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Cu superlattices
Journal of Magnetism and Magnetic Materials 125 (1993) 330-334 North-Holland
Hard magnetic Co/Cu superlattices V.M. Fedosyuk and O.I. Kasyutich Institute of Solid State Physics Semiconductors of the Belarussian Academy of Sciences, P. Brovki 17, 220072 Minsk, Belarus
Received 23 October 1992; in revised form 22 December 1992
Multilayer Co/Cu films produced by the method of pulse electrodeposition are studied. Investigations of their structure and magnetic properties have shown the possibility of obtaining very good hard magnetic properties and perpendicular magnetic anisotropy.
1. Introduction In recent year much attention has b e e n paid to investigation of artificially modulated structures (AMLs), which are remarkable from both the fundamental and practical points of view. In a n u m b e r of reports [1-3], promising soft and hard magnetic properties, unique electrical, mechanical and other properties of multilayers, including structures obtained by electrodeposition [4-6], have been illustrated, but we are not aware of any reports on electrodeposited multilayers showing the realization of hard magnetic properties. We have conducted experiments on electrodeposited films, including hard magnetic properties [7-8], so we were interested in analyzing the possibility of obtaining high coercivity together with perpendicular orientation of the magnetization vector in multilayer films. In our opinion, this would be easier to achieve with the C o / C u system than with others such as N i / C u or F e / C u . This was the aim of the present work. Multilayer C o / C u films were obtained using the pulse electrodeposition method, similar to that described in ref. [5] and in more detail in ref. [9]. The periodic and crystallographic struc-
Correspondence to: Dr V.M. Fedosyuk, Institute of Solid State
Physics and semiconductors of the Belarussian Academy of Sciences, P. Brovki 17, 220072 Minsk, Belarus.
tures of the obtained samples were studied using a D R O N - 3 M diffractometer in Co K~ radiation, in conjunction with a graphite monochromator. The magnetic m e a s u r e m e n t s were m a d e with the vibrating-sample magnetometers P A R C 155, PARC-4500 and a torque magnetometer.
2. Structure and internal stresses of C o / C u multilayers The X-ray diffraction patterns of the A M L s reveal two features: the absence of any reflections in a small-angle range and satellite reflections near the basic structure reflections of Co. When the thickness of the Co layers was changed, with a constant Cu layer thickness (dcu = 1.5 nm), the roentgenographic picture was the following. The first reflections a p p e a r e d at T = 80 nm (dco ~ 1.5 nm) and at a cobalt deposition pulse time Tco---200 ms ( d c o ~ 2.5 nm) there were three obvious reflections of different intensities. The reflections at 20 = 49.5 ° and 53.8 ° were the satellite reflections of the structure peak at 20 = 52.1 ° for (111) fcc Co (tabulated 20 = 51.95°). With further increases in Tco (up to 400 ms), the picture becomes more distinct. A p a r t from the satellite reflections of the p e a k for (111) fcc Co there appears one more reflection for (200) fcc Co (20 = 60.7 °) with satellite reflections ( 2 0 - =
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
331
V.M. Fedosyuk, O.L Kasyutich / Hard magnetic Co / Cu superlattices
58.5 °, 20 += 62.5°), as shown in fig. 1. In all the investigated Co and Cu layer thickness ranges (upto 10 nm) the multilayer C o / C u structures (obtained by electrodeposition from a single solution) were structures on a base of fcc Co and fcc Cu. In addition, the conjugation of Co and Cu layer structures took place on more compact planes ( ( l i d fcc Co I1(111) fcc Cu). The presence of a satellite reflection is confirmed by the fact that the modulation period of the interplane distances of the AMLs, calculated from the satellites for different reflections, are the same and equal to the compositional modulation obtained using Auger spectroscopy (fig. 2). According to the profile in fig. 2 some amount of one element may be present in the layers of the other element (5-15%), but this experimental result can have another explanation. The diameter of the Auger spectroscope probe was 1 ~m. Since the thickness of the alternating layers is very small (1-2 nm), parts of the Co and Cu layers will be irregular (uncovered by one another) and displayed on the surface limited by the probe diameter. The irregularities of the thin layers in periodic structure can be estimated from fig. 2 [11] to be upto ~ 15%. From an analysis of these figures and the magnetic behavior reported below, it is shown that the ultrathin alternating C o / C u multilayers are 'island'-type layers [11]. When the layer thickness is less than 1 nm, the irregularity of the
d
SO
~'
~'~'~
'~'~ ' ~
~0,~,j.
