The Microstructure And Perpendicular Magnetic Anisotropy Of Sputtered NdCo And NdFeCo Films

In recent years, the continuous emergence of new principles, new materials and newdevices based on the magnetic thin films has greatly promoted the development ofinformation technology and internet technology, leading to the birth of a new discipline of“spintronics". The magnetic film with a high perpendicular magnetic anisotropy (PMA)and high stability is a key material for a new generation of super high density informationstorage medium. It is also an important subject in the field of “spintronics".In this thesis, a systematic study has been carried on the origin of the PMA inamorphous film. In order to deepen the understanding of the microscopic mechanism ofPMA in amorphous film, it is necessary to systematically explore the correlation betweenPMA and the key technical parameters in film preparation process. A series of NdFeCo andNdCo films were prepared on single crystal of Si(111) by a magnetron sputtering machine.The microstructure of the films was regulated by rapid thermal annealing (RTA)technology, and examined by using X-Ray diffraction (XRD), scanning electronmicroscope (SEM) and atom force microscope (AFM), respectively. The magneticproperties are characterized by vibrating sample magnetometer (VSM). The magneticdomain patterns are obtained by using magnetic force microscopy (MFM). The regulationof structure and magnetic properties for NdFeCo and NdCo films are explored by varyingparameters of the substrate temperature (Ts), the sputtering Ar pressure (PAr), annealingtemperature (Ta) and so on. The main conclusions are introduced as follows:1. The structure and magnetic properties of Si(111)/NdFeCo/Cr filmsSeries of Si (111)/Nd43Fe40Co17(600nm)/Cr(20nm) thin films were prepared atsubstrate temperature of Ts=25，310℃, respectively. The working argon pressure was setas PAr=0.17,0.3,0.9, and1.5Pa, respectively. It is observed that the in-plane M-H loopsfor the samples deposited at Ts=310℃show a typical transcritical hysteresis loop.However, the M-H curves of films prepared at RT display the occurrence of a substantiallyin-plane magnetization process without any out-of-plane component. The MFM picturestaken at the magnetic remanence exhibit a stripe domain structure for samples deposited atTs=310℃. However, no domain contrast is observed for samples prepared at RT, whichimplies that the magnetization is aligned in the film plane. A regular stripe domain is observed for samples deposited at low Ar pressure of PAr=0.1Pa and at high substratetemperature of Ts=310℃. The bright and dark contrasts are due to magnetization cantedupward or downward out of the film plane. With argon pressure increased, the stripedomains become irregular maze domain. At argon pressure PAr=0.9Pa, the depositedsamples at Ts=310℃show maximum value of RMS (root mean square) of MFM phaseshift of Sqphase=2.28°. The maximum of the out-of-plane anisotropy energy (K⊥) isestimated to be124kJ/m3. Furthermore, the quality factor Q (defined by Q=K⊥/2πMs2) iscalculated to be0.8, which is larger than the values reported in the literature in otheramorphous films, suggesting a strong out-of-plane anisotropy.The RTA experiments were conducted on two sets of Si(111)/Nd43Fe40Co17(at.%)(610nm)/Cr(20nm) samples prepared at PAr=0.3Pa, and Ts=25,310℃, respectively. Theresults show that the in-pane saturation field Hs//and the value of RMS of MFM phase shiftSqphaseof samples deposited at Ts=310℃decreased rapidly with increase of annealingtemperature Ta(Ta≤310℃)，indicating the relaxation of residual internal stress quenchedin the films during deposition. After RTA annealing at Ta=500℃, the sample deposited atTs=310℃precipitates Nd6Fe13Si interface alloy phase which make in-plane coercivity(Hc//=750Oe) higher than that (Hc//=480Oe) of the sample deposited at Ts=25℃.Because the linear thermal expansion coefficients of Nd6Fe13Si and Nd2(FeCo) areopposite, the magneto-elastic anisotropy under stress is major cause of the PMA observedin films deposited at high substrate temperature of Ts=310℃.2. The structure and magnetic properties of Si(111)/Cr/NdCox(x=4,5)/Cr filmsIn order to clarify the role of anisotropic atomic environment of rare Nd or Cosublattice on the PMA in the amorphous NdCo films, series of NdCox (x=4,5) films withtwo different alloy composition were prepared. The MFM results show that the film ofSi(111)/Cr/NdCo5(400nm)/Cr-I(RT) deposited at RT exhibits stripe domains structure withbright and dark contrasts.The magnetization measurements show that the out-of-planeanisotropy energy (K⊥) and the quality factor Q of sample Si(111)/Cr/NdCo5(400nm)/Cr-Iprepared at PAr=0.17Pa and Ts=25℃are K⊥=82.6kJ/m3, and Q=0.094, respectively,which are almost double of the sample with low Co content ofSi(111)/Cr/NdCo4(400nm)/Cr-I prepared at PAr=0.9Pa and Ts=25℃,K⊥=32.4kJ/m3and Q=0.044, respectively. The measured average domain width (wexp.=201nm) is quiteconsistent with the calculated value of the ideal stripe domains width of NdCo5alloy (wcal.=226nm) according to the Landau domain structure theory. However, the as-depositedSi(111)/Cr/NdCo5(400nm)/Cr-I (HT) sample at RT display the occurrence of a substantially in-plane magnetization process without any out-of-plane component.The above results show that both of magnetoelastic anisotropy caused by internal stressand anisotropic atomic pair ordering induced during film preparation process plays animportant role in PMA of NdCo amorphous alloy films.These experimental results have theoretical and practical significance on theperpendicular magnetic anisotropy of the mechanism and the further development ofmicroelectronic components.