Antiferromagnetic Component Research Articles

Polycrystalline MnGe2 thin films on InAs(001) substrates

We report on the growth of MnGe2 thin films on InAs(001) substrates using radio frequency-magnetron co-sputtering. Polycrystalline thin films were obtained at a substrate temperature of 353 K. X-ray diffractometry was used to identify the tetragonal MnGe2 phase (space group I4/mcm). Measurements of the magnetic field and temperature dependence of the magnetization revealed that the MnGe2 thin films are ferromagnetic with a magnetization of 280 kAm−1 and a Curie temperature of 62 K. Additionally, an antiferromagnetic component is observed at low temperatures, which may arise atomic disorder at the grain boundaries between MnGe2 crystallites.

Giant exchange bias effect with low-coercivity in YbBaCo4O7

We observe significant exchange-bias effect below ∼ 76 K in antiferromagnetic YbBaCo4O7. Exchange-bias field (HE) is ∼ 10 kOe for 50 kOe cooling-field (Hcool) at 4 K. The HE increases considerably with Hcool associated with the nominal increase of coercivity (HC). Low HC provides high value of HE/HC (∼ 15) at 4 K. High HE/HC ratio and nominal increase of HC are appealing. The results suggest robust EB effect associated with the minimal occurrence of inhomogeneous micro-domains, which is potential for the applications. Magnetic phase separation between antiferromagnetic components with k1 = (0, 0, 0) and k2 = (1/2, 0, 0) propagation vectors of the Pbn21 space group is correlated with the exchange-bias effect.

Magnetic Structure and Magnetotransport Properties of La0.7Sr0.3Mn1 – xNi x O3

La0.7Sr0.3Mn1 – xNi x O3 (0.12 ≤ x ≤ 0.35) compositions have been studied using neutron diffraction, magnetometry, and measurements of magnetotransport properties. At temperatures of 5–300 K, these compounds were found to have a rhombohedral crystal structure. The substitution of nickel for manganese has been shown to result in a decrease in the Curie temperature from 278 K (х = 0.12) to 60 K (х = 0.3); in this case, the spontaneous magnetization of the compositions decreases to zero (x = 0.33). The magnetoresistive effect for the semimetals with 0.12 ≤ x < 0.18 increases near the Curie temperature, whereas the magnetoresistance of semiconducting compositions with х ≥ 0.2 progressively decreases as the temperature increases. For compositions with х ≥ 0.25, an antiferromagnetic G-type component has been found by neutron diffraction, the Neel temperature of which reaches 260 K (at х = 0.35). The study of the La1–ySr y Mn0.65Ni0.35O3 (y ≤ 0.3) system showed that the content of ferromagnetic component decreases with increasing Sr content. It has been inferred that the antiferromagnetism of the compositions with х > 0.25 is due to the strong negative exchange interactions Ni2+–О–Ni2+ and Mn4+–О–Mn4+ and the absence of ionic order. The obtained data have been used to construct the magnetic phase diagram of the La0.7Sr0.3Mn1–xNi x O3 (0.12 ≤ x ≤ 0.35) system.

Magnetic properties of Heusler compound Fe1.3Mn1.7Si

The magnetic properties of two Heusler compounds Fe1.3Mn1.7Si and Fe1.4Mn1.6Si are compared. Fe1.4Mn1.6Si exhibits a ferromagnetic transition at TC ∼ 117 K and another transition to a phase with antiferromagnetic components at TA ∼ 66 K, which is the same as the behavior of Fe2MnSi. Further, it is known that for Fe3−xMnxSi, with increasing x, TC decreases but TA remains almost constant, and they appear to meet around x ∼1.7. The careful measurements of magnetization at various magnetic fields reveal that Fe1.3Mn1.7Si has three transitions at low fields; for instance, at 0.01 T, the transition temperatures are found to be 85 K, 67 K, and 60 K. The lowest transition at 60 K is characterized by a rapid decrease in magnetization with decreasing temperature and significant magnetic field dependence, in contrast to the transition at TA, which is relatively independent of the magnetic field, for x = 1 and 1.6. The critical field at 0 K is ∼3 T. The transition at the lowest temperature was not found for x ≤ 1.6, and it is considered a different transition from that at TA. While the transition at the highest temperature 85 K is a ferromagnetic transition, the transition at 67 K has the characteristics of the transition at TA, and it is found to be insensitive to magnetic fields. They are revealed to be the same transitions as those at TC and TA for x ≤ 1.6.

