Macroscopic Magnetic Measurements Research Articles

Spectroscopic evidence of spin-state excitation in d-electron correlated semiconductor FeSb2

Iron antimonide (FeSb2) has been investigated for decades due to its puzzling electronic properties. It undergoes the temperature-controlled transition from an insulator to an ill-defined metal, with a cross-over from diamagnetism to paramagnetism. Extensive efforts have been made to uncover the underlying mechanism, but a consensus has yet to be reached. While macroscopic transport and magnetic measurements can be explained by different theoretical proposals, the essential spectroscopic evidence required to distinguish the physical origin is missing. In this paper, through the use of X-ray absorption spectroscopy and atomic multiplet simulations, we have observed the mixed spin states of 3d 6 configuration in FeSb2. Furthermore, we reveal that the enhancement of the conductivity, whether induced by temperature or doping, is characterized by populating the high-spin state from the low-spin state. Our work constitutes vital spectroscopic evidence that the electrical/magnetical transition in FeSb2 is directly associated with the spin-state excitation.

Evidence of correlated incommensurate structural and magnetic order in highly oxygen-doped layered nickelate Nd2NiO4.23

$\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ is a nonstoichiometric oxide, crystallizing in the Ruddlesden-Popper-type framework and exhibiting a wide range of oxygen nonstoichiometry $\ensuremath{\delta}$ with a complex structural phase diagram. Excess oxygen atoms insert holes, directly influencing the Ni valence state and associated electronic structure. We report here the existence of a correlated incommensurate structural and antiferromagnetic order below $T{}_{N}\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}$ 150 K in highly oxygen-doped $\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ with $\ensuremath{\delta} \ensuremath{\approx}$ 0.23. The crystal and magnetic structures are investigated by polarized and unpolarized single-crystal neutron diffraction studies together with synchrotron x-ray powder diffraction and macroscopic magnetization measurements. Due to the long-range ordering of excess oxygen atoms, the real structure of the compound is incommensurately modulated, represented by $\mathbf{Q}{}_{n}=\ifmmode\pm\else\textpm\fi{}0.813{\mathbf{a}}^{*}\ifmmode\pm\else\textpm\fi{}0.519{\mathbf{b}}^{*}$ wave vectors. The antiferromagnetic order is characterized by the presence of incommensurate Bragg peaks of type ($h\phantom{\rule{4pt}{0ex}}\ifmmode\pm\else\textpm\fi{}\phantom{\rule{4pt}{0ex}}\ensuremath{\varepsilon}$, 0, $l$/2), with $h$ and $l$ being odd-integer numbers and the magnetic incommensurability $\ensuremath{\varepsilon}\ensuremath{\approx}$ 0.36. The in-plane magnetic correlation length is found to be $\ensuremath{\xi}{}_{ab}$ = 184(1) \AA{} but is significantly reduced along the $c$ axis to $\ensuremath{\xi}{}_{c}$ = 39(2) \AA{}, indicating the quasi-two-dimensional nature of the antiferromagnetic correlations. Our study indicates that the excess oxygen atoms nevertheless strongly enhance the magnetic correlations between the ${\mathrm{NiO}}_{2}$ planes, resulting in a doubling of the magnetic unit cell along the $c$ axis. In contrast to $\mathrm{Nd}{}_{2}\mathrm{NiO}{}_{4.1}$, in which the incommensurate periodicity of oxygen and magnetic ordering was found to be identical, the structural and electronic orderings in heavily oxygen-doped ${\mathrm{Nd}}_{2}{\mathrm{NiO}}_{4.23}$ show two independent modulation vectors. Our study thus unravels a strong correlation between structural and electronic orderings in the highly oxygen-doped $\mathrm{Ln}{}_{2}\mathrm{NiO}{}_{4+\ensuremath{\delta}}$ system.

