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Serrated faces in [110] iron whiskers
Journal of Crystal Growth 77 (1986) 185—191 North-Holland, Amsterdam
185
SERRATED FACES IN LI1OJ IRON WHISKERS M. TEJEDOR and J.F. FUERTES Departamento de F,sica, Facultades de Ciencias, Universidad de Oviedo, Oviedo, Spain Received 16 October 1985; manuscript received in final form 5 March 1986
A saw-shape irregularity on the faces of some [1101iron whiskers is reported in this work. The conditions of growth to obtain this kind of whiskers are given. An attempt is made to explain the qualitative relationship between these “serrated” faces and the magnetic domain structures.
1. Introduction After the early work concerning the growth of iron whiskers [1], many workers have studied exhaustively their growth mechanism (e.g. refs. [1—3]), internal structure (e.g. refs. [4,5]), and mechanical [6] and magnetic [7—10] properties. Mainly, three kinds of whiskers are obtained [4,11] that exhibit an almost perfect crystal lattice and natural polyhedral form, called [100], [1111 and [110] iron whiskers since their long axis is parallel to the [1001,[111] and [110] crystallographic directions, respectively. They also show a different cross-sectional area: generally square for [100], hexagonal for [111] and rectangular for [110], although they exhibit other different external forms. There are also [210] and [211] oriented crystals [11,12]: nevertheless, they appear in a small amount and there is very little information about them. The four faces of the [100] iron whiskers are (100) perfect and reflective surfaces, whereas the [111] whiskers are bounded by six (110) perfect and generally bright faces. These are the most perfect whiskers, and the works concerning mechamcal and magnetic properties in these kinds of crystals are undoubtedly the most numerous attributed to their perfect surfaces and internal crystalline structure. Nevertheless, the [110] iron whiskers that appear in considerable numbers have not been
studied in much detail, and there is little information about them. We have reported in a previous paper the magnetic domain structures at zero applied field in these [110] whiskers [10], but works concerning their internal structure and surface perfection are unknown to the authors. Although [110] iron whiskers are bounded by two broad (100) perfect and reflective surfaces, tapered in triangular shape, and two (110) narrow faces never completely smooth, we have very often found special irregularities in the (110) narrow faces. The purpose of this work is to present a special case of irregularity in the (110) faces, that we call “serrated faces” due to their singular shape, and we make an attempt to explain a qualitative relationship between the crystal shape and magnetic domain structures. 2. Expenmental The iron whiskers were obtained by hydrogen reduction of ferrous chloride at 700°C using Brenner’s classical method [1]. Ferrous chloride used in the experiments was hydrolyzed salt of 99.9% purity grade, dehydrated during the first stage of the process at 400°Cand the vapor water from the quartz tube was removed by means of argon gas rapidly flowing through the tube. Boats were made of commercial iron, and hydrogen and argon were both 99.9% purity grade without fur-
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
186
M. Tejedor, J.F. Fuertes
/ Serrated faces in [110] iron whiskers
ther purifications. No special care was taken to highly isolate the reduction chamber, so allowing the presence of a small concentration of oxygen that has not been measured accurately, although it has been estimated to be no greater than 0.1 wt%. The hydrogen flow-rate and temperature were kept unvariable during the reduction process, which has not been stopped until the end of the reaction, about 2 h for 40 g of initial ferrous chloride. Visual examination has been made firstly without removing whiskers from the boat, with a low power microscope to select crystals to be tested. Photographs were obtained with a commercial camera added to a Zeiss metallograph microscope. X-ray diffraction studies were difficult to interpret in this case; nevertheless, magnetic domain structures obtained by the Bitter technique reveal clearly a crystal orientation in these particular cases, as we can see below. The colloid used in the Bitter technique has been made following the classical method described in the specific works (see, e.g., ref. [13]). 3. Results and discussion Iron whiskers with varying shapes and crystallographic orientations were obtained ranging in
Fig. 1. Perfect [1101 iron whisker.
