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Regulation of electrical and magnetic properties in amorphous CoFeTaBO films
Thin Solid Films 669 (2019) 114–119
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Regulation of electrical and magnetic properties in amorphous CoFeTaBO films
T
⁎
Zhengang Guoa, , Chengyue Xiongb, Zhaochu Luob, Xiaozhong Zhangb a b
School of Materials Science and Engineering, Tianjin Chengjian University, Tianjin 300384, China School of Materials Science and Engineering, Beijing National Center for Electron Microscopy, Tsinghua University, Beijing 100084, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CoFeTaBO Amorphous Resistivity Magnetic properties
The electrical and magnetic properties of amorphous CoFeTaBO material were modulated and investigated by controlling the oxygen content of thin films. The resistivity of the prepared CoFeTaBO films increases with the increase of oxygen content, which can be regulated from 10−3 Ω·cm to insulator. It exhibits good semiconductor properties when the oxygen content is < 43%, and there are mainly two different conduction mechanisms in CoFeTaBO materials, the variable range hopping model in low temperature and the long range interaction between electrons at a relatively high temperature. Meanwhile, the CoFeTaBO films maintain the intrinsic ferromagnetism, exhibiting a Curie temperature higher than 400 K and a coercive field of 40–80 Oe, and they are magnetized to saturation at a lower field in comparison with the oxygen-free CoFeTaB material, which is very meaningful to the application of magnetic semiconductors. The regulation of electrical and magnetic behaviors for ferromagnetic amorphous CoFeTaBO films provides a new way to design and fabricate the novel magnetic semiconductor with desirable properties, probably extending their applications in a wide variety of electronic devices.
1. Introduction A combination of magnetic and electrical properties makes magnetic semiconductor having a very extensive application prospect in the future of electronics. Through controlling both the spin and charge of electrons, magnetic semiconductors can realize the information processing, storage and transport, which provides a new conducting mode and concept device [1–4]. The application of magnetic semiconductor could promote the development of spin valve, spin field effect transistor (SFET), spin light emitting diode (SLED), nonvolatile magnetoresistive memories, and quantum computing transistor [5,6], which will reduce the energy consumption greatly, increase the integration density and improve the speed of data operation. At present, the research of magnetic semiconductor is mainly about diluted magnetic semiconductor, in which the magnetism is obtained by adding magnetic elements in traditional semiconductor materials. However, almost all diluted magnetic semiconductors prepared by this way have a shortage: the Curie temperature is far lower than room temperature, which makes them failing to meet the actual requirement of applications. It has been demonstrated that the Curie temperature of diluted magnetic semiconductors can be enhanced with the increase of the doping content of magnetic element, but it is very difficult to achieve a significant
⁎
increase of Curie temperature in such a way because there is an upper limit to the solid solubility of the added element in the crystal structure of material, and the upper limit is often small. Consequently, it is necessary to explore a new way to increase Curie temperature of magnetic semiconductors and develop room temperature magnetic semiconductor materials, which is always the hot area and breakthrough goal in this field. As we know, ferromagnetic metals exhibit intrinsic magnetism at room temperature, and the magnetic anisotropy can be modified by the external applied electric field, which could be used in memory devices [7,8]. It is very meaningful to control and regulate the electrical and magnetic properties of ferromagnetic materials during the fabrication processing. In a variety of regulatory means, it is a powerful means to modulate the oxygen ions in ferromagnetic materials, and then induce the phase transformations resulting in the evolution of electrical and magnetic properties. For instance, the modulation of oxygen content can be achieved through different heat treatment methods, making LuFe2O4+x material exhibit a switching magnetic behavior from ferromagnetic state to nonmagnetic state [9]. The electric field controlled ionic liquid gating is also an effective method to modulate the electrical and magnetic properties of materials through the insertion and extraction of O2– and H+ ions [10,11], which produces different phases with distinguished electrical and magnetic
Corresponding author. E-mail address: [email protected] (Z. Guo).
https://doi.org/10.1016/j.tsf.2018.10.034 Received 28 June 2018; Received in revised form 29 September 2018; Accepted 23 October 2018 Available online 24 October 2018 0040-6090/ © 2018 Elsevier B.V. All rights reserved.
Thin Solid Films 669 (2019) 114–119
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Table 1 The element concentration obtained by XPS and corresponding resistivity of CoFeTaBO films fabricated under different oxygen flow rate, as well as the thickness (Tf) and coercivity (HC) of each sample. Sample Number
Co (at.%)
Fe (at.%)
Ta (at.%)
B (at.%)
O (at.%)
ρ (Ω·cm)
ln(ρ)
Tf (nm)
HC (Oe)
0 1 2 3 4 5 6 7
35.20 29.47 30.32 25.60 30.89 21.87 22.06 18.99
22.92 20.68 18.45 17.67 14.42 16.22 16.13 10.12
14.42 12.61 10.21 8.41 6.91 7.89 8.71 6.10
17.98 21.42 22.06 24.62 18.90 21.37 16.54 21.76
9.47 15.82 18.96 23.70 26.88 32.65 36.56 43.06
0.0027 0.0225 0.054 0.53 2.48 14.94 440 2110
−5.91 −3.79 −2.92 −0.64 0.91 2.70 6.09 7.65
428 369 381 364 375 376 332 308
35 40 46 55 65 80 80 84
also regulated by adjusting the flow rate of oxygen. The electrical properties of CoFeTaBO samples were measured by classical fourterminal method at room temperature, in which Keithley 2400 was used as the current source, and Keithley 2182 was used to measure the voltage. According to the acquired IeV curves, the resistances of CoFeTaBO films were obtained, and the resistivity of the corresponding sample was calculated using the measured geometrical parameters. The magnetic properties of CoFeTaBO films were measured by Physical Property Measurement System (PPMS) of Quantum Design. The composition of each element in CoFeTaBO films was measured by X-ray photoelectron spectroscopy (XPS), using a focused monochromatized Al-Kα radiation (hv = 1486.6 eV). The spot of the irradiated area was about 600 μm and the residual pressure in the chamber was 1–2 × 10−8 Pa. Before the measurement, the surface of samples was plasma cleaned to avoid the contamination.
