Near room temperature spin reorientation and temperature evolution of magnetic structure in Mn substituted HoFeO3

Journal Pre-proofs Near room temperature spin reorientation and temperature evolution of magnetic structure in Mn substituted HoFeO3 Karthika Chandran, Pulkit Prakash, P. Neenu Lekshmi, Amitabh Das, P.N. Santhosh PII: DOI: Reference:

S0304-8853(19)32680-0 https://doi.org/10.1016/j.jmmm.2020.166415 MAGMA 166415

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Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

3 August 2019 31 December 2019 7 January 2020

Please cite this article as: K. Chandran, P. Prakash, P. Neenu Lekshmi, A. Das, P.N. Santhosh, Near room temperature spin reorientation and temperature evolution of magnetic structure in Mn substituted HoFeO3, Journal of Magnetism and Magnetic Materials (2020), doi: https://doi.org/10.1016/j.jmmm.2020.166415

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Near room temperature spin reorientation and temperature evolution of magnetic structure in Mn substituted HoFeO3 Karthika Chandran1, Pulkit Prakash2, P. Neenu Lekshmi1, Amitabh Das2,3, P. N. Santhosh1* 1Department

of Physics, Indian Institute of Technology Madras, Chennai 600036, India

2Solid

State Physics Division, Bhabha Atomic Research Centre, Mumbai 400085, India

3Homi

Bhabha National Institute, Mumbai 400094, India

Abstract Magnetic structure and spin reorientation transition of 45% Mn substituted HoFeO3 have been investigated using bulk magnetization, neutron powder diffraction and specific heat capacity techniques. These studies confirm the presence of antiferromagnetic ordering of Fe/Mn sublattice below 336 K. The spin reorientation (SR) transition (TSR~290 K) where the magnetic structure changes from 4(Ax, Fy, Gz) to 1(Gx, Cy, Az) is confirmed by Rietveld analysis of neutron diffraction patterns taken at various temperatures. Though the spin reorientation (SR) transition is happening at 290 K, the system is found to be in mixed 4(Ax, Fy, Gz) + 1(Gx, Cy, Az) phase from room temperature to 275 K. Below 275 K, the sample exhibits a pure 1(Gx, Cy, Az) structure in which the weak ferromagnetic component is absent. Bulk magnetization and specific heat capacity measurements also confirm the Néel temperature and spin reorientation transitions and support the neutron diffraction analysis. Keywords: Spin reorientation, neutron powder diffraction, magnetic structure, Rietveld analysis. *corresponding author: Santhosh P. N Email: [email protected]

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Introduction The recent development in material science and engineering has increased the demand for functional materials with interesting properties like multiferroicity, magnetocaloric effect, magnetoresistance and thermoelectricity. The materials which exhibit temperature, field and light-induced magnetic structural changes are of importance for various spintronics applications. Given these, rare earth orthoferrites have been recently studied extensively for their ultrafast spin dynamics [1], improper multiferroicity [2], magnetocaloric effect [3, 4], exchange bias [5], spin switching [6], temperature and magnetic field induced spin reorientation transition [7, 8]. In RFeO3 compounds with magnetic rare earth ions, three different magnetic exchange interactions lead to complex magnetic properties. The strongest Fe3+-Fe3+ interaction leads to a canted antiferromagnetic (c-AFM) structure due to the presence of Dzyaloshinskii-Moriya interaction [9, 10]. The magnetic structure of the RFeO3 below the Néel temperature (typically in the range of 650 K to 750 K) is mostly identified as a G-type antiferromagnetic structure with a weak ferromagnetic component. In Bertaut’s notation [12] for the Pnma space group, the magnetic structure can be identified as Г4(Ax,Fy,Gz). At relatively lower temperatures, the antisymmetric and anisotropic symmetric exchange interactions between R3+-Fe3+ lead to the reorientation of Fe3+ weak ferromagnetic moments from b-axis to the c-axis leading to a 2(Cx, Gy, Fz) structure [13]. In most of the orthoferrites with magnetic R ions, the spin reorientation happens as a continuous rotation of spins from 4(Ax, Fy, Gz) to 2(Cx, Gy, Fz) structure except for DyFeO3 where the spin reorientation is a sharp first order Morin transition from 4(Ax, Fy, Gz) to 1(Gx, Cy, Az) [14]. The weakest interaction among the three, R3+-R3+ exchange interaction comes into the picture at very low temperatures (<10 K) and decides the ground state magnetic structure of the material. The relative arrangement of R3+sublattice spins w.r.t that of Fe3+sublattice spins lead to magnetization reversal and exchange bias in some rare earth orthoferrites [15, 5]. On the other hand, RMnO3 are well-known multiferroics with a good magnetoelectric coupling (e.g., DyMnO3~2500µC/m2 at 2 K for 2 T) [16, 17, 18]. They also exhibit different properties like orbital ordering, Giant magnetoresistance and magnetocaloric effect, arising from the interplay of spin, charge and lattice degrees of freedom [19, 20]. The Mn substitution in the B site is found to affect the magnetism of RFeO3 interestingly. Spin reorientation transition is not observed in RFeO3 materials with non-magnetic R ions. However, in these materials, the substitution of Mn in the Fe site is found to be effective in

