Local Magnetic Fluctuations Research Articles

Structural, magnetic, and transport properties of quantum well GaAs/δ-Mn/GaAs/In x Ga1–x As/GaAs heterostructures

Abstract The results of high-resolution X-ray diffraction and X-ray glancing-incidence mirror reflection studies of structural characteristics of the GaAs/Ga1–x In x As/GaAs diluted magnetic semiconductors, with an Mn δ-layer well separated from the Ga1–x In x As quantum by the GaAs spacer, are presented. A strong correlation between the structural, magnetic, electro-physical and optical properties of the samples gives evidence that the samples consist of a distribution of ferromagnetic and paramagnetic regions. Such a phase separation can be understood by a non-random distribution of the local magnetic moments and fluctuations of the exchange interaction in the samples having semiconductor-type conductivity.

Local magnetization fluctuations in superconducting glasses resolved by Hall sensors

We report on magnetization measurements performed on a series of Fe$_{x}$Ni$_{1-x}$Zr$_{2}$ superconducting metallic glasses with $0\leq x\leq0.5$ using the Hall effect of a nearby 2-dimensional electron gas (2DEG) in a GaAs/Al$_{0.33}$Ga$_{0.67}$As heterostructure as a local probe. The great sensitivity of the Hall effect of the 2DEG in such heterostructure is exploited to determine the magnetization of the superconductor due to the Meissner effect and flux trapping. The data is used to determine the lower critical field B$_{c1}$ of the superconductors as a function of temperature. Surprisingly large fluctuations in the magnetization are also observed and attributed to the presence of large flux clusters in the superconductor.

Micromagnetic structures of nanocrystalline ferromagnetic materials – comparison of simulation and experiment

AbstractThe micromagnetic structure of nanocrystalline ferromagnetic materials is characterized by local fluctuations induced by the randomly varying magnetocrystalline anisotropy of the individual grains. Micromagnetic simulations may improve the understanding of the relevant features and the interpretation of the experimental observations of the magnetic structure. In this study we investigate the magnetic structure of nanocrystalline FeSi and Co model systems with average grain sizes of 20 nm to 60 nm. Small angle neutron scattering (SANS) is a suitable technique to study the micromagnetic structures of nanocrystalline materials at the nanometer scale. The simulated SANS curves for the FeSi model systems are compared with the experimental SANS curves of Vitroperm samples. It is found that the scattering at high fields is dominated by the local anisotropy fluctuations which can be well reproduced by the simulations. The simulations of the nanocrystalline hcp Co show that the stray fields resulting from the local magnetic fluctuations should lead to an observable contrast in magnetic force microscopy (MFM) images. We show that the contrast observed in the real MFM images of thin nanocrystalline hcp Co films compares well with the simulated image contrast with respect to magnitude and length scale of the observed fluctuations. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Evaluation of local magnetic field fluctuation in a toroidal plasma with heavy ion beam probe

A method to measure magnetic field fluctuation using a heavy ion beam probe for a toroidal magnetically confined plasma is proposed by providing a mathematical formula to connect the probing beam movement with the change in the magnetic field. The first challenge of the method has been faced in an electron-cyclotron resonance heated plasma in the compact helical system (CHS). The results reveal the existence of three characteristic frequency regimes of the magnetic field fluctuations: a low frequency regime showing a long-distance correlation, an intermediate frequency regime obeying a power law and the turbulence regime. Discussion is held on the degree of the contaminations in the local magnetic field fluctuation that are caused by the magnetic field fluctuations integrated along the beam orbit and the local electric field fluctuations.

Favorable magnetic fluctuation anisotropy for unconventional superconductivity inf-electron systems

We report spin-lattice relaxation time measurements in isostructural ($\mathrm{Ho}\mathrm{Co}{\mathrm{Ga}}_{5}$-type) actinide compounds with different ground states (paramagnet, antiferromagnet, and superconductor) using single-crystal samples. The anisotropy of local magnetic fluctuations due to $LS$ coupling on the actinide (magnetic) site is evaluated. In the superconductor $\mathrm{Pu}\mathrm{Rh}{\mathrm{Ga}}_{5}$, the $XY$-type magnetic anisotropy is strong compared with nonsuperconducting compounds. Favorable magnetic anisotropy for $d$-wave superconductivity in $f$-electron systems is discussed.

