稀有金属(英文版) 2020,39(01),76-83

Development of high-coercivity state in melt-spun Fe₄₁Pd₄₁B₈Si₆P₄ ribbons

Oksana Aleksandrovna Golovnia Nina Ivanovna Vlasova Aleksandr Gervasievich Popov Vasiliy Semenovich Gaviko Vladimir Vladimirovich Popov Jr. Andrey Vladimirovich Protasov Arti Kashyap

M.N.Miheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences

Institute of Natural Sciences and Mathematics,Ural Federal University

Israel Institute of Metals,Technion R&D Foundation

Indian Institute of Technology Mandi

作者简介：*Oksana Aleksandrovna Golovnia e-mail:golovnya@imp.uran.ru;

收稿日期：29 September 2018

基金：financially supported by the Indian-Russian Collaborative Project(Nos.17-52-45097 and INT/ RUS/RFBR/P-267);the State Assignment of Ministry of Science and Education of Russia(No.AAAA-A18-118020290129-5);

Development of high-coercivity state in melt-spun Fe₄₁Pd₄₁B₈Si₆P₄ ribbons

Oksana Aleksandrovna Golovnia Nina Ivanovna Vlasova Aleksandr Gervasievich Popov Vasiliy Semenovich Gaviko Vladimir Vladimirovich Popov Jr. Andrey Vladimirovich Protasov Arti Kashyap

M.N.Miheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences

Institute of Natural Sciences and Mathematics,Ural Federal University

Israel Institute of Metals,Technion R&D Foundation

Indian Institute of Technology Mandi

Abstract：

The phase transformation and magnetic hysteresis properties of melt-spun Fe41 Pd41 B8 Si6 P4 ribbons subjected to the annealing at temperatures of 500-550℃were studied after holding for 0.1-60.0 h by transmission electron microscopy(TEM),X-ray diffraction(XRD) and thermomagnetic analysis.The additions of P,B and Si to the FePd alloy allowed us to achieve the coercivity of124 kA·m^-1,which is 2.6 times higher than that of the melt-spun ribbons of the binary equiatomic FePd alloy.The high-coercivity Fe₄₁Pd₄₁B₈ Si₆ P₄ alloy is nanocrystalline and is composed of the ordered L1₀-phase grains approximately 40 nm in size and inclusions of the Fe2(P,B) and Pd2(Si,B) phases.The coercivity kinetics is controlled by the phase transformation which can be pided into three stages:transformation from the bcc structure to nanosized regions of the fee and Fe2 P phases;transformation from the fee to L1₀ nanosized regions with somewhat different degrees of tetragonality and their ordering;and extensive growth of the weight fraction of L1₀ phase from the fee nanosized regions.P and B atoms occupy interstitial sites in the iron plane of L1₀ lattice,thus decreasing its Curie temperature(T_C).

Keyword：

Fe-Pd-P-B-Si; Melt spinning; L1₀; Nanostructured materials; Phase transformation; Magnetic measurements;

Received： 29 September 2018

1 Introduction

CoPt,FePt and FePd alloys which undergo ordering into the L10 structure at temperatures T<Tcr (where Tcr is the critical temperature of the A1→L10 phase transformation)have been extensively studied in the last decades.These high-anisotropy,corrosion-resistant and easily deformed compounds are regarded as potential materials for permanent magnets and carriers for high-density information recording.However,attempts to achieve high hysteresis properties in the cheapest of these alloys,namely,FePd,which has the magnetic anisotropy field of Ha=2.8 MA·m-1,high saturation magnetization of1.37 T and theoretical maximum energy product of(BH)max=374 kJ·m-3,have not been successful.Nowadays,only certain materials demonstrate high properties,e.g.,among the Fe-Pd alloys,the nanocomposite (L10-FePd)82(α-Fe)18 which was prepared by annealing of the Pd/γ-Fe2O3 and elemental Pd nanoparticles [ 1] has the high value of (BH)max=82 kJ·m-3.FePd films prepared by an electron beam deposition on MgO (100) [ 2] are known to possess high values of coercivity Hc=0.48 MA·m-1.In Ref. [ 3] ,the authors managed to achieve a unique combination of high coercivity,magnetic energy product and remanence ratio at room temperature,i.e.,0.28 MA·m-1,140 kJ·m-3 and 0.94,respectively,for polycrystallineFe51Pd49 films with a thicknesses of (47±2) nm prepared by direct current (DC) magnetron sputtering on quartzglass substrates and subsequent annealing at 550℃for3 h.

