- Email: [email protected]
Thermomagnetic study of Finemet type nanocrystalline alloy by in situ hysteresis measurements
~i
ELSEVIER
journal of magnetism and magnetic materials
Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38
Letter to the Editor
Thermomagnetic study of Finemet type nanocrystalline alloy by in situ hysteresis measurements F. Mazaleyrat *, J.C. Faugi~res, J.F. Rialland LESiR-CNAM-ENSC, 61 ate. du Pdt Wilson, 94 235 Cachan cedex, France
Received 9 February 1996; revised 15 April 1996
Abstract The origin of the excellent magnetic properties of nanocrystallized Fe73.sCu~Nb3Sij35B 9 have been explained essentially by its nanograin weak anisotropy and its global zero magnetostriction. However, it has been suspected that a more complex problem of two-phase magnetic coupling must be taken into consideration. It is proposed herein to investigate in this way by in situ hysteresis measurements up to saturation at high temperature (reaching 700°C). Details of the Thermo-Magnetic Hysteresis method (TMH) are given. Dynamic and isothermal crystallization are investigated. Isothermal tests point out that induction arising could result from crystallization and from a slower mechanism such as atomic diffusion. It is also shown that the crystallization stage occurs, at its beginning, in a superparamagnetic phase and reaches a superferromagnetic phase. During cooling, the complex superferro-ferromagnetic transition shows that the magnetic coupling between grains and the amorphous matrix plays a leading role in Finemet magnetic properties. PACS:
75.60.Nt; 75.50.Kj; 75.30.Kz; 85.80.Lp
Keywords:
Nanocrystallization;Thermomagnetism;Magnetic coupling; Magnetic transitions; Crystallization kinetics; Superferromagnetism
1. Introduction
The most current methods to investigate nanocrystallization are Differential Thermal Analysis (DTA), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD), but these methods provide no information about magnetic quantities. DSC gives the Curie point of amorphous but not that of crystallites since it is too close to the FeB crystallization stage. Various authors have presented recordings of saturation magnetization during annealing by Ther-
* Corresponding author. Fax: + 33-1-4740-2199; email: [email protected].
momagnetic Gravimetry (TMG). This method is based on the measurement of the magnetic force induced by a magnet on the material which is set in a thermobalance. This method yields T~ry precisely, but the recordings at low heating rates are just a qualitative image of the saturation magnetization M~, since it is very sensitive to thermal drift. This obviously shows the usefulness of a device which is able to give absolute values of M~ and more magnetic quantities, such as squareness and coercivity. For the purpose of resolving this problem, an annealing furnace is fitted out with a magnetic device which allows measuring hysteresis loops up to saturation (even for as-cast material) at temperatures up to
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0304-8853(96)00372-1
L34
k. 1,1~:( levral et al /Journal of Magnetisn and Magnetic Materials 159 (1996) L33 L38
700°C. In this paper a description of the experiment is given. These Thermomagnetic Hysteresis tests (TMH) are compared to DTA and TMG. It is also demonstrated that the TMH method produces its most interesting results about crystallization processes during isothermal treatment.
2. Experimental From a magnetic point of view, the best configuration is a toms geometry because the demagnetizing field is zero. In our case, this configuration has three major problems. Firstly, high temperature imposes the use of mineral electric insulators, which have a poor space factor. So, only a large toms could be used (of about 5 cm inner diameter). Secondly, excitation and flux coils must be wound for each sample; and thirdly, the rolling of ribbons generates flexion stresses. For these reasons, a solenoid configuration is used as described below. The trickiest problem with this configuration is the evaluation of the internal field, thus H-coils must be used to measure it. With the aim of minimizing the demagnetizing field, the ribbons are cut 20% longer than the excitation coil (Fig. 1). They are laid on a 35 cm ceramic stage and surrounded by a mineral-insulated coil which picks up the flux. Two H-coils, made from stripped wire wound on ceramic sheets, are placed on the stage at each end of the
B-coil. The whole is slipped in a 25 cm solenoid composed of mineral-insulated wire wound on a quartz tube. The total outer diameter of this cell is 42 ram. The magnetic cell is then slipped in a tubular ceramic furnace which is wrapped in a magnetic shielding to protect the system from electromagnetic interferences and the Earth's magnetic field. In this device as few metallic parts as possible are being used. It must be underscored that this method only gives the flux density, B, through the material cross-section. Hence, saturation magnetization of crystallites cannot be directly evaluated because the crystallized volume fraction is not known. However, the squareness M e / M s is equal to B R / B s. Even in this open coil configuration, the evaluation of the demagnetizing factor is of no interest because the tangential component of the magnetic field is directly measured near the sample surface. However, it is necessary to evaluate the biased error on induction and field value due to their heterogeneity along the axis. This is performed experimentally with 10 mm long ultra-fine winding coils made of 50 ~ m copper wire. This test is run at ambient temperature on as-cast ribbons. Distribution of induction and field is shown in Fig. 2 at 50 and at 1200 A / r e . These curves are plotted with respect to the centre values B(0), H(0) and are compared to the distribution of the magnetic field along the solenoid axis = when no material is laid in. It is seen that this
Thermocouple monitor
Ribbons
/' /
H-Coil
/
~ /
Solenoid
B-c/oil
llIHlllJ
~
e Amp.