size between 10 and 300 ~smin diameter, although the most common were about 100 tim, and 1 cm long. First, only [110] perfect whiskers were selected to investigate their magnetic properties; fig. 1 shows such a well-finished [110] whisker. However, when occasionally the application of isolating grease in the stoppers was neglected, a very small amount of perfect whiskers were obtamed. Due to this fact, we have made several runs under these especial condition to study the possible influence of oxygen in the crystal growth. As it is well known, by changing the growth conditions, it is possible to alter the nature of the whiskers obtained. Since the change due to the presence of oxygen was very strong, no attempt was made to differentiate the influence of boats, temperature and purity of materials; furthermore, this influence has been reported by several workers (e.g. ref. [11]). The majority of whiskers so found were irregular, with no well-finished faces, and with defects which are distributed in a rather complex manner. A great number also exhibit changes in the growth direction and generally show oxide kinks. Dendntic whiskers were seldom found and were very thin. We only pay special attention to whiskers that
M. Tejedor, J. t Fuertes
___
~
‘
/ Serratedfaces in 1110] iron whiskers
~
187
__
Fig. 2. Serrated whisker with oxide kinks.
pp
Fig. 3. Perfect serrated whisker.
show very regular saw-shaped edges, which appear in a considerable amount under these conditions of growth. These whiskers were very often straight and showed perfect external serrated form. Figs. 2 and 3 are examples of these serrated whiskers. One may note in fig. 2 the oxide kinks, whereas the whisker of fig. 3 shows a perfect external serrated form and smooth broad faces. All the whiskers exhibit a marked rectangular cross-sectional area ranging in thickness between 20 and 40 tim. The thickness was estimated with a micrometer in the microscope, but was difficult to measure
Fig. 4. Magnetic domain ideal structure for a [1101iron whisker.
188
M. Tejedor, J.F. Fuertes
/ Serratedfaces in [1101 iron whiskers
I
Fig. 5. Magnetic domain pattern in a serrated whisker.
...
Fig. 6. Magnification of fig. 5.
accurately since the narrow surface was imperfect to focus perfectly. The direction of growth in these whiskers was the [110]. Only one attempt has been made to confirm the [110] growth direction by X-ray topographs, but it was very distorted and difficult to interpret, so the possibility of this kind of proof has been neglected. However, the magnetic domain patterns obtained in the broad side of the whiskers constitute a very easy manner to clarify
the growth direction in these cases. In fact, the magnetic domain structure of minimum energy, at zero applied field, for the [110] oriented crystals with this particular shape is shown in fig 4 [10], where we can see the main domains whose magnetization vectors lie alternatively directed along [010] and [010] directions (<100) are the easy directions in iron) with 180° domain walls making an angle of 45° with the edge of the crystal. A kind of secondary “echelon” domains near the edges reduce the width of the
alternating N and S poles that appear at the narrow surface. In this way, the magnetostatic energy due to the presence of free poles at the surfaces decreases notably. This particular domain structure is unmistakably shown by the [110] perfect crystals, allowing us to identify the growth direction by means of the domain patterns. Fig. 5 shows the magnetic domain patterns obtained in a serrated whisker which is in good
-
.
~“
.....
“/,‘
/
/‘/
...,
,/ ...‘
.
I
.
/ /,/
/ ‘
.,
~
/‘
(100)
H ~0 ~-~(110> Fig. 7. Ideal magnetization distribution of domains in a [110] serrated whisker.
M. Tejedor, J.F. Fuertes
/ Serratedfaces in 1110] iron whiskers
____________________________________
•
.
.
~
,.
