Room-temperature Anomalous Hall Research Articles

Robust room temperature perpendicular magnetic anisotropy and anomalous Hall effect of sputtered NiCo2O4 film

Inverse spinel ferrimagnetic NiCo2O4 (NCO) exhibits volatile physical properties due to the complex ion/valence occupation, which complicates the study its intrinsic properties. In this work, robust room temperature perpendicular magnetic anisotropy (PMA) is distinctly observed in high-quality RF-sputtered NCO film down to 3 uc (2.4 nm), confirmed by the room temperature anomalous Hall effect. The NCO films show a good metallic conductivity with a dimensional driven metal-insulator transition. The scaling relation between anomalous Hall conductivity ( σxy ) and the longitudinal conductivity ( σxx ) reveals the dirty metal behavior in conjunction with the contribution of intrinsic Berry phase or disorder-enhanced electron correlation contribute to the anomalous Hall effect for thick films while the dirty scaling law dominates for the thin films. This work introduces an oxide candidate with robust room temperature PMA as well as massive production ability for the functional spintronic applications.

Room-temperature anomalous Hall effect in graphene in interfacial magnetic proximity to EuO grown by topotactic reduction

Room-temperature anomalous Hall effect in graphene in interfacial magnetic proximity to EuO grown by topotactic reduction

Topological Hall Effect in Thin Films of an Antiferromagnetic Weyl Semimetal Integrated on Si.

Since the large room-temperature anomalous Hall effect was discovered in noncollinear antiferromagnets, Mn3Sn has received immense research interest as it exhibits abundant exotic physical properties including Weyl points and enormous potential for antiferromagnetic spintronic device applications. In this work, we report the emergence of the topological Hall effect in Mn3Sn films grown on Si that is the workhorse for the modern highly integrated information technology. Importantly, through a series of systematic comparative experiments, the intriguing topological Hall effect phenomenon related to the appearance of the noncoplanar chiral spin structure is found to be induced by the Mn3Sn/SiO2 interface. Furthermore, it was found that the current injection to a Pt/Mn3Sn bilayer Hall bar device can effectively manipulate the chiral spin structure of Mn3Sn, which demonstrates the feasibility of Si-based noncollinear antiferromagnetic spintronics.

Chiral domain walls of Mn3Sn and their memory

Magnetic domain walls are topological solitons whose internal structure is set by competing energies which sculpt them. In common ferromagnets, domain walls are known to be of either Bloch or Néel types. Little is established in the case of Mn3Sn, a triangular antiferromagnet with a large room-temperature anomalous Hall effect, where domain nucleation is triggered by a well-defined threshold magnetic field. Here, we show that the domain walls of this system generate an additional contribution to the Hall conductivity tensor and a transverse magnetization. The former is an electric field lying in the same plane with the magnetic field and electric current and therefore a planar Hall effect. We demonstrate that in-plane rotation of spins inside the domain wall would explain both observations and the clockwise or anticlockwise chirality of the walls depends on the history of the field orientation and can be controlled.

Transition from Anomalous Hall Effect to Topological Hall Effect in Hexagonal Non-Collinear Magnet Mn3Ga

We report experimental observation of large anomalous Hall effect exhibited in non-collinear triangular antiferromagnet D019-type Mn3Ga with coplanar spin structure at temperatures higher than 100 K. The value of anomalous Hall resistivity increases with increasing temperature, which reaches 1.25 μΩ · cm at a low field of ~300 Oe at room temperature. The corresponding room-temperature anomalous Hall conductivity is about 17 (Ω · cm)−1. Most interestingly, as temperature falls below 100 K, a temperature-independent topological-like Hall effect was observed. The maximum peak value of topological Hall resistivity is about 0.255 μΩ · cm. The appearance of the topological Hall effect is attributed to the change of spin texture as a result of weak structural distortion from hexagonal to orthorhombic symmetry in Mn3Ga. Present study suggests that Mn3Ga shows promising possibility to be antiferromagnetic spintronics or topological Hall effect-based data storage devices.

