- Email: [email protected]
Magnetic properties and phase stability of Co2Cr(Ga,Si) Heusler alloys
Journal of Alloys and Compounds 588 (2014) 153–157
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Magnetic properties and phase stability of Co2Cr(Ga,Si) Heusler alloys R.Y. Umetsu a,b,⇑, A. Okubo c, X. Xu c, R. Kainuma c a
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Japan Science and Technology Agency-Precursory Research for Embryonic Science and Technology (JST-PREST), Saitama 332-0012, Japan c Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan b
a r t i c l e
i n f o
Article history: Received 12 July 2013 Received in revised form 26 October 2013 Accepted 28 October 2013 Available online 5 November 2013 Keywords: Co-based Heusler alloys Half-metal-type ferromagnets The Curie temperature Order–disorder phase transition Degree of order
a b s t r a c t The phase diagram, magnetic properties and the region over which the Heusler alloys Co2Cr(Ga1xSix) occur as a single phase have been established. A single phase was obtained in the composition range of x 6 0.5, in which the order–disorder phase transition temperature from the L21 to the B2 phase, , increased almost linearly with increasing x. The value of T L21=B2 for the L21-type Co2CrSi Heusler T L21=B2 t t alloy, estimated by linear extrapolation from its concentration dependence, was about 1450 K, and therefore higher than that of Co2CrAl and Co2CrGa. The Curie temperature, TC, also increased with increasing x becoming 600 K at x = 0.5, thus reflecting an increase in the magnetic moment caused by the change in the number of the valence electrons. The concentration dependence of the spontaneous magnetic moment, Ms, measured at 5 K increased with increasing x, almost following the generalized Slater–Pauling (S.P.) rule predicted by Galanakis et al. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Half-metallic ferromagnets (HMFs) have been intensively investigated during the last decade because of their potential application in spintronics devices [1]. Since Kübler et al. reported that the density of states in the minority band nearly vanishes at the Fermi energy in Co2MnAl and Co2MnSn [2], many Co-based Heusler alloys have from first principle calculations been predicted to show such peculiar electronic structures. Due to their high Curie temperatures, TC, and easy growth in the fabrication of multi-layered thin films, Co-based Heusler alloys with a high spin polarization are good candidates for applications. An extremely high value of the tunneling magneto-resistance (TMR) ratio has been reported in magnetic tunneling junctions (MTJ) when Co-based Heusler alloys, such as Co2MnSi [3,4] and Co2Fe(Al1xSix) [5,6], are used as the ferromagnetic electrodes. For applications, not only a high Curie temperature and high spin polarization but also high stability of the L21 phase is desired. Theoretical studies of the L21-type Co2CrSi Heusler alloy indicate complete spin polarization around the Fermi energy [7–9]. In addition, the investigation of the transport properties of multilayers suggests that the high spin polarization is kept at the interface in Co2CrSi/GaAs [7], and Co2CrSi/Cu2CrAl [10]. However from ab initio calculations of the formation enthalpy [8], Chen et al. have suggested that the Co2CrSi is thermodynamically metastable, and in fact no Co2CrSi Heusler phase has been reported
in the Co–Cr–Si ternary system [11]. Consequently, there is no experimental data on the physical properties of Co2CrSi Heusler alloy. On the other hand, it has been reported by the present authors that for Co2CrGa a single phase can be obtained by quenching [12], but owing to inevitable spinodal decomposition this is not possible for Co2CrAl [13]. In the present study, the phase stability, magnetic properties and order–disorder phase transition temperatures were investigated in the Heulser alloys Co2Cr(Ga1xSix), in order to quantify the effects of Si substitution for Ga. From these results the properties of Co2CrSi were obtained by extrapolation. 2. Experimental procedure Polycrystalline specimens of Co2Cr(Ga1xSix) were made by induction melting and the resultant ingots were annealed at 1373 K for 3 days and then quenched into water. The microstructure and composition of the specimens were checked using an optical microscope, an electron probe micro analyzer and an inductively coupled plasma atomic emission spectroscopy, respectively. It was confirmed that the composition was well-controlled and within 0.5 at.% of the stoichiometric composition. The crystal structure was confirmed by X-ray powder diffraction (XRD) measurements at room temperature. After grinding the powdered specimens of Co2Cr(Ga,Si) alloys were annealed at 1373 K for 1 min in order to remove the induced strain. Differential scanning calorimetric (DSC) measurements were made with cooling and heating rates of 10 K/min and magnetic measurements were carried out with a superconducting quantum interference device (SQUID) magnetometer and a vibration sample magnetometer (VSM) using a heating rate of 2 K/min. For the magnetization measurements specimens were also prepared by air cooling from 1373 K.
