Low-temperature Neutron Powder Diffraction Research Articles

Fe Site Order and Magnetic Properties of Fe1/4NbS2.

Transition-metal dichalcogenides (TMDs) have long been attractive to researchers for their diverse properties and high degree of tunability. Most recently, interest in magnetically intercalated TMDs has resurged due to their potential applications in spintronic devices. While certain compositions featuring the absence of inversion symmetry such as Fe1/3NbS2 and Cr1/3NbS2 have garnered the most attention, the diverse compositional space afforded through the host matrix composition as well as intercalant identity and concentration is large and remains relatively underexplored. Here, we report the magnetic ground state of Fe1/4NbS2 that was determined from low-temperature neutron powder diffraction as an A-type antiferromagnet. Despite the presence of overall inversion symmetry, the pristine compound manifests spin polarization induced by the antiferromagnetic order at generic k points, based on density functional theory band-structure calculations. Furthermore, by combining synchrotron diffraction, pair distribution function, and magnetic susceptibility measurements, we find that the magnetic properties of Fe1/4NbS2 are sensitive to the Fe site order, which can be tuned via electrochemical lithiation and thermal history.

Short-range magnetic correlation, metamagnetism, and coincident dielectric anomaly in Na5Co15.5Te6O36

Here we explore the structural, magnetic and dielectric properties of Co based compound Na$_5$Co$_{15.5}$Te$_6$O$_{36}$ as a candidate of short-range magnetic correlations driven development of dielectric anomaly above N$\acute{e}$el temperature of ($T_N$=) 50 K. Low temperature neutron powder diffraction (NPD) in zero applied magnetic field clearly indicates that the canted spin structure is responsible for the antiferromagnetic transition and partially occupied Co form short range magnetic correlation with other Co, which further facilitates the structural distortion and consequent development of dielectric anomaly above antiferromagnetic transition. Additionally, the temperature dependent magnetic heat capacity and electron spin resonance measurements reveal the presence of short-range magnetic correlations which coincides with an anomaly in the dielectric constant vs temperature curve. Moreover, significant changes in the lattice parameters are also observed around the same temperature, indicating presence of noticeable spin-lattice coupling. Further, sharp jump in the magnetic field dependent magnetization clearly indicates the presence of metamagnetic transition and magnetic field dependent NPD confirms that rotations of Co spins with applied magnetic field are responsible for this metamagnetic phase transition. As a result, this transition causes the magnetocaloric effect to be developed in the system, which is suitable for the application in low temperature refrigeration.

Temperature Dependent Crystal Structure of Nd2CuTiO6: An In Situ Low Temperature Powder Neutron Diffraction Study

Herein we reported the crystal structure and crystal chemistry of orthorhombic perovskite type Nd2CuTiO6 in between 2 K and 290 K as observed from the in situ temperature-dependent powder neutron diffraction (PND) studies. It is observed that the cations in octahedral sites are statistically occupied, and the ambient temperature orthorhombic structure is retained throughout the temperature range of the study. Absence of any long-range magnetic ordering down to 2 K is confirmed by both low-temperature PND and magnetization studies. The lattice shows strong anisotropic thermal expansion with increasing temperature, viz. almost no or feeble negative expansion along the a-axis while appreciably larger expansion along the other two axes (αb = 10.6 × 10−6 K−1 and αc = 9.8 × 10−6 K−1). A systematic change in the rotation of octahedral units with temperature was observed in the studied temperature range, while the expansion of unit cells is predominantly associated with the polyhedral units around the Nd3Ions. The temperature-dependent relative change in unit cell parameters as well as coefficients of axial thermal expansion show anomalous behavior at lower temperatures, and that seems to be related to the electronic contributions to lattice expansion.

