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A Mössbauer study of ferromagnetic domain orientation in amorphous Fe75P15C10 foils
Journal of Magnetism and Magnetic Materials 0 North-Holland Publishing Company
A M&SBAUER
7 (1978)
182-
184
STUDY OF FERROMAGNETIC
DOMAlN ORIENTATION
IN AMORPHOUS Fe75P1,C10 FOILS
D.K. BROWN, I. NOWIK * and D.I. PAUL Columbia University, New York, N. Y., 10027 **, USA
MGssbauer studies of splat cooled amorphous Fe75P 1 SC I,-, foils show that in “as prepared” samples, the magnetization is essentially in the plane of the foil. However, by annealing at 600 K for 1 h, the magnetization becomes predominantly perpendicular to the surface (>85%). Thus, splat cooled materials may support the formation of bubble domains.
One of the simplest methods by which the magnetization orientation in iron containing materials can be studied is the MBssbauer effect. From the 57Fe spectrum, the relative amount of the magnetization component perpendicular to the foil’s plane can be derived. We have utilized this method to determine the magnetization direction in “splat cooled” amorphous samples [ 11. The system we have studied, Fe75P15C10, was in the form of thin foils (50 pm in thickness). Spectra were taken of both “as prepared” and “stress relieved” foils. Removal of stresses was accomplished via annealing both with and without an external magnetic field. Fe7sP,,Clo has been examined in detail by a variety of techniques [2] including the Massbauer effect [3,4]. Berry and Pritchet [2] have found that amorphous Fe,sP,sClo responds reversibly to magnetic annealing treatments. They show that magnetic annealing in a given direction in the plane of the foil introduces uniaxial anisotropy in that direction - i.e. the easy axis of magnetization becomes that of the magnetic annealing direction. Chien and Hasagawa [3] have studied the Mbssbauer spectra of Fe,,PIsClo as a function of temperature. They noticed that above -530 K the magnetization axis switches directions to become nearly perpendicular to the sample plane. We have verified this fact and found that the magnetic switching remains upon cooling and is thus the result of relaxing frozen strains by the annealing process.
The relative intensities of the six Mijssbauer absorption lines of s7Fe can be expressed in the form 3 : p(B) : 1 : 1 : p(B) : 3, where the functional form of p(B) depends on the angles the y-ray makes with the various orientations of the hyperfine fields in the absorber - 0 being the complement of the angle between the y-ray and the normal to the plane of the absorber. In the case where the magnetization is randomly distributed within the plane, p(e) is given by 4(1 - icos*0)/(1 t $os*Q. If, however, a fraction of the iron nuclei, /3, has hyperfine fields perpendicular to the plane (the remainder have fields randomly distributed in the plane of the foil), then p(0) is given by: 411 -
&OS28 - p(i - $0s*e)]/[i
+ p(i - &0s*e)]
.
