- Email: [email protected]
Ni multilayer structure
Applied Surface Science 238 (2004) 254–261
Study of interfacial properties and its effect on magnetization behaviour of Fe/Ni multilayer structure Rachana Guptaa,*, Ajay Guptaa, S.M. Chaudharia, Mukul Guptab, P. Allenspachb a
Inter-University Consortium for DAE Facilities, Khandwa Road, Indore 452 017, India b Laboratory for Neutron Scattering, ETHZ & PSI, Paul Scherrer Institute, Villigen CH-5232, Switzerland Available online 21 July 2004
Abstract This paper deals with the investigation of interfacial properties and their influence on magnetization behaviour of Fe/Ni ˚ )/Ni(86 A ˚ )]10 and multilayer (ML) structures. Two types of multilayer structures with nominal composition of [Fe(29 A ˚ ˚ [Fe(50 A)/Ni(50 A)]10 were prepared with e-beam evaporation technique under ultra high vacuum (UHV) conditions. Grazing incidence X-ray reflectivity (GIXRR) and X-ray diffraction (GIXRD) techniques have been employed to determine the microstructural parameters and indicates formation of FeNi3 alloy phase at the interface. GIXRD measurements on as-deposited Fe50/ Ni50 ML samples show highly textured growth and its structure remains the same upto annealing temperature of 350 8C. The magnetization behavior of the MLs has been obtained using extraction dc magnetometry. The M–H loops corresponding to the Fe50/Ni50 ML structure of as-deposited sample show gradual changes with applied field whereas the as-deposited Fe29/ Ni86 ML shows a perfectly square loop. Upon annealing of the Fe29/Ni86 MLs upto 300 8C show change in the loop shape from square to the smoother one. This is mainly attributed to release of stress in deposited layer and the change in magnetostriction. However, square loops observed in this case at higher annealing temperature of 300 and 400 8C is result of magnetically soft FeNi3 phase formation at interface. On the other hand, Fe50/Ni50 ML samples annealed upto 400 8C show no change in the shape even though there is variation in HC values. The observed changes in the HC values are mainly attributed in this case to the different magnetostriction effects involved due to the increase in grain size with temperature. # 2004 Elsevier B.V. All rights reserved. PACS: 61.10.Kw; 68.03.Fg; 68.35.Ct; 68.65.Ac; 75.70.-i Keywords: e-beam evaporation; Fe/Ni ML structure; Thermal study; Magnetization study co-relation with structure parameters
1. Introduction *
Corresponding author. Tel.: þ91 731 463913/762267/462265; fax: þ91 731 462294/465437. E-mail addresses: [email protected] (R. Gupta), [email protected] (S.M. Chaudhari), [email protected] (M. Gupta).
Magnetic thin films and multilayers (ML) are of current scientific and technological interest due to their unique magnetic properties. They are the key elements for their application in several devices like
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.230
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
low-field magnetic sensor, recording media, etc. [1,2]. These ML structures can be truly new materials whose properties may be totally different either from their constituents or even when they form a alloy of them. Particularly, Fe/Ni (ferromagnetic (FM)/FM) bilayer and multilayers system are very attractive because of their soft magnetic properties such as low coercivity and magnetostriction, high remanence and low saturation field. In addition to this, they also exhibit uniaxial anisotropy as well as have sufficient thermal stability. Therefore, considerable effort has been put-in [3–9], in order to understand and optimize magnetic properties of these ML structure combining either FM layer with anti-FM or other FM layer. However, magnetic properties of such structures are very sensitive to the quality of interface as well as interface interaction between the layers and with the substrate. In the present work, we have prepared Fe/Ni multilayers and studied their interfacial and magnetization behaviour as a function of annealing temperature.
2. Experimental Two types of Fe/Ni ML structures have been prepared using electron beam evaporation technique under UHV conditions on float glass substrates at room temperature. The periodicity of the bilayer in ˚ , with the ratio of Fe/Ni one type is chosen to be 115 A ˚ )/Ni(86 A ˚ )]10. In thickness equal to 1:3 [Fe(29 A another type, MLs were prepared having equal thick˚ each [Fe(50 A ˚ )/Ni(50 A ˚ )]10. The deposiness of 50 A tion of both the structures were carried out at base pressure of the order of 5 1010 mbar with the ˚ /s for both the eleconstant deposition rate of 0.1 A ments. During deposition, thickness of layers was measured using quartz crystal thickness monitor. The structural characterization of as-deposited as well as the annealed ML samples were carried out using grazing incidence X-ray diffraction (GIXRD) and reflectivity (GIXRR) techniques. All these measurements were carried out using a standard diffractometer ˚ ) in the equipped using Cu Ka radiation (g ¼ 1:542 A Bragg–Brantano geometry with the qz resolution of ˚ 1. Isochronal annealing of MLs samples was 0.003 A done under high vacuum condition for 1 h in an interval of 100 8C upto 400 8C. The corresponding
255
magnetic measurements were carried out using an extraction magnetization measurement system (Physical Property Measurement System, PPMS) with the applied magnetic fields parallel to the plane of the sample.
