H. P. Kunkel

A criterion for enhancing the giant magnetocaloric effect: (Ni?Mn?Ga)?a promising new system for magnetic refrigeration

This letter reports a giant magnetocaloric effect (GMCE) in a novel series of materials based on the shape memory alloy Ni2MnGa. The origin of an enhanced GMCE is traced to the coincidence of a first-order magnetic transition and its attendant structural phase transition with a second-order magnetic transition. This coincidence is achieved by careful compositional tuning and is a technique which provides a criterion for enhancing the GMCE in this system. Thus, for Ni55.2Mn18.6Ga26.2, we report an entropy change ?Sm = ?20.4?J?kg?1?K?1 at 317?K in a field of 5?T. This shape memory system also has the added advantage of being formed from inexpensive, non-toxic constituents. With a working temperature at and above room temperature, it appears to be a most promising candidate for practical room temperature magnetic refrigeration.

Site preference of Fe in nanoparticles of ZnFe2O4

ZnFe 2O 4 nanoparticles have been prepared by sol-gel methods.Mssbauer spectra from 4.2 K to RT, showing that their magnetic ordering took place at a much higher temperature than that of bulk zinc ferrite.The large increase of T N for nanoparticles can be attributed mainly to the redistribution of Fe and Zn cations on the A and B sites.It was found that there are different hyperfine interactions between nanoparticles and bulk material.Nanoparticles with smaller size have bigger θ YK(Yaffet-Kittel angle).

Mössbauer study of Fe-Co nanowires

Arrays of Fe1-xCox (0.0≤x≤0.92) nanowires have been prepared by an electrochemical process, co-depositing Fe and Co atoms into the pores of anodic aluminium; their compositions were determined by atomic absorption spectroscopy. Transmission electron microscope results show that the nanowires are regularly spaced and uniform in shape with lengths of about 7.5 µm and diameters of 20 nm. The x-ray diffraction indicates a texture in the deposited nanowires. For the composition below 82 at.% cobalt, the nanowires had a body-centred-cubic structure with a [110] preferred orientation. For the 92 at.% cobalt sample, the alloy exhibited a mixture of bcc and face-centred-cubic structure. The room temperature 57Fe Mossbauer spectra of the arrays of Fe1-xCox nanowires have second and fifth absorption lines of the six-line pattern with almost zero intensity, indicating that the internal magnetic field in the nanowires lies along the long axis of the nanowire. The maximum values of the hyperfine field (Bhf = 36.6±0.1 T) and isomer shift (IS = 0.06±0.01 mm s-1) occur for 44 at.% cobalt. The variations of the isomer shift and the linewidths with composition indicate that the Fe1-xCox alloy nanowires around the equiatomic composition are in an atomistic disordered state.

Magnetic texture in iron nanowire arrays

Highly ordered iron arrays of nanowires with different diameters have been prepared by the electrochemical deposition method, with the intention of studying magnetic texture in this system. Crystal texture with the [110] direction along the axis of the nanowires is demonstrated by x-ray diffraction measurements. The orientation of the magnetic moments is along the long axis of the nanowire, a result deduced from the transmission Mossbauer spectra of the as-prepared sample and the remanent states. However, as the intensity of the second and fifth lines is larger in conversion electron Mossbauer spectra than that in transmission Mossbauer spectra for the same diameter samples, the orientation of the magnetic moments appears to deviate from the axis more at the ends than that in the middle of the nanowires. On increasing the nanowire diameter, the magnetic texture becomes weaker. Large coercive force can be found for the small diameter samples when the external magnetic field is applied along the long axis of the nanowires.

Field-dependent magnetic and transport properties and anisotropic magnetoresistance in ceramic La0.67Pb0.33MnO3

A summary of detailed measurements of the field and temperature dependence of the ac susceptibility, magnetization and the longitudinal and transverse magnetoresistivities of ceramic La0.67Pb0.33MnO3 is presented. Detailed analysis of the magnetic data provides an estimate of 340.5±0.5 K for the paramagnetic to ferromagnetic transition temperature Tc, with the data closest to the critical point yielding = 1.39±0.06 (from the temperature dependence of the susceptibility along the crossover line), = 0.41±0.02 (from the field induced variation of the temperature of the same line) and = 4.20±0.15 (from the field dependence of the critical isotherm). Nevertheless the presence of disorder - a variance in the distribution of spin-spin coupling strengths - means that Heisenberg model (asymptotic) exponents cannot be excluded; the universality class for this system cannot therefore be definitively identified. The transport data yield Tc = 339.6±0.4 K, and while the isotropic magnetoresistance peaks near Tc, as expected, the magnitude of the spontaneous resistive anisotropy (SRA) (the difference between the longitudinal and transverse magnetoresistance extrapolated to zero induction) increases linearly with decreasing temperature below Tc, peaks near 30 K and then falls to a smaller value (-0.2±0.03%) in the liquid helium range. While this latter value is in reasonable agreement with itinerant model predictions, the temperature variation in the SRA above 30 K is more convincingly reproduced by a localized model. Furthermore, the mechanisms controlling the isotropic magnetoresistance and the SRA in this system appear to be different.

