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PubMed2007

Ligand design for heterobimetallic single-chain magnets: synthesis, crystal structures, and magnetic properties of MIICuII (M=Mn, Co) chains with sterically hindered methyl-substituted phenyloxamate bridging ligands.

Two new series of neutral oxamato-bridged heterobimetallic chains of general formula [MCu(L(x))(2)] m DMSO (m=0-4) (L(1)=N-2-methylphenyloxamate, M=Mn (1 a) and Co (1 b); L(2)=N-2,6-dimethylphenyloxamate, M=Mn (2 a) and Co (2 b); L(3)=N-2,4,6-trimethylphenyloxamate, M=Mn (3 a) and Co (3 b)) have been prepared by reaction between the corresponding anionic oxamatocopper(II) complexes [Cu(Lx)2](2-) with Mn(2+) or Co(2+) cations in DMSO. The crystal structures of [CoCu(L2)2(H2O)2] (2 b') and [CoCu(L3)2(H2O)2]4 H2O (3 b') have been solved by single-crystal X-ray diffraction methods. Compounds 2 b' and 3 b' adopt zigzag and linear chain structures, respectively. The intrachain Cu...Co distance through the oxamate bridge is 5.296(1) A in 2 b' and 5.301(2) A in 3 b', while the shortest interchain Co...Co distance is 5.995(5) A in 2 b' and 8.702(3) A in 3 b', that is, the chains are well isolated in the crystal lattice due to the presence of the bulky methyl-substituted phenyl groups. Although both Mn(II)Cu(II) and Co(II)Cu(II) chains exhibit ferrimagnetic behaviour with moderately strong intrachain antiferromagnetic coupling (-J(Mn,Cu)=24.7-27.9 cm(-1) and -J(Co,Cu)=35.0-45.8 cm(-1); H= Sigma -J(M,Cu)S(M,i)S(Cu,i)), only the Co(II)Cu(II) chains show slow magnetic relaxation at low temperatures (T(B)<3.5 K), which is characteristic of single-chain magnets (SCMs) because of the high magnetic anisotropy of the Co(II) ion. The blocking temperatures T(B) along this series of chains vary according to the steric hindrance of the aromatic substituent of the oxamate ligand in the series L(1)<L(2)<L(3). Analysis of the SCM behaviour for 3 b and 3 b' on the basis of Glauber's theory for a one-dimensional Ising system showed a thermally activated mechanism for the magnetic relaxation (Arrhenius law dependence). The activation energies E(a) to reverse the magnetisation direction are 38.0 (3 b) and 16.3 cm(-1) (3 b'), while the preexponential factors tau(0) are 2.3x10(-11) (3 b) and 4.0x10(-9) s (3 b').

Journal Article↗
PubMed1998

Characterization of Concentrated Magnetic Colloids by Measurements of Frequency Dependence of AC Magnetic Susceptibility.

The AC magnetic susceptibility frequency dependence in the range 10 Hz-10 kHz is used to study concentrated magnetic colloids. Different theoretical and empirical models are used to describe the frequency dependence. The magnetic analogue of the Cole-Cole model is found to be most suitable. Values of the model fitting parameters and the relaxation time distribution provide information about the dispersability, viscoelastic properties, and interparticle interactions in the magnetic colloids. Copyright 1998 Academic Press.

Journal Article↗
PubMed2000

Physical Properties of Magnetic Fluids and Nanoparticles from Magnetic and Magneto-rheological Measurements.

Static magnetization curves and the magnetorheological effect were used to study the microstructural properties (agglomerate formation) of magnetic fluids and the properties of dispersed nanoparticles. Improved techniques for magnetogranulometry analysis and a formula for the magnetoviscous effect were proposed. The area of applicability of some existing models was studied. The density, distribution, and dimension of particles, as well as the thickness of the nonmagnetic layer were accurately determined from magnetic measurements. The Shliomis diameter and the effective anisotropy constant were determined from rheological and magnetorheological measurements using information obtained from magnetization curves. Copyright 2000 Academic Press.

