Surface spin canting probed by EELS

Surface spin canting in Fe3O4 and CoFe2O4 NP probed by high-resolution electron energy loss spectroscopy.

D. S. Negi, H. Sharona, U. Bhat, S. Palchoudhury, A. Gupta, and R. Datta
Phys. Rev. B 95, 174444 (2017)

Experimental L3 spectra of CFO recorded (a) and (c) at room temperature and (b) and (d) at liquid nitrogen temperature (77 K) for Fe and Co atoms, respectively. The spectra from core and edge of nanoparticles are colored with green and red, respectively. Dominating features from Td and Oh atomic sites are marked. Kindly note the fine features are only sharper for Co atoms, but not for Fe atoms, suggesting possible formation of uniformly oriented spin canting configuration for Fe atoms but core-shell morphology for Co atoms.

Thermal Decomposition Synthesis of Iron Oxide Nanoparticles with Diminished Magnetic Dead Layer by Controlled Addition of Oxygen - ACS Nano (ACS Publications)

Thermal Decomposition Synthesis of Iron Oxide Nanoparticles with Diminished Magnetic Dead Layer by Controlled Addition of Oxygen.

Mythreyi Unni,Amanda M. Uhl, Shehaab Savliwala, Benjamin H. Savitzky, Rohan Dhavalikar, Nicolas Garraud, David P Arnold, Lena F. Kourkoutis, Jennifer S. Andrew, and Carlos Rinaldi

ACS Nano 11, 2284 (2017)

Decades of research focused on size and shape control of iron oxide nanoparticles have led to methods of synthesis that afford excellent control over physical size and shape but comparatively poor control over magnetic properties. Popular synthesis methods based on thermal decomposition of organometallic precursors in the absence of oxygen have yielded particles with mixed iron oxide phases, crystal defects, and poorer than expected magnetic properties, including the existence of a thick “magnetically dead layer” experimentally evidenced by a magnetic diameter significantly smaller than the physical diameter. Here, we show how single-crystalline iron oxide nanoparticles with few defects and similar physical and magetic diameter distributions can be obtained by introducing molecular oxygen as one of the reactive species in the thermal decomposition synthesis. This is achieved without the need for any postsynthesis oxidation or thermal annealing. These results address a significant challenge in the synthesis of nanoparticles with predictable magnetic properties and could lead to advances in applications of magnetic nanoparticles.

Surface vacancy mediated pinning in maghemite NP

Surface vacancy mediated pinning of the magnetization in γ−Fe2O3 nanoparticles: A micromagnetic simulation study

Bassel Alkadour, J. I. Mercer, J. P. Whitehead, J. van Lierop, and B. W. Southern

Phys. Rev. B 93, 140411(R) (2016)

The energy landscape for a nanoparticle selected at random from the K10 enemble. Each point on the surface of the sphere represents the energy associated with the alignment of the magnetic moment. The energy is calculated using a mean field approximation based on the distribution of surface vacancies and the average angular distribution of the energy per spins at T=0 shown in Fig. 4. The energy scale associated with the color map shown on the right is given in K.

Colloidal magnetic nanocrystal clusters: Review

Colloidal magnetic nanocrystal clusters: variable length-scale interaction mechanisms, synergetic functionalities and technological advantages.

Athanasia Kostopoulou, Alexandros Lappas

Nanotechnology Reviews 4, 595(2015)

Scheme summarizing common parameters, including, size and shape [surfaces] anisotropies, as well as exchange [interfaces] and dipolar [particle clustering] interactions. Each one of them alone or in synergy with one another may act in favor of enhanced magnetic properties (M_S, H_C) for surface-stabilized nanoscale structures. Nanoclusters may provide a modular carrier engineered to combine all such parameters for the benefit of their emerging application-specific tasks, for example, such as those required for contrast generation (r₂) in magnetic resonance imaging (MRI) and heat dissipation (SLP) in magnetic hyperthermia.

Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles

Spin-glass-like freezing of inner and outer surface layers in hollow γ-Fe2O3 nanoparticles.

Hafsa Khurshid, Paula Lampen-Kelley, Òscar Iglesias, Javier Alonso, Manh-Huong Phan, Cheng-Jun Sun, Marie-Louise Saboungi & Hariharan Srikanth

Scientific Reports 5, 15054 (2015) 

Snapshots of the outer and inner surface spin configurations subject to varying magnetic fields (a) h = 100 (the maximum positive applied field), (b) h = 0 (remanence at the upper branch), (c) h = −25 (near the negative coercive field), and (d) h = −100 (the maximum negative applied field). Spins have been colored with a gradient from dark-red/dark-blue (outer/inner surface) for spins along the field direction to yellow/green (outer/inner surface) for spins transverse to the field direction. Only a slice of the spin configurations of a hollow nanoparticle close to the central plane and perpendicular to the field direction is shown.

