Neutron diffraction study on the magnetic structure of (Fe0.70Co0.30)2P

Journal of Alloys and Compounds 439 (2007) 13–17

Neutron diffraction study on the magnetic structure of (Fe0.70Co0.30)2P S.K. Jain a , Sudhish Kumar b,∗ , P.S.R. Krishna c , A.B. Shinde c , Anjali Krishnamurthy a , Bipin K. Srivastava a a Department of Physics, University of Rajasthan, Jaipur-302004, India Department of Physics, M L Sukhadia University, Udaipur-313001, India c Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India b

Received 7 June 2006; received in revised form 25 August 2006; accepted 28 August 2006 Available online 9 October 2006

Abstract Ferromagnetic compound (Fe0.70 Co0.30 )2 P (Tc ∼ 450 K) has been studied by means of powder neutron diffraction at temperatures between 12 and 300 K. Rietveld analysis shows that the compound has Co2 P type orthorhombic structure with all the atoms occupying 4c positions in Pnma space group. Positional parameters are quite close to those in Co2 P. Cobalt atoms occupy the tetrahedral site with ∼95% preference. The compound, unlike Fe2 P, is a non-collinear ferromagnet with moments orienting at ϑ = 45◦ and φ = 30◦ for tetrahedral MI site and ϑ = 15◦ and φ = 5◦ for pyramidal MII site. Average values of the magnetic moments at 12 K are 0.82(18)μB and 1.65(5)μB at the MI and MII sites, respectively. These values are much less than the corresponding ones in the hexagonal Fe2 P. © 2006 Elsevier B.V. All rights reserved. Keywords: Neutron diffraction; Magnetic structure; Ferromagnetic alloys; Transition metal metalloid systems

1. Introduction Magnetic behaviour of hexagonal ferromagnetic compound Fe2 P (Curie temperature Tc ∼ 216 K) is quite sensitive to parameters such as temperature, pressure, external magnetic field and also the concentration of alloying elements [1–15 and references therein]. Interesting magnetic behaviour/structures and magneto-elastic properties have been reported for the substitutions of Mn, Cr and Ni for Fe in Fe2 P using magnetization, M¨ossbauer spectroscopy and neutron diffraction techniques [2,5–7,11–15]. As regards the effect of substitution of Co, reports show that up to 16 at% substitution, the alloys have hexagonal Fe2 P like structure and those with higher substitutions are Co2 P type orthorhombic [2,14–15]. For putting the crystal structures in perspective, in Fig. 1 we depict (a) the basic building block of the hexagonal and orthorhombic structures which is a rhombohedral cell MI MII P, (b) the hexagonal Fe2 P structure projected on to ab plane and the (c) orthorhombic Co2 P structure projected on to ac plane [2]. Metal atoms occupy two types of sites—a tetrahedral one where metal atom MI is surrounded by four phosphorous atoms in near tetrahedral arrangement and a

pyramidal one where the metal atom MII is surrounded by five phosphorous atoms in near pyramidal arrangement. Talking of the effect of substitution of Co on magnetic structure, we note that in the ferromagnetic compound Fe2 P the moments orient along c-axis and orthorhombic Co2 P is paramagnetic. Beginning from the Fe2 P end, initially the substitution of Co raises Tc peaking at ∼460 K for x = 0.38 in (Fe1−x Cox )2 P and a spin re-orientation is reported to occur in systems with x > 0.25 [2]. However, no neutron diffraction study has been reported so far in the orthorhombic phase. Our M¨ossbauer study on the alloy series (Fe1−x Cox )2 P showed that in going from x = 0.10–0.30, when the crystalline symmetry changes from Fe2 P like hexagonal one to Co2 P like orthorhombic one, the magnetic moment on the pyramidal metal site tilts off the principal electric field gradient axis [14]. For obtaining details of the magnetic structure in the orthorhombic phase, we have now undertaken a neutron diffraction study on the alloy (Fe0.70 Co0.30 )2 P. It is also to be noted that our earlier study on the series (Fe1−x Cox )2 P showed this composition to have the highest magnetization [15]. 2. Experimental details

∗

Corresponding author. E-mail address: sudhish [email protected] (S. Kumar).

