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Co bilayers grown on Ge(1 0 0)
Applied Surface Science 354 (2015) 95–99
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Pinning of magnetic moments at the interfacial region of ultrathin CoO/Co bilayers grown on Ge(1 0 0) Shin-Chen Chang a , Jyh-Shen Tsay a,∗ , Cheng-Hsun-Tony Chang a , Yeong-Der Yao b a b
Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan Institute of Physics, Academia Sinica, Nakang, Taipei 11529, Taiwan
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 25 September 2014 Received in revised form 27 March 2015 Accepted 3 April 2015 Available online 11 April 2015 Keywords: Ultrathin films Tunable exchange bias Magnetic thin films Oxide
For CoO overlayers prepared by evaporating Co atoms in an oxygen atmosphere, both the oxidation of the Co atoms at the interface and the segregation of oxygen atoms into the Co surface occur. Parts of Co atoms at the interface become nonferromagnetic and this causes the reduction of Kerr intensity at 300 K. After field cooling treatments, further reduction of the Kerr intensity is detected. The change of the Kerr intensity is an indicator of the pinned magnetic moments. In the case of forming thicker interfacial region, more pinned magnetic moments are observed and result in the larger exchange bias field for CoO on thicker Co/Ge(1 0 0). This tunable pinning provides a practical way of increasing exchange bias field by controlling the thickness of the interfacial region for ultrathin CoO/Co bilayers on semiconductor substrates. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to the conception and optimization of applicative devices for spintronics, exchange bias (EB) phenomena have attracted considerable attentions [1–16], especially on the unidirectional anisotropy which is caused by the exchange coupling between antiferromagnetic (AFM) and ferromagnetic (FM) layered materials and the pinned magnetic moments [6–16]. The exchange coupling is sensitive to the AFM/FM interface and relies on some microscopic parameters, such as crystallographic order, surface roughness, interfacial strain, and spin orientation [14–16]. From the literatures, great efforts have also been devoted to the nature of the pinned magnetic moments [6–13]. In an intuitive picture of the EB effect, the spin configuration of the AFM layer is assumed to be pinned and all the interfacial AFM spins are responsible for the unidirectional anisotropy via exchange coupling between AFM and FM layers [3,7]. In the last decade, more experimental evidences show that some pinned uncompensated spins at the interface, resulting from the intrinsic AFM spin structure or imperfections like step edges, are the key element of the EB [2,3,8–10]. X-ray magnetic circular dichroism (XMCD) studies show that the pinned uncompensated interfacial spins constitute a fraction of a monolayer (ML) tightly locked to the AFM lattice [8]. Soft X-ray
∗ Corresponding author. Tel.: +886 2 77346031. E-mail address: [email protected] (J.-S. Tsay). http://dx.doi.org/10.1016/j.apsusc.2015.04.019 0169-4332/© 2015 Elsevier B.V. All rights reserved.
resonant reflectivity studies on exchange-biased bilayer of permalloy/CoO identified 0.5-nm-thick layer containing uncompensated Co moments at the interface where a small fraction of the uncompensated Co moments pinned antiparallel to the cooling field is used to bias the sample [9]. Polarized neutron reflectivity measurements on annealed MnIr/CoFe show an extended pinned spin area at the interface due to the increased roughness and atomic interdiffusion [10]. Magneto-optical Kerr effect (MOKE) measurements of epitaxial AFM/FM structures show that the pinned magnetic moments inside the bulk of the AFM layer coexisting independently for orthogonal spin directions are responsible for the emergence of EB [6]. However, there is very limited knowledge about the relation between the pinned magnetic moments and exchange bias field (HE ) for ultrathin CoO/Co bilayers supported on semiconductor substrates. In our previous reports, the formations of interfacial compounds occurs for Co grown on semiconductor substrates [17,18]. Thickness dependent reactivity of ultrathin metal/semiconductor has been reported while the segregation of oxygen atoms is observed for oxygen exposure on Co/Ge [19,20]. After oxygen exposure on Co/Ge(1 1 1), the stress anisotropy is modified by the O/Co interface and causes the increased coercive force [21]. The purposefully partially oxidized Co nanoparticles exhibits a thick CoO shell and is exchange biased [22]. In granular Co-CoO EB system, high-fluence O-implanted thin films show reduced relative training values and no asymmetry in magnetization reversal while low-fluence O ion implantation results in an increased relative training and a
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Kerr signals (arb. units)
x ML CoO/25 ML Co/Ge(100)
x = 40 x = 25 x = 10 x=0 170 K
300 K -2.0
0.0
2.0
-2.0
0.0
2.0
Magnetic field (kOe) Fig. 2. Kerr signals versus the magnetic field for x ML CoO grown on 25 ML Co/Ge(1 0 0) measured at 300 K (left panel) and 170 K (right panel).
Fig. 1. (a) The Auger intensity ratio IO /ICo and the intensity of the RHEED spot versus CoO thickness for x ML CoO/25 ML Co/Ge(1 0 0). (b) RHEED pattern of 5 ML CoO/25 ML Co/Ge(1 0 0), (c) RHEED pattern of 25 ML CoO/25 ML Co/Ge(1 0 0), and (d) RHEED pattern of 35 ML CoO/25 ML Co/Ge(1 0 0).
