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This article reviews the 40+ year old spin-glass field and one of its earliest model interpretations as a spin density wave. Our description is from an experimental phenomenological point of view with emphasis on new spin glass materials and their relation to topical problems and strongly correlated materials in condensed matter physics. We first simply define a spin glass (SG), give its basic ingredients and explain how the spin glasses enter into the statistical mechanics of classical phase transitions. We then consider the four basic experimental properties to solidly characterize canonical spin glass behavior and introduce the early theories and models. Here the spin density wave (SDW) concept is used to explain the difference between a short-range SDW, i.e. a SG and, in contrast, a long-range SDW, i.e. a conventional magnetic phase transition. We continue with the present state of SG, its massive computer simulations and recent proposals of chiral glasses and quantum SG. We then collect and mention the various SG 'spin-off's'. A major section uncovers the fashionable unconventional materials that display SG-like freezing and glassy ground states, such as (high temperature) superconductors, heavy fermions, intermetallics and Heuslers, pyrochlor and spinels, oxides and chalogenides and exotics, e.g. quasicrystals. Some conclusions and future directions complete the review.
Spontaneous, collective ordering of electronic degrees of freedom leads to second-order phase transitions that are characterized by an order parameter driving the transition. The notion of a 'hidden order' has recently been used for a variety of materials where a clear phase transition occurs without a known order parameter. The prototype example is the heavy-fermion compound URu(2)Si(2), where a mysterious hidden-order transition occurs at 17.5 K. For more than twenty years this system has been studied theoretically and experimentally without a firm grasp of the underlying physics. Here, we provide a microscopic explanation of the hidden order using density-functional theory calculations. We identify the Fermi surface 'hot spots' where degeneracy induces a Fermi surface instability and quantify how symmetry breaking lifts the degeneracy, causing a surprisingly large Fermi surface gapping. As the mechanism for the hidden order, we deduce spontaneous symmetry breaking through a dynamic mode of antiferromagnetic moment excitations.
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).
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