Power sinus wave Amplifier generator
Fig. 1. Schematic diagram of magnetic cell and acquisition system.
F. Maz.aleyrat et ul. / Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38 i
'B-Coil
H.Cnil
Ribbons
Illllllllllllllllllll
SolenoidI
,_/1_
_
~
~
~
-
-~
1.0
--1
;0
,, v 1.0 "(3 (D
Q. 0.8
-
{" o Q.
iT 0.8-
-4
t-" -
0.6
0.6-
~6
0.4
E
0.4 13
8 "0 r~
0.2 ,~
0.2-
t-" o
'
0
i
,
20
i
,
40
i
,
60
i
,
80
i
,
~
]
,
i
,
100 120 140 160 180
Position on axis (mm) Fig.
2. Distribution along the
solenoid axis of induction
B(z)/B(O) ( I 1 2 0 0 A / m ; • 50 A / m ) , internal field H(z)/H(O) ([q 1200 A / m ; O 50 A / m ) and field with no ferromagnetic material (- • -). Maximum temperature gap during continuous heating ( • ) and during isotherm (*).
configuration gives rise to a good uniformity. On the other hand, the distribution depends on the excitation field value. It is noticeable that the best uniformity is obtained when the material is saturated, but in each case, the homogeneity of induction in its measuring area is better than 1%. For the magnetic field, the problem is most sensitive. However, since the H-coil is 50 mm long in the furnace, the field is evaluated by its mean value, and a maximum of 5% biased error is ultimately found. Temperature gradient is also shown in this figure. The temperature gap is calculated relative to the one inside the B-coil. During dynamic tests, the thermal gradient is positive along the axis because the electric insulator of the solenoid and the B-coil play the role of thermal insulator. During isothermal tests (thermal steady state), this gradient is negative because the ends of the tubular furnace are cooled. The maximum temperature gap in the measuring zone is 10% at the beginning of heating and 4.5% during isothermal treatment• Since section variations of coils are not significant, this method is practically free of thermal drift. Dynamic testing is performed at a 5°C/rain con-
-5
1.2 °o
o o o
1.0
....
t.-
.O
~-o. o
-,
-7 0.8-
°
o
I I
", o i o~
--,1 e--t- 0.6 o
~ 0.4-
/
ol ol
//i
o/ i
0
-9
\
\
/
0.2-
0
,
~'~
-10 ~ "
\
, "'~oO qo
200
~=~
# f~
,o
v
-11 -12
40O
600
Temperature (°C) Fig. 3. Comparison between TMH (O), DTA (
L35
) TMG at 5°C/min (- - -) and 40°C/rain ( . . . ) .
1~36
F.
Macaleyral et al./ Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38
stant heating rate up to 700°C for TMH and up to 850°C for DTA (stainless steel inert reference). In the case of isothermal testing, the maximum heating rate is used (25°C/rain) until reaching 525°C and the temperature is held constant for 4 h. Tests are run under a sinusoidal excitation field at 83.3 Hz. This frequency is chosen to be as small as possible, in order to keep the coercive field as a significant quantity and high enough to have a 5 txV minimum H-coil voltage per A/re. This signal is amplified by a 40 dB low noise amplifier (15 nV/,/Hz). Signal acquisition is carried out by a 10 bit numeric oscilloscope; integration and air flux compensation are computed via an IEEE bus (see Fig. 1).
