arXiv subjects

Øystein S. Fjellvåg

Publications and source records attributed to Øystein S. Fjellvåg.

6 recordsLinked to original sources

arXiv2026

Influence of Ni substitution on the phase transitions and magnetocaloric effect of NdCo2 at cryogenic temperatures

We have investigated NdCo2-xNix cubic Laves compounds with 0 <= x <= 1 using neutron diffraction and bulk magnetization measurements to study the influence of partial Ni substitutions of Co on the phase transitions and the magnetocaloric effect. Upon cooling, NdCo2 undergoes a cubic to tetragonal transition at 100 K, and a tetragonal to orthorhombic transition at 42 K. The transitions are associated with long-range ferromagnetic ordering of the magnetic moments along the c axis and spin reorientation into the ab plane, respectively. Both transitions shift to lower temperatures as the Ni content x increases. For x >= 0.5, the orthorhombic phase is suppressed. Additionally, there was a reduction in the magnetic moment upon increasing the Ni substitution of Co. The magnetocaloric effect was determined both indirectly and directly, with good agreement between the methods. NdCo2 exhibits an adiabatic temperature change of 6.3 K for a field of 20 T, which is decreased to 4.9 K for NdCoNi for the same field strength due to the reduced magnetic moment upon Ni substitution.

cond-mat.mtrl-sci↗

arXiv2025

Electronic and magnetic properties of light rare-earth cubic Laves compounds derived from XMCD experiments

This work presents electronic and magnetic properties of selected members in the cubic Laves phase series Nd1-xPrxCoNi and Ce0.25Pr0.75CoNi, together with the corresponding binary compositions (NdCo2, NdNi2, PrCo2, PrNi2, CeCo2, CeNi2), using soft x-ray absorption spectroscopy, x-ray magnetic circular dichroism (XMCD), density-functional theory, and crystal field multiplet calculations. All transition-metal moments saturate below 1 T, while the rare-earth moments do not saturate even at 5 T, consistent with van Vleck paramagnetic contributions and crystal field suppression. While the sum rules are widely used to extract element-specific magnetic moments from XMCD, we show that for 3d transition metals, their application requires accurate estimates of the number of unoccupied 3d states. We observe a finite magnetic moment on Ni, challenging the common assumption of its nonmagnetic character in Laves phases. The orbital magnetic moments were determined using the spin rules, while the spin moments were estimated from single-ion values from multiplet calculations, due to the invalidity of the spin sum rule for light rare-earth elements. The magnetic moments of Nd and Pr are found to be suppressed relative to their free-ion values, with multiplet theory indicating that this is due to crystal field effects. Our results confirm that Nd and Pr maintain localized 4f3 and 4f2 configurations, respectively, and that their element-specific magnetic moments are robust to rare-earth substitution. Ce, on the other hand, exhibits a tunable mixed-valent ground state with both magnetic 4f1 and nonmagnetic 4f0 components. The relative fraction of these states varies with the electronegativity of the surrounding 3d transition metals, revealing a pathway to tune Ce magnetism via composition. This work establishes a framework for accurately interpreting XMCD in light rare-earth-based intermetallics.

cond-mat.mtrl-sci↗

arXiv2025

Anomalous Electrical Transport in the Kagome Magnet YbFe$_6$Ge$_6$

Two-dimensional (2D) kagome metals offer a unique platform for exploring electron correlation phenomena derived from quantum many-body effects. Here, we report a combined study of electrical magnetotransport and neutron scattering on YbFe$_6$Ge$_6$, where the Fe moments in the 2D kagome layers exhibit an $A$-type collinear antiferromagnetic order below $T_{\rm{N}} \approx 500$ K. Interactions between the Fe ions in the layers and the localized Yb magnetic ions in between reorient the $c$-axis aligned Fe moments to the kagome plane below $T_{\rm{SR}} \approx 63$ K. Our magnetotransport measurements show an intriguing anomalous Hall effect (AHE) that emerges in the spin-reorientated collinear state, accompanied by the closing of the spin anisotropy gap as revealed from inelastic neutron scattering. The gapless spin excitations and the Yb-Fe interaction are able to support a dynamic scalar spin chirality, which explains the observed AHE. Therefore, our study demonstrates spin fluctuations may provide an additional scattering channel for the conduction electrons and give rise to AHE even in a collinear antiferromagnet.

cond-mat.str-el↗

arXiv2024

The magnetic structure of Ce$_3$TiBi$_5$ and its relation to current-induced magnetization