properties in SrCoOx compounds by controlling the content of oxygen and hydrogen, including ferromagnetic insulator, ferromagnetic metal and antiferromagnetic insulator [12]. Recently, a novel magnetic semiconductor, amorphous (CoFeTaB)100–xOx system, has been reported, which exhibits the electric-field-control of ferromagnetism at room temperature, and the modulation of electrical and magnetic properties by adjusting the oxidization states of the containing elements [13–15]. Amorphous material is lack of the long range order in atomic arrangement, and without the restriction of crystalline structure, therefore it is feasible and accessible to add the oxygen element into the metallic glass to form the amorphous oxide, coupling with different combinations of electrical and magnetic properties. These studies showed that the oxygen content plays a important role in modulating and determining the electrical and magnetic properties of materials. In this paper, we prepared CoFeTaBO thin films from a bulk metallic glass CoFeTaB by using RF (radio-frequency) magnetron sputtering, and investigated their electrical and magnetic behaviors detailedly. The material exhibits an amorphous structure composed of ultra-fine grains, as we have previously reported in reference [16]. Here, the oxygen content of CoFeTaBO films was controlled by modulating the oxygen flow rate in the magnetron sputtering, and the modulation of electrical and magnetic properties were realized successfully. It is found that the resistivity varies in a large range with the change of the oxygen content, and the samples always hold an intrinsic ferromagnetism above room temperature, which would open a promising way for the research and potential application of magnetic semiconductors.
3. Results and discussion 3.1. Regulation of electrical properties In order to determine the as-prepared CoFeTaBO films and study the effect of oxygen content on the electrical and magnetic properties of thin films, the compositions of the as-prepared CoFeTaBO films with different oxygen content were analyzed by X-ray photoelectron spectroscopy (XPS), and summarized in Table 1. It shows that there is a remarkable increase of oxygen content with the increase of O2 flow rate in CoFeTaBO films, from 9.47% when there is no oxygen introduced to 43.06% at 2.5 sccm of O2 flow rate. It should be noted that there is about 9% of oxygen content observed despite no oxygen flow introduced in the sputtering for sample 0. That may not be due to the surface oxidation in the air because the surface of samples was plasma cleaned to remove the contamination. There are two possible reasons for the increase in oxygen content. For one thing, there is some inevitable residual oxygen in the chamber at room temperature, which will be released into the chamber after the sputtering system is turned on because the temperature of the system will increase gradually; For another, the metal atoms sputtered from the target will combine with a small amount of oxygen to form an oxide and deposit on the substrate, which results in the same oxygen content in the surface and interior of samples, as illustrated in Ref. [13]. Additionally, the relative content of metal elements, including Co, Fe and Ta, decreases with the increase of oxygen content, while the boron content doesn't seem to change much, keeping around 20% for all samples. The electrical properties of the as-prepared CoFeTaBO films, mainly resistivity, were studied by using the IeV curves. The film thickness and continuity have important influence on the film resistivity. For a continuous film, the resistivity is a constant when the thickness is large enough. Fig. 1 shows the sectional SEM image of as-prepared CoFeTaBO film with 15.82% oxygen content. It can be seen that the CoFeTaBO film is in a well continuous state, and the thickness of the film is about 370 nm. In this case, the resistivity of the sample can be considered as constant as the bulk material. The thickness of the different samples
2. Experimental CoFeTaBO films with different oxygen content were deposited on SiO2/Si substrate by RF magnetron sputtering from a ferromagnetic Co32Fe14Ta7B21 alloy target. For better film quality, the SiO2/Si substrate (500 nm thickness thermal oxide SiO2 on n-type Si substrate) was cleaned ultrasonically by acetone and then by ethanol to remove organic pollutants, and rinsed by deionized water to remove the hydrosoluble impurities. The deposition was performed using a sputtering system with the cylindrical chamber, which has the diameter of 450 mm and the height of 350 mm. The diameter of the sputtering target is 60 mm and the thickness is 3 mm. The distance between the target and the substrate is controlled at 70 mm. For the homogeneity of sample structure, the substrate rotates constantly at the speed of 10 rpm during the deposition. The films were fabricated at room temperature and the high-purity (99.99%) argon was used as the main sputtering gas. The oxygen was injected into the chamber to grow CoFeTaBO films during the deposition. By adjusting the flow rate of oxygen, CoFeTaBO films with different composition were prepared, in which the value of oxygen flow rate was chosen at 0, 1.1sccm, 1.2sccm, 1.5sccm, 1.7sccm, 2.0sccm, 2.3sccm and 2.5sccm, respectively. The base pressure of the sputtering system is better than 9.0 × 10−5 Pa, and the working pressure was set as 0.5 Pa during the deposition. The sputtering power of 150 W was fixed and applied to the target. The electrical and magnetic properties of CoFeTaBO materials were 115
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Fig. 1. The sectional SEM image of CoFeTaBO film with oxygen content of 15.82%. The inset shows the high-resolution TEM image of the sample. Fig. 3. The temperature dependence of resistivity of CoFeTaBO film with 15.82% oxygen content.
