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Exchange Bias and Inverted Hysteresis in Monolithic Oxide Films by Structural Gradient

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Exchange Bias and Inverted Hysteresis in Monolithic Oxide Films by Structural Gradient PHYSICAL REVIEW RESEARCH 1, 033160 (2019) Exchange bias and inverted hysteresis in monolithic oxide films by structural gradient Mohammad Saghayezhian ,1 Zhen Wang,1,2 Hangwen Guo,1 Rongying Jin,1 Yimei Zhu,2 Jiandi Zhang,1 and E. W. Plummer 1,* 1Department of Physics and Astronomy, Louisiana State University, Baton Rouge, Louisiana 70803, USA 2Condensed Matter Physics and Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973, USA (Received 23 September 2019; published 9 December 2019) Interface-driven magnetic properties such as exchange bias and inverted hysteresis are highly sought after in modern functional materials, where at least two magnetically active layers are required. The ability to achieve these functionalities in a single-layer thin film (monolithic) reduces the dimensionality while enriching the magnetism. Here we uncover a previously unseen part of the phase diagram of a monolithic epitaxial thin film of La0.67Sr0.33MnO3 on SrTiO3 which exhibits inverted hysteresis, spontaneous magnetic reversal, and exchange bias due to a structural gradient in the oxygen-octahedral network. Varying the growth conditions, we have mapped the phase diagram of this material and discovered that at a specific oxygen pressure and above a critical thickness, a complex magnetic behavior appears. Atomic-scale characterization shows that this peculiar magnetism is closely linked to a continuous structural gradient that creates three distinct regions within the monolithic film, each with a different magnetism onset. Extracting oxygen-octahedral geometry by electron microscopy, we found that the Curie temperature is directly correlated with the metal-oxygen bond angle. This study illustrates the importance of octahedral geometry in shaping the physical properties of the materials. DOI: 10.1103/PhysRevResearch.1.033160 Introduction. The structure and composition of transition Recently, it has been shown that the interfacial structure metal oxides (TMOs) are two deciding factors in determining coupling together with composition variations near the in- their physical properties, providing geometrical constraints terface leads to complex magnetic interactions [26]. In fact, on the oxygen-octahedral network that leads to a rich phase the collaborating effects of change in bond geometry and diagram with a variety of physical properties such as super- composition are both necessary for observing spontaneous conductivity, ferroelectricity, or complex magnetic ordering magnetic reversal and inverted hysteresis. Therefore, it im- [1–7]. In the bulk, the structure-composition duo is often mediately becomes clear that the controlled variation of these modified via tuning the growth conditions, doping, or external two quantities via growth conditions, across a single-layer stimuli such as pressure [8–11]. Compared to the bulk, in thin film, can create intralayer magnetic couplings. This can epitaxial TMO heterostructures, various additional pathways provide a platform to not only access unexplored area of the are available to systematically amend this duo [12–14], such material’s magnetic phase diagram but also quantify the role as strain doping via inert atom implantation [15], strain of octahedral geometry on magnetism. Realizing an atomic- engineering [16,17], interfacing materials with octahedral- scale understanding of this type of monolithic thin film is symmetry mismatch [18–20], or structural migration via elec- remarkable for future devices where one-step growth, use of a trical gating [21,22]. Another approach in diversifying the single material, and reduced dimensionality all serve in favor physical properties is the creation of gradient phases in which of simplifying fabrication while enriching functionality. the composition or the corner-connected oxygen-octahedral In this paper we show that by controlled monolithic thin network continuously changes across the thin film. Examples film growth under different growth conditions, the magnetic are monolithic thin films of LaCoO3-x,La2/3Sr1/3MnO3-x, properties of the material undergo drastic changes, while the and SrCoO3-x, where large-scale oxygen vacancy migration films retain their structural coherency. Using La2/3Sr1/3MnO3 or creation, topotactic structural phase transition [21,23,24], (LSMO) as the target material, we systematically change and spin state manipulation [17,24,25] have been observed. the oxygen pressure and film thickness to manipulate the Change related to oxygen octahedra is the one key factor that structure and magnetism. Magnetic characterization reveals is shared between all mentioned physical phenomena. a new metamagnetic phase that appears above a