Phase Dynamics and Growth of Co - American Chemical Society

Article pubs.acs.org/crystal

Phase Dynamics and Growth of Co2Cr1−xFexAl Heusler Compounds: A Key to Understand Their Anomalous Physical Properties A. Omar, M. Dimitrakopoulou, C. G. F. Blum, H. Wendrock, S. Rodan, S. Hampel, W. Löser, B. Büchner, and S. Wurmehl* Leibniz Institute for Solid State and Materials Research, D-01171 Dresden, Germany S Supporting Information *

ABSTRACT: Many Heusler compounds are predicted to be half-metallic ferromagnets and thus find extensive interest as materials for spintronic applications, an example of which is the Co2Cr1−xFexAl series. So far, bulk samples of that series, in particular, Crrich samples, do not verify those predictions, and various studies have yielded results that are fraught with anomalies. Thin films as well do not meet expectations, neither in magnetoresistance ratio nor in degree of spin polarization and magnetic moments. Polycrystalline samples of Co2CrAl, Co2FeAl, and Co2Cr0.6Fe0.4Al were found to be phase segregated, a hurdle easily tackled by using the optical floating zone (FZ) technique. Hence, in this work, we have carried out crystal growth of Co2Cr1−xFexAl Heusler compounds, as a step toward understanding the intrinsic material properties of this series. Our results demonstrate that, although the phase segregation is eliminated in the FZ grown samples, an unexpected phase transformation via spinodal decomposition takes place in Cr-rich samples. This phase decomposition is found to strongly affect the magnetic properties and as well as other physical properties, e.g., spin polarization. Our finding presents new insight into the phase dynamics of this system and provides answers to many of the issues in this series of half-metallic ferromagnets.

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INTRODUCTION Since their discovery by Friedrich Heusler in 1903,1,2 Heusler compounds have been known to display a multitude of phenomena such as shape memory, thermoelectric effect, giant magnetoresistance (GMR),3 etc. They find enormous potential for application in spintronics owing to their predicted high spin polarization. As an example, the Co2Cr1−xFexAl series has attracted much attention in the research community as a potential half-metallic ferromagnet.4−6 In particular, the composition Co2Cr0.6Fe0.4Al is predicted to be 100% spin polarized with the most robust half-metallicity in the whole Co2Cr1−xFexAl series.7 Past experimental work has shown relatively high powder magnetoresistance (PMR)8,9 as well as relatively high tunnel magnetoresistance (TMR).10,11 Unfortunately, there have been numerous difficulties and inconsistencies with respect to the predicted properties, as a spin polarization only up to 81% has been reported.12 This may be related to the phase dynamics in this series, as, for example, the bulk polycrystalline Co2Cr1−xFexAl samples have been reported to be phase segregated with reduction in inhomogeneity as the Fe content is increased.13 Although this segregation was into cubic phases, some additional noncubic impurity peaks have been reported on the Cr-rich side postannealing.13 A transmission electron microscopy (TEM) study of polycrystalline Co2Cr1−xFexAl samples shows phase segregation even at the nanoscale.14,15 Element selective X-ray absorption spectroscopy photoelectron emission microscopy (XAS-PEEM) imaging of the Co2Cr0.6Fe0.4Al surface shows a similar Cr-rich inhomogeneity.16 On the other hand, a hexagonal closed © 2013 American Chemical Society

packed (hcp) phase has been observed in Co2Cr1−xFexAl thin films,17 and a phase segregation is likewise observed through TEM.18 Unidentified reflections or shoulders on certain reflections have also been observed by X-ray diffractometry in films with similar stoichiometry, although they have not been addressed.19−21 Large deviations between the band structure calculations and the measured magnetic properties and electronic structure of bulk Co2Cr1−xFexAl samples are found and have been attributed to antisite disorder.13,22 As an alternative explanation for the reduction of the magnetic moments, the formation of paramagnetic clusters observed in Mößbauer spectroscopy for annealed and subsequently quenched polycrystalline Co2Cr0.6Fe0.4Al samples was suggested.9 An even further reduction of magnetic moment was observed in annealed samples that were slowly cooled rather than quenched after annealing. This reduction may be related to secondary phases observed by neutron diffraction in those samples.9 Elementspecific magnetic moments from X-ray magnetic circular dichroism (XMCD) measurements on annealed polycrystalline Co2Cr0.6Fe0.4Al samples show a considerable difference from theoretical values which has also been attributed to antisite disorder.23 A similar mismatch in the magnetic data, especially at the Cr-rich side, has also been noticed for Co2Cr1−xFexAl films.19,10 Received: April 23, 2013 Revised: June 20, 2013 Published: July 22, 2013 3925

dx.doi.org/10.1021/cg4006136 | Cryst. Growth Des. 2013, 13, 3925−3934

Crystal Growth & Design

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Figure 1. SEM (BSE) data for polycrystalline sample.

