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Accepted Manuscript Magnetization reversal of antiferromagnetically coupled (Co/Ni) and (Co/Pt) multilayers A. Al Subhi, R. Sbiaa, M. Ranjbar, J. Akerman PII: DOI: Reference:
S0304-8853(18)34191-X https://doi.org/10.1016/j.jmmm.2019.02.022 MAGMA 64941
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
26 December 2018 23 January 2019 5 February 2019
Please cite this article as: A. Al Subhi, R. Sbiaa, M. Ranjbar, J. Akerman, Magnetization reversal of antiferromagnetically coupled (Co/Ni) and (Co/Pt) multilayers, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.02.022
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Magnetization reversal of antiferromagnetically coupled (Co/Ni) and (Co/Pt) multilayers
A. Al Subhi1, R. Sbiaa1*, M. Ranjbar2 and J. Akerman2 1
Department of Physics, Sultan Qaboos University, P.O. Box 36, PC 123, Muscat, Oman
2
Physics Department, University of Gothenburg, 412 96 Gothenburg, Sweden
PACS 75.60.Ch– magnetic properties and materials PACS 75.76. +j– Spin transport (magnetoelectronics) * Corresponding author: [email protected]
Abstract – Magnetization reversal and magnetic exchange coupling of (Co/Ni)×N/Ru/ (Co/Pt)×12 were investigated as a function of the temperature. The number of repeats N of the soft multilayer (Co/Ni) was varied from 4 to 8 bilayers while the number of repeats of the hard bilayers (Co/Pt) was fixed to 12. Two steps hysteresis loops were observed for coupled structure with only 4 repeats of (Co/Ni) in a wide range of temperature (25 to 300 K). From the shift of the minor hysteresis loop, the antiferromagnetic exchange coupling Hex was measured and then the interlayer exchange coupling Jex was calculated. A non-monotonous dependence of Jex with temperature was observed for N = 4 with a maximum Jex of 0.13 erg/cm2 at 150 K. The annealing process performed on the same structure confirms the unusual behavior of interlayer exchange coupling Jex. As the repetition number N increases to 8 bilayers the two steps hysteresis loops disappeared in the investigated temperature range, however a small kink appeared in the range of 125 and 225 K for the case of 6 bilayers. From the analysis of the coupled and uncoupled structures, it seems that the dipolar energy overcomes the antiferromagnetic coupling for thicker (Co/Ni) multilayer.
1. Introduction Materials with perpendicular magnetic anisotropy (PMA) have been the subject of intensive studies as they could be the basis of magnetic recording media [17] and spintronics devices such as magnetic random access memory (MRAM) [821]. They have better thermal stability compared to in-plane anisotropy type materials [22]. Among several materials which exhibit high PMA, multilayers based on alteration of two thin ferromagnetic layers such as Co, Fe, Ni, CoFe, CoFeB or a thin ferromagnetic layer and a noble metal like Pt, Pd and Au have been widely investigated. In fact, it is relatively easy to achieve a perpendicular orientation of the multilayer magnetization and tailor the materials properties by adjusting the thickness and the number of repeats of each sublayer. The PMA of the magnetic multilayers is mainly originating from the interface anisotropy contribution [23]. Thus, a perfect control of the thickness of each layer is important. For their application to magnetic recording or MRAM, although PMA materials are beneficial for the stability of the data/bit, they still represent a challenge for the adjacent or neighboring bit. This is due to the strong stray field that could reverse the magnetization direction of closest bits. To overcome this issue, antiferromagnetically coupled (AFC) structures have been proposed [24,25]. The AFC structure consists of a lamination of two ferromagnetic layers separated by a non-magnetic spacer which is Ru or Rh. The AF coupling can be achieved by manipulating the thickness of each layer in the structure. In magnetic recording, it is necessary that the coercivity HC1 of the stabilizing soft layer (SL) should be smaller than the interlayer exchange coupling field [2628]. Several studies were conducted on AFC systems using Ru spacer and ferromagnetic layers such as Co, CoFeB, Co/Pt and Co/Pd [2934]. In the present work, exchange coupling between a soft [Co (0.3 nm)/Ni (0.6 nm)]×N multilayer and a hard [Co (0.3 nm)/Pt (0.8 nm)]×12 multilayer was investigated. The number of repeats of the soft multilayer was varied from 4 to 8 and the numbers in brackets indicates the thickness value of the respective layer. The antiferromagnetic coupling is induced by a 0.8 nm thick Ru inserted between the two multilayers. In this paper, it is found that the number of bilayer repeats affects the antiferromagnetic coupling and magnetization reversal under an external magnetic field. The antiferromagnetic coupling is discussed as a function of temperature showing unusual behavior for the case of (Co/Ni)×4 antiferromagnetically coupled to (Co/Pt)×12.
