- Email: [email protected]
Pd multilayers
Journal of Magnetism and Magnetic Materials 177-181 (1998) 1191-1192
~
,~
Iourna] of
magnetism
and
magnetic
, i ~ materials
ELSEVIER
Structural and magnetic properties of NiO/Pd multilayers Takashi Manago a'b'*, Hideki Miyajima a, Kenji Kawaguchi b, Mitsugu Sohma b, Iwao Yamaguchi b aDepartment of Physics, Keio University, 3-14-1 HiyoshL Kohoku, Yokohama 223, Japan bNational h~stitute of Materials and Chemical Research 1-1 Higashi, Tsukuba, Ibaraki 305, Japan
Abstract
Magnetic properties of paramagnetic Pd on antiferromagnetm NiO have been investigated for NiO/Pd multiIayers. Most of the samples show ferromagnetic behavior. The origin is considered to be weak antiferromagnetic coupling in the extremely thin NiO layer and the induced magnetic moment of Pd at the interface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Multilayers; Antiferromagnetism
The magnetic moment of Ni 2+ in NiO with NaCI structure is ferromagnetically ordered in the (1 1 1) plane, whereas the inter-plane coupling is antiferromagnetic. Since Pd is considered nearly ferromagnetic, a strong exchange field may turn it ferromagnetic. Thus, the Pd layer on the (1 1 1) plane of NiO is expected to be ferromagnetic by a magnetic proximity effect. The magnetic properties of Pd will depend on the preferred orientation of the NiO layer on the basis of this idea. The primary purpose of this study is to investigate the structural and magnetic properties of these films from a viewpoint of the layer orientation and NiO-layer thickness. The NiO/Pd multilayers were prepared by a reactive deposition method in an oxygen atmosphere. Source metals of Ni (99.993% purity) and Pd (99.9% purity) were evaporated by electron-beam guns. Base pressure of the deposition chamber was below 5x 10 -7 Torr and 1 x 10-4Torr oxygen atmosphere was introduced. Growth rates were 0.15 ]V's for NiO and 0.25 A/s for Pd. Three different substrates, ~-AI;O3(0 0' 1), MgO(1 0 0) and polyimide film were employed at the same time and heated at 300~C. The NiO-layer thickness of the examined samples ranges from 9 to 50 A, while the Pd layer was kept at 29 A for all samples. The structural properties and crystallinity were investigated by X-ray diffractometry and a pole-figure method. The magnetic properties were measured using a SQUID magnetometer. *Corresponding author. Tel.: + 81 298 54 4552; fax: + 81 298 54 4551; e-mail: [email protected].
The X-ray diffraction (XRD) patterns of the samples show a preferred orientation of the (i 1 i) and (i 0 0) planes for A1203(0 0' 1) and MgO(1 0 0) substrates, respectively, while the films on the polyimide substrate are polycrystalline. Neither metal Ni nor Pd oxide reflection was observed. The XRD patterns and pole figures (inset) of [-NiO(9 ,~)/Pd(29 A,)] x 20 samples on A1203 and on MgO substrates are shown in Fig. 1. Higher-order Bragg peaks at low angle and satellite peaks around Pd(1 1 1) for A1203 substrate and around Pd(2 0 0) for MgO substrate indicate an epitaxial growth with well-regulated artificial structure. The epitaxial relationships out of the stacking plane obtained from the pole figures are AI2OJ1 1.07 ]lNiO/Pd[-1 - 1 0~ and MgO[-1 1 1~ ]]NiO/ Pd [-1 1 1]. Multilayers with NiO layers thicker than 33 ,~ on A12Oa substrate, however, come to be a textured structure with (1 1 1) preferred orientation instead of the single-crystaMike structure. Such a tendency is more significant for MgO substrate. The XRD results exhibit imperfect (1 0 0) texture accompanied with a slight (1 1 1) one for thick NiO (~> 19 ,~) samples. Most of the films show ferromagnetic behavior. Fig. 2 shows magnetization curves of [NiO(9 A)/Pd(29 ]~)] x 20 on three different substrates at 4.5 K. The (1 1 1) epitaxial film on the A1203 substrate exhibits a square-like hysteresis curve, while the magnetization of polycrystalline sample on polyimide saturates slowly and the coercivity is larger. Though the film on MgO is a (1 0 0) epitaxial film, it shows almost the same curve as the polyimide one. It indicates that easy magnetization direction lies in the
0304-8853/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 5 6 3 - 5
t192
T. Manago etal./Journal of Magnetism and Magnetic Materials 177-181 (1998) 1191-1192 20
106
, . .
