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Coercivity enhancement and magnetic property evaluation of Bi doped Mn2Sb
Journal of Magnetism and Magnetic Materials 476 (2019) 29–34
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Coercivity enhancement and magnetic property evaluation of Bi doped Mn2Sb Kritika Ananda,b, Nithya Christophera,b, Jagdish Kumarc, Anurag Guptaa,b, Nidhi Singha,b,
T
⁎
a
CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 012, India Academy of Scientific & Innovative Research (AcSIR), Dr. K. S. Krishnan Road, New Delhi 110 012, India c School of Physical and Material Sciences, Central University of Himachal Pradesh, Shahpur, H.P, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Mn2Sb Magnetization Coercivity Density functional theory
Mn2Sb is a well-known ferrimagnetic material, which has been investigated thoroughly to understand the effect of doping on its magnetic properties. Present work reports the synthesis of Bi doped Mn2Sb employing arcmelting and melt spinning followed by characterization with a view to establish a correlation in the structure and magnetic properties. Bi doping in Mn2Sb lattice produces a significant change in the latter’s magnetic properties, giving rise to a saturation magnetization of 27.2 emu/g (27.2 Am2/Kg) and a high coercivity value of 3.4 kOe (279.2 kA/m), in comparison to Mn2Sb, which has a coercivity of 0.1 kOe (7.96 kA/m). Theoretical studies were carried out to understand the changes in magnetic properties. Ab-initio electronic and atomic structure calculations show that Bi atom occupies equally the MnI and MnII sites and this leads to ferromagnetic ordering, which results in enhancement of the saturation magnetisation. The coercivity mechanism has also been studied in detail; the large coercivity is attributed to the nucleation mechanism achieved via the inclusion of diamagnetic Bi particles in Mn2Sb.
1. Introduction The compound Mn2Sb has been studied extensively because of its intriguing magnetic properties; it undergoes a first-order magnetic transition accompanied by large magnetoresistance and magneto-calorific effect. Also, this compound has been of fundamental interest as it helps in understanding phenomena such as phase-separation, metastability and co-existence of competing phases. Mn2Sb crystallizes in Cu2Sb-type structure with a space group of P4/nmm (space group no.129) [1–3]. It has two crystallographically different sites of Mn atoms, which carry different magnetic moments (direction and magnitude): MnI (+2.88 µB) and MnII (−3.82 µB) [4], as shown in Fig. 1(a). These two Mn atoms have unequal magnetic moments and are oppositely aligned thus making it ferrimagnetic (FRI), since the perfect compensation that occurs in an antiferromagnet (AFM) is lacking here [2,5,6]. The magnetic structure of Mn2Sb can be considered as a stacking of three ferromagnetic layers of MnII-MnI-MnII, structure, repeating along the tetragonal c-axis in such a way that adjacent MnIIMnII layers are parallel to each other but adjacent MnI-MnII layers are antiparallel [2,5]. Another interesting feature of Mn2Sb is that, it is known to exhibit magnetic ordering of different kinds as the environment around MnI
⁎
and MnII atoms plays a crucial role and is a deciding factor for the magnetic state of the compound. Studies suggest that the substitution of various elements (Cr, Co, Zn, and Cu) for Mn and (As, Ge, Sn) for Sb have resulted in a first-order FRI to AFM transition [6–12]. Kushwaha et. al studied the magnetic field induced AFM–FRI phase transition and metastability in cobalt-doped Mn2Sb [13]. The observed AFM to FRI transition in doped Mn2Sb based compound was interpreted in terms of exchange inversion model of Kittel [14], according to which, the exchange integral depends on the inter-atomic distance which changes sign at a critical value [15]. Based on this model many studies have concentrated on the magnetization of Mn2Sb and its ferrimagnetic state [13]. Further, boron doping in Mn2Sb at the interstitial site produced changes in Mn2Sb, both structurally and magnetically, leading to the ferromagnetic state of the compound [16]. In the present study we explore the effect of Bi doping on the magnetic properties of Mn2Sb. Elemental Bi has a diamagnetic, semimetallic character same as Sb, and has half-filled 6p orbital ([Xe] 4f14 5d10 6s2 6p3). To the best of our knowledge, Bi as a dopant at different lattice sites in the Mn2Sb system has not been explored earlier. Therefore, in the present work we have studied the possibilities of Bi substitution at different lattice sites, and its effect on the magnetic structure of Mn2Sb. By employing first principle calculations based
Corresponding author at: CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 012, India. E-mail address: [email protected] (N. Singh).
https://doi.org/10.1016/j.jmmm.2018.12.040 Received 26 July 2018; Received in revised form 1 November 2018; Accepted 11 December 2018 Available online 11 December 2018 0304-8853/ © 2018 Elsevier B.V. All rights reserved.
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arc-melted ingot and the melt-spun ribbons were measured by the vibrating sample magnetometer (VSM, VSM-7410, Lakeshore). Magnetic hysteresis (M-H) loop was recorded at room temperature and magnetization versus temperature (M-T) measurement was done from room temperature to 350 °C. 2.2. Theoretical calculations The DFT calculations have been performed by using density functional theory as implemented using Green's function based KorringaKohn-Rostoker approach in SPRKKR [21] code. The disorder in the unit cell has been studied within coherent potential approximation (CPA) as implemented in SPRKKR [21] code. A dense k-grid of 20 × 20 × 12 was used to perform Brillouin zone integrations to ensure accuracy of the calculations. For modeling electron-electron exchange and correlation effects, generalized gradient approximation (GGA) by PerdewBurke-Ernzerhof (PBE) was implemented [22]. Local density approximation (LDA) [23] and GGA results were compared to study the magnetic ground state of the compound.
Fig. 1. (a) Crystal structure of Mn2Sb, with two distinct sites of Mn atoms as MnI (purple) and MnII (brown) (b) Probable positions for the doping of Bi atom at the various lattice sites, marked as ‘a’, ‘b’, ‘c’, ‘d’ and ‘e’.
upon density functional theory we demonstrate that Bi prefers to substitute at the Mn sites and this result in a ferrimagnetic to ferromagnetic transition in doped Mn2Sb. Also, one of the key properties of a ferromagnetic material is its coercivity, which distinguishes a hard magnet from a soft magnet. Understanding and tuning the coercivity has a great impact on developing novel permanent magnet materials. The models to explain coercivity can be classified into three types [17,18]; (i) Nucleation of domain model, which demonstrates that the nucleation of reverse domains is due to surface irregularities and defects with low anisotropy; (ii) The domain wall pinning model, which shows that the inhomogeneities in the sample prevent domain wall motion resulting in high coercivity and (iii) Exchange hardening, which illustrates that a reasonable high coercivity is obtained resulting from an increase of the nucleation field by the exchange coupling between soft and hard magnetic phase in nanocomposite alloys. Also, high coercivity is usually attributed to the combination of nanostructuring and high magnetocrystalline anisotropy in the compounds. It can also be easily achieved by making the nucleation of new domains difficult [19]. In the present study, we have explored how Bi doping in Mn2Sb lattice affects its coercivity and which of the above model is important. First principle calculations suggest that the observed magnetic properties can be understood in terms of the spin reversal leading to a ferromagnetic state of the compound. The magnetization reversal process which is correlated with the microstructure and interaction between hard magnetic grains has been investigated in detail. Also, the magnetization relaxation behavior of Mn2Sb and Mn2Sb:Bi is studied to get a thorough understanding of the stability of the compound and the demagnetization process it undergoes.
