Local structure and dynamic properties of Mn

Local structure and dynamic properties of Mn-substituted manganites studied by EXAFS and anelastic spectroscopy Carlo Castellano Dipartimento di Fisica Università degli Studi di Roma “La Sapienza” Rome, Italy

Coworkers: –A. Martinelli (LAMIA-CNR-INFM, Genova) –M. Ferretti (LAMIA-CNR-INFM and Università di Genova) –M.R. Cimberle (IMEM-CNR, Genova) –R. Cantelli (Università degli Studi di Roma “La Sapienza”) –F. Cordero (CNR–ISC, Istituto dei Sistemi Complessi, Roma)

Summary -Short introduction on general properties of manganites -Our approach to the “problem” of phase separation nature -Local structure study (EXAFS) -Study of microscopic dynamics (Anelastic spectroscopy) -Results and Conclusions

General Properties of Manganites Perovskite manganites R1-xAxMnO3 where R is a trivalent rare earth (La, Pr or Nd) while A is a divalent alkaline metal (Ca, Sr or Ba), present very interesting conductive and magnetic properties : 1) A transition from a metallic-ferromagnetic state at low temperatures to an insulating-paramagnetic phase at higher temperatures (for 0.25 < x < 0.50 in La/Ca manganites); 2) A huge change in electrical resistivity when a magnetic field is applied. This effect is called colossal magnetoresistance and can be greater than 90 % (H ≥ 1 T; Jin et al., Science 264, 313 (1994)); 3) A strong dependence of resistivity from external pressure, ionic radius of dopants, oxygen stoichiometry and sinterization or deposition procedures; 4) Technological interest in magnetic memories reading – writing devices.

Jahn-Teller distortion (formulated in 1937) of the simple cubic cell to an orthorhombic one due to a degeneracy of the crystalline field orbitals eg in the Mn3+ ions (note: Mn atom is 3 d5 4 s2).

PHASE DIAGRAM of La1-xCaxMnO3 ) 0.00 < x < 0.20 : the system is insulating at every temperature, paramagnetic at high temperatures and spin canted antiferromagnetic or ferromagnetic insulating at low temperature; ) 0.20 < x < 0.50 : the system presents a metal-insulator transition around the Curie temperature and a magnetoresistance phenomenon; ) x = 0.50 : insulating phase with a double transition PM-FM-AFM; )

0 50

h

d

bit l

MI transition as percolation transition • Sample is naturally separated into ferromagnetic metallic regions and (charge localized) insulating domains • Sample becomes globally (DC) metallic when the metallic domains percolate the sample as a function of temperature and external magnetic field Uehara et al., Nature 399, 560 (1999); Moreo et al., Science 283, 2034 (1999); Fäth et al., 1999; Billinge et al., 1999.

Some open questions Which is the nature of the low temperature percolating insulating phase component for x < 0.50 ? Could it be charge ordered (CO) with a very low coherence length and then not visible by diffraction or by other spectroscopic techniques ? Is there the possibility to modify or control the magnetic and electronic mesoscopic texture (spatial phase variations over a wide range of length scales) recently observed by modern imaging techniques in manganites ? How we could emphasize the presence of a low temperature short-range charge ordered component in the x < 0.50 Ca doped compounds ? How we could amplify the presence of a low T metallic/FM component in the x > 0.50 Ca doped manganites ?

Mn is the magnetic ion therefore: Why do not substitute Mn with other ions in order to amplify the CO low temperature component inside the metallic one (in La1-xCaxMn1-y(Cr/Ni)yO3 with xCa < 0.50) or the metallic component inside the long-range CO one (xCa > 0.50) ? 1) We could enhance FM substituting Mn with Cr3+ or Ni2+ that are not associated to a lattice J-T distortion (e.g. Cr3+ (t2g3 eg0) has not a Jahn-Teller electronic configuration). The substituting ions act as random impurities determining an effective or site-random magnetic field (disordered J-T effect). 2) Otherwise Mn/Ni substitution can determine a different internal pressure on the conduction bandwidth along the Mn-O-Mn bond length due to the Ni different ionic radius respect to those of Mn or Cr and can influence the related magnetic interaction.

Selected samples: 1) La0.63Ca0.37Mn(0.97)Ni(0.03)O3 2) La0.63Ca0.37Mn(0.92)Ni(0.08)O3 3) La0.63Ca0.37Mn(0.92)Cr(0.08)O3 4) La0.25Ca0.75Mn(0.92)Cr(0.08)O3 0.07

La0.63Ca0.37MnO3 0.06

Polycrystalline samples

ρ (Ω cm)

0.05 0.04 0.03 0.02 0.01 0.00 0

50

100

150 T (K)

200

250

300

EXAFS EXAFS is the ideal technique to probe the local structure around Mn sites as a function of temperature. This synchrotron radiation technique is based on the modulation of the absorption coefficient due to the interference between the wave function of the electron photoemitted and of the backscattered one; In particular it is possible to study : 1) The local disorder parameter σ2(Å2) i.e. the mean square relative displacement of the photoabsorber (Mn) and of the backscattering atoms (oxygens in the case of the first coordination shell) as a function of temperature; 2) The interatomic distances between Mn and oxygen atoms.

