Special temperatures in frustrated ferromagnets.

The description and detection of unconventional magnetic states, such as spin liquids, is a recurring topic in condensed matter physics. While much of the efforts have traditionally been directed at geometrically frustrated antiferromagnets, recent studies reveal that systems featuring competing antiferromagnetic and ferromagnetic interactions are also promising candidate materials. We find that this competition leads to the notion of special temperatures, analogous to those of gases, at which the competing interactions balance, and the system is quasi-ideal. Although induced by weak perturbing interactions, these special temperatures are surprisingly high and constitute an accessible experimental diagnostic of eventual order or spin-liquid properties. The well characterised Hamiltonian and extended low-temperature susceptibility measurement of the canonical frustrated ferromagnet Dy2Ti2O7 enables us to formulate both a phenomenological and microscopic theory of special temperatures for magnets. Other members of this class of magnets include kapellasite Cu3Zn(OH)6Cl2 and the spinel GeCo2O4.

Magnetisation process in the rare earth tetraborides, NdB4 and HoB4.

A field-induced magnetisation process in the frustrated antiferromagnets is often much richer compared to the materials without competing interactions. The applied field tends to stabilise unusual spin configurations which frequently results in the appearance of magnetisation plateaux. Here we report a study into the field-induced magnetisation of the two frustrated rare earth tetraborides, HoB4 and NdB4. NdB4 shows a fractional magnetisation plateau occurring at M/M sat ≈ [Formula: see text] before saturating in a field of 33 kOe. On cooling down to 0.5 K the temperature dependent susceptibility of NdB4 shows an unconventional transition where the system returns to the zero field antiferromagnetic state from a higher-temperature ferrimagnetic state. We are able to reconstruct the magnetic phase diagram of NdB4 from the magnetisation, susceptibility and resistivity measurements for both H [Formula: see text] c and H ⊥ c. For HoB4, the most interesting behaviour is found at the lowest temperature of 0.5 K, where the field dependent magnetisation demonstrates a new fractional [Formula: see text]-magnetisation plateau. Further insight into the relations between the exchange interactions and single ion effects is gained through high-field magnetisation measurements in both HoB4 and NdB4.

Order by Quenched Disorder in the Model Triangular Antiferromagnet RbFe(MoO_{4})_{2}.

We observe a disappearance of the 1/3 magnetization plateau and a striking change of the magnetic configuration under a moderate doping of the model triangular antiferromagnet RbFe(MoO_{4})_{2}. The reason is an effective lifting of degeneracy of mean-field ground states by a random potential of impurities, which compensates, in the low-temperature limit, the fluctuation contribution to free energy. These results provide a direct experimental confirmation of the fluctuation origin of the ground state in a real frustrated system. The change of the ground state to a least collinear configuration reveals an effective positive biquadratic exchange provided by the structural disorder. On heating, doped samples regain the structure of a pure compound, thus allowing for an investigation of the remarkable competition between thermal and structural disorder.

Dispersionless Spin Waves and Underlying Field-Induced Magnetic Order in Gadolinium Gallium Garnet.

We report the results of neutron diffraction and inelastic neutron scattering on a powder sample of Gd_{3}Ga_{5}O_{12} at high magnetic fields. Analysis of the diffraction data shows that in high fields (B≳1.8 T) the spins are not fully aligned, but are canted slightly as a result of the dipolar interaction. The magnetic phase for fields ≲1.8 T is characterized by antiferromagnetic peaks at (210) and an incommensurate wave vector. The dominant contribution to inelastic scattering at large momentum transfers is from a band of almost dispersionless excitations. We show that these correspond to the spin waves localized on ten site rings, expected on the basis of nearest neighbor exchange interaction, and that the spectrum at high fields B≳1.8 T is well described by a spin wave theory.

Magnetic field-induced ordering in SrDy2O4.

Heat capacity measurements were used to investigate the magnetic ordering processes in single-crystal samples of SrDy2O4 in a magnetic field applied along the [010] and [001] directions. In zero field this compound appears to be magnetically disordered down to at least 0.39 K. A magnetic field applied along the [010] direction induces a very sharp transition at 20 kOe, seen as a strong peak in the heat capacity versus field, C(H) curves, while for H // [001], the magnetization process is accompanied by the development of only broad features in the C(H) curves. The process of field-induced ordering in SrDy2O4 appears to be rather remarkable even in the context of the unusual phase transitions observed in other geometrically frustrated magnetic systems consisting of hexagons and triangles.

