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Hydrogen-induced changes of magnetic properties of iron–chromium multilayers
Journal of Alloys and Compounds 288 (1999) 13–24
L
Hydrogen-induced changes of magnetic properties of iron–chromium multilayers 1
K. Kandasamy , M. Masuda, Y. Hayashi* Department of Material Science and Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan Received 4 December 1998; received in revised form 3 February 1999
Abstract The influence of hydrogen implantation on the structural, magnetic and electrical properties of iron–chromium (Fe / Cr) multilayers was investigated. Hydrogen implantation increases the bi-layer thickness, lattice constant, remanent magnetic moment and saturation resistivity but reduces the saturation magnetic field, magnetoresistance and the giant magnetoresistance of multilayers. Changes in the saturation magnetic moments of multilayers after hydrogen implantation are also observed. 1999 Elsevier Science S.A. All rights reserved. Keywords: Fe / Cr multilayers; GMR; Hydrogen implantation; Interlayer coupling; Magnetoresistance
1. Introduction The electrical resistance of antiferromagnetically coupled multilayers drop sharply when the magnetization of adjacent magnetic layers are aligned parallel by applying an external magnetic field. This phenomenon has been referred to as the giant magnetoresistance (GMR) and was first discovered in Fe / Cr / Fe sandwiches grown by molecular beam epitaxy [1]. Subsequently the same effect was found in single crystal [2] and polycrystalline [3] Fe / Cr multilayers. The GMR effect has generally been attributed to the spin-dependent scattering of conduction electrons at the interfaces of antiferromagnetically coupled multilayers. Since the discovery of the GMR in Fe / Cr multilayers, the basic academic interest in understanding the nature of the exchange coupling between the magnetic layers and the technological interest due to the possibilities of using them in magnetic sensors and electronic switching elements have attracted the attention of researchers. Many other magnetic multilayers have also been investigated [4] and theoretical models based on microscopic mechanism of interface scattering were proposed in order to explain the results of experimental investigations [5]. In spite of the large quantity of work on different magnetic multilayered
*Corresponding author. Fax: 181-92-632-0434. 1 Permanent address: Department of Physics, University of Jaffna, Jaffna, Sri Lanka.
structures during the past 10 years the present understanding of the GMR phenomenon is still incomplete, In this report the magnetoresistance (MR), magnetoresistance ratio (MRR) and the GMR of a multilayer are defined as ( r – rs ), ( r – rs ) /rs and maximum of ( r – rs ) /rs , respectively, where r is the resistivity of the multilayer, which is field dependent, and rs the saturation resistivity. It was reported that both the GMR and saturation field of Fe / Cr multilayers oscillate as a function of Cr spacer layer ˚ respectively, thickness with the first peaks at 9 and 7 A, ˚ [6]. A strong deposition temperaand the period of 18 A ture dependence of GMR [7] and the independent nature of the saturation field on the number of bi-layers [8] in Fe / Cr multilayered system were also reported. The reported room temperature GMR corresponding to a Cr spacer layer ˚ in sputter deposited Fe / Cr multilayers thickness of 9 A varies from values as low as 0.42% [9] to values as high as 7.7% [7]. The deviations between the GMR values observed in different experimental studies have generally been attributed to variation in interface quality of Fe / Cr multilayers used in these studies. Fullerton et al. found that in sputter deposited Fe / Cr multilayers the GMR value increases with increasing interfacial roughness [10]. On the contrary Rensing et al. found that in sputter deposited Fe / Cr multilayers the GMR value decreases with increasing interfacial roughness [11]. Lucinski et al. [12] stressed that a good crystalline quality of multilayers with smooth interfaces is important for obtaining large GMR. Further depending on the annealing temperature and time of annealing both a rise and fall in the GMR of sputter
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00081-X
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K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
deposited Fe / Cr multilayers were also reported [9,12]. However, the interfacial quality that influence the GMR of multilayers in general is not yet clearly identified. It was reported that inclusion of hydrogen into the metallic multilayers offers possibilities to change structural properties such as flatness of the layers, roughness of the interfaces and electrical properties of multilayers [13]. Therefore, one could expect that the inclusion of hydrogen into the magnetic multilayers will have an influence on the GMR and may throw some light on the properties which influence the GMR of magnetic multilayers. Previously such influence of hydrogen on the properties of Fe / Ce [14], Fe / Nb [15,16] and Co / Cu [17] multilayers was reported. In the first two systems the layers of nonmagnetic atoms are capable of absorbing hydrogen exothermically. It was reported that in such multilayers hydrogen predominantly dissolves in the non-magnetic layers [15] and affects the GMR properties, such as inter layer coupling and magnetoresistance via changing the electronic properties of the non-magnetic layers. In the third multilayer system, which is similar to the Fe / Cr system investigated in this work, both magnetic and nonmagnetic layers absorb hydrogen endothermically. It was reported that hydrogen segregates at the interfaces in layered structures formed by two metals, which absorb hydrogen endothermically (like Fe and Cr) [18]. The hydrogen at the interfaces is capable of influencing its structure [19] and thus may influence the GMR effect dominantly via altering the interfacial properties. It was reported in the Co / Cu multilayer study that hydrogen implantation in Co / Cu multilayers increases the remanence and saturation resistivity but reduces the magnetoresistance ratio, coercive force, saturation magnetic field and saturation magnetic moment [17]. Furthermore, this study reports that the magnetoresistance of some multilayers was reduced while that of others was relatively unaffected by hydrogen implantation. In this work, hydrogen inclusion into Fe / Cr multilayers with a range of Cr ˚ spanning the first peak of the layers thicknesses (7–15 A) GMR oscillation was performed by two methods, and the influence on the GMR effect was investigated. In one method a group of Fe / Cr multilayers were sputter deposited in an atmosphere of argon gas and later hydrogen was implanted. In the other method another group of Fe / Cr multilayers were sputter deposited in a mixed atmosphere of argon and hydrogen gases and later hydrogen was implanted.
previously on a glass substrate. The pressure of the residual gas in the sputtering chamber prior to sputtering was better than 1.5310 26 Torr. Research grade iron, and chromium target sources were used. Both the iron and chromium targets were sputter cleaned in an argon gas atmosphere for over 45 min before the preparation of multilayers. Two groups of multilayers were prepared. In one group (here after referred to as group 1) 30 multilayers were sputter deposited in an atmosphere of argon gas of pressure 5310 23 Torr. In the other group (here after referred to as group 2) over 30 multilayers were sputter deposited in a mixture of argon and hydrogen gas with a hydrogen partial pressure of 10 – 4 Torr and a total pressure of 5310 23 Torr. In both groups the Fe layers were ˚ / s but the Cr layers deposited at a growth rate of 0.18 A ˚ / s in group 1 and were deposited at a growth rate of 0.2 A ˚ / s in group 2. The growth rates were recorded using 0.5 A quartz crystal monitors. The hydrogen implantation was performed in another high vacuum chamber with an ion gun using purified hydrogen gas. The pressure of the residual gas in this system prior to the implantation was better than 1.5310 27 Torr. The ion current and the energy during the implantation were kept constant at 1.6310 26 A and 4 keV, respectively. During the implantation the pressure of hydrogen gas in the specimen chamber was about 1.5310 26 Torr. The crystallographic structure and periodicity of the layers in the multilayered structures were studied in a rotating sample type X-ray diffractometer ˚ of a cobalt source. The (XRD) using the Ka line (1.790 A) topographical images of multilayers were recorded in an atomic force microscope (AFM) using a silicon tip on a cantilever with a spring constant of 3 N / m. Magnetization measurements were performed in a vibrating sample magnetometer (VSM). The electrical resistivity was measured using a conventional four-terminal probe with inplane direct current and placed in an in-plane magnetic field. The magnitude of the direct current used in the measurement was usually 1 mA. In the case of hydrogenimplanted multilayers the electrical resistivity measurements and X-ray spectra were usually recorded nearly 24 h after implantation. All measurements were carried out at room temperature, which is about 295 K.
3. Result and discussion
3.1. Structural characterization
2. Experimental The Fe / Cr multilayers, with an Fe layer thickness of 22 ˚ and Cr layer thicknesses ranging from 7 to 15 A ˚ were A deposited in a three-target r.f. magnetron sputtering system ˚ thickness deposited onto a chromium-buffer layer of 50 A
In the XRD studies u –2u spectra were recorded at 2u ranges 8–1 and 80–508. In general the small angle (8–18) spectra and large angle (80–508) spectra recorded for group 1 multilayers are qualitatively identical, respectively, to that of group 2 multilayers. The small angle spectra contain a strong first-order peak and a weak second-order peak. A typical small angle spectrum is shown in Fig. 1. A
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
Fig. 1. Small angle X-ray diffraction spectra of the multilayer Cr(50 ˚ / [Fe(22 A) ˚ / Cr(9 A)]330 ˚ A) before (solid line) and after (dotted line) the implantation of 10 17 hydrogen atoms.
