Magnetization of Co-Fe-Ta-B-O Amorphous Thin Films

Chengyue Xiong(熊成悦)$^{1,2,3}$, Cheng Chen(陈澄)$^{4}$, Wen Sun(孙雯)$^{5}$, Ziyao Lu(卢子尧)$^{1,2,3}$, \\ Hongming Mou(牟鸿铭)$^{1,2,3}$, Xiaozhong Zhang(章晓中)$^{1,2,3\hyperlink{s}{**}}$

doi:10.1088/0256-307X/36/7/077502

⇦Chinese Physics Letters, 2019, Vol. 36, No. 7, Article code 077502⇨ Magnetization of Co-Fe-Ta-B-O Amorphous Thin Films * Chengyue Xiong (熊成悦)1,2,3, Cheng Chen (陈澄)4, Wen Sun (孙雯)5, Ziyao Lu (卢子尧)1,2,3, Hongming Mou (牟鸿铭)1,2,3, Xiaozhong Zhang (章晓中)1,2,3** Affiliations 1Key Laboratory of Advanced Materials (MOE), School of Materials Science and Engineering, Tsinghua University, Beijing 100084 2Beijing National Center for Electron Microscopy, Tsinghua University, Beijing 100084 3Centre for Brain-Inspired Computing Research, Tsinghua University, Beijing 100084 4Research Institute for Magnetoelectronics & Weak Magnetic-field Detection, College of Science, China Three Gorges of University, Yichang 443002 5Institute of Microelectronics, Tsinghua University, Beijing 100084 Received 20 March 2019, online 20 June 2019 ^*Supported by the National Key Research and Development Program of China under Grant No 2017YFA0206202, and the National Natural Science Foundation of China under Grant Nos 51471093 and 116741901.
^**Corresponding author. Email: xzzhang@tsinghua.edu.cn Citation Text: Xiong C Y, Chen C, Sun W, Lu Z Y and Mou H M et al 2019 Chin. Phys. Lett. 36 077502 Abstract An amorphous magnetic material system (Co$_{20}$Fe$_{47}$Ta$_{20}$B$_{13})_{1-x}$O$_{x}$ is fabricated by magneto sputtering. Three stages of magnetization behavior exist when oxygen content changes in the system. As the oxygen increases, the absence of percolation effect of magnetic nano-particles makes the multi-domain structure broken so that high coercivity appears in the samples with proper oxygen content. A temperature-dependent Stoner–Wohlfarth model is used to explain the magnetization properties at relatively high temperature. Magnetizations with magnetic field in and out of the sample plane are also investigated to prove the mechanisms. This work provides a systematic study of a new kind ofv amorphous magnetic system and is helpful for us to know more about this type of material. DOI:10.1088/0256-307X/36/7/077502 PACS:75.20.-g, 75.50.Lk, 75.60.-d, 64.60.A- © 2019 Chinese Physics Society Article Text Amorphous magnetic materials exhibit special properties in both structure and function. Therefore, many promising applications were proposed in various fields such as surface coating materials, sensors and memories.[1–4] Among them, the Co- and Fe-based amorphous magnetic material family has been focused on because of its better amorphous forming ability.[5,6] Especially, the quaternary alloy Co-Fe-Ta-B was reported with very high Young's modulus and fracture strength.[7] Furthermore, this material has extraordinary soft magnetic properties and unusual magnetic behaviors like spin reorientation transition.[8,9] Recently, some researchers found that introducing oxygen into this amorphous material could bring numerous new electrical, magnetic, and optical properties.[10] Liu et al. found the effect of electric field control of ferromagnetism at room temperature and the p-type semiconducting character, which may pave a new way to realize high Curie temperature magnetic semiconductors.[11] Sun et al. investigated the magnetic properties by analyzing the chemical environment of magnetic elements, showed that the magnetism conversion results from the decrease of ferromagnetic Fe$^{3+}$ and the increase of paramagnetic Co$^{2+}$ in the grown films.[12] All these rise the scientific and technological interests in this kind of materials and prompt investigation of their physical bases, especially magnetic properties. In this Letter, we systematically investigate magnetization properties in (Co$_{20}$Fe$_{47}$Ta$_{20}$B$_{13})_{1-x}$O$_{x}$ systems. The magnetic properties are quite sensitive to the oxygen content of the samples. Three typical types of magnetization exist in three different oxygen ranges. A large increase of magnetic coercivity at temperature 2 K is found to be more than 20 times larger than the samples with slight oxygen content. The result may be caused by the absence of percolation effect at certain oxygen content materials. The amorphous (Co$_{20}$Fe$_{47}$Ta$_{20}$B$_{13})_{1-x}$O$_{x}$ samples were fabricated by a magneto sputtering system on SiO$_{2}$/Si substrates. The background pressure in the sputtering chamber was $8\times 10^{-5}$ Pa. The oxygen content in the samples was adjusted by the ratio of Ar to O$_{2}$ while sputtering. The thickness of the sample was controlled close to 100 nm. X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250Xi) was used to measure the element content and chemical valence of each element. By calculating the ratio of characteristic peak area and sensitivity factor, we obtained the content of each element. Here the composition can be written as (Co$_{20}$Fe$_{47}$Ta$_{20}$B$_{13})_{1-x}$O$_{x}$, where $x$ is the oxygen content. Four-terminal measurement of film resistivity was carried out for the samples with different oxygen contents. The relationship of resistivity and oxygen content is shown in Fig. 1(a). From the results, we can divide the samples into three regions. The first is the metal region, when oxygen content is less than 35%, the resistivity does not change much as oxygen content increases. The second is the hybrid region, the oxygen content lays between 35% to 45%, the resistivity is very sensitive to the oxygen. The third is the insulator region, i.e., the oxygen content gets larger than 45%, the resistivity is too large for regular measurement, so that the sample becomes insulator. A transmission electron microscope (TEM, JEOL 2100) was used to examine the microstructures of the samples. Typical TEM images in the three oxygen regions are shown in Figs. 1(b)–1(d). From the samples we can see that for the metal and insulator regions there is no obvious contrast in the images, but for the sample in the hybrid region we can clearly see two different areas with a large contrast. As a simplified consideration, there exist two kinds of amorphous nano-particles in the system. One is metal, and the other is insulator. In metal or insulator region, one kind of nano-particle was dominated in quantity. However, in the hybrid region, the contents of two kinds are almost equivalent, and the percolation threshold is in this region. Since the nano-particles have distribution in both size and shape so that the percolation threshold is not a critical value but a range.

