Tunable high-field magnetization in strongly exchange-coupled freestanding Co/CoO core/shell coaxial nanowires

German Salazar-Alvarez, Julian Geshev, Sebastia Agramunt-Puig, Carles Navau, Alvar Sanchez, Jordi Sort, and Josep Nogués

ACS Applied Materials and Interfaces, 2016
DOI: 10.1021/acsami.6b05588

Abstract:
The exchange bias properties of Co/CoO coaxial core/shell nanowires have been investigated with cooling and applied fields perpendicular to the wire axis. This configuration leads to unexpected exchange-bias effects. Firstly, the magnetization value at high fields is found to depend on the field-cooling conditions. This effect arises from the competition between the magnetic anisotropy and the Zeeman energies for cooling fields perpendicular to the wire axis. This allows imprinting pre-defined magnetization states to the AFM, as corroborated by micromagnetic simulations. Secondly, the system exhibits a high-field magnetic irreversibility, leading to open hysteresis loops, attributed to the AFM easy-axis reorientation during the reversal (effect similar to athermal training). A distinct way to manipulate the high-field magnetization in exchange-biased systems, beyond the archetypical effects, is thus experimentally and theoretically demonstrated.

[OPEN ACCESS] Origin of the Large Dispersion of Magnetic Properties in Nanostructured Oxides: FexO/Fe3O4 Nanoparticles as a Case Study

M. Estrader, A. López-Ortega, I. V. Golosovsky, S. Estradé, G. Salazar-Alvarez, Ll. López- Conesa, D. Tobia, E. Winkler, J. D. Ardisson, W.A.A. Macedo, M. Vasilakaki, K. N. Trohidou, R. D. Zysler, F. Peiró, L. Bergström, and J. Nogués

Nanoscale (2015)
DOI: 10.1039/C4NR06351A

Abstract:
The intimate relationship in transition-metal oxides between stoichiometry and physiochemical properties makes them appealing as tunable materials. These features become exacerbated when dealing with nanostructures. However, due to the complexity of nanoscale materials, establishing a distinct relationship between structure-morphology and functionalities is often complicated. In this regard, in the FexO/Fe3O4 system a largely unexplained broad dispersion of magnetic properties has been observed. Here we show, thanks to a comprehensive multi-technique approach, a clear correlation between magneto-structural properties in large (45 nm) and small (9 nm) FexO/Fe3O4 core/shell nanoparticles that can explain the spread of magnetic behaviors. The results reveal that while the FexO core in the large nanoparticles is antiferromagnetic and has bulk-like stoichiometry and unit-cell parameters, the FexO core in the small particles is highly non-stoichiometric and strained, displaying no significant antiferromagnetism. These results highlight the importance of ample characterization to fully understand the properties of nanostructured metal oxides.

Robust antiferromagnetic coupling in hard-soft bi-magnetic core/shell nanoparticles

Nature Communications 4 (2013) 2960.

doi:10.1038/ncomms3960

M. Estrader, A. López-Ortega, S. Estradé, I. V. Golosovsky, G. Salazar-Alvarez, M. Vasilakaki, K. N. Trohidou, M. Varela, D. C. Stanley, M. Sinko, M. J. Pechan, D. J. Keavney, F. Peiró, S. Suriñach, M. D. Baró & J. Nogués

Abstract:

The growing miniaturization demand of magnetic devices is fuelling the recent interest in bi-magnetic nanoparticles as ultimate small components. One of the main goals has been to reproduce practical magnetic properties observed so far in layered systems. In this context, although useful effects such as exchange bias or spring magnets have been demonstrated in core/shell nanoparticles, other interesting key properties for devices remain elusive. Here we show a robust antiferromagnetic (AFM) coupling in core/shell nanoparticles which, in turn, leads to the foremost elucidation of positive exchange bias in bi-magnetic hard-soft systems and the remarkable regulation of the resonance field and amplitude. The AFM coupling in iron oxide—manganese oxide based, soft/hard and hard/soft, core/shell nanoparticles is demonstrated by magnetometry, ferromagnetic resonance and X-ray magnetic circular dichroism. Monte Carlo simulations prove the consistency of the AFM coupling. This unique coupling could give rise to more advanced applications of bi-magnetic core/shell nanoparticles.

