arXiv subjects

David P. Young

Publications and source records attributed to David P. Young.

11 recordsLinked to original sources

arXiv2024

Structural and magnetic properties of CoTeMoO$_6$ revisited

We have conducted a comprehensive investigation into the magnetic properties of the chiral multiferroic material CoTeMoO$_6$. In contrast with the previous claim of canted antiferromagnetic order with ferromagnetic components, our investigation reveals an antiferromagnetic ground state with compensated moments, providing an interesting platform for exploring exotic material properties. Through careful measurements of magnetization under a series of applied field, we demonstrate that there exist two sequential field-induced magnetic transitions in CoTeMoO$_6$, with one occurring at $H_{c1}$=460 Oe along the a-axis, and the other at $H_{c2}$=1.16 T with the field along the b-axis. The values of $H_{c1}$ and $H_{c2}$ exhibit strong angular dependence and diverge with different rates as the applied field is rotated 90 degrees within the ab plane. This reflects the distinct nature of these transitions, which is further supported by the different critical behavior of $H_{c1}$ and $H_{c2}$, characterized by the values of $γ$,in the function of $H_c=H_0\times(1-\frac{T}{T_c})^n$. Furthermore, we have demonstrated that there exist structural and magnetic twin domains in CoTeMoO$_6$ that strongly affect the experimental measurement of their macroscopic properties. Intriguingly, these twin domains can be related to the orthorhombicity/chirality of the crystal structure with the space group $P2_1 2_1 2$. We further explored the magnetic and structural domains with uniaxial pressure and polarized light microscopy. Our results suggest that CoTeMoO$_6$ could be used as a unique platform for investigating the intriguing physics involving intertwined degrees of freedom. The tunability of the underlying domain distribution and its strong anisotropy could also be useful for developing functional devices and applications.

cond-mat.mtrl-sci↗

arXiv2021

Annihilation and Control of Chiral Domain Walls with Magnetic Fields

The control of domain walls is central to nearly all magnetic technologies, particularly for information storage and spintronics. Creative attempts to increase storage density need to overcome volatility due to thermal fluctuations of nanoscopic domains and heating limitations. Topological defects, such as solitons, skyrmions, and merons, may be much less susceptible to fluctuations, owing to topological constraints, while also being controllable with low current densities. Here, we present the first evidence for soliton/soliton and soliton/antisoliton domain walls in the hexagonal chiral magnet Mn1/3NbS2 that respond asymmetrically to magnetic fields and exhibit pair-annihilation. This is important because it suggests the possibility of controlling the occurrence of soliton pairs and the use of small fields or small currents to control nanoscopic magnetic domains. Specifically, our data suggest that either soliton/soliton or soliton/antisoliton pairs can be stabilized by tuning the balance between intrinsic exchange interactions and long-range magnetostatics in restricted geometries

cond-mat.mes-hall↗

arXiv2020

Magnetic order and fluctuations in quasi-two-dimensional planar magnet Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$

We use neutron scattering to investigate spin excitations in Sr(Co$_{1-x}$Ni$_{x})_2$As$_2$, which has a $c$-axis incommensurate helical structure of the two-dimensional (2D) in-plane ferromagnetic (FM) ordered layers for $0.013\leq x \leq 0.25$. By comparing the wave vector and energy dependent spin excitations in helical ordered Sr(Co$_{0.9}$Ni$_{0.1}$)$_2$As$_2$ and paramagnetic SrCo$_2$As$_2$, we find that Ni-doping, while increasing lattice disorder in Sr(Co$_{1-x}$Ni$_{x})_2$As$_2$, enhances quasi-2D FM spin fluctuations. However, our band structure calculations within the combined density functional theory and dynamic mean field theory (DFT+DMFT) failed to generate a correct incommensurate wave vector for the observed helical order from nested Fermi surfaces. Since transport measurements reveal increased in-plane and $c$-axis electrical resistivity with increasing Ni-doping and associated lattice disorder, we conclude that the helical magnetic order in Sr(Co$_{1-x}$Ni$_{x})_2$As$_2$ may arise from a quantum order-by-disorder mechanism through the itinerant electron mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions.

