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Ir(111) ultrathin films
Thin Solid Films 548 (2013) 475–479
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Ag-related alloy formation and magnetic phases for Ag/Co/Ir(111) ultrathin films Jyh-Shen Tsay ⁎, Du-Cheng Tsai, Cheng-Hsun-Tony Chang, Wei-Hsiang Chen Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan
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Article history: Received 26 October 2012 Received in revised form 12 September 2013 Accepted 17 September 2013 Available online 24 September 2013 Keywords: Magnetic phases Alloy Surface magneto-optic Kerr effect Auger electron spectroscopy Silver Cobalt Iridium
a b s t r a c t The Kerr intensity versus the Ag thickness for Ag grown on the top of Co/Ir(111) exhibits an oscillating behavior with a period around one monolayer which should be due to the morphological change related electronic structure differences of the Ag layer. From systematical investigations of Ag/Co/Ir(111) films with the Co layer thinner than 4 monolayers at temperatures below 900 K, a magnetic phase diagram has been established. As the annealing temperature increases for Ag/Co/Ir(111) films, enhancements of the coercive force occur in both the polar and longitudinal configurations due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy. The disappearance of ferromagnetism is mainly attributed to the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of the thickness of magnetic layers, and the intermixing of Ag and Co at the Ag/Co interface. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Investigations of magnetic thin films on metallic substrates have drawn much attention in the past decade because of the involved interesting science and of great technological importance for the development of magnetic recording media, microelectronic devices, and spintronics [1–4]. Effects of interfaces on the physical properties of nanometer-scaled materials become key issues for the fabrication of layered structures that are widely used in industry [4–7]. For ultrathin films, the effective magnetic anisotropy can be affected by the surface and interfacial conditions because of the large surface-to-volume ratio. Their origins may be related to the changes of the electronic structure owing to the interaction between the atoms of different species at the interface, strain for lattice mismatched systems, and morphological changes induced by the introduction of an overlayer [8]. As an example for one monolayer (ML) of Co adatoms on Pt(111), perpendicular magnetic anisotropy (PMA) has been observed due to the positive magnetocrystalline anisotropy [9]. PMA shows advantages in the application for high-density magneto-optical recording [10]. After annealing treatments at 710 K for Co/Pt(111) films, a great enhancement of the polar magneto-optic Kerr signal is reported due to the hybridization of atomic levels of Co–Pt alloy [9]. Iridium shows high chemical stability and is therefore a preferred electrode material that is widely used in electrochemical environment [11,12]. Co and Co-based alloys are widely used in magnetic recording devices. On an Ir(111) surface, the easy axis ⁎ Corresponding author. Fax: +886 2 29326408. E-mail address: [email protected] (J.-S. Tsay). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.09.051
of magnetization of Co films could be stabilized in the direction of the surface normal by thermal annealing treatments [13]. As compared to the elements with high melting temperatures, Ag shows a smaller surface free energy [14]. In the binary phase diagram, Ag is bulkimmiscible with both Co and Ir [15]. As a capping layer, Ag-induced spin orientation transition (SRT) has been reported for both Co/ Pt(111) and Fe/Pt(111) films [16,17]. Magnetic measurements by means of X-ray magnetic circular dichroism show a reduction in the PMA of Co films by a thin Ag overlayer [18] and what has been interpreted recently for the Co/Au system in terms of magneto-elastic contributions [19]. In this report, Ag-related alloy formation of Co/ Ir(111) and effects of annealing on Ag/Co/Ir(111) have been investigated. After deposition of Ag overlayer on the top of Co/Ir(111), the longitudinal coercive force is always larger than the polar one. From systematical investigations of Ag/Co/Ir(111) films at elevated temperatures, a magnetic phase diagram has been established. Enhancements of the coercive forces occur for annealed Ag/Co/Ir(111) films due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy. 2. Experimental All experiments were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure of 2.7 × 10−8 Pa [13,20]. An iridium crystal was oriented within 0.5° of the [111] direction checked by X-ray Laue back reflection. The Ir(111) surface was cleaned by cycles of Ar+ ion bombardment and annealing treatments until a well-ordered p(1 × 1) pattern of low-energy electron diffraction (LEED) with bright, sharp
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spots and a low background intensity was observed. A K-type thermocouple spot-welded to the edge of the Ir(111) crystal was used for temperature measurements. The chemical purity of the surface was checked by Auger electron spectroscopy (AES) while Auger electrons were collected by an OMICRON hemispherical analyzer with an angular resolution of 2°. High purity cobalt (99.997%) and silver (99.999%) were deposited on the Ir(111) surface at ambient temperature. The thickness of the overlayer was determined by Auger signal ratio, Auger signals versus time (AS–t) plot and oscillations of the specular-beam intensity of LEED, and double checked by a thickness monitor [9,13,20]. The Ir(111) surface was cleaned after each experiment employing with cobalt and silver overlayers. A He–Ne laser with a wavelength of 632.8 nm was used as the light source for surface magneto-optic Kerr effect (SMOKE) measurements. A linear polarizer of Glan–Thompson polarizing prism polarized incident light with an extinction ratio of 1 × 10−6. The magnetic field was applied in-plane and perpendicular to the specimen surface in the longitudinal and polar configurations, respectively. The various components are described in detail elsewhere [13,20].