strong first-order peak and a second-order peak originating from the layered structure of the multilayers confirms the good periodicity of the bi-layers and flatness of the interfaces. The values estimated for the bi-layer thickness of the multilayers from the position of the first-order peaks were typically within 10% of that inferred from the growth rate recorded using quartz crystal monitors. The width at half height of the first-order peak accounts for an average fluctuation of 8% in the thickness of the bi-layers. The peaks in the small angle spectra were shifted to lower angles by hydrogen implantation. This suggests an increase of the bi-layer thickness on hydrogen implantation. For example, in the case of the spectrum shown in Fig. 1 the first-order peak was shifted from 3.20 to 3.148. Correspondingly the bi-layer thickness increases from 31.8 to ˚ This may be due to the solution of hydrogen in the 32.6 A. lattice of multilayers and the accompanying expansion. Furthermore, the comparison of the small angle spectra reveals that the peaks broaden and decrease in intensity due to hydrogen implantation. This may be due to structural damages introduced by the hydrogen implantation. The large angle spectra contain only a strong peak around 2u value of 51.78 for multilayers of group 1 and 51.658 for multilayers of group 2. These values are slightly lower than the 2u value expected for the (110) reflection in the bcc structure of bulk Cr or Fe. This observation suggests that the multilayers are crystalline, probably with a bcc structure, and that the lattice constants are larger than that of bulk Cr and Fe. Expanded lattice of metals in solid films has been a commonly reported phenomenon. A further shift of the (110) peak towards lower angle in the spectra of group 2 multilayers indicates that the (110) planes of these multilayers are separated more than in the group 1 multilayers. This may be due to solution of hydrogen in the interstitial sites of the lattice and the accompanying lattice expansion during the growth of these multilayers. The excess 2u values of the (110) peaks lead ˚ for the lattice constants in to values of 2.901 and 2.903 A groups 1 and 2, respectively. The width at half height of the (110) peak varies in the range 0.6–0.8 8 for multilayers of group 1 and in the range
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0.7–0.98 for group 2; but the intensity of the (110) peak for group 2 is nearly two times larger than that of group 1. The small but significant difference in the width at half height of the (110) peak suggests that the grain size in multilayers of group 2 is smaller than in group 1 multilayers, if the broadening would only be due to the size of the grains. Scherrer’s formula for the grain size predicts an ˚ for multilayers of group 1 and average grain size of 145 A ˚ 133 A for group 2. In other words the presence of hydrogen gas in the sputtering atmosphere has resulted in the growth of small grains in the multilayers of group 2. Furthermore, the relatively large intensity of the (110) peak of the multilayers of group 2 indicates contributions to the reflected X-ray beam from a large number of grains having (110) texture parallel to the plane of these multilayers. This may be due to the increased coherence of these multilayers and may form an indication for a relatively perfect structure. The implantation of 10 17 hydrogen atoms shifts the (110) peaks in the XRD spectra of both groups of multilayers to a 2u value around 51.68. This indicates that the separation of the (110) crystal planes increases on hydrogen implantation. This may be due to the occupation of interstitial sites by the implanted hydrogen and the accompanying expansion of these lattice sites. Again the comparison of the large angle XRD spectra reveals that in addition to the shift towards lower angles, the (110) peak broadens and decreases in intensity due to hydrogen implantation. AFM pictures are shown in Fig. 2. Fig. 2a,b are typical examples of the surface topography of as-deposited multilayers of groups 1 and 2, respectively. The root mean square (rms) roughness of the surface that was imaged in these figures are 1.5 and 0.47 nm, respectively. In a previous study a rms roughness of 0.7 nm was reported for Fe / Cr multilayers sputter deposited in a slightly low pressure argon gas atmosphere onto an Fe-buffer layer [12]. It is clear from these figures that the presence of hydrogen in the sputtering gas atmosphere produces growth of multilayers with less rough surfaces. As a consequence, one could, therefore, expect relatively smooth interfaces in the multilayers of group 2. This should give rise to relatively high intensity peaks in the small angle XRD spectra of these multilayers; but no significant difference is observed for the intensity of the peaks in the small angle spectra between the multilayers belonging to groups 1 and 2. Fig. 2c,d represent the surface topographies of the multilayers (whose surface topography was shown in Fig. 2a,b, respectively) after the implantation of 10 17 hydrogen atoms. The rms roughness of the surfaces that were imaged in Fig. 2c,d are 0.81 and 0.75 nm, respectively. This shows that the hydrogen implantation makes a rougher surface smoother and a smoother surface rougher. It also suggests that prolonged implantation of hydrogen may result in multilayer surfaces with some characteristic roughness.
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Fig. 2. The surface topography of a group 1 multilayer before (a), and after the implantation of 10 17 hydrogen atoms (c). The surface topography of a group 2 multilayer before (b), and after the implantation of 10 17 hydrogen atoms (d).
˚ / [Fe(22 A) ˚ / Cr(9 A)]330 ˚ Fig. 3. Magnetic moment versus in-plane magnetic field loops for the multilayer Cr(50 A) of group 1 (a) and the multilayer ˚ / [Fe(22 A) ˚ / Cr(9.5 A)]330 ˚ Cr(50 A) of group 2 (b).
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
3.2. Magnetization The in-plane magnetic moment versus magnetic field curves shown in Fig. 3a,b are typical examples for the magnetization hysteresis loops recorded for multilayers of groups 1 and 2, respectively. These examples are those which exhibit the largest GMR in their respective group. The magnetization feature (i.e. gradual increase of the magnetic moment up to the saturation moment, large saturation field and small remanent magnetic moment) displayed in these curves show that the Fe layers in the multilayers are coupled antiferromagnetically. The variations of the saturation magnetic field and remanent magnetic moment with Cr layer thickness for the groups 1 and 2 multilayers are shown in Fig. 4a,b, respectively. The variation of the saturation magnetic field with Cr layer thickness within the range investigated is in agreement with previously reported results except for the position of the peak value [6]. Comparison of the results shown for
Fig. 4. Variation of saturation magnetic field (solid line) and remanent magnetic moment (dotted line) as a function of Cr layers thickness in multilayers of the group 1 (a) and multilayers of the group 2 (b).
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groups 1 and 2 in Figs. 3 and 4 reveals that the nature of the magnetization in the Fe / Cr multilayers belonging to both groups is qualitatively identical; but the saturation magnetic moment of the group 2 multilayers is relatively lower than that of the group 1 multilayers. The average magnetization of 30 multilayers studied in each group (i.e. groups 1 and 2) are, respectively, 1515 and 1360 emu cm 23 (1 emu cm 23 51000 A m 21 ). The magnetization of bulk Fe is about 1740 emu cm 23 . Due to the expanded lattice of the Fe layers in the multilayers with respect to bulk Fe, their expected magnetization is about 1680 emu cm 23 . Therefore, the percentage of reduction of the average saturation magnetic moment in the group 1 multilayers is about 10% and about 19% in the group 2 multilayers. It was reported that Fe / Cr alloys containing more than 25% Cr atoms are non-magnetic [20]. Alloying of magnetic and non-magnetic atoms at interfaces of multilayers has been a widely reported phenomenon. Therefore, the above reduction of the magnetic moment from the expected value may be due to the alloying of Fe and Cr atoms at the interfaces of the multilayers. If a monolayer of FeCr alloy is formed at each interface, the expected reduction in the saturation magnetization would be about 9.3% for a flat interface. This suggests, on average, the formation of about one Fe / Cr alloy monolayer at the interfaces of the group 1 multilayers and two atomic layers at the interfaces of the group 2 multilayers. It ˚ alloy layer is worth to note here that the formation of 9 A (four atomic layers) at the interfaces of Fe / Cr multilayers was previously suggested by Fullerton et al. [10]. Furthermore, an increase of the interface mixing in multilayers sputter deposited in light sputtering gas was also suggested by Scheursser et al. [21]. The enhanced formation of alloy in the case of the multilayers belonging to group 2 may result from the effect of the hydrogen present in the sputtering atmosphere or / and form the relatively high growth rate (high sputtering power) of the Cr layers in these multilayers. A set of multilayers was selected for hydrogen implantation study from both groups of multilayers. The magnetic properties of these multilayers before and after the implantation of 10 17 hydrogen atoms are given in Table 1. A typical example for the changes in the magnetic characteristics of these multilayers due to hydrogen implantation is shown in Fig. 5. The results displayed in Fig. 5 and the data given in Table 1 show an increase of the remanent magnetic moment and a decrease of the saturation magnetic field on hydrogen implantation. Furthermore, upon hydrogen implantation the saturation magnetic moment decreases in the case of the group 1 multilayers; but in the cases of the group 2 multilayers it decreases only in a few cases and in other cases it either increases or remains unchanged. However, the changes are relatively small. The XRD studies for the as-deposited multilayers of group 2 have suggested a solution of hydrogen in them during their growth; but there is no supporting evidence from Figs. 3
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
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Table 1 Magnetic properties of Fe / Cr multilayers before and after the implantation of 10 17 hydrogen atoms, where t Cr is the Cr layer thickness, Hs the saturation magnetic field, Hc the coercive force, m s the saturation magnetic moment and m r the remanent magnetic moment a ˚ t Cr (A)
7 8 9 10 11 12 13 14 15 8.5* 9* 9.5* 10* 11* 11.5* 12* 13.5* 14* 15*
Hs (kOe)
21
HC (Oe)
21
m s (mJ T )
m r (mJ T )
A
B
A
B
A
B
A
B
0.2 4.5 5.5 4 3.5 3 2.5 1.5 1.25 2.25 5.75 4.5 4 4 3.75 3.25 2 1.5 1
0.2 4 5 3.5 3 2.5 2.25 1.5 1.25 1.8 5.25 4 3.5 3.5 3.25 2.75 2 1.5 1.25
50 80 95 90 70 85 100 100 85 50 70 80 75 85 75 100 80 80 80
55 75 85 85 80 85 100 100 85 55 75 80 75 85 80 85 80 80 85
12 13.5 12 11.7 11.5 11.4 12 15 13 11.2 11.4 11.7 10.7 10.4 12.3 11.1 10.5 10.5 12.5
11.5 12.7 11.7 11.5 11.3 10.6 11.7 14 11.6 11.5 11.2 11.6 11.5 10.4 13 11.5 10.5 10.5 12.8
10.7 6 0.75 2.25 1 0.5 0.3 4.5 3.5 8 5 1.5 1.6 1.5 1.2 3 0.7 2 4
11.3 7.5 3.4 4.7 3 1.6 1.75 4.5 3.75 9 7 3.5 4.25 3.6 3.7 5.2 2 2.2 4.6
a The data given in the rows corresponding to the values of t Cr with asterisks are for multilayers belong to group 2 and others are for multilayers belong to group 1. Column A gives data of as-deposited multilayers and column B gives data of hydrogen-implanted multilayers. 1 Oe5(1000 / 4p ) A m 21 .
and 4 for the effect of dissolved hydrogen on the remanent magnetic moment and saturation magnetic field of these multilayers. This suggests that the increase of the remanent magnetic moment and the decrease of the saturation magnetic field after hydrogen implantation may not be due to the solution of hydrogen in the metal lattice. Previously an increase of the remanent magnetic moment and a decrease of saturation magnetic field of Fe / Cr multilayers after Xe 1 ion irradiation was reported [22]. Therefore, we suggest that the increase of the remanent magnetic moment and the decrease of the saturation magnetic field can be due to structural changes (damages) created in the multilayers by the implanted hydrogen. The saturation magnetic moment shows a time-dependent variation, similar to the variations previously reported for Co / Cu multilayers after hydrogen implantation [17]. That is, immediately after hydrogen implantation the saturation magnetic moment of the multilayers is higher than their magnetic moment before hydrogen implantation. Subsequently over a period of time it drops to a lower value. Creation of metal vacancies and occupation of interstitial sites by hydrogen during implantation and subsequent atomic mixing at the interfaces due to vacancy diffusion and redistribution of hydrogen to energetically favorable interfacial sites were previously suggested (in order to explain the time-dependent changes observed in Co / Cu multilayer studies) [17]. Furthermore, the different behaviour of the overall changes of the saturation magnetic moment of the multilayers belonging to group 2 may be due to the presence of thick (two atomic layers) Fe / Cr alloy layers at their interfaces. If there is a thick layer of
Fe / Cr alloy at the interfaces, the magnetic moment of these multilayers may be insensitive to atomic mixing by vacancy diffusion at the interfaces. Therefore, the present observations are also in agreement with our previous suggestions.