[OPEN ACCESS] Anomalous Magnetic Properties of Nanoparticles Arising from Defect Structures: Topotaxial Oxidation of Fe1-xO|Fe3-δO4 Core|Shell Nanocubes to Single-Phase Particles

ACS Nano, 2013

DOI: 10.1021/nn402487q

Erik Wetterskog , Cheuk-Wai Tai , Jekabs Grins , Lennart Bergström, and German Salazar-Alvarez

Abstract:

Here we demonstrate that the anomalous magnetic properties of iron oxide nanoparticles are correlated with defects in their interior. We studied the evolution of microstructure and magnetic properties of biphasic core|shell Fe1-xO|Fe3-δO4 nanoparticles synthesized by thermal decomposition during their topotaxial oxidation to single-phase nanoparticles. Geometric phase analysis of high-resolution electron microscopy images reveals a large interfacial strain at the core|shell interface and the development of anti-phase boundaries. Dark-field transmission electron microscopy and powder x-ray diffraction concur that, as the oxidation proceeds, the interfacial strain is released as the Fe1-xO core is removed, but that the anti-phase boundaries remain. The anti-phase boundaries result in anomalous magnetic behavior, i.e., a reduced saturation magnetization and exchange bias effects in single-phase nanoparticles. Our results indicate that internal defects play an important role in dictating the magnetic properties of iron oxide nanoparticles.

Strongly exchange coupled inverse ferrimagnetic soft|hard, MnxFe3-xO4|FexMn3-xO4, core|shell heterostructured nanoparticles

http://pubs.rsc.org/en/content/articlelanding/2012/nr/c2nr30986f

Nanoscale, 2012,4, 5138-5147

Marta Estrader ,  Alberto López-Ortega ,  German Salazar-Alvarez,  Igor Golosovsky ,  Marianna Vasilakaki ,  Kalliopi Trohidou , David Keavney ,  Randy Dumas ,  Jordi Sort ,  Sonia Estrade , Dolors Baro ,  Santiago Suriñach ,  Francesca Peiro and Josep Nogués

Inverted soft|hard, in contrast to conventional hard|soft, bi-magnetic core|shell nanoparticles of Mn_xFe_3-xO₄|Fe_xMn_3-xO₄ with two different core sizes (7.5 and 11.5 nm) and fixed shell thickness (~ 0.6 nm) have been synthesized. The structural characterization suggests that the particles have an interface with a graded composition. The magnetic characterization confirms the inverted soft|hard structure and evidences a strong exchange coupling between the core and the shell. Moreover, larger soft core sizes exhibit smaller coercivities and loop shifts, but larger blocking temperatures, as expected from spring-magnet or graded anisotropy structures. The results indicate that, similar to thin film systems, the magnetic properties of soft|hard core|shell nanoparticles can be fine tuned to match specific applications.

Size-Dependent Passivation Shell and Magnetic Properties in Antiferromagnetic/Ferrimagnetic Core/Shell MnO Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/ja1021798

J. Am. Chem. Soc., 2010, 132 (27), pp 9398–9407
Alberto López-Ortega, Dina Tobia, Elin Winkler, Igor V. Golosovsky, German Salazar-Alvarez, Sònia Estradé, Marta Estrader, Jordi Sort, Miguel Angel González, Santiago Suriñach, Jordi Arbiol, Francesca Peiró, Roberto D. Zysler, Maria Dolors Baró, Josep Nogués

DOI: 10.1021/ja1021798

Abstract:

The magnetic properties of bimagnetic core/shell nanoparticles consisting of an antiferromagnetic MnO core and a ferrimagnetic passivation shell have been investigated. It is found that the phase of the passivation shell (γ-Mn₂O₃ or Mn₃O₄) depends on the size of the nanoparticles. Structural and magnetic characterizations concur that while the smallest nanoparticles have a predominantly γ-Mn₂O₃ shell, larger ones have increasing amounts of Mn₃O₄. A considerable enhancement of the Nel temperature, T_N, and the magnetic anisotropy of the MnO core for decreasing core sizes has been observed. The size reduction also leads to other phenomena such as persistent magnetic moment in MnO up to high temperatures and an unusual temperature behavior of the magnetic domains.