studied in this paper is between 300 and 450 nm, all of which are listed in Table 1. The inset of Fig. 1 shows the high-resolution TEM image of the corresponding sample, indicating that the microstructure of the film is comprised of ultra-fine grains in amorphous matrix. The calculated resistivity (ρ) and ln(ρ) of each sample are listed in Table 1. It is obvious that the resistivity of CoFeTaBO films is increased with the increase of oxygen content. In the IeV measurements, all the curves have a linear relationship between the current and voltage, indicating the good ohmic contact between the thin films and electrodes, which benefits from the treated electrodes made by pressing the metal indium at the contacts. In our experiments, with the increase of oxygen content, the resistivity of films increases from 2.7 × 10−3 Ω·cm to 2.11 × 103 Ω·cm. When continuing to increase the oxygen content of CoFeTaBO films, the resistivity will increase dramatically and the films become insulators. Therefore, the resistivity of CoFeTaBO films can be controlled and regulated in a large range by adjusting the oxygen content in the sample, making the films varies from conductor to insulator. In particular, the samples we have prepared at present are all examples of semiconductors in terms of resistivity, which commonly covers the range of 10−3–1012 Ω·cm. Fig. 2 shows the evolution of the resistivity of CoFeTaBO films with the increasing oxygen flow rate, which can be divided into three regions. In the first, the resistivity of the samples increases slowly when the oxygen flow rate is < 2.0 sccm, and the slope of the curve is about 7.47 Ω·cm/sccm; in the second stage, between 2.0 and 2.5 sccm of the oxygen flow rate, the resistivity increases rapidly by two orders of magnitude, and the slope of the curve is about 4190 Ω·cm/sccm; when the oxygen flow rate is > 2.5 sccm, the oxygen content of the CoFeTaBO films will exceed 43%, making the film a insulator. During the first stage, the oxygen content increases by 23.18%, while the content of Co, Fe, Ta decreases by 13.33%, 6.7% and 6.53% respectively. In the
second stage, the oxygen content continued to increase by 10.41%, roughly equaling to the reduction in metal element contents. The amount of boron element have not substantially changed with the increasing oxygen flow rate in the sputtering. These results indicate that the electrical properties of CoFeTaBO films depend mainly on the change in relative content of the metal and oxygen elements. For the different regions of resistivity, the temperature dependence of sample resistivity was measured to investigate the conduction mechanism of CoFeTaBO films with different oxygen content. In the low resistivity region, the film with 15.82% oxygen content (samples 1) was chosen to measure the resistivity-temperature curves, as shown in Fig. 3. The resistivity of the thin film is reduced gradually with the increase of temperature, which is the characteristic of typical semiconductor resistivity. This indicates that the introduction of oxygen element leads to a metal-semiconductor transition in CoFeTaBO films. Similar phenomena happened in other samples of the first region. For the sample 1, the resistivity is about 10−2 Ω·cm at room temperature, which is two orders of magnitude higher than that of ordinary metallic glass [17], belonging to the semiconductor resistivity range. As we know, based on the Davis-Mott Model, the extended resistivity of amorphous semiconductor conforms to the following relation:
lnρ ∝ 1/ T
(1)
However, the conductive mechanism described by formula (1) is not suitable for the CoFeTaBO films in this study, as shown in Fig. 4(a). The resistivity is analyzed carefully and it is believed that the material has different conduction mechanisms at different temperature ranges. Amorphous semiconductor can be considered as the Anderson insulator in low temperature because of no long-range order in the crystal structure, in which the localized electrons at the Fermi surface can only jump between the neighbouring localized states. In this case, the conductive mechanism of electrons becomes the variable range hopping model and can be fitted by the relation [18]:
lnρ ∝ T −1/4
(2)
As for sample 1 of CoFeTaBO films, the resistivity below 10 K can be better described by the above formula, as shown in Fig. 4(b), which is consistent with the research report on non-crystalline materials by assigning a metal-insulator transition at the temperature < 20 K driven by Anderson localization [19,20]. Above 10 K, the resistivity of the thin film shows a very well linear decreasing relationship with the temperature, as shown in Fig. 4(c), likely associated with the effect of long range electron-electron interaction on the density of states. With the increase of oxygen content in the sample, the resistivity of CoFeTaBO films has a significant increase. For example, the resistivity of sample 5, corresponding to 2.0 sccm of oxygen flow rate, is about 15 Ω·cm, three orders of magnitude higher than that of sample 1. Fig. 5
Fig. 2. The resistivity of thin films versus oxygen flow rate used in the samples deposition. 116
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Fig. 6. The magnetic hysteresis of CoFeTaBO film with 15.82% oxygen content at room temperature. The inset shows the enlarged detail of the curve, and the coercive field of 40 Oe is determined.
3.2. Regulation of magnetic properties On the condition that implementing the metal-semiconductor transition successfully by introducing oxygen into the CoFeTaB material, CoFeTaBO films still retains the intrinsic magnetism. Fig. 6 shows the magnetic hysteresis of the CoFeTaBO films with 15.82% oxygen content (sample 1) measured at room temperature. The sample is ferromagnetic at room temperature, reaching saturation state under the magnetic field of 2 T with a coercivity of 40 Oe. Fig. 7 shows the temperature dependence of magnetization for sample 1. There is no Curie transition from ferromagnetic to paramagnetic observed below 400 K, indicating that the Curie temperature of the sample is higher than 400 K. In addition, the divergence between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curve is observed at low temperature (below 45 K), which reveals that the sample shows a spin glass behavior at low temperature. This phenomenon is completely different from the unoxidized metallic glass target CoFeTaB [22], which probably should be attributed to the ultrafine-grained amorphous structure of the sample. The inset of Fig. 7 shows the relationship of magnetization versus temperature fitted by Bloch's law of ferromagnetic metals, conforming to the relation of ΔM = [M(0)-M(T)]∝T3/2, which also indicates the CoFeTaBO films is ferromagnetic at room temperature. In contrast, the magnetic properties of CoFeTaB target were measured and illustrated in Fig. 8. Apparently, CoFeTaB material is ferromagnetic at
Fig. 4. The different fitting relationship of electrical transport properties for CoFeTaBO film with 15.82% oxygen content: (a) ln(ρ/ρ0)~T−1; (b) ln(ρ/ρ0) ~T−1/4 below 10 K; (c) ρ/ρ0~T1/2 above 10 K.
