Sae Hwan Chun

Realization of giant magnetoelectricity in helimagnets.

We show that low field magnetoelectric (ME) properties of helimagnets Ba0.5Sr1.5Zn2(Fe1-xAlx)12O22 can be efficiently tailored by the Al-substitution level. As x increases, the critical magnetic field for switching electric polarization is systematically reduced from approximately 1 T down to approximately 1 mT, and the ME susceptibility is greatly enhanced to reach a giant value of 2.0x10{4} ps/m at an optimum x=0.08. We find that control of the nontrivial orbital moment in the octahedral Fe sites through the Al substitution is crucial for fine-tuning the magnetic anisotropy and obtaining the conspicuously improved ME characteristics.

Electric field control of nonvolatile four-state magnetization at room temperature.

We find the realization of large converse magnetoelectric (ME) effects at room temperature in a magnetoelectric hexaferrite Ba0.52Sr2.48Co2Fe24O41 single crystal, in which rapid change of electric polarization in low magnetic fields (about 5 mT) is coined to a large ME susceptibility of 3200 ps/m. The modulation of magnetization then reaches up to 0.62μ(B)/f.u. in an electric field of 1.14 MV/m. We find further that four ME states induced by different ME poling exhibit unique, nonvolatile magnetization versus electric field curves, which can be approximately described by an effective free energy with a distinct set of ME coefficients.

Electrical control of large magnetization reversal in a helimagnet

Reversal of magnetization M by an electrical field E has been a long-sought phenomenon in materials science because of its potential for applications such as memory devices. However, the phenomenon has rarely been achieved and remains a considerable challenge. Here we report the large M reversal by E in a multiferroic Ba0.5Sr1.5Zn2(Fe0.92Al0.08)12O22 crystal without any external magnetic field. Upon sweeping E through the range of ±2 MV m(-1), M varied quasi-linearly in the range of ±2 μB per f.u., resulting in the M reversal. Strong electrical modulation of M at zero magnetic field were observable up to ~\n150 K. Nuclear magnetic resonance measurements provided microscopic evidence that the electric field and the magnetic field play equivalent roles in modulating the volume of magnetic domains. Our results suggest that the soft ferrimagnetism and the associated transverse conical state are key ingredients to achieve the large magnetization reversal at fairly high temperatures.

Electric polarization enhancement in multiferroic CoCr2O4 crystals with Cr-site mixing

Single crystals of multiferroic cobalt chromite Co(Cr2−xCox)O4 have been grown via several methods to have different Co3+ doping levels (x=0.0, 0.14, and 0.18). Under magnetic fields, all the crystals display electric polarization reversal below their spiral spin ordering temperatures. We find that both saturated electric polarization and magnetization under magnetic fields increase significantly with the increase in x. This result can be qualitatively explained by a broken balance between at least two electric polarization contributions existing in CoCr2O4 and is expected to be useful in tailoring electric polarization in similar kinds of multiferroics.

Low-magnetic-field control of dielectric constant at room temperature realized in Ba0.5Sr1.5Zn2Fe12O22

We show that the room temperature resistivity of Ba0.5Sr1.5Zn2Fe12O22 single crystals increases by more than three orders of magnitude upon being subjected to optimized heat treatments. The increase in the resistivity allows the determination of magnetic field (H)-induced ferroelectric phase boundaries up to 310?K through measurements of dielectric constant at a frequency of 10?MHz. Between 280 and 310?K, the dielectric constant curve shows a peak centered at zero magnetic field and thereafter decreases monotonically up to 0.1?T, exhibiting a magnetodielectric effect of 1.1%. This effect is ascribed to the realization of magnetic field-induced ferroelectricity at an H value of less than 0.1?T near room temperature. Comparison between electric and magnetic phase diagrams in wide temperature- and field-windows suggests that the magnetic field for inducing ferroelectricity has decreased near its helical spin ordering temperature around 315?K due to the reduction of spin anisotropy in Ba0.5Sr1.5Zn2Fe12O22.

Interplay between carrier and impurity concentrations in annealed Ga1- xMnxAs : Intrinsic anomalous hall effect

Investigating the scaling behavior of annealed Ga1-xMnxAs anomalous Hall coefficients, we note a universal crossover regime where the scaling behavior changes from quadratic to linear. Furthermore, measured anomalous Hall conductivities in the quadratic regime when properly scaled by carrier concentration remain constant, spanning nearly a decade in conductivity as well as over 100 K in T_[C] and comparing favorably to theoretically predicated values for the intrinsic origins of the anomalous Hall effect. Both qualitative and quantitative agreements strongly point to the validity of new equations of motion including the Berry phase contributions as well as the tunability of the anomalous Hall effect.

