Bando, Yoshichika

Shigematsu, Toshihiko

Shinjo, Teruya

Takada, Toshio

Kyoto University Research Information Repository

Title Formation of Metastable Phase in Vapor-Deposited FeSb FilmsAuthor(s)Shigematsu, Toshihiko; Shinjo, Teruya; Bando, Yoshichika;Takada, ToshioCitationBulletin of the Institute for Chemical Research, KyotoUniversity (1979), 57(4): 310-317Issue Date1979-10-15URL http://hdl.handle.net/2433/76840RightType Departmental Bulletin PaperTextversionpublisherKyoto UniversityBull. Inst. Chem. Res., Kyoto Univ., Vol. 57, No. 4, 1979      Formation of Metastable Phase in Vapor-Deposited                    FeSb Films                   Toshihiko SHIGEMATSU*,Teruya  SHINJO*,                 Yoshichika BANDO**, and Toshio TAKADA*                               Received July 9, 1979    FeSb alloy films with various compositions were produced by simultaneous vapor depositions of Fe and Sb onto acetylcellulose sheets at room temperature. By electron diffraction and Mossbauer effect measurements, it was identified that 0-34 at%Sb is bcc alloy and 52-90 at%Sb is amorphous. In the supersaturated bcc alloys, the Mossbauer line width increases significantly with increasing Sb content. The amorphous alloys were stable up to about 405 K and they transformed to crystalline phases such as Fe,Sbz and FeSb,. The magnetic properties of the amorphous alloys are reported here.    KEY WORDS : Electron diffraction / Mossbauer effect / Magnetic property /                 Stability of metastable phase /                            INTRODUCTION    Metastable alloy films such as supersaturated solid solution and amorphous alloy can be produced by simultaneous vapor depositions of two components onto cold substrates. Several binary systems have been investigated, for example, super-saturated solid solutions of Cu-Co,1> Fe-Cu2,3) and Cu-Mg,' and amorphous metallic alloys of Co-Ag,1) Fe-Au,4) and Fe-Si5> and many interesting properties were observed which differ greatly from the bulk properties.    Mader et al.6) classified the metastable phases obtained in the co-deposited alloy films into the following three types. 1) Amorphous phases may be produced when the two metals are normally insoluble in each other and the atomic size difference is large. 2) The co-deposited alloy films consist of ultra fine grains with the same crystal structure as that of the stable phase; in this case also diffuse halos are shown in their diffraction patterns. 3) The extension of the solid solution range, namely supersaturation.    If the co-deposited films show diffuse halos in their diffraction patterns, it is difficult to distinguish the Type 1) from 2). Mossbauer effect is well suited to the study of local atomic and electronic structure of alloys. Both crystallographic and magnetic properties of co-deposited alloy films will be elucidated by Mossbauer spectroscopic studies.    In this report, the results on the system of Fe-Sb are described. The equilibrium  * Ilkfgg , f)L A.B3f : Laboratory of Solid State Chemistry, Institute for Chemical    Research, Kyoto University, Uji, Kyoto 611. ** Facility for Inorganic Synthesis, Institute for Chemical Research, Kyoto University,   Uji, Kyoto 611. (310)              Formation  of  Metastable Phases in Vapor-Deposited FeSb Films solubilities are limited (Sb in Fe is about 4 at% and Fe in Sb is practically ni17)). The atomic size difference is fairly large (Fe: 1.25 A and Sb: 1.59 A). Therefore, the appearance of the metastable phases in co-deposited alloy films is expected. In the present study, using the electron diffraction, Mossbauer effect, magnetic suscep-tibility and electrical resistivity measurements, metastable phases in FeSb co-deposited alloy films are detected and the magnetic properties are investigated.                      