Tribute to Prof. Nguyen Phu Thuy Giant Magneto-Caloric Effect Research in Vietnam - The Personal Impact of NP Thuy Tribute to Prof. Nguyen Phu Thuy

Development of magnetic refrigeration Motivations Development of magnetic refrigeration Demonstration of an Active Magnetic Refrigerator [C. Zimm et al. (1996), Zimm et al., Adv. Cryogen. Eng. 43(1998)1759]: 3 kg pure Gd metal 5 T superconducting magnet Cooling power: 600W, Efficiency: 60%. New design (possibly a Stirling Cycle prototype with Nd2Fe14B magnet) Discovery of the Giant MCE [Gschneidner, Pecharsky et al. (1997), IOWA State Univ. News Release (2001)]: GMCE material Gd5(Si1-xGex)4 Magneto-Caloric Effect - (MCE) is the change in temperature of a magnetic material due to the adiabatic change of the external applied magnetic field. Application of the MCE: Magnetic refrigeration.

VNU Key Research Project (QGTD-00-01) (NP Thuy formulated, proposed and got approved) Title: Fabrication and investigation of materials with giant magneto-caloric effect for use in a new generation of magnetic refrigerators. Proposer and Coordinator: Nguyen Phu Thuy Duration: 2 (two) years Budget: 300 Millions VND (about 20,000 US$ at that time)

Contents of the Research Investigation on the off-stoichiometric Gd5(SiGe)4 systems Investigation of the R5(SiGe)4 systems Other intermetallic alloy systems Perovskite and other like compounds Experimental methodology and techniques Magnetic Cooling Effect demonstration (Cryo-Lab, CMS, Solid State Dep, Faculty of Chemistry, ITIMS, HUT Chemistry Faculty, IMS and many others participated)

Results in R5(Si,Ge)4 systems Gd5(SixGe1-x)4 system with excess Gd, off-stoichiometric compounds, data measured on SQUID and Pulsed Field Magnetometer (PFM) for comparison -Smag[J/kg·K] B = 5T

MCE Determination: Isothermal magnetisation curves measured on PFM do not show enough details from the magnetisation of the materials in the temperature range around the Curie Transition Temperature. 148 K 77 K 170 K §­êng liÒn nÐt: PFM §­êng cã ®iÓm: SQUID Gd5(SixGe1-x)4 x=0.2525

The R5Si2Ge2 Compounds (proceedings of APPC 2000 conference) Replacing Gd by Rare Earths in R5Si2Ge2 All compounds formed in similar crystal structure as that of Gd5Si2Ge2. Magnetic Properties varied with R: either ferro- or antiferromagnetic structure, complex magnetic structure at low temperatures. Spin reorientation transition observed in Er5Si2Ge2. Giant MCE in Tb5(SixGe1-x)4, associated with structural and other field-induced phase transitions.

Spin reorientation in Er5Si2Ge2 (Collaboration with Ho of Cincinatti University)

Stoichiometric Tb5(SixGe1-x)4 compounds, measured on SQUID and PFM 50 100 150 200 250 300 350 4 8 12 16 20 24 28 32 36 40 D S mag [J/kg.K] T [K] x = 0.6 x = 0.6 - PFM x = 1 1 2 3 5 6 7 9 10 Tb (Si x Ge 1-x ) B = 5 T x = 0.4 DSMag

Giant MCE in the Tb5(SixGe1-x)4 system 10 100 150 200 250 300 4 8 12 16 20 24 28 32 ΔSmag [J/kg·K] Temperature [K] 1 2 3 5 6 7 9 Tb5(SixGe1-x)4 ΔB = 5 T x = 0.6 x = 1 x = 0.5 x = 0.75 x = 0.4

Results on the Tb5(SixGe1-x)4 system Magnetic Order 50 100 150 200 250 20 40 60 80 120 140 300 0.0 0.1 0.2 0.3 B = 1 T T [K] B = 0.01 T x13 Magnetisation [A·m2·kg-1] Temperature [K] FC ZFC 1/c[Tkg/Am2] 350 25 75 125 175 B = 0.01T B = 5T x 50 Tb5Si2.4Ge1.6 Tb5Si2Ge2

Field-induced transition in Tb5(SixGe1-x)4 10 20 30 40 100 150 200 250 Magnetization [Am2/kg] Magnetic field [T] Tb5Si3Ge Tb5Si2Ge2

Collaboration with TU Dresden Structural Phase Transition in the vicinity of the Curie Temperature (TC) in the Tb5(SixGe1-x)4 system Intensity [arb. units] 2 [degree] 20 22 24 26 28 30 32 34 36 38 40 42 100K 110K 115K 120K 130K 150K Tb5Si2.4Ge1.6 Collaboration with TU Dresden

