Koichiro Noguchi
email: noguchi@cs26.scitec.kobe-u.ac.jp

Makoto Nagata
email: nagata@cs26.scitec.kobe-u.ac.jp

On-Chip Multi-Channel Waveform Monitoring for Diagnostics of Mixed-Signal VLSI Circuits

Multi-channel waveform monitoring technique enhances built-in test and diagnostic capability of mixed-signal VLSI

circuits. An 8-channel prototype system incorporates adaptive sample time generation with a 10-bit variable step delay generator and algorithmic digitization with a 10-bit incremental reference voltage generator. The prototype in a 0.18-µm CMOS technology demonstrated on-chip waveform acquisition at 40-ps and 200-µV resolutions. The waveforms were as accurate as those by an off-chip measurement technique, while more than 95 % reduction of the waste time in waveform monitoring was achieved. The area of 700µm × 600µm was occupied by a single waveform acquisition kernel that was shared with 8 front-end modules of 60µm × 200µm each. The developed on-chip multi-channel waveform monitoring technique is waveform accurate, area efficient, and low cost, which are all requisite factors for diagnosing methodology toward mixed analog and digital signal integrity in a systems-on-a-chip era.

Introduction

Capturing signal waveforms on-chip is the most straightforward measure of in-depth diagnostics in a mixed-signal integrated circuit. As recent growth of systems-on-a-chip (SoC) markets forces to locate much more functionality and performance on a single die, most of chips appear to be mixed-signal. However, there are various physical issues that influence dynamic circuit behaviors in advanced SoCs, which finally degrades chip performance or even leads to malfunction. Examples include spurs in analog-to-digital or digital-to-analog conversion, jitters in phase-locked loops, and unstable skew variations in clock distribution network or among critical paths. Embedding signal monitoring circuitry within SoC chips provides the functionality of runtime test of circuit performance as a part of built-in self-test mechanisms as well as diagnostics of invasive factors such as noise, which is strongly helpful for calibration, validation, and improvement of physical design flow. This approach will be getting common as the integration capacity increases, where die area occupied by the embedding circuitry can trade off directly with the cost of mixed-signal test facility and also indirectly with the effective use of electric design automation (EDA) tools.

This paper proposes an architecture of on-chip waveform monitoring that realizes multi-channel waveform-accurate probing at low area consumption and with minimum offchip measurement equipments. Enabled waveform acquisition at various locations on-chip provides extended diagnostics and testability over previous works. On-chip analog test signal generation and analog signal capturing in enhancing testability of analog / mixed-signal circuits was discussed in [START_REF] Roberts | Improving the Testability of Mixed-Signal Integrated Circuits[END_REF]. As for more diagnostic purposes, on-chip oscilloscope macros targeting wide-bandwidth signal capturing within high-speed digital signaling have appeared in [START_REF] Zheng | On-Chip Oscilloscopes for Noninvasive Time-Domain Measurement of Waveforms in Digital Integrated Circuits[END_REF] [START_REF] Takamiya | An On-chip 100GHz-Sampling Rate 8-channel Sampling Oscilloscope with Embedded Sampling Clock Generator[END_REF]. In addition, on-chip measurements of power-supply and ground dynamic noises [START_REF] Fukuda | Voltage-Comparator-Based Measurement of Equivalently Sampled Substrate Noise Waveforms in Mixed-Signal Integrated Circuits[END_REF]- [START_REF] Okumoto | A Built-in Technique for Probing Power-Supply Noise Distribution within Large-Scale Digital Integrated Circuits[END_REF], signals [START_REF] Ho | Applications of On-Chip Samplers for Test and Measurement of Integrated Circuits[END_REF], and clock jitters [START_REF] Kuppuswamy | On-die Clock Jitter Detector for High Speed Microprocessors[END_REF][12] has also been reported.

Section 2 describes a proposed architecture of on-chip waveform monitoring and discusses flow and cost of measurements, followed by details of enabling circuit techniques in Section 3. Section 4 presents design and measurement results of a 0.18-µm CMOS prototype chip, and finally, conclusions will be given in Section 5.

Architecture

Overview

Figure 1 shows a chip structure embedding a multi-channel waveform monitoring function, where tiny probing frontend modules locate adjacent to each circuit block to monitor and probe wiring is drawn from the module to the point of interest within the block, and every module shares a single waveform acquisition kernel that locates on the top. This ar- In order to materialize the chip structure, we have partitioned an entire monitor circuitry as shown in Figure 2. A probing front-end module (PFE) consists of a source follower (SF) and a latch comparator (LC), where probe wiring from a target signal of interest is input to SF and its DClevel-shifted output voltage V sf is compared with reference voltage V ref by LC, immediately within PFE when sampling clock T ck is given. Therefore, output of PFE, D out , is a single bit binary digital stream. Another input signal to PFE is bias voltage V bsf given to SF. On the other hand, a waveform acquisition kernel includes a reference level generator (VG) supplying V ref from a 10-bit R-2R ladder, a sampling timing generator (TG) providing T ck from a 10bit variable step delay generator, and a data processing unit (DPU) producing a series 10-bit digital word from Dout.

This partitioning allows to connect multiple PFEs to the single kernel with multiplexing only digital signals, T ck and D out , while distributing common analog signals, V ref and V bsf . Compared with a conventional architecture consisting of compatible circuit components given in Figure 3 where S/H signals are multiplexed at the input to analog-to-digital converter (ADC) with a successive approximation register (SAR), the proposed architecture eliminates the necessity of multiplexing analog signals of interest and therefore achieves multiple channel probing without degrading waveform acquisition performance. Here, while one of PFEs is activated during waveform acquisition, all the others are cut off from power-supply through logical control accordingly to a state register. Another key difference from SAR-ADC is found in the monotonously incremental DAC operation of VG, which also eliminates the feed-back processing needed by SA operation. Finally, since our waveform acquisition works on the basis of sampling principle, an explicit sam- pling capacitor was equivalently replaced with repetitive comparison of V sf against V ref at LC with statistical processing, as will be discussed in the next section. and VG is also reset and resumes V ref loop. The operation continues until user-specified loop count, and then a signal waveform at the probe is reproduced from time-series data of the digitized V sf . Here, one can also acquire an on-chip signal waveform by embedding a PFE alone as has been achieved in previous works [START_REF] Nagata | Measurements and Analyses of Substrate Noise Waveform in Mixed-Signal IC Environment[END_REF][8], if the necessary input signals to PFE is provided from off-chip measurement equipments that are automatically controlled by PC. However, since the total loop count in the proposed waveformacquisition flow reaches the order of 10 9 , accumulated access time to the measurement equipments from the PC can not be negligible. Figure 6 roughly compares the total time of acquiring 1024 measurement points. For the cases with embedding PFE alone, the time for the simple incremental algorithm same as Figure 4 explodes while successiveapproximation like algorithm reduces it in 1/10, however, still more than 5 hours is required. On the other hand, the native time of 4 minutes is only left when embedding the whole of the system of Figure 2 that is running at 4 MHz. This is the clear motivation of the development of on-chip multi-channel waveform monitoring, where the cost of measurement time can be traded-off with that of die area.

