Effect of Ni doping on the magnetic and electronic properties of half heusler Cu1-xNixMnSb alloys

Accepted Manuscript Effect of Ni doping on the magnetic and electronic properties of half heusler Cu1-xNixMnSb alloys A. Bandyopadhyay, S.K. Neogi, A. Paul, C. Meneghini, I. Dasgupta, Sugata Ray PII:

S0925-8388(18)32183-2

DOI:

10.1016/j.jallcom.2018.06.065

Reference:

JALCOM 46401

To appear in:

Journal of Alloys and Compounds

Received Date: 2 May 2018 Accepted Date: 6 June 2018

Please cite this article as: A. Bandyopadhyay, S.K. Neogi, A. Paul, C. Meneghini, I. Dasgupta, S. Ray, Effect of Ni doping on the magnetic and electronic properties of half heusler Cu1-xNixMnSb alloys, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.06.065. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

E ect of Ni doping on the magnetic and electronic properties of Half Heusler Cu1-xNixMnSb alloys

RI PT

A. Bandyopadhyay a,*; S. K. Neogi a,*; A. Paul b; C. Meneghini c; I. Dasgupta b,d; Sugata Ray a,d . a

Department of Materials Science, Indian Association for the Cultivation of Science, 2 A & B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, INDIA b

d

Dipartimento di Scienze, Universita Roma Tre, Via della Vasca Navale, 84 I-00146 Roma, Italy

M AN U

c

SC

Department of Solid State Physics, Indian Association for the Cultivation of Science, 2 A & B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, INDIA

Center for Advanced Materials, Indian Association for the Cultivation of Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, India

*

Abstract

TE D

These two authors equally contribute to this work.

AC C

EP

The e ect of Ni-doping on the magnetic and electronic properties of cubic half heusler Cu1-xNixMnSb (x = 0.04, 0.07, 0.1, 0.125) compounds have been investigated both experimentally and theoretically in light of the development of half-metallic ferromagnetism (HMFM) in x = 1, NiMnSb. Our findings reveal that Ni-substitution introduces ferromagnetic (FM) correlations within the parent AFM matrix of CuMnSb. This is followed by the reduction in spin-down density of states (DOS) at the Fermi energy (EF), verified from the gradual suppression of T2-dependency in the low T resistivity variation upon increasing doping content. The ab-initio electronic structure calculations further suggest that the Sb p-holes, produced upon Ni doping, mediate the RKKY-type indirect FM exchange between the distant Mn atoms, and consequently, the spin-down DOS starts to get depleted at the Fermi energy. Further the importance of the Sb p-holes in mediating ferromagnetic (FM) exchange interaction, is illustrated theoretically on Fe-doped Cu1-xFexMnSb systems having identical crystal structure, where appreciable Sb holes stabilizes FM correlations at much lower concentration of Fe.

Key words: Half Heusler alloys, Half-metallic ferromagnet, Mixed magnetic phase, Resistivity variation, Density Functional Theory PACS: 71.20.Be, 75.50.Ee, 78.70.Dm, 72.25.Ba, 71.15.Mb

ACCEPTED MANUSCRIPT 1 May, 2018

AC C

EP

TE D

M AN U

SC

RI PT

Preprint submitted to Journal of Alloys and Compounds

ACCEPTED MANUSCRIPT 1

Introduction

M AN U

SC

RI PT

Heusler and Half-heusler compounds have attracted considerable attention both theoretically and experimentally in recent times due to their various exciting physical properties [1-3]. Among them, Half-heusler alloys of general chemical formulae XYZ (X and Y are transition metal atoms while Z is an sp atom) have become potential candidate in the context of half metallicity. In half-metallic (HM) magnetic materials (carrying a net integer moment), one of the spin channels crosses the Fermi energy (EF) while the other is energy gapped at EF [2,4]. Therefore synthesis of this kind of material capable of producing electron spin selective current is of immense interest both for spintronic applications and fundamental understandings [5]. On top of that, HM fully compensated ferrimagnets (HMFiM), o ering unique possibility of net zero moment within HM band structure [6,7], have more potential in technology as these materials are insensitive to applied external magnetic fields. But experimental realization of such a HMFiM state, among a wide variety of crystal structures e.g. metal alloys [8,9], metal pnictides [10], organic polymers [11], and even double perovskites [12-14], remained elusive till date.

EP

TE D

At this point, Mn-based half-heusler alloys of general chemical formulae XMnZ, crystallizing in cubic C1b phase, especially became promising due to the tunablity between half-metallic ferromagnetism (HMFM) and semi-metallic antiferromagnetism (AFM) [15,4,16-19]. The fairly large Mn-Mn distance (> 4Å) makes the direct overlap between neighboring Mn 3d states impossible and hence, the resulting magnetic state crucially depends on the competition between the RKKY-type indirect exchange stabilizing ferromagnetism and the superexchange promoting antiferromagnetism. On the other hand, choice of X and doping at the X-site change the number of valence electron count and therefore the electron density of states (DOS) at the EF. Consequently, the magnetic exchange interactions: the relative strength of superexchange with respect to the RKKY-type FM exchange might be influenced. So the magnetic ground state in these Mn-based half-heusler systems might have an intrinsic connection with the electronic structure and therefore, the half metallic nature.

AC C

The systems Cu1-xYxMnSb (Y = Ni, Fe) are of significant interest as the end member alloys: CuMnSb (Cu+1:3d10; nonmagnetic) is the only semi-metallic AFM [20] while both NiMnSb and FeMnSb (Ni, Fe; magnetic) are ferromagnetic half-metal [2,17,21,22]. So there is a clear possibility of tuning magnetic ordering and electronic structures between them as a function of the Ni/Fe and Cu

Corresponding author. Tel.: +91 33 24734971 (Ext:1226); fax: +91 33 24732805 Email address: [email protected] (Sugata Ray ).