Temperature dependence of magnetic anisotropy: Anab initioapproach

We present a first-principles theory of the variation of magnetic anisotropy, K, with temperature, T ,i n metallic ferromagnets. It is based on relativistic electronic structure theory and calculation of magnetic torque. Thermally induced local moment magnetic fluctuations are described within the relativistic generalization of the disordered local moment theory from which the T dependence of the magnetization, m, is found. We apply the theory to a uniaxial magnetic material with tetragonal crystal symmetry, L10-ordered FePd, and find its uniaxial K consistent with a magnetic easy axis perpendicular to the Fe/Pd layers for all m and proportional to m 2 for a broad range of values of m. This is the same trend that we have previously found in L10-ordered FePt and which agrees with experiment. We also study a magnetically soft cubic magnet, the Fe50Pt50 solid solution, and find that its small magnetic anisotropy constant K1 rapidly diminishes from 8 eV to zero. K1 evolves from being proportional to m 7 at low T to m 4 near the Curie temperature. The accounts of both the tetragonal and cubic itinerant electron magnets differ from those extracted from single ion anisotropy models and instead receive clear interpretations in terms of two ion anisotropic exchange.

Solvent Effect on Intramolecular Electron Transfer Rates of Mixed-Valence Biferrocene Monocation Derivatives

Intramolecular electron transfer (ET) rates in various solvents of mixed-valence biferrocene monocation (Fe(II), Fe(III)) and the 1',1' ''-diiodo and 1',1' ''-diethyl derivatives (respectively abbreviated as BFC(+), I(2)BFC(+), and Et(2)BFC(+)) were determined by means of the spin-lattice relaxation times of the protons, taking into account the local magnetic field fluctuation caused by the electron hopping between the two ferrocene units. We also determined the ET rates of a mixed-valence diferrocenylacetylene monocation (DFA(+)) in order to examine the effect of the insertion of an acetylene bridge between the two ferrocene units. The insertion of the bridge decreased the ET rate, while the effect of substitution on the cyclopentadienyl rings on the rate was minor. The observed rates for each mixed-valence monocation in various solvents did not correlate with the reorganization energies, but we did find a significant contribution of the solvent dynamics. The observed rates were considerably higher than those expected on the basis of the Sumi-Marcus-Nalder model in which the solvents were regarded as dielectric continua. The slope of the logarithm plot of the pre-exponential factors in various solvents for each mixed-valence monocation versus the inverse of the longitudinal dielectric relaxation times of the solvents was significantly smaller than unity, and the slope for DFA(+) was larger than those for BFC(+), I(2)BFC(+), and Et(2)BFC(+). These results were ascribed to a partial contribution of the dielectric friction to the dynamics along the solvent coordinate; the extent of the contribution decreased with a reduction in the ET distance. For the dynamics along the solvent coordinate of the ET reactions in methanol, the observed rates indicated an important contribution by the minor dielectric relaxation components with faster relaxation times, rather than the major component with an extraordinarily long relaxation time.

Normal-state electronic structure in the heavily overdoped regime ofBi1.74Pb0.38Sr1.88CuO6+δsingle-layer cuprate superconductors: An angle-resolved photoemission study

We explore the electronic structure in the heavily overdoped regime of the single layer cuprate superconductor Bi1.74Pb0.38Sr1.88CuO6+delta. We found that the nodal quasiparticle behavior is dominated mostly by phonons, while the antinodal quasiparticle lineshape is dominated by spin fluctuations. Moreover, while long range spin fluctuations diminish at very high doping, the local magnetic fluctuations still dominate the quasiparticle dispersion, and the system exhibits a strange metal behavior in the entire overdoped regime.

Consideration of magnetic field fluctuation measurements in torus plasma with a heavy ion beam probe

The article proposes a method of magnetic fluctuation measurement using a heavy ion beam probe (HIBP) in an axisymmetric torus configuration. The method is based on the detection of the toroidal position (not velocity) of the secondary beam in the analyzer. However, the method needs careful consideration with respect to path integral fluctuations along the probing beam orbit to evaluate local magnetic fluctuation, similarly to density fluctuation measurements with a HIBP. Here, we present an analytic formula to estimate and calculate the path integral effects for different fluctuation patterns in the profile, the correlation length, the radial wavelength, and the poloidal mode number. As a result, it is found that a large distance between the plasma and the detector lessens the importance of the path integral effect, and that local fluctuation of the magnetic field can be properly detected with a HIBP.

