A magnetoresistive sensor and a manufacturing method thereof are provided. The method includes: forming an initial reference layer in an annular shape, wherein the initial reference layer includes an anti-ferromagnetic layer and a ferromagnetic layer; performing a heat treatment on the initial reference layer, wherein the ferromagnetic layer is magnetized to have a magnetization direction oriented along a vortex path during a heating step of the heat treatment, and an exchange bias oriented along the vortex path is induced at an interface of the anti-ferromagnetic layer and the ferromagnetic layer during a cooling step of the heat treatment; patterning the initial reference layer to form separated reference layers, wherein the reference layers are respectively formed in a annular sector shape, and the reference layers are arranged along the vortex path; forming spacer layers and free layers to form magnetoresistive devices; routing the magnetoresistive devices to form the magnetoresistive sensor.

A seed layer stack with a uniform top surface having a peak to peak roughness of 0.5 nm is formed by sputter depositing an amorphous layer on a smoothing layer such as Mg where the latter has a resputtering rate 2 to 30 that of the amorphous layer. The uppermost seed layer is NiW, NiMo, or one or more of NiCr, NiFeCr, and Hf while the bottommost seed layer is one or more of Ta, TaN, Zr, ZrN, Nb, NbN, Mo, MON, TIN, W, WN, and Ru. Accordingly, perpendicular magnetic anisotropy in an overlying magnetic layer is substantially maintained during high temperature processing up to 400 C. and is advantageous for magnetic tunnel junctions in embedded MRAMs, spintronic devices, or in read head sensors. The amorphous seed layer is SiN, TaN, or CoFeM where M is B or another element with a content that makes CoFeM amorphous as deposited.

A method of forming a seed layer stack for a magnetic device may include depositing a bottom seed layer, forming at least one pair of smoothing layers over the bottom seed layer, and depositing a top seed layer over and abutting the at least one pair of smoothing layers. Forming at least one pair of smoothing layers may include sputter depositing a sub-smoothing layer over the bottom seed layer and sputter depositing an amorphous sub-smoothing layer over and abutting the sub-smoothing layer. A top surface of as-sputter deposited sub-smoothing layer has a first top surface roughness, and the sputter depositing of the amorphous sub-smoothing layer causes re-sputtering of the sub-smoothing layer, such that the top surface of the as-sputter deposited sub-smoothing layer has a second top surface roughness less than the first top surface roughness. The sub-smoothing layer, the bottom seed layer, and the top seed layer include different materials.