Table 1. SEM-EDX Data along with Standard Deviation for Polycrystalline Samples (atom %) sample Co2FeAl

Co2CrAl

Co2Cr0.6Fe0.4Al

Co nominal Al-rich phase Fe-rich phase average nominal Cr-rich phase Al-rich phase average nominal Cr-rich phase Al-rich phase average

50 47 49 48 50 48 49 48 50 48 48 48

Cr

25 21 ± 0.1 26 ± 0.2 24

± 0.1 ± 0.6 25 31 17 24 15 19 13 16

± 0.2 ± 0.7

± 0.3 ± 0.3

± 0.3 ± 0.4

± 0.5 ± 1.0

10 12 ± 0.4 10 ± 0.3 11

Al 25 32 25 28 25 21 34 28 25 21 29 25

± 0.5 ± 0.4

± 0.2 ± 0.6

± 1.1 ± 1.5

Samples were studied from different positions of the growth, and the frozen zones were also analyzed. To do so, the corresponding parts of the grown samples were cut and polished and analyzed using a NOVA-NANOSEM (FEI) high resolution scanning electron microscope with an energy-dispersive X-ray (EDX) analyzer. Landscape images of the frozen zone were taken using Philips XL30 electron microscope. Structural characterization was done by X-ray diffraction (XRD) on powdered samples using a STOE STADI diffractometer in transmission geometry with Mo Kα1 radiation equipped with a Germanium monochromator and a DECTRIS MYTHEN 1K detector. For powder investigations, the samples were crushed by hand using a stainless steel mortar. For the purpose of Rietveld refinement of the XRD data, FullProf software package was used.28 The electron backscatter diffraction (EBSD) measurements were carried out using a FEG-SEM Ultra 55 Plus (Zeiss) with a Nordlys F detector and Oxford Instruments HKL Channel 5 data acquisition software package. TEM was performed using a CM 20 field emission gun (FEG) TEM operating at 200 kV equipped with an energy-dispersive X-ray (EDX) analyzer. Sample preparation for TEM was carried out using focused ion beam (FIB) milling by means of a Crossbeam SEM/FIB 1450 XB (Zeiss) with Ga ion-beam for milling at 5−30 kV. The magnetic properties were investigated by a superconducting quantum interference device (SQUID, Quantum Design MPMS-XL-5).

So far, all these issues have not been fully addressed, nor have the real intrinsic material properties in samples free of this chemical segregation ever been investigated. However, for any physical property measurement, a phase-pure sample is mandatory. The optical floating zone (FZ) technique is an optimal technique for growing incongruently melting compounds.24−26 Hence, we have used the FZ technique to grow selected compositions in the Co2Cr1−xFexAl series and made a step toward understanding the solidification and solid state phase transformations in the system.

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Fe

EXPERIMENTAL METHODS

As-cast polycrystalline bulk samples were prepared by arc-melting stoichiometric quantities of at least 4N pure constituents in argon atmosphere. Care was taken to avoid oxygen contamination. The chamber was evacuated to 10−5 mbar pressure before backfilling with argon. Additional steps for achieving lower oxygen content consisted of melting a Ti piece inside the vacuum chamber before melting the compound as well as through additional purification of the process gas. The samples were flipped and remelted 3−4 times to achieve homogeneity. Polycrystalline rods of the intended stoichiometries were cast using a radio frequency (rf) induction melting Hukin-type copper cold-crucible, in an argon atmosphere and cast to 6 mm diameter rods, 50−70 mm in length. The induction melting ensured homogeneous mixing of the constituents. Similar steps as during the arc-melting process were employed to minimize oxygen contamination. The rods were used as seed and feed rods for crystal growth using the optical FZ technique. A vertical two-mirror Smart Floating Zone facility, designed and constructed at IFW Dresden,24,27 was used for the growth process. All growth experiments were carried out under a constant argon flow of 0.15 L/min with additional gas cleaning by a Ti-getter furnace to minimize oxidation. The growth speeds employed were 2−5 mm/h with counter-rotation of seed and feed rod of the order of ∼0.3 Hz. The final zone was quenched in order to study the zone composition.

