"Quantum sensing" describes the use of a quantum system, quantum properties or quantum phenomena to perform a measurement of a physical quantity Historical examples of quantum sensors include magnetometers based on superconducting quantum interference devices and atomic vapors, or atomic clocks More recently, quantum sensing has become a distinct and rapidly growing branch of research within the area of quantum science and technology, with the most common platforms being spin qubits, trapped ions and flux qubits The field is expected to provide new opportunities - especially with regard to high sensitivity and precision - in applied physics and other areas of science In this review, we provide an introduction to the basic principles, methods and concepts of quantum sensing from the viewpoint of the interested experimentalist

Quantum sensing

Antiferromagnetic materials could represent the future of spintronic applications thanks to the numerous interesting features they combine: they are robust against perturbation due to magnetic fields, produce no stray fields, display ultrafast dynamics and are capable of generating large magneto-transport effects Intense research efforts over the past decade have been invested in unraveling spin transport properties in antiferromagnetic materials Whether spin transport can be used to drive the antiferromagnetic order and how subsequent variations can be detected are some of the thrilling challenges currently being addressed Antiferromagnetic spintronics started out with studies on spin transfer, and has undergone a definite revival in the last few years with the publication of pioneering articles on the use of spin-orbit interactions in antiferromagnets This paradigm shift offers possibilities for radically new concepts for spin manipulation in electronics Central to these endeavors are the need for predictive models, relevant disruptive materials and new experimental designs This paper reviews the most prominent spintronic effects described based on theoretical and experimental analysis of antiferromagnetic materials It also details some of the remaining bottlenecks and suggests possible avenues for future research

Antiferromagnetic spintronics

Spins of impurities in solids provide a unique architecture to realize quantum technologies. A quantum register of electron and nearby nuclear spins in the lattice encompasses high-fidelity state manipulation and readout, long-lived quantum memory, and long-distance transmission of quantum states by optical transitions that coherently connect spins and photons. These features, combined with solid-state device engineering, establish impurity spins as promising resources for quantum networks, information processing and sensing. Focusing on optical methods for the access and connectivity of single spins, we review recent progress in impurity systems such as colour centres in diamond and silicon carbide, rare-earth ions in solids and donors in silicon. We project a possible path to chip-scale quantum technologies through sustained advances in nanofabrication, quantum control and materials engineering.

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Quantum technologies with optically interfaced solid-state spins

Stripline (SL), vector network analyzer (VNA), and pulsed inductive microwave magnetometer (PIMM) techniques were used to measure the ferromagnetic resonance (FMR) linewidth for a series of Permalloy films with thicknesses of 50 and 100nm. The SL-FMR measurements were made for fixed frequencies from 1.5to5.5GHz. The VNA-FMR and PIMM measurements were made for fixed in-plane fields from 1.6to8kA∕m (20–100Oe). The results provide a confirmation, lacking until now, that the linewidths measured by these three methods are consistent and compatible. In the field format, the linewidths are a linear function of frequency, with a slope that corresponds to a nominal Landau-Lifshitz phenomenological damping parameter α value of 0.007 and zero frequency intercepts in the 160–320A∕m (2–4Oe) range. In the frequency format, the corresponding linewidth versus frequency response shows a weak upward curvature at the lowest measurement frequencies and a leveling off at high frequencies.

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Ferromagnetic resonance linewidth in metallic thin films: Comparison of measurement methods | NIST

We have combined ultrasensitive magnetic resonance force microscopy (MRFM) with 3D image reconstruction to achieve magnetic resonance imaging (MRI) with resolution <10 nm. The image reconstruction converts measured magnetic force data into a 3D map of nuclear spin density, taking advantage of the unique characteristics of the “resonant slice” that is projected outward from a nanoscale magnetic tip. The basic principles are demonstrated by imaging the 1H spin density within individual tobacco mosaic virus particles sitting on a nanometer-thick layer of adsorbed hydrocarbons. This result, which represents a 100 million-fold improvement in volume resolution over conventional MRI, demonstrates the potential of MRFM as a tool for 3D, elementally selective imaging on the nanometer scale.

http://absimage.aps.org/image/MAR11/MWS_MAR11-2010-020116.pdf

Nanoscale Magnetic Resonance Imaging

We have investigated spin pumping from ${\mathrm{Y}}_{3}{\mathrm{Fe}}_{5}{\mathrm{O}}_{12}$ thin films into Cu, Ag, Ta, W, Pt, and Au with varying spin-orbit coupling strengths. From measurements of Gilbert damping enhancement and inverse spin Hall signals spanning 3 orders of magnitude, we determine the spin Hall angles and interfacial spin mixing conductances for the six metals. The spin Hall angles largely vary as ${Z}^{4}$ ($Z$: atomic number), corroborating the role of spin-orbit coupling. Amongst the four 5d metals, the variation of the spin Hall angle is dominated by the sensitivity of the $d$-orbital moment to the $d$-electron count, confirming theoretical predictions.