Fig. 1. (a) Resistivity changes with oxygen in the three regions. TEM images in (b) the metal region, (c) hybrid region and (d) insulator region.

Fig. 2. Typical $M$–$H$ curve of (a) the metal region, (b) hybrid region and (c) insulator region, and (d) schematic of magnetic structure.

Magnetic measurements were performed using a magnetic properties measurement system (MPMS) with a temperature range of 2–300 K and a field range of 0–70 kOe. Typical magnetic hysteresis loops of (Co$_{20}$Fe$_{47}$Ta$_{20}$B$_{13})_{1-x}$O$_{x}$ films in metal, hybrid and insulator regions with magnetic field parallel to the films are shown in Figs. 2(a)–2(c), respectively. The sample in the insulator region has almost lost its ferromagnetic properties even at 2 K, which will be explained in the following. The rest two $M$–$H$ curves have both similarity and difference. According to the Stoner–Wohlfarth model, for a large amount of non-interacting magnetic particles with random easy axis, the squareness, i.e., the ratio of remaining magnetization to the saturated magnetization ($M_{\rm r}/M_{\rm s}$) is 0.5 at 0 K. In our results, the samples in the hybrid region are more close to this value but the metal region samples have a large squareness. This is caused by the interactions between magnetic particles especially the magnetostatic interaction. As we can see in Fig. 2(d), the sample in the metal region is more or less like a large magnetic film, under the work of demagnetic field, a multi-domain structure is formed. When the samples are located in the hybrid region, magnetic particles do not have so large density and overlapping, and the properties are more closed to the large numbers of non-interaction particles. However, the samples in the insulator region have too small size and low density so that the thermal fluctuation will make them become superparamagnetic and even paramagnetic. Based on this, temperature dependence of magnetization is also investigated. For hysteresis loops in the metal and hybrid regions, the loop shrinks as the temperature increases, that is, the coercivity and squareness are both decreased. This is due to the thermal fluctuation of spin in magnetic nano-particles. In Fig. 3(a), for a uni-axial single particle, there exist two energy valleys under external magnetic field.[13] If the temperature gets higher than a certain value, the energy of thermal fluctuation will be greater than the energy barrier. That temperature is called the blocking temperature. When the temperature is lower than the blocking temperature, these particles will approximately obey the Boltzmann distribution according to the energy map since there are large numbers of nano-particles in the system. The relationship of $H_{\rm c}$ ($M_{\rm r}/M_{\rm s}$) and temperature is shown in Figs. 3(b) and 3(c). It is found that the larger the oxygen content is, the more obvious the influence of temperature is. There are two reasons for this. One is that the interaction strength is larger for the low oxygen samples due to the percolation effect, and the other is that the particle size gets smaller as the oxygen content increases. The blocking temperature can be measured by the field cooling-zero field cooling (FC-ZFC) measurement. The relationship of blocking temperature and oxygen content is shown in Fig. 3(d).

