We observe an unusual behavior of the spin Hall magnetoresistance (SMR) in Pt deposited on a tensile-strained LaCoO3 (LCO) thin film, which is a ferromagnetic insulator with the Curie temperature Tc=85K. The SMR displays a strong magnetic-field dependence below Tc, with the SMR amplitude continuing to increase (linearly) with increasing the field far beyond the saturation value of the ferromagnet. The SMR amplitude decreases gradually with raising the temperature across Tc and remains measurable even above Tc. Moreover, no hysteresis is observed in the field dependence of the SMR. These results indicate that a novel lowdimensional magnetic system forms on the surface of LCO and that the LCO/Pt interface decouples magnetically from the rest of the LCO thin film. To explain the experiment, we revisit the derivation of the SMR corrections and relate the spin-mixing conductances to the microscopic quantities describing the magnetism at the interface. Our results can be used as a technique to probe quantum magnetism on the surface of a magnetic insulator.Introduction.-Magnetoresistance has been key for understanding spin-dependent transport in solids [1]. In the last years, new magnetoresistance phenomena were discovered in thin ferromagnetic/normal metal(FM/NM)-based heterostructures [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18], which originate from the interplay of the spin currents generated in the heterostructure (via the spin Hall effect [19][20][21][22] or the Rashba-Edelstein effect [23,24]) with the magnetic moments of the FM layer. Among 2 many applications, these magnetoresistance effects have been used for quantifying spin transport properties such as the spin diffusion length and the spin Hall angle SH of different NM layers, or the spin-mixing conductance ↑↓ of FM/NM interfaces. More interestingly, unlike other surface-sensitive techniques that suffer from a bulk contribution due to a finite penetration depth, the spin Hall magnetoresistance (SMR) [4][5][6][7][8][9][10][11] uses the spin accumulation at interfaces for sensing the magnetic properties of the very first atomic layer of magnetic insulators (MIs) [25,26]. For instance, SMR has been employed for probing the surface of complex magnetic systems such as ferrimagnetic spinel oxides [11,27], spin-spiral multiferroics [28,29], canted ferrimagnets [30], Y3Fe5O12/antiferromagnetic (YIG/AFM) bilayers [31,32], and synthetic AFMs [33].LaCoO3 (LCO) presents an intriguing magnetic behavior, which has been studied for decades and is still under debate [34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49]. Bulk LCO is a diamagnetic insulator at low temperature, owing to the low-spin (LS) configuration of Co 3+ . The relatively small crystal-field splitting of the Co 3+ 3d-shell results in an increasing population of high-spin (HS) Co 3+ with temperature, reaching 1:1 (LS:HS) above ~150K. The close proximity between crystal-field splitting and exchange energy makes the magnetic properties of LCO particularly susceptible to small c...