Session C6: Magnetic Characterization and Imaging

Exploring 360 domain walls in ferromagnetic nanostructures using circular magnetic fields

Ferromagnetic nanostructures can exhibit intriguing magnetic states, such as the metastable 360 domain wall (DW), in which two 180 DWs combine to form a nearly flux closed state in sufficiently thin structures. These composite structures have potential to maximize storage densities due to their minimal stray fields. We study a straightforward method to nucleate 360 DWs in nanorings, nanowires, using in-plane circular fields, as if from a current carrying wire passing through the substrate in close proximity to the nanostructures. Our simulations, using OOMMF, predict that the vortex state of a ring with appropriate geometry will reverse from CW to CCW through an intermediate state consisting of pairs of 360 DWs. We examine the dependence of the switching field and intermediate states on geometric properties such as the diameter, thickness, and width of the ring. Using the local circular field, we can also nucleate 360 DWs in nanowires, pinning the location of the DWs at notches spaced as close as 100 nm apart, suggesting high density storage. We are currently studying these structures experimentally using AFM/MFM. We generate the circular field by passing current through AFM tip and image the resulting magnetic states with MFM.   

Quantitative X-Ray Magnetic Microscopy: from parallel stripe domains to buried topological defects

Magnetic transmission X-ray microscopy (TXM) is a powerful imaging technique that can produce element specific images of magnetic domains with nanometric lateral resolution. Here we present a novel imaging method in which the angular dependence of the magnetic contrast in a series of high resolution TXM images is used to obtain quantitative descriptions of the magnetization (canting angles and sense). This has been applied first to analyze parallel stripe domains in weak perpendicular anisotropy ferromagnetic NdCo5 layers of different thickness, and in NdCo5/Permalloy bilayers. Then, our method has been used to identify complex topological defects (merons or 1/2 skyrmions) in a NdCo5 film that are only partially replicated by the Permalloy overlayer [1]. Meron propagation in trilayers (across the thickness) and in hexagonal networks (across bifurcations) will be discussed in terms of their topological characteristics (chirality and polarity). [1] C. Blanco-Roldan et al. Nature Communications 6 (2015) 8196   

Soft x-ray ptychography studies of nanoscale magnetic and structural correlations in thin SmCo5 films

Soft x-ray ptychographic imaging was applied to probe an amorphous 50 nm thin SmCo5 film prepared by off-axis pulsed laser deposition and exhibiting a strong perpendicular magnetic anisotropy. Amplitude and phase contrast images, retrieved at photon energies near the cobalt L3 resonance, were used to identify and characterize magnetic and structural features with a spatial resolution of about10 nm. Aside from the common magnetic labyrinth domain pattern, nanoscale structural inclusions were identified that are primarily located in close proximity to the magnetic domain walls. X-ray absorption spectroscopy suggests that these inclusions are nanocrystalline Sm2Co17 phases with nominally in-plane magnetic anisotropy. Our results indicate that x-ray ptychographic imaging enables fruitful studies of magnetic and structural correlations at length scales relevant to emerging magnetic and spintronic devices.   

Asymmetric and Stochastic Behavior in Magnetic Vortices Studied by Soft X-ray Microscopy \newline

Asymmetry and stochasticity in spin processes are not only long-standing fundamental issues but also highly relevant to technological applications of nanomagnetic structures to memory and storage nanodevices. Those nontrivial phenomena have been studied by direct imaging of spin structures in magnetic vortices utilizing magnetic transmission soft x-ray microscopy (BL6.1.2 at ALS). Magnetic vortices have attracted enormous scientific interests due to their fascinating spin structures consisting of circularity rotating clockwise ($c=+$1) or counter-clockwise ($c=$-1) and polarity pointing either up ($p=+$1) or down ($p=$-1). We observed a symmetry breaking in the formation process of vortex structures in circular permalloy (Ni$_{80}$Fe$_{20})$ disks. The generation rates of two different vortex groups with the signature of \textit{cp}$=+$1 and \textit{cp}$=$-1 are completely asymmetric. The asymmetric nature was interpreted to be triggered by “intrinsic” Dzyaloshinskii-Moriya interaction (DMI) arising from the spin-orbit coupling due to the lack of inversion symmetry near the disk surface and “extrinsic” factors such as roughness and defects. We also investigated the stochastic behavior of vortex creation in the arrays of asymmetric disks. The stochasticity was found to be very sensitive to the geometry of disk arrays, particularly interdisk distance. The experimentally observed phenomenon couldn't be explained by thermal fluctuation effect, which has been considered as a main reason for the stochastic behavior in spin processes. We demonstrated for the first time that the ultrafast dynamics at the early stage of vortex creation, which has a character of classical chaos significantly affects the stochastic nature observed at the steady state in asymmetric disks. This work provided the new perspective of dynamics as a critical factor contributing to the stochasticity in spin processes and also the possibility for the control of the intrinsic stochastic nature by optimizing the design of asymmetric disk arrays.   

