Influence of Cobalt Substitution on the Magnetic Properties of Fe5PB2

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Influence of Cobalt Substitution on the Magnetic Properties of Fe5PB2 Johan Cedervall,*,† Elise Nonnet,† Daniel Hedlund,‡ Lennart Hag̈ gström,§ Tore Ericsson,§ Mirosław Werwiński,∥ Alexander Edström,§,⊥ Ján Rusz,§ Peter Svedlindh,‡ Klas Gunnarsson,‡ and Martin Sahlberg† †

Department of ChemistryÅngström Laboratory, Uppsala University, Box 538, 751 21 Uppsala, Sweden Department of Engineering Sciences, Uppsala University, Box 534, 751 21 Uppsala, Sweden § Department of Physics and Astronomy, Uppsala University, Box 516, 751 20 Uppsala, Sweden ∥ Institute of Molecular Physics Polish Academy of Sciences, M. Smoluchowskiego 17, 60-179 Poznań, Poland ⊥ Department of Materials Theory, ETH Zürich, Wolfgang-Pauli-Str. 27, 8093 Zürich, Switzerland ‡

S Supporting Information *

ABSTRACT: The substitutional effects of cobalt in (Fe1−xCox)5PB2 have been studied with respect to crystalline structure and chemical order with X-ray diffraction and Mössbauer spectroscopy. The magnetic properties have been determined from magnetic measurements, and density functional theory calculations have been performed for the magnetic properties of both the end compounds, as well as the chemically disordered intermediate compounds. The crystal structure of (Fe1−xCox)5PB2 is tetragonal (space group I4/mcm) with two different metal sites, with a preference for cobalt atoms in the M(2) position (4c) at higher cobalt contents. The substitution also affects the magnetic properties with a decrease of the Curie temperature (TC) with increasing cobalt content, from 622 to 152 K for Fe5PB2 and (Fe0.3Co0.7)5PB2, respectively. Thus, the Curie temperature is dependent on composition, and it is possible to tune TC to a temperature near room temperature, which is one prerequisite for magnetic cooling materials.

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mcm).3,4 This compound belongs to a larger class of structures with the general formula M5XB2 (M = Fe, Co, Mn and X = Si, P), which is represented in Figure 1. Fe5SiB2 was investigated by neutron diffraction which reported a uniaxial magnetic structure at room temperature, with the magnetic moments aligned along the c-axis.5 However, at 172 K, a spin reorientation occurs where the magnetic moments reorient and order in the ab-plane. The magnetocrystalline anisotropy energy (MAE) is, however, too low relative to saturation magnetisation (Msat) for permanent magnet applications. The MAE can be increased by phosphorus substitutions on the silicon position; this was studied experimentally6 and with firstprinciples calculations7 for Fe5Si1−xPxB2 in the complete range

INTRODUCTION The 3d-elements manganese, iron, cobalt, and nickel are all interesting for magnetic applications, because of their high abundance. Their spin angular momentum due to the valence electrons gives the material its magnetic properties.1 Uniaxial structures (one unique axis in the crystal structure) can possess the high anisotropy needed to gain a suitable coercivity, necessary for permanent magnetic materials. Some materials based on this approach are compounds, like MnAl and FeNi, with L10 crystalline phases, an ordered body centered tetragonal structure. However, MnAl is thermodynamically metastable whereas FeNi needs extremely slow cooling (millions of years)1 (although it is claimed to be possible to make from amorphous ribbons2). Another studied tetragonal compound is Fe5SiB2, which crystallizes in the Cr5B3-type structure (space group I4/ © XXXX American Chemical Society

Received: October 19, 2017

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DOI: 10.1021/acs.inorgchem.7b02663 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

example, big temperature and entropy changes upon magnetization.15 In this study the crystal structures and magnetic properties, including the ability to tune TC, over an extended range of cobalt substitutions are presented. A combination of X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements as well as first-principle calculations have been employed to show this.

