1. Collective Dynamics of Nanoscale Magnets
Per Nordblad, Uppsala University, Sweden

2. Collaborators
Petra Jönsson, Peter Svedlindh, ,Uppsala
Mikkel Fought-Hansen; Denmark
Sarbeswar Sahoo; Germany
Maxim Odnoblyudov; Russia
Jose Angel de Torro Sánches; Spain

3. OUTLINE: Particular magnetic media Isolated particle systems
Ferrofluids
Mechanially alloyed systems
Discontinuous metal insulator multilayers
Ni-particles
Isolated particle systems
Monodipersed particle size distribution
Polydispersed particle size distribution
Interacting particle systems – collective dynamics
Relaxation times and relaxation function
Non-equilibrium dynamics
Dimensionality
Conclusions

4. Monodipersed particles Fe(C) particles of size 5 nm

5. Monodipersed amorphous Ni-particles of size 2 nm
PLASMA
MICRODROPS
LASER
BEAM
SUBSTRATE
TARGET
DEPOSITED
NANOPARTICLES

6. Discontinuous Co-layers

7. Discontinuous

8. Relaxation times
0=10-10 s
Tc=40 K

9. Dipolar interaction - Ferrofluids
Particle size
Size distribution
Interaction strength
Isolated particles
Interacting particles

10. MC-simulations of a mono-dispersed particle system
Ms = A/m
K = J/m3
r = 3.5 nm
J.O. Andersson et al. Phys.Rev. B 56, (1997)

11. Time dependence from MC
T=20 K

12. Non-inteacting polydispersed
-Fe2O3 particles
c  0.03%
rav  3.5 nm
cf Monte Carlo monodispersed

13. Broadening of the Relaxation time spectrum

14. -Fe2O3-particles of size 70 nm
cf. MC-simulations from
Andersson et al
T. Jonsson et al. Phys. Rev.
Lett. 75, 4138 (1995)

15. Aging experiment
Thermal procedure in simple aging experiments on glassy systems.

16. ISOTHERMAL AGING ZFC Magnetization after different wait times
Isothermal Relaxation of c”

17. Influence of dipolar interaction on the relaxation rate
c=17 and 0.03 %, T=20 and 35 K
MC, T=20 K

18. Fe(C) ’monodispersed’ particles
Properties:
d = 5.3±0.3 nm
c = 0.06, 5 and 17%
K = J/m3
Ms = A/m

19. Frequency dependence ac-susceptibility Fe(C)
f = Hz
ZFC: t = 10 – 104 s
f = – 170 Hz

20. Model Systems
Ag(Mn)
Heisenberg
Fe0.5Mn0.5TiO3
ISING

21. Critical slowing down The slowing down of the relaxation times
with decreasing temperature can for the
two dense samples be described by critical
slowing down. The dilute sample follows an
Arrhenius law.
Parameters for the critical slowing down:
17%: z  10, Tg  50 K, 0  s
5%: z  11, Tg  35 K, 0  s

22. The Relaxation rate
FeC
particles
cf MC-simulations

23. Discontinuous layers
W. Kleemann et al. Phys. Rev. B 63, (2001)

24. Discontinuous Co-layers

25. Continuous
H=1mT
H (T)

26. Particles in metallic matrices

27. Model Systems
Ag(Mn)
Heisenberg
Fe0.5Mn0.5TiO3
ISING

28. Memory in an FeAgW mechanically alloyed system

29. Dimensionality: 3D or 2D
Myron B Salamon

30. Conclusions Interaction in nanoparticle systems introduce:
Collective dynamics
Spin glass relaxation
Re-inforcement of a frozen in spin (magnetic moment) structure.