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MgO)10 multilayers at low temperature
Journal of Magnetism and Magnetic Materials 499 (2020) 166318
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Research articles
Magnetic properties and magnetization reversal process in (Pt/CoFe/ MgO)10 multilayers at low temperature
T
Xiangqian Wanga,b, Yueyue Liua, YuRui Weia, Kaizhou Heb, Yi Huanga, Jianbo Wanga,c, ⁎ Qingfang Liua, , Genliang Hanb a
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China Key Laboratory of Sensor and Sensor Technology, Institute of Sensor Technology, Gansu Academy of Sciences, Lanzhou 730000, China c Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, Lanzhou 730000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: PMA Magnetization reversal FORC Low temperature
The magnetic properties and magnetization reversal processes in a (Pt/CoFe/MgO)10 film were studied as a function of temperature (100–300 K) using hysteresis loops and the first-order reversal curve (FORC) method. The coercivity, saturation magnetization, remanence and effective anisotropy field are enhanced as the temperature decreases. Also, as the temperature changes from 300 to 100 K, the distribution of the irreversible reversal magnetization field shifts from 100 to −20 Oe along the reversal field Hr axis. Two different sets of “hysterons” and interparticle magnetostatic interactions were clearly observed in the sample at 100 K.
1. Introduction In the last few decades, multilayers with perpendicular magnetic anisotropy (PMA), such as (Pt/CoFe/MgO)n, have been intensively investigated due to their applications in spin-transfer torque magnetic random access memory (STT-MRAM) and other magneto-electronic devices [1–4]. Because of the additional asymmetric exchange interaction at the interfaces, this kind of stack is commonly found in research focusing on perpendicular magnetic anisotropy, magnetic tunnel junction (MTJ) [5,6] and Dzyaloshinskii-Moriya interaction [7–9]. For example, Jinjun Qiu et al. [10] investigated the effect of roughness on PMA in (Co90Fe10/Pt)n superlattices. Shouzhong Peng et al. [11] found that interfacial PMA can be enhanced by inserting a thin Fe layer at the CoFe/W interface, allowing a sufficient thermal stability in small-node MTJs. Satoru Emori et al. [12] showed that the spin torque and domain wall (DW) dynamics in out-of-plane magnetized Pt/CoFe/MgO and Ta/ CoFe/MgO stacks have an opposite sign. J. F. Feng et al. [13] found that the PMA, coercivity and saturated magnetization also depend on the metallic and magnetic layer thickness in ultrathin (CoFe/Pt)n multilayers. In addition, magnetic multilayer films are good candidate to investigate the magnetization reversal process. It has been demonstrated that the first-order reversal curve (FORC) method is more accurate for characterizing the magnetization reversal of magnetic multilayer films [14–17]. Compared to major hysteresis loops, FORCs provide more information about the magnetization reversal, e.g. the switching field
⁎
distribution (SFD) and the degree of reversible and irreversible switching [18–20]. Furthermore, this method enables the study of coercive field and magnetostatic interaction distributions [21,22]. FORC distribution also provides information on the weight function of hysterons, or elements of hysteresis; and in certain systems, it is equivalent to the Preisach distribution. However, most studies about FORCs focus on the results at room temperature, there are only a few reports at low temperatures. Although the magnetic properties at low temperature have been reported in previous works, [23,24], the detail of magnetization reversal process of magnetic multilayers at low temperature is not entirely clear. The change of the domain nucleation field, annihilation field and domain wall motion of magnetic multilayers at low temperature are not easy to observe by other methods, e.g. magnetic force microscopy (MFM) and magneto-optical Kerr effect (MOKE) microscopy. (The MOKE is a useful method to study the dynamic variation process of magnetic domain, whether at room temperature or low temperature. But we found when the repeat number more than 7 in our samples, the magnetic domain cannot be observed by MOKE. It means that the method should apply only to relatively thin films.) However, it can be easily and clearly obtained through FORC distribution. The FORCs can be employed to investigate the switching field distribution, domain nucleation field, irreversible switching component and spin reorientation transition [20] of magnetic multilayer films at low temperature. Therefore, the low temperature FORCs is an important method in the research and industrial application of magnetic multilayer films. In this work we investigated the magnetic
Corresponding author.