3.3. Magnetoresistance Fig. 6 shows the variation of MRR and MR with applied ˚ Cr and 17 A ˚ Fe magnetic field for a multilayer with 9 A layers and sputter deposited in an argon gas atmosphere. This multilayer possesses a saturation field of 6.5 kOe and exhibits a GMR value of 7.5%. Both these values are the largest values observed in this study and the value of GMR is nearly equal to the highest room temperature GMR values reported for Fe / Cr multilayers. The exchange coupling strength of this multilayer is 0.29 erg cm 22 (erg cm 22 510 23 J m 22 ), which is comparable to the corresponding value of 0.24 erg cm 22 published previously [23]. However, this multilayer is not included in group 1 ˚ but the curves because its Fe layer thickness is not 22 A; shown in Fig. 6 are representative for the variations of MRR and MR with applied magnetic field recorded in this study. The general feature of the variations of MRR is in agreement with previously reported studies for Fe / Cr multilayers [23]. The electrical resistivity of multilayers drops rapidly with increasing magnetic field up to the field at which the magnetization of the multilayer saturates. Then the resistivity continues to decrease at reduced rate with increasing magnetic field up to 7.5 kOe, at which field
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
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Fig. 6. Magnetic field-dependent variation of magnetoresistance ratio ˚ / [Fe(17 (MRR) and magnetoresistance (MR) of the multilayer Cr (50 A) ˚ / Cr(9 A)]330. ˚ A) (d) MRR; (s) MR.
Fig. 5. Magnetic moment versus in-plane magnetic field loops for the ˚ / [Fe(22 A) ˚ / Cr(9 A)]330 ˚ multilayer Cr(50 A) of group 1 (a) before (b) after the implantation of 3310 16 hydrogen atoms and (c) after the implantation of 10 17 hydrogen atoms.
saturation of the electrical resistivity was assumed. The saturation electrical resistivity for as-deposited samples of the group 1 multilayers lies within the range from 42 to 52 mV cm with an average of 47.5 mV cm. For as-deposited samples of the group 2 multilayers it lies within the range from 45 to 56 mV cm, with an average of 51.3 mV cm. These values are slightly larger than the previously published values of the room temperature saturation electrical resistivity [7]. The deviation of the electrical resistivity of the individual multilayers belong to a particular group seems to depend on the structural quality of the multi-
layers. Solution of hydrogen in metal lattices is usually accompanied by an increase of their electrical resistivity. Therefore, the large average electrical resistivity of the group 2 multilayers compared to that of the group 1 multilayers is due to the solution of hydrogen in the metal lattice of these multilayers during their growth. This is also in agreement with the shift of the (110) peak in the large angle XRD spectra of these multilayers. In general, the variation of MR and MRR with magnetic field has a bell shape, with a maximum at zero field; but in the case of multilayers with large remanent magnetic moment the variation of MR with magnetic field shows a tendency to exhibit a double peak similar to that commonly observed in the study of Co / Cu multilayers [17]. The magnitude of the coercive force observed for most of the multilayers in this study (Table 1) is comparable to that reported in the study of Co / Cu multilayers [17]. The remanent magnetic moment of Co / Cu multilayers is usually large. This suggests that a reasonably large remanent magnetic moment of the multilayers is necessary for the appearance of a double peak in the variations of MRR and MR with magnetic field. A typical example of the effect of hydrogen implantation on the variation of MRR and MR with magnetic field is shown in Fig. 7a,b, respectively. The variation of GMR and maximum MR (MMR) with Cr layer thickness and the effect of hydrogen implantation on these variations are displayed in Fig. 8a,b, respectively. In the case of the as-deposited multilayers the variation of GMR and MMR ˚ for the with Cr layer thickness has a maximum around 9 A ˚ for those of group multilayers of group 1 but around 9.5 A 2. The significance of this shift is not clear because the shift is within the experimental uncertainty. However, in
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˚ / [Fe(22 Fig. 7. Magnetic field-dependent variation of the magnetoresistance ratio, MRR (a) and the magnetoresistance, MR (b) of the multilayer Cr (50 A) 16 ˚ / Cr(9 A)]330 ˚ A) before and after hydrogen implantation. (s) Before implantation; (^) after the implantation of 3310 hydrogen atoms; (h) after the implantation of 10 17 hydrogen atoms.
the as-deposited multilayers of group 2 the maximum saturation field and maximum GMR were observed for ˚ Cr layers, respectively. In multilayers with 9 and 9.5 A group 1 both maxima were observed in one and the same ˚ Cr layers. In other words, both multilayer with 9 A ˚ of Cr layer thickness; but in a maxima appear around 9 A previous study conducted at 4.5 K the maxima of the saturation field and the GMR were well separated and ˚ thick Cr layers, respectively [6]. observed in 7 and 9 A The reason for the coincidence (or near coincidence) of the ˚ maxima of saturation magnetic field and GMR around 9 A thick Cr layers in this study is not clear. The variation of MMR with Cr layer thickness shows a broad maximum ˚ for both groups of multilayers. The maximum around 10 A values of MMR for multilayers of both groups are also nearly the same. Both MMR and GMR are reduced by hydrogen implantation. The reduction of MMR decreases as Cr layer ˚ Previously it was reported that thickness approaches 15 A. the MMR dropped by hydrogen implantation in a Co / Cu ˚ but it multilayer with Cu layers of thickness 11.1 A, remained unchanged in a Co / Cu multilayer with Cu layers ˚ [17]. The former multilayer belongs to of thickness 23.8 A the first peak of antiferromagnetic coupling of Co / Cu multilayers, where the coupling is strong. The latter multilayer belongs to the second peak of antiferromagnetic coupling, where the coupling is relatively weak. This
means that the reduction of MMR by hydrogen implantation seems to depend on the strength of antiferromagnetic coupling. In general, a reduction of GMR is due to a reduction of MMR and an increase of the saturation resistivity. In the case of the multilayers investigated, hydrogen implantation increased the saturation resistivities of all multilayers. This increase may be partially due to solution of hydrogen in the interstitial sites of the multilayer lattices and (partially due to the) structural damages created by hydrogen implantation. The average values of the saturation resistivity for multilayers of group 1 after implantation of 3310 16 and 10 17 hydrogen atoms were 50.1 and 52.1 mV cm, respectively. For multilayers of group 2 the average values were 54.3 and 56.4 mV cm, respectively. Some multilayers after hydrogen implantation studies were placed in a vacuum of 2310 27 Torr for nearly 40 h and no significant changes in their magnetoresistance, saturation electrical resistivity and magnetization was found. In general the magnetization of magnetic multilayers can be divided into two portions. One portion is from zero magnetic moment to remanent magnetic moment, m r , and the other portion is from remanent magnetic moment to saturation magnetic moment, m s . The first portion is due to ferromagnetically coupled volume of the multilayer. The second portion is due to antiferromagnetically coupled volume of the multilayer. The antiferromagnetically cou-
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
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Fig. 8. Variation of the giant magnetoresistance, GMR (a,c) and maximum of magnetoresistance, MMR (b,d) as a function of Cr layer thickness in multilayers of the group 1 (a,b) and multilayers of the group 2 (c,d). (d) Before implantation; (s) after the implantation of 3310 16 hydrogen atoms; (j) after the implantation of 10 17 hydrogen atoms.