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inducing a spin reorientation and also in tuning the transition temperature with Mn concentration (e.g. YFe1-xMnxO3) [21]. Whereas in RFeO3 with magnetic R ions, the Mn substitution is very useful in tuning the SR transition temperatures [22, 23, 24]. Density functional theory calculations carried out on Fe/Mn-based double perovskites suggest the presence of ferrimagnetism, half-metallicity and ferroelectricity in these compounds [25]. It is recently understood that changing the Mn doping levels in RFeO3 is beneficial in tuning the Néel temperature and spin reorientation temperature to room temperature [24, 23, 21]. While the magnetic structures of both HoFeO3 [26] and HoMnO3 [27, 28] have been reported by many researchers based on neutron powder diffraction and single crystal polarized neutron diffraction results, the magnetic structure and transitions in mixed Ho(Fe/Mn)O3 systems are not well understood. Based on our previous work and other available reports [23], it is concluded that Mn substitution in HoFeO3 effectively tunes the spin reorientation and Néel temperature. It is also understood that for 45% Mn substitution, both the transitions are observed near room temperature. Spin reorientation transition is not observed in samples with 50% Mn concentration and above. Hence, in this work, 45% Mn substituted HoFeO3 polycrystalline samples are synthesized, and a detailed crystal and magnetic structure analysis have been carried out by using magnetization, neutron diffraction and heat capacity measurements. This study aims to understand the effect of 45% Mn substitution on the magnetic ordering and spin reorientation transition temperatures. Experimental details Polycrystalline samples of HoFe0.55Mn0.45O3 (HFMO45) were prepared by a solid state reaction. The stoichiometric amounts of Ho2O3, MnO2 and Fe3O4 were ground for 4 hours in an agate mortar and pestle. The mixed powders were heat treated from 1073 K to 1273 K for 24 hours with intermediate grinding. Finally, the powders were pressed into a cylindrical pellet and sintered at 1373 K for 24 hours. The magnetization measurements were carried out in a SQUID-based vibrating sample magnetometer (Quantum Design). The neutron powder diffraction patterns, at various temperatures between 7 and 300K, were recorded on the PD2 powder neutron diffractometer ( = 1.2443 Å) in Dhruva reactor, Bhabha Atomic Research Centre, Mumbai. The magnetic and crystal structure refinement was carried out by using FULLPROF [29]. The irreducible representations and the basis vectors were generated by using SARAh [30] program. The X-ray photoelectron spectrum

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was recorded in an instrument with Mg-K as the X-ray source, a PHOIBOS 100MCD analyser (SPECS) and the sample maintained in ultra-high vacuum (10-9 mbar). Results and discussion The Rietveld refinement of neutron powder diffraction (NPD) pattern recorded at room temperature is given in Figure 1. The pattern was refined by considering both nuclear and magnetic unit cells. Since Mn3+ and Fe3+ have the same ionic radii (0.645 Å), the Goldschmidt’s tolerance factor is the same (0.858) for all the three compounds (HoFeO3, HoMnO3 and HoFe0.55Mn0.45O3). From the crystal structure analysis, it is identified that the sample HFMO45 crystallizes in an orthorhombic structure with Pnma space group having ab+a- octahedral tilting as given by the Glazer’s notation [31]. The refined crystal structure parameters are given in Table I in comparison with the structural parameters of the parent compounds HoFeO3 [32] and HoMnO3 [33]. HoFeO3 falls under the O type orthorhombic structure where the lattice parameters obey the condition 𝑐 <