2

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

concentrations. It has been previously reported that both the magnetization and electrical resistivity show a sudden transition below x=0.3 for NixCu1-xMnSb [17,18] due to disorder in spin-orientations on a structurally perfect Mn-sublattice induced by chemical disorder on the Cu/Ni sublattice. Although there is no clear understanding on the intrinsic mechanism that drives the resulting state, DFT calculation further predicts a complex magnetic phase arising from competition between the two kind of indirect exchange interactions [19]. M. Halder et al [15] have shown that in the composition range 0.05 ≤ x ≤ 0.2 of Cu1-xNixMnSb, a mixed magnetic phase consisting of both FM and AFM regions exist, but they did not shed any light on the change of electronic DOS and hence the half metallic nature upon doping. So there was always an open question whether the electronic mechanism of half-metal to metal/semi-metal transition and FM to AFM transition is same and therefore they are bound to coincide in x of Cu1-xYxMnSb (Y = Ni, Co, Fe) compounds. Recently, we have discussed the generation of Sb pholes (upon Co-doping in Cu1-xCoxMnSb) being responsible for the development of both half metallicity and FM state [23]. Larger the number of holes, higher would be the strength of ferromagnetism and due to strong p-d hybridization the Fermi surface topology might be changed. In this paper, we aim to explore more systematically the role of Sb-p holes on the electronic DOS and the magnetic exchange interactions as a function of changing d-occupancy of the Y-cation. We therefore performed detailed experimental and theoretical studies on a set of Cu1xNixMnSb (x = 0.04, 0.07, 0.1, 0.125) compounds. We have also carried out abinitio DFT calculations on another set of isomorphous alloys Cu1-xFexMnSb (x = 0.031, 0.063, 0.094), could not be synthesized. Based on these, it is understood that there is a gradual change over from pure AFM state (as in parent CuMnSb) to mixed AFM/FM phase to a completely FM phase which is concomitant with lowering in spin-down DOS at the EF upon increasing impurity substitution. The only di erence between the two systems lies in the strength of FM polarization of the Mn-spins (around every dopant atom) caused by the number of Sb-p holes produced upon Ni-/Fe-doping. As a result, Cu1-xFexMnSb systems show stronger FM correlation in comparison to Cu1-xNixMnSb alloys due to larger Sb-p holes. At the same time, depopulation of the spin-down channel at the EF is much faster in Fe-doped compositions compared to the Ni-doped samples. Consequently, half-metallic nature is expected to be stabilized for much lower doping concentrations in Fe-CuMnSb systems compared to the Ni-CuMnSb ones.

3

ACCEPTED MANUSCRIPT 2

Methodology

2.1 Experimental Techniques

TE D

M AN U

SC

RI PT

Bulk polycrystalline samples of Cu1-xNixMnSb (x = 0.04, 0.07, 0.1, 0.125) have been synthesized by melt-quenching technique using an argon-arc furnace from stoichiometric amounts of constituent metals of very high purity (>99.99%). All the samples have been remelted 5-6 times to achieve better chemical homogeneity and finally the molten ingots were annealed at 650◦ C in vacuum-sealed quartz tube in order to reduce atomic disorder. [24] The X-ray di raction (XRD) patterns, collected from MCX Beam line of ELETTRA (Trieste, Italy) synchrotron radiation facility at room temperature and Indian beamline (BL-18B) at Photon Factory, KEK, Japan, were refined by Rietveld method using FULLPROF [25]. The exact stoichiometry of all the alloys are additionally verified by inductively coupled plasma-optical emission spectroscopy (ICP-OES) using a Perkin Elmer Optima 2100 DV instrument. Mn K-edge Extended X-ray Absorption Fine Structure (EXAFS) measurement of the 10% Ni-doped sample was further carried out at the XAFS beam line of ELETTRA (Trieste, Italy) synchrotron radiation facility at room temperature in transmission geometry using a double crystal Si (311) monochromator [26]. Data treatment and quantitative analysis of EXAFS spectra were performed using ARTEMIS [27]. Transport (both with and without field) properties of all the alloys have been measured by standard four probe method within a temperature range of 2 K to 300 K in a physical property measurement system (PPMS, Cryogenics). The temperature and magnetic field dependence of magnetization of all these samples were measured in a SQUID magnetometer (Quantum Design, USA).

EP

2.2 Theoretical Methods

AC C

The electronic structure calculations of Cu1-xMxMnSb (M = Ni, Fe) are carried out using density functional theory (DFT) as implemented in the Vienna ab-initio simulation package (VASP) in the framework of local density approximation (LDA). The Brillouin-zone integration was performed using 4X4X4 k-mesh for the Cu1-xMxMnSb (M = Ni, Fe) with a plane wave cut-o energy 600 eV. In order to model the experimentally synthesized Cu1-xNixMnSb (x = 0.07, 0.1, 0.125) and theoretically constructed Cu1-xFexMnSb (x = 0.031, 0.063, 0.094), a 2X2X2 supercell of the conventional cell of CuMnSb was prepared containing 96 atoms. We have substituted two, three and four Ni atoms for Cu in order to realize Cu1xNixMnSb systems with x = 0.063, 0.094 and 0.125 respectively, very close to the experimentally synthesized concentrations. A Cu1-xNixMnSb (x = 0.219) structure

4

ACCEPTED MANUSCRIPT

3

Results and Discussion

SC

3.1 Structure from powder X-ray di raction

RI PT

was further constructed theoretically by substituting seven Ni atoms in place of Cu atoms. All the calculations for Cu1-xNixMnSb systems have been carried out using experimentally obtained lattice constants while the interpolated lattice constant between the two end members CuMnSb and NiMnSb has been taken for the x = 0.219 composition. Due to the nonavailablity of the lattice constant for Cu1-xFexMnSb (x = 0.031, 0.063, 0.094), we again considered the interpolated values between the two end members CuMnSb and FeMnSb, substituting one, two and three Fe atoms in place of Cu respectively.

