A. Asenjo

Magnetic behavior of an array of cobalt nanowires

Cobalt nanowires have been electrodeposited into the pores of Anodisc™ alumina membranes after placing on one side a layer of sputtered copper, which acts as electrode and substrate during the electrodeposition. Nanowires are 60 μm long, 170–220 nm in diameter depending on the size of the pores of the alumina membrane. This array of nanowires exhibits uniaxial magnetic anisotropy related to the particular shape of each individual nanowire. On the contrary to the expected behavior in a uniaxial magnetic system, the coercivity of the array exhibits a maximum when the applied field is in a perpendicular direction with respect to the easy axis. This magnetic behavior is analyzed considering dipolar interactions among nanowires, and the magnetization of the array is obtained as a function of the magnetic characteristics of each nanowire using an iterative method.

Remanence of Ni nanowire arrays: Influence of size and labyrinth magnetic structure

The influence of the macroscopic size of the Ni nanowire array system on their remanence state has been investigated. A simple magnetic phenomenological model has been developed to obtain the remanence as a function of the magnetostatic interactions in the array. We observe that, due to the long range of the dipolar interactions between the wires, the size of the sample strongly influence the remanence of the array. On the other hand, the magnetic state of nanowires has been studied by variable field magnetic force microscopy for different remanent states. The distribution of nanowires with the magnetization in up or down directions and the subsequent remanent magnetization has been deduced from the magnetic images. The existence of two short-range magnetic orderings with similar energies can explain the typical labyrinth pattern observed in magnetic force microscopy images of the nanowire arrays.

Frequency dependence of the magnetoimpedance in amorphous CoP electrodeposited layers

Magnetic properties and changes of impedance upon external field (MI) are studied in amorphous CoP magnetic layers obtained by galvanostatic electrodeposition over cylindrical Cu substrates. The magnetic layer thickness is controlled by deposition time and varies between 3 and 7 μm. Due to the columnar growth of Co, thicker layers have stronger perpendicular radial anisotropy. The field and frequency dependence of the impedance is measured in the kHz/MHz range. Although it is generally accepted that a radial anisotropy should be unfavorable to the MI effect, an increase of the MI ratio with the thickness of the magnetic layer, and thus with anisotropy, is observed. Results are explained in terms of a model considering the current distribution along the sample thickness with two well-defined regions having different transport and magnetic properties.

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

Summary 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.

Magnetic structure of a single-crystal hcp electrodeposited cobalt nanowire

We report on the magnetic and structural properties of an individual 40 nm diameter magnetic nanowire, prepared by electrodeposition into anodic alumina templates. The high-resolution transmission electron microscopy and the X-ray diffraction reveal the monocrystalline hcp structure along the whole 10 μm nanowire length with the c-axis almost perpendicular to the nanowire axis. This observation allows the understanding of magnetic properties of the nanowire. The magnetic state observed by magnetic force microscopy and modelled by micromagnetic simulations is interpreted as consisting of tilted vortices with alternating chiralities.

Calibration of Coercive and Stray Fields of Commercial Magnetic Force Microscope Probes

Variable-field magnetic force microscope (MFM) is introduced to characterize the magnetic behavior of commercially available MFM probes that is relevant to interpret MFM imaging. A Nanotec Electronica S.L. microscope has been conveniently modified to apply magnetic fields in axial direction (up to 1.5 kOe) and in-plane direction (up to 2.0 kOe). Axial and transeverse hysteresis loops of the probes have been generated by measuring the changes in the MFM contrast observed when the magnetic field is applied. The variation of the MFM signal is ascribed to the modification of the magnetic state of the tips. This is enabled by the large coercitivity (~1.7 kOe) of the checked longitudinal recording media. The properties of the probes depend on the coating material, the macroscopic tip shape, and tip radius. In only a few cases, the magnetization of the probe can be oriented along in-plane orientation. In addition, the stray field of the tips has been deduced by measuring the influence of the probe in the magnetic state of the checked samples.

Nanoscale magnetic structure and properties of solution-derived self-assembled La0.7Sr0.3MnO3 islands

Strain-induced self-assembled La0.7Sr0.3MnO3 nanoislands of lateral size 50−150 nm and height 10−40 nm have been grown on yttria-stabilized zirconia (001)-substrates from ultradiluted chemical solutions based on metal propionates. The nanoislands grow highly relaxed withstanding the epitaxial relation (001)LSMO[110]//(001)YSZ[010] and show bulk-like average magnetic properties in terms of Curie temperature and saturation magnetization. The interplay of the magnetocrystalline and shape anisotropy within the nanoisland ensemble results in an in-plane magnetic anisotropy with a magnetocrystalline constant K1(150  K)=-(5±1)  kJ/m3 and in-plane easy axis along the [110]-La0.7Sr0.3MnO3 direction as measured, for the first time, through ferromagnetic resonance experiments. Magnetic force microscopy studies reveal the correlation between nanoisland size and its magnetic domain structure in agreement with micromagnetic simulations. In particular, we have established the required geometric conditions for single dom...

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.

Magnetic domain structure of nanohole arrays in Ni films

Nanohole arrays in Ni films have been prepared by a replica/antireplica method based on anodic alumina membranes. The nanohole arrays exhibited long range ordering with hexagonal symmetry, the hole distance was kept constant (105nm), and the hole diameter and the film thickness were varied between 50 and 70nm and 55 and 600nm, respectively. The magnetic domain structures of such samples have been studied by analyzing magnetic force microscopy images at remanent state. Different domain structures have been observed depending on the geometrical characteristics of the films. The experimental results have been interpreted with the help of micromagnetic simulations.

MFM imaging of FePd stripe domains. Evolution with Pt buffer layer thickness

Epitaxial FePd (0 0 1) thin films grown by UHV sputtering onto different Pt buffer layers have been studied by magnetic force microscopy. Samples with buffer thickness down to 150 A present high perpendicular magnetic anisotropy. Both the stripe domain width and the magnetic contrast increase with the buffer thickness, in agreement with the anisotropy deduced from the hysteresis loops.