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Al2O3)
Journal of Magnetism and Mapt%c Materi&
149-144 (1995) 381-382
EISEVIER
Magneticpropertiesof granularfilms for the co (permalloy/Al,Q,) F. Guevara a**, H. Hurdequint a, E. Vincent b, A. Vaur6s a a Laboratoire de Physique des Sakaks, Llnicersit6 Park-Suci, 91405 &say, Frame b SPEC, CEA, Onne des Merisiem, 91191 Gif-sur-Yvette, France
Abstract For the system fpermaIIoy/aIumina), granular films prepared by rf co-sputtering have bee magnetization measurements. A decrease of the blocking temperature with the measuring field which is in agreement with theory. The particle anisotropy is dominated by the shape anisotropy
We report the results of detailed magnetization measurements, performed with a SQUID magnetometer, on ~anulzlr films corresponding to the (metal/dielectric) composite system: permalloy (Ni,Fe~)/AI,O,. h these granular magnetic films, where a relatively low metallic volume fraction f =: 25% has been used, one of the main goals of the present work is to study the magnetic anisotropy of individual particles as a function of their shape and of their size. The fiims, corresponding to a multilayer structure (composite Iayer/AI,O, spacer),, have been deposited on mica substrates, by rf co-sputtering, at various substrate temperatures T, in the range [50-4OO”C]. The deposition temperature Z’s determines the size of the particles. We tried to influence the shape of these fine permaIIoy (hereafter Py) partictes by changing the composite layer thickness t. We designate in short by ‘sphere’ and ‘disc’ the shape obtained when, respectively, t > 20 and t < D where D refers to the sphere diameter of the Ni particles obtained [l] at the same T,. In the following we present and analyze the results of our measurements for hvo specific samples: (i) Sampte 1: multilayer [N = 20; t = 30 $; Ts = 22OT1, (ii) SampIe 2: multilayer [N = 10; t = 98 A; T, = 22O”Cl. The results obtained for these two samples (same deposition temperature Ts) are characteristic of the twr, categories of films we have produced (sphere and disc particle shape). The magnetic behavior of an ensemble of independent small magnetic particles (monodomains) may be theoretically discussed as follows. If one describes the magnetic
l
Corresponding author. Fax: +33-l-69416086.
E, = KV[l
- ( /4’2KV)]‘.
Due to thermal activation the magnetic overcome this barrier leading for its relaxa the following Arrhenius law: l/~=
I/T~
exp( -En/AT).
For an experiment of the characteristic measuring ti there is a temperature TB (called the ‘blocking ture) defined by I (Tn) = T,,, below which the appears frozen in time TV: kT, = EB( Iog( 7,/q,))
- ’.
For T > Ts, instead, a superparamagnetic sta where the variation (versus If, T) of the particle described by a Langevin law.
correspond
0304-8853/95/$09.50 8 1995 Etsevier Science B.V. Au rights reserved SSDI 0304-8853(94)01508-2
to a sphere diameter of 46 A.
(2)
and Magnetic Materials 140-144 @95) 381-382
1of kgn&m
---
H
10
20
30
40
$12
' I 2 o 4.2143-0.0027113
H
P 4.6803-0.0095062
H
50
f(K)
3.5
with ~rn~~~re, after zero field cooling (ZFC), ~~Q~~~~ 1 forH=lOand3OG.
oar samples)we proceedto the
re generAy, of a wider distributionof
25
' 0
' ' ' 10 20
'
' ' ' ' ' ' ' ' ' 30 40 50 60 70 H(Causs)
'
A 60
Fig.3. Variationof theblockingtemperature Ts withthemeasuring field H for samples1 and 2. The dashedand solid lines correspondto linearfitsof TL” versus If. of the barrier EB may be adequately accountedfor in termsof the theoreticalexpectationgiven bv Eq. (1). Using F& 3, from the extrapolatedvalue (at H = 0) of the blocking temperature,one deducesthe zero field energy barrier (En =KV) and hence gets the anisotropy energy density K. One has taken T, = lo* s (SQUID measurements)and T,,= lo-” s. One obtains for the 2 samplesdiscussed: (8 Sample 1: T&H= 0) = 18 K; E, = 7 X lo-l4 ergs; K = 0.83 X lo6 ergs/cm3,and (ii) Sample 2: T&f= 0) = 23.8 #; E, = 9.2 X lo-‘” ergs; K= 2.1 X lo6 ergs/cm3. For the granular films studied, it appears that the magneticanisotropy of the Py particles is dominatedby two contributions(of the same order of magnitude):the shapeanisotropy(demagnetizingenergy)and a surface-inducedanisotropywhich, in the case[4] of the (Py/Al,O,) interface,turnsout to be a stress-inducedanisotropycorresponding to a value of KS = 0.2 ergs/cm2. in order to separateout thesetwo contributionsa systematicstudyas a function of the dimensionsof the particlesis needed(as well as an adequatetheoreticaldescriptionof the contribution induced by surfaceanisotropy)and we are presently focusingour work on sucha study. Refereaces H. Craighead,Ph.D. Thesis, CornellUniversity,Ithaca,NY (1980). [2] A. Aharoni. J. Appl. Phys.75 (1994)5893. [3] E. Chudoovsky, J. Appl.Phys.73 (1993)6597. [4] N. Pquterfas,H. HurdequintandA. Vauris,abstractD.LI.4 submittedto ICM’94. [l]
Q Q
10
20
30
40
50
60
70
T(K)
ratwe, after ZFC, of the magnetizaand 75 G.