magnetization reversal asymmetry [23]. In this paper, the interfacial region caused by the segregation of oxygen atoms into the Co layer is observed and its width depends on the preparation procedure for ultrathin CoO/Co bilayers on Ge(1 0 0). A model of the distribution of pinned magnetic moments in the CoO/Co EB system is proposed. In thicker interfacial region, more pinned magnetic moments result in the larger HE value. This finding provides a practical way of increasing HE for ultrathin CoO/Co bilayers on semiconductor substrates and are valuable for future applications. 2. Experimental All experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 2 × 10−10 Torr. The UHV chamber was equipped with facilities for MOKE, Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and lowenergy electron diffraction (LEED) measurements. The equipment components have been described in detail elsewhere [17–21]. The Ge(1 0 0) surface was cleaned by cycles of Ar+ ion bombardment and subsequent annealing treatments at 1100 K. Co atoms were evaporated from a resistively heated cobalt coil with a high purity (99.997%). Oxygen was introduced into the chamber via a leak valve to control the gas flow. After oxygen exposure for Co/Ge, only a small amount of physisorbed oxygen is detected and no AFM CoO layer forms [21]. CoO overlayers were prepared by evaporating Co atoms in an oxygen atmosphere at a pressure of 5 × 10−7 Torr. This is an in situ study. The growth and all the characterizations of the specimens are performed in the UHV chamber. 3. Results and discussion The compositions and structures of CoO/Co/Ge(1 0 0) were systematically investigated. As an example, Fig. 1a shows the Auger intensity ratio IO /ICo (O KL2 L2 vs Co L3 M45 M45 ) as a function of the CoO thickness for x ML CoO/25 ML Co/Ge(1 0 0). On the top of 25 ML Co/Ge(1 0 0), CoO overlayers were prepared. For Auger intensity ratio in Fig. 1a, it was taken for CoO/25 ML Co/Ge(1 0 0) at each CoO thickness. As the CoO thickness increases, the Auger intensity ratio IO /ICo increases until reaching a saturated value around 1.8. From the investigations of optimized exchange biasing of CoO/Co bilayers by controlled oxidation of Co films, an equal concentration of Co and
O corresponds to an Auger intensity ratio IO /ICo being 1.8–1.9 [24]. The saturation value of the Auger intensity ratio IO /ICo around 1.8 in Fig. 1a is consistent with that in Ref. [24] where the concentration ratio of Co and O is close to 1:1 for all layers within the electron escape depth. The surface structures of CoO/Co/Ge(1 0 0) were investigated using RHEED technique. With grazing incidence condition, the change of the RHEED intensity should be attributed to the ordered/disordered structure of the top MLs of CoO rather than to the buried CoO/Co interface. For the RHEED intensity in Fig. 1a, it was taken for CoO/25 ML Co/Ge(1 0 0) at each CoO thickness. Some typical RHEED patterns are shown in Fig. 1b–d. As shown in Fig. 1a, the intensity of the RHEED spot increases to the maximum value at 25 ML CoO followed by an attenuation as the CoO thickness increases. The initial increase for the intensity of the RHEED spot is attributed to the formation of ordered CoO overlayers. For CoO layers thicker than 25 ML, the reduction of the RHEED intensity is due to the introduction of defects at surface layers since the penetration depth of RHEED electrons is only few MLs for the grazing incidence condition. Fig. 2 shows the Kerr signals versus the magnetic field for x ML CoO on 25 ML Co/Ge(1 0 0). At 300 K, both a slight reduction of the Kerr intensity and an enhanced coercive force (HC ) are observed as the CoO thickness increases. Under conditions of cooling in a magnetic field of +2 kOe down to 170 K, both an enhanced HC and the shift of the hysteresis loop are observed for x > 0 showing the EB phenomenon. The Neel temperature of CoO is around 290 K [2,3]. For ultrathin films, the blocking temperature, at or above which the exchange bias effect is no longer present, is lower than the Neel temperature. So the cooled temperature must be well below 290 K. With the design of the cryogenic system of the specimen in our UHV chamber by adding liquid nitrogen, the lowest sample temperature is around 120 K within reasonable cooling time around few hours [25] In this study, a series of CoO/Co/Ge(1 0 0) films with different CoO thicknesses have been grown and at each CoO thickness the MOKE measurements have been performed at room temperature as well as after field cooling to low temperatures for comparative purposes. To shorten the experimental time avoiding the contamination on the specimens, we choose 170 K as the common temperature for cooling. To detailed investigate the magnetic properties of CoO/Co/Ge(1 0 0), Fig. 3a shows the Kerr intensity and HC versus the CoO thickness at 300 K. As the CoO thickness increases, a slight reduction of the Kerr intensity is observed. From previous studies of Co/Ge and Co/Si systems, by increasing the Co thickness, Co adatoms show enhanced chemical reactivity for oxidation due to the change of the chemical state [19,21]. The reduction of the Kerr intensities for oxygen exposure on Co/Ge and Co/Si is due to the
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is related to the pinning of magnetic moments as a result of the exchange coupling interactions at the CoO/Co interface. The CoO thickness dependence of the reduction of the Kerr intensity and related HE are shown in Fig. 3c for CoO on 25 ML Co/Ge(1 0 0). The K is calculated from the difference of the Kerr intensities at 300 K (Fig. 3a) and at 170 K (Fig. 3b). As the CoO thickness increases, HE increases from zero at 5 ML CoO to the maximum value of 0.46 kOe at 25 ML CoO followed by a monotonous decrease to 0.37 kOe for 40 ML CoO. The onset of the EB could be influenced by the finite size effects, crystallinity, and surface effects [26,30] and is between 5 and 10 ML for x ML CoO/25 ML Co/Ge(1 0 0). Assuming coherent rotation of the magnetization of an EB system, the relations of the HE , the magnetic anisotropy KAFM of the AFM layer and the exchange stiffness AAFM of the AFM, can be expressed by
HE ∝
KAFM AAFM
MFM tFM
(1)
where MFM is the saturation magnetization of the FM layer, and tFM is the thickness of the FM layer [3,4,26]. For CoO layers thinner than 25 ML, the increased RHEED intensity in Fig. 1a shows the formation of an ordered CoO overlayer. The increase of HE for CoO thinner than 25 ML in Fig. 3c is therefore attributed to the enhancement of the magnetic anisotropy KAFM as a result of the formation of an ordered CoO layer. For CoO layers thicker than 25 ML, the reduction of HE is attributed to the increase of the AFM domain size that is often observed in continuous FM/AFM layers [3,31]. In Fig. 3c, similar evolutions of HE and K can be clear seem as the CoO thickness increases. We may write down the relation for the exchange bias field HE and K as: HE = ˛ K
Fig. 3. Kerr intensity and coercive force for x ML CoO/25 ML Co/Ge(1 0 0) measured at (a) 300 K and (b) 170 K. (c) HE and K versus the CoO thickness for x ML CoO/25 ML Co/Ge(1 0 0).
