Emergent Magnetism with Continuous Control in the Ultrahigh-Conductivity Layered Oxide PdCoO2.

The current challenge to realizing continuously tunable magnetism lies in our inability to systematically change properties, such as valence, spin, and orbital degrees of freedom, as well as crystallographic geometry. Here, we demonstrate that ferromagnetism can be externally turned on with the application of low-energy helium implantation and can be subsequently erased and returned to the pristine state via annealing. This high level of continuous control is made possible by targeting magnetic metastability in the ultrahigh-conductivity, nonmagnetic layered oxide PdCoO2 where local lattice distortions generated by helium implantation induce the emergence of a net moment on the surrounding transition metal octahedral sites. These highly localized moments communicate through the itinerant metal states, which trigger the onset of percolated long-range ferromagnetism. The ability to continuously tune competing interactions enables tailoring precise magnetic and magnetotransport responses in an ultrahigh-conductivity film and will be critical to applications across spintronics.

Controlling the Magneto-Optical Response in Ultrathin Films of EuO1-x via Interface Engineering with Ferroelectric BaTi2O5.

Utilizing pulsed laser deposition, a film of EuO1-x was deposited onto a Si(001) substrate with MgO buffer and compared to the same heterostructure with an additional BaTi2O5 thin film on top of the EuO1-x surface. X-ray diffraction (XRD) indicates the films crystallize into a preferred EuO(111) orientation; it also reveals the clear presence of EuSi2, which suggests Si or Eu diffuses across the MgO buffer layer. EuO1-x films exhibit a ferromagnetic (FM) signature and temperature-dependent exchange bias, indicated by MOKE measurements, suggesting the presence of a magnetic order well above the EuO Curie temperature with possible origins in charge carrier density near the interface. In comparison, an antiferromagnetic character persists well above the EuO Curie temperature of 69 K and the enhanced Curie temperature of 150 K for BaTi2O5 films grown on the EuO1-x films. The antiferromagnetic behavior is not seen in thicker EuO1-x thin films when integrated with other ferroelectric (FE) phases of the BaO-TiO2 system, suggesting an origin in the perturbed charge population at the BaTi2O5/EuO1-x interface.

Topological Hall Effect in a Topological Insulator Interfaced with a Magnetic Insulator.

A topological insulator (TI) interfaced with a magnetic insulator (MI) may host an anomalous Hall effect (AHE), a quantum AHE, and a topological Hall effect (THE). Recent studies, however, suggest that coexisting magnetic phases in TI/MI heterostructures may result in an AHE-associated response that resembles a THE but in fact is not. This Letter reports a genuine THE in a TI/MI structure that has only one magnetic phase. The structure shows a THE in the temperature range of T = 2-3 K and an AHE at T = 80-300 K. Over T = 3-80 K, the two effects coexist but show opposite temperature dependencies. Control measurements, calculations, and simulations together suggest that the observed THE originates from skyrmions, rather than the coexistence of two AHE responses. The skyrmions are formed due to a Dzyaloshinskii-Moriya interaction (DMI) at the interface; the DMI strength estimated is substantially higher than that in heavy metal-based systems.

Magnetization switching using topological surface states.

Topological surface states (TSSs) in a topological insulator are expected to be able to produce a spin-orbit torque that can switch a neighboring ferromagnet. This effect may be absent if the ferromagnet is conductive because it can completely suppress the TSSs, but it should be present if the ferromagnet is insulating. This study reports TSS-induced switching in a bilayer consisting of a topological insulator Bi2Se3 and an insulating ferromagnet BaFe12O19. A charge current in Bi2Se3 can switch the magnetization in BaFe12O19 up and down. When the magnetization is switched by a field, a current in Bi2Se3 can reduce the switching field by ~4000 Oe. The switching efficiency at 3 K is 300 times higher than at room temperature; it is ~30 times higher than in Pt/BaFe12O19. These strong effects originate from the presence of more pronounced TSSs at low temperatures due to enhanced surface conductivity and reduced bulk conductivity.

Room temperature d0 ferromagnetism in PbS films: nonuniform distribution of Pb vacancies.

Because of the importance of ferromagnetism at room temperature, we search for new materials that can exhibit a non-vanishing magnetic moment at room temperature and at the same time can be used in spintronics. The experimental results indicate that d0 ferromagnetism without any magnetic impurities takes place in PbS films made of close-packed lead sulfide nanoparticles of 30 nm. To explain the existence of the d0 ferromagnetism, we propose a model where various PbS bulk and surface configurations of Pb-vacancies are analyzed. The bulk configurations have a zero magnetic moment while the two surface configurations with Pb vacancies with the same non-vanishing magnetic moments and lowest ground state energies contribute to the total magnetization. Based on the experimental value of the saturation magnetization, 0.2 emu g-1, we have found that the calculated Pb vacancy concentration should be about 3.5%, which is close to typical experimental values. Besides being very important for applications, there is one feature of PbS d0 ferromagnetism that makes this material special for fundamental research: PbS ferromagnetism can exhibit topologically driven spatial magnetic moment distributions (e.g., magnetic skyrmions) due to large spin-orbit coupling.

Magnetic hard gap due to bound magnetic polarons in the localized regime.

We investigate the low temperature electron transport properties of manganese doped lead sulfide films. The system shows variable range hopping at low temperatures that crosses over into an activation regime at even lower temperatures. This crossover is destroyed by an applied magnetic field which suggests a magnetic origin of the hard gap, associated with bound magnetic polarons. Even though the gap forms around the superconducting transition temperature of lead, we do not find evidence of this being due to insulator-superconductor transition. Comparison with undoped PbS films, which do not show the activated transport behavior, suggests that bound magnetic polarons create the hard gap in the system that can be closed by magnetic fields.

Spin-orbit torque-assisted switching in magnetic insulator thin films with perpendicular magnetic anisotropy.

As an in-plane charge current flows in a heavy metal film with spin-orbit coupling, it produces a torque on and thereby switches the magnetization in a neighbouring ferromagnetic metal film. Such spin-orbit torque (SOT)-induced switching has been studied extensively in recent years and has shown higher efficiency than switching using conventional spin-transfer torque. Here we report the SOT-assisted switching in heavy metal/magnetic insulator systems. The experiments used a Pt/BaFe12O19 bilayer where the BaFe12O19 layer exhibits perpendicular magnetic anisotropy. As a charge current is passed through the Pt film, it produces a SOT that can control the up and down states of the remnant magnetization in the BaFe12O19 film when the film is magnetized by an in-plane magnetic field. It can reduce or increase the switching field of the BaFe12O19 film by as much as about 500 Oe when the film is switched with an out-of-plane field.