The relation between magnetic hysteresis and the micromagnetic state explored by quantitative magnetic force microscopy

Abstract

Since the discovery of magnetic lodestones in 600 BC described by ancient Greek philosophers, the mystery of magnetism has continued to intrigue. For thousands of years, these lodestones were more of a curiousity until medieval explorers discovered how to use them to produce a magnetic compass. The compass was studied and explained by William Gilbert in his 1600 treatise on magnetism called de Magnete\1{3. In his work, Gilbert proposed the Earth itself to be magnetic, helping to open up the modern ¯eld of magnetism. Only at the start of the nineteenth century was magnetism discovered to be due to elementary magnets analogous to how matter is made up of atoms and molecules. This idea was then developed further by Pierre-Ernest Weiss in 19074, who discovered that these elementary magnets grouped together in ferromagnets to form magnetic domains. In 1933, these domains were exper- imentally con¯rmed by Barkhausen5 after observing that magnetisation was a discontinuous process. He ampli¯ed the sound produced by these discontin- uous jumps and attributed it to domain switching. In a contemporary study, Bitter developed a technique using a suspension of magnetic colloids to visu- alise the domains6. Domain switching was subsequently found to be lacking as an explanation of magnetisation by Langmuir in 19317, who proposed that the propagation of the walls surrounding the domains was responsible for the magnetisation reversal. This domain wall motion also explained the appear- ance of hysteresis which characterises the tendency of a magnetic system to behave di®erently depending on its ¯eld history. The study of hysteresis re- ally took o® with the advent of magnetometry. These experimental techniques are capable of measuring the average magnetisation of a system as a function of applied ¯eld. One of these, vibrating sample magnetometry (VSM), was invented in 19598 and has been used ever since as the principle means of ac- cessing a systems magnetic parameters. Since VSM is an volume averaging technique, the microscopic nature of domains remained elusive until the ad- vent of magnetic force microscopy (MFM) in 19879. This powerful technique can image the magnetic ¯eld emanating from the surface which gives much needed information about the magnetisation for 100'snm below the surface. This is therefore especially useful when studying magnetic thin ¯lms. The widespread use of magnetic thin ¯lms in contemporary data storage technology, sensors of magnetic ¯eld, strain and acceleration, has been made possible by the optimisation of the materials involved, aimed at attaining speci¯c magnetisation structures and hysteresis curve characteristics. However the understanding at the microscopic scale of the reversal process remains a challenge. This thesis presents a study of magnetism in thin ¯lms, more speci¯cally thin ¯lms with perpendicular magnetic anisotropy (PMA). These mag- netic thin ¯lms with an easy axis of magnetisation perpendicular to the surface have attracted particular attention due to their potential advantage in high- density magnetic recording media10{13. Magnetic thin ¯lms with PMA also provide a model system of domain behaviour in a wide variety of materials. More particulary, thin ¯lms of nickel have been shown to be of great interest because of their intrinsic magnetic properties favouring PMA14{17. The origin, advantages and removal of PMA in thin ¯lms are investigated in this thesis. While both Chapters 3 and 4 present studies of the origin of PMA in nickel ¯lms, Chapter 3 explains how the conventional interpretation of the magnetisation loops can give an incorrect picture of both the magnetisa- tion processes and magnetic domain structures in the ¯lms. The observations defy explanation based on volume averaged magnetometry measurements but come to light when magnetometry is combined with quantitative magnetic force microscopy (qMFM). qMFM revealed a speci¯c magnetisation pattern, namely perpendicular stripe domains with closure caps. Furthermore, the do- main wall is shown to be as important as the domain itself in the average mag- netisation. These new ¯ndings come from using the well established method of magnetometry complimented by the latest qMFM techniques. Chapter 4 concentrates on how ion irradiation can be used to remove PMA. Furthermore, it shows how temperature variation can be used to control the perpendicular component of the magnetisation. A common theme through this thesis is the coercivity of the ferromagnetic ¯lm. Hysteresis lies at the very foundation of the magnetic recording indus- try10,12. Hysteretic systems are employed as recording media because they retain their magnetic state for a long period after a writing operation, namely they exhibit magnetic memory. This memory has been extensively studied and exploited. However, despite decades of intense study and signi¯cant re- cent advances18,19, a fully satisfactory microscopic understanding of magnetic hysteresis is still lacking20,21. The origin of hysteresis and its relation to the micromagnetic state has been studied in Chapters 3, 4 and 5. Our novel results show how domain wall motion can be hysteresis free (Chapter 3 and 4). Domain nucleation and wall motion were studied in more detail in Chap- ter 4, in which we show that increasing the defect density does not necessarily increase the hysteresis. Indeed, it can even decrease it by acting on the domain nucleation and therefore on the domain density. The low pinning action of the domain wall is shown to be due to the small size of the domain, implying short distance to travel for the domain wall to allow the magnetisation rever- sal. Thus, the hysteresis is shown to depend strongly on the micromagnetic domain structure and particularly on the domain width. The reproducibility of the domain nucleation and wall motion is a key factor in microscopic memory20 and is studied in Chapter 6. The e®ect of defects on this microscopic reproducibility is shown to be more complicated than the common understanding. We show that defects act as nucleation centres for domains during the magnetisation reversal. This implies a good reproducibility of the way domains nucleate after saturation. On the other hand, defects also act to break up the domains while a demagnetised sample is brought towards saturation. This decreases the reproducibility of the domain evolution on a minor loop excursion. Chapter 7 presents a study of a perpendicular magnetised array of nanois- lands. This has been previously proposed with a view to high density recording media10. The hysteresis of such systems has to be characterised and controlled. We show that decreasing the island size under 50nm increases the coercivity distribution, to detriment of their archival potential. In summary, this thesis presents novel information about the mechanism leading to hysteresis and the related micromagnetic state. Yet, it also im- proves the understanding of magnetic measurements two folds. It highlights the danger of quick interpretation of magnetometry. Moreover, it presents a novel way of using state-of-the-art qMFM.