Journal Article↗
PubMed1998

Investigation of the anisotropy in frozen nickel ferrite ionic magnetic fluid using magnetic resonance

Magnetic resonance is used to obtain the temperature dependence of the magnetic anisotropy of noninteracting NiFe2O4 nanoparticles from 100 to 250 K. The 10.3 nm particles are dispersed as a stable ionic magnetic fluid which is frozen under the action of an external field to perform angular variation measurements. The thermal fluctuation of the easy axis and magnetic moment about the direction of the external field is included in order to obtain the anisotropy from the angular dependence of the resonance field. Copyright 1998 Academic Press.

Journal Article↗
PubMed2000

Relaxation of nuclear magnetization in a nonuniform magnetic field gradient and in a restricted geometry

We study the influence of restriction on Carr-Purcell-Meiboom-Gill spin echo response of magnetization of spins diffusing in a bounded region in the presence of a nonuniform magnetic field gradient. We consider two fields in detail-a parabolic field which, like the uniform-gradient field, scales with the system size, and a cosine field which remains bounded. Corresponding to three main length scales, the pore size, L(S), the dephasing length, L(G), and the diffusion length during half-echo time, L(D), we identify three main regimes of decay of the total magnetization: motionally averaged, localization, and short-time. In the short-time regime (L(D) << L(S), L(G)), we confirm that the leading order behavior is controlled by the average of the square of the gradient, (nablaB(z))(2), and in the motionally averaged regime (MAv), where L(S) << L(D), L(G), by (integral dxB(z))(2). We verify numerically that two different fields for which those two averages are identical result in very similar decay profiles not only in the limits of short and long times but also in the intermediate times, with important practical implications. In the motionally averaged regime we found that previous estimates of the decay exponent for the parabolic field, based on a soft-boundary condition, are significantly altered in the presence of a more realistic, hard wall. We find the scaling of the decay exponent in the MAv regime with pore size to be L(2)(S) for the cosine field and L(6)(S) for the parabolic field, as contrasted with the linear gradient scaling of L(4)(S). In the localization regime, for both the cosine and the parabolic fields, the decay exponent depends on a fractional power of the gradient, implying a breakdown of the second cumulant or the Gaussian phase approximation. We also examined the validity of time-evolving the total magnetization according to a distribution of effective local gradients and found that such approximation works well only in the short-time regime and breaks down strongly for long times. Copyright 2000 Academic Press.

Journal Article↗
PubMed1987

Morphine-induced analgesia and exposure to low-intensity 60-Hz magnetic fields: inhibition of nocturnal analgesia in mice is a function of magnetic field intensity.

In 2 experiments male CF-1 mice were exposed for 60 min, during the mid-dark period of the day-night cycle, to low-intensity (0.5-1.5 gauss, rms) 60-Hz magnetic fields and then tested for levels of analgesia induced by morphine (10 mg/kg) injections. The magnetic field exposures inhibited the degree of morphine-induced analgesia in a field intensity-dependent manner in both experiments (P less than 0.01) with the largest inhibitory effect after exposure to the 1.5-gauss field. Analysis of the combined data from the two experiments revealed a significant (P less than 0.001) linear relationship between level of analgesia and magnetic field intensity. Thus, these data demonstrated a functional relationship between the behavioral effects of morphine in mice and the strength of the 60-Hz magnetic field. Possible mechanisms underlying these effects are discussed.

Analgesia↗
PubMed1996

Magnetic fields and pineal function in humans: evaluation of nocturnal acute exposure to extremely low frequency magnetic fields on serum melatonin and urinary 6-sulfatoxymelatonin circadian rhythms.

