- Email: [email protected]
Co) multilayers coupled to NiO antiferromagnetic layer
Solid State Communications 301 (2019) 113703
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Perpendicular exchange bias at room temperature in (Pt/Co) multilayers coupled to NiO antiferromagnetic layer
T
R. Belhia,b a b
LMOP, LR99ES17, Faculté des Sciences de Tunis, Université de Tunis El Manar, Tunis, Tunisia Faculté des Sciences de Bizerte Université de Carthage, Tunis, Tunisia
A R T I C LE I N FO
A B S T R A C T
Communicated by E.V. Sampathkumaran
(Pt/Co) multilayers coupled to NiO exhibit a small exchange bias field HEB which is consistent with a NiO blocking temperature TB of about the room temperature. Direct deposition of nickel oxide on cobalt increased coercivity and induced a weak exchange bias field. The reversal process has also been affected. It changes from a reversal clearly governed by wall propagation initiated by only a few domains to a reversal dominated by wall propagation that is initiated by a larger number of domains leading to an important role in the nucleation. The domains are larger in the sample without NiO. The insertion of a platinum layer with thickness range 0–0.6 nm between Co and NiO acts on the coercivity and the exchange field. The optimal values of HC and HEB are observed for tpt = 0.2 nm. We found that the magnetization reversal process changes from a wall propagationdominated process for tpt ≤0.2 nm to a domain nucleation dominated process for tpt > 0.2 nm.
1. Introduction The coupling between magnetic layers with different magnetic properties is of special interest for modern technologies in information storage. The storage media as well as the data readout sensors consist of magnetic layers coupled to each other via their interface. For a ferromagnet (FM)/antiferromagnet (AFM) interface a unidirectional anisotropy can occur which is called exchange bias (EB). The phenomenon of exchange bias anisotropy was discovered by Meiklejohn and Bean in 1956 by studying ferromagnetic Co particles covered by their antiferromagnetic oxide CoO [1,2]. This phenomenon has been of great interest for the last twenty years because of its applications in spintronic devices [3] such as hard disk read heads and MRAM memories (Magnetic Random Access Memories). Spin valves and tunnel junctions are used in these devices and behave like magnetoresistive sensors whose operation is based on the principle of giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) [4]. The exchange anisotropy is macroscopically manifested by a shift of the hysteresis loop along the field axis, called the exchange bias field HEB , and the reinforcement of the coercive field HC . It only appears for temperatures below the Néel temperature of the antiferromagnetic material. It is therefore necessary, in the context of the applications that this order temperature is well above the room temperature. The exchange anisotropy is extremely sensitive to parameters such
as the thicknesses of the FM and AFM layers, the interfacial roughness and the crystalline structure of the interface which are difficult to control during the preparation of thin films. The first studies dealing with magnetic exchange anisotropy in bilayers concerned planar anisotropic FM and AFM layers. Current studies concern FM/AFM multilayers with perpendicular anisotropy. The perpendicular configuration allows reducing the size of tunnel junctions and spin valves. This leads to an increase in the integration density of the spinctronic devices and gives rise to an increase in the storage density of MRAMs. It also increases the storage density of magnetic media. Among the FM/AFM systems which exhibit perpendicular magnetic exchange anisotropy, it is possible to use the FM (Pt/Co) or (Pd/Co) multilayers coupled to AFM compounds or alloys such as FeF2, FeMn, IrMn, CoO, NiO [5–8]. The influence of structural parameters such as layer thickness of Co, Pt and Pd, or number of (Pt/Co) or (Pd/Co) repeats on exchange anisotropy has been extensively studied. It has been suggested that a strong perpendicular magnetic anisotropy of multilayers favors a strong shift of the hysteresis loop [9–11]. Several studies have shown that the exchange field increases with the number of (Pt/Co) repeats [10,11]. In addition, it has been observed that for very thin Co layers, the insertion of a thin layer of Pt between the FM layer and the AFM layer increases the exchange field and the coercive field. This effect was attributed to a reinforcement of the perpendicular anisotropy of cobalt due to the insertion of the Pt spacer layer [12,13]. In this work, we investigate (Pt/Co)5/Pt(tPt)/NiO multilayers in
E-mail address: [email protected]. https://doi.org/10.1016/j.ssc.2019.113703 Received 6 July 2019; Received in revised form 11 August 2019; Accepted 12 August 2019 Available online 13 August 2019 0038-1098/ © 2019 Elsevier Ltd. All rights reserved.