3. Results and discussion
In Fig. 3, TMH at 5°C/min is compared to a DTA plot at 5°C/rain performed on material derived from the same casting. TMG at 5°C/rain (after Leu [1]) and 20°C/rain (after Conde [2]) can also be seen. It is noticed that Curie temperatures of amorphous and crystalline phases as well as the crystallization temperature (T x) are in good accordance with TMG if we assume the fact that alloys are developed from different castings; the amorphous ferromagnetic Curie point is determined as T~c = 326°C (399 K), the crystallites' Curie point as T~ry = 600°C (873 K) and Tx = 520°C (793 K), which is confirmed by DTA. This method provides also the paramagnetic Curie point, T~p = 350°C (423 K). The granulation of the FeB compound is not witnessed herein. It can be noticed that the Curie transition is more abrupt in TMG records. This difference could be explained by the fact that in the TMG method, the constant field (about 2 k A / m in the material [1]) is not sufficient to saturate the as-quenched material. In the TMH trial, the excitation field is controlled to be just enough to yield technical saturation (3.5 k A / m for as-cast and 150 A / m during crystallization). This field monitoring is essential for a good evaluation of coercivity, squareness and saturation induction. A validation of this method is obtained by resolving the molecular field equation using the quantum-mechanical relation [3], namely the Brillouin function, with a total angular momentum of J = 1/2, which means that the orbital contribution is
negligible. It is visible in Fig. 4 that experimental results found with as-quenched amorphous material are in good accordance with the theoretical temperature dependence of magnetization. The saturation induction, determined from the theoretical Brillouin function, at 0 K is found to be Bo = 1.19 T. However, it is difficult to interpret the continuous heating tests for two reasons. Firstly, this test is not equivalent to annealing conditions. Secondly, the crystallization process is partly hidden by a progressive approach towards the Curie point, because the decrease of magnetization induced by the heating rate and by crystallization kinetics are comparable. The fact that the crystallization peak is lower on the TMH plot than on TMG is due to the dilution of the ferromagnetic phase in the ribbon's cross-section (cf. Section 2). In Fig. 5, the results of the isothermal tests are presented. Temperature and magnetic quantities are plotted as functions of time. Generally, very high heating rates must be used for isothermal testing [4]; however, the test could be considered as isothermal because crystallization begins when the step temperature is reached, even if amorphous relaxation has begun during heating. The drop in coercivity at 280°C demonstrates the relaxation mechanism. The saturation induction increases up to 0. l 1 T during the first 20 min and then it takes 4 h to reach the maximum value of 0.3 T. During cooling, B~ in1 O
0.9 0.8
~; 07 0 0.6 ._ 0.5 t0'~ 0.4
E
0.3 "0 ID ~ 0.2 "o ~ 0.1 0
0
0,2
0.4
0.6
0.8
Reduced temperature TFFc
Fig. 4. Reduced magnetizationof as-cast amorphous (normalized at 0 K) as function of reduced temperature (normalized with respect to the Curie temperature). Points are experimental. Curve is calculated.
F. Maz.aleyrat et al. / Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38 [
600
12
I
L37
- 160 e-
140
500
(,~
1.0 o"
/
-~.
,0.8 .~
400
/
~
300
0.6 ~
(D
E 200
04~
\
g ,
0 -100
-,,,i
\- o.2 ~
100
.
.
.
.
.
I
'
~oo
I
200
'
'
300
~
4 o
'
C
2120 -
~
-100 ~Q.
-~o ~ -~o ~ 24o g - 20
-0
~.
Time (min) Fig. 5. Evolution of saturation induction (C)), remanence ratio ( 0 ) and coercivity ( l l ) during isotherm and cooling vs. time with regard to temperature ( - • -).