Exposure to a 50/60-Hz electromagnetic field can decrease the nocturnal production of melatonin in rodents. Melatonin is considered to be a marker of circadian rhythms, and abnormalities in its secretion are associated with clinical disorders, including fatigue, sleep disruption, mood swings, impaired performance, and depression, which are consequences of desynchronisation. Interestingly, some epidemiological studies have been reported finding most of these clinical disorders in individuals living or working in an environment exposed to electromagnetic fields. This experiment was designed to look for the possible effects of acute exposure (9 hours) to 50-Hz linearly polarized magnetic fields (10 mu T) on the pineal function. Thirty-two young men (20-30 years old) were divided into two groups (control group, i.e., sham-exposed: 16 subjects; exposed group: 16 subjects). All subjects participated in two 24-hour experiments to evaluate the effects of both continuous and intermittent exposure to linearly polarized magnetic fields. They were synchronized with a diurnal activity from 08:00 to 23:00 and nocturnal rest. The experiment lasted two months (mid-February to mid-April). The subjects were exposed to the magnetic fields (generated by three Helmholtz coils per bed) from 23:00 to 08:00, while lying down. Blood samples were collected during each session at 3-hour intervals from 11:00 to 20:00 and hourly from 22:00 to 08:00. Total urine was collected every 3 hours from 08:00 to 23:00 and once during the night, from 23:00 to 08:00. The levels of serum melatonin and its metabolite in urine (6-sulfatoxymelatonin) in exposed men did not differ significantly from those in control (sham-exposed) subjects. This study shows that nocturnal acute exposure to either continuous or intermittent 50-Hz linearly polarized magnetic fields of 10 mu T does not affect melatonin secretion in humans.

Adult↗
PubMed1998

Magnetic studies of the trinuclear center in laccase and ascorbate oxidase approached by EPR spectroscopy and magnetic susceptibility measurements.

The trinuclear centers in Rhus vernicifera laccase and Cucumis sativus ascorbate oxidase have been studied by EPR spectroscopy and magnetic susceptibility measurements over the wide range of 5 K to 300 K. The EPR spectra showed that type II copper receives increasing tetrahedral distortion with raising temperature. Magnetic susceptibilities of laccase showed that both of type I and type II coppers are almost fully paramagnetic since the antiferromagnetic interaction between type III coppers is extremely strong from 5 K to 300 K. On the other hand, the effective magnetic moment of ascorbate oxidase is contributed by ca. 1.7 Cu2+ even below ca. 100 K, since type II Cu is partly in the reduced form. The effective magnetic moment continuously increased with raising temperature because the antiferromagnetic interaction between type III coppers is not as strong as in the case of laccase. The simulation of the SQUID measurement results suggested that the conformational change of the ascorbate oxidase molecule caused the temperature dependence of the antiferromagnetic interaction. The type II Cu EPR signals in laccase and ascorbate oxidase were conspicuously broadened with raising temperature because of the increasing contribution of the triplet state by type III Cu's and/or of the rapid relaxation which finally led to only ca. 30% detection of the type II Cu signals at room temperature. The stepwise binding of azide to the trinuclear center made one of type III Cu's to be EPR detectable. SQUID measurements indicated that only one Cu in the trinuclear center is paramagnetic and other two Cu's are antiferromagnetically coupled for both of the one- and two-azide bound forms. The binding mode of azide to the trinuclear center was discussed based on some models.

Ascorbate Oxidase↗
PubMed2001

Single-molecule magnets: Jahn-Teller isomerism and the origin of two magnetization relaxation processes in Mn12 complexes.

