Session L16: Focus Session: Magnetic Nanostructures: Probing Using Advanced Methods

Soft X-ray Spectroscopy of Magnetic Nanostructures: New Phenomena and Applications

The delicate balance between charge, spin, orbital, and lattice degrees of freedom in transition metal oxides leads to unique phenomena such as colossal magnetoresistance, high temperature superconductivity, as well as a remarkable diversity of charge, spin, and orbital ordered phases. The rich phase diagrams are determined by the strong local interaction of electrons in transition metal $d$ orbitals. Subtle changes in $d$ occupancy and overlap---and thereby phase transitions---can be induced by variations in temperature, by external fields, through doping and lattice distortions. In particular, interfaces can hold surprising electronic and magnetic properties that differ remarkably from the adjacent layers. Soft x-ray based techniques are ideal tools to study these systems as they are inherently element-specific, allow characterizing the valence state and the symmetry of lattice sites and provide detailed information about the electronic and magnetic structure with nanometer spatial resolution and on ultrafast time scales. Here we show that the to-date little explored angular dependence of the x-ray magnetic dichroism provides unique insights in the correlation between atomic, magnetic and electronic structure in these systems [1-4]. Taking advantage of this approach will prove invaluable for the engineering of novel nanoarchitectures to be used in low cost and energy efficient devices with improved performance and multiple functionalities. \\[4pt] [1] G. van der Laan \textit{et al}., Phys. Rev. Lett. \textbf{105}, 067405 (2010).\\[0pt] [2] E. Arenholz \textit{et al}., Phys. Rev. B \textbf{82}, 140103(R) (2010).\\[0pt] [3] G. van der Laan \textit{et al}., Phys. Rev. Lett. \textbf{100}, 067403 (2008).\\[0pt] [4] E. Arenholz \textit{et al}., Phys. Rev. Lett. \textbf{98}, 197201 (2007).   

Investigation of ferromagnetic/antiferromagnetic nanostructures using X-ray magnetic dichroism

The spin structure of epitaxially grown antiferromagnetic/ferromagnetic bilayer was investigated using X-ray Magnetic Circular Dichroism (XMCD) and X-ray Magnetic Linear Dichroism (XMLD) techniques. The XMLD measurement on the antiferomagnetic film (NiO or CoO) gives a direct probe of the spin orientation of the compensated antiferromagnetic spins. This capability enables us to give a clear clue to the mechanism of the exchange bias in the bilayer and to give a direct observation of the antiferromagnetic vortex.   

XPCS Study of Antiferromagnetic Domain Fluctuation

We have studied magnetic domain fluctuations in Yttrium-Dysprosium-Yttrium trilayer films using X-ray Photon Correlation Spectroscopy (XPCS) in conjunction with resonant soft X-ray magnetic scattering. Dysprosium thin film possesses a helical antiferromagnetic phase below Tn = 180 K and a ferromagnetic phase below Tc = 64 K. With coherent soft x-rays we observed magnetic speckle in the scattering from the antiferromagnetic domains. We determined critical points and found a shift of Curie temperature from the bulk value. Hysteresis associated with a first order phase transition was also observed. We observed magnetic speckle around the magnetic satellite peak at (0,0,q$_{m})$ due to static disorder as well as magnetic domains. At temperatures above Tn the system showed static behavior on times scales up to $\sim $ 300 sec which is indicative of non-fluctuating static disorder that persists above Tn. Close to Tc, there appears to be time-dependent fluctuations due to slow domain wall motion and these will be discussed.   