Flow and cost of waveform acquisition

Circuit description 3.1 Probing front-end module

One from two types of PFEs using P-channel or N-channel SF shown in Figure 7 is selectively applied to the signal of interest, depending on the DC voltage level of the signal. P-channel SF works for voltage ranging from below 0 V to V dd -V th p , where V dd and V th p are the supply voltage and the threshold voltage of P-type MOSFET, respectively. On the other hand, N-channel SF works for voltage ranging from above V dd to V th n , where V th n represents the threshold voltage of N-type MOSFET. While AC small signals with DC voltage at half V dd, which is often equal to the bias point of analog circuits, can be covered either Pchannel or N-channel SF, the full voltage range in a lowvoltage digital circuit with V dd of the degree of 1.2 V can be sensed by a single P-channel SF based PFE using highvoltage, for instance, 3.3-V MOSFETs. A simple topology of Figure 7(c) was chosen as LC in order to minimize the area of PFE. The entire PFE was designed to retain the gain of 0 dB from DC to 1.0 GHz. It should be noticed that the unexpected interaction with the circuit under monitoring must be minimized in the design of on-chip waveform monitoring circuit. The SF not only buffers the signal of interest to LC but also isolates the waveform monitoring circuit from the circuit to monitor. Therefore, the proposed PFE can apply for monitoring various kinds of signals or even noises in a mixed-signal VLSI circuit.

Reference voltage generator

Reference voltage generator, VG, showing in Figure 8(

Sampling timing generator

The most straightforward implementation of TG uses voltage-controlled delay line (VCDL) for timing generation, however, the size of circuit can easily explode as the number of bits of delay step increases, and moreover, it can create significant environmental noises since all delay cells are activated at every input edge. These inappropriateness can not be mitigated even if interpolaters between delay cells or Vernier topology are applied. Therefore, we have developed a variable step delay generator (VSDG) shown in Figure 9, where bias current, I b , is divided by an integer, n, corresponding to a digital code and thus the delay time, T delay , follows to:

T delay (n) = n × T delay (0), (1)
where T delay is measured from negative edge of input clock, Mck, to positive edge of output clock, T ck , and T delay (0) is the minimum delay. Here, I b is regulated by a replica DLL that equates the maximum delay of T delay (2 m ) to the clock period of system clock, Sck. Figures 10 and11 show actual implementation of 10-bit VSDG and replica DLL, respectively. VSDG is a currentmode circuit and 6-bit MSB delay step is finely tuned by injecting 4-bit LSB currents. The net maximum delay of T delay (2 10 ) is defined by subtracting offset delay (T of f set delay ) from the maximum delay (T max delay ) of the 6-bit MSB delay steps as shown in Figure 11, and equated to the clock period of Sck by the replica DLL. Therefore, TG generates sampling time adaptively to Sck of a target circuit. The proposed VSDG-based TG is very quiet since only a single transition occurs for every timing generation along with twice transitions in replica DLL for every phase adjustment, and therefore suitable for on-chip waveform monitoring.

Experiments

Prototype chip design

A prototype chip shown in Figure 12 was developed using a 0.18-µm CMOS technology, where eight PFEs sharing a single waveform acquisition kernel probed various wirings within a programmable 24-bit digital shift register file (SR). Another identical waveform acquisition kernel is also included for evaluation. Here, the occupied areas are 700µm × 600µm for the kernel and 60µm × 200µm for each PFE in a 2.8mm × 2.8mm chip. The kernel and PFEs were designed with 2.5-V I/O MOSFETs and located within a deep N-type well that was capacitively isolated from a common P-type substrate, on the other hand, all the other circuits used 1.8-V CMOS devices in a conventional twin well structure.

Measurement results

Measurement setup is described in Figure 13, consisting of a logic analyzer (LA) fully controlled by PC through Figure 14 shows waveforms on power-supply and ground wirings in SR running at Sck of 100 MHz. Two from the eight PFEs were applied, and moreover, waveform acquisition using the on-chip kernel and also that with the full use of external equipments as in a conventional way were compared. Obviously, waveforms in both on-chip and off-chip signal generation are quite consistent, thus proposed architecture of Figure 2 eration with SA algorithm for 1024 point waveform acquisition is summarized in Figure 15, which is larger than estimation in Figure 6 due mainly to additional time of data transfer from LA to PC in this measurement setup. However, more than 95% reduction of measurement time was achieved in on-chip waveform monitoring system, as expected. Figure 16 shows an example of long-time waveform monitoring on the same power-supply and ground wirings, where Mck input to VSDG relatively moved in 40 ns whenever the 10-bit step delay generation completed, as was shown in Figure 5, until waveform acquisition of the total of 24 clock cycles of Sck. Each waveform includes 6144 data points, which is only possible with the on-chip signal generation, in terms of the measurement time. Apparent 4-clock cycle patterns are seen in the waveforms, resulting from a 4-bit repetitive pattern of "0011" written in SR.

Conclusion

Multi-channel waveform monitoring technique was proposed and implemented with a 10-bit incremental reference voltage generator using R-2R ladder and a 10-bit incremental sample timing generator incorporating a variable step delay generator. The prototype in a 0.18-µm CMOS technology demonstrated on-chip multi-point waveform acquisition at 40-ps and 200-µV resolutions synchronously to a digital test circuit running at 100 MHz. The waveforms were as accurate as those by an off-chip measurement technique, while more than 95% reduction of the waste time in waveform monitoring was achieved. It was obviously proven that the developed technique was waveform accurate, area efficient, low cost, and capable of monitoring analog and digital signals, although limited experiments on a digital test circuit were described. These factors are all requisite for diagnosing methodology toward mixed analog and digital signal integrity in a systems-on-a-chip era.

Figure 1 .

 1 Figure 1. Chip embedding on-chip multichannel probing.

Figure 2 .