T=4,SK
~
J
15
105
!/I:, i
104
,
* z *
.
.
,
AI20~ sub / ~ 0 aub
p,N~m~ sub
$', 5
103
/
10 102 .~
substrate
I0 4
÷2
//
~3\
I
Io 2
10
20
29
30
40
50
Fig. 1. The XRD patterns and the pole figures (inset) of [NiO(9 A)/Pd(29 A)] x 20 sample with the thinnest NiO layer: (a) on ~-AI203(0 0' I) substrate; (b) on MgO(1 0 0) substrates. • [NiO(9)tPd(29)lx20 T=4.5 K
5-
-5 ~" ~ r ~
--o
AI=O sub
L ~ - Polyimlde sUb
-to -'~5
40
50
Fig. 3. The NiO-layer thickness dependence of Ms at 4.5 K.
10 3
10
30
tN,o (A)
(b) MgO sub.
lo s
~
20
-lO
-5
0 H (kOe)
5
10
15
Fig. 2. The magnetization curves at 4.5K of [NiO(gA)/ Pd(29 ,~)] x 20 films on AIzO3(00" 1), MgO(1 0 0) and polyimide substrates at 4.5 K. (1 1 1) plane. Thick NiO samples (>t33 A.) on A1203, however, show similar magnetization curves as polycrystalline films. The in-plane structure of the (1 1 1) texture for the thick NiO samples seems to be polycrystalline like. The NiO-layer thickness dependence of the saturation magnetization Ms at 4.5 K is shown in Fig. 3. We employed the magnetization value at 70 kOe as the Ms. It increases with NiO-layer thickness t and has a maximum (about 18 emu/g (g: net mass)) around t = 19 .A. We estimated the mass from a sample surface area, each layer thickness of NiO and Pd and each bulk density. Above the thickness, the magnetization decreases and downs to zero at t = 50 A.. The Ms behavior is roughly the same for three different substrate samples. Thus, no significant dependence was observed for the preferred orientation. At the present stage, it is not concluded whether the ferromagnetic behavior arises from the Pd layer or the
NiO layer. There might be some alloy phases of Ni and Pd at the interfaces. Since the number of the interfaces is constant, we converted the magnetization into one per area (emu/cm2). Ms decreases with NiO-layer thickness (>i 19 A). If the alloy is an origin of the ferromagnetism, the magnetization should be constant of the NiO thickness against the observed result. Therefore, such an interface alloying is negligible. Since non-stoichiometric Ni~_~O was reported to be ferromagnetic like [1], we examined the magnetic properties of a thick NiO film (about 400 A) prepared under the same conditions as multilayers. No such ferromagnetic behavior was observed and, consequently, the stoichiometry of the NiO layer was not taken as a primary reason for the ferromagnetic behavior. The antiferromagnetic coupling between Ni 2 + ions in extremely thin NiO layers seems to be weak and imperfect due to the finite thickness effect and a structured distortion. These weakly coupled magnetic spins will tend to be aligned with a small applied field and the magnetic susceptibility will increase. The relation between Ms and tN~ofor rNio > 19 * in Fig. 3 will be explained by this mechanism. However, it cannot explain the observed finite value of coercive force. We consider induced Pd moments by a magnetic proximity effect at the interfaces as another additional origin of the ferromagnetic behavior. It was reported that in fine Ni particles coated with Pd [2], the magnetic moment of Pd is induced at the interface between the Ni core and the Pd layer even at room temperature and a coercivity enhancement is observed. Most samples showing ferromagnetic behavior give a Curie temperature ranging from 310 to 360 K, while for the samples with extremely thin NiO layers (9 ~-) it decreases to 250 K. This work was partly supported by Keio University Special Grant-in-Aid for Innovative Collaborative Research Project.
References
[1] Y. Shimomura, I. Tsubokawa, M. Kojima, J. Phys. Soc. Japan 9 (t954) 521. [2] T. Manago, H. Miyajima, Y. Otani, E. Aldba, J. Phys. Soc. Japan 66 (1997) 831.
