Copyright (c) 2013 University of Nebraska - Lincoln All rights reserved. http://digitalcommons.unl.edu/physicsbinek Recent documents in Christian Binek Publications en-us Thu, 24 Jan 2013 20:14:02 PST 3600 http://digitalcommons.unl.edu/physicsbinek/77 http://digitalcommons.unl.edu/physicsbinek/77 Fri, 04 May 2012 14:10:53 PDT We report the direct observation of surface magnetization domains of the magnetoelectric Cr₂O₃ using photoemission electron microscopy with magnetic circular dichroism contrast and magnetic force microscopy. The domain pattern is strongly affected by the applied electric field conditions. Zero-field cooling results in an equal representation of the two domain types, while electric-field cooling selects one dominant domain type. These observations confirm the existence of surface magnetization, required by symmetry in magnetoelectric antiferromagnets.

]]> Ning Wu et al. http://digitalcommons.unl.edu/physicsbinek/76 http://digitalcommons.unl.edu/physicsbinek/76 Fri, 04 May 2012 14:05:54 PDT Exchange coupled magnetic hard layer/soft layer thin films show a variety of complex magnetization reversal mechanisms depending on the hierarchy of interaction strengths within and between the films. Magnetization reversal can include uniform rotation, soft layer biasing, as well as exchange spring behavior. We investigate the magnetization reversal of a CoPt/Permalloy/Ta/Permalloy heterostructure. Here, Stoner-Wohlfarth-type uniform magnetization rotation of the virtually free Permalloy layer and exchange spring behavior of the strongly pinned Permalloy layer are found in the same sample. We investigate the complex magnetization reversal by polarized neutron reflectometry, magnetometry, and magneto-transport. The synergy of combining these experimental methods together with theoretical modeling is key to obtain the complete quantitative depth resolved information of the magnetization reversal processes for a multilayer of mesoscopic thickness.

]]> Yi Wang et al. http://digitalcommons.unl.edu/physicsbinek/75 http://digitalcommons.unl.edu/physicsbinek/75 Fri, 04 May 2012 14:01:38 PDT Artificial environment-friendly Gd-free magnetic nanostructures for magnetic cooling are investigated by temperature-dependent magnetic measurements. We consider two-phase nanocomposites where nanoclusters (Fe) are embedded in a Ni₆₁Cu₃₉ matrix. Several composite films are produced by cluster deposition. The average Fe cluster size depends on the deposition conditions and can be tuned by varying the deposition conditions. The quasiequilibrium Curie temperature of the Fe particles is high, but slightly lower than that of bulk Fe due to finite-size effects. Our experiments have focused on ensembles of 7.7 nm Fe clusters in a matrix with a composition close to Ni₆₁Cu₃₉, which has a T_C of 180 K. The materials are magnetically soft, with coercivities of order 16 Oe even at relatively low temperature of 100 K. The entropy changes are modest, -DS¼0.05 J/kg K in a field change of 1 T and 0.30 J/kg K in a field change of 7 T at a temperature of 180 K, which should improve if the cluster size is reduced.

]]> Steven A. Michalski et al. http://digitalcommons.unl.edu/physicsbinek/74 http://digitalcommons.unl.edu/physicsbinek/74 Fri, 04 May 2012 13:55:32 PDT Spin polarization is an unusually ambiguous scientific idiom and, as such, is rarely well defined. A given experimental methodology may allow one to quantify a spin polarization but only in its particular context. As one might expect, these ambiguities sometimes give rise to inappropriate interpretations when comparing the spin polarizations determined through different methods. The spin polarization of CrO₂ and Cr₂O₃ illustrate some of the complications which hinders comparisons of spin polarization values.