Fig. 1. X-raydiagramof Co/Cu multilayerfilmwith Tco= 200 ms and Tc~= 5 s (dco= 2.5 nm, dc~ = 1.5 nm).
Co,Cu, at To I00
•..
° °
. o
•
o •
•
,/'Cu
o
o
•
o o
50
:
•
•
o
CO
• 0•
0
o
•
2
~
•
6
•
8
o ¢ , /
10
12 d,nm
Fig. 2. Auger profile of Co/Cu film with Tco= 200 ms and Tcu= 5 s.
layers is of the order of ~ 50%. Structural investigations using transmission electron microscopy confirmed the island-like type layer structure. At Co deposition pulse times of less than 200 ms, the Co layer forms islands 15-30 nm in diameter. Microdiffraction studies show the presence of crystallites of fcc Co [11]. In addition to the determination of the periodic structure of the Co/Cu-modulated films, internal macrostress measurements provided information on the mechanism and kinetics of layer growth. The measurements were made using a flexible cathode method with simultaneous automatic registration. It is known [10] that most of the refractory metals (Co, Fe, Cr, Ni) expand during electrodeposition. The internal macrostress value for copper is close to zero within the limit of measurement precision. This is illustrated in fig. 3 for the samples obtained with T c o > 100 ms. If the Co deposition pulse time is less than 100 ms, the internal expansion macrostress changes into internal compression stress, i.e. the deposit will have to increase in volume; this can be explained as follows. As indicated above, the Co layers are small-size grain phase, characterized by large free surfaces, and thus they are highly energetic. This could explain the experimentally observed expansion stress. At the initial growth stage the deposit is of the island-type with large irregularities. The volume growth and thus compression stress are the result of the low island packing density. The thermodynamic principle to decrease the volume results in the consolidation
V.M. Fedosyuk, 0.I. Kasyutich / Hard magnetic Co / Cu superlattices
332
short (Tco< 100 ms), the compression stress does not change into expansion stress, because it is 'frozen' by the following copper layer (Co and Cu are mutually insoluble under normal conditions). The dual-stage nature of the internal macrostress during the electrodeposition process also confirms that the C o / C u multilayer films have an island-type structure at Cobalt deposition pulse times of less than 100 ms.
E V
e./
¢/\
--'rt,,,~
cZ
3.
Fig. 3. Deviation of the Cu substrate during electrodeposition of C o / C u multilayers.
of the structure, thus decreasing the volume, and therefore the expansion stress with increasing Co layer thickness. If the Co deposition pulse time is
Magnetic properties of Co/Cu multilayers
The value of the total magnetic anisotropy constant of an ultrathin magnetic layer in C o / C u AMLs can be given as the sum of the crystallo. graphic strain, form and surface anisotropy components. The contribution of the surface anisotropy constant to the perpendicular magnetic anisotropy becomes predominant [9] when the magnetic layer thickness in the multilayers is of
M/Mmo~
,M/M,~,~
~(a) //
II
(b)
14.~0e
M/M,,o~
I M/Mm~
I o
i II
e
~
6
a H, d ~
o
2
~
~
a ~,~Oe
Fig. 4. Magnetization curves of C o / C u multilayers with time of pulse Co deposition. (a) Tco= 100 ms; (b) 40 ms; (c) 15 ms; and (d) 10 ms. Tcu = 5 s = const.
