1

Anti-ferromagnetic interaction in double perovskites probed

by Raman Spectroscopy

R. X. Silva1, H. Reichlova2, X. Marti2,3 R. Paniago4 and C. W. A. Paschoal5

1Coordenação de Ciências Naturais, Universidade Federal do Maranhão, Campus VII,

65400-000, Codó-MA, Brazil

2 Institute of Physics ASCR, v.v.i., Cukrovarnická 10, 162 53 Praha 6, Czech Republic

3 Centre d’Investigacions en Nanociencia I Nanotechnologia (CIN2), CSIC-ICN,

Bellaterra 08193, Barcelona, Spain

4Departamento de Física, Universidade Federal de Minas Gerais, ICEx, 31270-901 Belo

Horizonte-MG, Brazil

5 Departamento de Física, Universidade Federal do Ceará, Campus do Pici, PO Box

6030, 60455-970, Fortaleza - CE, Brazil.

* Corresponding author. Tel: +55 85 3366 9908

E-mail address: paschoal.william@fisica.ufc.br (C. W. A Paschoal)

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Abstract

In this paper we show that Raman spectroscopy is a powerfull technique to

detect antisite disorder into A2B’B’’O6 magnetic double poerovskites whose

ferromagnetic properties are driven by superexchange interactions. We could detect low

antisite disorder levels by monitoring the coupling between the magnetic order and the

phonons in low-level disordered La2CoMnO6 double perovskite.

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I. INTRODUCTION

Multiferroics with crystalline structure derived from double perovskite RE2B’MnO6

(B’ = Ni, Co), in which RE is a rare earth ion, has attracted a lot of attention recently due

to their peculiar electric and magnetic properties, which involve magnetocapacitance1,

magnetoresistance1, magnetodielectric effect1,2 and relaxor behavior3,4, among others.

In addition, La2CoMnO6 (LCMO) and La2NiMnO6 (LNMO) has a Curie point near the

room temperature , around 230K and 280K respectively, which possibilities its

application in spintronics and electrically readable magnetic data storage devices1,5,6.

Structural disorder in complex perovskites plays an essential role because it

drives strongly theirs physical and chemical properties. For example, phonons, and

consequently, the dielectric response of 1:2 perovskites in microwave electromagnetic

range are strongly influenced by the antisite disorder 9–14. Also, in Ba3CaNb2O9 high

antisite disorder is interesting because it favors a higher conductivity enabling its

application as protonic conductors15.

Particularly, in RE2Me2+MnO6 perovskites the order is very important because in

these materials the ferromagnetic behavior is driven by super-exchange interaction

between Me2+ (partially filled eg orbital) and Mn4+ (filled t2g orbital) mediated by oxygen,

i. e. it comes from the Me2+ – O – Mn4+ bonds. Thus, disorder generates antisite defects

of the type Me2+ – O – Me2+ and Mn4+ – O – Mn4+, which induce short-range

antiferromagnetic interactions decreasing the overall ferromagnetic state. High ordered

RE2Me2+MnO6 single crystals exhibit narrow hysteresis (or low coercive field) and only

one ferromagnetic transition, observing only a smooth magnetization at low

temperatures. Commonly, two phases coexistence, oxygen vacancies, mixed valence

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and anti-site disorder promote multiple magnetic transitions at low temperatures as well

as a low magnetization of saturation and high coercive field. In the case of LCMO, the

most ordered sample reported had a magnetization of 5.89 μB/f.u. and a coercive field of

2.9 KOe with only one magnetic transition at 235 K16. While a typical disordered sample

showed two transitions (a new one at ~150 K) 17 and high coectivity (~10 kOe) 16,17.

Also, structural ordering influences dielectric18,19, vibrational16,20 properties and the band

gap21. Wherein, oxygen content, charges, and anti-site defects can be tuned by the

synthesis conditions16–19,22,23.

