H.-U. Habermeier and X.-H. Li
The observation of a colossal magnetoresistance (CMR) close to the spin ordering temper-ature, Tc, in doped rare earth manganites of the type Ln1 xBxMnO3(with Ln = La, Pr, Nd and B = Ca, Sr, Ba, Pb) has generated considerable research activities due to their peculiar electronic and magnetic properties caused by strong electron correlation. The CMR mate-rials have a perovskite-type crystal structure in which the corner-sharing oxygen octahedra surrounding the Mn ions can cooperatively rotate around the cubic [110] or [111] direction, causing a reduction of the original cubic to an orthorhombic or rhombohedral symmetry. It is now accepted that both, double exchange and Jahn-Teller (JT) lattice distortions, play an important role in determining their electronic and magnetic properties. If the JT distorted oxygen octahedra surrounding the Mn3+ ions are not distributed randomly in the mate-rial, an anisotropy of the transport properties can be expected in addition to the anisotropy introduced by spin ordering at temperatures below the spin ordering temperature.
In principle, experimental studies of anisotropy effects in transport properties can directly be performed using sufficiently large single crystals or – in the case of unavailability of the appropriate crystals – with single-crystalline thin films. In reality, using the thin film approach, the technological problems for the measurement of the out-of-plane properties can only be overcome if, e. g., the electrical conductivity in plane and out-of-plane differ by orders of magnitude. In our experiments, we use an unconventional thin film approach to study anisotropy effects and describe experimental evidence of a pronounced anisotropy in the transport properties of doped LaMnO3that is not compatible with a cubic symmetry or faintly distorted cubic symmetry. an YBCO film deposited on a vicinal cut STO substrate.
Our experiment is designed in analogy to the well established so called thermopile effect in YBa2Cu3O7(YBCO) single crystal thin films grown on vicinal cut STO single crystal sub-strates. This approach probes the anisotropic thermoelectric properties of a material rather than the dc conductivity. The origin of the thermopile effect lies in the generation of ther-moelectric fields transverse to a temperature gradient, OT. The thermoelectric fields are caused by the nonzero off-diagonal elements of the Seebeck tensor Sij. Off-diagonal See-beck coefficients occur in tetragonal, trigonal, hexagonal and orthorhombic crystals only, ifOT has an orientation not along the low indexed crystallographic axes. In the thermopile experiment, the non-crystallographic system orientation is realized by depositing YBCO thin films onto SrTiO3(001) single crystal substrates intentionally miscut by an angle to-wards the [010] direction as shown schematically in Fig. 40. This causes the CuO2planes of the YBCO to grow with a tilt anglewith respect to the substrate surface plane. An illu-mination of the film with a short laser pulse generates a transient temperature gradientOT, which gives rise to a voltage, U. Quantitatively, the integration of the Seebeck equation
O
(l = length of illuminated film strip, t = film thickness,OT = temperature difference of top and bottom of the film,OS = Sab– Sc, the difference of the Seebeck coefficients in the ab-plane and along the c-axis). In YBCO the signals detected at room temperature even for
moderate laser fluence of 50 mJ/cm2can be as large as 18 V for a sample with t = 250 nm and an illuminated length of 2 mm. The experiments reported here, demonstrate the exis-tence of an unexpected laser-induced voltage (LIV) in La2=3Ca1=3MnO3 thin films in the order of 1/10 of those measured in YBCO. The existence of these laser-induced signals suggests a microscopic mechanism comparable to that in YBCO implying a substantial anisotropy of the thermoelectric properties of La2=3Ca1=3MnO3.
Figure 41: a) Laser-induced voltage signals measured at room temperature for La2=3Ca1=3MnO3 thin films of thickness 180 nm in [100] and [010] direction, b) as a function of illuminated length, c) as a function of illuminating laser energy density, d) and the vicinal angle of the substrate.
Expitaxial La2=3Ca1=3MnO3 films were prepared on STO (001)-oriented substrates with intentional miscut angles using the standard pulsed laser deposition technique, subse-quently patterned and contacted by the usual lithographic and deposition procedures. The films consist of single-phase material with properties typical of epitaxial CMR thin films.
The thermoelectric experiment is performed using an excimer laser (KrF, = 248 nm,
data as recorded for a specimen deposited on a perfectly oriented (001) substrate together with the signals for a specimen deposited on a substrate with 100 tilt vs. the [010] direc-tion. As shown in Fig. 41a), the existence of a laser-induced voltage in La2=3Ca1=3MnO3 thin films oriented not along the principal crystal axes, suggests its thermoelectric origin.
Consequently, the predictions of Eq.(10) such as its dependence on the crystallographic orientation, length, thickness and laser fluence, i. e., OT must hold. The results of the most crucial experiment are given in Figs. 41a) – d) for La2=3Ca1=3MnO3films deposited on vicinal cut substrates and patterned in a L-shape with branches oriented along the [010]
and [100] directions, respectively. According to Eq.(10), U must vanish. for the strip oriented along [100]. Along the tilt direction [010], however, U should be nonzero and scale linearly with the illuminated length, laser fluence and the tilt angle. The data given in Fig. 41a) – d) are in accordance with the predictions of the Seebeck equation. A careful study of the temperature dependenceOS(T) and(T) suggests a close relationship between these quantities supporting the validity of the predictions of Eq.(10).
In conclusion, we have demonstrated the existence of a laser-induced voltage in La-Ca-Mn-O thin films deposited on vicinal cut substrates which is compatible with the predictions given by a model based on the existence of off-diagonal elements of the See-beck tensor. This implies the presence of an anisotropy of the material. We ascribe the origin of this anisotropy to the interplay of the Jahn-Teller distortion caused by the Mn3+ ions and biaxial interfacial strain of the films.