As the number of repeats N of the soft (Co/Ni)×N multilayer increases the coupling becomes very weak. The unusual temperature dependence of the AFC structure observed reveals a maximum Jex at around 150 K. This is different from what was reported in most metallic type AFC structures.
2. Experiment: The samples have been deposited by DC-sputtering in a high vacuum chamber on thermally oxidized silicon. The investigated stack consists of [Co (0.3 nm)/Ni (0.6 nm)]×N/Ru (0.8 nm)/[Co (0.3 nm)/Pt (0.8 nm)]×12 where N is the number of repeats of each bilayer and was varied from 4 to 8 by a step of 2. In this study, the number of repeats of the hard (Co/Pt) multilayer was fixed to 12. The capping layer is a lamination of 3 nm Pt and 5 nm Ta to protect the whole stack from oxidation. A schematic representation of the whole stack can be seen in Fig. 1. In this work, the magnetic reversal of three AFC structures and their respective non-coupled multilayers was studied as a function of the temperature. The samples investigated are reported in Tab. 1. The three soft multilayers with 4, 6 and 8 bilayers are denoted by S1, S2 and S3, respectively while the hard multilayer (Co/Pt) is denoted by S4. The three AFC structures are S5, S6 and S7. Magnetic measurements were performed using Physical Properties Measurements System (PPMS) in a magnetic field range of ± 2 kOe and varying temperature (25 K to 300 K). Annealing process was performed under high vacuum on sample S5 to confirm the unusual behavior of interlayer exchange coupling (Jex) with temperature. S5 was annealed for 30 min at 200 oC with a ramping rate of 5 oC/min, which requires about 35 min to reach the desired annealing temperature.
Fig. 1. Schematic representation of the antiferromagnetic structure studied. The repetition number of the soft bilayer was changed from 4 to 8 bilayers
Results and discussion: Fig.1 represents the structures of the stacks investigated in this work. The out-of-plane hysteresis loops of the soft multilayer (Co/Ni)×4, the hard multilayer (Co/Pt)×12 and their exchange coupled structure S5 are shown in Fig. 2. The loops exhibit perpendicular magnetic anisotropy of both soft and hard layers with a sharp switching of the soft layer. Fig.2 (c) shows the major and minor loops of the antiferromagnetically coupled structure (S5) at room temperature. In this case, a clear two steps switching is observed with an off-set of the minor loop. At remanence state (H= 0), the magnetizations of the soft and hard multilayers are aligned in the antiparallel directions. From the major and minor loops, Hex and the coercivities of the soft and hard multilayers under the exchange coupling can be obtained [Fig. 2(c)]. The AF coupling through Ru spacer is attributed to indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The coercivities of the (Co/Ni)×4 soft multilayer HC1 and (Co/Pt)×12 hard multilayer HC2 were measured for both single and exchange coupled structures as a function of the temperature (Fig. 3). For both multilayers, an increase of their intrinsic coercivities (non-coupled case) by their mutual exchange coupling was observed. The difference in coercivities for the soft multilayer (HC1) remains almost constant in the investigated temperature range (25 K to 300 K) with a value of about 50 Oe. In the case of (Co/Pt), the difference HC2 increases from about 75 Oe at 25 K to 140 Oe at 300 K. The minor hysteresis loops were plotted for different temperature values to analyze the effect of the Table 1: The indicated symbols and structures of the investigated samples. Sample Structure S1
(Co/Ni)×4
S2
(Co/Ni)×6
S3
(Co/Ni)×8
S4
(Co/Pt)×12
S5
(Co/Ni)×4/Ru/(Co0.3/Pt)×12
S6
(Co/Ni)×6/Ru/(Co/Pt)×12
S7
(Co/Ni)×8/Ru/(Co/Pt)×12
Fig. 2: Out-of-plane hysteresis loops of (a) (Co/Ni)×4 multilayer, (b) (Co/Pt)×12 multilayer and (c) major and minor hysteresis loops of antiferromagnetically coupled (Co/ Ni)×4/Ru/ (Co/ Pt)×12 at room temperature.
antiferromagnetic coupling on the soft layer in the field range of 0 ≤ H ≤ +1000 Oe. Fig. 4(a) shows the minor hysteresis loops of the soft layer at different temperatures with a magnetic field applied perpendicular to film plane. The exchange coupling Hex measured from the shift of the minor hysteresis loops at different temperatures can also be seen in Fig. 4(b). It is important to note that Hex has a minimum value of about 380 Oe at 25 K and increases until reaching a
Fig. 3: Temperature dependence of the coercive fields for (a) the soft (Co/Ni)×4 as single and antiferromagnetically coupled to the hard layer and (b) the hard (Co/Pt)×12 as a single multilayer and coupled to the soft (Co/Ni)×4.
