Orbital order and ferromagnetism in LaMnO 3doped with Ga C. AutieriID1M. CuocoID2 3G. CuonoID1yS. PicozziID4and C. NoceID3 2 1International Research Centre Magtop Institute of Physics

2025-04-29 0 0 739.76KB 9 页 10玖币

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Orbital order and ferromagnetism in LaMnO3doped with Ga

C. Autieri ID ,1, ∗M. Cuoco ID ,2, 3 G. Cuono ID ,1, †S. Picozzi ID ,4and C. Noce ID 3, 2

1International Research Centre Magtop, Institute of Physics,

Polish Academy of Sciences, Aleja Lotnik´ow 32/46, PL-02668 Warsaw, Poland

2Consiglio Nazionale delle Ricerche (CNR-SPIN), I-84084 Fisciano (SA), Italy

3Dipartimento di Fisica “E. R. Caianiello”, Universit`a di Salerno, I-84084 Fisciano (SA), Italy

4Consiglio Nazionale delle Ricerche (CNR-SPIN), Via Vetoio, I-67010 L’Aquila, Italy

We study from ﬁrst principles the magnetic, electronic, orbital and structural properties of the

LaMnO3doped with gallium replacing the Mn-site. The gallium doping reduces the Jahn-Teller

eﬀect, and consequently the bandgap. Surprisingly, the system does not go towards a metallic phase

because of the Mn-bandwidth reduction. The Ga-doping tends to reduce the orbital order typical

of bulk antiferromagnetic LaMnO3and consequently weakens the antiferromagnetic phase. The

Ga-doping favors the G-type orbital order and layered-ordered ferromagnetic perovskite at x=0.50,

both eﬀects contribute to the formation of the insulating ferromagnetic phase in LaMn1−xGaxO3.

I. INTRODUCTION

In recent years the series La1−xAxMnO3, where A

is a divalent metal, has been the object of systematic

investigations1–3. This is not only due to the discovery

of the giant magnetoresistance in several members of the

series but also for the strong interplay among orbital,

lattice, spin and charge degrees of freedom exhibited by

the compounds of the series. This interplay results in a

large variety of magnetic arrangements and phase tran-

sitions depending on the hole doping. Concerning the

giant magnetoresistance phenomena, it is observed in

the ferromagnetic metallic phase4. The correlation be-

tween ferromagnetism and metallic behaviour has been

explained by the double exchange mechanism5whereas

the electron-phonon coupling is mainly due to the Jahn-

Teller (JT) eﬀect6,7. Interestingly, the ferromagnetic

phase of La1−xAxMnO3has been used to study the inter-

play between ferromagnetism and ferroelectricity at the

oxide interface8–10 and to create high-temperature ferro-

magnetism in the 2D limit11,12.

Looking at the mother compound LaMnO3, it has been

considered as the prototype example of a cooperative JT

system and orbital-order state13,14. Indeed, LaMnO3

shows an orthorhombic unit cell with a cooperative

tetragonal deformation of the MnO6octahedra, JT like,

at room temperature. Moreover, the system is charac-

terized by a large octahedral tilt/rotation pattern of the

a−a−c+type15, following the Glazer notation, leading

to the Pnma (or Pbnm in non-standard setting) space

group, whose structural parameters are shown in Table

I. This compound develops long-range antiferromagnetic

(AFM) ordering of type A below TN=140 K. Speciﬁ-

cally, the manganese moments are aligned in the [010]

direction with the spins coupled ferromagnetically in the

ab plane and antiferromagnetically along the c-axis16. An

exotic ferromagnetic state was found theoretically in het-

erostructures based on LaMnO3, showing that the inter-

play between the band reduction due to the dimensional-

ity with strain and interface eﬀects can lead the LaMnO3

to the ferromagnetic phase.17,18 However, this band re-

TABLE I. Structural parameters of the P bnm (No. 62 in

the International Tables) structure of the parent compound

LaMnO3as reported by Elemans et al.19 ,a= 5.532 ˚

A, b=

5.742 ˚

A, c= 7.668 ˚

A at 4.2 K.

