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Muon Spin Rotation is employed to investigate the spin dynamics of ferritin proteins isolated from the brain of an Alzheimer's disease (AD) patient and of a healthy control, using a sample of horse-spleen ferritin as a reference. A model based on the N\'eel theory of superparamagnetism is developed in order to interpret the spin relaxation rate of the muons stopped by the core of the protein. Using this model, our preliminary observations show that ferritins from the healthy control are filled with a mineral compatible with ferrihydrite, while ferritins from the AD patient contain a crystalline phase with a larger magnetocrystalline anisotropy, possibly compatible with magnetite or maghemite., Comment: 16 pages, 9 figures

Nuclear spin-lattice relaxation times are measured on copper using magnetic resonance force microscopy performed at temperatures down to 42 mK. The low temperature is verified by comparison with the Korringa relation. Measuring spin-lattice relaxation times locally at very low temperatures opens up the possibility to measure the magnetic properties of inhomogeneous electron systems realized in oxide interfaces, topological insulators and other strongly correlated electron systems such as high-Tc superconductors., Comment: We revised the manuscript by including the supplemental material. The manuscript is changed from a Letter to a Research Article after change of journal

We present nuclear magnetic resonance evidence that very slow $(\ensuremath{\le}1$ MHz) spin fluctuations persist into the overdoped regime of $\mathrm{Ba}({\mathrm{Fe}}_{1\ensuremath{-}x}{\mathrm{Rh}}_{x}){}_{2}{\mathrm{As}}_{2}$ superconductors. Measurements of the $^{75}\mathrm{As}$ spin echo decay rate, obtained both with Hahn Echo and Carr Purcell Meiboom Gill pulse sequences, show that the slowing down of spin fluctuations can be described by short-range diffusive dynamics, likely involving domain walls motions separating $(\ensuremath{\pi}/a,0)$ from $(0,\ensuremath{\pi}/a)$ correlated regions. This slowing down of the fluctuations is weakly sensitive to the external magnetic field and, although fading away with doping, it extends deeply into the overdoped regime.

$^{19}\mathrm{F}$ NMR measurements in optimally electron-doped ${\mathrm{La}}_{1\text{\ensuremath{-}}y}{\mathrm{Y}}_{y}{\mathrm{Fe}}_{1\text{\ensuremath{-}}x}{\mathrm{Mn}}_{x}{\mathrm{AsO}}_{0.89}{\mathrm{F}}_{0.11}$ superconductors are presented. The effect of Mn doping on the superconducting phase is studied for two series of compounds $(y=0$ and $y=0.2)$ where the chemical pressure is varied by substituting La with Y. In the $y=0.2$ series superconductivity is suppressed for Mn contents an order of magnitude larger than for $y=0$. For both series a peak in the $^{19}\mathrm{F}$ NMR nuclear spin-lattice relaxation rate $1/{T}_{1}$ emerges upon Mn doping and becomes significantly enhanced on approaching the quantum phase transition between the superconducting and magnetic phases. $^{19}\mathrm{F}$ NMR linewidth measurements show that for similar Mn contents magnetic correlations are more pronounced in the $y=0$ series, at variance with what one would expect for $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ spin correlations. These observations suggest that Mn doping tends to reduce fluctuations at $\stackrel{P\vec}{Q}=(\ensuremath{\pi}/a,0)$ and to enhance other low-frequency modes. The effect of this transfer of spectral weight on the superconducting pairing is discussed along with the charge localization induced by Mn.

We investigate the degradation of the magnetic moment of a 300 nm thick FePt film induced by Focused Ion Beam (FIB) milling. A $1~\mu \mathrm{m} \times 8~\mu \mathrm{m}$ rod is milled out of a film by a FIB process and is attached to a cantilever by electron beam induced deposition. Its magnetic moment is determined by frequency-shift cantilever magnetometry. We find that the magnetic moment of the rod is $\mu = 1.1 \pm 0.1 \times 10 ^{-12} \mathrm{Am}^2$, which implies that 70% of the magnetic moment is preserved during the FIB milling process. This result has important implications for atom trapping and magnetic resonance force microscopy (MRFM), that are addressed in this paper., Comment: 4 pages, 4 figures

Using dynamic cantilever magnetometry, we study the vortex lattice and its corresponding melting transition in a micrometer-size crystallite of superconducting NbSe2. Measurements of the cantilever resonance frequency as a function of magnetic field and temperature respond to the magnetization of the vortex-lattice. The cantilever dissipation depends on thermally activated vortex creep motion, whose pinning energy barrier is found to be in good agreement with transport measurements on bulk samples. This approach reveals the phase diagram of the crystallite, and is applicable to other micro- or nanometer-scale superconducting samples., 5 pages, 4 figures

