Ferromagnetic resonance (FMR) and the measurement of magnetization dynamics in general have become sophisticated tools for the study of magnetic systems at the nanoscale. In this work, we present a detailed investigation of Co/Ag nanodots with a 200 nm diameter arranged in a square pitch array with a periodicity of 400 nm, which, due to their size, can support standing spin-wave modes with complex spectral responses. To interpret the experimentally measured broadband FMR, we compare the spectra of the nanoarray structure with those of the unpatterned Co/Ag film of identical thickness, which serves as a baseline for obtaining the general magnetic parameters of the system. Using state-of-the-art simulations of the dynamic response to identify the nature of the excitation modes allows us to assess the boundary conditions for the nanodots. We then proceed to calculate the spectral response of our system, for which we obtained good agreement. A full description of the theoretical framework and its application to our system is provided and the novel frequency domain, matrix-free simulation method used is described in detail.

We present an experimental study on hybrid magnetic systems based on hardmagnetic NdCo x films with weak perpendicular magnetic anisotropy, which are dipolar coupled to a softmagnetic layer with in-plane magnetic anisotropy, such as permalloy, via a nonmagnetic Al spacer. Using broadband ferromagnetic resonance, we have investigated the magnetization dynamics in this sample system, which exhibits a host of unique magnetic properties including a frequency hysteresis. We provide an explanation for the origin of this hysteresis, which is intimately related to the regular magnetic hysteresis, by accounting for the switching between acoustic and optical resonances, that naturally occur in unsaturated systems with different magnetic domains. Our model is ultimately supported by a simple expression relating the frequency difference between these two types of modes at their transition field to the periodicity of the stripe domain pattern imprinted into the permalloy film. Moreover, we show that these unique features in the FMR spectra can also be observed when substituting the ferromagnetic metal permalloy for a ferrimagnetic insulator such as yttrium iron garnet.

Reconfigurable magnetization textures offer control of spin waves with promising properties for future low-power beyond-CMOS systems. However, materials with perpendicular magnetic anisotropy (PMA) suitable for stable magnetization-texture formation are characterized by high damping, which limits their applicability in magnonic devices. Here, we propose to overcome this limitation by using hybrid structures, i.e., a PMA layer magnetostatically coupled to a low-damping soft ferromagnetic film. We experimentally show that a periodic stripe-domain texture from a PMA layer is imprinted upon the soft layer and induces a nonreciprocal dispersion relation of the spin waves confined to the low-damping film. Moreover, an asymmetric bandgap features the spin-wave band diagram, which is a clear demonstration of collective spin-wave dynamics, a property characteristic for magnonic crystals with broken time-reversal symmetry. The composite character of the hybrid structure allows for stabilization of two magnetic states at remanence, with parallel and antiparallel orientation of net magnetization in hard and soft layers. The states can be switched using a low external magnetic field; therefore, the proposed system obtains an additional functionality of state reconfigurability. This study offers a link between reconfigurable magnetization textures and low-damping spin-wave dynamics, providing an opportunity to create miniaturized, programmable, and energy-efficient signal processing devices operating at high frequencies.

Ferromagnetic resonance is a powerful method for the study of all classes of magnetic materials. The experimental technique has been used for many decades and is based on the excitation of a magnetic spin system via a microwave (or rf) field. While earlier methods were based on the use of a microwave spectrometer, more recent developments have seen the widespread use of the vector network analyzer (VNA), which provides a more versatile measurement system at almost comparable sensitivity. While the former is based on a fixed frequency of excitation, the VNA enables frequency-dependent measurements, allowing more in-depth analysis. We have applied this technique to the study of nanostructured thin films or nanodots and coupled magnetic layer systems comprised of exchange-coupled ferromagnetic layers with in-plane and perpendicular magnetic anisotropies. In the first system, we have investigated the magnetization dynamics in Co/Ag bilayers and nanodots. In the second system, we have studied Permalloy (Ni80Fe20, hereafter Py) thin films coupled via an intervening Al layer of varying thickness to a NdCo film which has perpendicular magnetic anisotropy.