Temperature Dependence of the Lattice Parameters of Tb5Si2. 4Ge1. 6: Temperature Dependence of the Lattice Parameters of Tb5Si2.4Ge1.6: Orthorhombic below TC=150K Monoclinic above TC= 150K 50 100 150 200 250 300 7.44 7.46 7.48 7.50 Lattice parameters [Å] Temperature [K] 7.70 7.72 7.74 14.64 14.66 14.68 bo (T) am (T) bm (T) ao (T) co (T) cm (T) TC = 150 K

Lattice parameters [Å] Orthorhombic Lattice parameters of Tb5Si3Ge in the temperature range from 50 to 300 K. An anomaly (a kink in the curves) occurs at 210 K, its Curie Temperature. 100 150 200 250 300 7.42 7.43 a (T) Lattice parameters [Å] Temperature [K] 7.69 7.70 7.71 7.72 c (T) b (T) 14.62 14.63 14.64 50 TC = 210 K

Effect of Cooling Cycle Temperature dependence of the electrical resistivity and the magnetization in magnetic field up to 1 T between 10 and 300 K of the compounds Tb5(SixGe1−x)4 with x=0.6, 0.75, and 1.0 were studied. All samples exhibit irreversible changes in the electrical resistance on thermal cycling between room temperature and 10 K. The irreversible effect is reduced with increasing x from 0.6 to 1.0, and it is not observed in the magnetisation studies. The resistance irreversible effect may be due to micro-cracks formed during the thermal cycling associated with the induced structural phase transition). Important to application: working substance can be destroyed! Collaboration with D. Yao (Taiwan)

MCE in the La1-xNdx(Fe,Si)13 sytems FWHM = 40 K FWHM = 26 K T2 T1 - Giant MCE. TC above 200K. Metamagnetic phase transition. Main part of material is Fe, therefore cost advantage.

MCE in RCo2 system -ΔSMag [J/kg ·K] T [K] Er(Co1-xSix)2 T [K] -ΔSMag [J/kg ·K] Er(Co1-xSix)2 RCo2 (R=Dy, Tb, Gd) 0.10 100 200 300 10 20 30 40 0.15 x = 0 0.05 0.075 0.02

MCE in Perovskite Compounds La1-xAxBO3 (A= Ca,Sr,Pb,Ag,Na; B=Mn,Co,Cu,Cr) -Smag [J/kgּK] TC [K] 100 150 200 250 300 350 0.05 0.15 0.25 0.35 Smag sol-gel samples Smag ceramic samples TC sol-gel samples TC ceramic samples 1 1.5 2 2.5 3 3.5 4 La1-xAgxMnO3 system x - Ceramic materials, simple preparation technology. Low cost of materials. - Reasonably high MCE, comparable to pure Gd. - High TC from 260K to above room temperature.

Other perovskite compounds - SMag[J/kg ·K] T [K] La1-xCaxMnO3-δ system, B=1, 3 vµ 5 T

MCE demonstration device Schematic working principle: Gd metal plates Pneumatic sample handling system Keithley Microvoltmeter to record the temperature changes using a thermo -couple (glued in between the Gd plates) Computer to display the sample temperature changes N S Magnet Pole

Results The magnet arrangement B(d): Induction as dependent on the clearance between the NdFeB magnet poles d [cm] B [mT] 200 400 600 800 1000 1200 1 2 3 4 5 6 Measured on the magnet pole surface Measured in the gap in between the magnet poles Prepared in LT Tai responsibility with NdFe14B material

Demonstration Experimental Setup

Temperature of the sample + holder + thermocouple in the absence of the magnetic field

Temperature of the sample holder and the thermo couple during magnetisation and demagnetisation process

Time [arbitary reference in seconds] Actual sample temperature changes during magnetisation and demagnetisation cycles 291.0 291.2 291.4 291.6 291.8 292.0 292.2 292.4 20 40 60 80 100 Time [arbitary reference in seconds] Sample Temperature [K]

Time [arbitary reference in seconds] Absolute sample temperature change at room temperature (291 K) during magnetisation and demagnetisation cycles 0.2 0.4 0.6 0.8 1.0 1.2 1.4 20 40 60 80 100 Time [arbitary reference in seconds] Sample Temperature [K]

Concluding remarks: Well designed, organised and sucessfully implemented Publications: Total number of papers published: 42 Number of papers published internationally: 16 Education: Number of Bachelor Theses: 15 Number of Master Theses: 02 PhD Project proposed and started: 01 Research continued after this project: Cryo-Lab, Solid State Department, CMS, ITIMS, IMS etc. (about 20 % of international scientific publications of the Vietnamese Physics Community in the last 8 years deal with MCE) International Cooperation promotion: extension, lot of new relationships and programmes established

NP Thuy: A decent and modest person, a bright and intelligent physicist with a sucessful career, featured by deep insight in every topic he approached, long term vision and sharp sense, great scientific imagination, intuitivity and creativity, always with an open mind and endless enthousiasm to catch up and to start with new topics, ideas and technologies. A kind colleague with strong attraction and capability to link together and to encourage both students and co-workers