Giant low-field magnetocaloric effect in ferromagnetically ordered Er1–xTmxAl2 (0 ≤ x ≤ 1) compounds

Magnetocaloric material is the key working substance for magnetic refrigerant technology, for which the low-field and low-temperature magnetocaloric effect (MCE) performance is of great importance for practical applications at low temperatures. Here, a giant low-field magnetocaloric effect in ferromagnetically ordered Er1–xTmxAl2 (0 ≤ x ≤ 1) compounds was reported, and the magnetic structure was characterized based on low-temperature neutron powder diffraction. With increasing Tm content from 0 to 1, the Curie temperature of Er1–xTmxAl2 (0 ≤ x ≤ 1) compounds decreases from 16.0 K to 3.6 K. For Er0.7Tm0.3Al2 compound, it showed the largest low-field magnetic entropy change (–ΔSM) with the peak value of 17.2 and 25.7 J/(kg K) for 0–1 T and 0–2 T, respectively. The (–ΔSM)max up to 17.2 J/(kg K) of Er0.7Tm0.3Al2 compound for 0–1 T is the largest among the intermetallic magnetocaloric materials ever reported at temperatures below 20 K. The peak value of adiabatic temperature change (ΔTad)max was determined as 4.13 K and 6.87 K for 0–1 T and 0–2 T, respectively. The characteristic of second-order magnetic transitions was confirmed on basis of Arrott plots, the quantitative criterion of exponent n, rescaled universal curves, and the mean-field theory criterion. The outstanding low-field MCE performance with low working temperatures indicates that Er1–xTmxAl2 (0 ≤ x ≤ 1) compounds are promising candidates for magnetic cooling materials at liquid hydrogen and liquid helium temperatures.

Magnetic structure and properties of the vanthoffite mineral Na6Mn(SO4)4

A detailed analysis of the magnetic properties of a vanthoffite type mineral Na6Mn(SO4)4 basedon dc magnetization, low temperature neutron powder diffraction and theoretical calculations is reported. The mineral crystallizes in a monoclinic system with space group P21/c, where MnO6 octahedra are linked via SO4 tetrahedra. This gives rise to super-exchange interaction between two Mn2+ ions mediated by two nonmagnetic bridging anions and leads to an antiferromagnetic ordering below 3 K. The magnetic structure derived from neutron powder diffraction at 1.7 K depicts an antiferromagnetic spin arrangement in the bc plane of the crystal. The magnetic properties are modelled by numerical calculations using exact diagonalization technique, which fits the experimental results and provides antiferromagnetic ground state of Na6Mn(SO4)4.

Evolution of crystal structure of PbMoO4 between 5 and 300 K: A low temperature powder neutron diffraction study

Evolution of crystal structure of PbMoO4 (Wulfenite, tetragonal, I41/a) from 5 to 300 K by in situ low temperature powder neutron diffraction studies is reported. The study revealed no structural transition in the complete range of temperature. The unit cell parameters show anisotropic expansion with about 2.2 times higher expansion along c-axis compared to that along a-axis. From the refined structural parameters, it is emphasized that the expansion of Pb–O bonds in PbO8 (bisdisphenoid) units and their orientations along a- and c-axes are the controlling factors. The MoO4 tetrahedra behaves like a rigid unit in the temperature range of this study. The expansion of unit cell volume is fully arising from the expansion of PbO8 bisdisphenoids. The analyses of anisotropic displacement parameters also show almost smooth variation with temperature down to 5 K. The evolutions of structural parameters of PbMoO4 with temperature suggest it for a promising material for cryogenic applications.

Structural, magnetic and electronic properties of Ti-doped BaFeO3-δ exhibiting colossal dielectric permittivity

We report a systematical investigation of the structural, magnetic, electronic, and dielectric properties of BaFe1-xTixO3-δ (x = 0.05, 0.10, 0.15 and 0.20) samples synthesized via the solid-state reaction method. Rietveld refinement using a combination of room-temperature X-ray (XRD) and neutron powder diffraction (NPD) data confirms single phases with a 6H-type structure for all the samples. X-ray absorption spectroscopy (XAS) study reveals a coexistence of Fe3+ and Fe4+, whose ratio remains almost unchanged with increasing Ti concentration. Magnetization measurements reveal a low-temperature inhomogeneous magnetic state with coexisting ferromagnetic (FM), AFM clusters and a paramagnetic phase. Moreover, magnetization of x = 0.10 shows a pinched hysteresis loop behavior, suggesting the coexistence of hard and soft FM phases in samples with high Ti concentrations. Low-temperature NPD measurements also confirm the absence of any long-range magnetic order in the samples. A colossal dielectric permittivity and two relaxation processes are observed for most of the studied samples. There is a change in electric conduction mechanism from the small polaron hopping to the Mott's variable-range hopping with increasing Ti concentration.

Large Positive Zero-Field Splitting in the Cluster Magnet Ba3CeRu2O9.

