Nowak, Ulrich

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## Suchergebnisse Publikationen

#### Geometric control of the magnetization reversal in antidot lattices with perpendicular magnetic anisotropy

2016-03-28, Gräfe, Joachim, Weigand, Markus, Träger, Nick, Schütz, Gisela, Goering, Eberhard J., Skripnik, Maxim, Nowak, Ulrich, Haering, Felix, Ziemann, Paul, Wiedwald, Ulf

While the magnetic properties of nanoscaled antidot lattices in in-plane magnetized materials have widely been investigated, much less is known about the microscopic effect of hexagonal antidot lattice patterning on materials with perpendicular magnetic anisotropy. By using a combination of first-order reversal curve measurements, magnetic x-ray microscopy, and micromagnetic simulations we elucidate the microscopic origins of the switching field distributions that arise from the introduction of antidot lattices into out-of-plane magnetized GdFe thin films. Depending on the geometric parameters of the antidot lattice we find two regimes with different magnetization reversal processes. For small antidots, the reversal process is dominated by the exchange interaction and domain wall pinning at the antidots drives up the coercivity of the system. On the other hand, for large antidots the dipolar interaction is dominating which leads to fragmentation of the system into very small domains that can be envisaged as a basis for a bit patterned media.

#### Atomistic spin model simulation of magnetic reversal modes near the Curie point

2010, Barker, Joe, Evans, Richard Francis L., Chantrell, Roy W., Hinzke, Denise, Nowak, Ulrich

The so-called linear reversal mode is demonstrated in spin model simulations of the high anisotropy material L10 FePt. Reversal of the magnetization is found to readily occur in the linear regime despite an energy barrier (KV/kBT) that would conventionally ensure stability on this timescale. The timescale for the reversal is also established with a comparison to the Landau Lifshitz Bloch equation showing good agreement.

#### Domain state model for exchange bias : II. Experiments

2002, Keller, Janine, Miltényi, Peter, Beschoten, Bernd, Güntherodt, Gernot, Nowak, Ulrich, Usadel, Klaus-Dieter

The exchange bias coupling at ferro-/antiferromagnetic interfaces of epitaxially grown Co/CoO bilayers can be intentionally enhanced and controlled by diluting the antiferromagnetic CoO layer, i.e., by introducing (i) nonmagnetic substitutions (Co1-xMgxO) or (ii) Co deficiencies (Co1-yO). All intentional nonmagnetic cations or defects were placed away from the interface throughout the whole volume part of the antiferromagnetic layer. This way the roughness at the Co/CoO interface was kept practically the same. For both types of defects, the exchange bias field can be increased by a factor of 3 to 4. Hence, exchange bias is primarily not due to roughness at the interface but rather can be controlled by the defects in the volume part of the antiferromagnetic layer. We systematically investigate the dilution dependence of various phenomena of exchange bias, such as the vertical magnetization shift of the hysteresis loop, temperature dependence, training effect, cooling field dependence, and antiferromagnetic layer thickness dependence. All these phenomena are directly compared to results from Monte Carlo simulations and are shown to be consistently described by the domain state model for exchange bias. The combined experimental and theoretical findings suggest that the origin of exchange bias in Co/CoO results from a domain state in the volume part of the antiferromagnet stabilized by the defects.

#### Atomistic spin model based on a spin-cluster expansion technique: Application to the IrMn3/Co interface

2011, Szunyogh, Laszlo, Udvardi, László, Jackson, Jerome, Nowak, Ulrich, Chantrell, Roy W.

In order to derive tensorial exchange interactions and local magnetic anisotropies in itinerant magnetic systems, an approach combining the spin-cluster expansion with the relativistic disordered local moment scheme is introduced. The theoretical background and computational aspects of the method are described in detail. The exchange interactions and site-resolved anisotropy contributions for the IrMn3/Co(111) interface, a prototype for an exchange bias system, are calculated including a large number of magnetic sites from both the antiferromagnet and ferromagnet. Our calculations reveal that the coupling between the two subsystems is fairly limited to the vicinity of the interface. The magnetic anisotropy of the interface system is discussed, including effects of the Dzyaloshinskii-Moriya interactions that appear due to symmetry breaking at the interface.

#### Spin configuration of ferromagnetic/antiferromagnetic nano-composite particles

2007, Phuoc, Nguyen N., Suzuki, Takao, Chantrell, Roy W., Nowak, Ulrich

A systematic study of the spin configuration of ferromagneti (core)/antiferromagnetic(shell) nanocomposite particles was carried out by two approaches: micromagnetic and atomistic simulations. In the case of strong interlayer exchange coupling between ferromagnet and antiferromagnet, the micromagnetic simulation result is consistent with the atomistic simulation result. However, in the case of weak interlayer exchange coupling the micromagnetic simulation predicts a multi-domain structure of the ferromagnet, which is inconsistent with the atomistic simulation. The failure of micromagnetic simulation in describing the spin structure of the nano-composite particles might be interpreted in terms of the under-estimation of the exchange energy for rapid spatial fluctuations of the magnetization by the continuum exchange formalism or might be due to the cooling process procedure. Also, the variation of spin structure as a function of the uniaxial anisotropy of the core is presented and discussed.

#### Constrained Monte Carlo Method and Calculation of the Temperature Dependence of Magnetic Anisotropy

2010, Asselin, Pierre, Evans, Richard Francis L., Barker, Joe, Chantrell, Roy W., Yanes Díaz, Rocio, Chubykalo-Fesenko, Oksana, Hinzke, Denise, Nowak, Ulrich

We introduce a constrained Monte Carlo method which allows us to traverse the phase space of a classical spin system while fixing the magnetization direction. Subsequently we show the method's capability to model the temperature dependence of magnetic anisotropy, and for bulk uniaxial and cubic anisotropies we recover the low-temperature Callen-Callen power laws in M. We also calculate the temperature scaling of the 2-ion anisotropy in L10 FePt, and recover the experimentally observed M2.1 scaling. The method is newly applied to evaluate the temperature dependent effective anisotropy in the presence of the N'eel surface anisotropy in thin films with different easy axis configurations. In systems having different surface and bulk easy axes, we show the capability to model the temperature-induced reorientation transition. The intrinsic surface anisotropy is found to follow a linear temperature behavior in a large range of temperatures.

#### Exchange bias in ferromagnetic/antiferromagnetic bilayers with imperfect interfaces

2006, Spray, J., Nowak, Ulrich

The influence of an imperfect interface on exchange bias (EB) properties is investigated. Within the framework of the domain state model, the EB field HEB and the coercive field HC are determined using computer simulations, and they are found to depend strongly on the details of the interface structure. This dependence is sensitive to the dilution of the antiferromagnet (AFM) with non-magnetic defects in the bulk. For the optimal interface structure, giving greatest EB, the optimal dilution is found to be much less than that for an ideal-interface system, taking a value in better agreement with experimental results. Even without any defects in the bulk of the AFM the interface roughness leads to EB for thin antiferromagnetic layers, in accordance with the model by Malozemoff. Finally, the thickness dependence of rough-interface systems is found to differ significantly from that of ideal-interface systems.