Critical thickness for ferromagnetism in LaAlO3/SrTiO3 heterostructures (1201.1063v1)

Abstract: In heterostructures of LaAlO3 (LAO) and SrTiO3 (STO), two nonmagnetic insulators, various forms of magnetism have been observed [1-7], which may [8, 9] or may not [10] arise from interface charge carriers that migrate from the LAO to the interface in an electronic reconstruction [11]. We image the magnetic landscape [5] in a series of n-type samples of varying LAO thickness. We find ferromagnetic patches that appear only above a critical thickness, similar to that for conductivity [12]. Consequently we conclude that an interface reconstruction is necessary for the formation of magnetism. We observe no change in ferromagnetism with gate voltage, and detect ferromagnetism in a non-conducting p-type sample, indicating that the carriers at the interface do not need to be itinerant to generate magnetism. The fact that the ferromagnetism appears in isolated patches whose density varies greatly between samples strongly suggests that disorder or local strain induce magnetism in a population of the interface carriers.

Critical Thickness for Ferromagnetism in LaAlO₃/SrTiO₃ Heterostructures

This paper presents a detailed investigation into the ferromagnetic behavior observed in LaAlO₃/SrTiO₃ (LAO/STO) heterostructures, focusing on the correlation between ferromagnetism and the critical thickness of LaAlO₃ layers. LAO/STO interfaces have received significant attention due to their unique electronic properties, notably conductivity and magnetism, despite consisting of two nominally nonmagnetic and insulating materials.

Key Insights and Findings

The research conducted utilizing scanning Superconducting QUantum Interference Device (SQUID) microscopy reveals that ferromagnetic patches appear only above a specific thickness threshold of LaAlO₃—approximately three unit cells (3 uc). This finding parallels earlier observations regarding the onset of interface conductivity, suggesting an electronic reconstruction at this critical thickness is essential for magnetism manifestation. However, notably, ferromagnetism is detected in non-conducting p-type samples, indicating that the charge carriers at the interface need not be itinerant for magnetism to occur.

Through a comprehensive analysis spanning samples with varying LaAlO₃ thicknesses, the paper observes substantial variability in ferromagnetic patch density and distribution. Such heterogeneity strongly implies that local disorder or strain at the interface induces magnetism in a subset of carriers.

Theoretical Implications

The paper explores theoretical frameworks explaining the observed phenomena, categorizing the underlying mechanisms into band structure effects and defect-driven models. Density functional theory (DFT) calculations have suggested the possibility of spin polarization in certain near-interface states. However, the presence of localized states, possibly stabilized by disorder, complicates this picture. Meanwhile, other models propose that cation or oxygen vacancies might serve as sources of localized magnetic moments.

This heterogeneity and variability across samples underscore the role of defects and interface disorder as crucial factors influencing magnetic behavior. Defect-driven models receive support from empirical observations, highlighting the potential impact of strain, oxygen vacancies, and dislocations on the interface's electronic properties.

Practical Implications and Future Directions

The interface ferromagnetism identified in this paper may have profound implications for the design of next-generation electronic devices, where magnetism and conductivity coexist at nanoscale interfaces. Specifically, the intersection of ferromagnetism with known superconducting phases at such interfaces hints at potential applications in topological quantum computation, particularly through the stabilization of Majorana fermions.

The paper suggests further explorations into the interplay between ferromagnetism and superconductivity, possibly paving the way for novel devices exhibiting exotic superconducting properties. Consequently, future research could focus on refining theoretical models to better account for disorder's role and exploring practical methodologies for controlling magnetic and conductive properties through interface engineering.

In conclusion, while the paper deepens our understanding of LAO/STO heterostructures, it also opens avenues for continued exploration within condensed matter physics and materials science. The heterogeneous nature of ferromagnetism within these systems holds potential for advancing functional device applications, provided the complex interdependencies between thickness, interface reconstruction, and disorder are meticulously deciphered.