Reorientation transition of the magnetic proximity polarization in Fe/(Ga,Mn)As bilayers

Abstract

Recently, it has been observed that thin ferromagnetic Fe films deposited on top of (Ga,Mn)As layers induce a significant proximity polarization in the (Ga,Mn)As film even at room temperature. Furthermore, it was found that a thin interfacial region of the (Ga,Mn)As film is coupled antiferromagnetically to the Fe layer. Here we report a series of combined x-ray magnetic dichroism and superconducting quantum interference device magnetometer measurements for Fe/(Ga,Mn)As bilayers where the (Ga,Mn)As layer thickness is varied between 5 and 50 nm. We find a reorientation transition of the magnetic proximity polarization as a function of the (Ga,Mn)As thickness. The data are compared to results obtained performing ab initio calculations. A varying concentration of Mn interstitials as a function of (Ga,Mn)As layer thickness is responsible for this reorientation. Furthermore, exchange bias is studied in the fully epitaxial bilayer system. We find a rather strong ferromagnetic exchange bias. The strength of the exchange bias can be estimated by using a simple partial domain wall model. The diluted magnetic semiconductor (DMS) GaMnAs is a promising material for semiconducting spintronic devices. However, the record values of the Curie temperature seem to stagnate below 200 K 6 with no clear strategy to further increase it above room temperature.A novel route toward increasing the ordering temperature is to engineer the material using ferromagnetic metal overlayers. Tuning the magnetic properties at FM metal/DMS-based interfaces based on the most representative DMS system (Ga,Mn)As 7 has recently been explored in a variety of experiments. For example, exchange bias is observed in MnAs/(Ga,Mn)As bilayers. [8][9][10] In contrast, NiFe/(Ga,Mn)As bilayers show an independent magnetization behavior 11 with no exchange bias. In the Fe/(Ga,Mn)As system, the Fe overlayer induces a proximity polarization antiparallel to the Fe moment within a 1-2 nm (Ga,Mn)As region, which is promising for improving the ferromagnetic properties of very thin (Ga,Mn)As films. [12][13][14] In fact, we have recently demonstrated that the proximity effect effectively enhances the operation temperature of Fe/(Ga,Mn)As hybrid spin injection devices when very thin (Ga,Mn)As injector and detector contacts are capped by an iron layer. 15 However, to take full advantage of the control of the magnetization of the (Ga,Mn)As injector/detector contacts, it is of importance to understand the coupling mechanism in this epitaxial FM metal/DMS bilayer system. Here we map out the influence of the (Ga,Mn)As thickness in fully epitaxial Fe/(Ga,Mn)As heterostructures on the interfacial coupling as well as on the magnetization properties of the bulk layer by a combined study using element-specific x-ray magnetic circular dichroism (XMCD) measurements and bulk-sensitive superconducting quantum interference device (SQUID) magnetometry. By means of XMCD we are able to identify a thickness dependent reorientation transition of the Mn magnetization from antiparallel to paral...

Interfacial exchange coupling in Fe/(Ga,Mn)As bilayers

“…reaches a maximum of 30%, which is in good agreement with reported values at room temperature [10][11][12]. The Mn XMCD asymmetry value is about 15% at 4 K, considerably larger than results reported earlier [13,23], indicating that a larger number of the Mn ions is contributing to the magnetic order in our samples than expected from the nominal concentration of Mn in the (Ga,Mn)As layer, with excellent agreement with published results [21,[26][27][28][29][30][31].…”

“…This observation differs from several previous studies of Fe/(Ga,Mn)As bilayers [10][11][12], where an antiparallel alignment was observed between the Fe layer and the Mn moments in the bulk of the GaMnAs layer. However, a recent study of exchange coupling in such bilayers also revealed that the type of coupling [antiferromagnetic (AFM) or FM] depends strongly on the thickness of the (Ga,Mn)As layer [13], AFM coupling of bulk (Ga,Mn)As occurring in thick (Ga,Mn)As layers, and FM alignment becoming dominant when the (Ga,Mn)As layers become ultrathin (below about 10 nm in Ref. [13]).…”

“…However, a recent study of exchange coupling in such bilayers also revealed that the type of coupling [antiferromagnetic (AFM) or FM] depends strongly on the thickness of the (Ga,Mn)As layer [13], AFM coupling of bulk (Ga,Mn)As occurring in thick (Ga,Mn)As layers, and FM alignment becoming dominant when the (Ga,Mn)As layers become ultrathin (below about 10 nm in Ref. [13]). …”

“…This combination is unique in that it is made possible by the fortuitous lattice match between the (001) faces of the bcc Fe and the fcc semiconductor GaAs [7][8][9] [and therefore also (Ga,Mn)As], thus allowing fabrication of thin Fe/(Ga,Mn)As heterostructures of exceptional crystalline quality. Although hybrid systems such as Fe/GaMnAs, consisting of a canonical ferromagnet and an important FM semiconductor, have not been as thoroughly explored as their fully metallic or fully semiconductor counterparts [10][11][12][13], they have already attracted considerable interest both because of the basic properties of FM bilayers created in this way, and because of the potential for spintronic applications of this monolithic metal/semiconductor system [14,15]. * Corresponding author: abdel.alsmadi@ku.edu.kw…”

Strain-induced magnetization control in an oxide multiferroic heterostructure

“…To overcome this limitation, the use of artificial 35 heterostructures combining ferromagnetic films with ferro-or 36 piezo-electric substrates has been explored [8][9][10][11][12].…”