3. Results 3.1. GIXRD and GIXRR measurements The GIXRD patterns corresponding to as-deposited ˚ )/Ni(86 A ˚ )]10 ML samples show intense peak [Fe(29 A at 2y ¼ 44:68 corresponding to the Ni (1 1 1). The Fe (1 1 0) peak is also having 2y values very close to the Ni (1 1 1) and could not be resolved clearly. Other GIXRD peaks are identified and marked in Fig. 1. Similar GIXRD patterns were also observed for the samples annealed at 100 8C and 200 8C indicating that upto this annealing temperature has no effect on structural changes. Whereas, the samples annealed at 300 8C and above show appearance of peaks corresponding to the FeNi3 phase. Since the peaks corresponding FeNi3 alloy phase are overlapping with Ni and Fe peaks, the formation of the FeNi3 phase is judged from shift of the peaks towards lower angle side. Corresponding reflectivity measurements carried out on the ML samples are shown in Fig. 2. The off specular scattering contribution (with an off-set 0.07 8) is subtracted from the specular data to yield the true specular nature. For the as-deposited multilayer, the Bragg peaks are observed upto the 5th order indicating good quality of ML. Similar reflectivity pattern are observed for the samples annealed upto 200 8C. The reflectivity behavior of samples annealed at 300, 350 and 400 8C shows different nature. At 400 8C the reflectivity pattern shows a single layer like pattern. This indicates that for the sample annealed at 300 8C and above, there is a substantial mixing at the interface. In order to determine the microstructural parameters such as bilayer thickness, total ML thickness, interface roughness, electron density, etc. in each case these spectra were fitted using the Parratt formulism [10]. The calculated parameters are given in Table 1. The best GIXRR fitting in the case of sample annealed at 300, 350 and 400 8C is achieved by considering a formation of FeNi3 alloy phase of proper thickness at the interface. GIXRR measurements done
256
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
˚ )/Ni(86 A ˚ )]10 ML at different annealing temperatures. Fig. 1. GIXRD pattern of [Fe(29 A
on respective samples also show formation of FeNi3 alloy phase. Fig. 3 shows GIXRD result of as-deposited as well as the annealed [Fe(50)/Ni(50)]10 multilayers. The recorded spectra show a highly texture growth in this case. The peak at 2y ¼ 44:82 corresponds to Fe (1 1 0) and Ni (1 1 1) and no other peaks are observed. The intensity of peak at 2y ¼ 44:82 increases up on annealing upto 350 8C, with no additional appearance
of peaks. However, at 400 8C new peaks at 2y ¼ 51:12 and 75.198 start appearing. Further the peak at 44.82 8 shifts towards the lower angle value. The new peaks correspond to the FeNi3 phase formation. Fig. 4 shows the corresponding GIXRR pattern of as-deposited as well as annealed MLs samples. From these patterns it is clear that there are no significant changes in the structure of the ML up to 350 8C. However, the samples annealed at 400 8C show the
˚ )/Ni(86 A ˚ )]10 ML at different annealing temperatures. Fig. 2. GIXRR pattern of [Fe(29 A
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
257
Table 1 ˚ )/Ni(86 A ˚ )]10 MLa Fitted GIXRR parameters as a function of annealing temperature for [Fe(29 A As-deposited
100 8C
200 8C
dFe dInter dNi sFe sInt sNi
29 – 89 14 – 12
30 – 90 11 – 9
25 8 83 14 8 13
a
˚ 1). All in (A
400
350
dFeNi3 [dNi dFeNi3 ]9 dNi sFeNi3 sNi sFeNi3 sNi
400°C
Intensity(arb. unit)
280
100
350 8C
400 8C
26 24 90 60 12 12 11 10
31 21 94 37 11 12 17 8
31 16 101 4 12 14 10 16
single layer behavior. The structural parameters calculated from the fitting of GIXRR data are given in Table 2. In order to achieve a good fitting of the data for sample annealed at 350 and 400 8C an interface layer was required to fit the data.
Exp. Fitted
300 200
300 8C
350°C 300°C 200°C 100°C
0 AS Dep. 42 43 44 45 46 47 48
210
0
400 C
3.2. Magnetic measurements
0
350 C
140
0
300 C 0
200 C
70 0
100 C AS Dep. 0 35 40 45 50 55 60 65 70 75 80 85
2θ ˚ )/Ni(50 A ˚ )]10 C(20 A ˚) Fig. 3. GIXRD pattern of the of [Fe(50 A multilayer with different annealing temperatures. Insert shows the fitted peak of Fe, Ni around 44.528 and FeNi3 at 43.928 for annealed sample at 400 8C.
