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  • 1
    Digitale Medien
    Digitale Medien
    Microstructure and magnetization reversal in nanocomposite SmCo5/Sm2Co17 magnets (2002)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 91 (2002), S. 2192-2196 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: A comparative investigation has been made of the microstructure and magnetization reversal behavior of nanocomposite SmCo5/Sm2Co17 powders prepared by intensive milling and subsequent annealing. It was found that the saturation magnetization increases monotonically with the increase of the volume fraction of Sm2Co17 at the expense of the coercivity. However, the remanence and the energy product first increase and then decrease with further increase of the fraction of Sm2Co17. The highest remanence of 0.72 T and energy product of 78 kJ/m3 are obtained in the powders with 80% and 60% Sm2Co17, respectively. All the hysteresis loops exhibit a magnetically single-phase behavior. X-ray diffraction results reveal that the coexistence of two phases is found in the mixture of SmCo5/Sm2Co17 powders. The demagnetization processes of the SmCo5/Sm2Co17 powders are similar to those of nanocomposites consisting of hard and soft phases, in which the exchange-spring magnet behavior was observed. Positive as well as negative deviations in Wohlfarth's remanence analysis (δM-plot) have been observed, indicating complex magnetic interactions in these materials. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1433940
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  • 2
    Digitale Medien
    Digitale Medien
    Magnetic properties of heteroepitaxial La0.7Sr0.3MnO3/SrTiO3 superlattices (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 6834-6836 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: We have investigated the magnetic properties of La0.7Sr0.3MnO3(LSMO)/SrTiO3(STO) superlattices on a number of samples with the thickness dLSMO≥dSTO in order to understand the interface magnetism and the possible interlayer magnetic coupling. A series of multilayers with several thickness combinations of dLSMO and dSTO, varying from 1.9 to 10 nm was fabricated on SrTiO3(100) substrates by pulsed laser deposition. A ferromagnetic transition was observed for all studied films and the transition temperature decreased with dLSMO, without showing a clear dependence on dSTO. The spontaneous magnetization was found to decrease with dLSMO though the decrease is not as strong as expected for a magnetically disordered surface of the LSMO layers. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1359467
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  • 3
    Digitale Medien
    Digitale Medien
    Large magnetoresistance of the polycrystalline metamagnetic layered manganite La1.2Nd0.2Sr1.6Mn2O7 (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 814-816 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Magnetization and magnetotransport have been investigated for polycrystalline samples of the bilayer Ruddlesden–Popper compound La1.2Nd0.2Sr1.6Mn2O7 that shows the colossal magnetoresistance effect connected with ferromagnetic ordering at TC=74 K. At T2=38 K the compound reveals a transition from the ferromagnetic to an antiferromagnetic spin arrangement of alternating ferromagnetic bilayers, which undergoes a first order metamagnetic transition towards ferromagnetism in a magnetic field of ∼3.5 kOe at 20 K. In measurements of resistance vs magnetic field, this transition is accompanied by a large resistance drop of [R(5 kOe)−R(0)]/R(0)=−60% at 5 K. The tunneling-like magnetoresistance (MR) known for current transport perpendicular to the layers in (La, Nd)1.4Sr1.6Mn2O7 single crystals contributes to this large MR observed in a polycrystalline sample. While current is expected to mainly follow the bilayers within the grains due to the anisotropy of conductivity, the high value of resistivity of our sample (∼8 Ω cm at 5 K) suggests that current paths are partially directed perpendicular to the layers. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.371946
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  • 4
    Digitale Medien
    Digitale Medien
