arXiv subjects

K. G. Sandeman

Publications and source records attributed to K. G. Sandeman.

17 recordsLinked to original sources

arXiv2025

Atomistic Description of Spin Crossover Under Pressure and its Giant Barocaloric Effect

The pressure-dependent evolution of the spin crossover (SCO) transition has garnered significant interest due to its connection to the giant barocaloric effect (BCE) near room temperature. Pressure alters both the molecular and solid-state structures of SCO materials, affecting the relative stability of low- and high-spin states and, consequently, the transition temperature ($T_{1/2}$). Crucially, the shape of the $T_{1/2}$ vs. pressure curve dictates the magnitude of the BCE, making its accurate characterization essential for identifying high-performance materials. In this work, we investigate the nonlinear $T_{1/2}$ vs. pressure behavior of the prototypical SCO complex [FeL$_2$][BF$_4$]$_2$ [L = 2,6-di(pyrazol-1-yl)pyridine] using solid-state PBE+U computations. Our results unveil the mechanisms by which pressure influences its SCO transition, including the onset of a phase transition, as well as the key role of low-frequency phonons in the BCE. Furthermore, we establish a computational protocol for accurately modeling the BCE in SCO crystals, providing a powerful tool for the rapid and efficient discovery of new materials with enhanced barocaloric performance.

cond-mat.mtrl-sci↗

arXiv2020

Fabrication of magnetocaloric La(Fe,Si)$_{13}$ thick films

La(Fe,Si)$_{13}$-based compounds are considered to be very promising magnetocaloric materials for magnetic refrigeration applications. Many studies have focused on this material family but only in bulk form. In this paper, we report the fabrication of thick films of La(Fe,Si)$_{13}$, both with and without post-hydriding. These films exhibit magnetic and structural properties comparable to those of bulk materials. We also observe that the ferromagnetic phase transition has a negative thermal hysteresis, a phenomenon not previously found in this material but which may have its origins in the availability of a strain energy reservoir, as in the cases of other materials in which negative thermal hysteresis has been found. Here, it appears that the substrate acts to store strain energy. Our exploratory study demonstrates the viability of thick films of the La(Fe,Si)$_{13}$ phase and motivates further work in the area, while showing that additional perspectives can be gained from reducing the dimensionality of magnetocaloric materials in which the magneto-volume effect is large.

cond-mat.mtrl-sci↗

arXiv2017

Electronic structure, metamagnetism and thermopower of LaSiFe$_{12}$ and interstitially doped LaSiFe$_{12}$

We present a systematic investigation of the effect of H, B, C, and N interstitials on the electronic, lattice and magnetic properties of La(Fe,Si)$_{13}$ using density functional theory. The parent LaSiFe$_{12}$ alloy has a shallow, double-well free energy function that is the basis of itinerant metamagnetism. On increasing the dopant concentration, the resulting lattice expansion causes an initial increase in magnetisation for all interstitials that is only maintained at higher levels of doping in the case of hydrogen. Strong s-p band hybridisation occurs at high B,C and N concentrations. We thus find that the electronic effects of hydrogen doping are much less pronounced than those of other interstitials, and result in the double-well structure of the free energy function being least sensitive to the amount of hydrogen. This microscopic picture accounts for the vanishing first order nature of the transition by B,C, and N dopants as observed experimentally. We use our calculated electronic density of states for LaSiFe$_{12}$ and the hydrogenated alloy to infer changes in magneto-elastic coupling and in phonon entropy on heating through $T_C$ by calculating the fermionic entropy due to the itinerant electrons. Lastly, we predict the electron thermopower in a spin-mixing, high temperature limit and compare our findings to recent literature data.

cond-mat.mtrl-sci↗

arXiv2017

Frustrated Magnetism and Caloric Effects in Mn-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 Mn$_3$AN (A = Rh, Pd, Ag, Co, Ni, Zn, Ga, In, Sn) at zero temperature. Building on the robustness of the effect we focus on Mn$_3$GaN 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).

cond-mat.str-el↗

arXiv2016

The mechanocaloric potential of spin crossover compounds

We present a first evaluation of the potential for spin crossover (SCO) compounds to be considered as a new class of giant mechanocaloric effect material. From literature data on the variation of the spin crossover temperature with pressure, we estimate the maximum available adiabatic temperature change for several compounds and the relatively low pressures that may be required to observe these effects.

cond-mat.mtrl-sci↗

arXiv2014

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.