Ultrasensitive Scanning Transmission X-ray Microscopy: Pushing the Limits of Time Resolution and Magnetic Sensitivity

Understanding magnetic properties at ultrafast timescales is crucial for the development of new magnetic devices. Samples of interest are often thin film magnetic multilayers with thicknesses in the range of a few atomic layers. This fact alone presents a sensitivity challenge in STXM microscopy, which is more suited toward studying thicker samples. In addition the relevant time scale is of the order of 10 ps, which is well below the typical x-ray pulse length of 50 – 100 ps. The SSRL STXM is equipped with a single photon counting electronics that effectively allows using a double lock-in detection at 476MHz (the x-ray pulse frequency) and 1.28MHz (the synchrotron revelation frequency) to provide the required sensitivity. In the first year of operation the excellent spatial resolution, temporal stability and sensitivity of the detection electronics of this microscope has enabled researchers to acquire time resolved images of standing as well as traveling spin waves in a spin torque oscillator in real space as well as detect the real time spin accumulation in non magnetic Copper once a spin polarized current is injected into this material. The total magnetic moment is comparable to that of a single nanocube of magnetic Fe buried under a micron of non-magnetic material.   

Magnetorheology of hybrid colloids measured by spin coating and classical rheometry.

Hybrid colloids composed of micron-sized ferromagnetic and diamagnetic particles constitute a promising category of magnetorheological fluids with enhanced field-induced apparent yield stress. However, the physical mechanism explaining this stress enhancement is currently lacking. For the first time, we measure and compare the magnetic field-dependent viscosity of hybrid diluted colloids using spin-coating [] and magnetorheometry []. In the former technique, a magnetic field is applied during the spin coating of the colloidal suspension involving evaporation of the solvent. The viscosity of the colloidal suspension at applied field can be derived from the surface coverage of the dry spin-coated deposits and from the viscosity of the colloid at zero field. In the latter, its viscosity is measured with a torsional parallel plate magnetorheometer under uniaxial magnetic fields aligned in the gradient direction of a steady shearing flow. The experimental results under different conditions and the effect of each component on the magnetorheological properties of the resulting colloid will be discussed. [] M. Pichumani et al., Soft Matter, 2013, 9, 3220-3229 [] Juan de Vicente \textit{et al.}, Soft Matter, 2011, 7, 3701-3710   

Accuracy of MRI-based Magnetic Susceptibility Measurements

Magnetic Resonance Imaging (MRI) is increasingly used to map tissue susceptibility to identify microbleeds associated with brain injury and pathologic iron deposits associated with neurologic diseases such as Parkinson's and Alzheimer's disease. Field distortions with a resolution of a few parts per billion can be measured using MRI phase maps. The field distortion map can be inverted to obtain a quantitative susceptibility map. To determine the accuracy of MRI-based susceptibility measurements, a set of phantoms with paramagnetic salts and nano-iron gels were fabricated. The shapes and orientations of features were varied. Measured susceptibility of 1.0 mM GdCl$_{\mathrm{3}}$ solution in water as a function of temperature agreed well with the theoretical predictions, assuming Gd$^{\mathrm{+3}}$ is spin 7/2. The MRI susceptibility measurements were compared with SQUID magnetometry. The paramagnetic susceptibility sits on top of the much larger diamagnetic susceptibility of water (-9.04 x 10-6), which leads to errors in the SQUID measurements. To extract out the paramagnetic contribution using standard magnetometry, measurements must be made down to low temperature (2K). MRI-based susceptometry is shown to be as or more accurate than standard magnetometry and susceptometry techniques.   