critical thickness, exhibiting a variety of magnetic properties such as large exchange bias, magnetization reversal, and inverted hysteresis. These properties are not a product of defects or *[email protected] imperfections and they are absent in bulk form, and thin films grown above or below a critical oxygen pressure. Published by the American Physical Society under the terms of the Using atomically resolved transmission electron microscopy Creative Commons Attribution 4.0 International license. Further (STEM) and electron energy loss spectroscopy (EELS) we distribution of this work must maintain attribution to the author(s) have studied the structure and composition across the thin and the published article’s title, journal citation, and DOI. films. Counterintuitively, STEM and EELS analyses show 2643-1564/2019/1(3)/033160(8) 033160-1 Published by the American Physical Society M. SAGHAYEZHIAN et al. PHYSICAL REVIEW RESEARCH 1, 033160 (2019) that oxygen pressure and oxygen content in the film do not The first FM transition (FM-1) occurs at 301 K and the have a linear relationship, where less oxygen pressure does second (FM-2) at 198 K. Figures 1(f)–1(i) show the magnetic not necessarily create more oxygen deficiency. By measuring hysteresis measured at T = 50, 100, 150, and 200 K, respec- the metal-oxygen-metal bond length and angle for three films tively. As discussed earlier, LSMO40 shows exchange bias up with different magnetic properties that were grown under dif- to 40 K. At 50 K the magnetic hysteresis is reminiscent of ferent oxygen pressures, we find that the bandwidth calculated a normal FM. As temperature increases, the left and right from bond length and angle [27] does not change, proving that coercive fields start to move toward each other so that they the commonly accepted bandwidth-ferromagnetism relation overlap at zero magnetic field at 100 K, while still having is oversimplified, and not valid in these materials. Rather, hysteresis. At higher temperatures, the right (left) coercive the continuous gradient of the bond angle in corner-shared field continues to move to the left (right), which is seen at octahedra is responsible for intriguing magnetism. Also, we temperatures 150 and 200 K, in Figs. 1(h) and 1(i). This trend show that macroscopic structural coherence determined from leads to the appearance of partially inverted hysteresis. The x-ray diffraction (XRD), while being a good measure of film very rich magnetic phase diagram of LSMO40 is illustrated in quality, is not sufficient to probe detailed structural changes. Fig. 1(e) totally unexpected in a monolithic thin film and it In addition, our data suggest that change in the oxidation state is not observed in the thin films grown under other oxygen of metal and oxygen is related to the structural gradient that pressures. modifies the bond hybridization, where without the atomic- To map the new magnetic phase diagram, we have sys- scale structural data, the oxidation state change is often justi- tematically grown LSMO thin films, varying oxygen pressure fied within a charge transfer scenario. This study illustrates the and film thickness. Figure 1 shows the false-color plot of potential of monolithic films for technological applications the coercivity [panel (j)] and exchange bias [panel (k)] as a and exemplifies the crucial role of atomic-scale measurements function of oxygen pressure and film thickness. The white for proper understanding of the underlying physics. filled circles are our data points. Interestingly, we observe that Magnetic characterization and phase diagram.Thinfilms above and below PO = 40 mTorr, the coercivity is small with of LSMO are grown on SrTiO3 (001) substrate (STO) under small or no exchange bias, while at PO = 40 mTorr, coercivity different oxygen pressures and film thicknesses using pulsed enhancement and exchange bias only appear in films with laser deposition. The thin-film growth details are presented thicknesses above 15 u.c. At 40 u.c. the exchange bias appears in the Supplemental Material [28]. Magnetic properties of as high as 622 Oe. Coercivity and exchange bias maps show the thin films are measured using a superconducting quantum that both dimensionality and growth conditions are crucial in interference device (SQUID). Figures 1(a) and 1(b) show the observing the complex magnetic behavior and that it is limited magnetization as a function of magnetic field, M(H), and to a narrow growth condition, only above a critical thickness. temperature, M(T), for 50 unit cell (u.c.) LSMO thin films Atomic-scale structure and composition characteristics. grown under 10, 40, and 80 mTorr oxygen pressure, labeled LSMO has a very diverse phase diagram in terms of spin LSMO10,LSMO40, and LSMO80, respectively. The magnetic and lattice coupling, often leading to coupled magnetic and hysteresis of LSMO80 and that of LSMO10 are qualitatively
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