Viewing spin structures with soft X-ray microscopy

Abstract

Viewing spin structures Viewing spin structures with soft x-ray microscopy Peter Fischer Center for X-ray Optics E. O. Lawrence Berkeley National Laboratory 1 Cyclotron Road, Berkeley, CA 94720 The spin of the electron and it’s associated magnetic moment marks the basic unit for magnetic properties of matter . Magnetism, in particular ferromagnetism and antiferromagnetism is described by a collective order of these spins, where the interaction between individual spins reflects a competition between exchange, anisotropy and dipolar energy terms. As a result the energetically favored ground state of a ferromagnetic system is a rather complex spin configuration, the magnetic domain structure . Magnetism is one of the eldest scientific phenomena, yet it is one of the most powerful and versatile utilized physical effects in modern technologies, such as in magnetic storage and sensor devices. To achieve highest storage density, the relevant length scales, such as the bit size in disk drives is now approaching the nanoscale and as such further developments have to deal with nanoscience phenomena . Advanced characterization tools are required to fully understand the underlying physical principles. Magnetic microscopes using polarized soft X-rays offer a close-up view into magnetism with unique features, these include elemental sensitivity due to X-ray magnetic dichroism effects as contrast mechanism, high spatial resolution provided by state-of-the-art X-ray optics and fast time resolution limited by the inherent time structure of current X- ray sources, which will be overcome with the introduction of ultrafast and high brilliant X-ray sources. The primary goal of magnetic microscopies is to image the static domain structure, i.e. the spin configuration in thermal equilibrium viewed at highest spatial resolution. A large variety of magnetic imaging techniques has been developed which are available to study magnetic systems. They can be classified to the probes they use. There are optical microscopes using the magneto- optical Kerr effect (MOKE), where a contrast is generated by the rotation of the polarization vector of the incoming light by a magnetic moment; there are electron microscopes, such as the Lorentz transmission electron microscope (TEM), which utilizes the Lorentz force diverting the electrons as they propagate through the magnetic specimen , or the Scanning Electron Microscope with subsequent Polarization Analysis of the electrons (SEMPA) . There are also several scanning probe microscopes, such as the Magnetic Force Microscope (MFM), which senses the interaction of the stray field emanating from the sample onto its magnetic tip or the Spin Polarized Scanning Tunneling Microscope (SP-STM), which detects the tunneling current between the atomic tip and the sample’s surface . Among magnetic microscopy techniques the SP-STM leads in spatial resolution as it has so far achieved almost atomic resolution. Whereas Kerr microscopy is diffraction limited in its spatial resolution, by using ultrafast laser systems, it can study ultrafast spin dynamics down to fsec time scales . Studies of ultrafast spin dynamics have gained significant interest in the recent past, since ultimately the temporal evolution of spin structures provides insight into the basic mechanism of the magnetic interactions and also determines the functionalities of magnetic devices. There are several ways to control magnetization on a nanoscale. The traditional mechanism is to apply an external magnetic field, which goes back to the more than 150 year old experiments by Oersted, but is still the technique of choice to write for example information in a magnetic hard drive . However, this mechanism runs into inherent limitations when the characteristic feature sizes approach the nanoscale. The long ranging dipolar field has a strong impact to neighboring cells, leading to significant cross-talk problems in devices. Further, the writing speed, i.e. the reversal of the magnetic moment as the external field is pointing in opposite direction to the moment is relatively slow, since the torque exerted on the moment is small and relies on thermal fluctuations of the magnetic moment to initiate the reversal process. Finally, there is the problem of the superparamagnetic limit , which requires on one hand high anisotropy media to compensate the counteracting spontaneous (Boltzmann) driven reversal, which on the other hand requires strong magnetic fields acting on short length scales, which is technologically very challenging to realize. In addition to the charge of the electron the concept of spintronics considers also its spin as a new degree of freedom . This opens new avenues to manipulate spins on the nanoscale . By injecting a pin polarized current into a ferromagnet, a torque is exerted onto a non-uniform spin configuration in the system, which can reverse the magnetization , move a domain wall or excite the dynamics of a vortex core . This spin torque effect scales in a more favorable way than the Oersted field since its efficiency increases with decreasing length scale.