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Effect of thermomagnetic treatment on magnetoresistive properties of trilayer thin films based on FexNi100-x and Cu
Intermetallics 93 (2018) 1–5
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Effect of thermomagnetic treatment on magnetoresistive properties of trilayer thin films based on FexNi100-x and Cu
MARK
Yu.O. Shkurdoda, I.M. Pazukha∗, A.M. Chornous Department of Applied Physics, Sumy State University, Rymskogo-Korsakova 2, 40007 Sumy, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: Trilayer thin films Magnetoresistive properties Anisotropic magnetoresistance Spin-dependent electron scattering
We investigated magnetoresistive properties of as-deposited and annealed in magnetic field at different temperatures F/N/F trilayer films based on ferromagnetic FexNi100-x (F-layer) and nonmagnetic Cu (N-layer). The results demonstrate that spin-dependent scattering is realized both in as-deposited and annealed at 400 K trilayer films within the range of thicknesses dF = 15–40 nm and dN = 6–15 nm and at the Ni concentration in magnetic layers cNi ≤ 90 wt %. The annealing at 550 K leads to the decrease of isotropic magnetoresistance value and to the appearance of anisotropy in thin films at the cNi ≥ 50 wt %. It was found that the maximum value of isotropic magnetoresistance has been observed for as-deposited Fe50Ni50(30 nm)/Cu(6 nm)/Fe50Ni50(30 nm)/S thin film at room temperature. The value of isotropic magnetoresistance increases in 1.3–2.2 times with the decline of the measurement temperature from 293 to 120 К depending on components concentration in magnetic layers.
1. Introduction Physical properties of nanostructures based on ferromagnetic alloy FeхNi100-х and nonmagnetic Cu have been actively investigated during the last decade because spin-dependent electron scattering is realized in them [1–3]. The interest in such film systems is associated with their wide application in magnetic sensors, random access memory units, automotive electronics, biomedical technologies, etc. However, the necessity of the further research and experimental investigation of thin film structures, which corresponds to the additional requirements of functional aspect (reducing the sensors size, increasing their sensitivity, ensuring parameters reproducibility), arises [4]. The main basic requirements for such structures are high thermal stability of their electrophysical and magnetic parameters; the predictability of electrophysical and magnetoresistive properties behavior within temperature change. At present, there is a huge amount of experimental investigations concerning the methods of preparation and magnetoresistive properties analysis of multilayer film systems based on FeхNi100-х [5,6]. For, example, in Ref. [7] has been reported experimental investigation on giant magnetoresistance of NiFe/Cu multilayer stacks with 2 nm single layer thickness. Magnetic properties have been shown [7] to be depend on the annealing temperature. Annealing at temperatures up to 520 K leads to a dramatic decrease of magnetoresistance. However, the majority of the papers have been devoted to studies of the thin film structures based on ferromagnetic alloys Fe20Ni80 and Fe50Ni50
∗
(permalloy) [7–9]. Meanwhile, there is no report on investigation of physical properties of multilayer film structures based on FexNi100-x within the wide range of concentrations prepared at the same conditions. In this paper, we have investigated the annealing effect on the value of transverse and longitudinal magnetoresistance (MR) of magnetic FeхNi100-х/Cu/FeхNi100-х/S trilayer thin films (S is substrate) within the wide range both layers thicknesses and components concentration of magnetic alloy. 2. Experimental details Trilayers film systems with thickness of the layers 1–50 nm were prepared in the vacuum chamber with a base pressure of 10−4 Pa. The layer by layer condensation was carried out by metals evaporation from independent sources (Cu by the method of thermal evaporation and FeхNi100-х by electron-beam evaporation). The bulk alloys of corresponding composition were used as initial materials for obtaining FeхNi100-х layers. The condensation was conducted at room temperature with a deposition rate ω = 0.5–1 nm/s that corresponds evaporator operating conditions. The thickness of the layers during the condensation was controlled with the quartz resonator. Glass plates with previously deposited pads were used as substrates for subsequent investigation of the system's magnetoresistive properties. Geometrical sizes (2 mm × 10 mm) of thin films for their resistance measurement
Corresponding author. E-mail address: [email protected] (I.M. Pazukha).
https://doi.org/10.1016/j.intermet.2017.10.007 Received 11 July 2017; Received in revised form 22 September 2017; Accepted 12 October 2017 0966-9795/ © 2017 Elsevier Ltd. All rights reserved.
Intermetallics 93 (2018) 1–5
Y.O. Shkurdoda et al.