(a)
(b)
3. Results and discussion On the top of Co/Ir(111) films, Ag overlayers have been deposited to investigate the corresponding magnetic properties. Fig. 1a shows the hysteresis loops for 4 ML Co/Ir(111), 1.0 and 2.0 ML Ag/4 ML Co/ Ir(111). For 4 ML Co/Ir(111), hysteresis occurs in both the polar and longitudinal configurations while longitudinal coercive force (HC) is larger than 5 times of the polar one. From the investigations of Co/ Ir(111) films, the polar coercive force is smaller than the longitudinal one for the Co layer thinner than 4 ML [13]. No clear SRT is observed for Co/Ir(111). After the deposition of Ag overlayer on 4 ML Co/ Ir(111), the hysteresis loops became slimmer in both the polar and longitudinal configurations while the longitudinal coercive force is larger than the polar one for each Ag thickness. Ag thickness dependencies of the coercive forces in both the polar and longitudinal configurations are shown in Fig. 1b for x ML Ag/4 ML Co/Ir(111). As the Ag thickness increases, monotonous decreases of the coercive forces have been observed. For Ag thinner than 2 ML, polar coercive force is always smaller than the longitudinal one. This shows that the easy axis of magnetization is relatively preferable in the out-of-plane direction. As the Ag thickness increases from 0 to 2 ML, a greater decrease of the longitudinal coercive force from 115 to 24 kA/m is observed and infers a reduced PMA of Co/Ir(111) by the Ag capping layer. Magnetic measurements by means of X-ray magnetic circular dichroism show a reduction in the PMA of Co films by a thin Ag overlayer [18] and this phenomenon has been interpreted recently for the Co/Au system in terms of magnetoelastic contributions [19]. The Kerr intensity versus the Ag thickness for x ML Ag/4 ML Co/Ir(111) exhibits an oscillating behavior in both the polar and longitudinal configurations as shown in Fig. 1c. For a half-filled Ag atomic layer, a larger Kerr intensity is observed. The period of the oscillation is around 1 ML. The influences of the magnetic properties of ultrathin films by nonmagnetic overlayers have been reported before [21–24]. For the oscillation of the magnetic parameter with a period of several MLs, it is more likely related to the quantum well states [21,22]. For the oscillation of the magnetic parameter with a period of a ML, it has been explained by changes of the electronic structures for filled and unfilled MLs related to the morphological change of the nonmagnetic layer [23,24]. We attribute the oscillation of the Kerr intensity versus the Ag thickness in Fig. 1c to the morphological change related electronic structure differences of the Ag layer. After capping 2 ML Ag on the top of Co/Ir(111) films with the Co layer thinner than 4 ML, a series of annealing experiments have been performed. The specimen is annealed at the indicated temperature for 3 min and then is cooled down to 350 K for carrying out AES and SMOKE measurements. As an example for 2 ML Ag/4 ML Co/Ir(111), Fig. 2a shows the intensities of Ag M5N45N45, Co L3M45M45, and Ir
(c)
Fig. 1. (a) Hysteresis loops, (b) coercive forces, and (c) Kerr intensities versus the Ag thickness for x ML Ag/4 ML Co/Ir(111) in both the polar and longitudinal configurations.
N5N7O4 Auger signals versus the annealing temperature. At 300 K, no Ir N5N7O4 Auger signal is detectable because of the limited inelastic mean free path (IMFP) of the Auger electrons [25,26]. As the annealing temperature increases, an attenuation of the Ag M5N45N45 signal and an enhancement of the Co L3M45M45 signal occur at 450 K. We will show that the changes of the Ag M5N45N45 and Co L3M45M45 signals are due to the interdiffusion of Ag and Co atoms at the interface in the next paragraph. Between 450 and 700 K, both the intensities of Ag M5N45N45 and Co L3M45M45 Auger signals remain nearly unchanged. At higher temperatures, the intensity of the Co L3M45M45 signal decreases while the Ir N5N7O4 signal appears. This shows the formation of Co–Ir alloy and the segregation of Ir atoms to the surface layers. The intensity of Ag M5N45N45 Auger signal decreases at temperatures above 700 K. No Ag M5N45N45 Auger signal is detectable above 800 K. Possible reasons are the diffusion of Ag into the Ir(111) substrate or the desorption of Ag atoms from the surface. We have performed depth profiling measurements for the specimen of 2 ML Ag/4 ML Co/ Ir(111) annealed at 850 K. As the sputtering time increases, no Ag Auger signal is detectable. It has been reported that thermal desorption of Ag overlayer from an Ir(111) surface occurs at temperatures above 800 K [20,27]. From these discussions, one can conclude with the
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(a) 2 ML Ag/4 ML Co/Ir(111)
(b) annealed at 600 K
Fig. 2. (a) Intensities of Ag M5N45N45 (open circles), Co L3M45M45 (open squares), and Ir N5N7O4 (open triangles) Auger signals versus the annealing temperature for 2 ML Ag/ 4 ML Co/Ir(111). (b) Intensities of Ag M5N45N45 (open circles), Co L3M45M45 (open squares), and Ir N5N7O4 (open triangles) Auger signals versus the sputtering time for 2 ML Ag/4 ML Co/Ir(111) annealed at 600 K. The inset shows the logarithms of the intensities of Ag M5N45N45 signals versus the sputtering time for as-deposited (300 K) and annealed (600 K) films.
