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The effect of Co or Ag addition on magnetotransport and magnetic properties of Ni80Fe20 thin films
Vacuum 116 (2015) 31e35
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The effect of Co or Ag addition on magnetotransport and magnetic properties of Ni80Fe20 thin films Ia.M. Lytvynenko a, *, I.M. Pazukha a, V.V. Bibyk b a b
Sumy State University, Rymskogo-Korsakova 2, 40007 Sumy, Ukraine Konotop Institute of Sumy State University, Myru 24., 41615 Konotop, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2015 Received in revised form 24 February 2015 Accepted 27 February 2015 Available online 6 March 2015
The magnetotransport and magnetic properties of Ni80Fe20, (Ni80Fe20)50Co50 and (Ni80Fe20)50Ag50 thin films prepared by the thermal evaporation method are presented. Effect of magnetic (Co) and nonmagnetic (Ag) elements addition on magnetoresistance, saturation field, coercivity and sensitivity to the magnetic field of permalloy films were described. Structure and magnetic properties of thin film alloys demonstrated a strong dependence from annealing temperature. Also was shown that structure of all types can be used as sensitive elements of magnetic sensors for low magnetic field detecting. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Permalloy Alloys Thin film Magnetoresistance Saturation field Coercivity
In recent few decades, considerable attention has been devoted to the investigation of the physical properties of thin magnetic films of 3d transition metals and their alloys as well as thin film systems based on it because of their application in nanoelectronics. So great interest to the thin film systems based on alloys mostly related with possibility in simple way create the wide range of new material with different basic properties (structural, electrical or magnetic) by changing of components concentration or adding a couple dopes. One of the most investigated such kind materials is the thin films of soft magnetic permalloy (alloy of nominally 80% nickel and 20% iron (Ni80Fe20)), which exhibit applicable electrical, magnetic and mechanical features [1e3]. For example, thin permalloy films are characterized by relatively large anisotropic magnitoresistance (AMR) [4], high permeability, near zero magnetostriction [5], small coercivity [6] with structure phase reliability [7,20] and have been used as sensing elements in rotation and magnetic field sensors [1,4]. However, the practical application of thin permalloy films is limited. Thus, the third elements addition to Ni80Fe20 composition is necessary to modify the properties of sensing elements to enhance of the application area. For example, among multitudinous systems based on permalloy, the FeNieCo alloys have received
* Corresponding author. E-mail address: [email protected] (Ia.M. Lytvynenko). http://dx.doi.org/10.1016/j.vacuum.2015.02.035 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
attention because of their high saturation magnetization and thermal stability [8]. Heterogeneous FeNieAg films characterized by large changes of resistivity in magnetic fields and may be used as magnetic field sensors [9]. Despite the fact that permalloy-based alloys are widely studied we could not find a lot of data about application the high rate (1e5 nm/s) thermoresistive coevaporation technique for alloy deposition and its effect on thin films properties. In this report, an experimental study on the structure and magnetic properties for three type of structures based on thin permalloy films prepared by thermoresistive sputtering has been carried out. Emphasize has been made on adding magnetic (Co) and non-magnetic (Ag) third elements at concentration 50 at.%. The concentration choice has been executed for the reason that film systems (Ni80Fe20)50X50 (where X ¼ Ag or Co) are almost not researched. It is well-known fact that the phase state, structure, magnetic and magnetotransport properties of thin film systems substantially depend on preparation condition: condensation rate, pressure of residual gases, material and substrate temperature etc. Chemical interaction evaporated materials with residual gases in deposition chamber causes the samples contamination by the undesirable phases, especially by oxides of sputtered metals. As a result it may cause a deterioration of magnetotransport properties due with
32
Ia.M. Lytvynenko et al. / Vacuum 116 (2015) 31e35