~
,~
Iourna] of
magnetism
and
magnetic
, i ~ materials
ELSEVIER
Structural and magnetic properties of NiO/Pd multilayers Takashi Manago a'b'*, Hideki Miyajima a, Kenji Kawaguchi b, Mitsugu Sohma b, Iwao Yamaguchi b aDepartment of Physics, Keio University, 3-14-1 HiyoshL Kohoku, Yokohama 223, Japan bNational h~stitute of Materials and Chemical Research 1-1 Higashi, Tsukuba, Ibaraki 305, Japan
Abstract
Magnetic properties of paramagnetic Pd on antiferromagnetm NiO have been investigated for NiO/Pd multiIayers. Most of the samples show ferromagnetic behavior. The origin is considered to be weak antiferromagnetic coupling in the extremely thin NiO layer and the induced magnetic moment of Pd at the interface. © 1998 Elsevier Science B.V. All rights reserved. Keywords: Multilayers; Antiferromagnetism
The magnetic moment of Ni 2+ in NiO with NaCI structure is ferromagnetically ordered in the (1 1 1) plane, whereas the inter-plane coupling is antiferromagnetic. Since Pd is considered nearly ferromagnetic, a strong exchange field may turn it ferromagnetic. Thus, the Pd layer on the (1 1 1) plane of NiO is expected to be ferromagnetic by a magnetic proximity effect. The magnetic properties of Pd will depend on the preferred orientation of the NiO layer on the basis of this idea. The primary purpose of this study is to investigate the structural and magnetic properties of these films from a viewpoint of the layer orientation and NiO-layer thickness. The NiO/Pd multilayers were prepared by a reactive deposition method in an oxygen atmosphere. Source metals of Ni (99.993% purity) and Pd (99.9% purity) were evaporated by electron-beam guns. Base pressure of the deposition chamber was below 5x 10 -7 Torr and 1 x 10-4Torr oxygen atmosphere was introduced. Growth rates were 0.15 ]V's for NiO and 0.25 A/s for Pd. Three different substrates, ~-AI;O3(0 0' 1), MgO(1 0 0) and polyimide film were employed at the same time and heated at 300~C. The NiO-layer thickness of the examined samples ranges from 9 to 50 A, while the Pd layer was kept at 29 A for all samples. The structural properties and crystallinity were investigated by X-ray diffractometry and a pole-figure method. The magnetic properties were measured using a SQUID magnetometer. *Corresponding author. Tel.: + 81 298 54 4552; fax: + 81 298 54 4551; e-mail: [email protected].
The X-ray diffraction (XRD) patterns of the samples show a preferred orientation of the (i 1 i) and (i 0 0) planes for A1203(0 0' 1) and MgO(1 0 0) substrates, respectively, while the films on the polyimide substrate are polycrystalline. Neither metal Ni nor Pd oxide reflection was observed. The XRD patterns and pole figures (inset) of [-NiO(9 ,~)/Pd(29 A,)] x 20 samples on A1203 and on MgO substrates are shown in Fig. 1. Higher-order Bragg peaks at low angle and satellite peaks around Pd(1 1 1) for A1203 substrate and around Pd(2 0 0) for MgO substrate indicate an epitaxial growth with well-regulated artificial structure. The epitaxial relationships out of the stacking plane obtained from the pole figures are AI2OJ1 1.07 ]lNiO/Pd[-1 - 1 0~ and MgO[-1 1 1~ ]]NiO/ Pd [-1 1 1]. Multilayers with NiO layers thicker than 33 ,~ on A12Oa substrate, however, come to be a textured structure with (1 1 1) preferred orientation instead of the single-crystaMike structure. Such a tendency is more significant for MgO substrate. The XRD results exhibit imperfect (1 0 0) texture accompanied with a slight (1 1 1) one for thick NiO (~> 19 ,~) samples. Most of the films show ferromagnetic behavior. Fig. 2 shows magnetization curves of [NiO(9 A)/Pd(29 ]~)] x 20 on three different substrates at 4.5 K. The (1 1 1) epitaxial film on the A1203 substrate exhibits a square-like hysteresis curve, while the magnetization of polycrystalline sample on polyimide saturates slowly and the coercivity is larger. Though the film on MgO is a (1 0 0) epitaxial film, it shows almost the same curve as the polyimide one. It indicates that easy magnetization direction lies in the
0304-8853/98/$19.00 ~ 1998 Elsevier Science B.V. All rights reserved PII S 0 3 0 4 - 8 8 5 3 ( 9 7 ) 0 0 5 6 3 - 5
t192
T. Manago etal./Journal of Magnetism and Magnetic Materials 177-181 (1998) 1191-1192 20
106
, . .
