Magnetization measurements on frozen ferrofluids: an attempt to separate interaction and anisotropy influences

Abstract

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.

Magnetic measurements on frozen ferrofluids as a method for estimating the magnetoviscous effect

“…Besides this, the local random anisotropy and dipole-dipole interaction were experimentally considered in [18]. Several years ago, novel strategies for magnetization measurements on frozen ferrofluids were presented [19,20]. One should expect that, if a frozen ferrofluid is heated under a constant field, its magnetization M(T ) should display a change at a certain temperature forced by structural transformations of internal magnetic structures.…”

Dependence of the Magnetization of an Ensemble of Single-Domain Particles on the Measurement Time within Various Experimental and Computational Methods

“…This equation has the same form as formula (9), in which the effective blocking temperature T r b * is substituted by T r b * * . The latter coincides with neither (10) nor (15); it equals…”

“…to the formula which was obtained by means of reduction of (10) to the dimensionless units. It is seen that T r b (see ( 13)) and T r b * (see (15)) are connected by a simple relation…”

“…( 18), we had to use assumption (16) which is not completely accurate in the strict sense. Moreover, it should be remembered that, to obtain (15), we used one more approximation which consisted in the extrapolation of the low-temperature high-field part of the τ * red vs h dependence to the zero field. In fact, the measuring time, which goes into these equations, can be interpreted as the time, during which the system relaxes from the magnetosaturated state after the instantaneous switching-on of the given field.…”

“…It should be noted that the calculation, which would be able to account for the mode of carrying out the ex-periment, of the magnetization curves for the ensemble of single-domain particles has remained a problem, for a solution of which various approaches continue to be proposed (see, for example, [14,15]).…”