Neutron elastic scattering experiments have been performed on the spin gap system TlCuCl 3 in magnetic fields parallel to the b-axis. The magnetic Bragg peaks which indicate the fieldinduced Néel ordering were observed for the magnetic field higher than the gap field H g ≈ 5.5 T at Q = (h, 0, l) with odd l in the a * − c * plane. The spin structure in the ordered phase was determined. The temperature and field dependence of the Bragg peak intensities and the phase boundary obtained were discussed in connection with a recent theory which describes the field-induced Néel ordering as a Bose-Einstein condensation of magnons.KEYWORDS: TlCuCl 3 , spin gap, field-induced magnetic ordering, spin structure, neutron elastic scattering, BoseEinstein condensation of magnonsThe singlet ground state with the excitation gap (spin gap) is a notable realization of the macroscopic quantum effect in quantum spin systems. When a magnetic field is applied in the spin gap system, the gap Δ is suppressed and closes completely at the gap field H g = Δ/gμ B . For H > H g the system can undergo magnetic ordering due to three-dimensional (3D) interactions. Such field-induced magnetic ordering was studied first for Cu(NO 3 ) 2 · 5 2 H 2 O. 1) However, the magnetic properties near H = H g have not been investigated because of the very low ordering temperature, the maximum of which is 0.18 K. Recently, the study of fieldinduced magnetic ordering has been revived, because the spin gap has been found in many quantum spin systems. Field-induced 3D ordering has been observed in several quasi-one-dimensional spin gap systems. [2][3][4][5][6] This paper is concerned with field-induced magnetic ordering in TlCuCl 3 . This compound has a monoclinic structure (space group P 2 1 /c). 7) TlCuCl 3 contains planar dimers of Cu 2 Cl 6 , in which Cu 2+ ions have spin-1 2 . These dimers are stacked on top of one another to form infinite double chains parallel to the crystallographic aaxis. These double chains are located at the corners and center of the unit cell in the b−c plane, and are separated by Tl + ions. The magnetic ground state is the spin singlet with the excitation gap Δ/k B ≈ 7.5 K. 8, 9) The magnetic excitations in TlCuCl 3 were investigated by Oosawa et al., 10) who found that the lowest excitation occurs at Q = (0, 0, 1) and its equivalent reciprocal points, as observed in KCuCl 3 . 11, 12) The origin of the gap is the strong antiferromagnetic interaction J = 5.26 meV on the planar dimer Cu 2 Cl 6 in the double chain. The * E-mail: tanaka@lee.phys.titech.ac.jp neighboring dimers couple magnetically along the chain and in the (1, 0, −2) plane.Our previous magnetic measurements revealed that TlCuCl 3 undergoes 3D magnetic ordering in magnetic fields higher than the gap field H g ≈ 5.5 T. 9) The magnetization exhibits a cusplike minimum at the ordering temperature T N . The phase boundary on the temperature vs field diagram is independent of the field direction when normalized by the g-factor, and can be represented by the power lawwith φ = 2.2...

X-ray and Neutron diffraction as well as muon spin relaxation and Mössbauer experiments performed on SrFe 2 As 2 polycrystalls confirm a sharp first order transition at T 0 = 205 K corresponding to an orthorhombic phase distortion and to a columnar antiferromagnetic Fe ordering with a propagation vector (1,0,1), and a larger distortion and larger size of the ordered moment than reported for BaFe 2 As 2 . The structural and the magnetic order parameters present an remarkable similarity in their temperature dependence from T 0 down to low temperatures, showing that both phenomena are intimately connected. Accordingly, the size of the ordered Fe moments scale with the lattice distortion when going from SrFe 2 As 2 to BaFe 2 As 2 . Full-potential band structure calculations confirm that the columnar magnetic order and the orthorhombic lattice distortion are intrinsically tied to each other.

The semiconductor Sr2FeOsO6, depending on temperature, adopts two types of spin structures that differ in the spin sequence of ferrimagnetic iron-osmium layers along the tetragonal c axis. Neutron powder diffraction experiments, 57Fe Mössbauer spectra, and density functional theory calculations suggest that this behavior arises because a lattice instability resulting in alternating iron-osmium distances fine-tunes the balance of competing exchange interactions. Thus, Sr2FeOsO6 is an example of a double perovskite, in which the electronic phases are controlled by the interplay of spin, orbital, and lattice degrees of freedom.

No abstract

Citation for published item:nD finghi nd ulD evijit uumr nd unungoD udipt nd eehuisD wnfred nd roserD endres nd ¤ oensD hniel wF nd hnelleD lter nd illimsD oert gF nd vnsterD om nd ioD pn nd w¤ ollerD tohnnes F nd flundellD tephen tF nd ryesD illim nd pelser D gludi nd tnsenD wrtin @PHIRA 9vttieEsiteEspei( spin dynmis in doule perovskite rPgoysyTF9D hysil review lettersFD IIP @IRAF pF IRUPHPF Further information on publisher's website: eprinted with permission from the emerin hysil oietyX finghi nD evijit uumr ulD udipt unungoD wnfred eehuisD endres roserD hniel wF ¤ oensD lter hnelleD oert gF illimsD om vnsterD pn ioD tohnnes F w¤ ollerD tephen tF flundellD illim ryesD gludi pelserD nd wrtin tnsenD hysil eview vettersD IIPD IRUPHPD PHIRF PHIR y the emerin hysil oietyF eders my viewD rowseD ndGor downlod mteril for temporry opying purposes onlyD provided these uses re for nonommeril personl purposesF ixept s provided y lwD this mteril my not e further reproduedD distriutedD trnsmittedD modi(edD dptedD performedD displyedD pulishedD or sold in whole or prtD without prior written permission from the emerin hysil oietyF Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Magnetic properties and spin dynamics have been studied for the structurally ordered double perovskite Sr 2 CoOsO 6 . Neutron diffraction, muon-spin relaxation, and ac-susceptibility measurements reveal two antiferromagnetic (AFM) phases on cooling from room temperature down to 2 K. In the first AFM phase, with transition temperature T N1 ¼ 108 K, cobalt (3d 7 , S ¼ 3=2) and osmium (5d 2 , S ¼ 1) moments fluctuate dynamically, while their average effective moments undergo long-range order. In the second AFM phase below T N2 ¼ 67 K, cobalt moments first become frozen and induce a noncollinear spin-canted AFM state, while dynamically fluctuating osmium moments are later frozen into a randomly canted state at T ≈ 5 K. Ab initio calculations indicate that the effective exchange coupling between cobalt and osmium sites is rather weak, so that cobalt and osmium sublattices exhibit different ground states and spin dynamics, making Sr 2 CoOsO 6 distinct from previously reported double-perovskite compounds.