- Email: [email protected]
Fe20Ni80 Films
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 82 (2016) 63 – 68
International Baltic Conference on Magnetism: Focus on Biomedical Aspects, IBCM 2015, 30 August – 3 September 2015, Kaliningrad, Russia
Exchange Coupling in NixMn100-x/Fe20Ni80 Films Vladimir Lepalovskij*, Andrey Svalov, Konstantin Balymov and Vladimir Vaskovskiy Ural Federal University, Lenin Av., 51, Ekaterinburg 620083, Russian Federation
Abstract The properties of NixMn100-x/Fe20Ni80 multilayer structures with exchange bias were systematically studied. The influence of composition of NixMn100-x pinning layers, annealing temperature on crystalline film structures, exchange bias field and coercive force of Fe20Ni80 pinned layers were comparatively analyzed. It was found that the multilayer films with Ni-Mn pinning layer containing some 30 wt.% Ni have the most suitable combination of exchange bias and magnetic hysteresis properties for their technical applications. © Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of IBCM 2015. Peer-review under responsibility of the organizing committee of IBCM 2015
Keywords: exchange bias, magnetic nanostructures, annealing
1. Introduction The multilayered film structures containing exchange coupled magnetic layers are among most actively investigated magnetic functional materials by [Giri et al. (2011)]. In many cases, the principal property of such materials is the presence of magnetic bias in the layers of certain type, so-called “pinned layers”, due to their exchange interaction with the layers of the different type, so-called “pinning layers”. The most common material of pinning layers is FeMn equiatomic alloy characterized by antiferromagnetic ordering [Nogués et al. (1999); Vas’kovskiy et al. (2015) and Savin et al. (2016)]. However, it has a significant shortcoming – a relatively low ordering temperature and, consequently, a relatively low blocking temperature (Tb) ~ 130 °C, above which the interlayer exchange coupling disappears. An alternative material may be Ni-Mn alloy, for which Tb reaches the ___________ *Corresponding author. Tel.: +7-343-261-75-28; fax: +7-261-68-23.
1875-3892 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of IBCM 2015 doi:10.1016/j.phpro.2016.05.012
64
Vladimir Lepalovskij et al. / Physics Procedia 82 (2016) 63 – 68
E-mail address: [email protected]
temperature of 400 °C [Lin et al. (1994)]. The conditions of the appearance of the exchange bias in the film structures with Ni-Mn pinning layer are the subject of active interest of the researches [Mao et all. (1996); Groudeva-Zotova et all. (2004); Teixeira et al. (2007) and Wienecke et al. (2015)]. Among other factors affecting the value of the exchange bias are chemical composition [Groudeva-Zotova et al. (2003)], deposition parameters [Dai et al. (2003)] and heat treatment conditions [Lai et al. (2001)]. In this work, we have prepared NixMn100-x/ Fe20Ni80 multilayered structures and comparatively analyzed the effectiveness of different control parameters for these factors. 2. Experimental methods The structures for the studies were Ta/(1)Fe20Ni80/NixMn100-x/(2)Fe20Ni80/Ta multilayered films prepared by magnetron sputtering using ATC Orion 8 by AJA International, Inc. In these film structures, the Ta 5 nm thick layers performed a protective function. The first Fe20Ni80 layer (1) of 5 nm thick played an additional role contributing to the formation of the crystallographic structure of the pinning NixMn100-x layer of 20 nm thick deposited onto the first permalloy layer. The top Fe20Ni80 layer (2) of 40 nm thickness played role of the pinned layer. This layer is very important in the present study because it plays role of the test layer and for this layer we only discuss its magnetic hysteresis properties. The multilayered film deposition was carried out at Ar pressure of 1.6 mTorr in the presence of magnetic in a plane field of 250 Oe. The radio frequency electrical bias was applied toward the glass substrates (Corning). Ta and Fe20Ni80 targets were used for preparation of the corresponding layers. Formation of NixMn100-x layers occurred in the course of co-sputtering of Ni and Mn targets. Their chemical compositions were controlled by the changes of the ratio between the deposition