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FeSi multilayers
Physica B 284}288 (2000) 1241}1242
Low-temperature studies of magnetic Fe/FeSi multilayers Ivo VaH vra *, JaH n Bydz\ ovskyH , Peter S[ vec , JaH n DeH rer , VladimmH r KamberskyH , Zdene\ k Frait, Radek Lopus\ nmH k, Peter S[ turc, Gerfried Hilscher Institute of Electrical Engineering, SAS, Du& bravska& cesta 9, SK-842 39 Bratislava, Slovakia, Slovak Republic Institute of Physics, AS CzR, Na Slovance 2, CZ-180 40 Prague, Czech Republic Institute of Experimental Physics, TU Vienna, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria
Abstract Magnetic coupling and magnetoresistance of Fe/FeSi multilayers were investigated in the range 4.2}300 K. From M(¹, H) data and FMR measurements the magnetization arrangement of Fe sublayers is discussed. GMR contribution increases with decreasing temperature, but its value is very low. 2000 Elsevier Science B.V. All rights reserved. Keywords: Magnetic multilayer; Magnetoresistance
The nature of exchange coupling in Fe/FeSi multilayers is still unclear [1,2]. In order to understand better the behavior of this type of multilayers the detailed structural investigations and low-temperature magnetic measurements were performed. The Fe/FeSi multilayers were deposited by DC magnetron sputtering. The substrate temperature was !203C, the Ar pressure 10\ Pa. For FeSi deposition an alloy target was used. Thickness of Fe layers was always 4 nm. The FeSi spacer layer thickness was varied from 0.7 up to 15 nm. Magnetic properties were investigated by magnetooptical Kerr e!ect and by SQUID magnetometry. Ferromagnetic resonance (FMR) at 69 GHz was used. Magnetoresistance was measured at 77 and 300 K. The typical microstructure of our Fe/FeSi multilayers is shown in Fig. 1. Electron di!raction shows the preferential [1 1 0] orientation of BCC Fe. No other spots were observed except BCC Fe. The second characteristic feature of Fe/FeSi multilayers is the structural correlation of neighboring Fe layers (the contrast of Fe grains in adjacent layers is frequently correlated). We conclude that the FeSi layers grow pseudomorphically on the (1 1 0)Fe crystallites. Subsequent deposition of Fe follows the [1 1 0] orientation of BCC FeSi. Similar
* Corresponding author. E-mail address: [email protected] (I. VaH vra)
pseudomorphic growth of BCC FeSi was observed by other authors [3]. Magnetic properties of Fe/FeSi multilayers are very sensitive to the thickness of spacer layer. In a rather narrow range of spacer thickness in the vicinity of 1.6 nm, the hysteresis loops displayed a strong decrease of both coercivity and remanence and also perminvar constriction (Fig. 2). The shape of hysteresis loops is characteristic for interlayer exchange coupling. The presence of coupling was con"rmed also from splitting of FMR curve (derivative of absorption). The absolute value of slopes of tangents to MR dependencies at B"0 are identical for both N and "" orientations of magnetic "eld at 300 K. The slope at 77 K is higher for magnetic "eld perpendicular (N) to the "lm plane than for parallel "eld ("") because of GMR contribution (Fig. 3). The temperature dependence of
Fig. 1. Cross-sectional TEM micrograph and electron di!raction of [Fe(4 nm/FeSi(1.7 nm)];10 multilayer.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 4 9 0 - 4
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I. Va& vra et al. / Physica B 284}288 (2000) 1241}1242
Fig. 2. Hysteresis loop of the [Fe(4 nm)/FeSi(1.7 nm)];10 multilayer measured by longitudinal Kerr method. Fig. 4. Temperature dependence of magnetic moment of [Fe(4 nm)/FeSi(1.7 nm)];10 multilayer at di!erent "elds.
Fig. 3. Magnetoresistive dependencies of [Fe(4 nm)/ FeSi(1.7 nm)];10 multilayer measured at 300 and 77 K. The current #ow is always in the "lm plane.
magnetic moment at di!erent constant applied "elds is shown in Fig. 4. Fig. 5 shows the temperature dependence of the saturation "eld H of Fe/FeSi multilayer determined from the m(¹) plots at "xed H. The steep increase of H on cooling is evident. Fig. 5 also shows tentative decomposition of H based on the measured temperature dependence of remanent magnetization, and on the model of bilinear (antiparallel magnetization arrangement) and biquadratic (perpendicular magnetization arrangement) exchange [1}3]. In this model, H and H are the e!ective "elds of these two interac tions (H "H #H ); the relative remanence, M /M increases with the ratio H /H . The increase of H at low temperatures is mostly ascribed to biquadratic component H (in accordance with Refs. [1,2]). This interpreta tion of high remanence is, however, not corroborated by FMR measurements at low temperatures. The splitting of the two FMR modes should be equal to H in this model,
Fig. 5. Temperature dependence of H and its model compo1 nents H (bilinear) and H (biquadratic).
i.e. it should also increase rapidly at low temperatures. Our measurements show only a slight increase of the FMR mode splitting on cooling, however, a large variation of the mode intensity and line width was observed. This indicates that the increasing remanence may have other sources than the biquadratic exchange.
References [1] E.E. Fullerton, S.D. Bader, Phys. Rev. B 53 (1996) 5112. [2] Y. Saito, K. Inomata, J. Phys. Soc. Jpn. 67 (1998) 1138. [3] Y. Endo, O. Kitakami, Y. Shimada, Phys. Rev. B 59 (1999) 4279.