Raman scattering is a powerful probe to detect disorder in 1:2 perovskites

because disorder induces a two-phonon behavior, characterizing the presence of the

antisite defects. However, antisite occupation in 1:1 samples, mainly in RE2Me2+MnO6

perovskites, it is complicated to observe because those does not exhibit a two-phonon

behavior. In fact, due to the charge difference between Me2+ and Mn4+ ions, the

octahedral modes are related mainly to the Mn4+ ions as reported by Silva et al24. Just

when there are a high difference in mass of the metallic ions, the two-mode behavior is

observed25, which is not the case of RE2Me2+MnO6 perovskites. This is clearly observed

in ordered and disordered LCMO samples, which exhibit very similar spectra26.

However, these compounds usually show coupling between magnetic order and

phonons, which is easily probed by Raman spectroscopy, being observed in position or

linewidth of the phonon associated with the MnO6 stretching27–33.

In this paper, we report that the antisite defect due to structural disorder in

magnetic double perovskites can be probed by Raman spectroscopy even when it is

small and not detected by magnetization measurements, by following the coupling

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between phonons and magnetic order. As a prototype sample we used a partially

disordered LCMO ceramic obtained by polymeric precursors method. To confirm our

assumption, magnetic, X-ray Photoelectron Spectroscopy (XPS) and X-ray diffraction

techniques were employed.

II. EXPERIMENTAL PROCEDURES

Polycrystalline samples of LCMO were synthesized by polymeric precursor

method34 using cobalt acetate tetrahydrate (C4H6CoO4∙4H2O, Sigma Aldrich),

manganese nitrate hydrate (MnN2O6∙xH2O, Sigma Aldrich) and lanthanum oxide (La2O3,

Sigma, Aldrich) as high purity metal sources. The resin and puff (powder obtained by

the resin pre-calcination) were obtained following the steps described elsewhere26. The

puff was lightly ground using an agate mortar and pestle, and calcined at 1100 °C for

16h to obtain LCMO sample. The crystalline structure of the sample was probed by

powder X-ray diffraction using a Bruker D8 Advance with Cu-Kα radiation (40 kV, 40

mA) over a range from 20° to 100° (0.02°/step with 0.3s/step). The powder XRD pattern

was compared with data from ICSD (Inorganic Crystal Structure Database, FIZ

Kalsruhe and NIST) International diffraction database (ICSD# 98240) 35. The structure was

refined using the GSAS code36,37. X-ray Photoelectron Spectroscopy (XPS) measurements

were carried out in a VG ESCALAB 220i-XL system, using Al-Ka radiation and base

pressure of 2x10-10 mbar. Survey XPS spectra were collected with pass energy of 50

eV and detailed spectra around the Co 2p and Mn 2p regions were taken with 20 eV

pass energy. Magnetic measurements were performed into a Quantum Design (QD)

superconducting quantum interference device (SQUID). Temperature sweeps were

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collected with 4 cm long Reciprocating Sample Option scans. Raman spectroscopy

measurements were performed using a Jobin-Yvon T64000 Triple Spectrometer

configured in a backscattering geometry coupled to an Olympus Microscope model

BX41 with a 20x achromatic lens. The 514.5 nm line of an Innova Coherent laser

operating at 18 mW was used to excite the Raman signal, which was collected in a N2-

cooled CCD detector. All slits were set up to achieve a resolution lower than 1 cm-1.

Low-temperature measurements were performed by using a closed-cycle He cryostat

where the temperature was controlled to within 0.1 K.

III. RESULTS AND DISCUSSIONS

The powder X-ray diffraction pattern obtained for the LCMO sample at room

temperature is shown in Figure 1 (a). It was indexed based on a monoclinic unit cell with

symmetry belonging to the space group , that is coherent with a quite ordered

sample17,38. The refined structure parameters are in excellent agreement with those

obtained early by solid state reaction 39 and theoretically calculated by the SPuDS

program40.