Fig. 8. Magnetic domain structure in a serrated whisker with an applied field parallel to the long axis.
agreement — qualitatively with the ideal one expected. Fig. 6 is a magnification of a zone in fig. 5, and in fig. 7 is sketched the ideal magnetization distribution in a [110] serrated whisker. One may note an interesting feature in fig. 7: when [110] crystals show (110) perfect narrow faces, free poles appear, increasing the magnetostatic energy, whereas in the [110] ideal serrated —
/ ‘“
~-‘
\ / “ ‘\/
H 5 Ce
\ /“
(100) (110)
Fig. 9. Ideal magnetization distribution of the pattern of fig. 8.
189
whisker the free poles disappear and thereby give rise to zero magnetostatic energy, decreasing in this way the whole energy of the crystal. According to Gardner’s investigations [11] to [110] iron whiskers grown under oxygen addition are expected to be with larger (100) faces and in ribbon shape, but there is not any assumption to explain the serrated faces that we found. If this is so, it seems to be possible to suggest that there is some magnetocrystalline contribution to energies involving the crystal growth. The values for magnetocrystalline constants for iron at the temperature of crystal growth are low, but not zero [13], although the data for iron that we know are quite old. It would be useful to have accurate values to make quantitative evaluations, or instead, estimate the anisotropy constants by means of crystalline energies and domain structures, but it seems to be feasible due to the difficulty to evaluate all the energies involved. It is interesting to_remark that, whereas only one main [010] (or [010]) domain appears in the ideal structure of fig. 7 for each tooth-saw, one main domain appears for every two or three tooth-saws in the real domain structure of the serrated whisker, as it can be seen clearly in fig. 6. This fact showed not be surprising since at the temperature of the reduction process, the amsotropy constants are lower than at the room temperature at which the domain structures were obtained, whereliy magnetocrystalline energy increases at the conditions of domain observations, and therefore increases the magnetic domain wall energy, as it is well known in basic domain theories (see, e.g., ref. [14]). However, only qualitative explanations can be suggested, due to the uncertainty of the fact that the magnetocrystalline energy can lead to the formation of serrated faces. One may also pay attention to the fact that the edges of the whisker in fig. 5 are not perfectly
expected, due probably to iniernal dislotj
or oxide inclusions. This is not an imperfection on handling since the whisker has been carefully manipulated; furthermore, because of the fact that magnetic domain structures are very sensitive to plastic deformations, external imperfections added during the handling of the whisker would distort
M. Tejedor, J.F. Fuertes
190
/ Serratedfaces in [110] iron
whiskers
Fig. 10. Magnetic domain pattern in a whisker showing a change in the growth direction.
the magnetic domain patterns, and in this case they are very regular structures, as we can clearly see in figs. 5 and 6. Fig. 8 shows a magnetic domain patterns in a serrated whisker with an applied field of 5 Oe parallel to the long axis. An ideal domain structure in. a serrated whisker with a weak longitudinal applied field is depicted in fig. 9. This is another
/
\
~‘
~
\
/
<010)
/
<0) ((00>
Fig. 11. Magnetization distribution of the pattern of fig. 10 (dashed lines are added to virgin domain patterns to complete domain structure).
proof which clarifies the [110] growth direction. Note that serrated faces are not very regular in the bottom surface. Finally, as a clear example of magnetic domain pattern examinations in iron whisker to elucidate the growth direction, we present two kinds of whiskers that show changes in the growth direction. Fig. 10 is the magnetic domain pattern in such a whisker which changes from [110] direction to [100]. Note that the domain structure in the [100] zone does not differ from the classical domain structures in [100] whiskers [71, although it is about 5 inclined from the ideal [100] direction: in this way, free poles appear at the edges and there is a strong accumulation of colloid in them. The domain structure in the [110] zone is not visible~it is drawn in dashed lines in fig. 11. Fig. 12 shows a whisker with two successive changes in the growth direction. This whisker is always a (100) crystal. Note how the domain structures also do not differ from the ideal ones. Some investigations in whiskers showing other kinds of changes in growth directions have been reported previously [15].
M. Tejedor, J.F. Fuertes / Serratedfaces in 11101 iron whiskers
191
Fig. 12. Iron whisker showing two successive 90°changes in the growth direction and magnetic domain structure.
