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Magnetotransport and magnetocaloric effects in intermetallic compounds
Duijn, H.G.M.
Publication date
2000
Link to publication
Citation for published version (APA):
Duijn, H. G. M. (2000). Magnetotransport and magnetocaloric effects in intermetallic
compounds.
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Summary y 179 9
Summary y
Inn bulk intermetallic compounds, temperature- and field-induced magnetic phase transitionss from antiferromagnetic to ferromagnetic alignment of the magnetic moments may bee accompanied by a substantial change of the electrical resistivity. The origin of the large (magneto)resistancee effects observed lies in spin-dependent scattering of the conduction electrons.. In intermetallic compounds, spin-polarised transport may arise from band-structure effects,, as well as from magnetic interactions. In the former case, the spin-dependent scatteringg probability is due to a different density of possible final states for up and spin-downn electrons (e.g. the Mott model), while in the latter case it is due to a difference in the scatteringg amplitudes (e.g. the s-d model of Van Peski-Tinbergen and co-workers). In order too gain a deeper insight in the mechanism responsible for the (magneto)resistance effects in intermetallicc compounds, a detailed knowledge of the magnetic properties is a prerequisite.
Thiss thesis deals with an experimental study of the electrical-transport and magnetic propertiess of several systems of intermetallic compounds: (Hf,Ta)Fe2, Fe3(Ga,Al)4, GdT2Si2
(TT = transition element), and RMn6Ge6 (R = rare-earth element). The common property of thesee systems is an antiferromagnetic(-like) phase that can be modified into a ferromagnetic phasee by changing the temperature and/or by application of a magnetic field. Additionally, we havee investigated some physical properties of the system Gd5(Ge,Si)4, for which an
extraordinarilyy large magnetocaloric effect has been reported. Below, we summarise the main resultss obtained on the various systems.
Withh decreasing temperature, for 0 . 1 0 ^ x ^ 0 . 2 5 , the pseudobinary system Hfi.jtTaJCFe22 undergoes a first-order phase transition from an antiferromagnetic state to a
ferromagneticc state. The magnetic structures have been determined by means of magnetisationn measurements, powder-neutron diffraction and single-crystal neutron-diffractionn experiments. Additionally, application of the theory of group representations to the systemm (Hf,Ta)Fe2 has proved to be elucidative in the magnetic-structure analysis. The
measuredd magnetisation curves and the corresponding magnetic phase diagram of Hfi.*TaJFe2 aree well described in terms of the theory of magnetic phase transitions of Moriya and Usami. Inn this theory, the competition between ferromagnetic and antiferromagnetic spin fluctuations givess rise to temperature- and field-induced magnetic phase transitions. Evidence for the presencee of spin fluctuations in Hfi.jTaJ^ is found in the quadratic temperature dependence off the electrical resistivity below 40 K. For Hfi.^TaJ^, the magnetic interactions within a Fe 6/ii plane give rise to ferromagnetic ordering. In the high-temperature antiferromagnetic state, thee 6h planes are coupled antiferromagnetically, while the Fe la site is not magnetically ordered.. With decreasing temperature, the competition between the la—la interaction and thee 6/i—6h interplane interaction gives rise to a first-order phase transition to a ferromagnetic state.. In both magnetic structures, the magnetic moments are directed within the basal plane.
180 0 Summary y Thee magnetic-field dependence of the electrical resistivity is well described with the model of Vann Peski-Tinbergen and co-workers, indicating that in the system Hf[_jTa;tFe2
spin-dependentt scattering contributes substantially to the resistivity and is due to a difference in scatteringg amplitudes of the spin-up and spin-down conduction electrons.
