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Imaging and Control of Ferromagnetism in a Polar Antiferromagnet

Atomically sharp oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. The interplay between electrostatic boundary conditions, strain and dimensionality in ultrathin epitaxial films can result in monolayer-scale transitions in electronic or magnetic properties. Here we report an atomically sharp antiferromagnetic-to-ferromagnetic phase transition when atomically growing polar antiferromagnetic LaMnO3 (001) films on SrTiO3 substrates. For a thickness of five unit cells or less, the films are antiferromagnetic, but for six unit cells or more, the LaMnO3 film undergoes a phase transition to a ferromagnetic state over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO3 (001) films. Our results demonstrate how new emergent functionalities can be visualized and engineered in atomically thick oxide films at the atomic level.

Atomically sharp oxide heterostructures often exhibit unusual physical properties that are absent in the constituent bulk materials. The interplay between electrostatic boundary conditions, strain and dimensionality in ultrathin epitaxial films can result in monolayer-scale transitions in electronic or magnetic properties. Here we report an atomically sharp antiferromagnetic-to-ferromagnetic phase transition when atomically growing polar antiferromagnetic LaMnO 3 (001) films on SrTiO 3 substrates. For a thickness of five unit cells or less, the films are antiferromagnetic, but for six unit cells or more, the LaMnO 3 film undergoes a phase transition to a ferromagnetic state over its entire area, which is visualized by scanning superconducting quantum interference device microscopy. The transition is explained in terms of electronic reconstruction originating from the polar nature of the LaMnO 3 (001) films. Our results demonstrate how new emergent functionalities can be visualized and engineered in atomically thick oxide films at the atomic level.
Modern thin film deposition techniques enable the synthesis of complex oxide thin films with unit cell (uc) level control over the thickness. Remarkably sharp phase transitions have been discovered in several systems upon increasing film thickness [1][2][3][4][5][6]. The most prominent example is the 2-dimensional electron gas formed between insulating thin films of LaAlO 3 and insulating TiO 2 -terminated SrTiO 3 (STO) substrates, which occurs at a critical LaAlO 3 thickness of 4 uc [2]. The possibility to select a different electronic/magnetic phase by adding a single layer of perovskite unit cells, with a typical lattice parameter of about 4 Å, offers tantalizing opportunities for nanostructured electronic/spin devices.
Since various interesting properties have been demonstrated in LaMnO 3 (LMO) bulk, thin films and multilayers, ranging from the occurrence of orbital waves to its use as a catalyst for water splitting [7][8][9][10][11], LMO is an ideal candidate for control of its functionalities. LMO is a Mott insulator with an orthorhombic structure, based on a √2a0, √2a0, 2a0 unit cell where a0 ≈ 3.9 Å is the elementary perovskite unit cell parameter. In stoichiometric LMO, La and Mn are both 3+ ions. Therefore, LMO is a polar material which contains alternatively charged (LaO) 1+ and (MnO 2 ) 1-. Mn 3+ , with electronic configuration t 2g 3 e g 1 and spin S = 2, is a Jahn-Teller ion. If LMO had a perfect cubic perovskite structure one might expect a conducting ground state due to the mobility of the unpaired electron in the degenerate e g band. However the e g orbital degeneracy is lifted by the Jahn-Teller effect, and distorted MnO 6 octrahedra line up with alternating long and short Mn-O bonds in the a-b plane, leading to orbital ordering, which contributes to the electron localization [12]. As a result of superexchange coupling [13], the compound is an A-type antiferromagnet, with Mn spins ferromagnetically aligned in-plane, and alternate planes aligned antiferromagnetically. The Néel temperature is ~ 140 K. In bulk, the antiferromagnetism can be slightly canted to produce a weak ferromagnetism of ~ 0.18 µ B per uc [14][15][16], which is attributed to the antisymmetric Dzyaloshinskii-Moriya interaction introduced by rotation of the MnO 6 octahedra. In thin films, ferromagnetism accompanied with insulating behavior is often observed, with a Curie temperature of ~ 115 K [17]. The origin of this ferromagnetism is still unclear, but in addition to the above-mentioned mechanism, defects and epitaxial strain can be important factors [17][18][19][20][21]. Here, we report a controllable monolayer-critical magnetic effect, whereby a uniform ferromagnetic state appears in LMO at a critical thickness of 6 uc.
