Magnetic Peak Research Articles

Achieving better glass-forming ability and magnetocaloric effect in the minor Zr-substituted Gd50Co50 amorphous alloy

The glass-forming ability (GFA) and magnetic properties of the minor Zr-substituted Gd[Formula: see text]Co[Formula: see text] amorphous alloy were investigated. The Gd[Formula: see text]Co[Formula: see text]Zr2amorphous ribbons prepared by melt-spinning show better GFA. With increasing minor Zr addition, the Gd[Formula: see text]Co[Formula: see text]Zr2amorphous ribbons possessed higher magnetic entropy change peak ([Formula: see text]S[Formula: see text] = 3.9 Jkg[Formula: see text]K[Formula: see text], under 5 T) but lower Curie temperature (T[Formula: see text] = 231 K) than the Gd[Formula: see text]Co[Formula: see text] amorphous alloy. The mechanism for the improved GFA, magnetocaloric effect (MCE) and the decrease in T[Formula: see text] was investigated. Finally, combined with other Gd-based amorphous ribbons, the Gd[Formula: see text]Co[Formula: see text]Zr2amorphous alloy was employed to construct amorphous composites to achieve a table-like magnetic entropy change profile, which can provide optimal efficiency when utilizing an Ericsson thermodynamic cycle.

Evidence for a dynamical ground state in the frustrated pyrohafnate Tb2Hf2O7

We report the physical properties of Tb2Hf2O7 based on ac magnetic susceptibility \chi_ac(T), dc magnetic susceptibility \chi(T), isothermal magnetization M(H), and heat capacity C_p(T) measurements combined with muon spin relaxation (\muSR) and neutron powder diffraction measurements. No evidence for long-range magnetic order is found down to 0.1 K. However, \chi_ac(T) data present a frequency-dependent broad peak (near 0.9 K at 16 Hz) indicating slow spin dynamics. The slow spin dynamics is further evidenced from the \muSR data (characterized by a stretched exponential behavior) which show persistent spin fluctuations down to 0.3 K. The neutron powder diffraction data collected at 0.1 K show a broad peak of magnetic origin (diffuse scattering) but no magnetic Bragg peaks. The analysis of the diffuse scattering data reveals a dominant antiferromagnetic interaction in agreement with the negative Weiss temperature. The absence of long-range magnetic order and the presence of slow spin dynamics and persistent spin fluctuations together reflect a dynamical ground state in Tb2Hf2O7.

Flow hydrogen absorption of LaFe10.9Co0.8Si1.3 compound under constant low hydrogen gas pressure

The hydrogen absorption of the LaFe10.9Co0.8Si1.3 compound under constant 1.01 × 105 Pa H2 gas in a flow hydrogen atmosphere was studied. The effects of hydrogen absorption on structure, Curie temperature, phase transition and magnetic property were investigated by X-ray diffraction (XRD), differential scanning calorimeter (DSC) and superconducting quantum interference device, respectively. The hydrides of LaFe10.9Co0.8Si1.3 crystallize into NaZn13-type structural phase after hydrogen absorption at temperature from 548 to 623 K. Lower hydrogen absorption temperature is of no advantage for pure 1:13 phase formation in a flow H2 atmosphere. The Curie temperature (TC) of LaFe10.9Co0.8Si1.3 compound increases by 70 K or more after hydrogen absorption. For LaFe10.9Co0.8Si1.3H1.8 compound, the maximum magnetic entropy change and the relative cooling power under a magnetic field change of 0–2 T are 6.1 J·kg−1·K−1 and 170 J·kg−1, respectively. Large refrigerant capacity, low hysteresis loss and wide temperature span of magnetic entropy change peak make it a competitive practical candidate for magnetic refrigerant.