maximum value of about 440 Oe at around 200 K. This behavior was not seen in structures like (Co/Pd)/Ru/(Co/Pd) previously studied [35] in which Hex could be expressed as [36,37]:
(1) where H0 is the antiferromagnetic exchange coupling field at 0 K and T0 is a characteristic temperature. From Hex, we could obtain the interlayer coupling strength (Jex) using: (2) where MS and tF are the saturation magnetization and thickness of the soft layer, respectively. For the application purpose, it is necessary to investigate the temperature dependence of interlayer exchange coupling Jex. For sample S5, a maximum value of 0.13 erg/cm2 was obtained at around 150 K as shown in Fig 5. Although the value of Ms of the Co/Ni multilayer decreases as the temperature rises (not shown here), the temperature dependence of Jex follows the same trend than Hex shown in Fig. 4(b); i. e., for low temperatures, a gradual increase of Jex was revealed. The non-monotonous dependence of Jex with temperature is unusual in such films. Recently, Xiao et al. reported a similar behavior in antiferromagnetically coupled (Pt/CoFeB)×N1/ Ru/(CoFeB/Pt)×N2, where N1 and N2 are the number of repetitions of each bilayer in the structure [34]. They explained the observed behavior by a change from out-of-plane to in-plane magnetic anisotropy with temperature. From the measurements conducted on each single multilayer (samples S1 and S4), we did not see a change of magnetization direction with temperature. In addition, we made an AFC structure where (Co/Ni)×4 and (Co/Pt)×12 multilayers were substituted by (Co/Pd)×3 and (Co/Pd)×15, respectively. As expected, a monotonous decrease of Hex with temperature was observed as plotted in Fig. 6. Inset of Fig. 6 are the major and minor hysteresis loops obtained at low and high temperatures. Similar behavior of Jex with temperature shown in Fig. 5 has been reported by Bellouard et al. for the case Fe/MgO/Fe structure where the spacer is a relatively a thick insulator layer of 3 to 4 nm [38]. In our investigated structure, all the layers are metallic. To confirm this unusual behavior of Jex with temperature, the same sample S5 has been annealed at 200 C for 30 min. From Fig. 5, it can be seen that Jex for the annealed sample has the same thermal
Fig. 4. (a) Minor hysteresis loops of (Co/Ni)×4 multilayer antiferromagnetically coupled (Co/Pt)×12 multilayer at different temperature values. The magnetic field is applied perpendicular to film plane. (b) the exchange coupling field as a function of temperature deduced from the shift of hysteresis loops shown in (a).
Fig. 5. Temperature dependence of the interlayer exchange antiferromagnetic coupling Jex of (Co/ Ni)×4/Ru/(Co/ Pt)×12.
Fig. 6. Temperature dependence of the exchange coupling field of (Co/Pd)N1/Ru/(Co/Pd)N2 . The inset shows OOP hysteresis loops at RT and 25 K.
behavior than as deposited with a clear reduction of its magnitude in low temperature range. The maximum of Jex for both samples reached its maximum value at around ~ 150 K. The observed behavior of Jex as a function of temperature for all metal AFC structure (S5) remains unexplained by Eq. (1). Chen et al. studied antiferromagnetically coupled (Co/Ni) multilayers through Ru spacer, it was reported that when Ru thickness is less than 0.85 nm a non-monotonous dependence of Hex with temperature was observed. They attribute the observed behavior of Hex to the competition between the ferromagnetic coupling caused by the existence of pinholes and the antiferromagnetic coupling induced by the RKKY interaction. The two types of interactions are sensitive to the temperature, which affects the spontaneous magnetization. In our study, it is worthy to note that the investigated sample S5 shows a weak Jex with a maximum value of 0.13 erg/cm2 making thus the FM pinhole coupling important at low temperatures [39]. As previously discussed, the annealed sample reveals the same behavior of Jex with temperature but with lower values than as deposited (Fig. 5). The reason could be due to the interface diffusion during the annealing process and hence an increase in the formation of pinholes contacts which leads to a reduction of the antiferromagnetic coupling [40]. A second sample, S6, which is a combination of S2 and S4 has been investigated in the same manner (Tab. 1). From the hysteresis loops taken at different temperatures and plotted in Fig. 7(a), no clear two steps reversal of magnetization was observed; except a small kink indicated by the red dotted circle for T between 125 K and 225 K. In most of the measured temperatures, there is a fast reversal of magnetization at low magnetic field followed by a continuous rotation of magnetization. It is know that as the number of multilayer increases, the magnetostatic energy becomes higher and it is reflected in the shape of hysteresis loop [41]. For the coupled structure shown in Fig. 7(a), it is clear that as the number of bilayers of the soft (Co/Ni) increases to 6, the hysteresis loops show a bow-tie shape with no steps indicating that the antiferromagnetic coupling energy is strongly competing with the magnetostatic energy as result of the total
thickness increase. A similar behavior was observed by Hellwig et al. for [(Co/Pt)/Ru]×N multilayers [42]. For (Co/Ni)×6 multilayer (Sample S2), a square hysteresis loops could be observed in the temperature range investigated (25 K to 300 K). To understand the competition between the magnetostatic coupling, which becomes important as the thickness of (Co/Ni) multilayer increases, and Hex which in contrast becomes weaker as the number N becomes larger, we plotted the hysteresis loops for the coupled structure S6 and uncoupled structure at 200 K. The latter structure is only an addition of the hysteresis loops of each multilayer (S2 and S4). In the uncoupled structure, one could see a two steps reversal of magnetization of each multilayer (blue hysteresis loop). In the case of coupled S2 and S4 through Ru, Hex still induced a shift of the reversal field of (Co/Ni)×6 multilayer from 125 Oe to +175 Oe (~300 Oe). For comparison, at the same temperature, the
Ms/M
0.5
0.0
(b)
4K 50 K 100 K 150 K 200 K 250 K 300 K
1.0
T = 200 K
0.5
Ms/M
(a) 1.0
0.0
-0.5
-0.5
-1.0 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-1.0
H (kOe)
coupled uncoupled
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
H (kOe)
Fig. 7: Out-of-plane hysteresis loops of antiferromagnetically coupled [(Co/Ni)×6/ Ru/[(Co/Pt)×12] (a) at different temperatures and (b) the hysteresis loop measured at 200 K for coupled and uncoupled structure shown in (a).
