A. Handstein

Influence of the substrate on growth and magnetoresistance of La0.7Ca0.3MnOz thin films deposited by magnetron sputtering

Off-axis radio frequency magnetron sputtering was employed to grow La0.7Ca0.3MnOz (LCMO) thin films onto three different types of substrates. The substrate strongly influences the structure and the colossal magnetoresistance effect of the obtained films. Single-crystalline thin films were prepared on LaAlO3 (100) substrates, showing a low value of residual resistivity and a metal–insulator transition at a temperature of up to Tpeak=290 K. The latter value of the transition temperature is one of the highest reported so far on thin films of the La–Ca–Mn–O system. Films deposited onto Y-stabilized ZrO2 substrates and onto MgO substrates are polycrystalline and less textured. These films are characterized by a large negative magnetoresistance ratio MR=[R(H)−R(0)]/R(0) measured for small values of the magnetic field H. For H=1.5 kOe, the MR was found to be approximately −30%, −20%, and −8% at temperatures of 20, 77, and 180 K, respectively. The magnetoresistance of polycrystalline LCMO films shows two contribu...

The study of magnetocaloric effect in R2Fe17 (R = Y, Pr) alloys

Magnetocaloric effect (MCE) in low-cost iron-based binary alloys R2Fe17 (R = Y, Pr) has been investigated by measuring their magnetic properties. The Curie temperature of these alloys is found to be close to room temperature. The specific heat measurement of these materials indicates a second order ferromagnetic/paramagnetic phase transition. The MCE properties in these alloys are comparable to that of gadolinium, which is very expensive for domestic use. Therefore these binary alloys are suitable as room temperature MCE materials. The effect of pulsed magnetic field on the MCE properties has also been studied up to 25 T.

Nanocrystalline high performance permanent magnets

Abstract Recent developments in nanocrystalline rare earth–transition metal magnets are reviewed and emphasis is placed on research work at IFW Dresden. Principal synthesis methods include high energy ball milling, melt spinning and hydrogen assisted methods such as reactive milling and hydrogenation-disproportionation-desorption-recombination. These techniques are applied to NdFeB-, PrFeB- and SmCo-type systems with the aim to produce high remanence magnets with high coercivity. Concepts of maximizing the energy density in nanostructured magnets by either inducing a texture via anisotropic HDDR or hot deformation or enhancing the remanence via magnetic exchange coupling are evaluated.

Heat and charge transport properties of MgB2

Abstract A polycrystalline sample of the MgB 2 superconductor was investigated by measurements of the electrical resistivity, the thermopower and the thermal conductivity in the temperature range between 1.8 and 300 K in zero magnetic field. The electrical resistivity shows a superconducting transition at T c =38.7 K and, similarly to borocarbides, a T 2.4 behaviour up to 200 K. The electron diffusion thermopower and its band-structure-derived value indicate the dominant hole character of the charge carriers. The total thermopower can be explained by the diffusion term renormalized by a significant electron–phonon interaction and a phonon drag term. In the thermal conductivity, for decreasing temperature, a significant decrease below T c is observed resulting in a T 3 behaviour below 7 K. The reduced Lorenz number exhibits values smaller than 1 and a characteristic minimum which resembles the behaviour of non-magnetic borocarbides.

The influence of particle size on the coercivity of sintered NdFeB magnets

Abstract Using smaller milling balls the deterioration of coercivity JHc, which is associated with a certain mean grain size D cr of the sintered material, was shifted to larger values of JHc. The oxygen content steadily increasing during the milling process does not depend on the milling-ball diameter.

Hydrogenation disproportionation desorption recombination in Sm–Co alloys by means of reactive milling

Sm–Co-type alloys were disproportionated by milling in hydrogen at enhanced temperatures. X-ray diffraction confirmed the disproportionation of the SmCo5 and Sm2Co17 phases into Sm hydride and α-Co. This “reactive milling” procedure facilitates the disproportionation of these alloys which are characterized by a very high thermodynamic stability, and therefore are not available for a standard hydrogenation disproportionation desorption recombination treatment. Recombination of the reactively milled powders leads to the formation of the original phases, now with dramatically refined grain sizes of around 25 nm and significant coercivities such as μ0JHC=3.7 T in the case of the SmCo5 alloy. Exchange coupling between the nanoscaled grains resulted in magnetically single phase behavior despite a multiphase microstructure. In particular, for the Sm2Co17 alloy, a remanence enhancement was observed for recombination temperatures ⩽700 °C.