EP

TE D

M AN U

The XRD patterns [Fig. 1(a) to (c)] were satisfactorily fitted within single cubic F-43m space group and considering that Cu/Ni atoms will occupy the (0,0,0) site while Mn and Sb atoms should occupy the (¼,¼,¼) and (¾,¾,¾) sites respectively. Further, the presence of any (Cu, Ni)-Mn site-disorder between (0,0,0) and (¼,¼,¼) sites and accidental occupancy to the vacant (½,½,½) site of the cubic C1b phase have been clearly excluded from our XRD analysis for all the synthesized samples. A systematic shift in the di raction peaks towards higher 2θ-side with respect to the parent CuMnSb [inset to Fig. 1(c)] suggests decrease in lattice constant and hence the unit cell volume with increasing Niconcentration. This scenario is consistent with the size di erence between Cu and Ni. The lattice constants, unit cell volume, Mn-Mn distances, χ2-values and the stoichiometric formulaes obtained from structural refinements for all the alloys are listed in Table-I. Since melt quench synthesis technique is very prone to stoichiometric mismatch and evaporative loss, we did perform ICP-OES analysis to quantify the exact stoichiometries which are indeed found to be very close to the desired compositions for all the compounds (not shown in this paper).

AC C

3.2 Local structure from EXAFS

Mn K-edge EXAFS analysis of CuMnSb (left panel of Fig. 2) shows structurally perfect Mn-sublattice [23], i.e., each Mn sees four Cu as nearest, six Sb as nextnearest and twelve Mn as next-to-next-nearest-neighbors [20]. Due to similar atomic numbers of Cu and Ni and very less doping percentage of Ni in these systems, it is almost impossible to distinguish them in the form of coordination numbers around Mn. Therefore, Mn K-edge EXAFS data for 10% Ni-doped sample was further analyzed (right panel of Fig. 2) by assuming 4 Cu as nearest-

5

ACCEPTED MANUSCRIPT Table I All the samples are refined within a single cubic C1b phase with F-43m space group to achieve reasonable R-factors and χ2.

Sample

Lattice constant a (Å)

V (Å)3

χ2

Cu0.96Ni0.04MnSb

6.098

226.76

3.77

4.312

Cu0.93Ni0.07MnSb

6.095

226.42

1.82

4.31

Cu0.9Ni0.1MnSb

6.089

225.76

2.87

4.306

Cu0.875Ni0.125MnSb

6.078

224.53

1.49

4.298

RI PT

Mn-Mn distance (Å) Stoichiometry from XRD

Cu0.96Ni0.04Mn1.00Sb1.00

Cu0.93Ni0.07Mn1.00Sb1.00 Cu0.9Ni0.1Mn0.99Sb1.00

SC

Cu0.87Ni0.13Mn0.98Sb0.99

M AN U

Table II

Local structure parameters as obtained from the EXAFS analysis of Mn K-edge for two selected samples. The fixed or constrained values are labeled by `*'. The absolute mismatch between the experimental data and the best fit is R2=0.014 for both the samples. Further bond distances between Mn and its nearest/nextnearest/next-to-next-nearest-neighbors have been compared to that obtained from structural refinements.

TE D

Sample

σ2 (x N 102Å2)

Shell

R (Å)

RXRD (Å)

4.0* 6.0* 12.0* 12.0*

0.92(6) 1.44(9) 2.34(2) 1.64(6)

2.637(3) 3.002(2) 4.303(2) 5.025(3)

2.642 3.051 4.314 5.059

Cu0:9Ni0:1MnSb Mn-Cu1/Ni1 4.0* Mn-Sb1 6.0* Mn-Mn1 12.0* Mn-Cu2/Ni2 12.0*

0.92(8) 1.33(4) 1.21(5) 1.58(7)

2.627(4) 2.997(3) 4.291(2) 5.023(4)

2.637 3.002 4.306 5.045

Mn-Cu1 Mn-Sb1 Mn-Mn1 Mn-Cu2

AC C

EP

CuMnSb

neighbors around Mn as well as using di erent Cu/Ni-combination as nearestneighbors in order to comment on the local atomic distribution at the Cu-site after Ni-doping. This combinational fit (not shown in the figure) did not provide any better agreement, instead we had arrived with serious correlation among the structural parameters. This comparative study allows us to conclude that the local structure around Mn in the doped samples is homogeneously preserved without any impurity clustering. The extracted parameters obtained from fitting are listed in Table-II. 6

