Size dependent Verwey transition

Size Dependence of Metal–Insulator Transition in Stoichiometric Fe3O4 Nanocrystals.
Jisoo Lee, Soon Gu Kwon, Je-Geun Park, and Taeghwan Hyeon
Nano Lett. 15, 4337 (2015)

Size dependence of T_V for Fe₃O₄ NCs. The contour plot represents the heat capacity data after removing the contribution of surfactant. The symbols mark the Verwey transition temperature (T_V) determined from three different types of measurements: heat capacity (green, C_P/T), conductance (blue, G), and magnetic moment (red, m). The size dependence of blocking temperature (T_B) is also plotted from the same magnetization measurement (black). The gray line is a guide to the eye for the size dependence of T_B.

 

Magnetic Interactions and Energy Barrier Enhancement in Core/Shell Bimagnetic Nanoparticles - The Journal of Physical Chemistry C (ACS Publications)

Magnetic Interactions and Energy Barrier Enhancement in Core/Shell Bimagnetic Nanoparticles.

Gabriel C. Lavorato, Davide Peddis, Enio Lima , Jr., Horacio E. Troiani, Elisabetta Agostinelli, Dino Fiorani, Roberto D. Zysler, and Elin L. Winkler

J. Phys. Chem. C 119, 15755 (2015) 

In this work, we studied the dynamic and static magnetic properties of ZnO-core/CoFe₂O₄-shell and CoO-core/CoFe₂O₄-shell nanoparticles. Both systems are formed by a core of ∼4 nm of diameter encapsulated in a shell of ∼2 nm of thickness. The mean blocking temperature changes from 106(7) to 276(5) K when the core is diamagnetic or antiferromagnetic, respectively. Magnetic remanence studies revealed the presence of weak dipolar interparticle interactions, where H_int is approximately −0.1 kOe for ZnO/CoFe₂O₄ and −0.9 kOe for CoO/CoFe₂O₄, playing a minor role in the magnetic behavior of the materials. Relaxation experiments provided evidence that the magnetization reversal process of CoFe₂O₄ is strongly dependent on the magnetic order of the core. At 10 K, activation volumes of ∼46(6) and ∼69(5) nm³ were found for CoO/CoFe₂O₄ and ZnO/CoFe₂O₄ nanoparticles, respectively, corresponding to one-third and one-fifth of the total shell volume. While the magnetic behavior of ZnO/CoFe₂O₄ nanoparticles is strongly affected by the surface disorder, the exchange coupling at the CoO/CoFe₂O₄ interface rules the magnetization reversal and the nanoparticles’ thermal stability by inducing a larger energy barrier and promoting smaller switching volume.

Diversity of Hyperthermia Response

A Single Picture Explains Diversity of Hyperthermia Response of Magnetic Nanoparticles.

Ivan Conde-Leboran , Daniel Baldomir, Carlos Martinez-Boubeta, Oksana Chubykalo-Fesenko, María del Puerto Morales, Gorka Salas, David Cabrera, Julio Camarero, Francisco J. Teran, and David Serantes

Journal of Physical Chemistry C 119, 15698 (2015)

 

Addressing the Control of NP arrangement and performance in Hyperthermia

Nano-objects for Addressing the Control of Nanoparticle Arrangement and Performance in Magnetic Hyperthermia.
Irene Andreu, Eva Natividad, Laura Solozábal, and Olivier Roubeau
ACS Nano, 9 1408 (2015)

One current challenge of magnetic hyperthermia is achieving therapeutic effects with a minimal amount of nanoparticles, for which improved heating abilities are continuously pursued. However, it is demonstrated here that the performance of magnetite nanocubes in a colloidal solution is reduced by 84% when they are densely packed in three-dimensional arrangements similar to those found in cell vesicles after nanoparticle internalization. This result highlights the essential role played by the nanoparticle arrangement in heating performance, uncontrolled in applications. A strategy based on the elaboration of nano-objects able to confine nanocubes in a fixed arrangement is thus considered here to improve the level of control. The obtained specific absorption rate results show that nanoworms and nanospheres with fixed one- and two-dimensional nanocube arrangements, respectively, succeed in reducing the loss of heating power upon agglomeration, suggesting a change in the kind of nano-object to be used in magnetic hyperthermia.

