简介概要

Room-temperature and high-temperature magnetic permeability of Co-doped nanocrystalline alloys

来源期刊：Rare Metals2018年第5期

论文作者：Bin Fu Jie Han Sheng-Qi Guo Zhi Wang Pan Zhang Zi-Jiao Pan Qiang Xu

文章页码：427 - 432

摘要：Influence of composition and annealing temperature on structure and magnetic properties of amorphous and nanocrystalline Fe_78.4—xCo_xSi₉ B₉ Nb_2.6Cu₁（x=27.4,40.0,51.0,78.4） alloys was investigated by X-ray diffraction（XRD） and the temperature dependence of permeability. According to the initial crystallization temperature(T_x1) from differential scanning calorimetry（DSC） curves of as-quenched amorphous alloys,490-700 ℃ isothermal annealing was carried out to obtain the characteristic nanocrystalline structure. Furthermore,the soft magnetic properties were measured by temperature evolution of magnetic permeability to obtain the correlation between Co content, annealing temperature and magnetic permeability. The results show that, on the one hand,the annealing temperature exerts a significant effect on phase structure and initial permeability（μ_i）. The highertemperature（from 550 to 610 ℃） annealed Co content nanocrystalline samples can remain higher μ_i at elevated temperature. On the other hand, partial substitution Fe by Co can improve the high-temperature magnetic stability;however, the room-temperature permeability of higher Co content alloys decreases obviously at the same time. This phenomenon was analyzed from the viewpoint of the saturation magnetic induction（B_s）, magnetic anisotropy（） and magnetostriction（λ_s）.

稀有金属(英文版) 2018,37(05),427-432

Room-temperature and high-temperature magnetic permeability of Co-doped nanocrystalline alloys

Bin Fu Jie Han Sheng-Qi Guo Zhi Wang Pan Zhang Zi-Jiao Pan Qiang Xu

Tianjin Key Laboratory for Photoelectric Materials and Devices,School of Materials Science and Engineering, Tianjin University of Technology

College of Science, Tianjin University of Technology

School of Science,Tianjin University

School of Materials Science and Engineering,Tianjin University of Technology

收稿日期：23 January 2018

基金：financially supported by the National Natural Science Foundation of China (No. 11604242);Tianjin Research Program of Application Foundation and Advanced Technology, China (No. 14JCQNJC04000);

Room-temperature and high-temperature magnetic permeability of Co-doped nanocrystalline alloys

Bin Fu Jie Han Sheng-Qi Guo Zhi Wang Pan Zhang Zi-Jiao Pan Qiang Xu

Tianjin Key Laboratory for Photoelectric Materials and Devices,School of Materials Science and Engineering, Tianjin University of Technology

College of Science, Tianjin University of Technology

School of Science,Tianjin University

School of Materials Science and Engineering,Tianjin University of Technology

Abstract：

Influence of composition and annealing temperature on structure and magnetic properties of amorphous and nanocrystalline Fe_78.4—xCo_xSi₉ B₉ Nb_2.6Cu₁(x=27.4,40.0,51.0,78.4) alloys was investigated by X-ray diffraction(XRD) and the temperature dependence of permeability. According to the initial crystallization temperature(T_x1) from differential scanning calorimetry(DSC) curves of as-quenched amorphous alloys,490-700 ℃ isothermal annealing was carried out to obtain the characteristic nanocrystalline structure. Furthermore,the soft magnetic properties were measured by temperature evolution of magnetic permeability to obtain the correlation between Co content, annealing temperature and magnetic permeability. The results show that, on the one hand,the annealing temperature exerts a significant effect on phase structure and initial permeability(μ_i). The highertemperature(from 550 to 610 ℃) annealed Co content nanocrystalline samples can remain higher μ_i at elevated temperature. On the other hand, partial substitution Fe by Co can improve the high-temperature magnetic stability;however, the room-temperature permeability of higher Co content alloys decreases obviously at the same time. This phenomenon was analyzed from the viewpoint of the saturation magnetic induction(B_s), magnetic anisotropy(<K>) and magnetostriction(λ_s).