Enhancing the magnetic anisotropy of maghemite nanoparticles via the surface coordination of molecular complexes : Nature Communications : Nature Publishing Group

Enhancing the magnetic anisotropy of maghemite nanoparticles via the surface coordination of molecular complexes.

Yoann Prado, Niéli Daffé, Aude Michel, Thomas Georgelin, Nader Yaacoub, Jean-Marc Grenèche, Fadi Choueikani, Edwige Otero, Philippe Ohresser, Marie-Anne Arrio, Christophe Cartier-dit-Moulin, Philippe Sainctavit, Benoit Fleury, Vincent Dupuis, Laurent Lisnard, Jérôme Fresnais

Nature Communications 6, 10139 (2015)

(a) Field-cooled and zero-field-cooled (FC/ZFC) magnetization curves measured in the 5–80 K temperature range under an applied field of 50 Oe and (b) magnetization vs field curves measured at 5 K for 0b and 1 in diluted solutions.

Enhancing the magnetic anisotropy of maghemite nanoparticles via the surface coordination of molecular complexes : Nature Communications : Nature Publishing Group

Enhancing the magnetic anisotropy of maghemite nanoparticles via the surface coordination of molecular complexes.

Yoann Prado, Niéli Daffé, Aude Michel, Thomas Georgelin, Nader Yaacoub, Jean-Marc Grenèche, Fadi Choueikani, Edwige Otero, Philippe Ohresser, Marie-Anne Arrio, Christophe Cartier-dit-Moulin, Philippe Sainctavit, Benoit Fleury, Vincent Dupuis, Laurent Lisnard; Jérôme Fresnais

Nature Communications 6, 10139 (2015)

(a) Representation of the [Co(TPMA)Cl₂] complex used to enhance the magnetic anisotropy of the γ-Fe₂O₃ nanoparticles. (b) TEM image of the γ-Fe₂O₃ nanoparticles functionalized with the cobalt(II) complex: 1 (5.0 nm, σ=0.09) and (c) schematic view of the coordination of the complex with the iron ions. (d) Synthesis scheme with measured pH values, hydrodynamic diameters (Z_av), sizes (D) and distributions (σ).

Tuning the magnetic anisotropy in coordination nanoparticles: random distribution versus core–shell architecture.

Yoann Prado, Nada Dia, Laurent Lisnard, Guillaume Rogez, François Brisset, Laure Catala and   Talal Mallah

Chem. Commun. 48, 11455 (2012)

Core–shell magnetic coordination nanoparticles made of a soft core and a hard magnetic shell, containing anisotropic Co(II) ions, display a dramatic increase in their average blocking temperature with a coercive field value 25 times larger than that of the soft core, due to a large enhancement of the magnetic anisotropy.

From shape to surface effects in magnetic anisotropy of Co NPs

Size effects in the magnetic anisotropy of embedded cobalt nanoparticles: from shape to surface.
Simón Oyarzún, Alexandre Tamion, Florent Tournus, Véronique Dupuis, Matthias Hillenkamp

Scientific Reports 5, 14749 (2015)

Comparison of the experimental values for with the magnetic surface anisotropy calculated using the Néel model for different example cluster sizes and single facets added along the [100] (blue, lower curve) and [111] (green, upper curve) directions.

Magnetism on a curved nano-surface

Evolution of magnetism on a curved nano-surface.

D. G. Merkel, D. Bessas, Z. Zolnai, R. Rüffer, A. I. Chumakov, H. Paddubrouskaya, C. Van Haesendonck, N. Nagy, A. L. Tóth and A. Deák

Nanoscale 7, 12878 (2015)

Magnetic moment configuration calculated by micromagnetic simulation for (a) 26 Å, (b) 34.5 Å, (c) 48.5 Å and (d) 70.5 Å evaporated iron thickness on 25 nm diameter spheres. The blue color represents the magnitude of the z component while the red stands for the x–y in-plane component.

 

Distribution of magnetic anisotropy in nanograins

Universal distribution of magnetic anisotropy of impurities in ordered and disordered nanograins
A. Szilva, P. Balla, O. Eriksson, G. Zaránd, and L. Szunyogh
Phys. Rev. B 91, 134421 (2015)

Radial distribution of the magnetic anisotropy parameters (dots) in the E plane in the case of NS=50 samples with N=225+25 atoms. The continuous line presents the predictions of the Gaussian orthogonal ensemble. Inset: Distribution of (K1,K2) in the E plane for these 50 nanograins. At this level of disorder the triangular structure is almost entirely lost.