4. Conclusions Whiskers growing under the influence of a small concentration of oxygen show very often sawshaped irregularities on their faces. This fact can be due to some magnetocrystalline contribution to crystal growth, as it seems to be possible from examining magnetic domain structures in these whiskers. Magnetic domain patterns also enable one to examine the perfection and crystalline orientation in the iron whiskers.
Acknowledgements The authors would like to acknowledge the help of B. Hernando and A. Fernandez. One of the authors (J.F.F.) also acknowledges the encouragement of L. Cuervo.
References [1] S.S. Brenner, Acta Met. 4 (1956) 62. [2) K. Wokulska and Z. Wokulski, Ada Phys. Polon. A39 (1971) 180.
[3] S.S. Brenner and G.W. Sears, Acta met. 4 (1956) 268. [4] Z. Bojarki and M. Suroviec, J. Crystal Growth 46 (1979) 43.
[5] P.D. Gorsuch, J. Appl. Phys. 30 (1959) 837. [6] S.S. Brenner, J. Appl. Phys. 27 (1956) 1484. [7] R.W. Dc Blois and G.D. Grahan, Jr., J. Appl. Phys. 29 (1958) 931. [8] R.V. Coleman and G.G. Scott, Phys. Rev. 107 (1957) 1276. [9] L. Dewrello and H.H. Mende, J. Magnetism Magnetic Mater. 13 (1979) 231. [10] M. Tejedor and J.F. Fuertes, J. Appl. Phys. 55 (1984) 1226. [11] R.N. Gardner, J. Crystal Growth 43 (1978) 425. [12] M. Tejedor and J.F. Fuertes, J. AppI. Phys., submitted. [13] C. Kittel, Rev. Mod. Phys. 21 (1949) 541. [14] R.M. Bozorth, J. Appl. Phys. 8 (1937) 575. [15] M. Tejedor and J.F. Fuertes, J. Magnetism Magnetic Mater., in press.
185
SERRATED FACES IN LI1OJ IRON WHISKERS M. TEJEDOR and J.F. FUERTES Departamento de F,sica, Facultades de Ciencias, Universidad de Oviedo, Oviedo, Spain Received 16 October 1985; manuscript received in final form 5 March 1986
A saw-shape irregularity on the faces of some [1101iron whiskers is reported in this work. The conditions of growth to obtain this kind of whiskers are given. An attempt is made to explain the qualitative relationship between these “serrated” faces and the magnetic domain structures.
1. Introduction After the early work concerning the growth of iron whiskers [1], many workers have studied exhaustively their growth mechanism (e.g. refs. [1—3]), internal structure (e.g. refs. [4,5]), and mechanical [6] and magnetic [7—10] properties. Mainly, three kinds of whiskers are obtained [4,11] that exhibit an almost perfect crystal lattice and natural polyhedral form, called [100], [1111 and [110] iron whiskers since their long axis is parallel to the [1001,[111] and [110] crystallographic directions, respectively. They also show a different cross-sectional area: generally square for [100], hexagonal for [111] and rectangular for [110], although they exhibit other different external forms. There are also [210] and [211] oriented crystals [11,12]: nevertheless, they appear in a small amount and there is very little information about them. The four faces of the [100] iron whiskers are (100) perfect and reflective surfaces, whereas the [111] whiskers are bounded by six (110) perfect and generally bright faces. These are the most perfect whiskers, and the works concerning mechamcal and magnetic properties in these kinds of crystals are undoubtedly the most numerous attributed to their perfect surfaces and internal crystalline structure. Nevertheless, the [110] iron whiskers that appear in considerable numbers have not been
studied in much detail, and there is little information about them. We have reported in a previous paper the magnetic domain structures at zero applied field in these [110] whiskers [10], but works concerning their internal structure and surface perfection are unknown to the authors. Although [110] iron whiskers are bounded by two broad (100) perfect and reflective surfaces, tapered in triangular shape, and two (110) narrow faces never completely smooth, we have very often found special irregularities in the (110) narrow faces. The purpose of this work is to present a special case of irregularity in the (110) faces, that we call “serrated faces” due to their singular shape, and we make an attempt to explain a qualitative relationship between the crystal shape and magnetic domain structures. 2. Expenmental The iron whiskers were obtained by hydrogen reduction of ferrous chloride at 700°C using Brenner’s classical method [1]. Ferrous chloride used in the experiments was hydrolyzed salt of 99.9% purity grade, dehydrated during the first stage of the process at 400°Cand the vapor water from the quartz tube was removed by means of argon gas rapidly flowing through the tube. Boats were made of commercial iron, and hydrogen and argon were both 99.9% purity grade without fur-
