Hydrostatic pressure study of the paramagnetic-ferromagnetic phase transition in (Ga,Mn)As

The effect of hydrostatic pressure on the paramagnetic-ferromagnetic phase transition has been studied in (Ga,Mn)As. The variation in the Curie temperature $\left(T_C\right)$ with pressure was monitored by two transport methods: (1) measurement of zero-field resistivity versus temperature $\rho \left(T\right)$ and (2) dependence on temperature of the Hall voltage hysteresis loop. Two specimens of different resistivity characteristics were examined. The measured pressure-induced changes in $T_C$ were relatively small (on the order of 1 K/GPa) for both samples, however they were opposite for the two.

Figure 1

(Color online) Temperature dependence of magnetic moments measured by SQUID at $B=2\text{mT}$ at ambient pressure for both samples (diamonds—A963 and squares—A777). The dotted and solid vertical lines depict the values of $T_C$ extracted from the maximum of $d\rho /dT$ and from the Hall voltage hysteresis, respectively (see text).

Figure 2

(Color online) Resistivity as a function of temperature $\rho \left(T\right)$ measured for the two samples at ambient pressure, before pressure cycles.

Figure 3

Resistivity evolution as a function of pressure for the A777 sample. Arrows show the evolution of the position and magnitude of the maximum in $\rho \left(T\right)$.

Figure 4

Temperature derivative of the resistivity, $d\rho /dT$ calculated for both samples from the experimental data measured at ambient pressure and high pressure.

Figure 5

(Color online) Runs of the Hall voltage versus magnetic field revealing hysteresis loops for both our samples: (a) A777 and (b) A963. Magnetic field was perpendicular to the sample plane. For the sample A963 the shape of the hysteresis loop indicates that an easy axis was perpendicular to the sample plane.

Figure 6

(Color online) Temperature dependence of the width of the hysteresis loop measured for the sample A963 in a series of experiments made at various pressures.

Figure 7

Pressure dependence of Curie temperature deduced [(a) and (c)] from the hysteresis loops detected by Hall resistivity and [(b) and (d)] from the maximum of $d\rho /dT$. (a) and (b) refer to the sample A777 and (c) and (d) to the sample A963. The points give the values averaged over different pressure cycles. The two sets of points visible in Hall resistivity results (a) and (c) correspond to different Hall probes. Although there is a difference in the absolute value of the $T_C$, the observed pressure trends are identical. The solid lines show the predictions of the model, described in the text, assuming that the free hole concentration changes due to volume changes only.