物質・材料研究機 共用基盤部門 強磁場共用ステーション 2008 年度 ...

FM order system. In addition, the remanent
magnetization shows a long time relaxation
behavior below T C (not shown here), which can be
been fitted very well using a logarithmic function,
M t)
= M − S ln( t + ) .
( t
0 0
In order to explore the possible SG effect,
we have measured the frequency dependence of
the real and imaginary parts of the ac
susceptibility around the transition temperature.
The results at frequency range 0.1 Hz≤ω/2π≤1000
Hz are illustrated in Fig. 4. It is clear from this
figure that the in-phase component of the ac
susceptibility, χ ac ', exhibits a pronounced
maximum at a frequency dependent temperature
T C (ω) very close to the inflection point of χ ZFC (T)
curve shown in Fig. 2. The T C value is determined
to be 12.36 K at ω/2π=0.1 Hz, which shifts to
12.62 K at ω/2π=1000 Hz. The upward-shift of
the peak position in χ ac '(T) curve with rising
frequency is a typical feature of SG material,
which can be considered as an important evidence
for the existence of random spin freezing effect in
our Dy 2 AuSi 3 sample
[2] . However, using
expression δT C =ΔT C /(T C Δlogω) the frequency
shift rate of TC is estimated to be δT C =0.005. This
value is much smaller than the δT f values
(frequency shift rate of freezing temperature)
reported for the typical 2:1:3 NMAD SG systems,
but evidently larger than the δT m values
(frequency shift rate of peak temperature in χ ac '(T)
curve) reported for the “almost long rang
ferromagnetic ordering” compound Nd 2 PtSi 3
(δT f =0.002). In this sense, the Dy 2 AuSi 3 sample
could be considered as a FM cluster glass system,
which reveals both FM-like characters and
SG-like behaviours. The presence of FM cluster
glass effect in Dy 2 AuSi 3 is further confirmed by a
dynamic analysis of the obtained T C (ω) data. We
have fitted the T C (ω) data to the standard
expression (critical slowing down),
− zν
τ = τ [( T − T ) / T ]
max 0 f s s
, and to the Vogel-Fulcher
law, ω = ω 0
exp[ −E a
/ k
B
( T
f
−Tvf
)] , respectively.
Following Tholence, τ 0 = 1/ω 0 =10
−13 s was kept
fixed, the following parameters are obtained from
the best fitting results: a static freezing
temperature T S =12.3 K, a critical (dynamical)
exponent zν =6.6, a Vogel-Fulcher temperature
T 0 =11.7 K and an average activation energy
E a ≈1.75 k B
TB
S. Note that zν is determined around 2
for conventional phase transitions of long-range
magnetic ordering systems, and around 10 for SG
[2]
transitions of typical 2:1:3 NMAD SG systems.
The zν value obtained for Dy2AuSi 3 is also
situated between them.
On the other hand, another evidence of the
presence of large FM cluster in the Dy 2 AuSi 3
sample is given by the temperature dependence of
electrical resistivity ρ(T) (not shown here), which
manifests a sudden bent at T C and rapid decrease
below T C due to the FM ordering within the large
χ'ac ( emu/g )
χ"ac ( emu/g )
0.016
0.012
0.008
0.004
magnetic clusters.
4. Conclusion
To summarize, magnetic properties of ternary
intermetallic compound Dy2AuSi 3 have been
systematically studied by magnetic measurements.
This compound shows a sharp peak in ac and dc
susceptibility curves near a transition temperature
T C =12.5 K. The upward-shift of the ac
susceptibility peak with increasing frequency, the
downward-shift of the dc susceptibility peak with
increasing magnetic field, the long-time magnetic
relaxation behavior and the clear irreversible
magnetism below T C are observed. In addition, the
magnetization measurement measured at 5 K shows
a sharp increase at low fields, and electrical
resistivity measurement reveals a sudden decrease
at T C . These features suggest the metastable
properties of the magnetic ground state for the
Dy 2 AuSi 3 sample, and can be explained by using an
extended short-range FM order model. According
to this model, larger FM clusters could exist in the
Dy 2 AuSi 3 sample and magnetic exchange
interactions between the clusters occur at low
temperature, which leads to the observed FM-like
and SG-like features, i.e. FM cluster glass
behaviors. This consequence is further confirmed
by a dynamic analysis of the ac susceptibility data,
which yields the values of frequency shift rate δT C
and the dynamical critical exponent zν
being typical for a magnetic cluster system.
References
0.0018
0.0012
0.0006
0.0000
11.0 11.5 12.0 12.5 13.0 13.5 14.0
[1] T . V. Chandrasekhar Rao, P. Raj, Sk.
Mohammad Yusur, L. Madhav Rao, A.
Sathyamoorthy, V. C. Sahni, Philosophical
Magazine B 74, 275 (1996).
[2] J. A. Mydosh, Spin Glass: An Experimental
Introduction (Taylar & Francis, London, 1993).
1000Hz
500Hz
300Hz
100Hz
10Hz
1Hz
0.1Hz
1000Hz
500Hz
300Hz
100Hz
10Hz
1Hz
0.1Hz
T ( K )
Fig. 4 Temperature dependences of real (χ ac ') and
imaginary (χ ac ") components of the ac susceptibility of
Dy 2 AuSi 3 measured at various frequencies with an
oscillating field of 5 Oe.
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