Effect of double layer thickness on magnetoelectric coupling in multiferroic BaTiO₃-Bi_0.95Gd_0.05FeO₃multilayers

Abstract

The effect of double-layer thickness and partial substitution of Bi3+ by Gd3+ is demonstrated for multiferroic BaTiO3–BiFeO3 2–2 heterostructures. Multilayers of 15 double layers of BaTiO3 and Bi0.95Gd0.05FeO3 were deposited onto (0 0 1) oriented SrTiO3 substrates by pulsed laser deposition with various double layer thicknesses. X-ray diffraction and high resolution transmission electron microscopy investigations revealed a systematic strain tuning with layer thickness via coherently strained interfaces. The multilayers show increasingly enhanced magnetoelectric coupling with reduced double layer thickness. The maximum magnetoelectric coupling coefficient was measured to be as high as 50.8 V cm−1 Oe−1 in 0 T DC bias magnetic field at room temperature, and 54.9 V cm−1 Oe−1 above 3 T for the sample with the thinnest double layer thickness of 22.5 nm. This enhancement is accompanied by progressively increasing perpendicular magnetic anisotropy and compressive out-of-plane strain. To understand the origin of the enhanced magnetoelectric coupling in such multilayers, the temperature and magnetic field dependency of is discussed. The magnetoelectric performance of the Gd3+ substituted samples is found to be slightly enhanced when compared to unsubstituted BaTiO3–BiFeO3 multilayers of comparable double-layer thickness.

Enhanced Magnetoelectric Coupling in BaTiO3-BiFeO3 Multilayers—An Interface Effect

“…The average in-plane lattice constants of the multilayers a ave , as extracted from (103) RSMs are tabulated in Table 2, and additionally illustrated in Figure S5. Similarly to the trend reported in [13], the thickness ratio of BTO to BFO leads to a modulation of a ave . For low BTO-content (R01) a ave is close to the bulk a p.c.…”

“…This is further evidenced by the appearance of the negative switching peak in Figure S7a P-E and Figure S7b I-E measurements at increasing maximum voltages) only after the voltage required to switch the polarization in the positive direction is reached. Typical saturation polarizations range from 20 to 30 µC cm −2 , comparable to values obtained for BTO-BFO bulk composites [20], but lower than some of our previous multilayers [13]. Overall many samples suffered from premature electrode breakdown at fields ≤1.2 MV cm −1 , which makes a systematic comparison difficult.…”

“…While the BFO single layers and composite films show a maximum and subsequent decrease in α ME when H bias is increased from 0 T to 6 T, similar to the behavior of bulk samples [7], multilayers show a saturating behavior [6]. This field dependency was observed for all BTO-BFO multilayer samples since [8][9][10][11][12][13]. By variation of the p O 2 pressure during growth we found an increase of oxygen octahedral tilt [8] and micro-strain [9] with lowered p O 2 that correlated with a decrease of the respective α ME values.…”

“…As noted above, our more recent investigations suggested a certain influence of the double layer thickness, or alternatively of the BFO sublayer thickness of the multilayer stack on the magnitude of α ME [12,13]. This called into question the verifiability of the p O 2 dependence of α ME previously reported in [8,9], where an invariant number of pulses led to a roughly exponential increase of d dl with decreasing p O 2 .…”

“…Note, however, that the pulse numbers were kept constant for these experiments, yielding increasingly larger double layer thickness (d dl ) values with decreasing p O 2 . A decrease of the BFO sublayer thickness with constant BTO thickness leads to a significant increase of the measured α ME value [12][13][14]. Jochum et al showed that lowering d BFO from 50 nm to 5 nm led to an increase of α ME from 11 Vcm −1 Oe −1 to 56 Vcm −1 Oe −1 , which was accompanied by an increasing asymmetry of the hyperfine field distribution [12].…”