Schottky anomaly inNd2CuO4observed by muon spin relaxation

We report zero-field muon spin relaxation experiments on ${\mathrm{Nd}}_{2}{\mathrm{CuO}}_{4}$ focused on the low-temperature range for studying the Nd Schottky anomaly. In the magnetically ordered state we observed a significant increase and saturation of the muon spin precession frequency below $T=15\mathrm{K}.$ A fit based on the temperature-dependent population of the ${\mathrm{Nd}}^{3+}$ Kramers ground-state doublet reveals a splitting of the doublet of $\ensuremath{\Delta}=4.2(6)\mathrm{K}$ in good agreement with the work of Adelmann et al. [Phys. Rev. B 46, 3619 (1992)]. The same model was applied to account for the temperature dependence of the dynamic relaxation rate resulting in $\ensuremath{\Delta}=4.6(5)\mathrm{K}.$ A quantitative evaluation of the temperature-dependent local magnetic field fluctuation rate is presented.

Mediated exchange interaction in Fe-Nb-B nanocrystalline soft magnetic materials

The magnetic interactions between bcc Fe nanocrystalline particles in ${\mathrm{Fe}}_{84}{\mathrm{Nb}}_{7}{\mathrm{B}}_{9}$ nanocrystalline soft magnetic alloys are in situ studied by electron holography. Wavy lines of magnetic flux inside magnetic domains reveal local magnetization fluctuations of magnetic ripple structure, which result from the interplay between the exchange and magnetostatic interactions. The longitudinal wavelength decreases with an increase in temperature. It is found that the local stray field greatly weakens the intergranular exchange coupling and reduces the coupled volume at elevated temperatures. The thermal magnetic behaviors of nanocrystalline soft magnetic alloys are explained.

What are the mesoscopic magnetic inhomogeneities in the dilute PdFeMn alloy? Polarized neutron study

The 3D analysis of neutron depolarization was carried out for different thermomagnetic treatment of the dilute PdFeMn alloy versus temperature and magnetic field applied in magnetizing/demagnetizing cycles. Both the macroscopic magnetization and the mean fluctuation of local magnetization behavior were subtracted from experimental data. A complicated behavior of the latter was observed. The hysteresis of local magnetization fluctuations is found out but that of macroscopic magnetization is practically absent. The effort to apply the simple model for the description of magnetic inhomogeneities was made in order to understand the mesostructure of this alloy.

μSR study of organic systems: ferromagnetism, antiferromagnetism, the spin-crossover effect, and fluctuations in magnetic nanodiscs

We present the results of recent μSR experiments on a variety of novel organic and molecular magnetic systems. Muons are sensitive to local static fields and magnetic fluctuations, but can probe much more than just the onset of long-range magnetic order. We review our work on nitronyl nitroxide organic ferromagnets and antiferromagnets. We describe a muon study of the spin-crossover phenomenon which has been studied in Fe(PM-PEA) 2(NCS) 2, and which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. Experiments on a disc-shaped molecular complex containing Fe 19 (with spin 31 2 ) reveal the effects of quantum tunneling of magnetization and allow an estimate of the quantum tunneling rate.

Muon study of molecular magnets, spin crossover and magnetic nanodiscs

We present the results of recent muon-spin rotation ( μSR) experiments on a variety of novel molecular magnetic systems. Muons are sensitive to local static fields and magnetic fluctuations, but can probe much more than just the onset of long-range magnetic order. The spin-crossover phenomenon has been studied in Fe(PM-PEA) 2(NCS) 2, which shows Gaussian and root-exponential muon relaxation in the high-spin and low-spin phases, respectively. Dy spin fluctuations have been studied with μSR in the ferrimagnetic chain Dy(hfac) 3(NITEt). Experiments on a disc-shaped molecular complex containing Fe 19 (with spin 31 2 ) reveal the effects of quantum tunneling of magnetization and allow an estimate of the quantum tunneling rate.

