First experimental evidence of non-collinear spin structure in CaCu2.3Ti3.3Fe1.4O12 quadruple perovskite through low-temperature, in-field 57Fe Mossbauer spectroscopy

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First experimental evidence of non-collinear spin structure in CaCu2.3Ti3.3Fe1.4O12 quadruple perovskite through low-temperature, infield 57Fe Mossbauer spectroscopy Pooja R. Pansaraa, Pooja Y. Ravala, Rabia Panditb, Jagdish Nehrac, Satya N. Doliac, Kunal B. Modia,∗ a b c

Department of Physics, Saurashtra University, Rajkot, 360005, India Department of Physics, I. K. Gujral Punjab Technical University, Jalandhar, 144603, India Department of Physics, University of Rajasthan, Jaipur, 302004, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Calcium-copper-titanate Mossbauer spectroscopy Canted spin structure Hyperfine interaction parameters

A polycrystalline sample of quadruple perovskite oxide, CaCu2.3Ti3.3Fe1.4O12, was synthesized and characterized by zero-field and in-field (H̄ app = 50 kOe) 57Fe Mossbauer spectroscopy at 5 K. The presence of ΔmI = 0 spectral lines in Mossbauer spectrum recorded with H̄ app parallel to the direction of γ-ray transmission provides unambiguous affirmation for the presence of canting angles among individual moments and the direction of netsublattice magnetization. The observed non-collinear spin structure is well supported by the M(H) loop characteristic. The canting angle, θc = 67.8°, determined from Mossbauer spectral analysis is in accordance with the canting angle, θM = 71.2°, calculated from the site magnetic moments. This led to cluster-spin-glass like magnetic ordering well reflected in the M(T) curve. The effects of low-temperature and applied magnetic field on the line shape of the Mossbauer spectra and nuclear hyperfine interaction parameters have also been discussed in detail.

1. Introduction In the simple perovskite (ABO3) derivate quadruple perovskite compounds with general formula AA′3B4O12, A mainly exemplifies a large cation (Ca2+, Cd2+, Sr2+, etc.) and twelve-fold coordinated A′site is occupied by Jahn-Teller Cu2+ or Mn3+ ions. A remarkable tilting (~141°) of the corner-sharing BO6 octahedral network generates fundamentally square-planar coordination for the stabilization of this smaller Cu2+ and Mn3+ ions on the A′- site. This makes the structure of the 2a × 2a × 2a unit cell distinct from that in a simple perovskite, hence responsible for a variety of intriguing and functional properties indispensable from the basic besides applied research perspectives. Calcium-copper-titanate, CaCu3Ti4O12, also belongs to one such family of perovskites. Since the last two-decades, CaCu3Ti4O12 and their isostructural systems in pristine as well as substituted with various metallic cations are well investigated for their fascinating dielectric, electric, photocatalytic, magnetic and photoluminescence properties and applications such as supercapacitor, sensors, memory cell, varistors, etc. [1,2]. In accordance with the powder neutron diffraction investigation on

∗

a polycrystalline sample of quadruple perovskite, CaCu3Ti4O12, Collomb et al. [3] have demonstrated that a non-collinear (canted) spin ordering can describe noted observed magnetic intensity. Unfortunately and surprisingly since then, no attempt has been made to investigate this intriguing phenomenon of canted spin structure in AA′3B4O12 type cubic perovskites. Later on, Kim et al. [4] in their work on singlecrystalline CaCu3Ti4O12 proposed that the non-collinear spin ordering model, as well as a collinear magnetic order along the crystallographic [111] direction, can equally well describe the integrated intensity of magnetic Bragg peaks. They have clearly suggested that elastic neutron diffraction measurements under the magnetic field are essential to determine the spin ordering direction unambiguously. Byeon et al. [5,6] have observed low-temperature magnetic moments significantly smaller than the calculated spin only moments in ferrimagnetic oxide, CaCu3Cr2Sb2O12. They speculated the reduced magnetic moment of ~1.4 μB inferred from the spontaneous magnetization (T = 5 K) which is considerably lower than the anticipated value of 3 μB based on the assumption that the moment of the A′-site (Cu2+) and B-site (Cr3+) are antiparallel to each other. A double exchange mechanism permits a net ferromagnetic moment in perovskite systems in which spin canting is