Anelastic Spectroscopy ynamic experiment: vibrating sample 2ωτ Debye Peak 2 ( ) 1 + ωτ complex dynamic elastic

Q −1 = Δ

retarded response =

φ

= imaginary part

elastic energy loss Q

strain stress

lastic energy loss

Q-1

ω= 2 π f

Relaxation processes occur with a time τ(T) generally following the Arrhenius law τ=τ0exp(Ea/KBT).

time

ωτ(T)=1

Δ temperature

modulus, E = E’ + i E’’ σ=Eε

Anelastic spectroscopy measurements Electrostatically exciting the flexural vibration modes Prismatic Sample suspended on two nodal lines by thin wires

electrode: alternating stress

Flexural vibration

The vibration amplitude is detected by a FM circuit, in which sample + electrode are a variable capacitance. The vibration frequencies (kHz range) of the 1st, 3rd and 5th modes are in the ratios 1 : 5.4 : 13.3 and are excited during the same run. We measure the vibration frequency, f ∝ E ' , and the width of the resonance peak, Q ∝ 1 .

E ''

We investigate the low frequency dynamics of any defect or excitation coupled to strain: atomic diffusion, polarons, dislocations, phase transitions, domain walls…

Relation with other spectroscopies Most of the spectroscopic measurements can be expressed in terms of the time- (and in some case space-) Fourier transform of the correlation function of some microscopic x(r,t) or macroscopic x(t), where x is an atomic displacement for X-ray, neutron and NQR spectroscopies, an electric dipole or uniform polarization for optical spectroscopies and dielectric susceptibility, a spin variable for NMR etc… Similarly, the mechanical analogous of these well known susceptibilities is the dynamic compliance or strain susceptibility χε(ω) = εω/σω and through the fluctuation-dissipation theorem its imaginary part can be expressed as

ω ω i ωt χ ε = ∫ dt e ε (t )ε (0) = J ε (ω , T ) T T ''

Jε = spectral density of strain is related to the spectral density of the defects and excitations coupled to strain; e.g. Jε for the motion of an atom between two sites is proportional to the spectral density of the atomic motion,

J r = ∫ dt eiωt r (t )r (0) Since ω ~ kHz is much smaller than any excitation, vibration or tunneling frequency, only diffusive-like motions or incoherent transitions can be probed by anelastic spectroscopy.

Magnetization of La0.25Ca0.75Mn0.92Cr0.08O3 La0.25Ca0.75MnO3 Zero Field Cooled Field Cooled

9 m (emu / g Tesla)

La0.25Ca0.75Mn0.92Cr0.08O3

Low fields

μ0H = 0.05 T

6

Charge ordering transition drop at TCO ~ 240 K.

(μ0H=0.05T) 3

La0.25Ca0.75MnO3

La0.25Ca0.75Mn0.92Cr0.08O3

0 0

50

100

150 T(K)

200

250

300

350

The CO transition drop appears only as a small shoulder at a lower temperature 160 K overwhelmed by the enhanced ferromagnetism.

From a macroscopic point of view our hypotheses are verified: the substituting ions act as random impurities determining an effective or site-random magnetic field and favour the formation of a significant ferromagnetic-metallic component over the residual short-range CO one. The large hysteresis between ZFC and FC curves at low fields suggests an enhanced ferromagnetism attributed to the formation of ferromagnetic clusters. We have obtained similar results on the CO manganites Pr0.55Ca0.45MnO3 with Mn/Ni substitution y = 0.03 and y = 0.06.

EXAFS on charge ordered La0.25Ca0.75MnO3 0,006

N

1.98

Mn-O

=4

1.95

2

R(Å)

2

σ (Å )

0,005

T

0,004

1.89

2

2

σS = 0.00183 (Å ) Θ D = 500 K

0,003 0

50

100

150 T(K)

200

250

CO

1.92 N

Mn-O

=2

1.86 300

0

40

80

120

160 T(K)

200

240

280

We have evidenced an enhanced local disorder of the first Mn-O shell in the charge ordered phase also at very low temperatures (CO is compatible with a lattice disorder at low temperature). A greater cooperative lattice distortion can be allocated in the charge ordered phase. The breaking of symmetries of the high T state induces also a splitting of the Mn-O interatomic distances: 4 distances around 1.95 Å and 2 shorter distances around 1.89 Å as a function of temperature (C. Castellano et al., SSC 129, 143 (2004)).