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RESULTS Polycrystalline As-Cast Samples. All polycrystalline samples were prepared as a basis for comparison with grown samples. After casting, the microstructure of the polycrystalline samples was investigated by SEM. Figure 1 shows the microstructure of the as-cast Co2FeAl sample which is found to melt incongruently. The sample solidifies into Fe-rich dendrites surrounded by an Al-rich phase with only a minor difference in composition similar to that reported in the literature13 (compare EDX results given in Table 1). Both phases have the same structure (Pm3̅m, B2-type) and similar lattice constants leading to overlapped reflections in the XRD 3926



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Figure 2. FZ grown samples (a) Co2FeAl, (b) Co2CrAl, and (c) Co2Cr0.6Fe0.4Al and the corresponding frozen zones of (d) Co2FeAl, (e) Co2CrAl, and (f) Co2Cr0.6Fe0.4Al. The data reported here onward are from samples taken from the end of the growth as marked by the corresponding white lines in a−c. The interfaces for the zones have been marked with white lines for clarity in d−f.

an average lattice constant of 2.8633 Å can be determined. Details of the refinement are given in the Supporting Information. Co2CrAl is reported to be incongruently melting,13 and the same is observed for the present as-cast samples. Phase segregation is into a Cr-rich phase and an Al-rich phase through dendritic solidification (Figure 1). Unlike for Co2FeAl, the two phases are significantly different in composition as listed in Table 1, which can also be seen in the strong chemical contrast using the backscattered electron (BSE) mode. As in the case of Co2FeAl, both phases are structurally similar (Pm3̅m, B2-type) and thus indistinguishable in XRD (see Supporting Information for details). Here as well, the individual lattice constants could not be accurately deconvoluted through refinement, and only

Table 2. Average SEM-EDX Data along with Standard Deviation for FZ Grown Samples (atom %)a sample

Co

Cr

Co2FeAl Co2CrAl Co2Cr0.6Fe0.4Al

50 ± 0.5 49 ± 0.3 47 ± 0.4

25 ± 0.7 15 ± 0.3

Fe

Al

26 ± 0.7

24 ± 0.4 26 ± 1.0 27 ± 0.1

11 ± 0.2

a

Please note that SEM-EDX could not resolve the compositions of the different phases in Cr-rich samples.

pattern. This can be easily misunderstood as a sample consisting of a single phase (see XRD data in the Supporting Information). It is not possible to accurately deconvolute the individual lattice constants during refinement, and hence the XRD data are refined with a single Pm3̅m phase, and thus only 3927



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Figure 3. FZ grown Co2FeAl (a) SEM (BSE) image, (b) XRD pattern with refinement.

Figure 4. FZ grown Co2FeAl (a) high resolution TEM (HRTEM) image, (b) selected area (electron) diffraction (SAED) pattern.

an average lattice constant of 2.8659 Å is determined (refer to Supporting Information for details). Co2Cr0.6Fe0.4Al has also been reported to be incongruently melting.13 The as-cast samples segregate into a Cr-rich phase and an Al-rich phase, with respect to the nominal stoichiometry, through dendritic solidification (Figure 1). The difference in the composition of the two phases is smaller than in Co2CrAl (Table 1), which can also be seen in the lower contrast between the phases. Here as well, the structure is B2type for both phases, with an average lattice constant of 2.8671 Å (refer to Supporting Information for details). Thus, as-cast samples do not allow one to study the intrinsic material properties of the Co2Cr1−xFexAl series. In order to overcome this issue, crystal growth was undertaken for the three selected compositions using the FZ technique, which gives us the possibility to grow phase-pure samples of incongruently melting compounds. In the following section, we present the results on samples grown using the FZ process. Grown Samples. The FZ growth process was sufficiently stable for all three compositions. Despite the high purity atmosphere during the growth, surface oxidation is observed as can be seen in Figure 2a−c, except near the end of the growth where a shiny surface is obtained. No significant Al loss is observed in any of the samples, and average compositions are approximately nominal within the error of the EDX measurement (Table 2). Furthermore, no traces of oxygen are observed in any of the samples beyond the resolution of the equipment. The final zone was quenched, and the growth interface is found