Fig. 5. The electrical properties of CoFeTaBO thin film with 32.65% oxygen content. The temperature dependence of the resistivity can be fitted by two different mechanisms, as shown in the inset, lnρ/ρ0 versus T−1/4 and ρ/ρ0 versus T1/2 respectively.
shows the temperature dependence of the resistivity of CoFeTaBO film with 32.65% oxygen content (sample 5). It is seen that the resistivity of the film increases dramatically at the low temperature with the decrease of temperature, and below 100 K, the logarithm of resistivity is proportional to T−1/4, as shown in the inset of Fig. 5, suggesting that the variable range hopping of electrons normally observed in magnetic semiconductor predominates in a large low temperature range [18,21]; above 100 K, the resistivity of thin film still shows a nearly linearly relationship with T1/2, indicating the long range interaction between electrons plays a dominant role in the relatively high temperature area.
Fig. 7. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of CoFeTaBO film with 15.82% oxygen content measured under an in-plane magnetic field of 10 mT. The inset shows the FC curve fitted by Bloch's law of ferromagnetic metals. 117
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0.5 T, and the coercive field is about 80 Oe, as shown in the inset of Fig. 9. The in-plane remanence accounts for 50% of saturation magnetization of the thin film. The temperature dependence of magnetization also shows a obvious split between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curve at low temperature of below 60 K, corresponding to a spin glass behavior. All the results indicate the introduction of oxygen increases the phase transition temperature of the systems from ferromagnetic to paramagnetic state, which is higher than 400 K. As a magnetic semiconductor, the high Curie temperature has the potential advantages and practical implication for the application of CoFeTaBO materials. 4. Conclusion In conclusion, the oxygen element introduced to CoFeTaBO thin films has a distinctive modulation effect on the electrical and magnetic properties of the samples prepared by magnetron sputtering. It is found that the resistivity and magnetic behavior of CoFeTaBO films can be manipulated by adjusting the relative content of metal and oxygen elements. The resistivity of CoFeTaBO films increases with the increase of oxygen content, covering a large range from 10−3 Ω·cm to insulators, in which CoFeTaBO films can be regarded as a ferromagnetic semiconductor when the oxygen content is < 43%. The electrical properties of films show that there are mainly two different conduction mechanisms in CoFeTaBO materials, the variable range hopping model in low temperature and the long range interaction between electrons at a relatively high temperature. In the meantime, CoFeTaBO films retain the intrinsic ferromagnetism. Compared with the CoFeTaB material, the Curie temperature of CoFeTaBO films increases obviously probably because of the structural relaxation and nucleation of nanocrystals, and they are magnetized to saturation under a smaller magnetic field, all of which indicate CoFeTaBO materials have a great potential to be a room temperature magnetic semiconductor. Therefore, it is feasible to prepare the ferromagnetic CoFeTaBO films with the specific electrical properties through modulating the content of oxygen, and balancing the composition of metal elements in the target, and that would be very valuable for the development and application of magnetic semiconductor and related devices.
Fig. 8. The magnetic properties of CoFeTaB metallic glass material. (a) The enlarged drawing of magnetic hysteresis indicates a coercive field of 35 Oe. (b) The temperature dependence of magnetization and derivative of magnetization, which shows a ferromagnetic-paramagnetic transition at about 274 K.
Fig. 9. The magnetic properties of CoFeTaBO film with 32.65% oxygen content. The ZFC and FC curves measured at 10 mT indicate the material is ferromagnetic and exhibits a spin glass behavior at low temperature. The inset shows the magnetic hysteresis of thin film at room temperature.
Acknowledgements
room temperature, and the coercive field is about 35 Oe, which indicates that the CoFeTaBO films inherit the magnetism of the target. The inset (b) of Fig. 8 shows the temperature dependence of magnetization and its derivative of CoFeTaB target. Note that the magnetization decreases with the increase of temperature, and it shows a ferromagnetic-paramagnetic transition at or near room temperature, which is absent in CoFeTaBO films. This indicates that the magnetic behavior of CoFeTaBO films can be tuned by introducing and adjusting the oxygen flow rate during magnetron sputtering, and the transition of magnetism shifts to higher temperature observably. This is probably related to the preparation process, as reported in the literature [13], which probably originates from the structural relaxation and nucleation of crystals before apparent crystal growth occurring in the amorphous CoFeTaBO thin films. In the literature, above 705 K, the magnetization of the sample starts to increase due to such crystallization, consistent with early findings that nanocrystallized CoFeMB (M = Ta, Hf) thin film has a higher Curie temperature than the corresponding amorphous CoFeMB sample. Therefore, the nanocrystallization phenomena of CoFeTaBO films occurred in the magnetron sputtering, makes the films have a higher Curie temperature. Fig. 9 shows the magnetic properties of CoFeTaBO film with 32.65% oxygen content (sample 5), including the temperature dependence of magnetization and hysteresis at room temperature. It is found that the magnetization of CoFeTaBO film decreases when the oxygen content increases, which is magnetized to saturation at a low magnetic field of
