Thomas Saerbeck

Coercivity enhancement in V2O3/Ni bilayers driven by nanoscale phase coexistence

We studied the temperature dependence of coercivity and magnetization of V2O3/Ni bilayers across the Structural Phase Transition in V2O3. We found a coercivity peak that coincides with the V2O3 phase transition on top of an overall increase of the coercivity with decreasing temperature. We propose that this sharp increase arises from a length scale competition between magnetic domains of Ni and phase coexistence during the V2O3 phase transition. This model is supported by micromagnetic simulations and shows that magnetic properties of ferromagnetic films are strongly affected by a proximal first order phase transition.

Exchange bias: The antiferromagnetic bulk matters

Using controlled ion bombardment, the contribution of interface and bulk antiferromagnetic spins to exchange bias (EB) is investigated. Several sets of ferromagnetic (FM)/antiferromagnetic (AFM) (Ni/FeF2) bilayers capped with a nonmagnetic and inert Au layer of varying thickness were grown simultaneously. He-ion bombardment was employed to selectively create defects in the EB structure at the FM/AFM interface or in the AFM bulk. Numerical simulations provide the depth profile of the ion damage. Quantitative structural and magnetic characterizations were compared before and after the bombardment revealing the relationship between interfacial and bulk located defects. These studies show that the creation of defects in the bulk of the antiferromagnet crucially affects the magnitude of EB.

Unveiling structural, chemical and magnetic interfacial peculiarities in ε -Fe 2 O 3 /GaN (0001) epitaxial films

The metastable ε-Fe2O3 is known to be the most intriguing ferrimagnetic and multiferroic iron oxide phase exhibiting a bunch of exciting physical properties both below and above room temperature. The present paper unveils the structural and magnetic peculiarities of a few nm thick interface layer discovered in these films by a number of techniques. The polarized neutron reflectometry data suggests that the interface layer resembles GaFeO3 in composition and density and is magnetically softer than the rest of the ε-Fe2O3 film. While the in-depth density variation is in agreement with the transmission electron microscopy measurements, the layer-resolved magnetization profiles are qualitatively consistent with the unusual wasp-waist magnetization curves observed by superconducting quantum interference device magnetometry. Interestingly a noticeable Ga diffusion into the ε-Fe2O3 films has been detected by secondary ion mass spectroscopy providing a clue to the mechanisms guiding the nucleation of exotic metastable epsilon ferrite phase on GaN at high growth temperature and influencing the interfacial properties of the studied films.

Chapter Three – Magnetic Exchange Phenomena Probed by Neutron Scattering

Neutron scattering techniques are powerful tools for investigation of magnetic materials and magnetic exchange interactions on the nanoscale. Owing to the weak interaction of the neutron, results are representative of the bulk and not only comprised of surface properties. Nevertheless, due to the high atomic and magnetic contrast, high interface sensitivity is easily achieved. This chapter will review the applicability of neutron scattering techniques to the investigation magnetic exchange interactions based on two examples, interlayer exchange coupling and exchange bias in metallic multilayers. PNR will be employed to study the magnetic state of a Cu0.94Mn0.06/Co multilayer showing a temperature-dependent coupling originating from the dilute magnetic impurities. Exchange bias within mono-stoichiometric FePt3 thin films based on chemical order modulation will be investigated with PNR and diffraction techniques to elucidate the magnetic ordering on nanometer and atomic length scales. During the individual discussion of the phenomena, both interlayer exchange coupling and exchange bias will be reviewed.

Growth-Induced In-Plane Uniaxial Anisotropy in V 2 O 3 /Ni Films

We report on a strain-induced and temperature dependent uniaxial anisotropy in V2O3/Ni hybrid thin films, manifested through the interfacial strain and sample microstructure, and its consequences on the angular dependent magnetization reversal. X-ray diffraction and reciprocal space maps identify the in-plane crystalline axes of the V2O3; atomic force and scanning electron microscopy reveal oriented rips in the film microstructure. Quasi-static magnetometry and dynamic ferromagnetic resonance measurements identify a uniaxial magnetic easy axis along the rips. Comparison with films grown on sapphire without rips shows a combined contribution from strain and microstructure in the V2O3/Ni films. Magnetization reversal characteristics captured by angular-dependent first order reversal curve measurements indicate a strong domain wall pinning along the direction orthogonal to the rips, inducing an angular-dependent change in the reversal mechanism. The resultant anisotropy is tunable with temperature and is most pronounced at room temperature, which is beneficial for potential device applications.

Two state coercivity driven by phase coexistence in vanadium sesquioxide/nickel bulk hybrid material

We developed a bulk hybrid material consisting of a vanadium sesquioxide (V2O3) matrix with nickel (Ni) rich inclusions that exhibit a switchable two-state magnetic coercivity. The V2O3 matrix magnetoelastically couples with the Ni-rich inclusions and its structural phase transition causes two possible magnetic coercivity states. Differences of up to 13% in the temperature window of 14 K are observed, depending whether the transition occurs from rhombohedral to monoclinic or vice versa. These findings provide a pathway for the development of bulk switchable coercivity materials. We present routes to further enhance the magnetoelastic effect by increasing the oxide/ferromagnetic material coupling.

Simple transition metal oxides(Conference Presentation)

Hybrid materials allow the engineering of new material properties by creative uses of proximity effects. When two dissimilar materials are in close physical proximity the properties of each one may be radically modified or occasionally a completely new material emerges. In the area of magnetism, controlling the magnetic properties of ferromagnetic thin films without magnetic fields is an on- going challenge with multiple technological implications for low- energy consumption memory and logic devices. Interesting possibilities include ferromagnets in proximity to dissimilar materials such as antiferromagnets or oxides that undergo metal-insulator transitions. The proximity of ferromagnets to antiferromagnets has given rise to the extensively studied Exchange Bias[1]. Our recent investigations in this field have addressed crucial issues regarding the importance of the antiferromagnetic [2-3] and ferromagnetic [4] bulk for the Exchange Bias and the unusual short time dynamics [5]. In a series of recent studies, we have investigated the magnetic properties of different hybrids of ferromagnets (Ni, Co and Fe) and oxides, which undergo metal-insulator and structural phase transitions. Both the static as well as dynamical properties of the ferromagnets are drastically affected. Static properties such as the coercivity, anisotropy and magnetization [6-8] and dynamical properties such as the microwave response are clearly modified by the proximity effect and give raise to interesting perhaps useful properties. Work supported by US-AFOSR and US-DOE

Charge injection across a metal-organic interface suppressed by thermal diffusion

We find that the ohmic conductance of Co-phthalocyanine (CoPc) vertical capacitive devices is irreversibly suppressed by orders of magnitude when they are heated above 340 K. Detailed structural and transport studies imply that the changes in the conductance are due to diffusion of the top Pd electrode into the CoPc layer. This leads to a decrease in Pd electrode effective work function, which increases the potential barrier for hole injection.