In this contribution we focus on a novel way to perform and evaluate magnetization experiments on frozen ferrofluids or magnetic nanoparticles fixed in space. The basic idea is to project the behaviour of a real particle system onto a Stoner-Wohlfarth (SW) particle ensemble behaviour. Therefore first an ideal SW particle system acting as a reference system with unique particle sizes and orientation of easy axis was studied numerically by introducing a particle-particle interaction via a demagnetizing field. We have shown that for non-interacting SW particles a universal mean value rule holds that the magnetization measured after ZFC (zero field cooling) can be expressed as a mean value of the magnetization measured after PHFC and NHFC (positive and negative high field cooling) processes. Any deviation from this mean value rule is therefore due to non-SW behaviour (multidirectional anisotropy and interaction) only and can be derived from the magnetization measurements and be compared with simulations.By this new method it is possible to determine the mean value of the particle's anisotropy and the mean interaction which is expressed here as a magnetic field.We will report on experiments performed on frozen ferrofluids because these systems offer the possibility of complete thermal demagnetization after melting. In this contribution we describe experiments performed on two different frozen Co-based ferrofluids by SQUID magnetometry.

Ensembles consisting of Co nanoparticles fixed in space by freezing ferrofluids are investigated. In frozen ferrofluids a well defined low temperature magnetic state of the nanoparticle system can be achieved. We distinguish between textured samples (preferred orientation of easy axes) and non-textured with randomly oriented easy axes. Considering the magnetization curves of such systems bears in principle the possibility to distinguish between influences arising from particle anisotropy and particle-particle interaction. Our intention is to present new experimental strategies aiming to separate these influences from magnetization measurements versus temperature. In another approach we analysed the susceptibility derived from magnetization measurements and determined the mean magnetic moment of the particles and an additional field which may be caused by the anisotropy of the particles or the interaction between them. Transmission electron microscopy was used for the direct measurement of the particles' sizes and their size distribution.

PACS 75.50.Mm, 75.75.+a The best way to achieve a virgin state of a magnetized sample is to heat it up to a temperature high enough to destroy every magnetic order and thereafter cooling it down in a zero magnetic field. This procedure is often not possible because high temperatures may cause irreversible changes in the sample's constitution e. g. by crystallization in amorphous systems. Because of magnetic history and memory effects it is difficult to measure the same initial magnetization curves after different demagnetizing procedures for materials consisting of nanoparticles with high Curie temperatur TC . Very often the demagnetization by alternating decreasing magnetic fields (AC demagnetization) is used instead. Ferrofluids offer the possibility to demagnetize the sample by heating it above the melting point of the liquid in zero external field. The brownian motion destroys any "frozen in" magnetic order and relieves single domain particles in the fluid. After zero field cooling the virgin state may be reproducibly achieved. We present an experimental study showing the influence of different demagnetization sequences on the initial magnetization curves in frozen ferrofluids. In this contribution we show the deviation between thermally demagnetized and AC demagnetized samples by substracting both initial curves from each other. We detect a "frozen in" pattern which shows the "steps" of the AC demagnetization sequences. In a further step we focus on the experimental determination of the switching field distribution (SFD) of the particles. We compare the SFD with a plot proposed by Thamm and Hesse (1996). ExperimentsMagnetization experiments were performed in a Quantum Design SQUID Magnetometer (MPMSR2) localized at the Physikalisch-Technische Bundesanstalt (PTB) Braunschweig, Germany. We present investigations on one ferrofluid consisting of Co-particles with o-dichlorobenzene (ODCB) as solvent. The ferrofluid was prepared using high-temperature solution-phase synthesis ([1] and references herein). The particle radii derived from room temperature magnetization measurements can be fitted very well by a lognormal distribution. The expectation value of the fitted log-normal distribution is approximately 1.6 nm and the standard deviation 0.5 nm. These values are in very good agreement with results from electron microscopy. For high resolution electron microscopy (HRTEM) the samples were prepared by dipping the electron transparent TEM carbon grids into the highly diluted ferrofluid. During this procedure the particles tend to self-organize and form a hexagonal superlattice of up to 1µm×1µm in size. The ODCB has a melting point of around 220 K. The further discussed measurements are all performed at 5 K which implies that the solution is frozen and the particles are fixed in space.

International audienceWe have investigated the contribution of stochastic thermally activated processes to the electric-field effects on coercivity in FePt. Coercive field distributions were measured under different gate voltages in solid-state field-effect structures. For low voltages a shift in the coercive field distribution can be observed, however, it is not larger than the width of the distribution. Higher oltages are needed to obtain the splitting from the negative (zero) voltage distribution allowing for the unambiguous characterization of the electric-field effect. A virtual unipolarity in the electric-field effect has been identified as a feature introduced by the dielectric layer that disappears upon annealing

Magnetization measurements are the "classic" experimental method to characterize systems of single domain magnetic particles. We consider such particles fixed in space so that the orientation of the magnetization vector of each particle can be influenced by an external magnetic field with respect to characteristic particle directions, i.e. easy directions in the case of anisotropy energy. The aim of this contribution is directed towards experimentalists. We will enhance the well-known idea of the "preparation" of different low temperature magnetic states (LTMS), introduce the quasi-paramagnetic state which can be reached by the PHFC (positive high field cooling) process, briefly discuss the NHFC (negative high field cooling) and other ways to reach the LTMS. Additionally, our aim is to persuade researchers to consider the susceptibility rather than the magnetization. The most interesting topic is here the pronounced susceptibility maximum which appears in a system of small single domain particles prepared in a LTMS by a PHFC process when measuring in a constant external field versus temperature. We use the susceptibility for an ideal super-paramagnet as a limiting case and consider step by step what are the influences on the susceptibility if the real particle systems exhibit magnetic anisotropy energy, particle-particle interactions and last but not least a particle size distribution.