Magnetic nanodot structures have significant potential in spintronic applications. The magnetism of nanodots are predominately influenced by external conditions such as temperature. While the magnetization reversal modes of sub-100 nm Fe nanodots has been reported, currently there is a lack of information about the temperature evolution of the magnetic properties of larger Fe nanodots. In this work, the magnetism of Fe/Ag thin films and nanodot arrays were comparatively analyzed over the temperature range of 2-300 K. A natively-oxidized alpha-Fe2O3 layer was formed at the Fe/SiO2 interface, and this oxide resulted in an exchange bias at low temperatures. The inplane and out-of-plane magnetization reversal modes of nanodot arrays were revealed through the magnetic hysteresis loops' temperature dependence. The contribution of exchange coupling between Fe and alpha-Fe2O3 was also identified through the temperature dependence of the coercivity, exchange bias, and DC susceptibility. Our results provide new insights towards understanding the magnetization dynamics in nanosized magnetic elements.

Resumen del poster presentado al 10th International Conference on Metamaterials, Photonic Crystals and Plasmonics (META), celebrado en Lisboa (Portugal) del 23 al 26 de julio de 2019.

A novel approach to tune the ferromagnetic resonance frequency of a soft magnetic Ni$_{80}$Fe$_{20}$ (Permalloy = Py) film with in-plane magnetic anisotropy (IMA) based on the controlled coupling to a hard magnetic NdCo$_\text{x}$ film with perpendicular magnetic anisotropy (PMA) through a non-magnetic Al spacer is studied. Using transverse magneto-optical Kerr effect (TMOKE), alternating gradient magnetometry (AGM) as well as vector network analyzer ferromagnetic resonance (VNA-FMR) spectroscopy, the influence of both Co concentration and Al spacer thickness on the static and dynamic magnetic properties of the coupled IMA/PMA system is investigated. Compared to a single Py film, two striking effects of the coupling between IMA and PMA layers can be observed in their FMR spectra. First, there is a significant increase in the zero-field resonance frequency from 1.3 GHz up to 6.6 GHz, and second, an additional frequency hysteresis occurs at low magnetic fields applied along the hard axis. The maximum frequency difference between the frequency branches for increasing and decreasing magnetic field is as high as 1 GHz, corresponding to a tunability of about 20% at external fields of typically less than $\pm$70 mT. The origin of the observed features in the FMR spectra is discussed by means of magnetization reversal curves.

Characterizing the dynamic magnetic properties of nanoscale magnetic thin films, multilayers, and nanostructures is crucial for exploiting their potential for practical applications such as in logic and microwave devices operating in the GHz region. Among the various techniques suitable for high-frequency characterization, ferromagnetic resonance spectroscopy (FMR) is widely considered as one of the gold standards. In its most advanced version, broadband vector network analyzer (VNA) FMR, it represents the perfect tool for detailed and accurate analysis of magnetic damping processes. In the first part of this talk, a compact introduction to FMR including underlying physics as well as technical details will be given, followed by a short presentation of a recently built VNA-FMR setup at UVSQ. In the second and longer part, both static and dynamic magnetic properties of two selected material systems will be discussed in depth, with focus on data obtained from VNA-FMR measurements. The first study involves Fe/Ag continuous films and nanodot arrays of various sizes prepared for elucidating the mechanisms of a potentially existing magneto-plasmonic coupling. The second sample system consisting of trilayers of Py/Al/NdCo is investigated with support from micromagnetic simulations with the purpose of studying the influence of competing anisotropies, in-plane for Permalloy (Py) and out-of-plane for NdCo, on its magnetic properties.

The magnetic domain contrast in wide-field Kerr microscopy on bulk specimens can be substantially distorted by non-linear, field-dependent Faraday rotations in the objective lens that are caused by stray-field components emerging from the specimen. These Faraday contributions, which were detected by Kerr-magnetometry on grain-oriented iron–silicon steel samples, are thoroughly elaborated and characterized. They express themselves as a field-dependent gray-scale offset to the domain contrast and in highly distorted surface magnetization curves if optically measured in a wide field Kerr microscope. An experimental method to avoid such distortions is suggested. In the course of these studies, a low-permeability part in the surface magnetization loop of slightly misoriented (110)-surfaces in iron–silicon sheets was discovered that is attributed to demagnetization effects in direction perpendicular to the sheet surface.