We present the synthesis and magnetic characterization of a polycrystalline sample of the 6H-perovskite Ba3CeRu2O9, which consists of Ru dimers based on face-sharing RuO6 octahedra. Our low-temperature magnetic susceptibility, magnetization, and neutron powder diffraction results reveal a nonmagnetic singlet ground state for the dimers. Inelastic neutron scattering, infrared spectroscopy, and the magnetic susceptibility over a wide temperature range are best explained by a molecular orbital model with a zero-field splitting parameter D = 85 meV for the Stot = 1 electronic ground-state multiplet. This large value is likely due to strong mixing between this ground-state multiplet and low-lying excited multiplets, arising from a sizable spin molecular orbital coupling combined with an axial distortion of the Ru2O9 units. Although the positive sign for the splitting ensures that Ba3CeRu2O9 is not a single molecule magnet, our work suggests that the search for these interesting materials should be extended beyond Ba3CeRu2O9 to other molecular magnets based on metal-metal bonding.

Strong magnetic exchange and frustrated ferrimagnetic order in a weberite-type inorganic-organic hybrid fluoride.

We combine powder neutron diffraction, magnetometry and 57Fe Mössbauer spectrometry to determine the nuclear and magnetic structures of a strongly interacting weberite-type inorganic-organic hybrid fluoride, Fe2F5(H taz). In this structure, Fe2+ and Fe3+ cations form magnetically frustrated hexagonal tungsten bronze layers of corner-sharing octahedra. Our powder neutron diffraction data reveal that, unlike its purely inorganic fluoride weberite counterparts which adopt a centrosymmetric Imma structure, the room-temperature nuclear structure of Fe2F5(H taz) is best described by a non-centrosymmetric Ima2 model with refined lattice parameters a = 9.1467(2) Å, b = 9.4641(2) Å and c = 7.4829(2) Å. Magnetic susceptibility and magnetization measurements reveal that strong antiferromagnetic exchange interactions prevail in Fe2F5(H taz) leading to a magnetic ordering transition at TN = 93 K. Analysis of low-temperature powder neutron diffraction data indicates that below TN, the Fe2+ sublattice is ferromagnetic, with a moment of 4.1(1) µB per Fe2+ at 2 K, but that an antiferromagnetic component of 0.6(3) µB cants the main ferromagnetic component of Fe3+, which aligns antiferromagnetically to the Fe2+ sublattice. The zero-field and in-field Mössbauer spectra give clear evidence of an excess of high-spin Fe3+ species within the structure and a non-collinear magnetic structure. This article is part of the theme issue 'Mineralomimesis: natural and synthetic frameworks in science and technology'.

Structural and magnetic studies of the ruthenium perovskites Ba2-xSrxHoRuO6

The series of ruthenate double perovskites Ba2-xSrxHoRuO6 (0 ≤ x ≤ 2) have been synthesized using solid state methods. The crystal structures of the series have been determined by high resolution synchrotron X-ray diffraction and show the sequence of structures to be Fm3̅m (a0a0a0) (0 ≤ x ≤ 0.6) → I4/m (a0a0c-) (x = 0.8) → I2/m (a0b-b-) (1.0 ≤ x ≤ 1.2) → P21/n (a-a-c+) (1.4 ≤ x ≤ 2.0). A similar progression of structures is observed in the sample BaSrHoRuO6 with increasing temperature. Magnetic characterisation of these materials was undertaken utilising variable temperature bulk magnetic susceptibility, isothermal magnetisation and low temperature neutron powder diffraction measurements. All members of the series order antiferromagnetically with the Ru and Ho sublattices ordering at different temperatures between 50 and 16 K. The presence of the magnetic Ho3+ at the perovskite B site stabilises the antiferromagnetic (AFM) ordering of the Ru sublattice. The addition of Sr for Ba beyond x = 1.2 weakens the AFM superexchange interactions through induced structural distortions, resulting in a canted AFM ground state.