˚ )/Ni(50 A ˚ )]10 multilayer with different Fig. 4. GIXRR of [Fe(50 A annealing temperatures.
The M–H loop measurements carried out on Fe(29)/ Ni(86) ML samples annealed at 100, 200, 300, 350, and 400 8C are plotted in Fig. 5. As it is seen that M–H loop corresponding to the as-deposited sample is square in shape. Upon subsequent annealing at 100 8C shape changes to smother one indicating the rotation of the loop in the easy direction of applied magnetic field. This observed magnetization resembles mostly that of with pure Ni thin film, since the thickness of Ni is three times than Fe. Similar type of behavior has been also reported by Cai et al. [8] in their study conducted in as-deposited Fe/Ni MLs. However, in the present case change in the M–H loop shape observed after annealing the ML samples cannot be attributed to the higher Ni layer thickness. In this case change may be due to the annealing of points defects and release of stresses in the deposited layers of MLs. It is known that Ni has negative magnetostriction, where as Fe has positive magnetostriction at the lower field, and it shows different magnetostriction behavior when thin film is studied separately. However, if we combine this film to form a ML, the observed magnetization will be due to the combine effect of magnetostriction of both the constituent elements. Such types of studies have been reported in the literature [7,3]. The observed change in M–H curve from as-deposited to sample annealed at 100 8C
258
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
Table 2 ˚ )/Ni(50 A ˚ )]10 ML Fitted GIXRR parameters as a function of annealing temperature for [Fe(50 A Sample
˚ d (Fe) 1 A
˚ d (FeNi3) 1 A
˚ d (Ni) 1 A
˚ s (Fe) 1 A
˚ s (FeNi3) 1 A
˚ s (Ni) 1 A
As-deposited 100 8C 200 8C 300 8C 350 8C 400 8C
53 54 55 55 48 40
– – – – 14 33
54.4 51 52 52 44 32
14 15 14 14 14 24
– – – – 14 33
16 17 12 14 12 25
is mainly due to the magnetostriction effects. Upon further annealing upto 300 8C overall shape of M–H loop remains same. However, HC and HS value varies from 100 to 300 8C annealed sample (see Table 3). ML sample annealed at 350 8C again change in shape to the square one having corresponding HC and HS values 5Oe and 12Oe, respectively. At this temperature of annealing our GIXRD and GIXRR result clearly shows the formation of FeNi3 phase, which is magnetically softer. This phase further grows with anneal-
ing temperature of 400 8C. From fitted GIXRR patterns it is clear that as-deposited Fe/Ni ML structure is modified to FeNi3/Ni ML. Hence, observed magnetization behaviour corresponding to ML sample annealed at 300, 350 and 400 8C is a consequence of FeNi3 phase formation and certainly not due to the grain size effect [11]. The magnetizations as function of magnetic field of as-deposited as well as the annealed Fe50/Ni50 ML structures at different temperatures are shown in Fig. 6.
˚ )/Ni(86 A ˚ )]10 ML for as-deposited as well as the annealed samples. Fig. 5. Hysteresis loop of [Fe(29 A
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
259
Table 3 ˚ )/Ni(86 A ˚ )]10 ML Coercive field, HS, MS and grain size as the function of the annealing temperature for [Fe(29 A Temperature
HC (Oe) ˚) Grain size (A HS (Oe) MS (emu/cm3)
As-deposited
100 8C
200 8C
300 8C
350 8C
400 8C
10 75 5 23 5 837
10 70 5 55 5 560
4 83 5 47 5 820
4 281 5 47 5 820
5 380 5 12 5 773
3 404 5 20 5 688
All samples show well saturation magnetisation behaviour. The typical smother M–H loop shape obtained in the as-deposited Fe50/Ni50 ML may be due to the combined effect of highly textured growth, stress present in the deposited layers and the nature of magnetostriction evolved. With the annealing at 100 8C the measured M–H loop become like square shape. This opposite behaviour obtained in M–H loop as compared to the previous case mainly due to the highly textured nature of Fe50/Ni50 ML and the different response of magnetostriction with the
annealing. At the 200 8C temperature the M–H loop become more perfectly square, this shape is maintained upto 400 8C annealed sample. The calculated HC and MS values corresponding to the sample annealed at different temperatures are given in Table 4. Upto 200 8C annealed sample, the observed changes in HC and MS values are mainly results of inplane grain growth. The increment of HC with annealing upto 200 8C may be explained on the basis of the increase of grain size in these samples calculated from measured GIXRD patterns. For nano crystalline size
