Session U32: Focus Session: Magnetic Multilayers and Nanostructures

Unusual resonant response in [Fe(001)/Cr(001)]$_{10 }$/ MgO(001) magnetic multilayers in magnetic field

We report on experimental observation of unusual electromotive resonances in [Fe/Cr]$_{10}$ multilayers epitaxially grown on MgO(001) substrates and measured by using balanced excitation and detection schemes. Electric voltage resonances with quality factor exceeding 10$^{3}$ induce strong enhancement of Hall resistance for specific frequencies. Surprisingly, the continuum mechanics model for suspended Fe/Cr layers accounts well for the observed phenomena. Cross-sectional electron microscopy analysis of the multilayers confirms that we could be dealing with non-suspended nanoelectromechanical system.   

Field Dependence of the Magnetic Roughness of CoFe(B)/MgO.

We have measured the field dependent roughness of the magnetic interface of CoFe and CoFeB films covered by MgO by using diffuse X-ray resonant magnetic scattering (diffuse-XRMS) of circular polarized light. The samples studied were 3.0 nm of either a Co(70)Fe(30) or a Co(60)Fe(20)B(20) film covered by 1.8 nm MgO created by UHV sputtering system (ANELVA C-7100). By comparing the specular scattering map and the diffuse scattering map for a large range of incidence angles for photons resonantly tuned near the Co L3-edge as the applied magnetic field is swept through the coercive field, we have determined the chemical and magnetic roughness as a function of applied field. For the CoFe films, the magnetic scattering of the X-rays increases significantly as the film passes through the coercive field where the magnetic in-plane correlations are relatively short range, indicating the presence of small magnetic domains during moment reversal. For the CoFeB films, there is no significant increase in magnetic scattering at the coercive field, consistent with large domain switching.   

Enhanced Electronic Density of States Observed in Fe-Cr Magnetic Multi-Layers

Magnetic multi-layer structures have garnered much interest over the past two decades particularly because of the giant magnetoresistance (GMR) effect and its application to information storage technology. Iron-chromium multi-layer structures have been studied extensively, but there remain many questions in the field due to the complex behavior of the anti-ferromagnetic layer (Cr). Using our silicon micro-machined calorimeters, we examine the low temperature specific heat for a range of sputtered MML films grown under similar conditions to those used in industry. We have observed an enhanced electronic density of states in the Fe-Cr MMLs far beyond that of the iron or chromium individually. We compare this enhancement to the observed GMR behavior through a systematic study varying the thickness of the spacer layer.   

High bias voltage effect on spin-dependent conductivity and low frequency noise in epitaxial Fe/MgO/Fe magnetic tunnel junctions

Low temperature (10K) high voltage bias dynamic conductivity (up to 2.7V) and shot noise (up to 1V) were studied in epitaxial Fe(100)/Fe-C/MgO(100)/Fe(100) magnetic tunnel junctions, as a function of the magnetic state. The MTJs show large TMR (185{\%} at 300K and 330{\%} at 4K). Multiple sign inversion of the magnetoresistance is observed for bias polarity when the electrons scan the electronic structure of the bottom Fe-C interface. The shot-noise shows a Poissonian character validating the high structural quality of the MgO barrier [1]. We have found that the normalized 1/f noise (Hooge factor) asymmetry between parallel and antiparallel states may strongly depend on the applied bias and its polarity. These MTJs exhibit record low Hooge factors being at least one order of magnitude smaller than previously reported [2]. ( [1] R. Guerrero, et al., Appl. Phys. Lett. \textbf{91}, 132504 (2007); [2] F.G. Aliev, et. al., accepted to Appl. Phys. Lett.).   

Giant Magnetoresistance in Nanogranular Magnets

I discuss the giant magnetoresistance of nanogranular magnets in the presence of an external magnetic field and finite temperature. It is shown that the magnetization of arrays of nanogranular magnets has hysteretic behavior at low temperatures leading to a double peak in the magnetoresistance which coalesces at high temperatures into a single peak. The magnetization of magnetic domains and the motion of domain walls in this system is calculated numerically using a combined mean-field approach and a model for an elastic membrane moving in a random medium, respectively. From the obtained results, the electric resistivity as a function of magnetic field and temperature is obtained. The findings show excellent agreement with various experimental data.   