Corresponding author. E-mail address: [email protected] (K.B. Modi).

https://doi.org/10.1016/j.ceramint.2019.12.268 Received 7 October 2019; Received in revised form 13 December 2019; Accepted 30 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Pooja R. Pansara, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.268

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In the last couple of years (2018–2019), we have investigated the effect of simultaneous replacement of weak magnetic Cu2+ (1 μB) ion and non-magnetic Ti4+ (0 μB) ion by highly magnetic Fe3+ (5 μB) ion in quadruple perovskite CaCu3Ti4O12 with common chemical formulary, CaCu3-xTi4-xFe2xO12 (x = 0.0, 0.1, 0.3, 0.5 and 0.7), on structural, and magnetic properties [32], electronic properties [33,34], defect transformation study by means of positron annihilation lifetime spectroscopy [35]. At the same time, consequences of high-energy ball milling generated strain as well as reduction of particle size, and influence of thermal history, slow-cooling, quenching, and microwave heating, on structural, microstructural, magnetic, electric, optical, and photoluminescence properties including defect structure have also been carried out recently [36–39]. A specific composition, CaCu2.3Ti3.3Fe1.4O12, of the system, CaCu3-xTi4-xFe2xO12 with x = 0.7, was chosen for the present investigation owing to the fact that this composition possesses maximum concentration Fe3+ ions and that exclusively reside on the A′-site. Not only that but also this composition has shown interesting features (i) sudden enhancement in the magnetic moment at 300 K and 5 K (ii) indication of cluster-spin-glass-like magnetic ordering in the M(H) curve and (iii) a broad paramagnetic singlet at 300 K in Mossbauer spectrum significantly different from the other compositions. This scientific background and lack of such investigations have motivated us to carry out zero-field and in-field (H̄ app = 50 kOe) 57Fe Mossbauer spectroscopic measurements at T = 5 K on a typical quadruple perovskite composition, CaCu2.3Ti3.3Fe1.4O12. The first experimental evidence of canted spin structure in the system has been observed successfully which is well supported by our magnetization measurements. The effects of low-temperature and applied magnetic field on the line shape of Mossbauer spectra and hyperfine interaction parameters have been discussed in detail.