Tuning shape, exchange and surface anisotropy of core/shell NPs

Nanoscale Magnetism Control via Surface and Exchange Anisotropy for Optimized Ferrimagnetic Hysteresis.
Seung-hyun Noh, Wonjun Na, Jung-tak Jang, Jae-Hyun Lee, Eun Jung Lee, Seung Ho Moon,
Yongjun Lim, Jeon-Soo Shin, and Jinwoo Cheon
Nano Letters 12, 3716 (2012)

Morphological and structural evolution of magnetic nanoparticle and correlated tunability of nanomagnetism. (a) Magnetic NPs with various structural motifs exhibiting differences in size, surface anisotropy, and exchange anisotropy. (b) Magnetism tuning by the systematicchanges of magnetic nanoparticles. Graphs i−iv correspond to the nanoparticles shown in part a where modulation of structural motifs is needed to control parameters such as K, Hc, Ms, or Mr.

Images and magnetization behaviors of cube and sphere nanoparticles. (a) TEM images of cube (18 nm (σ ≈ 5%) in edge length) and (b) sphere nanoparticle (22 nm (σ ≈ 7%) in diameter). Nanoparticles have identical composition (Zn_0.4Fe_2.6O₄) and magnetic volume (5.8 × 10^–24 m³). (c) High resolution TEM image of cube exhibiting well-defined lattice fringes of {100} faces. (d) M-H curves of cube and sphere measured at 300 K using SQUID. M_s of cube is 165 emu/g_(Fe+Zn), and that of sphere is 145 emu/g_(Fe+Zn). Simulated magnetic spin states of (e) cube and (f) sphere by using OOMMF program. The color map indicates the degree of spin canting against external magnetic field (B₀) where red indicates nondeviated spins and blue indicates highly canted spins. Local spin states on the surfaces of nanoparticles are depicted on the right corners of parts e and f. Cube exhibits lower spin disorder rate of 4% than sphere of 8%.

Spin-phonon coupling signature of SP in NPs

Spectroscopic Signature of the Superparamagnetic Transition and Surface Spin Disorder in CoFe2O4 Nanoparticles.
Qi -C. Sun, Christina S. Birkel, Jinbo Cao, Wolfgang Tremel, and Janice L. Musfeldt
ACS Nano 6, 4876 (2012)

Phonons are exquisitely sensitive to finite length scale effects in a wide variety of materials. To investigate confinement in combination with strong magnetoelastic interactions, we measured the infrared vibrational properties of CoFe₂O₄ nanoparticles and compared our results to trends in the coercivity over the same size range and to the response of the bulk material. Remarkably, the spectroscopic response is sensitive to the size-induced crossover to the superparamagnetic state, which occurs between 7 and 10 nm. A spin–phonon coupling analysis supports the core–shell model. Moreover, it provides an estimate of the magnetically disordered shell thickness, which increases from 0.4 nm in the 14 nm particles to 0.8 nm in the 5 nm particles, demonstrating that the associated local lattice distortions take place on the length scale of the unit cell. These findings are important for understanding finite length scale effects in this and other magnetic oxides where magnetoelastic interactions are important.

Superantiferromagnetism in NPs

Size-induced superantiferromagnetism with reentrant spin-glass behavior in metallic nanoparticles of TbCu2.
C. Echevarria-Bonet, D. P. Rojas, J. I. Espeso, J. Rodríguez Fernández,1 L. Rodríguez Fernández, P. Gorria, J. A. Blanco, M. L. Fdez-Gubieda, E. Bauer, G. André, and L. Fernández Barquín
Phys. Rev. B 87, 180407 (2013)

Thermomagnetic DC-susceptibility ZFC curves obtained with magnetic fields, 1 kOe H 10 kOe, where
the evolution from AF to SG state can be observed. A FC curve is also plotted at H = 1 kOe to evidence the irreversibility. Inset (bottom): Linear Tf variation vs H2/3. Inset (top): ZFC DC susceptibility of the alloy at a pressure P = 1.1 GPa.