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EXPERIMENTAL SECTION

Synthesis. Master alloys of the nominal composition Fe5PB2 and Co5PB2 were synthesized from elemental iron (Leico Industries, purity 99.995%; surface oxides were reduced in H2-gas), cobalt (Johnson Matthey, purity 99.999%), phosphorus (Cerac, purity 99.999%), and boron (Wacher-Chemie, purity 99.995%). The metals were reacted with boron using a conventional arc furnace to form M2B. The drop synthesis technique16 was then used to form the master alloys, where phosphorus was dropped in a melt of the previously formed metal boride to reach the correct composition. Samples of intermediate compositions ((Fe1−xCox)5PB2 with 0 ≤ x ≤ 0.7) were formed by mixing appropriate amounts of powder from the master alloys in an agate mortar. The samples were then pressed into pellets before they were heat treated in evacuated silica ampules for 14 days at 1273 K and subsequently quenched in cold water. X-ray Diffraction. X-ray powder diffraction (XRD) was performed on all samples to determine the unit cell parameters and evaluate their phase compositions. All samples were measured with a Bruker D8 diffractometer equipped with a LynxEye position sensitive detector (PSD, 4° opening) using Cu Kα1 radiation (λ = 1.540598 Å). All measurements were performed at 298 K in a 2θ range 20−90°. To precisely determine the unit cell parameters of the main phase the software UNITCELL17 was used, which uses least-squares refinements from the peak positions. The phase composition was studied in the samples with whole pattern refinements using the Rietveld method18 implemented in the software FullProf.19 There were 20 parameters varied in the refinements: atomic coordinates (5), atomic occupancies (5), isotropic temperature factor and unit cell parameters (2), as well as the zero point, background, scale factor, peak shape, and half width parameters (3). Mö ssbauer Spectroscopy. Mö ssbauer measurements were performed on a spectrometer with a constant acceleration type of vibrator and a 57CoRh source. Absorbers were made from powder (Fe1−xCox)5PB2 samples (∼17 mg/cm2 for x < 0.8) mixed with BN which were pressed to pellets. Measurements were performed in the temperature range 80−580 K under vacuum utilizing a flow cryostat and a furnace. Calibration spectra were recorded on a reference absorber made from a natural iron foil at room temperature. The obtained spectra were folded and analyzed using a least-squares Mössbauer fitting program. Magnetic Measurements. The magnetic properties in the (Fe1−xCox)5PB2 series were studied using different vibrating sample magnetometers (VSM) and a Quantum Design (QD) magnetic property measurement system (MPMS). Magnetic moments above 300 K were measured using a LakeShore VSM equipped with a high temperature option, whereas the magnetic moments below 300 K were studied using the QD physical property measurement system (PPMS) with the VSM option or the QD MPMS. Samples used for measurement below 300 K were made by fixing the samples in gelatin capsules using varnish. The varnish and gelatin capsules are paramagnetic and account for 0.7) the main phase decomposes, and these are therefore not studied here. The phase compositions were calculated from the refinements performed in FullProf for all samples, and reveal that there are small amounts of secondary phases in all samples, mainly Fe3P0.64B0.36 but also Co2P for compositions with x ≥ 0.4. However, the amount of secondary phases is less than 10 wt % over the whole studied composition range. In all refinements the structure model for Fe5PB2 in Table 1 was used, and all information regarding the refinements can be found in Supporting Information (Figure S1 and Tables S1 and S2). Unit cell refinements were also performed for all samples, and the values for pure Fe5PB2 (a = 5.4923(1) Å and c = 10.3654(4) Å) correspond well with those from previous reports.8,12 The a lattice parameter decreases with increasing Co concentration. This is also true for the c lattice parameter after an initial increase (Figure 3). The values for (Fe0.3Co0.7)5PB2 are 5.4020(2) and 10.3324(6) Å for a and c,

