Easy C-axis Research Articles

Two-dimensional quantum universality in the spin-1/2 triangular-lattice quantum antiferromagnet Na2BaCo(PO4)2.

An interplay of geometrical frustration and strong quantum fluctuations in a spin-1/2 triangular-lattice antiferromagnet (TAF) can lead to exotic quantum states. Here, we report the neutron-scattering, magnetization, specific heat, and magnetocaloric studies of the recently discovered spin-1/2 TAF Na2BaCo(PO4)2, which can be described by a spin-1/2 easy axis XXZ model. The zero-field neutron diffraction experiment reveals an incommensurate antiferromagnetic ground state with a significantly reduced ordered moment of about 0.54(2) μB/Co. Different magnetic phase diagrams with magnetic fields in the ab plane and along the easy c-axis were extracted based on the magnetic susceptibility, specific heat, and elastic neutron-scattering results. In addition, two-dimensional (2D) spin dispersion in the triangular plane was observed in the high-field polarized state, and microscopic exchange parameters of the spin Hamiltonian have been determined through the linear spin wave theory. Consistently, quantum critical behaviors with the universality class of d = 2 and νz = 1 were established in the vicinity of the saturation field, where a Bose-Einstein condensation (BEC) of diluted magnons occurs. The newly discovered quantum criticality and fractional magnetization phase in this ideal spin-1/2 TAF present exciting opportunities for exploring exotic quantum phenomena.

Spin-flip-driven reversal of the angle-dependent magnetic torque in layered antiferromagnetic Ca0.9Sr0.1Co2As2

Spin-flip transition can occur in antiferromagnets with strong magnetocrystalline anisotropy, inducing a significant modification of the anisotropic magnetic properties through phase conversion. In contrast to ferromagnets, antiferromagnets have not been thoroughly examined in terms of their anisotropic characteristics. We investigated the magnetic-field and angle-dependent magnetic properties of Ising-type antiferromagnetic Ca0.9Sr0.1Co2As2 using magnetic torque measurements. An A-type antiferromagnetic order emerges below TN = 97 K aligned along the magnetically easy c-axis. The reversal of the angle-dependent torque across the spin-flip transition was observed, revealing the strong influence of the magnetocrystalline anisotropy on the magnetic properties. Based on the easy-axis anisotropic spin model, we theoretically generated torque data and identified specific spin configurations associated with the magnetic torque variation in the presence of a rotating magnetic field. Our results enrich fundamental and applied research on diverse antiferromagnetic compounds by shedding new light on the distinct magnetic features of the Ising-type antiferromagnet.

Large magnetic anisotropy and field-induced spin-flop transition in Fe2(TeO3)2(SO4)·3H2O single crystals

Single crystals of Fe2(TeO3)2(SO4)·3H2O with an alternating spin-chain structure are successfully grown by a hydrothermal method. Magnetic and heat capacity measurements show that this compound possesses a short-range antiferromagetic order at ~55 K and a long-range antiferromagetic order at 32.5 K. A large magnetic anisotropy with magnetization easy c-axis is observed, in which a weak ferromagnetic component is observed along the a- and b-axes, while a spin flop transition is observed at applied field along the c-axis. Also, the magnetic anisotropy energy at 2 K is estimated to be 4.78(3) × 108 ergs/cm3. Such magnetic anisotropy may be due to a strong nonsymmetric distortion in the FeO6 octahedra.

The effect of samarium substitution on magnetic properties and microwave absorption of the rare earth-iron-boron compounds and composites

Soft magnetic composites (SMCs), consisting of micrometer-scale magnetic particles and insulating materials, are extensively used as microwave absorbing materials (MAMs) due to their advantages concerning the lightweight, low cost, manufacturing flexibility, and adjustable microwave properties. SMCs composed of rare earth-iron intermetallics for MAMs are rarely studied. Here we show that Sm substituted (Nd1-xSmx)2Fe14B/paraffin composites exhibit excellent microwave absorption properties with an average value of reflection loss (RL) being below −10 dB at 6–18 GHz. The Sm substitution for Nd not only increases the complex permeability by 23%, decreases the permittivity by 24%, but also improves the characteristic impedance matching, due to their easy magnetization directions (EMDs) change from easy c-axis to c-cone, then to ab-plane with Sm concentration increasing. Thus the microwave absorption properties of these composites are greatly enhanced. Besides, (Nd0.7Sm0.3)2Fe14B/paraffin composite exhibits the strongest microwave absorption with an RL value of −43.8 dB and a small thickness of 1.5 mm at a frequency of 9.1 GHz, which can be one of the promising microwave absorbing materials.