Atoms & Wyckoﬀ site x y z

La (4c) -0.010 0.049 0.250

Mn (4b) 0.500 0.000 0.000

O(1) (4c) 0.070 0.486 0.250

O(2) (8d) 0.724 0.309 0.039

duction due to the dimensionality is absent in bulk sys-

tems.

Surprisingly, the replacement of the magnetic JT Mn3+

ion by an isoelectronic non-magnetic non Jahn-Teller ion

such as Ga3+ induces long-range ferromagnetism20–24

without carrying hole doping. Without hole doping,

the double-exchange mechanism is not active. Never-

theless, the ferromagnetic interactions can not be as-

cribed to any of the previous mechanisms. It was shown

indeed the relationship between the static Jahn-Teller

distortion of the MnO6octahedron and the orthorhom-

bic distortion of the unit cell in the LaMn1−xGaxO3

series21. The Mn and Ga ionic radii are 0.785 and

0.760 ˚

A, respectively25, so that the replacement of man-

ganese by the smaller gallium reduces the octahedral

distortions and produces the appearance of a sponta-

neous magnetization24. The distribution of the Ga atoms

is uniform due to the missing of the bond formation

for d10 closed-shell26, while in the limit of strong Ga-

concentration the Mn atoms tend to segregate due to the

open shells d4that create bonds at the Fermi level27.

In the limit of strong Ga-concentration, the perovskite

systems with Ga3+ and Mn3+ behave like dilute mag-

netic semiconductors (DMS)27,28 where the d4-d4mag-

netic interaction is usually ferromagnetic also in the in-

sulating phase. Diﬀerent mechanisms were proposed to

describe the ferromagnetism in DMS with Mn-d4elec-

tronic conﬁguration29–32, with the superexchange that

arXiv:2210.04516v1 [cond-mat.str-el] 10 Oct 2022

dominates in absence of band carriers33,34.

The maximum total magnetic moment in

LaMn1−xGaxO3is achieved for x=0.5. For x>0.6

the lack of Jahn-Teller eﬀect makes the system cubic20.

Unusually for ferromagnetic compounds, these samples

are also electrically insulators. The gallium doping

has dramatic eﬀect at concentrations lower than 5%,

since one Ga atom increases the magnetic moment in

an applied magnetic ﬁeld up to 16 µBper Ga atom23.

Investigating similar ferromagnetism in LaMn1−xScxO3,

it was shown that the ordering of the Mn3+ Jahn-

Teller distortion is not disrupted in the ab-plane for

any Sc concentration. This contrasts with the results

of the LaMn1−xGaxO3, where a regular MnO6is

found for x>0.5. Therefore, both LaMn0.5Sc0.5O3and

LaMn0.5Ga0.5O3show a similar ferromagnetic behavior

independently of the presence or not of the Jahn-Teller

distorted Mn3+, pointing to the Mn-sublattice dilution

as the main eﬀect in driving ferromagnetism over the

local structural eﬀects35.

Here, using ﬁrst principles calculations, we study

the electronic, magnetic and orbital properties of the

LaMn1−xGaxO3focusing on the interplay between the

Jahn-Teller and the octahedral distortions in the antifer-

romagnetic phase. The paper is organized as follows: we

present the computational details for the ab-initio cal-

culations in Section II, while in Section III we show the

results from ﬁrst principles studies, focusing on the oc-

tahedral distortions, the density of state (DOS) and the

orbital order. Finally, in Section IV we propose the pos-

sible origin of the experimentally detected ferromagnetic

phase.

II. COMPUTATIONAL DETAILS

We perform spin-polarized ﬁrst-principles density func-

tional theory (DFT) calculations36 using the Quan-

tum Espresso program package37, the GGA exchange-

correlation functional of Perdew, Burke, and Ernzerhof38,

and the Vanderbilt ultrasoft pseudopotentials39 in which

the La(5s, 5p) and Mn(3s, 3p) semicore states are in-

cluded in the valence. We used a plane-wave energy cut-

oﬀ of 35 Ry and a Gaussian broadening of 0.01 Ry as in

the reference 40. These values for the plane-wave cutoﬀ

and the Gaussian broadening are used in all calculations

presented in this chapter. A 10 ×10 ×10 k-point grid

is used in all DOS calculations, while a 8 ×8×8 grid is

used for the relaxation of the internal degrees of freedom.