We have determined the material parameters of optimized synthetic ferrimagnet (SyF) free layers in magnetic tunnel junctions by means of magneto-resistance loops as well as microwave noise spectroscopy under constant voltage, and the field dependence thereof. By comparing the experimental data with calculated loops and spin wave modes from a 2-macrospin model, we have deduced the saturation magnetization, anisotropy, damping, and interlayer exchange coupling. From waiting time experiments of field-induced switching, the energy barrier relevant for the thermally activated switching of the free SyF has been experimentally evaluated and compared to an existing model in order to assess its consistency.

We report on the coupling of spin vortices in magnetic multilayer elements. The magnetization distribution in thin film disks consisting of two ferromagnetic layers separated by a nonmagnetic spacer is imaged layer-resolved by using x-ray microscopy. We directly observe two fundamentally different vortex coupling states, namely antiferromagnetic and ferromagnetic orientation of the flux directions. It is found that these states are predetermined for systems that involve a sufficiently strong interlayer exchange coupling, whereas for the case of a purely dipolar interaction both states are transformable into each other.

Based on polar magneto-optical Kerr effect and frequency dependent ferromagnetic resonance measurements, a method has been found that allows for the quantitative determination of the saturation magnetization of samples with unknown effective magnetic volume. Conventional magnetometry cannot be used for this purpose. Thin Py/Ta multilayers with an overall Py thickness of 20 nm but different number of interfaces are used as test systems. By means of Ne ion irradiation the magnetic moment and the saturation magnetization are affected due to interfacial mixing. With both increasing ion fluence and increasing number of Py/Ta interfaces, a decrease of saturation magnetization is observed.

We present a method to determine the saturation magnetization of samples for which the magnetic volume is unknown and thus cannot be calculated from the magnetic moment. This can happen, e.g., in multilayers, where the spacer material is likely to cause intermixing or whenever ion irradiation is used to modify the magnetic properties of samples on purpose. In both cases the effective magnetic volume is altered from its nominal value in an unknown manner. Therefore magnetometry like superconducting quantum interference devices (SQUID) or vibrating sample magnetometry (VSM) fail, because they detect the magnetic moment but do not provide information on the respective magnetic volume. We show, how ferromagnetic resonance (FMR) can be used instead to circumvent this problem. As a test system we have used thin films of permalloy (Py) and Py/Ta multilayers. Some of the Py/Ta samples were irradiated with Ne ions in order to modify the magnetic properties by altering the interface due to intermixing.

Pulsed temporal structure of synchrotron radiation (SR) at 3rd generation light sources allows for time-resolved studies of dynamic processes with sub-ns (∼100 ps) time resolution, given mainly by ...

