Short-range Magnetic Order Research Articles

Spin reorientation behaviour and dielectric properties of Fe-doped h-HoMnO3

We have studied the magnetic structure, spin reorientation behaviour and dielectric properties of polycrystalline HoMn1−x Fe x O3 (0.0 ⩽ x ⩽ 0.25) compounds using magnetization, neutron diffraction and dielectric measurements. These compounds crystallize predominantly in the hexagonal phase (P63 cm) with a small phase fraction of the orthorhombic phase (Pnma) which increases with increase in dopant concentration and a total suppression of the hexagonal phase is observed at x = 0.25. Doping Fe at the Mn site leads to an increase in the spin reorientation temperature (T SR) from 33 K (x = 0) to 55 K (x = 0.1) while the T N remains nearly constant at 72 K. The magnetic structure of the hexagonal phase was found to be Γ4 (P63′c′m) below T N and Γ3 (P63′cm′) below T SR. The magnetic ordering temperature of Ho3+ ions at 2(a) site appears to coincide with the T SR only in the case of x = 0 sample. The Ho ions at 4(b) site are found to magnetically order below 8 K. The T N of the Ho ions at both 4(b) and 2(a) sites do not appear to be affected by doping at the Mn site. The temperature variation of the Mn and Ho moments follow the Brillioun function dependence albeit with differing values of the molecular field constant λ 0 and λ 1. Short range magnetic order alone was found for the completely orthorhombic sample (x = 0.25). An anomalous suppression of the dielectric constant (ε) at T N is observed in the case of hexagonal samples. Further, a linear correlation between Δε (= ε(T) − ε(0)) and the square of the antiferromagnetic moment M, is observed in these compounds.

Emergent Magnetic Phases in Pressure-Tuned van der Waals Antiferromagnet FePS3

Layered van der Waals 2D magnetic materials are of great interest in fundamental condensed-matter physics research, as well as for potential applications in spintronics and device physics. We present neutron powder diffraction data using new ultrahigh-pressure techniques to measure the magnetic structure of Mott-insulating 2D honeycomb antiferromagnet FePS3 at pressures up to 183 kbar and temperatures down to 80 K. These data are complemented by high-pressure magnetometry and reverse Monte Carlo modeling of the spin configurations. As pressure is applied, the previously measured ambient-pressure magnetic order switches from an antiferromagnetic to a ferromagnetic interplanar interaction and from 2D-like to 3D-like character. The overall antiferromagnetic structure within the ab planes, ferromagnetic chains antiferromagnetically coupled, is preserved, but the magnetic propagation vector is altered from k=(0,1,12) to k=(0,1,0), a halving of the magnetic unit cell size. At higher pressures, coincident with the second structural transition and the insulator-metal transition in this compound, we observe a suppression of this long-range order and emergence of a form of magnetic short-range order which survives above room temperature. Reverse Monte Carlo fitting suggests this phase to be a short-ranged version of the original ambient-pressure structure—with the Fe moment size remaining of similar magnitude and with a return to antiferromagnetic interplanar correlations. The persistence of magnetism well into the HP-II metallic state is an observation in contradiction with previous x-ray spectroscopy results which suggest a spin-crossover transition.1 MoreReceived 9 April 2020Revised 15 September 2020Accepted 28 October 2020DOI:https://doi.org/10.1103/PhysRevX.11.011024Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasMagnetic orderMagnetic phase transitionsMagnetismQuantum phase transitionsPhysical Systems2-dimensional systemsAntiferromagnetsHoneycomb latticeMagnetic systemsMott insulatorsStrongly correlated systemsTechniquesMagnetic techniquesMethods in magnetismNeutron diffractionNeutron techniquesSusceptibility measurementsCondensed Matter, Materials & Applied Physics

Magnetic properties and ferrimagnetic structures of Mn self-doped perovskite solid solutions (Ho1−xMnx)MnO3

A high-pressure synthesis method was employed to prepare (Ho1-xMnx)MnO3 solid solutions with x = 0.2 and 0.3 (at about 6 GPa and 1,670 K) and magnetic properties and structures were investigated by magnetic, dielectric and neutron diffraction measurements. Both samples crystallize in the GdFeO3-type Pnma perovskite structure, and both samples show magnetization reversal behavior below the compensation temperature of about 35 K (at H = 100 Oe). The magnetization reversal phenomena are originated from a ferrimagnetic (FiM) structure which takes place below TC = 76 K (x = 0.2) and 102 K (x = 0.3) and is built from ferromagnetic (FM) ordering of Mn3+ and Mn4+ cations at the B site which are antiferromagnetically (AFM) coupled with Ho3+ and Mn2+ cations at the A site. The magnetic moment of Ho3+ cations increases significantly with deceasing temperature, and it overcomes the saturated magnetic moment of Mn3+ and Mn4+ cations at the B site. Field-induced first-order transitions were found in both compounds below about 35 K. Neutron diffraction measurements on the x = 0.2 sample found a collinear FiM structure between 76 K and 40 K, and the development of spin canting at the A site below 40 K. The coexistence of Ho3+ and Mn2+ at the A-site gives rise to additional short-range magnetic ordering of Ho3+ and to strain effects along the a-direction which are stronger than in (R1−xMnx)MnO3 materials with heavier and smaller rare-earths cations.