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
186
M. Tejedor, J.F. Fuertes
/ Serrated faces in [110] iron whiskers
ther purifications. No special care was taken to highly isolate the reduction chamber, so allowing the presence of a small concentration of oxygen that has not been measured accurately, although it has been estimated to be no greater than 0.1 wt%. The hydrogen flow-rate and temperature were kept unvariable during the reduction process, which has not been stopped until the end of the reaction, about 2 h for 40 g of initial ferrous chloride. Visual examination has been made firstly without removing whiskers from the boat, with a low power microscope to select crystals to be tested. Photographs were obtained with a commercial camera added to a Zeiss metallograph microscope. X-ray diffraction studies were difficult to interpret in this case; nevertheless, magnetic domain structures obtained by the Bitter technique reveal clearly a crystal orientation in these particular cases, as we can see below. The colloid used in the Bitter technique has been made following the classical method described in the specific works (see, e.g., ref. [13]). 3. Results and discussion Iron whiskers with varying shapes and crystallographic orientations were obtained ranging in
Fig. 1. Perfect [1101 iron whisker.
size between 10 and 300 ~smin diameter, although the most common were about 100 tim, and 1 cm long. First, only [110] perfect whiskers were selected to investigate their magnetic properties; fig. 1 shows such a well-finished [110] whisker. However, when occasionally the application of isolating grease in the stoppers was neglected, a very small amount of perfect whiskers were obtamed. Due to this fact, we have made several runs under these especial condition to study the possible influence of oxygen in the crystal growth. As it is well known, by changing the growth conditions, it is possible to alter the nature of the whiskers obtained. Since the change due to the presence of oxygen was very strong, no attempt was made to differentiate the influence of boats, temperature and purity of materials; furthermore, this influence has been reported by several workers (e.g. ref. [11]). The majority of whiskers so found were irregular, with no well-finished faces, and with defects which are distributed in a rather complex manner. A great number also exhibit changes in the growth direction and generally show oxide kinks. Dendntic whiskers were seldom found and were very thin. We only pay special attention to whiskers that
M. Tejedor, J. t Fuertes
___
~
‘
/ Serratedfaces in 1110] iron whiskers
~
187
__
Fig. 2. Serrated whisker with oxide kinks.
pp
Fig. 3. Perfect serrated whisker.
show very regular saw-shaped edges, which appear in a considerable amount under these conditions of growth. These whiskers were very often straight and showed perfect external serrated form. Figs. 2 and 3 are examples of these serrated whiskers. One may note in fig. 2 the oxide kinks, whereas the whisker of fig. 3 shows a perfect external serrated form and smooth broad faces. All the whiskers exhibit a marked rectangular cross-sectional area ranging in thickness between 20 and 40 tim. The thickness was estimated with a micrometer in the microscope, but was difficult to measure
Fig. 4. Magnetic domain ideal structure for a [1101iron whisker.
188
M. Tejedor, J.F. Fuertes
/ Serratedfaces in [1101 iron whiskers
I
Fig. 5. Magnetic domain pattern in a serrated whisker.
