Abstract. We report the multiferroic behaviour of MnWO 4 , a magnetic oxide with monoclinic crystal structure and spiral long-range magnetic order. Based upon recent theoretical predictions MnWO 4 should exhibit ferroelectric polarization coexisting with the proper magnetic structure. We have confirmed the multiferroic state below 13 K by observing a finite electrical polarization in the magnetically ordered state via pyroelectric current measurements.Multiferroic materials which combine magnetism and ferroelectricity currently attract considerable attention [1][2][3][4]. There are already several multiferroic materials recently discovered among transition metal oxides:. Nevertheless, the search for novel systems with multiferroic properties presents a definite interest. In this letter we report that yet another transition metal oxide, MnWO 4 , belongs to the same class of materials and develops spontaneous electric polarization in a spiral magnetically ordered state [8].There exist several different microscopic mechanisms which may cause multiferroic behavior [3]. One of the most interesting cases is when a spontaneous polarization exists only in a magnetically ordered phase with a particular type of ordering. This is e.g. the case in TbMnO 3 and TbMn 2 O 5 . Microscopic [9] and phenomenological [10] treatments have shown that this happens particularly in spiral magnetic structures with the spin rotation axis − → e not coinciding with the magnetic propagation vector − → Q : theoretical treatment shows that in this case a finite spontaneous polarization perpendicular to the plane spanned by − → e and − → Q may appearThis is not the only source for a magnetically driven ferroelectricity [11,12], but perhaps the most common one. Accordingly, one strategy to search for new multiferroic materials is to look for magnetic systems with proper magnetic structures. MnWO 4 (also known as the mineral hübnerite) appears to be just such a system. Detailed studies of the magnetic ordering in this material have shown [13,14] that below 12.3 K a spiral magnetic ordering develops which seems to satisfy the criterion of Eq. (1). In order to test this we carried out measurements of the dielectric response and of spontaneous polarization of MnWO 4 using single-crystalline samples. The crystals of MnWO4 were grown from melt solution. On the basis of earlier work [15] we applied a modified flux technique, using a melt solvent from the system Na 2 WO 4 -WO 3 . The resulting crystals are of dimensions up to 15 x 5 x 3 mm 3 and of 3 Author to whom correspondence should be addressed (khomskii@ph2.uni-koeln.de).

Using Co-L2,3 and O-K x-ray absorption spectroscopy, we reveal that the charge ordering in La1.5Sr0.5CoO4 involves high spin (S=3/2) Co 2+ and low spin (S=0) Co 3+ ions. This provides evidence for the spin blockade phenomenon as a source for the extremely insulating nature of the La2−xSrxCoO4 series. The associated e 2 g and e 0 g orbital occupation accounts for the large contrast in the Co-O bond lengths, and in turn, the high charge ordering temperature. Yet, the low magnetic ordering temperature is naturally explained by the presence of the non-magnetic (S=0) Co 3+ ions. From the identification of the bands we infer that La1.5Sr0.5CoO4 is a narrow band material.PACS numbers: 71.28.+d, 78.70.Dm Considerable research effort has been put in cobaltate materials during the last decade in search for new phenomena and extraordinary properties. A key aspect of cobaltates that distinguish them from e.g. the manganates and cuprates [1], is the spin state degree of freedom of the Co 3+/III ions: it can be low spin (LS, S=0), high spin (HS, S=2) and even intermediate spin (IS, S=1) [2,3]. This aspect comes on top of the orbital, spin (up/down) and charge degrees of freedom that already make the manganates and cuprates so exciting. Indeed, numerous cobaltate systems have been discovered with properties that include giant magneto resistance [4,5], superconductivity [6] and ferro-ferri-antiferro-magnetic transitions with various forms of charge, orbital and spin ordering [7,8,9,10,11,12,13,14]. A new and exciting aspect in here is the recognition that the so-called spin blockade mechanism could be present and responsible for several of those unusual properties [15]. If true, this would open up new research opportunities since one could envision exploiting explicitly this mechanism in materials design.Here we focus on the La 2−x Sr x CoO 4 system, which shows quite peculiar transport and magnetic properties [16,17,18,19,20,21,22,23,24,25]. This material is extremely insulating for a very wide range of x values with anomalously high activation energies for conductivity, very much unlike the Mn, Ni, or Cu compounds [1,18,26]. The commensurate antiferromagnetic (AF) state remains stable up to a surprisingly high value of x=0.3 [24,25]. Charge ordering (CO) and spin ordering (SO) at half doping involve quite different transition temperatures, namely T CO ∼ 750 K and T SO ≤ 30 K, respectively. This constitutes a ratio of 25, which is extraordinary since it is an order of magnitude larger than in the Mn and Ni materials [1,21,27].It was already reported that the SO in the La 1.5 Sr 0.5 CoO 4 composition involves non-magnetic Co 3+ ions with the claim that these Co 3+ ions are in the IS state and become non-magnetic due to strong planar anisotropy driven quenching of the spin angular momentum below the T SO [21,22]. Here we go one step further. Using soft x-ray absorption spectroscopy (XAS) we are able to show unambiguously that the Co 3+ ions are in the LS (S=0) state, both below and above T SO . Together with the verification...

We have succeeded in preparing high-quality Gd-doped single-crystalline EuO films. Using Eudistillation-assisted molecular beam epitaxy and a systematic variation in the Gd and oxygen deposition rates, we have been able to observe sustained layer-by-layer epitaxial growth on yttriastabilized cubic zirconia (001). The presence of Gd helps to stabilize the layer-by-layer growth mode. We used soft x-ray absorption spectroscopy at the Eu and Gd M4,5 edges to confirm the absence of Eu 3+ contaminants and to determine the actual Gd concentration. The distillation process ensures the absence of oxygen vacancies in the films. From magnetization measurements we found the Curie temperature to increase smoothly as a function of doping from 70 K up to a maximum of 125 K. A threshold behavior was not observed for concentrations as low as 0.2%.

Linear-polarized soft-x-ray absorption ͑XAS͒ and inelastic neutron scattering ͑INS͒ experiments have been performed on CeMIn 5 with M = Rh, Ir, and Co to determine the crystal-field scheme and characteristic Kondo temperatures T ‫ء‬ for the hybridization between 4f and conduction electrons. The ground-state wave functions are determined from the polarization-dependent soft-XAS data at the cerium M 4,5 edge and the crystal-field splittings from INS. The characteristic temperature T ‫ء‬ has been determined from the line widths of the neutron scattering data. We find that the quasielastic linewidths of the superconducting compounds CeCoIn 5 and CeIrIn 5 are comparable with the low-energy crystal-field splitting.

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