Influence of nature of the substrate on the soft magnetic properties of pulsed laser ablated-deposited amorphous Co

Amorphous Co films, ≈ 270nm thick, were prepared by pulsed laser ablation-deposition. Substrates of glass, monocrystalline Si(1 1 1), copper and tungsten were used. The surface morphology of the films, studied by means of scanning tunneling
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  Journal of Magnetism and Magnetic Materials 254–255 (2003) 140–142 Influence of nature of the substrate on the soft magneticproperties of pulsed laser ablated-deposited amorphous Co V. Madurga*, J. Vergara, C. Favieres Departamento de F  !  ısica, Universidad P  ! ublica de Navarra, Campus De Arrosadia s/n, E-31006 Pamplona, Spain Abstract Amorphous Co films,  E 270nm thick, were prepared by pulsed laser ablation-deposition. Substrates of glass,monocrystalline Si(111), copper and tungsten were used. The surface morphology of the films, studied by means of scanning tunneling microscopy, STM, as well as the coercive field,  H  c ;  and room temperature spontaneousmagnetization,  M  s ;  was determined. The  M  s  values were approximately the same for all films independent of thesubstrate; on the contrary, significant variation of   H  c  was found for the films grown on different substrates. The lowestcoercive field,  H  c E 1 : 2Oe, corresponds to the samples grown on glass and  H  c E 1 : 6Oe to the films deposited overSi(111) substrates, whereas  H  c E 2 : 2 and 5Oe were measured for the samples deposited over tungsten and coppersubstrates, respectively. The STM studies allowed us to correlate the different magnetic softness of the films—from bothmagneto-optical transverse Kerr effect and vibrating sample magnetometry—with their surface morphology. r 2002 Elsevier Science B.V. All rights reserved. Keywords:  Amorphous systems—metals; Thin films—amorphous; Pulsed laser deposition; Coercivity-field Amorphous and nanocrystalline ferromagnetic mate-rials [1–6] have attracted great scientific interest fromboth the theoretical and the applied points of view, i.e.for inductors for ultra-high frequency [4,5]. Hiroshimaet al. have reported results on the amorphous structureof pulsed laser ablated-deposited (PLAD) pure Co films[1]. Besides, we have reported the first results on themagnetic properties exhibited by amorphous PLADpure Co films [2–6]. In this paper, we show the influenceof the nature of the substrate, through the surfacemorphology, on this soft magnetic behavior of ourPLAD pure Co amorphous films. The films wereprepared using a stainless-steel chamber (Neocera) at10  5 mbar pressure. A pulsed Nd:YAG laser (QuantelBrilliant),  l ¼ 1024nm, 20Hz repetition rate, with 4nspulses, energy 300mJ/pulse, was used. A polishedcircular disk, 20mm in diameter, of pure Co (Good-fellow metals 99.999%) was used as the target, whichwas rotated at 32rpm. The area of the laser beam on thetarget was 3mm 2 . Substrates, 8mm long, 4mm wide, of glass, monocrystalline Si(111) (Topsil) tungsten andcopper, (Goodfellow metals) were used for depositingthe Co films. The copper and the tungsten substrateswere mechanically polished during 20min. before thedeposition to reduce possible surface irregularities. Allthe substrates, which were rotated at 120rpm during thedeposition, were situated at a distance of 75mm fromthe target. The deposition time was 30min, thusobtaining samples E 270nm thick.The magneto-optical transverse Kerr effect (MOKE)was used to measure the hysteresis loops  M  2 H   of thesesamples. A polarized beam laser was used for illuminat-ing the samples and a photodiode was utilized to pick upthe signal reflected by them. A rotating sample holderallowed to find the in-plane easy direction for the magnet-ization. The loops were monitored in an oscilloscope andtransferred to a computer. The samples were alsomeasured in a vibrating sample magnetometer (VSM) atroom temperature and in magnetic fields up to 1T.The surface morphology of the films was measuredwith a Metris-1000 (Burleigh) scanning tunnelingmicroscope. *Corresponding author. Tel.: +34-948-169571; fax: +34-948-169565. E-mail address: (V. Madurga).0304-8853/03/$-see front matter r 2002 Elsevier Science B.V. All rights reserved.PII: S 0304 - 8853(02 )0 0808- 9  From X-ray analysis and high resolution transmissionelectron microscopy, the amorphous nature of our filmswas confirmed [2,4–6].Fig. 1 shows the different surface morphology and thesurface roughness of the films obtained from the STMmeasurements. For the films deposited over glass andSi(111) high homogeneity and low surface roughnesswere found: the surface roughness is almost the same atthe 3 m m  3 m m scale as that at 300nm  300nm scale;however, a difference between these two films wasfound: the root mean square of the surface, roughness is R q E 10nm for the Co deposited over glass, whereas