Magnetic Nanoparticles

Magnetic nanoparticles and thin films exhibiting large magnetic anisotropy energy densities (MAE) have attracted enormous attention over the last years due to their potential use in magnetic data storage or sensing applications. FePt alloy nanoparticles and films have a huge MAE in the chemically ordered L10 phase. This ordered phase is typically obtained by annealing the as-prepared chemically disordered fcc FePt nanoparticles at temperatures above 600°C. Besides standard colloidal approaches [1], such fcc FePt nanoparticles can be fabricated by a micellar technique resulting in regular arrays on various substrates [2,3]. This approach offers the advantage that the interparticle distance can be tuned between 20-100 nm. Due to the larger separation particle agglomeration can be avoided during the annealing induced transformation into the L10 phase. Moreover, the particles are magnetostatically decoupled.Preparation and characterization of such particles are  tasks within the framework  Collaborative Research Centre SFB 569 "Hierarchic Structure Formation and Function of Organic-Inorganic Nanosystems"


Fig. 1 Phase diagram of Fe und Pt. Cubic FCC ordered Fe50Pt50 changes under annealing temperatures of around 600°C to the tretragonal L10 phase which is ferromagnetic.


Sample preparation

Micellar FePt particles are prepared by dip coating of Fe and Pt salt loaded reverse micelles and following plasma etching treatment to remove organic molecules and to reduce oxides [4].
The chain length of micelle building blocks defines the size and distance [5].


Fig. 2 Process of micellar FePt particle preparation by dip coating.


Transmission Electron Microscopy

Electron microscopy especially transmission electron microscopy is essential for characterization of nanoparticles. Size, shape, faceting and crystal structure can be determined by bright-field, dark-field and HRTEM imaging. The composition of the nanoparticles can be determined by spectroscopy using EDX-, EELS-STEM and energy filtering (EFTEM). 


Fig. 3 Cross sectional bright-field TEM image of FePt particles on MgO (left: BF-TEM image, right: Aberration corrected high resolution image of FePt particles on MgO [100]. Larger particles exhibits defects and are not perfectly aligned along the MgO lattice.


[1] C. Antoniak, J. Lindner, M. Spasova, D. Sudfeld, M. Acet, M. Farle, K. Fauth, U. Wiedwald, H.-G. Boyen, P. Ziemann, F. Wilhelm, A. Rogalev and Shouheng Sun
Enhanced Orbital Magnetism in Fe50Pt50 Nanoparticles
Phys. Rev. Lett. 97, 117201 (2006)

[2] A. Ethirajan, U. Wiedwald, H.-G. Boyen,  B. Kern, L. Han, A. Klimmer, F. Weigl, G. Kästle, P. Ziemann, K. Fauth, J. Cai, R. J. Behm, A. Romanyuk, P. Oelhafen, P. Walther, J. Biskupek and U. Kaiser
A Micellar Approach to Magnetic Ultrahigh-Density Data-Storage Media: Extending the Limits of Current Colloidal Methods
Adv. Mater. 19, 406 (2007)

[3] U. Wiedwald, A. Klimmer, B. Kern, L. Han, H.-G. Boyen, P. Ziemann, and K. Fauth
Lowering of the L10 ordering temperature of FePt nanoparticles by He+ ion irradiation
Appl. Phys. Lett. 90, 062508 (2007)

[4] U. Wiedwald, K. Fauth, M. Heßler, H.-G. Boyen, F. Weigl, M. Hilgendorff, M. Giersig, G. Schütz, P. Ziemann and M. Farle
From Colloidal Co/CoO Core/Shell Nanoparticles to Arrays of Metallic Nanomagnets: Surface Modification and Magnetic Properties
ChemPhysChem 6, 2522 (2005)

[5] G. Kästle, H.-G. Boyen, F. Weigl, G. Lengl, T. Herzog, P. Ziemann, S. Riethmüller, O. Mayer, C. Hartmann, J. P. Spatz, M. Möller, M. Ozawa, F. Banhart, M. G. Garnier and P. Oelhafen
Micellar Nanoreactors—Preparation and Characterization of Hexagonally Ordered Arrays of Metallic Nanodots
Adv. Funct. Mater. 13, 853 (2003)