change of reversal field of (Co/Ni)×4 due to Hex was about 385 Oe. Although there is a two-step reversal of S2 and S4 in the uncoupled case with a difference of about 425 Oe (blue hysteresis loop in Fig. 7(b)), it seems that the magnetostatic field has a strong effect on the magnetization orientation of both multilayers. It is most likely that the magnetic moments closer to the interfaces have their directions tilted from out-of-plane direction. It is possible to have an estimate to the affected interface from both S2 and S4 and indicated by the dashed square [Fig. 7(b)]. Above and below this dashed zone, the magnetizations of the multilayers (Co/Ni)×6 and (Co/Pt)×12 are still keeping their reversal in the out-of-plane direction. The third exchange AFC structure S7 investigated in this paper is based on a combination of S3 and S4. The hysteresis loops are plotted in Fig. 8(a) at different temperatures and almost all the loops are overlapped with a slight opening as the temperature is reduced. This type of hysteresis loop is a characteristic of a cooperative reversal of the two multilayers instead of one by one [42,43]. In this case, as the number of Co/Ni soft multilayer is further increased to 8, the magnetostatic coupling becomes even stronger than in S6 shown in Fig. 7 and the small kink was
(b) 1.0
(a)
Ms/M
0.5
T = 25 K (Co/Pt)x12
0.0 (Co/Ni)x4 -0.5
coupled uncoupled
-1.0
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
H (kOe)
Ms/M
0.5
(d) 1.0
T = 25 K
0.5
(Co/Pt)x12
Ms/M
(c) 1.0
0.0 (Co/Ni)x6 -0.5
coupled uncoupled
-1.0
T = 25 K (Co/Pt)x12
Coupled
0.0 (Co/Ni)x8
Uncoupled
-0.5 -1.0
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
H (kOe)
H (kOe)
Fig. 8: Out-of-plane Hysteresis loops of (a) antiferromagnetically coupled S7 structure at different temperatures, (b), (c), (d) coupled and uncoupled hysteresis loops of AFC structures S5, S6 and S7 at 25 K. not observed in the whole temperature range investigated. Fig. 8(bd) shows the coupled and uncoupled hysteresis loops measured at 25 K for the three AFC structures S5, S6 and S7 (Tab. 1). Although all the single multilayers show a clear square hysteresis loops as indicated by the sharp and two steps reversal (blue hysteresis loops), the competition between the magnetostatic and exchange energies has a strong impact on the coupled structures. The (Co/Ni) multilayer with only four repeats still reveals a clear two steps reversal. For the cases of 6 and 8 repeats, the AFC structures look like a single multilayer with a large number of repeats as a result of a strong magnetostatic coupling.
Conclusion Antiferromagnetically coupled systems consisting of (Co/Ni) and (Co/Pt) multilayers through Ru spacer were investigated as a function of temperature. It was found that AFC structure with
(Co/Ni)×4 soft multilayer shows two steps hysteresis loops. The interlayer exchange coupling Jex exhibits a strong dependence on temperature with non-monotonous behavior. The maximum of Hex was obtained at 200 K. By increasing the number of repeats in the (Co/Ni) multilayer to 6 and 8, the two steps hysteresis loops disappear which indicated that the magnetostatic coupling overcome the antiferromagnetic coupling.
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Highlights
Antiferromagnetic coupling between (Co/Ni) and (Co/Pt) multilayers with perpendicular magnetic anisotropy is investigated A non-monotonous exchange coupling field was observed as a function of temperature The observed behavior could be an indication of interface quality between different layers.