˚ )/Ni(50 A ˚ )]10 MLs. Fig. 6. Hysteresis loop of as-deposited as well as the annealed [Fe(50 A
260
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
Table 4 ˚ )/Ni(50 A ˚ )]10 ML Variation of MS, grain size and HC with the annealing temperature of [Fe(50 A Temperature
˚ (1) Grain size, A HC (Oe) MS (emu/cm3)
As-deposited
100 8C
200 8C
300 8C
350 8C
400 8C
57 6 998
60 8 1177
66 10 1272
66 4 1173
63 10 1450
78, 285 7 1010
grain (grain size < 20 nm), it is reported that HC increases with increasing grain size [12]. It can be mentioned here that grain size obtained from GIXRD measurements gives grain size perpendicular to film direction only. Because of 2D nature of the structure, in-plane size of grain can be much larger than the perpendicular to the plane. In such case crystallographic grains of nanometer size can be treated as single magnetic domain. The observed decrease in HC value on further annealing upto 300 8C is due to the inplane crystallographic domains may grow bigger in size and hence, single domain types magnetic structure can change to the multi domain type even though perpendicular grain size remains same. The other reason may be the segregation of small amount of FeNi3 alloy formation at the interface, which is not detected in the corresponding GIXRD and GIXRR measurements. However, the GIXRR measurements show the presence of FeNi3 alloy phase formation ˚ at the interface. Upon having layer thickness of 14 A further annealing at 400 8C, the thickness of FeNi3 ˚ . The observed increase alloy layer increases upto 33 A in the HC values corresponding to the sample annealed at 350 8C can be explained on as follows. In this case ML structure can be modelled as try layer with FeNi3 layer at interface, with corresponding calculated layer thickness as given in Table 2. Upon further annealing HC values again decrease and one can attribute this to further growth of FeNi3 phase at interface. In this case the individual layer thickness in try layer structure is modified as listed in Table 2. The contribution of interface layer seems to be more dominating in lowering the HC values.
4. Conclusion GIXRD measurements carried out on both types of ML structures show different growth behavior. Parti-
˚ )/Ni(50 A ˚ )]10 ML structure is highly cularly [Fe(50 A textured. The GIXRR measurements indicated alloy phase formation at interface. A FeNi3 phase having ˚ is easily formed in thickness of the order of the 100 A ˚ ˚ [Fe(29 A)/Ni(86 A)]10 ML sample annealed above ˚ )/ 300 8C. However, in case of ML sample [Fe(50 A ˚ ˚ Ni(50 A)]10 a very thin layer of the order of 14–33 A is formed above 300 8C. The measured M–H loop of as-deposited sample of both MLs structures are distinctly different. In one case there is square loop indicating switching action with applied magnetic field, while in the second case the gradual variation in the magnetization is observed. This difference is attributed to magnetostriction and different growth morphology. The observed M–H behavior of ML structure annealed at higher temperature of 300 8C is mainly due to the FeNi3 phase formation at the interface.
Acknowledgements The author would like to thank Mr. Satish Potdar for the deposition of multilayer samples.
References [1] T. Veres, M. Chai, R.W. Cochrane, M. Rouabhi, S. Roorda, P. Desjardins, Thin Solid Films 382 (2001) 167. [2] J. Neamtu, J. Magn. Magn. Mater. 157/158 (1996) 461. [3] Y. Nagai, M. Senda, T. Thosima, J. Appl. Phys. 63 (1988) 1136. [4] M. Takahashi, X. Hu, Y. Kawazoe, Solid State Com. 95 (1995) 127. [5] Y. Li, C. Polaczyk, J. Erxmeyer, J. Kapoor, D. Riegel, J. Magn. Magn. Mater. 140–144 (1995) 551. [6] T. Veres, R. Morel, R.W. Cochrane, R. Abdouche, M. Sutton, J. Appl. Phys. 81 (1997) 4758. [7] Y. Nagai, K. Yanagisawa, M. Senda, J. Appl. Phys. 64 (1988) 1343.
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261 [8] M. Chai, T. Veres, R. Morel, R.W. Cochrane, J. Appl. Phys. 79 (1996) 6289. [9] A.C. Erlich, D.J. Gillespie, T.M. Edelstein, C. Tritt, A.S. Kim, S.B. Qadri, J. Appl. Phys. 70 (1991) 5819. [10] L.G. Parratt, Phys. Rev. 95 (1954) 359.
261
[11] F. Pfeirfer, C. Radeloff, J. Magn. Magn. Mater. 19 (1980) 190. [12] G.C. Hadijipanayis (Ed.), Magnetic Hysteresis in Novel Magnetic Material, Kluwer Academic Publishers, Dordrecht, 1996, p. 713.