Several single-molecule magnets with the composition [Mn12O12(O2CR)16(H2O)x] (x = 3 or 4) exhibit two out-of-phase ac magnetic susceptibility signals, one in the 4-7 K region and the other in the 2-3 K region. New Mn12 complexes were prepared and structurally characterized, and the origin of the two magnetization relaxation processes was systematically examined. Different crystallographic forms of a Mn12 complex with a given R substituent exist where the two forms have different compositions of solvent molecules of crystallization and this results in two different arrangements of bound H2O and carboxylate ligands for the two crystallographically different forms with the same R substituent. The X-ray structure of cubic crystals of [Mn12O12(O2CEt)16(H2O)3]. 4H2O (space group P1) (complex 2a) has been reported previously. The more prevalent needle-form of [Mn12O12(O2CEt)16(H2O)3] (complex 2b) crystallizes in the monoclinic space group P2(1)/c, which at -170 degrees C has a = 16.462(7) A, b = 22.401(9) A, c = 20.766(9) A, beta = 103.85(2) degrees, and Z = 4. The arrangements of H2O and carboxylate ligands on the Mn12 molecule are different in the two crystal forms. The complex [Mn12O12-(O2)CC6H4-p-Cl)16(H2O)4].8CH2Cl2 (5) crystallizes in the monoclinic space group C2/c, which at -172 degrees C has a = 29.697(9) A, b = 17.708(4) A, c = 30.204(8) A, beta = 102.12(2) degrees, and Z = 4. The ac susceptibility data for complex 5 show that it has out-of-phase signals in both the 2-3 K and the 4-7 K ranges. X-ray structures are also reported for two isomeric forms of the p-methylbenzoate complex. [Mn12O12(O2CC6H4-p-Me)16(H2O)4]. (HO2CC6H4-p-Me) (6) crystallizes in the monoclinic space group C2/c, which at 193 K has a = 40.4589(5) A, b = 18.2288(2) A, c = 26.5882(4) A, beta = 125.8359(2) degrees, and Z = 4. [Mn12O12(O2CC6H4-p-Me)16(H2O)4].3(H2O) (7) crystallizes in the monoclinic space group I2/a, which at 223 K has a = 29.2794(4) A, b = 32.2371(4) A, c = 29.8738(6) A, beta = 99.2650(10) degrees, and Z = 8. The Mn12 molecules in complexes 6 and 7 differ in their arrangements of the four bound H2O ligands. Complex 6 exhibits an out-of-phase ac peak (chi(M)' ') in the 2-3 K region, whereas the hydrate complex 7 has a chi(M)' ' signal in the 4-7 K region. In addition, however, in complex 6, one Mn(III) ion has an abnormal Jahn-Teller distortion axis oriented at an oxide ion, and thus 6 and 7 are Jahn-Teller isomers. This reduces the symmetry of the core of complex 6 compared with complex 7. Thus, complex 6 likely has a larger tunneling matrix element and this explains why this complex shows a chi(M)' ' signal in the 2-3 K region, whereas complex 7 has its chi(M)' ' peak in the 4-7 K region, i.e., the rate of tunneling of magnetization is greater in complex 6 than complex 7. Detailed 1H NMR experiments (2-D COSY and TOCSY) lead to the assignment of all proton resonances for the benzoate and p-methyl-benzoate Mn12 complexes and confirm the structural integrity of the (Mn12O12) complexes upon dissolution. In solution there is rapid ligand exchange and no evidence for the different isomeric forms of Mn12 complexes seen in the solid state.

Journal Article↗
PubMed2001

Observation of a large magnetic anisotropy in the new 2H-perovskite related oxide Ba8CoRh6O21: magnetic measurements on aligned single crystals.

Single crystals of Ba8CoRh6O21, grown out of a potassium carbonate flux, were characterized by single-crystal X-ray diffraction and magnetic measurements. X-ray data were collected in a superspace group approach and solved using the JANA2000 software package. Ba8CoRh6O21 represents the first example of a structurally characterized m = 5, n = 3 member of the A3n+3mA'nB3m+nO9m+6n family of 2H hexagonal perovskite related oxides and contains chains consisting of six consecutive RhO6 octahedra followed by one distorted CoO6 trigonal prism. Magnetic measurements were carried out on large aligned single crystals, and a very large magnetic anisotropy in the magnetic susceptibility, persisting up to room temperature, was observed.

Journal Article↗
PubMed2002

Magnetic semiconductor: structural, magnetic, and conducting properties of the salts of the 6-oxoverdazyl radical cation with M(dmit)(2) anions (M = Ni, Zn, Pd, and Pt, dmit = 1,3-dithiol-2-thione-4,5-dithiolate).

