Ferromagnetic Moment Research Articles

Anomalous magnetic properties of RCrTiO5 (R = Dy and Ho) compounds

We have investigated the nature of magnetic ground state of RCrTiO5 (R = Dy and Ho) through dc magnetization and heat capacity measurements. Due to the strong competition between the Cr3+ and R3+ sublattice moments, several intriguing phenomena have been observed when the magnetic state is probed at low field. In both the systems, the Cr3+ sublattice undergoes a long-range antiferromagnetic ordering below ∼139 K with a weak ferromagnetic (FM) moment perpendicular to c axis as evident from the hysteresis in M(H) curve. At low fields (150 Oe), the zero-field-cooled magnetization shows that the FM component of Cr3+ spin and R3+ moments align in the opposite direction with respect to each other and the net moment aligns in the opposite direction to the applied field in the temperature range 136–16 K for DyCrTiO5 and below 128 K for HoCrTiO5. For both the samples, the strong coupling between the two magnetic sublattices is manifested in the temperature dependence of coercive field. Another interesting phenomenon, the spin reorientation transition, has been observed below K, where the easy axis of FM moment of Cr3+ starts to rotate from one crystallographic axis toward another in DyCrTiO5 but no such transition has been observed in HoCrTiO5. The other members of RCrTiO5 series do not show such kinds of interesting magnetic properties.

Spin switching and unusual exchange bias in the single-crystalline GdCrO3 compensated ferrimagnet

An unusual exchange bias (EB) effect in single-crystalline $\mathrm{GdCr}{\mathrm{O}}_{3}$ compensated ferrimagnet (fM), composed of two antiferromagnetically coupled Gd and Cr sublattices, presenting two opposite ferromagnetic (FM) moments, is reported. It is shown that the temperature- or field-induced fast reversal of the net FM moment in $\mathrm{GdCr}{\mathrm{O}}_{3}$ may be identified as a switching between two opposite spin configurations, attendant with abrupt drop in Zeeman energy $\ensuremath{\approx}{k}_{\mathrm{B}}(7\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}\phantom{\rule{0.16em}{0ex}}\mathrm{K})$ per formula unit. It was found that the FM moment reversal is exchange biased in the narrow temperature interval above the compensation temperature ${T}_{\mathrm{comp}}=144\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Namely, the EB field emerges and diverges upon approaching ${T}_{\mathrm{comp}}$ at temperatures $Tg{T}_{\mathrm{comp}}$ while unexpectedly it collapses to zero below ${T}_{\mathrm{comp}}$. This is in contrast to experimental results obtained for compensated fMs $R\mathrm{Fe}{\mathrm{O}}_{3}\phantom{\rule{4pt}{0ex}}(R=\mathrm{Nd}$, Sm, Er) orthoferrites showing EB in a narrow range, both above and below ${T}_{\mathrm{comp}}$. We suppose that breakdown of EB in $\mathrm{GdCr}{\mathrm{O}}_{3}$ may be linked to the lack of anisotropy of the spin-only Gd $S$-ion magnetic moment dominating below ${T}_{\mathrm{comp}}$.

A new class of spinel high-entropy oxides with controllable magnetic properties

A new class of high-entropy spinel oxides, (Cr0.2Fe0.2Mn0.2Ni0.2Zn0.2)3O4 and (Cr0.2Fe0.2Mn0.2Co0.2Zn0.2)3O4 nanocrystalline powders, has been synthesized by solution combustion synthesis. Their microstructures and magnetic properties were compared with those of (Cr0.2Fe0.2Mn0.2Co0.2Ni0.2)3O4. All the oxides were identified to have the spinel crystal structure (Fd-3m) with average crystallite size of about 24 nm, and their lattice parameters found to decrease with reduction in the electronegativity of Ni2+, Co2+ and Zn2+. Furthermore, nonmagnetic Zn2+ substitution for either magnetic Co2+ or Ni2+ weakened the ferromagnetic ordering and magnetic moments, showing the controllable magnetic property of this new class of high-entropy oxides.

