Visible range colossal magnetorefractive effect in (La1 − y Pr y )2/3Ca1/3MnO3 films

We report a colossal magnetorefractive effect (MRE) in epitaxial thin films of a classical colossal magnetoresistance (CMR) manganite, (La1 − y Pr y )2/3Ca1/3MnO3 (y = 0.375 and 0.7). Close to the ferromagnetic (FM) phase transition a moderate applied magnetic field, H ∼ 10 kOe, results in a reduction of the optical reflectance by ∼18% for the photon energy E ∼ 2.7 eV. The MRE spectral behavior with three pronounced maxima at E = 1.6, 2.7 and 4.0 eV points out an inter-site nature of the involved optical transitions. The results are discussed within a phase separation scenario with coexisting FM metallic nanodomains antiferromagnetically coupled by correlated polarons. The probability of MRE optical transitions is maximal for antiparallel alignment of Mn3+/Mn4+-spins realized for the coercive field, H c ∼ 200–800 Oe, and is suppressed by stronger fields, which favor FM metallic behavior. As a result, both the optical reflectivity and the electrical resistance decrease, yielding a close similarity between the CMR and MRE behavior.


Introduction
Colossal magnetoresistant (CMR) [1] perovskite manganites remain in the focus of modern correlation physics due to a number of phase transitions, like metal-insulator (MI), with hysteretic field and temperature dependencies close to phase transition was demonstrated. This indicates that colossal MRE behavior is caused by the existence of a short-range ordered polaronic AFM COI phase, which provides the optically active inter-site (inter-band) transitions between Mn 3+ /Mn 4 ions with antiparallel aligned spins.

Samples and experimental techniques
For the optical study LPCMO films with Pr-doping y = 0.375 and 0.7 and thicknesses d ≃ 70 nm were chosen. The films were prepared by a metalorganic aerosol deposition technique [21,22]. A heteroepitaxial growth on MgO(100) substrates and a strain-free state of LPCMO was evidenced by transmission electron microscopy and x-ray diffraction [7]. The films were characterized by magnetization and four-point resistance, measured by means of MPMS (SQUID) and PPMS from 'Quantum design', respectively. The optical reflectivity at normal incidence was monitored for photon energies, E = 1.5-4.1 eV, and in-plane magnetic fields, H = 0-10 kOe, created by an electromagnet. Simultaneously with the reflectivity, measured within the diameter of the light spot,  ∼ 3 mm, the dc resistance from the same illuminated region was recorded by 4-point technique. The relative change of the reflectivity, Ref, in applied field is a measure of MRE, therefore we define: to quantify the observed changes. In the following we use the term MRE to refer to changes in the reflectivity induced by applied magnetic field. To quantify the changes of the resistance, R, we define: These quantities were measured for temperatures, T = 10-300 K, and magnetic fields, H = 0-10 kOe, as well as in the spectral range, E = 1.5-4.1 eV (MRE).

Results
Temperature dependences of the resistance, R(T), shown in figure 1(a), reveal sharp MI transitions at temperatures T MI ∼ 190 and 100 K for films with y = 0.375 and 0.7, respectively. The sharpness of MI transitions is characterized by large values of the dimensionless parameter, α R = (T/R)(dR/dT) = 80 and 250 for films with y = 0.375 and 0.7, respectively. The magnetic and MI transitions are coupled, T MI ∼ T C (see figure 1(b)), indicating an intrinsic nature of the first order phase transition in the LPCMO films under study and the minor role of extrinsic effects (chemical disorder and/or grain boundaries). A huge cooling-warming hysteresis in R(T) and M (T) curves, ΔT = 30 K, in the heavily doped sample (y = 0.7) is in line with the enhancement of the phase separation tendency by increasing of Pr-doping. In contrast, a small hysteresis, ΔT < 1 K, was observed in y = 0.375 film.
