Ferromagnetic resonance study of Fe/FePt coupled films with perpendicular anisotropy

Exchange spring magnets with perpendicular magnetic anisotropy represent new magnetic properties with respect to their constituent components. These systems typically consist of a hard magnetic layer and a soft magnetic layer which are strongly coupled. The modification of their bulk magnetic properties arises from this strong ferromagnetic exchange coupling, interfacial effects and competing magnetic anisotropies of the two magnetic layers. We have studied the magnetic bilayer system which consists of an Fe (soft) film exchange coupled to an FePt (hard) layer which has an easy axis aligned along the direction perpendicular to the film plane. The entire structure has the form: MgO/FePt (10 nm)/Fe (2nm or 3.5nm)/ Ag (2nm), where the Ag overlayer acts as protection against oxidation. The epitaxial FePt layers were deposit- ed on MgO (100) substrates using the RF sputtering technique at a substrate temperature of about 390 C. The epitaxy of this layer was studied using x-ray and electron diffraction techniques. Layer morphologies were further studied using atomic force microscopy (AFM), these studies reveal a granular morphology with grain sizes of the order of 40 - 50 nm. We have made detailed angular measurements using the ferromagnetic resonance (FMR) at room temperature. This angular FMR study, which includes the orientations of in-plane and out-of-plane, was performed in order to study the magnetic anisotropies as well as the exchange coupling between the magnetic layers and interfacial effects. In particular, we have chosen to study two samples with 2 nm and 3.5 nm of Fe, which effectively constitute the rigid magnet (RM) and exchange spring (ES) regimes, respectively. The RM and ES regimes depend implicitly on the magnetic anositropies and properties of the two coupled layers [1]. In figure 1 (a) we show an example of an FMR spectrum for the RM (2 nm Fe) sample. Of the various resonances observed, only the three low field lines are due to the Fe layer. It will be noted that the FePt does not have any FMR signature in the field range studied due to it very high magnetocrystalline anisotropy. The other resonance features evident in the spectrum arise from the MgO substrate and show no significant angular variations. As such the only FMR signals observed in our samples will arise from the Fe layer. In figure 1 (b) we show the angular variation of the resonance field of the three Fe resonance lines. Of these, two resonances display a uniaxial anisotropy with the easy axes aligned along the direction perpendicular to the film plane and will be directly related to the exchange coupling with the hard (FePt) layer. The third resonance, while also manifesting a uniaxial anisotropy, displays an easy axis direction which is canted by about 50 degrees from the film normal. While the origin of this resonance is not entirely clear, we suspect it may arise from the interfacial region between the FePt and Fe layers. In figure 2 we show the corresponding FMR results for the ES (3.5 nm Fe) sample. It will be noted that in addition to the resonances observed in the RM sample, there are a further two resonance, whose angular dependences are illustrated in figure 2 (b). These also display a uniaxial like behaviour with easy axes close to the film normal. In all spectra lines were fit using a home made programme which allows multiple peak fitting of Lorentzian and Gaussian lines. We develop a model of FMR based on the magnetic free energy of the coupled layers which is required to interpret the angular dependences of the resonance fields [1]. Existing models fall short of a full explanation of all the resonance lines and we are working to bridge this gap by considering the effects of boundary conditions and spin wave modes. [1] G. Asti et al., Phys. Rev. B, 73, 094406 (2006

Exploiting magnetic properties of Fe doping in zirconia

In this study we explore, both from theoretical and experimental side, the effect of Fe doping in ZrO2 (ZrO2:Fe). By means of first principles simulation we study the magnetization density and the magnetic interaction between Fe atoms. We also consider how this is affected by the presence of oxygen vacancies and compare our findings with models based on impurity band and carrier mediated magnetic interaction. Experimentally thin films (~ 20 nm) of ZrO2:Fe at high doping concentration are grown by atomic layer deposition. We provide experimental evidence that Fe is uniformly distributed in the ZrO2 by transmission electron microscopy and energy dispersive X-ray mapping, while X-ray diffraction evidences the presence of the fluorite crystal structure. Alternating gradient force magnetometer measurements show magnetic signal at room temperature, however with low magnetic moment per atom. Results from experimental measures and theoretical simulations are compared.Comment: 8 pages, 9 figures. JEMS 201

Magneto-optical circular dichroism properties of fept layers with perpendicular anisotropy

