Studies On Magnetic Properties Of Ferromagnet/Antiferromagnet Thin Films With Curie Temperature Less Than Néel Temperature

Exchange coupling in ferromagnet（FM）/antiferromagnet（AFM）thin films plays a significant role in magnetic sensors,magnetic recordings and other fields.In the recent decades,researchers all over the world have carried out intensive investigations on the exchange bias filed（HE）,the coercivity（H_C）and giant magnetoresistance effect in FM/AFM film systems,and obtained a number of excellent results.In most researches on exchange bias systems,the Curie temperature（T_C）of FM is higher than the Néel temperature（T_N）of AFM.However,studies on the exchange bias systems with T_C < T_N have been rarely reported,which can cause some novel magnetic properties and enhance understanding of the mechanism of exchange coupling or the inducing effect from AFM net spins.In this paper,the Tb/Cr multilayers,Gd/FeMn bilayers and Gd/Cr multilayers were deposited on the glasses or silicon wafers by using DC magnetron sputtering.The Tb（T_C = 220 K）and Gd（T_C = 293 K）are the ferromagnetic layers,and the Cr（T_N = 311 K）and FeMn（T_N = 425 K）are the antiferromagnetic layers.The crystalline structure,cross-section structure,surface morphology and interfacial roughness is characterized by X-ray diffraction,scanning electron microscope and X-ray reflectivity,respectively.The magnetic properties are measured by vibrating sample magnetometer and magnetic property measure system.The results show that: the exchange bias effect in the FM/AFM film systems with T_C < T_N is obviously different from the traditional FM/AFM systems with T_C >T_N.H_C and HE varies monotonically with temperature;the net AFM spins at the FM/AFM interface can induce FM layers to be magnetized at T_C < T < T_N,hence giving rise to H_C and HE in this system.（1）For the Tb/Cr multilayers,as temperature increases,H_C initially decreasesmonotonically,and then increases rapidly to the the maximum.Moreover,at 156 K <T < 216 K,the hysteresis loops exhibit “double hysteresis loops” behaviors.（2）For the Gd/FeMn bilayers,H_C varies non-monotonically with temperature,and disappears at T ≈ T_N.HE crosses from negative value to positive value with increasing temperature,and vanishes at T ≈ T_C.Additionally,H_C and HE exhibit linear dependence on the inverse Gd thickness in Gd/FeMn bilayers when Gd thickness is large.（3）For the [Gd/Cr]₅ multilayers,the exchange bias is dependent on the deposition temperature,the measured temperature and the magnitude of cooling field.When the multilayers deposited at 25 ℃,the sign of HE is negative in the whole temperature range.When the multilayers deposited at 200 ℃,the sign of HE changes from negative to positive with increasing temperature.When the multilayers deposited at 400 ℃,the sign of HE is positive in the whole temperature range.Additionally,as the magnitude of cooling field increases,HE can change from negative value to positive value.