creases as the thermal energy decreases. The shapes of squareness and coercivity are characteristic of the grain diameter dependence of a single-domain fine particle system in a paramagnetic matrix [5], namely superferromagnetism [6]. The peak of coercivity during cooling is characteristic of the superferromagnetic state. The blocking temperature T B can be estimated by extrapolation from experimental data using the theoretical model [5]: Hc(r)/Hc(O)
:
1 - v:r/rB
T B is found to be about 600°C, but this evaluation is very imprecise since superferromagnetism is observed here in a small temperature range. Above 450°C, the drop in coercivity is probably a consequence of other thermal effects, such as increasing particle magnetization and progressive exchange coupling between grains via the amorphous phase this part of the H c curve is in good agreement with Herzer's and Zbroszczyk's measurements between 20 and 400°C [7,8]. This could be interpreted as a transition from superferromagnetism to ferromagnetism. The Curie transition of the residual amorphous phase is seen neither on the B s curve nor in TMG recordings [9] but on the B a / B s curve whose drop during cooling is due to the paramagnetic-ferromagnetic transition of the matrix and leads to
composite ferromagnetism. T~m is determined at about 330°C, which is close to the as-cast value, but this temperature cannot be precisely defined. It is possible that this relatively slow transition, as proposed by Varga et al., is due to a distribution of Tflm [10] in the amorphous matrix. It is noticeable that ultimately, this thermal treatment gives rise to excellent soft magnetic properties and is equivalent to optimum annealing (550°C, 60 rain) [11]. A maximum permeability of 200 000, a 68% squareness and a I A / m dc coercive field were measured.
4. C o n c l u s i o n
The interest of the TMH method is clearly demonstrated by the entire extent of the information it provides, especially in terms of remanence and coercivity. It should be stressed that these quantitative results are validated by the good agreement of the amorphous Curie transition with the theory of ferromagnetism. The most interpretable results are given by isothermal tests and could be summarized as follows: At 520°C, the saturation induction of crystallites is 0.3 T. The shoulder of the B s curve at about 20 min could be related to the DSC experiments con-
1,38
I-~ Macaleyrat et al. / Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38
ducted by P&ron et al. [4], who found a crystallization enthalpy after 16 min at 525°C of close to zero. Thus, the increase from 0.11 to 0.3 T could be explained by the slow diffusion of niobium atoms [12] which must be weakly energetic. Furthermore, investigations using this method and DSC have to be performed in order to validate this interpretation. According to He et al. [13], these exceptional magnetic properties are not only a result of small anisotropy and zero magnetostriction, but also of the magnetic coupling between phases, i.e. in the minimization of the parameter A M s = ,,1 tlArcry s - M sa m . T h i s point of view seems to be confirmed by our investigations and, in particular, by the notch in the B R / B s curve above the amorphous matrix Curie point. The minimum at 285°C shows the transition to composite ferromagnetism which leads to low coercivity and high permeability. It is obvious that these initial results have to be completed by further investigations on the transition from superferromagnetism to composite ferromagnetism. It would also be interesting to investigate the superferro-superparamagnetic transition in order to have an experimental evaluation of blocking temperature by precise measurements of the temperature dependence of coercivity. Lastly, an attempt is made to evaluate the crystalline fraction by carefully fitting of the B ( T ) curve
of annealed samples, using the Brillouin function, with the aim of obtaining correlation with XRD results.
References [1] M.S. Leu, W.K. Wang and S.C. Jang, Mat. Sci. Eng. A 181/182 (1994) 997. [2] C.F. Conde and A. Conde, Mat. Sci. Forum 179/181 (1995) 581. [3] S. Chikazumi and S.H. Charap, in: Physics of Magnetism (R.E. Krieger, Malabar, FL, 1978) p. 69. [4] N. Lecaude and J.C. Perron, J. Magn. Magn. Mater, 160 (1996) to appear. [5] B.D. Cullity, Introductionto Magnetic Materials (AddisonWesley, Reading, MA) pp. 410-417. [6] H. K. Lachowicz and A. Slawska-Waniewska, J. Magn. Magn. Mater. 133 (1994) 238. [7] G. Herzer, IEEE Trans. Magn. 25 (1989) 3327. [8] J. Zbroszczyk, Phys. Stat. Sol. (a) 136 (1993) 545. [9] C. Conde, M. MillS.nand A. Conde, J. Magn. Magn. Mater. 138 (1994) 314. [10] L.K. Varga, E. Kisdi-Koszo,V. Strom and K.V. Rao, SSM12, Krakov, Sept. 1995. [11] J. Bigot, N. Lecaude, J.C. Perron, C. Milan, C. Ramiarinjaona and J.F. Rialland, J. Magn. Magn. Mater. 133 (1994) 299. [12] A.R. Yavari and O. Drbohlav, Mat. Trans. JIM 36 (1995) p. 896. [13] K. Y. He, J. Zhi, L.Z. Cheng and M.L. Sui, Mater. Sci. Eng. A 181/182 (1994) 880.
