Structural transformation induced by magnetic field and "colossal-like" magnetoresistance response above 313 K in MnAs

Theoretical prediction of two-dimensional ferromagnetic Mn2X2 (X = As, Sb) with strain-controlled magnetocrystalline anisotropy

Two-dimensional (2D) magnetic materials with large and tunable magnetocrystalline anisotropy (MCA) provide unique opportunities to develop various spintronic devices. We, herein, propose an experimentally feasible 2D material platform, Mn2X2 (X = As, Sb), which is a family of intrinsic ferromagnet. Using first-principles calculations, we show that 2D Mn2X2 (X = As, Sb) with a robust ferromagnetic ground state exhibits not only a large perpendicular magnetic anisotropy (PMA), but also significant strain-driven modulation behaviors under external biaxial strain. The analysis of the results demonstrates that the dominant contribution to the change of MCA of Mn2As2 and Mn2Sb2 primarily arises from the Mn and Sb atoms, respectively. Moreover, we reveal that the underlying origin is the competitive mechanism for the spin-orbit coupling (SOC) between different orbitals and spin channels. These findings indicate that 2D Mn2X2 (X = As, Sb) provides a promising material platform for the next generation of ultra-low energy memory devices.

Lattice instability and magnetic phase transitions in strongly correlated MnAs

Using variable temperature x-ray total scattering in magnetic field, we study the interaction between lattice and magnetic degrees of freedom in MnAs, which loses its ferromagnetic order and hexagonal ('H') lattice symmetry at 318 K to recover the latter and become a true paramagnet when the temperature is increased to 400 K. Our results show that the 318 K transition is accompanied by highly anisotropic displacements of Mn atoms that appear as a lattice degree of freedom bridging the 'H' and orthorhombic phases of MnAs. This is a rare example of a lowering of an average crystal symmetry due to an increased displacive disorder emerging on heating. Our results also show that magnetic and lattice degrees of freedom appear coupled but not necessarily equivalent control variables for triggering phase transitions in strongly correlated systems in general and in particular in MnAs.

Homogeneous Nanoparticles of Multimetallic Phosphides via Precursor Tuning: Ternary and Quaternary M2P Phases (M = Fe, Co, Ni)

Transition metal phosphides (TMPs) are a highly investigated class of nanomaterials due to their unique magnetic and catalytic properties. Although robust and reproducible synthetic routes to narrow polydispersity monometallic phosphide nanoparticles (M₂P; M = Fe, Co, Ni) have been established, the preparation of multimetallic nanoparticle phases (M_2-x M' _x P; M, M' = Fe, Co, Ni) remains a significant challenge. Colloidal syntheses employ zero-valent metal carbonyl or multivalent acetylacetonate salt precursors in combination with trioctylphosphine as the source of phosphorus, oleylamine as the reducing agent, and additional solvents such as octadecene or octyl ether as "noncoordinating" cosolvents. Understanding how these different metal precursors behave in identical reaction environments is critical to assessing the role the relative reactivity of the metal precursor plays in synthesizing complex, homogeneous multimetallic TMP phases. In this study, phosphorus incorporation as a function of temperature and time was evaluated to probe how the relative rate of phosphidation of organometallic carbonyl and acetylacetonate salt precursors influences the homogeneous formation of bimetallic phosphide phases (M_2-x M' _x P; M, M' = Fe, Co, Ni). From the relative rate of phosphidation studies, we found that where reactivity with TOP for the various metal precursors differs significantly, prealloying steps are necessary to isolate the desired bimetallic phosphide phase. These insights were then translated to establish streamlined synthetic protocols for the formation of new trimetallic Fe_2-x-y Ni _x Co _y P phases.

Anisotropic manganese antimonide nanoparticle formation by solution-solid-solid growth mechanism: consequence of sodium borohydride addition towards reduced surface oxidation and enhanced magnetic moment

A new approach to the solution-phase synthesis of manganese antimonide nanoparticles was developed to reduce competitive oxide formation by exploitation of sodium borohydride (NaBH4) (0.53-2.64 mmol) as a sacrificial reductant. However, in the presence of near-stoichiometric precursor amounts of manganese carbonyl and triphenyl antimony, the introduction of NaBH4 results in a different growth mechanism, Solution-Solid-Solid (SSS), leading to tadpole-shaped manganese antimonide nanoparticles with antimony-rich heads and stoichiometric manganese antimonide tails. We hypothesize that a solid antimony-rich manganese antimonide cluster acts as an initiator to tail growth in solution. Notably, the length of the tail correlated with the amount of NaBH4 used. Interestingly, these anisotropic particles can be transformed progressively into spherical-shaped nanoparticles upon the addition of excess manganese carbonyl. The anisotropic manganese antimonide particles possess saturation magnetizations ca. twenty times higher than that reported for MnSb nanoparticles prepared without NaBH4, attributed to limitation of oxidation.