The characteristic feature in LPCMO films was the reduction of optical reflectance while cooling down across T C into the FM state. As shown in figure 1(b), the reflectivity of the y = 0.375 sample reduces by about 40% for T ≪ T C as compared to that in the PM state. Qualitatively the change of reflectance resembles M(T), signaling the magnetic origin of optical changes. Note that M(T) was measured in applied field, H = 100 Oe, whereas the reflectance in figure 1(b) was taken in an ambient field. Magnetic-field-induced changes of the reflectance, i.e. MRE, are found to be maximal at the phase transition (see inset to figure 1(b)) and are similar to the temperature behavior of CMR. In addition, the y = 0.375 sample reveals MRE = 17% and CMR = 1300%, which are significantly larger than those measured for the y = 0.7 sample, i.e. MRE = 4% and CMR = 900% (H = 10 kOe), obtained in the field-cooling (FC) mode.
In figure 2 one can see magnetic field dependences of the resistance (CMR) and reflectivity (MRE for λ = 827 nm) for a LPCMO-film with y = 0.375. For T < T C and T > T C the CMR(H) and MRE(H) scale quite well with each other. Very close to T C (figure 2(b)) and for low fields, −5 kOe < H < 5 kOe, both MRE and CMR show a similar hysteretic behavior. For very low (T < 50 K) and for high temperatures (T ≫ T C ) no magnetic-field-induced changes in the optical The reflectance (red curve, λ = 827 nm) for sample y = 0.375 is shown in (b). The inset in (b) demonstrates a similarity between the temperature behavior of CMR and MRE: both quantities are maximal close to T C . For the y = 0.7 sample a small MRE is due to the FC state. reflectivity and in the resistance were detected (not shown). For the heavily Pr-doped sample (y = 0.7), a huge temperature hysteresis, ΔT ∼ 30 K, in M(T) and R(T) in the vicinity of phase transition (see figure 1), correlates well with the pronounced field hysteresis in MRE and CMR, shown in figure 3. A FC in H = 10 kOe pushes the system into a low-resistance state, which can also be obtained by warming up in zero field. The strong hysteretic changes in the reflectivity for the y = 0.7 sample were obtained under zero-field-cooling (ZFC) down to T = 98 K, at which the resistance is maximal. By applying a magnetic field both resistance and reflectivity drop as shown in figure 3(a). During the following reduction of magnetic field the resistance remains almost constant, whereas the reflectivity increases slightly. Remarkably, the CMR(H) and MRE(H) dependences in the FC mode (inset of figure 3(a), figure 3(b)) reveal a characteristic 'butterfly-like' form with two maxima at the coercive field H c . This behavior is characteristic for both samples (y = 0.375 and 0.7), but it is especially pronounced for the heavily doped sample in the hysteretic region close to the phase transition.
To shed light on the microscopic origin of the colossal MRE the spectral dependence of the effect was measured for both LPCMO films at T ∼ T C in the range of photon energies, E = 1.2−4.1 eV. For the sample with y = 0.7, the spectrum represents irreversible changes of reflectivity after ZFC to T MI = 98 K (figure 3). As shown in figure 4 MRE spectra of both films reveal three maxima at phonon energies close to 1.6 (E 1 ), 2.7 (E 2 ) and 4.0 (E 3 ) eV. At these energies no special features were observed in the raw spectra of the reflected intensities (see figure 5). Furthermore, the distinct maxima and minima in the MRE spectrum cannot be caused by the multi-reflection interference since the phase shift between the rays, reflected at the interfaces air/LPCMO and LPCMO/MgO, changes only by about 3π/2 for the considered spectral range E = 1.2-4.1 eV (the LPCMO refractive index was approximated by the values for LSMO, taken from [25]). This could then only explain the occurrence of one broad maximum. The normalized presentations of MRE(H) curves, measured for E 1 , E 2 and E = 2.1 eV photon energies, show the same field behavior independent on the photon energy (see the inset in figure 4). Thus, by applying a moderate magnetic field, H = 10 kOe, it is possible to reduce the optical reflectivity by ∼18% for a blue light E 2 = 2.7 eV (λ = 460 nm). To the best of our knowledge, such large changes of the visible light reflectance in a magnetic material were not reported until now.

