The Evolution of Magnetostriction and Coercivity with Temperature in the Early Stages of Nanocrystallisation in Feconbb(Cu) Alloys

Journal of Magnetism and Magnetic Materials 250 (2002) 260–266

The evolution of magnetostriction and coercivity with temperature in the early stages of nanocrystallisation in FeCoNbB(Cu) alloys

J.S. Blazquez! a, V. Francoa, A. Condea,*, M.R.J. Gibbsb, H.A. Daviesc, Z.C. Wangc a Departamento de F!ısica de la Materia Condensada, Instituto de Ciencia de Materiales, C.S.I.C., Universidad de Sevilla, P.O. Box 1065, 41080 Sevilla, Spain b Department of Physics and Astronomy, University of Sheffield, Sheffield, S3 7RH, UK c Department of Engineering Materials, University of Sheffield, Sheffield, S1 3JD, UK

Received 5 February 2002

Abstract

Saturation magnetostriction and coercivity of Fe78xCoxNb6B16yCuy (x ¼ 18; 39; 60; y ¼ 0; 1) alloys annealed at different temperatures have been studied. Coercivity behaviour at the early stages of nanocrystallisation is described on the basis of the contribution to the overall magnetostriction of the residual amorphous and nanocrystalline phases. r 2002 Elsevier Science B.V. All rights reserved.

PACS: 75.30.Gw; 75.50.Tt; 75.80+q

Keywords: Nanocrystalline materials; Magnetostriction; Coercivity; Anisotropy magnetic

1. Introduction [4]. Moreover, for optimal annealing conditions (minimum in coercivity), the different sign in the Nanocrystalline alloy systems, consisting of saturation magnetostriction constant (lS) between nanosize ferromagnetic crystals magnetically the amorphous and the crystalline phase, volume coupled via a ferromagnetic amorphous matrix averages the net magnetostriction to a value close phase, within which they are embedded, have to zero [1]. Recently, a new family of alloys, outstanding soft magnetic properties which make known as HITPERM and composed of FeCoMB- them suitable for industrial applications [1]. In Cu [5], has been developed with the aim of FINEMET-FeSiBNbCu- [2] and NANOPERM- extending the outstanding soft magnetic properties FeMB(Cu) (M=Zr, Nb, Hfy)- [3], the ultra- of FINEMET and NANOPERM to higher small grain size results in averaging of the temperatures. This is based on the higher Curie magnetocrystalline anisotropy to very low values temperature of the amorphous phase in HIT- PERM alloys, due to the partial substitution of Fe *Corresponding author. Tel.: +34-95-4552-885; fax: +34- by Co, which allows the magnetic coupling 95-4612-097. between the nanocrystals through the ferromag- E-mail address: [email protected] (A. Conde). netic matrix over a broader temperature range. In

0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0304-8853(02)00406-7 J.S. Blazquez! et al. / Journal of Magnetism and Magnetic Materials 250 (2002) 260–266 261

1 these alloys, the sign of lS does not change from where DHDC (A m ) is the change in the DC the amorphous phase to the crystalline phase [6], applied field needed to compensate for the change 1 so it is not expected to have an averaged value of the induced voltage, MS (A m ) is the close to zero. saturation magnetisation and DT (N m2) is the This non-negligible value of the magnetostriction stress applied to the sample. The SAMR experi- constant must affect the soft magnetic properties of ments were performed at 621 Hz (chosen as a low these alloys through the magnetoelastic anisotropy, noise window) and applying tensioning weights especially in the early stages of the nanocrystallisa- ranging from 10 to 700 g, depending on the sample tion, when the magnetocrystalline anisotropy is still brittleness. Owing to the difficulty of measuring low. Therefore, a detailed study of the magnetos- accurately the cross-sectional area of the ribbons, triction evolution with the microstructure and of its which is necessary for obtaining DT; Eq. (1) can be dependency on the phase compositions is necessary modified to [8] to understand the behaviour of the magnetic 1 DHDC properties in these materials. lS ¼ sSWm ; ð2Þ 3 Dm 0 In a previous study, a description of the coercivity behaviour of Nb-containing HIT- where Dm is the mass which is used to apply stress PERM-type alloys at the start of the nanocrys- to the sample (kg), sS is the magnetisation per unit 1 1 tallisation was proposed in terms of the published mass (A m kg ), W is the weight per unit length 1 1 magnetostriction data for the residual amorphous of the ribbon (N m ) and DHDC (A m )isas and the emerging crystalline phases [7]. In the above. present study, magnetostriction and DCcoercivity Room temperature saturation magnetisation measurements have been performed on isother- values were obtained in an Oxford Instruments mally annealed Fe78xCoxNb6B15Cu1 (x ¼ 18; 39 vibrating sample magnetometer at an applied and 60) samples with the aim of investigating induction of 0.4 T. Room temperature DC further the applicability of the previous analysis. (0.01 Hz) coercivity values were obtained with a The influence of the Cu addition has also been hysteresis loop tracer [9]. Transmission electron studied for these alloys, comparisons being drawn microscopy (TEM) studies of the sample micro- with the corresponding Fe78xCoxNb6B16 alloy structure were performed with a Philips CM200 series. (200 kV) instrument.