https://doi.org/10.1016/j.jmmm.2019.166318 Received 17 October 2019; Received in revised form 4 December 2019; Accepted 17 December 2019 Available online 19 December 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 499 (2020) 166318
X. Wang, et al.
properties and magnetization reversal behavior of a (Pt/CoFe/MgO)10 film at low temperature by FORCs. We found that the PMA of the sample dramatically increases when the temperature decreases from 300 to 100 K. Also, the irreversible reversal process constantly shifts toward the negative reversal field (Hr) when the temperature decreases. Moreover, the horizontal ridges of irreversible reversal separate and the magnetostatic interaction between interparticles is enhanced at 100 K. 2. Materials and methods A series of Ta/[Pt(1 nm)/CoFe(0.8 nm)/MgO(2 nm)]10/Ta multilayer films were deposited on Si (1 0 0) substrates with a 0.5 µm-thick thermal-oxide using a magnetron sputtering system (base pressure 1 × 10−7 Torr). The Ta, CoFe and Pt layers were deposited by direct current (DC) magnetron sputtering, and the MgO layer by radio frequency sputtering. The Ar flow rate was 20 standard cubic centimeters per minute during sputtering and its pressure was 0.3 Pa during deposition. The magnetization reversal processes of the sample were characterized by the hysteresis loops and FORC by vibrating sample magnetometers (VSM, Lakeshore 8600). The FORC was measured by first saturating the sample at a large positive field, then decreasing the field to a reversal field Hr, and measuring the magnetization M as the applied field H is increased back to saturation. A subsequent FORC is measured after re-saturating the sample and then ramping the field to a lower reversal field Hr. As the reversal fields are changed, the interior of the major loop is filled with the FORCs [19]. The measurements were carried out with a cryostat (86-SSVT) at temperatures ranging from 100 to 300 K. Fig. 2. Temperature dependence of (a) Hc, (b) Ms, (c) Mr/Ms and (d) Keff.
3. Results and discussion using the following expression [10]: Fig. 1 displays a series of out-of-plane hysteresis loops (M-H) measured by VSM for the (Pt/CoFe/MgO)10 film at temperatures ranging from 100 to 300 K, the inset shows an enlargement of the −300 to 300 Oe region. All the curves exhibit a bowknot shape. The coercivity (Hc) decreases from 112 to 12 Oe when increasing the temperature from 100 to 300 K, as shown in Fig. 2(a). Fig. 2(b) shows the data extracted from the Ms vs T plot in the range from 100 to 300 K. Also, there is a remarkable increase in the saturation magnetization Ms with decreasing temperature. Additionally, the remanence Mr gradually increases with decreasing T. The reduced Mr/Ms of the loops are presented as a function of temperature T in Fig. 2(c). It can be observed that Mr/Ms drops from 0.92 (100 K) to 0.004 (300 K). The effective anisotropy Keff can be evaluated by measuring the area between the hard axis and easy axis magnetization curves for a given sample by
K eff =
∫0
H max
[M (OP ) − M (IP )] dH
which could be roughly estimated from Keff = MsHk/2, where Ms is the saturation magnetization and Hk is the saturation field obtained from the in-plane M-H curve. The Keff as a function of the temperature T is shown in Fig. 2(d). The maximum Keff of the sample was approximately 5.68 × 106erg/cc at 100 K. Table 1 lists all the measured magnetic parameters of the (Pt/CoFe/MgO)10 film at different temperature T. The magnetization reversal process of the (Pt/CoFe/MgO)10 film was quantitatively analyzed at various temperatures. For this purpose, out-of-plane FORC measurements were carried out to determine the distribution of the reversible and irreversible components of the process. A series of FORCs measured at 300 K are presented in Fig. 3(a). The FORC distribution is defined by a mixed second-order derivative [15]:
ρ (Hr , H ) = −
1 ∂2M (Hr , H )/ Ms 2 ∂Hr ∂H
where H is the applied magnetic field and Hr is the starting reversal field. The second order derivation eliminates the reversible magnetization process; thus, a plot of the FORC distribution ρ(Hr,H) can be used to probe the details of the irreversible magnetization reversal process Table 1 Magnetic parameters of the (Pt/CoFe/MgO)10 film at different temperatures.
