The phenomenon of heat dissipation by magnetic materials interacting with an alternating magnetic field, known as magnetic hyperthermia, is an emergent and promising therapy for many diseases, mainly cancer. Here, a magnetic hyperthermia model for core–shell nanoparticles is developed. The theoretical calculation, different from previous models, highlights the importance of heterogeneity by identifying the role of surface and core spins on nanoparticle heat generation. We found that the most efficient nanoparticles should be obtained by selecting materials to reduce the surface to core damping factor ratio, increasing the interface exchange parameter and tuning the surface to core anisotropy ratio for each material combination. From our results we propose a novel heat-based hyperthermia strategy with the focus on improving the heating efficiency of small sized nanoparticles instead of larger ones. This approach might have important implications for cancer treatment and could help improving clinical efficacy.

Mean-field and linear regime approach to magnetic hyperthermia of core–shell nanoparticles: can tiny nanostructures fight cancer?

Despite decades of advances in magnetic imaging, obtaining direct, quantitative information with nanometer scale spatial resolution remains an outstanding challenge. Current approaches, for example, Hall micromagnetometer and nitrogen-vacancy magnetometer, are limited by highly complex experimental apparatus and a dedicated sample preparation process. Here we present a new AC field-modulated magnetic force microscopy (MFM) and report the local and quantitative measurements of the magnetic information of individual magnetic nanoparticles (MNPs), which is one of the most iconic objects of nanomagnetism. This technique provides simultaneously a direct visualization of the magnetization process of the individual MNPs, with spatial resolution and magnetic sensitivity of about 4.8 nm and 1.85 × 10−20 A m2, respectively, enabling us to separately estimate the distributions of the dipolar fields and the local switching fields of individual MNPs. Moreover, we demonstrate that quantitative magnetization moment of individual MNPs can be routinely obtained using MFM signals. Therefore, it underscores the power of the AC field-modulated MFM for biological and biomedical applications of MNPs and opens up the possibility for directly and quantitatively probing the weak magnetic stray fields from nanoscale magnetic systems with superior spatial resolution.

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Quantitatively probing the magnetic behavior of individual nanoparticles by an AC field-modulated magnetic force microscopy.

Spin and orbital properties of perpendicular magnetic anisotropy for spin-orbit torque material devices

This thesis deals with a systematic study of magnetic properties, i.e. saturation magnetization, exchange stiffness, anisotropy, interlayer exchange coupling, and average domains size, of ultrathin Co-based multilayer systems. The multilayers were prepared by DC-magnetron sputtering and the Co-layers are sandwiched either symmetrically or antisymmetrically between nonmagnetic spacer layers of Pt or Ir.
Various experimental techniques were employed to study the different properties and their Co-thickness-driven evolution. The saturation magnetization was measured using ferromagnetic resonance spectroscopy (FMR), while anisotropies and interlayer exchange coupling were deter-mined from measurements of the magneto-optic Kerr effect (MOKE). The exchange stiffness is extracted from domain wall profiles acquired with X-ray holographic microscopy (XHM). To this end, the fabrication scheme of the X-ray optics used in the microscope was improved and a lateral resolution of sub-12 nm experimentally proven. The average domain size was obtained from intensity profiles acquired via X-ray resonant magnetic scattering (XRMS). From the comparison of measured domain sizes to ones calculated from domain spacing models, the strength of the Dzyaloshinskii-Moriya interaction (DMI) can be estimated. It is shown that this method, which is frequently applied in literature, is prone to problems, such as the strong dependence on all other magnetic parameters, as well as on the geometry of the domain pattern, the influence of domain wall pinning, and the various models producing differing results. This aggravates the comparison of values between different groups and strongly limits the applicability.
Unlike in recent publications concerning similar multilayer systems, the values measured for the saturation magnetization down to layer thicknesses of 2 nm are in line with the literature value for bulk Co. MOKE data suggest that bulk-like values persist down to ≈ 1 nm. The exchange stiffness was found to be considerably reduced in ultrathin layers compared to the bulk value of fcc Co. The reduction is attributed to interdiffusion at the interfaces and a model is developed that describes the reduction in dependence of the Co thickness and thewidth of the interdiffusion zone.
A spin reorientation transition (SRT) from perpendicular to easy-plane magnetization was ob-served, which is driven by the Co thickness. The transition, however, is strongly influenced by the number of repetitions N and the thickness of the non-magnetic spacer layers. Perpendicular magnetization was observed far beyond the zero crossing of the effective anisotropy for N ≥ 6, which is attributed to the formation of a three-dimensional magnetic microstructure, the so-called vortex state.
In the antisymmetric systems, the stacking sequence plays a crucial role. Both, anisotropy and interlayer exchange coupling are altered by inversion of the sequence. The interlayer exchange coupling changes from a strictly ferromagnetic to an oscillatory behavior, like it is found for a pure Ir spacer layer, but with a longer period length, which is dependent on the thickness of both spacer layers. Finally, perpendicular domains were observed also in antiferromagnetically coupled multilayers, starting from a specific Co-layer thickness, and extending far beyond the zero crossing of the effective anisotropy. While such domains were proposed and observed in such systems for weak effective anisotropies favoring perpendicular magnetization, their presence beyond the zero crossing has not been described before.
 