Influence of dipolar interactions on the formation of domains in layered Ni/Al$_{2}$O$_{3}$ nanocomposites

Pulsed laser deposition has been used to fabricate Ni/Al$_{2}$0$_{3}$ multilayer composites in which Ni nanoparticles of uniform size in the range of 3-60 nm are embedded as layers in an insulating Al$_{2}$0$_{3}$ host. At fixed temperatures, the coercive fields show well-defined peaks which define a critical size that delineates a crossover from single domain (SD) to multiple domain (MD) behavior. Most applications require that the particles be single domain with a uniform magnetization that remains stable with a sufficiently large anisotropy energy to overcome thermal fluctuations and beat the superparamagnetic limit, which establishes a temperature-dependent \textit{lower bound} to the particle size (superparamagnetic limit). These considerations must take into account the effect of interactions on magnetic properties as is evident for high-density recording media where particles are very close to each other. The effect of dipolar interactions on the establishment of an \textit{upper bound} to particle size ($d_{c})$, which defines the crossover from SD to MD behavior will be discussed. We show using coercivity measurements that, with increasing temperature, $d_{c}$ increases and then saturates due to attenuated dipolar interactions from thermally induced motions of neighboring randomly oriented particles.   

An investigation of quantum well states and magnetic properties of Co/Au/Ru(0001)

Quantum well state of Au/Ru(0001) and its effect on the magnetic properties of Co/Au/Ru(0001) was investigated using Spin Polarized Low Energy Electron Microscopy (SPLEEM). Epitaxially grown Au on Ru(0001) at room temperature was annealed to $\sim $300\r{ }C. Upon annealing, Au forms islands with atomically flat tops across stepped regions of Ru, forming local wedges of different Au thickness. Energy scans reveal clearly the existence of quantum well states in Au/Ru(0001). After depositing Co film on top of the Au, we found that the Curie temperature and the spin reorientation transition of Co film on the flat-top Au islands are different from on the Au wetting layer. However the quantum well states of the Au layer have no effect on the Curie temperature and the spin reorientation transition of Co film.   

Angular Dependent Magnetic Susceptibility with Photoexcitation Studies on Prussian Blue Analog Thin Films

Prussian blue analog systems are the topic of ongoing research because of their novel physical effects. One such effect is persistent photoinduced magnetism, found in CoFe analogs. For such an optical system, in an attempt to maximize the material's interaction with incident photons, a thin film geometry is often utilized; this geometry can produce new effects [1]. Samples of different starting materials have been characterized with respect to photoinduced states, angular dependent susceptibility, film thickness, and chemical formula. Notably, magnetic anisotropies present in the systems show a dependence on the studied factors. One class of interesting starting materials are Rb$_{j}$Ni$_{k}$[Cr(CN)$_{6}$]$_{l}$.nH$_{2}$O and Rb$_{j}$Co$_{k}$[Fe(CN)$_{6}$]$_{l}$.nH$_{2}$O heterostructures, generated by sequential adsorption on a Melinex substrate, that display behavior different than a noninteracting admixture of the two materials by themselves. \newline [1] J.-H. Park, E. Cizmar, M. W. Meisel, Y. D. Huh, F. Frye, S. Lane, and D. R. Talham, Appl. Phys. Lett. 85, 3797 (2004).   

Ferroelectric control of magnetism in BaTiO$_{3}$/Fe heterostructures

Multiferroics can offer the possibility to manipulate the cross coupled order parameters by conjugate electric and magnetic fields. Switching off ferromagnetic order by an electric field for instance promises significant impact in the design of novel spintronic devices. Here we report on the reversible control of magnetism for a Fe thin film in proximity of a BaTiO$_{3}$ single-crystal. Large magnetization changes emerge in response to ferroelectric switching and structural transitions of BaTiO$_{3}$ controlled by applied electric fields and temperature, respectively.$^{\dag }$ Interface strain coupling is the primary mechanism altering the induced magnetic anisotropy. As a result, coercivity changes up to 120{\%} occur between the various structural states of BaTiO$_{3}$. Up to 20{\%} coercivity change is achieved via electrical control at room temperature. Our all solid state ferroelectric-ferromagnetic heterostructures open viable possibilities for new technological applications. $^{\dag }$S. Sahoo, S. Polisetty, C.-G. Duan, S. S. Jaswal, E. Y. Tsymbal, and Ch. Binek, Phys. Rev. B \textbf{76}, 092108 (2007).   