Solid State Communications 301 (2019) 113703
R. Belhi
hysteresis loops as well as domain structures imaging were performed by a polar Kerr microscope. For the measurement of the loops, the samples with Pt layer spacer were heated at a temperature of 423 K for 15 min then cooled down to room temperature under a perpendicular magnetic field of magnitude of 2.5 KOe. The procedure used for imaging consists in first saturating the sample with a strong enough magnetic field (H~ 2HC), then starting from the saturated state, pulses of opposite field of amplitude close to the coercive field and of Δτ duration are applied before taking images of domains structure. All measurements were carried out at room temperature, which is lower than the Néel temperature of NiO (TN~523 K). 3. Results and discussions Fig. 1 shows the magnetic hysteresis (M − H) loop measured in the polar configuration on Pt(10 nm)/[Pt(4 nm)/Co(0.6 nm)]5. The loop clearly shows that the magnetization of the different cobalt layer is strongly ferromagnetically coupled. The fully remanence (Mr/Ms = 1) recorded on the loops testifies of perpendicular anisotropy. The coercivity is estimated from the loop to 290 Oe. For this sample the magnetization reversal is abrupt which suggests a reversal dominated by wall propagation. This deduction is confirmed by the images shown in Fig. 2 on which we see some domains that grow significantly by wall motion. Fig. 3 shows the polar magnetic hysteresis (M − H) loops measured on Pt(10 nm)/[Pt(4 nm)/Co(0.6 nm)]5/Pt(tPt)/NiO(4 nm) samples as a function of Pt spacer thickness (tPt). For all the multilayers the squarness shape of loops and the fully remanence testify of a perpendicular magnetic anisotropy. We note that the different layers of cobalt in each multilayer remain ferromagnetically coupled in the presence of Pt spacer layer and/or the NiO layer. The hysteresis loop for the sample with tpt = 0 is slightly shifted away from the zero field axis by ~14 Oe . This means that the direct deposition of NiO on Co induces a very small exchange bias field (HEB ~14 Oe) . The loop also shows significantly enhanced coercivity of HC = 412 Oe in contrast to HC = 290 Oe for the sample without NiO. Z. Y. Liu et al. found on [Pt/Co]3/NiO (11 A°) that the direct deposition of nickel oxide on cobalt considerably increases the coercivity without however induces any shift in hysteresis loop [6]. The hysteresis loops for samples with tpt≠0 show a slight asymmetry which varies with tpt. Table 1 presents the values of the coercive
Fig. 1. Polar magnetic hysteresis (M − H) loop measured at room temperature on Pt (10 nm)/[Pt (4 nm)/Co (0.6 nm)]5/Pt(2 nm).
order to study the influence of the Pt spacer layer insertion at the Co/ NiO interface on magnetic properties. We especially focus on the coercivity and on the magnetization reversal process. The Kerr microscopy allowed us to record the hysteresis loop as well as to directly observe the evolutions of the domain structure during reversal along the ascending and descending branches.
2. Experiments The Pt(10 nm)/[Pt(4 nm)/Co(0.6 nm)]5 multilayer and Pt(10 nm)/ [Pt(4 nm)/Co(0.6 nm)]5//Pt(tPt)/NiO(4 nm) multilayers with tPt = 0, 0.2, 0.4 and 0.6 nm were deposited by Dc and Rf magnetron sputtering at room temperature onto thermally oxidized silicon substrates. All the multilayers were covered with 2 nm thick Pt layer for prevention of oxidation [14]. The base pressure of sputtering chamber is of 5 10−7 mbar. Argon gas is introduced in the chamber. The Ar pressure was 1.5 10−3 mbar during deposition. No magnetic field was applied during deposition. The magnetic properties of the samples were characterized by using the polar magneto-optical Kerr effect (PMOKE). The
Fig. 2. Domains structure evolution of Pt (10 nm)/[Pt (4 nm)/Co (0.6 nm)]5/Pt(2 nm) multilayer recorded under applied magnetic field is of −160 Oe. The duration of the magnetic pulse is Δτ = 20 ms. 2
Solid State Communications 301 (2019) 113703
R. Belhi
Fig. 3. Normalized polar MOKE hysteresis loops of (Pt (4 nm)/Co(0.6 nm))5/Pt(tPt)/NiO(4 nm).
written as
Table 1 Coercive and exchange bias fields values at room temperature of Pt (10 nm)/[Pt (4 nm)/Co (0.6 nm)]5//Pt(tPt)/NiO(4 nm) as function as tpt thickness. Values are extracted from the loops of Fig. 3 HC = (HC+ − HC−)/2 and HBE = (HC+ + HC−)/2 where HC+ and HC− are respectively the coercive fields for the ascending and descending branches of the hysteresis loop. tPt(nm)
0
0.2
0.4
0.6
HC(Oe) HBE(Oe)
412 14
440 37
360 27
370 6
HEB = −
JFM/AFM cos(ϕ) μ 0MFM tFM
(1)
Where ϕ is the angle between the ferromagnetic layer magnetization MFM and the antiferromagnetic layer magnetization MAFM, tFM is the thicknesses of the FM layer and JFM/AFM is the interface exchange coupling constant. Equation (1) indicates that the HEB decreases with increasing angle ϕ, proportional to cos(ϕ ). Indeed, a tilt of the Co spins away from the surface normal increases the angle ϕ. Thanks to Co 3d-Pt 5d hybridization, the insertion of a thin Pt layer at the Co/NiO interface reorients the Co spins to a more perpendicular position decreasing the angle ϕ and thus enhancing the exchange bias. The improvement of the orientation of the spins of the last layer of Co in the multilayers (Pt/ Co)5, results in an additional interfacial perpendicular anisotropy energy [20,21] which explain the coercivity increase. Thus, the perpendicular anisotropy induced by a Co/Pt interface is stronger than that induced by a Co/NiO interface [22]. From a structural point of view, the Co/NiO interface suggests the existence of a mixture between the Co, Ni and O atoms at the interface which can lead to a very thin CoNiO alloy layer at the interface which reduces the perpendicular magnetic anisotropy. So the Pt spacer probably has the effect of preventing such a phenomenon. Several studies have shown that exchange bias is proportional to the FM–AFM spin projection at the interface and results indicate that HEB is optimized when the FM and AFM easy axes are completely parallel to each other. This occurs at tPt = 0.2 nm in our samples. The HEB decrease is due to the fact that FM-AFM exchange coupling is a short range interaction. A thick Pt spacer disrupts the exchange interaction and induces an exponential decay of the coercivity and exchange bias [13–23]. Although our results are qualitatively consistent with what has been found on these systems we point out that the exchange bias field
field HC and the exchange bias field HEB according to the Pt spacer thickness. Data of Table 1 show that the coercivity and the exchange bias field both depend on the thickness of the platinum spacer layer. We notice that the coercive field HC increases when inserting the Pt spacer at the FM/AFM interface from 412 Oe for the multilayer without platinum spacer (tpt = 0) to a maximum value of 440 Oe for tpt = 0.2 nm then decreases with the platinum spacer thickness increasing. The exchange bias field HEB varies in the same way as the coercive field by recording an optimal value of 37 Oe at tpt = 0.2 nm. In a previous work we found a similar behavior on (Pt/Co (0.6 nm))5/Pt/FeMn multilayers with a maximum values of HEB (120 Oe ) and Hc (430 Oe) at tpt = 0.4 nm [15]. Similar behavior was also observed by Garcia et al. [16] in (Pt/Co(0.4 nm))n (n = 2, 3 and 5)/Pt/IrMn multilayers where tpt corresponding to maximum values of HEB and HC varies in opposite way to n. For n = 5, the authors found an optimal values of 340 Oe and 250 Oe respectively for HEB and HC at tpt of 0.2 nm. In (Pt/Co (0.5 nm))3/Pt/IrMn multilayers a maximum value HEB of 270 Oe was recorded at tpt = 0.1 nm [17]. In order to explain the HEB behavior we discuss the relation between FM moment direction and HEB within the framework of the Meiklejohn and Bean model for FM/AFM coupling [2,18,19]. In this model, HEB is 3
Solid State Communications 301 (2019) 113703
R. Belhi
Fig. 4. Domains structure evolution of Pt (10 nm)/[Pt (4 nm)/Co (0.6 nm)]5 Pt(tPt)/NiO(4 nm)/Pt(2 nm) multilayers recorded under applied field. (a) For tPt = 0 nm, (b) for tPt = 0.2 nm, (c) for tPt = 0.4 nm and (d) for tPt = 0.6 nm. The duration of the magnetic pulse is Δτ = 20 ms. The image area is 345 × 264 μm2.