AC susceptibility of the re-entrant system (PdFe)Mn☆

Abstract We report the first measurements of both the linear and non-linear magnetic response of a “re-entrant” system, (Pd+0.35 at% Fe)+5 at % Mn, in the vicinity of both the upper and lower “candidate” transitions. The upper transition has been identified as a well defined paramagnetic-ferromagnetic transition; the associated critical exponents have been found from the field and temperature dependence of the cross-over line above T c . An anomaly in the non-linear response has been observed for the first time in the vicinity of the lower transition (located near the temperature at which the initial susceptibility falls abruptly). This anomaly takes the form of a distinct, but clearly not divergent, peak in the temperature dependence of the coefficient of the H 2 term in the susceptibility. Such behaviour is a strong indication of a re-entrant ferromagnetic-spin glass transition.

Relationship between the magnetocaloric effect and sequential magnetic phase transitions in Ni-Mn-Ga alloys

The fundamental nature of the magnetic phase transitions in the Ni-Mn-Ga system is investigated quantitatively. By compositional tuning, the first-order metamagnetic/martensitic structural transition (occurring at TM≈200K in the parent compound Ni2MnGa) and the second-order/continuous transition (at TC≈375K in the parent) can be merged. This occurs for Ni55.2Mn18.6Ga26.2, where the entropy changes by ΔSM=−20.4Jkg−1K−1 (in a field of 5T near 317K), at a transition that displays both first- and second-order characteristics simultaneously.

Magnetic and transport properties, and the phase diagram of hole-doped La1−xMgxMnO3 (x ≤ 0.4)

A summary of detailed measurements of the field- and temperature-dependent ac susceptibility, the field-cooled and zero-field cooled magnetization, coercive field and the resistivity in zero field and applied field of 1.5 T is presented for hole-doped La1−xMgxMnO3 (0.05 ≤ x ≤ 0.4). Measurements of the ac susceptibility enable estimates of the (effective) critical exponents γ, β and δ to be made, which, when combined with the magnetization data, enable a phase diagram to be proposed. The transport data highlight the suppression of a metal–insulator transition in these systems with small average A-site radius. Furthermore, the transport behaviour is shown to be consistent with the predictions for charge transport by conventional small polaron hopping in both the paramagnetic and ferromagnetic phases, but inconsistent with recent quantitative predictions for magnetic small polaron-mediated conduction in a phase-separated picture.

AC susceptibility of the dilute PdFe system

The authors present a detailed study of the field- and temperature-dependent AC susceptibility of Pd containing between 0.35 and 2.4 at.% Fe which reveals a number of unusual features of this previously well studied system. While all of the samples examined become ferromagnetic at low temperature, there are marked differences in the critical behaviour at different compositions. Near 1.4 at.% Fe this system displays near-Heisenberg-model exponents ( gamma =1.39, beta =0.365, delta =4.8) with little exchange-bond disorder. By contrast, at both higher and lower composition, effective exponent values that vary with both temperature and field are found, a result which is consistent with significant exchange-bond disorder. At lower composition, fluctuations in the exchange-coupling strength can arise due to oscillations that occur in the conduction-electron polarization at large distances, but at higher composition an alternative mechanism for exchange competition must exist. The authors suggest that direct Fe-Fe near-neighbour exchange is the source of this competition, and discuss other properties of the PdFe system that support this suggestion.

Spin-glass ordering in a spinel ferrite, Mg(Al, Fe)2O4

Non-linear susceptibility (χnl) measurements on MgAl1.4Fe0.6O4 show spin-glass ordering in a cubic spinel ferrite, for the first time. The ordering gives a peak in the temperature dependence of χnl, which shows that the spin ordering is a cooperative phase transition. The values of the critical exponents, obtained from χnl as the temperature approaches the transition temperature, are found to lie in the range obtained from measurements on other spin glasses. We also investigate the static and dynamical effects of the spin-glass ordering on Mossbauer spectra. Dynamical spin freezing as a result of spin-glass ordering is observable in Mossbauer spectra.