commonly observed. In contrast, in quadruple perovskite compound, CaCu3Cr2Sb2O12, the double-exchange interactions are totally absent, thus the observed large difference in spontaneous magnetization is unusual in such systems. Very recently, in the case of CaCu3-xTi4xMn2xO12 (x = 0.0, 0.5 and 1.0) system, spin canting is found responsible for the observed lower value of effective moment as compared to the expected spin only moment as well as a sudden increase in the magnetic moment and observed features of FC and ZFC curves for the substituted compositions [6]. The same is the case with many other isostructural systems. In the case of cupromanganite, CaCu3Mn4O12, the net magnetic moment of 9 μB calculated assuming ferrimagnetically aligned formal moments is higher than the magnetic moment of 7.1 μB derived from magnetization measurement at 20 K [7]. The impurity effects or defects are suggested causes for this kind of discrepancy. The saturation magnetization value for an ordered perovskite, CaCu3Fe2Sb2O12, is found to be 5.35 μB per formula unit at T = 5 K, smaller than 7 μB per formula unit predicted based on a model of an ideal collinear ferrimagnetic coupling between Fe3+ (S = 5/2) and Cu2+ (S = 1/2) [8]. The experimental value of magneton number, 9.7 μB per formula unit estimated for CaCu3Fe4O12 at 5 K from the M(H) loop characteristic is quite smaller than the ferrimagnetic moment of 13 μB per formula unit calculated assuming magnetic model, Cu2+ (S = 1/ 2↓)3 Fe4+ (S = 2↑)4 [9]. A typical composition of a quadruple iron perovskite system, Ca1-xSrxCu3Fe4O12 with x = 0.4, has a saturation magnetization of ~3.5 μB per formula unit at 5 K, that is much smaller than the theoretically expected magnetic moment of 13 μB per formula unit. They have attributed such difference to the decrease in the ferromagnetic fraction of the charge-ordered phase in a two-phase coexistence state [9]. The high field slope to the magnetization curve is an indication of the presence of significant canting in the composition and also contributes to such a lower value of the magnetic moment. Based on the extensive literature survey, the in-depth experimental study on the non-collinear spin arrangement, canting angle, etc. is highly essential to resolve this questionable area of the scientific interest. Mossbauer spectrometry probes local environments about a nucleus. Moreover, it has also been expected that Mossbauer spectra must be responsive to the local atomic structure at grain boundaries and defects, for instance, dislocations, vacancies, and paramagnetic centers [10,11]. It also provides a unique measurement of electronic, magnetic and structural properties within materials [12] owing to its high-resolution power better than NMR spectroscopy [13]. 57Fe Mossbauer spectroscopy is a multipurpose microscopic technique employed for the wide varieties of Fe-containing systems to explore certain aspects such as (i) valence state and precise concentration of Fe-ion on a particular crystallographic lattice sites [10,14] (ii) nuclear hyperfine interaction parameters such as isomer shift, quadrupole shift/splitting, hyperfine field, linewidth, etc. [15] (iii) type of magnetic ordering including Neel's temperature (temperature at which system transfers from ferrimagnetic to paramagnetic ordering) (iv) specific heat determination [16], (v) direct [17], and indirect check [14,18] of canted spin structure in the system (vi) when several crystallographic phases exist in a57Fe containing materials, it is generally feasible to deduce the phase fraction [19,20] at least semi-quantitatively by Mossbauer spectral analysis, (vii) particle size estimation for nanocrystalline systems etc. [21,22]. To the extent of our knowledge, a couple of research articles are available on 57Fe Mossbauer spectroscopy study on Fe4+ ion-containing ACu3Fe4O12 (A = Ca, Ce, Sr, La, Y, Tb) type quadruple perovskites [23–30]. The compositions are synthesized at high pressure (15–20 GPa) and elevated temperature (1273–1373 K) conditions and Mossbauer spectra are carried out in a 4–400 K temperature range. Throughout the discussion, specific emphasis is given to study the electronic phase transformations namely disproportionation of charge and the transfer of charge mechanisms along with valence states of Feion. Low-temperature, in-field (magnetic field applied parallel to γ-ray emission) 57Fe Mossbauer measurements effectively reveal non-collinear spin arrangement in the system [31].

2. Experimental details The experimental particulars concerning the preparation of the bulk polycrystalline composition, CaCu2.3Ti3.3Fe1.4O12, by customary double sintering ceramic route, stoichiometry confirmation by energy dispersive analysis of X-rays, structural and microstructural characterizations by means of powder X-ray diffractometry and scanning electron microscopy, and magnetic properties study by dc magnetization, M(H) loop characteristics and 57Fe Mossbauer spectroscopy at 300 K are given in our earlier communications [32,33,35]. The Rietveld analysis of the X-ray powder diffraction pattern of the composition, CaCu2.3Ti3.3Fe1.4O12, recorded at 300 K revealed that the composition possesses monophasic cubic perovskite crystal symmetry having space group Im3 (No. 204) and point group Th. The Rietveld refinement derived cation distribution for the composition is given by: A 2+ 4+ 3+ A 4+ 3+ 2+ B (Ca2+ 1.0 ) [Cu1.53 Ti0.12 Fe1.35] ′ {Ti3.18 Fe0.05 Cu0.77} O12

(1)

The details regarding the refinement process, phase analysis, and cation distribution determination are given elsewhere [33]. Mossbauer spectra at T = 5 K without magnetic field and with the applied magnetic field (H̄ app = 50 kOe) parallel to the gamma (γ)-rays direction were recorded in transmission mode employing a constant acceleration drive and a personal computer analyzer based Weissel velocity drive. Low temperature-high field 57Fe Mossbauer measurements were performed using superconducting magnets (Janis Research Company Inc., USA). The optimum weight of absorber was roughly 10 mg/cm2 of natural iron. The radioactive source was 57Co in Rhmatrix. The metallic iron spectrum was used for the calibration of both observed velocities and hyperfine magnetic fields. Quantitative analysis of the Mossbauer spectra was carried out by WINNORMOS software.