S0304-8853(18)34191-X https://doi.org/10.1016/j.jmmm.2019.02.022 MAGMA 64941
To appear in:
Journal of Magnetism and Magnetic Materials
Received Date: Revised Date: Accepted Date:
26 December 2018 23 January 2019 5 February 2019
Please cite this article as: A. Al Subhi, R. Sbiaa, M. Ranjbar, J. Akerman, Magnetization reversal of antiferromagnetically coupled (Co/Ni) and (Co/Pt) multilayers, Journal of Magnetism and Magnetic Materials (2019), doi: https://doi.org/10.1016/j.jmmm.2019.02.022
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Magnetization reversal of antiferromagnetically coupled (Co/Ni) and (Co/Pt) multilayers
A. Al Subhi1, R. Sbiaa1*, M. Ranjbar2 and J. Akerman2 1
Department of Physics, Sultan Qaboos University, P.O. Box 36, PC 123, Muscat, Oman
2
Physics Department, University of Gothenburg, 412 96 Gothenburg, Sweden
PACS 75.60.Ch– magnetic properties and materials PACS 75.76. +j– Spin transport (magnetoelectronics) * Corresponding author: [email protected]
Abstract – Magnetization reversal and magnetic exchange coupling of (Co/Ni)×N/Ru/ (Co/Pt)×12 were investigated as a function of the temperature. The number of repeats N of the soft multilayer (Co/Ni) was varied from 4 to 8 bilayers while the number of repeats of the hard bilayers (Co/Pt) was fixed to 12. Two steps hysteresis loops were observed for coupled structure with only 4 repeats of (Co/Ni) in a wide range of temperature (25 to 300 K). From the shift of the minor hysteresis loop, the antiferromagnetic exchange coupling Hex was measured and then the interlayer exchange coupling Jex was calculated. A non-monotonous dependence of Jex with temperature was observed for N = 4 with a maximum Jex of 0.13 erg/cm2 at 150 K. The annealing process performed on the same structure confirms the unusual behavior of interlayer exchange coupling Jex. As the repetition number N increases to 8 bilayers the two steps hysteresis loops disappeared in the investigated temperature range, however a small kink appeared in the range of 125 and 225 K for the case of 6 bilayers. From the analysis of the coupled and uncoupled structures, it seems that the dipolar energy overcomes the antiferromagnetic coupling for thicker (Co/Ni) multilayer.
1. Introduction Materials with perpendicular magnetic anisotropy (PMA) have been the subject of intensive studies as they could be the basis of magnetic recording media [17] and spintronics devices such as magnetic random access memory (MRAM) [821]. They have better thermal stability compared to in-plane anisotropy type materials [22]. Among several materials which exhibit high PMA, multilayers based on alteration of two thin ferromagnetic layers such as Co, Fe, Ni, CoFe, CoFeB or a thin ferromagnetic layer and a noble metal like Pt, Pd and Au have been widely investigated. In fact, it is relatively easy to achieve a perpendicular orientation of the multilayer magnetization and tailor the materials properties by adjusting the thickness and the number of repeats of each sublayer. The PMA of the magnetic multilayers is mainly originating from the interface anisotropy contribution [23]. Thus, a perfect control of the thickness of each layer is important. For their application to magnetic recording or MRAM, although PMA materials are beneficial for the stability of the data/bit, they still represent a challenge for the adjacent or neighboring bit. This is due to the strong stray field that could reverse the magnetization direction of closest bits. To overcome this issue, antiferromagnetically coupled (AFC) structures have been proposed [24,25]. The AFC structure consists of a lamination of two ferromagnetic layers separated by a non-magnetic spacer which is Ru or Rh. The AF coupling can be achieved by manipulating the thickness of each layer in the structure. In magnetic recording, it is necessary that the coercivity HC1 of the stabilizing soft layer (SL) should be smaller than the interlayer exchange coupling field [2628]. Several studies were conducted on AFC systems using Ru spacer and ferromagnetic layers such as Co, CoFeB, Co/Pt and Co/Pd [2934]. In the present work, exchange coupling between a soft [Co (0.3 nm)/Ni (0.6 nm)]×N multilayer and a hard [Co (0.3 nm)/Pt (0.8 nm)]×12 multilayer was investigated. The number of repeats of the soft multilayer was varied from 4 to 8 and the numbers in brackets indicates the thickness value of the respective layer. The antiferromagnetic coupling is induced by a 0.8 nm thick Ru inserted between the two multilayers. In this paper, it is found that the number of bilayer repeats affects the antiferromagnetic coupling and magnetization reversal under an external magnetic field. The antiferromagnetic coupling is discussed as a function of temperature showing unusual behavior for the case of (Co/Ni)×4 antiferromagnetically coupled to (Co/Pt)×12.
