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

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

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

Monodipersed particles Fe(C) particles of size 5 nm

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

Discontinuous Co-layers

Discontinuous

Relaxation times 0=10-10 s Tc=40 K

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

MC-simulations of a mono-dispersed particle system Ms = 4.2 105 A/m K = 1.6 104 J/m3 r = 3.5 nm J.O. Andersson et al. Phys.Rev. B 56, 13983 (1997)

Time dependence from MC T=20 K

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

Broadening of the Relaxation time spectrum

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

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

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

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

Fe(C) ’monodispersed’ particles Properties: d = 5.3±0.3 nm c = 0.06, 5 and 17% K = 0.9 105 J/m3 Ms = 1.0 106 A/m

Frequency dependence ac-susceptibility Fe(C) f = 0.01 - .....9100 Hz ZFC: t = 10 – 104 s f = 0.017 – 170 Hz

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

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  2 10-8 s 5%: z  11, Tg  35 K, 0  2 10-5 s

The Relaxation rate FeC particles cf MC-simulations

Discontinuous layers W. Kleemann et al. Phys. Rev. B 63, 134423 (2001)

Discontinuous Co-layers

Continuous H=1mT H (T)

Particles in metallic matrices

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

Memory in an FeAgW mechanically alloyed system

Dimensionality: 3D or 2D Myron B Salamon

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