37 research outputs found
Piezomagnetic effect as a counterpart of negative thermal expansion in magnetically frustrated Mn-based antiperovskite nitrides
The interplay of magnetic and elastic properties due to geometrical frustration in antiferromagnetic Mn-aniperovskite nitrides manifests itself in a range of phenomena such as the barocaloric (BCE), piezomagnetic (PME), magnetovolume effect (MVE), and the related negative thermal expansion (NTE). This systematic computational study uses density functional theory across a wide range of cubic antiperovskites MnAN (A = Rh, Pd, Ag, Co, Ni, Zn, Ga, In, Sn) in order to account for variations in the magnetic frustration based on features of the electronic structure. It focuses on PME - the linear dependence of magnetisation on applied biaxial strain. The PME in MnSnN predicted here is an order of magnitude larger than PME modelled so far in MnGaN,cite{lukashev2008theory} which opens the way to composite magnetoelectric effect in piezomagnetic/piezoelectric heterostructures. Moreover, the simulated PME as a zero temperature property is shown to be inversely proportional to the measured spontaneous volume expansion at a phase transition from paramagnetic (PM) to antiferromagnetic (AFM) state.cite{takenaka2014magnetovolume} On the fundamental level, such relation implies a significant suppression of spin fluctuations by the strong frustration in these systems. At the same time it can be used as a tool in search for materials with large negative thermal expansion and barocaloric effect
Magnetic structures of Mn3-xFexSn2: an experimental and theoretical study
We investigate the magnetic structure of Mn3-xFexSn2 using neutron powder
diffraction experiments and electronic structure calculations. These alloys
crystallize in the orthorhombic Ni3Sn2 type of structure (Pnma) and comprise
two inequivalent sites for the transition metal atoms (4c and 8d) and two Sn
sites (4c and 4c). The neutron data show that the substituting Fe atoms
predominantly occupy the 4c transition metal site and carry a lower magnetic
moment than Mn atoms. Four kinds of magnetic structures are encountered as a
function of temperature and composition: two simple ferromagnetic structures
(with the magnetic moments pointing along the b or c axis) and two canted
ferromagnetic arrangements (with the ferromagnetic component pointing along the
b or c axis). Electronic structure calculations results agree well with the
low-temperature experimental magnetic moments and canting angles throughout the
series. Comparisons between collinear and non-collinear computations show that
the canted state is stabilized by a band mechanism through the opening of a
hybridization gap. Synchrotron powder diffraction experiments on Mn3Sn2 reveal
a weak monoclinic distortion at low temperature (90.08 deg at 175 K). This
lowering of symmetry could explain the stabilization of the c-axis canted
ferromagnetic structure, which mixes two orthorhombic magnetic space groups, a
circumstance that would otherwise require unusually large high-order terms in
the spin Hamiltonian.Comment: 11 pages, 13 figure
Tuning the metamagnetism of an antiferromagnetic metal
We describe a `disordered local moment' (DLM) first-principles electronic structure theory which demonstrates that tricritical metamagnetism can arise in an antiferromagnetic metal due to the dependence of local moment interactions on the magnetisation state. Itinerant electrons can therefore play a defining role in metamagnetism in the absence of large magnetic anisotropy. Our model is used to accurately predict the temperature dependence of the metamagnetic critical fields in CoMnSi-based alloys, explaining the sensitivity of metamagnetism to Mn-Mn separations and compositional variations found previously. We thus provide a
finite-temperature framework for modelling and predicting new metamagnets of interest in applications such as magnetic cooling
A combined thermodynamics and first principles study of the electronic, lattice and magnetic contributions to the magnetocaloric effect in La0.75Ca0.25MnO3
Manganites with the formula La1−x Ca x MnO3 for 0.2 < x < 0.5 undergo a magnetic field driven transition from a paramagnetic to ferromagnetic state, which is accompanied by changes in the lattice and electronic structure. An isotropic expansion of the La0.75Ca0.25MnO3 cell at the phase transition has been observed experimentally. It is expected that there will be a large entropy change at the transition due to its first order nature. Doped lanthanum manganite (LMO) is therefore of interest as the active component in a magnetocaloric cooling device. However, the maximum obtained value for the entropy change in Ca-doped manganites merely reaches a moderate value in the field of a permanent magnet. The present theoretical work aims to shed light on this discrepancy. A combination of finite temperature statistical mechanics and first principles theory is applied to determine individual contributions to the total entropy change of the system by treating the electronic, lattice and magnetic components independently. Hybrid-exchange density functional (B3LYP) calculations and Monte Carlo simulations are performed for La0.75Ca0.25MnO3. Through the analysis of individual entropy contributions, it is found that the electronic and lattice entropy changes oppose the magnetic entropy change. The results highlighted in the present work demonstrate how the electronic and vibrational entropy contributions can have a deleterious effect on the total entropy change and thus the potential cooling power of doped LMO in a magnetocaloric device
Frustrated magnetism and caloric effects in Mn-based antiperovskite Nitrides : Ab Initio theory
We model changes of magnetic ordering in Mn-antiperovskite nitrides driven by biaxial lattice strain at zero and at finite temperature. We employ a non-collinear spin-polarised density functional theory to compare the response of the geometrically frustrated exchange interactions to a tetragonal symmetry breaking (the so called piezomagnetic effect) across a range of Mn3AN (A = Rh, Pd, Ag, Co, Ni, Zn, Ga, In, Sn) at zero temperature. Building on the robustness of the effect we focus on Mn3GaN and extend our study to finite temperature using the disordered local moment (DLM) first-principles electronic structure theory to model the interplay between the ordering of Mn magnetic moments and itinerant electron states. We discover a rich temperature-strain magnetic phase diagram with two previously unreported phases stabilised by strains larger than 0.75\% and with transition temperatures strongly dependent on strain. We propose an elastocaloric cooling cycle crossing two of the available phase transitions to achieve simultaneously a large isothermal entropy change (due to the first order transition) and a large adiabatic temperature change (due to the second order transition)
Magneto-elastic coupling and competing entropy changes in substituted CoMnSi metamagnets
We use neutron diffraction, magnetometry and low temperature heat capacity to
probe giant magneto-elastic coupling in CoMnSi-based antiferromagnets and to
establish the origin of the entropy change that occurs at the metamagnetic
transition in such compounds. We find a large difference between the electronic
density of states of the antiferromagnetic and high magnetisation states. The
magnetic field-induced entropy change is composed of this contribution and a
significant counteracting lattice component, deduced from the presence of
negative magnetostriction. In calculating the electronic entropy change, we
note the importance of using an accurate model of the electronic density of
states, which here varies rapidly close to the Fermi energy.Comment: 11 pages, 9 figures. Figures 4 and 6 were updated in v2 of this
preprint. In v3, figures 1 and 2 have been updated, while Table II and the
abstract have been extended. In v4, Table I has updated with relevant neutron
diffraction dat