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Spin polarization in Cu2MnSn Heusler alloy produced by melt-spinning
Intermetallics 85 (2017) 139e143
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Intermetallics journal homepage: www.elsevier.com/locate/intermet
Spin polarization in Cu2MnSn Heusler alloy produced by melt-spinning M. Obaida a, b, L. Galdun b, T. Ryba b, V. Komanicky b, K. Saksl c, M. Durisin c, J. Kovac d, V. Haskova b, d, P. Szabo d, Z. Vargova e, R. Varga b, * a
Solid State Physics Department, National Research Centre, 33 EL Bohouth St. (former El Tahrir St.), Dokki, Giza, P.O. 12622, Egypt rik University, Park Angelinum 9, 041 54, Kosice, Slovakia Institute of Physics, Faculty of Science, P.J. Safa IMR SAS, Watsonova 47, 04001, Kosice, Slovakia d IEF SAS, Watsonova 47, 04001, Kosice, Slovakia e rik University, Park Angelinum 9, 041 54, Kosice, Slovakia Institute of Chemistry, Faculty of Science, P.J. Safa b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2016 Received in revised form 24 October 2016 Accepted 11 February 2017
We report on magnetism, transport and spin polarization characteristics of the melt-spun Cu2MnSn alloy prepared by the rapid quenching technique. The as- cast ribbons showed a relatively well ordered chemical composition (Cu ¼ 50.4%, Mn ¼ 27.1%, Sn ¼ 22.5%). The structural characterization by using X-ray diffraction shows L21/B2 crystalline structure with the lattice parameter a ¼ 6.196 Å. The magnetic and transport measurements show a metallic behavior with the Curie temperature of 530 K and reveal anisotropic character with the easy magnetization plane parallel with respect to the ribbon plane. As-cast Cu2MnSn ribbons show the spin polarization measured by using Andreev reflection technique within the range 68e75%. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Heusler alloys Spin polarization Rapid quenching
1. Introduction Since the discovery of Heusler alloys in 1903 by Friedrich Heusler they have attracted interest of many scientists and engineers because of their interesting and promising possibilities of application. The theoretical prediction of half-metallic character of some Heusler alloys have created intensive progress in the field of spintronics such as Tunneling Magnetoresistance(TMR), Giant Magnetoresistance (GMR), Magnetic Tunnel Junction(MTJ),etc. Heusler evidenced the ferromagnetic behavior of some ternary (that time assumed “nonferromagnetic”) metal-based alloys, such as CuMnAl and CuMnSn, in which the ferromagnetic order is associated with the formation of an ordered cubic phase [1]. The prediction of half-metallic ferromagnetism in MnNiSb by de Groot et al., in 1983 initialized new theoretical and experimental interest in this group of materials [2]. Since that time, a number of Heusler compositions were shown to exhibit high spin-polarization [3]. This makes them very interesting candidates for spin electronic applications, even a long time after their discovery in 1903.
* Corresponding author. E-mail address: [email protected] (R. Varga). http://dx.doi.org/10.1016/j.intermet.2017.02.014 0966-9795/© 2017 Elsevier Ltd. All rights reserved.
The characteristic feature of half-metallic materials is a gap in one of the spin channels in the density of states (DOS) at the Fermi level. This gap leads to a full spin polarization, i.e. at this energy only the majority states are occupied. As only the electrons with energies close to the Fermi level contribute to the transport processes, the conductivity of a half-metallic material is accounted for only by the electrons of one spin orientation. According to Galanakis and coworkers [4], the origin of half-metallicity in Heusler alloys with the composition X2YZ can be found in a hybridization process between the orbitals of X- and Y constituent atoms. Additionally there are many theoretical works predicting the halfmetallic properties in full-Heusler compounds [4e9]. The materials that exhibit a high spin polarization are key materials for a spintronics exploitation. The efficiency of spintronics devices depends on the extent, to which a current is spin-polarized [10]. However, the spin polarization critically depends on the local disorder. Hence it is almost impossible to produce a half-metallic Heusler alloy that exhibit 100% spin polarization [11]. In order to obtain correct L21 structure to get at least elevated spin polarization, long-term post production processes at high temperatures are employed, which is one of the disadvantage of Heusler alloys [12]. On the other hand, there are the chemical compositions that can be produced easily and quickly (within few minutes) according to
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simple rules [13] and they show correct phase without the necessity of post-production process. One of the original Heusler alloys Cu2MnSn belongs to this group. Cu2MnSn is the Heusler compounds that have magnetic moment value ranges from 3.41 to 4.14 mB/f.u. [14]. In these alloys, Sn atoms do not carry any magnetic moment but could contribute to the magnetization according to Slater postulate [15,16]. So, the origin of the ferromagnetism in Cu2MnSn is related to the indirect superexchange mechanism via nonmagnetic Sn atom [17]. Even though, no pure halfemetallic character of Cu2MnZ (Z ¼ Al, Sn, In.) is predicted (due to a lack of energy gap in the one band), there is a strong asymmetry between minority and majority band in DOS close to the Fermi level reported for these alloy, which should lead to a reasonable spin polarization [18,19]. In the present work, we are dealing with the study on spin polarization of Cu2MnSn Heusler alloy produced by a rapid quenching. We show, that easy and fast production process allows to produce Cu2MnSn ribbons with L21 structure in a single step without the necessity of further annealing. We provide basic structural, magnetic and electric analysis together with the measurement of spin polarization by Andreev reflection technique. It was found that such alloy can exhibit the spin polarization up to 80%. 