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Magnetic Field Melting of the Charge-Ordered State of La0.5Ca0.5MnO3: A Local Structure Perspective

The local structure about the Mn site in the half doped system La0.5Ca0.5MnO3 was measured in magnetic fields up 10 T to probe the melting of the charge ordered state. Examination of the Mn-O and Mn-Mn correlations reveal three distinct regions in the structure-field diagram. A broad region with weak field dependence (mainly antiferromatnetic phase below 7.5 T), a narrow-mixed phase region near ~ 8.5 T followed by a ferromagnetic phase region with strong field-structure coupling. At high field the Mn-O radial distribution becomes Gaussian and the Mn-Mn correlations are enhanced - consistent with the dominance of a ferromagnetic phase. The exponential change in resistivity in the first region (observed in transport measurements) is dominated by the reordering of the moments on the Mn sites from CE type antiferromagnetic to ferromagnetic order with only a weak change in the local distortions of the MnO6 octahedra.


Introduction
The CMR system (Re 1-x A x MnO 3 , where A is a divalent cation (such as Ca, Sr or Ba) and Re is a trivalent rare earth metal) has intriguing properties manifested by complex phase diagrams which an be tuned by doping, internal and external pressure and electric and magnetic fields [1, 2, 3]. As a function of these parameters and x (doping level), the system can be forced between metallic and insulating regions. For the classic La 1-x Ca x MnO 3 system, the reference state (x=0) is an antiferromagnetic insulator. Ca doping system produces a ferromagnetic conducting system for x~0.2 to x~0.5 and for x > 0.5 and antiferromagnetic insulating phases are found. These materials are rich in intriguing phenomena and are an excellent test bed for studies of systems in which electron-electron and electron phonon interactions can not be neglected [4].
The half-doped system systems Re 0.5 A 0.5 MnO 3 are at the boundary of the ferromagnetic and charge ordered (CO) insulating region of the phase diagram. These systems are characterized by the onset of ferromagnetic ordering (dominated by the double exchange) followed by a significant loss of bulk magnetization at lower temperature where super exchange dominates. The balance between superexchange and double exchange can be shifted by application magnetic field [5,6,7,8,9,10,11] and Mn site doping [12]. In La 0.5 Ca 0.5 MnO 3 , neutron diffraction measurements found evidence for distinct structural phases corresponding to ferromagnetic and antiferromagneric regions co-existing at low temperature [13,14]. Cooling in high magnetic field is found to stabilize the ferromagnetic phase and reverse the volume ratios of the two phases [13,15].
The coupling to magnetic field suggest very closely lying energy states of different magnetic structures. Indeed, in La 0.5 Ca 0.5 MnO 3 , the coexistence of mixed phases of insulating and metallic region has been found to be a common feature of manganites and other strongly correlated systems due to the small atomic changes requires to move from one phase to another [16,17].

II. Experimental Methods
The polycrystalline samples of La 0.5 Ca 0.5 MnO 3 and CaMnO 3 were prepared by solid state reaction in air as described in Ref. [19] and extensive studies of the transport, magnetic and zero field structural studies were presented in Ref. [20]. The Neél and Curie temperatures in for the samples were 225 K and 155 K, respectively. X-ray absorption samples (XAFS) were prepared by grinding the sample to form a 400 mesh powder and brushing it onto scotch tape. Layers of tape were stacked to produce a uniform sample for transmission measurements with jump µt ~ 1. A custom designed Oxford Instruments 10T split-coil horizontal field superconducting magnet was used for the measurements [21]. All measurements reported were performed with the x-ray beam parallel to the magnetic field. Measurements were made by cooling the sample to 115±0.01 K (into the charge ordered phase) in the vapor from the liquid He reservoir at zero magnetic field.
Measurements were conducted at fixed field with persistent currents.

Spectra were measured at the National Synchrotron Light Source (NSLS) at
Brookhaven National Laboratory utilizing beam line X23B and a Si(111) monochromator and focusing torroidal mirror with an energy cutoff of 11 KeV were used. Spectra were taken in transmision mode using nitrogen filled ion chambers. A Mn foil reference was employed for energy calibration. The reduction of the XAFS data was performed using standard procedures [22]. Calibration was accomplished by defining the first inflection point in the spectrum of Mn metal as 6539 eV. The ionization threshold, E 0 , was set at 6554.5 eV based on the zero field spectrum. The photoelectron momentum is defined as . XAFS were extracted from the spectra as the difference between the normalized spectra and an adjustable spline function fit through the postedge region, the parameters of which were adjusted to minimize low frequency residuals in the Fourier transform below R= 0.75 Å [23]. After comparison, two to four individual scans were averaged (see Fig. 1(a)).
Fits to the first shell Mn-O distribution were performed by Fourier transforming the XAFS data over the range 2.54 < k < 14.5 Å -1 to R-space [24]. The R-space fits (Fig. 2) to theoretical signals [25] based on the low temperature structure [26] of La 0.5 Ca 0.5 MnO 3 were performed over the range 0.75 < R < 1.95 Å assuming a cumulant expansion [27] for the radial distribution ( Fig. 1(b)). The simplest model which fit the data well consists of an amplitude reduction factor S 0 2 = 0.7 and variable parameters, R (average bond distance), σ 2 (the DW Factor (<(R -<R>) 2 > mean squared relative displacement) representing the width of the distribution) and C4 (kurtosis or flatness of the distribution < (R -<R>) 4 > -3 σ 4 ). The first shell coordination number was held fixed at 6. The total number of free parameters in the fit was 3, compared to the theoretical maximum number of independent parameters 2∆k∆r/p + 2 = 7 [28].
Parameter errors were obtained by determining the stability of the fits with respect to changes in each parameter.

III. Results and Discussion
Measurements were made at 115K, deep within the charge ordered (T N ~ 155 K) region of this half doped system. Fourier transform spectra (qualitatively analogous to radial distributions with the caveat that the amplitude is scattering atom dependent and the peak position are shifted toward the origin by the phase of the scattering factor) of the field dependent XAFS data from 0 to 10 T are shown in Fig. 2  characterizing the flatness (due to multiple Mn-O bond lengths). In Fig. 4 we see that at high field C4 vanishes indicating the disappearance of the Jahn Teller distortion (resulting in a narrow Gaussian distribution of distances).
In Fig. 5 we show the average distance as a function of magnetic field, The average Mn-O bond distance approaches 1.96Å, the value measured in the optimally ferromagnetic region of the La 1-x Ca x MnO3 phase diagram near x = 0.33 [30]. Again the increase in bond distance above 7.5 T signals the existence of a mixed phase region occurring before transformation to the ferromagnetic phase.
In Fig 6. we plot the DW Factor (σ 2 ) as a function of the magnetization. One can clearly see from this figure that in the charge ordered region that Jahn-Teller distortion decreases approximately linearly with the spin polarization. On crossing into the ferromagnetic metal (FMM) phase, the decrease in the distortion occurs at a rate 100 times higher than in the CO region.

IV. Summary
The local structure about the Mn site in the half doped system La 0.5 Ca 0.5 MnO 3 was measured in magnetic fields up 10 T to probe the melting of the charge ordered state.