 2 Figure 2. Architecture of on-chip multichannel probing.

Figure 3 .

 3 Figure 3. Conventional architecture multiplexing inputs to ADC.

Figure 4

 4 shows the flow of waveform monitoring by the proposed architecture, based on sampling principle. The flow has dual loop operations where reference voltage loop is nested by sample timing loop, and premises that VG and TG generate incrementally stepwise voltage and sampling timing, respectively. Here, TG generates T ck for every negative edge of Mck that is initially located at a certain cycle of system clock (Sck), T sck , as shown in Figure 5(a). Then, VG generates V ref and LC repetitively compares it with V sf thus the probed voltage at this timing. After 2 10 iteration, average of LC decision is output from DPU as probability, P cmp , and then V ref increased step-wisely. When VG goes through every 10-bit steps, digitized V sf is determined as the bit number of V ref at the highest slope of ∆P cmp / ∆V ref in the comparator transition region. Then, TG sets forward T ck as shown in Figure 5(b) and again the 2nd loop is going. When TG goes through every 10-bit steps, TG is reset and Mck thus the first edge from TG is placed relatively to the next cycle of Sck as shown in Figure 5(c),

Figure 4 .

 4 Figure 4. Flow of waveform acquisition.

Figure 5 .

 5 Figure 5. Timing generation.

Figure 6 .

 6 Figure 6. Estimated cost of waveform acquisition.

Figure 7 .

 7 Figure 7. Probing front-end module, (a) Nchannel SF, (b) P-channel SF, (c) latch comparator.

Figure 8 .

 8 Figure 8. Reference voltage generator. (a) schematic and (b) voltage steps.

Figure 9 .

 9 Figure 9. Variable step delay generator.

  a) consists of R-2R ladder dividing external reference DC voltages of V ref p and V ref m and a 10-bit binary digital counter that increments the output value at every positive edge of DACinc. The switch network of R-2R ladder is designed to generate V ref that is linear to the output digital code from the 10-bit counter. Therefore, VG works as a 10-bit incremental digital-to-analog converter (DAC) as shown in Figure 8(b).

Figure 10 .

 10 Figure 10. Detail of 10-bit variable step delay generator.

Figure 11 .

 11 Figure 11. Replica delay locked loop.

Figure 12 .Figure 13 .

 1213 Figure 12. Die photo.

Figure 14 .

 14 Figure 14. Acquired waveforms on (a) VDD and (b) GND wirings.

Figure 15 .

 15 Figure 15. Actual cost of waveform acquisition.

Figure 16 .

 16 Figure 16. Results of long-time waveform acquisition.

Proceedings of the Design, Automation and Test in Europe Conference and Exhibition (DATE'05) 1530-1591/05 $ 20.00 IEEE

Proceedings of the Design, Automation and Test in Europe Conference and Exhibition (DATE'05)

Acknowledgments

This work is supported by Semiconductor Technology Academic Research Center (STARC). The authors would like to thank Masaki Hirata, Shin ichiro Azuma, Toshihiko Mori, Shiro Doushoh, and Hiroaki Ohkubo for helpful discussions. Test chips were fabricated by Hitachi Ltd. in the chip fabrication program provided by VLSI Design and Education Center (VDEC) of the University of Tokyo, in collaboration with Dai Nippon Printing Corporation.


Thomas Hérisson De Beauvoir

F Molinari

U-Chan C Chung-Seu

Dominique Michau

Dominique Denux

Michaël Josse

T Herisson De Beauvoir

Densification of MnSO4 ceramics by Cool-SPS: Evidences for a complex sintering mechanism and magnetoelectric coupling

Introduction

Since their revival in the early 2000's [START_REF] Hill | Why are there so few magnetic ferroelectrics ?[END_REF][START_REF] Kimura | Magnetic control of ferroelectric polarization[END_REF][START_REF] Hur | Electric polarization reversal and memory in a multiferroic material induced by magnetic fields[END_REF][START_REF] Spaldin | The Renaissance of magnetoelectric multiferroics[END_REF], magnetoelectric (ME) and multiferroic (MF) materials have strongly aroused the interest of materials chemistry and physics communities. A large part of their work in this field is focused on the research of new MF and/or ME phases [START_REF] Bos | Magnetoelectric coupling in the cubic ferrimagnet Cu2OSeO3[END_REF][START_REF] Bai | Dielectric responses and scaling behaviors in Aurivillius Bi 6Ti 3Fe 2O 18 multiferroic thin films[END_REF][START_REF] Alarcon-Suesca | Ferroelectric, magnetic and structural studies of the Bi4LaSmFe2Ti3O18 multiferroic material[END_REF]. However, for single phased materials, the scarcity of good candidates for multiferroism, all the more at room temperature, is well documented. This is, in part, due to the mutually excluding conditions for proper ferromagnetism and ferroelectricity, in transition metal oxides based on octahedral frameworks, among which most ferroelectric materials are found. This essentially prevents the appearance of multiferroic properties in a single phase material [START_REF] Hill | Why are there so few magnetic ferroelectrics ?[END_REF], the unique example of a ferroelectric ferromagnet being BiMnO3 [START_REF] Seshadri | Visualizing the role of Bi 6s "lone pairs" in the off-center distortion in ferromagnetic BiMnO3[END_REF]. Starting from that point, Kimura postulated that improper ferroelectrics could be interesting candidates as ME materials, especially electronic ferroelectrics [START_REF] Kimura | Magnetic control of ferroelectric polarization[END_REF] such as those associated with a spiral magnetic structure [START_REF] Kimura | Spiral Magnets as Magnetoelectrics[END_REF]. In these magnetically driven ferroelectrics, a helical magnetic structure can, in specific conditions [START_REF] Tokura | Multiferroics with spiral spin orders[END_REF], break the inversion symmetry and generate an electric polarization along a specific direction, for example. Materials displaying such a behavior are classified as type II multiferroics [START_REF] Khomskii | Classifying multiferroics: Mechanisms and effects[END_REF], and among them is the prototypical MnWO4 [START_REF] Arkenbout | Ferroelectricity in the cycloidal spiral magnetic phase of MnWO4[END_REF], which orders magnetically below 15K and displays a ferroelectric polarization four orders of magnitude smaller than classical ferroelectrics (BaTiO3 ~ 25 µC.cm -2 ).