As the number of repeats N of the soft (Co/Ni)×N multilayer increases the coupling becomes very weak. The unusual temperature dependence of the AFC structure observed reveals a maximum Jex at around 150 K. This is different from what was reported in most metallic type AFC structures.
2. Experiment: The samples have been deposited by DC-sputtering in a high vacuum chamber on thermally oxidized silicon. The investigated stack consists of [Co (0.3 nm)/Ni (0.6 nm)]×N/Ru (0.8 nm)/[Co (0.3 nm)/Pt (0.8 nm)]×12 where N is the number of repeats of each bilayer and was varied from 4 to 8 by a step of 2. In this study, the number of repeats of the hard (Co/Pt) multilayer was fixed to 12. The capping layer is a lamination of 3 nm Pt and 5 nm Ta to protect the whole stack from oxidation. A schematic representation of the whole stack can be seen in Fig. 1. In this work, the magnetic reversal of three AFC structures and their respective non-coupled multilayers was studied as a function of the temperature. The samples investigated are reported in Tab. 1. The three soft multilayers with 4, 6 and 8 bilayers are denoted by S1, S2 and S3, respectively while the hard multilayer (Co/Pt) is denoted by S4. The three AFC structures are S5, S6 and S7. Magnetic measurements were performed using Physical Properties Measurements System (PPMS) in a magnetic field range of ± 2 kOe and varying temperature (25 K to 300 K). Annealing process was performed under high vacuum on sample S5 to confirm the unusual behavior of interlayer exchange coupling (Jex) with temperature. S5 was annealed for 30 min at 200 oC with a ramping rate of 5 oC/min, which requires about 35 min to reach the desired annealing temperature.
Fig. 1. Schematic representation of the antiferromagnetic structure studied. The repetition number of the soft bilayer was changed from 4 to 8 bilayers
Results and discussion: Fig.1 represents the structures of the stacks investigated in this work. The out-of-plane hysteresis loops of the soft multilayer (Co/Ni)×4, the hard multilayer (Co/Pt)×12 and their exchange coupled structure S5 are shown in Fig. 2. The loops exhibit perpendicular magnetic anisotropy of both soft and hard layers with a sharp switching of the soft layer. Fig.2 (c) shows the major and minor loops of the antiferromagnetically coupled structure (S5) at room temperature. In this case, a clear two steps switching is observed with an off-set of the minor loop. At remanence state (H= 0), the magnetizations of the soft and hard multilayers are aligned in the antiparallel directions. From the major and minor loops, Hex and the coercivities of the soft and hard multilayers under the exchange coupling can be obtained [Fig. 2(c)]. The AF coupling through Ru spacer is attributed to indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. The coercivities of the (Co/Ni)×4 soft multilayer HC1 and (Co/Pt)×12 hard multilayer HC2 were measured for both single and exchange coupled structures as a function of the temperature (Fig. 3). For both multilayers, an increase of their intrinsic coercivities (non-coupled case) by their mutual exchange coupling was observed. The difference in coercivities for the soft multilayer (HC1) remains almost constant in the investigated temperature range (25 K to 300 K) with a value of about 50 Oe. In the case of (Co/Pt), the difference HC2 increases from about 75 Oe at 25 K to 140 Oe at 300 K. The minor hysteresis loops were plotted for different temperature values to analyze the effect of the Table 1: The indicated symbols and structures of the investigated samples. Sample Structure S1
(Co/Ni)×4
S2
(Co/Ni)×6
S3
(Co/Ni)×8
S4
(Co/Pt)×12
S5
(Co/Ni)×4/Ru/(Co0.3/Pt)×12
S6
(Co/Ni)×6/Ru/(Co/Pt)×12
S7
(Co/Ni)×8/Ru/(Co/Pt)×12
Fig. 2: Out-of-plane hysteresis loops of (a) (Co/Ni)×4 multilayer, (b) (Co/Pt)×12 multilayer and (c) major and minor hysteresis loops of antiferromagnetically coupled (Co/ Ni)×4/Ru/ (Co/ Pt)×12 at room temperature.
antiferromagnetic coupling on the soft layer in the field range of 0 ≤ H ≤ +1000 Oe. Fig. 4(a) shows the minor hysteresis loops of the soft layer at different temperatures with a magnetic field applied perpendicular to film plane. The exchange coupling Hex measured from the shift of the minor hysteresis loops at different temperatures can also be seen in Fig. 4(b). It is important to note that Hex has a minimum value of about 380 Oe at 25 K and increases until reaching a
Fig. 3: Temperature dependence of the coercive fields for (a) the soft (Co/Ni)×4 as single and antiferromagnetically coupled to the hard layer and (b) the hard (Co/Pt)×12 as a single multilayer and coupled to the soft (Co/Ni)×4.