...
Fig. 6. Magnification of fig. 5.
accurately since the narrow surface was imperfect to focus perfectly. The direction of growth in these whiskers was the [110]. Only one attempt has been made to confirm the [110] growth direction by X-ray topographs, but it was very distorted and difficult to interpret, so the possibility of this kind of proof has been neglected. However, the magnetic domain patterns obtained in the broad side of the whiskers constitute a very easy manner to clarify
the growth direction in these cases. In fact, the magnetic domain structure of minimum energy, at zero applied field, for the [110] oriented crystals with this particular shape is shown in fig 4 [10], where we can see the main domains whose magnetization vectors lie alternatively directed along [010] and [010] directions (<100) are the easy directions in iron) with 180° domain walls making an angle of 45° with the edge of the crystal. A kind of secondary “echelon” domains near the edges reduce the width of the
alternating N and S poles that appear at the narrow surface. In this way, the magnetostatic energy due to the presence of free poles at the surfaces decreases notably. This particular domain structure is unmistakably shown by the [110] perfect crystals, allowing us to identify the growth direction by means of the domain patterns. Fig. 5 shows the magnetic domain patterns obtained in a serrated whisker which is in good
-
.
~“
.....
“/,‘
/
/‘/
...,
,/ ...‘
.
I
.
/ /,/
/ ‘
.,
~
/‘
(100)
H ~0 ~-~(110> Fig. 7. Ideal magnetization distribution of domains in a [110] serrated whisker.
M. Tejedor, J.F. Fuertes
/ Serratedfaces in 1110] iron whiskers
____________________________________
•
.
.
~
,.
Fig. 8. Magnetic domain structure in a serrated whisker with an applied field parallel to the long axis.
agreement — qualitatively with the ideal one expected. Fig. 6 is a magnification of a zone in fig. 5, and in fig. 7 is sketched the ideal magnetization distribution in a [110] serrated whisker. One may note an interesting feature in fig. 7: when [110] crystals show (110) perfect narrow faces, free poles appear, increasing the magnetostatic energy, whereas in the [110] ideal serrated —
/ ‘“
~-‘
\ / “ ‘\/
H 5 Ce
\ /“
(100) (110)
Fig. 9. Ideal magnetization distribution of the pattern of fig. 8.
189
whisker the free poles disappear and thereby give rise to zero magnetostatic energy, decreasing in this way the whole energy of the crystal. According to Gardner’s investigations [11] to [110] iron whiskers grown under oxygen addition are expected to be with larger (100) faces and in ribbon shape, but there is not any assumption to explain the serrated faces that we found. If this is so, it seems to be possible to suggest that there is some magnetocrystalline contribution to energies involving the crystal growth. The values for magnetocrystalline constants for iron at the temperature of crystal growth are low, but not zero [13], although the data for iron that we know are quite old. It would be useful to have accurate values to make quantitative evaluations, or instead, estimate the anisotropy constants by means of crystalline energies and domain structures, but it seems to be feasible due to the difficulty to evaluate all the energies involved. It is interesting to_remark that, whereas only one main [010] (or [010]) domain appears in the ideal structure of fig. 7 for each tooth-saw, one main domain appears for every two or three tooth-saws in the real domain structure of the serrated whisker, as it can be seen clearly in fig. 6. This fact showed not be surprising since at the temperature of the reduction process, the amsotropy constants are lower than at the room temperature at which the domain structures were obtained, whereliy magnetocrystalline energy increases at the conditions of domain observations, and therefore increases the magnetic domain wall energy, as it is well known in basic domain theories (see, e.g., ref. [14]). However, only qualitative explanations can be suggested, due to the uncertainty of the fact that the magnetocrystalline energy can lead to the formation of serrated faces. One may also pay attention to the fact that the edges of the whisker in fig. 5 are not perfectly
expected, due probably to iniernal dislotj
or oxide inclusions. This is not an imperfection on handling since the whisker has been carefully manipulated; furthermore, because of the fact that magnetic domain structures are very sensitive to plastic deformations, external imperfections added during the handling of the whisker would distort
M. Tejedor, J.F. Fuertes
190
/ Serratedfaces in [110] iron
whiskers
Fig. 10. Magnetic domain pattern in a whisker showing a change in the growth direction.
