Thermally assisted resonant quantum tunneling of magnetization in Fe8 clusters

Abstract

Magnetic ac susceptibility of randomly arranged crystallites ͑powder sample͒ and well-oriented Fe 8 crystallites has been measured as a function of applied magnetic field, temperature, and frequency. The Fe 8 clusters, made of eight iron ions and with formula ͓͑tacn͒ 6 Fe 8 O 2 ͑OH͒ 12 ͔ 8ϩ , have a ground state Sϭ10. From the magnetization data M (H) with the dc field, H, applied parallel and perpendicular to the easy axis of the aligned sample and Љ(T) data for different frequencies, the anisotropy field H an ϳ5 T and energy barrier U/k B ϭ26.9 K were obtained. Peaks in the curves of Ј(H) were clearly observed at fields H n ϭnH 0 , with nϭ0,Ϯ1 for powder sample where H 0 ϭ2.4 kOe; and nϭ0, Ϯ1, Ϯ2, for an oriented sample where H 0 ϭ2.2 kOe. These peaks appearing at H n ϭnH 0 in the curves of Ј(H) can be well described by the thermally assisted resonant quantum tunneling of magnetization.

Anisotropic Magnetocaloric Effect in Nanostructured Magnetic Clusters

“…The magnetic interactions among the clusters are therefore negligible for the large enough lattice parameters. The magnetic anisotropy field of the clusters was found to be ϳ5 T from the aligned samples [17]. The molecular crystals of Fe 8 can therefore be considered to be a collection of a huge number of identical, aligned, noninteracting single domain particles with a moment of 20m B .…”

“…The Fe 8 crystallites of an average size of 0.2 3 0.1 3 0.1 mm used here are the same as those for the study of resonant tunneling [17]. The aligned sample is a solid cylinder with a diameter of 3 mm and a length of 6 mm, in which the easy axis (c axes) of the Fe 8 clusters is along the axis of the column [17].…”

“…The aligned sample is a solid cylinder with a diameter of 3 mm and a length of 6 mm, in which the easy axis (c axes) of the Fe 8 clusters is along the axis of the column [17]. By changing the angle u between the c axis and applied field, it is possible to study the anisotropy effects.…”

“…The molecular crystals of Fe 8 can therefore be considered to be a collection of a huge number of identical, aligned, noninteracting single domain particles with a moment of 20m B . This material, as well as another magnetic molecular crystal Mn 12 , has been studied very intensively, both experimentally and theoretically, in the context of resonant spin tunneling [16,17,[19][20][21][22][23]. The Fe 8 and Mn 12 crystals constitute a model system for testing many theoretical predictions on the macroscopic tunneling.…”

“…The magnetic characteristics of the clusters, studied by using EPR, ac, and dc magnetic techniques [15][16][17] ± [14,19]. The magnetic interactions among the clusters are therefore negligible for the large enough lattice parameters.…”