No abstract

Room-temperature ferromagnetism in Mn-doped chalcopyrites is a desire aspect when applying those materials to spin electronics. However, dominance of high Curie-temperatures due to cluster formation or inhomogeneities limited their consideration. Here we report how an external perturbation such as applied hydrostatic pressure in CdGeP2:Mn induces a two serial magnetic transitions from ferromagnet to non-magnet state at room temperature. This effect is related to the unconventional properties of created MnP magnetic clusters within the host material. Such behavior is also discussed in connection with ab initio density functional calculations, where the structural properties of MnP indicate magnetic transitions as function of pressure as observed experimentally. Our results point out new ways to obtain controlled response of embedded magnetic clusters.

The effect of hydrostatic pressure on resistivity and magnetic ac susceptibility has been studied in Mn-doped CdGeAs 2 room-temperature (RT) ferromagnetic chalcopyrite with two types of MnAs micro-clusters. The slight increase of temperature by about 30 K in the region between RT and Curie temperature T C causes a significant change in the positions of pressure-induced semiconductor-metal transition and magnetic phase transitions in low pressure area. By conducting measurements of the anomalous Hall resistance in the field H≤5 kOe, we present experimental evidence for pressure-induced metamagnetic-like state during the paramagnetic phase at pressure P≈5 GPa.

An unusual change in the hysteresis direction is believed as rare phenomenon associated with perovskite-type structure. Such 'anomalous' magnetization hysteresis could possess a direct impact on the giant magnetoresistance (MR). Here we demonstrate that the room-temperature magnetization versus pressure for chalcopyrite semiconductor Zn 1−x Mn x GeAs 2 with x = 0.01 follows a usual direction of hysteresis, while the direction turns into anomalous for x = 0.07. Both these phenomena are results of a pressure-induced structural transition occurring in the host material, as is evident from volumetric measurements and ab initio calculations. This structural transition gives rise to the pressure-enhanced large MR and changes it drastically. Unlike the case of x = 0.01 where MR can be well reproduced within a theoretical approach, the presence of magnetic inhomogeneities for x = 0.07 induces an unexpected crossover from large positive to non-saturating negative MR (~92% at H = 5 kOe) in the new high-pressure phase. These results suggest that Zn 1−x Mn x GeAs 2 provides an example of a chalcopyrite-based material whose functional possibilities could be expanded through a new type of 'structuredriven' MR.

A study on the electrical behavior of polymer composites based on multi-walled carbon nanotubes (MWCNT) under the application of hydrostatic pressure up to 9 GPa and at room temperature is reported. A higher resistance, with values of order of kΩ, is demonstrated for MWCNT with an aspect ratio R9GPa/Rint ≈7. Our observations also show that pressure induced a structural change of the MWCNT to an ellipsoid shape at P ∼ 1–1.5 GPa—a measurement that correlates rather well with theoretical predictions. By direct and reverse high-pressure measurements of resistance, as well as current-voltage characteristics, we have identified the reversibility of electrophysical properties. Our observations suggest that the polymer composite based on MWCNT is a promising material for pressure sensing devices.