Magneto-optical techniques allow the investigation of the reversal process in magnetic surfaces and granular systems and of their electronic structure. In the case of magnetic metals and their surfaces the use of VIS or nIR lights allow to explore the interband and intrabands transitions that involves the 3d band. Due to the magneto-optical effect is related with the spin-orbit coupling, this technique is quite sensible to structural and chemical orders which determine also the magnetic anisotropy [1]. In this work we investigate the magneto-optical properties at different wavelengths of nanometric films based in epitaxial FePt and Fe-FePt bilayer that exhibit perpendicular anisotropy. Magnetic circular dichroism technique (MCD) is used because it allows to investigate the magneto-optical properties and the reversal process of the entire layers. FePt films of 10 nm were deposited by RF sputtering directly on a MgO (100) single-crystal in order to obtain the epitaxial growth. The growth was performed at substrate temperatures in the range 415?C and 430 ?C. The films were obtained by alternating the deposition of very thin Fe and Pt layers with nominal thickness of about 0.2 nm. The chosen ratio between the individual thickness corresponds to a nominal atomic composition of Fe53Pt47. The ordered L10 phase growths epitaxially [2] with the c-axis perpendicular to the substrate. Lower chemical order was observed in the film annealed at 430?C. On this film a second layer of 5 nm of Fe was deposited which constitutes the bilayer Fe-FePt. The MCD hysteresis loops at 1.7 K were recorded using different continuous lasers covering the VIS-nIR spectrum range (476 nm - 904 nm). Details of the experimental set-up are described in [3]. The MCD hysteresis loop of the FePt film annealed at 415?C and measured with a wavelength 476.5 nm is represented in the figure 1. A square hysteresis loop is observed with a negative saturation MCD (-5.3 mrad) for positive magnetic fields. The large squareness ratio, near 1, and the large coercive field of 2.9 T confirm the high quality of the ordered c-axis epitaxial film and the orientation of the easy axis in this direction. The shape of MCD hysteresis loop measured using 632.8 nm is very similar to the measured with 476.5 nm, but the saturation MCD is positive and approximately 5 times smaller (+1.18 mrad). In the figure 2 the MCD hysteresis of the Fe-FePt film measured with 514.5 nm, 632.8 nm and 904.0 nm are represented. The hysteresis loop measured with the blue beam exhibits positive MCD in the saturation while with the red and n-IR beams that values are negative. Moreover the absolute saturation MCD increases with the increase of the wavelength being 13.9 mrad, -20,6 mrad and -23,4 mrad for the beams with wavelength 514.5 nm, 632.8 nm and 904.0 nm, respectively. The obtained hysteresis indicate the presence of two critical field, HC1 ≈1.3 T and HC2 ≈0.64 T being the coercive field 0.13 T. The reversal process does not indicate a full exchange coupling between the hard and soft layers. In fact micromagnetic calculations [4] indicate that 5 nm Fe layer is a thick- ness for which decoupling could be possible. Finally the shape of the hysteresis loop measured with 514,5 nm is slightly different of the loops measured with largest wavelengths, which are equal. The MCD values measured with 514,5 nm in the magnetic field range between HC1 and HC2 are small- er than the measured with larger wavelength. This suggests that modification of the MO signal due to the change of the wavelength is not similar in the Fe and FePt layers. Comparing the results, quantitatively the Fe-FePt film shows largest MCD signal than the FePt film. This difference can be due to larger Fe contain but it is not enough for explain the differences. Moreover in the Fe-FePt film the MCD changes from positive to negative values for largest wavelengths and the absolute MCD increases. The opposite behaviours are observed in the FePt film. Spectroscopic measurements are in progress to clarify these results. [1] A. Cebollada et al. Phys. Rev. B 50 (1994) 3419; H. Ebert,G.Y. Guo, G. Sch?tz IEEE Trans. Magn. 31 (1995) 3301. [2] F. Casoli, et al. IEEE Trans. Magn. 41 (2005) 3223. [3] L. Cavigli et al. J. Magn. Magn. Mater. 316 (2007) 798. [4] G. Asti et al. Phys. Rev. B 73 (2006) 09440

Analysis of temporal expression of HTLV-2 reveals similarities and functional differences from HTLV-1

In the present study, we developed a robust splice site-specific real-time RT-PCR method to quantitate all HTLV-2 transcripts. Results of this analysis conducted on three different infected cell lines (HTLV-2A Mo-T , C344 and HTLV-2B BJAB-Gu) showed that the most abundant mRNA was gag/pol followed by the accessory transcript 1-3, coding for the p28 and for p22/p20 proteins. The third most abundant mRNA was tax/rex. To investigate if different mRNAs produced by HTLV-2 are expressed at different levels upon viral reactivation, we studied the kinetics of viral expression in PBMCs from three subjects infected with HTLV-2B and cultured in vitro for 48 hours. The level of expression of the full length gag/pol transcript was the highest in all samples. The tax/rex mRNA was detected already at time zero and increased very rapidly following in vitro culture, reaching the highest copy number between zero and 2-4 hours. The minus-strand APH-2 mRNA, was expressed at high level. As observed in the infected cell lines, the 1-3 mRNA was expressed at high levels in all subjects. This finding is particularly intriguing, as it encodes two proteins that were shown to exert a powerful control on Tax and Rex function. This peculiar pattern of expression, which is in striking contrast with that of HTLV-1, might in part explain the differential pathogenicity of the two viruses