Fig. 1. Major hysteresis loops for (Pt/CoFe/MgO)10 film at different temperatures, the inset shows an enlargement of the hysteresis loops. 2
T(K)
Hc(Oe)
Ms (emu/cc)
Mr/Ms
Keff (106erg/cc)
300 260 220 180 140 100
12 19 33 53 82 112
2524 2750 2870 3040 3210 3520
0.004 0.10 0.19 0.31 0.60 0.92
3.12 3.91 4.36 4.73 5.02 5.68
Journal of Magnetism and Magnetic Materials 499 (2020) 166318
X. Wang, et al.
Fig. 3. (a) A series of out-of-plane FORCs for the (Pt/CoFe/MgO)10 film at 300 K; (b) contour and (c) three-dimensional plots of the corresponding FORC distribution. The color scale in (b) represents the strength of ρ. (d) Cross-sectional view of the FORC diagrams along the Hr axes along the maximum distribution.
[15]. Any nonzero ρ corresponds to an irreversible component. The corresponding FORC distribution plotted as a function of H and Hr is shown in Fig. 3(b). Here, the reversal behaviors, such as domain nucleation, propagation, and annihilation process, are clearly defined by the contour maps of the FORC distributions. The white dashed lines in Fig. 3(b) indicate three obvious features in the contour plot. First, a horizontal ridge in region I (20 Oe < Hr < 180 Oe) indicating the irreversible domain propagation from already nucleated sites. Second, a shallow plateau with ρ ≈ 80 in region II (−80 Oe < Hr < 20 Oe) corresponds to the most reversible domain expansion-contraction that does not change the domain topography. Finally, a vertical valley-peak pair in region III (−320 Oe < Hr < −80 Oe) is assigned to an irreversible domain annihilation process. These three features respectively correspond to the steep, gradual and again gradual decrease in magnetization toward the positive saturation in the FORCs. Fig. 3(c) shows a three-dimensional plot from data in Fig. 3(b). The rise of the ridge corresponds to the nonzero FORC distribution, indicating the onset of irreversible processes. This change is clearly observed in the projection of the FORC distribution onto the Hr axis, which can be obtained by integrating ρ over H, as shown in Fig. 3(d). We also obtained the out-of-plane FORC diagrams of the (Pt/CoFe/ MgO)10 film at low temperatures. A series of FORCs and their corresponding FORC distributions at 260, 180 and 100 K are shown in Fig. 4. Compared to the distribution at 300 K in Fig. 3(b), the area of irreversible reversal magnetization (−80 Oe < Hr < 180 Oe) and domain annihilation (−480 Oe < Hr < −100 Oe) regions at 260 K (Fig. 4(d)) are slightly extended. When the temperature decreases to 180 K, Fig. 4(e), the irreversible reversal distribution significantly broadens along the H axis and continues to shift to the negative Hr axis (−100 Oe < Hr < 100 Oe). This trend is also reflected in the domain annihilation portion, depicted with a valley-peak pair. At 100 K, the shape of the irreversible reversal distribution elongates along the
positive H axis, which is consistent with the previous report [25]. In addition, the horizontal ridge splits into two parts, one of which is located around H = 50 Oe, and the other around H = −20 Oe. This can be explained by the classical Preisach model [26]: the two distinct peaks may be associated with two different sets of “hysterons” with their respective local coercive field Hc and interaction field Hu. The distance from one peak to another corresponds to the difference between the “hysterons.” When T goes from 180 to 100 K, the difference between the two sets of “hysterons” increases, as indicated by the increased distance. It is worth noting that when the temperature is 180 K, a negative peak (around Hr = 130 Oe) in the horizontal ridge appears, depicted by red circle A in Fig. 4(e). As the temperature decreases, the negative peak becomes more evident, and another negative peak rises at Hr = 10 Oe in Fig. 4(f). This phenomenon is associated with the magnetostatic interaction between interparticles [22], which is enhanced at low temperatures (100 K). The projection of the horizontal FORC ridge onto the Hr axis at different temperatures are displayed in Fig. 4(g-i). The distribution of the irreversible reversal field shifts from 100 to −20 Oe along the Hr axis when the temperature decreases from 260 to 100 K. It can be found the PMA more stronger, the Hr of irreversible reversal process more negative. In the (Pt/CoFe/MgO)10 multilayers, the Keff can be described by the relation: Keff = −2πMS2 + Ki/ teff, where 2πMS2 is the shape anisotropy, and Ki is the interface anisotropy, teff is the effective Co layer thickness. The interlayer coupling is one of origin of Ki. As the temperature falls, the interlayer coupling will be enhanced [27]. Therefore, the Keff of (Pt/CoFe/MgO)10 multilayers increased with a dropping temperature. Furthermore, at low temperature, the strong exchange interaction between magnetic moments and weakening thermal disturbance will lead to the magnetic moment reversal more difficult and need to lager reversal field Hr. Thus the irreversible reversal process shifts toward the negative reversal field with increasing PMA. Thus the irreversible reversal process shifts toward the negative reversal field with increasing PMA. 3