Restoration of bulk magnetism in Fe oxide NPs by surfactant molecules

Surfactant Organic Molecules Restore Magnetism in Metal-Oxide Nanoparticle Surfaces.
Juan Salafranca, Jaume Gazquez, Nicolás Pérez, Amílcar Labarta, Sokrates T. Pantelides, Stephen J. Pennycook, Xavier Batlle, and Maria Varela
Nano Lett., 12, 2499 (2012)

Left: Density of states (DOS) projected over the majority spin d orbitals in the octahedral iron sites within different crystal environments. Organic acid bonded to magnetite surface. DOS projected over octahedral iron bonded to the organic acid. Because of the influence of the oxygen ions in the carboxylic group, occupancies are very similar to the bulk case, and the reduction of magnetization at the surface is partially lifted. Middle:  High-resolution Z-contrast STEM images of a Fe3O4 NP showing high crystal quality. Right: Top: L2,3 profile along the direction of the blue arrow in panel d (in red and in black for I+ and I− L23 ratio maps respectively). Bottom: difference between I+ and I− L2,3 ratios along the NP. The scale bar represents 5 nm in all panels. 

 

 

Surface stability of maghemite by Cu shell

Increased surface spin stability in  gamma-Fe2O3 nanoparticles with a Cu shell.R D Desautels, E Skoropata, Y-Y Chen, H Ouyang, J W Freeland and J van Lierop
J. Phys.: Condens. Matter 24, 146001 (2012)

(a) Temperature dependence of the Fe magnetization determined from the TEY XMCD signaland the Cu2C interface coating magnetization from the TFY XMCD signal. Cu(0.5 nm)/Fe2O3 and Cu(1.0 nm)/Fe2O3 NPs (b) Temperature dependence of the Fe A- and B-site magnetization.

Altered magnetism in maghemite NP by Cu shell

Tuning the surface magnetism of gamma-Fe2O3 nanoparticles with a Cu shell.
R. D. Desautels, E. Skoropata, Y.-Y. Chen, H. Ouyang, J. W. Freeland, and J. van Lierop
Appl. Phys. Lett. 99, 262501 (2011)

Increased surface spin stability in gamma -Fe2O3 nanoparticles with a Cu shell
R. D. Desautels, E. Skoropata, Y.-Y. Chen, H. Ouyang, J. W. Freeland, and J. van Lierop 
J. Phys. CM 24, 146001 (2012) 

Field dependence of the interfacial Cu in Cu-coated c-Fe2O3 nanoparticles.
R. D. Desautels, Y.-Y. Chen, H. Ouyang, S.-C. Lo, J. W. Freeland, and J. van Lierop
J. Appl. Phys. 111, 07B518 (2012) 

Temperature dependence of the saturation magnetization (Ms, top) and the exchange bias loop shift, Hex, of the bare gamma-Fe2O3 and Cu(0.5 nm)/Fe2O3 nanoparticles. The inset shows typical M vs mu0 H behavior of the Cu(0.5 nm)/gamma-Fe2O3 nanoparticles

Size effect on manganite NP

Nanometer Size Effect on Magnetic Properties of Sm0.8Ca0.2MnO3 Nanoparticles.
Vladimir Markovich, Ivan Fita, Andrzej Wisniewski, Roman Puzniak, Dmitrii Mogilyansky, Przemyslaw Iwanowski, Piotr Dluzewski, and Gad Gorodetsky
J. Phys. Chem. 116, 435 (2012)

(a-d) Temperature dependence of real component of ac susceptibility (χ0) measured during heating at different frequencies and magnetic ac field of 10 Oe for SCMO samples. Insets show the imaginary part (χ00) of ac susceptibility measured at different frequencies and magnetic ac field of 10 Oe.

Surface effects and dipolar interaction in NP

Surface effects on the magnetic behavior of nanoparticle assemblies.
G. Margaris, K. Trohidou, H. Kachkachi
Phys. Rev. B 85, 024419 (2012)

Magnetization as function of the applied field for a random anisotropy interacting assembly (g = 0.1) and for different
values of the surface anisotropy coefficients. (a) The value of the quadratic term (σ = 1) is smaller than the absolute value of the quartic term (w = ±2,±5,±8) together with the curve with only the core (quadratic) anisotropy term (w = 0). (b) The quadratic term (σ = 1) is equal or bigger than the absolute value of the quartic term (w = ±0.5,±1).

LaFeO3 NPs

Surface and shape anisotropy effects in LaFeO3 nanoparticles.
Dan Wang and Menglian Gong
JAP 109, 114304 (2011)

Hysteresis loops for the LaFeO3 nanospheres and nanotubes measured
at 2 K (a) and 293 K (b). The inset shows the enlargement of the low field data.

Finite-size effect in MnO NPs by neutrons

Antiferromagnetic order in MnO spherical nanoparticles.
C. H. Wang, S. N. Baker, M. D. Lumsden, S. E. Nagler,  W. T. Heller,  G. A. Baker, P. D. Deen, L. M. D. Cranswick, Y. Su, and A. D. Christianson
Phys. Rev. B 83, 214418 (2011)

The temperature-dependent magnetic moment
of theMnO bulk and nanoparticles of sample A