Scaling of Spin Hall Angle in 3d, 4d, and 5d Metals from Y 3 Fe 5 O 12 /Metal Spin Pumping

Photoluminescence measurements show that the spin state of an $N\phantom{\rule{0}{0ex}}V$ center can be modified nonresonantly (i.e. at frequencies far from the $N\phantom{\rule{0}{0ex}}V$ Larmor frequencies) via coupling to the dynamic magnetization of a ferromagnet---an unexpected result that presents a new approach to transferring ferromagnetic spin information into a paramagnet.

Off-resonant manipulation of spins in diamond via precessing magnetization of a proximal ferromagnet

Epitaxial Y${}_{3}$Fe${}_{5}$O${}_{12}$ thin films deposited by off-axis sputtering exhibit excellent crystalline quality enabling observation of large spin pumping signals in Pt/Y${}_{3}$Fe${}_{5}$O${}_{12}$ and W/Y${}_{3}$Fe${}_{5}$O${}_{12}$ bilayers driven by cavity ferromagnetic resonance. The inverse spin Hall voltages reach 2.10 and \ensuremath{-}5.26 mV in 5-mm-long Pt/Y${}_{3}$Fe${}_{5}$O${}_{12}$ and W/Y${}_{3}$Fe${}_{5}$O${}_{12}$ bilayers, respectively, excited by a radio-frequency magnetic field of 0.3 Oe. From the ferromagnetic resonance linewidth broadening, we obtain high interfacial spin mixing conductances of 4.56\ifmmode\times\else\texttimes\fi{}10${}^{14}$ and 2.30\ifmmode\times\else\texttimes\fi{}10${}^{14}$ ${\ensuremath{\Omega}}^{\ensuremath{-}1}$ m${}^{\ensuremath{-}2}$ for Pt/Y${}_{3}$Fe${}_{5}$O${}_{12}$ and W/Y${}_{3}$Fe${}_{5}$O${}_{12}$ bilayers, respectively.

Large spin pumping from epitaxial Y 3 Fe 5 O 12 thin films to Pt and W layers

We demonstrate tuning of magnetocrystalline anisotropy in high-quality ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ epitaxial films over a range of several thousand Gauss using strain induced by epitaxial growth on substrates of varying lattice constants. Spectroscopic measurements reveal a striking, linear dependence of the out-of-plane anisotropy on the strain-induced tetragonal distortion of the ${\mathrm{Sr}}_{2}{\mathrm{FeMoO}}_{6}$ lattice. This anisotropy can be tuned from $+2000$ to $\ensuremath{-}3300\text{ }\text{ }\mathrm{Oe}$, a range sufficient to rotate the easy axis from in plane to out of plane. Combined with its half-metallicity and high Curie temperature, this result implies a broad range of scientific and technological applications for this novel spintronic material.

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Control of magnetocrystalline anisotropy by epitaxial strain in double perovskite Sr(2)FeMoO(6) films.

The saturation magnetization of Y3Fe5O12 (YIG) epitaxial films 4 to 250 nm in thickness has been determined by complementary measurements including the angular and frequency dependencies of the ferromagnetic resonance fields as well as magnetometry measurements. The YIG films exhibit state-of-the-art crystalline quality, proper stoichiometry, and pure Fe3+ valence state. The values of YIG magnetization obtained from all the techniques significantly exceed previously reported values for single crystal YIG and the theoretical maximum. This enhancement of magnetization, not attributable to off-stoichiometry or other defects in YIG, opens opportunities for tuning magnetic properties in epitaxial films of magnetic insulators.

Rohan Adur

Papers

Scaling of Spin Hall Angle in 3d, 4d, and 5d Metals from Y 3 Fe 5 O 12 /Metal Spin Pumping

Off-resonant manipulation of spins in diamond via precessing magnetization of a proximal ferromagnet

Large spin pumping from epitaxial Y 3 Fe 5 O 12 thin films to Pt and W layers

Control of magnetocrystalline anisotropy by epitaxial strain in double perovskite Sr(2)FeMoO(6) films.

Exceptionally high magnetization of stoichiometric Y3Fe5O12 epitaxial films grown on Gd3Ga5O12