N. Mikuszeit

Accurate determination of the specific absorption rate in superparamagnetic nanoparticles under non-adiabatic conditions

We report on a general description of non-adiabatic calorimetry measurements for improving the accuracy on the determination of the specific absorption rate of superparamagnetic nanoparticles subjected to alternating magnetic fields. We perform experiments on reduced volumes of iron oxide nanoparticles dispersed in aqueous media under different thermal equilibrium conditions. We introduce a simple model, which considers linear thermal losses to precisely reproduce the complete time evolution of temperature. The control and the quantification of heat losses lead to higher accuracy for determining the specific absorption rate in superparamagnetic nanoparticles.

Tailoring magnetic anisotropy in epitaxial half metallic La0.7Sr0.3MnO3 thin films

We present a detailed study on the magnetic properties, including anisotropy, reversal fields, and magnetization reversal processes, of well characterized half-metallic epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto SrTiO3 (STO) substrates with three different surface orientations, i.e., (001), (110), and (11−8). The latter shows step edges oriented parallel to the [110] (in-plane) crystallographic direction. Room temperature high resolution vectorial Kerr magnetometry measurements have been performed at different applied magnetic field directions in the whole angular range. In general, the magnetic properties of the LSMO films can be interpreted with just the uniaxial term, with the anisotropy axis given by the film morphology, whereas the strength of this anisotropy depends on both structure and film thickness. In particular, LSMO films grown on nominally flat (110)-oriented STO substrates presents a well defined uniaxial anisotropy originated from the existence of elongated in-plane [001]-orient...

Role of anisotropy configuration in exchange-biased systems

We present a systematic study of the anisotropy configuration effects on the magnetic properties of exchange-biased ferromagnetic/antiferromagnetic (FM/AFM) Co/IrMn bilayers. The interfacial unidirectional anisotropy is set extrinsically via a field cooling procedure with the magnetic field misaligned by an angle bFC with respect to the intrinsic FM uniaxial anisotropy. High resolution angular dependence in-plane resolved Kerr magnetometry measurements have been performed for three different anisotropy arrangements, including collinear bFC ¼ 0 � and two opposite noncollinear cases. The symmetry breaking of the induced noncollinear configurations results in a peculiar nonsymmetric magnetic behavior of the angular dependence of magnetization reversal, coercivity, and exchange bias. The experimental results are well reproduced without any fitting parameter by using a simple model including the induced anisotropy configuration. Our finding highlights the importance of the relative angle between anisotropies in order to properly account for the magnetic properties of exchange-biased FM/AFM systems. V C 2011 American Institute of

Magneto-optic kerr effect : Incorporating the nonlinearities of the analyzer into static photometric ellipsometry analysis

A static photometric ellipsometer allows the measurement of the magneto-optic Kerr effect at a very high signal-to-noise ratio. To gain information about the state of magnetization a mathematical description of the optical components is required. This involves the use of trigonometric functions, e.g., within the Muller calculus. As these are transcendental functions the inversion of the formulae defining the final state of polarization is impossible. The commonly applied linearization is only valid for thin samples with small Kerr angles. For large Kerr angles the nonlinearity of the analyzer setup becomes important. The influence of these nonlinearities on static ellipsometry is studied analytically and numerically. A method is developed to reveal the magnetic properties with high accuracy despite of the nonlinearities due to the optical setup. The applicability of the method is demonstrated for the experimental data obtained from an ion-irradiated iron film on silicon, revealing the magnetic behavior of...

Multipole moments of in-plane magnetized disks

The multipole moments of in-plane magnetized disks have been calculated based on the fundamental theory of magnetostatics. Analytical solutions for disks with uniform magnetization or an onion state are given explicitly. It is demonstrated that depending on the polarization configuration, higher-order multipole moments beyond the dipole moment appear. The strength of the multipole moments can be of the same order of magnitude as the dipolar moment. The higher-order moments give rise to an additional energy contribution in arrays of close-packed polarized disks.