changes of crystal structure and phase state of thin film samples. Therefore, to obtain the samples with relatively high purity without oxide or hydride phases and correct interpretation expected results, all experiments were carried out in the same vacuum condition in HV chamber. Three series of samples were prepared: samples of A type are thin permalloy films with composition Ni80Fe20; samples of B type are (Ni80Fe20)50Co50 thin films; samples of C type are (Ni80Fe20)50Ag50 thin films. The samples of A type were deposited on amorphous glass-ceramic substrate by thermoresistive sputtering using 79 NM (79e80 wt.% nikell, 2e5 wt.% molybdenum, 13e16 wt.% iron) in HV chamber with a base pressure of 104 Pa at room temperature. The samples of B and C types were received by co-evaporation technique using permalloy 79 NM and Co or Ag independent sources respectively. The condensation rate was in the range 1e5 nm/s. Thickness of the samples was measured by МІІ-4 optical interferometer. To research annealing temperature effect on structure and magnetic properties of thin films, samples of all types were annealed up to 700 and 900 К under high vacuum for 20 min. The chemical composition of the films was calculated
on the basis of deposited components weight, and further controlled by energy-dispersive X-ray spectroscopy (EDX). Calculated data correspond to EDX-results, which revealed the material composition of the sputtered films: A type is 80 at.% nickel and 20 at.% iron; B type e 40 at.% nickel, 10 at.% iron and 50 at.% cobalt; C type e 40 at.% nickel, 10 at.% iron and 50 at.% silver. The transport properties were measured using software-hardware complex with current-in-plane geometries in an external magnetic field from 0 to 500 mТ at room temperature according the method described at work [10]. The magnetoresistive measurement (MR) was carried out in two geometries: “out of plane” (⊥) and “in plane” (jj) when external magnetic field is applied perpendicular or parallel to sample plane respectively. MR value has been calculated as DR/ Rs ¼ [(R(B) Rs)/Rs] 100%, where Rs is resistance of thin film samples at saturated or maximum field and R(B) is current value resistance in a magnetic field. Magnetic measurements were performed at 300 K using commercial vibrating sample magnetometer VSM Lake Shore Model 7400 (Institute Jean Lamour, University of Lorraine, Nancy, France) at “in plane” and “out of plane” applied magnetic field.
Fig. 1. Magnetoresistive curves measured at applied magnetic field out of plane (a, b, c) and in plane of the samples (d, e, f) for the Ni80Fe20, (Ni80Fe20)50Ag50 and (Ni80Fe20)50Co50 single layer alloys before (a, d) and after annealing to 700 K (b, e) and 900 K (c, f).
Ia.M. Lytvynenko et al. / Vacuum 116 (2015) 31e35
The research of samples phase state was carried in our previous works [11,12]. According to [11], the phase state of thin permalloy films corresponds to fcc-Ni3Fe structure with lattice parameter a ¼ 0.353e0.355 nm in annealing temperature range from 300 to 900 K. Besides, as-deposited samples have highly dispersed (mean grain size L ¼ 5 nm) labyrinthine structure. The annealing leads to gradual increasing of grain size, which reaches 15 nm without labyrinthine structure changing. Results presented in work [11] are consistent with the data reported by Tyschenko et al. [13] where the structural and phase state analysis for thin NiхFe1х film alloys with different components concentration received at almost similar conditions was presented. In previous work [12] we presented the results of detailed analysis of structure and phase state of thin film (Ni80Fe20)50Co50 alloy (samples of B type). As-deposited films have two-phase state (fcc-Ni3Fe þ hcp-Со) [13], which stayed unchanged after annealing in temperature range up to 700 K. The formation of disordered solid solution on the base of ternary alloy NiFeeCo with fcc-phase and lattice parameter а ¼ 0.354 nm was observed after annealing up to 900 K. B type samples after condensation have nanoscale structure with mean grain size L ¼ 6e7 nm. The rise of temperature up to 900 K leads to increasing of mean grain size value in 4e5 times (L ¼ 25e30 nm). The results of diffraction data analysis for (Ni80Fe20)50Ag50 thin film alloy show that samples have two-phase state after condensation and after annealing up to 900 K. It corresponds to fcc-Ni3Fe with a ¼ 0.353e0.355 nm and fcc-Ag with a ¼ 0.408e0.409 nm. The structure of the B type films at equal concentration of magnetic and nonmagnetic components consists of separated NiFe ferromagnetic particles embedded into nonmagnetic Ag matrix. These results completely agreed with data presented by Pohorilyi et al. [14]. Annealing to 900 K also causes an increasing of the mean size of Ag crystallites. The lack of additional phases with gas atoms inclusions testified the ability of the vacuum of 104 Pa ensures cleanliness of deposited samples. Figs. 1 and 2 show typical magnetoresistive curves and normalized magnetization vs. field for as-deposited and annealed to 700 and 900 K samples: Ni80Fe20, (Ni80Fe20)50Co50 and (Ni80Fe20)50Ag50. The results of parameters calculation (magnetoresistance (MR), saturation field (BS), coercive field (ВC) and sensitivity to the magnetic field (SB)) for Ni80Fe20 and permalloy-based thin film alloys are summarized in Table 1. The study of MR curves for A type samples allows to conclude