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Microstructural and magnetic properties of amorphous TbFe films
Journal
of Magnetism
and Magnetic
MICROSTRUCTURAL
Materials
AND
54-57
227
(1986) 227-228
MAGNETIC
PROPERTIES
OF AMORPHOUS
TbFe
FILMS
P.J. GRUNDY, E.T.M. LACEY and C.D. WRIGHT of Physics, Unruersity of Salford,Salford A45 4WT, UK
Dept
The temperature dependence of the magnetisation and domain structure of amorphous TbFe films has been investigated. Loss of DerDendicular anisotrogy__ on cycling changes in the films . - to the Curie temperature is associated with microstructural . . observed by electron microscopy and electron diffraction
1. Introduction
Although
memory systems using medium were proposed nearly ten years ago [l] it is only the development of small diode lasers that has made compact working devices a reality [2]. A continuing materials problem is the long term stability of the medium in the face of possible aging and corrosion effects. In this work we have been concerned with an investigation of the magnetisation. domain structure and perpendicular anisotropy in co-sputtered TbFe, Co films subject to thermal cycles through the Curie temperature. RE-TM
films
2. Experimental
magneto-optic as the
storage
methods
TbFe and TbFeCo films have been prepared by dc triode sputtering from a composite, mosaic target onto glass, carbon and rocksalt substrates. A low base pressure (- 5 X lo-’ Torr) and low sputtering pressure (5 4 mTorr) at a deposition rate of 200 nm/min were used. Magnetic properties of 1 pm thick films were measured on a VSM and 50 nm thick films were observed by transmission electron microscopy (TEM). Both instruments have in-situ heating facilities. The composition of the thin, as-deposited films was determined by Rutherford back scattering analysis. Oxygen impurity was present at the level of 2 or 3 at%.
MS = 75 emu/cm3 and perpendicular coercivity Hcp = 6.5 kOe. Film 2 had a narrower loop with MS = 210 emu/cm3 and Hcp = 300 Oe. These values agree reasonably well with previous work [1,2]. Fig. 1 shows the temperature dependence of the magnetisation of the two films measured in a vacuum of about 1 x 10 5 Torr. The rate of decrease of magnetisation towards the apparent Curie temperatures of the two films is followed by a slower reduction associated, we believe, with the beginnings of phase segregation in the films. The subsequent rapid rise in magnetisation corresponds to the segregation and crystallization of iron and a shift of the composition further from the RTCC. The changes in the film structure with heating are reflected in the electron diffraction patterns of the thin films. Fig. 2 gives patterns from film 1 taken at room temperature and at 300°C. The dissociation of the overlapping diffraction maxima from Tb-Tb and Fe-Fe correlations with increasing temperature is obvious. Their separation increases until at 600°C the second maximum is very near to the scattering parameter value of d-spacing for a (110) iron reflection. The TEM micrograph of fig. 2c shows the segregation of crystalline iron (the dark spots) at 600°C quite clearly. These magnetisation and microstructural changes should result in a loss of perpendicular anisotropy. This is indeed observed in the thinner 50 nm thick films even
3. Results and discussion
1
In a previous investigation it has been shown that changes in the Curie temperature, coercivity and magnetisation of TbFe films at a composition near to the room temperature compensation composition (RTCC Tb,,Fe,,) are caused by oxidation and phase segragation when the films contain oxygen [3]. These effects occured in the as-deposited evaporated films and were due simply to various levels of oxygen contamination in the films. Here we present data on two sputtered films of compsition Tb,,Fe,, (near to the RTCC and with a dominant iron moment) and Tb,,Fe,, (dominant terbium moment). Film 1 (Tb,,Fe,,) had a square perpendicular hysteresis loop at room temperature with
0304-8853/86/$03.50
0 Elsevier Science Publishers
Fig. 1. The normalised magnetisation of 1 pm thick amorphous Tb,,Fe,, (curve 1) and Tb,,Fe,, (curve 2) films as a function of temperature. T, is the apparent Curie temperature.
B.V.
l ---.---*-
a-C-cc----,
0
200
,
1
LOO
T’C
.