2. Experimental Ingot of Cu2MnSn master alloy was prepared by arc-melting in an argon atmosphere from high-purity elements of (Cu- 99.99%, Mn- 99.99% and Sn- 99.99%). The ingot was melted three times to guarantee a good homogeneity. Ribbons of Cu2MnSn with a length of 20 mm, width 0.8 mm and thickness of 15 mm were produced using melt- spinner technique in a helium atmosphere with a tangential velocity of Cu wheel 20 m/s. As-cast ribbons were milled into the powder and investigated by X- ray diffraction analysis (XRD) using Cu Ka radiation at room temperature to determine the crystalline structure and crystalline phases. Scanning electron microscope (SEM)with Energydispersive X-ray spectroscopy (EDX) analysis techniques were used to determine the grain size, preferred crystalline growth directions and chemical composition of Cu2MnSn ribbons. The temperature dependence, electrical resistivity (four- probe method) and the magnetization measurements of the Cu2MnSn ribbons were carried out by the Magnetic Property Measurement System (MPMS, Quantum Design) and the Physical Property Measurement System (PPMS, Quantum Design). The spin polarization of the quasiparticles has been studied applying Point-Contact Andreev Reflection spectroscopy (PCAR) measurements [20e22]. The ballistic point-contacts were prepared in situ by pressing a superconducting Nb tip (sharpened mechanically) on the freshly polished surface of the sample in a 4He variable temperature insert. The effect of Andreev-reflection has been studied through the differential conductance measured on the point-contact using standard lock-in method. The spin polarization parameter (P) was extracted by fitting the experimental PCAR spectra for the Blonder-Tinkham-Klapwijk model modified for the spin polarized systems (MBTK) [20,21].
Fig. 1. X-ray diffraction (XRD) patterns of Cu2MnSn ribbon.
be a result of strong tensile stress induced by rapid quenching. The presence of the (200) and (400) patterns with higher intensities than the (111) peak reflects that the crystalline structure of the ascast Cu2MnSn ribbons is not completely characterized by L21 structure but contain some mixture of L21/B2 phase, which could be a result of out-of-stoichiometric composition (as given by EDX analysis - see below). Moreover, X-ray diffractogram reveals the traces of other phases which could be referred to Cu2O and MnO that overlaps with the Heusler phase in Fig. 1. The presence of such phases may relate to the excess of Cu and Mn percentage during the preparation of the Cu2MnSn ingot. This coincide with the previous documented studies that points to the difficulty to produce Cu2MnSn alloy in a well ordered state of L21 structure without some impurity phases [26e29]. Fig. 2 shows the SEM micrograph of the ribbon cross-section which reveals the sample thickness of 30 mm. It points to the fine crystalline structure with the crystal diameter ~1.9 mm and height of ~4 mm that appears at the bottom of the ribbons (which was in contact with Cu wheel where highest cooling rate were achieved e bottom part in Fig. 2). Above it, columnar structure growth perpendicularly along entire cross section up to the surface with the column width of 4 mm (since lower cooling rate allows higher
3. Results and discussion Firstly, the structural analysis of the produced samples has been performed. The X-ray patterns for the as casted Cu2MnSn Heusler ribbons in Fig. 1 shows the (111) and (311) reflection peaks, which indicate the highly ordered L21 crystalline structure. The lattice parameter a ¼ 6.196 Å is slightly higher than that reported for bulk Cu2MnSn sample (a ¼ 6.176 Å [23e25]). The small variation could
Fig. 2. SEM micrograph of cross-section for Cu2MnSn ribbon. Bottom part of a ribbon is a wheel side.
M. Obaida et al. / Intermetallics 85 (2017) 139e143
crystals growth). EDX analysis denotes that the ribbon has a homogenous chemical composition with the stoichiometric formula (Cu ¼ 50.4%, Mn ¼ 27.1%, Sn ¼ 22.5%) and one can notice that there is the excess of Cu and Mn, which affects the crystalline structure. The large percentage of Mn is attributed to the difficulty in the controlling of its evaporation process during the ingot preparation. Although, the Cu2MnSn alloy consists of non-ferromagnetic elements, it shows ferromagnetic behavior. There is a general agreement that the magnetization of the copper-based Heusler alloys is mainly concentrated on the Mn atom and that the ferromagnetism occurs only within a value of R/r ratio (R is half of internuclear distance and r is the electron shell radius for incomplete shell) [30]. So, Zener has suggested a mechanism that describes the presence of ferromagnetism in Heusler alloys in relation to the polarization of the conduction electrons, which interact with the d spins and determine the ferromagnetic properties of the alloy [31]. Also, Yosida considered that the s-d spins interactions in CuMn alloys is dominant and has shown that the polarized electrons are concentrated near the Mn ions [32]. As a result, Cu2MnSn present ferromagnetism up to the elevated temperatures. In order to determine the Curie temperature Tc of the prepared ribbon, the temperature dependence of the magnetization was measured by VSM in a magnetic field of 2.5 kOe. As shown in Fig. 3, a typical ferromagnetic behavior for the temperature dependence of magnetization is observed, which decreases gradually with increasing the temperature up to the Curie temperature Tc of 530 K (consistent with the previous reported results [18,23,33]. The hysteresis loops were measured at a room temperature (RT) for the prepared ribbon in both the parallel and perpendicular directions in a magnetic field up to 50 kOe (Fig. 4). Coercive field in the parallel direction is HC ¼ 0.05 kOe and saturation field HS > 1 kOe. On the other side, coercive field in the perpendicular direction is smaller (HC ¼ 0.02 kOe) and saturation field exceeds our measurement range (5 kOe). These facts reveal a well-defined anisotropy of Cu2MnSn Heusler ribbons oriented in the ribbon plane, which is one of the crucial parameters for the spintronic applications. The value of the saturation magnetization estimated from the hysteresis loop is 2.8 mB/f.u. Howeverer, the hysteresis loops were measured at room temperature, hence the value is lower than that expected for Cu2MnSn (3.41e4.14 mB/f.u.) [14]. Extrapolation of
Fig. 3. Temperature dependence of saturation magnetization for Cu2MnSn ribbon.
