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Fast contributions to the magnetic permeability aftereffect in amorphous ferromagnetic ribbons
Jmumal of Magnetism and Magnetic Materials
54-57
(1986) 273-274
‘73
FAST CONTRIBUTIONS TO THE MAGNETIC AMORPHOUS FERROMAGNETIC RIBBONS P. ALLIA, C. BEATRICE Istrtuto Elettrotrc~nrc~o Nuziotdr
*, P. MAZZETTI “Gulrko
Ferrur,.~‘:
PERMEABILITY
AFTEREFFECT
IN
* and F. VINAI
Gruppo
Nu;roncrle
Strutturu
de//r
Mutericr
del C‘NR. I IO/75
Tor~w
/to/,
The aftereffect of the initial magnetic permeahility of amorphous ferromagnetic alloys i\ studied h> using a new impulsive method. specifically developed for the analysis of fast relaxation times (down to 10~ ’ s after an instantaneous change of sample’s magnetization state). A strong decay of p,. whose kinetics contrasts with the usual long-time behavior. has been detected in the explored short-time region.
Magnetic aftereffect phenomena occurring in amorphous ferromagnetic alloys are of considerable interest in theory and applications of this class of materials. Increasing efforts have been made in order to clarify the nature of the aftereffect of the initial magnetic permeability (disaccomodation), a property extremely sensitive to the degree of disorder of the amorphous structure [1.2]. In amorphous ferromagnets, the reversible decay of the permeability observed after a rearrangement of magnetic domains. is generally characterized by a superposition of relaxation times 7 distributed at each temperature over an extended time interval. Usually. the existence of such a distribution is ascribed to the presence of a spectrum of activation energies Q for the ordering processes responsible for the aftereffect, the relation between each 7 and the corresponding Q being given. in the simplest model. by an Arrhenius law [3]. In various approaches, the aftereffect is assumed to originate from directional ordering processes involving structural defects (atoms or more probably small atomic clusters) which can reversibly reorient in dependence of changes of the local magnetization vector. and have been pictured as classical two-level systems [2,3]. Although the nature of the interaction between defects and magnetization is still rather controversial [4.5], their directional ordering in generally assumed to be characterized by an extended range of activation energies. which consequently provides a natural explanation for the observed distribution of relaxation times. However. the actual location of the boundaries of such a distribution is unknown. even at room temperature. A relevant fraction of short-time effects are in fact lost in the aftereffect measurements which are currently performed, involving either a conventional demagnetization of samples. which comports a loss of information on relaxation times shorter than 7, = 1 s [5]. or just a redistribution of walls in new equilibrium positions. as formerly proposed by Allis et al. [6] (in this case 7, is of
order IO-~’ s). Our present aim is to add new information about the considered relaxation process, by substantially lowering this value of 7,. Actually, 7, cannot be reduced below an intrinsic limiting value, corresponding to the time T,, involved in the development and stabilization of a new domain structure after an instantaneous ( < 10mx s) magnetization change. This time was found to be of the order 10 -’ 5 in the present case. The present technique precisely allows times as short as 10~ ’ s to be explored after a change of the domain structure. consequently increasing by at least three orders of magnitude the range of relaxation times which can be studied in a single measurement. Possible spurious contributions to the signal have been carefully checked and suppressed in order to extract reliable and reproducible data. The method consists in replacing the conventional demagnetization procedure by a fast saturation of the sample. induced by a periodic pulsed field. A train of rectangular H pulses of suitable frequency. alternatively opposite in sign. is applied to the sample. The (variable) pulse width is = IO-’ s. the pulse amplitude being always sufficient to saturate the material. After each pulse, a new domain structure develops in a time f,, = IO-’ s. The changes in permeability occurring in the interval (0. I,,) are automatlcally suppressed by insertion of a suitable window allowing only times t 2 I,, to be explored. As a consequence. the analyzed signal is not affected by spurious contributions arising from transient effect. During each measurement. the walls are driven by a radiofrequency sinusoidal field. having an amplitude which can be varied from zero to values of the order ti. allowing for the study of the disaccom-
the
odation The
as a function induced
properly
amplified.
radiofrequency proportional
accommodation *
Physics Department.
0304-8853/85/$03.30
Politecnico of Torino,
Itall.
