Mössbauer study of FeCoNbBCu hitperm-type alloys

Mössbauer study of FeCoNbBCu hitperm-type alloys J. S. Blázquez, A. Conde, and J. M. Grenèche Citation: Applied Physics Letters 81, 1612 (2002); doi: 10.1063/1.1504164 View online: http://dx.doi.org/10.1063/1.1504164 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/81/9?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of Co addition on the atomic ordering of FeCo-phase in nanocrystalline Fe81-xCoxNb7B12 alloys (x = 20.25, 27, 40.5, 54, 60.75): An anomalous diffraction and Mössbauer study J. Appl. Phys. 114, 083516 (2013); 10.1063/1.4819398 The influence of isochronal annealing on the crystallization and magnetic properties of Fe40.5Co40.5Nb7B12 alloy AIP Conf. Proc. 1447, 265 (2012); 10.1063/1.4709981 Influence of isochronal annealing on the microstructure and magnetic properties of Cu-free HITPERM Fe40.5Co40.5Nb7B12 alloy J. Appl. Phys. 111, 113518 (2012); 10.1063/1.4728161 Crystallization behavior and high temperature magnetic phase transitions of Nb-substituted FeCoSiBCu nanocomposites Appl. Phys. Lett. 99, 192506 (2011); 10.1063/1.3660245 Thermomagnetic detection of recrystallization in FeCoNbBCu nanocrystalline alloys Appl. Phys. Lett. 79, 2898 (2001); 10.1063/1.1413957 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 150.214.182.17 On: Wed, 27 Jan 2016 12:07:45 Mo ¨ssbauer study of FeCoNbBCu hitperm-type alloys J. S. Bla ´zquez and A. Condea) Departamento Fı ´sica Materia Condensada, CSIC-Universidad Sevilla, Apartado 1065, 41080 Sevilla, Spain J. M. Grene `che Laboratoire de Physique de L’Etat Condense ´, Universite ´du Maine, 72085 Le Mans, France 共Received 17 April 2002; accepted for publication 10 July 2002兲 FeCoNbBCu alloys have been studied by Mo ¨ssbauer spectrometry. A high contribution of the interfacial region in the nanocrystalline samples to the hyperfine field distribution is detected. The possible presence of B in the nanocrystalline grains is also discussed. Low values of the hyperfine field with respect to the disordered ␣ -FeCo phase suggest the presence of the ordered ␣ ⬘-FeCo phase. Recrystallization phenomena at the second crystallization stage are confirmed. © 2002 American Institute of Physics. 关DOI: 10.1063/1.1504164兴 Nanocrystalline Fe-based alloys in which nanosized grains of a soft ferromagnetic crystalline phase are embedded in a soft ferromagnetic amorphous matrix have been found to be excellent soft magnetic materials. FINEMET alloys 共derived from precursor FeSiNbBCu1兲and NANOPERM alloys2关FeMB(Cu) with M⫽Zr, Nb, Hf...兴have been widely studied and are among the softer magnetic materials known up to now. In both series the addition of about 1 at.% Cu is useful to improve the nucleation process, refining the grain size and thus improving the soft magnetic properties.3 These outstanding magnetic properties are reduced by increasing the temperature above the Curie temperature of the amorphous phase. HITPERM alloys (FeCoMBCu),4due to the partial substitution of Fe by Co, exhibit a higher Curie temperature of the amorphous phase. 57Fe Mo ¨ssbauer spectrometry is a nondestructive and effective tool to probe the Fe environment in these FeCo-based alloys. Recently, a few studies have been published on Zrcontaining HITPERM alloys5,6 and CoFeNbB alloys.7 In the present work a series of melt-spun ribbons 共⬃20 ␮ m thick and 5 mm wide兲of nominal composition Fe78⫺xCoxNb6B15Cu1共x⫽18, 39, and 60兲has been studied at room temperature by transmission Mo ¨ssbauer spectrometry 共57Co source diffused into a Rh matrix兲共1兲in the as-cast state, 共2兲at the end of the nanocrystallization process 共annealed under argon atmosphere at 10 K/min up to 873 K for the alloys with 18 and 39 at.% Co, or 823 K for the 60 at.% Co containing alloy兲, and 共3兲after the second crystallization process 共annealed at 10 K/min up to 948 K for the alloy with 60 at.% Co兲, in which the ␣ -FeCo phase and the amorphous matrix partially recrystallizes into the (FeCo)23B6phase.8 The hyperfine parameters were refined using the MOSFIT program. The values of the isomer shifts 共IS兲are quoted relative to the ␣ -Fe foil at 300 K. Figure 1 shows the Mo ¨ssbauer spectra for the studied samples. For the as-cast samples they exhibit magnetic sextets with broadened and overlapped lines, assigned to the structural disorder of the amorphous state. The fitting procedure thus requires a distribution of sextets. In the 18 at.