MAGNETIC INTERACTIONS AT INTERFACES AND CONFINED SYSTEMS
People
Sergio D’Addato
Alessandro di Bona
Paola Luches
Sergio Valeri
In modern society, applications of magnetic materials can be found almost everywhere and our daily life is intimately tied to magnetism and magnetic materials. The most noteworthy impact of magnetism occurs via information transport and data storage devices, which mostly consist of artificially nanostructured magnetic materials. Here artificially structured materials refer to the materials which are either made into reduced dimensions such as two-dimensional (2D) ultrathin films, one-dimensional (1D) wires and zero-dimensional (0D) particles, or assemblies of these low-dimensional structures. Interesting phenomena come about by imposing spatial confinement at lengths that are comparable to some internal length scale of the material used, as spin diffusion length, carrier mean free path, magnetic domain extension and domain wall width. Magnetic nanostructures by virtue of their extremely small size possess therefore very different properties from their parent bulk materials.
Confined magnetic materials have been utilized in technologies with a large and growing economic impact. The magnetic recording industry is continuously pushing the technology as evidenced by the fact that the density of information storage has been steadily increasing at a compound annual rate exceeding 60% per year. This progress has been made possible by a series of scientific and technological advances, mostly marked by the synthesis of artificially nanostructured magnetic materials. With the ever improved knowledge of low-dimensional magnetism, particularly the correlation between the magnetic, electronic and structural properties, we are achieving an ability to design and to fabricate low-dimensional materials with the desired magnetic properties. The ability to tailor magnetism is therefore strictly related to the ability to fabricate low dimensional objects.
At CnrNano S3, various aspects of the physics of confined magnetic systems are studied. The research focuses on 2D, 1D, and 0D systems. A few examples are given in the followings.
FePt graded media obtained by ion irradiation
FePt L10 is an ordered alloy constituted of Fe and Pt sublattices, where the latter plays a primary role in determining the huge values of magnetocrystalline anisotropy (>106 J/m3). This property makes the material very promising for a variety of advanced application, including perpendicular magnetic media for ultrahigh density recording . If high anisotropy is beneficial for thermal stability at small grain size, however it introduces the problem of writability of the media.
Anisotropy graded media, i.e., magnetic materials where the magnetocrystalline anisotropy changes continuously as a function of the spatial coordinate switching, has therefore been introduced to respond to the increasing demand of storage density in magnetic recording. Continuously graded systems are expected to yield the largest reduction of the switching field with respect to layered systems without reducing the thermal stability of the grain. On the experimental side, an intentionally controlled anisotropy gradient is quite hard to obtain.
At CnrNano S3, in collaboration with Cnr Imem, a novel procedure for the fabrication of anisotropy graded perpendicular media based on low energy (0.5 ÷ 5 keV range), low dose (1013 ÷ 1015 ions/cm2), noble-gas ion irradiation has been developed.
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Fig 1: Hysteresis loops of FePt L10 samples before (top) and after (bottom) irradiation. Film thickness is 10 nm, 15 nm, 15 nm, 20 nm from left to right; irradiation angle is 85° for g), 40° for all the other samples. Magnetic field is applied perpendicular to the film plane (black line), and parallel to the film plane (red line).
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Interfacial magnetic structure in Fe/NiO(001)
Nanocomposite magnetic systems composed of different phases, each exhibiting a specific magnetic behavior and coupled by their mutual interaction, can be designed to obtain artificial materials with novel and potentially useful magnetic properties.
Among these systems of particular interest are coupled ferromagnetic (FM)/antiferromagnetic (AFM) materials, which present a unidirectional anisotropy, known as exchange bias.
An insightful understanding and control of the magnetic properties of nanocomposite systems requires a description of the materials at the atomic scale. A general quantitative explanation of the exchange bias is still not available, in spite of the great effort by many researchers and the numerous studies performed; there is a consensus that this is mostly due to a poor understanding of the interface between the two materials in terms of its local atomic structure and magnetic couplings.
In general, the magnetic structure of FM/AFM interfaces has to be taken into consideration in the models to correctly predict the overall magnetic behavior of the investigated system. By using NRS in grazing incidence with a probe layer approach and ab initio calculation of hyperfine fields, we have resolved the magnetic structure at and near the Fe/NiO(001) interface, a well-characterized ferromagnetic/antiferromagnetic coupled system.
