Phase Transition In MnAs Research Articles

Mechanism of the phase transitions in MnAs

Based on first-principles calculations, the structural, magnetic, dynamic properties, and the $\ensuremath{\alpha}$-hexagonal $\ensuremath{\rightarrow}$ $\ensuremath{\beta}$-orthorhombic $\ensuremath{\rightarrow}$ $\ensuremath{\gamma}$-hexagonal phase transitions of ferromagnetic, antiferromagnetic, and paramagnetic MnAs are studied. The phonon-dispersion curves for some of the phases were derived using the direct method. Soft modes were found in ferromagnetic and antiferromagnetic structures in a wide range of pressures. A strong dependence of the soft-mode energy on the magnetic moment and magnetic order was revealed. The double-well potential was found as a function of the soft-mode amplitude at the reduced crystal volume. In the $\ensuremath{\beta}$-orthorhombic phase, a new antiferromagnetic configuration consisting of linear chains of alternating spins was found. Therefore, the mechanism of the magnetostructural phase transitions confirms the antiferromagnetic or paramagnetic (with antiferromagnetic fluctuations) state of the $\ensuremath{\beta}$-orthorhombic phase. The paramagnetic $\ensuremath{\gamma}$-hexagonal phase is stabilized at high temperature in the displacive second-order phase transition owing to the disappearance of the soft mode.

Phonon Mechanism of the Magnetostructural Phase Transition in MnAs

Density functional theory was used to study structural and dynamical changes related to the magnetostructural phase transition in MnAs. The soft mode inducing the transition from the high-symmetry hexagonal to the low-symmetry orthorhombic phase was revealed. A giant coupling between the soft mode and magnetic moments was found and its crucial role in the magnetostructural transition was established. The estimated phonon contribution to the total entropy change has the opposite sign to the magnetic entropy change.

Fabrication of MnAs microstructures on [formula omitted] substrates and their electrical properties

We explore wet and dry etching processes of thin MnAs layers grown on GaAs(0 0 1) substrates for microstructuring. Most of the common wet chemical etch solutions for GaAs react with MnAs strongly and in a peculiar manner. Unidirectional cracks are generated when the MnAs layers are thicker than 100 nm. We demonstrate that the crack generation can be avoided by choosing a suitable etch solution or etch temperature. We fabricate submicrometer-wide MnAs wires using Ar ion milling. The resistivity of the narrow channels is measured over a temperature range covering the phase transitions in MnAs between the α , β , and γ phases. The resistivity along the MnAs[0 0 0 1] direction is found to be smaller than that along the MnAs [ 1 1 2 ¯ 0 ] direction regardless of the phase. A nearly linear temperature variation of the phase fraction is deduced in the α – β phase coexistence regime. The temperature coefficients of the resistivities are negative for the nonmagnetic phases.

Structural and magnetic study of the MnAs magnetocaloric compound

The temperature induced phase transition in MnAs is analyzed in this work using X ray Rietveld refinement. The results show the presence of the hexagonal phase (P6(3)/mmc) at room temperature and the first-order structural-magnetic transition to the orthorhombic phase (Pnma) around 318 K was followed in detail. The MnAs magnetic characterization allowed to obtain the transition temperature and a maximum value of 47 J/(kg.K) for the measured magnetocaloric effect for a magnetic field variation of 5 T.

X-ray diffraction study of the structural phase transition in MnAs under high magnetic fields

The structural phase transition induced by magnetic fields on MnAs was observed by the X-ray diffraction measurements in high magnetic fields up to 4 T . Magnetization measurements showed that Curie temperature T C was 315.5 K for increasing temperatures and a metamagnetic transition from the paramagnetic to the ferromagnetic state occurred above T C. The structural transformation from the hexagonal NiAs-type to the orthorhombic MnP-type structure was confirmed at T C with increasing temperature from 290 to 319 K via two-phase coexistence region. The X-ray diffraction profiles at 319 K showed the single phase of the MnP-type structure in zero field and applying magnetic field of 3 T caused appearance of the Brag peak of the hexagonal structure. On further increase of magnetic fields, the single phase of the hexagonal structure was observed above 3.5 T in the forced ferromagnetic state. Both the magnetic and structural transitions induced by magnetic fields above T C were first order with a hysteresis and had a close relationship between each other.