3. Results and discussion Fig. 1(a) shows the crystal structure of Mn2Sb compound. To determine the most favourable site for the Bi atom, formation energies of Bi doped Mn2Sb were calculated for varying compositions, at different lattice sites. Fig. 1(b) shows various possibilities for Bi substitution in Mn2Sb unit cell. The following possibilities were considered: (a) Bi going completely to MnI site, (b) Bi going completely to MnII site, (c) Bi going completely to Sb site, (d) Bi distributing equally among MnI and MnII sites (e) Bi distribution at all three sites equally. The change in the total formation energy and the magnetization is determined as Bi occupies various possible sites. The formation energy (Ef) for various configurations of MnI1−aMnII1−bSb1−cBix (a + b + c = x) has been obtained by using Eq. (1):
Ef = EMnI1 − aMnII1 − bSb1 − c − [2 − (a + b)] EMn − (1 − c ) ESb − xEBi
(1)
where x is total amount of Bi substituted into unit cell. The values of x range from 0.0, 0.02, 0.04, 0.06, 0.08, 0.10 corresponding to 0%, 2%, 4%, 6%, 8% and 10% atomic substitution of Bi in unit cell. The values of formation energies, EMn , EBi and ESb have been computed by first optimizing Mn, Sb and Bi in FCC structure using ELK code [24] and then using optimized lattice constant for calculating the energy by SPRKKR. Fig. 2(a) shows the plot of formation energy as a function of Bi at different lattice sites, for varying compositions. As it can be seen from Fig. 2(a) and (b), the formation energy for equal distribution of Bi at MnI and MnII site (as marked by possibility ‘d’ in Fig. 1) is lowest among all the studied configurations and for all the studied concentrations of Bi in Mn2Sb unit cell. This indicates the possibility of Bi substituting uniformly at MnI and MnII sites rather than completely going to one particular site. It is interesting to mention here that Bi is iso-electronic with Sb but still it prefers to distribute uniformly at MnI and MnII sites in Mn2Sb unit cell. For phase identification, the structural characterization of Mn2Sb:Bi arc-melted ingot and melt-spun ribbons were carried out by XRD. Fig. 3(a) displays the XRD pattern of Pristine Mn2Sb, Mn2Sb:Bi arcmelted and melt-spun ribbon. Detailed XRD analysis for pristine Mn2Sb is also discussed in detail by Singh et al. [16]. The diffraction pattern of doped Mn2Sb compound indicates that Bi doping has not produced a significant change in the lattice symmetry. As a consequence of preferential orientation of crystallites and possible fractional occupation of the dopant atom at the crystallographic site, there is a slight deviation of the experimental XRD pattern with respect to the calculated diffraction pattern. The presence of elemental Bi peaks indicates precipitation of Bi in the Mn2Sb:Bi matrix in free form. XRD pattern of the melt-spun ribbon, shown in Fig. 3(a) indicates the improvement in the
2. Experimental details 2.1. Materials and methods An alloy Mn2Sb:Bi was prepared by arc melting, where the constituent elements of purity better than 99.99%, were weighed in the stochiometric ratios and were arc-melted in a mini arc-melter (MAM-1, Edmund Buhler GmbH). The ingots were re-melted several times to ensure homogeneity. The arc-melted ingots were then used as a precursor for melt spinning (Melt Spinner HV, Edmund Buhler GmbH) in order to form nano-crystalline ribbons. The rapid cooling during the melt-spinning process is advantageous for controlling the phase purity. X-ray diffraction (XRD) measurements were performed in a Rigaku diffractometer using Cu-kα radiation to determine the phase purity and lattice parameters. Rietveld refinement of the obtained XRD pattern is also carried out using Fullprof suite [20] to confirm the constituent phases. The microstructural characterisation of the nanostructured melt-spun ribbons was done with the help of field emission scanning electron microscopy (FE-SEM, Carl Zeiss Supra 40VP), to determine the morphology. The elemental composition was confirmed with the help of energy dispersive spectroscopy (EDS). The magnetic properties of the 30
Journal of Magnetism and Magnetic Materials 476 (2019) 29–34
K. Anand et al.
Fig. 4. Room temperature M-H curve for Mn2Sb and Mn2Sb:Bi MS ribbon at an applied field of 2T. Inset shows the differential susceptibility curve of demagnetization branch of Mn2Sb:Bi. Fig. 2. Formation energy Ef as a function of (a) varying Bi concentration and (b) at different lattice sites in Mn2Sb, for Bi concentration of 0.1.
to inhomogeneous particle size distribution that gives rise to different coercivities [25]. It has been reported that a heterogeneous grain-size distribution, such as a bimodal distribution, causes kinks to appear in the second quadrant of the hysteresis loop [26]. The nature of the particles is also understood from the field dependence of differential susceptibility (dM/dH) at RT, as shown in the inset of Fig. 4. The double peak in the derivative suggests the heterogeneous microstructure, as evidenced by the XRD measurements. The high value of coercivity could be attributed to the presence of diamagnetic Bi particles in Mn2Sb:Bi matrix, which inhibits the movement of magnetic domain wall and thereby induces magnetic hardening, due to nucleation effects. This was further corroborated by minor loop measurements for the Mn2Sb:Bi ribbons, shown in the inset of Fig. 5 (a). The reversible domain wall movement is most prominent at small applied fields. Thus, the primary magnetization measurements could most precisely be accessed by minor loop measurements, as opposed to major loop measurements where domains undergo a substantial amount of irreversible magnetization. As shown in Fig. 5(a), which is a plot of magnetization (M/M2T) and coercivity (H/Hc) dependence on the applied magnetic field, the magnetization increases fast with the magnetic field, this suggests that the magnetization reversal process is dominated by the nucleation mechanism. According to the nucleation field dependence of coercivity, the nucleation of the reversed domain is necessary in magnetization reversal, and the pinning effect on domain wall motion makes the nucleation of reversed domain independent among grains and leads to a more non uniform magnetization reversal. Normally, in
Bi/Mn2Sb:Bi 100% peak intensity ratio, which is suggestive of the fact that the phase fraction of the Mn2Sb:Bi increased upon melt spinning. This is attributed to the fact that melt spinning significantly modifies the micro-structures and promotes enhanced solid-solubility. Further it helps to nanostructure the arc melted ingots which due to a reduction in grain size enhances coercivity. Also, the presence of Mn in elemental form in melt spun ribbon is indicative of the fact that the Bi occupies Mn site, leaving aside free Mn, which is in conformity with DFT results. To better understand the detailed crystal structure, Rietveld refinement was carried out using FullProf Suite to confirm the phase formation (Fig. 3(b)) as well as to determine the phase fraction of the alloy formed. Two phases were used to fit the diffraction pattern, viz., Mn2Sb:Bi (P4/nmm, a = b = 4.078 Å, c = 6.56 Å) and Bi (R3̅m, a = b = 4.531, c = 11.824). The Cu2Sb type crystal structure is still preserved with the lattice parameters a = b = 4.072 Å and c = 6.528 Å. Although there are reflections from elemental impurities, no indications of secondary alloy phases have been found. Room temperature MH plot of Mn2Sb and Mn2Sb:Bi melt spun ribbon is shown in Fig. 4. As can be seen from the figure, the as meltspun ribbon shows a coercivity value of 3.4 kOe (279.2 kA/m) and a magnetization of 27.2 emu/g (27.2 Am2/kg) (∼2.4 µB), at a maximum field of 2 T, as compared to Mn2Sb which has a magnetization of 22 emu/g (22 Am2/kg) and a coercivity of 0.1 kOe (7.96 kA/m). A constriction in the hysteresis loop of Mn2Sb:Bi is observed which is due
Fig. 3. (a) XRD pattern of pristine Mn2Sb, Mn2Sb:Bi arc-melted ingot and melt-spun ribbon (b) Rietveld refinement of the XRD pattern of Mn2Sb:Bi, fitted in two phases viz., Mn2Sb:Bi and Bi. The bottom line curve shows the difference between the measured and fitted curves, and the vertical line indicates the Bragg position of both the phases; upper bars corresponding to the Mn2Sb phase and the lower bars to Bi phase. 31
Journal of Magnetism and Magnetic Materials 476 (2019) 29–34
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Fig. 5. (a) Coercivity and magnetization dependence on the applied field. Inset shows the minor loop of Mn2Sb:Bi MS ribbon (b) FE-SEM micrograph showing surface morphology of as-melt spun Mn2Sb:Bi ribbon.
