Session E47: Frontiers in Magnetic Imaging

Local X-ray Magnetic Circular Dichroism of Fe/Cu(111) using Synchrotron STM

Synchrotron X-ray Scanning Tunneling Microscopy (SX-STM) combines two of the most powerful materials characterization methods; synchrotron X-ray and STM, and it has a great potential to revolutionize X-ray chemical imaging at the atomic limits. Here, we use SX-STM to study the x-ray magnetic circular dichroism (XMCD) on the Fe L2 and L3 edges of a thin iron film deposited on Cu (111) surface in room temperature. Polarization dependent x-ray absorption spectra have been obtained through a specially fabricated tip that captures photo-electrons. Unlike conventional spin-polarized STM, x-ray excitations provide magnetic contrast even with a non-magnetic tip. Intensity changes in the photo-excited current indicate chemical variations within a single magnetic Fe domain thereby opening up exciting opportunities to study local magnetic properties of materials using the SX-STM-XMCD method.   

Magnetic Order In Metallic Copper Induced By Proximity To Cobalt: A Detailed X-Ray Spectromicroscopy Study

We observe localized, magnetic 3d Cu states in Co/Cu alloys using high resolution X-Ray Absorption Spectroscopy (XAS) and X-Ray Magnetic Circular Dichorism (XMCD) spectroscopy at the Copper L3 resonance. We show that increased Co concentration in our alloy leads to three spectroscopic effects consistent with these localized Cu states: (1) down-wards shift of the Cu L3 XAS peak by 0.5eV relative to the Cu L3 XMCD peak, resulting in the alignment of these two peaks, (2) nonlinear narrowing of the XAS peak, and (3) merging of the satellite d-to-s Cu L3 transitions peaks in both the XAS and XMCD channels. We compare our results to XAS spectra taken of multilayered samples, and show that our alloyed Cu atoms behave analogously to Cu atoms near a Cu/Co interface. Our results thus provide novel, key insight into the behavior of Cu when placed near a ferromagnetic interface, which is crucial for modern spintronics research where Cu is not only often used as a spacer in F/N/F multilayer devices, but also is often tuned to such low thicknesses that interfacial effects begin to dominate the physics. References: R. Kukreja et al., Phys. Rev. Lett. 115, 096601 (2015); M. G. Samant et al., Phys. Rev. Lett. 72, 1112 (1994); O. Karis et al., Phys. Rev. B 62, R16239 (2000); J. Stoehr and H. C. Siegmann, Magnetism: From Fundamental to Nanoscale Dynamics (Springer, 2006).   

Measurement of ultrafast magnetization dynamic in iron, using a novel imaging spin detector, in combination with a compact pulsed VUV source

As ultrafast demagnetization is partially a spin transport effect, the continuously growing field of spintronics is related to femtosecond magnetization dynamics. Energy resolved photoemission experiments in principle provide the possibility for direct observation of the electrons that contribute to the macroscopic magnetization. However, for a long time spin detectors have not been efficient enough for spin and time resolved electron spectroscopy of the entire valence band. The development of imaging spin detectors increase the detection efficiency by several orders of magnitude compared to the well-known Mott spin detector. A spin detector has been developed, where spin selectivity is achieved by low energetic electron scattering at a Iridium crystal. In addition, a very compact and stable high harmonic source has been designed, with the goal of delivering short VUV pulses for just one single harmonic of the driving laser. We present first measurements of spin and time resolved photoemission spectra of ultrafast demagnetization in iron. We intend to compare the spin dynamics of electrons at the Fermi energy to the laser-induced change of the exchange splitting. This will lead to deeper insights into transport phenomena within the valence band.   