Structural and magnetic phase transition in MnAs(0001)/GaAs(111) epitaxial films

The ferromagnetic phase transition in a MnAs film on GaAs(111), where the MnAs unit cell is epitaxially fixed in its hexagonal plane, proceeds under conditions qualitatively different from the transition in bulk MnAs crystals or in MnAs films on GaAs(001). We present experimental evidence for the coexistence between ferromagnetic and paramagnetic phases in a temperature interval of 10°C. Temperature dependencies of the phase fractions and the in-plane lattice parameters obtained by grazing incidence x-ray diffraction are compared with magnetization measurements.

Magnetic and electrical characteristics of MnAs films grown by metalorganic chemical vapour deposition

The magnetic and electrical property of changes associated with the phase transition in MnAs have been measured in uncracked material by using thin films grown on to sapphire substrates by metal organic chemical vapour deposition. The films exhibited a first-order transition with Curie points in the range 35–47±1°C. An increase of approximately 60% in the electrical resistivity of the films was measured on heating through the phase transition. The Curie temperature of the films increased with an applied magnetic field by approximately 1.2°C in a field of 600 kA m −1. This shift in Curie temperature with applied magnetic field also produced a magnetoresistance effect with a maximum value of 2.24% at 600 kA m −1. The values of these parameters are shown to be dependent upon the growth conditions of the films.

Effect of external pressure and chemical substitution on the phase transitions in MnAs

The evolution of the phase transitions in Mn 1− t T t As ( T = Ti, V, Cr, Fe, Co and Ni) with t at low substitution levels is correlated with the external pressure, temperature ( P, T) phase diagram of MnAs using the chemical pressure ( P ∗) concept. The study is based on data for transition temperatures and unit cell dimensions at atmospheric pressure and P, T phase diagrams of Mn 1− t T t ,As with different T and t. Chemical pressure coefficients (ranging between ϵ = −52 ± 2 kbar for T = Ti and ϵ = 133 ± 12 kbar for T = Fe, assuming P ∗ = ϵt) are estimated from the location of the boundary of the essentially pressure driven NiAs, F to MnP, P or MnP, H a type transition in the individual P, T phase diagrams. Different features of the chemical pressure concept are examined, such as the relation P ∗ = ϵt and its break-down above a certain substitution level, the correlation between unit cell volumes and compressibility, and the non-hydrostatic character of P ∗ for these non-cubic phases.

Peculiarities of the first-order phase transition in MnAs

Peculiarities of the first-order phase transition in MnAs

Hall effect anomalies at the magnetic phase transitions in MnAs1-xPxcompounds

The specific Hall resistivity rho H of MnAs1-xPx crystals (x=0.1, 0.123) has been measured at stationary conditions for temperatures between 50 and 250K using the double-AC method with Bpeak<or=0.4 T. The combination of experimentally determined magnetisation values with the isothermal curves rho H(B) allowed for the separation of the normal and anomalous Hall constants R0 and RS, which show different anomalies at the phase transition temperatures TN, Tl and Tk. The dependences on R0(T) are interpreted in terms of a two-band model leading to the concept of charge-carrier localisation at Tk i.e. at the low-temperature magnetic phase transition beta to or from gamma .

Quenching of the phase transitions in MnAs and related compounds

The first- and second-order phase transition in MnAs and related compounds can be suppressed by rapid roller quenching. The magnetic behaviour of the resulting new metastable phases indicates a mixed double exchange-superexchange magnetic coupling for these compounds.

A model of magneto-structural phase transition in MnAs

Two magneto-structural phase transitions of MnAs between three phases; ferromagnetic and undistorted phase, paramagnetic and distorted phase, paramagnetic and undistorted phase; are consistently explained by a simple model Hamiltonian without an assumption of high- and low-spin states for Mn atoms. An operator is introduced to describe the distortion of the crystal, and an exchange interaction of each Mn atom pair is expressed in terms of the operator. Thus a coupled Hamiltonian for the crystal distortion and the magnetic ordering is obtained in a molecular field approximation. It is shown from the calculated result that the three phases are realised and an anomalous temperature dependence of a paramagnetic susceptibility is explained. In the presence of an external magnetic field, this model yields a field-induced transition, which is qualitatively in agreement with experimental results. Significant features of temperature dependences of the specific heat and the lattice constant are also explained by the present calculation.