rates of these metals. Technologically it was achieved by regulation of the electrical power supplied for functioning of the respective magnetrons. The relationship between the composition of layers and technological parameters was previously defined by means of Nanohunter instrument using total reflection X-ray fluorescence spectroscopy. Annealing of the films was carried out in vacuum in the presence of a magnetic field of 250 Oe and the annealing time was one hour. The deposition rates were determined from the height of specially formed sharp steps in the films employing a stylus Dektak-150 profiler and were 0.39; 1.61; 0.62; 0.04÷0.55 Å/c for Ta, Fe20Ni80, Mn and Ni, respectively. The crystalline structure of the film was characterized by characterized by X-ray diffractometer of Philips X'PertPRO with CuKĮ radiation. Magnetic measurements were carried out with vibrational LakeShore magnetometer (VSM). 3. Results and discussion Fig. 1 shows the dependences of coercive force (Hc) and exchange bias field (He) on composition of NixMn100-x pinning layer. These characteristics describe a half-width and shift from the center of hysteresis loops along the magnetic field axis (see inset of Fig. 1) for (2)Fe20Ni80 layer. It can be seen that the magnetic bias effect occurs in the confined region of the compositions near x = 30 wt.%. According to [Spenato et al. (2001) and Pearson (1965)] in binary Ni-Mn alloy the antiferromagnetic ordering, providing a magnetic pinning, appears for the case of a face centered tetragonal crystalline structure (fct). Another possible modification of crystalline structure is a face centered cubic (fcc) but it does not possess magnetic ordering and cannot be responsible for exchange bias effect. In this regard, it could be assumed that non-
Fig. 1: The dependences of exchange bias field (1) and coercive force (2) of Fe20Ni80 pinned layer (2) on composition of
Vladimir Lepalovskij et al. / Physics Procedia 82 (2016) 63 – 68
monotonic mode of He(x) dependence correlates with corresponding changes in the crystal structure.
Fig. 2. X-ray diffraction patterns for Ta/(1)Fe20Ni80/NixMn100-x/ (2)Fe20Ni80/Ta with different concentrations of Ni in the pinning layer: 1 – 15; 2 – 26; 3 – 30; 4 – 38 wt.%.
Fig. 3. The dependences of exchange bias field (1) and coercive force (2) of (2)Fe20Ni80 pinned layer on annealing temperature of Ta/(1)Fe20Ni80/Ni30Mn70/(2)Fe20Ni80/Ta structure.
Fig. 2 shows the examples of diffraction patterns for selected structure in the considered composition range. The intensive peak near the angle of 2θ = 44.3° is identified as a diffraction peak corresponding to polycrystalline permalloy layer with the (111) texture. Furthermore, in the diffraction patterns there is another peak, the intensity of which varies depending on Ni-Mn layer composition. A corresponding analysis shows that precisely this peak may correspond to Ni-Mn system but in the case of fcc phase. It gives the basis to search for another source of an antiferromagnetiɫ ordering which can be attributed either to Fe-Mn or to Fe-NiMn compositions [Vas’kovskiy et al. (2015)]. These compositions can be formed during the preparation of multilayered films as a consequence of partial mixing of Fe-Ni and Ni-Mn layers. Annealing is an efficient way to change the properties of the film structures, in particular, properties of NiMn system [Lai et al. (2001)]. Fig. 3 shows the change of coercive force and exchange bias field on the annealing temperature for the films with Ni30Mn70 layers. Note that the annealing at each one of the indicated temperatures was carried out for the individual sample. It can be seen that, the dependence of exchange bias field on annealing temperature Ta is substantially non-monotonic that point out fairly complex structural transformations. Fig. 4 shows the diffraction patterns of the samples in the initial state and after annealing at 400 °C. They indicate that the heat treatment leads to a noticeable shift of Ni-Mn peak towards the larger diffraction angles. The new position can be associated with a tetragonal phase, which after the annealing appears to be a main
Fig. 4: X-ray diffraction data for Ta/(1)Fe20Ni80/Ni30Mn70/ (2)Fe20Ni80/Ta structure in the initial state (1) and after annealing at Ta = 400 °ɋ (2).