600
Fig. 2. Results for a SO nm thick Tb?,,Fe,,, film (film 1). (a) Electron diffraction patterns at 20°C and 300°C: (h) scattering parameter values s ( = 471 sin 8/A) of the two diffraction mar;ima IS a function of temperature and (c) TEM micrograph of the film at 600°C showing segregation effects.
after cycling to relatively low temperatures. Fig. 3a shows the domain structure in film 1 at room temperature. The labyrinth. maze structure of closely spaced black and white domain walls is typical of perpendicular magnetiaation in a film of composition some distance from the RTCC. Heating to 60°C showed an Increase in domain width as the magnetisation decreases (fig. 3b). At 13O’C. the apparent q in the TEM, the domain structure disappeared apart from some widely spaced vestigial in-plane domains. The domain structure at room temperature after several cycles to just above r, is shown in fig. 3c. Here a narrow stripe structure is associated with contrasty black and white walls showing a considerable in-plane component of magnetisation. This loss of perpendicular anisotropy after cycling to T is a common observation. Fig. 3d shows the domain structure at 300°C where considerable microstructural changes occur. The “ripple” micromagnetic structure was retained at room temperature with a complete lob\ of perpendicular anisotropy. Similar results were ohtained for film 2 but the loss of perpendicular ansotropy occured more readily in this film with ;I greater iron
Fig. 3. Lorentz TEM
micrographs of the domain \tructurc 11, Film I (a) at 20°C. (h) at 60°C. (c) at 20°C after xvcral cvcle\ to 140°C’ and (d) at 300°C.
content. by
4. Summq The results presented in this paper show that the domain structure of free-standing TbFc films of a thickness used in some magneto-optic recording structures is unstable with temperature excursions above the Curie temperature. These changes are caused if compositional segregation and perhaps oxidation effects occur. How +,,.>~‘a,\hL.zzr\,.,t;,,nc rol..ts. +r\ the I,,,.,>l;c,d ho.rt;no ,-.,,,~a4
Iascr
clear.
pulses
coated
on
Microstructural
layered diffraction
structures
and
and domain by electron
are called
for
protected
films
observation\
microscopy
is
not
of huch
and electron
of Magnetism
and Magnetic
MICROSTRUCTURAL
Materials
AND
54-57
227
(1986) 227-228
MAGNETIC
PROPERTIES
OF AMORPHOUS
TbFe
FILMS
P.J. GRUNDY, E.T.M. LACEY and C.D. WRIGHT of Physics, Unruersity of Salford,Salford A45 4WT, UK
Dept
The temperature dependence of the magnetisation and domain structure of amorphous TbFe films has been investigated. Loss of DerDendicular anisotrogy__ on cycling changes in the films . - to the Curie temperature is associated with microstructural . . observed by electron microscopy and electron diffraction
1. Introduction
Although
memory systems using medium were proposed nearly ten years ago [l] it is only the development of small diode lasers that has made compact working devices a reality [2]. A continuing materials problem is the long term stability of the medium in the face of possible aging and corrosion effects. In this work we have been concerned with an investigation of the magnetisation. domain structure and perpendicular anisotropy in co-sputtered TbFe, Co films subject to thermal cycles through the Curie temperature. RE-TM
films
2. Experimental
magneto-optic as the
storage
methods
TbFe and TbFeCo films have been prepared by dc triode sputtering from a composite, mosaic target onto glass, carbon and rocksalt substrates. A low base pressure (- 5 X lo-’ Torr) and low sputtering pressure (5 4 mTorr) at a deposition rate of 200 nm/min were used. Magnetic properties of 1 pm thick films were measured on a VSM and 50 nm thick films were observed by transmission electron microscopy (TEM). Both instruments have in-situ heating facilities. The composition of the thin, as-deposited films was determined by Rutherford back scattering analysis. Oxygen impurity was present at the level of 2 or 3 at%.