4
Solid State Communications 301 (2019) 113703
R. Belhi
Fig. 4. (continued)
antiferromagnetic anisotropy, KAFM, is a critical condition for observing the exchange bias. Indeed it has been established in such systems that the bias exchange field depends on the magnetic anisotropy KAFM and the blocking temperature TB of the antiferromagnetic layer according to the law [25]:
measured on our samples is very low compared to the values found on the system (Pt/Co)n coupled to other antiferromagnetic such as FeMn [15,24] and IrMn [16]. The interfacial nature of the coupling implies that the exchange field HEB must be strongly dependent on the spin configuration at the interface which strongly depend on antiferromagnetic anisotropy, KAFM and blocking temperature TB of the AFM layer. In fact when there is no net magnetization at the interface of antiferromagnetic layer no HEB is expected. In contrast, when antiferromagnetic interface presents a net interface magnetization exchange bias is induced, HEB ≠ 0 . This situation corresponds to a strong antiferromagnetic anisotropy and T < TB. Let us note that strong
HEB ≈
T⎞ AAF KAF (0) ⎛1 − TB ⎠ ⎝ ⎜
⎟
(2)
whereAAFM and KAFM are the exchange constant and the magnetocrystalline anisotropy constant at 0 K respectively. Based on this overview we can associate the discrepancy in HEB values to a weaker anisotropy 5
Solid State Communications 301 (2019) 113703
R. Belhi
induced by the NiO layer. Indeed, the state of the last Co/Pt interface seems to be more perturbed by the presence of NiO and that this perturbation associated with a reduction of interface exchange anisotropy caused by the magnetic decoupling could explain the observed reversal process for tpt≥ 0.4 nm.
and a weaker blocking temperature of the NiO layer with respect to the anisotropies and blocking temperatures of FeMn and IrMn. Moreover we can conclude that according to the small values of HEB the blocking temperature of sputtered NiO layers is of about the room temperature. This deduction seems to disagree with the result presented in Ref. 6, which shows the evolution of TB as a function of the thickness of the NiO layer, where TB is of the order of 100 K for tNiO = 4 nm. Admittedly, the blocking temperature in NiO thin layers must be lower than TN. However, the blocking temperature is by no means an intrinsic property. This temperature depends on the AFM grain size distribution as well as on the roughness of the FM/AFM interface. On the hysteresis loops of Fig. 3 we can easily see that the reversal of the magnetization is not abrupt, it ends for the two ascending and descending branches, at a field a little larger than the coercive field. This signifies that in all the samples nucleation plays an important role in the reversal process. To investigate the effect of the antiferromagnetic layer on the magnetization reversal as a function of the platinum spacer layer thickness we proceeded to the magnetooptical imaging of the domain structure and its evolution with applied field close to the coercive field. Starting from a magnetically saturated state generated under a large enough field H~ 2HC , applied perpendicular to the sample, subsequent 20 ms long magnetic pulses were applied along the opposite direction. For each sample, magnetic domain structure images were recorded after applying each pulse. Fig. 4 shows the evolution of the domain structure of Pt(10 nm)/[Pt (4 nm)/Co (0.6 nm)]5//Pt(tPt)/NiO(4 nm) samples as a function of Pt spacer thickness. On the images of Fig. 4 (a) corresponding to the absence of Pt spacer (tpt = 0) it is clear that the reversal of magnetization is initiated by a fairly large number of nucleation centers that grow in size over time in a very irregular structure. From one image to the next, very few new domains appear in time. Consequently, in this sample the reversal of magnetization is essentially ensured by a wall propagation process. The shape of the domains reveals a strong domain walls pinning phenomenon. Such a phenomenon is consistent with a rough interface [26,27]. As a result, the Co/NiO interface is rougher than the Co/Pt interface. Moreover this pinning phenomenon makes it possible to explain in part the increase of the coercive field compared to the sample without AFM layer. Let us note that despite the similarity between the two evolutions of domains structure during reversals of the magnetization from up to down and from down to up where many domains that initiate the reversal are the same some differences are noticeable such as domain size and number of domains which result in branch asymmetry. Looking