    Magnetotransport of La0.7Sr0.3MnO3/SrTiO3 multilayers with ultrathin manganite layers (2001)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 89 (2001), S. 6973-6975 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The thickness dependence of electrical transport and magnetization of ultrathin La0.7Sr0.3MnO3 films has been investigated using epitaxial La0.7Sr0.3MnO3(LSMO)/SrTiO3 multilayers prepared by pulsed laser deposition. Layer thicknesses range from 1.9 to 10 nm for both constituents. Microstructural characterization by x-ray diffraction and transmission electron microscopy has confirmed a well-defined superlattice structure, coherent growth, and fairly smooth interfaces of the samples. Magnetization measurements show a pronounced drop of the ferromagnetic Curie temperature below a LSMO thickness of d=3 nm, accompanied by a strong rise of the coercive field. The in-plane electrical transport changes from metallic (dρ/dT〉0) to insulating at d∼2.3 nm. Three different types of magnetoresistance (MR) have been observed for d〈3 nm. In the sequence of decreasing d, the first resembles the grain boundary MR of polycrystalline LSMO, the second is positive, and the third is associated with a magnetic field-induced insulator-to-metal transition. Magnetic and transport results are discussed in the framework of a mixed-phase state of ferromagnetic metallic and less magnetically ordered insulating clusters in the ultrathin LSMO films. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1362649
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  • 5
    Digitale Medien
    Digitale Medien
    Hard magnetic properties of melt-spun R(Fe,M)12 nitrides (R=Nd or Pr; M=Mo or V) (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 88 (2000), S. 988-992 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: The hard magnetic properties of nitrided melt-spun R(Fe,M)12 (R=Pr and Nd, M=V and Mo) ribbons have been investigated. Hard magnetic powders were prepared by vacuum annealing of ground ribbons in the temperature range from 700 to 950 °C and subsequent nitrogenation. For NdFe10.5Mo1.5Nx and PrFe10.5Mo1.5Nx powders, coercivities as high as 10.4 and 4.5 kOe at room temperature, respectively, were achieved by carefully controlling the nitrogenation process. The coercivity strongly depends on the preparation route, i.e., annealing temperature as well as nitrogenation temperature and time. By the detailed investigation of the influence of these conditions, the optimum process parameters were determined. The coercivity mechanism of melt-spun NdFe10.5Mo1.5Nx powders shows characteristics of a randomly oriented single domain particle system. Poor coercivities have been obtained for Nd(Fe,V)12 and Pr(Fe,V)12. In these cases as well as for not optimally nitrogenated Nd(Fe,Mo)12, the low coercivity of the nitrided powders is related to (i) too large grain sizes, (ii) the presence of α-Fe additional to the ThMn12-type phase, or (iii) a nonhomogeneous nitrogenation of the powder. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.373766
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  • 6
    Digitale Medien
    Digitale Medien
    Weak ferromagnetism in low-dimensional cuprates Ba2Cu3O4Cl2 and Ba3Cu2O4Cl2 (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 6022-6024 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Ba2Cu3O4Cl2 is a two-dimensional quantum antiferromagnet with two Cu sublattices showing a small spontaneous magnetization M0. A group theoretical analysis shows that M0 may be due to (i) induced magnetization in the Cu II sublattice or (ii) spin canting of the Cu I moments, i.e., weak ferromagnetism (WFM). In case (i) the interaction is of pseudodipolar type. In case (ii) the Cu I moments are subject to some kind of anisotropy. The main reason for the occurrence of M0, compared to similar high-Tc cuprates related structures, is that the center between two Cu I moments is not an inversion center. Ba3Cu2O4Cl2 behaves like a classical antiferromagnet with a spin-flop transition. For this compound our analysis shows that, in the case where the magnetic and the crystallographic unit cells are identical, the Cu moments of equivalent sites should be parallel and the antiferromagnetic order is related to opposite directions of the moments at different sites. Weak antiferromagnetism but no WFM is permitted, in agreement with the experimental results. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.372600
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  • 7
    Digitale Medien
    Digitale Medien
    First order magnetization processes in Nd(Fe, M)12Cy compounds (M=V, Ti, Mo) (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 5278-5280 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Magnetization measurements on Nd(Fe, M)12Cy (M=Ti, V, Mo) with ThMn12-type structure have revealed well developed first order magnetization processes (FOMPs) when an external field is applied perpendicular to the c axis. The FOMPs depend on carbon concentration, temperature, and the type of M element. The FOMPs appear at a certain carbon concentration and at temperatures below 150–100 K. The FOMP fields of NdFe12−xVxCy are independent of the V content in the range x=1.25–2.0, which is due to the small change of the anisotropy field within this range. No FOMP has been found in the nitrides of the same parent alloys. The evaluation of the crystal field (CF) parameters shows that the higher-order CF parameters of the Nd ions play a key role in producing the observed type-II FOMP. The temperature dependence of the FOMPs is due to the thermal evolution of the anisotropy constants Ki (i=1,2,3) of the carbides. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.373320