Study of interfacial properties and its effect on magnetization behaviour of Fe/Ni multilayer structure Rachana Guptaa,*, Ajay Guptaa, S.M. Chaudharia, Mukul Guptab, P. Allenspachb a
Inter-University Consortium for DAE Facilities, Khandwa Road, Indore 452 017, India b Laboratory for Neutron Scattering, ETHZ & PSI, Paul Scherrer Institute, Villigen CH-5232, Switzerland Available online 21 July 2004
Abstract This paper deals with the investigation of interfacial properties and their influence on magnetization behaviour of Fe/Ni ˚ )/Ni(86 A ˚ )]10 and multilayer (ML) structures. Two types of multilayer structures with nominal composition of [Fe(29 A ˚ ˚ [Fe(50 A)/Ni(50 A)]10 were prepared with e-beam evaporation technique under ultra high vacuum (UHV) conditions. Grazing incidence X-ray reflectivity (GIXRR) and X-ray diffraction (GIXRD) techniques have been employed to determine the microstructural parameters and indicates formation of FeNi3 alloy phase at the interface. GIXRD measurements on as-deposited Fe50/ Ni50 ML samples show highly textured growth and its structure remains the same upto annealing temperature of 350 8C. The magnetization behavior of the MLs has been obtained using extraction dc magnetometry. The M–H loops corresponding to the Fe50/Ni50 ML structure of as-deposited sample show gradual changes with applied field whereas the as-deposited Fe29/ Ni86 ML shows a perfectly square loop. Upon annealing of the Fe29/Ni86 MLs upto 300 8C show change in the loop shape from square to the smoother one. This is mainly attributed to release of stress in deposited layer and the change in magnetostriction. However, square loops observed in this case at higher annealing temperature of 300 and 400 8C is result of magnetically soft FeNi3 phase formation at interface. On the other hand, Fe50/Ni50 ML samples annealed upto 400 8C show no change in the shape even though there is variation in HC values. The observed changes in the HC values are mainly attributed in this case to the different magnetostriction effects involved due to the increase in grain size with temperature. # 2004 Elsevier B.V. All rights reserved. PACS: 61.10.Kw; 68.03.Fg; 68.35.Ct; 68.65.Ac; 75.70.-i Keywords: e-beam evaporation; Fe/Ni ML structure; Thermal study; Magnetization study co-relation with structure parameters
1. Introduction *
Corresponding author. Tel.: þ91 731 463913/762267/462265; fax: þ91 731 462294/465437. E-mail addresses: [email protected] (R. Gupta), [email protected] (S.M. Chaudhari), [email protected] (M. Gupta).
Magnetic thin films and multilayers (ML) are of current scientific and technological interest due to their unique magnetic properties. They are the key elements for their application in several devices like
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.230
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
low-field magnetic sensor, recording media, etc. [1,2]. These ML structures can be truly new materials whose properties may be totally different either from their constituents or even when they form a alloy of them. Particularly, Fe/Ni (ferromagnetic (FM)/FM) bilayer and multilayers system are very attractive because of their soft magnetic properties such as low coercivity and magnetostriction, high remanence and low saturation field. In addition to this, they also exhibit uniaxial anisotropy as well as have sufficient thermal stability. Therefore, considerable effort has been put-in [3–9], in order to understand and optimize magnetic properties of these ML structure combining either FM layer with anti-FM or other FM layer. However, magnetic properties of such structures are very sensitive to the quality of interface as well as interface interaction between the layers and with the substrate. In the present work, we have prepared Fe/Ni multilayers and studied their interfacial and magnetization behaviour as a function of annealing temperature.
2. Experimental Two types of Fe/Ni ML structures have been prepared using electron beam evaporation technique under UHV conditions on float glass substrates at room temperature. The periodicity of the bilayer in ˚ , with the ratio of Fe/Ni one type is chosen to be 115 A ˚ )/Ni(86 A ˚ )]10. In thickness equal to 1:3 [Fe(29 A another type, MLs were prepared having equal thick˚ each [Fe(50 A ˚ )/Ni(50 A ˚ )]10. The deposiness of 50 A tion of both the structures were carried out at base pressure of the order of 5 1010 mbar with the ˚ /s for both the eleconstant deposition rate of 0.1 A ments. During deposition, thickness of layers was measured using quartz crystal thickness monitor. The structural characterization of as-deposited as well as the annealed ML samples were carried out using grazing incidence X-ray diffraction (GIXRD) and reflectivity (GIXRR) techniques. All these measurements were carried out using a standard diffractometer ˚ ) in the equipped using Cu Ka radiation (g ¼ 1:542 A Bragg–Brantano geometry with the qz resolution of ˚ 1. Isochronal annealing of MLs samples was 0.003 A done under high vacuum condition for 1 h in an interval of 100 8C upto 400 8C. The corresponding
255
magnetic measurements were carried out using an extraction magnetization measurement system (Physical Property Measurement System, PPMS) with the applied magnetic fields parallel to the plane of the sample.