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.

 

Size effects on EB in NiO NPs

Scrutinizing the role of size reduction on the exchange bias and dynamic magnetic behavior in NiO nanoparticles.
N Rinaldi-Montes, P Gorria, D Martínez-Blanco, A B Fuertes, L Fernández Barquín, I Puente-Orench and J A Blanco
Nanotechnology 26, 305705 (2015)

(a)–(d) Magnetic hysteresis loops for samples D2, D4 and D9 measured at selected temperatures, (a) 2, (b) 20, (c) 140 and (d) 300 K (that corresponding to a bulk NiO standard is included for the sake of comparison). (e)–(h) Enlarged views of the central part of the loops, evidencing the decrease in H_C and M_r with increasing temperature. The pictures inside (a)–(d) indicate the blocked/frozen (blue) or unblocked/unfrozen (red) regimes for the cores and shells, and correspond to samples D2 (top), D4 (middle) and D9 (bottom)

 

Heusler alloys for giant exchange bias

Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias.
Ajaya K. Nayak, Michael Nicklas, Stanislav Chadov, Panchanana Khuntia, Chandra Shekhar, Adel Kalache, Michael Baenitz, Yurii Skourski, Veerendra K. Guduru, Alessandro Puri, Uli Zeitler, J. M. D. Coey & Claudia Felser
Nature Materials 14 679 (2015)

a, Uncompensated magnetization, M, in Mn_3−xPt_xGa going through a compensation point (M = 0) at x = 0.59 (upper panel): squares are results from theoretical calculations. The line is a guide to eye. Lower panel: Experimental EB field H_EB…

 

Skyrmion deformation in strained crystals

Large anisotropic deformation of skyrmions in strained crystal.

K. Shibata, J. Iwasaki, N. Kanazawa, S. Aizawa, T. Tanigaki, M. Shirai, T. Nakajima, M. Kubota, M. Kawasaki, H. S. Park, D. Shindo, N. Nagaosa & Y. Tokura

Nature Nanotechnology 10, 589 (2015)

Evaluation of SkX deformation in reciprocal space. a,b, FFT patterns of the over-focused Lorentz TEM images in Fig. 1c (at 260 K) and
Fig. 1d (94 K). c,d, FFT patterns of simulated SkX spin configurations under isotropic and anisotropic DMIs, respectively. Fitting ellipses of the six spots corresponding to magnetic modulation vectors are denoted by dashed curves in a–d. e, SkX deformation parameters a (long axis of the fitting ellipse), b (short axis) and f = 1 − b/a as functions of temperature T (upper abscissa) and estimated strain ε (lower abscissa). f, SkX deformation parameters a, b and f in simulated SkX spin configurations as a function of the degree of introduced DMI anisotropy η = 1 − Dx/Dy.

 

Universal dependence of SW in magnonic crystals

Universal dependence of the spin wave band structure on the geometrical characteristics of two-dimensional magnonic crystals.

S. Tacchi, P. Gruszecki, M. Madami, G. Carlotti, J. W. Kłos, M. Krawczyk, A. Adeyeye, G. Gubbiotti 

Scientific Reports 5, 10367 (2015)

(a) BLS spectra taken at the Γ-point for the series S1 ADLs with different thicknesses applying a magnetic field μ₀H₀ = 0.1 T. (b) Calculated SW spatial profiles for the edge (E), the fundamental (F) and the fundamental-localized (F_loc) modes. The intensity of the color denotes the amplitude of the excitation, while the red and blue colors indicate opposite phase.