Controlling the strength of ferromagnetic order in YBa2Cu3O7/La2/3Ca1/3MnO3 multilayers

With dc magnetisation and polarized neutron reflectometry we studied the ferromagnetic response of YBa$_2$Cu$_3$O$_7$/La$_{2/3}$Ca$_{1/3}$MnO$_3$ (YBCO/LCMO) multilayers that are grown with pulsed laser deposition. We found that whereas for certain growth conditions (denoted as A-type) the ferromagnetic moment of the LCMO layer is strongly dependent on the structural details of the YBCO layer on which it is deposited, for others (B-type) the ferromagnetism of LCMO is much more robust. Both kinds of multilayers are of similar structural quality, but electron energy-loss spectroscopy (EELS) studies with a scanning transmission electron microscope reveal an enhanced average Mn oxidation state of +3.5 for the A-type as opposed to the B-type samples for which it is close to the nominal value of +3.33. The related, additional hole doping of the A-type LCMO layers, which likely originates from La and/or Mn vacancies, can explain their fragile ferromagnetic order since it places them close to the boundary of the ferromagnetic order at which even weak perturbations can induce an antiferromagnetic or glassy state. On the other hand, we show that the B-type samples allow one to obtain YBCO/LCMO heterostructures with very thick YBCO layers and, yet, strongly ferromagnetic LCMO layers.

Fe-doped ZnO nanoparticles as novel photonic and multiferroic semiconductor

Fe-doped ZnO nanoparticles were synthesized through the chemical method of sol-gel technique. XRD and its Rietveld refinement, high resolution TEM, Electron Diffraction Spectroscopy (EDS) and FTIR spectroscopy have been carried out to obtain the crystal structure, crystal growth direction, surface morphology and chemical composition of the samples. The UV–Visible spectra exhibit blue shift in optical band gap for iron incorporated ZnO nanoparticles. The excitonic peak at 367 nm relating to near band edge emission has been obtained through luminescence spectrum. Dielectric and ferroelectric properties of the studied samples have shown the Debye relaxation and 9% Fe-doped ZnO possess the highest remnant polarization among all the nano-compounds. The Zn0.97Fe0.03O sample exhibits ferromagnetic hysteresis loop with a magnetic moment of 0.01 Bohr Magneton per Fe ion. The Arrot-Belov-Kouvel plot for the samples favour antiferromagnetic character suggesting canted antiferromagnetic order along an axis leading to a weak ferromagnetic moment in the Fe-doped ZnO samples.

Size effects in the formation of an uncompensated ferromagnetic moment in NiO nanoparticles

The magnetic properties of samples of NiO nanoparticles with average sizes of 23, 8.5, and 4.5 nm were investigated. Using the magnetization curves measured in strong (up to 250 kOe) pulsed magnetic fields, the contributions of the free spin and ferromagnetic subsystems were extracted. It has been found that the ferromagnetic contribution increases with a decrease in the nanoparticle size and is proportional to the fraction of uncompensated exchange-coupled spins. It is demonstrated that the uncompensated spins form in the antiferromagnetic NiO oxide due to an increase in the fraction of surface atoms in the nanoparticles with decreasing particle size and defects in the bulk of particles.

Paramagnetic gold in a highly disordered Au-Ni-O alloy

Magnetic materials are usually classified into a distinct category such as diamagnets, paramagnets or ferromagnets. The enormous progress in materials science allows one nowadays, however, to change the magnetic nature of an element in a material. Gold, in bulk form, is traditionally a diamagnet. But in a ferromagnetic environment, it can adopt an induced ferromagnetic moment. Moreover, the growth of gold under certain conditions may lead to a spontaneous ferromagnetic or paramagnetic response. Here, we report on paramagnetic gold in a highly disordered Au–Ni–O alloy and focus on the unusual magnetic response. Such materials are mainly considered for plasmonic applications. Thin films containing Au, Ni and NiO are fabricated by co-deposition of Ni and Au in a medium vacuum of 2 × 10−2 mbar. As a result, Au is in a fully disordered state forming in some cases isolated nanocrystallites of up to 4 nm in diameter as revealed by high resolution transmission electron microscopy. The disorder and the environment, which is rich in oxygen, lead to remarkable magnetic properties of Au: an induced ferromagnetic and a paramagnetic state. This can be proven by measuring the x-ray magnetic circular dichroism. Our experiments show a way to establish and monitor Au paramagnetism in alloys.