(1)

where EDLM and EFM are total energies for DLM and ferromagnetic configurations, kB is the Boltzmann constant, and c is a total concentration of magnetic atoms (here c = 5/8). DLM was used to model the paramagnetic state, and the thermal disorder among the magnetic moments was described using CPA. C


Article

Inorganic Chemistry Table 1. Atomic Positions for All Atoms Used in the Refinements of Fe5PB2 (Space Group I4/mcm)a atom

site

x

y

z

M1 M2 P B

16l 4c 4a 8h

0.17011(2) 0 0 0.61878(22)

0.67011(2) 0 0 0.11877(22)

0.14064(2) 0 0.25 0

a

The same structural model with full occupation for all atoms were used in the refinements for the whole range in (Fe1−xCox)5PB2.

Figure 5. Mössbauer spectra of Fe5PB2 (top) and Fe4.5Co0.5PB2 (x = 0.1) (bottom) at 295 K. Individual patterns emanating from Fe(1)a and Fe(1)b in blue, Fe(2) in green, and the summed envelope in red.

has a more regular near surrounding of 4 B, 2 P, and 8 Fe(1) atoms closer than 2.58 Å. Assuming a fraction r of B and a fraction (1 − r) on the P site (4a) and randomized occupation (in Fe5P1−rB2+r), this gives the probabilities (1 − r)2, 2r, and r2 for Fe (at both 16l and 4c sites) having 2 P, 1 P, and 1 and 2 B atoms in the near neighborhood. For r < 0.1, the probability r2 can be neglected. This implies that patterns from Fe(1)a and Fe(2)a with regular surroundings (2 P) and Fe(1)b and Fe(2)b with (1 P and 1 B) as near surroundings would be observed. The fact that the Mössbauer data can be described with three six line patterns and that the occupation probability for Fe(1) is 4 times as high compared to that for Fe(2) means that the patterns can be ascribed to Fe(1)a, Fe(1)b, and Fe(2). The resonance areas for the fitted patterns are 70.7%, 8.1%, and 21.2%, respectively. Assuming full occupancies for the three Fe sites, the area for Fe(2) would in the thin absorber approximation be 20%, slightly lower than the experimental value found here of 21.2% for a thicker absorber. With application of the same reduction factor for Fe(1)b, a thin absorber approximation value gives 7.5%. The intensities for the three patterns can therefore be approximated to 72.5%, 7.5%, and 20%, respectively. This finally gives a value r = 0.047. For a decreasing value of r (more stoichiometric compound) it has previously been observed that the unit cell volume is also decreasing,11 and the present unit cell volume 312.7 Å3 and r ≈ 0.047 fit very well into that trend. The hyperfine parameters from the fitting of the present sample are very close to the previously reported results.11 For Co substituted samples as, e.g., x = 0.1, broadening of the three Mössbauer patterns occurs besides the effect of P/B substitution. This is an effect of the influence of Co occurring in the first coordination sphere of the Fe atoms. Assuming x = 0.1 and random occupation of Co atoms on the M(1) and M(2) sites, the probability of each of the Fe(1)a and Fe(1)b having 11 Fe, 10 Fe and 1 Co, 9 Fe and 2 Co, 8 Fe and 3 Co, etc., in the neighborhood would be 0.31, 0.38, 0.21, 0.07, etc. This would lead to a severe broadening of the Mössbauer patterns. Similar consideration would also lead to broadening of the Fe(2) pattern. Indeed, this is observed, and the broadening becomes unmanageable in any fitting process. Nevertheless, regarding x = 0.1 a reasonably good fitting could be made (Figure 5), with the results presented in Table S3 in the Supporting Information.

Figure 3. Evolution of the refined unit cell parameters for cobalt content up to x = 0.7 in (Fe1−xCox)5PB2.

respectively. That the refined unit cell values decrease is further evidence of the possibility of cobalt substitution in Fe5PB2 given that the same behavior was observed with phosphorus substitutions in Fe5Si1−xPxB2.6 The c/a-ratio increases whereas the unit cell volume decreases with increasing Co concentration (Figure 4). These trends might be of importance for the magnetic properties, since they cause changes the interatomic distances between the magnetic atoms in the crystalline structure.