Keyword：

Nanocrystalline alloys; High-temperature soft magnetic properties; Initial permeability; FeCo-based alloy;

Author： Bin Fu,e-mail:fubin@tjut.edu.cn;

Received： 23 January 2018

1 Introduction

The Fe-based nanocrystalline alloys firstly reported by Yosizawa et al. [ 1] are considered to be potential soft magnetic materials for practical application.In the past few years,they have been extensively studied due to their high saturation magnetization,high permeability and low core loss [ 2, 3, 4, 5, 6, 7, 8, 9] .In these materials,the random magneto-crystalline anisotropy is fluctuating on a scale much smaller than the domain wall width and is therefore averaged out by exchange interaction [ 10] ,so they have excellent soft magnetic properties at room temperature.However,due to lower Curie point of the residual amorphous phase in dualphase nanocrystalline alloys,the initial permeability deteriorated sharply at a higher temperature,so they can only be used under the temperature of 300℃ [ 11, 12, 13] .To overcome this limitation of the existing Fe-based alloys,partial substitution of Co for Fe was carried out to meet the desire for high-temperature applications [ 14, 15, 16, 17, 18] .However,when Co was added to FeCuNbSiB alloys,although both the Curie temperature and the high-temperature permeability were enhanced [ 19] ,the room-temperature permeability was reduced in a large part especially in higher Co content nanocrystalline alloys.Some people attribute the above phenomenon to the increase in the magnetic anisotropy constant [ 20] .But it is well known that the magnetostriction and the saturation magnetic induction also have influence on permeability of amorphous and nanocrystalline alloys [ 21, 22] .Study of the origin of above magnetic phenomena is in favor of exploitation new-type high-permeability nanocrystalline alloys in both roomtemperature and high-temperature applications.

In this paper,Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0,78.4) alloys with different annealing temperatures were investigated to obtain a relationship between the magnetic permeability and annealing temperature.And furthermore,the change of initial permeability with Co content at room temperature was also analyzed from the viewpoint of magnetic anisotropy,magnetostriction and saturation magnetic induction.

2 Experimental

Experiments were carried out on samples of amorphous alloys with nominal composition of Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0,78.4) prepared by single-roller melt spinning method.The as-quenched amorphous ribbons,with 3-4 mm in width and 20μm in thickness,were wound into 16-mm-inner-diameter and20-mm-outer-diameter toroidal cores and then were annealed under vacuum atmosphere (1×10^-3 Pa) at temperatures ranging from 490 to 700℃.X-ray diffractometer (XRD,Rigaku Dmax-RB) with Cu Kαradiation(λ=0.154056 nm) was used to determine the structure of the alloys.Differential scanning calorimetry (DSC,SII DSC6220) with a heating rate of 20℃,min^-1 was used to determine the thermal processes of as-quenched alloy studied.The permeability was determined by an impedance analyzer (HP4294A) through a conventional induction method at 10 kHz,with a maximum AC magnetic field of0.4A·m^-1 in a temperature range from 20 to 800℃,and the heating rate was about 10℃.min^-1.The saturation magnetic induction (B_s) was evaluated from the hysteresis loops obtained at room temperature,and the saturation magnetostriction (λ_s) was measured by a small-angle magnetization rotation [ 23] ,respectively.

3 Results and discussion

DSC curves of as-quenched Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁alloys are presented in Fig.1.The two crystalline exothermic peaks of each curve are corresponding to initial crystallization temperature ofα-FeCo soft magnetic phase(T_x1) and the secondary crystallization temperature of Fe(Co)B hard magnetic phase (T_x2),respectively.It can be seen that theΔT_x (=T_x2-T_x1) are increasing with Co content (x),which indicates that higher Co content is in favor of forming a single soft magnetic crystalline phase in a broad temperature range.