pled volume possesses a magnetic moment of (m s 2m r ) and has been identified to have some influence on the magnetoresistance of the multilayer [21]. A linear correlation between the GMR and the fraction of the antiferromagnetically coupled volume of a multilayer was previously proposed [24]. According to Hall et al. [25] the GMR of a multilayer is related to the fraction of the antiferromagnetically coupled volume, (m s 2m r ) /m s , of the multilayer by the equation GMR 5 h1 2 (m r /m s )jGMR ideal
(1)
Where GMR ideal is the value of GMR that would have been expected if the entire volume of the multilayer were coupled antiferromagnetically. Hydrogen implantation into a multilayer generally increases the saturation electrical resistivity and reduces the fraction of the antiferromagnet-
ically coupled volume. It is possible to introduce a correction term into Eq. (1) to account for the effect of the saturation resistivity increase on the GMR. With such a correction term in Eq. (1), the GMR will be given by GMR 5 h1 2 (m r /m s )j( r 0S /r *S )GMR ideal
(2)
Here r s0 is the saturation electrical resistivity of the as-deposited multilayer and r *s is that of the hydrogenimplanted multilayer. The estimated values of GMR ideal using the Eq. (1) for as-deposited multilayers and Eq. (2) for hydrogen-implanted multilayers are given in Table 2 together with other relevant quantities. The relatively good agreement between the estimated GMR ideal for as-deposited and hydrogen-implanted samples of the individual multilayer supports the importance of the fraction of antiferromagnetically coupled volume in GMR effects of
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
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Table 2 Magnetoelectrical properties for Fe / Cr multilayers before and after the implantation of 3310 16 hydrogen atoms, where t Cr is the Cr layer thickness, m s the saturation magnetic moment, m r the remanent magnetic moment, GMR ex the experimentally estimated values for the GMR and GMR ideal the estimated values for the GMR of ideal multilayer a ˚ t Cr (A) A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B
8 9 10 12 13 14 15 8.5* 9* 9.5* 10* 10.5* 11* 11.5* 12* 13* 13.5* 14* 15*
rsat (mV cm)
(12m r /m s )
GMR ex
GMR ideal
45.12 47.03 45.62 47.03 51.82 53.46 47.48 50.67 46.83 49.82 42.57 44.98 44.57 48.5 55.36 56.99 50.98 54.11 49.51 54.05 53.71 56.62 53.39 57.05 52.31 55.98 51.6 53.47 55.53 58 48.49 50.65 50.46 54 48.6 53.42 45.61 48.16
0.5555 0.4297 0.9375 0.7917 0.8077 0.693 0.9561 0.9273 0.975 0.9149 0.7 0.7 0.731 0.727 0.2857 0.2609 0.561 0.469 0.8718 0.784 0.8505 0.712 0.8372 0.686 0.8558 0.7228 0.9 0.784 0.73 0.617 0.945 0.9027 0.933 0.8878 0.81 0.842 0.68 0.68
5.23 3.82 6.65 5.43 6.02 5.02 5.1 4.46 5.16 4.47 3.7 3.33 3.41 3.16 2.5 1.72 4.9 3.52 5.9 4.9 5.4 4.4 5.63 4.41 5.75 4.58 5.4 4.33 4.9 3.85 4.85 3.87 4.28 3.6 3.8 3.35 3.12 2.75
9.47 9.27 7.01 7.43 7.45 7.47 5.33 5.13 5.29 5.2 5.28 5.03 4.71 4.73 8.75 6.79 8.73 7.97 6.77 6.82 6.35 6.51 6.73 6.87 6.72 6.78 6 5.72 6.71 6.52 5.13 4.48 4.59 4.35 4.69 4.37 4.58 4.27
a
The data given in the rows corresponding to the values of t Cr with asterisks are for multilayers belong to group 2 and others are for multilayers belong to group 1. Row A gives data of as-deposited multilayers and row B gives data of hydrogen-implanted multilayers.
multilayers. Furthermore, it helps to recognize the effect of the saturation electrical resistivity of multilayers on the GMR. Our results suggest that the effect of hydrogen inclusion on the variation of GMR with Cr layer thickness is related to the product of the magnetic moment of the antiferromagnetically coupled volume and the saturation magnetic field of multilayers. In order to analyse this further, the GMR values of as-deposited multilayers are plotted against the respective dimensionless quantity h(m s 2m r )Hs /m s H Is j in Fig. 9. Here H sI is the saturation magnetic field of the multilayer with the largest GMR for the same Fe layer thickness. The quantity h(m s 2m r )Hs /m s H Is j can be considered as a measure of the relative antiferromagnetic coupling strength of multilayers and will here after be denoted by JR . In Fig. 9 the GMR of as-deposited
multilayers of group 1 (solid) and group 2 (open) are plotted against JR . The GMR values used for multilayers of group 2 are corrected for the increase of the average saturation electrical resistivity with respect to that of the group 1 multilayers using the expression GMR( r 2s /r 1s ), where r 1s and r 2s are, respectively, the average saturation electrical resistivity of the group 1 and 2 multilayers. The approximate curves drawn to illustrate the variation of GMR with JR for the group 1 and 2 multilayers are very similar, and this suggests no major influence of dissolved hydrogen on the magnetic properties of Fe / Cr multilayers. Furthermore, both curves illustrate the tendency to saturation of GMR at large values of JR . The value of JR at which the GMR saturates for group 2 is slightly lower than that of group 1. This may be due to the easiness of the interfacial magnetization of group 2 compared to that of
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
Fig. 9. Variations of GMR as a function of the quantity, JR 5h12(m r / m s )j(Hs /H Is ) for as-deposited multilayers of group 1 and 2. (d) Group 1; (s) group 2.
23
approximate curves are drawn for each set of data to illustrate the variation of GMR with JR and the effect of hydrogen implantation. In all cases the general feature of the variation of GMR with JR is qualitatively the same. Hydrogen implantation reduced the saturation value of the GMR in all cases. The amount of reduction increases with increasing doses of hydrogen implantation. Furthermore, the value of JR at which the GMR saturates also decreases with increasing doses of hydrogen implantation. These observations suggest that the hydrogen implantation, in general, reduces the antiferromagnetic interlayer coupling and the antiferromagnetically coupled domains at the interfaces. The changes in slope of the approximate curves for small values of JR may be related to the effect of hydrogen implantation on the nature of the interfacial magnetization.
4. Conclusion group 1. The relatively large saturation value of GMR for the group 1 multilayers may be due to their relatively rougher interfaces. In Fig. 10a,b the corrected GMR values of as-deposited and hydrogen-implanted multilayers of groups 1 and 2 are plotted against the corresponding quantity JR . Separate
Fig. 10. Variations of GMR as a function of the quantity, JR 5h12(m r / m s )j(Hs /H Is ) for as-deposited and hydrogen-implanted multilayers of group 1 (a) and group 2 (b). (s) Before implantation; (^) after the implantation of 3310 16 hydrogen atoms; (h) after the implantation of 10 17 hydrogen atoms.
The multilayers sputter deposited in this study possess a bcc lattice with (110) texture parallel to the plane of the multilayers and lattice constants larger than that of bcc bulk Cr. The presence of hydrogen gas in the sputtering atmosphere-induced layer growth with relatively small grain sizes, large lattice constants and smooth surfaces. Hydrogen implantation increases the bi-layer thickness and the lattice constant of the multilayers due to solution of hydrogen in the lattice of these multilayers. Hydrogen implantation reduces the intensity of the peaks in the XRD spectra and changes the surface roughness of the multilayers. The magnetization behaviour of multilayers sputter deposited in argon gas and in a mixture of argon and hydrogen gases are identical. This suggest that the dissolved hydrogen in the multilayers during their growth has no major effect on the magnetic properties of the multilayers. The saturation magnetic moment of multilayers is smaller than that expected from the Fe content of the multilayers. This is due to the formation of Fe / Cr alloy at the interfaces. The hydrogen implantation increases the remanent magnetic moment but reduces the saturation magnetic field. The causes of these changes are not clear. The saturation magnetic moment of multilayers exhibits time-dependent changes after hydrogen implantation. These changes are due to the formation of metal atom vacancies and subsequent atomic mixing at the interfaces by vacancy diffusion and solution of hydrogen at interstitial sites and subsequent diffusion of hydrogen to the energetically favourable interfacial sites. The observed values for maximum GMR and corresponding antiferromagnetic coupling strength are in agreement with previously published data [7]; but in contrast to previously published results [6] we observed a coincidence of the maxima of the GMR and the saturation magnetic field variations with Cr layer thickness. The reason for this coincidence has not been identified. The magnetoresistance
24
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
and GMR of multilayers were reduced but the saturation resistivity was increased by hydrogen implantation. The decrease of the magnetoresistance may be due to the reduction of antiferromagnetically coupled magnetic domains at the interfaces by the implanted hydrogen. The magnitude of the decrease in magnetoresistance is relatively larger for multilayers with strong antiferromagnetic coupling. The reduction of the GMR is due to a decrease of the magnetoresistance and an increase of the saturation resistivity upon hydrogen implantation. There is fair agreement between the experimental data and Eq. (2), which relates the GMR, saturation resistivities of as-deposited and hydrogen-implanted multilayers to the fraction of antiferromagnetically coupled volume of the multilayers. This indicates the importance of the fraction of antiferromagnetically coupled volume and saturation resistivity for the GMR of multilayers.
Acknowledgements One of the authors (K.K.) would like to acknowledge a Fellowship from the Japan Society for the Promotion of Science (JSPS) during the course of this work. This work has been supported in part by a Grant–in Aid for Scientific Research on Priority Areas A of ‘New Protium Function’ from the Ministry of Education, Science, Sports and Culture.
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[3] S.S.P. Parkin, S. Fan, N. More, K.P. Roche, J. Appl. Phys. 67 (1990) 5931. [4] S.S.P. Parkin, Phys. Rev. Lett. 67 (1991) 3598. [5] R.K. Nesbet, J. Magn. Magn. Mater. 159 (1996) L.17. [6] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [7] S.S.P. Parkin, B.R. York, Appl. Phys. Lett. 62 (1993) 1842. [8] S.S.P. Parkin, A. Mansour, G.P. Felcher, Appl. Phys. Lett. 58 (1991) 1473. [9] E.M. Ho, A.K. Petford-Long, J. Magn. Magn. Mater. 156 (1996) 65. [10] E.E. Fullerton, D.M. Kelly, J. Guimpel, I.K. Schuller, Y. Bruynseraede, Phys. Rev. Lett. 68 (1992) 859. [11] N.M. Rensing, A.P. Payne, B.M. Clemens, J. Magn. Magn. Mater. 121 (1993) 436. [12] T. Lucinski, D. Elefant, G. Reiss, P. Verges, J. Magn. Magn. Mater. 162 (1996) 29. [13] A. Weidinger, D. Nagengast, Ch. Rehm, F. Klose, B. Pietzak, Thin Solid Films 275 (1996) 48. [14] F. Klose, J. Thiele, A. Schurian, O. Schulte, M. Steins, O. Bremert, W. Felsch, Z. Phys. B 90 (1993) 79. [15] D. Nagengast, J. Erxmeyer, F. Klose, Ch. Rehm, P. Kuschnerus, G. Dortmann, A. Weidinger, J. Alloys Compounds 231 (1995) 307. [16] D. Nagengast, Ch. Rehm, F. Klose, A. Weidinger, J. Alloys Compounds 253–254 (1997) 347. [17] K. Kandasamy, M. Masuda, Y. Hayashi, J. Alloys Compounds (accepted). [18] S. Yamaguchi, K. Takahiro, S. Nagata, S. Yamaguchi, in: Research Report on Metallic Artificial Superlattice, Ministry of Education, Science and Culture, Japan, 1992, p. 108. [19] S. Yamamoto, P. Goppelt-Langer, H. Naramoto, Y. Aoki, H. Takeshita, J. Alloys Compounds 231 (1995) 310. [20] D.M. Kelly, I.K. Schuller, V. Korenivski, K.V. Rao, K.K. Larsen, J. Bottiger, E.M. Gyorgy, R.B. van Dover, Phys. Rev. B 50 (1994) 3481. [21] S. Schuersser, G. Rupp, A. Hubert, J. Magn. Magn. Mater. 166 (1997) 267. [22] V. Korenivski, K.V. Rao, D.M. Kelly, I.K. Schuller, K.K. Larsen, J. Bottiger, J. Magn. Magn. Mater. 140–144 (1995) 549. [23] S.S.P. Parkin, B. Heinrich, J.A.C. Bland, in: Ultrathin Magnetic Structures, Vol. 11, 1994, p. 148. [24] G. Rupp, K. Schuster, J. Magn. Magn. Mater. 121 (1993) 416. [25] M.J. Hall, E.D. Whitton, D.B. Jardine, R.E. Somekh, J.E. Evetts, J.A. Leake, Thin Solid Films 275 (1996) 195.
