Ferromagnetic-Induced Component in Piezoresistance of GaMnAs


Koji Onomitsu1, 2, Imran Mahboob1, Hajime Okamoto1, Yoshiharu Krockenberger2, and Hiroshi Yamaguchi1
1Physical Science Laboratory, 2Materials Science Laboratory

   The piezoresistive effect (PR) is defined as the change in resistance of a material due to an applied stress and is a fundamental property of semiconductors and metals. In conventional semiconductors, PR originates from a change in either carrier density or mobility. PR can also be observed in metals, including ferromagnetic materials, due to geometrical effects. PR provides a powerful tool for the investigation of electronic transport properties and can be used for sensing applications. In this study, we report a new mechanism of PR in GaMnAs where the ferromagnetic ordering plays an essential role. We characterized PR by incorporating a GaMnAs piezoresistor into a micromechanical cantilever and investigated its temperature and magnetic field dependence at the ferromagnetic transition. A clear ferromagnetic-induced piezoresistance component (FMPR) was found below the Curie temperature (Tc) of our GaMnAs (Tc ~ 48 K). Figure 1 shows the schematic illustration of the cantilever together with electrode on PZT and measurements setup. The frequency response of PR is detected via down-mixing technique. The reference frequency can be tuned by adjusting bias-current frequency (f1) and actuation frequency (f2). Figure 2 shows the temperature dependence of PR around Tc without external magnetic field: (a) real part and (b) imaginary part. Data (dots) show the measured frequency response, and the straight lines were obtained by calculations. Data and calculated lines have been offset by = 0.04 for clarity.
   These results indicate that the FMPR of GaMnAs arises from the perturbation of spontaneous spin ordering by the applied strain. Moreover, the change in resistance is delayed with respect to mechanical strain. We deduced a delay time of 230 ± 35 ns. The experimental results presented indicate that the micromechanical method of characterizing spin dynamics is complementary to conventional electrical and optical methods [1].

[1] K. Onomitsu et al., Phys. Rev. B 87 (2013) 060410 (R).

Fig. 1. Schematic illustration of the cantilever on PZT and measurements setup.
Fig. 2. Temperature dependence of PR around Tc.