We investigate magnetic, thermal, and dielectric properties of SrCuTe2O6, which is isostructural to PbCuTe2O6, a recently found, Cu-based 3D frustrated magnet with a corner-sharing triangular spin network having dominant first and second nearest neighbor (nn) couplings [B. Koteswararao, et al. Phys. Rev. B 90, 035141 (2014)]. Although SrCuTe2O6 has a structurally similar spin network, but the magnetic data exhibit the characteristic features of a typical quasi-one-dimensional magnet, which mainly resulted from the magnetically dominant third nn coupling, uniform chains. The magnetic properties of this system are studied via magnetization (M), heat capacity (Cp), dielectric constant (ε'), measurements along with ab-initio band structure calculations. Magnetic susceptibility (T) data show a broad maximum at 32 K and the system orders at low temperatures TN1  5.5 K and TN2  4.5 K, respectively. The analysis of (T) data gives an intra-chain coupling, J3/kB, to be about  -42 K with non-negligible frustrated inter-chain couplings (J1/kB and J2/kB). The hopping parameters obtained from LDA band structure calculations also suggest the presence of coupled uniform chains. The observation of simultaneous anomalies in ε'(T) at TN1 and TN2 suggests the presence of magneto-dielectric effect in SrCuTe2O6. A magnetic phase diagram is also built based on M, Cp, and ε' results.

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The magnetoelectric effects are investigated in a cubic compound SrCuTe2O6, in which uniform Cu2+ (S = 1/2) spin chains with considerable spin frustration exhibit a concomitant antiferromagnetic transition and dielectric constant peak at TN ≈ 5.5 K. Pyroelectric Jp(T) and magnetoelectric current JME(H) measurements in the presence of a bias electric field are used to reveal that SrCuTe2O6 shows clear variations of Jp(T) across TN at constant magnetic fields. Furthermore, isothermal measurements of JME(H) also develop clear peaks at finite magnetic fields, of which traces are consistent with the spin-flop transitions observed in the magnetization studies. As a result, the anomalies observed in Jp(T) and JME(H) curves match well with the field-temperature phase diagram constructed from magnetization and dielectric constant measurements, demonstrating that SrCuTe2O6 is a new magnetoelectric compound with S = 1/2 spin chains.

Bulk magnetite (Fe 3 O 4 ), the loadstone used in magnetic compasses, has been known to exhibit magnetoelectric (ME) properties below ~10 K; however, corresponding ME effects in Fe 3 O 4 nanoparticles have been enigmatic. We investigate quantitatively the ME coupling of spherical Fe 3 O 4 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 Fe 3 O 4 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. KeywordsMagnetoelectric, magnetite nanoparticle, size effect, magnetic nanoparticle, iron oxide Magnetic nanoparticles have been extensively studied for the last decade due to their diverse biomedical applications, such as contrast agents for magnetic resonance imaging, hyperthermia, drug delivery, and bio separation. 1-4 However, their magnetoelectric (ME) properties, which allow for the modulation of electric polarization P (magnetization M) by a magnetic field H (electric field E) 5-6 , have seldom been explored. The lack of work on ME properties mainly originates from the difficulties in determining the intrinsic ME coupling, particularly in the case of nanoparticles, due to their high electrical leakage and weak ME signals. Most previous studies on ME properties have thus been limited to bulk materials and thin composite films 7-9 . Meanwhile, ME nanoparticles, once realized, are expected to be useful for various applications, e.g., applying electrical stimuli for cell proliferation 10 , preparing electrically responsive surface for functionalization 11 , developing stimuliresponsive photonic crystals 12 , and so on.To search for possible ME nanoparticles, it is first necessary tounderstand the behavior of an archetypal ME material, namely the magnetite (Fe 3 O 4 ), in nanoparticle form. As the oldest known loadstone material and with a ferrimagnetic Curie temperature of 858 K, Fe 3 O 4 exhibits an intriguing metal-insulator transition, known as the Verwey transition, at T v =~123 K. The Verwey transition is accompanied by an intriguing charge ordering between Fe 2+ and Fe 3+ ions in the inverse spinel structure and a concomitant structural transition from a cubic to a monoclinic phase. [13][14][15] However, this unique but complex phase transition, involving entangled spin-charge-lattice degrees of freedom, is not fully understood, and remains one of the long-standing conundrums in material physics. 15 Apart from this Verwey transition, bulk Fe 3 O 4 has been known ...

Ni 3 V 2 O 8 is an archetypical multiferroic material with a kagome staircase structure of S = 1 (Ni 2+ ) spins whose complex interplay between spin ordering and ferroelectricity has been studied for more than a decade. Here, we report a new kagome staircase compound PbCu 3 TeO 7 with Cu 2+ (S = 1/2) spins that exhibits two Néel temperatures at T N1 = 36 K and T N2 = 24 K, and a magnetic field (H)-induced electric polarization (P) below T N2 . Pyroelectric and magnetoelectric current measurements in magnetic fields up to 60 T reveal that for H||c of~8.3 T, a spin-flop transition induces a transverse P||a with a magnitude of 15 µC/m 2 below T N2 . Furthermore, for a parallel configuration with P//H//a, two spin-flop transitions occur, the first at~16 T with P//a of 14 µC/m 2 , and the second at~38 T, where P disappears. Monte Carlo simulations based on 12 major exchange interactions uncover that a sinusoidal amplitude modulation of the spins occurs along the b-axis below T N1 and an incommensurate, proper screw-type spin order occurs in the ac-plane below T N2 . The simulation results show that P//a under H//c stems from a spin-flop transition facilitating an ab-plane-type spiral order, while the two successive spin-flop transitions for H//a result in spiral spin orders in the aband bc-planes. Based on the experimental and theoretical results, we establish field-induced magnetic and electric phase diagrams for the two H directions, demonstrating that the distorted kagome staircase structure with competing intra-interlayer interactions and lifted frustration creates a plethora of different noncollinear spin textures of S = 1/2 spins that in turn induce electric polarization.