Fig. 5: Temperature dependences of exchange bias field (1) and coercive force (2) of (2)Fe20Ni80 pinned layer for Ta/(1)Fe20Ni80/ Ni30Mn70/(2)Fe20Ni80/Ta structure in the initial state.
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phase of the pinning layer and providing the maximum value of exchange bias. The reduction of exchange bias field after annealing at Ta > 400 °C can be explained by the changes in the composition of the layers as a result of thermally activated mutual diffusion. The above mentioned data refer to a samples type showing the greatest exchange bias in the initial state. Annealing of the films with a different composition of the pinning layer was also effective. The corresponding results for the films with x = 26 and 33 wt.% are shown in Table 1. It is important to mention the following facts. In all cases under consideration, the change of He as a result of annealing is non-monotonic. Maximum exchange bias was implemented in the interval of the Ta of 300 to 400 °C. The maximum value of He for the samples with different x were close to each other (30–35 Oe). This is not the same for the coercive force. Hc values corresponding to the maximum value of He, go down from 25 Oe for x = 26 wt.% to 6 Oe for x = 33 wt.%. This may reflect the increase in the structural homogeneity of layers of Ni-Mn with increasing Ni content. It also has practical significance for development of the functional materials with an exchange bias. Table 1. Exchange bias field He and coercive force Hc of (2)Fe20Ni80 pinned layer for Ta/(1)Fe20Ni80/NixMn100-x/(2)Fe20Ni80/Ta films with different composition of the pinning layer after annealing at various temperatures Ɍɚ. x (wt.%)Ni
Ta (°C)
26
400
30
33
He (Oe)
Hc (Oe)
20
5
2.5
300
35
25
3
55
20
16
4.6
300
7.5
5.4
400
32.5
16
500
24
56
20
9.7
7.5
300
31
6
400
4
39
500
0
76
Important properties of the pinning layer are temperature sensitivity of exchange bias field, and the value of the blocking temperature Tb (especially critical), above which the exchange bias effect disappears. Fig. 5 shows the temperature dependencies of He and Hc in the initial state, for the NixMn100-x/Fe20Ni80 multilayered structure with x = 30 wt.%. It is clear that for all temperature range under consideration the value of He changes quite dramatically and Tb appears to be as high as about 400 °C. In contrast, Hc varies just very slightly. This suggests that the temperature dependence of the exchange bias field, primarily related to a change in the efficiency of the magnetic ordering in the pinning layer. Moreover, the quantitative characteristics of the dependence of He(T) are typical for antiferromagnetic Fe-Mn [Vas’kovskiy et al. (2015) and Savin et al. (2014)]. It can be viewed as an indirect confirmation of the above proposed assumptions about formation of this phase in the course of the preparation of the samples.