the melt-spun ribbons coercivity fields are normally governed by the domain-wall pinning [27] but, in our samples it is primarily controlled by nucleation-hardening, which has been confirmed by the magnetic field dependence of coercivity. Another point worth emphasising is that the coercivity mechanism is sensitive to phase composition as well as the microstructure. Melt spinning promotes the reduction of grain size in nanometre scale which leads to the exchange coupling between the grains resulting in a high value of coercivity. To study the morphology of the nanocrystalline melt spun ribbon, FE-SEM was carried out. The melt-spun ribbon shows Bi in elemental form, spread throughout the matrix at the grain boundaries (Fig. 5(b)), which is in correlation with the XRD data. Also, we observe the inhomogeneous particle size distribution of Mn2Sb:Bi, leading to different values of coercivities, as is evident from the kink in the MH loop. The image shows dark and light contrast, marked by a well-defined boundary, where dark regions correspond to Mn2Sb:Bi composition and lighter regions are rich in Bi. The average composition as determined from EDAX analysis is found to be Mn(47)Sb(33)Bi(20) in at%. This emphasises that Bi goes to the Mn site, deviating composition from Mn (66.6)Sb(33.3). Thus, EDAX measurement confirms the stoichiometry of the compound. The thermo-magnetic measurements were undertaken to determine the Curie temperature (TC) of the alloy. M-T measurements were carried out at a constant field of 1 kOe. Fig. 6 shows the M-T curve for Mn2Sb and Mn2Sb:Bi melt spun ribbon. The Curie temperature is determined from the dM/dT curve versus temperature, as seen in the inset of Fig. 6. As is evident from the graph, Bi doping in Mn2Sb does not alter the TC of the compound. It shows that the compound has TC of ∼280 °C,
implying its usability in mid-temperature range. To further validate the effect of Bi doping on magnetism and to account for the ferromagnetic state of the compound, the magnetic ground state of the unit cell was computed by DFT calculations, substituting Bi equally at both the sites of Mn. Table 1 shows total energy and magnetic moments of MnI and MnII atoms as a function of Bi concentration in Mn2−xSbBix lattice calculated for GGA and LDA exchange correlations. As it can be seen from the table, GGA calculation suggests that the ferrimagnetic state is the ground state. Also, for all the concentrations of Bi i.e. × = 0.02, 0.04 and 0.06, the ferromagnetic state retains its lower energy in comparison to ferrimagnetic energy. However, the relative stability of this state in comparison to ferrimagnetic state decreases which can be seen from the difference in energy between ferromagnetic and ferrimagnetic state (Table 1). Whereas, the local density approximation as proposed by U von Barth and L Hedin [23], reproduces correct ferrimagnetic state, as the ground state, for pure Mn2Sb with magnetic moment of 2.44 and 3.57 µB on MnI and MnII atoms respectively. The net magnetic moment for pure Mn2Sb is found to be 2.13 µB/unit cell (containing two formula units). It was also observed that with the substitution of even 2% (x = 0.02) Bi at Mn site, the magnetic moment of MnI decreases significantly from 2.44 µB to 1.36 µB whereas the magnetic moment on MnII atom changes only slightly from 3.57 µB to 3.52 µB. This leads to a rise in the net magnetic moment on unit cell, which is almost doubled and found 4.40 µB/unit cell. With further rise in concentration of Bi at MnI site there is no significant change in saturation magnetization of the unit cell up to a studied concentration of x = 0.06. Such a rise in saturation magnetization is also observed in our experimental results. It is worth mentioning here that our calculations can be validated further by a detailed study on the magnetic structure of Bi-substituted Mn2Sb by techniques such as neutron diffraction. To further understand the stability of the Bi doped Mn2Sb compounds, magnetic relaxation studies were undertaken. The relaxation behavior of magnetic materials is characterized by a time constant (τ), which is influenced by both the environment of the particle and the properties of the particle itself. It is also dependent upon temperature. The parameter τ describes the period of time over which the magnetic systems approaches equilibrium or a steady-state. It has been reported that there are two distinct behaviors of relaxation; either through the nucleation of few domains followed by a rapid growth of the magnetic domains, or through the nucleation of many domains at random positions. The magnetic relaxation was measured after an application of saturating field of 10 kOe, keeping it on for a wait time of 300 s, and then switching it off to measure the evolution of remanent magnetization as a function of time. The temporal profiles of magnetic relaxation can be represented by a form:
Fig. 6. Magnetization (M) versus temperature (MT measurement curve) for Mn2Sb and Mn2Sb:Bi melt spun ribbon, measured at a field of 1 kOe. Inset shows the Tc obtained from the plot of dM/dT versus temperature.
M (t ) = Mr + Mrl e−(t / τ ) 32
β
(2)
Journal of Magnetism and Magnetic Materials 476 (2019) 29–34
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Table 1 Magnetic moment and the total energy for different concentration of Bi in Mn2Sb lattice, calculated by GGA and LDA. X
0.0
0.02
0.04
0.06
GGA
Energy (Ry) MnI (µB) MnII (µB) Total Moment Energy (Ry) MnI MnII Total Moment Energy (Ry) MnI MnII Total Moment Energy (Ry) MnI MnII Total Moment
(µB/u.c)
(µB/u.c)
(µB/u.c)
(µB/u.c)
LDA
Ferromagnetic
Ferrimagnetic
ΔE = EFerro − EFerri (mRy)
Ferromagnetic
Ferrimagnetic
ΔE = EFerro − EFerri (mRy)
−35203.8121 3.0525 3.6860 13.1204 −36837.2862 1.9299 3.5720 10.6815 −38471.0945 1.9433 3.5732 10.6008 −40104.9028 1.9576 3.5739 10.5195
−35203.7443 2.8727 3.9197 2.1319 −36837.2435 2.1712 3.8477 3.4396 −38471.0559 2.1538 3.8470 3.4351 −40104.8681 2.1418 3.8448 3.4152
−67.8124
−35171.7792 2.3006 3.2521 10.8272 −36804.6078 1.7414 3.3605 9.9928 −38437.7376 1.7245 3.3565 9.8498 −40070.8672 1.7033 3.3522 9.6983
−35171.7829 2.4457 3.5767 2.2840 −36804.6293 1.3635 3.5192 4.4028 −38437.7626 1.3527 3.5091 4.3512 −40070.8956 1.3352 3.4990 4.3135
3.7163
−42.6169
−38.5514
−34.6629
21.5358
25.0534
28.4054
deduced by GGA calculations. Mn2Sb:Bi exhibits appreciable coercivity value of 3.4 kOe (279.2 kA/m) and magnetization of 27.2 emu/g (27.2 Am2/Kg). The high value of coercivity obtained is attributed to the nucleation mechanism achieved via inclusion of diamagnetic Bi particles which obstructs the domain wall motion. It has been observed that the inclusion of dopants in Mn2Sb, hinders the magnetization reversal process, leading to a high value of coercivity. This coercivity enhancement has been achieved employing melt-spinning which also resulted in nanocrystalline microstructure in addition to Bi segregation at the grain boundaries. Hence, the approach of suitable doping along with optimized processing can be employed to design innovative magnetic materials with custom-made magnetic properties. Conflicts of interest Fig. 7. Time evolution of magnetization of Mn2Sb and Mn2Sb:Bi. Eq. (2) is used to fit the data to determine the respective time constants.
