Independent Magnetization Behavior of a Ferromagnetic Metal-Semiconductor Hybrid System

Abstract

We report the discovery of an effect where two ferromagnetic materials, one semiconductor ((Ga,Mn)As) and one metal (permalloy), can be directly deposited on each other and still switch their magnetization independently. We use this independent magnetization behavior to create various resistance states dependent on the magnetization direction of the individual layers. At zero magnetic field a two layer device can reach up to four non-volatile resistance states.PACS numbers: 75.50. Pp, 75.30.Gw, 75.70.Cn, Devices whose functioning is based on the relative magnetization state of two controllable magnetic elements, such as GMR (giant magneto resistance) [1] [2] based read heads [3] and TMR (tunnel magneto resistance) [4] based MRAM [5] are crucial to the modern information technology industry. So far, all such devices have been comprised of at least three layers: the two magnetic layers and a spacer layer to break the direct coupling between them and allow them to reorientate their relative magnetization. In this letter we show that, unlike the case of two ferromagnetic (FM) metals, the bringing together of a FM metal with a FM semiconductor (SC) can allow the layers to remain magnetically independent and thus permit the fabrication of devices without the need of a non magnetic interlayer. We demonstrate a first such device, which because of the strong anisotropies in the FM semiconductor layer has not only two, but up to four stable resistance states in the absence of a magnetic field.To prepare these structures, a 100 nm (Ga,Mn)As layer is grown by low-temperature molecular beam epitaxy on a GaAs buffer and substrate. Subsequently, without breaking the vacuum, the sample is transferred to a UHV magnetron sputtering chamber, and a permalloy (Ni 80 Fe 20 ) film with a thickness of 7 nm (and in some cases a 3 nm thick magnesium oxide (MgO) capping film) is deposited on top of the (Ga,Mn)As layer ( fig. 1a). Using optical lithography and chemically assisted ion beam etching (CAIBE), this layer stack is patterned into a 40 µm wide Hall bar oriented along the (Ga,Mn)As [010] crystal direction. Ti/Au contacts are established through metal evaporation and lift-off.For an initial study of the layer system, we include an MgO film on top of the permalloy layer to prevent the permalloy from naturally oxidizing in air. Without a cap layer the natural oxide consisting of NiO on an Fe oxide layer [6] produces an exchange bias coupling [7] with a magnetic field dependent anisotropy [8] [9] below its Néel temperature [10] which unduly complicates the layer characterization. We find that sputtering MgO on Py creates a well-defined uniform antiferromagnetic layer which couples antiferromagnetically to the Py film. • (light gray) and 20• (red) relative to the (Ga,Mn)As [010] easy axis after cooling the sample to 4.2 K in a field of 300 mT. The measurements exhibit clear double-step switching of the (Ga,Mn)As layer and a shifted magnetization contribution of the Py/MgO system due to exchange bias. Inset: projection of the magnetizat...

Exchange bias of a ferromagnetic semiconductor by a ferromagnetic metal

“…Here, we demonstrate an antiferromagnetic coupling and exchange bias in Fe/(Ga,Mn)As bilayer films, by combining element-specific XMCD measurements and bulk-sensitive superconducting quantum interference device (SQUID) magnetometry. As with previous studies of FM metal/FM semiconductor bilayers 4,5 (and in contrast to AFM coupled FM metal/FM metal exchange bias structures 10,11 ) the layers are in direct contact without a non-magnetic spacer in between. We distinguish interface and bulk (Ga,Mn)As layers that are respectively strongly and weakly antiferromagnetically coupled to the Fe overlayer.…”

“…The development of FM metal/FM semiconductor heterostructures has the potential to bring together the benefits of metal and semiconductor based spintronics, offering access to new functionalities and physical phenomena. Recent studies of MnAs/(Ga,Mn)As and NiFe/(Ga,Mn)As bilayer films have shown FM interlayer coupling and independent magnetization behavior, respectively 4,5 . Of particular interest is the Fe/(Ga,Mn)As system, since the growth of epitaxial Fe/GaAs(001) films is well-established 6 .…”

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

“…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.…”

Identifying the character of ferromagnetic Mn in epitaxial Fe/(Ga,Mn)As heterostructures

“…In the case of direct exchangecoupled HS, the investigation of spin valve effect in MnAs/ ͑Ga,Mn͒As at low temperature ͑4.2 K͒ found the apparent formation of a inhomogeneous magnetic spring in the ͑Ga,Mn͒As region 10 and a decrease in exchange coupling after insertion of a spacer layer ͓MnAs/ p-GaAs/ ͑Ga, Mn͒As͔. 11 Differently, HS with 3d FM metals revealed ͑i͒ independent magnetization switching in ͑Ni 80 Fe 20 ͒ / ͑Ga, Mn͒As, despite direct contact 12 and ͑ii͒ FM behavior of Mn at RT at nonepitaxial Fe/͑Ga,Mn͒As interfaces, with antiparallel alignment of the Fe and Mn moments. 13 A firm understanding of the mechanisms involved in these effects requires on one side a full control of HS growth and on the other side the ability to probe the electronic and magnetic properties in a chemical and depth sensitive way.…”