desorption of all the Ag atoms from the surface at temperatures above 800 K for the Ag/Co/Ir(111) system. At the intermediate temperatures, depth profiling experiments have been performed to investigate the compositions at the interface. The specimen is prepared by annealing 2 ML Ag/4 ML Co/Ir(111) at 600 K for 3 min and then cooled to room temperature. Fig. 2b shows the intensities of Ag M5N45N45, Co L3M45M45, and Ir N5N7O4 Auger signals versus the sputtering time. At the first half minute of sputtering, the intensity of Ag M5N45N45 signal decreases dramatically while slight increase of the intensity of Co L3M45M45 Auger signal is detected. After 1 min of the sputtering treatment, the intensity of Ir N5N7O4 Auger signal increases and the intensity of Co L3M45M45 Auger signal decreases until reaching a zero value at 12 min. These results are reasonable for the layered distribution of a Ag/Co/Ir(111) system. In order to obtain the information of compositions at the Ag/Co interface, depth profiling measurements of the as-deposited (300 K) and annealed (600 K) films have been performed and the inset in Fig. 2b shows the logarithms of the Ag M5N45N45 signals versus the sputtering time. We adopt a model reported before to determine the sputtering rate of an adsorbate in a depth profiling experiment [19,28]. After sputtering for t minutes, the intensity of the Ag Auger signal IAg can be expressed as t IAg ¼ IAg ð1−RÞ 0
ð1Þ
where (IAg)0 is the intensity of the Ag Auger signal before the sputtering experiments; R is the sputtering rate that is the removed percentage of Ag adatoms per minute. From Eq. (1), the sputtering rate R can be calculated by S
R ¼ 1−e
ð2Þ
477
where S is the slope of the linear fitting curve for ln(IAg) versus sputtering time. In the inset of Fig. 2b for the as-deposited film with 2 ML Ag on the top, ln(IAg) decreases linearly as the sputtering time increases up to 2 min followed by a linear decrease with a smaller slope. The slope change is due to the difference of the uncovered and covered Ag layer of the films. The sputtering rates of Ag atoms are evaluated to be (67.2 + 1.3)%/min and (50.5 + 1.5)%/min before and after 2 min, respectively. For the annealed film, the behavior of the ln(IAg) versus sputtering time is similar. The sputtering rates are determined to be (69.7 + 1.7)%/min and (40.3 + 1.2)%/min before and after 2 min, respectively. Comparing the sputtering rates after 2 min for the annealed and as-deposited films, a significantly reduced value of (40.3 + 1.2)%/min is obtained for the annealed film. The different sputtering rate shows the change of the chemical environment of Ag atoms at the Ag/Co interface by the annealing treatment. From Fig. 2a, an attenuation of Ag M5N45N45 signal and an enhancement of Co L3M45M45 signal are observed between 450 and 700 K. One can conclude that after the annealing treatment at 600 K, most of the Ag adatoms remain on the top two layers. Some Ag atoms intermix with the underlying Co layer to form an interfacial alloy. The hysteresis loops at different annealing temperatures for 2 ML Ag/4 ML Co/Ir(111) are shown in Fig. 3a in both the polar and longitudinal configurations. After the annealing treatments at 700 K, the hysteresis loops became broader. After annealed at 800 K, no hysteresis is detectable. The coercive force versus annealing temperature for 2 ML Ag/4 ML Co/Ir(111) is shown in Fig. 3b in both the polar and longitudinal configurations. As the annealing temperature increases below 700 K, an enhancement of the coercive force occurs in both the polar and longitudinal configurations. Referring to the AES and depthprofiling studies in Fig. 2, the intermixing of Ag atoms with the underlying Co layer to form an interfacial alloy occurs above 400 K. The alloy formation may modify the stress anisotropy and impedes the domain wall motion during magnetization reversal process. This results in the enhancement of the coercive force for the annealed Ag/Co/Ir(111) films. In Fig. 3b, for Co/Ir(111) the coercive force increases after 500 K, while for Ag/Co/Ir(111) the coercive force increases after 700 K. Ag seems to delay the Co–Ir alloy formation from 500 to 700 K. For Co/ Ir(111) at high temperatures, the enhancement of the polar coercive force is verified to be associated with the formation of Co–Ir alloys [13]. This is also one of the reasons for the change of the coercive forces for annealed Ag/Co/Ir(111) films while the limited IMFP is disadvantage for detecting the compositional changes by AES at the buried Co/Ir interface. As compared to a Co/Ir(111) system, a depression of the longitudinal coercive force for Ag/Co/Ir(111) is observed at high temperatures. Via deposition of Ag overlayer, a significant decrease of the longitudinal coercive force is observed as shown in Fig. 1b. As compared to Co/ Ir(111), the depression of the longitudinal coercive force in Fig. 3b for Ag/Co/Ir(111) could be due to the introduction of an Ag overlayer. In addition a drastic increase of the longitudinal coercive force for Ag/Co/ Ir(111) occurs at 700 K where the desorption of Ag atoms and the formation of Co–Ir alloy start. The increase of the longitudinal coercive force at 700 K should be related to both the desorption of Ag atoms and the formation of Co–Ir alloy. After the annealing treatment at temperatures higher than 750 K, no hysteresis is observed. The annealing temperatures for the disappearance of the hysteresis for Ag/Co/Ir(111) is lower as compared to that for Co/Ir(111). Since the intermixing of Ag atoms with the underlying Co layer to form an interfacial alloy occurs above 400 K, this may play a significant role on the thermal stability of the Ag/Co/Ir(111) system. Fig. 3c shows the Kerr intensity versus the annealing temperature for 2 ML Ag/4 ML Co/Ir(111) in both the polar and longitudinal configurations. As the annealing temperature increases, Kerr intensities increase with an oscillatory behavior. This is associated with the intermixing of Ag with Co. For 2 ML Ag/y ML Co/Ir(111) with the Co thickness y between 1 and 4 ML, the compositional and magnetic properties are similar in many aspects as discussed in Figs. 2 and 3. As an example for the compositions of Ag in the surface layers, Fig. 4a shows the intensities of Ag M5N45N45
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(a)
(a)
(b) (b)
(c)
Fig. 3. (a) Hysteresis loops in both the polar and longitudinal configurations for 2 ML Ag/ 4 ML Co/Ir(111) at different temperatures. (b) Coercive force versus the annealing temperature for 2 ML Ag/4 ML Co/Ir(111) in both the polar and longitudinal configurations. For a comparative purpose, the longitudinal coercive force for 4 ML Co/Ir(111) at elevated temperatures is also shown here (triangles). (c) Kerr intensity versus the annealing temperature for 2 ML Ag/4 ML Co/Ir(111) in both the polar and longitudinal configurations.
