Large exchange bias effect in LaCr0.9Ru0.1O3

Journal of Magnetism and Magnetic Materials 417 (2016) 160–164

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Large exchange bias effect in LaCr0.9Ru0.1O3 Babusona Sarkar, Biswajit Dalal, S.K. De n Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Kolkata 700032, India

art ic l e i nf o

a b s t r a c t

Article history: Received 22 September 2015 Received in revised form 13 May 2016 Accepted 22 May 2016 Available online 24 May 2016

The incorporation of tetravalent Ru (10%) into antiferromagnetic spin structure of LaCrO3 leads to mixed valence states of Cr (Cr2 þ and Cr3 þ ). Highly delocalized 4d orbital of Ru induces prominent ferromagnetic (FM) component in antiferromagnetic (AFM) matrix of parent compound. The complex magnetic interaction across the interface of FM and AFM regions gives rise to large exchange bias field (HEB) of about 10 kOe. The inverse and normal magnetocaloric effect for magnetic field up to 50 kOe coexists in a single material due to multiple magnetic phase transitions with temperature. & 2016 Elsevier B.V. All rights reserved.

Keywords: Exchange bias Magnetocaloric effect

1. Introduction Exchange bias (EB) phenomenon is generally observed in the heterogeneous magnetic system consisting of ferromagnetic (FM) and antiferromagnetic (AFM) phases due to the exchange anisotropy of the interface of two phases [1–3]. Such unidirectional anisotropy leads to the shift of magnetic hysteresis loop along the field direction while magnetization is measured after cooling down the sample in the presence of external magnetic field below magnetic ordering temperatures. This effect was first discovered in 1956 by Meiklejohn and Bean while studying the core/shell nanoparticles containing FM Co core and AFM CoO shell [4]. Generally magnetic interface is produced in thin film (artificial layered structure), core–shell nanostructure and composite system. In these procedures individual FM and AFM components are fabricated to induce interface interaction. The spontaneous formation of FM region in AFM matrix gives rise to the interfacial exchange coupling in a variety of oxide materials, such as manganites [5,6], cobaltates [7], and chromites [8,9]. In recent years exchange bias effect attracts many researchers due to its scientific interest to understand the unusual magnetic behavior at the interface as well as potential application in spintronic devices. The rare earth (R) chromites (RCrO3) belong to a class of complex magnetic system due to a number of magnetic phases depending on the ionic radius of rare earth element. The RCrO3 is G-type antiferromagnetic system in which Cr3 þ ions are antiferromagnetically coupled to its nearest neighbors. In the series of RCrO3, LaCrO3 (LCO) is the most attractive from the application point of view due to the highest Neel temperature, TN ¼ 292 K. n

Corresponding author. E-mail address: [email protected] (S.K. De).

http://dx.doi.org/10.1016/j.jmmm.2016.05.076 0304-8853/& 2016 Elsevier B.V. All rights reserved.

The canting of antiferromagnetically ordered Cr gives rise to weak ferromagnetism [9]. The modification of magnetic exchange interaction in LCO through the replacement of La by Ca and Sr weakens the AFM interaction and simultaneously increases the fraction of FM phase [10]. The substitution of Cr for Mn and Fe also enhances the FM phase due to the double exchange FM interaction [11,12]. These observations suggest that the mixed magnetic phase can be introduced by proper doping either at R site or Cr site. A few pure RCrO3 (R¼Nd and Sm) exhibit EB effect [13,14]. An exchange bias effect below 200 K is observed for Pr doped LCO [8]. Tunable exchange bias field in LCO is investigated in Ce doped LCO nanoparticles which is explained on the basis of core–shell micromagnetic structure. [15] The internal magnetic field in Mn3 þ doped at Cr3 þ controls the sign and magnitude of exchange bias field (2.2 kOe) [16]. All these exciting results motivated us to tune magnetic phases of LCO by incorporating highly delocalized 4d orbitals in place of more localized 3d electrons of Cr element. In this paper, we have investigated the EB effect in 10% Ru doped LCO as a function of temperature and magnetic field. The coexistence of inverse and normal magnetocaloric effect (MCE) is also observed.

