The exchange-bias properties of ferromagnetic, either Co or Ni, thin films deposited onto polycrystalline multiferroic CuO are investigated. After field cooling, the CuO/Co magnetization hysteresis loops show exchange bias at temperatures lower than 200 K, while the CuO/Ni system exhibits bias below about 5 K only. It is suggested that the exchange bias of CuO/Co is determined mainly by the magnetization reversal that takes place on the descending branch of the loop. Rather high values of both the interface coupling energy, 0.89 erg/cm(2), and coercivity, 2.44 kOe, of the CuO/Co film are obtained at 5 K. (C) 2017 Elsevier B.V. All rights reserved.

Proper estimation of magnetization curve parameters is vital in studying magnetic systems. In the present article, criteria for discrimination non-saturated (minor) from saturated (major) hysteresis loops are proposed. These employ the analysis of (i) derivatives of both ascending and descending branches of the loop, (ii) remanent magnetization curves, and (iii) thermomagnetic curves. Computational simulations are used in order to demonstrate their validity. Examples illustrating the applicability of these criteria to well-known real systems, namely Fe3O4 and Ni fine particles, are provided. We demonstrate that the anisotropy-field value estimated from a visual examination of an only apparently major hysteresis loop could be more than two times lower than the real one.

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.

This work investigates the effects of inserting an ultrathin ferromagnetic Py or Co spacer layer (SL) at the interface of ferromagnet/antiferromagnet films, namely Co/SL(Py)/IrMn and Py/SL(Co)/IrMn. The exchange bias field, HEB, of the two series of films changes with the spacer thickness in very different manners. Whereas HEB of the Py/SL(Co)/IrMn series decreases monotonously with the Co layer thickness, a fourfold initial bias enhancement is obtained for Py spacers with thicknesses up to 1 nm. Moreover, the Co/SL(Py)/IrMn series shows a maximum HEB more than 50% higher than that of the Py/IrMn film which, in turn, is almost three times higher than the HEB of the control Co/IrMn film. Simulations of experimental magnetization curves through a polycrystalline model for exchange bias showed that these variations, which could yield improvement in the performance of modern spintronic devices, are predominantly governed by modifications of the interfacial exchange interactions. Several mechanisms that could be responsible for the observed behavior are discussed.

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 an Angstrom 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.

This paper reports experimental and model magnetization results obtained on exchange-coupled ferromagnet/antiferromagnet (FM/AF) bilayers that show zero net bias. The coercivity of the films, either irradiated with He or implanted with Ge ions at 40 keV, varies significantly with the fluence used. We employed the remanence plots technique in order to estimate the nature of the interactions present and check if there exists a correlation between their type and the coercivity variations. The analysis of the remanence plots through numerical simulations based on the Landau-Lifshitz-Gilbert equation demonstrated that outcomes of interactions within the FM layer could be distinguished from those coming from coupling at the FM/AF interface and that demagnetizing interaction effects could be achieved without the presence of dipolar interactions. Our findings indicate that such experiments could give selective information on modifications caused by a post-deposition treatment in each layer of the film.

Efforts have been recently made to use remanence plots in exchange-bias studies. However, since the two remnant magnetizations of a biased loop may differ, this technique cannot be applied in its classical form. This work extends it to systems with shifted loops and shows that the number of distinct plots is significantly increased. The approach was probed on Co/IrMn exchange bias films. Possible discrepancies between experiment and theory are pointed out and discussed. The adaptation of the model presented here enables it to become one of the few accessible techniques for rapid and accurate evaluation of magnetic interactions in biased systems.

S. Nicolodi,1 L. G. Pereira,1 A. Harres,1 G. M. Azevedo,1 J. E. Schmidt,1 I. Garcı́a-Aguilar,2 N. M. Souza-Neto,2 C. Deranlot,3 F. Petroff,3 and J. Geshev1 1Instituto de Fı́sica, URFGS, Porto Alegre, 91501-970 Rio Grande do Sul, Brazil 2Laboratório Nacional de Luz Sı́ncrotron (LNLS), Campinas, 13083-970 São Paulo, Brazil 3Unité Mixte de Physique CNRS/Thales, 91767 Palaiseau and Université Paris-Sud, 91405 Orsay, France (Received 18 March 2012; published 29 June 2012)

The present work reports experimental and theoretical results for electrodeposited Co/Au(111) ultrathin layers with very specific magnetic behavior. We show that the observed two peaks in the out-of-plane magnetization versus deposition time variation could be explained by the remarkably high perpendicular anisotropy of the perimeter atoms of low-dimensional islands formed during the layer-by-layer growth, as compared to that of the surface atoms. Our results indicate that it is possible to sustain high anisotropy in very small grains without coming across the superparamagnetic limit, opening excellent opportunities for materials engineering.