Successive phase transitions and magnetization plateau in the spin-1 triangular-lattice antiferromagnet Ba2La2NiTe2O12 with small easy-axis anisotropy

The crystal structure and magnetic properties of the spin-1 triangular-lattice antiferromagnet Ba$_2$La$_2$NiTe$_2$O$_{12}$ are reported. Its crystal structure is trigonal $R\bar{3}$, which is the same as that of Ba$_2$La$_2$NiW$_2$O$_{12}$ [Y. Doi et al., J. Phys.: Condens. Matter 29, 365802 (2017)]. However, the exchange interaction $J/k_{\mathrm{B}}\simeq19$ K is much greater than that observed in the tungsten system. At zero magnetic field, Ba$_2$La$_2$NiTe$_2$O$_{12}$ undergoes successive magnetic phase transitions at $T_{\mathrm{N}1}=9.8$ K and $T_{\mathrm{N}2}=8.9$ K. The ground state is accompanied by a weak ferromagnetic moment. These results indicate that the ground-state spin structure is a triangular structure in a plane perpendicular to the triangular lattice owing to the small easy-axis-type anisotropy. The magnetization curve exhibits the one-third plateau characteristic of a two-dimensional triangular-lattice Heisenberg-like antiferromagnet. Exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate the large difference in the exchange constants between tellurium and tungsten systems and the good two-dimensionality of the tellurium system.

Size-dependent bistability in multiferroic nanoparticles

Most multiferroic materials with coexisting ferroelectric and magnetic order exhibit cycloidal antiferromagnetism with wavelength of several nanometers. The prototypical example is bismuth ferrite (${\mathrm{BiFeO}}_{3}$ or BFO), a room-temperature multiferroic considered for a number of technological applications. While most applications require small sizes such as nanoparticles, little is known about the state of these materials when their sizes are comparable to the cycloid wavelength. This work describes a microscopic theory of cycloidal magnetism in nanoparticles based on Hamiltonian calculations. It is demonstrated that magnetic anisotropy close to the surface has a huge impact on the multiferroic ground state. For certain nanoparticle sizes the modulus of the ferromagnetic and ferroelectric moments are bistable, an effect that may be used in the design of ideal memory bits that can be switched electrically and read out magnetically.

Interface assisted high magnetoresistance in BiFeO3/Fe97Si3 thin film at room temperature

Bilayer thin films of multiferroic oxide BiFeO3 and metallic alloy Fe97Si3 deposited by RF-magnetron sputtering have been investigated to understand spin polarized electron transport across bilayer interface. Effect of BiFeO3 thickness variation on magnetic and magnetotransport properties of bilayer thin films has been analyzed. An induced unidirectional magnetic anisotropy in bilayer thin films has been observed by interface coupling between antiferromagnetic BiFeO3 and ferromagnetic Fe97Si3 spin magnetic moments. Increasing interface roughness with increase in BiFeO3 film thickness has been found to be a regulatory factor for inducing in-plane unidirectional anisotropy and exchange interaction at bilayer interface. A high magnetic moment ∼315 emu/cc originated at room temperature by uncompensated interface spin density has been measured in 30 nm thick BiFeO3 film. Magnetic inhomogeneities at interface give rise to a spin glass like phase responsible for spin flip scattering at bilayer interface. Enhanced low field magnetoresistance ∼30% in 60 nm thick BiFeO3 film is ascribed to intensified scattering from disordered interface states with increasing BiFeO3 thickness. Magnetodielectric measurements (applied magnetic field 1 kOe) have revealed five times increase in relaxation time of charge carriers inside grains confirming intergrain charge transport dominated by alignment of magnetic moments of nearby grains via exchange coupling. High value of magnetoresistance ∼46% in 60 nm thick film of increased grain size is due to enhanced critical length for charge transport. Observed room temperature magnetoresistive properties of BiFeO3/Fe97Si3 thin films are critical for developing multiferroic antiferromagnetic based spintronic devices.

Spin switching temperature modulated by the magnetic field and spontaneous exchange bias effect in single crystal SmFeO3

The spin switching and exchange bias effect were investigated in the rare earth orthoferrite SmFeO3 composed of two antiferromagnetically coupled sublattices Sm3+ and Fe3+ with canted ferromagnetic moments and a temperature induced spin switching in single crystal SmFeO3 was observed. The spin switching temperature was found to be modulated by exerting different magnetic fields below the compensation temperature (). This effect could be explained as the changes of energy barrier related to the magnetization direction under different magnetic fields. In the meantime, the coercivity displayed strong dependence on the maximum applied magnetic fields in the hysteresis measurement. In addition, spontaneous exchange bias effect (EB) was observed with the largest EB field value of 1.2 T, and the EB field changed its sign across the compensation point. Our results indicate that the magnetic properties of SmFeO3 can be strongly affected and controlled by the temperature or the applied magnetic field during the measurement process, and it might lead to novel applications in magneto-optics, ultrafast switching, and magnetic sensing devices.

