Negative dissipation gradients in hysteretic materials.

Measuring energy dissipation on the nanoscale is of great interest not only for nanomechanics but also to understand important energy transformation and loss mechanisms that determine the efficiency of energy of data storage devices. Fully understanding the magnetic dynamics and dissipation processes in nanomagnets is of major relevance for a number of basic and applied issues from magnetic recording to spin-based sensor devices to biomedical magnetic-based hyperthermia treatments. Here we present experimental evidence for a counter-intuitive monotonical reduction of energy dissipation as the interaction between two nanomagnets is enhanced. This behavior, which takes place when spins are parallel, can be understood in terms of hysteresis phenomena involved in the reorientation of these spins. The measured magnetic losses of about a few femtowatts are in agreement with quasi-static micromagnetic numerical simulations.

Customized MFM probes with high lateral resolution.

Magnetic force microscopy (MFM) is a widely used technique for magnetic imaging. Besides its advantages such as the high spatial resolution and the easy use in the characterization of relevant applied materials, the main handicaps of the technique are the lack of control over the tip stray field and poor lateral resolution when working under standard conditions. In this work, we present a convenient route to prepare high-performance MFM probes with sub-10 nm (sub-25 nm) topographic (magnetic) lateral resolution by following an easy and quick low-cost approach. This allows one to not only customize the tip stray field, avoiding tip-induced changes in the sample magnetization, but also to optimize MFM imaging in vacuum (or liquid media) by choosing tips mounted on hard (or soft) cantilevers, a technology that is currently not available on the market.

Superparamagnetic versus blocked states in aggregates of Fe(3-x)O4 nanoparticles studied by MFM.

Magnetic domain configurations in two samples containing small aggregates of Fe(3-x)O4 nanoparticles of about 11 and 49 nm in size, respectively, were characterized by magnetic force microscopy (MFM). Two distinct magnetic behaviors were observed depending on the particle size. The aggregates constituted of nanoparticles of about 11 nm in size showed a uniform dark contrast on MFM images, reflecting the predominant superparamagnetic character of these particles and arising from the coherent rotation of the spins within the aggregate as the latter align along the tip stray-field. By applying a variable in-plane field, it is possible to induce magnetic polarization yielding an increasing dark/bright contrast as the strength of the applied field overcomes the stray-field of the tip, although this polarization completely disappears as the remanent state is recovered when the magnetic field is switched off. On the contrary, for aggregates of NPs of about 49 nm in size, dark/bright contrast associated with the existence of magnetic domains and magnetic polarization prevails in MFM images all along the magnetic cycle due to the blocking state of the magnetization of these larger particles, even in the absence of an applied field. All in all, we unambiguously demonstrate the capabilities of magnetic force microscopy to distinguish between blocked and superparamagnetic states in the aggregates of magnetic nanoparticles. Micromagnetic simulations strongly support the conclusions stated from the MFM experiments.

Spin configuration in isolated FeCoCu nanowires modulated in diameter.

Cylindrical Fe28Co67Cu5 nanowires modulated in diameter between 22 and 35 nm are synthesized by electroplating into the nanopores of alumina membranes. High-sensitivity MFM imaging (with a detection noise of 1 µN m(-1)) reveals the presence of single-domain structures in remanence with strong contrast at the ends of the nanowires, as well as at the transition regions where the diameter is modulated. Micromagnetic simulations suggest that curling of the magnetization takes place at these transition sites, extending over 10-20 nm and giving rise to stray fields measurable with our MFM. An additional weaker contrast is imaged, which is interpreted to arise from inhomogeneities in the nanowire diameter.

Direct imaging of the magnetic polarity and reversal mechanism in individual Fe(3-x)O4 nanoparticles.

This work reports on the experimental characterization of the magnetic domain configurations in cubic, isolated Fe3-xO4 nanoparticles with a lateral size of 25-30 nm. The magnetic polarity at remanence of single domain ferrimagnetic Fe3-xO4 nanoparticles deposited onto a carbon-silicon wafer is observed by magnetic force microscopy. The orientations of these domains provide a direct observation of the magneto-crystalline easy axes in each individual nanoparticle. Furthermore, the change in the domain orientation with an external magnetic field gives evidence of particle magnetization reversal mediated by a coherent rotation process that is also theoretically predicted by micromagnetic calculations.

Thin film multiferroic nanocomposites by ion implantation.

Thin film multiferroic nanocomposites might enable a range of potentially disruptive integrated magnetoelectric devices for information storage, spintronics, microwave telecommunications, and magnetic sensing. With this aim, we have investigated ion implantation of magnetic species into ferroelectric single crystal targets as a radically novel approach to prepare film nanoparticulate magnetic-metal ferroelectric-oxide composites. These materials are an alternative to multiferroic oxide epitaxial columnar nanostructures that are under intensive research, but whose magnetoelectric response is far from expectations. Here, we unambiguously demonstrate the preparation of such a thin film multiferroic nanocomposite of Co and BaTiO3 by ion implantation of a high dose of the magnetic species, followed by rapid thermal processing under tailored conditions. Results thus constitute a proof of concept for the feasibility of obtaining the materials by this alternative approach. Ion implantation is a standard technique for the microelectronic industry in combination with well-established patterning procedures.

Distinguishing magnetic and electrostatic interactions by a Kelvin probe force microscopy-magnetic force microscopy combination.

The most outstanding feature of scanning force microscopy (SFM) is its capability to detect various different short and long range interactions. In particular, magnetic force microscopy (MFM) is used to characterize the domain configuration in ferromagnetic materials such as thin films grown by physical techniques or ferromagnetic nanostructures. It is a usual procedure to separate the topography and the magnetic signal by scanning at a lift distance of 25-50 nm such that the long range tip-sample interactions dominate. Nowadays, MFM is becoming a valuable technique to detect weak magnetic fields arising from low dimensional complex systems such as organic nanomagnets, superparamagnetic nanoparticles, carbon-based materials, etc. In all these cases, the magnetic nanocomponents and the substrate supporting them present quite different electronic behavior, i.e., they exhibit large surface potential differences causing heterogeneous electrostatic interaction between the tip and the sample that could be interpreted as a magnetic interaction. To distinguish clearly the origin of the tip-sample forces we propose to use a combination of Kelvin probe force microscopy (KPFM) and MFM. The KPFM technique allows us to compensate in real time the electrostatic forces between the tip and the sample by minimizing the electrostatic contribution to the frequency shift signal. This is a great challenge in samples with low magnetic moment. In this work we studied an array of Co nanostructures that exhibit high electrostatic interaction with the MFM tip. Thanks to the use of the KPFM/MFM system we were able to separate the electric and magnetic interactions between the tip and the sample.

Hysteresis loops of individual Co nanostripes measured by magnetic force microscopy.

High-resolution magnetic imaging is of utmost importance to understand magnetism at the nanoscale. In the present work, we use a magnetic force microscope (MFM) operating under in-plane magnetic field in order to observe with high accuracy the domain configuration changes in Co nanowires as a function of the externally applied magnetic field. The main result is the quantitative evaluation of the coercive field of the individual nanostructures. Such characterization is performed by using an MFM-based technique in which a map of the magnetic signal is obtained as a function of both the lateral displacement and the magnetic field.