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Figure 1 – (a) Powder X-ray diffraction pattern of LCMO ceramic. The red solid line is the fitting

using the Rietveld method and the green line is the residual between the experimental and

calculated patterns. (b) C and ZFC magnetization at field of 100 Oe. The inset shows the

hysteresis curve at 10 K.(c) Comparison of XPS spectra of LCMO well ordered (red filled

circles) and partially disordered (black filled squares) for the (c) Co 2p and (d) Mn 2p energy

regions.

Figure 1 (b) shows the temperature-dependent magnetization of LCMO. The

inset shows the hysteresis obtained for LCMO at 10 K. FC and ZFC magnetization were

measured at field of 100 Oe and has been irreversible below T< 226K, this divergence

between FC and ZFC curves are an indication of magnetic frustration due to

competition between the ferromagnetic and antiferromagnetic interactions, which imply

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in a spin-glass behavior 41. At this point the magnetization curve reveals the onset of the

net magnetic moment at the Curie temperature near 226 K, consistent with previous

magnetic measurements in LCMO 42. The magnetic measurements has been used to

estimate the B-site structural order in LCMO. Previous works has shown that the

structural order level of site B is very sensitive to the synthesis conditions and can be

adjusted as a function of the calcination temperature in air atmosphere. 16,43 wherein the

sample obtained at 1000 C had the best order level, with Tc 235K and saturation

magnetization of up to 5.89 μB/f.u16,43 (matching to a nearly stoichiometric ordered

sample that contains Co2+ and Mn4+ cations). According to Blasse’s model to estimate

order at site B44, this magnetization would correspond to δ =0.98, or about 1% of anti-

site defects. Thus, for observed saturation magnetization of 3.49 μB/f.u we have a

δ=0.58, i.e., it is estimated that there are 21% of anti-site defects. However, despite the

high number of anti-site defects, the obtained magnetization curve shows only a

prominent ferromagnetic transition at 226 K, usually attributed to long-range

ferromagnetic (FM) interaction due to the superexchange interaction between Mn4+-O-

Co2+. This interaction is stabilized by a stoichiometric oxygen concentration. Dass et al17

showed that the stoichiometric compound has a single evident magnetic phase with Tc

= 226K (high Tc) and Ms= 5.78(2) μB/f.u.. Oxygen deficiencies stimulate the appearance

of Co3+ (t5e1) and Jahn-Teller Mn3+ (t3e1) ions, which generate a vibrionic

superexchange FM interaction, but with a lower exchange stabilization and a lower

critical temperature (low Tc). Multiple magnetic transitions are common in compounds

heated above 1200°C in ar17,21,23,45–47.

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Comparative XPS mesurementes shows that the charge states of partilly and

well ordered reported samples is almost the same, as observed in Figure 1 (c) and (d).

The spectra show a main peak located at 780.2 eV for Co 2p3/2, while Co 2p1/2 peak is

centered at 796.3 eV. Whereas the Co 2p3/2 peak is observed at 780.5 eV for CoO 48

and the weaker satellite peak observed at 787.1 eV is also characteristic of CoO 49.

Therefore, this analysis confirms the predominance of Co2+ species 50,51. Figure 1(d)

shows the region around the Mn 2p energies. The peak related to Mn 2p3/2 is centered

at around 642.4 eV, while for Mn 2p1/2 it is observed at 654.0 eV. Whereas the main

XPS peak of MnO2, which is related to Mn 2p3/2, is observed at 642.2 eV 48,51,52. This

ruled out the possibility of significant amount of Co3+ and Mn3+ and indicates that the

observed low saturation magnetization is due to the presence of other magnetic

interactions, in particular, the antiferromagnetic interactions (AFM) due to defects of

anti-site Mn4+−O−Mn4+ or Co2+−O−Co2+, and interaction between antiphase boundaries

that occur between regions with AFM, Mn4+−O−Mn4+ or Co2+−O−Co2 interactions, or

between FM regions promoting AFM interactions in the contours.

