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(a) The resistivity of the annealed GaMnAs film used to fabricate samples in this study, showing a peak at around 135 K, and metallic conduction down to 6 K. The inset is a cartoon showing the sample structure. (b)Low field magnetization of the GaMnAs film used in this study as a function of temperature, showing a Curie temperature of about 125 K. There is a small ferromagnetic contribution that persists above 250 K, suggesting the presence of MnAs clusters in the film.
(a) Low-magnification SEM image of a sample with a nano-constriction. (b) High-magnification SEM image of the area of a 40 nm nano-constriction. The current flows in the x-direction, and the magnetic field is applied along the y-axis. (c) Current through the nano-constriction as a function of the applied AC voltage V. (d) Resistance across the nano-constriction as a function of applied voltage V. The resistance is an order of magnitude larger than the resistance quantum, and decreases with V, suggesting that conduction through the constriction occurs through tunneling.
(a) The resistance across the constriction as a function of the magnetic field. The field is applied in the sample plane and perpendicular to the current flow direction. Red solid circles are for magnetic field swept from a large negative value toward positive values, while blue squares are for the opposite sweep direction. The measuring current is 3 nA. (b) The same as in (a) with a measuring current of 6 nA, but after running a large current of 300 nA through the sample. Note that both the resistance and the magnetoresistance increase after applying the large current. (c) An optical microscope image of a sample fabricated in the form of a Hall bar of width 200 μm. (d)Resistance and MR of the Hall bar. Note that both R and MR are much smaller than those measured on the nano-constriction. The lines in the data are a guide for the eye.
(a) Antisymmetric magnetoresistance measured on a sample with a nano-constriction using a measuring current of 3 nA. The local magnetization in the nano-constriction area has likely pointed out of the plane of the film, leading to this antisymmetric behavior. (b) Applying a larger voltage across the nano-constriction area gives an antisymmetric magnetoresistance curve but with a smaller resistance than that in (a) (see Fig. 2(d) ). The spike in magnetoresistance shows that the magnetization of the nano-islands is “frustrated,” and fluctuates in multiple directions as it switches. The measuring current in this figure is about 8 nA. (c) A cartoon showing a model of the sample. The magnetization is in the plane on either side of the nano-constriction, but can be quite different within the nano-constriction area due to strain relaxation. Both magnetic and electric properties of the constriction area are altered due to the fabrication process.
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