Griffiths phase and parimagnetism in ErCo$_{2}$

A systematic study of the paramagnetic phase of ErCo$_{2}$ has recently allowed to identify the inversion of the net magnetization of the Co net moment with respect to the applied field well above the ferrimagnetic ordering temperature, $T_{C}$. This phenomenon, which we have denoted \textit{parimagnetism}, may be related with the onset of a Griffiths-like phase in paramagnetic ErCo$_{2}$. We have measured SANS and ac susceptibility on ErCo$_{2}$ as a function of temperature, applied field, and excitation frequency. Several characteristics shared by systems showing a Griffiths phase are present in ErCo$_{2}$, namely the formation of ferromagnetic clusters in the disordered phase, the loss of analyticity of the magnetic susceptibility and its extreme sensitivity to an applied field. Our XMCD study of the Co magnetic moment flipping process show the ocurrence of a pseudo-violation of the third Hund's rule at the para- to parimagnetic ``transition''.   

Magnetostriction close to the phase transition in Gd$_{5}$(Si$_{x}$Ge$_{1-x}$)$_{4}$

Gd$_{5}$(Si$_{x}$Ge$_{1-x}$)$_{4}$ is a potential material for magnetic refrigeration. It has the highest magnetocaloric effect observed for the composition 0.41$\leq$x$\leq$0.5 near its first order coupled magnetic-structural phase transition. We have investigated the relation between the magnetic transition from ferromagnetic to paramagnetic phase and the structural transition from monoclinic to orthorhombic. A series of measurements have been carried out showing magnetostrictive strain as a function of temperature at various magnetic field strengths and magnetostrictive strain as a function of magnetic field at various temperatures with a magnetic field of up to 7 Tesla. There was fine structure observed in the magnetostriction curve $\lambda$ vs. H near the critical point. The magnetostriction measurements show that close to the critical temperature there is a sudden increase in the magnetostriction of about 100 ppm just before the field induced first order phase transition. This anomaly was observed for both strain vs. magnetic field at various temperatures and for strain vs. temperature at various magnetic field strengths measurements.   

Magnetostatic interaction between two thin nanotubes

We consider here the magnetic interaction between two identical tubes, characterized by: total magnetization M, length 2L, external radius Re, internal radius Ri. Following most of the experimental realizations we consider very thin tubes, namely, (Re-Ri)$<<$Ri. We begin by considering the two nanotubes in perfect parallel alignment and we vary the separation distance D. The continuous magnetization approach is invoked using different methods to compute the interacting energy: analytic expression valid for D$<$2L, analytic integration over the interaction of elements on each tube, and numeric integration for general cases. These results are compared with two independent results: a) the tubes are far apart so they can be considered solid nanowires: b)each tube is considered as a set of elementary nanowires and a series expansion is obtained and truncated. The advantages and disadvantages of each method are discussed. The ranges of applicability of the ``handy'' approximate expressions are obtained.   

Kondo-like features in chemically pure magnetic atomic-size contacts

The influence of magnetism in the electronic transport in atomic sized contacts is not yet clear. However, certain features systematically appear in the conductance measurements of magnetic atomic contacts. Specifically, the spectroscopy of atomic size contacts of Ni, Fe or Co reveal the existence of a characteristic significant peak or dip at zero bias that is not present in the case of non-magnetic materials. We have measured the differential conductance as a function of bias at 4K on two houndred monoatomic contacts of Ni, Fe and Co fabricated by STM. The zero bias anomaly has been analyzed as using the Kondo- Fano lineshape typical of magnetic adatoms in non-magnetic surfaces. The statistical analysis of the data results in Kondo temperatures around 250 K, 120 K and 80 K for Ni, Co and Fe respectively. A Kondo-like behaviour could arise in chemically pure magnetic contacts if tip atoms behave different due to their smaller coordination.   

Spin-density wave in polycrystalline Cr films from infrared reflectivity

The spin-density wave properties of polycrystalline chromium thin films were determined by using infrared reflectivity to determine the gap energies. The incommensurate spin density wave of bulk chromium is highly sensitive to perturbations from stress, disorder, and finite size effects, such as those found in polycrystalline films. Films prepared under various conditions display three different types of spin density wave behavior: incommensurate, commensurate, and mixed. Unexpectedly, the mixed phase includes the incommensurate spin density wave and two different forms of commensurate spin density wave. A phenomenologically determined low temperature phase diagram is created to describe the spin density wave in chromium in the stress-disorder plane. The effects of stress and disorder on the spin density wave of chromium films are analogous to the effects of dilute alloying in bulk chromium. In this case, tensile stress has a similar effect to Mn impurities while disorder has a similar effect to Al.