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Fig. 1. Mossbauer spectrum of quadruple perovskite, CaCu2.3Ti3.3Fe1.4O12, recorded at 300 K [32].

3. Results and discussion 57 Fe Mossbauer spectrum taken at 300 K for the composition, CaCu2.3Ti3.3Fe1.4O12, is displayed in Fig. 1, which shows a broad paramagnetic singlet. This is quite expected because the magnetic ordering temperature (TB = 55 K) (Fig. 2(a)) for this composition is much lower than 300 K [32]. The paramagnetic singlet is also attributed to superparamagnetic grains in the material [16,40] which is in good agreement with the zero coercivity observed in the M(H) curve at 300 K as shown in Fig. 2(b). The hyperfine interaction parameters, isomers shift (IS), quadrupole splitting (QS), line width (Γ), thus determined [32], are given in Table 1. Based on the percentage population of Fe3+ ions, the concentration of ferric ion on the respective crystallographic sites has been calculated. It was found that Fe3+ ions prefer to reside at the A′-site only. The distribution of cations proposed for the composition under investigation is given by Ref. [32]: A 2+ 3+ A 4+ 2+ B −2 (Ca2+ 1.0 ) [Cu1.6 Fe1.4 ] ′ {Ti3.3 Cu0.7 } O12

(2)

57

Zero-field and in-field Fe Mossbauer spectroscopy is a very powerful and functional characterization technique for the examination of the local surroundings of iron atoms and the study of dynamic effects. Mossbauer spectra recorded for the composition CaCu2.3Ti3.3Fe1.4O12 at T = 5 K without magnetic field and with the applied magnetic field (H̄ app) of 50 kOe are shown in Fig. 3(a) and (b). In the figure, the dots present the experimental data points and the solid line through the data points is the result of computer fits of spectra obtained assuming equal line width for the A′- site and B-site of the cubic perovskite structure. The spectra exhibit two well-resolved Zeeman split sextets; one owing to Fe3+ ions at A′-site and other because of Fe3+ ions at the B-site, which stipulate ferrimagnetic response of the sample. The quite satisfactory goodness of fit (χ2) values for the spectra, zero-field (χ2 = 1.56) and in-field (χ2 = 1.49), suggest quality refinement and fitting of the experimental data. On the application of the magnetic field, well split spectral lines have been observed. This can be clarified along these lines. In the presence of a high magnetic field, the effective magnetization of the individual particle tends to align along the applied magnetic field direction. Due to the ferromagnetism in perovskite, the direction of H̄ app is opposite to the hyperfine field at the B-site while it is in the same direction of the hyperfine field at the A′-site, thereby creating an effective separation of overlapped sub-patterns (Fig. 3(b)). The externally applied magnetic field is parallel to the γ-rays direction. It is wherefore presumed that the spectral lines 2 and 5 correspond to ΔmI = 0 transitions, that is, the transition of an atomic nucleus from the ground state to an excited one with no change in the nuclear magnetic quantum number (mI), would be missing as the atomic spin moments on the A′- and B-sites of collinear ferromagnet align parallel and anti-parallel to the applied magnetic field, respectively. Surprisingly, in the resulting spectrum, these lines

Fig. 2. (a) thermal variation of magnetization in field-cooled and zero-fieldcooled modes registered at the applied magnetic field of 100 Oe (b) M(H) loop characteristic at T = 300 K (c) M(H) loop characteristic for CaCu2.3Ti3.3Fe1.4O12 composition at T = 5 K [32].