Discussion
Before we discuss the microscopic origin of the colossal MRE some general remarks on the studied magnetooptical phenomena can be made. Firstly, the observed field-induced changes in the reflectivity are caused by diagonal elements of the dielectric tensor. The non-diagonal elements (linear or quadratic Kerr-effects) are not allowed [23,24] in the used geometry with perpendicular incidence of light and magnetic field parallel to the [100] crystallographic direction of the film. Secondly, the observed MRE is a volume effect since light penetration depth [25], t = 1/α ∼ 100 nm, (α is absorption coefficient) exceeds the film thickness, d = 70 nm. This is also illustrated by the similar field dependences of the optical transmittance and reflectance (see inset in figure 5). Thirdly, a counterintuitive reduction of optical reflectivity for a metallic state (optical conductivity σ(ω) should be proportional to the dc conductivity, σ(0), in the Drude picture [26]) is well-known for correlated metals [17,18] undergoing a MI transition and was explained within the spectral weight transfer from the inter-band to the intra-band optical transitions.  (100)) at T = 188 K with no applied field and H = 10 kOe. Light intensity is reduced below E = 2.0 eV as a high-pass filter is inserted to block the second order light from the monochromator. The inset depicts the normalized reflectance and transmittance, Trans, for the same sample at T = 186 K and E = 1.5 eV.
A specific feature of LPCMO is related to the fact that MI and FM transitions develop through a phase separated state, in which FM nanodomains are AFM coupled by correlated JT polarons [7]. Close to phase transition the existence of short range spin (AFM) correlations between Mn ions may affect the inter-band optical transitions between spin polarized d-bands. The temperature-and/or field-induced appearance of long range FM order would suppress the probability of such transitions. Indeed, a reduction of optical reflectivity, ΔRef(T)/Ref (T = 240 K > T C ), was observed (see figure 1(b)) by cooling the film into the FM state; the reflectivity scales apparently with M(T) and most changes occur in the vicinity of T C . Similarly, the field-induced effect (MRE) is also maximal close to T C (see figures 2, 3) and vanishes at low temperatures (T ≪ T C ). Thus, optical conductivity in LPCMO is not sensitive to the magnetic domain structure in a homogeneous FM metallic phase in agreement with considerations of Bennet and Stern [27] that diagonal elements of the optical conductivity tensor should be quadratic functions of the magnetization. In contrast, close to T C the observed large hysteretic MRE and CMR (see figures 2 and 3) for relatively low magnetic fields, H = 0-10 kOe, point out the relevance of the magnetic domain structure. Moreover, the existence of two MRE minima (maxima in reflectivity) at coercive fields H c ≈ ±200 Oe and ±800 Oe for y = 0.375 and y = 0.7, respectively, infers that the optical conductivity is maximal in a demagnetized state with M = 0.
Recently we have shown [7], that in LPCMO films with y = 0.4 an intrinsic AFM coupling, actuated by correlated polarons, stabilizes a highly insulating state close to T C due to the antiparallel alignment of spins of adjacent magnetic nanodomains, R ∼ 6-8 nm, as sketched in the inset in figure 3(b). As a result, two CMR(H c ) minima appear in close correlation with two minima in MRE(H c ). Hence, the AFM correlations between Mn 3+ and Mn 4+ ions are important not only for CMR but also for MRE.