In order to obtain high-coercivity state in the bulk Fe-Pd alloys,nanocrystalline structure is required.In a number of works,in order to obtain a nanocrystalline state,the severe plastic deformation was employed [ 4, 5, 6, 7] .In bulk samples of the equiatomic FePd alloy,after the severe plastic deformation by cold rolling [ 6] and even by high-pressure torsion (HPT) with subsequent low-temperature annealing for ordering (Tann=350-450℃),values of the hysteresis properties () also remain low [ 7] .Nevertheless,the HPT allows one to achieve the coercivity approximately three times higher than that of the as-cast alloy.In addition,in order to achieve maximum value of coercivity by low-temperature annealing,long-term holdings are required.

A more simple and fast approach to obtain the nanocrystalline state is melt spinning.However,this method does not help in the case of the equiatomic FePd alloy.According to Refs. [ 7, 8] ,after the annealing at450-500℃,the coercivity (Hc) of melt-spun samples of FePd does not exceed 39.8 kA·m-.Another approach is amorphization of an alloy.If an alloy consists of at least three unlike sorts of elements with significantly different atomic sizes and negative mixing energy,amorphous state can form.Moreover,if an alloy consists of more than three elements,the amorphous-forming ability and the stability of the amorphous state enhance [ 9] .These conditions were successfully used in order to produce amorphous and nanocrystalline states and gain high coercivity in the L10CoPt and FePt alloys [ 10, 11, 12] ,as well as to obtain the L10-type phase in the FeNi alloy [ 13] .Usually,the following elements were chosen as doping elements:B,C,P and Si.The literature contains data on doping of FePd with Co,Cu,Ag and other metals [ 14, 15, 16, 17] .However,the formation of the L10 structure in FePd doped with nonmetallic elements was not studied,except for Refs. [ 18, 19] where FePd thin films doped with B were considered.

In the experimental phase diagram of the binary FePd,the eutectoid point falls into the concentration range of the existence of L10 structure and corresponds to～40 at%Pd.The amorphous Pd40Ni40P20 and Pd40Fe17.5Ni22.5P20alloys belong to a large and extensively studied group of amorphous materials based on Pd [ 9] .The solidification of these alloys was studied in detail in Refs. [ 20, 21] .However,both alloys are not ferromagnetic.Magnetic properties of amorphous FexPd80-xP20 alloys(13<x<44)were studied in Refs. [ 22, 23] .In this work,we study the phase transformation and magnetic hysteresis properties of meltspun ribbons of Fe41Pd41B8Si6P4 subjected to the annealing at 500-550℃.

2 Experimental

Fe41Pd41B8Si6P4 ribbons were prepared by the induction melting and melt spinning on a copper wheel rotating at a speed of 40 m·s-1 in the argon atmosphere.The ribbons were annealed at temperatures of 500-550℃with different holdings from 5 min to 60 h.X-ray diffraction analysis(XRD) investigations were performed using a multifunctional Empyrean (PANalytical) diffractometer with Cu Kαradiation.Diffraction patterns were analyzed with the software“High Score Plus”,“FullProf” [ 24] and“PowderCell”.