are present with a substantial intensity (Fig. 3(b)). This observed feature unambiguously suggests the evidence for the non-collinear spin structure. We have observed equal contribution to this ΔmI = 0 lines correspond to the A′-site and the B-site Fe3+ spins. The most likely explanation is that Fe3+ ions are separated into two sets with spins directed at an angle to each other and with the spins of Cu2+ ions

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Table 1 57 Fe Mossbauer parameters of quadruple perovskite CaCu2.3Ti3.3Fe1.4O12. T(K)

H̄ app (kOe)

Site

ISa

QSb

Γ

H̄ hf ± 0.7 (kOe)

Area ( ± 1%)

0.91 0.560 0.519 0.685 0.504

– 452.5 512.0 450.0 534.0

100 79 21 76 24

± 0.01 mm/s 300 5

– –

5

50

a b

A′ A′ B A′ B

0.32 0.395 0.450 0.388 0.520

0.31 00.00 00.00 00.00 00.00

With respect to Fe-metal. QS: quadrupole shift for magnetic sextet at 5 K.

directed anti-parallel to the resultant. In pure CaCu3Ti4O12, Cu spin is coupled antiferromagnetically through Cu↑-O-Ti-O-Cu↓ superexchange pathway. On simultaneous replacement of Cu2+ and Ti4+ ions by Fe3+ ions in the composition, CaCu2.3Ti3.3Fe1.4O12, switches the Cu spins to align ferromagnetically but antiferromagnetically with Fe-spins in a cooperative fashion, Cu↑-O-Fe↓-O-Cu↑. The Fe3+ and Fe3+ spins at the B-site align ferromagnetically. The competition between the antiferromagnetic ordering and ferromagnetic ordering leads to a canted spin structure. With a view to describe the phenomenon of spin canting and its correlation with specific lines in the Mossbauer spectrum, the following situation needs to be assumed. The effective magnetic field (H̄ eff) at the 57 Fe nucleus is given by a vector summation of the hyperfine magnetic field (H̄ hf) and externally applied magnetic field (H̄ app): H̄ eff = H̄ hf + H̄ app

Fig. 4. The resultant magnetic field (H̄ eff) at the nucleus of the iron atom.

Accordingly, it turns out to be: H̄ hf2 = H̄ eff

2

- 2 H̄ eff H̄ app + H̄ app2

(4)

The direction of H̄ hf is although, anti-parallel to the orientation of the magnetic moment of the iron atom. Since it is crucial to ascertain the spin canting angle theoretically, an experimental investigation through neutron diffraction [41] or any other technique is highly essential. The average canting angle, θc, is then calculated from the following formula [42]:

(3)

The H̄ eff is thus inclined at the angle θc to the γ-ray direction (Fig. 4).

Fig. 3. (a) 57Fe Mossbauer spectrum recorded at T = 5 K (b) Mossbauer spectrum recorded at T = 5 K with an external magnetic field of 50 kOe parallel to the direction of propagation of the gamma-rays. The circles represent the observed data and the fine line represents fitting results. 4

