High coercivity and perpendicular anisotropy in Co–Cugranular films Nguyen Anh Tuana, Nguyen Hoang Luongb,*, Nguyen Chaub, Vuong Van Hiepb, a International Training Institute for Material
Trang 1High coercivity and perpendicular anisotropy in Co–Cu
granular films Nguyen Anh Tuana, Nguyen Hoang Luongb,*, Nguyen Chaub, Vuong Van Hiepb,
a International Training Institute for Materials Science (ITIMS), 1 Dai Co Viet, Hanoi, Viet Nam
b Center for Materials Science, Faculty of Physics, Viet Nam National University, 334 Nguyen Trai, Hanoi, Viet Nam
Abstract
High coercivity was obtained in Co–Cu granular films, RF sputtered on Si(1 0 0) substrates, when the Co content is less than about 40 at% and annealed at high temperatures Perpendicular anisotropy was observed in the Co-rich films, where the Co content is not less than about 40 at% atom The reasons for the high coercivity and the perpendicular anisotropy in these Co–Cu films are discussed
r2002 Elsevier Science B.V All rights reserved
Keywords: Granular films; Superparamagnetic; High coercivity; Perpendicular anisotropy
Magnetic granular films have been known about
a decade ago for their giant magneto-resistance
(GMR) [1] Recently, these materials have again
got considerable attention, not only from a
view-point of fundamental physics[2,3]but also because
of their potential application as high-density
magnetic recording media[4] These media require
small magnetic particles having a high coercivity
and perpendicular anisotropy In this work we
report some results of observations of the high
coercivity and perpendicular anisotropy in Co–Cu
granular films prepared by RF sputtering These
magnetic properties were investigated as a
func-tion of the Co fracfunc-tion as well as of the annealing
temperature, because these factors affect
sensi-tively the structure characteristics and the
mag-netic properties
The CoxCu1x films (x=0.12, 0.16, 0.20, 0.26, 0.34, 0.42, 0.59 and 0.77) were deposited on Si(1 0 0) substrates at room temperature by RF sputtering using Ar gas The composition target was prepared from a Co target on which Cu pieces were attached Sputtering power was 400 W, the basic pressure was 106mbar, and the Ar pressure for discharging was 103mbar The thickness of the samples was fixed at 5000 (A, measured by an Alpha Step apparatus The Co fraction is deter-mined by energy dispersive X-ray spectroscopy (EDS), and the structure of the films was characterized by X-ray diffraction (XRD) mea-surements using the radiation of CuKa: The magnetic properties were measured in a vibrating sample magnetometer (VSM) The thermal treat-ment of the samples was carried out in vacuum (B105mbar) for 30 min at 1001C, 2001C, 3001C, 4001C and 5001C
Analysis of the XRD measurements for the samples showed that the Cu matrix has the FCC
*Corresponding author Tel./Fax: +84-4-8589496.
E-mail address: luongnh@vnu.edu.vn (N.H Luong).
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PII: S 0 9 2 1 - 4 5 2 6 ( 0 2 ) 0 1 7 5 7 - X
Trang 2structure and a so-called metastable phase of Co–Cu alloy is formed at low Co fraction The XRD patterns for some selected samples are pre-sented in Fig 1 The Cu(1 1 1) and (2 0 0) peaks are very close to standard lines of bulk Cu (2yCuð1 1 1ÞE43:41) for Co-poor films (see, for example, diagram a for x ¼ 0:12 in Fig 1), and these peaks are shifted to the larger 2y angle, closed to standard lines of bulk Co (2yCoð1 1 1ÞE44:21) for Co-richer films (see, for example, diagram d for x ¼ 0:59 in Fig 1) Simultaneously, the intensity of these peaks decreases with increasing x These results suggest that Co atoms form small clusters or fine particles
at low Co fractions, and that these particles become larger when the Co fraction increases[5]
Cu(111)
Cu(111)
Cu(200) Co(111)
40 45 50 55
2 θ (degree) →
a b c d
Fig 1 XRD diagrams for the Co x Cu 1x films (a) x=0.12,
(b) x=0.26, (c) x=0.42, (d) x=0.59.