T=4,SK
~
J
15
105
!/I:, i
104
,
* z *
.
.
,
AI20~ sub / ~ 0 aub
p,N~m~ sub
$', 5
103
/
10 102 .~
substrate
I0 4
÷2
//
~3\
I
Io 2
10
20
29
30
40
50
Fig. 1. The XRD patterns and the pole figures (inset) of [NiO(9 A)/Pd(29 A)] x 20 sample with the thinnest NiO layer: (a) on ~-AI203(0 0' I) substrate; (b) on MgO(1 0 0) substrates. • [NiO(9)tPd(29)lx20 T=4.5 K
5-
-5 ~" ~ r ~
--o
AI=O sub
L ~ - Polyimlde sUb
-to -'~5
40
50
Fig. 3. The NiO-layer thickness dependence of Ms at 4.5 K.
10 3
10
30
tN,o (A)
(b) MgO sub.
lo s
~
20
-lO
-5
0 H (kOe)
5
10
15
Fig. 2. The magnetization curves at 4.5K of [NiO(gA)/ Pd(29 ,~)] x 20 films on AIzO3(00" 1), MgO(1 0 0) and polyimide substrates at 4.5 K. (1 1 1) plane. Thick NiO samples (>t33 A.) on A1203, however, show similar magnetization curves as polycrystalline films. The in-plane structure of the (1 1 1) texture for the thick NiO samples seems to be polycrystalline like. The NiO-layer thickness dependence of the saturation magnetization Ms at 4.5 K is shown in Fig. 3. We employed the magnetization value at 70 kOe as the Ms. It increases with NiO-layer thickness t and has a maximum (about 18 emu/g (g: net mass)) around t = 19 .A. We estimated the mass from a sample surface area, each layer thickness of NiO and Pd and each bulk density. Above the thickness, the magnetization decreases and downs to zero at t = 50 A.. The Ms behavior is roughly the same for three different substrate samples. Thus, no significant dependence was observed for the preferred orientation. At the present stage, it is not concluded whether the ferromagnetic behavior arises from the Pd layer or the
NiO layer. There might be some alloy phases of Ni and Pd at the interfaces. Since the number of the interfaces is constant, we converted the magnetization into one per area (emu/cm2). Ms decreases with NiO-layer thickness (>i 19 A). If the alloy is an origin of the ferromagnetism, the magnetization should be constant of the NiO thickness against the observed result. Therefore, such an interface alloying is negligible. Since non-stoichiometric Ni~_~O was reported to be ferromagnetic like [1], we examined the magnetic properties of a thick NiO film (about 400 A) prepared under the same conditions as multilayers. No such ferromagnetic behavior was observed and, consequently, the stoichiometry of the NiO layer was not taken as a primary reason for the ferromagnetic behavior. The antiferromagnetic coupling between Ni 2 + ions in extremely thin NiO layers seems to be weak and imperfect due to the finite thickness effect and a structured distortion. These weakly coupled magnetic spins will tend to be aligned with a small applied field and the magnetic susceptibility will increase. The relation between Ms and tN~ofor rNio > 19 * in Fig. 3 will be explained by this mechanism. However, it cannot explain the observed finite value of coercive force. We consider induced Pd moments by a magnetic proximity effect at the interfaces as another additional origin of the ferromagnetic behavior. It was reported that in fine Ni particles coated with Pd [2], the magnetic moment of Pd is induced at the interface between the Ni core and the Pd layer even at room temperature and a coercivity enhancement is observed. Most samples showing ferromagnetic behavior give a Curie temperature ranging from 310 to 360 K, while for the samples with extremely thin NiO layers (9 ~-) it decreases to 250 K. This work was partly supported by Keio University Special Grant-in-Aid for Innovative Collaborative Research Project.
References
[1] Y. Shimomura, I. Tsubokawa, M. Kojima, J. Phys. Soc. Japan 9 (t954) 521. [2] T. Manago, H. Miyajima, Y. Otani, E. Aldba, J. Phys. Soc. Japan 66 (1997) 831.