were specified by windows in the nichrome foil mechanical masks. The results of X-ray spectrometry analysis have shown that chemical composition of prepared thin films coincides with chemical composition of initial bulk alloys. The measurement error did not exceed 2%. The measurements of longitudinal (magnetic field in the sample plane and parallel to current) and transverse (magnetic field in the sample plane and perpendicular to current) magnetoresistance and thermomagnetic treatment of the thin films were carried out in magnetic field with an amplitude 200 mT in special ultrahigh vacuum apparatus with the base pressure of 10−6-10−7 Pa. The measurement error of magnetoresitance is 0.02%. The heat treatment of the samples at 400, 500, 600 and 700 К for 15 min was carried out under a magnetic field of magnetic induction 50 mT applied in the direction of the current density vector. The value of longitudinal and transverse MR has been calculated by equation (R(B) – R0)/R0, where R(B) is the current value of resistance in the magnetic field B; R0 is the resistance of the sample in the field of the coercivity Вc. 3. Results and discussion It is well-known fact that conditions of heat treatment of thin film systems affect their phase state, crystal structure, magnetic and magnetoresistive properties etc. (see, for example, Ref. [10–13]). This correlates with results of our investigations too. Typical field dependences at room temperature in the longitudinal and transverse geometries for as-deposited and annealed at 700 К FeхNi100-х/Cu/FeхNi100-х trilayer thin films with dF = 20 nm, dN = 6 nm, х = 80 wt% and dF = 35 nm, dN = 6 nm, х = 50 wt % are shown in Fig. 1. The figure presents the variation of MR for annealed samples in comparison with as-deposited. As can see from Fig. 1, Fe80Ni20(20)/Cu(6)/Fe80Ni20(20)/S film system can be characterized by isotropic MR before and after annealing. At the same time, the shape of MR curves for Fe50Ni50(35)/Cu(6)/Fe50Ni50(35)/S film system changes from isotropic to anisotropic after annealing. Temperature dependences of isotropic magnetorisistance for thin film systems with different layer thicknesses and different component concentration in magnetic layers are presented at Fig. 2. Experimental data for FexNi100-x/Cu/FexNi100-x/S triple layer films show effect of thermomagnetic treatment on the value of isotropic magnetoresistance. It should be noted that the isotropic MR is conditional by spin-dependent electron scattering, so in this case the effect of giant magnetoresistance (GMR) is observed. The analysis of these dependences allows to conclude that effects of thermal treatment depend not only on the component concentration in the magnetic layers and their thickness dF,
Fig. 2. Isotropic magnetoresistance as a function of the annealing temperature for FexNi100-x(dF)/Cu(dN)/FexNi100-x(dF)/S trilayer thin films at dF = 35 (a) and 15 nm (b), different Ni concentration in the magnetic layers and different thicknesses of nonmagnetic layer dN.
but on the thickness of nonmagnetic layer dN. Under this condition the influence of thermal treatment can be significantly different. Thus, the annealing at temperatures 400, 500, 600 and 700 К does not change MR isotropic field dependences behavior for FexNi100-x/Cu/FexNi100-x/S thin films with dF = 20–30 nm, dCu = 5–15 nm and cNi ≤ 40%. The variation of effect amplitude and widening of magnetoresistive loops are observed only. The high-temperature annealing at 600, 700 К leads to the appearance of MR anisotropic character for thin film systems with the concentration of Ni within the range cNi = 50–60 wt % and at dF = 20–30 nm and dN = 8–15 nm. The decrease of effective thickness of nonmagnetic layer dN to 5–8 nm reduces the heat treatment temperature to 500 K at which anisotropic magnetoresistance (AMR) emerges. For film systems with thinner separate layers dN = 3–5 nm, the appearance of anisotropic magnetoresistance is observed after annealing at 400 K. In all cases denoted previously, the emerge of magnetoresistance anisotropy is caused by breaking of structural integrity Fig. 1. Field dependences of longitudinal (||) and transverse (+) magnetoresistance for as-deposited (a, c) and annealed at 700 К (b, d) FeхNi100-х/Cu/FeхNi100-х trilayer thin films with dF = 20 nm, dN = 6 nm, cNi = 20 wt% (a, b) and dF = 35 nm, dN = 6 nm, cNi = 50 wt % (c, d). The measurement temperature is 300 К.
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Y.O. Shkurdoda et al.
of copper interlayer. As a result, the magnetostatic couple between magnetic layers significantly increases. This excludes the possibility of separate remagnetization of magnetic layers that is the necessary condition for realization electron spin-dependent scattering. For thin film with higher nickel concentration cNi > 80 wt %, the isotropy of MR field dependences remains at the low-temperature heat treatment only (Тann = 400 К). It should be mentioned that for Ni/Cu/ Ni/S trilayer thin film systems, that we investigated previously, regardless of layers thickness and temperature of heat treatment the anisotropic behavior of MR is observed only. Consider more detail the peculiarity of annealing temperature effect on the value of isotropic magnetoresistance. It can be spectated at Fig. 2a (curves 1, 2) that the dependences at the Ni concentration in magnetic layers up to 40 wt% and at the relatively large thickness of magnetic layers (dF = 30–35 nm) are similar and described by nonmonotonic character. The maximum value of GMR is observed after heat treatment at 500 K. With