Journal of Magnetism and Magnetic Materials 499 (2020) 166318
X. Wang, et al.
Fig. 4. (a-c) Series of out-of-plane FORCs, (d-f) their corresponding FORC distributions and (g-i) their projection on the Hr axis at different temperatures (260–100 K). The color scale represents the ρ strength. The red circle in (e) and (f) indicate a negative peak. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions
Supervision. Genliang Han: Funding acquisition.
In summary, we characterized the magnetic properties and magnetization reversal process of a (Pt/CoFe/MgO)10 film based on the hysteresis loops and FORC at temperatures ranging from 300 to 100 K. The PMA shows a strong temperature-dependence; the Keff increases from 3.12 to 5.68 erg/cc when the temperature decreases from 300 to 100 K. According the FORC diagrams, the irreversible reversal magnetization distribution constantly elongates along the positive H axis and shifts to negative Hr values with decreasing temperature. In addition, there are two areas with different coercive field and interaction field where the magnetostatic interaction between interparticles is enhanced at 100 K. The results of this work should be useful for the practical application of Pt/CoFe/MgO films in MTJs.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported in part by the National Natural Science Foundation of China (Nos. 51771086 and 11574121), in part by the Fund for Less Developed Regions of the National Natural Science Foundation of China (Nos. 51761001), in part by the Cooperation Project of Gansu Academy of Science under Grant (Nos. 2019HZ-04), and in part by the Applied Research and Development Project of Gansu Academy of Science under Grant (Nos. 2017JK-13).
CRediT authorship contribution statement Xiangqian Wang: Conceptualization, Formal analysis, Writing original draft. Yueyue Liu: Visualization. YuRui Wei: Data curation. Kaizhou He: Methodology. Yi Huang: Investigation. Jianbo Wang: Project administration. Qingfang Liu: Writing - review & editing,
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Journal of Magnetism and Magnetic Materials 499 (2020) 166318
