Strain-driven antiferromagnetic exchange interaction in SrMnO3 probed by phase-shifted spin Hall magnetoresistance

Abstract

Multiferroics have found renewed interest in topological magnetism and for logic-in-memory applications. Among them, SrMnO 3 , possessing strong magnetoelectric coupling, is gaining attention for the design of coexisting magnetic and polar orders upon straining. Here we demonstrate antiferromagnetic exchange interactions in strained SMO thin films extracted from a new feature in the phase response of spin Hall magnetoresistance, which has not been explored in earlier works, such as in magnetic insulators. We explain our findings with a model that incorporates magnetic anisotropy along the [110] direction, corroborates with density functional theory studies, and is consistent with the direction of ferroelectric polarization in SrMnO 3 . The fundamental insights obtained from our studies establishes the potential of this material in magnetoelectrically coupled devices for different logic and memory applications.

Probing the Spatial Variation of Magnetic Order in Strained SrMnO3 Thin Films Using Spin Hall Magnetoresistance

“…Second, oxygen defects modify locally the structure such that the bond angles between the Mn and oxygen can alter up to 30°, further weakening the antiferromagnetic interaction in accordance with the Goodenough–Kanamori–Anderson rules. [ 24 ] From this we infer that the ferromagnetic hysteresis loops are well explained by the formation of oxygen vacancies. From first‐principle calculations, the formation energy of oxygen vacancies is lowest in structural domain walls [ 25 ] and additionally at the surface.…”

“…[ 26,27 ] The 135° phase shift with a quadratic dependence is explained by taking into account the preferential magnetic anisotropy axes [110] and [001] for oxygen‐deficient SMO and domain formation. [ 24 ] Here, we note that no spin‐flop and spin‐flip effects are observed since there is no abrupt change of the SMR signal with field, nor a saturation of the SMR amplitude. The [110] magnetic easy axis is stabilized by the symmetry breaking introduced by the oxygen vacancy formation and the resulting Pt resistivity for an in‐plane magnetic field rotation is given by…”

“…From first‐principle calculations, the formation energy of oxygen vacancies is lowest in structural domain walls [ 25 ] and additionally at the surface. [ 23,24 ] Since the amount of oxygen deficiency‐induced magnetization is independent of the thickness, we argue that a deficient layer forms directly after growth rather than during growth and is inevitable in uncapped strained SMO films. Since the surface magnetization is similar for the three films, we focus on the 6 nm film for probing the local magnetic structure further.…”

“…With temperature, both devices show a gradual decrease in amplitude which signifies the reduction in magnetic moment with temperature, saturating between 100 and 150 K, in line with the predicted Néel temperature of 121 K, for strained oxygen‐deficient SMO films. [ 24 ] The phase of both devices is temperature dependent and transitions toward a phase of 90° at temperatures above 100 K. We attribute the shift of the phase to temperature‐dependent magnetocrystalline anisotropy. Above the Néel temperature, the phase of both devices stabilizes.…”

Tunable magnetic anisotropy in SrMnO3/SrRuO3 bilayers studied by angle and temperature dependence of magneto-transport

“…16 Typically the Ru-O-Ru bond angle in the ab plane for SRO is 158 for such thin films of SRO as was shown in our earlier work 22 and is similar to the Mn-O-Mn bond angle with oxygen vacancies in strained SMO. 33 Oxygen vacancies in SMO also lead to varying exchange interactions ranging from ferromagnetic to antiferromagnetic, 21,33 which can influence the magnetic coupling with SRO and will be interesting to study with spectroscopic techniques.…”