interaction of oxygen with the top Co layers by modifying the electronic density of states of Co [19,21]. In this study, CoO is prepared by evaporating Co atoms in an oxygen atmosphere. The top layer of the 25 ML Co/Ge(1 0 0) is therefore exposure to oxygen. Parts of Co atoms at the interface lost the FM properties and this causes the reduction of the Kerr intensity at 300 K. After preparation of CoO on Co/Ge(1 0 0), the enhancement of HC is attributed to the imperfection at the CoO/Co interface introduced by oxygen to impede the magnetization reversal (Fig. 3a). After field cooling down to 170 K, further enhancements of HC are detected as shown in Fig. 3b. The onset of the HC increase in FM/AFM systems can be considered as an indirect measure of the AFM ordering temperature [26–29]. The HC usually increases below the blocking temperature, which is linked to the anisotropy of the AFM layer [3]. By comparing the Kerr intensities in Fig. 3a and b, further reduction of the Kerr intensity after field cooling to 170 K is detected. The pinned uncompensated interfacial spins tightly locked to the AFM lattice have been reported and they do not rotate in an external magnetic field [8]. For CoO/Co/Ge(1 1 1), the pinning of magnetic moments at the AFM/FM interface results in that some Co atoms do not contribute to the magneto-optical responses [18]. The reduction of the Kerr intensity after field cooling to low temperatures in Fig. 3b
(2)
where ˛ is a constant. From the experimental data, the value can be estimated to be around 32 kOe/degree. The coincidence of the trends of HE and K in Fig. 3c could be attributed to the pinned magnetic moments in the interfacial region and will be discussed in the following paragraphs. The pinned uncompensated spins have been observed at the AFM/FM interfaces of different forms such as bilayers and nanostructures [10,32–34]. For CoO/Fe/Ag(001) films, CoO spins are rotatable at smaller thicknesses and frozen at larger thicknesses evidenced by X-ray magnetic circular dichroism [32,33]. For the MnIr/CoFe system, pinned Mn spins are found in a wide area with a width of 3 nm due to the atomic interdiffusion and increased roughness as revealed from polarized neutron reflectivity measurements [10]. The pinned magnetic moments are responsible for the EB phenomenon while the higher EB is the result of the extended distribution of pinned magnetic moments at the AFM/FM interface [10,32]. From the modified equation concerning the HE and pinned interfacial magnetizations [8,35], the relation between HE and the nominal thickness of the pinned uncompensated layer in fractions of a ML is HE =
· J · SAFM SFM a2AFM MFM tFM
(3)
where J is the interface exchange energy; SAFM and SFM are the magnetic moments of the AFM and FM, respectively; aAFM is the size of the unit cell of the AFM. In our MOKE measurements, geometric parameters and optical components were kept the same. Although the absolute magnitude of magnetization cannot be obtained, the Kerr intensity is proportional to the magnetization from the analysis in terms of molecular-orbital energy levels [28,29]. Since the amount of pinned magnetic moments Mpin is proportional to [8,35], by recalling the proportional relation between HE and K in Eq. (2), the evolution of K is reasonable as an indicator of the pinned magnetic moments. The preparation of CoO may introduce oxygen atoms into the Co layers to form an interfacial region
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x ML CoO/ y ML Co/Ge(100) y = 25 ML y = 15 ML
HE (kOe)
0.6
0.3
0.0 0
10
20
30
40
CoO thickness (ML) Fig. 4. HE versus the CoO thickness for x ML CoO/y ML Co/Ge(1 0 0).
