Anomalous Hall effect inL10−MnAlfilms with controllable orbital two-channel Kondo effect

Abstract

Abstract：The anomalous Hall effect (AHE) in strongly disordered magnetic systems has been buried in persistent confusion despite its long history. We report the AHE in perpendicularly magnetized L10-MnAl epitaxial films with variable orbital two-channel Kondo (2CK) effect arising from the strong coupling of conduction electrons and the structural disorders of two-level systems. The AHE is observed to excellently scale with ρAH/f=a0ρxx0+bρxx 2 at high temperatures where phonon scattering prevails. In contrast, significant deviation occurs at low temperatures where the orbital 2CK effect becomes important, suggesting a negative AHE contribution. The deviation of the scaling agrees with the orbital 2CK effect in the breakdown temperatures and deviation magnitudes. The scaling between longitudinal resistivity (ρxx) and anomalous Hall resistivity (ρAH) can provide access to the detailed mechanisms of the AHE. Present theories predict a scaling of ρAH~ρxx n , with n = 2 for side jump and intrinsic contribution [4,5], and n = 1 for skew scattering [6]. The intrinsic contribution is related to the topological Berry curvature of band structure [4], while the extrinsic side jump and skew scattering result from spin-orbit interaction-induced asymmetric impurity scattering of conduction electrons [5,6]. Notably, in contrast to the conventional picture that ρAH scales with the total resistivity irrespective of its sources [3], recent experimental studies have revealed that both ρAH and the scaling relation are qualitatively different for various types of electron scattering. Phonon scattering was found to have no distinguishable contribution to extrinsic part of ρAH in ferromagnetic Fe, Co, Ni, L10-Mn1.5Ga films, Co/Pt multilayers, and paramagnetic Ni34Cu66 films [7][8][9][10][11][12]. When ρxx is dominated by phonon and static defect scattering, the AHE has a scaling of ρAH = a0 ρxx0+bρxx 2 , where ρxx0 is the residual resistivity, a0ρxx0 is the extrinsic contribution from both side jump and skew scattering, and b is the intrinsic anomalous Hall conductivity (AHC). In magnetic materials with relatively low Curie temperature, the temperature (T) induced variation of the magnetization must be taken into account [13]. For the simplest case that ρAH scales proportionally with magnetization (M) which is not the fact in some magnetic systems [14,15], the AHE scaling can be modified into a more general expression explicitly as ρAH/f = a0 ρxx0+bρxx 2 . Here, f = M / M0, where M0 is magnetization at 0 K; the Tindependence of a0 and b is not considered. In the presence of strong disorder effects (dirty metal [1]), the AHE scaling is more complex. In the hopping conduction regime, the AHE was reported to scale as ρAH~ρxx 0.4 [19][20][21]. Therefore, further experimental and theoretical attempts are crucial for better understanding of the AHE, especially in disordered magnetic systems. Key words：AnomalousThe Kondo effect is a striking consequence of conduction electrons coupling with localized spin or "pseudospin" impurities. The on...

Anomalous resistivity upturn in epitaxial L21-Co2MnAl films

“…The orbital 2CK fixed point and the resultant T 1/2 dependence also appear to be robust against a weak population asymmetry of the two spin channels (e.g. L10-MnGa and L10-MnAl films with an estimated bulk P of <12.5%) [21][22][23] . However, the P appears not to be low in these L21-Co2MnAl films (P~60%-75%) as discussed above.…”

“…I 004 first drops remarkably by a factor of 10 as T s increases from 50 to 150 °C, and then gradually increases as T s further increases towards 350 °C, the tendency of which excellently coincides with those of homogeneity and strain. Intriguingly, in L 1 0 -MnGa and L 1 0 -MnAl films192122, if we use ρ xx 0 (approximately the residual resistivity due to static impurity scattering) and the thermal contribution to the resistivity ( ρ them  =  ρ xx  −  ρ xx 0 ) to quantify the strengths of structural disorder and the phonon and magnon scattering, respectively, the phonon and magnon scattering can be found to become increasingly suppressed with enhancing structural disorder. Therefore, ρ them also reflects the degree of structural perfection.…”

“…Structural disorder such as impurities, strain, and dislocations in Mn-based alloys has been proved to account for the large variations in magnetooptical Kerr effects1516, Gilbert damping17, magnetic anisotropy18, anomalous Hall effect19, and for the occurrence of electronic disorder physics (e.g. weak localization20 and atomic tunneling effects212223). However, so far, the disorder effects on the transport properties of Co 2 MnAl films have been barely investigated.…”

Anomalous Hall effect in ferromagneticWeyl semimetal candidate Zr1−x V x Co1.6Sn

“…In order to shed light on the mechanisms of AHE, the experimental data are fitted with the scaling laws in which the influence of M(T ) is also taken into account. [29][30][31][32] We find that the data can be well fitted by a modified TYJ scaling [1,33,34]…”

Observation of orbital two-channel Kondo effect in a ferromagnetic L10-MnGa film

“…A magnetic field ( H ) of 5 T was reported to result in a breakdown of the NFL behavior at low energies in an early Cu PC experiment 24 . Some recent experiments argue for a negligible influence of strong magnetic field of up to 14 T in ThAsSe glasses 22 23 or even a slight spin polarization in L 1 0 -MnAl films 14 27 , suggesting a considerable robustness of the orbital 2CK fixed points at finite temperatures. One reason was suggested to be that the electron spins are not directly involved in the Hamiltonian of the TLS coupling to the conduction electrons in the two spin channels 22 .…”