Deposition-order dependent magnetization reversal of graded anisotropy Co/Pd films

We report deposition-order-dependent, and depth-dependent, magnetization reversal in Co/Pd with graded anisotropy, which are technologically important as they address both writability and thermal stability challenges. Multilayers of [Co(0.4nm)/Pd(0.6nm)]$_{60}$ have been deposited by sputtering, where the Ar pressure has been varied from 5 to 12 and 20 mtorr in type A samples and in the reverse order in type B samples. An extensive set of structural and magnetization reversal studies with depth-resolution has been performed using XRD, cross-sectional TEM, magnetometry, PNR and XMCD. In type A samples, due to the larger grain size, lower interfacial roughness and less disorder in the magnetically softer layer, magnetization reversal proceeds via domain nucleation, propagation, and annihilation. Type B samples show a more localized reversal. Layers grown at higher pressure contain more disorder and rougher interfaces, which is carried into the magnetically softer layers deposited on top, thus impeding domain movement.   

Magnetization reversal and magnetic imprinting in a giant exchange bias system

We present compelling experimental evidence, obtained with small angle neutron scattering (SANS) that magnetization reversal of an exchange biased DyFe$_{2}$/YFe$_{2}$ superlattice occurs via reversal of magnetic domains with at least two different length scales. Our SANS studies used both unpolarized neutron beams (with a high field magnet) and polarized neutron beams using $^{3}$He filter polarization analysis. Magnetic length scales ranging from tens of nanometers to greater than several hundreds of nanometers were observed. The magnetization contained within nanometer large domains constituted a significant fraction of the total magnetization at the exchange bias field. During magnetization reversal some of the domains were arranged in a quasi-periodic manner. Because the ferromagnetic domains are so small, they exchange couple to relatively small portions of the pinned magnetic layer (i.e., DyFe$_{2})$, which at the nm length scale may appear relatively perfect. This reasoning may explain why the DyFe$_{2}$/YFe$_{2}$ system is a system that exhibits among the largest exchange bias observed to date. This work was supported by the Office of Basic Energy Science, U.S. Department of Energy and the National Science Foundation. ML acknowledges support from DanScatt.   

Polarized Neutron Reflectivity and Electron Microscopy Analysis of the Magnetic Microstructure in Antiparallel-Coupled Co Multilayers

Antiparallel exchange-coupled thin films are a convenient way to provide a ferromagnetic surface in situations where zero net magnetization is required, for example, when studying superconducting-ferromagnetic proximity effects with spin-triplet superconducting correlations.\footnote{T. Khaire, etal. Phys. Rev. Lett. 104, 137002 (2010)} We use the complementary techniques of polarized neutron reflectivity (PNR) and scanning electron microscopy with polarization analysis (SEMPA) to characterize the magnetic structure of such an antiferromagnetically coupled Co/Ru/Co multilayer. We find that, although the average macroscopic magnetization follows the simple antiparallel coupling picture, at the nanoscale the 3-dimensional magnetic structure is much more complex. The films are mostly antiparallel, but the magnetization directions fluctuate by as much as $\pm $ 40\r{ } over lengths as small as 100 nm. This structure has significant implications when trying to understand the local spin-dependent transport properties at the ferromagnetic interface. Applying magnetic fields further complicates the structures, leading to spin-flop related magnetic arrangements.   

Distinguishing the ultrafast dynamics of orbital and spin magnetic moments in solids

Ultrafast magnetization dynamics is an important issue for both fundamental science and for applications in order to optimize spin manipulation on a microscopic level. Since the first observation of laser induced spin dynamics,\footnote{Beaurepaire, E., Merle, J.C., Daunois, A., and Bigot, J.-Y. \textit{Phys. Rev. Lett.} \textbf{76}, 4250 (1996).} the mechanisms of angular momentum dissipation at picosecond timescales have been widely debated. In order to progress in the understanding of such microscopic ultrafast mechanisms, it is now possible to probe absolute values of magnetization with a high temporal resolution (100 fs). In this context, we have used ultrashort optical laser pulses (60 fs duration) to induce changes of the magnetization in a ferromagnetic CoPd alloy film with perpendicular anisotropy. The dynamics was probed with ultrashort circularly polarized femtosecond X-ray pulses, measuring the X-ray magnetic circular dichroism (XMCD) at Co L$_{2,3}$ edges.\footnote{Stamm, C. Kachel, T., Pontius, N., Mitnzer, R., Quast, T., Holldack, Khan, S., Lupulescu, C., Aziz, E. F., Wietstruk, M., D\"{u}rr, H. A., and Eberhardt, W. \textit{Nature Mater.} \textbf{6}, 740-743 (2007).}$^,$\footnote{C. Boeglin, E. Beaurepaire, V. Halt\'{e}, V. Lopez-Flores, C. Stamm, N. Pontius, H. D\"{u}rr, J. -Y. Bigot ``Observing how fast the spin-orbit interaction branches spin and orbital moments in solids'' Nature 465, 458 (2010).} We observe that the two components of the magnetic moments (L and S) show different ultrafast dynamics and that the spin-orbit coupling related to the magneto-crystalline anisotropy in solids is strongly affected by fs laser pulses in the ultrashort time scales. These dynamics can be compared to the purely electronic effect at the CoL$_{3}$ edge. Electronic excitations and their response to the laser pump pulse will be discussed and related to the modifications in the spin-orbit coupling. We will compare our results with time resolved MOKE experiments recently performed on CoPd alloys.   