High trapped fields in bulk MgB2 prepared by hot-pressing of ball-milled precursor powder

Bulk superconducting MgB2 samples, 20?mm in diameter, were prepared by hot-pressing of ball-milled Mg and B powders using fine-grained boron powders. High maximum trapped fields of B0?=?5.4?T were obtained at 12?K in one of the investigated trapped field magnets (height 8?mm) at the centre of the bulk surface. Investigating the temperature dependence of the trapped field for short MgB2 samples (height ?1.6?mm), trapped fields of up to B0?=?3.2?T at 15?K were achieved. These high trapped fields are related to extremely high critical current densities of up to 106?A?cm?2 at 15?K, indicating strong pinning due to nanocrystalline MgB2 grains. Expected trapped field data for long trapped field magnets prepared from the available MgB2 material are estimated.

Microstructure, microchemistry, and magnetic properties of melt-spun Sm(Co,Fe,Cu,Zr)z magnets

The evolution of the microstructure, microchemistry, and magnetic properties during slow cooling of melt-spun Sm(Co,Fe,Cu,Zr)z magnets was investigated. It was found that uniform cellular and lamellar structures are formed upon isothermal aging the as-spun ribbons at 850 °C for 3 h, without subsequent slow cooling. No microstructural changes and no obvious difference in the Cu content in the 2:17 matrix phase were observed after slow cooling but the coercivity was significantly enhanced from 0.32 to 3 T. A large gradient of the Cu content in the cell boundary phase was detected in the highly coercive melt-spun Sm(Co,Fe,Cu,Zr)z ribbons with slow cooling by nanoprobe chemical analysis, in contrast to a homogeneous Cu distribution in the cell boundary phase of the ribbons without slow cooling. Further investigation revealed that a spinodal structure is developed in the Cu-rich Sm(Co,Cu)5 cell boundary phase of 2:17 SmCo magnets during slow cooling and the high coercivity of the 2:17 type magnets could result...

Thermodynamics of Fe–Sm, Fe–H, and H–Sm systems and its application to the hydrogen–disproportionation–desorption–recombination (HDDR) process for the system Fe17Sm2–H2

Abstract Thermodynamic properties of the Fe–Sm, Fe–H, and H–Sm systems have been analyzed by means of the CALPHAD method. Thermodynamic models have been defined to describe the Gibbs energy of the individual phases and the model parameters have been optimized using different experimental information: phase diagram data, calorimetric data, and equilibrium partial pressures of hydrogen. The entropy of Fe17Sm2 has been derived from experimental data on low-temperature heat capacity. The thermodynamic parameters for Fe17Sm2 have been evaluated using binary (Fe–Sm) and ternary (Fe17Sm2–H2) data simultaneously. The calculated phase diagrams and thermodynamic properties are in agreement with experiments. The thermodynamics of hydrogen in the interstitial solid solution Fe17Sm2Hδ have been described by compound energy formalism. Combining the obtained thermodynamic descriptions of the individual phases, the equilibrium temperatures of the recombination reaction: Fe+SmH2±x⇒Fe17Sm2+H2, which is an important part of the HDDR process, have been calculated as a function of hydrogen pressure.

The upper critical field in superconducting MgB2

The upper critical field Hc2(T) of sintered pellets of the recently discovered MgB2 superconductor was investigated by transport, ac susceptibility and dc magnetization measurements in magnetic fields up to 16 T covering a temperature range between Tc∼39 K and T=3 K∼0.1Tc. The Hc2 data from ac susceptibility are consistent with resistance data and represent the upper critical field of the major fraction of the investigated sample which increases up to Hc2(0)=13 T at T=0 corresponding to a coherence length of ξo=5.0 nm. A small fraction of the sample exhibits higher upper critical fields which were measured both resistively and by dc magnetization measurements. The temperature dependence of the upper critical field, Hc2(T), shows a positive curvature near Tc and at medium temperatures indicating that MgB2 is in the clean limit. The Hc2(T) dependence can be described within a broad temperature region 0.3Tc<T≤Tc by a simple empirical expression Hc2(T)∝(1−T/Tc)1+α, where the parameter α specifies the positive curvature of Hc2(T). This positive curvature of Hc2(T) is similar to that found for the borocarbides YNi2B2C and LuNi2B2C.