]]> Peter Dowben et al. http://digitalcommons.unl.edu/physicsbinek/73 http://digitalcommons.unl.edu/physicsbinek/73 Tue, 24 May 2011 12:49:03 PDT We report the direct observation of surface magnetization domains of the magnetoelectric Cr₂O₃ using photoemission electron microscopy with magnetic circular dichroism contrast and magnetic force microscopy. The domain pattern is strongly affected by the applied electric field conditions. Zero-field cooling results in an equal representation of the two domain types, while electric-field cooling selects one dominant domain type. These observations confirm the existence of surface magnetization, required by symmetry in magnetoelectric antiferromagnets.

]]> Ning Wu et al. http://digitalcommons.unl.edu/physicsbinek/72 http://digitalcommons.unl.edu/physicsbinek/72 Tue, 24 May 2011 12:42:27 PDT Spin polarization is an unusually ambiguous scientific idiom and, as such, is rarely well defined. A given experimental methodology may allow one to quantify a spin polarization but only in its particular context. As one might expect, these ambiguities sometimes give rise to inappropriate interpretations when comparing the spin polarizations determined through different methods. The spin polarization of CrO₂ and Cr₂O₃ illustrate some of the complications which hinders comparisons of spin polarization values.

]]> Peter Dowben et al. http://digitalcommons.unl.edu/physicsbinek/71 http://digitalcommons.unl.edu/physicsbinek/71 Tue, 24 May 2011 12:38:25 PDT Artificial environment-friendly Gd-free magnetic nanostructures for magnetic cooling are investigated by temperature-dependent magnetic measurements. We consider two-phase nanocomposites where nanoclusters (Fe) are embedded in a Ni₆₁Cu₃₉ matrix. Several composite films are produced by cluster deposition. The average Fe cluster size depends on the deposition conditions and can be tuned by varying the deposition conditions. The quasiequilibrium Curie temperature of the Fe particles is high, but slightly lower than that of bulk Fe due to finite-size effects. Our experiments have focused on ensembles of 7.7 nm Fe clusters in a matrix with a composition close to Ni₆₁Cu₃₉, which has a T_C of 180 K. The materials are magnetically soft, with coercivities of order 16 Oe even at relatively low temperature of 100 K. The entropy changes are modest, –ΔS = 0.05 J/kg K in a field change of 1 T and 0.30 J/kg K in a field change of 7 T at a temperature of 180 K, which should improve if the cluster size is reduced.

]]> Steven A. Michalski et al. http://digitalcommons.unl.edu/physicsbinek/70 http://digitalcommons.unl.edu/physicsbinek/70 Fri, 01 Apr 2011 12:07:49 PDT Magnetization relaxation is investigated in a structurally ordered magnetic Co/Cr superlattice. Tailored nanoscale periodicity creates mesoscopic spatial magnetic correlations with slow relaxation dynamics when quenching the system into a nonequilibrium state. Magnetization transients are measured after exposing the heterostructure to a magnetic set field for various waiting times. Scaling analysis reveals an asymptotic powerlaw behavior in accordance with a full aging scenario. The temperature dependence of the relaxation exponent shows pronounced anomalies at the equilibrium phase transitions of the antiferromagnetic superstructure and the ferromagnetic to paramagnetic transition of the Co layers. The latter leaves only weak fingerprints in the equilibrium magnetic behavior but gives rise to a prominent change in nonequilibrium properties. Our findings suggest scaling analysis of nonequilibrium data as a probe for weak equilibrium phase transitions.

]]> Tathagata Mukherjee et al. http://digitalcommons.unl.edu/physicsbinek/69 http://digitalcommons.unl.edu/physicsbinek/69 Fri, 01 Apr 2011 12:03:39 PDT Piezoelectrically controlled strain is used for electric tuning of exchange-bias fields. A generic exchange-bias Co/CoO bilayer is deposited on the surface of a ferroelectric and thus piezoelectric BaTiO₃ substrate which allows to apply electrically and thermally tunable stress in the adjacent ferromagnetic Co thin film. The stress-induced strain alters foremost the magnetic anisotropy of the Co film and by that the magnetization orientation at the Co/CoO interface modifying the exchange-bias field. This results in a pronounced electrically induced weakening of the exchange bias but also includes the possibility of tuning the exchange-bias field through a subtle sign change from regular negative to positive values. The electrically controlled crossover from negative to positive exchange bias is consistently observed at various temperatures in the rhombohedral phase of BaTiO₃. This complex electric field dependence of the exchange-bias field is the result of the long-range nature of strain and interpreted through competition between ferromagnetic and antiferromagnetic exchange at the Co/CoO interface. Our data suggest competition between regular negative and positive exchange bias. Weakening of negative exchange bias originates from noncollinear alignment of the Co and CoO interface magnetizations. Positive exchange bias is activated when stress induces antiferromagnetic exchange through atomic displacements changing the exchange paths at the Co/CoO interface.