Five kinds of (1:1), (1:3), and (2:1) salts of 3-[4-(diethylmethylammonio)phenyl]-1,5-diphenyl-6-oxoverdazyl radical cation [V](+) with M(dmit)(2) anions (M = Ni, Zn, Pd, and Pt, dmit = 1,3-dithiol-2-thione-4,5-dithiolate) ([V](+)[Ni(dmit)(2)](-) (1), [V](+)[Ni(dmit)(2)](3)(-) (2), [V](+)(2)[Zn(dmit)(2)](2-) (3), [V](+)(2)[Pd(dmit)(2)](2-) (4), and [V](+)(2)[Pt(dmit)(2)](2-) (5)) and an iodide salt of [V](+) ([V](+)[I](-) (6)) have been prepared, and the magnetic susceptibilities (chi(M) values) have been measured between 1.8 and 300 K. The chi(M) of the (1:1) Ni salt (1) can be well reproduced by the sum of the contributions from (i) a Curie-Weiss system with a Curie constant (C) of 0.376 K emu/mol and a negative Weiss constant (theta) of -1.5 K and (ii) the one-dimensional Heisenberg antiferromagnetic alternating chain system with 2J(A-B)/k(B) = -274 K (alternation parameter alpha = J(A-C)/J(A-B) = 0.2). The chi(M) of the (1:3) Ni salt (2) can be well explained by the two-term contributions from (i) the Curie-Weiss system with C = 0.376 K emu/mol and theta = -5.0 K and (ii) the dimer system with 2J/k(B) = -258 K. The magnetic properties of 1 and 2 were discussed based on the results obtained by crystal structure analysis and ESR measurements of 1 and 2. The chi(M) values of the (2:1) Zn, Pd, Pt salts 3, 4, and 5 and [V](+)[I](-) salt 6 follow the Curie-Weiss law with C = 0.723, 0.713, 0.712, and 0.342 K emu/mol and theta = -2.8, -3.1, -2.6, and +0.02 K, respectively, indicating that only the spins of the verdazyl radical cation contribute to the magnetic property of these salts. The salts 1, 3, and 5 are insulators. On the other hand, the conductivity (sigma) of the Ni salt 2 and Pd salt 4 at 20 degrees C was sigma = 8.9 x 10(-2) and 1.3 x 10(-4) S cm(-)(1) with an activation energy E(A) = 0.11 and 0.40 eV, respectively. The salts 2 and 4 are new molecular magnetic semiconductors.

Journal Article↗
PubMed2003

Magnetic studies on hexaiodorhenate(IV) salts of univalent cations. Spin canting and magnetic ordering in K2[ReI6] with Tc = 24 K.

The ionic salts of rhenium(IV) of formula (Cat)(2)[ReI(6)] with Cat = Li(+) (1), Na(+) (2), K(+) (3), Rb(+) (4), Cs(+) (5), NH(4)(+) (6), and AsPh(4)(+) (7) [AsPh(4)(+) = tetraphenylarsonium cation] have been synthesized, and the structures of two of them (namely, 3 and 6) were determined by single-crystal X-ray diffraction. 3 crystallizes in the monoclinic system, space group Pn, with a = 7.815(1) A, b = 7.874(1) A, c = 11.335(1) A, beta = 90.38(1) degrees, and Z = 2 whereas 6 crystallizes in the tetragonal system, space group P4/mnc, with a = 7.881(1) A, b = 7.881(1) A, c = 11.474(2) A, and Z = 2. The structures of 3 and 6 are made up of discrete [ReI(6)](2)(-) anions and K(+) (3) or NH(4)(+) (6) cations held together by electrostatic forces (3 and 6) and N-H.I hydrogen bonds (6). The rhenium(IV) cation in 3 and 6 is surrounded by six iodide ligands in an octahedral environment with the Re-I bond lengths varying in a very narrow range [2.704(3)-2.738(3) and 2.716(1)-2.722(2) A for 3 and 6, respectively]. The [ReI(6)](2)(-) anions in 6 describe a tetragonally distorted body-centered cubic structure. In 3, the arrangement of these units is similar but more distorted. The different arrangement of the anions in 3 and 6 accounts for the centrosymmetric (6) and non-centrosymmetric (3) structures observed. The magnetic properties of 1-7 were investigated in the temperature range 2.0-300 K. The magnetic behavior of 7 is that of a magnetically diluted Re(IV) complex with a large value of the zero-field splitting of the ground level (|2D| = 49.8 cm(-)(1)) whereas those of 1, 2, and 4-6 are typical of antiferromagnetically coupled systems with susceptibility maxima at 28 (1), 27 (2), 21 (4), 16 (5), and 20 K (6). In the case of compound 3, its magnetic behavior in the high-temperature range is parallel to that observed in the parent salts but below 24 K it is a weak ferromagnet with a canting angle of ca. 1.2 degrees.