Tuning the magnetostructural transformation by wheel speed in Mn-Fe-Ni-Ge-Si alloy ribbons

The Mn-Fe-Ni-Ge-Si alloy ribbons are prepared by the melt spinning technique with different wheel speeds and the microstructure, magnetic and magnetocaloric properties are systematically investigated. All the samples reveal the coincident magnetostructural transformation from the ferromagnetic TiNiSi-type martensite to paramagnetic Ni2In-type austenite on heating. The magnetostructural transformation temperature decreases remarkably from 380 K to 320 K with the increase in wheel speed from 10 m/s to 30 m/s. Giant magnetocaloric effect with excellent magnetic reversibility is obtained and the effective refrigeration capacity shows an evident increase, namely 103.9, 125.3, and 185.9 J/kg for three ribbons (under a field change of 0–5 T), though the maximal magnetic entropy change peak value of 27.4 J/kg K is achieved for the sample at 20 m/s. All these features suggest that the wheel speed is an effective way to manipulate the magnetostructural transformation temperature and magnetocaloric properties for MnNiGe-based alloys. The underlying mechanisms are discussed.

Doping evolution of the second magnetization peak and magnetic relaxation in (Ba1−xKx)Fe2As2 single crystals

We present a thorough study of doping dependent magnetic hysteresis and relaxation characteristics in single crystals of $(\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_{x})\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ ($0.18\ensuremath{\le}x\ensuremath{\le}1$). The critical current density ${J}_{c}$ reaches maximum in the underdoped sample $x=0.26$ and then decreases in the optimally doped and overdoped samples. Meanwhile, the magnetic relaxation rate S rapidly increases and the flux creep activation barrier ${U}_{0}$ sharply decreases in the overdoped sample $x=0.70$. These results suggest that vortex pinning is very strong in the underdoped regime, but it is greatly reduced in the optimally doped and overdoped regime. Transmission electron microscope (TEM) measurements reveal the existence of dislocations and inclusions in all three studied samples $x=0.38$, 0.46, and 0.65. An investigation of the paramagnetic Meissner effect (PME) suggests that spatial variations in ${T}_{c}$ become small in the samples $x=0.43$ and 0.46, slightly above the optimal doping levels. Our results support that two types of pinning sources dominate the $(\mathrm{B}{\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{K}}_{x})\mathrm{F}{\mathrm{e}}_{2}\mathrm{A}{\mathrm{s}}_{2}$ crystals: (i) strong \ensuremath{\delta}l pinning, which results from the fluctuations in the mean free path $l$ and $\ensuremath{\delta}{T}_{c}$ pinning from the spatial variations in ${T}_{c}$ in the underdoped regime, and (ii) weak $\ensuremath{\delta}{T}_{c}$ pinning in the optimally doped and overdoped regime.

Engineered Gd-Co based multilayer stack to enhanced magneto-caloric effect and relative cooling power

Magnetic refrigeration based on the magneto-caloric effect is one of the best alternatives to compete with vapor-compression technology. The viability of a magnetic refrigeration system for magnetic cooling can be tested by exploiting the materials in various forms, ranging from bulk to nanostructured materials. In order to achieve a wide refrigerating temperature range in magnetic refrigeration, we study in this paper a 100 nm-thick Gd-Co alloys-based multilayer stack. The stack is made of four individual Gd-Co alloy layers with different values of concentration and Curie temperature (TC). A magnetic entropy change associated with the second-order magnetic phase transition was determined from the magnetic isotherms. Moreover, the relative cooling power (RCP) of the studied Gd-Co-based multilayer is enhanced compared to the one of bulk Gd, and reaches a value of 200 J/kg. Such an enhancement of the RCP is not due to an enhanced maximum variation of entropy, but this is due to a much broader magnetic entropy peak. This study demonstrates the potential of nanostructured Gd-Co multilayer stack for magnetic cooling applications.

Using correlated tephras to refine radiocarbon-based age models, upper and lower Whitshed Lakes, south-central Alaska

Abstract Tephra deposits correlated between nearby lakes provide the opportunity to improve age estimates of the sediment sequences, even if the ages of the tephras are previously unknown. We explore this potential using cryptotephras and visible tephra deposits in sediment cores from Upper and Lower Whitshed Lakes near Cordova, Alaska. Each tephra was described in terms of visual stratigraphy and shard morphology, and the major-oxide glass geochemistry was analyzed. Independent age models were developed for the cores using radiocarbon ages and profiles of short-lived radioisotopes for the near-surface sediments. Four tephras were correlated between the two lakes based on the magnitude and spacing of magnetic susceptibility peaks and glass major-oxide geochemistry. These correlations confirm agreement of the age models because the independently modeled confidence intervals overlap for each correlated tephra. The stratigraphic correlations were subsequently used to improve the age models by extracting the subset of possible age-model iterations that produce similar ages for each of the four correlated tephras at the two lakes. The iterations that agree within 25 years for each correlated tephra were used to create tephra-matched age models for both lakes, which narrowed the width of the 95% confidence intervals of the age models by 3% overall and reduced the uncertainty in age estimates of the correlated tephras by 34% on average. This synchronization technique may be useful in other studies that have multiple independently dated records with confident stratigraphic correlations.

