Direct Observation of Interactions between Nanoparticles and Nanoparticle Self-Assembly in Solution

Direct Observation of Interactions between Nanoparticles and Nanoparticle Self-Assembly in Solution.
Shu Fen Tan, See Wee Chee, Guanhua Lin, and Utkur MirsaidovAcc. Chem. Res., 50, 1303 (2017)

Magnetic Möbius stripe

Magnetic Möbius stripe without frustration: Noncollinear metastable states.
S. Castillo-Sepúlveda, R. A. Escobar, D. Altbir, M. Krizanac, and E. Y. Vedmedenko
Phys. Rev. B 96, 024426 (2017)

Equilibrium MC configuration of a chain consisting of 100 moments for $D=1,K=0.4$ meV, $J=-40$ meV: (a) closed KB configuration and (b) the same shown with open ends for clarity.

Competition between interactions and anisotropy in NPs

Understanding particle size and distance driven competition of interparticle interactions and effective single-particle anisotropy.

B Pacakova, A Mantlikova, D Niznansky, S Kubickova and J Vejpravova

Journal of Physics: Condensed Matter 28, 206004 (2016)

 Comparison of volume dependence of TMAX, representing the blocking temperature of the ensemble of the NPs with size distribution and interparticle interactions;  attributed to blocking temperature of Vm affected by interparticle interactions and finally , which is the blocking temperature attributed to Vm after decoupling the effect of dipole–dipole interactions and external DC magnetic field  mT (equation (4)). . Line corresponds to the fit encountering the finite size effect with  Jm−3,  Jm−2.

Dipolar helices

Structure and cohesive energy of dipolar helices.
Igor Stankovic,Miljan Dasic and René Messina
Soft Matter 12, 3056 (2016)

This paper deals with the investigation of cohesive energy in dipolar helices made up of hard spheres. Such tubular helical structures are ubiquitous objects in biological systems. We observe a complex dependence of cohesive energy on surface packing fraction and dipole moment distribution. As far as single helices are concerned, the lowest cohesive energy is achieved at the highest surface packing fraction.

Infinite geometric frustration in a cubic dipole cluster

Infinite geometric frustration in a cubic dipole cluster.

Johannes Schönke, Tobias M. Schneider, and Ingo Rehberg

Phys. Rev. B 91, 020410(R) (2015)

(a) Rendering of eight dipoles as arrows located at the corners of a cube. The colored and translucent arrows show selected orientations in the ground state of the system which is a continuum of infinitely many configurations. The continuum corresponds to rotations of the dipoles in planes perpendicular to the cube's volume diagonal of the respective corner in analogy to the mechanical system in Fig. 2. Pairs of dipoles in opposing corners are always parallel. (b) Magnetic energy landscape for eight dipoles located at the corners of a cube. The energy is shown as a function of the two orientation angles of one dipole, covering all possible orientations. The other seven dipoles adjust to the respective minimum energy configuration. The white line through the “valley” marks the continuous ground state. (c) Rendering of eight bevel gears located at the corners of a cube. The rotation axes point to the cube's center and every gear interlocks with its three edge neighbors. The motion of this mechanical system, indicated by the red arrows, corresponds to the motion of the ground state continuum in the magnetic system.

Smooth Teeth: Why Multipoles Are Perfect Gears

Smooth Teeth: Why Multipoles Are Perfect Gears.
Johannes Schönke
Phys. Rev. Applied 4, 064007 (2015)

Definition of the angles which describe the rotation of the dipoles (red). The upper panel introduces the two angles θ and φ which set the positions of the rotation axes (light gray). The rotation axes are restricted to lie in the x−y plane. The angles θ and φ (and, therefore, the rotations axes) are fixed for a given configuration. The lower panel introduces the angles α and β which correspond to the 2 degrees of freedom the system possesses. α and β describe the actual orientation of the two dipoles during rotation.

Effects of the individual particle relaxation time on superspin glass dynamics

Effects of the individual particle relaxation time on superspin glass dynamics.

Mikael Svante Andersson, Jose Angel De Toro, Su Seong Lee, Peter S. Normile, Per Nordblad, and Roland Mathieu

Phys. Rev. B 93, 054407 (2016)

MIRM vs temperature using a perturbation field of 40 A/m and th=300 s for several halting temperatures; Th=50, 60, 70, 80, 90, 100, and 110 K for (a) RCP6 and (b) RCP8. (c) Selected MIRM(T) curves for RCP6 and RCP8 plotted together. Data for RCP8 is adapted from Ref. [15].