that a value of magnetoresistance of thin permalloy film depends on researched geometries and annealing temperature. For asdeposited samples MR vs. applied field curves partly overlap in the both geometries, so observed hysteretic behavior of magnetoresistance. The MR value is relatively small with maximum 0.35% in out of plane geometry that correlate with data obtained in Ref. [15]. Annealing of the films does not change the curves shape but increase MR value from 0.35 to 0.42 % and from 0.08 to 0.1 % in ⊥ and jj geometries respectively. This fact can be explained as a result of healing of defects and thermal stabilization in grains and domains. A further increase of the MR value due to the fact that the maximum grain size of NiFe has been achieved. The research of magnetoresistive properties of B type film samples showed that thin film alloy (Ni80Fe20)50Co50 appears anisotropic magnetoresistance. The maximum value has been received in ⊥ geometry (0.35%). For MR vs. magnetic field curves saturation in out of plane geometry for as-deposited and annealing up to 700 K was not observed. The saturation appears after annealing up to 900 K (~200 mT, it is in 3e4 times more than in jj geometry). This result suggests about existence of easy plane reversal in jj geometry. The processes of defects healing and thermal stabilization after annealing up to 700 K cause the decreasing
33
of MR values in both geometries. The formation of disordered solid solution on the base of NiFeeCo ternary alloy causes the changes of shape of MR curves and the increasing of MR value in four times. Addition of Ag as third element with 50 at.% concentration transforms the shape of MR curves into the bell-like in out of plane geometry. Besides, the saturation in magnetic field up to 500 mТ was not observed. The MR values for sample of C type increase to 0.45% and 0.23% in ⊥ and jj geometries respectively (in comparison with MR values for A type sample). These magnetoresistance values are close to values of similar systems prepared by sputtering techniques [9,15] and rather less compared to data for heterogeneous FeNieAg films prepared by beam evaporated technique [14]. Under the assumption that films of C type are granular structured the conclusion about appearing spin-dependent scattering on an isolation of the NiFe grains can be done. The presence of mechanism scattering on interfaces between magnetic grain and non-magnetic matrix causes the increasing of magnetoresistance
Fig. 2. Normalized magnetization vs field measurements of the Ni80Fe20, (Ni80Fe20)50Ag50 and (Ni80Fe20)50Co50 single layer alloy samples before (a) and after annealing to 700 K (b) and 900 K (c) at in plane applied magnetic field.
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Ia.M. Lytvynenko et al. / Vacuum 116 (2015) 31e35
Table 1 Calculated parameters for Ni80Fe20 and permalloy-based thin films. MR (%), ⊥
A B C
BS (mT), jj
MR (%), jj
BC (mT), jj
SB%/T, ⊥
SB%/T, jj
300
700
900
300
700
900
300
700
900
300
700
900
300
700
900
300
700
900
0.35 0.35 0.45
0.33 0.07 0.36
0.42 0.27 0.26
0.08 0.06 0.23
0.09 0.08 0.14
0.10 0.14 0.07
3.24 12.2 1.52
2.22 19.7 4.32
7.74 24.1 4.65
0.45 3.71 0.25
0.39 6.90 0.55
0.74 9.88 0.58
0.69 0.88 0.66
0.63 0.14 0.67
0.83 0.46 0.51
0.17 0.12 0.50
0.18 0.16 0.28
0.20 0.27 0.14
[16,17]. The magnitude of MR vs. applied field decreases after heat treated at 700 K and then increase again after annealing up to 900 К. The process of annealing up to 700 K increase occur gradual coalescence of ferromagnetic NiFe grains, reorientation of Ag crystallites and, as a result, blurring of interfaces magnetic grain/ nonmagnetic matrix. The difference in the shape of MR curves in out of plane and in plane geometries for (Ni80Fe20)50Ag50 alloy indicates an easy remagnetization of the sample mostly as a consequence of the shape anisotropy predominance. The typical normalized magnetization curves for as-deposited thin permalloy films, measured at applied magnetic field in plane are presented at Fig. 2a (black line). The samples of A type exhibit a small saturation field and a coercivity with values 3.24 and 0.45 mT at 300 K, respectively. This hysteresis loop is typical for magnetic films with in-plane magnetization. Similar result was received by Peng et al. [18] for thin permalloy films with 20 nm thickness. The evolution of magnetization curves of the Ni80Fe10 film after annealing up to 700 and 900 K are presented at Fig. 2b and c (black line). The permalloy film shows a similar magnetization behavior after annealing up to 700 K. The values of BS and BC change a little (decreases to 2.22 and 0.39 mT). However, normalized magnetization curves of the samples annealed up to 900 K are quite different from the as-deposited. The relative changes of magnetic parameter are BS ¼ 139% and BC ¼ 64%. This result can be explained by structured characterization of the samples if noticed that annealing up to 