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.

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.

We show that room temperature resistivity of Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 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 the 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 Ba 0.5 Sr 1.5 Zn 2 Fe 12 O 22 .

The study of abrupt increases in magnetization with magnetic field known as metamagnetic transitions has opened a rich vein of new physics in itinerant electron systems, including the discovery of quantum critical end points with a marked propensity to develop new kinds of order. However, the electric analogue of the metamagnetic critical end point, a "metaelectric" critical end point, has been rarely studied. Multiferroic materials wherein magnetism and ferroelectricity are cross-coupled are ideal candidates for the exploration of this novel possibility using magnetic-field (H) as a tuning parameter. Herein, we report the discovery of a magnetic-fieldinduced metaelectric transition in multiferroic BiMn 2 O 5 , in which the electric polarization (P) switches polarity along with a concomitant Mn spin-flop transition at a critical magnetic field H c . The simultaneous metaelectric and spin-flop transitions become sharper upon cooling but remain a continuous cross-over even down to 0.5 K. Near the P = 0 line realized at μ 0 H c ≈18 T below 20 K, the dielectric constant (ε) increases significantly over wide field and temperature (T ) ranges. Furthermore, a characteristic powerlaw behavior is found in the P(H) and ε(H) curves at T = 0.66 K. These findings indicate that a magnetic-field-induced metaelectric critical end point is realized in BiMn 2 O 5 near zero temperature. T he term "critical end point" refers to a singular point in the phase diagram of matter at the end of a first order phase line, as for example, the liquid-gas critical point of water. The importance of this special point for broad classes of matter has rapidly increased in recent years (1, 2). Not only can it provide large thermal fluctuations as a necessary ingredient for displaying universal power-law of physical quantities, but in the special case where the phase transition is suppressed to zero temperature, it can give rise to intense quantum mechanical fluctuations with a marked propensity to develop instabilities into new kinds of ground state. The latter case has been recently realized in itinerant metamagnets such as Sr 3 Ru 2 O 7 (3) and URu 2 Si 2 (4). These developments motivate a parallel search for a metaelectric critical end point. By analogy with magnetism, one might expect sudden cross-overs in electric polarization in the vicinity of a metaelectric critical endpoint to be a rather general phenomenon in (anti)ferroelectric materials under application of an electric field (E). However, to date, such metaelectric transitions induced by electric fields have been limitedly observed in specific systems such as relaxor ferroelectrics (5) and DyVO 4 with the Jahn-Teller structural distortion (6). One possible reason for the scarcity of the phenomenon is the practical difficulty of applying the large voltages required ( 1 kV) without inducing electrical breakdown. Magnetic fields may in fact be better candidates for inducing metaelectric transitions, because they not only avoid the problem of electrical breakdown but also provide a reversi...

Comparative studies of magnetoelectric susceptibility (α), magnetization (M ), and magnetostriction (u) in TbMn 2 O 5 reveal that the increment of M owing to the field-induced Tb 3+ spin alignment coins a field-asymmetric line shape in the α(H) curve, being conspicuous in a low temperature incommensurate phase but persistently subsisting in the entire ferroelectric phase. Correlations among electric polarization, u, and M 2 variation represent linear relationships, unambiguously showing the significant role of Tb magnetoelastic effects on the low field magnetoelectric phenomena of TbMn 2 O 5 . An effective free energy capturing the observed experimental features is also suggested.