Tuning interfacial ferromagnetism in LaNiO3/CaMnO3 superlattices by stabilizing nonequilibrium crystal symmetry

Abstract

Perovskite oxide heterostructures offer an important path forward for stabilizing and controlling low-dimensional magnetism. One of the guiding design principles for these materials systems is octahedral connectivity. In superlattices composed of perovskites with different crystal symmetries, variation of the relative ratio of the constituent layers as well as the individual layer thicknesses gives rise to nonequilibrium crystal symmetries that, in turn, lead to unprecedented control of interfacial ferromagnetism. We have found that in superlattices of CaMnO 3 (CMO) and LaNiO 3 (LNO), interfacial ferromagnetism can be modulated by a factor of three depending on LNO and CMO layer thicknesses as well as their relative ratio. Such an effect is only possible due to the non-equilibrium crystal symmetries at the interfaces and can be understood in terms of the anisotropy of the exchange interactions and modifications in the interfacial Ni-O-Mn and Mn-O-Mn bond angles and lengths with increasing LNO layer thickness. These results demonstrate the potential of engineering non-equilibrium crystal symmetries in designing ferromagnetism.Transition metal perovskite oxides exhibit a wide range of ground states which are a manifestation of the delicate balance of the lattice, charge, and spin degrees of freedom in these materials. Competing interactions with similar energy scales mean that small perturbations, be they external fields, pressure or other parameters, can give rise to large changes in magnetic and electronic properties. In a transition metal perovskite oxide with the ABO 3 structure, BO 6 octahedra form building blocks and their relative connectivity can dramatically change its properties. In bulk single crystals, high pressure has been used to substantially modify the ground states of some of these transition metal perovksite oxides. 1 More recently, there have been theoretical studies indicating that stabilizing new crystal symmetries via octahedra rotation patterns in oxide heterostructures may give rise to unexpected emergent behavior. 2,3 For example, Rondinelli and Fennie have predicted ferroelectricity in cation-ordered LaGaO 3 /YGaO 3 superlattices due to stabilization of unique octahedral rotation patterns. 4

Depth-resolved charge reconstruction at the LaNiO3/CaMnO3 interface

“…Among the many complex-oxide heterostructures studied to date, there has been a class of heterostructures where the interfaces give rise to functional properties not observed in the constituent materials [6].With many such emergent properties, ranging from interfacial metallicity [7][8][9] to interfacial superconductivity [10,11], there has been only a handful of successful efforts demonstrating new magnetic ground states at interfaces [12,13]. One such example is the LaNiO 3 /CaMnO 3 system where ferromagnetic ground state emerges at the interface, although LaNiO 3 is a paramagnetic metal and CaMnO 3 is an antiferromagnetic insulator in the bulk [13][14][15].…”

“…The emergence of interfacial ferromagnetism in the LaNiO 3 /CaMnO 3 system has been attributed to two distinct mechanisms: a Mn 4+ -Mn 3+ double exchange interaction in the interfacial CaMnO 3 layer and a Ni 2+ -O-Mn 4+ superexchange interaction at the interface between the LaNiO 3 and CaMnO 3 [13][14][15]. The relative importance of the two mechanisms depends on the thickness of the individual layers due to a thickness-dependent metal-insulator transition in the LaNiO 3 layer [16][17][18].…”

“…Both of the above mentioned mechanisms require an interfacial charge reconstruction, which results in the presence of the transition-metal cations with the valence states other than the formal valence state of the stoichiometric compounds (LaNi 3+ O 3 and CaMn 4+ O 3 ). Such charge reconstruction, resulting in the formation of off-stoichiometric cations (Ni 2+ and Mn 3+ ) at the interface, can be explained in terms of the charge leakage from LaNiO 3 to CaMnO 3 [13][14][15], as well as the presence of oxygen vacancies driven to the interface by polar compensation [21][22][23][24].…”

“…Both scenarios are plausible in the LaNiO 3 /CaMnO heterostructures. Specifically, in metallic superlattices with near-and above-critical LaNiO 3 thicknesses, leakage of itinerant Ni 3d e g electrons into the interfacial CaMnO 3 layer is expected to reduce the valence state of some of the interfacial Mn cations from 4+ to 3+, leading to the emergence of interfacial ferromagnetism mediated by the Mn 4+ -Mn 3+ double exchange interaction [13][14][15]25]. This scenario has been theoretically predicted and discussed in-depth in prior experimental studies [13][14][15] and could explain the observed link between the thickness-dependent metal-insulator transition in ultrathin LaNiO 3 and the observed suppression of ferromagnetism in the superlattices with near-and below-critical-thickness LaNiO 3 layers (see Fig.…”

Emergentc-axis magnetic helix in manganite-nickelate superlattices

“…Superlattices composed of manganites and nickelates are a paradigmatic venue for efforts to discover and understand emergent interface magnetism, motivated by the complex magnetic order and phase diagram of its bulk constituents [10,11]. In fact, this system harbors fascinating phenomena, such as exchange bias and interfacial electronic reconstructions [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26]. Recently, a highly unusual magnetic coupling between ferromagnetic (LSMO) 9 layers was observed in [001]-oriented (La 2/3 Sr 1/3 MnO 3 ) 9 /(LaNiO 3 ) n [(LSMO) 9 /(LNO) n , n = 1 − 9], in which the coupling angle between (LSMO) 9 layers varies between zero and 130 • as a function of n ( Fig.…”