This work was funded by Tianjin Education Committee Scientific Research Project (Grant No.: 2016CJ06). The authors also would like to acknowledge the financial support from the research start-up fund of Tianjin Chengjian University. References [1] H. Ohno, D. Chiba, F. Matsukura, T. Omiya, E. Abe, T. Dietl, Y. Ohno, K. Ohtani, Electric-field control of ferromagnetism, Nature 408 (2000) 944–946. [2] D.D. Awschalom, M.E. Flatté, Challenges for semiconductor spintronics, Nat. Phys. 3 (2007) 153–159. [3] T. Dietl, A ten-year perspective on dilute magnetic semiconductors and oxides, Nat. Mater. 9 (2010) 965. [4] Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, H. Koinuma, Room-temperature ferromagnetism in transparent transition metal-doped titanium dioxide, Science 291 (2001) 854–856. [5] W. Yu, Z. Zhu, C.Y. Niu, C. Li, J.H. Cho, Y. Jia, Dilute magnetic semiconductor and half-metal behaviors in 3d transition-metal doped black and blue phosphorenes: a first-principles study, Nanoscale Res. Lett. 11 (2016) 77. [6] Y. Ohno, D. Young, F. Beschoten, H. Matsukura, D. Awschalom Ohno, Electrical spin injection in a ferromagnetic semiconductor heterostructure, Nature 402 (1999) 790. [7] M. Weisheit, S. Fähler, A. Marty, Y. Souche, C. Poinsignon, D. Givord, Electric fieldinduced modification of magnetism in thin-film ferromagnets, Science 315 (2007) 349–351. [8] T. Maruyama, Y. Shiota, T. Nozaki, K. Ohta, N. Toda, M. Mizuguchi, A. Tulapurkar, T. Shinjo, M. Shiraishi, S. Mizukami, Large voltage-induced magnetic anisotropy change in a few atomic layers of iron, Nat. Nanotechnol. 4 (2009) 158. [9] M. Hervieu, A. Guesdon, J. Bourgeois, E. Elkaïm, M. Poienar, F. Damay,
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Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Regulation of electrical and magnetic properties in amorphous CoFeTaBO films
T
⁎
Zhengang Guoa, , Chengyue Xiongb, Zhaochu Luob, Xiaozhong Zhangb a b
School of Materials Science and Engineering, Tianjin Chengjian University, Tianjin 300384, China School of Materials Science and Engineering, Beijing National Center for Electron Microscopy, Tsinghua University, Beijing 100084, China
A R T I C LE I N FO
A B S T R A C T
Keywords: CoFeTaBO Amorphous Resistivity Magnetic properties
The electrical and magnetic properties of amorphous CoFeTaBO material were modulated and investigated by controlling the oxygen content of thin films. The resistivity of the prepared CoFeTaBO films increases with the increase of oxygen content, which can be regulated from 10−3 Ω·cm to insulator. It exhibits good semiconductor properties when the oxygen content is < 43%, and there are mainly two different conduction mechanisms in CoFeTaBO materials, the variable range hopping model in low temperature and the long range interaction between electrons at a relatively high temperature. Meanwhile, the CoFeTaBO films maintain the intrinsic ferromagnetism, exhibiting a Curie temperature higher than 400 K and a coercive field of 40–80 Oe, and they are magnetized to saturation at a lower field in comparison with the oxygen-free CoFeTaB material, which is very meaningful to the application of magnetic semiconductors. The regulation of electrical and magnetic behaviors for ferromagnetic amorphous CoFeTaBO films provides a new way to design and fabricate the novel magnetic semiconductor with desirable properties, probably extending their applications in a wide variety of electronic devices.
1. Introduction A combination of magnetic and electrical properties makes magnetic semiconductor having a very extensive application prospect in the future of electronics. Through controlling both the spin and charge of electrons, magnetic semiconductors can realize the information processing, storage and transport, which provides a new conducting mode and concept device [1–4]. The application of magnetic semiconductor could promote the development of spin valve, spin field effect transistor (SFET), spin light emitting diode (SLED), nonvolatile magnetoresistive memories, and quantum computing transistor [5,6], which will reduce the energy consumption greatly, increase the integration density and improve the speed of data operation. At present, the research of magnetic semiconductor is mainly about diluted magnetic semiconductor, in which the magnetism is obtained by adding magnetic elements in traditional semiconductor materials. However, almost all diluted magnetic semiconductors prepared by this way have a shortage: the Curie temperature is far lower than room temperature, which makes them failing to meet the actual requirement of applications. It has been demonstrated that the Curie temperature of diluted magnetic semiconductors can be enhanced with the increase of the doping content of magnetic element, but it is very difficult to achieve a significant
⁎
increase of Curie temperature in such a way because there is an upper limit to the solid solubility of the added element in the crystal structure of material, and the upper limit is often small. Consequently, it is necessary to explore a new way to increase Curie temperature of magnetic semiconductors and develop room temperature magnetic semiconductor materials, which is always the hot area and breakthrough goal in this field. As we know, ferromagnetic metals exhibit intrinsic magnetism at room temperature, and the magnetic anisotropy can be modified by the external applied electric field, which could be used in memory devices [7,8]. It is very meaningful to control and regulate the electrical and magnetic properties of ferromagnetic materials during the fabrication processing. In a variety of regulatory means, it is a powerful means to modulate the oxygen ions in ferromagnetic materials, and then induce the phase transformations resulting in the evolution of electrical and magnetic properties. For instance, the modulation of oxygen content can be achieved through different heat treatment methods, making LuFe2O4+x material exhibit a switching magnetic behavior from ferromagnetic state to nonmagnetic state [9]. The electric field controlled ionic liquid gating is also an effective method to modulate the electrical and magnetic properties of materials through the insertion and extraction of O2– and H+ ions [10,11], which produces different phases with distinguished electrical and magnetic
Corresponding author. E-mail address: [email protected] (Z. Guo).
https://doi.org/10.1016/j.tsf.2018.10.034 Received 28 June 2018; Received in revised form 29 September 2018; Accepted 23 October 2018 Available online 24 October 2018 0040-6090/ © 2018 Elsevier B.V. All rights reserved.