cond-mat.str-el↗

arXiv2020

Topological Hall effect and the magnetic states of Nowotney chimney ladder compound Cr$_{11}$Ge$_{19}$}

We have investigated the magnetic and charge transport properties of single crystals of Nowotney Chimney Ladder compound Cr$_{11}$Ge$_{19}$ and mapped out a comprehensive phase diagram reflecting the complicated interplay between the Dzyaloshinskii-Moriya (DM) interaction, the dipolar interaction, and the magnetic anisotropy. We have identified a set of interesting magnetic phases and attributed a finite topological Hall effect to the recently discovered bi-skyrmion phase. These data also suggest the existence of an anti-skyrmion state at finite fields for temperatures just below the magnetic ordering temperature, $T_c$, as indicated by a distinct change in sign of the topological Hall effect. Above $T_c$, we discovered a region of enhanced magnetic response corresponding to a disordered phase likely existing near the ferromagnetic critical point under small magnetic fields. Strong spin chirality fluctuations are demonstrated by the large value of the topological Hall resistivity persisting up to 1 T which is most likely due to the existence of the DM interaction. We argue that changes to the topological Hall effect correspond to different topological spin textures that are controlled by magnetic dipolar and DM interactions that vary in importance with temperature.

cond-mat.mtrl-sci↗

arXiv2019

Flat band magnetism and helical magnetic order in Ni-doped SrCo$_2$As$_2$

A series of Sr(Co$_{1-x}$Ni$_x$)$_2$As$_2$ single crystals was synthesized allowing a comprehensive phase diagram with respect to field, temperature, and chemical substitution to be established. Our neutron diffraction experiments revealed a helimagnetic order with magnetic moments ferromagnetically (FM) aligned in the $ab$ plane and a helimagnetic wavevector of $q=(0,0,0.56)$ for $x$ = 0.1. The combination of neutron diffraction and angle-resolved photoemission spectroscopy (ARPES) measurements show that the tuning of a flat band with $d_{x^2-y^2}$ orbital character drives the helimagnetism and indicates the possibility of a quantum order-by-disorder mechanism.

cond-mat.str-el↗

arXiv2017

Exploring the origins of the Dzyalloshinski-Moria interaction in MnSi

By using magnetization and small-angle neutron scattering (SANS) measurements, we have investigated the magnetic behavior of Mn_{1-x}Ir_{x}Si system to explore the effect of increased carrier density and spin-orbit interaction on the magnetic properties of MnSi. We determine estimates of the spin wave stiffness and the Dzyalloshinski-Moria, DM, interaction strength and compare with Mn_{1-x}Co_{x}Si and Mn_{1-x}Fe_{x}Si. Despite the large differences in atomic mass and size of the substituted elements, Mn_{1-x}Co_{x}Si and Mn_{1-x}Ir_{x}Si show nearly identical variations in their magnetic properties with substitution. We find a systematic dependence of the transition temperature, the ordered moment, the helix period and the DM interaction strength with electron count for Mn{1-x}Ir{x}Si, Mn_{1-x}Co_{x}Si, and Mn_{1-x}Fe_{x}Si indicating that the magnetic behavior is primarily dependent upon the additional carrier density rather than on the mass or size of the substituting species. This indicates that the variation in magnetic properties, including the DM interaction strength, are primarily controlled by the electronic structure as Co and Ir are isovalent. Our work suggests that although the rigid band model of electronic structure along with Moira's model of weak itinerant magnetism describe this system surprisingly well, phenomenological models for the DM interaction strength are not adequate to describe this system.

