416
M. Reissner
the name mictomagnet—from the greek syllabel micto for mixed—because of the
simultaneous existence of ferro- and antiferro-magnetic correlations. At very low
concentrations of magnetic impurity atoms, concentration independent scaling laws
are present. When concentration increase, the possibility that two impurity atoms
are nearest neighbours increases, and due to direct exchange interaction between dorbitals magnetic clusters are formed. Such materials are called cluster glasses [115].
Cluster glasses are also possible due to chemical clusters, which may form during
thermal treatment [116–118]. Moments of such clusters can have several thousand
Bohr magnetons μ B . If the density of the impurities is such, that the possibility that
each impurity has at least one impurity in the nearest neighbour shell, the percolation limit is reached, where a path of neighbouring magnetic atoms goes from one
side of the sample to the other. The probability that a moment is part of an infinite
cluster is then larger than zero. The sample becomes long range ordered, but the
order is strongly inhomogeneous. The main characteristic feature of spin glasses is
the freezing of the moments in random orientations below a well defined freezing
temperature T f , without appearance of long range order. This freezing temperature
was first discovered by a sharp peak in ac-susceptibility measurements [102] and
later on confirmed also by dc-magnetization measurements [119–122]. For some
spin glasses T f is frequency dependent like for AuFe and CuMn, whereas for others it is not. Below the freezing temperature strong irreversibilities are present in
magnetization measurements, visible in large differences between field-cooled (FC)
and zero-field cooled (ZFC) curves of temperature dependence of magnetization
(Fig. 8.37). In this temperature regime also time dependence of magnetic moments is
observed. Maxima are also present in specific heat and resistivity measurements, but
not always at the same temperature as found in susceptibility measurements. Field
dependence curves of magnetization M(H) are strongly curved above T f . They cannot be fitted by simple Brillouin function, but a fit assuming the existence of magnetic
clusters of different size overlapped by a linear term from the single moments can
explain the curvature M(H, T ) = χ 0 H + ¯
μcB( ¯
μ, (H + λ(M − χ H ))/T ), with χ 0
a field independent susceptibility, ¯
μ the mean moment with concentration c, λ the
molecular field constant and B the Brillouin function. The fit shows that the mean
moments of the clusters decrease with increasing temperature, whereas the number
of clusters increase. This is a clear indication of dynamic magnetic behaviour. The
existence of magnetic correlations above the ordering temperature is confirmed by
specific heat measurements, which show that entropy at T f is only 20 to 30% in
case of CuMn [124] of the value expected in case of fully spin disorder in paramagnetic state. Under the first experiments proving a magnetic phase transition are
Mössbauer experiments [125, 126], which have shown that below a temperature T 0
hyperfine splitting appears. The obtained hyperfine fields could be set in relation
to local magnetic moments, with temperature dependence reminiscent to ferromagnetism. To determine, if the orientation of the spins is ferro- or antiferromagnetic,
in-field Mössbauer spectra were performed on Fe 0,5 Au 0,5 [127]. The result pointed
to a weak canted antiferromagnet. Detailed analyses showed that hyperfine fields
are statistical distributed in magnitude and orientation [128]. Whereas for AuFe T 0
matches T f obtained from susceptibility measurements, for other spin glasses like
M. Reissner
the name mictomagnet—from the greek syllabel micto for mixed—because of the
simultaneous existence of ferro- and antiferro-magnetic correlations. At very low
concentrations of magnetic impurity atoms, concentration independent scaling laws
are present. When concentration increase, the possibility that two impurity atoms
are nearest neighbours increases, and due to direct exchange interaction between dorbitals magnetic clusters are formed. Such materials are called cluster glasses [115].
Cluster glasses are also possible due to chemical clusters, which may form during
thermal treatment [116–118]. Moments of such clusters can have several thousand
Bohr magnetons μ B . If the density of the impurities is such, that the possibility that
each impurity has at least one impurity in the nearest neighbour shell, the percolation limit is reached, where a path of neighbouring magnetic atoms goes from one
side of the sample to the other. The probability that a moment is part of an infinite
cluster is then larger than zero. The sample becomes long range ordered, but the
order is strongly inhomogeneous. The main characteristic feature of spin glasses is
the freezing of the moments in random orientations below a well defined freezing
temperature T f , without appearance of long range order. This freezing temperature
was first discovered by a sharp peak in ac-susceptibility measurements [102] and
later on confirmed also by dc-magnetization measurements [119–122]. For some
spin glasses T f is frequency dependent like for AuFe and CuMn, whereas for others it is not. Below the freezing temperature strong irreversibilities are present in
magnetization measurements, visible in large differences between field-cooled (FC)
and zero-field cooled (ZFC) curves of temperature dependence of magnetization
(Fig. 8.37). In this temperature regime also time dependence of magnetic moments is
observed. Maxima are also present in specific heat and resistivity measurements, but
not always at the same temperature as found in susceptibility measurements. Field
dependence curves of magnetization M(H) are strongly curved above T f . They cannot be fitted by simple Brillouin function, but a fit assuming the existence of magnetic
clusters of different size overlapped by a linear term from the single moments can
explain the curvature M(H, T ) = χ 0 H + ¯
μcB( ¯
μ, (H + λ(M − χ H ))/T ), with χ 0
a field independent susceptibility, ¯
μ the mean moment with concentration c, λ the
molecular field constant and B the Brillouin function. The fit shows that the mean
moments of the clusters decrease with increasing temperature, whereas the number
of clusters increase. This is a clear indication of dynamic magnetic behaviour. The
existence of magnetic correlations above the ordering temperature is confirmed by
specific heat measurements, which show that entropy at T f is only 20 to 30% in
case of CuMn [124] of the value expected in case of fully spin disorder in paramagnetic state. Under the first experiments proving a magnetic phase transition are
Mössbauer experiments [125, 126], which have shown that below a temperature T 0
hyperfine splitting appears. The obtained hyperfine fields could be set in relation
to local magnetic moments, with temperature dependence reminiscent to ferromagnetism. To determine, if the orientation of the spins is ferro- or antiferromagnetic,
in-field Mössbauer spectra were performed on Fe 0,5 Au 0,5 [127]. The result pointed
to a weak canted antiferromagnet. Detailed analyses showed that hyperfine fields
are statistical distributed in magnitude and orientation [128]. Whereas for AuFe T 0
matches T f obtained from susceptibility measurements, for other spin glasses like