The room-temperature Raman spectrum obtained of LCMO is shown in Figure

4(a). The spectrum shows the characteristic spectrum of a manganite double

perovskite, in which the stronger phonon corresponding to (BO6) octahedral symmetrical

stretching (S) at 646 cm-1, whereas a mode at 499 cm-1 is assigned as a combination of

both antistretching (AS) and bending motions53. Low-intensity modes were also

observed at 426 and 254 cm-1, which correspond to the octahedra bending with La

moves in xz plane, and octahedra out-of-phase tilt along x axes with La and apical O

move in xz plane, respectively. Also, two modes at high wavenumbers, at 1160 and

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1283 cm-1, were observed and correspond to symmetrical stretching and bending

combination, and symmetrical stretching overtone, respectively. The low-intensity

modes are shown in the inset of the Figure 4(a). The spectrum obtained at room

temperature is in excellent agreement with preceding works involving Raman

spectroscopy on LCMO22,54.

Figure 2 – Top: Raman spectrum of LCMO at room temperature. The inset shows the low

wavenumber region of the spectrum. Bottom: temperature-dependent Raman-active phonon

spectra of LCMO in the wavenumber range of (b) (Co/Mn)O6 antistretching (AS) and bending

mode and (c) (Co/Mn)O6 symmetrical strectching (S) region. The temperature measurements

were performed in temperature steps of 10 K far from the magnetic transition and of 5 K near

the transition.

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The temperature dependence of the Raman spectra of LCMO between room

temperature and 40 K is shown in Figures 2 (b-c). Usually, a spin-phonon coupling in

manganite double perovskites is observed in octahedral stretchings modes 27,29–31. The

temperature-dependence of those modes is shown in Figure 3 (a-b). The common

anharmonic contribution to the temperature dependence of a phonon position, which is

modeled by Balkanski model 55, is given by

( ) [

( )]

with and being fitting parameters. In the absence of structural phase transitions, as

it happens in LCMO, the temperature dependence of the phonons must follow this

behavior. However, clearly the stretching phonon exhibits a typical deviation from the

anharmonic model, which is similar to that recently observed in LCMO 56, as well as for

other double perovskites with magnetic transitions, and it is associated to the

renormalization of the phonons induced by the magnetic ordering27,29–32,57–60. The

coupling occurs at the temperature on the onset of the net magnetic moment occurs, as

showed by the spontaneous magnetization of LCMO, which was shown in Figure 1 (b).

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Figure 3 – (a-b) Temperature dependence of the symmetric (a) and antisymmetric stretching (b)

position observed for LCMO (blue spheres). The red lines show the usual behavior of the

anharmonic effect contribution to the temperature dependence according to the Balkanski’s

model. This line is not a fit in the temperature range in which the sample is ferromagnetic (in this

region it is an extrapolation following the Balkanski’s model). (c) Detachment between AS and S

Raman stretching phonons. (d-e) Temperature dependence of stretching phonons FWHM (d)

symmetric (S) (e) antisymmetric (AS) phonons. (f) Temperature dependence of the departure

from the anharmonic behavior of the mode that exhibits the spin-phonon coupling compared

with ( ( ) ) .

The AS mode also exhibited an anomalous softening near the major magnetic

transition (high-Tc) which is consistent with reported spin-phonon coupling. However,

observing carefully its behavior (see Figure 3 (b)) we can note a small second

discontinuity around 135K. In addition, the detachment between symmetric (S) and

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antisymmetric (AS) stretching modes was reported previously as a

comparative parameter of ordering 16. Our results evidence that the temperature

dependence of detachment showed three distinct regions, with changes around 226

and 135 K (see Figure 3 (c)).