2. Experiment Polycrystalline sample of 10% Ru doped LaCrO3 (LaCr0.9Ru0.1 O3) was prepared using conventional solid state reaction method. Stoichiometric ratio of La2O3 (Sigma Aldrich, 99.9%), Cr2O3 (Sigma Aldrich, 99.9%) and RuO2 (Sigma Aldrich, 99.8%) were mixed in an agate mortar for 1 h and calcined at 600 °C for 24 h. This sample was reground thoroughly for homogeneity of the sample, pressed into pellet using uniaxial hydraulic press and sintered thrice through intermediate grindings at 1150 °C for 24 h. Crystalline

Intensity (a.u)

pure phase was identified by X-ray diffraction method using X-ray powder diffractometer (PANalytical, Xpert Pro model) in the range of 20°–80° using Cu Kα radiation ( λ = 1.54 Å ). The diffraction pattern was analyzed with the Rietveld refinement of the sample using software MAUD [17]. The analysis of chemical states of the constituent atoms was performed from X-ray photoelectron spectroscopy (XPS) recorded with Omicron 0571 spectrometer using MgKα, 1253.6 eV line, and spot size of 800 μm. DC magnetization measurements both as a function of temperature (M–T) and magnetic field (M–H) were performed by Quantum Design SQUID magnetometer (MPMS XL Evercool model).

Intensity (a.u)

B. Sarkar et al. / Journal of Magnetism and Magnetic Materials 417 (2016) 160–164

161

Cr 2p3/2

O 1s

534

532

530

528

526

Binding Energy (eV)

Cr3+

Cr 2p1/2

Cr2+

3. Results and discussion

Experimental Calculated Bragg Reflection

LaCr0.9Ru0.1O3 Orthorhombic

Intensity (a.u)

Difference Rp = 8.25 % Rwp = 7.34 %

Space Group- Pnma

GOF = 1.21

594

591

588

585

582

579

576

573

Binding Energy (eV) Fig. 2. Deconvolution of X-ray photoelectron spectra (XPS) for Cr 2p region. Inset shows the deconvolution of O 1s XPS pattern.

0.10

(a)

H = 0.1 kOe LaCr0.9Ru0.1O3

0.06

Moment (emu/gm)

Moment (emu/gm)

0.08

0.04 0.02

0.0785 0.0780 0.0775 0.0770

0.00

0

20

40

60

80

100

T (K) 0

50

100

150

200

250

300

250

300

T (K) 0.6

(b) H = 50 kOe

Moment (emu/gm)

Fig. 1 shows the Rietveld refinement of room temperature X-ray diffraction (XRD) pattern of LaCr0.9Ru0.1O3. Absence of any extra peaks indicates the pure crystalline phase of LaCrO3. The value of reliability factors Rwp, Rp and goodness of fit (GOF) reveal the best fitted XRD data. The crystal structure obtained from Rietveld analysis is orthorhombic, space group Pnma (No. 62) and lattice parameters are a = 5.481(3) Å , b = 7.767(1) Å and c = 5.519(2) Å . To explore the valence states of Cr and oxygen vacancy, core level X-ray photoelectron spectra (XPS) of Cr 2p and O 1s regions have been investigated. The peaks of Cr 2p pattern are asymmetry enough to deconvolute the peaks into multiple peaks. Deconvolution of Cr 2p region clearly reveals two peaks as shown in Fig. 2. The peak positions derived from deconvolution of Cr 2p3/2 are 575.81 eV and 576.93 eV for LaCr0.9Ru0.1O3. The peak of higher binding energy is associated to the Cr3 þ as reported earlier in LaCrO3 [18]. In Cr 2p XPS pattern higher binding energy peak is related to higher valency of Cr. Hence, it is obvious that the lower binding energy peak corresponds to Cr2 þ ion [19]. The concentration of Cr2 þ and Cr3 þ for LaCr0.9Ru0.1O3 are 37% and 63% respectively. The substitution of Cr3 þ by Ru4 þ gives rise to the mixed valency of Cr (Cr2 þ and Cr3 þ ) due to charge neutrality of the system. The asymmetry in O 1s spectrum clearly reveals the two peaks as shown in inset of Fig. 2. The lower binding energy is ascribed to the intrinsic lattice site of LCO and the higher binding energy peak originates from the oxygen vacancy in the lattice. The oxygen vacancy concentration is 31%. The formation of Cr2 þ ion in LaCr0.9Ru0.1O3 is due to the cumulative effect of the incorporation of Ru4 þ and oxygen vacancy in lattice. Temperature variation of field cooled (FC) magnetization M (T )

0.5 0.4

H = 30 kOe

0.3

H = 15 kOe H = 5 kOe

0.2

H = 2 kOe

0.1

H = 0.1 kOe

0.0 0

50

100

150

200

T (K) 20

30

40

50

2θ (Degree)

60

70

Fig. 1. Room temperature Rietveld refinement of LaCr0.9Ru0.1 O3.