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Fig. 4. Parallel and perpendicular hysteresis loops for as-cast Cu2MnSn ribbon at RT.
temperature dependence of magnetization to low temperature leads to value 3.55 mB/f.u., which is within the range of values measured before [14]. Moreover, the maximum applied field was quite low (5 kOe) which does not led to the total saturation (see Fig. 4). Hence the correct saturation magnetization could be even higher. The temperature dependence of the electrical resistivity for the as-cast ribbons was measured from 300 K down to 2 K (Fig. 5). Cu2MnSn ribbons showed a typical metallic behavior for the resistivity measurement with a low resistance of 7U and residual resistivity ratio (RRR), which indicates the relative portion of the phonon scattering and the defect scattering in the electrical resistivity, is 1.97. The formation of scattering centers due to the structural disorder causes reduction of RRR values. The highest value reported so far with RRR of 6.5 is for single crystal Co2MnSi [13]. The RRR value for the case of our Cu2MnSn ribbon was found much lower most probably due to the weak atomic disorder or
Fig. 5. Temperature dependent electrical resistivity for Cu2MnSn ribbon. Full lines corresponds to the fit (for T5 below 75 K (blue line) and TþT2 for T > 75 K (red line) see the text). Inset confirms the T5 dependence of resistance at low temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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polycrystalline character of the ribbon(due to grain boundary scattering). On the other hand, RRR is slightly higher than that observed in the thin films of similar composition confirming a higher quality of structural long-range order than that for thin films [35] Moreover, below 20 K the resistivity exhibits a shallow minimum. The low temperature minimum is of the same type as observed in many amorphous or crystalline metals with structural disorder and is interpreted as due to the weak localization or electronic correlations [34]. Above 20 K, the temperature dependence of resistivity can be divided into the two regions. Below 85 K, it can be fit with T5 dependence (blue line) indicating the small angle scattering of the conduction electrons with the phonons. Above 85 K, the fitting gives dependence of aTþbT2 (red line), which points to an electronphonon (that corresponds to the linear part) or one magnon scattering (for quadratic part of the fit) as the main contributions to the electrical resistivity in Cu2MnSn Heusler alloy [35,36]. As given in the introduction, the Cu2MnSn alloy is not predicted to be fully half-metallic (i.e. It has no energy gap for minority band of electrons). However, there is an asymmetry in the density of states (DOS) assumed for minority and majority band of electrons that could leads to the spin polarization between 30 and 50% [18,19]. When such asymmetry is well developed (as in case of Cu2MnSn), the interesting values of spin polarization can be achieved. Point-contact Andreev reflection (PCAR) spectroscopy measurements have been performed on the different pieces of Cu2MnSn ribbons. The tens of PCAR spectra have been measured on the each sample at T ¼ 1.32 K using a high purity superconducting Nb tip. Ballistic PCAR spectra with two symmetric maxima near the value of the superconducting energy gap of Nb have been measured at contact resistances in the range RPC ¼ 10e40 U. The spin polarization parameter has been determined from fitting the PCAR spectra to the thermally smeared MBTK model [20,21]. This model is a two channel model, where the PCAR spectrum is calculated as the weighted sum of the Andreev reflection conductance and the spin polarized conductance. The weight of the spin polarized channel defines the value of the spin polarization parameter P. The model can be described as a function of the superconducting energy gap D, point-contact barrier strength Z and spectral broadening parameter G, which represents the imaginary part of the energy E ¼ E’ þ iG [20,21]. When the spin polarization parameter is determined from PCAR, it's value reveals a weak increase with a decreasing barrier strength and saturates at Z -> 0 [21,37e39]. This
dependence is related to enhanced spin-flip scattering at the pointcontact area. Thus, the correct value of the parameter P should be estimated from point-contacts with the low barrier strengths (Z < 0.4) [37e40]. Our PCAR spectra measured at the different Nb-Cu2MnSn pointcontact resistances revealed large variation of the spectral broadening and small changes of the barrier strength Z ¼ 0.2e0.4 at the known value of the Nb energy gap DNb ¼ 1.53 meV. The highest spectral broadenings G > 0.6DNb have been observed on the polished Cu2MnSn surfaces where ballistic PCAR spectra have been observed only at the higher point-contact resistances RPC ¼ 30e60 U. Below this range we lost the ballistic conditions for the quasiparticle transport through the point-contact and the nonspectroscopic thermal features dominated. Point-contacts formed on the freshly cleaved surfaces revealed spectra with much lower smearing G ¼ 0.3DNb e 0.5DNb at typical resistances RPC ¼ 10e30 U. At the lower resistances, the non-spectroscopic thermal effects were observed again. The increase of the spectral resolution (decrease of G) on the freshly cleaved surfaces points to the importance of extrinsic inelastic scattering centers on the surfaces of the studied ribbons. The high transparency of our point-contacts (Z < 0.4) allowed the direct estimation of the spin polarization parameter P from