In this paper we present the results of an experimental investigation of the magnetocaloric properties of hydrogenated La(Fe-Mn-Si) 13 -H with Mn substituting Fe to finely tune the transition temperature. We measured the specific heat under magnetic field c p (H, T ) and the magnetic field induced isothermal entropy change ∆s(H, T ) of a series of compounds by direct Peltier calorimetry. Results show that increasing Mn from 0.06 to 0.46 reduces the transition temperature from 339 K to 270 K whilst the total entropy change due to a 1.5 T field is depressed from 18.7 Jkg −1 K −1 to 10.2 Jkg −1 K −1 and the thermal hysteresis similarly is reduced from 1.5 K to zero. In the paper we interpret the results in terms of a magnetic phase transition changing from the first to the second order with increasing Mn content and we discuss the value of the results for magnetic cooling applications.

Microcalorimetry has proven to be a versatile tool to investigate first order magnetic phase transitions as it can be used in different experimental modes to separate the latent heat from heat capacity. However, the methodology fails if the latent heat contribution is below instrumental resolution of 10 nJ. If the nucleation size of the new phase is much less than 100 μm, the typical size of the fragment measured, the latent heat could appear to be too distributed in temperature or magnetic field to be detected. Here, we show that for certain classes of magnetic transition, our microcalorimetry technique can be extended to enable an estimate of the latent heat to be obtained from a combination of heat capacity and magnetic measurements. This technique is best suited for material systems with weakly first order phase transitions, or highly distributed due to inhomogeneity.

Definitive determination of first order character of the magnetocaloric magnetic transition remains elusive. Here we use a microcalorimetry technique in two modes of operation to determine the contributions to entropy change from latent heat and heat capacity separately in an engineered set of La(Fe, Mn, Si)13 samples. We compare the properties extracted by this method with those determined using magnetometry and propose a model independent parameter that would allow the degree of first order character to be defined across different families of materials. The microcalorimetry method is sufficiently sensitive to allow observation of an additional peak feature in the low field heat capacity associated with the presence of Mn in these samples. The feature is of magnetic origin but is insensitive to magnetic field, explicable in terms of inhomogeneous occupancy of Mn within the lattice resulting in antiferromagnetic ordered Mn clusters.

We use a calorimetric technique operating in sweeping magnetic field to study the thermomagnetic historydependence of the magnetocaloric effect (MCE) in Mn 0.985 Fe 0.015 As. 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.The first order nature of the Curie transition in MnAs has been of interest for many years and was the motivation behind Bean and Rodbell's well-known model of first order magnetic phase transitions 1 . Recently MnAs has been in the spotlight again after the report of a "colossal" magnetocaloric effect (MCE)-an order of magnitude larger than usual 2 . It, along with anomalous peaks in the MCE of other systems, was inferred indirectly from magnetization measurements via a Maxwell relation and disputed because of a comparison with calorimetric measurements showing no colossal effect 3,4 . The calorimetric measurements, however, follow a significantly different thermomagnetic history. Here we use a calorimetric technique operating in a field-sweeping mode to reproduce several thermomagnetic histories and thus investigate the possibility of observing an MCE that depends on the specific magnetization history.The MCE can be quantified as an adiabatic temperature change, ∆T ad , or an isothermal entropy change, ∆S, for a certain magnetic field variation. The latter can be estimated via a Maxwell relation:where M is the magnetization and B is the magnetic flux density, which we later assume to be equal to µ 0 H. The use of this relation requires a knowledge of magnetization as a function of B and T and it is usually obtained from isothermal magnetization measurements taken in discrete temperature steps. It is important to note that the Maxwell relation is based on equilibrium thermodynamics, i.e. assuming that magnetization and entropy are single-valued functions of state. This is true in materials with a continuous phase transition, however, first order phase transitions usually display finite hysteresis due to the presence of multiple local minima in the free a) m.bratko09@imperial.ac.uk energy separated by an energy barrier. This enables the existance of metastable, non-equilibrium states. Nevertheless, the Maxwell relation is often used, assuming that the effect of a small and finite region of irreversibility is negligible.A "colossal" MCE was first reported in MnAs under hydrostatic pressure 2 followed by its observation in Mn 1−x Fe x As 5 and Mn 1−x Cu x As 6 at ambient pressure. The effect was later reported even in MnAs at ambient pressure 7 , in disagreement with previous reports 2,8 . The authors in Ref. 7 claimed the capture of a colossal effect was possible due to smaller t...

It is now well established that the paramagnetic to ferromagnetic transition in the magnetocaloric La(FeSi)13 is a cooperative effect involving spin, charge and lattice degrees of freedom. However, the influence of this correlated behaviour on the ferromagnetic state is as yet little studied. Here we measure the specific heat at low temperatures in a systematic set of LaFexMnySiz samples with and without hydrogen, to extract the Sommerfeld coefficient, the Debye temperature and the spin wave stiffness. Substantial and systematic changes in magnitude of the Sommerfeld coefficient are observed with Mn substitution and introduction of hydrogen, showing that over and above the changes to the density of states at the Fermi energy there are significant enhanced d band electronic interactions, at play. The Sommerfeld coefficient is found to be 90-210 mJmol -1 K -2 unusually high compared to that expected from band structure calculations. The Debye temperature determined from the specific heat measurement is insensitive to Mn and Si doping, but increases when hydrogen is introduced into the system. The Sommerfeld coefficient is reduced in magnetic field for all compositions that have a measurable spin wave contribution. These results move our understanding of the cooperative effects forward in this important and interesting class of materials significantly, and provides a basis for future theoretical development.