% Co alloy, a small crystalline contribution clearly occurs, which may be associated with surface crystallization of the ribbon,9 not detected neither by x-ray diffraction 共XRD兲nor by transmission electron microscopy experiments, because of its small content (⬃2%) and because of the electropolishing treatment of the two faces of the ribbon, respectively. The hyperfine field (Bhyp) distribution for the nanocrystalline samples 共Fig. 2兲can be roughly divided into three contributions: crystalline, interface, and amorphous.10 Results are summarized in Table I. The lower values of the averaged hyperfine field of the residual amorphous matrix in the nanocrystalline samples compared to the as-cast samples must be due to the enrichment of the matrix in Nb and B.11 The angle 共 ␪ 兲between the incident ␥ radiation and the hyperfine field shows similar trends with the Co content of the alloy to the saturation magnetostriction (␭s).12 In fact, both ␭sand ␪ decrease as the Co content increases in the alloy for the as-cast samples, but during nanocrystallization this dependency is reversed, increasing both ␭sand ␪ as the Co content increases in the alloy. However, no direct correlation between ␭sand ␪ can be found because ␭sincreases after nanocrystallization 共except for the 18 at.% Co alloy兲 but the angle is lower in the nanocrystalline samples than in the as-cast samples, indicating the important effect of the microstructure. The Bhyp range for the crystalline contribution in the nanocrystalline grains is 37.5–33.5 T for the 18 at.% Co alloy, 35.2–32.7 T for the 39 at.% Co, and 34.2–33.0 T for the 60 at.% Co. The broader distribution for the lowest Co content is in agreement with the behavior found in bcc FeCo binary alloys,13,14 in which a steeper variation of Bhyp can be found for the lowest Co content, until 30 at.% Co, where a maximum is reached. After this maximum the decrease is less marked, reaching values lower than 33.0 T for the highest Co concentration explored (⬃75 at. %).13 Assuming for the crystalline phase the composition estimated by atomprobe experiments,11 in which the Co concentration is homogeneous throughout the nanocrystalline grains and the amorphous matrix, we can observe the Bhyp dependence with the Co content of the nanocrystalline grains. The values obtained are in agreement with those reported in Ref. 14 for binary alloys with 18 and 60 at.% Co. For the 39 at.% Co alloy, we have obtained a lower value. For this alloy, even the full a兲Author to whom correspondence should be addressed; electronic mail: [email protected] APPLIED PHYSICS LETTERS VOLUME 81, NUMBER 9 26 AUGUST 2002 16120003-6951/2002/81(9)/1612/3/$19.00 © 2002 American Institute of Physics This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 150.214.182.17 On: Wed, 27 Jan 2016 12:07:45 range of Bhyp distribution 共32.7–35.2 T兲corresponding to the crystalline phase has lower values than those corresponding to the binary alloy. This could be due to the experimental errors, but it is important to note that the values of Ref. 14 correspond to a disordered ␣ -FeCo phase. It was previously observed that the ordered ␣ ⬘-FeCo phase exhibits lower values for Bhyp .13 In fact, if an ordered phase is formed with the nanocrystalline grain composition Fe61Co39 共which is in the range of stability for the ordered phase at room temperature15兲, the mean Fe environment changes from the ordered to the disordered phase. As the number of Co first neighbors in the ordered phase is larger, this would result in a lower value of Bhyp for this phase. In addition, the behavior of 具 IS 典 for the crystalline contribution in the nanocrystallized samples with 39 and 60 at.