𝑏 2

< 𝑎 (for Pnma) while

HoMnO3 (in orthorhombic phase) has an O´-type orthorhombic structure where 𝑏/√2 < 𝑐 < 𝑎 because of the presence of static Jahn-Teller (JT) distortions due to JT active Mn3+. The lattice parameters of HFMO45 obey the condition 𝑐 < 𝑏/√2 < 𝑎 (Table I), making it an O type orthorhombic structure. Comparing the lattice parameters, it is noticed that, the change in HFMO45 lattice parameters with respect to HoFeO3 (Δa = 0.072 Å, Δb = -0.086 Å, Δc = -0.013 Å) is much less than that with respect to HoMnO3 (Δa = -0.172 Å, Δb = 0.164 Å, Δc = 0.013 Å) [Both HoFeO3 and HoMnO3 lattice parameters were converted to the Pnma setting before calculating the difference].

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Figure 1. The Rietveld refinement of neutron powder diffraction pattern of HFMO45 taken at 300 K. In this figure, the symbols indicate the data points, the continuous line through the data points is the fit to the model (discussed in the text) and the continuous line at the bottom indicate the difference ( = Iobs – Icalc ) curve. The three sets of tick marks correspond to Bragg positions representing reflections from nuclear (top) and magnetic (middle Γ4 and bottom Γ1) structures, respectively. In orthorhombic HoMnO3, due to the presence of orbital ordering and Jahn-Teller distortions, the difference between the bond lengths is high. On the other hand, in HoFeO3, the bond lengths are almost same in the ac plane (for Pnma). The geometrical parameters associated with the MnO6 octahedra of HFMO45 derived from the refinement is also compared in Table I. The difference in M-O bond lengths in ac-plane and along the bdirection is an indication of the presence of octahedral distortions. Further, HFMO45 shows bond angle ~ 144o, which is largely deviated from 180o, also points to structural distortion through octahedral tilting. To find the distortion of the orthorhombic unit cell from the ideal cubic perovskite structure, the angle <ω> is calculated from the formula: <ω>=180 ― <θ>where <θ> is the average bond angle . Further, the average octahedral tilt angle [<φ> = (φ1+φ2)/2] with respect to the pseudocubic axis [111]pc is calculated by using the formulas cos θ1 =

2 ― 5cos2 φ1 2 + cos2 φ1

and cos θ2 =

1 ― 4cos2 φ2 3

given by

O’Keefe and Hyde [34], where θ1 and θ2 are the M-O1-M and M-O2-M bond angles, respectively. The values of <ω> and <φ> calculated for HFMO45 fall in between the reported values for non-distorted HoFeO3 and highly distorted HoMnO3. The values deviate from the

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non-distorted structure but are found still closer to HoFeO3 confirming that the distortion is not as much prominent in HFMO45 as in HoMnO3. Further, a quantitative measure of the magnitude of JT distortion is obtained by calculating the octahedral distortion parameter, ∆𝑑 1

6

[

= 6∑𝑛 = 1

𝑑𝑛 ―< 𝑑 > 2 <𝑑>

]