Field-induced magnetic transitions and strong anisotropy in α-CoV2O6 single crystal

The Ising-like antiferromagnet α-CoV2O6 has received considerable interests because of stabilized 1/3 magnetization plateau around 5 K under magnetic field applied along magnetic easy c-axis. In this work, this magnetization plateau was studied by varying temperature or rotating magnetic field. As temperature decreased, this stabilized plateau collapsed, and additional magnetic transitions were observed. As a result, a rich magnetic phase diagram was constructed and extended to temperature lower than previously reported. When magnetic field moved from the c to b (or a) axis, the magnetization plateau developed with field directions and vanished finally when the field was restricted in the ab plane. An impressive observation is that this 1/3-plateau can be stabilized and remain robust even when magnetic field deviated from the c axis, accompanied by the evolutions of the magnetic moments and the critical transition fields. We suppose that the origins of these temperature and angular dependences of the 1/3 magnetization plateau are related to strong spin-orbital coupling. Indeed, electron spin resonance (ESR) measurement gives large Landé factor of 8.9, evidencing that there exists strong spin-orbital coupling.

Large anomalous Hall effect in ferromagnetic Weyl semimetal candidate PrAlGe

We present the structure and anisotropic magnetic and electrical transport properties on the flux-grown PrAlGe single crystal, a recently proposed magnetic Weyl semimetal candidate. From the powder and single-crystal x-ray diffraction analysis, the grown crystal is revealed to crystallize in tetragonal LaPtSi-type structure with space group of I41md. The PrAlGe exhibits strong Ising-type magnetic anisotropy with ferromagnetic moments ∼2.32μB/Pr along easy c-axis below transition temperature Tc ∼ 15 K. Accordingly, anomalous Hall effect (AHE) is observed for field (H) along c axis in contrast to a axis, and large anomalous Hall conductivity value reaches up to ∼680 Ω−1 cm−1 close to the theoretical expected value based on intrinsic Berry-curvature mechanism. Moreover, the linear scaling behaviors between the anomalous Hall resistivity and longitudinal resistivity also support the intrinsic Karplus-Luttinger mechanism as a dominant role on the observed AHE rather than extrinsic scattering mechanism.

Structural, magnetic, and magnetocaloric properties of Fe7Se8 single crystals

The magnetocaloric effect has been studied in high quality single crystals of Fe7Se8 (3c type) grown by using Bridgman’s method. Magnetization and magnetocaloric effect measurements have been carried out in a magnetic field up to 5 T over the temperature range from 2 to 490 K. The spin reorientation transition from the easy c-axis to the easy c-plane, proceeding in an abrupt fashion, as a first-order phase transition, has been observed near the temperature TR ≈ 125 K. The magnetization curves in the vicinity of this transition were shown to have an S-shape with a clear hysteresis. The first order metamagnetic field induced transitions have been identified above and below TR. The conventional magnetocaloric effect related to the metamagnetic transitions has been found above TR, while below TR the inverse magnetocaloric effect was clearly seen. The existence of both kinds of magnetocaloric effect is important from the point of view of large rotating field entropy change in Fe7Se8 single crystals. The refrigeration capacity associated with a second order phase transition from the ferrimagnetic to the paramagnetic state at the Néel temperature TN ≈ 450 K was found to be weaker than that appearing near TR. The giant anisotropy of the magnetocaloric effect was related to the magnetic anisotropy of Fe7Se8 crystals. The one-ion model of the magnetocaloric effect has been developed and its predictions have been compared with experimental data.

