Magnetocaloric effect of YbRh2Si2 at field-induced QCP

We develop a system to measure a magneto-caloric effect, dT/dH, at low temperatures down to ∝50 mK to study the field-induced QCP of YbRh2Si2. The key quantity for a study of field-induced QCP called magnetic Grueneisen ratio, Γmag=1/T(dT/dH)=1/T(dT/dH)=-(dM/dT)/C (M: magnetization, C: specific heat), for this compound was previously obtained by measuring both M and C at the critical field Hcr=0.06 T and above. We obtained Γmag by measurements of dT/dH, which have the following advantages over the combination of M and C measurements. (1) Γmag can be obtained at very low fields (and also at zero-field), while it is very difficult with M and C because M becomes very small at low fields (impossible at zero-field because magnetization is zero). (2) Since it is a single measurement, possible error in Γmag is expected to be smaller compared to the two separated measurements combined. We discuss the details of the measurement technique and the results on YbRh2Si2 around Hcr, including the un-explored field-range below Hcr.