ACCEPTED MANUSCRIPT 3.3 Magnetization

EP

TE D

M AN U

SC

RI PT

The temperature dependence of Zero-field-cooled (ZFC) and Field-cooled (FC) magnetization has been shown in Fig. 3 for all the doped samples. It has been reported in our previous work that CuMnSb is AFM having its Neel temperature TN = 62 K [23]. Upon Ni-doping the AFM transition temperature TN gets marginally shifted towards the lower values [TN = 57 K for x=0.04 to TN =49.5 K for x=0.125, indicated in Fig. 3 (a) to (d)]. In addition to single AFM to paramagnetic transition another FM-like transition starts to develop for the higher doping concentrations (x = 0.1, 0.125). A bifurcation in FC and ZFC curves below AFM transition temperature (TN) and the constant value of FCmagnetization below TN possibly suggest some FM-like cluster spins appearing in the AFM background of CuMnSb upon Ni-doping for all the doped alloys. These FM clusters during ZFC, freeze in random directions, hence do not contribute in a collective way to the net magnetization. While during FC process, these FM clusters tend to align along field, hence contributing to a higher and constant value of magnetization below TN. Interestingly, unlike the previously investigated Co-doped CuMnSb samples [23], the AFM peak does not show any tendency of being weak neither the ferromagnetic TC goes significantly higher at least up to 12.5% Ni-doping. This establishes the fact that the strength of FM polarization of the Mn-spins caused by Ni-doping is relatively weak compared to its Co counterpart. Further, M(T) measurements at several higher magnetic fields have been performed [see Fig. 3 (e)] for the 10%-doped sample to understand the nature of its magnetic ground state. It is observed that the AFM peak starts to get shifted to lower temperatures with H and finally disappears without any ZFC/FC bifurcation at the maximum applied 2 Tesla field, while the ferromagnetic TC is gradually shifted to higher values, as determined from the temperature derivative of magnetization dM/dT at the respective dc magnetic fields. This represents a closer analogy to those of typical systems, where competing magnetic interactions coexist.

AC C

M(H) isotherms at 5 K are shown in Figure 4. Hysteresis behavior is evident for all the compositions. Relatively larger coercivity (HC) for low doped samples (evident from bottom right inset) could be due to larger intrinsic anisotropy of the isolated FM-like clusters, embedded in the AFM background. The decrease in coercivity (same inset) with further increase of Ni-concentration suggests a soft FM nature of the higher doped samples. It could be assumed here that the FM regions are growing in volume within the sample with the increase of doping. The maximum obtained moment (Mmax at the highest field and lowest temperature) for the 7, 10 and 12.5% Ni-doped samples are 0.98 µ B/f.u, 2.28 µ B/f.u. and 2.48 µ B/f.u. respectively, whereas the expected moment is 3.85 µ B/f.u. for 100% FM Cu1-xNixMnSb samples [15,28], considering that only Mn atoms carry the moment. So the volume fractions of the FM and AFM phases have been

7

ACCEPTED MANUSCRIPT estimated to be 64% and 36% for 12.5%-doping while 25%-75% and 60%-40% for the x = 0.07 and 0.1 samples respectively.

3.4 Resistivity variations

TE D

M AN U

SC

RI PT

The comparative resistivity variation of Cu1-xNixMnSb (shown in Fig. 5 (a)) indicates good metallic character of them. Sharp change in the resistivity-slop (dρ/dT ) near the respective magnetic transition temperatures establishes intrinsic connection between magnetic ordering and electronic transport in all the doped samples. It has been previously reported that the spin ip scattering of charge carriers causes quadratic T-dependence of the resistivity in the low temperature region of parent CuMnSb, ruling out the possibility of half metallicity [23,24,29]. In the present work, we show that with increasing Ni-concentration a gradual suppression of this T2-dependency [as indicated by the n-values obtained from low temperature resistivity fitting using the equation, ρ = ρ0 + BT n, shown in Fig. 5 (b) to (d)] occurs, suggesting gradual reduction in spin-down DOS at the EF with increasing doping percentage [30-32]. But unlike 10% Co-doped CuMnSb sample [23], 12.5% Ni-doping in CuMnSb (the maximum doping percentage we have achieved experimentally) is not su cient to establish a complete T-linear dependence of the resistivity at low temperature, and hence, one of the spin channels will not be completely energy gapped at the EF in these Ni-doped systems at least up to x = 0.125. So higher doping concentration in Cu1-xNixMnSb is required for complete realization of HM nature. Thus, we may conclude that Ni-doping brings a gradual depletion of the spin-down DOS at the EF (close to desired half-metallicity) within the mixed magnetic (AFM plus FM) phase.

EP

3.5 Field dependent Resistivity measurements:

AC C

The e ect of applied field on the ρ(T) variation of the two doped samples has been shown in Fig. 6 (a) and (b). Inset to Fig. 6(a) represents the variation of percentage change in resistivity (i.e., Magnetoresistance or MR%= [[ρ(H)-ρ(0)]/ ρ(0)]x100%) of the 10% Ni-doped sample. Presence of two extremum around the respective magnetic transition temperatures of both the samples (figure is not shown for 12.5%-doped sample) further supports the existence of mixed magnetic phase (FM plus AFM). Unlike 10% Co-doped CuMnSb case, the resistivity does not undergo a change from positive to negative temperature dependence at the onset of magnetic FM-PM transition, thereby again supporting qualitatively the absence of HM band structure [23,30]. Finally we show the MR%(H) variations of Cu1-xNixMnSb (x = 0.1, 0.125) samples at several constant temperatures [see Fig. 6 (c) and (d) where figures

8

ACCEPTED MANUSCRIPT

TE D

3.6 Electronic structure calculation

M AN U

SC

RI PT

are not included for the 4 and 7% doping]. Below their respective TN , positive MR in the low field region and then a cross over from positive to negative MR with H are evident for all the compositions. Development of positive MR is possibly due to spin fluctuations caused by the applied field on the competing magnetic structure (coexisting AFM and FM) [33,34], and hence verifies the presence of mixed magnetic state once again. The strength of positive MR decreases with increasing temperature and finally it completely disappears as the measurement temperature is shifted from TN to TC for all the compositions. Further, presence of field-induced irreversibility (the zero field value at the beginning of first field increasing path (path 1) is slightly di erent from the same as the field is cycled in the decreasing path (path 2) as well as slight mismatch in the MR value at the magnetic field where cross over from positive MR to negative MR is taking place) below TN, is another interesting feature in the MR behavior for these compositions. Such irreversibility completely disappears as the temperature moves towards the FM transition (see Fig. 6(c)-[iii] and 6(d)-[iii]). Such field-induced irreversibility arises from two competing magnetic interactions (FM and AFM) [35-37]. Similar to the CoxCu1-xMnSb case [23], the most likely scenario behind such kind of MR behavior is that the FM clusters get embedded in the parent AFM background upon Ni-doping, external magnetic field increases the e ective strength of FM regions in the sample, thereby suppressing spin fluctuation and hence the overall resistivity decreases.