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Table 1 The element concentration obtained by XPS and corresponding resistivity of CoFeTaBO films fabricated under different oxygen flow rate, as well as the thickness (Tf) and coercivity (HC) of each sample. Sample Number
Co (at.%)
Fe (at.%)
Ta (at.%)
B (at.%)
O (at.%)
ρ (Ω·cm)
ln(ρ)
Tf (nm)
HC (Oe)
0 1 2 3 4 5 6 7
35.20 29.47 30.32 25.60 30.89 21.87 22.06 18.99
22.92 20.68 18.45 17.67 14.42 16.22 16.13 10.12
14.42 12.61 10.21 8.41 6.91 7.89 8.71 6.10
17.98 21.42 22.06 24.62 18.90 21.37 16.54 21.76
9.47 15.82 18.96 23.70 26.88 32.65 36.56 43.06
0.0027 0.0225 0.054 0.53 2.48 14.94 440 2110
−5.91 −3.79 −2.92 −0.64 0.91 2.70 6.09 7.65
428 369 381 364 375 376 332 308
35 40 46 55 65 80 80 84
also regulated by adjusting the flow rate of oxygen. The electrical properties of CoFeTaBO samples were measured by classical fourterminal method at room temperature, in which Keithley 2400 was used as the current source, and Keithley 2182 was used to measure the voltage. According to the acquired IeV curves, the resistances of CoFeTaBO films were obtained, and the resistivity of the corresponding sample was calculated using the measured geometrical parameters. The magnetic properties of CoFeTaBO films were measured by Physical Property Measurement System (PPMS) of Quantum Design. The composition of each element in CoFeTaBO films was measured by X-ray photoelectron spectroscopy (XPS), using a focused monochromatized Al-Kα radiation (hv = 1486.6 eV). The spot of the irradiated area was about 600 μm and the residual pressure in the chamber was 1–2 × 10−8 Pa. Before the measurement, the surface of samples was plasma cleaned to avoid the contamination.
properties in SrCoOx compounds by controlling the content of oxygen and hydrogen, including ferromagnetic insulator, ferromagnetic metal and antiferromagnetic insulator [12]. Recently, a novel magnetic semiconductor, amorphous (CoFeTaB)100–xOx system, has been reported, which exhibits the electric-field-control of ferromagnetism at room temperature, and the modulation of electrical and magnetic properties by adjusting the oxidization states of the containing elements [13–15]. Amorphous material is lack of the long range order in atomic arrangement, and without the restriction of crystalline structure, therefore it is feasible and accessible to add the oxygen element into the metallic glass to form the amorphous oxide, coupling with different combinations of electrical and magnetic properties. These studies showed that the oxygen content plays a important role in modulating and determining the electrical and magnetic properties of materials. In this paper, we prepared CoFeTaBO thin films from a bulk metallic glass CoFeTaB by using RF (radio-frequency) magnetron sputtering, and investigated their electrical and magnetic behaviors detailedly. The material exhibits an amorphous structure composed of ultra-fine grains, as we have previously reported in reference [16]. Here, the oxygen content of CoFeTaBO films was controlled by modulating the oxygen flow rate in the magnetron sputtering, and the modulation of electrical and magnetic properties were realized successfully. It is found that the resistivity varies in a large range with the change of the oxygen content, and the samples always hold an intrinsic ferromagnetism above room temperature, which would open a promising way for the research and potential application of magnetic semiconductors.
3. Results and discussion 3.1. Regulation of electrical properties In order to determine the as-prepared CoFeTaBO films and study the effect of oxygen content on the electrical and magnetic properties of thin films, the compositions of the as-prepared CoFeTaBO films with different oxygen content were analyzed by X-ray photoelectron spectroscopy (XPS), and summarized in Table 1. It shows that there is a remarkable increase of oxygen content with the increase of O2 flow rate in CoFeTaBO films, from 9.47% when there is no oxygen introduced to 43.06% at 2.5 sccm of O2 flow rate. It should be noted that there is about 9% of oxygen content observed despite no oxygen flow introduced in the sputtering for sample 0. That may not be due to the surface oxidation in the air because the surface of samples was plasma cleaned to remove the contamination. There are two possible reasons for the increase in oxygen content. For one thing, there is some inevitable residual oxygen in the chamber at room temperature, which will be released into the chamber after the sputtering system is turned on because the temperature of the system will increase gradually; For another, the metal atoms sputtered from the target will combine with a small amount of oxygen to form an oxide and deposit on the substrate, which results in the same oxygen content in the surface and interior of samples, as illustrated in Ref. [13]. Additionally, the relative content of metal elements, including Co, Fe and Ta, decreases with the increase of oxygen content, while the boron content doesn't seem to change much, keeping around 20% for all samples. The electrical properties of the as-prepared CoFeTaBO films, mainly resistivity, were studied by using the IeV curves. The film thickness and continuity have important influence on the film resistivity. For a continuous film, the resistivity is a constant when the thickness is large enough. Fig. 1 shows the sectional SEM image of as-prepared CoFeTaBO film with 15.82% oxygen content. It can be seen that the CoFeTaBO film is in a well continuous state, and the thickness of the film is about 370 nm. In this case, the resistivity of the sample can be considered as constant as the bulk material. The thickness of the different samples
2. Experimental CoFeTaBO films with different oxygen content were deposited on SiO2/Si substrate by RF magnetron sputtering from a ferromagnetic Co32Fe14Ta7B21 alloy target. For better film quality, the SiO2/Si substrate (500 nm thickness thermal oxide SiO2 on n-type Si substrate) was cleaned ultrasonically by acetone and then by ethanol to remove organic pollutants, and rinsed by deionized water to remove the hydrosoluble impurities. The deposition was performed using a sputtering system with the cylindrical chamber, which has the diameter of 450 mm and the height of 350 mm. The diameter of the sputtering target is 60 mm and the thickness is 3 mm. The distance between the target and the substrate is controlled at 70 mm. For the homogeneity of sample structure, the substrate rotates constantly at the speed of 10 rpm during the deposition. The films were fabricated at room temperature and the high-purity (99.99%) argon was used as the main sputtering gas. The oxygen was injected into the chamber to grow CoFeTaBO films during the deposition. By adjusting the flow rate of oxygen, CoFeTaBO films with different composition were prepared, in which the value of oxygen flow rate was chosen at 0, 1.1sccm, 1.2sccm, 1.5sccm, 1.7sccm, 2.0sccm, 2.3sccm and 2.5sccm, respectively. The base pressure of the sputtering system is better than 9.0 × 10−5 Pa, and the working pressure was set as 0.5 Pa during the deposition. The sputtering power of 150 W was fixed and applied to the target. The electrical and magnetic properties of CoFeTaBO materials were 115
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Fig. 1. The sectional SEM image of CoFeTaBO film with oxygen content of 15.82%. The inset shows the high-resolution TEM image of the sample. Fig. 3. The temperature dependence of resistivity of CoFeTaBO film with 15.82% oxygen content.
