PT Unknown
AU Bedanta, S
TI Supermagnetism in magnetic nanoparticle systems
PD 02
PY 2007
LA en
DE magnetic nanoparticles; superparamagnetism; superspin glass; superferromagnetism; domain wall motion; magnetic inter-layer coupling; ferrofluids
AB Nanoscale magnetic materials are of interest for applications in ferrofluids, high-density magnetic storage, high-frequency electronics, high performance permanent magnets, and,
magnetic refrigerants. Magnetic single-domain nanoparticles (“superspins) are very interesting not only for potential applications, e.g. high density storage devices, but also for
fundamental research in magnetism. In an ensemble of nanoparticles in which the interparticle magnetic interactions are sufficiently small, the system shows superparamagnetic (SPM) behavior as described by the Néel-Brown model. On the contrary, when interparticle
interactions are non-negligible, the system eventually shows collective behavior, which overcomes the individual anisotropy properties of the particles. In order to address the effect of interactions, we have investigated two different magnetic nanoparticle
systems.
The first part of this thesis focuses on the magnetic properties of ensembles of magnetic single-domain nanoparticles in an insulating matrix. The samples have a granular multilayer structure prepared as discontinuous metal-insulator multilayers (DMIM) [Co80Fe20 (tn)/Al2O3 (3nm)]m where the nominal thickness of CoFe is varied in the range
0.5 £ tn £ 1.8 nm, and the number of bilayers m is varied between 1- 10. The DMIMs represent a model system to study the effect of inter-particle interactions by varying the nominal thickness which corresponds to the magnetic particle concentration. The structural
properties are investigated by transmission electron microscopy, small angle X-ray reflectivity and electric conductivity measurements. It is found that CoFe forms wellseparated and quasi-spherical nanoparticles in the Al2O3 matrix, and the samples exhibit a regular multilayer structure. The magnetic properties are investigated by means of dc
magnetization, ac susceptibility, polarized neutron reflectometry (PNR), magneto-optic Kerr effect and ferromagnetic resonance. The DMIM system with the lowest tn = 0.5 nm, in which the inter-particle interaction is almost negligible, single particle blocking has been
observed. When increasing the nominal thickness to tn = 0.7 nm and, hence, increasing the inter-particle interaction, the system shows spin glasslike cooperative freezing of magnetic
moments at low temperatures. Superspin glass properties have been evidenced by static and dynamic criticality studies such as memory and rejuvenation. With further increase of nominal thickness and hence stronger interaction, the system shows a superferromagnetic
(SFM) state, e.g., at tn = 1.3 nm. A SFM domain state has been evidenced by Cole-Cole analysis of the ac susceptibility and polarized neutron reflectivity measurements. Finally, the SFM domains have been imaged by synchrotron based photoemission electron microscopy (PEEM) and magneto-optic Kerr microscopy. Stripe domains stretched along
the easy in-plane axis, but exhibiting irregular walls and hole- like internal structures (“domains in domains”) are revealed. They shrink and expand, respectively, preferentially by sideways motion of the long domain walls as expected in a longitudinal field. The SFM
domain state is explained by dipolar interaction and tunneling exchange between the large particles mediated by ultrasmall atomically small magnetic clusters. These have been evidenced by their sizable paramagnetic contributions, first in systems referring to tn = 0.5
nm and 0.7 nm, but later on also at SFM coverages, tn = 1.3 nm and at higher coverages. These ultrasmall particles (atoms?) are undetectable in transmission electron microscopy. At tn = 1.4 nm, physical percolation occurs and a conventional three-dimensional
(3D) ferromagnetic phase with Ohmic conduction is encountered. Polarized neutron reflectivity and magnetometry studies have been performed on the DMIM sample with tn = 1.6 nm which exhibits dominant dipolar coupling between the ferromagnetic layers. Our PNR measurements at the coercive field reveal a novel and unexpected magnetization state of the sample exhibiting a modulated magnetization depth profile from CoFe layer to layer with a period of five bilayers along the multilayer stack. With the help of micromagnetic
simulations we demonstrate that competition between long and short-ranged dipolar interactions apparently gives rise to this unusual phenomenon.
In the second part of the thesis the structural and magnetic properties of FeCo nanoparticles in liquid hexane will be analyzed for two different concentrations of the ferrofluids. Inter-particle SFM ordering between FeCo nanoparticles are evidenced by magnetization measurements and ac susceptibility measurements. Mössbauer spectroscopy measurements are shown to evidence collective inter-particle correlations between the nanoparticles.
ER