Figure 4. Change of volume and the c/a-ratio of the (Fe1−xCox)5PB2 unit cell for x up to 0.7.

Mö ssbauer Results. Measurements below TC. The Mössbauer spectrum of Fe5PB2 at 295 K, Figure 5, resembles the spectrum reported previously.11 Three six line patterns, each due to magnetic hyperfine interactions of 57Fe, are needed to fit the experimental data despite there being only two iron sites in the crystal structure, the 16-fold Fe(1) and the 4-fold Fe(2) (Figure 1). A plausible reason for this is that Fe5PB2 is slightly nonstoichiometric, where partial P/B substitution is common.11 Fe(1) has a very irregular surrounding of 3 B, 2 P, 2 Fe(2), and 9 Fe(1) atoms all closer than 2.94 Å, while Fe(2) D


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Inorganic Chemistry Measurements above TC. The isomer shift and electric quadrupole interaction for Fe are not as sensitive to the Co/Fe substitution in the paramagnetic state. Thus, throughout the whole series of (Fe1−xCox)5PB2 the Mössbauer spectra can be characterized by two doublets, each due to electric quadrupole interaction, emanating from Fe(1) and Fe(2) which makes it possible to study if any site preference for Co substitution exists. The obtained Mössbauer spectra are presented in Figure 6 with the corresponding fitting parameters in the Supporting Information Table S4.

Table 2. Spectral Intensities of Fe(1) and Fe(2) Recalculated by Omitting the Intensity of the Impurity Phases and Also Approximating the Thickness Effect x

T (K)

I(Fe(1))

I(Fe(2))

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

900 580 524 437 402 295 295 295

80(2) 82(2) 78(2) 80(2) 88(2) 88(2) 94(2) 96(2)

20(2) 18(2) 22(2) 20(2) 12(3) 12(2) 6(2) 4(2)

Figure 7. Saturation magnetization at 3 K (black ■) and 300 K (red ●) for (Fe1−xCox)5PB2.

μB/f.u., respectively (f.u. = formula unit). These values can be compared with previously reported results, 8.00 μB/f.u. (Fe5PB2) and 6.71 μB/f.u. (Fe4CoPB2),13 showing good agreement given the impurity content of the samples. The more pressing question is why the moments are suppressed when cobalt is added to the system. Cobalt itself carries a smaller magnetic moment than iron,29 and Co5PB2 has been suggested to be ferromagnetic with very small magnetic moments which agrees with this. In this system, no other substitutions of iron with cobalt above 20% have been reported previously. As Mössbauer spectroscopy indicates, Co occupies the M(2) site preferably for x ≥ 0.4, and it is therefore likely that Fe and Co have a weaker coupling than for pure iron. If the magnetic moment on Co is assumed to be very small, and furthermore omits the small induced moments of P and B, then the linear decrease observed here fits well with this trend. It should be noted that the x = 0.5 compound is paramagnetic at room temperature from both Mössbauer and magnetometry results. However, even at temperatures more than 20 K above its TC the moment gives rise to a volume magnetization of roughly 200 kA/m. Such a large volume magnetization is not likely to stem from a paramagnetic compound without any rare earth elements. Therefore, it is likely that this large magnetization is at least partially explained by impurities or high anisotropy. Figure 8 shows the Curie temperature versus cobalt substitution with the applied field of μ0H = 0.01 T. TC decreases linearly from 622 K (Fe 5 PB 2 ) to 152 K ((Fe0.3Co0.7)5PB2). A similar linear trend is seen with a higher applied field (Table S5). However, it should be noted that the inflection point was used to determine the TC and that there are deviations from pure Curie−Weiss behavior. This is most prominently seen as a bump in the magnetization versus

Figure 6. Mössbauer spectra of (Fe1−xCox)5PB2 in the paramagnetic state. Individual patterns emanating from Fe(1) and Fe(2) in blue and green, respectively, and the summed envelope in red. Patterns due to secondary phases are shown in orange.