2. Experimental 3. Results

Amorphous ribbons, B20 mm thick and B5mm Fig. 1 shows the evolution of the DCcoercivity with increasing annealing temperature for the Cu- wide, of nominal compositions Fe78xCox- free and Cu-containing alloys. From kinetic data Nb6B16yCuy (x ¼ 18; 39, 60; y ¼ 0; 1) were produced at Warsaw University of Technology [10], the incubation time for the onset of the by the single roller melt-spinning technique. nanocrystallisation process for the 60% Co alloys Samples B90 mm long were annealed in an argon has been estimated to be less than 15 min, at atmosphere for 15 min at various temperatures. temperatures 45 K below the continuous heating The magnetostriction measurements were per- crystallisation onset temperature for the alloy formed using the small angle magnetisation rota- without Cu, and 30 K below the onset for the tion (SAMR) technique [8]. The net saturation alloy with Cu (measured from DSC runs at 10 K/ min [11]). These estimated nanocrystallisation magnetostriction constant, lS; of the sample can be obtained from onset temperatures are indicated in Fig. 1. Taking account of the crystallisation onset temperatures 1 DHDC in these isothermal treatments, it is possible to l ¼ M m ; ð1Þ S 3 DT S 0 divide the explored temperature range into two 262 J.S. Blazquez! et al. / Journal of Magnetism and Magnetic Materials 250 (2002) 260–266