Journal Article↗
PubMed2003

Structure and magnetic properties of the two-dimensional ferrimagnet (NEt4)[[Mn(salen)]2Fe(CN)6]: investigation of magnetic anisotropy on a single crystal.

The title compound, (NEt(4))[[Mn(salen)](2)Fe(CN)(6)] (1), was synthesized via a 1:1 reaction of [Mn(salen)(H(2)O)]ClO(4) with (NEt(4))(3)[Fe(CN)(6)] in a methanol/ethanol medium (NEt(4)(+) = tetraethylammonium cation, salen(2)(-) = N,N'-ethylenebis(salicylidene)iminate). The two-dimensional layered structure of 1 was revealed by X-ray crystallographic analysis: 1 crystallizes in monoclinic space group P2(1)/c with cell dimensions of a = 12.3660(8) A, b = 15.311(1) A, c = 12.918(1) A, beta = 110.971(4) degrees, Z = 2 and is isostructural to the previously synthesized compound, (NEt(4))[[Mn(5-Clsalen)](2)Fe(CN)(6)] (5-Clsalen(2-) = N,N'-ethylenebis(5-chlorosalicylidene)iminate; Miyasaka, H.; Matsumoto, N.; Re, N.; Gallo, E.; Floriani, C. Inorg. Chem. 1997, 36, 670). The Mn ion is surrounded by an equatorial salen quadridentate ligand and two axial nitrogen atoms from the [Fe(CN)(6)](3-) unit, the four Fe[bond]CN groups of which coordinate to the Mn ions of [Mn(salen)](+) units, forming a two-dimensional network having [[bond]Mn[bond]NC[bond]Fe[bond]CN[bond]](4) cyclic repeating units. The network is spread over the bc-plane of the unit cell, and the layers are stacked along the a-axis. The countercation NEt(4)(+) is located between the layers. Compound 1 is a ferrimagnet with T(c) = 7.7 K and exhibits hysteresis with a remnant magnetization of 13.44 cm(3).mol(-1) (M/N mu(B) = 2.4) at zero field and a coercivity of 1000 Oe when the powder sample was measured at 1.9 K. Magnetic measurements of a direction-arranged single crystal were also carried out. The orientation of the crystallographic axes of a selected single crystal was determined by X-ray analysis, and magnetization was measured when an external field was applied in the a*, b, and c directions. The magnetization in the a* direction increased more easily than those in the b and c directions below the critical temperature. No hysteresis was observed only for the measurement in the a* direction, indicating the presence of strong structural anisotropy with potential anisotropy on Mn(III) ions.

Journal Article↗
PubMed2004

Structural characterization, magnetic properties, and electrospray mass spectrometry of two Jahn-Teller isomers of the single-molecule magnet [Mn12O12(CF3COO)16(H2O)4].