80

Fig. 3. (a) Field cooled (FC) magnetization at applied magnetic field H¼ 0.1 kOe and inset shows the FC magnetization in reduced scale. Red line represents the theoretical data fitted to Eq. (1). (b) FC magnetization at various magnetic fields. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this paper.)

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at an applied magnetic field H¼ 0.1 kOe is depicted in Fig. 3(a) for LaCr0.9Ru0.1O3 to describe the nature of phase transition. The M (T ) behavior demonstrates the FM like phase transition at applied field H ¼0.1 kOe. The FM transition (TC) appears at 287 K which is near to TN (292 K) of AFM LCO. The M (T ) curve below transition temperature is fitted based on the following power law:

M (T ) = A(TC − T ) β

(1)

where β describes the divergence of magnetization near TC. The M (T ) curve is fitted at low temperature up to 120 K due to the low temperature anomaly at T ¼50 K. The calculated value of β ¼ 0.328 describes that the system is supported by standard Hiesenberg spin–spin coupling model. At low temperature magnetization initially increases up to 50 K then decreases with decreasing of temperature as shown in inset of Fig. 3(a) with a reduced scale. This anomaly is attributed to the presence of some short range AFM interactions which vanishes with increase of magnetic field. The FC magnetization M (T ) at various magnetic field 0.1 kOe ≤ H ≤ 50 kOe is displayed in Fig. 3(b). The magnetic field variation of M (T ) has been investigated to understand the field induced magnetic phase transition from FM to AFM. The observation of FM phase into the AFM spin structure of Cr is attributed to the presence of mixed valency of Cr. The Cr3 þ (d3)–O– Cr3 þ (d3) AFM superexchange interaction is weaken with the doping of Ru ions. The canting of Cr spins or noncollinear FM in LCO is reported by several authors [10,20]. Beside this effect the existence of Cr2 þ (d4) and Ru4 þ (d4) introduce the double exchange (DE) FM interaction. The major contribution of FM phase is Cr2 þ (d4)–O–Cr3 þ (d3) and Cr3 þ (d3)–O–Ru4 þ (d4) double exchange FM interaction [11,12,21]. The inclusion of tetravalent Ru ions destroys G-type AFM order of LCO due to large overlap between Ru 4d and O 2p orbitals. In order to investigate the irreversibility of the system we have measured both zero field cooled (ZFC) and field cooled (FC) M(T) at magnetic field of 2 kOe. The irreversibility temperature of the system at 2 kOe is 288.4 K as clearly shown in Fig. 4. A large irreversibility between FC and ZFC occurs below 288 K. Magnetic field dependence of magnetization (M–H) at temperature T¼ 5, 100, 200 and 250 K are shown in Fig. 5(a–d) under both zero field cooled (ZFC) and FC mode for LaCr0.9Ru0.1O3. The sample cooling was done from 320 K and at 50 kOe of magnetic field for FC M–H. The hysteresis curves were measured between 750 kOe but displayed in Fig. 5 up to 730 kOe. The ZFC M–H

0.25

Moment (emu/gm)

