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Magnetic and 57Fe Mössbauer studies of hydrided Ti(Fe1-xCox
T E M P E R A T U R E AND FIELD D E P E N D E N C E O F T H E D O M A I N S T R U C T U R E O F FeSi AND CoPd S I N G L E CRYSTALS UNGEMACH
V.
Physikalisch-Technische Bz.ndesanstalt, 3300 Braunschweig, Fed. Rep. Germany
The domain structure was investigated as a function of temperature a n d field up to the Curie-point. For both materials the platea n d wedge-shaped domains strongly depend on the thermal treatment, the Bloch walls being movable or immovable a n d the d o m a i n width varying by a factor of up to 10.
On prismatic single crystals of Fe-4.8 at.% Si ( T c = 755°C; easy directions (100)) and C o - 1 6 at.% Pd ( T c = 1020°C; easy directions (111) [1]) the domain structure was analyzed with the Kerreffect up to 745°C for FeSi and up to 900°C for CoPd. The crystallographic orientations and the dimensions of the samples are shown in fig. 1 and fig. 3. The preparation of the crystals as well as the experimental procedure of the measurements have been described in [1-3]. The closure domains were studied at room temperature in [1] (CoPd) and [2] (FeSi). In this paper measurements of the main structure are reported. For both FeSi and CoPd in low axial fields the main structure consists of plate-shaped domains perpendicular to the axis (fig. 1 and fig. 3). At 1~4 950
.[mo]
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,
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IIi
"
."
/ "~'®~ / /
f,,,o, _
/ ;" ?" /7+---H ;
T+ool,, •
-
-
/2.5,~ ,"
V ~
80ram
'4®ti 300]~ --c 2uu~ \ Lx.
20*E
~----~.
0
o
-- ~_:_,___..
2b
4'0
....
6'o ± ~b cm
H,
Fig. 1. FeSi. Width of the plate-shaped domains as a ,function of the effective field Hj at 20"C. Notation a, b, c as in fig. 2. Insert: orientation a n d dimensions of the crystals, plate-shap(xt main domains.
higher field strengths additional wedge-shaped domains appear from the sides and grow through the crystal thus subdividing each plate [l(CoPd), 2-4(FeSi)]. This behaviour can be observed for both materials at all temperatures. Despite idealization by an alternating circular magnetic field produced by a decreasing axial current (50 s- t, lm~x 45 A [3]) the mobility of the Bloch walls and the width of the plates strongly depend on the prior thermal treatment: (1) For FeSi after cooling the sample from temperatures above 450°C down to room temperature without any field, i.e. without Bloch wall movement, the 90°-walls are totally anchored. With increasing field the plate width decreases only by wedges. From this the step-like d/2(Hi)-curve a in fig. 1 results. With a rise in temperature this behaviour doesn't change up to about 320°C. Above 320°C the Bloch walls become movable and the plate width varies in low fields continuously by displacements of the walls, in higher fields by wedge-shaped domains [3]. The corresponding d/2(T)-curve (fig. 2, curve a) shows three characteristic temperatures. At 320°C the plate-width begins to increase, at approximately 450°C a minimum appears and at 500°C there is a maximum. Above 500°C the plate width decreases with rising temperature [3] (see also [4]). An analogous behaviour occurs in the case of curves b (fig. 1 and fig. 2), which result from the following: at approximately 500°C without idealization very broad plates with a width of up to 1 mm sometimes appear after switching off a field. These broad plates are unstable and not reproducible at 500°C. After cooling down to room temperature they are stable and reproducible. Just as in case a, the cooled 90°-Bloch walls are anchored and the plate width decreases only by wedgeshaped domains. Between curves a and b any plate width with immovable Bloch walls can be produced.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 1503-1504 ©North Holland
1503
F. Ungemach / Domain structure of FeSi and CoPd single crystals
1504
_i
b-
d ?
iJ m
o) stool[ plates cooled from 730°C wdhout field
:, r~ t----'L_t__ -
C/,
~l
Fig. 3. CoPd. Width of the plate-domains as a function of the effective field H i at 20°C and 500°C. Insert: orientation and dimensions of the crystals.
200
O'~F 0
:,
500°I --~
4O
:o.8 A
100
i _ ~..q_
50
400
,:"
jJ
7o
r:cooled from 600°01r superimposed : rcuior end o~,~i olfern@mg fields
,t'.5,
itll
8o
b} ey:r~me~y thcK piolee o@ed from 500°C w,t'~ou" fie;d
500-
300
,-d :,
90
1080~ .......
..,.
8 . . . . ZOO 400 r
i ' 600 °C '800 ~
,~
755
Fig. 2. FeSi. Width of the plate-domains as a function of temperature at H~ = 0.8 A cm ~.