MS = 75 emu/cm3 and perpendicular coercivity Hcp = 6.5 kOe. Film 2 had a narrower loop with MS = 210 emu/cm3 and Hcp = 300 Oe. These values agree reasonably well with previous work [1,2]. Fig. 1 shows the temperature dependence of the magnetisation of the two films measured in a vacuum of about 1 x 10 5 Torr. The rate of decrease of magnetisation towards the apparent Curie temperatures of the two films is followed by a slower reduction associated, we believe, with the beginnings of phase segregation in the films. The subsequent rapid rise in magnetisation corresponds to the segregation and crystallization of iron and a shift of the composition further from the RTCC. The changes in the film structure with heating are reflected in the electron diffraction patterns of the thin films. Fig. 2 gives patterns from film 1 taken at room temperature and at 300°C. The dissociation of the overlapping diffraction maxima from Tb-Tb and Fe-Fe correlations with increasing temperature is obvious. Their separation increases until at 600°C the second maximum is very near to the scattering parameter value of d-spacing for a (110) iron reflection. The TEM micrograph of fig. 2c shows the segregation of crystalline iron (the dark spots) at 600°C quite clearly. These magnetisation and microstructural changes should result in a loss of perpendicular anisotropy. This is indeed observed in the thinner 50 nm thick films even
3. Results and discussion
1
In a previous investigation it has been shown that changes in the Curie temperature, coercivity and magnetisation of TbFe films at a composition near to the room temperature compensation composition (RTCC Tb,,Fe,,) are caused by oxidation and phase segragation when the films contain oxygen [3]. These effects occured in the as-deposited evaporated films and were due simply to various levels of oxygen contamination in the films. Here we present data on two sputtered films of compsition Tb,,Fe,, (near to the RTCC and with a dominant iron moment) and Tb,,Fe,, (dominant terbium moment). Film 1 (Tb,,Fe,,) had a square perpendicular hysteresis loop at room temperature with
0304-8853/86/$03.50
0 Elsevier Science Publishers
Fig. 1. The normalised magnetisation of 1 pm thick amorphous Tb,,Fe,, (curve 1) and Tb,,Fe,, (curve 2) films as a function of temperature. T, is the apparent Curie temperature.
B.V.
l ---.---*-
a-C-cc----,
0
200
,
1
LOO
T’C
.
600
Fig. 2. Results for a SO nm thick Tb?,,Fe,,, film (film 1). (a) Electron diffraction patterns at 20°C and 300°C: (h) scattering parameter values s ( = 471 sin 8/A) of the two diffraction mar;ima IS a function of temperature and (c) TEM micrograph of the film at 600°C showing segregation effects.
after cycling to relatively low temperatures. Fig. 3a shows the domain structure in film 1 at room temperature. The labyrinth. maze structure of closely spaced black and white domain walls is typical of perpendicular magnetiaation in a film of composition some distance from the RTCC. Heating to 60°C showed an Increase in domain width as the magnetisation decreases (fig. 3b). At 13O’C. the apparent q in the TEM, the domain structure disappeared apart from some widely spaced vestigial in-plane domains. The domain structure at room temperature after several cycles to just above r, is shown in fig. 3c. Here a narrow stripe structure is associated with contrasty black and white walls showing a considerable in-plane component of magnetisation. This loss of perpendicular anisotropy after cycling to T is a common observation. Fig. 3d shows the domain structure at 300°C where considerable microstructural changes occur. The “ripple” micromagnetic structure was retained at room temperature with a complete lob\ of perpendicular anisotropy. Similar results were ohtained for film 2 but the loss of perpendicular ansotropy occured more readily in this film with ;I greater iron
Fig. 3. Lorentz TEM
micrographs of the domain \tructurc 11, Film I (a) at 20°C. (h) at 60°C. (c) at 20°C after xvcral cvcle\ to 140°C’ and (d) at 300°C.
content. by
4. Summq The results presented in this paper show that the domain structure of free-standing TbFc films of a thickness used in some magneto-optic recording structures is unstable with temperature excursions above the Curie temperature. These changes are caused if compositional segregation and perhaps oxidation effects occur. How +,,.>~‘a,\hL.zzr\,.,t;,,nc rol..ts. +r\ the I,,,.,>l;c,d ho.rt;no ,-.,,,~a4
Iascr
clear.
pulses
coated
on
Microstructural
layered diffraction
structures
and
and domain by electron
are called
for
protected
films
observation\
microscopy
is
not
of huch
and electron