Magnetization and magnetic phase diagram of Heusler compounds Fe3−y(Mn1−xVx)ySi (y = 1 and 1.5)

Fe2MnSi exhibits a ferromagnetic transition at TC and another transition to a phase with antiferromagnetic components at a low temperature of TA. By substituting V for Mn, so as to obtain Fe2Mn1−xVxSi, TA decreases with x and then vanishes around x ∼ 0.2. In this study, the magnetic transitions are investigated by magnetization measurements for Fe2Mn1−xVxSi (y = 1, 0 ≤ x ≤ 0.2) in high magnetic fields up to ∼70 T and for Fe1.5(Mn1−xVx)1.5Si (y = 1.5, 0 ≤ x ≤ 0.1) up to 5 T. For y = 1.5, with increasing x, TC increases and TA decreases as for y = 1, and the rate of the decrease in TA with x is slightly smaller than for y=1. In the low magnetic field region, TA for y = 1 does not significantly change due to the magnetic field but a distinct decrease is observed in high fields. The critical field at 0 K of this transition in Fe2MnSi is found to be larger than 70 T, and with increasing x, the critical field decreases corresponding to the decrease of TA at zero field.

Magnetization reversal and tunable exchange bias behavior in Mn-substituted NiCr2O4

Single phase samples of Ni(Cr1−xMn x )2O4 (x = 0–0.50) were synthesized by using sol–gel route. Investigation of structural, magnetic, exchange bias and magnetization reversal properties was carried out in the bulk samples of Ni(Cr1−xMn x )2O4. Rietveld refinement of the X-ray diffraction patterns recorded at room temperature reveals the tetragonal structure for x = 0 sample with I41/amd space group and cubic structure for x ≥ 0.05 samples with \( {\text{Fd}\bar{3}\text{m}} \) space group. Magnetization measurements show that all samples exhibit ferrimagnetic behavior, and the transition temperature (TC) is found to increase from 73 K for x = 0 to 138 K for x = 0.50. Mn substitution induces magnetization reversal behavior especially for 30 at% of Mn in NiCr2O4 system with a magnetic compensation temperature of 45 K. This magnetization reversal is explained in terms of different site occupation of Mn ions and the different temperature dependence of the magnetic moments of different sublattices. Study of exchange bias behavior in x = 0.10 and 0.30 samples reveals that they exhibit negative and tunable positive and negative exchange bias behavior, respectively. The magnitudes of maximum exchange bias field of these samples are found to be 640 and 5306 Oe, respectively. Exchange bias in x = 0.10 sample originates from the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic moment. The tunable exchange bias behavior in x = 0.30 sample is explained in terms of change in domination of one sublattice moment over the other as the temperature is varied.

Revival of Metal-Insulator and Ferromagnetic-Paramagnetic Transitions by Ni Substitution at Mn Site in Charge-Ordered Monovalent Doped Nd0.75Na0.25MnO3 Manganites

Nd0.75Na0.25Mn1−xNi x O3 (x = 0 − 0.07) were synthesized using a conventional solid-state synthesis method to investigate the effect of Ni substitution on their magnetic and electrical transport properties. XRD analysis using the Rietvield refinement method showed an increase in unit cell volume with increasing Ni content, indicating the possibility of substituting Ni for the Mn site. For x = 0, magnetization measurements showed paramagnetic (PM) to antiferromagnetic (AFM) transition at the transition temperature, TN ∼ 185 K, which may be related to the presence of charge-ordering (CO) state. For Ni-substituted samples, ferromagnetic (FM) to PM transition was induced with Curie temperature (TC), increasing from 91 K (x = 0.01) to 115 K (x = 0.03), followed by decreases in TC to 60 K (x = 0.07). Electrical resistivity measurements showed that samples for x ≤ 0.01 exhibited insulator behavior while both x = 0.03 and 0.05 samples exhibited metal-insulator transition suggesting that the CO state is weakened. For x = 0.07, the sample shows re-entrant of insulating characteristic. Inducement of FM metallic (FMM) state, as well as increased of TC for x = 0.03 is suggested to be related to FM SE between Ni2+ and Mn4+, which suppressed the CO state, hence inducing double-exchange (DE)-like mechanism. In addition, an increase in resistivity was observed as temperature decreased below the resistivity minimum (Rmin) at Tmin ∼ 40 K for x = 0.05 is possibly due to the Kondo-like effect mechanism. On one hand, the M(H) curve showed the presence of a hysteresis loop for x = 0 and 0.01 at T= 60 and 80 K, indicating the presence of an unstable phase, which may be related with the CO-AFM phase. For x = 0.03, no hysteresis loop was observed, indicating the suppression of the CO-AFM. Intriguingly, for x = 0.05 and 0.07, small hysteresis loop was observed, indicating the possible presence of a minor AFM component. Further, the observed MR behavior in present studies can be understood as a result of a decreased scattering mechanism in the presence of magnetic field.

