Spintronic devices make use of the electron spin to store, recall and process data. Examples include the spin-valve read head, which has produced orders-of-magnitude improvements in magnetic data storage densities, and cutting-edge nonvolatile magnetic random access memory. Over the past decade, ferromagnetic (FM) semiconductors have emerged as new candidates for spintronic applications, offering the prospect of combining high density storage and gate-controlled logic in a single material.1 The development of FM metal/FM semiconductor heterostructures brings together the benefits of metal and semiconductor based spintronics, offering access to new functionalities and physical phenomena. Recently, it was shown that a metal (Fe) deposited on a semiconductor (Mn-doped GaAs) can induce a polarization of the interface semiconductor states, lying antiparallel to the Fe magnetization and persisting above room temperature.2 We have studied hybrid Fe/(Ga,Mn)As layered structures using a combination of X-ray magnetic circular dichroism (XMCD) on beamline I06, and sensitive bulk magnetometry measurements performed at the University of Nottingham. This combined study allows us to distinguish the magnetic response of the interface and bulk regions of the magnetic semiconductor, revealing a different exchange-coupling of each region to the Fe layer.
In earlier studies, the hybrid structures were produced by a process involving exposure of the (Ga,Mn)As underlayer to air, followed by Ar ion sputtering and annealing prior to the Fe layer deposition2. In contrast, in the present study the (Ga,Mn)As and Fe layers were grown by molecular beam epitaxy in the same ultra-high vacuum chamber, to ensure a clean interface with minimal disorder. The structure was then capped with a thin layer of Al, to prevent oxidation.
Figure 1 shows the magnetization hysteresis loop for a structure consisting of a 2nm Fe layer and a 20nm Ga1-xMnxAs layer with Mn concentration x=3%. A two-step magnetization reversal is observed, indicating different behaviour of the Fe and (Ga,Mn)As layers, with the smaller loop attributed to the dilute moment (Ga,Mn)As film. The minor hysteresis loop clearly shows a shift from zero field. The bias magnetic field HE is found to be inversely proportional to the thickness of the FM semiconductor layer, signifying that the shift is due to the exchange coupling at the interface between the two layers.
Figure 1: Magnetization hysteresis loop for an Fe/(Ga,Mn)As bilayer film (black) and a (Ga,Mn)As single layer (blue). The green curve is a minor loop, showing a clear exchange bias effect. The bias field HE is inversely proportional to the (Ga,Mn)As layer thickness d, as shown in the inset.
To shed more light on this effect, we performed X-ray magnetic circular dichroism measurements at the Mn and Fe L2,3 absorption edges. In L2,3 XMCD, electrons are excited from a 2p core level to the unoccupied 3d valence states of the element of interest by circularly polarized X-rays. The difference in absorption for opposite polarizations gives a direct and element-specific measurement of the projected 3d magnetic moment along the X-ray polarization vector. By measuring the XMCD asymmetry as a function of magnetic field, the magnetization hysteresis loops of the Fe and (Ga,Mn)As layers can be separately determined. Furthermore, by comparing the XMCD signals obtained by the surface-sensitive total electron yield (TEY) and bulk-sensitive fluorescent yield (FY) methods, the magnetic response of the near-interface and deeper-lying regions of the (Ga,Mn)As layer can be distinguished.
Fig 2 compares the XMCD hysteresis loops of Fe and Mn for a bilayer film with a 2nm Fe layer and a 10nm (Ga,Mn)As layer. The Fe hysteresis loop shows a single magnetization switch at a coercive field of around 100 Oe. The effect of the interlayer coupling can be seen for the Mn loop: at low fields, the projected Mn moment aligns antiparallel to the Fe moment and undergoes a magnetization reversal of opposite sign to the Fe. With further increase in the magnetic field, the FY signal shows that the bulk component of the Mn moment rotates away from antiparallel alignment with the Fe layer and into the external field direction.
Figure 2: Element-specific XMCD hysteresis loops for (a) Fe and for (b) Mn with detection by total electron yield (TEY) and (c) Mn with detection by fluorescence yield (FY). Black and red points are for increasing and decreasing fields, respectively. Black and red lines are a single-domain model calculation.
A comparison of the surface-sensitive TEY and bulk-sensitive FY Mn XMCD measurements allows us to identify an interface layer which is aligned antiparallel to the Fe layer, even for large external magnetic fields. This is deduced from the reduced Mn signal in TEY at high field. The strongly exchange-coupled interface layer is associated with a proximity polarization of the (Ga,Mn)As surface by the neighbouring Fe layer, which, consistent with previous studies2, is found to be present well above the bulk Curie temperature of the (Ga,Mn)As layer. The thickness of the strongly coupled interface layer is estimated to be ~0.7 nm or 2-3 monolayers, assuming a uniform distribution of Mn ions and magnetic moments throughout the (Ga,Mn)As film.
Our results shed light on the magnetic coupling in Fe/(Ga,Mn)As hybrid layers which are important for room-temperature spintronics, and also offer a means of controlling the spin orientation in a FM semiconductor. The magnitude of the exchange bias is larger than has been previously observed in (Ga,Mn)As heterostructures with an antiferromagnetic pinning layer and furthermore is readily controllable by reorientation of the Fe magnetization. Such layers are relevant for future applications in heterostructures for spin injection or the investigation of magnetoresistance effects. Future efforts will be directed at determining the underlying mechanism of interfacial coupling and proximity polarization, and their influence on spin transport in devices.
Olejnik, K., Wadley, P., Haigh, J.A., Edmonds, K.W., Campion, R.P., Rushforth, A.W., Gallagher, B.L., Foxon, C.T., Jungwirth, T., Wunderlich, J., Dhesi, S.S., Cavill, S.A., van der Laan, G., and Arenholz, E. Exchange bias in a ferromagnetic semiconductor induced by a ferromagnetic metal: Fe/(Ga,Mn)As bilayer films studied by XMCD measurements and SQUID magnetometry. Phys. Rev. B 81, 104402 (2010)
References
- Jungwirth, T. et al. Theory of ferromagnetic (III,Mn)V semiconductors. Rev. Mod. Phys. 78, 809 (2006).
- Maccherozzi, F. et al.. Evidence for a magnetic proximity effect up to room temperature at Fe/(Ga,Mn)As interfaces. Phys. Rev. Lett. 101, 267201 (2008).
Funding Acknowledgement
The EU under Grants No. SemiSpinNet-215368 and NAMASTE-214499.
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