+ $0s*e (1)
In fig. 1 we show Mossbauer spectra at room temperature of a Fe75P,sC,o foil, before and after annealing as a function of the angle between the sample plane and the y-ray. We observe that the spectra are very sensitive to this angle, proving, without any analysis, that in no case are the magnetization directions randomly distributed over the entire foil. We observe in these spectra the presence of a distribution of absolute values of the hyperfine field and probably also of the quadrupole interactions [4]. We have analyzed the spectra assuming no quadrupole interaction (its contribution only leading to a broader absorption line) and an asymmetric gaussian distribution of hyperfine fields. In table 1 we give the values of the parameters of
* On leave from Hebrew University, Jerusalem. ** Supported in part by NSF Grant DMR 72-03 118AO2, Division of Materials Science. 182
183
D. K. Brown et al. / Mssbauer studies of amorphous Fe 75P15Clo foils Amorphous
Velocity
Fe,,
H, = 320 + 10 kOe, which is very close to the value of that observed in pure iron metal. This indicates that the highest hyperfine field is obtained when the local environment around an iron nucleus resembles that of pure iron metal. Our results for p(0), the relative intensity of the m = 0 Mossbauer absorption line in the varous samples, is also given in table 1. Using eq. (1) one can obtain the relative amount fl of iron nuclei which have their hyperfine fields perpendicular to the plane. Sample no. 1 was studied both as prepared and after annealing in zero magnetic field. The as prepared sample had its magnetization preferentially in the plane of the foil ((3 = 0.16). The annealing process switched the magnetization such that is was predominantly perpendicular to the plane (/3 = 0.85). The same phenomenon was observed for sample no. 2, where the annealing was done in a magnetic field of -200 Oe perpendicular to the plane of the foil. To summarize our observations we may say that the detailed analysis of the Mossbauer spectra of Fe,sPrsCre amorphous foild lead to the following conclusions: (1) In as prepared samples, strain frozen during the rapid quenching process force the magnetization to lie predominantly in the plane of the foil. (2) In annealed samples (600 K for 1 h) the magnetization is predominantly perpendicular to the plane independent of whether the annealing was in a finite (-200 Oe) or zero magnetic field. The fact that both annealed samples yield approximately the same value
P,, C,,
(mmlsec)
Fig. 1. Massbauer spectra of Fe,,Pt&ro foil no. 2 at room temperature before and after magnetic annealing.
the hyperfine field distributions for the various spectra. We observe that they are almost the same in all spectra studied. Another surprising result is that
Table 1 Characteristic values of parameters used in fitting the Mossbauer spectra of Fe75Pt &to e
Foil
-t H kOe
-* H kOe
* Ze
& kOe
p(e)
P
1 “as prepared” annealed
90.0 90.0
245 259
313 322
88 98
53 44
2.90 5 0.03 0.32 r 0.03
0.159 f 0.002 0.852 fr 0.013
2 “as prepared”
90.0 90.0 45.0
250 255 259
322 330 339
91 98 101
52 54 48
2.45 r 0.02 0.32 * 0.02 1.31 f 0.02
0.240 f 0.002 0.852 r 0.010
annealed
The quantities Hi, H,, q_, and 0~ represent best fits for the average hyperfine field, maximum hyperfine field, left line width, and right line width, respectively, for the asymmetric gaussian distribution of hyperfine fields. p(0) is the relative intensity of the Am = 0 line and p represents the fraction of nuclei with hyperfine fields perpendicular to the foil plane. * Annealed in zero magnetic field at 600 K for 1 h. $ Annealed in -200 Oe field at 600 K for 1 h. *Errors onR, o~and 0~ are *5 kOe and on H, are +lO kOe.
184
D.K. Brown et al. / Mtissbauer studies of amorphous Fe75P15Cl
of &3 = 0.85) indicates that a finite fraction of the magnetization will remain parallel to the foil plane at the surface in order to reduce the strong demagnetizing fields resulting from the high magnetic moment of iron. (3) Finally, it can be remarked that the origin of the perpendicular orientation of the magnetization in annealed Fe,sPIsCro foils is not clear from basic theoretical considerations. Whatever the origin, it appears that splat cooled amorphous materials subject to stress relief through annealing exhibit a uniaxial anisotropy and magnetization perpendicular to the plane of the foil. This implies that similar preparation of ferromagnetic material possessing a smaller magnetic moment than Fe,sP,sCro may provide a substance capable of supporting bubble domains [S] and therefore be of considerable technological importance.
,J foils
Acknowledgement The author wish to thank Prof. Pol Duwez for generously supplying the amorphous material.
References [l] P. Duwez and SCH. Lin, J. Appl. Phys. 38 (1967) 4096. [2] B.S. Berry and W.C. Pritchet, Phys. Rev. Lett. 34 (1975) 1022. [3] C.L. Chien and R. Hascgawa, J. de Phys. 37 (1976) C6759. [4] C.C. Tsuei, G. Longworth and S.C.H. Lin, Phys. Rev. 170 (1968) 603. [S] P. Chaudhari et al., IBM J. Res. Develop. 17 (1973) 66.
