Enhanced Curie temperature and nonvolatile switching of ferromagnetism in ultrathin (Ga,Mn)As channels

The integration of ferroelectric polymer gates on a Mn-doped GaAs magnetic channel provides a promising route for the persistent field-effect control of magnetic properties in high-quality diluted magnetic semiconductors (DMSs) that are otherwise incompatible with traditional oxide ferroelectrics. That control demands the thinnest possible DMS layers, for which to date the Curie temperature ($T_C$) is severely depressed. Here we show that reducing the channel thickness from 7 to 3–4 nm by etching, followed by a brief 135 °C anneal, does not degrade the $T_C$ (∼70 K) of the 7-nm film. The channel thinning results in a dramatic threefold increase of the $T_C$ shift controlled by the ferroelectric polarization reversal. Furthermore, we obtain the same exponent $(\partial \ln \hspace{0.5em}{T}_C/\partial \ln \hspace{0.5em}R)\equiv \gamma \approx -0.3$ for all channels with different thicknesses, regardless of the technique used for $T_C$ determination. These results suggest that the ferromagnetic coupling in an ultrathin 3-nm channel is far from the two-dimensional limit and shows a rather bulklike behavior, similar to well-established 7-nm films.

Figure 1

Change of the (Ga,Mn)As magnetic channel thickness by chemical etching. (a) Normalized room-temperature conductivity measured on the reference Hall bar after etching and reoxidizing as a function of number of etching steps. (b) TEM cross-section image of the (Ga,Mn)As channel taken after four consecutive etching runs. The image is taken at the border of the electrode in order to visualize the step between the part protected by electrode and the etched area. (c) Temperature derivative of the measured resistivity for the channel with thickness reduced to <3.5 nm. $T_C$ signaled by cusp of the resistance derivative is 37 K.

Figure 2

Persistent gate effect on the (Ga,Mn)As magnetic channel resistance and $T_C$. (a) Temperature-dependent sheet resistance of the channel in the depletion and accumulation poled states. (b) Example of Arrott plots for the moderately etched sample (II). (c) and (d) Sample I: Determination of $T_C$ in the accumulation and depletion states using the cusp of the resistance derivative (c) and intercepts of Arrott graphs plotted vs temperature (d). (e) and (f) Same analysis for sample II. (g) and (h) Same analysis for sample III.