0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.08.258

The title alloy has been synthesized using the technique of solid state diffusion at ∼1000 ◦ C. Iron powder, cobalt powder and red phosphorous lumps (all of

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S.K. Jain et al. / Journal of Alloys and Compounds 439 (2007) 13–17

Fig. 1. (a) Basic building block of the hexagonal and orthorhombic structures—a rhombohedral cell MI MII P containing tetrahedrally coordinated MI and pyramidally coordinated MII metal atoms, (b) the hexagonal Fe2 P structure projected onto ab plane and (c) orthorhombic Co2 P structure projected onto ac plane.

4N purity) were the starting materials [for details cf. literature [14]]. For confirmation of the formation of single phase alloy, powder X-ray diffraction (XRD) measurements were made using Mn filtered monochromatic Fe K␣ radiation in the 2θ range of 10–90◦ on Philips make X-ray powder diffractometer (PW 1840). All the Bragg reflections of the XRD pattern were indexed in Co2 P type orthorhombic structure. Lattice parameters obtained using least square method ˚ b = 3.542(2) A ˚ and c = 6.621(3) A. ˚ These are in agreement are a = 5.775(3) A, with earlier reported values [2,14]. Neutron diffraction (ND) measurements were carried out at the 100 MW ‘Dhruva Reactor’ at Bhabha Atomic Research Centre, Mumbai, on a powder diffractometer equipped with position sensitive detectors. Neutrons of wave˚ were used. For measurements at different temperatures down to length 1.2495 A 12 K, a closed helium cycle refrigerator cryostat was used. Data were recorded with 7 g of the powdered sample placed in a cylindrical vanadium can. Measurements were made at 300, 80, 40 and 12 K in 2θ range of 12–121◦ with step size of 0.05◦ .

3. Results and discussion Rietveld profile refinements of the ND patterns were carried out using FullProf Program [16]. In this program magnetic part

of the diffraction pattern is treated as corresponding to a ‘phase’ additional to the nuclear one. Values of cell constants as obtained from the analysis of XRD pattern and of the positional parameters as reported in literature for Co2 P [1] were used as initial parameters for carrying out structural refinement. Neutron scattering lengths of 9.45, 2.49 and 5.13 fm were used for Fe, Co and P, respectively. Initial guess parameters for the values of magnetic moments were estimated using low temperature magnetization data and the ratio of internal hyperfine fields as seen in M¨ossbauer spectra [14,15] at the two iron sites. Profile refinements were carried out in the space group Pnma (with Z = 4). In this compound, all the atoms occupy the following 4c sites: (x, 1/4, z); (−x, 3/4, −z); (1/2 − x, 3/4, 1/2 + z) and (1/2 + x, 1/4, 1/2 − z) with different x and z coordinates. Crystalline structure is built up of alternating layers of two nonequivalent metal sites at the symmetry positions 4c, which are referred to as tetrahedral MI and pyramidal MII . Since at room temperature the compound is ferromagnetic, in the profile refinement of the 300 K data, in addition to