ELSEVIER
journal of magnetism and magnetic materials
Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38
Letter to the Editor
Thermomagnetic study of Finemet type nanocrystalline alloy by in situ hysteresis measurements F. Mazaleyrat *, J.C. Faugi~res, J.F. Rialland LESiR-CNAM-ENSC, 61 ate. du Pdt Wilson, 94 235 Cachan cedex, France
Received 9 February 1996; revised 15 April 1996
Abstract The origin of the excellent magnetic properties of nanocrystallized Fe73.sCu~Nb3Sij35B 9 have been explained essentially by its nanograin weak anisotropy and its global zero magnetostriction. However, it has been suspected that a more complex problem of two-phase magnetic coupling must be taken into consideration. It is proposed herein to investigate in this way by in situ hysteresis measurements up to saturation at high temperature (reaching 700°C). Details of the Thermo-Magnetic Hysteresis method (TMH) are given. Dynamic and isothermal crystallization are investigated. Isothermal tests point out that induction arising could result from crystallization and from a slower mechanism such as atomic diffusion. It is also shown that the crystallization stage occurs, at its beginning, in a superparamagnetic phase and reaches a superferromagnetic phase. During cooling, the complex superferro-ferromagnetic transition shows that the magnetic coupling between grains and the amorphous matrix plays a leading role in Finemet magnetic properties. PACS:
75.60.Nt; 75.50.Kj; 75.30.Kz; 85.80.Lp
Keywords:
Nanocrystallization;Thermomagnetism;Magnetic coupling; Magnetic transitions; Crystallization kinetics; Superferromagnetism
1. Introduction
The most current methods to investigate nanocrystallization are Differential Thermal Analysis (DTA), Differential Scanning Calorimetry (DSC) and X-Ray Diffraction (XRD), but these methods provide no information about magnetic quantities. DSC gives the Curie point of amorphous but not that of crystallites since it is too close to the FeB crystallization stage. Various authors have presented recordings of saturation magnetization during annealing by Ther-
* Corresponding author. Fax: + 33-1-4740-2199; email: [email protected].
momagnetic Gravimetry (TMG). This method is based on the measurement of the magnetic force induced by a magnet on the material which is set in a thermobalance. This method yields T~ry precisely, but the recordings at low heating rates are just a qualitative image of the saturation magnetization M~, since it is very sensitive to thermal drift. This obviously shows the usefulness of a device which is able to give absolute values of M~ and more magnetic quantities, such as squareness and coercivity. For the purpose of resolving this problem, an annealing furnace is fitted out with a magnetic device which allows measuring hysteresis loops up to saturation (even for as-cast material) at temperatures up to
0304-8853/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved. PII S0304-8853(96)00372-1
L34
k. 1,1~:( levral et al /Journal of Magnetisn and Magnetic Materials 159 (1996) L33 L38
700°C. In this paper a description of the experiment is given. These Thermomagnetic Hysteresis tests (TMH) are compared to DTA and TMG. It is also demonstrated that the TMH method produces its most interesting results about crystallization processes during isothermal treatment.
2. Experimental From a magnetic point of view, the best configuration is a toms geometry because the demagnetizing field is zero. In our case, this configuration has three major problems. Firstly, high temperature imposes the use of mineral electric insulators, which have a poor space factor. So, only a large toms could be used (of about 5 cm inner diameter). Secondly, excitation and flux coils must be wound for each sample; and thirdly, the rolling of ribbons generates flexion stresses. For these reasons, a solenoid configuration is used as described below. The trickiest problem with this configuration is the evaluation of the internal field, thus H-coils must be used to measure it. With the aim of minimizing the demagnetizing field, the ribbons are cut 20% longer than the excitation coil (Fig. 1). They are laid on a 35 cm ceramic stage and surrounded by a mineral-insulated coil which picks up the flux. Two H-coils, made from stripped wire wound on ceramic sheets, are placed on the stage at each end of the
B-coil. The whole is slipped in a 25 cm solenoid composed of mineral-insulated wire wound on a quartz tube. The total outer diameter of this cell is 42 ram. The magnetic cell is then slipped in a tubular ceramic furnace which is wrapped in a magnetic shielding to protect the system from electromagnetic interferences and the Earth's magnetic field. In this device as few metallic parts as possible are being used. It must be underscored that this method only gives the flux density, B, through the material cross-section. Hence, saturation magnetization of crystallites cannot be directly evaluated because the crystallized volume fraction is not known. However, the squareness M e / M s is equal to B R / B s. Even in this open coil configuration, the evaluation of the demagnetizing factor is of no interest because the tangential component of the magnetic field is directly measured near the sample surface. However, it is necessary to evaluate the biased error on induction and field value due to their heterogeneity along the axis. This is performed experimentally with 10 mm long ultra-fine winding coils made of 50 ~ m copper wire. This test is run at ambient temperature on as-cast ribbons. Distribution of induction and field is shown in Fig. 2 at 50 and at 1200 A / r e . These curves are plotted with respect to the centre values B(0), H(0) and are compared to the distribution of the magnetic field along the solenoid axis = when no material is laid in. It is seen that this
Thermocouple monitor
Ribbons
/' /
H-Coil
/
~ /
Solenoid
B-c/oil
llIHlllJ
~
e Amp.