Auger signals versus the annealing temperature for 2 ML Ag/y ML Co/ Ir(111). For each Co thickness, an attenuation of the Ag M5N45N45 Auger signal is observed around 450 K owing to the intermixing of Ag atoms with the underlying Co layer to form an interfacial alloy as discussed in Fig. 2. At temperatures above 700 K, a decrease of the intensity of Ag M5N45N45 Auger signal until zero value is observed due to the desorption of Ag adatoms from the surface. The temperature for the complete desorption of Ag atoms decreases from 950 to 800 K as the Co thickness increases. Ultrathin films with different thicknesses may show different thermal stability as reported before [29,30]. The observation of different temperatures for the complete desorption of Ag for 2 ML Ag/y ML Co/Ir(111) is reasonable because of the different compositions of underlying CoxIr1 − x layer. One interesting phenomenon as shown in Fig. 4b is that polar Kerr intensities vanish at the same annealing temperature at 750 K. In order to find the relation between compositions and the vanishing of the polar Kerr intensities, we use a compositional model reported before [13,29]. Since Ag exhibits the smallest surface free energy among Ag, Co, and Ir [14], it is reasonable that most Ag atoms remain on the top
Fig. 4. (a) Intensities of Ag M5N45N45 Auger signals versus the annealing temperature for 2 ML Ag/y ML Co/Ir(111). (b) Kerr intensities versus the annealing temperature for 2 ML Ag/y ML Co/Ir(111) in the polar configuration.
surface. The attenuation factors of the Co and Ir signals by the Ag capping layers are exp(−dAg/λCo) and exp(−dAg/λIr), respectively, and have to be considered in the calculation of the Auger intensity ratios (ICo/IIr); where dAg is the thickness of Ag overlayer and λ is the IMFP. Assume that a homogenous alloy of CoxIr1 − x forms. After substituting the experimental data of Auger signals, the Co composition x in CoxIr1 − x, where ferromagnetism vanishes, is estimated to be around 0.81. For Co/Ir(111), polar Kerr intensities vanish at annealing temperatures higher than 750 K while the Co composition x for ferromagnetism vanishing has been determined to be a smaller value around 0.76 [13]. The disappearance of ferromagnetism of Co/Ir(111) films is mainly due to the reduced atomic percent of cobalt in Co–Ir alloy and the lowering of the Curie temperature by a reduction of the thickness of magnetic layers [13]. These reasons should be also applicable for Ag/ Co/Ir(111) while a larger Co composition x around 0.81 for ferromagnetism vanishing is observed. For Ag/Co/Ir(111), the intermixing of Ag and Co at the Ag/Co interface occurs between 450 and 700 K. It has been reported that segregation of Ag and grain boundary diffusion may exist at Ag/Co interface [31,32]. The formation of small grains in magnetic layer results in a lower thermal stability due to the more possibility of a contact between magnetic atoms and the interlayer atoms [32,33]. This causes that the annealing temperature where ferromagnetism vanishes for Ag/Co/Ir(111) is lower than that for Co/Ir(111). In addition to the reduced atomic percent of cobalt in Co–Ir alloy and the lowering of the Curie temperature, one can conclude that the intermixing of Ag and Co at the Ag/Co interface plays an important role on the disappearance of ferromagnetism of the films. From systematical investigations of 2 ML Ag/y ML Co/Ir(111) films with the Co thickness y between 1 and 4 ML and the annealing temperatures below 900 K, a magnetic phase diagram is summarized in Fig. 5. From the discussions of surface compositions and magnetic properties, the phase diagram can be divided to three regions. In region I with the annealing temperatures below 400 K, no significant interdiffusion at both the Ag/Co and Co/Ir interfaces occurs. Ag/Co/Ir(111) films are
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the thickness of magnetic layers, and intermixing of Ag and Co at the Ag/Co interface. Acknowledgments The authors thank Professor Wen-Chin Lin for his valuable comments and suggestions on the preparation of the manuscript. Authors acknowledge the support from the National Science Council of ROC through Grant No. NSC 100-2112-M-003-004-MY3. References
Fig. 5. Magnetic phases for Ag/Co/Ir(111) with the Co layer thinner 4 ML at annealing temperatures below 900 K. In region I represented by black circles, Ag/Co/Ir(111) films are ferromagnetic. In region II represented by gray circles, an enhancement of the polar coercive force is observed. In region III represented by open circles, a nonferromagnetic state is observed.