Diese Doktorarbeit befasst sich mit der systematischen Untersuchung von sehr dunnen kobaltbasierten Multilagensystemen im Hinblick auf ihre magnetischen Eigenschaften, d.h. Sattigungsmagnetisierung, Austauschsteiffigkeit, magnetische Anisotropie, Zwischenlagenaus-tauschkopplung und mittlere Domanengrose. Die Kobaltlagen sind symmetrisch oder antisym-metrisch von nichtmagnetischen Zwischenlagen aus Platin oder Iridium eingefasst und wurden mittels Gleichspannungs-Kathodenzerstaubung prapariert.
Die verschiedenen Eigenschaften wurden mittels unterschiedlicher experimenteller Techniken auf ihre kobaltschichtdickenabhangige Entwicklung untersucht. Fur die Untersuchung der Satti-gungsmagnetisierung wurde ferromagnetische Resonanzspektroskopie eingesetzt, wahrend Anisotropie und Zwischenlagenkopplung durch Messung des magnetooptischen Kerr-Effektes (MOKE) untersucht wurden. Die Austauschsteiffigkeit wurde aus Domanenwandprofilen extrahiert, welche mit rontgenholographischer Mikroskopie (XHM) aufgenommen wurden. Um dies zu ermoglichen wurde der Herstellungsprozess eines fur dieses Mikroskop essentiellen rontgenoptischen Bauteils verbessert und eine raumliche Auflosung von unter 12 nm experimentell nachgewiesen. Die Bestimmung der mittleren Domanengrose erfolgte aus Intensitatsprofilen, welche mittels resonanter Rontgen-Kleinwinkelstreuung (XRMS) gemessen wurden. Aus dem Vergleich der experimentell bestimmten und mit Modellen berechneten Domanengrosen kann die Starke der Dzyaloshinskii-Moriya Wechselwirkung (DMI) bestimmt werden. Es wird gezeigt, dass diese haufig in der Literatur verwendete Methode sehr problemanfallig ist, was den Vergleich von Werten zwischen verschidenen Arbeitsgruppen erschwert und die Verlasslichkeit so gewonnener Ergebnisse in Frage stellt. Diese Anfalligkeit ist der starken Abhangigkeit von den ubrigen magnetischen Parametern sowie dem Einfluss von Pinning, der Geometrie des Domanenmusters, sowie abweichenden Vorhersagen der unterschiedlichen verwendeten Modelle geschuldet.
Im Gegensatz zu mehreren kurzlich erschienen Publikationen zu ahnlichen Multilagensystemen, wurden fur Kobaltschichtdicken bis zu 2 nm Sattigungsmagnetisierungswerte gemessen die in Ubereinstimmung mit dem Literaturwert fur Kobaltvolumenkristalle sind. Aus MOKE Messun-gen wurde abgeschatzt, dass volumenartige Werte bis Schichtdicken von 1 nm vorherschen. Fur die Austauschsteiffigkeit wurde ein im Vergleich zu fcc Volumenkobalt stark verringerter Wert gemessen. Die Reduktion wird auf Durchmischung an den Grenzflachen zuruckgefuhrt und ein Modell entwickelt, welches die Reduktion in Abhangigkeit der Kobaltschichtdicke und der Breite der Durchmischungszone an der Grenzflache beschreibt.
Es wurde ein kobaltdickengetriebener Spinreorientierungsubergang (SRT) beobachtet, bei wel-chem sich die Vorzugsrichtung der Magnetisierung von senkrecht zu in der Ebene verandert. Die Art des Ubergangs ist allerdings stark von der Anzahl der Multilagen N und der Dicke der nichtmagnetischen Zwischenlagen abhangig. Auch noch in Bereichen jenseits des Null-durchgangs der effektiven Anisotropiewurden fur N ≥ 6 senkrechte Domanen beobachtet, welche der Entstehung einer dreidimensionalen Magnetisierungsverteilung zugeschrieben wird. 
Es hat sich gezeigt, dass in den antisymmetrischen Systemen die Reihenfolge der Schichten eine entscheidende Rolle fur die resultierenden Eigenschaften spielt. Sowohl die Anisotropie wie auch die Zwischenlagenaustauschkopplung werden davon entscheidend beeinflusst. Durch die Umkehrung der Schichtenfolge wird ein Wechsel von ausschlieslich ferromagnetischer Kopplung zu einem periodisch zwischen ferro- und antiferromagnetisch oszillierenden Verhalten hervorgerufen – ein Verhalten, welches bereitsvon Zwischenlagen aus reinem Iridium bekannt ist. Die Oszillation erfolgt jedoch mit einer langeren Periodenlange welche sowohl von der Dicke wie auch der Zusammensetzung der Zwischenlagen abhangig ist. Des Weiteren wurden senkrecht magnetisierte Domanen auch in antiferromagnetisch gekoppelten Multilagen oberhalb einer spezifischen Kobaltschichtdicke beobachtet, die sich bis weit uber den Nulldurchgang der effektiven Anisotropie hinaus beobachteten liesen. Wahrend derartige Domanen fur solche Systeme im Bereich von schwacher senkrechter Anisotropie vorhergesagt und bereits beobachtet wurden, ist deren Prasenz jenseits des Nulldurchgangs bisher nicht berichtet worden.