The hysteresis curve and XPS measurements already clarified that the LCMO

sample is partially disordered with Co2+ and Mn4+ cations. However, it is well known that

LCMO depending on structural order or oxygen content can show two or more

remarkable magnetic transitions due: (I) a long range FM superexchange Mn4+-O-Co2+,

with high K; (II) a second FM interaction between the intermediate-spin

Co3+(t5e1) and the Jahn-Teller Mn3+ cations, creates by introduction of oxygen

vacancies, with low K 17,61,62; (III) a AFM interactions due antisite disorder

around 133 K 21,63 and (IV) a glassy behavior observed below 80 K43,63,64 owing

coexistence of FM and AFM interactions. The bulk sintered in this paper shown only a

strong high K in the magnetization measurement, which evidences a quite

ordered phase. However, a careful analysis of the linewidth (FWHM) of the stretching

modes point out three different regions in FWHM with discontinuities near 135 K and

226 K, as observed in Figure 3 (d) and (e), as observed in AS position, we can see

those anomalies at the same temperatures in the linewidth of the stretching (S) anti-

stretching (AS) modes.

This result shows that, in spite of the FC and ZFC magnetic curves does not

evidences antisite disorder in LCMO, there are at one new magnetic transition in LCMO,

which can be characterized as an inhomogeneous magnetic phase 65. This kind of

changes in phonon lifetimes was already observed as evidence of spin-phonon coupling

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32,66. This result is consistent with the existence of a majority ferromagnetic (FM) phase,

basically a ordered region, and similar regions with disordered clusters wherein

predominate the antiferromagnetic (AFM) interaction from Co2+-O-Co2+ , Mn4+-O-Mn4+

and antiphase boundaries. This result shows the powerful of Raman spectroscopy to

indicate magnetic inhomogeneity via spin phonon coupling monitoring in these

perovskites. Even not observing the effect in the magnetization curve, we could detect it

in Raman phonon parameters. This observation also possibilities to analyze the order

evolution through the spin-phonon coupling using others phonon parameters like

linewidth of double perovskites during thermal treatments, which are a standard

procedure to get ordered double perovskites.

Finally, it is expected the phonon renormalization due to the spin-phonon

coupling depart of the measured position with relation to the expected position due only

to the anharmonic temperature dependence is proportional to ( ( ) ) , where ( )

and are the magnetization at temperatures T and 0 K, respectively, as predicted by

the mean field theory67. In Figure 3(f) we shown that this model agree very well down to

140 K, temperature in which is observed the second transition, clearly showing we have

a new renormalization of the frequency at this transition that comes from the spin-

phonon coupling. This unconventional behavior exhibited by LCMO can be associated

to a competition between FM and AFM magnetic states promoted by anti-site disorder.

Different value and signals of magnetic couplings Jij can yield different contributions to

the phonon renormalization induced by magnetic ordering. Interestingly, the S mode

renormalization in partially disordered LCMO was very similar to observed for

Y2CoMnO6 (YCMO)66, other multiferroic double perovskite whose stretching mode

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renormalization has been associated to competing between FM and AFM magnetic

states due to their frustrated Ising spin chain (E*-type) with ↑↑-↓↓spin pattern 68. In both

compounds, LCMO and YCMO the different value of Jij caused a S softening behavior

that has a dependence with the square of the temperature, as observed in Figure 3(f).

IV. CONCLUSIONS

Summarizing, in this paper we showed that Raman spectroscopy can be used as

tool to detect low disorder levels in in super-exchange driven ferromagnetic double

perovskites. By monitoring the spin-phonon coupling in LCMO ceramic obtained by

temperature-dependent Raman spectra, we showed an anomaly at 135K due to

disorder in LCMO is probed, even when the disorder levels are low, while magnetization

measurements observed only one magnetic transition. This result shows that Raman

spectroscopy as a powerful tool to probe disorder in double perovskites.

Acknowledgments

The authors are grateful to CNPq, CAPES, Fapemig, FUNCAP and Fapema for

co-funding this work.

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