[1] [2] [3] [4] [5] [6] [7] [8]
ferromagnetic. In region II, intermixing of Ag and Co at the Ag/Co interface and the formation of Co–Ir alloy occur. Due to the compositional change in the surface layers, an enhancement of the polar coercive force is observed. In region III for thinner Co layer or after high temperature annealing, a nonferromagnetic state is observed. The phase transition from phases II to III is driven by the annealing treatment by way of the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of the thickness of magnetic layers, and the intermixing of Ag and Co at the Ag/Co interface. 4. Conclusion
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
After deposition of Ag overlayer on the top of Co/Ir(111), the Kerr intensity versus the Ag thickness exhibits an oscillating behavior with a period around 1 ML which should be due to the morphological change related electronic structure differences of the Ag layer. After annealing treatment for Ag/Co/Ir(111) films, depth profiling measurements show that some Ag atoms intermix with the underlying Co layer to form an interfacial alloy. From systematical investigations of 2 ML Ag/ y ML Co/Ir(111) films with the Co thinner than 4 ML at annealing temperatures below 900 K, a magnetic phase diagram has been established. Ferromagnetism is observed below 400 K (region I). As the annealing temperature increases, enhancements of the coercive force occur in both the polar and longitudinal configurations due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy (region II). For thinner Co layers (≤1 ML) or at high temperatures (≥700 K), Ag/Co/Ir(111) is nonferromagnetic (region III). The disappearance of ferromagnetism is mainly attributed to the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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Ag-related alloy formation and magnetic phases for Ag/Co/Ir(111) ultrathin films Jyh-Shen Tsay ⁎, Du-Cheng Tsai, Cheng-Hsun-Tony Chang, Wei-Hsiang Chen Department of Physics, National Taiwan Normal University, Taipei 116, Taiwan
a r t i c l e
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Article history: Received 26 October 2012 Received in revised form 12 September 2013 Accepted 17 September 2013 Available online 24 September 2013 Keywords: Magnetic phases Alloy Surface magneto-optic Kerr effect Auger electron spectroscopy Silver Cobalt Iridium
a b s t r a c t The Kerr intensity versus the Ag thickness for Ag grown on the top of Co/Ir(111) exhibits an oscillating behavior with a period around one monolayer which should be due to the morphological change related electronic structure differences of the Ag layer. From systematical investigations of Ag/Co/Ir(111) films with the Co layer thinner than 4 monolayers at temperatures below 900 K, a magnetic phase diagram has been established. As the annealing temperature increases for Ag/Co/Ir(111) films, enhancements of the coercive force occur in both the polar and longitudinal configurations due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy. The disappearance of ferromagnetism is mainly attributed to the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of the thickness of magnetic layers, and the intermixing of Ag and Co at the Ag/Co interface. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Investigations of magnetic thin films on metallic substrates have drawn much attention in the past decade because of the involved interesting science and of great technological importance for the development of magnetic recording media, microelectronic devices, and spintronics [1–4]. Effects of interfaces on the physical properties of nanometer-scaled materials become key issues for the fabrication of layered structures that are widely used in industry [4–7]. For ultrathin films, the effective magnetic anisotropy can be affected by the surface and interfacial conditions because of the large surface-to-volume ratio. Their origins may be related to the changes of the electronic structure owing to the interaction between the atoms of different species at the interface, strain for lattice mismatched systems, and morphological changes induced by the introduction of an overlayer [8]. As an example for one monolayer (ML) of Co adatoms on Pt(111), perpendicular magnetic anisotropy (PMA) has been observed due to the positive magnetocrystalline anisotropy [9]. PMA shows advantages in the application for high-density magneto-optical recording [10]. After annealing treatments at 710 K for Co/Pt(111) films, a great enhancement of the polar magneto-optic Kerr signal is reported due to the hybridization of atomic levels of Co–Pt alloy [9]. Iridium shows high chemical stability and is therefore a preferred electrode material that is widely used in electrochemical environment [11,12]. Co and Co-based alloys are widely used in magnetic recording devices. On an Ir(111) surface, the easy axis ⁎ Corresponding author. Fax: +886 2 29326408. E-mail address: [email protected] (J.-S. Tsay). 0040-6090/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2013.09.051
of magnetization of Co films could be stabilized in the direction of the surface normal by thermal annealing treatments [13]. As compared to the elements with high melting temperatures, Ag shows a smaller surface free energy [14]. In the binary phase diagram, Ag is bulkimmiscible with both Co and Ir [15]. As a capping layer, Ag-induced spin orientation transition (SRT) has been reported for both Co/ Pt(111) and Fe/Pt(111) films [16,17]. Magnetic measurements by means of X-ray magnetic circular dichroism show a reduction in the PMA of Co films by a thin Ag overlayer [18] and what has been interpreted recently for the Co/Au system in terms of magneto-elastic contributions [19]. In this report, Ag-related alloy formation of Co/ Ir(111) and effects of annealing on Ag/Co/Ir(111) have been investigated. After deposition of Ag overlayer on the top of Co/Ir(111), the longitudinal coercive force is always larger than the polar one. From systematical investigations of Ag/Co/Ir(111) films at elevated temperatures, a magnetic phase diagram has been established. Enhancements of the coercive forces occur for annealed Ag/Co/Ir(111) films due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy. 2. Experimental All experiments were carried out in an ultrahigh vacuum (UHV) chamber with a base pressure of 2.7 × 10−8 Pa [13,20]. An iridium crystal was oriented within 0.5° of the [111] direction checked by X-ray Laue back reflection. The Ir(111) surface was cleaned by cycles of Ar+ ion bombardment and annealing treatments until a well-ordered p(1 × 1) pattern of low-energy electron diffraction (LEED) with bright, sharp
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spots and a low background intensity was observed. A K-type thermocouple spot-welded to the edge of the Ir(111) crystal was used for temperature measurements. The chemical purity of the surface was checked by Auger electron spectroscopy (AES) while Auger electrons were collected by an OMICRON hemispherical analyzer with an angular resolution of 2°. High purity cobalt (99.997%) and silver (99.999%) were deposited on the Ir(111) surface at ambient temperature. The thickness of the overlayer was determined by Auger signal ratio, Auger signals versus time (AS–t) plot and oscillations of the specular-beam intensity of LEED, and double checked by a thickness monitor [9,13,20]. The Ir(111) surface was cleaned after each experiment employing with cobalt and silver overlayers. A He–Ne laser with a wavelength of 632.8 nm was used as the light source for surface magneto-optic Kerr effect (SMOKE) measurements. A linear polarizer of Glan–Thompson polarizing prism polarized incident light with an extinction ratio of 1 × 10−6. The magnetic field was applied in-plane and perpendicular to the specimen surface in the longitudinal and polar configurations, respectively. The various components are described in detail elsewhere [13,20].