700 K caused grain size increasing [19]. Fig. 2 shows the normalized magnetizations vs. field measurements of B type sample (blue (in the web version) curves). VSM investigation of magnetic properties of thin film (Ni80Fe20)50Co50 alloy showed that hysteresis loop has close squared shape with sharp remagnetization at applied magnetic field parallel to the sample plane as for A tape sample, but values of BS and BC increase in 3.8 and 8.2 times respectively. The film magnetization changed completely in the ranges ±13 mТ at asdeposited state and ±23 mТ after annealing to 900 K. The square shape of hysteresis loop is typical for easy reversal process. Besides, the increasing of remanent magnetization, coercivity and saturation field at increasing of heat treatment temperature is observed. Relative changes of BS and BC are 88% and 154% respectively in temperature range 300e900 K. The increasing of saturated field in the case of ferromagnetic alloy obtained after finishing of remagnetization processes, namely domain growth with magnetic moment, which oriented in the direction of easy magnetization axes as a result of process of domain boundaries propagation and rotation of the sample magnetization vector in the direction of applied magnetic field. The sample of C type shows a similar magnetization behavior as a permalloy film (Fig. 2, red line). The hysteresis squareness at in-plane applied field keeps high. Addition a non-magnetic material as third element does not change significantly the values of BS and BC in contrast to the adding of magnetic materials. The values of saturation field and coercivity decrease from 3.24 to 1.52 and from 0.45 to 0.25 mT, respectively, in compare with permalloy film. Saturation field decreasing might be related with the
structure disorder. The annealing causes the increasing of BS and BC values; furthermore it significantly changed the hysteresis loops shape. Saturation field reaches a maximum 4.65 mT and coercivity 0.58 mT after annealing up to 900 K. These results similar to experimental investigations reported by Weber et al. [21,22]. One of the parameters, which defines areas of practical applications, is film system sensitivity to applied magnetic field (Table 1). In this work the maximum value of SB was determined by the equation (DR/R(Bs))max/DВ, where (DR/R(Bs))max e the maximum MR value; DВ e a magnetic induction change from saturation (or the maximum value) BS to demagnetization BC. The analysis of presented data allows determining that film structures of all types can be used as sensitive elements of magnetic sensors for low magnetic field detecting. In summary, we have studied the magnetic properties of permalloy thin films in dependence on the adding of magnetic (Co) and nonmagnetic (Ag) third elements. In the case of Co addition, the curves of MR vs. applied field appear anisotropic character with maximum value in ⊥ geometry. Upon annealing, the anisotropy in plane of samples vanishes, the values of BS and BC increase as a result of formation disordered solid solution based on the NiFeeCo ternary alloy. For permalloy-Ag system, films structure changes from highly dispersed labyrinthine to granular and MR value increases in both geometries because of spin-dependent scattering on the NiFe grains. However, the magnetization behavior is similar for systems of A and C types. The further annealing leads to blurring of interfaces magnetic grain/nonmagnetic matrix and as a result to decreasing of MR value and increasing of BS and BC values. Acknowledgments The authors thank to the “nanomagnetism and spintronics” team at Institute of Jean Lamour (University of Lorraine, France) for technical assistance and providing equipments for experiments. This work was partially funded by the State Program of the Ministry of Education and Science of Ukraine concerning research training abroad. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
Volmer M, Neamtu J. J Magn Magn Mater 2010;322:1631. Bartok A, Laurent D, Razek A. J Magn Magn Mater 2013;326:116. Tran QH, Sunjong O, Jong-Ryul J, CheolGi K. Sens Actuators A 2010;157:42. Shalygina EE, Perepelova EV, Kozlovski LV, Tamanis E, Mukasheva MA, Shalygin AN. Tech Phys Lett 2007;33:456. Singh J, Gupta SK, Singh AK, Kothari P, Kotnala RK, Akhtar J. J Magn Magn Mater 2012;324:999. Neamtu BV, Chicinas I, Isnard O, Popa F, Pop V. Intermetallics 2011;19:19. Kim S-G, Yun U-J, Kim J-Y, Kim J, Cho K. J Appl Phys 2001;90:3533. Dalavi SB, Theerthagiri J, Raja MM, Panda RN. J Magn Magn Mat 2013;344:30. Dieny B, Teixiera SR, Rodmacq B, Cowache C, Auffret S, Redon O, et al. J Magn Magn Mat 1994;130:197. Demydenko MH, Protsenko SI, Kostyuk DM, Cheshko IV. J Nano Electron Phys 2011;3:1063. Makukha ZM, Synashenko OV, Lytvynenko IaM, Shabelnyk YuM, Protsenko IYu. Proc Int Conf Promis Innov Sci Educ Manuf Transp 2012;3:64. Lytvynenko IaM, Pazukha IM, Bibyk VV. J Nano Electron Phys 2014;6:02014.
