Vertical Strain-Driven Antiferromagnetic to Ferromagnetic Phase Transition in EuTiO₃ Nanocomposite Thin Films

Abstract

Three-dimensional (3D) strain induced in self-assembled vertically aligned nanocomposite (VAN) epitaxial films provides an unrivaled method to induce very large strains in thin films. Here, by growing VAN films of EuTiO3 (ETO)–Eu2O3 (EO) with different EO fractions, the vertical strain was systematically increased in ETO, up to 3.15%, and the Eu–Ti–Eu bond angle along ⟨111⟩ decreased by up to 1°, leading to a weakening of the antiferromagnetic interactions and switching from antiferromagnetic to ferromagnetic behavior. Our work has shown for the first time that Eu–Ti–Eu superexchange interactions play a key role in determining the magnetic ground state of ETO. More broadly, our work serves as an exemplar to show that multifunctionalities in strong spin–lattice coupling perovskite oxides can be uniquely tuned at the atomic scale using simple VAN structures.

Defect engineering of magnetic ground state in EuTiO3 epitaxial thin films

“…In ETO, the Eu–Ti–Eu super‐exchange interaction, which is known to stabilize the AFM state in bulk, is suppressed by the reduction of the Eu–Ti–Eu bond angle from 180°. Indeed, a similar interpretation has also been qualitatively given for the FM ground state of vertically strained ETO thin films 50 . To validate this scenario, we calculated the Eu–Ti–Eu bond angle in the defect‐engineered ETO.…”

“…As shown in Figure 5D, the Eu–Ti–Eu bond angle near the Eu and O vacancies in both the high‐temperature (cubic) and low‐temperature (tetragonal) phases deviates from the initial value of 180° by 2‐4°. These values are larger than that of ~1° expected in the vertically strained ETO thin films mentioned earlier 50 . Hence, the reduced Eu–Ti–Eu bond angle suppresses the AFM J 3 via Eu–Ti–Eu super‐exchange in defect‐engineered ETO thin films.…”

“…The magnetic field‐dependent magnetization, M ( H ), of the ETO thin films grown at the base pressure and the post‐annealed thin films measured at 2 K (Figure 3B) also demonstrate the FM phase. The magnetization shows clear hysteretic behaviors (see the inset of Figure 3B) with a large M s value approaching ~7 μ B /Eu and a finite coercive field ( H c ) of 9.6 Oe 39,40,49,50 . On the other hand, the as‐grown thin film at P (O 2 ) = 10 −4 Torr shows highly suppressed magnetization with a M s value of ~1.3 μ B /Eu and a H c of 0 Oe.…”

On the resistance minimum in LaAlO$_3$/Eu$_{1-x}$La$_x$TiO$_3$/SrTiO$_3$ heterostructures

“…At the same time, experiments revealed that antiferromagnetism only occurs for post-annealed PLD films 26,27 , and films grown by MBE 37 . Here we used PLD films without annealing, which can become ferromagnetic 23,[26][27][28] because of either the formation of oxygen vacancies 27,38 or a longer out-of-plane lattice constant, which leads to a bigger lattice volume 23,26,39 . The films tend to be ferromagnetic if the ratio between out-of-plane constant and in-plane constant is larger than 1.02 or less than 0.99.…”

On the origin of supertetragonality in BaTiO$_3$

“…This tunability has strongly motivated the technological interest in these metal oxides, and although past decades have witnessed great efforts in fundamental research, there remains a great need for novel materials designs and manipulations. The former has been an essential pathway to find new multiferroic systems, and to the exploration of double perovskites [3,4] as well as the idea of ferroelectric materials driven by polar octahedral rotations [5,6], while for the latter the development of vertically aligned nanocomposites (VANs) has recently attracted great attention [7][8][9][10][11][12]. For example, an enhanced Curie temperature of 616 • C in BaTiO 3 has been induced through negative pressure [11].…”