THz magneto-optic study of a clean and a bond-disordered $S=1$ quantum magnet DTN

We report THz absorption spectroscopy results from 3 to 40\,cm$^{-1}$ (0.09 to 1.2\,THz) for a spin-1 quantum magnet NiCl$_2\cdot$4SC(NH$_2$)$_2$ (DTN) and its magnetic bond-disordered derivate Ni(Cl$_{0.87}$Br$_{0.13}$)$_2\cdot$4SC(NH$_2$)$_2$ (DTNX) at temperatures down to 0.3\,K in magnetic fields from 0 to 12\,T. In DTN the single optically active mode at 9 cm\,$^{-1}$ in zero field corresponds to the top of the magnon band excitation. In DTNX, an additional broad feature is observed at about 1.5\,cm$^{-1}$ above the 9\,cm$^{-1}$ mode in agreement with the recent inelastic neutron scattering data [PRB92, 024429, (2015)]. Both modes undergo Zeeman splitting in magnetic field with similar g-factors, but the weak mode loses intensity as the systems order antiferromagnetically in 2\,T at 0.3\,K. We are able to resolve several modes around 15\,cm$^{-1}$ in the ordered phase, which above ordering temperature merge into the broad mode reported earlier by ESR. As DTNX approaches the second critical field 11.5\,T of the induced ferromagnetic transition at 0.3K, an additional mode, not present in DTN, emerges at 14 cm$^{-1}$. These new data assist in constructing a detailed microscopic Hamiltonian for DTN and will be discussed in the context of Bose glass physics reported for DTNX.   

Scanning-SQUID investigation of spin-orbit torque acting on yttrium iron garnet devices

Successful manipulation of electrically insulating magnets, such as yttrium iron garnet, by by current-driven spin-orbit torques could provide a highly efficient platform for spintronic memory. Compared to devices fabricated using magnetic metals, magnetic insulators have the advantage of the ultra-low magnetic damping and the elimination of shunting currents in the magnet that reduce the torque efficiency. Here, we apply current in the spin Hall metal $\beta$-Ta to manipulate the magnetic orientation of micron-sized, electrically-insulating yttrium iron garnet devices. We do not observe spin-torque switching even for applied currents well above the critical current expected in a macrospin switching model. This suggests either inefficient transfer of spin torque at our Ta/YIG interface or a breakdown of the macrospin approximation.   

Off-Resonant, Broadband Paramagnetic Resonance Spectroscopy via NV Centers in Diamond

Nitrogen-Vacancy (NV) centers in diamond are attractive probes for ultrasensitive, room temperature, optically detected magnetic resonance at the nanoscale. Coherent pulsed magnetic resonance manipulation of an NV center spin enables sensitive target spin spectroscopy. We have detected the hyperfine spectrum of substitutional nitrogen (P1) centers in diamond over a broad field-frequency range using simple continuous-wave NV magnetic resonance. In contrast to previous work [1-2], we are able to measure the paramagnetic electron spin resonance spectra over a broad field-frequency range and far from level crossings of the NV and P1 spins. We report on progress towards understanding the physical mechanisms underlying this phenomenon. We modify the NV-P1 dipole coupling strength by changing the doping concentrations and measure this strength independently using pulsed magnetic resonance techniques. We show how the effect depends on the orientation of the external field relative to the NV axis. [1.] H.-J. Wang, C.S. Shin, S.J. Seltzer, C.E. Avalos, A. Pines and V.S. Bajaj, Nat. Comm. \textbf{5}, 4135 (2014). [2.] L.T. Hall, P. Kehayias, D.A. Simpson, A. Jarmola, A. Stacey, D. Budker and L.C.L. Hollenberg, Nat. Comm. \textbf{7}, 10211 (2016).   

