Itinerant and Localized Magnetization Dynamics in Antiferromagnetic Ho.

Using femtosecond time-resolved resonant magnetic x-ray diffraction at the Ho L_{3} absorption edge, we investigate the demagnetization dynamics in antiferromagnetically ordered metallic Ho after femtosecond optical excitation. Tuning the x-ray energy to the electric dipole (E1, 2p→5d) or quadrupole (E2, 2p→4f) transition allows us to selectively and independently study the spin dynamics of the itinerant 5d and localized 4f electronic subsystems via the suppression of the magnetic (2 1 3-τ) satellite peak. We find demagnetization time scales very similar to ferromagnetic 4f systems, suggesting that the loss of magnetic order occurs via a similar spin-flip process in both cases. The simultaneous demagnetization of both subsystems demonstrates strong intra-atomic 4f-5d exchange coupling. In addition, an ultrafast lattice contraction due to the release of magneto-striction leads to a transient shift of the magnetic satellite peak.

Growth modes of nanoparticle superlattice thin films.

We report on the fabrication and characterization of iron oxide nanoparticle thin film superlattices. The formation into different film morphologies is controlled by tuning the particle plus solvent-to-substrate interaction. It turns out that the wetting vs dewetting properties of the solvent before the self-assembly process during solvent evaporation plays a major role in determining the resulting film morphology. In addition to layerwise growth three-dimensional mesocrystalline growth is also evidenced. The understanding of the mechanisms ruling nanoparticle self-assembly represents an important step towards the fabrication of novel materials with tailored optical, magnetic or electrical transport properties.

Self-assembled iron oxide nanoparticle multilayer: x-ray and polarized neutron reflectivity.

We have investigated the structure and magnetism of self-assembled, 20 nm diameter iron oxide nanoparticles covered by an oleic acid shell for scrutinizing their structural and magnetic correlations. The nanoparticles were spin-coated on an Si substrate as a single monolayer and as a stack of 5 ML forming a multilayer. X-ray scattering (reflectivity and grazing incidence small-angle scattering) confirms high in-plane hexagonal correlation and a good layering property of the nanoparticles. Using polarized neutron reflectivity we have also determined the long range magnetic correlations parallel and perpendicular to the layers in addition to the structural ones. In a field of 5 kOe we determine a magnetization value of about 80% of the saturation value. At remanence the global magnetization is close to zero. However, polarized neutron reflectivity reveals the existence of regions in which magnetic moments of nanoparticles are well aligned, while losing order over longer distances. These findings confirm that in the nanoparticle assembly the magnetic dipole-dipole interaction is rather strong, dominating the collective magnetic properties at room temperature.

Magnetic domain fluctuations in an antiferromagnetic film observed with coherent resonant soft x-ray scattering.

We report the direct observation of slow fluctuations of helical antiferromagnetic domains in an ultrathin holmium film using coherent resonant magnetic x-ray scattering. We observe a gradual increase of the fluctuations in the speckle pattern with increasing temperature, while at the same time a static contribution to the speckle pattern remains. This finding indicates that domain-wall fluctuations occur over a large range of time scales. We ascribe this nonergodic behavior to the strong dependence of the fluctuation rate on the local thickness of the film.

Template-assisted self-assembly of individual and clusters of magnetic nanoparticles.

The deliberate control over the spatial arrangement of nanostructures is the desired goal for many applications such as, for example, in data storage, plasmonics or sensor arrays. Here we present a novel method to assist the self-assembly process of magnetic nanoparticles. The method makes use of nanostructured aluminum templates obtained after anodization of aluminum discs and the subsequent growth and removal of the newly formed alumina layer, resulting in a regular honeycomb-type array of hexagonally shaped valleys. The iron oxide nanoparticles, 20 nm in diameter, are spin-coated onto the surface of honeycomb nanostructured Al templates. Depending on the size, each hexagon site can host up to 30 nanoparticles. These nanoparticles form clusters of different arrangements within the valleys, such as collars, chains and hexagonally closed islands. Ultimately, it is possible to isolate individual nanoparticles. The strengths of the magnetic interaction between particles in a cluster are probed using the memory effect known from the coupled state in superspin glass systems.

Conductance features in point contact Andreev reflection spectra.

Point contact Andreev reflection (PCAR) spectroscopy is a common technique for determining the spin polarization of a ferromagnetic sample. The polarization is extracted by measuring the bias dependence of the conductance of a metallic/superconducting point contact. Under ideal conditions, the conductance is dominated by Andreev reflection and the Blonder-Tinkham-Klapwijk (BTK) model can be used to extract a value for the polarization. However, PCAR spectra often exhibit unwanted features in the conductance that cannot be appropriately modelled with the BTK theory. In this paper we isolate some of these unwanted features and show that any further extraction of the spin polarization from these non-ideal spectra proves unreliable. Understanding the origin of these features provides an objective criterion for rejection of PCAR spectra unsuitable for fitting with the modified BTK model.

