In a pioneer work, tunneling anisotropic magnetoresistance (TAMR) effect has very recently been used to probe the motion of the AFM spins in AFM spintronic devices 26, 27. X-ray magnetic circular dichroism and x-ray magnetic linear dichroism can detect FM and AFM spins due to their element-specific advantage 12, 16, 25. Very few methods can be implemented to directly probe the AFM spins due to almost zero net magnetic moment of the AFM layers. In most studies, the information of AFM spins is indirectly explored from the hysteresis loops of the FM layers by micromagnetic simulations and Monte Carlo calculations 15, 17, 21, 23. There is still a lack of direct experimental evidence. The EB training effect is attributed to the relaxation of the Δ m AFM towards the equilibrium state during consecutive hysteresis loops 19, 20, 21, 22, 23, 24. Moreover, for FM/AFM bilayers, the rotational hysteresis loss at external magnetic field H larger than the saturation field is ascribed to the irreversible switching of AFM spins during clock wise (CW) and counter clock wise (CCW) rotations 11, 17, 18. Therefore, the behavior of AFM spins is still under debate. thought that EB can be established without Δ m AFM 12. found that a nonzero AFM net magnetic moment Δ m AFM is necessary to establish EB 16. Rotatable and frozen AFM spins are often thought to be responsible for the coercivity enhancement and shift of the magnetization hysteresis loops 5, 11, 12, 13, 14, 15. Nevertheless manipulation and characterization of the AFM spins are important to understand and control the EB phenomena 10. FM/AFM bilayers are now commonly integrated in spintronic devices 9. After the FM/AFM bilayers are cooled under an external magnetic field H FC from high temperatures to below the Néel temperature of the AFM layers, hysteresis loops are simultaneously shifted and broadened 8.

Exchange bias (EB) in ferromagnetic (FM)/antiferromagnetic (AFM) systems has attracted much attention because of its intriguing physics and technological importance in spin valve based magnetic devices 1, 2, 3, 4, 5, 6, 7.

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