(a)
(b)
3. Results and discussion On the top of Co/Ir(111) films, Ag overlayers have been deposited to investigate the corresponding magnetic properties. Fig. 1a shows the hysteresis loops for 4 ML Co/Ir(111), 1.0 and 2.0 ML Ag/4 ML Co/ Ir(111). For 4 ML Co/Ir(111), hysteresis occurs in both the polar and longitudinal configurations while longitudinal coercive force (HC) is larger than 5 times of the polar one. From the investigations of Co/ Ir(111) films, the polar coercive force is smaller than the longitudinal one for the Co layer thinner than 4 ML [13]. No clear SRT is observed for Co/Ir(111). After the deposition of Ag overlayer on 4 ML Co/ Ir(111), the hysteresis loops became slimmer in both the polar and longitudinal configurations while the longitudinal coercive force is larger than the polar one for each Ag thickness. Ag thickness dependencies of the coercive forces in both the polar and longitudinal configurations are shown in Fig. 1b for x ML Ag/4 ML Co/Ir(111). As the Ag thickness increases, monotonous decreases of the coercive forces have been observed. For Ag thinner than 2 ML, polar coercive force is always smaller than the longitudinal one. This shows that the easy axis of magnetization is relatively preferable in the out-of-plane direction. As the Ag thickness increases from 0 to 2 ML, a greater decrease of the longitudinal coercive force from 115 to 24 kA/m is observed and infers a reduced PMA of Co/Ir(111) by the Ag capping layer. Magnetic measurements by means of X-ray magnetic circular dichroism show a reduction in the PMA of Co films by a thin Ag overlayer [18] and this phenomenon has been interpreted recently for the Co/Au system in terms of magnetoelastic contributions [19]. The Kerr intensity versus the Ag thickness for x ML Ag/4 ML Co/Ir(111) exhibits an oscillating behavior in both the polar and longitudinal configurations as shown in Fig. 1c. For a half-filled Ag atomic layer, a larger Kerr intensity is observed. The period of the oscillation is around 1 ML. The influences of the magnetic properties of ultrathin films by nonmagnetic overlayers have been reported before [21–24]. For the oscillation of the magnetic parameter with a period of several MLs, it is more likely related to the quantum well states [21,22]. For the oscillation of the magnetic parameter with a period of a ML, it has been explained by changes of the electronic structures for filled and unfilled MLs related to the morphological change of the nonmagnetic layer [23,24]. We attribute the oscillation of the Kerr intensity versus the Ag thickness in Fig. 1c to the morphological change related electronic structure differences of the Ag layer. After capping 2 ML Ag on the top of Co/Ir(111) films with the Co layer thinner than 4 ML, a series of annealing experiments have been performed. The specimen is annealed at the indicated temperature for 3 min and then is cooled down to 350 K for carrying out AES and SMOKE measurements. As an example for 2 ML Ag/4 ML Co/Ir(111), Fig. 2a shows the intensities of Ag M5N45N45, Co L3M45M45, and Ir
(c)
Fig. 1. (a) Hysteresis loops, (b) coercive forces, and (c) Kerr intensities versus the Ag thickness for x ML Ag/4 ML Co/Ir(111) in both the polar and longitudinal configurations.
N5N7O4 Auger signals versus the annealing temperature. At 300 K, no Ir N5N7O4 Auger signal is detectable because of the limited inelastic mean free path (IMFP) of the Auger electrons [25,26]. As the annealing temperature increases, an attenuation of the Ag M5N45N45 signal and an enhancement of the Co L3M45M45 signal occur at 450 K. We will show that the changes of the Ag M5N45N45 and Co L3M45M45 signals are due to the interdiffusion of Ag and Co atoms at the interface in the next paragraph. Between 450 and 700 K, both the intensities of Ag M5N45N45 and Co L3M45M45 Auger signals remain nearly unchanged. At higher temperatures, the intensity of the Co L3M45M45 signal decreases while the Ir N5N7O4 signal appears. This shows the formation of Co–Ir alloy and the segregation of Ir atoms to the surface layers. The intensity of Ag M5N45N45 Auger signal decreases at temperatures above 700 K. No Ag M5N45N45 Auger signal is detectable above 800 K. Possible reasons are the diffusion of Ag into the Ir(111) substrate or the desorption of Ag atoms from the surface. We have performed depth profiling measurements for the specimen of 2 ML Ag/4 ML Co/ Ir(111) annealed at 850 K. As the sputtering time increases, no Ag Auger signal is detectable. It has been reported that thermal desorption of Ag overlayer from an Ir(111) surface occurs at temperatures above 800 K [20,27]. From these discussions, one can conclude with the
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(a) 2 ML Ag/4 ML Co/Ir(111)
(b) annealed at 600 K
Fig. 2. (a) Intensities of Ag M5N45N45 (open circles), Co L3M45M45 (open squares), and Ir N5N7O4 (open triangles) Auger signals versus the annealing temperature for 2 ML Ag/ 4 ML Co/Ir(111). (b) Intensities of Ag M5N45N45 (open circles), Co L3M45M45 (open squares), and Ir N5N7O4 (open triangles) Auger signals versus the sputtering time for 2 ML Ag/4 ML Co/Ir(111) annealed at 600 K. The inset shows the logarithms of the intensities of Ag M5N45N45 signals versus the sputtering time for as-deposited (300 K) and annealed (600 K) films.