to be convex in all three cases, as indicated by white lines in Figure 2d−f. For Co2FeAl, the final zone has no substantial enrichment of elements. The average melt temperature of the stabilized zone during growth was ∼1540 °C. On the other hand, the overall composition in the frozen zone for Co2CrAl growth is found to have excess Cr (∼5 atom % at the cost of Al), whereas samples from the beginning of the growth are found to be enriched in Al to the same extent, which is in agreement with the liquidus projection.29 This can be understood from the similar chemical inhomogeneity due to element segregation seen in the case of polycrystalline as-cast samples (see above). The average melt temperature of the stabilized zone during growth was ∼1510 °C. The zone for Co2Cr0.6Fe0.4Al is also enriched in Cr but to a lesser extent (excess of ∼2 atom % at the cost of Al), and samples from the beginning of the growth have an excess of Al overall. Samples from the later part of the growth have an overall nominal composition confirming a stable growth. The average melt temperature of the stabilized zone during growth was ∼1540 °C. The data reported here onward are from samples taken from the end of the growth as marked by the corresponding white lines in Figure 2a−c. Co2FeAl. The as-grown sample is found to be phase-pure but not a single crystal as shown in Figure 3a, and the composition is nominal within the error of the SEM-EDX (Table 2). The grain selection during growth is not optimal, probably due to the convex interface, although millimeter-sized coarse grains are observed in the final cross-section. Crystal structure, same as as3928



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Figure 5. FZ grown Co2CrAl (a) SEM (BSE) image, (b) SEM (BSE) image in tilted position for EBSD, (c) EBSD pattern of the secondary phase consistent with tetragonal symmetry, (d) EBSD pattern of the matrix consistent with cubic symmetry, (e) XRD pattern with refinement.

cast samples, is Pm3̅m, B2-type as seen in Figure 3b. No superlattice reflections unique to L21 ordering are observed. Lattice constant is found to be 2.8628 Å using Rietveld refinement, which is summarized in Supporting Information. The sample homogeneity and the B2-type structure are further confirmed by TEM (Figure 4). Co2CrAl. The as-grown sample is found to have an unexpected microstructure (Figure 5a). It consists of an interconnected basket-weave pattern of secondary phase, in a matrix similar to the microstructure typically observed in the case of a phase transformation via spinodal decomposition. The corresponding XRD pattern (Figure 5e) indicates tetragonal splitting of the main reflection (220) which is similar to a σCoCr phase. A similar tetragonal splitting has also been observed earlier, albeit in an annealed polycrystalline sample.13

The feature size of the corresponding phases is too small to accurately measure individual composition using SEM-EDX without signal mixing from the other phase, and so only an average measurement can be done. Hence, we performed EBSD measurements to shed further light on the structure and orientation of those phases (Figure 5b−d). The data are consistent with a tetragonal σ-CoCr secondary phase in a CoAltype B2 matrix. The matrix is single crystalline, oriented along [6̅21̅ ], whereas the secondary phase has also one orientation everywhere, along [104]. The structure is confirmed using TEM (Figure 6). The corresponding phase compositions are listed in Table 3. It is to be noted that further element segregation in the matrix is also observed, as shown in the inset of Figure 6 and in the EDX-linescan across phase boundary of secondary phase showing modulations in the matrix especially 3929



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Figure 6. FZ grown Co2CrAl (a) high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image; inset: nanoscale segregation; (b) EDX-linescan across phase boundary of secondary phase showing modulations in matrix due to segregation; (c) nanobeam diffraction for the secondary phase.