the PCAR spectra with higher resolution (G ¼ 0.3DNb e 0.5DNb). The obtained P reveals values in the range P ¼ 0.6e0.77, the uncertainty at the determination of P were below 10%. Fig. 6 a) shows typical PCAR spectra measured in Nb-Cu2MnSn point-contacts at T ¼ 1.32 K (open symbols). The MBTK model describes our PCAR curves in a very good agreement (see solid lines) with the spin polarization parameters P ¼ 0.68, 0.76 and 0.71. Fig. 6 b) show the dependence of the spin polarization parameter P on the barrier strength Z. The saturation of P to the value of 0.75 at a decreasing barrier strength points to the correctness of our estimations [21,37e39]. The distribution of the values of the spin polarization parameter is sample independent, most probably reflecting the local fluctuation of chemical elements in the crystallographic structure along the sample. The measured values of spin polarization are higher in comparison to the theoretical prediction that is are quite spread [18,19]. They point out to the differences in the minority and majority bands in DOS at Fermi level that could lead to the values of spin polarization from 30% to 50%. One of the possible explanation for the difference between the theory and the experiment may consist
Fig. 6. a) PCAR conductance curves dI/dV(V) (open symbols) measured in Nb-Cu2MnSn point-contacts at T ¼ 1.32 K. The curves are normalized for the conductance value at V z 10 meV and shifted for the clarity. The solid lines plot fitting curves to MBTK model with DNb ¼ 1.53 meV, barrier transparency Z ¼ 0.26, 0.207, 0.25, spectral smearing G ¼ 0.73, 0,6, 0,55 meV and spin polarization parameter P ¼ 0.68, 0.76 and 0.71 from top to bottom, resp. b) The polarization parameter P determined from the MBTK model as a function of the Z parameter. The dashed line (parabolic fit of the data) is a guide to the eye.
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in the different values of the lattice constants, which are used in the theoretical calculations (6.13 Å egiving P~30% [18], 6.18 Å e giving P~50% [19]) and our experimental value (6.196 Å). It should be mentioned that besides the theoretical investigation of the influence of the crystalline structure on the spin polarization there exist also other theories. For example, one of them deals with the influence of the Mn content on spin polarization in off-stoichiometric Co2MnAl Heusler alloy [41]. The investigation points to the fact, that the increase of Mn atoms in Co2-xMn1þxAl alloys, results in the increase of spin polarization from 75% (for Co2MnAl) to 100% (for Co1.125Mn1.875Al). In our case the content of Mn is slightly higher (Cu ¼ 50.4%, Mn ¼ 27.1%, Sn ¼ 22.5%). By following this model we may expect an increase of spin polarization with the increase of Mn content in the alloy. However, such prediction must be confirmed by experiment. 4. Conclusions We report on the preparation of full Heusler Cu2MnSn ribbon by the rapid-quenching technique. The sample shows a homogenous chemical composition with a dominant L21 structure. Preferential crystal growth in perpendicular direction to the ribbon surface results in a well-defined anisotropy with an easy-plane parallel to the ribbon surface. Although, Cu2MnSn is not predicted to be halfmetallic, it exhibits high spin polarization (between 68 and 75%) due to the strong asymmetry in the minority and majority DOS. These facts, together with its high Curie temperature (530 K) and easy production process avoiding post-production long-term annealing, indicates that rapidly quenched Cu2MnSn could be promising materials for spintronic applications. Acknowledgements M. Obaida would like to acknowledge the grant from Slovak Academic Information Agency (SAIA) under National Scholarship program. This work was supported partially by the NanoCEXmat Project under Grant ITMS 26220120019, Extrem1 project under Grant ITMS 26220120005 and the COST action MP1201 as well as by the U.S. Steel Kosice. Also part of the project was supported by Slovak Grant Agency VEGA under Grants 1/0164/16, VEGA 2/0149/ 16 and VEGA 1/0409/15 and Slovak Grant Agency APVV under Grants APVV-0027-11, APVV-0605-14 and APVV-0036-11. References [1] F. Heusler, Verh. Dtsch. Phys. Ges. 5 (1903) 219. [2] R. de Groot, F. Mueller, P. van Engen, K. Buschow, Phys. Rew. Lett. 50 (1983) 2024e2027. [3] K.H.J. Buschow, Handbook of Magnetic Materilas, first ed., Elsevier,
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Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Spin polarization in Cu2MnSn Heusler alloy produced by melt-spinning M. Obaida a, b, L. Galdun b, T. Ryba b, V. Komanicky b, K. Saksl c, M. Durisin c, J. Kovac d, V. Haskova b, d, P. Szabo d, Z. Vargova e, R. Varga b, * a
Solid State Physics Department, National Research Centre, 33 EL Bohouth St. (former El Tahrir St.), Dokki, Giza, P.O. 12622, Egypt rik University, Park Angelinum 9, 041 54, Kosice, Slovakia Institute of Physics, Faculty of Science, P.J. Safa IMR SAS, Watsonova 47, 04001, Kosice, Slovakia d IEF SAS, Watsonova 47, 04001, Kosice, Slovakia e rik University, Park Angelinum 9, 041 54, Kosice, Slovakia Institute of Chemistry, Faculty of Science, P.J. Safa b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 July 2016 Received in revised form 24 October 2016 Accepted 11 February 2017