studied in this paper is between 300 and 450 nm, all of which are listed in Table 1. The inset of Fig. 1 shows the high-resolution TEM image of the corresponding sample, indicating that the microstructure of the film is comprised of ultra-fine grains in amorphous matrix. The calculated resistivity (ρ) and ln(ρ) of each sample are listed in Table 1. It is obvious that the resistivity of CoFeTaBO films is increased with the increase of oxygen content. In the IeV measurements, all the curves have a linear relationship between the current and voltage, indicating the good ohmic contact between the thin films and electrodes, which benefits from the treated electrodes made by pressing the metal indium at the contacts. In our experiments, with the increase of oxygen content, the resistivity of films increases from 2.7 × 10−3 Ω·cm to 2.11 × 103 Ω·cm. When continuing to increase the oxygen content of CoFeTaBO films, the resistivity will increase dramatically and the films become insulators. Therefore, the resistivity of CoFeTaBO films can be controlled and regulated in a large range by adjusting the oxygen content in the sample, making the films varies from conductor to insulator. In particular, the samples we have prepared at present are all examples of semiconductors in terms of resistivity, which commonly covers the range of 10−3–1012 Ω·cm. Fig. 2 shows the evolution of the resistivity of CoFeTaBO films with the increasing oxygen flow rate, which can be divided into three regions. In the first, the resistivity of the samples increases slowly when the oxygen flow rate is < 2.0 sccm, and the slope of the curve is about 7.47 Ω·cm/sccm; in the second stage, between 2.0 and 2.5 sccm of the oxygen flow rate, the resistivity increases rapidly by two orders of magnitude, and the slope of the curve is about 4190 Ω·cm/sccm; when the oxygen flow rate is > 2.5 sccm, the oxygen content of the CoFeTaBO films will exceed 43%, making the film a insulator. During the first stage, the oxygen content increases by 23.18%, while the content of Co, Fe, Ta decreases by 13.33%, 6.7% and 6.53% respectively. In the
second stage, the oxygen content continued to increase by 10.41%, roughly equaling to the reduction in metal element contents. The amount of boron element have not substantially changed with the increasing oxygen flow rate in the sputtering. These results indicate that the electrical properties of CoFeTaBO films depend mainly on the change in relative content of the metal and oxygen elements. For the different regions of resistivity, the temperature dependence of sample resistivity was measured to investigate the conduction mechanism of CoFeTaBO films with different oxygen content. In the low resistivity region, the film with 15.82% oxygen content (samples 1) was chosen to measure the resistivity-temperature curves, as shown in Fig. 3. The resistivity of the thin film is reduced gradually with the increase of temperature, which is the characteristic of typical semiconductor resistivity. This indicates that the introduction of oxygen element leads to a metal-semiconductor transition in CoFeTaBO films. Similar phenomena happened in other samples of the first region. For the sample 1, the resistivity is about 10−2 Ω·cm at room temperature, which is two orders of magnitude higher than that of ordinary metallic glass [17], belonging to the semiconductor resistivity range. As we know, based on the Davis-Mott Model, the extended resistivity of amorphous semiconductor conforms to the following relation:
lnρ ∝ 1/ T
(1)
However, the conductive mechanism described by formula (1) is not suitable for the CoFeTaBO films in this study, as shown in Fig. 4(a). The resistivity is analyzed carefully and it is believed that the material has different conduction mechanisms at different temperature ranges. Amorphous semiconductor can be considered as the Anderson insulator in low temperature because of no long-range order in the crystal structure, in which the localized electrons at the Fermi surface can only jump between the neighbouring localized states. In this case, the conductive mechanism of electrons becomes the variable range hopping model and can be fitted by the relation [18]:
lnρ ∝ T −1/4
(2)
As for sample 1 of CoFeTaBO films, the resistivity below 10 K can be better described by the above formula, as shown in Fig. 4(b), which is consistent with the research report on non-crystalline materials by assigning a metal-insulator transition at the temperature < 20 K driven by Anderson localization [19,20]. Above 10 K, the resistivity of the thin film shows a very well linear decreasing relationship with the temperature, as shown in Fig. 4(c), likely associated with the effect of long range electron-electron interaction on the density of states. With the increase of oxygen content in the sample, the resistivity of CoFeTaBO films has a significant increase. For example, the resistivity of sample 5, corresponding to 2.0 sccm of oxygen flow rate, is about 15 Ω·cm, three orders of magnitude higher than that of sample 1. Fig. 5
Fig. 2. The resistivity of thin films versus oxygen flow rate used in the samples deposition. 116
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Fig. 6. The magnetic hysteresis of CoFeTaBO film with 15.82% oxygen content at room temperature. The inset shows the enlarged detail of the curve, and the coercive field of 40 Oe is determined.
3.2. Regulation of magnetic properties On the condition that implementing the metal-semiconductor transition successfully by introducing oxygen into the CoFeTaB material, CoFeTaBO films still retains the intrinsic magnetism. Fig. 6 shows the magnetic hysteresis of the CoFeTaBO films with 15.82% oxygen content (sample 1) measured at room temperature. The sample is ferromagnetic at room temperature, reaching saturation state under the magnetic field of 2 T with a coercivity of 40 Oe. Fig. 7 shows the temperature dependence of magnetization for sample 1. There is no Curie transition from ferromagnetic to paramagnetic observed below 400 K, indicating that the Curie temperature of the sample is higher than 400 K. In addition, the divergence between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curve is observed at low temperature (below 45 K), which reveals that the sample shows a spin glass behavior at low temperature. This phenomenon is completely different from the unoxidized metallic glass target CoFeTaB [22], which probably should be attributed to the ultrafine-grained amorphous structure of the sample. The inset of Fig. 7 shows the relationship of magnetization versus temperature fitted by Bloch's law of ferromagnetic metals, conforming to the relation of ΔM = [M(0)-M(T)]∝T3/2, which also indicates the CoFeTaBO films is ferromagnetic at room temperature. In contrast, the magnetic properties of CoFeTaB target were measured and illustrated in Fig. 8. Apparently, CoFeTaB material is ferromagnetic at
Fig. 4. The different fitting relationship of electrical transport properties for CoFeTaBO film with 15.82% oxygen content: (a) ln(ρ/ρ0)~T−1; (b) ln(ρ/ρ0) ~T−1/4 below 10 K; (c) ρ/ρ0~T1/2 above 10 K.