0 Elsevier Science Publishers
of the applied
signal,
detected
rectified
oscillation. to
the
relaxing
and The
field by
[7].
a pickup
filtered output permeability.
coil.
to suppress signal
is the
is then Each
dis-
measurement actually consists in an average over several independent. consecutive decays of the permeability. In this way. it is possible to signifi-
B.V.
cantly ent
reduce the statistical
decays.
arising
the process
of domain
may then be analyzed in the latter
a train
independent
simply
proportional.
modulus
submitted order
permeability
plot is
to
reported
a small
composed walls. here
of
The
advantage
is emphasised
lower
limits
Lation at very
involving that
short
aftereffect
is
(1.3
kg/mm’)
structure.
using regions
which
domain
after
observed.
decay of the permeability
I,,.
On
sample
from
the in
change (b).
a
an “initial”
thL1s shown time
tail
tail
by
h>
with
observed rihhons
it
is
contrihutlons
[I ] 121 [3] (41
1
:0-*
0-1
lC3
10lt(5, lo2
[S] [6]
Fag. I. Behavwr with time of the room temperature initial permeability p,(r)/p,(t,,) for the Fc~,,CO,~B,~S~~ alloy: lli = I
x10 5s.
the short-
measurements
to the
[7]
This
are
different not
indicate is
the
methods.
procease\ similar
long-time state,
and
tall.
or
different strlt
the
Iempcraturc.
of
to Jccide
I ).
that
substanttatlv
again
reproduced
permeability
to a completely
not
is
c<>mposition.
possible the
to ordering
explain
decay.
conventional
results
ha\ing
the amorphou\
$3
function.
with such a LICW (see fig.
phenomenolog\
50 far with
the
distrlhution
treatments.
the
dc-
uith
constant\.
short-time
similar
moment.
to
k-4
agrermcnt 01‘ time
preliminary
contrast
the I~>ng-
a spectrum
contrasts
annealing
10 arc
to the inttial
while
by ;I change of the measurement
or
’ 10 ’
be approximatelv
law. in
a logarithmic of
respect
addition.
may
IO
50 Car. which
of
strong
value (actu-
In
at
comple-
corresponding
investigated weak uith
the ones
measured After
modified
closeI>
dramatic
than
intensrty
acrihed
:c -5
precisely
the aftereffect
this
generally
time of the order
;I
to be rather
by ;I suitable
Moreover.
higher
are
appears.
LI’~LI;~~ assumption weighted
much
demagnetization.
phenomena
of
scribed
rrtatsd
4i
after
01‘ the fast decay in
constant a
fast.
times
time decay clearly
demagneti-
the contrary. in
in
described
indicating
does not stay
resulting
was
180”
are explored
pattern
the permeability times
method
lines.
conventional
by
long
deca? of the permeability.
sub&mtiall>
separated
the
by the dotted
(a) and impulsive
Notice
stress
which
the aftereffect
r,) in as-cast sample
tion
t = t,,).
is
initial
1. The
domains
of
after
rather
at
reported.
5. 11long-time
decay.
temperature
the value
currently
of
to the squared
fig.
domain
of the time
experiments
in
by
which
of a single
of time
tensile
antiparallel
signal
spectrum
decays.
of the room
to get a simple
of
in time or in frequency.
p, as a function
Fe,,C‘o,,B,,Si,.
The
case. the frequency
in these conditions,
A representative
nature
formation.
transform
ally.
between differ-
the random
structure
of the Fourier
permeability
from
either
evaluating, of
fluctuations
mostly
in
strong
aftereffects Notice
that
in
amorphou\
At
the prezcnt
\*hrther
aftereffect
the may
I’a\t hc
to the ones in\c,hetl if
thq
have
mechanl\rn.
to
bc
typical
01
to he .specified.
P. Ailia. l:.k. Lutx~rsk~. R. Sat0 ~urtclh. (;.I’. Soar&l .rnd F. Vinai, IEbt Tram. Magn. MAGI7 (19X1) 2615 P. Allia and F. Vmai. Phya. Kc\. BZh (10x1) 6141. 11. KronmCiller. Phll. Mag. B4X (19X3) 127. I’. All13 and F. Vlnai. Proc. RQ.5 C‘onfurrnce. S. Stwh ,~nd Il. Warllmonr. ai\. (North-Holl;ind. Rm\tcrd;im. IYXi I p. II’). H Kronmuller. N. Moser and F. Kerttmmelcr. II-.I-,I, Train\. Magn. MAC;-20 ( 1‘9x4) 13Xx. I’. Allis. P. Ma/ettl and t:. VI~II. .I Magn. Magn. Mar. I4 (IYXO) 2x1. I’. 4lh;1 and I’. V1nai. IEEE -rrnn\. Magn. LlACi-I7 (19x1 ) 14x1.