We report on magnetism, transport and spin polarization characteristics of the melt-spun Cu2MnSn alloy prepared by the rapid quenching technique. The as- cast ribbons showed a relatively well ordered chemical composition (Cu ¼ 50.4%, Mn ¼ 27.1%, Sn ¼ 22.5%). The structural characterization by using X-ray diffraction shows L21/B2 crystalline structure with the lattice parameter a ¼ 6.196 Å. The magnetic and transport measurements show a metallic behavior with the Curie temperature of 530 K and reveal anisotropic character with the easy magnetization plane parallel with respect to the ribbon plane. As-cast Cu2MnSn ribbons show the spin polarization measured by using Andreev reflection technique within the range 68e75%. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Heusler alloys Spin polarization Rapid quenching
1. Introduction Since the discovery of Heusler alloys in 1903 by Friedrich Heusler they have attracted interest of many scientists and engineers because of their interesting and promising possibilities of application. The theoretical prediction of half-metallic character of some Heusler alloys have created intensive progress in the field of spintronics such as Tunneling Magnetoresistance(TMR), Giant Magnetoresistance (GMR), Magnetic Tunnel Junction(MTJ),etc. Heusler evidenced the ferromagnetic behavior of some ternary (that time assumed “nonferromagnetic”) metal-based alloys, such as CuMnAl and CuMnSn, in which the ferromagnetic order is associated with the formation of an ordered cubic phase [1]. The prediction of half-metallic ferromagnetism in MnNiSb by de Groot et al., in 1983 initialized new theoretical and experimental interest in this group of materials [2]. Since that time, a number of Heusler compositions were shown to exhibit high spin-polarization [3]. This makes them very interesting candidates for spin electronic applications, even a long time after their discovery in 1903.
* Corresponding author. E-mail address: [email protected] (R. Varga). http://dx.doi.org/10.1016/j.intermet.2017.02.014 0966-9795/© 2017 Elsevier Ltd. All rights reserved.
The characteristic feature of half-metallic materials is a gap in one of the spin channels in the density of states (DOS) at the Fermi level. This gap leads to a full spin polarization, i.e. at this energy only the majority states are occupied. As only the electrons with energies close to the Fermi level contribute to the transport processes, the conductivity of a half-metallic material is accounted for only by the electrons of one spin orientation. According to Galanakis and coworkers [4], the origin of half-metallicity in Heusler alloys with the composition X2YZ can be found in a hybridization process between the orbitals of X- and Y constituent atoms. Additionally there are many theoretical works predicting the halfmetallic properties in full-Heusler compounds [4e9]. The materials that exhibit a high spin polarization are key materials for a spintronics exploitation. The efficiency of spintronics devices depends on the extent, to which a current is spin-polarized [10]. However, the spin polarization critically depends on the local disorder. Hence it is almost impossible to produce a half-metallic Heusler alloy that exhibit 100% spin polarization [11]. In order to obtain correct L21 structure to get at least elevated spin polarization, long-term post production processes at high temperatures are employed, which is one of the disadvantage of Heusler alloys [12]. On the other hand, there are the chemical compositions that can be produced easily and quickly (within few minutes) according to
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simple rules [13] and they show correct phase without the necessity of post-production process. One of the original Heusler alloys Cu2MnSn belongs to this group. Cu2MnSn is the Heusler compounds that have magnetic moment value ranges from 3.41 to 4.14 mB/f.u. [14]. In these alloys, Sn atoms do not carry any magnetic moment but could contribute to the magnetization according to Slater postulate [15,16]. So, the origin of the ferromagnetism in Cu2MnSn is related to the indirect superexchange mechanism via nonmagnetic Sn atom [17]. Even though, no pure halfemetallic character of Cu2MnZ (Z ¼ Al, Sn, In.) is predicted (due to a lack of energy gap in the one band), there is a strong asymmetry between minority and majority band in DOS close to the Fermi level reported for these alloy, which should lead to a reasonable spin polarization [18,19]. In the present work, we are dealing with the study on spin polarization of Cu2MnSn Heusler alloy produced by a rapid quenching. We show, that easy and fast production process allows to produce Cu2MnSn ribbons with L21 structure in a single step without the necessity of further annealing. We provide basic structural, magnetic and electric analysis together with the measurement of spin polarization by Andreev reflection technique. It was found that such alloy can exhibit the spin polarization up to 80%. 2. Experimental Ingot of Cu2MnSn master alloy was prepared by arc-melting in an argon atmosphere from high-purity elements of (Cu- 99.99%, Mn- 99.99% and Sn- 99.99%). The ingot was melted three times to guarantee a good homogeneity. Ribbons of Cu2MnSn with a length of 20 mm, width 0.8 mm and thickness of 15 mm were produced using melt- spinner technique in a helium atmosphere with a tangential velocity of Cu wheel 20 m/s. As-cast ribbons were milled into the powder and investigated by X- ray diffraction analysis (XRD) using Cu Ka radiation at room temperature to determine the crystalline structure and crystalline phases. Scanning electron microscope (SEM)with Energydispersive X-ray spectroscopy (EDX) analysis techniques were used to determine the grain size, preferred crystalline growth directions and chemical composition of Cu2MnSn ribbons. The temperature