Power sinus wave Amplifier generator
Fig. 1. Schematic diagram of magnetic cell and acquisition system.
F. Maz.aleyrat et ul. / Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38 i
'B-Coil
H.Cnil
Ribbons
Illllllllllllllllllll
SolenoidI
,_/1_
_
~
~
~
-
-~
1.0
--1
;0
,, v 1.0 "(3 (D
Q. 0.8
-
{" o Q.
iT 0.8-
-4
t-" -
0.6
0.6-
~6
0.4
E
0.4 13
8 "0 r~
0.2 ,~
0.2-
t-" o
'
0
i
,
20
i
,
40
i
,
60
i
,
80
i
,
~
]
,
i
,
100 120 140 160 180
Position on axis (mm) Fig.
2. Distribution along the
solenoid axis of induction
B(z)/B(O) ( I 1 2 0 0 A / m ; • 50 A / m ) , internal field H(z)/H(O) ([q 1200 A / m ; O 50 A / m ) and field with no ferromagnetic material (- • -). Maximum temperature gap during continuous heating ( • ) and during isotherm (*).
configuration gives rise to a good uniformity. On the other hand, the distribution depends on the excitation field value. It is noticeable that the best uniformity is obtained when the material is saturated, but in each case, the homogeneity of induction in its measuring area is better than 1%. For the magnetic field, the problem is most sensitive. However, since the H-coil is 50 mm long in the furnace, the field is evaluated by its mean value, and a maximum of 5% biased error is ultimately found. Temperature gradient is also shown in this figure. The temperature gap is calculated relative to the one inside the B-coil. During dynamic tests, the thermal gradient is positive along the axis because the electric insulator of the solenoid and the B-coil play the role of thermal insulator. During isothermal tests (thermal steady state), this gradient is negative because the ends of the tubular furnace are cooled. The maximum temperature gap in the measuring zone is 10% at the beginning of heating and 4.5% during isothermal treatment• Since section variations of coils are not significant, this method is practically free of thermal drift. Dynamic testing is performed at a 5°C/rain con-
-5
1.2 °o
o o o
1.0
....
t.-
.O
~-o. o
-,
-7 0.8-
°
o
I I
", o i o~
--,1 e--t- 0.6 o
~ 0.4-
/
ol ol
//i
o/ i
0
-9
\
\
/
0.2-
0
,
~'~
-10 ~ "
\
, "'~oO qo
200
~=~
# f~
,o
v
-11 -12
40O
600
Temperature (°C) Fig. 3. Comparison between TMH (O), DTA (
L35
) TMG at 5°C/min (- - -) and 40°C/rain ( . . . ) .
1~36
F.
Macaleyral et al./ Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38
stant heating rate up to 700°C for TMH and up to 850°C for DTA (stainless steel inert reference). In the case of isothermal testing, the maximum heating rate is used (25°C/rain) until reaching 525°C and the temperature is held constant for 4 h. Tests are run under a sinusoidal excitation field at 83.3 Hz. This frequency is chosen to be as small as possible, in order to keep the coercive field as a significant quantity and high enough to have a 5 txV minimum H-coil voltage per A/re. This signal is amplified by a 40 dB low noise amplifier (15 nV/,/Hz). Signal acquisition is carried out by a 10 bit numeric oscilloscope; integration and air flux compensation are computed via an IEEE bus (see Fig. 1).
