Physical Properties Of Magnetic Thin Film Systems Deposited On Smooth And Rough Substrates

The principal of the exchange bias (EB) is one of the essential physical mechanisms for some basic devices, for example, spin valves giant magnetic resistance, hard disk driver head and magnetic random access memory etc. The study on the EB effect in ferromagnet(FM)/antiferromagnet(AFM) film systems is an important issue for the magnetic electronics. Although it has been extensively investigated during the last several decades, some aspects still remain unclear. Up to now, works have mainly focused on FM/AFM film systems in which the Curie temperature Tc of the FM layer is much higher than the Neel temperature TN of the AFM phase, however, very few results on FM/AFM exchange bias systems with TC< TN is available.Cr(TN= 311 K)/Gd(TC= 293 K), Cr/Tb(TC= 216 K) bilayers and multilayers systems, where the Tc of both Gd and Tb is lower than the TN of Cr, have been prepared by dc magnetron sputtering method on glasses and Si(100) substrates, respectively. The temperature dependences of the exchange bias field (HE) and the coercivity (HC), as well as the annealing influences on the structural and magnetic properties of the systems, have been studied systematically in this dissertation. In addition, an iron film percolation system has been fabricated successfully by vapor-phase deposition on fracture surfaces ofα-Al2O3 ceramics, its microstructure and low temperature magnetic properties have been investigated in detail.The results show that the temperature dependences of HE and HC, i.e., HE(T) and HC(T), in the Cr/Gd and Cr/Tb film systems depend strongly on the relation between Tc and TN, the characteristic magnetic structures of the ferromagnetic layers(Gd,Tb) and antiferromagnetic Cr layer.For the Cr/Gd bilayers and multilayers systems, where TC is slightly less than TN, some anomalous behaviors have been found:(1) the HE(T) and HC(T) are quite different from those of the traditional FM/AFM films with TC> TN; (2) it is the first time that positive exchange bias phenomenon is observed in FM/AFM films with Tc< TN; (3) the exchange bias field persists to T> TN. Due to TC is near to TN, the spins of the Gd layer are affected by the spins of the Cr layer at Tc< T< TN and at T> TN, and thus leads to the anomalous HE(T), Hc(T) behaviors. It is proved that the positive exchange bias field results from the temperature dependent wavelength of the spin-density wave in Cr. When T> TN, Gd and Cr are in the ferromagnetic and commensurate spin-density wave phases, respectively, resulting in the persistence of the exchange coupling between Gd and Cr layers.It is demonstrated that the He (T) of the Cr/Gd system relates to the magnetic structure of Gd, the thickness of Cr layer and the interfacial exchange coupling between Gd and Cr. The enhancement of He has not been observed which results from the inconsistent reverse of the moments. For Cr/Gd bilayer, Cr(102nm)/Gd(90nm)/Cr(102nm) trilayer and [Cr(20.4nm)/Gd(18nm)]5 multilayers, HC varies nonmonotonously with the temperature. HC first decreases with the temperature at 80< T< 205 K, and then makes local minima and maxima at T≈205 K; at 205< T< 255 K, He increase with T, which originates from the fact that the orientation of the easy axis of Gd turns continuously from parallel to perpendicular to the c axis; at 255 K< T< TC, HC falls rapidly; when T> Tc, due to the influence of Cr layer, the anisotropy of Gd is enhanced and leads to the increase of HC with T. For the [Cr(3.4nm)/Gd(3nm)]3o multilayer, however, He decreases linearly with the temperature in the temperature range of 80 K< T< TC, resulting from the competition between the interfacial exchange coupling and the adjacent Gd layers interaction.The heat treatment influences strongly on the microstructure and the magnetic properties of Cr/Gd/Cr trilayers. The X-ray diffraction (XRD) patterns show that the as-prepared sample shows amorphous or microcrystalline structure, whereas the annealed sample exhibits polycrystalline structure. The magnetic measurement shows that:(1) He is decreased after annealing. (2) For the as-prepared sample, when T< 260 K, it exhibits magnetic hysteresis behavior, it can not be saturated magnetized even at a strong field (H= 15000 Oe); when 260 K< T< TC, it is superparamagnetic. For the annealed sample, it exhibits ferromagnetic characteristics at T< TC and can be easily saturated.The HE(T) and HC(T) for the Cr/Tb bilayers and multilayers with TN >> Tc are quite different from those of the Cr/Gd system above. For the Cr/Tb multilayers, the negative exchange bias field is observed, the absolute value of HE decreases monotonously with T and falls to zero at Tc. For the Cr/Tb bilayers, He changes from negative to positive values with the temperature, demonstrating the expansion of the wave length of spin-density wave of the Cr layer with T. The magnetic measurement results show that Hc decreases linearly with the temperature for all the samples. On the other hand, as the Cr thickness increases, a maximum peak in both the He and He curves is observed, respectively.The XRD patterns show that the iron films on fracture surfaces ofα-Al2O3 ceramics exhibit microcrystalline structure. The scanning electron microscopy (SEM) photograph of the sample displays an uneven surface morphology of the film sample. The magnetic measurement results show that, due to the disorder of anisotropy and the surface oxidation, a spin-glass-like state and a ferromagnetic phase coexist in the iron films. It is exhibited that the temperature dependence of the coercivity Hc(T) of the iron film possesses a maximum peak around T= 10 K, which is related to the spin-glass like state.The exchange coupling between the freezing spins and reversible spins causes the exchange bias phenomena in the iron films. The temperature dependence of HE exhibits a minimum peak around T= 5 K, which is independent of the magnitude of the cooling field HCF.However, for T> 10 K, (1) HE is always negative when HCF= 2000 Oe and (2) for HCF= 20000 Oe, HE changes from negative to positive values with T.This dissertation is organized as following:In chapterⅠ, we present a brief review on the studies of the basic characters, related effects, the theoretical models and experimental results about the exchange bias. An introduction about the research progress on magnetic thin films is also given.In chapterⅡ, the experimental principles and methods for the preparation of the films and characterization for the sample properties are introduced.In chapter III, the temperature behavior of both the exchange bias field and coercivity for the Cr/Gd bilayers and Cr/Gd/Cr trilayers, where Tc is slightly less than TN, is studied in detail. In addition, the annealing effect on the microstructure and magnetic properties of the Cr/Gd/Cr trilayers is studied systematically.In chapter IV, the temperature dependence of both the exchange bias field and coercivity for the Cr/Tb bilayers and multilayers is revealed.In chapter V, we present the characteristic microstructures and anomalous low temperature magnetic properties of the iron film system deposited on fracture surfaces ofα-Al2O3 ceramics.