Magnetospheric Multiscale Observations of Electron Scale Magnetic Peak

AbstractThe sudden enhancements of magnetic strength, named magnetic peaks (MPs), are often observed in the magnetosheath of magnetized planets. They are usually identified as flux ropes (FRs) or magnetic mirror mode structures. Previous studies of MPs are mostly on the magnetohydrodynamics (MHD) scale. In this study, an electron scale MP is reported in the Earth magnetosheath. We present a typical case with a scale of ~7 electron gyroradii and a duration of ~0.18 s. A strong magnetic disturbance and associated electrical current are detected. Electron vortex is found perpendicular to the magnetic field line and is self‐consist with the peak. We use multipoint spacecraft techniques to determine the propagation velocity of the MP structure and find that the magnetic peak does propagate relative to the plasma (ion) flow. This is very different from the magnetic mirror mode that does not propagate relative to the plasma flow. Furthermore, we developed an efficient method that can effectively distinguish “magnetic bottle like” and “FRs like” structures. The MP presented in this study is identified as magnetic bottle like type. The mechanism to generate the electron scale magnetic bottle like structure is still unclear, suggesting that new theory needs to be developed to understand such small‐scale phenomena.

A Spin-Canted Antiferromagnetic Ground State in CeRu2Al10

Resonant polarised soft x-ray scattering at the cerium M-edge has been used to refine the magnetic structure of CeRu2Al10. A strong resonant feature at the cerium MIV-edge was observed at the disallowed \((0,1,0)\) Bragg position, consistent with previous neutron diffraction refinement of the moment pointing along the c-axis. The magnetic peak was found to have a temperature dependence expected for the paramagnetic–antiferromagnetic transition, disappearing above around 30 K. The polarisation dependence of the scattered x-rays conclusively shows that the low-temperature antiferromagnetic structure is non-collinear in nature. Fitting the polarisation dependence of the obtained Stokes parameters was undertaken with models for canting along either the a-axis or the b-axis. The experimental data agrees better with the model involving canting towards the a-axis. However, this is inconsistent with the Cmcm space group, suggestive of a symmetry lowering to either Pmnm or Cm2m. The resulting model is then achieve...

Experimental study of microwave properties of composites filled with Permalloy powder

Frequency dependencies of permittivity and permeability of composites filled with Permalloy powder with various concentrations and shapes of particles have been measured and discussed. The powder particles of various shapes were obtained by mechanical milling of industrial Permalloy powder in organic media. Paraffin wax was used as a matrix for producing the composites. A shape data of the particles was obtained with an electron microscope. The permittivity and permeability were measured in the frequency range of 0.13 to 10 GHz. The shape of the particles changes from sphere to platelet with increase of milling time. The change leads to a shift of the ferromagnetic resonance (FMR) frequency and magnetic loss peak. It is shown that for larger particles, a decrease of the Q-factor of the ferromagnetic resonance is observed, which is explained by the strong influence of the skin effect. With increase of the concentration of the Permalloy particles in composite, the magnetic loss peak shifts to low frequencies. When the spherical particles are flattened, the resonance frequency shifts to high frequencies, which indicates that the shift to high frequencies due to the decrease of the skin effect and the change in the magnetic structure is stronger than the shift to the low frequencies by changing the shape and reducing the demagnetization.