where dn is the individual M-O bond length (M=Fe/Mn) and is the

average bond length. The octahedral distortion parameter 104Δd for HoFeO3 is much less (0.88) compared to that of HoMnO3 (48.9), whereas for HFMO45, the value of 104Δd is calculated as 11.33. In order to further understand the effect of JT distortion, the octahedral (Q2) and tetragonal (Q3) modes of MnO6 distortion are calculated by using the formulas: Q2 = 2(l ― s) √2 and Q3 = 2(2m ― l ― s) √6, where l, m and s are the long (M-O2), medium (M-O1) and short (M-O2) bond lengths, respectively, obtained during structural refinement [33]. Finally, the values obtained for 104Δd, Q2, and Q3 confirm that only a marginal distortion is present in HFMO45 with respect to HoMnO3. Hence, it can be concluded that even though the orbital ordering and cooperative Jahn-Teller distortion are not evident in HFMO45, a weak Jahn-Teller distortion is responsible for the octahedral distortions observed. The random distribution of Mn ions in the B site might be the reason for the weak Jahn-Teller distortion observed in this system. Table I. Results of Rietveld refinement of NPD pattern of HFMO45 taken at room temperature in comparison with HoFeO3 and HoMnO3. Parameter

HoFe0.55Mn0.45O3

HoFeO3

HoMnO3

Space group

Pnma

Pbnm

Pbnm

a (Å)

5.6626(3)

5.2834(1)

5.2572(1)

b (Å)

7.5241(3)

5.5910(1)

5.8354(1)

c (Å)

5.2704(2)

7.6098(1)

7.3606(1)

V (Å3)

224.55(2)

224.79

225.804(4)

M-O1(x2) (Å)

1.9762(1)

1.9990(30)

1.9435(17)

M-O2(x2) (Å)

2.1083(1)

2.037(9)

2.2224(17)

M-O2(x2) (Å)

1.9559(1)

----

1.9046(16)

(Å)

2.0136(1)

2.0180

2.0235(13)

7

M-O1-M

144.28(1)º

144.2º

142.47º

M-O2-M

144.21(1)º

144.7º

144.08º



144.25(1)º

144.45º

143.54º

<ω>

35.76(1)º

35.55º

36.46º

<φ>

21.8º

21.6º

22.4º

104Δd

11.33(3)

0.88

48.90

Q2

0.21(1)

0

0.3944(24)

Q3

-0.091(2)

-0.062(9)

-0.196(2)

Reference

This work

[32]

[33]

Figure 2. Thermal evolution of lattice parameters and volume of the unit cell. The dashed line through the data points is a guide to the eye. The neutron diffraction patterns were collected at various temperatures from 7 K to 300 K, and Rietveld analysis of these patterns confirm the absence of crystal structural

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transition in the measured temperature range. The variation of lattice parameters and volume of the unit cell with respect to temperature is given in Figure 2. The lattice parameters a, b and c follow a similar trend, where the parameters increase with increasing temperature. The percentage increase in b with respect to temperature (0.25 %) is much more than that in a (0.09 %) and c (0.18 %) lattice parameters. The change in volume follows a similar trend as b. The M-O bond lengths calculated from the Rietveld refinement at different temperatures are given in Table II. Table II. The refinement results of NPD patterns of HFMO45 taken at different temperatures.

Parameter

300 K

175 K

50 K

7K

a(Å)

5.6626(3)

5.6606(4)

5.6576(3)

5.6570(3)

b(Å)

7.5241(3)

7.5150(5)

7.5045(4)

7.5054(4)

c(Å)

5.2704(2)

5.2672(3)

5.2629(3)

5.2626(3)

V(Å3)

224.549(17)

224.065(24)

223.449(21)

223.441(21)

M-O1(x2)

1.9763(1)

1.9759(1)

1.9725(1)

1.9735(1)

2.1083(1)

2.1097(1)

2.1078(1)

2.1152(1)

1.9559(1)

1.9519(1)

1.9530(1)

1.9521(1)

104Δd

11.3(3)

11.9(3)

11.7(3)

12.9(3)

Q2

0.21(1)

0.22321(18)

0.21888(15)

0.23078(15)

Q3

-0.091(2)

-0.089(1)

-0.095(1)

-0.098(1)

RBragg(%)

4.65

4.99

4.7

4.35

Rmagnetic(%)

5.58 (Γ4)

4.32

4.99

5.92

For Γ1

Γ1

Γ1

Γ1

c1=0.2696(481)

c1 = 1.2300(136)

c1 = 1.6370(108)

c1 = 1.6865(109)

(Å) M-O2(x2) (Å) M-O2(x2) (Å)