3. Results 3.1. GIXRD and GIXRR measurements The GIXRD patterns corresponding to as-deposited ˚ )/Ni(86 A ˚ )]10 ML samples show intense peak [Fe(29 A at 2y ¼ 44:68 corresponding to the Ni (1 1 1). The Fe (1 1 0) peak is also having 2y values very close to the Ni (1 1 1) and could not be resolved clearly. Other GIXRD peaks are identified and marked in Fig. 1. Similar GIXRD patterns were also observed for the samples annealed at 100 8C and 200 8C indicating that upto this annealing temperature has no effect on structural changes. Whereas, the samples annealed at 300 8C and above show appearance of peaks corresponding to the FeNi3 phase. Since the peaks corresponding FeNi3 alloy phase are overlapping with Ni and Fe peaks, the formation of the FeNi3 phase is judged from shift of the peaks towards lower angle side. Corresponding reflectivity measurements carried out on the ML samples are shown in Fig. 2. The off specular scattering contribution (with an off-set 0.07 8) is subtracted from the specular data to yield the true specular nature. For the as-deposited multilayer, the Bragg peaks are observed upto the 5th order indicating good quality of ML. Similar reflectivity pattern are observed for the samples annealed upto 200 8C. The reflectivity behavior of samples annealed at 300, 350 and 400 8C shows different nature. At 400 8C the reflectivity pattern shows a single layer like pattern. This indicates that for the sample annealed at 300 8C and above, there is a substantial mixing at the interface. In order to determine the microstructural parameters such as bilayer thickness, total ML thickness, interface roughness, electron density, etc. in each case these spectra were fitted using the Parratt formulism [10]. The calculated parameters are given in Table 1. The best GIXRR fitting in the case of sample annealed at 300, 350 and 400 8C is achieved by considering a formation of FeNi3 alloy phase of proper thickness at the interface. GIXRR measurements done
256
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
˚ )/Ni(86 A ˚ )]10 ML at different annealing temperatures. Fig. 1. GIXRD pattern of [Fe(29 A
on respective samples also show formation of FeNi3 alloy phase. Fig. 3 shows GIXRD result of as-deposited as well as the annealed [Fe(50)/Ni(50)]10 multilayers. The recorded spectra show a highly texture growth in this case. The peak at 2y ¼ 44:82 corresponds to Fe (1 1 0) and Ni (1 1 1) and no other peaks are observed. The intensity of peak at 2y ¼ 44:82 increases up on annealing upto 350 8C, with no additional appearance
of peaks. However, at 400 8C new peaks at 2y ¼ 51:12 and 75.198 start appearing. Further the peak at 44.82 8 shifts towards the lower angle value. The new peaks correspond to the FeNi3 phase formation. Fig. 4 shows the corresponding GIXRR pattern of as-deposited as well as annealed MLs samples. From these patterns it is clear that there are no significant changes in the structure of the ML up to 350 8C. However, the samples annealed at 400 8C show the
˚ )/Ni(86 A ˚ )]10 ML at different annealing temperatures. Fig. 2. GIXRR pattern of [Fe(29 A
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
257
Table 1 ˚ )/Ni(86 A ˚ )]10 MLa Fitted GIXRR parameters as a function of annealing temperature for [Fe(29 A As-deposited
100 8C
200 8C
dFe dInter dNi sFe sInt sNi
29 – 89 14 – 12
30 – 90 11 – 9
25 8 83 14 8 13
a
˚ 1). All in (A
400
350
dFeNi3 [dNi dFeNi3 ]9 dNi sFeNi3 sNi sFeNi3 sNi
400°C
Intensity(arb. unit)
280
100
350 8C
400 8C
26 24 90 60 12 12 11 10
31 21 94 37 11 12 17 8
31 16 101 4 12 14 10 16
single layer behavior. The structural parameters calculated from the fitting of GIXRR data are given in Table 2. In order to achieve a good fitting of the data for sample annealed at 350 and 400 8C an interface layer was required to fit the data.
Exp. Fitted
300 200
300 8C
350°C 300°C 200°C 100°C
0 AS Dep. 42 43 44 45 46 47 48
210
0
400 C
3.2. Magnetic measurements
0
350 C
140
0
300 C 0
200 C
70 0
100 C AS Dep. 0 35 40 45 50 55 60 65 70 75 80 85
2θ ˚ )/Ni(50 A ˚ )]10 C(20 A ˚) Fig. 3. GIXRD pattern of the of [Fe(50 A multilayer with different annealing temperatures. Insert shows the fitted peak of Fe, Ni around 44.528 and FeNi3 at 43.928 for annealed sample at 400 8C.
