2018-09-13, Simon, Eszter, Yanes Díaz, Rocio, Khmelevskyi, Sergii, Palotás, Krisztián, Szunyogh, László, Nowak, Ulrich

Based on ab initio calculations and spin dynamics simulations, we perform a detailed study on the magnetic properties of bulk MnN and the MnN/Fe interface. We determine the spin model parameters for the θ-phase of bulk MnN, and we find that the competition between the nearest and the next-nearest-neighbor interactions leads to antiferromagnetic ordering of the Mn spins, in agreement with previous theoretical and experimental results. At the MnN/Fe interface, a sizable Dzyaloshinskii-Moriya interaction appears leading to a stable exchange-bias effect. We study the dependences of the exchange-bias effect on the thicknesses of the ferromagnetic and the antiferromagnetic layers, and we compare them to experimentally obtained results [Meinert et al., Phys. Rev. B 92, 144408 (2015)].

2017, Kleibert, Armin, Balan, Ana, Yanes Díaz, Rocio, Derlet, Peter M., Vaz, Carlos A. F., Timm, Martin, Fraile Rodríguez, Arantxa, Béché, Armand, Verbeeck, Jo, Nowak, Ulrich

Magnetic nanoparticles are important building blocks for future technologies ranging from nano-medicine to spintronics. Many related applications require nanoparticles with tailored magnetic properties. However, despite significant efforts undertaken towards this goal, a broad and poorly-understood dispersion of magnetic properties is reported, even within mono-disperse samples of the canonical ferromagnetic 3d transition metals. We address this issue by investigating the magnetism of a large number of size- and shape-selected, individual nanoparticles of Fe, Co, and Ni using a unique set of complementary characterization techniques. At room temperature only superparamagnetic behavior is observed in our experiments for all Ni nanoparticles within the investigated sizes, which range from 8 to 20 nm. However, Fe and Co nanoparticles can exist in two distinct magnetic states at any size in this range: (i) a superparamagnetic state as expected from the bulk and surface anisotropies known for the respective materials and as observed for Ni; and (ii) a state with unexpected stable magnetization at room temperature. This striking state is assigned to significant modifications of the magnetic properties arising from metastable lattice defects in the core of the nanoparticles as concluded by calculations and atomic structural characterization. Also related with the structural defects, we find that the magnetic state of Fe and Co nanoparticles can be tuned by thermal treatment enabling one to tailor their magnetic properties for applications. This work demonstrates the importance of complementary single particle investigations for a better understanding of nanoparticle magnetism and for full exploration of their potential for applications.

2015, Hinzke, Denise, Schlickeiser, Frank, Yanes Díaz, Rocio, Ritzmann, Ulrike, Selzer, Severin, Nowak, Ulrich

2013, Yanes Díaz, Rocio, Jackson, J., Udvardi, László, Szunyogh, László, Nowak, Ulrich

The exchange bias effect in a compensated IrMn3/Co(111) system is studied using multiscale modeling from ab initio to atomistic spin model calculations. We evaluate numerically the out-of-plane hysteresis loops of the bilayer for different thicknesses of the ferromagnetic layer. The results show the existence of a perpendicular exchange bias and an enhancement of the coercivity of the system. To identify the origin of the exchange bias, we analyze the hysteresis loops of a selected bilayer by tuning the different contributions to the exchange interaction across the interface. Our results indicate that the exchange bias is primarily induced by Dzyaloshinskii-Moriya interactions, while the coercivity is increased mainly due to a spin-flop mechanism.

2017-09-27, Hirohata, Atsufumi, Huminiuc, Teodor, Sinclair, John, Wu, Haokaifeng, Samiepour, Marjan, Vallejo-Fernandez, Gonzalo, O’Grady, Kevin, Balluf, Jan, Yanes Díaz, Rocio, Nowak, Ulrich

As a platinum group metal, iridium (Ir) is the scarcest element on the earth but it has been widely used as an antiferromagnetic layer in magnetic recording, crucibles and spark plugs due to its high melting point. In magnetic recording, antiferromagnetic layers have been used to pin its neighbouring ferromagnetic layer in a spin-valve read head in a hard disk drive for example. Recently, antiferromagnetic layers have also been found to induce a spin-polarised electrical current. In these devices, the most commonly used antiferromagnet is an Ir–Mn alloy because of its corrosion resistance and the reliable magnetic pinning of adjacent ferromagnetic layers. It is therefore crucial to explore new antiferromagnetic materials without critical raw materials. In this review, recent research on new antiferromagnetic Heusler alloys and their exchange interactions along the plane normal is discussed. These new antiferromagnets are characterised by very sensitive magnetic and electrical measurement techniques recently developed to determine their characteristic temperatures together with atomic structural analysis. Mn-based alloys and compounds are found to be most promising based on their robustness against atomic disordering and large pinning strength up to 1.4 kOe, which is comparable with that for Ir–Mn. The search for new antiferromagnetic films and their characterisation are useful for further miniaturisation and development of spintronic devices in a sustainable manner.