L
Hydrogen-induced changes of magnetic properties of iron–chromium multilayers 1
K. Kandasamy , M. Masuda, Y. Hayashi* Department of Material Science and Engineering, Kyushu University, Hakozaki, Fukuoka 812, Japan Received 4 December 1998; received in revised form 3 February 1999
Abstract The influence of hydrogen implantation on the structural, magnetic and electrical properties of iron–chromium (Fe / Cr) multilayers was investigated. Hydrogen implantation increases the bi-layer thickness, lattice constant, remanent magnetic moment and saturation resistivity but reduces the saturation magnetic field, magnetoresistance and the giant magnetoresistance of multilayers. Changes in the saturation magnetic moments of multilayers after hydrogen implantation are also observed. 1999 Elsevier Science S.A. All rights reserved. Keywords: Fe / Cr multilayers; GMR; Hydrogen implantation; Interlayer coupling; Magnetoresistance
1. Introduction The electrical resistance of antiferromagnetically coupled multilayers drop sharply when the magnetization of adjacent magnetic layers are aligned parallel by applying an external magnetic field. This phenomenon has been referred to as the giant magnetoresistance (GMR) and was first discovered in Fe / Cr / Fe sandwiches grown by molecular beam epitaxy [1]. Subsequently the same effect was found in single crystal [2] and polycrystalline [3] Fe / Cr multilayers. The GMR effect has generally been attributed to the spin-dependent scattering of conduction electrons at the interfaces of antiferromagnetically coupled multilayers. Since the discovery of the GMR in Fe / Cr multilayers, the basic academic interest in understanding the nature of the exchange coupling between the magnetic layers and the technological interest due to the possibilities of using them in magnetic sensors and electronic switching elements have attracted the attention of researchers. Many other magnetic multilayers have also been investigated [4] and theoretical models based on microscopic mechanism of interface scattering were proposed in order to explain the results of experimental investigations [5]. In spite of the large quantity of work on different magnetic multilayered
*Corresponding author. Fax: 181-92-632-0434. 1 Permanent address: Department of Physics, University of Jaffna, Jaffna, Sri Lanka.
structures during the past 10 years the present understanding of the GMR phenomenon is still incomplete, In this report the magnetoresistance (MR), magnetoresistance ratio (MRR) and the GMR of a multilayer are defined as ( r – rs ), ( r – rs ) /rs and maximum of ( r – rs ) /rs , respectively, where r is the resistivity of the multilayer, which is field dependent, and rs the saturation resistivity. It was reported that both the GMR and saturation field of Fe / Cr multilayers oscillate as a function of Cr spacer layer ˚ respectively, thickness with the first peaks at 9 and 7 A, ˚ [6]. A strong deposition temperaand the period of 18 A ture dependence of GMR [7] and the independent nature of the saturation field on the number of bi-layers [8] in Fe / Cr multilayered system were also reported. The reported room temperature GMR corresponding to a Cr spacer layer ˚ in sputter deposited Fe / Cr multilayers thickness of 9 A varies from values as low as 0.42% [9] to values as high as 7.7% [7]. The deviations between the GMR values observed in different experimental studies have generally been attributed to variation in interface quality of Fe / Cr multilayers used in these studies. Fullerton et al. found that in sputter deposited Fe / Cr multilayers the GMR value increases with increasing interfacial roughness [10]. On the contrary Rensing et al. found that in sputter deposited Fe / Cr multilayers the GMR value decreases with increasing interfacial roughness [11]. Lucinski et al. [12] stressed that a good crystalline quality of multilayers with smooth interfaces is important for obtaining large GMR. Further depending on the annealing temperature and time of annealing both a rise and fall in the GMR of sputter
0925-8388 / 99 / $ – see front matter 1999 Elsevier Science S.A. All rights reserved. PII: S0925-8388( 99 )00081-X
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K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
deposited Fe / Cr multilayers were also reported [9,12]. However, the interfacial quality that influence the GMR of multilayers in general is not yet clearly identified. It was reported that inclusion of hydrogen into the metallic multilayers offers possibilities to change structural properties such as flatness of the layers, roughness of the interfaces and electrical properties of multilayers [13]. Therefore, one could expect that the inclusion of hydrogen into the magnetic multilayers will have an influence on the GMR and may throw some light on the properties which influence the GMR of magnetic multilayers. Previously such influence of hydrogen on the properties of Fe / Ce [14], Fe / Nb [15,16] and Co / Cu [17] multilayers was reported. In the first two systems the layers of nonmagnetic atoms are capable of absorbing hydrogen exothermically. It was reported that in such multilayers hydrogen predominantly dissolves in the non-magnetic layers [15] and affects the GMR properties, such as inter layer coupling and magnetoresistance via changing the electronic properties of the non-magnetic layers. In the third multilayer system, which is similar to the Fe / Cr system investigated in this work, both magnetic and nonmagnetic layers absorb hydrogen endothermically. It was reported that hydrogen segregates at the interfaces in layered structures formed by two metals, which absorb hydrogen endothermically (like Fe and Cr) [18]. The hydrogen at the interfaces is capable of influencing its structure [19] and thus may influence the GMR effect dominantly via altering the interfacial properties. It was reported in the Co / Cu multilayer study that hydrogen implantation in Co / Cu multilayers increases the remanence and saturation resistivity but reduces the magnetoresistance ratio, coercive force, saturation magnetic field and saturation magnetic moment [17]. Furthermore, this study reports that the magnetoresistance of some multilayers was reduced while that of others was relatively unaffected by hydrogen implantation. In this work, hydrogen inclusion into Fe / Cr multilayers with a range of Cr ˚ spanning the first peak of the layers thicknesses (7–15 A) GMR oscillation was performed by two methods, and the influence on the GMR effect was investigated. In one method a group of Fe / Cr multilayers were sputter deposited in an atmosphere of argon gas and later hydrogen was implanted. In the other method another group of Fe / Cr multilayers were sputter deposited in a mixed atmosphere of argon and hydrogen gases and later hydrogen was implanted.
previously on a glass substrate. The pressure of the residual gas in the sputtering chamber prior to sputtering was better than 1.5310 26 Torr. Research grade iron, and chromium target sources were used. Both the iron and chromium targets were sputter cleaned in an argon gas atmosphere for over 45 min before the preparation of multilayers. Two groups of multilayers were prepared. In one group (here after referred to as group 1) 30 multilayers were sputter deposited in an atmosphere of argon gas of pressure 5310 23 Torr. In the other group (here after referred to as group 2) over 30 multilayers were sputter deposited in a mixture of argon and hydrogen gas with a hydrogen partial pressure of 10 – 4 Torr and a total pressure of 5310 23 Torr. In both groups the Fe layers were ˚ / s but the Cr layers deposited at a growth rate of 0.18 A ˚ / s in group 1 and were deposited at a growth rate of 0.2 A ˚ / s in group 2. The growth rates were recorded using 0.5 A quartz crystal monitors. The hydrogen implantation was performed in another high vacuum chamber with an ion gun using purified hydrogen gas. The pressure of the residual gas in this system prior to the implantation was better than 1.5310 27 Torr. The ion current and the energy during the implantation were kept constant at 1.6310 26 A and 4 keV, respectively. During the implantation the pressure of hydrogen gas in the specimen chamber was about 1.5310 26 Torr. The crystallographic structure and periodicity of the layers in the multilayered structures were studied in a rotating sample type X-ray diffractometer ˚ of a cobalt source. The (XRD) using the Ka line (1.790 A) topographical images of multilayers were recorded in an atomic force microscope (AFM) using a silicon tip on a cantilever with a spring constant of 3 N / m. Magnetization measurements were performed in a vibrating sample magnetometer (VSM). The electrical resistivity was measured using a conventional four-terminal probe with inplane direct current and placed in an in-plane magnetic field. The magnitude of the direct current used in the measurement was usually 1 mA. In the case of hydrogenimplanted multilayers the electrical resistivity measurements and X-ray spectra were usually recorded nearly 24 h after implantation. All measurements were carried out at room temperature, which is about 295 K.