3. Results and discussion ⁄ Corresponding author at: Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Tel.: +81 22 215 2470; fax: +81 22 215 2381. E-mail address: [email protected] (R.Y. Umetsu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.209
Fig. 1 indicates the microstructures observed with an optical microscope for Co2Cr(Ga1xSix) alloys quenched from 1373 K with
154
R.Y. Umetsu et al. / Journal of Alloys and Compounds 588 (2014) 153–157
x = 0.2, 0.3, 0.5, 0.6, 0.8 and 1.0. It was clear that a single phase is obtained in the concentration region of x 6 0.5, whereas precipitates are observed in specimens with x P 0.6. The XRD powder patterns for the compositions of 0.1 6 x 6 0.5 are shown in Fig. 2(a), together with the calculated patterns for x = 0.1 and 0.5 assuming a fully ordered L21-type structure. The Ga and Si atoms are assumed to randomly occupy the 4b site in the Wyckoff position. Owing to the weak super lattice reflections it is difficult to estimate the degree of order from the observed intensities. Fig. 2(b) shows the XRD pattern for x = 1.0 (Co2CrSi), together with the calculated patterns for the Co2Si-type (Pnma) and the Ti5Re24 m) Cr0.3Co0.5Si0.2 structures. The pattern can be indexed as type (I43 a mixed phase of these structures with lattice constants of a = 0.5002, b = 0.7086 and c = 0.3772 nm (Co2Si-type), and a = 0.8704 nm (Ti5Re24-type). This conclusion is in accordance with the ternary phase diagram of the Co–Cr–Si system reported by Borusevich and Gladyshevskii [11]. The concentration dependence of the lattice constant for the Co2Cr(Ga1xSix) alloys in the single phase concentration region of x 6 0.5, is shown in Fig. 3, together with the reported data for x = 0.0 [12]. The lattice constant linearly decreases with increasing x, reflecting the difference in the atomic radius between the Ga and Si atoms. The value for x = 1.0 (Co2CrSi) estimated by linear extrapolation using the concentration dependence is about 0.565 nm. This is closer to the value of 0.563 nm, calculated assuming a ferromagnetic state than that of 0.559 nm for the nonmagnetic state [8].
In order to determine the Curie temperature, TC thermomagnetization (M–T) measurements for Co2Cr(Ga1xSix) alloys were performed using a VSM in a magnetic field of 5 kOe up to about 700 K. In the M–T curves of Fig. 4(a), the arrows indicate TC which is defined as the minimum point in the dM/dT plots. It is seen that TC increases with increasing x as indicated by the arrows. The values of TC were also confirmed by the DSC measurements, shown in Fig. 4(b) for the heating and cooling cycles. For the composition x 6 0.4, two endothermic peaks and two exothermic peaks are observed at the same temperatures in the heating and in the cooling processes in the DSC measurements. The peaks observed at low temperatures are associated with TC, and accord with the data in the M–T curves. The peaks at higher temperature are due to the order–disorder phase transition from the L21 to the B2 phase, since T L21=B2 for x = 0.0 (Co2CrGa) has been already confirmed and ret ported to be about 1050 K in the literature [12]. In the DSC heating curve for x = 0.5, an exothermic reaction is observed around 1050 K and evidence of T L21=B2 is not observed. From the fact that the pret cipitates exist in specimens with x P 0.6 as shown in Fig. 1, it seems that in x = 0.5 the second phase precipitates during heating. Based on the data obtained from the magnetic and the DSC measurements in the concentration region of x 6 0.5, together with those for x = 0.0 [12] the concentration dependence of TC and T L21=B2 t for the Co2Cr(Ga1xSix) alloys is plotted in Fig. 5(a). The Curie temperature TC shows a tendency to saturate with increasing x. This behavior reflects the increase in the magnetic moment caused by the change in the number of valence electrons with the value of
Fig. 1. Microstructures observed with an optical microscope for Co2Cr(Ga1xSix) alloys with x = 0.2, 0.3, 0.5, 0.6, 0.8 and 1.0 annealed at 1373 K for 3 days and then quenched into water.
155
R.Y. Umetsu et al. / Journal of Alloys and Compounds 588 (2014) 153–157
Fig. 4. (a) Thermomagnetization curves in a magnetic field of 5 kOe obtained by VSM measurements. (b) DSC heating and cooling curves of Co2Cr(Ga1xSix) alloys for 0.0 6 x 6 0.5.
Fig. 2. (a) X-ray powder diffraction patterns measured at room temperature for Co2Cr(Ga1xSix) alloys with x = 0.1, 0.2, 0.3, 0.4 and 0.5, together with the calculated patterns for x = 0.1 and 0.5 assuming a fully ordered L21-type structure. (b) X-ray powder diffraction patterns measured at room temperature for x = 1.0 (Co2CrSi), together with the calculated patterns for Co2Si-type and Ti5Re24-type structures.
the interchange energy of the YAZ bonds in X2YZ Heusler alloys. This means that the phase stability of the L21-type phase is simply given by the following relation: Xð2Þ
T tL21 =B2 ¼
3W YZ ; 2kB
ð1Þ Xð2Þ
where kB is the Boltzmann constant and W YZ is the interchange energy between Y and Z atoms which are second nearest neighbors and surrounded by X atoms as first nearest neighbors. The interacXð2Þ
Xð2Þ
Xð2Þ
Xð2Þ
tion energy is defined by W YZ ¼ eYY þ eZZ 2eYZ bonding energy between i and j atoms given by
with the pair
eXð2Þ [17]. Eq. (1) ij Xð2Þ
means that there is a linear relation between T L21=B2 and W YZ , t and that
T L21=B2 t
is only a function of
Xð2Þ W YZ ,
independent of other
Xð2Þ
pairwise interactions. If W YZ of the substituted alloys is given by the weighted mean a linear concentration dependence of the T L21=B2 is expected. In the case of the Co2Cr(Ga1xSix) alloys, the t actually exhibits a linear dependence with x in the range T L21=B2 t Fig. 3. The concentration dependence of the lattice constant of Co2Cr(Ga1xSix) alloys, together with previous data [12].
