- Email: [email protected]
Microstructural and magnetic studies of granular GdW films
MATEI~LLS SCIENCE & ENGINEERING ELSEVIER
Materials Science and Engineering A204 (1995) 39-42
A
Microstructural and magnetic studies of granular G d - W films N.B. Shevchenko*~ A.S. Murthy,
G.C. Hadjipanayis
Department qf Physics and Astronomy, Universi O' qf Delaware, Newark, DE 19716, USA
Abstract
We have studied the microstructural and magnetic properties of sputtered nanocrystalline granular Gd-W thin films as a function of annealing temperature. Granular G d - W films were formed by annealing the as-deposited amorphous films in the temperature range 700-950 °C for 15 rain. The TEM studies on the annealed samples indicate that the Gd and W phases crystallize as separate phases around 825 °C. The grain sizes of these granular phases were estimated to be about 10-30 nm. Magnetic measurements of samples annealed at 875 °C indicate the coexistence of superparamagnetic Gd with a blocking temperature near 50 K and ferromagnetic Gd with a Curie temperature near 293 K. Higher temperature annealings led to bulk-like ferromagnetic Gd behavior. Kevwords: Magnetic properties; Granular; Gadolinium; Tungsten
1. Introduction
Phase separated materials continue to be a subject of interest and investigation [1,2]. Among such materials are granular solids. The fine grain structure in granular solids gives rise to a variety of physical phenomena such as the size dependence of the Curie temperature, the ferromagnetic single domain particles, and superparamagnetism [3,4]. Most studies to date have been done on systems involving a ferromagnetic transition metal (Fe, Co, Ni) in a non-magnetic matrix material (SiO2, A1203, BN, Cu, Ag) [5,6]. The main criteria for material selection are that they do not alloy with each other and that they do not react to form compounds. Very little work has been done outside of these said systems, especially for granular solids in which the magnetic constituent is a rare earth metal. Thus, in this study we have investigated the possibility of forming a granular solid with a rare earth metal. The G d - W system has been chosen for this study. It is known that Gd and W form no chemical compounds, and have very low mutual solubility. Furthermore, Gd orders ferromagnetically below its Curie temperature of 293 K while W remains paramagnetic down to very low temperatures. Thus it seems that Gd and W are ideal
* Corresponding author. 0921-5093/95/$09.50 ¢~ 1995 - - Elsevier Science S.A. All rights reserved
S S D I 0921-5093(95)09934-4
ingredients for a granular solid. In this study, we have investigated the synthesis of sputtered thin film granular solids of the G d - W system, and correlated the microstructure and magnetic properties of the films in the as-deposited state and at several stages of crystallization.
2. Preparation and measurement
G d - W thin films were deposited on Ta foil and carbon supported Ni grids by d.c. magnetron sputtering. The tandem method of deposition was used and sputtering occurred from high purity solid metal targets. The system base pressure was 10-8 Torr, and sputtering was done with 5 mTorr of Ar gas pressure. A high deposition rate was used ( ~ 5 A s - ] ) to ensure an amorphous as-deposited film. A 20 A protective overlayer of W was applied to prevent oxidation of Gd near the film surface while handling samples in air. A total film thickness of 300 A resulted. The as-deposited films were then annealed in vacuum for 15 min at temperatures between 700 °C and 950 °C to separate the two metals. Bright field, dark field, and selected area diffraction patterns were obtained using a Jeol JEM 2000 FX transmission electron microscope. The magnetization of the films was measured as a function of temperature and magnetic field via SQUID magnetometry in the temperature range 5 300 K, with ap-
40
N.B. Shet,chenko et a/. / Materials Science and Engineering A204 (1995) 39 42
plied fields of up to 50 kOe. The data were corrected for the paramagnetic contributions by Ta and W to the magnetization. This was done by preparing a W only film on Ta foil, and then subtracting proportionate amounts of its magnetic signal from the signals of the Gd-containing films. Thus the magnetization value reported for the films are due only to the Gd component. The film composition was determined by the inductively coupled plasma emission technique (ICP), and found to be Gd25W75.
9
.
8-".