the increasing of Ni concentration in magnetic layer (curves 3, 4) the dependences become monotonous and the GMR value decreases with the rise of the heat treatment temperature. It should be noted that our results on trilayer film systems with cNi = 80 wt% (curve 4) are close to those that reported in Ref. [5] on sputtered Ni80Fe20/Cu multilayers. According to Ref. [5], the breakdown of the GMR occurs in the Ni80Fe20/Cu multilayers after Тann = 500 K. This result explained by the processes of interdiffusion between magnetic and nonmagnetic layers in above 500 K [14]. Consider the dependences (ΔR/R0)max = f(Тann) for thin film systems with relatively thin magnetic layers dF = 15–20 nm and relatively thick nonmagnetic interlayer dN = 10–15 nm (Fig. 2b). As can see from Fig. 2b, for FexNi100-x/Cu/FexNi100-x/S trilayer thin film systems at cNi = 10–20 wt% in magnetic layers the inverse tendency is observed. The low-temperature annealing leads to the reduction of the isotropic magnetoresistance value, whereas the high-temperature annealing leads to the rise of the MR value. Such behavior of dependences (ΔR/R0)max = f(Тann) can be explained by the fact that low-temperature annealing at Тann = 400–500 К is attended by interdiffusion of atoms and, as a result, by disruption of structural integrity both magnetic layers and nonmagnetic interlayer (simple diffusive mixing of magnetic and nonmagnetic components). The following increasing of annealing temperature (600, 700 К) can lead to granular state formation and, as a consequence, to intensification of electron spin-dependent scattering role. According to Ref. [15,16], there is a possibility that in this case the effect of electron grain boundary scattering influences behavior of GMR value, as well as, the aforesaid reasons. So, with the increase of annealing temperature the grain boundary atom penetration from copper interlayer to the volume of magnetic layer goes up. That is set conditions for the rise of electron spin-dependent scattering at the grain boundaries. The annealing stimulates diffusion processes and leads to appearing of redundant vacancies in the volume of magnetic layer. This is one of the reasons of GMR insignificant growth, whereas the flow of spin-polarized electron that moves to interface between layers increases. It should be noted that in our previous work [17,18], we reported that a phase state of the film samples (dF = 10–50 nm and dN = 5–20 nm, х = 20–40 wt %) is fcc-Ni3Fe (а = 0.355–0.358 nm) and fcc-Cu phases. The samples with Ni concentration in magnetic layers х = 50–60 wt% have a two-phase structure – fcc-NiFe (а = 0.358–0.362 nm) + fcc-Cu. This result matches those reported in Ref. [1] on NiFe-Cu films prepared using the magnetron-controlled sputtering method. After annealing at 700 K, in these systems as a result of interdiffusion between the individual layers are formed solid solutions (Ni3Fe, Cu) and (NiFe, Cu), respectively [14]. For the as-deposited and annealed at 700 K samples with х > 60 wt% the phase state is bcc(Fe-Ni) + fcc-Cu. In order to establish the causes of isotropic magnetoresistance changes, it is necessary to analyze the annealing temperature
Fig. 3. Resistivity ρ (a) and ΔRmax value (b) as a functions of the annealing temperature for FexNi100-x(dF)/Cu(dN)/FexNi100-x(dF)/S trilayer thin films at the different Ni concentration in the magnetic layers and different thickness of magnetic and nonmagnetic layers: 1 – dF = 15 nm, dN = 10 nm; 2 – dF = 15 nm, dN = 10 nm; 3 – dF = 35 nm, dN = 5 nm; 4 – dF = 35 nm, dN = 5 nm; 5 – dF = 35 nm, dN = 6 nm; 6 – dF = 35 nm, dN = 8 nm.
Fig. 4. Maximum value of isotropic magnetoresistance as a function of the Ni concentration for as-deposited FexNi100-x/Cu/FexNi100-x trilayer thin films (1) and annealed up to 400 (2), 550 (3) and 700 К (4).
dependences of resistivity ρ and the value of ΔRmax for the first cycle of heating. To start with, the resistivity of all investigated samples in the process of annealing irreversibly decreases (Fig. 3а). It should be noted that the resistivity of as-deposited samples significantly depends on layers thickness and component concentration in the magnetic layers. Thus, the values of resistivity are (40–55)·10−7 Оhm·m for film systems with dF = 10–20 nm, dN = 5–10 nm; and (10–20)·10−7 Оhm·m for film systems with dF = 30–40 nm, dN = 10–20 nm. The large values of resistivity of trilayer systems (in comparison with values of resistivity for pure metal in bulk state) can be explained by more defect structures of thinnest layers and occurrence of defects at the interfaces of layers on the one hand; and by the small size of crystallites (compared to the bulk state) and size effects in electrical conductivity on the other hand. The low-temperature annealing (Тann = 400 К) sequences the irreversible decline of resistance in 1.2–1.5 times that is conditioned by healing of structural defects. Simultaneously, the change of the ΔRmax value depends on both layers thickness and component concentration in magnetic layers. Thus, the ΔRmax value increases in 1.2–1.5 times for samples with cNi ≤ 40 wt % and dF = 30–40 nm after annealing at 3
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Fig. 5. Value of isotropic magnetoresistance as a function of the measurement temperature for as-deposited (а) and annealed at 400 (b), 550 (c) and 700 К (d) FexNi100-x(dF)/Cu(dN)/FexNi100-x(dF) trilayer thin films at the different Ni concentration in magnetic layer: 1 – dF = 30 nm, dN = 3 nm; 2 – dF = 30 nm, dN = 5 nm; 3 – dF = 30 nm, dN = 6 nm; 4 – dF = 30 nm, dN = 8 nm.