Ia.M. Lytvynenko et al. / Vacuum 116 (2015) 31e35 [13] Tyschenko KV, Protsenko IYu. Metallofiz Noveishie Tekhnol 2012;34:907. [14] Pohorilyi AN, Kravets AF, Shypil EV, Pod’yalovsky DY, Vovk AYa, Kim CS, et al. Thin Solid Films 2003;423:218. [15] Miyazaki T, Azima T, Sato F. J Magn Magn Mat 1989;81:86. [16] Watson ML, Barnard JA, Hossain S, Parker MR. J Appl Phys 1993;70:5506. [17] Bakonyi I, Peter L. Prog Mater Sci 2010;52:107. [18] Peng Ch, Zhang Sh, Li G, Dai D. J Appl Phys 1994;76:998.
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[19] Svalov AV, Aseguinolaza IR, Garcia-Arribas A, Orue I, Barandiaran JM, Alonso J, et al. IEEE Trans Magn 2010;46:333. [20] Nahrwold G, Scholtyssek JM, Motl-Ziegler S, Albrecht O, Merkt U, Meier G. J Appl Phys 2010;108:013907. [21] Weber W, Back CH, Bischof A, Wursch Ch, Allenspach R. Phys Rev Lett 1996;76:1940. [22] Weber W, Allenspach R, Bischof A. Appl Phys Lett 1997;70:520.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Rapid communication
The effect of Co or Ag addition on magnetotransport and magnetic properties of Ni80Fe20 thin films Ia.M. Lytvynenko a, *, I.M. Pazukha a, V.V. Bibyk b a b
Sumy State University, Rymskogo-Korsakova 2, 40007 Sumy, Ukraine Konotop Institute of Sumy State University, Myru 24., 41615 Konotop, Ukraine
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 February 2015 Received in revised form 24 February 2015 Accepted 27 February 2015 Available online 6 March 2015
The magnetotransport and magnetic properties of Ni80Fe20, (Ni80Fe20)50Co50 and (Ni80Fe20)50Ag50 thin films prepared by the thermal evaporation method are presented. Effect of magnetic (Co) and nonmagnetic (Ag) elements addition on magnetoresistance, saturation field, coercivity and sensitivity to the magnetic field of permalloy films were described. Structure and magnetic properties of thin film alloys demonstrated a strong dependence from annealing temperature. Also was shown that structure of all types can be used as sensitive elements of magnetic sensors for low magnetic field detecting. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Permalloy Alloys Thin film Magnetoresistance Saturation field Coercivity
In recent few decades, considerable attention has been devoted to the investigation of the physical properties of thin magnetic films of 3d transition metals and their alloys as well as thin film systems based on it because of their application in nanoelectronics. So great interest to the thin film systems based on alloys mostly related with possibility in simple way create the wide range of new material with different basic properties (structural, electrical or magnetic) by changing of components concentration or adding a couple dopes. One of the most investigated such kind materials is the thin films of soft magnetic permalloy (alloy of nominally 80% nickel and 20% iron (Ni80Fe20)), which exhibit applicable electrical, magnetic and mechanical features [1e3]. For example, thin permalloy films are characterized by relatively large anisotropic magnitoresistance (AMR) [4], high permeability, near zero magnetostriction [5], small coercivity [6] with structure phase reliability [7,20] and have been used as sensing elements in rotation and magnetic field sensors [1,4]. However, the practical application of thin permalloy films is limited. Thus, the third elements addition to Ni80Fe20 composition is necessary to modify the properties of sensing elements to enhance of the application area. For example, among multitudinous systems based on permalloy, the FeNieCo alloys have received
* Corresponding author. E-mail address: [email protected] (Ia.M. Lytvynenko). http://dx.doi.org/10.1016/j.vacuum.2015.02.035 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
attention because of their high saturation magnetization and thermal stability [8]. Heterogeneous FeNieAg films characterized by large changes of resistivity in magnetic fields and may be used as magnetic field sensors [9]. Despite the fact that permalloy-based alloys are widely studied we could not find a lot of data about application the high rate (1e5 nm/s) thermoresistive coevaporation technique for alloy deposition and its effect on thin films properties. In this report, an experimental study on the structure and magnetic properties for three type of structures based on thin permalloy films prepared by thermoresistive sputtering has been carried out. Emphasize has been made on adding magnetic (Co) and non-magnetic (Ag) third elements at concentration 50 at.