The magnetic properties of the samples were investigated using a vibrating sample magnetometer (VSM,Lake Shore 7407).The maximum field was 1.39 MA·m-1.The Curie temperatures were determined from the first derivatives of the temperature dependences of magnetic susceptibility,x(T),which were measured by the method of compensated transformer in an alternating magnetic field with an amplitude of 318 A·m-1 and a frequency of800 Hz in the temperature range of 18-800℃.

For the transmission electron microscopy (TEM)investigation,samples with the average size of 3.0 mm×1.5 mm were prepared from the melt-spun ribbons by subsequent ion polishing and plasma cleaning.The highresolution transmission electron microscopy (HRTEM)images,high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images and electron dispersive spectroscopy (EDS) maps were taken using a monochromated and double-corrected Titan Themis G2 60-300 (FEI/Thermo Fisher) operated at 200 keVand equipped with a DualX detector (Bruker).The quantitative analysis of the EDS maps was performed with the Velox software (FEI/Thermo Fisher).

3 Results and discussion

3.1 Magnetic hysteresis properties

For the sake of comparison,Fig.1 demonstrates dependences of the specific magnetization(σ1.2)measured in the magnetic field of 1.2 MA·m-1 and coercivity (Hc) of the melt-spun ribbons of FePd and Fe41Pd41B8Si6P4 versus annealing time (t) at 500℃and at 500,515 and 550℃,respectively.The specific magnetization of Fe41Pd41B8-Si6P4 is significantly lower than that of FePd.In addition,it does not depend on the temperature and time of annealing(Fig.1a).

The maximum value of coercivity of the doped alloy is three times higher than that of FePd and reaches124 kA·m-1.In order to achieve the same value of,either short-term annealing (for 15 min) at 515-550℃orlong-term annealing (for 12 h) at 500℃(Fig.1b) is required.As shown in Fig.1b,the formation of the highcoercivity state in Fe41Pd41B8Si6P4 at 500℃can be pided into three stages:negligible increase of Hc in the course of the first 15 min;slow and almost linear increase of Hc up to 10-h annealing;and after abrupt increase from10 to 12h annealing,the stable high-coercivity state is observed up to 30 h.On the contrary,after the short-term annealing at higher temperatures of 515 and 550℃,the coercivity lacks thermal stability (Fig.1b).

Fig.1 Dependences of a specific magnetization (σ1.2) and b coercivity (Hc) of FePd and Fe41Pd41B8Si6P4 ribbons on time of annealing at500,515 and 550℃

Figure 2 demonstrates hysteresis loops of Fe41Pd41B8-Si,P4 and FePd ribbons after various holdings at 500℃.The hysteresis loops that do not have any kinks evidence that even though the samples are multiphase (as will be demonstrated below),their magnetic behavior is typical of single-phase materials.This can take place if a multiphase material consists of exchange-coupled or magnetostatically interacting grains or phases.However,the relative remanence of the alloyσr/σs (whereσs is the specific saturationmagnetization) does not exceed 0.5,which is true even for the samples with the best hysteresis properties.

Fig.2 Hysteresis loops of FePd and Fe41Pd41B8Si6P4 ribbons after annealing at 500℃for 5,10 and 15 h

3.2 Phase composition

Figure 3 shows XRD patterns for the as-spun and as-annealed ribbons of the Fe41Pd41B8Si6P4.Phase compositions and lattice parameters of the samples are listed in Table 1.After the melt spinning,the alloy becomes nanocrystalline.This is proved by the broadened base of structural peaks(Fig.3(1)).XRD analysis demonstrates that the as-spun alloy consists of several solid solutions:bcc with the lattice parameter a=0.3024 nm and the largest content(75 wt%),fcc-A1 with a=0.3801 nm and small content(5wt%) and Fe,P-type phases.The solid solution with bcc structure is likely to be an interstitial phase of B and P inα-Fe.Besides,Pd and Si partially substitute for Fe,since the lattice parameter a of a-Fe increases in comparison with that of pureα-Fe (a=0.2866 nm [ 25] ;Table 1).

