High Temperature Multiferroicity in Cupric Oxide

Multiferroic materials where magnetic and electrical orders coexist have been subject of extensive studies during last few years. Non-collinear/spiral magnetic structures are expected to exhibit concurrent ferroelectricity. CuO which adopts the structure as mineral Tenorite, with monoclinic structure (space group C2/c), has been reported to possess two high temperature antiferromagnetic (AFM) transitions, TN1 ∼ 213 K & TN2 ∼ 230 K. Below the first AFM transition, magnetic structure is reported to be collinear while below later one magnetic structure is non-collinear. In the present study observation of strong anomalies in dielectric and heat capacity measurements around both the TN’sconfirm magnetically induced ferroelectricity in CuO.


Introduction
Materials which exhibit co-existence of magnetic and electrical orderings foresee tremendous technological applications and have attracted considerable attention in recent years [1]. Magnetic order and ferroelectric polarization in spiral magnets are expected to exhibit strong magneto-electric (ME) coupling effects; recent observation of large magneto-electric and magneto-capacitive effects in TbMnO 3 that provides a novel approach to the mutual control of magnetization and electric polarization in magnetic ferroelectrics (FE) materials [2]. However, the multiferroicity in these materials typically occurs at low temperatures, at ~ 40K. Recently, significant progress has been achieved in some hexaferrites, where spiral magnetic order (SMO) induced multiferroics and ME effects were discovered at or near room temperature [3]. In addition to the complex compounds mentioned above, a prominent exception was recently discovered in the very well-known oxide, cupric oxide CuO, where SMO-induced polarization is observed in the temperature range of 215-230K [4][5]. Such discoveries extend the family of high-temperature multiferroics. Copper (II) oxide, occurs in nature as the mineral Tenorite, and has a monoclinic structure with the space group C2/c [6].
There are two well-established magnetic transitions in CuO, one at T N1 ~ 213 K and another atT N2 ~ 230K [4][5]. A commensurate (CM) collinear magnetic structure develops below T N1 . In this phase, magnetic moments are aligned collinear along the b-axis, such as parallel and anti-parallel zigzag chains of Cu-O along the [101] and [101̅ ], respectively. With increasing temperature, an incommensurate (ICM) spiral magnetic structure develops, exists between T N1 and T N2 , due to the competition between antiferromagnetic (AFM) and ferromagnetic (FM) interactions. This noncollinear structure results in a net electric polarization through the inverseDzyaloshinskii-Moriya (D-M) interaction and transverse spin-lattice coupling [7]. Above T N2 , the magnetic susceptibility of CuO undergoes an anomalous continuous rise, instead of a peak at its Nèel temperature, and rises to a broad maximum at about 540K [5]. Even in the strongest magnetic field, the transition temperatures and the associated dielectric anomalies were found to be unaffected by Wang et.al. [8].Considering the SMO as very robust, these authors concluded that the ME effects are nominal in CuO at both T N1 and T N2 . In the present work we have performed structural, dielectric spectroscopy and heat capacity, measurements and investigated the above mentioned transitions.

Experimental Details
High-purity CuO (99.995%) powder was pelletized and then heat treated at 800°C for few hours. Phase purity of sintered material was checked by powder X-ray diffraction (XRD) at room temperature using Cu K α radiation by Bruker D-8 advanced diffractometer. The dielectric measurements were performed using Novo control Alpha-A High Frequency Analyzer, with a homemade test-cell, across 10Hz-10MHz, from room temperature down to 10K. Heat capacity was measured using Modulated Differential Scanning Calorimeter (MDSC) from TA-2910 instruments with temperature range 150-300K in Nitrogen atmosphere, with the scanning rate of 10K/min. Figure 1shows the room temperature Rietveld fitted XRD pattern of CuO, confirming the sample having monoclinic structure with the space group C2/c. A smallunavoidable secondaryphase of cubic Cu 2 O was detected (a small peak at ~ 36.4deg.) during Rietveld fitting. Wang et al. [8] also observed the small quantity of same secondary phase. Lattice parameters of synthesized CuO,obtained from fitting, are as follows; a = 4.6878 Å, b = 3.4228 Å, c = 5.1316 Å, α = 90°, β = 99.51°, γ = 90°.These are similar to those reported elsewhere [8].

Results and Discussions
Further, we explored the dielectric behaviour of this material in wide frequency range, across magnetic transitions reported in CuO. Zhenget.al. [9] reported both the AFM transitions (T N1 &T N2 ), and unlike other antiferromagnets, they found that its magnetization increases with temperature above T N2 , and transformation to proper paramagnetic state occurs only at higher temperatures (630K >>T N2 = 230K). Fig.2 (a) shows the temperature dependence of real part of dielectric permittivity in the temperature region 200-250K, at two different high frequencies (0.6 &0.8 MHz). The dielectric constant shows small FE-like peak at T N2 and a decreasing step-like behaviour atT N1 . It shows no substantial ferroelectric polarization (P) exists below T N1 and above T N2 . Observation of a finite P between T N1 and T N2 is a signature of magnetically induced ferroelectricity.It is to be highlighted here that in an earlier study on CuO single crystal, though the existence of magnetically induced polarization was confirmed in between T N1 and T N2 through pyroelectric current measurement [4], but the anomaly in dielectric constant around T N1 was not as evident as here.
The manifestation of these transitionsis also clear in our heat capacity measurement and it show sharp peak at both the commensurate-incommensurate antiferromagnetic transition temperatures ( fig.2b).The anomalies observed in the heat capacity are reported here for the first time. Also the anomalies observed here in the dielectric data are much clearer than the previous reports [4,9].

Conclusions
Dielectric anomalies across the incommensurate magnetic phase boundaries, atT N2 ~ 230K and T N1 ~ 213Kin CuO evidence magnetically-induced ferroelectricity. Anomalies in the heat capacity are observed for the first time, confirming the AFM transitions.