[The incidence of postoperative myocardial infarction and the left ventricular function in patients with type 2 diabetes mellitus after recanalization of chronic coronary occlusions].

AIM: To study the effects of intracoronary administration of the ischemic preconditioning (IP) trigger adenosine on the reduced incidence of percutaneous coronary intervention (PCI)-associated myocardial infarction (MI) and left ventricular (LV) systolic and contractile function in patients with type 2 diabetes mellitus (DM) during recanalization of chronic coronary occlusions (CCO). SUBJECTS AND METHODS: The patients were divided into 4 groups: 1) 45 patients without DM who received intracoronary placebo infusion; 2) 51 patients without DM who had 10 mg intracoronary adenosine during PCI; 3) 34 patients with type 2 DM who had intracoronary adenosine during PCI; 4) 37 with type 2 DM who received intracoronary placebo. Troponin I and the MB fraction of creatine phosphokinase were measured an hour before and 18-24 hours and 5 days after PCI. The authors estimated the incidence of PCI-associated MI according to the ESC/ACCF/AHA/WHF (2007) criteria and the time course of changes in LV ejection fraction (EF) and end-systolic volume (ESV) and end-diastolic volume, impaired LV local contractility index (ILVLCI) in 167 patients with CCO 1 day and 1 and 12 months after PCI. RESULTS: There was an improvement in myocardial systolic and contractile function after recanalization and stenting for CCO. The intracoronary adenosine groups showed significantly reduced incidence of PCI-associated MI, increased LV EF, and decreased LV ESV and ILVLCI as compared to the placebo groups. CONCLUSION: The intracoronary injection of the IP trigger adenosine is an effective and safe method to improve LV systolic and contractile function in patients with CCO and type 2 DM.

Slow magnetic order-order transition in the spin chain antiferromagnet Ca3Co2O6.

Using powder neutron diffraction, we have discovered an unusual magnetic order-order transition in the Ising spin chain compound Ca3Co2O6. On lowering the temperature, an antiferromagnetic phase with a propagation vector k=(0.5,-0.5,1) emerges from a higher temperature spin density wave structure with k=(0,0,1.01). This transition occurs over an unprecedented time scale of several hours and is never complete.

Titanium pyrochlore magnets: how much can be learned from magnetization measurements?

We report magnetization data for several titanium pyrochlore systems measured down to 0.5 K. The measurements, performed on single crystal samples in fields of up to 7 kOe, have captured the essential elements of the previously reported phase transitions in these compounds and have also revealed additional important features overlooked previously either because of the insufficiently low temperatures used, or due to limitations imposed by polycrystalline samples. For the spin-ice pyrochlores Dy(2)Ti(2)O(7) and Ho(2)Ti(2)O(7), an unusually slow relaxation of the magnetization has been observed in lower fields, while the magnetization process in higher fields is essentially hysteresis-free and does not depend on sample history. For the XY pyrochlore Er(2)Ti(2)O(7), the magnetic susceptibility shows nearly diverging behaviour on approach to a critical field, H(C) = 13.5 kOe, above which the magnetization does not saturate but continues to grow at a significant rate. For the Heisenberg pyrochlore Gd(2)Ti(2)O(7), the magnetic susceptibility shows a pronounced change of slope at both transition temperatures, T(N1) = 1.02 K and T(N2) = 0.74 K, contrary to the earlier reports.

Chiral and collinear ordering in a distorted triangular antiferromagnet.

Magnetization, specific heat, and neutron diffraction measurements are used to map out the entire magnetic phase diagram of KFe(MoO4)2. This stacked triangular antiferromagnet is structurally similar to the famous multiferroic system RbFe(MoO4)2. Because of an additional small crystallographic distortion, it contains two sets of inequivalent distorted magnetic triangular lattices. As a result, the spin network breaks down into two intercalated yet almost independent magnetic subsystems. One is a collinear antiferromagnet that shows a simple spin-flop behavior in applied magnetic fields. The other is a helimagnet that instead goes through a series of exotic commensurate-incommensurate phase transformations. In the various phases one observes either true three-dimensional or unconventional quasi-two-dimensional ordering.