A method of measuring micro-displacement based on spin magnetic resonance effect of diamond color center

As one of the excellent piezoelectric materials, piezoelectric ceramic has been widely used to develop a highly precise displacement measurement system, which is the key part of the scanning probe system of the high-precision measuring instrument.Based on the high-precision scanning probe system, the micro/nano structures can be easily and accurately detected by the instrument system.However, due to the limitations caused by the character of hysteresis and nonlinearity, it is difficult to further improve the precision of highly precise displacement measurement system.In this work, we present a novel method to develop the highly precise displacement measurement system based on the quantum spin effect.The nitrogen vacancy (NV) color center of single crystal diamond as a sensitive element senses the change of the micro-displacement.Based on the electron spin magnetic resonance effect of diamond nitrogen vacancy color center, the variation of the magnetic field generated from the magnetic steel can be detected with high precision by the electron spin.The relative relation between the displacement and the magnetic gradient field can be used to establish the correlation model between the displacement and the electron spin resonance peak.In the experiment, a corresponding micro-displacement measurement system is established based on the cylindrical permanent magnet, according to the correlation model between the electron spin resonance effect and micro-displacement.The linear region of magnetic field gradient is designed to detect the micro-displacement.Firstly, the intensity distribution of magnetic field gradient is measured by the gauss meter.As the measurement results show, the gradient value is -7.77 Gauss/mm along the core axis of cylindrical permanent magnet, and the intensity of magnetic field gradient distribution region is linear in the millimeter range.Meanwhile, the electron spin magnetic resonance peak of diamond nitrogen vacancy color center is achieved by the optically detected magnetic resonance technology.The electron spin magnetic resonance peak is approximately 2.79 MHz/Gauss in the magnetic field achieved by the fluorescence spectrum of diamond nitrogen vacancy color center, attributed to the relation model between Zeeman splitting effect and magnetic field. In the experiment, the electron spin magnetic resonance signal of diamond nitrogen vacancy color center is lockedin by the demodulation method to achieve the change of micro-displacement.As the results show, the sensitivity is about 16.67 V/mm at the corresponding demodulation frequency of 3000.56 MHz.By the calculation, the resolution of micro-displacement measurement system is about 60 nm based on our method.It proves out a high precision and well reliability method to detect the micro-displacement.By the further theoretical calculation, based on the electron spin effect, the detection resolution of our method can be enhanced up to sub-nanometer scale by reducing the distance between the NV color center and the magnet.It presents a new research direction and field for the micro-displacement detection system.

Alignment of hooked-end fibres in matrices with similar rheological behaviour to cementitious composites through homogeneous magnetic fields

The following study aim is to analyse the parameters affecting the alignment of hooked-end steel fibres (HSF) in fluid matrices with Yield Stress (τo), using homogeneous magnetic fields as a main source to modify their orientation. HSF are commonly used as reinforcement in composites in the construction industry, mainly in cementitious materials which also show τo. The parameters studied here are: on one hand those referring to the rheological properties of the matrix, mainly τo, and on the other hand those associated to the HSF: aspect ratio, geometry and magnetic dipole moment (μf). To do so, a homogenous Magnetic Field (MF) with controlled strength and time pulse was used to induce a torque to the HSF and hence to control their orientation by mean of turning fibres. The source to obtain the MF was designed to reach up to 10 mTeslas, using a Helmholtz coil of 10.5 cm radius.The results obtained here assess how a MF with intensities adjusted to a known τo of a matrix can induce fibres orientation. Some steel fibre parameters such as geometry (specially length) and mass, affect considerably the total induced torque needed to rotate the fibres, and hence to align them in a matrix. Besides, rheological parameters can be also adjusted to achieve the fibre alignment, as well as the magnetic pulse shape and the magnetic field peak intensity.

Pinning-Induced Vortex-System Disordering at the Origin of the Second Magnetization Peak in Superconducting Single Crystals

The nature of the second magnetization peak (SMP) appearing on the dc magnetic hysteresis curves of superconducting single crystals with random pinning is still under debate. Many interesting SMP models and mechanisms were proposed and considered so far, and it is believed at present that this effect is system dependent. We analyzed the dc magnetization curves and the magnetic relaxation in the SMP domain for various single-crystal specimens (superconducting cuprates and iron-based superconductors), of different pinning strengths, with the external magnetic field H oriented along the crystallographic c axis or perpendicular to it. The sample independent aspects revealed by the relaxation results are the absence of single-vortex collective pinning around the SMP onset field and the sign changing of the vortex creep exponent between the onset field and the peak field. This general behavior supports strongly the pinning-induced disordering of the low-H quasi-ordered vortex solid as the actual scenario for the SMP.