Neutron diffraction study on exotic magnetic properties of Mn substituted spinel cobalt chromite

We report the evolution of temperature and magnetic field dependent magnetic properties of Mn substituted spinel cobalt chromite Co(Cr0.7Mn0.3)2O4 compound. It is observed that 30% ‘Mn’ substitution for Cr-site in CoCr2O4 results in magnetic compensation and huge reversal of temperature dependent magnetization below compensation temperature, Tcomp∼83 K under low applied fields which was absent in the parent compound CoCr2O4. For large applied fields magnetization reversal disappears but ends in field induced transition across Tcomp. Magnetic hysteresis loops exhibit gigantic coercivity at 3 K, and dramatically drops on increasing temperature. The exchange-bias-like behavior is observed below 10 K and as well as in the vicinity of compensation point, Tcomp(∼83 K). Low temperature neutron powder diffraction (NPD) experiment shows the possible magnetic structure and ordering in Co(Cr0.7Mn0.3)2O4. Non-monotonic change in bond lengths and bond angles around the compensation temperature (Tcomp) revealed that there is a correlation between crystal and magnetic structure. Possible origin behind such exotic magnetic properties has been discussed in detail in the paper.

Structural and Magnetic Properties of the Trirutile-type 1D-Heisenberg Anti-Ferromagnet CuTa2O6.

We prepared trirutile-type polycrystalline samples of CuTa2O6 by low-temperature decomposition of a Cu-Ta-oxalate precursor. Diffraction studies at room temperature identified a slight monoclinic distortion of the hitherto surmised tetragonal trirutile crystal structure. Detailed high-temperature X-ray and neutron powder diffraction investigations as well as Raman scattering spectroscopy revealed a structural phase transition at 503(3) K from the monoclinic structure to the tetragonal trirutile structure. GGA+U density functional calculations of the spin-exchange parameters as well as magnetic susceptibility and isothermal magnetization measurements reveal that CuTa2O6 is a new 1D Heisenberg magnet with predominant anti-ferromagnetic nearest-neighbor intrachain spin-exchange interaction of ∼50 K. Interchain exchange is a factor of ∼5 smaller. Heat capacity and low-temperature high-intensity neutron powder diffraction studies could not detect long-range order down to 0.45 K.

Ising-like antiferromagnetism on the octahedral sublattice of a cobalt-containing garnet and the potential for quantum criticality

In this contribution, we report that ${\mathrm{CaY}}_{2}{\mathrm{Co}}_{2}{\mathrm{Ge}}_{3}{\mathrm{O}}_{12}$ exhibits an unusual anisotropic and chainlike antiferromagnetic arrangement of spins despite crystallizing in the highly symmetric garnet structure. Using low-temperature powder neutron diffraction and symmetry analysis, we identify a magnetic structure consisting of chainlike motifs oriented along the body diagonals of the cubic unit cell with moments pointing parallel to the chain direction due to the strong Ising character of the Co ions. Antiferromagnetic order sets in below 6 K and exhibits both temperature- and field-induced magnetic transitions at high fields. Combining the results, we present a magnetic phase diagram that suggests ${\mathrm{CaY}}_{2}{\mathrm{Co}}_{2}{\mathrm{Ge}}_{3}{\mathrm{O}}_{12}$ undergoes a quantum phase transition at low temperatures and moderate fields.

Enhanced magnetoresistance in CaCu3(Mn4−xRex)O12 (x= 0, 0.1, 0.2) complex perovskites prepared at moderate pressures

New complex perovskites of the series CaCu3(Mn4−xRex)O12 have been prepared from citrate precursors under moderate pressure conditions of 2 GPa and 1000 °C, in the presence of KClO4 as oxidizing agent to stabilize Mn3+/Mn4+ mixed valence. The polycrystalline samples have been characterized by x-ray diffraction, neutron powder diffraction (NPD), magnetic, and magnetotransport measurements. The crystal structures are cubic, space group Im-3. The unit-cell parameters increase from a = 7.2379(2) Å for the parent (x = 0) compound to a = 7.2420(4) Å for CaCu3(Mn3.9Re0.1)O12. Both oxides adopt a superstructure of the perovskites ABO3 with long-range 1:3 ordering for Ca2+ and Cu2+ ions at the A sublattice. For the compound doped with Re (x = 0.1), the result of a NPD study shows that Re6+ ions are randomly located at the octahedral positions, being the (Mn,Re)O6 octahedra strongly tilted, with superexchange (Mn,Re)-O-(Mn,Re) angles of 142.03°. Neutron diffraction data clearly show that some Mn3+ ions are located together with Cu2+ at the square-planar 6b positions. The magnetic structure determined from low-temperature NPD data unveils a ferromagnetic coupling between (Mn,Re)8c spins at octahedral positions and weak antiferromagnetism with (Cu2+,Mn3+)6b spins. Interestingly, an enhancement of the magnetoresistance effect is observed for the Re-doped compound, well beyond that found for the parent perovskite.