There are no conflicts to declare. Author contributions
where Mr is the remnant magnetization; Mrl is the initial part of the relaxing magnetization, τ is the relaxation time constant, and β is an exponent, which reflects the changes of relaxation rate with the evolution of time. Fig. 7 shows the experimentally obtained curves of M-t for Mn2Sb and Mn2Sb:Bi. These curves were fitted using Eq. (2), to determine the time constant. It clearly shows that the relaxation is slower in Mn2Sb:Bi, with a relaxation time constant of 4474 s as opposed to the value of 2690 s for Mn2Sb. This could be explained as the relaxation at constant applied field and temperature proceeds through the nucleation and propagation of domain wall-like structures [28]. Also, it was observed that the relaxation in Mn2Sb:Bi is slowing down whereas it is speeding up for Mn2Sb, as indicated by the value of β which is less than 1 for Mn2Sb:Bi (0.8659) and greater than 1 for Mn2Sb (1.6179). The magnetic relaxation is dominated by larger spin domains at later times, giving rise to slowing down of relaxation for Mn2Sb:Bi. The long relaxation times for Mn2Sb:Bi reflect the existence of strong coupling among the magnetic domains, which is due to the appropriate doping by Bi.
NS supervised the overall work, conceived the problem related experiments and analyzed the results. JK carried out the DFT calculations and NS, KA and NC performed the experimental work. NS and KA wrote the manuscript. All the authors contributed to the interpretation of the results. Funding This work was carried out under CSIR (India) Network Project PSC0109. Acknowledgements This work was carried out under CSIR (India) Network Project PSC0109. KA and NC acknowledge Council of Scientific and Industrial Research (CSIR), India for financial assistance. Authors would like to thank Radhey Shyam and Naval Kishor for their technical support. References
4. Conclusion
[1] O. Beckman, L. Lundgren, Handbook of Magnetic Materials, Elsevier, 1991, p. 6. [2] M.K. Wilkinson, N.S. Gingrich, C.G. Shull, The magnetic structure of Mn2Sb, J. Phys. Chem. Solids 2 (1957) 289–300. [3] V.M. Ryzhkovskii, N.D. Zhigadlo, I.L. Pashkovskii, Crystal structure and magnetic properties of some Mn2Sb-based alloys, Cryst. Res. Technol. 25 (1990) 165–169. [4] R.W. Houghton, W. Weyhmann, NMR Study of the spin-reorientation behavior of Mn2-xCrxSb, Phys. Rev. Lett. 20 (1968) 842–844. [5] M. Suzuki, M. Shirai, K. Motizuki, Electronic band structure in the ferrimagnetic
To conclude, the structural and magnetic changes induced by Bi doping in Mn2Sb system were studied. Doping Bi in Mn2Sb, leads to significant changes in the crystallographic structure, with remarkable effects on the magnetic properties of the compound. From the first principle calculations, it was inferred that Bi occupies both the MnI and MnII site, which leads to ferromagnetic state of the compound as 33
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magnetic materials, J. Mater. Chem. C 5 (2017) 11832–11836. [17] M.F. de Campos, The coercivity mechanisms in Sm(CoFeCuZr)z nanocrystalline magnets: nucleation x pinning, Mater. Sci. Forum 591–593 (2008) 5. [18] Seiji Miyashita, H. Satoshi, N. Masamichi, Perspectives for high-performance permanent magnets: applications, coercivity, and new materials, Adv. Nat. Sci.: Nanosci. Nanotechnol. (2017) 8. [19] R.S. Turtelli, D. Triyono, R. Grössinger, H. Michor, J.H. Espina, J.P. Sinnecker, et al., Coercivity mechanism in Nd60Fe30Al10 and Nd60Fe20Co10Al10 alloys, Phys. Rev. B. 66 (2002) 054441. [20] Rodríguez-Carvajal J. developments of the program FULLPROF Commission on Powder Diffraction (IUCr). Newsletter, 26 (2001). [21] H. Ebert, D. Ködderitzsch, J. Minár, Calculating condensed matter properties using the KKR-Green's function method—recent developments and applications, Rep. Prog. Phys. 74 (2011) 096501. [22] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [23] L. Hedin, Barth U. Von, J. Phys. C: Solid State Phys. 5 (1972) 13. [24] < http://elk.sourceforge.net/ > . [25] N.V. Rama Rao, G.C. Hadjipanayis, Influence of jet milling process parameters on particle size, phase formation and magnetic properties of MnBi alloy, J. Alloy. Compd. 629 (2015) 80. [26] R. Ramesh, G. Thomas, B.M. Ma, Magnetization reversal in nucleation controlled magnets. II. effect of grain size and size distribution on intrinsic coercivity of Fe-NdB magnets, J. Appl. Phys. 64 (1988) 6416–6423. [27] D.T. Zhang, S. Cao, M. Yue, W.Q. Liu, J.X. Zhang, Y. Qiang, Structural and magnetic properties of bulk MnBi permanent magnets, J. Appl. Phys. 109 (2011) 07A722. [28] K. Hika, L.V. Panina, K. Mohri, Magneto-impedance in sandwich film for magnetic sensor heads, IEEE Trans. Magn. 32 (1996) 4594–4596.
state of Mn2Sb, J. Phys. Condens. Matter (1992) 4. [6] T. Kanomata, H. Ido, Magnetic transitions in Mn2−xMxSb (M=3d metals), J. Appl. Phys. 55 (1984) 2039–2041. [7] T.J. Swoboda, W.H. Cloud, T.A. Bither, M.S. Sadler, H.S. Jarrett, Evidence for an antiferromagnetic-ferrimagnetic transition in Cr-modified Mn2Sb, Phys. Rev. Lett. 4 (1960) 509–511. [8] T.A. Bither, P.H.L. Walter, W.H. Cloud, T.J. Swoboda, P.E. Bierstedt, New modified Mn2Sb compositions showing exchange inversion, J. Appl. Phys. 33 (1962) 1346–1347. [9] F.J. Darnell, W.H. Cloud, H.S. Jarrett, X-ray and magnetization studies of Crmodified Mn2Sb, Phys. Rev. 130 (1963) 647–655. [10] J.H. Wijngaard, C. Haas, R.A. de Groot, Ferrimagnetic-antiferromagnetic phase transition in Mn2-xCrxSb: electronic structure and electrical and magnetic properties, Phys. Rev. B. 45 (1992) 5395–5405. [11] M. Bartashevich, T. Goto, N. Baranov, V. Gaviko, Volume magnetostriction at the AF– FRI metamagnetic transition in the itinerant-electron system Mn2−xTxSb (T=Co, Cr), Phys. B: Condens. Matter 351 (2004) 71. [12] Y.Q. Zhang, Zhi-Dong Zhang, Metamagnetic-transition-induced giant magnetoresistance inMn2Sb1-xSnx (0 < x < 0.4) compounds, Phys. Rev. B. 67 (2003) 132405. [13] K. Pallavi, R. Rawat, P. Chaddah, Metastability in the ferrimagnetic–antiferromagnetic phase transition in Co substituted Mn2Sb, J. Phys.: Condens. Matter 20 (2008) 022204. [14] C. Kittel, Model of exchange-inversion magnetization, Phys. Rev. 120 (1960) 335–342. [15] S.A. Nikitin, T.I. Ivanova, Y.A. Ovchenkova, M.V. Maslennikova, G.S. Burkhanov, O.D. Chistyakov, The influence of interatomic distances on magnetic ordering in RMnSi compounds (R = La, Y, Sm, and Gd), Phys. Solid State 44 (2002) 4. [16] N. Singh, K. Anand, N. Christopher, A. Bhattacharya, A.K. Srivastava, Anomalous enhancement in magnetization upon interstitial doping due to spin reversal in
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Coercivity enhancement and magnetic property evaluation of Bi doped Mn2Sb Kritika Ananda,b, Nithya Christophera,b, Jagdish Kumarc, Anurag Guptaa,b, Nidhi Singha,b,
T
⁎
a
CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 012, India Academy of Scientific & Innovative Research (AcSIR), Dr. K. S. Krishnan Road, New Delhi 110 012, India c School of Physical and Material Sciences, Central University of Himachal Pradesh, Shahpur, H.P, India b
A R T I C LE I N FO
A B S T R A C T
Keywords: Mn2Sb Magnetization Coercivity Density functional theory
Mn2Sb is a well-known ferrimagnetic material, which has been investigated thoroughly to understand the effect of doping on its magnetic properties. Present work reports the synthesis of Bi doped Mn2Sb employing arcmelting and melt spinning followed by characterization with a view to establish a correlation in the structure and magnetic properties. Bi doping in Mn2Sb lattice produces a significant change in the latter’s magnetic properties, giving rise to a saturation magnetization of 27.2 emu/g (27.2 Am2/Kg) and a high coercivity value of 3.4 kOe (279.2 kA/m), in comparison to Mn2Sb, which has a coercivity of 0.1 kOe (7.96 kA/m). Theoretical studies were carried out to understand the changes in magnetic properties. Ab-initio electronic and atomic structure calculations show that Bi atom occupies equally the MnI and MnII sites and this leads to ferromagnetic ordering, which results in enhancement of the saturation magnetisation. The coercivity mechanism has also been studied in detail; the large coercivity is attributed to the nucleation mechanism achieved via the inclusion of diamagnetic Bi particles in Mn2Sb.
