#### Year

2014

#### Degree Name

Doctor of Philosophy

#### Department

Institute for Superconducting and Electronic Materials

#### Recommended Citation

Md Din, Muhamad Faiz, Magnetic phase transitions and novel materials for magnetocaloric effect, Doctor of Philosophy thesis, Institute for Superconducting and Electronic Materials, University of Wollongong, 2014. http://ro.uow.edu.au/theses/4371

#### Abstract

The magnetic and structural properties of some selected magnetocaloric materials such as La (Fe, Si)_{13} series compounds, RMn_{2}X_{2} series (R=rare earth and X=Si or Ge) compounds and typical MM′X series (M, M′ = transition metal, X = Si, Ge, Sn) have been systematically investigated in this thesis with total 7 chapters being included. After a general introduction in Chapter 1, the description of theoretical aspects of the research and experimental techniques are given in Chapters 2 and 3, respectively.

In Chapter 4, the investigation on structure and magnetic properties of the La_{0.7}Pr_{0.3}Fe_{11.4- x}Cu_{x}Si_{1.6} and La_{0.7}Pr_{0.3}Fe_{11.4-x}Cr_{x}Si_{1.6} compounds is presented. Cu substitution for Fe in La_{0.7}Pr_{0.3}Fe_{11.4-x}Cu_{x}Si_{1.6} (x = 0, 0.06, 0.12, 0.23, 0.34) leads to a reduction in hysteresis loss, a decrease in magnetic entropy change but an increase in Curie temperature (T_{C}). The influences of annealing processes at different temperatures on T_{C}, magnetic hysteresis, and the magnetocaloric effect (MCE) of La_{0.7}Pr_{0.3}Fe_{11.4}Si_{1.6} are also investigated in detail. It has been found that a short-time and high temperature annealing process has benefits for the formation of the NaZn_{13} types as phase compared to a long-time and low temperature annealing process.

Furthermore, the effects of substitution Fe by Cr in NaZn_{13}-type La_{0.7}Pr_{0.3}Fe_{11.4-x}Cr_{x}Si_{1.6} (x=0, 0.06, 0.12, 0.26, and 0.34) compounds have been investigated by high intensity of X-ray and neutron diffraction, scanning electron microscopy, specific heat, and magnetization measurement. It has been found that a substitution of Cr for Fe in this compounds leads to a decrease in the lattice parameter a at room temperature and a variation on T_{C}. While the first order nature of magnetic phase transition around T_{C} does not change with increasing Cr content up to x=0.34. High intensity X-ray and neutron diffraction study at variable temperatures for highest Cr concentration x=0.34 confirmed the presence of strong magnetovolume effect around T_{C} and indicated the direct evidence of coexistence of two magnetic phases across magnetic transition as characteristic of first order nature. The values of -ΔS_{M} around T_{C} found to decrease from 17 J kg^{-1}K^{-1} for x=0 to 12 J kg^{-1}K^{-1} for x=0.06 and then increases with further increasing Cr content up to 17.5 J kg^{-1}K^{-1} for x=0.34 under a change of 0–5 T magnetic field. The relative cooling power (RCP) also indicated the similar behaviour which is decrease from 390 J kg^{-1} for x=0 to 365 J kg^{-1} for x=0.06 at the beginning and then increases up to 400 J kg^{-1} for x=0.34 at the same field applied.

Chapter 5 describes the investigation on magnetic behaviour and magnetocaloric effects of RMn_{2}X_{2}-based materials (R=rare earth and X=Si or Ge). The RMn_{2}X_{2} series has attracted significant interest in recent years due primarily to their natural layered structure in which R and Mn atoms lie in alternate layers, separated by layers of X atoms. The strong dependence of the Mn–Mn magnetic exchange interaction on the intralayer near neighbour distance, and the interplay between the magnetism of the Mn and R layers lead to a fascinating arrangement of magnetic phases for these compounds. Firstly, in order to clarify the effect of substitution Mn with other transition metal (T) in NdMn_{2}Si_{2} compound, structural and magnetic properties of the intermetallic compounds NdMn_{2−x}T_{x}Si_{2} (T=Ti, Cr, Cu and V) have been studied.