Self-Assembly of Superparamagnets: Review

Predicting the Self-Assembly of Superparamagnetic Colloids under Magnetic Fields.
Jordi Faraudo, Jordi S. Andreu, Carles Calero and Juan Camacho

Adv. Funct Materials 22, 3837 (2016)

a) Motion of colloids in a chain in an LD simulation; b) effective ellipsoid representing a chain of s colloids, which has anisotropic diffusion with different coefficients in the parallel and perpendicular directions. c) Map of magnetic potential energy of interaction between an imaginary colloid and a chain of five particles, all magnetized in the z-direction. The calculation corresponds to Γ = 40 and the magnetic energy is measured in units of k_BT. The dashed line indicates the regions at which the magnetic energy is equal (in absolute value) to the thermal energy. d) Coarse grain representation of the chain in panel (c) as employed in MagChain simulations in which the attractive region inside the dashed lines in panel (c) is replaced by a spherical region.

Colloidal Dimers on a Honeycomb Magnetic Lattice

Geometric Frustration of Colloidal Dimers on a Honeycomb Magnetic Lattice. 
Pietro Tierno
Phys. Rev. Lett. 116, 038303 (2016)

(a) Schematic of the FGF film with a magnetic bubble lattice subjected to an external field Hz. One Wigner-Seitz unit cell is shaded in blue with one paramagnetic colloid. For Hz=0, the lattice constant is a=11.6  μm and the radius R=4.2  μm. (b) Normalized distance r/a of one paramagnetic colloid from the center of a magnetic bubble versus Hz. (c) Normalized energy landscape for a FGF under a static field Hz=0.17Ms. The inset shows a 3D view of one triangular minimum. (d) Snapshots of a small section (48×42  μm2) of a magnetic bubble lattice filled with n=1, 2, 3 particles per pinning site (scale bar is 10  μm). Schematics at the bottom show the corresponding configurations in a triangular minimum.

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.

Dipolar Interactions and Size Distribution on Blocking Temperature NPs

Role of Dipolar Interactions and Volume Particle Size Distribution on the Nonmonotonic Magnetic Field Dependence of the Blocking Temperature in Magnetic Nanoparticles.
Sueli H. Masunaga, Renato F. Jardim, Marcos J. Correia, and Wagner Figueiredo
J. Phys. Chem. C 120, 765 (2016)

The nonmonotonic behavior of the magnetic field dependence H of the superparamagnetic blocking temperature T_B(H) (or T_max(H)) of an assembly of magnetic nanoparticles NPs was studied both experimentally and theoretically. We have combined the measurements of zero-field cooling ZFC magnetization curves under H performed on samples with increasing concentration of Ni nanoparticles and Monte Carlo simulations. As a result, we have found that increasing the strength of the dipolar interaction between granules and the occurrence of a very narrow width of a log-normal volume particle size distribution of the NPs suppress the nonmonotonic behavior of T_max(H).

Dipole interactions in crystals

Magnetic dipole interactions in crystals
David C. Johnston
Phys. Rev. B 93, 014421 (2016)

Eigenvalues (a) ${\lambda }_{(1/2,0,1/2)}$ for AFM wave vector k = (1/2,0,1/2)  r.l.u. and (b) ${\lambda }_{(1/2,1/2,1/2)}$ for AFM wave vector k = (1/2,1/2,1/2)  r.l.u. versus the $c/a$ ratio for a simple tetragonal lattice with the moments aligned along [010] ($b$ axis, solid red circles), [001] ($c$ axis, solid green diamonds), and [100] ($a$ axis, solid blue squares).

 

Domain wall formation in dipolar NP assemblies

Longitudinal domain wall formation in elongated assemblies of ferromagnetic nanoparticles.
Miriam Varón, Marco Beleggia, Jelena Jordanovic, Jakob Schiøtz, Takeshi Kasama, Victor F. Puntes & Cathrine Frandsen
Scientific Reports 5, 14536 (2015)

Simulation of the relaxed F-state (a–c) and the relaxed L-state (d–f) with increasing reverse fields of (a,d) 0 mT, (b,e) 35 mT, and (c,f) 50 mT. The colors denote the orientation of the magnetic moment (cf. the color wheel in Fig. 3). Panels on the right hand side indicate the net moment in the length direction over the widths of the assemblies.

 

Phase transitions in spin ice of macroscopic dimensions

Thermodynamic phase transitions in a frustrated magnetic metamaterial.
L. Anghinolfi, H. Luetkens, J. Perron, M. G. Flokstra, O. Sendetskyi, A. Suter, T. Prokscha, P. M. Derlet, S. L. Lee & L. J. Heyderman
Nature Communications 6, 8278 (2015) 

(a) Scanning electron microscope (SEM) micrograph showing a small part of an artificial kagome spin ice array with nanomagnets arranged on a kagome lattice (indicated in red). The whole array extends over an area of 25 mm² and comprises 10⁹ nanomagnets. Thermally active over a wide range of temperatures, such a large system is able to emulate the thermodynamics characteristic of bulk systems. Scale bar, 170 nm. (b) Highlights the predicted magnetic phase transitions. Snapshots of possible spin configurations for each phase are included, with magnetic charges of opposite sign indicated in red and blue at the vertices

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.

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.