Control and Local Measurement of the Spin Chemical Potential in a Magnetic Insulator

In recent decades, a large scientific effort has focused on harnessing spin transport for providing insights into novel materials and low-dissipation information processing. We introduce single spin magnetometry based on nitrogen-vacancy (NV) centers in diamond as a new and generic platform to locally probe spin chemical potential which essentially determines the flow of spin currents. We use this platform to investigate magnons in a magnetic insulator yttrium iron garnet (YIG) on a 100 nanometer length scale. We demonstrate that the local magnon chemical potential can be systematically controlled through both ferromagnetic resonance and electrical spin excitations, which agrees well with the theoretical analysis of the underlying multi-magnon processes. Our results open up new possibilities for nanoscale imaging and manipulation of spin-related phenomena in condensed matter systems.   

Field noise near ferromagnetic films.

Thermally driven magnetization fluctuations can be viewed as a nuisance noise source or as interesting physics. For example, “mag noise” in a field sensor may set the minimum detectable field of that sensor. On the other hand, the field noise spectrum reflects the dynamics of the magnetic components, which are essential for device operation. Here, we model the field noise spectrum near the surface of a magnetic film due to thermal spin waves, and we calculate its effect on the $T_1$ relaxation rate of a nearby nitrogen-vacancy (NV) center spin[1]. The model incorporates four components: the spin wave dispersion of the magnetization in a finite-thickness film, thermal excitation of spin waves, the coupling geometry between waves in the film and an external point dipole and finally, the relaxation dynamics of the NV spin. At a distance of 100 nm above a 50 nm thick permalloy film, we find that the strongest stray fields are along the film normal and parallel to the magnetization, on the order of 1 mA m$^{-1}$ Hz$^{-1/2} $ or 1 nT Hz$^{-1/2}$, yielding relaxation times on the order of 10 $\mu$s. The spin wave field noise can dominate the intrinsic relaxation, ($T_1$ $\approx$ 1 ms) of the NV center spin. [1] T. van der Sar, et al., Nat. Commun. 6, 7886 (2015).   

Is MOKE a Viable Method for Probing Spin Hall Effect in Metals?.

In a recent publication, van`t Erve et al. (Appl. Phys. Lett. 104, 172402(2014)) reported observation of the magneto-optical Kerr effect (MOKE) from the spin Hall effect (SHE) in beta-tungsten ($\beta $-W) and platinum (Pt) films. This is most interesting, as it would provide an alternative means to probe SHE in metals. However, despite repeated attempts on different samples, we were unable to find a true SHE-induced MOKE signal from $\beta $-W and Pt. Both our theoretical estimate and experimental results indicate that the MOKE signal from SHE in metals ought to be very weak, below the detection limit of currently available MOKE setups. The false MOKE signal observed by van't Erve et al. likely came from the unbalanced ac heating effect.   

Probing the spin-dynamics in reentrant spin-glass phase with giant magnetoimpedance

In a relatively narrow concentration range amorphous FeZr alloys undergo a phase transition from a ferromagnet state to a spin-glass phase at low temperatures. To investigate the high frequency dynamics of this reentrant transition we have used measurements of the giant magnetoimpedance (GMI). In general the GMI is a measurement of the transverse magnetic permeability, $\mu_{T}$, as a function of temperature, $T$, and frequency, $f$, through their relation to the skin-depth $\delta$ $(=[\rho/\pi f \mu_{T}]^{1/2})$. In our recent work [Appl. Phys. Lett. 109, 102404 (2016)], the GMI was used to explore the relation between the scaling-law of the relaxation time,$\tau$, $(\tau =1/f)$ and the reduced freezing temperature $(t=1-T_f/T_G)$ in the MHz $f$-regime for a Fe$_{90}$Zr$_{10}$ sample with a $T_{G}$ of 14.0 K. We will review this work and present GMI and $\chi_{ac}$ data for a Fe$_{91}$Zr$_{9}$ sample with $T_{G}$ equal to 38.4 K. For the Fe$_{91}$Zr$_{9}$ sample it was found that the product of the critical exponents $z\nu$ $(=7.8)$ is comparable to the Fe$_{90}$Zr$_{10}$ sample $(=7.4)$ despite the different values of $T_{G}$. We conclude the alloys belong to the short-range Ising class of universality up to the MHz $f$-regime.   