Magnetism in Mn-nanowires and -clusters as δ-doped layers in group IV semiconductors (Si, Ge)

Mn doping of group-IV semiconductors (Si/Ge) is achieved by embedding nanostructured Mn-layers in group-IV matrix. The Mn-nanostructures are monoatomic Mn-wires or Mn-clusters and capped with an amorphous Si or Ge layer. The precise fabrication of δ-doped Mn-layers is combined with element-specific detection of the magnetic signature with x-ray magnetic circular dichroism. The largest moment (2.5 μB/Mn) is measured for Mn-wires with ionic bonding character and a-Ge overlayer cap; a-Si capping reduces the moment due to variations of bonding in agreement with theoretical predictions. The moments in δ-doped layers dominated by clusters is quenched with an antiferromagnetic component from Mn–Mn bonding.

Synthesis, structural characterization, and magnetic properties of mixed ternary spinel-type Mn-Ni-Co oxides

Mixed ternary spinel-type oxides MnxNi1-xCo2O4 (x = 0, 0.5, 1) were synthesized using a citrate sol-gel route. The morphologies and structures of the mixed oxide nanoparticles were characterized using Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD) and Fourier-Transform Infrared Spectroscopy (FTIR). Structural analysis from XRD and FTIR confirm the formation of single phase cubic spinels. TEM and SEM images show the formation of interconnected nanoparticles. The particle sizes obtained from TEM and XRD are consistent with each other. It was observed that the lattice constant increases with increasing Mn concentration from x=0 to x=0.5 and then decreases with further increase in Mn concentration to x=1. Magnetization measurements were carried out using Vibrating Sample Magnetometery (VSM), which showed that the oxides have weak ferromagnetic behavior with the presence of an antiferromagnetic component. The oxide with the highest Mn substitution was observed to be superparamagnetic at room temperature.

Neutron diffraction study of the inverse spinels Co2TiO4 and Co2SnO4

We report a detailed single-crystal and powder neutron diffraction study of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$ between the temperature 1.6 and 80 K to probe the spin structure in the ground state. For both compounds the strongest magnetic intensity was observed for the (111)${}_{\text{M}}$ reflection due to ferrimagnetic ordering, which sets in below ${T}_{\text{N}}=48.6$ and 41 K for ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ and ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$, respectively. An additional low intensity magnetic reflection (200)${}_{\text{M}}$ was noticed in ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ due to the presence of an additional weak antiferromagnetic component. Interestingly, from both the powder and single-crystal neutron data of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$, we noticed a significant broadening of the magnetic (111)${}_{\text{M}}$ reflection, which possibly results from the disordered character of the Ti and Co atoms on the $B$ site. Practically, the same peak broadening was found for the neutron powder data of ${\mathrm{Co}}_{2}{\mathrm{SnO}}_{4}$. On the other hand, from our single-crystal neutron diffraction data of ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$, we found a spontaneous increase of particular nuclear Bragg reflections below the magnetic ordering temperature. Our data analysis showed that this unusual effect can be ascribed to the presence of anisotropic extinction, which is associated to a change of the mosaicity of the crystal. In this case, it can be expected that competing Jahn-Teller effects acting along different crystallographic axes can induce anisotropic local strain. In fact, for both ions ${\mathrm{Ti}}^{3+}$ and ${\mathrm{Co}}^{3+}$, the $2{t}_{g}$ levels split into a lower ${d}_{xy}$ level yielding a higher twofold degenerate ${d}_{xz}/{d}_{yz}$ level. As a consequence, one can expect a tetragonal distortion in ${\mathrm{Co}}_{2}{\mathrm{TiO}}_{4}$ with $c/a&lt;1$, which we could not significantly detect in the present work.

