The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spu... more The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spun Co 10 Cu 90 granular alloys were investigated. The interaction effects were studied from both remanent magnetization and magnetotransport data, using two different methods to reach the demagnetized state, ac and dc demagnetization. The analysis of the structural evolution and interaction strength between the magnetic clusters clearly shows the role of some structural parameters ͑particle size and density, interparticle distance͒ and the degree of magnetic correlation in the magnetic field response of the resistance in these inhomogeneous systems.
We report here the results of magnetotransport and electrical resistivity (ρ) measurements in the... more We report here the results of magnetotransport and electrical resistivity (ρ) measurements in the temperature range of 4.2-320 K and in the presence of magnetic fields up to 10 T on the Ru-doped, bilayered manganite system, La 1.2 Ca 1.8 Mn 2-x Ru x O 7 (0 Յ x Յ 1). We find that the Ru doping affects the magnetotransport properties considerably. The ρ versus H data were analyzed by fitting the data to the power-law equation, ρ ϭ ρ 0-αH n. The isothermal magnetoresistance (MR) versus H curves taken up to Ϯ10 T are highly symmetrical, and their curvature changes from concave up to concave down as the temperature increases. The MR, defined as [ρ(H)-ρ(0)]/ρ(0), is found to increase with Ru doping from 58% to 64% up to x ϭ 0.1 and to decrease to 45% for the x ϭ 1 sample at 10 K. Analysis of the ρ-T data below 30 K shows that, at low temperature, the system behaves like a disordered metal.
We present depth-resolved experimental results on the atomic and electronic structures of the Co-... more We present depth-resolved experimental results on the atomic and electronic structures of the Co-Cr interface on four IrMn/Cr/Co thin films with variable thickness of the Cr layer. Grazing incidence X-ray absorption near edge structure near the Cr K-edge was used, and anÅngstrom resolved depth-profile for this layer was obtained. An interdiffusion between chromium and cobalt layers was observed in all films, being more pronounced for samples with thinner Cr layers , where Cr behaves as an amorphous material. This causes a contraction in coordination distances in Cr near the interface with Co. In this region, a change in the electronic structure of chromium's 3d orbitals is also observed, and it appears that Cr and Co form a covalent bond resulting in a CrCo alloy. Ab initio numerical simulations support such an interpretation of the obtained experimental results.
Cu 1−x FexO nanoparticles were prepared using a freeze-drying process followed by heat treatment.... more Cu 1−x FexO nanoparticles were prepared using a freeze-drying process followed by heat treatment. The particles were then characterized using Mössbauer spectroscopy and magnetization techniques. The results revealed complex magnetic behavior, which can be attributed to the presence of two different magnetic regions: the particle core, which has antiferromagnetic fluctuations, and the particle shell, where uncompensated spins are responsible for their superparamagnetic characteristics. At low temperatures, the moments freeze, revealing a ferromagnetic order for the shells and a dipolar magnetic interaction among the nanoparticles. In addition, an exchange-bias field revealed magnetic interactions between the core and the shell of the nanoparticles. The ferromagnetism observed in this system suggests that antiferromagnetic oxide matrices can be used for diluted magnetic semiconductor applications, if suitably doped.
Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straight... more Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the δM plots of γ-Fe 2 O 3 nanoparticles isolated by thick SiO 2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions, as well as offer new perspectives in the use of Henkel-and δM-plots to quantify the rather elusive inhomogeneous magnetizations states in nanoparticles.
The exchange bias properties of Co/CoO coaxial core/shell nanowires have been investigated with c... more The exchange bias properties of Co/CoO coaxial core/shell nanowires have been investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. Firstly, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting pre-defined magnetization states to the AFM, as corroborated by micromagnetic simulations. Secondly, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops, attributed to the AFM easy-axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, is thus experimentally and theoretically demonstrated. Post-print of: Salazar-Alvarez, German et al. "Tunable high-field magnetization in strongly exchange-coupled freestanding Co/CoO core/shell coaxial nanowires" in ACS applied materials & interfaces, Vol. 8 núm 34 (Agost 2016
Apart from being so far the only known binary multiferroic compound, CuO has a much higher transi... more Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of ≈50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric featu...