%. The concentration choice has been executed for the reason that film systems (Ni80Fe20)50X50 (where X ¼ Ag or Co) are almost not researched. It is well-known fact that the phase state, structure, magnetic and magnetotransport properties of thin film systems substantially depend on preparation condition: condensation rate, pressure of residual gases, material and substrate temperature etc. Chemical interaction evaporated materials with residual gases in deposition chamber causes the samples contamination by the undesirable phases, especially by oxides of sputtered metals. As a result it may cause a deterioration of magnetotransport properties due with
32
Ia.M. Lytvynenko et al. / Vacuum 116 (2015) 31e35
changes of crystal structure and phase state of thin film samples. Therefore, to obtain the samples with relatively high purity without oxide or hydride phases and correct interpretation expected results, all experiments were carried out in the same vacuum condition in HV chamber. Three series of samples were prepared: samples of A type are thin permalloy films with composition Ni80Fe20; samples of B type are (Ni80Fe20)50Co50 thin films; samples of C type are (Ni80Fe20)50Ag50 thin films. The samples of A type were deposited on amorphous glass-ceramic substrate by thermoresistive sputtering using 79 NM (79e80 wt.% nikell, 2e5 wt.% molybdenum, 13e16 wt.% iron) in HV chamber with a base pressure of 104 Pa at room temperature. The samples of B and C types were received by co-evaporation technique using permalloy 79 NM and Co or Ag independent sources respectively. The condensation rate was in the range 1e5 nm/s. Thickness of the samples was measured by МІІ-4 optical interferometer. To research annealing temperature effect on structure and magnetic properties of thin films, samples of all types were annealed up to 700 and 900 К under high vacuum for 20 min. The chemical composition of the films was calculated
on the basis of deposited components weight, and further controlled by energy-dispersive X-ray spectroscopy (EDX). Calculated data correspond to EDX-results, which revealed the material composition of the sputtered films: A type is 80 at.% nickel and 20 at.% iron; B type e 40 at.% nickel, 10 at.% iron and 50 at.% cobalt; C type e 40 at.% nickel, 10 at.% iron and 50 at.% silver. The transport properties were measured using software-hardware complex with current-in-plane geometries in an external magnetic field from 0 to 500 mТ at room temperature according the method described at work [10]. The magnetoresistive measurement (MR) was carried out in two geometries: “out of plane” (⊥) and “in plane” (jj) when external magnetic field is applied perpendicular or parallel to sample plane respectively. MR value has been calculated as DR/ Rs ¼ [(R(B) Rs)/Rs] 100%, where Rs is resistance of thin film samples at saturated or maximum field and R(B) is current value resistance in a magnetic field. Magnetic measurements were performed at 300 K using commercial vibrating sample magnetometer VSM Lake Shore Model 7400 (Institute Jean Lamour, University of Lorraine, Nancy, France) at “in plane” and “out of plane” applied magnetic field.
Fig. 1. Magnetoresistive curves measured at applied magnetic field out of plane (a, b, c) and in plane of the samples (d, e, f) for the Ni80Fe20, (Ni80Fe20)50Ag50 and (Ni80Fe20)50Co50 single layer alloys before (a, d) and after annealing to 700 K (b, e) and 900 K (c, f).
Ia.M. Lytvynenko et al. / Vacuum 116 (2015) 31e35
The research of samples phase state was carried in our previous works [11,12]. According to [11], the phase state of thin permalloy films corresponds to fcc-Ni3Fe structure with lattice parameter a ¼ 0.353e0.355 nm in annealing temperature range from 300 to 900 K. Besides, as-deposited samples have highly dispersed (mean grain size L ¼ 5 nm) labyrinthine structure. The annealing leads to gradual increasing of grain size, which reaches 15 nm without labyrinthine structure changing. Results presented in work [11] are consistent with the data reported by Tyschenko et al. [13] where the structural and phase state analysis for thin NiхFe1х film alloys with different components concentration received at almost similar conditions was presented. In previous work [12] we presented the results of detailed analysis of structure and phase state of thin film (Ni80Fe20)50Co50 alloy (samples of B