Hard magnetism in novel glassy bulk Nd-Fe-Al

Abstract: This thesis focuses on the magnetic properties of a newclass of Nd-based bulk glassyhard magnet, by measuring themagnetization in AC- and DC- fields up to 30 tesla, in thetemperature range 1 - 570 K. The main results of ourinvestigations and analyses in the framework of existing modelsare summarized as follows:1) The bulk glassy Nd-Fe-Al is found to be magneticallyisotropic, with hysteresis losses Wh?120 kJ/m3and maximum energy product (BH)max?2.1 kJ/m3, normalized to the volume of the alloy, at 300 K.Because of the existence of Fe-, and Nd-rich regions,evidencedby TEM structural analyses, the energies corresponding to the(Fe-rich) Fe-Nd hard magnetic entities evaluates to be Wh?460 kJ/m3and (BH)max?26 kJ/m3.2) The magnetization at very low temperatures and at highmagnetic fields (<= 30 Tesla) provides evidence for twodifferent magnetic behavior of the main short range orderedentities which may be described as: A cluster-glass hardmagnetic Fe-Nd phase, exhibiting square-like hysteresis loops(&&micro;0Hc=8.4 tesla,&&micro;0MS=1.4 tesla, Wh?20 MJ/m3and(BH)max?96 kJ/m3, at 1.26 K), embedded in the Nd-rich matrix, whichresults in unsaturated sigmoidal loops of vanishing remanenceand coercivity at low temperatures. We thus have a magneticallygranular and yet structurally glassy hard magnetic materialThe Nd-matrix3) The low field AC-susceptibility measurements suggests amultiphase type of magnetic structure: The existence of variousatomic environments for the Nd-matrix, reminiscent of a dhcp(antiferromagnetic, with two Néel temperatures 7.5 and 20K for two unequivalent crystallographic sites), and fcc-(ferromagnetic, with Curie temperature 30 K) allotropic formsof crystalline Nd, in addition to another Nd-Fe-(Al) phase withtransition temperature ?70 K.4) Assuming Nd contributes with its free ion moment, viz.3.27&micro;B, it is found from our data that 5/6 fraction ofthe Nd content forms the matrix, leaving the composition Fe3Nd for the hard magnetic phase. The mean fieldmicroscopic exchange coupling constant and atomic single ionanisotropy constant are found to be JNd-Nd?2.0x10-23J and D?8.1x10-23J,respectively, in reasonable agreement withdata in literature. The comparable exchange and anisotropyenergies of a Nd moment suggests the competition among the twoin the low temperature magnetic behavior, which isqualitatively explained by models of random anisotropymagnetism with antiferromagnetic exchange interactions.The Fe3Nd phase5) The temperature dependence of the coercive field of theFe-Nd hard magneticphase increases exponentially withdecreasing temperature below ?70 K. This temperature marks thetransition between two distinct magnetic characteristics:a) low temperature behaviorBelow 70 K, the Stoner-Wolfarth, SW, type of loop evolvestowards a random-axis Ising type of loop, which stems from thefreezing of the Fe3Nd cluster moment alongits local anisotropy axisbelow ?4 K. Because the easy axes are distributedisotropically, the ensemble of clusters form asperomagnetic-like structure of frozen moments in the thermallydemagnetized state below 4 K.A step-like magnetization process is observed below 3.5 K,whose pattern is found to depend upon the temperature, initialmagnetic state, and the external field sweep rate. Thissuggests flux reversal along the easy axes of frozen momentstowards the external field direction. The quantitativeinterpretation of our experimental data suggest a dispersion inanisotropy in the range 5 - 20 MJ/m3, with a mean value 10.5 MJ/m3, at 4 K.b)high temperature behaviorAbove ?70 K, the paramagnetism of the interfacial Nd-matrixisolates magnetically theFe3Nd clusters embedded in it. The hysteresis loopsare well described by SW type of loops which depends on 1)random orientation of anisotropy easy axes, 2) dispersion inanisotropy in the range 5 - 20 MJ/m3, with mean 9.0 MJ/m3, at 80 K, and 3) thermal activation effects overanisotropy energy barriers, with activation volume ?6.5 nm3, at 80 K,assuming Arrhenius kinetics of therelaxation process.6) Our data suggest that thermal activation effects are themain cause for the decrease of the coercivity with increasingtemperature in the range 1 - 80 K.7) Magnetic relaxation data above room temperature areanalyzed within existing models, as well as within a modeldeveloped in this thesis, which considers coherent rotation asthe process of magnetization. These results are in reasonableagreement among the models discussed for the derived activationvolume and anisotropy energy barriers.8) Analyses of the critical behavior indicates a magnetictransition at 465 K from a high temperature paramagnetic phaseto a low temperature ferromagnetic-like orasperomagnetic phase,with critical exponents ? =1.5(1), ? =0.65(5) and,from the scaling relation ? =1+? /? , d=3.3(3).These exponents correlate with the universality class proposedby a model that considers long range order ferromagnetismdestabilized by weak randommagnetic anisotropy.