However, research on type II multiferroics has been quite successful, unveiling the multiferroic properties of CuO at a rather high temperature (Tc~230K) [START_REF] Kimura | Cupric oxide as an inducedmultiferroic with high-TC[END_REF] and a polarization reaching 0,15 µC.cm - 2 in CaBaCo4O7 [START_REF] Caignaert | Gigantic magnetic-field-induced polarization and magnetoelectric coupling in a ferrimagnetic oxide CaBaCo4O7[END_REF]. When looking for phases presenting such a helical magnetic structure, one can notice that the ferroelectric properties of some of the candidate phases have not been explored yet because of the difficulty to obtain dense ceramic samples by sintering. Usually, the thermal stability of these candidate phases is limited by low temperature decompositions, phase transitions... that prevent the use of conventional, pressureless sintering for their densification. However, the emergence of new sintering techniques opens opportunities to densify and study these thermodynamically fragile materials (referred to as fragile materials in the following). In the present work, we relied on the spark plasma sintering (SPS) technique for the densification of MnSO4 which displays a helical magnetic structure at low temperature (TN=11K) [START_REF] Will | Cycloidal Spin Configuration of Orthorhombic MnSO4[END_REF] but decomposes at 780°C [START_REF] Narayanchar | DTA studies on preparation of MnSO4 by reacting pyrite and pyrolusite at high temperatures[END_REF].

On the one hand, this relatively low decomposition temperature forbids any successful sintering by conventional solid-state technique (which we unsuccessfully attempted). On the other, SPS is a powerful technique that allows for rapid sintering of refractory materials at temperatures much lower than those requested for any conventional sintering , as for Al2O3 which can be sintered at 1050 °C [START_REF] Shen | Spark Plasma Sintering of Alumina[END_REF], while conventional sintering requires temperatures as high as 1800 °C [START_REF] Coble | Sintering Alumina: Effect of Atmospheres[END_REF]. If the efficiency of SPS allow for such a lowering of the sintering temperature for refractory materials, it becomes tempting to also envision a significant efficiency of SPS at low temperature (typically T<700°C), for the sintering of a fragile materials such as MnSO4 depicted above. At such low temperatures, the application of larger pressures may become necessary, to maintain the efficiency of SPS densification. Indeed, we recently reported [START_REF] Herisson De Beauvoir | Cool-SPS: an opportunity for low temperature sintering of thermodynamically fragile materials[END_REF] how Cool-SPS emerges as an original and powerful method for the sintering of thermodynamically fragile materials at low temperature. The present work brings further experimental proofs to support this concept, while also offering some insight on the sintering mechanism at work during the densification of MnSO4 by Cool-SPS. Through the evaluation of several experimental parameters, we aim at establishing a reproducible sintering route for the elaboration of dense (>90% compactness) MnSO4 pellets. The role of the in situ dehydration and the transition to the high temperature form in the sintering mechanism will be investigated. Ultimately, dielectric characterizations enabled by the disposal of dense ceramics will allow to evaluate the potential multiferroic/magnetoelectric properties of MnSO4.

Experimental

Commercial MnSO4•H2O powders were used (Sigma Aldrich 99.9% purity). Dehydration has been performed through two different ways, either through heat treatment in an oven under air for 24h at 300 °C, or during SPS processing. Spark plasma sintering (Dr. Sinter Lab spark plasma sintering system, Model SPS-511S/SPS-515S) experiments were performed with WC:Co 10 mm die using 0.5 g of powder for each sintering attempt. Temperature was controlled by a thermocouple in contact with the inner part of the die. The pulse sequence for the SPS applied voltage for all the samples was 12 ON/2 OFF.

The densification was continuously monitored by the displacement of a punch rod. Heating rate was kept constant over the different sintering experiments and was set to 50 °C/min, while temperature and uniaxial pressure were varied from 400 °C to 600 °C and from 40 to 500 MPa respectively. Sintering attempts were performed under primary vacuum of about 10 Pa. Powder and pellet diffraction were performed with a Phillips X'Pert MPD X-ray diffractometer with the Bragg-Brentano geometry using Cu Kα1/Kα2 radiation (10° < 2θ < 80° range, step of 0.017°). The ceramics' relative densities were evaluated by geometrical measurements considering a theoretical density of 3.46 g/cm 3 for MnSO4 [START_REF] Will | The crystal structure of MnSO4[END_REF] and 2.94 g/cm 3 for MnSO4•H2O [START_REF] Le Fur | Structure des sulfates monohydrates, monocliniques, des metaux de transition, MSO4 (H2O)[END_REF]. The pellets' microstructures were observed on fracture surfaces, with a scanning electron microscope (JEOL 840 SEM). TGA measurements were performed with a Setaram tag2400 apparatus coupled to a Balzer Thermostar mass spectrometer. DSC datasets were collected with a Perkin Elmer DSC8000. Magnetic measurements were performed with a Quantum Design MPMS-5 (S.Q.U.I.D) magnetometer on powder and ceramics. Zero-field cooled and field cooled data were collected with an applied field of 1 kOe and with increasing temperature from 2K to 300K. Silver electrodes were deposited on both surfaces of the pellets and silver wires were subsequently used for sake of electrical connection. Once prepared for dielectric measurements, the ceramics were placed in a Quantum Design Physical Properties Measurement System (PPMS) from which a 9 T magnetic field can be applied. The dielectric measurements were carried out in the frequency range of 10 2 -10 6 kHz (the amplitude of the applied ac electric field being 1V) using an HP4194a impedance bridge, at heating and cooling rates between 0.2 and 1 K/min. Measurements of capacitance and dielectric losses were first performed by scanning temperature with and without application of a magnetic field of 9T. In a second step, the magnetic field was swept from -9T to 9T at 2K to evaluate magnetoelectric behavior of the samples. No dielectric dispersion was observed in the investigated frequency range, for all 8 various measured samples, thus a single frequency of 300 kHz is presented in the following.

Results and discussion

Phase stability

MnSO4 phase in ambient conditions is only stable under hydrated form. The one used here is a monohydrated phase MnSO4•H2O (Hyd-MnSO4) crystallizing in the monoclinic A2/a space group (n˚15). It can be dehydrated at 300°C to form the anhydrous MnSO4 (Anh-MnSO4), which crystallizes in the orthorhombic Cmcm space group (n˚63) corresponding to the α form, and transitions to the β form around 400°C [START_REF] Kirfel | New high temperature phase of MnSO4[END_REF] (in the following, the α form will be referred to as Anh-MnSO4, and the β form as β-MnSO4). The β form crystallizes in the space group Pbnm (n˚62) and corresponds to a 45° rotation of oxygen octahedra around (001) axis. After dehydration, when the powder is put in ambient conditions, rehydration takes place within one or two days. Thus, for experiments performed on Anh-MnSO4, powders and pellets were kept in an oven (at 120 °C) after thermal treatment and directly placed in the SPS die (for sintering) or the measurement setup (for characterization of the pellets). This way, the average ambient air exposure does not exceed 10-20min, and the sample is considered anhydrous. The two phases Hyd-MnSO4 and Anh-MnSO4 were both used for sintering experiments and compared.