at the images of Fig. 4 (b), (c) and (d) we see that the insertion of the platinum between the ferromagnetic layer (Co) and the antiferromagnetic layer (NiO) strongly affected the domain structure and consequently the magnetization reversal process. Indeed, for the sample with tPt = 0.2 nm where the exchange bias is maximum, we still observe a reversal process ensured by the wall propagation after nucleation of a much larger number of domains than in the case tPt = 0 nm. For samples with tpt > 0.2 nm the situation is quite different. The reversal of magnetization in these samples seems to be dominated by the nucleation phenomenon. This is confirmed by the increase in the nucleation density as shown in Fig. 4 (c) and (d). This result is unexpected since inserting a platinum layer of sufficient thickness to decouple the FM from AFM one would expect behavior similar to that of the sample without AFM where the magnetization reversal is due essentially to a wall propagation phenomenon. However, the situation seems different for the reason that the inserted separating layer magnetically decouples FM and AFM even if it is not totally (HEB ≠ 0) but does not prevent the effects of interfaces that could be
4. Conclusion We found that the NiO deposited on (Pt/Co) induces a small exchange bias field, which means that the blocking temperature of a 4 nm thick NiO layer is slightly higher than the room temperature. The behavior of the coercivity and of the exchange bias field shows an increase as a function of pt spacer thickness followed by a significant decrease where HEB ~0 . The optimal values of HEB and HC are observed at tpt = 0.2 nm. Using magneto-optical Kerr microscopy, we have investigated the magnetic domain structure and its temporal evolution as a function of Pt spacer for applied field close to the coercive field. It was revealed that the magnetization reversal process changes from a wall propagation-dominated process tpt ≤0.2 nm to a domain nucleated dominated process for tpt > 0.2 nm. Acknowledgments I acknowledge for their help in this work the Équipe Micro-et Nanomagnetisme of the Institut Néel de Grenoble, especially A. Fassatoui and J. Vogel. References [1] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. [2] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 105 (1957) 904. [3] B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, D. Mauri, Phys. Rev. B 43 (1991) 1297. [4] X. Marti, B.G. Park, J. Wunderlich, H. Reichlova, Y. Kurosaki, M. Yamada, H. Yamamoto, A. Nishide, J. Hayakawa, H. Takahashi, T. Jungwirth, Phys. Rev. Lett. 108 (2012) 017201. [5] S.S. Kim, J.Y. Hwang, J.R. Rhee, J. Magn. Magn. Mater. 310 (2007) 2310. [6] Z.Y. Liu, S. Adenwalla, J. Appl. Phys. 94 (2003) 1105. [7] S. Maat, K. Takano, S.S.P. Parkin, E.E. Fullerton, Phys. Rev. Lett. 87 (2001) 087202. [8] C.H. Marrows, Phys. Rev. B 68 (2003) 012405. [9] F. Garcia, G. Casali, S. Auffret, B. Rodmacq, B. Dieny, J. Appl. Phys. 91 (2002) 6905. [10] S. Hashimoto, Y. Ochiai, K. Aso, J. Appl. Phys. 66 (1989) 4909. [11] M.T. Johnson, P.J.H. Bloemen, F.J. A.d. Broeder, J.J.d. Vries, Rep. Prog. Phys. 59 (1996) 1409. [12] F. Garcia, J. Sort, B. Rodmacq, S. Auffret, B. Dieny, Appl. Phys. Lett. 83 (2003) 3537. [13] N.J. Gökemeijer, T. Ambrose, C.L. Chien, Phys. Rev. Lett. 79 (1997) 4270. [14] R. Belhi, S. Jomni, N. Mliki, K. Abdelmoula, M. Ayadi, G. Clugnet, A. Charai, C. Leroux, G. Nihoul, Can. J. Phys. 79 (2001) 1011. [15] A. Fassatoui, R. Belhi, J. Vogel, S. Pizzini, P. David, K. Abdelmoula, J. Magn. Magn. Mater. 449 (2018) 475. [16] A. Zarefy, L. Lechevallier, R. Lardé, H. Chiron, J.-M. Le Breton, V. Baltz, B. Rodmacq, B. Dieny, J. Appl. Phys. D 43 (2010) 215004. [17] M. Czapkiewiez, S. van Dijken, T. Stobiecki, R. Rak, M. Moladz, P. Mietniowski, Phys. Status Solidi 3 (1) (2006) 48–52. [18] W.H. Meiklejohn, J. Appl. Phys. 33 (1962) 1328. [19] J. Nogués, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [20] J. Sort, V. Baltz, F. Garcia, B. Rodmacq, B. Dieny, Phys. Rev. B 71 (2005) 054411. [21] J. Sort, B. Dieny, J. Nogués, Phys. Rev. B 72 (2005) 104412. [22] J. Moritz, S. van Dijken, J.M.D. Coey, Eur. Phys. J. B 45 (2005) 191. [23] T. Mewes, B.F.P. Roos, S.O. Demokritov, B. Hillebrands, J. Appl. Phys. 87 (2000) 5064. [24] F. Garcia, J. Sort, B. Rodmacq, S. Auffret, B. Dieny, Appl. Phys. Lett. 83 (2003) 3537. [25] M.J. Carey, A.E. Berkowitz, J. Appl. Phys. 73 (1993) 6892. [26] R. Belhi, A.A. Adanlété Adjanoh, K. Abdelmoula, J. Magn. Magn. Mater. 339 (2013). [27] R. Belhi, A. Adanlété Adjanoh, J. Vogel, M. Ayadi, K. Abdelmoula, J. Appl. Phys. 108 (2010) 093924.