desorption of all the Ag atoms from the surface at temperatures above 800 K for the Ag/Co/Ir(111) system. At the intermediate temperatures, depth profiling experiments have been performed to investigate the compositions at the interface. The specimen is prepared by annealing 2 ML Ag/4 ML Co/Ir(111) at 600 K for 3 min and then cooled to room temperature. Fig. 2b shows the intensities of Ag M5N45N45, Co L3M45M45, and Ir N5N7O4 Auger signals versus the sputtering time. At the first half minute of sputtering, the intensity of Ag M5N45N45 signal decreases dramatically while slight increase of the intensity of Co L3M45M45 Auger signal is detected. After 1 min of the sputtering treatment, the intensity of Ir N5N7O4 Auger signal increases and the intensity of Co L3M45M45 Auger signal decreases until reaching a zero value at 12 min. These results are reasonable for the layered distribution of a Ag/Co/Ir(111) system. In order to obtain the information of compositions at the Ag/Co interface, depth profiling measurements of the as-deposited (300 K) and annealed (600 K) films have been performed and the inset in Fig. 2b shows the logarithms of the Ag M5N45N45 signals versus the sputtering time. We adopt a model reported before to determine the sputtering rate of an adsorbate in a depth profiling experiment [19,28]. After sputtering for t minutes, the intensity of the Ag Auger signal IAg can be expressed as t IAg ¼ IAg ð1−RÞ 0
ð1Þ
where (IAg)0 is the intensity of the Ag Auger signal before the sputtering experiments; R is the sputtering rate that is the removed percentage of Ag adatoms per minute. From Eq. (1), the sputtering rate R can be calculated by S
R ¼ 1−e
ð2Þ
477
where S is the slope of the linear fitting curve for ln(IAg) versus sputtering time. In the inset of Fig. 2b for the as-deposited film with 2 ML Ag on the top, ln(IAg) decreases linearly as the sputtering time increases up to 2 min followed by a linear decrease with a smaller slope. The slope change is due to the difference of the uncovered and covered Ag layer of the films. The sputtering rates of Ag atoms are evaluated to be (67.2 + 1.3)%/min and (50.5 + 1.5)%/min before and after 2 min, respectively. For the annealed film, the behavior of the ln(IAg) versus sputtering time is similar. The sputtering rates are determined to be (69.7 + 1.7)%/min and (40.3 + 1.2)%/min before and after 2 min, respectively. Comparing the sputtering rates after 2 min for the annealed and as-deposited films, a significantly reduced value of (40.3 + 1.2)%/min is obtained for the annealed film. The different sputtering rate shows the change of the chemical environment of Ag atoms at the Ag/Co interface by the annealing treatment. From Fig. 2a, an attenuation of Ag M5N45N45 signal and an enhancement of Co L3M45M45 signal are observed between 450 and 700 K. One can conclude that after the annealing treatment at 600 K, most of the Ag adatoms remain on the top two layers. Some Ag atoms intermix with the underlying Co layer to form an interfacial alloy. The hysteresis loops at different annealing temperatures for 2 ML Ag/4 ML Co/Ir(111) are shown in Fig. 3a in both the polar and longitudinal configurations. After the annealing treatments at 700 K, the hysteresis loops became broader. After annealed at 800 K, no hysteresis is detectable. The coercive force versus annealing temperature for 2 ML Ag/4 ML Co/Ir(111) is shown in Fig. 3b in both the polar and longitudinal configurations. As the annealing temperature increases below 700 K, an enhancement of the coercive force occurs in both the polar and longitudinal configurations. Referring to the AES and depthprofiling studies in Fig. 2, the intermixing of Ag atoms with the underlying Co layer to form an interfacial alloy occurs above 400 K. The alloy formation may modify the stress anisotropy and impedes the domain wall motion during magnetization reversal process. This results in the enhancement of the coercive force for the annealed Ag/Co/Ir(111) films. In Fig. 3b, for Co/Ir(111) the coercive force increases after 500 K, while for Ag/Co/Ir(111) the coercive force increases after 700 K. Ag seems to delay the Co–Ir alloy formation from 500 to 700 K. For Co/ Ir(111) at high temperatures, the enhancement of the polar coercive force is verified to be associated with the formation of Co–Ir alloys [13]. This is also one of the reasons for the change of the coercive forces for annealed Ag/Co/Ir(111) films while the limited IMFP is disadvantage for detecting the compositional changes by AES at the buried Co/Ir interface. As compared to a Co/Ir(111) system, a depression of the longitudinal coercive force for Ag/Co/Ir(111) is observed at high temperatures. Via deposition of Ag overlayer, a significant decrease of the longitudinal coercive force is observed as shown in Fig. 1b. As compared to Co/ Ir(111), the depression of the longitudinal coercive force in Fig. 3b for Ag/Co/Ir(111) could be due to the introduction of an Ag overlayer. In addition a drastic increase of the longitudinal coercive force for Ag/Co/ Ir(111) occurs at 700 K where the desorption of Ag atoms and the formation of Co–Ir alloy start. The increase of the longitudinal coercive force at 700 K should be related to both the desorption of Ag atoms and the formation of Co–Ir alloy. After the annealing treatment at temperatures higher than 750 K, no hysteresis is observed. The annealing temperatures for the disappearance of the hysteresis for Ag/Co/Ir(111) is lower as compared to that for Co/Ir(111). Since the intermixing of Ag atoms with the underlying Co layer to form an interfacial alloy occurs above 400 K, this may play a significant role on the thermal stability of the Ag/Co/Ir(111) system. Fig. 3c shows the Kerr intensity versus the annealing temperature for 2 ML Ag/4 ML Co/Ir(111) in both the polar and longitudinal configurations. As the annealing temperature increases, Kerr intensities increase with an oscillatory behavior. This is associated with the intermixing of Ag with Co. For 2 ML Ag/y ML Co/Ir(111) with the Co thickness y between 1 and 4 ML, the compositional and magnetic properties are similar in many aspects as discussed in Figs. 2 and 3. As an example for the compositions of Ag in the surface layers, Fig. 4a shows the intensities of Ag M5N45N45
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(a)
(a)
(b) (b)
(c)
Fig. 3. (a) Hysteresis loops in both the polar and longitudinal configurations for 2 ML Ag/ 4 ML Co/Ir(111) at different temperatures. (b) Coercive force versus the annealing temperature for 2 ML Ag/4 ML Co/Ir(111) in both the polar and longitudinal configurations. For a comparative purpose, the longitudinal coercive force for 4 ML Co/Ir(111) at elevated temperatures is also shown here (triangles). (c) Kerr intensity versus the annealing temperature for 2 ML Ag/4 ML Co/Ir(111) in both the polar and longitudinal configurations.