R q E 20nm for the Co deposited on Si(111). Ratherdifferent was the surface topography of the films grownover the other substrates. Surface ‘‘stripes’’ wereobserved in both films, probably due to the fabricationand preparation processes of the substrates. The stripesobserved in the film grown over tungsten were  E 3 m mwide with  R q E 33nm and the corresponding values forthe film deposited over copper substrate surface were E 1 m m and 48nm. A very low surface roughness at300nm  300nm scale was found for both films grownover tungsten and copper substrates with  R q  values of 4and 3.8nm, respectively.Fig. 2 shows the easy axis MOKE hysteresis loops, M  2 H  ;  for the films grown onto the different substrates,revealing the different value of the coercive field, as isshown in Table 1. The easy direction for the magnetiza-tion was found to be the diagonal direction in all thesamples. This is probably due to a shape magneticanisotropy, because of the geometry of the samples:8mm long, 4mm wide. A magnetic anisotropy K  E 1 : 2  10 3 J/m 3 has been measured. At low magneticfield applied in this easy direction, first magnetic wallsnucleation took place, followed by small jumps in themagnetization processes, according to the evolution of the magnetic domain structures already observed inthese films [3,4]. Fig. 1. STM analysis corresponding the surface topography of the films deposited over the four different substrates. V. Madurga et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 140–142  141  The VSM measurements shown in Fig. 3 exhibitedsimilar values for the coercive field as well as similar easydirection for the magnetization as the MOKE results,see Table 1. In the hysteresis loops measured at 1Tmagnetic field, the magnetization achieved the samevalue as those measured at 7Oe; the films were alreadysaturated for this low field, according to the MOKEresults. Only a small approach to saturation up to100Oe was observed in the samples deposited on copperand tungsten substrates. These saturation magnetizationvalues agreed with those obtained from SQUIDmeasurements at 5T magnetic field [5],  M  s E 860emu/cm 3 , at room temperature. Table 1 shows the values of the magnetization saturation for the four differentsamples.As it can be seen from Table 1 an increase of thecoercive field of the films, from the glass to the coppersubstrates, is simultaneous to the increase in the rootmean square of the surface of these samples. These CoPLAD films exhibit a strong increase of the coercivefield when their thickness decreases as a consequence of predominant role of the magnetic wall nucleation field inthe irreversible magnetization processes. Since the sur-face roughness of these films is between a few tenths andsome hundredths of their thickness, an increase of thecoercive field for the samples with these large surfacesroughness must take place. Furthermore, these largesurface roughnesses will be hindrances to wall motion.This work was supported by the Spanish CICYTunder project MAT98-0404. References [1] Y. Hiroshima, T. Ishiguro, I. Urata, H. Ohta, M. Tohogi,Y. Ichinose, J. Appl. Phys. 79 (1996) 3572.[2] V. Madurga, J. Vergara, R.J. Ortega, I.P. Landaz ! abal,C. Favieres, Magnetic and structural properties of laser-ablated planar and cylindrical Co thin films, in: M. Coey,et al. (Eds.), Advanced Hard and Soft Magnetic Materials,Vol. 577, Material Research Society, Pittsburgh, PA, 1999,p. 599.[3] C. Favieres, V. Madurga, J. Non-Cryst. Solids 287 (2001)390.[4] V. Madurga, J. Vergara, C. Favieres, Laser-ablated non-crystalline pure Co thin films for inductors for ultra highfrequencies, in: W. C. Black, et al., (Ed.), Applicationof Ferromagnetics and Optical materials, Storage andMagnetoelectronic, Vol. 674, Material Research Society,Pittsburgh, PA, 2001, P.T. 302.[5] V. Madurga, J. Vergara, C. Favieres. Work Presentedat the 2001 Spring Meeting of the Materials ResearchSociety.[6] V. Madurga, J. Vergara, C. Favieres, Europhys. Lett.,submitted for publication.Fig. 2. MOKE hysteresis loops,  M  2 H  ;  corresponding to thefilms grown onto the different substrates, revealing the differentvalue of the coercive field.Fig. 3. VSM hysteresis loops,  M  2 H  ;  corresponding to the fourdifferent samples obtained in this work.Table 1Values of magnetization saturation for 1T magnetic field,  M  s ; magnetization in reduced units,  M  ;  from MOKE; coercivefields,  H  c ;  obtained from VSM and MOKE measurements; androot mean square of the surface,  R q ;  for the four differentsamplesSubstrate  M  s ; VSM(emu/cm 3 ) M  ; MOKE(r.u.) H  c ; VSM(Oe) H  c ; MOKE(Oe) R q (nm)Glass 860 1.0 1.2 1.2 10Si (111) 860 1.0 1.6 1.6 20Tungsten 860 0.80 2.4 2.2 33Copper 780 0.92 4.5 5.0 48 V. Madurga et al. / Journal of Magnetism and Magnetic Materials 254–255 (2003) 140–142 142
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