˚ )/Ni(50 A ˚ )]10 multilayer with different Fig. 4. GIXRR of [Fe(50 A annealing temperatures.
The M–H loop measurements carried out on Fe(29)/ Ni(86) ML samples annealed at 100, 200, 300, 350, and 400 8C are plotted in Fig. 5. As it is seen that M–H loop corresponding to the as-deposited sample is square in shape. Upon subsequent annealing at 100 8C shape changes to smother one indicating the rotation of the loop in the easy direction of applied magnetic field. This observed magnetization resembles mostly that of with pure Ni thin film, since the thickness of Ni is three times than Fe. Similar type of behavior has been also reported by Cai et al. [8] in their study conducted in as-deposited Fe/Ni MLs. However, in the present case change in the M–H loop shape observed after annealing the ML samples cannot be attributed to the higher Ni layer thickness. In this case change may be due to the annealing of points defects and release of stresses in the deposited layers of MLs. It is known that Ni has negative magnetostriction, where as Fe has positive magnetostriction at the lower field, and it shows different magnetostriction behavior when thin film is studied separately. However, if we combine this film to form a ML, the observed magnetization will be due to the combine effect of magnetostriction of both the constituent elements. Such types of studies have been reported in the literature [7,3]. The observed change in M–H curve from as-deposited to sample annealed at 100 8C
258
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
Table 2 ˚ )/Ni(50 A ˚ )]10 ML Fitted GIXRR parameters as a function of annealing temperature for [Fe(50 A Sample
˚ d (Fe) 1 A
˚ d (FeNi3) 1 A
˚ d (Ni) 1 A
˚ s (Fe) 1 A
˚ s (FeNi3) 1 A
˚ s (Ni) 1 A
As-deposited 100 8C 200 8C 300 8C 350 8C 400 8C
53 54 55 55 48 40
– – – – 14 33
54.4 51 52 52 44 32
14 15 14 14 14 24
– – – – 14 33
16 17 12 14 12 25
is mainly due to the magnetostriction effects. Upon further annealing upto 300 8C overall shape of M–H loop remains same. However, HC and HS value varies from 100 to 300 8C annealed sample (see Table 3). ML sample annealed at 350 8C again change in shape to the square one having corresponding HC and HS values 5Oe and 12Oe, respectively. At this temperature of annealing our GIXRD and GIXRR result clearly shows the formation of FeNi3 phase, which is magnetically softer. This phase further grows with anneal-
ing temperature of 400 8C. From fitted GIXRR patterns it is clear that as-deposited Fe/Ni ML structure is modified to FeNi3/Ni ML. Hence, observed magnetization behaviour corresponding to ML sample annealed at 300, 350 and 400 8C is a consequence of FeNi3 phase formation and certainly not due to the grain size effect [11]. The magnetizations as function of magnetic field of as-deposited as well as the annealed Fe50/Ni50 ML structures at different temperatures are shown in Fig. 6.
˚ )/Ni(86 A ˚ )]10 ML for as-deposited as well as the annealed samples. Fig. 5. Hysteresis loop of [Fe(29 A
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
259
Table 3 ˚ )/Ni(86 A ˚ )]10 ML Coercive field, HS, MS and grain size as the function of the annealing temperature for [Fe(29 A Temperature
HC (Oe) ˚) Grain size (A HS (Oe) MS (emu/cm3)
As-deposited
100 8C
200 8C
300 8C
350 8C
400 8C
10 75 5 23 5 837
10 70 5 55 5 560
4 83 5 47 5 820
4 281 5 47 5 820
5 380 5 12 5 773
3 404 5 20 5 688
All samples show well saturation magnetisation behaviour. The typical smother M–H loop shape obtained in the as-deposited Fe50/Ni50 ML may be due to the combined effect of highly textured growth, stress present in the deposited layers and the nature of magnetostriction evolved. With the annealing at 100 8C the measured M–H loop become like square shape. This opposite behaviour obtained in M–H loop as compared to the previous case mainly due to the highly textured nature of Fe50/Ni50 ML and the different response of magnetostriction with the
annealing. At the 200 8C temperature the M–H loop become more perfectly square, this shape is maintained upto 400 8C annealed sample. The calculated HC and MS values corresponding to the sample annealed at different temperatures are given in Table 4. Upto 200 8C annealed sample, the observed changes in HC and MS values are mainly results of inplane grain growth. The increment of HC with annealing upto 200 8C may be explained on the basis of the increase of grain size in these samples calculated from measured GIXRD patterns. For nano crystalline size