Magnetic susceptibility in metals above the Curie temperature

We calculate the magnetic susceptibility in ferromagnetic metals above the Curie temperature TC using the dynamic spin fluctuation theory and make a detailed comparison with experiment. Calculation results for Fe, Co and Ni are found in good agreement with experiment over a wide temperature range. The paramagnetic Curie temperature is found 7–8% larger than the ferromagnetic one in agreement with experiment and various approximations of the Heisenberg model. We calculate the spin-density correlator to show that magnetic short-range order leads to deviation of the susceptibility from the Curie-Weiss law in the temperature interval up to1.1–1.2TC.

Size- and temperature-dependent magnetization of iron nanoclusters

The magnetic behavior of bcc iron nanoclusters, with diameters between 2 and 8 nm, is investigated by means of spin dynamics (SD) simulations coupled to molecular dynamics (MD-SD), using a distance-dependent exchange interaction. Finite-size effects in the total magnetization as well as the influence of the free surface and the surface/core proportion of the nanoclusters are analyzed in detail for a wide temperature range, going beyond the cluster and bulk Curie temperatures. Comparison is made with experimental data and with theoretical models based on the mean-field Ising model adapted to small clusters, and taking into account the influence of low coordinated spins at free surfaces. Our results for the temperature dependence of the average magnetization per atom M(T), including the thermalization of the transnational lattice degrees of freedom, are in very good agreement with available experimental measurements on small Fe nanoclusters. In contrast, significant discrepancies with experiment are observed if the translational degrees of freedom are artificially frozen. The finite-size effects on M(T) are found to be particularly important near the cluster Curie temperature. Simulated magnetization above the Curie temperature scales with cluster size as predicted by models assuming short-range magnetic ordering (SRMO). Analytical approximations to the magnetization as a function of temperature and size are proposed.

Putative quantum critical point in the itinerant magnet ZrFe4Si2 with a frustrated quasi-one-dimensional structure

The Fe sublattice in the compound ZrFe$_4$Si$_2$ features geometrical frustration and quasi-one-dimensionality. We therefore investigated the magnetic behavior in ZrFe$_4$Si$_2$ and its evolution upon substituting Ge for Si and under the application of hydrostatic pressure using structural, magnetic, thermodynamic, and electrical-transport probes. Magnetic measurements reveal that ZrFe$_4$Si$_2$ holds paramagnetic Fe moments with an effective moment $\mu_{\rm eff}= 2.18~\mu_{B}$. At low temperatures the compound shows a weak short-range magnetic order below 6 K. Our studies demonstrate that substituting Ge for Si increases the unit-cell volume and stabilizes the short-range order into a long-range spin-density wave type magnetic order. On the other hand, hydrostatic pressure studies using electrical-resistivity measurements on ZrFe$_4$(Si$_{0.88}$Ge$_{0.12}$)$_2$ indicate a continuous suppression of the magnetic ordering upon increasing pressure. Therefore, our combined chemical substitution and hydrostatic pressure studies suggest the existence of a lattice-volume-controlled quantum critical point in ZrFe$_4$Si$_2$.

Long- and Short-Range Magnetic Order in Titanomagnetite

Abstract—It is shown that the method of random exchange-interaction fields is suitable for identifying the temperature interval where the short-range magnetic order is still present while the long-range order is already absent. On this interval, the state of the magnetic system is a set of ferromagnetic clusters whose magnetic moment can vary with time and ensure magnetic viscosity. In the paper, the Curie point and paramagnetic Curie point are determined for a two-sublattice ferrimagnetic with the distribution of iron ions corresponding to titanomagnetite.