Fig. 5. The electrical properties of CoFeTaBO thin film with 32.65% oxygen content. The temperature dependence of the resistivity can be fitted by two different mechanisms, as shown in the inset, lnρ/ρ0 versus T−1/4 and ρ/ρ0 versus T1/2 respectively.
shows the temperature dependence of the resistivity of CoFeTaBO film with 32.65% oxygen content (sample 5). It is seen that the resistivity of the film increases dramatically at the low temperature with the decrease of temperature, and below 100 K, the logarithm of resistivity is proportional to T−1/4, as shown in the inset of Fig. 5, suggesting that the variable range hopping of electrons normally observed in magnetic semiconductor predominates in a large low temperature range [18,21]; above 100 K, the resistivity of thin film still shows a nearly linearly relationship with T1/2, indicating the long range interaction between electrons plays a dominant role in the relatively high temperature area.
Fig. 7. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of CoFeTaBO film with 15.82% oxygen content measured under an in-plane magnetic field of 10 mT. The inset shows the FC curve fitted by Bloch's law of ferromagnetic metals. 117
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0.5 T, and the coercive field is about 80 Oe, as shown in the inset of Fig. 9. The in-plane remanence accounts for 50% of saturation magnetization of the thin film. The temperature dependence of magnetization also shows a obvious split between the zero-field-cooled (ZFC) and field-cooled (FC) magnetization curve at low temperature of below 60 K, corresponding to a spin glass behavior. All the results indicate the introduction of oxygen increases the phase transition temperature of the systems from ferromagnetic to paramagnetic state, which is higher than 400 K. As a magnetic semiconductor, the high Curie temperature has the potential advantages and practical implication for the application of CoFeTaBO materials. 4. Conclusion In conclusion, the oxygen element introduced to CoFeTaBO thin films has a distinctive modulation effect on the electrical and magnetic properties of the samples prepared by magnetron sputtering. It is found that the resistivity and magnetic behavior of CoFeTaBO films can be manipulated by adjusting the relative content of metal and oxygen elements. The resistivity of CoFeTaBO films increases with the increase of oxygen content, covering a large range from 10−3 Ω·cm to insulators, in which CoFeTaBO films can be regarded as a ferromagnetic semiconductor when the oxygen content is < 43%. The electrical properties of films show that there are mainly two different conduction mechanisms in CoFeTaBO materials, the variable range hopping model in low temperature and the long range interaction between electrons at a relatively high temperature. In the meantime, CoFeTaBO films retain the intrinsic ferromagnetism. Compared with the CoFeTaB material, the Curie temperature of CoFeTaBO films increases obviously probably because of the structural relaxation and nucleation of nanocrystals, and they are magnetized to saturation under a smaller magnetic field, all of which indicate CoFeTaBO materials have a great potential to be a room temperature magnetic semiconductor. Therefore, it is feasible to prepare the ferromagnetic CoFeTaBO films with the specific electrical properties through modulating the content of oxygen, and balancing the composition of metal elements in the target, and that would be very valuable for the development and application of magnetic semiconductor and related devices.
Fig. 8. The magnetic properties of CoFeTaB metallic glass material. (a) The enlarged drawing of magnetic hysteresis indicates a coercive field of 35 Oe. (b) The temperature dependence of magnetization and derivative of magnetization, which shows a ferromagnetic-paramagnetic transition at about 274 K.
Fig. 9. The magnetic properties of CoFeTaBO film with 32.65% oxygen content. The ZFC and FC curves measured at 10 mT indicate the material is ferromagnetic and exhibits a spin glass behavior at low temperature. The inset shows the magnetic hysteresis of thin film at room temperature.
Acknowledgements
room temperature, and the coercive field is about 35 Oe, which indicates that the CoFeTaBO films inherit the magnetism of the target. The inset (b) of Fig. 8 shows the temperature dependence of magnetization and its derivative of CoFeTaB target. Note that the magnetization decreases with the increase of temperature, and it shows a ferromagnetic-paramagnetic transition at or near room temperature, which is absent in CoFeTaBO films. This indicates that the magnetic behavior of CoFeTaBO films can be tuned by introducing and adjusting the oxygen flow rate during magnetron sputtering, and the transition of magnetism shifts to higher temperature observably. This is probably related to the preparation process, as reported in the literature [13], which probably originates from the structural relaxation and nucleation of crystals before apparent crystal growth occurring in the amorphous CoFeTaBO thin films. In the literature, above 705 K, the magnetization of the sample starts to increase due to such crystallization, consistent with early findings that nanocrystallized CoFeMB (M = Ta, Hf) thin film has a higher Curie temperature than the corresponding amorphous CoFeMB sample. Therefore, the nanocrystallization phenomena of CoFeTaBO films occurred in the magnetron sputtering, makes the films have a higher Curie temperature. Fig. 9 shows the magnetic properties of CoFeTaBO film with 32.65% oxygen content (sample 5), including the temperature dependence of magnetization and hysteresis at room temperature. It is found that the magnetization of CoFeTaBO film decreases when the oxygen content increases, which is magnetized to saturation at a low magnetic field of
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