Neutron interferometry with cold stage

Neutron interferometry (NI) is amongst the most precise methods for characterizing neutron interactions by measuring the relative difference between two neutron paths, one of which contains a sample-of-interest. Because neutrons carry magnetic moment and are deeply penetrating, they are excellent probes to investigate properties of magnetic materials. The advantage of NI is its unique sensitivity which allows to directly measure magnetic and structural transitions in materials. Up to now NI has been sparingly used in material research due to its sensitivity to environmental noise. However, recent successes in implementing Quantum Error Correction principles lead to an improved NI design making it robust against mechanical vibrations. Following these advances, a new user facility at the National Institute for Standards and Technology was built to study condensed matter applications, biology and quantum physics. Incorporating cold sample stage inside NI is the first of its kind experiment which can be carried out on large range of temperatures down to 4K. Upon successful realization, it will open new frontiers to characterize magnetic domains, phase transitions and spin properties in a variety of materials such as, for example, iron-based superconductors and spintronic materials.   

Relaxometry imaging of superparamagnetic magnetite nanoparticles at ambient conditions

We present a novel technique to image superparamagnetic iron oxide nanoparticles via their fluctuating magnetic fields. The detection is based on the nitrogen-vacancy (NV) color center in diamond, which allows optically detected magnetic resonance (ODMR) measurements on its electron spin structure. In combination with an atomic-force-microscope, this atomic-sized color center maps ambient magnetic fields in a wide frequency range from DC up to several GHz [1], while retaining a high spatial resolution in the sub-nanometer range [2]. We demonstrate imaging of single 10 nm sized magnetite nanoparticles using this spin noise detection technique. By fitting simulations (Ornstein-Uhlenbeck process) to the data, we are able to infer additional information on such a particle and its dynamics, like the attempt frequency and the anisotropy constant [3]. This is of high interest to the proposed application of magnetite nanoparticles as an alternative MRI contrast agent or to the field of particle-aided tumor hyperthermia. [1] E. Sch\"{a}fer-Nolte et al., Phys. Rev. Lett. \textbf{113}, 217204 (2014) [2] P. Maletinsky et al., Nat. Nanotech. \textbf{7}, 320 (2012) [3] D. Schmid-Lorch et al., Nano Lett. \textbf{15}, 4942 (2015)   

Measurements of magnetic spin excitations in Permalloy microstructures using nitrogen-vacancy magnetometry

The magnetic properties of nitrogen-vacancy (NV) centers in diamond have enabled emerging applications in fields ranging from cell biology to quantum computing. An NV center is a lattice defect, which behaves like a spin-1 system. NV centers can be prepared in the m$_{\mathrm{z}} \quad =$ 0 state by excitation with green light, and the spin state can be detected by the center's fluorescence of red light. The Zeeman splitting of the m$_{\mathrm{z}} \quad =$ \textpm 1 state, combined with a spin coherence time that can approach 1 ms, makes the NV center a sensitive, atom-sized magnetometer. Recently, NV centers have been used to measure spin wave excitations and vortex core dynamics in a Permalloy microdisk. In this talk, we present current NV center measurements on Permalloy micro and nanostructures that build on previous work. Permalloy structures were fabricated on top of a microstrip antenna and the measurements were conducted on a home-built confocal microscope. Preliminary measurements show photoluminescence contrast of \textasciitilde 12{\%} and field detectivity on the order of \textmu T/Hz$^{\mathrm{1/2}}$. This allows for fine field mapping of stray magnetic fields produced by micro and nanostructures, which are typically a few milliteslas in magnitude.   

Using NV-centers in diamond for optical magnetic sensing in superconductors

Magnetic field--dependent fluorescence of nitrogen vacancy (NV) centers in diamond has recently emerged as a promising technology for nanoscale sensing including non-invasive sensitive magnetometry and mapping of the magnetic field distribution. In particular, NV-sensing can be used to study magnetic phenomena in superconductors. After detailed introduction of this novel magneto-sensing technique, we will present results of magnetic measurements on several superconductors, including Ba$_{\mathrm{1-x}}$K$_{\mathrm{x}}$Fe$_{\mathrm{2}}$As$_{\mathrm{2}}$ and type-I materials. Details of the superconducting phase transition, the Meissner state, magnetic flux distribution upon field penetration, exit and trapping will be discussed.