Transmission electron microscopy studies on the assputtered and heat-treated samples indicated that the G d - W films remained amorphous up to 800 °C. Selected area diffraction (SAD) patterns show the characteristic halo indicative of amorphous structure (Fig. 1). Magnetic measurements show an ordering temperature near 40 K (Fig. 2(a)), and a magnetically soft behavior at cryogenic temperatures as expected for Gd alloys (Fig. 2(b)). The crystallization of both Gd and W was observed to occur around 825-850 °C. SAD patterns for the crystallized samples show two sets of diffractions, one corresponding to Gd and the other to W. Particles of Gd and W could be clearly observed in samples annealed at 850, 875, and 900 °C. The sample annealed at 850 °C showed homogeneously distributed particles of 10-20 nm size (Fig. 3(a)). Annealing at higher temperature had the effect of increasing particle size, 20-30 nm particles were observed in the 900 °C sample (Fig. 3(b)). An occasional coalescence of particles could be observed in the sample annealed at 900 °C. A percolated structure of Gd and W particles is evident in the images obtained from the samples annealed at 925 and 950 °C (Fig. 4). This is an indication
Fig. 1. Selected area diffraction image of as-deposited ad~sWT~.
cl
. . . . . .
se{ g4-: ~3"
''
3. Results and discussion
-.
7-"
''
50
'
'
"~" '
I
100
. . . .
150
(a)
I
. . . .
200 T (K)
I
' ' '
250
' l '
300
' ' '
350
200 -r T ~---I--- 10K --~- 150K / 150-~ .--e-- 77._~K ~
22OK j
I
°01 ' -
-10000 (b )
5
0
0
~
10000 20000
30000
40000
50000
H (Oe)
Fig. 2. (a) Temperaturedependenceof magnetization for as-deposited Gd2~WT5. (b) Magnetization curves at different temperatures. that an atomic concentration of 25% Gd is greater than the percolation threshold for thin films of the Gd W system. Thus, the particulate state of these films is merely a transition to the state in which continuous Gd paths form. Magnetic measurements taken for the 875 °C sample show a Curie temperature of about 250 K, and a blocking temperature of about 50 K (Fig. 5). Magnetic measurements taken for the samples annealed at 925 °C show a Curie temperature of about 293 K and a ferromagnetic response to applied fields. A description and explanation of the evolution of these effects is as follows. Upon the initial crystallization of the system from the amorphous state, very fine particles of Gd form. These particles exhibit superparamagnetism. A range of particle sizes are present in the 850-900 °C samples, 10 30 nm. While some of these particles are small enough to be superparamagnetic, the larger particles behave ferromagnetically. These larger particles possibly show the size effect of depressing the Curie temperature from the bulk value of 293 K to 250 K. For samples annealed at 900 °C, the average grain size has increased above that which exhibits superparamagnetic behavior, and the magnetic response is that of bulk Gd. At 900 °C the onset of particle coalescence is evident. This behavior continues to form the percolated
N.B. Shecchenko et al.
,'
Materials Science and Enghwering A204 (1995) 39 42
structures seen for the 925 °C and 950 °C samples, which also displayed magnetic properties of bulk Gd. The measured blocking temperature (TB) of 50 K is in agreement with the value predicted for uniaxial Gd particles. Using the expression T~ = KV/25k, where K is the anisotropy value for Gd (1.3 x 105 erg c m )) and k is the Boltzmann constant, with an observed particle volume V of 10 ~ cm 3, a temperature of 37 K is obtained. Using the information in Fig. 5(b), and applying the Curie law C = C/T, where C = N/12/3km,/l is the particle moment and m is the mass of Gd, the number N of superparamagnetic particles can be predicted. A value of 3.7 x 10') particles has been obtained for the sample annealed at 875 °C. This number falls short of that predicted if all the Gd was in the form of superparamagnetic particles (2.7 x 10t3). This result in combination with the observed Curie temperature, near that of
41
Fig. 4. Dark field image of Gd,sWT~ annealed at 925
°C
for 15 rain.
bulk Gd, supports the assertion that both bulk Gd and superparamagnetic Gd particles coexist in the 875 °C sample.
4. Conclusions In conclusion it has been seen that it is possible to form a granular solid with the Gd:fWTs system. The 1.6
F
1.4,
__m__ ZFC 1
1.2-. .~
1:
E 0.8-" ~" 0.6 " 0.4-
(a)
0.20
',,,
0
(a)
40
, i , , ~
, i , ,
50
100
- 1 0 ]
Fig. 3. (a) Bright field image with SAD inset of Gd:sW7> annealed at 850 °C for 15 min. (b) Dark field image with SAD inset of Gd,~W75 annealed at 90(1 o(, for 15 min.