reached, the structure spinodally decomposes and then transforms into a tetragonal CoCr-type phase in a CoAl-type B2 matrix. The EBSD confirms the precipitation of the secondary phase from a single phase matrix. Furthermore, the orientation-relationship between the matrix and secondary phase leads to the phase having the same orientation everywhere. The final microstructure is in very good agreement with the simulations of spinodal decomposition in thin films done by Seol et al.30 This agreement further supports our interpretation of having a phase transformation via spinodal decomposition in the Cr-rich compounds of the Co2Cr1−xFexAl series as explained in the following: The simulation is based on an elastically anisotropic film with a cubic structure. The film is also elastically constrained by the substrate. Similar conditions can be assumed at the crystallization front during the FZ process. During the growth, the crystallization front moves unidirectionally at a very slow speed (∼2−5 mm/h). This can be approximated as a layer-by-layer growth process. Since a uniform thermal gradient is present, the grown sample enters the spinodal temperature regime layer-by-layer. This layer can also be assumed to be constrained by the grown sample below which is already at a lower temperature. Thus, the spinodal decomposition does not progress any further, being a diffusioncontrolled process, albeit locally. Hence, the present crystal growth process, including a phase transformation via spinodal decomposition, can be nicely represented by the simulations in thin films and further corroborates the existence of a solid-state

Table 3. TEM-EDX Data along with Standard Deviation for Grown Co2CrAl (atom %) phase

Co

Cr

Al

nominal CoAl type B2 σ-CoCr type

50 55 ± 0.7 46 ± 0.7

25 17 ± 0.7 53 ± 0.7

25 28 ± 0.7 01 ± 0.7

for the Cr content. This segregation is on the order of 10−20 nm especially with respect to the Cr content and is similar to that observed in an earlier work,14 albeit in an annealed polycrystalline sample. This nanoscale segregation may be the starting point of the σ-phase precipitation. The lattice parameters and the phase fractions of the two phases are found using Rietveld refinement of the XRD data and are summarized in the Supporting Information. The main phase is cubic (Pm3̅m, B2 type structure) with a lattice constant of 2.8672 Å, while the secondary phase is tetragonal (P42/mnm, a = b = 8.7650 Å, c = 4.5360 Å). The phase fraction of the secondary phase from the refinement is 21.7 ± 0.5 vol %, which is similar to that obtained from analysis of SEM-BSE images (24 ± 3 vol %) within experimental uncertainty. Our overall analysis supports the idea of a spinodal decomposition taking place in the material. The spatially ordered nature of the secondary phase, as observed in the microstructure, lacks the solidification characteristics of a directional-growth process. Using the FZ technique, a pure cubic phase originally solidifies from the melt. As the temperature is lowered and the solid state miscibility gap is 3930



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Figure 7. FZ grown Co2Cr0.6Fe0.4Al (a) SEM (BSE) image, (b) SEM (BSE) image in tilted position for EBSD, (c) XRD pattern with refinement.

transformation in both as-cast and FZ grown samples, and the calculated phase diagram data should be corrected accordingly. Co2Cr0.6Fe0.4Al. In this case as well, the as-grown sample is observed to have undergone a spinodal decomposition (Figure 7a), although the extent of decomposition is much less as compared to that in Co2CrAl. At low magnification, the sample appears to be phase-pure, and the phase contrast was only discernible at high magnification. The secondary phase is more globular and not interconnected, indicating early stages of decomposition. The microstructure is similar to one simulated for the early stages of spinodal decomposition in thin elastic films mentioned earlier.30 This further supports the approximation of the crystal growth process of Co2Cr0.6Fe0.4Al to be similar to that in thin films. The feature size of the secondary phase is on the order of 50−100 nm. Hence, the structure and composition of this phase cannot be accurately determined using SEM or even EBSD, although the matrix is found to be a CoAl-type B2 phase and the phase fraction of the secondary phase is determined to be 9 ± 2 vol % by an analysis of the corresponding BSE images (Figure 7b). XRD shows a B2 phase with additional reflections which are indexed with a hcp Co phase (Figure 7c). The structure of the secondary phase is confirmed using TEM to be hexagonal ε-Co-type unlike the FZ grown Co2CrAl (Figure 8). The composition was found to be mainly Co with dissolution of small amounts of Cr and Al (see Table 4 for the EDX data). From the refinement, the matrix orders Pm3̅m B2 cubic with a lattice constant of 2.8661 Å, while