Preliminary exploration of sintering conditions

Multiple sintering attempts have been performed under different experimental conditions. Selected parameters were temperature, time, pressure, and the use of anhydrous or hydrated precursor, which will be discussed in the following. Figure 1 summarizes some selected samples sintering conditions and densification results. In this figure, to some samples are assigned 2 different pressures. The first pressure corresponds to the applied pressure during heating ramp, while the second one corresponds to the pressure applied during dwell time. This specific protocol will be described in detail in section 3.2.2. The first observation from Figure 1 is that all the presented samples, except Anh1, show a high density above 90 %, for sintering temperature down to 400 °C. It is the first report on the sintering of MnSO4, which decomposition occurs at 780 °C. Secondly, one can notice that the final compactness is higher when starting from Hyd-MnSO4 than from Anh-MnSO4. Moreover, it was necessary to increase both temperature and time to 450 °C and 30 min respectively to obtain dense samples from anhydrous precursors. This suggests that structural water is involved in the sintering process. In the case of Hyd1, dehydration did not occur, therefore the hydrated form is preserved after sintering, and the resulting pellet displays a density close to 98% which is remarkable. This observation points out the importance of temperature-pressure profile, which will be detailed in section 3.2.2.

-Role of hydration

TGA measurement of Hyd-MnSO4 raw powder and of Anh-MnSO4 pellet (Hyd3, sintered at 400 °C) are displayed on Figure 2. The hydrated powder loses 10.8 wt.%, which is expected from the dehydration of MnSO4•H2O, while the pellet contains about 0.5 wt.%, corresponding to a composition of MnSO4•0.04H2O. This residual water likely results from the adsorption of water, or partial rehydration, that occurred between pellet removal from the die and the beginning of TGA measurement (i.e. in a span of 30 min in total). This confirms efficiency of the in situ dehydration during Cool-SPS treatment at 400 °C, and suggests a form a reactive sintering, the water lost during the sintering being structural, and thus bonded to other constituting species in the precursor powder MnSO4,H2O. Indeed, when using dehydrated powders, no pellet can be obtained below 450 °C, and a 30 min dwell time is needed anyway, whereas for hydrated powders, sintering at 400 °C for 5 min is enough to reach densification above 90 % (cf Figure 1). Similar conclusions have recently been drawn for cold sintering process and hydrothermal sintering [START_REF] Guo | Cold Sintering : A Paradigm Shift for Processing and Integration of Ceramics[END_REF][24], especially in the case of ZnO [START_REF] Funahashi | Demonstration of the Cold Sintering Process Study for the Densification and Grain Growth of ZnO Ceramics[END_REF], which can also be processed by SPS at 400 °C when water is added to the powder [START_REF] Dargatz | FAST/SPS sintering of nanocrystalline zinc oxide-Part I: Enhanced densification and formation of hydrogen-related defects in presence of adsorbed water[END_REF][START_REF] Dargatz | FAST/SPS sintering of nanocrystalline zinc oxide-Part II: Abnormal grain growth, texture and grain anisotropy[END_REF]. However, in the case of ZnO, the optimal content of water added for sintering has been determined to be 1.7 wt.% and increasing it, even to 7.7 wt.%, strongly hinders densification. Recent study on flash sintering of ZnO powder with addition of water also confirmed these observations [START_REF] Nie | Water-assisted flash sintering: Flashing ZnO at room temperature to achieve ~ 98% density in seconds[END_REF]. In the present case, water content represents 10.8 wt.% which is far above the optimal content determined for ZnO, although it consists exclusively in structural water, as no water is added to the Hyd-MnSO4 powder. Moreover, water is released specifically during in situ dehydration in the present study, while added water is lost continuously during the processing in the afore-mentioned examples on ZnO. This suggests that there is an added value to the use of hydrated precursors, in Cool-SPS but also likely in alternative water assisted sintering methods. Indeed, being able to control the release of water, as in the present work, allows for further optimization of the experimental conditions and, ultimately, for a better densification. Thus, the present results show that a water-assisted sintering mechanism can be triggered in SPS conditions without water addition, by the simple use of hydrated precursors and the control of in situ dehydration. This constitutes a valuable processing commodity allowing for simpler and easier processing protocols.

Pre-dwell pressure application

Most of the time, thermal and mechanical pressure profiles are as follows: pressure is applied first on the powder while room temperature is maintained, then temperature is raised once the targeted pressure is reached. Dwell time starts when temperature reaches its maximum. After dwell time is over, temperature and pressure are both released (Figure 3 shows a strong influence of the pre-dwell pressure, especially when one considers that the sample Anh1 was maintained for a longer period at the dwell temperature (i.e. the time needed to increase pressure from 40 to 400 MPa is not considered as dwell time, although the dwell temperature of 450 °C is applied). In this case, it means that even if the total sintering time was longer for Anh1 than for Anh2, the latter sample is better densified. Interestingly, results from Hyd-MnSO4 powders show the opposite behavior. During SPS experiments on Hyd-MnSO4 powders, an increase of the gas pressure in the chamber, which is maintained under dynamic vacuum, was observed, signaling the release of gaseous species.

Since subsequent characterization of the pellets excluded decomposition of manganese sulfate, it is At 300 MPa, the two main peaks at 350 and 380 °C differ in their amplitude, the first one being slightly more intense than the second one. When sintering is performed under a pressure of 300 MPa, the two peaks are again moved to higher temperatures, respectively ~370 and 415 °C (the location of this second peak being difficult to determine due to gauge saturation), and the latter one is now the more intense; i.e. the major part of dehydration is moved to higher temperature. It is noteworthy also that dehydration is barely completed at 400°C under 100 MPa, while for higher pressures it is necessary to go beyond 400 °C for dehydration to be complete. Thus, the sequence in which the thermal and pressure cycles are applied has in an impact on the dehydration temperature, which increases with pressure. Moreover, the span of temperature within which this dehydration occurs narrows with increasing pressure. On one hand, this explains the observation on Figure 4b, displaying XRD patterns for Hyd1, Hyd2 and Hyd3, of the preservation of the hydrated phase in the case of a 400°C dwell temperature and 400 MPa pre-dwell pressure, the latter forbidding the dehydration before or at the dwell temperature. On the other, in the cases in which dehydration occurred (100 and 40 MPa), the final phase is Anh-MnSO4. One can observe the incomplete dehydration of the sample sintered at 100-400 MPa on Figure 4, for which both chamber pressure (a) and XRD (b) are displayed. These observations confirm the possibility to control dehydration through pressure monitoring. In the present case, the starting powders always contain 10 wt. % of (structural) water, and densification is enhanced when dehydration is completed in situ, the final Anh-MnSO4 samples encompassing less than 0.05 wt.% of water (see Figure 2). This proves the presence of a different sintering mechanism, driven (partially at least) by dehydration. Results obtained on Hyd1 demonstrates the possibility to obtain a dense ceramic from a hydrated phase, thus confirming the presence of additional sintering mechanism, or of a combination of different mechanisms.