by the partial transformation of Co to CoO while the width of this region depends on the preparation procedure. For the preparation of a thicker CoO layer, the oxygen exposure time is longer and a thicker interfacial region is obtained. Both the formation of CoO layer and the segregation of oxygen atoms into the Co surface introduce pinned magnetic moments in the interfacial region. They are responsible for the coincidence for HE and K in Fig. 3c. Systematical investigations of HE for x ML CoO on y ML Co/Ge(1 0 0) have been performed and the results are shown in Fig. 4. As the CoO thickness increases, HE increases from zero to the maximum value followed by a monotonous decrease. The similar trend of the HE versus CoO thickness is observed for different Co thicknesses y while the HE value is smaller for CoO/Co/Ge(1 0 0) with a thinner Co layer. For FM layers of a few nm, the discontinuity of the films and related microstructure may play important roles on the HE value [3]. The preparation of CoO on thinner Co/Ge(1 0 0) could be more influenced by the Co/Ge interfaces and therefore more disordered structures of the CoO layer are often detected for CoO on thinner Co/Ge(1 0 0) as revealed by smaller RHEED intensity. In addition, for ultrathin metal/semiconductor, oxygen exposure experiments show thickness dependent reactivity of the films [19,21]. Solid surfaces of Co–Si compounds are resistive against oxidation while the segregation of oxygen atoms is observed for oxygen exposure on Co/Ge(1 1 1) [19,20]. The preparation of CoO can introduce oxygen atoms into the Co layer to form an interfacial region while its thickness depends on the preparation procedure. To reveal the thickness of the interfacial region, depth profiling measurements performed by Ar+ ion sputtering combined with AES are shown in Fig. 5a for CoO/Co/Ge(1 0 0). As the sputtering time increases for 40 ML CoO/15 ML Co/Ge(1 0 0), the Auger intensity ratio IO /ICo around 1.8 remains unchanged within 3.75 min followed by a significant reduction until zero value at 6.0 min. Within 3.75 min of sputtering, the constant value of the Auger intensity ratio IO /ICo around 1.8 corresponds to both the formation of 1:1 concentration ratio of Co and O [24] and the topmost CoO MLs thicker than the inelastic mean free path (IMFP) of the Auger electrons [36,37]. Between 3.75 and 6.0 min of sputtering time, the decrease of the Auger intensity ratio IO /ICo is related to both the CoO overlayer and the oxygen atoms distributed in the topmost Co MLs. As a comparison to 40 ML CoO/25 ML Co/Ge(1 0 0), the Auger intensity ratio IO /ICo shows a similar behavior of a near constant value before 3.75 min followed by a significant reduction until zero value around 6.5 min. Due to the same CoO thicknesses, the sputtering time for the constant IO /ICo value is the same, i.e., 3.75 min. However, the sputtering time for IO /ICo decreasing to zero value is longer for CoO on thicker Co/Ge(1 0 0). This shows the extended interfacial region and more pinned magnetic moments are produced. A model
Fig. 5. (a) The Auger intensity ratio (IO /ICo ) versus the sputtering time for 40 ML CoO on 25 and 15 ML Co/Ge(1 0 0). (b) A model of the distribution of pinned magnetic moments in the CoO/Co EB system.
of the distribution of pinned magnetic moments in the CoO/Co EB system is schematically illustrated in Fig. 5b. The related EB mechanism is discussed below. After field cooling to low temperatures for CoO/Co/Ge(1 0 0), EB occurs. Pinned magnetic moments in the AFM layer are developed [10,32]. In addition to the formation of CoO layer, the segregation of oxygen atoms into the Co surface introduces more pinned magnetic moments in the interfacial region. The pinned magnetic moments are responsible for the coincidence for HE and K in Fig. 3c. For CoO prepared on thicker Co/Ge(1 0 0), the thicker interfacial region is evidenced by depth profiling measurements in Fig. 5a. The thicker the interfacial region is, the more pinned magnetic moments results in the larger HE . By controlling the thickness of the interfacial region, it shows a possible way of increasing HE for ultrathin CoO/Co bilayers on semiconductor substrates. 4. Conclusions In summary, we report on the interfacial region and tunable pinning of magnetic moments of ultrathin CoO/Co/Ge(1 0 0) films. CoO overlayers were prepared by evaporating Co atoms in an oxygen atmosphere. As the CoO thickness increases, the Auger intensity ratio IO /ICo increases until reaching a saturated value around 1.8 corresponding to the concentration ratio of Co and O close to 1:1. Both the oxidation of the Co atoms at the interface and the segregation of oxygen atoms into the Co surface occur. Parts of Co atoms at the interface lost the FM properties and this causes the reduction of Kerr intensity at 300 K. Under conditions of cooling in a magnetic field, further reduction of the Kerr intensity is detected. The segregation of oxygen atoms into the Co layer introduces pinned magnetic moments in the interfacial region that are responsible for the coincidence for HE and K . Depth profiling measurements show that the width of the interfacial region depends on the preparation procedure. A model of the distribution of pinned magnetic moments in the CoO/Co EB system is proposed. Because of the
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Pinning of magnetic moments at the interfacial region of ultrathin CoO/Co bilayers grown on Ge(1 0 0) Shin-Chen Chang a , Jyh-Shen Tsay a,∗ , Cheng-Hsun-Tony Chang a , Yeong-Der Yao b a b
Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan Institute of Physics, Academia Sinica, Nakang, Taipei 11529, Taiwan
a r t i c l e
a b s t r a c t
i n f o
Article history: Received 25 September 2014 Received in revised form 27 March 2015 Accepted 3 April 2015 Available online 11 April 2015 Keywords: Ultrathin films Tunable exchange bias Magnetic thin films Oxide
For CoO overlayers prepared by evaporating Co atoms in an oxygen atmosphere, both the oxidation of the Co atoms at the interface and the segregation of oxygen atoms into the Co surface occur. Parts of Co atoms at the interface become nonferromagnetic and this causes the reduction of Kerr intensity at 300 K. After field cooling treatments, further reduction of the Kerr intensity is detected. The change of the Kerr intensity is an indicator of the pinned magnetic moments. In the case of forming thicker interfacial region, more pinned magnetic moments are observed and result in the larger exchange bias field for CoO on thicker Co/Ge(1 0 0). This tunable pinning provides a practical way of increasing exchange bias field by controlling the thickness of the interfacial region for ultrathin CoO/Co bilayers on semiconductor substrates. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Due to the conception and optimization of applicative devices for spintronics, exchange bias (EB) phenomena have attracted considerable attentions [1–16], especially on the unidirectional anisotropy which is caused by the exchange coupling between antiferromagnetic (AFM) and ferromagnetic (FM) layered materials and the pinned magnetic moments [6–16]. The exchange coupling is sensitive to the AFM/FM interface and relies on some microscopic parameters, such as crystallographic order, surface roughness, interfacial strain, and spin orientation [14–16]. From the literatures, great efforts have also been devoted to the nature of the pinned magnetic moments [6–13]. In an intuitive picture of the EB effect, the spin configuration of the AFM layer is assumed to be pinned and all the interfacial AFM spins are responsible for the unidirectional anisotropy via exchange coupling between AFM and FM layers [3,7]. In the last decade, more experimental evidences show that some pinned uncompensated spins at the interface, resulting from the intrinsic AFM spin structure or imperfections like step edges, are the key element of the EB [2,3,8–10]. X-ray magnetic circular dichroism (XMCD) studies show that the pinned uncompensated interfacial spins constitute a fraction of a monolayer (ML) tightly locked to the AFM lattice [8]. Soft X-ray
∗ Corresponding author. Tel.: +886 2 77346031. E-mail address: [email protected] (J.-S. Tsay). http://dx.doi.org/10.1016/j.apsusc.2015.04.019 0169-4332/© 2015 Elsevier B.V. All rights reserved.