Physical properties and field-induced metamagnetic transitions in UAu0.8Sb2

We have successfully synthesized single crystals of UAu0.8Sb2 using a flux method and present a comprehensive study of its physical properties by measuring the magnetic susceptibility, electrical resistivity and specific heat. Evidence for at least three magnetic phases is observed in the field-temperature phase diagram of UAu0.8Sb2. In zero field, the system undergoes an antiferromagnetic transition at 71 K, and upon further cooling it passes through another antiferromagnetic phase with a ferromagnetic component, before reaching a ferromagnetic ground state. A complex magnetic field-temperature phase diagram is obtained for fields along the easy c-axis, where the antiferromagnetic order eventually becomes polarized upon applying a magnetic field.

Magnetic structure of La1-xTbxMn2Si2 compounds

The magnetic structure of La1-xTbxMn2Si2 compounds with 0 ≤ x ≤ 1 has been studied by powder neutron diffraction and magnetization measurements on single-crystal samples. We demonstrate that at 4.2 K, with increasing the Tb concentration, a canted ferromagnetic structure of LaMn2Si2 changes at x > 0.2 to a canted antiferromagnetic structure. The antiferromagnetic in-plane component of Mn moment decreases with increasing concentration x and vanishes at x > 0.5. TbMn2Si2 is characterized by a collinear in-plane Mn ordering and ferrimagnetic structure, in which the Mn sublattice possesses the ferromagnetic interlayer alignment along the easy c-axis. Neutron diffraction study does not reveal a long-range magnetic order of Tb moments for the compounds with x ≤ 0.4. Our results show that for concentrations 0.2 < x ≤ 0.4, a competition of the interlayer Tb-Mn, Mn-Mn exchange interactions and strong uniaxial magnetic anisotropy leads to formation of a frustrated magnetic state of Tb ions, which prevents magnetic ordering in the Tb sublattice.

In-depth study of the [formula omitted] phase diagram of [formula omitted] by magnetization experiments

We present magnetization measurements on Sr4Ru3O10 as a function of temperature and magnetic field applied perpendicular to the magnetic easy c-axis inside the ferromagnetic phase. Peculiar metamagnetism evolves in Sr4Ru3O10below the ferromagnetic transition TC as a double step in the magnetization at two critical fields Hc1 and Hc2. We map the H−T phase diagram with special focus on the temperature range 50K ≤T≤TC. We find that the critical field Hc1(T) connects the field and temperature axes of the phase diagram, whereas the Hc2 boundary starts at 2.8T for the lowest temperatures and ends in a critical endpoint at (1T; 80K). We conclude from the temperature dependence of the ratio Hc1Hc2(T) that the double metamagnetic transition is an intrinsic effect of the material and it is not caused by sample stacking faults such as twinning or partial in-plane rotation between layers.

Micromagnetic investigation of domain and domain wall evolution through the spin-reorientation transition of an epitaxial NdCo5 film

The domain pattern and the domain wall microstructure throughout the spin-reorientation transition of an epitaxial NdCo5 thin film are investigated by micromagnetic simulations. The temperature-dependent anisotropy constants K1 and K2, which define the anisotropy energy term in the model, are chosen to reflect the easy axis—easy cone—easy plane spin-reorientation transition observed in epitaxial NdCo5 thin films. Starting at the high-temperature easy c-axis regime, the anisotropy constants are changed systematically corresponding to a lowering of the temperature of the system. The character of the domain walls and their profiles are analysed. The calculated domain configurations are compared to the experimentally observed temperature-dependent domain structure of an in-plane textured NdCo5 thin film.

Magnetostriction of La0.75Sm0.25Mn2Si2 compound

The magnetostriction and magnetization are studied for a quasi-single crystalline sample of the La0.75Sm0.25Mn2Si2 compound which has the spontaneous first-order antiferromagnetic to ferromagnetic (AF–F) phase transition at a temperature of 160 K. For the AF state, the field-induced AF–F transition is observed in magnetic fields directed both along the easy c-axis and in the basal plane. The transition is accompanied by both the volume change ΔV/V = 2.8 × 10−3 and an anisotropic lattice distortion. The anisotropic magnetostriction is negative in magnetic fields applied along the c-axis and positive, in the basal plane. In small magnetic fields, both the volume and anisotropic magnetostrictions are proportional to the square of magnetization. In magnetic fields applied along the c-axis, the field-induced canting of the magnetic moments leads to much larger magnetoelastic deformations than in the basal plane. The values of the magnetovolume coupling constants both along the c-axis and in the basal plane are in agreement with those estimated from the thermodynamic relationships. Very small volume and linear strains are observed for the ferromagnetic state at 250 K. The obtained results suggest that the magnetoelastic effects in these compounds are mainly due to the change of mutual orientations of the Mn magnetic moments of adjacent layers.