Тann = 400 К. Such changes of ρ and ΔRmax values lead to increasing (ΔRmax)/R0 in 1.5–2 times. The main reasons of (ΔRmax)/R0 ratio growth at the low-temperature annealing are the healing of structural defects and changing of surface roughness. The rise of annealing temperature up to 500 К causes the reduction of resistivity and the value of ΔRmax, so ratio (ΔRmax)/R0 goes up. The growth of annealing temperature up to 600–700 К leads to the decline of ρ and ΔRmax values, thus the GMR value falls. Rather different situation is observed for film systems with 40 < cNi < 90 wt %, in particular, the change of ρ and ΔRmax values during annealing leads to the decline of isotropic magnetoresistance and to the emerge of MR anisotropic character. These results correlate those reported in Refs. [4,19] for multilayer film systems Fe19Ni81/Cu. The reason of isotropic MR increasing is an interdiffusion of atoms that causes the rise of interface length, formation of solid solution at the interfaces and the reduction of uniaxial magnetic anisotropy. Summarizing the results of the investigation of annealing effect on the value of the isotropic magnetoresistance, the following conclusion can be made. Firstly, film systems with cNi = 50 wt %, dF = 30–40 nm and dN = 6 nm (Fig. 4, curves 1, 2) reveal the maximum values of GMR for as-deposited (1.2%) and annealed at 400 К (0.7%) samples. Secondly, the maximum at the dependences (ΔR/R0)max = f(Тann) for annealed at 550 К thin films (Fig. 3, curves 3) is displaced in the area of smaller Ni concentration (cNi = 20 wt %). Thirdly, the pattern (ΔR/ R0)max = f(Тann) has monotonic character. The value of isotropic MR decreases with the increasing Ni concentration only and obtains anisotropic character of magnetoresistance at cNi = 40–50 wt %. The results of isotropic magnetoresistance measurements at the different annealing temperatures within the range 120–700 К are presented at Fig. 4. For as-deposited thin films (Fig. 5a) the temperature dependences of magnetoresistance have monotonous character and for Fe60Ni40/Cu/Fe60Ni40/S thin films with dN = 4 nm (Fig. 5a, curve 2) the GMR value is 1.7% at 120 K (overtakes in 2–2.2 times). Whereas, for Fe50Ni50/Cu/Fe50Ni50/S thin films (Fig. 5a, curve 3), the GMR value increases in 1.3–1.5 times only with the temperature reduction to 120 K. For thin films annealed at 400, 550 and 700 К (Fig. 5 b–d) the temperature dependences character does not change. The fall of effect amplitude caused by the rise of measurement temperature is associated with process of electron-phonon scattering, especially in the non-magnetic interlayer. The emerge of this mechanism of scattering leads to the decline part of processes of electron spin-dependent scattering that causes GMR effect and prevent electron transmission from one ferromagnetic layer to another [20,21].
4. Conclusion Thereby, it has been demonstrated experimentally that for trilayer thin films FexNi100-x/Cu/FexNi100-x/S (0 ≤ cNi ≤ 100 wt %) prepared by layer by layer condensation the MR behavior and their value depend on the magnetic and nonmagnetic layers thickness, component concentration in the magnetic layers and heat treatment condition. It was determined: 1. The isotropic MR field dependences are observed for as-deposited thin film systems at cNi < 90 wt%. The maximum value of isotropic magnetoresistance at the room temperature is 1.2% for as-deposited Fe50Ni50/Cu/Fe50Ni50/S thin film with dF = 30 nm and dN = 6 nm. 2. The heat treatment of samples with concentration cNi < 40 wt % and dF = 20–30 nm, dN = 4–15 nm at 400, 500 К leads to increase of isotropic MR. In the case of samples with 40 < cNi < 90 wt % the heat treatment leads to the decline of isotropic MR only and appearance of anisotropic magnetoresistance. The maximum value of isotropic MR of thermally stabilized films is 0.6% for Fe80Ni20/Cu/ Fe80Ni20/S thin films with dF = 30 nm and dN = 5 nm. 3. Temperature dependences of isotropic magnetoresistance have a monotonous character. GMR Notably, increase in the GMR value in 1.3–2.2 times (depending on the Ni concentration in the magnetic layers and thickness of magnetic and nonmagnetic layers) was observed with decreasing measurement temperature from 300 to 120 K. 4. The achieved experimental results denote an opportunity of heat treatment application for thermal stabilization characteristics of magnetoresistive elements based on FexNi100-x/Cu/FexNi100-x/S trilayer thin films (cNi < 40 wt%) in the vacuum at the temperature 500 K directly after condensation. Funding This work was funded by the State Program of the Ministry of Education and Science of Ukraine 0116U002623. References [1] Changzheng Wang, Xiaoguang Xiao, Haiguan Hu, Yonghua Rong, T.Y. Hsu, Nanoparticle morphology in FeNi-Cu granular films with giant magnetoresistance, Phys. B 392 (2007) 72–78. [2] Yuan-Tsung Chen, Jiun-Yi Tseng, S.H. Lin, T.S. Sheu, Effect of grain size on optical and electrical properties of Ni80Fe20 thin films, J. Magn. Magn. Mater 360 (2014)
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[13] YuO. Shkurdoda, А.М. Chornous, V.B. Loboda, YuМ. Shabelnyk, V.О. Kravchenko, L.V. Dekhtyaruk, Structure and magnetoresistive properties of thee-layer film systems based on permalloy and copper, J. Nano- Electron. Phys. 8 (2016) 02056. [14] W. Bruckner, S. Baunack, M. Hecker, J.-I.M. onch, L.v. Loyen, C.M. Schneider, Interdiffusion in NiFe/Cu/NiFe trilayers: possible failure mechanism for magnetoelectronic devices, Appl. Phys. Lett. 77 (2000) 358. [15] N.N. Svirkova, Effect of grain-boundary electron scattering in magnetic layers on the magnetoresistive ratio of a polycrystalline sandwich under transverse charge transfer, Techn. Phys. 74 (2004) 14–19. [16] L.A. Chebotkevich, YuD. Vorob’ev, I.N. Burkova, A.V. Kornilov, Structure and magnetic properties of annealed Co/Cu/Co films, Phys. Metal. Metallogr. 89 (2000) 263–268. [17] Y.O. Shkurdoda, A.M. Chornous, Y.M. Shabelnyk, V.B. Loboda, The influence of the concentration of components in magnetic layers on the magnetoresistive properties of three-layer film systems based on FexNi1−x and Cu, J. Magn. Magn. Mater 443 (2017) 190–194. [18] Y.O. Shkurdoda, The influence of structural and phase state on the magnetoresistive properties of film systems based on FexNi100-x and Cu, J. Nano- Electron. Phys. 9 (2017) 04008. [19] K. Wakoh, T. Hihara, T.J. Konno, K. Sumiyama, K. Suzuki, Comparative GMR study of Fe/Cu granular films deposited by co-evaporation and cluster beam techniques, Mater. Sci. Eng. A 217/218 (1996) 326–330. [20] Y.K. Yang, L.H. Chen, Y.H. Chang, Y.D. Yao, Microstructure, magnetic and transport properties of melt-spun Cu60(Ni0.8Fe0.2)40 alloys, J. Magn. Magn. Mater 189 (1998) 195–201. [21] P.D. Kim, D.L. Khalyapin, I.A. Turpanov, L.A. Li, A. Ya, Beten’kova, S.V. Kan, Anomalous temperature dependence of the magnetoresistance in Co/Cu multilayers, Phys. Solid State 49 (2000) 1688–1690.