cond-mat.mtrl-sci↗

arXiv2013

Magnetoelastic effects in doped Fe2P

We use combine high resolution neutron diffraction (HRPD) with density functional theory (DFT) to investigate the exchange striction at the Curie temperature (TC) of Fe2P and to examine the effect of boron and carbon doping on the P site. We find a significant contraction of the basal plane on heating through TC with a simultaneous increase of the c-axis that results in a small overall volume change of ~0.01%. At the magnetic transition the FeI-FeI distance drops significantly and becomes shorter than FeI-FeII . The shortest metal-metalloid (FeI-PI) distance also decreases sharply. Our DFT model reveals the importance of the latter as this structural change causes a redistribution of the FeI moment along the c-axis (Fe-P chain). We are able to understand the site preference of the dopants, the effect of which can be linked to the increased moment on the FeI-site, brought about by strong magneto-elasticity and changes in the electronic band structure.

cond-mat.mtrl-sci↗

arXiv2013

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.

cond-mat.str-el↗

arXiv2012

Microstructural control and tuning of thermal conductivity in La0.67Ca0.33MnO3+/-δ

Manganites are one of only a small number of material families currently being trialled as room temperature magnetic refrigerants. Here we examine the dependence of the thermal conductivity, K, of La0.67Ca0.33MnO3+/-δ as a function of density, grain size and silver impregnation around room temperature. We use a simple effective medium model to extract relevant trends in the data and demonstrate a threefold increase in thermal conductivity by silver impregnation.

cond-mat.str-el↗

arXiv2012

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.

cond-mat.mtrl-sci↗

arXiv2010

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.

cond-mat.mtrl-sci↗

arXiv2010

Giant magneto-elastic 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 inter-atomic 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.

cond-mat.mtrl-sci↗

arXiv2007

Phase diagram and magnetocaloric effect of CoMnGe_{1-x}Sn_{x} alloys

We propose the phase diagram of a new pseudo-ternary compound, CoMnGe_{1-x}Sn_{x}, in the range x less than or equal to 0.1. Our phase diagram is a result of magnetic and calometric measurements. We demonstrate the appearance of a hysteretic magnetostructural phase transition in the range x=0.04 to x=0.055, similar to that observed in CoMnGe under hydrostatic pressure. From magnetisation measurements, we show that the isothermal entropy change associated with the magnetostructural transition can be as high as 4.5 J/(K kg) in a field of 1 Tesla. However, the large thermal hysteresis in this transition (~20 K) will limit its straightforward use in a magnetocaloric device.

cond-mat.mtrl-sci↗

arXiv2006

Negative magnetocaloric effect from highly sensitive metamagnetism in CoMnSi_{1-x}Ge_{x}

We report a novel negative magnetocaloric effect in CoMnSi_{1-x}Ge_{x} arising from a metamagnetic magnetoelastic transition. The effect is of relevance to magnetic refrigeration over a wide range of temperature, including room temperature. In addition we report a very high shift in the metamagnetic transition temperature with applied magnetic field. This is driven by competition between antiferromagnetic and ferromagnetic order which can be readily tuned by applied pressure and compositional changes.

cond-mat.str-el↗

arXiv2003

The Normal State Resistivity of Grain Boundaries in YBa2Cu3O7-delta

Using an optimized bridge geometry we have been able to make accurate measurements of the properties of YBa2Cu3O7-delta grain boundaries above Tc. The results show a strong dependence of the change of resistance with temperature on grain boundary angle. Analysis of our results in the context of band-bending allows us to estimate the height of the potential barrier present at the grain boundary interface.

cond-mat.mtrl-sci↗

arXiv2002

Ferromagnetic superconductivity driven by changing Fermi surface topology

We introduce a simple but powerful zero temperature Stoner model to explain the unusual phase diagram of the ferromagnetic superconductor, UGe2. Triplet superconductivity is driven in the ferromagnetic phase by tuning the majority spin Fermi level through one of two peaks in the paramagnetic density of states (DOS). Each peak is associated with a metamagnetic jump in magnetisation. The twin peak DOS may be derived from a tight-binding, quasi-one-dimensional bandstructure, inspired by previous bandstructure calculations.

cond-mat.str-el↗

arXiv2000

Measuring anisotropic scattering in the cuprates

A simple model of anisotropic scattering in a quasi two-dimensional metal is studied. Its simplicity allows an analytic calculation of transport properties using the Boltzmann equation and relaxation time approximation. We argue that the c-axis magnetoresistance provides the key test of this model of transport. We compare this model with experiments on overdoped Tl-2201 and find reasonable agreement using only weak scattering anisotropy. We argue that optimally doped Tl-2201 should show strong angular-dependent magnetoresistance within this model and would provide a robust way of determining the in-plane scattering anisotropy in the cuprates.

cond-mat.str-el↗