Observation of a spontaneous anomalous Hall response in the Mn5Si3 d-wave altermagnet candidate

Phases with spontaneous time-reversal (T ) symmetry breaking are sought after for their anomalous physical properties, low-dissipation electronic and spin responses, and information-technology applications. Recently predicted altermagnetic phase features an unconventional and attractive combination of a strong T -symmetry breaking in the electronic structure and a zero or only weak-relativistic magnetization. In this work, we experimentally observe the anomalous Hall effect, a prominent representative of the T -symmetry breaking responses, in the absence of an external magnetic field in epitaxial thin-film Mn5Si3 with a vanishingly small net magnetic moment. By symmetry analysis and first-principles calculations we demonstrate that the unconventional d-wave altermagnetic phase is consistent with the experimental structural and magnetic characterization of the Mn5Si3 epilayers, and that the theoretical anomalous Hall conductivity generated by the phase is sizable, in agreement with experiment. An analogy with unconventional d-wave superconductivity suggests that our identification of a candidate of unconventional d-wave altermagnetism points towards a new chapter of research and applications of magnetic phases

Competitive actions of MnSi in the epitaxial growth of Mn5Si3 thin films on Si(111)

International audienceSome magnetically ordered phases of the Mn5Si3 crystal are proving to be prototypes for the study of thenew fundamental spin physics related to the spontaneous breaking of the time-reversal symmetry despite a zeronet magnetization. Here, we report on a route to grow epitaxial Mn5Si3 thin films on Si(111). To this end, weuse Mn and Si codeposition in a molecular beam epitaxy system and carefully tune the deposition rates, thegrowth temperature, and the annealing temperature. We assessed the silicide phase-formation and morphologyusing reflection high-energy electron diffraction, x-ray diffraction, high-resolution transmission electron mi-croscopy (HRTEM) and atomic force microscopy. Layers containing only Mn5Si3 could be stabilized undervery restrictive conditions, by tuning the Mn/Si flux ratio to match the compound stoichiometry and adjustingthe substrate temperature during growth to 443 K. HRTEM imaging revealed the existence of an interfacialamorphous layer of few nanometers thickness. Annealing the heterostructure up to 573 K led to the formationof MnSi at the vicinity of the Mn5Si3/Si(111) interface, which significantly reduced the nucleation barrier ofMn5Si3. High-quality crystalline Mn5Si3 thin films were then formed with the following epitaxial relationships:Mn5Si3 (0001)[011̄0]//MnSi(111)[2̄11]//Si(111)[11̄0]. Our experiments showed that the formation of MnSi isenhanced at a growth temperature above 473 K or for a longer annealing step, while the crystalline quality of theMn5Si3 overlayer is correspondingly degraded leading to textured thin films. The growth pathways and structuralproperties of the manganese silicides can be rationalized in terms of reactions maximizing the free-energylowering rate. Moreover, we found that the magnetic and the magnetotransport properties can be used as anefficient tool to track both Mn5Si3 crystallinity and proportion in the deposited layers

Mechanism of Spin‐Orbit Torques in Platinum Oxide Systems

International audienceSpin-Orbit Torque (SOT) Magnetic Random-Access Memories (MRAM) have shown promising results toward the realization of fast, non-volatile memory systems. Oxidation of the heavy-metal (HM) layer of the SOT-MRAM has been proposed as a method to increase its energy efficiency. But the results are widely divergent due to the difficulty in controlling the HM oxidation because of its low enthalpy of formation. Here, these differences are reconciled by performing a gradual oxidation procedure, which allows correlating the chemical structure to the physical properties of the stack. As an HM layer, Pt is chosen because of the strong SOT and the low enthalpy of formation of its oxides. The evidence of an oxide inversion layer at the ferromagnet (FM)/HM interface is found: the oxygen is drawn into the FM, while the HM remains metallic near the interface. Moreover, the oxygen migrates in the volume of the FM layer rather than being concentrated at the interface. Consequently, it is found that the intrinsic magnitude of the SOT is unchanged compared to the fully metallic structure. The previously reported apparent increase of SOTs is not intrinsic to platinum oxide and instead arises from systemic changes produced by oxidation

Macroscopic time reversal symmetry breaking arising from antiferromagnetic Zeeman effect

Time-reversal (T) symmetry breaking is a fundamental physics concept underpinning a broad science and technology area, including topological magnets, axion physics, dissipationless Hall currents, or spintronic memories. A central role in the field has been played by ferromagnets with spontaneously Zeeman-split bands and corresponding macroscopic T-symmetry breaking phenomena observable in the absence of an external magnetic field. In contrast, the Neel antiferromagnetism with anti-parallel atomic moments was not considered to generate the Zeeman splitting, leaving this abundant materials family outside of the focus of research of macroscopic T-symmetry breaking. Here, we discover a T-symmetry breaking mechanism in a compensated collinear antiferromagnet Mn5Si3, with a Zeeman splitting in the momentum space whose sign alternates across the electronic band structure. We identify the antiferromagnetic Zeeman effect using ab initio electronic structure calculations and from an analysis of spin-symmetries which were previously omitted in relativistic physics classifications of spin-splittings and topological quasiparticles. To experimentally demonstrate the macroscopic T-symmetry breaking in a Zeeman-split antiferromagnet, we measure the spontaneous Hall effect in Mn5Si3 epilayers exhibiting a negligible net magnetic moment. The experimental Hall conductivities are consistent with our ab initio calculations of the intrinsic disorder-independent contribution, proportional to the topological Berry curvature. Our study of the multi-sublattice antiferromagnet Mn5Si3 illustrates that a robust macroscopic T-symmetry breaking from the antiferromagnetic Zeeman effect is compatible with a unique set of material properties, including low atomic numbers, collinear magnetism with weak spin-decoherence, and vanishing net magnetization

Observation of a spontaneous anomalous Hall response in the Mn5Si3 d-wave altermagnet candidate

Phases with spontaneous time-reversal (T) symmetry breaking are sought after for their anomalous physical properties, low-dissipation electronic and spin responses, and information-technology applications. Recently predicted altermagnetic phase features an unconventional and attractive combination of a strong T-symmetry breaking in the electronic structure and a zero or only weak-relativistic magnetization. In this work, we experimentally observe the anomalous Hall effect, a prominent representative of the T-symmetry breaking responses, in the absence of an external magnetic field in epitaxial thin-film Mn5Si3 with a vanishingly small net magnetic moment. By symmetry analysis and first-principles calculations we demonstrate that the unconventional d-wave altermagnetic phase is consistent with the experimental structural and magnetic characterization of the Mn5Si3 epilayers, and that the theoretical anomalous Hall conductivity generated by the phase is sizable, in agreement with experiment. An analogy with unconventional d-wave superconductivity suggests that our identification of a candidate of unconventional d-wave altermagnetism points towards a new chapter of research and applications of magnetic phases