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.

Ferromagnetic–antiferromagnetic transition and magnetotransport properties of La0.7Sr0.3Mn1−xNixO3 perovskites

The crystal structure and magnetic properties of polycrystalline compounds La0.7Sr0.3Mn1−xNixO3 (x ⩽ 0.37) were studied by neutron powder diffraction and magnetometry measurements, and the magnetotransport properties were also investigated, focusing on the magnetic state of the compounds. The crystal structure of the compounds remains rhombohedral upon chemical doping with Ni ions in the mentioned concentration range within the temperature interval 5–300 K. Doping with nickel ions causes a decrease in magnetization, and the Curie point drops from ~290 K for x = 0.1 compound down to ~70 K for x = 0.3 wherein the electrical resistivity of the compounds increases. The compounds with the dopant concentration x < 0.2 are characterized by pronounced magnetoresistance near the Curie points; the compounds with x ⩾ 0.2 show a gradual decrease in the magnetoresistance upon temperature increase. Neutron diffraction measurements of the compounds with x ⩾ 0.25 have showed a gradual reduction in ferromagnetic component, while an antiferromagnetic component of the G-type appears and progressively strengthens with an increase in the dopant concentration. The inhomogeneous magnetic state of the compounds with 0.25 < x ⩽ 0.3, attested by magnetization and neutron diffraction measurements, is governed by a competition between positive and negative exchange interactions formed between Ni and Mn ions depending on their oxidation state and electron configuration.

Magnetic order, hysteresis, and phase coexistence in magnetoelectric LiCoPO4

The magnetic phase diagram of magnetoelectric LiCoPO$_4$ is established using neutron diffraction and magnetometry in fields up to 25.9T applied along the crystallographic $b$-axis. For fields greater than 11.9T the magnetic unit cell triples in size with propagation vector Q = (0, 1/3, 0). A magnetized elliptic cycloid is formed with spins in the $(b,c)$-plane and the major axis oriented along $b$. Such a structure allows for the magnetoelectric effect with an electric polarization along $c$ induced by magnetic fields applied along $b$. Intriguingly, additional ordering vectors Q $\approx$ (0, 1/4, 0) and Q $\approx$ (0, 1/2, 0) appear for increasing fields in the hysteresis region below the transition field. Traces of this behavior are also observed in the magnetization. A simple model based on a mean-field approach is proposed to explain these additional ordering vectors. In the field interval 20.5-21.0T, the propagation vector Q = (0, 1/3, 0) remains but the spins orient differently compared to the cycloid phase. Above 21.0T and up until saturation a commensurate magnetic structure exists with a ferromagnetic component along $b$ and an antiferromagnetic component along $c$.

Single crystal polarized neutron diffraction study of the magnetic structure of HoFeO3

Polarised neutron diffraction measurements have been made on HoFeO3 single crystals magnetised in both the [0 0 1] and [1 0 0] directions (Pbnm setting). The polarisation dependencies of Bragg reflection intensities were measured both with a high field of T parallel to [0 0 1] at K and with the lower field T parallel to [1 0 0] at K. A Fourier projection of magnetization induced parallel to [0 0 1], made using the hk0 reflections measured in 9 T, indicates that almost all of it is due to alignment of Ho moments. Further analysis of the asymmetries of general reflections in these data showed that although, at 70 K, 9 T applied parallel to [0 0 1] hardly perturbs the antiferromagnetic order of the Fe sublattices, it induces significant antiferromagnetic order of the Ho sublattices in the plane, with the antiferromagnetic components of moment having the same order of magnitude as the induced ferromagnetic ones. Strong intensity asymmetries measured in the low temperature structure with a lower field, 0.5 T [1 0 0] allowed the variation of the ordered components of the Ho and Fe moments to be followed. Their absolute orientations, in the domain stabilised by the field were determined relative to the distorted perovskite structure. This relationship fixes the sign of the Dzyalshinski–Moriya (D-M) interaction which leads to the weak ferromagnetism. Our results indicate that the combination of strong y-axis anisotropy of the Ho moments and Ho-Fe exchange interactions breaks the centrosymmetry of the structure and could lead to ferroelectric polarization.