3.54 (Γ1) c-coefficients

For Γ4 c2 = 0.1929(608)

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c3 = 0.7096(178)

The X-ray photoelectron spectroscopy technique was utilized to confirm the oxidation states of the transition metal ions in HFMO45. The XPS spectra corresponding to Fe 2p and Mn 2p are analysed by using CasaXPS software (Casa Software Ltd.). The Fe 2p3/2 and Mn 2p3/2 core level spectra along with the fitted peaks are shown in Figure 3. Both the peaks are fitted in single peak elements with peak centres 711.03 eV and 641.78 eV corresponding to the Fe3+ and Mn3+ oxidation states, respectively [35, 36].

Figure 3. XPS spectra of (a) Fe 2p3/2 and (b) Mn 2p3/2 of HFMO45 along with the fitted peak

(a)

(b)

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(c)

(d)

Figure 4. a) Temperature dependence of magnetization with an applied field of 100 Oe in ZFC, FCC and FCW modes (inset) dM/dT around the transitions, b) Temperature dependence of magnetization at higher temperature region for an applied field of 100 Oe (inset) CurieWeiss fit for inverse susceptibility, c) Isothermal magnetization vs. field curves taken at 300 K and at 5 K and (d) The ferromagnetic component of M vs. H curve taken at 300 K showing weak ferromagnetic nature. The temperature dependent Zero field cooled (ZFC), Field cooled cooling (FCC), and Field cooled warming (FCW) magnetization studies, M(T), taken for HFMO45 under an applied field of 100 Oe is shown in Figure 4. The derivative of M(T) curve portraits two clear transitions around 336 and 290 K. As per previous reports, the transition observed around 336 K is considered as the Néel temperature (TN) where the Fe/Mn sublattice orders antiferromagnetically. Also, the inverse susceptibility vs. T plot [inset, Figure 4(b)] obeys the Curie-Weiss law throughout the high temperature region (360 K to 780 K) and the negative value of Curie temperature (θp= -34.9 K) obtained from the Curie-Weiss fit confirms the presence of predominant antiferromagnetic correlations in HFMO45. The increase in bulk magnetization just below the TN suggests the presence of weak ferromagnetism in the system. The total magnetic moment per unit cell calculated from the C-W fit is 11.52 µB. The theoretical total magnetic moment is calculated by using the formula 𝜇𝑡ℎ𝑒𝑜𝑟𝑦 = 𝑒𝑓𝑓 𝜇2𝑒𝑓𝑓(𝐻𝑜3 + ) + 0.55 × 𝜇2𝑒𝑓𝑓(𝐹𝑒3 + ) + 0.45 × 𝜇2𝑒𝑓𝑓(𝑀𝑛3 + ) where µeff for Ho3+, Fe3+ and Mn3+ are calculated as 10.61µB, 5.92 µB and 4.90 µB respectively. The value (11.75 µB) calculated theoretically by assuming a high spin state for both Fe3+ and Mn3+ agrees well with the experimentally observed value (11.52 µB). XPS data also confirms the trivalent states of

