Z. Gercsi

Thermal-history dependent magnetoelastic transition in (Mn,Fe)2(P,Si)

The thermal-history dependence of the magnetoelastic transition in (Mn,Fe)2(P,Si) compounds has been investigated using high-resolution neutron diffraction. As-prepared samples display a large difference in paramagnetic-ferromagnetic (PM-FM) transition temperature compared to cycled samples. The initial metastable state transforms into a lower-energy stable state when the as-prepared sample crosses the PM-FM transition for the first time. This additional transformation is irreversible around the transition temperature and increases the energy barrier which needs to be overcome through the PM-FM transition. Consequently, the transition temperature on first cooling is found to be lower than on subsequent cycles characterizing the so-called “virgin effect.” High-temperature annealing can restore the cycled sample to the high-temperature metastable state, which leads to the recovery of the virgin effect. A model is proposed to interpret the formation and recovery of the virgin effect.

Giant Magnetoelastic Coupling in a Metallic Helical Metamagnet

Using high resolution neutron diffraction and capacitance dilatometry we show that the thermal evolution of the helimagnetic state in CoMnSi is accompanied by a change in interatomic distances of up to 2%, the largest ever found in a metallic magnet. Our results and the picture of competing exchange and strongly anisotropic thermal expansion that we use to understand them sheds light on a new mechanism for large magnetoelastic effects that does not require large spin-orbit coupling.

Designed metamagnetism in CoMnGe1−xPx

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.

Structurally driven metamagnetism in MnP and related Pnma compounds

We investigate the structural conditions for metamagnetism in MnP and related materials using Density Functional Theory. A magnetic stability plot is constructed taking into account the two shortest Mn-Mn distances. We find that a particular Mn-Mn separation plays the dominant role in determining the change from antiferromagnetic to ferromagnetic order in such systems. We establish a good correlation between our calculations and structural and magnetic data from the literature. Based on our approach it should be possible to find new Mn-containing alloys that possess field-induced metamagnetism and associated magnetocaloric effects.

Magnetoelastic 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.

Magnetic structures of Mn3-xFexSn2: an experimental and theoretical study

We investigate the magnetic structure of Mn3-xFexSn2 using neutron powder diraction 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 diraction experiments on Mn3Sn2 reveal a weak monoclinic distortion at low temperature (90.08 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.

Frustrated magnetism and caloric effects in Mn-based antiperovskite Nitrides : Ab Initio theory

We model changes of magnetic ordering in Mn-based antiperovskite nitrides driven by biaxial lattice strain at zero and at finite temperature. We employ a noncollinear spin-polarized density functio ...

Magnetic coupling in transition-metal–doped LaSiFe11.5 TM0.5 (TM=Cr, Mn, Co and Ni)

This study describes the effect of small concentration of transition metal (TM) dopants on the nature of magnetic and electronic structure of the metamagnetic La-Si-Fe. In agreement with experimental results, all of the investigated dopants are found to reduce the overall magnetisation of the parent compound, regardeless of the nature of actual spin arrangement. The large magnetic moment of Cr (?1.87? B ) and Mn (?2.35? B ) prefers to couple antiferromagnetically to the magnetic moments of Fe (2.22? B ), whilst the Co and Ni spins are aligned ferromagnetically but with a lower magnetic moment of 1.40 and , respectively. The variation of electronic entropy due to the change in with the dopants suggests a reshaped metamagnetic energy landscape that can lead to an altered itinerant-electron metamagnetic (IEM) transition.

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 LaFe

A hybrid-exchange density functional study of Ca-doped LaMnO3

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