149-144 (1995) 381-382
EISEVIER
Magneticpropertiesof granularfilms for the co (permalloy/Al,Q,) F. Guevara a**, H. Hurdequint a, E. Vincent b, A. Vaur6s a a Laboratoire de Physique des Sakaks, Llnicersit6 Park-Suci, 91405 &say, Frame b SPEC, CEA, Onne des Merisiem, 91191 Gif-sur-Yvette, France
Abstract For the system fpermaIIoy/aIumina), granular films prepared by rf co-sputtering have bee magnetization measurements. A decrease of the blocking temperature with the measuring field which is in agreement with theory. The particle anisotropy is dominated by the shape anisotropy
We report the results of detailed magnetization measurements, performed with a SQUID magnetometer, on ~anulzlr films corresponding to the (metal/dielectric) composite system: permalloy (Ni,Fe~)/AI,O,. h these granular magnetic films, where a relatively low metallic volume fraction f =: 25% has been used, one of the main goals of the present work is to study the magnetic anisotropy of individual particles as a function of their shape and of their size. The fiims, corresponding to a multilayer structure (composite Iayer/AI,O, spacer),, have been deposited on mica substrates, by rf co-sputtering, at various substrate temperatures T, in the range [50-4OO”C]. The deposition temperature Z’s determines the size of the particles. We tried to influence the shape of these fine permaIIoy (hereafter Py) partictes by changing the composite layer thickness t. We designate in short by ‘sphere’ and ‘disc’ the shape obtained when, respectively, t > 20 and t < D where D refers to the sphere diameter of the Ni particles obtained [l] at the same T,. In the following we present and analyze the results of our measurements for hvo specific samples: (i) Sampte 1: multilayer [N = 20; t = 30 $; Ts = 22OT1, (ii) SampIe 2: multilayer [N = 10; t = 98 A; T, = 22O”Cl. The results obtained for these two samples (same deposition temperature Ts) are characteristic of the twr, categories of films we have produced (sphere and disc particle shape). The magnetic behavior of an ensemble of independent small magnetic particles (monodomains) may be theoretically discussed as follows. If one describes the magnetic
l
Corresponding author. Fax: +33-l-69416086.
E, = KV[l
- ( /4’2KV)]‘.
Due to thermal activation the magnetic overcome this barrier leading for its relaxa the following Arrhenius law: l/~=
I/T~
exp( -En/AT).
For an experiment of the characteristic measuring ti there is a temperature TB (called the ‘blocking ture) defined by I (Tn) = T,,, below which the appears frozen in time TV: kT, = EB( Iog( 7,/q,))
- ’.
For T > Ts, instead, a superparamagnetic sta where the variation (versus If, T) of the particle described by a Langevin law.
correspond
0304-8853/95/$09.50 8 1995 Etsevier Science B.V. Au rights reserved SSDI 0304-8853(94)01508-2
to a sphere diameter of 46 A.
(2)
and Magnetic Materials 140-144 @95) 381-382
1of kgn&m
---
H
10
20
30
40
$12
' I 2 o 4.2143-0.0027113
H
P 4.6803-0.0095062
H
50
f(K)
3.5
with ~rn~~~re, after zero field cooling (ZFC), ~~Q~~~~ 1 forH=lOand3OG.
oar samples)we proceedto the
re generAy, of a wider distributionof
25
' 0
' ' ' 10 20
'
' ' ' ' ' ' ' ' ' 30 40 50 60 70 H(Causs)
'
A 60
Fig.3. Variationof theblockingtemperature Ts withthemeasuring field H for samples1 and 2. The dashedand solid lines correspondto linearfitsof TL” versus If. of the barrier EB may be adequately accountedfor in termsof the theoreticalexpectationgiven bv Eq. (1). Using F& 3, from the extrapolatedvalue (at H = 0) of the blocking temperature,one deducesthe zero field energy barrier (En =KV) and hence gets the anisotropy energy density K. One has taken T, = lo* s (SQUID measurements)and T,,= lo-” s. One obtains for the 2 samplesdiscussed: (8 Sample 1: T&H= 0) = 18 K; E, = 7 X lo-l4 ergs; K = 0.83 X lo6 ergs/cm3,and (ii) Sample 2: T&f= 0) = 23.8 #; E, = 9.2 X lo-‘” ergs; K= 2.1 X lo6 ergs/cm3. For the granular films studied, it appears that the magneticanisotropy of the Py particles is dominatedby two contributions(of the same order of magnitude):the shapeanisotropy(demagnetizingenergy)and a surface-inducedanisotropywhich, in the case[4] of the (Py/Al,O,) interface,turnsout to be a stress-inducedanisotropycorresponding to a value of KS = 0.2 ergs/cm2. in order to separateout thesetwo contributionsa systematicstudyas a function of the dimensionsof the particlesis needed(as well as an adequatetheoreticaldescriptionof the contribution induced by surfaceanisotropy)and we are presently focusingour work on sucha study. Refereaces H. Craighead,Ph.D. Thesis, CornellUniversity,Ithaca,NY (1980). [2] A. Aharoni. J. Appl. Phys.75 (1994)5893. [3] E. Chudoovsky, J. Appl.Phys.73 (1993)6597. [4] N. Pquterfas,H. HurdequintandA. Vauris,abstractD.LI.4 submittedto ICM’94. [l]
Q Q
10
20
30
40
50
60
70
T(K)
ratwe, after ZFC, of the magnetizaand 75 G.