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Fe and Mn. The other transition observed at 290 K can be assigned to the spin reorientation (TSR) transition. Below spin reorientation transition down to 5 K, the M (T) curves show a paramagnetic like behaviour dominated by Ho3+ moments. The magnetism in RFeO3 are generally decided by the three competing interactions:(i) Fe3+-Fe3+, (ii) Fe3+-R3+ and (iii) R3+-R3+ interactions. According to Good enough – Kanamori rules, Fe3+-Fe3+ exchange interaction leads to an antiferromagnetic ordering [37, 38] and the strong Fe3+-Fe3+ interactions in RFeO3 compounds account for the high TN. Further, weak ferromagnetism is also observed in RFeO3 compounds, arising from the canted antiferromagnetic structure (C-AFM). This canting is determined by the antisymmetric exchange interaction or Dzyaloshinskii-Moriya interaction (DMI) [39, 40], given by the term ― 𝐃𝐢,𝐣 ∙ (𝐒𝐢 × 𝐒𝐣), where Si and Sj are the neighbouring spins and Di,j is the Dzyaloshinskii vector. This term is responsible for the canting of spins observed in rare earth orthoferrites [11]. In HFMO45, 45% of Fe3+ ions are replaced by Mn3+ ions. The random distribution of Mn in the Fe sublattice gives rise to three types of interactions (Fe3+-Fe3+, Mn3+-Mn3+ and Fe3+-Mn3+) that make the system to have a complex magnetic phenomenon. Since the Fe and Mn ions are almost equal in number, the mixed Fe3+-Mn3+ interactions are expected to be predominant [21]. Due to the Mn3+ substitution, a much lower TN is observed, which has a value of ~336 K compared to that of the parent compound HoFeO3 with TN~640 K [26]. In RFe1-xMnxO3 with non-magnetic rare earth ions, these three interactions and their temperature dependence introduce spin reorientation in the system [21]. However, for RFe1-xMnxO3 with magnetic rare earth ions, the magnetism gets even more complicated because of the interaction between the R3+ and the Fe/Mn sublattice moments. The long-range ordering of R3+ ions happens at very low temperatures, typically less than 10 K. So, in pure HoFeO3, above Ho3+ ordering temperature the magnetism is decided by the Fe3+-Fe3+ and Fe3+-Ho3+ interaction. Since Mn substitution dilutes the Fe sublattice, it will influence the spin reorientation transition too. In HFMO45, the spin reorientation has been shifted to 290 K from 55 K observed in HoFeO3 [26]. The isothermal magnetization plots, M (H) obtained at 5 and 300 K are shown in Figure 4, which exhibits a purely linear behaviour at room temperature. The paramagnetic component was calculated by fitting the high field magnetization values with a straight line and by multiplying the field with the slope. This paramagnetic contribution was subtracted

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from the overall magnetization to find the ferromagnetic contribution [41]. The ferromagnetic component calculated for M(H) curve taken at 300 K is shown in Figure 4 (d). There is a small hysteresis and remanent magnetization observed in the FM component of M vs. H curves as an indication of the weak ferromagnetic behaviour at room temperature [Figure 4(d)]. This analysis suggests that the system transfers from a C-AFM with a weak ferromagnetic component to an antiferromagnetic structure with no net ferromagnetic moments below the spin reorientation temperature. Since it is not possible to conclude about the nature of magnetic transitions based on bulk magnetization measurements, a detailed magnetic structure analysis has been carried out based on neutron diffraction measurements. To further investigate the observed magnetism, the specific heat data of HFMO45 is taken in a wide temperature range and is shown in Figure 5. The specific heat data corroborates well with the magnetization data, and the enlarged view, given in Figure 5 [inset (b)] shows anomalies around magnetic transitions TN and TSR. This agrees well with the magnetization and the neutron diffraction results. Our low temperature specific heat data shows a broad peak centred at 4.5 K, which can be a Schottky anomaly owing to the Ho3+ ions.

Figure 5. Specific heat capacity of HFMO45 [inset (a)] low temperature specific heat data showing the Schottky anomaly, [inset (b)] enlarged view of specific heat near TN and TSR.

Magnetic structure

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In order to understand the magnetic structure as well as the nature of magnetic transitions observed in dc magnetization, NPD patterns were recorded at several temperatures below 300 K. Selected patterns along with Rietveld refinement results, are shown in Figure 6. The pattern obtained at 300 K shows a superlattice reflection corresponding to antiferromagnetic ordering, confirming the presence of ordered magnetic moments at room temperature. This corroborates the dc magnetization measurements (TN = 333 K). The magnetic unit cell is commensurate with the crystal structure; thus, a k = 0 propagation vector was used for the refinement. The basis vectors corresponding to the irreducible representations were generated using SARAh programme [30].

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Figure 6. Rietveld refinement of NPD patterns of HFMO45 taken at different temperatures. The black circles represent the observed intensity; the red line represents the intensity calculated by FULLPROF, The Bragg positions corresponding to both nuclear (top) and magnetic (bottom) unit cell reflections are given by green symbols, blue lines give the difference between the observed and calculated intensity.