In-plane magnetic anisotropy and magnetization reversal in phase-separated (La0.5Pr0.5)0.625Ca0.375MnO3 thin films

The dynamics and interaction of different electronic phases near the metal-to-insulator transition of the phase-separated ${({\mathrm{La}}_{0.5}{\mathrm{Pr}}_{0.5})}_{0.625}{\mathrm{Ca}}_{0.375}\mathrm{Mn}{\mathrm{O}}_{3}$ (LPCMO) thin film grown on NGO substrate was studied using the first-order reversal curves (FORC) diagram method for electric transport measurements. The in-plane angle-dependent remanence and coercivity field in the region of the ferromagnet metallic phase was measured using the macroscopic magnetization technique. These measurements suggest an in-plane uniaxial magnetic anisotropy for the film with a uniaxial anisotropic constant (Ku) of $\ensuremath{\sim}1.2\ifmmode\times\else\texttimes\fi{}{10}^{6}\phantom{\rule{0.16em}{0ex}}\mathrm{erg}/\mathrm{c}{\mathrm{m}}^{3}$ at 20 K. The angle dependence of the coercivity is best described by 1/cos\ensuremath{\theta} dependence indicating that the magnetization reversal occurs mainly through the depinning of the domain wall, a signature of nucleation and propagation mechanism. The correlation of FORC measurements, resistance relaxation, and macroscopic magnetic measurements indicate a fast reversal of electronic and magnetic phases towards the denser phase region and strong interaction of different phases for the LPCMO system.

Magnetic structure and spin dynamics of the quasi-two-dimensional antiferromagnet Zn-doped copper pyrovanadate

Magnetic properties of the antiferromagnet Zn$_{0.15}$Cu$_{1.85}$V$_2$O$_7$ (ZnCVO) have been thoroughly investigated on powder and single-crystal samples. The crystal structure determination using powder x-ray and neutron diffraction confirms that ZnCVO with Zn = 0.15 is isostructural with $\beta$-Cu$_{2}$V$_2$O$_7$ ($\beta$-CVO) with small deviation in the lattice parameters. Macroscopic magnetic properties measurements also confirm the similarity between the two compounds. The Cu$^{2+}$ spins were found to align along the crystallographic $c$-axis, antiparallel to their nearest neighbors connected by the leading exchange interaction $J_1$. Spin dynamics reveals a typical symmetric spin-wave dispersion with strong interactions in the $bc$-plane and weak interplane coupling. The exchange interaction analysis indicates that the spin network of ZnCVO is topologically consistent with the previous DFT prediction but the values of leading exchange interactions are contradictory. Furthermore, rather than the predicted 2D honeycomb structure, the spin network in ZnCVO could be better described by the anisotropic 2D spin network composing of $J_1$, $J_5$, and $J_6$ interactions, four bonds per one spin site, coupled by weak interplane interactions.

Field-induced spin cycloidal modulation to antiferromagnetic transition and possible flexomagnetic effect in BiFeO3 nanoparticles

Beyond its various properties, the model multiferroic BiFeO3 (BFO) displays a rich magnetic structure illustrated in the bulk by its long period (∼62 nm) spin cycloidal modulation. Here, BFO nanoparticles are produced by a facile hydrothermal method and show average size of 8 nm and a narrow size distribution, as determined using x-ray diffraction analysis and transmission electron microscopy images. Mössbauer spectrometry (MS) unambiguously reveals that a cycloidal modulation does still exists with particles about 5 times smaller than the bulk cycloid. Combining macroscopic magnetic measurements and in situ Mössbauer spectrometry, we demonstrate that a critical magnetic field of ∼0.2 T destabilizes the cycloidal modulation to lead to a homogenous antiferromagnetic state, as the result of magnetic anisotropy due to magnetoelastic and surface-confinement effects. More interestingly, further increasing of the external magnetic field up to 8 T does not change the average magnetic hyperfine field and results into multiple Mössbauer sextets we propose to explain by a flexomagnetic effect i.e. magnetic anisotropies resulting from strain gradients due to a continuous variation of the coupling between magnetization and the structural distortion from the surface to the particle core.