Scaling of the thickness dependent anomalous Hall effect in amorphous ferromagnetic thin films

We testified the scaling theory of room temperature anomalous Hall effect in amorphous Co90Zr10 ferromagnetic thin films with the thickness between 5 nm to 600 nm. The experiment results did not meet the theory expectation if we simply put σxy and σxx into a log-log plot, because a negative exponent in σxy = σxxβ appeared for the films thicker than 100 nm. We removed the thickness independent bulk contribution to the scattering, and rebuilt the scaling relation by using only the surface/interface contribution. A crossover from the intrinsic moderately dirty regime, where β = 0.82 to the dirty regime, where β = 1.76, had been observed, which matched previous results and theory expectations. Therefore, it is necessary to remove the bulk contribution in establishing the AHE scaling relation when using the thickness to tune the transportation properties of ferromagnetic thin films.

Room-temperature anomalous Hall effect and magnetroresistance in (Ga, Co)-codoped ZnO diluted magnetic semiconductor films**Project supported by the National Natural Science Foundation of China (Grant No. 50772122) and the Young Scientists Fund of the National Natural Science Foundation of China (Grant No. 51002176).

This paper reports that the (Ga, Co)-codoped ZnO thin films have been grown by inductively coupled plasma enhanced physical vapour deposition. Room-temperature ferromagnetism is observed for the as-grown thin films. The x-ray absorption fine structure characterization reveals that Co2+ and Ga3+ ions substitute for Zn2+ ions in the ZnO lattice and exclude the possibility of extrinsic ferromagnetism origin. The ferromagnetic (Ga, Co)-codoped ZnO thin films exhibit carrier concentration dependent anomalous Hall effect and positive magnetoresistance at room temperature. The mechanism of anomalous Hall effect and magneto-transport in ferromagnetic ZnO-based diluted magnetic semiconductors is discussed.

Anomalous Hall effect suppression in anatase Co:TiO2 by the insertion of an interfacial TiO2 buffer layer

We present the effect of introducing a TiO2 buffer layer at the SrTiO3/Co:TiO2 interface on the magnetic and structural properties of anatase Co:TiO2 (1.4 at. % Co). Inserting the buffer layer leads to suppression of the room-temperature anomalous Hall effect, accompanied by a reduced density of Co clusters, and a different depth distribution of such clusters. Co clusters in Co:TiO2 with a buffer layer are mostly formed at the surface, such that they are situated outside the current path and cannot contribute to the transverse anomalous Hall resistivity. These results indicate extrinsic origins of magnetism in anatase Co:TiO2.

Room temperature anomalous Hall effect in Co doped ZnO thin films in the semiconductor regime

Observation of the room temperature (RT) anomalous Hall effect (AHE) and ferromagnetism in semiconducting like (carrier concentration ∼1019cm−3) Co-doped ZnO samples is reported. These small AHE signals match quantitatively with the magnetic hysteresis and can be correspondent to the intrinsic diluted magnetic oxide (DMO) effect with spin polarized carriers. The contribution to the DMO effect depends on the types of carriers and how they incorporated into the electric conduction, magnetic coupling, and the coupling between them. These findings can provide useful information in the study of the origin of RT ferromagnetism in ZnO-based DMO and for further application in spintronics.

Two Magnetic Regimes in Doped ZnO Corresponding to a Dilute Magnetic Semiconductor and a Dilute Magnetic Insulator

Films of ZnO doped with magnetic ions Mn and Co and, in some cases, with Al have been fabricated with a very wide range of carrier densities. Ferromagnetic behavior is observed in both insulating and metallic films, but not when the carrier density is intermediate. Insulating films exhibit variable range hopping at low temperatures and are ferromagnetic at room temperature due to the interaction of the localized spins with static localized states. The magnetism is quenched when carriers in the localized states become mobile. In the metallic (degenerate semiconductor) range, robust ferromagnetism reappears together with very strong magneto-optic signals and room temperature anomalous Hall data. This demonstrates the polarization of the conduction bands and indicates that, when ZnO is doped into the metallic regime, it behaves as a genuine magnetic semiconductor.