The Curie temperature and N´eel temperature of NdMn_{2}Si_{2} decrease from T_{C} = 36 K and T_{N} = 380 K to T_{C} = 14 K and T_{N} = 360 K, respectively, on substitution of Ti (x = 0.3) for Mn. The magnetocaloric effect around T_{C}, has been investigated in detail. Under a change of magnetic field of 0–5 T, the maximum value of the magnetic entropy change is 27 J kg^{-1}K^{-1} for x = 0, reducing to 15.3 J kg^{-1}K^{-1} for x = 0.1 and 10 J kg^{-1}K^{-1} for x = 0.3; importantly, no thermal or field hysteresis losses occur with increase in Ti concentration. Combined with the lack of any hysteresis effects, these findings indicate that NdMn_{1.9}Ti_{0.1}Si_{2} compound offers potential as a candidate for magnetic refrigerator applications in the temperature region below 35 K.

In substitution Mn with Cr in NdMn_{2−x}Cr_{x}Si_{2} compound, a giant magnetocaloric effect has been observed around Curie temperature, T_{C}~42 K, in NdMn_{1.7}Cr_{0.3}Si_{2} with no discernible thermal and magnetic hysteresis losses. Detailed study shown that below 400 K, three magnetic phase transitions take place around 380 K, 320 K and 42 K. High resolution synchrotron and neutron powder diffraction (10–400 K) analysis confirmed the magnetic phases transitions as follows: T_{N} ^{intra}~380 K denotes the transition from paramagnetism to intralayer antiferromagnetism (AFl), T_{N} ^{inter}~320 K represents the transition from the AFl structure to the canted antiferromagnetic spin structure (AFmc), while T_{C}~42 K denotes the first order magnetic transition from AFmc to canted ferromagnetism (Fmc+F(Nd)) due to ordering of the Mn and Nd sub-lattices. The maximum values of the magnetic entropy change and the adiabatic temperature change, around T_{C} for a field change of 5 T are evaluated to be -ΔS_{M}~15.9 J kg^{-1}K^{-1} and ΔT_{ad}~5 K, respectively. The first order magnetic transition associated with the low levels of hysteresis losses (thermal 1.7Cr_{0.3}Si_{2} offers potential as a candidate for magnetic refrigerator applications in the temperature region below 45 K.

Furthermore, the structural and magnetic properties of NdMn_{2-x}Cu_{x}Si_{2} compounds (x=0–1.0) also have been investigated. Substitution of Cu for Mn leads to an increase in the lattice parameter a but a decrease in *c* at room temperature. Two magnetic phase transitions have been found for NdMn_{2-x}Cu_{x}Si_{2} compounds with T_{N} for the antiferromagnetic ordering of Mnsublatttice and T_{C} for the Nd-sublattice ferromagnetic ordering. T_{C} increases significantly with increasing Cu content from 36 K at x=0 to 100 K at x=1.0. Moreover, it is found that the order of magnetic phase transition around T_{C} also changes from first order at xM around T_{C} decrease with increasing x from 27 J kg^{-1}K^{-1} for x=0 to 0.5 J kg^{-1}K^{-1} for x=1.0 under 0–5 T field. Refinement of neutron diffraction patterns for x=0.2 confirms the magnetic states detected by magnetic study and also indicates that the lattice constants a and c show a distinct variation around T_{C}. Moreover, further study on substitution Mn with V in NdMn_{2}Si_{2} compound shown the similar behaviour with the replacement Mn by Ti. Both T_{C} and T_{N} are found decrease with increasing V concentration accompany with decreasing magnetic entropy change as discussed in more detail in Chapter 5.

Secondly, we carry out investigations of the Pr_{1-x}Y_{x}Mn_{2}Ge_{2} magnetic phase diagram as functions of both composition and Mn–Mn spacing using X-ray and neutron diffraction, magnetization and differential scanning calorimetry measurements. Pr_{1-x}Y_{x}Mn_{2}Ge_{2} exhibits an extended region of re-entrant ferromagnetism around x=0.5 with re-entrant ferromagnetism at T_{C}^{Pr}~ 50 K for Pr_{0.5}Y_{0.5}Mn_{2}Ge_{2}. The entropy values -ΔS_{M} around the ferromagnetic transition temperatures T_{C}^{inter} from the layered antiferromagnetic AFl structure to the canted ferromagnetic structure Fmc (typically T_{C}^{inter}~330–340 K) have been derived for Pr_{1-x}Y_{x}Mn_{2}Ge_{2} with x=0.0, 0.2, and 0.5 for ΔB=0–5 T. The changes in magnetic states due toY substitution for Pr are also discussed in terms of chemical pressure, external pressure, and electronic effects.

