Magnetism and magnetic neutron scattering

 Contact: Jörg Weissmüller

In conventional ferromagnets, the magnetic moments of each domain are aligned along one of the energetically preferred crystal directions ("easy axes"). However, in nanocrystalline ferromagnets, the direction of the easy axes jumps on the scale of grain size (i.e., a few nanometers). If the ferromagnetic exchange length is greater than this scale, the magnetization cannot follow the jumps. This has significant consequences for the macroscopic magnetic behavior. Magnetic neutron small-angle scattering is often the only way to study the variation in spin (mis-)orientation in volume and with the required resolution of nanometers. In addition, the method of field-dependent magnetic neutron small-angle scattering developed by us allows the quantitative investigation of magnetic interaction terms such as exchange coupling as well as the strength and spatial structure of the anisotropy field. 

This work is done in cooperation with  PD Dr. Andreas Michels.


Selected Publications:


J. Weissmüller, R.D. McMichael, A. Michels, and R.D. Shull
Small-Angle Neutron Scattering by the Magnetic Microstructure of Nanocrystalline Ferromagnets near Saturation
J. Res. NIST 104 (1999), 261

A. Michels, R.N. Viswanath, J.G. Barker, R. Birringer and J. Weissmüller
Range of Magnetic Correlations in Nanocrystalline Soft Magnets
Phys. Rev. Lett. 91 (2003) 267204

A. Michels and J. Weissmüller
Magnetic-field-dependent Small-angle Neutron Scattering on Random Anisotropy Ferromagnets
Rep. Prog. Phys. 71 (2008) 066501

G. Balaji, S. Ghosh, F. Döbrich, H. Eckerlebe and J. Weissmüller
Small-Angle Neutron Scattering of Nanocrystalline Terbium with Random Paramagnetic Susceptibility
Phys. Rev. Lett. 100  (2008) 227202.
 

Differenzieller Streuquerschnitt einer nanokristallinen Nickelprobe bei verschiedenen angelegten Magnetfeldern. Die Linien sind Fits mit einem mikromagnetischen Modell. Aus den Fitparametern können räumliche Struktur und Stärke des Anisotropiefeldes bestimmt werden. Diese Größen sind anderen Methoden typisch nicht zugänglich.

Differential scattering cross section of a nanocrystalline nickel sample at different applied magnetic fields. The lines are fits with a micromagnetic model. From the fit parameters, the spatial structure and strength of the anisotropy field can be determined. These quantities are typically inaccessible to other methods.

 

Modellunabhängig aus Neutronenstreudaten bestimmte Werte für die Korrelationslänge der Spinfehlorientierung in nanokristallinem Kobalt als Funktion des Magnetfeldes. Die Linie ist die Vorhersage des ‚Random Anisotropy’ Modells (ohne freie Parameter).

Independent of model neutron scattering data determined values for the correlation length of the spin misorientation in nanocrystalline cobalt as a function of the magnetic field. The line is the prediction of the "Random Anisotropy" model (without free parameters).