Vladimir Lepalovskij et al. / Physics Procedia 82 (2016) 63 – 68
Fig. 6. The temperature dependences of exchange bias field (a) and coercive force (b) of (2)Fe20Ni80 pinned layer for annealing Ta/(1)Fe20Ni80/ NixMn100-x/(2)Fe20Ni80/Ta films with different Ni concentrations in the pinning layer: 1 – 26; 2 – 30; 3 – 33 wt.% Ni
Fig. 6 shows He(T) and Hc(T) dependences for the structures with different Ni concentrations of the pinning layer after annealing at the temperatures which insure the maximum value of the exchange bias. It is seen that annealing resulted in a number of significant modifications in the exchange interaction for Ta/(1) Fe20Ni80/Ni30Mn70/(2)Fe20Ni80/Ta structure. In addition, the exchange field has a high stability at low temperatures, increases almost twice at room temperature and gives a weak growth in the temperature interval of 300–400 K. The blocking temperature increases nearly 200 degrees and becomes above 600 K. All these features can be interpreted as a consequence of the formation of the tetragonal phase Ni-Mn with high Neel temperature of the pinning layer. Noteworthy that other structures with different x show almost identical He(T) dependences. The coercive force behaved in a different way. The Hc values and its general behavior with the change of the temperature depended on Ni concentration. In addition, it looks like at first glance, that Hc(T) dependences included a large element of contingency. At the same time, the comparison of the curves for different structures revealed certain correlation in the tendencies. From this observation we can conclude that the sign variation in the changes of Hc(T) may be due to the phase inhomogeneity of the pinning layer. In the other words, the pinning layer may contain antiferromagnetic phases, characterized by different structural and chemical compositions and having different blocking temperatures. However, for the better understanding of the matter further studies are necessary. It is also important that a low value of the coercive force remains constant in a wide temperature range down to room temperature for the sample with x = 33 wt.% Ni in the pinning layer. 4. Results and discussion In conclusion, the obtained results confirm the data previously reported in the literature on the possible use of the Ni-Mn as the material for pinning layers in the films with an exchange bias and clarify the effect of the chemical composition of the pinning layer, heat treatment and temperature on hysteretic properties of the pinned layer. In particular, it is noted that in the initial state and after heat treatments a variety of structural and chemical phases can be responsible for the exchange bias. Furthermore, it was shown, that pinning layers with Ni content of 30 wt.% had certain advantages from the point of view of applications. After heat treatments, they insure the high value and thermally stable exchange bias, and also they provide a relatively low coercive force of the pinned layer.
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Acknowledgements The equipment of the Ural Center for Shared Use “Modern nanotechnology” of the Ural Federal University was used. This work was supported by The Ministry of Education and Science of the Russian Federation, project ʋ 2582. References Giri, S., Patra, M., Majumdar S., 2011. Exchange bias effect in alloys and compounds. Journal of Physics: Condensed Matter 23, 073201. Nogués, J., Schuller, I.K., 1999. Exchange bias. Journal of Magnetism and Magnetic Materials 192, 203–232. Vas’kovskiy, V. O., Lepalovskij, V. N., Gor’kovenko, A. N., Kulesh, N. A., Savin, P. A., Svalov, A. V., Stepanova, E. A., Shchegoleva, N. N., Yuvchenko, A. A., 2015. Fe20Ni80/Fe50Mn50 film magnetoresistive medium. Technical Physics 60, 116–122. Savin, P. A., Guzmán, J., Lepalovskij, V. N., Svalov, A. V., Kurlyandskaya, G. V., Asenjo, A., Vas’kovskiy, V. O., Vazquez, M., 2016. Exchange bias in sputtered FeNi/FeMn systems: Effect of short low-temperature heat treatments. Journal of Magnetism and Magnetic Materials 402, 49–54. Lin, T., Mauri, D., Staud, N., Hwang, C., Howard, J.K., Gorman, G.L., 1994. Improved exchange coupling between ferromagnetic Ni-Fe and antiferromagnetic Ni-Mn-based films. Applied Physics Letters 65, 1183–1185. Mao, S., Gangopadhyay, S., Amin, N., Murdock, E., 1996. NiMn-pinned spin valves with high pinning field made by ion beam sputtering. Applied Physics Letters 69, 