This paper reports experimental results obtained on unconventional exchange-coupled ferromagnet/antiferromagnet (FM/AF) system showing zero net bias. The Curie temperature of the FM (NiCu) is lower than the blocking temperature of the AF (IrMn). Samples were either annealed or irradiated with He, Ar, or Ge ions at 40 keV. Due to the exchange coupling at the FM/AF interface, the coercivity (HC) of the as-deposited FM/AF bilayer is rather higher than that of the corresponding FM single layer. We found that by choosing a proper ion fluence or annealing temperature, it is possible to controllably vary HC. Ion irradiation of the FM single layer has lead to only a decrease of HC and annealing or He ion irradiation has not caused important changes at the FM/AF interface; nevertheless, a twofold increase of HC was obtained after these treatments. Even more significant enhancement of HC was attained after Ge ion irradiation and attributed to ion-implantation-induced modification of only the FM layer; damages of the FM/AF interface, on the other hand, decrease the coercivity.

We investigate the importance of using nanotips on a point contact spin-transfer torque (STT) experiment. A systematic analysis comparing the STT in a magnetic thin film in current-perpendicular-to-plane (CPP) geometry sample for magnetic coated and uncoated tungsten nanotips is shown. The STT effect presents a reverse resistance to current behavior when using a magnetic coating layer on the nanotips. We demonstrate that the magnetic layer on the tip may assume the role of a polarizer layer. This effect opens up the possibility of exploiting simpler architectures in STT-based devices, such as STT-random access memory (STT-RAM).

In this work we studied the magnetic, resistive and magnetoresistive properties of thin iron films deposited on vicinal silicon substrates. The film surfaces were characterized by atomic force microscopy which revealed formation of elongated stripes. We show that the morphological anisotropic structure of the films influences significantly both their magnetic and electronic transport.

Electrical, magnetic and magnetotransport properties were measured in a Co–Al2O3 granular film. Electrical resistance was obtained in the low-field regime (e ΔV⪡kBT) under variation of injected current and bias and as a function of temperature and bias. Electrical properties were best described with the model of variable range hopping, where the electrical resistance decreases and the electronic localization length increases with increase of the applied bias and/or current. We associate this behavior to the activation of new electronic paths between more distant grains, reducing the total resistance whilst additional parallel paths are formed. This behavior is similar to results obtained with Fe–Al2O3 granular thin films, which however have a higher range of resistance variation.

This paper reports results obtained on exchange-biased IrMn/Al2O3/Co films deposited by magnetron sputtering, where the thickness of the non-magnetic insulator layer, tAl2O3, was varied. Ferromagnetic resonance and static magnetization measurements were used to study the exchange interaction between the antiferromagnet (IrMn) and ferromagnet (Co) layers. X-ray diffractometry and x-ray reflectometry as well as high-resolution transmission electron microscopy were employed for structural characterization of the films. It was found that the IrMn/Co exchange coupling decreases very abruptly with tAl2O3 being the exponential decay length at least four times smaller than the values previously reported for exchange-bias systems. Such a rapid suppression of the coupling is explained in terms of the prompt loss of the direct contact between the antiferromagnet and the ferromagnet with tAl2O3.

The present study discusses on modification of the exchange-bias parameters due to magnetic annealing during 15min of sputtered IrMn/Co and IrMn/Cu(0.25nm)/Co films. The annealing temperature Tann has been systematically varied in the presence of sufficiently strong magnetic fields to ensure saturation of the ferromagnetic layers. Broad maxima at Tann≈200°C have been observed in the in-plane exchange-bias field Heb versus Tann variations for both series of samples. Numerical simulations have been employed in order to fit the magnetization curves in both easy and hard axis configurations from which the magnetic anisotropy parameters were extracted. This work indicates that, in the films studied, an improvement in the bulk IrMn crystallinity and/or grain growth, which may lessen the number of uncompensated spins at the last-grown IrMn sublattice, seems to be a plausible candidate to account, in part, for the observed decrease of Heb after rapid anneal for Tann between 200 and 300°C. This mechanism might be considered as an alternative and/or adding to the commonly observed IrMn and Co interdiffusion.