Optical and magnetic investigation of multiferroic and magnetocaloric properties of Nd0.8Tb0.2Mn2O5

The multiferroic and the rotating magnetocaloric properties of ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ are investigated by microscopic optical probes and macroscopic magnetic measurements. Raman-active phonons as a function of temperature, and ${\mathrm{Nd}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ infrared active crystal-field (CF) excitations as a function of temperature and under magnetic fields up to 11 T have been studied in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$. The obtained results are compared to those of ${\mathrm{NdMn}}_{2}{\mathrm{O}}_{5}$ and ${\mathrm{TbMn}}_{2}{\mathrm{O}}_{5}$ reference compounds. The observation of one set of Raman-active phonons and CF excitations rule out possible twinning while their energy positions and thermal evolutions indicate noticeable changes of Mn1-O3-Mn1 and ${\mathrm{TbO}}_{8}$ structural units. This would explain the nature of separated magnetic phases in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$. The degeneracy of the ground-state Kramers doublet is lifted $({\mathrm{\ensuremath{\Delta}}}_{0}\ensuremath{\sim}9\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\text{--}1})$, indicating that the ${\mathrm{Nd}}^{3+}\text{--}{\mathrm{Mn}}^{3+}$ interaction impacts the magnetic and ferroelectric properties of ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$. The Zeeman splitting of excited crystal-field levels of the ${\mathrm{Nd}}^{3+}$ ions at low temperatures shows that $\mathrm{the}\phantom{\rule{4pt}{0ex}}{g}_{z}$ factor is weak compared to that in ${\mathrm{NdMn}}_{2}{\mathrm{O}}_{5}$. This indicates that the ${R}^{3+}$ spins in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ are mostly aligned within the $ab$-plane. The nature of magnetocrystalline anisotropy in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ as well as in all $R{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ compounds is quantitatively investigated by studying the anisotropy of paramagnetic Curie temperatures along (${\ensuremath{\theta}}_{||}$) and perpendicular (${\ensuremath{\theta}}_{\ensuremath{\perp}}$) to the $c$ axis, $({\ensuremath{\theta}}_{||}\ensuremath{-}{\ensuremath{\theta}}_{\ensuremath{\perp}})$, as a function of the rare-earth atomic number. It is particularly found that the magnetocrystalline anisotropy is mainly determined by the quadrupolar charge distribution of $4f$ shells. The rotating magnetocaloric effect in ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$ is also evaluated and compared to that in ${\mathrm{NdMn}}_{2}{\mathrm{O}}_{5}$ and ${\mathrm{TbMn}}_{2}{\mathrm{O}}_{5}$. Our findings show that Nd- and Tb- separated magnetic phases independently contribute to the magnetocaloric effect of ${\mathrm{Nd}}_{0.8}{\mathrm{Tb}}_{0.2}{\mathrm{Mn}}_{2}{\mathrm{O}}_{5}$.

Magnetic properties of the intercalated compoundLu2Fe3O7

Complex spin and charge order in the frustrated Fe/O bilayers of rare-earth ferrites $R$Fe${}_{2}$O${}_{4}$ and proposed multiferroicity for $R$ = Lu have attracted a lot of interest. The authors focus on $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}r\phantom{\rule{0}{0ex}}c\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}l\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}e\phantom{\rule{0}{0ex}}d$ Lu${}_{2}$Fe${}_{3}$O${}_{7}$, with additional Fe/O single layers inserted between the bilayers. Macroscopic magnetization measurements, polarized neutron scattering, and x-ray magnetic circular dichroism indicate that spin-charge coupling and the local spin arrangement in each individual bilayer is the same as in the nonintercalated compound, with the Fe spins in the single layers giving rise to an additional paramagneticlike contribution.

Atomic Order Along the Half‐ to Full‐Heusler Transition in Ni1+xMnSb

Structural voids are a defining feature of the half‐Heusler structure. Using temperature‐dependent neutron diffraction, complemented by room‐temperature X‐ray diffraction and high‐resolution neutron diffraction as well as calorimetry and macroscopic magnetization measurements, the disordering of these vacancies is followed across the Clb–L21 transition in samples of for various Ni excesses x. The thermal vacancy concentration increases toward high temperatures, and excess vacancy concentrations large enough to increase the low‐temperature macroscopic magnetization can be retained by quenching. Finally, the consequences of the strongly differing relaxation rates of order and vacancy concentrations are demonstrated, showing that in these and related systems the thermal history potentially determines material properties such as ordering temperatures much more strongly than usually.