45° sign switching of effective exchange bias due to competing anisotropies in fully epitaxial Co3FeN/MnN bilayers.

We report an unusual angular-dependent exchange bias effect in ferromagnet/antiferromagnet bilayers, where both ferromagnet and antiferromagnet are epitaxially grown. Numerical model calculations predict an approximately 45° period for the sign switching of the exchange-bias field, depending on the ratio between magnetocrystalline anisotropy and exchange-coupling constant. The switching of the sign is indicative of a competition between a fourfold magnetocrystalline anisotropy of the ferromagnet and a unidirectional anisotropy field of the exchange coupling. This predicted unusual angular-dependent exchange bias and its magnetization switching process are confirmed by measurements on fully epitaxial Co3FeN/MnN bilayers by longitudinal and transverse magneto-optic Kerr effect magnetometry. These results provide a deeper understanding of the exchange coupling phenomena in fully epitaxial bilayers with tailored materials and open up a complex switching energy landscape engineering by anisotropies.

Invited article: Vector and Bragg Magneto-optical Kerr effect for the analysis of nanostructured magnetic arrays.

Experimental and theoretical aspects of obtaining the magnetic information carried by laser beams diffracted from an array of micro- or nanosized magnetic objects are reviewed. We report on the fundamentals of vector magneto-optic Kerr effect (MOKE), Bragg-MOKE, and second-order effects in the Kerr signal in longitudinal Kerr geometry as well as on an experimental setup used for vector and Bragg-MOKE experiments. The vector and Bragg-MOKE technique in combination with micromagnetic simulation is a reliable tool for measuring the complete magnetization vector and for characterizing the reversal mechanism of lateral magnetic nanostructures. We discuss the Bragg-MOKE effect for three standard domain configurations during the magnetization reversal process and present the expected behavior of the magnetic hysteresis loops.

Probing ultrafast changes of spin and charge density profiles with resonant XUV magnetic reflectivity at the free-electron laser FERMI.

We report the results of resonant magnetic XUV reflectivity experiments performed at the XUV free-electron laser FERMI. Circularly polarized XUV light with the photon energy tuned to the Fe M2,3 edge is used to measure resonant magnetic reflectivities and the corresponding Q-resolved asymmetry of a Permalloy/Ta/Permalloy trilayer film. The asymmetry exhibits ultrafast changes on 240 fs time scales upon pumping with ultrashort IR laser pulses. Depending on the value of the wavevector transfer Qz , we observe both decreasing and increasing values of the asymmetry parameter, which is attributed to ultrafast changes in the vertical spin and charge density profiles of the trilayer film.

Measurements of ultrafast spin-profiles and spin-diffusion properties in the domain wall area at a metal/ferromagnetic film interface.

Exciting a ferromagnetic material with an ultrashort IR laser pulse is known to induce spin dynamics by heating the spin system and by ultrafast spin diffusion processes. Here, we report on measurements of spin-profiles and spin diffusion properties in the vicinity of domain walls in the interface region between a metallic Al layer and a ferromagnetic Co/Pd thin film upon IR excitation. We followed the ultrafast temporal evolution by means of an ultrafast resonant magnetic scattering experiment in surface scattering geometry, which enables us to exploit the evolution of the domain network within a 1/e distance of 3 nm to 5 nm from the Al/FM film interface. We observe a magnetization-reversal close to the domain wall boundaries that becomes more pronounced closer to the Al/FM film interface. This magnetization-reversal is driven by the different transport properties of majority and minority carriers through a magnetically disordered domain network. Its finite lateral extension has allowed us to measure the ultrafast spin-diffusion coefficients and ultrafast spin velocities for majority and minority carriers upon IR excitation.

ALICE­An advanced reflectometer for static and dynamic experiments in magnetism at synchrotron radiation facilities.

We report on significant developments of a high vacuum reflectometer (diffractometer) and spectrometer for soft x-ray synchrotron experiments which allows conducting a wide range of static and dynamic experiments. Although the chamber named ALICE was designed for the analysis of magnetic hetero- and nanostructures via resonant magnetic x-ray scattering, the instrument is not limited to this technique. The versatility of the instrument was testified by a series of pilot experiments. Static measurements involve the possibility to use scattering and spectroscopy synchrotron based techniques (photon-in photon-out, photon-in electron-out, and coherent scattering). Dynamic experiments require either laser or magnetic field pulses to excite the spin system followed by x-ray probe in the time domain from nano- to femtosecond delay times. In this temporal range, the demagnetization/remagnetization dynamics and magnetization precession in a number of magnetic materials (metals, alloys, and magnetic multilayers) can be probed in an element specific manner. We demonstrate here the capabilities of the system to host a variety of experiments, featuring ALICE as one of the most versatile and demanded instruments at the Helmholtz Center in Berlin-BESSY II synchrotron center in Berlin, Germany.

