Ordering Temperature Tc Research Articles

Visible Light-Induced Reversible Photomagnetism in Copper(II) Octacyanomolybdate(IV)

Abstract We report a visible light-induced reversible photomagnetism in CuII2[MoIV(CN)8]·8H2O (1). By the irradiation with a 480-nm laser light, the compound 1 showed a spontaneous magnetization with a magnetic ordering temperature (Tc) of 30 K. Conversely, the magnetization and Tc values were reduced by the irradiation with a 600-nm laser light. This photomagnetism is due to the reversible photo-induced electron transfer between paramagnetic 1 and its valence isomer exhibiting ferromagnetism.

Ferromagnetism in carbon-doped Mn5Si3 films

The effect of carbon doping on the structural and magnetic properties of Mn5Si3Cx films is investigated for different substrate temperatures TS and concentrations x. Samples with x≈0.75 prepared at TS=650–750 K exhibit ferromagnetic order with enhanced ordering temperatures TC well above room temperature in contrast to the undoped antiferromagnetic Mn5Si3 compound. Structural analysis shows that C is incorporated interstitially in the hexagonal Mn5Si3-type structure with a lattice expansion with respect to the undoped compound. The enhanced ferromagnetic order is presumably not simply due to a change of the interatomic Mn–Mn distances but due to a change of the electronic structure and/or exchange interactions.

Magnetic properties of thin film and granular Dy50Fe50 as a function of size

Thin amorphous Dy50Fe50/A layers are prepared by sputtering to understand the influence of the spacer layer A on magnetic properties in the DyFe thickness regime where size effects are also important. The spacer layer A in this work is either Ti or Ca and we compare to our earlier work involving a spacer layer of Mo. The Dy50Fe50 layer is amorphous and Mössbauer spectroscopy indicates that two types of Fe site are present. Finite size effects manifest themselves below a size of 10 nm in a reduction of magnetic ordering temperature TC and coercivity HC. For equal values of d, the reduction of TC and HC is larger for the system with a Ti or Mo spacer layer as compared to the Ca spacer layer. We argue that both size and the structure of the interface are important in lowering HC and TC. The temperature dependence of coercivity is also studied and suggests that magnetic reversal via domain wall movement occurs in our thinnest films.

Mössbauer studies of the alloys Fe3Ga4, (Fe1-xTix)3Ga4 and (Fe1-yCry)3Ga4

The crystal structure of the alloy Fe3Ga has four inequivalent crystallographic iron sites. Mössbauer spectra recorded at 4.2 K were able to be fitted with four sextets corresponding to these sites. Measurement of the hyperfine fields Bhf gave values 271 kG, 191 kG, 152 kG and 195 kG respectively. For the series (Fe1-xTix)3Ga4 spectra were recorded at 4.2 and 300 K for x = 0.05, x = 0.10 and x = 0.15. These spectra were fitted to five sextets, each associated with a distinct iron site. The magnetic ordering temperature TC was observed to be essentially unchanged for 0 x0.15. Site preference for Ti atoms was observed for x = 0.10 and x = 0.15 causing a large change in hyperfine field values at a neighbouring site. Similar Mössbauer studies were made on the series (Fe1-yCry)3Ga4 (0 y0.20). Spectra at 4.2 K could be fitted with four sextets but continuous reductions in hyperfine field values and ordering temperature TC were observed with increasing y. The contrast in the magnetic behaviour of the two series is associated with different patterns of substitutional site preference.

Ordered stack of spin valves in a layered magnetoresistive perovskite

The layered compound La2-2xSr1+2xMn2O7 (x=0.3) consists of bilayers of metallic MnO2 sheets separated by insulating material. The compound exhibits markedly anisotropic magnetoresistance at temperatures well below the three-dimensional magnetic ordering temperature TC=90 K in addition to colossal magnetoresistance around TC. We present neutron diffraction data which show that the magnetic structure of this material switches from antiferromagnetic stacking of the (ferromagnetically ordered) sheets in zero field to ferromagnetic stacking in a field of 1.5 Tesla. The data are the first to be collected on any manganite as a function of applied field, exactly as the magnetoresistance data themselves are collected. They provide a natural explanation of the low-field magnetoresistance in the ordered phase in terms of spin-polarised tunnelling between the magnetic layers and suggest that the material is a bulk stack of spin-valve devices.

Low field magnetotransport in La0.7Sr0.3MnO3 films

We have observed a strong correlation between the degree of epitaxy and the low field resistance versus field R(H) behavior in highly textured La0.7Sr0.3MnO3 films grown by pulsed laser deposition on SrTiO3 substrates. Highly epitaxial films show a small temperature dependent low field hysteretic R(H) peak near the magnetic ordering temperature (Tc), when the applied field is parallel to the transport current (J∥H). In contrast, films with reduced epitaxy show a much larger low field hysteretic R(H) behavior with a pronounced in-plane angular dependence. The magnitude of this response grows with decreasing temperature below Tc. Our data and analysis suggest that even low angle grain boundaries play a fundamental role in transport properties in colossal magnetoresistive materials.