Skyrmion magnonic crystals

Skyrmion-Based Dynamic Magnonic Crystal.
Fusheng Ma, Yan Zhou, H. B. Braun, and W. S. Lew

Nano Letters 15, 4029 (2015)

A linear array of periodically spaced and individually controllable skyrmions is introduced as a magnonic crystal. It is numerically demonstrated that skyrmion nucleation and annihilation can be accurately controlled by a nanosecond spin polarized current pulse through a nanocontact. Arranged in a periodic array, such nanocontacts allow the creation of a skyrmion lattice that causes a periodic modulation of the waveguide’s magnetization, which can be dynamically controlled by changing either the strength of an applied external magnetic field or the density of the injected spin current through the nanocontacts. The skyrmion diameter is highly dependent on both the applied field and the injected current. This implies tunability of the lowest band gap as the skyrmion diameter directly affects the strength of the pinning potential. The calculated magnonic spectra thus exhibit tunable allowed frequency bands and forbidden frequency bandgaps analogous to that of conventional magnonic crystals where, in contrast, the periodicity is structurally induced and static. In the dynamic magnetic crystal studied here, it is possible to dynamically turn on and off the artificial periodic structure, which allows switching between full rejection and full transmission of spin waves in the waveguide. These findings should stimulate further research activities on multiple functionalities offered by magnonic crystals based on periodic skyrmion lattices.

  

The design and verification of MuMax3

The design and verification of MuMax3
Arne Vansteenkiste, Jonathan Leliaert, Mykola Dvornik, Mathias Helsen, Felipe Garcia-Sanchez, and Bartel Van Waeyenberge
AIP Advances 4, 107133 (2014)

MUMAX3 (dots) and OOMMF (lines) solution to standard problem #4a (top graph) and #4b (bottom graph), as well as space-dependent magnetization snapshots when <mx > crosses zero, for fields (a) and (b). All use a 200 × 50 × 1 grid.

Cobalt-Ferrite Nanoparticles for permanent magnets

Exploring the Magnetic Properties of Cobalt-Ferrite Nanoparticles for the Development of aRare-Earth-Free Permanent Magnet.
Alberto López-Ortega, Elisabetta Lottini, César de Julián Fernández, and Claudio Sangregorio

Chemistry of Materials 27, 4048(2015)

We present for the first time an in-depth magnetic characterization of a family of monodisperse cobaltferrite nanoparticles (NPs) with average size covering a broad range of particles sizes (from 4 to 60 nm), synthesized by thermal decomposition of metal−organic precursors. Metal precursors, surfactants, and synthetic parameters were settled in order to fine-tune the particle size, which preserves a narrow particle size distribution.

Magnonics: Special Reviews in Nature Phys.

Reconfigurable magnonics heats up.

Dirk Grundler
Nature Physics 11, 438(2015) 

Applying heat with a specific spatial periodicity to a magnet gives rise to periodically modulated magnetic properties. Stop bands (forbidden frequency gaps) for spin waves (cyan) occur. As a consequence, signal transmission is not allowed.

Magnon spintronics:  Review

A. V. Chumak, V. I. Vasyuchka, A. A. Serga & B. Hillebrands

Nature Physics 11, 453 (2015)

Information coded into charge or spin currents is converted into magnon currents, processed within the magnonic system and converted back.

Atomistic spin dynamics: Review

Atomistic spin dynamics and surface magnons.
Corina Etz, Lars Bergqvist, Anders Bergman, Andrea Taroni and Olle Eriksson
Journal of Physics: Condensed Matter 27, 243202 

 Bridging the gap: the ASD link between ab initio methods and micromagnetics simulations.