˚ )/Ni(50 A ˚ )]10 MLs. Fig. 6. Hysteresis loop of as-deposited as well as the annealed [Fe(50 A
260
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261
Table 4 ˚ )/Ni(50 A ˚ )]10 ML Variation of MS, grain size and HC with the annealing temperature of [Fe(50 A Temperature
˚ (1) Grain size, A HC (Oe) MS (emu/cm3)
As-deposited
100 8C
200 8C
300 8C
350 8C
400 8C
57 6 998
60 8 1177
66 10 1272
66 4 1173
63 10 1450
78, 285 7 1010
grain (grain size < 20 nm), it is reported that HC increases with increasing grain size [12]. It can be mentioned here that grain size obtained from GIXRD measurements gives grain size perpendicular to film direction only. Because of 2D nature of the structure, in-plane size of grain can be much larger than the perpendicular to the plane. In such case crystallographic grains of nanometer size can be treated as single magnetic domain. The observed decrease in HC value on further annealing upto 300 8C is due to the inplane crystallographic domains may grow bigger in size and hence, single domain types magnetic structure can change to the multi domain type even though perpendicular grain size remains same. The other reason may be the segregation of small amount of FeNi3 alloy formation at the interface, which is not detected in the corresponding GIXRD and GIXRR measurements. However, the GIXRR measurements show the presence of FeNi3 alloy phase formation ˚ at the interface. Upon having layer thickness of 14 A further annealing at 400 8C, the thickness of FeNi3 ˚ . The observed increase alloy layer increases upto 33 A in the HC values corresponding to the sample annealed at 350 8C can be explained on as follows. In this case ML structure can be modelled as try layer with FeNi3 layer at interface, with corresponding calculated layer thickness as given in Table 2. Upon further annealing HC values again decrease and one can attribute this to further growth of FeNi3 phase at interface. In this case the individual layer thickness in try layer structure is modified as listed in Table 2. The contribution of interface layer seems to be more dominating in lowering the HC values.
4. Conclusion GIXRD measurements carried out on both types of ML structures show different growth behavior. Parti-
˚ )/Ni(50 A ˚ )]10 ML structure is highly cularly [Fe(50 A textured. The GIXRR measurements indicated alloy phase formation at interface. A FeNi3 phase having ˚ is easily formed in thickness of the order of the 100 A ˚ ˚ [Fe(29 A)/Ni(86 A)]10 ML sample annealed above ˚ )/ 300 8C. However, in case of ML sample [Fe(50 A ˚ ˚ Ni(50 A)]10 a very thin layer of the order of 14–33 A is formed above 300 8C. The measured M–H loop of as-deposited sample of both MLs structures are distinctly different. In one case there is square loop indicating switching action with applied magnetic field, while in the second case the gradual variation in the magnetization is observed. This difference is attributed to magnetostriction and different growth morphology. The observed M–H behavior of ML structure annealed at higher temperature of 300 8C is mainly due to the FeNi3 phase formation at the interface.
Acknowledgements The author would like to thank Mr. Satish Potdar for the deposition of multilayer samples.
References [1] T. Veres, M. Chai, R.W. Cochrane, M. Rouabhi, S. Roorda, P. Desjardins, Thin Solid Films 382 (2001) 167. [2] J. Neamtu, J. Magn. Magn. Mater. 157/158 (1996) 461. [3] Y. Nagai, M. Senda, T. Thosima, J. Appl. Phys. 63 (1988) 1136. [4] M. Takahashi, X. Hu, Y. Kawazoe, Solid State Com. 95 (1995) 127. [5] Y. Li, C. Polaczyk, J. Erxmeyer, J. Kapoor, D. Riegel, J. Magn. Magn. Mater. 140–144 (1995) 551. [6] T. Veres, R. Morel, R.W. Cochrane, R. Abdouche, M. Sutton, J. Appl. Phys. 81 (1997) 4758. [7] Y. Nagai, K. Yanagisawa, M. Senda, J. Appl. Phys. 64 (1988) 1343.
R. Gupta et al. / Applied Surface Science 238 (2004) 254–261 [8] M. Chai, T. Veres, R. Morel, R.W. Cochrane, J. Appl. Phys. 79 (1996) 6289. [9] A.C. Erlich, D.J. Gillespie, T.M. Edelstein, C. Tritt, A.S. Kim, S.B. Qadri, J. Appl. Phys. 70 (1991) 5819. [10] L.G. Parratt, Phys. Rev. 95 (1954) 359.
261
[11] F. Pfeirfer, C. Radeloff, J. Magn. Magn. Mater. 19 (1980) 190. [12] G.C. Hadijipanayis (Ed.), Magnetic Hysteresis in Novel Magnetic Material, Kluwer Academic Publishers, Dordrecht, 1996, p. 713.