]]> S. Polisetty et al. http://digitalcommons.unl.edu/physicsbinek/68 http://digitalcommons.unl.edu/physicsbinek/68 Mon, 02 Aug 2010 12:41:41 PDT Voltage-controlled spin electronics is crucial for continued progress in information technology. It aims at reduced power consumption, increased integration density and enhanced functionality where non-volatile memory is combined with highspeed logical processing. Promising spintronic device concepts use the electric control of interface and surface magnetization. From the combination of magnetometry, spin-polarized photoemission spectroscopy, symmetry arguments and first-principles calculations, we show that the (0001) surface of magnetoelectric Cr2O3 has a roughness-insensitive, electrically switchable magnetization. Using a ferromagnetic Pd/Co multilayer deposited on the (0001) surface of a Cr2O3 single crystal, we achieve reversible, room-temperature isothermal switching of the exchange-bias field between positive and negative values by reversing the electric field while maintaining a permanent magnetic field. This effect reflects the switching of the bulk antiferromagnetic domain state and the interface magnetization coupled to it. The switchable exchange bias sets in exactly at the bulk Néel temperature.

]]> Xi He et al. http://digitalcommons.unl.edu/physicsbinek/66 http://digitalcommons.unl.edu/physicsbinek/66 Fri, 19 Jun 2009 09:18:15 PDT Nanostructured materials for refrigeration applications are experimentally realized by molecular beam epitaxial growth of Co/Cr superlattices using mean-field theoretical concepts as guiding principles. Magnetocaloric properties are deduced from measurements of the temperature and field dependence of the magnetization of our samples. More generally, the potential of artificial antiferromagnets for near room-temperature refrigeration is explored. The effects of intraplane and interplane exchange interactions on the magnetic phase diagram in Ising-type model systems are revisited in mean-field considerations with special emphasis on tailoring magnetocaloric properties. The experimental results are discussed in light of our theoretical findings, and extrapolations for future improved nanostructures are provided.

]]> Tathagata Mukherjee et al. http://digitalcommons.unl.edu/physicsbinek/65 http://digitalcommons.unl.edu/physicsbinek/65 Fri, 19 Jun 2009 09:18:14 PDT Manipulation of magnetically ordered states by electrical means is a promising approach towards novel spintronics devices. We report on the electric control of surface magnetism in Cr2O3 thin films and uniaxial anisotropy in ferroelectric/ferromagnetic heterostructures, respectively. Artificial magnetoelectricity is realized in a BaTiO3/Fe heterostructure. Here, thermally induced coercivity changes of the Fe hysteresis loop are used to derive the stress imposed by the ferroelectric BaTiO3 substrate on the adjacent Fe film. Electrically induced coercivity changes give rise to a giant magnetoelectric susceptibility in the vicinity of the magnetic coercive field.