We obtained direct evidence for the presence of an antiferromagnetic FeO-like phase, confined over one or two layers at the interface, with a significantly increased magnetic moment compared to the case of a sharp interface. The layers inside, although metallic with bulk-like hyperfine parameters, are strongly coupled to the antiferromagnetic phase. As a consequence, the magnetization reversal of the metallic component at the interface and inside the film is very similar.
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Fig 2: NRS time spectra measured at selected values of the applied magnetic field H along the decreasing-field branch of the hysteresis loop for (a) sample with Fe57 marker layer in the Fe film and (b) sample with Fe57 marker layer at the Fe/NiO interface at RT. The topmost spectrum is acquired at +200 mT along the increasing-field branch of the loop. The magnetic field H is applied collinear to the photon wave vector k.
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Morphology and magnetic properties of size-selected Ni nanoparticles
Metal nanostructured films are intensively studied because of their fascinating physical properties and their potential in various applications, like magnetic recording, catalysis, and tribology.
Different methods for producing nanoparticles assemblies have been developed: chemical synthesis, lithography-based top-down and self-assembling bottom-up techniques, atom deposition, and thermally assisted precipitation in matrix. More recently, the possibility of depositing preformed gas-phase nanoparticles onto surfaces allowed a systematic study of magnetic anisotropy, interparticle exchange interaction, superparamagnetic and ferromagnetic behavior as a function of particle mass and of film thickness. This investigation is necessary for fundamental and technological reasons, as control of the size and of the shape of the nanoparticles is highly desirable, in order to understand their basic magnetic properties and to obtain new functional materials. Deposition of preformed gas-phase nanoparticles has been used extensively to produce Fe and Co nanostrucured and granular thin films, giving rise to systems that show a variety of complex magnetic phases depending on particle size, volume fraction, and temperature. Here we present the results of an investigation on preformed and mass-selected Ni nanoparticles films grown on inert substrates.
We chose Si(100) for its low surface roughness, and because it does not affect the magnetic properties of the nanoparticles films, which makes it an ideal substrate for our experiments. The deposition was obtained by making use of a gas aggregation cluster source and a quadrupole mass filter (QMF). We produced Ni nanoparticles films having different average thickness, corresponding to regimes where the nanoparticles are dispersed or they constitute a nanostructured film.
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Fig 3: SEM image of Ni nanoparticles deposited on Si/SiOx substrate. SEM images from the same sample, with a higher magnification. Lateral size distribution of the particles shown in the histogram. The histogram was fitted with a log-normal function. The average lateral size is estimated to be 5.5 nm.
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Magneto-crystalline and configurational anisotropy in micromagnets
n single-crystal Fe square elements the interplay between the magnetocrystalline and configurational anisotropies is not trivial. The symmetry of the overall anisotropy is obtained by measuring the transverse magnetic susceptibility along different directions, as shown in the figure. In such plots, the maxima and minima correspond to easy and hard axes directions, respectively.
We demonstrated that the total anisotropy of single crystal square elements is not a mere summation of independent contributions. The interplay between magneto-crystalline and configurational anisotropy can give rise to enhancement or reduction of the configurational contributions and provide guidance about how to play with the size and orientation of single crystal square magnets to obtain confined systems with peculiar anisotropy symmetries.