333
V.M. Fedosyuk, 0.I. Kasyutich / H a r d magnetic Co / Cu superlattices
the order of a few nm. As the thickness of the Co layer decreases, the perpendicular anisotropy and coercive force increase (fig. 4). Additional evidence of the development of perpendicular magnetic anisotropy can be seen from the magnetization reversal curves parallel and perpendicular to the coating directions. As the thickness of the magnetic layers of Co decreases, the remanent magnetization and H c in the direction perpendicular to the surface of films also increases. Simultaneously, these parameters decrease in the film plane. This is typical of magnetic structures with increasing perpendicular anisotropy. Analyses of these figures show that the decrease in the Co layer thickness leads to an increase in the perpendicular magnetic anisotropy constant. Furthermore, in our opinion, the increase in the discontinuity of these layers is the reason for the increase in coercivity of the island-type structure. This fact is an important result for the use of the investigated multilayer films as materials for vertical magnetic recording. Additional information on the value of the magnetic anisotropy and the mechanism of magnetization reversal in C o / C u multilayer films has been obtained from a study of the rotational hysteresis losses using a torque magnetometer. It was determined that the torque curves of the films in their plane (dco < 2.5 nm) are completely reversible in all regions of the used magnetic fields up to 10 kOe. Thus, the torque hysteresis losses in the film plane are absent. This confirms that the orientation of the Co layer magnetization vector in multilayer C o / C u structures is perpendicular [13]. When the magnetization reversal in the direction perpendicular to the coating was used, the torque curves were reversible and proportional to sin 0 in magnetic fields up to 100-150 Oe. Upon increasing the magnetic field in this direction the torque curves become irreversible and proportional to sin 20. When the magnitude of the magnetic field is H > 8-9 kOe, the torque curves remain irreversible. In order to determine the value of the PMA constant (K), the half-maximum torque curve amplitude was used. The obtained values of K are closer to the those determined by traditional methods. The measured val-
ues of the torque hysteresis losses (Wr//Wrmax) as a function of applied magnetic field are shown in fig. 5. The torque hysteresis losses are not closer to zero. The curve shapes are typical of a magnetic material with magnetization reversal by means of domain wall displacement. In addition, t h e calculated values of the torque hysteresis integral R H=
f:WJMs
d(1/H)
confirm this assumption because the resulting values of R n for magnetic C o / C u films are closer to 4. The magnetic results reported here show that if the thickness of magnetic Co layers is of the order of 3-4 nm, the surface effect is sufficient and high values of the PMA constant are obtained. Using the extensive structural data presented in ref. [11], a more detailed analysis of the magnetic behavior is possible. As reported [11], the Co layers with thicknesses of less than 3 nm are 'island'-type layers. Such a structural pecularity affects the temperature-magnetization dependence of the investigated C o / C u multilayers. As a result, if the Co layer thickness is less than 3 nm, the Curie temperature falls to less than that of the Co volume (Tc = 1395 K). The Curie temperature is 543 K at dco = 2.5 nm (fig. 6). As reported in ref. [12], the diffusional changes in such structures begin when the annealing temperature is 723-743 K. Consequently, a considerable decrease in T c can not be explained by the formation of the nonmagnetic C o / C u alloy.
0
15
~.5
~
7.5
~
H,xOe
Fig. 5. Rotational hysteresis losses of C o / C u films. 1, Tco = 100 ms; 2, 60 ms; and 3, 15 ms.
334
V.M. Fedosyuk, 0.I. Kasyutich /Hard magnetic Co/Cu superlattices tions, such as an e x p l a n a t i o n o f t e m p e r a t u r e d e pendence of magnetization remain and further investigations a r e n e e d e d .
0,6 o
o
0~4
• ° o
Og
Acknowledgements
o
o
ioo
~
~oo
~oo.~.°c
This w o r k was s u p p o r t e d by t h e F u n d o f F u n d a m e n t a l R e s e a r c h o f B e l a r u s ( c o n t r a c t No. 4F047).
Fig. 6. Magnetization of Co/Cu multilayers versus temperature. 1, dco = 6.0 nm; 2, 4.5 nm; 3, 3.0 nm (dc, = 1.5 nm).