A patient was referred for radiotherapy of histologically confirmed squamous cell carcinoma in the left upper lobe and a subcarinal lymph node, stage IIIA (T3N2M0). He tested human immunodeficiency virus-positive 16 years before (current CD4 count of 58 cells/μL). His antiretroviral regimen included 1250 mg nelfinavir every 12 hours and five other antiretrovirals. After extensive discussion regarding chemoradiotherapy versus radiation therapy alone, he declined chemotherapy. From his own review of the preclinical literature on nelfinavir as a radiosensitizer, he chose to discontinue his antiretroviral regimen and take nelfinavir alone at 3750 mg every 12 hours. Treatment planning positron emission tomography-computed tomography revealed a 5.4 × 3.4 × 4.0 cm primary tumor with a 1.6 × 0.5 × 2.2 cm central necrotic cavity (Figure 1A). Intensity-modulated radiation therapy was planned to 74 Gy total in 37 daily fractions (Figure 1B). Radiotherapy began 5 days after increasing the nelfinavir dose. The patient received nine treatments (18 Gy) without toxicity. Positron emission tomography-computed tomography for adaptive replanning revealed primary tumor size decrease to 4.6 × 3.9 × 3.9 cm and maximum standardized uptake value decrease from 11 to 5.8. The central cavity now measured 2.7 × 1.7 × 2.2 cm and communicated with the left bronchus (Figure 1C). On that day, he experienced mild hemoptysis not requiring intervention. He received one more fraction on the following day, completing 20 Gy, after which he suffered fatal massive hemoptysis. Autopsy indicated hemorrhage into the left lower lung parenchyma. Gross examination identified no residual subcarinal node or primary tumor, which was replaced by a 7 × 4 × 4 cm cavity containing necrotic debris. A 1.5 × 1.5 cm erosion tracked from the cavity into the left main bronchus (Figure 2C). Histology revealed microscopic foci of squamous cell carcinoma with extensive necrosis around the cavity and in the erosion track (Figure 2B). The adjacent lung parenchyma, bronchial mucosa, blood vessels, and nerves lacked histologic evidence of radiation injury. Thus, it appeared that rapid tumor regression allowed bleeding through the erosion into the lung parenchyma, leading to asphyxiation. After 20 Gy radiation without concurrent chemotherapy, our patient exhibited complete response grossly with only microscopic residual disease. In the landmark Intergroup 0139 trial for stage IIIA non-small cell lung cancer, only 14% of subjects achieved complete pathologic responses despite 45 Gy and concurrent chemotherapy.1Albain KS Swann RS Rusch VR et al.Radiotherapy plus chemotherapy with or without surgical resection for stage III non-small-cell lung cancer: a phase III randomised controlled trial.Lancet. 2009; 374: 379-386Abstract Full Text Full Text PDF PubMed Scopus (1018) Google Scholar Although an imperfect comparison, this highlights that marked early responses are uncommon even with more intensive therapy and that we may have observed tumor-specific radiosensitization by nelfinavir. Preclinically, nelfinavir demonstrated tumor-specific radiosensitization through downregulation of the phosphatidylinositol 3-kinase (PI3K)-Akt pathway.2Plastaras JP Vapiwala N Ahmed MS et al.Validation and toxicity of PI3K/Akt pathway inhibition by HIV protease inhibitors in humans.Cancer Biol Ther. 2008; 7: 628-635Crossref PubMed Scopus (37) Google Scholar, 3Brunner TB Geiger M Grabenbauer GG et al.Phase I trial of the human immunodeficiency virus protease inhibitor nelfinavir and chemoradiation for locally advanced pancreatic cancer.J Clin Oncol. 2008; 26: 2699-2706Crossref PubMed Scopus (138) Google Scholar Tumor hypoxia is an important radioresistance mechanism,4Tatum JL Kelloff GJ Gillies RJ et al.Hypoxia: importance in tumor biology, noninvasive measurement by imaging, and value of its measurement in the management of cancer therapy.Int J Radiat Biol. 2006; 82: 699-757Crossref PubMed Scopus (481) Google Scholar and PI3K-Akt inhibition increases tumor oxygenation through antiangiogenic “normalization” of vasculature.5Jain RK Normalization of tumor vasculature: an emerging concept in antiangiogenic therapy.Science. 2005; 307: 58-62Crossref PubMed Scopus (3996) Google Scholar, 6Pore N Gupta AK Cerniglia GJ et al.Nelfinavir down-regulates hypoxia-inducible factor 1alpha and VEGF expression and increases tumor oxygenation: implications for radiotherapy.Cancer Res. 2006; 66: 9252-9259Crossref PubMed Scopus (132) Google Scholar Nelfinavir is well tolerated in patients with acquired immune deficiency syndrome, and at the common dose of 1250 mg every 12 hours, clinically relevant PI3K-Akt inhibition is measured.2Plastaras JP Vapiwala N Ahmed MS et al.Validation and toxicity of PI3K/Akt pathway inhibition by HIV protease inhibitors in humans.Cancer Biol Ther. 2008; 7: 628-635Crossref PubMed Scopus (37) Google Scholar For these reasons, nelfinavir was studied in a recent phase I trial for pancreatic cancer, showing promising activity as a radiosensitizer.3Brunner TB Geiger M Grabenbauer GG et al.Phase I trial of the human immunodeficiency virus protease inhibitor nelfinavir and chemoradiation for locally advanced pancreatic cancer.J Clin Oncol. 2008; 26: 2699-2706Crossref PubMed Scopus (138) Google Scholar Toxicities were not observed in a case series that included three patients treated with thoracic radiation and nelfinavir.2Plastaras JP Vapiwala N Ahmed MS et al.Validation and toxicity of PI3K/Akt pathway inhibition by HIV protease inhibitors in humans.Cancer Biol Ther. 2008; 7: 628-635Crossref PubMed Scopus (37) Google Scholar However, hemoptysis has been described with another antiangiogenic drug, bevacizumab, when treating lung tumors with cavitation,7Sandler AB Schiller JH Gray R et al.Retrospective evaluation of the clinical and radiographic risk factors associated with severe pulmonary hemorrhage in first-line advanced, unresectable non-small-cell lung cancer treated with Carboplatin and Paclitaxel plus bevacizumab.J Clin Oncol. 2009; 27: 1405-1412Crossref PubMed Scopus (144) Google Scholar and given our experience with this patient, we recommend using nelfinavir with thoracic radiation only in clinical trials until the optimal use of this promising combination has been elucidated.