dependence, electrical resistivity (four- probe method) and the magnetization measurements of the Cu2MnSn ribbons were carried out by the Magnetic Property Measurement System (MPMS, Quantum Design) and the Physical Property Measurement System (PPMS, Quantum Design). The spin polarization of the quasiparticles has been studied applying Point-Contact Andreev Reflection spectroscopy (PCAR) measurements [20e22]. The ballistic point-contacts were prepared in situ by pressing a superconducting Nb tip (sharpened mechanically) on the freshly polished surface of the sample in a 4He variable temperature insert. The effect of Andreev-reflection has been studied through the differential conductance measured on the point-contact using standard lock-in method. The spin polarization parameter (P) was extracted by fitting the experimental PCAR spectra for the Blonder-Tinkham-Klapwijk model modified for the spin polarized systems (MBTK) [20,21].
Fig. 1. X-ray diffraction (XRD) patterns of Cu2MnSn ribbon.
be a result of strong tensile stress induced by rapid quenching. The presence of the (200) and (400) patterns with higher intensities than the (111) peak reflects that the crystalline structure of the ascast Cu2MnSn ribbons is not completely characterized by L21 structure but contain some mixture of L21/B2 phase, which could be a result of out-of-stoichiometric composition (as given by EDX analysis - see below). Moreover, X-ray diffractogram reveals the traces of other phases which could be referred to Cu2O and MnO that overlaps with the Heusler phase in Fig. 1. The presence of such phases may relate to the excess of Cu and Mn percentage during the preparation of the Cu2MnSn ingot. This coincide with the previous documented studies that points to the difficulty to produce Cu2MnSn alloy in a well ordered state of L21 structure without some impurity phases [26e29]. Fig. 2 shows the SEM micrograph of the ribbon cross-section which reveals the sample thickness of 30 mm. It points to the fine crystalline structure with the crystal diameter ~1.9 mm and height of ~4 mm that appears at the bottom of the ribbons (which was in contact with Cu wheel where highest cooling rate were achieved e bottom part in Fig. 2). Above it, columnar structure growth perpendicularly along entire cross section up to the surface with the column width of 4 mm (since lower cooling rate allows higher
3. Results and discussion Firstly, the structural analysis of the produced samples has been performed. The X-ray patterns for the as casted Cu2MnSn Heusler ribbons in Fig. 1 shows the (111) and (311) reflection peaks, which indicate the highly ordered L21 crystalline structure. The lattice parameter a ¼ 6.196 Å is slightly higher than that reported for bulk Cu2MnSn sample (a ¼ 6.176 Å [23e25]). The small variation could
Fig. 2. SEM micrograph of cross-section for Cu2MnSn ribbon. Bottom part of a ribbon is a wheel side.
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crystals growth). EDX analysis denotes that the ribbon has a homogenous chemical composition with the stoichiometric formula (Cu ¼ 50.4%, Mn ¼ 27.1%, Sn ¼ 22.5%) and one can notice that there is the excess of Cu and Mn, which affects the crystalline structure. The large percentage of Mn is attributed to the difficulty in the controlling of its evaporation process during the ingot preparation. Although, the Cu2MnSn alloy consists of non-ferromagnetic elements, it shows ferromagnetic behavior. There is a general agreement that the magnetization of the copper-based Heusler alloys is mainly concentrated on the Mn atom and that the ferromagnetism occurs only within a value of R/r ratio (R is half of internuclear distance and r is the electron shell radius for incomplete shell) [30]. So, Zener has suggested a mechanism that describes the presence of ferromagnetism in Heusler alloys in relation to the polarization of the conduction electrons, which interact with the d spins and determine the ferromagnetic properties of the alloy [31]. Also, Yosida considered that the s-d spins interactions in CuMn alloys is dominant and has shown that the polarized electrons are concentrated near the Mn ions [32]. As a result, Cu2MnSn present ferromagnetism up to the elevated temperatures. In order to determine the Curie temperature Tc of the prepared ribbon, the temperature dependence of the magnetization was measured by VSM in a magnetic field of 2.5 kOe. As shown in Fig. 3, a typical ferromagnetic behavior for the temperature dependence of magnetization is observed, which decreases gradually with increasing the temperature up to the Curie temperature Tc of 530 K (consistent with the previous reported results [18,23,33]. The hysteresis loops were measured at a room temperature (RT) for the prepared ribbon in both the parallel and perpendicular directions in a magnetic field up to 50 kOe (Fig. 4). Coercive field in the parallel direction is HC ¼ 0.05 kOe and saturation field HS > 1 kOe. On the other side, coercive field in the perpendicular direction is smaller (HC ¼ 0.02 kOe) and saturation field exceeds our measurement range (5 kOe). These facts reveal a well-defined anisotropy of Cu2MnSn Heusler ribbons oriented in the ribbon plane, which is one of the crucial parameters for the spintronic applications. The value of the saturation magnetization estimated from the hysteresis loop is 2.8 mB/f.u. Howeverer, the hysteresis loops were measured at room temperature, hence the value is lower than that expected for Cu2MnSn (3.41e4.14 mB/f.u.) [14]. Extrapolation of
Fig. 3. Temperature dependence of saturation magnetization for Cu2MnSn ribbon.