resonant reflectivity studies on exchange-biased bilayer of permalloy/CoO identified 0.5-nm-thick layer containing uncompensated Co moments at the interface where a small fraction of the uncompensated Co moments pinned antiparallel to the cooling field is used to bias the sample [9]. Polarized neutron reflectivity measurements on annealed MnIr/CoFe show an extended pinned spin area at the interface due to the increased roughness and atomic interdiffusion [10]. Magneto-optical Kerr effect (MOKE) measurements of epitaxial AFM/FM structures show that the pinned magnetic moments inside the bulk of the AFM layer coexisting independently for orthogonal spin directions are responsible for the emergence of EB [6]. However, there is very limited knowledge about the relation between the pinned magnetic moments and exchange bias field (HE ) for ultrathin CoO/Co bilayers supported on semiconductor substrates. In our previous reports, the formations of interfacial compounds occurs for Co grown on semiconductor substrates [17,18]. Thickness dependent reactivity of ultrathin metal/semiconductor has been reported while the segregation of oxygen atoms is observed for oxygen exposure on Co/Ge [19,20]. After oxygen exposure on Co/Ge(1 1 1), the stress anisotropy is modified by the O/Co interface and causes the increased coercive force [21]. The purposefully partially oxidized Co nanoparticles exhibits a thick CoO shell and is exchange biased [22]. In granular Co-CoO EB system, high-fluence O-implanted thin films show reduced relative training values and no asymmetry in magnetization reversal while low-fluence O ion implantation results in an increased relative training and a
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Kerr signals (arb. units)
x ML CoO/25 ML Co/Ge(100)
x = 40 x = 25 x = 10 x=0 170 K
300 K -2.0
0.0
2.0
-2.0
0.0
2.0
Magnetic field (kOe) Fig. 2. Kerr signals versus the magnetic field for x ML CoO grown on 25 ML Co/Ge(1 0 0) measured at 300 K (left panel) and 170 K (right panel).
Fig. 1. (a) The Auger intensity ratio IO /ICo and the intensity of the RHEED spot versus CoO thickness for x ML CoO/25 ML Co/Ge(1 0 0). (b) RHEED pattern of 5 ML CoO/25 ML Co/Ge(1 0 0), (c) RHEED pattern of 25 ML CoO/25 ML Co/Ge(1 0 0), and (d) RHEED pattern of 35 ML CoO/25 ML Co/Ge(1 0 0).
magnetization reversal asymmetry [23]. In this paper, the interfacial region caused by the segregation of oxygen atoms into the Co layer is observed and its width depends on the preparation procedure for ultrathin CoO/Co bilayers on Ge(1 0 0). A model of the distribution of pinned magnetic moments in the CoO/Co EB system is proposed. In thicker interfacial region, more pinned magnetic moments result in the larger HE value. This finding provides a practical way of increasing HE for ultrathin CoO/Co bilayers on semiconductor substrates and are valuable for future applications. 2. Experimental All experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 2 × 10−10 Torr. The UHV chamber was equipped with facilities for MOKE, Auger electron spectroscopy (AES), reflection high energy electron diffraction (RHEED), and lowenergy electron diffraction (LEED) measurements. The equipment components have been described in detail elsewhere [17–21]. The Ge(1 0 0) surface was cleaned by cycles of Ar+ ion bombardment and subsequent annealing treatments at 1100 K. Co atoms were evaporated from a resistively heated cobalt coil with a high purity (99.997%). Oxygen was introduced into the chamber via a leak valve to control the gas flow. After oxygen exposure for Co/Ge, only a small amount of physisorbed oxygen is detected and no AFM CoO layer forms [21]. CoO overlayers were prepared by evaporating Co atoms in an oxygen atmosphere at a pressure of 5 × 10−7 Torr. This is an in situ study. The growth and all the characterizations of the specimens are performed in the UHV chamber. 3. Results and discussion The compositions and structures of CoO/Co/Ge(1 0 0) were systematically investigated. As an example, Fig. 1a shows the Auger intensity ratio IO /ICo (O KL2 L2 vs Co L3 M45 M45 ) as a function of the CoO thickness for x ML CoO/25 ML Co/Ge(1 0 0). On the top of 25 ML Co/Ge(1 0 0), CoO overlayers were prepared. For Auger intensity ratio in Fig. 1a, it was taken for CoO/25 ML Co/Ge(1 0 0) at each CoO thickness. As the CoO thickness increases, the Auger intensity ratio IO /ICo increases until reaching a saturated value around 1.8. From the investigations of optimized exchange biasing of CoO/Co bilayers by controlled oxidation of Co films, an equal concentration of Co and