Influence of Polymorphism on the Magnetic Properties of Co5TeO8 Spinel

This study presents the influence of polymorphism on the magnetic properties of Co5TeO8. This compound with a spinel-like structure [Co2]A[Co3Te]BO8 was synthesized into two polymorphs: one disordered within a cubic Fd3̅m structure, where Co2+ and Te6+ ions are randomly distributed on the octahedral B sites [the disordered polymorph can also be presented as an inverse spinel of the formula Co(Co1.5Te0.5)O4] and the other ordered with a cubic P4332 structure where Co2+ and Te6+ ions are ordered on the B sites. The macroscopic magnetic measurements showed that both polymorphs present a ferrimagnetic ordering, below ∼40 K, and a second transition is also observed at 27 K for the ordered polymorph. Neutron powder diffraction data between room temperature and 1.7 K showed as well the presence of short-range magnetic ordered clusters, which appears for both polymorphs below 200 K. At lower temperature, these short-range orders are transformed into long-range ferrimagnetic orders. Below TC = 40 K, the colinear ferrimagnetic structure of the disordered polymorph is described with the I41/am'd' space group. The ordered polymorph undergoes an incommensurate ferrimagnetic spiral spin ordering below TC1 = 45 K, followed by a second magnetic phase transition at TC2 = 27 K. This last transition is associated with the emergence of an additional ferrimagnetic component and an abrupt change in the magnitude of the magnetic propagation vector k = [0, 0, γ] from γ = 0.086 at T = 30 K to γ ≈ 0.14 in the range between 27 and 1.7 K. The magnetic symmetry of the ordered polymorph is described with the P43(00γ)0 magnetic superspace group. We evidenced that the ordering of Co2+/Te6+ on the B sites changes all of the Co-Co and Co-O distances and thus all JAB, JAA, and JBB exchange interactions, between the A and B sites, which are able to stabilize the incommensurate spin modulation in the ordered polymorph.

Interface-driven static and dynamic magnetic properties of ultrathin Fe/Ge multilayers

Ferromagnet-semiconductor (FM-SC) alloy phase with room temperature ferromagnetic properties at the interfaces of FM/SC heterostructures are believed to improve the electron spin injection from the FM to the SC for the realization of spintronic devices. Here we report the formation of a crystalline alloy phase (Fe3Ge) at the interfaces on growing ultrathin Fe/Ge multilayers at an elevated substrate temperature of ∼ 523 K as well as on annealing the multilayer at a temperature > 523 K. The alloy phase formation at interfaces is achieved at a much lower temperature than the Fe3Ge phase formation in bulk material (∼973 K), mainly due to rapid interdiffusion of Fe and Ge at the interfaces which is about 4 (∼2) order higher in magnitude than the diffusion of Ge (Fe) in the bulk crystalline phase. We find a large magnetic anisotropy constant (K ∼ 3 × 106 to 7 × 107 erg/cm3) on the formation of the nanocrystalline alloy phase at interfaces. Ultrathin Fe/Ge multilayers were characterized using x-ray scattering, macroscopic magnetization measurement, and polarized neutron reflectivity (PNR). The temperature-induced diffusion process indicates that the alloy phase is structurally stable up to higher temperature. PNR provided a detailed depth profile of the magnetization in the system and suggested that the alloy phase is ferromagnetic at room temperature. We have also studied the dynamic magnetic properties of these systems by employing ferromagnetic resonance measurements.

Suppression of the superconducting proximity effect in ferromagnetic-superconducting oxide heterostructures with ion-irradiation

The effect of ion irradiation on the proximity effect in YBa2Cu3O7−δ/La0.67Sr0.33MnO3 and YBa2Cu3O7−δ/SrTiO3/La0.67Sr0.33MnO3 heterostructures has been investigated using spin-polarized neutron reflectivity experiments. We demonstrate that the magnetization in the ferromagnetic (La0.67Sr0.33MnO3) layer at the interface is correlated with the suppression of the superconductivity in the YBa2Cu3O7−δ layer after irradiation, while the layer structure of the heterostructures remains intact. The evolution of the magnetization of the interfacial ferromagnetic layer studied as a function of temperature for both the irradiated heterostructures shows the absence of the proximity effect observed in the un-irradiated samples. The absence of a proximity effect is attributed to the suppression of the superconductivity, as seen in macroscopic magnetization measurements of the heterostructures after ion irradiation.