1. Introduction The compound Mn2Sb has been studied extensively because of its intriguing magnetic properties; it undergoes a first-order magnetic transition accompanied by large magnetoresistance and magneto-calorific effect. Also, this compound has been of fundamental interest as it helps in understanding phenomena such as phase-separation, metastability and co-existence of competing phases. Mn2Sb crystallizes in Cu2Sb-type structure with a space group of P4/nmm (space group no.129) [1–3]. It has two crystallographically different sites of Mn atoms, which carry different magnetic moments (direction and magnitude): MnI (+2.88 µB) and MnII (−3.82 µB) [4], as shown in Fig. 1(a). These two Mn atoms have unequal magnetic moments and are oppositely aligned thus making it ferrimagnetic (FRI), since the perfect compensation that occurs in an antiferromagnet (AFM) is lacking here [2,5,6]. The magnetic structure of Mn2Sb can be considered as a stacking of three ferromagnetic layers of MnII-MnI-MnII, structure, repeating along the tetragonal c-axis in such a way that adjacent MnIIMnII layers are parallel to each other but adjacent MnI-MnII layers are antiparallel [2,5]. Another interesting feature of Mn2Sb is that, it is known to exhibit magnetic ordering of different kinds as the environment around MnI
⁎
and MnII atoms plays a crucial role and is a deciding factor for the magnetic state of the compound. Studies suggest that the substitution of various elements (Cr, Co, Zn, and Cu) for Mn and (As, Ge, Sn) for Sb have resulted in a first-order FRI to AFM transition [6–12]. Kushwaha et. al studied the magnetic field induced AFM–FRI phase transition and metastability in cobalt-doped Mn2Sb [13]. The observed AFM to FRI transition in doped Mn2Sb based compound was interpreted in terms of exchange inversion model of Kittel [14], according to which, the exchange integral depends on the inter-atomic distance which changes sign at a critical value [15]. Based on this model many studies have concentrated on the magnetization of Mn2Sb and its ferrimagnetic state [13]. Further, boron doping in Mn2Sb at the interstitial site produced changes in Mn2Sb, both structurally and magnetically, leading to the ferromagnetic state of the compound [16]. In the present study we explore the effect of Bi doping on the magnetic properties of Mn2Sb. Elemental Bi has a diamagnetic, semimetallic character same as Sb, and has half-filled 6p orbital ([Xe] 4f14 5d10 6s2 6p3). To the best of our knowledge, Bi as a dopant at different lattice sites in the Mn2Sb system has not been explored earlier. Therefore, in the present work we have studied the possibilities of Bi substitution at different lattice sites, and its effect on the magnetic structure of Mn2Sb. By employing first principle calculations based
Corresponding author at: CSIR-National Physical Laboratory, Dr. K. S. Krishnan Road, New Delhi 110 012, India. E-mail address: [email protected] (N. Singh).
https://doi.org/10.1016/j.jmmm.2018.12.040 Received 26 July 2018; Received in revised form 1 November 2018; Accepted 11 December 2018 Available online 11 December 2018 0304-8853/ © 2018 Elsevier B.V. All rights reserved.
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arc-melted ingot and the melt-spun ribbons were measured by the vibrating sample magnetometer (VSM, VSM-7410, Lakeshore). Magnetic hysteresis (M-H) loop was recorded at room temperature and magnetization versus temperature (M-T) measurement was done from room temperature to 350 °C. 2.2. Theoretical calculations The DFT calculations have been performed by using density functional theory as implemented using Green's function based KorringaKohn-Rostoker approach in SPRKKR [21] code. The disorder in the unit cell has been studied within coherent potential approximation (CPA) as implemented in SPRKKR [21] code. A dense k-grid of 20 × 20 × 12 was used to perform Brillouin zone integrations to ensure accuracy of the calculations. For modeling electron-electron exchange and correlation effects, generalized gradient approximation (GGA) by PerdewBurke-Ernzerhof (PBE) was implemented [22]. Local density approximation (LDA) [23] and GGA results were compared to study the magnetic ground state of the compound.
Fig. 1. (a) Crystal structure of Mn2Sb, with two distinct sites of Mn atoms as MnI (purple) and MnII (brown) (b) Probable positions for the doping of Bi atom at the various lattice sites, marked as ‘a’, ‘b’, ‘c’, ‘d’ and ‘e’.
upon density functional theory we demonstrate that Bi prefers to substitute at the Mn sites and this result in a ferrimagnetic to ferromagnetic transition in doped Mn2Sb. Also, one of the key properties of a ferromagnetic material is its coercivity, which distinguishes a hard magnet from a soft magnet. Understanding and tuning the coercivity has a great impact on developing novel permanent magnet materials. The models to explain coercivity can be classified into three types [17,18]; (i) Nucleation of domain model, which demonstrates that the nucleation of reverse domains is due to surface irregularities and defects with low anisotropy; (ii) The domain wall pinning model, which shows that the inhomogeneities in the sample prevent domain wall motion resulting in high coercivity and (iii) Exchange hardening, which illustrates that a reasonable high coercivity is obtained resulting from an increase of the nucleation field by the exchange coupling between soft and hard magnetic phase in nanocomposite alloys. Also, high coercivity is usually attributed to the combination of nanostructuring and high magnetocrystalline anisotropy in the compounds. It can also be easily achieved by making the nucleation of new domains difficult [19]. In the present study, we have explored how Bi doping in Mn2Sb lattice affects its coercivity and which of the above model is important. First principle calculations suggest that the observed magnetic properties can be understood in terms of the spin reversal leading to a ferromagnetic state of the compound. The magnetization reversal process which is correlated with the microstructure and interaction between hard magnetic grains has been investigated in detail. Also, the magnetization relaxation behavior of Mn2Sb and Mn2Sb:Bi is studied to get a thorough understanding of the stability of the compound and the demagnetization process it undergoes.