S.K. Jain et al. / Journal of Alloys and Compounds 439 (2007) 13–17

the nuclear phase, we also considered a ferromagnetic phase. Initially the pattern has been analyzed only in the higher ˚ −1 ) range (where magnetic contribu2θ (above sin θ/λ = 0.5 A tion is negligible). Experimental profile was fitted with Gaussian function. In the first step of refinement, scale factor, 2θ zero-point, three cell constants, three half-width parameters, asymmetry parameter and six background coefficients were refined. In the second step, six positional parameters, an overall isotropic temperature factor and site occupancies were refined in sequence. Site occupancies, for Fe and Co, at the two metallic sites (tetrahedral MI and pyramidal MII ) were constrained according to the stoichiometry of the alloy. In the final cycles when the agreement factor Rwp attained its minimum value, all the parameters were simultaneously refined. Best fit is obtained for 95% occupancy of tetrahedral sites for Co atoms. As concerns the magnetic phase, initially it was refined in ˚ −1 ) where magnetic the low angle region (below sin θ/λ = 0.5 A scattering would dominate and after fixing all the parameters arrived at in refinement of the nuclear phase, magnetic moments and their orientations were refined for fitting of the magnetic phase. Only average values of magnetic moments, and not individually of Fe and Co, could be fitted. For form factors we put in weighted values corresponding to occupancy of Fe and Co at the two metallic sites. Best fitting was obtained when for magnetic form factors of iron atoms we used the values corresponding to Fe0 and Fe4+ at MI and MII sites, respectively. This is in accordance with the procedure followed by Fujii et al. in their analysis of polarized neutron scattering data on single crystal of Fe2 P [17]. For cobalt atoms, we used the form factor values corresponding to Co0 and Co3+ at the two respective sites. Values for form factors were taken as those input in the fitting program as default. For estimating the orientations of magnetic moments, refinements of the patterns were tried with different sets of values plugged in for the angles ϑ and φ. These values were kept constrained and varied in steps of 5◦ . After arriving at the best possible fit of the magnetic phase in low angle region, whole of the data were merged and magnetic phase was refined over the complete set of data. Figs. 2 and 3 show fitted patterns at 300 and 12 K. Fittings confirm Co2 P type crystal structure (space group Pnma with Z = 4) for this compound. Table 1 presents the results of the refinements. Obtained values of atomic positions for the different sites are very close to those reported for Co2 P. With cooling, there is no appreciable change in the atomic positions but the cell parameters do show a change; a- and c-axes are reduced, bshows a little enhancement. The inter-atomic distances in the alloy (Fe0.70 Co0.30 )2 P at 12 and 300 K, computed using the atomic position coordinates in the space group Pnma with Z = 4 and the lattice constants at the two temperatures, are listed in Table 2. Also listed are the distances reported in the end members Co2 P and Fe2 P [18,19]. It is noted that the distances in the composition under study are quite close to those in Co2 P and are smaller than those in Fe2 P. This is in conformity with the fact that the alloy under study crystallizes in Co2 P type orthorhombic structure. Further, d(MI − P) are smaller than d(MII − P). Thus,

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Fig. 2. Neutron diffraction patterns of (Fe0.70 Co0.30 )2 P at 300 K. Observed (calculated) profiles are shown by dotted (solid) lines. The short vertical marks represent Bragg reflections. The lower curve is the difference plot.

the electron transfer from P to MI is larger than to MII atoms implying a greater mixing of p bands of P atoms with d bands of MI than with d bands of MII . These facts and that Co has more valence electrons than has Fe, would explain the observed large preference of Co atoms for MI site [1,18]. As regards magnetic structure, magnitudes of the refined values of the moments at 12 K are 0.82(18)μB and 1.65(5)μB for the tetrahedral MI and pyramidal MII sites, respectively. At 300 K, the moments are slightly lower at 0.53(10)μB for MI and 1.47(3)μB for MII site. These values are to be compared with the values of 0.90μB and 1.72μB , respectively, for the two sites in Fe2 P at 77 K [17]. As for orientations of the magnetic moments, best fittings are obtained taking ϑ = 45◦ and φ = 30◦ for MI site

Fig. 3. Neutron diffraction patterns of (Fe0.70 Co0.30 )2 P at 12 K. Observed (calculated) profiles are shown by dotted (solid) lines. The short vertical marks represent Bragg reflections. The lower curve is the difference plot.

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Table 1 ˚ Space group Pnma (No. 62, Z = 4) Magnetic structure and crystallographic data for (Fe0.70 Co0.30 )2 P obtained using powder neutron diffraction (λ = 1.2495 A). T = 300 K

T = 12 K

˚ Cell parameters (A) a b c

5.7716(3) 3.5431(2) 6.6145(4)

5.7258(2) 3.5513(2) 6.5947(2)

˚ 2) Overall temperature factor (A

0.2182(10)

0.0159(8)

Reliability factors (%) Rp Rwp Rexp RBragg Rmag

5.53 7.16 3.12 5.11 8.60

6.82 8.78 3.16 6.31 5.69

Positional parameters Site

MI MII P

300 K

12 K

Occupancy

x

y

z

x

y

z

0.8586(2) 0.9701(2) 0.2422(2)

0.25 0.25 0.25

0.0647(2) 0.6670(2) 0.1207(2)

0.8592(2) 0.9699(2) 0.2416(2)

0.25 0.25 0.25

0.0630(2) 0.6674(2) 0.1206(2)

0.432(4)Fe + 0.568(3)Co 0.968(2)Fe + 0.032(4)Co 0.985(6)P

Magnetic structure Atoms

Moment (μB ) 300 K

Moment (μB ) 12 K

ϑ (◦ )