TC reaching about 600 K at x = 0.5. The magnetic moment of these alloys will be discussed later. In Fig. 5(a), it is confirmed that the T L21=B2 increases almost linearly with increasing x. Under the t assumption that the degree of order of the X element is perfect then according to the Bragg–Williams–Gorsky (BWG) approximation [14–16], there is a proportional relation between T L21=B2 and t
for the L21-type x 6 0.5. By linear extrapolation, the value of T L21=B2 t Co2CrSi Heusler alloy is estimated to be about 1450 K, which is higher than that of 800 and 1050 K obtained for the Co2CrAl and Co2CrGa alloys, respectively [12,18]. The T L21=B2 for various Co-based t Heusler alloys Co2YZ (Y = Cr, Mn, Fe and Z = Al, Ga, Si) is shown in Fig. 5(b) [18–22]. The dependence of T L21=B2 on the Y elements for t for Co2YGa just shifts Co2YAl and Co2YGa is similar, that is, T L21=B2 t to higher temperature compared to that for Co2YAl. An even higher value is observed for Co2YSi for which T L21=B2 also has a similar t
156
R.Y. Umetsu et al. / Journal of Alloys and Compounds 588 (2014) 153–157
Fig. 5. (a) The concentration dependence of the order–disorder phase transition temperature from the L21 phase to the B2 phase, T L21=B2 , and the Curie temperature, t TC, of Co2Cr(Ga1xSix) alloys, together with previous data [12]. (b) The order– disorder phase transition temperature from the L21 phase to the B2 phase, T L21=B2 , in t the paramagnetic state of Co2YZ (Y = Ti, V, Cr, Mn, Fe, Z = Al, Ga, Si) Heusler alloys [18–22].
variation. As shown in the figure the systematic investigations of the various Co-based Heusler alloys have enabled T L21=B2 of the t L21 phase to be established. Focusing on the dependence of T L21=B2 on the Y element reveals t that there is a trend, except for Y = Cr, in which T L21=B2 decreases with t increasing number of valence electrons. Thus it appears that T L21=B2 of t the Y = Cr series is exceptionally low. Although the reason for this is not yet clear, the instability of the phase maybe correlated to the features of the electronic structure. In the theoretical investigations of the electronic structures of Co2YGa (Y = Ti, V, Cr, Mn and Fe) alloys by Ishida et al., it was found that the shape of the density of states in the minority-spin state are similar for all Y elements [23]. The Fermi energy locates in a pseudo-energy gap in each density of states and the occupied minority-spin states are approximately equal. On the other hand, the shape of the density of states in the majority-spin states is different and the occupied states increase with increasing the number of the valence electrons via the Y atoms. The replacement of the Z (Z = Al, Si, Ge, etc.) atom for Ga does not so affect the whole of shape of the electronic structure it only brings about a slight change in bandwidth. In this scenario, there seems to be a common feature in the Co2CrZ alloys, in that the large density of states due to the Cr-3d bands locates in the majority-spin state around the Fermi energy. The Fermi energy is close to the maximum in the 3d density of states, which implies an instability of the L21-type structure in the Co2CrZ alloys [7–9,23,24]. Fig. 6(a) shows the magnetization as a function of the magnetic field (M–H curve) at 5 K obtained using a SQUID for the air-cooled (AC) and water-quenched (WQ) specimens with x = 0.0 and 0.5. The values of the magnetization for the AC specimens are larger than those for the WQ specimens, similar behavior is also seen in other compositions with 0 6 x 6 0.5. The concentration
Fig. 6. (a) Magnetization curves at 5 K obtained by SQUID measurements for the air-cooled (AC) and water-quenched (WQ) from 1373 K specimens with x = 0.0 and 0.5. (b) The concentration dependence of the spontaneous magnetic moment for the AC and WQ Co2Cr(Ga1xSix) specimens, together with the reported data [24–26].
dependence of the spontaneous magnetic moment, Ms, which is obtained by the extrapolation to H/M = 0 in the H/M versus M2 plot (Arrott plot), is plotted in Fig. 6(b) for Co2Cr(Ga1-xSix) alloys together with some reported data [24–26]. In the figure, the solid line represents the value obtained using the generalized Slater– Pauling (S.P.) rule proposed by Galanakis et al. [27]. Here, total magnetic moment, Mt, is expressed as Mt = Zt 24 where Zt is number of the valence electrons. If the total magnetic moment is close to the expected value is one indication of HMFs [27]. Based on the S.P. rule, Mt for Co2CrGa and Co2CrSi is 3 and 4 lB/f.u., respectively. It is shown in the figure that the Ms of the AC specimens are closer to the expected Mt than that of the WQ specimens and that Ms increases linearly following the S.P. rule due to the increase of Zt. The difference in Ms for the AC and WQ specimens with the same composition arises from the different degree of order. The slower cooling rate of the AC specimens results in a higher degree of order. However, it should be noted that annealing at low temperatures for a long time will cause phase precipitation. In our previous studies, precipitation was confirmed by microstructural observation and XRD in Co2CrGa annealed at 973 K. Ms of Co2CrGa is reported to be 2.4 lB/f.u., which is significantly lower than the expected value of 3 lB/f.u. [24]. The present magnetization measurements suggest that the L21-type Co2Cr(Ga1xSix) alloys will be half-metal ferromagnets, consistent with theoretical results for Co2CrGa and Co2CrSi alloys, if the degree of order is completely controlled within a single phase. 4. Conclusions Microstructural observation, X-ray powder diffraction, thermal and magnetic measurements were performed on Co2Cr(Ga1xSix)
R.Y. Umetsu et al. / Journal of Alloys and Compounds 588 (2014) 153–157
Heulser alloys in order to investigate the phase stability, magnetic properties and the order–disorder phase transition temperature. A single phase was obtained in the range of x 6 0.5, and the order– disorder phase transition temperature from the L21 to the B2 phase, T L21=B2 , increases almost linearly with increasing x. The t T L21=B2 of the L21-type Co2CrSi Heusler alloy estimated by linear t extrapolation using its concentration dependence is about 1450 K, which is higher than that of the Co2CrAl and Co2CrGa alloys. The Curie temperature, TC, also increases with increasing x, reaching about 600 K for x = 0.5, reflecting the increase in the magnetic moment caused by the change in the number of valence electrons. The concentration dependence of the spontaneous magnetic moment, Ms, measured at 5 K increases with increasing x, following the generalized Slater–Pauling (S.P.) rule predicted by Galanakis et al. [27]. The values of Ms for the air-cooled specimen were closer to the value expected from the S.P. rule than those for the water-quenched ones, suggesting that the L21-type Co2Cr(Ga1xSix) alloys are half-metal ferromagnets as proposed by the theoretical reports, if the degree of order is sufficiently high. Acknowledgments This study was supported by Grant-in-Aids from the Japanese Society for the Promotion of Science and a Grant for Excellent Graduate Schools from MEXT, Japan. Parts of this work were performed at the Center for Low Temperature Science, Institute for Materials Research, Tohoku University. References [1] R.A. de Groot, F.M. Mueller, P.G. van Engen, K.H.J. Buschow, Phys. Rev. Lett. 50 (1983) 2024.