Magnetic field induced reversed (Negative) magnetization for electrochemically deposited Tc = 260 K Oxidized Films of Chromium Cyanide Magnets

Molecule‐based magnets with high ordering temperatures Tc (above room temperature) include some mixed‐oxide hexacyanometallates. The example studied here, Cr2.12(CN)6 · ½ H2O (the Figure shows its crystalline and amorphous phases), is shown to have Tc equals; 260 K and exhibit negative (reverse) magnetization. An explanation is suggested for this behavior and the temperature and field dependences of the magnetization. magnified image

Competing magnetic interactions in manganese perovskites

Structural and magnetotransport properties of L2/3A1/3MnO3 (A=Ca, Sr) oxides reveal the gradual increase of the magnetic frustration when bending the Mn–O–Mn bonds. The relative strength of competing magnetic interactions and thus the ferromagnetic ordering temperature TC are controlled not only by R0 (R0 is the mean radius of the lanthanides) but also by the electronegativity of the divalent cation. Consequently, the curve TC(R0) is not universal but it is sensitive to the alkaline ions.

Magnetic properties of liquid quenched R3Co alloys

The magnetization and ac-susceptibility of liquid quenched R3Co (R =Nd, Tb, Dy) alloys were determined. The magnetic ordering temperatures Tc of the alloys are about 30, 84 and 38 K, respectively. Their magnetic state below Tc can be associated with an asperomagnetic structure. The alloys show large hysteresis loops at low temperature. The coercive field (Hc) depends on the Tc value and increases with increasing Tc. Large discontinuous magnetization jumps were observed for Tb3Co in the vicinity of Hc below 3.5 K.

A room-temperature organometallic magnet based on Prussian blue

THE rational design of molecular compounds that exhibit spontaneous magnetic ordering might enable one to tailor magnetic properties for specific applications in magnetic memory devices1–4. In such materials synthesized previously5–17, however, the underlying weak magnetic interactions are incapable of maintaining ordering at ambient temperatures. One remarkable exception is a compound derived from vanadium and tetracyanoethylene18, but the material is amorphous and fragile, and consequently the molecular interactions responsible for its striking properties are not understood. Here we demonstrate another route to the synthesis of a room-temperature organometallic magnet, in which we combine a hexa-cyanometalate [M(CN)6]q− with a Lewis acid Lp+ If L and M are transition-metal ions, then the orbital interactions in the resulting compound can be described by well understood principles21–24, and it is therefore possible to choose the metals to tune the compound's magnetic properties–in particular, the magnetic ordering (Curie) temperature Tc (refs 21–26). We have synthesized a room-temperature magnetic material (TC = 315 K) that belongs to the Prussian blue family of compounds27 (where M is chromium and L is vanadium), demonstrating that transition-metal hexacyano complexes are promising components for the construction of molecule-based high-Tc magnets.

Magnetic transition and coercivity in TbMn6Sn6

In the present paper, the structure and magnetic properties of TbMn6Sn6 have been investigated. TbMn6Sn6 is found to be of HfFe6Ge6-type structure (space group P6/mmm) with a=5.5461 AA, c=9.0477 AA and V=241 AA3. This compound shows ferrimagnetic behaviour with the ordering temperature Tc=421 K. A peak is also observed in the thermal magnetic curve at 330 K. Under magnetic fields of 100, 300 and 400 Oe, the magnetization values first increase with increasing temperature, next exhibit a peak and then decrease with further increase in temperature. When the applied magnetic field is sufficiently large, e.g. 1000 and 2100 Oe, the magnetization of the whole sample decreases with increasing temperature. With increasing temperature from the range 77-300 K, the coercivity first increases, next undergoes a peak at about 200 K and then decreases. At room temperature a magnetization jump is observed in the magnetization curves at a field of about 4 kOe in both the aligned and the free powder samples. This jump may be attributed to the spin-flop or metamagnetic transition. The magnetic anisotropy field at room temperature is found to be 8.33 kOe.

Influence of exchange bond disorder on the magnetic properties of (Pd1−xFex)95Mn5 near T c

A summary of a detailed study of the field and temperature dependent ac susceptibility of a series of ternary (Pd1−xFex)95Mn5 alloys near the ferromagnetic ordering temperature Tc is presented. By following the behavior of the crossover line (a line of maxima in the field dependent susceptibility above Tc, the maxima moving upwards in temperature but decreasing in amplitude with increasing field) it is possible to catalog the influence of exchange bond disorder on aspects of the critical behavior. These ternary systems are not soft ferromagnets and thus asymptotic critical exponents are difficult to estimate using this technique. Nevertheless, effective exponents (i.e., obtained away from the critical point) clearly reflect the presence of significant variance in the distribution of exchange bonds.