AC C

EP

For the purpose of understanding the strength of ferromagnetic correlation upon substitution of Ni and Fe atoms at Cu sites in CuMnSb, three model spin configuration have been considered, (A) pure FM interaction between Mn atoms, (B) AFM configuration as in CuMnSb and (C) mixed phase (FM clustering around Ni (or Fe) within an overall CuMnSb-like AFM connection). The Fig. 7 shows the model spin configurations for Cu1-xMxMnSb (M = Ni, Fe; x = 0.063). Here, Fe-doping results have also been included in order to understand a general trend or criteria (Fe
Co
Ni), but no experimental results could be acquired as none of the Fe-doped samples could be synthesized in pure form. The energy di erences between the model (A) and (C) with respect to the AFM configuration (model B) for the doping concentrations considered in this work are shown in Fig. 8 (a) and (b), for the Ni- and Fe-doped systems respectively. Our calculations reveal that for Cu1-xNixMnSb, AFM phase remains stable for small doping concentrations considered in the work, but ferromagnetic correlations become dominant above x = 0.219 concentration. However for Cu1xFexMnSb, while the AFM phase is stable for extremely small concentrations of

9

ACCEPTED MANUSCRIPT Fe (not taken in our model calculations), eventually upon increasing Fe concentrations, the mixed phase (the four tetrahedrally coordinated Mn spins around every dopant Fe are ferromagnetically aligned while the rest of the Mn atoms maintain the AFM spin arrangement similar to CuMnSb) starts to appear and the FM phase starts to dominate. This finding suggests that upon small doping the FM correlations strongly dominate in Fe doped systems in comparison to Ni doped systems.

4

Conclusion

TE D

M AN U

SC

RI PT

Upon substitution of Ni and Fe for Cu in Cu1-xMxMnSb (M = Ni, Fe), the valence electrons reduces to (23 - x) and (23 - 3x) respectively from 23 (for CuMnSb). In order to understand the di erence in electronic structure of Cu1-xMxMnSb (M = Ni, Fe), we have calculated DOS for Cu1-xMxMnSb (M = Ni, Fe; x = 0.063) in their FM configurations, as shown in Fig. 9. It can be seen that Cu-d and also Ni/Fe-d occupies both the spin-up and spin-down channels at the expense of Sb-p holes. The Sb-p holes are more pronounced in case of Cu1-xFexMnSb (see Fig. 9(b)) because of more unoccupied electronic states in Fe-d to be occupied by Sbp electrons due to p-d hybridization. The Mn-d is largely exchange split, possessing large moment of ~ 4.15 µ B in both the systems. Therefore Sb-p holes produced upon Ni/Fe-doping induces RKKY interactions between the distant Mn atoms over superexchange interaction in Cu1-xMxMnSb (M = Ni, Fe) and stabilizes FM correlation in these systems. The ferromagnetic correlation is even stronger in case of Cu1-xFexMnSb due to more Sb-p holes and therefore, halfmetallicity is expected to be developed much faster via the depopulation of the spin-down DOS at the Fermi energy in Fe-doped system compared to the Nicounterparts. The regular trend of electronic and magnetic structure development as a result of hole concentration (Fe2+; 3d6, Co2+; 3d7, Ni2+; 3d8) confirms our description and understanding.

AC C

EP

In brief, we have investigated the Ni-doped CuMnSb systems in the low doped region both experimentally and theoretically while for Fe-doped CuMnSb compounds only ab-initio electronic structure calculations were carried out in order to make a comparative study on their electronic structure and magnetism. Impurity substitution (Ni or Fe) in place of Cu gradually introduces FM correlations at the expense of AFM phase in these half heusler systems. Actually Fe/Ni locally polarizes its surrounding Mn-spins ferromagnetically, i.e, Mnspins around every dopant atom align parallel to each other forming FM-like clusters. These clusters grow in volume via percolation upon increasing the impurity content, as more number of Mn-spins around dopant ion align parallel which finally drives the system to a FM state. These results are consistent with ab-initio electronic structure calculations where we argue that Sb-p holes produced upon impurity doping tend to stabilize the ferromagnetism via RKKY

10

ACCEPTED MANUSCRIPT

Acknowledgement:

SC

5

RI PT

mechanism, while strong hybridization between occupied Fe-/Ni-d and unoccupied Mn-d states in the minority spin channel paves the way for HM band structure. The amount of Sb-p holes is considerably larger in Cu1-xFexMnSb systems compared to the Ni-counterpart as the number of unoccupied spin states is higher in Fe-doped (Fe:3d6; four empty spin up/spin down states) samples than the Ni-doped (Ni:3d8; two unoccupied spin up/spin down states) ones. So, we may conclude that Sb-p holes, produced upon doping, triggers a competition between the RKKY interactions stabilizing ferromagnetism and the superexchange mechanism establishing antiferromagnetism in all these doped alloys, while DOS of the minority spin channel shows a gradual reduction at the EF , pushing these systems close to the half-metallicity.