157
[2] J.J. Kübler, A.R. Williams, C.B. Sommers, Phys. Rev. B 28 (1983) 1745. [3] T. Kubota, J. Hamrle, Y. Sakuraba, O. Gaier, M. Oogane, A. Sakuma, B. Hillebrands, K. Takanashi, Y. Ando, J. Appl. Phys. 106 (2009) 113907. [4] M. Yamamoto, T. Ishikawa, T. Taira, G. Li, K. Matsuda, T. Uemura, J. Phys.: Condens. Matter 22 (2010) 164212. [5] K. Inomata, S. Okamoto, A. Miyazaki, K. Kikuchi, N. Tezuka, M. Wojcik, E. Jedryka, J. Phys. D: Appl. Phys. 39 (2006) 816. [6] N. Tezuka, N. Ikeda, F. Mitsuhashi, S. Sugimoto, Appl. Phys. Lett. 94 (2009) 162504. [7] K. Nagao, Y. Miura, M. Shirai, Phys. Rev. B 73 (2006) 104447. [8] X.-Q. Chen, R. Podloucky, P. Rogl, J. Appl. Phys. 100 (2006) 113901. [9] K. Özdog˘an, I. Galanakis, E. Sß asßiog˘lu, B. Aktasß, Phys. Stat. Sol. (RRL) 95–97 (2007) 1. [10] Z.Q. Bai, Y.H. Lu, L. Shen, V. Ko, G.C. Han, Y.P. Feng, J Appl. Phys. 111 (2012) 093911. [11] L.K. Borusevich, E.I. Gladyshevskii, Russ. Metall. 6 (1965) 83. [12] R.Y. Umetsu, K. Kobayashi, R. Kainuma, K. Ishida, A. Fujita, K. Fukamichi, A. Sakuma, Appl. Phys. Lett. 85 (2004) 2011. [13] K. Kobayashi, R.Y. Umetsu, R. Kainuma, K. Ishida, T. Oyamada, A. Fujita, K. Fukamichi, Appl. Phys. Lett. 85 (2004) 4684. [14] W.L. Bragg, E.J. Williams, Proc. Roy. Soc. A 145 (1934) 699. [15] W.L. Bragg, E.J. Williams, Proc. Roy. Soc. A 151 (1935) 540. [16] V.S. Gorsky, Z. Physk. 50 (1928) 64. [17] G.Z. Inden, Metallkde. 66 (1975) 577. [18] K. Kobayashi, R. Kainuma, K. Ishida, Mater. Trans. 47 (2006) 20. [19] K. Ishikawa, R. Kainuma, I. Ohnuma, K. Aoki, K. Ishida, Acta Mater. 50 (2002) 2233. [20] K. Kobayashi, K. Ishikawa, R.Y. Umetsu, R. Kainuma, K. Aoki, K. Ishida, J. Magn. Magn. Mater. 310 (2007) 1794. [21] R.Y. Umetsu, K. Kobayashi, A. Fujita, R. Kainuma, K. Ishida, Scr. Mater. 58 (2008) 723. [22] R.Y. Umetsu, A. Okubo, R. Kainuma, J. Appl. Phys. 111 (2012) 073909. [23] S. Ishida, S. Sugiyama, S. Fujii, S. Asano, J. Phys.: Condens. Matter 3 (1991) 5793. [24] R.Y. Umetsu, K. Kobayashi, A. Fujita, K. Oikawa, R. Kainuma, K. Ishida, N. Endo, K. Fukamichi, A. Sakuma, Phys. Rev. B 72 (2005) 214412. [25] K.H.J. Buschow, P.G. van Engen, J. Magn. Magn. Mater. 25 (1981) 90. [26] T.M. Nakatani, Z. Gercsi, A. Rajanikanth, Y.K. Takahashi, K. Hono, J. Phys. D: Appl. Phys. 41 (2008) 225002. [27] I. Galanakis, P.H. Dederichs, N. Papanikolaou, Phys. Rev. B 66 (2002) 174429.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Magnetic properties and phase stability of Co2Cr(Ga,Si) Heusler alloys R.Y. Umetsu a,b,⇑, A. Okubo c, X. Xu c, R. Kainuma c a
Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan Japan Science and Technology Agency-Precursory Research for Embryonic Science and Technology (JST-PREST), Saitama 332-0012, Japan c Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan b
a r t i c l e
i n f o
Article history: Received 12 July 2013 Received in revised form 26 October 2013 Accepted 28 October 2013 Available online 5 November 2013 Keywords: Co-based Heusler alloys Half-metal-type ferromagnets The Curie temperature Order–disorder phase transition Degree of order
a b s t r a c t The phase diagram, magnetic properties and the region over which the Heusler alloys Co2Cr(Ga1xSix) occur as a single phase have been established. A single phase was obtained in the composition range of x 6 0.5, in which the order–disorder phase transition temperature from the L21 to the B2 phase, , increased almost linearly with increasing x. The value of T L21=B2 for the L21-type Co2CrSi Heusler T L21=B2 t t alloy, estimated by linear extrapolation from its concentration dependence, was about 1450 K, and therefore higher than that of Co2CrAl and Co2CrGa. The Curie temperature, TC, also increased with increasing x becoming 600 K at x = 0.5, thus reflecting an increase in the magnetic moment caused by the change in the number of the valence electrons. The concentration dependence of the spontaneous magnetic moment, Ms, measured at 5 K increased with increasing x, almost following the generalized Slater–Pauling (S.P.) rule predicted by Galanakis et al. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Half-metallic ferromagnets (HMFs) have been intensively investigated during the last decade because of their potential application in spintronics devices [1]. Since Kübler et al. reported that the density of states in the minority band nearly vanishes at the Fermi energy in Co2MnAl and Co2MnSn [2], many Co-based Heusler alloys have from first principle calculations been predicted to show such peculiar electronic structures. Due to their high Curie temperatures, TC, and easy growth in the fabrication of multi-layered thin films, Co-based Heusler alloys with a high spin polarization are good candidates for applications. An extremely high value of the tunneling magneto-resistance (TMR) ratio has been reported in magnetic tunneling junctions (MTJ) when Co-based Heusler alloys, such as Co2MnSi [3,4] and Co2Fe(Al1xSix) [5,6], are used as the ferromagnetic electrodes. For applications, not only a high Curie temperature and high spin polarization but also high stability of the L21 phase is desired. Theoretical studies of the L21-type Co2CrSi Heusler alloy indicate complete spin polarization around the Fermi energy [7–9]. In addition, the investigation of the transport properties of multilayers suggests that the high spin polarization is kept at the interface in Co2CrSi/GaAs [7], and Co2CrSi/Cu2CrAl [10]. However from ab initio calculations of the formation enthalpy [8], Chen et al. have suggested that the Co2CrSi is thermodynamically metastable, and in fact no Co2CrSi Heusler phase has been reported
in the Co–Cr–Si ternary system [11]. Consequently, there is no experimental data on the physical properties of Co2CrSi Heusler alloy. On the other hand, it has been reported by the present authors that for Co2CrGa a single phase can be obtained by quenching [12], but owing to inevitable spinodal decomposition this is not possible for Co2CrAl [13]. In the present study, the phase stability, magnetic properties and order–disorder phase transition temperatures were investigated in the Heulser alloys Co2Cr(Ga1xSix), in order to quantify the effects of Si substitution for Ga. From these results the properties of Co2CrSi were obtained by extrapolation. 2. Experimental procedure Polycrystalline specimens of Co2Cr(Ga1xSix) were made by induction melting and the resultant ingots were annealed at 1373 K for 3 days and then quenched into water. The microstructure and composition of the specimens were checked using an optical microscope, an electron probe micro analyzer and an inductively coupled plasma atomic emission spectroscopy, respectively. It was confirmed that the composition was well-controlled and within 0.5 at.% of the stoichiometric composition. The crystal structure was confirmed by X-ray powder diffraction (XRD) measurements at room temperature. After grinding the powdered specimens of Co2Cr(Ga,Si) alloys were annealed at 1373 K for 1 min in order to remove the induced strain. Differential scanning calorimetric (DSC) measurements were made with cooling and heating rates of 10 K/min and magnetic measurements were carried out with a superconducting quantum interference device (SQUID) magnetometer and a vibration sample magnetometer (VSM) using a heating rate of 2 K/min. For the magnetization measurements specimens were also prepared by air cooling from 1373 K.