Co-doped ZnO films with various electron concentrations up to 4.61 x 10(19) cm(-3) at room temperature were prepared by pulsed laser deposition on a-plane sapphire substrates. Only paramagnetism was observed down to 2K for all samples. X-ray magnetic circular dichroism measurements at 30K confirmed the paramagnetic properties of the doped Co(2+) ions. The average magnetic moment is significantly smaller than the expected moment for Co(2+) ions (L = 1.07, S = 3/2), mainly due to the antiferromagnetic exchange interaction between the neighbouring Co(2+) ions in the ZnO matrix. Also clustering instead of a uniform distribution of Co(2+) ions may play a role. The formation of Co clusters is hindered at higher substrate temperature during the thin film growth. A clear anomalous Hall effect was observed in the highly conducting Co-doped ZnO films at low temperatures up to 100 K.

In this paper we show that spinel ferrite nanocrystals (NiFe2O4, and CoFe2O4) can be texturally embedded inside a ZnO matrix by ion implantation and postannealing. The two kinds of ferrites show different magnetic properties, e.g., coercivity and magnetization. Anomalous Hall effect and positive magnetoresistance have been observed. Our study suggests a ferrimagnet/semiconductor hybrid system for potential applications in magnetoelectronics. This hybrid system can be tuned by selecting different transition-metal ions (from Mn to Zn) to obtain various magnetic and electronic properties.

Magnetism is a collective phenomenon. Hence, a local variation on the nanoscale of material properties, which act on the magnetic properties, affects the overall magnetism in an intriguing way. Of particular importance are the length scales on which a material property changes. These might be related to the exchange length, the domain wall width, a typical roughness correlation length, or a length scale introduced by patterning of the material. Here we report on the influence of two artificially created length scales: (i) ion erosion templates that serve as a source of a predefined surface morphology (ripple structure) and hence allow for the investigation of roughness phenomena. It is demonstrated that the ripple wave length can be easily tuned over a wide range (25-175 nm) by varying the primary ion erosion energy. The effect of this ripple morphology on the induced uniaxial magnetic anisotropy in soft magnetic Permalloy films is studied. Only below a ripple wavelength threshold (approximate to 60 nm) is a significant induced magnetic anisotropy found. Above this threshold the corrugated Permalloy film acts as a flat film. This cross-over is discussed in the frame of dipolar interactions giving rise to the induced anisotropies. (ii) Ion implantation through a lithographically defined mask, which is used for a magnetic property patterning on various length scales. The resulting magnetic properties are neither present in non-implanted nor in homogeneously implanted films. Here new insight is gained by the comparison of different stripe patterning widths ranging from 1 to 10 mu m. In addition, the appearance of more complicated magnetic domain structures, i.e. spin-flop domain configurations and head-on domain walls, during hard axis magnetization reversal is demonstrated. In both cases the magnetic properties, the magnetization reversal process as well as the magnetic domain configurations depend sensitively on the artificially introduced length scale.