Revisiting the pinning sites in 2:17-type Sm-Co-Fe-Cu-Zr permanent magnets

It is still an open debate whether the 1:5H cell boundaries (CBs) or the intersections of 1:3R platelets and 1:5H CBs are the strong pining sites for the cellular nanostructured 2:17-type Sm-Co-Fe-Cu-Zr high temperature permanent magnets despite that they have been widely applied in advanced industries since 1970s. Herein, through tuning the volume fraction of Zr-enriched 1:3R platelets by varying the second-step aging time, the pinning behavior in a model magnet Sm25Co44.9Fe21.5Cu5.6Zr3.0 (wt%) was investigated. The results show that the volume fraction of 1:3R platelets can be effectively enlarged without changing the cell size (i.e. the volume fraction of CBs) by extending the aging time at 400 °C. Microscopic TEM characterizations combined with macroscopic magnetic measurements reveals that the locally thickened 1:3R platelets after long-term second-step aging reduce the effective pinning area by interrupting the magnetic domain walls at CBs, weakening the average pinning strength and the coercivity of the magnet. Consequently, our work supports that the 1:5H CBs act as the dominating pinning sites instead of the intersections of 1:3R platelets and 1:5H CBs, which may provide an important insight towards understanding the hard magnetism of pinning-controlled permanent magnets.

Polarity and charge redistribution induced emergent interfacial ferromagnetism in non-magnetic LaNiO3/SrMnO3 superlattices

Artificial heterostructures composed of strongly correlated oxides can host many up-and-coming phenomena. The interfacial ferromagnetism can be observed at the interface of paramagnetic LaNiO3 (LNO) and G-type antiferromagnetic SrMnO3 (SMO). Through the macroscopic magnetic and transport measurements and the microscopic x-ray magnetic circular dichroism signals, the obvious interfacial ferromagnetic behavior exhibited in the thicker superlattices. Using x-ray absorption spectroscopy and x-ray photoelectron spectroscopy measurements, the polar mismatch-induced charge redistribution in Ni and Mn ions accurately demonstrates the dependence on the varying thickness of LNO/SMO superlattices. These results suggest that the ferromagnetic double-exchange interaction of Mn3+–Mn4+ increases and the ferromagnetic super-exchange interaction of Ni2+–Mn4+ decreases with the increasing thickness of LNO/SMO superlattices. Consequently, the obvious ferromagnetic behavior in the thicker superlattices is induced by the double-exchange interaction. For the thinner LNO/SMO superlattice, the interfacial super-exchange ferromagnetic state is offset by the double-exchange ferromagnetism. This is the crucial reason behind the fact that ferromagnetism is hard to exist in the prior LNO/SMO superlattices. Our findings highlight the intrinsic mechanism responsible for the interfacial ferromagnetism and the significance of understanding polar mismatch at the interface of heterostructures.

Relation among Oxygen Stoichiometry, Structure, and Co Valence and Spin State in Single-Layer La2-xAxCoO4±δ (A = Ca, Sr) Perovskites.

We have investigated the role of oxygen stoichiometry and structural properties in the modulation of Co valence and spin state in single-layer La2-xAxCoO4±δ (A = Sr, Ca; 0 ≤ x ≤ 1) perovskites as well as the interplay between their local structural properties and the magnetic and charge-ordering phenomena. We show the results of high angular resolution powder X-ray diffraction and Co K-edge X-ray absorption and emission spectroscopy experiments on polycrystalline and single-crystal samples. The different doping-induced changes in the Co valence and spin state by Ca (or Sr) substitution can be understood in terms of the evolving oxygen stoichiometry. For Ca doping, the interstitial oxygen excess around the La/Ca atoms in underdoped samples is rapidly lost upon increasing the Ca content. The creation of oxygen vacancies leads to the stabilization of a mixed-valence Co2.5+ independently of the Ca content. In contrast, Sr substitution leads to almost stoichiometric samples and a lower oxygen vacancy concentration, which allows higher mixed-valence states for Co up to Co2.9+. The Co mixed-valence state along the two series is fluctuating between two valence states, Co2.4+ as in La2CoO4.2 and Co2.9+ as in LaSrCoO3.91, that become periodically ordered for the charge-ordered phases around the half-doping. The X-ray emission derived spin states agree well with the Co fluctuating mixed-valence state derived from X-ray absorption spectroscopy on consideration of a distribution of high-spin Co2+ and low-spin Co3+. Furthermore, there is no quenching of the orbital contribution for the high-spin Co2+, as concluded from a comparison with macroscopic magnetization measurements. Doping holes are mainly located in the ab plane and have a strong oxygen 2p character. The major lattice distortions, which are different for Sr and Ca doping, occur along the c axis, where changes in the oxygen stoichiometry take place. Moreover, charge-order transitions are clearly shown from the anomalous increase of the c lattice parameter with an increase in the temperature above 500 K but there is no signature for a temperature-dependent spin-state transition.