A M&SBAUER
7 (1978)
182-
184
STUDY OF FERROMAGNETIC
DOMAlN ORIENTATION
IN AMORPHOUS Fe75P1,C10 FOILS
D.K. BROWN, I. NOWIK * and D.I. PAUL Columbia University, New York, N. Y., 10027 **, USA
MGssbauer studies of splat cooled amorphous Fe75P 1 SC I,-, foils show that in “as prepared” samples, the magnetization is essentially in the plane of the foil. However, by annealing at 600 K for 1 h, the magnetization becomes predominantly perpendicular to the surface (>85%). Thus, splat cooled materials may support the formation of bubble domains.
One of the simplest methods by which the magnetization orientation in iron containing materials can be studied is the MBssbauer effect. From the 57Fe spectrum, the relative amount of the magnetization component perpendicular to the foil’s plane can be derived. We have utilized this method to determine the magnetization direction in “splat cooled” amorphous samples [ 11. The system we have studied, Fe75P15C10, was in the form of thin foils (50 pm in thickness). Spectra were taken of both “as prepared” and “stress relieved” foils. Removal of stresses was accomplished via annealing both with and without an external magnetic field. Fe7sP,,Clo has been examined in detail by a variety of techniques [2] including the Massbauer effect [3,4]. Berry and Pritchet [2] have found that amorphous Fe,sP,sClo responds reversibly to magnetic annealing treatments. They show that magnetic annealing in a given direction in the plane of the foil introduces uniaxial anisotropy in that direction - i.e. the easy axis of magnetization becomes that of the magnetic annealing direction. Chien and Hasagawa [3] have studied the Mbssbauer spectra of Fe,,PIsClo as a function of temperature. They noticed that above -530 K the magnetization axis switches directions to become nearly perpendicular to the sample plane. We have verified this fact and found that the magnetic switching remains upon cooling and is thus the result of relaxing frozen strains by the annealing process.
The relative intensities of the six Mijssbauer absorption lines of s7Fe can be expressed in the form 3 : p(B) : 1 : 1 : p(B) : 3, where the functional form of p(B) depends on the angles the y-ray makes with the various orientations of the hyperfine fields in the absorber - 0 being the complement of the angle between the y-ray and the normal to the plane of the absorber. In the case where the magnetization is randomly distributed within the plane, p(e) is given by 4(1 - icos*0)/(1 t $os*Q. If, however, a fraction of the iron nuclei, /3, has hyperfine fields perpendicular to the plane (the remainder have fields randomly distributed in the plane of the foil), then p(0) is given by: 411 -
&OS28 - p(i - $0s*e)]/[i
+ p(i - &0s*e)]
.
+ $0s*e (1)
In fig. 1 we show Mossbauer spectra at room temperature of a Fe75P,sC,o foil, before and after annealing as a function of the angle between the sample plane and the y-ray. We observe that the spectra are very sensitive to this angle, proving, without any analysis, that in no case are the magnetization directions randomly distributed over the entire foil. We observe in these spectra the presence of a distribution of absolute values of the hyperfine field and probably also of the quadrupole interactions [4]. We have analyzed the spectra assuming no quadrupole interaction (its contribution only leading to a broader absorption line) and an asymmetric gaussian distribution of hyperfine fields. In table 1 we give the values of the parameters of
* On leave from Hebrew University, Jerusalem. ** Supported in part by NSF Grant DMR 72-03 118AO2, Division of Materials Science. 182
183
D. K. Brown et al. / Mssbauer studies of amorphous Fe 75P15Clo foils Amorphous
Velocity
Fe,,
