Exchange spring in a heterostructure with Pd$_{0.96}$Fe$_{0.04}$ low-temperature ferromagnet

The exchange bias of the magnetic hysteresis loop and the exchange-spring effect have been observed in thin-film heterostructures comprising layers of Pd$_{0.96}$Fe$_{0.04}$ low-temperature ferromagnet, metallic cobalt, and cobalt monoxide (CoO) grown on single-crystalline sapphire by methods of magnetron sputtering and molecular beam epitaxy in an ultrahigh-vacuum system. It is established that the exchange anisotropy of Pd$_{0.96}$Fe$_{0.04}$ layer in Pd$_{0.96}$Fe$_{0.04}$/CoО/Co/Al$_{2}$O$_{3}$ thin-film heterostructure is absent and the exchange-spring effect in this system is not manifested. In the case of a Pd$_{0.96}$Fe$_{0.04}$/Co/CoО/Al$_{2}$O$_{3}$ structure, it is shown that, according to the results of magnetometric measurements, the inversion of the order of antiferromagnetic and ferromagnetic layers leads to pinning of the magnetic moment of Pd$_{0.96}$Fe$_{0.04}$ layer at the interface with cobalt and to realization of the exchange-spring effect. Additional evidence in favor of this interpretation was obtained using the micromagnetic modeling of magnetic hysteresis loops.