0at.%Cu 1at.%Cu 0at.%Cu 1at.%Cu

30 100 60 at. 20 60 at. % Co %Co 10 10

100 30

39 at. (ppm) 20 39 at. (A/m) S C 10 %Co λ %Co H 10

30 18 at. 20 18 at. 10 %Co % Co 10

400 600 800 400 600 800 400 600 800 400 600 800 Ta (K) Ta (K) Fig. 1. Coercivity values versus annealing temperature for Cu- Fig. 2. Magnetostriction values versus annealing temperature free and Cu-containing alloys. Arrows indicate the onset of for Cu-free and Cu-containing alloys. Arrows indicate the onset crystallisation for the different alloys. The error bar, estimated of crystallisation for the different alloys. The error bars are from the instrument performance at 70.5 A m1 is similar to generated from statistical analysis of the SAMR data sets used the size of the symbols, and is omitted for clarity. to derive the saturation magnetostriction constants. Dotted arrow for 60 at% Co, 0 at% Cu alloy indicates the expected behaviour of lS; out of the measuring range. zones. The first zone, up to 45 K below the crystallisation onset for the Cu-free alloys (30 K for the Cu-containing alloys), corresponds to the nanocrystallisation detected. It has been shown occurrence of relaxation phenomena in the amor- that DCcoercivity techniques are the most phous phase. The second zone, above these sensitive way of detecting the onset of crystal- respective temperatures, corresponds to the nano- lisation (better than DSCor TEM) [12]. crystallisation process itself. Fig. 2 shows the magnetostriction results for the For annealing temperatures below the crystal- Cu-free (left) and Cu-containing alloys (right). For lisation onset (o675 K for the 60 at% Co alloys the Cu-free alloys a continuous increase in the and o700 K for the 39 and 18 at% Co alloys), in saturation magnetostriction constant occurs for the relaxation zone, the 60 at% Co alloys show a the 39 and 60 at% Co alloys after annealing at slight increase in coercivity. The 39 at% Co alloys temperatures above the crystallisation onset. In show a slight decrease towards the end of the contrast, for the 18 at% Co alloy, the behaviour is relaxation zone, more evident in the case of the more complex, with an initial slight rise followed alloy without Cu. For the 18 at% Co alloys a more by a decrease. significant decrease is observed for the alloy The behaviour of the Cu-containing alloys is without Cu, but a nearly constant value in the similar to that observed for the Cu-free alloys. lS relaxation zone can be observed for the alloy is larger after the nanocrystallisation onset than with Cu. for the respective as-cast sample for the alloys with For annealing temperatures above the crystal- 39 and 60 at% Co, whereas the value for lS for lisation onset the coercivity increases as the samples of the 18 at% Co alloy annealed at annealing temperature increases. Only in the temperatures above the nanocrystallisation onset 18 at% Co alloys is a decrease at the start of are similar to that for the as-cast sample. However, J.S. Blazquez! et al. / Journal of Magnetism and Magnetic Materials 250 (2002) 260–266 263 for the 18 and 39 at% Co- and Cu-containing Fig. 3 shows hysteresis loops for the samples alloys annealed at 650 K, an increase in the annealed below their corresponding crystallisation saturation magnetostriction can be observed. This onset. The hysteresis loops for the 39 and 60 at% increase is larger for the 18 at% than for the Co alloys, with or without Cu, show a wasp- 39 at% Co alloy. waisted shape, characteristic of the domain wall stabilisation effect [6,14–16] associated with annealing below the Curie temperature. For the 4. Discussion 18 at% Co alloy, the hysteresis loop is smoother, without the step. We interpret this in terms of the The coercivity values for the Cu-containing lower Curie temperature of these alloys (o650 K). alloys agree with those reported previously for Samples of the 39 at% Co-containing and Cu-free samples submitted to continuous heating treat- alloys annealed at temperatures below crystal- ments [7] and only for the 18 at% Co alloy does a lisation onset (and consequently below the Curie minimum in coercivity occur at the onset of temperature of the amorphous phase) had low nanocrystallisation (725 K). The coercivity values coercivity values, but the domain wall stabilisation in the relaxation zone are lower than those effect in the hysteresis loop shape is clearly reported from continuous heating experiments observed (Fig. 3). [7]. We believe that this is due to a greater degree The microstructure of heat-treated samples of relaxation being achieved under the conditions (Fig. 4) consists of nanocrystals of mean size applied in this study. 5–20 nm, dispersed in a residual amorphous The behaviour of the coercivity of the present matrix. Selected area diffraction patterns (inset alloys for samples annealed in the relaxation zone, of Fig. 4) reveal the presence of these two phases. differs from the continuous decrease observed for As can be observed the presence of Cu refines the FINEMET (FeSiBNbCu) alloys [13], and can be microstructure (Fig. 4a). A similar nanostructure explained in terms of the Curie temperature of the type in FINEMET alloys is invoked to explain amorphous phase. It has been reported that the reduction in the net magnetic anisotropy [4]. annealing at temperatures below the Curie The observed trends of the saturation magneto- point can induce the stabilisation of domain walls striction and the coercivity with annealing [6,14–16]. This effect can be avoided by annealing history are consistent with the proposed depen- in a saturating field or by ACfield annealing [16]. dence of the coercive field on the magnetoelastic anisotropy in the early stages of nanocrystallisation [7]. For the Cu-free alloys containing 39 and 60 at% 0at.%Cu 1at.%Cu Co, lS is nearly constant in the relaxation zone but increases monotonically as the annealing tempera- 60 at. ture increases in the nanocrystallisation zone. %Co Atom-probe analytical data indicate that the compositions of the nanocrystals are Fe61Co39

.) and Fe40Co60 for the 39 and 60 at% Co alloys, 39 at. respectively [17], and the reported value of lS for %Co

