78
R. Mathieu and P. Nordblad
Fig. 3.12 Superspin dimensionality and relation to T B for RCP6 and RCP8 (T g ~ 140 K in both
cases). m IRM (T ) is recorded on reheating after halts made at T h = 50, 60, 70, 80, 90, 100 and 110 K
(h = 0.5 Oe, t h = 300 s) during the cooling. The inset shows m IRM (T ) for Ising and Heisenberg
spin glasses, for T h /T g ~ 0.6–0.7 (h = 10 Oe, t h = 3000 s) [28]
at lower temperatures, whereas the curves at higher temperatures exhibit a clear
maximum (Heisenberg like) before the decay. All m IRM (T ) curves for the compact
of 6 nm particles on the other hand exhibit a clear maximum (Heisenberg like) before
rapidly decaying to zero at temperatures above T h . The significant dynamic difference
between the two systems is that the blocking temperatures of the non-interacting
systems are significantly different although the superspin glass temperatures are the
same (140 K) for the two compacts. T B = 12.5 K for the non-interacting system 6 nm
particles and T B = 35 K for the compact 8 nm particle assembly. This difference
between the two compacts hints that the crossover is related to the evolution of the
relaxation times of the particles with temperature and that the particles behave Ising
like at temperatures T < T B and Heisenberg like at T T B . At low temperatures,
the particles are mainly confined to point along the anisotropy direction, whereas
at higher temperature, T B , they very rapidly switch between the two energy
minima. This type of experiments reflects properties of the collective dynamics of
the particle system and is only viable in the temperature region between T B and T g .
At temperatures below T B of the non-interacting system, the particles are essentially
blocked on the time scales of the experiment and no remanence is induced from a
field pulse of duration minutes.
The spin dimensionality has been found to affect the anisotropy and reversal of
the magnetization of spin glasses [25]. For example strikingly different hysteresis
curves are displayed by spin glasses such as Au(Fe) with an Ising character—brought
forth by the single ion anisotropy of the Fe spins—and the above-discussed Cu(Mn)
Heisenberg spin glass. As seen in Fig. 3.13 (main frames), the ZFC M(H) curves of the
Heisenberg system include an excess moment ( Furthermore, as seen in inset,
these curves, like the field-cooled ones recorded after cooling the system in H FC , are
exchange biased. The FC M(H) data recorded after sweeping the magnetic field from
H FC to −H FC and back includes an excess moment and defines two switching fields.
R. Mathieu and P. Nordblad
Fig. 3.12 Superspin dimensionality and relation to T B for RCP6 and RCP8 (T g ~ 140 K in both
cases). m IRM (T ) is recorded on reheating after halts made at T h = 50, 60, 70, 80, 90, 100 and 110 K
(h = 0.5 Oe, t h = 300 s) during the cooling. The inset shows m IRM (T ) for Ising and Heisenberg
spin glasses, for T h /T g ~ 0.6–0.7 (h = 10 Oe, t h = 3000 s) [28]
at lower temperatures, whereas the curves at higher temperatures exhibit a clear
maximum (Heisenberg like) before the decay. All m IRM (T ) curves for the compact
of 6 nm particles on the other hand exhibit a clear maximum (Heisenberg like) before
rapidly decaying to zero at temperatures above T h . The significant dynamic difference
between the two systems is that the blocking temperatures of the non-interacting
systems are significantly different although the superspin glass temperatures are the
same (140 K) for the two compacts. T B = 12.5 K for the non-interacting system 6 nm
particles and T B = 35 K for the compact 8 nm particle assembly. This difference
between the two compacts hints that the crossover is related to the evolution of the
relaxation times of the particles with temperature and that the particles behave Ising
like at temperatures T < T B and Heisenberg like at T T B . At low temperatures,
the particles are mainly confined to point along the anisotropy direction, whereas
at higher temperature, T B , they very rapidly switch between the two energy
minima. This type of experiments reflects properties of the collective dynamics of
the particle system and is only viable in the temperature region between T B and T g .
At temperatures below T B of the non-interacting system, the particles are essentially
blocked on the time scales of the experiment and no remanence is induced from a
field pulse of duration minutes.
The spin dimensionality has been found to affect the anisotropy and reversal of
the magnetization of spin glasses [25]. For example strikingly different hysteresis
curves are displayed by spin glasses such as Au(Fe) with an Ising character—brought
forth by the single ion anisotropy of the Fe spins—and the above-discussed Cu(Mn)
Heisenberg spin glass. As seen in Fig. 3.13 (main frames), the ZFC M(H) curves of the
Heisenberg system include an excess moment ( Furthermore, as seen in inset,
these curves, like the field-cooled ones recorded after cooling the system in H FC , are
exchange biased. The FC M(H) data recorded after sweeping the magnetic field from
H FC to −H FC and back includes an excess moment and defines two switching fields.