O corresponds to an Auger intensity ratio IO /ICo being 1.8–1.9 [24]. The saturation value of the Auger intensity ratio IO /ICo around 1.8 in Fig. 1a is consistent with that in Ref. [24] where the concentration ratio of Co and O is close to 1:1 for all layers within the electron escape depth. The surface structures of CoO/Co/Ge(1 0 0) were investigated using RHEED technique. With grazing incidence condition, the change of the RHEED intensity should be attributed to the ordered/disordered structure of the top MLs of CoO rather than to the buried CoO/Co interface. For the RHEED intensity in Fig. 1a, it was taken for CoO/25 ML Co/Ge(1 0 0) at each CoO thickness. Some typical RHEED patterns are shown in Fig. 1b–d. As shown in Fig. 1a, the intensity of the RHEED spot increases to the maximum value at 25 ML CoO followed by an attenuation as the CoO thickness increases. The initial increase for the intensity of the RHEED spot is attributed to the formation of ordered CoO overlayers. For CoO layers thicker than 25 ML, the reduction of the RHEED intensity is due to the introduction of defects at surface layers since the penetration depth of RHEED electrons is only few MLs for the grazing incidence condition. Fig. 2 shows the Kerr signals versus the magnetic field for x ML CoO on 25 ML Co/Ge(1 0 0). At 300 K, both a slight reduction of the Kerr intensity and an enhanced coercive force (HC ) are observed as the CoO thickness increases. Under conditions of cooling in a magnetic field of +2 kOe down to 170 K, both an enhanced HC and the shift of the hysteresis loop are observed for x > 0 showing the EB phenomenon. The Neel temperature of CoO is around 290 K [2,3]. For ultrathin films, the blocking temperature, at or above which the exchange bias effect is no longer present, is lower than the Neel temperature. So the cooled temperature must be well below 290 K. With the design of the cryogenic system of the specimen in our UHV chamber by adding liquid nitrogen, the lowest sample temperature is around 120 K within reasonable cooling time around few hours [25] In this study, a series of CoO/Co/Ge(1 0 0) films with different CoO thicknesses have been grown and at each CoO thickness the MOKE measurements have been performed at room temperature as well as after field cooling to low temperatures for comparative purposes. To shorten the experimental time avoiding the contamination on the specimens, we choose 170 K as the common temperature for cooling. To detailed investigate the magnetic properties of CoO/Co/Ge(1 0 0), Fig. 3a shows the Kerr intensity and HC versus the CoO thickness at 300 K. As the CoO thickness increases, a slight reduction of the Kerr intensity is observed. From previous studies of Co/Ge and Co/Si systems, by increasing the Co thickness, Co adatoms show enhanced chemical reactivity for oxidation due to the change of the chemical state [19,21]. The reduction of the Kerr intensities for oxygen exposure on Co/Ge and Co/Si is due to the
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is related to the pinning of magnetic moments as a result of the exchange coupling interactions at the CoO/Co interface. The CoO thickness dependence of the reduction of the Kerr intensity and related HE are shown in Fig. 3c for CoO on 25 ML Co/Ge(1 0 0). The K is calculated from the difference of the Kerr intensities at 300 K (Fig. 3a) and at 170 K (Fig. 3b). As the CoO thickness increases, HE increases from zero at 5 ML CoO to the maximum value of 0.46 kOe at 25 ML CoO followed by a monotonous decrease to 0.37 kOe for 40 ML CoO. The onset of the EB could be influenced by the finite size effects, crystallinity, and surface effects [26,30] and is between 5 and 10 ML for x ML CoO/25 ML Co/Ge(1 0 0). Assuming coherent rotation of the magnetization of an EB system, the relations of the HE , the magnetic anisotropy KAFM of the AFM layer and the exchange stiffness AAFM of the AFM, can be expressed by
HE ∝
KAFM AAFM
MFM tFM
(1)
where MFM is the saturation magnetization of the FM layer, and tFM is the thickness of the FM layer [3,4,26]. For CoO layers thinner than 25 ML, the increased RHEED intensity in Fig. 1a shows the formation of an ordered CoO overlayer. The increase of HE for CoO thinner than 25 ML in Fig. 3c is therefore attributed to the enhancement of the magnetic anisotropy KAFM as a result of the formation of an ordered CoO layer. For CoO layers thicker than 25 ML, the reduction of HE is attributed to the increase of the AFM domain size that is often observed in continuous FM/AFM layers [3,31]. In Fig. 3c, similar evolutions of HE and K can be clear seem as the CoO thickness increases. We may write down the relation for the exchange bias field HE and K as: HE = ˛ K
Fig. 3. Kerr intensity and coercive force for x ML CoO/25 ML Co/Ge(1 0 0) measured at (a) 300 K and (b) 170 K. (c) HE and K versus the CoO thickness for x ML CoO/25 ML Co/Ge(1 0 0).