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Fig. 4. Parallel and perpendicular hysteresis loops for as-cast Cu2MnSn ribbon at RT.
temperature dependence of magnetization to low temperature leads to value 3.55 mB/f.u., which is within the range of values measured before [14]. Moreover, the maximum applied field was quite low (5 kOe) which does not led to the total saturation (see Fig. 4). Hence the correct saturation magnetization could be even higher. The temperature dependence of the electrical resistivity for the as-cast ribbons was measured from 300 K down to 2 K (Fig. 5). Cu2MnSn ribbons showed a typical metallic behavior for the resistivity measurement with a low resistance of 7U and residual resistivity ratio (RRR), which indicates the relative portion of the phonon scattering and the defect scattering in the electrical resistivity, is 1.97. The formation of scattering centers due to the structural disorder causes reduction of RRR values. The highest value reported so far with RRR of 6.5 is for single crystal Co2MnSi [13]. The RRR value for the case of our Cu2MnSn ribbon was found much lower most probably due to the weak atomic disorder or
Fig. 5. Temperature dependent electrical resistivity for Cu2MnSn ribbon. Full lines corresponds to the fit (for T5 below 75 K (blue line) and TþT2 for T > 75 K (red line) see the text). Inset confirms the T5 dependence of resistance at low temperature. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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polycrystalline character of the ribbon(due to grain boundary scattering). On the other hand, RRR is slightly higher than that observed in the thin films of similar composition confirming a higher quality of structural long-range order than that for thin films [35] Moreover, below 20 K the resistivity exhibits a shallow minimum. The low temperature minimum is of the same type as observed in many amorphous or crystalline metals with structural disorder and is interpreted as due to the weak localization or electronic correlations [34]. Above 20 K, the temperature dependence of resistivity can be divided into the two regions. Below 85 K, it can be fit with T5 dependence (blue line) indicating the small angle scattering of the conduction electrons with the phonons. Above 85 K, the fitting gives dependence of aTþbT2 (red line), which points to an electronphonon (that corresponds to the linear part) or one magnon scattering (for quadratic part of the fit) as the main contributions to the electrical resistivity in Cu2MnSn Heusler alloy [35,36]. As given in the introduction, the Cu2MnSn alloy is not predicted to be fully half-metallic (i.e. It has no energy gap for minority band of electrons). However, there is an asymmetry in the density of states (DOS) assumed for minority and majority band of electrons that could leads to the spin polarization between 30 and 50% [18,19]. When such asymmetry is well developed (as in case of Cu2MnSn), the interesting values of spin polarization can be achieved. Point-contact Andreev reflection (PCAR) spectroscopy measurements have been performed on the different pieces of Cu2MnSn ribbons. The tens of PCAR spectra have been measured on the each sample at T ¼ 1.32 K using a high purity superconducting Nb tip. Ballistic PCAR spectra with two symmetric maxima near the value of the superconducting energy gap of Nb have been measured at contact resistances in the range RPC ¼ 10e40 U. The spin polarization parameter has been determined from fitting the PCAR spectra to the thermally smeared MBTK model [20,21]. This model is a two channel model, where the PCAR spectrum is calculated as the weighted sum of the Andreev reflection conductance and the spin polarized conductance. The weight of the spin polarized channel defines the value of the spin polarization parameter P. The model can be described as a function of the superconducting energy gap D, point-contact barrier strength Z and spectral broadening parameter G, which represents the imaginary part of the energy E ¼ E’ þ iG [20,21]. When the spin polarization parameter is determined from PCAR, it's value reveals a weak increase with a decreasing barrier strength and saturates at Z -> 0 [21,37e39]. This
dependence is related to enhanced spin-flip scattering at the pointcontact area. Thus, the correct value of the parameter P should be estimated from point-contacts with the low barrier strengths (Z < 0.4) [37e40]. Our PCAR spectra measured at the different Nb-Cu2MnSn pointcontact resistances revealed large variation of the spectral broadening and small changes of the barrier strength Z ¼ 0.2e0.4 at the known value of the Nb energy gap DNb ¼ 1.53 meV. The highest spectral broadenings G > 0.6DNb have been observed on the polished Cu2MnSn surfaces where ballistic PCAR spectra have been observed only at the higher point-contact resistances RPC ¼ 30e60 U. Below this range we lost the ballistic conditions for the quasiparticle transport through the point-contact and the nonspectroscopic thermal