HAEMOPHILUS INFLUENZAE TIPO B (HIB)

The summary of the results of the field dependent magnetisationM(H)and its derivative

dM/dH, withHapplied along thea,b, andcaxes of SrHo2O4at 2 K and 0.5 K are shown

0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 2.5 H = 100 Oe / H || a ZFC / FC / H || b ZFC / FC / H || c ZFC / FC ( e m u / O e m o l H o ) T (K) 0.5 1.0 1.5 2.0 0.5 1.0 1.5 2.0 2.5 3.0 H = 1 kOe H || a H || b H || c ( e m u / O e m o l H o ) T (K)

Figure 5.13: Low temperature magnetic susceptibility obtained in the temperature range of 0.5 to 2 K for the three principal axes of SrHo2O4 in applied fields of (top) 100 Oe and

(bottom) 1 kOe. Dashed line represents 0.62 K, where a cusp is observed in the susceptibil- ity at low fields, and also the temperature at which a difference between Zero-Field-Cooled (ZFC) and Field-Cooled (FC) measurements becomes apparent. In higher applied fields, there is very little difference between ZFC and FCχ(T)measurements for SrHo2O4.

0 2 4 6 0 1 2 3 4 5 6 0 2 4 6 8 0 1 2 3 4 5 6 7 M ( B / H o ) H || a H || b H || c d M / d H ( B / H o T ) 0 H (T) T = 2.0 K 0 H (T) T = 0.5 K 0.0 0.5 1.0 1.5 0 2 4 6 8 d M / d H ( B / H o T ) 0 H (T)

Figure 5.14: Field-dependent magnetisation curves (top panels) for SrHo2O4 obtained at

2.0 K (left) and 0.5 K (right) in the field range of 0 to 7 T. (Bottom panels) The field derivatives of the magnetisation at 2.0 K (left) and 0.5 K (right). Inset: the low field part of the 0.5 K dataset for the field applied along theb axis in more detail. The dashed line indicates 0.8 T, the value of the field around which the plateau in dM/dH is observed at approximately one third of the value for the fully saturated moment. In this case, this observation suggests that a collineartwo-spins-up-one-spin-down(uud) structure is being stabilised around 0.8 T when the field is applied along thebaxis.

nature of this rare-earth oxide.

Theaaxis is clearly a hard magnetisation direction, anddM/dHfor a field applied along this direction remains small and nearly flat at all fields and at all measured temper- atures. At 0.5 K, forH ∥ b, initially, the magnetisation rises sharply with a maximum in

dM/dHatµ0Hc1≈0.59T, and so for a small region of the applied field the magnetisation

shows much slower growth (with a minimum indM/dHseen at 0.8 T), and then another sharp rise up to a second maximum indM/dHatHc2 ≈1.2T.Hc1andHc2thus confine a narrow plateau with an average magnetisation value of 2.5µB, which equates to roughly a third of the saturation magnetisation value observed aboveHc2. These types of plateaux are a sign of a field induced stabilisation of a collineartwo-spins-up-one-spin-down(uud) magnetic structure, in which on each triangle of spins, two are pointing up along the field and the third spin pointing down anti-parallel to the field direction [102, 103]. On warming the sample above 0.5 K the magnetisation plateau betweenHc1 andHc2 forH ∥bgradu- ally disappears, and by 2 K, shown in the lefthand panels of Fig. 5.14,dM/dHonly shows a single broad maximum. This temperature behaviour implies that on further cooling below 0.5 K, the plateau in magnetisation is expected to become even better defined and perhaps occupy a more extensive region betweenHc1andHc2.

ForH∥c, at 0.5 K, the magnetisation process is characterised by a single transition (seen as a sharp maximum indM/dH) atµ0Hc = 0.4 T. The nature of this field-induced transition cannot be known using only magnetisation measurements, and instead will be discussed in Section 5.4.1 which reports the relevant single crystal neutron scattering data. Upon raising the temperature, the maximum indM/dHforH ∥cgradually becomes less pronounced. By 2 K, this maximum is significantly broader and shifts in field to 0.5 T.

Above 4 T the magnetisation curves for thea,bandcaxes of SrHo2O4look rather

featureless, however, no complete saturation of magnetisation is observed at any temper- ature for any of the directions studied, as thedM/dH values remain nonzero even in the highest applied field of 7 T. This implies that the spins are still not fully aligned at this field, which is not surprising given the fact that the observed values of magnetisation re- main much lower than what is expected from Hunds rules predictions for Ho3+ ions [3]. No hysteresis with the applied field is observed for the field applied along any of the prin-

cipal crystal axes of SrHo2O4. The demagnetisation corrections only have a small impact

on the position of the field-induced phase transitions in SrHo2O4, limited to shifts of a few

percent at most.