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  • 8
    Digitale Medien
    Digitale Medien
    Magnetization processes in two different types of anisotropic, fully dense NdFeB hydrogenation, disproportionation, desorption, and recombination magnets (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 6119-6121 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Two types of textured, fully dense NdFeB hydrogenation, disproportionation, desorption, and recombination (HDDR) magnets were produced. The first type was produced by hot pressing isotropic HDDR powder followed by die upsetting; the second, by hot pressing prealigned, anisotropic HDDR powder (MQA-T). Studies of the magnetization processes revealed that for isotropic HDDR powder and its hot pressed and die-upset magnets a much larger initial susceptibility is found after thermal demagnetization than after reverse dc-field demagnetization. Prealigned, hot pressed magnets made from MQA-T material showed a different virgin magnetization curve, indicating a unique coercivity mechanism. Interaction domains larger than the average grain size can be observed in both cases by Kerr microscopy, with the MQA-T type showing significantly broader interaction domains. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.372628
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  • 9
    Digitale Medien
    Digitale Medien
    Influence of Fe and Co additions on magnetization and magnetoresistivity of MnAu2 melt-spun alloys (2000)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 87 (2000), S. 5789-5791 
    Details
    ISSN: 1089-7550
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Melt-spun MnAu2 ribbons modified by ternary magnetic additions (Fe and Co) show a large giant magnetoresistance (GMR) effect up to a magnetoresistance ratio MR={[R(H=7T)−R(0)/R(0)]} of the order of −10% at room temperature. Pure MnAu2 samples show a transition from positive magnetoresistance at low temperatures (e.g., MR=+23% at 10 K and μ0H=5 T) to a GMR behavior with values of about −8% at 295 K and 5 T. At 135 K the resistivity is nearly independent of the applied magnetic field. The strong increase of |MR| at room temperature is only found with applied fields larger than a threshold field, where a metamagnetic transition occurs. Alloying the compound with Co or Fe this threshold field can be reduced or suppressed. This effect is due to softening of the antiferromagnetic behavior of the (Mn; Co or Fe)Au2 samples. For an alloy with 5 at. % Fe as substitutes for Mn, no metamagnetic transition is observed in the temperature range between 10 and 300 K. For both 5 at. % Co or Fe additions a GMR behavior is observed in this temperature range with higher sensitivity at low fields, e.g., (Mn0.95Fe0.05)Au2 has values of MR=−12% and −1% in a field of 1 T at 10 and 295 K, respectively. At low temperatures, an anomalous increase of resistance at higher fields (above 1–2 T) is observed for these alloys. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.372523
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  • 10
    Digitale Medien
    Digitale Medien
    Superconducting bulk magnets: Very high trapped fields and cracking (2001)
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 79 (2001), S. 3131-3133 
    Details
    ISSN: 1077-3118
    Quelle: AIP Digital Archive
    Thema: Physik
    Notizen: Improved trapped fields are reported for bulk melt-textured YBa2Cu3O7−δ (YBCO) material in the temperature range between 20 and 50 K. Trapped fields up to 12.2 T were obtained at 22 K on the surface of single YBCO disks (with Ag and Zn additions). In YBCO minimagnets, maximum trapped fields of 16 T (at 24 K) and of 11,2 T (at 47 K) were achieved using (Zn+Ag) and Zn additions, respectively. In all cases, the YBCO disks were encapsulated in steel tubes in order to reinforce the material against the large tensile stress acting during the magnetizing process and to avoid cracking. We observed cracking not only during the magnetizing process, but also as a consequence of flux jumps due to thermomagnetic instabilities in the temperature range betweeen 20 and 30 K. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1413502
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