, , i ,
,oK _ . _
l 0K
'-"-
77 K - - , , -
220 K j
,,,I
, , , t l
250
.
, , , ,
300
350
j
,.i . . . .
-10000 b)
,
150 T (200K)
_._
I (b)
~,1,
I
0
. . . .
i
. . . .
,
. . . .
I
. . . .
1 0 0 0 0 20000 30000 H (Oe)
i .... 40000 50000
" Fig. 5. (a) Thermomagnetic data and (b) My(H) curves for Gd2~WT, annealed at 875 °C for 15 rain.
42
N.B. Shevehenko et al. / Materials Science and Engineering A204 (1995) 39 42
amorphous as-deposited film segregates into a finegrained mixture of pure Gd and W upon annealing. The small Gd particles (10-20 nm) exhibit superparamagnetic behavior. As particle sizes increase, the Gd component takes on properties similar to bulk Gd but with size effects (depressed To). Larger particles behave like bulk with T c ~ 295°C.
Acknowledgement This work was supported by NSF DMR-9307676.
References [1] A. Tsoukatos, H. Wan, G.C. Hadjipanayis and Z.G. Li, Appl. Phys. Lett., 61 0992) 3059. [2] A.K. Petford-Long, R.C. Doole and P.E. Donovan, J. Magn. Magn. Mater., 126 (1993) 41. [3] D.P. Landau and K. Binder, J. Appl. Phys., 63(8) (1988) 3077. [4] G.G. Kenning, J.M. Slaughter and J.A. Cowen, Phys. Rev. Lett., 59 (1987) 2596. [5] A.S. Edelstein, B.N. Das, R.L. Holtz, N.C. Koon, M. Rubinstein, S.A. Wolf and K.E. Kihlstrom, J. Appl. Phys., 6•(8) (1987) 3320. [6] J.Q. Xiao, J.S, Jiang and C.L. Chien, Phys. Ret. B46 (1992) 9266.
Materials Science and Engineering A204 (1995) 39-42
A
Microstructural and magnetic studies of granular G d - W films N.B. Shevchenko*~ A.S. Murthy,
G.C. Hadjipanayis
Department qf Physics and Astronomy, Universi O' qf Delaware, Newark, DE 19716, USA
Abstract
We have studied the microstructural and magnetic properties of sputtered nanocrystalline granular Gd-W thin films as a function of annealing temperature. Granular G d - W films were formed by annealing the as-deposited amorphous films in the temperature range 700-950 °C for 15 rain. The TEM studies on the annealed samples indicate that the Gd and W phases crystallize as separate phases around 825 °C. The grain sizes of these granular phases were estimated to be about 10-30 nm. Magnetic measurements of samples annealed at 875 °C indicate the coexistence of superparamagnetic Gd with a blocking temperature near 50 K and ferromagnetic Gd with a Curie temperature near 293 K. Higher temperature annealings led to bulk-like ferromagnetic Gd behavior. Kevwords: Magnetic properties; Granular; Gadolinium; Tungsten
1. Introduction
Phase separated materials continue to be a subject of interest and investigation [1,2]. Among such materials are granular solids. The fine grain structure in granular solids gives rise to a variety of physical phenomena such as the size dependence of the Curie temperature, the ferromagnetic single domain particles, and superparamagnetism [3,4]. Most studies to date have been done on systems involving a ferromagnetic transition metal (Fe, Co, Ni) in a non-magnetic matrix material (SiO2, A1203, BN, Cu, Ag) [5,6]. The main criteria for material selection are that they do not alloy with each other and that they do not react to form compounds. Very little work has been done outside of these said systems, especially for granular solids in which the magnetic constituent is a rare earth metal. Thus, in this study we have investigated the possibility of forming a granular solid with a rare earth metal. The G d - W system has been chosen for this study. It is known that Gd and W form no chemical compounds, and have very low mutual solubility. Furthermore, Gd orders ferromagnetically below its Curie temperature of 293 K while W remains paramagnetic down to very low temperatures. Thus it seems that Gd and W are ideal
* Corresponding author. 0921-5093/95/$09.50 ¢~ 1995 - - Elsevier Science S.A. All rights reserved
S S D I 0921-5093(95)09934-4
ingredients for a granular solid. In this study, we have investigated the synthesis of sputtered thin film granular solids of the G d - W system, and correlated the microstructure and magnetic properties of the films in the as-deposited state and at several stages of crystallization.
