IEEE TRANSACTIONS ON MAGNETICS VOL

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46 NO 2 FEBRUARY 2010 333 Structure and Magnetic Properties of Thin Permalloy Films Near the Transcritical State A V Svalov I R Aseguinolaza A GarciaArribas I Orue J M Barandiaran J Alonso M L Fern57569ndezGubieda and G ID: 30221 Download Pdf

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IEEE TRANSACTIONS ON MAGNETICS VOL

46 NO 2 FEBRUARY 2010 333 Structure and Magnetic Properties of Thin Permalloy Films Near the Transcritical State A V Svalov I R Aseguinolaza A GarciaArribas I Orue J M Barandiaran J Alonso M L Fern57569ndezGubieda and G

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 2, FEBRUARY 2010 333 Structure and Magnetic Properties of Thin Permalloy Films Near the “Transcritical” State A. V. Svalov , I. R. Aseguinolaza , A. Garcia-Arribas , I. Orue , J. M. Barandiaran , J. Alonso M. L. Fernndez-Gubieda , and G. V. Kurlyandskaya Departamento de Electricidad y Electrnica, Universidad del Pas Vasco (UPV/EHU), 48080 Bilbao, Spain Ural State University, Ekaterinburg, Russia SGIker, Servicios Generales de Investigacin, Universidad del Pas Vasco, Bilbao, Spain Various

series of permalloy thin films were grown by dc-sputtering on Si (100) and glass substrates at room temperature and different argon pressure values using a Fe  Ni  target. The increase of argon pressure leads to a decrease of the Fe concentration in the films from 17 at.% to 15 at.%, an increase of the root mean square roughness of film surfaces, and a decrease of the sharpness of the crystalline texture of the samples. The increase of the film thickness leads to an increase of the coercive field. The transition to the “transcritical state was observed at a

critical thickness that decreases from 220 to 50 nm as the argon pressure in the chamber increases. This state was confirmed by the characteristic shapes of hysteresis loops, rotatable magnetic anisotropy, and the appearance of stripe domains. Index Terms Magnetic domains, magnetic force microscopy, permalloy films, perpendicular magnetic anisotropy. I. I NTRODUCTION HIN magnetic films are of great interest due to their present and potential applications in various magnetic technologies. In the case of magnetic sensors these devices need to incorporate a sense layer that is

magnetically soft. Generally, thin films are magnetically soft at thickness of a few tens of nanometers. However, certain technological applications, like thin film magnetoimpedance (MI) devices require the thickness of the film to be of the order of microns for high MI ratio [1]. Another example is the reduction of the thermal magnetic noise in magnetic tunnel junction sensors which requires a sense-layer thickness of hundreds of nanometers [2]. On the other hand, the experiment demonstrates that it is not easy to achieve good softness in films, when their thickness is

greater than nm, for both polycrystalline (e.g., -metal [2], FeNi [3], FeAlN [4]), and amorphous (e.g., FeSiB [5]) films. Usually, softness is associated with a magnetization arrangement in the plane of the thin-film sample, originating from a relatively strong shape anisotropy. However, under some conditions, an out-of-plane magnetization component, accompanied by the formation of stripe domains [6] and increased coercivity, can appear. This is the so-called “transcritical” state of thin magnetic films [5], [7]. The perpendicular magnetic anisotropy in these films

might be caused by microshape anisotropy, magnetocrystalline anisotropy and/or magnetoelastic anisotropy [8], [9]. The mi- croshape anisotropy appears as a result of columnar structure of the film when the columns are separated by a nonmagnetic phase or voids. For polycrystalline films with random oriented grains usually the magnetocrystalline anisotropy is averaged out [10]. However, for films with crystallographic texture a net magnetocrystalline anisotropy may remain. The magnetoelastic contribution to the perpendicular magnetic anisotropy might Manuscript received June