Dwell temperature and pressure

Various pressures have been explored to identify a range in which the densification could be enhanced without exceeding upper limits above which the mechanical cohesion is put in jeopardy. A few of them is sufficient to explain the influence of pressure on the sintering and are presented in Figure 5.

Temperature also has been tested to evaluate its influence on sintering. The most significant results of this survey are summarized in the same figure. In these experiments, for the same temperature (500 °C) and time conditions (5' dwell time), but also same powder quantity, the only difference is the different pressures applied during sintering. From 300 to 400 MPa, the density increases from 89 % to 95 % of theoretical density. When the pressure reached 500 MPa, the resulting pellet broke down during pellet's manipulation by delamination, which is indicative of an excessive applied pressure. These results have been reproduced on several occasions. As a general result, we never reached 90 % density for pellets sintered at 300 MPa, even for higher temperatures and longer dwell times. Therefore, not only pressure plays a role during the dehydration step, but also has a huge impact on densification during dwell, with a necessity to apply an important load of 400 MPa. Pressure has a complex impact on densification mechanisms, promoting rearrangement in the early sintering stages, enhancing plastic deformation, and increasing solid-solid surface tensions, possibly resulting in higher solid-state diffusion through grain boundaries.

Surprisingly, no improvement of relative density is observed from 400 (95.5%) to 500°C (95.1%), and only 1.5 % difference between 500 and 600 °C (96.6%). temperatures. The observation of grain shape shows that samples sintered at 400 and 500 °C have similar size and shape, although some faceting seems to occur at 500°C. However, these two samples present the same compact microstructure, while the one sintered at 600 °C looks different. The first two are composed of grains of about 3-5 µm, with curved shapes, whereas the last one looks to be composed of small grains of about 1μm with nice facets. This observation is in line with the relative density increase and suggests that a process (diffusion in the solid state, grain growth…) is triggered between 500 and 600 °C that induces a density increase and a change in the grains shape.

α-β Phase transition

At ambient temperature, Anh-MnSO4 crystallizes in the Cmcm space group (n°63), and this form is called α-MnSO4 (but Anh-MnSO4 throughout the manuscript). It undergoes a reversible transition to a β phase at 380 °C under Ar and reverts to the α phase at 150 °C on cooling (Figure 7a). On Figure 7b is represented the temperature of maximum dehydration as detected by monitoring the gas pressure in the SPS chamber, with respect to the mechanical pressure applied on the sample (from Figure 4). The first point at 0 MPa corresponds to the dehydration measured from TGA, used as a reference. After a first increase of the pressure to 40 MPa, a 20 °C increase of the maximal water release temperature is observed. This can be explained by the application of pressure on the sample that slightly stabilizes the monohydrate form. The evolution of dehydration temperature under pressures ranging from 100 to 400 MPa shows a different scheme, as it seemingly reaches a plateau at 100 MPa, and after this point, remains essentially constant (it should be reminded here that the dehydration typically takes place over a 20-30° range at the least). The red ribbon, with a width of about 50°C, represents the temperature range in which the phase transition from α to β form takes place, as extracted from the DSC measurement represented on Figure 7a. At about the same temperature (380 °C), we demonstrated that the dehydration of the powder (under 40-400 MPa) and its sintering occur. When larger pre-dwell pressures are applied, the dehydration still occurs around 400°C. The unique range of temperature in which both dehydration/sintering and phase transition take place raises the question of the involvement of the α-β phase transition in the sintering mechanism, and particularly as a triggering event for dehydration, which is itself critical to the sintering as illustrated earlier. It has been discussed elsewhere that phase transitions taking place during the sintering process can have an impact on the resulting densification [START_REF] Bousquet | Tuning Al2O3 crystallinity under supercritical fluid conditions: Effect on sintering[END_REF]. It appears that fast dehydration, associated to phase transition, imply an important stress on the grains, which is even assisted by an external mechanical pressure during sintering, and results in the onset of sintering. Moreover, the applied pressure may influence the stabilization of either α or β phases, depending on their respective density. However, literature data for β phase are given for a temperature of 450 °C, whereas for α phase, density is given at room temperature. In situ XRD measurement has been performed on MnSO4 powder at room temperature and at 350 °C, both showing a pure α phase. Lattice parameters refinements for these two patterns lead to lattice volume evolution from 288.54(1) Å 3 to 295.51(1) Å 3 for 25 and 350 °C respectively. Linear extrapolation of these values can give a hypothetical cell volume at 450 °C for the α phase which would be 298 Å 3 , which would result in a density of 3.37 g/cm 3 at 450 °C. Considering the density of β phase at the same temperature, being 3.35 g/cm 3 , it means α phase would be 0.6 % more dense, therefore, the application of external mechanical pressure on the sample may slightly favor its stabilization. However, considering this small density variation, and the variation of applied pressure (from 40 to 400 MPa at dwell temperature) on one hand, it is difficult to formally affirm that pressure has an influence on the α-β phase transition. On the other hand, considering the dependence of the dehydration temperature on applied pressure, and the plateau it reaches in the region of the α-β phase transition, it is likely that this transition triggers the dehydration.