type). As-deposited films have two-phase state (fcc-Ni3Fe þ hcp-Со) [13], which stayed unchanged after annealing in temperature range up to 700 K. The formation of disordered solid solution on the base of ternary alloy NiFeeCo with fcc-phase and lattice parameter а ¼ 0.354 nm was observed after annealing up to 900 K. B type samples after condensation have nanoscale structure with mean grain size L ¼ 6e7 nm. The rise of temperature up to 900 K leads to increasing of mean grain size value in 4e5 times (L ¼ 25e30 nm). The results of diffraction data analysis for (Ni80Fe20)50Ag50 thin film alloy show that samples have two-phase state after condensation and after annealing up to 900 K. It corresponds to fcc-Ni3Fe with a ¼ 0.353e0.355 nm and fcc-Ag with a ¼ 0.408e0.409 nm. The structure of the B type films at equal concentration of magnetic and nonmagnetic components consists of separated NiFe ferromagnetic particles embedded into nonmagnetic Ag matrix. These results completely agreed with data presented by Pohorilyi et al. [14]. Annealing to 900 K also causes an increasing of the mean size of Ag crystallites. The lack of additional phases with gas atoms inclusions testified the ability of the vacuum of 104 Pa ensures cleanliness of deposited samples. Figs. 1 and 2 show typical magnetoresistive curves and normalized magnetization vs. field for as-deposited and annealed to 700 and 900 K samples: Ni80Fe20, (Ni80Fe20)50Co50 and (Ni80Fe20)50Ag50. The results of parameters calculation (magnetoresistance (MR), saturation field (BS), coercive field (ВC) and sensitivity to the magnetic field (SB)) for Ni80Fe20 and permalloy-based thin film alloys are summarized in Table 1. The study of MR curves for A type samples allows to conclude that a value of magnetoresistance of thin permalloy film depends on researched geometries and annealing temperature. For asdeposited samples MR vs. applied field curves partly overlap in the both geometries, so observed hysteretic behavior of magnetoresistance. The MR value is relatively small with maximum 0.35% in out of plane geometry that correlate with data obtained in Ref. [15]. Annealing of the films does not change the curves shape but increase MR value from 0.35 to 0.42 % and from 0.08 to 0.1 % in ⊥ and jj geometries respectively. This fact can be explained as a result of healing of defects and thermal stabilization in grains and domains. A further increase of the MR value due to the fact that the maximum grain size of NiFe has been achieved. The research of magnetoresistive properties of B type film samples showed that thin film alloy (Ni80Fe20)50Co50 appears anisotropic magnetoresistance. The maximum value has been received in ⊥ geometry (0.35%). For MR vs. magnetic field curves saturation in out of plane geometry for as-deposited and annealing up to 700 K was not observed. The saturation appears after annealing up to 900 K (~200 mT, it is in 3e4 times more than in jj geometry). This result suggests about existence of easy plane reversal in jj geometry. The processes of defects healing and thermal stabilization after annealing up to 700 K cause the decreasing
33
of MR values in both geometries. The formation of disordered solid solution on the base of NiFeeCo ternary alloy causes the changes of shape of MR curves and the increasing of MR value in four times. Addition of Ag as third element with 50 at.% concentration transforms the shape of MR curves into the bell-like in out of plane geometry. Besides, the saturation in magnetic field up to 500 mТ was not observed. The MR values for sample of C type increase to 0.45% and 0.23% in ⊥ and jj geometries respectively (in comparison with MR values for A type sample). These magnetoresistance values are close to values of similar systems prepared by sputtering techniques [9,15] and rather less compared to data for heterogeneous FeNieAg films prepared by beam evaporated technique [14]. Under the assumption that films of C type are granular structured the conclusion about appearing spin-dependent scattering on an isolation of the NiFe grains can be done. The presence of mechanism scattering on interfaces between magnetic grain and non-magnetic matrix causes the increasing of magnetoresistance
Fig. 2. Normalized magnetization vs field measurements of the Ni80Fe20, (Ni80Fe20)50Ag50 and (Ni80Fe20)50Co50 single layer alloy samples before (a) and after annealing to 700 K (b) and 900 K (c) at in plane applied magnetic field.