FC 0.20

H = 2 kOe

0.15 0.10

Tirr = 288.4 K

0.05 0.00

ZFC

curves at all temperatures are symmetric about both axes. The FM like behavior at all temperatures are also observed from M–H loops with large coercivity. The magnetization does not saturate even at high magnetic field H ¼50 kOe due to the presence of AFM interactions or high magnetocrystalline anisotropy induced by Ru ion. Saturation magnetization of FM phase has been estimated by linear extrapolation of high field magnetization M (H ) to zero field as shown in inset of Fig. 5(c). The estimated value of the saturation magnetization of ferromagnetic phase (Msat) at 200 K is 0.1713 emu/gm (0.0075 μB/f.u). We have also theoretically determined the saturation moment if all the spins (Cr3 þ , Cr2 þ and Ru4 þ ) are aligned with the magnetic field. The percentages of Cr3 þ , Cr2 þ and Ru4 þ ions are 56.7%, 33.3% and 10% respectively from our XPS analysis. So, fully saturated moment in Bohr magneton per formula unit would be 3  (0.57) þ 4  (0.33) þ 2  (0.10) ¼ 3.23. On the basis of these data, the computed FM fraction is 0.23%. The FC M–H curves are shifted significantly towards negative field axis as well as positive magnetization axis indicating large exchange bias and asymmetry in remanent magnetization. The EB is defined as H EB ¼ – (H1 − H2) , where H 1 and H 2 are the intersections 2

of positive and negative field axis of M–H loops respectively. Similarly, the asymmetry in remanence is defined as M EB ¼ – (M1 − M2) , 2

where M 1 and M 2 denote the positive and negative remanent magnetization respectively. Temperature variation of EB field and M EB at cooling field H cool ¼50 kOe is depicted in Fig. 6. At temperature T¼5 K exchange bias field is 10.075 kOe and MEB ¼0.1035 emu/gm. This EB field is very large with comparison of other previous reports [8,2]. The EB field gradually decreases with increasing temperature up to 250 K. The variation is unique as EB field slowly decreases up to 100 K and then drastically decreases to 400 Oe at 250 K. The sample is cooled down from T > TN with applied field 50 kOe. The FM spins start to orient along the applied field with decreasing the temperature. Hence, EB effect arises due to the competition of FM/AFM spin below ordering temperature. The existence of both FM and AFM phases in a single compound gives rise to a large EB in LaCr0.9Ru0.1O3. The microscopic mechanism for EB is still unknown due to complex magnetic exchange interaction across AFM and FM interface. Among various tuning parameters, the AFM to FM volume ratio in magnetic heterostructures plays a crucial role to influence the magnitude of EB effect. Computer simulation based on Monte–Carlo method, it is predicted that significant exchange bias field occurs in systems with small fraction of FM phase [22]. A smaller fraction of FM phase as estimated from the saturation magnetization gives rise to large exchange bias effect. Fig. 7(a) shows the effect of cooling field on EB field at temperature T¼ 100 K for LaCr0.9Ru0.1O3. The EB field increases with increasing the cooling field up to H cool ¼15 kOe and then decreases with further increase of cooling field. The low magnetic field is not enough to saturate the FM spins along direction. With increase of cooling field the orientation of FM spins along a preferential direction is enhanced. Therefore, EB field is initially increased with increasing cooling field [23]. The further increase of cooling field gives the significance effect on field induced magnetic phase transition that results the decrease of H EB. The temperature dependence of coercivity field (HC) is shown in Fig. 7(b). The value of HC at T¼5 K is 16.65 kOe and decreases with increasing temperature. According to Stoner–Wohlfarth law, 2K HC = M , where K is the magnetocrystalline anisotropy constant S

0

40

80

120

160

200

240

280

T (K) Fig. 4. Zero field cooled (ZFC) and field cooled (FC) magnetization at applied magnetic field H¼ 2 kOe.

and MS is the saturation magnetization. MS is not varied significantly with Ru doping than pure LCO, so the maximum contribution of large HC is due to the large value of anisotropy constant K. Furthermore, the reason behind large coercivity in this materials may be due to the competition between FM and AFM

0.4

(a)

Moment (emu/gm)

Moment (emu/gm)

B. Sarkar et al. / Journal of Magnetism and Magnetic Materials 417 (2016) 160–164

ZFC FC

0.2 0.0 Hcool = 50 kOe

-0.2

T=5K

-0.4 -30

-15

0

15

0.4

(b) ZFC FC

0.2 0.0

Hcool = 50 kOe

-0.2

T = 100 K

-0.4 -30

30

-15

Hcool = 50 kOe

Moment (emu/gm)

T = 200 K

ZFC FC

0.2 0.0

0.5

M (emu/gm)

Moment (emu/gm)

(c)