maximum value of about 440 Oe at around 200 K. This behavior was not seen in structures like (Co/Pd)/Ru/(Co/Pd) previously studied [35] in which Hex could be expressed as [36,37]:
(1) where H0 is the antiferromagnetic exchange coupling field at 0 K and T0 is a characteristic temperature. From Hex, we could obtain the interlayer coupling strength (Jex) using: (2) where MS and tF are the saturation magnetization and thickness of the soft layer, respectively. For the application purpose, it is necessary to investigate the temperature dependence of interlayer exchange coupling Jex. For sample S5, a maximum value of 0.13 erg/cm2 was obtained at around 150 K as shown in Fig 5. Although the value of Ms of the Co/Ni multilayer decreases as the temperature rises (not shown here), the temperature dependence of Jex follows the same trend than Hex shown in Fig. 4(b); i. e., for low temperatures, a gradual increase of Jex was revealed. The non-monotonous dependence of Jex with temperature is unusual in such films. Recently, Xiao et al. reported a similar behavior in antiferromagnetically coupled (Pt/CoFeB)×N1/ Ru/(CoFeB/Pt)×N2, where N1 and N2 are the number of repetitions of each bilayer in the structure [34]. They explained the observed behavior by a change from out-of-plane to in-plane magnetic anisotropy with temperature. From the measurements conducted on each single multilayer (samples S1 and S4), we did not see a change of magnetization direction with temperature. In addition, we made an AFC structure where (Co/Ni)×4 and (Co/Pt)×12 multilayers were substituted by (Co/Pd)×3 and (Co/Pd)×15, respectively. As expected, a monotonous decrease of Hex with temperature was observed as plotted in Fig. 6. Inset of Fig. 6 are the major and minor hysteresis loops obtained at low and high temperatures. Similar behavior of Jex with temperature shown in Fig. 5 has been reported by Bellouard et al. for the case Fe/MgO/Fe structure where the spacer is a relatively a thick insulator layer of 3 to 4 nm [38]. In our investigated structure, all the layers are metallic. To confirm this unusual behavior of Jex with temperature, the same sample S5 has been annealed at 200 C for 30 min. From Fig. 5, it can be seen that Jex for the annealed sample has the same thermal
Fig. 4. (a) Minor hysteresis loops of (Co/Ni)×4 multilayer antiferromagnetically coupled (Co/Pt)×12 multilayer at different temperature values. The magnetic field is applied perpendicular to film plane. (b) the exchange coupling field as a function of temperature deduced from the shift of hysteresis loops shown in (a).
Fig. 5. Temperature dependence of the interlayer exchange antiferromagnetic coupling Jex of (Co/ Ni)×4/Ru/(Co/ Pt)×12.
Fig. 6. Temperature dependence of the exchange coupling field of (Co/Pd)N1/Ru/(Co/Pd)N2 . The inset shows OOP hysteresis loops at RT and 25 K.
behavior than as deposited with a clear reduction of its magnitude in low temperature range. The maximum of Jex for both samples reached its maximum value at around ~ 150 K. The observed behavior of Jex as a function of temperature for all metal AFC structure (S5) remains unexplained by Eq. (1). Chen et al. studied antiferromagnetically coupled (Co/Ni) multilayers through Ru spacer, it was reported that when Ru thickness is less than 0.85 nm a non-monotonous dependence of Hex with temperature was observed. They attribute the observed behavior of Hex to the competition between the ferromagnetic coupling caused by the existence of pinholes and the antiferromagnetic coupling induced by the RKKY interaction. The two types of interactions are sensitive to the temperature, which affects the spontaneous magnetization. In our study, it is worthy to note that the investigated sample S5 shows a weak Jex with a maximum value of 0.13 erg/cm2 making thus the FM pinhole coupling important at low temperatures [39]. As previously discussed, the annealed sample reveals the same behavior of Jex with temperature but with lower values than as deposited (Fig. 5). The reason could be due to the interface diffusion during the annealing process and hence an increase in the formation of pinholes contacts which leads to a reduction of the antiferromagnetic coupling [40]. A second sample, S6, which is a combination of S2 and S4 has been investigated in the same manner (Tab. 1). From the hysteresis loops taken at different temperatures and plotted in Fig. 7(a), no clear two steps reversal of magnetization was observed; except a small kink indicated by the red dotted circle for T between 125 K and 225 K. In most of the measured temperatures, there is a fast reversal of magnetization at low magnetic field followed by a continuous rotation of magnetization. It is know that as the number of multilayer increases, the magnetostatic energy becomes higher and it is reflected in the shape of hysteresis loop [41]. For the coupled structure shown in Fig. 7(a), it is clear that as the number of bilayers of the soft (Co/Ni) increases to 6, the hysteresis loops show a bow-tie shape with no steps indicating that the antiferromagnetic coupling energy is strongly competing with the magnetostatic energy as result of the total
thickness increase. A similar behavior was observed by Hellwig et al. for [(Co/Pt)/Ru]×N multilayers [42]. For (Co/Ni)×6 multilayer (Sample S2), a square hysteresis loops could be observed in the temperature range investigated (25 K to 300 K). To understand the competition between the magnetostatic coupling, which becomes important as the thickness of (Co/Ni) multilayer increases, and Hex which in contrast becomes weaker as the number N becomes larger, we plotted the hysteresis loops for the coupled structure S6 and uncoupled structure at 200 K. The latter structure is only an addition of the hysteresis loops of each multilayer (S2 and S4). In the uncoupled structure, one could see a two steps reversal of magnetization of each multilayer (blue hysteresis loop). In the case of coupled S2 and S4 through Ru, Hex still induced a shift of the reversal field of (Co/Ni)×6 multilayer from 125 Oe to +175 Oe (~300 Oe). For comparison, at the same temperature, the
Ms/M
0.5
0.0
(b)
4K 50 K 100 K 150 K 200 K 250 K 300 K
1.0
T = 200 K
0.5
Ms/M
(a) 1.0
0.0
-0.5
-0.5
-1.0 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-1.0
H (kOe)
coupled uncoupled
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
H (kOe)
Fig. 7: Out-of-plane hysteresis loops of antiferromagnetically coupled [(Co/Ni)×6/ Ru/[(Co/Pt)×12] (a) at different temperatures and (b) the hysteresis loop measured at 200 K for coupled and uncoupled structure shown in (a).