Fig.1 DSC curves of as-quenched Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁(x=27.4,40.0,51.0) alloys

According to the initial crystallization temperature (T_x1),490-700℃isothermal annealing was selected to obtain the characteristic nanocrystalline structure.Figure 2 gives theμ_i-T curves of as-quenched and annealed Fe_27.4Co_51-Si₉B₉Nb_2.6Cu₁ alloys.A characteristic Hopkinson peak is observed at the Curie temperature of the amorphous phase inμ_i-T curves of as-quenched sample,which is due to a faster decrease in magnetic anisotropy than that of saturation magnetization with temperature increasing,and thus,the permeability perges at the Curie point [ 24] .However,the sharp Hopkinson peak vanishes in annealed samples,which indicates that the dual-phase structure has formed when annealing temperature (T_a)>490℃ [ 25] ,and the variation ofμ_i with temperature depends on T_a.When T_a=490℃,μ_i decreases with T increasing and falls nearly to zero at about 470℃which corresponds to the Curie temperature of the intergranular amorphous phase( ),but when T_a≥550℃,μ_i no longer reduces to zero at the Curie temperature of the residual amorphous phase,which suggests that when annealing temperature is much higher than the crystallization temperature (T_x1),a lot of ferromagnetic crystallites are precipitated from amorphous matrix during the annealing progress,so the thickness of the amorphous layer gradually gets smaller,which is in favor of forming a stronger exchange coupling betweenα-FeCo nanograins andμ_i remains a higher value even at the temperature of much higher than ,and eventually,μ_ivalues of all samples quickly decrease above 700℃,which is attributed to the formation of a hard magnetic boride other than to the magnetic transition from ferromagnetic to paramagnetic state [ 26] .

Fig.2μ_i curves for as-quenched and annealed Fe_27.4Co_51-Si₉B₉Nb_2.6Cu₁ alloys

To analyze the change of initial magnetic permeability with T,XRD was used to determine the structure of Fe_27.4Co₅₁Si₉B₉Nb_2.6Cu₁ alloys annealed at different temperatures,as shown in Figs.3 and 4.It can be seen that XRD patterns reflect a successive crystallization process which is correlated initially with the crystal growth of the bcc-FeCo phase (Fig.3) and finally with the formation of fcc-FeCo,hcp-Co and Fe₂B phases at the annealing temperature of higher than 640℃(Fig.4).When T_a is lower than 610℃,the patterns are mainly composed of characteristic (110),(200) and (211) diffraction peaks which correspond to bccα-FeCo crystalline precipitates.Owing to the ultrafine structure of ferromagnetic grains in the crystalline phase,the nanocrystalline alloys have very higher magnetic permeability.However,with T_a increasing,the peaks ofα-FeCo become more intense and narrower,indicating an increase in the average grain size (D),so the effective anisotropy increases,which makes roomtemperature permeability decrease.However,when T>640℃,due to the precipitation of small fractions of Fe₂B,hcp-Co and fcc-FeCo,soft magnetic properties drastically deteriorate although bcc-FeCo crystallites remain existence.By contrasting XRD patterns of 670℃annealed sample with that of 700℃annealed one in Fig.4,one can easily see that the diffraction peak intensity of bcc-FeCo is gradually weakening and that of fcc-FeCo is gradually getting stronger with T_a increasing,which indicates that the bcc-FeCo phase is transformed into the fccFeCo phase through high-temperature annealing.So,the fcc-Fe with antiferromagnetism and Fe₂B with hard magnetic properties make excellent soft magnetic properties vanish.

Fig.3 XRD patterns of Fe_27.4C0₅₁Si₉B₉Nb_2.6Cu₁ alloys annealed at490-610℃

Fig.4 XRD patterns of Fe_27.4Co₅₁Si₉B₉Nb_2.6Cu₁ alloys annealed at640-700℃

Usually in dual-phase Fe-based nanocrystalline alloys,a single bcc-Fe nanocrystalline phase precipitates from the amorphous matrix after the first stage of crystallization,then a coarse bcc-Fe grains appears,and small fractions of FeB boride phase precipitates simultaneously after a second stage [ 27] .This crystallization process is different from that of the present study;the formation of the fccFeCo phase and hcp-Co phase in the second stage of crystallization in this experiment may be related to a large content of Co.

For clarifying the evolution of initial permeability with Co content,μ_i-T curves for Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁(x=27.4,40.0,51.0,78.4) annealed at 610℃were measured,as shown in Fig.5.The initial permeability declines significantly with the increase in Co content at room temperature;however,it decreases slowly at elevated temperature.That is to say appropriate substitution of Fe by Co can improve high-temperature magnetic stability,but excessive Co content can make soft magnetic properties deteriorate,especially at room temperature.This may be related to the effective magnetic anisotropy (<K>),the saturation magnetic induction (B_s) and the saturation magnetostriction (λ_s).