19, 2009; revised August 27, 2009; accepted September 02, 2009. Current version published January 20, 2010. Corre- sponding author: A. V. Svalov (e-mail: andrey.svalov@ehu.es). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2009.2032519 take place in films with a negative magnetostriction constant and a planar tensile stress [11]. Generally speaking any of the three mechanisms may be dominant, depending on the film composition and parameters of the film

fabrication processes. In this work we study the microstructure and magnetic properties of a series of thin permalloy films prepared by dc magnetron sputtering at different values of argon pressure, in order to get insight on the origin of the perpendicular magnetic anisotropy. II. E XPERIMENT The samples were deposited by dc magnetron sputtering on Si (100) and glass substrates at room temperature using a Fe Ni target. A constant magnetic field of 250 Oe was applied parallel to the film plane during deposition in order to induce a uniaxial anisotropy. The background pressure

was mbar. Three series of samples were prepared at different argon pressures: samples of “A” series at mbar, sam- ples of “B” series at mbar, and samples of “C series at mbar. The thickness of the thin films was between 20 and 300 nm. Their compositions were deter- mined by energy dispersive X-ray analysis. The microstructure was studied by X-ray diffraction (XRD) using radiation. The atomic and magnetic force microscopy (AFM/MFM) mea- surements were performed with a commercial scanning probe microscope (Nanotec DSP classic) equipped with WSxM pro- gramme [12] to determine the

topographical characteristics of the films and their magnetic domain structure. Topographic and magnetic images were obtained in tapping and tapping-lift mode using standard nonmagnetic and magnetic tips. The magnetic domain structure was studied in remanence state by the Bitter technique using a commercial EMG 507 ferrofluid [11]. The hysteresis loops were recorded by means of the magneto-optic Kerr effect. Magnetization was obtained by using a vi- brating sample magnetometer. III. R ESULT AND ISCUSSION It was found that structure and magnetic properties of permalloy films

are affected by the Ar pressure in the sputtering 0018-9464/$26.00  2010 IEEE
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334 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 2, FEBRUARY 2010 TABLE I EPOSITION ARAMETERS AND ROPERTIES FOR HREE ERIES OF AMPLES chamber. No significant difference was observed for films deposited onto glass or Si substrates. Table I lists sputter pres- sure, deposition rate , composition, and critical thickness for transition into “transcritical” state of different types of samples deposited at different The hysteresis loops of films of all (A, B, C) series at nm [Fig.

1(a), (c), (e)] are typical for magnetic films with in-plane magnetization. However, already at this thickness the increase of the coercivity and gradual vanishing of the preferred magnetic axis in the film plane with were ob- served. The thicker samples of all series are characterized by hysteresis loops which are constituted by two magnetization phases: abruptly reverse at fields close to the coercive field, and a linear approach to saturation [Fig. 1(b), (d), (f)]. The change of the loop shape is accompanied by an increase of (Fig. 2). Moreover, for these samples the

so-called rotatable magnetic anisotropy was observed—the hysteresis loops remain unchanged whatever the orientation of the applied in-plane mag- netic field. These facts indicate that thicker samples are in “tran- scritical” state. This transition which occurred with the increase of film thick- ness is verified by a change of the domain structure as well. For the thinnest sample of A series, the static magnetic config- uration observed by Bitter technique consists of large domains with antiparallel magnetization oriented along the in-plane easy axis [Fig. 3(e)]. The MFM

images for this sample exhibit low contrast (not shown here), which indicates that magnetization mainly lies in the film plane. The MFM image of the thicker sample in series A shows a stripe domain structure, with mag- netization canted up or down out of the film plane [Fig. 3(f)], and confirms the presence of perpendicular anisotropy in the sample. Similar changes in domain structure were observed for the samples of type B [Fig. 4(e), (f)]. For samples of the type C a more complex behavior was observed. For a thin sample in series C, Bitter technique did not revealed the

domain bound- aries. For C sample with nm it was observed to be in a “transcritical” state according to the hysteresis loops [Fig. 1(f)]. MFM image did not reveal the stripe domain structure. It might be connected with the small size of the stripe domains (in this case the domain width must be of order of the film thickness [11]) and their very low contrast. The transition into “transcritical” state occurs at critical thick- ness of the film . Its value can be estimated from any of the above mentioned phenomena. It was found that decreases with increasing (Table I). Taking into

account that [13], where is perpendicular magnetic anisotropy constant, one can conclude that high values of help to the increase of the perpendicular anisotropy constant value. Fig. 1. Magnetization curves of FeNi films with different thickness : (a), (b) samples A; (c), (d) samples B; and (e), (f) samples C. Loops were measured in film plane. and represent directions parallel and perpendicular to the dc magnetic field applied during film deposition, respectively. Fig. 2. Thickness dependence of coercivity for FeNi films deposited at different pressure of the Ar,