interaction of oxygen with the top Co layers by modifying the electronic density of states of Co [19,21]. In this study, CoO is prepared by evaporating Co atoms in an oxygen atmosphere. The top layer of the 25 ML Co/Ge(1 0 0) is therefore exposure to oxygen. Parts of Co atoms at the interface lost the FM properties and this causes the reduction of the Kerr intensity at 300 K. After preparation of CoO on Co/Ge(1 0 0), the enhancement of HC is attributed to the imperfection at the CoO/Co interface introduced by oxygen to impede the magnetization reversal (Fig. 3a). After field cooling down to 170 K, further enhancements of HC are detected as shown in Fig. 3b. The onset of the HC increase in FM/AFM systems can be considered as an indirect measure of the AFM ordering temperature [26–29]. The HC usually increases below the blocking temperature, which is linked to the anisotropy of the AFM layer [3]. By comparing the Kerr intensities in Fig. 3a and b, further reduction of the Kerr intensity after field cooling to 170 K is detected. The pinned uncompensated interfacial spins tightly locked to the AFM lattice have been reported and they do not rotate in an external magnetic field [8]. For CoO/Co/Ge(1 1 1), the pinning of magnetic moments at the AFM/FM interface results in that some Co atoms do not contribute to the magneto-optical responses [18]. The reduction of the Kerr intensity after field cooling to low temperatures in Fig. 3b
(2)
where ˛ is a constant. From the experimental data, the value can be estimated to be around 32 kOe/degree. The coincidence of the trends of HE and K in Fig. 3c could be attributed to the pinned magnetic moments in the interfacial region and will be discussed in the following paragraphs. The pinned uncompensated spins have been observed at the AFM/FM interfaces of different forms such as bilayers and nanostructures [10,32–34]. For CoO/Fe/Ag(001) films, CoO spins are rotatable at smaller thicknesses and frozen at larger thicknesses evidenced by X-ray magnetic circular dichroism [32,33]. For the MnIr/CoFe system, pinned Mn spins are found in a wide area with a width of 3 nm due to the atomic interdiffusion and increased roughness as revealed from polarized neutron reflectivity measurements [10]. The pinned magnetic moments are responsible for the EB phenomenon while the higher EB is the result of the extended distribution of pinned magnetic moments at the AFM/FM interface [10,32]. From the modified equation concerning the HE and pinned interfacial magnetizations [8,35], the relation between HE and the nominal thickness of the pinned uncompensated layer in fractions of a ML is HE =
· J · SAFM SFM a2AFM MFM tFM
(3)
where J is the interface exchange energy; SAFM and SFM are the magnetic moments of the AFM and FM, respectively; aAFM is the size of the unit cell of the AFM. In our MOKE measurements, geometric parameters and optical components were kept the same. Although the absolute magnitude of magnetization cannot be obtained, the Kerr intensity is proportional to the magnetization from the analysis in terms of molecular-orbital energy levels [28,29]. Since the amount of pinned magnetic moments Mpin is proportional to [8,35], by recalling the proportional relation between HE and K in Eq. (2), the evolution of K is reasonable as an indicator of the pinned magnetic moments. The preparation of CoO may introduce oxygen atoms into the Co layers to form an interfacial region
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x ML CoO/ y ML Co/Ge(100) y = 25 ML y = 15 ML
HE (kOe)
0.6
0.3
0.0 0
10
20
30
40
CoO thickness (ML) Fig. 4. HE versus the CoO thickness for x ML CoO/y ML Co/Ge(1 0 0).
by the partial transformation of Co to CoO while the width of this region depends on the preparation procedure. For the preparation of a thicker CoO layer, the oxygen exposure time is longer and a thicker interfacial region is obtained. Both the formation of CoO layer and the segregation of oxygen atoms into the Co surface introduce pinned magnetic moments in the interfacial region. They are responsible for the coincidence for HE and K in Fig. 3c. Systematical investigations of HE for x ML CoO on y ML Co/Ge(1 0 0) have been performed and the results are shown in Fig. 4. As the CoO thickness increases, HE increases from zero to the maximum value followed by a monotonous decrease. The similar trend of the HE versus CoO thickness is observed for different Co thicknesses y while the HE value is smaller for CoO/Co/Ge(1 0 0) with a thinner Co layer. For FM layers of a few nm, the discontinuity of the films and related microstructure may play important roles on the HE value [3]. The preparation of CoO on thinner Co/Ge(1 0 0) could be more influenced by the Co/Ge interfaces and therefore more disordered structures of the CoO layer are often detected for CoO on thinner Co/Ge(1 0 0) as revealed by smaller RHEED intensity. In addition, for ultrathin metal/semiconductor, oxygen exposure experiments show thickness dependent reactivity of the films [19,21]. Solid surfaces of Co–Si compounds are resistive against oxidation while the segregation of oxygen atoms is observed for oxygen exposure on Co/Ge(1 1 1) [19,20]. The preparation of CoO can introduce oxygen atoms into the Co layer to form an interfacial region while its thickness depends on the preparation procedure. To reveal the thickness of the interfacial region, depth profiling measurements performed by Ar+ ion sputtering combined with AES are shown in Fig. 5a for CoO/Co/Ge(1 0 0). As the sputtering time increases for 40 ML CoO/15 ML Co/Ge(1 0 0), the Auger intensity ratio IO /ICo around 1.8 remains unchanged within 3.75 min followed by a significant reduction until zero value at 6.0 min. Within 3.75 min of sputtering, the constant value of the Auger intensity ratio IO /ICo around 1.8 corresponds to both the formation of 1:1 concentration ratio of Co and O [24] and the topmost CoO MLs thicker than the inelastic mean free path (IMFP) of the Auger electrons [36,37]. Between 3.75 and 6.0 min of sputtering time, the decrease of the Auger intensity ratio IO /ICo is related to both the CoO overlayer and the oxygen atoms distributed in the topmost Co MLs. As a comparison to 40 ML CoO/25 ML Co/Ge(1 0 0), the Auger intensity ratio IO /ICo shows a similar behavior of a near constant value before 3.75 min followed by a significant reduction until zero value around 6.5 min. Due to the same CoO thicknesses, the sputtering time for the constant IO /ICo value is the same, i.e., 3.75 min. However, the sputtering time for IO /ICo decreasing to zero value is longer for CoO on thicker Co/Ge(1 0 0). This shows the extended interfacial region and more pinned magnetic moments are produced. A model
Fig. 5. (a) The Auger intensity ratio (IO /ICo ) versus the sputtering time for 40 ML CoO on 25 and 15 ML Co/Ge(1 0 0). (b) A model of the distribution of pinned magnetic moments in the CoO/Co EB system.