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Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Effect of thermomagnetic treatment on magnetoresistive properties of trilayer thin films based on FexNi100-x and Cu
MARK
Yu.O. Shkurdoda, I.M. Pazukha∗, A.M. Chornous Department of Applied Physics, Sumy State University, Rymskogo-Korsakova 2, 40007 Sumy, Ukraine
A R T I C L E I N F O
A B S T R A C T
Keywords: Trilayer thin films Magnetoresistive properties Anisotropic magnetoresistance Spin-dependent electron scattering
We investigated magnetoresistive properties of as-deposited and annealed in magnetic field at different temperatures F/N/F trilayer films based on ferromagnetic FexNi100-x (F-layer) and nonmagnetic Cu (N-layer). The results demonstrate that spin-dependent scattering is realized both in as-deposited and annealed at 400 K trilayer films within the range of thicknesses dF = 15–40 nm and dN = 6–15 nm and at the Ni concentration in magnetic layers cNi ≤ 90 wt %. The annealing at 550 K leads to the decrease of isotropic magnetoresistance value and to the appearance of anisotropy in thin films at the cNi ≥ 50 wt %. It was found that the maximum value of isotropic magnetoresistance has been observed for as-deposited Fe50Ni50(30 nm)/Cu(6 nm)/Fe50Ni50(30 nm)/S thin film at room temperature. The value of isotropic magnetoresistance increases in 1.3–2.2 times with the decline of the measurement temperature from 293 to 120 К depending on components concentration in magnetic layers.
1. Introduction Physical properties of nanostructures based on ferromagnetic alloy FeхNi100-х and nonmagnetic Cu have been actively investigated during the last decade because spin-dependent electron scattering is realized in them [1–3]. The interest in such film systems is associated with their wide application in magnetic sensors, random access memory units, automotive electronics, biomedical technologies, etc. However, the necessity of the further research and experimental investigation of thin film structures, which corresponds to the additional requirements of functional aspect (reducing the sensors size, increasing their sensitivity, ensuring parameters reproducibility), arises [4]. The main basic requirements for such structures are high thermal stability of their electrophysical and magnetic parameters; the predictability of electrophysical and magnetoresistive properties behavior within temperature change. At present, there is a huge amount of experimental investigations concerning the methods of preparation and magnetoresistive properties analysis of multilayer film systems based on FeхNi100-х [5,6]. For, example, in Ref. [7] has been reported experimental investigation on giant magnetoresistance of NiFe/Cu multilayer stacks with 2 nm single layer thickness. Magnetic properties have been shown [7] to be depend on the annealing temperature. Annealing at temperatures up to 520 K leads to a dramatic decrease of magnetoresistance. However, the majority of the papers have been devoted to studies of the thin film structures based on ferromagnetic alloys Fe20Ni80 and Fe50Ni50
∗
(permalloy) [7–9]. Meanwhile, there is no report on investigation of physical properties of multilayer film structures based on FexNi100-x within the wide range of concentrations prepared at the same conditions. In this paper, we have investigated the annealing effect on the value of transverse and longitudinal magnetoresistance (MR) of magnetic FeхNi100-х/Cu/FeхNi100-х/S trilayer thin films (S is substrate) within the wide range both layers thicknesses and components concentration of magnetic alloy. 2. Experimental details Trilayers film systems with thickness of the layers 1–50 nm were prepared in the vacuum chamber with a base pressure of 10−4 Pa. The layer by layer condensation was carried out by metals evaporation from independent sources (Cu by the method of thermal evaporation and FeхNi100-х by electron-beam evaporation). The bulk alloys of corresponding composition were used as initial materials for obtaining FeхNi100-х layers. The condensation was conducted at room temperature with a deposition rate ω = 0.5–1 nm/s that corresponds evaporator operating conditions. The thickness of the layers during the condensation was controlled with the quartz resonator. Glass plates with previously deposited pads were used as substrates for subsequent investigation of the system's magnetoresistive properties. Geometrical sizes (2 mm × 10 mm) of thin films for their resistance measurement
Corresponding author. E-mail address: [email protected] (I.M. Pazukha).
https://doi.org/10.1016/j.intermet.2017.10.007 Received 11 July 2017; Received in revised form 22 September 2017; Accepted 12 October 2017 0966-9795/ © 2017 Elsevier Ltd. All rights reserved.