The control of magnetization using electric fields has been extensively studied in magnetoelectric multiferroic insulator materials. Changes in magnetization in bulk metals caused by electric currents have attracted less attention. The recently discovered metallic magnet Ce$_3$TiBi$_5$ has been reported to exhibit current-induced magnetization. Here we determined the magnetic structure of Ce$_3$TiBi$_5$ using neutron diffraction, aiming to understand the microscopic origin of this magnetoelectric phenomenon in a metal. We established that the antiferromagnetic order emerging below $T_N=5$ K is a cycloid order described by $P6_3/mcm.1'(0,0,g)00sss$ with small moment sizes of $0.50(2)~μ_B$ and propagation vector ${\bf k}=(0,0,0.386)$. Surprisingly, the symmetry of this magnetic structure is inconsistent with the presence of current-induced magnetization and potential origins of this inconsistency with previous results are discussed. Additionally, our results suggest that moments order along their hard magnetic direction in Ce$_3$TiBi$_5$, a phenomenon which has been observed in other Kondo systems.

cond-mat.str-el↗

arXiv2020

Canted antiferromagnetism in high purity $\mathrm{NaFeF_3}$ prepared by a novel wet-chemical synthesis method

We report a novel synthesis method for, and structural and magnetic characterization of the fluoroperovskite $\mathrm{NaFeF_3}$. We have developed a wet-chemical method that allows preparation of large volumes of air-sensitive fluoroperovskites with high purity. $\mathrm{NaFeF_3}$ has a Néel temperature ($T_N$) of 90 K and a Weiss constant ($θ$) of -124 K, corresponding to dominant antiferromagnetic interactions. Below $T_N$, a slight difference is observed between zero-field and field cooled samples, indicating spin-canting and weak ferromagnetism. AC magnetometry confirms that weak ferromagnetism is inherent to $\mathrm{NaFeF_3}$ and not due to impurities. From powder neutron diffraction data, we describe the magnetic structure precisely as a weakly canted G-type (magnetic space group $Pn'ma'$). A ferromagnetic component is allowed in $Pn'ma'$, however, this component may be absent in zero magnetic fields and is too small to be confirmed on the basis of powder neutron diffraction data.

cond-mat.mtrl-sci↗

arXiv2020

Electron doping of the layered nickelate La$_4$Ni$_3$O$_{10}$ by aluminum substitution: A combined experimental and DFT study

The physical properties of La$_4$Ni$_3$O$_{10}$ with a 2D-like Ruddlesden-Popper-type crystal structure are extraordinarily dependent on temperature and chemical substitution. By introducing Al$^{3+}$ atoms ($x$) randomly at the Ni-sites, the average oxidation state for the two non-equivalent Ni-cations is tuned and adopt values below the average of +2.67 in La$_4$Ni$_3$O$_{10}$. La$_4$Ni$_{3-x}$Al$_x$O$_{10}$ is a solid solution for $x=0.00$ to 1.00, and are prepared by the citric acid method. The samples adopt a slightly distorted monoclinic structure (P21/a), evidenced by peak broadening of the (117) reflection. We report on a remarkable effect on the electronic properties induced by tiny amounts of homogeneously distributed Al-cations, with clear correspondence between resistivity, magnetization, diffraction, and DFT data. DFT shows that electronically there is no significant difference between the non-equivalent Ni atoms and no tendency towards any Ni$^{3+}$/Ni$^{2+}$ charge ordering. The electron doping via Al-substitution has a profound effect on electric and magnetic properties. The resistivity changes from metallic to semiconducting with increasing band-gap at higher Al-levels, consistent with results from DFT. The metal-to-metal transition reported for La$_4$Ni$_3$O$_{10}$, which is often interpreted as a charge density wave, is maintained until $x = 0.15$ Al-level. However, the temperature characteristics of the resistivity change already at very low Al-levels (below 0.03). A coupling of the metal-to-metal transition to the lattice is evidenced by an anomaly in the unit cell dimensions. No long-range magnetic order is detected by powder neutron diffraction. The introduction of the non-magnetic Al$^{3+}$ changes the Ni$^{3+}$/Ni$^{2+}$ ratio and is likely to block double-exchange pathways by means of -Ni-O-Al-O-Ni- fragments into the network of corner shared octahedra with the emergence of possible short-range order in ferromagnetic like islands.

cond-mat.mtrl-sci↗