Magnetoelastic coupling in La${}_{0.7}$Ca${}_{0.3}$MnO${}_3$/BaTiO${}_3$ ultrathin films

The magnetism of La${}_{0.7}$Ca${}_{0.3}$MnO${}_3$ (LCMO) epitaxial thin films grown on SrTiO${}_3$ (STO) and BaTiO${}_3$ (BTO) substrates is studied using polarized neutron reflectometry (PNR) and ferromagnetic resonance (FMR) techniques. In LCMO/BTO, PNR reveals a strongly suppressed magnetization of 300 kA/m, equivalent to a magnetic moment of 2 μB/Mn, throughout the LCMO layer, amounting to half the expected value. The largest suppression occurs near the interface with BTO, with magnetization values of 50 kA/m, equivalent to 0.3 μB/Mn. FMR is observable at 8.9 GHz only around the [110] crystallographic direction in thin LCMO/BTO. The resonance barely shifts as the applied field is rotated away from [110]. The FMR results are analyzed in terms of magnetoelastic anisotropy and compared to LCMO/STO grown under the same conditions. A two-layer magnetization model is proposed, based on strong out-of-plane anisotropy near the BTO interface and shown to qualitatively explain the main characteristics of the FMR results.

Figure 1

(a)–(c) NSF reflectivities measured on Asterix (Los Alamos Neutron Science Center) and experimental conditions given. (d) Magnetization and NSLD profiles obtained with co_nevot_croce from the experimental data. Note the absence of a plateau in the magnetization profile of LCMO/BTO at 120 K, which is present both for LCMO/STO and LCMO/BTO at 30 K.

Figure 2

Characteristic magnetic hysteresis loop of a Matteucci LCMO/BTO sample, at 100 K. No FMR was observed for this sample at 8.9 GHz.

Figure 3

Schematics of the geometry of magnetic field with respect to the LCMO film.

Figure 4

FMR spectra with various magnetic field orientations in plane (a) and out of plane (b) of a 120-Å-thick LCMO film on STO at 77 K. The spectra are offset for clarity, and their baseline value corresponds to the orientation angle (φ in plane and θ out of plane; see Fig. 3).

Figure 5

FMR spectra with various magnetic field orientations in plane (a) and out of plane (b) of a 150-Å-thick LCMO film on BTO at 77 K. The spectra are shifted vertically, and their baseline value corresponds to the orientation angle. The resonance around 0.3 T is DPPH used as a reference.

Figure 6

FMR spectra with various magnetic field orientations in plane (a) and out of plane (b) of a 120-Å-thick LCMO film on BTO at 77 K. The spectra are shifted vertically, and their baseline value corresponds to the orientation angle. The resonance around 0.3 T is DPPH used as a reference.

Figure 7

FMR fields for LCMO/STO 120 Å in plane (a) and out of plane (d), LCMO/BTO 150 Å in plane (b) and out of plane (e), and LCMO/BTO 120 Å in plane (c) and out of plane (f), as extracted from FMR spectra shown in Figs. 4, 5, 6. Lines correspond to simulations using Eq. (1) and parameters listed in Table .

Figure 8

FMR maps calculated for the LCMO/STO 120-Å layer [upper panels (a)–(c)] and the LCMO/BTO 150-Å layer [central panels (d)–(f)] in three selected applied field orientations B//[100], B//[110], and B//[001]. For the LCMO/STO layer, a one-layer model described by Eq. (1) was used. For LCMO/BTO, the two-layer model described by Eq. (5) was used. Green lines indicate the experimental microwave excitation frequency of 8.9 GHz. Lower panels [(g)–(i)] show the simulated magnetization in the direction of the applied magnetic field for LCMO/STO (black), and LCMO/BTO top sublayer A (red), and bottom sublayer B (blue).

Figure 9

Overlayed FMR maps where B is rotated in plane from B//[100] (φ $=$ 0°) to B//[110] (φ $=$ 45°). Resonance B${}_0$(1) (∼B${}_{\max }$) is only observed after a certain angle is reached (φ $=$ 35° in the model), then it moves to lower fields.

Figure 10

Overlayed FMR maps at two different orientations B//[100] and B//[110] of the two-layer model for three different model thickness: a) 120 Å, b) 500 Å, and c) 1000 Å. Green horizontal line indicates 8.9 GHz.

Figure 11

Simulated FMR spectra of the two-layer model with parameters of Table  for $t$ $=$ 150-Å LCMO/BTO in out-of-plane field rotation between B//[110] (θ $=$ 0°) and B//[001] (θ $=$ 90°).

Figure 12

Illustration of the two-layer model. With no applied field, the magnetization near the substrate is out of plane due to out-of-plane anisotropy possibly induced by coupling to AF clusters (a). Under saturating fields $M^{\mathrm{B}}$ turns in plane (b).