cond-mat.mtrl-sci↗

arXiv2017

Ir d-band Derived Superconductivity in the Lanthanum-Iridium System LaIr3

The electronic properties of the heavy metal superconductor LaIr3 are reported. The estimated superconducting parameters obtained from physical properties measurements indicate that LaIr3 is a BCS-type superconductor. Electronic band structure calculations show that Ir d- states dominate the Fermi level. A comparison of electronic band structures of LaIr3 and LaRh3 shows that the Ir-compound has a strong spin-orbit-coupling effect, which creates a complex Fermi surface.

cond-mat.supr-con↗

arXiv2016

The emergence of giant multicaloric phenomena near room temperature

Caloric responses (temperature changes) can be induced in solid-state materials by applying external stimuli such as stress, pressure, and electric and magnetic fields. The magnetic-field-stimulated response is called the magnetocaloric effect, and materials that exhibit this property have long been sought for applications in room temperature magnetic cooling due to their potentially superior efficiency and low impact on the environment. Other solid-state caloric phenomena are less developed, but are likewise under intense investigation. Here we introduce a new material that not only displays giant barocaloric (hydrostatic-pressure-induced) properties, but also a large magnetocaloric response near room temperature. It is unprecedented that two caloric effects of such extreme magnitude occur in the same material and at the same temperature. These effects originate from a magnetostructural transition and a magneto-volume (magnetostriction) effect where the volume change is large enough to force the system from a localized ordered state into an itinerant paramagnetic state.

cond-mat.mtrl-sci↗

arXiv2015

Effects of pressure on the magnetostructural and magnetocaloric properties of isostructurally alloyed (MnNiSi)1-x(FeCoGe)x

The isostructural alloying of two compounds with extremely different magnetic and thermo-structural properties has resulted in a new system, (MnNiSi)1-x(FeCoGe)x, that exhibits extraordinary magnetocaloric properties with an acute sensitivity to applied hydrostatic pressure (P). Application of hydrostatic pressure shifts the first-order phase transition to lower temperature ($Δ$ T=-41 K with P=3.43 kbar) but preserves the giant value of isothermal entropy change (-$Δ$S$\max$=143.7 J/kg K for a field change of ΔB=5 T at atmospheric pressure). Together with the magnetic field, this pressure-induced temperature shift can be used to significantly increase the effective relative cooling power.

cond-mat.mtrl-sci↗

arXiv2014

Pressure-induced giant enhancement of magnetocaloric effects in MnNiSi-based systems

A remarkable decrease of the structural transition temperature of MnNiSi from 1200 K to <300 K by chemically alloying it with MnFeGe results in a coupling of the magnetic and structural transitions, leading to a large magnetocaloric effect near room temperature. It was found that the magnetostructural transition is highly sensitive to external (hydrostatic) pressure: relatively low hydrostatic pressures (~2.4 kbar) lead to an extraordinary enhancement of the isothermal entropy change from $-$Δ$S$ = 44 to 89 J/kg K at ambient (atmospheric) and 2.4 kbar applied pressures, respectively, for a field change of Δ$H$ = 5 T. This giant entropy change is associated with a large relative volume change of about 7% induced by 2.4 kbar applied pressure during the magnetostructural transition. The pressure-enhanced magnetocaloric effects are accompanied by a shift in transition temperature, an effect that may be exploited to tune the transition to the required working temperature, and thereby eliminate the need for a given material to possess a large magnetocaloric effect (i.e., entropy change) over a wide temperature range. Furthermore, this material also possesses negligible hysteresis losses.

cond-mat.mtrl-sci↗

arXiv2003

New Aspects of the Temperature-Magnetic Field Phase Diagram of CeB6

We have measured the magnetic field dependence of the paramagnetic to the field-induced high temperature antiferroquadrupolar magnetically ordered phase transition is CeB6 from 0 to 60 T using a variety of techniques. It is found that the field-dependent phase separation line becomes re-entrant above 35 T and below 10 K. Measurements of resonant ultra-sound, specific heat and neutron diffraction show conclusively that the zero-field temperature-dependent phase transition is to a state with no ordered dipole moments, but with second order transition signatures in both sound attenuation and specific heat.

cond-mat.str-el↗