Quantum criticality of spinons

The free fermion nature of interacting spins in one dimensional (1D) spin chains still lacks a rigorous study. In this letter we show that the length-$1$ spin strings significantly dominate critical properties of spinons, magnons and free fermions in the 1D antiferromagnetic spin-1/2 chain. Using the Bethe ansatz solution we analytically calculate exact scaling functions of thermal and magnetic properties of the model, providing a rigorous understanding of the quantum criticality of spinons. It turns out that the double peaks in specific heat elegantly mark two crossover temperatures fanning out from the critical point, indicating three quantum phases: the Tomonaga-Luttinger liquid (TLL), quantum critical and fully polarized ferromagnetic phases. For the TLL phase, the Wilson ratio $R_W=4K_s$ remains almost temperature-independent, here $K_s$ is the Luttinger parameter. Furthermore, applying our results we precisely determine the quantum scalings and critical exponents of all magnetic properties in the ideal 1D spin-1/2 antiferromagnet Cu(C${}_4$H${}_4$N${}_2$)(NO${}_3$)${}_2$ recently studied in Phys. Rev. Lett. {\bf 114}, 037202 (2015)]. We further find that the magnetization peak used in experiments is not a good quantity to map out the finite temperature TLL phase boundary.

Simulation of magnetic holes formation in the magnetosheath

Magnetic holes have been frequently observed in the Earth's magnetosheath and are believed to be the consequence of the nonlinear evolution of the mirror instability. Mirror mode perturbations mainly form as magnetic holes in regions where the plasma is marginally mirror stable with respect to the linear instability criterion. We present an expanding box particle-in-cell simulation to mimic the changing conditions in the magnetosheath as the plasma is convected through it that produces mirror mode magnetic holes. We show that in the initial nonlinear evolution, where the plasma conditions are mirror unstable, the magnetic peaks are dominant, while later, as the plasma relaxes toward marginal stability, the fluctuations evolve into deep magnetic holes. While the averaged plasma parameters in the simulation remain close to the mirror instability threshold, the local plasma in the magnetic holes is highly unstable to mirror instability and locally mirror stable in the magnetic peaks.

Pseudo-Goldstone Magnons in the Frustrated S=3/2 Heisenberg Helimagnet ZnCr2Se4 with a Pyrochlore Magnetic Sublattice

Low-energy spin excitations in any long-range ordered magnetic system in the absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating from the ordering wave vectors. In helimagnets, these modes hybridize into the so-called helimagnon excitations. Here we employ neutron spectroscopy supported by theoretical calculations to investigate the magnetic excitation spectrum of the isotropic Heisenberg helimagnet ZnCr2Se4 with a cubic spinel structure, in which spin-3/2 magnetic Cr3+ ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the (0 0 q) ordering vector, low-energy magnetic excitations in the single-domain proper-screw spiral phase show soft helimagnon modes with a small energy gap of ~0.17 meV, emerging from two orthogonal wave vectors (q 0 0) and (0 q 0) where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spin-wave theory and only acquire a finite gap due to higher-order quantum-fluctuation corrections. Our results are likely universal for a broad class of symmetric helimagnets, opening up a new way of studying weak magnon-magnon interactions with accessible spectroscopic methods.

Weak-ferromagnetism of CoF3 and FeF3

Cobalt trifluoride CoF3 and Iron trifluoride FeF3 are known as a G-type antiferromagnetic structure in the R-3c rhombohedral crystal structure. The neutron powder diffraction study had been reported that Co antiferromagnetic spin direction is parallel to 3-fold crystal axis in CoF3 whereas Fe antiferromagnetic spin direction is perpendicular to 3-fold crystal axis in FeF3. However, the resolution of early neutron powder diffractometer is not enough to distinguish between (003)h and (101)h rhombohedral magnetic peaks definitely.We employed high-resolution powder neutron diffractometer(SuperHRPD) at room temperature in MLF, J-PARC. The time-of-flight neutron powder diffractometer distinguishes clearly between (003)h and (101)h magnetic peaks. Then, we confirmed the proposed magnetic structural models of CoF3 and FeF3 at room temperature. In a magnetic susceptibility, CoF3 displays the additional magnetic anomaly at T* = 40 K. We discussed possible weak-ferromagnetism by the interplay of spin tilting and Dzyaloshinski-Moriya interactions.