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θc

1/2

( ) ( ) ⎤⎥ ( ) ( ) ⎥⎦

⎡ 1.5 II2 / II1 5 6 = Sin−1 ⎢ ⎢ 1 + 0.75 I2 / I5 ⎣

I1 I6

increased magnetic moment for this composition (Fig. 2(b) and (c)). Above all, the non-collinear or ferrimagnetic/ferromagnetic clusters formation at the cost of the antiferromagnetic state of pristine CaCu3Ti4O12 is apparent from the isothermal magnetization measurements at 5 K (Fig. 2(b)). This may reveal the existence of a magnetic cluster phase in the sample. This is because of the highly magnetic Fe3+(5 μB) ions being isolated from non-magnetic Ti4+ ions and weak magnetic Cu2+ (1 μB) ions, which hinders the long-range interactions i.e. clustering. Further, an anomaly noticed at blocking temperature, TB = 55 K, demonstrates that the magnetic order consists of coexistence of long-range order and short-range (clustering) order at all temperatures up to a temperature of irreversibility (Tirr). Short-range order gets transformed into a longrange order at TB. The existence of cluster spin glass magnetic ordering suggested by a previous study discussed above [32] is well supported by the present findings. The canting angle determined from Mossbauer spectra analysis and magnetization data is found to be much greater than 60°. If the canting angle is more than 60°, it is rather anticipated that the transverse component is substantially longer than the longitudinal component. Consequently, there is a successive increase of spin-spin correlation on lowering the temperature and which supports cluster spin-glass-like magnetic ordering. 57 Fe Mossbauer spectroscopy is a sensitive probe for numerical investigation of oxidation state at the restricted environment of the Mossbauer nuclei by means of hyperfine interaction parameters, for instance, isomer shift (IS), quadrupole shifting/quadrupole splitting (QS), magnetic hyperfine field (H̄ hf), and linewidth (Γ), etc.. The change in S-electron density by any means at the Mossbauer nucleus is reflected in isomer shift or chemical shift (IS) value. For α-Fe (~0.15 mm/s at T = 300 K), ferrous ion, Fe2+(0.90–1.06 mm/s at 300 K and 1.7–1.26 mm/s at T = 4.2 K) Fe5+ (0.05 mm/s at T = 4.2 K), Fe4+ (0.07 mm/s at T = 300 K) are reported in the literature [32,44]. In the present investigation, IS values vary between 0.39-0.52 mm/s at 5 K corresponding to the A′- and B-site of zero-field and in-field Mossbauer spectra (Table 1). These IS values suggest that Fe-ion remains in the +3 state only. The IS values at 5 K are higher than the IS value at 300 K, which can be explained based on the second-order Doppler shift or temperature shift which arises out of the correlative motion of the emitting nuclei and contingent on temperature. The reason for this difference between IS value at 300 K and 5 K can be attributed to the marginal overlap amongst the 57Fe nucleus and S-electron wave function at low temperatures, and therefore the free thermal energy is not sufficient to ease the transfer of charge from 4d to 3S orbital [13]. The observed changes (increase or decrease) in IS value at 300 K and 5 K, without magnetic field and with the applied magnetic field, do not imply any intrinsic valence changes (decrease or increase) in the Feions but tells about changes in the Fe3+- O−2 bonding character. On application of the magnetic field, the IS value increases for the A′-site and decreases for the B-site (Table 1) that suggests the Fe3+- O−2 covalency decreases and increases for the A′-site and B-site respectively [25]. The elongation of the Fe3+- O−2 bond decreases the overlap of Fe 3d- O 2p orbitals resulting in less covalent (or more ionic) character of the Fe3+- O−2 bonds. Owing to the inhomogeneous electric field at the nucleus departure from an ideal cubic symmetry arises. Quadrupole shifting (QS) is a quantitative estimate of the degree of such derivation around the Mossbauer nucleus. No observable QS (i.e. QS ≈ 0) is found for both, without-field and with-field Mossbauer spectra registered at 5 K due to the concurrence of chemical disorder and overall cubic symmetry between Fe3+ ion and its surrounding comprising Ca2+, Cu2+ and Ti4+ ions on the A′-site and B-site. This is an outcome of one or the other summation of the quadrupole interactions for indiscriminate distributions of the magnetic hyperfine field vector (along < 100 >) and the main axes of the electric field gradient (EFG) (along < 111 >) tensor or a constant angle, α, equals to 54.7° which causes the term (3cos2α-1) existing in the expression for the quadrupole shift to vanish [42].

(5)