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Usually, orthoferrites are associated with 4 magnetic structure below TN with the magnetic components (Pnma): G-type antiferromagnetic component along the z-direction with a weak ferromagnetic component (WFM) denoted by F along the y-direction arising due to spin canting and a negligible A-type antiferromagnetic component along the x-direction [42]. The nuclear structure was refined well with Pnma at 300 K, but the magnetic structure could not give a satisfactory fit only with 4 alone (Rmagnetic = 8.63%). However, a mixed domain (4+1) magnetic structure gave the best fit (Rmagnetic = 5.58% for 4 and Rmagnetic = 3.54% for 1) at 300 K, assuming only the ordering of Fe/Mn spins in 4c positions. The mixed magnetic phase model (4+1) was observed in the range 300K - 275 K, which might be due to the coexistence of both magnetic phases near the spin reorientation temperature 290 K. The refinement results show that the phase fraction of 4 phase is 87.86 % and that of 1 phase 12.13 % at 300 K. The refinement of NPD pattern at 275 K shows that the moments of 1(Gx) increases and the moments of 4 (Gz) decreases [Figure 7(b)]. The phase fraction of 4 phase 30.7 % and that of 1 phase is 69.3 % at 275 K. This agrees well with the magnetization measurements which shows a spin reorientation at 290 K. This transition is reflected in the relative intensity [Figure 7(a)] of the (110) and (011) reflections observed at all temperatures. Above the spin reorientation transition, the relative intensity I(110)/I(011) > 1 at 300 K, and at 7 K the relative intensity I(110)/I(011) becomes much less than 1. This is also evident from Figure 7(b) where the Fe/Mn magnetic sublattice moments are presented as a function of temperature. Even though the spin reorientation from 4 to 1 is considered to be a first order Morin type of transition [43], there are some domains of 4 found in the 1 matrix below the spin reorientation. As the temperature reduces further, 1 grows in concentration. It can also be seen in the magnetic moment vs. temperature in Figure 7(b). For the diffraction patterns taken below 275 K, the mixed magnetic domains considering both 4(Gz) and 1(Gx) magnetic moments did not give a good fit (Rmagnetic = 7.7 % for the pattern taken at 200 K). So below this temperature, only 1 (Rmagnetic = 4.4 % for the pattern taken at 200 K) phase was considered to be present in the system. The moments arising due to the long-range ordering of Ho3+ ions were also considered (especially at low temperatures) in the refinement. The possible magnetic structure of Ho3+ ions in 4b positions compatible with 1 and 4 structures (not shown here) were considered during refinement, but they did not yield

16

a good fit (Rmagnetic = 6.9 % for the pattern taken at 7K). Thus, NPD analysis confirms the absence of long-range ordering of Ho3+ moments down to 7 K.

(a)

(b) Figure 7. (a) Normalised intensity of (110) and (011) magnetic reflections at different temperatures (b) Fe/Mn moments vs. temperature obtained from the refinement of NPD patterns.

17

(a)

(b)

Figure 8. Schematic representation of (a) 4(AxFyGz) structure and (b) 1(GxCyAz) structure obtained from the refinement of neutron diffraction pattern taken at 300 K. The blue spheres represent Fe/Mn ions; Red arrows represent the spin direction. Based on our unreported work and some earlier studies on Mn substituted HoFeO3 [23] it is understood that the Mn substitution brings down the antiferromagnetic ordering temperature and moves the spin reorientation transition to higher temperatures. 45 % Mn substitution brings down the Néel temperature from 640 K to 333 K. Reduction in Néel temperature is the consequence of the dilution of Fe sublattice by Mn ions which weakens the strong three dimensional Fe3+-O2--Fe3+ interaction. Moreover, Mn substitution also has a huge impact on both the temperature and nature of the spin reorientation transition. The drastic increase in spin reorientation transition temperature from 55 K (in HoFeO3) to 290 K (in HoFe0.55Mn0.45O3) can be understood by considering the different magnetic interactions present in the sample. Spin reorientation is explained as a result of competition between Fe3+Fe3+ long range interactions and Ho3+-Fe3+ exchange interactions in HoFeO3. Since Mn substitution weakens the Fe3+-Fe3+ long range interactions, Ho3+-Fe3+ interaction can overcome the former at much high temperatures, leading to an increase in spin reorientation transition [23]. There are two kinds of spin reorientation transitions generally observed in rare earth orthoferrites: (i) a slow rotation of Fe spins from Γ4 to a Γ2 structure (ii) a sudden jump of Fe spins from Γ4 to a Γ1 structure (Morin transition, DyFeO3). In most of the orthoferrites including HoFeO3, the spin reorientation happens as a continuous rotation from a Γ4 to a Γ2 phase as temperature decreases below spin reorientation [e.g. GdFeO3, ErFeO3 and HoFeO3]. However, in HFMO45, as evident from the neutron diffraction results, the transition occurs

