Universal Conductance Fluctuations in Epitaxial GaMnAs Ferromagnets: Dephasing by Structural and Spin Disorder

Mesoscopic transport measurements reveal a large effective phase coherence length in epitaxial GaMnAs ferromagnets, contrary to usual $3d$-metal ferromagnets. Universal conductance fluctuations of single nanowires are compared for epilayers with a tailored anisotropy. At large magnetic fields, quantum interferences are due to structural disorder only, and an unusual behavior related to hole-induced ferromagnetism is evidenced, for both quantum interferences and decoherence. At small magnetic fields, phase coherence is shown to persist down to zero field, even in presence of magnons, and an additional spin disorder contribution to quantum interferences is observed under domain walls nucleation.

Figure 1

(Top) Temperature dependence of the square resistance measured with a GaMnAs nano-Hall bar ($L=220\mathrm{nm}$, $W=50\mathrm{nm}$), a common behavior for both anisotropy. (Bottom left) Anisotropic magnetoresistance at $T_{\mathrm{cryo}}=25\mathrm{mK}$, for an in-plane anisotropy. (Bottom right) Extraordinary Hall resistance at $T_{\mathrm{cryo}}=25\mathrm{mK}$, for a perpendicular anisotropy. All measurements were done with a large current $I=100\mathrm{nA}$, leading to vanishingly small quantum corrections (see text).

Figure 2

(Bottom thick line) UCF of a nanowire with a perpendicular anisotropy ($L=1200\mathrm{nm}$, $W=150\mathrm{nm}$) measured at base temperature $T_{\mathrm{cryo}}\approx 25\mathrm{mK}$, with a small current $I=1\mathrm{nA}$ corresponding to an electronic temperature $T_{\mathrm{el}}\approx 100\mathrm{mK}$ [23]. (Bottom thin line) Same measurement, 24 h later. (Top thick line) Same measurement, after a thermal cycling up to $T=4.2\mathrm{K}$. (Top thin line) Same measurement, after a thermal cycling up to $T=300\mathrm{K}$.

Figure 3

(Top) Temperature dependence of UCF for a short nanowire with a perpendicular anisotropy ($L=220\mathrm{nm}$, $W=50\mathrm{nm}$). (Bottom) Temperature dependence of the normalized root mean square UCF amplitude $\delta G^{\mathrm{rms}}$ $L^{3/2}$, showing a universal scaling law $1/T^{3/4}$. Dashed lines define the accuracy for the determination of the prefactor. All results correspond to a perpendicular anisotropy, but open squares (planar anisotropy, $L=1200\mathrm{nm}$ and 600 nm, $W=150\mathrm{nm}$).

Figure 4

UCF measured at base temperature for a nanowire with either a perpendicular or a planar anisotropy. (Inset) Crossover below the anisotropy field for a planar anisotropy, showing the effect of domain walls on quantum interferences. Each field sweep begins from 7 T.