Magnetic susceptibility anomaly connected with a displacive phase transition in MnAs 1− xP x and Cr xMn 1− xas compounds

With the second order B31 [rlhar2] B8 1 phase transition T t in MnAs 1- x P x and Cr x Mn 1- x As compounds a high-spi low-spin transition occurs simultaneously with a drastic change in the magnetic coupling. An interpolation scheme is proposed which yields C and T c for each temperature below T t separately.

Elastic anomalies at the second-order phase transition in MnAs

The elastic modulus combinations s11, 2s13 + s44, and s44 — 2s12 of MnAs are measured for temperatures between 290 and 420 K. At Tt = 400 K divergent behaviour of s11 and s13 is found, indicating that a soft phonon mode exists. Structure in the sij(T) near 350 K, in particular for s44(T), is assigned to the high-spin-low-spin transition at Thl. A relation between the first order ferromagnetic-paramagnetic phase transition at Tu = 313 K and the displacive phase transition at Tt is suggested as the energy and entropy of the soft lattice mode competes with those of the spin system. Die elastischen Moduln s11, 2s13 + s44 und s44 — 2s12 von MnAs werden im Temperaturbereich von 290 bis 420 K gemessen. Das divergente Verhalten der Moduln bei der Phasenumwandlung 2. Art (Tt = 400 K) weist auf eine weiche optische Phononenmode hin. Weitere Struktur in den sij(T) nahe 350 K wird der high-spin-low-spin-Umwandlung zugeordnet. Ein Zusammenhang zwischen der Phasenumwandlung 1. Art bei Tu und der Phasenumwandlung 2. Art bei Tt aufgrund des Wechselspiels zwischen Energie und Entropie der weichen Mode und des Spinsystems wird vorgeschlagen.

Phase Transitions in MnAs.

Phase Transitions in MnAs.

NMR Study of Pressure-Induced Phase Transition in MnAs

The Mn 55 NMR in MnAs has been observed as a function of pressure up to 7 kbar at 77 K. The NMR is observed at 235 MHz in the hexagonal phase below about 4 kbar, while it is observed between 140 and 175 MHz in the orthorhombic phase above about 4 kbar. In the orthorhombic phase the NMR result is interpreted on the basis of a double spiral spin structure, and the magnetic moment estimated from the hyperfine field suggests a low-spin configulation of Mn atom.

High-Pressure Study of the First-Order Phase Transition in MnAs

The first-order magnetic-transition temperature in MnAs has been measured as a function of both increasing and decreasing pressure. A critical pressure ${P}_{c}\ensuremath{\sim}4.6$ kbar has been obtained for the range of stability of the hexagonal phase. The pressure hysteresis increases with decreasing temperatures. Cooling to 77\ifmmode^\circ\else\textdegree\fi{}K under 5 kbar and then releasing pressure gives the $B31$ phase at atmospheric pressure, and it remains stable on heating to 138\ifmmode^\circ\else\textdegree\fi{}K, where there is an increase of magnetization of over a factor of 50. These data, together with earlier magnetic measurements on the system $\mathrm{Mn}{\mathrm{As}}_{1\ensuremath{-}x}{\mathrm{P}}_{x}$, demonstrate that the origin of the first-order phase transition is a large exchange striction in the basal planes plus a volume-dependent Weiss molecular field and manganese moment. This volume dependence is due to electron rearrangements associated with a high-spin-to-low-spin transition. The sharpness of the high-spin-to-low-spin transition cannot be accounted for by variations in crystal-field splitting with volume. It is suggested that a bandwidth is increasing with decreasing volume through the maximum bandwidth for spontaneous band ferromagnetism. The critical band appears to consist of ${t}_{\ifmmode\pm\else\textpm\fi{}}$ orbitals, which are primarily influenced by the Mn-Mn interactions within basal planes.