EXAFS on La0.25Ca0.75Mn0.92Cr0.08O3 EXAFS evidences a local disorder σ2 similar to that present in the unsubstituted manganite (anomalous enhancement below the transition). Locally short-range charge ordering still exists and its effect on local order is slightly lowered (C. Castellano et al., SSC 136, 244 (2005)). N=4 N=2 N=4 N=2

x=0.75 x=0.75 Cr0.08 x=0.75 Cr0.08 x=0.75

sigma^2 x=0.75 Cr0.08 sigma^2 x=0.75

La

0.25

Ca

0 .75

Mn

0 .92

Cr

0.08

O

3

0.006

1.98 1.96

0.0055

2

2

σ (Å )

R(Å)

1.94 1.92

0.005

1.9

8% 0.0045

1.88 1.86 0

40

80

120

160 200 T(K)

240

280

320

0.004 0

50

100

150 200 T(K)

250

300

350

Anelastic Spectroscopy of La0.25Ca0.75Mn0.92Cr0.08O3 0.4 0.3 0.2 0.1

-0.1 -0.2 -0.3 -0.4

-2

Q

-1

10

-0.5

LCMO-75 #1 R30p35+36 R30p35 1442 Hz R30p35 18600 Hz

-3

10

-4

10

0

100

200

300

400

500 T (K)

600

700

800

900

2 2

LCMCO 75/8 #1 R30P42 1471.2 Hz [f/2=735.6] 7713 Hz [f/2=3856.5] 19055 Hz [f/2=9527.5] 1471.2 Hz R30P41 7693.4 Hz 19055 Hz

df /f

0.0

Magnetization of La0.63Ca0.37Mn1-y(Ni/Cr)yO3 600

m (emu / g Tesla)

500

400

300

La0.63Ca0.37MnO3 La0.63Ca0.37Mn0.97Ni0.03O3 La0.63Ca0.37Mn0.92Ni0.08O3 La0.63Ca0.37Mn0.92Cr0.08O3

200

100

0

0

50

100

150

200

250

300

350

T(K)

For x < 0.50 the ferromagnetic metallic phase maintains its long range character even after Mn substitution, but both the doping species (Ni or Cr) lower TC and broaden the magnetic transition.

Some Mn3+-O2--Mn4+ conduction paths become e.g. Mn3+-O2--Ni2+ or Cr3+-O2--Cr3+ (at high dopings), where a superexchange interaction can be induced between ions having different magnetic moments, influencing the PM-FM transition. In addition Ni2+ and Cr3+ doping induces a reequilibration of the [Mn3+]/[Mn4+] ratio. The decrease in TC associated with the Mn substitution is anyway mainly determined by the variation of the electronic contribution to TC with the structural disorder introduced by the presence of cations of different sizes at B sites (e.g. in this case Mn4+ and Ni2+).

El-Fadli et al., 2002

Ni2+ determines a different chemical pressure because of its greater ionic radius with respect to those of Mn or Cr3+ (considering the ionic species in octahedral coordination the following ionic radii are reported: Mn3+ = 0.645 Å; Mn4+ = 0.53 Å; Cr3+ = 0.615 Å; Ni2+ = 0.69 Å). The substitution of atoms in specific lattice sites and the presence of cations of different sizes at B sites acts on the structural disorder or on the tolerance factor of the ABO3 perovskite structure and therefore on double exchange (t = dA-O/ 2 dB-O, where dA-O and dB-O are the A-O and B-O bond lengths in the ABO3 perovskite-type structure).

0.008 y(Ni) = 0.08 y(Cr) = 0.08 y(Ni) = 0.03 y = 0.00

MI x=0.00

0.006

2

2

σ (Å )

0.007

T

0.005 0.004 0.003

50

100

150

200 T(K)

250

300

350

Also EXAFS local order variations as a function of Mn substitution reveal a progressive broadening and lowering of the PM-FM transition (C. Castellano et al., SSC 136, 244 (2005)). Due to the larger ionic radius of Ni2+, the effect induced by the substitution of 3% of Mn ions with Ni is of the same magnitude as that obtained with 8% of Cr; the [Mn3+]/[Mn4+] ratio in both compounds is nearly the same for both compositions ([Mn3+] = 0.59 and 0.60 for 3% of Ni2+ and 8% of Cr3+, respectively).