Quantitative Measurements of Size-Dependent Magnetoelectric Coupling in Fe3O4 Nanoparticles

Bulk magnetite (Fe3O4), the loadstone used in magnetic compasses, has been known to exhibit magnetoelectric (ME) properties below ∼10 K; however, corresponding ME effects in Fe3O4 nanoparticles have been enigmatic. We investigate quantitatively the ME coupling of spherical Fe3O4 nanoparticles with uniform diameters (d) from 3 to 15 nm embedded in an insulating host, using a sensitive ME susceptometer. The intrinsic ME susceptibility (MES) of the Fe3O4 nanoparticles is measured, exhibiting a maximum value of ∼0.6 ps/m at 5 K for d = 15 nm. We found that the MES is reduced with reduced d but remains finite until d = ∼5 nm, which is close to the critical thickness for observing the Verwey transition. Moreover, with reduced diameter the critical temperature below which the MES becomes conspicuous increased systematically from 9.8 K in the bulk to 19.7 K in the nanoparticles with d = 7 nm, reflecting the core-shell effect on the ME properties. These results point to a new pathway for investigating ME effect in various nanomaterials.

Field- and temperature-induced evolution of the magnetocaloric effect in Ba0.3Sr1.7Co2Fe12O22 single crystals with heliconical magnetism

The magnetocaloric effect (MCE) associated with the spin transitions of alternating longitudinal conical (ALC)-mixed conical (MC) and MC-ferrimagnetic (FIM) states in a Ba0.3Sr1.7Co2Fe12O22 single crystal has been investigated. For magnetic field directions applied along either the [120] or [001] directions, the crystal is found to exhibit the conventional and inverse MCE near the ALC-MC (T(N1) = 235 K) and MC-FIM (T(N2) = 348 K) states, respectively. The dependence of the magnetic entropy on the magnetic field also exhibits such sign change behaviors in the MCE, which is attributed to the magnetic field induced gradual collapse of heliconical magnetic order.

Competing magnetic anisotropy fields and double polarization flops in multiferroic mn1-xCoxWO4

We report the cobalt doping dependencies of the ferroelectric polarization and underlying incommensurate spiral magnetic ordering in multiferroic Mn1-xCoxWO4 (0 ≤x 0.15). These consecutive changes were accompanied by simultaneous 90° flops of ferroelectric polarizations. Such doping dependencies are driven by the competitions between the anisotropy fields of MnWO4 and CoWO4, the spatial distributions of which are nearly orthogonal to each other. We propose a model that can consistently describe the two consecutive flops based on a combined picture of magnetic anisotropy and biquadratic exchange.

Temperature- and Magnetic-Field-Induced Change of Electric Polarization in a Multiferroic Mn0.93Co0.07W0.93O4-delta Single Crystal

We have investigated magnetic and electrical properties in a multiferroic Mn0.93Co0.07W0.93O4 (delta) single crystal grown using the flux method, in which spiral spin orderings are known to be stabilized and thus magnetically induced electric polarization develops consistent with the spin current model. Upon temperature being lowered, two successive magnetic transitions appeared at T-N1 = 13.0 and T-N2 = 12.2 K. While there was no development of ferroelectric polarization P below T-N1, a dominant P along the a-axis (P-a) and a small P along the b-axis (P-b) clearly developed below T-N2, suggesting the stabilization of a spiral order with its basal plane close to the ac-plane. The magnetization measurements also support the existence of the spiral order with ac-basal plane below T-N2 by exhibiting the largest spin susceptibility along the b-axis and the smaller ones along the a- and c-axes. Upon further lowering temperature below T* = 10.1 K, P-a starts to decrease significantly accompanied by a small increase of P-b while the magnetic susceptibility along the c-axis gradually becomes the largest and concomitantly shows a saturation. We have also found that the magnitude of P-a or P-b can be controlled smoothly at low temperatures as a function of magnetic field less than 9 T. These findings show that the magnitude and direction of the electric polarization vector in Mn0.93Co0.07W0.93O4-delta can be varied as a function of temperature and magnetic field via the changes in the properties of the associated spiral phases.