X. Wang, et al.
[14] C.R. Pike, Phys. Rev. B 68 (10) (2003) 185–192. [15] J.E. Davies, O. Hellwig, E.E. Fullerton, G. Denbeaux, K. Liu, Phys. Rev. B 70 (22) (2004) 155–163. [16] J.H. Yin, H.W. Zhang, F.X. Hu, B.G. Shen, L.Q. Pan, J. Appl. Phys. 106 (10) (2009) 103901. [17] A. Markou, I. Panagiotopoulos, L. Stoleriu, A. Stancu, F. Ott, J. Magn. Magn. Mater. 485 (2019) 205–211. [18] J.E. Davies, O. Hellwig, E.E. Fullerton, K. Liu, Phys. Rev. B 77 (1) (2008) 014421. [19] M. Almasi-Kashi, A. Ramazani, E. Golafshan, M. Arefpour, E. Jafari-Khamse, Physica B 429 (2013) 46–51. [20] Y. Li, W. He, J.W. Cai, Z.H. Cheng, J. Magn. Magn. Mater. 485 (2019) 369–373. [21] L. Alonso, T.R.F. Peixoto, D.R. Cornejo, J. Phys. D: Appl. Phys. 43 (46) (2010) 465001. [22] A. Stancu, C. Pike, L. Stoleriu, P. Postolache, D. Cimpoesu, J. Appl. Phys. 93 (10) (2003) 6620. [23] J.G. Alzate, P.K. Amiri, G.Q. Yu, P. Upadhyaya, J.A. Katine, J. Langer, B. Ocker, I.N. Krivorotov, K.L. Wang, Appl. Phys. Lett. 104 (11) (2014) 813–815. [24] T. Nozaki, M. Oida, T. Ashida, N. Shimomura, M. Sahashi, Appl. Phys. Lett. 103 (2013) 242418. [25] D. Cimpoesu, L. Spinu, A. Stancu, IEEE Trans. Magn. 42 (10) (2006) 3165–3167. [26] Z. Diao, N. Decorde, P. Stamenov, K. Rode, G. Feng, J.M.D. Coey, J. Appl. Phys. 111 (7) (2012) 1409. [27] X.M. Liu, P. Ho, J.S. Chen, A.O. Adeyeye, J. Appl. Phys. 112 (2012) 073902.
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Research articles
Magnetic properties and magnetization reversal process in (Pt/CoFe/ MgO)10 multilayers at low temperature
T
Xiangqian Wanga,b, Yueyue Liua, YuRui Weia, Kaizhou Heb, Yi Huanga, Jianbo Wanga,c, ⁎ Qingfang Liua, , Genliang Hanb a
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, China Key Laboratory of Sensor and Sensor Technology, Institute of Sensor Technology, Gansu Academy of Sciences, Lanzhou 730000, China c Key Laboratory of Special Function Materials and Structure Design, Ministry of Education, Lanzhou University, Lanzhou 730000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: PMA Magnetization reversal FORC Low temperature
The magnetic properties and magnetization reversal processes in a (Pt/CoFe/MgO)10 film were studied as a function of temperature (100–300 K) using hysteresis loops and the first-order reversal curve (FORC) method. The coercivity, saturation magnetization, remanence and effective anisotropy field are enhanced as the temperature decreases. Also, as the temperature changes from 300 to 100 K, the distribution of the irreversible reversal magnetization field shifts from 100 to −20 Oe along the reversal field Hr axis. Two different sets of “hysterons” and interparticle magnetostatic interactions were clearly observed in the sample at 100 K.
1. Introduction In the last few decades, multilayers with perpendicular magnetic anisotropy (PMA), such as (Pt/CoFe/MgO)n, have been intensively investigated due to their applications in spin-transfer torque magnetic random access memory (STT-MRAM) and other magneto-electronic devices [1–4]. Because of the additional asymmetric exchange interaction at the interfaces, this kind of stack is commonly found in research focusing on perpendicular magnetic anisotropy, magnetic tunnel junction (MTJ) [5,6] and Dzyaloshinskii-Moriya interaction [7–9]. For example, Jinjun Qiu et al. [10] investigated the effect of roughness on PMA in (Co90Fe10/Pt)n superlattices. Shouzhong Peng et al. [11] found that interfacial PMA can be enhanced by inserting a thin Fe layer at the CoFe/W interface, allowing a sufficient thermal stability in small-node MTJs. Satoru Emori et al. [12] showed that the spin torque and domain wall (DW) dynamics in out-of-plane magnetized Pt/CoFe/MgO and Ta/ CoFe/MgO stacks have an opposite sign. J. F. Feng et al. [13] found that the PMA, coercivity and saturated magnetization also depend on the metallic and magnetic layer thickness in ultrathin (CoFe/Pt)n multilayers. In addition, magnetic multilayer films are good candidate to investigate the magnetization reversal process. It has been demonstrated that the first-order reversal curve (FORC) method is more accurate for characterizing the magnetization reversal of magnetic multilayer films [14–17]. Compared to major hysteresis loops, FORCs provide more information about the magnetization reversal, e.g. the switching field