-Sintering mechanism

Beyond the previous observations, no formal proof of the influence of the phase transition can be collected in our experimental conditions, since it is impossible to prove the presence of the β phase during SPS processing. Indeed, the α phase is restored at the end of the experiment, and no signature of the α-β transition could be detected from SPS monitoring data. Yet, based on the fact that dehydration and α-β phase transition (as well as sintering) occur in the same temperature range, it is difficult to exclude the α-β phase transition from consideration as no alternative explanation can be provided for the observed plateau of the dehydration temperature One could thus consider that the dehydration observed at 400°C under pressure of 100 MPa and beyond, is actually forced by the transition to β-MnSO4, a phase for which no hydrated form is known. In this case, the role of this phase transition, and that of the dehydration, could be interwoven in the sintering mechanism. Therefore, the question is to know whether the role of the α-β phase transition is o a direct one, in which sintering occurs thanks to the transition o an indirect one, if this transition triggers the dehydration that allows for the sintering

The results obtained on Anh-MnSO4 clearly exclude the first option, as relying solely on the α-β transition does not allow to obtain highly dense ceramics at 400°C.

The results obtained on Hyd-MnSO4 at 350°C also exclude the second option, as relying solely on dehydration does not allow to obtain cohesive, mechanically strong pellets.

As a consequence, we propose that the sintering mechanism for MnSO4 actually involves a coupling of the α-β transition and the dehydration, in which:

o dehydration allows for a very efficient densification (i.e. increase of relative density) o dehydration is triggered by the α-β phase transition o α-β transition and coupled dehydration allows for a simultaneous and efficient densification and consolidation (i.e. good cohesiveness of the final ceramic, intergranular bonding) Indeed, considering the efficient densification provided by dehydration, the volume per formula unit of Hyd-MnSO4 is 95.3 Å 3 , while that of Anh-MnSO4 is 72.7 Å 3 . This represents a diminution of 23% of the volume per formula unit that likely accounts significantly for the densification associated to dehydration. Moreover, as highlighted above, it is difficult to justify the plateau of the dehydration temperature without consideration of the α-β phase transition. Finally, when relying on conditions allowing only for dehydration or α-β phase transition, no efficient sintering can be obtained. Thus, such a mechanism is fully consistent with all our experimental observations, and especially with the characteristics of pellets sintered in optimal conditions (Hyd-MnSO4, 40-400 MPa, 400°C), which are perfectly anhydrous and single phase, very cohesive (as experienced when breaking the pellets to investigate fracture surfaces) and highly densified.

Characterization

Magnetic characterization

Although XRD clearly points towards the successful densification of α-MnSO4, magnetic measurements were nonetheless performed to confirm the conservation of the magnetic properties after SPS sintering. Figure 8a displays the magnetization of both the anhydrous powder and the corresponding SPS pellet (sample Hyd3 in Figure 1). Both exhibit the same response, although a slight decrease of magnetization between powder and pellet from 0.70 to 0.67 emu/g can be noted. The Neel temperature TN=11K corresponds to the reported magnetic transition of α-MnSO4 [START_REF] Allain | Magnetic study of the manganous sulfates MnSO4 and MnSO4.H2O[END_REF] and no transition is visible at 16K, which is the Neel temperature of the MnSO4•H2O phase [START_REF] Allain | Magnetic study of the manganous sulfates MnSO4 and MnSO4.H2O[END_REF]. This result further confirms the purity of the phase, and validates SPS as an effective route for the sintering of this thermodynamically fragile phase.

Capacitance and magnetocapacitance characterization

Capacitance measurements were performed on several samples exhibiting relative densities higher than 90%. Figure 8b displays the thermal evolution of capacitance at 300kHz with (red curve) and without (black curve) magnetic field for Hyd3 sample (sintered 400 °C, 40-400 MPa). Whatever the magnetic field, a similar decrease of the capacitance from 300K to ca. 18-20K is observed, followed by an increase from 11K (i.e. TN) down to 2K. This dielectric anomaly, being observed around TN, can be considered as an intrinsic feature of α-MnSO4, and this is further supported by the dielectric loss factor tanδ (not displayed) that remains inferior to 0.05% up to 175K, thus ruling out extrinsic contributions such as those associated to defects or electrodes, for example. Upon application of a 9T magnetic field, the onset of this second signal is repelled to 10K, and the permittivity decreases (∆ε'/ε'(max) = 0,15 %), claiming for an intrinsic magnetocapacitive coupling. Considering the helicoidal magnetic structure of α-MnSO4, investigated both theoretically [START_REF] Solyom | On the theory of magnetic phase transitions I : Many-step phase transitions[END_REF] and experimentally [START_REF] Will | The crystal structure of MnSO4[END_REF], these observations could be the signature of a type II multiferroic behaviour, with a dielectric anomaly associated with the antiferromagnetic transition and a magnetoelectric coupling related to the polarization possibly generated by this specific magnetic structure. However, the anomaly detected in the case of MnSO4 cannot be directly ascribed to a ferroelectric transition (contrary to MnWO4 [START_REF] Arkenbout | Ferroelectricity in the cycloidal spiral magnetic phase of MnWO4[END_REF]), although the transverse conical magnetic structure is indeed expected to induce a spontaneous polarization [START_REF] Tokura | Multiferroics of spin origin[END_REF].

This discrepancy remains unexplained at present, but one should keep in mind that the magnetic structure model was (necessarily) built on a limited amount of data fifty years ago, and some subtleties hindering the onset of a polarization may have remained hidden. Nonetheless there is a clear connection between the onset of the antiferromagnetic state, the corresponding dielectric anomaly M (emu/g) Capacitance (pF)

Capacitance (pF)

and the magnetocapacitance effect observed that confirms the potential of α-MnSO4 as a multiferroic/magnetoelectric. To further characterize the magnetoelectric behavior of α-MnSO4, magnetic field dependent capacitance measurements were performed.

Isothermal magnetocapacitance measurements, performed by sweeping the magnetic field at 2K, are displayed on Figure 9. The capacitance presents a clear dependence on magnetic field, with different regimes. From 0 to ±2T the capacitance increases, while from (±)7 to (±)9T it decreases, but a more complex behaviour is found for magnetic field ranging from (±)2 to (±)7T. Indeed, a hysteretic dependence of the capacitance is observed in these conditions, and could be related to a magnetically induced polarization, possibly a ferroelectric phase induced by the applied magnetic field. This behaviour is confirmed on all the α-MnSO4 ceramics measured (> 10 samples), and strongly supports the existence of a magnetoelectric coupling.

Conclusion

Cool-SPS was recently established as an efficient and reliable route to explore the ferroic properties of thermodynamically fragile materials in ceramic form [START_REF] Herisson De Beauvoir | Cool-SPS: an opportunity for low temperature sintering of thermodynamically fragile materials[END_REF]. The present work on α-MnSO4 pushes further the proof of concept of this approach. This thermodynamically fragile material was successfully sintered, with relative densities up to 95.5 % while operating at 400°C. Investigation of various processing parameters highlights a complex sintering mechanism involving in situ dehydration and a reversible phase transition which are coupled, through adequate experimental conditions.