6
Contents lists available at ScienceDirect
Solid State Communications journal homepage: www.elsevier.com/locate/ssc
Perpendicular exchange bias at room temperature in (Pt/Co) multilayers coupled to NiO antiferromagnetic layer
T
R. Belhia,b a b
LMOP, LR99ES17, Faculté des Sciences de Tunis, Université de Tunis El Manar, Tunis, Tunisia Faculté des Sciences de Bizerte Université de Carthage, Tunis, Tunisia
A R T I C LE I N FO
A B S T R A C T
Communicated by E.V. Sampathkumaran
(Pt/Co) multilayers coupled to NiO exhibit a small exchange bias field HEB which is consistent with a NiO blocking temperature TB of about the room temperature. Direct deposition of nickel oxide on cobalt increased coercivity and induced a weak exchange bias field. The reversal process has also been affected. It changes from a reversal clearly governed by wall propagation initiated by only a few domains to a reversal dominated by wall propagation that is initiated by a larger number of domains leading to an important role in the nucleation. The domains are larger in the sample without NiO. The insertion of a platinum layer with thickness range 0–0.6 nm between Co and NiO acts on the coercivity and the exchange field. The optimal values of HC and HEB are observed for tpt = 0.2 nm. We found that the magnetization reversal process changes from a wall propagationdominated process for tpt ≤0.2 nm to a domain nucleation dominated process for tpt > 0.2 nm.
1. Introduction The coupling between magnetic layers with different magnetic properties is of special interest for modern technologies in information storage. The storage media as well as the data readout sensors consist of magnetic layers coupled to each other via their interface. For a ferromagnet (FM)/antiferromagnet (AFM) interface a unidirectional anisotropy can occur which is called exchange bias (EB). The phenomenon of exchange bias anisotropy was discovered by Meiklejohn and Bean in 1956 by studying ferromagnetic Co particles covered by their antiferromagnetic oxide CoO [1,2]. This phenomenon has been of great interest for the last twenty years because of its applications in spintronic devices [3] such as hard disk read heads and MRAM memories (Magnetic Random Access Memories). Spin valves and tunnel junctions are used in these devices and behave like magnetoresistive sensors whose operation is based on the principle of giant magnetoresistance (GMR) or tunnel magnetoresistance (TMR) [4]. The exchange anisotropy is macroscopically manifested by a shift of the hysteresis loop along the field axis, called the exchange bias field HEB , and the reinforcement of the coercive field HC . It only appears for temperatures below the Néel temperature of the antiferromagnetic material. It is therefore necessary, in the context of the applications that this order temperature is well above the room temperature. The exchange anisotropy is extremely sensitive to parameters such
as the thicknesses of the FM and AFM layers, the interfacial roughness and the crystalline structure of the interface which are difficult to control during the preparation of thin films. The first studies dealing with magnetic exchange anisotropy in bilayers concerned planar anisotropic FM and AFM layers. Current studies concern FM/AFM multilayers with perpendicular anisotropy. The perpendicular configuration allows reducing the size of tunnel junctions and spin valves. This leads to an increase in the integration density of the spinctronic devices and gives rise to an increase in the storage density of MRAMs. It also increases the storage density of magnetic media. Among the FM/AFM systems which exhibit perpendicular magnetic exchange anisotropy, it is possible to use the FM (Pt/Co) or (Pd/Co) multilayers coupled to AFM compounds or alloys such as FeF2, FeMn, IrMn, CoO, NiO [5–8]. The influence of structural parameters such as layer thickness of Co, Pt and Pd, or number of (Pt/Co) or (Pd/Co) repeats on exchange anisotropy has been extensively studied. It has been suggested that a strong perpendicular magnetic anisotropy of multilayers favors a strong shift of the hysteresis loop [9–11]. Several studies have shown that the exchange field increases with the number of (Pt/Co) repeats [10,11]. In addition, it has been observed that for very thin Co layers, the insertion of a thin layer of Pt between the FM layer and the AFM layer increases the exchange field and the coercive field. This effect was attributed to a reinforcement of the perpendicular anisotropy of cobalt due to the insertion of the Pt spacer layer [12,13]. In this work, we investigate (Pt/Co)5/Pt(tPt)/NiO multilayers in
E-mail address: [email protected]. https://doi.org/10.1016/j.ssc.2019.113703 Received 6 July 2019; Received in revised form 11 August 2019; Accepted 12 August 2019 Available online 13 August 2019 0038-1098/ © 2019 Elsevier Ltd. All rights reserved.