In our study, LMO (001) films with a thickness varying from 1 uc to 24 uc were grown using pulsed laser deposition monitored by reflection high energy electron diffraction (RHEED), on TiO 2 -terminated (001)-oriented STO substrates, in a 10 -2 mbar oxygen pressure. LMO films are coherently strained up to 20 uc, as verified by X-ray diffraction and RHEED. All the films were found to be insulating by conductance measurement.
The distribution of local magnetic stray field emanating from the LMO films was imaged by scanning Superconducting QUantum Interference Device (SQUID) microscopy [22], called SSM, in zero applied field at 4.2 K. The essential part of the SSM is the square pickup loop with an inner size of ~ 3 x 5 µm (Fig. 1a). During the measurement, the pickup loop was scanned ~ 2 µm above the sample surface at a contact angle of approximately 10 degrees.     To find out whether the ferromagnetism is indeed solely dependent on the LMO thickness, we fabricated and measured a sample that consisted partly of a 5 uc-thick and partly of a 7 uc-thick LMO film. The sample was prepared first by growing the 5 uc LMO film, then an ex situ a shadow mask was mounted to protect half of the sample, after which 2 extra uc of LMO were grown on the non-covered part. The sample was subsequently imaged by SSM following the usual measurement procedure. As seen from Fig. 3a, the 5 uc area does not display any signal within the noise level and the 7 uc area reveals an inhomogeneous magnetic field distribution comparable to the regular 7 uc LMO films (see Fig. 2c). Figure 3b demonstrates the line profile of the magnetic field-value difference between the 7 uc area and the 5 uc area. This experiment demonstrates the ability to modify the magnetic properties of this oxide compound by the controllable addition of less than 1 nm of the same material.  Such transfer then decreases the electric field in LMO. The transferred charge as a function of thickness is zero below t c , and increases asymptotically with increasing thickness to 0.5e [26]. The electron transfer makes the interface region of LMO electron-doped and the top surface region of LMO hole-doped. This is analogous to the electronic phase separation known for bulk manganites [27], which in our case is stabilized by an intrinsic electric field of the polar LMO film. The doping of LMO tilts the exchange interaction between Mn ions from superexhange to double exchange and lead to ferromagnetism when the sufficient number of dopants is available. Our density-functional calculations predict that hole-doping favours the ferromagnetic ground state when the doping exceeds 0.08 e/Mn, in agreement with the experimentally reported bulk phase diagram of LMO [28,29]. Thus, it is expected that electronic reconstruction will favour ferromagnetism above a certain critical thickness of LMO.
We note that, contrary to the LAO/STO system, where the interface becomes metallic above the critical thickness, our films remain insulating. This difference is due to the large band gap of LAO which leads to electron transfer to the conduction band of STO, whereas in the LMO/STO case the electronic reconstruction is revealed in the self-doping of LMO.
The exact thickness at which the ferromagnetism occurs depends on the spread of the charge. Figure 5b shows an estimated doping charge per Mn atom when all the charge is projected onto just one uc (black curve) and when the charge is spread over 2 uc (red curve).
We see that in the latter case the bulk phase diagram predicts phase transition from the insulting antiferromagnetic phase to the insulating ferromagnetic phase when LMO thickness exceeds 6 uc, which is consistent with our experimental observations. The assumption of the charge spread over 2 uc agrees with results of first-principles calculations [30] that indicate screening of the interface charge in doped LMO.
In order to verify the feasibility of this electronic reconstruction mechanism, we compared the