The effect of tilted edges on the shape anisotropy and stray field coupling of uniformly magnetized rectangular elements

The influence of tilted edges on the magnetostatic properties of uniformly magnetized thin rectangular elements is studied. To calculate the magnetostatic energy, the Poisson equation is solved. The shape of the magnetic element is approximated by horizontally assembled thin cuboids and the solutions of Rhodes and Rowlands [Proc. Leeds Phil. Soc. 6, 191 (1954)] are utilized. A second approach is the straightforward integration of the Poisson equation taking into account the trapezoidal shape of the side faces due to the tilted edges. For an adequate number of cuboids, both methods agree very well. It is found that the shape anisotropy of a single magnetic element with tilted edges is reduced compared to that of an ideal cuboid. For a two element system the shape anisotropy competes with the magnetostatic interaction favoring a magnetization orientation parallel to the connecting line of the elements. If the elements are oriented in-line with their short axes, the easy magnetization axis switches at a critical distance between the elements. This distance increases when the elements have tilted edges.The influence of tilted edges on the magnetostatic properties of uniformly magnetized thin rectangular elements is studied. To calculate the magnetostatic energy, the Poisson equation is solved. The shape of the magnetic element is approximated by horizontally assembled thin cuboids and the solutions of Rhodes and Rowlands [Proc. Leeds Phil. Soc. 6, 191 (1954)] are utilized. A second approach is the straightforward integration of the Poisson equation taking into account the trapezoidal shape of the side faces due to the tilted edges. For an adequate number of cuboids, both methods agree very well. It is found that the shape anisotropy of a single magnetic element with tilted edges is reduced compared to that of an ideal cuboid. For a two element system the shape anisotropy competes with the magnetostatic interaction favoring a magnetization orientation parallel to the connecting line of the elements. If the elements are oriented in-line with their short axes, the easy magnetization axis switches at a crit...

Room Temperature Instability of Exchange Anisotropy in FeMn/FeCo System

Ferromagnetic (FM) FeCo is investigated in exchange bias systems. The ferromagnetic layer is grown on a FeMn antiferromagnetic (AFM) layer. Partial superficial oxidation of FeCo is observed. The standard field cooling procedure results in a large room temperature exchange bias effect. However, the training effect observed when the hysteresis loops are repeated does not have a saturating trend. This behavior is related to the evolution of pinned moments at the FM/AFM interface. X-ray circular magnetic dichroism technique is used to clarify this mechanism.

Substrate polarization effects in two-dimensional magnetic arrays

We acknowledge funding by Consolider-Ingenio en Nanociencia Molecular Ref. No. CSD2007-00010, by the Comunidad de Madrid through Project No. S2009/MAT-1726, and Project No. FIS 2010-18847 from MICINN

Substrate-induced magnetic anisotropy in La0.7Sr0.3MnO3 epitaxial thin films grown onto (110) and (1bar 18) SrTiO3 substrates

We show a detailed magneto-optical Kerr study at room temperature of well characterized epitaxial La0.7Sr0.3MnO3 (LSMO) thin films grown onto (110) and (18) SrTiO3 substrates. The films present a well-defined uniaxial (two-fold) magnetic anisotropy ascribed to substrate-induced anisotropy. In particular, the in-plane uniaxial anisotropy in the(110)-oriented LSMO films originates from the existence of elongated in-plane [001]-oriented structures. Similar elongated structures, parallel to the [110] crystallographic direction, are found for LSMO films grown on (18) STO surfaces. In all films, such a uniaxial magnetic anisotropy is characterized by an easy axis lying along the elongated structures. Furthermore, the vectorial-resolved hysteresis loops as a function of the in-plane applied field direction are interpreted in terms of rotation and propagation and nucleation of magnetic domains processes. Our results demonstrate the tailoring of magnetic anisotropy by exploiting the substrate-induced anisotropy in epitaxial thin films.

Spin-spin correlations in ferromagnetic nanosystems

Abstract. Using exact diagonalization, Monte-Carlo, and mean-field techniques, characteristic temperature scales for ferromagnetic order are discussed for the Ising and the classical anisotropic Heisenberg model on finite lattices in one and two dimensions. The interplay between nearest-neighbor exchange, anisotropy and the presence of surfaces leads, as a function of temperature, to a complex behavior of the distance-dependent spin-spin correlation function, which is very different from what is commonly expected. A finite experimental observation time is considered in addition, which is simulated within the Monte-Carlo approach by an incomplete statistical average. We find strong surface effects for small nanoparticles, which cannot be explained within a simple Landau or mean-field concept and which give rise to characteristic trends of the spin-correlation function in different temperature regimes. Unambiguous definitions of crossover temperatures for finite systems and an effective method to estimate the critical temperature of corresponding infinite systems are given.