Fig. 3. (a) Schematic of thermal fluctuation of magnetization, (b) $H_{\rm c}$ and (c) $M_{\rm r}/M_{\rm s}$ versus temperature, and (d) blocking temperature of different oxygen contents.

Figures 4(a) and 4(b) are the $H_{\rm c}$ and $M_{\rm r}/M_{\rm s}$ values of the (Co$_{20}$Fe$_{47}$Ta$_{20}$B$_{13})_{1-x}$O$_{x}$ system at temperatures of 2, 20, and 300 K with different oxygen contents, respectively. The results of 2 K are more closed to the state with no thermal fluctuation. Due to the magnetostatic interaction, multi-domain structures are formed so that the samples in metal regions have very low $H_{\rm c}$ value, e.g., 30 Oe. The hybrid region samples remain the single-domain structure. As a result, $H_{\rm c}$ is larger. However, in the insulator region, the domain is too small to remain ferromagnetic at 2 K so that $H_{\rm c}$ is decreased. The results at 20 K is similar to 2 K, but when temperature gets higher, the samples with high oxygen contents will reach their blocking temperature. At 300 K, only the samples in the metal region will stay at the ferromagnetic state, which is the trend of $M_{\rm r}/M_{\rm s}$ values.

Fig. 4. Values of (a) $H_{\rm c}$ and (b) $M_{\rm r}/M_{\rm s}$ versus oxygen content.

Fig. 5. (a) The $M$–$H$ curve in the metal region when magnetic field is out of plane. (b) Comparison of the $M$–$H$ curve in plane and out of plane at 2 K in the metal region. [(c), (d)] The case in the hybrid region.

The magnetization at different magnetic field directions is also investigated, as shown in Figs. 5(a) and 5(b). For the thin film sample, the demagnetizing field is large when the external field is perpendicular to the thin film. It is easy to predict that the magnetic permeability will be largely decreased in comparison with the in-plane results. Moreover, if we only consider the magnetostatic interaction and exchange interaction in the single domain system, $H_{\rm c}$ will remain the same no matter how the external field directions change. This is proved by experiments in Fig. 5(c). However, it is not correct in the multi-domain system. In Fig. 5(d), there is a small loop in the center of the $M$–$H$ curve, which means that when the external magnetic field is small, the domain structure has been changed so that the magnetization is more complicated than the single-domain case. As to the details of multi-domain formation and evolution, our future work is making progress on this. In conclusion, we have systematically investigated the magnetization of amorphous magnetic materials of (Co$_{20}$Fe$_{47}$Ta$_{20}$B$_{13})_{1-x}$O$_{x}$. They are classified into metal, hybrid and insulator regions according to the electrical properties and microstructures. These three regions also present different magnetization properties. The magnetic structure is varied in multi-domain structure, single-domain structure and superparamagnetic structure. There is a large increase of coercivity at a certain oxygen range because of the broken multi-domain structure. Sensitivity to oxygen content will make this amorphous material system diversified and more applications will be achieved. References Designing metallic glass matrix composites with high toughness and tensile ductilityDuctile Bulk Metallic GlassModern soft magnets: Amorphous and nanocrystalline materialsEffect of a Cr Underlayer on the Magnetic Properties of TbCo Amorphous Films with Perpendicular AnisotropySuperhigh strength and good soft-magnetic properties of (Fe,Co)–B–Si–Nb bulk glassy alloys with high glass-forming abilityEnhancement of the fracture strength and glass-forming ability of CoFeTaB bulk glassy alloyCobalt-based bulk glassy alloy with ultrahigh strength and soft magnetic propertiesObservation of unusual magnetic behavior: Spin reorientation transition in thick Co–Fe–Ta–B glassy filmsL1 ₀ FePt(111)/glassy CoFeTaB bilayered structure for patterned mediaTransparent magnetic semiconductor with embedded metallic glass nano-granulesA room-temperature magnetic semiconductor from a ferromagnetic metallic glassElectric and magnetic properties of magnetic (CoFeTaB) _(100−x) O _x filmsA Mechanism of Magnetic Hysteresis in Heterogeneous Alloys

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