Room-temperature scanning Hall probe microscope (RT-SHPM) imaging of garnet films using new high-performance InSb sensors

High-performance InSb micro-Hall sensors were fabricated by optical lithography and incorporated in a room-temperature scanning Hall probe microscope for imaging of localized magnetic fluctuations in close proximity to the surfaces of crystalline uniaxial garnet films. The room-temperature noise figure of the InSb sensors was 6-10 mG//spl radic/Hz, which is an order of magnitude. better than GaAs-AlGaAs two-dimensional electron gas sensors used to date.

Comparison of hard magnetic electrodes for magneto-electronics by magnetic force microscopy

Both spin valve sensor elements and magnetic tunnel junctions usually consist of a magnetically fixed hard magnetic layer and a soft magnetic counter electrode which can rotate freely in an external driving field. In order to yield reliable sensor signals, the hard magnetic electrode should be magnetically rigid against small fields, i.e., must not be influenced during switching of the sensing electrode. Furthermore, in order to provide a homogeneous switching behavior, the electrodes should show uniformity in their magnetic properties, especially if scaled and patterned down to sub-micron lateral sizes. In this paper, magnetic force microscopy is used to compare the magnetic properties of different hard magnetic layer stacks commonly used in magneto-electronic device technology, i.e. an artificial antiferrimagnet (CoFe/Ru/CoFe and Co/Cu/Co) and exchange biased stacks (MnIr/CoFe/Ru/CoFe). The domain pattern of a polycrystalline artificial ferrimagnet (AFi) shows a strong magnetization ripple, i.e., local variations of the magnetization directions, which are maintained in patterned submicron elements. These statistical local fluctuations can be quantitatively correlated with a reduction of the tunneling magnetoresistance (TMR) of 3%–5% in the minor loop. In addition for small area junctions, the ripple can cause different switching fields in neighboring elements. An additional exchange biasing by a thin natural antiferromagnet is shown to rigidly pin the local magnetization fluctuations of the AFi. The process used for the preparation of this system, however, creates 360° domain walls, which again deteriorate the TMR signal.

Room temperature scanning Hall probe microscopy of localized magnetic field fluctuations on the surfaces of magnetic recording media, permanent magnets and crystalline garnet films in external bias fields

A sub-micron room temperature scanning Hall probe microscope (RT-SHPM) was used for real-time imaging of surface magnetic domains of floppy disks, Sr ferrite magnets and Bi-substituted iron garnets placed in large external bias fields. Domain wall nucleation was observed in the garnets where bubble lattices expanded, collapsed and transformed into stripe domains in cyclic bias fields. Evolution of RT-SHPM images was compared with conventional vibrating sample magnetometer measurements.

Noise characteristics in low-impedance NiFe/Al 2O 3/NiFe tunnel junctions

We have studied the low-frequency voltage noise in micron-scale magnetic tunnel junctions. Random telegraph noise (RTN) dominates over 1/ f noise in the switching region of magnetic hysteresis loops, suggesting that RTN comes mostly from local magnetization fluctuations. The temperature dependence of the 1/ f noise and the size of the magnetic fluctuators responsible for the RTN are discussed.

Room Temperature Sub-Micron Magnetic Imaging by Scanning Hall Probe Microscopy

An ultra-high sensitive room temperature scanning Hall probe microscope (RT-SHPM) system incorporating a GaAs/AlGaAs micro-Hall probe was used for the direct magnetic imaging of localized magnetic field fluctuations in very close proximity to the surface of ferromagnetic materials. The active area, Hall coefficient and field sensitivity of the Hall probe were 0.8 µm×0.8 µm, 0.3 Ω/G and 0.04 G/√Hz, respectively. The use of a semiconducting Hall probe sensor enabled measurements in the presence of externally applied magnetic fields. Samples studied included magnetic recording media, demagnetized strontium ferrite permanent magnets, and low coercivity perpendicular garnet thin films. The RT-SHPM offers a simple means for quantitatively monitoring sub-micron magnetic domain structures at room temperature.

Organic Magnetic Materials Studied by Positive Muons

Positive muons can be implanted into organic and molecular magnets in order to study their internal magnetic field distribution and any associated dynamics. The muon behaves essentially as a “microscopic magnetometer”, sensitive to local magnetic order and magnetic fluctuations. We describe some recent experiments using this technique which were performed on a variety of organic systems, including nitronyl nitroxide magnets and materials with spin-Peierls ground states, MEM(TCNQ)2 and DEM(TCNQ)2, and demonstrate how the technique can give information concerning their ground states.