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Observation of Exchange Bias in Antiferromagnetic Cr$_{0.79}$Se due to the Coexistence of Itinerant Weak Ferromagnetism at Low-temperatures

We report on the structural, electrical transport, and magnetic properties of antiferromagnetic transition-metal monochalcogenide Cr$_{0.79}$Se. Different from the existing off-stoichiometric compositions, Cr$_{0.79}$Se is found to be synthesised into the same NiAs-type hexagonal crystal structure of CrSe. Resistivity data suggest Cr$_{0.79}$Se to be a Fermi-liquid-type metal at low temperatures, while at intermediate temperatures the resistivity depends sublinearly on the temperature. Eventually, at the elevated temperatures the rate of change of resistivity rapidly decreases with increasing temperature. Magnetic measurements suggest a transition from paramagnetic phase to an antiferromagnetic phase at a N$\acute{e}$el temperature of 225 K. Further reduction of the sample temperature results into coexistance of weak ferromagnetism along with the antiferromagnetic phase below 100 K. As a result, below 100 K, we identify significant exchange bias due to the interaction between the ferro- and antiferromagnetic phases. In addition, from the temperature dependent X-ray diffraction measurements we observe that the NiAs-type structure is stable up to as high as 600$^o$C.