Signatures of magnetostriction and spin-phonon coupling in magnetoelectric hexagonal15R−BaMnO3

Spin-phonon coupling, the interaction of spins with surrounding lattice is a key parameter to understand the underlying physics of multiferroics and engineer their magnetization dynamics. Elementary excitations in multiferroic materials are strongly influenced by spin-phonon interaction, making Raman spectroscopy a unique tool to probe these coupling(s). Recently, it has been suggested that the dielectric and magnetic properties of 15R-type hexagonal BaMnO3 are correlated through the spin-lattice coupling. Here, we report the observation of an extensive renormalization of the Raman spectrum of 15R-BaMnO3 at 230 K, 280 K, and 330 K. Magnetic measurements reveal the presence of a long-range and a short-range magnetic ordering in 15R-BaMnO3 at 230 K and 330 K, respectively. The Raman spectrum shows the appearance of new Raman modes in the magnetically ordered phases. Furthermore, an additional Raman phonon appears below ~ 280 K, possibly arising from a local lattice-distortion due to the displacement of Mn-ions, that exhibits anomalous shift with temperature. The origin of the observed renormalization and phonon anomalies in Raman spectra are discussed based on the evidences from temperature- and magnetic-field-dependent Raman spectra, temperature-dependent x-ray diffraction, magnetization, and specific heat measurements. Our results indicate the presence of magnetostriction and spin-phonon coupling in 15R-BaMnO3 thus suggesting that the optical phonons are strongly correlated to its magnetoelectric properties.

Element-Specific Detection of Sub-Nanosecond Spin-Transfer Torque in a Nanomagnet Ensemble.

Spin currents can exert spin-transfer torques on magnetic systems even in the limit of vanishingly small net magnetization, as recently shown for antiferromagnets. Here, we experimentally show that a spin-transfer torque is operative in a macroscopic ensemble of weakly interacting, randomly magnetized Co nanomagnets. We employ element- and time-resolved X-ray ferromagnetic resonance (XFMR) spectroscopy to directly detect subnanosecond dynamics of the Co nanomagnets, excited into precession with cone angle ≳0.003° by an oscillating spin current. XFMR measurements reveal that as the net moment of the ensemble decreases, the strength of the spin-transfer torque increases relative to those of magnetic field torques. Our findings point to spin-transfer torque as an effective way to manipulate the state of nanomagnet ensembles at subnanosecond time scales.

Anharmonicity of the lattice properties of strongly correlated ferromagnet UGe2

Based on ab initio calculations of the ground state energy as a function of volume E(V) and thermodynamic model of the crystal lattice, we study the role of phonon anharmonicity in the formation of a complex of thermal and elastic properties of UGe2. In the framework of the developed model, an explanation is given for the first time of the experimentally observed abnormal growth of the bulk modulus of UGe2 as a function of temperature. The reassessment of the lattice anharmonicity parameters led to an indication of a significant contribution of optical phonons to the thermal expansion coefficient (CTE) of UGe2, which has not been considered previously. Taking this contribution into account leads to the conclusion that there is a negative non-lattice component of the CTE not only in the region of magnetic ordering but also above the Curie temperature, which indicates the effects of short-range magnetic order.

Short-range magnetic order in the paramagnetic phase of cubic SrMnO3−x ( x<0.005 ): An O17 and Sr87 NMR study

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Magnetic and cryogenic magnetocaloric properties of GdPO4 nanorods

A sample of GdPO4 nanorods (NRs) with average diameter about 20 nm and length 150 nm has been prepared by a hydrothermal method. The analyses of the XRD pattern and transmission electron microscopy images have proved that the NRs are single crystals with a hexagonal structure. The variations of the magnetization versus temperature and magnetic field have confirmed magnetic ordering at low temperatures. The assessment of the magnetocaloric effect from the magnetization isotherms, M(H), in the temperature range T = 2 – 25 K has revealed a strong increase of the magnetic entropy change (|ΔSm|) below T = 15 K with increasing H. The magnetic entropy change reaches maximum (|ΔSmmax|) at temperatures T = 2 – 2.7 K, around a ferromagnetic–paramagnetic transition. It has been found that the relative cooling power (RCP) and refrigerant cooling (RC) increase with increasing H by the broadened linewidth of the |ΔSm(T)| curve. At H = 60 kOe, the values of ΔSmmax and RCP (or RC) are about 56.2 J/kg⋅K and 415.9 (296.5) J/kg, respectively. Our analyses also have inferred that the GdPO4 NRs exhibit a short-range magnetic order with coexisting ferromagnetic and antiferromagnetic interactions.

Magnetocaloric Effect in a Frustrated Gd-Garnet with No Long-Range Magnetic Order.