2017, Rózsa, Levente, Palotás, Krisztián, Deák, András, Simon, Eszter, Yanes Díaz, Rocio, Udvardi, László, Szunyogh, László, Nowak, Ulrich

We observe metastable localized spin configurations with topological charges ranging from Q=-3 to Q=2 in a (Pt_0.95Ir_0.05)/Fe bilayer on Pd(111) surface by performing spin dynamics simulations, using a classical Hamiltonian parametrized by ab initio calculations. We demonstrate that the frustration of the isotropic exchange interactions is responsible for the creation of these various types of skyrmionic structures. The Dzyaloshinsky--Moriya interaction present due to the breaking of inversion symmetry at the surface energetically favors skyrmions with Q=-1, distorts the shape of the other objects, and defines a preferred orientation for them with respect to the underlying lattice.

2015, Hasselberg, Georg, Yanes Díaz, Rocio, Hinzke, Denise, Sessi, Paolo, Bode, Matthias, Szunyogh, Laszlo, Nowak, Ulrich

We report a detailed study of the magnetic properties of a monoatomic layer of Mn on W(110). By comparing multiscale numerical calculations with measurements we evaluate the magnetic ground state of the system and its temperature-dependent evolution. We find that the ground state consists of a cycloidal spin spiral (CSS) that persists up to the Néel temperature with a temperature-independent wavelength. However, by continuously increasing the temperature, that CSS becomes thermally depinned. This results in a time-averaged absence of magnetic order, a process that can be viewed as the antiferromagnetic analog of superparamagnetism.

2017-08-29, Yanes Díaz, Rocio, Simon, Eszter, Keller, Sebastian, Nagyfalusi, Balazs, Khmelevsky, Sergii, Szunyogh, Laszlo, Nowak, Ulrich

Based on a multi-scale calculations, combining ab-initio methods with spin dynamics simulations, we perform a detailed study of the magnetic behavior of Ni₂MnAl/Fe bilayers. Our simulations show that such a bilayer exhibits a small exchange bias effect when the Ni₂MnAl Heusler alloy is in a disordered B2 phase. Additionally, we present an effective way to control the magnetic structure of the Ni₂MnAl antiferromagnet, in the pseudo-ordered B2-I as well as the disordered B2 phases, via a spin-flop coupling to the Fe layer.

2016-06-08T09:15:19Z, Rózsa, Levente, Deák, András, Simon, Eszter, Yanes Díaz, Rocio, Udvardi, László, Szunyogh, László, Nowak, Ulrich

We determined the parameters of a classical spin Hamiltonian describing an Fe monolayer on Pd(111) surface with a Pt_1-xIr_x alloy overlayer from ab initio calculations. While the ground state of the system is ferromagnetic for x=0.00, it becomes a spin spiral state as Ir is intermixed into the overlayer. Although the Dzyaloshinsky-Moriya interaction is present in the system, we will demonstrate that the frustrated isotropic exchange interactions play a prominent role in creating the spin spiral state, and these frustrated couplings lead to an attractive interaction between skyrmions at short distances. Using spin dynamics simulations, we show that under these conditions the individual skyrmions form clusters, and that these clusters remain stable at finite temperature.

2014, Balan, Ana, Derlet, Peter M., Rodríguez, Arantxa Fraile, Bansmann, Joachim, Yanes Díaz, Rocio, Nowak, Ulrich, Kleibert, Armin, Nolting, Frithjof

X-ray photoemission electron microscopy combined with x-ray magnetic circular dichroism is used to study the magnetic properties of individual iron nanoparticles with sizes ranging from 20 down to 8 nm. While the magnetocrystalline anisotropy of bulk iron suggests superparamagnetic behavior in this size range, ferromagnetically blocked particles are also found at all sizes. Spontaneous transitions from the blocked state to the superparamagnetic state are observed in single particles and suggest that the enhanced magnetic energy barriers in the ferromagnetic particles are due to metastable, structurally excited states with unexpected life times.