The ratios, I2/I5 and I1/I6, are the line intensities from each subspectrum (i.e. the area of the peaks in a magnetic sextet) (Fig. 3(b)). A significant canting, θc = 67.8°, corresponds to both sub-spectrum in Mossbauer spectrum of CaCu2.3Ti3.3Fe1.4O12 composition is found. This suggests that the spins located on the two sublattices are found to behave similarly in the presence of a magnetic field of 50 kOe applied externally. The knowledge of Fe3+-ion distribution and resultant distribution of other cations amongst the crystallographic sites, A, A′- and B, is very useful to explain many physical properties of the system. Here, the accuracy of Fe3+-ion distribution is not limited by difference in ratio of recoilless fraction at the A′-site i.e. ƒA′ and the B-site i.e. ƒB, since ƒA′/ƒB is supposed to be 1 at 5 K [13]. To determine the distribution of Fe-ions from zero-field and in-field Mossbauer spectra, the area ratio of the A′site and the B-site spectra is calculated (Table 1), which is then directly connected to the concentration of Fe3+ ions on the respective crystallographic sites and distribution of cations (Ca2+, Cu2+, and Ti4+) among the A, A′ and B-sites is proposed as given by: A 3+ 2+ A 3+ 2+ 4+ B T = 5 K, H̄ app = 0 kOe: (Ca2+ 1.0 ) [ Fe1.10 Cu1.90] ′ {Fe0.30 Cu0.40 Ti3.30} −2 O12 6(a) A 3+ 2+ A 3+ 2+ T = 5 K, H̄ app = 50 kOe: (Ca2+ 1.0 ) [ Fe1.06 Cu1.94] ′ {Fe0.34 Cu0.36 4+ B −2 Ti3.30} O12 6(b)

Fig. 2(c) shows the plot of magnetic moment (emu/g) versus magnetic field (Oe) (M(H) loop) in the range of ± 70 kOe, recorded at 5 K. The M(H) loop indicates that the composition CaCu2.3Ti3.3Fe1.4O12 shows the onset of ferromagnetism. The high field slope (the lack of saturation magnetization) to the magnetization curve manifests canted spin structure and the canting angle changes with an applied magnetic field [43]. The value of saturation magnetization (σs) through an approximation of Stener- Wohlfarth theory by extrapolating the magnetization against the 1/H2 curve to approach zero (i.e. H̄ = ∞) [36] at 5 K is found to be 15 emu/g. That successively used to workout magneton number, nB (μB) (saturation magnetization per formula unit in Bohr magneton (μB)), using the following formula: nB (μB) = σs × MW/ 5585, where Mw is the molecular weight of the composition. The nB value has been found to be 1.65 μB. To check the extent of canting in the quadruple perovskite, CaCu2.3Ti3.3Fe1.4O12, the average canting angle (θM) is calculated considering the experimental value of magnetic moment at 5 K by employing the two sub-lattice model of ferrimagnetism [43]: nB (μB) = MA′ Cos θM - MB

(7)

where MA’ and MB are the A′ and B- sub-lattice magnetization deduced from the cation distribution formula (Equation (1)). The calculated value of the canting angle using this model is found to be, θM = 71.2°. The observed difference between the canting angle calculated from the in-field Mossbauer spectral analysis and magnetic moment is attributed to the different techniques used, M(H) study being bulk technique while Mossbauer spectroscopy is a microscopic technique. Fig. 2(a) displays the thermal variation of magnetization registered in field-cooled (FC) and zero-field cooled (ZFC) modes in an externally applied magnetic field of 100 Oe for the composition, CaCu2.3Ti3.3Fe1.4O12. The FC- ZFC data sets show ramifications for the whole range of temperature and the absence of a characteristic peak corresponding to antiferromagnetic ordering in FC data. The substitution of Fe3+ - ion on Cu2+ and Ti4+- sites seemingly destroys the longrange antiferromagnetic ordering. According to Pal et al. [6], such characteristics can be associated to the magnetic phase separation, where the presence of Fe3+ results in local nonpercolated canted antiferromagnetic state, and such spin canting could be the origin of the 5

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field components and spin canting. The changes in linewidth reflect the corresponding changes in the environment of Fe3+ ions residing at the A′- and B-sites. On the application of magnetic field linewidth, (Γ), for the A′-site increases while for the B-site decreases slightly. Further, ΓA′ is greater than ΓB at 5 K in the absence and presence of a magnetic field (Table 1). This is probably owing to the increased distribution value of hyperfine field (field gradients) for the A′-site in contrast to that of the B-site, ensuing from the B-site magnetic enfeebling by small concentration of highly magnetic Fe3+ ions (5 μB) being isolated by non-magnetic Ti4+ ions (0 μB) and weak magnetic Cu2+ ions (1 μB).