Intermetallics 93 (2018) 1–5
Y.O. Shkurdoda et al.
were specified by windows in the nichrome foil mechanical masks. The results of X-ray spectrometry analysis have shown that chemical composition of prepared thin films coincides with chemical composition of initial bulk alloys. The measurement error did not exceed 2%. The measurements of longitudinal (magnetic field in the sample plane and parallel to current) and transverse (magnetic field in the sample plane and perpendicular to current) magnetoresistance and thermomagnetic treatment of the thin films were carried out in magnetic field with an amplitude 200 mT in special ultrahigh vacuum apparatus with the base pressure of 10−6-10−7 Pa. The measurement error of magnetoresitance is 0.02%. The heat treatment of the samples at 400, 500, 600 and 700 К for 15 min was carried out under a magnetic field of magnetic induction 50 mT applied in the direction of the current density vector. The value of longitudinal and transverse MR has been calculated by equation (R(B) – R0)/R0, where R(B) is the current value of resistance in the magnetic field B; R0 is the resistance of the sample in the field of the coercivity Вc. 3. Results and discussion It is well-known fact that conditions of heat treatment of thin film systems affect their phase state, crystal structure, magnetic and magnetoresistive properties etc. (see, for example, Ref. [10–13]). This correlates with results of our investigations too. Typical field dependences at room temperature in the longitudinal and transverse geometries for as-deposited and annealed at 700 К FeхNi100-х/Cu/FeхNi100-х trilayer thin films with dF = 20 nm, dN = 6 nm, х = 80 wt% and dF = 35 nm, dN = 6 nm, х = 50 wt % are shown in Fig. 1. The figure presents the variation of MR for annealed samples in comparison with as-deposited. As can see from Fig. 1, Fe80Ni20(20)/Cu(6)/Fe80Ni20(20)/S film system can be characterized by isotropic MR before and after annealing. At the same time, the shape of MR curves for Fe50Ni50(35)/Cu(6)/Fe50Ni50(35)/S film system changes from isotropic to anisotropic after annealing. Temperature dependences of isotropic magnetorisistance for thin film systems with different layer thicknesses and different component concentration in magnetic layers are presented at Fig. 2. Experimental data for FexNi100-x/Cu/FexNi100-x/S triple layer films show effect of thermomagnetic treatment on the value of isotropic magnetoresistance. It should be noted that the isotropic MR is conditional by spin-dependent electron scattering, so in this case the effect of giant magnetoresistance (GMR) is observed. The analysis of these dependences allows to conclude that effects of thermal treatment depend not only on the component concentration in the magnetic layers and their thickness dF,
Fig. 2. Isotropic magnetoresistance as a function of the annealing temperature for FexNi100-x(dF)/Cu(dN)/FexNi100-x(dF)/S trilayer thin films at dF = 35 (a) and 15 nm (b), different Ni concentration in the magnetic layers and different thicknesses of nonmagnetic layer dN.
but on the thickness of nonmagnetic layer dN. Under this condition the influence of thermal treatment can be significantly different. Thus, the annealing at temperatures 400, 500, 600 and 700 К does not change MR isotropic field dependences behavior for FexNi100-x/Cu/FexNi100-x/S thin films with dF = 20–30 nm, dCu = 5–15 nm and cNi ≤ 40%. The variation of effect amplitude and widening of magnetoresistive loops are observed only. The high-temperature annealing at 600, 700 К leads to the appearance of MR anisotropic character for thin film systems with the concentration of Ni within the range cNi = 50–60 wt % and at dF = 20–30 nm and dN = 8–15 nm. The decrease of effective thickness of nonmagnetic layer dN to 5–8 nm reduces the heat treatment temperature to 500 K at which anisotropic magnetoresistance (AMR) emerges. For film systems with thinner separate layers dN = 3–5 nm, the appearance of anisotropic magnetoresistance is observed after annealing at 400 K. In all cases denoted previously, the emerge of magnetoresistance anisotropy is caused by breaking of structural integrity Fig. 1. Field dependences of longitudinal (||) and transverse (+) magnetoresistance for as-deposited (a, c) and annealed at 700 К (b, d) FeхNi100-х/Cu/FeхNi100-х trilayer thin films with dF = 20 nm, dN = 6 nm, cNi = 20 wt% (a, b) and dF = 35 nm, dN = 6 nm, cNi = 50 wt % (c, d). The measurement temperature is 300 К.