This work reports on the evolution of the exchange-bias (EB) field and the shape of the magnetization curves with the thickness of the non-magnetic spacer layer (SL, either Cu or Al2O3) of polycrystalline Co/IrMn/SL/Co films as well as their modifications caused by different post-deposition annealing treatments. Conventional x-ray diffractometry, small-angle x-ray reflectivity and cross-section transmission electron microscopy were used for the structural characterization. The hysteresis loops traced on the as-made films without SL present a pattern of two oppositely displaced subloops; it was observed that the positively shifted subloop gradually vanishes with the increase in the SL thickness. Our study points out that the significant decrease in the EB field of the top-pinned Co layers after magnetic annealing could be attributed to relaxation of the bottom interfacial IrMn spin structure caused by the heating. This relaxation mechanism might be considered as an alternative to the commonly accepted IrMn and Co interdiffusion and defect creation at the interface. Models for the uncompensated spins' configurations at each of the ferromagnet (FM)/antiferromagnet (AF) and AF/FM interfaces are proposed in order to explain the modifications of the magnetic behaviour with the SL thickness and with the annealing.

This paper reports experimental results obtained on an unconventional exchange bias (EB) system where the ferromagnetic layer, Ni0.75Cu0.25, has lower ordering temperature (TC) than that (TN) of the antiferromagnetic one, NiO, with emphasis on modifying EB through either magnetic annealing or light-ion irradiation. Samples were cooled from temperatures higher than TN or in between TC and TN to room temperature with magnetic field applied in different in-plane directions. Upon ion irradiation, magnetic fields, parallel or antiparallel to the orientation of the field present during the films deposition, were applied to explore different effects on EB. We found that the EB direction can be completely reversed by means of either annealing or ion bombardment; however, both postdeposition treatments provide very little variation of the EB field value over that produced during the film’s growth. The importance of the annealing field strength was also discussed. The results were interpreted based on a mechanism which assumes that the interfacial moments adjacent to the antiferromagnetic layer are responsible for establishing the exchange biasing in the paramagnetic state.

Co ( 3 nm ) / IrMn ( 15 nm ) / Cu ( d Cu ) / Co ( 7 nm ) films were subjected to magnetic annealing where its temperature and duration as well as the direction and amplitude of the applied field were varied. We demonstrate that the exchange-bias field magnitude and sign of the subloop of the bottom-pinned Co layer can be tailored in a controlled manner allowing the whole hysteresis loop to be tuned from a double negatively/negatively shifted to a double negatively/positively shifted with the shifts of the subloops in antiphase.

The perpendicular exchange bias and magnetic anisotropy were investigated in IrMn/Pt/[Co/Pt]3 multilayers through the analysis of in-plane and out-of-plane magnetization hysteresis loops. A phenomenological model was used to simulate the in-plane curves and the effective perpendicular anisotropies were obtained employing the area method. The canted state anisotropy was introduced by taking into account the first and second uniaxial anisotropy terms of the ferromagnet with the corresponding uniaxial anisotropy direction allowed to make a nonzero angle with the film's normal. This angle, obtained from the fittings, was of approximately 15° for IrMn/[Co/Pt]3 film and decreases with the introduction of Pt in the IrMn/Pt/ [Co/Pt]3 system, indicating that the Pt interlayer leads to a predominant perpendicular anisotropy. A maximum of the out-of-plane anisotropy was found between 0.5 and 0.6nm of Pt, whereas a maximum of the perpendicular exchange bias was found at 0.3nm. These results are very similar to those obtained for IrMn/Cu/[Co/Pt]3 system; however, the decrease of the exchange bias with the spacer thickness is more abrupt and the enhacement of the perpendicular anisotropy is higher for the case of Cu spacer as compared with that of Pt spacer. The existence of a maximum in the perpendicular exchange bias as a function of the Pt layer thickness was attributed to the predominance of the enhancement of exchange bias due to more perpendicular Co moment orientation over the exponential decrease of the ferromagnetic/antiferromagnetic exchange coupling and, consequently, of the exchange-bias field.

The present work reports on Fe thin films grown on vicinal Si(111) substrates via rf magnetron sputtering. The dependencies of the growth mode and magnetic properties of the obtained iron nanostructures on both crystallographic surface orientation and on the direction of the very weak stray magnetic field from the magnetron gun were studied. Scanning tunneling microscopy images showed strong dependence of the Fe grains' orientation on the stray field direction in relation to the substrate's steps demonstrating that, under appropriately directed magnetic field, Si surfaces can be used as templates for well-defined self-assembled iron nanostructures. Magneto-optical Kerr effect hysteresis loops showed an easy-axis coercivity almost one order of magnitude smaller for the film deposited with stray field applied along the steps, accompanied with a change in the magnetization reversal mode. Phenomenological models involving coherent rotation and/or domain-wall unpinning were used for the interpretation of these results. (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3172926]