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

Waste of batteries management: Synthesis of magnetocaloric manganite compound from the REEs mixture generated during hydrometallurgical processing of NiMH batteries

In the present study, rare earth elements (REEs, i.e., La, Ce, Nd, and Pr) were hydrometallurgically recovered in oxalate form with presence of very low concentration of Co, Al, Zn and Ni from solution after processing of spent Nickel metal hydride (Ni-MH) batteries. The recovered mixture was used as alternative source in the synthesis of magnetocaloric materials. In this study, a manganite sample with general formula ABO3 was selected to be prepared since it is relatively easy to synthesize and is tuneable by adjustment of the doping concentration. The conventional solid-state reaction method was used to prepare an orthorhombic structure of manganite with presence of REE2O3 and MnO2 as secondary phases reported from x-ray pattern at room temperature. The thermomagnetic measurements showed a PM to FM transition at 184 K in a 0.01 T magnetic field that shifts to 194 K by increasing the magnetic field to 1.8 T. The magnetocaloric properties were determined by calculating the isothermal entropy change and directly measuring the adiabatic temperature change. A reversible magnetocaloric effect was observed

Direct and indirect measurement of magnetocaloric effect in NiCoMnGa alloys

The Co-substitution for Ni in the Mn-rich NiMnGa Heusler alloys changes substantially their structural and magnetic ordering. The results will be discussed within the basic thermodynamic relations

Co and In Doped Ni-Mn-Ga Magnetic Shape Memory Alloys: A Thorough Structural, Magnetic and Magnetocaloric Study

In Ni-Mn-Ga ferromagnetic shape memory alloys, Co-doping plays a major role in determining a peculiar phase diagram where, besides a change in the critical temperatures, a change of number, order and nature of phase transitions (e.g., from ferromagnetic to paramagnetic or from paramagnetic to ferromagnetic, on heating) can be obtained, together with a change in the giant magnetocaloric effect from direct to inverse. Here we present a thorough study of the intrinsic magnetic and structural properties, including their dependence on hydrostatic pressure, that are at the basis of the multifunctional behavior of Co and In-doped alloys. We study in depth their magnetocaloric properties, taking advantage of complementary calorimetric and magnetic techniques, and show that if a proper measurement protocol is adopted they all merge to the same values, even in case of first order transitions. A simplified model for the estimation of the adiabatic temperature change that relies only on indirect measurements is proposed, allowing for the quick and reliable evaluation of the magnetocaloric potentiality of new materials starting from readily available magnetic measurements

Pressure dependence of magnetism and martensitic properties in Co-doped NiMnGa alloys

NiMnGa alloys display several "giant" effects due to the interplay between magnetic and structural degrees of freedom. A large magnetization difference, between martensitic phases and austenitic phases iso f great importance to obtain higher performances due to the improbe possibilit? of driving structural trasformations by magnetic fields. A deltaM enhancement was recently achieved by adding Co to Mn-rich NiMnGa alloy

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

Role of interface and morphology in the magnetic behaviour of perpendicular thin films based on L10 FePt

FePt L10 ordered alloy is a promising material for high-density magnetic recording, since it allows the ferromagnetic stability in particles of few nanometers. Here we present our recent studies on the correlation between magnetic and morphological/interfacial properties of FePt -based thin films, nanostructures, and nano-composite bilayers. L10 FePt (001) epitaxial thin films with high structural quality were grown on (100) MgO by sputtering r.f., using the alternate-layer deposition method. By playing with growth temperature on the one hand and post-annealing temperature and time on the other, we have been able to finely control epitaxy, structural order, and morphology from 3D laterally confined structures to continuous film, with desired grain size. In particular we have been able to decrease grain size and to optimise magnetic properties (increase of anisotropy/coercivity ratio) at the same time, by post-annealing in situ [1]. Laterally confined magnetic structures were also obtained by focused ion beam (FIB). We have shown that for suitable Ga+ doses (1?1014 ion/cm2), it is possible to transform the L10 ordered phase to the A1 disordered one, without affecting morphology, giving rise to substantial modifications of magnetic properties from hard to soft. Perpendicular 2D magnetic patterns (dots, stripes) in a soft easy-plane matrix were realized in films of continuous morphology [2]. FePt L10 has also been exploited as the hard layer of nanostructured hard-soft nanocomposite bilayers. The exploitation of the exchange-coupling between hard and soft layers in exchange-coupled media represents a possible approach to overcome the so-called "recording trilemma" [3]. The samples were prepared by growing a magnetically soft Fe layer (2 and 3.5 nm) over a hard FePt(001) layer (10 nm). Three bilayers series have been grown based on FePt epitaxial layers with high degree of chemical order (S≥0.76) and different morphologies, corresponding to different interface characteristics. The resulting hard layer anisotropy is high (K>1?107 erg/cm3), and the coercivity is increased by the grains separation (from 1.7 to 3 T). In the Fe/FePt bilayers the coercivity HC is strongly reduced compared to the hard layer value (HC/HChard down to 0.37), indicating that high anisotropy perpendicular systems with moderate coercivity can be obtained [4]. Moreover, the control of the interface morphology allows to modify the magnetic regime at fixed Fe thickness (Rigid Magnet to Exchange-Spring), due to the nanoscale structure effect on the hard/soft coupling, and to tailor the hysteresis loop characteristics