miscibility gap in the Co2Cr1−xFexAl series, although a dedicated modeling of the FZ process and the possible spinodal decomposition needs to be done. Analysis of the available phase diagram data is necessary to understand the phase transformations. Ternary phase diagram data for the Co−Cr−Al system are only available for 900 °C and above.29,31,32 We believe that below the liquidus, the Co2CrAl composition lies in the region of B2 CoAl solid solubility, unlike the available data, since using the FZ technique we are able to obtain a phase-pure Co2CrAl. At slightly lower temperatures, e.g., at 1250 °C,29 the Co2CrAl composition lies in the three-phase field of a B2 type CoAl (cubic), σ-CoCr (tetragonal), and α-Co (ht, cubic) phases but very close to the two-phase field of B2 CoAl (cubic) phase and σ-CoCr (tetragonal) phase. Also, since this is an incongruently melting system, phase segregation into Al-rich and Cr-rich phases during a fast solidification process, such as arc-melting, can be explained. As the temperature is lowered, the three-phase field becomes smaller, and at 1000 °C (Figure 10) the Co2CrAl composition is at the boundary between the three-phase field and the twophase field. Most likely below 1000 °C, the Co2CrAl composition lies in the two-phase regime of CoAl (cubic) phase and σ-CoCr (tetragonal) phase, leading to the existence of a solid state miscibility gap for metastable Co2CrAl. This scenario gives a coherent picture of the solidification and 3931



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Figure 8. FZ grown Co2Cr0.6Fe0.4Al (a) HAADF-STEM image, (b) EDX-linescan across phase boundary of secondary phase, (c) nanobeam diffraction for the secondary phase.

Information. The phase fraction of the secondary phase was calculated from refinement to be 7.4 ± 0.5 vol %. No nanoscale segregation was observed in this case, as can be seen in the EDX-linescan in Figure 8, further corroborating the smaller extent of decomposition. It is not viable to visualize the phase diagram for a quaternary system such as Co−Cr−Fe−Al. However, it is possible to gain an understanding through available ternary and binary phase diagrams between the various elements, although it is purely suggestive in nature. We have focused on Co−Cr−Al and Co− Fe−Al systems since Co and Al are the major elements. Although calculated phase diagram data are available only for Co−Fe−Al, the projection of the Co2Cr0.6Fe0.4Al composition (approximately Co59Fe12Al29) lies in the large solid solubility region of CoAl and FeAl cubic phases until room temperature33 and can account for the large solid solubility of CoAl. As discussed earlier, phase diagram data for Co−Cr−Al are only available at and above 900 °C. Here the projection of the Co2Cr0.6Fe0.4Al composition (approximately Co55.5Cr16.5Al28) lies in the two-phase field of CoAl (cubic) and Co (ht, cubic), unlike the case of Co2CrAl (Figure 10). Also, pure Co is known to undergo a transformation at ∼430 °C from α-Co (cubic) to ε-Co (hexagonal). Therefore, in that case, it is likely that the αCo precipitates from the B2 CoAl with a subsequent transformation to ε-Co at lower temperatures. The stability of ε-Co until only ∼430 °C could limit the extent of the miscibility gap, which may in turn account for the lesser extent of the spinodal decomposition observed. Magnetic Properties. Magnetic measurements were performed on as-cast and as-grown samples for all three

Table 4. TEM-EDX Data along with Standard Deviation for Grown Co2Cr0.6Fe0.4Al (atom %) phase

Co

Cr

Fe

Al

nominal CoAl type B2 ε-Co type

50 55 ± 0.7 55 ± 0.7

15 09 ± 0.7 32 ± 0.7

10 10 ± 0.7 12 ± 0.7

25 26 ± 0.7 01 ± 0.7

Figure 9. Magnetization data for as-cast and FZ grown samples with calculated magnetic moments; inset: M(H) curve for as-cast Co2FeAl.

the secondary phase is hexagonal (P63/mmc phase; a = b = 2.5295 Å, c = 4.0725 Å), details of which are in the Supporting 3932



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Figure 10. Isothermal section for Co−Cr−Al at 1000 °C (adapted from ref 29).