Publications
Interfacial magnetic structure in Fe/NiO(001)
P. Luches, L. Pasquini, S. Benedetti, V. Bellini, S. Valeri, F. Manghi, R. Rüffer, and F. Boscherini
Phys. Rev. B 83, 094413 (2011)
Role of interface morphology in the exchange-spring behavior of FePt/Fe perpendicular bilayers
F. Casoli, L. Albertini, S. Nasi, R. Fabbrici, R. Cabassi, F. Bolzoni, C. Bocchi, and P. Luches
Acta Materialia 58, 3594 (2010)
Morphology and magnetic properties of size-selected Ni nanoparticle films
S. D’Addato, L. Gragnaniello, S. Valeri, A. Rota, A. di Bona, F. Spizzo, T. Panozaqi, and S.F. Schifano
Journ Appl Phys 107, 104318 (2010)
Magnetic couplings and exchange bias in Fe/NiO epitaxial layers
P. Luches, S. Benedetti, A. di Bona, and S. Valeri
Phys Rev B 81, 054431 (2010)
Depth-dependent magnetic characterization of Fe films on NiO(001)
P. Luches, S. Benedetti, L. Pasquini, F. Boscherini, M. Zajac, J. Korecki, R. Rüffer, and S. Valeri
Nuclear Instruments & Methods in Physics Research B 268, 361 (2010)
Fe/NiO(100) and Fe/MgO(100) interfaces studied by X-ray absorption spectroscopy and non-linear Kerr effect
S. Colonna, A. Cricenti, P. Luches, S. Valeri, F. Boscherini, J. Qi, Y. Xu, N. Tolk
Superlattices and microstructures 46, 107 (2009)
Morphology-induced magnetic phase transitions in Fe deposits on MgO films investigated with XMCD and STM
P. Torelli, S. Benedetti, P. Luches, L. Gragnaniello, J. Fujii, and S. Valeri
Phys Rev B 79, 035408 (2009)
Local modifications of magnetism and structure in FePt (001) epitaxial thin films by focused ion beam: Two-dimensional perpendicular patterns
F. Albertini, L. Nasi, F. Casoli, S. Fabbrici, P. Luches, G.C. Gazzadi, A. di Bona, P. Vavassori, S. Valeri, and S.F. Contri
J. Appl. Phys. 104, 053907 (2008)
Morphology evolution and magnetic properties improvement in FePt epitaxial films by in situ annealing after growth
F. Casoli, L. Nasi, F. Albertini, S. Fabbrici, C. Bocchi, F. Germini, P. Luches, A. Rota, and S. Valeri
J. Appl. Phys. 103, 043912 (2008)
Self-organized growth of Ni nanoparticles on a cobalt-oxide thin film induced by a buried misfit dislocation network
P. Torelli, E. A. Soares, G. Renaud, L. Gragnaniello, S. Valeri, X. X. Guo, and P. Luches
Phys. Rev. B 77, 081409(R) (2008)
Arrays of metal nanostructures produced by focussed ion beam
P. Luches, A. Di Bona, S.F. Contri, et al.
Acta Physica Polonica A 112, 1297 (2007)
Ferromagnetic-antiferromagnetic Fe/NiO (100) interface studied by non-linear Kerr effect
S. Colonna, F. Ronci, A. Cricenti, P. Luches, S. Valeri, J. Qi, Y. Xu, J.K. Miller, and N. Tolk
Surface Science 601, 4362 (2007)
Magnetic anisotropy engineering in square magnetic elements
A. di Bona, S.F. Contri, G.C. Gazzadi, S. Valeri, and P. Vavassori
J Magn Magn Mater 316, 106 (2007)
Grain size reduction and magnetic properties improvement by in situ annealing of FePt epitaxial thin films
F. Albertini, L. Nasi, F. Casoli, S. Fabbrici, P. Luches, A. Rota, and S. Valeri
J Magn Magn Mater 316, E158 (2007)
Magnetoresistance of single Permalloy circular rings
P. Vavassori, A. Busato, A. Chiapatti, A. di Bona, S. Valeri, V. Metlushko, and B. Ilic
J Magn Magn Mater 316, E944 (2007)
Magnetic linear dichroism studies of in situ grown NiO thin films
S.R. Krishnakumar, M. Liberati, C. Grazioli, M. Veronese, S. Turchini, P. Luches, S. Valeri, C. Carbone
J Magn Magn Mater 310, 8 (2007)
Effects of structural nonplanarity on the magnetoresistance of Permalloy circular rings
P. Vavassori, A. Busato, A. Chiapatti, A. di Bona, S. Valeri, V. Metlushko, and B. Ilic
J. Appl. Phys. 101, 043901 (2007)
Interplay between magnetocrystalline and configurational anisotropies in Fe(001) square nanostructures
P. Vavassori, D. Bisero, F. Carace, A. di Bona, G.C. Gazzadi, M. Liberati, and S. Valeri
Phys. Rev. B 72, 054405 (2005)
Magnetocrystalline and configurational anisotropies in Fe nanostructures
P. Vavassori, D. Bisero, F. Carace, M. Liberati, A. di Bona, G.C. Gazzadi, and S. Valeri
J Magn Magn Mater 290, 183 (2005)
Submicron-scale patterns on ferromagnetic-antiferromagnetic Fe/NiO layers by focused ion beam (FIB) milling
G.C. Gazzadi, P. Luches, S.F. Contri, A. di Bona, and S. Valeri
Nuclear Instruments & Methods in Physics Research B 230, 512 (2005)