The AFM images indicate that the topography of the sample surface changes appreciably, and rms roughness increases with argon pressure (Figs. 3–5). For thinner samples the average height of the topography features has become 2–3 times bigger [Figs. 3(a)–5(a)]. For thicker films the topography features height reaches a few nanometers [Figs. 3(b)–5(b)]. It is possible that the Ar pressure increase results in a lower average energy
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SVALOV et al. : STRUCTURE AND MAGNETIC PROPERTIES OF THIN PERMALLOY FILMS 335 Fig. 3. AFM images (a), (b) and corresponding line scans (c), (d);

zero-field Bitter image of domain structure (e) and MFM image of the stripe domain struc- ture (f) for A samples with  nm (left column) and  nm (right column). The values of rms roughness are indicated too (a), (b). of the particles reaching of the substrate. The reduced energy leads to less surface mobility of the deposited adatoms, an increased amount of absorbed Ar atoms, and a higher voids density. In this way, the high values of facilitates the formation of column structure in the films. The X-ray diagrams for the 100 nm thick samples deposited at different exhibited the

diffraction spots of the (111) fcc crystalline structure (Fig. 6). For lowest the small and bright (200) peak was observed as well. The intensity of the (111) peak decreases and (200) peak disappears completely, as the in- creases. The grain sizes calculated by Scherrer method remain in the 9–12 nm range. Thus, the observed intensity decrease reflects, most likely, the degradation of the crystallinity of the films. The (111) texture helps to the formation of the perpen- dicular anisotropy in FeNi films because in fcc crystal of FeNi alloys the direction is the easy

magnetization axis [14]. But in the present case, the increase of destroys the texture sharpness. EDX analysis shows that the iron content in the samples de- creases with an increase of the working gas pressure (Table I). It might be connected, in particular, with the fact that the sput- tered atoms of different types take the rather different mean free path length under collisions with the gas atoms. So they have different probability to arrive on the substrate [15]. It can be sug- gested that composition variation results in film magnetization Fig. 4. AFM images and corresponding line

scans (c), (d); zero-field Bitter image of domain structure (e) and MFM image of the stripe domain structure (f) for B samples with  nm (left column) and  nm (right column). The values of rms roughness are indicated too (a), (b). Fig. 5. AFM images (a), (b) and corresponding line scans (c), (d) for C samples with  nm (left column) and  nm (right column). The values of rms roughness are also indicated (a), (b). change. Obtained values for A and B samples are satisfac- torily consistent with known dependence of on permalloy composition [16]. However, the saturation magnetization

of the
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336 IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 2, FEBRUARY 2010 Fig. 6. X-ray diffraction spectra for FeNi films deposited at different 1—A sample, 2—B sample, 3—C sample;  nm (see also Table I). sample C decreases more than expected from the above men- tioned dependence. Probably, it was caused by intensive cap- ture of argon atoms by depositing film at high , formation of the voids, and the effective decrease of the film density. In any case, the decrease reduces the value of magnetic shape anisotropy, that, in turn, facilitates the deviation of

the magne- tization from the sample plane. Moreover, in this composition interval the decrease of the Fe content leads to an increase of the value of negative magnetostriction constant [14], that con- tributes to the development of perpendicular anisotropy. IV. C ONCLUSION In summary, we suggest that the main origin of the perpendic- ular magnetic anisotropy in investigated FeNi films is the com- bination of negative magnetostriction and planar tensile stresses. The increase leads to an increase of the magnetostriction constant, through the change in the film composition, and more

imperfections in the films produce larger stresses. Moreover, high values of facilitate the formation of column structure in the films. In addition, it was shown that the variation of the working pressure is an effective instrument for the tuning of magnetic properties of thin magnetic films. CKNOWLEDGMENT This work was supported in part by RFBR under Grant 08-02-99063-r_ofi, Spanish MEC under Project MAT2008-06542-C02-02_MAT, and the Basque Govern- ment Departments of Education and Industry under Projects IT-347-07 and SAIOTEK S-PE08UN03. The magnetic and X-ray

measurements were performed at General Research Services (SGIker) of the University of the Basque Country UPV-EHU. The authors thank Dr. J. Feuchtwanger for special support in the design and development of the rf-sputtering magnetic system. EFERENCES [1] C. Tannous and J. Gieraltowski, “Giant magneto-impedance and its ap- plication, J.Mater. Sci.: Mater.Electr. , vol. 15, pp. 125–133, 2004. [2] W. F. Egelhoff, Jr., J. Bonevich, P. Pong, C. R. Beauchamp, G. R. Stafford, J. Unguris, and R. D. McMichael, “400-fold reduction in sat- uration field by interlayering, J. Appl. Phys. , vol. 105,

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