Thee compound Fe3Ga4 shows a behaviour similar to (Hf,Ta)Fe2: it orders antiferromagneticallyy at about 390 K, while below about 50 K it behaves like a ferromagnet. Fe3Ga44 crystallises in a complex monoclinic structure, which brings about ferromagnetic as welll as antiferromagnetic interactions. The relative values of these interactions are strongly temperature,, field and pressure dependent, giving rise to magnetic phase transitions. We have foundd that the temperature at which the ferromagnetic to antiferromagnetic transition takes place,, strongly increases upon application of hydrostatic pressure, and upon the substitution off Al for Ga. Furthermore, we have constructed a magnetic phase diagram of Fe3(Gao.98Alo.o2)4,, which is more complex than the theoretical phase diagrams calculated by
Moriyaa and Usami. The electrical-transport properties of Fe3(Gai_^Alj)4 are dominated by the effectss of spin fluctuations. This is clear, for example, from the quadratic temperature dependencee of the electrical resistivity at low temperatures, and the characteristic maximum inn the magnetoresistance at the field-induced transition. We have found several indications thatt band-structure effects substantially influence the magnetic and transport properties of Fe3(Gai.(Ay44 compounds. In conclusion, it seems that the occurrence of the interesting
phenomenaa in the Fe3(Gai.^Al^)4 compounds is intimately related to the complexity of the system,, which limits a detailed analysis of the physical properties.
Thee Gd sublattice of GdT2Si2 compounds orders antiferromagnetically at low temperatures.. The temperature dependence of the electrical resistivity arising from magnetic interactionss in GdT2Si2 compounds generally shows a pronounced kink at the magnetic orderingg temperature TN. The compounds GdRu2Si2, GdAg2Si2 and GdPt2Si2 show a
maximumm in the electrical resistivity around TN. This behaviour is attributed to the opening of
aa gap at the Fermi surface due to the formation of superzone boundaries arising from the antiferromagneticc structure. Furthermore, for most of the GdT2Si2 compounds we have
observedd additional anomalies in the electrical resistivity, which we attribute to magnetic phasee transitions. Also, various field-induced transitions are found in the field dependence of thee electrical resistivity. The effects in the magnetoresistance can often be attributed to the removall or opening of gaps at the Fermi surface, yielding resistance effects up to 70 %. Often thee field-induced transitions show up more pronounced in the magnetoresistance than in the magnetisation.. Hence, measurement of the magnetoresistance proves to be a valuable technique,, because it often serves as a sort of 'magnifying glass' for determining transition fields. .
RMn6Ge66 compounds possess complex magnetic structures due to the existence of
magneticc moments on the R and Mn sublattices that interact in a complex way. Furthermore, mostt of the RMnöGee compounds undergo spin-reorientation transitions as a function of temperature.. Electrical-resistivity measurements on these compounds have proved that there aree in principle only moderate (magneto)resistance effects arising from the magnetic interactions.. As the R and Mn magnetic moments are thought to be well localised, we
Summary y 181 1 attributee this to the only limited interaction between the electrons carrying a magnetic momentt and the conduction electrons. Nevertheless, considerable (magneto)resistance effects mayy arise due to the formation of superzone boundaries at the Fermi surface. In RMriéGee compounds,, this situation is met for R = Er.
Forr 0.25 £ x £ 0.50, the system Gd5(Gei.jSi^)4 shows a magnetic phase transition, that iss accompanied by an extraordinarily large magnetocaloric effect. X-ray-diffraction measurementss as a function of temperature have pointed out that the magnetic phase transitionn is accompanied by a simultaneous structural phase transition. The entropy change associatedd with the magnetic/crystallographic phase transition has been calculated from magnetisationn data using a thermodynamic Maxwell relation, and is found to be close to or evenn larger than the upper limit R ln(2J+l). Since the magnetic-ordering transition is accompaniedd by a crystallographic phase transition, latent heat gives rise to an additional contributionn to the magnetocaloric effect. Magnetisation curves measured on a single crystal off GdsGe2.4Sii.6 display anisotropic behaviour, i.e.: the magnetisation measured in a field of 55 T applied along the b axis is lower than the magnetisation measured in a field of 5 T applied alongg the a and c axes. A clue on the origin of this anisotropy is given by the results of symmetryy considerations. We have generated the magnetic-moment configurations of Gd5(Gei.xSiJt)44 that are allowed by symmetry by means of the theory of group representations.
Theree are several magnetic-moment configurations for which no component of the magnetic momentt along the b axis is allowed at one specific Gd site, while the components of the magneticc moment along the a and c axes may have a non-vanishing size. Hence, we conclude thatt the symmetry of the Gd5(Gei.^SU)4 lattice imposes confinements upon the magnetic structure,, that yields a measurable anisotropy in the magnetic properties. Finally, the electricall resistivity of Gd5Ge2.4Sit.6 changes 20 % at the magnetic/crystallographic phase transition,, and can be interpreted as being due to changes in the band structure.