Effect of Co Substitution on the Crystal and Magnetic Structure of SrFeO2.75-δ: Stabilization of the "314-Type" Oxygen Vacancy Ordered Structure without A-Site Ordering.

A study of the structure-composition-properties correlation is reported for the oxygen-deficient SrFe1-xCoxO2.75-δ (x = 0.1-0.85) materials. The introduction of Co in the parent SrFeO2.75 (Sr4Fe4O11) structure revealed an interesting structural transformation. At room temperature (RT), an orthorhombic (space group Cmmm, 2√2ap × 2ap × √2ap type, ap = lattice parameter of the cubic perovskite) → tetragonal (space group P4/mmm, ap × ap × 2ap type) → tetragonal (space group I4/mmm, 2ap × 2ap × 4ap type) structural transformation is observed in parallel with increasing Co content and decreasing oxygen content in the structure. At the same time, a rich variation in the magnetic properties is explored. The samples with x = 0.25, 0.3 show temperature-induced magnetization reversal. With increasing Co content in the structure, magnetic interactions start to weaken due to the random distribution of Fe and Co in the structure; the x = 0.5 sample shows frustration in the magnetic behavior with much smaller magnetization value. With a further increase in the Co content in the structure, RT ferrimagnetic-type behavior is observed for the sample with x = 0.85. The nuclear and magnetic structure refinements using RT and low-temperature neutron powder diffraction (NPD, 10 K) patterns confirm the formation of a "314-type" novel oxygen vacancy ordered phase for the sample with x = 0.85, which is the first case of "314-type" novel oxygen vacancy ordering without A-site (ABO3-δ type perovskite) ordering. The magnetic structure is G-type antiferromagnetic starting at room temperature. Further, the stabilization of the "314-type" complex superstructure is related to the ordering of oxygen vacancies in the oxygen-deficient Co-O layers, and the same assists in building a network of Co ions with different coordination environments, each with different spin states, and forms the spin-state ordering.

Zig-zag magnetic ordering in honeycomb-layered Na3Co2SbO6

Na3Co2SbO6is a layered oxide with a hexagonal O3-type structure, in which CdI2-type edge-sharing MO6 octahedral layers are intercalated by Na. The MO6 octahedral layer in turn adopts a honeycomb ordering pattern of magnetic (S=3/2) Co2+ sites surrounding isolated non-magnetic Sb5+ sites. Magnetic susceptibility measurements show that Na3Co2SbO6orders antiferromagnetically below TN=8.3K, with an effective magnetic moment of 5.22μB (indicating a strong orbital contribution above the expected spin-only value of 3.87μB). While a honeycomb arrangement of magnetic cations could, in principle, support a co-operative long-range-ordered magnetic structure in which all nearest neighbors are antiferromagnetic with respect to one another, symmetry analysis of low-temperature neutron powder diffraction data shows that it instead adopts a partially frustrated ‘zig-zag’ ordering in which 2/3 of nearest-neighbor interactions are ferromagnetic and 1/3 are antiferromagnetic. The low Néel temperature and Weiss constant of θ=2.2K underlines the presence of significant frustration of the expected strong superexchange interactions among Co2+.

Stabilization of the tetragonal structure in(Ba1−xSrx)CuSi2O6

We present a structural analysis of the substituted system (Ba$_{1-x}$Sr$_{x}$)CuSi$_{2}$O$_{6}$, which reveals a stable tetragonal crystal structure down to 1.5 K. We explore the structural details with lowtemperature neutron and synchrotron powder diffraction, room-temperature and cryogenic highresolution NMR, as well as magnetic- and specific-heat measurements and verify that a structural phase transition into the orthorhombic structure which occurs in the parent compound BaCuSi2O6, is absent for the x = 0.1 sample. Furthermore, synchrotron powder-diffraction patterns show a reduction of the unit cell for x = 0.1 and magnetic measurements prove that the Cu-dimers are preserved, yet with a slightly reduced intradimer coupling Jintra. Pulse-field magnetization measurements reveal the emergence of a field-induced ordered state, tantamount to Bose-Einsteincondensation (BEC) of triplons, within the tetragonal crystal structure of $I\,4_{1}/acd$. This material offers the opportunity to study the critical properties of triplon condensation in a simple crystal structure.