Auger signals versus the annealing temperature for 2 ML Ag/y ML Co/ Ir(111). For each Co thickness, an attenuation of the Ag M5N45N45 Auger signal is observed around 450 K owing to the intermixing of Ag atoms with the underlying Co layer to form an interfacial alloy as discussed in Fig. 2. At temperatures above 700 K, a decrease of the intensity of Ag M5N45N45 Auger signal until zero value is observed due to the desorption of Ag adatoms from the surface. The temperature for the complete desorption of Ag atoms decreases from 950 to 800 K as the Co thickness increases. Ultrathin films with different thicknesses may show different thermal stability as reported before [29,30]. The observation of different temperatures for the complete desorption of Ag for 2 ML Ag/y ML Co/Ir(111) is reasonable because of the different compositions of underlying CoxIr1 − x layer. One interesting phenomenon as shown in Fig. 4b is that polar Kerr intensities vanish at the same annealing temperature at 750 K. In order to find the relation between compositions and the vanishing of the polar Kerr intensities, we use a compositional model reported before [13,29]. Since Ag exhibits the smallest surface free energy among Ag, Co, and Ir [14], it is reasonable that most Ag atoms remain on the top
Fig. 4. (a) Intensities of Ag M5N45N45 Auger signals versus the annealing temperature for 2 ML Ag/y ML Co/Ir(111). (b) Kerr intensities versus the annealing temperature for 2 ML Ag/y ML Co/Ir(111) in the polar configuration.
surface. The attenuation factors of the Co and Ir signals by the Ag capping layers are exp(−dAg/λCo) and exp(−dAg/λIr), respectively, and have to be considered in the calculation of the Auger intensity ratios (ICo/IIr); where dAg is the thickness of Ag overlayer and λ is the IMFP. Assume that a homogenous alloy of CoxIr1 − x forms. After substituting the experimental data of Auger signals, the Co composition x in CoxIr1 − x, where ferromagnetism vanishes, is estimated to be around 0.81. For Co/Ir(111), polar Kerr intensities vanish at annealing temperatures higher than 750 K while the Co composition x for ferromagnetism vanishing has been determined to be a smaller value around 0.76 [13]. The disappearance of ferromagnetism of Co/Ir(111) films is mainly due to the reduced atomic percent of cobalt in Co–Ir alloy and the lowering of the Curie temperature by a reduction of the thickness of magnetic layers [13]. These reasons should be also applicable for Ag/ Co/Ir(111) while a larger Co composition x around 0.81 for ferromagnetism vanishing is observed. For Ag/Co/Ir(111), the intermixing of Ag and Co at the Ag/Co interface occurs between 450 and 700 K. It has been reported that segregation of Ag and grain boundary diffusion may exist at Ag/Co interface [31,32]. The formation of small grains in magnetic layer results in a lower thermal stability due to the more possibility of a contact between magnetic atoms and the interlayer atoms [32,33]. This causes that the annealing temperature where ferromagnetism vanishes for Ag/Co/Ir(111) is lower than that for Co/Ir(111). In addition to the reduced atomic percent of cobalt in Co–Ir alloy and the lowering of the Curie temperature, one can conclude that the intermixing of Ag and Co at the Ag/Co interface plays an important role on the disappearance of ferromagnetism of the films. From systematical investigations of 2 ML Ag/y ML Co/Ir(111) films with the Co thickness y between 1 and 4 ML and the annealing temperatures below 900 K, a magnetic phase diagram is summarized in Fig. 5. From the discussions of surface compositions and magnetic properties, the phase diagram can be divided to three regions. In region I with the annealing temperatures below 400 K, no significant interdiffusion at both the Ag/Co and Co/Ir interfaces occurs. Ag/Co/Ir(111) films are
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the thickness of magnetic layers, and intermixing of Ag and Co at the Ag/Co interface. Acknowledgments The authors thank Professor Wen-Chin Lin for his valuable comments and suggestions on the preparation of the manuscript. Authors acknowledge the support from the National Science Council of ROC through Grant No. NSC 100-2112-M-003-004-MY3. References
Fig. 5. Magnetic phases for Ag/Co/Ir(111) with the Co layer thinner 4 ML at annealing temperatures below 900 K. In region I represented by black circles, Ag/Co/Ir(111) films are ferromagnetic. In region II represented by gray circles, an enhancement of the polar coercive force is observed. In region III represented by open circles, a nonferromagnetic state is observed.
[1] [2] [3] [4] [5] [6] [7] [8]
ferromagnetic. In region II, intermixing of Ag and Co at the Ag/Co interface and the formation of Co–Ir alloy occur. Due to the compositional change in the surface layers, an enhancement of the polar coercive force is observed. In region III for thinner Co layer or after high temperature annealing, a nonferromagnetic state is observed. The phase transition from phases II to III is driven by the annealing treatment by way of the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of the thickness of magnetic layers, and the intermixing of Ag and Co at the Ag/Co interface. 4. Conclusion
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
After deposition of Ag overlayer on the top of Co/Ir(111), the Kerr intensity versus the Ag thickness exhibits an oscillating behavior with a period around 1 ML which should be due to the morphological change related electronic structure differences of the Ag layer. After annealing treatment for Ag/Co/Ir(111) films, depth profiling measurements show that some Ag atoms intermix with the underlying Co layer to form an interfacial alloy. From systematical investigations of 2 ML Ag/ y ML Co/Ir(111) films with the Co thinner than 4 ML at annealing temperatures below 900 K, a magnetic phase diagram has been established. Ferromagnetism is observed below 400 K (region I). As the annealing temperature increases, enhancements of the coercive force occur in both the polar and longitudinal configurations due to the intermixing of Ag and Co at the interface and the formation of Co–Ir alloy (region II). For thinner Co layers (≤1 ML) or at high temperatures (≥700 K), Ag/Co/Ir(111) is nonferromagnetic (region III). The disappearance of ferromagnetism is mainly attributed to the reduced atomic percent of cobalt in Co–Ir alloy, the lowered Curie temperature by a reduction of
[20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]
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