EXPERIMENTAL PROCEDURE    The films with thicknesses of 400-900 A were prepared by simultaneous deposi-tions of Fe (purity 99.9%) and Sb (purity 99.999%). The deposition sources on tungsten boats are heated separately and deposited onto an acetylcellulose sheets maintained at room temperature. The vacuum during the deposition was in the order of 10-5 torr. The deposition rate of 1-5 A/s was checked with quartz crystal thickness monitor and was constant during the deposition-run within a few percent. The film composition was determined as follows. The films were dissolved into 3-4 M hydrochloric acid and the concentrations of iron and antimony were measured by atomic absorption spectrometry.     For the electron diffraction measurements, the acetyl-cellulose sheets were removed by methyl acetate and then the films were examine by JEM-7 electron microscope. Mossbauer effect measurements were made using a conventional spectrometer. 57Co diffused in copper plate was used as a source. The velocity scale was calibrated by using an iron foil as the standard absorder.    The electrical resistivity measurements were carried out from 2 K to 300 K by means of the usual four-point method using a dc potentiometer.     Magnetic susceptibility and magnetization were measured between 4.2 K and 300 K with a magnetic torsion balance. Those measurements were made after removing the acetylcellulose in order to avoid the diamagnetic contribution.    Aging behavior of these alloy films was also studied by using Mossbauer effect. The alloy films were sealed in evacuated pyrex tubes and annealed at different tem-peratures for 30 minutes.                       RESULTS AND DISCUSSION    From electron diffraction measurements, the obtained films were classified into four groups. Alloys with up to 34 at%Sb concentration had bcc structure and alloys with 52-.90 at%Sb had amorphous structure. While, alloys with 34-52 at%Sb were mixture of bcc solid solution and amorphous alloy, and those with 90-100 at%Sb were also mixture of amorphous alloy and Sb. No effect of deposition rate on the formation of alloy phases was observed in this experiment. Figure 1 shows typical electron diffraction patterns for FeSb alloy films.    The 57Fe Mossbauer spectra at 295 K for FeSb alloy films with 4, 17, and 28 at%Sb shown in Fig. 2 are examples of those observed for the bcc phase. The line intensity ratio of nearly 3:4:1 in each hyperfine spectrum indicates that the magne-tization lies in the film plane. The line width increases significantly with increasing (311)                  T.  SHIGEMATSU, T. SHINJO, T. BANDO, and T. TAKADA ab ~~.. • \   •,s 4 cd s~a • IP • F• •      rig. 1. Electron on-traction patterns o1 FeSb co-deposited films: (a) bcc phase;             (b) mixture of bcc and amorphous phase; (c) amorphous phase; (d)              mixture of amorphous phase and Sb. 4 17 .L' . •                                              28   II                -8 -6 -4-2 0 2 4 6 8 V(mm/s)              Fig. 2. Mossbauer spectra of bcc Feloo_xSbx films at 295 K. (312)              Formation of Metastable Phases in Vapor-Deposited FeSb Films Sb content. Since the broadening of the outer lines are the most remarkable, the distribution of the hyperfine field should be the main reason of the broadenings. The hyperfine field of Fe depends on the number of Sb neighbors and their  configura-tions. It is reasonable that the distribution becomes wider with increasing of Sb concentration. The mean hyperfine field at 295 K decreases with increasing Sb content, namely 330, 311, and 300 kOe for alloys with 4, 17, and 28 at%Sb, respec-tively. These facts indicate that a high degree of atomic cluster formation, observed in co-deposited FeCu alloy films,8) does not occur in these alloy films. Sb atoms may be solved randomly in iron matrix.    