3. Results and discussion Fig. 1(a) shows the crystal structure of Mn2Sb compound. To determine the most favourable site for the Bi atom, formation energies of Bi doped Mn2Sb were calculated for varying compositions, at different lattice sites. Fig. 1(b) shows various possibilities for Bi substitution in Mn2Sb unit cell. The following possibilities were considered: (a) Bi going completely to MnI site, (b) Bi going completely to MnII site, (c) Bi going completely to Sb site, (d) Bi distributing equally among MnI and MnII sites (e) Bi distribution at all three sites equally. The change in the total formation energy and the magnetization is determined as Bi occupies various possible sites. The formation energy (Ef) for various configurations of MnI1−aMnII1−bSb1−cBix (a + b + c = x) has been obtained by using Eq. (1):
Ef = EMnI1 − aMnII1 − bSb1 − c − [2 − (a + b)] EMn − (1 − c ) ESb − xEBi
(1)
where x is total amount of Bi substituted into unit cell. The values of x range from 0.0, 0.02, 0.04, 0.06, 0.08, 0.10 corresponding to 0%, 2%, 4%, 6%, 8% and 10% atomic substitution of Bi in unit cell. The values of formation energies, EMn , EBi and ESb have been computed by first optimizing Mn, Sb and Bi in FCC structure using ELK code [24] and then using optimized lattice constant for calculating the energy by SPRKKR. Fig. 2(a) shows the plot of formation energy as a function of Bi at different lattice sites, for varying compositions. As it can be seen from Fig. 2(a) and (b), the formation energy for equal distribution of Bi at MnI and MnII site (as marked by possibility ‘d’ in Fig. 1) is lowest among all the studied configurations and for all the studied concentrations of Bi in Mn2Sb unit cell. This indicates the possibility of Bi substituting uniformly at MnI and MnII sites rather than completely going to one particular site. It is interesting to mention here that Bi is iso-electronic with Sb but still it prefers to distribute uniformly at MnI and MnII sites in Mn2Sb unit cell. For phase identification, the structural characterization of Mn2Sb:Bi arc-melted ingot and melt-spun ribbons were carried out by XRD. Fig. 3(a) displays the XRD pattern of Pristine Mn2Sb, Mn2Sb:Bi arcmelted and melt-spun ribbon. Detailed XRD analysis for pristine Mn2Sb is also discussed in detail by Singh et al. [16]. The diffraction pattern of doped Mn2Sb compound indicates that Bi doping has not produced a significant change in the lattice symmetry. As a consequence of preferential orientation of crystallites and possible fractional occupation of the dopant atom at the crystallographic site, there is a slight deviation of the experimental XRD pattern with respect to the calculated diffraction pattern. The presence of elemental Bi peaks indicates precipitation of Bi in the Mn2Sb:Bi matrix in free form. XRD pattern of the melt-spun ribbon, shown in Fig. 3(a) indicates the improvement in the
2. Experimental details 2.1. Materials and methods An alloy Mn2Sb:Bi was prepared by arc melting, where the constituent elements of purity better than 99.99%, were weighed in the stochiometric ratios and were arc-melted in a mini arc-melter (MAM-1, Edmund Buhler GmbH). The ingots were re-melted several times to ensure homogeneity. The arc-melted ingots were then used as a precursor for melt spinning (Melt Spinner HV, Edmund Buhler GmbH) in order to form nano-crystalline ribbons. The rapid cooling during the melt-spinning process is advantageous for controlling the phase purity. X-ray diffraction (XRD) measurements were performed in a Rigaku diffractometer using Cu-kα radiation to determine the phase purity and lattice parameters. Rietveld refinement of the obtained XRD pattern is also carried out using Fullprof suite [20] to confirm the constituent phases. The microstructural characterisation of the nanostructured melt-spun ribbons was done with the help of field emission scanning electron microscopy (FE-SEM, Carl Zeiss Supra 40VP), to determine the morphology. The elemental composition was confirmed with the help of energy dispersive spectroscopy (EDS). The magnetic properties of the 30
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Fig. 4. Room temperature M-H curve for Mn2Sb and Mn2Sb:Bi MS ribbon at an applied field of 2T. Inset shows the differential susceptibility curve of demagnetization branch of Mn2Sb:Bi. Fig. 2. Formation energy Ef as a function of (a) varying Bi concentration and (b) at different lattice sites in Mn2Sb, for Bi concentration of 0.1.
to inhomogeneous particle size distribution that gives rise to different coercivities [25]. It has been reported that a heterogeneous grain-size distribution, such as a bimodal distribution, causes kinks to appear in the second quadrant of the hysteresis loop [26]. The nature of the particles is also understood from the field dependence of differential susceptibility (dM/dH) at RT, as shown in the inset of Fig. 4. The double peak in the derivative suggests the heterogeneous microstructure, as evidenced by the XRD measurements. The high value of coercivity could be attributed to the presence of diamagnetic Bi particles in Mn2Sb:Bi matrix, which inhibits the movement of magnetic domain wall and thereby induces magnetic hardening, due to nucleation effects. This was further corroborated by minor loop measurements for the Mn2Sb:Bi ribbons, shown in the inset of Fig. 5 (a). The reversible domain wall movement is most prominent at small applied fields. Thus, the primary magnetization measurements could most precisely be accessed by minor loop measurements, as opposed to major loop measurements where domains undergo a substantial amount of irreversible magnetization. As shown in Fig. 5(a), which is a plot of magnetization (M/M2T) and coercivity (H/Hc) dependence on the applied magnetic field, the magnetization increases fast with the magnetic field, this suggests that the magnetization reversal process is dominated by the nucleation mechanism. According to the nucleation field dependence of coercivity, the nucleation of the reversed domain is necessary in magnetization reversal, and the pinning effect on domain wall motion makes the nucleation of reversed domain independent among grains and leads to a more non uniform magnetization reversal. Normally, in
Bi/Mn2Sb:Bi 100% peak intensity ratio, which is suggestive of the fact that the phase fraction of the Mn2Sb:Bi increased upon melt spinning. This is attributed to the fact that melt spinning significantly modifies the micro-structures and promotes enhanced solid-solubility. Further it helps to nanostructure the arc melted ingots which due to a reduction in grain size enhances coercivity. Also, the presence of Mn in elemental form in melt spun ribbon is indicative of the fact that the Bi occupies Mn site, leaving aside free Mn, which is in conformity with DFT results. To better understand the detailed crystal structure, Rietveld refinement was carried out using FullProf Suite to confirm the phase formation (Fig. 3(b)) as well as to determine the phase fraction of the alloy formed. Two phases were used to fit the diffraction pattern, viz., Mn2Sb:Bi (P4/nmm, a = b = 4.078 Å, c = 6.56 Å) and Bi (R3̅m, a = b = 4.531, c = 11.824). The Cu2Sb type crystal structure is still preserved with the lattice parameters a = b = 4.072 Å and c = 6.528 Å. Although there are reflections from elemental impurities, no indications of secondary alloy phases have been found. Room temperature MH plot of Mn2Sb and Mn2Sb:Bi melt spun ribbon is shown in Fig. 4. As can be seen from the figure, the as meltspun ribbon shows a coercivity value of 3.4 kOe (279.2 kA/m) and a magnetization of 27.2 emu/g (27.2 Am2/kg) (∼2.4 µB), at a maximum field of 2 T, as compared to Mn2Sb which has a magnetization of 22 emu/g (22 Am2/kg) and a coercivity of 0.1 kOe (7.96 kA/m). A constriction in the hysteresis loop of Mn2Sb:Bi is observed which is due
Fig. 3. (a) XRD pattern of pristine Mn2Sb, Mn2Sb:Bi arc-melted ingot and melt-spun ribbon (b) Rietveld refinement of the XRD pattern of Mn2Sb:Bi, fitted in two phases viz., Mn2Sb:Bi and Bi. The bottom line curve shows the difference between the measured and fitted curves, and the vertical line indicates the Bragg position of both the phases; upper bars corresponding to the Mn2Sb phase and the lower bars to Bi phase. 31
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Fig. 5. (a) Coercivity and magnetization dependence on the applied field. Inset shows the minor loop of Mn2Sb:Bi MS ribbon (b) FE-SEM micrograph showing surface morphology of as-melt spun Mn2Sb:Bi ribbon.