References

TE D

M AN U

AB and AP thank CSIR, India for fellowship. SKN thanks CSIR for funding. SR thanks CSIR for funding (project no. 03(1269)/13/EMR-II), Department of Science and Technology (DST) [Project No. WTI/2K15/74] and Indo-Italian POC for support to carry out experiments in Elettra, Italy. Authors also thank TRC-DST of IACS and Centre for Research in Nanoscience and Nanotechnology, University of Calcutta for providing experimental facilities. Authors also thank Dr. Sudipta Bandyopadhyay for providing experimental facilities.

EP

[1] T. Krenke, E. Duman, M. Acet, E. F. Wassermann, X. Moya, L. Manosa, and A. Planes, Nat. Mater. 4 (2005) 450.

AC C

[2] I. Galanakis, P. H. Dederichs, and N. Papanikolaou, Phys. Rev. B. 66 (2002) 134428. [3] I. Takeuchi, O. O. Famodu, J. C. Read, M. A. Aronova, K.-S. Chang, C. Craciunescu, S. E. Lo and, M. Wuttig, F. C. Wellstood, L. Knauss, and A. Orozco, Nat. Mater. 2 2003 180. [4] E. Şaşioğlu, L. M. Sandratskii, and P. Bruno, Phys. Rev. B. 77 (2008) 064417.

11

ACCEPTED MANUSCRIPT [5] M. I. Katsnelson, V. Yu. Irkhin, L. Chioncel, A. I. Lichtenstein, and R. A. de Groot, Rev. Mod. Phys. 80 (2008) 315. [6] K. W. Lee, and W. E. Pickett, Phys. Rev. B. 77 (2008) 115101.

RI PT

[7] S. Wurmeh, H. C. Kandpal, G. H. Fecher, and C. Felser, J. Phys.: Condens. Matter. 18 (2006) 6171 [8] H. v. Leuken, and R. A. de Groot, Phys. Rev. Lett. 74 (1995) 1171.

[9] K. Özdoğan, and I. Galanakis, J. Magn. Magn. Mater. 321 (2009) L34.

SC

[10] N. H. Long, M. Ogura M, and H. Akai, J. Phys.: Condens. Matter 21 (2009) 064241. [11] L. Zhu, K. L. Yao, and Z. L. Liu, 2009 J. Chem. Phys. 131 (2009) 204702.

M AN U

[12] S. Jana, V. Singh, S. D. Kaushik, C. Meneghini, P. Pal, R. Knut, O. Karis, I. Dasgupta, V. Siruguri, and S. Ray, Phys. Rev. B. 82(R) (2010) 180407. [13] S. Jana, V. Singh, A. Nag, C. Meneghini, I. Dasgupta, G. Aquilanti, and S. Ray, Phys. Rev. B. 86 (2012) 014203.

TE D

[14] A. Bandyopadhyay, S. K. Neogi, A. Paul, C. Meneghini, I. Dasgupta, S. Bandyopadhyay, and S. Ray, Phys. Rev. B. 95 (2017) 024432. [15] M. Halder, S. M. Yusuf, A. Kumar, A. K. Nigam, and L. Keller, Phys. Rev B. 84 (2011) 094435.

EP

[16] A. K. Nayak, M. Nicklas, S. Chadov, P. Khuntia, C. Shekhar, A. Kalache, M. Baenitz, Y. Skourski, V. K. Guduru, A. Puri, U. Zeitler, J. M. D. Coey, and C. Felser, Nat. Mater. 14 (2015) 679.

AC C

[17] J. Kudrnovsky, V. Drchal, I. Turek, and P. Weinberger, Phys. Rev. B. 78 (2008) 054441. [18] S. K. Ren, W. Q. Zou, J. Gao, X. L. Liang, F. M. Zhang, and Y. W. Du, J. Magn. Magn. Mater. 288 (2005) 276. [19] I. Galanakis, E. Şaşioğlu, and K. Özdoğan, Phys. Rev. B. 77 (2008) 214417. [20] T. Jeong, R. Weht, and W. E. Pickett, Phys. Rev. B. 71 (2005) 184103. [21] R. A. de Groot, F. M. Mueller, P. G. van Engen, and K. H. J. Buschow, Phys. Rev. Lett. 50 (1983) 25.

12

ACCEPTED MANUSCRIPT [22] R. A. de Groot, A. M. van der Kraan, and K. H. J. Buschow, J. Magn. Magn. Mater. 61 (1986) 330. [23] A. Bandyopadhyay, S. K. Neogi, A. Paul, C. Meneghini, I. Dasgupta, S. Bandyopadhyay, and S. Ray, J. Phys.: Condens. Matter 30 (2018) 205802.

RI PT

[24] M. J. Otto, R. A. M. van Woerden, P. J. van der Valk, J. Wijngaard, C. F. van Bruggen, and C. Haas, J. Phys.: Condens. Matter. 1 (1989) 2351.

[25] J. Rodriguez Carvajal, Physica B 192 (1993) 55.

[27] M. Newville, J. Synchrotron Rad. 8 (2001) 322.

SC

[26] A. Di Cicco, G. Aquilanti, M. Minicucci, E. Principi, N. Novello, A. Cognigni, and L. Olivi, J. Phys. Conf. Ser. 190, (2009) 012043.

M AN U

[28] A. Szytula, Z. Dimitrijevic, J. Todorovic, A. Kolodziejczyk, J. Szelag, and A. Wanic, Phys. Status Solidi(a) 9 (1972) 97. [29] M.J. Otto, H. Feil, R.A.M. Van Woerden, J. Wijngaard, P.J. Van Der Valk, C.F. Van Bruggen, and C. Haas, J. Magn. Magn. Mater. 70 (1987) 33.