The impurity phases observed from the XRD patterns (Figure 2) are also seen in the Mössbauer spectra for x = 0.3, 0.6, and 0.7, and give observable spectral intensities. In order to distinguish any Co preference in (Fe1−xCox)5PB2 the impurity spectral areas have to be disregarded. Furthermore, the thickness effect has to be taken into account where a small (high) absorption line gives a spectral intensity which is an overestimation (underestimation) of the nominal occupation value. This is a more delicate and difficult task but using the reduction value 20/21 for the Fe(2) spectral area the nominal value 20% for x = 0 can be obtained. By approximating the same reduction value for other x-values the results in Table 2 are obtained. Any deviation from 80/20 spectral area gives a Co preference for one of the metal sites M(1) or M(2). Table 2 shows that, within experimental uncertainty, no preference for Co/Fe substitution can be found for x up to 0.3. For x ≥ 0.4 it suggests that Co species prefer to occupy the M(2) site since the Fe(2) intensity decreases below the expected 20%. Magnetic Properties. Magnetization measurements performed at 3 K revealed that Msat decreases linearly (Figure 7) with increased value of x. At 3 K the experimentally obtained Msat values for Fe5PB2 and Fe4CoPB2 are 8.29 μB/f.u. and 6.56 E


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Inorganic Chemistry

crystallographic metal sites in the crystal structure of (Fe1−xCox)5PB2; M(1) in 16l and M(2) in 4c (Figure 1). The two compositions considered with CPA, both as ordered and disordered compounds, are Fe(1)4Co(2)PB2 (x = 0.2) and Fe(2)Co(1)4PB2 (x = 0.8). The energy difference between the ordered and disordered structures (with CPA) is ∼21 meV/ atom for Fe4CoPB2 and ∼7.5 meV/atom for FeCo4PB2 and in both cases favors the ordered counterpart. These energy values correspond to temperatures of about 239 and 87 K, respectively. FPLO14-GGA calculations were performed with opposite signs of an initial spin splitting on different Co sublattices to study whether the coupling in Co5PB2 is antiferro- or ferromagnetic. These calculations were based upon the coupling between the cobalt atoms at the different metal sites in the studied structure. The self-consistent calculations converged into the ferromagnetic state indicating the antiferromagnetic solution as unstable. The total energy difference between the ferromagnetic and disordered local moments (DLM) configurations indicates a ferromagnetic ground state for the whole concentration range (0 ≤ x ≤ 1). The mean-field Curie temperature for different concentrations of Co has been calculated using eq 1. For Fe5PB2 the calculated TMFA = 547 K underestimates the C experimental value of 622 K. Similarly, like in the experiment, the calculated Curie temperature drops significantly with Co concentration, reaching 71 K at x = 0.8. Theory also suggests an ordered magnetic ground state for Co5PB2 with TMFA = 37 K. C The magnetic moments for the whole concentration range of (Fe1−xCox)5PB2 were calculated with two different methods, VCA-GGA and CPA-LDA. VCA gives only an averaged value of the magnetic moment on the virtual atom, while CPA can reproduce the magnetic moments for the Fe and Co atoms separately. The drawback for CPA within FPLO5 code implementation is the necessitated application of LDA, which significantly underestimates (∼15%) the total magnetic moment of (Fe1−xCox)5PB2 compositions. The total magnetic moment for Fe5PB2 calculated with LDA is equal to 7.30 μB/f.u. (1.46 μB/Fe atom), and the one obtained with GGA is equal to 8.85 μB/f.u. (1.77 μB/Fe atom). The saturation magnetization measured for single crystals at 2 K is 8.6 μB/f.u. (1.72 μB/Fe atom).12 The GGA result 1.77 μB/Fe atom, calculated for the optimized Fe5PB2 structure (Table 3), is even slightly closer to the experimental magnetic moment than the previously reported first-principles result 1.79 μB/Fe atom.12 The small difference between the two theoretical values of the magnetic moments originates from the small variations in the assumed lattice parameters and from the 0.01 μB/Fe atom lower orbital