References A s r e p o r t e d above, t h e obvious i n f l u e n c e o f s u r f a c e a n i s o t r o p y is o b s e r v e d in C o / C u m u l t i layers. T h e r e is a l a r g e i n c r e a s e in t h e p e r p e n d i c u l a r m a g n e t i c a n i s o t r o p y with d e c r e a s i n g C o l a y e r thickness. T h u s it was n a t u r a l to e x p e c t a n inc r e a s e in t h e m a g n e t i c m o m e n t o f C o atoms, b u t this was n o t o b s e r v e d [13], p e r h a p s b e c a u s e o f t h e ' i s l a n d ' - t y p e layers w h e n t h e i r t h i c k n e s s is less t h a n a few nm, a n d t h e r e f o r e t h e p r e c i s i o n o f t h e d e t e r m i n a t i o n o f M s was n o t sufficient. So, it was n a t u r a l to expect, at least, an u n c h a n g e d C u r i e t e m p e r a t u r e , b u t t h e s i t u a t i o n was q u i t e t h e o p p o s i t e (fig. 6). S e r i o u s t h e o r e t i c a l efforts in this d i r e c t i o n a r e necessary.
4. Conclusions W e h a v e i l l u s t r a t e d t h e possibility o f o b t a i n i n g h a r d m a g n e t i c C o / C u m u l t i l a y e r s using t h e p u l s e e l e c t r o d e p o s i t i o n m e t h o d w i t h a high p e r p e n d i c u l a r m a g n e t i c a n i s o t r o p y . H o w e v e r , s o m e ques-
[1] S.B. Qadri, C. Kim, P. Lubitz and M. Twiss, J. Vac. Sci. Technol. A9 (1991) 430. [2] R.F.C. Farrow, C.H. Lee and S.S.P. Parkin, IBM J. Res. Dev. 434 (1990) 903. [3] S. Ohnuma, N. Yano and M. Mayashide, Jpn. J. Appl. Phys. 15 (1991) 391. [4] M.P. Yahalom and O. Zadok, J. Mater. Sci. 27 (1987) 497. [5] M.P. Dariel, J. Appl. Phys. 61 (1987) 4067. [6] D.S. Lasmore and O. Zadok, Proc. Electrochem. Soc. Int. Conf., Honolulu (1987) p. 421. [7] V.M. Fedosyuk, L.F. Iiyushenko and M.U. Sheleg, Thin Solid Films 158 (1988) 7. [8] V.M. Fedosyuk, O.I. Kasyutich and T.A. Tochitskii, Proc. ECS'89, Hollywood (1990) p. 591. [9] V.M. Fedosyuk, O.I. Kasyutich and N.N. Kozich, J. Mater. Chem. 1 (1991) 795. [10] Vnutrennie napryazhenia electroliticheski osazhdaemih metallov (Zap.-sib. izd. Novosibirsk, 1966) p. 335. [11] T.A. Tochitskii, O.I. Kasyutich and V.M. Fedosyuk, Phys. Stat. Solidi (URSS) 34 (1992) 1348. [12] V.M. Fedosyuk, A.V. Boltuchkin and O.I. Kasyutich, Phys. Met. Met. (URSS) 4 (1993) 59. [13] V.M. Fedosyuk, O.I. Kasyutich and N.N. Kozich, Phys. Met. Met. (URSS) 12 (1991) 43.
Hard magnetic Co/Cu superlattices V.M. Fedosyuk and O.I. Kasyutich Institute of Solid State Physics Semiconductors of the Belarussian Academy of Sciences, P. Brovki 17, 220072 Minsk, Belarus
Received 23 October 1992; in revised form 22 December 1992
Multilayer Co/Cu films produced by the method of pulse electrodeposition are studied. Investigations of their structure and magnetic properties have shown the possibility of obtaining very good hard magnetic properties and perpendicular magnetic anisotropy.
