atmosphere at T=500 0 С during 6 hours (  CoO 2 ). Reduction in H 2 atmosphere at T=300-500 0 С with magnetic moment control (CoO 2 – >Co).

6. Krakow, NGCM2004 The pore size distribution for the different silica gel types.

7. Krakow, NGCM2004 Exerimental setup 1 – electromagnet 2 – pick-up coil 3 – reactor (quartz tube) 4 - sample 1 2 3 4

8. Krakow, NGCM2004 The time dependence of the magnetic moment of the sample during the reduction in the pure hydrogen atmosphere.

9. Krakow, NGCM2004 Approximation was made with Langeven function: M/M s =L(  )=coth  - 1/  Here (d – diameter of the particle, M co – magnetization) Superparamagnetic particle remagnetization Re-magnetization curve of the superparamagnetic particles

10. Krakow, NGCM2004 Remagnetization of the system of the single domain particles Uniform distribution on the orientation

11. Krakow, NGCM2004 External conditions The size distributions of Co particles for the sample reduced in mixure of H 2 with water vapor (b) and sample reduced in pure H 2 at 300 0 C (a).

12. Krakow, NGCM2004 External conditions The size distributions of Co particles for the sample reduced in pure H 2 at 500 0 C (a) and sample reduced in 95%Ar+5%H 2 (b).

13. Krakow, NGCM2004 The hysteresis loops for samples of Co nanoparticles produced using the porous material (silica gel) with different porous size.

14. Krakow, NGCM2004 The coercive force dependence on the reduction degree for the silica gel with different porous size (T=773K).

15. Krakow, NGCM2004 The coercive force dependence on the oxidation degree for the silica gel with different porous size (T=293K).

16. Krakow, NGCM2004 The change of the particles size during oxidation (based Q-6 silica gel) S1 – before oxidation S2 – after oxidation 50%

17. Krakow, NGCM2004 Conclusions The coercivity stabilization occurs in the silica gel with the smaller size of the pores. As a result of oxidation the coercivity increases if the size of the Co particles is larger and decreases if the size of the particles is smaller, that is due to non-linear dependence of the coercivity on the particle size.

18. Krakow, NGCM2004 Thank you!