the melt-spun ribbons coercivity fields are normally governed by the domain-wall pinning [27] but, in our samples it is primarily controlled by nucleation-hardening, which has been confirmed by the magnetic field dependence of coercivity. Another point worth emphasising is that the coercivity mechanism is sensitive to phase composition as well as the microstructure. Melt spinning promotes the reduction of grain size in nanometre scale which leads to the exchange coupling between the grains resulting in a high value of coercivity. To study the morphology of the nanocrystalline melt spun ribbon, FE-SEM was carried out. The melt-spun ribbon shows Bi in elemental form, spread throughout the matrix at the grain boundaries (Fig. 5(b)), which is in correlation with the XRD data. Also, we observe the inhomogeneous particle size distribution of Mn2Sb:Bi, leading to different values of coercivities, as is evident from the kink in the MH loop. The image shows dark and light contrast, marked by a well-defined boundary, where dark regions correspond to Mn2Sb:Bi composition and lighter regions are rich in Bi. The average composition as determined from EDAX analysis is found to be Mn(47)Sb(33)Bi(20) in at%. This emphasises that Bi goes to the Mn site, deviating composition from Mn (66.6)Sb(33.3). Thus, EDAX measurement confirms the stoichiometry of the compound. The thermo-magnetic measurements were undertaken to determine the Curie temperature (TC) of the alloy. M-T measurements were carried out at a constant field of 1 kOe. Fig. 6 shows the M-T curve for Mn2Sb and Mn2Sb:Bi melt spun ribbon. The Curie temperature is determined from the dM/dT curve versus temperature, as seen in the inset of Fig. 6. As is evident from the graph, Bi doping in Mn2Sb does not alter the TC of the compound. It shows that the compound has TC of ∼280 °C,
implying its usability in mid-temperature range. To further validate the effect of Bi doping on magnetism and to account for the ferromagnetic state of the compound, the magnetic ground state of the unit cell was computed by DFT calculations, substituting Bi equally at both the sites of Mn. Table 1 shows total energy and magnetic moments of MnI and MnII atoms as a function of Bi concentration in Mn2−xSbBix lattice calculated for GGA and LDA exchange correlations. As it can be seen from the table, GGA calculation suggests that the ferrimagnetic state is the ground state. Also, for all the concentrations of Bi i.e. × = 0.02, 0.04 and 0.06, the ferromagnetic state retains its lower energy in comparison to ferrimagnetic energy. However, the relative stability of this state in comparison to ferrimagnetic state decreases which can be seen from the difference in energy between ferromagnetic and ferrimagnetic state (Table 1). Whereas, the local density approximation as proposed by U von Barth and L Hedin [23], reproduces correct ferrimagnetic state, as the ground state, for pure Mn2Sb with magnetic moment of 2.44 and 3.57 µB on MnI and MnII atoms respectively. The net magnetic moment for pure Mn2Sb is found to be 2.13 µB/unit cell (containing two formula units). It was also observed that with the substitution of even 2% (x = 0.02) Bi at Mn site, the magnetic moment of MnI decreases significantly from 2.44 µB to 1.36 µB whereas the magnetic moment on MnII atom changes only slightly from 3.57 µB to 3.52 µB. This leads to a rise in the net magnetic moment on unit cell, which is almost doubled and found 4.40 µB/unit cell. With further rise in concentration of Bi at MnI site there is no significant change in saturation magnetization of the unit cell up to a studied concentration of x = 0.06. Such a rise in saturation magnetization is also observed in our experimental results. It is worth mentioning here that our calculations can be validated further by a detailed study on the magnetic structure of Bi-substituted Mn2Sb by techniques such as neutron diffraction. To further understand the stability of the Bi doped Mn2Sb compounds, magnetic relaxation studies were undertaken. The relaxation behavior of magnetic materials is characterized by a time constant (τ), which is influenced by both the environment of the particle and the properties of the particle itself. It is also dependent upon temperature. The parameter τ describes the period of time over which the magnetic systems approaches equilibrium or a steady-state. It has been reported that there are two distinct behaviors of relaxation; either through the nucleation of few domains followed by a rapid growth of the magnetic domains, or through the nucleation of many domains at random positions. The magnetic relaxation was measured after an application of saturating field of 10 kOe, keeping it on for a wait time of 300 s, and then switching it off to measure the evolution of remanent magnetization as a function of time. The temporal profiles of magnetic relaxation can be represented by a form:
Fig. 6. Magnetization (M) versus temperature (MT measurement curve) for Mn2Sb and Mn2Sb:Bi melt spun ribbon, measured at a field of 1 kOe. Inset shows the Tc obtained from the plot of dM/dT versus temperature.
M (t ) = Mr + Mrl e−(t / τ ) 32
β
(2)
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Table 1 Magnetic moment and the total energy for different concentration of Bi in Mn2Sb lattice, calculated by GGA and LDA. X
0.0
0.02
0.04
0.06
GGA
Energy (Ry) MnI (µB) MnII (µB) Total Moment Energy (Ry) MnI MnII Total Moment Energy (Ry) MnI MnII Total Moment Energy (Ry) MnI MnII Total Moment
(µB/u.c)
(µB/u.c)
(µB/u.c)
(µB/u.c)
LDA
Ferromagnetic
Ferrimagnetic
ΔE = EFerro − EFerri (mRy)
Ferromagnetic
Ferrimagnetic
ΔE = EFerro − EFerri (mRy)
−35203.8121 3.0525 3.6860 13.1204 −36837.2862 1.9299 3.5720 10.6815 −38471.0945 1.9433 3.5732 10.6008 −40104.9028 1.9576 3.5739 10.5195
−35203.7443 2.8727 3.9197 2.1319 −36837.2435 2.1712 3.8477 3.4396 −38471.0559 2.1538 3.8470 3.4351 −40104.8681 2.1418 3.8448 3.4152
−67.8124
−35171.7792 2.3006 3.2521 10.8272 −36804.6078 1.7414 3.3605 9.9928 −38437.7376 1.7245 3.3565 9.8498 −40070.8672 1.7033 3.3522 9.6983
−35171.7829 2.4457 3.5767 2.2840 −36804.6293 1.3635 3.5192 4.4028 −38437.7626 1.3527 3.5091 4.3512 −40070.8956 1.3352 3.4990 4.3135
3.7163
−42.6169
−38.5514
−34.6629
21.5358
25.0534
28.4054
deduced by GGA calculations. Mn2Sb:Bi exhibits appreciable coercivity value of 3.4 kOe (279.2 kA/m) and magnetization of 27.2 emu/g (27.2 Am2/Kg). The high value of coercivity obtained is attributed to the nucleation mechanism achieved via inclusion of diamagnetic Bi particles which obstructs the domain wall motion. It has been observed that the inclusion of dopants in Mn2Sb, hinders the magnetization reversal process, leading to a high value of coercivity. This coercivity enhancement has been achieved employing melt-spinning which also resulted in nanocrystalline microstructure in addition to Bi segregation at the grain boundaries. Hence, the approach of suitable doping along with optimized processing can be employed to design innovative magnetic materials with custom-made magnetic properties. Conflicts of interest Fig. 7. Time evolution of magnetization of Mn2Sb and Mn2Sb:Bi. Eq. (2) is used to fit the data to determine the respective time constants.