3593–3595. Groudeva-Zotova, S., Elefant, D., Kaltofen, R., Thomas, J., Schneider, C., 2004. NiMn/FeNi exchange biasing systems – magnetic and structural characteristics after short annealing close to the phase transition point of the AFM layer. Journal of Magnetism and Magnetic Materials 278, 379–391. Teixeira, J. M., Ventura, J., Negulescu, B., Araújo, J. P., Fermento, R., Sousa, J. B., Freitas, P. P., 2007. Temperature dependence of transport properties and exchange field of NiMn based spin valves. Journal of Magnetism and Magnetic Materials 316, e973–e976. Wienecke, A., Kruppe, R., Rissing, L., 2015. Influence of growth conditions on exchange bias of NiMn-based spin valves. Journal of Applied Physics 117, 17C108–4. Groudeva-Zotova, S., Elefan, D., Kaltofen, R., Tietjen, D., Thomas, J., Hoffmann, V., Schneider, C. M., 2003. Magnetic and structural characteristics of exchange biasing systems based on NiMn antiferromagnetic films. Journal of Magnetism and Magnetic Materials 263, 57–71. Dai, B., Cai, J. W., Lai, W. Y., 2003. Structural and magnetic properties of NiFe/NiMn bilayers with different seed and cap layers. Journal of Magnetism and Magnetic Materials 257, 190–194. Lai, C.-H., Lien, W. C., Chen, F. R., Kai, J. J., Mao, S., 2001. Effects of phase transformation and interdiffusion on the exchange bias of NiFe/NiMn. Journal of Applied Physics 89, 6600–6602. Pearson, W. B., 1965. Equiatomic transition metal alloys of manganese. III. The tetragonal NiMn phase. Acta Chemica Scandinavica 19, 477–484. Spenato, D., Ben, Y.J., Le Gall, H., Ostoréro, J., 2001. From ferromagnetic – ferromagnetic to ferromagnetic – antiferromagnetic exchange coupling in NiFe/MnNi bilayers. Journal of Applied Physics 89, 6898–6900. Vas’kovskiy, V. O., Adanakova, O. A., Gorkovenko, A. N., Lepalovskij, V. N., Svalov, A. V., Stepanova, E. A., 2015. The effect of temperature on the characteristics of the magnetization reversal of the ferromagnetic layers of 3d-metals as part of exchange-based structures FeMn. The Physics of Metals and Metallography 116, 1–7. Savin, P. A., Lepalovskiij, V. N., Svalov, A. V., Vas’kovskiy, V. O., Kurlyandskaya, G. V., 2014. Effect of phase separation in an Fe20Ni80/Fe50Mn50 structure with exchange coupling. The Physics of Metals and Metallography 115, 856–863.
ScienceDirect Physics Procedia 82 (2016) 63 – 68
International Baltic Conference on Magnetism: Focus on Biomedical Aspects, IBCM 2015, 30 August – 3 September 2015, Kaliningrad, Russia
Exchange Coupling in NixMn100-x/Fe20Ni80 Films Vladimir Lepalovskij*, Andrey Svalov, Konstantin Balymov and Vladimir Vaskovskiy Ural Federal University, Lenin Av., 51, Ekaterinburg 620083, Russian Federation
Abstract The properties of NixMn100-x/Fe20Ni80 multilayer structures with exchange bias were systematically studied. The influence of composition of NixMn100-x pinning layers, annealing temperature on crystalline film structures, exchange bias field and coercive force of Fe20Ni80 pinned layers were comparatively analyzed. It was found that the multilayer films with Ni-Mn pinning layer containing some 30 wt.% Ni have the most suitable combination of exchange bias and magnetic hysteresis properties for their technical applications. © Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license © 2016 2016The TheAuthors. Authors. Published by Elsevier B.V. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of IBCM 2015. Peer-review under responsibility of the organizing committee of IBCM 2015
Keywords: exchange bias, magnetic nanostructures, annealing
1. Introduction The multilayered film structures containing exchange coupled magnetic layers are among most actively investigated magnetic functional materials by [Giri et al. (2011)]. In many cases, the principal property of such materials is the presence of magnetic bias in the layers of certain type, so-called “pinned layers”, due to their exchange interaction with the layers of the different type, so-called “pinning layers”. The most common material of pinning layers is FeMn equiatomic alloy characterized by antiferromagnetic ordering [Nogués et al. (1999); Vas’kovskiy et al. (2015) and Savin et al. (2016)]. However, it has a significant shortcoming – a relatively low ordering temperature and, consequently, a relatively low blocking temperature (Tb) ~ 130 °C, above which the interlayer exchange coupling disappears. An alternative material may be Ni-Mn alloy, for which Tb reaches the ___________ *Corresponding author. Tel.: +7-343-261-75-28; fax: +7-261-68-23.