% Co, showing approximately equal values, agrees with the results found for the ordered ␣ ⬘-FeCo phase, whereas for the disordered phase a continuous decrease is found.13 The ␣ ⬘-FeCo phase has been experimentally detected in Fe44Co44Zr7B4Cu1HITPERM alloy using synchrotron radiation.4 In the nanocrystalline samples an important contribution of the interface is observed 共between 14%–20% of the total Fe兲. This might be due to the small size of the nanocrystalline grains in these alloys (⬃5 nm).16 Indeed, if we consider the ratio between the Fe percentage in the pure crystalline phase and the Fe content in the combined crystalline phase and interface, the interface thickness can be estimated at around 0.4–0.6 nm, which is roughly twice the lattice parameter of the bcc ␣ -FeCo phase. Other authors consider that this region of Bhyp ranged from ⬃20 to ⬃32 T, corresponds to the presence of nonferromagnetic atoms in the nanocrystals 共B, Nb兲.5However, our results indicate that the region is more important in these alloys with smaller crystalline grains, that is, higher surface/volume ratio. Atom-probe experiments show rejection of Nb to the matrix, but seems to detect ⬃5 at.% B inside the nanocrystals,11 a value much higher than the solubility of B expected in the ␣ -FeCo phase and in contradiction with theoretical predictions.17 This result, detected in other nanocrystalline compositions by atom probe,3could be an artifact, due to the diffusion of B into the nanocrystalline grains during the atom-probe experiment. From Mo ¨ssbauer experiments it is possible to obtain data which might help to elucidate this problem. Assuming the homogeneity of Co distribution in the material, we have assumed two possible nanocrystalline grain compositions: 共a兲B and Nb are completely rejected from the nanocrystalline grains to the matrix. Thus, the composition of the nanocrystals would be Fe100⫺xCox, with xthe Co content in the alloy. 共b兲The Nb is completely rejected from the nanocrystalline grains, but a residual 5 at.% B remains into the nanocrystals. Thus, the compositions would be Fe95⫺xCoxB5. Usually the crystalline atomic fraction (aC) is assumed to be equal to the crystalline volume fraction (xC) and is compared directly with XRD results. However, in these alloys, due to the high B content, the averaged atomic radii in the amorphous matrix and in the crystalline phase are very different. In other nanocrystalline alloys this difference is much smaller: due either to a lower B content in the case of HITPERM4and NANOPERM2compositions with Zr, to the presence of small atoms in the crystalline phase, such as Si in FINEMET alloys.1The expression: xC⫽aC关 具 RC 典 / 具 R 典 兴3 共where 具 RC 典 and 具 R 典 are the averaged radii in the crystalline and in the whole alloy, respectively兲, relates xCand aC.If we assume that half of the Fe atoms in the interface correspond to the crystalline contribution we can obtain xCfor the two assumptions proposed 共0 and 5 at.% B content in the nanocrystalline grains兲. The comparison with the XRD data, in which the scattering powers of the two different phases FIG. 1. Mo ¨ssbauer spectra for as-cast samples 共a兲18 at. % Co, 共b兲39 at. % Co and 共c兲60 at. % Co containing alloys. For nanocrystalline samples 共d兲18 at. % Co, 共e兲39 at. % Co, 共f兲60 at. % Co containing alloys previously heated at 10 K/min up to 873, 873, and 823 K, respectively, and 共g兲for fully crystallized sample of 60 at. % Co containing alloy, previously heated up to 948Kat10K/min. FIG. 2. Hyperfine field distribution for nanocrystalline samples of 18 共squares兲,39共circles兲and 60 共crosses兲at. % Co alloys, previously heated at 10 K/min up to 873, 873, and 823 K, respectively. 1613Appl. Phys. Lett., Vol. 81, No. 9, 26 August 2002 Bla ´zquez, Conde, and Grene `che This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 150.214.182.17 On: Wed, 27 Jan 2016 12:07:45 共amorphous and crystalline兲have been taken into account,16 is presented in Figs. 3共a兲and 3共b兲, assuming 0 and 5 at.% B in the nanocrystalline grains, respectively. It can be observed that a better agreement is obtained under the assumption of complete B rejection from the nanocrystalline grains, but lower values for the Mo ¨ssbauer experiments have been found. This could be due to an overestimation of xCby XRD technique, or because the contribution of the interface to the crystalline diffraction phase is more than half of the atoms. In the two cases we have a smaller crystalline fraction for the 60 at.% Co alloy at the end of the nanocrystallization process compared to the other alloys, in agreement with our previous results.16 This difference was explained in terms of an amorphous matrix exhausted in Fe at the end of nanocrystallization for the 60 at.