54-57
(1986) 273-274
‘73
FAST CONTRIBUTIONS TO THE MAGNETIC AMORPHOUS FERROMAGNETIC RIBBONS P. ALLIA, C. BEATRICE Istrtuto Elettrotrc~nrc~o Nuziotdr
*, P. MAZZETTI “Gulrko
Ferrur,.~‘:
PERMEABILITY
AFTEREFFECT
IN
* and F. VINAI
Gruppo
Nu;roncrle
Strutturu
de//r
Mutericr
del C‘NR. I IO/75
Tor~w
/to/,
The aftereffect of the initial magnetic permeahility of amorphous ferromagnetic alloys i\ studied h> using a new impulsive method. specifically developed for the analysis of fast relaxation times (down to 10~ ’ s after an instantaneous change of sample’s magnetization state). A strong decay of p,. whose kinetics contrasts with the usual long-time behavior. has been detected in the explored short-time region.
Magnetic aftereffect phenomena occurring in amorphous ferromagnetic alloys are of considerable interest in theory and applications of this class of materials. Increasing efforts have been made in order to clarify the nature of the aftereffect of the initial magnetic permeability (disaccomodation), a property extremely sensitive to the degree of disorder of the amorphous structure [1.2]. In amorphous ferromagnets, the reversible decay of the permeability observed after a rearrangement of magnetic domains. is generally characterized by a superposition of relaxation times 7 distributed at each temperature over an extended time interval. Usually. the existence of such a distribution is ascribed to the presence of a spectrum of activation energies Q for the ordering processes responsible for the aftereffect, the relation between each 7 and the corresponding Q being given. in the simplest model. by an Arrhenius law [3]. In various approaches, the aftereffect is assumed to originate from directional ordering processes involving structural defects (atoms or more probably small atomic clusters) which can reversibly reorient in dependence of changes of the local magnetization vector. and have been pictured as classical two-level systems [2,3]. Although the nature of the interaction between defects and magnetization is still rather controversial [4.5], their directional ordering in generally assumed to be characterized by an extended range of activation energies. which consequently provides a natural explanation for the observed distribution of relaxation times. However. the actual location of the boundaries of such a distribution is unknown. even at room temperature. A relevant fraction of short-time effects are in fact lost in the aftereffect measurements which are currently performed, involving either a conventional demagnetization of samples. which comports a loss of information on relaxation times shorter than 7, = 1 s [5]. or just a redistribution of walls in new equilibrium positions. as formerly proposed by Allis et al. [6] (in this case 7, is of
order IO-~’ s). Our present aim is to add new information about the considered relaxation process, by substantially lowering this value of 7,. Actually, 7, cannot be reduced below an intrinsic limiting value, corresponding to the time T,, involved in the development and stabilization of a new domain structure after an instantaneous ( < 10mx s) magnetization change. This time was found to be of the order 10 -’ 5 in the present case. The present technique precisely allows times as short as 10~ ’ s to be explored after a change of the domain structure. consequently increasing by at least three orders of magnitude the range of relaxation times which can be studied in a single measurement. Possible spurious contributions to the signal have been carefully checked and suppressed in order to extract reliable and reproducible data. The method consists in replacing the conventional demagnetization procedure by a fast saturation of the sample. induced by a periodic pulsed field. A train of rectangular H pulses of suitable frequency. alternatively opposite in sign. is applied to the sample. The (variable) pulse width is = IO-’ s. the pulse amplitude being always sufficient to saturate the material. After each pulse, a new domain structure develops in a time f,, = IO-’ s. The changes in permeability occurring in the interval (0. I,,) are automatlcally suppressed by insertion of a suitable window allowing only times t 2 I,, to be explored. As a consequence. the analyzed signal is not affected by spurious contributions arising from transient effect. During each measurement. the walls are driven by a radiofrequency sinusoidal field. having an amplitude which can be varied from zero to values of the order ti. allowing for the study of the disaccom-
the
odation The
as a function induced
properly
amplified.
radiofrequency proportional
accommodation *
Physics Department.
0304-8853/85/$03.30
Politecnico of Torino,
Itall.