The related inter-site optical transitions should involve the t 2g and e g states of Mn with different oxidation states. Being mediated by (p-d) hybridization they are not forbidden by dipole selection rules as on-site transitions [28]. Actually, on-site transitions do not depend on the magnetic state, since the relative orientation of t 2g -and e g -spins is fixed by strong on-site Hund's coupling. Moreover, charge transfer transitions involving the O 2p states should also not depend on the spin order due to their double occupancy. We suggest the following inter-site transitions are responsible for the measured MRE spectra, shown in figure 4. The peak centered around E 1 = 1.5 eV is ascribed to (e g -e g ) transitions between Mn 3+ and Mn 4+ ions with parallel to t 2g core spins [14,20,28,29]. Above T C the excitation energy is determined by E 1 = E B + 1/2E JT ≈ 1.2 eV (the values E B ∼ E JT = 0.8 eV are taken from [2]). Since the octahedron of the Mn 4+ ion can be compressed according to a breathing mode, E B , so that the state after the electron transfer is excited, the additional excitation energy of E B ≈ E JT has to be added [28,30]. The temperature-and/or magnetic-field-induced ordering of t 2g spins favors delocalization of e g electrons by reducing the number of JT distorted octahedra [19] as observed by neutron scattering [8,9]. Thus, with an increase of spin order the spectral weight of this transition is shifted to lower frequencies. The second feature, centered around E 2 = 2.7 eV, is assigned to the (e g -e g ) transition between Mn 3+ and Mn 4+ ions with antiparallel t 2g spins with excitation energy of E 2 = E B + 1/2E JT + J H ≈ 3.2 eV [28,31]. Due to the antiparallel orientation of e g -and t 2g electron spins in the final state, Hund's coupling energy J H has to be added. Finally, the maximum around 4.0 eV can be caused by (e g -e g ) transitions between two Mn 3+ ions with antiparallel t 2g spins where the excitation energy, E 3 = E JT + J H + U ≈ 4.1 eV, is additionally increased by the Coulomb repulsion U. For a FM ordering the probability of E 2 and E 3 transitions is apparently reduced. Note that MRE in the UV-range is much smaller (see figure 4), since for high photon energies the charge transfer transitions involving O 2p states, which are not sensitive to the spin order, also contribute to the absorption of photons [27,28]. Earlier theoretical calculations [14,15] predicted a magnetic-field-induced reduction of optical conductivity for E > 1.0 eV. However, the pronounced MRE spectral features were not present most probably because electronic correlations, i.e. Hund's coupling and Coulomb repulsion, were neglected.
The interpretation given above allows one to establish a clear connection between the simultaneously recorded CMR(H) and MRE(H) curves (see figures 2 and 3) in LPCMO films. The coexisting FM metallic and AFM COI polaronic phases close to a first order phase transition [7] result in a highly resistive state for H = 0. An applied magnetic field melts the AFM polaronic phase, favoring FM metallic domains and yielding a decrease of both resistance (MRE) and reflectivity (MRE). Besides this phase transformation the effect of magnetic field on the intrinsic electronic properties of the FM metallic phase can be neglected, since magnetic field energy is too small to compensate thermal fluctuations in the FMM phase. Indeed, a very small MRE < 0.5% was observed in LSMO films [12], which is a classical double-exchange material with second-order (continuous) phase transition and without phase separation. However, both fieldand temperature-induced changes of the reflectance in LPCMO close T C are large and comparable with each other ∼10-20% (see figures 1(b) and 2). This evidences a highly susceptible phase separated state (or mixed phase) close to a very sharp first-order phase transition-a low magnetic field of a few kOe is able to actuate a significant reduction of optical reflectivity.
In summary, a 'colossal MRE' with reflectivity changes up to −18% for visible light in applied field H = 10 kOe was observed in thin LPCMO films with Pr-doping, y = 0.375 and 0.7. A close similarity between the field-induced changes of the reflectivity (MRE) and of the resistance (CMR) points out their same origin, i.e. phase separation. A spectral behavior of MRE could be rationalized within the inter-site (e g -e g ) transitions, the probability of which and, hence, optical conductivity depends on the short range ordering of t 2g spins of Mn 3+ and Mn 4+ ions.