1. Introduction In recent year much attention has b e e n paid to investigation of artificially modulated structures (AMLs), which are remarkable from both the fundamental and practical points of view. In a n u m b e r of reports [1-3], promising soft and hard magnetic properties, unique electrical, mechanical and other properties of multilayers, including structures obtained by electrodeposition [4-6], have been illustrated, but we are not aware of any reports on electrodeposited multilayers showing the realization of hard magnetic properties. We have conducted experiments on electrodeposited films, including hard magnetic properties [7-8], so we were interested in analyzing the possibility of obtaining high coercivity together with perpendicular orientation of the magnetization vector in multilayer films. In our opinion, this would be easier to achieve with the C o / C u system than with others such as N i / C u or F e / C u . This was the aim of the present work. Multilayer C o / C u films were obtained using the pulse electrodeposition method, similar to that described in ref. [5] and in more detail in ref. [9]. The periodic and crystallographic struc-
Correspondence to: Dr V.M. Fedosyuk, Institute of Solid State
Physics and semiconductors of the Belarussian Academy of Sciences, P. Brovki 17, 220072 Minsk, Belarus.
tures of the obtained samples were studied using a D R O N - 3 M diffractometer in Co K~ radiation, in conjunction with a graphite monochromator. The magnetic m e a s u r e m e n t s were m a d e with the vibrating-sample magnetometers P A R C 155, PARC-4500 and a torque magnetometer.
2. Structure and internal stresses of C o / C u multilayers The X-ray diffraction patterns of the A M L s reveal two features: the absence of any reflections in a small-angle range and satellite reflections near the basic structure reflections of Co. When the thickness of the Co layers was changed, with a constant Cu layer thickness (dcu = 1.5 nm), the roentgenographic picture was the following. The first reflections a p p e a r e d at T = 80 nm (dco ~ 1.5 nm) and at a cobalt deposition pulse time Tco---200 ms ( d c o ~ 2.5 nm) there were three obvious reflections of different intensities. The reflections at 20 = 49.5 ° and 53.8 ° were the satellite reflections of the structure peak at 20 = 52.1 ° for (111) fcc Co (tabulated 20 = 51.95°). With further increases in Tco (up to 400 ms), the picture becomes more distinct. A p a r t from the satellite reflections of the p e a k for (111) fcc Co there appears one more reflection for (200) fcc Co (20 = 60.7 °) with satellite reflections ( 2 0 - =
0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
331
V.M. Fedosyuk, O.L Kasyutich / Hard magnetic Co / Cu superlattices
58.5 °, 20 += 62.5°), as shown in fig. 1. In all the investigated Co and Cu layer thickness ranges (upto 10 nm) the multilayer C o / C u structures (obtained by electrodeposition from a single solution) were structures on a base of fcc Co and fcc Cu. In addition, the conjugation of Co and Cu layer structures took place on more compact planes ( ( l i d fcc Co I1(111) fcc Cu). The presence of a satellite reflection is confirmed by the fact that the modulation period of the interplane distances of the AMLs, calculated from the satellites for different reflections, are the same and equal to the compositional modulation obtained using Auger spectroscopy (fig. 2). According to the profile in fig. 2 some amount of one element may be present in the layers of the other element (5-15%), but this experimental result can have another explanation. The diameter of the Auger spectroscope probe was 1 ~m. Since the thickness of the alternating layers is very small (1-2 nm), parts of the Co and Cu layers will be irregular (uncovered by one another) and displayed on the surface limited by the probe diameter. The irregularities of the thin layers in periodic structure can be estimated from fig. 2 [11] to be upto ~ 15%. From an analysis of these figures and the magnetic behavior reported below, it is shown that the ultrathin alternating C o / C u multilayers are 'island'-type layers [11]. When the layer thickness is less than 1 nm, the irregularity of the
d
SO
~'
~'~'~
'~'~ ' ~
~0,~,j.
Fig. 1. X-raydiagramof Co/Cu multilayerfilmwith Tco= 200 ms and Tc~= 5 s (dco= 2.5 nm, dc~ = 1.5 nm).
Co,Cu, at To I00
•..