The central broad doublet line observed in the spectra of 17 and 28 at%Sb is due to the oxidized fraction. At 4.2 K, these oxide phases show magnetic hyperfine spectra. The observed Mossbauer parameters, I.S. (vs a-Fe)=0.32 mm/s, dE=1.10 mm/s and FWHM=1.00 mm/s at 295 K and H (mean hyperfine field) =480 kOe at 4.2 K are similar to those observed in amorphous iron (III) oxide films obtained by reactive evaporation method.6)    After aging the specimen of Fe83Sb17 for 1 hour at 530 K, no remarkable change was observed in the Mossbauer spectra, indicating that the most of the iron atoms remained in the metastable bcc FeSb alloy and the local Sb configuration around the Fe atoms was the same as those in the as-deposited alloy. After aging the same specimen at 580 K for 1 hour, the proportion of oxide phase increased and a part of it shows magnetic hyperfine structure even at 295 K. Because of the oxide formation and grain growth of the oxide phase after aging at higher temperatures, Mossbauer spectrum becomes very complicated. It is difficult to know the segregation-start temperature and what phase is obtained after aging.    The Mossbauer spectra at 295 K for FeSb alloy films with 38 and 44 at%Sb are shown in Fig. 3. In the spectra, a quadrupole splitting paramagnetic doublet due to amorphous phase is superposed to broad 6-line spectra. Based on the integrated intensities of the two patterns, 82% and 50% of the iron atoms are contained in the bcc phase in the Fefi2Sb38 and Fe56Sb44, respectively. We may assume here that the solubility of Sb into bcc phase is the same in these two alloys and also the composi-tion of amorphous phase is the same. We can thus estimate the solubility limit of Sb in bcc phase and concentration of amorphous phase from these results. Then, solubility limit of Sb in bcc phase is obtained as 34 at%Sb and alloys above 52 at%Sb are amorphous phase.    The Mossbauer spectra at 295 K for FeSb alloy films with 53, 68, 77, and 90 at%-Sb shown in Fig. 4 are typical of those observed for amorphous alloys. Mossbauer spectra show a well-resolved quadrupole splitting at 295 K, except for the case of. 53 at%Sb9). The values of I.S. and dE were 0.42 mm/s and 0.43 mm/s at 295 K independently of Sb content. In the case of 53 at%Sb, the Mossbauer spectrum was formed by two doublets. The major one has similar I.S. and dE as those of the films with compositions more than 62 at%Sb. The minor one has a larger dE (0.84 mm/s) and a smaller I.S. (0.37 mm/s), which means higher d electron density than the major part. The alloys Fe23Sb77 and Fe10Sb90 are Pauli paramagnetic with almost temperature independent susceptibilities (x=28 x 10-6 emu/g and 12 x 10-6 (313)                          T.  SHIGEMATSU, T. SHINJO, T. BANDO, and T. TAKADA 90 77                                                                                                                1L•                                                                                   • .                                                                                                                                                                                            • 38  _...,........ 68, 44•                                                                     53           \\if it I I I I 1 I I  II I     -8 -6 -4 -2 0 2 4 6 8 -101 V(MM/S)Velocity(mm/s)      Fig. 3. Mossbauer spectra of Feloo_xSbg alloy Fig. 4. Mossbauer spectra of amorphous             films at 295 K. These films are mixture Feloo_xSbx films at 295 K.              of bcc and amorphous phase. emu/g, respectively). All the other alloys which have more Fe content show a    magnetization which increases with decreasing temperature. Figure 5 shows the    temperature dependence of magnetization. At 4.2 K, the Mossbauer spectrum of Fe47    Sb53 shows unresolved magnetic hyperfine splitting which is superposed to the central    quadrupole doublet as is shown in Fig. 6. While, the Mossbauer spectra at 4.2 K      50 —  oH=8KG D 40—   E 0   •0 cG30—o0.y+,..,~,,••L420 -  00 00•                      000.0000 OO53   P3•10—000000000 0 0 0 0         0000000p000000100000001000000010000     00100 0001I I I I I    0100, 200 300-4 -20 2 4                      Temperature (°K)Velocity (mm/s)        Fig. 5. Temperature dependence of theFig. 