This work presents 75As NMR spin echo decay rate (1/T2) measurements in Ba(Fe1-xRhx)2As2 superconductors, for 0.041 < x < 0.094. It is shown that 1/T2 increases upon cooling, in the normal phase, suggesting the onset of an unconventional very low-frequency activated dynamic. The correlation times of the fluctuations and their energy barriers are derived. The motion is favored at large Rh content, while it is hindered by the application of a magnetic field perpendicular to the FeAs layers. The same dynamic is observed in the spin-lattice relaxation rate, in a quantitatively consistent manner. These results are discussed in the light of nematic fluctuations involving domain wall motion. The analogies with the behaviour observed in the cuprates are also outlined.

We measured the static uniform spin susceptibility of Ba(Fe$_{1-x}$Rh$_x$)$_2$As$_2$ iron-based superconductors, over a broad range of doping ($0.041\leq x\leq 0.094$) and magnetic fields. At small fields ($H \le$ 1 kOe) we observed, above the transition temperature $T_c$, the occurrence of precursor diamagnetism, which is not ascribable to the Ginzburg-Landau theory. On the contrary, our data fit a phase fluctuation model, which has been used to interpret a similar phenomenology occurring in the high-$T_c$ cuprate superconductors. On the other hand, in presence of strong fields the unconventional fluctuating diamagnetism is suppressed, whereas 3D fluctuations are found, in agreement with literature.

${}^{75}\phantom{\rule{-0.16em}{0ex}}$As NMR spin-lattice relaxation ($1/{T}_{1}$) and spin-echo decay ($1/{T}_{2}$) rate measurements were performed in a single crystal of Ba(Fe${}_{0.93}$Rh${}_{0.07}$)${}_{2}$As${}_{2}$ superconductor. Below the superconducting transition temperature ${T}_{c}$, when the magnetic field $\mathbf{H}$ is applied along the $c$ axes, a peak in both relaxation rates is observed. Remarkably that peak is suppressed for $\mathbf{H}\ensuremath{\perp}c$. Those maxima in $1/{T}_{1}$ and $1/{T}_{2}$ have been ascribed to the flux lines lattice motions and the corresponding correlation times and pinning energy barriers have been derived on the basis of a heuristic model. Further information on the flux lines motion was derived from the narrowing of ${}^{75}\phantom{\rule{-0.16em}{0ex}}$As NMR linewidth below ${T}_{c}$ and found to be consistent with that obtained from $1/{T}_{2}$ measurements. All the experimental results are described in the framework of thermally activated vortices motions.

$^{31}\mathrm{P}$ nuclear and muon spin-lattice relaxation-rate measurements in SrZnVO(PO${}_{4}$)${}_{2}$, a $S=1/2$ frustrated magnet on a square lattice, are presented. The temperature ($T$) dependence of the in-plane correlation length $\ensuremath{\xi}$ is derived and it is shown that the overall behavior is analogous to the one found for nonfrustrated systems but with a reduced spin stiffness. The temperature dependence of $\ensuremath{\xi}$ in SrZnVO(PO${}_{4}$)${}_{2}$ is compared to the one of other frustrated magnets on a square lattice with competing nearest neighbor (${J}_{1}$) and next-nearest-neighbor (${J}_{2}$) exchange couplings and it is shown that $\ensuremath{\xi}$ progressively decreases as the ratio ${J}_{2}/{J}_{1}$ approaches the critical value leading to the suppression of long-range magnetic order. In spite of the differences in the functional form of $\ensuremath{\xi}(T)$ found in different vanadates, it is pointed out that the characteristic energy scale describing spin correlations in all those compounds appears to scale as $|2{J}_{2}+{J}_{1}|$.

By using nuclear magnetic resonance and ac-susceptibility, the characteristic correlation times for the vortex dynamics, in an iron-based superconductor, have been derived. Upon cooling, the vortex dynamics displays a crossover consistent with a vortex glass transition. The correlation times, in the fast motions regime, merge onto a universal curve which is fit by the Vogel-Fulcher law, rather than by an Arrhenius law. Moreover, the pinning barrier shows a weak dependence on the magnetic field which can be heuristically justified within a fragile glass scenario. In addition, the glass freezing temperatures obtained by the two techniques merge onto the de Almeida-Thouless line. Finally the phase diagram for the mixed phase has been derived.