Gómez, M. E., G. Campillo, J. - G. Ramirez, A. Hoffmann, and J. Guimpel. "Detailed magnetic and structural properties of exchange-biased La1-xCaxMnO3." physica status solidi (c) 4, no. 11 (2007): 4181–4187.
Abstract: We measured structural, magnetic and, magneto-transport properties of heterostructures consisting of La1–xCaxMnO3 ferromagnetic (FM) layers (x = 0.33) and antiferromagnetic (AF) layers (x = 0.67). We grew FM/AF superlattices via a high-pressure sputtering technique on (001) oriented SrTiO3 substrates. We systematically varied the thickness of the ferromagnetic layers, while maintaining a fixed thickness of the antiferromagnetic layers. Alternatively, we also varied the thickness of the antiferromagnetic layers, keeping a fixed ferromagnetic layer thickness. The total superlattice thickness was kept approximately constant. XRD analysis confirms the existence of the superlattice structure by the observation of multiple satellite peaks around the (00l) manganite Bragg reflections. We extracted the average lattice parameter of the superlattice and its dependence on the thicknesses of the ferro- and antiferromagnetic layers. We conducted field cooling (FC) and zero field cooling (ZFC), magneto-thermal, and magneto-resistance measurements. The existence of an exchange bias effect at temperatures below the Néel temperature of the AF layer was revealed by magnetization loops measured after FC. We studied the dependence of magnetic parameters on the thicknesses of the ferro- and antiferromagnetic layers. We found that the structural, magnetization, and magneto transport properties as functions of F-layer thickness are all correlated. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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Prieto, P., M. E. Gomez, G. Campillo, A. Berger, E. Baca, R. Escudero, F. Morales, J. Guimpel, and N. Haberkorn. "Exchange-coupling effect and magnetotransport properties in epitaxial La2/3Ca1/3MnO3/La1/3Ca2/3MnO3 superlattices." PHYSICA STATUS SOLIDI A-APPLIED RESEARCH 201, no. 10 (2004): 2343–2346.
Abstract: We have measured structural, magnetic and, magneto transport properties of heterostructures consisting of La1-xCaxMnO3 ferromagnetic (FM) layers (x = 0.33) and antiferromagnetic (AF) layers (x = 0.67). FM/AF superlattices were grown by a high-pressure sputtering technique on (001) oriented SrTiO3 substrates. We have systematically varied the thickness of the ferromagnetic layers, while maintaining the thickness of the antiferromagnetic layers fixed. The total superlattice thickness was held approximately constant. The XRD analysis confirmed the existence of the superlattice structure by the multiple satellite peaks around the 001 manganite Bragg reflections. We have done field cooling (FC) and zero field cooling (ZFC) magnetization, and magneto resistance measurements at temperatures between 10 K and 280 K. The existence of an exchange bias effect at temperatures below the Neel temperature of the AF layer was revealed by magnetization loops after field cooling. The exchange bias field magnitude H-ex exhibited exponential temperature dependence below the blocking temperature, as well as an inverse proportionality with the ferromagnetic layer thickness. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
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Monton, C., F. de la Cruz, and J. Guimpel. "Magnetic state modification induced by superconducting response in ferromagnet/superconductor Nb/Co superlattices." PHYSICAL REVIEW B 77, no. 10 (2008).
Abstract: Magnetization measurements in superconductor/ferromagnet Nb/Co superlattices show a complex behavior as a function of temperature, applied field, and sample history. Based on a simple model, it is shown that this behavior is due to an interplay between the superconductor magnetization temperature dependence, the ferromagnet magnetization time dependence, and the stray fields of both materials. It is also shown that the magnetic state of the Co layers is modified by the Nb superconducting response, implying that the problem of a superconductor/ferromagnetic heterogeneous sample has to be solved in a self-consistent manner.
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Monton, C., F. de la Cruz, and J. Guimpel. "Magnetic behavior of superconductor/ferromagnet superlattices." PHYSICAL REVIEW B 75, no. 6 (2007).
Abstract: A study of the electromagnetic response of superconductor/ferromagnet Nb/Co superlattices is presented. The dc magnetization is measured as a function of Co layer thickness, applied field H-a, and temperature. For very thin Co layers, no ferromagnetism is detected and the observed response is essentially that of a collection of uncoupled Nb layers. When the Co layer is thick enough for ferromagnetism to be established, prior to the field-cooled magnetization measurements, the Co layers are magnetically saturated in the parallel or antiparallel directions to H-a. In this case, the magnetic response of the otherwise uncoupled Nb layers is found to depend on both H-a and the stray fields.
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Haberkorn, N., F. Lovey, A. M. Condo, G. Nieva, and J. Guimpel. "Glasslike behavior at the PrBa2Cu3O7/La0.75Sr0.25MnO3 interface." PHYSICAL REVIEW B 75, no. 2 (2007): 024427.