Solid State Communications 301 (2019) 113703
R. Belhi
hysteresis loops as well as domain structures imaging were performed by a polar Kerr microscope. For the measurement of the loops, the samples with Pt layer spacer were heated at a temperature of 423 K for 15 min then cooled down to room temperature under a perpendicular magnetic field of magnitude of 2.5 KOe. The procedure used for imaging consists in first saturating the sample with a strong enough magnetic field (H~ 2HC), then starting from the saturated state, pulses of opposite field of amplitude close to the coercive field and of Δτ duration are applied before taking images of domains structure. All measurements were carried out at room temperature, which is lower than the Néel temperature of NiO (TN~523 K). 3. Results and discussions Fig. 1 shows the magnetic hysteresis (M − H) loop measured in the polar configuration on Pt(10 nm)/[Pt(4 nm)/Co(0.6 nm)]5. The loop clearly shows that the magnetization of the different cobalt layer is strongly ferromagnetically coupled. The fully remanence (Mr/Ms = 1) recorded on the loops testifies of perpendicular anisotropy. The coercivity is estimated from the loop to 290 Oe. For this sample the magnetization reversal is abrupt which suggests a reversal dominated by wall propagation. This deduction is confirmed by the images shown in Fig. 2 on which we see some domains that grow significantly by wall motion. Fig. 3 shows the polar magnetic hysteresis (M − H) loops measured on Pt(10 nm)/[Pt(4 nm)/Co(0.6 nm)]5/Pt(tPt)/NiO(4 nm) samples as a function of Pt spacer thickness (tPt). For all the multilayers the squarness shape of loops and the fully remanence testify of a perpendicular magnetic anisotropy. We note that the different layers of cobalt in each multilayer remain ferromagnetically coupled in the presence of Pt spacer layer and/or the NiO layer. The hysteresis loop for the sample with tpt = 0 is slightly shifted away from the zero field axis by ~14 Oe . This means that the direct deposition of NiO on Co induces a very small exchange bias field (HEB ~14 Oe) . The loop also shows significantly enhanced coercivity of HC = 412 Oe in contrast to HC = 290 Oe for the sample without NiO. Z. Y. Liu et al. found on [Pt/Co]3/NiO (11 A°) that the direct deposition of nickel oxide on cobalt considerably increases the coercivity without however induces any shift in hysteresis loop [6]. The hysteresis loops for samples with tpt≠0 show a slight asymmetry which varies with tpt. Table 1 presents the values of the coercive
Fig. 1. Polar magnetic hysteresis (M − H) loop measured at room temperature on Pt (10 nm)/[Pt (4 nm)/Co (0.6 nm)]5/Pt(2 nm).
order to study the influence of the Pt spacer layer insertion at the Co/ NiO interface on magnetic properties. We especially focus on the coercivity and on the magnetization reversal process. The Kerr microscopy allowed us to record the hysteresis loop as well as to directly observe the evolutions of the domain structure during reversal along the ascending and descending branches.
2. Experiments The Pt(10 nm)/[Pt(4 nm)/Co(0.6 nm)]5 multilayer and Pt(10 nm)/ [Pt(4 nm)/Co(0.6 nm)]5//Pt(tPt)/NiO(4 nm) multilayers with tPt = 0, 0.2, 0.4 and 0.6 nm were deposited by Dc and Rf magnetron sputtering at room temperature onto thermally oxidized silicon substrates. All the multilayers were covered with 2 nm thick Pt layer for prevention of oxidation [14]. The base pressure of sputtering chamber is of 5 10−7 mbar. Argon gas is introduced in the chamber. The Ar pressure was 1.5 10−3 mbar during deposition. No magnetic field was applied during deposition. The magnetic properties of the samples were characterized by using the polar magneto-optical Kerr effect (PMOKE). The
Fig. 2. Domains structure evolution of Pt (10 nm)/[Pt (4 nm)/Co (0.6 nm)]5/Pt(2 nm) multilayer recorded under applied magnetic field is of −160 Oe. The duration of the magnetic pulse is Δτ = 20 ms. 2
Solid State Communications 301 (2019) 113703
R. Belhi
Fig. 3. Normalized polar MOKE hysteresis loops of (Pt (4 nm)/Co(0.6 nm))5/Pt(tPt)/NiO(4 nm).
written as
Table 1 Coercive and exchange bias fields values at room temperature of Pt (10 nm)/[Pt (4 nm)/Co (0.6 nm)]5//Pt(tPt)/NiO(4 nm) as function as tpt thickness. Values are extracted from the loops of Fig. 3 HC = (HC+ − HC−)/2 and HBE = (HC+ + HC−)/2 where HC+ and HC− are respectively the coercive fields for the ascending and descending branches of the hysteresis loop. tPt(nm)
0
0.2
0.4
0.6
HC(Oe) HBE(Oe)
412 14
440 37
360 27
370 6
HEB = −
JFM/AFM cos(ϕ) μ 0MFM tFM
(1)
Where ϕ is the angle between the ferromagnetic layer magnetization MFM and the antiferromagnetic layer magnetization MAFM, tFM is the thicknesses of the FM layer and JFM/AFM is the interface exchange coupling constant. Equation (1) indicates that the HEB decreases with increasing angle ϕ, proportional to cos(ϕ ). Indeed, a tilt of the Co spins away from the surface normal increases the angle ϕ. Thanks to Co 3d-Pt 5d hybridization, the insertion of a thin Pt layer at the Co/NiO interface reorients the Co spins to a more perpendicular position decreasing the angle ϕ and thus enhancing the exchange bias. The improvement of the orientation of the spins of the last layer of Co in the multilayers (Pt/ Co)5, results in an additional interfacial perpendicular anisotropy energy [20,21] which explain the coercivity increase. Thus, the perpendicular anisotropy induced by a Co/Pt interface is stronger than that induced by a Co/NiO interface [22]. From a structural point of view, the Co/NiO interface suggests the existence of a mixture between the Co, Ni and O atoms at the interface which can lead to a very thin CoNiO alloy layer at the interface which reduces the perpendicular magnetic anisotropy. So the Pt spacer probably has the effect of preventing such a phenomenon. Several studies have shown that exchange bias is proportional to the FM–AFM spin projection at the interface and results indicate that HEB is optimized when the FM and AFM easy axes are completely parallel to each other. This occurs at tPt = 0.2 nm in our samples. The HEB decrease is due to the fact that FM-AFM exchange coupling is a short range interaction. A thick Pt spacer disrupts the exchange interaction and induces an exponential decay of the coercivity and exchange bias [13–23]. Although our results are qualitatively consistent with what has been found on these systems we point out that the exchange bias field
field HC and the exchange bias field HEB according to the Pt spacer thickness. Data of Table 1 show that the coercivity and the exchange bias field both depend on the thickness of the platinum spacer layer. We notice that the coercive field HC increases when inserting the Pt spacer at the FM/AFM interface from 412 Oe for the multilayer without platinum spacer (tpt = 0) to a maximum value of 440 Oe for tpt = 0.2 nm then decreases with the platinum spacer thickness increasing. The exchange bias field HEB varies in the same way as the coercive field by recording an optimal value of 37 Oe at tpt = 0.2 nm. In a previous work we found a similar behavior on (Pt/Co (0.6 nm))5/Pt/FeMn multilayers with a maximum values of HEB (120 Oe ) and Hc (430 Oe) at tpt = 0.4 nm [15]. Similar behavior was also observed by Garcia et al. [16] in (Pt/Co(0.4 nm))n (n = 2, 3 and 5)/Pt/IrMn multilayers where tpt corresponding to maximum values of HEB and HC varies in opposite way to n. For n = 5, the authors found an optimal values of 340 Oe and 250 Oe respectively for HEB and HC at tpt of 0.2 nm. In (Pt/Co (0.5 nm))3/Pt/IrMn multilayers a maximum value HEB of 270 Oe was recorded at tpt = 0.1 nm [17]. In order to explain the HEB behavior we discuss the relation between FM moment direction and HEB within the framework of the Meiklejohn and Bean model for FM/AFM coupling [2,18,19]. In this model, HEB is 3
Solid State Communications 301 (2019) 113703
R. Belhi
Fig. 4. Domains structure evolution of Pt (10 nm)/[Pt (4 nm)/Co (0.6 nm)]5 Pt(tPt)/NiO(4 nm)/Pt(2 nm) multilayers recorded under applied field. (a) For tPt = 0 nm, (b) for tPt = 0.2 nm, (c) for tPt = 0.4 nm and (d) for tPt = 0.6 nm. The duration of the magnetic pulse is Δτ = 20 ms. The image area is 345 × 264 μm2.