3. Results and discussion ⁄ Corresponding author at: Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan. Tel.: +81 22 215 2470; fax: +81 22 215 2381. E-mail address: [email protected] (R.Y. Umetsu). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.209
Fig. 1 indicates the microstructures observed with an optical microscope for Co2Cr(Ga1xSix) alloys quenched from 1373 K with
154
R.Y. Umetsu et al. / Journal of Alloys and Compounds 588 (2014) 153–157
x = 0.2, 0.3, 0.5, 0.6, 0.8 and 1.0. It was clear that a single phase is obtained in the concentration region of x 6 0.5, whereas precipitates are observed in specimens with x P 0.6. The XRD powder patterns for the compositions of 0.1 6 x 6 0.5 are shown in Fig. 2(a), together with the calculated patterns for x = 0.1 and 0.5 assuming a fully ordered L21-type structure. The Ga and Si atoms are assumed to randomly occupy the 4b site in the Wyckoff position. Owing to the weak super lattice reflections it is difficult to estimate the degree of order from the observed intensities. Fig. 2(b) shows the XRD pattern for x = 1.0 (Co2CrSi), together with the calculated patterns for the Co2Si-type (Pnma) and the Ti5Re24 m) Cr0.3Co0.5Si0.2 structures. The pattern can be indexed as type (I43 a mixed phase of these structures with lattice constants of a = 0.5002, b = 0.7086 and c = 0.3772 nm (Co2Si-type), and a = 0.8704 nm (Ti5Re24-type). This conclusion is in accordance with the ternary phase diagram of the Co–Cr–Si system reported by Borusevich and Gladyshevskii [11]. The concentration dependence of the lattice constant for the Co2Cr(Ga1xSix) alloys in the single phase concentration region of x 6 0.5, is shown in Fig. 3, together with the reported data for x = 0.0 [12]. The lattice constant linearly decreases with increasing x, reflecting the difference in the atomic radius between the Ga and Si atoms. The value for x = 1.0 (Co2CrSi) estimated by linear extrapolation using the concentration dependence is about 0.565 nm. This is closer to the value of 0.563 nm, calculated assuming a ferromagnetic state than that of 0.559 nm for the nonmagnetic state [8].
In order to determine the Curie temperature, TC thermomagnetization (M–T) measurements for Co2Cr(Ga1xSix) alloys were performed using a VSM in a magnetic field of 5 kOe up to about 700 K. In the M–T curves of Fig. 4(a), the arrows indicate TC which is defined as the minimum point in the dM/dT plots. It is seen that TC increases with increasing x as indicated by the arrows. The values of TC were also confirmed by the DSC measurements, shown in Fig. 4(b) for the heating and cooling cycles. For the composition x 6 0.4, two endothermic peaks and two exothermic peaks are observed at the same temperatures in the heating and in the cooling processes in the DSC measurements. The peaks observed at low temperatures are associated with TC, and accord with the data in the M–T curves. The peaks at higher temperature are due to the order–disorder phase transition from the L21 to the B2 phase, since T L21=B2 for x = 0.0 (Co2CrGa) has been already confirmed and ret ported to be about 1050 K in the literature [12]. In the DSC heating curve for x = 0.5, an exothermic reaction is observed around 1050 K and evidence of T L21=B2 is not observed. From the fact that the pret cipitates exist in specimens with x P 0.6 as shown in Fig. 1, it seems that in x = 0.5 the second phase precipitates during heating. Based on the data obtained from the magnetic and the DSC measurements in the concentration region of x 6 0.5, together with those for x = 0.0 [12] the concentration dependence of TC and T L21=B2 t for the Co2Cr(Ga1xSix) alloys is plotted in Fig. 5(a). The Curie temperature TC shows a tendency to saturate with increasing x. This behavior reflects the increase in the magnetic moment caused by the change in the number of valence electrons with the value of
Fig. 1. Microstructures observed with an optical microscope for Co2Cr(Ga1xSix) alloys with x = 0.2, 0.3, 0.5, 0.6, 0.8 and 1.0 annealed at 1373 K for 3 days and then quenched into water.
155
R.Y. Umetsu et al. / Journal of Alloys and Compounds 588 (2014) 153–157
Fig. 4. (a) Thermomagnetization curves in a magnetic field of 5 kOe obtained by VSM measurements. (b) DSC heating and cooling curves of Co2Cr(Ga1xSix) alloys for 0.0 6 x 6 0.5.
Fig. 2. (a) X-ray powder diffraction patterns measured at room temperature for Co2Cr(Ga1xSix) alloys with x = 0.1, 0.2, 0.3, 0.4 and 0.5, together with the calculated patterns for x = 0.1 and 0.5 assuming a fully ordered L21-type structure. (b) X-ray powder diffraction patterns measured at room temperature for x = 1.0 (Co2CrSi), together with the calculated patterns for Co2Si-type and Ti5Re24-type structures.
the interchange energy of the YAZ bonds in X2YZ Heusler alloys. This means that the phase stability of the L21-type phase is simply given by the following relation: Xð2Þ
T tL21 =B2 ¼
3W YZ ; 2kB
ð1Þ Xð2Þ
where kB is the Boltzmann constant and W YZ is the interchange energy between Y and Z atoms which are second nearest neighbors and surrounded by X atoms as first nearest neighbors. The interacXð2Þ
Xð2Þ
Xð2Þ
Xð2Þ
tion energy is defined by W YZ ¼ eYY þ eZZ 2eYZ bonding energy between i and j atoms given by
with the pair
eXð2Þ [17]. Eq. (1) ij Xð2Þ
means that there is a linear relation between T L21=B2 and W YZ , t and that
T L21=B2 t
is only a function of
Xð2Þ W YZ ,
independent of other
Xð2Þ
pairwise interactions. If W YZ of the substituted alloys is given by the weighted mean a linear concentration dependence of the T L21=B2 is expected. In the case of the Co2Cr(Ga1xSix) alloys, the t actually exhibits a linear dependence with x in the range T L21=B2 t Fig. 3. The concentration dependence of the lattice constant of Co2Cr(Ga1xSix) alloys, together with previous data [12].