° °
. o
•
o •
•
,/'Cu
o
o
•
o o
50
:
•
•
o
CO
• 0•
0
o
•
2
~
•
6
•
8
o ¢ , /
10
12 d,nm
Fig. 2. Auger profile of Co/Cu film with Tco= 200 ms and Tcu= 5 s.
layers is of the order of ~ 50%. Structural investigations using transmission electron microscopy confirmed the island-like type layer structure. At Co deposition pulse times of less than 200 ms, the Co layer forms islands 15-30 nm in diameter. Microdiffraction studies show the presence of crystallites of fcc Co [11]. In addition to the determination of the periodic structure of the Co/Cu-modulated films, internal macrostress measurements provided information on the mechanism and kinetics of layer growth. The measurements were made using a flexible cathode method with simultaneous automatic registration. It is known [10] that most of the refractory metals (Co, Fe, Cr, Ni) expand during electrodeposition. The internal macrostress value for copper is close to zero within the limit of measurement precision. This is illustrated in fig. 3 for the samples obtained with T c o > 100 ms. If the Co deposition pulse time is less than 100 ms, the internal expansion macrostress changes into internal compression stress, i.e. the deposit will have to increase in volume; this can be explained as follows. As indicated above, the Co layers are small-size grain phase, characterized by large free surfaces, and thus they are highly energetic. This could explain the experimentally observed expansion stress. At the initial growth stage the deposit is of the island-type with large irregularities. The volume growth and thus compression stress are the result of the low island packing density. The thermodynamic principle to decrease the volume results in the consolidation
V.M. Fedosyuk, 0.I. Kasyutich / Hard magnetic Co / Cu superlattices
332
short (Tco< 100 ms), the compression stress does not change into expansion stress, because it is 'frozen' by the following copper layer (Co and Cu are mutually insoluble under normal conditions). The dual-stage nature of the internal macrostress during the electrodeposition process also confirms that the C o / C u multilayer films have an island-type structure at Cobalt deposition pulse times of less than 100 ms.
E V
e./
¢/\
--'rt,,,~
cZ
3.
Fig. 3. Deviation of the Cu substrate during electrodeposition of C o / C u multilayers.
of the structure, thus decreasing the volume, and therefore the expansion stress with increasing Co layer thickness. If the Co deposition pulse time is
Magnetic properties of Co/Cu multilayers
The value of the total magnetic anisotropy constant of an ultrathin magnetic layer in C o / C u AMLs can be given as the sum of the crystallo. graphic strain, form and surface anisotropy components. The contribution of the surface anisotropy constant to the perpendicular magnetic anisotropy becomes predominant [9] when the magnetic layer thickness in the multilayers is of
M/Mmo~
,M/M,~,~
~(a) //
II
(b)
14.~0e
M/M,,o~
I M/Mm~
I o
i II
e
~
6
a H, d ~
o
2
~
~
a ~,~Oe
Fig. 4. Magnetization curves of C o / C u multilayers with time of pulse Co deposition. (a) Tco= 100 ms; (b) 40 ms; (c) 15 ms; and (d) 10 ms. Tcu = 5 s = const.
333
V.M. Fedosyuk, 0.I. Kasyutich / H a r d magnetic Co / Cu superlattices
the order of a few nm. As the thickness of the Co layer decreases, the perpendicular anisotropy and coercive force increase (fig. 4). Additional evidence of the development of perpendicular magnetic anisotropy can be seen from the magnetization reversal curves parallel and perpendicular to the coating directions. As the thickness of the magnetic layers of Co decreases, the remanent magnetization and H c in the direction perpendicular to the surface of films also increases. Simultaneously, these parameters decrease in the film plane. This is typical of magnetic structures with increasing perpendicular anisotropy. Analyses of these figures show that the decrease in the Co layer thickness leads to an increase in the perpendicular magnetic anisotropy constant. Furthermore, in our opinion, the increase in the discontinuity of these layers is the reason for the increase in coercivity of the island-type structure. This fact is an important result for the use of the investigated multilayer films as materials for vertical magnetic recording. Additional information on the value of the magnetic anisotropy and the mechanism of magnetization reversal in C o / C u multilayer films has been obtained from a study of the rotational hysteresis