Effect of Ag content on magnetic properties of (FePt)-Ag sputtered thin films

Ordered FePt thin films deserved particular attention owing to their very large magnetocrystalline anisotropy making them attractive in high-density magnetic recording. The addiction o fan immiscibile elements such Ag promotes the formation of a granular FePt phase displaying a significant magnetoresistence effect (MR). The effect of Ag addiction on the morphological and magnetic properties of the starting Fe33Pt47 system will be clarified

Magnetoresistance and energy model of Alq3-based spintronic devices

Spin transport in organic semiconductors has been receiving widespread attention since the first experimental demonstration of magnetoresistive effects (change in resistance under an applied magnetic field) in hybrid ferromagnetic/organic/ferromagnetic structures [1]. Continuous effort in the field has led to the realization, for example, of vertical organic spintronic devices with differ- ent organic semiconductor layers [2,3] or organic tunnel barriers [4]. However, there is still a lack of understanding on the mechanism that governs spin injection and transport in organics, leading to general disagreement even on the expected sign of the devices output magnetoresistance. With the aim to clarify the spin transport behaviour in organic semiconductors, we present new results on hybrid inorganic/organic spin valves with the most successful up-to-date combination of materials [2-6]. The highly spin polarized manganite La2/3Sr1/3MnO3 and Cobalt have been used as ferromagnetic electrodes for spin injection into thick layers (up to 200 nm) of tris(8-hydrox- yquinoline)aluminum(III) (Alq3). In a critical design improvement, we have for the first time intro- duced an artificial tunnel barrier (Al2O3 or LiF) between the organic and the Co top electrode to study its influence on spin injection into organic semiconductors and to improve the chemical sta- bility and reproducibility of the devices. In our manuscript we: explore the importance of artificial tunnel barriers for spin injection in organics, record room temperature magnetoresistance, demonstrate that only ferromagnetic elec- trodes and not organic semiconductor limit device output and, finally, sketch an energy diagram able to explain negative magnetoresistance in LSMO/Alq3/Co spin valves. Our work is a new step forward in organic spintronics, as we prove that organic semiconductors do not have a clear limit for room temperature performance with the adequate ferromagnets, and we present a reliable model that could be easily extrapolated to predict the output of different materi- als combinations in hybrid spin valves. [1] Dediu, V., Murgia, M., Matacotta, F.C., Taliani, C. & Barbanera, S. Sol. State Commun. 122, 181-184 (2002). [2] Xiong, Z.H., Wu, D., Vardeny, Z.V. & Shi, J. Nature 427, 821-824 (2004). [3] Majumdar, S., Majumdar, H.S., Laiho, R. & Osterbacka, R. J. Alloy & Compounds 423, 169-171 (2006). [4] Santos, T.S., Lee, J.S., Migdal, P., Lekshmi, I.C., Satpati, B. & Moodera, J.S. Phys. Rev. Lett. 98, 016601 (2007). [5] Xu, W., Szulczewski, G.J., LeClair, P., Navarrete, I., Schad, R., Miao, G., Guo, H. & Gupta, A. Appl. Phys. Lett. 90, 072506 (2007). [6] Hueso, L.E., Riminucci, A., Bergenti, I., Zhan, Y. & Dediu, V. Adv. Mater. 19, 2639-2642 (2007)

Molecular and phylogenetic analysis of HIV-1 variants circulating in Italy

<p>Abstract</p> <p>Objective</p> <p>The continuous identification of HIV-1 non-B subtypes and recombinant forms in Italy indicates the need of constant molecular epidemiology survey of genetic forms circulating and transmitted in the resident population.</p> <p>Methods</p> <p>The distribution of HIV-1 subtypes has been evaluated in 25 seropositive individuals residing in Italy, most of whom were infected through a sexual route during the 1995–2005 period. Each sample has been characterized by detailed molecular and phylogenetic analyses.</p> <p>Results</p> <p>18 of the 25 samples were positive at HIV-1 PCR amplification. Three samples showed a nucleotide divergence compatible with a non-B subtype classification. The phylogenetic analysis, performed on both HIV-1 <it>env </it>and <it>gag </it>regions, confirms the molecular sub-typing prediction, given that 1 sample falls into the C subtype and 2 into the G subtype. The B subtype isolates show high levels of <it>intra</it>-subtype nucleotide divergence, compatible with a long-lasting epidemic and a progressive HIV-1 molecular diversification.</p> <p>Conclusion</p> <p>The Italian HIV-1 epidemic is still mostly attributable to the B subtype, regardless the transmission route, which shows an increasing nucleotide heterogeneity. Heterosexual transmission and the interracial blending, however, are slowly introducing novel HIV-1 subtypes. Therefore, a molecular monitoring is needed to follow the constant evolution of the HIV-1 epidemic.</p