In this paper, the hyperkagome lattice of Gd spins in a garnet compound, Gd3CrGa4O12, is studied using bulk measurements and density functional computations, and the observation of large magnetocaloric effect corresponding to an entropy change, ΔSm = 45 J kg-1K-1 (≈ 45 J mol-1K-1) at 2 K, 8 T is reported. Though the compound defies long-range magnetic order down to 0.4 K, a broad feature below 10 K is observed in the specific heat with two low temperature anomalies at T* ≈ 0.7 K and TS ≈ 2.45 K. The anomaly at T* is reminiscent of one in Gd3Ga5O12, where it is related to the development of a complex magnetic phase, whereas the TS-peak is accounted for by a multilevel Schottky-like model. The spin-lattice relaxation times studied by nuclear magnetic resonance experiments show that the relaxation is dominated by the magnetic fluctuations in Cr which has a longer relaxation time compared to that of the garnet, Lu3CrGa4O12 containing a nonmagnetic rare earth. Our first-principles density functional theory calculations agree well with the experimental results and support short-range magnetic order in the Gd-sublattice and antiferromagnetism in the Cr-sublattice. The importance of spin fluctuations and short-range order in the rare earth and transition metal lattices in garnets resulting in large magnetocaloric effect is brought out through this work.

Structure, phonons, and orbital degrees of freedom in Fe2Mo3O8

We report on the structural and spectroscopic characterization of the multiferroic Fe$_2$Mo$_3$O$_8$. Synchrotron x-ray and neutron diffraction, as well as thermal expansion measurements reveal a lattice anomaly at $T_{\mathrm{N}}\simeq 60\,$K but do not show any symmetry lowering in the magnetically ordered state. The lattice parameter $c$ exhibits a non-monotonic behavior with a pronounced minimum around $200\,$K, which is also reflected in an anomalous behavior of some of the observed infrared-active optical excitations and parallels the onset of short-range magnetic order. The infrared reflectivity spectra measured between 5 and 300$\,$K in the frequency range of $100-8000\,$cm$^{-1}$ reveal most of the expected phonon modes in comparison with the eigenfrequencies obtained by density-functional calculations. The $A_1$ phonons show an overall hardening upon cooling, whereas a non-monotonic behavior is observed for some of the $E_1$ modes. These modes also show a strongly increased phonon lifetime below $T_\mathrm{N}$, which we associate with the quenched direction of the orbital moment in the magnetically ordered state. A similar increase is observed in the lifetime of the higher-lying $d$-$d$ excitations of the tetrahedral Fe$^{2+}$ site, which become clearly visible below $T_\mathrm{N}$ only.

Magnetic features near filled impurity band in diluted magnetic semiconductors

The antiferromagnetic-ferromagnetic competition in a diluted magnetic semiconductor is discussed. In a virtual-crystal approximation, the active magnetic ions in the system are assumed to distribute homogeneously, and the quantum magnetic correlations are described in the Kondo lattice model involving local disorder. In the framework of the dynamical mean-field theory, we deliver the signatures of the static magnetic susceptibility function and the ${B}_{1g}$ channel Raman response adapting to the model. At low temperatures, one finds the antiferromagnetic instability against the ferromagnetic state when the impurity band is nearly filled. For a completely filled impurity band system, the antiferromagnetic state is stabilized in the case of sufficiently large magnetic exchanges. The effect of the thermal fluctuations on the phase competition is also discussed. Analyzing the ${B}_{1g}$ channel Raman spectrum attributes a competition between the short-range magnetic order and the short-wavelength excitation in the filled impurity band paramagnetic state.

Evolution of thermoelectric properties in EuxYb1−xB6 family

Seebeck effect in the crystalline samples of EuxYb1−xB6 (x = 0, 0.082, 0.127, 0.9, 1) was investigated at temperatures 2–300 K. For all the compounds thermopower is shown to be well described by the sum of diffusion (Sd = AT) and phonon drag components. The latter contribution is induced by quasilocal (Einstein) modes of ytterbium and europium ions with characteristic temperatures ΘE(YbB6) ≈ 91 K and ΘE(EuB6) ≈ 122 K. The estimation of effective mass m* of the charge carriers proves that increasing of Eu content induces crossover from ‘heavy’ holes with mh*(x ⩽ 0.127) ≈ 0.3–0.36 m0 to ‘light’ electrons with me*(x ⩾ 0.9) ≈ 0.12–0.13 m0 (m0−free electron mass). For the Eu-rich compounds we propose the existence of additional point on the phase diagram, which corresponds to short-range magnetic order with enhanced spin fluctuations preceding the stabilization of magnetic polarons.