of the distribution of pinned magnetic moments in the CoO/Co EB system is schematically illustrated in Fig. 5b. The related EB mechanism is discussed below. After field cooling to low temperatures for CoO/Co/Ge(1 0 0), EB occurs. Pinned magnetic moments in the AFM layer are developed [10,32]. In addition to the formation of CoO layer, the segregation of oxygen atoms into the Co surface introduces more pinned magnetic moments in the interfacial region. The pinned magnetic moments are responsible for the coincidence for HE and K in Fig. 3c. For CoO prepared on thicker Co/Ge(1 0 0), the thicker interfacial region is evidenced by depth profiling measurements in Fig. 5a. The thicker the interfacial region is, the more pinned magnetic moments results in the larger HE . By controlling the thickness of the interfacial region, it shows a possible way of increasing HE for ultrathin CoO/Co bilayers on semiconductor substrates. 4. Conclusions In summary, we report on the interfacial region and tunable pinning of magnetic moments of ultrathin CoO/Co/Ge(1 0 0) films. CoO overlayers were prepared by evaporating Co atoms in an oxygen atmosphere. As the CoO thickness increases, the Auger intensity ratio IO /ICo increases until reaching a saturated value around 1.8 corresponding to the concentration ratio of Co and O close to 1:1. Both the oxidation of the Co atoms at the interface and the segregation of oxygen atoms into the Co surface occur. Parts of Co atoms at the interface lost the FM properties and this causes the reduction of Kerr intensity at 300 K. Under conditions of cooling in a magnetic field, further reduction of the Kerr intensity is detected. The segregation of oxygen atoms into the Co layer introduces pinned magnetic moments in the interfacial region that are responsible for the coincidence for HE and K . Depth profiling measurements show that the width of the interfacial region depends on the preparation procedure. A model of the distribution of pinned magnetic moments in the CoO/Co EB system is proposed. Because of the
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thicker interfacial region, more pinned magnetic moments result in the larger HE value for CoO on thicker Co/Ge(1 0 0). These results reveal the correlation between pinned magnetic moments and HE and are valuable for the applications of ultrathin AFM/FM bilayers supported on semiconductor substrates. Acknowledgments The authors acknowledge supports from National Science Council of Taiwan under Contract No. NSC-100-2112-M-003-004-MY3 and Ministry of Science and Technology of Taiwan under Contract No. MOST-103-2112-M-003-006. References [1] Y. Fan, K.J. Smith, G. Luepke, A.T. Hanbicki, R. Goswami, C.H. Li, H.B. Zhao, B.T. Jonker, Exchange bias of the interface spin system at the Fe/MgO interface, Nat. Nanotechnol. 8 (2013) 438–444. [2] P.K. Manna, S.M. Yusuf, Two interface effects: exchange bias and magnetic proximity, Phys. Rep. 535 (2014) 61–99. [3] J. Nogués, I.K. Schuller, Exchange bias, J. Magn. Magn. Mater. 192 (1999) 203–232. [4] S. Giri, M. Patra, S. Majumdar, Exchange bias effect in alloys and compounds, J. Phys. Condens. Matter 23 (2011) 073201. [5] A.D. Lamirand, M.M. Soares, A.Y. Ramos, H.C.N. Tolentino, M.D. Santis, J.C. Cezar, A. de Siervo, M. Jamet, Robust perpendicular exchange coupling in an ultrathin CoO/PtFe double layer: strain and spin orientation, Phys. Rev. B 88 (2013), 140401(R). [6] M.Y. Khan, C.B. Wu, W. Kuch, Pinned magnetic moments in exchange bias: role of the antiferromagnetic bulk spin structure, Phys. Rev. B 89 (2014) 094427. [7] W.H. Meiklejohn, C.P. Bean, New magnetic anisotropy, Phys. Rev. 105 (1957) 904–913. [8] H. Ohldag, A. Scholl, F. Nolting, E. Arenholz, S. Maat, A.T. Young, M. Carey, J. Stöhr, Correlation between exchange bias and pinned interfacial spins, Phys. Rev. Lett. 91 (2003) 017203. [9] E. Blackburn, C. Sanchez-Hanke, S. Roy, D.J. Smith, J.I. Hong, K.T. Chan, A.E. Berkowitz, S.K. Sinha, Pinned Co moments in a polycrystalline permalloy/CoO exchange-biased bilayer, Phys. Rev. B 78 (2008), 180408(R). [10] N. Awaji, T. Miyajima, S. Doi, K. Nomura, Polarized neutron reflectivity study of a thermally treated MnIr/CoFe exchange bias system, J. Phys. Condens. Matter 22 (2010) 474012. [11] A.L. Rizzini, C. Krull, T. Balashov, A. Mugarza, C. Nistor, F. Yakhou, V. Sessi, S. Klyatskaya, M. Ruben, S. Stepanow, P. Gambardella, Exchange biasing single molecule magnets: coupling of TbPc2 to antiferromagnetic layers, Nano Lett. 12 (2012) 5703. [12] S. Chawla, S. Sharma, R.K. Kotnala, Tailoring magnetic and photoluminescence properties in ZnS/ZnO core/shell nanostructures through Cr doping, Appl. Surf. Sci. 284 (2013) 33–39. [13] Q.R. Xu, Y. Liu, Y.D. Zheng, W. Rui, Y. Sheng, X. Shen, J. Du, M. Xu, S. Dong, D. Wu, Q. Xu, Low-temperature synthesis of K0.5 FeF3 with tunable exchange bias, Appl. Phys. Lett. 103 (2013) 102405. [14] R. Bali, M.M. Soares, A.Y. Ramos, H.C.N. Tolentino, F. Yildiz, C. Boudot, O. Proux, M.D. Santis, M. Przybylski, J. Kirschner, Magnetic and structural properties of the Fe layers in CoO/Fe/Ag(0 0 1) heterostructure, Appl. Phys. Lett. 100 (2012) 132403. [15] J.C. Read, P.M. Braganca, N. Robertson, J.R. Childress, Magnetic degradation of thin film multilayers during ion milling, APL Mater. 2 (2014) 046109. [16] O.V. Billoni, F.A. Tamarit, S.A. Cannas, Disorder-induced mechanism for positive exchange bias fields, Phys. Rev. B 88 (2013) 020405.
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