Anisotropic spin-relaxation in mesoscopic copper wires

Spin-orbital (SO) effects play important roles in spintronics. The SO effects not only generate spin currents and spin torques, but also provide ways to modulate and control spin currents. However, SO effects induce higher rates of spin relaxation and therefore lead to shorter spin-relaxation length in materials, which is incompatible with the general desire for a longer spin-relaxation length to transport a spin current over distance. We demonstrate that substantial SO effects and a long spin diffusion length can coexist in a mesoscopic Cu channel. Anisotropic spin signals are observed in nonlocal spin valves. The spin signals are higher when the spins are aligned parallel to the Cu channel and lower when aligned perpendicular to it. The percentage of the anisotropic change increases with Cu channel length between the spin injector and detector, indicating that the Cu spin-relaxation length is anisotropic. The anisotropic differences of spin-relaxation lengths, estimated from two sets of samples, are 5{\%} and 9{\%}. The experiments can be explained by Rashba effects on the surfaces of Cu channels. The spin-flip probabilities of surface scatterings are larger for spins perpendicular to the surface than for spins parallel to it.   

Resistivity tensor in Permalloy films by modified van der Pauw method

We have applied a modified van der Pauw four point measurement method to measure the resistivity in thin ferromagnetic films of sputter deposited Permalloy (Ni$_{80}$Fe$_{20}$). Adding a fifth contact point allows determination of the full $2\times 2$ in-plane resistivity tensor. We did a series of measurements, at zero field, with a saturation field applied along the easy axis and saturation along the hard axis of films in the range between 10 and 250~nm thick. From the results we can confirm that the principle resistivity axes are aligned with the magnetic easy and hard axes, that were set by the growth conditions (affected both by the deposition angle and in situ applied magnetic field). We also extract the anisotropic magnetoresistance, that starts dropping significantly for films thinner than about 50 nm, in reasonable agreement with other studies. We also investigated magnetic anisotropy and coercivity for these films. The coercivity remains relatively stable around 1~Gauss, while the anisotropy field is stable for the thicker films in the range between 50-250~nm, but rises to almost double the value in the range from 50~nm down to 10~nm.   

Nanoscale magnetic imaging using picosecond thermal gradients

Research and development in spintronics is challenged by the lack of table-top magnetic imaging technologies that posses the simultaneous temporal resolution and spatial resolution to characterize magnetization dynamics in emerging spintronic devices. In addition, many of the most exciting magnetic material systems for spintronics are difficult to image with any method. To address this challenge, we developed a spatiotemporal magnetic microscope based on picosecond heat pulses that stroboscopically transduces an in-plane magnetization into a voltage signal. When the magnetic device contains a magnetic metal like FeCoB or NiFe, we use the time-resolved anomalous Nernst effect. When it contains a magnetic insulator/normal metal bilayer like yttrium iron garnet/platinum, we use the combination of the time-resolved longitudinal spin Seebeck effect and the inverse spin Hall effect. We demonstrate that these imaging modalities have time resolutions in the range of 10-100 ps and sensitivities in the range of 0.1 -- 0.3$^{\circ}/\sqrt{\text{Hz}}$, which enables not only static magnetic imaging, but also phase-sensitive ferromagnetic resonance imaging. One application of this technology is for magnetic torque vector imaging, which we apply to a spin Hall device. We find an unexpected variation in the spin torque vector that suggests conventional, all-electrical FMR measurements of spin torque vectors can produce a systematic error as large as 30\% when quantifying the spin Hall efficiency. Finally, I will describe how time-resolved magnetic imaging can greatly exceed the spatial resolution of optical diffraction. We demonstrate scanning a sharp gold tip to create near-field thermal transfer from a picosecond laser pulse to a magnetic sample as the basis of a nanoscale spatiotemporal microscope.