6. Mossbauer spectra of amorphous              magnetization in amorphousFe47Sb53 film at 4.2 K. Feloo_xSbx films. ( 314 )                Formation of Metastable Phases in Vapor-Deposited FeSb Films   for 68, 77, and 90 at%Sb show only a quadrupole doublet. From these results, we   may  conclude that Fe atoms in amorphous alloys of 68, 77, and 90 at%Sb do not   carry a magnetic moment. Only a few Fe atoms which have many Fe nearest neighbor   atoms can bear a magnetic moment.      All amorphous alloys investigated here have a high electrical resistivity with a   very low temperature coefficient. The value and temperature coefficient of electrical   resistivity obtained in this experiment change remarkably with the film thickness.   It is difficult to obtain a concentration dependence of the value of electrical resistivity.   Experimental results for Fe23Sb77 with thickness of 720 A and Fe32Sb68 with 440 A shown   in Fig. 7 are typical for amorphous alloy films. The values at 2.2 K are 560 fsf •cm   for Fe23Sb77 and 2,446 ten • cm for Fe32Sb68. All of the films investigated here have   negative temperature coefficients. These negative temperature coefficients are often   observed in metallic amorphous alloys.10) These experimental results show that the   amorphous alloy films obtained here have a metallic conduction.                                                                                                                      • • cEi 2000-Fe32Sb68 440A 1000-                                        Fe23Sb77 720A           0100 200 300                                                 Temperature (K)             Fig. 7. Temperature dependence of the electrical resistivity of amorphous                    FeSb films.      Two intermetallic compounds, Fe3Sb2 (42-48 at%Sb) and FeSb2 (66.7 at%Sb)   were found in the equilibrium phase diagram of Fe-Sb system.7) Fe3Sb2 is isomor-  phous with NiAs (B8 type). This compound is antiferromagnetic with TN=210 K.11)   The observed Mossbauer parameters are I.S.=0.3 mm/s and AE=0.3 mm/s at 295 K   and H=100 kOe at 4.2 K. FeSb2 is isomorphous with marcasite FeS2 (C18 type).   FeSb2 appears to be more like a semimetal than a semiconductor.12) FeSb2 has an   extremely narrow band gap (-0.026 eV) and shows unusual electric and magnetic properties.12,13) At low temperatures (<80 K), the electrons are in the d4 low spin   state and FeSb2 is diamagnetic. As the temperature increases, a partial unpairing   of the 3d electrons due to promotion to the conduction band occurs and magnetic   susceptibility increased with increasing temperature. Above 280 K, van Vleck   temperature independent paramagnetism (x=1.8 x 10-6 emu/g) is observed. The   observed Mossbauer parameters are I.S. =0.450 mm/s and dE=1.268mm/s at 296   K.13) From the comparison of the magnetic and electrical properties of amorphous                                (315)                     T.  SHIGEMATSU, T. SHINJO, T. BANDO, and T. TAKADA 53 408K 95           77 415K , • • •+. ( I (   -2 -1 0 1 2-10 1 V (mm/s)V(mm/s)  Fig. 8. Mossbauer spectra of Fe47Sb53 and Fig. 9. Mossbauer spectrum of Fe5Sb95 at Fe23Sb77 films after crystallization. 295 K. This film is a mixture                                                      of amorphous phase and Sb.FeSb alloys with those of the intermetallic compounds, we can conclude that amorphous FeSb alloys obtained here are not the assembly of ultrafine grains of Fe3Sb2 and/or FeSb2.    