Effect of pressure on magnetic properties of magnetic nanoparticles, based on Prussian blue analogues, were studied in pressures up to 1.2 GPa. The Mn 3 [Cr(CN) 6 ] 2 · nH 2 O and Ni 3 [Cr(CN) 6 ] 2 · nH 2 O nanoparticles were prepared by reverse micelle technique. Transmission electron microscopy images show nanoparticles with average diameter of about 3.5 nm embedded in an organic matrix. The characteristic X-ray peaks of nanoparticles are more diffused and broader. Systems of nanoparticles behave as systems of interacting magnetic particles. The Curie temperature TC is reduced from T C = 56 K for Ni-Prussian blue analogues to T C = 21 K for Ni-nanoparticles system and from TC = 65 K for Mn-Prussian blue analogues to T C = 38 K for Mn-nanoparticles system. One can explain this reduction of the Curie temperature and of the saturated magnetization µs by dispersion of nanoparticles in an organic matrix i.e. by a dilution effect. Applied pressure leads to a remarkable increase in T C for system of Mn-nanoparticles (∆T C /∆p = +13 K/GPa) and to only slight decrease in T C for system of Ni-nanoparticles (∆T C /∆p = −3 K/GPa). The pressure effect follows behavior of the mother Prussian blue analogues under pressure. The increase in saturated magnetization, attributed to compression of the organic matrix, is very small. PACS numbers: 75.30.Cr, 75.50.Ee, 75.50.Gg, 75.50.Xx (489) 490 A. Zentko et al.

No abstract

Positive muon spin relaxation measurements performed on the ferromagnet UGe2 reveal, in addition to the well known localized 5f -electron density responsible for the bulk magnetic properties, the existence of itinerant quasi-static magnetic correlations. Their critical dynamics is well described by the conventional dipolar Heisenberg model. These correlations involve small magnetic moments.PACS numbers : 74.70. Tx, 76.75.+i The discovery of superconductivity below 1 K within a limited pressure range in the ferromagnet UGe 2 [1-4] provides an unanticipated example of coexistence of superconductivity and strong ferromagnetism. The electronic pairing mechanism needed for superconductivity is believed to be magnetic in origin. However, it is amazing that ferromagnetically ordered uranium magnetic moments with so large magnitude (∼ 1.4 µ B at ambient pressure as deduced from magnetization measurements) are directly involved [5]. Since the pairing must involve the conduction electrons, it is important to characterize their magnetic properties. Because of the restrictions imposed by the magnetic form factor, this can not be done by diffraction techniques. As the muons localize in interstitial sites, they have the potentiality to yield information on the conduction electrons. Here we show, using the muon spin relaxation technique, that UGe 2 is actually a dual system where two sub-states of f electrons coexist. We indeed report the existence at ambient pressure of itinerant long-range magnetic correlations with magnetic moments of ∼ 0.02 µ B and a spectral weight in the megahertz range. A quantitative understanding of this state is moreover reached assuming that these correlations involve only long wavelength fluctuation modes.UGe 2 is a ferromagnet with a Curie temperature T C ≃ 52 K which crystallizes in the orthorhombic ZrGa 2 crystal structure (space group Cmmm) [6,7]. Magnetic measurements indicate a strong magnetocrystalline anisotropy [8,9,3] with easy magnetization axis along the a axis.We present results obtained by the muon spin relaxation (µSR) technique. Fully polarized muons are implanted into the studied sample. Their spin (1/2) evolves in the local magnetic field, B loc , until they decay into positrons. Since the positron is emitted preferentially in the direction of the muon spin at the decay time, it is possible to follow the evolution of the muon spin polarization [10,11]. The measured physical parameter is the so-called asymmetry which characterizes the anisotropy of the positron emission. Below T C , if B loc has a component perpendicular to the initial muon beam polarization, S µ (taken parallel to Z), we expect the asymmetry to display spontaneous oscillations with an amplitude maximum for B loc ⊥ S µ . On the other hand, if B loc S µ , the asymmetry can be written as the product of an initial asymmetry related to sample, a s , and the muon spin relaxation function, P Z (t), which monitors the dynamics of B loc .UGe 2 crystals were grown from a polycrystalline ingot using a Czochralski tri-arc ...

No abstract

We report a study of magnetocaloric effect (MCE) in cyanido-bridged {[M(II)(pyrazole)(4)](2)[Nb(IV)(CN)(8)]·4H(2)O}(n) molecular compounds where M = Ni, Mn, pyrazole = C(3)H(4)N(2). The substances show a sharp phase transition to a long range magnetically ordered state, with ferromagnetic coupling between M and Nb sublattices in the case of the Ni-based sample 1 (T(c) = 13.4 K) and ferrimagnetic coupling for the Mn-based sample 2 (T(c) = 23.8 K). The magnetic entropy change ΔS due to applied field change ΔH as a function of temperature was determined by the magnetization and heat capacity measurements. The maximum value of ΔS at μ(0)ΔH = 5 T is 6.1 J mol(-1) K(-1) (5.9 J kg(-1) K(-1)) for 1 at T = 14 K and 6.7 J mol(-1) K(-1) (6.5 J kg(-1) K(-1)) for 2 at T = 25 K. MCE data at different applied fields have been presented as one universal curve, which confirms magnetic transitions in 1 and 2 to be of second order. The temperature dependences of the n exponent characterizing the dependence of ΔS on ΔH have been obtained. The n(T(c)) values, consistent with the shape of the magnetization curves, pointed to the 3D Heisenberg behaviour for 2 and some anisotropy, probably of the XY type, for 1. The (H/T(c))(2/3) dependence of the maximum entropy change has been tested in the ferrimagnetic Mn(2)-L-[Nb(CN)(8)] (L = C(3)H(4)N(2), C(4)H(4)N(2)) series.

We present a detailed study of the field ͑B͒ -temperature ͑T͒ phase diagram of the heavy-fermion antiferromagnet Ce 7 Ni 3 that crystallizes in the hexagonal Th 7 Fe 3 -type structure with three nonequivalent Ce sites (Ce I , Ce II , and Ce III ). This compound undergoes two magnetic transitions at T N1 ϭ1.9 K and T N2 ϭ0.7 K in zero field. Below T N1 , an incommensurate spin density wave ͑SDW͒ develops, and below T N2 a commensurate SDW appears independently. By applying fields along the c axis, both T N1 and T N2 , are suppressed and vanish at 0.3 T. For BʈcϾ0.7 T, however, another magnetic phase appears below 0.5 K, which was found by magnetoresistance, specific heat, and magnetization measurements. The separation of the field induced phase from the SDW phase is attributed to large spin fluctuations of Ce III , which originate from a geometrical frustration in the quasiregular tetrahedron made of Ce I and Ce III .

We report the magnetic field dependence of the dc susceptibility and resistivity in superconducting single crystals of the hexagonal heavy-fermion superconductor UNi 2 Al 3. From the susceptibility we derive a crystalline-electric-field scheme, which closely resembles that of UPd 2 Al 3. Furthermore, we examine the magnetic phase transition into the incommensurably ordered state at T N = 4.1 K and construct the magnetic phase diagram for the three crystallographic directions. While for fields B parallel to the a-or c-axis T N is monotonically lowered, this is not the case for B b. Instead, here we find a field-induced magnetic transition into a, presumably, commensurate ordered magnetic state. We discuss our results on UNi 2 Al 3 in comparison to those for the related system UPd 2 Al 3 .

No abstract