φ (◦ )

MI MII

0.53(10) 1.47(3)

0.82(18) 1.65(5)

45 15

30 5

Total number of reflections = 220; The numbers in the parentheses are estimated standard deviations referred to the last significant digit.

and ϑ = 15◦ and φ = 5◦ for MII site. Choosing the angles 1◦ off these values started giving poorer fittings. Thus, the magnetic structure is a tilted ferromagnetic one unlike in hexagonal Fe2 P in which the moments at both the sites are along c-axis. In Fig. 4(a), showing an elementary ferromagnetic sub-cell in the orthorhombic structure, are depicted the

orientations of the magnetic moments with the c-axis, viz., 45◦ and 15◦ for MI and MII , respectively. Orientations vis-a-vis aaxis on the ac plane, viz., φ = 30◦ and 5◦ for the two respective sites are depicted in Fig. 4(b). Inference of the MI site moment tilting by as much as 45◦ with the c-axis is in line with the prediction in our earlier M¨ossbauer study [14].

Table 2 ˚ in the alloy (Fe0.70 Co0.30 )2 P at 300 and 12 K (also for comparison are given the distances, as reported in literature [18,19] for the end Inter-atomic distances (A) ˚ are listed. The figures in parentheses show the numbers of atoms) members, viz., orthorhombic Co2 P at RT and hexagonal Fe2 P at RT. Distances shorter than 3.10 A Atoms

MI

MII

P

(Fe0.70 Co0.30 )2 P at 300 K MI 2.556(2) 2.685(2), 2.693(1), 2.701(2), 2.711(1) Mii

2.685(2), 2.693(1), 2.701(2), 2.711(1) 2.850(2), 3.087(2),

2.187(1), 2.231(2), 2.244(1) 2.316(1), 2.443(2), 2.573(2)

(Fe0.70 Co0.30 )2 P at 12 K 2.537(2) MI 2.678(2), 2.687(1), 2.695(2), 2.697(1) MII

2.678(2), 2.687(1), 2.695(2), 2.697(1) 2.853(2), 3.062(2)

2.191(1), 2.224(2), 2.225(1), 2.330(1), 2.440(2), 2.560(2)

Atoms

CoI

CoII

P

Co2 P CoI CoII

2.54(2) 2.62(2), 2.66(1), 2.69(2), 2.71(1)

2.62(2), 2.66(1), 2.69(2), 2.71(1) 2.83(2), 3.04(2)

2.14(1), 2.23(2), 2.24(1) 2.29(1), 2.40(2), 2.54(2)

Atoms

FeI

FeII

PI

PII

Fe2 P FeI FeII

2.610(2) 2.630(2), 2.708(4)

2.630(2), 2.708(4) 3.087(4)

2.2151(2) 2.483(4)

2.294(2) 2.379(1)

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Fig. 4. Orientations of magnetic moments of tetrahedrally coordinated MI and pyramidally coordinated MII atoms (a) with respect to c-axis (angle θ) shown on the rhombohedral cell and (b) with respect to a-axis (angle φ) shown on the orthorhombic Co2 P structure projected onto ac plane.

4. Conclusions Crystal structure of (Fe0.70 Co0.30 )2 P has been refined and the obtained values of lattice parameters are in agreement with the reported results and the positional parameters are found to be close to those for Co2 P at 300 K. With cooling, there is no appreciable change in the atomic positions but the cell parameters do show a change; a- and c-axes are reduced, b-axis shows a little enhancement. About 95% of Co atoms preferentially occupy the tetrahedral site (MI ). The refined average values of magnetic moments at MI and MII sites are comparable to those in Fe2 P. The alloy is a non-collinear ferromagnet and the moments at MI and MII sites are inclined to the c-axis at 45◦ and 15◦ , respectively. Acknowledgements Part of the work was done with the financial assistance of erstwhile Inter University Consortium for DAE Facilities (now UGC-DAE Consortium for Scientific Research), Mumbai Centre. The same is gratefully acknowledged. References [1] O. Beckman, L. Lundgren, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, 6, Elsevier Publ. Co., Amsterdam, 1991. [2] R. Fruchart, A. Roger, J.P. Senateur, J. Appl. Phys. 40 (1969) 1250.

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