change of reversal field of (Co/Ni)×4 due to Hex was about 385 Oe. Although there is a two-step reversal of S2 and S4 in the uncoupled case with a difference of about 425 Oe (blue hysteresis loop in Fig. 7(b)), it seems that the magnetostatic field has a strong effect on the magnetization orientation of both multilayers. It is most likely that the magnetic moments closer to the interfaces have their directions tilted from out-of-plane direction. It is possible to have an estimate to the affected interface from both S2 and S4 and indicated by the dashed square [Fig. 7(b)]. Above and below this dashed zone, the magnetizations of the multilayers (Co/Ni)×6 and (Co/Pt)×12 are still keeping their reversal in the out-of-plane direction. The third exchange AFC structure S7 investigated in this paper is based on a combination of S3 and S4. The hysteresis loops are plotted in Fig. 8(a) at different temperatures and almost all the loops are overlapped with a slight opening as the temperature is reduced. This type of hysteresis loop is a characteristic of a cooperative reversal of the two multilayers instead of one by one [42,43]. In this case, as the number of Co/Ni soft multilayer is further increased to 8, the magnetostatic coupling becomes even stronger than in S6 shown in Fig. 7 and the small kink was
(b) 1.0
(a)
Ms/M
0.5
T = 25 K (Co/Pt)x12
0.0 (Co/Ni)x4 -0.5
coupled uncoupled
-1.0
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
H (kOe)
Ms/M
0.5
(d) 1.0
T = 25 K
0.5
(Co/Pt)x12
Ms/M
(c) 1.0
0.0 (Co/Ni)x6 -0.5
coupled uncoupled
-1.0
T = 25 K (Co/Pt)x12
Coupled
0.0 (Co/Ni)x8
Uncoupled
-0.5 -1.0
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0
H (kOe)
H (kOe)
Fig. 8: Out-of-plane Hysteresis loops of (a) antiferromagnetically coupled S7 structure at different temperatures, (b), (c), (d) coupled and uncoupled hysteresis loops of AFC structures S5, S6 and S7 at 25 K. not observed in the whole temperature range investigated. Fig. 8(bd) shows the coupled and uncoupled hysteresis loops measured at 25 K for the three AFC structures S5, S6 and S7 (Tab. 1). Although all the single multilayers show a clear square hysteresis loops as indicated by the sharp and two steps reversal (blue hysteresis loops), the competition between the magnetostatic and exchange energies has a strong impact on the coupled structures. The (Co/Ni) multilayer with only four repeats still reveals a clear two steps reversal. For the cases of 6 and 8 repeats, the AFC structures look like a single multilayer with a large number of repeats as a result of a strong magnetostatic coupling.
Conclusion Antiferromagnetically coupled systems consisting of (Co/Ni) and (Co/Pt) multilayers through Ru spacer were investigated as a function of temperature. It was found that AFC structure with
(Co/Ni)×4 soft multilayer shows two steps hysteresis loops. The interlayer exchange coupling Jex exhibits a strong dependence on temperature with non-monotonous behavior. The maximum of Hex was obtained at 200 K. By increasing the number of repeats in the (Co/Ni) multilayer to 6 and 8, the two steps hysteresis loops disappear which indicated that the magnetostatic coupling overcome the antiferromagnetic coupling.
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Highlights
Antiferromagnetic coupling between (Co/Ni) and (Co/Pt) multilayers with perpendicular magnetic anisotropy is investigated A non-monotonous exchange coupling field was observed as a function of temperature The observed behavior could be an indication of interface quality between different layers.