-0.2 -0.4

M(0) = 0.1713 emu/gm

-30

-15

0.4 0.3 0.2 0

0

20

0

15

30

Magnetic field (kOe)

Magnetic field (kOe) 0.4

163

40

H (kOe)

15

30

0.4

(d) ZFC FC

0.2 0.0

Hcool = 50 kOe

-0.2

T = 250 K -0.4 -30

-15

0

15

Magnetic field (kOe)

Magnetic field (kOe)

30

Fig. 5. Magnetic field dependence of magnetization under zero field cooled and field cooled mode at temperature: (a) T ¼ 5 K, (b) T ¼ 100 K, (c) T ¼200 K and (d) T ¼250 K with cooling field Hcool ¼50 kOe. Inset of (c) shows the linear extrapolation of high field magnetization at T ¼200 K.

12

HEB (kOe)

0.08

8

0.06

6

0.04

4 2 0

0.02

Hcool = 50 kOe 0

50

100

150

200

250

ΔS(T , ΔH ) =

MEB (emu/gm)

HEB MEB

10

temperature increment ( ΔT = 5 K ) using the integral in Maxwell's equation,

0.10

0.00

T (K) Fig. 6. Temperature dependence of exchange bias field (HEB) and remanence magnetization (MEB) at cooling field Hcool ¼ 50 kOe.

interactions [11]. The magnetocaloric effect (MCE) has been investigated to make the materials applicable in refrigeration technology. The MCE is defined as the change in magnetic entropy ( ΔS ) with the variation of magnetic field. The ΔS is calculated from the isothermal magnetization (M–H) curve collected from 230 K to 320 K in equal

∫0

Hmax

⎛ ∂M ⎞ ⎜ ⎟ dH ⎝ ∂T ⎠H

(2)

This equation is approximated a sum as Hmax

ΔS(Ti, Hmax) =

∑ j=0

M (Ti, Hj ) − M (Ti − 1, Hj ) Ti − Ti − 1

ΔHj

(3)

where Mi and Mi − 1 are the magnetization measured at Ti and Ti − 1 respectively, in a magnetic field Hj. The variation of −ΔS with temperature at different magnetic field is displayed in Fig. 8. The −ΔS is positive at high temperature for all magnetic field, increases with decreasing temperature and finally attains a maximum value at around ordering temperature (T¼290 K). With further decreasing temperature −ΔS decreases and becomes negative below T¼ 270 K. The value of entropy change depends on the temperature derivative of sample magnetization. The maximum value of −ΔS is 5  10  2 J kg  1 K  1 at applied magnetic field 5 T which is almost two order of magnitude smaller than that obtained by Pecharsky et al. [24] in magnetocaloric materials Gd5(Si2Ge2). Weak ferromagnetic component arising from the canted AFM order of Cr3 þ in LaCrO3 results in small MCE around magnetic transition [25]. The small value of MCE in the present system may be due to the presence of extremely small volume fraction of

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B. Sarkar et al. / Journal of Magnetism and Magnetic Materials 417 (2016) 160–164

8.45

(a)

T = 100 K

8.40

HEB (kOe)

ferromagnetic phase. The positive −ΔS value indicate the FM ordering and cooling in refrigeration process whereas negative −ΔS gives the heating in this process. Hence, the coexistence of inverse and normal MCE in same materials makes the materials more interesting and can be used as constant temperature bath at T¼ 270 K.

HEB

8.35 8.30

4. Conclusion

8.25 8.20 8.15 0 18 17

10

20

30

40

50

Hcool (kOe) (b)

HC

HC (kOe)

16

Acknowledgments Babusona Sarkar and Biswajit Dalal are thankful to the Council of Scientific and Industrial Research (CSIR), Government of India, for providing fellowship. This work is funded by the Council of Scientific and Industrial Research, Government of India, Scheme No: 03(1210)/12/EMR-II.

15 14 13 12

References

11 0

50

100

150

200

250

T (K) Fig. 7. (a) Cooling field dependence of HEB at temperature T ¼ 100 K and (b) temperature variation of coercivity (HC).