According to XRD analysis,even after the short-term annealing at 550℃,the Fe41Pd41B8Si6P4 alloy contains more than 60%of the tetragonal L10 phase (Table 1).The lattice parameter a of the L10 phase increases by 1%,and the lattice parameter c somewhat decreases in comparison with the corresponding lattice parameters of the melt-spun ordered equiatomic FePd (Table 1).Hence,the cla ratio decreases,which enhances the degree of tetragonality of the L10 lattice by 2.5%.The change in the lattice parameters of the L10 phase as well as the bcc structure can originate from the presence of B and P interstitials in their lattices.

The phase transformation proceeds more slowly at500℃.It allowed us to establish the course of multiphase state formation and its correlation with the coercivity kinetics observed.After annealing for 15 min (Fig.3(2)),the transformation from bcc structure to nanosized regions of certain structures (NRCS) with the total content of 60wt%and distinguishable fcc and Fe2P phases (Table 1)terminates.It can be assumed that nanosized regions of each structure of the NRCS have close but different values of the lattice parameters.The superposition of the reflections from such regions results in the smearing of the XRDpattern in the range of 39°-50°(Fig.3),which complicates the structure identification.However,peak positions after annealing for 15 min (Fig.3(2)) and peak positions after prolonged annealing (Fig.3(3)-(5)) suggest that the main phases of the NRCS are probably fcc and Fe2P.Plausibly,this phase transformation is the reason for the enhanced coercivity after the annealing for 15 min (Fig.1b).

Fig.3 XRD patterns for Fe41Pd41B8Si6P4 ribbons after various treatments

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Table 1 Phase composition and lattice parameters of melt-spun ribbons before and after annealing of FePd and Fe41Pd41B8Si6P4 alloys

V volume of unit cell aRCS designates regions of coherent scattering bIn brackets of c/a ratio being fct (face-centered tetragonal) representation cA number of close but different lattice parameters

In the course of further annealing for up to 10 h,the nanosized regions of the ordered L10 phase are formed.According to XRD analysis,the ordered regions have different values of c/a ratio.The latter is indicated by the large broadening of the superstructure reflections (Fig.3).The difference in the c/a ratio can be caused by the misfit of the lattice parameters of the phases forming in the course of the phase transformation.It is possible that in the alloy,a coherent bonding is formed between the L10,fcc and other phases,which induces the elastic stresses that slow down the formation of the high-anisotropy L10 phase and coercivity growth.The similar incubation stage was observed and theoretically analyzed earlier for the case of the A1→L12 ordering in Cu3Au [ 26] .This stage is characterized by the increase in c/a.

The stage ends with the stable values of the L10 lattice parameters corresponding to c/a=1.331,which are equal to the lattice parameters of this phase after the annealing at550℃.In this case,the content of L10 phase is～25wt%.After the annealing for 10-12 h,an abrupt increase in Hc is observed.As given in Table 1,this corresponds to the increase in the content of L10 and Fe2P phases together with the decrease in the content of the NRCS down to26 wt%,which confirms our suggestion on the phase composition of the NRCS.At the end of this stage,the Fe2B phase is formed (Fig.3(5)).Feasibly,it is formed from the remaining free elements of the NRCS (indicated by arrows in Fig.3).Moreover,the L10 content reaches 60wt%,and its lattice parameters nearly have not changed.The further annealing for up to 60 h does not affect the phase composition of the alloy which is the same as that after the annealing at 550℃.

3.3 Microstructure

Figure 4 demonstrates elemental mapping for the as-spun ribbons.Table 2 lists the main regions that can be found in elemental mapping with their chemical compositions after various treatments.Microstructure of the alloy consists of equiaxial grains of the bcc phase with the size of 100 nm and chains of bcc grains with the predominant Fe content(Fig.4,Table 2).