Direct evidence of imprinted vortex states in the antiferromagnet of exchange biased microdisks

http://apl.aip.org/resource/1/applab/v95/i1/p012510_s1
Appl. Phys. Lett. 95, 012510 (2009)
G. Salazar-Alvarez, J. J. Kavich, J. Sort, A. Mugarza, S. Stepanow, A. Potenza, H. Marchetto, S. S. Dhesi, V. Baltz, B. Dieny, A. Weber, L. J. Heyderman, J. Nogués, and P. Gambardella doi:10.1063/1.3168515

Abstract:
The magnetic domain structure of patterned antiferromagnetic/ferromagnetic Ir₂₀Mn₈₀/Ni₈₀Fe₂₀ bilayer microdisk arrays has been investigated using layer-specific polarized x-ray photoemission electron microscopy and magnetic circular dichroism. Magnetic imaging at the Fe and Mn L-edge resonances provided direct evidence of a vortex state imprinted into the antiferromagnet at the interface. The opposite magnetic contrast between the layers indicated a reversed chirality of the imprinted vortex state, and a quantitative analysis of the magnetic moment from the dichroism spectra showed that uncompensated Mn spins equivalent to about 60% of a monolayer of bulk Ir₂₀Mn₈₀ contributed to the imprinted information at the interface.

Magnetic Proximity Effect Features in Antiferromagnetic/Ferrimagnetic Core-Shell Nanoparticles

Phys. Rev. Lett. 102, 247201 (2009)

URL:

http://link.aps.org/doi/10.1103/PhysRevLett.102.247201

I. V. Golosovsky, G. Salazar-Alvarez, A. López-Ortega, M. A. González, J. Sort, M. Estrader, S. Suriñach, M. D. Baró, and J. Nogués DOI:

10.1103/PhysRevLett.102.247201

Abstract:
A study of “inverted” core-shell, MnO/γ-Mn₂O₃, nanoparticles is presented. Crystal and magnetic structures and characteristic sizes have been determined by neutron diffraction for the antiferromagnetic core (MnO) and the ferrimagnetic shell (γ-Mn₂O₃). Remarkably, while the MnO core is found to have a T_N not far from its bulk value, the magnetic order of the γ-Mn₂O₃ shell is stable far above T_C, exhibiting two characteristic temperatures, at T∼40  K [T_C(γ-Mn₂O₃)] and at T∼120  K [∼T_N(MnO)]. Magnetization measurements are consistent with these results. The stabilization of the shell moment up to T_N of the core can be tentatively attributed to core-shell exchange interactions, hinting at a possible magnetic proximity effect.

Synthesis and Size-Dependent Exchange Bias in Inverted Core−Shell MnO|Mn3O4 Nanoparticles

http://pubs.acs.org/doi/abs/10.1021/ja0714282

J. Am. Chem. Soc., 2007, 129 (29), pp 9102–9108
German Salazar-Alvarez, Jordi Sort, Santiago Suriñach, M. Dolors Baró, and Josep Nogués

DOI: 10.1021/ja0714282

Abstract:
Core−shell nanoparticles of MnO|Mn₃O₄ with average particle sizes of 5−60 nm, composed of an antiferromagnetic (AFM) core and a ferrimagnetic (FiM) shell, have been synthesized and their magnetic properties investigated. The core−shell structure has been generated by the passivation of the MnO cores, yielding an inverted AFM-core|FiM-shell system, as opposed to the typical FM-core|AFM-shell. The exchange-coupling between AFM and FiM gives rise to an enhanced coercivity of 8 kOe and a loop shift of 2 kOe at 10 K, i.e., exchange bias. The coercivity and loop shift show a non-monotonic variation with the core diameter. The large coercivity and the loop shift are ascribed to the highly anisotropic Mn₃O₄ and size effects of the AFM (i.e., uncompensated spins, AFM domains, and size-dependent transition temperature).