5T 4T 3T 2T 1T

5

-1 -1 -2 -ΔS (× 10 JKg K )

In conclusion, we have presented the observation of ferromagnetism (FM) in G-type antiferromagnetic (AFM) LaCrO3 due to double exchange interaction Cr2 þ (d4)–O–Cr3 þ (d3) and Cr3 þ (d3)– O–Ru4 þ (d4). Coexistence of FM and AFM phases in single compound leads to the large exchange bias field and asymmetry in remanence magnetization. Both inverse and normal magnetocaloric effect is observed at all magnetic field up to 50 kOe. Valency mismatch between Cr and Ru, oxygen vacancy and stronger overlap between 4d(Ru) and 2p(O) influence the magnetic phases of LCO. Large exchange bias field can be used in memory devices.

4 3 2 1 0 240

260

280

300

320

T (K) Fig. 8. Magnetic entropy change( ΔS ) as a function of temperature under different magnetic fields.

[1] Òscar Iglesias, Amílcar Labarta, Xavier Batlle, J. Nanosci. Nanotechnol. 8 (2008) 2761. [2] J. Nogués, Ivan K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [3] M. Kiwi, J. Magn. Magn. Mater. 234 (2001) 584. [4] W.H. Meiklejohn, C.P. Bean, Phys. Rev. 102 (1956) 1413. [5] S. Karmakar, S. Taran, E. Bose, B.K. Chaudhuri, C.P. Sun, C.L. Huang, H.D. Yang, Phys. Rev. B 77 (2008) 144409. [6] V. Markovich, I. Fita, A. Wisniewski, R. Puzniak, D. Mogilyansky, S.V. Naumov, E.V. Mostovshchikova, S.V. Telegin, G. Gorodetsky, G. Jung, J. Appl. Phys. 116 (2014) 093903. [7] Y.-K. Tang, Y. Sun, Z.-H. Cheng, Phys. Rev. B 73 (2006) 174419. [8] K. Yoshii, Appl. Phys. Lett. 99 (2011) 142501. [9] T. Bora, S. Ravi, J. Appl. Phys. 114 (2013) 183902. [10] J.J. Neumeier, H. Terashita, Phys. Rev. B 70 (2004) 214435. [11] T. Bora, S. Ravi, Physica B 448 (2014) 233. [12] J. Yang, Y.Q. Maa, B.C. Zhaoa, R.L. Zhang a, W.H. Songa, Y.P. Suna, Phys. Lett. A 346 (2005) 217. [13] T. Bora, S. Ravi, J. Appl. Phys. 116 (2014) 063901. [14] S. Lei, L. Liu, Chunying Wang, Chuanning Wang, D. Guo, S. Zeng, B. Cheng, Y. Xiao, L. Zhou, J. Mater. Chem. A 1 (2013) 11982. [15] P.K. Manna, S.M. Yusuf, R. Shukla, A.K. Tyagi, Appl. Phys. Lett. 96 (2010) 242508. [16] T. Bora, S. Ravi, J. Appl. Phys. 114 (2013) 033906. [17] See 〈http://www.ing.unitn.it/maud/S〉 for information about MAUD version 2.046, 2006. [18] Y. Jeon, D.-H. Park, J.-I. Park, S.-H. Yoon, I. Mochida, J.-H. Choy, Y.-G. Shul, Sci. Rep. 3 (2013) 2902. [19] S.S. Li, Y.M. Hu, J. Phys.: Conf. Ser. 266 (2011) 012018. [20] G.A. Alvarez, X.L. Wang, G. Peleckis, S.X. Dou, J.G. Zhu, Z.W. Lin, J. Appl. Phys. 103 (2008) 07B916. [21] B. Dabrowski, S. Kolesnik, O. Chmaissem, T. Maxwell, M. Avdeev, P.W. Barnes, J. D. Jorgensen, Phys. Rev. B 72 (2005) 054428. [22] Yong Hu, Guo-Zhen Wu, Yan Liu, An Du, J. Magn. Magn. Mater. 324 (2012) 3204. [23] Y.-K. Tang, Y. Sun, Z.-H. Cheng, J. Appl. Phys. 100 (2006) 023914. [24] V.K. Pecharsky, K.A. Gschneidner Jr., Phys. Rev. Lett. 78 (1997) 4494. [25] B. Tiwari, A. Dixit, R. Naik, G. Lawes, M.S. Ramachandra Rao, Mater. Res. Expr. 2 (2015) 026103.