Christian Robert Haffenden Bahl
Technical University of Denmark
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Publication
Featured researches published by Christian Robert Haffenden Bahl.
Journal of Physics: Condensed Matter | 2007
Steen Mørup; Daniel Esmarch Madsen; Cathrine Frandsen; Christian Robert Haffenden Bahl; Mikkel Fougt Hansen
The magnetic properties of nanoparticles of antiferromagnetic materials are reviewed. The magnetic structure is often similar to the bulk structure, but there are several examples of size-dependent magnetic structures. Owing to the small magnetic moments of antiferromagnetic nanoparticles, the commonly used analysis of magnetization curves above the superparamagnetic blocking temperature may give erroneous results, because the distribution in magnetic moments and the magnetic anisotropy are not taken into account. We discuss how the magnetic dynamics can be studied by use of magnetization measurements, Mossbauer spectroscopy and neutron scattering. Below the blocking temperature, the magnetic dynamics in nanoparticles is dominated by thermal excitations of the uniform mode. In antiferromagnetic nanoparticles, the frequency of this mode is much higher than in ferromagnetic and ferrimagnetic nanoparticles, but it depends crucially on the size of the uncompensated moment. Excitation of the uniform mode results in a so-called thermoinduced moment, because the two sublattices are not strictly antiparallel when this mode is excited. The magnetic dipole interaction between antiferromagnetic nanoparticles is usually negligible, and therefore such particles present a unique possibility to study exchange interactions between magnetic particles. The interactions can have a significant influence on both the magnetic dynamics and the magnetic structure. Nanoparticles can be attached with a common crystallographic orientation such that both the crystallographic and the magnetic order continue across the interfaces.
International Journal of Refrigeration-revue Internationale Du Froid | 2010
Rasmus Bjørk; Christian Robert Haffenden Bahl; Anders Smith; Nini Pryds
One of the key issues in magnetic refrigeration is generating the magnetic field that the magnetocaloric material must be subjected to. The magnet constitutes a major part of the expense of a complete magnetic refrigeration system and a large effort should therefore be invested in improving the magnet design. A detailed analysis of the efficiency of different published permanent magnet designs used in magnetic refrigeration applications is presented in this paper. Each design is analyzed based on the generated magnetic flux density, the volume of the region where this flux is generated and the amount of magnet material used. This is done by characterizing each design by a figure of merit magnet design efficiency parameter, Λcool. The designs are then compared and the best design found. Finally recommendations for designing the ideal magnet design are presented based on the analysis of the reviewed designs.
Journal of Magnetism and Magnetic Materials | 2010
Rasmus Bjørk; Christian Robert Haffenden Bahl; M. Katter
The magnetocaloric properties of three samples of LaFe
Applied Physics Letters | 2012
Christian Robert Haffenden Bahl; David Velázquez; Kaspar Kirstein Nielsen; Kurt Engelbrecht; Kjeld Bøhm Andersen; Regina Bulatova; Nini Pryds
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Journal of Applied Physics | 2008
Rasmus Bjørk; Christian Robert Haffenden Bahl; Anders Smith; Nini Pryds
Co
Review of Scientific Instruments | 2008
Christian Robert Haffenden Bahl; Thomas Frank Petersen; Nini Pryds; Anders Smith
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Journal of Applied Physics | 2009
Christian Robert Haffenden Bahl; Kaspar Kirstein Nielsen
Si
Journal of Applied Physics | 2010
Kurt Engelbrecht; Christian Robert Haffenden Bahl
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Journal of Applied Physics | 2010
Anders Smith; Kaspar Kirstein Nielsen; Dennis Valbjørn Christensen; Christian Robert Haffenden Bahl; Rasmus Bjørk; Jesper Henri Hattel
have been measured and compared to measurements of commercial grade Gd. The samples have (x=0.86, y=1.08), (x=0.94, y=1.01) and (x=0.97, y=1.07) yielding Curie temperatures in the range 276-288 K. The magnetization, specific heat capacity and adiabatic temperature change have been measured over a broad temperature interval. Importantly, all measurements were corrected for demagnetization, allowing the data to be directly compared. In an internal field of 1 T the maximum specific entropy changes were 6.2, 5.1 and 5.0 J/kg K, the specific heat capacities were 910, 840 and 835 J/kg K and the adiabatic temperature changes were 2.3, 2.1 and 2.1 K for the three LaFeCoSi samples respectively. For Gd in an internal field of 1 T the maximum specific entropy change was 3.1 J/kg K, the specific heat capacity was 340 J/kg K and the adiabatic temperature change was 3.3 K. The adiabatic temperature change was also calculated from the measured values of the specific heat capacity and specific magnetization and compared to the directly measured values. In general an excellent agreement was seen.
Journal of Magnetism and Magnetic Materials | 2010
Rasmus Bjørk; Christian Robert Haffenden Bahl; Anders Smith; Nini Pryds
We have applied mixed valance manganite perovskites as magnetocaloric materials in a magnetic refrigeration device. Relying on exact control of the composition and a technique to process the materials into single adjoined pieces, we have observed temperature spans above 9 K with two materials. Reasonable correspondence is found between experiments and a 2D numerical model, using the measured magnetocaloric properties of the two materials as input.