Anelastic Spectroscopy on La0.63Ca0.37Mn0.92(Ni/Cr)0.08O3

-3

1.2x10

Tc

La0.63Ca0.37MnO3

0.0

-3

1.0x10

-4

8.0x10

r30p10.1.DAT 1497 Hz r30p10.5.DAT 19410 Hz

-2

-6.0x10

2 2

df /f

-4

Q

-1

6.0x10

-1

-1.2x10 -4

4.0x10

-4

2.0x10

-1

-1.8x10 0.0 100

150

200 T (K)

250

300

Anelastic spectroscopy on La1-xCaxMnO3 (x < 0.50) 1x10

-3

La 0.6Ca 0.4MnO 3 0.2

-4

Q

-1

8x10

I modo (1220 Hz) V modo (15788 Hz)

2 2

-4

df /f

6x10

0.1 4x10

-4

2x10

-4

0.0 0 0

50

100

150 T (K)

200

250

300

Anelastic starting remarks on CMR manganites We have shown by anelastic spectroscopy measurements: -The presence of a high temperature relaxation process connected to the orthorhombic-trigonal phase transition; -An elastic energy loss peak at the conductive phase transition and another maximum below this temperature; -This secondary peak slightly below the Curie temperature has a temperature and a frequency dependence typical of frustrated and inhomogeneous systems like relaxor ferroelectrics or spin glasses; -Such a fluctuation in the anelastic spectra could be driven by a strong magnetoelastic coupling determined by the formation of magnetic polarons with sizes of tens of Ångstroem (F. Cordero, C. Castellano et al., PRB 65, 012403 (2002)); -It could also be due to the fluctuation in the magnetization of the nanometer ferromagnetic domains observed previously by STM and NMR or to the formation and disappearance of a charge ordered component and to the reorientation of the Jahn-Teller distorted stripes present in this insulating state.

1.2

x10

-3

La0.63Ca0.37Mn0.92Cr0.08O 3

r30p39.1 757.54 Hz r30p39.5 9563.98 Hz

x10

-2

18.0

1.0

0.8 12.0 Q

2 2

-1

df /f

0.6

6.0

0.4

0.2 0.0 100

150

200 T (K)

250

300

18.0

x10

-3

La0.63Ca0.37Mn0.92Ni0.08O 3

0.9

x10

-2

r30p41.1 911.62 Hz r30p41.5 11235.44 Hz 12.0

Q

-1

2 2

df /f

0.6

6.0

0.3

0.0 100

150

200 T (K)

250

300

Summary of anelastic results 1.5

x10

LCMO-37 1351.14 Hz LCMO-37 17526 Hz LCMCO 63/92 757.54 Hz LCMCO 63/92 9563.98 Hz LCMNO 63/92 911.62 Hz LCMNO 63/92 11235.44 Hz

-3

1.2

0.25

0.20

0.15

0.9 Q

2 2

-1

df /f 0.10

0.6

0.05

0.3

0.00 0.0 100

150

200 T (K)

250

300

The Jahn-Teller Q3 vibration mode of the MnO6 octahedron (apical bond stretching and basal plane compression), associated with the 3dz2 orbital electronic occupation, is most probably the main mode involved in the anelastic relaxation processes. This attitude is due to its asymmetry or anisotropy and therefore to the presence of at least two non-equivalent elastic dipole states (basic condition for anelastic relaxation)

By substituting the Mn3+ ion with Cr3+ or with the larger Ni2+ (nonJahn-Teller) the Q-1 peak intensity decreases and disappears: the Jahn-Teller distortion related to the charge and orbital ordered (3dz2 orbitals) percolating phase is weakened ⇒ the Q3 mode and the associated anelastic relaxation process are weakened.

….observing that EXAFS, anelastic spectroscopy and magnetic measurements are in mutual agreement in all cases, thus confirming the correlation between local disorder related to charge localization, dynamic and magnetic degrees of freedom. There is an extraordinary influence of the partial Mn substitution on the strong temperature dependence of magnetoresistance and resistivity. Y. Sun et al., 2001 250

La0.63Ca0.37Mn0.92Cr0.08O3

rho (mohm cm)

200 Rho Zero Gauss Rho 50 Gauss Rho 500 Gauss Rho 1T Rho 2T Rho 4T Rho 7T Rho 9T

150

100

50

0 0

50

100

150

200

T (K)

250

300

350

La0.63Ca0.37Mn1-xCrxO3

Conclusions We are studying the characteristics of the intrinsic phase separation of manganites not only in presence of external perturbations (i.e. magnetic field, pressure etc...) but also: 1) modifying the conductive and magnetic properties directly changing their chemical composition; 2) performing different and complementary experimental measurements in order to characterize the different aspects of this complex phenomenon; 3) playing physically and chemically on the balance between the different components of the inhomogeneous low temperature phase with the challenge to tune and to optimise the physical properties like magnetoresistance temperature dependence, magnetic transition temperature, magnetic field intensity etc…also for technological applications.