⁎
distribution (SFD) and the degree of reversible and irreversible switching [18–20]. Furthermore, this method enables the study of coercive field and magnetostatic interaction distributions [21,22]. FORC distribution also provides information on the weight function of hysterons, or elements of hysteresis; and in certain systems, it is equivalent to the Preisach distribution. However, most studies about FORCs focus on the results at room temperature, there are only a few reports at low temperatures. Although the magnetic properties at low temperature have been reported in previous works, [23,24], the detail of magnetization reversal process of magnetic multilayers at low temperature is not entirely clear. The change of the domain nucleation field, annihilation field and domain wall motion of magnetic multilayers at low temperature are not easy to observe by other methods, e.g. magnetic force microscopy (MFM) and magneto-optical Kerr effect (MOKE) microscopy. (The MOKE is a useful method to study the dynamic variation process of magnetic domain, whether at room temperature or low temperature. But we found when the repeat number more than 7 in our samples, the magnetic domain cannot be observed by MOKE. It means that the method should apply only to relatively thin films.) However, it can be easily and clearly obtained through FORC distribution. The FORCs can be employed to investigate the switching field distribution, domain nucleation field, irreversible switching component and spin reorientation transition [20] of magnetic multilayer films at low temperature. Therefore, the low temperature FORCs is an important method in the research and industrial application of magnetic multilayer films. In this work we investigated the magnetic
Corresponding author.
https://doi.org/10.1016/j.jmmm.2019.166318 Received 17 October 2019; Received in revised form 4 December 2019; Accepted 17 December 2019 Available online 19 December 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.
Journal of Magnetism and Magnetic Materials 499 (2020) 166318
X. Wang, et al.
properties and magnetization reversal behavior of a (Pt/CoFe/MgO)10 film at low temperature by FORCs. We found that the PMA of the sample dramatically increases when the temperature decreases from 300 to 100 K. Also, the irreversible reversal process constantly shifts toward the negative reversal field (Hr) when the temperature decreases. Moreover, the horizontal ridges of irreversible reversal separate and the magnetostatic interaction between interparticles is enhanced at 100 K. 2. Materials and methods A series of Ta/[Pt(1 nm)/CoFe(0.8 nm)/MgO(2 nm)]10/Ta multilayer films were deposited on Si (1 0 0) substrates with a 0.5 µm-thick thermal-oxide using a magnetron sputtering system (base pressure 1 × 10−7 Torr). The Ta, CoFe and Pt layers were deposited by direct current (DC) magnetron sputtering, and the MgO layer by radio frequency sputtering. The Ar flow rate was 20 standard cubic centimeters per minute during sputtering and its pressure was 0.3 Pa during deposition. The magnetization reversal processes of the sample were characterized by the hysteresis loops and FORC by vibrating sample magnetometers (VSM, Lakeshore 8600). The FORC was measured by first saturating the sample at a large positive field, then decreasing the field to a reversal field Hr, and measuring the magnetization M as the applied field H is increased back to saturation. A subsequent FORC is measured after re-saturating the sample and then ramping the field to a lower reversal field Hr. As the reversal fields are changed, the interior of the major loop is filled with the FORCs [19]. The measurements were carried out with a cryostat (86-SSVT) at temperatures ranging from 100 to 300 K. Fig. 2. Temperature dependence of (a) Hc, (b) Ms, (c) Mr/Ms and (d) Keff.