Anisotropic magnetic behaviors of the spin-3/2 chain system γ-CoV2O6

Abstract In this paper, anisotropic magnetic properties of γ-CoV2O6 are investigated by means of susceptibility and magnetization measurements. Our results confirm that γ-CoV2O6 exhibits Ising-spin magnetic behaviors with TN=6 K, in which the c-axis is magnetic easy axis. A 1/3 magnetization plateau is observed at the critical field of ~2500 Oe applied along the c-axis, while two field-induced spin-flop-like transitions are observed along the a- and b-axes. Compared to α-CoV2O6, we suggest that the spin-arrangements in γ-CoV2O6 may not be completely parallel to magnetic easy c-axis, but shift slightly to the a-axis.

Magnetic phase diagram of an Ising spin-chain system α-CoV2O6 with 1/3 magnetization plateau

α-CoV2O6 exhibits a linear chain structure with monoclinic system which does not show any ingredient for geometrical frustration. Magnetic properties of α-CoV2O6 are investigated by means of magnetic and heat capacity measurements. 1/3 magnetization plateau occurs at low temperature while magnetic field is applied along the easy c-axis. Magnetic phase diagram (H vs. T) is constructed by low-temperature susceptibility and magnetization data. On the basis of the model of one-spin flip transition for a two-sublattice antiferromagnet, the spin arrangements of antiferromagnetic, ferrimagnetic and ferromagnetic states are also suggested.

High magnetic field evolution of ferroelectricity inCuCrO2

CuCrO2 offers insights into the different types of spiral magnetic orderings that can form spontaneously due to frustration in triangular-lattice antiferromagnets. We explore the magnetic phase diagram up to 65 T along all the principle axes, and also use electric polarization to probe changes in the spiral order at high magnetic fields. It is known that at zero magnetic field a proper-screw spiral of the Cr S = 3/2 spins forms that in turn induces electric polarization with six possible orientations ab-plane. Applied magnetic fields in the (hard) ab-plane have been shown to induce a transition to cycloidal spiral magnetic order above 5.3 T in those domains that have spins perpendicular to the applied magnetic field. We show that the cycloidal order remains unchanged all the way up to 65 T, which is one quarter of the extrapolated saturation magnetization. On the other hand for magnetic fields along the (easy) c-axis, we observe a transition in the electric polarization near 45 T, and it is followed by a series of steps and/or oscillations in the electric polarization. The data is consistent with the a proper-screw-to-cycloidal transition that is pushed from 5.3 to 45 T by easy-axis anisotropy, and is in turn followed by stretching of the magnetic spiral through commensurate and incommensurate wave vectors. This work also highlights the ability of the magnetically-induced electric polarization to probe complex magnetic orders in regimes of phase space that are difficult to reach with neutron diffraction.

Domain evolution during the spin-reorientation transition in epitaxial NdCo5 thin films

The domain structure and its changes with temperature were investigated for an epitaxial NdCo5 thin film with in-plane texture in which a spin-reorientation transition takes place from the easy c-axis via the easy cone to the easy plane. Scanning electron microscopy with polarization analysis reveals a transition from a two-domain state at temperatures above 318 K via a four-domain state back to a 90°-rotated two-domain state at temperatures below 252 K. The transition temperatures correspond well to those determined by global magnetization measurements. The magnetization configuration at the three different regimes of magnetic anisotropy and its transition with temperature were analysed in detail. From the local measurements, the spin-reorientation angle and the magnetocrystalline anisotropy constants of first and second order were derived.