Antiferromagnet-ferromagnet transition in La1–xSrxMn0.5Ni0.5O3 (0 ≤ x ≤ 0.2) ceramics

Ceramics samples of La1–xSrxMn0.5Ni0.5O3 (0 ≤ x ≤ 0.2) with perovskite structure have been studied by neutron diffraction method, magnetometry and magnetoresistance measurements. Structural data testify a partial order of the Ni and Mn ions in all the compounds in spite of chemical substitution of the La3+ ions by Sr2+ ones and an increase of the oxidation state of the Ni ions from 2+ towards to 3+ one. Magnetic structure of the compounds changes from ferromagnetic one (specific for the compound with x = 0) to antiferromagnetic (compounds with x ≥ 0.1) while the temperature of a transition into paramagnetic state remains about 270 K for all the samples. Magnetoresistance gradually decreases with temperature increase and strontium content. The results are discussed assuming nearly equal ferromagnetic and antiferromagnetic components of the superexchange interaction Ni2+–O–Mn4+ while the interaction between Ni3+ and Mn4 ions is considered to be strongly antiferromagnetic.

W3CoB3-type {Y, Gd - Ho}3Co4−xAlx (x=0.5−1) rare earth compounds: Specific features of crystal structure and magnetic ordering

The crystal structure of new W3CoB3-type {Y, Gd - Ho}3Co3.25Al0.75, Gd3Co3.5Al0.5 and Tb3Co3Al compounds (Cmcm. N 63, oC28) has been established using powder X-ray diffraction studies. The magnetic properties of Gd3Co3.5Al0.5, Gd3Co3.25Al0.75 and Tb3Co3.25Al0.75 were determined by bulk magnetization measurements and neutron diffraction studies. Gd3Co0.5Al0.5, Gd3Co3.25Al0.75 and Tb3Co3.25Al0.75 exhibit ferrimagnetic ordering below 196K, 161K and 151K, respectively. Tb3Co3.25Al0.75 shows a spin-reorientation transition at ~42K. Below the ferrimagnetic ordering temperature Gd3Co3.25Al0.75 and Tb3Co3.25Al0.75 are soft ferrimagnets, meanwhile Tb3Co3.25Al0.75 shows magnetic hardness below the spin-reorientation transition with remanent magnetization per formula unit of 9.7 μB and coercive field of 15kOe at 2K.The magnetocaloric effects of Gd3Co3.25Al0.75 and Tb3Co3.25Al0.75 were calculated in terms of isothermal magnetic entropy change and they reach maximum values of −4.9J/kg·K at 135–145K and −3.7J/kg·K at 115–135K, respectively, for a field change of 0–50kOe. Low temperature magnetic ordering in Tb3Co3.25Al0.75 is accompanied by a positive magnetocaloric effect with isothermal magnetic entropy changes of +13.6J/kg·K at 10K for a field change of 0–50kOe and +0.9J/kgK at 45K for a field change of 0–10kOe.Neutron diffraction study in zero applied field shows mixed ferro-antiferromagnetic ordering of Tb3Co3.25Al0.75 with a wave vector K0=[0, 0, 0]. Below ~ 137K Tb3Co3.25Al0.75 exhibits non-collinear ferrimagnetic ordering of terbium and cobalt sublattices with resulting of b-axis ferromagnetic and c-axis antiferromagnetic components of Cmc′m={1, mx}×{1, mz/[0, 0, 1/2]}×{1, i′}×{1, 1/[1/2, 1/2, 0]} magnetic space group. The spin-reorientation transition in Tb3Co3.25Al0.75 below ~38K corresponds to appearance of additional a-axis ferromagnetic component and decreasing of symmetry of magnetic ordering down to C2′/c={1, mz/[0, 0, 1/2]}×{1, i′}×{1, 1/[1/2, 1/2, 0]} magnetic space group.