After aging the specimens of amorphous alloys for 30 minutes. up to 365 K, no remarkable change was observed in the Mossbauer spectra. These amorphous alloys were stable for aging at 365 K. A remarkable change in the spectra occurred after aging the specimens at 408 K for 30 minutes as is shown in Fig. 8. Fe47Sb53 changes to mixture of crystalline Fe3Sb2 and FeSb2. At temperatures above 408 K, crystallization takes place in amorphous FeSb alloys with 52-66.7 at%Sb. Using Mossbauer effect, it is found that both Fe3Sb2 and FeSb2 are present after crystalliza-tion takes place. In the specimen of Fe23Sb77 aged at 415 K for 30 minutes, only a well-resolved quadrupole splitting due to FeSb2 was observed in the spectrum (Fig. 8). From this result and the fact that the solubility of Fe in Sb is practically nil in bulk alloy, we can conclude that the amorphous alloys with 66.7-90   at%Sb crystallize to the mixture of FeSb2 and Sb after aging at above 415 K. The melting tempera-ture (Tm) of Fe3Sb2 is about 1290 K and FeSb2 transforms to the mixture of Fe3Sb2 and liquid at 1000 K. From these values we can estimate a reduced crystallization temperature Tc/Tm for amorphous FeSb alloy. These values are from 0.32 to 0.40, which are similar to those obtained in other amorphous alloys.    The Mossbauer spectrum at 295 K for FeSb alloy film with 95 at%Sb shown in Fig. 9 is typical of those observed for mixture of amorphous alloy and Sb. Mossbauer spectrum show a quadrupole splitting at 295 K. This spectrum is similar to those observed in amorphous FeSb alloys. From this spectrum, we may conclude that all of iron atoms are contained in the amorphous phase in the FeSb alloy films with 90-.-100 at%Sb. The solubility of Fe in Sb in co-deposited FeSb alloy films is almost nil.    Two types of metastable alloy phases, supersaturated bcc solid solution and (316)              Formation of Metastable Phases in Vapor-Deposited FeSb Films metallic amorphous alloy were produced in co-deposited FeSb alloy films, as described before. The supersaturated solid solution exists in the composition range of  0-34 at%Sb. From the equilibrium phase diagram,7) the solubility of Sb in a-Fe is about 4 at% at room temperature and about 20 at% at 1275 K. This more extended equilibrium solid solubility at higher temperature may produce the supersaturated solid solution of 34 at%Sb. The metallic amorphous alloy exists in the composi-tion range of 52-.90 at%Sb. According to Mader,14) the factors which favor the appearance of an amorphous phase in co-deposited alloy films are a) a relatively large difference (-10%) in the atomic radii of the components and b) the absence of an extend equilibrium solid solubility. The difference in the atomic radii of the components is 21.4% for FeSb. This fact is favorable to obtain the amorphous alloy films with wide composition range. While, the solid solubility of Sb in Fe is about 4 at% at room temperature. This leads to the rather narrow composition range, 52-.90 at%Sb in co-deposited FeSb alloy films.                        ACKNOWLEDGMENT    The authors would like to thank to Professor M. Matsui and Dr. 0. Fujino for their help in the atomic absorption spectrometry.                             REFERENCES( 1) S. Mader, J. Vac. Sci. Technol., 2, 35 (1965). ( 2) E. Kneller, J. Appl. Phys., 35, 2210 (1964). ( 3) W. Keune, J. Lauer, and D. L. Williamson, J. de Phys., 35, C6-473 (1974). ( 4) W. Felsch, Z. Angew. Phys., 29, 217 (1970). ( 5) G. Marchal, Ph. Mangin, and Chr. Janot, Solid State Commun., 18, 739 (1976). ( 6) S. Mader, A. S. Nowick, and H. Widmer, Acta Met., 15, 203 (1967). ( 7) M. Hansen, "Constitution of Binary Alloys," 2nd ed, McGraw-Hill, New York, 1958, pp. 708-     710. ( 8) T. Shigematsu, Y. Bando, and T. Takada, J. de Phys., 40, C2-153 (1979). ( 9) T. Shigematsu, T. Shinjo, Y. Bando, and T. Takada, Submitted for publication in J. Mag-     netism Magn. Mat. (10) K. H. J. Buschow, A. M. van Diepen, N. M. Beekmans, and J. W. M. Biesterbos, Solid State     Phys., 28, 181 (1978). (11) F. W. Richter and K. Schmidt, Z. Naturforsch., 30a, 1621 (1975). (12) A. K. L. Fan, G. H. Gerald, H. Rosenthal, H. L. McKinzie, and A. Wold, J. Solid State Chem.,     5, 136 (1972). (13) J. Steger and E. Kostiner, J. Solid State Chem., 5, 131 (1972). (14) S. Mader, J. Vac. Sci. Technol., 7, 175 (1970). (317)

Formation of Metastable Phase in Vapor-Deposited FeSb Films

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Formation of Metastable Phase in Vapor-Deposited FeSb Films

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