Figure 8. Experimentally determined Curie temperatures vs x for (Fe1−xCox)5PB2.

temperature data close to TC for an applied field of 0.01 T (Figures S2 and S3) for x ≥ 0.3. When using a 0.1 T probing field (Figures S4 and S5) the bump seen in the field cooled curve is less pronounced. Preliminary results on these in fields lower than 0.01 T show that this feature seems to be stronger for a smaller field. It could argued that, since the bump moves in temperature with cobalt content, it cannot be attributed to an impurity but rather to a particular temperature dependence of the magnetic anisotropy. For instance, the x = 0.6 sample shows a TC of 209 K by the inflection point, and yet almost 300 K is necessary for the magnetization to become diminishingly small. The findings when using the inflection point method are however consistent with Mössbauer results presented earlier in this paper. The Curie temperature of Fe5PB2 has been reported previously with large variation: 615 K,6 628 K,10 640 K,13 and 655 ± 2 K (single crystals).12 For the silicon analogue (Fe5SiB2) which has the same crystal structure, it has been shown that TC can vary depending on the stoichiometry.11,30 This might explain the large discrepancy from the values reported here for x = 0.20, TC = 481 K, compared to 517 K by McGuire and Parker.13 TC may also be sensitive to lattice distortions from mechanical treatments, e.g., milling, grinding, or crushing if these affect the lattice parameters. For instance, in an extreme case for GdAl2, milling for 60 h decreased TC by more than 50 K.31 The experimental and calculated TC both quantitatively show a reduction in TC and the magnetic exchange in the rest of the series. The results of the calculated TC are presented in more detail in the next section. Density Functional Theory Calculations. Table 3 presents the calculated crystal structure parameters of Fe5PB2 and Co5PB2 together with the corresponding experimental results. The ordered compounds can be formed due to the two

Table 3. Optimized Crystallographic Parameters for Fe5PB2 and Co5PB2 As Calculated with FPLO14-GGA Together with the Corresponding Experimental Results system

a [Å]

c [Å]

xM(1)

zM(2)

xB

Fe5PB2 (experimental, this work) at RT Fe5PB2 at RT8,11 Fe5PB2 at RT13 Fe5PB2 (Fe5P0.95B2.05) at RT8 Fe5PB2 (Fe5P0.88B2.12) at RT12 Fe5PB2 (theory, this work) Fe5PB2 at 20 K13 Co5PB2 (theory, this work) Co5PB28

5.4923(1) 5.482 5.4867(1) 5.485 5.485(3) 5.456 5.4766(1) 5.284 5.42

10.3654(4) 10.332 10.3532(2) 10.346 10.348(6) 10.296 10.3488(2) 10.541 10.20

0.1701(2) 0.1695(4) 0.1701(4)

0.1406(2) 0.1400(2) 0.1400(3)

0.381(2) 0.384(6) 0.384(4)

0.1701(1) 0.170 0.1698(3) 0.169

0.1403(1) 0.139 0.1407(2) 0.142

0.3825(2) 0.381 0.388(2) 0.376

F


Article

Inorganic Chemistry moment obtained in earlier studies,12 which most probably comes from missing the orbital contribution from outside the muffin tin radii in the LAPW method. The spin, orbital, and total magnetic moments calculated with GGA for Fe5PB2 and Co5PB2 are collected in Table 4. For Fe5PB2 the calculated spin