1875-3892 © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the organizing committee of IBCM 2015 doi:10.1016/j.phpro.2016.05.012
64
Vladimir Lepalovskij et al. / Physics Procedia 82 (2016) 63 – 68
E-mail address: [email protected]
temperature of 400 °C [Lin et al. (1994)]. The conditions of the appearance of the exchange bias in the film structures with Ni-Mn pinning layer are the subject of active interest of the researches [Mao et all. (1996); Groudeva-Zotova et all. (2004); Teixeira et al. (2007) and Wienecke et al. (2015)]. Among other factors affecting the value of the exchange bias are chemical composition [Groudeva-Zotova et al. (2003)], deposition parameters [Dai et al. (2003)] and heat treatment conditions [Lai et al. (2001)]. In this work, we have prepared NixMn100-x/ Fe20Ni80 multilayered structures and comparatively analyzed the effectiveness of different control parameters for these factors. 2. Experimental methods The structures for the studies were Ta/(1)Fe20Ni80/NixMn100-x/(2)Fe20Ni80/Ta multilayered films prepared by magnetron sputtering using ATC Orion 8 by AJA International, Inc. In these film structures, the Ta 5 nm thick layers performed a protective function. The first Fe20Ni80 layer (1) of 5 nm thick played an additional role contributing to the formation of the crystallographic structure of the pinning NixMn100-x layer of 20 nm thick deposited onto the first permalloy layer. The top Fe20Ni80 layer (2) of 40 nm thickness played role of the pinned layer. This layer is very important in the present study because it plays role of the test layer and for this layer we only discuss its magnetic hysteresis properties. The multilayered film deposition was carried out at Ar pressure of 1.6 mTorr in the presence of magnetic in a plane field of 250 Oe. The radio frequency electrical bias was applied toward the glass substrates (Corning). Ta and Fe20Ni80 targets were used for preparation of the corresponding layers. Formation of NixMn100-x layers occurred in the course of co-sputtering of Ni and Mn targets. Their chemical compositions were controlled by the changes of the ratio between the deposition rates of these metals. Technologically it was achieved by regulation of the electrical power supplied for functioning of the respective magnetrons. The relationship between the composition of layers and technological parameters was previously defined by means of Nanohunter instrument using total reflection X-ray fluorescence spectroscopy. Annealing of the films was carried out in vacuum in the presence of a magnetic field of 250 Oe and the annealing time was one hour. The deposition rates were determined from the height of specially formed sharp steps in the films employing a stylus Dektak-150 profiler and were 0.39; 1.61; 0.62; 0.04÷0.55 Å/c for Ta, Fe20Ni80, Mn and Ni, respectively. The crystalline structure of the film was characterized by characterized by X-ray diffractometer of Philips X'PertPRO with CuKĮ radiation. Magnetic measurements were carried out with vibrational LakeShore magnetometer (VSM). 3. Results and discussion Fig. 1 shows the dependences of coercive force (Hc) and exchange bias field (He) on composition of NixMn100-x pinning layer. These characteristics describe a half-width and shift from the center of hysteresis loops along the magnetic field axis (see inset of Fig. 1) for (2)Fe20Ni80 layer. It can be seen that the magnetic bias effect occurs in the confined region of the compositions near x = 30 wt.%. According to [Spenato et al. (2001) and Pearson (1965)] in binary Ni-Mn alloy the antiferromagnetic ordering, providing a magnetic pinning, appears for the case of a face centered tetragonal crystalline structure (fct). Another possible modification of crystalline structure is a face centered cubic (fcc) but it does not possess magnetic ordering and cannot be responsible for exchange bias effect. In this regard, it could be assumed that non-
Fig. 1: The dependences of exchange bias field (1) and coercive force (2) of Fe20Ni80 pinned layer (2) on composition of
Vladimir Lepalovskij et al. / Physics Procedia 82 (2016) 63 – 68
monotonic mode of He(x) dependence correlates with corresponding changes in the crystal structure.