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of copper interlayer. As a result, the magnetostatic couple between magnetic layers significantly increases. This excludes the possibility of separate remagnetization of magnetic layers that is the necessary condition for realization electron spin-dependent scattering. For thin film with higher nickel concentration cNi > 80 wt %, the isotropy of MR field dependences remains at the low-temperature heat treatment only (Тann = 400 К). It should be mentioned that for Ni/Cu/ Ni/S trilayer thin film systems, that we investigated previously, regardless of layers thickness and temperature of heat treatment the anisotropic behavior of MR is observed only. Consider more detail the peculiarity of annealing temperature effect on the value of isotropic magnetoresistance. It can be spectated at Fig. 2a (curves 1, 2) that the dependences at the Ni concentration in magnetic layers up to 40 wt% and at the relatively large thickness of magnetic layers (dF = 30–35 nm) are similar and described by nonmonotonic character. The maximum value of GMR is observed after heat treatment at 500 K. With the increasing of Ni concentration in magnetic layer (curves 3, 4) the dependences become monotonous and the GMR value decreases with the rise of the heat treatment temperature. It should be noted that our results on trilayer film systems with cNi = 80 wt% (curve 4) are close to those that reported in Ref. [5] on sputtered Ni80Fe20/Cu multilayers. According to Ref. [5], the breakdown of the GMR occurs in the Ni80Fe20/Cu multilayers after Тann = 500 K. This result explained by the processes of interdiffusion between magnetic and nonmagnetic layers in above 500 K [14]. Consider the dependences (ΔR/R0)max = f(Тann) for thin film systems with relatively thin magnetic layers dF = 15–20 nm and relatively thick nonmagnetic interlayer dN = 10–15 nm (Fig. 2b). As can see from Fig. 2b, for FexNi100-x/Cu/FexNi100-x/S trilayer thin film systems at cNi = 10–20 wt% in magnetic layers the inverse tendency is observed. The low-temperature annealing leads to the reduction of the isotropic magnetoresistance value, whereas the high-temperature annealing leads to the rise of the MR value. Such behavior of dependences (ΔR/R0)max = f(Тann) can be explained by the fact that low-temperature annealing at Тann = 400–500 К is attended by interdiffusion of atoms and, as a result, by disruption of structural integrity both magnetic layers and nonmagnetic interlayer (simple diffusive mixing of magnetic and nonmagnetic components). The following increasing of annealing temperature (600, 700 К) can lead to granular state formation and, as a consequence, to intensification of electron spin-dependent scattering role. According to Ref. [15,16], there is a possibility that in this case the effect of electron grain boundary scattering influences behavior of GMR value, as well as, the aforesaid reasons. So, with the increase of annealing temperature the grain boundary atom penetration from copper interlayer to the volume of magnetic layer goes up. That is set conditions for the rise of electron spin-dependent scattering at the grain boundaries. The annealing stimulates diffusion processes and leads to appearing of redundant vacancies in the volume of magnetic layer. This is one of the reasons of GMR insignificant growth, whereas the flow of spin-polarized electron that moves to interface between layers increases. It should be noted that in our previous work [17,18], we reported that a phase state of the film samples (dF = 10–50 nm and dN = 5–20 nm, х = 20–40 wt %) is fcc-Ni3Fe (а = 0.355–0.358 nm) and fcc-Cu phases. The samples with Ni concentration in magnetic layers х = 50–60 wt% have a two-phase structure – fcc-NiFe (а = 0.358–0.362 nm) + fcc-Cu. This result matches those reported in Ref. [1] on NiFe-Cu films prepared using the magnetron-controlled sputtering method. After annealing at 700 K, in these systems as a result of interdiffusion between the individual layers are formed solid solutions (Ni3Fe, Cu) and (NiFe, Cu), respectively [14]. For the as-deposited and annealed at 700 K samples with х > 60 wt% the phase state is bcc(Fe-Ni) + fcc-Cu. In order to establish the causes of isotropic magnetoresistance changes, it is necessary to analyze the annealing temperature
Fig. 3. Resistivity ρ (a) and ΔRmax value (b) as a functions of the annealing temperature for FexNi100-x(dF)/Cu(dN)/FexNi100-x(dF)/S trilayer thin films at the different Ni concentration in the magnetic layers and different thickness of magnetic and nonmagnetic layers: 1 – dF = 15 nm, dN = 10 nm; 2 – dF = 15 nm, dN = 10 nm; 3 – dF = 35 nm, dN = 5 nm; 4 – dF = 35 nm, dN = 5 nm; 5 – dF = 35 nm, dN = 6 nm; 6 – dF = 35 nm, dN = 8 nm.
Fig. 4. Maximum value of isotropic magnetoresistance as a function of the Ni concentration for as-deposited FexNi100-x/Cu/FexNi100-x trilayer thin films (1) and annealed up to 400 (2), 550 (3) and 700 К (4).
dependences of resistivity ρ and the value of ΔRmax for the first cycle of heating. To start with, the resistivity of all investigated samples in the process of annealing irreversibly decreases (Fig. 3а). It should be noted that the resistivity of as-deposited samples significantly depends on layers thickness and component concentration in the magnetic layers. Thus, the values of resistivity are (40–55)·10−7 Оhm·m for film systems with dF = 10–20 nm, dN = 5–10 nm; and (10–20)·10−7 Оhm·m for film systems with dF = 30–40 nm, dN = 10–20 nm. The large values of resistivity of trilayer systems (in comparison with values of resistivity for pure metal in bulk state) can be explained by more defect structures of thinnest layers and occurrence of defects at the interfaces of layers on the one hand; and by the small size of crystallites (compared to the bulk state) and size effects in electrical conductivity on the other hand. The low-temperature annealing (Тann = 400 К) sequences the irreversible decline of resistance in 1.2–1.5 times that is conditioned by healing of structural defects. Simultaneously, the change of the ΔRmax value depends on both layers thickness and component concentration in magnetic layers. Thus, the ΔRmax value increases in 1.2–1.5 times for samples with cNi ≤ 40 wt % and dF = 30–40 nm after annealing at 3
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Fig. 5. Value of isotropic magnetoresistance as a function of the measurement temperature for as-deposited (а) and annealed at 400 (b), 550 (c) and 700 К (d) FexNi100-x(dF)/Cu(dN)/FexNi100-x(dF) trilayer thin films at the different Ni concentration in magnetic layer: 1 – dF = 30 nm, dN = 3 nm; 2 – dF = 30 nm, dN = 5 nm; 3 – dF = 30 nm, dN = 6 nm; 4 – dF = 30 nm, dN = 8 nm.
