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Multifrequency magnetic resonance and blocking behavior of FexPt1-x nanoparticles

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Abstract

Multifrequency ferromagnetic resonance well above the blocking temperature was used to determine the composition dependent g-factor in wet-chemically synthesized FexPt1-x nanoparticles with mean diameters around 3 nm. The magnetic relaxation process in these experiments is found to be non-Gilbert like. To obtain the blocking temperature and the effective anisotropy, zero-field-cooled magnetometry data are simulated using a model of non-interacting particles. In this simulation, the temperature dependence of the effective anisotropy has to be taken into account for accurate results.

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... This behavior depends in an exponential fashion on the particle's magnetic anisotropy , volume and the temperature . It is characterized by a " blocking temperature " below which the magnetization is thermally stable over the duration of the magnetic measurement [134]. In an ensemble measurement the magnetization of smaller particles may fluctuate so quickly that over the time window of the measurement only bigger particles dominate the effectively measured magnetization and magnetic anisotropy while the signal of the small particles is negligible. ...
... Phase contrast techniques have been demonstrated which yield a magnetic contrast [149], [150] on the nm length scale and may be applicable to individual particles. The dynamic magnetic properties of nanoparticle ensembles have been studied by ferromagnetic resonance (FMR) in the Gigahertz regime [134], and FMR techniques approaching 30 nm lateral resolution using a scanning tunnelling microscopy approach have been described [151]. With respect to synchrotron radiation first results on element-specific FMR detection [152]–[156] have been published and one may realistically hope that future developments will allow the imagery and element-specific investigation of dynamic and static magnetic properties of individual and dipolar coupled core-shell nanoparticles in the frequency and time domain with a spatial resolution on the 10 nm scale. ...
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• M Respaud
• J M Broto
• H Rakoto
• A R Fert
• L Thomas
• B Barbara
• M Verelst
• E Snoek
• P Lecante
• A Mosset
• J Osuna
• T Ely
M. Respaud, J. M. Broto, H. Rakoto, A. R. Fert, L. Thomas, B. Barbara, M. Verelst, E. Snoek, P. Lecante, A. Mosset, J. Osuna, T. Ould Ely, C. Amiens, and B. Chaudret, Phys. Rev. B 57, 2925 (1998).
• V Salgueiriño-Maceira
• M Spasova
• M Farle
V. Salgueiriño-Maceira, M. Spasova, and M. Farle, Adv. Funct. Mater. 15, 1036 (2005).
• O N Mryasov
• U Nowak
• K Y Guslienko
O. N. Mryasov, U. Nowak, K. Y. Guslienko, and R. W. Chantrell, Europhys. Lett. 69, 805 (2005).
• U Wiedwald
• J Lindner
• M Spasova
U. Wiedwald, J. Lindner, M. Spasova, Z. Frait, and M. Farle, Phase Trans. 78, 85 (2005).
• U Wiedwald
• K Fauth
• M Heßler
• H.-G Boyen
• F Weigl
• M Hilgendorff
• M Giersig
• G Schütz
• P Ziemann
• M Farle
U. Wiedwald, K. Fauth, M. Heßler, H.-G. Boyen, F. Weigl, M. Hilgendorff, M. Giersig, G. Schütz, P. Ziemann, and M. Farle, Chem. Phys. Chem. 6, 2522 (2005).
• N Shukla
• J Ahner
• D Weller
N. Shukla, J. Ahner, and D. Weller, J. Magn. Magn. Mater. 272–276, E1349 (2004).
• M Ulmeanu
• C Antoniak
• U Wiedwald
• M Farle
M. Ulmeanu, C. Antoniak, U. Wiedwald, M. Farle, Z. Frait, and S. Sun, Phys. Rev. B 69, 054417 (2004).
• S Sun
• C B Murray
• D Weller
• L Folks
• A Moser
S. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, Science 287, 1989 (2000).
• V Salgueiriño-Maceira
• M A Correa-Duarte
• M Spasova
• L M Liz-Marzán
• M Farle
V. Salgueiriño-Maceira, M. A. Correa-Duarte, M. Spasova, L. M. Liz-Marzán, and M. Farle, Adv. Funct. Mater. 16, 509 (2006).
• J Lindner
• K Lenz
• E Kosubek
• K Baberschke
• D Spoddig
• R Meckenstock
• J Pelzl
• Z Frait
• D L Mills
J. Lindner, K. Lenz, E. Kosubek, K. Baberschke, D. Spoddig, R. Meckenstock, J. Pelzl, Z. Frait, and D. L. Mills, Phys. Rev. B 68, 060102(R) (2003).
• M Spasova
• V Salgueiriño-Maceira
• A Schlachter
• M Hilgendorff
• M Giersig
• L M Liz-Marzán
• M Farle
M. Spasova, V. Salgueiriño-Maceira, A. Schlachter, M. Hilgendorff, M. Giersig, L. M. Liz-Marzán, and M. Farle, J. Mater. Chem. 15, 2095 (2005).
• U Wiedwald
• M Spasova
• E L Salabas
• M Ulmeanu
• M Farle
• Z Frait
• A Fraile Rodriguez
• D Arvanitis
• N S Sobal
• M Hilgendorff
• M Giersig
U. Wiedwald, M. Spasova, E. L. Salabas, M. Ulmeanu, M. Farle, Z. Frait, A. Fraile Rodriguez, D. Arvanitis, N. S. Sobal, M. Hilgendorff, and M. Giersig, Phys. Rev. B 68, 064424 (2003).