3. Result and discussion
3.1. Structural characterization
2. Experimental The Fe / Cr multilayers, with an Fe layer thickness of 22 ˚ and Cr layer thicknesses ranging from 7 to 15 A ˚ were A deposited in a three-target r.f. magnetron sputtering system ˚ thickness deposited onto a chromium-buffer layer of 50 A
In the XRD studies u –2u spectra were recorded at 2u ranges 8–1 and 80–508. In general the small angle (8–18) spectra and large angle (80–508) spectra recorded for group 1 multilayers are qualitatively identical, respectively, to that of group 2 multilayers. The small angle spectra contain a strong first-order peak and a weak second-order peak. A typical small angle spectrum is shown in Fig. 1. A
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
Fig. 1. Small angle X-ray diffraction spectra of the multilayer Cr(50 ˚ / [Fe(22 A) ˚ / Cr(9 A)]330 ˚ A) before (solid line) and after (dotted line) the implantation of 10 17 hydrogen atoms.
strong first-order peak and a second-order peak originating from the layered structure of the multilayers confirms the good periodicity of the bi-layers and flatness of the interfaces. The values estimated for the bi-layer thickness of the multilayers from the position of the first-order peaks were typically within 10% of that inferred from the growth rate recorded using quartz crystal monitors. The width at half height of the first-order peak accounts for an average fluctuation of 8% in the thickness of the bi-layers. The peaks in the small angle spectra were shifted to lower angles by hydrogen implantation. This suggests an increase of the bi-layer thickness on hydrogen implantation. For example, in the case of the spectrum shown in Fig. 1 the first-order peak was shifted from 3.20 to 3.148. Correspondingly the bi-layer thickness increases from 31.8 to ˚ This may be due to the solution of hydrogen in the 32.6 A. lattice of multilayers and the accompanying expansion. Furthermore, the comparison of the small angle spectra reveals that the peaks broaden and decrease in intensity due to hydrogen implantation. This may be due to structural damages introduced by the hydrogen implantation. The large angle spectra contain only a strong peak around 2u value of 51.78 for multilayers of group 1 and 51.658 for multilayers of group 2. These values are slightly lower than the 2u value expected for the (110) reflection in the bcc structure of bulk Cr or Fe. This observation suggests that the multilayers are crystalline, probably with a bcc structure, and that the lattice constants are larger than that of bulk Cr and Fe. Expanded lattice of metals in solid films has been a commonly reported phenomenon. A further shift of the (110) peak towards lower angle in the spectra of group 2 multilayers indicates that the (110) planes of these multilayers are separated more than in the group 1 multilayers. This may be due to solution of hydrogen in the interstitial sites of the lattice and the accompanying lattice expansion during the growth of these multilayers. The excess 2u values of the (110) peaks lead ˚ for the lattice constants in to values of 2.901 and 2.903 A groups 1 and 2, respectively. The width at half height of the (110) peak varies in the range 0.6–0.8 8 for multilayers of group 1 and in the range
15
0.7–0.98 for group 2; but the intensity of the (110) peak for group 2 is nearly two times larger than that of group 1. The small but significant difference in the width at half height of the (110) peak suggests that the grain size in multilayers of group 2 is smaller than in group 1 multilayers, if the broadening would only be due to the size of the grains. Scherrer’s formula for the grain size predicts an ˚ for multilayers of group 1 and average grain size of 145 A ˚ 133 A for group 2. In other words the presence of hydrogen gas in the sputtering atmosphere has resulted in the growth of small grains in the multilayers of group 2. Furthermore, the relatively large intensity of the (110) peak of the multilayers of group 2 indicates contributions to the reflected X-ray beam from a large number of grains having (110) texture parallel to the plane of these multilayers. This may be due to the increased coherence of these multilayers and may form an indication for a relatively perfect structure. The implantation of 10 17 hydrogen atoms shifts the (110) peaks in the XRD spectra of both groups of multilayers to a 2u value around 51.68. This indicates that the separation of the (110) crystal planes increases on hydrogen implantation. This may be due to the occupation of interstitial sites by the implanted hydrogen and the accompanying expansion of these lattice sites. Again the comparison of the large angle XRD spectra reveals that in addition to the shift towards lower angles, the (110) peak broadens and decreases in intensity due to hydrogen implantation. AFM pictures are shown in Fig. 2. Fig. 2a,b are typical examples of the surface topography of as-deposited multilayers of groups 1 and 2, respectively. The root mean square (rms) roughness of the surface that was imaged in these figures are 1.5 and 0.47 nm, respectively. In a previous study a rms roughness of 0.7 nm was reported for Fe / Cr multilayers sputter deposited in a slightly low pressure argon gas atmosphere onto an Fe-buffer layer [12]. It is clear from these figures that the presence of hydrogen in the sputtering gas atmosphere produces growth of multilayers with less rough surfaces. As a consequence, one could, therefore, expect relatively smooth interfaces in the multilayers of group 2. This should give rise to relatively high intensity peaks in the small angle XRD spectra of these multilayers; but no significant difference is observed for the intensity of the peaks in the small angle spectra between the multilayers belonging to groups 1 and 2. Fig. 2c,d represent the surface topographies of the multilayers (whose surface topography was shown in Fig. 2a,b, respectively) after the implantation of 10 17 hydrogen atoms. The rms roughness of the surfaces that were imaged in Fig. 2c,d are 0.81 and 0.75 nm, respectively. This shows that the hydrogen implantation makes a rougher surface smoother and a smoother surface rougher. It also suggests that prolonged implantation of hydrogen may result in multilayer surfaces with some characteristic roughness.
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K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
Fig. 2. The surface topography of a group 1 multilayer before (a), and after the implantation of 10 17 hydrogen atoms (c). The surface topography of a group 2 multilayer before (b), and after the implantation of 10 17 hydrogen atoms (d).
˚ / [Fe(22 A) ˚ / Cr(9 A)]330 ˚ Fig. 3. Magnetic moment versus in-plane magnetic field loops for the multilayer Cr(50 A) of group 1 (a) and the multilayer ˚ / [Fe(22 A) ˚ / Cr(9.5 A)]330 ˚ Cr(50 A) of group 2 (b).
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
3.2. Magnetization The in-plane magnetic moment versus magnetic field curves shown in Fig. 3a,b are typical examples for the magnetization hysteresis loops recorded for multilayers of groups 1 and 2, respectively. These examples are those which exhibit the largest GMR in their respective group. The magnetization feature (i.e. gradual increase of the magnetic moment up to the saturation moment, large saturation field and small remanent magnetic moment) displayed in these curves show that the Fe layers in the multilayers are coupled antiferromagnetically. The variations of the saturation magnetic field and remanent magnetic moment with Cr layer thickness for the groups 1 and 2 multilayers are shown in Fig. 4a,b, respectively. The variation of the saturation magnetic field with Cr layer thickness within the range investigated is in agreement with previously reported results except for the position of the peak value [6]. Comparison of the results shown for
Fig. 4. Variation of saturation magnetic field (solid line) and remanent magnetic moment (dotted line) as a function of Cr layers thickness in multilayers of the group 1 (a) and multilayers of the group 2 (b).
17
groups 1 and 2 in Figs. 3 and 4 reveals that the nature of the magnetization in the Fe / Cr multilayers belonging to both groups is qualitatively identical; but the saturation magnetic moment of the group 2 multilayers is relatively lower than that of the group 1 multilayers. The average magnetization of 30 multilayers studied in each group (i.e. groups 1 and 2) are, respectively, 1515 and 1360 emu cm 23 (1 emu cm 23 51000 A m 21 ). The magnetization of bulk Fe is about 1740 emu cm 23 . Due to the expanded lattice of the Fe layers in the multilayers with respect to bulk Fe, their expected magnetization is about 1680 emu cm 23 . Therefore, the percentage of reduction of the average saturation magnetic moment in the group 1 multilayers is about 10% and about 19% in the group 2 multilayers. It was reported that Fe / Cr alloys containing more than 25% Cr atoms are non-magnetic [20]. Alloying of magnetic and non-magnetic atoms at interfaces of multilayers has been a widely reported phenomenon. Therefore, the above reduction of the magnetic moment from the expected value may be due to the alloying of Fe and Cr atoms at the interfaces of the multilayers. If a monolayer of FeCr alloy is formed at each interface, the expected reduction in the saturation magnetization would be about 9.3% for a flat interface. This suggests, on average, the formation of about one Fe / Cr alloy monolayer at the interfaces of the group 1 multilayers and two atomic layers at the interfaces of the group 2 multilayers. It ˚ alloy layer is worth to note here that the formation of 9 A (four atomic layers) at the interfaces of Fe / Cr multilayers was previously suggested by Fullerton et al. [10]. Furthermore, an increase of the interface mixing in multilayers sputter deposited in light sputtering gas was also suggested by Scheursser et al. [21]. The enhanced formation of alloy in the case of the multilayers belonging to group 2 may result from the effect of the hydrogen present in the sputtering atmosphere or / and form the relatively high growth rate (high sputtering power) of the Cr layers in these multilayers. A set of multilayers was selected for hydrogen implantation study from both groups of multilayers. The magnetic properties of these multilayers before and after the implantation of 10 17 hydrogen atoms are given in Table 1. A typical example for the changes in the magnetic characteristics of these multilayers due to hydrogen implantation is shown in Fig. 5. The results displayed in Fig. 5 and the data given in Table 1 show an increase of the remanent magnetic moment and a decrease of the saturation magnetic field on hydrogen implantation. Furthermore, upon hydrogen implantation the saturation magnetic moment decreases in the case of the group 1 multilayers; but in the cases of the group 2 multilayers it decreases only in a few cases and in other cases it either increases or remains unchanged. However, the changes are relatively small. The XRD studies for the as-deposited multilayers of group 2 have suggested a solution of hydrogen in them during their growth; but there is no supporting evidence from Figs. 3
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
18
Table 1 Magnetic properties of Fe / Cr multilayers before and after the implantation of 10 17 hydrogen atoms, where t Cr is the Cr layer thickness, Hs the saturation magnetic field, Hc the coercive force, m s the saturation magnetic moment and m r the remanent magnetic moment a ˚ t Cr (A)
7 8 9 10 11 12 13 14 15 8.5* 9* 9.5* 10* 11* 11.5* 12* 13.5* 14* 15*
Hs (kOe)
21
HC (Oe)
21
m s (mJ T )
m r (mJ T )
A
B
A
B
A
B
A
B
0.2 4.5 5.5 4 3.5 3 2.5 1.5 1.25 2.25 5.75 4.5 4 4 3.75 3.25 2 1.5 1
0.2 4 5 3.5 3 2.5 2.25 1.5 1.25 1.8 5.25 4 3.5 3.5 3.25 2.75 2 1.5 1.25
50 80 95 90 70 85 100 100 85 50 70 80 75 85 75 100 80 80 80
55 75 85 85 80 85 100 100 85 55 75 80 75 85 80 85 80 80 85
12 13.5 12 11.7 11.5 11.4 12 15 13 11.2 11.4 11.7 10.7 10.4 12.3 11.1 10.5 10.5 12.5
11.5 12.7 11.7 11.5 11.3 10.6 11.7 14 11.6 11.5 11.2 11.6 11.5 10.4 13 11.5 10.5 10.5 12.8
10.7 6 0.75 2.25 1 0.5 0.3 4.5 3.5 8 5 1.5 1.6 1.5 1.2 3 0.7 2 4
11.3 7.5 3.4 4.7 3 1.6 1.75 4.5 3.75 9 7 3.5 4.25 3.6 3.7 5.2 2 2.2 4.6
a The data given in the rows corresponding to the values of t Cr with asterisks are for multilayers belong to group 2 and others are for multilayers belong to group 1. Column A gives data of as-deposited multilayers and column B gives data of hydrogen-implanted multilayers. 1 Oe5(1000 / 4p ) A m 21 .