features dominated. Point-contacts formed on the freshly cleaved surfaces revealed spectra with much lower smearing G ¼ 0.3DNb e 0.5DNb at typical resistances RPC ¼ 10e30 U. At the lower resistances, the non-spectroscopic thermal effects were observed again. The increase of the spectral resolution (decrease of G) on the freshly cleaved surfaces points to the importance of extrinsic inelastic scattering centers on the surfaces of the studied ribbons. The high transparency of our point-contacts (Z < 0.4) allowed the direct estimation of the spin polarization parameter P from the PCAR spectra with higher resolution (G ¼ 0.3DNb e 0.5DNb). The obtained P reveals values in the range P ¼ 0.6e0.77, the uncertainty at the determination of P were below 10%. Fig. 6 a) shows typical PCAR spectra measured in Nb-Cu2MnSn point-contacts at T ¼ 1.32 K (open symbols). The MBTK model describes our PCAR curves in a very good agreement (see solid lines) with the spin polarization parameters P ¼ 0.68, 0.76 and 0.71. Fig. 6 b) show the dependence of the spin polarization parameter P on the barrier strength Z. The saturation of P to the value of 0.75 at a decreasing barrier strength points to the correctness of our estimations [21,37e39]. The distribution of the values of the spin polarization parameter is sample independent, most probably reflecting the local fluctuation of chemical elements in the crystallographic structure along the sample. The measured values of spin polarization are higher in comparison to the theoretical prediction that is are quite spread [18,19]. They point out to the differences in the minority and majority bands in DOS at Fermi level that could lead to the values of spin polarization from 30% to 50%. One of the possible explanation for the difference between the theory and the experiment may consist
Fig. 6. a) PCAR conductance curves dI/dV(V) (open symbols) measured in Nb-Cu2MnSn point-contacts at T ¼ 1.32 K. The curves are normalized for the conductance value at V z 10 meV and shifted for the clarity. The solid lines plot fitting curves to MBTK model with DNb ¼ 1.53 meV, barrier transparency Z ¼ 0.26, 0.207, 0.25, spectral smearing G ¼ 0.73, 0,6, 0,55 meV and spin polarization parameter P ¼ 0.68, 0.76 and 0.71 from top to bottom, resp. b) The polarization parameter P determined from the MBTK model as a function of the Z parameter. The dashed line (parabolic fit of the data) is a guide to the eye.
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in the different values of the lattice constants, which are used in the theoretical calculations (6.13 Å egiving P~30% [18], 6.18 Å e giving P~50% [19]) and our experimental value (6.196 Å). It should be mentioned that besides the theoretical investigation of the influence of the crystalline structure on the spin polarization there exist also other theories. For example, one of them deals with the influence of the Mn content on spin polarization in off-stoichiometric Co2MnAl Heusler alloy [41]. The investigation points to the fact, that the increase of Mn atoms in Co2-xMn1þxAl alloys, results in the increase of spin polarization from 75% (for Co2MnAl) to 100% (for Co1.125Mn1.875Al). In our case the content of Mn is slightly higher (Cu ¼ 50.4%, Mn ¼ 27.1%, Sn ¼ 22.5%). By following this model we may expect an increase of spin polarization with the increase of Mn content in the alloy. However, such prediction must be confirmed by experiment. 4. Conclusions We report on the preparation of full Heusler Cu2MnSn ribbon by the rapid-quenching technique. The sample shows a homogenous chemical composition with a dominant L21 structure. Preferential crystal growth in perpendicular direction to the ribbon surface results in a well-defined anisotropy with an easy-plane parallel to the ribbon surface. Although, Cu2MnSn is not predicted to be halfmetallic, it exhibits high spin polarization (between 68 and 75%) due to the strong asymmetry in the minority and majority DOS. These facts, together with its high Curie temperature (530 K) and easy production process avoiding post-production long-term annealing, indicates that rapidly quenched Cu2MnSn could be promising materials for spintronic applications. Acknowledgements M. Obaida would like to acknowledge the grant from Slovak Academic Information Agency (SAIA) under National Scholarship program. This work was supported partially by the NanoCEXmat Project under Grant ITMS 26220120019, Extrem1 project under Grant ITMS 26220120005 and the COST action MP1201 as well as by the U.S. Steel Kosice. Also part of the project was supported by Slovak Grant Agency VEGA under Grants 1/0164/16, VEGA 2/0149/ 16 and VEGA 1/0409/15 and Slovak Grant Agency APVV under Grants APVV-0027-11, APVV-0605-14 and APVV-0036-11. References [1] F. Heusler, Verh. Dtsch. Phys. Ges. 5 (1903) 219. [2] R. de Groot, F. Mueller, P. van Engen, K. Buschow, Phys. Rew. Lett. 50 (1983) 2024e2027. [3] K.H.J. Buschow, Handbook of Magnetic Materilas, first ed., Elsevier,
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