M(a.u these compositions is B60 ppm [6]. This value is larger than that measured for the amorphous 18 at. phase. The measured magnetostriction is the net %Co value for the amorphous and nanocrystalline composite represented by the sample, and H(a.u.) some form of volume averaging will give a value Fig. 3. Hysteresis loops for samples of the various alloys lower than that for the nanocrystal composition studied, following annealing for 15 min at 650 K. alone. 264 J.S. Blazquez! et al. / Journal of Magnetism and Magnetic Materials 250 (2002) 260–266

alloy, whereas it occurs after the crystallisation onset for the Cu-free alloy. This could explain the fact that coercivity decreases with increasing annealing temperature below the nanocrystallisation onset in the Cu-free alloy, in contrast to the Cu-containing alloy where coercivity increases. After the nanocrystallisation onset, the Cu-containing alloy shows a pronounced minimum in coercivity, in contrast to the Cu-free alloy, although this might be also affected by the larger grain size observed for the latter alloy (Fig. 4b). The increase in coercivity detected towards the end of the temperature range investigated can be explained on the basis of crystallite (a) coarsening [4]. The observed increase of magnetostriction in the 20 nm relaxation range with respect to the as-cast sample for the Cu-containing alloys (DlS ¼ 17; 11 and 5 ppm for 18, 39 and 60 at% Co alloys, respectively) increases as the Co content of the alloy decreases. The main effect of the Cu addition in the nanocrystalline alloys is the formation of Cu clusters that enhances the nucleation of the nanocrystalline phase [18,19]. These clusters are formed at temperatures below the crystallisation onset and will change the composition of the amorphous phase slightly. This could be respon- sible for the appearance of higher magnetostric- (b) tion in the alloy before the nanocrystallisation in the Cu-containing alloys. The decrease of the Fig. 4. Bright field TEM image of Fe Co Nb B Cu (a) and 60 18 6 15 1 effect as the Co content increases in the alloy can Fe60Co18Nb6B16 (b) annealed at 703 and 716 K, respectively, for 10 min. Selected area diffraction patterns are included as an be interpreted in terms of a diminishing Cu cluster inset. density as the Co content increases, which is in agreement with recent results on the influence of Co reported for FINEMET [20] and for Zr- For the 18 at% Co alloy the expected value for containing HITPERM-type alloys [21]. lS of the crystalline phase is B10–20 ppm [6], The Cu-free 18 at% Co alloy shows a maximum similar to the value found for the amorphous in lS for samples annealed after the nanocrystalli- phase. Therefore, this could explain the lack of sation onset, unlike the Cu-containing 18 at% Co significant change in magnetostriction for samples alloy. This could be explained from the different annealed after the nanocrystallisation onset (1472 microstructures of the two 18 at% Co alloys and 1372 ppm for the nanocrystalline and as-cast (Fig. 4) induced by the Cu cluster formation. In sample, respectively). However, an increase in net the Cu-free alloy, the mean distance between lS can be observed at the beginning of nanocrys- nanocrystals is larger than for the Cu-containing tallisation (700 and 750 K). The simple volume alloy and, therefore, the amorphous matrix might averaging model is inadequate to account for this. be less homogeneous, particularly as these alloys For the 18 at% Co alloys the maximum lS contain a relatively high concentration (6 at% occurs in the relaxation zone for the Cu-containing overall) of the slow diffusing element Nb, which J.S. Blazquez! et al. / Journal of Magnetism and Magnetic Materials 250 (2002) 260–266 265 is being completely ‘ejected’ from the FeCo by the large distance between nanocrystals in the crystallites during the devitrification and thus Cu-free alloy with 18 at% Co. increasing further in concentration in the amorphous matrix. This inhomogeneity might be reduced as the nanocrystallisation progresses, because the crystal growth reduces the mean Acknowledgements thickness of the amorphous layer between the nanocrystals, which could explain the decrease in This work was supported by the Spanish Government and EU-FEDER (Projects PB97- lS as the nanocrystallisation progresses. These results agree with the detected maximum in 1119-C02-01 and MAT 2001-3175) and by the ! ! magnetisation for FINEMET alloys at the very PAI of the Junta de Andalucıa. J.S. Blazquez beginning of nanocrystallisation [22]. acknowledges a research fellowship of the There are a number of examples in the literature DGES. The research at Sheffield has been [6,23] where the compositional dependence of supported in part by the UK Engineering & saturation magnetostriction constant and satura- Physical Research Council under the Advanced tion magnetisation follow similar trends. 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