The syntheses and characterization of two new, highly soluble, single-molecule magnets [Mn12O12(CF3COO16(H2O)4].2CF3COOH.4H2O (1) and Mn12O12(CF3COO16(H2O)4].CF3COOH.7H2O (2) are reported. Compound 1 was isolated from the reaction of Mn12O12(CF3COO16(H2O)4] with trifluoroacetic acid in CH2Cl2. Compound 1 crystallizes in the tetragonal space group Ifourmacr; (No. 82) with unit cell parameters a = b = 18.128(3) A, c = 13.048(3) A, V = 4287.9(19) A3, Z = 2 and is isostructural to [Mn12O12(CH3COO)16(H2O)4]. Compound 2 was prepared from the reaction of Mn12O12(CF3COO16(H2O)4] with neat trifluoroacetic acid, and crystallizes in the monoclinic space group P2(1)/n (No. 14) with unit cell parameters a = 15.221(8) A, b = 21.870(12) A, c = 27.217(15) A, beta = 90.53(1) degrees, V = 9060(9) A3, and Z = 4. The dc magnetic susceptibility measurements in the 2-300 K temperature range support a high-spin ground state. The magnetization data collected in the 1-7 T field range from 1.8 to 4.0 K were best fit to the parameters S = 10, g = 2.15, D = -0.65 cm(-1), and E = 0 cm(-1) for 1 and S = 10, g = 1.87, D = -0.34 cm(-1), and E = -0.10 cm(-1) for 2. The ac susceptibility data for compound 1 reveal out-of-phase (chi(m)") signals in the 4-7 K temperature range, whereas the chi(m)" signals for compound 2 appear below temperatures of 4 K. This variation in blocking temperatures is a consequence of the two different crystallographic forms of compounds 1 and 2. Compound 1 exhibits the same structural geometry and distortions found in [Mn12O12(CH3COO)16(H2O)4], while compound 2 is of lower molecular symmetry with two Jahn-Teller axes of distortion being oriented along oxide ligands. This different structural arrangement facilitates a different tunneling pathway that leads to a lower effective barrier for magnetization reorientation for compound 2. The substitution of the acetate ligands by trifluoroacetic acid was monitored by mass spectrometry, which is a convenient tool for judging completion of the substitution process.

Journal Article↗
PubMed2004

Crystallographic, electronic, and magnetic studies of zeta(2)-GaM (M = Cr, Mn or Fe): trends in itinerant magnetism.

This study of the crystal structure, electronic structure, and magnetic properties of the zeta(2)-GaM (M = Cr, Mn or Fe) alloys is motivated by the recent reinvestigation of the crystallographic Al(8)Cr(5) structure type of zeta(2)-GaMn. The isostructural compounds zeta(2)-GaFe and zeta(2)-GaCr have been refined using X-ray powder diffraction as well as neutron powder diffraction for zeta(2)-GaFe. Their structures have been refined using the space group Rm, with cell parameters a = 12.625(8) A and c = 7.785(10) A for zeta(2)-GaCr and a = 12.4368(11) A and c = 7.7642(10) A for zeta(2)-GaFe. Band structure calculations using the self-consistent, spin-polarized TB-LMTO method were performed to understand their electronic structure and magnetic properties. Band calculations show that from GaCr to GaFe the magnetic interactions change from weakly antiferromagnetic coupling to ferromagnetic coupling. Magnetic measurements confirm ferromagnetism for GaFe and show a weak paramagnetic response for GaCr.

Journal Article↗
PubMed2005

Magnetic properties of isostructural BaCoP2O7, BaNiP2O7, and BaCuP2O7 studied with dc and ac magnetization and specific heat.