Тann = 400 К. Such changes of ρ and ΔRmax values lead to increasing (ΔRmax)/R0 in 1.5–2 times. The main reasons of (ΔRmax)/R0 ratio growth at the low-temperature annealing are the healing of structural defects and changing of surface roughness. The rise of annealing temperature up to 500 К causes the reduction of resistivity and the value of ΔRmax, so ratio (ΔRmax)/R0 goes up. The growth of annealing temperature up to 600–700 К leads to the decline of ρ and ΔRmax values, thus the GMR value falls. Rather different situation is observed for film systems with 40 < cNi < 90 wt %, in particular, the change of ρ and ΔRmax values during annealing leads to the decline of isotropic magnetoresistance and to the emerge of MR anisotropic character. These results correlate those reported in Refs. [4,19] for multilayer film systems Fe19Ni81/Cu. The reason of isotropic MR increasing is an interdiffusion of atoms that causes the rise of interface length, formation of solid solution at the interfaces and the reduction of uniaxial magnetic anisotropy. Summarizing the results of the investigation of annealing effect on the value of the isotropic magnetoresistance, the following conclusion can be made. Firstly, film systems with cNi = 50 wt %, dF = 30–40 nm and dN = 6 nm (Fig. 4, curves 1, 2) reveal the maximum values of GMR for as-deposited (1.2%) and annealed at 400 К (0.7%) samples. Secondly, the maximum at the dependences (ΔR/R0)max = f(Тann) for annealed at 550 К thin films (Fig. 3, curves 3) is displaced in the area of smaller Ni concentration (cNi = 20 wt %). Thirdly, the pattern (ΔR/ R0)max = f(Тann) has monotonic character. The value of isotropic MR decreases with the increasing Ni concentration only and obtains anisotropic character of magnetoresistance at cNi = 40–50 wt %. The results of isotropic magnetoresistance measurements at the different annealing temperatures within the range 120–700 К are presented at Fig. 4. For as-deposited thin films (Fig. 5a) the temperature dependences of magnetoresistance have monotonous character and for Fe60Ni40/Cu/Fe60Ni40/S thin films with dN = 4 nm (Fig. 5a, curve 2) the GMR value is 1.7% at 120 K (overtakes in 2–2.2 times). Whereas, for Fe50Ni50/Cu/Fe50Ni50/S thin films (Fig. 5a, curve 3), the GMR value increases in 1.3–1.5 times only with the temperature reduction to 120 K. For thin films annealed at 400, 550 and 700 К (Fig. 5 b–d) the temperature dependences character does not change. The fall of effect amplitude caused by the rise of measurement temperature is associated with process of electron-phonon scattering, especially in the non-magnetic interlayer. The emerge of this mechanism of scattering leads to the decline part of processes of electron spin-dependent scattering that causes GMR effect and prevent electron transmission from one ferromagnetic layer to another [20,21].
4. Conclusion Thereby, it has been demonstrated experimentally that for trilayer thin films FexNi100-x/Cu/FexNi100-x/S (0 ≤ cNi ≤ 100 wt %) prepared by layer by layer condensation the MR behavior and their value depend on the magnetic and nonmagnetic layers thickness, component concentration in the magnetic layers and heat treatment condition. It was determined: 1. The isotropic MR field dependences are observed for as-deposited thin film systems at cNi < 90 wt%. The maximum value of isotropic magnetoresistance at the room temperature is 1.2% for as-deposited Fe50Ni50/Cu/Fe50Ni50/S thin film with dF = 30 nm and dN = 6 nm. 2. The heat treatment of samples with concentration cNi < 40 wt % and dF = 20–30 nm, dN = 4–15 nm at 400, 500 К leads to increase of isotropic MR. In the case of samples with 40 < cNi < 90 wt % the heat treatment leads to the decline of isotropic MR only and appearance of anisotropic magnetoresistance. The maximum value of isotropic MR of thermally stabilized films is 0.6% for Fe80Ni20/Cu/ Fe80Ni20/S thin films with dF = 30 nm and dN = 5 nm. 3. Temperature dependences of isotropic magnetoresistance have a monotonous character. GMR Notably, increase in the GMR value in 1.3–2.2 times (depending on the Ni concentration in the magnetic layers and thickness of magnetic and nonmagnetic layers) was observed with decreasing measurement temperature from 300 to 120 K. 4. The achieved experimental results denote an opportunity of heat treatment application for thermal stabilization characteristics of magnetoresistive elements based on FexNi100-x/Cu/FexNi100-x/S trilayer thin films (cNi < 40 wt%) in the vacuum at the temperature 500 K directly after condensation. Funding This work was funded by the State Program of the Ministry of Education and Science of Ukraine 0116U002623. References [1] Changzheng Wang, Xiaoguang Xiao, Haiguan Hu, Yonghua Rong, T.Y. Hsu, Nanoparticle morphology in FeNi-Cu granular films with giant magnetoresistance, Phys. B 392 (2007) 72–78. [2] Yuan-Tsung Chen, Jiun-Yi Tseng, S.H. Lin, T.S. Sheu, Effect of grain size on optical and electrical properties of Ni80Fe20 thin films, J. Magn. Magn. Mater 360 (2014)
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