Prepared and Study of Low Cost and High Performance Nanocomposite (Nd,Dy,Pr)-(Fe,Nb,Zr)-B/α-Fe

The microstructure and magnetic properties of melt-spun nanocomposite (Nd, Dy, Pr)-(Fe,Nb,Zr)-B /α-Fe ribbons have been investigated by XRD, SPM and VSM. The ribbons were prepared at a wheel speed range of 10-30 m/s. The multigrain domain or interaction domain was observed in the samples prepared at 20 m/s wheel speed. The easy magnetized direction c-axis parallel to the ribbon plane for the sample prepared at the wheel speed of 20m/s. The average gain size in this sample is about 18nm and the sample consists of hard magnetic phase (Nd, Dy, Pr) 2(Fe,Nb,Zr)14B and sofe magnetic phase α-Fe, there is strong exchange-coupling interaction between them. The magnetic properties obtained are Br=1.28T，Hci= 504.2kA•m-1，(BH)max= 157.8 kJ•m-3，Mr/Ms =0.76.

Effects of deposition temperature and in-situ annealing time on structure and magnetic properties of (001) orientation FePt films

FePt films were prepared on (100) oriented single crystal MgO substrates at high temperature ranging from 620 until 800°C and in-situ annealed for different times ranging from 0 to 60min to obtain ordered FePt films. The structural analysis indicates that FePt films grow epitaxially on MgO (100) substrates. Both increasing deposition temperature and in-situ annealing time enhance the (001) texture and ordering of FePt films. The magnetic analysis shows that these L10 FePt films have perpendicular anisotropy and the easy magnetization c-axis is perpendicular to the film plane. Magnetization reversal is controlled by a rotational mechanism. The hard magnetic properties of the films are improved with increasing deposition temperature or in-situ annealing time.

Thermal strain and magnetization of the ferromagnetic shape memory alloy Ni52Mn25Ga23 in a magnetic field

The objective of this paper is the investigation of the correlations between crystal structure and the magnetization of Ni–Mn–Ga Heusler alloy. Thermal strain, permeability, and magnetization measurements of the ferromagnetic shape memory alloy Ni52Mn25Ga23 were performed across the martensite transition temperature TM and reverse martensite transition temperature TR at atmospheric pressure. When cooling from the austenite phase, thermal strain steeply decreases because of martensite transition. Permeability suddenly increases at the Curie temperature TC=358K, indicating ferromagnetism, and suddenly decreases to around TM=328K. This indicates that the lattice transformation and magnetic phase transition correspond to each other. The percentage of contraction by martensite transition at TM and in a magnetic field is twice that in zero fields. Considering with other Ni–Mn–Ga alloys, it is supposed that the magnetic field influences the orientation of the easy c-axis along the magnetic field, and then the variant rearrangement occurs, and consequently, the variation in the strain between zero fields and non-zero field is observed. The measurement results indicate that the regions above and below TM or TR are the ferromagnetic-austenite (Ferro-A) and ferromagnetic-martensite (Ferro-M) phases, respectively. Magnetic phase diagrams were constructed from the results of the temperature dependence of thermal strain. TM and TR increase gradually with magnetic field. TM shift in magnetic fields (B) around zero magnetic field was estimated as dTM/dB=0.46K/T, indicating that magnetization influences martensite transition and the dTM/dB value is the same as that of Ni52Mn12.5Fe12.5Ga23, thereby suggesting the Ferro-M to Ferro-A transition.

Dimensionality Driven Spin-Flop Transition in Layered Iridates

Using resonant x-ray diffraction, we observe an easy c-axis collinear antiferromagnetic structure for the bilayer Sr3Ir2O7, a significant contrast to the single layer Sr2IrO4 with in-plane canted moments. Based on a microscopic model Hamiltonian, we show that the observed spin-flop transition as a function of number of IrO2 layers is due to strong competition among intra- and interlayer bond-directional pseudodipolar interactions of the spin-orbit entangled J(eff)=1/2 moments. With this we unravel the origin of anisotropic exchange interactions in a Mott insulator in the strong spin-orbit coupling regime, which holds the key to the various types of unconventional magnetism proposed in 5d transition metal oxides.