Magnetization reversal mechanism in strongly exchange-coupled double layers of Co/Pt and TbFe

Since the pioneering work in rare earth-transition metal (RE-TM) by Mimura et al. and K.H.J. Buschow et al., amorphous TbFe ferrimagnets and other rare earth-transition metal (RE-TM) films gained attention for their strong perpendicular magnetic anisotropy (PMA) and good magneto-optical properties. Recently, the development of all-optical switching has triggered a renewed interest in rare earth-ferrimagnets. For example, Radu et al. reported X-ray Magnetic Circular Dichroism (XMCD) measurements that probe the optically excited nonequilibrium spin dynamics on nanometre length scales and femtosecond timescales in GdFeCo. Liu et al. demonstrated that single femto-second optical laser pulses of suffcient intensity were able to reproducibly reverse the magnetization in TbFeCo thin films, which can be a model system for all-optical switching-based
recording technologies.
The focus of my thesis is on the study of the RE-TM ferrimagnet TbxFey thin films and the ferri/ferro-magnet TbxFey/[Co/Pt]n exchange-coupled systems,
where x and y designate the atomic ratio of Tb and Fe, and n designate the number of [Co/Pt] bi-layers in the multilayer. These exchange-coupled double layer structures have many potential applications, e.g. as candidate systems for
a hard RE-TM storage layer coupled to a softer read/write layer for heat-assisted magnetic recording (HAMR). The details of the exchange-coupling and the magnetization
reversal mechanism are not fully studied and understood in this strong coupling regime. This thesis is a first study of these systems with high spatial resolution to understand the physics that determines the reversal.
The samples were fabricated using DC magnetron sputtering at room temperature under ultra high vacuum (UHV) conditions. For the imaging of the stray fields emanating from a sample surface and the magnetization reversal of magnetic thin films, magnetic force microscope (MFM) is the technique of choice since it probes the local stray field of materials with high spatial resolution and in applied magnetic fields. Therefore MFM is used as a main technique in my work to investigate the micro-magnetic state of the sample.
An outline of this thesis is given below, with each chapter giving a short overview providing the necessary context.
Chapter 2 provides an introduction to the principles of MFM. For the quantitative analysis and modeling of MFM data, a transfer function relating the MFM contrast to the stray field emanating from the sample surface is necessary. It is
obtained through the tip calibration procedures. Our MFM instrumentation and the important aspects of sample and probe preparation and handling are discussed as well.
The measured MFM magnetic contrast arises from the magnetic forces between tip and sample, due to the stray field emanating from the sample surface. The stray field decays rapidly with increasing distance from the surface. In order to obtain high resolution MFM images and the subsequent quantitative analysis, the magnetic tip needs to scan very close to the sample surface and be kept at a
constant (average) distance during image scan even in a large applied magnetic field. Therefore, a robust method for active tip-sample distance control based on frequency modulation of the cantilever oscillation has been developed. With this method, a tip-sample distance of the order of 10nm can be controlled with a precision better than 0.4 nm. This frequency-modulated capacitive tip-sample distance control method is presented in Chapter 3.
Chapter 4 presents the results for the study of TbFe thin films. The magnetization reversal was studied by MFM measurements, and the magnetization loops measured by vibrating sample magnetometry (VSM) and superconducting quantum interference device (SQUID) magnetometry. The MFM-scans were performed at 10.5K and in external magnetic fields ranging from 0 to 7 T, at a constant average tip sample distance of 7 nm. The topographical and magnetic contributions in the MFM frequency shift contrast were separated by scanning with up and down tip magnetizations. Magnetic contrast and magnetic pattern evolution as function of field were evaluated for the original and the zoomed MFM images. We used the transfer function to simulate the MFM contrast measured on TbFe thin films. In addition, Rutherford backscattering spectrometry (RBS) and transmission electron microscopy (TEM) were used for composition and chemical analyses of the samples.
The magnetization reversal of exchange-coupled TbFe/[Co/Pt] double layers is addressed in Chapter 5. The magnetometry and MFM measurements, as well as the quantitative MFM data analysis were carried out in a similar way to the TbFe thin films presented in Chapter 4. The reversal processes can be classified into three field regions, e.g. the rotation of [Co/Pt] local magnetic moments in low fields, the reversal of [Co/Pt] via nucleation of 'sub-domains' accompanied by the formation of interfacial domain walls (iDW) in intermediate fields, and the compression of the iDW in high fields. In addition, TbxFey/[Co/Pt] samples with Pt interlayers exhibit lower exchange-bias field with larger interlayer thickness, showing the possibility of tuning the exchange-coupling by the Pt interlayer.
The above studies attempted to understand the micromagnetic state of the amorphous TbFe alloy thin films and the magnetization reversal of TbFe/[Co/Pt] based exchange-coupled double-layer structures. Chapter 6 gives a summary of the presented work and provides an outlook on the envisaged experiments and simulations on the above mentioned systems and methods.