In [3] it is shown that this stabilization of the Bloch walls results from reorientation of Si-Siatom pairs in parallel with the magnetization in the domains (induced anisotropy [5, 6]). The minimum of the plate width at 450°C can be explained by an additional long-range diffusion of the atom pairs out of the walls into the adjacent domains. To avoid this reorientation of Si-Si-pairs the sample was cooled in superimposed circular (36 s - I ) and axial (50 s - t ) alternating fields. In this case at room temperature movable 90°-Bloch walls result. Up to about 15 A cm -~ the plate width changes continuously by displacements of the walls, at higher field strengths again by wedges (curves c, fig. 1 and fig. 2). The corresponding d/2(T)-curve shows a minimum at 450°C and a maximum at 500°C, too. All d/2(T)-curves coincide near the maximum at 500°C. Above 500°C the structure is reversible independent of the prior thermal treatment, i.e. above 500°C the influence of the Siatoms is negligible. (2) For CoPd there is an even stronger influence on the Bloch wall mobility. The 71°-walls are totally anchored after cooling the sample. Just as for FeSi the decrease of the plate width with increasing field results only from wedges. With rising temperature
this behaviour doesn't change up to approximately 450°C. Above 450°C the wedge mechanism disappears and up to 900°C no influence of the temperature and field c a n be observed. A constant plate width of 52 ffm independent of the field was found (in fig. 3 shown for 500°C). If the sample was cooled down in this state also at room temperature the plate width cannot be varied. In this case at all temperatures and at all field strengths there is no change of the domain structure. It is still unknown, whether in CoPd movable 71°-Bloch walls exist at all for those crystals. Indeed the investigations on this material are just beginning. The described results show that for both FeSi and CoPd prior thermal treatment strongly influences the Bloch wall mobility and the domain width. Therefore one has to assume that analogous effects might exist for other ferromagnetic alloys, too.
This work was performed at the Institute A for Physics of the Technical University Braunschweig. I thank Prof. Schwink for his support and m a n y helpful discussions. References [1] V. Ungemach and Ch. Schwink, Physica 80B (1975) 381. [2] G. Dedi~, J. Niemeyer and Ch. Schwink, Phys. Stat. Sol. (b)43 (1972) 163. [3] V. Ungemach, Thesis work, University Braunschweig (1977). [4] V. Ungemach and Ch. Schwink, J. Magn. Magn. Mat. 2 (1976) 167. [5] K. Forsch, Phys. Stat. Sol. 42 (1970) 329. [6] P. E. Brommer and H. A. 't Hooft, J. de Phys., suppl 2-3, C1-105 (1971) T32.
V.
Physikalisch-Technische Bz.ndesanstalt, 3300 Braunschweig, Fed. Rep. Germany
The domain structure was investigated as a function of temperature a n d field up to the Curie-point. For both materials the platea n d wedge-shaped domains strongly depend on the thermal treatment, the Bloch walls being movable or immovable a n d the d o m a i n width varying by a factor of up to 10.
On prismatic single crystals of Fe-4.8 at.% Si ( T c = 755°C; easy directions (100)) and C o - 1 6 at.% Pd ( T c = 1020°C; easy directions (111) [1]) the domain structure was analyzed with the Kerreffect up to 745°C for FeSi and up to 900°C for CoPd. The crystallographic orientations and the dimensions of the samples are shown in fig. 1 and fig. 3. The preparation of the crystals as well as the experimental procedure of the measurements have been described in [1-3]. The closure domains were studied at room temperature in [1] (CoPd) and [2] (FeSi). In this paper measurements of the main structure are reported. For both FeSi and CoPd in low axial fields the main structure consists of plate-shaped domains perpendicular to the axis (fig. 1 and fig. 3). At 1~4 950
.[mo]
,m~
+4', I
,
,Z- d-./
t0mJ
IIi
"
."
/ "~'®~ / /
f,,,o, _
/ ;" ?" /7+---H ;
T+ool,, •
-
-
/2.5,~ ,"
V ~
80ram
'4®ti 300]~ --c 2uu~ \ Lx.
20*E
~----~.
0
o
-- ~_:_,___..
2b
4'0
....
6'o ± ~b cm
H,
Fig. 1. FeSi. Width of the plate-shaped domains as a ,function of the effective field Hj at 20"C. Notation a, b, c as in fig. 2. Insert: orientation a n d dimensions of the crystals, plate-shap(xt main domains.