High quality single crystals of the SrR(2)O(4) family of frustrated magnets.

Large high quality single crystals of several compounds of the new family of frustrated magnetic oxides SrR(2)O(4) where R = Dy, Er, Ho and their nonmagnetic analogues with R = Lu, Y have been synthesized by the floating zone technique. The magnetic rare earth ions in these compounds are linked to each other through a network of hexagons and triangles reminiscent of the honeycomb lattice. Initial characterization measurements show that geometrical frustration plays an important role in the formation of the magnetic ground states in these systems. The single crystals grown are suitable for more detailed investigations, especially those using neutron scattering techniques.

Nature of the magnetic order in Ca3Co2O6.

We present a detailed powder and single-crystal neutron diffraction study of the spin chain compound Ca3Co2O6. Below 25 K, the system orders magnetically with a modulated partially disordered antiferromagnetic structure. We give a description of the magnetic interactions in the system which is consistent with this magnetic structure. Our study also reveals that the long-range magnetic order coexists with a shorter-range order with a correlation length scale of approximately 180 angstroms in the ab plane. Remarkably, on cooling, the volume of material exhibiting short-range order increases at the expense of the long-range order.

Field induced ordering in highly frustrated antiferromagnets.

We predict that an external field can induce a spin ordering in highly frustrated classical Heisenberg magnets. We find analytically stabilization of collinear states by thermal fluctuations at a one-third of the saturation field for kagome and garnet lattices and at a half of the saturation field for pyrochlore and frustrated square lattices. This effect is studied numerically for the frustrated square-lattice antiferromagnet by Monte Carlo simulations for classical spins and by exact diagonalization for S = 1/2. The field induced collinear states have a spin gap and produce magnetization plateaus.

[Characteristics of human plasminogen activator and of a fraction of fibrinolytically active plasma enriched with this enzyme]. / Kharakteristika aktivatora plazminogena cheloveka i fraktsii fibrinoliticheski aktivnoi plazmy, obogashchennoi étim fermentom.

An activator of plasminogen with specific activity 203 AU/mg was isolated from blood plasma with fibrinolytic activity, obtained from blood of suddenly decreased patients. Specific activity of the plasminogen activator exceeded 88-fold the activity of the initial blood plasma. The protein was identified with a glycoprotein, similar to beta-globulin; its molecular weight was 70,000 as shown by gel filtration; the isoelectric point was at pH 6.2. The plasminogen activator remained stable after heating up to 50 degrees. Effects of pH in an incubation media and of cations on the activity of the plasminogen activator were studied. A fraction containing the fibrinolytic activity and enriched with plasminogen activator was obtained from the blood plasma after fractionation at low temperature with ethanol at definite pH value. The specific fibrinolytic activity in the fraction exceeded 17.6-fold the activity of blood plasma. The fraction exhibited high thrombolytic and antithrombin activities in vitro. It was similar to streptase and streptokinase preparations in its throm-bolytic effect. Relative species specificity was found in studies of the fibrinolytic and antithrombin effects of the fraction containing fibrinolytic activity.

[Purification and properties of plasminogen activator from human blood plasma after sudden death]. / Ochistka i svoistva aktivatora plazminogena iz plazmy krovi vnezapno umershikh liudei.

Plasminogen activator from human blood plasma after sudden death was isolated and purified 60-90-fold by precipitation with ammonium sulfate, ZnSO4 and ethanol as well as by chromatography on DEAE-Sephadex A-50 and gel--filtration through Sephadex G-200. The resulting enzyme had specific activity of 110-210 units per mg of protein. The enzyme prepartion possessed no plasmin activity; total content of carbohydrates was 2.4-2.5%; that of syalic acids--1.2-1.3%. The enzyme was found heterogeneous during disc electrophoresis in 7.0% polyacrylamide gel and corresponded in its mobility to beta-globulins of blood plasma. Molecular weight of enzyme as determined by gel-filtration through Sephadex G-200 is 70000. The isoelectric point lies at pH 6.2.