H, = 320 + 10 kOe, which is very close to the value of that observed in pure iron metal. This indicates that the highest hyperfine field is obtained when the local environment around an iron nucleus resembles that of pure iron metal. Our results for p(0), the relative intensity of the m = 0 Mossbauer absorption line in the varous samples, is also given in table 1. Using eq. (1) one can obtain the relative amount fl of iron nuclei which have their hyperfine fields perpendicular to the plane. Sample no. 1 was studied both as prepared and after annealing in zero magnetic field. The as prepared sample had its magnetization preferentially in the plane of the foil ((3 = 0.16). The annealing process switched the magnetization such that is was predominantly perpendicular to the plane (/3 = 0.85). The same phenomenon was observed for sample no. 2, where the annealing was done in a magnetic field of -200 Oe perpendicular to the plane of the foil. To summarize our observations we may say that the detailed analysis of the Mossbauer spectra of Fe,sPrsCre amorphous foild lead to the following conclusions: (1) In as prepared samples, strain frozen during the rapid quenching process force the magnetization to lie predominantly in the plane of the foil. (2) In annealed samples (600 K for 1 h) the magnetization is predominantly perpendicular to the plane independent of whether the annealing was in a finite (-200 Oe) or zero magnetic field. The fact that both annealed samples yield approximately the same value
P,, C,,
(mmlsec)
Fig. 1. Massbauer spectra of Fe,,Pt&ro foil no. 2 at room temperature before and after magnetic annealing.
the hyperfine field distributions for the various spectra. We observe that they are almost the same in all spectra studied. Another surprising result is that
Table 1 Characteristic values of parameters used in fitting the Mossbauer spectra of Fe75Pt &to e
Foil
-t H kOe
-* H kOe
* Ze
& kOe
p(e)
P
1 “as prepared” annealed
90.0 90.0
245 259
313 322
88 98
53 44
2.90 5 0.03 0.32 r 0.03
0.159 f 0.002 0.852 fr 0.013
2 “as prepared”
90.0 90.0 45.0
250 255 259
322 330 339
91 98 101
52 54 48
2.45 r 0.02 0.32 * 0.02 1.31 f 0.02
0.240 f 0.002 0.852 r 0.010
annealed
The quantities Hi, H,, q_, and 0~ represent best fits for the average hyperfine field, maximum hyperfine field, left line width, and right line width, respectively, for the asymmetric gaussian distribution of hyperfine fields. p(0) is the relative intensity of the Am = 0 line and p represents the fraction of nuclei with hyperfine fields perpendicular to the foil plane. * Annealed in zero magnetic field at 600 K for 1 h. $ Annealed in -200 Oe field at 600 K for 1 h. *Errors onR, o~and 0~ are *5 kOe and on H, are +lO kOe.
184
D.K. Brown et al. / Mtissbauer studies of amorphous Fe75P15Cl
of &3 = 0.85) indicates that a finite fraction of the magnetization will remain parallel to the foil plane at the surface in order to reduce the strong demagnetizing fields resulting from the high magnetic moment of iron. (3) Finally, it can be remarked that the origin of the perpendicular orientation of the magnetization in annealed Fe,sPIsCro foils is not clear from basic theoretical considerations. Whatever the origin, it appears that splat cooled amorphous materials subject to stress relief through annealing exhibit a uniaxial anisotropy and magnetization perpendicular to the plane of the foil. This implies that similar preparation of ferromagnetic material possessing a smaller magnetic moment than Fe,sP,sCro may provide a substance capable of supporting bubble domains [S] and therefore be of considerable technological importance.
,J foils
Acknowledgement The author wish to thank Prof. Pol Duwez for generously supplying the amorphous material.
References [l] P. Duwez and SCH. Lin, J. Appl. Phys. 38 (1967) 4096. [2] B.S. Berry and W.C. Pritchet, Phys. Rev. Lett. 34 (1975) 1022. [3] C.L. Chien and R. Hascgawa, J. de Phys. 37 (1976) C6759. [4] C.C. Tsuei, G. Longworth and S.C.H. Lin, Phys. Rev. 170 (1968) 603. [S] P. Chaudhari et al., IBM J. Res. Develop. 17 (1973) 66.