and 4 for the effect of dissolved hydrogen on the remanent magnetic moment and saturation magnetic field of these multilayers. This suggests that the increase of the remanent magnetic moment and the decrease of the saturation magnetic field after hydrogen implantation may not be due to the solution of hydrogen in the metal lattice. Previously an increase of the remanent magnetic moment and a decrease of saturation magnetic field of Fe / Cr multilayers after Xe 1 ion irradiation was reported [22]. Therefore, we suggest that the increase of the remanent magnetic moment and the decrease of the saturation magnetic field can be due to structural changes (damages) created in the multilayers by the implanted hydrogen. The saturation magnetic moment shows a time-dependent variation, similar to the variations previously reported for Co / Cu multilayers after hydrogen implantation [17]. That is, immediately after hydrogen implantation the saturation magnetic moment of the multilayers is higher than their magnetic moment before hydrogen implantation. Subsequently over a period of time it drops to a lower value. Creation of metal vacancies and occupation of interstitial sites by hydrogen during implantation and subsequent atomic mixing at the interfaces due to vacancy diffusion and redistribution of hydrogen to energetically favorable interfacial sites were previously suggested (in order to explain the time-dependent changes observed in Co / Cu multilayer studies) [17]. Furthermore, the different behaviour of the overall changes of the saturation magnetic moment of the multilayers belonging to group 2 may be due to the presence of thick (two atomic layers) Fe / Cr alloy layers at their interfaces. If there is a thick layer of
Fe / Cr alloy at the interfaces, the magnetic moment of these multilayers may be insensitive to atomic mixing by vacancy diffusion at the interfaces. Therefore, the present observations are also in agreement with our previous suggestions.
3.3. Magnetoresistance Fig. 6 shows the variation of MRR and MR with applied ˚ Cr and 17 A ˚ Fe magnetic field for a multilayer with 9 A layers and sputter deposited in an argon gas atmosphere. This multilayer possesses a saturation field of 6.5 kOe and exhibits a GMR value of 7.5%. Both these values are the largest values observed in this study and the value of GMR is nearly equal to the highest room temperature GMR values reported for Fe / Cr multilayers. The exchange coupling strength of this multilayer is 0.29 erg cm 22 (erg cm 22 510 23 J m 22 ), which is comparable to the corresponding value of 0.24 erg cm 22 published previously [23]. However, this multilayer is not included in group 1 ˚ but the curves because its Fe layer thickness is not 22 A; shown in Fig. 6 are representative for the variations of MRR and MR with applied magnetic field recorded in this study. The general feature of the variations of MRR is in agreement with previously reported studies for Fe / Cr multilayers [23]. The electrical resistivity of multilayers drops rapidly with increasing magnetic field up to the field at which the magnetization of the multilayer saturates. Then the resistivity continues to decrease at reduced rate with increasing magnetic field up to 7.5 kOe, at which field
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
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Fig. 6. Magnetic field-dependent variation of magnetoresistance ratio ˚ / [Fe(17 (MRR) and magnetoresistance (MR) of the multilayer Cr (50 A) ˚ / Cr(9 A)]330. ˚ A) (d) MRR; (s) MR.
Fig. 5. Magnetic moment versus in-plane magnetic field loops for the ˚ / [Fe(22 A) ˚ / Cr(9 A)]330 ˚ multilayer Cr(50 A) of group 1 (a) before (b) after the implantation of 3310 16 hydrogen atoms and (c) after the implantation of 10 17 hydrogen atoms.
saturation of the electrical resistivity was assumed. The saturation electrical resistivity for as-deposited samples of the group 1 multilayers lies within the range from 42 to 52 mV cm with an average of 47.5 mV cm. For as-deposited samples of the group 2 multilayers it lies within the range from 45 to 56 mV cm, with an average of 51.3 mV cm. These values are slightly larger than the previously published values of the room temperature saturation electrical resistivity [7]. The deviation of the electrical resistivity of the individual multilayers belong to a particular group seems to depend on the structural quality of the multi-
layers. Solution of hydrogen in metal lattices is usually accompanied by an increase of their electrical resistivity. Therefore, the large average electrical resistivity of the group 2 multilayers compared to that of the group 1 multilayers is due to the solution of hydrogen in the metal lattice of these multilayers during their growth. This is also in agreement with the shift of the (110) peak in the large angle XRD spectra of these multilayers. In general, the variation of MR and MRR with magnetic field has a bell shape, with a maximum at zero field; but in the case of multilayers with large remanent magnetic moment the variation of MR with magnetic field shows a tendency to exhibit a double peak similar to that commonly observed in the study of Co / Cu multilayers [17]. The magnitude of the coercive force observed for most of the multilayers in this study (Table 1) is comparable to that reported in the study of Co / Cu multilayers [17]. The remanent magnetic moment of Co / Cu multilayers is usually large. This suggests that a reasonably large remanent magnetic moment of the multilayers is necessary for the appearance of a double peak in the variations of MRR and MR with magnetic field. A typical example of the effect of hydrogen implantation on the variation of MRR and MR with magnetic field is shown in Fig. 7a,b, respectively. The variation of GMR and maximum MR (MMR) with Cr layer thickness and the effect of hydrogen implantation on these variations are displayed in Fig. 8a,b, respectively. In the case of the as-deposited multilayers the variation of GMR and MMR ˚ for the with Cr layer thickness has a maximum around 9 A ˚ for those of group multilayers of group 1 but around 9.5 A 2. The significance of this shift is not clear because the shift is within the experimental uncertainty. However, in
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˚ / [Fe(22 Fig. 7. Magnetic field-dependent variation of the magnetoresistance ratio, MRR (a) and the magnetoresistance, MR (b) of the multilayer Cr (50 A) 16 ˚ / Cr(9 A)]330 ˚ A) before and after hydrogen implantation. (s) Before implantation; (^) after the implantation of 3310 hydrogen atoms; (h) after the implantation of 10 17 hydrogen atoms.
the as-deposited multilayers of group 2 the maximum saturation field and maximum GMR were observed for ˚ Cr layers, respectively. In multilayers with 9 and 9.5 A group 1 both maxima were observed in one and the same ˚ Cr layers. In other words, both multilayer with 9 A ˚ of Cr layer thickness; but in a maxima appear around 9 A previous study conducted at 4.5 K the maxima of the saturation field and the GMR were well separated and ˚ thick Cr layers, respectively [6]. observed in 7 and 9 A The reason for the coincidence (or near coincidence) of the ˚ maxima of saturation magnetic field and GMR around 9 A thick Cr layers in this study is not clear. The variation of MMR with Cr layer thickness shows a broad maximum ˚ for both groups of multilayers. The maximum around 10 A values of MMR for multilayers of both groups are also nearly the same. Both MMR and GMR are reduced by hydrogen implantation. The reduction of MMR decreases as Cr layer ˚ Previously it was reported that thickness approaches 15 A. the MMR dropped by hydrogen implantation in a Co / Cu ˚ but it multilayer with Cu layers of thickness 11.1 A, remained unchanged in a Co / Cu multilayer with Cu layers ˚ [17]. The former multilayer belongs to of thickness 23.8 A the first peak of antiferromagnetic coupling of Co / Cu multilayers, where the coupling is strong. The latter multilayer belongs to the second peak of antiferromagnetic coupling, where the coupling is relatively weak. This
means that the reduction of MMR by hydrogen implantation seems to depend on the strength of antiferromagnetic coupling. In general, a reduction of GMR is due to a reduction of MMR and an increase of the saturation resistivity. In the case of the multilayers investigated, hydrogen implantation increased the saturation resistivities of all multilayers. This increase may be partially due to solution of hydrogen in the interstitial sites of the multilayer lattices and (partially due to the) structural damages created by hydrogen implantation. The average values of the saturation resistivity for multilayers of group 1 after implantation of 3310 16 and 10 17 hydrogen atoms were 50.1 and 52.1 mV cm, respectively. For multilayers of group 2 the average values were 54.3 and 56.4 mV cm, respectively. Some multilayers after hydrogen implantation studies were placed in a vacuum of 2310 27 Torr for nearly 40 h and no significant changes in their magnetoresistance, saturation electrical resistivity and magnetization was found. In general the magnetization of magnetic multilayers can be divided into two portions. One portion is from zero magnetic moment to remanent magnetic moment, m r , and the other portion is from remanent magnetic moment to saturation magnetic moment, m s . The first portion is due to ferromagnetically coupled volume of the multilayer. The second portion is due to antiferromagnetically coupled volume of the multilayer. The antiferromagnetically cou-
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
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Fig. 8. Variation of the giant magnetoresistance, GMR (a,c) and maximum of magnetoresistance, MMR (b,d) as a function of Cr layer thickness in multilayers of the group 1 (a,b) and multilayers of the group 2 (c,d). (d) Before implantation; (s) after the implantation of 3310 16 hydrogen atoms; (j) after the implantation of 10 17 hydrogen atoms.