3. Results and discussion
In Fig. 3, TMH at 5°C/min is compared to a DTA plot at 5°C/rain performed on material derived from the same casting. TMG at 5°C/rain (after Leu [1]) and 20°C/rain (after Conde [2]) can also be seen. It is noticed that Curie temperatures of amorphous and crystalline phases as well as the crystallization temperature (T x) are in good accordance with TMG if we assume the fact that alloys are developed from different castings; the amorphous ferromagnetic Curie point is determined as T~c = 326°C (399 K), the crystallites' Curie point as T~ry = 600°C (873 K) and Tx = 520°C (793 K), which is confirmed by DTA. This method provides also the paramagnetic Curie point, T~p = 350°C (423 K). The granulation of the FeB compound is not witnessed herein. It can be noticed that the Curie transition is more abrupt in TMG records. This difference could be explained by the fact that in the TMG method, the constant field (about 2 k A / m in the material [1]) is not sufficient to saturate the as-quenched material. In the TMH trial, the excitation field is controlled to be just enough to yield technical saturation (3.5 k A / m for as-cast and 150 A / m during crystallization). This field monitoring is essential for a good evaluation of coercivity, squareness and saturation induction. A validation of this method is obtained by resolving the molecular field equation using the quantum-mechanical relation [3], namely the Brillouin function, with a total angular momentum of J = 1/2, which means that the orbital contribution is
negligible. It is visible in Fig. 4 that experimental results found with as-quenched amorphous material are in good accordance with the theoretical temperature dependence of magnetization. The saturation induction, determined from the theoretical Brillouin function, at 0 K is found to be Bo = 1.19 T. However, it is difficult to interpret the continuous heating tests for two reasons. Firstly, this test is not equivalent to annealing conditions. Secondly, the crystallization process is partly hidden by a progressive approach towards the Curie point, because the decrease of magnetization induced by the heating rate and by crystallization kinetics are comparable. The fact that the crystallization peak is lower on the TMH plot than on TMG is due to the dilution of the ferromagnetic phase in the ribbon's cross-section (cf. Section 2). In Fig. 5, the results of the isothermal tests are presented. Temperature and magnetic quantities are plotted as functions of time. Generally, very high heating rates must be used for isothermal testing [4]; however, the test could be considered as isothermal because crystallization begins when the step temperature is reached, even if amorphous relaxation has begun during heating. The drop in coercivity at 280°C demonstrates the relaxation mechanism. The saturation induction increases up to 0. l 1 T during the first 20 min and then it takes 4 h to reach the maximum value of 0.3 T. During cooling, B~ in1 O
0.9 0.8
~; 07 0 0.6 ._ 0.5 t0'~ 0.4
E
0.3 "0 ID ~ 0.2 "o ~ 0.1 0
0
0,2
0.4
0.6
0.8
Reduced temperature TFFc
Fig. 4. Reduced magnetizationof as-cast amorphous (normalized at 0 K) as function of reduced temperature (normalized with respect to the Curie temperature). Points are experimental. Curve is calculated.
F. Maz.aleyrat et al. / Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38 [
600
12
I
L37
- 160 e-
140
500
(,~
1.0 o"
/
-~.
,0.8 .~
400
/
~
300
0.6 ~
(D
E 200
04~
\
g ,
0 -100
-,,,i
\- o.2 ~
100
.
.
.
.
.
I
'
~oo
I
200
'
'
300
~
4 o
'
C
2120 -
~
-100 ~Q.
-~o ~ -~o ~ 24o g - 20
-0
~.
Time (min) Fig. 5. Evolution of saturation induction (C)), remanence ratio ( 0 ) and coercivity ( l l ) during isotherm and cooling vs. time with regard to temperature ( - • -).