Abstract: We study the crystalline structure and the magnetic properties of two different PrBa2Cu3O7-delta/La0.75Sr0.25MnO3 (PBCO/LSr0.25MO) superlattices grown by dc magnetron sputtering on (100) SrTiO3. The microstructure was studied by high-resolution transmission electron microscopy (HRTEM). Magnetic, magnetotransport, and time dependence measurements indicate a behavior compatible with phase separation at the interface, strain induced by the lattice parameter difference, and the presence of a structural transition in the manganite material. Spin glasslike blocking behavior was found at low temperatures, below 70 K.
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Stoll, O. M., M. I. Montero, J. Guimpel, J. J. Akerman, and I. K. Schuller. "Hysteresis and fractional matching in thin Nb films with rectangular arrays of nanoscaled magnetic dots." PHYSICAL REVIEW B 65, no. 10 (2002).
Abstract: We have investigated the periodic pinning of magnetic flux quanta in thin Nb films with rectangular arrays of magnetic dots. In this type of pinning geometry, a change in the periodicity and shape of the minima in the magnetoresistance occurs for magnetic fields exceeding a certain threshold value. This was explained recently in terms of a reconfiguration transition of the vortex lattice due to an increasing vortex-vortex interaction with increasing magnetic field. In this picture the dominating elastic energy at high fields forces the vortex lattice to form a square symmetry, rather than being commensurate with the rectangular geometry of the pinning array. In this paper we present a comparative study of rectangular arrays with Ni dots, Co dots, and holes. In the magnetic dot arrays we found a strong fractional matching effect up to the second-order matching field. In contrast, no clear fractional matching is seen after the reconfiguration. Additionally, we discover the existence of hysteresis in the magnetoresistance in the crossover between the low- and high-field regimes. We find evidence that this effect is correlated with the reconfiguration phenomenon rather than to the magnetic state of the dots. The temperature and angular dependences of the effect are measured, and possible models are discussed to explain this behavior.
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Steren, L. B., M. Sirena, and J. Guimpel. "Magnetic ordered phase in La0.6Sr0.4MnO3 ferromagnets." PHYSICAL REVIEW B 65, no. 9 (2002).
Abstract: The ferromagnetic phase of La0.6Sr0.4MnO3 thin films has been investigated through measurements of magnetization loops at different temperatures, zero-field-cooled and field-cooled magnetization curves obtained under different magnetic fields. We have found that the main sources of the “bulk” coercivity in manganite films are the film/substrate interface and film surface. The temperature dependence of the coercivity is described by a “strong domain-wall pinning” model, independently of the thickness and substrate. The magnetization-vs-temperature curves, measured under different magnetic fields (10 Oe<H<2.5 kOe), have been explained in terms of the magnetic hysteresis of the films. We find no evidence of glass states or of the existence of single-domain clusters, as suggested by other authors.
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Sirena, M., L. B. Steren, and J. Guimpel. "Magnetic relaxation in bulk and film manganite compounds." PHYSICAL REVIEW B 64, no. 10 (2001).
Abstract: We have investigated the time dependence of the magnetic and transport properties of La0.6Sr0.4MnO3 films and bulk samples. A significant magnetic aftereffect has been observed in all the samples through magnetization and resistivity measurements. The relaxation effects can be described by the addition of a logarithmic to an exponential contribution. The weight of each contribution is strongly temperature dependent, with a relaxation that is predominantly logarithmic at low temperatures (T<30 K) and exponential at high temperatures (T>100 K). As the film thickness is decreased the magnetic relaxation becomes more important and the magnetic viscosity of the system increases. These results point out the important role of structural defects in the magnetic relaxation of the samples.
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Fainstein, A., P. Etchegoin, and J. Guimpel. "Raman study of photoinduced chain-fragment ordering in GdBa2Cu3Ox thin films." PHYSICAL REVIEW B 58, no. 14 (1998): 9433–9439.
Abstract: Raman scattering experiments on oxygen deficient GdBa2Cu3Ox thin films (x=6.53, 6.7, 6.8, and 6.93) as a function of photoexcitation and annealing induced oxygen disorder are presented. Raman lines, unambiguously assigned to copper and oxygen vibrations involving atoms at the end of Cu-O chains, are used as markers for the existence of short chain fragments. The dynamics of chain conjunction and fragmentation was monitored by the peak intensity dependences with photoexcitation time and annealing temperature. These results demonstrate the presence of photoassisted oxygen ordering in these high-T-c superconductors, and give definitive support for its ultimate role in the phenomena of persistent photoconductivity and photoinduced superconductivity. [S0163-1829(98)04037-5].
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Pardo, F., A. P. Mackenzie, F. delaCruz, and J. Guimpel. "Effect of the reversibility region on the low-temperature vortex structure imaged by Bitter magnetic decoration." PHYSICAL REVIEW B 55, no. 21 (1997): 14610–14613.
Abstract: We have combined magnetic measurements and imaging of the vortex lattice at low fields in Tl2Ba2CuO6-delta single crystals with different oxygen content. Those with high critical temperature T-c > 80 K have a reversible region while those with low T-c < 30 K have an irreversible magnetization in the whole range of temperatures, up to T-c. The presence of the reversible region allows the formation of ordered vortex structures, while amorphous structures are obtained when no reversible region is present below T-c. The results are discussed in terms of vortex nucleation processes in the individual pinning regime.
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