losses using a torque magnetometer. It was determined that the torque curves of the films in their plane (dco < 2.5 nm) are completely reversible in all regions of the used magnetic fields up to 10 kOe. Thus, the torque hysteresis losses in the film plane are absent. This confirms that the orientation of the Co layer magnetization vector in multilayer C o / C u structures is perpendicular [13]. When the magnetization reversal in the direction perpendicular to the coating was used, the torque curves were reversible and proportional to sin 0 in magnetic fields up to 100-150 Oe. Upon increasing the magnetic field in this direction the torque curves become irreversible and proportional to sin 20. When the magnitude of the magnetic field is H > 8-9 kOe, the torque curves remain irreversible. In order to determine the value of the PMA constant (K), the half-maximum torque curve amplitude was used. The obtained values of K are closer to the those determined by traditional methods. The measured val-
ues of the torque hysteresis losses (Wr//Wrmax) as a function of applied magnetic field are shown in fig. 5. The torque hysteresis losses are not closer to zero. The curve shapes are typical of a magnetic material with magnetization reversal by means of domain wall displacement. In addition, t h e calculated values of the torque hysteresis integral R H=
f:WJMs
d(1/H)
confirm this assumption because the resulting values of R n for magnetic C o / C u films are closer to 4. The magnetic results reported here show that if the thickness of magnetic Co layers is of the order of 3-4 nm, the surface effect is sufficient and high values of the PMA constant are obtained. Using the extensive structural data presented in ref. [11], a more detailed analysis of the magnetic behavior is possible. As reported [11], the Co layers with thicknesses of less than 3 nm are 'island'-type layers. Such a structural pecularity affects the temperature-magnetization dependence of the investigated C o / C u multilayers. As a result, if the Co layer thickness is less than 3 nm, the Curie temperature falls to less than that of the Co volume (Tc = 1395 K). The Curie temperature is 543 K at dco = 2.5 nm (fig. 6). As reported in ref. [12], the diffusional changes in such structures begin when the annealing temperature is 723-743 K. Consequently, a considerable decrease in T c can not be explained by the formation of the nonmagnetic C o / C u alloy.
0
15
~.5
~
7.5
~
H,xOe
Fig. 5. Rotational hysteresis losses of C o / C u films. 1, Tco = 100 ms; 2, 60 ms; and 3, 15 ms.
334
V.M. Fedosyuk, 0.I. Kasyutich /Hard magnetic Co/Cu superlattices tions, such as an e x p l a n a t i o n o f t e m p e r a t u r e d e pendence of magnetization remain and further investigations a r e n e e d e d .
0,6 o
o
0~4
• ° o
Og
Acknowledgements
o
o
ioo
~
~oo
~oo.~.°c
This w o r k was s u p p o r t e d by t h e F u n d o f F u n d a m e n t a l R e s e a r c h o f B e l a r u s ( c o n t r a c t No. 4F047).
Fig. 6. Magnetization of Co/Cu multilayers versus temperature. 1, dco = 6.0 nm; 2, 4.5 nm; 3, 3.0 nm (dc, = 1.5 nm).
References A s r e p o r t e d above, t h e obvious i n f l u e n c e o f s u r f a c e a n i s o t r o p y is o b s e r v e d in C o / C u m u l t i layers. T h e r e is a l a r g e i n c r e a s e in t h e p e r p e n d i c u l a r m a g n e t i c a n i s o t r o p y with d e c r e a s i n g C o l a y e r thickness. T h u s it was n a t u r a l to e x p e c t a n inc r e a s e in t h e m a g n e t i c m o m e n t o f C o atoms, b u t this was n o t o b s e r v e d [13], p e r h a p s b e c a u s e o f t h e ' i s l a n d ' - t y p e layers w h e n t h e i r t h i c k n e s s is less t h a n a few nm, a n d t h e r e f o r e t h e p r e c i s i o n o f t h e d e t e r m i n a t i o n o f M s was n o t sufficient. So, it was n a t u r a l to expect, at least, an u n c h a n g e d C u r i e t e m p e r a t u r e , b u t t h e s i t u a t i o n was q u i t e t h e o p p o s i t e (fig. 6). S e r i o u s t h e o r e t i c a l efforts in this d i r e c t i o n a r e necessary.
4. Conclusions W e h a v e i l l u s t r a t e d t h e possibility o f o b t a i n i n g h a r d m a g n e t i c C o / C u m u l t i l a y e r s using t h e p u l s e e l e c t r o d e p o s i t i o n m e t h o d w i t h a high p e r p e n d i c u l a r m a g n e t i c a n i s o t r o p y . H o w e v e r , s o m e ques-
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