2. Preparation and measurement
G d - W thin films were deposited on Ta foil and carbon supported Ni grids by d.c. magnetron sputtering. The tandem method of deposition was used and sputtering occurred from high purity solid metal targets. The system base pressure was 10-8 Torr, and sputtering was done with 5 mTorr of Ar gas pressure. A high deposition rate was used ( ~ 5 A s - ] ) to ensure an amorphous as-deposited film. A 20 A protective overlayer of W was applied to prevent oxidation of Gd near the film surface while handling samples in air. A total film thickness of 300 A resulted. The as-deposited films were then annealed in vacuum for 15 min at temperatures between 700 °C and 950 °C to separate the two metals. Bright field, dark field, and selected area diffraction patterns were obtained using a Jeol JEM 2000 FX transmission electron microscope. The magnetization of the films was measured as a function of temperature and magnetic field via SQUID magnetometry in the temperature range 5 300 K, with ap-
40
N.B. Shet,chenko et a/. / Materials Science and Engineering A204 (1995) 39 42
plied fields of up to 50 kOe. The data were corrected for the paramagnetic contributions by Ta and W to the magnetization. This was done by preparing a W only film on Ta foil, and then subtracting proportionate amounts of its magnetic signal from the signals of the Gd-containing films. Thus the magnetization value reported for the films are due only to the Gd component. The film composition was determined by the inductively coupled plasma emission technique (ICP), and found to be Gd25W75.
9
.
8-".
Transmission electron microscopy studies on the assputtered and heat-treated samples indicated that the G d - W films remained amorphous up to 800 °C. Selected area diffraction (SAD) patterns show the characteristic halo indicative of amorphous structure (Fig. 1). Magnetic measurements show an ordering temperature near 40 K (Fig. 2(a)), and a magnetically soft behavior at cryogenic temperatures as expected for Gd alloys (Fig. 2(b)). The crystallization of both Gd and W was observed to occur around 825-850 °C. SAD patterns for the crystallized samples show two sets of diffractions, one corresponding to Gd and the other to W. Particles of Gd and W could be clearly observed in samples annealed at 850, 875, and 900 °C. The sample annealed at 850 °C showed homogeneously distributed particles of 10-20 nm size (Fig. 3(a)). Annealing at higher temperature had the effect of increasing particle size, 20-30 nm particles were observed in the 900 °C sample (Fig. 3(b)). An occasional coalescence of particles could be observed in the sample annealed at 900 °C. A percolated structure of Gd and W particles is evident in the images obtained from the samples annealed at 925 and 950 °C (Fig. 4). This is an indication
Fig. 1. Selected area diffraction image of as-deposited ad~sWT~.
cl
. . . . . .
se{ g4-: ~3"
''
3. Results and discussion
-.
7-"
''
50
'
'
"~" '
I
100
. . . .
150
(a)
I
. . . .
200 T (K)
I
' ' '
250
' l '
300
' ' '
350
200 -r T ~---I--- 10K --~- 150K / 150-~ .--e-- 77._~K ~
22OK j
I
°01 ' -
-10000 (b )
5
0
0
~
10000 20000
30000
40000
50000
H (Oe)
Fig. 2. (a) Temperaturedependenceof magnetization for as-deposited Gd2~WT5. (b) Magnetization curves at different temperatures. that an atomic concentration of 25% Gd is greater than the percolation threshold for thin films of the Gd W system. Thus, the particulate state of these films is merely a transition to the state in which continuous Gd paths form. Magnetic measurements taken for the 875 °C sample show a Curie temperature of about 250 K, and a blocking temperature of about 50 K (Fig. 5). Magnetic measurements taken for the samples annealed at 925 °C show a Curie temperature of about 293 K and a ferromagnetic response to applied fields. A description and explanation of the evolution of these effects is as follows. Upon the initial crystallization of the system from the amorphous state, very fine particles of Gd form. These particles exhibit superparamagnetism. A range of particle sizes are present in the 850-900 °C samples, 10 30 nm. While some of these particles are small enough to be superparamagnetic, the larger particles behave ferromagnetically. These larger particles possibly show the size effect of depressing the Curie temperature from the bulk value of 293 K to 250 K. For samples annealed at 900 °C, the average grain size has increased above that which exhibits superparamagnetic behavior, and the magnetic response is that of bulk Gd. At 900 °C the onset of particle coalescence is evident. This behavior continues to form the percolated
N.B. Shecchenko et al.