Emergent Interfacial Ferromagnetism and Exchange Bias Effect in Paramagnetic/Ferromagnetic Oxide Heterostructures

AbstractThe experimental evidence of the exchange bias (EB) effect in epitaxial heterostructures of ferromagnetic La0.67Sr0.33MnO3 (LSMO) and paramagnetic LaNiO3 (LNO) grown on MgO(001) substrates is reported. The macroscopic magnetization measurements show a clear indication of EB in LNO/LSMO and LSMO/LNO/LSMO heterostructures. The strength of the exchange coupling rapidly reduces as the temperature is increased and almost vanishes above 50 K. Using spin dependent polarized neutron reflectometry (PNR) detailed depth dependent magnetization profiles of the heterostructures are investigated to understand the role of interface magnetism in the observed EB. PNR results establish the emergence of the interfacial ferromagnetic LNO layer of thickness ≈22–35 Å (≈6–9 unit cell) and magnetization ≈80 emu cc−1 at the LNO/LSMO interface for T < 100 K. The PNR results also show both ferromagnetic and anti‐ferromagnetic coupling between the LSMO and interfacial LNO layers, which may be dependent on the stacking order. The emergence of ferromagnetism in the interfacial paramagnetic LNO layer with unexpected EB in these heterostructures suggests a promising platform toward the effective realization of magnetic devices.

Field dependent helical magnetic structure in a compensated Gd/Co multilayer

Rare earth –transition metal heterostructures are known to exhibit a number of magnetic structures at different temperatures and magnetic fields due to the strong antiferromagnetic interaction at the interfaces. Using spin-dependent specular and off-specular polarized neutron reflectivity (PNR), we observed the field-dependent twisted helical magnetic structure with a planar 2π domain wall (DW) in a Gd/Co multilayer. Macroscopic magnetization measurements revealed the field-dependent compensation temperature, which increases with an increase in the applied field. Specular PNR at 5 K, after field cooling the multilayer in an applied field of 100 Oe, reveals the formation of planar 2π DWs and antiferromagnetic coupling at the interfaces. Off-specular PNR data from multilayer at 5 K also revealed the development of magnetic inhomogeneities for the central part (thickness ~ 25 Å) of each Gd layer in the multilayer. In contrast, a similar magnetic property of the multilayer was observed at a higher temperature (125 K) on applying a field of 500 Oe. The field-dependent tunability of twisted helical magnetic structures with planar 2π DWs and the development of highly correlated magnetic inhomogeneities may be key for application in spin-based technology.

Critical size limits for collinear and spin-spiral magnetism in CoCr2O4

The multiferroic behavior of CoCr$_2$O$_4$ results from the appearance of conical spin-spiral magnetic ordering, which induces electric polarization. The magnetic ground state has a complex size dependent behavior, which collapses when reaching a critical particle size. Here, the magnetic phase stability of CoCr$_2$O$_4$ in the size range of 3.6 - 14.0 nm is presented in detail using the combination of neutron diffraction with XYZ polarization analysis and macroscopic magnetization measurements. We establish critical coherent domain sizes for the formation of the spin spiral and ferrimagnetic structure and reveal the evolution of the incommensurate spin spiral vector with particle size. We further confirm the presence of ferroelectric polarization in the spin spiral phase for nanocrystalline CoCr$_2$O$_4$.