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DISCUSSION We have been able to confirm the presence of a low temperature solid state miscibility gap in the Co−Cr−Fe−Al system in the vicinity of Co2Cr1−xFexAl, which leads to a spinodal decomposition in the Cr-rich side of the substitution series. We believe that a similar solid state phase transformation via spinodal decomposition, as seen in our bulk data, very well occurs in thin films of respective compositions, which may account for various observations in the literature. As an example, a TEM study on Co2Cr0.6Fe0.4Al films17 showed a similar cross-type pattern as the microstructure observed for the grown samples. Furthermore, most films are annealed at slightly elevated temperatures postdeposition, for phase formation, homogenization, and ordering. Such an annealing process is sufficient to cause the decomposition, as the time scales for decomposition are much shorter compared with other diffusion-based processes, since the segregation is very localized. A similar evolution of hexagonal and tetragonal phases on sequential annealing has been reported for Co2Cr1−xFexAl films.37 Since the low temperature phase dynamics leads to formation of noncubic phases, the additional reflections in the XRD pattern, seen in some reports mentioned earlier,19,21 can also be accounted for. The phase transformation via spinodal decomposition would also very well explain the anomalous magnetic data for thin films due to formation of nonmagnetic phases.

compositions. The saturation moments are summarized in Figure 9. All the samples show soft magnetic behavior along with high saturation moments characteristic for Heusler compounds. For all the as-cast samples, the saturation moments do not match the Slater−Pauling values mainly due to the phase segregated nature of the samples. The trend is similar to an earlier work on annealed polycrystalline samples.22 In the case of Co2FeAl, the as-cast sample has a slightly higher saturation moment than the Slater−Pauling value which could be due to the phase segregated nature. The FZ grown sample, on the other hand, has a slightly higher saturation moment than the Slater−Pauling value which is most likely a result of antisite disorder, as has been calculated for some specific cases.22,34 In the cases of as-cast Co2CrAl and Co2Cr0.6Fe0.4Al as well, the saturation moments are lower than the respective Slater− Pauling values. As-grown Co2CrAl and Co2Cr0.6Fe0.4Al, unlike Co2FeAl, show a further lowering in the saturation moment as compared to the corresponding as-cast sample. This is likely due to the presence of the secondary phase and thus a loss of phase purity. The effect is even stronger in the case of Co2CrAl, where the spinodal decomposition is much more pronounced. It is to be noted that the secondary phase considerably affects the magnetic properties, even when the extent of decomposition is much less, as in the case in Co2Cr0.6Fe0.4Al where the secondary phase fraction is only ∼8 vol %. Earlier work on Co−Cr thin films35 asserts that the deviations from the theoretical saturation moment arise due to existence of two nonmagnetic phases, specifically the hexagonal ε-Co phase and σ-CoCr phase which become stable at lower temperatures,36 which is consistent with our results and analysis.

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SUMMARY Selected compositions in the Co2Cr1−xFexAl series have been grown and studied. In the case of Co2FeAl, the as-cast sample melts incongruently, but it was possible to obtain a phase-pure sample using the FZ technique. On the other hand, secondary phases were observed in as-grown samples containing Cr. The existence of a solid state miscibility gap in the Co−Cr−Al 3933



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ternary system along with the quaternary Co−Cr−Fe−Al is reported. With the partial substitution of Cr with Fe, the extent of spinodal decomposition is reduced. This spinodal decomposition has a strong effect on the magnetic properties and likely also on the spin polarization. The possibility of phase transformations via spinodal decomposition in thin films, especially those annealed, can easily explain the various structure and magnetic anomalies reported in the literature. Further efforts in film deposition, keeping in mind our results on the phase dynamics, should lead to improved functionality of the Co2Cr1−xFexAl Heusler series. As shown here, the FZ technique, with its possibility to grow incongruently melting systems and thus allowing one to further explore the phase diagram and transformations, is a promising way to understand, tune, and control the intrinsic qualities of such intermetallic Heusler systems exhibiting interesting functional properties.

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ASSOCIATED CONTENT

S Supporting Information *

Summary of Rietveld refinement for polycrystalline samples. XRD patterns with refinement for polycrystalline samples. Summary of Rietveld refinement for FZ grown samples. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS S.W. gratefully acknowledges the financial support from the Deutsche Forschungsgemeinschaft DFG in Project WU595/3-1, and M.D. thanks Deutscher Akademischer Austauschdienst (DAAD) for funding. The authors would like to thank Dr. J. Thomas and Dr. T. Gemming for help with the TEM equipment and Dr. L. Giebeler for assistance with the XRD measurement. The authors also thank Sabine MüllerLitvanyi and Gesine Kreutzer for support with metallography and zone images, as well as Dina Lohse and Tina Sturm for the TEM sample preparation.

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REFERENCES

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Phase Dynamics and Growth of Co - American Chemical Society

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