Characterization of the processed ceramics confirmed the complete preservation of the structural and physical-chemical properties of the parent (anhydrous) powder. Moreover, a highly dense ceramic of MnSO4•H2O has been obtained (98 % theoretical density), which is, to our knowledge, the only example of ceramic made of a hydrated phase. The use of such hydrated precursor allows a better control over dehydration process and a highly homogeneous concentration of water over the sample compared to classical low temperature sintering processes involving the presence of added water.

Beyond the successful processing of these ceramics, physical characterizations revealed a dielectric anomaly associated with the antiferromagnetic transition observed at 11K, a magnetocapacitive coupling, and a hysteretic behavior of the magnetocapacitance. This latter observation is associated to an abrupt modification of the capacitance for a magnetic field around 2T, which likely signals a transition induced by the magnetic field. All these observations point towards a probable magnetoelectric coupling and demonstrate that elaborated functional properties can be found in thermodynamically fragile materials, which are thus much worthy of further exploration. Ultimately, these results open a new route for the densification of thermodynamically fragile materials with potential multiferroic or magnetoelectric properties, and allow exploring known or new thermodynamically fragile phases for ferroic or other properties requiring ceramic samples.

Figure 1 :

 1 Figure 1: Densification results after SPS sintering of hydrated (hyd) and anhydrous (anh) precursors in various conditions of temperature and pressure. Anhydrous samples sintering had to be extended to 30 min to obtain cohesive ceramics. For some samples, two pressures are indicated, corresponding to pre-dwell pressure and dwell pressure respectively, detailed in section 3.2.2.

Figure 2 :

 2 Figure 2: TGA measurement under air at 3 K/min of a) Hyd-MnSO4 powder and b) Hyd3 pellet sintered at 400 °C under 40-400 MPa

  a). In several of the experiments presented in this paper, temperature is raised first while a pre-dwell pressure is applied. Due to the use of a 10 mm diameter die, and the application of a minimal force of 3kN to ensure electrical contact, this predwell pressure is at least 40 MPa. Thus, this pre-dwell pressure can vary within the range 40 MPa < Ppre-dwell < Pdwell. Once the dwell temperature is reached, the mechanical pressure is raised, and dwell time starts when the applied pressure reaches dwell pressure (Figure3 b).

Figure 3 :

 3 Figure 3: Schematic view of the two different strategies of SPS treatment. In the first case (a) pressure is set first and temperature increases when pressure is stable. In the second case (b), pressure is set lower pressure of the machine (>40 MPa) during heating. Once the temperature is stable at dwell value, pressure can be applied In the case of Anh-MnSO4 samples Anh1 and Anh2, introduction of a pre-dwell pressure of 40 MPa has a negative impact on the final density of the pellet, which is limited to 88.3(3) % (Anh1), while a relative density of 92,8(3) % (Anh2) is obtained when applying 400 MPa through the whole thermal cycle. This

  Among the three samples sintered from hydrated powder (Hyd1, Hyd2 and Hyd3) at 400 °C with different pre-dwell pressure, Hyd1 lead to hydrated structure whereas Hyd2 and Hyd3 where dehydrated in situ. Hyd2, with a pre-dwell pressure of 100 MPa, shows a density of 93.8(3), while Hyd3 sample, on which 40 MPa pre-dwell was used, reached 95.5(3) % relative density (i.e. an extra 1.6 %).

4 .

 4 this gas release to the in situ dehydration of the precursor powder. To highlight the influence of pre-dwell pressure, specific experiments were conducted that are summarized in Figure Figure4a) displays the SPS chamber pressure during heating for Hyd-MnSO4 samples under various pressures (maintained constant throughout the whole SPS processing). Although the heating ramp and atmosphere of TGA are different from that used during SPS (3 °C/min under air vs. 30 °C/min under vacuum, respectively), TGA under air is also displayed as a pressureless reference concerning dehydration temperature, for a qualitative comparison. With increasing mechanical pressure, dehydration is shifted to higher temperatures, and this phenomenon already occurs under a pressure of 40 MPa. At this stage, a single maximum of water release is located at 290 °C, which is 60 °C above the maximum of water release observed in TGA, and the dehydration apparently occurs in a single broad step, instead of 2 steps in TGA conditions. When the pressure is raised up to 100 MPa, dehydration again occurs in two steps, and the temperature of the maximum water release increases.

Figure 4 :

 4 Figure 4: a) Chamber pressure during SPS experiment on MnSO4, H2O for various applied pressures; TGA under air in atmospheric pressure is used as reference. b) XRD patterns of Hyd1, Hyd2, Hyd3 corresponding to pre-dwell pressures of 400, 100 and 40 MPa. Blue arrows highlight the presence of MnSO4.H2O phase in 100-400 and 400 pellets.

Figure 5 :

 5 Figure 5: Densification level obtained for various sintering temperature and pressure conditions. All samples were sintered from a MnSO4•H2O powder, during a soaking time of 5 min, and a pre-dwell pressure of 40 MPa was applied before the sintering pressure was set

Figure 6 :

 6 Figure 6: SEM images of MnSO4 pellets sintered at (a) 400 °C, (b) 500 °C and (c.1 & c.2) 600 °C On figure 6 are displayed SEM images of the fractures of three different pellets sintered at different

Figure 7 :

 7 Figure 7: a) DSC measurement of Anh-MnSO4 powder under Ar at 10K/min, the peak below 300 °C corresponds to adsorbed water release (inset correspond to α and β structure view along (001) direction; b) Green dots represent the temperature of maximal water release during SPS sintering and red box represents the temperature range of α to β phase transition in ambient pressure. Data at 0 MPa is extracted from TGA.

Figure 8 :

 8 Figure 8: Temperature scan of a) magnetization under 1000 Oe in ZFC and b) capacitance measurement both while heating and cooling with and without magnetic field applied, at constant heating rate of 1 K/min and frequency measurement of at 300 kHz. Inset represents scan without magnetic field on 2-300 K scale. Pellets made in the same conditions as Hyd3 (400 °C, 40-400 MPa, 5 min) have been used for these measurements.

Figure 9 :

 9 Figure 9: Magnetocapacitance measurement at 2K, 300 kHz. Pellet sintered at 400 °C, 40-400 MPa for 5 min was used for measurement. Δε/ε is calculated considering the ε at 0 zero magnetic field.