TC reaching about 600 K at x = 0.5. The magnetic moment of these alloys will be discussed later. In Fig. 5(a), it is confirmed that the T L21=B2 increases almost linearly with increasing x. Under the t assumption that the degree of order of the X element is perfect then according to the Bragg–Williams–Gorsky (BWG) approximation [14–16], there is a proportional relation between T L21=B2 and t
for the L21-type x 6 0.5. By linear extrapolation, the value of T L21=B2 t Co2CrSi Heusler alloy is estimated to be about 1450 K, which is higher than that of 800 and 1050 K obtained for the Co2CrAl and Co2CrGa alloys, respectively [12,18]. The T L21=B2 for various Co-based t Heusler alloys Co2YZ (Y = Cr, Mn, Fe and Z = Al, Ga, Si) is shown in Fig. 5(b) [18–22]. The dependence of T L21=B2 on the Y elements for t for Co2YGa just shifts Co2YAl and Co2YGa is similar, that is, T L21=B2 t to higher temperature compared to that for Co2YAl. An even higher value is observed for Co2YSi for which T L21=B2 also has a similar t
156
R.Y. Umetsu et al. / Journal of Alloys and Compounds 588 (2014) 153–157
Fig. 5. (a) The concentration dependence of the order–disorder phase transition temperature from the L21 phase to the B2 phase, T L21=B2 , and the Curie temperature, t TC, of Co2Cr(Ga1xSix) alloys, together with previous data [12]. (b) The order– disorder phase transition temperature from the L21 phase to the B2 phase, T L21=B2 , in t the paramagnetic state of Co2YZ (Y = Ti, V, Cr, Mn, Fe, Z = Al, Ga, Si) Heusler alloys [18–22].
variation. As shown in the figure the systematic investigations of the various Co-based Heusler alloys have enabled T L21=B2 of the t L21 phase to be established. Focusing on the dependence of T L21=B2 on the Y element reveals t that there is a trend, except for Y = Cr, in which T L21=B2 decreases with t increasing number of valence electrons. Thus it appears that T L21=B2 of t the Y = Cr series is exceptionally low. Although the reason for this is not yet clear, the instability of the phase maybe correlated to the features of the electronic structure. In the theoretical investigations of the electronic structures of Co2YGa (Y = Ti, V, Cr, Mn and Fe) alloys by Ishida et al., it was found that the shape of the density of states in the minority-spin state are similar for all Y elements [23]. The Fermi energy locates in a pseudo-energy gap in each density of states and the occupied minority-spin states are approximately equal. On the other hand, the shape of the density of states in the majority-spin states is different and the occupied states increase with increasing the number of the valence electrons via the Y atoms. The replacement of the Z (Z = Al, Si, Ge, etc.) atom for Ga does not so affect the whole of shape of the electronic structure it only brings about a slight change in bandwidth. In this scenario, there seems to be a common feature in the Co2CrZ alloys, in that the large density of states due to the Cr-3d bands locates in the majority-spin state around the Fermi energy. The Fermi energy is close to the maximum in the 3d density of states, which implies an instability of the L21-type structure in the Co2CrZ alloys [7–9,23,24]. Fig. 6(a) shows the magnetization as a function of the magnetic field (M–H curve) at 5 K obtained using a SQUID for the air-cooled (AC) and water-quenched (WQ) specimens with x = 0.0 and 0.5. The values of the magnetization for the AC specimens are larger than those for the WQ specimens, similar behavior is also seen in other compositions with 0 6 x 6 0.5. The concentration
Fig. 6. (a) Magnetization curves at 5 K obtained by SQUID measurements for the air-cooled (AC) and water-quenched (WQ) from 1373 K specimens with x = 0.0 and 0.5. (b) The concentration dependence of the spontaneous magnetic moment for the AC and WQ Co2Cr(Ga1xSix) specimens, together with the reported data [24–26].
dependence of the spontaneous magnetic moment, Ms, which is obtained by the extrapolation to H/M = 0 in the H/M versus M2 plot (Arrott plot), is plotted in Fig. 6(b) for Co2Cr(Ga1-xSix) alloys together with some reported data [24–26]. In the figure, the solid line represents the value obtained using the generalized Slater– Pauling (S.P.) rule proposed by Galanakis et al. [27]. Here, total magnetic moment, Mt, is expressed as Mt = Zt 24 where Zt is number of the valence electrons. If the total magnetic moment is close to the expected value is one indication of HMFs [27]. Based on the S.P. rule, Mt for Co2CrGa and Co2CrSi is 3 and 4 lB/f.u., respectively. It is shown in the figure that the Ms of the AC specimens are closer to the expected Mt than that of the WQ specimens and that Ms increases linearly following the S.P. rule due to the increase of Zt. The difference in Ms for the AC and WQ specimens with the same composition arises from the different degree of order. The slower cooling rate of the AC specimens results in a higher degree of order. However, it should be noted that annealing at low temperatures for a long time will cause phase precipitation. In our previous studies, precipitation was confirmed by microstructural observation and XRD in Co2CrGa annealed at 973 K. Ms of Co2CrGa is reported to be 2.4 lB/f.u., which is significantly lower than the expected value of 3 lB/f.u. [24]. The present magnetization measurements suggest that the L21-type Co2Cr(Ga1xSix) alloys will be half-metal ferromagnets, consistent with theoretical results for Co2CrGa and Co2CrSi alloys, if the degree of order is completely controlled within a single phase. 4. Conclusions Microstructural observation, X-ray powder diffraction, thermal and magnetic measurements were performed on Co2Cr(Ga1xSix)
R.Y. Umetsu et al. / Journal of Alloys and Compounds 588 (2014) 153–157
Heulser alloys in order to investigate the phase stability, magnetic properties and the order–disorder phase transition temperature. A single phase was obtained in the range of x 6 0.5, and the order– disorder phase transition temperature from the L21 to the B2 phase, T L21=B2 , increases almost linearly with increasing x. The t T L21=B2 of the L21-type Co2CrSi Heusler alloy estimated by linear t extrapolation using its concentration dependence is about 1450 K, which is higher than that of the Co2CrAl and Co2CrGa alloys. The Curie temperature, TC, also increases with increasing x, reaching about 600 K for x = 0.5, reflecting the increase in the magnetic moment caused by the change in the number of valence electrons. The concentration dependence of the spontaneous magnetic moment, Ms, measured at 5 K increases with increasing x, following the generalized Slater–Pauling (S.P.) rule predicted by Galanakis et al. [27]. The values of Ms for the air-cooled specimen were closer to the value expected from the S.P. rule than those for the water-quenched ones, suggesting that the L21-type Co2Cr(Ga1xSix) alloys are half-metal ferromagnets as proposed by the theoretical reports, if the degree of order is sufficiently high. Acknowledgments This study was supported by Grant-in-Aids from the Japanese Society for the Promotion of Science and a Grant for Excellent Graduate Schools from MEXT, Japan. Parts of this work were performed at the Center for Low Temperature Science, Institute for Materials Research, Tohoku University. References [1] R.A. de Groot, F.M. Mueller, P.G. van Engen, K.H.J. Buschow, Phys. Rev. Lett. 50 (1983) 2024.