Magnetization, Young’s moduli, and magnetostriction of rare-earth–iron eutectic alloys with R=Tb0.6Dy0.4

Rare-earth–iron alloys, R0.9Fe0.1, R0.72Fe0.28, and R0.42Fe0.58 (R=Tb0.6Dy0.4), containing the R/RFe2 eutectic composition were prepared by Bridgman and free-standing zone-melting techniques. Magnetization measurements were made in fields up to 800 kA/m between 55 and 300 K. A huge increase in magnetization below 210 K occurs as the R component becomes ordered. At low applied magnetic fields there is clear identification of both the ferromagnetic ordering temperature TC and the Néel spiral ordering temperature TN of R. (For Tb0.6Dy0.4, TC=165 K, TN=210 K.) Magnetization and magnetostriction measurements reveal very large magnetocrystalline anisotropies for both the R and the RFe2 components. Unexpectedly, at 77 K, were the rare-earth component of the eutectic system is ordered and the magnetostriction is large (λhγ≳0.6%), the magnetostriction is largest in the samples containing the largest amount of the RFe2 phase. Young’s modulus measurements reveal the reduction in the stiffness with the addition of the softer rare earth to the stiff RFe2 compound.

Magnetic properties of some TbFe11TiHx Hydrides

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Magnetic Properties of Metal Halides-Graphite Intercalation Compounds Under Pressure

Abstract Magnetization measurements under pressure of 0–10 kbar have been carried out on GIC's with CoCl2 (stage 1), NiCl2 (stage 2), MnBr2 (stage 3), CrCl3 (stage 3) and biintercalated CrCl3-CoCl2 (pseudo stage 1). For the three former ones, the ordering temperature Tc increases with increasing pressure in agreement with the smaller interlayer distance and the corresponding J' increase. However, for GIC's containing CrCl3, an unexpected decrease of Tc is observed.

Ferromagnetic domain size of Tb

The temperature dependence of ferromagnetic domain size for Tb is calculated over the range 0 K to the ferromagnetic ordering temperature Tc∼221 K and compared with earlier calculations and experimental data. The present results show a decrease in domain size with increasing temperature in broad agreement with the earlier calculations. Discrepancies between sets of calculations and experiments at high temperatures (T ⪆ 180 K) may be due to domain nucleation and/or thermal history effects.

Phase transitions and critical behavior of spin 1/2 quasi 2-D systems (abstract)

Zero-field ac susceptibility and isothermal magnetization of the quasi-2D alkanediammonium copper tetrahalide series, [NH3(CH2)nNH3]CuX4, where n=4, 5, 6, and 10 with X=Cl and Br, are reported. The 3D antiferromagnetic ordering temperature Tc for the Cl compounds is determined. It is shown that the critical susceptibilities decay exponentially as the temperature increases (T≳Tc). A power-law divergence at Tc is seen in the Br compounds with n=7 and 10. This behavior is characteristic of 3D ferromagnetic ordering at Tc. The critical exponent γ for the initial susceptibility (T≳Tc) has been obtained for the Br compounds. It is found that there is a second (minor) peak below Tc in the Br compounds with n=5 and 7. The transition associated with this peak may be interpreted as a long range (spontaneous) ordering due to very small spin anisotropies, such as a spin canting between the layers. In the Br compounds the isothermal magnetization is suppressed in low fields, and the value of the critical exponent δ estimated from the isothermal data is considerably smaller than that given by standard models. Spatial- and spin-dimensionality crossovers are apparent in the both initial susceptibility and isothermal magnetization data.

MAGNETIC PROPERTIES OF AMORPHOUS NdxFe1-x FILMS

The magnetic properties are reported for amorphous NdxFe1-x films prepared by flash e-vaporation at 77K. The result indicates that Nd is nearly random distributed with a small net moment parallel to the Fe moments which posseses a ferromagnetic alignment. The distribution angle of the Nd-atomic moments increases slightly with increasing x. The ordering temperature Tc is dependent on applied field and the values of Tc from Arrott-plot or other evaluation methods are different. This can be attributed to exsistence of clusters with different Tc in the film. The exchange interaction, but not the spontaneous magnetisation, is strongly influenced by the preparation conditions.

Structural dependence of the ordering temperature on nanostructured magnetic materials

The variation of the ordering temperature Tc as compared to the bulk value is studied for very thin ferromagnetic films in early states of film growth, deposited on a nonmagnetic substrate. Using a mean field model, including thermal fluctuations, T c is estimated for different sizes and shapes of ferromagnetic islands, and also for inhomogeneous but coherent films with various spatial atomic arrangements. Some results will be presented. A simple rule emerges: the larger the average coordination number the larger the ordering temperature. Thus a higher T c is expected for a compact island than for an extended monolayer. However, the details of the island shape is found to play an important role, especially for small islands. This approach allows to estimate the growth mode (e.g. layer-by-layer vs. islandic) from the experimental value of T c and the deposition rate.

Local atomic environment parameters and magnetic properties of disordered crystalline and amorphous iron-silicon alloys

The dependences of the specific saturation magnetization sigma (C), magnetic ordering temperature TC(C), average hyperfine field H(C) and average isomer shift delta (C) on Si concentration are measured in crystalline Fe-Si alloys disordered by mechanical crushing. It is shown that topological disordering does not determine the magnetic properties. The peculiarities found are explained in terms of the local atomic structure parameters depending on the number of Si nearest neighbours of an Fe atom.