3. Results and discussion using the following expression [10]: Fig. 1 displays a series of out-of-plane hysteresis loops (M-H) measured by VSM for the (Pt/CoFe/MgO)10 film at temperatures ranging from 100 to 300 K, the inset shows an enlargement of the −300 to 300 Oe region. All the curves exhibit a bowknot shape. The coercivity (Hc) decreases from 112 to 12 Oe when increasing the temperature from 100 to 300 K, as shown in Fig. 2(a). Fig. 2(b) shows the data extracted from the Ms vs T plot in the range from 100 to 300 K. Also, there is a remarkable increase in the saturation magnetization Ms with decreasing temperature. Additionally, the remanence Mr gradually increases with decreasing T. The reduced Mr/Ms of the loops are presented as a function of temperature T in Fig. 2(c). It can be observed that Mr/Ms drops from 0.92 (100 K) to 0.004 (300 K). The effective anisotropy Keff can be evaluated by measuring the area between the hard axis and easy axis magnetization curves for a given sample by
K eff =
∫0
H max
[M (OP ) − M (IP )] dH
which could be roughly estimated from Keff = MsHk/2, where Ms is the saturation magnetization and Hk is the saturation field obtained from the in-plane M-H curve. The Keff as a function of the temperature T is shown in Fig. 2(d). The maximum Keff of the sample was approximately 5.68 × 106erg/cc at 100 K. Table 1 lists all the measured magnetic parameters of the (Pt/CoFe/MgO)10 film at different temperature T. The magnetization reversal process of the (Pt/CoFe/MgO)10 film was quantitatively analyzed at various temperatures. For this purpose, out-of-plane FORC measurements were carried out to determine the distribution of the reversible and irreversible components of the process. A series of FORCs measured at 300 K are presented in Fig. 3(a). The FORC distribution is defined by a mixed second-order derivative [15]:
ρ (Hr , H ) = −
1 ∂2M (Hr , H )/ Ms 2 ∂Hr ∂H
where H is the applied magnetic field and Hr is the starting reversal field. The second order derivation eliminates the reversible magnetization process; thus, a plot of the FORC distribution ρ(Hr,H) can be used to probe the details of the irreversible magnetization reversal process Table 1 Magnetic parameters of the (Pt/CoFe/MgO)10 film at different temperatures.
Fig. 1. Major hysteresis loops for (Pt/CoFe/MgO)10 film at different temperatures, the inset shows an enlargement of the hysteresis loops. 2
T(K)
Hc(Oe)
Ms (emu/cc)
Mr/Ms
Keff (106erg/cc)
300 260 220 180 140 100
12 19 33 53 82 112
2524 2750 2870 3040 3210 3520
0.004 0.10 0.19 0.31 0.60 0.92
3.12 3.91 4.36 4.73 5.02 5.68
Journal of Magnetism and Magnetic Materials 499 (2020) 166318
X. Wang, et al.
Fig. 3. (a) A series of out-of-plane FORCs for the (Pt/CoFe/MgO)10 film at 300 K; (b) contour and (c) three-dimensional plots of the corresponding FORC distribution. The color scale in (b) represents the strength of ρ. (d) Cross-sectional view of the FORC diagrams along the Hr axes along the maximum distribution.
[15]. Any nonzero ρ corresponds to an irreversible component. The corresponding FORC distribution plotted as a function of H and Hr is shown in Fig. 3(b). Here, the reversal behaviors, such as domain nucleation, propagation, and annihilation process, are clearly defined by the contour maps of the FORC distributions. The white dashed lines in Fig. 3(b) indicate three obvious features in the contour plot. First, a horizontal ridge in region I (20 Oe < Hr < 180 Oe) indicating the irreversible domain propagation from already nucleated sites. Second, a shallow plateau with ρ ≈ 80 in region II (−80 Oe < Hr < 20 Oe) corresponds to the most reversible domain expansion-contraction that does not change the domain topography. Finally, a vertical valley-peak pair in region III (−320 Oe < Hr < −80 Oe) is assigned to an irreversible domain annihilation process. These three features respectively correspond to the steep, gradual and again gradual decrease in magnetization toward the positive saturation in the FORCs. Fig. 3(c) shows a three-dimensional plot from data in Fig. 3(b). The rise of the ridge corresponds to the nonzero FORC distribution, indicating the onset of irreversible processes. This change is clearly observed in the projection of the FORC distribution onto the Hr axis, which can be obtained by integrating ρ over H, as shown in Fig. 3(d). We also obtained the out-of-plane FORC diagrams of the (Pt/CoFe/ MgO)10 film at low temperatures. A series of FORCs and their corresponding FORC distributions at 260, 180 and 100 K are shown in Fig. 4. Compared to the distribution at 300 K in Fig. 3(b), the area of irreversible reversal magnetization (−80 Oe < Hr < 180 Oe) and domain annihilation (−480 Oe < Hr < −100 Oe) regions at 260 K (Fig. 4(d)) are slightly extended. When the temperature decreases to 180 K, Fig. 4(e), the irreversible reversal distribution significantly broadens along the H axis and continues to shift to the negative Hr axis (−100 Oe < Hr < 100 Oe). This trend is also reflected in the domain annihilation portion, depicted with a valley-peak pair. At 100 K, the shape of the irreversible reversal distribution elongates along the