]]> Christian Binek et al. http://digitalcommons.unl.edu/physicsbinek/64 http://digitalcommons.unl.edu/physicsbinek/64 Fri, 19 Jun 2009 09:18:12 PDT Exchange bias (EB) and its associated training effects are studied in an epitaxial Fe(10 nm)/Cr2O3(2.7 nm)/Fe(10 nm) trilayer heterostructure grown by molecular beam epitaxy. The EB decreases linearly with increasing temperature from T = 5 K to T = 50 K. It changes sign and becomes positive within 50 K < T < 200 K, finally changing back to regular EB for T<200K up to the highest measured temperature of T = 395 K. Remarkably, the latter is far above the bulk Néel temperature TN = 307 K. EB training effects occur only at 5 K < T < 50 K. We show that this training can be quenched by subjecting the system to DC magnetic field, µ0HDC <= 7 T. The applied field most likely induces a temperature dependent spin-flop transition. Upon its removal the antiferromagnetic Cr2O3 pinning layer evolves uniformly into its quasi-equilibrium spin configuration thus leading to quasi-equilibrium EB.

]]> S. Sahoo et al. http://digitalcommons.unl.edu/physicsbinek/63 http://digitalcommons.unl.edu/physicsbinek/63 Fri, 19 Jun 2009 09:18:11 PDT In this paper, we report on the magnetic properties of isolated nanoparticles and interacting nanochains formed by the self-assembly of Ni nanoparticles. The magnetic properties were studied using superconducting quantum interference device (SQUID) magnetometry and magnetic force microscopy (MFM). We demonstrate that single-domain Ni nanoparticles spontaneously form one-dimensional (1D) chains under the influence of an external magnetic field. Furthermore, such magnetic field-driven self-assembly in conjunction with surface templating produces regular arrays of 1D nanochains with antiferromagnetic intra-chain order. The antiferromagnetic order, which is in striking contrast to what is found for non-interacting nanoparticle assemblies within the chains, can be evidenced from MFM and SQUID measurements.

]]> V. Bliznyuk et al. http://digitalcommons.unl.edu/physicsbinek/62 http://digitalcommons.unl.edu/physicsbinek/62 Tue, 07 Apr 2009 09:08:28 PDT The temperature dependence of the training effect is studied in an exchange coupled thin-film bilayer composed of a hard ferromagnetic pinning (CoPtCrB) layer in proximity of a soft ferromagnetic pinned (CoCr) layer. Interlayer exchange shifts the hysteresis loops of the soft layer along the magnetic-field axis. This shift is quantified by the bias field in far reaching analogy to the exchange bias field of conventional antiferromagnetic/ferromagnetic heterostructures. A ferromagnetic domain state induced in the hard layer experiences aging very similar to the training behavior of the antiferromagnetic domain state in conventional exchange bias systems. Training originates from changes in the spin structure of the pinning layer with consecutive magnetization reversals of the pinned layer. Here we perform a detailed investigation of the temperature dependence of the bias field and its training effect. Consecutively cycled hysteresis loops of the soft layer are measured at various temperatures. We also derive a theoretical description of the temperature dependence of the training effect which is in agreement with the experimental data.

]]> Christian Binek et al. http://digitalcommons.unl.edu/physicsbinek/61 http://digitalcommons.unl.edu/physicsbinek/61 Tue, 07 Apr 2009 09:08:27 PDT The temperature dependence of the training effect is studied in an exchange coupled thin-film bilayer composed of a hard ferromagnetic pinning (CoPtCrB) layer in proximity of a soft ferromagnetic pinned (CoCr) layer. Interlayer exchange shifts the hysteresis loops of the soft layer along the magnetic-field axis. This shift is quantified by the bias field in far reaching analogy to the exchange bias field of conventional antiferromagnetic/ferromagnetic heterostructures. A ferromagnetic domain state induced in the hard layer experiences aging very similar to the training behavior of the antiferromagnetic domain state in conventional exchange bias systems. Training originates from changes in the spin structure of the pinning layer with consecutive magnetization reversals of the pinned layer. Here we perform a detailed investigation of the temperature dependence of the bias field and its training effect. Consecutively cycled hysteresis loops of the soft layer are measured at various temperatures. We also derive a theoretical description of the temperature dependence of the training effect which is in agreement with the experimental data.