TE D

[30] S. Majumdar, M. K. Chattopadhyay, V. K. Sharma, K. J. S. Sokhey, S. B. Roy and P. Chaddah, Phys. Rev. B. 72 (2005) 012417. [31] J. Pierre, R. V. Skolozdra, Yu. K. Gorelenko, and M. Kouacou, J. Magn. Magn. Mater. 134 (1994) 95.

EP

[32] L. Pal, S. Gupta, K. G. Suresh, and A. K. Nigam, J. Appl. Phys. 115 (2014) 17C303.

AC C

[33] S. Yoon, and J. G. Booth, Phys. Lett. A. 48 (1974) 381. [34] V. Niculescu, T. J. Burch, and J. I. Budnick, J. Magn. Magn. Mater. 39 (1983) 223. [35] L. Bainsla, M. Manivel Raja, A. K. Nigam, B. S. D. Ch. S. Varaprasad, Y. K. Takahashi, K. G. Suresh, and K. Hono, J. Phys. D: Appl. Phys. 48 (2015) 125002. [36] A. K. Nayak, K. G. Suresh, and A. K. Nigam, Appl. Phys. Lett. 96 (2010) 112503. [37] B. Maji, K. G. Suresh, and A. K. Nigam, Europhys. Lett. 91 (2010) 37007.

13

ACCEPTED MANUSCRIPT FIGURE CAPTIONS

Figure 1 (color online) (a)-(c) The refined curves are laid over the experimental data points for all the synthesized samples. Inset to (c) shows the expanded views of the most intense peak of the corresponding samples.

SC

RI PT

Figure 2 (color online) (Upper Panel) Mn K-edge k3 weighted experimental EXAFS data (shaded black circles) and respective best fits (red solid line) are presented for both CuMnSb (a) and 10% Ni-doped sample (c). The contributions from individual single scattering path (solid colored line) and the residual [k2χexp k2χth] (open black dots) are also presented for both the samples, vertically shifted for clarity. (Lower Panel) Fourier Transform of the experimental (shaded black circles) and theoretical (solid red line) curves for CuMnSb (b) and 10%-doped (d) cases; the magnitude (|FT|) and imaginary parts (Imm) are also plotted in the respective figures.

M AN U

Figure 3 (color online) The ZFC (shaded black circles) and FC (shaded red circles) magnetization (M) variations have been plotted as a function of temperature (T) for all the doped alloys (a-d) at low constant magnetic fields. Further the M-T variations (e) have been presented for the 10%-doped sample during both ZFC (open circles) and FC (shaded circles) modes at several constant higher magnetic fields.

TE D

Figure 4 (color online) The M(H) isotherms at 5 K for all the doped samples. Bottom right inset of which shows the enlarged views of the M(H) curves for the x=0.07, 0.1 and 0.125 samples in the low field region.

EP

Figure 5 (color online) (a) Normalized zero field resistivity variations (ρ(T)/ρ300K) of the doped samples along with the parent CuMnSb. Fitting of the resistivity data in the low T region has been shown for 4%- (b), 10%- (c) and 12.5% (d) Ni-doped samples.

AC C

Figure 6 (color online) Temperature dependent resistivity variations for x = 0.1 (a) and x = 0.125 (b) samples both with 5 Tesla fields (shaded red circles) and without field (shaded black circles). Inset to (a) shows the MR%(T ) variation of the corresponding sample, while the enlarged view of the zero field ρ(T ) variation in the high-T region has been presented for x = 0.125 sample in the inset to (b). Further the MR% versus H curves (shaded coloured circles) have been shown at several constant temperatures for Cu0.9Ni0.1MnSb (c-[i] to [iii]) and Cu0.875Ni0.125MnSb (d-[i] to [iii]) alloys. Figure 7 (color online) The (a) FM, (b) AFM, and (c) mixed phase (FM clustering within AFM matrix) configurations for Cu1-xMxMnSb (M = Ni, Fe ; x = 0.063) alloys. The up (blue) and down (red) arrows represent Mn spins. The spherical green balls are either Ni or Fe.

14

ACCEPTED MANUSCRIPT Figure 8 (color online) Energies of the FM and mixed phase configurations with respect to the AFM configuration for di erent doping concentrations in (a) Cu1xNixMnSb and (b) Cu1-xFexMnSb systems.

AC C

EP

TE D

M AN U

SC

RI PT

Figure 9 (color online) (a) The DOS for the Cu1-xNixMnSb (top) and Cu1xFexMnSb (bottom) systems for x = 0.063 concentration. The solid black, while the dotted blue and red lines corresponds to the Cu-d, Ni-d and Fe-d DOS respectively. The filled region (cyan) is for partial Mn-3d DOS. (b) The DOS for Sb-p (blue and red solid lines are for the Ni- and Fe-doped cases). The zero of the energy is at Fermi energy.

15

ACCEPTED MANUSCRIPT

Figure 1:

(2 2 0)

λ = 0.827

(1 1 1)

Å

(a) 7% Ni-doped

(2 0 0)

(3 1 1)

(4 2 2) (4 0 0) (2 2 2) (3 3 1)(4 2 0) (3 3 3) (4 4 0)(5 3 1) (4 4 2)

15

20

25

30

20

25

Figure 2:

TE D

-3

3

0 -1

6 k (A )

8

8

|FT|

6

-4

FT [k .χ(k)] [ ]

Å

3

3

0

-3 |Imm|

-6

Mn-Sb (ss) Mn-Mn (ss)

4

9 (b) R-space fitting of CuMnSb 6

Mn-Cu1 (ss)

Residual

10

AC C

-4

45

Mn-Cu2 (ss)

EP

Mn-Mn (ss)

4

3

40

-3

k . χ(k) [ ]

Mn-Sb (ss)

3

k . χ(k) [ ]

Mn-Cu1 (ss)

Residual

FT [k .χ(k)] [ ]