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from a Γ4 to a Γ1 phase [14]. Based on these results, it is clear that 45% Mn substitution in HoFeO3 not only brings the spin reorientation to room temperature and also changes the nature of spin reorientation transition. Conclusion The temperature dependent magnetization, crystal and magnetic structure of HoFe0.55Mn0.45O3 were analysed by using neutron powder diffraction, bulk magnetization, and specific heat capacity studies. It is concluded from the above studies that 45% Mn substituted HoFeO3 crystallizes in an orthorhombic structure with Pnma space group. From the magnetization and neutron diffraction results, it is concluded that HFMO45 orders antiferromagnetically at 336 K in 4(AxFyGz) phase as given by Bertaut’s notation. A spin reorientation transition is observed near room temperature (290 K) where the magnetic structure changes from 4(AxFyGz) to 1(GxCyAz) as the temperature decreases. It is observed that from 300 K to 275 K, the system exists in a mixed phase 4+1. Below 275 K, the system goes into a pure 1 phase which has the G-type antiferromagnetic moments along the x-axis. The long-range ordering of Ho3+ is not observed until 7 K from NPD patterns. Acknowledgements Authors thank DST sanctioned project (Number: EMR/2014/000592) for financial support. Authors acknowledge IIT Madras for specific heat measurements (DST FIST phase II project for PPMS). PNL thanks Science and Engineering Research Board, India for National Post-Doctoral Fellowship (PDF/2017/001826). Authors acknowledge Professor M. S. R. Rao for providing XPS facility (DST-SR/NM/Nat-02/5). References 1. Jin Tang, YajiaoKe, Wei He, Xiangqun Zhang, Wei Zhang, Na Li, Yongsheng Zhang, Yan Li, and Zhaohua Cheng, Adv. Mater. 30, 1706439 (2018). 2. Y. Tokunaga, S. Iguchi, T. Arima and Y. Tokura, Phys. Rev. Lett. 101, 097205 (20008). 3. M. Das, S. Roy, and P. Mandal, Phys. Rev. B 96, 174405 (2017). 4. Mingjie Shao, Shixun Cao, Yabin Wang, Shujuan Yuan, Baojuan Kang and Jincang Zhang, Solid state Comm. 152, 947 (2012).

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43. F. Hong, Z. Cheng, H. Zhao, H. Kimura and X. Wang, Appl. Phys. Lett.99, 092502 (2011). Highlights



Magnetic structure and transitions of 45% Mn substituted HoFeO3 are investigated



HoFe0.55Mn0.45O3 crystallizes in an orthorhombic structure (Pnma)



Magnetization measurements and specific heat studies show that the sample has a Néel temperature TN~336 K



A spin reorientation transition is observed at 290 K



The sample is found to be in mixed 4(Ax, Fy, Gz) + 1(Gx, Cy, Az) phase from room temperature to 275 K.

 Below 275 K, the sample exhibits a pure 1(Gx, Cy, Az) structure 44. Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☒The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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45. Karthika Chandran: Investigation, Methodology, Validation, Writing – Original Draft, Visualization. Pulkit Prakash: Investigation, Writing - Review & Editing. P. Neenu Lekshmi: Formal analysis, Writing - Review & Editing. Amitabh Das: Investigation, Writing - Review & Editing. P. N. Santhosh: Conceptualization, Supervision, Writing - Review & Editing, Funding acquisition. 46.