Polarized neutron reflectivity from monolayers of self-assembled magnetic nanoparticles.

We prepared monolayers of iron oxide nanoparticles via self-assembly on a bare silicon wafer and on a vanadium film sputter deposited onto a plane sapphire substrate. The magnetic configuration of nanoparticles in such a dense assembly was investigated by polarized neutron reflectivity. A theoretical model fit shows that the magnetic moments of nanoparticles form quasi domain-like configurations at remanence. This is attributed to the dipolar coupling amongst the nanoparticles.

Radiotherapy treatment planning of basal meningiomas: improved tumor localization by correlation of CT and MR imaging data.

A localization technique, based on three-dimensional CT and MR imaging data for precision radiotherapy of basal meningiomas, is presented. Indications for radiotherapy included unresected tumors, gross disease remaining despite surgery, and recurrences. The patient's head was fixed in a stereotactic localization system which is usable at the CT, MR and the linear accelerator installations. The geometrical distortion of MR imaging data was evaluated in three dimensions by phantom measurements. The geometrical distortion was "corrected" (reducing displacements to the size of a pixel) by calculations based on modelling the distortion as a fourth order two-dimensional polynomial. The target volume was defined in three-dimensional MR imaging data after application of 0.1 mmol/kg b.w. Gd-DTPA solution and transferred precisely from MR onto CT data to provide a map of the radiation attenuation coefficient for dose calculation. The superior soft tissue contrast of MR showed an excellent tumor delineation especially when the bony base of the skull obscured the target in CT images. Target volume, calculated dose distribution, and critical structures could be transferred between CT and MR imaging data and displayed as three-dimensional shaded structures for better assessment for matching of target volume and dose distribution. With the described planning system a more precise target definition of basal meningiomas was possible by integration of the superior tumor delineation in MR compared with CT.

Monitoring local disposition kinetics of carboplatin in vivo after subcutaneous injection in rats by means of 195Pt NMR.

The anticancer drug carboplatin has been monitored in rats during treatment by means of in vivo 195Pt NMR spectroscopy at 2.0 T. The purpose of the study was to assess local disposition kinetics in intact tissue following subcutaneous injection of a platinum-containing drug. Serial 195Pt NMR measurements have been carried out in four animals after administration of carboplatin solutions with doses ranging from 37.1 to 59.4 mg per kg body weight. A surface coil of 2 cm diameter tuned to 18.3 MHz was placed over the injection site (back of the neck of the animals). To optimize measurement parameters of the single-pulse-acquire sequence and to determine chemical shifts and the detection threshold, in vitro 195Pt NMR experiments have been performed on model solutions of potassium tetrachloroplatinate(II), carboplatin, and cisplatin with different solvents such as H2O, DMSO, and DMF. Resonances of PtCl2-4, carboplatin, cisplatin, and cis-[Pt(NH2)Cl(DMSO)]+ were observed at chemical shift positions delta = -1623 ppm, -1705 ppm, -2060 ppm (cisplatin in DMSO), and -3120 ppm, respectively, relative to the reference signal of Na2PtCl6 at delta = 0 ppm. A spin-lattice relaxation time of carboplatin of T1 = (0.103 +/- 0.02) s was measured. The threshold for NMR detection of platinum-containing compounds estimated from the in vitro experiments was 10 micromol (corresponding to approximately 4.8 mM). In vivo 195Pt NMR spectra obtained in four rats after administration of carboplatin showed a broad resonance at delta = -(1715 +/- 8) ppm. The signal-to-noise ratio of this peak (starting 2 min after the injection) was approximately 9:1 for a measurement time of 6 min (TR= 13 ms, 28672 transients). The elimination rate constant of local disposition of carboplatin was kel = 0.017 (0.008-0.025) min-1 (median and range).

Combined use of stereotaxic CT and angiography for brain biopsies and stereotaxic irradiation.

A Riechert Mundinger stereotaxic device was modified to enable artifact-free computed tomographic (CT) scanning with the stereotaxic frame attached to the patient's head. A localization system was developed allowing determination of the XYZ coordinates of the target point directly from the CT cut. Angiography was performed intraoperatively with the stereotaxic frame attached. Coronal and sagittal CT reconstructions were enlarged to the radiographic magnification to allow direct comparison with angiography. CT offered optimum localization of the target, whereas angiography determined the safest approach. Computer programs were developed to enable three-dimensional radiotherapy planning. 125I seeds were implanted for treatment of low-grade gliomas and solid craniopharyngiomas. Yttrium-90 was applied in cystic craniopharyngiomas. Intracavitary rhenium-186 application was abandoned because of frequent cyst recurrence and leakage from the cyst.