Colossal magnetoresistance in amino-functionalized graphene quantum dots at room temperature: manifestation of weak anti-localization and doorway to spintronics

In this work, we have demonstrated the signatures of localized surface distortions and disorders in functionalized graphene quantum dots (fGQD) and consequences in magneto-transport under weak field regime (∼1 Tesla) at room temperature. Observed positive colossal magnetoresistance (MR) and its suppression is primarily explained by weak anti-localization phenomenon where competitive valley (inter and intra) dependent scattering takes place at room temperature under low magnetic field; analogous to low mobility disordered graphene samples. Furthermore, using ab-initio analysis we show that sub-lattice sensitive spin-polarized ground state exists in the GQD as a result of pz orbital asymmetry in GQD carbon atoms with amino functional groups. This spin polarized ground state is believed to help the weak anti-localization dependent magneto transport by generating more disorder and strain in a GQD lattice under applied magnetic field and lays the premise for future graphene quantum dot based spintronic applications.

The role of magnetoelastic and magnetostrictive energies in the magnetization process of MnAs/GaAs epilayers

In this work we present a detailed study of the anisotropic magnetic behavior of MnAs epilayers grown by molecular beam epitaxy on GaAs(001) and GaAs(111)B substrates. An extended approach of the Stoner-Wohlfarth model is used to simulate magnetic hysteresis loops of MnAs epilayers for temperatures around the magnetostructural phase transition. We demonstrate that magnetoelastic and magnetostrictive energy contributions to the magnetic free energy density are crucial to correctly describe the magnetization of both kinds of MnAs epilayers.

Diverse structural and magnetic properties of differently prepared MnAs nanoparticles

Discrete nanoparticles of MnAs with distinct magnetostructural properties have been prepared by small modifications of solution-phase arrested precipitation reactions. Rietveld and X-ray atomic pair distribution function based approaches were used to explore the evolution of the structure of the samples with temperature, and these data were compared to the magnetic response measured with ac susceptibility. Relative to a bulk standard, one type of MnAs nanoparticles was found to demonstrate similar but smaller structural transitions and corresponding magnetic changes. However, both magnetic and structural transitions in the second type of nanoparticles are strongly suppressed.

Swingback in magnetization reversal in MnAs-GaAs coaxial nanowire heterostructures

The reversal processes of magnetization in epitaxial MnAs nanotubes prepared by an overgrowth on the sidewall of GaAs nanowires having a diameter of 26 nm are investigated. While the magnetic hard axis is aligned in the direction of the nanowire axis, we apply an external magnetic field perpendicular to the axis to examine the flipping characteristics of magnetic moments. We determine the contributions from the substrate by a direct measurement in order to extract the magnetization of the core-shell heterostructures. The abrupt change in the thus-obtained magnetization due to a flip when the field is varied exhibits an overshoot at about 0.4 kOe for samples with a thickness of the ferromagnetic shell (40-50 nm) larger than the diameter of the core. Moreover, the peak value exceeds the value when the field is swept in the opposite direction. The magnetic hysteresis loop consequently involves line crossings. We speculate that the spin textures of domain walls in such thick hollow cylinders and their movement at the magnetization flip are affected by the geometry and magnetostatic interactions of various origins, giving rise to the anomalous behaviour.

Giant magnetoresistance in oxypnictides (La,Nd)OMnAs

A sizeable negative magnetoresistance (MR) has been observed for oxypnictides LnOMnAs (Ln = La,Nd). MR up to -24% is observed at 200 K for LaOMnAs which is unprecedented for divalent Mn(2+). Both materials are weak ferromagnets with transition temperatures above room temperature.

Strain engineering of the magnetocaloric effect in MnAs epilayers

By using heteroepitaxy on two different GaAs templates, we have investigated the impact of anisotropic strain on the magnetocaloric effect (MCE) of MnAs. The temperature range, spread around room temperature, and the maximal MCE position are markedly different in the two epitaxial systems. Simulated MCE curves, obtained from a model based on the mean-field approximation, are in good agreement with the experimental data, indicating that the entropy variation is magnetic in origin. These results illustrate how strain can be used to tune the MCE in materials with coupled structural and magnetic phase transition and suggest that the MCE of MnAs may find applications in microelectronic circuitry.

Massive magnetic-field-induced structural transformation in Gd5Ge4 and the nature of the giant magnetocaloric effect

A massive magnetic-field-induced structural transformation in Gd5Ge4, which occurs below 30 K, was imaged at the atomic level by uniquely coupling high-resolution x-ray powder diffraction with magnetic fields up to 35 kOe. In addition to uncovering the nature of the magnetic field induced structural transition, our data demonstrate that the giant magnetocaloric effect, observed in low magnetic fields, arises from the amplification of a conventional magnetic entropy-driven mechanism by the difference in the entropies of two phases, borne by the concomitant structural transformation.