Spin canting across core/shell Fe3O4/MnxFe3−xO4 nanoparticles

Magnetic nanoparticles (MNPs) have become increasingly important in biomedical applications like magnetic imaging and hyperthermia based cancer treatment. Understanding their magnetic spin configurations is important for optimizing these applications. The measured magnetization of MNPs can be significantly lower than bulk counterparts, often due to canted spins. This has previously been presumed to be a surface effect, where reduced exchange allows spins closest to the nanoparticle surface to deviate locally from collinear structures. We demonstrate that intraparticle effects can induce spin canting throughout a MNP via the Dzyaloshinskii-Moriya interaction (DMI). We study ~7.4 nm diameter, core/shell Fe3O4/MnxFe3−xO4 MNPs with a 0.5 nm Mn-ferrite shell. Mössbauer spectroscopy, x-ray absorption spectroscopy and x-ray magnetic circular dichroism are used to determine chemical structure of core and shell. Polarized small angle neutron scattering shows parallel and perpendicular magnetic correlations, suggesting multiparticle coherent spin canting in an applied field. Atomistic simulations reveal the underlying mechanism of the observed spin canting. These show that strong DMI can lead to magnetic frustration within the shell and cause canting of the net particle moment. These results illuminate how core/shell nanoparticle systems can be engineered for spin canting across the whole of the particle, rather than solely at the surface

Using different Mn-oxides to influence the magnetic anisotropy of FePt in bilayers with little change of the exchange bias field

We have investigated the exchange coupling between a bottom FePt thin film layer capped withdifferent Mn-oxides. Results have shown that the magnetization reversal of the soft FePt layer isinfluenced strongly by the capped Mn-oxide layer (Mn, MnO, and Mn3O4), as revealed by theenhanced coercivities. Typical temperature dependent magnetization between zero-field cooled(ZFC) and field cooled (FC) scans was observed in the Mn-oxide (8%O2/Ar)/FePt bilayer thatexhibited a blocking temperature (TB 120 K) close to the Ne`el temperature, TN, of MnO.However, the Mn/FePt bilayer exhibited unusual temperature dependent of M vs. T, implying thatintermixing between Mn and FePt interfaces formed an AF FeMn that may have enabled a highirreversibility temperature (Tirr. 400 K) compared to almost identical ZFC and FC curves fromweaker exchange coupling between FePt and the Mn3O4 created with 21 and 30%O2/Ar depositionconditions

Altering the exchange bias in Co90Fe10/(Co,Fe)O bilayers by changing the antiferromagnet's magnetism via interfacial ion-beam bombardment and different single crystalline MgO substrates

In this study, we investigated the exchange bias (coupling) effects in CoFe/(Co,Fe)O bilayers byusing different single crystal substrates of MgO(100) and MgO(110) and Ar ion-beambombardment on the surface of the bottom antiferromagnet (Co,Fe)O layer before capping withferromagnet CoFe. In the CoFe/(Co,Fe)O/MgO(110) bilayer, above the irreversibility temperature(Tirr. 170 K), there was a rapid decrease in M(T) with increasing temperature, unlike the CoFe/(Co,Fe)O/MgO(100) film that showed an increased Tirr. 300K and no observable decrease inM(T) above Tirr. The different M vs T zero-field-cooled/field-cooled behavior of the CoFe/(Co,Fe)O bilayers on MgO(100) and MgO(110) indicated that the FM CoFe spin orientationswere affected by the different substrates used via exchange coupling to the AF (Co,Fe)O layeraltered by MgO

Comment on “Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe₂O₄ Nanoparticles”

Comment on “Colossal Reduction in Curie Temperature Due to Finite-Size Effects in CoFe₂O₄ Nanoparticles

Interface mixing and its impact on exchange coupling in exchange biased systems

Exchange bias and interlayer exchange coupling are interface driven phenomena. Since an ideal interface is very challenging to achieve, a clear understanding of the chemical and magnetic natures of interfaces is pivotal to identify their influence on the magnetism. We have chosen Ni80Fe20/CoO(t CoO)/Co trilayers as a model system, and identified non-stoichiometric Ni-ferrite and Co-ferrite at the surface and interface, respectively. These ferrites, being ferrimagnets typically, should influence the exchange coupling. However, in our trilayers the interface ferrites were found not to be ferro- or ferri-magnetic; thus having no observable influence on the exchange coupling. Our analysis also revealed that (i) interlayer exchange coupling was present between Ni80Fe20 and Co even though the interlayer thickness was significantly larger than expected for this phenomenon to happen, and (ii) the majority of the CoO layer (except some portion near the interface) did not contribute to the observed exchange bias. We also identified that the interlayer exchange coupling and the exchange bias properties were not interdependent

Core/Shell Bimagnetic Nanoparticles

The advances in the physical and chemical fabrication methods have enabled the possibility to produce artificial nanostructures whose properties are different from that of their constituent materials. The presence of interfaces in core/shell bimagnetic nanoparticles introduces additional interactions that could radically modify the static and dynamic magnetic behavior of the systems. The number of parameters that governs the magnetic behavior grows enormously and the opportunity to manipulate, control, and understand the role played by each one of them, opens a wide range of possibilities to design novel materials with suited properties. The magnetic response changes depend on the magnetic ordering and anisotropy of the phases, the core size and shell thickness, the quality of the interface, and the strength of the interface exchange coupling. In this chapter, we discuss the new properties found in core/shell bimagnetic nanoparticles and analyze the main characteristics that have to be taken into account to design a system with a particular response.Fil: Winkler, Elin Lilian. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física. Laboratorio de Resonancias Magnéticas; Argentina. Comisión Nacional de Energía Atómica. Gerencia del Área de Energía Nuclear. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro | Universidad Nacional de Cuyo. Instituto Balseiro. Archivo Histórico del Centro Atómico Bariloche e Instituto Balseiro; ArgentinaFil: Zysler, Roberto Daniel. Comisión Nacional de Energía Atómica. Gerencia del Área de Investigación y Aplicaciones No Nucleares. Gerencia de Física. Laboratorio de Resonancias Magnéticas; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Oficina de Coordinacion Administrativa Ciudad Universitaria. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche | Comision Nacional de Energia Atomica. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia. Unidad Ejecutora Instituto de Nanociencia y Nanotecnologia - Nodo Bariloche.; Argentin