Fig. 2. X-ray diffraction patterns for Ta/(1)Fe20Ni80/NixMn100-x/ (2)Fe20Ni80/Ta with different concentrations of Ni in the pinning layer: 1 – 15; 2 – 26; 3 – 30; 4 – 38 wt.%.
Fig. 3. The dependences of exchange bias field (1) and coercive force (2) of (2)Fe20Ni80 pinned layer on annealing temperature of Ta/(1)Fe20Ni80/Ni30Mn70/(2)Fe20Ni80/Ta structure.
Fig. 2 shows the examples of diffraction patterns for selected structure in the considered composition range. The intensive peak near the angle of 2θ = 44.3° is identified as a diffraction peak corresponding to polycrystalline permalloy layer with the (111) texture. Furthermore, in the diffraction patterns there is another peak, the intensity of which varies depending on Ni-Mn layer composition. A corresponding analysis shows that precisely this peak may correspond to Ni-Mn system but in the case of fcc phase. It gives the basis to search for another source of an antiferromagnetiɫ ordering which can be attributed either to Fe-Mn or to Fe-NiMn compositions [Vas’kovskiy et al. (2015)]. These compositions can be formed during the preparation of multilayered films as a consequence of partial mixing of Fe-Ni and Ni-Mn layers. Annealing is an efficient way to change the properties of the film structures, in particular, properties of NiMn system [Lai et al. (2001)]. Fig. 3 shows the change of coercive force and exchange bias field on the annealing temperature for the films with Ni30Mn70 layers. Note that the annealing at each one of the indicated temperatures was carried out for the individual sample. It can be seen that, the dependence of exchange bias field on annealing temperature Ta is substantially non-monotonic that point out fairly complex structural transformations. Fig. 4 shows the diffraction patterns of the samples in the initial state and after annealing at 400 °C. They indicate that the heat treatment leads to a noticeable shift of Ni-Mn peak towards the larger diffraction angles. The new position can be associated with a tetragonal phase, which after the annealing appears to be a main
Fig. 4: X-ray diffraction data for Ta/(1)Fe20Ni80/Ni30Mn70/ (2)Fe20Ni80/Ta structure in the initial state (1) and after annealing at Ta = 400 °ɋ (2).
Fig. 5: Temperature dependences of exchange bias field (1) and coercive force (2) of (2)Fe20Ni80 pinned layer for Ta/(1)Fe20Ni80/ Ni30Mn70/(2)Fe20Ni80/Ta structure in the initial state.
