11 research outputs found
Designed Metamagnetism in CoMnGe_{1-x}P_{x}
We extend our previous theoretical study of Mn-based orthorhombic metamagnets
to those that possess large nearest neighbour Mn-Mn separations (d1>3.22A).
Based on our calculations, we design and synthesize a series of alloys,
CoMnGe_{1-x}P_{x}, to experimentally demonstrate the validity of the model.
Unusually, we predict and prepare several metamagnets from two ferromagnetic
end-members, thus demonstrating a new example of how to vary crystal structure,
within the Pnma symmetry group, to provide highly tunable metamagnetism
Research Update: The Mechanocaloric Potential of Spin Crossover Compounds
We present a first evaluation of the potential for spin crossover (SCO) compounds to be considered as a new class of giant mechanocaloric effect materials. From literature data on the variation of the spin crossover temperature with pressure, we estimate the maximum available adiabatic temperature change for several compounds and the relatively low pressures that may be required to observe these effects
History dependence of directly observed magnetocaloric effects in (Mn, Fe)As
We use a calorimetric technique operating in sweeping magnetic field to study
the thermomagnetic history- dependence of the magnetocaloric effect (MCE) in
Mn0.985Fe0.015As. We study the magnetization history for which a "colossal" MCE
has been reported when inferred indirectly via a Maxwell relation. We observe
no colossal effect in the direct calorimetric measurement. We further examine
the impact of mixed-phase state on the MCE and show that the first order
contribution scales linearly with the phase fraction. This validates various
phase-fraction based methods developed to remove the colossal peak anomaly from
Maxwell-based estimates.Comment: 4 pages, 2 figure
Magnetocaloric effect at the reorientation of the magnetization in ferromagnetic multilayers with perpendicular anisotropy
We investigate the magnetocaloric effect obtained by the rotation of a
magnetic field applied to an exchange-coupled multilayer system composed of two
different ferromagnetic (FM) materials. We specifically consider a system in
which the two FMs have perpendicular uniaxial anisotropy axes and utilise
conditions which yield a reorientation of the total magnetization when
compensation between the anisotropies of the two layers occurs. We calculate
the consequent entropy change associated with the "artificial" reorientation.
By using known parameters from MnBi and Co we predict an entropy change of
JkgK for perfect coupling. Lastly, we study the
behavior of the multilayer under a rotating magnetic field via a micromagnetic
model. When the layer thicknesses are of the order of the local domain wall
width, the magnetic field-induced entropy change can be obtained with magnetic
fields one order of magnitude lower than in the uncoupled case.Comment: 7 pages, 7 figure
Fabrication of magnetocaloric La(Fe,Si)13 thick films
La(Fe,Si)13–based compounds are considered to be very promising magnetocaloric materials for magnetic refrigeration applications. Many studies have focused on this material family but only in bulk form. In this paper we report on the fabrication of thick films of La(Fe,Si)13, both with and without post-hydriding. These films exhibit magnetic and structural properties comparable to bulk materials. We also observe that the ferromagnetic phase transition has a negative thermal hysteresis, a phenomenon not previously found in this material but which may have its origins in the availability of a strain energy reservoir, as in the cases of other materials in which negative thermal hysteresis has been found. Here, it appears that the substrate acts to store strain energy. Our exploratory study demonstrates the viability of thick films of the La(Fe,Si)13 phase and motivates further work in the area while showing that additional perspectives can be gained from reducing the dimensionality of magnetocaloric materials in which the magneto-volume effect is large
Effect of direct-current magnetic field on the specific absorption rate of metamagnetic CoMnSi: A potential approach to switchable hyperthermia therapy
Materials with 1st order antiferromagnetic (AFM) to high-magnetization (MM) phase transition known for their inverse magnetocaloric effect, abrupt rise in magnetization and magnetoelastic coupling, are promising for application in combined simultaneous diagnosis and targeted cancer therapy. A therapy that combines alternating-current (ac) and direct-current (dc) magnetic fields for simultaneous magnetic hyperthermia therapy (MHT) and magnetic resonance imaging (MRI), using same magnetic particles for heating and as con- trast agents. We report a proof-of-concept study on the induction heating ability of 1st order metamagnetic material with moderate specific absorption rates (SAR) and no tendency for agglomeration, for potential MHT and MRI cancer therapy. CoMnSi, a metam- agnetic antiferromagnet (MM) was used in this study because of its desirable ability to rapidly switch from a low to high magnetiza- tion state in an applied dc bias field condition without particle agglomeration on field removal. The results showed that the magne- tization switched from \u3c 20 Am2kg-1 at 0.75 T to about 53.31 Am2kg-1 at 1.0 T applied dc field, a field large enough for magnetic resonance imaging. An SAR value of 10.7 Wg-1 was obtained under an ac field of 31.0 kAm-1 at 212.0 kHz. When combined with a dc bias field of 1.0 T, SAR values of 9.83 Wg-1 and 6.65 Wg-1 were obtained in the directions 45â—‹ and 90â—‹ away from the dc bias field direction respectively. These SAR values obtained from CoMnSi particles in the presence of simultaneous ac and dc magnetic field bias are in comparison, at least 25 times greater than those obtained from 2nd order magnetic phase transition Fe3O4 suspension. It is observed that Fe3O4 particles showed large suppression of SAR, and agglomeration under the same experimental conditions. This study shows the great potential of 1st order phase transition metamagnets for simultaneous MHT and MRI cancer therapy using MRI equipment
The dynamics of spontaneous hydrogen segregation in LaFe13−xSixHy
By means of time-and temperature-dependent magnetization measurements, we demonstrate that the timescale of hydrogen diffusion in partially hydrogenated LaFe13-xSixHy is of the order of hours, when the material is held at temperatures close to its as-prepared Curie temperature, T-C0. The diffusion constant is estimated to be D approximate to 10(-15)-10(-16) m(2) s(-1) at room temperature. We examine the evolution of a magnetically phase-separated state upon annealing for 3 days at a range of temperatures around T-C0 and show that the thermodynamic driving force behind hydrogen diffusion and phase segregation may be attributed to the lower free energy of hydrogen interstitials in the ferromagnetic state relative to the paramagnetic state