The magnetic properties of ultrathin Cobalt multilayers symmetrically and antisymmetrically sandwiched between layers of platinum and iridium

We present a magneto-optical technique to spatially amplify and image fringe fields from single ferromagnetic nanorods. The fringe fields nucleate magnetic domains in a low-coercivity, perpendicularly magnetized indicator film, which are expanded by an applied out-of-plane field from the nanoscale to the microscale for measurement with polar Kerr microscopy. The nucleation location and therefore magnetic orientation of the sample nanorod are detected as spatially dependent field biases in locally measured hysteresis loops of the indicator film. We first discuss our method to fabricate the high-sensitivity indicator film with low energy argon ion irradiation. We then present a map of the amplified signal produced from a single nanorod as measured by the indicator film and compare it with a simultaneously obtained, unamplified fringe field map. The comparison demonstrates the advantage of the amplification mechanism and the capability of the technique to be performed with single-spot magneto-optical Kerr effect magnetometers. Our signal-to-noise ratio determines a minimum measureable particle diameter of tens of nanometers for typical transition metals. We finally use our method to obtain hysteresis loops from multiple nanorods in parallel. Our technique is unperturbed by applied in-plane fields for magnetic manipulation of nanoparticles, is robust against many common noise sources, and is applicable in a variety of test environments. We conclude with a discussion of the future optimization and application of our indicator film technique.

Magnetometry of single ferromagnetic nanoparticles using magneto-optical indicator films with spatial amplification

In this paper, a method is presented that has the sensitivity to measure magnetization behavior of single nanostructures. It is demonstrated that the technique gives the ability to separate different signals of single nanodots from a small ensemble of structures. Our method is based on the anomalous Hall-Effect and allows for resolving signals from spherical nanoparticles with diameter down to 3.5 nm. The method gives access to magnetic properties of particles in a wide thermal and dynamical range. The potential of the technique is demonstrated utilizing particles that are created from Co films sandwiched by Pt layers.

Probing the magnetic behavior of single nanodots.

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Probing the magnetic behavior of single nanodots.