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Ia.M. Lytvynenko et al. / Vacuum 116 (2015) 31e35
Table 1 Calculated parameters for Ni80Fe20 and permalloy-based thin films. MR (%), ⊥
A B C
BS (mT), jj
MR (%), jj
BC (mT), jj
SB%/T, ⊥
SB%/T, jj
300
700
900
300
700
900
300
700
900
300
700
900
300
700
900
300
700
900
0.35 0.35 0.45
0.33 0.07 0.36
0.42 0.27 0.26
0.08 0.06 0.23
0.09 0.08 0.14
0.10 0.14 0.07
3.24 12.2 1.52
2.22 19.7 4.32
7.74 24.1 4.65
0.45 3.71 0.25
0.39 6.90 0.55
0.74 9.88 0.58
0.69 0.88 0.66
0.63 0.14 0.67
0.83 0.46 0.51
0.17 0.12 0.50
0.18 0.16 0.28
0.20 0.27 0.14
[16,17]. The magnitude of MR vs. applied field decreases after heat treated at 700 K and then increase again after annealing up to 900 К. The process of annealing up to 700 K increase occur gradual coalescence of ferromagnetic NiFe grains, reorientation of Ag crystallites and, as a result, blurring of interfaces magnetic grain/ nonmagnetic matrix. The difference in the shape of MR curves in out of plane and in plane geometries for (Ni80Fe20)50Ag50 alloy indicates an easy remagnetization of the sample mostly as a consequence of the shape anisotropy predominance. The typical normalized magnetization curves for as-deposited thin permalloy films, measured at applied magnetic field in plane are presented at Fig. 2a (black line). The samples of A type exhibit a small saturation field and a coercivity with values 3.24 and 0.45 mT at 300 K, respectively. This hysteresis loop is typical for magnetic films with in-plane magnetization. Similar result was received by Peng et al. [18] for thin permalloy films with 20 nm thickness. The evolution of magnetization curves of the Ni80Fe10 film after annealing up to 700 and 900 K are presented at Fig. 2b and c (black line). The permalloy film shows a similar magnetization behavior after annealing up to 700 K. The values of BS and BC change a little (decreases to 2.22 and 0.39 mT). However, normalized magnetization curves of the samples annealed up to 900 K are quite different from the as-deposited. The relative changes of magnetic parameter are BS ¼ 139% and BC ¼ 64%. This result can be explained by structured characterization of the samples if noticed that annealing up to 700 K caused grain size increasing [19]. Fig. 2 shows the normalized magnetizations vs. field measurements of B type sample (blue (in the web version) curves). VSM investigation of magnetic properties of thin film (Ni80Fe20)50Co50 alloy showed that hysteresis loop has close squared shape with sharp remagnetization at applied magnetic field parallel to the sample plane as for A tape sample, but values of BS and BC increase in 3.8 and 8.2 times respectively. The film magnetization changed completely in the ranges ±13 mТ at asdeposited state and ±23 mТ after annealing to 900 K. The square shape of hysteresis loop is typical for easy reversal process. Besides, the increasing of remanent magnetization, coercivity and saturation field at increasing of heat treatment temperature is observed. Relative changes of BS and BC are 88% and 154% respectively in temperature range 300e900 K. The increasing of saturated field in the case of ferromagnetic alloy obtained after finishing of remagnetization processes, namely domain growth with magnetic moment, which oriented in the direction of easy magnetization axes as a result of process of domain boundaries propagation and rotation of the sample magnetization vector in the direction of applied magnetic field. The sample of C type shows a similar magnetization behavior as a permalloy film (Fig. 2, red line). The hysteresis squareness at in-plane applied field keeps high. Addition a non-magnetic material as third element does not change significantly the values of BS and BC in contrast to the adding of magnetic materials. The values of saturation field and coercivity decrease from 3.24 to 1.52 and from 0.45 to 0.25 mT, respectively, in compare with permalloy film. Saturation field decreasing might be related with the
structure disorder. The annealing causes the increasing of BS and BC values; furthermore it significantly changed the hysteresis loops shape. Saturation field reaches a maximum 4.65 mT and coercivity 0.58 mT after annealing up to 900 K. These results similar to experimental investigations reported by Weber et al. [21,22]. One of the parameters, which defines areas of practical applications, is film system sensitivity to applied magnetic field (Table 1). In this work the maximum value of SB was determined by the equation (DR/R(Bs))max/DВ, where (DR/R(Bs))max e the maximum MR value; DВ e a magnetic induction change from saturation (or the maximum value) BS to demagnetization BC. The analysis of presented data allows determining that film structures of all types can be used as sensitive elements of magnetic sensors for low magnetic field detecting. In summary, we have studied the magnetic properties of permalloy thin films in dependence on the adding of magnetic (Co) and nonmagnetic (Ag) third elements. In the case of Co addition, the curves of MR vs. applied field appear anisotropic character with maximum value in ⊥ geometry. Upon annealing, the anisotropy in plane of samples vanishes, the values of BS and BC increase as a result of formation disordered solid solution based on the NiFeeCo ternary alloy. For permalloy-Ag system, films structure changes from highly dispersed labyrinthine to granular and MR value increases in both geometries because of spin-dependent scattering on the NiFe grains. However, the magnetization behavior is similar for systems of A and C types. The further annealing leads to blurring of interfaces magnetic grain/nonmagnetic matrix and as a result to decreasing of MR value and increasing of BS and BC values. Acknowledgments The authors thank to the “nanomagnetism and spintronics” team at Institute of Jean Lamour (University of Lorraine, France) for technical assistance and providing equipments for experiments. This work was partially funded by the State Program of the Ministry of Education and Science of Ukraine concerning research training abroad. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
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