the LDA-CPA and GGA-VCA approaches. Both methods predict a nearly linear decrease of the total magnetic moment with x agreeing well with the measurements made at 3 K, given that the calculations do not take the impurity phases into account. However, the LDA values are about 17% lower than the GGA ones for both boundary compositions where the variation in magnetic moments with x can be related to variations in unit cell volume. The linear regression of experimental values suggests the possible existence of a weak ferromagnetic state for Co5PB2. The element-resolved spin magnetic moments calculated within LDA-CPA give the picture of stable spin magnetic moments on Co atoms, a characteristic maximum for Fe atoms at 4c sites, and a reduction of moments for Fe atoms at 16l sites. As four out of five metal atoms per formula unit occupy 16l sites (M(1)), the observed reduction of the total magnetic moment originates, mainly, from the decrease of percentage contribution of M(1) moments combined with the reduction of the Fe magnetic moment at 16l sites.

Table 4. Spin and Orbital Moment (μB/Atom) as Well as the Total Magnetic Moments (μB/f.u.) for Fe5PB2 and Co5PB2 As Calculated with FPLO14-GGA Fe5PB2

Co5PB2

atom

ms

ml

ms

ml

Fe(1)/Co(1) Fe(2)/Co(2) P B mtot

1.78 2.11 −0.13 −0.21

0.033 0.052 0.002 0.001

0.41 0.64 −0.02 −0.05

0.011 0.013 0.001 0.000

8.85

2.20

magnetic moments at Fe(1) (16l) and Fe(2) (4c) sites are equal to 1.78 and 2.11 μB, respectively. They are in agreement with the values 1.62 and 2.16 μB, evaluated from the magnetic hyperfine fields.11 The orbital contributions to the magnetic moments on Fe atoms are 0.033 and 0.052 μB, respectively. These values are close to 0.044 μB on the Fe atom for Fe2B22 and 0.043 μB/atom for bcc iron, calculated within the same framework. However, the theoretical values of the orbital magnetic moment are all reduced in comparison to the experimentally measured value 0.086 μB.32 The small induced spin magnetic moments on P and B are opposite to the spin moments on Fe and equal to −0.13 and −0.21 μB, respectively. Table 4 also presents the partial magnetic moments of the ferromagnetic Co5PB2, with the spin magnetic moments at Co(1) (16l) and Co(2) (4c) sites being equal to 0.41 and 0.64 μB, respectively, both with orbital contributions of about 0.01 μB. Together with small induced moments on P and B, they lead to the total magnetic moment of 2.20 μB/f.u of Co5PB2 (0.44 μB/Co atom). Figure 9 presents the total and element-resolved spin magnetic moments as a function of x in (Fe1−xCox)5PB2 in

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CONCLUSIONS

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ASSOCIATED CONTENT

In this study the effects of cobalt substitutions in Fe5PB2 have been studied with X-ray diffraction, Mössbauer spectroscopy, and magnetometry as well as with first-principles density functional theory calculations. It has been seen that an increased cobalt content reduces the magnetic exchange interactions. This has been concluded from a large, linear decrease in both the Curie temperature as well as the saturated magnetic moment. For the chemical order, Mö ssbauer spectroscopy shows that the cobalt atoms prefer to occupy the M(2) position in the crystal structure at higher Co concentrations. A tunable Curie transition with cobalt content, as presented here, has some prerequisites for magnetic cooling. It would be interesting to study this specific application for compositions with transition temperatures close to room temperature, i.e., x ≈ 0.4. A way of limiting the loss of magnetization with increasing substitution content is however needed if the material is to become a realistic alternative in any real application.

S Supporting Information *

In the Supporting Information, the following tables and figures are available to support the statements presented in this work. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02663. • Tables of values from XRD patterns, unit cell parameters, Mossbauer fitting parameters, and saturation magnetization and Curie temperature values; figures showing XRD patterns, field cooled magnetic measurements, and magnetic measurements as a funtion of applied field (PDF)

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AUTHOR INFORMATION

Corresponding Author

Figure 9. (a) Total and (b) species-resolved spin magnetic moments as functions of Co concentration x in (Fe1−xCox)5PB2. The total magnetic moments are calculated with the FPLO treating disorder by the VCA and CPA; species-resolved magnetic moments are based on CPA. Experimental values of magnetization were measured at 3 K.

