Effect In MnAs Research Articles

Anomalous Hall effect in MnAs: Intrinsic contribution due to Berry curvature

We present an experimental and theoretical study of the anomalous Hall effect (AHE) in MnAs epilayers grown over GaAs, with the aim to identify the intrinsic contribution to the AHE, which can be accurately evaluated using ab initio electronic structure calculations. Our magnetotransport measurements show a quadratic behavior of the Hall resistivity with longitudinal resistivity, characteristic of scattering-independent processes, thus enabling the comparison with our ab initio calculations. The calculated Berry phase contribution to the AHE is in quantitative agreement with the measured AHE in these epilayers. Moreover, the predicted anisotropic dependence of the experimental AHE on the magnetization is well reproduced.

Phase transitions and magnetocaloric effect in MnAs, MnAs0.99P0.01, and MnAs0.98P0.02 single crystals

We have examined the influence of external magnetic fields up to H = 14 T on the phase transitions of MnAs, MnAs0.99P0.01, and MnAs0.98P0.02 single crystals. The results indicate that there is structural instability of the crystal lattice, associated with anomalous compression at temperatures from \(T_{f_1 }\) ≃ 250 K to the phase transition temperature T u and anomalous lattice expansion in the basal plane between T u and \(T_{f_2 }\) ≃ 350 K. Near the first-order phase transition, the magnetic entropy change at a magnetic field change from 0 to 14 T is about 40 J/(kg K) in single-crystal MnAs and approaches 50 J/(kg K) in single-crystal MnAs0.98P0.02.

Ultrasonic triggering of giant magnetocaloric effect in MnAs thin films

Mechanical control of magnetic properties in magnetostrictive thin films offers the unexplored opportunity to employ surface wave acoustics in such a way that acoustic triggers dynamic magnetic effects. The strain-induced modulation of the magnetic anisotropy can play the role of a high frequency varying effective magnetic field leading to ultrasonic tuning of electronic and magnetic properties of nanostructured materials, eventually integrated in semiconductor technology. Here, we report about the opportunity to employ surface acoustic waves to trigger magnetocaloric effect in MnAs(100nm)/GaAs(001) thin films. During the MnAs magnetostructural phase transition, in an interval range around room temperature (0{\deg}C - 60{\deg}C), ultrasonic waves (170 MHz) are strongly attenuated by the phase coexistence (up to 150 dB/cm). We show that the giant magnetocaloric effect of MnAs is responsible of the observed phenomenon. By a simple anelastic model we describe the temperature and the external magnetic field dependence of such a huge ultrasound attenuation. Strain-manipulation of the magnetocaloric effect could be a further interesting route for dynamic and static caloritronics and spintronics applications in semiconductor technology.

Theoretical investigation on the magnetocaloric effect in MnAs using a microscopic model to describe the magnetic and thermal hysteresis

We report the thermal and magnetic hysteresis diagram for MnAs that comes from a microscopic description of a magnetic system through a model Hamiltonian that takes into account the magnetoelastic interaction. The temperature and magnetic hysteresis intervals are governed by the magnetoelastic interaction parameter, which leads to the energy barrier between stable and metastable minima in the exact free energy, obtained from our microscopic model. Application of the model to the MnAs first-order magnetic material, which presents high hysteresis effect, leads to a good agreement with the experimental magnetic and magnetocaloric data.

Entropy determinations and magnetocaloric parameters in systems with first-order transitions: Study of MnAs

We present a study of the giant magnetocaloric effect in MnAs produced by a magnetostructural first-order phase transition. Results deduced from magnetization, M, and heat capacity, Cp,B(T), are compared and discussed. Some spurious effects are explained, and especially a spike in the isothermal entropy change, ΔST, occurring at TC when obtained via the Maxwell relation (∂S/∂B)T=(∂M/∂T)B. Alternative determination methods are given to circumvent this problem. The spike is explained as an artifact due to the incorrect application of the Maxwell relation to path dependent thermodynamic functions that are not state functions. The added wrong contribution to ΔST has been calculated using calorimetric data, giving a good agreement with the result from the magnetization measurements.

Adiabatic measurement of the giant magnetocaloric effect in MnAs

We have studied the compound MnAs with a giant magnetocaloric effect, which has a magneto-structural transition at 316 K. The magnetic phase diagram has been deduced from adiabatic heat-capacity measurements and from cooling and heating curves. The ferroto paramagnetic transition changes with field from 316 K at 0 T to 335.3 K at 6 T. There is a strong thermal hysteresis between 10 and 5 K, depending on field. Direct measurements of the adiabatic temperature change caused by the application of magnetic field were made and compared with the values deduced from heat-capacity data.Moreover, adiabatic field cycles were performed quasi-statically between 0 and 6 T around the phase coexistence region, showing the strength of the effect in each phase and on the coexistence lines.

The magnetic and magnetocaloric properties of Gd5Ge2Si2 compound under hydrostatic pressure

The Gd5Ge2Si2 compound presents a giant magnetocaloric effect with transition temperature at around 276 K and is a very good candidate for application as an active regenerator material in room temperature magnetic refrigerators. Recently it has been shown that pressure induces a colossal magnetocaloric effect in MnAs, a material that presents a giant magnetocaloric effect and a strong magnetoelastic coupling, as also happens with the Gd5Ge2Si2 compound. This motivated a search of the colossal effect in the Gd5Ge2Si2 compound. This work reports our measurements on the magnetic properties and the magnetocaloric effect of Gd5Ge2Si2 under hydrostatic pressures up to 9.2 kbar and as a function of temperature. Contrary to what happens with MnAs, pressure increases the Curie temperature of the compound, does not affect the saturation magnetization and decreases markedly its magnetocaloric effect.

Pressure-Induced Colossal Magnetocaloric Effect in MnAs

To present day, the maximum magnetocaloric effect (MCE) at room temperature for a magnetic field change of 5 T is 40 J/(kg K) for MnAs. In this Letter we present colossal MCE measurements on MnAs under pressure, reaching values up to 267 J/(kg K), far greater than the magnetic limit arising from the assumption of magnetic field independence of the lattice and electronic entropy contributions. The origin of the effect is the contribution to the entropy variation coming from the lattice through the magnetoelastic coupling.

Theoretical investigations on giant magnetocaloric effect in MnAs 1− xSb x

In this work we apply a model to describe the magnetocaloric effect for the MnAs 1− x Sb x series of compounds, 0⩽ x⩽0.4. The behavior of the material under first order phase transitions is well described, and we are able to obtain the magnetocaloric potential for the series of compounds presenting first order magnetic phase transitions. Based on these results we predict the performance of a composite comprising a combination of compositions of this compound to work as active element in a magnetic refrigerator using an Erickson cycle spanning a great temperature range down from room temperature.

Enchanced magnetooptical response of magnetic nanoclusters embedded in semiconductor

In the present work, we report the results of an experimental study of magnetooptical Kerr effect and optical properties in MnAs:GaAs granular films in the spectral range 0.5–4.5 eV at room temperature and at 4.2 K. The possible mechanisms of the giant magnetooptical effect in MnAs:GaAs granular films are discussed on the basis of the obtained results.

Anisotropy of electrical conductivity and hall effect in MnAs 0.95P 0.05

The resistivity tensor ϱ and the anisotropic Hall resistance ϱ H are measured for 70 K ⩽ T ⩽ 320K. ϱ is assigned to the anisotropy of the Fermi surface while ϱ H is related to the magnetization ellipsoid for T < T N = 228 K (anomalous Hall effect).