Writing and Deleting Single Magnetic Skyrmions

Writing and Deleting Single Magnetic Skyrmions.
Niklas Romming, Christian Hanneken, Matthias Menzel, Jessica E. Bickel, Boris Wolter,

Kirsten von Bergmann, André Kubetzka, Roland Wiesendanger

Science 341, 636 (2015)

Magnetic field dependence of the PdFe bilayer on the Ir(111) surface at T = 8 K. (A to C) Perspective sketches of the magnetic phases. (D) Overview SP-STM image, perspective view of constant-current image colorized with its derivative. (E to G) PdFe bilayer at different magnetic fields (U = +50 mV, I = 0.2 nA, magnetically out-of-plane sensitive tip). (E) Coexistence of spin spiral and skyrmion phase. (F) Pure skyrmion phase. (G) Ferromagnetic phase. A remaining skyrmion is marked by the white circle.

Spins on a Möbius ring

Coupling of Chiralities in Spin and Physical Spaces: The Möbius Ring as a Case Study.
Oleksandr V. Pylypovskyi, Volodymyr P. Kravchuk, Denis D. Sheka, Denys Makarov, Oliver G. Schmidt, and Yuri Gaididei
Phys. Rev. Lett. 114, 197204 (2015)

Diagram of ground states of magnetic Möbius rings with fixed radius R=100  nm and width w=80, see (a). The magnetization distributions of possible ground states are shown in the middle part in a large scale: (b) vortex state (marked by disks on the diagram), (c) state with single transversal Bloch domain wall (open squares), (d) state with three transversal Bloch walls (filled triangles), (e) states with longitudinal domain wall (filled squares). The detailed structures of the transverse and longitudinal domain walls are shown in (c′) and (e′), respectively.

Vortex dynamics in arrays of nanomagnets

Observation of vortex dynamics in arrays of nanomagnets.
W. Yu, P. S. Keatley, P. Gangmei, M. K. Marcham, T. H. J. Loughran, R. J. Hicken, S. A. Cavill, G. van der Laan, J. R. Childress, and J. A. Katine
Phys. Rev. B 91, 174425 (2015)

TRSKM images, XPEEM images, and SEM images of arrays of 250-nm squares with separation of 500 nm in (a) and (b), and 50 nm in (c). After acquisition of the TRSKM images in (a), the equilibrium state was reinitialized before acquiring the TRSKM images in (b). In all TRSKM images the horizontal white bar has length of 1 μm. In (c) the white arrows in the TRSKM images denote regions of contrast with size comparable to, or larger than, that of a single element, while the dashed white rectangle in the XPEEM image highlights a cluster of elements with vortex equilibrium states. (d) Enlarged view of (c), in which the black grid position is a guide to the eye to show where the individual elements lie, and the white boxes highlight regions of contrast

Dipolar Kagome lattice

Classical dipoles on the kagome lattice.
Mykola Maksymenko, V. Ravi Chandra, and Roderich Moessner

Phys. Rev. B 91, 184407 (2015)

The kagome lattice (left) and the ground-state phase diagram of the model consisting of nearest-neighbor exchange J=sinθ and dipolar interactions of strength D=cosθ (right). A1 and A2 are the basis vectors of the triangular Bravais lattice, and a dark green triangle denotes the unit cell of three sites labeled by r0=(0,0),r1=(1/2,0), and r2=(0,1/2) in this basis. The system exhibits long-range 120∘ order for π/2≥θ>θ1 with θ1=10.01∘ and ferrimagnetic order for −π/2<θ<θ2 where θ2=1.03∘. The latter has two spins inclined with respect to one of the unit-cell edges by angle ±ϕ(θ). The area between two phases possibly contains an incommensurate intermediate regime.

Quantitative simulation of temperature-dependent magnetization dynamics

Quantitative simulation of temperature-dependent magnetization dynamics and equilibrium properties of elemental ferromagnets.
R. F. L. Evans, U. Atxitia, and R. W. Chantrell

Phys. Rev. B 91, 144425 (2015)

Temperature-dependent magnetization for the elemental ferromagnets (a) Co, (b) Fe, (c) Ni, and (d) Gd. Circles give the simulated mean magnetization, and dark solid lines show the corresponding fit according to Eq. (4) for the classical case α=1. Light solid lines give the experimentally measured temperature-dependent magnetization as fitted by Kuz'min's equation. Triangles give the simulated data after the temperature rescaling has been applied, showing excellent agreement with the experimentally measured magnetizations for all studied materials.

