Characterization of electrodeposited magnetic nanostructures

Characterization of electrodeposited magnetic nanostructures

Journal of Magnetism and Magnetic Materials 198}199 (1999) 239}242 Characterization of electrodeposited magnetic nanostructures Yukimi Jyoko *, Sato...

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Journal of Magnetism and Magnetic Materials 198}199 (1999) 239}242

Characterization of electrodeposited magnetic nanostructures Yukimi Jyoko *, Satoshi Kashiwabara , Yasunori Hayashi , Walther Schwarzacher Department of Materials Science & Engineering, Faculty of Engineering, Kyushu University, Fukuoka 812-8581, Japan H.H. Wills Physics Laboratory, University of Bristol, Bristol BS8 1TL, UK

Abstract Electrodeposition will prove to be a promising candidate for the preparation of magnetic nanostructures. Multilayered FCC Co/Pt and CoNi/Pt nanostructures grown on a Cu (1 1 1) substrate by electrodeposition under potential control, respectively, exhibit a remanent perpendicular magnetization and a large coercivity, which depend on the deposition overpotential and hence the multilayer growth mechanism. Giant magnetoresistance and oscillatory antiferromagnetic interlayer coupling have been observed in an FCC (1 1 1) textured Co/Cu multilayered nanostructure. Moreover, a large saturation magnetoresistance of more than 20% has been achieved at room temperature for a heterogeneous Co}Cu alloy, which consists of ultra"ne FCC Co-rich clusters in a nonmagnetic Cu matrix.  1999 Elsevier Science B.V. All rights reserved. Keywords: Electrodeposition; Multilayer; Perpendicular magnetic anisotropy; Giant magnetoresistance

The discovery of perpendicular magnetic anisotropy, magneto-optical Kerr e!ect [1}3] and giant magnetoresistance [4}6] in metallic multilayers has greatly stimulated interest in magnetic multilayered nanostructures. Currently, the development of new magnetic nanostructures is attracting considerable attention for potential applicability to high-density magneto-optical recording media or magnetoresistive sensor devices. Most nanostructured "lms have been fabricated by sputtering or molecular beam epitaxy (MBE), which have proved to be the most suitable for the controlled preparation of high-quality structures on an atomic scale. Electrodeposition may also be a promising candidate for this purpose, given the simplicity of the required equipment. Few papers directly addressing this subject, however, have been published to date, except for recently published reports on giant magnetoresistance CoNi/Cu,

* Corresponding author. Tel.: #81-92-642-3681; fax: #8192-632-0434; e-mail: [email protected]

Co/Cu multilayers and nanowires [7]. Recently we have presented the "rst evidence for composition modulation across successive layers in a Co/Pt nanometer-multilayered structure grown by electrodeposition [8]. In this paper we have demonstrated the presence of perpendicular magnetic anisotropy in an electrodeposited Co/Pt or CoNi/Pt multilayered nanostructure, as well as giant magnetoresistance in Co/Cu nanostructures. Multilayered Co/Pt, CoNi/Pt, and Co/Cu thin "lms were grown on a Cu (1 1 1) substrate, respectively, from two separate electrolytes for the deposition of the constituents of the multilayer by transferring the substrate repeatedly from one to the other under potential control, and from a single electrolyte by repeatedly controlling the electrode (substrate) potentials for the alternate deposition of both constituents [8,9]. The electrode potentials were measured and quoted relative to a saturated Ag/AgCl reference electrode at 298 K in a standard three-electrode electrolytic cell. The microstructure and compositionally modulated repeat length of the "lms were evaluated by cross-sectional transmission electron microscopy, X-ray di!raction, energy dispersive X-ray

0304-8853/99/$ } see front matter  1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 8 ) 0 1 0 5 9 - 2

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analysis, Auger electron spectroscopy, and magnetization measurements using a vibrating sample magnetometer. Resistance measurements were made with a standard low-frequency lock-in technique using a four in-line contact geometry. The magnetoresistance is measured with respect to the resistance in an applied high magnetic "eld. The saturation magnetoresistance is the highest resistance in the experimental "eld range normalized with respect to the resistance at the highest "eld used, as de"ned by 䉭R/R (%)"[R(H)!R(H )]/R(H ).



 As discussed in the previous paper [8], the growth and microstructure of electrodeposited ultrathin "lms and multilayers depend signi"cantly on the deposition overpotential. Fig. 1 shows typical X-ray di!raction pro"les obtained from Co/Pt multilayered "lms with repeat lengths down to a few nanometers, [Co(t nm)/ ! Pt(2.0 nm)] , grown on a Cu (1 1 1) substrate by elec trodeposition of the Co and Pt layers, respectively, at electrode potentials of !0.85 and !0.65 V. Since the Pt deposition potential is much less negative than that of Co, deposits obtained at !0.65 V are almost pure Pt layers, with Co tending to dissolve into CoOH or Co>  at the beginning of the Pt deposition on top of the previously grown Co layers. Indeed, the multilayered "lms, as characterized by the X-ray di!raction data, have mainly an FCC Co/Pt multilayered nanostructure, which is compositionally modulated over nanometer length scales with distinctly pure Pt and Co layers. In this case, it is probable that the deposition of Pt onto the Co layer surface proceeds with the preceding formation of a complex hydroxide, such as [Pt(OH) ) nCoOH] , as an ad  sorbed intermediate, leading to an interfacial alloying below the Pt overlayers, as expressed in the following reaction equation:

This indicates a more complicated growth process for the monolayer than simple adsorption}desorption. Possibly interfacial alloy formation induces a magnetic moment on Pt atoms, which would also have implications on such models for perpendicular magnetic anisotropy as already discussed in MBE-grown highly oriented Co/Pt (1 1 1) superlattices [2]. This view is also supported by re#ection electron microscopy (REM-RHEED) of electrodeposited Co/Pt (1 1 1) ultrathin layers and bilayers [8], and will be discussed in a separate paper. Typical room-temperature magnetization curves for the electrodeposited [Co(t nm)/Pt(2.0 nm)] multi!  layered "lms, taken with the applied "eld parallel (dotted curves) and perpendicular (solid curves) to the "lm plane, are shown in Fig. 2. As can be seen from the magnetic hysteresis loops, the FCC Co/Pt multilayered nanostructures, with decreasing Co layer thickness, tend to exhibit a remanent perpendicular magnetization and a large coercivity. On the other hand, for a [Co Ni (1.0 nm)/     Pt(0.5 nm)] multilayered FCC structure, as well as  a Co Ni Pt alloy with a mixture structure of FCC       and HCP, as characterized by the in-plane magnetization

[Pt(OH) ) nCoOH] #(2#n)H>#(2#n)e   PPtCo #(2#n)H O. L 

Fig. 1. Typical X-ray di!raction pro"les of [Co(t nm)/ ! Pt(2.0 nm)] multilayers grown on a Cu (1 1 1) substrate by  electrodeposition under potential control.

Fig. 2. Normalized room-temperature magnetization curves for electrodeposited [Co(0.5 nm)/Pt(2.0 nm)] (a) and [Co(1.0 nm)/  Pt(2.0 nm)] (b) multilayers, taken with the applied "eld paral lel (dotted curves) and perpendicular (solid curves) to the "lm plane.

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Fig. 3. Normalized in-plane magnetization curves for electrodeposited Co Pt (a) and Co Ni Pt (b) alloy single           layers (dotted curves), and [Co(1.0 nm)/Pt(0.5 nm)] (a) and  [Co Ni (1.0 nm)/Pt(0.5 nm)] (b) multilayers (solid      curves).

curves summarized in Fig. 3, a large coercivity value of up to 1.25 kOe is observed. This suggests the presence of a nonferromagnetic or weakly ferromagnetic Pt-rich phase localized at grain boundaries in the CoNi/Pt multilayerd nanostructure, which would pin domain wall motion. In order to achieve perpendicular magnetic anisotropy together with a much larger coercivity in electrodeposited Co/Pt and CoNi/Pt multilayers, further e!ort is needed for the precise control of the coherence and uniformity of the multilayered nanostructures. Fig. 4a shows typical room-temperature transverse magnetoresistance versus in-plane "eld data and the dependence of saturation magnetoresistance on the Cu spacer layer thickness for electrodeposited [Co(2.0 nm)/ Cu(t nm)] multilayered "lms. As can be seen from !  Fig. 4a, such oscillatory antiferromagnetic interlayer coupling as discussed in sputtered Co/Cu multilayers [5] and MBE-grown Co/Cu (1 1 1) superlattices [6], and large magnetoresistance values are evident for an FCC (1 1 1) textured Co/Cu multilayered nanostructure. Moreover, for a heterogeneous Co}Cu alloy, which consists of ultra"ne FCC Co-rich clusters in a nonmagnetic Cu matrix, as shown in Fig. 4b, a large saturation mag-

Fig. 4. Typical room-temperature transverse magnetoresistance versus in-plane "eld curves for electrodeposited [Co(2.0 nm)/ Cu(t nm)] multilayers (a), and heterogeneous Co}Cu alloy !  "lms (b).

netoresistance of more than 20% at room temperature is obtained together with a much higher saturation "eld and a smaller remanent magnetization suggesting the presence of large antiferromagnetic coupling. This value is comparable to those previously reported in vapordeposited superlattices as well as heterogeneous alloys. In conclusion, we have revealed that electrodeposition will prove to be a highly competitive technique for the preparation of magnetic nanostructures. Further studies on the crystallization kinetics of nucleus growth and its growth mechanism are necessary to achieve improved structural and magnetic qualities in the electrodeposited nanostructures. This work was supported in part by a Visiting Fellowship Research Grant from the United Kingdom Engineering and Physical Sciences Research Council and by a Grant-in-Aid for Scienti"c Research from the Ministry of Education, Science, Sports and Culture of Japan.

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