,'
Materials Science and Enghwering A204 (1995) 39 42
structures seen for the 925 °C and 950 °C samples, which also displayed magnetic properties of bulk Gd. The measured blocking temperature (TB) of 50 K is in agreement with the value predicted for uniaxial Gd particles. Using the expression T~ = KV/25k, where K is the anisotropy value for Gd (1.3 x 105 erg c m )) and k is the Boltzmann constant, with an observed particle volume V of 10 ~ cm 3, a temperature of 37 K is obtained. Using the information in Fig. 5(b), and applying the Curie law C = C/T, where C = N/12/3km,/l is the particle moment and m is the mass of Gd, the number N of superparamagnetic particles can be predicted. A value of 3.7 x 10') particles has been obtained for the sample annealed at 875 °C. This number falls short of that predicted if all the Gd was in the form of superparamagnetic particles (2.7 x 10t3). This result in combination with the observed Curie temperature, near that of
41
Fig. 4. Dark field image of Gd,sWT~ annealed at 925
°C
for 15 rain.
bulk Gd, supports the assertion that both bulk Gd and superparamagnetic Gd particles coexist in the 875 °C sample.
4. Conclusions In conclusion it has been seen that it is possible to form a granular solid with the Gd:fWTs system. The 1.6
F
1.4,
__m__ ZFC 1
1.2-. .~
1:
E 0.8-" ~" 0.6 " 0.4-
(a)
0.20
',,,
0
(a)
40
, i , , ~
, i , ,
50
100
- 1 0 ]
Fig. 3. (a) Bright field image with SAD inset of Gd:sW7> annealed at 850 °C for 15 min. (b) Dark field image with SAD inset of Gd,~W75 annealed at 90(1 o(, for 15 min.
, , i ,
,oK _ . _
l 0K
'-"-
77 K - - , , -
220 K j
,,,I
, , , t l
250
.
, , , ,
300
350
j
,.i . . . .
-10000 b)
,
150 T (200K)
_._
I (b)
~,1,
I
0
. . . .
i
. . . .
,
. . . .
I
. . . .
1 0 0 0 0 20000 30000 H (Oe)
i .... 40000 50000
" Fig. 5. (a) Thermomagnetic data and (b) My(H) curves for Gd2~WT, annealed at 875 °C for 15 rain.
42
N.B. Shevehenko et al. / Materials Science and Engineering A204 (1995) 39 42
amorphous as-deposited film segregates into a finegrained mixture of pure Gd and W upon annealing. The small Gd particles (10-20 nm) exhibit superparamagnetic behavior. As particle sizes increase, the Gd component takes on properties similar to bulk Gd but with size effects (depressed To). Larger particles behave like bulk with T c ~ 295°C.
Acknowledgement This work was supported by NSF DMR-9307676.
References [1] A. Tsoukatos, H. Wan, G.C. Hadjipanayis and Z.G. Li, Appl. Phys. Lett., 61 0992) 3059. [2] A.K. Petford-Long, R.C. Doole and P.E. Donovan, J. Magn. Magn. Mater., 126 (1993) 41. [3] D.P. Landau and K. Binder, J. Appl. Phys., 63(8) (1988) 3077. [4] G.G. Kenning, J.M. Slaughter and J.A. Cowen, Phys. Rev. Lett., 59 (1987) 2596. [5] A.S. Edelstein, B.N. Das, R.L. Holtz, N.C. Koon, M. Rubinstein, S.A. Wolf and K.E. Kihlstrom, J. Appl. Phys., 6•(8) (1987) 3320. [6] J.Q. Xiao, J.S, Jiang and C.L. Chien, Phys. Ret. B46 (1992) 9266.