4
Solid State Communications 301 (2019) 113703
R. Belhi
Fig. 4. (continued)
antiferromagnetic anisotropy, KAFM, is a critical condition for observing the exchange bias. Indeed it has been established in such systems that the bias exchange field depends on the magnetic anisotropy KAFM and the blocking temperature TB of the antiferromagnetic layer according to the law [25]:
measured on our samples is very low compared to the values found on the system (Pt/Co)n coupled to other antiferromagnetic such as FeMn [15,24] and IrMn [16]. The interfacial nature of the coupling implies that the exchange field HEB must be strongly dependent on the spin configuration at the interface which strongly depend on antiferromagnetic anisotropy, KAFM and blocking temperature TB of the AFM layer. In fact when there is no net magnetization at the interface of antiferromagnetic layer no HEB is expected. In contrast, when antiferromagnetic interface presents a net interface magnetization exchange bias is induced, HEB ≠ 0 . This situation corresponds to a strong antiferromagnetic anisotropy and T < TB. Let us note that strong
HEB ≈
T⎞ AAF KAF (0) ⎛1 − TB ⎠ ⎝ ⎜
⎟
(2)
whereAAFM and KAFM are the exchange constant and the magnetocrystalline anisotropy constant at 0 K respectively. Based on this overview we can associate the discrepancy in HEB values to a weaker anisotropy 5
Solid State Communications 301 (2019) 113703
R. Belhi
induced by the NiO layer. Indeed, the state of the last Co/Pt interface seems to be more perturbed by the presence of NiO and that this perturbation associated with a reduction of interface exchange anisotropy caused by the magnetic decoupling could explain the observed reversal process for tpt≥ 0.4 nm.
and a weaker blocking temperature of the NiO layer with respect to the anisotropies and blocking temperatures of FeMn and IrMn. Moreover we can conclude that according to the small values of HEB the blocking temperature of sputtered NiO layers is of about the room temperature. This deduction seems to disagree with the result presented in Ref. 6, which shows the evolution of TB as a function of the thickness of the NiO layer, where TB is of the order of 100 K for tNiO = 4 nm. Admittedly, the blocking temperature in NiO thin layers must be lower than TN. However, the blocking temperature is by no means an intrinsic property. This temperature depends on the AFM grain size distribution as well as on the roughness of the FM/AFM interface. On the hysteresis loops of Fig. 3 we can easily see that the reversal of the magnetization is not abrupt, it ends for the two ascending and descending branches, at a field a little larger than the coercive field. This signifies that in all the samples nucleation plays an important role in the reversal process. To investigate the effect of the antiferromagnetic layer on the magnetization reversal as a function of the platinum spacer layer thickness we proceeded to the magnetooptical imaging of the domain structure and its evolution with applied field close to the coercive field. Starting from a magnetically saturated state generated under a large enough field H~ 2HC , applied perpendicular to the sample, subsequent 20 ms long magnetic pulses were applied along the opposite direction. For each sample, magnetic domain structure images were recorded after applying each pulse. Fig. 4 shows the evolution of the domain structure of Pt(10 nm)/[Pt (4 nm)/Co (0.6 nm)]5//Pt(tPt)/NiO(4 nm) samples as a function of Pt spacer thickness. On the images of Fig. 4 (a) corresponding to the absence of Pt spacer (tpt = 0) it is clear that the reversal of magnetization is initiated by a fairly large number of nucleation centers that grow in size over time in a very irregular structure. From one image to the next, very few new domains appear in time. Consequently, in this sample the reversal of magnetization is essentially ensured by a wall propagation process. The shape of the domains reveals a strong domain walls pinning phenomenon. Such a phenomenon is consistent with a rough interface [26,27]. As a result, the Co/NiO interface is rougher than the Co/Pt interface. Moreover this pinning phenomenon makes it possible to explain in part the increase of the coercive field compared to the sample without AFM layer. Let us note that despite the similarity between the two evolutions of domains structure during reversals of the magnetization from up to down and from down to up where many domains that initiate the reversal are the same some differences are noticeable such as domain size and number of domains which result in branch asymmetry. Looking