Magnetic properties of three isostructural compounds BaMP2O7 (M = Co, Ni, and Cu) were investigated by dc and ac magnetization and specific heat measurements. BaCuP2O7 was shown to be an excellent quasi-one-dimensional linear-chain Heisenberg antiferromagnet with an exchange constant (J/kB) of 103.8 K (Hamiltonian H = J Sigma SiS(i+1)) and a temperature for the long-range magnetic order (TN) of 0.81 K giving the ratio kBTN/J = 0.78%. BaCoP2O7 and BaNiP2O7 exhibited long-range antiferromagnetic order at TN = 10.4 and 10.1 K, respectively. BaCoP2O7 and BaNiP2O7 showed a large contribution of the short-range correlation above TN. BaNiP2O7 remained in the antiferromagnetic state up to 90 kOe at 2 K, whereas BaCoP2O7 demonstrated two metamagnetic phase transitions at about 52 and 71 kOe at 2 K if the magnetic field was parallel to the easy direction. BaMP2O7 melted incongruently at 1323 K (M = Co), 1344 K (M = Ni), and 1338 K (M = Cu).

Journal Article↗
PubMed2006

Solvent-enhanced magnetic ordering temperature for mixed-valent chromium hexacyanovanadate(II), Cr(II)0.5Cr(III)[V(II)(CN)6].zMeCN, magnetic materials.

The reaction of V(III)(THF)3Cl3 with NEt(4)CN in acetonitrile (MeCN) forms (NEt4)3[V(III)(CN)6].4MeCN (1), which after characterization was used as a molecular building block toward the synthesis of Prussian blue structured magnets. The reaction of 1 with [Cr(II)(NCMe)4](BF4)2 forms Cr(II)(0.5)Cr(III)[V(II)(CN)6].zMeCN via internal electron transfer, whose structure and magnetic properties are dependent on the degree of solvation, z. When solvated, Cr(II)(0.5)Cr(III)[V(II)(CN)6].1.2MeCN (2) is a mixture of crystalline and amorphous fractions that yield a material with two magnetic phases: bulk ferrimagnetic phase/crystalline [faced-centered-cubic lattice with a = 10.55(2) A] and cluster-glass phase/amorphous. The bulk ferrimagnetic phase exhibits a critical temperature, Tc, of 110 K, while the amorphous cluster-glass phase exhibits a freezing temperature, Tf, of approximately 25 K. Amorphous Cr(II)(0.5)Cr(III)[V(II)(CN)6].0.1MeCN (3) was determined to be the pure cluster-glass phase. This is an overall enhancement of 85 K (350%) in the magnetic ordering temperature via solvation, z. The coercivity was also increased 4-fold from 890 (2) and 3900 Oe (3) via desolvation.

Journal Article↗
PubMed2006

Magnetic structure and magnetic properties of synthetic lindgrenite, Cu3(OH)2(MoO4)2.

Synthetic Cu3(OH)2(MoO4)2 consists of Cu3(OH)2 brucite ribbons of edge-sharing copper octahedra connected by MoO4 into a 3D network as in the mineral, lindgrenite, for all temperatures between 1.5 and 300 K. Each ribbon consists of a triangular connection between two different types of copper atom (Cu(1) and 2 Cu(2)) via mu3-OH. The MoO4 acts both as one- and three-atom bridges to connect six Cu atoms belonging to three adjacent ribbons. The magnetic properties are consistent with those of ferrimagnetic chains, and the resulting moment of each chain is parallel below the long-range magnetic ordering at 13 K. The Curie constant is 0.468(1) emu K mol-1 of Cu; the Weiss temperature is -14.2(2) K, and the saturation magnetization at 2 K in 50 kOe is 0.41 N muB mol-1 of Cu. Analyses of the neutron powder diffraction reveal an ordered magnetic state where the moment of Cu(1) is antiparallel to those of the two Cu(2); all of them point along the a axis without any sign of geometrical frustration. Any degeneracy that may be present because of the triangular topology of the Cu atoms (s = 1/2) appears to be lifted by the distortion from an ideal equilateral geometry of the triangle. The entropy, estimated from the heat capacity measurements, attains 50% of the total of 17.7 J K-1 mol-1, close to that expected for three Cu atoms (3R ln 2), up to the long range ordering temperature, and the remaining is associated with the low dimensionality of the material.

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