The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Prev... more The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Previous experiments on the same system have shown that, starting from the ascending branch of the first hysteresis loop, coherent magnetization rotation is the dominant reversal mechanism. This is confirmed by the performed numerical simulations, which also indicate that the training is predominantly caused by changes of the rotatable anisotropy parameters of uncompensated spins at the Co/CoO interface. Moreover, in contrast with what is commonly assumed, the exchange coupling between the rotatable spins and the ferromagnetic layer is stronger than the coupling between the ferromagnet and the spins responsible for the bias. Thus, uncompensated spins strongly coupled to the ferromagnet contribute to the coercivity rather than to the bias, whatever the strength of their magnetic anisotropy. V
The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spu... more The effects of annealing on the structural, magnetic, and magnetotransport properties of melt-spun Co 10 Cu 90 granular alloys were investigated. The interaction effects were studied from both remanent magnetization and magnetotransport data, using two different methods to reach the demagnetized state, ac and dc demagnetization. The analysis of the structural evolution and interaction strength between the magnetic clusters clearly shows the role of some structural parameters ͑particle size and density, interparticle distance͒ and the degree of magnetic correlation in the magnetic field response of the resistance in these inhomogeneous systems.
We report here the results of magnetotransport and electrical resistivity (ρ) measurements in the... more We report here the results of magnetotransport and electrical resistivity (ρ) measurements in the temperature range of 4.2-320 K and in the presence of magnetic fields up to 10 T on the Ru-doped, bilayered manganite system, La 1.2 Ca 1.8 Mn 2-x Ru x O 7 (0 Յ x Յ 1). We find that the Ru doping affects the magnetotransport properties considerably. The ρ versus H data were analyzed by fitting the data to the power-law equation, ρ ϭ ρ 0-αH n. The isothermal magnetoresistance (MR) versus H curves taken up to Ϯ10 T are highly symmetrical, and their curvature changes from concave up to concave down as the temperature increases. The MR, defined as [ρ(H)-ρ(0)]/ρ(0), is found to increase with Ru doping from 58% to 64% up to x ϭ 0.1 and to decrease to 45% for the x ϭ 1 sample at 10 K. Analysis of the ρ-T data below 30 K shows that, at low temperature, the system behaves like a disordered metal.
We present depth-resolved experimental results on the atomic and electronic structures of the Co-... more We present depth-resolved experimental results on the atomic and electronic structures of the Co-Cr interface on four IrMn/Cr/Co thin films with variable thickness of the Cr layer. Grazing incidence X-ray absorption near edge structure near the Cr K-edge was used, and anÅngstrom resolved depth-profile for this layer was obtained. An interdiffusion between chromium and cobalt layers was observed in all films, being more pronounced for samples with thinner Cr layers , where Cr behaves as an amorphous material. This causes a contraction in coordination distances in Cr near the interface with Co. In this region, a change in the electronic structure of chromium's 3d orbitals is also observed, and it appears that Cr and Co form a covalent bond resulting in a CrCo alloy. Ab initio numerical simulations support such an interpretation of the obtained experimental results.
Cu 1−x FexO nanoparticles were prepared using a freeze-drying process followed by heat treatment.... more Cu 1−x FexO nanoparticles were prepared using a freeze-drying process followed by heat treatment. The particles were then characterized using Mössbauer spectroscopy and magnetization techniques. The results revealed complex magnetic behavior, which can be attributed to the presence of two different magnetic regions: the particle core, which has antiferromagnetic fluctuations, and the particle shell, where uncompensated spins are responsible for their superparamagnetic characteristics. At low temperatures, the moments freeze, revealing a ferromagnetic order for the shells and a dipolar magnetic interaction among the nanoparticles. In addition, an exchange-bias field revealed magnetic interactions between the core and the shell of the nanoparticles. The ferromagnetism observed in this system suggests that antiferromagnetic oxide matrices can be used for diluted magnetic semiconductor applications, if suitably doped.
Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straight... more Remanence magnetization plots (e.g., Henkel or δM plots) have been extensively used as a straightforward way to determine the presence and intensity of dipolar and exchange interactions in assemblies of magnetic nanoparticles or single domain grains. Their evaluation is particularly important in functional materials whose performance is strongly affected by the intensity of interparticle interactions, such as patterned recording media and nanostructured permanent magnets, as well as in applications such as hyperthermia and magnetic resonance imaging. Here we demonstrate that δM plots may be misleading when the nanoparticles do not have a homogeneous internal magnetic configuration. Substantial dips in the δM plots of γ-Fe 2 O 3 nanoparticles isolated by thick SiO 2 shells indicate the presence of demagnetizing interactions, usually identified as dipolar interactions. Our results, however, demonstrate that it is the inhomogeneous spin structure of the nanoparticles, as most clearly evidenced by Mössbauer measurements, that has a pronounced effect on the δM plots, leading to features remarkably similar to those produced by dipolar interactions. X-ray diffraction results combined with magnetic characterization indicate that this inhomogeneity is due to the presence of surface structural (and spin) disorder. Monte Carlo simulations unambiguously corroborate the critical role of the internal magnetic structure in the δM plots. Our findings constitute a cautionary tale on the widespread use of remanence plots to assess interparticle interactions, as well as offer new perspectives in the use of Henkel-and δM-plots to quantify the rather elusive inhomogeneous magnetizations states in nanoparticles.
The exchange bias properties of Co/CoO coaxial core/shell nanowires have been investigated with c... more The exchange bias properties of Co/CoO coaxial core/shell nanowires have been investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. Firstly, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting pre-defined magnetization states to the AFM, as corroborated by micromagnetic simulations. Secondly, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops, attributed to the AFM easy-axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, is thus experimentally and theoretically demonstrated. Post-print of: Salazar-Alvarez, German et al. "Tunable high-field magnetization in strongly exchange-coupled freestanding Co/CoO core/shell coaxial nanowires" in ACS applied materials & interfaces, Vol. 8 núm 34 (Agost 2016
Apart from being so far the only known binary multiferroic compound, CuO has a much higher transi... more Apart from being so far the only known binary multiferroic compound, CuO has a much higher transition temperature into the multiferroic state, 230 K, than any other known material in which the electric polarization is induced by spontaneous magnetic order, typically lower than 100 K. Although the magnetically induced ferroelectricity of CuO is firmly established, no magnetoelectric effect has been observed so far as direct crosstalk between bulk magnetization and electric polarization counterparts. Here we demonstrate that high magnetic fields of ≈50 T are able to suppress the helical modulation of the spins in the multiferroic phase and dramatically affect the electric polarization. Furthermore, just below the spontaneous transition from commensurate (paraelectric) to incommensurate (ferroelectric) structures at 213 K, even modest magnetic fields induce a transition into the incommensurate structure and then suppress it at higher field. Thus, remarkable hidden magnetoelectric featu...
The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Prev... more The training effect for exchange bias in field-cooled Co/CoO bilayers films is investigated. Previous experiments on the same system have shown that, starting from the ascending branch of the first hysteresis loop, coherent magnetization rotation is the dominant reversal mechanism. This is confirmed by the performed numerical simulations, which also indicate that the training is predominantly caused by changes of the rotatable anisotropy parameters of uncompensated spins at the Co/CoO interface. Moreover, in contrast with what is commonly assumed, the exchange coupling between the rotatable spins and the ferromagnetic layer is stronger than the coupling between the ferromagnet and the spins responsible for the bias. Thus, uncompensated spins strongly coupled to the ferromagnet contribute to the coercivity rather than to the bias, whatever the strength of their magnetic anisotropy. V
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