Magnetic structure, magnetoelastic coupling, and thermal properties ofEuCrO3nanopowders

We carried out detailed studies of the magnetic structure, magnetoelastic coupling, and thermal properties of EuCrO$_3$ nano-powders from room temperature to liquid helium temperature. Our neutron powder diffraction and X-ray powder diffraction measurements provide precise atomic positions of all atoms in the cell, especially for the light oxygen atoms. The low-temperature neutron powder diffraction data revealed extra Bragg peaks of magnetic origin which can be attributed to a $G_x$ antiferromagnetic structure with an ordered moment of $\sim$ 2.4 $\mu_{\rm B}$ consistent with the $3d^3$ electronic configuration of the Cr$^{3+}$ cations. Apart from previously reported antiferromagnetic and ferromagnetic transitions in EuCrO$_3$ at low temperatures, we also observed an anomaly at about 100 K. This anomaly was observed in temperature dependence of sample's, lattice parameters, thermal expansion, Raman spectroscopy, permittivity and conductance measurements. This anomaly is attributed to the magnetoelastic distortion in the EuCrO$_3$ crystal.

Sc2Ga2CuO7: A possible quantum spin liquid near the percolation threshold

Sc$_{2}$Ga$_{2}$CuO$_{7}$ (SGCO) crystallizes in a hexagonal structure (space group: $P$$6$$_{3}/mmc$),\textcolor{black}{{} which can be seen as an alternating stacking of single and double triangular layers. Combining neutron, x-ray, and resonant x-ray diffraction we establish that the single triangular layers are mainly populated by non-magnetic Ga$^{3+}$ ions (85\% Ga and 15\% Cu), while the bi-layers have comparable population of Cu$^{2+}$ and Ga$^{3+}$} ions \textcolor{black}{(43\% Cu and 57\% Ga)}. Our susceptibility measurements in the temperature range 1.8 - 400 K give no indication of any spin-freezing or magnetic long-range order (LRO). We infer an effective paramagnetic moment $\mu_{eff}=1.79\pm0.09$ $\mu_{B}$ and a Curie-Weiss temperature $\theta_{CW}$ of about $-44$ K, suggesting antiferromagnetic interactions between the Cu$^{2+}$($S=1/2$) ions. Low-temperature neutron powder diffraction data showed no evidence for LRO down to 1.5 K. In our specific heat data as well, no anomalies were found down to 0.35 K, in the field range 0-140 kOe.\textcolor{black}{{} The magnetic specific heat, $C_{m}$, exhibits a broad maximum at around 2.5 K followed by a nearly power law $C$$_{m}$$\propto$ $T$$^{\alpha}$ behavior at lower temperatures, with $\alpha$ increasing from }0.3\textcolor{black}{{} to} 1.9\textcolor{black}{{} as a function of field for fields upto 90 kOe and then remaining at 1.9 for fields upto 140 kOe. Our results point to a disordered ground state in SGCO. }

Magnetic Interactions in the Double Perovskites R2NiMnO6 (R = Tb, Ho, Er, Tm) Investigated by Neutron Diffraction.

R2NiMnO6 (R = Tb, Ho, Er, Tm) perovskites have been prepared by soft-chemistry techniques followed by high oxygen-pressure treatments; they have been investigated by X-ray diffraction, neutron powder diffraction (NPD), and magnetic measurements. In all cases the crystal structure is defined in the monoclinic P21/n space group, with an almost complete order between Ni(2+) and Mn(4+) cations in the octahedral perovskite sublattice. The low temperature NPD data and the macroscopic magnetic measurements indicate that all the compounds are ferrimagnetic, with a net magnetic moment different from zero and a distinct alignment of Ni and Mn spins depending on the nature of the rare-earth cation. The magnetic structures are different from the one previously reported for La2NiMnO6, with a ferromagnetic structure involving Mn(4+) and Ni(2+) moments. This spin alignment can be rationalized taking into account the Goodenough-Kanamori rules. The magnetic ordering temperature (TCM) decreases abruptly as the size of the rare earth decreases, since TCM is mainly influenced by the superexchange interaction between Ni(2+) and Mn(4+) (Ni(2+)-O-Mn(4+) angle) and this angle decreases with the rare-earth size. The rare-earth magnetic moments participate in the magnetic structures immediately below TCM.