Single Magnetic Skyrmions

Field-Dependent Size and Shape of Single Magnetic Skyrmions.
Niklas Romming, André Kubetzka, Christian Hanneken, Kirsten von Bergmann, and Roland Wiesendanger
Phys. Rev. Lett. 114, 177203 (2015)

Spin structure of individual Skyrmion in PdFe/Ir(111). (a) Sketch of the experimental setup of a spin-polarized STM tip probing a magnetic Skyrmion. (b) Topographic constant-current SP-STM image measured with out-of-plane sensitive magnetic tip; each blue circular entity is a Skyrmion (U=+200  mV, I=1  nA, T=2.2  K, B=-1.5  T). (c) Magnetic signal (methods [22]) of two Skyrmions measured with an in-plane magnetization of the tip, m→T, revealing a two-lobe structure (U=+250  mV, I=1  nA, T=4.2  K). (d) Same area as in (c) with inverted magnetic field; due to the preserved rotational sense, the contrast is inverted. (e),(f) Line profiles across a Skyrmion along the rectangles in (c) and (d), respectively, and fits with Eq. (1) [(e) c=(0.90±0.01)  nm, w=(1.18±0.02)  nm; (f) c=(0.91±0.01)  nm, w=(1.17±0.01)  nm] and corresponding calculated out-of-plane magnetization mz. The sketches show spins with atomic distance, colorized according to the SP-STM contrast.

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.

Hyperthermia in core/shell NP by Monte Carlo

Susceptibility losses in heating of magnetic core/shell nanoparticles for hyperthermia: a Monte Carlo study of shape and size effects.

M. Vasilakaki, C. Binns and K. N. Trohidou

Nanoscale 7, 7753 (2015)

Antiferromagnetic-Core/Ferrimagnetic-Shell NP

Enhanced Magnetic Properties in Antiferromagnetic-Core/Ferrimagnetic-Shell Nanoparticles.

Marianna Vasilakaki, Kalliopi N. Trohidou, Josep Nogués

Scientific Reports 5, 9609 (2015) 

(left) Schematic representation of the inverse AFM/FiM core/shell structure and (right) hysteresis loops of the doubly inverted nanoparticles, for different AFM core sizes (D_core) and constant FiM shell thickness of four lattice spacings.

Control of NP Arrangement for Magnetic Hyperthermia

Nano-objects for Addressing the Control of Nanoparticle Arrangement and Performance in Magnetic Hyperthermia.Irene Andreu, Eva Natividad, Laura Solozábal, and Olivier Roubeau
ACS Nano 9, 1408 (2015)

One current challenge of magnetic hyperthermia is achieving therapeutic effects with a minimal amount of nanoparticles, for which improved heating abilities are continuously pursued. However, it is demonstrated here that the performance of magnetite nanocubes in a colloidal solution is reduced by 84% when they are densely packed in three-dimensional arrangements similar to those found in cell vesicles after nanoparticle internalization. This result highlights the essential role played by the nanoparticle arrangement in heating performance, uncontrolled in applications. A strategy based on the elaboration of nano-objects able to confine nanocubes in a fixed arrangement is thus considered here to improve the level of control. The obtained specific absorption rate results show that nanoworms and nanospheres with fixed one- and two-dimensional nanocube arrangements, respectively, succeed in reducing the loss of heating power upon agglomeration, suggesting a change in the kind of nano-object to be used in magnetic hyperthermia.

Scaling approach to MC simulations of magnetic models

Bridging the gap between discrete and continuous magnetic models in the scaling approach. E. A. Velásquez, J. Mazo-Zuluaga, P. Vargas, and J. Mejía-López
Phys. Rev. B 91, 134418 (2015)

Collapsed phase diagram for different scaling λ values. The inset shows the phase diagram for some selected scaling values, i.e., before the scaling up process.