Ferroelectric polymer gates for non-volatile field effect control of ferromagnetism in (Ga, Mn)As layers

(Ga, Mn)As and other diluted magnetic semiconductors (DMS) attract a great deal of attention for potential spintronic applications because of the possibility of controlling the magnetic properties via electrical gating. Integration of a ferroelectric gate on the DMS channel adds to the system a non-volatile memory functionality and permits nanopatterning via the polarization domain engineering. This topical review is focused on the multiferroic system, where the ferromagnetism in the (Ga, Mn)As DMS channel is controlled by the non-volatile field effect of the spontaneous polarization. Use of ferroelectric polymer gates in such heterostructures offers a viable alternative to the traditional oxide ferroelectrics generally incompatible with DMS. Here we review the proof-of-concept experiments demonstrating the ferroelectric control of ferromagnetism, analyze the performance issues of the ferroelectric gates and discuss prospects for further development of the ferroelectric/DMS heterostructures toward the multiferroic field effect transistor.

Ferroelectric Polymer Gates for Persistent Field Effect Control of Ferromagnetism in (Ga,Mn)As Layers

Sebastian Riester

The growing demand for higher computing power and element density continuously drives the development of novel device concepts. One group of materials currently attracting a lot of interest are the magnetoelectric multiferroics, due to their potential for creating devices with novel functionalities. These materials exhibit coupled ferroelectric and ferromagnetic behavior. Although the focus is on homogeneous multiferroics like bismuth ferrite, the stronger multiferroic coupling that is possible in composite multiferroics makes them attractive for many potential applications (e.g. spintronic logic and memory elements). In particular, electric field-mediated multiferroic composites promise worthwhile compatibility with CMOS circuitry. The present work explores a multiphase multiferroic system where the ferroelectric gate controls a dilute magnetic semiconductor (DMS) channel via the nonvolatile field effect associated with the spontaneous polarization. The system consists of a copolymer of vinylidene fluoride and trifluoroethylene [P(VDF-TrFE)] as the ferroelectric gate and an ultra-thin magnetic layer of (Ga,Mn)As which is one of the most well-established DMS and is considered a chief candidate for spintronic applications. The main outcomes of this research can be summarized as follows: A widely recognized incompatibility between ferroelectrics and group III-V semiconductors has been solved by the integration of a polymer ferroelectric onto a DMS in a multiferroic transistor configuration. Nonvolatile electric field control of ferromagnetism in a (Ga,Mn)As channel has been successfully demonstrated. Several pieces of evidence including the change of hysteretic properties, ferromagnetic Curie temperature shift and magnetoresistance behavior attest to the field effect-mediated multiferroic coupling in this system. Low ferroelectric gate operation voltages below 10 V were achieved through aggressive P(VDF-TrFE) thickness reduction without diminishing the gate effect strength. Polarization screening at the semiconductor/ferroelectric interface was identified as the main issue limiting the ferroelectric gate effect. A significant enhancement of the multiferroic coupling has been reached by thinning the (Ga,Mn)As channel down to 3 nm without compromising the ferromagnetic properties. From the Curie temperature response to ferroelectric gating, that follows the same trend for samples with thicknesses ranging from 3 to 7 nm, we conclude that the 2D limit has not been reached for the thinnest channels and the 3D models describing the ferromagnetic coupling are valid for this case. The ferroelectric control of ferromagnetism has been quantitatively interpreted in terms of existing models for hole-mediated exchange in (Ga,Mn)As.

EPFL2010

Towards room-temperature deterministic ferroelectric control of ferromagnetic thin films

Zhen Huang

The persisting demand of higher computing power and faster information processing keeps pushing scientists and engineers to explore novel materials and device structures. Within emerging functional materials, there is a focus on multiferroics materials and material-systems possessing both ferroelectric and ferromagnetic orders. Multiferroics with two order parameters coupled through a magnetoelectric interaction are of particular interest for novel information processing technologies. This thesis explores a promising concept of magnetoelectric multiferroic heterostructure incorporating separate ferroelectric and ferromagnetic thin layers with field-effect-mediated coupling through the heterointerface. The major issues related to ferroelectric control of ferromagnetism in multiferroic heterostructures addressed in this thesis include non-volatile magnetoelectric coupling at room temperature, deterministic switching of magnetization via polarization reversal, ferroelectric control of dynamics of magnetic domains and integration of ferroelectric gates on magnetic channels. The major accomplishments of this thesis are the following: Non-volatile ferroelectric control of magnetic properties has been demonstrated in ultra-thin Co layers at room temperature. The ferromagnetic transition Curie temperature, magnetic anisotropy energy and magnetic coercivity are shown to be switchable via the persistent field effect associated with the ferroelectric polarization. The magnitude of the effect is comparable or exceeds the results observed using the conventional (non-ferroelectric) gates. Local control of individual magnetic domain nucleation and propagation in Co channels by creating new ferroelectric domains has been achieved at the ambient conditions. The microscopic ferroelectric domains written on poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] ferroelectric polymer gate projected onto the magnetic channel changing locally the magnetic anisotropy energy, and consequently altering magnetic domain dynamics. Therefore it was possible to promote/impede magnetic domain nucleation and significantly change the domain wall velocity using a non-destructive and reversible procedure of ferroelectric domain writing. Non-volatile control of anisotropic magnetoresistance (AMR) has been demonstrated on diluted magnetic semiconductor (Ga,Mn)(As,P) thin films with integrated P(VDF-TrFE) ferroelectric gate. The field effect induced by the ferroelectric gate has shown the capability to strongly modulate the AMR behavior. Profound qualitative changes of the nature of AMR has been observed, including complete on/off switching of the crystalline component of AMR in (Ga,Mn)(As,P). A workflow for fabrication of integrated FET-type structures comprising a magnetic metal or semiconductor magnetic channel and ferroelectric polymer gate has been successfully developed. Different approaches for enhancement of ferroelectric gate operation were explored including integration of highly resistive hydrophobic interfacial layer and change of polymer composition for lower leakage. A significant increase of the non-volatile gate effect magnitude compared to the state of art was reached via improved quality of the ferroelectric/ferromagnetic interface.

EPFL2015

Non-volatile ferroelectric gating of magnetotransport anisotropy in (Ga,Mn)(As,P)

Zhouji Huang, Evgeny Mikheev, Nava Setter, Igor Stolichnov

We demonstrate charge-mediated and non-volatile control of anisotropic magnetoresistance (AMR) in a dilute magnetic semiconductor (Ga,Mn)(As, P) with an integrated polymer ferroelectric gate. The persistent electric field associated with switchable polarization in the ferroelectric layer is shown to be capable of strongly modulating the AMR magnitude. Furthermore, ferroelectric gate switching has a profound impact on the nature of AMR, changing the symmetry of the effect and enhancing/suppressing the crystalline component of AMR. Thus, in addition to a rather weak modulation of the ferromagnetic Curie temperature (4-5 K) reported previously, the ferroelectric gate can induce a strong deterministic switching of the magnetotransport anisotropy. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4731245]