Fig.5μ_i-T curves for Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0,78.4) alloys annealed at 610℃

In order to ascertain the main reasons for the change of room-temperature permeability with Co content,the average grain size (D),the saturation induction (B_s) and the saturation magnetostriction (λ_s) were measured.Table 1shows the average grain size of FINEMET and Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0) with different T_a.One can see that the average grain size of FINEMET is smaller than that of Co-contained alloys;on the other hand,Co content has little impact on D in FeCo-based alloys,which may be related to the kinetics of Cu clustering [ 20] .The clustering temperature (T_clust) shifts to a higher temperature when substituting Fe by Co,and it becomes higher than T_x1 when x>20.0.Cu clustering does not appear,which makes D remain roughly constant with Co content increasing.As all known that the effective magnetic anisotropy (<K>) is strongly influenced by D,so there will not have a noticeable increase in<K>with Co content increasing.

The dependence of B_s on Co content for Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0) alloys annealed at610℃is shown in Fig.6.B_s reduces sharply with the increase in x,and this can be explained from Slater-Pauling curve [ 28] .B_s increases when Co is added to Fe-based nanocrystalline alloys,but when Co content (the ratio of x/78.4 in this paragraph) of FeCo alloy is nearly 30 at%,the magnetic moment reaches a maximum value;then with adding Co continuing,the magnetic moment begins to decline,which is related to the energy band of Fe atoms.It can be seen from Fig.6 that B_s monotonically decreaseswith Co content increasing,and this is due to that the smallest Co content of the studied materials is 35 at%.

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Table 1 Average grain size (D) of FINEMET and Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0) alloys annealed at different temperatures

Fig.6 Dependence of saturation magnetic induction B_s on Co content x for Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0) alloys annealed at 610℃

Figure 7 shows the change of saturation magnetostriction with T_a for different Co content alloys.The increase inλ_s with T_a indicates thatλ_s is gradually increasing after nanocrystallization for Co-contained alloys.As magnetostriction of crystalline phase ( ) ofα-FeCo(Si) is positive and larger than magnetostriction of amorphous phase ( ) of the residual amorphous phase,with the increase in annealing temperature,the volume of crystalline phase becomes large,which results in the increase inλ_s.But the evolution ofλ_s with Co content does not change obviously.When Co content increases from x=27.4 to x=40.0 and 51.0,theλ_s values of all annealed samples increase by about 5×10^-6 and 6×10^-6,respectively,which indicates that Co content has little effect onλ_s in FeCo-based alloys,and this is different from Fe-based alloys [ 29] .Considering the evolution of<K>,B_s andλ_s,B_s is the main reason why room-temperature permeability decreases sharply with Co content in FeCo-based nanocrystalline alloys.

Fig.7 Saturation magnetostriction (λ_s) as a function of annealing temperatures T_a for Fe_784-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0)alloys

4 Conclusion

Influence of composition and annealing temperature on structure and magnetic properties of amorphous and nanocrystalline Fe_78.4-xCo_xSi₉B₉Nb_2.6Cu₁ (x=27.4,40.0,51.0,78.4) alloys was studied.Improved high-temperature magnetic softness is observed for the annealed Fe_27.4Co₅₁Si₉B₉Nb_2.6Cu₁ alloys.The initial permeability(μ_i-T) of higher-temperature (T_a>550℃) annealed samples remains a higher value even at the temperature of much higher than T ,and this may be ascribed to a large crystalline content and a stronger exchange coupling betweenα-FeCo nanograins.Owing to a large Co content,hcp-Co and fcc-FeCo phases are precipitated fromα-FeCo phase,and the high-temperature permeability quickly drops after T>650℃.On the other hand,the effective magnetic anisotropy and saturation magnetostriction are little influenced by Co content,but the saturation magnetic induction reduces sharply with the increase in Co content at room temperature,which may be the main reason for lower room-temperature initial permeability of higher Co content FeCo-based nanocrystalline alloys.