35

42.2

Å

Mn-Cu2 (ss)

Å

30

42.0

(c) k-space fitting of Cu0.9Ni0.1MnSb

(a) k-space fitting of CuMnSb

Å

41.8

2θ (in degree)

50

λ = 1.5406

M AN U

15

45

Å

Å

41.6

10

40

CuMnSb 7% Ni-doped 10% Ni-doped 12.5% Ni doped

(c) 12.5% Ni-doped λ = 0.786

35

2θ (in degree)

SC

10

RI PT

(b) 10% Ni-doped

6 k (A0)-1 8

10

(d) R-space fitting of Cu0.9Ni0.1MnSb

4

|FT|

2 0 -2 |Imm|

-4 -6

2

0

3 R (A ) 4

5

2

6

16

3 R (A0)

4

5

ACCEPTED MANUSCRIPT

Figure 3: 0.4

(a)

0.25

(b) x = 0.07

x = 0.04

0.20

M (µB/f.u)

M (µB/f.u)

0.3

0.15

0.2

0.10

TN= 57 K

0.1

TN= 52 K

0.05 0.00

0

50

100

150

200

0.60

250

0

300

(c) x = 0.1

100

150

M (µB/f.u)

0.6

TC= 147 K

0.30

0.2

0.00

0.0 50

100

150

200

T (K)

2.5

2.0 2000 Oe

M (µB/f.u)

300

0

50

100

150

T (K)

(e) x = 0.1

2 Tesla

1.5 1000 Oe 1.0

250

M AN U

0

TN= 49 K

SC

TN= 50.5 K

500 Oe

TE D

TN

0.5

150 Oe

0.0 0

50

100

150

AC C

EP

T (K)

17

250

300

TC= 152 K

0.4

0.15

200

(d) x = 0.125

0.8

0.45

M (µB/f.u)

50

RI PT

0.0

200

250

300

200

250

300

ACCEPTED MANUSCRIPT

Figure 4: 2.4

M(H) at 5 K x = 0.04 x = 0.07 x = 0.1 x = 0.125

1.8

0.6 0.0 1.05

x = 0.07 x = 0.1 0.125

0.70

M (µB/f.u)

-0.6 -1.2

0.35 0.00

-0.35

-1.8

-0.70

H (Oe)

-1.05

-2.4

TC

1.0

TE D

TN TN

TC

TN

0

50

100

150

15000

(a)

x=0 x = 0.04 x = 0.1 x = 0.125

EP

0.4

0.2

TN

AC C

ρ/ρ300K

0.8

0.6

0

H (Oe)

ρ (µΩ-cm)

Figure 5:

-15000

500 1000 1500

30000

45000

M AN U

-30000

0

SC

-1500-1000 -500

-45000

RI PT

M (µB/f.u)

1.2

12.5% Ni-doped 51 (d) n 48 Fitting : ρ = ρo + BT 45 n = 1.3 42 1.5K < T < 15K 39 36 0 8 16 24 32 40 48 56 48 10% Ni-doped (c) 45 n Fitting: ρ = ρo + BT 42 n = 1.4 39 5K < T < 20K 36 33

200

250

T (K)

300

8 16 24 32 40 48 56 64 65 4% Ni-doped 60 (b) n 55 Fitting : ρ = ρo + BT n = 1.6 50 45 2K < T < 20K 40 35 0 8 16 24 32 40 48 56 64

T (K)

18

ACCEPTED MANUSCRIPT

Figure 6: 60

56 (b) x = 0.125

(a) x = 0.1

TC

56

TC

52

TN

40 36

48

TC TN

0

50

100

54.4 53.6

40 0

50 100 150 200 250 300

150

200

36

250

0

300

50

0 -1

-2

-2

[ iii]

R (H ) a t T = 1 2 0 K

[ ii]

1 0

3 .0 2 .5 2 .0 1 .5 1 .0 0 .5 0 .0 5

(2 )

250

280

300

(1 )

[ i]

R (H ) a t T = 5 K 3

4

5

EP

- 5 - 4 - 3 -2 - 1 0 1 2 H (T e s la )

[ iii]

[ ii] R (H ) a t T = 2 5 K (2 )

4

TE D

2

200

240

R (H ) a t T = 1 0 5 K

-4

R (H ) a t T = 4 0 K

4 3

150

200

( d ) x = 0 .1 2 5

M AN U

-3

AC C

MR%

-4 2 .0 1 .5 1 .0 0 .5 0 .0 - 0 .5 - 1 .0

100

160

T (K)

(c ) x = 0 .1

-3

T (K)

120

SC

-1

H=0

80

T (K) 0

56.0 55.2

44

T (K) 32

TN

RI PT

44

MR% versus T at 5 Tesla

ρ (µΩ-cm)

48

0.0 -1.5 -3.0 -4.5 -6.0 -7.5

ρ (µΩ-cm)

56.8

MR%

ρ (µΩ-cm)

52

19

(1 )

3 2 1

[i] R (H ) a t T = 3 K

0

-5 -4 -3 -2 -1 0 1 2 H (T e s la )

3

4

5

ACCEPTED MANUSCRIPT

SC

RI PT

Figure 7:

AC C

EP

TE D

M AN U

Figure 8:

20

ACCEPTED MANUSCRIPT

AC C

EP

TE D

M AN U

SC

RI PT

Figure 9:

21

ACCEPTED MANUSCRIPT

RI PT SC M AN U TE D EP

• • •

Generalizing the mechanism of electronic and magnetic structure modification as a function of transition metal doping in CuMnSb, Development of half metallicity is electronically understood, Fe-doped CuMnSb is studied theoretically for the first time , Detailed experimental and theoretical study.

AC C

•