Structural disorder and magnetic correlations driven by oxygen doping in Nd2NiO4+δ ( δ∼0.11 )

We investigated the influence of oxygen over-stoichiometry on apical oxygen disorder and magnetic correlations in Nd2NiO4+d (d~0.11) in the temperature range of 2-300 K by means of synchrotron x-ray powder diffraction, neutron single crystal and powder diffraction studies, combined with macroscopic magnetic measurements. In the investigated temperature range, the compound crystalizes in a tetragonal commensurate structure with the P42/ncm space group with excess oxygen atoms occupy the 4b (3/4 1/4 1/4) interstitial sites, coordinated by four apical oxygen atoms. Large and anisotropic thermal displacement parameters are found for equatorial and apical oxygen atoms, which are strongly reduced on an absolute scale compared to the Nd2NiO4.23 phase. Maximum Entropy analysis of the neutron single crystal diffraction data uncovered anharmonic contributions to the displacement parameters of the apical oxygen atoms, toward the nearest vacant 4b interstitial site, related to the phonon assisted oxygen diffusion mechanism. Macroscopic magnetization measurements and neutron powder diffraction studies reveal long-range antiferromagnetic ordering of the Ni-sublattice at TN ~ 53 K with a weak ferromagnetic component along the c-axis, while the long-range magnetic ordering of the Nd-sublattice occurs below 10 K. Temperature dependent neutron diffraction patterns show the appearance of a commensurate magnetic order at TN with the propagation vector k = (100) and the emergence of an additional incommensurate phase below 30 K, while both phases coexist at 2 K. The commensurate magnetic structure is best described by the P42/nc`m` Shubnikov space group. Refined magnetic moments of the Ni and Nd-sites at 2 K are 1.144(76) muB and 1.632(52) muB respectively. A possible origin of the incommensurate phase is discussed and a tentative magnetic phase diagram is proposed.

Nanoscale measurement of giant saturation magnetization in α″-Fe16N2 by electron energy-loss magnetic chiral dichroism

Metastable α″-Fe16N2 thin films were reported to have a giant saturation magnetization of above 2200 emu/cm3 in 1972 and have been considered as candidates for next-generation rare-earth-free permanent magnetic materials. However, their magnetic properties have not been confirmed unequivocally. As a result of the limited spatial resolution of most magnetic characterization techniques, it is challenging to measure the saturation magnetization of the α″-Fe16N2 phase, as it is often mixed with the parent α′-Fe8N phase in thin films. Here, we use electron energy-loss magnetic chiral dichroism (EMCD), aberration-corrected transmission electron microscopy, X-ray diffraction and macroscopic magnetic measurements to study α″-Fe16N2 (containing ordered N atoms) and α′-Fe8N (containing disordered N atoms). The ratio of saturation magnetization in α″-Fe16N2 to that in α′-Fe8N is determined to be 1.31 ± 0.10 from quantitative EMCD measurements and dynamical diffraction calculations, confirming the giant saturation magnetization of α″-Fe16N2. Crystallographic information is also obtained about the two phases, which are mixed on the nanoscale.

Element selective magnetism in Ho0.5Nd0.5Fe3(BO3)4 single crystal probed with hard X-ray magnetic circular dichroism

Abstract We present a study of a Ho 0.5 Nd 0.5 Fe 3 ( BO 3 ) 4 single crystal aiming at understanding the roles of each magnetic sublattices in magnetic transitions at low temperatures. The crystal structure is determined to have the R32 symmetry in whole studied temperature range but the crystal appears to be racemic. Element selective magnetic properties studied with X-ray magnetic circular dichroism at the L 2 , 3 -edges of Ho and Nd, and at the Fe K-edge at T = 3 K under magnetic fields of ±17 T are compared with the results of macroscopic magnetization measurements in pure and substituted crystals. All three magnetic sublattices are shown to be strongly coupled and to undergo a spin-reorientation transition when magnetic field is applied along the trigonal c-axis. On the contrary, when magnetic field is applied in the ab-plane only the holmium atoms exhibit a sizeable magnetization jump. This result allows us to conclude that the spin-flop transitions observed in macroscopic magnetization curves and associated anomalies of electric polarization are due to the Ho3+ magnetic moment reorientation.