Thirdly, the structural and magnetic properties of CeMn_{2}Ge_{2-x}Si_{x} compounds with Si concentrations in the range x = 0.0–2.0 have been investigated. Substitution of Ge with Si leads to a monotonic decrease of both a and c along with concomitant contraction of the unit cell volume and significant modifications to the magnetic states - a crossover from ferromagnetism at room temperature for Ge-rich compounds to antiferromagnetism for Sirich compounds. The magnetic phase diagram has been constructed over the full range of CeMn_{2}Ge_{2-x}Si_{x} compositions and co-existence of ferromagnetism and antiferromagnetism has been observed in both CeMn_{2}Ge_{1.0}Si_{1.0} and CeMn_{2}Ge_{0.8}Si_{1.2} compounds with novel insight provided by high resolution X-ray synchrotron radiation studies. This study has enabled the large variety of magnetic structures and magnetic phase transitions of CeMn_{2}Ge_{2-x}Si_{x} compounds and their related magnetic properties to be determined by controlling chemical concentration.

Finally in Chapter 6, the MnCoGe-based materials, as a typical MM′X (M, M′ = transition metal, X = Si, Ge, Sn) compound which undergoes a second-order phase transition as well as a crystallographic phase transition from the low temperature orthorhombic TiNiSi-type to the high temperature hexagonal Ni_{2}In-type structure have been studied. An investigation on substituting Ge by other metalloids in MnCoGe_{1-x}T_{x} compounds (T = Al and Si) has been implemented in this thesis and it was found that an appropriate T concentration successfully shifted the structural change and magnetic phase transition into the temperature range of interest, leading to the attainment of a high contribution to the giant magnetocaloric effect (GMCE). MnCoGe_{1-x}T_{x} provides the best example for control of the temperature window in order to investigate the effects of the structural and magnetic transition on the total entropy change, providing an excellent vehicle for investigation of the field-induced martensitic transformation in GMCE materials. Thus, in an effort to understand the nature of the magnetic transition in MnTiGe_{0.97}Al_{0.03}, critical exponent analysis in the vicinity of the ferromagnetic (FM)–paramagnetic (PM) region has been performed. The outcomes revealed that this material undergoes a structural transition at ~ 420 K as well as a second-order ferromagnetic–paramagnetic transition at ~ 350 K. Finally, a temperature dependent neutron diffraction experiment has been performed and confirmed that there is a coupling of the structural transition and the magnetic phase transition.

Structural, magnetic and magnetocaloric properties of the Mn_{1-x}Ti_{x}CoGe also have been investigated using X-ray diffraction, DC magnetization and neutron diffraction measurements in order to define the effect of substitution Mn with Ti in MnCoGe compound. Substitution of Ti for Mn in the parent MnCoGe compound leads to a significant reduction in both structure change temperature, T_{str} (from ~ 645 K for MnCoGe to ~ 235 K for Mn_{0.94}Ti_{0.06}CoGe and ~ 178 K for Mn_{0.9}Ti_{0.1}CoGe) and Curie temperature, T_{C} (from ~ 345 K for MnCoGe to ~ 270 K for Mn_{0.94}Ti_{0.06}CoGe and ~ 280 K for Mn_{0.9}Ti_{0.1}CoGe). Moreover, all the critical exponents for Mn_{0.94}Ti_{0.06}CoGe and Mn_{0.9}Ti_{0.1}CoGe fulfil the Widom scaling law. The validity of the calculated critical exponents was confirmed by the scaling equation, with the magnetization, field, and temperature data obtained below and above T_{C} collapsing onto two different curves. Thus, the scaling of the magnetization data above and below T_{C} was obtained using the respective critical exponents, and the consistency of the values of the critical exponents determined by different methods confirm that the calculated exponents are unambiguous and intrinsic. The critical exponents determined are close to those predicted by the mean-field theory for long range interactions.