Spin-chain correlations in the frustrated triangular lattice material CuMnO2

The Ising triangular lattice remains the classic test-case for frustrated magnetism. Here we report neutron scattering measurements of short range magnetic order in CuMnO2, which consists of a distorted lattice of Mn3+ spins with single-ion anisotropy. Physical property measurements on CuMnO2 are consistent with 1D correlations caused by anisotropic orbital occupation. However the diffuse magnetic neutron scattering seen in powder measurements has previously been fitted by 2D Warren-type correlations. Using neutron spectroscopy, we show that paramagnetic fluctuations persist up to ∼25 meV above TN = 65 K. This is comparable to the incident energy of typical diffractometers, and results in a smearing of the energy integrated signal, which hence cannot be analysed in the quasi-static approximation. We use low energy XYZ polarised neutron scattering to extract the purely magnetic (quasi)-static signal. This is fitted by reverse Monte Carlo analysis, which reveals that two directions in the triangular layers are perfectly frustrated in the classical spin-liquid phase at 75 K. Strong antiferromagnetic correlations are only found along the b-axis, and our results hence unify the pictures seen by neutron scattering and macroscopic physical property measurements.

Influence of Ga3+ ion dopant on the structure and magnetic properties of YCrO3

In this work, Ga3+ doped YCr1-xGaxO3 (x = 0, 0.1, 0.2 and 0.3) polycrystalline samples were prepared by solid state reaction method and their crystal structure, morphology, internal defect, specific heat and magnetic properties were studied. The results show that the lattice constants, lattice volume and particle size increase, while Y/Cr–O bond constant decreases with replacing Cr3+ by Ga3+. The positron annihilation spectrum (PAS) measurement shows that cation vacancy defect exists in all samples and the internal defect concentration fluctuates slightly with increasing of Ga3+ ions doping concentration. For low doping content, an additional magnetization anomaly emerges in the zero-field-cooling curves measured in H = 1000Oe. With the increase of magnetic field, the anomaly can be well suppressed. Simultaneously, for the specific heat curve peak cannot be observed at the corresponding temperature. These results imply the establishment of short-range magnetic order above the antiferromagnetic transition temperature (TN) due to the dilution effect of nonmagnetic Ga3+ doping.

High-temperature short-range order in Mn3RhSi

Conventional phase transitions are well understood in terms of the order parameter, based on the Landau–Ginzburg–Wilson theory. However, unconventional magnetic orders have been observed in clean systems such as MnSi. The unconventional magnetic orders of conduction electrons in the metallic phase has been observed for high-temperature superconductors and heavy fermion compounds. However, these unconventional magnetic orders have been limited to relatively low temperatures as quantum phase transitions. Here high-temperature magnetic short-range order is observed as one of the unconventional magnetic orders at temperatures up to 720 K in a noncentrosymmetric intermetallic antiferromagnet Mn3RhSi with a well-ordered lattice. The magnetic Mn ions form a hyperkagome network of corner-sharing triangles, where the spins are geometrically frustrated. The spin network is equivalent to that of a spin liquid and non-Fermi-liquid material, β-Mn. Our observation indicates that a metallic phase with magnetic short-range order exists at high temperatures.

Néel-type antiferromagnetic order and magnetic field–temperature phase diagram in the spin- 12 rare-earth honeycomb compound YbCl3

Most of the searches for Kitaev materials deal with $4d/5d$ magnets with spin-orbit-coupled $J=1/2$ local moments such as iridates and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$. Here we propose the monoclinic ${\mathrm{YbCl}}_{3}$ with a ${\mathrm{Yb}}^{3+}$ honeycomb lattice for the exploration of Kitaev physics. We perform thermodynamic, $ac$ susceptibility, angle-dependent magnetic torque, and neutron diffraction measurements on ${\mathrm{YbCl}}_{3}$ single crystal. We find that the ${\mathrm{Yb}}^{3+}$ ion exhibits a Kramers doublet ground state that gives rise to an effective spin ${J}_{\text{eff}}=1/2$ local moment. The compound exhibits short-range magnetic order below 1.20 K, followed by a long-range N\'eel-type antiferromagnetic order at 0.60 K, below which the ordered ${\mathrm{Yb}}^{3+}$ spins lie in the $ac$ plane with an angle of 16(11)${}^{\ensuremath{\circ}}$ away from the $a$ axis. These orders can be suppressed by in-plane and out-of-plane magnetic fields at around 6 and 10 T, respectively. Moreover, the N\'eel temperature varies nonmonotonically under the out-of-plane magnetic fields, suggesting a reduced spin dimensionality. Together with the strong in-plane magnetic anisotropy and the reduced order moment 0.8(1) ${\ensuremath{\mu}}_{B}$ at 0.25 K, all indicate that ${\mathrm{YbCl}}_{3}$ could be a two-dimensional spin system to proximate the Kitaev physics.