Controllable Unidirectional Magnetoresistance In A Ferromagnetic CoPt Alloy Single Layer

Spintronics uses spin and charge properties of electrons to design and develop new functional materials and spintronics devices,which has shown great application value in information storage,communication,logical operation,sensing and neural networklike fields.Spin-related Unidirectional magnetoresistance(UMR)effects have received a lot of research interest in recent years,because of their nonreciprocal transport to charge currents or magnetization reversals,This provides potential applications for reading the direction of magnetization inside the surface directly using simple geometry at both ends.Because the UMR mechanism relies on current-induced spin accumulation,most current work has focused on heavy metal/ferromagnetic multilayer structures with strong spin orbit coupling.However,the multilayer structure not only complicates the structure design of UMR devices,but also restricts the change of the symbol of UMR,which is dominated by interfacial electron scattering,hindering the development of multifunctional devices.To solve this problem,the following two aspects of work are carried out in this paper,focusing on the regulation of one-way magnetoresistance and magnetoresistance symbol of single-layer ferromagnetic film:1.Unidirectional Magnetoresistance in ferromagnetic film broken by symmetry.CoPt alloy films with positive,negative and no gradient of Co were grown by magnetron sputtering.The three gradient samples all show in-plane magnetic anisotropy.The unidirectional magnetoresistance effect in single-layer CoPt alloy ferromagnetic film is proved by measuring the second harmonic resistance of the sample.Since the samples with positive and negative Co gradient show opposite one-way magnetoresistance symbols,combined with our previous work,we judge that the one-way magnetoresistance in monolayers is due to the bulk Rashba effect caused by the sample composition gradient.2.Control of unidirectional magnetoresistance.By testing the dependence of the current amplitude of the unidirectional magnetoresistance,we found that the unidirectional magnetoresistance of the sample with a positive Co component gradient decreases first with the increase of the current amplitude,then the sign is opposite,and then the reverse increase phenomenon,that is,the unidirectional magnetoresistance of this type of film with the change of the current amplitude is controllable.By changing the thickness of the magnetic layer,we find that the critical current density when the UMR symbol changes is related to the thickness of the magnetic layer.We reveal two competing one-way magnetoresistance mechanisms,namely the bulk Rashba effect dominated by component gradient at low current density and the anomalous Nernst effect dominated by high current density.This current amplitude-regulated UMR is obviously not available in conventional heavy metal/ferromagnetic multilayer structures.In conclusion,we found that single-layer ferromagnetic CoPt alloy films with symmetry breaking can also achieve unidirectional magnetoresistance,which not only simplifies the structure of UMR devices,but also contributes to our understanding of the spin transport dependent scattering properties.In addition,our study provides a method to control the unidirectional magnetoresistance of monolayer ferromagnetic CoPt alloy films by using bulk Rashba effect and anomalous Nernst effect.