at the images of Fig. 4 (b), (c) and (d) we see that the insertion of the platinum between the ferromagnetic layer (Co) and the antiferromagnetic layer (NiO) strongly affected the domain structure and consequently the magnetization reversal process. Indeed, for the sample with tPt = 0.2 nm where the exchange bias is maximum, we still observe a reversal process ensured by the wall propagation after nucleation of a much larger number of domains than in the case tPt = 0 nm. For samples with tpt > 0.2 nm the situation is quite different. The reversal of magnetization in these samples seems to be dominated by the nucleation phenomenon. This is confirmed by the increase in the nucleation density as shown in Fig. 4 (c) and (d). This result is unexpected since inserting a platinum layer of sufficient thickness to decouple the FM from AFM one would expect behavior similar to that of the sample without AFM where the magnetization reversal is due essentially to a wall propagation phenomenon. However, the situation seems different for the reason that the inserted separating layer magnetically decouples FM and AFM even if it is not totally (HEB ≠ 0) but does not prevent the effects of interfaces that could be
4. Conclusion We found that the NiO deposited on (Pt/Co) induces a small exchange bias field, which means that the blocking temperature of a 4 nm thick NiO layer is slightly higher than the room temperature. The behavior of the coercivity and of the exchange bias field shows an increase as a function of pt spacer thickness followed by a significant decrease where HEB ~0 . The optimal values of HEB and HC are observed at tpt = 0.2 nm. Using magneto-optical Kerr microscopy, we have investigated the magnetic domain structure and its temporal evolution as a function of Pt spacer for applied field close to the coercive field. It was revealed that the magnetization reversal process changes from a wall propagation-dominated process tpt ≤0.2 nm to a domain nucleated dominated process for tpt > 0.2 nm. Acknowledgments I acknowledge for their help in this work the Équipe Micro-et Nanomagnetisme of the Institut Néel de Grenoble, especially A. Fassatoui and J. Vogel. References [1] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. [2] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 105 (1957) 904. [3] B. Dieny, V.S. Speriosu, S.S.P. Parkin, B.A. Gurney, D.R. Wilhoit, D. Mauri, Phys. Rev. B 43 (1991) 1297. [4] X. Marti, B.G. Park, J. Wunderlich, H. Reichlova, Y. Kurosaki, M. Yamada, H. Yamamoto, A. Nishide, J. Hayakawa, H. Takahashi, T. Jungwirth, Phys. Rev. Lett. 108 (2012) 017201. [5] S.S. Kim, J.Y. Hwang, J.R. Rhee, J. Magn. Magn. Mater. 310 (2007) 2310. [6] Z.Y. Liu, S. Adenwalla, J. Appl. Phys. 94 (2003) 1105. [7] S. Maat, K. Takano, S.S.P. Parkin, E.E. Fullerton, Phys. Rev. Lett. 87 (2001) 087202. [8] C.H. Marrows, Phys. Rev. B 68 (2003) 012405. [9] F. Garcia, G. Casali, S. Auffret, B. Rodmacq, B. Dieny, J. Appl. Phys. 91 (2002) 6905. [10] S. Hashimoto, Y. Ochiai, K. Aso, J. Appl. Phys. 66 (1989) 4909. [11] M.T. Johnson, P.J.H. Bloemen, F.J. A.d. Broeder, J.J.d. Vries, Rep. Prog. Phys. 59 (1996) 1409. [12] F. Garcia, J. Sort, B. Rodmacq, S. Auffret, B. Dieny, Appl. Phys. Lett. 83 (2003) 3537. [13] N.J. Gökemeijer, T. Ambrose, C.L. Chien, Phys. Rev. Lett. 79 (1997) 4270. [14] R. Belhi, S. Jomni, N. Mliki, K. Abdelmoula, M. Ayadi, G. Clugnet, A. Charai, C. Leroux, G. Nihoul, Can. J. Phys. 79 (2001) 1011. [15] A. Fassatoui, R. Belhi, J. Vogel, S. Pizzini, P. David, K. Abdelmoula, J. Magn. Magn. Mater. 449 (2018) 475. [16] A. Zarefy, L. Lechevallier, R. Lardé, H. Chiron, J.-M. Le Breton, V. Baltz, B. Rodmacq, B. Dieny, J. Appl. Phys. D 43 (2010) 215004. [17] M. Czapkiewiez, S. van Dijken, T. Stobiecki, R. Rak, M. Moladz, P. Mietniowski, Phys. Status Solidi 3 (1) (2006) 48–52. [18] W.H. Meiklejohn, J. Appl. Phys. 33 (1962) 1328. [19] J. Nogués, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [20] J. Sort, V. Baltz, F. Garcia, B. Rodmacq, B. Dieny, Phys. Rev. B 71 (2005) 054411. [21] J. Sort, B. Dieny, J. Nogués, Phys. Rev. B 72 (2005) 104412. [22] J. Moritz, S. van Dijken, J.M.D. Coey, Eur. Phys. J. B 45 (2005) 191. [23] T. Mewes, B.F.P. Roos, S.O. Demokritov, B. Hillebrands, J. Appl. Phys. 87 (2000) 5064. [24] F. Garcia, J. Sort, B. Rodmacq, S. Auffret, B. Dieny, Appl. Phys. Lett. 83 (2003) 3537. [25] M.J. Carey, A.E. Berkowitz, J. Appl. Phys. 73 (1993) 6892. [26] R. Belhi, A.A. Adanlété Adjanoh, K. Abdelmoula, J. Magn. Magn. Mater. 339 (2013). [27] R. Belhi, A. Adanlété Adjanoh, J. Vogel, M. Ayadi, K. Abdelmoula, J. Appl. Phys. 108 (2010) 093924.
6