Biomedical subjects

S D Bader

Publications and source records attributed to S D Bader.

10 recordsLinked to original sources

PubMed2006

Magnetic vortex core dynamics in cylindrical ferromagnetic dots.

We report direct imaging by means of x-ray photoemission electron microscopy of the dynamics of magnetic vortices confined in micron-sized circular permalloy dots that are 30 nm thick. The vortex core positions oscillate on a 10 ns time scale in a self-induced magnetostatic potential well after the in-plane magnetic field is turned off. The measured oscillation frequencies as a function of the aspect ratio of the dots are in agreement with theoretical calculations presented for the same geometry.

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PubMed2005

Magnetic bistability of Co nanodots.

Size-dependent magnetic single-domain versus vortex state stability of Co/Ru(0001) nanodots is explored with spin-polarized low-energy electron microscopy, analytical modeling, and micromagnetic simulations. We show that both single-domain and vortex states can be stabilized in a broad region near the phase boundary. The calculated width of the bistability region and temperature dependent heights of the energy barriers between both states agree well with our experimental findings.

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PubMed2004

Nuclear resonant magnetometry and its application to Fe/Cr multilayers.

We introduce nuclear resonant magnetometry as a means to record the magnetization curve of isotopically enhanced regions of a sample. It is based on nuclear resonant scattering with circularly polarized synchrotron radiation and the use of a nuclear resonant reference sample. We apply this approach to study the interlayer coupling in Fe/Cr(100) multilayers and to obtain a layer-specific magnetization curve. Our measurements provide experimental evidence for the existence of a nontrivial interlayer-coupling angle in Fe/Cr/Fe.

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PubMed2003

Hysteretic spin-density-wave ordering in confined geometries.

We have measured the antiferromagnetic spin-density-wave (SDW) order in Cr/Cr(97.5)Mn2.5(001) superlattices. The Mn doping creates a high Néel temperature layer that confines the incommensurate SDW order within the Cr layers. With temperature cycling we observe a transition from commensurate to incommensurate SDW order and discrete changes in the SDW period. We find that these transitions show significant hysteresis (up to 75 K) when the number of SDW nodes within the Cr layer changes by an odd number, while there is no hysteresis for changes of an even number of nodes. This results from the competition between maintaining the spin structure at the interfaces and introducing a spin slip at the nodes of the Cr SDW.

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PubMed2002

Magnetization-orientation dependence of the superconducting transition temperature in the ferromagnet-superconductor-ferromagnet system: CuNi/Nb/CuNi.

The superconducting critical temperature (T(c)) of ferromagnet-superconductor-ferromagnet systems has been predicted to exhibit a dependence on the magnetization orientation of the ferromagnetic layers such that T(AP)(c)>T(P)(c) for parallel (P) and antiparallel (AP) configurations of the two ferromagnetic layers. We have grown CuNi/Nb/CuNi films via magnetron sputtering and confirmed the theoretical prediction by measuring the resistance of the system as a function of temperature and magnetic field. We find an approximately 25% resistance drop occurs near T(c) in Cu0.47Ni0.53(5 nm)/Nb(18)/CuNi(5) when the two CuNi layers change their magnetization directions from parallel to antiparallel, whereas there is no corresponding resistance change in the normal state.

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PubMed2002

Spin flop transition in a finite antiferromagnetic superlattice: evolution of the magnetic structure.

An antiferromagnetic (AF) superlattice of Fe/Cr(211) is used as a model system to study magnetic transitions in a finite-size geometry. With polarization neutron reflectometry the magnetic structure at the surface spin-flop transition and its evolution with field is determined. A domain wall created near the surface penetrates the superlattice with increasing field, splitting it into two antiphase, AF domains. After reaching the center the spin-flopped phase spreads throughout the superlattice. The experimental results are in substantial agreement with theoretical and numerical predictions.

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PubMed2001

Enhanced interfacial magnetic coupling of Gd /Fe multilayers.

The spatial extent zeta(AFM) and strength J(AFM) of the antiferromagnetic (AFM) exchange coupling at buried Gd /Fe interfaces in ferrimagnetic [Gd(50 A)Fe(15,35 A)](15) sputtered multilayers is obtained from combined x-ray resonance magnetic reflectivity and magnetic circular dichroism measurements. zeta(AFM) is 4.1(7) A or approximately 1-2 interatomic distances in bulk Gd and Fe; J(AFM) is 1050(90) K, comparable to the ferromagnetic exchange in bulk Fe.

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PubMed2001

Origin of biquadratic coupling in Fe/Cr(100) superlattices.

We investigate the magnetic properties of a (100) oriented [Fe(1.7 nm)/Cr(8.4 nm)](10) superlattice by means of perturbed angular correlation spectroscopy. The magnetic ordering in the Cr layers is obtained by measuring the magnetic hyperfine interaction at implanted 111Cd nuclear probes. We identify dynamic antiferromagnetic spin fluctuations in the Cr layers and show that it gives rise to the biquadratic interlayer coupling.

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PubMed2001

Field induced biquadratic exchange in hard/soft ferromagnetic bilayers.

The appearance of biquadratic exchange coupling between soft Fe and hard SmCo thin layers is found. The remanent magnetization in the Fe layer reorients from parallel to perpendicular with respect to the SmCo easy axis after application of large enough negative field. To explain such an unexpected behavior in contacting ferromagnetic layers a model is proposed based on Slonczewski's fluctuating exchange mechanism. In our samples a partial remagnetization of the hard layer creates fluctuations of the bilinear interactions. The intralayer exchange averaging of the resulting magnetization fluctuations in the soft layer causes the observed biquadratic coupling.

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PubMed2000

Experimental observation of disorder-driven hysteresis-loop criticality.

We have studied the effect of magnetic disorder on the magnetization reversal process in thin Co/CoO films. The antiferromagnetic CoO layer allows a reversible tuning of the magnetic disorder by simple temperature variation. For temperatures above a critical temperature T(c), we observe a discontinuous magnetization reversal, whereas smooth magnetization loops occur for T<T(c). Our measurements establish the existence of a disorder-driven critical point in the nonequilibrium phase diagram. In addition, we observe scaling behavior in the vicinity of the critical point and determine the critical exponents beta = 0.022+/-0.006 and betadelta = 0.30+/-0.03 for this two-dimensional system.

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