positive H axis, which is consistent with the previous report [25]. In addition, the horizontal ridge splits into two parts, one of which is located around H = 50 Oe, and the other around H = −20 Oe. This can be explained by the classical Preisach model [26]: the two distinct peaks may be associated with two different sets of “hysterons” with their respective local coercive field Hc and interaction field Hu. The distance from one peak to another corresponds to the difference between the “hysterons.” When T goes from 180 to 100 K, the difference between the two sets of “hysterons” increases, as indicated by the increased distance. It is worth noting that when the temperature is 180 K, a negative peak (around Hr = 130 Oe) in the horizontal ridge appears, depicted by red circle A in Fig. 4(e). As the temperature decreases, the negative peak becomes more evident, and another negative peak rises at Hr = 10 Oe in Fig. 4(f). This phenomenon is associated with the magnetostatic interaction between interparticles [22], which is enhanced at low temperatures (100 K). The projection of the horizontal FORC ridge onto the Hr axis at different temperatures are displayed in Fig. 4(g-i). The distribution of the irreversible reversal field shifts from 100 to −20 Oe along the Hr axis when the temperature decreases from 260 to 100 K. It can be found the PMA more stronger, the Hr of irreversible reversal process more negative. In the (Pt/CoFe/MgO)10 multilayers, the Keff can be described by the relation: Keff = −2πMS2 + Ki/ teff, where 2πMS2 is the shape anisotropy, and Ki is the interface anisotropy, teff is the effective Co layer thickness. The interlayer coupling is one of origin of Ki. As the temperature falls, the interlayer coupling will be enhanced [27]. Therefore, the Keff of (Pt/CoFe/MgO)10 multilayers increased with a dropping temperature. Furthermore, at low temperature, the strong exchange interaction between magnetic moments and weakening thermal disturbance will lead to the magnetic moment reversal more difficult and need to lager reversal field Hr. Thus the irreversible reversal process shifts toward the negative reversal field with increasing PMA. Thus the irreversible reversal process shifts toward the negative reversal field with increasing PMA. 3
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Fig. 4. (a-c) Series of out-of-plane FORCs, (d-f) their corresponding FORC distributions and (g-i) their projection on the Hr axis at different temperatures (260–100 K). The color scale represents the ρ strength. The red circle in (e) and (f) indicate a negative peak. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
4. Conclusions
Supervision. Genliang Han: Funding acquisition.
In summary, we characterized the magnetic properties and magnetization reversal process of a (Pt/CoFe/MgO)10 film based on the hysteresis loops and FORC at temperatures ranging from 300 to 100 K. The PMA shows a strong temperature-dependence; the Keff increases from 3.12 to 5.68 erg/cc when the temperature decreases from 300 to 100 K. According the FORC diagrams, the irreversible reversal magnetization distribution constantly elongates along the positive H axis and shifts to negative Hr values with decreasing temperature. In addition, there are two areas with different coercive field and interaction field where the magnetostatic interaction between interparticles is enhanced at 100 K. The results of this work should be useful for the practical application of Pt/CoFe/MgO films in MTJs.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported in part by the National Natural Science Foundation of China (Nos. 51771086 and 11574121), in part by the Fund for Less Developed Regions of the National Natural Science Foundation of China (Nos. 51761001), in part by the Cooperation Project of Gansu Academy of Science under Grant (Nos. 2019HZ-04), and in part by the Applied Research and Development Project of Gansu Academy of Science under Grant (Nos. 2017JK-13).
CRediT authorship contribution statement Xiangqian Wang: Conceptualization, Formal analysis, Writing original draft. Yueyue Liu: Visualization. YuRui Wei: Data curation. Kaizhou He: Methodology. Yi Huang: Investigation. Jianbo Wang: Project administration. Qingfang Liu: Writing - review & editing,
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