Selective 19F MR imaging of 5-fluorouracil and alpha-fluoro-beta-alanine.

A 19F MR chemical shift imaging (CSI) technique is presented which enables selective imaging of the antineoplastic drug 5-fluorouracil (5-FU) and its major catabolite alpha-fluoro-beta-alanine (FBAL). The CSI sequence employs a chemical shift selective (CHESS) saturation pulse to suppress either the 5-FU or the FBAL resonance before the other component of the two-line 19F MR spectrum is measured. Because the transmitter frequency can always be set to the Larmor frequency of the 19F resonance to be imaged, this approach yields 5-FU and FBAL MR images free of chemical shift artifacts in read-out and slice-selection direction. In phantom experiments, selective 5-FU and FBAL images with a spatial resolution of 15 x 15 x 20 mm3 (4.5 ml) were obtained in 30 min from a model solution, whose drug and catabolite concentrations were similar to those estimated in the liver of tumor patients undergoing IV chemotherapy with 5-FU. The drug-specific MR imaging technique developed is, therefore, well-suited for the direct and noninvasive monitoring of the up-take and trapping of 5-FU in liver tumors in vivo.

Correction of spatial distortion in magnetic resonance angiography for radiosurgical treatment planning of cerebral arteriovenous malformations.

A treatment planning system based on magnetic resonance (MR) angiographic imaging data for the radiosurgery of inoperable cerebral arteriovenous malformations is reported. MR angiography was performed using a three-dimensional (3D) velocity-compensated fast imaging with steady-state precession (FISP) sequence. Depending on the individual MR system, inhomogeneities and nonlinearities induced by eddy currents during the pulse sequence can distort the images and produce spurious displacements of the stereotactic coordinates in both the x-y plane and the z axis. If necessary, these errors in position can be assessed by means of two phantoms placed within the stereotactic guidance system--a "2D-phantom" displaying "pincushion" distortion in the image, and a "3D-phantom" displaying displacement, warp, and tilt of the image plane itself. The pincushion distortion can be "corrected" (reducing displacements from 2-3 mm to 1 mm) by calculations based on modeling the distortion as a fourth order 2D polynomial. Displacement, warp, and tilt of the image plane may be corrected by adjustment of the gradient shimming currents. After correction, the accuracy of the geometric information is limited only by the pixel resolution of the image (= 1 mm). Precise definition of the target volume could be performed by the therapist either directly in the MR images or in calculated projection MR angiograms obtained by a maximum intensity projection algorithm. MR angiography provides a sensitive, noninvasive 3D method for defining target volume and critical structures, and for calculating precise dose distributions for radiosurgery of cerebral arteriovenous malformations.

Impact of hyperthermic regional perfusion therapy on cell metabolism of malignant melanoma monitored by 31P MR spectroscopy.

For the first time, the influence of hyperthermic regional perfusion therapy on tumor cell metabolism has been monitored by means of 31P MR spectroscopy. Shortly after treatment, a complete depletion of high energy phosphates, a significant increase of inorganic phosphate, phosphomono- and phosphodiester resonances and a decrease of pH have been observed. Healthy muscle tissue showed only minor effects caused by this aggressive therapy. The time course of the transient variation of phosphate metabolite concentrations is explained in the framework of a simple pathophysiological model. The results demonstrate that the imediate effect of chemotherapy on tumor tissue can be detected by 31P MRS before any clinical signs of tumor response are visible.

Quantitative MR temperature monitoring of high-intensity focused ultrasound therapy.

A new quantitative method has been developed for real-time mapping of temperature changes induced by high intensity focused ultrasound (HIFU). It is based on the temperature dependence of the T1 relaxation time and the equilibrium magnetization. To calibrate the temperature measurement, the functional relationship between T1 and temperature was examined in different samples of porcine muscle and fatty tissue. The method was validated by a comparison of calculated temperature maps with fiber-optic measurements in heated muscle tissue. The experiment showed that the accuracy of the MR method for temperature measurements is better than 1 degree C. Since the acquisition time of the employed MR sequence takes only 3 s per slice and the calculation of the temperature map can be performed within seconds, the imaging technique works nearly in real-time. The temperature measurement could be realized during HIFU showing no disturbances by ultrasound sonication. In comparison to other MR approaches, the advantages of the introduced method lie in a sufficient accuracy and time resolution combined with a reasonable robustness against motion as well as the feasibility for temperature monitoring in fatty tissues.

Liver tissue characterization using computerized echography.

A digital echographic data acquisition and evaluation system was used in a clinical study including more than 100 patients with various local or diffuse liver tissue alterations. For diagnosis two decision methods are applied: first linear discriminant analysis and second a Bayesian method. Each A-scan is represented by a set of various numerical parameters from which only the most signal-distinctive ones are used for evaluation. The diagnostic results which will be presented and discussed have proved the method to be very sensitive and specific.