Neutron Reflectivity Study in Py/CoO Exchange Bias System

We have studied the permalloy-cobalt monoxide exchange bias system using polarized neutron reflectivity. Both polycrystalline and epitaxial single crystalline (with the (111) and (100) CoO planes at the interface) CoO films were studied. By fitting the reflectivity for both directions of the applied field relative to the cooling field, we are able to obtain both the nuclear and spin depth profiles, as well as locating the pinned spins which are responsible for the exchange bias effect. The pinned spins at the interface can be resolved in the polycrystalline sample, which is consistent with our previous study with resonant soft x-ray reflectivity. One could reasonably have expected a stronger exchange bias effect in the (111) single crystal CoO film, because it has more uncompensated spins at the interface. The neutron reflectivity, however, shows lesser pinned spins. In the presentation, we will show the difference between the magnetic density profiles of the samples involving polycrystalline, (111) and (100) single crystalline CoO films respectively.   

Effects of hydrogen/deuterium absorption on the magnetic properties of Co/Pd multilayers

The effects of hydrogen and deuterium absorption were studied in two Co/Pd multilayers with perpendicular magnetic anisotropy using Polarized Neutron Reflectivity (PNR). PNR measurements were performed with the field in the plane of the sample with the magnetization M saturated at H = 6.0 T and unsaturated at 0.65 T. The nominal thicknesses of the Co/Pd layers were 2.5 {\AA}/21 {\AA}. Therefore, the actual layer chemical composition, thickness, and interface width parameters were defined from the nuclear scattering length density (SLD) profile obtained from both x-ray reflectivity (XRR) and PNR and their derivatives. The nuclear PNR SLD profile showed that although deuterium absorption occurred throughout the sample, the multilayer stack did not expand. The magnetic SLD showed that M in both the Pd and Co layers was affected. At saturation, M decreased, while at H = 0.65 T M increased upon deuterium exposure. Magnetization measurements confirmed hydrogen absorption decreased the total M at saturation and increased the component of M parallel to the field when not at saturation. These results indicate that hydrogen or deuterium absorption decreases both the perpendicular anisotropy and total magnetization of the samples.   

Revisit of Magnetism of Fe overlayers on Cu(001)

Fe films on Cu(001) have drawn a great attention due to the complicated structural and magnetic properties. It is known from the previous investigations that depending on the Fe thickness, Fe/Cu system has three distinguishable regions in terms of the structural and magnetic properties. Especially the spin structure in region II, where the film thickness ranges from 5 ML to 11 ML, has been controversial for the last two decades. We have studied the spin structure of Fe films on Cu(001) in region II via x-ray magnetic circular dichroism (XMCD) and surface magneto-optic Kerr effect (SMOKE) measurements. Wedge-shaped Fe films, which ranged from 6 ML to 10 ML, have been grown on Cu(001) at room temperature by an e-beam evaporator. Our results suggest a new model, which is totally different from the previous models, based on the incommensurate spin density wave.