]]> S. Polisetty et al. http://digitalcommons.unl.edu/physicsbinek/60 http://digitalcommons.unl.edu/physicsbinek/60 Mon, 02 Jun 2008 12:44:14 PDT We present a comparative study on an experimental and theoretical optimization of magneto-optical Kerr setups based on photoelastic modulation and phase sensitive detector methodology. The first and second harmonics, Iomega,2omega, of the reflected light intensity are measured for a CoO/Co magnetic reference film. The magnetic field dependence of the optical off-diagonal Fresnel reflection coefficients rps and rsp follows the sample magnetization. Different Kerr setups provide various dependencies of Iomega,2omega on the reflection coefficients and, hence, on the Kerr ellipticity epsilonK and rotation thetaK. Jones matrix formalism has been used to analyze the impact of a systematic variation of relative analyzer and polarizer orientations with respect to each other and with respect to the retardation axis of the modulator involved in longitudinal Kerr setups for incoming s-polarized light. We find one particular setup which maximizes Iomega as well as I2omega and maximizes the signal-to-noise ratio. Inefficient setups are characterized by Iomega,2omega intensities involving large nonmagnetic contributions of rp and rs.

]]> Christian Binek http://digitalcommons.unl.edu/physicsbinek/58 http://digitalcommons.unl.edu/physicsbinek/58 Tue, 22 Jan 2008 09:34:20 PST Stress-induced perturbation of the antiferromagnetic long-range order in epitaxially grown (111) Cr2O3 thin films gives rise to pronounced piezomagnetism and a significant reduction of the antiferromagnetic ordering temperature. The temperature dependence of the piezomagnetic moment measured by superconducting quantum interference device magnetometry reveals a power law behaviour with a critical exponent 2
¼0.66 in accordance with the Ising anisotropy of a three-dimensional system. The observed shift of the Ne´el temperature allows estimating the internal lateral stress which is in excellent agreement with an independent estimate based on the elastic properties of Cr2O3 and the lattice mismatch at the interface between the sapphire substrate and the isostructural (111) Cr2O3 thin film. The isothermal freezing field dependence of the piezomoment is interpreted in terms of Zeeman energy arguments. Implications of the piezomagnetic effects for spintronic devices and the investigation of the piezomagnetoelectric effect are briefly discussed.

]]> Sarbeswar Sahoo et al. http://digitalcommons.unl.edu/physicsbinek/57 http://digitalcommons.unl.edu/physicsbinek/57 Tue, 11 Dec 2007 11:33:57 PST Reversible control of magnetism is reported for a Fe thin film in proximity of a BaTiO₃ single crystal. Large magnetization changes emerge in response to ferroelectric switching and structural transitions of BaTiO₃ controlled by applied electric fields and temperature, respectively. 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 BaTiO₃. Up to 20% coercivity change is achieved via electrical control at room temperature. Our all solid state ferroelectric-ferromagnetic heterostructures open viable possibilities for technological applications.

]]> S. Sahoo et al. http://digitalcommons.unl.edu/physicsbinek/56 http://digitalcommons.unl.edu/physicsbinek/56 Fri, 07 Dec 2007 11:18:39 PST The dependence of the exchange-bias training effect on temperature and ferromagnetic film thickness is studied in detail and scaling behavior of the data is presented. Thickness-dependent exchange bias and its training are measured using the magneto-optical Kerr effect. A focused laser beam is scanned across a Co wedge probing local hysteresis loops of the Co film which is pinned by an antiferromagnetic CoO layer of uniform thickness. A phenomenological theory is best fitted to the exchange-bias training data resembling the evolution of the exchange-bias field on subsequently cycled hysteresis loops. Best fits are done for various temperatures and Co thicknesses. Data collapse on respective master curves is achieved for the thickness and temperature-dependent fitting parameters as well as the exchange bias and coercive fields of the initial hysteresis loops. The scaling behavior is strong evidence for the validity and the universality of the underlying theoretical approach based on triggered relaxation of the pinning layer towards quasiequilibrium.

]]> Christian Binek et al.