higher field strengths additional wedge-shaped domains appear from the sides and grow through the crystal thus subdividing each plate [l(CoPd), 2-4(FeSi)]. This behaviour can be observed for both materials at all temperatures. Despite idealization by an alternating circular magnetic field produced by a decreasing axial current (50 s- t, lm~x 45 A [3]) the mobility of the Bloch walls and the width of the plates strongly depend on the prior thermal treatment: (1) For FeSi after cooling the sample from temperatures above 450°C down to room temperature without any field, i.e. without Bloch wall movement, the 90°-walls are totally anchored. With increasing field the plate width decreases only by wedges. From this the step-like d/2(Hi)-curve a in fig. 1 results. With a rise in temperature this behaviour doesn't change up to about 320°C. Above 320°C the Bloch walls become movable and the plate width varies in low fields continuously by displacements of the walls, in higher fields by wedge-shaped domains [3]. The corresponding d/2(T)-curve (fig. 2, curve a) shows three characteristic temperatures. At 320°C the plate-width begins to increase, at approximately 450°C a minimum appears and at 500°C there is a maximum. Above 500°C the plate width decreases with rising temperature [3] (see also [4]). An analogous behaviour occurs in the case of curves b (fig. 1 and fig. 2), which result from the following: at approximately 500°C without idealization very broad plates with a width of up to 1 mm sometimes appear after switching off a field. These broad plates are unstable and not reproducible at 500°C. After cooling down to room temperature they are stable and reproducible. Just as in case a, the cooled 90°-Bloch walls are anchored and the plate width decreases only by wedgeshaped domains. Between curves a and b any plate width with immovable Bloch walls can be produced.
Journal of Magnetism and Magnetic Materials 15-18 (1980) 1503-1504 ©North Holland
1503
F. Ungemach / Domain structure of FeSi and CoPd single crystals
1504
_i
b-
d ?
iJ m
o) stool[ plates cooled from 730°C wdhout field
:, r~ t----'L_t__ -
C/,
~l
Fig. 3. CoPd. Width of the plate-domains as a function of the effective field H i at 20°C and 500°C. Insert: orientation and dimensions of the crystals.
200
O'~F 0
:,
500°I --~
4O
:o.8 A
100
i _ ~..q_
50
400
,:"
jJ
7o
r:cooled from 600°01r superimposed : rcuior end o~,~i olfern@mg fields
,t'.5,
itll
8o
b} ey:r~me~y thcK piolee o@ed from 500°C w,t'~ou" fie;d
500-
300
,-d :,
90
1080~ .......
..,.
8 . . . . ZOO 400 r
i ' 600 °C '800 ~
,~
755
Fig. 2. FeSi. Width of the plate-domains as a function of temperature at H~ = 0.8 A cm ~.
In [3] it is shown that this stabilization of the Bloch walls results from reorientation of Si-Siatom pairs in parallel with the magnetization in the domains (induced anisotropy [5, 6]). The minimum of the plate width at 450°C can be explained by an additional long-range diffusion of the atom pairs out of the walls into the adjacent domains. To avoid this reorientation of Si-Si-pairs the sample was cooled in superimposed circular (36 s - I ) and axial (50 s - t ) alternating fields. In this case at room temperature movable 90°-Bloch walls result. Up to about 15 A cm -~ the plate width changes continuously by displacements of the walls, at higher field strengths again by wedges (curves c, fig. 1 and fig. 2). The corresponding d/2(T)-curve shows a minimum at 450°C and a maximum at 500°C, too. All d/2(T)-curves coincide near the maximum at 500°C. Above 500°C the structure is reversible independent of the prior thermal treatment, i.e. above 500°C the influence of the Siatoms is negligible. (2) For CoPd there is an even stronger influence on the Bloch wall mobility. The 71°-walls are totally anchored after cooling the sample. Just as for FeSi the decrease of the plate width with increasing field results only from wedges. With rising temperature
this behaviour doesn't change up to approximately 450°C. Above 450°C the wedge mechanism disappears and up to 900°C no influence of the temperature and field c a n be observed. A constant plate width of 52 ffm independent of the field was found (in fig. 3 shown for 500°C). If the sample was cooled down in this state also at room temperature the plate width cannot be varied. In this case at all temperatures and at all field strengths there is no change of the domain structure. It is still unknown, whether in CoPd movable 71°-Bloch walls exist at all for those crystals. Indeed the investigations on this material are just beginning. The described results show that for both FeSi and CoPd prior thermal treatment strongly influences the Bloch wall mobility and the domain width. Therefore one has to assume that analogous effects might exist for other ferromagnetic alloys, too.
This work was performed at the Institute A for Physics of the Technical University Braunschweig. I thank Prof. Schwink for his support and m a n y helpful discussions. References [1] V. Ungemach and Ch. Schwink, Physica 80B (1975) 381. [2] G. Dedi~, J. Niemeyer and Ch. Schwink, Phys. Stat. Sol. (b)43 (1972) 163. [3] V. Ungemach, Thesis work, University Braunschweig (1977). [4] V. Ungemach and Ch. Schwink, J. Magn. Magn. Mat. 2 (1976) 167. [5] K. Forsch, Phys. Stat. Sol. 42 (1970) 329. [6] P. E. Brommer and H. A. 't Hooft, J. de Phys., suppl 2-3, C1-105 (1971) T32.