Effect of chemical pressure induced by La3+/Y3+ substitution on the magnetic ordering of (AMn3)Mn4O12 quadruple perovskites

We report on the successful high-pressure synthesis of single-phase powders of the quadruple perovskite $({\mathrm{YMn}}_{3}){\mathrm{Mn}}_{4}{\mathrm{O}}_{12}$, where the chemical pressure exerted by the small ${\mathrm{Y}}^{3+}$ ion is expected to enhance the exchange interaction between ${\mathrm{Mn}}^{3+}$ ions. According to this expectation, powder neutron diffraction, specific heat, and magnetization measurements give evidence of a C-type antiferromagnetic ordering of the ${\mathrm{Mn}}^{3+}$ ions in the octahedrally coordinated $B$ sites at ${T}_{N,B}=108\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, 30 K higher than in the isostructural and isovalent compound, $({\mathrm{LaMn}}_{3}){\mathrm{Mn}}_{4}{\mathrm{O}}_{12}$. Surprisingly, we found no evidence of long-range magnetic order of the square-coordinated ${A}^{\ensuremath{'}}$ sites, although an indication of latent magnetic order is given by a broad peak of the DC magnetization between 70 and 40 K and by a sharp peak in the AC susceptibility at ${T}^{*}=70\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. A further unexpected feature of $({\mathrm{YMn}}_{3}){\mathrm{Mn}}_{4}{\mathrm{O}}_{12}$, not found in previously reported $(A{\mathrm{Mn}}_{3}){\mathrm{Mn}}_{4}{\mathrm{O}}_{12}$ compounds, is a second-order structural phase transition at ${T}_{s}=200\phantom{\rule{0.28em}{0ex}}\mathrm{K}$, attributed to a partial redistribution of charge involving both the $B$ and ${A}^{\ensuremath{'}}$-site ${\mathrm{Mn}}^{3+}$ ions. The present results suggest the existence of competing magnetic orderings, which may be linked to the occurrence of magnetic ferroelectricity in the $(A{\mathrm{Mn}}_{3}){\mathrm{Mn}}_{4}{\mathrm{O}}_{12}$ system.

Collective vortex pinning and merging of the irreversibility line and second peak effect in optimally doped Ba[formula omitted]KxBiO3 single crystals

Measurements on magnetization and relaxation have been carried out on an optimally doped Ba1−xKxBiO3+δ single crystal with Tc = 31.3 K. Detailed analysis is undertaken on the data. Both the dynamical relaxation and conventional relaxation have been measured leading to the self-consistent determination of the magnetization relaxation rate. It is found that the data are well described by the collective pinning model leading to the glassy exponent of about μ ≈ 1.64–1.68 with the magnetic fields of 1 and 3 T. The analysis based on Maley’s method combining with the conventional relaxation data allows us to determine the current dependent activation energy U which yields a μ value of about 1.23–1.29 for the magnetic fields of 1 and 3 T. The second magnetization peaks appear in wide temperature region from 2 K to 24 K. The separation between the second peak field and the irreversibility field becomes narrow when temperature is increased. When the two fields are close to each other, we find that the second peak evolves into a step-like transition of magnetization. Finally, we present a vortex phase diagram and demonstrate that the vortex dynamics in Ba1−xKxBiO3 can be used as a model system for studying the collective vortex pining.

Doping-induced quantum crossover in Er2Ti2−xSnxO7

We present the results of the investigation of magnetic properties of the Er$_2$Ti$_{2-x}$Sn$_x$O$_7$ series. For small doping values the ordering temperature decreases linearly with $x$ while the moment configuration remains the same as in the $x = 0$ parent compound. Around $x = 1.7$ doping level we observe a change in the behavior, where the ordering temperature starts to increase and new magnetic Bragg peaks appear. For the first time we present evidence of a long-range order (LRO) in Er$_2$Sn$_2$O$_7$ ($x = 2.0$) below $T_N = 130$ mK. It is revealed that the moment configuration corresponds to a Palmer-Chalker type with a value of the magnetic moment significantly renormalized compared to $x = 0$. We discuss our results in the framework of a possible quantum phase transition occurring close to $x = 1.7$.