Low-temperature studies of magnetic Fe/FeSi multilayers Ivo VaH vra *, JaH n Bydz\ ovskyH , Peter S[ vec , JaH n DeH rer , VladimmH r KamberskyH , Zdene\ k Frait, Radek Lopus\ nmH k, Peter S[ turc, Gerfried Hilscher Institute of Electrical Engineering, SAS, Du& bravska& cesta 9, SK-842 39 Bratislava, Slovakia, Slovak Republic Institute of Physics, AS CzR, Na Slovance 2, CZ-180 40 Prague, Czech Republic Institute of Experimental Physics, TU Vienna, Wiedner Hauptstrasse 8-10, A-1040 Wien, Austria
Abstract Magnetic coupling and magnetoresistance of Fe/FeSi multilayers were investigated in the range 4.2}300 K. From M(¹, H) data and FMR measurements the magnetization arrangement of Fe sublayers is discussed. GMR contribution increases with decreasing temperature, but its value is very low. 2000 Elsevier Science B.V. All rights reserved. Keywords: Magnetic multilayer; Magnetoresistance
The nature of exchange coupling in Fe/FeSi multilayers is still unclear [1,2]. In order to understand better the behavior of this type of multilayers the detailed structural investigations and low-temperature magnetic measurements were performed. The Fe/FeSi multilayers were deposited by DC magnetron sputtering. The substrate temperature was !203C, the Ar pressure 10\ Pa. For FeSi deposition an alloy target was used. Thickness of Fe layers was always 4 nm. The FeSi spacer layer thickness was varied from 0.7 up to 15 nm. Magnetic properties were investigated by magnetooptical Kerr e!ect and by SQUID magnetometry. Ferromagnetic resonance (FMR) at 69 GHz was used. Magnetoresistance was measured at 77 and 300 K. The typical microstructure of our Fe/FeSi multilayers is shown in Fig. 1. Electron di!raction shows the preferential [1 1 0] orientation of BCC Fe. No other spots were observed except BCC Fe. The second characteristic feature of Fe/FeSi multilayers is the structural correlation of neighboring Fe layers (the contrast of Fe grains in adjacent layers is frequently correlated). We conclude that the FeSi layers grow pseudomorphically on the (1 1 0)Fe crystallites. Subsequent deposition of Fe follows the [1 1 0] orientation of BCC FeSi. Similar
* Corresponding author. E-mail address: [email protected] (I. VaH vra)
pseudomorphic growth of BCC FeSi was observed by other authors [3]. Magnetic properties of Fe/FeSi multilayers are very sensitive to the thickness of spacer layer. In a rather narrow range of spacer thickness in the vicinity of 1.6 nm, the hysteresis loops displayed a strong decrease of both coercivity and remanence and also perminvar constriction (Fig. 2). The shape of hysteresis loops is characteristic for interlayer exchange coupling. The presence of coupling was con"rmed also from splitting of FMR curve (derivative of absorption). The absolute value of slopes of tangents to MR dependencies at B"0 are identical for both N and "" orientations of magnetic "eld at 300 K. The slope at 77 K is higher for magnetic "eld perpendicular (N) to the "lm plane than for parallel "eld ("") because of GMR contribution (Fig. 3). The temperature dependence of
Fig. 1. Cross-sectional TEM micrograph and electron di!raction of [Fe(4 nm/FeSi(1.7 nm)];10 multilayer.
0921-4526/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 2 6 ( 9 9 ) 0 2 4 9 0 - 4
1242
I. Va& vra et al. / Physica B 284}288 (2000) 1241}1242
Fig. 2. Hysteresis loop of the [Fe(4 nm)/FeSi(1.7 nm)];10 multilayer measured by longitudinal Kerr method. Fig. 4. Temperature dependence of magnetic moment of [Fe(4 nm)/FeSi(1.7 nm)];10 multilayer at di!erent "elds.
Fig. 3. Magnetoresistive dependencies of [Fe(4 nm)/ FeSi(1.7 nm)];10 multilayer measured at 300 and 77 K. The current #ow is always in the "lm plane.
magnetic moment at di!erent constant applied "elds is shown in Fig. 4. Fig. 5 shows the temperature dependence of the saturation "eld H of Fe/FeSi multilayer determined from the m(¹) plots at "xed H. The steep increase of H on cooling is evident. Fig. 5 also shows tentative decomposition of H based on the measured temperature dependence of remanent magnetization, and on the model of bilinear (antiparallel magnetization arrangement) and biquadratic (perpendicular magnetization arrangement) exchange [1}3]. In this model, H and H are the e!ective "elds of these two interac tions (H "H #H ); the relative remanence, M /M increases with the ratio H /H . The increase of H at low temperatures is mostly ascribed to biquadratic component H (in accordance with Refs. [1,2]). This interpreta tion of high remanence is, however, not corroborated by FMR measurements at low temperatures. The splitting of the two FMR modes should be equal to H in this model,
Fig. 5. Temperature dependence of H and its model compo1 nents H (bilinear) and H (biquadratic).
i.e. it should also increase rapidly at low temperatures. Our measurements show only a slight increase of the FMR mode splitting on cooling, however, a large variation of the mode intensity and line width was observed. This indicates that the increasing remanence may have other sources than the biquadratic exchange.
References [1] E.E. Fullerton, S.D. Bader, Phys. Rev. B 53 (1996) 5112. [2] Y. Saito, K. Inomata, J. Phys. Soc. Jpn. 67 (1998) 1138. [3] Y. Endo, O. Kitakami, Y. Shimada, Phys. Rev. B 59 (1999) 4279.