There are no conflicts to declare. Author contributions
where Mr is the remnant magnetization; Mrl is the initial part of the relaxing magnetization, τ is the relaxation time constant, and β is an exponent, which reflects the changes of relaxation rate with the evolution of time. Fig. 7 shows the experimentally obtained curves of M-t for Mn2Sb and Mn2Sb:Bi. These curves were fitted using Eq. (2), to determine the time constant. It clearly shows that the relaxation is slower in Mn2Sb:Bi, with a relaxation time constant of 4474 s as opposed to the value of 2690 s for Mn2Sb. This could be explained as the relaxation at constant applied field and temperature proceeds through the nucleation and propagation of domain wall-like structures [28]. Also, it was observed that the relaxation in Mn2Sb:Bi is slowing down whereas it is speeding up for Mn2Sb, as indicated by the value of β which is less than 1 for Mn2Sb:Bi (0.8659) and greater than 1 for Mn2Sb (1.6179). The magnetic relaxation is dominated by larger spin domains at later times, giving rise to slowing down of relaxation for Mn2Sb:Bi. The long relaxation times for Mn2Sb:Bi reflect the existence of strong coupling among the magnetic domains, which is due to the appropriate doping by Bi.
NS supervised the overall work, conceived the problem related experiments and analyzed the results. JK carried out the DFT calculations and NS, KA and NC performed the experimental work. NS and KA wrote the manuscript. All the authors contributed to the interpretation of the results. Funding This work was carried out under CSIR (India) Network Project PSC0109. Acknowledgements This work was carried out under CSIR (India) Network Project PSC0109. KA and NC acknowledge Council of Scientific and Industrial Research (CSIR), India for financial assistance. Authors would like to thank Radhey Shyam and Naval Kishor for their technical support. References
4. Conclusion
[1] O. Beckman, L. Lundgren, Handbook of Magnetic Materials, Elsevier, 1991, p. 6. [2] M.K. Wilkinson, N.S. Gingrich, C.G. Shull, The magnetic structure of Mn2Sb, J. Phys. Chem. Solids 2 (1957) 289–300. [3] V.M. Ryzhkovskii, N.D. Zhigadlo, I.L. Pashkovskii, Crystal structure and magnetic properties of some Mn2Sb-based alloys, Cryst. Res. Technol. 25 (1990) 165–169. [4] R.W. Houghton, W. Weyhmann, NMR Study of the spin-reorientation behavior of Mn2-xCrxSb, Phys. Rev. Lett. 20 (1968) 842–844. [5] M. Suzuki, M. Shirai, K. Motizuki, Electronic band structure in the ferrimagnetic
To conclude, the structural and magnetic changes induced by Bi doping in Mn2Sb system were studied. Doping Bi in Mn2Sb, leads to significant changes in the crystallographic structure, with remarkable effects on the magnetic properties of the compound. From the first principle calculations, it was inferred that Bi occupies both the MnI and MnII site, which leads to ferromagnetic state of the compound as 33
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magnetic materials, J. Mater. Chem. C 5 (2017) 11832–11836. [17] M.F. de Campos, The coercivity mechanisms in Sm(CoFeCuZr)z nanocrystalline magnets: nucleation x pinning, Mater. Sci. Forum 591–593 (2008) 5. [18] Seiji Miyashita, H. Satoshi, N. Masamichi, Perspectives for high-performance permanent magnets: applications, coercivity, and new materials, Adv. Nat. Sci.: Nanosci. Nanotechnol. (2017) 8. [19] R.S. Turtelli, D. Triyono, R. Grössinger, H. Michor, J.H. Espina, J.P. Sinnecker, et al., Coercivity mechanism in Nd60Fe30Al10 and Nd60Fe20Co10Al10 alloys, Phys. Rev. B. 66 (2002) 054441. [20] Rodríguez-Carvajal J. developments of the program FULLPROF Commission on Powder Diffraction (IUCr). Newsletter, 26 (2001). [21] H. Ebert, D. Ködderitzsch, J. Minár, Calculating condensed matter properties using the KKR-Green's function method—recent developments and applications, Rep. Prog. Phys. 74 (2011) 096501. [22] J.P. Perdew, K. Burke, M. Ernzerhof, Generalized gradient approximation made simple, Phys. Rev. Lett. 77 (1996) 3865–3868. [23] L. Hedin, Barth U. Von, J. Phys. C: Solid State Phys. 5 (1972) 13. [24] < http://elk.sourceforge.net/ > . [25] N.V. Rama Rao, G.C. Hadjipanayis, Influence of jet milling process parameters on particle size, phase formation and magnetic properties of MnBi alloy, J. Alloy. Compd. 629 (2015) 80. [26] R. Ramesh, G. Thomas, B.M. Ma, Magnetization reversal in nucleation controlled magnets. II. effect of grain size and size distribution on intrinsic coercivity of Fe-NdB magnets, J. Appl. Phys. 64 (1988) 6416–6423. [27] D.T. Zhang, S. Cao, M. Yue, W.Q. Liu, J.X. Zhang, Y. Qiang, Structural and magnetic properties of bulk MnBi permanent magnets, J. Appl. Phys. 109 (2011) 07A722. [28] K. Hika, L.V. Panina, K. Mohri, Magneto-impedance in sandwich film for magnetic sensor heads, IEEE Trans. Magn. 32 (1996) 4594–4596.
state of Mn2Sb, J. Phys. Condens. Matter (1992) 4. [6] T. Kanomata, H. Ido, Magnetic transitions in Mn2−xMxSb (M=3d metals), J. Appl. Phys. 55 (1984) 2039–2041. [7] T.J. Swoboda, W.H. Cloud, T.A. Bither, M.S. Sadler, H.S. Jarrett, Evidence for an antiferromagnetic-ferrimagnetic transition in Cr-modified Mn2Sb, Phys. Rev. Lett. 4 (1960) 509–511. [8] T.A. Bither, P.H.L. Walter, W.H. Cloud, T.J. Swoboda, P.E. Bierstedt, New modified Mn2Sb compositions showing exchange inversion, J. Appl. Phys. 33 (1962) 1346–1347. [9] F.J. Darnell, W.H. Cloud, H.S. Jarrett, X-ray and magnetization studies of Crmodified Mn2Sb, Phys. Rev. 130 (1963) 647–655. [10] J.H. Wijngaard, C. Haas, R.A. de Groot, Ferrimagnetic-antiferromagnetic phase transition in Mn2-xCrxSb: electronic structure and electrical and magnetic properties, Phys. Rev. B. 45 (1992) 5395–5405. [11] M. Bartashevich, T. Goto, N. Baranov, V. Gaviko, Volume magnetostriction at the AF– FRI metamagnetic transition in the itinerant-electron system Mn2−xTxSb (T=Co, Cr), Phys. B: Condens. Matter 351 (2004) 71. [12] Y.Q. Zhang, Zhi-Dong Zhang, Metamagnetic-transition-induced giant magnetoresistance inMn2Sb1-xSnx (0 < x < 0.4) compounds, Phys. Rev. B. 67 (2003) 132405. [13] K. Pallavi, R. Rawat, P. Chaddah, Metastability in the ferrimagnetic–antiferromagnetic phase transition in Co substituted Mn2Sb, J. Phys.: Condens. Matter 20 (2008) 022204. [14] C. Kittel, Model of exchange-inversion magnetization, Phys. Rev. 120 (1960) 335–342. [15] S.A. Nikitin, T.I. Ivanova, Y.A. Ovchenkova, M.V. Maslennikova, G.S. Burkhanov, O.D. Chistyakov, The influence of interatomic distances on magnetic ordering in RMnSi compounds (R = La, Y, Sm, and Gd), Phys. Solid State 44 (2002) 4. [16] N. Singh, K. Anand, N. Christopher, A. Bhattacharya, A.K. Srivastava, Anomalous enhancement in magnetization upon interstitial doping due to spin reversal in
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