0 Elsevier Science Publishers
of the applied
signal,
detected
rectified
oscillation. to
the
relaxing
and The
field by
[7].
a pickup
filtered output permeability.
coil.
to suppress signal
is the
is then Each
dis-
measurement actually consists in an average over several independent. consecutive decays of the permeability. In this way. it is possible to signifi-
B.V.
cantly ent
reduce the statistical
decays.
arising
the process
of domain
may then be analyzed in the latter
a train
independent
simply
proportional.
modulus
submitted order
permeability
plot is
to
reported
a small
composed walls. here
of
The
advantage
is emphasised
lower
limits
Lation at very
involving that
short
aftereffect
is
(1.3
kg/mm’)
structure.
using regions
which
domain
after
observed.
decay of the permeability
I,,.
On
sample
from
the in
change (b).
a
an “initial”
thL1s shown time
tail
tail
by
h>
with
observed rihhons
it
is
contrihutlons
[I ] 121 [3] (41
1
:0-*
0-1
lC3
10lt(5, lo2
[S] [6]
Fag. I. Behavwr with time of the room temperature initial permeability p,(r)/p,(t,,) for the Fc~,,CO,~B,~S~~ alloy: lli = I
x10 5s.
the short-
measurements
to the
[7]
This
are
different not
indicate is
the
methods.
procease\ similar
long-time state,
and
tall.
or
different strlt
the
Iempcraturc.
of
to Jccide
I ).
that
substanttatlv
again
reproduced
permeability
to a completely
not
is
c<>mposition.
possible the
to ordering
explain
decay.
conventional
results
ha\ing
the amorphou\
$3
function.
with such a LICW (see fig.
phenomenolog\
50 far with
the
distrlhution
treatments.
the
dc-
uith
constant\.
short-time
similar
moment.
to
k-4
agrermcnt 01‘ time
preliminary
contrast
the I~>ng-
a spectrum
contrasts
annealing
10 arc
to the inttial
while
by ;I change of the measurement
or
’ 10 ’
be approximatelv
law. in
a logarithmic of
respect
addition.
may
IO
50 Car. which
of
strong
value (actu-
In
at
comple-
corresponding
investigated weak uith
the ones
measured After
modified
closeI>
dramatic
than
intensrty
acrihed
:c -5
precisely
the aftereffect
this
generally
time of the order
;I
to be rather
by ;I suitable
Moreover.
higher
are
appears.
LI’~LI;~~ assumption weighted
much
demagnetization.
phenomena
of
scribed
rrtatsd
4i
after
01‘ the fast decay in
constant a
fast.
times
time decay clearly
demagneti-
the contrary. in
in
described
indicating
does not stay
resulting
was
180”
are explored
pattern
the permeability times
method
lines.
conventional
by
long
deca? of the permeability.
sub&mtiall>
separated
the
by the dotted
(a) and impulsive
Notice
stress
which
the aftereffect
r,) in as-cast sample
tion
t = t,,).
is
initial
1. The
domains
of
after
rather
at
reported.
5. 11long-time
decay.
temperature
the value
currently
of
to the squared
fig.
domain
of the time
experiments
in
by
which
of a single
of time
tensile
antiparallel
signal
spectrum
decays.
of the room
to get a simple
of
in time or in frequency.
p, as a function
Fe,,C‘o,,B,,Si,.
The
case. the frequency
in these conditions,
A representative
nature
formation.
transform
ally.
between differ-
the random
structure
of the Fourier
permeability
from
either
evaluating, of
fluctuations
mostly
in
strong
aftereffects Notice
that
in
amorphou\
At
the prezcnt
\*hrther
aftereffect
the may
I’a\t hc
to the ones in\c,hetl if
thq
have
mechanl\rn.
to
bc
typical
01
to he .specified.
P. Ailia. l:.k. Lutx~rsk~. R. Sat0 ~urtclh. (;.I’. Soar&l .rnd F. Vinai, IEbt Tram. Magn. MAGI7 (19X1) 2615 P. Allia and F. Vmai. Phya. Kc\. BZh (10x1) 6141. 11. KronmCiller. Phll. Mag. B4X (19X3) 127. I’. All13 and F. Vlnai. Proc. RQ.5 C‘onfurrnce. S. Stwh ,~nd Il. Warllmonr. ai\. (North-Holl;ind. Rm\tcrd;im. IYXi I p. II’). H Kronmuller. N. Moser and F. Kerttmmelcr. II-.I-,I, Train\. Magn. MAC;-20 ( 1‘9x4) 13Xx. I’. Allis. P. Ma/ettl and t:. VI~II. .I Magn. Magn. Mar. I4 (IYXO) 2x1. I’. 4lh;1 and I’. V1nai. IEEE -rrnn\. Magn. LlACi-I7 (19x1 ) 14x1.