*E-mail: [email protected]. ORCID

Johan Cedervall: 0000-0003-0336-2560 G


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Inorganic Chemistry Notes

(19) Rodriguez-Carvajal, J. Recent advances in magnetic structure determination by neutron powder diffraction. Phys. B 1993, 192, 55− 69. (20) Koepernik, K.; Eschrig, H. Full-potential nonorthogonal localorbital minimum-basis band-structure scheme. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1743. (21) Soven, P. Coherent-potential model of substitutional disordered alloys. Phys. Rev. 1967, 156, 809. (22) Edström, A.; Werwiński, M.; Iuşan, D.; Rusz, J.; Eriksson, O.; Skokov, K. P.; Radulov, I. A.; Ener, S.; Kuz’min, M. D.; Hong, J.; Fries, M.; Karpenkov, D. Y.; Gutfleisch, O.; Toson, P.; Fidler, J. Magnetic properties of (Fe1−xCox)2B alloys and the effect of doping by 5d elements. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 92, 174413. (23) Reichel, L.; Schultz, L.; Pohl, D.; Oswald, S.; Fähler, S.; Werwiński, M.; Edström, A.; Delczeg-Czirjak, E. K.; Rusz, J. From soft to hard magnetic Fe-Co-B by spontaneous strain: a combined first principles and thin film study. J. Phys.: Condens. Matter 2015, 27, 476002. (24) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865. (25) Koepernik, K.; Velický, B.; Hayn, R.; Eschrig, H. Self-consistent LCAO-CPA method for disordered alloys. Phys. Rev. B: Condens. Matter Mater. Phys. 1997, 55, 5717. (26) Perdew, J. P.; Wang, Y. Accurate and simple analytic representation of the electron-gas correlation energy. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 45, 13244. (27) Gyorffy, B. L.; Pindor, A. J.; Staunton, J.; Stocks, G. M.; Winter, H. A first-principles theory of ferromagnetic phase transitions in metals. J. Phys. F: Met. Phys. 1985, 15, 1337. (28) Bergqvist, L.; Dederichs, P. H. A theoretical study of halfmetallic antiferromagnetic diluted magnetic semiconductors. J. Phys.: Condens. Matter 2007, 19, 216220. (29) Coey, J. M. D. Magnetism and Magnetic Materials; Cambridge University Press, 2010. (30) Wäppling, R.; Ericsson, T.; Häggström, L.; Andersson, Y. Magnetic Properties of Fe5SiB2 and Related Compounds. J. Phys. Colloq. 1976, 37, 591−593. (31) Morales, M. A.; Williams, D. S.; Shand, P. M.; Stark, C.; Pekarek, T. M.; Yue, L. P.; Petkov, V.; Leslie-Pelecky, D. L. Disorderinduced depression of the Curie temperature in mechanically milled GdAl2. Phys. Rev. B: Condens. Matter Mater. Phys. 2004, 70, 184407. (32) Chen, C. T.; Idzerda, Y. U.; Lin, H.-J.; Smith, N. V.; Meigs, G.; Chaban, E.; Ho, G. H.; Pellegrin, E.; Sette, F. Experimental Confirmation of the X-Ray Magnetic Circular Dichroism Sum Rules for Iron and Cobalt. Phys. Rev. Lett. 1995, 75, 152−155.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Swedish Research Council and the Swedish Energy Agency for financing this project. M.W. would like to thank the Foundation of Polish Science grant HOMING for financial support. The HOMING programme is cofinanced by the European Union under the European Regional Development Fund.

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Influence of Cobalt Substitution on the Magnetic Properties of Fe5PB2

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