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Vladimir Lepalovskij et al. / Physics Procedia 82 (2016) 63 – 68
phase of the pinning layer and providing the maximum value of exchange bias. The reduction of exchange bias field after annealing at Ta > 400 °C can be explained by the changes in the composition of the layers as a result of thermally activated mutual diffusion. The above mentioned data refer to a samples type showing the greatest exchange bias in the initial state. Annealing of the films with a different composition of the pinning layer was also effective. The corresponding results for the films with x = 26 and 33 wt.% are shown in Table 1. It is important to mention the following facts. In all cases under consideration, the change of He as a result of annealing is non-monotonic. Maximum exchange bias was implemented in the interval of the Ta of 300 to 400 °C. The maximum value of He for the samples with different x were close to each other (30–35 Oe). This is not the same for the coercive force. Hc values corresponding to the maximum value of He, go down from 25 Oe for x = 26 wt.% to 6 Oe for x = 33 wt.%. This may reflect the increase in the structural homogeneity of layers of Ni-Mn with increasing Ni content. It also has practical significance for development of the functional materials with an exchange bias. Table 1. Exchange bias field He and coercive force Hc of (2)Fe20Ni80 pinned layer for Ta/(1)Fe20Ni80/NixMn100-x/(2)Fe20Ni80/Ta films with different composition of the pinning layer after annealing at various temperatures Ɍɚ. x (wt.%)Ni
Ta (°C)
26
400
30
33
He (Oe)
Hc (Oe)
20
5
2.5
300
35
25
3
55
20
16
4.6
300
7.5
5.4
400
32.5
16
500
24
56
20
9.7
7.5
300
31
6
400
4
39
500
0
76
Important properties of the pinning layer are temperature sensitivity of exchange bias field, and the value of the blocking temperature Tb (especially critical), above which the exchange bias effect disappears. Fig. 5 shows the temperature dependencies of He and Hc in the initial state, for the NixMn100-x/Fe20Ni80 multilayered structure with x = 30 wt.%. It is clear that for all temperature range under consideration the value of He changes quite dramatically and Tb appears to be as high as about 400 °C. In contrast, Hc varies just very slightly. This suggests that the temperature dependence of the exchange bias field, primarily related to a change in the efficiency of the magnetic ordering in the pinning layer. Moreover, the quantitative characteristics of the dependence of He(T) are typical for antiferromagnetic Fe-Mn [Vas’kovskiy et al. (2015) and Savin et al. (2014)]. It can be viewed as an indirect confirmation of the above proposed assumptions about formation of this phase in the course of the preparation of the samples.
Vladimir Lepalovskij et al. / Physics Procedia 82 (2016) 63 – 68
Fig. 6. The temperature dependences of exchange bias field (a) and coercive force (b) of (2)Fe20Ni80 pinned layer for annealing Ta/(1)Fe20Ni80/ NixMn100-x/(2)Fe20Ni80/Ta films with different Ni concentrations in the pinning layer: 1 – 26; 2 – 30; 3 – 33 wt.% Ni
Fig. 6 shows He(T) and Hc(T) dependences for the structures with different Ni concentrations of the pinning layer after annealing at the temperatures which insure the maximum value of the exchange bias. It is seen that annealing resulted in a number of significant modifications in the exchange interaction for Ta/(1) Fe20Ni80/Ni30Mn70/(2)Fe20Ni80/Ta structure. In addition, the exchange field has a high stability at low temperatures, increases almost twice at room temperature and gives a weak growth in the temperature interval of 300–400 K. The blocking temperature increases nearly 200 degrees and becomes above 600 K. All these features can be interpreted as a consequence of the formation of the tetragonal phase Ni-Mn with high Neel temperature of the pinning layer. Noteworthy that other structures with different x show almost identical He(T) dependences. The coercive force behaved in a different way. The Hc values and its general behavior with the change of the temperature depended on Ni concentration. In addition, it looks like at first glance, that Hc(T) dependences included a large element of contingency. At the same time, the comparison of the curves for different structures revealed certain correlation in the tendencies. From this observation we can conclude that the sign variation in the changes of Hc(T) may be due to the phase inhomogeneity of the pinning layer. In the other words, the pinning layer may contain antiferromagnetic phases, characterized by different structural and chemical compositions and having different blocking temperatures. However, for the better understanding of the matter further studies are necessary. It is also important that a low value of the coercive force remains constant in a wide temperature range down to room temperature for the sample with x = 33 wt.% Ni in the pinning layer. 4. Results and discussion In conclusion, the obtained results confirm the data previously reported in the literature on the possible use of the Ni-Mn as the material for pinning layers in the films with an exchange bias and clarify the effect of the chemical composition of the pinning layer, heat treatment and temperature on hysteretic properties of the pinned layer. In particular, it is noted that in the initial state and after heat treatments a variety of structural and chemical phases can be responsible for the exchange bias. Furthermore, it was shown, that pinning layers with Ni content of 30 wt.% had certain advantages from the point of view of applications. After heat treatments, they insure the high value and thermally stable exchange bias, and also they provide a relatively low coercive force of the pinned layer.
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