Antiferroelectricity and weak ferromagnetism in rare earth doped multiferroic BiFeO3

The remanent polarization Pr and remanent magnetization Mr for different Sm-concentrations x in Sm-doped BiFeO3 is investigated using a microscopic model and the Green's function technique. For pure BiFeO3 with a rhombohedral symmetry we obtain a hysteresis loop P(E) which is typical for a ferroelectric material. Further increase of x (x = 0.16) leads to a double hysteresis loop, which is typical for an antiferroelectric substance. Sm substitution induces a weak ferromagnetic moment at room-temperature. Pr and Mr versus x show a maximum at x = 0.05. The ferroelectric phase transition temperature TC decreases strongly whereas the magnetic phase transition temperature TN increases slightly with increasing the Sm concentration x.

First principle calculations of electronic and magnetic properties of Cr doped HgSe

Ab-initio calculations have been performed based on the Korringa-Kohn-Rostoker (KKR) Green's function method combined with coherent potential approximation (CPA) in order to investigate the electronic and magnetic properties of HgSe doped by chromium (Cr). The lattice parameter has been optimized to be 6.5 Å which is close to the experimental value of 6.085 Å. The pure HgSe compound is found to be a semimetal, whereas the Cr dopant introduces an itinerant ferromagnetic moment with the spin polarization at the Fermi surface of around 90%. The Curie temperature (TC) has been estimated using the mean field theory and is found to vary from 326/313 to 485/585 (K) for 8% and 24% of Cr concentration, against the lattice parameter of 6.5/6.085 (Å), respectively. The double exchange is suggested to be the most responsible interaction of ferromagnetism in the system.

Successive Symmetry Breaking in a Jeff = 3/2 Quartet in the Spin–Orbit Coupled Insulator Ba2MgReO6

We report on the cubic double perovskite Ba2MgReO6 containing Re6+ ions with the 5d1 electron configuration. Resistivity, magnetization, and heat capacity measurements using single crystals show that the compound is a Mott insulator with a magnetic transition at Tm = 18 K, which is accompanied by a weak ferromagnetic moment with [110] anisotropy. Another transition is observed at Tq = 33 K in heat capacity, where the inverse of magnetic susceptibility changes its slope, indicating a substantial change in the electronic state. The significance of spin-orbit coupling is revealed by the reduced effective magnetic moment of ~0.68{\mu}B at high temperature above Tq and the total electronic entropy close to Rln4. These features indicate that Ba2MgReO6 is a spin-orbit coupled Mott insulator possessing a Jeff = 3/2 quartet state, which exhibits quadrupolar and dipolar orders at Tq and Tm, respectively.

Comprehensive magnetic phase diagrams of the polar metal Ca3(Ru0.95Fe0.05)2O7

The authors report on comprehensive magnetic phase diagrams of the novel magnetic polar metal Ca${}_{3}$(Ru${}_{0.95}$Fe${}_{0.05}$)${}_{2}$O${}_{7}$. The small amount of Fe acts like a ``magnetic softener'', resulting in incommensurate spin order, in contrast to the collinear magnetic order seen in pure Ca${}_{3}$Ru${}_{2}$O${}_{7}$. The authors show that the complex magnetic order is extremely sensitive to magnetic field and that Ca${}_{3}$(Ru${}_{0.95}$Fe${}_{0.05}$)${}_{2}$O${}_{7}$ can exhibit a net ferromagnetic moment.

Magnetoelectric effects in superconductor/ferromagnet bilayers

We demonstrate that the hybrid structures consisting of a superconducting layer with an adjacent spin-textured ferromagnet demonstrate the variety of equilibrium magnetoelectric effects originating from coupling between the conduction electron spin and superconducting current. By deriving and solving the generalized Usadel equation which takes into account the spin-filtering effect we find that a supercurrent generates spin polarization in the superconducting film which is non-coplanar with the local ferromagnetic moment. The inverse magnetoelectric effect in such structures is shown to result in the spontaneous phase difference across the magnetic topological defects such as a domain wall and helical spin texture. The possibilities to obtain dissipationless spin torques and detect domain wall motion through the superconducting phase difference are discussed.