Co12Cu88
-1.0 -0.5 0.0 0.5 1.0
Co20Cu80
-1.0 -0.5 0.0 0.5 1.0
Co34Cu66
H (kOe)
-1.0 -0.5 0.0 0.5 1.0
Co42Cu58
Co59Cu41
Co77Cu23
H (kOe)
Fig 2 Hysteresis loops measured in-plane for the Co Cu films.
Trang 3Fig 2shows the hysteresis loops of the selected
CoxCu1x films As can be seen in this figure,
superparamagnetism or property of the fine
particles systems is dominant for Co-poor samples
(xo0:40) For the samples with xo0:20; the
magnetization process is rather similar to that of
a paramagnet, as can be seen for the Co12Cu88
sample This property is less prominent when the
Co content increases, and ferromagnetism
dom-inates at the Co-richer samples (x > 0:40) Such
behavior has been reported by some other authors
[6,7] Fig 3 presents the dependence of the
coercivity, HC; on the Co content As one can
see in this figure, the coercivity first increases with
increasing x; reaches a maximum value of about
250 Oe at x ¼ 0:34; and then decreases with further
increasing x: The enhancement of the coercivity in
granular films is known to be due to an increase in
size of the magnetic particles[8] The occurrence of
such an increase was proved by the XRD
measurements shown above (see Fig 1) The
hysteresis for fine particles system has been
attributed to blocking of particles whose size
exceeds the critical size for superparamagnetism
[9] However, the hysteresis of granular systems
can also be explained by an interaction of
magnetic particles rather than of blocked particles
[10] Another cause is surface anisotropy[3,11,12]
For the CoxCu1x films with xE0:2020:40; it
could be suggested that the Co particles embedded
in the Cu matrix are single-domain fine particles, whereas for x > 0:40 they may be multi-domain[9]
It could be said that the Co-content threshold at
x ¼ xp; where xplies between 0.34 and 0.40, is the magnetic percolation threshold [8] The high coercivity for the low Co content films was also observed after annealing Fig 4shows the depen-dence of the coercivity on annealing temperature,
Ta; for selected CoxCu1xfilms As can be seen in this figure, HCincreases with increasing Tafor the Co-poor samples (xo0:40), and decreases for Co-rich samples Considerable increase of HCfor Co-poor samples with increasing Ta from 3001C can also be explained by the growth of the Co particles
[12] This has been confirmed by XRD measure-ments for samples as-deposited and annealed (see
Fig 5 for Co26Cu74 film, as an example); it is indicated by the shift to higher 2y angle of Cu(1 1 1) and Cu(2 0 0) peaks with increasing annealing temperature
Another important phenomenon observed for Co-rich films (x > 0:40) is that the shape of the hysteresis loops manifests a partly perpendicular magnetic anisotropy, as seen from the graphics on the right-hand side of Fig 2 This has been observed in Co-rich granular films of Co–Ag systems, and has been suggested to be due to a preferential orientation of Co phase or preferential arrangement of the Co particles in the direction
x (at %)
HC
0
50
100
150
200
250
300
Fig 3 Coercivity as a function of Co fraction for the
Co x Cu 1x films.
Ta (oC)
HC
0 100 200 300 400
500
Fig 4 Coercivity as a function of annealing temperature for the Co x Cu 1x films.
Trang 4perpendicular to the film plane[13] However, the
perpendicular anisotropy may originate from the
surface magnetic anisotropy at the interfaces
between Co particles and the Cu matrix [3,14]
Acknowledgements
This work is supported by the State Program of
Science & Technology of Viet Nam, Project
KC-02-13, and State Program of Fundamental
Re-search, Project 420101
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2θ (degree) ) →
As-deposited
C
C
C
Cu(111)
C
Co(111)
Fig 5 XRD diagrams for Co 26 Cu 74 film as-deposited and annealed.