T. Eimüller

Element-specific imaging of magnetic domains at 25 nm spatial resolution using soft x-ray microscopy

The combination of magnetic circular dichroism as a magnetic contrast mechanism and a transmission x-ray microscope allows imaging of magnetic structures with lateral resolutions down to 25 nm. Results on magneto-optical Tb25(Fe75Co25)75 layers system with thermomagnetically written bits of various sizes were obtained at the x-ray microscope XM-1 at the Advanced Light Source in Berkeley, CA. The results prove the thermal stability of the bits in the recording process. Furthermore the capability of soft x-ray microscopy with respect to the achievable lateral resolution, element specificity and sensitivity to thin magnetic layers is demonstrated. The potential of imaging in applied magnetic fields for both out-of-plane and in-plane magnetized thin magnetic films is outlined.

Soft x-ray microscopy to 25 nm with applications to biology and magnetic materials

We report both technical advances in soft X-ray microscopy (XRM) and applications furthered by these advances. With new zone plate lenses we record test pattern features with good modulation to 25 nm and smaller. In combination with fast cryofixation, sub-cellular images show very fine detail previously seen only in electron microscopy, but seen here in thick, hydrated, and unstained samples. The magnetic domain structure is studied at high spatial resolution with X-ray magnetic circular dichroism (X-MCD) as a huge element-specific magnetic contrast mechanism, occurring e.g. at the L2,3 edges of transition metals. It can be used to distinguish between in-plane and out-of-plane contributions by tilting the sample. As XRM is a photon based technique, the magnetic images can be obtained in unlimited varying external magnetic fields. The images discussed have been obtained at the XM-1 soft X-ray microscope on beamline 6.1 at the Advanced Light Source in Berkeley. # 2001 Elsevier Science B.V. All rights reserved.

Uncompensated spins in a micro-patterned CoFeB/MnIr exchange bias system

The element specific domain configuration of a microstructured Co86Fe10B4∕Mn77Ir23∕Ni80Fe20∕Si3N4 exchange bias film was studied by photoemission electron microscopy using x-ray magnetic circular dichroism. In the dots the magnetization shows less preferred orientation along the exchange bias direction than in unstructured areas. Uncompensated Mn spins at the CoFeB/MnIr interface with an antiferromagnetic coupling between the Mn and the Co magnetic moments could be studied by imaging and spectromicroscopy.

Study of in-plane magnetic domains with magnetic transmission x-ray microscopy

Magnetic transmission x-ray microscopy is a novel technique to image element specifically magnetic domain structures. A lateral resolution down to 25 nm is provided by the Fresnel zone plates used as optical elements in soft x-ray microscopy. The magnetic contrast is given by x-ray magnetic circular dichroism, i.e., large magnetic contributions up to 25% to the absorption cross section of circularly polarized x rays that occur in the vicinity of, e.g., the Fe L3,2 edges (706 and 719 eV) and depend on the relative orientation of the projection of the magnetization of the sample onto the photon propagation direction. Thus, both in-plane and out-of-plane contributions to the magnetization are accessible. Here we present images of the magnetic domain structure of a (3 nm Cr/50 nm Fe/6 nm Cr) thin film system with a preferentially in-plane magnetization recorded at the Fe L edges. The samples have been prepared by thermal evaporation onto a 100 nm thin Si3N4 membrane and were mounted under a tilt of 30° with r...

Element-specific spin and orbital momentum dynamics of Fe∕Gd multilayers

The role of orbital magnetism in the laser-induced demagnetization of Fe/Gd multilayers was investigated using time-resolved X-ray magnetic circular dichroism at 2-ps time resolution given by an x-ray streak camera. An ultrafast transfer of angular momentum from the spin via the orbital momentum to the lattice was observed which was characterized by rapidly thermalizing spin and orbital momenta. Strong interlayer exchange coupling between Fe and Gd led to a simultaneous demagnetization of both layers.

Magnetization reversal study of CoCrPt alloy thin films on a nanogranular-length scale using magnetic transmission soft x-ray microscopy

We have investigated element-specific magnetization reversal behavior of (Co83Cr17)87Pt13 alloy thin films with a lateral resolution of 35-nm using magnetic transmission soft x-ray microscopy. We report that the magnetization switching is carried out by a random nucleation process that can be attributed to individual grains. We found evidence of a large distribution of the switching fields at the nanogranular-length scale, which has to be considered seriously for applications of CoCrPt systems as high-density magnetic storage materials.

Magnetic imaging with femtosecond temporal resolution

A scanning Kerr microscope with a temporal resolution of <230 fs and a spatial resolution of 210 nm is presented. Equipped with a large temporal and spatial scanning range of 8 ns and 320 microm, respectively, the microscope allows studying nonuniform magnetization dynamics on many different time scales over a large area. For demonstration, we study the magnetization dynamics in Fe/Gd multilayer dot arrays exhibiting a spin reorientation transition (SRT) on three different time scales, namely, femtosecond, picosecond, and nanosecond scales. The dynamics on all time scales varies from one dot to another. This is attributed to the high sensitivity of the SRT to the variations of the layer thicknesses and the Fe/Gd interface structure.

Magnetic imaging with soft X-ray microscopy

Recent achievements in magnetic transmission soft X-ray microscopy are reviewed. The magnetic contrast is given by X-ray magnetic circular dichroism, i.e., the dependence of the absorption coefficient of circularly polarized X-rays on the projection of the magnetization in a ferromagnetic system onto the photon propagation direction. A lateral resolution down at 25 nm is provided by Fresnel zone plates used as optical elements. Recording the images in varying external magnetic fields, inherent chemical specificity, a high sensitivity to thin magnetic layers due to the large contrast, and the possibility to distinguish between in-plane and out-of plane contributions allows detailed studies of magnetization reversal processes in magnetic patterned elements and thin films. Micromagnetic simulations support the experimental findings. The potential to study spin dynamics will be briefly outlined.

Magnetic domain imaging with a transmission X-ray microscope

Abstract The combination of X-ray magnetic circular dichroism being an element-specific local probe for the magnetic microstructure and the transmission X-ray microscope providing a spatial resolution of about 30 nm allows to image magnetic domains with a huge contrast. Special virtues of this technique are the applicability of varying magnetic fields thus allowing technologically relevant studies of the evolution and the switching behaviour of domain structures. Quantitative information on the local magnetization and in particular a separation of spin and orbital moments is possible. Results obtained at the Fe and Co L3 edges in GdFe and PtCo multilayered systems demonstrate the potential of this new technique.

Study of magnetic domains by magnetic soft x-ray transmission microscopy

X-ray magnetic circular dichroism serves as a strong element-specific magnetic contrast mechanism in full-field transmission soft x-ray microscopy to image micromagnetic domain structures. A lateral resolution down to 25 nm is provided by Fresnel zone plates used as optical elements. Recording the images in varying external magnetic fields and the sensitivity to the direction of the magnetization allows for detailed studies of static magnetization reversal processes in magnetic thin films and nanopatterned elements. Results on highly magnetostrictive Terfenol-D layers are reported. The experimental findings of the switching processes in soft magnetic permalloy rectangular structures are consistent with micromagnetic simulations. The pulsed time structure of polarized synchrotron radiation allows for a stroboscopic imaging of spin dynamics on a sub-nanosecond timescale.