Abstract
A simple model is developed to explain the often-observed diminished values of the magnetic hyperfine fields in microcrystals. It is shown that below the superparamagnetic blocking temperature thermally excited oscillations of the magnetization around an energy minimum reduce the average magnetization and the magnetic hyperfine splitting in the Mössbauer spectrum. In a microcrystal of volumeV these quantities are reduced by a factor of about 1−kT/(2κV), wherekT is the thermal energy and κ is related to the anisotropy constant. Mössbauer spectra of 60 Å, 100 Å, and 120 Å Fe3O4 particles and 120 Å α-Fe2O3 particles show excellent agreement with the theory.
This is a preview of subscription content, log in via an institution to check access.
Similar content being viewed by others
References
J.M.D. Coey, P.W. Readman: Nature246, 476 (1973)
M. Boudart, H. Topsøe, J.A. Dumesic: In The Physical Basic for Heterogeneous Catalysis, ed. by E. Drauglis and R.I. Jaffe (Plenum Press, New York 1975) p. 337
J.A. Dumesic, H. Topsøe, M. Boudart: J. Catal.37, 513 (1975)
A.E. Berkowitz, J.A. Lahut, I.S. Jacobs, L.M. Levison, D.W. Forester: Phys. Rev. Letters34, 594 (1975)
L. Néel: Ann. Geophys5, 99 (1949)
W.F. Brown, Jr.: Phys. Rev.130, 1677 (1963)
A. Aharoni: Phys. Rev.135, A447 (1964)
A. Aharoni: Phys. Rev.B7, 1103 (1973)
W. Kündig, H. Bömmel, G. Constabaris, R.H. Lindquist: Phys. Rev.142, 327 (1966)
T.K. McNab: Thesis 1968 (unpublished)
M.C. Hobson, Jr., H.M. Gager: J. Catal.16, 254 (1970)
A.M. van der Kraan: Thesis 1972 (unpublished)
H. Topsóe, M. Boudart: J. Catal.31, 346 (1973)
A. Simopoulos, A. Kostikas: In:Proc. 5th Intern. Conf. on Mössbauer Spectroscopy, Bratislava, Czechoslovakia (1973) ed. by M. Hucl and T. Zemčík (Nuclear Information Centre, Prague) p. 759
H. Kubsch, H.A. Schneider: InProc. 5th Intern. Conf. on Mössbauer Spectroscopy, Bratislava, Czechoslovakia (1973) ed. by M. Hucl and T. Zemčík (Nuclear Information Centre, Prague) p. 718
I.S. Jacobs, C.P. Bean: InMagnetism, Vol. 3, ed. by G.T. Rado and H. Suhl (Academic Press, New York 1963) p. 275
J. Smit, H.P.J. Wijn:Ferrites (John Wiley and Sons, New York 1959)
M. Blume, J.A. Tjon: Phys. Rev.165, 446 (1968)
L. Gerward, S. Mørup, H. Topsøe: J. Appl. Phys.47, 822 (1976)
H. Topsøe, J.A. Dumesic, M. Boudart: J. Physique35, C6–411 (1974)
H. Topsøe, S. Mørup: InProc. Intern. Conf. on Mössbauer Spectroscopy, Vol. 1, Poland, Cracow (1975) ed. by A.Z. Hrynkiewich and J.A. Sawicki, p. 321
D. Schroeer, R.C. Nininger, Jr.: Phys. Rev. Letters19, 632 (1967)
U. Gonser (ed.):Mössbauer Spectroscopy, Top. Appl. Phys.5 (Springer, Berlin, Heidelberg, New York 1975)