First,the comparison of maps for Fe,Pd,B,Si and Pproves the supposition that the bcc phase,which,according to XRD analysis (Table 1),is the main phase of the sample,contains all interstitial doping elements.The phase is highly inhomogeneous in chemical composition ranging from FePd doped with Si and B to almost pure Fe (Table 2).Boundary regions surrounding bcc grains contain Pd,P and Si,which can be seen from the corresponding elemental mapping.Apparently,in the course of the solidification of the bcc grains from liquid,the excessive amount of Si and P atoms are localized at the grain boundaries forming additional regions of the Fe2P-type phase enriched in Pd,P and Si anddepleted ofFe.The latter can be attributed to the Pd2Si phase.Such elemental distribution does not contradict the diffraction patterns for the as-spun ribbons (Table 1),becauseand Pd2Si structures are the same,and the lattice paramof the Fe2P-type phase are shifted to Pd2Si[a(Fe2P)=0.5870 nm and c(Fe2P)=0.3465 nm vs.a(Pd2Si)=0.6491 nm and c(Pd2Si)=0.3465 nm].Similar structure characteristic of the FePdP phase [ 27] ;however,in ourFe and Pd contents do not correspond to the compositionthis phase (Table 2).

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Table 2 Chemical compositions of regions of Fe41Pd41B8Si6P4 ribbons determined from EDS-HAADF mapping after various treatments (Figs.4,5)(at%)

Fig.4 EDS-HAADF mapping for as-spun Fe41Pd41B8Si6P4 ribbons:a HAADF image,b Fe map,c Pd map,d B map,e Si map and f P map(brighter color corresponds to higher element content;numbers (1-5) being bcc regions of different chemical compositions listed in Table 2)

Fig.5 EDS-HAADF mapping for Fe41Pd41B8Si6P4 ribbons annealed at 500℃for 10.75 h:a HAADF image,b Fe map,c Pd map,d B map,e Si map and f P map (brighter color correspons to higher element content)

Figure 5 demonstrates elemental maps for the melt-spun ribbons after the annealing at 500℃for 10.75 h.There are three types of grains that are distinguishable in the maps:L10-phase grains which can be regarded as a matrix(Fig.5a),Fe2(P,B)-type grains depleted of Pd and Si(Fig.5b),and Pd2(Si,B) grains depleted of Fe and P(Fig.5b).For instance,in Fig.5,the same L10 grain is outlined in all elemental maps.Apparently,the comparison of the maps evidences that the region ascribed to the L10phase contains Fe,Pd,B,Si and P (Fig.5a-e,Table 2).The size of the L10 phase is～40 nm and is close to the regions of coherent scattering (RCS) size determined from XRD analysis (Table 1).According to Fig.5,the grain sizes of Fe2(P,B) and Pd2(Si,B) change in the range(20-50 nm).Other regions that are depleted of Fe and enriched in Pd in comparison with the L10 regions can be attributed to 26%NRCS (Table 1).Since elemental maps of the ribbons annealed at 500℃for 15 h are qualitatively the same as those of the ribbons after the annealing for10.75 h,they are not shown here.The only difference that we have found is an increase of the L10 matrix grain size up to 60 nm and the existence of some regions that can be identified as Fe2B.

3.4 Thermomagnetic study

The differentiation of the temperature dependences of the susceptibility (Fig.6) allows determination of the Curie temperature (TC) of the phases which exist in the Fe41Pd41B8Si6P4 alloy at different stages of the phase transformation.First,the dependence which corresponds to the annealing for 15 h and is shown in Fig.6(5) should banalyzed,because the corresponding phase composition is the most easily analyzed.Starting from this time of annealing,the magnetic state of the alloy is stable (Fig.6 (5),(6)).Figure 6 shows that at the end phase transformation TC of the L10 phase is This TC value is considerably lower than that of FePd(～480℃) [ 28] .Such reduction in TC can be explained by the presence of P and B interstitials in the iron plane of thL10 lattice,which weakens the exchange interaction.On the contrary,TC of the Fe2P phase is 300℃,which is considerably higher than-60℃characteristic of pure Fe2P [ 29] .This is also caused by P,B and Si interstitials in the Fe2P lattice [ 30, 31] .