However, when Fe3+ ions are situated in a non-cubic circumambient, it shows the larger quadrupole splitting of the order of ~1.5 mm/s [32]. At T = 4.2 K, QS values for Fe2+ and Fe3+ ions are reported to be 2.02–2.07 mm/s and 0.035 mm/s respectively [44]. Fe3+ ion in the octahedral coordination exhibits high QS value than in the tetrahedral coordination [45]. In general, the QS values for Fe3+ containing fcc structured spinel ferrites and bcc structured garnets vary from 0.35 to 0.65 mm/s. For the certain biochemical compounds, a large QS value of the order of 2.2–4.2 mm/s has also been reported [46]. At low-temperature T = 5 K, (T ≪ 300 K) the iron ion is randomly positioned in either of the environments, energetically separated by a potential barrier, with the Wyckoff notation 6b (A′-site) and 8c (B-site). In quadruple perovskite structure (AA′3B4O12), the structure of 2a × 2a × 2a unit cell is dissimilar from that in simple perovskite (ABO3). At adequately high-temperature (T = 300 K) iron has sufficient energy to surmount the potential barrier with jump frequency higher than the inverse of the lifetime (≈107/s) of the meta-stable 14.4 keV state of 57Fe. It seems that this model is perhaps accountable for nearly zero value of quadrupole shift for Mossbauer spectra recorded at 5 K with and without magnetic field and the observed large quadrupole splitting value (0.31 ± 0.01 mm/s) for Mossbauer spectrum registered at 300 K. Such large QS value most plausibly demonstrates a strong anisotropy of the square-planar coordination (A′ - site). The high field low-temperature Mossbauer study is mandatory to make affirm calculations for the quantitative dispersal of the hyperfine field (H̄ hf) at the A′- and B-sites required for the exhaustive examination of magnetic interactions in such materials. A very small variation in the hyperfine field value for the A′-site (~2.5 kOe) is observed while the Bsite enhancement in hyperfine field value on the application of the magnetic field is quite large (~22 kOe) (Table 1). The A′-site hyperfine fields are normally 10% lower than the hyperfine fields of the B-site and the discrepancy is generally accredited to covalency. Such changes refer that the hyperfine field at the A′-site is not responsive to any changes in the concentration and type of metallic cations reside at the B-site, on the contrast, the B-site hyper field is responsive to any such changes on the A′-site. The super transferred hyperfine (STH) interactions induced non-dynamical effects are responsible for the changes in the hyperfine field. The STH field components are highly affected by the super-exchange coupling and magnetic moments of cations around Fe-ions. In quadruple perovskite structure, Fe3+ ions on the A′-site have persistent super-exchange coupling with the neighboring cations available on the B-sites. The change in the cations distribution does not provide adequate percentage modification in super-exchange interaction to produce noteworthy variation in the resultant magnetic moment and thus STH, which may be accountable for the noticed negligible change in the hyperfine field value at the A′-site. At the same time, each Fe3+ ion situated at the B-site is coupled via superexchange coupling to the neighboring metallic cations present at the A′-site. Thus, any change in the concentration of Fe3+-ion on the B-site is commonly anticipated to bring appreciable changes in STH and hence in hyperfine field value too. It is interesting to note that change in the cation distribution on the application of the magnetic field is very small thus above discussed cation disorder alone is not enough to figure out the observed variation in the hyperfine field at the A′- and B-sites. It seems that a non-collinear spin arrangement that produces an apparent hyperfine field distribution, under the influence of an external magnetic field, need to be taken into consideration. The STH field components are supposed to rely upon this spin canting and this canted spin structure will not allow STH field components to contribute effectively. The dissemination of the hyperfine field arises out of the competition between STH field interactions and the effects emerging from the external magnetic field. Owing to consequential canting, the nucleus continually experiences a smaller component of an external magnetic field, subsequently, resulting in a narrow hyperfine field distribution. Accordingly, a small change in the hyperfine field at the A′-site on the application of magnetic field can be interpreted in virtue of effects coming into existence from both STH

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