MnxZn1−xFe2O4-based magnetic fluids with x = 0.1–0.9 are synthesized by coprecipitation. The samples are heated in a radio frequency (rf) magnetic field using an rf generator at different powers, and the temperature is measured as function of time using an optical thermometer. The heating effect of the dispersed magnetic nanoparticles is proportional to the imaginary part of the dynamic magnetic susceptibility of the ferrofluid, a quantity that depends on the temperature through the magnetization of the ferrite nanoparticles and the Néel or Brownian relaxation times, respectively. We propose an extrapolation method to actuate the Curie temperatures of the dispersed magnetic nanoparticles. By means of appropriate fitting functions for (dT/dt) versus T for both the heating and the cooling process, we deduce the Curie temperature of the samples under investigation. For MnxZn1−xFe2O4-based magnetic nanoparticles the Curie temperatures decrease with increasing Zn content. They turn out to be lower than the literature values for bulk MnxZn1−xFe2O4, a phenomenon which is generally observed for phase transitions of nanocrystalline materials.

Summary Colloids made from ferrite nanoparticles are a good example of how the properties of materials change when scaled down to the nanometre scale: the continuous spectrum of magnetic spin wave excitations breaks up to a set of discrete absorption lines. In order to exploit this phenomenon, the chemical composition of the ferrite as well as the preparation method for the suspensions are essential: in nickel zinc ferrites, the magnetic anisotropy field and the life time of magnetic resonance states can be fine tuned by the zinc content to achieve optimal microwave absorption. This makes such colloids interesting for high-power microwave applications, such as bond/disbond-on-command, that are difficult to obtain with polycrystalline materials.

We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent small-angle neutron scattering (SANS) data are analyzed quantitatively in terms of the correlation function of the spin misalignment. We find that up to applied fields of several tesla the magnetization remains "locked in" to the basal planes of the hcp crystal lattice of each individual crystallite, but that the in-plane orientation of the spins is highly nonuniform within each grain. This spin disorder at the nanoscale can be suppressed by a large applied field, but in the remanent state the disorder reduces the magnetization to values considerably below the Stoner limit. In field-dependent SANS, the intragrain spin disorder gives rise to a crossover of the scattering curves, and to the unusual finding that the scattering cross section at small scattering vector increases with increasing magnetic field. As the origin of the internal spin disorder within the grains, we propose an extra magnetic anisotropy energy at small grain size, presumably due to microstrain, a suggestion which is supported by analysis of ac-susceptibility data in the paramagnetic state. Our finding of a reduced remanence at small grain size is contrary to the remanence enhancement that is observed in other nanocrystalline hard magnets. We also report an unusual logarithmic field dependence of the magnetization over wide ranges of the applied field and temperature.

Magnetic nanometer-sized crystallites dissolved in an aerosol have been condensed on a substrate in the presence of a homogeneous magnetic field. Dipolar interaction favours chain formation, which on further growth leads to the self-organized formation of nanoparticle chain arrays that spontaneously undergo structural instabilities. When a critical chain length is exceeded, spontaneous formation of regular patterns is observed, similar to the Rosensweig instability in ferrofluids. We present a phenomenological stability analysis enabling to predict the magnetic-field dependence of the modulation wavelength of the observed hexagonal patterns.

We present an experimental study of the magnetic microstructure in the nanocrystalline hard magnet Tb. Field-dependent SANS data are analyzed quantitatively in terms of the correlation function of the spin misalignment. We find that up to applied fields of several Tesla the magnetization remains ‘locked in’ to the basal planes of the hcp crystal lattice of each individual crystallite; But that the in-plane orientation of the spins is highly nonuniform within each particle. This internal structure can be suppressed by the applied field.

Electric resistivity measurements in nanocrystalline Cu-samples alloyed with Fe in the concentration regime of C Fe ∼ 0.17 - 0.37 at - % and nanocrystallite sizes of 6 nm to 24 nm show a Kondo minimum at ∼ 30 K. Resistivity does not saturate at low temperature but passes over a maximum at ∼ 10 K, which may be due to a spin glass transition. An applied magnetic field of 0.5T shifts the minimum to lower T, but does not destroy the maximum. The curves measured are rather spiky in contrast to curves obtained from polycrystalline samples which is possibly due to non-selfaveraging effects. Further indication for such effects is a non-monotonuous dependence of the resistivity minimum as a function of average crystallite size.

In samples of nanocrystalline tungsten with a typical grain size of about 10 nm a minimum in resistivity has been observed at a temperature of Ti = 35 K by Ramansani (1). It is shown that this effect has its origin in the existence of a new length scale defined by the medium grain size in the nanostructured metal which leads to corrections to the conductivity tensor. These correspond to lowest-order quantum interference effects of conduction electrons at the grain boundaries. The relation to geometric Berry phases and Anderson-localization of electron states is pointed out. The results are in reasonable agreement with experiment. The effect can clearly be distinguished from other resistivity mechanisms such as spin-flip-scattering (Kondo-effect) or Mott metal-insulator transitions.