Ronald Tackett

Magnetodielectric coupling in Mn3O4

We have investigated the dielectric anomalies associated with spin-ordering transitions in the tetragonal spinel

Evidence of low-temperature superparamagnetism in Mn3O4 nanoparticle ensembles

{\mathrm{Mn}}_{3}{\mathrm{O}}_{4}

Dynamic susceptibility evidence of surface spin freezing in ultrafine NiFe2O4 nanoparticles

using thermodynamic, magnetic, and dielectric measurements. We find that two of the three magnetic ordering transitions in

Magnetic relaxation and dissipative heating in ferrofluids

{\mathrm{Mn}}_{3}{\mathrm{O}}_{4}

Discrete, Dispersible MnAs Nanocrystals from Solution Methods: Phase Control on the Nanoscale and Magnetic Consequences

lead to decreases in the temperature-dependent dielectric constant at zero applied field. Applying a magnetic field to the polycrystalline sample leaves these two dielectric anomalies practically unchanged, but leads to an increase in the dielectric constant at the intermediate spin-ordering transition. We discuss possible origins for this magnetodielectric behavior in terms of spin-phonon coupling. Band structure calculations suggest that in its ferrimagnetic state,

Positive and negative magnetocapacitance in magnetic nanoparticle systems

{\mathrm{Mn}}_{3}{\mathrm{O}}_{4}

Large low-temperature specific heat in pyrochlore Bi2Ti2O7

corresponds to a semiconductor with no orbital degeneracy due to strong Jahn-Teller distortion.

Memory effects and magnetic interactions in a γ‐Fe2O3 nanoparticle system

Using ac-susceptibility, dc-magnetization, and transmission electron microscopy, we have investigated the magnetic behavior of Mn(3)O(4) nanoparticle ensembles at temperatures below the paramagnetic-to-ferrimagnetic transition of the title material (T(N) approximately equal 41 K). Our data show no evidence of the complex magnetic ordering exhibited by bulk Mn(3)O(4), or of a magnetic behavior around T(N) that has a dynamic (relaxation) origin. Instead, we find a low-temperature (at approximately 11 K) magnetic anomaly that manifests itself as a peak in the out-of-phase component of the ac-susceptibility. Analysis of the frequency and average-particle-size dependence of the peak temperature demonstrates that this behavior is due to the onset of superparamagnetic relaxation, and not to a previously hinted at spin-glass-like transition. Indeed, the relative peak temperature variation per frequency decade DeltaT/TDeltalog(f) is 0.11, an order of magnitude larger than the value expected for collective spin freezing, but within the range of values observed for superparamagnetic blocking. Furthermore, attempts to fit the frequency f/observation time tau = 1/2pif dependence of the peak temperature by a power law led to parameter values unexpected for a spin-glass transition. On the other hand, a Vogel-Fulcher law tau = tau(0)exp[E(B)/k(B)(T - T(0))]-where E(B) is the energy barrier to magnetization reversal, k(B) is the Boltzmann constant, tau(0) and T(0) are constants related to the attempt frequency and the interparticle interaction strength-correctly describes the peak shift and yields values consistent with the superparamagnetic behavior of a slightly interacting system of nanoparticles. In addition, the peak temperature T is sensitive to minute changes in the average particle size (D), and scales as (T - T(0) is proportional to(D)3, another signature of superparamagnetic relaxation.

Ordered spin-ice state in the geometrically frustrated metallic ferromagnet Sm 2 Mo 2 O 7

We investigated the dynamic behavior of ultrafine NiFe2O4 nanoparticles (average size D = 3.5 nm) that exhibit anomalous low temperature magnetic properties such as low saturation magnetization and high-field irreversibility in both M(H) and ZFC-FC processes. Besides the expected blocking of the superspin, observed at T1 approximately 45 K, the system undergoes a magnetic transition at T2 approximately 6 K. For the latter, frequency- and temperature-resolved dynamic susceptibility data reveal characteristics that are unambiguously related to collective spin freezing: the relative variation (per frequency decade) of the in-phase susceptibility peak temperature is approximately 0.025, critical dynamics analysis yields an exponent znu = 9.6 and a zero-field freezing temperature T(F) = 5.8 K, and, in a magnetic field, T(F)(H) is excellently described by the de Almeida-Thouless line delta T(F) = 1 - T(F)(H)/T(F) alpha H(2/3). Moreover, out-of-phase susceptibility versus temperature datasets collected at different frequencies collapse on a universal dynamic scaling curve. All these observations indicate the existence of a spin-glass-like surface layer that surrounds the superparamagnetic core and undergoes a transition to a frozen state upon cooling below 5.8 K.

Observation of 300K high energy magnetodielectric contrast in the bilayer manganite (La0.4Pr0.6)1.2Sr1.8Mn2O7

We have investigated the ac magnetic susceptibility and magnetic heating of aqueous suspensions of γ‐Fe2O3 nanoparticles embedded in alginate hydrogel matrix and isolated γ‐Fe2O3 and Fe3O4 nanoparticles coated with tetramethyl ammonium hydroxide. All three ferrofluids were characterized by measuring the dc magnetization, ac susceptibility, and magnetic heating. We found that significant Neel relaxation is present in all samples, but only the isolated nanoparticle ferrofluids show any significant feature associated with Brownian relaxation near the freezing temperature of the carrier liquid. The heating rate of the ferrofluids varies systematically with the magnitude of the Brownian relaxation peak, despite similar values of the absolute magnetization. These results highlight the importance of the Brownian relaxation for heating applications incorporating magnetic nanoparticles.