Anisotropic compression effects on nanocrystalline crystals of nickel oxide

Abstract Nickel oxide (NiO) with bulk crystalline size is an antiferromagnetic insulator correlated strongly with crystal structure. A reduction in crystalline size causes a change in the magnetic sublattice, resulting in the appearance of ferromagnetic moments. Furthermore, in the nano crystals fabricated in mesoporous silica, there is a lattice distortion at the unit cell level that brings about the change in the magnetic property, suggesting a prominent magneto-structural correlation. We investigate anisotropic compression effects on nanocrystalline NiO with a crystalline size (D) of 11.4 nm to observe this remarkable magneto-structural correlation. Magneto-crystalline anisotropy is at its maximum when negative contraction occurs. The negative contraction appears even in a bulk crystal under non-ideal hydrostatic compression, and its anomalous structural change is suppressed with decreasing D.

Magnetic inversion symmetry breaking and spin reorientation in Tb2MnNiO6 : A polar strong ferromagnet

We report a description and comprehensive study on four successive magnetic transitions in ferromagnetic $\mathrm{T}{\mathrm{b}}_{2}\mathrm{MnNi}{\mathrm{O}}_{6}$ double perovskite. In the ground state ($P{2}_{1}^{\ensuremath{'}}$), the moments of magnetic $A$ and $B$ sites order according to different nonpolar magnetic modes, but the coupling between them generates an overall polar symmetry which makes this oxide potentially multiferroic due to magnetic trilinear coupling, and therefore ferromagnetic and ferroelectric in its ground state. Its macroscopic magnetization is large ($5\phantom{\rule{0.16em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}/\mathrm{f}.\mathrm{u}$) and not related to a weak ferromagnetic component induced by Dzyaloshinskii-Moriya interaction. However, a sharp and severe spin reorientation of the ferromagnetic transition-metal moments has been observed which opens the door to the magnetic switching of the ferroelectric state in this perovskite, and conversely to the control of the magnetization direction by electrical fields applied parallel to $b$. We also anticipate that in this material the direction of the magnetization (towards $c/a$) could be used as the key to switch the polar/nonpolar (ferroelectric/antiferroelectric) transformation.

Magnetocaloric effect in Fe2P : Magnetic and phonon degrees of freedom

Devices based on magnetocaloric materials provide great hope for environmentally friendly and energy efficient cooling that does not rely on the use of harmful gasses. Fe2P based compounds are alloys that have shown great potential for magnetocaloric devices. The magnetic behavior in Fe2P is characterized by a strong magnetocaloric effect that coexists with a first-order magnetic transition (FOMT). Neutron diffraction and inelastic scattering, Mossbauer spectroscopy, and first-principles calculations have been used to determine the structural and magnetic state of Fe2P around the FOMT. The results reveal that ferromagnetic moments in the ordered phase are perturbed at the FOMT such that the moments cant away from the principle direction within a small temperature region. The acoustic-phonon modes reveal a temperature-dependent nonzero energy gap in the magnetically ordered phase that falls to zero at the FOMT. The interplay between the FOMT and the phonon energy gap indicates hybridization between magnetic modes strongly affected by spin-orbit coupling and phonon modes leading to magnon-phonon quasiparticles that drive the magnetocaloric effect.

Modulations in magnetostructural coupling in C and Sn deficient Mn3SnC

The structural and magnetic interactions in stoichiometric (Mn$_3$SnC), carbon deficient (Mn$_3$SnC$_{0.8}$) and tin deficient (Mn$_3$Sn$_{0.85}$C) antiperovskite compounds are studied using x-ray absorption fine structure spectroscopy and neutron diffraction. The study confirms the presence of local structural distortions only around Mn atoms in the antiperovskite compounds irrespective of their stoichiometry. The distortions in the Mn$_6$C octahedra are such that only Mn atoms are displaced from their crystallographic positions resulting in long and short Mn-Mn bonds. These long and short Mn-Mn bonds are responsible for presence of ferromagnetic and antiferromagnetic moments on Mn atoms. The C deficiency at the center of the octahedra increases the strain on the Mn$_6$C octahedra and results in a wide variation of Mn-Mn bond distances as a function of temperature and large hysteresis in magnetic properties. On the other hand, Sn deficiency tends to relax strain by giving more space for the octahedra to distort leading to temperature independent Mn-Mn bond distances.