creases as the thermal energy decreases. The shapes of squareness and coercivity are characteristic of the grain diameter dependence of a single-domain fine particle system in a paramagnetic matrix [5], namely superferromagnetism [6]. The peak of coercivity during cooling is characteristic of the superferromagnetic state. The blocking temperature T B can be estimated by extrapolation from experimental data using the theoretical model [5]: Hc(r)/Hc(O)
:
1 - v:r/rB
T B is found to be about 600°C, but this evaluation is very imprecise since superferromagnetism is observed here in a small temperature range. Above 450°C, the drop in coercivity is probably a consequence of other thermal effects, such as increasing particle magnetization and progressive exchange coupling between grains via the amorphous phase this part of the H c curve is in good agreement with Herzer's and Zbroszczyk's measurements between 20 and 400°C [7,8]. This could be interpreted as a transition from superferromagnetism to ferromagnetism. The Curie transition of the residual amorphous phase is seen neither on the B s curve nor in TMG recordings [9] but on the B a / B s curve whose drop during cooling is due to the paramagnetic-ferromagnetic transition of the matrix and leads to
composite ferromagnetism. T~m is determined at about 330°C, which is close to the as-cast value, but this temperature cannot be precisely defined. It is possible that this relatively slow transition, as proposed by Varga et al., is due to a distribution of Tflm [10] in the amorphous matrix. It is noticeable that ultimately, this thermal treatment gives rise to excellent soft magnetic properties and is equivalent to optimum annealing (550°C, 60 rain) [11]. A maximum permeability of 200 000, a 68% squareness and a I A / m dc coercive field were measured.
4. C o n c l u s i o n
The interest of the TMH method is clearly demonstrated by the entire extent of the information it provides, especially in terms of remanence and coercivity. It should be stressed that these quantitative results are validated by the good agreement of the amorphous Curie transition with the theory of ferromagnetism. The most interpretable results are given by isothermal tests and could be summarized as follows: At 520°C, the saturation induction of crystallites is 0.3 T. The shoulder of the B s curve at about 20 min could be related to the DSC experiments con-
1,38
I-~ Macaleyrat et al. / Journal of Magnetism and Magnetic Materials 159 (1996) L33-L38
ducted by P&ron et al. [4], who found a crystallization enthalpy after 16 min at 525°C of close to zero. Thus, the increase from 0.11 to 0.3 T could be explained by the slow diffusion of niobium atoms [12] which must be weakly energetic. Furthermore, investigations using this method and DSC have to be performed in order to validate this interpretation. According to He et al. [13], these exceptional magnetic properties are not only a result of small anisotropy and zero magnetostriction, but also of the magnetic coupling between phases, i.e. in the minimization of the parameter A M s = ,,1 tlArcry s - M sa m . T h i s point of view seems to be confirmed by our investigations and, in particular, by the notch in the B R / B s curve above the amorphous matrix Curie point. The minimum at 285°C shows the transition to composite ferromagnetism which leads to low coercivity and high permeability. It is obvious that these initial results have to be completed by further investigations on the transition from superferromagnetism to composite ferromagnetism. It would also be interesting to investigate the superferro-superparamagnetic transition in order to have an experimental evaluation of blocking temperature by precise measurements of the temperature dependence of coercivity. Lastly, an attempt is made to evaluate the crystalline fraction by carefully fitting of the B ( T ) curve
of annealed samples, using the Brillouin function, with the aim of obtaining correlation with XRD results.
References [1] M.S. Leu, W.K. Wang and S.C. Jang, Mat. Sci. Eng. A 181/182 (1994) 997. [2] C.F. Conde and A. Conde, Mat. Sci. Forum 179/181 (1995) 581. [3] S. Chikazumi and S.H. Charap, in: Physics of Magnetism (R.E. Krieger, Malabar, FL, 1978) p. 69. [4] N. Lecaude and J.C. Perron, J. Magn. Magn. Mater, 160 (1996) to appear. [5] B.D. Cullity, Introductionto Magnetic Materials (AddisonWesley, Reading, MA) pp. 410-417. [6] H. K. Lachowicz and A. Slawska-Waniewska, J. Magn. Magn. Mater. 133 (1994) 238. [7] G. Herzer, IEEE Trans. Magn. 25 (1989) 3327. [8] J. Zbroszczyk, Phys. Stat. Sol. (a) 136 (1993) 545. [9] C. Conde, M. MillS.nand A. Conde, J. Magn. Magn. Mater. 138 (1994) 314. [10] L.K. Varga, E. Kisdi-Koszo,V. Strom and K.V. Rao, SSM12, Krakov, Sept. 1995. [11] J. Bigot, N. Lecaude, J.C. Perron, C. Milan, C. Ramiarinjaona and J.F. Rialland, J. Magn. Magn. Mater. 133 (1994) 299. [12] A.R. Yavari and O. Drbohlav, Mat. Trans. JIM 36 (1995) p. 896. [13] K. Y. He, J. Zhi, L.Z. Cheng and M.L. Sui, Mater. Sci. Eng. A 181/182 (1994) 880.