Title :
Magnetorelaxation (MRX) Measurements With DC-SQUID Gradiometers
Author :
Büttner, M. ; Schmidl, F. ; Schiffler, M. ; Seidel, P.
Author_Institution :
Inst. fur Festkorperphys., Friedrich-Schiller-Univ. Jena, Jena, Germany
fDate :
6/1/2011 12:00:00 AM
Abstract :
Magnetic nanoparticles (MNP) show different relaxation mechanisms after switching off an external field. The Brownian relaxation of the free rotation of the whole particles and the Néel relaxation of the magnetization vector inside a fixed particle have different time scales. While the Brownian relaxation time is in the order of milliseconds the Néel relaxation time is in the second range. Thus a clear separation of free and fixed MNP is possible. To measure the very small magnetic relaxation fields we have developed MRX systems using low temperature superconductor thin film second order SQUID gradiometers working in unshielded environment. By standard MRX we are able to separate both relaxation processes to determine the amount of bounded MNP as results of binding reactions. By Néel measurements with a x-y scanning MRX system a spatial resolved imaging of the bounded particles is possible even for living objects. We try to improve the spatial resolution of our MRX system by a small ferromagnetic needle. Within an anti-cryostat we can measure the temperature dependence of small samples (TMRX) between 4 K and 300 K. From Néel signals using some theoretical modeling important parameters of the MNP such as energy barrier distribution, activation energy, anisotropy constant and diameter of the magnetic core were obtained.
Keywords :
SQUID magnetometers; magnetic anisotropy; magnetic relaxation; magnetic variables measurement; Brownian relaxation; DC SQUID gradiometer; Neel relaxation; activation energy; anisotropy constant; anti cryostat; energy barrier distribution; ferromagnetic needle; low temperature superconductor thin film second order SQUID gradiometers; magnetic core diameter; magnetic nanoparticle; magnetization vector; magnetorelaxation measurement; relaxation mechanism; relaxation process; temperature 4 K to 300 K; Magnetic cores; Magnetometers; Nanoparticles; Perpendicular magnetic anisotropy; SQUIDs; Superconducting magnets; Temperature measurement; Anisotropy constant; SQUIDs; biomagnetics; magnetic field measurements; relaxation measurements;
Journal_Title :
Applied Superconductivity, IEEE Transactions on
DOI :
10.1109/TASC.2010.2077616