pled volume possesses a magnetic moment of (m s 2m r ) and has been identified to have some influence on the magnetoresistance of the multilayer [21]. A linear correlation between the GMR and the fraction of the antiferromagnetically coupled volume of a multilayer was previously proposed [24]. According to Hall et al. [25] the GMR of a multilayer is related to the fraction of the antiferromagnetically coupled volume, (m s 2m r ) /m s , of the multilayer by the equation GMR 5 h1 2 (m r /m s )jGMR ideal
(1)
Where GMR ideal is the value of GMR that would have been expected if the entire volume of the multilayer were coupled antiferromagnetically. Hydrogen implantation into a multilayer generally increases the saturation electrical resistivity and reduces the fraction of the antiferromagnet-
ically coupled volume. It is possible to introduce a correction term into Eq. (1) to account for the effect of the saturation resistivity increase on the GMR. With such a correction term in Eq. (1), the GMR will be given by GMR 5 h1 2 (m r /m s )j( r 0S /r *S )GMR ideal
(2)
Here r s0 is the saturation electrical resistivity of the as-deposited multilayer and r *s is that of the hydrogenimplanted multilayer. The estimated values of GMR ideal using the Eq. (1) for as-deposited multilayers and Eq. (2) for hydrogen-implanted multilayers are given in Table 2 together with other relevant quantities. The relatively good agreement between the estimated GMR ideal for as-deposited and hydrogen-implanted samples of the individual multilayer supports the importance of the fraction of antiferromagnetically coupled volume in GMR effects of
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Table 2 Magnetoelectrical properties for Fe / Cr multilayers before and after the implantation of 3310 16 hydrogen atoms, where t Cr is the Cr layer thickness, m s the saturation magnetic moment, m r the remanent magnetic moment, GMR ex the experimentally estimated values for the GMR and GMR ideal the estimated values for the GMR of ideal multilayer a ˚ t Cr (A) A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B A B
8 9 10 12 13 14 15 8.5* 9* 9.5* 10* 10.5* 11* 11.5* 12* 13* 13.5* 14* 15*
rsat (mV cm)
(12m r /m s )
GMR ex
GMR ideal
45.12 47.03 45.62 47.03 51.82 53.46 47.48 50.67 46.83 49.82 42.57 44.98 44.57 48.5 55.36 56.99 50.98 54.11 49.51 54.05 53.71 56.62 53.39 57.05 52.31 55.98 51.6 53.47 55.53 58 48.49 50.65 50.46 54 48.6 53.42 45.61 48.16
0.5555 0.4297 0.9375 0.7917 0.8077 0.693 0.9561 0.9273 0.975 0.9149 0.7 0.7 0.731 0.727 0.2857 0.2609 0.561 0.469 0.8718 0.784 0.8505 0.712 0.8372 0.686 0.8558 0.7228 0.9 0.784 0.73 0.617 0.945 0.9027 0.933 0.8878 0.81 0.842 0.68 0.68
5.23 3.82 6.65 5.43 6.02 5.02 5.1 4.46 5.16 4.47 3.7 3.33 3.41 3.16 2.5 1.72 4.9 3.52 5.9 4.9 5.4 4.4 5.63 4.41 5.75 4.58 5.4 4.33 4.9 3.85 4.85 3.87 4.28 3.6 3.8 3.35 3.12 2.75
9.47 9.27 7.01 7.43 7.45 7.47 5.33 5.13 5.29 5.2 5.28 5.03 4.71 4.73 8.75 6.79 8.73 7.97 6.77 6.82 6.35 6.51 6.73 6.87 6.72 6.78 6 5.72 6.71 6.52 5.13 4.48 4.59 4.35 4.69 4.37 4.58 4.27
a
The data given in the rows corresponding to the values of t Cr with asterisks are for multilayers belong to group 2 and others are for multilayers belong to group 1. Row A gives data of as-deposited multilayers and row B gives data of hydrogen-implanted multilayers.
multilayers. Furthermore, it helps to recognize the effect of the saturation electrical resistivity of multilayers on the GMR. Our results suggest that the effect of hydrogen inclusion on the variation of GMR with Cr layer thickness is related to the product of the magnetic moment of the antiferromagnetically coupled volume and the saturation magnetic field of multilayers. In order to analyse this further, the GMR values of as-deposited multilayers are plotted against the respective dimensionless quantity h(m s 2m r )Hs /m s H Is j in Fig. 9. Here H sI is the saturation magnetic field of the multilayer with the largest GMR for the same Fe layer thickness. The quantity h(m s 2m r )Hs /m s H Is j can be considered as a measure of the relative antiferromagnetic coupling strength of multilayers and will here after be denoted by JR . In Fig. 9 the GMR of as-deposited
multilayers of group 1 (solid) and group 2 (open) are plotted against JR . The GMR values used for multilayers of group 2 are corrected for the increase of the average saturation electrical resistivity with respect to that of the group 1 multilayers using the expression GMR( r 2s /r 1s ), where r 1s and r 2s are, respectively, the average saturation electrical resistivity of the group 1 and 2 multilayers. The approximate curves drawn to illustrate the variation of GMR with JR for the group 1 and 2 multilayers are very similar, and this suggests no major influence of dissolved hydrogen on the magnetic properties of Fe / Cr multilayers. Furthermore, both curves illustrate the tendency to saturation of GMR at large values of JR . The value of JR at which the GMR saturates for group 2 is slightly lower than that of group 1. This may be due to the easiness of the interfacial magnetization of group 2 compared to that of
K. Kandasamy et al. / Journal of Alloys and Compounds 288 (1999) 13 – 24
Fig. 9. Variations of GMR as a function of the quantity, JR 5h12(m r / m s )j(Hs /H Is ) for as-deposited multilayers of group 1 and 2. (d) Group 1; (s) group 2.
23
approximate curves are drawn for each set of data to illustrate the variation of GMR with JR and the effect of hydrogen implantation. In all cases the general feature of the variation of GMR with JR is qualitatively the same. Hydrogen implantation reduced the saturation value of the GMR in all cases. The amount of reduction increases with increasing doses of hydrogen implantation. Furthermore, the value of JR at which the GMR saturates also decreases with increasing doses of hydrogen implantation. These observations suggest that the hydrogen implantation, in general, reduces the antiferromagnetic interlayer coupling and the antiferromagnetically coupled domains at the interfaces. The changes in slope of the approximate curves for small values of JR may be related to the effect of hydrogen implantation on the nature of the interfacial magnetization.
4. Conclusion group 1. The relatively large saturation value of GMR for the group 1 multilayers may be due to their relatively rougher interfaces. In Fig. 10a,b the corrected GMR values of as-deposited and hydrogen-implanted multilayers of groups 1 and 2 are plotted against the corresponding quantity JR . Separate
Fig. 10. Variations of GMR as a function of the quantity, JR 5h12(m r / m s )j(Hs /H Is ) for as-deposited and hydrogen-implanted multilayers of group 1 (a) and group 2 (b). (s) Before implantation; (^) after the implantation of 3310 16 hydrogen atoms; (h) after the implantation of 10 17 hydrogen atoms.
The multilayers sputter deposited in this study possess a bcc lattice with (110) texture parallel to the plane of the multilayers and lattice constants larger than that of bcc bulk Cr. The presence of hydrogen gas in the sputtering atmosphere-induced layer growth with relatively small grain sizes, large lattice constants and smooth surfaces. Hydrogen implantation increases the bi-layer thickness and the lattice constant of the multilayers due to solution of hydrogen in the lattice of these multilayers. Hydrogen implantation reduces the intensity of the peaks in the XRD spectra and changes the surface roughness of the multilayers. The magnetization behaviour of multilayers sputter deposited in argon gas and in a mixture of argon and hydrogen gases are identical. This suggest that the dissolved hydrogen in the multilayers during their growth has no major effect on the magnetic properties of the multilayers. The saturation magnetic moment of multilayers is smaller than that expected from the Fe content of the multilayers. This is due to the formation of Fe / Cr alloy at the interfaces. The hydrogen implantation increases the remanent magnetic moment but reduces the saturation magnetic field. The causes of these changes are not clear. The saturation magnetic moment of multilayers exhibits time-dependent changes after hydrogen implantation. These changes are due to the formation of metal atom vacancies and subsequent atomic mixing at the interfaces by vacancy diffusion and solution of hydrogen at interstitial sites and subsequent diffusion of hydrogen to the energetically favourable interfacial sites. The observed values for maximum GMR and corresponding antiferromagnetic coupling strength are in agreement with previously published data [7]; but in contrast to previously published results [6] we observed a coincidence of the maxima of the GMR and the saturation magnetic field variations with Cr layer thickness. The reason for this coincidence has not been identified. The magnetoresistance
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and GMR of multilayers were reduced but the saturation resistivity was increased by hydrogen implantation. The decrease of the magnetoresistance may be due to the reduction of antiferromagnetically coupled magnetic domains at the interfaces by the implanted hydrogen. The magnitude of the decrease in magnetoresistance is relatively larger for multilayers with strong antiferromagnetic coupling. The reduction of the GMR is due to a decrease of the magnetoresistance and an increase of the saturation resistivity upon hydrogen implantation. There is fair agreement between the experimental data and Eq. (2), which relates the GMR, saturation resistivities of as-deposited and hydrogen-implanted multilayers to the fraction of antiferromagnetically coupled volume of the multilayers. This indicates the importance of the fraction of antiferromagnetically coupled volume and saturation resistivity for the GMR of multilayers.
Acknowledgements One of the authors (K.K.) would like to acknowledge a Fellowship from the Japan Society for the Promotion of Science (JSPS) during the course of this work. This work has been supported in part by a Grant–in Aid for Scientific Research on Priority Areas A of ‘New Protium Function’ from the Ministry of Education, Science, Sports and Culture.
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