We optimized the internal degrees of freedom by mini-

mizing the total energy to be less than 10−4Hartree and

the remaining forces are smaller than 10−3Hartree/Bohr,

while ﬁxing the lattice parameter a,band cto the experi-

mental values19,24. After obtaining the DFT Bloch bands

within GGA, we use the occupation matrix to obtain the

orbital order parameter. For the DOS calculations, we

used a Gaussian broadening of 0.02 eV to have an accu-

rate measurement of the bandgap.

To go beyond GGA, it has been proposed to in-

clude the Coulomb repulsion Uinto the GGA theory

giving rise to the so-called GGA+Utheory. GGA+U

was ﬁrst introduced by Anisimov and his coworkers41,42.

Here, we use the rotational invariant form introduced by

Lichtenstein43 in its spherically averaged and simpliﬁed

approach44 where there is just an adjustable parameter

Ueff =U−J. A self-consistent method for the deter-

mination of Ueff was proposed by Cococcioni et al.45

Starting from the observation of the non piecewise be-

haviour of the energy as a function of the occupation

number42, they implemented a method to take into ac-

count the electron screening in the Hubbard repulsion.

We used the reﬁned approach suggested by Cococcioni

seeking internal consistency for the value of Uef f . Once

calculated the ﬁrst value of Ueff within the GGA scheme,

we performed the Cococcioni technique for the functional

GGA+Ueff obtaining a correction to the Uef f value and

repeating the procedure until the correction for the ﬁ-

nal value of Ueff vanishes. We constructed several su-

percells required by the Cococcioni method and we cal-

culated Ueff from the AFM conﬁguration using exper-

imental volume and atomic positions46. The well con-

verged value for the AFM conﬁguration of LaMnO3is

Ueff =6.3 eV. The Uef f obtained in the AFM phase is

in good agreement with Ueff values between 3.25 and

6.25 eV used for LaMnO36,47 and other Mn-based insu-

lating systems48–51. We use the value U=6.3 eV in all

the conﬁgurations and at all the doping concentrations

of LaMn1−xGaxO3.

The optimization of manganites within DFT is a very

delicate issue and no clear methodology (GGA, LSDA,

GGA+U, etc) has been assessed in the literature, each

having advantages and disadvantages, as for the descrip-

tion of tilting, JT distortion, lattice parameters etc. We

perform the relaxation of the atomic positions at ﬁxed

experimental volume for some doping concentrations in

GGA in the AFM spin conﬁguration using several su-

percells. After the relaxation, we calculate the density

of state, the distortion and the orbital order in GGA+U

using the Ucalculated by the Cococcioni method.

Here, we study the properties of the LaMn1−xGaxO3

at x=0.000, 0.125, 0.250, 0.500 considering a 2 ×2×2

supercell for x=0.125 and 0.250 while we consider a

√2×√2×2 supercell for the other cases. We substi-

tute the Ga atom to the Mn atom in the centre of the

octahedron. We use the experimental volume from 19

and 24 to construct the supercells. The net magnetic

moment of the supercell will be diﬀerent from zero at

x=0.125, 0.250 due to the presence of an odd number of

Mn atoms. The impurities distribution could be treated

with more advances techniques to get accurate quantita-

tive results, however, the results obtained within these

approximations return satisfactory qualitative results.

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OrbitalorderandferromagnetisminLaMnO3dopedwithGaC.AutieriID,1,M.CuocoID,2,3G.CuonoID,1,yS.PicozziID,4andC.NoceID3,21InternationalResearchCentreMagtop,InstituteofPhysics,PolishAcademyofSciences,AlejaLotnikow32/46,PL-02668Warsaw,Poland2ConsiglioNazionaledelleRicerche(CNR-SPIN),I-84084Fisciano(SA),It...

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Orbital order and ferromagnetism in LaMnO 3doped with Ga C. AutieriID1M. CuocoID2 3G. CuonoID1yS. PicozziID4and C. NoceID3 2 1International Research Centre Magtop Institute of Physics.pdf

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Orbital order and ferromagnetism in LaMnO 3doped with Ga C. AutieriID1M. CuocoID2 3G. CuonoID1yS. PicozziID4and C. NoceID3 2 1International Research Centre Magtop Institute of Physics

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