O. Petracic

Collective states of interacting ferromagnetic nanoparticles

Abstract Discontinuous magnetic multilayers [CoFe/Al 2 O 3 ] are studied by use of magnetometry, susceptometry and numeric simulations. Soft ferromagnetic Co 80 Fe 20 nanoparticles are embedded in a diamagnetic insulating a-Al 2 O 3 matrix and can be considered as homogeneously magnetized superspins exhibiting randomness of size (viz. moment), position and anisotropy. Lacking intra-particle core-surface ordering, generic freezing processes into collective states rather than individual particle blocking are encountered. With increasing particle density one observes first superspin glass and then superferromagnetic domain state behavior. The phase diagram resembles that of a dilute disordered ferromagnet. Criteria for the identification of the individual phases are given.

Cooperative versus superparamagnetic behavior of dense magnetic nanoparticles in Co80Fe20/Al2O3 multilayers

Spin glasslike cooperative freezing is evidenced at low temperatures in a magnetic nanoparticle system prepared as a discontinuous metal–insulator multilayer [Co80Fe20(0.9 nm)/Al2O3(3 nm)]10. The relaxational behavior clearly deviates from Arrhenius–Neel–Brown-type and rather hints at collective freezing into a spin glass phase below Tg≃61 K. Holelike memory imprinting during a stop-and-wait magnetization procedure corroborates the collective nature of the frozen state. Consequences for future magnetic terabit storage devices are briefly discussed.

Magnetic nanoparticles: a subject for both fundamental research and applications

Single domain magnetic nanoparticles (MNPs) have been a vivid subject of intense research for the last fifty years. Preparation of magnetic nanoparticles and nanostructures has been achieved by both bottom-up and top-down approaches. Single domain MNPs show Neel-Brown-like relaxation. The Stoner-Wohlfarth model describes the angular dependence of the switching of the magnetization of a single domain particle in applied magnetic fields. By varying the spacing between the particles, the interparticle interactions can be tuned. This leads to various supermagnetic states such as superparamagnetism, superspin glass, and superferromagnetism. Recently, the study of the magnetization dynamics of such single domain MNPs has attracted particular attention, and observations of various collective spin wave modes in patterned nanomagnet arrays have opened new avenues for on-chip microwave communications. MNPs have the potential for various other applications such as future recording media and in medicine. We will discuss the various aspects involved in the research on MNPs.

Single-particle blocking and collective magnetic states in discontinuous CoFe/Al2O3 multilayers

Discontinuous metal?insulator multilayers (DMIMs) of [CoFe(tn)/Al2O3]m containing soft ferromagnetic (FM) Co80Fe20 nanoparticles embedded discontinuously in a diamagnetic insulating Al2O3 matrix are ideal systems to study interparticle interaction effects. Here the CoFe nanoparticles are treated as superspins with random size, position and anisotropy. At low particle density, namely nominal layer thickness tn = 0.5?nm, single-particle blocking phenomena are observed due to the absence of large enough interparticle interactions. However at 0.5?nm 1.4?nm physical percolation occurs between the particles and the samples are no longer discontinuous and then termed as metal insulating multilayers. Competition between long- and short-ranged dipolar interactions leads to an oscillating magnetization depth profile from CoFe layer to CoFe layer with an incommensurate periodicity.

Cole-Cole Analysis of the Superspin Glass System Co 80 Fe 20 /Al 2 O 3

Ac susceptibility measurements were performed on discontinuous magnetic multilayers [Co 80 Fe 20 ( t )/Al 2 O 3 (3 nm)] 10 , t = 0.9 and 1.0 nm, by Superconducting Quantum Interference Device (SQUID) magnetometry. The CoFe forms nearly spherical ferromagnetic single-domain nanoparticles in the diamagnetic Al 2 O 3 matrix. Due to dipolar interactions and random distribution of anisotropy axes the system exhibits a spin-glass phase. We measured the ac susceptibility as a function of temperature 20 h T h 100 K at different dc fields and as a function of frequency 0.01 h f h 1000 Hz. The spectral data were successfully analyzed by use of the phenomenological Cole-Cole model, giving a power-law temperature dependence of the characteristic relaxation time c and a high value for the polydispersivity exponent, f , 0.8, typical of spin glass systems.

Competing Interactions in Patterned and Self-Assembled Magnetic Nanostructures

In this chapter we describe the static properties of different nanomagnetic systems, which have one thread in common: they consist of arrays of structures each containing a macrospin or a magnetic dipole interacting with its neighbors. We start with a general overview on rectangular and circular islands, their domain structures and interactions. In the second part we discuss the competing interactions and geometric frustration of magnetic dipoles arranged on square and honeycomb lattices, a new field that has become known as artificial spin-ice. The third part covers a technologically very important aspect of densely packed magnetic nanostructures with perpendicular anisotropy for use in hard disk drives in the form of bit patterned media. Here the interaction with neighboring islands may increase or decrease the switching field of an individual target island in the patterned array. The fourth part discusses magnetic clusters, which are arranged by self-organization rather than by lithography, providing further possibilities for investigating magnetic ordering and phase transitions.

Neutron scattering study of transverse magnetism in the metamagnet FeBr2

Abstract:In order to clarify the nature of the additional phase transition at H1(T) < Hc(T) of the layered antiferromagnetic (AF) insulator FeBr2 as found by Aruga Katori et al. (1996) we measured the intensity of different Bragg-peaks in different scattering geometries. Transverse AF ordering is observed in both AF phases, AF I and AF II. Its order parameter exhibits a peak at T1 = T (H1) in temperature scans and does not vanish in zero field. Possible origins of the step-like increase of the transverse ferromagnetic ordering induced by a weak in-plane field component when entering AF I below T1 are discussed.

METAMAGNETIC DOMAINS AND DYNAMIC FLUCTUATIONS IN FEBR2

The mixed phase and the regime of non-critical fluctuations of the magnetic phase diagram of FeBr2 is investigated by SQUID susceptometry and light diffraction techniques. The experiments seem to evidence instability of the tricritical point as conjectured recently. The observation of stripe domains and light diffraction below and above Tm=4.6 K and the virtual continuation of the phase transition line to above Tm are in agreement with the occurrence of a critical endpoint at Tm and of a bicritical endpoint at T>Tm.

Synthesis, Properties, and Applications of Single-Domain Magnetic Nanoparticles

1 School of Physical Sciences, National Institute of Science Education and Research (NISER), IOP Campus, Bhubaneswar 751005, India 2Department of Condensed Matter Physics and Material Sciences, S. N. Bose National Centre for Basic Sciences, Block JD, Sector III, Salt Lake City, Kolkata 700098, India 3 Department of Physics, University Duisburg-Essen, 47057 Duisburg, Germany 4 Juelich Centre for Neutron Science JCNS and Peter Gruenberg Institute (PGI), JARA-FIT, Forschungszentrum Juelich GmbH, 52425 Juelich, Germany 5 Institute for Materials Research, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan

Magnetic relaxation phenomena in the superspin-glass system [Co80Fe20/Al2O3]10

Relaxation and temperature cycles of thermoremanent magnetization, MTRM, in the superspin-glass phase of [Co80Fe20(0.9 nm)/Al2O3(3 nm)]10 have been investigated. The relaxation of MTRM exhibits ageing phenomena. In negative temperature cycles for temperature steps larger than 1 K the magnetic state is retrieved (memory effect) on returning to the measurement temperature. This property is independent of the application of a field step during intermediate cooling. In positive temperature cycles the relaxation is suppressed after temporary heating. The observations are discussed in the light of both the droplet and the hierarchical picture.