157
[2] J.J. Kübler, A.R. Williams, C.B. Sommers, Phys. Rev. B 28 (1983) 1745. [3] T. Kubota, J. Hamrle, Y. Sakuraba, O. Gaier, M. Oogane, A. Sakuma, B. Hillebrands, K. Takanashi, Y. Ando, J. Appl. Phys. 106 (2009) 113907. [4] M. Yamamoto, T. Ishikawa, T. Taira, G. Li, K. Matsuda, T. Uemura, J. Phys.: Condens. Matter 22 (2010) 164212. [5] K. Inomata, S. Okamoto, A. Miyazaki, K. Kikuchi, N. Tezuka, M. Wojcik, E. Jedryka, J. Phys. D: Appl. Phys. 39 (2006) 816. [6] N. Tezuka, N. Ikeda, F. Mitsuhashi, S. Sugimoto, Appl. Phys. Lett. 94 (2009) 162504. [7] K. Nagao, Y. Miura, M. Shirai, Phys. Rev. B 73 (2006) 104447. [8] X.-Q. Chen, R. Podloucky, P. Rogl, J. Appl. Phys. 100 (2006) 113901. [9] K. Özdog˘an, I. Galanakis, E. Sß asßiog˘lu, B. Aktasß, Phys. Stat. Sol. (RRL) 95–97 (2007) 1. [10] Z.Q. Bai, Y.H. Lu, L. Shen, V. Ko, G.C. Han, Y.P. Feng, J Appl. Phys. 111 (2012) 093911. [11] L.K. Borusevich, E.I. Gladyshevskii, Russ. Metall. 6 (1965) 83. [12] R.Y. Umetsu, K. Kobayashi, R. Kainuma, K. Ishida, A. Fujita, K. Fukamichi, A. Sakuma, Appl. Phys. Lett. 85 (2004) 2011. [13] K. Kobayashi, R.Y. Umetsu, R. Kainuma, K. Ishida, T. Oyamada, A. Fujita, K. Fukamichi, Appl. Phys. Lett. 85 (2004) 4684. [14] W.L. Bragg, E.J. Williams, Proc. Roy. Soc. A 145 (1934) 699. [15] W.L. Bragg, E.J. Williams, Proc. Roy. Soc. A 151 (1935) 540. [16] V.S. Gorsky, Z. Physk. 50 (1928) 64. [17] G.Z. Inden, Metallkde. 66 (1975) 577. [18] K. Kobayashi, R. Kainuma, K. Ishida, Mater. Trans. 47 (2006) 20. [19] K. Ishikawa, R. Kainuma, I. Ohnuma, K. Aoki, K. Ishida, Acta Mater. 50 (2002) 2233. [20] K. Kobayashi, K. Ishikawa, R.Y. Umetsu, R. Kainuma, K. Aoki, K. Ishida, J. Magn. Magn. Mater. 310 (2007) 1794. [21] R.Y. Umetsu, K. Kobayashi, A. Fujita, R. Kainuma, K. Ishida, Scr. Mater. 58 (2008) 723. [22] R.Y. Umetsu, A. Okubo, R. Kainuma, J. Appl. Phys. 111 (2012) 073909. [23] S. Ishida, S. Sugiyama, S. Fujii, S. Asano, J. Phys.: Condens. Matter 3 (1991) 5793. [24] R.Y. Umetsu, K. Kobayashi, A. Fujita, K. Oikawa, R. Kainuma, K. Ishida, N. Endo, K. Fukamichi, A. Sakuma, Phys. Rev. B 72 (2005) 214412. [25] K.H.J. Buschow, P.G. van Engen, J. Magn. Magn. Mater. 25 (1981) 90. [26] T.M. Nakatani, Z. Gercsi, A. Rajanikanth, Y.K. Takahashi, K. Hono, J. Phys. D: Appl. Phys. 41 (2008) 225002. [27] I. Galanakis, P.H. Dederichs, N. Papanikolaou, Phys. Rev. B 66 (2002) 174429.