Shell-ferromagnetic precipitation in martensitic off-stoichiometric Ni-Mn-In Heusler alloys produced by temper-annealing under magnetic field

The variety of the multifunctional properties of martensitic Ni-Mn based Heusler alloys are related to the presence of a magnetostructural transition. We report here on a new functionality based on a newly observed property. The observed property is that all off-stoichiometric Ni-Mn-based Heuslers, here in the form Ni50Mn50−xInx with 0<x<25, decompose into predominantly cubic ferromagnetic Ni50Mn25In25 and tetragonal antiferromagnetic NiMn components when temper-annealed. The new functionality is based on magnetic field assisted temper-annealing of a compound with stoichiometry x=5, whereby precipitates of Ni50Mn25In25 with a ferromagnetic shell are formed with spins in the field direction, strongly pinned by the surrounding antiferromagnetic anisotropy, even at temperatures as high as 500 K. The remanent pinning at high temperatures survives any thermal cycling between lowest temperatures and the annealing temperature and any magnetic field cycling between −9 and +9 T. The resulting product can serve as a thermally stable, magnetic-field-proof memory.

Structural transition and its effect in La, Zr co-substituted mono-domain BiFeO3

A new approach was employed in explaining the weak ferromagnetic behavior of conventionally synthesized Zr4+ modified Bi0.8La0.2FeO3. Rietveld refinement of XRD patterns revealed a polar-to-non-polar R3c → Pnma structural transition in Zr4+substituted samples. Magnetic properties were discovered to be remarkably enhanced, with extracted coercivity and remanence as high as 14 kOe and 0.2 emu/g, respectively. More importantly, an answer to the essential question of the magnetic domain state of the samples has been put forward. Our analysis established, nearly without doubt, the presence of grains consisting of a single magnetic domain. Separated ferromagnetic and anti-ferromagnetic components of the total M-H curves helped to reveal an immense effect of the structural transition on the shape of the hysteresis loops. The orthorhombic magnetocrystalline anisotropy of the Pnma phase has been primarily deemed responsible for the high coercivity and remanence of the Zr4+ modified samples. The effect of the grain size and Fe-O-Fe bond angle on the magnetic properties of the samples has also been discussed. Ferroelectricity was found to be absent in Zr-substituted samples because of their non-polar structure. However, the leakage current was reduced by up to two orders of magnitude compared to Bi0.8La0.2FeO3 upon addition of Zr4+. Dielectric measurements conformed to the ferroelectric and leakage current characteristics, and Zr4+ substitution was again found to be beneficial.

Sign reversal of magnetization and exchange bias in Ni(Cr1−xAlx)2O4 (x=0–0.50)

Ni(Cr1-xAlx)2O4 (x=0–0.50) samples were prepared in single phase form by using sol-gel method and their structural and magnetic properties were studied. Al substitution transforms the crystal structure of NiCr2O4 from tetragonal cell with space group I41/amd to cubic cell of Fd3¯m space group. Magnetization measurements by varying the temperature and magnetic field were carried out to investigate the interesting magnetization reversal and exchange bias behaviors. Magnetization reversal is observed for x=0.10 sample with a magnetic compensation temperature of 40K and it is explained by considering different temperature dependences of magnetic moments of the two sublattices. Shifting of magnetic hysteresis loops towards the negative magnetic field axis and hence the presence of negative exchange bias field is observed for x=0.15 sample. The x=0.10 sample exhibits the tunable positive and negative exchange bias field. Exchange bias in these samples is explained considering the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic spins. However, the sign reversal of exchange bias field is due to the change in domination of one ferrimagnetic sublattice over the other with variation in temperature. Both normal and inverse magnetocaloric effects are observed for x=0.10 sample.

Magnetic structure and effect of magnetic field on its domain structure in magnetoelectric Ba1.3Sr0.7CoZnFe11AlO22

The magnetic structure and the effect of a magnetic field on its domain structure were investigated in a magnetoelectric Y-type hexaferrite, Ba1.3Sr0.7CoZnFe11AlO22, by means of mapping with a micro-focused and circularly polarized X-ray beam in the resonant X-ray diffraction. It was revealed that this hexaferrite exhibits a magnetic order characterized by two distinct antiferromagnetic components: incommensurate helical and commensurate collinear ones, which can be explained as the development of the so-called alternating longitudinal conical structure. A multi-domain state due to the handedness of the helical component, i.e., spin-chirality, is transformed into nearly a mono-domain one by using only a magnetic field. Furthermore, the sign of the spin-chirality in the mono-domain state is reversed by reversing the sign of a magnetic field. These results demonstrate that the spin-chirality in this hexaferrite can be manipulated by a magnetic field alone at room temperature.