% Co alloy. In fact, the Fe content in the residual amorphous matrix is ⬃6% of the total atoms, assuming half of the interface contributing to the nanocrystalline grains and complete rejection of the B atoms. For the fully crystallized sample of the 60 at.% Co alloy it is clear that the number of Fe atoms belonging to the ␣ -FeCo phase decreases. In this process part of the ␣ -FeCo nanocrystalline grains, along with the surrounding amorphous matrix, recrystallizes forming a (FeCo)23B6phase,8 which is ferromagnetic at room temperature. This phase is the main boride phase present at this stage, so the parameters obtained from the distribution of Bhyp might correspond mainly to this phase. The similar values of 具 Bhyp 典 and 具 IS 典 found in the nanocrystallized and the fully crystallized samples of the 60 at.% Co alloy, agree with the lattice parameter behavior for the ␣ -FeCo phase, which remains constant after the transformation. The parameters found for the (FeCo)23B6phase are similar to those found for the as-cast sample. In fact, the composition in Fe, Co, and Nb of the boride phase, measured by windowless energy-dispersive X-ray analysis are very similar to that of the as-cast sample and, therefore, the chemical environment between the initial amorphous and the (FeCo)23B6phase might be similar. The presence of several different atoms in this phase explains the very broad hyperfine field distribution used, due to the large number of possible different Fe sites. This work was supported by the Spanish Government and EU FEDER 共Project PB97-1119-C02-01 and MAT 2001- 3175兲and by the PAI of the Junta de Andalucı ´a. J.S. Bla ´zquez acknowledges a research fellowship of the DGES. 1Y. Yoshizawa, S. Oguma, and K. Yamauchi, J. Appl. Phys. 64, 6044 共1988兲. 2K. Suzuki, A. Makino, N. Kataoka, A. Inoue, and T. Masumoto, Mater. Trans., JIM 32,93共1991兲. 3Y. Zhang, K. Hono, A. Inoue, and T. Sakurai, Mater. Sci. Eng., A 217Õ218, 407 共1996兲. 4M. A. Willard, D. E. Laughlin, M. E. McHenry, D. Thoma, K. Sickafus, J. O. Cross, and V. G. Harris, J. Appl. Phys. 84, 6773 共1998兲. 5T. Keme ´ny, D. Kapta ´s, L. F. Kiss, J. Balogh, L. Bujdoso ´, J. Gubicza, T. Unga ´r, and I. Vincze, Appl. Phys. Lett. 76,2110共2000兲. 6M. Kopcewicz, A. Grabias, M. A. Willard, D. E. Laughlin, and M. E. McHenry, IEEE Trans. Magn. 37, 2226 共2001兲. 7S. N. Kane, A. Gupta, L. Kraus, and P. Duhaj, J. Magn. Magn. Mater. 215–216, 375 共2000兲. 8J. S. Bla ´zquez, C. F. Conde, and A. Conde, Appl. Phys. Lett. 79,2898 共2001兲. 9Y. Q. Wu, T. Bitoh, K. Hono, A. Makino, and A. Inoue, Acta Mater. 49, 4069 共2001兲. 10M. Miglierini and J. M. Grene `che, J. Phys.: Condens. Matter 9, 2321 共1997兲. 11 Y. Zhang, J. S. Bla ´zquez, A. Conde, P. J. Warren, and A. Cerezo 共in press兲. 12J. S. Bla ´zquez, V. Franco, A. Conde, M. R. J. Gibbs, H. A. Davies, and Z. C. Wang 共in press兲. 13B. DeMayo, D. W. Forester, and S. Spooner, J. Appl. Phys. 41, 1319 共1970兲. 14H. H. Hamdeh, J. Okamoto, and B. Fultz, Phys. Rev. B 42, 6694 共1990兲. 15O. Kubaschewski, in Iron-Binary Phase Diagrams 共Springer, Berlin, 1982兲, pp. 27–31. 16J. S. Bla ´zquez, V. Franco, C. F. Conde, and A. Conde 共in press兲. 17A. R. Yavari and O. Drbohlav, Mater. Trans., JIM 36, 896 共1995兲. TABLE I. Hyperfine parameters obtained for the different phases detected 共samples identification as in Fig. 1兲. Sample Phase 具 Bhyp 典 共T兲 具IS典 共mm/s兲 ␪ 共degrees兲 a amorphous 22.1 0.023 63.6 b amorphous 24.2 0.054 62.5 c amorphous 25.2 0.085 59.2 amorphous 11.3 0.056 d interface 27.1 37.3 ␣ -FeCo 35.9 0.040 amorphous 10.5 0.047 e interface 28.0 41.7 ␣ -FeCo 34.6 0.023 amorphous 8.4 0.020 f interface 29.5 43.4 ␣ -FeCo 33.8 0.021 g ␣ -FeCo (FeCo)23B6 33.6 23.7 0.020 0.061 55 FIG. 3. Comparison between crystalline volume fraction obtained from XRD 共squares兲and from Mo ¨ssbauer spectroscopy 共circles兲. Crystalline atomic fraction 共crosses兲appears for comparison, assuming 共a兲0% and 共b兲5 at. % B in the nanocrystalline grains. 1614 Appl. Phys. Lett., Vol. 81, No. 9, 26 August 2002 Bla ´zquez, Conde, and Grene `che This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 150.214.182.17 On: Wed, 27 Jan 2016 12:07:45