Now,let us consider the annealing in a sequence.Apparently,Fig.6(2) corresponding to the annealing for15 min demonstrates the existence of L10 phase with Tc≈480℃,fcc phase with TC=425℃,bec solution with TC=250℃and Fe2P-type phase with TC=320℃.It is necessary to note that L10 reflections are not observein XRD patterns,which can be explained by its negligible fraction.The further annealing (Fig.6(3)-(5)) shows a gradual decrease in Tc of L10 phase and extinction of the fcc phase.TC of the Fe,P-type phase also decreases from320 to 303℃in the course of annealing.Such behavior indicates that in the course of the incubation stage,after leaving the Fe2P-type phase,B and P atoms occupyinterstices and Si substitutes for Pd or Fe in L10 lattice.Moreover,it can be seen that at the beginning of the third stage of the fast transformation,the active redistribution of the doping elements in L10 phase finishes,which correlates with the constant values of its lattice parameters.

Fig.6 Temperature dependence of susceptibility (x) of as-spun ribbons and of ribbons after annealing at 500℃for different time and Curie temperatures of forming phases versus time of annealing

3.5 Discussion of magnetization reversal mechanism

Summarizing the results,it can be concluded that the highcoercivity Fe41Pd41B8Si6P4 alloy is nanocrystalline.It is composed of the ordered L10-phase grains (60 wt%) with a size of 40 nm.The L10 lattice contains P and B interstitials and Si which substitutes for Fe or Pd.In the course of the L10-phase formation,its c/a ratio changes from 0.94 to0.92.(e.g.,after high-pressure torsion (HPT) [ 32] ,FePd has c/a=0.95 [ 33] ).In addition,the alloy contains inclusions of Fe2(P,B) and Pd2(Si,B)(total content of 34 wt%) and Fe2B (6 wt%).The size of the inclusions (20-50 nm) has the same order of magnitude as that of the L10 grains.It can be suggested that,on the one hand,inhomogeneities in the alloy composition and the sizes of grains,as well as their arrangement,facilitate the nucleation,and on the other hand,they bring about the gradient of the boundary energy,which leads to the pinning of domain walls,thus enhancing the coercivity.The size of the L10-phase grains is considerably smaller than the critical size (Dc) of single-domain particles of FePd,i.e.,Dc≈330 nm [ 34] .Therefore,the dependence of the critical single-domain size on the external field has to contribute to the magnitude of coercivity.However,to determine the magnetization reversal mechanism in the doped alloy,the microstructure investigations are necessary in more detail.

4 Conclusion

The additions of P,B and Si to the FePd alloy allowed us to achieve the coercivity of 124 kA·m-1,which is 2.6 times higher than that of the melt-spun ribbons of the binary equiatomic FePd alloy and close to the maximal coercivity obtained after the annealing of the FePd subjected to HPT.Development of the high-coercivity state is controlled by the phase transformation that can be pided into three stages:the transformation from the bcc structure to nanosized regions of the fcc and Fe2P phases;the transformation from the fcc to L10 nanosized regions with the c/a ratio changing from 0.94 to 0.92 and their ordering;and extensive growth of the weight fraction of the L10 phase from the fcc nanosized regions.The grain size of the high-coercivity melt-spun doped Fe41Pd41B8Si6P4 ribbons decreases in comparison with that of the melt-spun FePd alloy,i.e.,50 nm and 5μm,respectively,which enhances the coercivity.P and B atoms occupy interstitial sites in the L10 lattice in the iron plane,which can be a reason for the weakening of exchange interaction and decrease in the Curie temperature.

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