the magnetic domain patterns, and in this case they are very regular structures, as we can clearly see in figs. 5 and 6. Fig. 8 shows a magnetic domain patterns in a serrated whisker with an applied field of 5 Oe parallel to the long axis. An ideal domain structure in. a serrated whisker with a weak longitudinal applied field is depicted in fig. 9. This is another
/
\
~‘
~
\
/
<010)
/
<0) ((00>
Fig. 11. Magnetization distribution of the pattern of fig. 10 (dashed lines are added to virgin domain patterns to complete domain structure).
proof which clarifies the [110] growth direction. Note that serrated faces are not very regular in the bottom surface. Finally, as a clear example of magnetic domain pattern examinations in iron whisker to elucidate the growth direction, we present two kinds of whiskers that show changes in the growth direction. Fig. 10 is the magnetic domain pattern in such a whisker which changes from [110] direction to [100]. Note that the domain structure in the [100] zone does not differ from the classical domain structures in [100] whiskers [71, although it is about 5 inclined from the ideal [100] direction: in this way, free poles appear at the edges and there is a strong accumulation of colloid in them. The domain structure in the [110] zone is not visible~it is drawn in dashed lines in fig. 11. Fig. 12 shows a whisker with two successive changes in the growth direction. This whisker is always a (100) crystal. Note how the domain structures also do not differ from the ideal ones. Some investigations in whiskers showing other kinds of changes in growth directions have been reported previously [15].
M. Tejedor, J.F. Fuertes / Serratedfaces in 11101 iron whiskers
191
Fig. 12. Iron whisker showing two successive 90°changes in the growth direction and magnetic domain structure.
4. Conclusions Whiskers growing under the influence of a small concentration of oxygen show very often sawshaped irregularities on their faces. This fact can be due to some magnetocrystalline contribution to crystal growth, as it seems to be possible from examining magnetic domain structures in these whiskers. Magnetic domain patterns also enable one to examine the perfection and crystalline orientation in the iron whiskers.
Acknowledgements The authors would like to acknowledge the help of B. Hernando and A. Fernandez. One of the authors (J.F.F.) also acknowledges the encouragement of L. Cuervo.
References [1] S.S. Brenner, Acta Met. 4 (1956) 62. [2) K. Wokulska and Z. Wokulski, Ada Phys. Polon. A39 (1971) 180.
[3] S.S. Brenner and G.W. Sears, Acta met. 4 (1956) 268. [4] Z. Bojarki and M. Suroviec, J. Crystal Growth 46 (1979) 43.
[5] P.D. Gorsuch, J. Appl. Phys. 30 (1959) 837. [6] S.S. Brenner, J. Appl. Phys. 27 (1956) 1484. [7] R.W. Dc Blois and G.D. Grahan, Jr., J. Appl. Phys. 29 (1958) 931. [8] R.V. Coleman and G.G. Scott, Phys. Rev. 107 (1957) 1276. [9] L. Dewrello and H.H. Mende, J. Magnetism Magnetic Mater. 13 (1979) 231. [10] M. Tejedor and J.F. Fuertes, J. Appl. Phys. 55 (1984) 1226. [11] R.N. Gardner, J. Crystal Growth 43 (1978) 425. [12] M. Tejedor and J.F. Fuertes, J. AppI. Phys., submitted. [13] C. Kittel, Rev. Mod. Phys. 21 (1949) 541. [14] R.M. Bozorth, J. Appl. Phys. 8 (1937) 575. [15] M. Tejedor and J.F. Fuertes, J. Magnetism Magnetic Mater., in press.