Fig. 1. Axial hysteresis loop of a CoP microtube with 10 lmthickness. The inset shows the low "eld region of the loop.

*Corresponding author. Tel.: #34-91}3007173; fax: 34-91-3007176.

E-mail address: josemgarcia@renfe.es (J.M. Garcia).

Journal of Magnetism and Magnetic Materials 215}216 (2000) 352}354

Correlation between GMI e!ect and domain structurein electrodeposited Co}P tubes

J.M. Garcia!,*, A. Asenjo!, J.P. Sinnecker", M. Vazquez!

!Instituto de Magnetismo Aplicado (UCM-RENFE)-Instituto de Ciencia de Materiales de Madrid (CSIC), P.O. Box 155,Las Rozas 28230, Madrid, Spain

"Instituto de Fisica, Universidade Federal do Rio de Janeiro C.P. 68528, 21945-970 Rio de Janeiro, Brazil

Abstract

Magnetic microtubes of amorphous Co90

P10

have been electrodeposited over cylindrical Cu substrates. The samplesexhibit radial anisotropy, as deduced from hysteresis loops and magnetic force microscopy images. In this work, theevolution of both the domain structure and the giant magneto-impedance behavior in the MHz frequency range with themagnetic layer thickness is analyzed. ( 2000 Elsevier Science B.V. All rights reserved.

Keywords: Giant magneto-impedance; Electrodeposition; Magnetic force microscopy

Giant magneto-impedance (GMI) [1,2] is a phenom-enon of importance for applications in magnetic-"eldsensing devices. Recently, GMI has been studied in het-erogeneous samples that consist of non-magnetic wireselectroplated with magnetic layers exhibiting circular[3,4] or helical [5] anisotropy. In this work, we correlatethe GMI curves and the domain structure of Co

90P10

tubes that exhibit radial anisotropy.Thin layers of CoP with a thickness ranging from 2 to

20lm have been electrodeposited onto Cu wires (200lmdiameter) using the electrolyte given by Brenner et al. [6]at 743C with a constant current density of 200mA/cm2.The thickness of the CoP layer could be controlled usingdi!erent deposition times. The magnetic layer composi-tion was determined by atomic emission spectroscopy asbeing Co

90P10

and its amorphous character was veri"edby means of X-ray di!raction.

GMI has been measured in the MHz frequency rangeusing a setup previously described [7]. The GMI ratiohas been de"ned with respect to the maximum applied"eld as follows: *Z/Z"[Z(H)!Z(H

.!9)]/Z(H

.!9).

The domain structure has been studied by means ofmagnetic force microscopy (MFM) using a NanotecTMmicroscope operating in non-contact mode with com-mercial Co-covered tips (x

3&90 kHz, k&0.5N/m)

magnetized along the pyramid axis.As it has been previously reported [8,9], the elec-

trodeposition of CoP amorphous alloys gives rise toanisotropic materials. Due to the columnar growth ofCo, the anisotropy is mainly perpendicular to the surfacefor a thickness of several lm. Considering the cylindricalgeometry of our substrates, the out-of-plane anisotropy

0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 3 0 4 - 8 8 5 3 ( 0 0 ) 0 0 1 5 6 - 6

Fig. 2. MFM images (size: 11lm x 11 lm) of CoP microtubes.CoP layer thickness: 3 lm. The stripe domain width is 1.2lm.CoP layer thickness: 7 lm. The stripe domain width is 1.2lm.Notice the undulation of the structure.

Fig. 3. GMI curves corresponding to CoP microtubes withthickness of 2, 10 and 20 lm. The measurements have been madeat 1MHz using a 10 mA current. Low "eld region of the GMIcurve of the 10lm thick microtube. The solid symbols corres-pond to the measurements obtained when the "eld varied from#H

.!9down to !H

.!9, while the open ones where obtained

with the "eld varying from !H.!9

up to #H.!9

.

leads to an easy axis in the radial direction. Fig. 1 showsthe longitudinal hysteresis loop of a 10lm thick sample,where an anisotropy "eld of 6 kA/m can be deduced. Allthe samples here studied exhibit similar loops, indicatingthat all of them have an easy axis that is perpendicular tothe axial direction.

MFM images of the lateral surface of the microtubesshow the typical domain pattern for materials exhibiting

an out-of-plane anisotropy, with dark and bright areaswhich can be ascribed to domains with up and downmagnetization [10]. Samples with a thickness lower than7lm show stripe domains (see Fig. 2a). The domainwidth depends on the square root of the layer thickness,as expected for the so-called Kittel structure [11]. How-ever, with increasing thickness an undulation of the stripedomains appears to reduce the magnetostatic energy ofthe free poles on the surface (Fig. 2b). Such feature couldbe explained by the so-called wavy structure [12] andrami"cations due to closure domains [13,14].

Fig. 3a shows the GMI ratio for di!erent samplesmeasured using a 1MHz, 10 mA current. The maximumvalue of the GMI ratio increases with the layer thickness,which can be explained considering that GMI is relatedto the skin e!ect [2]. Since the penetration depth in-creases with the applied DC "eld, the change of theimpedance is higher for thicker magnetic tubes because oftheir larger cross section. Fig. 3b shows the low "eldregion in a GMI curve. It is worth noticing that themaximum of the GMI e!ect does not "t to the anisotropy"eld as in conventional wires with circular transverseanisotropy but to an applied "eld close to the coercivity(0.5 kA/m as shown in Fig. 1). Such peaks should be theconsequence of irreversible magnetization processes, typ-ically domain wall displacements [15].

In summary, electrodeposited Co90

P10

tubes withradial anisotropy exhibit GMI e!ect which increaseswith the cross section of the samples. The domain struc-ture also evolves with the magnetic layer thickness:

J.M. Garcia et al. / Journal of Magnetism and Magnetic Materials 215}216 (2000) 352}354 353

initially, the domain size increases, and "nally, an undu-lation of the domains is observed.

Authors want to thank the following institutions fortheir "nancial support: Volkswagen Audi-CSIC (J. M.Garcia), Comunidad Autonoma de Madrid (A. Asenjo)and the CNPq Brazilian agency (J. P. Sinnecker). Thiswork has been performed under projectsCAM/07N/0033/1998 and CICYT MAT98-0965-C04-C01.

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